Meaning
Degradation occurring throughout the entire volume of a polymer specimen characterizes the loss of physical integrity in specific hydrophilic materials. When a part undergoes bulk erosion, water penetrates the inner regions of the plastic faster than the chemical bonds can cleave. This leads to a gradual, homogenous reduction in molecular weight across the whole cross-section of the component before any visible weight loss occurs on the outer surfaces.
This uniform breakdown is common in amorphous polyesters with low hydrophobic character.
Degradation Mechanism
Hydrolytic reaction rates inside the material matrix exceed the rate of moisture diffusion to establish this specific mode of decay. In the process of bulk erosion, the polymer chains are cleaved uniformly, causing the part to remain geometrically intact while its mechanical strength declines sharply. This stands in contrast to surface-limited processes where the dimensions of the component shrink gradually from the outside inward.
Structural Breakdown
Internal void formation and micro-cracking occur as the polymer chains break down into soluble oligomers. Once these fragments are small enough, they diffuse out of the plastic, leaving behind a highly porous structure with minimal load-bearing capacity. These changes can cause sudden, catastrophic failure of the part when it is subjected to mechanical stress.
Moulding Application
Wall thickness determines the transition point between different modes of degradation in moulded components. Molders working with resins susceptible to bulk erosion must control the cooling rates during production to prevent internal stresses that would accelerate water uptake. Selecting the proper wall thickness ensures that the part performs its function for the required duration before degrading.
