High strength plastic from reactive blending of starch and polylactic acids

Reference Number: K 99-23

Inventors: Sun, Xiuzhi (Susan); Seib, Paul; Wang, Hua

USPTO Link: 6211325

Invention Summary

The present invention overcomes the problems of the prior art by providing novel polymer compositions useful for forming high-strength, degradable plastics. The inventive compositions broadly comprise starch reacted with polylactic acid via compatibilizing or linkage groups.

In more detail, polymer compositions according to the invention are prepared by forming a mixture comprising the starch, polylactic acid, and linkage group and causing the ingredients of the mixture to react such as by heating the mixture to a temperature of at least about 150.degree. C., and preferably at least about 175.degree. C. The heating step should be carried out for at least about 2 minutes, and more preferably from about 3-5 minutes.

The weight ratio of starch:polylactic acid in the mixture should be from about 1:99 to about 70:30, preferably from about 40:60 to about 60:40, and more preferably from about 45:55 to about 50:50. The average molecular weight of the polylactic acid used to prepare the mixture is preferably at least about 70,000 Daltons, and more preferably from about 90,000-140,000 Daltons. Suitable starches include those selected from the group consisting of corn starch, wheat starch, sorghum starch, potato starch, tapioca starch, or any other starch from crops and plants.

The linkage group should comprise at least one isocyanate moiety, and more preferably at least two such isocyanate moieties, with preferred linkage groups being selected from the group consisting of diphenylmethylene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate. The most preferred linkage group is diphenylmethylene diisocyanate. The linkage group should be mixed with the starch and polylactic acid at a level of less than about 4% by weight, preferably from about 0.1-2% by weight, and more preferably from about 0.2-0.5% by weight linkage group, based upon the total weight of the starch/polylactic acid/linkage group mixture taken as 100% by weight.

In forming the starch/polylactic acid/linkage group mixture, it is preferred that all of the ingredients simply be mixed together. Alternately, a precursor mixture comprising respective quantities of polylactic acid and of the linkage group is formed, and the precursor mixture is then mixed with the starch and the remainder of the polylactic acid. In these instances, the polylactic acid should be present in the precursor mixture at a level of from about 96-99.9% by weight, and preferably from about 98-99% by weight, based upon the total weight of the precursor mixture taken as 100% by weight.

In another embodiment, a precursor mixture comprising respective quantities of polylactic acid, starch, and the linkage group is formed, and the precursor mixture is then mixed with the remainder of the starch and polylactic acid. In this embodiment, the precursor mixture should comprise from about 30-99% by weight polylactic acid, from about 1-70% by weight starch, and from about 0.1-4% by weight of the linkage group. Even more preferably, the precursor mixture should comprise from about 30-70% by weight polylactic acid, from about 30-70% by weight starch, and from about 1-2% by weight of the linkage group, based upon the total weight of the precursor mixture taken as 100% by weight.

The final prepared polymer composition can then be used to form a plastic in the same manner as prior art plastic-forming processes, including utilizing known additives and plasticizers. For example, the polymer composition can be formed into disposable food utensils, packaging for food, and numerous other plastic items. Advantageously, the inventive methods allow smaller quantities of polylactic acid to be utilized, thus decreasing the cost of the final product compared to prior art plastic products derived from polylactic acid. Furthermore, by using starch with smaller quantities of polylactic acid rather than simply large quantities of polylactic acid alone, the biodegradability of the polylactic acid is not compromised.

The inventive compositions, and the plastics derived therefrom, have highly desirable mechanical properties in general, and have substantially improved mechanical properties when compared to pure polylactic acid or to prior art polylactic acid-derived plastics. For example, the ASTM D638-92 tensile strength of the inventive polymer composition is at least about 50 MPa, preferably at least about 60 MPa, and more preferably from about 40-75 MPa. The ASTM D638-92 percent elongation of the composition is at least about 3%, preferably at least about 4%, and more preferably from about 3-6%. Additionally, the ASTM D638-92 modulus of elasticity of the composition is at least about 1500 MPa, preferably at least about 1800 MPa, and more preferably at least about 1800-2000 MPa.

The crystallinity (X.sub.c, described in detail below) of the compositions is at least about 2 times, and more preferably at least about 5 times greater than the crystallinity of pure polylactic acid (i.e., polylactic acid which has not been blended with some other polymer or modifier). Finally, when a polymer composition according to the invention is subjected to four heating cycles (i.e., it is heated to its melting temperature followed by cooling to room temperature, and this cycle is then repeated three times for a total of four cycles) the melting point of the composition during the fourth heating cycle is within about 3.degree. C. of the composition melting point during the first heating cycle. Thus, the melting point of the composition during the fourth heating cycle is preferably 170-175.degree. C.