Standard Practice for Heat and Humidity Aging of Oxidatively Degradable Plastics

Importancia y uso:

4.1 Since the correlation between the conditions specified in this practice and actual disposal environments (for example, composting, soil or landfill) has not been determined or established, the test results are to be used only for comparative and ranking purposes in the laboratory. No extrapolation to real world disposal expectations or predictions are to be made from results obtained by this procedure. Real world evaluations and correlations are needed for such claims.

4.2 Degradable plastics exposed to heat and humidity are subject to many types of physical and chemical changes. The severity of the exposures in both time, temperature and humidity level, determines the extent and type of change that occurs. For example, short exposure times at elevated temperatures generally serve to shorten the induction period of oxidatively degradable plastics during which the depletion of antioxidants and stabilizers occurs. Physical properties, such as tensile and impact strength and elongation and modulus, sometimes change during this induction period; however, these changes are generally not due to molecular-weight degradation, but are merely a temperature-dependent response, such as increased crystallinity or loss of volatile material, or both. The effects of humidity are less well understood and are more difficult to predict and depend on the degradable plastics characteristics such as hydrophilicity, polarity and composition.

4.3 Generally, short exposures at elevated temperatures drive out volatiles such as moisture, solvents, or plasticizers; relieve molding stresses; advance the cure of thermosets; increase crystallinity; and cause some change in color of the plastic or coloring agent, or both. Normally, additional shrinkage is expected with a loss of volatiles or advance in polymerization.

4.4 Some plastic materials such as PVC become brittle due to loss of plasticizers or to molecular breakdown of the polymer. Polypropylene and its copolymers tend to become very brittle as molecular degradation occurs, whereas polyethylene tends to become soft and weak before it embrittles with resultant loss in tensile strength and elongation.

4.5 Embrittlement of a material is not necessarily commensurate with a decrease in molecular weight.

4.6 The degree of change observed will depend on the property measured. Different properties do not change at the same rate. In most cases, ultimate properties, such as break strength or break elongation, are more sensitive to degradation than bulk properties such as modulus.

4.7 Effects of exposure can be quite variable, especially when samples are exposed for long intervals of time. Factors that affect the reproducibility of data are the degree of temperature control of the enclosure, humidity level of the oven, air velocity over the specimen, and exposure period which are evaluated by this practice. Errors in exposure are cumulative with time; for example certain materials have the potential to be degraded due to the influence of humidity rather than oxidation in long-term tests and thus give misleading results. Materials susceptible to hydrolysis (that is, hydrolytically degradable plastics) undergo degradation when subjected to long-term thermal tests due to the presence of moisture rather than oxidation.

4.8 Do not infer that comparative material ranking is undesirable or unworkable. On the contrary, this practice is designed to provide information that can be used for such comparative purposes after appropriate physical property tests are performed following exposure. However, since it does not account for the influence of stress or environment that is involved in most real life applications, the information obtained from this practice must be used cautiously by the designer, who must inevitably make material choices using additional information, such as moisture, soil-type and composition, and mechanical-action effects that are consistent with the requirements of the particular application.

4.9 It is possible for many temperature indices to exist, in fact, one for each failure criterion (time to reach failure is dependent on the exposure temperature and humidity). Therefore, for any application of the temperature index to be valid, the thermal-aging program must duplicate the intended exposure conditions of the end product. If the plastic material is exposed in the end use in a manner not evaluated in the aging program, the temperature index thus derived is not applicable to the use of the plastic material.

4.10 In some situations, a material can be exposed to one temperature and humidity, for a particular period of time, followed by exposure to another temperature at the same humidity, for a particular period of time. This practice can be used for such applications. The heat-aging curve of the first temperature and humidity is derived, followed by derivation of the heat-aging curve for the second temperature at the same humidity, after exposure of samples to the first temperature and humidity.

4.11 There can be very large errors when Arrhenius plots or equations based on data from experiments at a series of temperatures and humidity are used to estimate time to produce a defined property change at some lower temperature. This estimate of time to produce the property change or failure must always be accompanied by a 95 % confidence interval for the range of times possible based on the calculation or estimate.

Subcomité:

D20.96

Volúmen:

08.03

Número ICS:

83.080.01 (Plastics in general)

Palabras clave:

age; degradable; embrittlement; humidity; oven; oxidation; property change;