CA2081105C - Method of improving the oxidative thermal stability of ethylene polymers - Google Patents
Method of improving the oxidative thermal stability of ethylene polymers Download PDFInfo
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- CA2081105C CA2081105C CA002081105A CA2081105A CA2081105C CA 2081105 C CA2081105 C CA 2081105C CA 002081105 A CA002081105 A CA 002081105A CA 2081105 A CA2081105 A CA 2081105A CA 2081105 C CA2081105 C CA 2081105C
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- polymer
- ethylene
- interpolymer
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- stripping agent
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F6/00—Post-polymerisation treatments
- C08F6/001—Removal of residual monomers by physical means
- C08F6/005—Removal of residual monomers by physical means from solid polymers
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
- B29C48/25—Component parts, details or accessories; Auxiliary operations
- B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
- B29C48/395—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using screws surrounded by a cooperating barrel, e.g. single screw extruders
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
- B29C48/25—Component parts, details or accessories; Auxiliary operations
- B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
- B29C48/50—Details of extruders
- B29C48/76—Venting, drying means; Degassing means
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
- B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
- B29C48/07—Flat, e.g. panels
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
- B29C48/25—Component parts, details or accessories; Auxiliary operations
- B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
- B29C48/50—Details of extruders
- B29C48/76—Venting, drying means; Degassing means
- B29C48/762—Vapour stripping
Abstract
A method of improving the oxidative thermal stability of ethylene polymers is disclosed. The method comprises removing residual unreacted monomer(s), solvent and thermally unstable species from the polymer using a stripping agent and subsequent devolatilization. The resulting polymer is characterized by having an oxidative exotherm of not more than 50 percent of the original polymer, as measured by differential scanning calorimetry, as well as improved taste and odor properties.
Description
2Q~~ ~.~~
METHOD OF IMPROVING THE OXIDATIVE THERMAL STABILITY OE
ETHYLENE POLYMERS
This invention relates to a method of improving the taste and odor properties and the oxidative thermal stability of thermoplastic homopolymers and'or interpolymers of ethylene and at least one comonamer in the presence of oxygen. The method comprises removing residual unreacted monomer(s), solvent and thermaa:n unstable species. The resultant polymer is more oxidatively thermally stable and has an oxidative exotherm of not more than about 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry. This method of improving oxidative thermal stability is especially effective for ethylene/acrylic acid interpolymers.
Volatile components can be removed from thermoplastic polymers in a variety of ways. one traditional method is extrusion devolatilization using vacuum equipped extruders. The use of vacuum alone, however, is generally insufficient to remove residual monomer and other-volatiles from the molten polymer to sufficiently improve the oxidative thermal stability of the polymer. Residual monomer and volatiles also contribute to odor deficiencies in the polymers, a fact 20~1i0~
which is particularly important when the polymers are used in food storage appii.cations. , US-A-4.09~1,9~2 (Nakai et al.) discloses a method for removing unreacted monomer from an ethylene .
polymer during pelletization by injecting water or steam into the nose portion of the mixing section of an extruder, intermixing the polymer with the water or steam and removing the unreacted monomer and water or steam using vacuum of 500 mm Hg absolute or less. Nakai et ai. teach that products thus produced have increased commercial value with respect to odor, but does nat teach or suggest how to improve the taste performance cr thermal stability of the polymer.
Thermal stability of such polymers is important ?5 because they are thermally processed at elevated temperatures where they are subject to accelerated degradation and/or cross-linking. especiaiiy in the presence of oxygen. Oxidative thermal degradation of ethylene polymers can cause gel formation and loss of physical properties in the processed polymers (e. g., poorer extrusion performance (such as neck-in and lower line speeds in extrusion coating) and reduced optical properties (such as haze and gloss or even color)), in addition to poor taste and odor properties from the oxidative by-products formed during processing.
Anti-oxidants (e.g., organic phosphites and ~ .y hindered phenols) have been added to various polymers.
including polyethylene. to improve their thermal ' stability. However, anti-oxidants can nave deleterious side effects, e.g., color formation, and they can also create food contact problems in themselves, thus ' negating the benefits.
I
METHOD OF IMPROVING THE OXIDATIVE THERMAL STABILITY OE
ETHYLENE POLYMERS
This invention relates to a method of improving the taste and odor properties and the oxidative thermal stability of thermoplastic homopolymers and'or interpolymers of ethylene and at least one comonamer in the presence of oxygen. The method comprises removing residual unreacted monomer(s), solvent and thermaa:n unstable species. The resultant polymer is more oxidatively thermally stable and has an oxidative exotherm of not more than about 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry. This method of improving oxidative thermal stability is especially effective for ethylene/acrylic acid interpolymers.
Volatile components can be removed from thermoplastic polymers in a variety of ways. one traditional method is extrusion devolatilization using vacuum equipped extruders. The use of vacuum alone, however, is generally insufficient to remove residual monomer and other-volatiles from the molten polymer to sufficiently improve the oxidative thermal stability of the polymer. Residual monomer and volatiles also contribute to odor deficiencies in the polymers, a fact 20~1i0~
which is particularly important when the polymers are used in food storage appii.cations. , US-A-4.09~1,9~2 (Nakai et al.) discloses a method for removing unreacted monomer from an ethylene .
polymer during pelletization by injecting water or steam into the nose portion of the mixing section of an extruder, intermixing the polymer with the water or steam and removing the unreacted monomer and water or steam using vacuum of 500 mm Hg absolute or less. Nakai et ai. teach that products thus produced have increased commercial value with respect to odor, but does nat teach or suggest how to improve the taste performance cr thermal stability of the polymer.
Thermal stability of such polymers is important ?5 because they are thermally processed at elevated temperatures where they are subject to accelerated degradation and/or cross-linking. especiaiiy in the presence of oxygen. Oxidative thermal degradation of ethylene polymers can cause gel formation and loss of physical properties in the processed polymers (e. g., poorer extrusion performance (such as neck-in and lower line speeds in extrusion coating) and reduced optical properties (such as haze and gloss or even color)), in addition to poor taste and odor properties from the oxidative by-products formed during processing.
Anti-oxidants (e.g., organic phosphites and ~ .y hindered phenols) have been added to various polymers.
including polyethylene. to improve their thermal ' stability. However, anti-oxidants can nave deleterious side effects, e.g., color formation, and they can also create food contact problems in themselves, thus ' negating the benefits.
I
The taste and odor performance and the oxidative thermal stability of thermoplastic polymers continue to be areas of commercial need.
A method of improving the taste and odor properties and the oxidative thermal stability of thermoplastic ethylene polymers has now been discovered.
The novel method is characterized by removing residual unreacted monomer or monomers, solvent and thermally unstable species from a first ethylene polymer, thereby forming a second more oxidatively thermally stable ethylene polymer having an oxidative exotherm of not more than 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry.
The novel method is particularly useful in improving the taste and odor properties and the oxidative thermal stability of ethylene/acrylic acid interpolymers which are widely used in the food packaging industry in single or multilayer laminate film structures for food wraps, beverage containers or condiment pouches.
In one aspect, the invention provides a method of improving the taste and odor properties and the oxidative thermal stability of a first thermoplastic ethylene polymer, comprising removing residual unreacted monomer or monomers, solvent and thermally unstable species from the first polymer, thereby forming a second more oxidatively thermally stable ethylene polymer having an oxidative exotherm of not more than about 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry, the method comprising: (a) feeding the first polymer into a devolatilization extrusion system having at least two vacuum zones and at least one barrel, the at least one barrel having:~i. a first vacuum zone, ii. at least one i -3a-stripping agent injection port, iii. at least one confined mixing zone, and iv. a second vacuum zone, wherein the at least one confined mixing zone is situated immediately downstream of the at least one stripping agent injection port, the second vacuum zone is situated downstream of the first vacuum zone, and the first vacuum zone is designed to operate partially-filled with the first polymer; (b) conveying the first polymer through the extrusion system;
(c) applying vacuum to the first vacuum zone; (d) concurrently injecting at least about 0.1 percent by weight based on total first polymer feed of at least one inert stripping agent to at least one stripping agent injection port; (e) mixing the first polymer and the inert stripping agent in the at least one confined mixing zone; (f) applying vacuum to the second vacuum zone; and (g) recovering the extruded second polymer.
Figure 1 schematically shows an extrusion system comprising two single screws operated in series which can be used to improve the oxidative thermal stability of ethylene polymers.
Figure 2 schematically shows a twin screw extrusion system which can be used to improve the oxidative thermal stability of ethylene polymers.
_q_ Figure 1 schematically shows the cross-section of an extrusion system comprising two single screws useful for improving the oxidative thermal stability of , ethylene polymers, as disclosed in Comparative Examples .
1-3 and Example 4 below. In Figure 1, the polymer is fed into the feed section (1) of a first extruder system (13) and is melted and, conveyed into zone 1 by a screw.
The temperature of,tlie entire extruder system is controlled by a hot oil system (not shown). Ethylene (as a stripping agent) is injected into zone 2 through a , port (2). The melt temperature of the °irst extruder is monitored by means of a thermocouple (3). A transfer line (4) is used to transfer the polymer/ethylene mixture to a second extruder system 1,12). Preferably, the screw in the first extruder operates at slower revolutions per minute (rpm) than the screw in the second extruder such that the feed to the second extruder is less than that required to fill the screw channels (i.e., the second extruder is "starve fed"). A
First injection port (5) is used to inject an additional ,.
stripping agent into the polymer melt stream in the second extruder and the polymer/ethylene/additional .
stripping agent mixture is conveyed into a mixing zone (10). The polymer/ethylene/additional stripping agent .
mixture is temporarily confined and controlled within the mixing area by a melt seal (11) and the compression side (up-stream side) of the screw. The polymer/ethylene/additional stripping agent mixture flows past the melt seal (11) and into the first vacuum , port area (6). Vacuum is applied to the first vacuum port (6) and the unreacted monomer(s), residual solvent and residual thermally unstable species are partially devolatilized. The polymer then enters a second WO 92/15621 PCT/US92/01.720 -7- 2~8~.~.~~
compression zone and an effective melt seal is formed.
