A research project was carried out in collaboration with researchers from both University of British Columbia and University of Toronto to develop and test a range of hollow core composite sandwich panels based on lignocellulosic materials that can extend the current applications of wood composite products such as high density particleboard and fibreboard (hardboard and MDF). With proper engineering design and unique light weight structural features, wood fibre resources will be more effectively used and the performance of each component can be maximized in these types of novel composite panels. The outcome of this project is the development of Canadian-made light weight panels containing various low density cores, including honeycomb, low density wood wool composites and cup-shaped thin fibreboard, and high density surface panels, including plywood, hardboard and high density fibreboard (HDF) for the applications in ready to assemble (RTA) modular furniture, home and commercial cabinetry and door panels.
The work completed at Forintek included:
Development of low density wood wool panels (LCD) as the core material for the sandwich panels.
Development of cup-shaped high density fibreboard (CHDF) as the core material
Evaluation of edgewise and flat compression strength and creep behaviour of honeycomb sandwich panels fabricated by UBC.
Development of book shelf panels using four different core materials.
Performance evaluation of the book shelves developed.
The results of the experimental work suggest that:
Low density composite core materials can be made by the technology developed at Forintek laboratory using low density poplar wood wool and high viscosity phenol and formaldehyde resin with steam injection hot pressing technology. However, the strength of the panels was relatively low comparing to conventional low density particleboard, OSB or fibreboard.
The experimental work carried out on the cup-shaped high density fibreboard (CHDF) show the potential for developing various light weight core materials using current MDF process technology. The internal bond strength (IB) and water absorption (WA) of the cup-shaped panels were strongly correlated with panel density. IB increased and WA reduced when increasing the panel density. The flexibility of the technology could optimize the properties and performance of CHDF through manipulating the fibre refining process, profile design, resin system and hot pressing strategy. It shows that CHDF is a good alternative material to Kraft paper honeycombs for the manufacture of sandwich panels for higher strength and performance applications.
Test results from sandwich panels made of cup-shaped fibreboard core and HDF surface show that the nominal density of the cup-shaped core was one of the most important process parameters to adjust for the improvement of the sandwich panel properties. The flat compressive modulus, flat tensile strength and short-beam strength increased when increasing the nominal density of the core panels. Furthermore, the overall density of the sandwich panels were only fractionally increased by increasing the nominal density of the core panels due to the cup-shaped shape of the core panels. It suggests that higher nominal core density should be used when higher mechanical strength of the panels is required.
To a lesser extent, fibre type in the core panels also affects the sandwich panel properties. Longer wood fibres are recommended for use in the manufacture of the core panels.
The results of the experiment also show that increasing the thickness of the surface HDF panels increased the bending strength of the sandwich panels substantially. However, the overall density also increased.
Comparing shear properties of the four different sandwich panels developed by Forintek, we can identify that the ultimate shear strengths were different for different core materials. The sandwich panel made from polycarbonate core had the highest shear strength (0.744 MPa) followed by the panel made with CHDF (0.497 MPa). The sandwich panel made from low density wood wool core had much lower shear strength (0.012 MPa) which is lower than the paper honeycomb sandwich panels previously made by UBC with the same surface and core thickness (0.024 MPa).
The sandwich panels made with high density cup-shaped fibreboard had significantly higher core shear modulus (92.0 MPa) than any other sandwich panel studied in this project.
Le tronçonnage demeure pour la majorité des scieurs de bois feuillus un domaine problématique possédant un potentiel d’amélioration significatif, tant au niveau du volume sciable que du rendement valeur de la ressource disponible. La récupération de la valeur optimale d’une tige est directement liée à l’efficacité du préposé au tronçonnage. De mauvaises décisions de sa part résultent en une perte de valeur. Les principales raisons entraînant de mauvaises décisions sont la complexité et l’imprécision des lignes directrices, le grand nombre de classe de qualité, les exigences de productivité, le manque de formation et d’outils d’aide à la prise de décision. De plus, le nombre possible de combinaisons de longueur de billes et de découpes pour une même tige est assez important. L’évaluation d’une partie seulement des solutions potentielles requiert déjà un effort mental important.
