Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Patent
1995-07-13
1996-07-09
Yoon, Tae
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
524599, 264216, C08J 518, B29D 700, C08L 6700
Patent
active
055346162
DESCRIPTION:
BRIEF SUMMARY
THIS INVENTION relates to polyhydroxyalkanaoates and the production of films therefrom, which may be free standing or supported.
Films of polymers have many uses. Free standing films of plastics materials have innumerable industrial and consumer applications. Supported films, for example coated paper, are also widely useful, particularly for imparting the qualities of moisture resistance and heat-sealability to the coated support.
In principle, films can be prepared from polymer either by using a preparation of molten polymer or by the use of a solution of polymer in a suitable solvent. In the latter case, it is clearly necessary to match a polymer with a suitable solvent for it; for polyhydroxyalkanoate polymers, which have the merit of being biodegradable, a problem exists in that the suitable solvents are generally chlorinated and therefore environmentally detrimental. Attention has therefore diverted to attempting to prepare films of these biodegradable polymers from molten preparations of the polymer; and it is to this area that the present invention is directed.
Extrusion coating of polymer films from molten polymer has been developed for a number of plastics, in particular low density polyethylene. The standard procedure is for molten polyethylene to be extrusion coated onto a chilled roller, known in the art as a chill-roll. Low density polyethylene crystallises very quickly, and at reasonably low temperatures, so that it is adequately crystalline before it is drawn off the chill-roll, even when the roll is operating at very high line speeds, such as 700 m/min. Polyhydroxyalkanoate polymers crystallise much more slowly than low density polyethylene at a typical chill-roll temperature.
The process of polyhydroxyalkanoate crystallisation from the molten state involves growth of crystallites which are often spherical regions of crystallised material (spherulites). A nucleation site for a crystallite may occur in the pure polymer or can be artificially introduced by the use of a nucleating agent. We have found that the optimum temperature for creating nucleating sites is considerably lower than the optimum temperature for growth of the crystallites.
It has now been found that it is possible to adapt a chill-roll process for the preparation of a polyhydroxy-alkanoate film if the film formation is allowed to take place on the roll at a relatively low temperature, so as to improve or optimise nucleation density and maintain the film in a glassy state which is of low tackiness and subsequently the film temperature is raised to enable crystallisation to take place. Acceptable line speeds are practicable.
The film so produced has smaller crystallites than that produced if the chill roll is at higher temperatures. This can have two advantages: firstly, it may lead to improved mechanical properties. Secondly, smaller crystallites can lead to reduced voiding/pinholing at the boundaries between crystallites. This improves the coatings barrier properties, and is an important feature of films whether free standing or supported for example on paper or board. If the coating tends to pinhole or crack, its efficiency is reduced as a barrier medium. Typically paper coatings are between 10 and 25 microns, and hence crystallites should be smaller than 10 microns. We have devised a process which produces crystallites <10 .mu.m and typically <7 .mu.m, mostly 3-5 .mu.m.
According to a first aspect of the present invention, there is provided a process for preparing a film of a polyhydroxyalkanoate polymer, the process comprising applying a layer of molten polyhydroxyalkanoate to a surface not substantially above the glass transition temperature of the polymer and for example below 30.degree. C., preferably below 20.degree. C. and more preferably below 15.degree. C. so as to form a film and subsequently raising the temperature of the film so formed to a temperature of 45.degree. to 100.degree. C. and preferably 50.degree. to 95.degree. C. and more preferably 60.degree. to 80.degree. C.
Without wishing to be bound by thi
REFERENCES:
patent: 4537738 (1985-08-01), Holmes
patent: 4880592 (1989-11-01), Martini et al.
Yoon Tae
Zeneca Limited
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