This work explores the foam development process under atmospheric pressure as applicable to rotational molding, investigating the influence of the processing parameters, and characterizing the morphology of the foamed structure. Detailed understanding of the bubble transformation during foaming of nonpressurized polymer melts provided an accurate basis for predicting the morphological structure and macroscopic properties of foams produced by rotational molding. The experimental results are based on visualization studies using a hot stage microscopy setup. The cellular structure developed during the foaming was analyzed for its bubble density, bubble size, and statistical parameters considering the combined effect of these two factors. It was found during this investigation that the foaming mechanism comprises four distinct stages. It included two major stages of bubble nucleation, primary nucleation in interstitial regions, and secondary nucleation in the polymer melt. Statistical analysis of the developing foamed structure revealed that primary nucleation in the interstitial regions was the controlling stage in determining the final cellular structure. Subsequently, the nucleation stages were followed by bubble growth and bubble coalescence/shrinkage. The microscopic observations were complemented by actual rotational foam molding experiments. C 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E809-E821, 2013; View this article online at wileyonlinelibrary.com.
The objective of this work is to investigate the effects of different chemical blowing agents (CBAs) and processing conditions on the cellular structure of foamed metallocene polyethylene and characterize an appropriate blowing agent. An experimental study was conducted to produce metallocene polyethylene foams in dry-blending-based rotational foam molding. The critical processing parameters that optimize the foam structure have been identified through modifications of the molding conditions. The physical properties and cellular structure of the final foamed parts were also examined. The foaming performance of exothermic and endothermic CBAs was studied. It was revealed that selecting a suitable CBA is crucial as the foam structure depends significantly on the properties of the blowing agent. Exothermic blowing agents resulted in greater foam density reduction and exhibited a wider processing window compared to endothermic blowing agents. It was found that a balance between different properties of the blowing agent is required to achieve control over the foam structure.
The mechanism of bubble nucleation in the foaming process under atmospheric pressure is investigated in the present study. The experimental observations using a plastic‐foaming visualization setup revealed two stages of nucleation, primary nucleation in interstitial regions and secondary nucleation in the polymer melt, which followed the sintering and densification of the polymer matrix. Statistical analysis of the evolving cellular structure during the nucleation stage was used to study the significance of rheology on the behavior of polymer materials during the nucleation. The role of viscosity on the nucleation rate was also investigated theoretically by using a modified form of the classical nucleation theory and it was verified with the experimentally observed data. POLYM. ENG. SCI., 54:1201–1210, 2014. © 2013 Society of Plastics Engineers
This work explores the influence of rheological properties on polymer foam development in nonpressurized systems. To understand the complex contributions of rheology on the mechanism of bubble growth during different stages of foam processing, visualization studies were conducted by using a polymer-foaming microscopy setup. The evolving cellular structure during foaming was analyzed for its bubble surface density, bubble size, total bubble projected area, and bubble size distribution. Morphological analysis was used to determine the rheological processing window in terms of shear viscosity, elastic modulus, melt strength and strain-hardening, intended for the production of foams with greater foam expansion, increased bubble density and reduced bubble size. A bubble growth model and simulation scheme was also developed to describe the bubble growth phenomena that occurred in nonpressurized foaming systems. Using thermophysical and rheological properties of polymer/gas mixtures, the growth profiles for bubbles were predicted and compared to experimentally observed data. It was verified that the viscous bubble growth model was capable of depicting the growth behaviors of bubbles under various processing conditions. Furthermore, the effects of thermophysical and rheological parameters on the bubble growth dynamics were demonstrated by a series of sensitivity studies.
In the present study, an evaluation of the “scale of production” on rotational foam molding was conducted and experimental observations from microscopic, lab‐ and pilot‐scale foam systems were used to investigate the impact of the rheology on the process. A systematic comparison looking at the influence of rheological properties of the polymer matrix on the expansion behavior of polyethylene foams indicated that the size of the foam system does not affect the fundamental mechanisms of the foaming process and that the effects of system size on foam techniques could be suppressed by tailoring the processing conditions. It was demonstrated that the impact of rheology on bubble development observed in microscopic studies can be practically extended to higher scales of foaming, and the results can provide guidelines for the selection of the polymer materials for customized foam applications.
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