Phenolic adhesive

These adhesives, more properly ealled phenol-formaldehyde adhesives, are eondensation products 

They dominate the field of wood adhesives and represent one of the largest volumes of any synthetic adhesive. Phenolics are also among the lowest-cost adhesives and may he formulated as water dispersions, to allow penetration into the cell structure of wood that is important for the formation of permanent bonds. Beyond the wood and wood products area, unmodified phenolics are used mainly as primers, to prepare metal surfaces for bonding, and as binders, for such varied products as glass wool insulation mats, foundry sand, abrasive wheels, and brake lining composites. Phenolics are supplied either as one-component, heat-curable liquid solution, as powder, or as liquid solution to which catalysts must be added. The curing mechanisms are different for these two groups.  

Phenolic adhesives are structural adhesives with specific applications, e.g. where wide gap bonding is required and where large structures need to be bonded. Phenolic resins are the product of a special reaction ratio of a phenol and formaldehyde in the presence of an organic catalyst. There are two main types of phenolic resins phenol or methylol terminated. The phenol terminated are called novalacs, while the methylol terminated are called resoles (one step resins). Modem phenolic resins are prepared in the presence of metal carboxylates and these resins contain a large number of benzylic ether linkages and have open para positions which have good temperature stability and are usually of low viscosity. 

The novalac phenolic resins react under weakly acidic and anhydrous conditions using metal catalysts of the divalent state, e.g. Ca, Mg, Zn, Pb, Cd, Co, Ni and Cu acetals, halides or sulphonates. The mechanism of this ortho-hydroxymethylation reaction has been attributed to the formation of chelate complexes as intermediates as proposed occurs at the initial and subsequent condensation reaction of the phenolic alcoholics. It has been shown that electropositive bivalent metals work best when pH is between 4 and 7. This may be described as follows  

Metal analysis associated with these adhesives involves monitoring raw materials for metal contamination and metal content of activators or initiators used as the second part 

Certain phenolic-formaldehyde resins can be made to cure in the absence of metals and they are ortho-ortho resoles, etc., and these adhesives will not be discussed further. 

Such compounds - although as a functional group, they also have C=C-double bonds - are not assigned to the acrylate adhesives. These products are mentioned, because experience shows that repairs ofboats, vehicle bodywork (e.g., caravans) and other plastic parts are frequently carried out, where unsaturated polyester resins as two-component systems with hardener component on styrene basis play an important role. 

In this context, unsaturated resins are usually compounds containing C=C double bonds that, by polymerization and crosslinkage with the respective monomers, can be transferred into saturated thermosetting compounds that no longer contain double bonds. Here again the basis are resin and hardener components that cure after mixing at room temperature or in the heat. 

Apart from polyaddition and polymerization adhesives, a third kind of adhesive that shows a special reaction mechanism during the curing process exists. They should be mentioned for the sake of completeness, although they are less important compared to the systems mentioned so far. Their characteristic is the fact that a byproduct arises in the formation of the polymers from the monomers, which has to be considered in curing. The central molecule of these adhesives is formaldehyde 

Due to this water condensation, such adhesives have to be cured at high temperature and under high pressure in so-called autoclaves, when impermeable materials are bonded, to avoid an increase in volume of the adhesive layer caused by the water-vapour expansion. 

Polycondensation adhesives based on phenol or phenol derivates with formaldehyde are mainly used in the production of wooden constructions (in woodworking one traditionally talks of glueing instead of bonding ), for example, in the manufacturing of laminated bonds (plywood, chipboard, fiber board, beams). Since in these applications, the adherends are able to absorb the water developing during the polycondensation reaction, the processing occurs in heated presses to enable the application of the necessary contact pressure at the required temperature (Section 7.2.4). For applications in nonindustrial fields these adhesive are practically unimportant. 

Phenolic resins are synthesised from phenol (or phenol substitutes) and formaldehyde. They first appeared in the early 1900 s, since when Bakelite has been a widely recognised name. Phenolic-based formulations are used as adhesives for structural applications, notably for wood products. Their main features are  

Phenolics are also used as additives in other synthetic adhesives, e.g. epoxies and contact neoprenes, where they lend some level of the above properties to the adhesive. 

Polyimide adhesives are mainly available as films, although some formulations are beginning to be available as pastes. 

Many of the design concepts that were used in the Hornet and Sea Hornet are shown in Fig. 16. Sheets of aluminium reinforcement were Redux -bonded to 

Ruonnated Ethiytane-Propylene Parting Film Pressure Bag 

In the mid-1940s, in order to improve its stability and performance, the resole was changed to one of lower alkalinity and a phenol to formaldehyde ratio of 1 1.43 (Redux K6) this has remained unaltered until the present day. As well as the expected improvement in stability, there was also an unexpected benefit. This was the marked improvement in the ambient temperature lap-shear performance on aluminium an increase from an average value of about 15 MPa to bond strengths which were consistently above 30 MPa. 

However, in spite of this innovation, the use of Redux Liquid E was retained by de Havilland until 1963 (well into the Comet programme) and by Chance Vought in the United States of America until the termination of the F-7 Cutlass programme. 

On a point of terminology, in 1955, Redux was qualified to a new military specification - DTD 775. Following that qualification all Redux products took the specification number into their official designation and became Redux 775. 

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WOOD-BASEDCOMPOSITESANDLAMINATES] (Supplement) -phenolic adhesive [PHENOLIC RESINS] (Vol 18)... 

