Dimensional Stabilisation Research Articles

Impact of a conditioning step during the treatment of wood with melamine-formaldehyde resin on dimensional stabilisation

Abstract The dimensional stabilisation of wood using thermosetting resins relies on the resin uptake into the cell walls. This study tested if a conditioning step after the impregnation and before the final heat-curing enhances the cell wall uptake to improve dimensional stabilisation without increasing the chemical consumption. Small blocks of Scots pine sapwood were vacuum-impregnated with an aqueous melamine formaldehyde solution and conditioned at 33, 70, or 95 % RH for up to 1 week before drying and curing the blocks at 103 °C. However, the conditioning step decreased the cell wall bulking and the moisture exclusion effect compared to the immediate heat curing of the impregnated samples. Analyses of the resin-treated samples by scanning electron microscopy, IR spectroscopy and confocal Raman microspectroscopy provided evidence of wood hydrolysis and polycondensation of the resin within the cell lumen during the conditioning step. Hydrolysis and removal of wood constituents may have counterbalanced the cell wall bulking of the resin. Polycondensation of the resin in the lumen increased its molecule size, which could have hindered the cell wall diffusion of the resin.

The effect of the drying method on the cell wall structure and sorption properties of waterlogged archaeological wood

Drying is a process affecting various wood properties, including its structure, moisture behaviour and mechanical properties. Since waterlogged wooden artefacts usually constitute priceless objects of cultural heritage, understanding the effect of drying on the complex interactions between the wood ultrastructure and the resulting properties is necessary to ensure their proper conservation. Hence, this was the aim of the present study, with a particular emphasis on the influence of drying conditions on the relations between the cell wall structure, dimensional stability and hygroscopicity of degraded archaeological wood. The choice of the particular drying methods was dictated by their final effect on wood appearance (dimensions). The results obtained clearly show that depending on the drying method applied, the resulting material differs significantly in structure, dimensions and sorption properties, despite the same degree of wood degradation. Air- and oven-drying resulted in the highest wood shrinkage, lower porosity, and a decreased number of free hydroxyls in the wood cell wall. The best wood dimensional stabilisation and the highest porosity were ensured by freeze- and supercritical drying. No correlations were found between wood structure and moisture behaviour. The outcome of the research may be useful for conservators who plan to provide the artefacts with proper storage conditions and effective conservation/reconservation.

Interactions between Different Organosilicons and Archaeological Waterlogged Wood Evaluated by Infrared Spectroscopy

The goal of the study was to characterise chemical interactions between waterlogged archaeological wood and organosilicon compounds applied for its conservation to shed lights on the mechanism of wood dimensional stabilisation by the chemicals. Two alkoxysilanes (methyltrimethoxysilane and (3-mercaptopropyl) trimethoxysilane) and a siloxane (1,3-bis(diethylamino)-3-propoxypropanol)-1,1,3,3-tetramethyldisiloxane) were selected for the research since they already have been proven to effectively stabilise waterlogged wood upon drying. Fourier transform infrared spectroscopy was used for structural characterisation of the degraded wood and evaluation of reactivity of the applied chemicals with polymers in the wooden cell wall. The results obtained clearly show much stronger interactions in the case of alkoxysilanes than the siloxane, suggesting a different mechanism of wood stabilisation by these compounds. The results of this study together with other data obtained in our previous research on stabilisation of waterlogged archaeological wood with organosilicon compounds allow the conclusion that the mechanism of waterlogged wood stabilisation by the used alkoxysilanes is based on bulking the cell wall by silane molecules and wood chemical modification, while in the case of the applied siloxane, it builds upon filling the cell lumina.

