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1. Definition of stress cracking:
Stress cracking refers to phenomenon in which a polymeric material cracks and fails when subjected to prolonged stress below its yield strength or stress below its short-term strength. This stress includes internal stress, external stress, and a combination of two stresses.
The most common internal stress is internal stress caused by injection pressure during injection molding. Internal stress is generated as a result of uneven cooling and shrinkage of material after processing.
2. Environmental cracking definition:
Environmental stress cracking, environmental stress cracking, abbreviated as ESC. It refers to cracking and failure of polymeric materials exposed to chemical environments and under long-term stress below its yield strength or stress below its short-term strength (including internal and external stress and a combination of two stresses). Phenomenon.
3. The classification of cracking of polymer materials mainly includes:
①Passion for solvents
②Cracking of active material,
③Oxidative stress cracking
④ Thermal stress cracking.
4. Main hacking mechanisms involved:
①Griffith microcrack strength theory,
In a real polymer material, there are tiny cracks and defects, and presence of active media reduces surface tension and requires less energy to form cracks.
②Maxwell and Ram's theory of molecular chain slip,
The active medium promotes plasticization and swelling of polymer, causing a decrease in glass transition temperature and viscosity, which ultimately leads to easier sliding between molecular chains.
③Theory of enhanced thermal motion
The active medium enhances thermal motion at crack tip, and stress contributes to mutual dissolution of polymer and active medium.
④Molecular crystallization, solidification stress theory.
The difference in crystallization behavior during cooling process of real polymers leads to stress. In addition, different polymers and fillers have different effects on crystallization.
5. Main cause of cracking in TPE-S
Typically, TEP-S formula consists of PP+SEBS+filler+softening oil. The styrene content of SEBS is about 30%. The combination of EB segments gives TEP-S a certain elasticity and resistance to a certain temperature. phase).
The author believesthat main cause of TPE-S cracking is physical styrene crosslink point, as they say, success is also Xiao He, and failure is Xiao He . In various cases, we have found that in TPE-S with a high SEBS content and when some actual test temperatures approach 100°C, TPE-S is especially prone to cracking at this time, especially when it is subjected to some external stress. What can be understood here is that PS in SEBS is actually completely incompatible with PP, but EB segment in SEBS is more compatible with PP. Once there are high temperature conditions, PS and PP, two guys who disagree with each other , will think about parting!
The plastic phase of PP in TPE-S forms an interpenetrating network with SEBS, but when PP decreases to a certain extent and entire continuous network consists of only SEBS, risk of cracking increases dramatically, and PS itself is subject to molding Then, even if there is no external stress and external high temperature, internal stress of PS will slowly cause this crack.
In presence of external stress, crack will accelerate, for example, in following two cases:
Stress cracking of flame retardant jacket of TPE-S cable
Cracking a garden hose under stress during a hydrostatic test
In presence of high temperature external conditions, even if there is no external stress, it is easy to cause cracking, as shown in following cases:
Medical device handle cracked during high and low temperature disinfection test
Summarizing above, main causes of TPE-S cracking are:
①Presence of internal stress;
②External stress and internal stress coexist;
③Existence of internal stress plus high temperature conditions.
Based on understanding of styrene in TPE-S, I think it should be relatively easy to solve this problem, but it is theoretically impossible to achieve crack resistance in thermosets.
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