Silane Pretreatments

Formulated primers. The use of formulated primers in which a silane (or silanes) is blended with a film former, a solvent, and perhaps a hydrolysis catalyst, has been described by Plueddemann [16] and in the Dow Corning trade literature [17]. Primers of this type which contain a film former should not be confused with the previously mentioned silane pretreatment primers, as they deposit a film of measureable thickness and strength. Such formulated primers are required to wet the substrate, be readily wet by the adhesive or coating and should provide a layer of intermediate modulus between substrate and resin system. As with the pretreatment primers a formulated primer can be tailored to both the substrate and the resin. Ageing of formulated primers may be a problem both in the storage container and on the substrate before subsequent coating. 

The good adhesion of membranes onto the surface of AI2O3 is illustrated by the fact that they withstand an ultrasonic bath treatment for several hours without any loss of adhesion. Without the silanization pretreatment, on the other hand, all membranes liftoff within 5 minutes of ultrasonic agitation. 

In either process, the rate of adsorption is expected to be extremely slow. Hence, the addition of an alkaline catalyst greatly aids in accelerating these adsorption processes. Heat and/or time can achieve the same viscosity reduction effect, because both increased temperature and prolonged time lead to more adsorbed silanes. Very few silane oligomers are produced in this method since the hydrolyzed silane molecules do not exist in quantity at any time of treatment. Unlike the mixing of a silane pretreated filler, it is essential to consider the kinetics of silane adsorption in this type of treatment method. 

Organic inhibitors, ion exchanged inhibitors and coated inhibitors such as plasma-polymerized inhibitors have also been investigated as pigments in silane pretreatments and/or superprimers. More on this subject can be found in Refs. [8, 61-63]. 

The presence of small amounts of silica or ceria nanoparticles in the silane pretreatment also improves the corrosion resistance of galvanised steel substrates. Ceria nanoparticles are very effective and work as active anti-corrosion inhibitors, being much more effective than silica. The previous activation of the nanoparticles with cerium ions seems to create a synergistic effect that improves the corrosion resistance of silane pre-treated metallic substrates. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Chemical pretreatments with amines, silanes, or addition of dispersants improve physical disaggregation of CNTs and help in better dispersion of the same in rubber matrices. Natural rubber (NR), ethylene-propylene-diene-methylene rubber, butyl rubber, EVA, etc. have been used as the rubber matrices so far. The resultant nanocomposites exhibit superiority in mechanical, thermal, flame retardancy, and processibility. George et al. [26] studied the effect of functionalized and unfunctionalized MWNT on various properties of high vinyl acetate (50 wt%) containing EVA-MWNT composites. Figure 4.5 displays the TEM image of functionalized nanombe-reinforced EVA nanocomposite. 

Abstract— The use of organosilanes as adhesion promoters for surface coatings, adhesives and syntactic foams is described and reviewed in the light of published work. Data are presented on the beneficial effect of silanes, when used as pretreatment primers and additives, on the bond strength of two pack epoxide and polyurethane paints applied to aluminium and mild steel. It is shown that silanes when used as additives to structural epoxide and polyurethane adhesives are less effective than when used as pretreatment primers on metals but are highly effective on glass substrates. The compressive properties of glass microballoon/epoxide syntactic foams are shown to be markedly improved by the addition of silanes. 

Application of silanes as pretreatments. Where silanes were used as pretreatment primers, they were applied from a 2 wt% solution in an 80/20 wt% ethyl alcohol/water mixture, by brush, and allowed to air dry for 4h before coating or bonding. No hydrolysis catalyst was used and the solution was applied at the natural pH. 

Pretreatment primers. In this method of use the silane may be applied from a solvent solution, by vapour phase deposition or by plasma deposition although solvent application is the more usual. The solution usually contains water and silane at a concentration of 1-2 wt%. The applied film may be water washed before subsequent coating/bonding and/or heat cured. The solvent(s) used may be important in both the stability of the solution and the performance, particularly in the wet adhesion. It has been shown that the presence of water either in the solution or as a final rinse is important, particularly in the case of AAMS and presumably other silanes [1]. Other factors which are important include the concentration of silane the pH of the solution the thickness of the silane film deposited. 

Pretreatment for fillers. When used as a surface treatment for fillers or reinforcing materials, in which the silane is applied to the filler or fibre before incorporation into a resin matrix, the same factors as for pretreatment primers apply. In addition, the particle size and the absence/presence of water are important, and in a sense this application is only a variation on the former. It should be noted that silane treated fillers may have, or impart, different rheological properties to non-treated fillers, particularly particulates. A major disadvantage of this approach is that a general purpose silane may have to be used by a manufacturer rather than one specifically tailored to the use of a particular resin type and less than optimum properties are likely to be achieved in some cases. 

The effect of using different silanes as pretreatment primers on the initial bond strength of a two pack polyamide cured epoxide to mild steel and aluminium is shown in Table 2. 

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. 

Reference to Table 12 will show the data for the structural polyurethane adhesive, from which it can be seen that the pattern for the epoxide adhesive is almost repeated, with all the silanes used as pretreatments producing marked... 

Comparison of silanes as pretreatments and additives for a structural epoxide adhesive Butt tensile, grit-blasted metal, 2 wt%as pretreatment and additive... 

The finding that the silanes were more effective as additives in coatings than as pretreatments may at first sight seem surprising as the reverse was true of adhesives. The explanation may be in the fact that in the case of pretreatments it is likely that the silanes were used under less than optimum conditions. All the silanes were used at their natural pH, no hydrolysis catalysts were added and no attempt was made to adjust the pH of the solutions befure use. It has been shown that the structures of APES films on iron are dependent on solution pH, as is the availability of reactive groups [23],... 

The results obtained here also indicate that the acidity of the oxidized surface of a metal depends on the pretreatment of the surface. For a take-off angle of 75°, about 42% of the amino groups were protonated when y-APS films were applied to mechanically polished 1100 aluminum. That increased to about 57% when the silane films were rinsed, probably because of the dissolution of a small amount of silane from the free surface of the film on which the amino groups were mostly not protonated. However, for a take-off angle of 75°, about 48% of the amino groups were protonated when y-APS was applied to FPL-etched aluminum substrates. That increased to about 65% when the silane films were rinsed. These results imply that the FPL-etched surface is more acidic than the polished surface. 

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