Лако-красочные материалы — производство

In chemically setting systems, a polymerization takes place in the bond-line during compression, followed by crosslinking of the polymer molecules. This results in an increase in the average molecular weight of the basic polymers, inducing in turn a considerable increase in the strength values compared to the noncrosslinked matrix. Furthermore, the crosslinking results in transfer from a thermoplastic to a thermosetting behavior, which is prerequisite for joints exposed to a consistent load. Under these circumstances, thermoplastics would be subject to creep, and this would result in failure of the bond-line in the course of time. Reactive systems adhere over a wide spectrum of materials and can be formulated under the aspect of very different mechanical properties (e. g. modulus of elasticity). Therefore, selec­tion of the correct proper adhesive requires consideration of the type of substrates to be bonded on the one hand, and the future technical performance of the bond­line on the other hand.

The most extensive class of chemically setting adhesives is the group of formalde­hyde condensation resins. The reaction partners of formaldehyde are ureas, phenols or melamines, as well as all possible combinations of these. The reaction partners are used in the form of two-part systems in aqueous solution. For industrial purposes, the components are mixed directly in front of the glue spreader, whereas for handicraft purposes powder premixes are used which are mixed with water to create a reactive product that can be applied to the substrates. An intermediate form of highly

crosslinking reactive resin and thermoplastic aqueous systems is that of postcuring dispersion adhesives. Here, shortly before application of the thermoplastic emulsion, a bifunctional isocyanate component is added which reacts with the hydroxyl groups of the polymers and protective colloids. With this adhesive system, the water resistance and thermal endurance achieved generally correspond to durability class D4 of DIN — EN 204. Suitable dispersions with a high content of crosslinking agent are also called emulsion polymer isocyanate (EPI) adhesives. These have almost the same properties as thermosetting resins, and so may be used for specific structural purposes.

One-part moisture-curing polyurethane adhesives have recently gained impor­tance for wood-to-wood bonding, especially in civil engineering when wood is used as a building material [24]. Compared to conventional formaldehyde condensation resins, this class of adhesives offers the following benefits:

• lower quantities need to be applied

• almost loss-less application

• no need for a mixing process, as it can be used as received from the manufacturer; no need for a glue room, and no waste-water

• reaction times may be chosen from short to extremely short

• unobtrusive bond-line, with an aging behavior similar to wood

• approval for indoor and outdoor use as well as for effective lengths

• ecologically harmless

When bonding wood to other materials, all known reactive systems may be used. Selection of the adhesive system depends on the required performance profile of the intended application. A high modulus may be required, for example, when fixing carbon fiber-reinforced lamellae to structural beams within their tensile loading area by means of two-part epoxy adhesives. Yet just the opposite would be needed when bonding parquet to mineral grounds by means of reactive systems based on silane or polyurethane. Owing to the toughened behavior of these joints, impact sounds are dampened, and important tensile and shrinking stresses which are induced by climatic changes in the wood parquet are absorbed.

For mechanical high-speed manufacture processes, hot melts are preferred because their adhesive strength builds up via solidification or crystallization; this is an advantage compared to other physically or chemically setting systems. Conven­tional hot melts are based on thermoplastic polymers that are adapted to the desired application by means of tackifiers, rheology modifiers and stabilizers. Polymers used for the bonding of wood are mostly EVAs (copolymers of ethylene and vinyl acetate), polyamides and atactic poly-alpha olefins (APAOs). Resins made from natural raw materials (colophony, terpenes, etc.) or of synthetic origin (hydrocar­bon resins, acrylate resins) are used as tackifiers. These hot melts are — and will remain to be — thermoplastic, which limits their potential thermal endurance. They are also subject to retardation and may therefore not be used for structural purposes.

The benefits of conventional hot melts and thermosetting adhesives, however, can be combined by means of reactive hot melts, which post-cure via a moisture effect following application. In PU resin hot melts, the basic polymers carry reactive

isocyanate groups which, after the thermoplastic hot melt-bonding process, undergo a crosslinking reaction initiated by the moisture contained in the substrates. The thermoplastic joint is strong enough to allow destacking and processing of the work pieces. The increased strength values induced by crosslinking of the isocyanates build up over the course of hours and days during subsequent storage. Hot melts containing reactive silane groups show a similar behavior but have a different adhesive profile.

8.4.1.4 Adhesion Promoters (Primers)

Today, inallfields ofapplication ofbonding technology, customers are demanding a universal adhesive and, moreover, an adhesive that develops infinite adhesive and cohesive forces on nonprepared surfaces without being sensitive to environmental or aging effects. According to current knowledge, there is no such ‘all-rounder’ in the offing, at least not for the purpose of refined professional bonding purposes.

The adhesion between material surfaces in general, and wood in particular, may not be sufficiently explained by a model that is based on the existence of a sharp interface between the substrate and the adhesive. As shown in Figure 8.19, there is rather an interphase than an interface — a zone of finite dimension — in which the substrate and the adhesive amalgamate to form a new material. The adhesive failure which is often referred to corresponds in fact to a failure within this interphase. Another impediment to the bonding ofwood is the fact that some wood species are rich in components that rediffuse to the surface shortly after a mechanical surface preparation. These components include low-molecular-weight resins and oils as well as aqueous solutions of sugar or peptides, and they may weaken the joint by acting as a release agent or by softening the wood structure. Several authors have therefore recommended increasing the durability of bonded wood joints by using adhesion promoters. It has been demonstrated for example that, following appropri­ate treatment with primers, the relevant delamination values of glulams obtained in the water test were clearly below the maximum prescribed by standards, even if difficult woods were bonded and if different adhesive systems were used [25]. The dilemma of primer treatment is that an additional manufacture step is required; hence, it will only be adopted for bonding purposes where quality is more important than quantity.

8.4.8