Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-06-18
2003-04-08
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
C264S115000, C264S124000, C264S320000
Reexamination Certificate
active
06544452
ABSTRACT:
The present invention relates generally to the manufacture of plastics materials, particularly, but not exclusively, from polymers which require compounding before manufacture, and/or polymers which are sensitive to heat treatment.
Many polymers are produced from a polymerisation reactor as a relatively coarse powder or granules. Before extrusion or moulding into a final plastics material product, the polymer usually needs to be compounded with other ingredients, for example lubricants, stabilizers, plasticizers, filler, foaming agents, colouring agents, flame retardant agents, flow improvers and the like.
It is possible to mix the ingredients together as powders immediately prior to extrusion or moulding the final product, or to add ingredients to the material as it is being extruded. However, it is often very difficult to achieve reliable dispersion of all additives throughout the powder to achieve a uniform product. In addition, the powdered material is not easy to work with, and can tend to form bridges and clog up machinery. The powdered material can also give rise to dust, so operators are required to wear masks and other protective clothing to handle the material. Thus, it is preferred to avoid the use of powdered material is possible.
An alternative method is to mix the desired ingredients and then melt them and extrude the mixture to form pellets. The pellets can be fed more reliably into an extruder by the end user, and require less specialised handling precautions. In addition, the ingredients may be more evenly mixed in the pellets. This method has been used successfully or some thermoplastics such as polyethylene.
A problem with this method is that a significant amount of energy is required to form the pellets, leading to an increased cost of the final product.
GB-1,397,127 discloses a method of producing pellets in which a thermoplastic polymer of an &agr;-olefin having a specified particle distribution and a melt index of less than 100 and glass fibres are mixed in a dry blender and cold compacted into pellets; this is disclosed to avoid problems stated to be associated with having water present in the formed pellets.
Another problem is that many compositions are not completely heat-stable, but are sensitive to the thermal history of the composition. Particularly problems arise with such polymers are PVA (polyvinyl alcohol) and PVC (polyvinyl chloride), but the problems may be present in a variety of plastics materials, depending on the nature of both the polymer and of the compounding ingredients. The result of this heat-sensitivity is that the product is affected by variations in both the initial pellet-forming process and the extrusion process.
Whilst satisfactory extrusion can normally be achieved by adjusting the final extrusion parameters, and also by controlling the pellet forming process (for example by melting the pellets at a relatively low temperature and rapidly cooling the pellets to minimise heat degradation), a significant proportion of the product may be wasted as the extrusion parameters are optimised. The problem is more acute for products which are particularly heat-sensitive, such as bio-degradable products or adhesives based on PVA, in which typically the first twenty minutes or so of production may be wasted every time the machine is started, and in which cleaning operations may be required when the machine is stopped. For a machine capable of extruding many tonnes every hour, this represents significant wastage.
As a result of the problems obtaining reliable extrusion, although the basic ingredients for PVA-based biodegradable plastics are comparatively cheap, the final cost of the product is at present significantly higher than polyethylene based products partly because of the above production problems; bio-degradable plastics might be more widely used if these problems could be alleviated.
The invention is generally, but not exclusively, concerned with providing a polymer feedstock which alleviates some or all of the above problems. By polymer feedstock is meant a substance containing polymer in a form suitable for feeding to a machine, preferably a conventional extrusion apparatus, for manufacturing a product of plastics material, preferably a thermoplastic material. References to compounding imply that the raw polymer is blended with additives to give desired properties in the finished product, or to affect the processing of the product.
According to a first aspect, the invention provides a method of producing a compounded polymer feedstock comprising mixing a polymer, preferably substantially uncompounded and in powder or granular form, with at least one compounding ingredient and cold pressing the mixture to form a tablet or pellet. The mixture preferably contains moisture serving to bind the tablets or pellets.
With this method, the ingredients can be uniformly mixed and compounded in the correct proportions for the final product, and made into a form that is easier to handle than powder. In addition, since the mixture is cold pressed rather than melted, problems of thermal degradation or variation in properties are reduced. The method need not rely on &agr;-olefin polymers of particular properties to ensure binding, but is most preferably employed in conjunction with polymers such as PVA which can be readily bound under pressure in the presence of small quantities of moisture.
References to “cold pressinng” are intended to imply applying pressure to the powder to cause agglomeration without substantially melting of the polymer. The temperature is preferably less than about 100 degrees celsius, more preferably less than about 70-80 degrees celsius. Preferably heat is not applied directly to the mixture. However, heat may be generated by mixing and/or pressing, and this may be allowed to warm the mixture, or cooling may be provided. It is permissible for some heat to be applied directly if necessary to facilitate binding. Some melting of the polymer or other ingredients may occur, particularly on a microscopic scale as a result of pressure at the interface between adjacent particles, but it is important that the bulk of the polymer granules do not liquify.
Preferably, the mixture is compressed in discrete quantities into tablets or pellets, for example in a table press; this may provide more reliable agglomeration.
However, it is possible for the mixture to be extruded under pressure through an orifice, the pellets forming following exit of the mixture from the orifice. In such a case, binder or moisture may be added to the mixture, preferably as the mixture is extruded, or after preliminary mixing of the constituents.
Preferably, the mixture is substantially dry, but includes sufficient residual moisture to bind the tablets or pellets on pressing. This avoids the need to add a separate binder, and allows the tablets or pellets to be broken more easily as they enter the extruder simply by drying them further. When water is relied upon for binding, it is desirable (although not absolutely essential) to ensure that the tablets or pellets are dried prior to extruding, for example at between 40 and 80 degrees celsius for 4-8 hours; this alleviates problems of vapour formation during extrusion. The moisture content is desirable to form the tablets, but after forming, the tablets should in most cases retain their shape even when dried.
Preferably, the moisture content is less than about 10% by weight, and preferably greater than 0.1% by weight, more preferably at least about 1% and preferably no more than about 5%. If the pellets or tablets are extruded or otherwise formed at relatively low pressures, however, the moisture content may be about 20% or even higher to ensure binding. If the pellets are too moist, they may tend to stick together, inhibiting flow, and drying after forming may be desirable.
The moisture may comprise residual moisture in the polymer or in one more of the compounding ingredients. In this way, moisture can be provided automatically simply by not drying the ingredients fully prior to mixing.
The at least one compou
Eashoo Mark
Hodgson & Russ LLP
PVAXX Technologies Ltd.
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