Poly(vinyl acetate), PVAc, adhesives are commonly used for wood bonding; however, they are fossil-based and the final products usually do not have a sufficient water resistance for more durable applications. In this study we prepared an adhesive formulation by grafting VAc from chitosan using emulsion polymerization, chitosan-graft-PVAc. Thereby, we could decrease the fossil-based content of the adhesive and at the same time significantly improve the water resistance. Chitosan by itself has very good bonding properties as a wood adhesive, especially regarding water resistance; however, very low solid contents of the adhesive formulation can be achieved due to a very high viscosity of chitosan adhesives. In our chitosan-graft-PVAc adhesives, we explored two chitosan samples with different molecular weights, by using as-received chitosan and hydrolyzing it to a lower molecular weight. The chitosan fractions in the adhesives prepared with a higher molecular weight chitosan were 15, 20 and 25 wt%. However, due to the high viscosity, a solid content higher than 17 wt% could not be achieved for these adhesives. Sufficient bond strengths were achieved when the adhesive was applied in 122 g/m2 solid spread rate. In order to decrease the viscosity, we used hydrolyzed chitosan, with a lower molecular weight, to allow for a higher adhesive solid content, 34 wt%, and for a higher chitosan fraction, 40 wt%. In the adhesive with 40 wt% chitosan and 17 wt% solid content, all VAc was grafted from chitosan. This decreased the molecular mobility of the chains, leading to a lower susceptibility to plastic creep in the adhesive which contributes to the final bond strength. The dry and wet strengths of the specimens bonded with adhesives containing chitosan were higher than the strength of the specimens bonded with the reference PVAc adhesive.

Locust bean gum, derived from the carob tree, was evaluated as a biobased adhesive in particleboard manufacturing, investigating the effect of adhesive amount, mat moisture content, and process parameters such as temperature and time. Single-layer particleboards prepared with locust bean gum showed that effective hydration of the polymer chains is necessary to achieve satisfactory interactions with wood and thus yield a sufficient particleboard strength. A mat moisture content below 30 % resulted in weak particleboards, which easily broke immediately after pressing. With increasing mat moisture content, while keeping the adhesive amount constant, the internal bond strength was increased. Moreover, with constant mat moisture contents (40 %), the internal bond strength increased when the adhesive amount was increased, even though not proportionally. With an increase from 9 % to 18 % adhesive, the internal bond strength was increased by more than 100 %. However, with a further increase in adhesive content from 18 % to 36 %, the increase in internal bond strength was statistically insignificant. Even with high mat moisture contents (35–45 %), larger lab-scale particleboards had internal bond strength that fulfilled standard requirements for P2 boards, commonly used for furniture in dry conditions (SS EN 312), when the pressing time was long enough (75 s/mm) to allow for water and vapors to be removed before releasing the pressure. Using biopolymers as adhesives, without extensive chemical modification and hazardous crosslinkers, could lead to a more benign and sustainable particleboard production. Since the chemistry and setting/curing processes of biopolymer-based adhesives differ from those of the fossil-based adhesives used today, increased understanding of how production parameters affect the properties of the particleboards prepared with biopolymers may pave the way for their better utilization in this field.

It is highly desirable to increase the use of renewable resources and reduce fossil-dependence due to sustainability concerns. As a consequence, it is expected that the use of the wood-based materials will increase in the future. Therefore, adhesives will have an important role since they allow for the use of lower-quality and recycled wood. At present, adhesives are mainly fossil-based and contain toxic components which can present a health risk. Extensive research is being conducted with the goal to replace fossil- with bio-based adhesives, and eliminate toxic components.

In this work, several polysaccharides have been investigated as wood adhesives, of which wood hemicelluloses are an interesting alternative. When applied between wood veneers, wood hemicelluloses alone do not exhibit sufficient strength, but combined with chitosan, they exhibit impressive strength and even sufficient water resistance. Since the molecular weight of hemicelluloses can affect the adhesive performance, locust bean gum has been used as a model in order to investigate this effect. An optimal molecular weight, 320-530 kDa in this case, is necessary for achieving the highest bond strength. These adhesives also show promising results in particleboards, and their internal bond strength fulfills the standard requirements for P2 boards (SS-EN 319), which are commonly used for furniture in dry conditions. High mat moisture contents are necessary for sufficient hydration of polysaccharides and internal bond strength; hence, a high mat moisture content (35-45 %), a pressing temperature of 150 °C, and pressing time of 15 min, yielded strong particleboards with no signs of steam blistering or delamination.

