Abstract
This study deals with the influence of chemical modification on elasto-mechanical properties of Scots pine (Pinus sylvestris L.). The elasto-mechanical properties examined were impact bending strength, determined by impact bending test; tensile strength; and work to maximum load in traction, determined by tensile tests. The modification agents used were one melamine-formaldehyde resin (MF), one low molecular weight phenol-formaldehyde resin, one higher molecular weight phenol-formaldehyde resin, and a dimethylol dihydroxyethyleneurea (DMDHEU). Special attention was paid to the influence of the solution concentration (0.5%, 5%, and 20%). With an increase in the concentration of each modification agent, the elasto-mechanical properties decreased as compared to the control specimens. Especially impact bending strength decreased greatly by modifications with the 0.5% solutions of each agent (by 37% to 47%). Modification with DMDHEU resulted in the highest overall reduction of the elasto-mechanical properties examined (up to 81% in work to maximum load in traction at 20% solution concentration). The results indicate that embrittlement is not primarily related to the degree of modification depended on used solution concentration. It is therefore assumed that molecular size and the resulting ability to penetrate into the cell wall could be crucial. The results show that, in the application of chemically modified wood, impact and tensile loads should be avoided even after treatment with low concentrations.
Highlights
Among others the main objectives of chemical modifications of wood are to improve dimensional stability and resistance against wood-decaying fungi in order to offer an alternative to tropical hardwoods
The reasons why chemical modification systems influence biological and physical properties of wood have been the subject of numerous studies [1,2,3,4]
Mechanisms via which impregnation modifications work according to Hill [5] are: (1) permanent swelling by incorporating the resin in the cell wall structure; (2) moisture content is reduced; (3) cell wall pores with a size of 2–4 nm are blocked and it is assumed that a chemical reactions between modification agents and cell wall polymers occurs
Summary
Among others the main objectives of chemical modifications of wood are to improve dimensional stability and resistance against wood-decaying fungi in order to offer an alternative to tropical hardwoods. The modification may, cause undesirable changes of other wood characteristics, mainly elasto-mechanical properties. The reasons why chemical modification systems influence biological and physical properties of wood have been the subject of numerous studies [1,2,3,4]. Mechanisms via which impregnation modifications work according to Hill [5] are: (1) permanent swelling by incorporating the resin in the cell wall structure (bulking); (2) moisture content is reduced; (3) cell wall pores with a size of 2–4 nm are blocked and it is assumed that a chemical reactions between modification agents and cell wall polymers (cross-linking) occurs. Impregnation modifications based on reactive resins have been shown to result in embrittlement of wood [6,7,8]. The embrittlement is most evident in a decrease of tensile properties and dynamic strength properties like impact bending strength [9,10]
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