Cell morphology and property relationships of microcellular foamed pvc/wood-fiber composites

Preparation of short carbon fiber/polypropylene fine-celled foams in supercritical CO2

This article reports an attempt to improve polypropylene fine-celled foaming with the incorporation of short carbon fibers. The carbon fibers with uniform dispersion in polypropylene matrix were obtained in reference to the way of papermaking. The composites were prepared via melt compounding in a twin-screw extruder followed by injection molding. Three concentrations of carbon fibers, 5, 15 and 25 wt.%, were used. The foaming was carried out by a batch foaming process with supercritical CO2 as the physical blowing agent. The results showed the foamed composites had well-defined closed cell structure, but large amount of unfoamed regions and non-uniform cell size distribution were observed at the lowest carbon fibers content. Further incorporating carbon fibers decreased the crystallinity and increased the melt viscoelasticity. As a result, the composites had higher CO2 solubility with the increment of carbon fibers content. It is found that the foamed composite with 25 wt.% carbon fibers had the most uniform size distribution and better cell morphology compared to the other two composites. The increment in the cell density with increasing carbon fibers content indicated that the bubble nucleation was dominated by the heterogeneous mechanism with the addition of carbon fibers. The poorly bonded interface between carbon fibers and polypropylene could serve well as nucleation sites.

Cellular biocomposites from polylactide and microfibrillated cellulose

This paper describes the production of “green” microfibrillated cellulose-reinforced polylactide cellular biocomposites using a wet mixing technique combined with supercritical carbon dioxide foaming. The effect of composition on the morphology, density and compression modulus of foams was investigated for different processing parameters, along with the use of a chain extender to modify the melt elasticity. Foams with mean densities ranging from 0.18 to 0.32 g/cm3 were obtained for the neat polylactide and polylactide/5 wt% microfibrillated cellulose, respectively, and there was a corresponding increase in compressive modulus from 25 to 47 MPa. The addition of the chain extender is argued to compensate the molar mass loss induced by the different processing steps, promoting more uniform foam structures and allowing a density reduction of up to 75%.

Effects of Nucleating Agents’ Shapes and Interfacial Properties on Cell Nucleation

The classical nucleation theory (CNT) provides the fundamentals to understand the cell nucleation phenomena theoretically during polymeric foaming processing. Since a variety of additives and impurities are ubiquitous in commercial plastic resins and crevices are omnipresent on the internal walls of processing equipment, heterogeneous nucleation plays a significant role in plastic foaming. According to the CNT, interfacial tensions at different interfaces and surface geometries at various heterogeneous nucleating sites are important factors that govern the heterogeneous nucleation rate. Therefore, the elucidation of their roles in cell nucleation will be beneficial to the foaming industry, which can use the information to optimize its foaming technologies and develop effective nucleating agents. In this study, a series of sensitivity analyses were performed; the results indicated that the contact angle ( θc) at the gas—liquid—solid junction and the semi-conical angle ( β) at the nucleating site are critical parameters that govern the free energy barrier to initiate heterogeneous nucleation ( Whet). Moreover, regardless of the surface geometries of the nucleating agents, Whet decreases as θc increases. For moderate values of θc (e.g., θc = 90°), heterogeneous nucleation is promoted in conical cavities with a smaller β. However, a smaller β is unfavorable for cell nucleation when θc is too small.

Some properties of composite panels made from wood flour and recycled polyethylene

This study investigated the effect of board type (unmodified vs. MAPE modified) on the surface quality and thickness swelling-water absorption properties of recycled high density polyethylene (HDPE) based wood plastic composites. Additionally, two commercially available coatings (cellulosic coating and polyurethane lacquer coating) were also applied to composite surfaces and their adhesion strength, abrasion and scratch resistance, and gloss values were determined. This study showed that modification of the composites with MAPE coupling agent increased the surface smoothness and reduced the water absorption and thickness swelling of the panels. Abrasion resistance of the composites was also improved through MAPE modification. Regardless of board type, higher scratch resistance and gloss values were observed for polyurethane lacquer coated samples compared to those of cellulosic varnish coated ones. Improvement of adhesion strength was also seen on SEM micrographs.

