Pretreatment for adhesion

Fig. 4. Example.s of rough surfaces pretreated for adhesive bonding (a) microtibrous oxide on copper (cf. 29J) (b) a dendritic zinc surface (cf. [30J).

Table V. Classification of Surface Pretreatments for Adhesive Bonding of Titanium"...

Phosphoric acid baths are used in the aircraft industry as a pretreatment for adhesive bonding. They are also very good treatments before plating onto aluminum. A typical bath might contain from 3 to 20 percent of phosphoric acid at about 32°C, with voltages as high as 60 V. 

The fluorocarbon surface may be made more wettable by exposing it for a brief moment to a hot flame to oxidize the surface. The most satisfactory surface treatment is achieved by immersing the plastic in a bath consisting of sodium-naphthalene dispersion in tetrahydrofuran. This process is believed to remove fluorine atoms, leaving a carbonized surface which can be wet easily. Fluorocarbon films pretreated for adhesive bonding are available from most suppliers. A formulation and description of the sodium-naphthalene process may be found in Table 7.16. Commercial chemical products for etching fluorocarbons are also listed. 

In a similar manner to the alloys of aluminium, titanium and its alloys can be pretreated for adhesive bonding by both mechanical and chemical methods. When chemical pretreatments are used, the oxide films so formed are not only thinner than those produced on aluminium but are significantly more stable to both heat... 

The chemical pretreatment methods mentioned are mostly used for TPOs, but in some cases can also be effective when used on polyamide (PA), polybutylene terephthalate (PBT), or other crystalline polymers, as well as some blends like poly(phenylene oxide)/polyamide (PPO/PA), are flamed. As a possible future trend, research is currently underway (plasma polymerization) attempting to combine a pretreatment for adhesion and to provide the surface conductivity on necessary on plastic parts for acceptable electrostatic application (15). 

Figure 3 241 illustrates that the durability of metal/polymer adhesion systems can greatly be influenced by the metal pretreatment chosen 1 K Therefore, it is very important to select the best pretreatment for a given system. 

Similarly, Allen, Alsalim and Wake 45,46 determined that alkaline hydrogen peroxide was the best pretreatment for titanium alloys. This pretreatment was found to preferentially etch the P phase, while also undercutting some of the a grains and redepositing needle-like crystals on the P grains. The very rough surfaces that resulted were found to enhance adhesion by mechanical aspects. 

Recently, many research efforts have been directed at developing pretreatments for metal surfaces which produce oxide layers with pores, fibrous projections, or microscopic roughness which can enhance metal/polymer adhesion by mechanical means. In order for the pretreatments to lead to an increase in durability, the oxide layers formed must be stable under environmental conditions. The bulk 3l-33 S2-128> of the research in this area has been completed in an attempt to increase the durability of... 

One might expect that the more developed porous layer produced by the PAA process would tend to provide a greater number of successful mechanical interlocking sites 54-86-135-136> and initiate a larger degree of plastic deformation in the resin upon failure than the FPL oxide. Test data i29) comparing PAA and FPL pretreated systems have supported this conclusion. Also, the oxides formed by the PAA pretreatment have exhibited better stability in wet environments 54). Hence, the PAA process has replaced the FPL etch as the method of choice for the pretreatment of aluminum for adhesion systems i32-133>. 

As it had been shown that silanes were effective as pretreatments for a variety of coatings and particularly so when used as additives, selected silanes were examined as pretreatments and additives in conjunction with a two pack polyamide cured epoxide adhesive (Epikote 828/Versamid 115, 1/1) and a structural polyurethane adhesive based on diphenylmethanediisocyanate and a polyester resin. 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

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. 

Cotter, J. L., and Mahoon, A., Development of New Surface Pretreatments Based on Alkaline Hydrogen Peroxide Solutions for Adhesive Bonding of Titanium, International Journal of Adhesion Adhesives, vol. 2, 1982, p. 47. 

Surface oxidation processes have also been used as pretreatments for improving the bonding strength of adhesives. Brink et al. [9] reported that the wet bonding strength of plywoods or particleboards manufactured using phenol formaldehyde increased after pretreatment of wood with nitric acid. Mari et al. [10] also reported that nitric acid oxidation reduced the amount of isocyanate resin adhesive required to manufacture particleboard and improved the mechanical properties and biological resistance of boards. 

More recent viscoelastic 2C acrylate adhesives often adhere to low-energy surfaces as well, making them suitable for adhesion to polyolefins without pretreatment (3 M Scotch-Weld DP 8005). The 2C acrylate adhesives are often supplied in double cartridges (side-by-side cartridges), whereby the flow-mix principle is used. Users process the adhesive like a 1C system. These acrylate adhesives are transparent and UV-stable, making them suitable for braiding glass. They can also be painted over. 

Independent of the chemical structure, industrially available adhesives are characterized by the formation of strong adhesive bonds on the respectively pretreated surfaces of the materials described. This results in the criteria for adhesive selection described in Chapter 8. 

Different surface treatments such as peel ply make it possible to roughen and clean composite surfaces and consequently to improve adhesion performance. However, like many other treatments, it is limited by the cohesive failure that may occur in the material. As an alternative, laser ablation appears to be quite an interesting surface pretreatment for polymer composites since it fadhtates efficient control of the surfaces to be adhesively bonded. Furthermore, surface properties must be suitably defined by taking into account the nature of the composite material and the adhesive used. 

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

Tritolyl phosphate (TTP) has been examined as a pretreatment for E-glass in epoxide laminates and thermoplastic adhesives for bonding poly(vinyl chloride) to aluminum, steel to zinc, and acrylonitrile butadiene styrene to aluminum [59]. Mono- and diphosphate esters have been claimed to be suitable adhesion-promoting primers for acrylic adhesives on metal [60,61], unsaturated acid phosphates have been suggested as primers for use on metals to be bonded with free radical initiated adhesives [42], and thiopho-sphate esters have been suggested for adhesives to be used on plastics, ceramics, and metals [62]. 

In the example shown in Fig. 7, a thin film of plasma-polymerized trimethylsilane had been deposited on cold-rolled steel as a pretreatment for improved adhesion and corrosion [30]. The film thickness was determined by ellipsometry to be 500 A. The composition was characterized by AES, XPS, and TOFSIMS. AES gave information on the bulk composition, surface enrichment, and interfacial oxide (Fig. 7b). Note that the C/Si ratio of the bulk of the film, after equilibrium sputtering conditions have been reached, is approximately 3, i.e., identical to that of the monomer from which the film was... 

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