Development, adhesives

Phenol—formaldehyde (PF) was the first of the synthetic adhesives developed. By combining phenol with formaldehyde, which has exceptional cross-linking abiHties with many chemicals and materials, and a small amount of sodium hydroxide, a resin was obtained. The first resins soHdified as they cooled, and it was discovered that if it was ground to a powder with a small amount of additional formaldehyde and the appHcation of more heat, the mixture would Hquify and then convert to a permanently hard material. Upon combination of the powdered resin mixture with a filler material such as wood flour, the result then being placed in a mold and pressed under heat and pressure, a hard, durable, black plastic material was found to result. For many years these resulting products were called BakeHte, the trade name of the inventor. BakeHte products are still produced today, but this use accounts for only a small portion of the PF resins used. 

Adhesion development depends on diffusion of the CPO component of the primer through the crystalline boundary layers followed by swelling and entanglement with the rubber rich layer [75]. 

As indicated above, one of the main attributes of acrylic PSAs is their broad formulation and processing latitude. While this is an important benefit to the end user, it also poses a significant challenge to the adhesive developer not only to understand what each component can do for the performance of the adhesive, but also on how to combine these components to get the best balance of properties. To help understand acrylic PSA formulation, one needs to know the major components and their impact on the PSA properties. 

Rubber base adhesives develop strength faster than most other polymeric types. Fig. 1 [3J shows the differences in the development of peel strength for several rubber polymers (without additional additives, except an antioxidant). Natural... 

It is noteworthy that an important industrial application is based on pure silicone network [9]. This is the organic PSA release technology where an uncured silicone is deposited as a thin coating to a flexible substrate. Strong adhesion develops at the silicone-substrate interface whilst the coating cures. 

Progress in research and development in the wood-based industry and in the adhesive industry has shown many successes during the last decades. On the other hand, many industrial requirements still require considerable and important developments in this area. The main driving forces today are cheaper , quicker and more complex . The first two are caused by the heightened competition in the above-mentioned industries and the attempt to minimize costs while maintaining a certain level of product quality and performance. The key word more complex stands for new and specialized products and processes. Adhesives play a central role in wood-based panel production. The quality of bonding, and hence the properties of the wood-based panels, are determined mainly by the type and quality of the adhesive. Development in wood-based panels, therefore, is always linked to development in adhesives and resins. 

Adhesives for sandwich bonding progressed similarly. Early low-flow adhesives were used with plastic foam cores and end-grain balsa to manufacture sandwich as.semblies. The development of less dense honeycomb cores necessitated a change in adhesive. Development of good bond strength to honeycomb core requires relatively high flow so that an adhesive fillet forms on the core walls (Fig. 6). 

J. Comyn, in Structural Adhesives—Developments in Resins and Primers, A. J. Kinloch (Ed.), Elsevier, London (1986). 

Figure 21. Rate of crosslink development vs. rate of fiber-matrix adhesion development for 12-volume fiber glass composite...

The same theories relevant to adhesion, developed to explain and predict the performance of glues, adhesives, and paints, have also been applied to bioadhesive systems [44], These include the electronic, absorption, wetting, diffusion, and fracture theories. 

Radiation cured epoxy adhesives significantly reduce the long cure times needed for conventional adhesives. These adhesives develop excellent physical and mechanical properties in a nonthermal cure process that requires a cycle time of only seconds to minutes, versus hours for conventional thermal curing. 

Perhaps the greatest success at hydrophobic epoxy adhesive development was the development of fluorinated epoxies together with compatible curing agents. These adhesives resulted in equilibrium water concentrations as low as 0.2%. However, for various reasons these adhesives have not been commercialized. 

Standard test methods are useful only if they can be reproduced. It is important that the same results be measured by both the adhesive developer and the end user. It is also important that the results be reproducible with time and with different testing personnel. The accuracy and reproducibility of test results depend on the conditions under which the bonding process is performed. The following variables must be strictly controlled. 

On the other hand, it is not always necessary that an interfacial agent be present. Polypropylene is available in impact-modified grades which are made by simply blending polypropylene with suitable olefin-based elastomers. Most often the elastomer is a suitably chosen ethylene-propylene-based rubber. Evidently, the required adhesion develops naturally in these systems without the need for an interfacial agent. However, proper control of phase morphology during mixing is essential. 

Finally, for those adhesive developments based on renewable resources that are immediately competitive and/or unique in performance, the wood products industry will do its share to assist with their evaluation and use. Commercial guidance toward this end will be freely provided. 

The shear and tension tests after submersion of specimens in boiling water for 2 hours are not tests accepted or specified by any regulatory or testing society or agency. Gluing experience over many years, however, has shown that important conclusions with respect to reasons for adhesive failure, as well as important pointers for adhesive development, can be obtained by careful interpretation of the comparison of test results after boil with those from dry and vacuum-pressure testing. 

Although, as previously pointed out, the many synthetic adhesives developed during the past two decades are replacing the natural adhesives to a large extent in a number of areas, starch-based adhesives are almost impossible to remove from some markets because of specific properties and cost savings. Some of those markets are discussed below. 

An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formalde-hyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resorcinol-formal-... 

Over the last 20 years, environmental concerns have become an important consideration in adhesive formulation and use. Firstly, in the plant the adhesive may require particular handling or the use of protective equipment. Secondly, volatile emissions arising from adhesive reactions both in the hot press and subsequently when the panels are in service are subject to tight regulatory control. In particular the reduction formaldehyde emissions from wood-based panels has been a major objective in adhesive development over the last few years, both because formaldehyde-based adhesives are the major type used and because these have been implicated in environmental and health concerns. 

Snrgical therapy is not withont risk. Potential complications in-clnde nterine prolapse, adhesion development with snhseqnent re-dnced fertility, and damage to or denervation of the pelvic stmcture. Risks and benefits of snrgery therefore shonld he discnssed on an in-dividnal patient basis. 

The casein adhesives developed for paper labels have proven to be readily soluble when the glass bottles are washed at defined pH levels. This has shortened the washing procedure and the labels have been able to comply with all further... 

As described above, adhesives on polymer basis have to be transferred into a processible state by using adequate solvents or water. Thus, the important group of solvent-based adhesives develops, in contrast to reactive adhesives processed in the form of monomers, which, due to their mostly liquid or pastelike state, do not require solvents. In the common language use, only products containing organic and, in the majority of cases, combustible solvents in their formulations are called solvent-based adhesives. 

The applied liquid adhesive develops into the adhesive layer according to the chemical reactions or physical processes described in Sections 2.2.1 and 2.2.2 as well as in Chapter 3. So for reasons of preciseness, we talk of... 

The thermal conductivity of adherends influences the temperature conditions in the glueline during adhesive curing. It plays a special role in the application of hot-melt adhesives on metals due to the quick solidification of the melt in the boundary layer zone and the possible impairment of the adhesion development. The thermal conductivity A is indicated in the dimension W/cm K (Watt per centimeter Kelvin). Values of certain materials ... 

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