Use of Primers

PTFE and other fluoropolymers have been treated using a solution of sodium in liquid ammonia and other etching solutions [3]. This method dramatically improves surface-wetting characteristics, and the plastic can then readily be bonded using a wide range of adhesives. 

In the late 1980s primers were introduced that considerably enhance the adhesion of cyanoacrylates to polyolefins. 

The introduction within the last few years of two-part acrylics for the bonding of polyolefins has given the design engineer another option for the bonding of the 

These 10 1 mix ratio acrylics show excellent adhesion to polyethylene and polypropylene with handling strengths in less than 10 minutes. The products contain glass beads or fillers to control the bond-line thickness to 0.2 mm or 0.25 mm and so the joint should be designed to accommodate these fillers. 

These two-part acrylics do not require any pre-treatment of the joint surfaces or any surface primer and will bond polyethylene, polypropylene and ethylene copolymers with shear strengths in the range 4-8 N/mm. They can be used on many other substrates and so can be used as a general-purpose adhesive, although they are not recommended for bonding PTFE or the fluoropolymers. The resistance to water and high humidity environments is good but the mix ratio is critical to avoid unpredictable results. 

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Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

Obviously, there are many combinations and variations of the use of primers and detonators. There are instances where primers are used to ignite detonators, the output of which is then used either to cause directly the operation of a main expl charge or to ignite a slow-burning... 

The use of primer solutions was investigated—chlorinated polypropylene and chlorinated ethylene-vinyl acetate, both in toluene these gave improvements in adhesion, the second being the superior. Such a primer, in conjunction with a moisture-curing polyurethane adhesive, gave a product where it was possible to remove small particles of grit and adhesive only by prising from the surface with a knife. 

Gas flaming may be applied in much the same way as with blown bottles (and subject to similar constraints), while the use of primers is dealt with in Chapter 11 of this book, Vacuum Metallizing . Some further details of dipping and vapour etching are given below ... 

Oxidative stability depends on the adherend surface as well as on the adhesive itself. Some metal adhesive interfaces are chemically capable of accelerating the rate of oxidation. For example, it has been found that nearly all types of structural adhesives exhibit better thermal stability when bonded to glass or aluminum than when bonded to stainless steel or titanium.12 For any given metal, the method of surface preparation can also determine oxide characteristics, and hence bond durability. Thus, the use of primers is common practice with high-temperature structural adhesives. 

Beneficial properties Excellent adhesion without the use of primers minimum surface preparation one-component system self-heahng excellent resistance to ultraviolet radiation good chemical resistance non-staining... 

Solvent-borne acrylic sealants have very good weathering properties and exhibit excellent adhesion to a wide variety of substrates without the use of primers. These sealants can even be applied to substrates where slight contamination occurs (Brieton, 1994). 

Beneficial properties One-component system good adhesion without the use of primer paintable good resistance to ultraviolet radiation easy application and clean-up good packing stability low cost fast skinning... 

Silicone products give excellent adhesion to many substrates used in the building industry (e.g. concrete, glass, ceramics and metals), and in most cases primers are not needed. However, the use of primers gives much stronger adhesion. 

The use of primers is also advisable if the bonded parts are to be subjected to temperature changes, damp conditions, tension compression, shear forces, etc. Some porous substrates and plastic materials also require primers. Primers can be applied to a thoroughly prepared surface by brushing, spraying or dipping to form a thin film. All primers contain flammable solvents and therefore safety precautions are essential. 

Like other coatings, excellent surface preparation is necessary to make them free from dust, grease, tar, bituminous substances, laitance and adhering friable particles. Application can be made by brush, roller or by spraying. A coating thickness of up to 1 mm does not cause any flow of material in vertical applications. The use of primer ensures excellent adhesion. Some important applications of silicone coatings are as follows. They can be used as ... 

Epoxy-polysulphide coatings exhibit excellent adhesion to steel, aluminium and concrete without the use of primers (Morton International,... 

Conventional PCR uses primers that are present in equal amounts, thereby ensuring that the majority of the products are double-stranded amplicons. A variant method uses different concentrations of the two primers to generate more of one strand than of the other (asymmetric PCR). For instance, the use of primer A at 0.5 jiM and primer B at 0.005 pM produces mostly single-stranded DNA extended off the more abundant primer. This is useful for sequencing purposes or making single-stranded probes. Yield of the product, however, may be low. With less extreme ratios (e.g., primer A at 0.5 pM and primer B at 0.2 pM), the yield is mostly preserved, with one strand produced in enough excess to make it more available for probe hybridization. 

With the use of primers that hybridize to the vector sequence, the ends of the cDNA insert are sequenced. Automatic DNA sequencers generate most EST data. If the cDNA has been directionally cloned into the vector, the sequences can be classified as deriving from the 5 or 3 end of the clone. In most cases, both the 5 and 3 sequences are determined, but some EST projects have concentrated only on 5 ESTs to maximize the amount of coding sequence determined. Because the sequence of each EST is generated only once, the sequences may (and often do) contain errors. Contaminating vector, mitochondrial, and bacterial sequences are routinely removed before the EST sequences are deposited into the public databases (Hillier et al., 1996). ESTs in the databases are identified by their clone number as well as their 5 or 3 orientation, if known. 

Use of Primer-Adapter and the Generation of Cohesive Termini of cDNA... 

The use of primers or coupling agents based upon alkoxide or silane chemistry has been demonstrated to be potentially useful when used in combination with grit blasting or acid etching, though results are highly variable (see Silane adhesion promoters). 

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. 

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