Un système de tronçonnage complètement optimisé demeurera probablement une solution inaccessible pour la majorité des industriels à moyen terme. Cependant, la technologie des lecteurs et des caméras progressant très rapidement, il existe une possibilité de développer un système hybride qui pourrait générer des bénéfices importants. La ressource disponible est bien souvent de piètre qualité et il est envisageable de maximiser le volume de fibre sciable en optimisant le tronçonnage selon la courbure et la géométrie des tiges. Ce projet vise à chiffrer les bénéfices potentiels de cette approche de tronçonnage et d’en valider la faisabilité économique.
English version available: https://library.fpinnovations.ca/en/permalink/fpipub7315
Le présent manuel se veut une ressource didactique et un instrument de travail pour ceux et celles qui participent activement au séchage de sciages du groupe Épinette-Pin-Sapin (EPS). Il couvre un vaste éventail de sujets, depuis les principes de base du séchage jusqu'à l'application de techniques de séchage propres à ce groupe d'essences. La portée du contenu et la profondeur de son traitement ont été déterminées à la lumière de ces objectifs.
La diversité de la matière première au sein du groue EPS et ses incidences sur les décisions concernant le séchage ont fait l'objet d'une attention particulière. Les auteurs ont tenté de formuler des solutions englobant un vaste éventail de conditions touchant la ressource et les opérations de séchage. Les premiers chapitres du manuel traitent des connaissances préalables pour l'analyse des solutions éventuelles que le lecteur pourra retenir à la lumière de sa situation particulière.
Des études menées dans nos laboratoires constituent une importante toile de fond pour l'information présentée dans ce manuel. Les rapports techniques énumérés à la section Suggestions de lecture contiennent les résultats détaillés d'essais effectués chez FPInnovations - Division Forintek sur le séchage des sciages d'EPS.
Le séchage du bois d'oeuvre ne se limite pas aux seules actions qui se déroulent dans les séchoirs ou dans le parc de séchage à l'air. Un des objectifs du présent manuel est de démontrer que l'opérateur de séchoir doit tenir compte d'un grand nombre de facteurs lors du processus de séchage. Selon Joseph M. Juran, pionner dans le domaine de la gestion de la qualité, un processus est "... une série systématique d'action axées sur l'atteinte d'un but. La performance du processus et des variations excessives auront des effets directs sur les résultats financiers d'une entreprise". [traduction] Il ne s'agit pas d'éliminer toute variation, mais d'en comprendre la nature et l'origine afin de minimiser ses incidences sur les résultats de l'opération.
Une revue des interventions techniques et travaux de recherche effectués aux cours des dix dernières années par le département de Technologie de fabrication du bois de sciage de FPInnovations – Forintek a été réalisée pour définir le niveau de performance des différentes technologies et procédés utilisés dans les scieries de résineux de l’est du Canada. Le but visé est de prioriser l’amélioration des activités offrant un rendement monétaire intéressant et mieux cibler les efforts des ingénieurs de procédé responsables de l’amélioration continue.
Il existe une grande variabilité dans le niveau de performance des différentes technologies et procédés de fabrication. Selon le cas, les pertes ou les gains liés à leur utilisation peuvent être importants. Il est donc impératif de cibler les interventions qui génèrent des bénéfices rapidement et à moindre coût.
Experimental work was carried out to investigate the effect of replacing different types of non-wood fibre sources in MDF panels and the effect of their substitution levels on the mechanical and physical properties of the panels. Three types of non-wood fibre sources, switch grass, reed canary grass and corn stalks, were studied with different substitution levels, in normally and commonly used refining and hot pressing processes. The conclusions are made based on the results of the experiment:
In general, the non-wood fibres studied in this experiment, namely, switch grass, reed canary grass, and corn stalk, have lower acidity as compared to wood expressed by higher value of pH and acid buffer capacity. It suggests that the resin system needs to be adjusted for the effective use of these lower acidity raw materials if UF or MUF resin being used.
In terms of the internal bond (IB) strength of the panel, replacing wood fibre with non-wood fibre generally did not affect the panel IB as the substitution level was low. There was no detrimental effect to the IB when adding 12.5% of switch grass content. At the 50% of non-wood fibre substitution level, IB strength was almost the same for the panels made with 100% wood, 50/50 wood/reed canary grass, 50/50 wood/corn stalks, and 50/50 wood/non-wood fibres (consisting in equal amounts of switch grass, reed canary grass and corn stalks). The experiment proved that non-wood fibre can be a viable fibre source for the manufacture of MDF.
There was no detrimental impact on the panel thickness swelling (TS) and water absorption (WA) when replacing wood with non-wood fibres. However, under the process condition used in the experiment, both TS and WA were higher than normally expected even for the control panels using 100% wood.
