Biodegradable compositions comprising starch and alkenol polymers

What we claim is:

1. A biodegradable composition obtained from a melt comprising converted starch, wherein said converted starch has a 8.8% calcium chloride viscosity within the range of about 5 seconds to about 60 seconds, a plasticizer and a member selected from the group consisting of alkenol hompolymers, alkenol copolymers and combinations thereof, which are combined under conditions sufficient to ensure uniform melt formation, wherein the member is present in the composition at a concentration of from about 10 to about 200 parts per 100 parts of dry converted starch.

2. A biodegradable composition according to claim 1 wherein said converted starch is derived from at least one member selected from the group consisting of native starches of vegetable origin, which starches are derived from potatoes, rice, tapioca, corn, pea, rye, oats, wheat.

3. A biodegradable composition according to claim 1 or claim 2 wherein said converted starch is acid hydrolysed fluidity starch and wherein said acid hydrolysis has been carried out below the gelatinization point of the starch.

4. A biodegradable composition according to claim 3, wherein said converted starch is acid hydrolysed corn starch.

5. A biodegradable composition according to claim 1, wherein the 8.8% calcium chloride viscosity of said converted starch is from about 10 seconds to about 40 seconds.

6. A biodegradable composition according to claim 5, wherein the 8.8% calcium chloride viscosity of said converted starch is from about 20 seconds to about 40 seconds.

7. A composition according to claim 1, wherein the converted starch has an amylose content of from about 10% up to about 95%.

8. A composition according to claim 7, wherein the converted starch has an amylose content of between about 60 and about 95%.

9. A composition according to claim 1, wherein the converted starch has an amylose content between about 65% and about 80%.

10. A composition according to claim 1, wherein the converted starch has an amylose content of from about 10% up to about 45%.

11. A composition according to claim 1, wherein the converted starch has an amylose content of from about 10% up to about 35%.

12. A composition according to claim 7, wherein the converted starch has an amylose content of at least about 5 to about 10%.

13. A composition according to claim 1, in which the composition comprises from about 10 to about 120 parts of said polymer per 100 parts of converted starch.

14. A composition according to claim 1, in which the composition comprises from about 20 to about 85 parts of said polymer per 100 parts of converted starch.

15. A composition according to claim 1, in which the composition comprises from about 20 to about 40 parts of said polymer per 100 parts of converted starch.

16. A composition according to claim 1, in which the alkenol homopolymer is polyvinyl alcohol.

17. A composition according to claim 16, in which the polyvinyl alcohol is substantially completely hydrolysed.

18. A composition according to claim 16, in which the polyvinyl alcohol is hydrolysed to an extent of from about 70 to about 100 mol %.

19. A composition according to claim 16, in which the polyvinyl alcohol is hydrolysed to an extent of from about 87 to about 98 mol %.

20. A composition according to claim 16, in which the polyvinyl alcohol is hydrolysed to an extent of from about 96 to about 98 mol %.

21. A composition according to claim 16, in which the polyvinyl alcohol has a number average molecular weight of at least 10,000.

22. A composition according to claim 16, in which the polyvinyl alcohol has a number average molecular weight of from 20,000 to 100,000.

23. A composition according to claim 17, in which the polyvinyl alcohol has a number average molecular weight of from 160,000 to 250,000.

24. A composition according to claim 16, wherein the polyvinyl alcohol has a degree of hydrolysis of at least about 96% and has a number average molecular weight of from about 20,000 to about 100,000.

25. A composition according to claim 1, wherein the alkenol copolymer containing vinyl alcohol units and aliphatic chain units as obtained by co-polymerization of vinyl acetate with ethylene and/or propylene with subsequent hydrolysis of the vinyl acetate groups.

26. A composition according to claim 25, in which the copolymer comprises an ethylene/vinyl alcohol copolymer.

27. A composition according to claim 26, in which the molar ratio of vinyl alcohol units to alkylene units is from about 40:60 to about 90:10.

28. A composition-according to the preceding claim, in said molar ratio is from about 45:55 to about 70:30.

29. A composition according to claim 28, in which the ethylene vinyl alcohol has an ethylene content of about 42 mol % to about 48 mol %.

