WO1997022634A1

WO1997022634A1 - Method for determining degradation rates of degradable materials

Info

Publication number: WO1997022634A1
Application number: PCT/US1996/020758
Authority: WO
Inventors: Georgette Siparsky; Kevin H. Schilling
Original assignee: Georgette Siparsky; Schilling Kevin H
Priority date: 1995-12-19
Filing date: 1996-12-19
Publication date: 1997-06-26
Also published as: AU1354397A

Abstract

The present invention is directed to a method for reducing waste accumulation by using an environmentally degradable disposable material. The disposable material, which includes a hydroxycarboxylic acid-containing polymer, degrades hydrolytically during operative and disposal stages in a controlled manner such that the disposal degradation rate of the material is accelerated relative to the operative degradation rate of the material.

Description

METHOD FOR DETERMINING DEGRADATION RATES OF DEGRADABLE MATERIALS

Field of the Invention The present invention relates generally to a method for formulating compositions of biodegradable disposable materials for specific environments and specifically to a method for predicting the degradation rates of the compositions based on the temperature and free water content of the environment of use.

Background

There is a need for an environmentally degradable disposable material as a potential replacement for the tremendous amount of conventional plastic materials which, when discarded, do not degrade well. A significant amount of these plastic materials are discarded and become pollutants that deface the landscape and threaten marine life. A further problem with the disposal of non- degradable plastics is the concern for dwindling landfill space. It has been estimated that most major cities will have used up available landfills for solid waste disposal during the 1990's. Plastics currently comprise approximately 8 percent of the weight of and about 17 percent of the volume of solid waste.

A number of biodegradable materials have been described, including polymers of some hydroxycarboxylic acids, such as lactic acid, which are attractive because of their bioco patible and thermoplastic nature. Polymers of hydroxycarboxylic acids can be degraded into monomers and small oligomers over time by hydrolysis under most environmental conditions. The resulting monomers and small oligomers are then readily taken up by organisms in the environment and can be aerobically converted to carbon dioxide and water or anaerobically converted to carbon dioxide and methane.

In formulating the composition and molecular weight of a degradable material for a specific application, it would be useful to have a methodology for predicting the degradation rate of the degradable material under specified conditions of storage and/or use. Based on the degradation rate, the composition and molecular weight of the degradable material would be selected to provide for stability of the material throughout its useful life, which spans from when the material was produced through storage and use stages, and instability after disposal.

Summary of the Invention The present invention is directed to a method for predicting the degradation rate of a hydrolytically degradable material, such as polymers of α- hydroxycarboxylie acids. The method permits a specific composition of the degradable material to be selected to yield a desired degradation rate and therefore a desired useful life under selected conditions of use and/or a desired life after disposal. The method reduces or entirely eliminates the expensive and time consuming techniques presently used to predict degradation rates. Such techniques place the degradable material either in the selected use conditions and periodically take measurements of the degree of degradation or in accelerated use conditions with the anticipated degradation under the selected use conditions being extrapolated from the degradation measurements under the accelerated conditions. In contrast, the present invention permits the degradation rate for a given degradable material to be accurately predicted based on the measured or estimated water content of the degradable material for selected material compositions and conditions of use (i.e., relative humidity and temperature) . The degradation rate is related to the water content of the material and can be determined from the water content. The relationship can be determined based on experimental measurements of the degradation rate and water content. For a given composition of degradable material, the degradation rate is estimated for the anticipated conditions of use by reference to previously measured water contents for that particular composition of material at various relative humidities and temperatures. Using previously measured water contents for that composition of material, it is also possible to extrapolate degradation rates for points not previously measured.

In one method to estimate the water content (which is generally the equilibrium water content) of the degradable material under specified conditions of temperature and the free water content of the ambient environment (e.g. , water vapor partial pressure, relative humidity, and the like) , the solubility coefficient is measured. As used herein, "solubility coefficient" refers to the thermodynamic term that describes the amount of permeant that will dissolve in a given quantity of polymer under a given driving force (e.g., free water content of the ambient environment). The solubility coefficient thus relates the equilibrium water content of the degradable material to the free water content of the surrounding environment. The equilibrium water content is related to the solubility coefficient and free water content of the ambient atmosphere.

In one embodiment of the present invention, a process is provided for determining the initial molecular weight of the degradable material for a specified useful life of the degradable material under specified conditions of use. The method includes the steps: (i) selecting a final molecular weight of the degradable material at the end of the desired useful life of the material; (ii) determining the water content of the degradable material during the period of use; (iii) projecting, based on the water content, the loss in molecular weight for the degradable material during the period of use; and (iv) selecting an initial molecular weight (at the beginning of the period of use) that is no less than the sum of the final molecular weight and the molecular weight loss during the period of use. If the initial and final molecular weights and therefore the maximum permissible molecular weight loss during the period of use are known, the preceding steps can be reversed to determine the maximum permissible water content of the degradable material during the period of use. This is particularly useful where it is being determined for what environment(s) and/or applications a given composition of material can be used. In another embodiment, a method is provided for manufacturing a hydrolytically degradable material having a desired degradation rate. The method includes the steps: (i) selecting the temperature and relative humidity of the atmosphere to be contacted by the degradable material during use; (ii) projecting the degradation rate of the degradable material based on the water content of the degradable material; and (iii) based on the difference between the projected degradation rate and the desired degradation rate, varying the amount of a hydrophilic or hydrophobic additive in the degradable material to yield the desired degradation rate.

The hydrophilic additive can include at least one of a hydrophilic comonomer, a hydrophilic polymer block, a hydrophilic polymer, a blotting compound, an activator compound, a hydrophilic plasticizer, other water-absorbing compounds, and mixtures thereof. The hydrophilic additive is normally used when the projected degradation rate is substantially less than the desired degradation rate.

The hydrophobic additive can include at least one of a hydrophobic comonomer, a hydrophobic polymer block, a hydrophobic polymer, a hydrophobic plasticizer, other water-repelling compounds, and mixtures thereof. The hydrophobic additive is normally used when the projected degradation rate is substantially more than the desired degradation rate.

Brief Description of the Figure Figure 1 is a schematic depicting the degradation of a film that contains environmentally degradable disposable materials of the present invention.

Detailed Description of the Invention One embodiment of the present invention is an environmentally degradable disposable material which includes a hydroxycarboxylic acid-containing polymer and which degrades hydrolytically during operative and disposal stages in a controlled manner such that the disposal degradation rate of the material is accelerated relative to the operative degradation rate of the material. This material is discussed in detail in U.S. Patent Application "Degradation Control of Degradable Polymers" having Serial No. 07/949,675 filed September 22, 1992, which is incorporated herein by reference in its entirety.

An environmentally degradable disposable material of the present invention includes any one-way hydroxycarboxylic acid polymer-containing plastic that does not undergo significant degradation until it is discarded. As used herein, disposable materials of the present invention exist through two stages. The operative stage, during which there is minimal degradation, begins immediately following production of the material and continues until the material is discarded, but excludes any processing performed during this time period. As such, the operative stage includes the time during which the material is stored (sometimes called shelf-life) and used. In one embodiment in which disposable materials of the present invention are packaging materials, the operative stage can also include a packaging stage during which products are packaged using disposable materials (e.g., films or moldings) of the present invention (e.g., when containers composed of disposable materials of the present invention are filled with products such as milk or when products such as meat are packaged by enclosing them in disposable packaging materials of the present invention) . However, the operative stage does not include any processing steps, such as when the disposable materials are exposed to harsh conditions, including exposure to heat and moisture (e.g., orientation processing steps or other thermal forming steps) .

The disposal stage is the period of time extending from when the disposable material is discarded until the material is at least about 98 percent, and preferably about 100 percent, hydrolytically degraded, as defined hereafter. During the disposal stage, the disposable material, which usually has been discarded in a landfill, in water, or on a landscape surface, can decompose to smaller molecules which are environmentally benign. In one embodiment, the disposal stage also includes the use of other mechanisms to further degrade the disposable materials. Disposable materials can be produced by a variety of methods, including but not limited to, extruding, molding, coating, calendaring, laminating, spraying, and other methods known in the art. Suitable disposable materials of the present invention include, but are not limited to: films; foams; coatings; molded articles, including injection moldings, blown moldings, and thermoformed moldings; extruded articles, including those that are cast or oriented; non-woven fibers; pellets; powders; laminates; and adhesives. Preferred disposable materials include films, foams, coatings, and molded articles. Examples of disposable materials suitable for use in the present invention include, but are not limited to, packaging materials, such as food product containers (e.g., egg cartons and fast-food containers) , beverage containers (e.g., cups, cans, cartons, and six-pack containers), and packages containing a variety of other products; utensils; hardware items; hospital supplies, such as sheets, gowns, packages, sample containers, and syringes; bags, such as for trash or cleaning; diaper backings; shipping materials; and coatings, such as for paperboard or other containers.

Disposable materials of the present invention can degrade in a variety of ways, including, but not limited to, hydrolytic, biological, chemical, mechanical, photo, and/or thermal degradation. In accordance with the present invention, disposable materials are preferably degraded hydrolytically, but other degradation methods can be used to further degrade the disposable materials. Preferred methods to further degradation of the materials include biological degradation, such as microbial or biochemical (e.g., enzymatic) action; and thermal induction of degradation, such as exposure of the disposable materials to steam which can trigger degradation by increasing the water content of the materials. A more preferred method to further degrade the materials is microbial degradation. Suitable microorganisms to conduct microbial degradation include, but are not limited to, bacteria, yeast and other fungi, algae, protozoa, and mixtures thereof.

As used herein, hydrolytic degradation is the process by which moisture penetrates a disposable material of the present invention and hydrolyzes, for example, ester bonds, thereby breaking down polymers in the material. Without being bound by theory, hydrolytic degradation is thought to proceed through a series of somewhat overlapping steps including: (1) diffusion of water into the material; (2) initial hydrolysis yielding polymers with reduced molecular weight (i.e., conversion of polymers to oligomers); (3) continued loss of molecular weight (i.e., formation of smaller oligomers) and gradual loss of plasticizers incorporated into the material; (4) initial loss of physical properties (e.g., pliability); (5) loss of further properties resulting in an opaque and hazy material; (6) major loss of physical properties, such as tensile strength and form-stability; (7) weight loss; and (8) volume loss, until the material is essentially degraded to monomers or small oligomers. Typically, the obvious loss of physical properties correlates with a reduction in molecular weight of the polymer down to a number average molecular weight of about 50,000 daltons.

An example of a polymer film degradation profile is depicted in Figure 1, which correlates hydrolysis reactions with physical property changes in the degrading polymer. Figure 1 shows that the degradation process begins when water, which may obtained from the atmosphere or a liquid, diffuses into (i.e., moistens) the film. In addition, Figure 1 shows a series of hydrolytic reactions by which polyesters are hydrolyzed into low molecular weight esters, leading to a partly hydrolyzed film, which is then subjected further to hydrolysis into oligomers and finally into monomers and small oligomers that can be consumed by microorganisms. The Figure also shows that microbial action can convert monomers and small oligomers to carbon dioxide and either water or methane.