Open-pore biodegradable foams prepared via gas foaming and microparticulate templating

Open-pore biodegradable foams with controlled porous architectures were prepared by combining gas foaming and microparticulate templating. Microparticulate composites of poly(epsilon-caprolactone) (PCL) and micrometric sodium chloride particles (NaCl), in concentrations ranging from 70/30 to 20/80 wt.-% of PCL/NaCl were melt-mixed and gas-foamed using carbon dioxide as physical blowing agent. The effects of microparticle concentration, foaming temperature, and pressure drop rate on foam microstructure were surveyed and related to the viscoelastic properties of the polymer/microparticle composite melt. Results showed that foams with open-pore networks can be obtained and that porosity, pore size, and interconnectivity may be finely modulated by optimizing the processing parameters. Furthermore, the ability to obtain a spatial gradient of porosity embossed within the three-dimensional polymer structure was exploited by using a heterogeneous microparticle filling. Results indicated that by foaming composites with microparticle concentration gradients, it was also possible to control the porosity and pore-size spatial distribution of the open-pore PCL foams.

Solid state microcellular foamed poly(lactic acid): morphology and property characterization

Poly(lactic acid) or PLA is a plant-based biodegrable plastic which exhibits many properties that are equivalent to or better than many petroleum-based plastics. However, there have been few commercial applications due to its lower impact resistance and higher cost than synthetic plastics. In this paper, the concept of creating microcellular foamed structures in PLA as a means to improve its shortcomings is presented. The effect of the foaming conditions (temperature and time) on the void fraction, volume expansion ratio, impact strength and tensile properties of foamed PLA is discussed. Each step of microcellular processing is addressed including: the manufacture of PLA film; the saturation of the samples with gas; the microcellular foaming of PLA; the void fraction determination, volume expansion ratio calculation, impact and tensile property characterization of foamed samples. The microcellular morphologies developed in PLA samples were a strong function of the foaming conditions. Due to the presence of foamed microcells, a twofold expansion ratio and significant improvements in the impact resistance (twofold increase over unfoamed PLA), strain at break (up to twofold increase over unfoamed PLA) and toughness (up to fourfold increase over unfoamed PLA) were achieved in PLA.

Determination of thermal properties and morphology of eucalyptus wood residue filled high density polyethylene composites

Thermal behaviors of eucalyptus wood residue (EWR) filled recycled high density polyethylene (HDPE) composites have been measured applying the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Morphology of the materials was also studied using scanning electron microscope (SEM). Addition of the EWR into the recycled HDPE matrix reduced the starting of degradation temperature. EWR filled recycled HDPE had two main decomposition peaks, one for EWR around 350 °C and one for recycled HDPE around 460 °C. Addition of EWR did not affect the melting temperature of the recycled HDPE. Morphological study showed that addition of coupling agent improved the compatibility between wood residue and recycled HDPE.

Thermal Degradation, Mechanical Properties and Morphology of Wheat Straw Flour Filled Recycled Thermoplastic Composites

Thermal behaviors of wheat straw flour (WF) filled thermoplastic compositeswere measured applying the thermogravimetric analysis and differential scanningcalorimetry. Morphology and mechanical properties were also studied using scanningelectron microscope and universal testing machine, respectively. Presence of WF inthermoplastic matrix reduced the degradation temperature of the composites. One for WFand one for thermoplastics, two main decomposition peaks were observed. Morphologicalstudy showed that addition of coupling agent improved the compatibility between WFs andthermoplastic. WFs were embedded into the thermoplastic matrix indicating improvedadhesion. However, the bonding was not perfect because some debonding can also be seenon the interface of WFs and thermoplastic matrix. In the case of mechanical properties ofWF filled recycled thermoplastic, HDPE and PP based composites provided similar tensileand flexural properties. The addition of coupling agents improved the properties ofthermoplastic composites. MAPE coupling agents performed better in HDPE while MAPPcoupling agents were superior in PP based composites. The composites produced with thecombination of 50-percent mixture of recycled HDPE and PP performed similar with theuse of both coupling agents. All produced composites provided flexural properties requiredby the ASTM standard for polyolefin-based plastic lumber decking boards.

Influence of Different Endothermic Foaming Agents on Microcellular Injection Moulded Wood Fibre Reinforced PP Composites

Microcellular wood fibre reinforced polypropylene composites, a new development using bio-fibre strengthened plastic, were prepared in an injection moulding process. The influence of three different endothermic chemical foaming agents was examined. The effects of various concentrations (1 to 4 wt.% of the composites) of the chemical foaming agent on the properties of the composites was studied with a view to establishing the concentration-structure-property relationships for these materials.