A second injection port (7) is utilized to inject a second stripping agent into the polymer melt s=ream.
immediately followed by a mixing zone (14), and a melt seal (15). The polymer/additional stripping agent mixture is temporarily confined between the melt seal and the compression side (up-stream side) of the scresu ~o ensure adequate mixing. A second vacuum port (8) is used to remove additional unreaeted monomer(s), residual solvent and residual thermally unstable species, thus .
forming a polymer with greater oxidative thermal stability. A melt thermocouple (9) monitors the temperature of the exiting melt stream.
Figure 2 schematically shows the cross-section 1~ of one screw of a co-rotating twin screw extrusion system useful for improving the oxidative thermal stability of ethylene polymers, as disclosed in Comparative Examples 5-7 and Example 8. In Figure 2, the polymer is fed into the feed section (16) of a twin screw extruder system (28) and is melted and conveyed into zone 1 by two intermeshing screws. The temperature of each extruder zone is controlled by a temperature controller and resistance heaters (not shown). The temperature of the extruder is monitored by means of a series of thermocouplesi(29-36). Preferably, the feed to the extruder is controlled such that it is less than that required to fill the screw channels of the devolatilization zone (i.e.. the vacuum zone of .the extruder is partially filled). A first injection port (17) is used to inject a first stripping agent into the polymer melt stream and the polymer/stripping agent mixture is. homogenized in a mixing zone (24). The mixed polymer/stripping agent mixture is temporarily confined and controlled within the mixing area by two melt seals WO 92/15621 ~ PCT/US92/01720 _ 2~~~.1~5 (38, 25). The polymer/stripping agent mixture flows past the melt seal (25) and into the first vacuum port area (18). Vacuum is applied to the first vacuum port (i8) and the unreacted monomer(s), residual solvent and . .
residual thermally unstable species are partially ' devolatilized. A second melt seal (37) separates the first vacuum zone from a second stripping agent injection port (19). The second injection port (19) is utilized to inject a second stripping agent into the Polymer melt stream. immediately followed by a mixing.
zone (33), and a melt seal (27).:w..The mixed polymer/stripping .agent mixture.... is temporarily confined and controlled within the mixing area by two melt seals (37, 27). The polymer/stripging agent mixture flows 1, past the melt seal (27) and into the second vacuum port area (20). The second vacuum port (20) is used to remove additional unreacted monomer(s), residual solvent and residual thermally unstable species. thus forming the more oxidatively thermally stable polymer. A motor .
(not shown) drives a gear pump (21) which pumps the thermally stable polymer into a pelletizer (23) which chops the more thermally stable polymer into pellets. A
liquid stream (22) cools and solidifies the polymer into pellets. The cooling liquid stream conveys the ehopped/pelletized polymer to a separation point (not shown) where the liquid is separated from the polymer pellets and, optionally, is recycled back through the process.
_7_ Detailed Description of the Invention The thermoplastic ethylene polymers used in this invention can be homopolymers or interpolymers of ethylene with at least one comonomer, such as alpha-olefins or substituted alpha-olefins.
Homopolymers of ethylene for use in the present invention include homopolymers of ethylene made using a high pressure, free radical generating process and are generally known as low density polyethylene. Low density polyethylene (LDPE), also known as High Pressure Polyethylene and historically as ICI-type polyethylene:
is a branched ethylene homopolymer made using free radical polymerization techniques under high pressures.
The ethylene is randomly polymerized into the polymer chains and forms numerous branch points. The density of LDPE is dictated by the degree and length of branches and is typically in the range,of 0.91-0.925 grams per cubic centimeter (g/em3). The molecular weight of the LDPE is indicated by melt index (MI) as measured using ASTM D-1238 (E) (190°C/2.16 kilograms). The MI range for LDPE for use in the present invention can be from 0.01 to 1000 grams/10 minutes, preferably 0.1 to 100.
Linear polyethylene is another type of polyethylene useful in practicing the present invention.
Manufacture of linear polyethylene is disclosed, e.g., in US-A-u,076,698 and involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type.
These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear"
polymers because of the substantial absence of. branched 2~8~ 1~~
chains of polymerized monomer un its pendant from the.
main polymer "backbone." Two types of linear polyethylene are also suitable for use in the present invention: linear high density polyethylene (HDPE) and .
linear low density polyethylene (LLDPE).
Linear high density polyethylene has a density from 0.941 to 0.965 g/cm~ while linear low density polyethylene typica_ly has a density from 0.880 g~'cm~ to 0.940 g/cm~. The density of the linear polyethylene is lowered by polymerizing ethylene with minor amounts of alpha. beta-ethylenically unsaturated alkenes (i.e., alpha-olefins) having from 3 to 20 carbons per alkene molecule, preferably 4,.to 8 (e.g., i-butene, 4-methyl 1- ., pentene, 1-hexene) and most preferably 8 carbons per alkene molecule (i.e., 1-octene). The amount of the .
alkene .comonomer is generally sufficient to cause the density of the linear low density polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification. These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion or cast films and the:~high "slip" characteristic inherently found in HDPE is diminished.
The use of coordination-type catalysts for polymerizing ethylene into homopolymers, either in solution or slurry type polymerization, or copolymerizing ethylene with higher alkenes to make copolymers having-densities above 0.94 g/cm3 as defined ' in ASTM D-1248 (i.e., HDPE polymers) and/or for copolymerizing ethylene with higher.alkenes to make ' copolymers having densities in the range of LDPE and WO 92/1562y PCT/US92101720 2~~'~ ~.0~
medium density polyethylene (.i.e., LLDPE copolymers) is disclosed variously in, e.g.. US-A-2,699,457;
US-A-2,862,917: US-A-2,905.645: US-A-2.846,425;
US-A-3,058.963 and US-A-4,076.698. Thus, the density of the~linear polyethylene useful in the present invention is from 0.88 g/cm3 to 0.965 g/cm'.
The molecular weight of the linear polyethylene is indicated and measured by melt index according to ASTM D-1238, Condition (E) (i.e.. 190°C/2.?6 kilograms).
The melt index:of the linear polyethylene useful in the present invention can be from 0.001 grams/10 minutes (g/10 min) to 1000 g/l0 min, preferably from 0.01 gi'?0 min to 100 g/10 min.
Ethylene can also be interpolymerized with substituted alpha-olefins by known techniques. :The substituted alpha-olefins are a known class of polymerizable monomers that include, for example, ethylenically unsaturated carboxylic acids, esters and anhydrides (e. g., acrylic acid, methaerylie acid, malefic acid, malefic anhydride, methyl methacrylate. butyl aerylate), vinyl esters~of alkanoic acids (e. g., vinyl acetate) and vinyl alcohol.
2~
Interpolymers of ethylene and at least one ethylenically unsaturated carboxylic acid are particularly useful in the present invention. The term "interpolymer" means that ethylene is copolymerized with at least one comonomer. Thus the term "interpolymer"
can indicate a copolymer, a terpolymer and so forth.
Such interpolymers can be made by any of the known techniques. For example, US-A-4,599,-392 (MeKinney et al.) and US-A-4,248,990 (Pieski et al.) disclose processes for manufacturing random and non-random interpolymers, respectively, of ethylene/acrylic acid and ethylene/methaerylie acid. The amount of unsaturated carboxylic acid can vary from 0 (i.e.. LDPE) to as high as X10 percent, dependent on the carboxylic .
acid used in the interpolymerization.
Ethylene/unsaturated carboxylic acids) interpolymers made by use of telogenic modifiers, as disclosed in US-A-u,988,781, are also suitable for practicing the present invention. The~inolecular weight of the , ethyleneiunsaturated carboxylic acid interpolymers is indicated by melt index and is also measured by ASTM D-1238. Condition E (i.e., 190°C/2.16 kilograms). The melt index of the ethylene/unsaturated carboxylic acid interpolymers can be from 0.1 g/10 min to 3000 g/10 min.
preferably from 1 g/10 min to 300 g/10 min.
Ionomers of various interpolymers (,e.g., metal ionomers of ethylene/aerylie acid or ethylene/methacrylic acid interpolymers) are also within the scope of this invention. US-A-3,404,13+ (Bees), for example, discloses a process for cross-linking polymers thereby creating ionomers which are useful in the present invention. In one embodiment of the present invention, aqueous solutions of metal hydroxides (e. g..
sodium hydroxide or potassium hydroxide . nitrogenous bases (e.g., ammonium hydroxide). or water-soluble a t strong base organic amines (e. g., mono-, di-and tri-methyl amines) can be added to a devolatilizing extruder in a first injection zone and mixed with ethylene/unsaturated carboxylic acids) interpolymers to effectively function as both a stripping agent and neutralizing/ionizing agent. The water is subsequently stripped under vacuum thus removing residual unreaeted .
2~~~.~~~
monomer or monomers, solvent and thermally unstable species forming a more thermally stable ionomer.
Additives can be included in the interpolymers useful in practicing the present invention. but are preferably added after forming the more thermally stable polymer. These additives include pigments (e. g., titanium dioxide), wetting agents, fillers, slip and anti-block compounds. Anti-oxidants can also be included in the interpolymers. but they need not be present for the present invention to be effectively used.
Solvents (or diluents) can be and generally are used in both the high pressure process (e.g., LDPE) or the low pressure process (e.g.. LLDPE) to make the ethylene polymers. Some of the typical solvents~used during interpolymerization include: paraffinic hydrocarbons such as pentane, hexane, heptane, 2~ cyclohexane. branched alkanes, alkenes, and alkynes (plus their various isomeric forms); or aromatic hydrocarbons such as benzene, toluene, chloro-substituted aromatic hydrocarbons such as dichlorobenzene; or polar solvents such as alcohols.
etzers, ketones and esters; or various naphtha distillates such as those sold under the trademark ISOPAR by EXXON. These solvents are difficult to remove from the interpolymer, even after pelletization.
