Basic Introduction to Bimodal Polyethylene Molecular Structure and Flow
Bimodal PE decouples strength from processability by pairing low MW matrix lubricant with branched high MW tie chains for high ESCR and low extrusion pressure.
Gel Content
Meaning
Insoluble polymer mass represents the fraction of a material that remains after extraction by a solvent, typically used to determine the degree of crosslinking within a resin. Gel content provides a quantitative metric for assessing the thermal or chemical curing progress of thermosets and modified thermoplastics. High values indicate extensive network formation while low values signal under-curing or degradation.
Polymer Structure
Crosslinked networks form when polymer chains covalently link, creating an infinite structure that resists dissolution. The gel content measurement relies on the difference between the initial dry weight of a sample and the weight of the remaining insoluble portion after exposure to a boiling solvent for a set duration. Proper extraction requires fine subdivision of the specimen to ensure solvent penetration reaches the innermost core.
Processing Variable
Moulding conditions dictate the resulting insoluble fraction through temperature, residence time and pressure during the transition from melt to solid. Deviations in these parameters shift the conversion rate and produce parts with variable physical performance. Excessive heat during processing triggers unwanted crosslinking in some resins, increasing the measurement beyond the intended specification.
Insufficient heat leaves the material partially uncured, creating weak spots that fail under mechanical load.
Economic Consequence
Virgin resin typically carries a consistent curing profile that allows moulders to forecast output performance reliably. Regrind material introduces secondary thermal histories, often leading to unpredictable fluctuations in crosslinking density that complicate quality control. Datasheet values represent a controlled laboratory environment that rarely mirrors the rapid cooling cycles of commercial injection machinery.
Relying on nominal values without adjusting for process variance leads to costly scrap rates and compromised structural integrity in finished components.