Cast Film Extrusion Research Articles

Modelling of the Cooling of Semi-crystalline Polymers during their Processing

Abstract As a first step for the modelling of the processing of semi-crystalline polymers, it is proposed a new overall kinetic model which allows to calculate the evolution of the transformed volume fraction whatever the cooling conditions are. This model derives from initial Avrami-Evans' theory. The transformed volume fraction is calculated as a function of parameters which are measured during crystallizations at a constant cooling rate. This model is easy to use and can be introduced into a more complete thermomechanical calculation. For this purpose we propose to express the energy equation in terms of the enthalpy function. Two applications of our model are presented: the cooling of a polymer sheet (non-flowing polymer) and the cooling of the film on the chill-roll during the cast film extrusion process (flowing polymer).

Influence of Stretching and Cooling Conditions in Cast Film Extrusion of PP Films

Abstract In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was to study the effect of the roll temperature on the polymer crystallization in the thickness of extruded films. A thermomechanical model of the whole process taking into account the polymer crystallization was used to determine the mean stretching stress in air, local temperature, local cooling rate and local crystallization development. The morphologies in the thickness of the films were observed by optical microscopy on microtomed slices and the crystalline structures were investigated by wide-angle X-ray diffraction and DSC measurements. From DSC melting curves of thin sections it is possible to know the local crystallization temperature and to check the thermomechanical model. The model is validated and explains the influence of the roll temperature on the formation of structures and morphologies. Nevertheless neither the existence of three different zones in the thickness nor the high p phase concentration are predicted by the model, which needs new developments.

Cast Film Extrusion of Polypropylene Films

Abstract In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was devoted to the development and application of a thermomechanical model of stretching in air and of a purely thermal model of cooling on the roll, taking polymer crystallization into account. The structures and morphologies in a series of extruded polypropylene films were investigated using density and sonic modulus measurements, wide-angle X-ray diffraction, small angle light scattering and optical microscopy. These experimental results combined with theoretical predictions obtained from the model allow to discuss the respective effects of stretching in air and cooling on the roll. Stretching in air essentially affects spherulite nucleation and growth during subsequent crystallization on the roll. Dissymmetrical cooling on the roll induces an inhomogeneous microstructure within the film. The presence of a zone with a finer microstructure near the roll has been correlated with the cooling rates and thermal gradients calculated by the model.

Étude de la cristallisation dans la mise en forme des polymères. Application à l’extrusion de films de polypropylène

Crystallization of a polymer in an industrial process is highly dependent on the mechanical (flow), thermal (cooling rates, thermal gradients) and geometrical (walls of the processing devices) conditions encountered by this polymer in the process. Cast film extrusion of polypropylene is chosen as an example of interrelations between processing conditions and microstructure. In this process, the polymer is extruded through a slit die, slightly stretched in air and then cooled on a chill roll. Our analysis is based on the combination of a thermomechanical model and morphological observations, which makes it possible to discuss the respective influences of stretching in air and cooling on the roll. Stretching in air essentially affects spherulite nucleation and growth during subsequent crystallization on the roll. The existence of a transcrystalline region is attributed to heterogeneous nucleation at the roll surface Finally, the observation of a zone near the roll with a finer microstructure is related to the cooling rates and the thermal gradients calculated by the thermo- mechanical model. A conclusion of this work is that a complete quantitative interpretation requires the development of crystallization models taking into account the actual processing conditions.

Polyethylene‐polycarbonate blends: Interface modification, phase morphology and measurement of orientation development during cast film and tubular extrusion by infrared dichroism

AbstractThe phase morphology and polymer chain orientation in polyethylene‐polycarbonte blends is investigated. A blend with a dispersed polyethylene phase was found to exhibit phase growth under quiescent conditions. Addition of a styrene‐(ethylene/butene) block copolymer controls the phase morphology and prevents phase growth. Melt spun tapes and tubular film have been prepared from polyethylene‐polycarbonate blends. Orientation of the crystallo‐graphic axes of the polyethylene and the chain axis of the polycarbonate were determined by infrared dichroism. The addition of polycarbonate to polyethylene is found to decrease chain orientation at the same drawdown stress.

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The effect of polypropylene/poly(lactic acid) weight ratios on the properties of blend films compatilized with polypropylene-grafted-maleic anhydride were investigated with 100:0, 60:40, 50:50, 40:60, and 0:100 of weight ratio. The blend films were prepared by melt mixing technique and cast film extrusion. The results shown that the FTIR spectrum was confirmed the interaction between compatibilizer and polymers. Morphological investigation was distinctly seen a two phases system between polypropylene and poly(lactic acid). Increasing of PLA content from 40 to 60 wt.% resulting in decreased melting temperature and crystallinity from 158 °C to 154 °C and 38% to 31%, respectively. For tensile properties, modulus and tensile strength increased with increasing the PLA content, while elongation at break was drastically decreased from 500% (polypropylene) to less than 50% (blends). The barrier properties indicated that incorporation of poly(lactic acid) into polypropylene tend to increased water vapor permeability while oxygen permeability was decreased. From the morphology, thermal, mechanical, and barrier results, the polypropylene/poly(lactic acid) blends showed a typical immiscible polymer blend.