Transformative Technologies - Development of "Green" Wood Adhesives for Wood Composite Products
Chitosan is an amino polysaccharide obtained from the deacetylation of chitin, which is naturally occurring in the shells of a large number of marine crustaceans. Chitosan is soluble in weakly acidic aqueous solutions and possesses adhesive properties. Chitosan has received much attention for medical and industrial applications; however, only limited studies have been conducted on the application of chitosan as a wood adhesive, because its bonding properties on wood are poor. To improve the adhesive quality of chitosan resin, an innovative study on chitosan adhesives has been conducted to use selected fungal species to modify chitosan and improve its bonding properties, to synthesize non-formaldehyde resins with the fungus-modified chitosan, and to enhance urea-formaldehyde (UF) and phenol-formaldehyde (PF) resin performance with the fungus-modified chitosan.
The bonding properties of wood composites made with these chitosan-based green wood adhesives were significantly improved, in terms of lap-shear strength. Unmodified chitosan solution was not compatible with ammonium lignosulfonate, liquid PF resin, soybean resin, powder PF resin, or soybean flour, but was compatible with UF resin, polyvinyl acetate (PVA) resin, and phenol. With the addition of chitosan in UF and PVA resins, both the dry and wet shear strengths of plywood panels were improved, compared with those of panels bonded with the control UF and PVA resins, i.e. without chitosan. A number of chitosan and chitosan-reinforced UF resins were prepared as a binder for particleboard panel manufacturing. Six (6) types of particleboard panels with different levels of resin loadings and press conditions were manufactured. The resulting boards were tested to evaluate the bond quality of the chitosan and chitosan-reinforced UF resins. The test results showed that particleboard panels with good visual quality could be produced with all formulations of chitosan-UF adhesives, even with resin systems made with 1% of chitosan resin only. All chitosan resins used alone or added to UF resins yielded panels with better internal bond (IB) strength than those made with the UF control resin. The panels made with 1% chitosan resin plus 66% UF resin in a 1:1 ratio yielded panels with the highest IB strength and the best overall mechanical properties.
A round robin study using the CSA O112.9-10 standard was conducted with three (3) participating test laboratories in Canada as recommended by the Standards Council of Canada (SCC). The study included only the block shear test under three test conditions (dry, vacuum-pressure, and 9-cycle boil-dry-freeze). Test specimens were prepared using black spruce as substrate and phenol-resorcinol formaldehyde adhesive as the bonding agent.
All participating test laboratories passed the shear test of the CSA O112.9-10 standard in which the requirements are based on quantile parameters (median shear strength, median and lower quartile wood failure). This finding showed that the block shear test method of this adhesive standard was reproducible among three well-known test laboratories in Canada, all of which have been qualified by the SCC for the ISO 17025 accreditation program.
The test laboratories, however, exhibited some differences in terms of average shear strength and wood failure in the three test conditions. Some possible reasons were suggested for these differences.
Chitosan is an amino polysaccharide deacetylated from chitin, which is naturally occurring in large amount in shells of marine crustaceans. Chitosan is soluble in weakly acidic aqueous solutions and possesses an adhesive property. Chitosan has received much attention for medical and industrial applications; however, only limited studies have been conducted on the application of chitosan as a wood adhesive because of its bonding properties on wood are poor. To improve the adhesive quality of chitosan resin, an innovative study on chitosan adhesives has been conducted to use selected fungal species to modify chitosan and improve its bonding property, to synthesize non-formaldehyde resin with the fungus-modified chitosan and to prepare UF and PF resins enhanced with the fungal modified chitosan. Bonding properties of wood composites made with these chitosan-based green wood adhesives in terms of lap-shear strength were significantly improved in this study. Unmodified chitosan solution was not compatible with ammonium lignosulfonate liquid, liquid PF resin, soybean resin, PF powder, or soybean flour, but was compatible with UF resin (liquid), PVA resin, or phenol. With addition of chitosan in UF and PVA resins, both dry and wet shear strengths of plywood panels were improved comparing with the use of the control UF and PVA resins without chitosan. A number of chitosan and chitosan-reinforced UF resins as binder for particleboard manufacturing have been prepared. Six (6) types of particleboards with different levels of resin loadings and press conditions were manufactured and evaluated for the bond quality of chitosan and chitosan-reinforced UF resins. The results showed that all formulations of chitosan-UF adhesives were able to produce particleboards with nice appearance, even those made of only with 1% of chitosan resin alone. All chitosan resins, alone or added to UF resins, had a better IB strength than UF control resin. The panels made of 1% of chitosan resin plus 66% of UF resin in a 1:1 ratio had the highest IB strength.