The influence of wood fibre type (hard wood and soft wood) on the microcellular structure and physico-mechanical properties of the composites was also investigated. Microcell morphology (cell size, shape and distribution) was observed using scanning electron microscope.

The chemical substance of different endothermic foaming agent affected the microcellular structure of hard and soft wood fibre-PP composites. Endothermic foaming agent with chemical substance polymeric microsphere (ESC 5313) showed finer microcellular structures compared to other foaming agents and 4 wt.% content of chemical foaming agent exhibits finer microcellular structures than the other contents. The reinforcing of soft wood fibre showed significantly finer microcellular structures than the hard wood fibre reinforcements. Density reduced maximum 30% and decreased up to 0.721 g/cm3 at soft wood fibre 30 wt.% content with coupling agent maleic anhydride grafted polypropylene (MAH-PP). With the addition of MAH-PP, specific tensile strength and specific flexural strength increased maximum 60% and 55% respectively with foaming agent ESC 5313 at soft wood fibre 30 wt.% content.

The Effect of Clay Content on PMMA-Clay Nanocomposite Foams

In this study the CO2 sorption at 45 °C in PMMA nanocomposite films containing 2 wt.% of nanoclay has been measured using an in-situ gravimetric technique. The films examined were prepared by compression moulding material obtained by dry-blend and solvent co-precipitation techniques. The CO2 diffusion coefficients were found to be higher for the dry-blended nanocomposite due to the larger agglomerations of the organoclay agglomerations, which prevented the polymer chains from fully wetting and intercalating the clay particles. The Tg-p profile for PMMA nanocomposite containing 2 wt.% nanoclay in the presence of CO2 was also measured using high-pressure DSC. The glass transition phase envelope was shifted vertically by approximately 10 °C when compared to the value reported in the literature for neat PMMA. This result suggests that the nanoclay affects the plasticization behaviour of PMMA under high-pressure CO2 conditions. The cellular morphologies obtained for these PMMA nanocomposite foams produced by batch processing with subcritical CO2 are strongly dependent upon the clay content and the dispersion of the nanoclay in the material. In the case of intercalated nanocomposites, most clay particles exist as agglomerated stacks of silicate sheets. On foaming the cells tend to form around the clay particles causing either irregular-shaped cells or layers to be produced. As a result, the cell density increases and the mean cell size decreases in the foamed nanocomposite on increasing the nanoclay content. Accordingly, the resulting cell structures are highly non-uniform and show large variations in cellular morphologies throughout the foam.

Polyethylene-Kevlar Composite Foams I: Morphology

Polyethylene-Kevlar composite foams were investigated in order to understand the effect of fibers. In this first part, cell sizes, cell density and cell size distribution of the foams were measured as well as the shear viscosity of the unfoamed composites and matrix. It is shown that reinforcing fibers can act as nucleation sites for foaming depending on the processing conditions of the composites. The results are discussed in terms of fiber content and polymer-fiber interface in relation with the rheological properties of the unfoamed materials.

A Factorial Design Applied to the Extrusion Foaming of Polypropylene/Wood-Flour Composites

A factorial design was performed to determine the statistical effects of material compositions and extrusion processing variables on the foamability of polypropylene (PP)/wood-flour composites. Two levels with centrepoint values of wood flour content, chemical foaming agent (CFA) content, extruder's die temperature and screw speed were employed. The isolated main and interaction effects of these variables on the void fraction of foamed composite samples were analysed using Design Expert software. Statistical analysis of data revealed that the void fraction data was best fit with a linear model. The extruder's screw speed showed no discernible effect within the narrow range studied (20 to 50 rpm) whereas the other three main factors showed significant effects (values of “Prob > F” less than 0.0001) on the void fraction. Wood flour content/CFA content and wood flour content/die temperature constitute the important interaction effects. The experimental results indicate that void fraction of extrusion foamed composites is a strong function of the extruder's die temperature. A large amount of gas molecules available for the cell growth is not the only requirement for the production of composite foams with a high void fraction. Processing at a high die temperature is also very important for the development of proper viscoelastic properties of the matrix suitable for cell growth.