Degradation PVDC

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometaUic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinyhdene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see Vinyl polymers). 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Figure 6.37 Photograph of a segment from a Maddock solidification experiment for a PVDC resin extrusion. The dark band is resin degraded due to a long residence time at the location...

Polyvinyiidenc chloride (PVDC) b a crystalline polymer with a low Tt and a low Tm This homopolymer tends to degrade at elevated temperatures and is difficult to process. 

The instability of PVDC is one of the reasons why ionic initiation of VDC polymerization has not been used extensively. Many of the common catalysts either react with the polymer or catalyze its degradation. 

During the pyrolysis of mixed waste plastic, one of the main problems associated with the feedstock recycling is the presence of plastic containing hetroatoms, e.g. PVC, ABS, PVDC, etc. Efforts are made to remove the heteroatoms before pyrolysis. Chlorine can be removed either by thermal degradation or by using a catalyst. The HCl generated in the process can be used as industrial hydrochloric acid. 

The halide-containing (co)polymers, such as PVC or PVDC, recovered from commingled polymer scrap were contaminated by low melting point PA S [e.g., PA-6, PA-1212 or PARA]. The recyclate was compatibilized and impact modified by addition of an acrylic copolymer of ethylene, alkyl(meth)acrylate, vinylacetate, (meth)acrylic acid, CO and MA. To prevent degradation, the blends were processed at T < 220°C, then formed by extrusion or injection molding into a variety of articles [Hofmann, 1994]. 

PVDC copolymers also tend to be subject to thermal degradation, undergoing the same general types of reaction as PVC, so they also require the use of heat stabilizers. 

Saran is resistant to oxidants, mineral acids, and solvents. In applications such as plating solutions, chlorides and certain other chemicals, polyvinylidene chloride is superior to pol)rpropylene and finds many applications in the handling of municipal water supplies and waste waters. Saran is also resistant to weathering and UV degradation. Refer to Table 2.33 for the compatibility of PVDC with selected corrodents. Reference [1] provides a more comprehensive listing. 

Heat stabilizers are used to prevent the thermal degradation of resins during periods of exposure to elevated temperatures. Almost all heat stabilizers are used to stabilize PVC, polyvinylidene chloride (PVDC), vinyl chloride copolymers (for example, vinyl chloride/vinyl acetate), and PVC blends (for example, PVC and ABS). Thermal degradation is prevented not only during processing but also during the useful life of the finished products. 

Chain Structure. The chemical composition of PVDC has been confirmed by various techniques, including elemental analysis, x-ray diffraction analysis, degradation studies, and infrared (ir), Raman, and nuclear magnetic resonance (nmr) spectroscopy. The polymer chain is made up of VDC imits added head-to-tail ... 

PVDC has improved strength, hardness, and chemical resistance, particularly to oxidants, mineral acids, and solvents, over that of PVC. It is resistant to all of the normal atmospheric pollutants including weather and UV degradation. Applications include piping and equipment components (pumps, valves, etc.). The operating temperature range is 0 to 175°F (—18 to 80°C). 

Polyvinylidene chloride (PVDC) Polytetrafluoroethylene (PTFE) Polychlorotrifluoroethylene (PCTFE) Polyvinylidene fluoride (PVDF) Ethylene tetrafluoroethylene (ETFE) Fluorinated ethylene/propylene (FEP) 50 200 1000 1000 50 Yellowing cleavage of hydrochloric acid Degrades very strongly... 

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