For the future the evolution of adhesive bonding as a joining technique in automobile production points in two directions. On the one hand, well-known applications have to be optimized and improved to make them cost-effective but nevertheless reliable and trustful processes enjoying increasing acceptance for adhesive bonding. On the other hand, there will be new applications with different adhesive requirements, and adhesive suppliers must anticipate these changes and develop compatible adhesive compounds to satisfy the new requirements.

As to the first point mentioned above, cost-cutting steps have to be taken seriously. Increasing automation of the adhesive application is imperative. Adhesive bonding pro­cesses without extra pretreatment of joint surfaces and without using a primer but with

Type of adhesive Method of curing

Window lifter rails Roofs and sun roofs Rigid roof linings Bonding of plastic components: Bonnets Tailgates

Multipiece spoiler Impact protection parts Bumper

Heating and ventilation systems Seat buckets Backrest linings Head and rear light housings Direct glazing

Adhesive bonding and sealing at
cabriolet hard and soft tops

Hot-melt adhesives Cool down (moisture) Bonding of wiring harnesses

(also cross-linking) Sealing of radiators

Rear view mirrors Laminating at interior fittings Assembly bonding of moldings Bonding of headlight lenses Adhesive bonding of sound systems Antiflutter bonding Bonding of insulation pads and sound deadeners Adhesive bonding at filters and filter housings, heating and ventilation channels Bonding of insignia parts at wheel caps Bonding of brackets at interior door panels Adhesive bonding and sealing at cabriolet hoods

reliable efficiency are required. Pregelling of body shop adhesives will be eliminated and oven temperatures for adhesive curing will be lowered to reduce energy costs. Increased use of reactive hot melts is conceivable. Multifunctional adhesives will be welcome: for example, hem flange adhesive bonding and seam sealing with only one material in one procedure. For ecological and personnel safety reasons, the use of harmful adhesives (e. g., solvent-based cements) will be reduced. Costs for toxic waste disposal, exhauster, reheat, or solvent recovery equipment will be reduced.

New applications of adhesive bonding can be expected where the specific advantages of this joining technique will be usable. Due to lightweight construction, which will be more and more important, outside panels must be used as supporting parts of the body structure. Conventional sheet steel constructions often show welded joints at the visible

outer skin of the car body, which should be avoided in a smooth aerodynamic body design. In hybrid constructions different materials must be bonded. For both techniques adhesive bonding is preferred to welding or soldering. Adhesive bonding can also be combined with two new joining techniques, clinchen and toxen.

Structural adhesive bonding processes could be transferred from the body shop into the assembly shop to get clean and better defined glue surfaces. Temperature loadings to glue joints in paint bake ovens could be dropped, which would be an additional advantage, Components could be manufactured in a subsystem production process and adhesive bonded to the car body in the assembly shop. Adhesive bonding processes separate from the assembly line, performed at special working sites with specific adhesive equip­ment, would have advantages.

Recycling aspects will get more attention. Components should be recoverable and the adhesives applied must not disturb the reprocessing. New improvements are being developed to manufacture laminated interior fittings, in which coverings and form sub­strates are made of the same or similar materials, so reprocessing can be done without prior delaminating of the layers. In this case the adhesives used had to fit with the sub­strate materials. The future number of adhesive bonding applications in the automotive industry will depend on the success of the adhesive bonding processes. The quality and the safety reproducibility, especially of high-performance structural adhesive bondings, will be more and more important for large-scale productions. A quality system including planning and surveillance should support these requirements.

[1]The treatment (Eqs. 10-14) requires that a be in dimensionless form. a then is a “normalized area,’’ i. e., a ratio of the cross-sectional area to some large, fixed area, such as the sample area.

[2]The IEP corresponds to the pH at which the zeta potential of the metal oxide is zero. If there is no specific adsorption of ions other than H+ or OH-, the IEP is simply the point of zero charge (PZC). The PZC is defined as the pH of the solution required to achieve zero net surface charge.