30. A composition according to claim 1, wherein the composition further comprises at least one member selected from the group consisting of extenders, fillers, lubricants, mold release agents, stabilizers, coloring agents, flame retardants, alkali or alkaline earth metal salts and boron-containing compounds.

31. A composition according to claim 30, in which said fillers are present in said composition at a concentration of between about 20% and about 40% by weight, bas ed on the weight of the composition.

32. A composition according to claim 30, in which said plasticisers are present in said composition at a concentration of between about 0.5% and about 40% by weight, based on the weight of the composition.

33. A composition according to claim 30, in which said plasticisers are present in said composition at a concentration of between about 0.5% and about 5% by weight, based on the weight of the composition.

34. A composition according to claim 32, further comprising water, wherein the sum of the plasticizer and water content of said composition does not exceed about 25% by weight, based on the weight of the composition.

35. A composition according to claim 31, in which said lubricants are selected from the group consisting of mono or diglycerides, lecithin and stearic acid.

36. A composition according to claim 35, in which the lubricant is stearic acid.

37. A composition according to claim 31, further comprising a nucleating agent, said nucleating agent having a particle size of from about 0.01 to about 5 microns, wherein said nucleating agent is selected from the group consisting of silica, titania, alumina, barium oxide, magnesium oxide, sodium chloride, potassium bromide, magnesium phosphate, barium sulphate, aluminum sulphate, boron nitrate and magnesium silicate or mixtures thereof.

38. A composition according to claim 37, wherein the nucleating agent is magnesium silicate (micro talcum).

39. A composition according to claim 30, in which the flame retardant is selected from the group consisting of guanidinium phosphate: diethyl-N,N-bis(2-hydroxyethyl)aminomethyl phosphonate; dimethyl methylphosphonate; phosphonic acid, methyl-, dimethylester, polymer with oxirane and phosphorous oxide; aliphatic phosphate/phosphonate oligomers; tributyl phosphate; triphenyl phosphate; tricresyl phosphate; 2-ethylhexyl diphenyl phosphate; and tributoxyethyl phosphate; bis (hydroxypropyl) sec-butyl phosphine oxide; polypropoxylated dibutyl pyrophosphoric acid; a mixture of phosphonic acid, methyl-,(5-ethyl-2-methyl-1,3,2-dioxaphosphorinan-5-yl)methyl ethyl ester, P-oxide and phosphonic acid, methyl-, bis [(5-ethyl-2-methyl-1,3,2-dioxaphosphorinan-5-yl)methyl] ester, P,P'-dioxide; ammonium polyphosphate; ethylendiamine polyphosphate; melamine phosphate; dimelamine phosphate; and microencapsulated red phosphorous.

40. A composition according to claim 30, wherein the flame retardant is ammonium polyphosphate or ethylendiamine polyphosphate or a mixture thereof.

41. A composition according to any claim 30, wherein the flame retardant is present in the composition in an amount of from about 0.1 to about 10%, by weight with respect to that of the converted starch component of the composition.

42. A composition according to claim 1, in which the bulk density of the composition is in the range of about 8.5 to about 30 kg/m.sup.3.

43. A composition according to claim 42, in which the bulk density of the composition is in the range of about 10.0 to about 15.5 kg/m.sup.3.

44. A composition according to claim 42, in which the resiliency of the composition is in the range of about 46 to 63%.

45. A composition according to claim 42, in which the compressibility of the composition is in the range of about 6 to 15%.

Description Submit all comments and votes

The present invention relates to biodegradable polymer compositions capable of being formed by heat and pressure into articles having dimensional stability. The invention relates particularly to biodegradable compositions comprising a converted starch and at least one member selected from alkenol homopolymers and/or alkenol copolymers. Such compositions are suitable for use, inter alia, in extrusion, in film formation, in the formation of foamed packaging materials or injection molding.

BACKGROUND OF THE INVENTION

It is known that natural starch which is found in vegetable products can be treated at elevated temperatures to form a melt.

Such a melt may preferably be formed by heating the starch material above the glass transition and melting temperatures of its components so that such undergo endothermic rearrangement. Preferably the starch material contains a defined amount of a plasticizer, which preferably is water, and melt formation is carried out at an elevated temperature in a closed volume, and hence at an elevated pressure.

It is possible to melt starch substantially in the absence of water, but in the presence of another suitable plasticizer, for example a liquid having a boiling point higher than the starch glass transition and melting temperature.

