Adhesion, chemical pretreatment

Some of these techniques using electrons and photons as probes of the surface chemistry have been described in this symposium by other authors. In this paper methods of surface analyses using beams of ions will be described. Emphasis is placed on ion scattering spectrometry (ISS) and secondary ion mass spectrometry (SIMS). Examples are shown for adhesive bonding applications including determination of locus of failure, contamination, cleaning and thermal and chemical pretreatments. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

Conventional coating approaches to TPO, which rely on the use of substrate pretreatments such as flaming, corona discharge, plasma or chemical pre-treatment, have led to a substantial oxidation of the surface, which enhances adhesion. These pretreatments, however, have a limited service life before the surface reverts to its unoxidized, apolar condition. 

The problem of adhesion between a polymer and a metal is strongly dependent on the specific type of polymer and metal involved, as well as on the deposition process under which the interface between the two is formed. In order to improve adhesion, different pretreatment methods can be used, but the development of such techniques requires detailed information about metal-polymer interfaces. Particularly, in the case of thin metal films deposited by physical vapor deposition (PVD) in ultra high vaccum (UHV), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used to obtain chemical information about initial film growth modes,... 

The adhesion or welding of wood surfaces to each other on the basis of a chemical pretreatment of the surfaces and probably heat activation. 

The components in adhesives and pretreatment chemicals are characterized and differentiated in terms of their volatility, Table 2. 

Grinding, brushing or sanding (with the exception of the above-mentioned Saco method) do not cause chemical modifications of the material s surface. A clean surface results with a characteristic structure corresponding to the composition of the material, as shown in Figure 7.6. Therefore, physical and chemical pretreatment methods are aimed at the chemical modification of the surfaces. Thus, on the one hand it is possible to further enhance the adhesive forces for extremely high demands on bonded joints, and on the other hand, to make poorly bondable material (e.g., plastics) bondable at all. Since physical methods are mainly used in bonding of plastics, they are described in Section 9.2.4. 

An aqueous chemical pretreatment for unfilled polyetherimide substrates is described. The process includes the use of a chemical impregnator to increase the mechanical adhesion component and avoid metal lymer delamination during the initial electroless metallization step. Excellent adhesion was achieved for copper or nickel to both injection-molded plaques and extruded films. The metalA>olymer bond was found to be durable in thermal environments (dry and moist) as well as in simulated solder tests. Failure occurred within the polymer substrate during 90° peel testing. The process was found to be applicable to filled substrates as welt. 

Recent advancements have yielded methodologies for chemically pretreating the polyetherimide surface thereby introducing a chemical component to the metal olymer bond b. These schemes were both nonaqueous and aqueous. FmthennoFe, the methods employed an adhesion promoter or else relied on chemical modiHcation of the polymer. ... 

Table I. Adhesion Of Electrolessly-Deposited Metals To Chemically Pretreated Unfilled Polyetherilmide Film. ...

Table IV. Effect of Chemical Pretreatment and Heat-Treatment Atmosphere on the Adhesion of Evaporated Copper to Polyetherimide. Film...

One of the most important factors in adhesive bonding is the surface condition of the substrate, i.e. the surfaces of the materials to be joined. Since adhesion takes place only at the interface between the component and the adhesive, it is evident that surface preparation or chemical pretreatment has a crucial bearing on the quality of the adhesive bond. 

Substrates with low surface energy usually have few or no polar groups, which provide only little or no interaction with the adhesive. Such substrates need either a physical surface preparation, e.g. a plasma treatment or a chemical pretreatment by use of an organic adhesion promoting layered system. 

It is always difficult to get durable bonds on many surfaces without surface preparation or chemical pretreatments. The use of primers and a complete portfolio of activators and primers for glass, ceramic, plastics, metals, wood, etc. improves the adhesion. 

The goal remains to improve the direct adhesion to substrates by the adhesive. However, this is complex. Chemical pretreatments generally provide good wettability due to their low viscosity. The adhesive s higher viscosity reduces the wettability and also the adhesion quality. 

Chemical pretreatment can be performed with an adhesion promoter, cleaners and activators providing layers of nanometer (10 m) thickness or with primers achieving coating thickness in the micrometer (10 m) range. 

Suitable methods of surface pretreatment for mass-production applications must be discussed and coordinated with the technical service department of the adhesive manufacturer. Adhesive manufacturers have established treatment methods for the main substrates encountered in industrial production and working instructions for users. They are in a position to offer their customers the best professional advice. This approach offers the right chemical pretreatment, which contributes best to the specific adhesive system. Table 4 lists the typical surface pretreatment options for a range of common substrates. 

Fluoropolymers. There are no known adhesives for fluoropolymers that give bonds of adequate strength without pretreatment. Simple, low-quality bonds can be obtained with contact adhesives. After chemical pretreatment, for example, with a solution of an alkali metal naphthalenide in THF, polyfluorocarbons can be bonded with high strength by using epoxy and polyurethane adhesives. 

Polyurethane adhesives also are suitable for bonding nonpolar elastomers, for example, natural rubber, styrene-butadiene rubber, or ethylene-propylene terpol-ymers, after chemical pretreatment of the surface. 

Depending on their chemical composition, they wUl require mechanical, chemical, and physical pretreatment or priming to enhance coating adhesion. Since mechanical pretreatment consists of abrasion, its effect on the substrate must be considered. Chemical pretreatments involve corrosive materials which etch the substrates and can be hazardous. Therefore, handhng and disposal must be considered. Physical pretreatments consist of plasma, corona discharge, and flame impingement. Process control must be considered. 

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