Cooling Environmental stress cracking

Isotactic polybutene crystallizes into three different forms. When it cools from the melt, it originally crystallizes into a metastable crystalline one. After several days, however, it transforms into a different form. Noticeable changes in melting point, density, flexural modulus, yield, and hardness accompany this transformation. The third crystalline form results from crystallization from solution. The polymer exhibits good impact and tear resistance. It is also resistant to environmental stress-cracking. 

Analysis of extruded and injection molded iPP shows that a skin-core structure is formed due to high shear and thermal gradients upon cooling. These frozen-in tensile stresses in the skin can act as the stress component in the environmental stress cracking process [4]. In injection moldings, molecular weight has a secondary effect. This effect is that residual orientation in the direction of flow increases with higher... 

All weldments in a reactor contain high residual stress and, unless they are suitably stress-relieved, they are focal points for environmental cracking. However, the circumferential welds by which nozzles are attached to the vessels are particularly vulnerable. The radius of a dished head also contains residual stress from the forming operation, and the top head is often the first point of failure when volatile species (e.g., traces of hydrogen or ferric chloride) are involved (Fig. 12.5) [7]. External jackets can cause problems, unrelated to the process, that arise from the use of cooling water or steam in the jacket which causes external SCC of the vessel projjer. 

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