Different degrees of uniformity in melt formation, which can be measured by various methods, are possible. One method, for example, is to microscopically determine the amount of granular structure remaining in a starch melt. It is preferred that the starch is destructurised, viz, that the melt is substantially uniform in character, that light microscopy at a magnification of about 500.times., indicates a substantial lack of, or reduction in, granular structure, that the starch so melted exhibits little or no birefringence and that X-ray studies indicate a substantial, reduction in, or lack of, starch crystallinity in the melt.

It is implicit in the art of forming thermoplastics that the major components thereof should be of high molecular weight. This is the case also for the use of native starch in melt formation processes and for articles obtained therefrom. However, when blending native starch, in many cases such native starch is difficult to process and also difficult to blend with an alkenol homopolymer and/or an alkenol copolymer, because a relatively high amounts of plasticizer as well as energy input is required in order to achieve uniform melt formation and maximum physical properties of the shaped articles obtained from such a melt. Neat converted starch exhibits enhanced processability but lower strength and elongation to break as expected. The lower molecular weight of converted starch leads to enhanced crystallysability and higher modulus in neat systems.

It has now been discovered that native starch can be replaced by "converted" starches, i.e. a starch with a much lower average molecular weight than native starch. The advantages of using converted starches are enhanced processability of the converted starch/polymer blends. Concomitant the converted starch reduces the amount of plasticizer as well as the energy input necessary to provide uniform melt formation. As a consequence higher production speeds are possible. Surprisingly an improved mixing behaviour in blends with other synthetic polymers is observed resulting in a very uniform and often single phase product. The improved physical properties in blends are novel and, in view of the reduced average molecular weight of the starch, also very surprising.

Converted starches are prepared by degradation of starch molecules yielding products of lower dispersion viscosities than the original starch. Such products are known. Although many of the properties of the original starch are changed during the conversion process the main purpose of said process is to reduce the viscosity of the raw starch. The conversion process involves breaking, rearranging and/or recombining the starch chains for example in the presence and through the action of acids, alkalies, enzymes, oxidizing agents and/or heat. An important effect is the cutting of the chain lengths to lower average molecular weights. Controlled acid hydrolysis yields "thin-boiling" or "fluidity" starches in a wide range of viscosities wherein this hydrolysis is carried out below the gelatinization point of the starch. Acid hydrolysed corn starch is most preferred. The higher the "fluidity" the more degraded is the starch and in consequence the less, viscous is the dispersion for a given concentration. Acid conversion is preferred due to the ease of handling and recovery afforded by a granular starch as for example opposed to starch in dispersed form as necessitated by enzyme conversion. However, the means of producing the converted starch is of no importance for carrying out the present invention. These starches are generally named "converted starch" and this term will be used herein.

The degree of conversion is given herein as a 8.8% solids calcium chloride viscosity in seconds. Such calcium chloride viscosity values are known in the art and are for example described in the U.S. Pat. No. 4,726,957, which procedure is especially suitable for high amylose starch. The procedure of U.S. Pat. No. 4,726,957 can, for example, be modified slightly in that the weight of anhydrous starch is 9.0 grams and 125 grams of 40% calcium chloride solution is used.

U.S. Pat. No. 4,207,355 describes a water fluidity test which is most suitable for all other starches, i.e. starches that are not classified as high amylose starch or contain less than about 30% amylose. In carrying out the present invention it is recommended that the procedure of U.S. Pat. No. 4,726,957 is used for high amylose starches and the procedure of U.S. Pat. No. 4,207,355, is used for all other starches. Of course it is possible to further modifiy these tests. It is no problem for the person skilled in the art to correlate the different viscosity values obtained by the different modified test procedures.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a biodegradable composition as obtained from a melt comprising converted starch, a plasticizer and at least one member selected from alkenol homopolymers and/or alkenol copolymers which are combined under conditions sufficient to ensure uniform melt formation, and wherein the at least one member is present in the composition at a concentration of from 10 to 200 parts per 100 parts of dry converted starch.

The composition may contain further plasticizers as well further additives. By plasticizer is meant a substance which can be incorporated into a material to increase its flexibility, workability or reduce the melt viscosity, or lower the elastic modulus of the product. The term plasticizer includes all known types of plasticizers such as solvent plasticisers and non-solvent plasticisers.

