Comparison of shear strength of linear welded beech wood and urea-formaldehyde adhesive bond

Les je naravni material, ki ga uporabljamo za izdelavo različnih izdelkov, tako v notranjih kot zunanjih prostorih. Varjenje lesa predstavlja tehniko spajanja lesa, brez uporabe lepil. Največja pomanjkljivost tako spojenega lesa je neodpornost spojev na povišano vlažnost. To lastnost smo poskusili izboljšati s termično in kemično modifikacijo bukovine pred varjenjem, pri čemer smo les tretirali z raztopino borove, citronske in ocetne kisline. Za primerjavo smo nekaj vzorcev lesa zlepili tudi z urea-formaldehidnim lepilom, ki je namenjeno za lepljenje izdelkov za uporabo v suhih pogojih. Vzorce spojenega lesa smo pripravili v skladu s standardom SIST EN 12765. Ugotovili smo, da vnos različnih raztopin kislin v les ni izboljšal trdnosti varjenih spojev in njihove odpornosti na povišano vlažnost. Med uporabljenimi kislinami sta se v suhih pogojih najbolje izkazali citronska in ocetna kislina. Zvarjen naravni in termično modificiran bukov les ima določeno strižno trdnost (6,6 oz. 9,1 N/mm2), ki pa je nižja od strižne trdnosti spoja pri lepljeni naravni in termično modificirani bukovini z urea-formaldehidnim lepilom (14,3 oz. 11,9 N/mm2).Wood is a natural material to manufacture a variety of products, for internal and external usage. Welding of wood is a technique for joining wood without use of any synthetic adhesive. The main imperfection of such wood is its low resistance to higher humidity. We tried to improve this property with thermal and chemical modification of welded beech wood, with solutions of boric, citric and acetic acid added. For comparison, several samples of wood were bonded using urea-formaldehyde adhesive quite often used to manufacture products for dry conditions. Samples of such wood were then prepared in accordance to standard SIST EN 12765. Among the acids used for dry conditions, citric and acetic acid showed the best results. Welded natural and heat treated beech wood had a certain strength (6.6 or 9.1 N/mm2), lower than shear strength of bonded natural and thermally modified beech wood with urea-formaldehyde adhesive (14.3 or 11.9 N/mm2)