WO1990001043A1

WO1990001043A1 - A hydrophilic polymer with increased resistance to hydrophilic solvents

Info

Publication number: WO1990001043A1
Application number: PCT/CH1989/000132
Authority: WO
Inventors: Ivan Tomka; Stefan Schmidlin
Original assignee: Ivan Tomka; Stefan Schmidlin
Priority date: 1988-07-20
Filing date: 1989-07-13
Publication date: 1990-02-08
Also published as: AU3860989A; HU894300D0; CN1039604A; IE892350L; HUT54719A; GR890100450A; EP0378646A1; ES2022769A6

Abstract

An aliphatic polyester with low water absorption is selected as a coating material for hydrophilic polymers like starch. The polyester can be degraded by bacteria or fungi. This polyester may, for example, be a polyhydroxycarbonic acid. It is dissolved and applied to the components, of starch, for instance, by one of the customary physical or chemical coating processes. Once the solvent has evaporated, a coating film is left on the surface. If the coating thickness is adequate, the starch thus treated can no longer dissolve or swell. Such combinations of coatings adhere poorly to one another. Improved adhesion may be achieved by various methods, e.g. by prior corona treatment of the surface for the starch and/or the addition of a solvent or swelling agent for the starch to the coating solution.

Description

Hydrophilic polymer with increased resistance to hydrophilic solvents

The present invention relates to a hydrophilic polymer with increased resistance to hydrophilic solvents, a method for increasing the resistance of a hydrophilic polymer to hydrophilic solvents, a method for improving adhesion between a hydrophilic polymer and a hydrophobic coating, and a use of the methods.

Hydrophilic polymers are becoming more and more important as so-called "engineering plastics", i.

for use as so-called plastics. This is mainly because the hydrophilic polymers are natural substances such as starch, gelatine, cellulose, etc., which are cheap on the one hand and help to reduce dependence on fossil raw materials on the other. In addition, as natural products, they are physiologically safe to process and easily biodegradable.

In particular, the hydrophilic polymers based on natural substances have the disadvantage that, due to the hydrophilicity of hydrophilic solvents, such as water and solvents of similar polarity to water, they are more or less partially dissolved on their surface, resp. the surface swells.

This causes forth from hydrophilic polymers Asked moldings, films, etc., for example, have insufficient resistance to water. This is reflected in the fact that the water sorption curve of the polymers passes through a so-called inflection point, ie the absorption of

"Free water" in the polymer exceeds the proportion of "bound water", which leads to the swelling mentioned. The swelling or partial dissolution of the polymers, for example by water, is so disadvantageous that the use of the above-mentioned polymers, which are largely produced on a natural basis, is severely restricted.

It is therefore an object of the invention to provide a hydrophilic polymer without the disadvantages mentioned above, resp. to propose a method for increasing the resistance of hydrophilic polymers to hydrophilic solvents.

According to the invention, the object is achieved by a polymer and a method, preferably according to at least one of the claims, in particular according to claim 1 or 9.

A hydrophilic polymer is proposed which comprises a coating of a hydrophobic material.

In particular, hydrophilic polymers produced largely on a natural basis are proposed, such as starch, plasticized starch, gelatin, animal or vegetable proteins, cellulose, derivatives of cellulose, carbohydrates, but also largely synthetically produced hydrophilic polymers such as vinyl acetate, acrylic acid, polyester and polyvinylpyrrolidine.

In particular, plasticized starch is proposed as the hydrophilic polymer, which is produced by supplying mechanical and/or thermal energy, the production of the essentially homogeneous amorphous mass, i.e. the supply of energy to the starch, being able to take place, for example, by means of a kneader.

All polymers listed are either obtained from natural raw materials in a known manner, resp. manufactured or produced by known methods.

All polymers can be processed into shaped bodies, films, hoses, etc. by at least one of the usual processing methods, such as injection molding, extrusion, etc.

The hydrophilic polymers proposed have increased resistance in particular to at least one of the following solvents: dimethyl sulfoxide

- Water, cold or hot

- caustic potash - Ammonia (liquid)

- formamide

- Aqueous solution of copper ethylene diamine

- 1:1 mixture of N-ethylenepyridinium chloride with dimethyl sulfoxide

- pyridine

- dimethylformamide.

