Original

Modification of wood with siliconcompounds. Treatment systems basedon organic silicon compounds — a review

Carsten Mai, Holger Militz

Abstract A wide variety of organo-silicon compounds has been described forapplication on wood. Some compounds such as organo-functional silanes whichare mostly applied in combination with tetraalkoxysilanes (sol-gel process) aswell as chlorosilanes and trimethylsilyl derivatives were proposed for a fullimpregnation treatment of wood. Other systems have been developed for surfacetreatment of wood such as plasma coating with hexametyldisiloxane and micro-emulsions which mainly contain silane/siloxane mixtures. The effects related tothe various treatments vary from an increase in dimensional stability, durabilityand fire resistance to an enhanced hydrophobation of wood. In the cases of decayand fire resistance a combination of silicon based systems with other chemicalswas required to obtain satisfactory results. Due to the excellent water repellentability and weathering stability of some treatments, application of silicon treatedwood under conditions of hazard class III (EN 335 outside above ground expo-sure) is recommended.

AbbreviationsASE anti-shrink efficiencyATR-FT-IR attenuated total reflectance — Fourier transform infrared

spectroscopyCP MAS-NMR cross polarisation magic angle spinning nuclear magnetic

resonance spectroscopyDTA differential thermal analysisDTMOS decyltrimethoxysilaneEDX energy dispersive x-ray analysisEETMOS b-(3,4 epoxycyclohexyl) ethyl trimethoxysilaneEMC equilibrium moisture contentESCA electron spectroscopy for chemical analysisHFOETMOS 2-heptadecafluorooctylethyltrimethoxysilane

Wood Sci Technol 37 (2004) 453–461

DOI 10.1007/s00226-004-0225-9

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Received: 15 January 2002Published online: 2 March 2004� Springer-Verlag 2004

C. Mai (&), H. MilitzInstitute of Wood Biology and Technology,University of Goettingen,Buesgenweg 4, 37077,Goettingen, GermanyTel.: +49-551392051Fax: +49-551399646E-mail: cmai@gwdg.de

HMDSO hexamethyldisiloxaneIPTEOS 3-isocyanatepropyl triethoxysilaneMPTMOS c-methacryloxypropyl trimethoxysilaneMTMOS methyltrimethoxysilanePDMS polydimethylsiloxanePTMOS propyltrimethoxysilaneTEOS tetraethoxysilane/triethyl orthosilicateTFPTMOS 3,3,3-trifluoropropyltrimethoxysilaneTGA thermo-gravimetric analysisTMSAH 3-(trimethoxysilyl)propyl (carboxymethyl) decylmethyl

ammonium hydroxide inner saltTMSCl trimethylsilyl chlorideTPT tetraisopropyl titanateVTMOS vinyl trimethoxysilaneWPG weight percent gainWRE water repellent efficiency

IntroductionDue to growing environmental concerns, the industry and research institutes aresearching for new methods of wood protection in order to substitute conventionalpreservatives based on creosotes or copper, chrome and arsenic (CCA). Chemicalwood modification is a promising approach not just to enhance the durability ofwood but also to improve further material properties.

In a recent publication, we reviewed various approaches of wood treatmentwith inorganic silicon compounds (Mai and Militz 2002). Such systems are mainlybased on condensation products of silicic acid (colloidal silicic acids, silicates,‘‘water glass’’) or tetraalkoxysilanes which undergo hydrolysis and condensationsteps to form sols and finally gels (sol-gel technology). When the sol-gel process iscompleted an inorganic silicate free of organic groups is formed (e.g. Saka et al.1992).

Wood treated with tetraalkoxysilanes showed enhanced dimensional stabilityespecially when the hydrolysis and the condensation of the silanes was steered toproceed within the cell wall. Durability and fire resistance were improved to acertain degree but could be significantly enhanced by addition of boron com-pounds.

This review describes various treatments of wood with organo-silicon com-pounds as well as a combination of organo-silicon and tetraalkoxysilanes whichform inorganic-organic gel systems. The outstanding property of organo-siliconcompounds is their hydrophobicity and water repellence caused by the organic(in most cases methyl) groups. In addition, organo-silicons possess high thermalstability and are good insulators (Anonymous 1989).

