Rotational foam molding has lately been brought into being a distinct plastic processing technology. It has been developed by deliberately modifying the conventional rotational molding process to accommodate the need for fabrication of rotomolded foamed plastic articles. This technology advantageously allows for creating a foam layer or core in the interior of hollow moldings and thereby offers the capacity to deliver reinforced, large-sized, complex-shaped, single-piece, foamed plastic articles that can satisfy severe service requirements and achieve improved strength-to-weight ratios that no other process can provide. The nature of the rotational molding process is cyclic. In its simplest form, the rotational molding processing cycle consists of subjecting a pre-charged (with plastic), separable (cast or fabricated), vented, metal mold to biaxial rotation (or at least a rocking motion) into a heated area (oven), and subsequently, into a cooled environment (forced air, water, or a combination of these), after which the mold is opened, the solidified part removed, and the cycle repeated.
Essentially, the manufacture of rotational moldings with a distinct non-foamed outer skin that encapsulates entirely a foamed core or layer requires both non-foamable and foamable plastic resins to be charged into the mold within the same rotational molding cycle. This could be achieved either by interrupting the molding process, or continuously, in a single-shot fashion by charging the mold with predetermined quantities of both non-foamable and foamable resins simultaneously at the outset of the cycle, so that the use of drop boxes or plastic bags become unnecessary. This processing approach assumes simultaneous processing of a mix of non-foamable pulverized resins and pre-decomposition-free foamable solidified pellets that have been normally pre-processed in a remote extrusion-based melt compounding operation involving a carrier resin and a chemical blowing agent (CBA). However, in addition to being extremely time consuming and very energy-intensive, due to the unavoidable thermal gradient formed across the mold during both heating and cooling, the single-charge technique suffers from difficulties of controlling the timely formation of the solid skin versus the formation of a foamed core or layer of controlled density. This is often demonstrated through a premature decomposition of the CBA compounded into the foamable pellets, thereby causing poor skin thickness uniformity, foam invasion/protrusion into the skin, undesired coarse-celled foam morphologies and a weak skin-foam interface.
The rotational molding technology is inherently disadvantaged by very lengthy and energy-intensive processing cycles, which are even further aggravated when processing integral-skin plastic foams, due to the insulative effect of the developed foam layer or core within the mold. Rotational molding production cycles are, undesirably, lengthy because the plastic material charged into the bi-axially rotating mold has to be indirectly heated from room temperature to beyond its melting point (which is traditionally conducted by using convection-based heath transfer while implementing an oven) and then cooled back to room temperature (which is traditionally achieved by forced airflow and/or water sprinklers) until it eventually solidifies. In addition, the foam developed within the mold during processing produces an undesired insulative effect which slows down and practically precludes any real-time process control of both the heating and cooling segment of the cycle even further.
Consequently, this prevents the highly-desired fine-celled bubbles, originally nucleated and existing in the polymer melt, to be retained (“frozen”) until cycle completion by deliberately inducing quick solidification of the melt, because of which the bubbles continue to grow beyond control and eventually shrink and disappear. These are the motivating factors for undertaking research aiming at developing a technology that will allow for fabricating high-quality fine-celled rotationally molded integral-skin polymeric cellular composites by enabling a more efficient control of the temperature of the melt within the mold than the currently achievable.
In any closed-cell polymeric foam production the ultimate goal is to achieve the highest possible cell size distribution uniformity, cell size reduction, and cell density augmentation. However, the control of the cell size of rotationally foam molded cellular structures formed based on the use of a chemical blowing agent (CBA) might be often aggravated by some inherent limitations that are unique to the rotational molding process such as lengthy processing cycles, which result in coarser-celled final cellular structures being yielded. Another reason is the fact that the polymer close to the internal mold surface continues to be heated even after its foaming is completed, simply because the interior polymer has yet to reach the decomposition temperature of the blowing agent. Even when cooling is applied to the mold, the recrystallization temperature in the melt is reached after several minutes, during which time bubble coalescence and collapsing begin to set out and accelerate. It is also inherent to the rotomolding process that, as the melt front progresses, the air pockets that have been entrapped inside the melt eventually become bubbles that will be subjected to diffusion controlled shrinkage and eventual disappearance. At a high enough melt temperature, the air in the bubbles begins to dissolve into the polymer. Since oxygen has about twice the solubility of nitrogen in polyethylene, at high temperatures, the oxygen is further depleted by direct oxidation reactions with polyethylene. The depletion of oxygen reduces the bubble diameter. Since the laws of surface tension dictate that the pressure inside the bubble has to increase as the diameter decreases, the increase in bubble pressure forces nitrogen to dissolve in the polymer thereby reducing the bubble diameter even further. This repeats until the bubble disappears.