The relationship between proof load level of fingerjoined lumber and degree of cure of adhesive bonds was investigated. Tension tests were completed for two different degrees of cure for two different adhesives. The proof load level determined for the partially cured joints did not cause damage to the joints that survived the proof test.
Preliminary guidelines for determining appropriate proof load levels for testing fingerjoined lumber with partially cured joints were proposed. The proposed guidelines will need to be validated through mill trials to demonstrate their efficacy and reliability to the manufacturer and third party inspection agency.
Keywords: fingerjoined lumber; tension proof testing/loading; partially cured adhesive bonds.
Lignin or lignin residue by-product can be produced from black liquor of pulping process or from cellulosic ethanol process. This project studied these lignin by-products as a substitute for corncob/Superbond fillers in PF glue mixes for plywood production. Two types of lignins were evaluated in the lab tests and one of them was used in the mill trial production. PF glue mixes with up to 50% corncob/Superbond substituted with lignin were produced and evaluated for plywood production. The test glue mixes were comparable to the control in viscosity, stability and bond performance. It is technically viable that lignin or lignin residue by-product be used as a substitute for corncob/Superbond fillers in the plywood industry. Caution must be taken to ensure that all particles of lignin pass a 100-mesh screen and the glue mix passes a 40-mesh screen in order for the glue mix to be smoothly transferred from the storage tank to the glue mix applicator.
Robatech’s glue application system for finger-jointer consists in a pumping unit and application heads with multiple nozzles intended for jetting glue directly into machined finger profiles. The novel system was tested onto a CRP 2000 finger-jointing machine located at the FPInnovation – Forintek laboratory with three types of glue onto 2 X 4 SPF blocks.
For the first run test, the system was found fairly complicated to adjust, as much in terms of its positioning in reference to the finger profiles as for the amount of glue applied. The trial with standard PVA glue resulted with a too important quantity of glue unevenly applied. There were also issues with clogging as the injectors are air tight, but not the nozzles. The test with the modified PVA glue was interrupted shortly after beginning when the mixture turned into a foamy substance that could barely be sucked by the pumping unit.
The delaminating test that was conducted for this run had 100% of the joints failing performance criteria. Following these results, the bending test scheduled was cancelled.
In its initial format, the glue application system assembled by Robatech is not suitable for industrial use. The application head required some modification to be less tedious to adjust and the manufacturer was compelled to provide an elegant solution for the issues regarding clogging and application quality of the initial nozzle configuration.
In the second trial, the system was much easier to set-up and the glue application was achieved in a more controlled fashion. There was no clogging issue as the type of glue used (Franklin Advantage 405) was more permissive in term of curing time. The delamination and bending tests for the second trial run had both positive outcomes.
Following the results of the tests performed at FPInnovations with Robatech’s gluing system; the latter has proven to be a potential substitution for existing applicators. More testing with different glues and some improvements are be needed to fully proof the system. Robatech has proven to provide adequate solutions to issues following the first testing session and came up with more ideas as a result of the second trial run.
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
Waferboard/oriented strandboard (OSB) has been traditionally manufactured almost exclusively with trembling aspen in Canada. With the declining availability of aspen wood resource, OSB mills have begun to use alternative species in their production, usually other hardwood species. Meanwhile the mills have also begun experiencing some constraints in the use of phenol-formaldehyde (PF) resins for bonding these species, such as poor resin distribution and low retention on the surfaces of strands, particularly for a powdered form. The change of binder systems for bonding dense hardwoods can be extremely costly to OSB producers. The objective of this study is to determine the optimum adhesive and process requirements for manufacturing OSB from high-density hardwood furnish.