Related to health aspects, polydimethylsiloxane (PDMS, silicone) has beenmost intensively studied among the organo-silicons. PDMS does not irritatehuman or animal skin and shows minor acute toxicity at oral, dermal and in-halative exposure. Under long term exposure, no special target organs displayingtoxic effects could be identified in animal tests. Further studies have revealed noteratogenic, mutagenic or cancerogenic effects on animals or aquatic life andthere is no evidence that silicones adversely affect ecosystems (Marquardt andSchafer 1994).

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Wood treatment with organo-functional silanes (sol-gel process)A variation of the sol-gel process, which applies tetraalkoxysilanes and producesinorganic glasses consisting of pure polymeric SiO2, is via the use of organo-silanes (Fig. 1). These are bifunctional molecules which contain three silicon-functional alcoxy groups, mostly methoxy and ethoxy groups, and an organo-functional group which increases, e.g., the hydrophobicity of the gel or forms acovalent bond with the cell wall polymers. Organo-silicons have a high variety ofapplications such as adhesion promoters, surface modifiers, or cross-linkingagents.

Organo-functional silanes were mainly applied in combination with tetraeth-oxysilane (TEOS) or other gel forming precursors. In one line of experiments,wood samples were impregnated with mixtures of TEOS and organo-functionalsilanes in one step. Various property enhancers such as 3,3,3-trifluoropropyltri-methoxysilane (TFPTMOS), 2-heptadecafluorooctylethyltrimethoxysilane(HFOETMOS) and decyltrimethoxysilane (DTMOS) (Fig. 1) were shown to pre-vent leaching from SiO2-P2O5-B2O3 wood components due to hydrophobation.Among the silanes tested HFOETMOS was most efficient in prevention of leachingand imparted the lowest water absorption ratio. In addition, it improved the fire

Fig. 1. Chemical structure of various organo-silanes applied for wood modification

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resisting properties which was probably due to the presence of fluorine (Saka andTanno 1996).

In a further study on SiO2-P2O5-B2O3 wood components (Saka and Tanno1996), TEOS as the silane component was compared to methyltrimethoxysilane(MTMOS) (Fig. 1). The fire retarding properties in both systems were very similarbut the leachability of the phosphorous and boron oxide was significantly lowerwith application of MTMOS. The weight percent gain (WPG) of MTMOS andTEOS treated wood was similar but MTMOS caused a higher bulking even thoughthe concentration in the impregnation solution was lower.

Several compounds known to improve fire resistance were tested in the TEOSand MTMOS system such as trimethylphosphite, diethylphospite, terakis(hy-droxylmethyl)phosphonium chloride, phenylphosphonic dichloride and dim-ethylphenylphosphonate (Saka and Ueno 1997). Wood-inorganic Na2O-SiO2

composites prepared by adding sodium methoxide or sodium acetate to thereaction system of MTMOS also revealed high fire resistance (Miyafuji and Saka2001). The composites prepared from sodium acetate appeared to be superior tothose prepared from sodium methoxide since, in the former composites, thesilicon part was mainly deposited specifically in the cell wall and produced ahigher bulking than in the latter. In addition, the sodium methoxide solution wasalkaline and caused a discoloration of the composites which was not the case forthe nearly neutral solution of sodium acetate.

The decay resistance against basidiomycete attack was enhanced by applyingthe amphoteric quaternary ammonium compound 3-(trimethoxysilyl)propyl(carboxymethyl) decylmethyl ammonium hydroxide inner salt (TMSAH) (Fig. 1)in the sol-gel process. SiO2-TMSAH and TMSAH composites were very resistantagainst brown-rot decay by Tyromyces palustris but displayed a low resistanceagainst Coriolus versicolor. The decay resistance was significantly enhanced byadditional application of HFOETMOS in the sol-gel system (Tanno et al. 1998;Saka et al. 1999, 2001). However, the durability of HFOETMOS-TMSAH-SiO2

wood was not compared to HFOETMOS-SiO2 wood in order to estimate thecontribution of TMSAH to durability.

Impregnation of beech and pine sapwood with propyltrimethoxysilane(PTMOS) (Fig. 1) without addition of a catalyst resulted in maximal anti-shrinkefficiency (ASE) of 35% (beech) and 27% (pine) while the equilibrium moisturecontent (EMC) was hardly reduced. Decay resistance tests revealed poor activityagainst basidiomycete fungi for both hardwood (against Coriolus versicolor) andsoftwood species (against Coniophora puteana) (Goethals and Stevens 1994).

