Environmental stress crack resistance dependence

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

Al-Saidi LF, Mortensen K, Almdal K (2003) Environmental stress cracking resistance behaviour of polycarbonate in different chemicals by determination of the time-dependence of stress at constant strains. Polym Degradat Stabil 82(3) 451—461... 

An example of the dependence of polymer properties on catalyst activation temperature is shown in Table 5. Polymers of various MW were made with a Cr/silica catalyst activated at various temperatures. The polymers were then tested for environmental stress crack resistance (ESCR), a measure of their ability to resist chemical attack when polymer strips are placed under stress (bent). The test is conducted in the presence of hot surfactant, and the time to failure is monitored. Like most other polyethylene physical properties, ESCR is usually improved when the MW is increased. Therefore, comparisons of catalysts are usually made at similar melt indices (a surrogate for MW). A broader MW distribution... 

TABLE 12 Dependence of Environmental Stress Crack Resistance (ESCR) on Catalyst Activation Temperature and on Comonomer Type... 

Environmental Stress Cracking Resistance (ESCR) of PE (D 1693) is the oldest and most conunon test method for SCG. Comj d to the PH test (1) it h poor precision with respect to inter-laboratory reproducibility being about 60%,(2) it takes longer to get a result and uses an Igepal enviroiunent that can contaminate the laboratory, (3) it involves different test conditions depending on the grade of the PE, (4) its stress intensi cannot be defined. It is recommended Aat ASTM conunittees in Table S that are concerned with the resistance to SCG of their PE product remove their ESCR requirement and insert the PENT test. Each committee can then more precisely specify the desired quality of their resin with the assurance that the PENT lifetime will be related to the performance of their... 

In the ESCR test (as described by the research group of Bledzki [268]), a test specimen in a three-point bending setup in a medium is put under constant strain by an acting force. This force decreases with time to the same extent that the stress decreases due to relaxation, beginning damage and crack formation. The extent of stress cracking resistance results from the time-dependent ratio of the force on the test specimen with media influence to the force without media influence. The closer this ESCR factor approaches 1 and remains dependent on time, the lower the tendency to environmentally related stress cracking [263]. 

---