Polyvinyl formal-phenolics adhesives

Table 3.5. Effect of pretreatment on lap shear strength of steel polyvinyl-formal-phenolic adhesive joints... 

TABLE 7.15 Effect of Pretreatment on the Shear Strength of Steel Joints Bonded with a Polyvinyl Formal Phenolic Adhesive (from Ref. 26)... 

Suggested adhesives include modified acrylics, epoxies, polyesters, resorcinol-formaldehyde, furane, phenol-formaldehyde, polyvinyl formal-phenolic, polyvinyl butyral, nitrile rubber-phenolic, polyisobutylene rubber, polyurethane rubber, reclaimed rubber, melamine-formaldehyde, epoxy-phenolic, and cyanoacrylates. For maximum adhesion primers should be used. Nitrile-phenoUcs give excellent bonds if cured under pressure at temperatures of 149 C. Lower-strength bonds are obtained with most rubber-based adhesives. 

Structural adhesives Polyvinyl formal-phenol formaldehyde Polyvinyl butyral-phenol formaldehyde Nitrile-phenolic Epoxy-polyamine Epoxy-anhydride Epoxy-nylon Epoxy-phenolic Epoxy-polyamide Epoxy-polyurethane... 

Peel resistance. The straight epoxies cured by either polyamines (amides) or anhydride are poor in peel. Polyvinyl formal-phenol formaldehyde (P/F) and butyral-P/F are moderate but the peel varies with the P/F content. As the P/F content increases, room temperature lap-shear and peel strength decrease but these properties improve at elevated temperatures. The epoxy-polyamides (epoxy-nylon) such as FM1000 (Cyanamid Corp.) give extremely good peel resistance under dry conditions. Hockney (1970) quotes 511N for an adhesive of this type with a strip 25 4 mm wide, peeled at 90°. The peeling... 

The viscosity of most adhesives increases with time as they set by cross-linking, cooling from the melt or loss of solvent. The cross-linking of a phenolic-polyvinyl formal adhesive and of cold-setting epoxies was found by de Bruyne [41] to be represented adequately by an exponential relationship ... 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

Heat-setting resins cannot be plasticized by low molecular weight plasticizers. Polyvinyl acetals have been claimed for these products. American Cyanamid Co. has suggested polyvinyl acetals or butyrals in an amount of 10 to 25% of the resin for urea and melamine resins. For varnishes and adhesives, combinations of phenolic resins and polyvinyl formal are recommended. Polyvinyl acetals with higher alkyl radicals are suitable for cellulose esters and improve elasticity as well as resistance against water. 

Uses Phenolic/polyvinyl formal resins (wire enamels, can coatings) mfg. of paints, adhesives, and films In food-pkg. adhesives In food-contact coatings In paper/paperboard in contact with aq./fatty foods Regulatory FDA 21CFR 175.105, 175.300, 176.170... 

In the waterborne phenolic adhesives, a new dispersion system (98) has been reported. This system uses the solubility of some thermoplastics in phenol, and the subsequent polymerization takes place upon the addition of formaldehyde, an emulsifier, and water. The dispersion is then formed in situ. The versatility of this process is indicated by the use of many polymers with a wide range of properties, e.g., polyvinyl formal, polyvinyl butyral, acrylonitrile-butadiene copolymer, bisphenol-A resins, polyvinyl acetate, polycarbonate, polymethyl methacrylate, and nylons. 

In most widely used two-polymer adhesives, the thermosetting component is phenolic. Phenolic resins are generally compatible, although not easily miscible, with a number of thermoplastic polymers. Particularly good compatibility is demonstrated between conventional alcohol-soluble phenolic resins and polyvinyl esters and acetals. Epoxies are important in two polymer adhesive systems. The most important thermoplastic components are the polyvinyl acetals (polyvinyl formal and butyral) and synthetic rubber, particularly nitrile rubber. Soluble nylons are also an important class. ... 

The largest commercial use for polyvinyl formal is as wire enamel, usually in conjunction with cresylic phenolic resin. Similar resins are used also with phenolics as structural adhesives. They are very resistant to greases and oils. Some grades have been used as can coatings and wash primers. Although they can be molded, extruded, or cast, these grades have achieved little commercial significance. 

Vinyl-phenolics are alloys composed of polyvinyl formal (PVF) - phenolics or polyvinyl butyral (PVB)-phenolics. They are available as liquids and films. In general, vinyl-phenolics have a maximum service temperature of SOT and are equal to phenolic - nitriles in strength. These adhesives are better than epoxies in sandwich structures that require high strength. 

Vinyl-phenolic. Vinyl-phenolic adhesives are based on a combination of phenolic resin with polyvinyl formal or polyvinyl butyral resins. They have excellent shear and peel strength. Room-temperature shear strength as high as 5,000 Ib/in is available. Maximum operating temperature, however, is only 200°F, because the thermoplastic constituent softens at elevated temperatures. Chemical resistance and impact strength are excellent. 

Historically, the first structural adhesive successfully used was used in the aircraft industry for bonding aluminium to wood (Garnish, 1977). It was a polyvinyl formal composition with a phenol formaldehyde resin. 

Adhesives which incorporate phenol-formaldehyde resins are much used as structural adhesives for metals. They comprise the original polyvinyl formal/PF, and polyvinyl butyral/PF together with nitrile phenolics and epoxy phenolics. The ratio of non-phenolic polymer to the PF resin in the structural adhesives determines the morphology of the material, whether it is two-phased, and which phase is continuous and which discrete. This, in turn, influences the flexibility of the... 

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