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- Date: 2023-05-07
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In fact, this is determined by the molecular structure of the polymer. Most people are not polymer-specific, so it is difficult to judge from the molecular structure level, but there are several simple methods that can be roughly inferred from the data sheet
There are basically several reasons for stress cracking:
A. Macromolecular chain orientation internal stress
B. Shrinkage internal stress
C. Environmental stress
1.) Macromolecular chain orientation stress
The flexibility of the macromolecular chain after freezing is different depending on the molecular chain. Most of the molecular chains containing amino groups are more flexible, and the molecular chains containing benzene rings are relatively less flexible, so look at the name of a polymer. If the engineering plastic contains an amino group, the possibility of macromolecular chain orientation stress cracking is basically removed.
PS, PPS is susceptible to stress cracking because the molecular backbone is a benzene ring;
PEI resists stress cracking because the molecular backbone is an amino group.
2.) Shrinkage internal stress
The smaller the thermal shrinkage expansion of a polymer, the more balanced the directional shrinkage and the less likely the stress cracking.
Please see the thermal expansion properties of PEI Ultem 1000 on data sheet:
CTE parallel 5.E-0.5 1/°C
CTE transverse 5E-0.5 1/°C
It can be seen that its coefficient of thermal expansion is very small, and it is also even in parallel and transeverse directions, so it is not easy to generate internal stress during cooling.
The following is the thermal expansion coefficient of PPS Fortron 1140L4
CTE parallel 4.E-0.26 1/°C
CTE transverse 4.E-0.62 1/°C
Its coefficient of thermal expansion is relatively large and un-even, so it is also prone to stress, of course, there are other reasons.
3.) Environmental stress
The occurrence of environmental stress is mainly caused by long-term exposure to chemical media substances. If a polymer itself has strong and extensive chemical resistance, it is not easy to destroy its molecular structure, resulting in failure and environmental stress. Chemical resistance can be seen clearly on the data sheet.
There are basically several reasons for stress cracking:
A. Macromolecular chain orientation internal stress
B. Shrinkage internal stress
C. Environmental stress
1.) Macromolecular chain orientation stress
The flexibility of the macromolecular chain after freezing is different depending on the molecular chain. Most of the molecular chains containing amino groups are more flexible, and the molecular chains containing benzene rings are relatively less flexible, so look at the name of a polymer. If the engineering plastic contains an amino group, the possibility of macromolecular chain orientation stress cracking is basically removed.
PS, PPS is susceptible to stress cracking because the molecular backbone is a benzene ring;
PEI resists stress cracking because the molecular backbone is an amino group.
2.) Shrinkage internal stress
The smaller the thermal shrinkage expansion of a polymer, the more balanced the directional shrinkage and the less likely the stress cracking.
Please see the thermal expansion properties of PEI Ultem 1000 on data sheet:
CTE parallel 5.E-0.5 1/°C
CTE transverse 5E-0.5 1/°C
It can be seen that its coefficient of thermal expansion is very small, and it is also even in parallel and transeverse directions, so it is not easy to generate internal stress during cooling.
The following is the thermal expansion coefficient of PPS Fortron 1140L4
CTE parallel 4.E-0.26 1/°C
CTE transverse 4.E-0.62 1/°C
Its coefficient of thermal expansion is relatively large and un-even, so it is also prone to stress, of course, there are other reasons.
3.) Environmental stress
The occurrence of environmental stress is mainly caused by long-term exposure to chemical media substances. If a polymer itself has strong and extensive chemical resistance, it is not easy to destroy its molecular structure, resulting in failure and environmental stress. Chemical resistance can be seen clearly on the data sheet.
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