NEW THIN FILM SINGLE SCREW VENTING MECHANISM TESTED ON 30 WOOD FLOUR/PELLETS,35 CALCIUM CARBONATE/PP PELLETS AND UNDRIED PMMA

摘要

A new single screw mixing/venting element was tested.This element combines unique properties.First,the mixing/venting element makes a thin film over a large surface area at near zera pressure.Such thin films transported under barrel vent openings can efficiently extract volatiles,air or water.Second,each mixing/venting element creates multiple elongational flow fields-the key to good compounding.Since the pressure is near zero,heat generated by mixing is very low.Third,the entire mixing/venting element melts polymer in an axial length of only 3 or 4 L/Ds.Therefore,a 36/1 single screw extruder can then have 3 vents and the first mixing/venting element substantially melts the polymer.The mechanism allows the removal of volatiles during the melting process.It should be noted that removing moisture from some materials during the melting process can limit hydrolysis.Fourth,this mixing/venting element is also a melt separation screw somewhat similar to a barrier screw but without the need of high pressure (and consequent heat generation) or high compression (causing agglomeration before mixing).Rather than restraining material by means of a barrier,the mixing/venting/melting element forwards material and high output is achieved.A 1 inch single screw extruder was built with three venting/mixing elements and one vents over each element.Three materials were tested.Undried PMMA pellets were processed for moisture removal by looking for any sign of bubbles in the strands and with an emphasis on achieving a high output rate.Output rates were 18.4 pounds per hour with one or two vents and 17.0 with three vents.There was no vent flow at these outputs and there were no indications of moisture in the clear strands or on the surface.35 MI PP pellets,2% black concentrate and 35% calcium carbonate powder were processed to make sure that entrained air could be removed and that the strand would be smooth-indicating good mixing.The mixture was processed into strands and pelletized.There were no signs of entrained air and the surface is smooth and shown at 100 times magnification.LDPE,PP and HDPE,all pellets,were processed with undried wood flour at up to 30% by weight.Three atmospheric vents were used to degas the wood of moisture and volatiles.Since this was a single screw extruder,no gear pump was necessary to achieve high pressures or stable flow.The materials were processed,without any processing aides,directly into smooth,30 mil,glossy sheet.This new element extends the utility of single screw extruders by allowing more vents in a shorter axial length than conventional screws; it allows for degassing even during the melting; it creates a large thin film for efficient degassing in each element; it is a high output melting mechanism and a melt separation element.Finally,it showed compounding of wood and calcium carbonate at up to 35%.The element does not change the primary advantages of the single screw extruder: economy,high output,high stable pressures and low cost but extends its usefulness into the range of multi-screw extruders.Multi-screw extruders sometimes call this,"direct-extrusion"- meaning compounding while making a shape other than pellets but this requires a gear pump.Since the precursor invention showed good mixing of immiscible blends in the submicron range (without a compatibilizer and the same size domains at multi-screw extruders) and with nano-particulates even at 50,000 to 100,000 times magnification (where pictures even from multi-screw machines are rare),the new screw elements should be of significant benefit.