An investigation into the edge gluing of green Eucalyptus grandis lumber using a one-component polyurethane adhesive

Thesis (MScFor)--Stellenbosch University, 2017.ENGLISH ABSTRACT: South Africa is a timber-scarce country with a relatively small portion of only about 1% of its total land area being used for commercial forestry. Due to an increasing demand for timber but restrictions regarding the expansion of plantation area, the country is expected to experience a shortage of softwood supply for saw logs in the near future. The predominant hardwood species Eucalyptus grandis is, despite good strength and stiffness properties, at present for the most part used for wood chip or pulp and paper production. This is mainly owing to growth stresses, splitting, and the low dimensional stability of the wood, which therefore could not comply with the requirements of the South African national standard for sawn eucalyptus timber (SANS 1707-1 2010). As wood defects often develop or aggravate during the drying stage, the edge-bonding of Eucalyptus grandis lumber in its wet state was considered to possibly inhibit this behaviour. This study consisted of two experiments. In the first experiment shear tests were used to determine the influence of various material and processing variables on the bonding quality of green Eucalyptus grandis wood with a moisture-curing one-component polyurethane adhesive. The experimental design for this investigation comprised 16 groups with different combinations of parameters for wood density, moisture content, adhesive spread rate and pressure. Ten samples per group were tested for shear strength. The penetration behaviour of the one-component polyurethane adhesive into the wood structure was additionally investigated on samples with extreme results, employing a micro CT scanner. All tested samples clearly exceeded the minimum shear strength for cross-laminated timber products according to EN 16351 (2015). Better results were generally obtained for samples with an increased moisture content of roughly 60% compared to specimens with a lower moisture content around fibre saturation point, which could be linked to an enhanced adhesive penetration. In the second experiment the potential of edge gluing green Eucalyptus grandis boards before kiln drying in order to inhibit the development of certain wood defects was investigated. Edge-glued panels were produced from wet material above fibre saturation point and kiln-dried together with non-edgebonded control boards from the same material source. After drying, the panels were sawn apart into single boards, graded regarding the development of check, split, bow, cup and twist and compared to the results obtained for the control boards. The ability of stress-relief grooves in boards to reduce the development of defects was also investigated. The results showed that the edge gluing of green Eucalyptus grandis lumber before kiln drying could not decrease the number of board rejections according to the SANS 1707-1 (2010) requirements for sawn eucalyptus timber. Cup could be significantly decreased, while twist was only reduced for boards without pith. Stress-relief grooves did not have a significant influence on the development of any of the investigated defects but caused severe deformation and damage in some of the boards. Further investigations should be carried out on mass timber products such as cross-laminated timber, where green edge-glued and kiln-dried Eucalyptus grandis panels could be used as a component.AFRIKAANSE OPSOMMING: Suid Afrika het min boshulpbronne met slegs ‘n klein gedeelte, van ongeveer 1%, van sy landoppervlak wat gebruik word vir bosbou. Daar word verwag dat ‘n groeiende vraag na soliede saaghout en krimpende plantasie-area sal lei tot ‘n tekort in naaldhoutprodukte in die nabye toekoms. Die mees aangeplante loofhoutspesie, Eucalyptus grandis, word ten spyte van goeie sterkte -en styfheidseienskappe hoofsaaklik vir pulp en spaanderproduksie gebruik. Dit is hoofsaaklik as gevolg van groeispannings, spleting, en swak dimensionele stabiliteit van die hout – wat veroorsaak dat dit nie voldoen aan die vereistes van soliede Eucalyptus saaghout nie (SANS 1707-1 2010). Houtdefekte ontwikkel of vererger dikswels gedurende die drogingsfase en hierdie studie ondersoek die inhiberende potensiaal op defekformasie van die aanmekaarlym van planke terwyl dit steeds nat is. Die studie bestaan uit twee eksperimente. In die eerste eksperiment is skuiftoetse gebruik om die invloed van verskeie material –en prosesseringsveranderlikes op die lymlaskwaliteit van groen (nat) Eucalyptus grandis planke wat met ‘n enkelkomponent poli-uretaanlym gelym is. Die eksperimentele ontwerp vir hierdie studie bestaan uit 16 groepe wat met verskillende kombinasies van houtdigtheid, voggehalte, lymhoeveelheid, en druk geheg is. Tien monsters per groep is getoets vir skuifsterkte. Die penetrasiegedrag van die lym in die houtstruktuur is ondersoek met mikro-CT skandering. Al die getoetsde monsters het die minimum skuifsterktevereiste volgens die EN16351 (2015) standaard behaal. Beter resultate is oor die algemeen verkry met hoër voggehalte van bykans 60% in vergelyking met monsters wat by veselsversadigingspunt was – waarskynlik as gevolg van beter lympenetrasie. In die tweede eksperiment is die potensiaal ondersoek van nat aanmekaarlym van planke in panele voor oonddroging om drogingsdefekte te minimeer . Kant-gelymde panele is vervaardig van groen (nat) Eucalyptus grandis en gedroog saam met individuele kontroleplanke . Na droging is die panele weer losgesny in planke en gegradeer ten opsigte van oppervlakkrake, spleting, boogtrek, koppievorming, en draaitrek en vergelyk met die individuele kontroleplanke. Die vermoë van spanningsgleuwe in die planke om defektvorming te inhibeer is ook ondersoek. Die resultate wys dat die kantlym van groen Eucaluptus grandis planke voor oonddroging nie die plank afgradering volgens SANS 1707-1 (2010) kon verminder nie. Koppievorming kon beduidend verminder word, terwyl draaitrek slegs verminder kon word in nie-pitplanke. Spanningsgleuwe het nie ‘n beduidende invloed gehad op die vorming van enige van die defekte nie, maar het erge deformasie en skade in sommige planke veroorsaak. Verdere ondersoeke moet gedoen word op groot-houtprodukte (“mass timber products”) soos kruisgelamineerde hout, waar heel panele gebruik kan word as ‘n komponent