Various approaches to achieve fixation of the SiO2 gels in the wood viacovalent bonds or polymerisable groups have been described. Treatment ofhardwoods and softwoods with c-methacryloxypropyl trimethoxysilane (MPT-MOS) (Fig. 1) decreased the EMC and increased the ASE by up to 70%. Thechange in the wood properties after leaching was relatively low (Schneider andBrebner 1985; Brebner and Schneider 1985).

In addition to the one-stage treatment systems described above, two-stageimpregnation procedures have been applied. In the first step, wood was treatedwith an organo-silane which is able to react with the cell wall polymers. Thus, acoupling agent is fixed in the cell wall. This can be linked in a second step bytetraalkoxy metal compounds such as TEOS or tetraisopropyl titanate (TPT). Inthe first step, coupling agent 3-isocyanatepropyl triethoxysilane (IPTEOS) (Fig. 1)was covalently fixed to the cell wall. In a second step, tetraisopropyl titanate wasused as a cross-linking agent (Saka and Yakake 1993).

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The highest ASE obtained with IPTEOS-modified wood was about 60% (20%WPG). However, additional treatment with TPT further increased the ASE up to80% (70% WPG) while specimens which were treated with TPT alone showednegative ASEs. The fire resistance was increased in the IPTEOS-TPT-treatedwood; the oxygen index amounted to 38 (23 in untreated wood) (Saka and Yakake1993).

A similar approach of a two-stage treatment combined organo-functionalsilanes (first step) with TEOS (second step) (Ogiso and Saka 1994). IPTEOS, b-(3,4-epoxycyclohexyl) ethyl trimethoxysilane (EETMOS), vinyl trimethoxysilane(VTMOS) and c-methacryloxypropyl trimethoxysilane (MPTMOS) were used tofix silanes covalently to the cell wall (VTMOS and MPTMOS were polymerised byinitiation with benzoyl peroxide). IPTEOS and EETMOS appeared to be cova-lently bound to the cell wall, while VTMOS and MPTMOS formed homopolymers.The ASEs of IPTEOS and EETMOS composites were between 30 and 40% (afterfirst impregnation) and increased linearly up to about 55% with increasingamount of TEOS. In the VTMOS composites the ASE increased linearly with theWPG to about 40% and was partly increased after TEOS treatment. However,cross-linking of the silicon-functional sites apparently did not occur. For theMPTMOS-SiO2 composites, ASEs up to 60% were achieved.

Micro-emulsion technologyCoatings and primers based on the micro-emulsion technology have beendeveloped for surface treatment of wood and masonry. The system consists ofdifferent silicon polymers in the form of a so-called micro-emulsion in water witha particle size from 10 to 80 nm (Gerhardinger et al. 1996; Hager 1995). Incomparison to ‘‘macro’’-emulsions of an oil phase in water which require anemulsifier, the micro-emulsion technology applies an additional co-emulsifierthat interferes with the quasi-crystalline monomolecular surfactant film (Fig. 2).In doing so, a particle size in the range of nanometres is obtained while that of‘‘macro’’-emulsions amounts to 1000 nm and more. Because of their size, themicro-emulsions are able to penetrate into the voids of wood which cannot bereached by conventional emulsions. The micro-emulsion typically consists of anagent to be emulsified (silane, siloxane or polysiloxane), an emulsifier (silane,siloxane) and a co-emulsifier (functional polysiloxane). Both emulsifier and co-emulsifier in the micro-emulsion technology are active ingredients at the sametime and lose their ability to emulsify after drying. When poured into water themicro-emulsions are activated since hydrolysis and condensation occur. There-fore, dilution should take place directly before the application due to a growingparticle size. The application of SMK micro-emulsions on wood caused high waterrepellence (reduction of water uptake up to 70% after two years of natural

Fig. 2. Composition of silicon micro-emulsions (Hager1995)

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exposure) and prevented micro-cracks (fibre separation) due to weathering(Hager 1995).

