Page 561 - Handbook of Adhesives and Sealants

482 Chapter Thirteen

13.2.2 Asphaltic and other
bituminous mastics
Mastics are formulated from bituminous materials obtained as by-
products of petroleum refining. These bitumens are hydrocarbon ma-
terials containing various polymeric materials of high molecular
weight and complexity. They are usually solids or semi-solids that
must be applied hot, or else the sealant is formulated from a solvent
or water-based emulsion of the asphaltic product.
Because of their hydrocarbon nature, these bituminous materials
have a low surface tension and wet most surfaces moderately well.
However, bitumens can be displaced from the substrate by water, or
they can be prevented from wetting a substrate by a film of moisture.
To overcome this problem, a surfactant is usually added to the for-
mulation, and the substrate must be completely dry before application
of the sealant.
Bitumen based sealants can become brittle at low temperatures.
This fault is overcome by the addition of rubber (e.g., scrap neoprene)
to the formulation to act as a fexibilizing agent. Drying oils (about 5%
by weight) are added to the formulations to prevent oxidation and
resulting embrittlement and surface crazing. A tendency to cold flow
is reduced by the incorporation of epoxy resins.
A major application for these sealants is in highway joints. Typically
the highway sealant will contain 50–60% asphalt, 20–30% ground
rubber (usually from scrap automobile tires), and 20% cyclic hydro-
carbons. This sealant is applied at 250–400 F. For applications where
fuel resistance is important, such as airfield joints, nitrile rubber is
generally added to the asphaltic formulation. Asphaltic-based mastics
are also used for automotive joints, construction sealing, pipe seals,
and marine joints.

13.2.3 Polyvinyl acetate
Polyvinyl acetate sealants have the same base latex resin that is used
for wood glues. These sealants are a low performance, inexpensive
consumer product that dries to a hard, rigid seal. A typical sealant
formulation consists of polyvinyl acetate latex, filler, plasticizer (for
improved flexibility), and water. Since these sealants set by evapora-
tion of water, their shrinkage is greater than that of 100% solids seal-
ants.
Polyvinyl acetate-based systems are generally used for interior ap-
plications such as, spackling compounds, sealing wall framing, or bath
tub caulk. Their advantages include ease of application, water clean-
up, paintability, and good adhesion to most substrates. Disadvantages
are low movement capability, high modulus, and shrinkage on curing.
Polyvinyl acetate sealants have a modulus that is very temperature