Residual monomer (e. g., ethylene and/or acrylic acid) ~ can also be retained in the interpolymer. Thermally unstable species (e.g.., free radical species) can react with oxygen and cause taste problems. Thermally unstable species, as defined herein, are compounds which thermally degrade in the presence of oxygen at elevated temperatures (i.e., at polymer melt processing WO 92/15621 PGT/US92/01.720 temperatures). fihese semi-volatile components can cause taste and/or odor problemsin fabricated parts made from the interpolymers, depending on the application, and surprisingly, can also cause the interpolymer to have less oxidative thermal stability.
Removal of the residual unreaeted monomer or monomers, solvent and thermally unstable species to form the oxidatively thermally stable interpolymers of the present invention can be accomplished by any conventional means.
Batch operation includes such pieces of equipment as a Banbury mixer, or a roll mill. In batch operation, a specific amount of polymer and stripping agents) is placed into the equipment. The polymer is then melted and fluxed or mixed for a pre-designated amount of time. The equipment is vented periodically to allow for devolatilization. The sample is then removed 20 for processing into pellets or other forms for final usage. Steam can be added during this processing to aid in devolatilization.
Continuous devolatilization can be performed on 25 a single or twin screw extruder as shown in the examples. Other continuous devolatilization equipment includes Bolling Mixtrumat'" continuous process or Farrel DiscpackT" and other types of continuous extrusion equipment which can be fitted with vacuum 30 ports/adapters. Many of these systems can be fitted with a mixing section prior to devolatilization to improve the contact of the stripping agent and the interpolymer. The devolatilization occurs by producing -'3- 2~~~ 34~
a thin melt which is continuously regenerating a new surface that is exposed to a vacuum.
A particularly effective embodiment for forming more oxidatively thermally stable ethylene polymers of the present invention comprises the consecutive steps of (a)feeding a first ethylene polymer to an extruder comprising an optional stripping agent injection zone. an optional~confined mixing zone, at least a first partially filled vacuum zone, at least a first stripping agent injection zone, at least one first confined mixing zone, at least a second partially filled vacuum zone;
(b)melting the first ethylene polymer:
(e)applying vacuum to the first partially filled vacuum zone;
(.d)cocurrently injecting at least 0.1 percent by weight, based on total ethylene polymer feed, of at least one stripping agent to the first stripping agent injection zone;
(e)mixing the stripping agent with the first ethylene polymer in the first confined mixing zone;
(f)apglying vacuum to the second partially filled vacuum zone; and (g)recovering the second more oxidatively thermally stable ethylene polymer.
The stripping agents useful in the present invention are preferably inert with respect to the interpolymerization process. When forming oxidatively thermally stable ionomers from interpolymers of ethylene and at least one unsaturated carboxylic acid, however, the stripping agent should react with the unsaturated carboxylic acid. The stripping agents) should also have a vapor pressure sufficient to either be a gas or form gas bubbles at the devolatiiizing operating conditions (i.e., when the interpolymer is molten).
Preferably, the stripping agents) is a light hydrocarbon (e. g., ethylene, propylene or isobutane).
water, steam, alcohol, carbon dioxide and/or nitrogen.
Super critical fluids (e. g., super critical carbon dioxide) are also useful as stripping agents for use in the present invention. Aqueous solutions of alkali metal hydroxides (e.g.:.., sodium hydroxide), nitrogenous bases (e. g., ammoniumwhydroxide), water-soluble organic strong base organic amines (e. g.. mono-, di-and tri-methyl amines) are particularly effective when simulataneously devolatilizing ethylene/unsaturated carboxylic acid interpolymers while forming the corresponding ionomer. Mixtures of stripping agents are also useful in the present invention. The stripping agent should be used in amounts effective to remove enough residual unreacted monomer(s), solvent and stripping agents) such that the oxidative exotherm of the interpolymer or. ionomer is reduced by at least 50 percent.
The devolatilization of the interpolymers is ~o preferably performed using a single or twin screw system with water injection as described in US-A-u,09~.9~2, with the added feature of using a screw which has a surface renewal ratio of at least about two pounds per square meter or less, preferably about one pound per square meter or less. When creating a more thermally stable interpolymer made using a high pressure process (e.g., LDPE or ethylene/acrylic acid interpolymers) and the devolatilizing screw is installed in-line with the process (i.e., before the pelletization.step), the extruders) vacuum devolatilization zones) should be partially-filled (i.e., the flow of the interpolymer is restricted such that the channels of the screw are partially filled; this can be accomplished, e.g.. using a device which controls/restricts flow, such as a slide ~.ralve or a gear pump). The extruder comprises an optional stripping agent injection zone. an optional confined mixing zone. at least a first partially filled vacuum zone, at least a first stripping agent injection zone, at least one first confined mixing zone, and at Ieast a second partially filled vacuum zone.
Preferably, a vacuum of 100 mm Hg absolute or less is applied to the first vacuum zone, at least one inert stripping agent is cocurrently injected at a concentration of at least 0.1 percent by weight. based on.
total interpolymer feed to the first stripping agent zone, and a vacuum of 100 mm Hg absolute or less is applied to the second vacuum zone. The stripping agent should form small gas bubbles when in the vacuum zone to effectively strip the interpolymer of volatiles thereby forming the more oxidatively thermally stable interpolymer.
The screw channels of the vacuum zone should be z5 partially filled with the stripping agent/polymer mix, creating frequent polymer surface renewal (i.e., a relatively low specific surface renewal ratio, measured in pounds of polymer per square meter of surface) which is exposed to the vacuum. Partially filling each vacuum zone of the extruder such that each vacuum zone has a specific surface renewal ratio of two pounds per square meter or less, preferably one pound per square meter or less, is especially effective when used in a devolatilizing extruder vacuum zone.
.,-..
~~~~ .~~~J _16_ Partially filling the channels can be controlled in different ways, including modifying the geometry of the vacuum zone. This can be accomplished by increasing flight depth, and/or by simultaneously increasing flight pitch while increasing the number of flight starts (i.e., using a multi-start screw design in the region of vacuum).
The stripping agent/polymer mix should be confined to adequately mix.ttie str9.pping agent with the polymer. Confiaemen~t of the stripping agent/polymer mix can be accomplished using various means, such as by using melt seals (i.e., blisters) on single or twin screw extruder systems, reverse flights on a twin screw extruder system, by incorporating high pressure compression zones along the screw, or by using axial, radial, and/or traps-axial throttle valves on single or twin screw extruder systems.
To be effective in the present invention, the devolatilization method used should remove enough residual unreaeted monomer(s), solvent, thermally unstable species and stripping agents) from the polymer such that the oxidative exotherm of -the interpolymer is reduced by at least 50 percent.
The more oxidativ.ely thermally stable polymers produced by the present invention are useful in a variety of thermal forming processes, including molding techniques (e.g., injection molding, rotomolding and blow molding), film forming techniques (e. g., cast, blown and extrusion coating) and.fiber forming techniques. The oxidatively thermally stable polymers and/or ionomers have particular utility when they are . used as at least one layer of a multilayered film i i structure having an inner layer and an outer layer, e.g., a food package. The ionomers have particular utility when they are used in skin packaging.
Comparative Examples 1-3 and Example 4 An ethylene/acrylic acid interpolymer having a melt index of 10 grams/10 minutes and about 10 percent by weight of interpolymer of acrylic acid was made according to the technique described in US-A-4,599,392. This interpolymer was pelletized for later use in a stripping experiment.
The stripping experiment was perfomed by feeding ethylene/acrylic acid interpolymer pellets into a first 90 mm single screw extruder having a length/diameter (L/D) ratio of 25 and a stripping agent injection port at a rate of 350 pounds/hour (160 kg/hour). This screw was operated according to the conditions outlined in Table 1:
Extrusion Conditions for the First Screw Extruder Zone 1 (C) 220 Ext rude Zone 2 ( C ) 2 2 0 r Transfer Line (C) 220 Melt Temp. 210 (C) Extruder rpm 95 Ethylene injection (weight percent, 0.2 based polymer fed) on The molten ethylene/acrylic acid interpolymer was conveyed to a second 90 mm single screw extruder .~
2~8'~ ~~ ~ -'$-having L/D of 40, two stripping agent injection ports and two vacuum ports operated according to the conditions described in Table 2:
Extrusion Conditions for the Second Screw Extruder Zone 1 (C) 220 Extruder Zone 2 (C) 220 Water Injection Zone 0.5 (weight percent) Melt Temp. (C). ' 220 Water Injection Zone 0.5 (weight percent) Extruder rpm 130 Vacuum Zone 1 (mm 45 Hg ) (60 mbars) Vacuum Zone 2 (mm 30 Hg ) (40 mbars) _ 19_ The screws were operating at differing speeds in order that the devolatilizing screw was only partially filled. The second screw was equipped with two injection ports and two vacuum ports along its length, as shown in Figure 1. Each injection section of the screw itself was equipped with two melt seals (i.e., a "blister" or restrictor ring) to confine she mixing zone. The melt seals confine the stripping agent to the area of the mixing zone to ensure that the stripping agent and the interpolymer were well mixed prior to devolatilization:
The interpolymer was extruded through the system described above. with varying degrees of stripping and devolatilization. Table 3 describes these 15 differences:
Table 3 Example Interpolymer processing description 1 * , Base interpolymer without further proce 2* Extruded interpolymer without stripping or vacuum 3* Extruded interpolymer with vacuum only 4 Extruded interpolymer with ethylene injection, dual water stripping injection and dual vacuum zones o ,.",o.,,",., WIII~.Ja~ mvm cnc....r." ......~, ......... ~.._...r._ _ _ _..
