Metal oxides, adhesion

While a non-phosphated topcoat/adhesive interface provided an excellent, moisture resistant, occlusive seal even under the most severe cycle testing, phosphated ZM adherends did not prove to be as durable in comparison (Figure 11). The reason for this lies in the fact that phosphate coverage on Zincrometal is incomplete. Partially crystalline phosphates are non-uniformly interspersed on randomly exposed zinc dust spheres at the surface. Consequently, the moisture resistance normally provided at the adhesive/topcoat interface was reduced due to the incomplete sealing between the topcoat/ adhesive surfaces. This became apparent as most of the failures examined after aging in these environments were concentrated at the adhesive/phosphate/paint interface. Results obtained on these samples were similar to those obtained for phosphated CRS joints, indicating that the locus of failure occurred at phosphate crystal sites. Note, however, that the durability of these joints was still considered to be very good in comparison to other metallic oxide/ adhesive interfaces. 

I For the case of copper, a mixture of cuprous and cupric oxides is present on the copper surface which acts as a defect semiconductor. Therefore, electrons can readily be transported from copper to its oxide surface allowing oxidation to continue at the metal oxide/adhesive interface ls. This continued oxidation reaction which involves the base metal can interfere with adhesion between the oxide and the adhesive. Hence, the underlying metal atoms can effect the adhesion forces in some cases 171... 

Equation (6.11) has been used to calculate the contribution of van der Waals interactions to the metal/oxide adhesion energy (McDonald and Eberhart 1965,... 

The failure mode of samples exposed for long times is gradual undercutting of the adhesive bond at the metal oxide/primer or metal oxide/adhesive interface. This is shown schematically in Pig. 14. This undercutting continues until the remaining bonded surface is not enough to support the load. 

Buchwalter, L. P., Adhesion of Polyimides to Metals and Metal Oxides, Adhesion Sci. Technol, 1(4) 341-347 (1987)... 

Another important aspect of testing the adhesive as part of an adhesive-joint system is that the joint presents a number of options for the location of the failure path. Failure may be cohesive in approximately the center of the adhesive layer. It may be cohesive but near the interface as is often seen in peel testing. It may be interfacial along the adhesive-substrate interface or it may run entirely within an interphase, for example, within a metal oxide/ adhesive interphase region. The failure path could run cohesively through the substrate, for example, the crack could run in the interlaminar region of a fiber-reinforced polymer composite substrate (Kinloch et al. 1992). Finally, some combination of the above could occur. Each of these options for the failure path may lead to a different fi-acture resistance being measured and thus adhesive-joint tests and their interpretation are necessarily more complex than bulk adhesive studies. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Aluminum, the most common material used for contacts, is easy to use, has low resistivity, and reduces surface Si02 to form interfacial metal-oxide bonds that promote adhesion to the substrate. However, as designs reach submicrometer dimensions, aluminum, Al, has been found to be a poor choice for metallization of contacts and via holes. Al has relatively poor step coverage, which is nonuniform layer thickness when deposited over right-angled geometric features. This leads to keyhole void formation when spaces between features are smaller than 0.7 p.m. New collimated sputtering techniques can extend the lower limit of Al use to 0.5-p.m appHcations. 

Neoprene—phenohc contact adhesives, known for thein high green strength and peel values, contain a resole-type resin prepared from 4-/-butylphenol. The alkyl group increases compatibiHty and reduces cross-linking. This resin reacts or complexes with the metal oxide, eg, MgO, contained in the formulation, and increases the cohesive strength of the adhesive. In fact, the reactivity with MgO is frequently measured to determine the effectiveness of heat-reactive phenoHcs in the formulation. 

Waterborne contact adhesives contain an elastomer in latex form, usually an acryflc or neoprene-based latex, and a heat-reactive, cross-linkable phenohc resin in the form of an aqueous dispersion. The phenoHc resin improves metal adhesion, green strength, and peel strength at elevated temperature. A typical formulation contains three parts latex and one part phenohc dispersion (dry weight bases). Although metal oxides may be added, reaction of the oxide with the phenohc resin does not occur readily. 

Phenohc/adhesives adhesives / sealants / gaskets abrasives and break pad binders printed circuit boards stmctural composites /laminates Metal/glass binders metal oxide binders retroreflective coatings glass optical coatings Hard copy printing inks toners... 

Once a metal surface has been conditioned by one of the above methods, a coupling agent composed of a bifimctional acid—methacrylate similar to a dentin adhesive is appHed. This coupling material is usually suppHed as a solvent solution that is painted over the conditioned metal surface. The acidic functional group of the coupling molecule interacts with the metal oxide surface while the methacrylate functional group of the molecule copolymerizes with the resin cement or restorative material placed over it (266,267). 

Carboxylated polymers such as AF use similar but not identical compounds. The higher strength, especially hot bond strength, is due to the interaction of the carboxyl groups on the polymer chain with the metal oxides. The crystallization rate of AF is low and does not contribute to bond strength. Manufacture of adhesive compounds from AF is more demanding than manufacture of those from AD. 

Polymeric surfaces are fundamentally different from metal oxide surfaces, and consequently the technical challenges to obtaining strong and durable adhesive... 

Priming to improve adhesion Table 7 Surface energies of polymeric and metal oxide surfaces 459... 

Neoprene AF ( 963). It is a polychloroprene modified with methacrylic acid. Although it is a slow-crystallizing elastomer, the cohesive strength develops very rapidly and it has improved creep resistance at high temperature compared with Neoprene AC or AD. The improved properties of Neoprene AF are derived from the interaction between the carboxyl functionality with the metal oxides added in the solvent-borne polychloroprene adhesives. 

Titanium dioxide used for adhesive applications should contain an inorganic coating to control polarity, improve its ease of dispersion, and improve its weather resistance. The inorganic coating (zirconium dioxide, silica, alumina) is applied in the aqueous sluny by precipitation of one or more hydrated metal oxides and by neutralization of acidic and alkaline compounds. 

Metal oxides. Magnesium oxide is used to cure polychloroprene by converting its few active allylic chloride from 1,2 addition into ether cross-links. There is a synergistic effect when magnesium oxide is used in combination with t-butyl phenolic resins in solvent-borne polychloroprene adhesives. When solvent is removed, the phenolic group in the resin reacts with the magnesium oxide to cross-link [49]. 

Formulation of a solvent-borne CR. A typical formulation of a solvent-borne CR adhesive may include the following components (fillers are not commonly added and curing agents are added to improve heat resistance) (1) polychloroprene elastomer (2) metal oxides (3) resins (4) antioxidants (5) solvents (6) fillers (7) curing agents (8) other modifiers. 

Metal oxides. Metal oxides provide several functions in solvent-borne polychloroprene adhesives. 

Acid acceptor. This is the main function of metal oxides in CR adhesive formulations. Upon age, small amounts of hydrochloric acid are released which may cause discolouration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergistically in the stabilization of solvent-borne polychloroprene adhesives against dehydrochlorination. 

Solutions of polychloroprene adhesives containing metal oxides and r-butyl phenolic resin may show phasing (e.g. clear upper layer and flocculated lower layer of metal oxides) on standing for days or months. To recover the full utility... 

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

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