With the goal to partially replace fossil content in PVAc adhesives chitosan has been used in emulsion polymerization of VAc, as a surfactant and grafting site. Besides lowering the fossil content to 60 %, the water resistance was significantly improved.

Ökad användning av förnybara resurser och minskat fossilberoendet är högst önskvärt ur miljösynpunkt. Som ett led i detta förväntas användningen av träbaserade material öka i framtiden och därmed kommer även lim att spela en viktig roll eftersom limning möjliggör användning av tex återvunnet trä av lägre kvalitet. De lim som används idag huvudsakligen fossilbaserade och de innehåller dessutom giftiga komponenter som kan utgöra en hälsorisk. Omfattande forskning bedrivs med målet att helt eller delvis ersätta fossila lim med biobaserade lim och att eliminera giftiga komponenter.

I detta arbete har flera polysackarider undersökts som trälim, varav trähemicellulosor är ett intressant alternativ. När trähemicellulosor appliceras mellan träfanér uppvisar de i sig själva inte tillräcklig bindningsstyrka, men i kombination med kitosan uppvisar de imponerande styrka och även godtagbar vattenbeständighet. Eftersom molekylvikten hos hemicellulosa kan påverka vidhäftningsförmågan har locust bean gum använts som modell för att undersöka denna effekt. En optimal molekylvikt, 320-530 kDa i detta fall, är nödvändig för att uppnå högsta bindningsstyrka. Dessa lim uppvisar också lovande resultat i spånskivor och deras inre bindningsstyrka uppfyller standardkraven för P2-skivor (SS-EN 319), vilka vanligtvis används för möbler i torra förhållanden. Höga fukthalter är nödvändiga för tillräcklig hydrering av polysackarider och intern bindningsstyrka; en hög spånskivefukthalt (35-45 %), en presstemperatur på 150 °C och presstid på 15 min gav därför starka spånskivor, utan tecken på ångblåsor eller delaminering.

Med målet att delvis ersätta mängden fossilbaserat material i PVAc-lim har kitosan använts i emulsionspolymerisation av VAc, som ytaktivt ämne och som substrat för ympning. Förutom att sänka fossilhalten till 60 % förbättrades också vattentåligheten avsevärt i det resulterande limmet.

Today, the most commonly used adhesives in the wood industry are fossil-based, but the search for new, renewable and less hazardous, resources for adhesives is intensifying. Hemicelluloses show promising bonding performance when used as a component in wood adhesives. Since batch-to-batch variations can affect the use of hemicelluloses in a large-scale production, we have investigated the effect of hemicellulose molecular weight on important adhesive properties, such as viscosity and bond strength, by using locust bean gum as a hemicellulose model and varying the molecular weight through hydrolysis. Results showed that there is a nonlinear proportionality between bond strength and molecular weight. In the molecular weight range used in the study, 70–1460 kDa, anoptimum in the adhesive performance was achieved with intermediate molecular weights, 320 and 530 kDa, especially when considering applicability and bond strength. Adhesives with lower molecular weights, 70 and 150 kDa, did not exhibit sufficient cohesive strength; therefore, the bond strength was lower. The adhesive with higher molecular weight, 1460 kDa, was difficult to apply, especially since its maximum solid content was very low, 5 wt %.

