Sol-Gel Wood Preservation
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€bert and Muhammad Shabir Mahr Thomas Hu
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wood and Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wood Impregnation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of Structure and Properties of Impregnated Wood . . . . . . . . . . . . . . . . . . . . . . . . . Structure of Sol-Gel Impregnated Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Sol-Gel Modified Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weathering Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol-Gel Based Wood Preservation for Improved Biodurability . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol-Gel-Based Wood Preservation for Improved Fire Retardancy . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract
The sol-gel-based modification of wood introduces chemical substances into wood in order to improve its characteristics and impart new properties. It stabilizes dimensions of wood (timber) components, increases its strength and resistance to water, and reduces cracking. Many sol-gel-based impregnations aim to protect against wood rot and fire. In most cases, the treatments are performed with alkoxysilanes, polysiloxanes, colloidal silica, or organically modified silica. In addition further substances such as titania, copper, and boron compounds have been applied on different types of wood. The precursor solutions were introduced
T. H€ ubert (*) · M. Shabir Mahr Federal Institute for Materials Research and Testing (BAM), Berlin, Germany e-mail:
[email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Klein et al. (eds.), Handbook of Sol-Gel Science and Technology, https://doi.org/10.1007/978-3-319-32101-1_106
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by immersion, painting, or spray coating of wood followed by a drying and heat treatment process. The structure of the impregnated wood was investigated by SEM, EDX, TEM, FT-IR, NMR, and XRD. Frequently, test procedures according to standards were applied in order to assess the improvement in properties. Investigations demonstrate that silica and titania impregnations change properties first of all with increasing the amount of absorbed inorganic material (WPG), which is determined by the concentration of precursors, amount and size of particles in the sol, and the impregnation method. Sol-gel impregnation can be considered as an environmentally friendly approach of wood preservation. Various improvements in wood properties can be achieved such as reduced water uptake and volume swelling, improved weather stability, resistance against biodegradation, and fire retardancy.
Introduction Wood is a porous and fibrous tissue of plants and is composed mostly of the biopolymers, cellulose, hemicellulose, and lignin. Additionally it contains so-called extractives, such as resins, fatty acids, waxes, and terpenes. One can distinguish sapwood as the younger, living outer wood part of a growing tree from heartwood which has formed as a result of naturally occurring transformation and consists in the final form of dead cells. Early wood is formed earlier in the growth season and has thin-walled cells with large cell cavities (so-called lumen), whereas latewood has thick-walled cells with very small cavities and it is denser, harder, smoother, and darker than early wood. These differences in morphology and widths of latewood have been shown to affect wood properties, weathering, and bio-decay. Furthermore, wood properties are anisotropic in relation to the main growth direction of the tree trunk and are axial (or longitudinal), radial, and tangential in 3D space. The anisotropy is due to the orientation of the wood cell, preferred in axial direction. Wood has been used by mankind for thousands of years due to its numerous favorable properties for building homes, bridges, fences, ships, and many other structures. Wood is available in many species, sizes, and shapes. Most people are familiar with its aesthetic inherent beauty. It is an easy to work and handle material. When dry, wood has good insulating properties against heat, cold, sound, and electricity. It is low cost, sustainable, environmentally friendly, and renewable. Unfortunately, many types of wood have some drawbacks in their application, such as instability against weathering due to moisture, rain, and ultraviolet radiation. Wood is combustible when provided with adequate heat and oxygen. In fact, it is the most widely used fuel in many parts of the world. Wood has a porous structure, and the biopolymers can be served as foodstuff for fungi, insects, bacteria, and marine borers resulting in biodegradation and loss of its functionality. Even though some types of wood are quite resistant against biodegradation due to their inherent components, usually, wood preservation is needed. The aim of contemporary wood preservation is to protect wood from weathering, photodegradation (graying), fire, and wood pests. This can be performed in a constructive way, e.g., by avoiding
