Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
1999-07-27
2001-03-27
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S357000, C528S361000, C428S221000, C428S323000, C428S325000, C428S326000, C428S329000, C428S331000, C428S338000, C428S411100, C428S480000
Reexamination Certificate
active
06207792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a film comprising a melt-stable, biodegradable amorphous lactide polymer composition and a process for manufacturing the film from a melt-stable, biodegradable lactide polymer.
2. Description of the Prior Art
The need for polymeric biodegradable films is well established. Films manufactured from blown or cast processes are well known. Typically in a blown film process, a plastic melt passes through a die which extrudes the molten plastic into an annular shape. Typically, the extruded film is extruded in an upward fashion. As the film moves upward, air is blown into the film which expands the film into a tubular shape. The tube is generally closed at some distance above the die, with a pair of nip rolls.
In a cast film process, a sheet is typically extruded from a slit die. The sheet is thereafter pulled through a series of rollers which cool the extruded sheet and may also elongate the length and width of the sheet to a desired dimension and thickness.
The use of films is widespread and well known in the art. The heaviest use of films occurs in the packaging and disposable article industries. Films employed in the packaging industry include films used in food and non-food packaging, merchandise bags and trash bags. In the disposable article industry, the general uses of films occur in the construction of diapers and personal hygiene articles, including tapes.
In light of depleting landfill space and adequate disposal sites, there is a need for biodegradable films. Currently, films comprising polymers such as polyethylene, polypropylene, polyethylene terephthlate, nylon, polystyrene, polyvinyl chloride and polyvinylidene chloride are popular for their superior extrusion and film-making properties. However, these films are not biodegradable. Furthermore, these films are generally noncompostable, which is undesirable from an environmental point of view.
Films have been developed which are generally considered to be biodegradable. These are films which purportedly have adequate properties to permit them to breakdown when exposed to conditions which lead to composting. Examples of such arguably biodegradable films include those made from polycaprolactone, starch biopolymers and polyvinyl alcohol.
Although films extruded from these materials have been employed in film containing articles, many problems have been encountered with their use. Often the films are not completely biodegradable or compostable. Furthermore, some biodegradable films may also be unduly sensitive to water, either limiting the use of the film or requiring some type of surface treatment to the film, often rendering the film nonbiodegradable. Others have inadequate heat resistance for wide spread use.
The use of lactic acid and lactide to manufacture a biodegradable polymer is known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. Processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatability in the final product. These processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield.
In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form may then be sold to end users who convert, i.e., extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.
Prior to the present invention, it is believed that there has been no disclosure of a combination of composition control and melt stability requirements which will lead to the production of commercially viable lactide polymer film.
It is generally known that lactide polymers or poly(lactide) are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation which occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during processing which must be overcome.
Lactide polymer degradation at elevated temperature has been the subject of several studies, including: I. C. McNeill and H. A. Leiper,
Polymer Degradation and Stability
, vol. 11, pp. 267-285 (1985); I. C. McNeill and H. A. Leiper,
Polymer Degradation and Stability
, vol. 11, pp. 309-326 (1985); M. C. Gupta and V. G. Deshmukh,
Colloid
&
Polymer Science
, vol. 260, pp. 308-311 (1982); M. C. Gupta and V. G. Deshmukh,
Colloid
&
Polymer Science
, vol. 260, pp. 514-517 (1982); Ingo Luderwald,
Dev. Polymer Degradation
, vol. 2, pp. 77-98 (1979); Domenico Garozzo, Mario Giuffrida, and Giorgio Montaudo,
Macromolecules
, vol. 19, pp. 1643-1649 (1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada,
Polymer
, vol. 29, pp. 2229-2234 (1988).
It is known that lactide polymers exhibit an equilibrium relationship with lactide as represented by the reaction below:
No consensus has been reached as to what the primary degradation pathways are at elevated processing temperatures. One of the proposed reaction pathways includes the reaction of a hydroxyl end group in a “back-biting” reaction to form lactide. This equilibrium reaction is illustrated above. Other proposed reaction pathways include: reaction of the hydroxyl end group in a “back-biting” reaction to form cyclic oligomers, chain scission through hydrolysis of the ester bonds, an intramolecular beta-elimination reaction producing a new acid end group and an unsaturated carbon-carbon bond, and radical chain decomposition reactions. Regardless of the mechanism or mechanisms involved, the fact that substantial degradation occurs at elevated temperatures, such as those used by melt-processors, creates an obstacle to use of lactide polymers as a replacement for petrochemical-based polymers. It is apparent that degradation of the polymer during melt-processing must be reduced to a commercially acceptable rate while the polymer maintains the qualities of biodegradation or compostability which make it so desirable. It is believed this problem has not been addressed prior to the developments disclosed herein.
As indicated above, poly(lactide)s have been produced in the past, but primarily for use in medical devices. These polymers exhibit biodegradability, but also a more stringent requirement of being bioresorbable or biocompatible. As disclosed by M. Vert,
Die Ingwandte Makromolekulare Chemie
, vol. 166-167, pp. 155-168 (1989), “The use of additives is precluded because they can leach out easily in body fluids and then be recognized as toxic, or, at least, they can be the source of fast aging with loss
Eichen Conn Robin Sue
Gruber Patrick Richard
Hall Eric Stanley
Kolstad Jeffrey John
Ryan Christopher M.
Cargill Incorporated
Hampton-Hightower P.
Merchant & Gould P.C.
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