WO 92/iS621 PGT/US92/01720 2 ~ ~ x.10 ~ -20-Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC) analysis was used to determine the thermal activity of resin samples in the presence of oxygen. For each sample, a known amount of resin (approximately 5 milligram) was placed in a very small glass ampule. The bottom of the glass ampul.e was cooled with liquid nitrogen and the top of the ampule was sealed. The sealed ampule containing the resin sample was placed in the DSC sample chamber at room temperature. The DSC
sample and reference chambers were then heated at a constant rate of 10°C per'minute to a final temperature of 300°C. The thermal activity of the sample resin was continuously monitored versus the reference. The reference was an empty glass ampule. The thermal activity (i.e., exotherm) of the resin sample up_to 300°C was calculated in terms of joules per gram.
_S_olvent Analysis Residual solvent concentrations in the resin samples were determined using multiple headspace gas chromatography. For each sample, a known amount of resin (approximately 0.5 gram) was placed in a septum sealed vial. The sample vial was then placed on an automated headspace analyzer connected to a gas chromatograph. The sample vial was then analyzed using a quantitative multiple headspace extraction procedure.
The concentration of the residual solvent in the sample vial was determined from the quantitative analysis of known solvent standards analyzed under identical multiple headspace extraction conditions.
WO 92/15b21 PCT/U592/01720 -21- 20~~~.~
Residual Comonomer Analysis Residual comonomer concentration in the resin samples was determined using a solvent extraction procedure followed by high performance liquid chromatography (HPLC) analysis. For each sample, a known amount of resin (approximately 0.5 grams) was placed in container along with 75 ml of a xylene/butanol (at a 3:1 ratio) extraction solvent. The container was sealed and heated for 4 hours at 160°F (71°C~. The container was allowed to cool and the extraction solvent was analyzed using HPLC. In the case of acrylic acid.
the concentration of residual acrylic acid in the sample was determined from the quantitative analysis of known acrylic acid standards.
Table 4 summarizes the melt index, total volatiles content and oxidative exotherm data of the interpolymer after treatment according to the trials outlined in Table 3. For Comparative Examples 1-3 and Example 4, the maximum oxidative exotherm was reduced by 73 percent over that of the the original polymer (Comparative Example 1*) and occured at about 240°C.
Ethylene was not detected in any of the samples, either prior to or after extrusion. The residual comonomer for this experiment was acrylic acid.
_22_ Table 4 Total Oxidative Percent idative O
Example Melt IndexVolatiles Exotherm x (gms/10 Exotherm min) (ppm) (joules/ Reduction m) g 1 * 11.2 950 27 NA
2 * 10.8 490 19 29.6 3* 11.1 293 21 22.2 4 10.9 45 10 73 ."" ,. ,e~+",~
,. ~ ,."
t.vm framvm cna...t..m "~, " .......,...~ ... ..._ ... _ _. _ _. _ . NA =- Not Applicable.
. ~.., Table 5 lists the components of the total volatiles for the four interpolymers tested after the interpoiymer sample was strand chopped and sealed for later analysis:
Table S
Residual Monomer and Solvent Level (PPm) l e Examp Total Unreacted Solvent monomer 1* 950 320 630 2* ND ND 490 3* 293 113 180 wW ~JaW VJW c~amyc v...~, not an example of the invention.
ND = Not determined.
Comparative Examples 5-7 and Example 8 ~ An ethylene/acrylic acid interpolymer having a melt index of 10 grams/10 minutes and 9.7 percent by weight of interpolymer of acrylic acid was made according to the technique described in US-A-4,599,392.
This interpolymer was pelletized for later use in the following stripping experiment.
-23- 2~~~ ~.~a The ethylene/acrylic acid interpolymer pellets were gravity fed into a co-rotating 90 mm twin screw extruder having L/D of 33. two stripping agent injection ports and two vacuum ports at a rate of 2000 pounds/hour (908 ~g/hour). This twin screw system was operated according to the conditions outlined in Table 6:
Table 6 ' Extrusion Conditions _2t~_ The twin screw was equipped with two injection ports and two vacuum ports along its length, as shown in Figure 2. The first injection section of the screw itself was equipped with two melt seals (i.e., blisters) to confine the mixing zone. The second injection section has a melt seal up-stream of the vacuum port to prevent the stripping agent from premature devolatilization. The melt seals confine the stripping agent to the area of the mixing zone to ensure that the Stripping agent and the interpolymer were well mixed prior to devolatilization.
The interpolymer was extruded through the system described above using a weigh belt feeder (not shown in Figure 2) to ensure that the extruder was 1~ partially filled. Table 7 describes processing differences for the extruded resins:
Table 7 Inter- Resin processing description polymer 5* Base resin without further processing 6* Extruded resin without stripping or vacuum 7* Extruded resin with vacuum only 8 Extruded resin with dual water stripping injection and dual vacuum zones ~, + o ,.",o"t,~., - W111rJOlUVmc.~~...'rm.......1~.._-_..~.__.__r___ Table 8 summarizes the melt index, total volatiles content and oxidative exotherm data of the interpolymer after treatment according to the trials ' outlined in Table 7.
Table 8 ,~----, Percent Melt Index Total Exotherm Oxidative Example (gms/10 min)Volatiles(~ouleslgm)Exotherm (ppm) Reduction 5* 10.3 -_ 950 27 NA
* 10.2 520 27 0 7* 10 440 16 - 40.7 g 9.8 200 8 70.3 ev~mn o n a W vPnt~(~n_ WIIIt.Iamvm.nv...l..... ......~, ..-_ _-- _.._ NA = Not Applicable.r Table 9 lists the components of the total volatiles for the four interpolymerstested:
20 Table 9 Volatiles (PPm) Example Total Unrearted Solvent z5 monomer 5* 950 320 X30 6* ND ND 520 7* 440 180 2so 3p g 200' 100 100 ompanson examp a on y, not an examp a o t a invention. ND = Not Determi ned The resultant interpolymers from trials 5*. 6*, !
7* and 8 were each extrusion coatedonto paper roll ' stock at a thickness of 1 mil (.025mm) on a Black c~~~'~'~1~
Clawson Extrusion Coater and monitored for performance.
The Extrusion Coater had a 30:1 L/D ratio, a 3.5 inch _ (8.9 cm) diameter screw and a 30 inch coathanger die.
The processing temperature was 550'F(2$8°C) and the screw speed was 85 revolutions per minute (RPM). The chill roll was operated at 440 feet (134 m) per minute for the neck-in determinations. To determine maximum line speed. the chill roll speed was increased until the web began to nip on each side of the die. Data for these tests appears in Table IO:
Table 10 Maximum InterpolymerNeck-in Line Speed (feet/min) from example(inches) number Side 1 Side 2 5* 1.875 1000 1200 (4.76 cm) (305 m/min) (366 m/min) 6* 1.625 900 900 (4.13 cm) (274 m/min) (274 m/min) 7* 1.875 1000 1000 (4.76 cm) (305 m/min) (305 m/min) g 1.875 1100 1250 (4.76 cm) (335 m/min) (381 m/min) nmn~ncnn xamo a t an example o a Wven ion.
P On v.
no These data show that more oxidatively thermally stable interpolymers processed according to the present invention exhibit equivalent if not superior neck-in and j0 line-speed performance to that of the base interpolymer.
- 2~'u'~ ~.~~
Film Fabrication Procedure for Taste and Odor Evaluations Cast films were prepared from Example 5* and from Example 8. The cast films were prepared on a small cast film unit. The extruder had a 1.5 inch (3.8 cm) diameter screw and a 20:1 L/D. The cast film unit Was equipped with a 12 inch (30 cm) slot die. The films were fabricated at a temperature of 550°F (288°C). The fabrication conditions used result in one mil (0.025 mm) cast films.
Odor Evaluation Procedure For each sample. 10 grams of cast film were placed in 16 ounce (473 ml) glass containers. These containers were sealed and placed in an oven at 60"C for 16 hours. The samples were removed and allowed; to cool.
The headspace odor in the samples was then compared using a group of test panelists. The panelists were asked to choose the sample which was preferred (i.e.
preference) and the results recorded.
Taste Evaluation Procedure For each sample, 20 grams of cast film were placed in a glass container with 800 milliliters of drinking water. The containers were th~n.sealed and placed in an oven set at 60°C for 16 hours. The samples were then removed and allowed to cool. The water from each sample was divided into aliquots and tasted by the test panelists. The panelists were asked to choose the sample which was preferred (i.e. preference) and the results recorded.
2 ~ ~ ~.1 ~ ~ -28-Taste and Odor Results The taste and odor results from the evaluation of the cast films are summarized in Tables 11 and 12, respectively.
Table 11 Taste Results Cast Film Number of PanelistsPercent Preference Example 5* 6 27 10(Control Sample) Example 8 16 73 (Vacuum Devolatilized Sample) ~ V~a~ W uW uci vyrvmcmw - ._._ Table 12 Odor Results ~ Cast Film Number of Panelists Percent Preference Example 5* 6 ~ 30 (Control Sample) Example 8 14 70 (Vacuum Devolatilized Sample) ota num er o pane is s =
A method of improving the taste and odor properties and the oxidative thermal stability of thermoplastic ethylene polymers has now been discovered.
The novel method is characterized by removing residual unreacted monomer or monomers, solvent and thermally unstable species from a first ethylene polymer, thereby forming a second more oxidatively thermally stable ethylene polymer having an oxidative exotherm of not more than 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry.
The novel method is particularly useful in improving the taste and odor properties and the oxidative thermal stability of ethylene/acrylic acid interpolymers which are widely used in the food packaging industry in single or multilayer laminate film structures for food wraps, beverage containers or condiment pouches.
