Bonding systems primers

Modern bonding systems usually consist of a primer coat, often with a secondary tie coat, plus a tacky solution to assist in the application of the rubber. The bonding systems currently in use are usually suitable both for autoclave vulcanisation and vulcanisation at 100°C with atmospheric pressure steam or hot water. Ambient vulcanisation bonding systems have to be chemically active at the lower temperatures and are therefore specialist in nature. 

The acidity of self-etching primers allows them not only to penetrate the smear layer and incorporate its component, but also to interact with the underlying intact dentine. In doing so, it forms a hybrid layer of the type previously identified with three-layer bonding systems [20,21], and which is essential in promoting adhesion. This acidity varies between formulations, and can be classified as either strong or weak, depending on the acid involved in the blend and also its concentration [25]. 

M. Ogata, N. Harada, S. Yamaguchi, M. Nakajima, P.N. Periera, J. Tagami, Effect of different burs on bond strengths of self-etching primer bonding systems. Open Dent. 26 (2001)375-382. 

Studies have been conducted on the effects of corrosive salt-spray environment on bondlines of different bonded systems. The system variations included clad and bare alloys, surface treatments, adhesive primers, and adhesives. Five specimens were fabricated for each of the bonded systems. The specimens were then placed in a salt-spray environment of 5% NaCl at 35 °C. The change in wedge-test crack length of each specimen was recorded periodically. At the end of 1 month, one specimen was randomly selected from each bonded system and opened for visual inspection of the bondline condition, both in the stressed zone (crack-tip zone) and in the unstressed zone. The same procedure was carried out after 2, 3, 6, and 12 months, when the last specimen was removed from test. The conclusions were as follows... 

The purpose of accelerated ageing is clearly to cause deterioration to the bonding system under investigation, that is, the adhesive-primer-pre-treatment-substrate combination. It is of critical importance that the ageing conditions should be selected to recreate the same failure mechanisms as wonld be seen in service conditions. 

The durability, or permanence, of a joint or structure is a function of the entire bonding system that is, the adherend-pre-treatment-adhesive/primer combination along with the conditions to which the joint or structure is being exposed. A change in any one of these parameters can radically influence the durability of the bonding system. In general terms, durability can be considered separately for both structural and non-structural bonds. 

Importantly, for a particular application, and where the metals to be joined have been specified, an overall metal bonding system should be considered. Typically, this will include selection of a suitable adhesive and possibly primer or coupling agent in combination with appropriate methods of surface preparation or pre-treatment. Organofunctional silanes (see Silane adhesion promoters) are commonly used coupling agents, which have been demonstrated to provide covalent chemical bonds with steel or aluminium surfaces and possess epoxide or amine functionality for reaction with Epoxide adhesives. 

Once coated with the bonding system, metal components should be stored in such a way that risk of contamination or mechanical damage is minimal. However, a minimum period of storage should be specified to ensure complete drying of the primer and the top coat typically, 30 min at room temperature should be adequate. 

The evaluation of adhesives by mechanical test methods is complex due to the state in which the adhesives are tested. Common adhesive tests require that the sample actually act as a bonded system formed by the adhesive in question and the appropriate substrates. Contrary to the usual testing of materials in their bulk form, adhesives are usually tested as one component of a system of many parts. An adhesive test actually tests the total bonded assembly, i.e., the substrates, the joint geometry, the interface, the primer coat, the surface preparation, the curing cycle, and the adhesive itself. 

If prolonged storage is necessary, a compatible primer may be used to coat the treated substrates after surface preparation. The primer wiU protect the surface during storage and interact with the adhesive during bonding. Many primer systems are sold with adhesives for this purpose. 

The basic components of a dental bonding system include a primer, the adhesive resin, an organic solvent and polymerization initiators. Primers contain hydrophilic blends of resin monomers/co-monomers. Adhesive resins contain blends of hydrophobic monomers/co-monomers. Solvents are added to the systems to enhance resin infiltration into the tissue, whereas photoinitiators are commonly used for convenient operator-controlled photopolymerization of the adhesive. Other ingredients such as fluoride, glutaraldehyde, antimicrobials are also commonly added to the mixture in an attempt to further protect or strengthen the adhesive interface. 

Although film adhesives account for the overwhelming bulk of stmctural adhesive used by the aerospace industry, expandable and syntactic adhesive foams, paste adhesives, and adhesive primers are all necessary parts of a complete bonding system. 

Aqueous dispersions are used in fiber bonding, paper coating, friction and abrasive appHcations, and laminates and wood bonding. PhenoHc dispersions improve the strength of latex-contact adhesive appHcations. Epoxy-modified phenoHc dispersions are prepared by dispersion of the phenoHc epoxy resin. The systems are used for baked primer appHcations and bonding requirements. Minimum baking conditions are 20 min at 150°C (25). 

With the exception of coupling agent technology, primers for structural adhesive bonding have received little theoretical treatment in the literature beyond a discussion of mechanisms of corrosion inhibition by primer additives and limited discussion about statistical techniques for primer formulation. Perhaps because of the much more widespread use and greater economic importance of corrosion-protective coatings, the design and function of primers for these systems have... 

P.V.D.F. Polyvinylidene fluoride (p.v.d.f. or p.v.f.2) dispersions are applied by the coil-coating process. They are blends of p.v.d.f. resin and acrylic. The combination produces a system which has excellent weather-ability and which can be bonded via an adhesive primer to a galvanised steel or aluminium substrate. 

These results Indicate that the degree of volume change due to the uptake of water is related to the resistance of the bond to cathodic action. Tentatively we can conclude from this test, pending further information, that a primer with high resistance to swelling in the presence of OH- and a topcoat with similar volume change behavior in the presence of OH-, will perform best as an adhesive system. 

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