High Performance Sealants For Building Joints
Today's modern high performance sealants have four primary functions, as barriers, as materials to join two or more surfaces, as protective coatings and as appearance enhancers. In their functions, they are used to exclude dirt, water, dust, moisture and chemicals. As barriers, they are used to contain both liquids and gases, or to exclude noise. As surface coatings, they protect against mechanical or chemical attack.
On a daily basis, contractors -- as well as architects, design engineers and specification writers -- often consider adhesives and sealants in the same vein because many formulations both adhere and seal. A sealant should be used for its ability to exclude and/or contain a material, whereas an adhesive should be chosen for its holding and binding power.
There is a wealth of product information available which makes these formulations difficult to classify. Sealant manufacturers are an excellent reference point. For this article, we will discuss only curing types of a flexible nature. These sealants are the only type that fit as "high performance" products when dealing with physically sealing or caulking of a structure.
Basic Parameters Required
Minor formulation changes often result in major changes in properties. For this reason, it is advisable to consult with a reputable sealant manufacturer. Contractors should consider the following factors before using a sealant:
- Temperature and pressure;
- Chemical compatibility and permeability;
- Weatherability;
- Mechanical properties;
- Adhesion;
- Dynamic performance;
- Color stability, flammability and toxicity;
- Repairability;
- Ease of application;
- Surface preparation;
- Cost; and
- Consequence of joint failure.
After considering these factors for a specific application, one needs to decide which type of sealant is desirable. Generally, sealants are either curing or non-curing. Curing is understood as the changing of the properties of a material by a chemical reaction such as condensation, polymerization or vulcanization. These type sealants are high performance, of an elastomeric nature, and are intended for joints that may experience movement on the order of +/-25 percent, +/-50 percent, +100 percent or -50 percent.
Chemical curing (or reactive) sealants are either one-part or two-part. One-part chemical curing sealants cure by the absorbing of moisture or oxygen. Two-part chemical-curing sealants are packaged in two separate containers, one a base and one an accelerator. The accelerator (or catalyst) is a substance that hastens the natural curing process of the sealant. One must mix these sealants either by a manual or automatic method just prior to use. Once one has mixed these two parts, generally A and B, these sealants allow a limited time before they begin to cure.
Classification Of Sealants
The high performance thermosetting sealants and caulks have predicted cyclic dynamic joint movement capability of +/-12.5 percent to +/-25 percent throughout their useful service life. Those with the lower movement capability are designated as Class B in Federal Specifications TT-S-00227, TT-S-0023 and TT-S-001542 and as Class 12 1/2 in ASTM C920. The higher capability is designated Class A in federal specifications and Class 25 in ASTM. There are now available neutral-cure silicone sealants that have movement capability of +/-50 percent. Note that there is no classification for +/-50 percent capability in either federal or ASTM specifications. Sealants in this class are either one or two-component systems that cure by chemical reaction to a solid state from the liquid form in which they were applied.
The generic types of sealant included in this high performance category include polymercaptans, polythioether, polysulfides, polyurethanes, silicones, and hybrid combinations of polymers. The properties that make them suitable as sealants are their resistance to weathering and ozone, flexibility and resilience at both high and low temperatures, and inertness to a wide range of chemicals, solvents and fuels.
The most critical concern in evaluating sealants of this range is their ability to perform satisfactorily under dynamic joint movement. As previously mentioned, it is no single characteristic, but a proper balance of several essential properties that determines sealant performance. The sealant will perform only as long as this balance is maintained.
Mechanical Properties
Understanding the mechanical properties of sealants also plays a role in determining a correct sealant choice. Important properties include strength, elongation, compressibility, elasticity and tear resistance. Each is described below:
Strength -- In some applications, strength is 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.
Elongation -- Elongation, when considered with elasticity, sometimes can predict the extension/compression characteristics of a sealant. (Note the difference between elongation and extension: elongation represents the maximum ability of a thin cross section of sealant to stretch a single time before breaking; extension is the capability of a sealant bead to stretch and compress repeatedly for sustained periods before breaking.)
Compressibility and Elasticity -- Some high performance sealants are formulated for a higher movement capability than a joint actually was designed to accommodate. This is done with good reason. The fact is, joints designed for about 25 percent extension/compression often must accommodate movement of 50 percent or more. Thus, higher performance sealants provide an added safety factor. Keep in mind that they were not formulated to provide a license to design narrower joints that utilize the sealant's improved capabilities.
Tear Resistance -- Because sealants are exposed to a lot of scuffing and mechanical wear, 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, with urethanes having the highest all-around tear resistance.
In addition to the factors above, dynamic loads, shock and rapid variations in stress also can cause seals to fail. Thus, evaluating more flexible, elastomeric sealants -- which can stretch and then return to their original length in a short time -- should be the beginning of the selection process for joints designed for dynamic loads.
In many situations, the appearance of seals is almost as important as their physical properties. Thus, most sealants are available in a variety of colors to match the environment in which they are used.
Weatherability Of Sealants
One of the factors that will affect one's choice is weatherability. Weatherability is the degree of resistance a sealant will withstand during exposure to moisture, heat, cold or solar radiation. Weatherability also may include resistance to acids or alkalis from airborne contamination and spills. If a sealant's weatherability is important to a facility, contact sealant manufacturers and ask for results of accelerated testing performed on their sealants.
Though a sealant may be formulated to resist attack by almost any single exposure factor, it also should be able to resist -- at least to some degree -- most other exposure factors. Why? Because one will find few situations where a sealant is exposed to only one element.
For that reason, today's sealants are compromise formulations. That is, manufacturers formulate their products to meet a variety of applications. These products, then, meet a majority of needs, while the final choice rests with the informed specifier.
Conclusion
Over the past several years, polyurethanes have become one of the mainstays for sealing of building joints. Silicones have had success in structural glazing applications due to their favorable performance in weathering, particularly UV resistance. Polysulfides and polymercaptans have been losing ground. All four generic materials have the capability of meeting a variety of standards relative to joint movement, with silicone performing at higher ranges.
User Comments:
No comments added
Add your comment
Fill out the fields below:
|
