Effects of clay dispersion on the foam morphology of LDPE/clay nanocomposites

Structure-Property Correlation in Natural Rubber Nanocomposite Foams: A Comparison between Nanoclay and Cellulose Nanofiber Used as Nanofillers

Nanocomposite foams of natural rubber (NR) with 5 phr of two kinds of nanofillers, nanoclay (NC) and cellulose nanofiber (CNF), were produced using the latex mixing method and foaming with azodicarbonamide. The effect of the nanofiller on the structure and mechanical properties of NR foams was investigated through SEM, TEM, tensile tests, WAXD, and compression set measurements. Smaller cells with a narrower distribution were attained in the NC/NR foam when compared to the NR and CNF/NR foams, and the expansion ratio was larger due to the suppression of the shrinkage in the NC/NR foam. The foaming of the NR nanocomposites reduced the size of the filler aggregates and improved the dispersion and alignment of nanofillers in the cell walls. The addition of NC and CNF enhanced the tensile strength of the NR foam by 139% and 62%, respectively, without sacrificing the excellent strain of the NR, due to the acceleration of the strain-induced crystallization and small size of the filler aggregates. The compression set of the NR foam could also be reduced in the NC/NR foam compared with the NR and CNF/NR foams.

Open-Celled Foams from Polyethersulfone/Poly(Ethylene Glycol) Blends Using Foam Extrusion

Polyethersulfone (PESU), as both a pristine polymer and a component of a blend, can be used to obtain highly porous foams through batch foaming. However, batch foaming is limited to a small scale and is a slow process. In our study, we used foam extrusion due to its capacity for large-scale continuous production and deployed carbon dioxide (CO₂) and water as physical foaming agents. PESU is a high-temperature thermoplastic polymer that requires processing temperatures of at least 320 °C. To lower the processing temperature and obtain foams with higher porosity, we produced PESU/poly(ethylene glycol) (PEG) blends using material penetration. In this way, without the use of organic solvents or a compounding extruder, a partially miscible PESU/PEG blend was prepared. The thermal and rheological properties of homopolymers and blends were characterized and the CO₂ sorption performance of selected blends was evaluated. By using these blends, we were able to significantly reduce the processing temperature required for the extrusion foaming process by approximately 100 °C without changing the duration of processing. This is a significant advancement that makes this process more energy-efficient and sustainable. Additionally, the effects of blend composition, nozzle temperature and foaming agent type were investigated, and we found that higher concentrations of PEG, lower nozzle temperatures, and a combination of CO₂ and water as the foaming agent delivered high porosity. The optimum blend process settings provided foams with a porosity of approximately 51% and an average foam cell diameter of 5 µm, which is the lowest yet reported for extruded polymer foams according to the literature.

Polyolefinic nanocomposite foams: Review of microstructure-property relationships, applications, and processing considerations

In this review, we survey the state of the art on polymeric foams incorporating nano-scale fillers. Particular focus of the review is on foams from polyolefinic nanocomposite formulations incorporating a wide variety of fillers. The nano-scale additives can influence the foam structure and properties in two ways: Firstly, they can act as composite reinforcement to enhance the mechanical properties and functionality of the matrix polymer; and secondly, they can act as foaming-processing aids through modification of the rheological, thermal and crystallization properties of the matrix as well as serving as heterogeneous nucleation sites. Through a combination of these influences, and using advanced processing techniques it is possible to achieve nanocomposite foams that have higher cell density, and more uniform cell size or controlled cell-size distribution. Such controlled foam morphologies, in turn, can yield better specific mechanical properties resulting in more effective light-weighting solutions. Further, the nano-scale additives can impart additional desired functionality resulting in multi-functional foams. In this article, we provide an overview of the mechanical, thermal and a few other relevant functional properties – such as piezoelectric sensitivity, acoustics, and filtration efficiency – of foams prepared using nanocomposite formulations, along with the processing considerations for achieving high quality foams using such materials.

