Batch Foaming Research Articles

Linking ethylene co-monomer content and stereostructure to polycrystallinity and foam density of random copolymers of polypropylene

Static batch foaming is used in a model investigation aiming to understand and predict the impact of resin molecular structure on the foamability of random copolymers of polypropylene (RCP) for bead foam applications. The foaming behavior of seven polypropylene based resins with ethylene content varying from 0 to 11 mol% can be categorized into two groups related to their respective degrees of regio-defects: below 0.1 and around 0.66 mol%. Increasing the ethylene content from 0 to 4.04 mol% is enough to decrease the foam density by a factor of 6.3, as well as the foaming temperature by 15 °C. For a specific number of stereo and regio-defects, the ethylene content has a linear response with the optimal foaming temperature. Regio-defect impact exhibits a similar linear behavior; however, it is decoupled from ethylene content effects. A hierarchical relationship between the polycrystallinity and the foam mechanism was then drawn. The stress induced by the foam cell growth was shown to provoke the generation of more ordered crystalline structure, adding another layer of complexity in obtaining foams with the desired density.

Thermoplastic polyurethane–graphene nanoplatelets microcellular foams for electromagnetic interference shielding

The incorporation of graphene-related materials as nanofiller can produce multifunctional foams with enhanced specific properties and density reduction. Herein we report on the preparation of microcellular thermoplastic polyurethane/graphene foams by batch foaming. Solution blending was first adopted to disperse graphene nanoplatelets (GNP) in the elastomeric matrix. Then, a foaming process based on the use of supercritical CO2 was adopted to produce the microcellular TPU/GNP composite foams with graphene content up to 1 wt%. The EMI shielding behaviour of the TPU/GNP foams has been assessed in the THz range, and has revealed their potential in comparison with other graphene-filled foams presented in the literature, that exhibit similar specific shielding effectiveness but at much higher content of graphene-related materials (10-30 wt%).

Striking effect of carbon nanotubes on adjusting sc-CO2 foaming performance of PS/LLDPE blends and forming semi-open cellular structure

The effect of carbon nanotubes (CNTs) on the phase structure and foaming performance of PS/LLDPE blends during batch foaming of sc-CO2 was studied. It was found that the phase structure of PS/LLDPE = 50/50 blends with CNTs could be transformed from bi-continuous to sea-island morphology, when a sufficient quantity of CNTs nanofillers was added. As a result, the foaming ability of PS/LLDPE = 50/50 blends was improved effectively. Meanwhile, the critical value of CNTs for significantly improving the foaming ability of blends decreased slightly with the increase of foaming temperature. In contrast, for the PS/LLDPE blends with sea-island phase structure (such as 20/80), the addition of CNTs did not significantly change the morphology and foaming capacity of the blends. Very interestingly, the foam with semi-open cellular structure (meaning that there were small pores within the large cell wall) was obtained in the foamed PS/LLDPE blends after tetrahydrofuran (THF) etching PS phase. Benefiting from the interfacial channel effect formed by the presence of CNTs, the etching speed and efficiency of PS were significantly improved, and the size for the small pore on the cell wall could be adjusted by the content of PS dispersed phase in the PS/LLDPE/CNTs nanocomposites.

Formation of novel “coral reef-like” structures for polycarbonate microcellular foam via asphalt-based microporous organic polymers and supercritical CO2

Polycarbonate microcellular foam (PCMF) with novel “coral reef-like” structures have successfully developed to improve the mechanical properties. By incorporating green asphalt-based microporous organic polymer (AMOP) into polycarbonate (PC) via melting mixing, AMOP/PC achieved improvements in better compatibility, lower complex viscosity and higher Tg due to the lubricant and physical adsorption effects of AMOP. Then AMOP/PC microcellular foams were prepared via supercritical CO2 batch foaming. The good compatibility between AMOP and PC facilitated the formation of “coral reef-like” structures during the foaming process, ascribed to strong interfacial bond between AMOP and PCMF matrix. Due to the toughening and strengthening effects of “coral reef-like” structures, the impact and compressive strength of AMOP/PCMF with 1.5 wt% AMOP achieved 3.2 and 1.2 times increases when PCMF used as control. This study paved a facile approach to fabricate novel “coral reef-like” structures of microcellular foams and extended the application of asphalt-based microporous organic polymer.

