PROCESSING AID

Acrylic process aids include copolymers of methyl methacrylate (MMA) and other types of acrylic monomers like ethyl acrylate, butyl acrylate and butyl methacrylate. Some even include styrene monomer, imparting to the polymer an antisticking effect due to its limited compatibility with PVC. The functions of these lubricating process aids are not described in this chapter. 

A first important factor in the behaviour of process aids is the choice of comonomers and their polymerisation sequence. MMA offers excellent compatibility with wood composites (WC) bnt has a higher Tg. Acrylate monomers are less compatible bnt have mnch lower Tg than MMA. When combined with MMA in a process aid, they 

Process aids are obtained through emulsion polymerisation, a process able to lead to extremely high molecular weights (MW), often above 1 million, when compared with WC MW, which typically range from 50,000 to 65,000 for films and bottles, and from 60,000 to 80,000 for rigid extrusion. 

The main types of processing aids are discussed next under separate headings  

Antiblock agents Antioxidants Antisplit agents Antistatic agents Heat stabilisers Lubricants 

Melt strength improvers Mould release agents Plasticisers 

Slip additives Other stabilisers Gas barrier agents 

General purpose PVC processing aids MMA/BA, EA, BMA MMA/Sty, SAN, SAN/MMA Promote PVC fusion, improve melt elasticity and strength, reduce melt fracture, and improve surface quality 15-53 

Lubricating processing aids BA/Sty/MMA, BA/MMA, EVA Promote PVC fusion, prevent polymer melt from sticking to hot surface, assist mold release, improve surface quality and throughput 54-59 

Foamed PVC processing aids MMA/BMA, EA, BA SAN/MMA, SAN Promote PVC fusion, reduce foam density, improve surface quality, provide good cell uniformity, increase process flexibility 51, 60-62, 157-183 

Melt rheology modifiers Methacrylate-based polymer Lower melt viscosity in PVC and ABS, improve melt strength in polyolefin and engineering resins, improve mixing and homogeneity in ABS/SAN blend 63-70, 146 

PVOH processing AIDS MMA/NVP/ methacrylic acid Enable melt processing, maintain rigidity and barrier properties of the polymer 71-73 

As raw materials prices go up, more attention is being paid to additives that make materials go further in processing - and there is every likelihood that this trend will continue. Processing aids, based on a variety of chemicals, are increasingly 

Calcium carbonates Surface-treated grades improve heat transfer, give faster set-up in raouiding, improve bubble stability for blown film 

Methyl styrenes Standard processing aids for PVC impact modifiers 

Styrenes, acrylics High molecular weight acrylic copolymers improve processability of PVC compounds, good weatherability lubricant grades reduce adherence of melt to processing equipment 

Clarifying/nucleating agents For polypropylene increase rate of crystal initiation, improved clarity, better flow, faster set-up 

Stearates, such as calcium and zinc stearates, are present in several commercial resins of both linear and long-chain branched polyethylenes. Stearates are not used as processing aids for linear low-density polyethylenes. They have strong evidence of promoting slip and aid in the reduction of instabilities with long chain branching material [24]. 

Processing aids, particularly acrylic types, have been used in rigid PVC to increase fusion, decrease jetting and 

In many instances it is difficult to process polyethylenes, especially where it comes to recycling. Studies were performed on the blending of different types of polyethylene in order to improve processing [28-32]. Typically, one would expect a processing agent (wax) to improve the melt flow of the polymer to be processed, without having a detrimental influence on the mechanical properties and thermal stability of that polymer. 

In order to overcome these difficulties and to render the processes economically feasible, processing aids (PAs) are frequently used. PAs eliminate flow instabilities or postpone them to higher flow rates. The end result is an increase of the productivity as well as an energy cost reduction, while high product quality is maintained. 

The processing aid polymer can significantly enhance product quality and processability, not only by eliminating surface defects, but also by reducing torque and power requirements as well as die plate pressure [26,33]. 

The oil yield and the quality of the oil can be improved by enzymes with pecti-nolytic and cellulosolytic activity (Di Giovacchino 1996). Micronised mineral talc is used in Spain for hard pastes to increase oil yields. Talc reduces oil/water emulsions and increases the recovery of free oil. However the use of coadjuvants is not in accordance with the legal definition of virgin olive oil and the oil must be obtained from the fruits solely by mechanical or other physical means. 

The increased viscosity from filler addition also raises resin pressure within the tool cavity further aiding elimination of air. 

Lastly, a major use of fillers is for flame-retardant applications, with aluminium hydroxide being extensively used, especially in unsaturated polyesters. This is dealt with in depth in Chapter 6. 

Another very important group of compounds are hindered phenols such as 2,6-di-te/t-butyl-p-cresol, i.e., butylated hydroxytoluene (BHT)  

For polyolefins, the primary antidegradents used are again alkylarylamines, and hindered phenols [156]. 

We treat the development and mechanism of antidegradents in greater detail in Chapter 4, notably in Sections 4.3 and 4.6. 

The advantages of using processing aids are improved product surface, improved production rate, reduction of die build-up, and reduction of gel formation. 

Lubricants are classified to internal and external lubricants. They are described in Fig. 1.24. Internal lubricants are soluble in the polymer matrix and are plasticizers. They are used for lowering melt viscosity and reducing heat generated from shear forces. External lubricants are immiscible in the polymer matrix and are used to produce boundary layer between matrix 

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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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