As coating materials for the hydrophilic

polymers, resp. for moldings, films and the like produced therefrom, those materials are particularly suitable which show a low absorption of hydrophobic solvents and are suitable for film formation, i.e. whose molecular weight is above the film formation limit and is at least almost dense

result in film. Suitable coating materials include aliphatic polyesters, such as polyhydroxycarboxylic acids in particular. Proposed are, for example, polyhydroxycarboxylic acids, which consist of at least one hydroxycarboxylic acid of the formula below

are made and / or at least one of the corresponding lactones.

Polyglycolic acid, polyhydroxybutyric acid, polyhydroxypropionic acid, polylactic acid or polyhydroxyvaleric acid, for example, or their copolymers or mixtures, are proposed as being particularly suitable. Natural fats and waxes, paraffin, zein, a hydrophobic protein, and chitin are also suggested. The examples mentioned are particularly suitable because, on the one hand, they have low water vapor permeability and low swellability and, on the other hand, they can be biologically treated in a simple manner for example by means of bacteria and fungi in the soil, are degradable. Another advantage of the examples mentioned is that some of them are so-called bacterial products, ie. that they can be produced using biotechnological methods.

Next, a method for increasing the resistance of hydrophilic polymers, respectively. of moldings, films, etc. produced therefrom, to hydrophilic solvents, such as in particular water, by the polymer being at least partially coated with a film-forming hydrophobic material. For the coating of the hydrophilic polymers, according to a proposed variant of the method according to the invention, film-forming coating materials are proposed which are readily soluble in a hydrophobic solvent such as chloroform or trichlorobenzene. The application of the coating to the polymer can now be achieved, for example, by dipping a cold or heated object consisting of the polymer into the solution. When the object is removed, a film remains on the object, which film is produced, for example, by evaporation of the solvent. Depending on the desired film thickness of the hydrophobic coating, the application process can be repeated several times.

However, the solution can also be applied by means of the other known customary application methods, such as pouring, spraying, for example with a spray gun, flooding, rolling, etc., take place. The solvent is also removed in a known manner, for example by evaporation by means of heat or the action of radiation. Film formation can result either from pure precipitation of the coating material or, for example, by further molecular build-up or a crosslinking reaction in the coating material enabling the formation of an at least almost dense film-like coating under the influence of the radiation mentioned.

If the solubility of the coating material is poor, which can often be the case in particular with high-molecular substances, it is also proposed to carry out the application to the polymer, for example by means of a dispersion. Another proposed possibility is that the coating material is applied to the polymer in a melt. This can be done, for example, by dipping a polymer object into the melt and after removing the object from the melt, the coating material solidifies with film formation.

Application by means of coextrusion, an application method known per se, is also suitable, particularly when coating film-like or foil-like objects made from hydrophilic polymers. According to a further proposed method, the coating is carried out by applying the coating material in powder form. All well-known application processes, such as fluidized bed sintering, electrostatic application, etc., can be used for this. The film can then be formed either by simply melting the powder or else by polymerizing and/or crosslinking the coating material.

The application processes mentioned are not exhaustively listed, since other processes, such as electro-dipping processes, application of so-called high solids, pastes, etc., can also be used for applying the coating materials.

It has been shown that coatings of hydrophobic materials on hydrophilic polymers exhibit insufficient to poor adhesion in most cases. It is therefore further proposed to improve the adhesion of the film-like coating using an adhesion promoter which both the polymer, resp. on its surface, as well as the coating material can be attached.

It is further proposed that at least one so-called adhesion promoter is added to at least the hydrophobic material and/or its solution before application, which at least partially dissolves or swells the polymer during application or else chemically reacts with the polymer. By adding the adhesion promoter, such as dimethyl sulfoxide or another solvent or swelling agent, such as a hydrophilic solvent, the hydrophilic polymer is swollen on the surface and thus allows molecular chains of the hydrophobic material to partially penetrate into the polymer or vice versa.

