Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-07-06
2002-06-25
Chen, Vivian (Department: 1773)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C528S361000, C524S845000, C427S372200, C427S384000
Reexamination Certificate
active
06410096
ABSTRACT:
DESCRIPTION
The present invention relates to a method for producing a biologically degradable polyhydroxyalkanoate coating, wherein an aqueous dispersion of polyhydroxyalkanoate is applied to the surface to be coated. In particular, the invention relates to the coating of surfaces which form part of a product to be supplied for consumption to a human being or animal and/or surfaces which form part of a product which is not resistant to high temperatures. A suitable example of a product to be coated is a foodstuff, in particular cheese. The invention also relates to objects coated with such a polyhydroxyalkanoate coating, and also to elastomeric, films of polyhydroxyalkanoate and latices thereof. Elastomeric means the polyhydroxyalkanoates behave at room temperature as elastomers, wherein the crystals serve as physical crosslinks providing a minimum elongation at break of 50%.
Poly(3-hydroxyalkanoates) (PHA) are biologically degradable polymers which can be accumulated by microorganisms as sources of carbon and energy (Anderson and Dawes 1990, Microbiol. Rev. 54:450-472). Poly(3-hydroxybutyrate) (PHB) and the copolymer poly(3-hydroxybutyrate)co-valerate (PHB/HV) are the most known and best studied forms of PHA and are classified as polyhydroxyalkanoates of the PHB type. In the past ten years, a large number of new types of PHA have been found. In particular, Pseudomonas strains have proved to be capable of synthesizing PHAs with diverse compositions (Lageveen et al. 1987, Appl. Environ. Microbiol. 54, 2924-2932). These polyhydroxyalkanoates comprise 3-hydroxy acids having a carbon-chain length of C
6
-C
14
and are termed polyhydroxyalkanoates of the PHA type.
In order to be able to use polyhydroxyalkanoates, they must be derived from biomass and separated from contaminating cell parts such as proteins and lipids. Two important working-up strategies have been developed for PHB and PHB/HV. The first method relates to an extraction with a solvent such as methylene chloride, dichloroethane, chloroform or trichlorobenzene (Lafferty et al. 1988, Biotechnology vol. 6b, 136-176). The disadvantage of this method is that the organic solvents used are toxic and environmentally unacceptable. The use of these solvents would not only make the working-up of PHAs expensive but also considerably limit the number of possible applications of PHAS. All the applications in which PHAs come into contact with products to be applied to or consumed by a human being or animal, such as foodstuffs, are ruled out in this case because small residues of the solvent which is harmful to health can, after all, always be present.
In the second working-up method, the bacteria cells containing PHB/HV are broken open by means of a combined physical, chemical and enzymatic procedure. In this way, the PHB/HV granules are released from the biomass and an aqueous dispersion is produced, viz. a latex. The aqueous PHB/HV dispersion is further purified of cell constituents by means of rinsing steps and centrifugation. A dry PHB/HV powder which can be processed further to produce, for example, foils, small bottles, fibres etc. can then be obtained by means of spray drying the dispersion (Holmes 1985, Phys. Technol. 16, 32-36).
With a view to the environmental requirements, great need has arisen for biologically degradable products. This certainly applies to products which comprise a solid support with a coating. Processing of PHA to form such a coating is an interesting field of application. It is known of PHB and the copolymer PHB/HV that they can be processed in various ways to form a coating. In the first place, by applying a PHB solution in an organic solvent such as chloroform to the surface of the coated agent and allowing the solvent to evaporate. It is clear that this is a very unacceptable method for environmental reasons.
In a second approach, use is made of an aqueous dispersion of PHB, PHB remaining behind as a powder on the surface of the agent to be coated after evaporation of the water. The PHB granules do not fuse and films can only be formed by exposing the surface coated with PHB powder to a solvent such as chloroform (Marchessault et al. 1990, in Novel Biodegradable Microbial Polymers) or by heating the surface coated with PHB powder to the melting point of PHB. This process is disclosed in international patent application WO91/13207 of Marchessault and Lepoutre. Marchessault and Lepoutre describe a method of preparing a coated or impregnated paper or other fibre construct, a latex for use in the preparation of paper or other fibre constructs and films and self supporting films. They describe a latex comprising a colloidal suspension in water of essentially non-crystalline particles of a poly-&bgr;-hydroxyalkanoate polymer or copolymer and illustrate butyrate and valerate as preferred hydroxyalkanoates. Their latex will comprise 10 to 50%, preferably 15 to 30%, more preferably 20 to 25% by weight of the PHA. Typically their latex will comprise 10 to 15 parts by weight. They describe that a film can be made from a latex by casting a dilute solution of 15-25% w/w solids on an impervious surface and allowing the water to evaporate slowly at room temperature. They state that heating the air dried film in a drying oven at a temperature 30 degrees below T
m
causes moderate fusion. These films have a microporous structure and can be hot pressed at 100-140° C. and 1000-5000 psi to produce flexible translucent films of average to high crystallinity. The hot fusion treating of PHB/V copolymer containing 21% by weight PHV provides water imperviousness. Dense transparent films are formed by exposing air dried films to solvent or liquid vapour atmospheres such as chloroform and other halogenated solvents. Explicitly Marchessault and Lepoutre describe “A few grams of 21% HV polymer latex was diluted to 20-30% w/w solids with distilled water. The resulting mixture was poured on a clean glass surface and allowed to air dry at room temperature overnight. A white uniform coat or film with little or no strength was obtained. It was then placed in a convection oven set at 100° C. for a period of not less than 10 min. A partially fused film was obtained”. Hot pressing the resulting film provided a film with at most elongation at break of 30%. Alternatively subjecting the partially fused film to chloroform provided an extremely smooth and tough film.
Disadvantages also apply to these known methods, firstly the use of an organic solvent and secondly, the performance of a heating step in which a temperature above 100° C. has to be used. The applications are therefore also clearly limited in this case. Marchessault and Lepoutre's main application is for impregnating paper. They disclose nothing with regard to preparing elastomeric films. Their coatings are either microporous and non-peelable or thermoplastic exhibiting at most elongation at break of 30%. They do not disclose elastomeric films suitable for appliance to or consumption by humans or animals.
DE-4 040 158 describes how the use of organic solvents is harmful to health and how the presence of the solvent in the coating makes it impossible to use them in the foodstuffs or pharmaceutical industry, in particular as packing material also for material for treating wounds. A further disadvantage mentioned is that the known coatings are available solely as impermeable films. For many applications, however, such as medical bandaging or other materials for absorbing body fluids, a porous coating, in particular a coating permeable to gas and air, is necessary, which materials cannot be obtained with the aid of the coatings with solvents. In DE 4 040 158 a dispersion of polyhydroxybutyrate or polyhydroxyvalerate, or copolymers thereof, in water is described for obtaining such a porous coating. The aqueous dispersions can be prepared by stirring PHA powders or directly in the processing of PHA-forming organisms. The coated objects are prepared, according to DE 4 040 158, by applying the dispersion of PHA to the object and then subsequently drying to remove the dispersing ag
Eggink Gerrit
Northolt Martin Dinant
Chen Vivian
Stichting Onderzoek en Ontwikkeling Noord-Nederland (SOONN)
Young & Thompson
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