The invention also includes the melt which is obtained from said composition as well as shaped articles, preferably films, laminates, injection moulded articles or foams made from said melt.

In one embodiment of the composition, the converted starch is derived from a high amylose variety and has an amylose content by weight of up to about 95%, and preferably of between 70 and 95%.

Said converted starch, however, does not have to be derived from a high amylose variety, and may have an amylose content, of up to about 65%, up to about 45%, and up to about 35%. It is possible that the amylose content of the converted starch is between 25 and 35%. The lower limit for the amylose content of the converted starch preferably is about 5 to about 10%, likewise by weight, including for example waxy maize starch.

The composition according to this invention may preferably comprise from about 10 to about 120 parts of said polymer and/or copolymer per 100 parts by weight of dry converted starch, and in a more preferred embodiment, the composition comprises from about 20 to about 85 parts of said polymer or copolymer per 100 parts of converted starch.

The composition may also comprise a polymer or copolymer content of from 10 to 65 parts, and particularly from 20 to 40 parts with respect to 100 parts of converted starch.

The alkenol homopolymer is preferably a polyvinyl alcohol which may be pre-plasticised with a polyhydric alcohol such as glycerol. The polyvinyl alcohol preferably is hydrolysed to an extent of from about 45 and about 100% and preferably has a number average molecular weight of about 15,000 to about 250,000, and more preferably has a number average molecular weight of from 10,000 to 150,000.

It is particularly preferred that the composition contains pre-treated polyvinyl alcohol in the form of a melt, obtained previously by adding sufficient energy to polyvinyl alcohol to melt it and substantially eliminate crystallinity in the melt. It is particularly preferred that the such crystallinity is substantially completely eliminated. Such pre-treatment of polyvinyl alcohol is disclosed in U.S. Pat. No. 5,051,222 and U.S. Pat. No. 5,137,969 and its contents is incorporated herewith by reference.

Alkenol copolymers as mentioned above are preferably synthetic copolymers containing vinyl alcohol units as well as aliphatic units as are obtained by copolymerization of vinyl esters, preferably vinyl acetate with monomers preferably ethylene, propylene, isobutylene and/or styrene with subsequent hydrolysis of the vinyl ester group.

Such copolymers are known and are described in "Encyclopedia of Polymer Science and Technology, Interscience Publ. Vol. 14, 1971".

The composition may further include compounds selected from the group consisting of nucleating agents, fillers, stabilisers, coloring agents and flame retardants and boron containing compounds. Said composition may further include known processing aids, such as lubricants, mould release agents and plasticisers.

It will be appreciated that the concentrations of the components in the composition can be derived according to a master-batching process, if desired.

The invention also provides a method for producing the composition in the form of a melt and optionally processing said melt, e.g. by extrusion, comprising:

a) providing a starting composition comprising converted starch, a plasticizer and at least one member selected from alkenol homopolymers and/or alkenol copolymers which are present in the composition at a concentration of from 10 to 200 parts per 100 parts of said converted starch;

b) adjusting the plasticizer content of the composition to between about 0.5 and about 40% by weight of the total composition during processing or plastification; c) heating the thereby adjusted composition to a temperature of between 100.degree. and 220.degree. C. and for a time at least sufficient to form a uniform melt of the composition;

d) removing any excess moisture before the extruder die to obtain a moisture content of between about 1% and about 20%; and optionally

e) extruding the thereby heated composition.

The present invention further includes a melt as obtained according to the method.

The present invention further refers to a method of working said composition under controlled plasticizer content, temperature and pressure conditions as a thermoplastic melt wherein said process is any known process, such as for example, foaming, filming, compression molding, injection molding, blow molding, vacuum forming, thermoforming, extrusion, coextrusion, and combinations thereof.

The invention will be further apparent from the following description, in conjunction with the following examples and the appended claims.

SPECIFIC DESCRIPTION

The present invention is defined in the appended claims. In particular, the invention refers to a biodegradable composition as obtained from a melt comprising converted starch, a plasticizer and at least one member selected from alkenol homopolymers and/or alkenol copolymers which are combined under conditions sufficient to ensure uniform melt formation, in which the at least one member is present in the composition at a concentration of from 10 to 200 parts per 100 parts of dry converted starch. Such a uniform melt is thermoplastic in character.

