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How Sealants Function

Introduction


Sealants are generally chosen for their ability to fill gaps, resist relative movement of the substrates, and exclude or contain another material. Sealants are generally lower in strength than adhesives, but have better flexibility. Usually, a sealant must effectively bond to a substrate in order to perform these functions.

 

Sealants are like adhesives in many ways. If fact, they are often considered together because some formulations can perform as either an adhesive or as a sealant, and some formulations actually provide both functions. Sealants, however, must perform distinctive tasks that stand them in a separate category from adhesives. It is important that the sealant formulator recognizes and supplies to these functional needs.

This article will review the special functions required of sealants. The methods by which the sealant performs these functions will be reviewed. Specifically, the application and performance characteristics of sealants will be addressed and correlated to the composition of the sealant formulation.

 

Functions of Sealants

Sealants are generally used as a barrier or a means of protection. In this way, sealants are used to exclude dust, dirt, moisture, and chemicals or to contain a liquid or gas. They are also often used as a coating to protect a surface or an article. They can exclude noise and vibration, improve appearance, and perform a joining function. Sealants can also be used as electrical or thermal insulators, fire barriers, and as products for smoothing or filleting. Most importantly, sealants are often used to serve multiple roles as they may be called upon to perform one or several of these functions.

No matter what the application, a sealant has three basic functions:

1. it fills a gap between two or more substrates,
2. it forms a barrier by the physical properties of the sealant itself and by adhesion to the substrate, and
3. it maintains sealing properties for the expected lifetime, service conditions, and environments.

 The sealant performs these functions via correct formulation to achieve specific application and performance properties. These properties are discussed in the sections below.

Unlike adhesives, there are not many functional alternatives to the sealing process. Soldering or welding can perhaps be used as a sealant in certain instances, depending on the substrates and the relative movement that the substrates will see in service. However, the simplicity and reliability offered by organic elastomers usually make them the apparent choice for performing these functions.

 Many sealants are designed for specific applications. Table 1 gives typical applications for various classes of sealants. The proper application of a sealant involves more than merely choosing a material with the correct physical and chemical properties. As with adhesives, the substrates to be sealed, the joint design, performance expectations, production requirements, and economic costs must all be considered. Table 2 is a partial list of considerations that are often used to select sealants in the construction industry.

 

Application Properties

The speed at which a sealant cures is critically important in some industries. Catalytic primers, two component systems, or controlled environments are used to increase the rate of cure. Depth of cure and the rate at which cure is achieved throughout the joint are other important considerations for a sealant. Single component sealants such as silicones, urethanes, and polysulfides, which depend on the diffusion of atmospheric moisture or oxygen into the sealant for curing, can take days or even weeks to cure entirely throughout the sealant bead. These sealants will develop a skin that will then inhibit the diffusion of water vapor to the center regions of the seal. In fact, some of these systems are limited to very small cross-sectional beads or else they will never fully cure throughout.


Shrinkage on curing can be a critical factor for sealant systems as well as adhesives. Excessive shrinkage can result in internal stresses and voids in the joint. Low shrinkage products are those with very high or 100% solids such as many of the two component systems. Medium shrinkage systems include hot melt sealants that shrink as they go from a molten state to a gelatinous state. High shrinkage systems are all solvent or aqueous base sealants in which shrinkage is due to evaporation of solvent or water.

For improved gap filling characteristics, some sealants are formulated to expand prior to curing. These sealants include automotive sealants with foaming agents in the formulation similar to the foaming agents used in packaging materials. Hot melt formulations are also prepared with gases dissolved into the resin that will expand when heated. All of these formulations will expand before cure to fill in the joint gap (Figure 1) and maintain positive pressure on the substrate as the sealant cures.

Foam-in-place gaskets promise to eliminate production bottlenecks in mass produced automobiles, appliances, electronics, and office equipment. These sealants are either hot melt thermoplastic (e.g., ethylene vinyl acetate, butyl) or multicomponent curable thermosets (e.g., urethane). These materials can be applied directly to the joint, thus eliminating the need for die-cut gaskets and the resulting large quantities of scrap. Preformed solid gaskets conserve material but are labor intensive and require secondary adhesives to install. In addition, because it can be applied in three dimensions, foam-in-place gasketing provides greater flexibility in joint design.


The sealant must wet the substrate and easily flow into the joint; therefore, it must be a liquid during application. However, if it is to remain in a vertical joint it must behave as a solid. The former condition is met by using a liquid polymer or a solution of a solid polymer. In either case, the flow properties of the liquid must be such that flow is reasonable under a moderately applied stress but ceases when the stress is small. This requirement is called thixotropy, flow control, or anti-slump. It is obtained in sealant formulations generally by fillers if the base polymer is a liquid or by solvents when the polymer is a solid.


When working with sealants, joint design considerations such as crack bridging, coverage rates, color, practicality of placement, order of placement, unusual movement conditions, and aesthetics must be addressed. One consideration that is required of sealants and not generally with adhesives is appearance. A sealant material may be acceptable in all respects, but appearance problems could make it aesthetically unacceptable. Usually, sealants are easily visible whether the application is in the automotive, construction, or appliance industries. Adhesives, on the other hand, are often hidden by the substrates. The sealant could also contain compounds that discolor surrounding areas. They can also be incorrectly applied so that the flow of liquids in contact with the sealant results in a residual buildup of extraneous matter at the joint.

An important consideration for any sealing operation is the relative ease of handling and applying the sealant. There are wide ranges of sealants available with varying degrees of application difficulty. There are single and two component sealants, primer and primerless sealant systems, hot melt application systems, and sealants containing solvents. As with adhesives, the time required for the sealant to harden from a liquid state into a semi-solid with some degree of handling strength is very important. Significant production properties are: cure rate, low temperature flow characteristics, paint-over ability, color, self-leveling properties, non-bubbling properties, and cost.

Safety and toxicity effects are a concern when using certain types of sealants. Chemical reactions can cause the release of toxic fumes, heat, or possibly both. Solvent evaporation may contribute to a safety or heath hazard in the working place. Metering and mixing of multiple component systems could bring personnel into direct contact with the components resulting in dermatitis or other health related problems.

 

References

1. Panek, J.R., and Cook, J.P., Chapter 2, Construction Adhesives and Sealants, John Wiley & Sons, New York, 1991.
2. Dunn, D.J., "Sealants and Sealant Technology", in Adhesives and Sealants, vol. 3, Engineered Materials Handbook, ASM International, 1990.

 


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