During the operative stage, disposable materials of the present invention are susceptible to moisture diffusion and, possibly, limited hydrolysis. During the disposal stage, as hydrolysis increases, additional carboxyl ends are formed which promote additional hydrolysis both by polarizing ester bonds and by serving as water reservoirs. As such, steps 2 through 5 (discussed herein before) of hydrolytic degradation proceed fairly rapidly, followed by the rest of the degradation process. For example, a disposable material containing uniaxially oriented L- and D,L-lactic acid-containing copolymers shows essentially no molecular weight change when stored in a dry room. However, when the same material is discarded into an environment of about 95°F and about 95 percent humidity, the molecular weight of the polymers begins to change within about 1 to about 3 weeks; the material becomes cloudy, opaque, and brittle within about 4 weeks. Between about 1 month and about 6 months, the material becomes extremely fragile and degrades to only about 1 percent to about 10 percent of its original molecular weight. The rate at which an environmental degradable disposable material degrades depends on a variety of factors including, but not limited to: polymer composition, including polymer type, molecular weight, amount and type of plasticizer, amount and type of residual monomer, crystallinity, orientation, intimacy of polymer/plasticizer mixture, and surface-to-volume ratio; as well as the moisture, pH, temperature, and biological activity of the environment into which the material is placed. Degradation rates can be measured using a variety of short- or long- term tests including, but not limited to, environmental chamber tests in which the temperature and humidity of the environment can be manipulated, boiling water tests, seawater tests, microbiologically active sewage sludge tests (such as the aerobic and anaerobic tests recommended by ASTM in the 1992 Annual Book of ASTM Standards, volume 08.03, Plastics fill) : D3100-latest. pages 815-822), and composing tests that simulate soil degradation. An advantage of environmental chamber tests is that the conditions are controlled and that degradation rates of disposable materials can be evaluated by accelerated short- term testing under which materials are exposed to harsh conditions. For example, when certain of the disposable materials of the present invention are subjected to conditions in which the temperature is about 95°F and the relative humidity is about 95 percent, the disposable materials typically degrade at about the same rate as the materials degrade in seawater in the summer, about 16 times as rapidly as the materials degrade in seawater in winter, about 2 to about 8 times as rapidly as materials degrade on a landfill, about 10 times as rapidly as the material degrades in a compost bed, and about 4 to about 100 times as rapidly as materials degrade when buried in a landfill, depending on the moisture content and microbial activity in the landfill.

The rate at which the material is degraded, i.e., the degradation rate, can be monitored by a variety of methods including changes in molecular weight, moisture penetration, stiffness, strength, hardness, weight, volume, shape, transparency, crystallinity, and glass transition temperature (Tg) of the disposable material. Preferably, the degradation rate is monitored by molecular weight changes or moisture penetration analyses of the disposable material. More preferably, the degradation rate is measured by monitoring changes in the average molecular weight (MW) of the polymers over time, using the formula:

(MW_t2 - MW_t1) / (t₂ - t,), wherein MW_t1 is the average molecular weight at a first time point t_t and MW_t2 is the average molecular weight of the material at a second time point t-. Molecular weight changes can be measured using a gel permeation chromatography device in which the molecular weight of the unknown is compared with polymer standards of known molecular weights. When the molecular weight becomes quite low (i.e., below about 2,000 daltons) , it is preferable to use high performance liquid chromatography to measure molecular weight changes.

Moisture penetration can be measured using the Karl Fischer method which is based on a pyridine reaction that can detect moisture at less than 100 ppm.

A preferred disposable material of the present invention is capable of being at least about 98 percent, and preferably about 100 percent, hydrolytically degraded after discard. As used herein, the term "about 100 percent hydrolytically degraded" means that, after being used and thrown away, the material can be degraded essentially to monomers and small oligomers that can be metabolized by microorganisms. (Microorganisms typically can consume hydroxycarboxylic acid-containing oligomers with molecular weights of up to about 1000 daltons, and preferably up to about 600 daltons, depending on the chemical and physical characteristics of the oligomer.) Preferably, disposable materials of the present invention are at least about 98 percent, and more preferably about 100 percent, hydrolytically degraded within about ten years after discard, more preferably within about five years after discard, and even more preferably within about three years after discard. Such time periods are preferable to the 500 to 1000 years that can be required for essentially non- degradable plastics now in use to break down. In addition, many such plastics contain environmentally toxic compounds in their formulation, such as ultraviolet light absorbers and heat stabilizers.

Exposure of the hydrolytically degraded material to microbial action leads to further degradation which can proceed until the microorganisms have converted the material essentially to carbon dioxide and either water or methane, depending on whether the environment is aerobic or anaerobic. As used herein, conversion of disposable materials of the present invention essentially to carbon dioxide and either water or methane refers to the ability of the microorganisms to either assimilate the degraded disposable material and/or to convert the disposable material to carbon dioxide and either water or methane, such that less than about 50 percent, and preferably less than about 1 percent of the original disposable material remains.

In accordance with the present invention, disposable materials of the present invention degrade in a "controlled manner", which refers to the material's ability to degrade more rapidly after having been discarded, or thrown away, than before; i.e., the material's disposal degradation rate is accelerated relative to the material's operative degradation rate. As used herein, an "operative degradation rate" is the average rate at which disposable materials of the present invention degrade during the time period spanning from the end of their production to the time at which they are discarded. As such, an operative degradation rate is influenced by the rate that a disposable material degrades during storage as well as during use (e.g., when items are being removed from a package or container, or when utensils are being used) . Thus, the operative degradation rate comprises the average of the storage degradation rate and the use degradation rate. The operative degradation rate does not include any processing steps that the disposable material might undergo during this stage, such as formation into a package, for example. Preferably, operative degradation rates are low and, as discussed above, are the result of water content potentially leading to limited hydrolysis. It is within the scope of the present invention that the operative stage may consist of more than one storage or use stage, in which case the operative degradation rate is determined by taking the average of the degradation rates of each stage. Degradation during processing is not taken into account.

As used herein, a "disposal degradation rate" refers to the rate at which a disposable material of the present invention degrades after the material is discarded. Preferably the disposal degradation rate is measured by determining the change in molecular weight of the disposable material during the disposal time period, which spans from the time the disposable material was discarded until the material is substantially degraded. It is desirable that disposable materials of the present invention degrade faster than non-degradable disposable materials and rapidly enough to avoid significant accumulation of discarded materials. In addition, the disposal degradation rate of disposable materials of the present invention is accelerated relative to the operative degradation rate of the disposable materials. That is, the average operative degradation rate, as measured by a decrease in the average molecular weight over time as described above, is less than about 50 percent of the disposal degradation rate, preferably less than about 33 percent, more preferably less than about 10 percent, and even more preferably less than about 5 percent of the disposal degradation rate. Preferably, the disposal degradation rate will be such that there is no appreciable net accumulation of waste at a disposal site.

One embodiment of the present invention is an environmentally degradable disposable material that includes a hydroxycarboxylic acid-containing polymer and that degrades hydrolytically during an operative stage and during a disposal stage in a controlled manner such that the disposal degradation rate of the material is accelerated relative to the operative degradation rate of the material. As used herein, a "hydroxycarboxylic acid- containing polymer" is a polymer that contains at least one type of hydroxycarboxylic acid. The polymer may also contain other materials, including those described in greater detail hereinafter. Preferred polymers are essentially non-toxic, odor-free, biocompatible, and biodegradable. A hydroxycarboxylic acid polymer can be produced by a number of methods including polymerization of: at least one type of hydroxycarboxylic acid; at least one type of cyclic ester of at least one hydroxycarboxylic acid; at least one type of polymer block, including an oligomer block, containing at least one type of hydroxycarboxylic acid or cyclic ester; and mixtures thereof. Polymers of the present invention may be copolymers of hydroxycarboxylic acids, cyclic esters, oligomers, or mixtures thereof. Polymers of the present invention can include other monomers or oligomers, including those that form non-degradable plastics. When copolymerized with hydroxycarboxylic acids, the "non- degradable" portion of the resulting material may, in fact, become degradable by virtue of shorter repeating lengths of the non-degradable portion. Disposable materials of the present invention can include internal or external plasticizers.

As used herein, a hydroxycarboxylic acid includes all of its derivatives that can form polyester linkages in whole or in part, such as esters, salts, and amides thereof. Preferred hydroxycarboxylic acids of the present invention are α-hydroxycarboxylic acids, but other hydroxycarboxylic acids in which the hydroxyl group is attached to a different carbon, such as, but not limited to, the beta-, gamma-, delta-, epsilon-, and/or omega- carbon, can also be used. Suitable α-hydroxycarboxylic acids include lactic acid, glycolic acid, tartaric acid, malic acid, mandelic acid, benzylic acid, hydroxy-valeric acid, 1-hydroxy-l-cyclo-hexane carboxylic acid, 2-hydroxy- 2-(2-tetrahydrofuranyl) ethanoic acid, 2-hydroxy-2-(2- furanyl) ethanoic acid, 2-hydroxy-2-phenylpropionic acid, 2-hydroxy-2-methylpropionic acid, 2-hydroxy-2-methyl- butanoic acid, 2-hydroxy-2-ethylhexylcarboxylic acid, α- hydroxybutyric acid, α-hydroxyisobutyric acid, α-hydroxy- pentanoic acid, α-hydroxyhexanoic acid, α-hydroxyheptanoic acid, α-hydroxyoctanoic acid, α-hydroxynonanoic acid, α- hydroxydecanoic acid, α-hydroxydodecanoic acid, α-hydroxy- myristic acid, α-hydroxypalmitic acid, α-hydroxystearic acid, α-hydroxyarachidic acid, α-hydroxybehenic acid, α- hydroxylignoceric acid, α-hydroxycerotic acid, α-hydroxy- oleic acid, α-hydroxylinoleic acid, α-hydroxylinolenic acid, α-hydroxyarachidonic acid, other α-hydroxycarboxylic acids having a carbon chain with an even number of carbon atoms, and mixtures thereof. Also suitable are α- hydroxycarboxylic acids with a carbon chain containing an odd number of carbon atoms. Examples include, but are not limited to, α-hydroxypelargonic acid, α-hydroxyundecanonic acid, α-hydroxytridecanoic acid, α-hydroxypentadecanonic acid, α-hydroxyheptadecanoic acid, and α-hydroxynonadec- anoic acid. Preferred α-hydroxycarboxylic acids include lactic acid, glycolic acid, tartaric acid, malic acid, mandelic acid, benzylic acid, valeric acid, α-hydroxy- butyric acid, α-hydroxyoctanoic acid, α-hydroxystearic acid, and mixtures thereof. More preferred α-hydroxy¬ carboxylic acids include lactic acid, glycolic acid, and mixtures thereof. Other preferred embodiments include lactones, such as caprolactone; aliphatic esters of glycols and dicarboxylic acids; and mixtures thereof.

