Maximum continuous use temperatur

The polyacetals have a good record of performance in uses involving hot air and hot water. Plumbing components have been used for hot water service in the range 60-80°C and applications in hot air well in excess of 90°C. Recommended maximum continuous use temperatures are given in Table 19.4... 

Table 19.4 Recommended maximum continuous use temperatures for polyacetals...

Figure 12-6. Maximum continuous use temperature of some engineering thermoplastics. ...

Other terms used in connection with the maximum service temperature are the Underwriters Laboratories index and the maximum continuous use temperature (MCUT) which is usually based on 100,000 hours. One criticism of MCUT (and other similar measures) is that the changes in properties are measured at ambient temperature rather than at the operating temperature (see Section 7.1). 

FEP injection and extrusion grades with the same advantages as PTFE but a slightly lower performance 200°C maximum continuous use temperature instead of 260°C. 

The temperature at which the threshold is reached in (usually) 20,000 hours is called the Temperature Index (TI) in ISO 2578. The Relative Temperature Index (RTI) is a comparative value with a reference material. The HIC is the halving interval - the temperature change needed to halve the time to the end point from TI. MCUT is the Maximum Continuous Use Temperature which is normally based on 100,000 hours. One criticism of MCUT and similar measures is that the changes in properties are made at... 

Polymer Maximum Continuous-Use Temperature, °C Typical Processing Temperature, °C... 

Nitrile rubber (NBR) is widely used in compounds designed for seals and gaskets, and in hoses for both aqueous and fatty foods. In particular, dairy hosing and milk liners are normally manufactured in nitrile rubber or nitrile rubber blends (e.g. with SBR). Nitrile mbber is better able to withstand heat ageing than natural rubber and so the maximum continuous use temperature is higher at 120 °C. In practice most applications involve flow or short-term static conditions at temperatures below 40 °C. 

Because of its high viscosity (10 °-10 poise at 380°C), PTFE cannot be fabricated by melt-processing techniques. Melt-processible fluoropolymers have been developed by copolymerization of TFE, and FEP, a copolymer of TFE and HFP, has a lower maximum continuous use temperature than PTFE (200° C vs. 260° C) because of the deterioration of mechanical properties. Whereas, PFA, a copolymer of TFE with PPVE or PEVE, offers thermal stability, melt-processibility, and a maximum continuous use temperature of 260°C. Both FEP and PFA are considered perfluoropolymers. 

The discovery of PTFE was a major leap forward in material science. Yet the new polymer could not be fabricated by melt-processing. The next two forms of PTFE, fine powder and dispersion, were also not melt-processible. The pursuit of a more easily processible polymer led to FEP, which could be melted in an extruder. Compared with PTFE, the major disadvantage of FEP is its reduced thermal stability and lower maximum continuous-use temperature (200°C) (Table 1.3). PFA, which was introduced in 1973, offers both melt-processing and the same upper continuous-use temperature as PTFE (260°C). 

PCTFE 1953 CTFE Melt-processible/Non melt-processible Maximum continuous-use temperature 180°C... 

PVF 1961 VF Thin flhn/weatherable Maximum continuous-use temperature 107°C... 

PVDF 1961 VDF Melt-processible Maximum continuous-use temperature 150°C... 

Figure 4. Maximum continuous use temperatures of some engineering thermoplastics based on Underwriters Laboratories ratings...

Fibre genus Second order temperature, °C Melting temperature, °C Onset of decomposition, °C Maximum, continuous use temperature, °C LOI,yo1%... 

Maximum continuous use temperature °C Coefficient of linear expansion cm/cmxlO °C Tensile strength, MPa % Elongation Flexural strength, MPa Compressive strength, MPa Notched Izod impact J/m Hardness Rockwell Specific gravity... 

Specific gravity, density Maximum continuous-use temperature Tensile properties of thin plastic sheeting Determining the bearing strength (bearing stress to cause 4% strain)... 

The properties of unmodified PP are compared with other competitive thermoplastics in Table 1. It can be seen from the table that PP offers advantages over most of its competitive materials on the basis of specific modulus (modulus to density ratio), heat deflection temperature (HDT), maximum continuous use temperature or modulus to cost ratio. Environmental and food legislation may further tip the balance in favour of PP. 

Maximum continuous use temperatures are based upon the Underwriters Laboratories (UL) rating for long-term (100,000 hours) continuous use, and specifically on the elevated temperature which causes the ambient temperature tensile strength of the material to fall to half its unexposed initial value following exposure to that elevated temperature for 100,000 hours. The tests provide a continuous use temperature for a plastic in the absence of stresses. The maximum use temperature of PP is compared with other thermoplastics in Table 13. It can be seen that other commodity plastics and some other engineering plastics have a significantly lower maximum continuous use temperature than PP. However, polycarbonate has a higher maximum continuous use temperature in comparison to PP. 

Occasionally it is required that the service life of the component is predicted at a temperature above its maximum continuous use temperature or vice-versa. As a rule of thumb, a 10 °C increase in temperature is equivalent to a decade increase in time. Since the maximum continuous use temperature of PP is 100,000 hours at 100 °C, this would be equivalent to 10,000 hours at 110 °C or 1,000,000 hours at 90 °C. Hence, certain grades of PP may be theoretically suitable for a very short-term use at 140 °C. However, the maximum use temperature of a polymer depends on the specific grade and its heat stabilisation system and should be carefully noted from the relevant trade literature. However, the functionality of the polymer for high temperature application might be quite limited in the presence of stresses. 

The data obtained by these tests eannot be used to predict the behaviour of plastic materials at elevated temperature nor ean it be used in designing a part or selecting and specifying material. If an article is subjeeted to high temperature in the absence of stresses, maximum continuous use temperature (Section 4.2.2) can provide a suitable... 

In applications involving no undue mechanical stresses, PP articles will withstand 100 °C for a long period of time, depending on the stabiliser systems. Consequently, the heat and thermal stability of PP is closely related to its maximum continuous use temperature (Section 4.2.2). Short-term exposure to 140 °C is also possible. It has been observed that a properly heat stabilised and properly processed material ean undergo up to five processing cycles without noticeable reduction in molecular weight or the level of antioxidant content. 

The Underwrites Laboratory (UL) index indicates the safe temperature of use of a material in air for up to 100,000 hours (11.4 years). Maximum continuous use temperatures for a range of rubbers with minimal stabilisation are shown in Table 2. 

Table 2 Generic maximum continuous use temperatures for rubbers (9) ...

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