Dimensional stabilisation of Scots pine (Pinus sylvestris L.) sapwood by reaction with maleic anhydride and sodium hypophosphite

Wood has the ability to absorb and desorb moisture, which can affect its dimensional size when in use. Limiting this can provide products with greater shape stability and less stresses on external coatings. One method that has been investigated for achieving this has been through chemical modification. In this work, the dimensional stabilisation imparted to Scots pine sapwood by chemical modification with maleic anhydride (MA) combined with sodium hypophosphite (SHP) was investigated. The influence of concentration of MA, treatment temperature and treatment period on weight percent gain (WPG) and bulking coefficient (BC) during treatment with MA and SHP of wood was studied. Furthermore, dimensional stability was determined by the water soak/oven dry method (wet-dry cycle) through five cycles in order to determine the hydrolytic stability of the ester bond and any potential cross-linking reactions. Wood blocks (20 × 20 × 10 mm) modified with MA combined with SHP exhibited lower weight loss following water soaking than unmodified blocks or MA-treated blocks. Wood blocks modified with MA and SHP showed the best anti-swelling efficiency and minimum wet-volume (water-saturated). However, as the concentration of SHP increased, dimensional stability was diminished without any increase in weight percentage gain after water soaking. When combined with FTIR results, it appeared that the modification with MA and SHP seemed to form cross-linking between wood constituents, though high concentration of SHP did not seem to result in additional cross-linking.

Biological effectiveness of archaeological oak wood treated with methyltrimethoxysilane and PEG against Brown-rot fungi and moulds

The results of the previous research have shown that methyltrimethoxysilane is an effective agent for dimensional stabilisation of archaeological waterlogged wood. This paper presents an assessment of the effectiveness of MTMS in improving the biological resistance of archaeological waterlogged oak (Quercus robur) against moulds and brown-rot fungus Coniophora puteana. Hygroscopic properties of the treated wood were also examined. Small wood samples were treated with MTMS using the oscillating pressure method. PEG-treated specimens were used as a reference and contemporary oak was used as a control. The results clearly show that both MTMS and PEG treatment efficiently protected waterlogged wood against brown-rot fungus, increasing its resistance by about 80% in comparison with untreated samples but they are not effective against moulds. Moreover, both chemicals reduced liquid water sorption by the treated wood. However, the mechanism of wood protection and hydrophobation seems different. PEG-treated wood was found to be much more hygroscopic (equilibrium moisture content about 35%) than the untreated wood (EMC about 20%), while MTMS significantly decreased wood hygroscopicity (EMC about 14%). The use of accelerated ageing revealed that PEG is highly susceptible to leaching, which reduces its effectiveness. MTMS is more resistant, therefore its antifungal and hydrophobising properties remain almost unchanged even after the ageing process.

Organosilicon compounds with various active groups as consolidants for the preservation of waterlogged archaeological wood

Abstract Waterlogged wood conservation focuses primarily on its dimensional stabilisation to prevent irreversible deformations that occur upon drying. It also aims to solve the concerns regarding wood degradation as well as its physico-mechanical properties, all of which are inextricably connected with wood hygroscopicity. Therefore, the reduction of wood hygroscopicity can provide a solution to some of the conservation problems. Due to the hydrophobising properties and the ability to cell wall bulking, a set of organosilicons with various active groups were selected to evaluate their stabilisation effect on highly degraded medieval waterlogged elm wood. Exceptionally satisfying results were obtained for (3-Mercaptopropyl)trimethoxysilane, 1,3-Bis(diethylamino)-3-propoxypropanol)-1,1,3,3-tetramethyldisiloxane and Methyltrimethoxysilane, for which the volumetric anti-shrink efficiency was around 98%, 90% and 81%, respectively. Additionally, the applied treatment resulted in a decrease of wood moisture content from 6.7% for untreated wood to 2.4–3.6% for wood treated with the above-mentioned compounds. Neither the alkoxysilanes with a long alkyl chain, nor those terminated with a highly hydrophilic pyridine ring were found effective for waterlogged wood stabilisation. It can be concluded that the organosilicon compounds with alkoxy groups and a relatively short alkyl chain or containing and additional chemical group enable to form stable bonds with wood components proved to be the most effective wood stabilisers.

