Meaning
Polymer swelling coupled with chain relaxation dictates the boundary kinetics where case II transport governs solvent penetration into glassy thermoplastics. Amorphous resins like polymethyl methacrylate absorb penetrants through a sharp advancing front rather than Fickian diffusion because polymer chains stretch and mobilize only behind the boundary. This velocity remains constant over time instead of decreasing with the square root of time, creating severe internal stresses during solvent bonding or chemical exposure.
Resin Selection
Polymethyl methacrylate and polyvinyl chloride display this distinct sorption behavior due to low segment mobility below their glass transition temperatures. Unreinforced amorphous grades react differently than semicrystalline counterparts because rigid domains prevent immediate chain disentanglement under chemical attack. Virgin pellets process reliably, whereas heavily degraded regrind materials alter local free volume and accelerate penetrant front speeds unpredictably.
Moulders specify exact molecular weight distributions to stabilize this kinetic rate during secondary solvent welding operations.
Moulding Parameter
Barrel temperature profiles and injection pressures dictate residual orientation levels that subsequently accelerate or retard chemical penetration rates in finished mouldings. High cooling rates freeze excessive mechanical stress into the part, creating preferential pathways for aggressive solvent agents. Tool designers adjust gate locations to minimize frozen-in birefringence and prevent localized stress concentration zones near structural ribs.
Production technicians verify annealing cycles to reduce internal stress before components encounter aggressive environments or secondary liquid assembly lines.
Dimensional Failure
Microcracking and catastrophic crazing emerge when solvent front velocities outpace the structural relaxation capacity of the host polymer matrix. Dimensional distortion occurs because swollen outer layers expand against unyielding core material, resulting in severe warpage after assembly. Part specifications demand strict limits on residual stress to prevent premature cracking during exposure to cleaning agents or lubricants.
Datasheet values for chemical resistance fail to predict these part failure modes unless evaluated under actual mechanical load conditions.