[3]For multifunctional solutes one should really refer to af and the ‘‘effective’’ hydrogen-

bond acidity or basicity. Indeed, af and refer to 1:1 complexation whilst it is by no means obvious that such values are relevant to the solvation situation in which a solute is surrounded by solvent molecules and hence undergoes multiple hydrogen-bonding [75]. yIn the literature, the acidity and basicity of the stationary phases (solvents) are defined by b and a, respectively which, in our opinion, can be misleading.

aRef. [162]; bRef [168]; cthis work; dRef. [167]; eRef. [59]; fRef. [173]; gRef. [172] (note that I3d5/2 BE from I2 decreased with the amount of sorbed probe); hRef. [46]; ‘Ref. [83].

PEMA: polyethylenemethacrylate; PCHMA: polycyclohexyl methacrylate); PBAC: (polybisphenol A carbon).

[5] Differential Thermal Analysis

Differential thermal analysis is a technique by which phase transitions or chemical reac­tions can be followed through observation of heat absorbed or evolved. It is especially suited to the study of structural changes within a solid adhesive at elevated temperatures. The temperature difference between a sample and an inert reference material is monitored while both are subjected to a linearly increasing environmental temperature. Figure 19

[6] See Table 1 for a definition of all acronyms used in the text.

[7] Glow Discharge Optical Spectroscopy (GDOS)

This technique and a variation, GDMS (glow discharge mass spectrometry), are essentially depth-profiling techniques [99]. However, there is a major difference between GDOS and depth profiling in typical surface analysis techniques such as AES and SIMS. The rate of profiling in GDOS is of the order of 1-5 pm/min. A high-intensity argon lamp (DC gas discharge) is used for sputtering the material. The sputtered elements are detected in the plasma by a spectrochemical analysis of the glow light via an optically transparent window. Because of the requirement of fast, simultaneous detection, a grating spectro­meter is used, so each element requires its own photomultiplier.

Sputtering in GDOS occurs by the positive ions generated in the gas discharge glow from where they are accelerated toward the sample. The electrically conductive sample is pressed against the cathode of which it forms a part. The high sputtering rate allows depth profiling through metallic coatings of industrial interest, e. g., zinc coatings on

[9] End Uses

As previously noted, the primary use for this elastomer has been as solvent-based adhe­sives and sealants. Solvent-based products are losing market share to water-based poly — chloroprenes, to other polymer types such as acrylics and polyurethanes, and to hot melt adhesives. However, where the processing facility is able to contain the vapor emissions, a solvent-based adhesive or sealant is preferred because of better wetting of surfaces, faster drying, and higher performance of the cured or dried product. Many rubber bonding

[10] Properties

The wide variety of grades available provides for an extremely diverse array of properties that can be developed for butyl rubber adhesives and sealants. However, the following general properties apply to varying degrees.

Superior water and moisture resistance The saturated polymer chain resists passage of water molecules through the chain structure.

Superior resistance to air and gas permeability Also a result of the saturated poly­mer chain.

[11] End Uses

While popular for many years as solvent-based pressure sensitive adhesives and sealant grade compounds, butyl rubber products have been losing favor to water-based acrylics in recent years. Adhesion performance of the thermoplastic acrylics is often quite simi­lar, and even though drying time is longer for water based versus solvent based, many uses have moved to the acrylics. There are, however, many areas where butyl rubber compounds excel, particularly where water resistance and low permeability are required. Specialty adhesive tapes for pipe wrap, surgical tapes, electrical tapes, and similar areas still often use a butyl rubber base. High solids content butyl rubber sealants are quite popular for many construction and repair operations. Hot melt butyls, either as a straight formulation or compounded with other polymers, are used for carton closing, insulated window sealing, appliance manufacture, and prefabricated metal buildings. Extruded tapes come in several grades and are extensively used for auto glass repair, mobile home and recreational vehicle manufacture, rubber roof installation, and marine applications.