In a further study, two SMK micro-emulsions were compared in which bothproducts imparted high water repellence but dimensional stability was not im-proved (Lukowsky et al. 1997). Moreover, both products displayed increasedwater repellent efficiency (WRE) after artificial weathering. This was explained byfurther condensation of siloxanes in the presence of water. A low bath stability(short pot life) in the presence of water due to hydrolysis and condensation isreportedly a drawback for vacuum/pressure or dip treatment of both SMKproducts (Lukowsky et al. 1997).

Chlorosilanes and trimethylsilyl derivativesChlorosilane treatment (SiCl4) of various wood species was first reported byOwens et al. (1980). The decay resistance against Coriolus versicolor (white-rot)and five brown-rotters (Lentizes saepiaria, Lentinus lepideus, Poria monticola,Poria vaillantii, Poria carbonica) was tested. All treated samples showed signifi-cantly lower weight loss than untreated controls.

Stevens (1981, 1985) tested tetrachlorosilane (SiCl4), methyltrichloro silane(CH3SiCl3), dimethyldichloro silane ((CH3)2SiCl2), methyldichlorohydrogensilane (CH3SiHCl2) and trimethylsilyl chloride (TMSCl, (CH3)3SiCl) using basichydrochloric acid acceptors (triethylamine, formamide, dimethylformamide) aswell as hexane as solvents. The decay resistance of wood treated with SiCl4,(CH3)3SiCl and CH3SiCl3 was very low while CH3SiHCl2 and (CH3)2SiCl2 caused aconsiderable reduction of weight loss (5–10% compared to 25–34% in the con-trols). The efficacy of chlorosilanes against blue stain fungi (Aureobasidiumpullulans, Sclerophoma pithyophila) and molds (Penicillium spp., Alternariaalternata, Cladosporium herbarum) was low. Further drawbacks of chlorosilanetreatment include the need of dry conditions during treatment and the suscep-tibility of the Si-O-C(wood)-bond to hydrolysis. In addition, the monomers areacidic and hydrochloric acid is formed in the course of the reaction. In order toenhance the yield of the reaction and to bind hydrochloric acid a basic acceptor inmolar ratios is required. All these disadvantages mentioned let chlorosilanesappear hardly suitable for the application in practice.

In more recent studies, beech wood meal and beech beads as well as smallpieces of white fir were silylated with TMSCl (Zollfrank 2001), 1-(trimethylsilyl)-imidazole and N-(trimethylsilyl)-acetamide (Zollfrank and Wegener 2002). FT-IRmicro-graphs further revealed that silylation did occur on the surface and withinthe wood particles. These findings were confirmed by x-ray mapping (EDX). Themorphology of the wood cell appeared to be not severely degraded. Transmissionelectron micro-graphs showed that silylation mainly took place on the lumen-faced side of the cell wall and that half of the S2 wall appeared to be silylated(Zollfrank 2001; Zollfrank and Wegener 2002).

Maritime pine sapwood was esterified with compounds bearing trimethylsilylgroups: 3-trimethylsilylpropanoic anhydride (I), 2-trimethylsilylmethylglutaricanhydride (II), trimethylsilylethenone (III) (Sebe and De Jeso 2000). Grafting ofthese compounds to the cell wall polymers was confirmed by FT-IR as well as 13Cand 29Si NMR-CP MAS spectroscopy. ASE of specimens treated with I and II washigh at about 75% and 70% respectively, and increased with WPG. These valueswere stable after three leaching cycles. Dimensional stability of wood treated withIII (ASE 59%, WPG 22%) decreased significantly after five leaching cycles (32%)(Sebe and De Jeso 2000). Although the decay resistance of treated samples was not

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reported, a high durability can be expected due to strong cell wall bulking.However, the approach will remain of academic value since the compounds ap-plied are no technical bulk products and are very costly to synthesise.

Surface modification and hexametyldisiloxane-plasma coatingThe main aim of surface modification is to impart hydrophobicity and stabili-sation against weathering (including UV-stability) to wood. While most con-ventional surface treatments such as coatings and paints generally interactphysically with the substrate, Sebe and Brook (2001) report a three-step treatmentbased on a chemical fixation as follows: esterification with maleic anhydride,etherification with allyl glycidyl ether and hydro-silylation with hydride-termi-nated silicones. The presence of silicon on the surface was confirmed by electronspectroscopy for chemical analysis (ESCA). Neither an increase in WPG orswelling was observed after hydrosilylation (last step), nor could silicon com-pounds be detected by infrared spectroscopy. The silylated wood showed excel-lent hydrophobicity. However, when 1.5 mm of the surface was removed thecontact angle was significantly reduced. The authors deduced that hydrosilylationoccurred preferentially at the external surface.

