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A study of wood adhesion and interactions using DMTA

The aim of the work presented in this thesis was to investigate whether the adhesion of glues and lacquers to wood could be studied using dynamic mechanical thermal analysis (DMTA). This method has been widely used to study interactions in polymer blends, copolymers and in polymeric composites but very few investigations had been done on solid wood. DMTA has the ability to give basic information on polymeric structures, which is an advantage over other test methods for adhesion. 

In this thesis pine wood (Pinus sylvestris) and several glues and lacquers have been studied using DMTA in the tensile mode. Both wood and pure polymers have been studied as well as polymers that have been polymerised in situ in wood, applied with a brush or as glue between wooden strips. The adhesion has then been studied and verified using fractography tests viewed with scanning electron microscopy (SEM). The adhesion has been characterised as "poor at the cell wall level" when there was a noticeable separation between the adhesive and the wood substrate as viewed in SEM. If the fracture occurred in the polymer or in wood cell walls, i.e. not at the interface, and no smooth polymer surfaces could be found, the adhesion was characterised as "good at the cell wall level".

The primary result was that the interaction as measured with DMTA between wood and polymers with high interaction and adhesion to wood showed a decrease in glass transition temperature (Tg) as compared to the Tg of the pure polymer material. Examples of high adhesion and interaction are the interaction between wood and a hydrophilic acrylate polymerised in situ from monomeric methacrylates. High interaction and adhesion was also found for two commercial polyurethane alkyd lacquers. No decrease in Tg was noted for polymers with low adhesion and interaction with wood. An example of low adhesion is polymethyl methacrylate (PMMA) polymerised in situ in wood. Low interaction and adhesion was also observed for a commercial polyvinyl acetate (PVAc) and wood. No change in interaction could be observed for the PMMA to wood when wood was made more unpolar (more lipophilic) by acetylation or silylation. The hydrophilic acrylate showed an increased interaction to silylated wood but unchanged interaction to acetylated wood.

The decrease in Tg for polymers showing high interaction and adhesion to wood is probably due to an increase of the free volume in the polymer. An increase in free volume can be due to tensile forces developing during polymerisation, drying or curing of the glues, lacquers and acrylate polymers.

This thesis shows that DMTA is an interesting method to obtain a basic understanding of adhesion phenomena and should be of interest for manufacturers of glues and paints and lacquers, and especially for thermoplastic adhesives. However, further experimental investigation is needed for the potential to test load-bearing glues. 

  • Anna C. Backman Sandlund
  • Luleå University of Technology, Division of Wood Material Science

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