Modifying wood with bio-polyesters: analysis and performance

Oligomers of lactic and glycolic acid (OLA, OGA) and poly(butylene succinate/adipate) (OBS, OBA) were impregnated and heated into wood for dimensional stabilisation. These bio-polyesters variously provide bulking and lumen filling treatments. Solvent extraction of treated wood showed that ca. 50% of the polymer remains in wood with in situ polymerisation evidenced by GPC. OGA readily penetrates the wood cell wall, which occurred only on heating OLA, whereas OBS/OBA was lumen filling with any cell wall penetration only evident after water soaking. Stability of OLA/OGA treated wood was improved, but did not withstand extended leaching, whereas OBS/OBA treated wood presented higher stability once leached. Bending resistance was influenced by OLA/OGA treatments, but original MOE values were recovered after leaching. Overall, the results show the bio-polyester treatment can confer specific properties of the modified wood to suit selected end use applications.

Investigations on wood treatment by eco-friendly hydroxypolyurethanes

Dimensional stabilisation is a crucial property for using wood in most applications. Therefore an isocyanate-free route is proposed, based on the reaction of various diamines with dicarbonates obtained by addition of two glycerol carbonates moieties on a linker. For the initial experiments, the mildest conditions to perform the reaction are researched with model mono- and polyamines without wood: solvent, temperature, number of equivalent and regioselectivity were investigated. Subsequently, beech wood blocks were impregnated by a double vacuum process involving cyclic carbonates and then polyamines. The key step was a polycondensation performed in the beech blocks after the monomers had been impregnated. Resistance of treatment to leaching, anti-swelling efficiency and decay durability versus Poria placenta were evaluated. Results were promising with ASE values of about 70% for a weight gain of approximately 30% after leaching. Biological tests indicated also an improvement of resistance to fungal decay of beech wood.

A methodological approach in the evaluation of the efficacy of treatments for the dimensional stabilisation of waterlogged archaeological wood

Abstract The aim of the work is to set up a methodological approach to verify the effectiveness of the treatments of decayed waterlogged archaeological wood and to point out the proper thermo-hygrometric conditions for its preservation after treatment. The treatments were performed on wood samples of maritime pine (Pinus pinaster Aiton), oak (Quercus sp. caducifolia), elm (Ulmus cf. minor) and strawberry tree (Arbutus unedo L.), obtained from stems pertaining to the original vegetation found in the excavation site of the Ancient Ships in Pisa (Italy), and dated from seventh century BC to second century AD The utilised products were: Polyethylene Glycols (PEG) of various molecular weights, a Polypropylene Glycol (PPG 425), Trehalose (α-D-glucopyranosyl-α-D-glucopyranoside), and their mixtures, and also a Hydroxypropylcellulose (Klucel). The main objective to be pursued by the treatments was considered the stabilisation of the original size and shape of samples. The various steps of this approach were: the execution of a preliminary diagnostic survey on untreated samples; the characterisation of treatment solutions ‘as such’ to establish the property to be monitored during the treatment; the evaluation of the main physical characteristics of wood after the treatment were determined. Among the latter, the coefficient of dimensional stability during the exposure to a series of selected thermo-hygrometric conditions and the retention of consolidants after the treatment. These two measurements allowed the definition of the ‘efficacy of a treatment’, φT, a new parameter firstly utilised in this work. It measures the stabilisation capability of the percent unit of retained product, and its value permits to put in evidence the consolidants that stabilise wood with the lowest amount of product. Conversely, it was not possible to measure the Anti-Shrink Efficiency (ASE), one of the most utilised parameters for the evaluation of treatments, because of the serious distortions and fractures observed in all the heavily degraded untreated samples.

Dimensional Stabilisation of Fast Growing Forest Species by Acetylation

Dimensional Stabilisation of Fast Growing Forest Species by Acetylation

Einsatz von modifiziertem Holz für Parkett | Use of modified wood for wooden flooring

Based on experience gained we can sum up the following points as given and proved. The hardness of the wood is increased many times; durability and thus the life span of a product considerably improved; only insignificant improvement has been achieved so far with regard to dimensional stabilisation;the natural character of the wood is preserved; the product(floor boards) is restorable, i.e. it can be smoothed and the new surface can be treated; its employment in heavily frequented rooms has furnished proof of its suitability.

