Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – To form particulate product
Patent
1997-01-10
2000-06-27
Tentoni, Leo B.
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
To form particulate product
26421121, 26421122, 26421124, 366 79, 366 89, 366318, 366322, 366323, 366343, 425143, 425200, 425203, 425208, 425209, 4253781, 4253782, 4253822, B29C 4700, B29C 4760
Patent
active
060803460
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an extrusion and moulding apparatus and method, and more particularly to an apparatus and method for use in the extrusion or moulding of polymeric articles and materials, e.g. polymeric sheets, films and other forms. It extends to a polymer per se with certain novel properties.
BACKGROUND OF THE INVENTION
Single and multiple (e.g. twin) screw feeder apparatus is known, whereby a composition of an organic polymer can be delivered to a die or mould assembly for extrusion or moulding. The screw or screws are housed in a barrel and rotate so that the screw flights carry the composition along the barrel. It is also known that the root diameter of the shaft of such screws can vary along the length of the screw, so that the composition can be subjected, during its journey along the barrel, to varying degrees of compression and heating, which are necessary to prepare the material for extrusion or moulding.
Some organic polymers are more susceptible than others to damage from such compression, especially if they are excessively chopped and broken up. For example, certain grades of polyvinyl alcohol can degrade, leading to a cloudy, hard product where a clear flexible film might have been intended. This degradation has been attributed to cross-linking. It would therefore be desirable to pass such material through such heating and working zones as are necessary but without breaking it up.
Multilayered co-extruded films can be made by providing a plurality of such apparatuses in parallel, with means for uniting the parallel newly extruded films, which would typically have different but carefully coordinated properties, as described in for example International Application PCT/WO93/22125.
SUMMARY OF THE INVENTION
The present invention provides in one aspect the improvement whereby a screw for delivering such compositions to a die or mould is coated over at least part of its surface with an adhesion-reducing coating.
According to the present invention, a barrel screw feeder apparatus for delivering an organic polymer composition (such as polyvinyl alcohol (PVOH) or polyalkylene with optional slip agents and optional plasticisers) to a die or mould is characterised by a (single or multiple) screw exhibiting less adhesion to the composition than does the barrel. The screw may, over at least part and preferably over substantially all of its surface, have a room temperature adhesion strength to polyvinyl alcohol cast thereon of less than 200 kPa, preferably not exceeding 100 kPa, more preferably not exceeding 50 kPa. Such an adhesion-reducing surface may be inherent to the material of which the screw is made or may be a coating, which is preferably a non-stick rather than a specifically slippery coating, and may be e.g. a fluorocarbon and is preferably serviceable up to at least 240.degree. C. Preferably the die and/or the barrel are also coated, with preferably a coating which is both non-stick and slippery, the barrel in such a case having a higher coefficient of friction against PVOH than has the screw, although the opposite is also conceivable (barrel more slippery than the screw). When, in this specification, PVOH is mentioned as a reference material, or as a specific feedstock, it is taken to have a molecular mass (number-average) of 100000-120000 and (weight-average) of 200000-300000.
The coefficient of dynamic friction of the screw surface measured against polyvinyl alcohol at room temperature may be less than 0.2 and is preferably less than 0.15, more preferably less than 0.12, most preferably less than 0.11, and can be as low as 0.06. (A screw of uncoated steel, by way of comparison, would have a coefficient of friction of 0.34 and bond strength 540 Newtons per square inch i.e. 865 kPa.)
The coefficient of dynamic friction of the die surface measured against polyvinyl alcohol at room temperature may be less than 0.2 and is preferably less than 0.15, e.g. less than 0.13, in order to minimise back-pressure at the die. The adhesion strength of PVOH cast on the die surface is preferably
REFERENCES:
patent: 3564651 (1971-02-01), Convington, Jr. et al.
patent: 4309115 (1982-01-01), Klein et al.
patent: 4478971 (1984-10-01), Ballard
patent: 4493807 (1985-01-01), Vyvial et al.
British Technology Group Inter-Corporation LIC LIM
Tentoni Leo B.
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