Activation of Wood Surface and Nonconventional Bonding

Zavarin, Eugene

doi:10.1021/ba-1984-0207.ch010

Cited by 37 publications

(35 citation statements)

References 25 publications

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“…Ether bonds are less receptive to hydrogen bonding with polar adhesives than the original hydroxyl groups (Christiansen 1991). A loss of hygroscopicity is assigned to a gradual loss of wood hydroxyl groups during drying (Zavarin 1984). This mechanism cannot completely explain poor adhesion of thermally inactivated wood.…”

Section: Elimination Of Surface Hydroxyl Bonding Sitesmentioning

confidence: 99%

Comparative analysis of inactivated wood surfaces

Šernek¹,

Kamke²,

Glasser³

2004

Holzforschung

132

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A wood surface, which is exposed to a high temperature condition, can experience inactivation. Surface inactivation reflects physical and chemical modifications of the wood surface. Consequently, these changes result in reduced ability of an adhesive to properly wet, flow, penetrate, and cure. Thus, an inactivated wood surface does not bond well with adhesives.The changes in surface chemistry, wettability, and adhesion of inactivated wood surfaces, including heartwood of yellow-poplar (Liriodendron tulipifera) and southern pine (Pinus taeda), were studied. Wood samples were dried from the green moisture content condition in a convection oven at five different temperature levels ranging from 50 to 200°C. The comparative characterization of the surface was done by X-ray photoelectron spectroscopy (XPS), sessile drop wettability, and fracture testing of adhesive bonds. Additionally, several chemical treatments were utilized to improve wettability and adhesion of inactivated wood surfaces.The comparative analysis helped elucidate clear relationships between surface chemistry, wettability, and bond performance in regard to surface inactivation. XPS results showed that wood drying caused modification in wood surface chemistry. The oxygen to carbon ratio (O/C) decreased and the C1/C2 ratio increased with drying temperature. The C1 component is related to carbon-carbon or carbon-hydrogen bonds, and the C2 component represents single carbonoxygen bond. A low O/C ratio and a high C1/C2 ratio reflected a high concentration of non-polar wood components (extractives/VOCs) on the wood surface, which modified the wood surface from hydrophilic to more hydrophobic. A hydrophobic wood surface repelled water and wettability of this surface was low (i.e., a high contact angle). Wettability was directly related to the O/C ratio and inversely related to the C1/C2 ratio.iii Contact angle decreased with time and increased with the temperature of exposure. A dependence of wood species was evident. Southern pine had a lower wettability than yellowpoplar, which was due to a greater concentration of non-polar hydrocarbon-type extractives and heat-generated volatiles on the surface. Solvent extraction prior to drying did not improved wettability, whereas, extraction after drying improved wettability. A contribution of extractives migration and pyrolysis products deposition played a significant role in the heat-induced inactivation process of southern pine.The maximum strain energy release rate (G max ) obtained by fracture testing showed that surface inactivation was insignificant for yellow-poplar when exposed to drying temperatures < 187°C. The southern pine was most susceptible to inactivation particularly when bonded with phenol-formaldehyde (PF) adhesive. A typical surface inactivation for southern pine occurred at drying temperatures > 156°C.Chemical treatments improved the wettability of inactivated wood surfaces, but an improvement in adhesion was not evident for specimens bonded with polyvinyl-acetate (PVA) adhesive. Of the chemica...

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Section: Elimination Of Surface Hydroxyl Bonding Sitesmentioning

confidence: 99%

Comparative analysis of inactivated wood surfaces

Šernek¹,

Kamke²,

Glasser³

2004

Holzforschung

132

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“…The heat causes plastification of the lignin in the middle lamellae and, in consequence, separation of the fibres during the defibration process. Upon cooling down, the lignin hardens again and forms a glassy crust on the surface of the fibres (Mjöberg 1981, Zavarin 1984, Wagenführ 1988, Kharazipour & Hüttermann 1993; Fig. 3A).…”

Section: Enzymes In Lignin Activationmentioning

confidence: 99%

“…Lignin is modified e.g. by hydrolytic cleavage of ether bonds and by homolytic cleavage of covalent bonds (Hon 1983, Lee & Sumimoto 1990, resulting in the generation of low molecular weight lignin fragments and relocation of lignin from inner cell wall layers to the fibre surface (Zavarin 1984, Widsten et al 2001. Homolytic cleavage of β-O-4 ether bonds in fibre lignin generates phenoxy radicals that may undergo various further chemical reactions affecting i. the molecular size distribution and content of functional groups of the lignin polymers and ii.…”

Section: Enzymes In Lignin Activationmentioning

confidence: 99%

“…Pressing at high forces and temperatures for long periods can create water-resistant bonds (Klauditz & Stegmann 1955, Back 1987, Unbehaun et al 2000. Pyrolytic degradation of hemicellulose sugars and thermal conversion to furan products have been discussed to contribute to the bonding and increase of free reactive sites on the aromatic ring of lignin units and to auto-condensation of lignin (Zavarin 1984, Ellis & Paszner 1994, Tjeerdsma et al 1998, Rowell et al 2002. Boards formed by just pressing fibres have however unfavourable properties: they show high thickness swelling in presence of water and have poor mechanical properties , Unbehaun et al 2000.…”

Section: Enzymes In Lignin Activationmentioning

confidence: 99%

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Wood production, wood technology, and biotechnological impacts

Kües¹

2007

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“…By applying hot air and afterwards hot steam, temperatures well over 100 °C are reached, which are required in the hardening of native wood fiber lignin (Euring and Kharazipour 2012;Euring et al 2015). Especially thermo-mechanical-pulping (TMP) wood fibers have the ability for self-bonding, also called auto-adhesion (Zavarin 1984;Back 1987;Suzuki et al 1998;Unbehaun et al 2000). Caused by the thermo-mechanical treatment of wood, the glass transition temperature (Tg) of native lignin is reached between 100 and 170 °C (Irvine 1985;Nada et al 2002).…”

Section: Introductionmentioning

confidence: 99%

Pre-pressing and Pre-heating via Hot-Air/Hot-Steam Process for the Production of Binderless Medium-Density Fiberboards

Euring¹,

Kirsch²,

Kharazipour³

2016

BioResources

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The hot-air/hot-steam process was used for the first time as a combined pre-pressing and pre-heating system for the production of medium-density fiberboards (MDF) at the pilot scale. Pre-heating systems are designed to pre-heat fiber mats before pressing by hot-presses. Using such techniques, pressing times are reduced significantly and the board properties are influenced positively; both are essential for effective MDF production. In recent years, industry has searched for alternatives to petrochemical binders. Primarily, MDF are bonded by urea-formaldehyde (UF) resins in Europe. To replace UF resins, a laccase-mediator-system (LMS) was used to activate the wood fibers' self-cohesion. It was found that the internal bond strength (IB) and thickness swelling (TS) were noticeably improved by applying the hot-air/hot-steam process before final hot-pressing for both LMS and 10% UF binding systems. Simultaneously, the total pressing time could be reduced by 25% when combining the hotair/hot-steam process with hot-pressing.

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Activation of Wood Surface and Nonconventional Bonding

Cited by 37 publications

References 25 publications

Comparative analysis of inactivated wood surfaces

Comparative analysis of inactivated wood surfaces

Wood production, wood technology, and biotechnological impacts

Pre-pressing and Pre-heating via Hot-Air/Hot-Steam Process for the Production of Binderless Medium-Density Fiberboards

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