If the coating material is applied repeatedly, for example dissolved, onto the hydrophilic polymer, the application of an adhesion promoter is only necessary for the first application. The subsequent increase in the layer thickness can take place without the addition of an adhesion promoter. To improve adhesion, an adhesion promoter does not necessarily have to be used, but according to a further proposed method, this can also be achieved in that at least the polymer, resp. subjecting the surface of an article made of hydrophilic polymer to electron and/or ionic action by means of a corona treatment, thereby physically and/or chemically activating the polymer substantially at its surface. Corona treatment to increase adhesion is a well known process that uses a high voltage generator and electrode assembly. The electrode distance between the object and the electrode, the duration of the treatment and the output of the generator are decisive for the intensity of the corona treatment. The under The difference between a treated and an untreated surface of the hydrophilic polymer can be achieved by sprinkling a powder mixture of lead oxide with

Sulfur, preferably in a ratio of about 2: 1, and then shaken off are made easily visible. Since the corona effect subsides after hours to days, the coating with the hydrophobic material should be carried out within a reasonable period of time. In principle, an adhesion promoter can be dispensed with if the corona treatment of the polymer is used, but a combination of the various methods to improve adhesion is also possible.

The methods described according to the invention are suitable very generally for improving adhesion between an essentially hydrophilic polymer and any essentially hydrophobic coating material.

The methods described above according to the invention are also particularly suitable for adjusting the speed at which a hydrophilic solvent attacks a hydrophilic polymer by selecting the layer thickness and/or the molecular weight of the hydrophobic material and the crystalline components and/or the chemical composition of this material. With a small layer thickness, resp. with a low density of the coating due to the lower molecular weight of the hydrophobic material, resp. lack of networking, the attack occurs hydrophilic solvent, such as water, faster, respectively. the hydrophilic polymer is dissolved faster, partially or completely than when applying a greater layer thickness or a greater density of the coating.

All of the polymers and methods according to the invention proposed above are not limited to hydrophilic polymers which have insufficient resistance to hydrophilic solvents, but also relate to polymers which have a relatively low absorption of a hydrophilic solvent. For example, a polyamide can also be coated to prevent water absorption. The same also applies to the hydrophobic coating, which is not limited to materials that completely exclude absorption or solubility in hydrophilic solvents. For example, coatings based on thermoplastic or duroplastic polymers can also be used which, for example, have low water absorption, but not to such an extent that the hydrophilic polymer to be protected is damaged as a result.

The examples according to the invention given below show possible variants of the coating of a hydrophilic polymer with a hydrophobic material.

The examples are described with reference to the accompanying figures. show:

1 shows a schematic diagram of a system for corona treatment of the surface of hydrophilic polymers,

Fig. 2 the electrode arrangement according to Fig. 1 in

Longitudinal cross-section, Fig. 3 shows the geometry of a specimen on a scale of 1:1, which was used to carry out the examples,

Fig. 4 shows the results of adhesion improvement with corona pretreatment of the test specimen in

table form,

5 shows a test arrangement for measuring the water resistance of the specimen,

Fig. 6 three specimens with different

Water resistance after performing the resistance test, Fig. 7 is a schematic diagram of an arrangement for

carrying out the adhesion test,

8 shows the results of adhesion improvement

use of an adhesion promoter in tabular form, 9 is a heat flow diagram of polyhydroxybutyric acid used in the examples, FIG. 10 is a heat flow diagram of a coating material used in another example, and

11 shows the results in tabular form of the measurement of the water resistance of the coating of the further example with different layer thicknesses.

First example:

Coating of starch with polyhydroxybutyric acid by corona treatment of the starch a) storage of the starch moldings - resp. Starch Samples:

Although the uncoated starch samples are water sensitive, they are stored at room temperature and normal room humidity. You don't change in this environment. b) Cleaning the samples: Is the starch surface

greasy or otherwise soiled, it must be cleaned before treatment. Essentially, this improves adhesion. The Reini This is done with acetone or another fat-dissolving solvent. Since the starch bodies quickly become brittle under the influence of acetone, care must be taken. After cleaning, wait a few minutes until the acetone has completely evaporated (sniff the nose).

