Important mechanical properties of sealants include elongation, compressibility, tensile strength, modulus of elasticity, tear resistance and fatigue resistance. Depending on the nature of the application, a sealant may require very little strength or great strength. The sealant must have sufficient mechanical characteristics to remain attached to substrates during service as well as to provide a barrier. The substrates could move considerably, requiring that the sealant expand and contract significantly without losing adhesion from the surface. Defining movement capability is a complex process. Temperature, rate of temperature change and joint configuration will influence the results.
In some applications, strength may be more important than elasticity. Low-strength — or, more precisely, low tensile modulus — may be the most important factor in a situation where a sealant joins one or more weak surfaces. Tensile strength is needed primarily to avoid cohesive failure under stress and so as not to transfer stress between substrates, as is the case with most adhesives.
Modulus can sometimes predict the extension or compression characteristics of a sealant. In general, low-to-medium-modulus sealants are able to take significant movement without putting much stress on the sealant or the substrate materials. Some high-performance sealants are formulated for a higher movement capability than a joint is actually designed to accommodate. In fact, joints designed for about 25% extension or compression must often accommodate movement of 50% or more. Thus, higher performance sealants provide an added safety factor. A change in elasticity or hardness on aging may be an indication that further curing or degradation is taking place.
Compressive strength is the maximum compressive stress that a sealant can withstand without breaking down or experiencing excessive extrusion from the joint. Compression set is the inability of a sealant to return to its original dimension after being compressed. High compression set is usually caused by further curing or degradative crosslinking of the material while under compression. Compression set is undesirable in a joint that needs to expand and contract. Stress relaxation is a condition in which the stress decays as the strain remains constant. Some very-low-modulus sealants literally get pulled apart when held at low elongation.
Sealants may be exposed to scuffing and mechanical wear. Examples include the sealant used as an expansion joint in highways and the sealant used in preparing stone walkways. Thus, they must offer good abrasion, puncture and tear resistance. Flexible sealants, which are available in either chemical curing or non-curing types, exhibit varying degrees of tear resistance. Urethanes have the highest tear resistance.
Dynamic loads, shock, and rapid variations in stress can also cause seals to fail. Thus, the consideration of tough and flexible elastomeric sealants that can stretch and then return to their original length in a short time should be the first step in the selection process for joints designed for mechanical loads.
Adhesion is also an important factor in determining a sealant’s performance. The same rules of adhesion that apply to adhesives also apply to sealants. Adhesion is primarily affected by the physio-chemical interaction between the sealant material and the surface to which it is applied. However, in certain joints where there is great movement, strong adhesion of a sealant to a specific substrate may not be desirable. In these situations, the adhesive strength is stronger than the cohesive strength of the sealant, and the sealant may tear apart when it expands or contracts. This requires the sealant to be applied so that it does not adhere to all surfaces. To achieve this affect, a bond-breaker or release material at the bottom of the joint is generally used.
Conditions that will influence the adhesion of sealants include water exposure, temperature extremes, movement considerations and surface cleanliness. Often, a surface-conditioning process or a priming step is necessary to make a substrate compatible with a specific sealant.
Weatherability is defined as a sealant’s degree of resistance when exposed to heat, moisture, cold, solar radiation, etc. The degree of weatherability is determined by the base polymer and the nature of the additives in the sealant formulation. Generally, sealants are formulated for maximum resistance to a single element, such as moisture.
Often, this chemistry will lend its resistance to other elements as well. In many situations, the appearance of the sealant is almost as important as its physical properties. Thus, most sealants are available in a variety of colors to match the environment in which they are used. Several questions must be considered when determining the appearance requirements of sealants.
- Does a sealant cause discoloration of surrounding areas initially or over a period of time?
- Does water runoff over the material cause unsightly residues?
- Does one product cause discoloration of another?
- Does the product itself change in appearance over time for any reason?
Sealants can have a chemical effect on the substrate. Chemical incompatibility could cause the sealant or substrate to soften, harden, crack, craze, inhibit cure, or cause other changes. An example of this would be the use of an acid cure sealant (such as a silicone sealant) on a surface like concrete, marble, or limestone. On these surfaces, an acid/base reaction can cause the formation of bond-breaking salts at the bond-line. Another example of chemical incompatibility is the bleed of plasticizers or other low-molecular-weight volatiles through sealants, causing them to discolor after exposure to sunlight. This happens frequently when sealants or coatings are applied over asphalt or organic rubber-based materials that are formulated with low-molecular-weight plasticizers.
Sealants may also need to be compatible with a specific environment for certain applications. Examples of this may be a requirement for a sealant to have USDA or FDA acceptance because food or drugs are to be processed in the area near the sealant. It may happen that, in an installation such as a food processor or clean room, the sealant cannot outgas or liberate certain chemical components either during or after cure. Another end-use requirement could be that the sealant must meet certain fire-resistance properties to meet code requirements in housing construction or another area of use.