Every book has a section that is more controversial than the others. In a book on sealants, any comparison of the various generic types of sealants is bound to generate some criticism. As of January 1986, when this chapter was written, no good statistical comparison, acceptable to U.S. Industry, seems to have appeared in the open literature, despite the publication of many comparisons through the years. ASTM Committee C-24 on Building Seals and Sealants put together an Ad-Hoc Committee in 1982 to prepare a factual , nonbiased paper comparing U.S. sealants. Their work could take 10 years or more. The Japan Sealant Industry Association published a booklet in January 1983 that did some comparing of sealants available in Japan. Although it is very general, this work does have some value. The most important feature of the Japanese work is that the comparisons were agreed to by the entire industry.

In the author's opinion, the single best comparison published to date in the United States is probably that of Bob Baldwin, Senior Editor of Plant Engineering magazine, Published in Plant Engineering in January 1976. This part of the chapter will paraphrase a good deal of his work, updating it in the light of new product introductions, new specifications, and industry thinking as of January 1987. A recent book by Julian Panek and John Cook is also a very valuable reference to types of sealants, as is ASTM Special Technical Publication 606, Building Seals and Sealants. A good comparison of weathering characteristics, with good pictures, and a good comparison on effects of temperature was shown by M H Miller in The Construction Specifier in January 1984. Another important source of information for this chapter was "Caulks and Sealants - short Course," sponsored jointly by the Adhesive and Sealants Council and the University of Akron, at the University of Akron, December 10-12, 1985.

Performance
There are many ways of describing or differentiating sealants, and almost any method of comparing performance of the generic type to any of the types of needed performance described in the previous chapter are acceptable. However, one property of sealants seems more important than others, and this is ability to take cyclic movement. This section is therefore divided into sealant classes by movement capabilities. It is interesting that service life or longevity and cost difference fall roughly into the same divisions. Low cost generally implies a short useful lifetime and low movement ability.

Low-Range Sealants
The sealants that fall into the low-range category can easily be identified by their low cost. They generally include the oil-based, resin-based, resinous caulks, bituminous-based mastics, and polybutene-based sealants. They are generally very highly filled with inexpensive principally mineral, fillers.

Some low-range sealants are now siliconized. This generally means that they contain between 0.20% and 2% of a silicone fluid for better and greater plasticity, or a silicone (silane) coupling agent for better adhesion, or a silicone (silica) filter for better thixatrophy or some other handling characteristic. A siliconized oleoresinous or other low-range sealant is close in performance to the parent material with no silicone, silane, or silica in it. The most noticeable difference between the siliconized and standard products is usually the very large size of the work "silicone" on the label.

Low-range caulks were the standard of the industry in the early part of the century and still enjoy some popularity in the over-the-counter market because of their low cost. Commercially (industrially) low-range caulks are generally not used except in easily accessible locations with very mild environments where re-caulking would require only minimal labour cost. The labour of re-caulking and the frequency of re-caulking needed with these materials, keep their usage in commercial construction to a minimum. It is this author's opinion that the low-grade caulks would be losing market share even faster if it were not for the exploitation of the work "siliconized" on the label, which gives the consumer the impression that the caulk is a low-cost silicone.

Medium-Range Sealants
Medium-range sealants have generally been limited to applications with movements from 0 to 5% of the joint width. In the 1970's however, some materials in this range were marketed with movement capabilities up to ±12.5% of the joint width. Such is the case in the mid 1980's as well. While the ability to take more movement is an important feature, the chief advantage of these materials over the low-range caulks is their longer service life. Manufacturers of these materials often advertise 10 year and 15 year lifetimes, if the environment is protected and the movement is low or non-existent. Actual effective lifetime in the range of 3 to 10 years is typically experienced.

A disadvantage common to most sealants in this category is shrinkage, which can approach 30% in some products. Plasticizers that discolour paints, walls, and windows have been a problem in some highly plasticized butyls, as well as in most plasticized systems. The nonskinning butyls, which are acceptable for buttering some splice joints, acoustical sealing, and other hidden joints are not considered to be part of this group.

Sealants in this class do include the better butyls and latex acrylics (but not PVA [vinyl] latex products, which are generally in the low range category). This category also includes the neoprenes as well as the solvent-release acrylics. The characteristics most noted by the consumer is the price, generally between the low- and high-range sealants. These sealants, with the exception of the odorous solvent acrylics, are generally easy to apply. As with the low-range sealants, many sealants of these general types were reintroduced to the market in the early 1980's as siliconized, or advertised as containing an additive of silicone and sometimes called modified silicones.

Extensive work by this author has shown the siliconized materials he studied to be very close in performance to the parent sealant and very far from the typical silicone sealants. Siliconizing medium-range sealants has no more benefit, other than marketing, than it did for the low-range caulks. Some specific properties, such as adhesion, extrudability, or consistency, were improved by the 1-2% of silicone or silane added. But the key performance properties of cyclic joint movement ability, tolerance to harsh weather conditions, tolerance to high temperature, and longevity showed little if any improvement. The best of the modified silicones studied by this author were really polyether chain-type sealants cured with a silane cross-linking mechanism and were called "modified silicone" and not "siliconized". Many of the standard polyurethanes have a polyether chain, and the performance of what are called modified silicones closely approaches that of the polyurethanes and will be discussed with the high-range sealants.

High-Range Sealants
High range sealants include polysulfides, urethanes, and silicones, and certain proprietary modifications of these materials. These sealants have many important attributes, but of primary importance is their ability to take joint movement of greater than ±12.5%. Sealants in the high-range group are advertised for from ±25% movement to ±50% or ±100%/-50% movement. It is generally from this class that commercial sealants are chosen. Some effort will be spent here to describe the key features of each of these high-range sealants. The property checklist developed in the first chapter can be compared to the performance values given here. Thus a better grasp on the variety of sealants available to satisfy a need should be achieved.

The high-range sealants are taking an ever-increasing part of the market in both the do-it-yourself, over-the-counter trade as well as in the commercial construction market, as evidenced by various industry reports. The heaviest emphasis will thus be on the high-performance sealants.

The fundamental properties evaluated when selecting a sealant were detailed in the checklist in Chapter 1, but there are a few key properties that are generally important in all sealant applications, and we will look at these properties in a general comparison of sealants.