Surface viscosity adhesion

Most often NR adhesives are supplied either as a solvent dispersion or as a latex for coating onto surfaces. Viscosity can range from very low viscosity solutions for... 

Plain slideways are preferred in the majority of applications. Only a thin film of lubricant is present, so its properties - especially its viscosity, adhesion and extreme-pressure characteristics - are of vital importance. If lubrication breaks down intermittently, a condition is created known as stick-slip , which affects surface finish, causes vibration and chatter and makes close limits difficult to hold. Special adhesive additives are incorporated into the lubricant to provide good bonding of the oil film to the sliding surfaces, which helps to overcome the problems of table and slideway lubrication. On long traverses, oil may be fed through grooves in the underside of the slideway. 

Pressure-Sensitive Adhesives—These are really viscous polymers which melt at room temperature, so the polymers must be used at temperatures above their Tg values to permit rapid flow. The adhesives flow because of the application of pressure. When the pressure is removed, the viscosity of the polymer is high enough to retain its adhesion to the surface. Many adhesive tapes are of this type the back is smooth and coated with a nonpolar coating which does not bond with the sticky surface. 

The wetting of surfaces by adhesives can be described by two activities (1) a lateral spreading of the film and (2) a penetration of the fluid adhesive into the surface cavities that are characteristic of the inherent surface roughness. The first activity is controlled by the relative surface energies of the adhesive and substrate as explained above. The second activity is controlled mainly by the viscosity of the adhesive and the time it is in the liquid state. 

Low-viscosity primers can also easily fill the irregularities on the substrate surface and displace air and fill hollows. This can improve the wetting properties of the adhesive system. For example, if the adhesive is a hot melt and it is applied to a bare metallic surface, the adhesive will gel before it gets a chance to efficiently wet the surface and mechanically interact with any surface roughness. However, if a dilute primer were first applied to the substrate and dried, the hot-melt adhesive could bond directly to the primer that in turn has bonded to the interstices of the substrate, thus providing excellent adhesion. 

On the assumption of a clean and, respectively, pretreated surface, the next step will be the application of the adhesive. It has to be ensured, however, that in areas with adhesive forces emanating from the adherend surface, the adhesive molecules are really able to get closer. Only then can the adhesive distribute itself on the surface, that is, wet the surface despite a, more or less, existing roughness. Furthermore, sufficient flowability of the adhesive is important. A complete and equal wetting of the surface to be bonded is therefore an indispensible prerequisite for the production of a strong bonded joint. Figure 6.5 demonstrates the difference between a low-viscosity and high-viscosity adhesive. 

Surfaces must be flat the low viscosity adhesive system has poor gap-filling properties Potential (and actual) toxicity problems, due to monomers and activators. 

Once the condition of wettability of the adherend surface is settled, the viscosity of the adhesive has to be considered. Low viscosity of the adhesive facilitates the spread of the adhesive, while high viscosity makes it difficult to apply the adhesive homogeneously over the surface. Viscosity decreases with increasing temperature and increases with increasing values of average molecular weight (MW). 

Furthermore, surface-mount adhesives (SMAs) used in pin transfer must have relatively low viscosities to facilitate pickup by the pins and transfer to the PCB. Because viscosity is so critical, the temperature of the adhesive reservoir must be controlled and preferably maintained between 25 °C and 30 Some SMAs that may be applied by pin transfer are listed in Table 4.17. 

Table 4.23 Viscosity changes vs pot life for a surface-mount adhesive ...

Rapeseed oil had rather limited industrial application until the development of steam power when it was found that rapeseed oil would cling to water and steam washed metal surfaces better than any other lubricant. This was due to its composition, providing the desired viscosity, adhesiveness, and solubility in mineral oil when processed for lubricants. For this reason rapeseed oil became an essential component of marine engine lubricants in naval and merchant ships. 

Normally a higher viscosity or thixotropic adhesive, a sealant, or a thin wood fascia is first applied to the surface to seal the fissures. Small diameter plastic tubes are left in place at regular intervals to be used later to inject in a lower viscosity adhesive, that will hopefully penetrate the whole depth of the fissure, and let the air out. Injection starts from the lower injection tubes until the adhesive starts coining out from the next lower tube, and moves up. Plastic tubes will be cut and disguised after cure. This is nevertheless a tricky operation as the right balance between gap filling and penetration ability of the adhesive must be found. 

Surface viscosity scales directly with the surfactant surface concentration, the intermolecular cohesion between the surface molecules and the intermolecular adhesion between the surfactant molecules and the underlying bulk liquid layer. 

Intermolecular adhesion between the polyether chains of a typical silicone surfactant and the underlying polyurethane liquid matrix could increase the surface viscosity contribution in PU films and foams. 

In order to maximise surface viscosity, and therefore minimise the drainage rate, the surfactant concentration, the intermolecular cohesion and adhesion should all be high. Later in the chapter evidence is presented correlating silicone surfactant concentration and structure (which influences the intermolecular cohesion and adhesion) to surface viscosity and film drainage rate. 

Table 4.23. Viscosity Changes vs Pot Life for a Surface-Mount Adhesive ...

The physical properties of the surface film relevant to foam formation are surface tension, surface elasticity, and surface viscosity. The surface tension should be low since it is a measure of the energy required to form a new surface. The film should be elastic so that local deformations do not need to create a new surface. With an elastic film the ingress and egress in the film is relatively slow so that the surface viscosity is high. The surface viscosity is a direct measure of the adhesion between molecules in a film and it increases in the presence of iso-a-acids [139]. 

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