The converted starch which is present in the composition of the present invention is derived from at least one member selected from the group consisting of native starches of vegetable origin, which starches are derived from potatoes, rice, tapioca, corn, pea, rye, oats, wheat.

Controlled acid hydrolysed starch, so called "thin-boiling" or "fluidity" starch is preferred in a wide range of viscosities wherein this hydrolysis has been carried out below the gelatinization point of the starch so that the granular structure is only weakened. Acid hydrolysed corn starch is most preferred.

The degree of conversion is given herein as a 8.8% solids calcium chloride viscosity in seconds and such specification for converted starch is known as mentioned above. The test procedure to determine the 8.8% calcium chloride viscosity or for example the 7.2% calcium chloride viscosity is as follows.

Calcium Chloride viscosity

A. 8.8% Solids Test

The calcium chloride viscosity of the converted starch is measured by using a Thomas Stormer Rotational Shear-Type Viscometer (manufactured by Arthur H. Thomas Co., Philadelphia, Pa.) standardized at 30.degree. C., with a standard oil having a viscosity of 24.73 cps, which oil requires 23.12.+-.0.05 seconds for 100 revolutions. As conversion of the starch increases, the viscosity of the starch decreases. Accurate and reproducible measurements of the calcium chloride viscosity are obtained by determining the time which elapses for 100 revolutions at a specific solids level. The general procedure is as follows: A total of 8.8 grams of the converted starch (anhydrous basis) is slurried in 100 grams of buffered 20% calcium chloride solution in a covered stainless steel cup, the slurry heated in a boiling water bath for 30 minutes with occasional stirring. Then, the starch solution is brought to the final weight (108.8 grams) with hot distilled water. The time required for 100 revolutions of the resultant solution at 81.degree.-83.degree. C., is measured three times in rapid succession and the average of the three measurements recorded.

The calcium chloride solution is prepared as follows: A total of 264.8 grams of reagent grade calcium chloride dihydrate is dissolved in 650 ml. of distilled water in a tared 1 liter glass beaker. Thereafter 7.2 grams of anhydrous sodium acetate is dissolved in the solution. The solution is allowed to cool and the pH is measured. If necessary, the solution is adjusted with hydrochloric acid to pH 5.6.+-.0.1. Finally, the solution is brought to weight (1007.2) grams) with distilled water.

B. 7.2% Solids Test

The calcium chloride viscosity test described above in Part A is employed with the exception that a 7.2 gram (anhydrous basic) sample of starch is cooked in the calcium chloride solution and the final weight of the starch solution to be evaluated is brought to a final weight of 107.2 grams.

A correlation of 8.8% solids versus 7.2% solids viscosity readings of a series of six acid hydrolysed high amylose starch samples is provided below:

______________________________________ CaCl.sub.2 Viscosity Measurement (sec) 8.8% Solids 7.2% Solids ______________________________________ 95.6 40.6 71.6 39.0 58.7 29.3 40.5 24.5 31.5 19.9 30.2 20.4 ______________________________________

The preferred 8.8% calcium chloride viscosity (fluidity level) for the use in the present invention is from about 5 seconds to about 60 seconds, preferably from about 10 seconds to about 40 seconds and most preferably from about 20 seconds to about 40 seconds.

For certain applications it is preferred that the converted starch is derived from a high amylose starch having an amylose content of between about 60% and about 95%, preferably between about 65% and about 80%, the percentages being by weight with respect to that of the converted starch. The lower limit for the amylose content of the preferably is about 10 to about 15%, likewise by weight.

For certain applications further it is preferred that the converted starch has an amylose content of between about up to about 45%, preferably between about 35%, preferably between about 25% and about 35%, the percentages being by weight with respect to that of the converted starch.

The starch component of the composition according to the invention includes converted starch melted in the absence of added water, but in the presence of another plasticizer--such as glycerol. The preferred plasticizer is, however, water.

The alkenol homopolymer is preferably polyvinyl alcohol (PVA) having a number average molecular weight of at least about 10,000 (which corresponds to a degree of polymerization of at least 240). It is more preferred that the PVA has a number average molecular weight of between about 20,000 and 100,000, and most preferred that it has a number average