Now the starch sample can be pre-treated. It must be ensured that the cleaned surface is no longer touched when handling the starch samples. Corona treatment: The apparatus used, for example, for the corona treatment is a laboratory device from SOFTAL Electronic GmbH, Hamburg, Germany. The plant is sketched in fig. A high-voltage generator of type 3003 is used.

A high-voltage generator 1 is connected to an electrode arrangement consisting of an electrode 3 and a counter-electrode 5 . The counter-electrode 5 is mounted on rollers and can be displaced transversely with respect to the electrode 3 . Between the two electrodes 3 and 5, the starch parts, respectively. Samples 7 arranged. The electrode 3 is mounted on a protective housing 9, which can be changed by four leveling screws 11 in height and thus the distance between the two electrodes 3 and 5. The counter-electrode 5 can by a Electric motor 13 are moved transversely to the electrode 3 with winding coil. Furthermore, the electrode 3 is connected to an extraction system 15 for extracting ozone.

Three parameters are varied for the treatment of the starch. They are:

1. Electrode distance between sample and electrode,

2. Duration of corona treatment,

3. Four power levels of the generator (0.1 - 0.5 kW).

The corona treatment and the subsequent coating are carried out, for example, with the aid of starch test bodies or specimens according to FIG. The specimen in Fig. 3 is shown on a scale of 1: 1, with the listed

Dimensions are given in mm.

The selected times, power levels and the electrode distances can be seen from FIG. 4, in which results relating to improvement in adhesion with corona treatment are tabulated. d) Procedure for corona treatment: This will

described below with reference to Figs. 1 and 2: 1. The starch samples 7 are cleaned and glued with double-sided adhesive tape to the counter-electrode 5, an aluminum plate that represents the mass. This bonding. is used for fixation and for maintaining plane parallelism to electrode 3. This ensures a definite distance d between sample 7 and electrode 3. 2. The electrode gap is set precisely as shown in Fig.2. 2 shows the electrode arrangement in longitudinal cross section. Between the electrode 3 and the counter-electrode 5, the starch samples 7 are arranged on the latter. A so-called dielectric 8 is applied to the electrode 3 below it. The distance between the dielectric 8 and the sample 7 is now referred to as the distance d. 3. The ozone extraction 15 is switched on.

4. The motor 13 for traversing the aluminum plate 5 with the sample 7 thereon is started. (It is also possible not to move the sample at all and to vary the treatment time. With this option, point 5 below does not apply.)

5. The generator is immediately switched on at the highest power level and switched off again after the sample 7 has been pulled through under the electrode 3. 6. Sample 7 is set aside without touching the surface for subsequent coating. Since the corona effect subsides after a few hours to days, the sample should be coated within a reasonable period of time. e) Coating: The coating solution consists

from between 1% to 15% polyhydroxybutyric acid dissolved in chloroform. A 5% polyhydroxybutyric acid solution is used to measure the improvement in adhesion with the corona treatment according to the table in FIG. In this example, the molecular weight of the polyhydroxybutyric acid is approximately 410,000.

The polyhydroxybutyric acid used in the present example is characterized in more detail in the accompanying heat flow diagram in FIG. 9 shows a heat flow diagram of the PHB preparation used in Examples 1 and 2. The recording originates from a commercial heat flow measuring device (DSC=differential scanning calorimetry) from Perkin Elmer.

This PHB shows an endotherm at about 5°C

Glass transition 30, at about 52°C an exothermic recrystallization transition 31 and at about 176°C an endothermic melting transition. The amount of heat required in the glass transition 30 is much smaller than in the

Melt conversion 32, from which are taken that this PHB preparation is a highly crystalline polymer.

With regard to polyhydroxybutyric acid, however, molecular weights of approximately 300,000 to a few million can also be used. With other coating materials used, smaller molecular weights of, for example, 50,000 are also possible. The film formation limit of the coating material used is particularly important.

In the present example, an immersion apparatus is used to coat the samples. The specimens are dipped into the coating solution and pulled out again at a speed of, for example, about 7 cm/sec. The dwell time of the test specimens in the solution can be selected as desired by switching an immersion mechanism used, for example, on and off. In the present example, the samples pre-treated with corona are only dipped quickly and then immediately withdrawn from the solution. Drying: If several immersion processes are necessary, you should wait between each immersion process until the film that has formed has dried.