Chemical reaction of maritime pine sapwood (Pinus pinaster Soland) with alkoxysilane molecules: A study of chemical pathways

Abstract The chemical modification of maritime pine sapwood (Pinus pinaster) with alkoxysilanes was studied according to three different pathways: carbamoylation with 3-isocyanatopropyltriethoxysilane, etherification with 3-glycidoxypropyltrimethoxysilane and alcoholysis of n-propyltrimethoxysilane. Grafting was confirmed by weight percent gain calculations (WPG), infrared spectroscopy (FTIR) as well as 13C and 29Si NMR CP MAS analysis. Signals of the grafted groups in the different spectra were assigned and the reactivity of the trialkoxysilane moieties towards wood was discussed. Experiments with model wood blocks showed that the reactions investigated occurred within the wood cell walls. Grafted chemicals were found to be relatively stable with regard to water leaching but only slight dimensional stabilisation was noted after treatment.

The use of organo alkoxysilane coupling agents for wood preservation

Abstract Corsican pine (Pinus nigra) sapwood was treated with methanolic solutions of two organo alkoxysilanes ([γ-(methacryloxy)propylx] trimethoxy silane (TMPS), or vinyl trimethoxy silane (VTMS)). Various treatment parameters were studied in order to determine the optimum method for obtaining a stable weight percentage gain. For TMPS, it was found that best results were obtained when a free-radical initiator was used in conjunction with the silane. This indicates that polymerisation of this species occurs via the pendant methacryloxy moeity, a result confirmed by solid-state nuclear magnetic resonance studies of the treated wood. For VTMS, wood moisture content had a profound influence upon the weight percentage gain due to treatment, with higher weight percentage gains obtained as wood moisture content was increased. This indicates that polymerisation in this case occurred via the formation of silanol groups to form siloxane linkages. Solid-state NMR confirmed that the double bond of the organo-functional group was intact after polymerisation had occurred. Modified wood samples were then prepared using optimised treatments for each silane. The wood was prepared to a variety of weight percentage gains and the effect of treatment on the dimensional stability and decay properties of the treated material studied. Treatment of Corsican pine sapwood with the two organo-alkoxy silanes, resulted in only moderate increases in dimensional stabilisation, with anti shrink efficiency values (after 5 water-soak/oven-dry cycles) no greater than 40%. Treatments were found to be relatively stable to hydrolysis. Some differences were observed in the effectiveness of the two silanes in providing protection. Full decay protection required high treatment levels with the silanes.

Dimensional stabilisation and strength of particleboard by chemical modification with propionic anhydride

Chemical modification of wood chips using propionic anhydride resulted in improved dimensional stability of particle boards manufactured with these chips. The TS values of the UF bonded boards were about 104% lower than controls and were in accordance with the stringent requirements of the ANSI standard. The internal bond strength, however, decreased significantly, but was still substantially higher than the requirements of the EN 312 and ANSI standards for P4 load-bearing boards or M3 boards. As in the case of acetylated chips, the majority of failure was due to the resin and not to wood.

Dimensional changes in Corsican and Scots pine sapwood due to reaction with crotonic anhydride

The effects of the reaction temperature and varying level of weight gain on the dimensional stabilisation of crotonic anhydride modified Corsican and Scots pine sapwood were investigated. With Corsican pine at low levels of substitution, the reagent occupied larger molar volumes in the cell wall than at higher weight gains. With Scots pine the molar volume occupied by reagent was found to be temperature dependent at low levels of substitution. Dimensional stabilisation was determined by the water-soak/oven-dry method through a total of ten cycles in order to determine the stability of the ester bond to hydrolysis at neutral pH. It was found that the reaction temperature has no significant effect on dimensional stability provided the same weight gain level is obtained. At weight gain levels in excess of 30%, values of anti-shrink efficiency of 90% were obtained.

