22 ноября, 2015 
Pokraskin The groundbreaking technology oflaminated wood constructions, invented by the carpenter Otto Karl Friedrich Hetzer (1846-1911) from Weimar in Germany, is the supreme discipline of wood bonding. Until the 1950s, Hetzer’s name was synonymous with beam constructions made from layers oflaminated wood boards, today called ‘glu(e)lam’. Several types of adhesive have been used for this technology; Hetzer himself invented — and was granted a patent for — formulations based on natural raw material, especially casein. The adhesive joints obtained were of good quality, but they had the disadvantage of not being resistant to water, so that they could not be used for outdoor applications. In 1926, I. G. Farben (today BASF) introduced synthetic resins based on urea/formaldehyde, called ‘Kaurit glue’, which overcame this disadvantage. Due to their brittleness, only perfectly fitting joints could be obtained with this glue, but the addition of different fillers such as industrial flour eliminated this shortcoming, without impairing the adhesive force.
In general, the strength of the adhesive joint is required to be higher than the perpendicular tensile strength and the shear strength. In practice, this becomes apparent in fracture tests when the wood ‘tears out’ (cohesional failure in the wood adherents), wherethe resultantfracture surfaces are seento haveahigh fiber content. This strength in a normal climate is considered tobea neces sary, but not sufficient, property ofanadhesive joint in structural wood elements. It is well known that the strength ofbonded woodjoints decreases under the influence of moisture and temperature changes. Therefore, standards have been developed for a classification of structural and nonstructural adhesively bonded wood according to the climatic conditions and fields ofapplications.
Urea-based adhesives already covered a wide range of applications as they were appropriate for elements not exposed to outdoor weathering or special climatic conditions. However, only synthetic resins based on resorcin and melamine are absolutely resistant to water and weathering. From the early 1990s, a new adhesive technology was introduced with the development of one-part polyurethane adhesives, the advantages of which have already been described. One problem which made their market entry difficult, however, was the fact that the existing — and very extensive — technical standards for wood and wood materials were tailored to conventional gluing procedures, and the new technology was only established through special approval (Radovic) [24]. Today, this still applies to some geographical regions — for example, in Japan, where modern polyurethane adhesives have not yet been generally approved for the structural gluing of wood as the relevant Japanese Industry Standard (JIS) does not include this class of adhesives. Moreover, there has been very little advancement in any striving for the standard to be modified.
Опубликовано в рубрике 