Лако-красочные материалы — производство

The adhesives to be used in making joints between timber members are specified in BS 5268: Part 2 (24). Permissible adhesive types based on the exposure conditions of the joint are reproduced here as Table 7.4. ‘Weather and Boil Proof (WBP) products such as resorcinol and phenol-formaldehyde are stipulated in damp con­ditions whereas melamine and urea-formaldehyde adhesives are permitted in permanently dry situations. Natural adhesives such as casein are now only allowed in the least hostile exposure category.

One particular structural application of adhesives is in the manufacture of glued laminated timber or ‘glulam’ members. This is normally manufactured by glueing together at least four planks of timber with their grain essentially parallel. The laminations are usually machined from 38 or 50 mm thick timber although thinner sections may be necessary in the production of curved members. End

jointing to obtain full length laminations is usually obtained by using finger joints.

Adhesives used in the construction of glulam are required to have a strength sufficient to provide a joint at least as strong as that of the timber in shear parallel to the grain. This implies a value normally not greater than 1 N/mm2 for most softwoods. Careful selection of adhesive type in accordance with the guidelines of Table 7.4 is necessary for longer term applications, particularly in damp conditions or outdoors. Fig. 7.12 shows the use of laminated hardwood for the main girders of a footbridge.

Glue spreading and clamping must be accurately and rapidly carried out as adhesives have limited pot-life and assembly times(26). To achieve a controlled spread of glue, laminates are coated using double roller glue spreaders, and jigs are used to assemble the glued laminations. They are then clamped under controlled pressure to achieve a thin glue-line, the clamping force being maintained by hydraulic, pneumatic or mechanical means. The clamped laminations are held at a steady temperature until the adhesive is fully cured, the curing time depending upon the type of adhesive and the temperature applied. After completion of the curing the member is conditioned for a period at room temperature before it is put imo service.

A major advantage of glulam arises from the use of graded laminates in the reconstitution process. This causes a reduction in the natural variability of the timber, thereby permitting higher design stresses than in an equivalent solid member. Further, the flexible members can be held in a bent form during the curing process allowing graceful curved members as shown in Fig. 7.13 to be produced. These technical advantages combined with high fire resistance and attractive appearance suggest an increased application in the future, particularly where clients are prepared to pay a little extra for improved aesthetics. Costs are likely to reduce now that some manufacturers maintain stocks of standard straight beams which are available for purchase ‘off the shelf.