In one aspect, the invention provides a method of improving the taste and odor properties and the oxidative thermal stability of a first thermoplastic ethylene polymer, comprising removing residual unreacted monomer or monomers, solvent and thermally unstable species from the first polymer, thereby forming a second more oxidatively thermally stable ethylene polymer having an oxidative exotherm of not more than about 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry, the method comprising: (a) feeding the first polymer into a devolatilization extrusion system having at least two vacuum zones and at least one barrel, the at least one barrel having:~i. a first vacuum zone, ii. at least one i -3a-stripping agent injection port, iii. at least one confined mixing zone, and iv. a second vacuum zone, wherein the at least one confined mixing zone is situated immediately downstream of the at least one stripping agent injection port, the second vacuum zone is situated downstream of the first vacuum zone, and the first vacuum zone is designed to operate partially-filled with the first polymer; (b) conveying the first polymer through the extrusion system;
(c) applying vacuum to the first vacuum zone; (d) concurrently injecting at least about 0.1 percent by weight based on total first polymer feed of at least one inert stripping agent to at least one stripping agent injection port; (e) mixing the first polymer and the inert stripping agent in the at least one confined mixing zone; (f) applying vacuum to the second vacuum zone; and (g) recovering the extruded second polymer.
Figure 1 schematically shows an extrusion system comprising two single screws operated in series which can be used to improve the oxidative thermal stability of ethylene polymers.
Figure 2 schematically shows a twin screw extrusion system which can be used to improve the oxidative thermal stability of ethylene polymers.
_q_ Figure 1 schematically shows the cross-section of an extrusion system comprising two single screws useful for improving the oxidative thermal stability of , ethylene polymers, as disclosed in Comparative Examples .
1-3 and Example 4 below. In Figure 1, the polymer is fed into the feed section (1) of a first extruder system (13) and is melted and, conveyed into zone 1 by a screw.
The temperature of,tlie entire extruder system is controlled by a hot oil system (not shown). Ethylene (as a stripping agent) is injected into zone 2 through a , port (2). The melt temperature of the °irst extruder is monitored by means of a thermocouple (3). A transfer line (4) is used to transfer the polymer/ethylene mixture to a second extruder system 1,12). Preferably, the screw in the first extruder operates at slower revolutions per minute (rpm) than the screw in the second extruder such that the feed to the second extruder is less than that required to fill the screw channels (i.e., the second extruder is "starve fed"). A
First injection port (5) is used to inject an additional ,.
stripping agent into the polymer melt stream in the second extruder and the polymer/ethylene/additional .
stripping agent mixture is conveyed into a mixing zone (10). The polymer/ethylene/additional stripping agent .
mixture is temporarily confined and controlled within the mixing area by a melt seal (11) and the compression side (up-stream side) of the screw. The polymer/ethylene/additional stripping agent mixture flows past the melt seal (11) and into the first vacuum , port area (6). Vacuum is applied to the first vacuum port (6) and the unreacted monomer(s), residual solvent and residual thermally unstable species are partially devolatilized. The polymer then enters a second WO 92/15621 PCT/US92/01.720 -7- 2~8~.~.~~
compression zone and an effective melt seal is formed.
A second injection port (7) is utilized to inject a second stripping agent into the polymer melt s=ream.
immediately followed by a mixing zone (14), and a melt seal (15). The polymer/additional stripping agent mixture is temporarily confined between the melt seal and the compression side (up-stream side) of the scresu ~o ensure adequate mixing. A second vacuum port (8) is used to remove additional unreaeted monomer(s), residual solvent and residual thermally unstable species, thus .
forming a polymer with greater oxidative thermal stability. A melt thermocouple (9) monitors the temperature of the exiting melt stream.
Figure 2 schematically shows the cross-section 1~ of one screw of a co-rotating twin screw extrusion system useful for improving the oxidative thermal stability of ethylene polymers, as disclosed in Comparative Examples 5-7 and Example 8. In Figure 2, the polymer is fed into the feed section (16) of a twin screw extruder system (28) and is melted and conveyed into zone 1 by two intermeshing screws. The temperature of each extruder zone is controlled by a temperature controller and resistance heaters (not shown). The temperature of the extruder is monitored by means of a series of thermocouplesi(29-36). Preferably, the feed to the extruder is controlled such that it is less than that required to fill the screw channels of the devolatilization zone (i.e.. the vacuum zone of .the extruder is partially filled). A first injection port (17) is used to inject a first stripping agent into the polymer melt stream and the polymer/stripping agent mixture is. homogenized in a mixing zone (24). The mixed polymer/stripping agent mixture is temporarily confined and controlled within the mixing area by two melt seals WO 92/15621 ~ PCT/US92/01720 _ 2~~~.1~5 (38, 25). The polymer/stripping agent mixture flows past the melt seal (25) and into the first vacuum port area (18). Vacuum is applied to the first vacuum port (i8) and the unreacted monomer(s), residual solvent and . .
residual thermally unstable species are partially ' devolatilized. A second melt seal (37) separates the first vacuum zone from a second stripping agent injection port (19). The second injection port (19) is utilized to inject a second stripping agent into the Polymer melt stream. immediately followed by a mixing.
zone (33), and a melt seal (27).:w..The mixed polymer/stripping .agent mixture.... is temporarily confined and controlled within the mixing area by two melt seals (37, 27). The polymer/stripging agent mixture flows 1, past the melt seal (27) and into the second vacuum port area (20). The second vacuum port (20) is used to remove additional unreacted monomer(s), residual solvent and residual thermally unstable species. thus forming the more oxidatively thermally stable polymer. A motor .
(not shown) drives a gear pump (21) which pumps the thermally stable polymer into a pelletizer (23) which chops the more thermally stable polymer into pellets. A
liquid stream (22) cools and solidifies the polymer into pellets. The cooling liquid stream conveys the ehopped/pelletized polymer to a separation point (not shown) where the liquid is separated from the polymer pellets and, optionally, is recycled back through the process.
_7_ Detailed Description of the Invention The thermoplastic ethylene polymers used in this invention can be homopolymers or interpolymers of ethylene with at least one comonomer, such as alpha-olefins or substituted alpha-olefins.
Homopolymers of ethylene for use in the present invention include homopolymers of ethylene made using a high pressure, free radical generating process and are generally known as low density polyethylene. Low density polyethylene (LDPE), also known as High Pressure Polyethylene and historically as ICI-type polyethylene:
is a branched ethylene homopolymer made using free radical polymerization techniques under high pressures.
The ethylene is randomly polymerized into the polymer chains and forms numerous branch points. The density of LDPE is dictated by the degree and length of branches and is typically in the range,of 0.91-0.925 grams per cubic centimeter (g/em3). The molecular weight of the LDPE is indicated by melt index (MI) as measured using ASTM D-1238 (E) (190°C/2.16 kilograms). The MI range for LDPE for use in the present invention can be from 0.01 to 1000 grams/10 minutes, preferably 0.1 to 100.
Linear polyethylene is another type of polyethylene useful in practicing the present invention.
Manufacture of linear polyethylene is disclosed, e.g., in US-A-u,076,698 and involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type.
These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear"
polymers because of the substantial absence of. branched 2~8~ 1~~
chains of polymerized monomer un its pendant from the.
main polymer "backbone." Two types of linear polyethylene are also suitable for use in the present invention: linear high density polyethylene (HDPE) and .
linear low density polyethylene (LLDPE).
Linear high density polyethylene has a density from 0.941 to 0.965 g/cm~ while linear low density polyethylene typica_ly has a density from 0.880 g~'cm~ to 0.940 g/cm~. The density of the linear polyethylene is lowered by polymerizing ethylene with minor amounts of alpha. beta-ethylenically unsaturated alkenes (i.e., alpha-olefins) having from 3 to 20 carbons per alkene molecule, preferably 4,.to 8 (e.g., i-butene, 4-methyl 1- ., pentene, 1-hexene) and most preferably 8 carbons per alkene molecule (i.e., 1-octene). The amount of the .
alkene .comonomer is generally sufficient to cause the density of the linear low density polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification. These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion or cast films and the:~high "slip" characteristic inherently found in HDPE is diminished.
The use of coordination-type catalysts for polymerizing ethylene into homopolymers, either in solution or slurry type polymerization, or copolymerizing ethylene with higher alkenes to make copolymers having-densities above 0.94 g/cm3 as defined ' in ASTM D-1248 (i.e., HDPE polymers) and/or for copolymerizing ethylene with higher.alkenes to make ' copolymers having densities in the range of LDPE and WO 92/1562y PCT/US92101720 2~~'~ ~.0~
medium density polyethylene (.i.e., LLDPE copolymers) is disclosed variously in, e.g.. US-A-2,699,457;
US-A-2,862,917: US-A-2,905.645: US-A-2.846,425;
US-A-3,058.963 and US-A-4,076.698. Thus, the density of the~linear polyethylene useful in the present invention is from 0.88 g/cm3 to 0.965 g/cm'.
The molecular weight of the linear polyethylene is indicated and measured by melt index according to ASTM D-1238, Condition (E) (i.e.. 190°C/2.?6 kilograms).
The melt index:of the linear polyethylene useful in the present invention can be from 0.001 grams/10 minutes (g/10 min) to 1000 g/l0 min, preferably from 0.01 gi'?0 min to 100 g/10 min.
Ethylene can also be interpolymerized with substituted alpha-olefins by known techniques. :The substituted alpha-olefins are a known class of polymerizable monomers that include, for example, ethylenically unsaturated carboxylic acids, esters and anhydrides (e. g., acrylic acid, methaerylie acid, malefic acid, malefic anhydride, methyl methacrylate. butyl aerylate), vinyl esters~of alkanoic acids (e. g., vinyl acetate) and vinyl alcohol.
2~
Interpolymers of ethylene and at least one ethylenically unsaturated carboxylic acid are particularly useful in the present invention. The term "interpolymer" means that ethylene is copolymerized with at least one comonomer. Thus the term "interpolymer"
can indicate a copolymer, a terpolymer and so forth.