Combining foam injection molding with batch foaming to improve cell density and control cellular orientation via multiple gas dissolution and desorption processes

In contrast to solid parts fabricated through conventional injection molding (CIM), foamed parts manufactured via foam injection molding (FIM) exhibit substantial variations in mechanical properties, which are attributed to differences in the cellular structure. In this study, parts with different cellular structures are fabricated via FIM, during which the gas dissolution and desorption processes are controlled by subjecting the gas‐laden melt to reciprocating compression and expansion operations. The results suggest that the cell density can be drastically improved by rapidly decreasing the pressure caused by the mold opening and that the cell orientation obviously occurs in the direction perpendicular to the mold‐opening direction. Moreover, the cell density and cellular orientation can be adjusted by utilizing appropriate mold opening and closing operations, leading to improvements in the resultant ultimate mechanical properties. In particular, the foamed samples fabricated with controlled mold opening‐closing operations exhibit excellent tensile strength and strain‐at‐break, indicating that samples containing a high density of cells oriented along the tensile test direction facilitate the formation of superductility and an increase in tensile strength. Hence, a method that combines FIM with batch foaming has been proposed for improving the cellular structure and controlling the cellular orientation.

Fabrication of poly (lactic acid) foams using supercritical nitrogen

In this article, poly (lactic acid) (PLA) was foamed via batch foaming using supercritical nitrogen as a physical blowing agent by two methods, conventional foaming process (CFP) and low-temperature foaming process (LTFP). The fabrication processes, cell morphologies, thermal properties, crystallization behavior, and electrical resistance of resulted foams were studied to investigate the effect of foaming on these properties of PLA. It was found that the foams resulted from CFP method have micrometric cell sizes, while LTFP method led to nanometric cell structure and high cell density. Also scanning electron microscopy showed that the PLA foams have a heterogeneous cellular structure. The results showed that the foaming process increased the melting point and degree of crystallinity of PLA, which led to decrease in the electrical resistance of samples.

Effect of silica nanoparticles on the impregnation process, foaming dynamics and cell microstructure of styrene-methyl methacrylate copolymer/n-pentane foams

In the present study, two-step foaming procedure with a designed in-situ foaming observation apparatus was used to study the foaming dynamics of styrene-methyl methacrylate (St-MMA) copolymer/nanosilica composites. For this purpose, the St-MMA copolymer was synthesized using suspension copolymerization and its nanocomposites were prepared using solution method. The foaming dynamics was studied through temperature-induced (two-step batch foaming) method. Furthermore, the effects of content, size and surface chemistry of silica nanoparticles on the impregnation process, the foaming dynamics and the final morphology of prepared foams were investigated. The impregnation data showed that the presence of silica nanoparticles in matrix prolonged the impregnation and decreased diffusion coefficient. This effect would be clearer where nanoparticle contents are high and the temperature is quite above Tg. The foaming dynamics results illustrated that the nucleation and foaming rate were enhanced with the nanoparticle addition and its size reduction. At the St-MMA copolymer foaming system, the silica nanoparticles with hydrophilic surface chemistry are more efficient in comparison to hydrophobic nanoparticles where all foaming dynamics and the final microstructure parameters were improved.

Adjusting cell structure of polypropylene composite foams by controlling the size and dispersed state of NaCl particles during CO2 batch foaming process

In this work, a novel method for regulating cellular structure of polypropylene (PP) foam under CO2 batch foaming with water as disperse medium was established by addition of hydrophilic fillers (Such as NaCl). The presence of NaCl increased adsorption amount of water in PP/NaCl composites. As a result, both CO2 and adsorbed water acts as blowing agents during CO2 batch foaming. Thus the dispersed state and size of NaCl particles directly affect the mixed state of two blowing agents (water and CO2) and the competition between two types of heterogeneous nucleation from NaCl particles and PP crystals during foaming, which are the fundamental reasons for the formation of the foamed materials with different cellular structures, such as isolated large cell surrounded by uniform small cell structure, bimodal cell structure and homogeneous cell structure. Another unexpected finding was that the water absorption of PP/NaCl composites increased with the decrease of NaCl particle size, which is beneficial to play the in-situ cooling effect and co-blowing agent of absorbed water to control the foaming behavior of PP, and avoid the damage of foam mechanical properties due to the high content of filler.