There was no obvious difference in panel MOR and MOE when replacing the wood with non-wood fibres.
In comparison of different types of non-wood fibres studied in the experiment, they generally performed similarly in terms of panel mechanical and physical properties.
Nine structural adhesive formulations were selected to evaluate the effect of different curing methods on pH and alkalinity and/or acidity of adhesives. These included four phenol-formaldehyde (PF) resins with high pH, one phenol-resorcinol-formaldehyde (PRF) resin with intermediate pH, two melamine-urea-formaldehyde resins (MUF) with low pH and two melamine-formaldehyde (MF) resins with low pH. Four curing methods were used to prepare cured resin samples for the study: 1) curing at 102-105oC for one hour based on the CSA O112.6-1977 Standard; 2) curing for four hours at 66oC, followed by one hour at 150oC based on the ASTM D 1583-01 Standard; 3) curing at room temperature overnight based on the ASTM D 1583-01 Standard; and 4) cured adhesive collected from glue line squeezed-out from block shear assembly.
The effect of the curing method on pH of the cured adhesive strongly depended on the individual adhesives. For the PF, the alkalinity observed was different for each resin tested in the liquid form, while in the cured form, the difference in the alkalinity depended on the curing method. The MUF and MF were the most affected by the curing method, particularly the MUF, which showed much higher cured film pH values when tested by method 2 compared to the other three methods, while both the cured MF and MUF exhibited quite variable acidity values when measured with the different methods. The PRF showed reasonably uniform cured film pH but varying acidity values when measured with the different methods.
A reasonable relationship was observed between pH and alkalinity and between pH and acidity when the adhesives were considered as a group (i.e., adhesives of high pH as one group and adhesives of low pH as another group). Such a relationship was weaker when the adhesives were considered individually.
The use of a liquid sample in the determination of alkalinity/acidity of adhesives by titration was more convenient than using a cured film sample.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive standards. Part I. Investigation of different test methods for measuring pH, alkalinity and/or acidity of cured adhesive films and cured adhesives
Nine structural adhesives with varying pH were selected to examine the effect of pH on wood-adhesive bond quality. These included four high pH phenol-formaldehyde (PF), one intermediate pH phenol-resorcinol-formaldehyde (PRF), two low pH melamine-urea-formaldehyde (MUF), and two low pH melamine-formaldehyde (MF) adhesives. Block shear specimens were prepared with these adhesives using Douglas fir and black spruce. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum-pressure / re-dry (VPD) conditions.
Adhesive pH, test condition, and wood species showed significant effects on the shear properties. Douglas fir yielded about 40% higher shear strength at the dry condition compared to the VPD condition. Black spruce showed smaller difference in shear strength between the dry and the VPD conditions, the difference being only about 6%.
The different adhesives performed differently at the dry and VPD conditions. The high pH adhesives showed similar wood failures at both test conditions. On the other hand, the low pH adhesives showed high wood failure at the dry condition, but dropped significantly at the VPD condition for both species. This indicates that the low pH adhesives were less durable than the high pH adhesives.
Some correlation was observed between shear properties (strength and wood failure) and cured adhesive pH in the VPD condition, but not in the dry condition. Such a correlation was stronger in Douglas-fir than in black spruce.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive standards. Part II. Evaluation of block shear properties of selected wood adhesives by short term exposure test
This is a continuation of the short-term testing performed in Phase II of this study to determine the effects of adhesive pH on wood-adhesive bond durability. In this phase, the Douglas-fir block shear specimens prepared in Phase II using the nine structural adhesives, viz. four high pH phenol-formaldehyde (PF), one intermediate pH phenol-resorcinol-formaldehyde (PRF), two low pH melamine-urea-formaldehyde (MUF), and two low pH melamine-formaldehyde (MF), were tested periodically for up to 12 months under long-term vacuum-pressure / re-dry (VPD) condition. The VPD consisted of vacuum-pressure treatment followed by 0, 4, 8, and 12-month exposure durations at 50°C. The specimens were dried, in each exposure period, to their original moisture content prior to testing for shear strength and wood failure.
Indications of the extent of degradation of the wood layer, adjacent to the glue line due to pH during the long-term exposure, were also examined by the 1 % sodium hydroxide solubility test. The results indicated that the wood-layer samples closest to the glue line, which contained included-glue, showed higher solubility compared to those farther from the glue line. This suggests that wood degradation and/or potential glue decomposition occurred and is considered to be induced by the adhesive alkalinity or acidity under the long-term exposure conditions.
