Page 476 - Handbook of Adhesives and Sealants

412 Chapter Ten

by water. Water resistance can be improved by applying suitable alu-
minum salts to substrates such as paper prior to bonding. Addition of
sugar, glycerin, and other materials promotes retention of moisture in
the film and increases its flexibility, tackiness, and toughness. Kaolin
clay is frequently added to raise the viscosity and prevent excessive
penetration into porous substrates. Sodium silicate adhesives are very
resistant to high temperatures. Some can resist up to 2,000 F. Their
main use is to bond porous substrates such as paper and cardboard
where an inexpensive, fast processing adhesive is required. Other ap-
plications for silicates include wood bonding, adhering glass to porous
substrates, glass fiber insulation bonding, fabrication of foundry
molds, and abrasive wheel cements.

10.4.3.2.2 Phosphate cements. These cements are based on the reaction
products of phosphoric acid with other materials such as sodium sili-
cate, metal oxides and hydroxides, and the salts of basic elements.
Zinc phosphate is one of the most important metal phosphate cements
and is used as a dental adhesive. Metallic phosphates of aluminum,
magnesium, chromium, and zirconium develop excellent thermal sta-
bility when heated to 570 F. They are virtually insoluble in hot water
once fully set. Other applications are the bonding of refractory mate-
rials.
10.4.3.2.3 Litharge cement. Mixtures of glycerin and litharge (lead ox-
ide) are used as adhesives in the repair of tubs and sinks, pipe valves,
glass, stoneware, and common gas conduits. A mixture of one part
slightly diluted glycerin with two to three parts of lead oxide requires
approximately one day to form a crystalline compound. The resulting
cement resists weak acids and nitric acid, but reacts with sulfuric acid.
These materials have been used as ceramic seals in potting electronic
equipment. They are not used much today because of the toxic effects
of lead.
10.4.3.2.4 Sulfur cement. Liquid sulfur (melting point 730 F) can re-
ally be considered as an inorganic hot melt adhesive. However, this
material should not be exposed at temperatures greater than 200 F
because of a marked change in the coefficient of expansion at 205 F
as a result of phase change. The addition of carbon black and poly-
sulfides improves the physical properties of this adhesive. Adhesion to
metals, particularly copper is good. Tensile strengths of about 580 psi
are possible. The principal use of sulfur cement is for acid tank con-
struction, where resistance to high temperature oxidizing acids are
required.

References
1. Austin, J. E., and Jackson, L. C., ‘‘Management: Teach Your Engineers to Design
Better with Adhesives’’, SAE J., October 1961.