Process aids

Process aids are often used to reduce compound viscosity. There are many proprietary chemicals and mixtures available, but their selection should be made with great care and use made of them only after carefully controlled experiments. The quantities recommended by the suppliers of these materials are not necessarily going to give the desired result in every formulation, and may not work at all in the required way Peptisers provide a useful way of significantly reducing compound viscosity and aid the process of incorporation of the other ingredients. 

Process aids are added primarily to PVC-U (including foam) to improve fusion characteristics and melt flow during the processing stage. Addition levels are typically from 1-6 phr with resultant improvements in output and surface finish. Transparency can also be improved in clear formulations. Chemically, these products are high molecular weight methyl methacrylate and alkyl acrylate copolymers. 

The role that acrylic processing aids play in compound processability, end product properties and manufacturing efficiency has been reviewed (79). The functions of acrylic process aids, with very high molecular weight, have also been discussed from a rheological point of view (290). Co-stabilising process aids have also been developed (335). 

Process aids are added in small quantities (0.01 to 0.1 phr) to minimise defects in finished products and to improve processing characteristics. Some lubricants and impact modifiers also have beneficial effects on processing. 

It is sometimes possible to achieve satisfactory processing conditions and production rates without using process aids, simply by altering other variables such as process temperature or polymer composition. (If the polymer contains some low molecular weight material it will provide some lubrication.) 

Process aids can sometimes interact with other additives and lose some of their effectiveness. Antiblocking agents such as silica and talc and certain pigments are known to have an adverse effect. Careful selection of processing aids is advisable with certain polyethylenes containing HALS additives. Different substances are used in different polymers and to combat different defects. 

Process aids are used chiefly in polyethylene (LLDPE, LDPE and HOPE) and in PVC. Some metallocene PP grades have a narrow molecular weight distribution and also benefit from process aids. Certain styrene polymers, polyamides, acrylics and thermoplastic elastomers may also benefit. 

Melt fracture is the breakage of molten polymer during processing. Melt fracture can be a problem in LLDPE, but not in LDPE because the latter has a broader molecular weight distribution. In cyclic melt fracture, the stress at the die wall becomes sufficient to separate the polymer from the metal surface. Repeated cycles of separation and adhesion can therefore occur in cyclic melt fracture. This allows an increased output rate, but it also reduces the pressure, and allows the polymer to adhere again to the die wall. 

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In addition to lowering the interfacial tension between a soil and water, a surfactant can play an equally important role by partitioning into the oily phase carrying water with it [232]. This reverse solubilization process aids hydrody-namically controlled removal mechanisms. The partitioning of surface-active agents between oil and water has been the subject of fundamental studies by Grieser and co-workers [197, 233]. 

The burnout stage invoives eiiminating the organic processing aids and any residuai organic impurities or water prior to sintering [, and ]. Minor concentrations of residuai iiquid used in fonning, and physicaiiy... 

Acrylics. Acetone is converted via the intermediate acetone cyanohydrin to the monomer methyl methacrylate (MMA) [80-62-6]. The MMA is polymerized to poly(methyl methacrylate) (PMMA) to make the familiar clear acryUc sheet. PMMA is also used in mol ding and extmsion powders. Hydrolysis of acetone cyanohydrin gives methacrylic acid (MAA), a monomer which goes direcdy into acryUc latexes, carboxylated styrene—butadiene polymers, or ethylene—MAA ionomers. As part of the methacrylic stmcture, acetone is found in the following major end use products acryUc sheet mol ding resins, impact modifiers and processing aids, acryUc film, ABS and polyester resin modifiers, surface coatings, acryUc lacquers, emulsion polymers, petroleum chemicals, and various copolymers (see METHACRYLIC ACID AND DERIVATIVES METHACRYLIC POLYMERS). 

Raw Materials. PVC is inherently a hard and brittle material and very sensitive to heat it thus must be modified with a variety of plasticizers, stabilizers, and other processing aids to form heat-stable flexible or semiflexible products or with lesser amounts of these processing aids for the manufacture of rigid products (see Vinyl polymers, vinyl chloride polymers). Plasticizer levels used to produce the desired softness and flexibihty in a finished product vary between 25 parts per hundred (pph) parts of PVC for flooring products to about 80—100 pph for apparel products (245). Numerous plasticizers (qv) are commercially available for PVC, although dioctyl phthalate (DOP) is by far the most widely used in industrial appHcations due to its excellent properties and low cost. For example, phosphates provide improved flame resistance, adipate esters enhance low temperature flexibihty, polymeric plasticizers such as glycol adipates and azelates improve the migration resistance, and phthalate esters provide compatibiUty and flexibihty (245). 

Lubricants. Process aids or lubricants promote smooth and rapid extmsion and calendering, prevent sticking to extmders or calender roUs, and impart good release properties to molding compounds. In some cases use of lubricants allow slightly lower processing temperatures (see Vinyl polymers). 

AH these appHcations underscore polymer safety. If articles made of HDPE contain fiUers, processing aids, and colorants, their toxic effects must be estimated separately. 

LLDPE by itself does not present any health-related hazard on account of its chemical inertness and low toxicity. Consequently, film, containers, and container Hds made from LLDPE are used on a large scale in food and dmg packaging. Some LLDPE grades produced with unsupported metallocene catalysts have an especially high purity due to high catalyst productivity and a low contamination level of resins with catalyst residue. FDA approved the use of film manufactured from these resins for food contact and for various medical appHcations (80). However, if LLDPE articles contain fillers, processing aids, or colorants, thek health factors must then be judged separately. 

Poly(ethylene terephthalate). PET is a crystalline material and hence difficult to plasticize. Additionally, since PET is used as a high strength film and textile fiber, plasticization is not usually required although esters showing plasticizing properties with PVC may be used in small amounts as processing aids and external lubricants. Plasticizers have also been used to aid the injection mol ding of PET, but only at low concentrations. 

Polystyrene. Polystyrene shows compatibiHty with common plasticizers but modification of properties produced is ofHtde value. Small amounts of plasticizer (eg, DBP) are used as a processing aid. 

Fluoroplastics. Conventional plasticizers are used as processing aids for duoroplastics up to a level of 25% plasticizer. However, certain grades of Kel-E (chlorotriduorethylene) contain up to 25 wt % plasticizer to improve elongation and increase softness the plasticizers used are usually low molecular weight oily chloroethylene polymers (5). 

Table 1. Oils and Processing Aids for Reclaiming Rubber...

Solvenol monocyclic terpenes processing aid nonstaining reclaim oil and solvent swells and penetrates mbber dissolves and disperses heavy oils... 

Corray 40 carbon black cyclop araffinic hydrocarbon processing aid softens, swells, and smooths reclaim reinforcing filler... 

Processing Aids. Petroleum oils are processing aids as well as softeners/extenders/plasticizers. This section concentrates on those materials which serve mainly as processing aids without a large contribution to vulcanized properties. They are most often used at much lower levels. 

Other processing aids utilized in tires are vegetable oils (fatty acids, fatty acid esters/alcohols, and metal salts of these oils), naturally occurring resins such as pine tar, hydrocarbon resins from petroleum stUlbottoms, and vulcanized vegetable oils (WOs). WOs were utilized heavily in the early 1900s, but are no longer used extensively in tires. 

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