Study in 1997-1999 has shown that a dense wood is more difficult to bind than a less dense wood with a powder phenolic resin due to the poor resin distribution and retention. A liquid resin appears to produce stronger panels compared to a powder resin. The powder resin blending and bonding efficiency could be apparently improved by different resin application methods: (1) enhancing the wax distribution; (2) separately applying wax and resin to individual species strands and then mixing them up; (3) spraying a small amount of liquid additives after resin application; and (4) finally using small particle-size powders. A series of strandboards were constructed with aspen, birch, southern yellow pine, and sweetgum, using powder PF resins in the face and powder PF, liquid PF or diphenylmethane diisocyanate (MDI) in the core. An overall comparison showed that the aspen panels performed best followed by the southern yellow pine panel while the birch and sweetgum panels performed similarly with regard to both physical and mechanical properties.
Work in 1999-2000 focused on some fundamental studies on wood chemical and physical properties. Seven wood species were characterised for pH, base and buffering capacities, bound and soluble acids, and water and ethanol-toluene solubility. The wood species included aspen, white birch, yellow birch, red maple, sugar maple, southern yellow pine, and sweetgum. The study was also extended to the mixed wood species, included white/yellow birch, aspen/birch, southern yellow pine/sweetgum, aspen/red maple, aspen/sugar maple, and aspen/red maple/birch. This work indicated that there were significant differences in the chemical characteristics between the species investigated. These wood species were also characterised for surface roughness using a surface roughness tester. It was found that aspen strands showed significantly rougher surfaces than did southern yellow pine, sweetgum, and sugar maple. Strand surface characteristics seem to be related to the wood anatomical structure. A species (like aspen) having low density appears to yield a rougher surface than does one having high density (like sweetgum).
In the coming year, the efforts will focus on characterisation of wood/resin interaction, modification of phenolic resin, and optimization of panel manufacturing process parameters in order to more efficiently utilise various high-density hardwood furnishes for OSB production based on the information obtained in the previous studies of this project. The detailed information on project plan and milestone is illustrated in Appendixes 1 and 2.
Technological innovation has proven to be the prime vehicule for the remarkable development of the wood composite panel industry in Canada over the last half century. The challenge for the future is to maintain the place and to make appropriate choices in setting the direction of technological innovation. The Roadmap is intended to help the industry meet this challenge by establishing the importance of innovation to the major sectors in the panel industry and identifies new and promising technologies along with suggested priorities for the future. This document also lays out recommendations related to infrastructure and organization that may help the various sectors of the industry shift into high gear and assume a leadership role. The panel types included are: softwood plywood, oriented strandboard (OSB), particleboard and medium density fibreboard (MDF). Hardwood plywood is covered in less detail. Hybrid products which combine two or more panels, or panels with other materials, into a single product are also mentioned.
Adhesion and Adhesives - Composite Materials
Building materials - Composites - Research
Oriented Strand Board (OSB)
Medium Density Fibreboard (MDF)
Part of a technology roadmap series from Industry Canada: http://www.ic.gc.ca/eic/site/trm-crt.nsf/eng/rm00094.html
L'innovation technologique a sans nul doute été l'élément moteur dans la croissance remarquable qu'a connu l'industrie canadienne du panneau de bois au cours des 50 dernières années. Le défi consiste maintenant à définir la direction qu'elle doit prendre pour respecter l'évolution du marché et de la consommation. La carte routière se veut un guide qui aidera l'industrie à relever ce défi en identifiant les techniques nouvelles et prometteuses et en suggérant certaines priorités pour l'avenir. Aussi cette publication propose des recommandations quant à l'infrastructure et à l'encadrement qui pourraient aider les divers secteurs de l'industrie à accélérer pour s'engager sur la voie rapide de l'innovation. La carte routière technologique porte sur les types de panneaux suivants: contreplaqués de résineux, panneaux de lamelles orientées (OSB), panneaux de particules et panneaux de fibres de moyenne densité (MDF). Les contreplaqués de feuillus sont couverts en moindre détail. Aussi quelques mentions sont faites sur les matériaux hybrides qui combinent plusieurs types de panneaux ou des composantes nouvelles.