Such interpolymers can be made by any of the known techniques. For example, US-A-4,599,-392 (MeKinney et al.) and US-A-4,248,990 (Pieski et al.) disclose processes for manufacturing random and non-random interpolymers, respectively, of ethylene/acrylic acid and ethylene/methaerylie acid. The amount of unsaturated carboxylic acid can vary from 0 (i.e.. LDPE) to as high as X10 percent, dependent on the carboxylic .
acid used in the interpolymerization.
Ethylene/unsaturated carboxylic acids) interpolymers made by use of telogenic modifiers, as disclosed in US-A-u,988,781, are also suitable for practicing the present invention. The~inolecular weight of the , ethyleneiunsaturated carboxylic acid interpolymers is indicated by melt index and is also measured by ASTM D-1238. Condition E (i.e., 190°C/2.16 kilograms). The melt index of the ethylene/unsaturated carboxylic acid interpolymers can be from 0.1 g/10 min to 3000 g/10 min.
preferably from 1 g/10 min to 300 g/10 min.
Ionomers of various interpolymers (,e.g., metal ionomers of ethylene/aerylie acid or ethylene/methacrylic acid interpolymers) are also within the scope of this invention. US-A-3,404,13+ (Bees), for example, discloses a process for cross-linking polymers thereby creating ionomers which are useful in the present invention. In one embodiment of the present invention, aqueous solutions of metal hydroxides (e. g..
sodium hydroxide or potassium hydroxide . nitrogenous bases (e.g., ammonium hydroxide). or water-soluble a t strong base organic amines (e. g., mono-, di-and tri-methyl amines) can be added to a devolatilizing extruder in a first injection zone and mixed with ethylene/unsaturated carboxylic acids) interpolymers to effectively function as both a stripping agent and neutralizing/ionizing agent. The water is subsequently stripped under vacuum thus removing residual unreaeted .
2~~~.~~~
monomer or monomers, solvent and thermally unstable species forming a more thermally stable ionomer.
Additives can be included in the interpolymers useful in practicing the present invention. but are preferably added after forming the more thermally stable polymer. These additives include pigments (e. g., titanium dioxide), wetting agents, fillers, slip and anti-block compounds. Anti-oxidants can also be included in the interpolymers. but they need not be present for the present invention to be effectively used.
Solvents (or diluents) can be and generally are used in both the high pressure process (e.g., LDPE) or the low pressure process (e.g.. LLDPE) to make the ethylene polymers. Some of the typical solvents~used during interpolymerization include: paraffinic hydrocarbons such as pentane, hexane, heptane, 2~ cyclohexane. branched alkanes, alkenes, and alkynes (plus their various isomeric forms); or aromatic hydrocarbons such as benzene, toluene, chloro-substituted aromatic hydrocarbons such as dichlorobenzene; or polar solvents such as alcohols.
etzers, ketones and esters; or various naphtha distillates such as those sold under the trademark ISOPAR by EXXON. These solvents are difficult to remove from the interpolymer, even after pelletization.
Residual monomer (e. g., ethylene and/or acrylic acid) ~ can also be retained in the interpolymer. Thermally unstable species (e.g.., free radical species) can react with oxygen and cause taste problems. Thermally unstable species, as defined herein, are compounds which thermally degrade in the presence of oxygen at elevated temperatures (i.e., at polymer melt processing WO 92/15621 PGT/US92/01.720 temperatures). fihese semi-volatile components can cause taste and/or odor problemsin fabricated parts made from the interpolymers, depending on the application, and surprisingly, can also cause the interpolymer to have less oxidative thermal stability.
Removal of the residual unreaeted monomer or monomers, solvent and thermally unstable species to form the oxidatively thermally stable interpolymers of the present invention can be accomplished by any conventional means.
Batch operation includes such pieces of equipment as a Banbury mixer, or a roll mill. In batch operation, a specific amount of polymer and stripping agents) is placed into the equipment. The polymer is then melted and fluxed or mixed for a pre-designated amount of time. The equipment is vented periodically to allow for devolatilization. The sample is then removed 20 for processing into pellets or other forms for final usage. Steam can be added during this processing to aid in devolatilization.
Continuous devolatilization can be performed on 25 a single or twin screw extruder as shown in the examples. Other continuous devolatilization equipment includes Bolling Mixtrumat'" continuous process or Farrel DiscpackT" and other types of continuous extrusion equipment which can be fitted with vacuum 30 ports/adapters. Many of these systems can be fitted with a mixing section prior to devolatilization to improve the contact of the stripping agent and the interpolymer. The devolatilization occurs by producing -'3- 2~~~ 34~
a thin melt which is continuously regenerating a new surface that is exposed to a vacuum.
A particularly effective embodiment for forming more oxidatively thermally stable ethylene polymers of the present invention comprises the consecutive steps of (a)feeding a first ethylene polymer to an extruder comprising an optional stripping agent injection zone. an optional~confined mixing zone, at least a first partially filled vacuum zone, at least a first stripping agent injection zone, at least one first confined mixing zone, at least a second partially filled vacuum zone;
(b)melting the first ethylene polymer:
(e)applying vacuum to the first partially filled vacuum zone;
(.d)cocurrently injecting at least 0.1 percent by weight, based on total ethylene polymer feed, of at least one stripping agent to the first stripping agent injection zone;
(e)mixing the stripping agent with the first ethylene polymer in the first confined mixing zone;
(f)apglying vacuum to the second partially filled vacuum zone; and (g)recovering the second more oxidatively thermally stable ethylene polymer.
The stripping agents useful in the present invention are preferably inert with respect to the interpolymerization process. When forming oxidatively thermally stable ionomers from interpolymers of ethylene and at least one unsaturated carboxylic acid, however, the stripping agent should react with the unsaturated carboxylic acid. The stripping agents) should also have a vapor pressure sufficient to either be a gas or form gas bubbles at the devolatiiizing operating conditions (i.e., when the interpolymer is molten).
Preferably, the stripping agents) is a light hydrocarbon (e. g., ethylene, propylene or isobutane).
water, steam, alcohol, carbon dioxide and/or nitrogen.
Super critical fluids (e. g., super critical carbon dioxide) are also useful as stripping agents for use in the present invention. Aqueous solutions of alkali metal hydroxides (e.g.:.., sodium hydroxide), nitrogenous bases (e. g., ammoniumwhydroxide), water-soluble organic strong base organic amines (e. g.. mono-, di-and tri-methyl amines) are particularly effective when simulataneously devolatilizing ethylene/unsaturated carboxylic acid interpolymers while forming the corresponding ionomer. Mixtures of stripping agents are also useful in the present invention. The stripping agent should be used in amounts effective to remove enough residual unreacted monomer(s), solvent and stripping agents) such that the oxidative exotherm of the interpolymer or. ionomer is reduced by at least 50 percent.
The devolatilization of the interpolymers is ~o preferably performed using a single or twin screw system with water injection as described in US-A-u,09~.9~2, with the added feature of using a screw which has a surface renewal ratio of at least about two pounds per square meter or less, preferably about one pound per square meter or less. When creating a more thermally stable interpolymer made using a high pressure process (e.g., LDPE or ethylene/acrylic acid interpolymers) and the devolatilizing screw is installed in-line with the process (i.e., before the pelletization.step), the extruders) vacuum devolatilization zones) should be partially-filled (i.e., the flow of the interpolymer is restricted such that the channels of the screw are partially filled; this can be accomplished, e.g.. using a device which controls/restricts flow, such as a slide ~.ralve or a gear pump). The extruder comprises an optional stripping agent injection zone. an optional confined mixing zone. at least a first partially filled vacuum zone, at least a first stripping agent injection zone, at least one first confined mixing zone, and at Ieast a second partially filled vacuum zone.
Preferably, a vacuum of 100 mm Hg absolute or less is applied to the first vacuum zone, at least one inert stripping agent is cocurrently injected at a concentration of at least 0.1 percent by weight. based on.
total interpolymer feed to the first stripping agent zone, and a vacuum of 100 mm Hg absolute or less is applied to the second vacuum zone. The stripping agent should form small gas bubbles when in the vacuum zone to effectively strip the interpolymer of volatiles thereby forming the more oxidatively thermally stable interpolymer.
The screw channels of the vacuum zone should be z5 partially filled with the stripping agent/polymer mix, creating frequent polymer surface renewal (i.e., a relatively low specific surface renewal ratio, measured in pounds of polymer per square meter of surface) which is exposed to the vacuum. Partially filling each vacuum zone of the extruder such that each vacuum zone has a specific surface renewal ratio of two pounds per square meter or less, preferably one pound per square meter or less, is especially effective when used in a devolatilizing extruder vacuum zone.
.,-..
~~~~ .~~~J _16_ Partially filling the channels can be controlled in different ways, including modifying the geometry of the vacuum zone. This can be accomplished by increasing flight depth, and/or by simultaneously increasing flight pitch while increasing the number of flight starts (i.e., using a multi-start screw design in the region of vacuum).
The stripping agent/polymer mix should be confined to adequately mix.ttie str9.pping agent with the polymer. Confiaemen~t of the stripping agent/polymer mix can be accomplished using various means, such as by using melt seals (i.e., blisters) on single or twin screw extruder systems, reverse flights on a twin screw extruder system, by incorporating high pressure compression zones along the screw, or by using axial, radial, and/or traps-axial throttle valves on single or twin screw extruder systems.
To be effective in the present invention, the devolatilization method used should remove enough residual unreaeted monomer(s), solvent, thermally unstable species and stripping agents) from the polymer such that the oxidative exotherm of -the interpolymer is reduced by at least 50 percent.
The more oxidativ.ely thermally stable polymers produced by the present invention are useful in a variety of thermal forming processes, including molding techniques (e.g., injection molding, rotomolding and blow molding), film forming techniques (e. g., cast, blown and extrusion coating) and.fiber forming techniques. The oxidatively thermally stable polymers and/or ionomers have particular utility when they are . used as at least one layer of a multilayered film i i structure having an inner layer and an outer layer, e.g., a food package. The ionomers have particular utility when they are used in skin packaging.