In a preferred embodiment, disposable materials of the present invention include polylactic acid disposable materials (including polylactide disposable materials) , polyglycolic acid disposable materials (including polyglycolide disposable materials) , substituted polyglycolic acid disposable materials, caprolactone polymer disposable materials, valerolactone polymer disposable materials, and copolymers of two or more of these types.

In accordance with one embodiment of the present invention, the hydrolytic degradation rate of an environmentally degradable disposable material of the present invention can be controlled by a method which includes at least one of the following steps: (a) adding at least one activator compound to the material that accelerates degradation; (b) adding at least one blotting compound to the material that absorbs water or other hydrolytic degradation products; (c) coating the material with at least one coating compound; (d) producing a material comprising a copolymer; (e) adding at least one plasticizer to the material, including varying the amount and type of plasticizer added; (f) modifying the crystallinity, free volume, orientation, molecular weight, and/or surface area of the material; (g) applying a stress to the material; (h) adding at least one hydrophobic compound to the material; (i) adding at least one end- capping agent to the material; and (j) cross-linking the material. If the material is further degraded by microbial action, at least one source of nutrients can be added to the material in order to augment microbial degradation. According to the scope of the present invention, one of the aforementioned steps can produce the desired disposable material; however a combination of more than one of the steps can also be used to produce desired disposable materials. This method is particularly useful in commercial production, and especially large-scale commercial production, of environmentally degradable disposable materials of the present invention.

One embodiment of the present invention is the use of at least one activator compound that promotes degradation to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. Suitable activator compounds include, but are not limited, to acidic compounds, basic compounds that generate hydroxyl ions when exposed to water, moisture-containing compounds, and water. An activator compound can be incorporated into a disposable material such that when the activator compound is released, it promotes degradation of the disposable material. This property enables a disposable material having activator compounds to demonstrate an accelerated disposal degradation rate.

Acidic activator compounds are thought to promote hydroxycarboxylic acid polymer degradation because the acids not only can polarize ester bonds but they also attract water molecules, both of which lead to accelerated hydrolysis. A preferred acidic activator compound is a strong acid which has suitable melting and boiling points such that the acid will not volatilize during polymerization. Examples of such acidic activator compounds include fumaric acid, succinic acid, tartaric acid, malic acid, adipic acid, citric acid, glutamic acid, methane sulfonic acid, phosphoric acid, polyphosphoric acid, lactic acid oligomers; including acidic salts, hydrated forms, and easily hydrolyzed derivatives thereof; and mixtures thereof. A more preferred acidic activator compound is fumaric acid which has a melting point of 286°C and which is approved for food use.

Basic activator compounds are thought to promote hydroxycarboxylic acid polymer degradation because the hydroxyl groups can interact with the polymer to promote hydrolysis, thereby reducing polymer molecular weight and crystallinity. Suitable basic activator compounds include, but are not limited to, sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium hydroxide, ammonium borate, and mixtures thereof. A preferred basic activator compound is sodium bicarbonate. Water is directly responsible for the hydrolysis of hydroxycarboxylic acid-containing polymers. As such, it is a preferred activator compound. However, it is also possible to use as activator compounds moisture sources (i.e., moisture-containing compounds) that provide water. Suitable moisture-containing activator compounds include, but are not limited to, amylose, other starch-based hydrophilic polymers, cellulose-based hydrophilic polymers, hydrates of inorganic acids or salts thereof, and hydrates of organic acids or salts thereof. Water and moisture sources are advantageous because they can initiate the disposable material degradation process even in an environment that has very little water, such as a dry landfill. In one embodiment, a moisture source such as amylose can provide water to begin degradation of polylactic acid. As polylactic acid degradation leads to acid formation, the acid can promote degradation of the moisture source and of the hydroxycarboxylic acid polymer.

Activator compounds of the present invention can be incorporated directly into a disposable material or coated onto a disposable material. Preferably, activator compounds are microencapsulated in a capsule consisting of a material that retains the activator compound until the capsule is disrupted as the result of, for example, abrasion, mechanical pressure, heat, exposure to acid, or exposure to water, as could be found at a disposal site. Disruption of the capsule may be instantaneous or gradual. If a transparent disposable material is desired, the size of a capsule should be less than about 1 micron. For foam or other non-transparent disposable materials, a wider range of capsule sizes is acceptable, for example from about 1 micron to about 50 microns. Preferred capsular formulations include hydroxyacid-containing polymers such as polylactides and polylactic acids; amyloses; ethyl cellulose, polyethylene terephthalate, aliphatic polyesters, and cellulose acetate butyrate. More preferred capsule materials include cellulose acetate butyrate, polylactides, and polyethylene terephthalate.

Disposable materials of the present invention can be blended with activator compounds, including microencapsulated activator compounds. Preferably, the disposable materials are coated or laminated with films that contain microencapsulated activator formulations of the present invention. As such, the microencapsulated activator compounds are more likely to be exposed to environmental impact, such as abrasion, pressure, heat, water, or acid that will trigger release of the activator compounds and accelerate disposable material degradation during the disposal stage.

Another embodiment of the present invention is the use of at least one blotting compound to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. Blotting compounds are compounds that attract and absorb substances that promote degradation, such as moisture and degradation products (e.g. , acidic groups) . As such, blotting compounds can retard the rate at which disposable materials degrade. However, once the blotting compounds are saturated, the degradation rate will accelerate. Use of an appropriate amount of a blotting compound allows for blotting compound saturation to occur soon after the beginning of the disposal stage. As such, use of an appropriate amount of a blotting compound leads to an accelerated disposal degradation rate relative to the operative degradation rate. Blotting compounds are particularly useful to stabilize disposable materials during processing stages, such as packaging manufacture, when the materials are exposed to high temperatures and/or water. Preferred blotting compounds are essentially not volatile at the temperatures at which polymer formation or packaging occurs. Blotting compounds can also be used to absorb moisture into the disposable material from the environment, such as rainfall in a landfill.

Suitable blotting compounds include, but are not limited to, water grabbers, alkaline compounds capable of neutralizing acid, dry mineral fillers, and mixtures thereof. Suitable water grabbers include dry silica, talc, clays, calcium sulfate, calcium chloride, sodium sulfate, carbodiimides, and mixtures thereof. Suitable alkaline blotting compounds include sodium bicarbonate, sodium acetate, sodium phosphate, and mixtures thereof. Preferred blotting compounds include dry silica gel and calcium sulfate. Blotting compounds can be incorporated directly into the disposable materials or can be microencapsulated in capsules that degrade in the presence of water and acid. For example, since soluble alkaline blotting compounds may actually promote polymer degradation if they are present in the disposable material in free form, such compounds can be incorporated into the disposable material as coated solids which solubilize in the presence of acid or be microencapsulated in a capsule that is susceptible to acid and/or water degradation, so that the blotting compound is released at a rate effective to promote neutralization of the acid. Suitable capsular formulations include hydroxyacid-containing polymers such as polylactides and polylactic acids; amyloses; ethyl cellulose, polyethylene terephthalate, aliphatic polyesters, and cellulose acetate butyrate. Blotting compounds can also be used to stabilize or heal polymers because they can remove water formed during an esterification reaction.

Yet another embodiment of the present invention is to produce a disposable material which is coated with at least one coating compound capable of retarding degradation in such a manner as to modify and/or control the rate at which the disposable material degrades such that the disposal degradation rate is accelerated relative to the operative degradation rate. The coating compound can be applied to the surface or can be mixed with the polymer and bloom (i.e., migrate) to the surface. Other examples of coating processes include, but are not limited to: surface halogenation in which small amounts of fluorine gas and a light catalyst can modify the surface of the disposable material by increasing the contact angle; and plasma polymerization, in which a hydroxycarboxylic acid monomer is exposed to an ionizing field in order to apply a thin surface layer of highly cross-linked material to a disposable material.

A coating compound of the present invention should be compatible with the hydroxycarboxylic acid-containing polymer it is to coat. The coating, which is obtained by coating a disposable material with the coating compound, is preferably prone to abrasion so that it can protect the disposable material during the operative stage, but will still be penetrable to moisture during the disposal stage. Also preferred is a coating that has a high contact angle when the objective is to delay the onset of hydrolytic degradation. A coating with a high contact angle (e.g., at least about 90 degrees) is hydrophobic; it causes water and other hydrophilic substances to bead on the surface, thereby reducing penetration of the disposable material by such substances. As described above, a particularly preferred coating is one that is prone to abrasion and hydrophobic. Suitable coating compounds include polyvinyl chloride, polyvinylidine chloride, nitrocellulose, polylactic acid, polylactide, polytetramethyl glycolide, polyurethanes, aliphatic polyamides, and polyethylene terephthalate. Preferred coating compounds are poly(L- lactide) and polytetramethyl glycolide. The coating can be multiple plies or laminates. For example, a less plasticized polymer layer may coat a more plasticized polymer layer. In addition, a more amorphous D,L-polylactic acid can be coated with a more moisture- resistant L-polylactic acid coating. As a further example, a coating can include several layers: one of which is a moisture barrier, one of which is a gas barrier, and one of which is abrasion prone. The layers can be held together with adhesives that contain activators or inhibitors of degradation.

It is within the scope of the present invention to produce a package on which the label of the package is imprinted into the coat of the disposable material. Rapid degradation of the coat, in accordance with the present invention, soon after the package has been disposed, thereby leads to rapid label disintegration.

Yet another embodiment of the present invention is a disposable material that contains at least one copolymer in order to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. Disposable materials of the present invention containing at least one secondary material, such as an additional monomer, polymer block, or polymer, can have very different degradation properties from those of a homopolymer. For example, as the concentration of a secondary material (e.g. , monomer, polymer block, or polymer) increases, the crystallinity of the resultant disposable material typically decreases compared to the original homopolymer. For example, polymers with increased amounts of D,L-lactide per amount of L-lactide are more susceptible to degradation than a L-lactide polymer since the copolymer composition is less crystalline. As used herein, D,L-lactide includes both isomers of racemic lactide, the meso isomer of lactide, and/or mixtures thereof. In addition, the resultant disposable material may be, for example, more or less hydrophobic, depending on the nature of the secondary material. For example, block copolymers with polyethylene terephthalate polymer blocks interspersed with polylactic acid polymer blocks increase the hydrophobicity of the resulting polymer, thereby decreasing its degradation rate compared to that of polylactic acid. Copolymers of the present invention have from about 3 percent to about 50 percent of a secondary material, and preferably eit least about 10 percent of the secondary material. Methods to produce copolymers are described, for example, in co- pending U.S. Patent Application Serial No. 07/579,005. Copolymers can be produced by polymerizing at least two monomers together. Suitable monomers include, but are not limited to, hydroxycarboxylic acids and cyclic esters thereof, cyclic carbonates, dicarboxylic acids, anhydrides, diisocyanates, glycols, oligomeric polyalkylene adipates, and mixtures thereof. Preferred monomers include cyclic esters of hydroxycarboxylic acids, dilactones, cyclic carbonates, and mixtures thereof. More preferred monomers include L-lactide, D-lactide, D,L-lactide, glycolide, 2- ethyl glycolide, tetramethyl glycolide, dioxanone, caprolactone, ethylene carbonate, propylene carbonate, and mixtures thereof.