Today, most wood adhesives are prepared from fossil-based polymers and contain hazardous components,e.g., formaldehyde. With the growing environmental concern there is an urge to develop bio-based and harmless substitutes. In this study, the ambition is to explore and valorise hemicelluloses, a biproduct from pulping, as the main component in wood adhesives. Wood adhesives were prepared from different sources: xylan from beech wood, hemicellulose-rich liquids obtained from hydrolysis of hardwood, and ultrafiltered softwood hemicellulose recovered from the process water of a thermomechanical pulp mill. Hemicelluloses themselves do not exhibit sufficient bonding performance, but excellent bond strength and water resistance were obtained in combination with poly(vinyl amine). It was also demonstrated that chitosan can be used as a bio-based amino-functional alternative to synthetic poly(vinyl amine), with similar or superior properties. Hemicelluloses alone show insufficient water resistance, but hemicelluloses in combination with chitosan exhibit exceptionally good bonding performance, especially regarding water resistance. Adhesives prepared from liquids rich in hardwood- and softwood hemicelluloses showed similar bond strength in combination with amino-functional polymers (poly(vinyl amine) and chitosan), regardless of their differences in structure. The current study constitutes an example on how sidestreams from the pulp industry in combination with chitosan can be used to substitute fossil-based materials in the quest for a more sustainable society.

The increasing sustainability concern encourages the society to use sustainable resources. In the wood industry the main component is from a renewable resource, but others are not. One of the fossil-based components is the adhesive. The wood adhesive industry is, besides the sustainability issue, facing environmental and health issues since most adhesives contain formaldehyde. Many biobased polymers show promising results as wood adhesives, and could therefore be used in the wood industry.

Wood hemicelluloses that originate from the pulp industry as a sidestream could be used in the adhesive production, but their bonding performances need to be improved. When used alone, wood hemicelluloses do not have sufficient bond strength. On the other hand, locust bean gum which has similar structure to hemicelluloses but substantially higher molecular weight gives a significantly higher bond strength. The investigation of the molecular-weight effect on bonding properties shows that solely low molecular weight hemicelluloses do not show sufficiently high strength to be used as wood adhesives. Hemicellulose-like compounds with very high molecular weight, over 1,000 kDa, show higher strength, but their dispersions are very viscous, which limits the applicability and the penetration of the adhesive into the wood. A proper balance between adhesive properties, such as molecular weight, dry content and viscosity, and the application method, leads to an improved bond strength.

When wood hemicelluloses are used together with high molecular weight amino-functional polymers, the bond strength is further increased. Additionally, the water resistance is significantly improved. The hypothesis is that these improvements are the results of synergetic effects of high molecular weight, electrostatic interactions and hydrogen bonding between the amino-functional polymer and the hemicelluloses in the adhesives, and between the adhesive and wood.

Det ökande engagemanget i hållbarhetsfrågan uppmuntrar samhället att använda hållbara resurser. Inom träindustrin kommer huvudkomponenten från en förnybar resurs, men andra inte. En av de fossilbaserade komponenterna är limmet. Förutom hållbarhetsfrågan, står trälimbranschen inför miljö- och hälsoutmaningar eftersom de flesta lim innehåller formaldehyd. Emellertid, många biobaserade polymerer visar lovande resultat som trälim och skulle därför kunna användas i träindustrin. 

Hemicellulosor som härrör från sidoströmmar i massaindustrin skulle kunna användas i limapplikationer, men deras bindningsförmåga måste förbättras. När de används som enda komponent i trälim, har dessa limfogar inte tillräcklig hållfasthet. Å andra sidan, ger Fruktkärnmjöl (locust bean gum) som har liknande struktur men signifikant högre molekylvikt en signifikant högre bindningsstyrka. Undersökningen av molekylviktseffekten på bindningsegenskaper visar att hemicellulosor med låg molekylvikt inte uppvisar tillräckligt hög hållfasthet för att användas som trälim. Hemicellulosaliknande föreningar med mycket hög molekylvikt, över 1,000 kDa, visar högre styrka, men deras dispersioner är mycket viskösa, vilket begränsar appliceringen och limmets penetration i träet. En korrekt balans mellan vidhäftande egenskaper, såsom molekylvikt, torrhalt och viskositet, och appliceringsmetoden leder till en förbättrad bindningsstyrka. 

När trähemicellulosor används tillsammans med aminofunktionella polymerer med hög molekylvikt, ökar bindningsstyrkan ytterligare. Dessutom förbättras vattentåligheten avsevärt. Hypotesen är att dessa förbättringar är resultatet av en synergistisk effekt av hög molekylvikt, elektrostatiska interaktioner och vätebindning mellan den aminofunktionella polymeren och hemicellulosorna i lim och mellan lim och trä.