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contact with water. However, this cannot always be excluded. Therefore, wood can be classified in respect to its uses into five classes: 1. Under cover, not exposed to the weather and wetting. 2. Under cover and not exposed to the weather (particularly rain and driven rain), but not persistent, wetting can occur. 3. Above ground and exposed to the weather (particularly rain). 4. Indirect contact with ground and/or fresh water. 5. Seawater contact (EN 335: 2013). These different application classes imply consequences for the needed protection of wood (EN599: 1996). Chemical wood preservation has been known since long term. Since ancient times, olive oil and tar were used for wood preservation. In the nineteenth century, railroad ties and telegraph poles were immersed in creosote, a product of coal tar distillation. Since then, various substances and their mixtures have been suggested, investigated, and commercialized. However, the use of wood preservatives is ambivalent, because poisoning substances are used for protection of wood which may leach out to the environment and have impact on animals and plants. Therefore, some substances are nowadays in part prohibited to use, such as chlorinated organics, arsenic, or chromium(VI) compounds. In order to protect wood from fire, inorganic substances such as aluminum hydroxide and organic compounds containing phosphor or halogens were used. Protection against graying can be achieved by the application of UV-absorbing substances, such as benzophenone or zinc oxide. This chapter focuses on wood protection by no or scant colored impregnations which preserve the original wood surface instead of hiding it under a thick opaque or in-transparent pigmented layer. Paints based on a polymeric matrix which can contain sol-gel-derived pigments and fillers are not considered in the following. Today’s impregnations for increasing stability toward photodegradation, bio-decay, and flammability are limited in durability and environmental compatibility. Therefore, further research and development has been carried out. Contemporary wood protection should pursue a multiple approach to improve the properties altogether in relation to the intended application of this material (see Fig. 1). A promising approach of a multifunctional protection and improvement of wood properties is sol-gel technology based on compounds which contain in particular nontoxic elements such as silicon or titanium. However, a benchmark for innovation is the approved commercial wood preservations. First attempts to improve fire retardancy of wood by sol-gel technology started already in the nineteenth century by application of potassium water glass (Fuchs 1825). Systematic investigations in this area were performed by Furuno in the 1980s of the last century who investigated the silica mineralization of wood and performed treatments with aqueous silicate solutions (Furuno et al. 1986, 1991). Schneider and Saka impregnated wood by several alkoxysilanes (Schneider and Brebner 1985; Saka et al. 1992). This approach is inspired by a natural process of the formation of
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Fig. 1 Targets of contemporary wood preservation
