Processing of PVC

Considerable effort has been expended in order to control porosity, surface area and diffusivity of PVC particles and this has led to great improvements over the years in the processability of PVC. 

BURGESS, R. H. (Ed.), Manufacture and Processing of PVC, Applied Science Publishers. London, New York (1981)... 

PVC on its own is extremely heat- and shear-sensitive and cannot be processed into finished goods, as it starts degrading at temperatures considerably lower than those required to process it. The processing of PVC requires a number of additives. These include heat stabilizers, impact modifiers, processing aids, and lubricants. 

Figure 7. Schematic representation of the overall laser-carbonization process of PVC...

Detailed methods are also specified in various countries for determining the fastness to bleeding [33], Plate-out frequently occurs in the coloring and processing of PVC particularly on calenders and roll mills. 

To evaluate extensively the processability of PVC/EPR and to compare it with that of PVC having roughly the same molecular weight, we carried out a set of measurements by a capillary extrusion rheometer and Brabender plastograph. Other runs were carried out by an industrial size extruder. 

In the photodegradation process of PVC, the involvement of free radicals has been extensively documented. The applications of electron spin resonance, ESR, spectroscopy have revealed some details in the degradation mechanism (5, 6). However, the potential applications of ESR spectroscopy in this area are far from being exhaustively exploited. For instance, in the initial step of the photodegradation of PVC, one would expect "Radical I" to be a predominant radical (8) ... 

Radical I". The purpose of polymerizing vinyl chloride under a variety of conditions was to prepare polymers with a wide range of branching, and molecular weight. The ESR spectra of all PVC glasses, however, exhibited the same six line absorption at LNT. This seems to indicate that the variation of the above properties in the present case did not affect the degradation process of PVC appreciably. 

Cince a wide variety of thermoplastic materials are fairly stable at processing temperatures, the incorporation into them of low molecular weight adjuvants, notably stabilizers and antioxidants, is almost superfluous. No one will say that such adjuvants are unnecessary, but in many cases the benefits are related mainly to end use and the retardation of aging effects. Poly (vinyl chloride) (PVC), however, is perhaps the most liable of the commercial thermoplastics. Processing of PVC is hardly possible without using stabilizers, and most practical innovations in PVC applications have been foreshadowed by advances in stabilizer technology. 

When PVC is pyrolyzed, the main decomposition product is hydrochloric acid, along with small amounts of saturated and unsaturated hydrocarbon side products. PVC is easily degraded through the effect of heat, light and mechanical energy. In order to improve the low stability of this plastic, a series of additives are incorporated into the PVC melt. The most important additives for the processing of PVC are the plasticizers, which may be incorporated at elevated temperatures to give mixtures stable at room temperature. 

The term lubricant is not used in a standard way. Synthetic lubricants in automotion and in machinery in general - mainly liquids - are designed to reduce the friction and wear of/or between (mainly) metallic parts in the widest sense. In a similar way, lubricants are understood in textile and fibre processing, where they are externally applied to reduce friction between textile fibres and metallic machinery parts. The same applies for example, to processing of PVC films on a calendar, where lubricants may be fed externally by a roller device. 

Very efficient stabilizers are used during the processing of PVC the mechanism of their action has become somewhat controversial. Extensive tabulation of these additives has been given [175]. Some of the most... 

Figure I. Relationship between the applied torque during the processing of PVC and the formation of solvent-insoluble gel...

Figure 2. Development of peroxide and olefinic unsaturation with time during the processing of PVC...

SMA copolymers and terpolymers have also been used for blending with PVC to improve the heat distortion temperature and processability of PVC. These blends also contain a rubbery component for impact modification that is usually a high rubber ABS or a polymethyl methacrylate grafted styrene-butadiene rubber (MBS). For improved weatherability, acrylic rubber modified PVC has been used for blending with SMA copolymers and terpolymers (Table 15.4). The market for SMA/PVC blends is still relatively low in volume with only a few applications such as in business machine housings as a low cost replacement for flame retarded ABS. 

Since PVC is known to be quite miscible with PMMA (miscibility with an LCST behavior) [Jager et al., 1983] and is also low in cost, some blends of PVC and PMMA have been used in sheet extrusion and thermoforming applications. However, the acrylic PVC compositions commercially used are invariably contain an acrylic coreshell rubber (PMMA-g-n-BuA or MBS type) to get high toughness), with some PMMA, to reduce the cost/impact performance balance. The role of PVC in these blends is to reduce cost and impart some degree of flame-retardancy. The acrylics definitely help in the processability of PVC. These blends have already been discussed under PVC heading. 

Processing of PVC is carried out by conventional methods such as injection molding, extrusion, blown film, and blow molding. Additives used in PVC (as well as other plastics) must be FDA-approved if the resin is used in food contact applications. 

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