Block copolymers can also be formed by mixing at least two polymer blocks (including oligomer blocks) together, under conditions such that the polymer blocks are joined to form a block copolymer. Suitable polymer blocks include, but are not limited to, polymers that will enhance degradation, such as hydroxycarboxylic acid polymers, polymers of cyclic esters of hydroxycarboxylic acids, alkyl ester polymers, alkyiene carbonate polymers, cyclic carbonate polymers, and polyethylene glycols, as well as polymers that retard degradation, such as poly-ethylene terephthalates, phthalate esters, polyethylenes, polystyrenes, polyvinyl chlorides, and polypropylenes. Mixtures of such polymer blocks can also be used. Preferred polymer blocks include α-hydroxycarboxylic acid polymers, polymers of cyclic esters of α-hydroxycarboxylic acids, lactone polymers, dilactone polymers, and cyclic carbonate polymers, and mixtures thereof. More preferred polymer blocks include lactic acid polymers, glycolic acid polymers, L-lactide polymers, D-lactide polymers, D,L- lactide polymers, glycolide polymers, polyethylene terephthalates, adipic acid ethylene-glycol polymers, epsilon-caprolactone polymers, delta-valerolactone polymers, and mixtures thereof. Preferably, hydrophobic polymer blocks contain from about 5 to about 50 hydrophobic monomer units. More preferably, hydrophobic polymer blocks contain from about 5 to about 10 hydrophobic monomer units. Hydrophilic polymer blocks can contain from about 5 to about 500 hydrophilic monomer units.

In addition, copolymers can be produced by combining polymer blocks, or oligomers, and monomers. For example, a copolymer can be produced that consists of a polymer which is difficult to degrade interspersed every about 5 to about 20 units with at least one hydroxycarboxylic acid unit. For example, a copolymer containing about 10 polyethylene terephthalate units per lactic acid unit can degrade hydrolytically to a size (i.e., polyethylene terephthalate decamerε) that is degradable by micro¬ organisms.

It is within the scope of this embodiment that at least two homopolymers or copolymers can be mixed together to produce a polymer blend for a disposable material with improved degradation characteristics. For example, a hydrophilic polymer can be mixed with a hydrophobic polymer to produce a physical blend that is more hydrophobic than the original hydrophilic polymer and less hydrophobic than the original hydrophobic polymer. Suitable hydrophilic polymers include, but are not limited to, polylactic acid, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl alcohol, polyvinyl acetate, polyethylene glycol, polypropylene glycol, maleic anhydride copolymers, polyanhydrides, polyureas, and mixtures thereof. Suitable hydrophobic polymers include, but are not limited to polyethylene, polystyrene, polyvinyl chloride, polypropyl-ene. acrylonitrile polymers, styrene-butadiene copolymers, polyisoprene, polyalkenoic acids, and mixtures thereof. Intermediate hydrophilic polymers include aliphatic polyamides, polyurethanes, polyalkylene carbonates, and mixtures thereof.

It is also within the scope of this embodiment that copolymers of the present invention can form inter¬ penetrating networks or composites such that the polymers can be intimately dispersed or even covalently attached. For example, a very hydrophobic monomer can be polymerized in the presence of a preformed hydrophilic polymer to form an intimate polymer network.

Another embodiment of the present invention is the use of at least one plasticizer to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. Plasticizers of the present invention are compounds that are incorporated into disposable materials of the present invention during, or after, polymerization. Plasticizers introduce pliability, flexibility and toughness into a polymer to an extent not typically found in a disposable material containing only a polymer or copolymer. Introduction of plasticizers into the polymer reduces the melt viscosity of the polymer and lowers the temperature, pressure, and shear rate required to melt-form the polymer. Plasticizers also prevent heat build-up and consequent discoloration and molecular weight decrease during processing steps, such as production and packaging. Further, plasticizers add impact resistance to the polymer. Plasticizers can increase or decrease polymer degradation rates and, thus, addition of plasticizers to a disposable material can be used to control the operative degradation rate of the material.

Plasticizers of the present invention can be either internal or external plasticizers. Internal plasticizers are part of the polymeric backbone itself or chemically bonded to the polymer backbone, whereas external plasticizers are discrete compounds that are not chemically bonded to the polymer. A method to produce polymers with plasticizers is described in co-pending U.S. Patent Application Serial No. 07/579,005. Suitable plasticizers of the present invention include, but are not limited to, hydroxycarboxylic acids and cyclic esters thereof, oligomers of hydroxycarboxylic acids and cyclic esters thereof, dibasic acid esters, polyesters, aromatic ethers, aromatic esters, esters of relatively long chain acids, esters of relatively long chain alcohols, sulfonamides, tertiary amines, alkyiene carbonates, keto-esters, ketones, compound with multiple ketone groups, ethers, other polar compounds, and mixtures thereof. Preferred plasticizers include α-hydroxy-carboxylic acids and cyclic esters thereof, oligomers of α-hydroxycarboxylic acids and cyclic esters thereof, lactones, dilactones, dibasic acid esters, low molecular weight polyesters, ketones, cyclic amides, diphenyl ether, diethyl phthalate, ethyl octoate, lauryl acetate, polypropylene glycol adipate, glyceryl dietcetate. glyceryl triacetate, cyclododecanone, isophorone, other polar, non-toxic, non-fugitive compounds, and mixtures thereof. Glucose or sucrose ethers and esters are included, as are polyethylene glycol ethers and esters, glycerine diacetate N,N'-substituted amino acid esters and oligomers, and amides. More preferred hydrophilic plasticizers include lactic acid, L-lactide, D-lactide, D,L-lactide (wherein D,L-lactide includes the racemic isomers, the meso isomer, and/or mixtures thereof) , glycolide, glycolic acid oligomers, lactic acid oligomers, glycolide oligomers, L-lactide oligomers, D-lactide oligomers, D,L-lactide oligomers, butyl lactate, ethyl lactate, diethyl adipate, polyethylene glycol succinate, epsilon-caprolactone, valerolactone, adipic acid esters, citric acid esters, glycol-alkyl esters, and mixtures thereof. More preferred hydrophobic plasticizers include stearyl esters, low-toxicity phthalates, phenyl ethers, phenyl esters, sebacic acid esters, and mixtures thereof. Oligomers are typically from about 2 to about 35 monomeric units. Typically, the disposable material contains from about 5 to about 50 weight percent plasticizer. A preferred amount of plasticizer in the disposable material is from about 10 to about 30 weight percent.

Another embodiment of the present invention is the modification of at least one structural characteristic of a disposable material in order to modify and/or control the rate at which the disposable material degrades such that the disposal degradation rate is accelerated relative to the operative degradation rate. The structure of the disposable material can be modified in a variety of ways including modifying the crystallinity, free volume, orientation, molecular weight (i.e., chain length), and/or surface area of the polymer. For example, a highly crystalline, oriented disposable material has a regular structure that is typically not very susceptible to degradation. As a disposable material loses crystallinity and orientation, the material becomes less ordered and has more "free volume" into which water can penetrate, leading to accelerated hydrolysis and degradation. For example, the rate at which a highly crystalline lactic acid polymer degrades is typically about two to three times longer than the rate at which an amorphous lactic acid polymer degrades.

Methods to obtain hydroxycarboxylic acid-containing disposable materials of varying degrees of crystallinity and orientation are described, for example, in U.S. Patent Application Serial Nos. 07/579,005 and 07/579,465. Polymers can be oriented either uniaxially (i.e., the polymers line up in one direction) or biaxially (i.e., the polymers line up in two directions) .

Crystallinity can be measured in a variety of ways including, but not limited, to X-ray diffraction and thermal analyses such as thermal gravimetric analysis and density scanning calorimetry. A preferred method is X-ray diffraction which gives quantitative measurements. Disposable materials of the present invention have crystallinities and orientations that are suitable for their applications; see, for example, co-pending, commonly assigned, patent application entitled "Degradable Polymer Composition," filed on September 22, 1992, U.S. Serial No. 07/950,854 (Attorney File No. 4042-26), which is incorporated herein by reference in its entirety.

The free volume of disposable materials of the present invention can be measured using, for example, a free volume microprobe. Disposable materials of the present invention can have macroscopic free volumes, such as is found in foams; microscopic free volumes, which are small voids that can be seen with a microscope; and nanoscopic free volumes, which are very small voids that are created by the shape of an object, such as gas, as it exits the polymer. For example, a soluble or volatile substance, such as carbon dioxide, can be mixed into a polymer and later be removed by, for example, volatilization.

One method to modify the crystalline structure of a disposable material is to control the amount of residual monomer remaining in the polymer after polymerization. A polymer having a low amount of residual monomer has fewer acidic end groups if the monomer is an acid and less low molecular weight esters that readily form compounds with acid end groups if the monomer is an ester and hence, is less susceptible to hydrolytic degradation activated by acid groups than is a polymer having a high amount of residual monomer. The concentration of residual monomer affects not only the rate at which water penetrates the surface of the disposable material but also the rate of water penetration inside the amorphous structure of the polymer. In addition, high residual monomer concentrations can impede crystallization. The concentration of residual monomer in typical disposable materials of the present invention is from about 10 ppm to about 40 weight percent residual monomer in the disposable material. Preferably, the concentration of residual monomer is less than about 20 weight percent. The degradation characteristics of a polymer can also be changed by modifying the size of the polymer. For example, a D,L-lactic acid polymer (PLA) having a molecular weight of over about 5,000 daltons is typically rigid and hard, whereas PLA having a molecular weight of less than about 5,000 daltons is typically semisolid and soft which allows increased water penetration compared to high molecular weight polymers. In addition, low molecular weight PLA polymers (i.e., polymers having shorter chain length) have more acid groups (i.e., free ends) per unit length than do high molecular weight polymers (i.e., polymers having a long chain length) ; such acid groups, as described above, can catalyze hydrolysis. The molecular weights of disposable materials of the present invention can depend on their applications; see, for example, co- pending, commonly assigned, patent application entitled "Degradable Polymer Composition," filed on September 22, 1992, U.S. Serial No. 07/950,854, which is incorporated herein by reference in its entirety. The structure of disposable materials of the present invention can also be modified by modifying the surface area of the disposable material. Materials with larger surface areas are typically more prone to degradation than are materials with smaller surface areas as there is an increased area through which compounds, such as water and acid, can penetrate. The surface area to volume ratio of disposable materials of the present invention depends on their use. For example, packaging materials usually have a thickness of about 0.5 to about 20 mil. Methods to increase the surface area include production of disposable materials with foamed structures (including foams with very small pores) through which water can penetrate.

