Polymer processing method and tablet-forming apparatus

Abstract

The invention provides a compounded polymer feedstock formed by cold-pressing of the ingredients into tablets or pellets. This avoids the need for heating associated with conventional pellet-forming processes, and is particularly suitable for compounding of heat-sensitive materials such as PVA. The invention also provides a novel tablet-forming apparatus, suitable for forming large volumes of pressed compounded polymer feedstock, together with tablets of the feedstock, and methods of manufacturing plastics products using the tablets.

1 . A method of producing a compounded polymer feedstock comprising mixing a polymer with at least one compounding ingredient and cold pressing the mixture to form tablets or pellets, wherein moisture is present in the mixture to bind the tablets or pellets.  
     
     
         2 . A method according to  claim 1 , wherein the temperature is maintained below about 100 degrees celsius during mixing and pressing.  
     
     
         3 . A method according to any preceding claim, wherein the moisture content is between about 0.1% and 10% by weight of the pellets.  
     
     
         4 . A method according to any preceding claim, wherein said at least one compounding ingredient includes a lubricant, slip agent or flow improver.  
     
     
         5 . A method according to any preceding claim, wherein the mixture is supplied to pressing means for forming the tablets or pellets as a free-flowing powder.  
     
     
         6 . A method according to any preceding claim, wherein the mixture includes at least one compounding ingredient selected from the group consisting of a stabiliser, a filler, a colouring agent, a plasticiser, a foaming agent, a flame-retardant agent.  
     
     
         7 . A method according to any preceding claim, wherein the polymer and all compounding ingredients are food-grade or bio-degradable.  
     
     
         8 . A method according to any preceding claim, wherein a force of at least about ¼ tonne is used to press each tablet or pellet.  
     
     
         9 . A method according to any preceding claim, wherein the tablet or pellet contains at least one heat-sensitive component.  
     
     
         10 . A method according to any preceding claim, wherein the polymer comprises PVA or PVC, preferably PVA.  
     
     
         11 . A method according to any preceding claim, further comprising melting the tablets or pellets and extruding or moulding them to form a plastics material product.  
     
     
         12 . A method of producing a product of plastics material comprising receiving a supply of cold-pressed tablets or pellets comprising a polymer and at least one compounding ingredient as polymer feedstock and melting and extruding or moulding the polymer feedstock to form the product.  
     
     
         13 . A method according to  claim 11  or  12 , further comprising adding further compounding ingredients or extrusion or moulding agents to the tablets or pellets.  
     
     
         14 . A method according to any of  claims 11  to  13  including the step of drying the tablets or pellets prior to extrusion or moulding.  
     
     
         15 . A cold-pressed pellet or tablet for use as a polymer feedstock, the pellet or tablet comprising a polymer and at least one compounding ingredient, the pellet or tablet containing moisture serving as a binder.  
     
     
         16 . A pressed pellet or tablet according to  claim 15 , wherein the polymer comprises PVA or PVC.  
     
     
         17 . A pressed pellet or tablet according to  claim 15  or  claim 16 , wherein the at least one compounding ingredient includes a lubricant, a slip agent, or a flow improver.  
     
     
         18 . A pressed pellet or tablet according to  claim 15 ,  16  or  17  including a heat-sensitive component.  
     
     
         19 . A pressed pellet or tablet according to any of  claims 15  to  18  in which all ingredients are food-grade and/or bio-degradable.  
     
     
         20 . Apparatus for forming tablets comprising roller means having a plurality of indentations on the rolling surface thereof and backing means, the roller means and backing means defining a nip therebetween; a supply of powder comprising polymer and at least one compounding agent as a feedstock, the powder containing moisture in sufficient quantity to serve as a binder; means for supplying said powder to the roller means at a feed side of said nip; means for advancing the roller means to draw the powder through said nip to compress the powder into said indentations to form compressed tablets whereby moisture in the powder serves to bind the tablets; and means for receiving said tablets at the other side of said nip.  
     
     
         21 . Apparatus according to  claim 20 , further comprising recirculating means for receiving powder that has not been successfully formed into tablets and providing it to the feed side.  
     
     
         22 . Apparatus according to  claim 20  or  21 , wherein the indentations are substantially hemi-spherical.  
     
     
         23 . Apparatus according to any of  claims 20  to  22 , wherein the roller means and backing means comprises a pair of opposed substantially cylindrical counter-rotating rollers.  
     
     
         24 . Apparatus according to any of  claims 20  to  23 , including means for separating formed tablets from the roller means.  
     
     
         25 . Apparatus according to any of  claims 20  to  24  further comprising first feed means for receiving a polymer in powder form; second feed means for receiving at least one compounding ingredient in powder form; mixing means for mixing the polymer and the or each compounding ingredient; delivery means for supplying the mixed powders to the feed side of the roller means; the roller means being arranged to compress the mixed powders to form a self-supporting substantially solid tablet, preferably without the addition of binder.  
     
     
         26 . Tablet or pellet forming apparatus comprising first feed means arranged to receive a polymer in powder form; second feed means arranged to receive at least one compounding ingredient in powder form; mixing means for mixing the polymer and the or each compounding ingredient to form mixed powder containing moisture in sufficient quantity to serve as a binder; pressing means arranged to compress the mixed powder to form a self-supporting substantially solid tablet or pellet, whereby the moisture in the mixed powder serves as a binder, preferably without the addition of further binder, to form the tablet or pellet.  
     
     
         27 . Use of moisture, preferably residual moisture, as a binder in the formation of cold-pressed tablets or pellets of a polymer feedstock comprising polymer and at least one compounding ingredient.