Surface primer, adherend

Surface Primers (Dick, 1987) - A surface primer is a coating applied to an adherend to improve an adhesive bond or subsequent coating. It is typically used where die adhesive and adherend have widely differing polarities, a butyl rubber adhesive on a steel surface for example. Primers are of two general types ... 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

In developing criteria for the ranking of adhesive formulations or adherend surface treatments or primers, it is necessary to distinguish between two different situations. In one case (contact adhesion), a true interface is believed to exist across which intermolecular forces are engaged, while in the other, an interphase is formed by diffusive interpenetration or interdigitation between the adhesive and the adherend (diffusion interphase adhesion). Even in the case of contact adhesion, more often than not, an mi vphase of macroscopic thickness forms on... 

The adhesive or primer must wet the adherend surface (Chapter 10). 

The surface preparation must enable and promote the formation of bonds across the adherend/primer-adhesive interface. These bonds may be chemical (covalent, acid-base, van der Waals, hydrogen, etc.), physical (mechanical interlocking), diffusional (not likely with adhesive bonding to metals), or some combination of these (Chapters 7-9). 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

In all adhesive joints, the interfacial region between the adhesive and the substrate plays an important role in the transfer of stress from one adherend to another [8]. The initial strength and stability of the joint depend on the molecular structure of the interphase after processing and environmental exposure, respectively. Characterization of the molecular structure near the interface is essential to model and, subsequently, to maximize the performance of an adhesive system in a given environment. When deposited on a substrate, the silane primers have a finite thickness and constitute separate phases. If there is interaction between the primer and the adherend surface or adhesive, a new interphase region is formed. This interphase has a molecular structure different from the molecular structure of either of the two primary phases from which it is formed. Thus, it is essential to characterize these interphases thoroughly. 

Abstract—The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive. 

IR data. Spectroscopic techniques were then employed to determine more about the composition of the fracture surfaces. ATR of the adhesive side of the fracture surfaces showed only slight differences in the composition of the organic phase near the interface as a result of applying primer to the adherend surface before applying the adhesive. The spectrum shown in Fig. 7C (obtained with the 45° KRS-5 reflection element) is the difference between a sample prepared from an unprimed adherend (Fig. 7B) and one prepared from a primed... 

The matching RAIR spectrum of the adherend side from the primed specimens (Fig. 9) was weak because of the small amount of organic material left on the surface. This spectrum shows a trace of epoxy, as suggested by the bands near 1510 and 1600 cm. The presence of primer is also indicated by the band near 1080 cm, but the weakness of this band in comparison with the bands due to the epoxy indicates that only a small amount of primer is present relative to the amount of epoxy present. 

The definition of an adhesion promoter in its most literal sense may be stated as any substance which when placed between two adherends results in a measurable increase in the force required to separate the two materials. This definition does not address the basic mechanism responsible for the increased adhesion nor does it concern itself with the mode by which the promoter is contacted with the adherend surface, i.e. as primer, by in situ incorporation into an adhesive or coating, or by other means. 

Protection of surfaces after treatment (primers can be used to extend the time between preparing the adherend surface and bonding)... 

The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should be used only when justified. The most likely occasions for a primer to be used are when (1) the adhesive or sealant cannot be applied immediately after surface preparation, (2) the substrate surface is weak or porous, or (3) the adhesive-adherend interface requires additional protection from service environments such as moisture. 

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. 

Primers. Primers tend to hinder adhesive strength degradation in moist environments by providing corrosion protection to the adherend surface. A fluid primer that easily wets the interface presumably tends to fill in minor discontinuities on the surface. Qrganosilane, organotitanate, and phenolic primers have been found to improve the bond strength of many adhesive systems. 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

For most adhesive bonded metal joints that must see outdoor service, corrosive environments are a more serious problem than the influence of moisture. The degradation mechanism is corrosion of the metal interface, resulting in a weak boundary layer. Surface preparation methods and primers that make the adherend less corrosive are commonly employed to retard the degradation of adhesive joints in these environments. 

Primers Substances enhancing adhesion between adherend surfaces and adhesive and delaying ageing processes. In contrast to adhesion promoters, primers are applied to the adherend surfaces. Adhesion promoters are usually admixed to the adhesive (q.v. adhesion promoter). 

Silanes and other coupling agents can be applied to various substrates or incorporated into an adhesive/primer to serve as hybrid chemical bridges to increase the bonding between organic adhesive and inorganic adherend surfaces [97-100]. Such bonding... 

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