Лако-красочные материалы — производство

A. Fast-Setting Adhesive for Fingerjointing and Glulam

Together with the more traditional fingerjointing adhesives that have just been discussed, a series of ambient-temperature fast-setting separate application systems have also been developed. These eliminate the long delays caused by the use of more conventional PRF adhesives, which require lengthy periods to set. These types of resorcinol adhesives are applied separately. They were first developed in the United States [17-20] to bond large components where presses were impractical. Kreibich [20] describes these separate appli­cation or ‘‘honeymoon’’ systems as follows: ‘‘Component A is a slow-reacting phenol- resorcinol-formaldehyde resin with a reactive hardener. Component B is a fast-reacting resin with a slow-reacting hardener. When A and B are mated, the reactive parts of the component react within minutes to form a joint which can be handled and processed further. Full curing of the slow-reacting part of the system takes place with time.’’ The m-aminophenol used for component B is a frightfully expensive chemical and for this reason these systems were discarded and not used industrially [14]. In their original con­cept component A is a traditional PRF cold-setting adhesive at its standard pH of between 8 to 8.5 to which formaldehyde hardener has been added. Flour fillers may be added or omitted from the glue mix. Component B is a phenol/meta-aminophenol/formaldehyde resin with a very high pH (and therefore a high reactivity) which contains no hardener or only a very slow hardener.

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Component A of the adhesive is again a stan­dard PRF cold-setting adhesive with powder hardener added at its standard pH. Component B can be either the same PRF adhesive with no hardener and the pH adjusted to 12 or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho or pine bark extract with no hardener [3, 21]. The results obtained with these two systems are good and the resin not only has all the advantages desired but also as a result of the use of vegetable tannins and of the halving of the resorcinol content of the entire adhesive system is considerably cheaper [3,21,22].

The adhesive works in the following manner. Once the component A glue mix is spread on one fingerjoint profile and component B on the other fingerjoint profile and the two profiles are joined under pressure, the reaction of component B with the hardener of part A is very fast. In 30 min at 25°C fingerjoints prepared with these adhesives generally reach the levels of strength that fingerjoints glued with more conventional phenolic adhe­sives are able to reach only after 6 h at 40 to 50°C or in 16 to 24 h at 25°C [3,22]. Clamping of laminated beams (glulam) bonded with these fast-set honeymoon adhesives is an aver­age of only 3 h at ambient temperature compared with the 16 to 24 h necessary with traditional PRF resins [21,22]. These adhesives present also two other advantages, namely (i) they are able to bond without any decrease of performance at temperatures down to 5°C and (ii) they are able to bond ‘‘green’’ timber at high moisture content, a feat which has been used in industrial glulam bonding since their commercial introduction in 1981. Several variations on the theme exist, such as the ‘‘Greenweld’’ system from New Zealand in which component B is a solution composed of just ammonia as a strong accelerator of the PRF plus hardener of component A, and of a thickener; this system, however, suffers from the presence of the odor of ammonia which is unacceptable in some sophisticated markets.