Surface interactions of wood with adhesives and coatings

Abstract: The theme of this thesis is the properties of wood surfaces,and particularly some important aspects of their interactionswith adhesives and coatings.The surface inactivation of wood due to the development ofchemical weak boundary layers was monitored by wettabilitystudies based on contact angle measurements. The timedependence of the formation of such weak boundary layers wasdetermined for Scots pine and Norway spruce. To evaluate thewettability data, aconstant wetting rate angle(cwra), representing thesituation when a liquid droplet spreads on the surface at aconstant rate, was determin-ed. For planed surfaces there wastypically a linear reduction in wettability during a week aftermachining, whereas sawn surfaces exhibited a time-lag of a fewdays before the wettability started to decrease. A comparisonof surface wettability data with quantitative and qualitativeanalytical determinations of lipophilic wood extractivesrevealed a rather complex picture, the wetting behaviour beingdependent on the storage time after the machining of thesurface. The time-dependent surface inactivation data presentedin the thesis can be used to assess performance requirementsfor optimum adhesion in industrial coating and gluingpro-cesses.In order to improve the adhesion properties of inactivatedwood surfaces, an oxidative activation technique based on flametreatment was employed. A substantial increase in surfacewettability was obtained by optimizing parameters such asspeed, flame burner output, distance between wood surface andflame burner, and the number of passes under the burner. Ahigher treatment level was needed for teak, having a highcontent of lipophilic extractives, than for species with alower content (pine and birch). The oxidative effect wasconfirmed by ESCA measurements. In addition to improvedwettability, flame treatment also resulted in a considerablesurface sterilization effect.The relation between glue adhesion properties and woodextractives content was evaluated by a standard shear strengthtest and a non-standard fracture test on pine heartwoodmaterial classified according to its content of lipophilicextractives. For specimens assembled with a water-resistantpolyvinyl acetate adhesive, glue adhesion properties were tosome extent negatively affected by the existence of resinousweak boundary layers. Failure was more like-ly to occur at highwood extractive contents, roughly above 7%.The ability of coating materials to penetrate a wood surfaceand facilitate mechanical interlocking has been evaluated forvarious wood primers. Penetration was mainly demonstrated bymicroautoradiography, a technique superior to most othermethods for studying how monomeric and polymeric substancesinteract with the wood cell structure. Coating viscosity had amajor effect on the penetration, which in the radial directionnormally varied between 0.1 and 0.5 mm into longitudinaltracheids of pine sapwood. Penetration into spruce was at asignificantly lower level. It was further established thatsolvent-borne alkyd primers and water-borne alkyd emulsionprimers of comparable viscosities had a similar ability topenetrate the wood.Keywords:adhesion, adhesives, alkyd, autoradiography,coatings, contact angles, extractives, flame treatment,fracture test, glue bond strength, inactivation, interaction,penetration, pine, spruce, surface activation, weak boundarylayer, wettability, wood