The drying, ie the evaporation of the solvent, takes place, for example, in air room temperature. In order to obtain a layer with good water resistance, the sample is coated three times, for example, with a 5% polyhydroxybutyric acid solution. After each immersion, wait at least 10 minutes until the layer is dry. Water Resistance Testing: In order to test the water resistance of the coating on the starch surface, the following test is proposed with reference to Figs. 5 and 6.

Small rings 17 are placed on the dried starch bodies 7 . Aqueous iodine-potassium iodide solution is filled inside the ring. Depending on the water resistance of the layer, the starch may swell and cracks form in the layer. The test solution with the iodine quickly penetrates there and small blue, clearly visible cracks form, as shown in FIG. 6b.

If testing is carried out over a longer period of time, care must be taken to ensure that the water does not evaporate. In the case of a waterproof film, only the film surface will be slightly yellowed by the iodine. However, no blue coloring is visible, as shown in FIG. 6a.

The test body according to FIG. 6c shows poor water resistance, resp. a distinct blue discoloration. If the samples have passed the water test after 48 hours, for example, i.e. there is no blue discoloration, the samples are tested for the adhesion of the layer. Adhesion testing: In order to measure the adhesion of the coating, the following test is proposed with reference to Figure 7: The coating to be tested on the thickness 7 is slightly loosened with a sharp knife from the untreated starch surface. This allows a clip 25 to be attached to the partially detached layer 23 .

The specimen 7 is clamped in a holder 21 . A container 29, connected via a wire 27, can now be attached to the terminal 25. In this container 29 weights can be filled. These serve the adhesion of the layer 23 with the launched

to correlate weight. The adhesive strength is then exceeded when the weight acting on the layer at the clamp 25 causes the layer to detach without any further addition of weight and without external influence.

If the adhesion improvement treatment is successful, a weight difference between the treated and untreated sample must be measurable. The

The results of the adhesion test can be found in the table according to Fig. 4 notice that all coatings are the same and that the layers were applied within almost the same time periods after the corona treatment. It can be seen from this table according to FIG. 4 that the adhesive strength of the hydrophobic polymer, such as starch, can be greatly increased with a corona treatment.

Second example:

Coating of Starch with Polyhydroxybutyric Acid Using dimethyl Sulfoxide as Adhesion Promoter The starch samples for this example are treated in the same way up to and including point b) of the previous example.

Points c) and d) do not apply, i.e. the corona treatment can be dispensed with. These two

Points are replaced by the following point i). i) Pre-treatment of the starch with an adhesion-promoting solution: dimethyl sulfoxide, or DMSO for short, is considered a swelling and solvent for the starch. If, for example, 0.1% to 10% DMSO is added to an ordinary 1% to 15% polyhydroxybutyric acid solution and the starch surface is coated with it, a strong improvement in adhesion is achieved. In this example, the sample is mixed with 0.5%, 1% and 3% DMSO in a 3% polyhydroxybutyric acid-chloroform solution. Instead of chloroform, another suitable solvent such as trichlorobenzene can be used for the polyhydroxybutyric acid solution.

Likewise, instead of DMSO, another solvent for starch, such as pyridine, can be used as an adhesion promoter. A possible parameter here is the time the sample remains in the DMSO-polyhydroxybutyric acid solution before it is withdrawn again. In the present example, immersion times of 1 and 30 seconds are selected. Only part of the sample is coated. After this first coating, the sample is dried for 24 hours. Thereafter, point f) of the previous example can be continued, i.e. further coating using adhesion promoter-free solution. In the present example, one layer each is applied to the sample twice more.

The exact details as well as the results of the water and adhesion tests can be found in the table according to FIG. As can be seen from this table, a significant increase in the adhesive strength between treated and untreated starch moldings can be observed. The water resistance lasts for at least 24 hours without any visible change in the surface. Third example:

Adjusting the resistance of starch to water by choosing the layer thickness of the coating

In this example, a 4 to 1 copolymer of polyhydroxybutyric acid (PHB) with polyhydroxyvaleric acid (PHV) is used as the new coating material. Starch is again used as the carrier material - the same material as in Examples 1 and 2.