Comparative Examples 1-3 and Example 4 An ethylene/acrylic acid interpolymer having a melt index of 10 grams/10 minutes and about 10 percent by weight of interpolymer of acrylic acid was made according to the technique described in US-A-4,599,392. This interpolymer was pelletized for later use in a stripping experiment.
The stripping experiment was perfomed by feeding ethylene/acrylic acid interpolymer pellets into a first 90 mm single screw extruder having a length/diameter (L/D) ratio of 25 and a stripping agent injection port at a rate of 350 pounds/hour (160 kg/hour). This screw was operated according to the conditions outlined in Table 1:
Extrusion Conditions for the First Screw Extruder Zone 1 (C) 220 Ext rude Zone 2 ( C ) 2 2 0 r Transfer Line (C) 220 Melt Temp. 210 (C) Extruder rpm 95 Ethylene injection (weight percent, 0.2 based polymer fed) on The molten ethylene/acrylic acid interpolymer was conveyed to a second 90 mm single screw extruder .~
2~8'~ ~~ ~ -'$-having L/D of 40, two stripping agent injection ports and two vacuum ports operated according to the conditions described in Table 2:
Extrusion Conditions for the Second Screw Extruder Zone 1 (C) 220 Extruder Zone 2 (C) 220 Water Injection Zone 0.5 (weight percent) Melt Temp. (C). ' 220 Water Injection Zone 0.5 (weight percent) Extruder rpm 130 Vacuum Zone 1 (mm 45 Hg ) (60 mbars) Vacuum Zone 2 (mm 30 Hg ) (40 mbars) _ 19_ The screws were operating at differing speeds in order that the devolatilizing screw was only partially filled. The second screw was equipped with two injection ports and two vacuum ports along its length, as shown in Figure 1. Each injection section of the screw itself was equipped with two melt seals (i.e., a "blister" or restrictor ring) to confine she mixing zone. The melt seals confine the stripping agent to the area of the mixing zone to ensure that the stripping agent and the interpolymer were well mixed prior to devolatilization:
The interpolymer was extruded through the system described above. with varying degrees of stripping and devolatilization. Table 3 describes these 15 differences:
Table 3 Example Interpolymer processing description 1 * , Base interpolymer without further proce 2* Extruded interpolymer without stripping or vacuum 3* Extruded interpolymer with vacuum only 4 Extruded interpolymer with ethylene injection, dual water stripping injection and dual vacuum zones o ,.",o.,,",., WIII~.Ja~ mvm cnc....r." ......~, ......... ~.._...r._ _ _ _..
WO 92/iS621 PGT/US92/01720 2 ~ ~ x.10 ~ -20-Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC) analysis was used to determine the thermal activity of resin samples in the presence of oxygen. For each sample, a known amount of resin (approximately 5 milligram) was placed in a very small glass ampule. The bottom of the glass ampul.e was cooled with liquid nitrogen and the top of the ampule was sealed. The sealed ampule containing the resin sample was placed in the DSC sample chamber at room temperature. The DSC
sample and reference chambers were then heated at a constant rate of 10°C per'minute to a final temperature of 300°C. The thermal activity of the sample resin was continuously monitored versus the reference. The reference was an empty glass ampule. The thermal activity (i.e., exotherm) of the resin sample up_to 300°C was calculated in terms of joules per gram.
_S_olvent Analysis Residual solvent concentrations in the resin samples were determined using multiple headspace gas chromatography. For each sample, a known amount of resin (approximately 0.5 gram) was placed in a septum sealed vial. The sample vial was then placed on an automated headspace analyzer connected to a gas chromatograph. The sample vial was then analyzed using a quantitative multiple headspace extraction procedure.
The concentration of the residual solvent in the sample vial was determined from the quantitative analysis of known solvent standards analyzed under identical multiple headspace extraction conditions.
WO 92/15b21 PCT/U592/01720 -21- 20~~~.~
Residual Comonomer Analysis Residual comonomer concentration in the resin samples was determined using a solvent extraction procedure followed by high performance liquid chromatography (HPLC) analysis. For each sample, a known amount of resin (approximately 0.5 grams) was placed in container along with 75 ml of a xylene/butanol (at a 3:1 ratio) extraction solvent. The container was sealed and heated for 4 hours at 160°F (71°C~. The container was allowed to cool and the extraction solvent was analyzed using HPLC. In the case of acrylic acid.
the concentration of residual acrylic acid in the sample was determined from the quantitative analysis of known acrylic acid standards.
Table 4 summarizes the melt index, total volatiles content and oxidative exotherm data of the interpolymer after treatment according to the trials outlined in Table 3. For Comparative Examples 1-3 and Example 4, the maximum oxidative exotherm was reduced by 73 percent over that of the the original polymer (Comparative Example 1*) and occured at about 240°C.
Ethylene was not detected in any of the samples, either prior to or after extrusion. The residual comonomer for this experiment was acrylic acid.
_22_ Table 4 Total Oxidative Percent idative O
Example Melt IndexVolatiles Exotherm x (gms/10 Exotherm min) (ppm) (joules/ Reduction m) g 1 * 11.2 950 27 NA
2 * 10.8 490 19 29.6 3* 11.1 293 21 22.2 4 10.9 45 10 73 ."" ,. ,e~+",~
,. ~ ,."
t.vm framvm cna...t..m "~, " .......,...~ ... ..._ ... _ _. _ _. _ . NA =- Not Applicable.
. ~.., Table 5 lists the components of the total volatiles for the four interpolymers tested after the interpoiymer sample was strand chopped and sealed for later analysis:
Table S
Residual Monomer and Solvent Level (PPm) l e Examp Total Unreacted Solvent monomer 1* 950 320 630 2* ND ND 490 3* 293 113 180 wW ~JaW VJW c~amyc v...~, not an example of the invention.
ND = Not determined.
Comparative Examples 5-7 and Example 8 ~ An ethylene/acrylic acid interpolymer having a melt index of 10 grams/10 minutes and 9.7 percent by weight of interpolymer of acrylic acid was made according to the technique described in US-A-4,599,392.
This interpolymer was pelletized for later use in the following stripping experiment.
-23- 2~~~ ~.~a The ethylene/acrylic acid interpolymer pellets were gravity fed into a co-rotating 90 mm twin screw extruder having L/D of 33. two stripping agent injection ports and two vacuum ports at a rate of 2000 pounds/hour (908 ~g/hour). This twin screw system was operated according to the conditions outlined in Table 6:
Table 6 ' Extrusion Conditions _2t~_ The twin screw was equipped with two injection ports and two vacuum ports along its length, as shown in Figure 2. The first injection section of the screw itself was equipped with two melt seals (i.e., blisters) to confine the mixing zone. The second injection section has a melt seal up-stream of the vacuum port to prevent the stripping agent from premature devolatilization. The melt seals confine the stripping agent to the area of the mixing zone to ensure that the Stripping agent and the interpolymer were well mixed prior to devolatilization.
The interpolymer was extruded through the system described above using a weigh belt feeder (not shown in Figure 2) to ensure that the extruder was 1~ partially filled. Table 7 describes processing differences for the extruded resins:
Table 7 Inter- Resin processing description polymer 5* Base resin without further processing 6* Extruded resin without stripping or vacuum 7* Extruded resin with vacuum only 8 Extruded resin with dual water stripping injection and dual vacuum zones ~, + o ,.",o"t,~., - W111rJOlUVmc.~~...'rm.......1~.._-_..~.__.__r___ Table 8 summarizes the melt index, total volatiles content and oxidative exotherm data of the interpolymer after treatment according to the trials ' outlined in Table 7.
Table 8 ,~----, Percent Melt Index Total Exotherm Oxidative Example (gms/10 min)Volatiles(~ouleslgm)Exotherm (ppm) Reduction 5* 10.3 -_ 950 27 NA
* 10.2 520 27 0 7* 10 440 16 - 40.7 g 9.8 200 8 70.3 ev~mn o n a W vPnt~(~n_ WIIIt.Iamvm.nv...l..... ......~, ..-_ _-- _.._ NA = Not Applicable.r Table 9 lists the components of the total volatiles for the four interpolymerstested:
20 Table 9 Volatiles (PPm) Example Total Unrearted Solvent z5 monomer 5* 950 320 X30 6* ND ND 520 7* 440 180 2so 3p g 200' 100 100 ompanson examp a on y, not an examp a o t a invention. ND = Not Determi ned The resultant interpolymers from trials 5*. 6*, !
7* and 8 were each extrusion coatedonto paper roll ' stock at a thickness of 1 mil (.025mm) on a Black c~~~'~'~1~
Clawson Extrusion Coater and monitored for performance.
The Extrusion Coater had a 30:1 L/D ratio, a 3.5 inch _ (8.9 cm) diameter screw and a 30 inch coathanger die.
The processing temperature was 550'F(2$8°C) and the screw speed was 85 revolutions per minute (RPM). The chill roll was operated at 440 feet (134 m) per minute for the neck-in determinations. To determine maximum line speed. the chill roll speed was increased until the web began to nip on each side of the die. Data for these tests appears in Table IO:
Table 10 Maximum InterpolymerNeck-in Line Speed (feet/min) from example(inches) number Side 1 Side 2 5* 1.875 1000 1200 (4.76 cm) (305 m/min) (366 m/min) 6* 1.625 900 900 (4.13 cm) (274 m/min) (274 m/min) 7* 1.875 1000 1000 (4.76 cm) (305 m/min) (305 m/min) g 1.875 1100 1250 (4.76 cm) (335 m/min) (381 m/min) nmn~ncnn xamo a t an example o a Wven ion.
P On v.
no These data show that more oxidatively thermally stable interpolymers processed according to the present invention exhibit equivalent if not superior neck-in and j0 line-speed performance to that of the base interpolymer.
