Introduction: Process aids (PA) are unique additives that dramatically improve the processability of PVC, at relatively lower concentration. Besides, they are used to achieve processing improvements, higher productivity, and better product quality. Nature of process aids: Process aids are random copolymers, whose major component is methyl methacrylate. Other component may be styrene or acrylonitrile. They are PVC compatible. Process aids are produced by emulsion polymerization. Processing aids have two categories – 1. That promotes fusion and adds rubber elasticity during processing, and 2. That adds lubricity during processing. Processing Aids that promotes fusion – They are acrylic polymer, high molecular weight MMA copolymers (MMA / styrene) or Acrylonitrile styrene. Modified rheology (Viscosity & Elasticity) and morphology (how primary particles arrange – fuse together) of PVC compound during processing is a function of molecular weight of process aid. No alt text provided for this image Selection of molecular weight of process aids depend upon the process: Lower the molecular weight of process aids and softening temperature, greater is the effect on fusion. They are used in rigid extrusion. Medium molecular weight processing aids are used for calendaring process. Medium – high molecular weight processing aids are used in injection moulding as well as thermoformed products. High molecular weight process aids are used in flexible PVC processing. Higher molecular weight process aids are used in foamed product extrusion at higher Phr level. Effect of fusion promoting acrylic process aids: Promotes fusion Improves dispersion–that is essential for fusion Improves surface gloss Promotes die swell Improves hot melt strength Increases melt elasticity Improves melt extensibility & reduces melt fracture Improves jetting or gate blush and weld line in injection molded products Eliminates distortion in rolling bank and flow marks in calendaring process, and Improves appearance of PVC foamed products (very high mole weight process aids.) PA that adds lubricity are- Low molecular weight MMA copolymers or acrylic polymers. It can be considered as acrylic waxes. These process aids consist of PVC compatible part and PVC incompatible part. Effect of lubricating acrylic process aids: Improves metal release Prevents plate out, and Are neutral to fusion SAN based process aids: They are based on Poly (styrene – Acrylonitrile) They have higher molecular weight and viscosity than acrylic process aids. They are relatively cheaper. They offer higher surface gloss in opaque products. And, they do not affect VST. Copolymer of styrene – MMA, on the other hand has high molecular weight and high glass transition temp than PVC. Some researchers recommend blend of Acrylic and SAN based processing aids for UPVC injection moulding. Fusion of PVC is promoted by process aids through two mechanisms: 1. By improving adhesion in the first step of fusion S-PVC grain (secondary particle) of size 90-250 micron has a hard skin of 0.2 – 1.0 micron. This hard skin makes fusion very difficult. Unless this skin breaks, dispersion of secondary particle to primary particle and in turn fusion cannot take place effectively. During extrusion, some degree of adhesion is necessary to facilitate shearing and convey the material along the flights of the screw. In the initial stages of fusion, PVC tends to exhibit reduced adhesion to it and to metal surfaces. This delays heating and breakdown of the grains. Process aids do not have hard skin like PVC. Due to this reason, when heated under shear and stress, process aids soften earlier than PVC and then adhere to the surrounding PVC particles. Owing to good compatibility with PVC, they transmit elongational and shearing forces to PVC grains. In this way process aids work as shear transfer agents, resulting in faster and uniform breakdown of PVC grains to primary particles (dispersion), leading to faster and uniform fusion. Generally, lower the molecular weight of process aid, lower is the temperature of onset of fusion. 2. By providing longer chains to entangle with PVC chains in the second stage of fusion: Acrylic processing aids have very good compatibility with PVC because of their main component PMMA. They also have higher molecular weight and longer chains than PVC. Thus, when heated under shear stress during processing, entanglement appear between PVC chains and process aid chains resulting in viscoelastic network. These entanglements work as cross links during processing and add rubbery characteristics to PVC compounds enabling increase in drawdown for extrudate or blow up ratio for blown films. Process aid chains are elastic at processing temperature. Their action depends upon molecular weight or chain length and evaluated through die swell. Higher molecular weight process aids give higher die swell.
FOR MORE, PLEASE CONTACT: MANDYZHANG@NOVISTAGROUP.COM
WECHAT:86 15662577378
