André Merlin

Modification of wood wettability by plasma and corona treatments

Abstract The durability of an outdoor wood-coating system is low, especially when the coating is a stain. The most important degradations come from the dimensional variations of wood which strain the coating and lead to its cracking. Thus the service life of a wood-coating system can be increased through different routes: • Improvement of the coating flexibility: it can be obtained by decreasing the glass transition temperature (Tg) below the temperature of use. But in this case the coating is more sensitive to water • Dimensional stabilization of wood: thermal and chemical treatments (acetylation) are efficient but most of them decrease the wettability of the wood surface and therefore the adhesion of the coating is no more guaranteed. Surface treatments are convenient either to improve wettability and therefore coating adhesion or to decrease wettability in order to provide an extra protection.That is why plasma and corona treatment have been applied to wood in two ways: • The first way consists in improving the wettability of wood surface to increase the coating adhesion especially after a thermal treatment. First experiments consist in selecting treatment parameters (type of gas, treatment duration, power) leading to better surface characteristics of wood, before and after thermal treatments. Plasma and corona treatments are effective to improve the wettability of wood but no improvement in the coating adhesion has been observed. • The second way consists in decreasing the wettability so that the surface becomes waterproof. Some tests have been performed to deposit thin films (based on ethylene or fluorine) by plasma or corona treatments. This kind of deposits can be considered either on bare wood or on coated wood with a low Tg stain, that is to say, a stain whose sensitivity against water will be solved because of this kind of films. This is the aim of an on-going research programme.

Antioxidant characteristics of hydrolysable and polyflavonoid tannins : An ESR kinetics study

As part of an investigation of the role of tannin as antioxidants, the radical formation and radical decay reactions of some polyflavonoid and hydrolysable tannins has been followed by electron spin resonance (ESR) techniques. Comparative kinetics were determined for both light-induced radicals and by radical transfer from a less stable chemical species for the tannin alone and when the tannin is in a methanol solution. The five parameters which appear to have a bearing on the very complex pattern of the rates of tannin radical formation and radical decay were found to be (1) the extent of the colloidal state of the tannin in solution, (2) the stereochemical structure at the interflavonoid units linkage, (3) the ease of heterocyclic pyran ring opening, (4) the relative numbers of A- and B-rings hydroxy groups, and (5) solvation effects when the tannin is in solution. It is the combination of these five factors that appears to determine the behavior as an antioxidant of a particular tannin under a set of application conditions.

Analysis of the Natural and Artificial Weathering of a Wood Coating by Measurement of the Glass Transition Temperature

Weathering of wood coating systems is often only followed by visual observations which are very subjective. This is the reason why the natural and artificial weathering of an alkyd based wood surface finish has been studied by measuring a quantifiable parameter. This parameter, namely the glass transition temperature (T g ) of the polymer constituting the finish, has been measured as a function of time. For both weathering cycles tested, T g variations are the same : an increase in T g during the first steps of weathering followed by a stabilization is observed. An equation is proposed to describe these variations. It allows the determination of time constants and also the acceleration produced by the artificial weathering. From this equation, reaction rates of curing can be calculated for both weathering cycles tested and for each wood species on which the coating has been applied. This work validates the performance of the artificial weathering device used.

Role of phenolic derivatives in photopolymerization of an acrylate coating

The photopolymerization of acrylate resins on wood surfaces suffers from retardation and inhibition effects due to the phenolic derivatives present at the interface. This article details the study of the effect of a set of phenolic compounds on the initiation step. The global effect was recorded by differential scanning calorimetry and photocalorimetry. A comparison between a direct photocleavable initiator such as 2,2-dimethyloxy-2-phenylacetophenone (DMPA) and a two-component system like benzophenone/N-methyldiethanolamine (BP/MDEA) suggests that the retardation effect observed in the latter case is due to the interaction between phenols and the triplet state of BP. Subsequently, nanosecond transient absorption (NTA) spectroscopy was used to measure in acetonitrile the quenching rate constants k Q . A hydrogen abstraction occurred, and the ketyl radical quantum yield was also determined by NTA experiments. In comparison with the photoreduction mechanisms proposed in the literature, the high k Q values obtained were tentatively correlated to the half-wave oxidation potentials of phenols in order to discuss the involvement of an electron transfer within the reaction. Some EPR experiments were done to confirm in situ the photoreduction process at the wood surface and the creation of phenoxyl radicals. The interaction of phenols with some initiating radicals was also studied.