Thermoplastic polyurethane foams: From autoclave batch foaming to bead foam extrusion

Two ester-based and one ether-based thermoplastic polyurethane grades have been used to produce thermoplastic polyurethane foams. The foaming process comprised pressure-induced batch foaming, foam extrusion, and bead foam extrusion by using an underwater granulator. Foam density and morphological properties, such as cell size, cell size distribution, and cell density, were measured through different analytical methods. Through autoclave batch foaming a minimum cell size of 10 µm and density of 202 kg/m3 is obtained, which is lower than the densities previously reported in the literature for thermoplastic polyurethane. Extrusion foaming however could not achieve the same range of foam expansion given that the lowest density achieved is 635 kg/m3 and a minimum cell size equal to 46 µm. The production of thermoplastic polyurethane bead foams is also reported for the first time. The minimum density of the obtained foamed beads is 306 kg/m3 and the lowest cell size is 55 µm.

Molecular origin of the foam structure in model linear and comb polystyrenes: II. Volume expansion ratio

The effect of the molecular architecture of a series of anionically synthesized linear and comb atactic polystyrenes (PS) on their foam properties including cell size, cell density, and volume expansion ratio (V.E.R.) was investigated. The comb-PS had the same molecular weight of the backbone, Mw,bb ≈ 290 kg/mol, Zbb ≈ 20 entanglements, and branches, Mw,br ≈ 44 kg/mol, Zbr ≈ 3 entanglements, but different numbers of branches, 3 ≤ Nbr ≤ 190. Batch foaming of well-purified linear and comb-PS using CO2 resulted in cell densities about 109 and 4 × 109 cell/cm3, respectively, which shows that LCB has no distinct effect on the cell density, whereas a small amount of residual impurity in linear PS reduced the cell density to ~108 cell/cm3. For a comb-PS series with the same entangled backbone, Mw,bb ≈ 290 kg/mol, Zbb ≈ 20 entanglements, and similar branches, Mw,br ≈ 44 kg/mol, Zbr ≈ 3 entanglements, but different numbers of branches, 3 ≤ Nbr ≤ 190, an increase in the Nbr to 120 with densely grafted comb conformation gradually increased the highest achievable V.E.R. close to a theoretical limit given by the CO2 solubility. The further increase of Nbr to 190 with a bottlebrush conformation reduced the V.E.R. of the foam. From a rheological point of view, this optimum Nbr was related to a comb-PS which showed the maximum strain hardening factor (SHF ≈ 200) in uniaxial extension, and simultaneously the minimum in the zero shear viscosity, η0. The optimum Nbr = 120 for this comb-PS series corresponds to an average spacing distance between two neighbor branch points of about Zs ≈ 0.2 entanglements.

Application of a CO2Pressure Swing Saturation Strategy in PP Semi-Solid-State Batch Foaming: Evaluation of Foamability by Experiments and Numerical Simulations

Compared with extrusion foaming, semi-solid-state batch foaming with CO2 can produce foams with small bubble size and high expansion ratio using the polymers with regular melt strength as the raw m...

Fundamental Influences of Crosslinking Structure on the Cell Morphology, Creep Property, Thermal Property, and Recycling Behavior of Microcellular EPDM Foams Blown with Compressed CO2

In this study, the crosslinked ethylene–propylene–diene monomer (EPDM) foams were prepared by batch foaming using compressed CO2 as the blowing agent. The various crosslinking structures, i.e., the...

Solubility and diffusivity of CO2 and N2 in TPU and their effects on cell nucleation in batch foaming

Abstracts Solubility and diffusivity of CO2 and N2 in thermoplastic polyurethane (TPU) were measured to distinguish the foaming behavior of TPU with different blowing agents. Measurements were conducted by magnetic suspension balance (MSB) and a high-pressure visualization chamber at temperatures from 190 °C to 210 °C and pressures up to 19.3 MPa. CO2 solubility was one magnitude higher than that of N2, and their solubilities had an inverse dependence on temperature. The foaming behavior of TPU with CO2 and N2 was observed in situ by a visualization batch foaming system, and was simulated by cell growth model. The cell density of foamed TPU increased with gas concentration, but it was higher with N2 than it was with CO2, despite the lower solubility of N2. This is different to the results for polypropylene (PP) and polystyrene (PS) foaming because of the low diffusivity and the low plasticization effect of N2 in TPU.