silicified wood. Silicified wood develops over millions of years through the infiltration of silicic acid into the wood tissue. A silica gel is formed by polycondensation, which further reacts to form quartz (chalcedony) and opal (wood opal). Several researchers have applied sol-gel technology to impregnate wood with inorganic (mostly oxide) or hybrid (organic/inorganic) materials. On this topic, more than 70 papers were published between 2010 and 2016. An overview of sol-gelderived approaches for wood modification is given in Table 1 (see page 2799). However, their technical use and commercialization are still pending. This is because of a long-term process for commercial approval. In many cases, these sol-gel-based impregnations were called wood composites because of the presence of relatively high amount of new ingredients in wood and the resulting change in properties. However, it should be considered that these impregnations are not always chemically modifying wood. A strong chemical bonding between the dispersant and the matrix is not likely; mostly the wood structure is preserved.
Experimental Wood and Chemicals Several types of wood (mostly sapwood) are used for sol-gel-based impregnations, often in respect to the sphere of activity of the investors. Examples are pine, spruce, larch, birch, beech, aspen (poplar), robinia, oak, and exotic wood species such as Japanese cedar and cypress, Chinese fir, teak, rubberwood, bamboo, white lauan, yemane, eucalyptus wood, and Brazilian pine.
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Table 1 Precursors for wood impregnation No 1
2
Precursor Silicates (water glass) Lithium, sodium, or potassium silicate solutions Additionally: Al2(SO4)3, CaCl2, BaCl2·2H2O, B2O3, H3BO3, Na2B4O7·10H2O, K2B4O7·4H2O, (NH4)2O·5B2O3·8H2O Lithium silicates + dibutyl amine phosphate + ZnSO4 Metyltrimethoxysilane (MTMOS), Sodium methoxide, Sodium acetate Pure silica sol, silica emulsion Silica sols, silicic acid
Tetraethoxysilane (TEOS)
3
Silica + salts or oxides Colloidal silicic acid + H3BO3 Silica + H3BO3 or + CuCl2, ZnCl2, FeCl2, CoCl2 SiO2 + ammonium salt (TMSAC)
4
Hydrolysis/ decomposition/ solvent
Wood
Reference (selection)
Biocide (fungi and termites) Fire retardant
Japanese cedar (Cryptomeria japonica)
Water
Fire retardant
Monterey pine (Pinus radiata)
Acidic (HAc) or alkaline
Fire retardant
Western hemlock (Tsuga heterophylla)
Miyafuji and Saka 2001
Basic, water
Biocide
Japanese cedar
Acidic Basic
Water uptake Dimensional stability Fire retardant Hardness Biocide
Yamaguchi 1994a Yamaguchi 1994b Yamaguchi 2002 Saka et al. 1992; Cookson et al. 2007 Mahltig et al. 2008 Unger et al. 2013
Basic, water, ethanol
Monterey pine Brazilian pine (Araucaria angustifolia)
Water, ethanol
Biocide (termites) Fire retardant
Water
Biocide (brown-rot fungi) Biocide
Silica + aluminum oxychloride
Acidic (HAc), ethanol Water
Organo-functional silane Decyltrimethoxysilane, Hexadecyltriethoxysilane HDTMOS + MTMOS
Acidic (TFA)
Metyltriethoxysilane (MTES) Propyltriethoxysilane (PTEO) Tetrapropoxysilane (TPOS)/ Propyltriethoxysilane (PTEO) Methyltrimethoxysilane (MTM) Octyltriethoxysilane (OTES) Triethoxysilane (TES) + polydimethylsiloxane (PDMS) Methoxy-terminated dimethylphenylsiloxane (DMS) + N-octyltriethoxysilane (n-OTES) PDMS/(n-O)TES, Methyltrimethoxysilane (MTM)
Application/ target property
n-heptane
Acidic (HCl), Ethanol, Water
Water uptake Biocide (blue strain) Water repellent Fire retardant Water repellant Weathering Water uptake, Anti-swelling
Poplar (Populus ssp.) Scots pine (Pinus sylvestris) Monterey pine, Castanospermum australe
Yamaguchi 2003
Hinoki (Chamaecyparis obtusa)
Tanno et al. 1998
Scots pine, beech
Pries and Mai 2013b
Loblolly pine (Pinus taeda L.) Scots pine, Beech (Fagus sylvatica)
Biocide Discoloration
Pine, Beech
Water, Solvent
Water repellent, Anti-swelling, Biocide (fungi) Water repellant Fire retardant
Pine
Basic (NH4OH), Water
Vinyl- or (3-mercaptopropyl)trimethoxysilane and an organically modified zirconium-oxocluster TEOS+MTMOS, 2-heptadecafluorooctyl-ethyltrimethoxysilane (HFOETMOS) TEOS + 2-heptadecafluorooctylethyltrimethoxysilane (HFOETMOS) + propyldimethyloctadecyl ammonium chloride (TMSAC) Silica + perfluoroalkyl methacrylic copolymer TEOS + dimethyldiethoxysilane, perfluoorooctyltriethoxysilane (PFOS) Phenyltriethoxysilane (PHTES) Chlorotrimethylsilane (CTMS) octadecyltrichlorosilane (OTS) dichlorodiphenylsilane (DPS) dichlorodimethylsilane (DDS) TEOS + octadecyltrichlorosilane (OTS) + polyvinyl acetate (PVA) TEOS + 3-aminopropyltriethoxysilane (APTES), + CuCl2/+ H3BO3 Aminopropylmethyldiethoxisilane Alkoxisilane + Amino/alkyl functionalized siloxane
Acidic, THF Acidic (HAc) Ethanol, Methanol Acidic (HAc), Ethanol Methanol
Marney et al. 2008 Pereyra and Giudice 2009 Canosa et al. 2011
Japanese black pine (Pinus thunbergii ) Western hemlock Pine Japanese cedar
Water, Solvent