Yet another embodiment of the present invention is the use of physical stress to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. A disposable material is produced which begins to degrade when exposed to a physical stress, such as pressure (e.g., crumpling a package), torsion (e.g., twisting so as to disrupt disposable material) , and stretching past the yield point. As long as yield stress is not applied to the disposable material, the disposable material's rate of degradation is very low. When stress is applied at the beginning of the disposal stage, such as twisting a package prior to throwing it away or breaking a barrier coating, the stress exposes the disposable material to degradation. Since a rate-dependent step for hydrolysis of disposable materials of the present invention is the water content of the polymer, one embodiment of the present invention is the use of at least one hydrophobic compound to modify and/or control the water content so that the disposal degradation rate is accelerated relative to the operative degradation rate. Hydrophobic compounds are compounds that have a low affinity for water. As such, they can protect hydroxycarboxylic acid-containing polymers from hydrolysis and, thus, reduce the degradation rate of disposable materials containing such polymers. Suitable hydrophobic compounds for use in the present invention include, but are not limited to: monomers and oligomers, including those comprising substituted hydroxycarboxylic acids and cyclic esters thereof; hydrophobic plasticizers, including hydroxy-carboxylic acids or cyclic esters or oligomers thereof; other hydrophobic monomers or polymers that can be used as comonomers, copolymers,, block copolymers or plasticizers; and mixtures thereof. Hydrophobic compounds can be incorporated directly into polymers, can be added in the form of internal or external plasticizers, and/or can be grafted (i.e., attached) to the polymer backbone. Methods to graft hydrophobic compounds to a polymer backbone include, but are not limited to, trifunctional monomers, grafting sites, and post- polymerization covalent attachment. Methods of polymerization, including copolymerization, both with and without plasticizers have been taught in co-pending U.S. Application Serial Nos. 07/579,000 by Sinclair, 07/579,005 by Sinclair, 07/579,460 by Sinclair et al., and 07/579,465 by Sinclair, all filed on September 6, 1990, and, as referred to above, are incorporated by reference herein in their entirety.

Preferred hydrophobic compounds of the present invention include hydroxycarboxylic acids with long aliphatic chains, esters having long chain aliphatic or olefinic acids, esters having long chain alcohols, polyesters, glycerides, ketones, aromatic acids, aromatic ethers, aromatic esters, hydrophobic mineral fillers, and mixtures thereof. Hydrophobic compounds usually have a high ratio of carbon and hydrogen atoms to oxygen atoms. Preferred hydrophobic compounds of the present invention have an average of at least about three carbon atoms per oxygen atom (i.e., a carbon-to-oxygen ratio of about 3) because such compounds exhibit little water solubility. Examples of such compounds include: one or more long chain aliphatic acids or fatty acids, which can have a carbon chain having an odd or even number of carbon atoms. Suitable examples include α-hydroxyoctanoic acid, α- hydroxynonanoic acid, α-hydroxydecanoic acid, α-hydroxy- undecanoic acid, α-hydroxydodecanoic acid, α-hydroxy- tridecanoic acid, α-hydroxymyristic acid, α-hydroxypenta- decanoic acid, α-hydroxy-palmitic acid, α-hydroxyhepta- decanoic acid, α-hydroxystearic acid, α-hydroxynonadeca- noic acid, α-hydroxyarachidic acid, α-hydroxybehenic acid, α-hydroxylignoceric acid, α-hydroxycerotic acid, α- hydroxyoleic acid, α-hydroxy-linoleic acid, α-hydroxy- linolenic acid, α-hydroxy-arachidonic acid, 2-ethylhexyl- carboxylic acid, and other naturally-occurring hydroxy¬ carboxylic acids; cyclic esters thereof; and a cyclic ester of α-hydroxyisobutyric acid. Other suitable hydrophobic compounds include one or more aromatic or dicarboxylic acids such as mandelic acid, benzylic acid, sebacic acid, azelaic acid, and their glycol esters. More preferred hydrophobic compounds are those with a carbon: oxygen ratio of at least about 6 which exhibit essentially no water solubility.

One embodiment of the present invention is the use of hydrophobic plasticizers to control the water content in the resulting disposable material. The plasticizers can be added at a variety of concentrations. Preferably, disposable materials containing hydrophobic plasticizers contain from about 5 percent to about 50 percent plasticizer. The plasticizers, depending on their characteristics, may be added before or during formulation of the disposable material. Hydrophobic plasticizers can be used in the presence of lactide and/or lactic acid oligomers as long as the overall hydrophilic/lipophilic balance is high enough to impart sufficient hydrophobicity to keep the water concentration below a critical concentration, generally less than about 200 ppm by weight. Suitable plasticizers include, but are not limited to: esters of relatively long chain acids, such as ethyl laurate; esters of relatively long chain alcohols, such as lauryl acetate; low molecular weight polyesters, such as polypropylene glycol adipate; glyceryl triacetate; ketones, such as 2-undecanone, isophorone, and cyclododecanone; aromatic ethers or esters, such as alkylated polyglycol ethers or polyethylene adipate; and mixtures thereof.

In addition, copolymers of hydroxycarboxylic acid and hydrophobic polymers, such as polystyrene, polyethylene terephthalate (PET) , diethyl phthalate (DEP) , polyvinyl chloride (PVC) , and polypropylene can be used to promote hydrophobicity. Copolymers can be produced by mixing at least two co-reactive monomers; at least two co-reactive polymer blocks; at least two polymers; two monomers or polymer blocks with a third monomer or polymer block that is reactive with each of the first two; or mixtures thereof. In order to enhance the compatibility of hydrophobic polymers and, for example, lactic acid or lactide polymers, during blending, compatibilizers may be used. Suitable compatibilizers include the use of a styrene-lactide copolymer with polystyrene, a PET- polylactide block copolymer with PET, a polyethylene- polylactide block copolymer with polyethylene, lactide with DEP, DEP-lactide with PVC, and a polypropylene-polylactide block copolymer with polypropylene. These can be grafted together or blocked by chain extension processes using reactive end groups.

If transparency of the resulting disposable material is not an issue, other hydrophobic compounds including minerals such as carbon blacks, mica, talc, silica, and titanium oxide can be used as fillers.

In a preferred embodiment, disposable materials of the present invention consist of an α-hydroxycarboxylic acid- containing polymer coated or laminated with a hydrophobic compound-containing material. Particularly preferred is a coating that is prone to abrasion. Such a coating protects the polymer from significant water penetration, and hence from significant degradation, during the operative stage of the disposable material. However, as the disposable material is used, it becomes scratched, and, upon disposal, the material is exposed to increased abrasion. Such abrasion allows water penetration of the coating which leads to accelerated degradation rate of the polymer, and hence, of the disposable material. An advantage of this embodiment is that the coating can be much more hydrophobic than can be a disposable material into which at least one hydrophobic compound has been blended. As increased amounts of hydrophobic compounds are blended into a disposable material, the material loses strength. Thus, the option of coating the material with a hydrophobic compound allows for the internal disposable material to be of desired strength and other characteristics. In one embodiment, a disposable material of the present invention is first coated with a compound that absorbs water and then is coated with a hydrophobic abrasion-prone coating.

Suitable hydrophobic coatings should be compatible with the hydroxycarboxylic acid-containing polymer. Examples of hydrophobic compounds with which to coat polymers include any of the compounds described above, and especially polyvinyl chloride, polyvinylidine chloride, polyethylene terephthalate, nitrocellulose, polystyrene, polyethylene, polypropylene, polyvinyl acetate, and mixtures thereof. Preferred hydrophobic compounds to use as a coating include polyvinyl chloride and polyethylene terephthalate.

The hydrophobicity of disposable materials of the present invention can be measured in a variety of ways, including Hydrophile-Lipophile Balance (HLB) and contact angle. The HLB is an expression of the relative amounts of the hydrophilic (water-loving or polar) and lipophilic (oil-loving or non-polar) groups in the disposable material without weighting their polarity strengths. A disposable material that is lipophilic (i.e., hydrophobic) in nature is assigned a low HLB (below 9.0, on a scale of 0 to 20 units) . HLB measurements are useful in estimating whether a candidate additive will help to increase or decrease water concentration. For example, the HLB value of polylactic acid is approximately 10 and that of polyglycolic acid is about 15. Lactide has an HLB of about 12 and glycolide has an HLB of about 15. A typical good plasticizer for polylactic acid is dimethyl adipate (HLB about 10) ; however this plasticizer does not function well with polyglycolic acid. A plasticizer that functions marginally with polylactic acid is lauronitrile, which has an HLB of about 3 and an extremely polar hydrophilic group. Preferably, the HLB values of a plasticizer should be within about 4 units, and more preferably within about 2 units, of the polymer to be plasticized. In certain circumstances, the range can be as broad as about 7 HLB units.

The contact angle is a measure of the ease with which the air that occupies the interface can be displaced by water. A standard method to measure contact angle is to place a droplet on the surface of the material to be tested and make a direct measurement with a microscope equipped with a goniometer (M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, 1978) . Contact angle measurements are particularly useful in determining how soon a disposable material will start to degrade. A material with a high contact angle causes water and other hydrophilic substances to bead on the surface, thereby reducing penetration of the disposable material by such substances. Materials with contact angles of less than about 90 degrees are prone to degradation, whereas materials with contact angles of at least about 90 degrees, and up to about 180 degrees, usually repel moisture. In a preferred embodiment in which a disposable material is coated with a hydrophobic compound, the inner disposable material will have a relatively low contact angle (e.g. , preferably less than about 90 degrees) , whereas the coating will have a relatively high contact angle (e.g., preferably at least about 90 degrees) . Another embodiment of the present invention is the use of at least one end-capping agent to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. While not being bound by theory, it is believed that polymers of hydroxycarboxylic acids degrade by two mechanisms: random scission within the polymer and back-biting of the terminal hydroxyl ends of the polymer. In addition, the carboxyl ends apparently promote degradation both by polarizing ester bonds and by providing acid groups that increase (i.e., accelerate) the rate of hydrolysis as it is believed that free carboxyl groups are surrounded by shells of water which promote degradation. As more carboxyl end groups are formed during hydrolysis, there are additional acid groups which trap and accumulate water. It is further believed that while reactive hydroxyl ends can be more responsible for enhancing degradation during melt processing, free carboxyl ends can be more responsible for degradation during the operative and disposal stages. As such, blocking carboxyl and/or hydroxyl groups of the disposable material is thought to retard degradation during the operative stage. As degradation proceeds during the disposal stage, however, new end groups will be exposed which will further enhance degradation.

In a preferred embodiment, carboxyl end groups of the disposable material are blocked by an end-capping agent which is typically added after polymer production. Suitable end-capping agents to block carboxyl groups include alcohols, chlorosilanes, alkyl chlorides, isocyanates, amines, methyl esters, and mixtures thereof. Preferred carboxyl end-capping agents include t-butyl alcohol, trimethyl chlorosilane, hexamethyldisilazane phenylisocyanate, and acetyl chloride.

