There is a need for an environmentally degradable packaging thermoplastic as an answer to the tremendous amounts of discarded plastic packaging materials. U.S. plastic sales in 1987 were 53.7 billion lbs of which 12.7 billion lbs were listed as plastics in packaging. A significant amount of this plastic is discarded and becomes a plastic pollution that is a blot on the landscape and a threat to marine life.
A further problem with the disposal of plastic packaging is the concern for dwindling landfill space. It has been estimated that most major cities will have used up available landfills for solid waste disposal by the early 1990's. Plastics comprise approximately 3 percent by weight and 6 percent of the volume of solid waste.
One other disadvantage of conventional plastics is that they are ultimately derived from petroleum, which leaves plastics dependent on the uncertainties of foreign crude oil imports. A better feedstock would be one that partially derives from renewable, domestic resources thus reducing reliance on imports.
However, there are good reasons for the use of packaging plastics. They provide appealing aesthetic qualities in the form of attractive packages which can be quickly fabricated and filled with specified units of products. The packages maintain cleanliness, storage stability, and other desirable qualities such as transparency for inspection of contents. These packages are known for their low cost of production and chemical stability. This stability, however leads to very long-life of the plastic, so that when its one-time use is completed, discarded packages remain on, and in, the environment for incalculably long times.
There are many citations in the prior art for the preparation of lactic acid polymers and copolymers. The earliest processes used lactic acid directly as the monomer, cf., eg., U.S. Pat. Nos. 1,995,970; 2,362,511; and 2,683,136. The poly(lactic acids) of these patents were of low molecular weights, tacky and without good physical properties. U.S. Pat. No. 2,668,162 (Lowe, DuPont) discloses the use of lactide as the monomer. Lactide is the dilactone of lactic acid and is an internal ester of lactic acid. When lactide is formed, byproduct water is eliminated, permitting the lactide subsequently to be ring-opened polymerized to linear polyester of high molecular weight without tedious condensation methods. Polymers and copolymers of excellent physical properties were obtained by using the intermediate, lactide, to form poly(lactic acid). Copolymers of lactide and glycolide are disclosed by the Lowe patent which are tough, clear, cold-drawable, stretchable, and capable of forming at 210 C into self-supporting films.
Other patents related to forming lactide polymers include U.S. Pat. Nos. 2,703,316; 2,758,987; 2,951,828; 3,297,033; 3,463,158; 3,531,561; 3,636,956; 3,736,646; 3,739,773; 3,773,929; 3,887,699; 3,797,499; 4,273,920; 4,471,077; and 4,578,384; German Offenlegungsschrift 2,118,127; Canadian Patents 808,731; 863,673; and 923,245. U.S. Pat. No. 3,636,956 teaches an interweaving of fibers that is not blending or melt blending of a composition to make a physical mixture.
U.S. Pat. No. 4,719,246 teaches the blending of homopolymers of L-lactide, D-lactide, polymers of mixtures thereof; and copolymers of L-lactide or D-lactide with at least one nonlactide comonomer. The blending is intended to produce compositions having interacting segments of poly(L-lactide) and poly(D-lactide).
U.S. Pat. No. 4,661,530, discloses the mixtures of a poly(L-lactic acid) and/or poly (D,L-lactic acid) and segmented polyester urethanes or polyether urethanes. Biodegradable materials are formed that are useful in synthetic replacements of biological tissues and organs in reconstructive surgery.
PCT publication W0 87/00419 to Barrows reveals a bone spacer comprising a blend or mixture of a nonabsorbable polymer and a bioabsorbable polymer, poly(lactic acid) is one of the preferred biodegradable polymers but plasticizers are not revealed thererin. PCT publication W0 84/00303 to Gogolewski et al suggests blends of polyesters and polyurethanes for preparing surgical filaments.