Critical processing temperature in the manufacture of fine-celled plastic/wood-fiber composite foams

Density graded polymer composite foams

This article reviews the main developments associated with density graded polymer composite foams. After a short introduction, a discussion is made on both density graded polymer-based composites and foams separately to better understand the challenges when both structures are combined together to produce composite foams. In all cases, the basic concepts, fabrication methods, main properties and general applications are presented. Opening for future works are also presented to conclude.

Low-Density Particleboards Modified with Blowing Agents—Characteristic and Properties

Although lightweight particleboards have been commercially available for years, they still have a number of disadvantages, including difficulty to process, brittleness, low impact strength, and other mechanical resistance. The aim of the paper was to determine the possibility of producing particleboards of reduced density (dedicated for furniture industry) as a result of using blowing agents from the group of hydrazides, dicarboxamides, or tetrazoles, which were modifiers of the adhesive resin used for bonding the particles of the core layer of three-layer particleboards. The concept presents the possibility of producing low-density particleboards in a standard technological process by modifying the adhesive resin, which has not been practiced by others until now. Analysis of the results of testing the particleboards properties with various types of modifiers (blowing agents), glue content (high 10%/12% and low 8%/10%), differing in glue dosing method, and different particle sizes allowed concluding that the most satisfactory effect was found in particleboards made of the variant modified with p-toluenesulfonyl hydrazide. This variant was characterised by the highest mechanical properties (bending strength, modulus elasticity, and internal bond strength) with high dimensional stability. The presented technology proposal can be applied in the industry.

Low-Density Particleboards Modified with Expanded and Unexpanded Fillers—Characteristics and Properties

Reducing the density of wood-based materials is a desirable research direction in the development of the wood-based materials sector. Even though lightweight wooden particleboards have been commercially available for many years, they still have a number of disadvantages, especially their low strength parameters. The aim of this paper was to determine the possibility of producing particleboards of reduced density for use in the furniture industry, as a result of using expanded polystyrene and two types of microspheres (expanded and unexpanded) to modify the core layer of three-layer particleboards. Analysis of the results of testing the particleboards’ properties when using various types of modifiers (expanded and unexpanded fillers), urea formaldehyde (UF) glue content (high: 10%/12% and low: 8%/10%), various glue-dosing methods, and different particle sizes, allows us to conclude that the most satisfactory effect was found when using EPS. One partly positive effect was observed when using the Expancel-type 031 DU 40 as a filler; therefore, it is recommended that research be continued in this area. Using microspheres that have not been used before as a filler in the production of wood-based panels is the novelty of the presented research. The proposed technology has potential for application in the industry.

3D Printable Filaments Made of Biobased Polyethylene Biocomposites

Two different series of biobased polyethylene (BioPE) were used for the manufacturing of biocomposites, complemented with thermomechanical pulp (TMP) fibers. The intrinsic hydrophilic character of the TMP fibers was previously modified by grafting hydrophobic compounds (octyl gallate and lauryl gallate) by means of an enzymatic-assisted treatment. BioPE with low melt flow index (MFI) yielded filaments with low void fraction and relatively low thickness variation. The water absorption of the biocomposites was remarkably improved when the enzymatically-hydrophobized TMP fibers were used. Importantly, the 3D printing of BioPE was improved by adding 10% and 20% TMP fibers to the composition. Thus, 3D printable biocomposites with low water uptake can be manufactured by using fully biobased materials and environmentally-friendly processes.

Foam extrusion of PP-based wood plastic composites with chemical blowing agents and the Celuka technique

Wood plastic composites have gained relevance in recent years as an alternative to wood boards. However, because the cavities in wood fibres are compressed by high processing pressure during the extrusion of wood plastic composites, the product densities show a range of up to 1.5 g/cm3 depending on wood content and base material. Particularly in large-sized products, this may be disadvantageous for processors and end users. Foaming of the plastic matrix is a promising approach to reduce the density of wood plastic composites products. This article discusses the foam extrusion of PP-based wood plastic composites with chemical blowing agents in combination with the Celuka technique. Integral wood plastic composites foam with a rigid and plain outer layer was produced using a parallel, counter rotating twin screw extruder. The profiles obtained were analysed with respect to foam structure and mechanical properties. It was possible to achieve a density reduction of up to 0.7 g/cm3 in the foamed wood plastic composites profiles. Furthermore, we demonstrate that wood fibre length and type of chemical blowing agent have a strong effect on the resulting foam morphology.

Foaming behavior of cellulose fiber-reinforced polypropylene composites in extrusion

This paper investigates the effect of polypropylene type and cellulose content on the foaming behavior of cellulose fiber-reinforced polypropylene composites in extrusion. Two types of polypropylene (linear and branched structures) were used as a polymer matrix. The thermal properties of the composites were characterized by a differential scanning calorimeter, and the viscosity of the composites was evaluated by a rotational rheometer. The foaming behavior of the composites was examined using an extrusion foaming system, in which carbon dioxide was used as a physical blowing agent for foams. The results suggested that the cell density increased with the increase of cellulose content. On the other hand, the void fraction decreased with the addition of cellulose, but the void fraction at the 40 wt% cellulose was higher than that at the 20 wt% cellulose. The results also indicated that the two types of polypropylene had a minimal effect on the foaming behaviors of the cellulose fiber composites.

The potential of wood fibers as reinforcement in cellular biopolymers

Wood fiber-reinforced polylactic acid composite foams have been successfully produced using supercritical carbon dioxide. The addition of fibers had a strong effect on microstructure of the foams. An increase in wood fiber content implied smaller average cell size and higher average cell wall thickness as estimated from image analysis of scanning electron microscopy micrographs. Addition of 10 wt% wood fibers seemed to be a limit to obtain foams, with the used processing conditions.

The stiffness properties of the foams in compression improved upon addition of wood fibers. A significant increase of specific stiffness was achieved by adding 5–10 wt% wood fibers. It was shown that the stiffness was about 50% higher in the transverse direction for reinforced foams. The strength in the transverse direction increased for foams with unmodified wood fibers but decreased for foams with two types of treated wood fibers as compared with the strength of the pure polylactic acid foam of similar density. A butyl tetracarboxylic acid treatment followed by an additional surfactant treatment results in reduced wood fiber network-forming ability and reduced fiber–matrix adhesion. This contributes to the inferior observed strength properties in this study.

The experimental stiffness was comparable with a superposed micromechanical model for a three-phase fiber-reinforced foam. The model shows that increasing the relative density, that is, the ratio of the density of the foam to the density of the composite material, by adding wood fibers results in a noteworthy increase in the transverse compression stiffness of the foams but only at relative density values above 0.2 for the used processing conditions in this study. The key factor for reinforcement is the relation between foam relative density and fiber volume fraction in the preform. The foaming conditions have to be adapted for each wood fiber content to obtain foams with the desired relative density.

Critical Issues in Extrusion Foaming of Plastic/Woodfiber Composites

Foaming of wood-fiber/plastic composites (WPC) with a fine-celled structure can offer benefits such as improved ductility and impact strength, lowered material cost, and lowered weight, which can improve their utility in many applications. However, foaming of WPC is still a poorly understood art. This paper presents a review of material published, which address the various critical issues particularly in extrusion foaming of WPC, and the proposed processing techniques and strategies, for producing artificial wood with enhanced properties.