HDTMOS + TEOS Methylmethoxydisiloxane + methyltrimethoxysilane + trimethylborate (TMB)/H3BO3 + trimethyphosphite (TEP)/H3PO4 trimethylphosphite (TMP)
Furuno et al. 1991 Furuno et al. 1992 Furuno and Imamura 1998
Beech (Fagus sylvatica) Chinese fir (Cunninghamia lanceolata) Western hemlock
Böttcher et al. 1999 Yamaguchi 2002
Saka and Ueno 1997; Miyafuji et al. 1998 Tshabalala and Gangstad 2003 Donath et al. 2004
DeVetter et al. 2009b
De Vetter et al. 2010 Pries et al. 2013 Chang et al. 2015 Miyafuji et al. 1998
Biocide (brown rot) Fire retardant, Discoloration Water repellant, Water uptake
Pine, Larch Western hemlock
Girardia et al. 2014 Miyafuji and Saka 1999
Water repellent Biocide
Hinoki
Tanno et al. 1998
Maggini et al. 2012
Water
Water repellent
Pine (Pinus taiwanensis)
Hsieh et al. 2011
Acidic (HCl)
Water repellent
Larch (Larix decidua), Pine
Cappelletto et al. 2013
Ethanol, water Acidic (TFA) n-heptane
Water repellent Water uptake
Apple (Malus sylvestris), Eucalyptus, Spruce (Picea abies)
Mohammed -Ziegler et al. 2008
Basic (NH4OH) Ethanol Ethanol
Water repellent
Poplar
Liu et al. 2013
Bio attack
Scots pine
Palanti 2012b
Basic (NH4OH)
Fire retardant
Brazilian pine (Araucaria angustifolia)
Giudice et al. 2013a
Acidic (HCl), Ethanol, Water
Water uptake
Scots pine
Donath 2006
(continued)
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Table 1 (continued) Alkoxysilane + trimethylborate (TMB) trimethylphosphite (TMP) TEOS + APTES + copper sulfate (CuSO4) or H3BO3 TEOS/silicic acid + H3BO3 MTMOS + HDTMOS + Aluminum isopropoxide (AIP) TEOS+ 2,2’,4’-trihydroxy-4- [2-hydroxy-3(3-trimethoxysilylpropoxy) propoxy] benzophenone (BP) TEOS + 3-(trimethoxysilyl) propyl octadecyl ammonium chloride (TMSAC) Polydimethylsiloxanes bearing amino and quaternary ammonium groups Methacryloxypropyltrimethoxysilane (TMPS)
Vinyltrimethoxysilane (VTMS) 3-trimethylsilylpropanoic anhydride, 2-trimethylsilylmethylglutaric anhydride, trimethylsilylethenone , bis(trimethylsiloxy)methylsilane, poly-dimethylsiloxane, 3-isocyanatepropyl triethoxysilane (IPTEOS) 3-glycidoxypropyltrimethoxysilane (GPTMS), n-propyltrimethoxysilane (PTMS) Silica epoxysilane surface modified 5
Non-silicon compounds TiO2 nano particles TiO2 dispersion + UV radiation TiO2 + (heptadecafluoro-1,1,2,2tetradecyl trimethoxosilane Tetrabutyl orthotitanate (TBOT), tetraisopropyl titanate (TPT) + Zinc nitrate Titanium(IV) n-butoxide (TBT) titanium isopropoxide (TIP)
Zn acetate, Zn nitrate, Trimethyl borate (TMB)
Fire retardant
Miyafuji and Saka 1996
Ethanol
Biocide (termites)
Scots pine
Feci et al. 2009
Acidic (TFA) Isopropanol Acidic (HAc)
Photodegradation, UV resistant UV stability
Western hemlock
Miyafuji 2004
Acidic (HAc) THF, n-butylacetate Isopropanol
Biocide
Hinoki
Tanno et al. 1998
Biocide (white, brown rot, blue stain)
Scots pine, Beech
Gosh et al. 2012a
Methanol
Biocide, Anti-swelling
Schneider and Brebner 1985
Acidic (HAc) DMF, Pyridine
Water repellent, Anti-swelling
Eastern white pine (Pinus strobus), White birch (Betula papyrifera), Trembling aspen (Populus tremuloides) Corsican pine (Pinus nigra) Maritime pine (Pinus pinaster)
Acidic, Water
Water repellant Biocide
Scots pine
Liu et al. 2015 Pries and Mai 2013
Water Water
Biocide (mould) Biocide
Paulownia Scots pine, silver fir, walnut, chestnut, wild cherry, sessile oak, beech, ash
Chen at al. 2009 De Filpo et al. 2013
Poplar (Populus ussienis) Poplar Scots pine Poplar Chinese fir
Gao et al. 2015 Sun et al. 2010 Sun et al. 2012 Hübert et al. 2010; Qin and Zhang 2012; Wang et al. 2012
Water repellant
Polar (Populus euramericana)
Fu et al. 2012
Leachablility
Poplar
Zhang 2015
Acidic
Water repellant
Basic, Ethanol
Water resistance Water resistance Fire retardant Biocide
Acidic (HAc, HCl), 2Methoxyethanol, Ethanol Basic (NaOH), Ethanol, water Acidic (HAc), Ethanol
Tshabalala 2007
Hill et al. 2004 Sèbe and De Jeso 2000 Sebe and Brook 2001 Sebe et al. 2004
For sol-gel-based investigations, commercial colloidal sols prepared from hydrolysis of alkoxides or originated on water glass were available. Alternatively, the investigators prepared their precursor solutions by themselves from a broad variety of available alkoxides, siloxanes, and further components1. Frequently used solvents are water, ethanol, or isopropanol. Hydrolysis was performed in acidic or alkaline media using, for example, acetic and hydrochloric acid, or sodium hydroxide or ammonia. Precursor compositions for silica-based sols have an alkoxide/water molar ratios of
