Cold flex temperature

It has been common practice to blend plasticisers with certain polymers since the early days of the plastics industry when Alexander Parkes introduced Parkesine. When they were first used their function was primarily to act as spacers between the polymer molecules. Less energy was therefore required for molecular bond rotation and polymers became capable of flow at temperatures below their decomposition temperature. It was subsequently found that plasticisers could serve two additional purposes, to lower the melt viscosity and to change physical properties of the product such as to increase softness and flexibility and decrease the cold flex temperature (a measure of the temperature below which the polymer compound loses its flexibility). 

Figure 12.20. Effect of change of plasticiser on the properties of polyvinyl chloride compounds. (a) Tensile strength, (b) Cold flex temperature, (c) BS softness number, (d) Elongation at break, (e) 100% modulus. (The Distillers Company Ltd.)...

BS 2782, Method 150B, Determination of cold flex temperature of flexible poixxinyl compound, 1976. 

Phosphate plasticizers such as tritolyl phosphate and trixylyl phosphate are generally used where good flame resistance is required, such as in insulation and mine belting. These materials, however, are toxic and give products with poor low-temperature resistance, i.e., with a high cold flex temperature (typically, -5°C). 

For applications where it is important to have a compound with good low-temperature resistance, aliphatic ester plasticizers are of great value. Dibutyl sebacate, dioctyl sebacate, and, more commonly, cheaper esters of similar effect derived from mixed acids produced by the petrochemical industry are used. These plasticizers give PVC products with a cold flex temperature of — 42°C. 

When this material is heated in a flame then, its behavior depends tremendously on the compound composition, for example, the amount and type of plasticizer. Most plasticizers burn easily and so their presence worsens flame resistance and allows the compound to burn more easily. However, some are naturally flame resistant (the phosphates, for example, trixylyl phosphate or TXP) and compounds based on such materials will not burn easily. Because of their high price, phosphates are only used where flame resistance is essential as they are toxic and, give high cold flex temperatures. In general therefore, plasticized PVC compounds burn when... 

FIGURE 3.10 Effect of plasticizers on cold flex temperature of PVC compounds. The temperature is that at which torsional stiffness reaches some arbitrary value. (Data from Lannon, D. A., and E. I. Hoskins, chap. 7 inP. D. Ritchie, ed.. Physics of Plastics,WiaiNostimd, Princeton, NJ, 1965.)... 

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