Hydroxyl end groups can also be blocked by adding end- capping agents such as anhydrides, acid chlorides, and isocyanates. Preferred end-capping agents to block hydroxyl groups are anhydrides, such as acetic anhydride and stearic anhydride. Stearic anhydride is more preferred as addition of stearic groups increases the hydrophobicity of the disposable material.

The use of reversible end-capping agents that, can be removed from the disposable material during the disposal stage allows for an accelerated disposal degradation rate of the disposable material upon removal of the agents. Such removal may be triggered by a change in the environment, such as an increase in moisture or temperature (e.g., steam autoclaving) . Preferred reversible carboxyl end-capping agents include phenylisocyanate, acetyl- chloride, hexamethyldisilazane and trimethyl chlorosilane. Preferred reversible hydroxyl end-capping agents include phenyl isocyanate, acetic anhydride, and acetyl chloride. Another embodiment of the present invention is cross- linking disposable materials of the present invention in order to modify and/or control the rate at which disposable materials degrade such that the disposal degradation rate is accelerated relative to the operative degradation rate. Cross-linking of polymers within the disposable material usually leads to slower degradation rates. While not being bound by theory, it is believed that cross-linking of polymers leads to tighter binding, thereby making it more difficult for water to penetrate the disposable material; in addition, cross-linking can reduce the rate at which polymer bonds are hydrolyzed.

Suitable cross-linking agents include, but are not limited to: tartaric acid; free-radical generators, such as peroxides and radiation; multifunctional chain extenders; trifunctional monomers; reactive pendant groups; and mixtures thereof.

Preferred cross-linking agents include tartaric acid, peroxides, hydroperoxides, trichloroisocyanurate, nadic anhydride, glycerine, pyromellitic dianhydride, tri- isocyanates, polyaniline, polyisocyanate, 1, 3, 5-triamino- benzene, bisphenol/diepoxide, polymerized allyl or vinyl- substituted dioxanedione, and mixtures thereof. Tartaric acid is a particularly preferred cross-linking agent because its structure leads to cross-link formation as it is being polymerized into tartaric acid polymers. As used herein, a tartaric acid polymer includes polymers containing tartaric acid, as well as salts, esters, amides, and cyclic esters thereof; the cyclic esters can contain two tartaric acids or one tartaric acid joined to another α-hydroxycarboxylic acid. Dihydroxy maleic anhydride is a preferred source of tartaric acid linkages in these cross- 1inked polymers. Tartaric acid can also be used to cross¬ link other hydroxycarboxylic acid-containing polymers.

The stage at which cross-linking is conducted depends on the cross-linking agent being used. For example, tartaric acid can be an integral part of the polymerization. Trifunctional monomers are usually incorporated during or after polymerization. Free-radical generators, such as peroxides and radiation, chain extenders, and pendant groups are typically used for crosslinking after polymer formation. In one embodiment, functional hydroxyl, isocyanate, or epoxy groups are added to polymers that are then cross-linked using, for example, melamine, epoxy resin, or polyols.

Yet another embodiment of the present invention is the addition of at least one source of microbial nutrients to a disposable material in order to modify and/or control the rate at which the disposable material degrades such that the disposal degradation rate is accelerated relative to the operative degradation rate. While the hydroxy- carboxylic acid-containing polymers of the present invention can be degraded to monomers by hydrolysis, typically microorganisms, or chemically- or microbially- produced enzymes, are used to accomplish complete degradation of the polymers to, for example in the case of lactic acid, carbon dioxide and either water or methane. Disposable materials deposited in a landfill can attract microbial degradation by containing nutrients that promote the activity of microorganisms capable of degrading the materials. As used herein, the phrase "promote the activity of microorganisms" refers to the ability of the nutrients to enhance the rate at which the microorganisms grow and degrade disposable materials. Suitable nutrients include, but are not limited to, sources of carbohydrate, nitrogen, phosphate, sulfate, metals, and other salts. Typically, microorganisms can obtain carbon from the disposable materials, but in most cases, the disposable materials do not supply nitrogen, metals or salts. Thus, a preferred source of nutrients includes at least one of the following: a nitrogen source, a salt source, and a metal source. Also preferred are complex nutrients, such as vitamins and growth promotants, particularly for microorganisms that are not capable of producing such compounds from simple nutrients. Sources of nutrients can, for example, be incorporated into the polymer, grafted to the polymeric backbone of the disposable material, be microencapsulated, or be coated onto the surface of the disposable material. In one embodiment, nutrients can include compounds, such as hydroxycarboxylic acids with long aliphatic or fatty acid chains and isocyanates, which are added to the disposable material to control degradation in other ways.

In one embodiment, disposable materials of the present invention include polymers that contain amide functional groups, such as those formed from alpha amino acids. Such poly(esteramides) have excellent strength and can have their flexibility controlled by plasticization. In addition, the poly(esteramide) contains nitrogen-containing nutrients in a form that can be utilized by the microorganisms that conduct biodegradation reactions. Incorporation of amide groups accelerates disposal degradation rates because the degradation microenvironment (e.g., a fragment of a plastic container) can support a large microbial population. The microorganisms consume the polyester part of the molecules for energy and use the nitrogenous parts to make microbial protein, nucleotides, and other nitrogen-containing products needed by the microorganisms for growth and reproduction.

Plasticizers can also be sources of nitrogen- containing nutrients. For example, N-methyl pyrrolidinone can be used in the formulation to plasticize polymers of the present invention. Such a plasticizer can also provide a nitrogen source that can be used by the microorganisms more rapidly than nitrogen contained in a polymer bcickbone.

Another embodiment of the present invention is the inclusion in the disposable material of an indicator to detect whether degradation has occurred. Suitable indicators include those which detect pH and/or moisture changes and which indicate hydrolytic degradation by changes in color or cloudiness.

Another embodiment of the present invention provides a method for selecting the molecular weight of a hydrolytically degradable material. The method includes the steps: (i) selecting a final molecular weight of a degradable material after exposure to specified conditions of use for a predetermined period of use; (ii) projecting the water content of the degradable material during the period of use; (iii) projecting, based on the projected water content, the loss in molecular weight for the degradable material during the period of use; and (iv) selecting an initial molecular weight of the degradable material at the beginning of the period of use that is no less than the sum of the final molecular weight and the molecular weight loss during the period of use. As will be appreciated, the molecular weight loss during the period of use is related to the degradation rate. The degradation rate is in turn related to the water content of the degradable material during the period of use.

The hydrolytically degradable material can be any polymer derived from a monomer having either of the following formulas:

is an inte

where R₁ and R₂ are hydrogen or a hydrocarbon, such as an alkyl group

Additionally, the polymer can be derived from a monomer that is an ester, salt, or amide of a monomer having either of the above formulas. The hydrolytically degradable material can be a copolymer or block copolymer having one or more monomers defined by either of the above formulas. The copolymer or block copolymer can also have one or more monomers that are not contained within this family. The degradable material can also include one or more blend compatible, non-hydrolytically or hydrolytically degradable polymers. By way of example, the material can include a blend of hydrolytically degradable polymers having different degradation rates or of a hydrolytically and non- nonhydrolytically degradable polymer. Although the degradable material will include non-hydrolytically degradable bonds, the hydrolytically degradable ester bonds in the degradable monomer(s) will hydrolyze and cause the material nonetheless to break down or degrade during use.

Preferred degradable materials include a polymer or copolymer derived from an α-hydroxycarboxylic acid. The most preferred degradable material includes a polymer or copolymer derived from lactide, glycolide, caprolactone, and acids, salts, and amides thereof. The degradable material preferably includes from about 40 to about 100% by weight of a polymer or copolymer derived from an α- hydroxycarboxylic acid.

The predetermined pericd of use is the time period from the end of the degradable material's production to the time at which the degradable material is discarded. Accordingly, the predetermined time period includes the time of storage of the degradable material (e.g., when intermediate polymer products are stored prior to fabrication into articles and when the articles are stored in suitable packaging prior to use) as well as actual use (e.g., after articles are removed from a package or container and used) . In packaging applications, the period of use typically ranges from about 3 to about 24 months.

The molecular weight loss is determined based on selected conditions of use of the degradable material during the period of use. The selected conditions of use generally refer to the temperature and free water content (e.g. , free water such as water vapor pressure, precipitation, relative humidity, etc.) of the environment that will contact the degradable material during use, whether the use is indoors or outdoors or both. As desired, the conditions of use can be the average conditions during the period of use (i.e., the time weighted average of temperature or free water content during the period of use) or the "worst case" conditions (i.e., the temperature and/or free water content during the period of use that lead to the highest operative degradation rates) for the area of interest or some other suitable computational method that considers fluctuations in free water content of the ambient environment and temperature over time. The use conditions can be determined based on the known seasonal climatic fluctua¬ tions over time (which are preferably available from many sources) for the geographical area of use of the degradable material, whether a region, country, or area within a country. For example, if the degradable material is a packaging material to be used in Brazil the use conditions would reflect the temperatures and relative humidities expected in Brazil and the period over which the various conditions exist.

The desired final molecular weight after the predetermined period of use is dependent upon a number of factors, including the desired physical properties of the degradable material at the end of the predetermined use period. For example, for packaging films the desired final molecular weight preferably ranges from about 75,000 to about 300,000 daltons. The procedures for determining the molecular weight of a material are set forth above..

The initial molecular weight is no less than the sum of the desired final molecular weight and the anticipated molecular weight loss during the period of use. In some applications, it may be desired to increase the sum by a safety factor to allow for unexpected temperature or relative humidity fluctuations or fluctuations in other parameters influencing the operative degradation rate.

The water content of the degradable material during the period of use (which is generally the equilibrium water content) is related to the molecular weight loss during the same period and therefore to the operative degradation rate. The relationships between these variables can be determined experimentally for a degradable material of a selected composition as described in more detail below. As will be appreciated, the nature of the relationship will vary depending upon the composition and structure of the degradable material.

By way of example, the relationship between the equilibrium water content on the one hand and the degradation rate and molecular weight loss on the other can be determined experimentally for a specific composition of degradable material by determining a first degradation rate for a first water content of the degradable material and a second degradation rate for a second water content of the degradable material. These steps can be repeated as required to generate sufficient data to determine the relationship. The relationship can be determined by extrapolation or other mathematical techniques based on the experimental data. As will be appreciated, the relationship will change for differing concentrations of degradable ester bonds in the degradable material. By way of example, the use of end-capping to replace the carboxylic and/or hydroxyl end groups can lower the degradation rate and decrease the rate of autocatalysis for a given water content. As noted above, incorporating organic or inorganic salts or bases in the degradable material can also influence the degradation rate for a given water content.