Self-curing solvent-based butyl rubber adhesives are used in bonding ethylene-pro — pene diene monomer (EPDM) rubber to itself in rubber roof seaming, laminating poly­ethylene film, and as a flocking adhesive for bonding short fibers to surfaces, as in auto window inner seals. Butyl rubber adhesives are also used for a variety of applications in belt and hose manufacture and repair. Water-based butyl rubber adhesives are not as popular as the solvent-based grades, but often are used in pressure sensitive adhesives for paper and foil laminating, and for packaging and laminating applications for many other substrates.

[12] Properties

Once again, the wide variety of grades and copolymers available in this polymer make generalizations somewhat difficult, except for the following select properties that are inherent to the polymer.

Superior resistance to oils Nitrile rubber has the highest resistance of any of the generally used elastomers to grease, oil, plasticizers, and most organic solvents, both aliphatic and aromatic. There is limited resistance to some polar solvents, and polymers are generally soluble in ketones.

Excellent high temperature resistance When properly cured at elevated tempera­tures, nitrile adhesives can withstand temperatures above 150° C, with some compounds capable of limited exposure to temperatures above 200°C.

High strength Lap shear can exceed 200 kg/cm2 on certain substrates, such as some metals, particularly if combined with other reactive resins, such as phenolic or epoxy.

[14]Current affiliation: Teledyne Analytical Instruments, City of Industry, California, U. S.A. Copyright © 2003 by Taylor & Francis Group, LLC

[15]Excluding the weight of the combined ethylene.

Process: Load the reactor with the initial charge, using a presolution of the poly(vinyl alcohol) in 300 g of the water. Rinse in the sodium bicarbonate and ammo­nium persulfate with the remaining water. Switch on the agitator and purge with nitro­gen. Then pump in the initial vinyl acetate and pressurize with ethylene gas. Raise the temperature to 35 to 40°C and maintain at this temperature. Start to add the reducing initiator feed to go in over about 8 h. After 1 h, start to add the continuous monomer feeds (1) and (2) to go in over about 7h. At all times the unreacted monomer should be kept at 1 to 2% of the reaction mixture to ensure even copolymerization of the N-methylol acrylamide.

[16] Bonding under conditions of high room moisture content (relative humidity), as may be the case during a rainy period.

[17]For example, two US distributors of Japanese heat seal connectors are Nippon Graphite Industries, and Nitto Denko America, Inc.

[18] Highly Reactive Adhesive Resins in Plywood, Parquet Flooring, and Door Production

Plywood, parquet flooring, and doors are usually produced using aminoplastic adhesives. The press time necessary for these applications depends on the press temperature, the total thickness of the wood layers which have to be heated through, and the reactivity of the resin glue mix. Traditional adhesive resin systems need rather long press times due to their

[19] glue resin tank

[20] hardener tank

[21] twin pumps

[22] filter unit

[23] flow sensor

[24] mixer

[25] switchbox

[26] distributor

[27] level sensor

[28] roll coater

[29] ventilation valve

[30] Addition of Isocyanates

Isocyanates [polymeric MDI (PMDI)] as a fortifier for phenolic resins have only been used in the past in rare cases. Deppe and Ernst [41] reported a precuring reaction between the isocyanate and the phenolic resin, even if both components had been applied separately to the particles. Hse et al. [36] also found good results with an isocyanate and a PF resin added separately to wood particles. Pizzi and Walton [191] reported on the reactions and their mechanisms of PF resins premixed in the glue mix with nonemulsifiable water-based diisocyanate adhesives for exterior plywood. Pizzi et al. [192] reported on the industrial applications of such systems (PF + PMDI + sometimes tannin accelerator; UF + PMDI)

[31] Modifications of the Wood Surface

Modifications of the wood surface can be implemented using various physical, mechanical, and chemical treatments. Chemical treatments are performed in particular to enhance dimensional stability of the panel, but also to improve physical and mechanical properties

[32]Current affiliation: University of Wisconsin, Madison, Wisconsin, U. S.A.

