In an article recently published in the open access journal Materialsresearchers discussed the preparation and characterization of wood-based rigid polyurethane composites.
Study: Synthesis and Characterization of Rigid Polyurethane Wood Composites. Image Credit: Valentina Dudnichenko/Shutterstock.com
Wood is one of Canada’s most valuable natural resources, and a significant amount of waste is generated through the use of this resource. For each of the categories of residues, there are specific recovery channels, mainly for bioenergy uses. Wood chips are generally used in the production of pulp and paper. However, due to their poor wettability and negative impact on pulp production and paper characteristics, needle-like shavings and sawdust are underutilized.
The decline of paper, pulp and cardboard producers is another indicator of the crisis in the paper industry. As of 2010, wood chip manufacturing continues to be a concern for sawmills that cannot find customers, driving down price and revenue. Finding new markets is crucial to add value to woodchips. Adding natural fibers to polymer matrices, especially wood chips, is becoming increasingly popular. Indeed, previous research has shown that their use can be advantageous from an economic and ecological point of view.
Compressive strength and dimensional stability have been improved by adding cellulose nanocrystals to palm oil-based polyurethane foams. For optimum compressive strength and improved thermal insulation, the rigid polyurethane has been reinforced with wheat straw lignin. Lignin has also been shown to work well in place of polyol. A high percentage of components from bio-based sources must be introduced before bio-based products can be widely used. This objective could be achieved by using lignocellulosic fibers to reinforce the foams.
Digital image of mechanically mixed foams with 10, 20 and 30% fine fibers. Image credit: Bradai, H et al., Materials
About the study
In this study, the authors discussed the usefulness of different wood particles as reinforcement in rigid polyurethane foams (RPUF). Ground wood particles of various shapes and sizes, along with commercial polyol and isocyanate, have been used to make reinforced rigid polyurethane foams. The impact of fiber treatments and mechanical agitation on foam characteristics was also investigated. Additional tests were performed on polyisocyanurate (PIR) foams to see how reinforcement affected their characteristics. The impact of the wood particle reinforcement on the foam was studied by measuring the mechanical characteristics.
The team revealed the interaction between wood fibers and the matrix using confocal microscopy and Fourier Transform Infrared (FTIR) spectroscopy. Wood fibers from ground wood chips of various sizes and a commercial formulation of polyurethane were used to fabricate polyurethane biocomposites using a mechanical and manual approach.
Researchers examined the ability of fibers to reinforce a polyisocyanurate foam and the effect of acetylation on the processing and characteristics of RPUF biocomposites. The use of wood particles in polyurethane composites has been used to transform wood scraps and by-products into bioproducts that are more profitable than bioenergy. The understanding of the interfacial adhesion between the polyurethane matrix and the wood fiber was also facilitated by fibers showing important morphological and chemical differences.
Cell morphology and fibre/matrix interaction: (a) Clean foam, (b) PUR-05, (vs,D) PUR-01. Image credit: Bradai, H et al., Materials
The thermal conductivity of the PIR_01/05-10 foams increased by 16%, going from 0.147 W/mK for PIR0 to 0.171 W/mK for PIR_05-10. PIR_05-10 had a compressive strength of 439 kPa, compared to 230 kPa for PIR0. A 10% increase in reinforcement resulted in a large increase in density. The largest substantial increase was observed in PIR_05-10, which increased by more than 50% from 41 kg/m3 in PIR0 at 62 kg/m3 in PIR 05-10. Compressive strength decreased with increasing percentage of reinforcement in PUM MC foams, with a 15% drop between PUM_MC-10 and PUM_MC-30, notwithstanding the density fluctuation. Foam with kraft fibers (PUM_KR) had the highest density at reinforcement rates of 10 and 20 php.
As the percentage of reinforcement increased, the compressive strength (Rc) of PUR_01 foams decreased. Indeed, Rc went from 103 kPa for PUR_01-10 to 83 kPa for PUR_01-30. For a percentage of 20%, which is equivalent to nearly 120 kPa, this grain size has the best resilience. This denoted the greater compressive strength achieved by the formulation. For PUR_03-30, however, this resistance decreased until it reached 104 kPa. For PUM_01-30, the foam density was 103 kg/m3while for PUM_03-30 it was 81 kg/m3. PUM_01-10 had a maximum compressive strength of 400 kPa, while PU_01-10 had a maximum compressive strength of 120 kPa.
Despite some fiber fixation, the hardwood fibers pierced most of the RPUF cell walls, which explains the lower compressive strength of the composites for the hand-mixed foams. Mechanical agitation has proven to be an effective means of increasing the reinforcing strength of untreated fibers. When untreated wood flour was added to the mix, the characteristics of the RPUF foams similarly changed, which significantly increased the compressive strength.
Effect of particle size and percentage on total heat release and total oxygen consumption. Image credit: Bradai, H et al., Materials
In conclusion, this study has elucidated the development of RPUF of different filler types and sizes. The addition of fillers validated the interaction of the wood fibers with the isocyanate as well as their adhesion to the matrix. Coarse particles were found to be ineffective in enhancing RPUFs. In addition, compared to manual stirring, mechanical steering improved the compressive strength. The fiber treatments could not improve the mechanical capabilities of the foam, which was mainly due to the aggregation of the treated fibers, which resulted in anomalies in the three-dimensional structure of the foam.
Wood fibers increased the compressive strength of polyisocyanurate and polyurethane foams when added at 10 php. The wood fibers slightly increased the thermal conductivity, thermal stability and flame retardancy of the foams.
Bradai, H., Koubaa, A., Bouafif, H., et al. Synthesis and Characterization of Rigid Polyurethane Wood Composites. Papers 15(12) 4316 (2022). https://www.mdpi.com/1996-1944/15/12/4316