Joint Sealants for Indiana Limestone

As the sealant industry continues to develop new and improved formula for joint sealants, comments by observers outside that industry become quickly dated. Although ILI updates this Technote from time to time, in consultation with sealant manufacturers, users are urged to review manufacturers' data sheets, and to consult with sales engineers, prior to specifying or approving joint sealant materials for use with Indiana Limestone. ILI personnel are happy to advise on the matter as well.

Indiana Limestone provides an excellent substrate for adhesion for most sealants in current use. While most materials do not require a primer, many manufacturers recommend their use for best adhesion to limestone.

This is a good point to confirm with the manufacturer, as some will not provide a guarantee unless primers are used. Care should be taken to not apply primers outside the joint area, as some may darken the stone.

Sometimes, the oils contained in sealant formulations cause stain at or under the surfaces of masonry materials. This condition is said to occur in Indiana Limestone with some formulations under some circumstances. In addition, some primers are said to cause stain. Materials which are well-known for their staining potential are oil-based caulks and butyl sealants, though not all butyls stain. ILI suggests oil-based caulks not be used with Indiana Limestone, and that staining tests be run on any butyls under consideration. The general question of stain potential should be part of the decision-making process in choosing joint closing materials. Users and specifiers may wish to pursue this question with manufacturers.

For all sealants, the contact surface of the stone must be dry, clean, dust-free, and frost-free. Some sealant formulations are temperature sensitive. All should be gunned against a backer rod or caulk-stop rope, and the rod or rope must be placed at the proper depth to assure good performance. Of all problems with sealants, failure to observe this rule is by far the most common source of trouble. The sealant bead must meet a ratio of depth-to-width described in the manufacturer's literature; usually that ratio is less than 1:1. Whatever the rule for the sealant being used, it should be followed.

One recurring complaint is the tendency of some formulations to attract dirt. Most manufacturers will have reduced this problem; this is a good question to raise with technical representatives.

Sealant systems are not necessarily mutually exclusive with mortar systems. In some cases, both mortar and sealant joints are needed on the same project. Or, it may be desirable to face an occasional mortar joint with sealant to match other joints on the job.

The mortar must be raked back to a proper depth, and a sealant tape placed against it. The tape prevents sealant adhesion to the mortar. Sealants tend to fail when gunned against mortar, because they cannot adhere for any length of time to three surfaces. The tape acts as a bond-breaker. Tapes require less rake-depth than rods, and thus leave more mortar in the joint for bearing.

The general feeling in the sealant industry is that multi-component job-mixed sealant materials provide higher quality than one-component formulations. As technology improves, it seems likely that one-part sealants will be developed to equal the adherence and long-life qualities of the two-part (job-mixed) sealants. Some types of one-part, high-performance systems now are the equal of two-part systems, their makers claim. Professional sealant applicators are used to the critical measuring and mixing requirements of the two-part sealants; improper measuring and mixing can result in failed joints. One-part sealants tend to be goof-proof, but as a rule they are more expensive than comparable quality, job-mixed formulations. In general, two-part, internal cure systems set and cure more rapidly than one-part, air- or moisture-cure systems. Some sealant types, such as urethanes, are available in both one- and two-part systems. As this issue is so complex, and the technology is ever-changing, ILI suggests that interested inquirers contact sealant manufacturers for the latest information on this subject.

1. Urethanes/Polyurethanes: Tack-free 2-24 hours. Excellent adhesion, with no primer required for most substrates, though often suggested for best longterm adhesion on limestone. 15-20 year life. Good resistance to UV, ozone, and acid atmospheres. Can accommodate movement in the joint up to 50% of design width. Good color selection. -40 to +180 degrees F service temperature range.
   
2. Acrylic polymeric: Tack-free 1-2 hours after installation, excellent adhesion; 5-10 year life; good for dissimilar materials; -30 to +180 degrees F service temperature range; good resistance to UV, ozone and acid. 10-15% movement capability; primers not required; broad color selection.
   
3. Silicones: Tack-free time 5-30 minutes; excellent adhesion; 20 year life; service temperature -60 to +300 degrees F; good for dissimilar materials; excellent resistance to attack agents; may require primers; 50% movement capability; broad color selection; may cause staining.
   
4. Acrylic Latex: Tack-free time 15-30 minutes. Fair adhesion. Fair resistance to UV and ozone. No primers, 12-15% joint movement capability; limited colors; sealant bead accepts paint. Shrinkage and exterior usage ability a consideration in some formulations. 2-10 year life. Often tend to become brittle with age, though some manufacturers indicate that many formulations do not.