Foaming of PLA Composites by Supercritical Fluid-Assisted Processes: A Review

Polylactic acid (PLA) is a well-known and commercially available biopolymer that can be produced from different sources. Its different characteristics generated a great deal of interest in various industrial fields. Besides, its use as a polymer matrix for foam production has increased in recent years. With the rise of technologies that seek to reduce the negative environmental impact of processes, chemical foaming agents are being substituted by physical agents, primarily supercritical fluids (SCFs). Currently, the mass production of low-density PLA foams with a uniform cell morphology using SCFs as blowing agents is a challenge. This is mainly due to the low melt strength of PLA and its slow crystallization kinetics. Among the different options to improve the PLA characteristics, compounding it with different types of fillers has great potential. This strategy does not only have foaming advantages, but can also improve the performances of the final composites, regardless of the implemented foaming process, i.e., batch, injection molding, and extrusion. In addition, the operating conditions and the characteristics of the fillers, such as their size, shape factor, and surface chemistry, play an important role in the final foam morphology. This article proposes a critical review on the different SCF-assisted processes and effects of operating conditions and fillers on foaming of PLA composites.

Compression behavior of soft PVC foams obtained by cardanol-derived plasticizer

This work is aimed to study the application of a bio-based plasticizer, obtained by acetylation and epoxydation of cardanol, for the production of soft PVC foams. The use of epoxidized cardanol acetate allowed obtaining a more efficient foaming of soft PVC compared to phthalate plasticizer bis(2-ethylhexyl) phthalate (DEHP), mainly due to the lower viscosity attained in the decomposition range of azodicarbonamide (AZDC). As a consequence, the foams produced by epoxidized cardanol acetate showed a lower density compared to those produced with DEHP. The lower density yielded lower values of compressive modulus. However, the modulus was shown to be not only dependent on the density, but also showed a direct dependence on the type of plasticizer used, in addition to processing temperature and AZDC content. As a consequence, the specific compressive modulus also showed a direct dependence on the type of plasticizer, processing temperature and AZDC content. Such dependence was explained by considering different cellular morphologies developed during foaming under different processing conditions, including type of plasticizer. In particular, it was shown that the lower viscosity attained by epoxidized cardanol acetate plasticized PVC involved an increase of the average pore size of the foam, which was shown to be the main cause of the variation of the specific compressive modulus.

Effects of CO2 and Talc Contents on Foaming Behavior of Recyclable High-melt-strength PP

This article presents an experimental study on the foaming behavior of recyclable high-melt-strength (HMS) branched polypropylene (PP) with CO2 as a blowing agent. The foamability of branched HMS PP has been evaluated using a tandem foaming extruder system. The effects of CO2 and nucleating agent contents on the final foam characteristics have been thoroughly investigated. Low density (i.e., 12-14-fold), fine-celled (i.e., 107-109 cells/cm3) PP foams were successfully produced using a small amount of talc (i.e., 0.8 wt%) and 5 wt% CO2.

Response surface optimization for producing microcellular polymethyl methacrylate foam using supercritical CO2

A regression model constructed by response surface methodology was employed to optimize the relationships between the cell density of microcellular polymethyl methacrylate foam and three independent variables: foaming pressure, temperature, and time. A Box–Behnken Design statistical approach was employed to fit the available response data to a second-order polynomial response surface model. The analysis of variance of the model indicated that the interactions between the foaming pressure and temperature, and that between the foaming temperature and the saturation time, both positively affect the cell density. Experimental verification of the predicted optimum conditions of foaming pressure = 21 MPa, foaming temperature = 313 K, and saturation time = 6.9 h gave an actual maximum cell density of 20.86 × 109 cells/cm3, which is close to the data predicted by the regression model.

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%.

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.

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.

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.