While not wishing to be bound by any theory, the relationship between degradation rate and water content is based on the discovery that the hydrolytic degradation of the degradable materials, and particularly polymers and copolymers of α-hydroxycarboxylic acids such as lactic acid, is reaction and not diffusion controlled. In other words, the permeability of such degradable materials to water vapor is relatively high such that the hydrolytic degradation of ester bonds is not limited by the diffusion of water in the degradable material. The water consumed by cleavage of the ester bonds is rapidly replenished by water diffusing through the degradable material in an attempt to maintain the equilibrium water content. Accordingly, the degradation rate is based upon the solubility coefficient and water vapor pressure in the ambient atmosphere and not the permeability coefficient, as in a diffusion controlled system. Moreover, it is not believed that a sufficient decrease in relative humidity or temperature will cause the degradable material to become a diffusion and not a reaction controlled system.

It has been discovered that for some degradable materials the relationship between water content and degradation rate can drastically change if the temperature of the degradable material exceeds its glass transition temperature. While not wishing to be bound by any theory, it is believed that the increased mobility and freedom of motion in the vicinity of the ester bonds allows the atomic rearrangement required for hydrolysis to be more easily attained when the temperature is greater than the glass transition temperature. Accordingly, it may be desirable for degradable materials having a glass transition temperature more or less than the selected temperature of use to be thermally or chemically treated by known techniques to decrease or increase, as desired, the glass transition temperature to acceptable levels. In some cases, such as where the degradable material will be above and below the material's glass transition temperature, it may be necessary to determine the relationships between water content of the material and degradation rate both above and below the glass transition temperature. These relationships can be used to determine the respective degradation rates and molecular weight losses both above and below the glass transition temperature and thereby provide a more accurate estimation of shelf life.

Another embodiment of the present invention provides a specific process to determine the degradation rate of the degradable material. The process includes the following steps: (i) selecting the temperature and relative humidity of the atmosphere to be contacted by the degradable material during use; (ii) determining the water content of the degradable material at the temperature and relative humidity; and (iii) projecting the degradation rate based on the water content of the degradable material.

The water content of the degradable material can be determined by measuring the solubility coefficient of the degradable material at the selected temperature and relative humidity. The relationship between the equilibrium water content and the solubility coefficient and vapor pressure of water depends upon a number of factors, such as the temperature, the relative humidity, and the composition of the degradable material. To facilitate the water content determination, a table or index of water contents measured for a specific composition of degradable material at various temperatures and relative humidities can be prepared. The table is generated by repeating steps (i) and (ii) above for a variety of temperatures and relative humidities while holding the composition of the degradable material constant. These steps can be repeated for a plurality of different compositions of degradable material to generate separate tables for different compositions of degradable materials.

The equilibrium water content can be measured by coulometric Karl-Fischer titration or gravimetrically. The water content, solubility coefficient, and water vapor pressure can be plotted and the relationship determined by extrapolation or other mathematical techniques.

Another embodiment of the present invention provides a method to alter the water content of the degradable material to yield a selected degradation rate under specific use conditions. The method includes the steps: (i) selecting the temperature and at least one of the relative humidity and water vapor pressure of the atmosphere to be contacted by the degradable material during use; (ii) projecting the degradation rate of the degradable material based on the water content of the degradable material at the selected temperature and at least one of the relative humidity and water vapor pressure; and (iii) based on the difference between the degradation rate and the selected degradation rate, varying the amount of at least one of a hydrophilic and hydrophobic additive in the degradable material to yield the selected degradation rate.

The hydrophilic additive can be a hydrophilic comonomer, a hydrophilic polymer, a blotting compound, an activator compound, a hydrophilic plasticizer, other water- absorbing compounds, and mixtures thereof. Hydrophilic additives are generally employed to accelerate the degradation rate when the degradation rate is significantly slower than the selected degradation rate. As will be appreciated, the hydrophilic additives will attract water into the degradable material and thereby increase the water content. The increased water content causes an increase in the degradation rate. The hydrophilic comonomer can be derived from a number of hydrophilic monomers, such as monomers derived from a hydroxycarboxylic acid or ester, salt, or amide thereof. Preferred hydrophilic comonomers for random copolymers include lactide, glycolide, ethylene oxide, propylene oxide, vinyl alcohols, amides, and acids or salts thereof. Preferred hydrophilic block copolymers include block copolymers derived from a variety of hydrophilic monomers, i.e., hydroxycarboxylic acid polymers, polymers of cyclic esters of hydroxycarboxylic acids, alkyl ester polymers, alkyiene carbonate polymers, cyclic carbonate polymers, and polyethylene glycols. The preferred concentration of the hydrophilic comonomer ranges from about 2 to about 40% by weight. The hydrophilic polymer can be a variety of water attracting and/or absorbing polymers that are blend compatible with the degradable material. Preferred hydrophilic polymers include poly(ethylene glycol) , poly(ethylene oxide), poly(vinyl alcohol), polyamides, poly(propylene glycol) and mixtures thereof. Less preferred intermediate hydrophilic polymers include aliphatic polyamides, polyurethanes, polyalkylene carbonates, and mixtures thereof. The preferred concentration of the hydrophilic polymer in the degradable material ranges from about 2 to about 40% by weight.

The blotting compound can be any blotting compound that attracts and thereby provides water for the hydrolytic degradation of the degradable material. Preferred blotting compounds are water grabbers, dry mineral fillers, and mixtures thereof. Suitable water grabbers include dry silica, talc, clays, calcium sulfate, calcium chloride, sodium sulfate, carbodiimides, magnesium sulfate, molecular sieves, and mixtures thereof. Suitable dry mineral fillers include talc. The preferred concentration of the blotting compound in the degradable material ranges from about 5 to about 40% by weight.

The activator compound can be any water-attracting acidic activator compounds, moisture-containing compounds or water. Preferred water-attracting acidic activator compounds include organic acids, such as lactic acid, lactic acid oligomers, fumaric acid, succinic acid, tartaric acid, malic acid, adipic acid, citric acid, glutamic acid, and methane sulfonic acid and inorganic acids, such as phosphoric acid, polyphosphoric acid, and esters, salts and amides thereof, and mixtures thereof. Preferred moisture-containing compounds include amylose and other starch-based hydrophilic polymers, cellulose-based hydrophilic polymers, hydrates of inorganic acids or salts thereof, and hydrates of organic acids or salts thereof, and mixtures thereof. The preferred concentration of the activator compound in the degradable material ranges from about 5 to about 40% by weight.

The hydrophilic plasticizer can be either internal or external water attracting and/or water absorbing plasticizers. Preferred hydrophilic plasticizers include lactic acid, L-lactide, D-lactide, D,L-lactide, glycolide, glycolic acid oligomers, lactic acid oligomers, glycolide oligomers, L-lactide oligomers, D-lactide oligomers, D,L- lactide oligomers, butyl lactate, ethyl lactate, diethyl lactate, diethyl adipate, polyethylene glycol succinate, valerolactone, adipic acid esters, citric acid esters, glycol-alkyl esters, soy oils and other naturally derived oils, and mixtures thereof. The preferred concentration of the hydrophilic plasticizer in the degradable material ranges from about 5 to about 40% by weight.

The hydrophobic additive can include a hydrophobic comonomer, a hydrophobic polymer block, a hydrophobic polymer, a hydrophobic plasticizer, other water-repelling additives, and mixtures thereof. The hydrophobic additive is used to retard the degradation rate when the degradation rate is significantly faster than the selected degradation rate. As will be appreciated, the hydrophobic additive will decrease the water content of the degradable material and thereby retard the degradation rate. Preferred hydrophobic comonomers for random and block copolymers include ethylene terephthalate, ethylene, styrene, vinyl chloride, propylene, acrylonitrile, styrene- butadiene, isoprene, alkenoic acids, caprolactone, and mixtures thereof. The preferred concentration of the hydrophobic comonomer in the degradable material ranges from about 5 to about 40% by weight.

Preferred hydrophobic polymers include polymers derived from hydrophobic monomers. Such polymers include poly(caprolactone) poly(ethylene terephthalate), polyethylene, polystyrene, poly(vinyl chloride) , polypropylene, acrylonitrile polymers, styrene-butadiene copolymers, polyisoprene, polyalkenoic acids, and mixtures thereof. The preferred concentration of the hydrophobic polymer in the degradable material ranges from about 5 to about 40% by weight.

Preferred hydrophobic plasticizers include caprolactone, stearyl esters, low-toxicity phthalates, phenyl esters, phenyl ethers, sebacic acid esters, caprolactone and mixtures thereof. The preferred concentration of the hydrophobic plasticizer in the degradable material ranges from about 5 to about 20% by weight. For best results, it is preferred that the hydrophilic and hydrophobic additives be physically blended such that they are intimately dispersed in the degradable material. It has been discovered that the formation of a separate phase by a hydrophilic additive that is blended into the degradable material can neutralize or adversely affect the impact of the additive on the degradation rate. In some cases, especially for additives more hydrophilic than the degradable material itself, the formation of separate phases can cause water to be removed from the degradable material by the additive, thereby decreasing, and not increasing, the degradation rate if the amount available to the system is limited. Accordingly, the degradable material can include a blend compatible agent to increase the solubility of the hydrophilic additive in the degradable material. Suitable blend compatible additives include compatibilizers (i.e., polyesters and polyamides), other additives that increase the solubility of the hydrophilic or hydrophobic additives in the degradable material, and mixtures thereof.

Another embodiment of the present invention provides a method and apparatus for determining the permeability of a polymeric material to water vapor. The permeability is important to determining the barrier properties of the material when used as a packaging material and, for hydrolytically degradable materials, whether the degradation of the material is diffusion or reaction controlled. The apparatus includes at least one surface of the polymeric material located between a water absorbing material positioned in a sealed environment that is substantially free of water vapor and a water-containing gas. The water vapor must pass through the surface to contact the water absorbing material. In one configuration, the apparatus is a container, such as a bag, formed from the polymeric material with the water absorbing material and sealed environment being contained in the container.

The water absorbing material can be a molecular sieve, desiccating minerals (i.e., calcium sulfate, magnesium sulfate, etc.), and mixtures thereof. Preferred molecular sieves are zeolites. The sealed environment is preferably substantially free of water. Normally, the sealed environment will be evacuated with an inert gas, such as argon or nitrogen prior to use. The sealed environment preferably contains no more than about 1% by volume water vapor. The water-containing gas preferably contains a known amount of water. In this manner, the water-containing gas can replicate a desired relative humidity encountered during use of the material. More preferably, the water- containing gas contains from about 50 to about 99% by weight water vapor.

In operation, the water absorbing material is weighed, placed in the sealed environment and the degradable material contacted with the water-containing gas for a selected length of time. After the selected time period, the water absorbing material is removed from the sealed environment and weighed. The difference in weight of the water absorbing material is the amount of water vapor that diffused through the degradable material during the selected time period. The permeability or diffusion rate can thus be readily determined.

The following examples are provided for purposes of illustration of the various embodiments described above and are not intended to limit the scope of the invention.

