Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-07-05
2001-03-13
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C560S127000, C528S295500, C528S300000, C528S308000
Reexamination Certificate
active
06201083
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention broadly concerns modified polymer compositions and methods for their use in the preparation of polymeric end-products. In particular, the invention relates to a process for covalently cross-linking polyhydroxyalkanoate (PHA) polymer molecules so as to produce branched PHA compositions suitable for use in the production of PHA coatings, films and moldings.
There has been considerable interest in recent years in the use of biodegradable polymers to address concerns over plastic waste accumulation. The potential worldwide market for biodegradable polymers is enormous (>10 B lbs/yr). Some of the markets and applications most amenable to the use of such biopolymers involve those having single, short use applications, including packaging, personal hygiene, garbage bags, and others. These applications, although poorly suited for recycling, are ideally suited for biodegradation through composting.
PHA biopolymers are polyesters produced by numerous microorganisms in response to nutrient limitation. The commercial potential for PHA spans many industries, and is derived primarily from certain advantageous properties which distinguish PHA polymers from petrochemical-derived polymers and other polymers derived from renewable resources, namely excellent biodegradability and/or natural renewability compared to the petrochemical-derived polymers, and hydrophobicity compared to other polymers derived from renewable resources.
Widespread use and acceptance of PHAs, however, has been hindered by difficulties in developing acceptable means by which these biodegradable polymers can be efficiently and economically processed into commercially attractive products. In addition, many characteristics of PHA-derived products, such as ductility, flexibility and elongation, are rapidly lost over time. This “aging” of PHA-derived products is unacceptable for many applications in that the products fail to maintain adequate structural integrity for their intended useful life. Thus, the success of PHA as a viable alternative to both petrochemical-derived polymers and to non-PHA biodegradable polymers, will depend upon novel approaches to overcome the unique difficulties associated with PHA polymer processing and with products derived therefrom.
One important application of biodegradable polymers is in the area of films and coatings for packaging and one-time use applications. Coatings, for example, are typically made by melting a polymeric material and extruding the melt through a die onto a substrate such as paper or board. Extrusion coating of PHA polymers, however, has typically required coextrusion with a non-PHA polymeric sacrificial layer such as polyethylene, resulting in a coated material which comprises a paper layer, a PHA layer, and a sacrificial layer. The use of a sacrificial layer during PHA processing has been necessary to provide sufficient melt stability and adhesion of the PHA coating to the paper surface and to allow the coatings to be processed at commercially desirable line speeds. Typically, the sacrificial layer is subsequently stripped from the PHA coated paper. It is important from a commercial standpoint to develop compositions and methods for cost-effective extrusion operations which do not depend on the use of a sacrificial non-PHA polymeric layer, and which provide PHA films and coatings having good physical and mechanical properties.
SUMMARY OF THE INVENTION
The present invention broadly concerns PHAs modified to contain branching, also known as cross-linking, in their polymer backbone. The branched PHA compositions of this invention are produced by melting PHAs in the presence of low levels of free-radical initiators under conditions which cause their decomposition into free radicals. The free radicals stimulate inter-chain cross-link formation between linear PHA molecules giving rise to the branched PHA compositions of the invention. Branched PHAs can provide significant cost benefits when used for production of PHA-derived articles, particularly coatings and films, by maximizing throughput while minimizing excessive waste of materials. Furthermore, products produced from the branched PHAs of the invention have improved physical, mechanical and aging characteristics relative to those produced from unmodified, linear PHAs.
In one aspect of the invention, there are provided polymeric compositions comprising PHA and 0.001 to 0.5% of a peroxide or other suitable free radical initiator. The compositions can be in any suitable form, for example, powder, granules, pellets, latex, solution etc. During processing into a desired PHA end-product, the compositions are exposed to temperatures above the melting point of the PHA and above the decomposition temperature of the peroxide for a length of time sufficient to cause PHA cross-linking to be present in the resulting product.
In accordance with another aspect of the present invention there are provided polymeric pellet compositions comprising branched PHA and methods of making the pellet compositions. The compositions can be produced by melting PHA in the presence of a free radical initiator in an amount sufficient to cause the desired PHA branching. Preferably, the free radical initiator is an organic peroxide present at between 0.001 to 0.5 wt. % of the PHA. During pellet production, the peroxide is exposed in the melt to temperatures above its decomposition temperature for a length of time sufficient to generate the free radical decomposition products of the peroxide and to thereby stimulate PHA cross-linking. The melt is typically extruded as a continuous strand, cooled and crystallized, and cut into pellets comprising branched PHA.
In accordance with another aspect of the invention, there are provided methods of using the PHA compositions of the present invention in the production of PHA end-products. Branched PHA compositions of the invention are advantageous for use in any thermal processing operation where good extensional rheology is important. The compositions are useful in extrusion coating, film and sheet formation, foams, thermoforming etc. In a particularly important application, there is provided a method of making PHA films and coatings which do not require the use of non-PHA polymeric sacrificial layers and which can be produced under efficient and economical processing conditions. The films and coatings can be made by melting a polymeric composition of the present invention and extruding the melt as an unsupported film or onto a substrate support. When the compositions of the present invention are used in melt extrusion operations such as these, they provide numerous advantages not achievable with linear PHAs. For example, they significantly reduce neck-in behavior typical of these processes, thereby increasing yield by reducing waste. In addition, the products so produced have improved ductility and display less age-related embrittlement over time.
In another aspect of the invention, there is provided an extrusion coating process wherein a stream of air or other suitable medium is directed against a polymer web after the web leaves the extrusion die but before it contacts the substrate to be coated. This method allows extrusion coating operations to be performed at significantly higher line speeds by reducing the draw resonance and neck-in of the polymeric material. In addition, the method reduces the critical coat weight of the polymer that is required for acceptable substrate adhesion.
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Cavallaro, Pasquale et al. (1993) “Reactive blending of bioaffine polyesters through free-radical processes,”Die Angewandie Makromolekulare Chemie210:129-141.
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Grimaldi, M. et al. (1996) “Reactive processing—property r
Asrar Jawed
D'Haene Pol
Beusen Jon
Howrey Simon Arnold & White , LLP
Monsanto Company
Nutter Nathan M.
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