Increasing cell density/decreasing cell size to produce microcellular and nanocellular thermoplastic foams: A review

Nowadays, polymeric foams have attracted particular attention in scientific and industrial societies due to their unique properties, such as high strength to weight ratio, excellent thermal and sound insulation, and low cost. Researchers have shown that the extraordinary properties of polymeric foams such as superior thermal insulation, can be achieved by increasing the cell density/decreasing the cell size. In this regard, firstly, the most important foaming processes, i.e. batch, extrusion, and injection molding are studied in the present research. Then, cell nucleation stage as the most crucial phenomenon for achieving high cell density/small cell size is investigated in detail. In the next step, the most important researches in the field of polymeric foams are introduced in which the largest cell densities/smallest cell sizes have been achieved. The investigations show that the most remarkable results (highest cell densities/smallest cell sizes) belong to the batch process. Also, the use of nucleating agents, increasing the solubility of blowing agent into the polymer, and the use of nanoparticles are the most efficient solutions to achieve microcellular and nanocellular structures.

Low-Density Polybutylene Terephthalate Foams with Enhanced Compressive Strength via a Reactive-Extrusion Process

Due to their appealing properties such as high-temperature dimensional stability, chemical resistance, compressive strength and recyclability, new-generation foams based on engineering thermoplastics such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have been gaining significant attention. Achieving low-density foams without sacrificing the mechanical properties is of vital importance for applications in the field of transportation and construction, where sufficient compressive strength is desired. In contrast to numerous research studies on PET foams, only a limited number of studies on PBT foams and in particular, on extruded PBT foams are known. Here we present a novel route to extruded PBT foams with densities as low as 80 kg/m³ and simultaneously with improved compressive properties manufactured by a tandem reactive-extrusion process. Improved rheological properties and therefore process stability were achieved using two selected 1,3,5-benzene-trisamides (BTA1 and BTA2), which are able to form supramolecular nanofibers in the PBT melt upon cooling. With only 0.08 wt % of BTA1 and 0.02 wt % of BTA2 the normalized compressive strength was increased by 28% and 15%, respectively. This improvement is assigned to the intrinsic reinforcing effect of BTA fibers in the cell walls and struts.

Microcellular foaming by using subcritical CO2 of crosslinked and non-crosslinked LDPE/clay nanocomposites

The semicrystalline character of low density polyethylene adds severe difficulties to its foamability by a batch process in which the gas is dissolved into the polymer matrix under subcritical conditions. To improve the low density polyethylene foamability, two strategies have been used: the addition of nanoclays and a partial crosslinking of the polymer matrix. On the one hand, the use of nanoparticles is suggested because they act as heterogeneous nucleating sites reducing the cell size and increasing the cell density. On the other hand, crosslinking is also adopted as a solution because both the crystallinity (and hence, the gas solubility and diffusivity) and the extensional rheological properties of the polymer matrix are highly influenced by the crosslinking degree achieved. Results indicate that despite the fact that the presence of nanoclays deteriorates the rheological behaviour of the nanocomposites and, hence, the later foaming behaviour, the use of partially crosslinked polymer matrices allows achieving high expansion ratios (around 7.5) as well as enhanced cellular structures with cell sizes of approximately 15 µm.

Optimization of Dispersion of Nanosilica Particles in a PP Matrix and Their Effect on Foaming

Nanocomposites based on isotactic polypropylene (PP) and nanosilica (SiO2) were prepared using a co-rotating twin-screw extruder (TSE). The effect of operating variables, such as screw speed and screw configuration on the dispersion of nanosilica in the polymer matrix has been studied, using TEM imaging. High shear stress, sufficient residence time, and high fill ratio in the melting section of the screw were the most important factors in achieving good nanosilica dispersion. Furthermore, the effects of filler loading and amount of a maleated polypropylene (PP-g-MA) compatibilizer on the degree of SiO2 dispersion were investigated. The foaming performance of the composites was evaluated using a batch foaming simulation system, and an extrusion foaming setup that employed respectively N2 and CO2 blowing agents. Well-dispersed surface-modified hydrophobic SiO2 particles acted as effective nucleating agents for foaming, when used at loadings below 1 phr.