Design of bi-modal pore structure polyarylene ether nitrile/SiO2 foams with ultralow-k dielectric and wave transparent properties by supercritical carbon dioxide

The introduction of the pores into polymers is an efficient method to prepare the materials with low dielectric constant (ε) and low dielectric loss tangent (tan δ). Herein, low-weight polyarylene ether nitrile (PEN)/SiO2 foams with bi-modal pore structure were prepared via supercritical carbon dioxide batch foaming. The effects of SiO2 as heterogeneous nucleation on the dielectric properties, wave-transparent properties, thermal conductivity properties and mechanical properties of PEN/SiO2 foams have been investigated. Compared with PEN/SiO2 films, PEN/SiO2 foams showed excellent properties, which was most probably due to their uniform bi-modal pore morphology. As cell density and the SiO2 weight content was increased, ε and tan δ of the PEN/SiO2 foams decreased to 1.71 and 0.0047, respectively. The PEN/SiO2 foams exhibited ultra-low dielectric constant, showing low temperature and stabilization dielectric properties from −100 °C to 120 °C. Furthermore, the wave-transparent properties of PEN/SiO2 films and foams, which are required for wave-transparent application, were also investigated. The thermal insulating properties of PEN/SiO2 foams were remarkably improved after the pores and SiO2 fillers were introduced into the PEN polymer, with thermal conductivity reaching as low as 0.073 W/m K. PEN/SiO2 foams showed markedly increased elongation at the break, compared with PEN/SiO2 films, whereas tensile strength ＞50 MPa. The excellent properties of PEN/SiO2 foams indicated their potential as dielectric materials for microelectronics and wave-transparent materials for aerospace technology.

Effects of Tourmaline Concentration on Natural Rubber Foam

Natural rubber foam/tourmaline composite was prepared from natural rubber latex and vulcanized by sulfur curing system. Tourmaline powder was dispersed in potassium oleate before foaming production. The batch foaming production was used in this research. In order to investigate the effects of tourmaline concentration in natural rubber foam, mechanical properties, microstructure, and thermal skin temperature analysis were performed. Increasing of tourmaline concentration affected on high density and small microstructure of natural rubber foam, and the increasing of skin temperature. EDS result confirmed the good distribution of tourmaline powder in natural foam.

Effect of Resin and Blocked/Unblocked Hardener Mixture on the Production of Epoxy Foams with CO2 Blocked Hardener in Batch Foaming Process.

The present study focuses on the processing and properties of epoxy foams by the use of CO2 blocked hardener N-aminoethylpiperazine (B-AEP) and different resins. Although some studies described the foaming with carbamates, little attention has been given to the interaction of resin properties (such as viscosity) on the foaming performance. Therefore, two resins, diglycidyl ether of bisphenol-A (DGEBA) and epoxy novolac (EN), as well as their 50:50 blend, were foamed with B-AEP and unblocked/blocked AEP hardener mixtures in a batch foaming process. Furthermore, the commercially available chemical blowing agent para-toluenesulfonyl hydrazide (TSH) was used as a benchmark for commonly used chemical blowing agents. The lowest density in this study was reached by the DGEBA+B-AEP system in the range of 215 kg/m3 with the drawback of an inhomogeneous cell structure and high cell size distribution. The best cell morphology and lowest cell size distribution was reached with the EN+15:85% unblocked:blocked hardener mixture, resulting in a density in the range of 394 kg/m3. A syntactic foam was achieved by a DGEBA+50:50% unblocked:blocked hardener mixture with a density of around 496 kg/m3. It was found that a higher viscosity of the resin lead to an increase in the density and a decrease in the cell size distribution range as a result of a closer expansion time window.

