Adhesives for highest stress levels
When wood species with limited durability are used for parts intended to be used outdoors, various chemical or thermal modification methods have been developed over the past years. With brand names like e.g. Accoya®, Lignia®, Keboni® and others, modified types of wood have increasingly appeared on the market over the past years which now complement the range of resistant wood species that had already been used for these applications, like Meranti, Bongossi, robinia and teak.
All these naturally resistant or modified types of wood have one thing in common: the high resistance against climate exposure, which is due to a much slower moisture absorption and release, among other factors. This performance, and also the natural composites of the wood or those that were introduced by the modification process, are of major impact on their bonding performance. For instance, using water-based PVAc adhesives is usually not economical, since these glues have a long pressing time. Direct sun exposure in exterior applications, especially on dark surfaces, also requires increased thermal resistance of the bondline, which can not be supplied by the use of thermoplastic PVAc adhesives.
In order to meet the special requirements on the bond, chemically crosslinking adhesives are avail-able from the Jowat portfolio:
- 2-component EPI adhesives
- 1-component PUR adhesives
Both adhesive systems have in common that they are highly resistant to moisture, achieving bonds that clearly exceed the durability levels of the grade D4 of the EN 204/205. In heat resistance testing under the EN 14257 (Watt 91), both the EPI system and the 1-component PUR adhesives reach tensile shear resistances >7 N/mm² at 80 °C. EPI (Emulsion polymer isocyanate) adhesives differ from reactive PVAc dispersions, based on their clearly higher solid content, which offers the advantage of fast setting, although the moisture absorption is slower for the resistant modified wood. The chemical reaction of the isocyanate crosslinker additionally minimises the thermoplastic behaviour of the adhesive, and a very heat-resistant bond develops. EPI adhesives are always processed in two-component form, with a 15 % crosslinker addition, and always require premixing. The pot life indicated must be observed. Application is possible by most standard units from dispersion processing. When down times of the machines exceed the indicated pot life, the unit must be emptied and cleaned.
Liquid PUR prepolymers are moisture-curing and are processed as one-component products. They are adhesives with a solid content of 100 %, so that the low moisture absorption of resistant timber will remain without impact on the setting process. The bondline, however, has to be adequately prepared for sufficient moisture exposure, to allow a complete chemical reaction of the polyurethane adhesive: This means that when the moisture level of the wood is less than 8 %, the bondline must additionally be fogged. PUR adhesives are processed with special applicators which prevent any contact of the adhesive with moisture or humidity before the application process itself takes place. Both for laminate assembly of modified wood, and for fingerjointing or dowelling, special solutions are available from the Jowat adhesives programme.