Interior-grade adhesives

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in stmctural appHcations. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

In this chapter, the development of a thermosetting adhesive from soda bagasse lignin is described. The research has concentrated on the development of interior-grade adhesives for particleboard. The local market for exterior boards is smaller than that for the interior panels, and adhesives for exterior boards are already covered by an excellent range of tannin-based adhesives. 

Hardwood plywood is sold in both decorative (thin boards, <6 mm) and construction (thick panels, >6mm) markets. Temperate hardwoods are used primarily for decorative purposes, although Finnish birch is an exception being used in specialized high value construction applications. Thin boards are manufactured from tropical hardwoods and are used for decorative or platform uses. Decorative uses include wall panelling and door faces. As a platform the thin board receives a decorative surface that is either printed or overlaid on the panel surface, at which point it is known as prefinished (ready to use), and these are the major items traded internationally. Thin tropical boards are manufactured with water resistant, interior grade adhesives, whereas the majority of other boards use phenolic-based resin that can be used in exterior situations. 

AH-acryHc (100%) latex emulsions are commonly recognized as the most durable paints for exterior use. Exterior grades are usuaHy copolymers of methyl methacrylate with butyl acrylate or 2-ethyIhexyl acrylate (see Acrylic ester polymers). Interior grades are based on methyl methacrylate copolymerized with butyl acrylate or ethyl acrylate. AcryHc latex emulsions are not commonly used in interior flat paints because these paints typicaHy do not require the kind of performance characteristics that acryHcs offer. However, for interior semigloss or gloss paints, aH-acryHc polymers and acryHc copolymers are used almost exclusively due to their exceUent gloss potential, adhesion characteristics, as weU as block and print resistance. 

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. 

Articles made from amino resins are water clear, hard, and strong, but they can crack. They have good electrical properties, and they have better colorability than phenolic resins. Amino resins are used as adhesives for plywood and particleboard but only in interior grades. They have low weather resistance and deteriorate when exposed to sun, heat, cold, and moisture. 

Adhesives from Diisocyanates and Proteins. Protein-based adhesives have been used as traditional binders for wood since the beginning of wood products manufacture. Glutin and casein binders can provide interior-grade wood products but with modified casein binders, even panels for exterior use are possible. For economic and technical reasons, protein-based wood adhesives have been replaced more and more by synthetic adhesives since the beginning of the century. 

Me I am in resin bonded adhesives are not yet widely used in North America, mainly due to their cost. In Europe, they have long been popular for making high quality interior-grade plywood. These products emit more formaldehyde than phenolic resins, but significantly less than UFR. 

As mentioned earlier, the single largest commercial use of soybean flour in wood glues during the recent past has been as a blend with other adhesive proteins, mainly blood and casein, for bonding interior-grade plywood, doors, and millwork. These blended formulations exploit several unique properties of the soybean glues themselves and incorporate useful adhesive characteristics from the other protein materials. 

Formaldehyde additions overcome the effect of phenolic extractives in certain hardwood species, which prevent proper cure or adhesion of PF resins. Free formaldehyde appears to react rapidly with these phenolic extractives before they can interfere with the phenolic resin curing mechanism. With certain wood species that are rich in extractives, this technique has been used to increase bond durability from interior-grade to true exterior-grade performance. 

On the basis of PF resin, PUF resin has been developed to decrease the cost and to accelerate curing. Similar to the standard PF resin, PUF resin is prepared under alkaline conditions. Application of nanoclays to urea-formaldehyde (UF) resins has recently shown that nanoclays can improve the properties of thermosetting resins for interior-grade wood adhesives [20], and urea in PF resin could possibly be useful for the preparation of PUF resin/MMT composites, too. 

Adhesives for wood particleboards were obtained giving results satisfying the relevant standard specifications for interior grade wood boards and the following were found ... 

For the first time mixed wood panel adhesive formulations for interior-grade applications were developed in which lignin is in considerable proportion, 50%, of the wood panel binder. 

D-3024. Specification for Protein-Base Adhesives for Structural Laminated Wood Products for Use Under Interior (Diy Use) Exposure Conditions. Employs the D-0>905 block shear, D-0906 plywood shear, and D-4300 mold resistance tests to evaluate mostly casein-base adhesives for use in interior grade laminated beams. 

Plywood, particle board and timber laminations - Interior grade plywood and particle board are made using urea formaldehyde adhesives due to their good bonding to wood and low cost. Their disadvantage is poor water resistance. Exterior grades of plywood use phenol formaldehyde adhesives due to their good water resistance and low cost. 

Major uses for mea formaldehyde adhesives are in the manufacture of particleboard and interior grade plywood. 

Protein-based adhesives have been used for centuries. The present primary use for these adhesives is as a stmctural material to generate interior-grade plywood. The source of the proteins for these adhesives can be used in their classification. Thus, the proteins for these adhesives come from blood, fish skin, casein (milk derived), soybeans, animal hides, bones, and connective tissue (collagen based). This classification is useful for predicting the durability of bonds made with protein-based adhesives. 

Urea-formaldehyde adhesives are widely used in the wood industry for joining wood to make useful forms such as interior-grade plywood and particle board. The chemistry of these adhesives is shown in Figure 8. The primary reaction is the addition of formaldehyde to urea to generate methylolated amines under basic conditions which can then further condense with other ureas to create a three-dimensional network under acidic conditions. These reactions can take place at room temperature or at elevated temperatures depending upon the... 

Adhesive raw materials come from two sources plant based and petroleum based. As was mentioned above, petroleum-based raw materials are becoming more expensive and harder to come by. This has led to a drive to find and manufacture adhesive components based on sustainable or renewable raw materials. Adhesives and sealants have a long history with natural product-based raw materials. The imfortu-nate situation is that adhesive bonds made with natural product-based raw materials are usually not dirrable. Thus, protein-based adhesives can be used to make interior-grade plywood, but synthetic adhesives are used to make exterior-grade plywood. Therefore, the drive to make adhesives cheaper is combined with the drive to make natural product-based adhesives more durable. 

---