The PF showed the best durability performance followed by PRF and MF/MUF. The MF/MUF degraded completely after the 12-month exposure period.
For the PF, there are indications that some degree of degradation occurred in the wood layer adjacent to the bond line during the 12-month exposure period, which could be attributed to the high pH of the adhesives. This observation was not apparent for the PRF, and is considered inconclusive for the MF/MUF since they degraded during the exposure period.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive CSA standards. Part III. Evaluation of block shear properties of selected wood adhesives by long term exposure test
In April 2008, the State of California adopted an airborne toxic control measure (ATCM) to reduce formaldehyde emissions from composite wood products, proposed by the California Air Resources Board (CARB), part of the California Environmental Protection Agency. Phase 1 started in January 2009, and at the end of the implementation, in July 2012, formaldehyde emission limits will range between 0.05 and 0.13 ppm, depending on the type of products, based on the ASTM E 1333 Large Chamber Method.
These new limits are in the order of the limits of detection of the current analytical methods presently used, and rendered the chromotropic acid reaction, on which the ASTM E 1333 is based, with a limit of detection of 0.01 ppm less precise.
An alternative method to determine formaldehyde concentration in air has been developed to be used as part of the ASTM E1333 Large Chamber Method. 60 L of air are sampled through an impinger containing an acetylacetone-ammonia solution. The solution is then heated, and analyzed by fluorimetry using a Turner Quantech filter fluorometer equipped with a NB430 excitation filter and a SC500 emission filter. The test method is inexpensive, easy to use, compatible with the Large Chamber, Perforator and Desiccator Methods, and is very sensitive. The minimum detection limit (MDL) and the limit of quantification (LOQ) of this analytical method are 0.0004 and 0.0013 ppm, respectively.
Ce guide fournit les éléments principaux permettant d’atteindre une meilleure qualité de surface et de copeaux avec les équarrisseuses de type conique. Un guide de résolution de problèmes contient de l’information complémentaire selon les problèmes rencontrés. Des formules de calculs sont aussi expliquées.
A review of the technical interventions and research work carried out over the past ten years by Forintek’s Lumber Manufacturing Technology Department was undertaken to determine the level of performance of the various technologies and processes used in eastern Canadian softwood lumber mills. The goal was to prioritize the improvement of activities that offer an attractive monetary return and to more accurately focus the efforts of process engineers who are responsible for continuous improvement.
The level of performance of the various technologies and manufacturing processes varies greatly. Losses or gains associated with their use can be significant. This is why it is critical to focus on interventions that generate benefits quickly and at lower cost.
Determining wood moisture content of is a key factor at all stages of the manufacturing process. This is a critical operation to obtain quality products, reduce raw material waste and minimize problems in the use of finished products. The objective of this project was to develop procedures for assessing the moisture content of appearance wood during each processing stage, that is, from the sawmill to the kiln, from the kiln to secondary processing facilities and from these facilities to the end-user.
Using reliable, accepted and proven moisture content assessment procedures will benefit manufacturers and their business relations. Client-supplier business relations (internal or external) are founded on mutual trust and moisture content measurement is one of the main causes of disagreement. In addition, poor assessment of the moisture content of semi-finished or finished products can have costly consequences for business.
Part of this report focuses on basic concepts because it is critical to understand certain physical properties of wood in order to interpret moisture content measurements. These properties include moisture in wood, equilibrium moisture content, moisture gradient and fibre saturation point. It is also important to understand how variations in wood moisture content affect the dimensional stability of wood products. This calls for a discussion on shrinkage and swelling.
Another section of this report describes the three main methods for assessing moisture content: the oven-drying method, the use of DC-resistance moisture meters and the use of dielectric moisture meters, and explains the various factors affecting each method. Basic procedures for each method are presented and adapted to account for the condition of the lumber (green or dry, rough or surfaced, stacked or unstacked, stickered or solid-stacked, etc.) and the stage of the manufacturing process (lumber stacking facility, kiln, processing mill entrance, shipping/reception docks, finished products, etc). Finally, the report deals briefly with basic statistical and sampling concepts.
Experimental work was carried out for the manufacture of UF resin bonded particleboards with hardwood residues generated from the mushroom production. The panels were made with different mixing ratios of the residues and virgin wood particles using different UF resin contents. The effect of different mixing ratio on the properties of the particleboards was investigated.
