Лако-красочные материалы — производство

There are adhesives and sealants in the body shop with basically four different functions (Table 2):

1. Sealants for body joints

2. Spot-welding sealants and tapes

3. Antiflutter bonding

4. Structural adhesive bonding and hem flange sealing

Examples of these applications are shown in Figs. 4 and 5. The sealants for body joints are applied after assembly. They are extruded over the welded joints and have to seal out dust and water and avoid corrosion. A typical application is shown in Fig. 6. The following materials are in use:

1. Moisture and/or heat curing one-component polyurethanes

2. PVC plastisols

3. Pregelling compounds based on synthetic rubber

4. Butyls

Spot-weld sealants and tapes are used in spot-welded flanges to protect against corrosion. They are applied to the flanges before joining, then the body parts are pressed together and spot-welded. Figure 4 shows the application points on a body shop car schematically. The application of spot-weld sealing on the flange between the front section

and the lower windshield and on the inner flange of the rear fender can be seen as an example. The extrusion of a spot-weld sealant on the inner side of the fender is shown in Fig. 7. In use are:

1. PVC plastisols

2. Acrylic plastisols

3. Warm-applied butyls

4. Butyl tapes

5. Rubber-based pastes

The solvent-based compounds used formerly are no longer used.

Outer car body panels are stiffened with reinforcements to avoid flutter and the so — called ‘‘oil can’’ effect. In this way the strength of the body component is also improved. In Fig. 4 the use of an expandable butyl as antiflutter material is shown. Figure 8 gives an example of the use of an intermediate layer for antiflutter bonding on a hood. The following materials are used:

1. Bituminous or acrylic-soaked foams

2. Warm-applied butyls

3. Vulcanizable expandable butyls

4. Hot-curing one-component polyurethanes

Metal-to-metal adhesives are used to bond and seal hem flanges as well as for structural bonding of body shop components. As an example, Fig. 5 shows the adhesive bonding of inner and outer door panels. In Fig. 9 the robotized application of an adhesive on a rotary table can be seen. The adhesive bonding of hem flanges enables a homoge­neous stress distribution along the bond line, through which the stiffness of the bonded component is better than in a conventional spot-welded joint. Often, adhesive bonding is combined with spot welding, which provides some advantages: the adhesive can replace a large number of spot-welding points, which reduces expensive surface finishing at outer panels; the components can be handled immediately after joining, before the adhesive is cured; and improved strength is achieved. Moreover, the spot-welding points hinder the attack of peel forces, which is harmful to the bond line. Instead of spot welding, other joining techniques (rivet fastening, screw fastening, ‘‘clinchen,’’ ‘‘toxen,’’ etc.) can be used in combination with the adhesive bonding.

Corrosion protection is often mentioned as a principal advantage of adhesive flange bonding, but today, coated sheet metal and aluminum are used more and more in the body shop, so this advantage is no longer the primary one. With the increasing use of coated sheet metal, the adhesive choice becomes more important. The bond strength is poor if the adhesive used is not adapted to the particular properties of the coatings. For adhesive metal bonding the following compounds are in use:

1. PVC or acrylic plastisols if no higher strength is required

2. One-component hot-curing or two-component cold-setting polyurethanes with medium strength

3. Epoxy-based adhesives, hot or cold curing, as one — or two-component pastes or as adhesive films for structural bonding with significant loads

4. Polyurethane/epoxy-based polymer blends (so-called ‘‘toughened’’ epoxies)

The conventional epoxy-resin adhesives give good sheer strength results, but they show brittle fracture under impact loads, particularly in cold conditions. Polyurethane compounds are more flexible and tougher, but they give lower strength in adhesive bond­ings. Adhesive applications in the body shop had usually been confined to low-loaded components. Nowadays adhesive bonding is also used more and more for parts that have to transmit significant structural loads (e. g., chassis components, floor panels, and side rails). The deformation ability of the adhesive-bonded components must be high to absorb the impact energy to give the car body good crash behavior. That requires a toughened adhesive with as high a strength as possible and was the reason for the development of polymer blends.

Worthwhile mentioning also are the adhesive applications in car body manufactur­ing, where the traditional sheet steel construction is replaced by steel or light metal space frames with plastic exterior body panels. A growing demand exists for the use of exterior body components made of plastic or metal-plastic composites, as doors, tailgates, trunk lids, hoods, roofs, and so on. Using these components to build a car body has forced the manufacturing process to be altered from the conventional flow of manufacture. That has influenced adhesive processing and caused modifications in the adhesives. Polyurethane — based adhesives, which are one-component moisture-curing or two-component cold­setting compounds, are generally used in these applications.