- 2~'u'~ ~.~~
Film Fabrication Procedure for Taste and Odor Evaluations Cast films were prepared from Example 5* and from Example 8. The cast films were prepared on a small cast film unit. The extruder had a 1.5 inch (3.8 cm) diameter screw and a 20:1 L/D. The cast film unit Was equipped with a 12 inch (30 cm) slot die. The films were fabricated at a temperature of 550°F (288°C). The fabrication conditions used result in one mil (0.025 mm) cast films.
Odor Evaluation Procedure For each sample. 10 grams of cast film were placed in 16 ounce (473 ml) glass containers. These containers were sealed and placed in an oven at 60"C for 16 hours. The samples were removed and allowed; to cool.
The headspace odor in the samples was then compared using a group of test panelists. The panelists were asked to choose the sample which was preferred (i.e.
preference) and the results recorded.
Taste Evaluation Procedure For each sample, 20 grams of cast film were placed in a glass container with 800 milliliters of drinking water. The containers were th~n.sealed and placed in an oven set at 60°C for 16 hours. The samples were then removed and allowed to cool. The water from each sample was divided into aliquots and tasted by the test panelists. The panelists were asked to choose the sample which was preferred (i.e. preference) and the results recorded.
2 ~ ~ ~.1 ~ ~ -28-Taste and Odor Results The taste and odor results from the evaluation of the cast films are summarized in Tables 11 and 12, respectively.
Table 11 Taste Results Cast Film Number of PanelistsPercent Preference Example 5* 6 27 10(Control Sample) Example 8 16 73 (Vacuum Devolatilized Sample) ~ V~a~ W uW uci vyrvmcmw - ._._ Table 12 Odor Results ~ Cast Film Number of Panelists Percent Preference Example 5* 6 ~ 30 (Control Sample) Example 8 14 70 (Vacuum Devolatilized Sample) ota num er o pane is s =
Claims (29)
1. ~A method of improving the taste and odor properties and the oxidative thermal stability of a first thermoplastic ethylene polymer, comprising removing residual unreacted monomer or monomers, solvent and thermally unstable species from the first polymer, thereby forming a second more oxidatively thermally stable ethylene polymer having an oxidative exotherm of not more than about 50 percent of the oxidative exotherm of the first polymer, as measured by differential scanning calorimetry, the method comprising:
(a) feeding the first polymer into a devolatilization extrusion system having at least two vacuum zones and at least one barrel, the at least one barrel having:
i. a first vacuum zone, ii. at least one stripping agent injection port, iii. at least one confined mixing zone, and iv. a second vacuum zone, wherein the at least one confined mixing zone is situated immediately downstream of the at least one stripping agent injection port, the second vacuum zone is situated downstream of the first vacuum zone, and the first vacuum zone is designed to operate partially-filled with the first polymer;
(b) conveying the first polymer through the extrusion system;
(c) applying vacuum to the first vacuum zone;
(d) concurrently injecting at least about 0.1 percent by weight based on total first polymer feed of at least one inert stripping agent to at least one stripping agent injection port;
(e) mixing the first polymer and the inert stripping agent in the at least one confined mixing zone;
(f) applying vacuum to the second vacuum zone; and (g) recovering the extruded second polymer.
(a) feeding the first polymer into a devolatilization extrusion system having at least two vacuum zones and at least one barrel, the at least one barrel having:
i. a first vacuum zone, ii. at least one stripping agent injection port, iii. at least one confined mixing zone, and iv. a second vacuum zone, wherein the at least one confined mixing zone is situated immediately downstream of the at least one stripping agent injection port, the second vacuum zone is situated downstream of the first vacuum zone, and the first vacuum zone is designed to operate partially-filled with the first polymer;
(b) conveying the first polymer through the extrusion system;
(c) applying vacuum to the first vacuum zone;
(d) concurrently injecting at least about 0.1 percent by weight based on total first polymer feed of at least one inert stripping agent to at least one stripping agent injection port;
(e) mixing the first polymer and the inert stripping agent in the at least one confined mixing zone;
(f) applying vacuum to the second vacuum zone; and (g) recovering the extruded second polymer.
2. The method of claim 1, wherein step (a) immediately follows an interpolymerization reaction of ethylene with at least one other comonomer.
3. The method of claim 1, wherein step (a) immediately follows an interpolymerization reaction of ethylene with an unsaturated carboxylic acid.
4. The method of claim 1, 2 or 3, wherein each vacuum zone of the extruder has a specific surface renewal ratio of about two pounds per square meter or less.
5. The method of any one of claims 1 to 4, wherein the extruder has a specific surface renewal ratio of about one pound per square meter or less.
6. The method of any one of claims 1 to 5, wherein the stripping agent is a C2-C18 hydrocarbon, methane, water, aqueous solutions of metal hydroxides, steam, alcohol, carbon dioxide, nitrogen or a mixture thereof.
7. The method of any one of claims 1 to 5, wherein the stripping agent is selected from the group consisting of aqueous solutions of metal hydroxides, nitrogenous bases and water-soluble, strong base organic amines.
8. The method of any one of claims 1 to 5, wherein the stripping agent is an aqueous solution of sodium hydroxide, thereby forming a more oxidatively thermally stable ionomer.
9. The method of any one of claims 1 to 8, wherein the first polymer is molten when entering the first vacuum zone.
10. The method of any one of claims 1 to 9, wherein a gear pump is located at the downstream end of the extruder barrel to maintain a constant feed pressure out of the extruder.
11. The method of any one of claims 1 to 10, wherein the diameter of the root of a screw of the extrusion system increases at the end of the barrel to increase pressure on the polymer.
12. The method of any one of claims 1 to 11, wherein the first polymer is low density polyethylene.
13. The method of any one of claims 1 to 12, wherein the first polymer is linear polyethylene.
14. The method of claim 13, wherein the linear polyethylene is ultra low density polyethylene.
15. The method of any one of claims 1 to 11, wherein the first polymer is an interpolymer of ethylene and acrylic or methacrylic acid.
16. The method of claim 15, wherein the first polymer is an interpolymer of ethylene anti acrylic acid.
17. The method of claim 15, wherein the first polymer is an interpolymer of ethylene and methacrylic acid.
18. The method of claim 1, wherein step (a) immediately follows a homopolymerization reaction of ethylene.
19. The ionomer produced by the method of claim 8.
20. The ethylene/unsaturated carboxylic acid interpolymer produced by the method of claim 3.
21. The ethylene/acrylic acid interpolymer produced by the method of claim 17.
22. The ethylene/methacrylic acid interpolymer produced by the method of claim 17.
23. A multilayered film structure having an inner layer and an outer layer, at least one layer of which comprises the interpolymer of claim 20.
24. A multilayered film structure having an inner layer and an outer layer, at least one layer of which comprises the interpolymer of claim 21.
25. A multilayered film structure having an inner layer and an outer layer, at least one layer of which comprises the interpolymer of claim 22.
26. The multilayered film structure of claim 23, 24 or 25, wherein the structure is used in food products wraps, beverage containers or condiment pouches.
27. An ethylene polymer produced by the method of any one of claims 1 to 7, 9 to 14 and 18.
28. A multilayered film structure having an inner layer and an outer layer, at least one layer of which comprises the ethylene polymer of claim 27.
29. Use of a multilayered film structure of claim 28, for food product wraps, beverage containers or condiment pouches.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US66399591A | 1991-03-04 | 1991-03-04 | |
| US663,995 | 1991-03-04 | ||
| PCT/US1992/001720 WO1992015621A1 (en) | 1991-03-04 | 1992-03-04 | Method of improving the oxidative thermal stability of ethylene polymers |
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| Publication Number | Publication Date |
|---|---|
| CA2081105A1 CA2081105A1 (en) | 1992-09-05 |
| CA2081105C true CA2081105C (en) | 2004-10-05 |
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| Application Number | Title | Priority Date | Filing Date |
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| CA002081105A Expired - Lifetime CA2081105C (en) | 1991-03-04 | 1992-03-04 | Method of improving the oxidative thermal stability of ethylene polymers |
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|---|---|
| US (1) | US5756659A (en) |
| EP (1) | EP0527225B1 (en) |
| JP (1) | JP3174915B2 (en) |
| CA (1) | CA2081105C (en) |
| DE (1) | DE69224242T2 (en) |
| ES (1) | ES2111635T3 (en) |
| FI (1) | FI924965A (en) |
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1992
- 1992-03-04 ES ES92908695T patent/ES2111635T3/en not_active Expired - Lifetime
- 1992-03-04 CA CA002081105A patent/CA2081105C/en not_active Expired - Lifetime
- 1992-03-04 EP EP92908695A patent/EP0527225B1/en not_active Revoked
- 1992-03-04 WO PCT/US1992/001720 patent/WO1992015621A1/en not_active Application Discontinuation
- 1992-03-04 JP JP50785592A patent/JP3174915B2/en not_active Expired - Lifetime
- 1992-03-04 DE DE69224242T patent/DE69224242T2/en not_active Revoked
- 1992-03-12 TW TW081101891A patent/TW246679B/zh not_active IP Right Cessation
- 1992-11-03 FI FI924965A patent/FI924965A/en unknown
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1994
- 1994-10-21 US US08/326,981 patent/US5756659A/en not_active Expired - Lifetime
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| TW246679B (en) | 1995-05-01 |
| DE69224242D1 (en) | 1998-03-05 |
| FI924965A0 (en) | 1992-11-03 |
| EP0527225A1 (en) | 1993-02-17 |
| WO1992015621A1 (en) | 1992-09-17 |
| ES2111635T3 (en) | 1998-03-16 |
| FI924965A (en) | 1992-11-03 |
| EP0527225B1 (en) | 1998-01-28 |
| US5756659A (en) | 1998-05-26 |
| JP3174915B2 (en) | 2001-06-11 |
| JPH05506692A (en) | 1993-09-30 |
| DE69224242T2 (en) | 1998-05-14 |
| CA2081105A1 (en) | 1992-09-05 |
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