Preparation and foaming extrusion behavior of polylactide acid/polybutylene succinate/montmorillonoid nanocomposite

Polylactic acid (PLA)/polybutylene succinate (PBS) and the PLA/PBS/montmorillonite (MMT) composites were prepared by melt extrusion. The morphology structure, mechanical, thermal, and dynamic rheological properties of the composites were characterized, as well as foaming extrusion behavior of the composites was also studied. The results showed that MMT particles were intercalated into the PLA/PBS matrix forming an ‘intercalation’ structure. The toughness of PLA could be improved by blending with PBS. The comprehensive evaluation of mechanical properties of PLA/PBS was further improved by adding MMT. MMT could play the role of heterogeneous nucleation to improve the crystallization effect of PLA/PBS composite. The melt strength of PLA could be significantly increased by the synergies of MMT and PBS. Meanwhile, MMT could act as a compatibilizer to enhance the interfacial interaction between PLA and PBS. The synergies of PBS and MMT effectively reduced the density of PLA blend foam products, and increased their cell density, uniformity of cell size, and shape distribution.

Synthesis and expansion characteristics of expandable styrene/methyl methacrylate copolymer beads: The effects of monomer composition and cell structure modifying aid

Parts cast of metals such as iron, using expandable polystyrene foams, may have an unacceptable amount of surface defects, such as lustrous carbon. The use of foams made of styrene/methyl methacrylate copolymers can improve the quality of foam molds and metal parts made using such molds. In this work, expandable copolymers were synthesized by suspension copolymerization of styrene and methyl methacrylate in the presence of blowing agents (pentane isomers). When the surrounding temperature was high enough to vaporize the blowing agent and soften the polymer shell, the beads were expanded. The effects of monomer composition and cell structure modifying aid (polyethylene wax) on the particle size, average molecular weight and molecular weight distribution of synthesized copolymer, pentane content and expansion properties (density, residual pentane of pre-expanded particles and cell structure) were investigated. The results showed that monomer composition had a significant effect on the copolymerization kinetic and the copolymer molecular weight. In addition, cell structure, cell size and cell size distribution of the pre-expanded particles were strongly influenced by cell structure modifying aid concentration. The effects of comonomer composition and copolymer particle size on pentane content of expandable particles were also studied.

Synergistic Effects of Modification with Nanoclay and Polystyrene on the Foaming Behavior of a Random Copolymerized Polypropylene

Microcellular polypropylene/polystyrene/nanoclay (PP/PS/nanoclay) composites were processed in a continuous extrusion foaming system using supercritical CO2 as the foaming agent. Neat PP foam showed quite a broad distribution of cell sizes and thick cell walls. Under the same foaming conditions, nanoclay and PS were introduced in the foaming process of PP. Consequently, foams with uniform cell size distribution, reduced cell size of 10~30 μm and enhanced cell-population density of 2.16×108 cells/cm3 were obtained. The heterogeneous nucleation of PS and nanoclay dramatically increased the nucleation rate, decreased the nucleation time interval, and hence facilitated an almost instantaneous growth of cell size. This method also provided new insight into the mechanism of improving the foaming performance of PP.

Foaming Behavior and Cellular Structure of Microcellular HDPE Nanocomposites Prepared by a High Temperature Process

This article discusses the development of microcellular HDPE and HDPE—clay nanocomposites via a batch high temperature process using supercritical N 2. The study incorporates the effects of clay content and nanocomposite microstructure on the foaming process performance and cellular morphology under investigation. It was possible to produce much nucleated and more expanded microcellular foams with the nanocomposites than with the pure HDPE, as over 14 vol% void fraction could be reached at 6 wt% clay containing nanocomposite as a good result for foaming by N2 gas via a batch foaming process. We found that the state of nanoparticle dispersion affects the microcellular morphology, so a better dispersed nanocomposite results in a more nucleated system in the microcellular foaming process. Moreover, the relationship between crystalline morphology and cell structure was investigated. Crystallinity and melting point were the important parameters for controlling the cell growth mechanism in this foaming method.