ELASTOMERS AND SEALANT FORMULATIONS

clarify the mechanics of sealant and seal perfor-

elastomer or elastomers together with additives.

mance associated with low temperature pavement

Various formulations with the same base elas-

applications, and to address the issue of low tem-

tomer can be developed for very different appli-

perature hardening that should be a dominant fac-

cations. For example, polyurethane formulations

tor in the selection of a sealant, this report presents

can be used in elastomeric applications ranging

background information on the formulation and

from shoe soles to joint sealants. Although elasto-

mechanical properties of elastomeric seal materials

meric formulations can be designed or advanced

and the structural behavior of field-molded joint

by theoretical modeling techniques, the develop-

and crack seals.

ment of most is primarily empirical.

Additives are used primarily to improve spe-

cific properties of a formulation. A "plasticizer" is

an additive that increases the flexibility of a com-

ELASTOMERS AND

pound. More specifically a plasticizer reduces the

SEALANT FORMULATIONS

stiffness of the base elastomer at a given tempera-

Commercially available sealants are produced

ture, lowers the temperature for which elastomeric

from a variety of materials. Because the ideal be-

behavior can occur, increases the strain at which

havior of a pavement joint seal is that of an elas-

failure occurs in the elastomer, and increases its

tomer, it is revealing to describe sealant formula-

impact strength at lower temperatures (Immergut

tions in the context of elastomers and elastomeric

and Mark 1965). In some sealant formulations,

compounds.

plasticizers are used in this way, i.e., to lower the

"Elastomer" is a term often used for rubber and

modulus of the cured material and to improve its

polymers that have properties similar to those of

low temperature capabilities. "Filler" refers to ma-

rubber (e.g., ASTM 1991b). Elastomers are poly-

terial that is used primarily to add bulk to the base

mers with a particular type of molecular structure.

elastomer, although the term is often used to desig-

Specifically they are linear, amorphous (i.e., non-

nate any additive other than plasticizers that has an

crystalline), high molecular weight polymers in

impact on the properties of the compound. At times

which the chains of the molecular structure are

fillers have been used to improve the profit mar-

flexible and cross-linked (e.g., Hearle 1982). The

gin of the manufacturer at the expense of the de-

flexibility and cross-linking of the polymer chains

sired elastic material properties. Panek and Cook

allow elastomers to behave elastically, as the name

(1984) have suggested that such economic-based

implies, such that they are capable of recovering

compounding has been the cause of the inadequate

from large deformations. Mark (1982) has listed

performance of some asphalt rubber highway joint

three molecular requirements for a material to ex-

seals in the past. Other additives that might be

hibit this rubber-like elastic behavior:

used include coloring agents, curing ingredients,

The material must consist of polymeric

adhesive agents, and ultraviolet light and oxygen

chains.

stabilizers. Usually exact formulations of elasto-

The chains must have a high degree of flex-

meric compounds are kept as trade secrets of the

ibility.

manufacturer. Table 1 includes several nonspe-

The chains must be joined into a network

cific sealant formulations that have been pub-

structure.

lished by Panek and Cook (1984).

In this context the terms elastomer and rubber are

Elastomers are used as the base polymer of com-

used as synonyms, and the broad range of natu-

mercially available sealants so that seals formed

ral and synthetic rubbers and rubber compounds

by these sealants will respond elastically to ser-

that behave like elastomers can be studied in a

vice loads and deformations. The base polymers of

single context because the behavior can be attrib-

sealants include natural rubber, styrene-butadiene

uted to this molecular structure. It is of interest to

rubber, neoprene, silicone, polyurethane, poly-

note that when the molecular chains lose their flex-

sulfide and polyvinyl chloride (Panek and Cook

ibility in response to a temperature reduction, there

1984). The sealant formulations range from asphalt

is a transition in the material behavior of the poly-

and rubber compounds to coal tar and polyvinyl

mer from a elastomeric behavior to that of a hard

chloride compounds to silicone sealants. Often cat-

material, e.g., glass. The transition is referred to

egorical distinctions are made in the applications

as the "glass transition."

literature between asphalt rubber sealants, coal

Elastomeric compounds are marketed com-

tar rubber sealants and other polymeric sealants.

mercially as formulations that consist of the base

In the context of elastomeric compounds, however,