Example 1 A series of experiments were conducted to generate, for different compositions of degradable materials, tables indexing the equilibrium water content against the temperature and relative humidity ("RH") . The degradable materials were (i) 100% polylactide; (ii) 30%/70% copolymers of caprolactone and lactide; (iii) 100% polycaprolactone; and (iv) 20%/80% physical blends of poly(ethylene glycol) and polylactide. The tables are presented below. Also presented below are the equations that were determined experimentally for generating the tables. In the equations, S is the solubility coefficient, S₀ and ΔH are constants that are dependent upon the composition of the degradable material, R is the gas constant, and T is the temperature. Table 1 Prediction of Water Content in Films at Specific Temperatures and %RH

PREDICTION OF WATER CONTENT IN FILMS 100/0 PLA

Ln S = Ln S_n - ΔH/RT Ln S„ = -21.9

T (°C) 1/T Ln S

293 0.003413 -5.55 313 0.003195 -6.57 323 0.003096 -7.08

C_w = 100*ΔP*S* (18g H₂0/22414cm³)(1cm³/1.25g PLA) = 0.064*ΔP*S

90% RH 75% RH

50%RH 25% RH

30/70 PCL/PLA COPOLYMERS Ln S = Ln S₀ - ΔH/RT Ln S₀ = -24.2 ΔH/R = 5351 °K

C_w = 100*ΔP*S* (18g H₂0/22414cm³)(1cm³/1.25g PU) = 0.064*ΔP*S

75% RH

50%RH 25% RH

100/0 PCL

Ln S = Ln S₀ - ΔH/RT Ln S₀ = -20.7 ΔH/R = 4199°K

C_w = 100*ΔP*S* (18g H₂0/22414cm³)(1cm³/1.25g PLA) = 0.064*ΔP*S 90% RH 75% RH

50%RH 25% RH

20/80 PEG/PLA

Ln S = Ln S„ - ΔH/RT Ln S₀ = -9.8 ΔH/R = 1542"K

C_w = 100*ΔP*S* (18g H₂0/22414cm³)(1cm³/1.25g PLA) = 0.064*ΔP*S

75% RH

50%RH 25% RH

Example 2 The degradation of PLA has been extensively studied in buffered systems for applications such as drug delivery and for implantable devices. It has also been examined at specific conditions of temperature and humidity with thin films. Several methods of controlling the rate of degradation have been documented; these include the control of crystallinity by the thermal history or the ratio of L and D iso eric fractions, the incorporation of oligomers in the polymer and the addition of basic compounds.

Poly-(lactic acid) degradation proceeds by hydrolysis of the ester bonds in the polymer backbone. Water is a reactant in the hydrolysis process, and it is expected that increasing the water content in the films would lead to an increase in the rate of degradation. This can be achieved by introducing a hydrophilic moiety, either as a blend or as a copolymer, in the final material.

The rate of degradation may be predicted and/or controlled by the water concentration in the film. An experiment to determine the validity of this statement is set up with polyethylene glycol (PEG) as the hydrophilic moiety to be used in the form of a blend with PLA. The addition of varying levels of PEG to PLA is expected to increase the water content in the films. Blends of PEG/PLA were prepared by mixing the various ratios of polymers in the Haake Blender. The studies were conducted in the environmental chamber at 30°C and 40°C and 90% relative humidity. The samples were weighed periodically. Films were prepared in the following ratios: 0% PEG (all PLA) 5% PEG 10% PEG The water content of the blends increased according to the PEG content in the films.

The experiment described above has confirmed the fact that increasing the content of a hydrophilic moiety in a PLA blend leads to an increase in the rate of degradation. Films with greater than 10% PEG did not exhibit the trends indicated in the tables above. It is believed that above 10% PEG phase separation occurs and therefore the water in the PEG is not available for hydrolysis of the ester bonds in the PLA. While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims:

Claims

What is claimed is:

1. A method for selecting the molecular weight of a hydrolytically degradable material, comprising: selecting a final molecular weight of a degradable material after a predetermined period of use; determining the water content of the degradable material during the period of use; projecting, based on the water content, the loss in molecular weight for the degradable material during the period of use; and preparing a degradable material having an initial molecular weight at the beginning of the period of use that is no less than the sum of the final molecular weight and the molecular weight loss during the period of use.

2. The method of Claim 1, wherein the degradable material is derived from a precursor selected from the group consisting of:

integ

from 2 to 6

where R₁ and R₂ are hydrogen or a hydrocarbon

3. The method of Claim 1, wherein the degradable material is selected from the group consisting of poly(lactic acid) , poly(glycolic acid) , polycaprolactone, and mixtures thereof.

4. The method of Claim 1, wherein the degradable material is a blend of a hydrolytically degradable polymer with a non-hydrolytically degradable polymer.

5. The method of Claim 1, wherein the degradable material is a copolymer of a monomer of a hydrolytically degradable polymer and a monomer of a non-hydrolytically degradable polymer.

6. The method of C'aim 1, wherein the degradable material is a copolymer of monomers of hydrolytically degradable polymers.

7. The method of Claim 1, wherein the degradable material is a blend of a first hydrolytically degradable polymer with a second hydrolytically degradable polymer with the first and second hydrolytically degradable polymers being different.

8. The method of Claim 1, wherein the final molecular weight ranges from about 75,000 to about 300,000 daltons.

9. The method of Claim 1, wherein the degradable material is a packaging material.

10. The method of Claim 1, wherein the determining step comprises: measuring the solubility coefficient of water in the degradable material at a selected temperature and water content of the atmosphere to be contacted by the degradable material during use.

11. The method of Claim 10, wherein the water content is the equilibrium water content of the degradable material and is computed based on the solubility coefficient and the water content.

12. The method of Claim 1, wherein the determining step comprises: determining the temperature and water content of the atmosphere contacting the degradable material during use; determining the solubility coefficient for the degradable material at the temperature and water content; determining the equilibrium water content of the degradable material based on the solubility coefficient and water content at the temperature; and determining at least one of the degradation rate and molecular weight loss of the degradable material based on the equilibrium water content.

13. The method of Claim 12, wherein the determining step comprises: performing the steps of Claim 10 for a first composition of degradable material; and performing the steps of Claim 10 for a second composition of degradable material, wherein the first composition differs from the second composition.

14. The method of Claim 1, wherein the projecting step comprises: determining a first degradation rate for a first water content of the degradable material; determining a second degradation rate for a second water content of the degradable material, wherein the relationship between water content of the degradable material and molecular weight loss is based on the first and second degradation rates and first and second water contents.

15. The method of Claim 1, wherein the projecting step comprises: determining the concentration of degradable ester bonds in the degradable material; and determining the rate at which the ester bonds hydrolyze based on the water content of the degradable material.

16. The method of Claim 1, wherein the initial molecular weight ranges from about 150,000 to about 450,000 daltons.

17. The method of Claim 1, wherein the period of use ranges from about 3 to about 24 months.

18. The method of Claim 1, wherein the determining step comprises: selecting the temperature and at least one of the relative humidity and water vapor pressure cjf the atmosphere to be contacted by the degradable material during use, and wherein the projecting step comprises; projecting the degradation rate of the degradable material based on the water content of the degradable material at the selected temperature and the at least one of the relative humidity and water vapor pressure; and further comprising: based on the difference between the degradation rate and the selected degradation rate, varying the amount of at least one of a hydrophilic and hydrophobic additive in the degradable material to yield the selected degradation rate.

19. The method of Claim 18, wherein the hydrophilic additive includes at least one of the following: a hydrophilic comonomer, a hydrophilic polymer, a blotting compound, an activator compound, a hydrophilic plasticizer, other water-attracting compounds, and mixtures thereof.

20. The method of Claim 19, wherein the hydrophilic comonomer includes at least one of the following: ethylene oxide, amides, propylene oxide, vinyl alcohols, glycolide, lactide, and acids or salts thereof, and mixtures thereof.

21. The method of Claim 19, wherein the hydrophilic polymer includes at least one of the following: poly(ethylene glycol), poly(ethylene oxide), poly(vinyl alcohol, polyamides, poly (propylene glycol) , and mixtures thereof.

22. The method of Claim 19, wherein the hydrophilic block polymer includes at least one of the following: hydroxycarboxylic acid polymers, polymers of cyclic esters of hydroxycarboxylic acids, alkyl ester polymers, alkyiene carbonate polymers, cyclic carbonate polymers, poly(ethylene glycol), and mixtures thereof.

23. The method of Claim 19, wherein the blotting compound includes at least one of the following: water grabbers, dry mineral fillers, and mixtures thereof.

24. The method of Claim 19, wherein the activator compound includes at least one of the following: fumaric acid, succinic acid, tartaric acid, malic acid, adipic acid, citric acid, glutamic acid, methane sulfonic acid, phosphoric acid, polyphosphoric acid, lactic acid oligomers, water, amylose, other starch-based hydrophilic polymers, cellulose-based hydrophilic polymers, hydrates of inorganic acids or salts thereof, and hydrates of organic acids or salts thereof, and mixtures thereof.

25. The method of Claim 19, wherein the hydrophilic plasticizer includes at least one of the following: lactic acid, lactide, glycolic acid oligomers, lactic acid oligomers, glycolide, glycolide oligomers, lactide oligomers, lactates, diethyl adipate, polyethylene glycol succinate, epsilon-caprolactone, valerolactone, adipic acid esters, citric acid esters, glycol-alkyl esters, soy oils, and mixtures thereof.

26. The method of Claim 18, wherein the hydrophobic additive includes at least one of the following: a hydrophobic comonomer, a hydrophobic block polymer, a hydrophobic polymer, a hydrophobic plasticizer, other water-repelling additives, and mixtures thereof.

27. The method of Claim 26, wherein the hydrophobic comonomer includes at least one of the following: caprolactone, ethylene terephthalate, ethylene, styrene, vinyl chloride, propylene, acrylonitrile, styrene- butadiene, isoprene, alkenoic acids, and mixtures thereof..

28. The method of Claim 26, wherein the hydrophobic polymer block includes at least one of the following: poly(ethylene terephthalate) , polyethylene, polystyrene, poly(vinyl chloride) , polypropylene, acrylonitrile polymers, styrene-butadiene copolymers, polyisoprene, polyalkenoic acids, and mixtures thereof.

29. The method of Claim 26, wherein the hydrophobic polymer includes at least one of the following: poly(ethylene terephthalate), polyethylene, polystyrene, poly(vinyl chloride), polypropylene, acrylonitrile polymers, styrene-butadiene copolymers, polyisoprene, polyalkenoic acids, and mixtures thereof.

30. The method of Claim 26, wherein the hydrophobic plasticizer includes at least one of the following: stearyl esters, low-toxicity phthalates, phenyl esters, phenyl ethers, sebacic acid esters, and mixtures thereof.

31. The method of Claim 18, wherein the degradable material includes a blend compatible agent to increase the solubility of the at least one of a hydrophilic additive and hydrophobic additive in the degradable material.