The results show that particleboard can be made with the hardwood residues with adequate properties. In general, with UF resin content of 12%, panel IB reduced and WA increased when reducing the weight ratio of the virgin wood particles. No obvious changes in TS, MOR and MOE were observed when reducing the weight ratio of the virgin wood. With 100% hardwood residues, panel IB, MOR and MOE reduced and TS and WA increased when UF resin content reduced from 12% to 6%. The quantity of mold growth on different types of panel was similar, but less mold species were involved in the panels made of hardwood residues after the mushroom production. In addition, both pH and acid buffer capacity were measured for different type of raw materials and the result shows that both pH and acid buffer capacity were lower in hardwood residues than in virgin wood particles.
The impact of formaldehyde on human health causes great concerns nowadays. As it was one of the cheapest cross linking agents and a by-product of bio-process, finding an adhesive or resin product or material without formaldehyde and its derivatives is very challenging. Thus, one has to use a systematic method to work out the issue of reducing formaldehyde emission. However, as more people understand the challenge of reducing formaldehyde emission and endorsing the cost increase in developing new products with no or low formaldehyde emission, it provides a great opportunity to upgrade product line and develop new products.
In this project, bio-polymer, epoxy, polyvinyl acetate (PVA), urea formaldehyde (UF), phenol formaldehyde (PF), melamine urea formaldehyde (MUF), diphenylmethane diisocyanate (pMDI) resins and a new resin formulation developed at Mississippi State University (MSU) together with bio-based resins derived from bark, soy bean protein, lignin and wood have been tested. Epoxy, MSU, MUF, PF, pMDI, PVA, UF resins and soy bean protein and bark based PF resins were used for panel performance evaluation.
PVA and epoxy resins did not show any advantages in panel performance. MSU resin has potential in reducing panel free formaldehyde emission. PF resin will help improve panel modulus of elasticity (MOE) and modulus of rupture (MOR), thickness swelling (TS), water absorption (WA) and linear expansion (LE). It was found that MSU, MUF, PF and MDI resins offer great possibilities for medium density fiberboard (MDF) and particleboard applications.
A commercially available MUF resin was found especially suitable for particleboard application, in terms of low free formaldehyde emission and panel strength.
Mixing MDI with UF resin in particleboard application has potentially in improving panel internal bond strength (IB), MOE, and MOR performances and reducing panel TS and LE. It also has the potential to improve panel productivity.
A new three-layer with high surface or face moisture content (MC) concept was developed in this project. This concept was evaluated using different types of panel, especially high density fiberboard (HDF), MDF, and particleboard. The following variables were evaluated: UF face resin contents, panel face layer MC contents, MDI resin contents in panel core layer, hot press times, hot press temperatures, panel face and core ratio and types of face resins. At 11.1 mm (7/16 inch) panel thickness, the concept can mainly be applied successfully in making particleboard panels at a face:core ratio of 40:60, at 6% resin in face and 9% resin in core layers, with UF, PF, MDI, a combination of UF and MDI and a combination of UF, MDI and PF resins, with face MC up to 20% and core MC 6%, hot pressed at 180°C and 130 seconds. The particleboard panels made had improved panel performance, especially in terms of surface quality, when compared with panels made using conventional processing. To better use the concept, resins with hydrolysis resistance may be required.
Further, the three-layer with high face MC process concept was used to make HDF without resin in face layers. The same concept has also been applied to MDF panel manufacturing. It was found that the concept can be more easily adopted with MDI resin in MDF operation. With high MC in the face and with a small amount of resin in the core layer, one should be able to make HDF to meet the performance requirement. Generally, the experiment has shown that the new process concept has big potential in reducing panel operation cost with improved performance.
It was recommended that FPInnovations should do further work with the panel industry to allow the research results to be transferred to industry as soon as possible. Any resin with a renewable feature has great potential. Bark and wood should be considered first throughout the process to convert soy bean protein, wood and bark into resin or parts of resin.
La détermination de la teneur en humidité du bois est un facteur clé lors de toutes les étapes de transformation afin d’obtenir des produits de qualité, diminuer les pertes de matière première et minimiser les problèmes lors de l’utilisation finale. L’objectif de ce projet est de développer des procédures d’évaluation de la teneur en humidité des bois d’apparence pour les différentes étapes de transformation, soit de l’usine de sciage au séchoir, du séchoir à l’entrée de l’usine de transformation, en cours de transformation et de l’usine de transformation aux utilisateurs finaux.