Cellular morphology evolution in nanocellular poly (lactic acid)/thermoplastic polyurethane blending foams in the presence of supercritical N2

Currently, the formation of nanocells and complex cellular structure (CCS) in semi-crystalline polymer using supercritical fluids as blowing agent had become a big and newly developing challenge worldwide. In this paper, a facile methodology of polymer blending and batch foaming was proposed to fabricate CCS with large cells in micro-size and small cells in nano-size in semi-crystalline poly (lactic acid) (PLA) foams. Thermoplastic polyurethane (TPU) was introduced into PLA through melt mixing method to improve the viscoelasticity and foaming behaviors of PLA. Differential scanning calorimetry results showed that with the addition of TPU, the cold crystallization temperature of various PLA samples increased and their crystallinity kept unchanged almost. Compared with those of pure PLA, the complex viscosity, and storage modulus of PLA/TPU blends increased. Interestingly, with the increasing TPU content, the CCS appeared and became distinct in PLA/TPU blending foams as well as the thermal conductivity of various PLA foams decreased, respectively. The formation mechanism of CCS in nanocellular PLA/TPU blending foams was proposed and explained by schematic diagram. With the foaming temperature rising from 135 to 140 °C, a transition from nano-cells to micro-cells was also observed in PLA/TPU 5 blending foam, due to the decreased melt viscosity.

Preparation of polymeric foams with bimodal cell size: An application of heterogeneous nucleation effect of nanofillers

A new method was proposed for fabricating the polymer foams with bimodal structure. Briefly, nanoparticles and polymer powders were dry-mixed by ball-milling and then hot-pressed into the composites in which nanoparticles were selectively distributed at the interfaces between polymer multi-facets. Upon batch foaming using scCO2 as blowing agent, bimodal structure was generated due to the selective distribution and heterogeneous nucleation effect of the nanoparticles. Using this method, several bimodal composite foams based on different nanoparticles, i.e. graphene, carbon nanotubes, and silica nanoparticles and different polymers, i.e. thermoplastic polyurethane (TPU) and polystyrene (PS) were successfully fabricated. Specifically, TPU/graphene system was systemically studied. The results showed that the foam morphologies could be easily tuned by changing nanoparticle content, particle size of polymer powder, hot-pressing temperature and pressure, as well as foaming conditions (e. g. foaming temperature and pressure), indicating the great simplicity and tune-ability of this method.

Chemical Modification and Foam Processing of Polylactide (PLA).

Polylactide (PLA) is known as one of the most promising biopolymers as it is derived from renewable feedstock and can be biodegraded. During the last two decades, it moved more and more into the focus of scientific research and industrial use. It is even considered as a suitable replacement for standard petroleum-based polymers, such as polystyrene (PS), which can be found in a wide range of applications—amongst others in foams for packaging and insulation applications—but cause strong environmental issues. PLA has comparable mechanical properties to PS. However, the lack of melt strength is often referred to as a drawback for most foaming processes. One way to overcome this issue is the incorporation of chemical modifiers which can induce chain extension, branching, or cross-linking. As such, a wide variety of substances were studied in the literature. This work should give an overview of the most commonly used chemical modifiers and their effects on rheological, thermal, and foaming behavior. Therefore, this review article summarizes the research conducted on neat and chemically modified PLA foamed with the conventional foaming methods (i.e., batch foaming, foam extrusion, foam injection molding, and bead foaming).

Carbon Dioxide-Blown Expanded Polyamide Bead Foams with Bimodal Cell Structure

Bead foam has received substantial attention in recent years because of its high expansion ratio and ease of formability. Currently, most commercially available bead foams are produced from commodity plastics, and not many engineering polymers are used for this. In this study, the potential of polyamide 6 (PA 6), a typical engineering polymer, for bead foam applications was investigated by batch foaming. PA 6 was blended with its own copolymer to create double melting peaks for foam sintering. Chain extenders such as styrene maleic anhydride and Joncryl ADR 4368 C were added to improve the melt strength. After foaming, a crystal structure with double melting peaks was formed. In addition, the foam possessed a bimodal/nanocellular structure, and the foamed beads could be sintered. This is the first report of bead foam manufacturing using engineering plastics without toxic solvents. These results provide insights for bead foam research.