Meaning
Liquid chromatographic separation separates polyolefin molecules based on hydrodynamic volume inside heated column beds containing porous gels. Operating at elevated temperatures between 135 and 160 degrees Celsius with solvents such as trichlorobenzene, high temperature size exclusion chromatography enables full dissolution of semi-crystalline polymers like polyethylene and polypropylene. The analytical procedure measures molecular weight distribution metrics including number-average and weight-average molecular weights.
The technique governs raw resin characterization, stopping at insoluble crosslinked structures or filled composites that foul column beds.
Molecular Separation
Separation column packed beds partition polymer chains as solvent carries the dissolved sample through calibrated pore networks. Larger polymer chains cannot enter small pores and elute first, whereas smaller chains penetrate deeper and elute later. Detectors measuring refractive index and infrared absorption quantify mass concentration and chain branching across the elution spectrum.
Raw material specifications rely on high temperature size exclusion chromatography to establish molecular weight limits that guarantee melt strength during blow molding or film extrusion.
Viscosity Prediction
Molder processing windows depend directly on the high molecular weight tail of a resin’s molecular weight distribution. Excessive high molecular weight fraction raises shear viscosity, leading to melt fracture and elevated cavity pressures. Conversely, excess low molecular weight species act as plasticizers but increase mold deposit formation and lower environmental stress crack resistance.
Degradation Tracking
Repeated thermal processing during regrind incorporation breaks long polymer chains through scission. Analytical runs using high temperature size exclusion chromatography reveal shifts toward lower average molecular weight values after multiple extrusion passes. Converters establish maximum allowable regrind percentages by tracking these molecular weight reductions against part impact strength drops.