L’utilisation de procédures d’évaluation de la teneur en humidité fiables, reconnues et éprouvées sera bénéfique aux manufacturiers et leurs relations d’affaires. Les relations entre les clients et les fournisseurs (internes ou externes) reposent sur la confiance mutuelle et la mesure de la teneur en humidité est une des principales causes de mésentente. De plus, une mauvaise évaluation de la teneur en humidité sur les produits semi-finis ou finis a des conséquences coûteuses pour les entreprises.
Une partie de ce rapport est consacrée aux notions de base car il est primordial de comprendre certaines propriétés physiques du bois pour interpréter des mesures de teneur en humidité, Parmi celles-ci, notons l’humidité dans le bois, la teneur en humidité d’équilibre, le gradient de teneur en humidité et le point de saturation des fibres. Il est aussi essentiel de connaître la relation entre les variations de la teneur en humidité du bois et la stabilité dimensionnelle des produits en bois, soit les notions de retrait et de gonflement.
Une autre partie décrit les trois (3) principales méthodes de détermination de la teneur en humidité, soit la méthode au four, par humidimètre à résistance et par humidimètre diélectrique et explique les différents facteurs qui affectent ces méthodes. Des procédures de base pour chaque méthode sont présentées et aussi adaptées pour tenir compte de l’état du bois (vert ou sec, brut ou raboté, empilé ou non, sur lattes ou solide, etc.) et de l’étape de production (usine de lattage, séchoirs, entrée de l’usine de transformation, réception de camion, produits finis, etc.). En dernier lieu, les notions de base de statistiques et d’échantillonnage sont abordées sommairement.
Supplementary fibre supplies for the manufacture of MDF and particleboard. Part VI. Process development for the manufacture of particleboard using steam explosion treated wheat straw and combination resin system
Experimental work was carried out for the manufacture of UF resin bonded wheat straw particleboards with various steam explosion treatments and a combination resin system. Panels with different combinations of wood and straw particles were also made. The effect of different treatment conditions (i.e., steam temperature and retention time), resin combination and mixing proportion of wood and straw materials on the properties of boards was investigated. To clarify the mechanism of bondability improvement, chemical characterization of wheat straws was conducted to identify the acidity, wettability, and silicon content of the raw material before and after treatment. Two panels were produced for each condition, and a total of 30 boards were made. The properties of panels were tested in accordance with ASTM D1037.
The acidity of the wheat straw was low and wettability was very poor before the treatment. Both acidity and wetttability were increased after steam explosion treatment. The ash and silicon contents of the wheat straw before the treatment were much higher than those of wood material. Part of ash and silicon can be removed or washed out by steam when the steam was released after the treatment, therefore resulted in the decreased silicon content. The increased acidity and wettability as well as the decreased silicon content contributed to the bondability improvement between straw particles and UF resin.
The results show that the steam explosion treatment and/or its combination with combination resin system can be a feasible approach to improve the bonding strength between wheat straw materials and adhesive binders. Generally, panel properties in terms of bending strength (MOR and MOE) and internal bond (IB) strength increased, thickness swelling (TS) and water absorption (WA) decreased for the panels with the steam explosion treated straws. This indicates that steam explosion treatment improved all the panel properties. The better performance of the boards with treated straws are attributed to the improved bondability between straw particles and UF resin due to increased acidity and wettability as well as the decreased silicon content after treatments.
The effect of the combination of resins and treatments on the panel properties shows that the MOR and MOE values for the panels from the straws after 12 h water soaking treatment were significantly improved at the UF/MDI combination of 10%/2%. The IB strength dramatically increased with the use of combination resins. The TS and WA showed the decreased values for all the panels made with the combination of UF and MDI resins. The results indicate that the combination of dual resin system and steam explosion treatment can significantly improve the panel performances of straw boards.
The properties of mixed straw and wood particleboards indicate that there was a decreasing trend of the MOR, MOE, and IB with increase of straw addition contents while a increasing trend in TS and WA. Pure wheat straw board had the lowest bending and IB values, and highest TS and WA. The inferior strength and dimensional stability properties of pure straw boards were due to the poor bonding between the straw particles caused by the outer waxy layer and high silica content of straw materials. The use of wood particles to replace part of straw materials can help improve the panel properties.