Slip and Antiblock Additives Migrating into a Sealed Film
Excess primary amide migration depresses hot tack and seal peel strength; control with secondary amides or non-migrating siloxanes verified via ATR-FTIR.
Amorphous Fractional Free Volume
Meaning
Amorphous fractional free volume is a structural polymer metric quantifying the ratio of unoccupied interstitial space to total volume in non-crystalline domains. Thermoplastic resin processors rely on this measurement to predict gas barrier permeability and physical aging rates in extruded films. Polycarbonate and amorphous polyamide batches exhibit severe dimensional instability during thermoforming when packing density parameters exceed standard production thresholds.
Unmodified polymer granules supplied by resin manufacturers establish a baseline value that shifts immediately once thermal histories alter molecular chain entanglement.
Polymer Density
Molecular packing efficiency governs how polymer chains slide past one another under thermal stress during injection moulding. Cooling rates dictate whether amorphous regions retain adequate intermolecular spacing or collapse into dense configurations that resist plasticizer absorption. Virgin compounds maintain predictable internal void ratios, whereas regrind batches accumulate thermal degradation history that reduces available interstitial gaps.
Barrel temperature profiles set during plastication determine the initial configuration of these microscopic voids before the molten polymer enters the cold steel cavity.
Moulding Variance
Datasheet property values published by resin producers rarely match the actual void fraction measured inside a finished moulded part after cooling. Shear forces generated inside narrow gate geometries stretch polymer chains and artificially inflate local spacing beyond laboratory test bar limits. Cavity pressure decay curves during the packing phase force polymer segments into tighter arrangements, reducing the final volume available for gas diffusion.
Operators adjust holding pressures to compensate for shrinkage, but excessive clamping tonnage crushes the internal void network and causes warp defects.
Part Performance
Gas transmission rates through packaging films escalate rapidly when the internal void ratio exceeds the specified resin threshold. Structural parts operating in high humidity environments absorb moisture into these unoccupied spaces, causing unexpected dimensional swelling and mechanical load failure. Component strength degrades over time as physical aging allows polymer chains to creep closer together, expelling residual free volume.
Strict control over barrel residence time prevents thermal crosslinking from permanently locking incorrect void distributions into the finished component.