Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
2000-04-14
2001-04-03
Sanders, Kriellion (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C528S073000, C528S361000
Reexamination Certificate
active
06211325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is broadly concerned with modified synthetic polymer compositions for use in forming high strength, biodegradable plastics and methods of forming such compositions. More particularly, the inventive compositions comprise polylactic acid joined or copolymerized with starch via a linkage or compatabilizing group which comprises a diisocyanate moiety. The invention allows lower quantities of polylactic acid to be utilized while maintaining the biodegradability of polylactic acid-derived plastics and improving their mechanical properties.
2. Description of the Prior Art
Plastics (synthetic resins) are widely used and an important material in current commercial products. As more plastics having varying mechanical properties are developed, industries are finding uses for plastics which only a few years ago would have been unexpected. For example, many automobiles which were previously formed entirely of metal now include plastic components such as plastic body panels. Furthermore, these plastic components are so well-designed that it is generally difficult to visually differentiate between the plastic and steel components on an automobile. In addition to automobile components, plastics find uses in innumerable products including children's toys, kitchen dishes and appliances, packaging materials, and medical products.
While plastics have generally been an inexpensive and efficient material for manufacturing products, they are derived in large part from petroleum resources which are finite and increasing in cost. Thus, it is important to develop new methods and materials for forming plastics as an alternative to the current methods.
Additionally, the environmental impact of discarded plastic objects is of growing global concern due to the fact that disposal methods for such wastes are quite limited. Incineration of the plastic wastes generates toxic air pollution. At the same time, satisfactory landfill sites are limited, and most durable plastics do not biodegrade. There is, thus, a need for durable and biodegradable plastic materials, particularly for short-term use items such as packaging materials and disposable utensils.
Fully biodegradable, synthetic polymers have been commercially available for several years. Such polymers include polylactic acid (PLA), polycaprolactone (PCL), and polyhydroxybutyratevalerate (PHBV). Among these polymers, PLA has been extensively studied in medical implants, sutures, and drug delivery systems. Unlike other available biodegradable synthetic polymers, PLA exhibits promising mechanical properties, thus making it appealing for use as a disposable and biodegradable plastic substitute. However, PLA is costly compared to conventional petroleum-based plastics, and its degradation rate is slow compared to the waste accumulation rate. Finally, another disadvantage of PLA is that its modulus of elasticity decreases by about 85% at temperatures above its glass transition temperatures (60° C.), where it becomes very soft, and thus it has only limited applications.
Starch is a renewable and degradable carbohydrate biopolymer that can be purified from various sources by environmentally sound processes. By itself however, starch has severe limitations in plastic applications due to its water solubility. That is, articles molded from starch will swell and deform upon exposure to moisture. To decrease interaction with water, starch is often blended with hydrophobic polymers which reduce the use of petroleum polymers while simultaneously increasing the biodegradability of the product.
Starch has been blended with synthetic biodegradable polymers such as PCL, PHBV, and poly(hydroxybutyrate) (PHB). However, starch and PLA have not previously been successfully blended because they are immiscible polymers. There is a need for a compatibilizer which can enhance the compatibility of starch and PLA to yield a high-strength biodegradable plastic.
SUMMARY OF THE INVENTION
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° C., and preferably at least about 175° 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
Seib Paul
Sun Xiuzhi S.
Wang Hua
Hovey Williams Timmons & Collins
Kansas State University Research Foundation
Sanders Kriellion
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