In 1932, the first plywood hot press was installed in the United States. This marked the advent of the large market for phenolic wood adhesives [51]. By 1962, the volume of phenolic wood adhesives had reached about 33 kt (solids) in the U.S. Growth was accelerated in 1962 with the development of Southern pine plywood. By 1979, the consumption of phenolic plywood adhesives exceeded 220 kt or about 25% of phenolic resin production [51]. Phenolic adhesive demand for wood products took another jump in 1964 with the commencement of waferboard production. The first oriented strandboard (OSB) plants were built in 1981 [52]. OSB soon replaced most of the waferboard production and began a period of... 

In addition to epoxy-phenolic adhesives three-part epoxy-phenolic-nitrile rubber systems are used in metal-metal edge joints and honeycomb constructions [208], These add toughness not available in most EP systems and improve peel strengths. When used on honeycomb, the NR-P is normally applied to the aluminum skin and the EP to the honeycomb for assembly. Service temperature limitations are those imposed by the NR-P part. 

Though toughened phenolic adhesives remain in use for specific applications, toughened epoxy adhesives have dominated metallic bonding on civil aircraft since their development in the 1960s. Advances since then have been incremental and mostly revolving around manufacturing issues such as handleability and allowed out-time. 

Phenol—formaldehyde resins, 10 408—409 Phenol—formaldehyde reactions, 18 760 Phenolic acetates, 20 45 Phenolic adhesives, 1 543—544 Phenolic antioxidants, 10 806... 

Benzylation has been performed on wood in order to impart thermoplastic properties to the substrate (Hon and Ou, 1989). Wood is pre-treated with aqueous NaOH solution, then with benzyl chloride. Benzylation of the surfaces of wood blocks and chips for selfbonding of wood surfaces has also been reported (Kiguchi, 1990a,b Kiguchi and Yamamoto, 1992). A vapour-phase benzylation method has also been developed (Kiguchi, 1993). Carboxymethylation of NaOH-treated wood using various solvent systems has been studied (Shiraishi and Kishi, 1986 Honma and Nakano, 1991). Wood modified in this way has been used to make wood-phenolic adhesives (Kishi and Shiraishi, 1986). 

Kishi, H. and Shiraishi, N. (1986). Wood-phenol adhesives prepared from carboxymethylated wood II. Mokuzai Gakkaishi, 32(7), 520-526. 

Other applications for phenolics are switchgears, handles, and appliance parts, such as washing machine agitators (that s why they re usually black). Phenolics are widely used to bond plywood, particularly exterior and marine grades. Although urea-formaldehyde resins are cheaper for this purpose, they were not nearly as water-resistant and have been limited to interior grades. Abrasive wheels and brake linings also are bonded with phenolic adhesives. 

Control boards made from bamboo particles using a phenolic adhesive swelled about 10 percent after 1 h, I5 percent after 6 h, and 20 percent after 5 days. Particleboards made from acetylated bamboo particles swelled about 2 percent after 1 h and only 3 percent after 5 days (35) ... 

Figure 5 (2J) shows changes in thickness of aspen flakeboards made from control and acetylated flakes using a phenolic adhesive at different relative humidities. After four cycles of 30 to 90 percent RH, control boards swelled 30 percent in thickness, while acetylated boards at 17 9 WPG swelled about 5 percent. 

The objective of this work was to demonstrate the utility of organosolv red oak lignin (a projected cheaper polyphenol than phenol) in phenolic adhesives for wood composites. This work involved three stages ... 

Cure Rate of the Phenolated SEL Resins. 13C NMR spectra of the phenolated SEL formaldehyde-treated resins revealed the formation of methylol groups. A similar cure reaction to resole type phenolic resins is expected to occur with the phenolated lignin-based resins. Since cure rate normally determines production capacity of a board mill, it is important that new types of adhesives have at least the same cure rate as the conventional phenolic adhesives. Cure analysis of resins has usually been examined by... 

Fiberboard -for food packaging [FOOD PACKAGING] (Vol 11) -medium-density [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement) -phenolic adhesive [PHENOLIC RESINS] (Vol 18) -sampling standards for [SAMPLING] (Vol 21) -structural [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement)... 

Polymerization kinetics have been used to question whether or not phenolic adhesives covalently bond to wood. Chow (23)... 

Thermal Inactivation. Heating the wood surface to temperatures above 150 to 200°C definitely has been shown to cause poor adhesion with phenolic adhesives and, possibly, urea adhesives. Early research (60) verified that overheated wood was less wettable and tended to absorb less water from the adhesive. Whether this is caused by extractives, pyrolysis, oxidation of hydroxyl groups, or other chemical reactions at the wood surface has been the subject of much debate. Further, the mechanisms of thermal or "surface" inactivation may vary from species to species. 

Phenol-formaldehyde adhesives are the only other adhesive system used in significant quantity in particleboard production. The increased durability of this class over that of the ureas results in phenolics as the adhesive of choice for exterior particleboard. However, phenolic adhesives are only used where the additional durability is required since they are more expensive and require longer curing times. 

The bark extract bonded particleboards met specifications requiring the inherent durability provided by phenolic adhesives. These products are used for floor decking for modular homes, specialized furniture uses, home siding, garage door panels and more recently, as a wall and roof sheathing and single layer floor decking in conventional home construction. Thus, phenol and phenol-resorcinol modified resins can be replaced by a low-cost bark product. This use of bark would be a profitable outlet for bark residues and could lead to virtual independence of the wood particleboard industry from the petrochemical industry. 

Another area of real concern deals with HEW Occupational and Health Standards on hazardous substances listing formaldehyde and phenol. With phenolic adhesives, this is not a problem since all of the resin solids and non-volatile solids are usually tied up in the set glue line. 

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