The Dimensional Stabilisation of Maritime Pine Sapwood (Pinus pinaster) by Chemical Reaction with Organosilicon Compounds

Summary The dimensional stabilisation imparted to Maritime pine sapwood by chemical modification with hydrophobic organosilicon compounds was investigated. Modification was performed via esterification, using different chemically synthesised models bearing trimethylsilyl groups: 3-trimethylsilylpropanoic anhydride (I), 2-trimethylsilylmethylglutaric anhydride (II) and trimethylsilylethenone (III). Grafting was confirmed by infrared spectroscopy as well as 13C and 29Si NMR CP MAS analysis and was located into the wood cell walls. The dimensional stabilisation induced by I and II appeared to be stable during three oven-dry/water-soak cycles and, for a given number of hydroxyl blocked, to be superior to that imparted by acetylation. The graft from III, however, was found to hydrolyse progressively into an acetyl group via a metallotropic rearrangement. Further investigation on the hygroscopicity and swelling behaviour of modified wood from I and II indicated that dimensional stabilisation was attributable to a bulking effect rather than a change in substrate hygroscopicity. The bulky grafts from I and II were, in particular, thought to locally over-swell the wood cell wall, revealing regions with new water-accessible hydroxyl groups.

Wood modification to improve the durability of coatings

The exterior durability of a wood-coating is low, especially when it is a protective stain. The severest degradation comes from the dimensional variations of the wood which strains the coating and leads to the formation of cracks. Thus, the service life of a wood coating system con be increased by various means: ▀ improvement of the coating flexibility and also by reducing the glass transition temperature (Tg) below the temperature of use. ▀ dimensional stabilisation of wood, for which thermal and chemical treatments (acetylation) are efficient, although most of them decrease the wettability of the wood surface and the adhesion of the coating. Polyethylene glycol (PEG) can be used to stabilise wood and con be added directly in the finishing primer. ▀ improvement of coating adhesion by a modification of the wood surface. For example, plasma treatments have been applied to wood, with and without thermal treatments, in order to improve its surface properties.

The Dimensional Stabilisation of Corsican Pine Sapwood by Reaction with Carboxylic Acid Anhydrides. The Effect of Chain Length

The dimensional stabilisation imparted to Corsican pine sapwood by chemical modification with carboxylic acid anhydrides of varying chain length was investigated. Dimensional stabilisation was determined by the water soak/oven dry method through a total of ten cycles in order to determine the stability of the ester bond to hydrolysis at neutral pH. These tests indicated that the dimensional stabilisation was attributable solely to a bulking effect. The loss of material due to hydrolysis was minimal, with modified wood blocks exhibiting lower weight loss following water soaking than unmodified blocks. Considerable variation in dimensional stabilisation (as determined by volumetric swelling coefficient and anti-shrink efficiency) was found throughout the water soak tests. A contribution to this variability was found to be due to changes in the water saturated volume caused by variations in the ambient air temperature. It was found that weight percent gain provided an excellent measure of dimensional stabilisation irrespective of the chain length of the anhydride involved. Increases in volume due to modification were found to be larger than theoretically expected.

A Theory of Wood Waterproofing by Complexes of Metal Oxides with Lignin Structural Units

SYNOPSIS The effect of various metallic oxides and hydroxides on wood waterproofing was investigated. The waterproofing effect was attributed to the type of complexes formed by the metallic oxide with the guaiacol units of the lignin network of wood. The type of structure of the complex formed, as well as its insolubility, contribute to the wood waterproofing, although to a different extent. Only Cr, Zn, Pb and Sn oxides formed insoluble complexes. Of these only Cr and Zn oxides are able to waterproof wood, although to a different extent. This was attributed to their capacity to further cross-link the lignin network in wood, which renders more difficult water penetration and improves wood dimensional stability. While it is necessary for the complexes formed to be insoluble, their insolubility is not the main contributing factor to wood waterproofing and its dimensional stabilisation.

Dimensional Stabilisation of Wood: Factors Influencing it and the Principles of Treatment

SUMMARY The factors that cause dimensional changes in wood and the principles of dimensional stabilisation treatments as well as the practical treatments employed, are discussed. Two treatments which appear promising for the future, water repellent application and the production of wood/plastic composites, are given special attention.