[33]The hard tissue of the tooth substance consists of a protective outer coat of enamel and an underlying dentin phase. The latter, in turn, connects to the inner core of soft tissue (pulp), which is interpenetrated by nerve strands and blood vessels. The enamel, which covers essentially the visible part of the tooth and indeed represents the hardest tissue in the body, is composed almost entirely (97% by weight) of mineral-type hydroxyapatite (a crystalline calcium phosphate) in addition to a few percent of water and organic, mostly proteinaceous, matter. Dentin, constituting the major proportion of tooth substance, contains less mineralized phase (69% hydroxyapatite) but a comparatively large proportion of organic matter and water. Compositional and physical property data for enamel and dentin [3] are summarized in Tables 1 and 2.

[34]The retention of most crown restorations and the older types of bride design is largely secured by the compressive forces exerted in vivo during mastication. Compressive strength data are therefore routinely specified for luting and cavity-lining cements. Representative data have been compiled in Table 3.

[35] Zinc Oxide-Eugenol Cements

Falling under the heading of metal chelate compounds, the zinc oxide-eugenol (ZOE) cements in the hardened state are essentially zinc phenolates formed by reaction of zinc oxide and eugenol (4-allyl-2-methoxyphenol) in the presence of moisture, embedding

[36]For optimal retention, the surfaces of the restoration require special pre-treatment generally performed in the technician’s workshop. For example, the bonding surfaces of porcelain restorations are commonly subjected to microsandblasting, followed by silanizing with a silane coupling agent, such as 3-methacryloyloxypropyl(trimethoxy)silane, or simply by acid etching with hydrofluoric acid. Combined acid-etching and silanizing procedures are also popular, as are the more recently developed methods of silica coating by various techniques [7]. Similar silanizing treatments have been proposed for composite resin inlays [8]. The mechanism of adhesion to silica — coated and/or silanized adherend surfaces includes chemical bonding through Si—O links and major or minor contributions by micromechanical interlocking.

[37]The release of fluoride ions to combat caries and encourage remineralization, although not directly pertinent to the adhesion problem, may affect bonding indirectly through creation of porous structures that would enhance leakage and ultimate weakening of the bond. The development of fluoride release mechanisms devoid of detrimental effects on existing bonds to restorations and restoratives or removable appliances is therefore a prime concern in dental materials research. See, for example, Cooley et al. [17,18].

[38]The divergence of test methods currently employed in different laboratories has prompted numerous calls for international standardization, exemplified by recent proposals to standardize methods for dentinal bond strength determination and, herewith related, for the evaluation of microleakage and marginal gap dimensions [33]. On a more universal scale, several years ago, with the aim of developing standardized test methods, a working group was convened by D. R. Beech of the Australian Dental Standards Laboratory under the auspices of the International Standards Organisation (ISO) Technical Committee 106 (Dentistry). A draft report, completed in 1991, CD TR 11405, entitled Dental Materials Guidance on Testing of Adhesion to Tooth Structure, presents precise details of screening tests, bond strength measurements, gap and microleakage tests, and clinical usage tests. A useful tool for assessment of the reliability of a bond is the Weibull analysis approach [34]. The method, utilized now in many laboratories, allows for determination of the probability of bond failure as a function of applied stress.

[39]Although not specifically indicated in the text, the techniques of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) represent indispensable tools in the study of bonding processes and are widely used for the qualitative and quantitative evaluation of adherend surfaces, wetting and penetration, gap dimensions, and fracture mechanisms. Roulet et al. [45] have discussed the use of SEM in margin analysis, and publications dealing with preparatory methods for TEM and SEM investigations have been referenced by Eick et al. [46].

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