Bond formation, adhesive

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

Two steps have been described in adhesive bond formation (1) intimate contact of the mucoadhesive agent and of mucus or mucosa consequent to wetting and (2) formation of physical or chemical bonds between the biological substrate and the mucoadhesive agent, preceded, in the case of polymeric materials, by interpenetration and diffusion between the bioadhesive and the mucin glycoprotein. 

I thank A1 Christiansen of the Forest Products Laboratoiy and Chip Frazier of the Wood Based Composite Center at Virginia Tech for their comments on wood adhesive bond formation and failure mechanisms. The experimental work reported in this paper was carried out by Rishawn Brandon, Jennal Chandler, and Daniel Yelle of the Forest Products Laboratoiy, with fluorescence microscopy assistance from Fred Kamke of the Wood Based Composite Center at Virginia Tech. 

We can classify acrylic adhesive bond formation into four different processes ... 

Increase rate and strength of adhesive bond formation. 

Cleaning, or removal of contamination, including process oils, dirt, waxes, mold release agent, and exuded plasticizer, is an important change that occurs as a result of surface treatment. Methods involving chemicals such as solvent cleaning and etching, if not properly used, can leave behind a residue that may interfere with adhesive bond formation. Clean surfaces must be protected because of rapid reacquisition of contamination from the ambient atmosphere. 

Resins are used to improve the tack of pressure sensitive adhesives. They must be compatible with the polymer (i.e. mixture has a single Tg) and modify its viscoelastic properties resulting in improved polymer flow characteristics, substrate wetting, and adhesive bond formation. 

The paster is a nonheated operation. The most common paster adhesive formulation consists of poly(vinyl alcohol)—clay—starch blends (10). A 100% area adhesive coverage is used. The rate of bond strength development of the adhesive is an important commercial concern and rapid bond formation rates are desirable. 

Surface energies are assoeiated with formation of the adhesive bond beeause they determine the extent to whieh, at equilibrium, a liquid adhesive will eome into eontaet with a solid surfaee. This is refleeted in the value of the eontaet angle, 6, whieh is related to the surfaee energies (written, following common usage, as y) by Young s equation [9]... 

Surface energies are again important in determining the practical adhesion, F, in the breaking of an adhesive bond. Eqs. 7 to 10 show how the two are related. Emphasis was placed on the important contribution to fracture energy of which represents energy absorbing processes other than those (VTa and Wcoh) directly associated with the actual formation of new surfaces. It must be remembered that... 

The high heat resistance produced by adding phenolic resins to solvent-borne CR adhesives is due to the formation of the infusible resinate, which reduces the thermoplasticity of the adhesive and provides good bond strength up to 80°C (Table 11). The resinate also increases the adhesive bond strength development by accelerating solvent release. 4 phr of magnesium oxide for 40 phr of phenolic resin are sufficient to produce a room temperature reaction. A small amount of water (1-2 phr) is necessary as a catalyst for the reaction. Furthermore, the solvent... 

Loss of adhesion occurs at the silicone substrate interface and two main mechanisms can be outlined the formation of a weak boundary layer (WBL) and the breaking of adhesive bonds. 

Weak boundary layer. WBL theory proposes that a cohesively weak region is present at the adhesive-substrate interface, which leads to poor adhesion. This layer can prevent the formation of adhesive bonds, or the adhesive can preferentially form bonds with the boundary layer rather that the surface it was intended for. Typically, the locus of failure is interfacial or in close proximity to the silicone-substrate interface. One of the most common causes of a WBL being formed is the presence of contaminants on the surface of the substrate. The formation of a WBL can also result from migration of additives from the bulk of the substrate, to the silicone-substrate interface. Alternatively, molecular... 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

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). 

Therefore, RSNOs appear to regulate PDI-dependent adhesion by competitively inhibiting integrin-ligand disulfide bond formation and, in the process, producing NO which prevents further activation of recruited platelets via the GC/G-kinase route. 

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