The current state-of-the-art in rotational foam molding technology is the single-charge technique for fabricating integral-skin cellular composites. Although the single-charge processing concept is beneficial for improving the efficacy of the molding process and the structural homogeneity of the moldings, it certainly suffers from inherently aggravating the fulfillment of crucial processing goals such as: (i) the execution of the adhesion of the non-foamable thermoplastic resin to the internal surface of the mold that should always take place prior to the thermal activation of the foaming resin (thereby avoiding skin protrusions), and (ii) obtaining a solid skin layer with a uniform thickness. In this context, the fundamental research of the lifespan of CBA-blown bubbles in non-pressurized non-isothermal polymer melts using hot-stage optical microscopy and digital imaging indicated that the lifespan of fine-celled bubbles is significantly shorter than the inherently lengthy heating portion of the rotational molding process, so that fine-celled bubbles seldom reach the solidification stage of the cycle, which implies that only coarser-celled bubbles live long enough to participate in the final cellular structure. One of the major progresses of this research includes the development of a two-step oven temperature profile that prevents the foamable resins invading the solid skin layer and ensures that skin formation always completes prior to the activation of the foamable resin. It was based on the fundamental study of the adherence behavior of powders and foamable pellets to a high-temperature rotating mold wall and a fundamental study of the rotofoamablility of polymeric resins using both dry blending and melt compounding based methods including the characterization of their respective rheological and thermal properties. This study clarified why in rotationally foam molded cellular structures based on the use of a CBA, a fine-celled morphology has been closely approached, but it has not been actually achieved yet. Thus, it was clearly indicated that it would be very difficult to generate the preferred kind of bubbles (fine-celled) in rotational foam molding unless the duration of the heating portion of the process is dramatically reduced, or else. This created an importunate need for overcoming the fundamental disadvantage of the rotational molding process which is its very lengthy processing cycle time in comparison with respective currently implemented technologies. Embodiments of the present invention suggest a solution to this fundamental problem.
In comparison with the relatively large volume of research studies associated with conventional rotomolding, not much work related to the rotational foam molding process has been published to date in the open literature. Particularly, very limited research has been conducted on the processing of integral-skin cellular polymeric composites in rotational foam molding, while even a smaller body of literature deals with the study of the single-charge rotational foam molding technology. Furthermore, while several authors have done an admirable job in studying the formation and removal of unwanted bubbles in conventional rotational molding, other than a few recent studies, no substantial work has been performed yet to explain the mechanisms governing the CBA-blown production and retention of controlled size bubbles and their lifespan in non-pressurized non-isothermal polymer melts, such as in rotational foam molding. Likewise, compared to PE foams, very little research has been accomplished to date on the production of PP foams in rotational foam molding. Unlike PE, successful processing of PP foams in extrusion melt compounding-based rotational foam molding, as well as the production of integral-skin PP foams encapsulated with PE skins, have been reported only recently.
The surveyed patent literature indicates that for the manufacture of integral-skin cellular polymeric composites using the single-charge rotational foam molding technology, it would be essential to charge the mold with predetermined quantities of non-foamable and foamable resins having a significant particle size difference. The non-foamable particles intended for forming the skin should be introduced into the mold in a powder form, whereas the foamable particles intended for forming the foamed core should be first extrusion melt-compounded with an adequate quantity of CBA and then introduced into the mold in a pellet form. The powder particles would sinter earlier than the pellets because of their greater total contact area and because their smaller size would allow them to migrate towards the internal surface of the rotating heated mold where the temperature is the highest. Thereby, the powder will preferably fuse and form the skin layer before the uninterrupted heating of the mold initiates pellet sintering and subsequently trigger the decomposition of the CBA particles dispersed within the foamable pellets to form the foamed core. This concept may be effectively improved by selecting non-foamable resins that have a lower melting point, density, and viscosity, and/or heat capacity than the respective properties pertaining to the foamable resins. In addition, the non-foamable skin-forming resin should preferably comprise a low and a high zero shear viscosity components. Yet, the mold could be charged with a blend of non-foamable and foamable powders (e.g., reground compounded pellets of a polymer with a CBA) of polymers that have significantly dissimilar melting points and/or significantly dissimilar particle sizes. Also, a more intensive mold rotation during the formation of the skin and an optimized oven temperature profile would be helpful in preventing the premature adherence of the foamable pellets on the skin.
Previously conducted related research work indicated evidence of a strong causality between the duration of the heating cycle and the morphology of the obtained polymeric foams produced in rotational molding. These studies implicitly indicated that the morphologies of these cellular structures might be dramatically improved if successfully inventing a processing strategy that will utilize synergistically and concurrently the advantages of both the melt extrusion and rotational foam molding technologies. The purpose of the embodiments of this invention is to deliberately combine these two technologies: (i) to develop a technology that would be capable of achieving fine cell density in rotationally molded integral skin foams in a much wider processing/materials/system window through increased process controllability in comparison with the currently achievable and (ii) to reduce the duration of the processing cycle to thereby increase the efficacy and utility of the process to levels not conceivable with the prior art.
An embodiment of invention herein exploits the synergistic effects resulting from the deliberate conjunction of extrusion melt compounding and rotational molding through the development of an innovative extrusion-assisted technology for the “rapid” manufacture of lightweight integral-skin fine-celled rotationally molded foamed articles having unique physical and mechanical properties and strength to weight ratios. An embodiment of the invention includes the design and development of an extrusion-assisted heavy-duty rotational foam molding experimental apparatus that was utilized to facilitate the understanding of this novel process and the experimental work intended to validate the extrusion-assisted rapid fabrication technology for lightweight integral-skin fine-celled rotationally molded foams as well as to determine the feasibility of successfully developing microcellular rotationally molded foams.