Adhesion setting

Attempts have been made to use cold-set adhesives in the cormgating operation, such as poly(vinyl acetate) and modified, precooked starch formulations, but these have not achieved any appreciable degree of commercial acceptance (20). The use of a polyethylene film appHed to the inside surface of the linerboard facing, which serves as a hot-melt cormgator adhesive, has achieved some commercial usage. However, its use is limited to the small, specialty product niche of fast-food hamburger cartons (see Olefin polymers, polyethylene). 

J. J. Bechei, G. R. Hoffman,. W. Swanson, and R. C. McKee, Development of Cold-Set Adhesive, Project 2696-11, Report One to the Eouidiiniei Kraft Board Institute from the Institute of Paper Science and Technology, Adanta, Ga., Nov. 1973. 

Urea resin adhesives, by the use of the proper hardener, may be set either by heat or at room temperature. For room temperature curing, the hardener may be ammonium chloride, together with basic materials like calcium phosphate to neutralize excess acid that might damage the wood. Cold set or room temperature set adhesives are those that set satisfactorily at 20 —30°C, whereas a hot set adhesive generally means one that is set above 99 °C. 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Phenolic resins also find use in varnishes, electrical insulation, and in other protective coatings. Heat-settings adhesives which are based on phenolics find use in producing plywood. These also find use in the production of ionexchange resins having amine, sulphonic acid, hydroxy or phosphoric acid functional groups. 

The most suitable in any particular instance is best established by trials in advance. Sometimes, fairly severe abrasion (by increasing the surface area available) is the most successful. On the other hand, excessive roughening may weaken the substrate and so give inferior results. With fast-setting adhesives like the cyanoacrylates, microscopic bubbles of air may be trapped between adhesives and substrate and so impair bonds. 

Perry, H. A., Room Temperature Setting Adhesive for Metals and Plastics, Adhesion and Adhesive Fundamentals and Practice, J. E. Rutzler and R. L. Savage, eds., Society of Chemical Industry, London, 1954. 

Lignin-based cold-setting adhesives evaluated on beech strips (74% lignin 26% resorcinol). 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

Cold-set adhesives 80% of success dependent upon adhesive formulation... 

In cold-setting adhesives, the combination of resorcinol with the tannin in some form, be it added or partly generated from the tannin itself, hides a multitude... 

Epoxy resins are used in cold setting or thermo setting adhesives, in production of industrial composites and films of high resistance and permanence. Over half of production is used in protective coatings. Electrical laminates and filament windings are also important uses, representing about 20% of consumption. 

The chances of adhesive technology could be exploited more fully if advances to counter these risk factors could be made. The reactive and physically setting adhesives will be used mainly in the field of construction, with natural adhesives used more often in the areas of consumer goods, foodstuffs, and packaging. Water-based adhesives will continue to replace solvent-based products, a process supported in particular by the food processing and packaging industries. 

Chemically curing Physically setting Adhesives with special setting characteristics... 

A typical characteristic of physically setting adhesives is the evaporation of solvents or heatup to a molten state. Application is then in liquid form (solvent adhesives, contact adhesives, dispersion adhesives, hotmelt adhesives) or from the solid phase (adhesive films, laminate adhesives). In all of these types there is a liquid state during processing from which emissions are always a possibility due to the volatility of the basic polymer or other adhesive components. 

Adhesives Curing without Chemical Reaction (Physically Setting Adhesives)... 

One-component reactive adhesives have to be distinguished from the physically setting adhesives described in Chapter 5, which generally occur only in the form of one component, namely, the already finished polymer, for example, in the case of hot-melt adhesives, dispersion adhesives and solvent-based adhesives. They are called one-component adhesives. 

In their processible mixture, these one-component products, also belonging to physically setting adhesives, consist of two components PVC (polyvinyl chloride-) particles and plasticizers (Section 9.2.9). The solid PVC particles are disperged in the high-viscosity plasticizer. The adhesive layer formation occurs by heating (120-180 °C), when the thermoplastic PVC swells and is thus able to absorb the plasticizer (no chemical reaction ). This process is called a sol-gel process. The formerly two-phase system (sol) is turned into a single-phase system (gel) by the inclusion of the plasticizer. 

In the case of fast-setting adhesives and large adherends, possibility of curing starting prior to fixation of the adherends... 

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