ABSTRACT:The effect of heat sealing variables (platen temperature and dwell time) on seal strength of a linear low-density polyethylene (LLDPE) was examined. In order to characterize the development of interfacial strength, blown films were heat-sealed for times from 1 to 100,000 s, much longer than the typical sealing times of less than 1 s. The seal temperature ranged from 100 to 130°C. From the differential scanning calorimetry thermogram, the LLDPE was determined to be completely melted at 130°C. Therefore, the films ranged from partially to fully melted when they were heat-sealed. The seal strength was measured in the T-peel configuration, and the peel fracture surfaces were examined in the scanning electron microscope. A temperature of 115°C or higher was required to form a good seal. The strong effect of seal temperature was related to the heterogeneous composition of the LLDPE studied. At 115°C, the lowermolecular-weight, more highly branched chains easily diffused across the interface. Crystallization upon cooling produced connections across the interface. However, because these chains represented a small fraction of the crystallinity and the molecular weight was low, they contributed much less than the full peel strength. Conversely, chains with less branching represented the main fraction of crystallinity (anchors for tie chains) and the highest molecular weights (more entanglements). Only at temperatures at which the higher-molecular-weight, less branched chains began to melt and diffuse across the interface could high peel strengths be achieved.

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The drawing behavior of a series of linear polyethylene homopolymers with weight‐average molecular weight (M̄w) ranging from 67,800 to ∼3,500,000 and variable distribution (M̄w/M̄n = 5.1−20.9) has been studied. Sheets were prepared by two distinct routes: either by quenching the molten polymer into cold water or by slow cooling below the crystallization temperature (∼120°C) followed by quenching into cold water.When the samples (2 cm long) were drawn in air at 75°C using a crosshead speed of 10 cm/min it was found that for low M̄w polymers the initial thermal treatment has a dramatic effect on the rate at which the local deformation proceeds in the necked region. At high M̄w such effects are negligible. An important result was that comparatively high draw ratios (λ > 17) and correspondingly high Young's moduli could be obtained for a polymer with M̄w as high as 312,000. It is shown how some of the structural features of the initial materials (mainly studied by optical microscopy, small‐angle x‐ray scattering and low‐frequency laser Raman spectroscopy) can be interpreted in terms of the molecular weight and molecular weight distribution of the polymers. Although crystallization and morphology can be important at low M̄w, it suggested that the concept of a molecular network which embraces both crystalline and noncrystalline material is more helpful in understanding the drawing behavior over the whole range of molecular weights.

Measurements of shrinkage and shrinkage force have been undertaken on drawn polyethylenes over a wide range of temperatures. The behaviour has been studied as a function of draw ratio and molecular weight for several linear polyethylenes and polyethylene copolymers. Although complete retraction is observed on heating even the most highly drawn samples above the melting range, at lower temperatures the shrinkage is less than that of low draw material. This is attributed to the formation of a substantial degree of crystal continuity during drawing, so that shrinkage requires mobility in the crystalline phase. The shrinkage forces are very high, being comparable with the drawing stresses at corresponding temperatures, and are molecular weight dependent, increasing with increasing molecular weight. It is considered that the results presented here on shrinkage and shrinkage force provide further support for the proposition that the drawing process relates to the extension of a molecular network.

An extensive series of homopolymers and copolymers has been investigated to test earlier indications of a possible link between resistance to slow crack growth and creep of macroscopic oriented samples. It was found that in the majority of cases a good correlation exists between a single creep parameter and slow crack growth. The existence of such a correlation supports the model presented, in which crack growth is primarily controlled by the creep of fibrils spanning the craze zone. The limit of improvement achievable by increasing molecular weight and the greater effectiveness of short-chain branching to improve fibril creep have been highlighted.