Surface modification with cold plasma is a dry process which alters only theoutermost layer of the wood surface (Denes et al. 1999). Plasma is a gas con-taining positive and negative ions, electrons, radicals, and excited and non-ex-cited neutral particles in parallel but with no electric charge. Cold plasmas operateat reaction temperatures at about room temperature and possess very low degreesof ionisation. The process is usually stimulated by using electric energy (Deneset al. 1999). Wood plasma coating with hexamethyldisiloxane (HMDSO) (Fig. 3)was able to impart a water contact angle higher than 120� (almost zero on un-treated, fresh wood) while water uptake in an immersion test was significantlyreduced (Cho and Sjoblom 1990; Mahlberg et al. 1998; Denes et al. 1999; Pod-gorski 2002). A cross-linked polymeric structure based on Si–O–Si and Si–O–Clinkages was detected on the surface by ESCA, ATR-FT-IR and pyrolysis massspectrometry; the structure could have been formed by selective bond breakingand recombination of the initial monomer HMDSO. The coating showed highthermal stability confirmed by thermo-gravimetric analysis (TGA) and differen-tial thermal analysis (DTA) (Denes et al. 1999).

Due to the hydrophobic nature of the HMDSO-treated surface the adhesion ofpaints was poor. However, the outermost surface could be hydrophilised by ac-rylic acid plasma post treatment without reduction of water penetration of thewhole treated wood blocks (Cho and Sjoblom 1990). HMDSO treatment did notresult in an improvement in the adhesion of polypropylene film to wood(Mahlberg et al. 1998).

ConclusionsSeveral studies on the application of silicon compounds for wood modificationhave been reported which describe the utilisation of a wide variety of chemicals.Some of the treatments reported address more academic and fundamental

Fig. 3. Chemical structure of hexamethyldisiloxane

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questions, e.g., related to the controlled alterations in specific wood properties(Sebe and De Jeso 2000; Zollfrank 2001; Zollfrank and Wegener 2002). Variouswood properties such as dimensional stability, moisture uptake, weathering andfire resistance as well as durability could be improved.

Organo-functional silanes have been described as property enhancers incombination with tetraalkoxysilanes in order to impart a hydrophobic nature tothe inorganic silicates derived from the sol-gel process. This hydrophobationreduces moisture uptake and leaching of chemicals from the wood composite.Moreover, the application of an organo-functional silane bearing a fungicidalquaternary ammonium group enhanced the durability of the resulting composites(Tanno et al. 1998; Saka et al. 1999, 2001).

Although some applications were able to improve various wood properties thetreatment procedures appear to be too complicated to be feasible in practice. Thisis especially true for modifications which are performed in several steps or re-quire expensive organic solvents and a costly curing procedure after each step ofthe treatment. An example includes the modifications with organo-silanes whichare condensed in a second step by applying sol-gel chemistry (Saka and Yakake1993; Ogiso and Saka 1994).

Some of the treatments described in this review including micro-emulsiontechnology, plasma coating or hydro-silylation of modified wood (Sebe andBrook 2001) are designed as surface treatments, not as full treatments for thewood.

A careful evaluation of the durability reported for the various treatments re-vealed that in many cases the decay resistance was improved insufficiently whensilicon compounds were applied alone. This tendency could also be observed fortreatments applying inorganic silicon compounds (Mai and Militz 2002). For thisreason, silicon treatment was combined with fungicides such as quaternaryammonium compounds (Tanno et al. 1998; Saka et al. 1999). A comparabletendency could be observed in respect to fire resistance (Mai and Militz 2002;Saka and Tanno 1996; Saka et al. 2001; Miyafuji and Saka 2001).

Several formulations which contain organo-silicon monomers or polymers areable to cause excellent water repellence without significantly reducing thedimensional stability and moisture uptake of wood (micro-emulsion technology,polyalkylsiloxanes) (Hager 1995; Belyi et al. 1985). Because of their high chemicaland weathering stability, the application of these formulations on wood exposedto conditions of hazard class III (EN 335, outside exposure without soil contact) isrecommended.

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