Temperature

Temperature is the thermal state of matter as measured by a specific scale. Basically it is a measure of the intensity of the molecular energy in a substance. The higher temperatures have more molecular movement. The temperature at which molecular movement ceases completely is absolute zero; it has been reached theoretically but not yet in actuality. Ambient temperature, usually synonymous with room temperature, denotes the surrounding environmental conditions such as pressure and temperature.

Plastics behave differendy when exposed to temperatures; most plastic can take greater heat than humans. There are some plastics that cannot take boiling water and others operate at 150°C (300°F) with a few up to 540°C (1000°F). Most are not effected by low temperature (below freezing). The flexible (elastomer) plasties at room temperature become less flexible as they are cooled, finally becoming britde at a certain low temperature. Then there are plastics that reach 1370°C (2500°F) with exposures in fractions of a second. Performance is influenced by short to long time static and dynamic mechanical requirements. An excellent test if a plastic can take heat is put in your automobile trunk or a railroad boxcar where temperatures can reach 55°C (130°F).

Important to understand that there is a temperature transition in plastics; also called ductile-to-britde transition temperature. It is temperature at which the properties of a material change. Depending on the material, the transition change may or may not be reversible. A few other characteristics are presented. The plastic softening range temperature is the temperature at which a plastic is sufficiendy soft to be distorted easily. A number of tests exist and the temperatures arrived at may vary according to the particular test method. Softening range is sometimes erroneously referred to as the softening point. Temperature stability identifies the percent change usually in tensile strength or in percent elongation as measured at a specified temperature and compared to values obtained at the standard conditions of testing.

Data obtained by testing different impact properties at various temperatures produces information that is similar to an elongation vs. temperature curve. As temperatures drop significandy below the ambient temperatures, most TPs lose much of their room-temperature impact strength. A few, however, are on the lower, almost horizontal portion of the curve at room temperature and thus show only a gradual decrease in impact properties with decreases in temperature. One major exception is provided by the glass fiber RPs, which have relatively high Izod impact values, down to at least -40°C (-40°F). The S-N (fatigue) curves for TPs at various temperatures show a decrease in strength values with increases in temperature. However the TSs, specifically the TS RPs, in comparison can have very low losses in strength.

Plastics can be affected in different ways by temperature. It can influence short- and long-time static and dynamic mechanical properties, aesthetics, dimensions, electronic properties, and other characteristics. Fig. 6.3 provides a guide relating time at temperature vs. 50% retention mechanical and physical properties. Testing temperature was at the exposure temperature of test specimens.

As the temperature rises thermoplastics (TPs) are effected. In comparison thermosets (TSs) are not affected. The maximum temperatures it Guide to temperature versus plastic properties (Courtesy of Plastics FALLO)

under which plasdcs can be employed are generally higher than the temperatures found in buildings, including walls and roofs, but there are those such as LDPE that are marginal and cannot carry appreciable stresses at these moderately elevated temperatures without undergoing nodceable creep. Many plasdcs can take shipping conditions that are more severe than their service conditions. With a closed automobile trunk or railroad boxcar temperatures reach at least 52°C (126°F); a temperature endurance test could be run in these closed containers or other containers.

Plastic strength and modulus will decrease and its elongation increase with increasing temperature at constant strain. Curves for creep isochronous stress and isometric stress are usually produced from measurements at a fixed temperature (Chapter 3). Complete sets of these curves are sometimes available at temperatures other than the ambient. It is common to obtain creep rupture or apparent modulus curves plotted against log time, with temperature as a parameter (Fig. 6.4).

A set of creep-rupture curves at various temperatures (Fig. 6.5) can be extended to provide data to obtain longer time data. With these data projecting the lowest-temperature curves to longer times as a straight line would produce a dangerously high prediction of rupture strength.

it Guide to temperature versus plastic properties (Courtesy of Plastics FALLO)

10,000 100,coo

. Effect of temperature on creep modulus

Temperature increasing -

Temperature increasing -

1 10 SO YEARS

1 10 SO YEARS

LOG TIME (HOURS)

LOG TIME (HOURS)

An advantage of conducting complete creep-rupture tesdng at elevated temperatures is that although such tesdng for endurance requires long times, the strength levels of the plasdc at different temperatures can be developed in a reladvely short dme of usually just 1,000 to 2,000 h. The Underwriters Laboratories and other such organizations have employed such a system for many decades.

0 0

Post a comment