Comparative kinetics of induced radical autocondensation of polyflavonoid tannins. I. Modified and nonmodified tannins

Comparative kinetics of the radical autocondensation induced by SiO2 of a polyflavonoid tannin, namely quebracho tannin a mostly profisetinidin/prorobinetinidin tannin, in its natural extract state, sulfited, carbohydrate free, and in its adhesive intermediate form were carried out by electron spin resonance (ESR). The results obtained not only confirmed the existence of strong radical mechanisms of tannin autocondensation in the presence of dissolved SiO2, but also pointed out new effects of interest in such a reaction. The SiO2 induced autocondensation proceeds at a faster rate, and radical surge and concentration decay appear to be more marked and more rapid the more colloidal the tannin solution is. This appears to indicate that a colloid-induced intramicellar radical mechanism of the reaction is at work. As a consequence the intensity and rate of the radicals surge and decay decreases passing from more colloidal to less colloidal tannin solutions. Thus, the decay rate decreases passing from natural tannin to adhesive intermediate, to carbohydrate-free tannin, to almost disappear for sulfited tannin. The colloidal state of the solution appears to depend mostly, but not only, on the presence of the polymeric carbohydrates in the extract.

Comparative kinetics of the induced radical autocondensation of polyflavonoid tannins. III. Micellar reactions vs. cellulose surface catalysis

Cellulose surface catalysis of the induced autocondensation of tannins has been found to occur also for the radical mechanism of the reaction. Equally effective in accelerating pyran ring opening of polyflavonoids by radical mechanisms appears to be the presence of any substance leading to micellar structures in the tannin solution. Thus, soaps and synthetic and natural polymeric colloids have been found to accelerate the reaction. The micellar effect is distinct and different from the surface catalysis of the process induced by cellulose. Amorphous and crystalline cellulose appear to differ somewhat as regards the extent of catalytic activation. As for ionic mechanisms, also for catalyzed radical mechanisms, procyanidins invert their favorite reaction of cleavage of the interflavonoid bond to favor, instead, pyran ring opening. This inversion of the favorite reaction is induced both by the presence of cellulose and by the level of the micellar state of the reaction.

Molecular Mechanics/Experimental Methods Applied to Varnish/Primer/Wood Interactions

Molecular mechanics modelling is used to calculate the energies of interaction, hence the molecular level energy of adhesion at the interface with crystalline cellulose I of three different photopolymerisable primers and of a polyester varnish at the interface with the primer/cellulose assembly. The energy of interaction for just one of the primers with the statistically most common conformation of amorphous cellulose has also been obtained for comparison. Experimental results of adhesion by a standard peel test and by thermomechanical analysis, in which the effect of energy dissipation by crack tip propagation has been respectively minimised or is not present, hence in which the energy of interfacial interaction S is nothing else than the work of adhesion, correlated well with the energies of interaction calculated by molecular mechanics. An equation correlating energy of interaction at each finish/cellulose interface with the deflection and flexibility derived by thermomechanical analysis, and with the number of bond rotational degrees of freedom as well as degree of networking of the finish has been derived and is presented. Discussion of the relationship of the effects found with the glass transition temperature of the finish is also presented.

An ESR study of the silica-induced autocondensation of polyflavonoid tannins

Radical mechanisms of silica-induced polyflavonoid tannin autocondensation and hardening through alkaline pyran ring cleavage are shown to be important, especially at different levels according to the reaction conditions and type of tannin. For prodelphinidin tannins, radical mechanisms are shown to be more important than are ionic ones even under mild alkaline reaction conditions. For procyanidin tannins, radical mechanisms appear to be of little or no importance under mild alkaline reaction conditions, but acquire more importance at more alkaline pHs, a result confirmed also with a (+)-catechin monomer as a model compound. Under the latter conditions, the radical mechanism of pyran ring cleavage inverts the relative importance of the interflavonoid bond cleavage and pyran ring opening for procyanidins. Relative intensities of phenoxide radical and radical anion ESR signals indicate that pyran ring opening and autocondensation are rapid under alkaline conditions.

Ionic polycondensation effects on the radical autocondensation of polyflavonoid tannins: An ESR study

An electron spin resonance (ESR) study of the presence or lack of interference by ionic hardening mechanisms and ionic coreactants on the polyflavonoid tannin radical autocondensation reaction indicated that in certain cases hardening by ionic coreactants can be coupled with the simultaneous hardening of the tannin by radical autocondensation. Some coreactants tend to depress the tannin radical autocondensation while still leaving a small contribution of this reaction to the formation of the final crosslinked network. Other coreactants instead appear to enhance formation of the final network by synergy between ionic and radical mechanisms, while still others do not show any interference between the two types of reaction. Mechanisms describing the interaction between the two reactions are proposed and discussed.

Wood-coating layer studies by X-ray imaging

The penetration depth into wood structure by different wood finishes (solvent-borne, waterborne or powder) was assessed using a non-destructive analysis method, that is, X-ray tomography. The coating thickness was also evaluated. The results obtained from the image analysis by two specific softwares, namely VGStudio MAX® and ImageJ®, are compared and discussed by taking into account the chemical properties and anatomy of wood materials and the nature of finishes. The degree of coating penetration is mainly determined by its ability to flow into capillaries, and it was observed that resin type, solid matter content and drying method influence this ability.