The heat flow diagram according to FIG. 10, which was created analogously to FIG. 9, characterizes the copolymer PHB/PHV. At about 3°C an endothermic glass transition 35 is visible, at about 74°C an exothermic recrystallization 36 and at about 125°C an endothermic melting transition is visible. Note now that the glass transition 35 relative to the melt transition 37 is no longer as small as in Figure 9, indicating a lower crystallinity of the copolymer relative to neat PHB. A consequence of this is, for example, a slightly improved solubility of the coating material in chloroform.

This example examines the influence of the layer thickness of the coating material on the water resistance of the coated starch. This is examined in the present example to the effect that the coating to be coated tending starch is, on the one hand, immersed once, twice or three times in the coating solution and, on the other hand, three different concentrations of the coating material in the coating solution are chosen.

To better understand the influence of

Depending on the layer thickness and the water resistance, the measures taken in Examples 1 and 2 to improve the adhesion of the coating to the starch are dispensed with. The non-corona-treated starch specimens are immersed one, two or three times in the dipping solution, which is a 1%, 3% or 5% PHB/PHV solution in chloroform. The coated test specimens are then dried as described in the previous examples, resp. the solvent evaporates. The now dry, coated test specimens are subjected to the water test described in Example 1. The results of the various water tests are summarized in the table shown in FIG. It is clearly visible that the water resistance increases with increasing layer thickness of the coating. The layer thickness is influenced on the one hand by the number of times the starch is dipped in the dipping solution and on the other hand by the concentration of the coating material in the dipping solution. This example clearly shows that the access of water to the carrier material increases with increasing solution concentration and increasing number of immersion processes is more hindered, thus the water resistance of the starch, resp. the attack speed of the water can be adjusted to the strength individually.

The three examples 1 to 3 selected here can be carried out in the same way with shaped bodies made of any hydrophilic polymer other than starch. As coating materials, in addition to the polyhydroxybutyric acid used, for example, resp. the copolymers essentially all aliphatic polyesters, such as in particular those based on polyhydroxycarboxylic acids, in question. The solvents used

Adhesion promoters, application conditions, etc. can be varied and changed in any number of ways.

In the case of the corona treatment in particular, it should also be mentioned that the system shown in FIG. 1 is a laboratory system which is particularly suitable for the corona treatment of test specimens. In the corona treatment of non-planar objects, such as hollow bodies, the electrode arrangement must of course be chosen differently. In the case of hollow bodies, for example, a plate-shaped electrode is not chosen, as shown, but a point or rod-shaped electrode that can be inserted into the hollow body. Sourced writings:

- J Branddrup, Polymer Handbook, J Wiley & Sons,

2nd ed., NY.

- Donovan J.W. (1979) Biopolymers 18:263

- J. Hansmann, methods for surface treatment

adhesion improvement. paper and plastic processing,

4 & 7, 1981

- E. Husemann and E. Siefert, Makrom. Chemistry. 128,

288 (1969)

- B. Vergnes, J.P. Villemaire, Rheological behavior of low moisture molten maize starch, Rheol. acta: 570-576 (1987)

- Winnacker, Küchler, Chemical Technology, 4th edition, Carl Hanser Verlag, 1986

- The theory and practice of corona treatment for

improving adhesion, Tappi, Vol. 65, no. 8, 1982

Claims

STATEMENT REFERRED TO IN ARTICLE 19

According to the search report and the documents cited therein, the subject matter of claims 1 and 10 is described in full. Correspondingly, hydrophilic polymers coated with hydrophobic materials are known, which means that claims 1 to 9 in the present form can no longer be upheld as protection of substances.

In order to maintain the increased resistance of the hydrophilic polymers, it is essential that auxiliary measures are taken during coating with the hydrophobic material in order to ensure that the adhesion of the coating to the hydrophilic material is at least adequate. These measures, originally claimed in claims 16 and 19, have been taken into account for the reformulation of claim 1. However, these auxiliary measures are not disclosed in any citation in connection with the task at hand.

All of the other features and measures that are essential to the invention during coating have now been made dependent on independent claim 1. All features and measures are included in the original application, so the new claims do not go beyond the subject matter of the original application.