Adhesion cause

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

The main adhesive causing the problem is poly(vinyl acetate) (PVA). Buckman Laboratories have developed an enzyme treatment called Optimyze using an esterase to hydrolyse the polymer to poly(vinyl alcohol). This is water soluble, not sticky and easy to remove. 

Finally, an important feature of gels made of adhesive emulsions arises from the deformation of the droplets. Indeed, as the temperature is lowered the contact angles between the droplets increase [27,28] (see Chapter 2, Section 2.3). Consequently, the structure of the final floes depends on the time evolution of the strength of the adhesion. Initially, the adhesion results in the formation of a random, solid gel network in the emulsion. Further increase of adhesion causes massive fracturing of the gel, disrupting the rigidity of the structure and leading to well separated, and more compact floes [27,28]. 

Some rubber compounds can cause corrosion of, and tend to stick to, metal surfaces with which they are in contact, and corrosion can even be caused to a metal in close proximity but not touching the rubber. Although not a very widespread problem, there has been sufficient concern, particularly for some military applications, for tests to be devised to assess the relative degree of corrosion and adhesion caused by different compounds. 

Fiirst W, Bancrjcc A. (2005) Release of glutaraldehyde from an albumin-glutaraldehyde tissue adhesive causes significant in vitro and in vivo toxicity. Ann. Thorac. Surg. 79, 1522- 8. 

Besides the shorter curing time, the spraying procedure also plays a role in determining tensile strength. The relatively high viscosity of the adhesive caused some problems that can, however, easily be overcome in a technical process. 

Figure 6. Because of the rapid increase in dynamic viscosity of Halthane 88-2 poly(urea-urethane) adhesive caused by a higher concentration of HMDI- aromatic diamine chain extender, the initial viscosities are difficult to determine accurately.

Atherosclerotic narrowing of the smallest coronary arteries is enormously increased in heavy and even in moderate smokers the increased platelet adhesiveness caused by smoking increases the readiness with which thrombi form. 

Fig. 7.3 Pericholecystitic adhesions causing years of abdominal pain ( serosa syndrome ) - without previous operative intervention in the abdominal cavity...

Missing dots result from insufficient line pressure, clogged needles, or air bubbles in the adhesive causing the adhesive to be dispensed intermittently. Missing dots result in improper parts placement, incorrect orientation, and degraded bond... 

The invention of sticky notes occurred over a period of about 10 years and involved a great deal of serendipity. The adhesive for Post-it Notes was discovered by Dr. Spencer F. Silver of 3M in 1968. Silver found that when an acrylate polymer material was made in a particular way, it formed crosslinked microspheres. When suspended in a solvent and sprayed on a sheet of paper, this substance formed a sparse monolayer of adhesive after the solvent evaporated. Scanning electron microscope images of the adhesive show that it has an irregular surface, a little like the surface of a gravel road. In contrast, the adhesive on cellophane tape looks smooth and uniform, like a superhighway. The bumpy surface of Silver s adhesive caused it to be sticky but not so sticky as to produce permanent adhesion because the number of contact points between the binding surfaces was limited. 

In this animal model for strabismus surgery, 20 Mr ad y-irradiation cross-linked poly (GVGVP), X -poly(GVGVP), presented a biocompatible barrier to the formation of adhesions, caused neither an inflammatory response nor the usual foreign body response of fibrous capsule formation, and appeared to be innocuous, its presence being ignored by the host. 

However, there were no differences in hardness between starch from hard and soft wheats or between matrices from hard and soft wheats. To rationalize this observation, it was postulated that variations in hardness were due to differences in adhesion between starch granules and protein matrix. In hard wheats, strong adhesion caused a more compact structure in soft wheats, lower adhesion strength resulted in a more porous structure with flaws in the contact between starch granule and matrix. 

The physiological activity of the solvents and the vapours and fumes released during the evaporation and vulcanisation stages of the heat-cured versions must be recognised. Solutions are toxic if swallowed, will cause skin irritation and the vapour will irritate the eyes and induce narcosis if inhaled in quantity. However, used sensibly, these adhesives cause few problems - hence the popularity of the simpler versions for domestic repairs. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

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