The Chemistry Behind Sealants

Chemistry 101: Caulks & Sealants for RV’s & Camping Trailers

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Speaking from experience, retail marketers have us at their mercy. A recent trip to the grocery store brought me to the cereal aisle. Simple enough I thought. However, arriving there I discovered that Cheerios comes in fourteen varieties. Does it have to be so difficult? Is life really so complicated that we need this many choices? The same overbaked branding that happens in the grocery store can be found in the caulking section of the RV parts store.

Luckily, the RV caulk selection is slightly less confusing than the cereal aisle, which coincidentally closely resembles the home improvement store in regards to splashy graphics and unending choices. At last count I came up with about twelve choices for RV sealants. With a little knowledge it’s not too difficult to understand what’s really in the cartridge and which sealant is right for your application.

The chemistry of sealants is a little more complicated than cereal. Looking beyond the marketing description and reading the information on the back of the tube is where you find the differences in the products. This article focuses on solvent-based, urethane, and silicone sealants as they are the most common products associated with RV’s.

Drying Verses Curing
Drying occurs through evaporation. Curing occurs through a chemical reaction. Solvent-based caulking products dry when the solvents evaporate into the air, leaving the remainder as a semi-solid substance. Sealants that cure through a reactive chemistry process, such as silicone and urethane, require a catalyst to initiate a chemical change. The chemical reaction changes the sealant from a liquid to semi-solid.

Typically, reactive chemistry sealants are higher performing than solvent-based caulks. This is due to the solvent-base products continued loss of material through evaporation as they age. Over time solvent-based products shrink, become brittle, and finally crack. The cracking and shrinking opens a path for water to get inside the wall.

Urethane and silicon sealants are two common reactive chemistry sealants. They cure through a reaction with moisture, which is the same cure mechanism for super glue. Moisture in the air and on the surface of the wall starts a chemical reaction that converts the liquid sealant to a rubber-like material. Once the material cures there is no drying, evaporation, or outgassing. These types of sealants typically outperform solvent-based caulks over the long haul.

Technical advances in product formulations by leading companies in the adhesive and sealant industry are bringing hybrid products to market. They can contain solvents as well as components of urethane, silicone, and rubber. Then there are water-based or latex-based based products which can be very good in certain situations, however are not typically associated with automotive or extreme conditions. This article focuses on what you can currently expect to find on the shelf at the RV store, which are traditional solvent-base, silicone, or urethanes. We will be testing hybrid products and share the results in the future.

When buying a sealant, identifying the chemistry type and whether it is the right choice is a challenge. A few tips can help you understand the products. All the silicone and urethane products I came across were readily marked on the front of the cartridge. The solvent-based products are typically not labeled “solvent-based” so you have to look at the back of the tube for clues such as being flammable. They also contain aromatic solvents, petroleum distillates, and other volatile organic compounds (VOC’s) such as toluene, benzene, acetone, naptha, and xylene. Hybrids typically use the word like modified, siliconized, polymer, elastomeric, silane, or acrylic in their titles.
The chart below highlights some of the attributes of common RV solvent-base, urethane, and silicone sealants.

In summary, we found urethanes offer the best overall performance with Sikaflex the leading brand. GE silicones are well respected, but in our opinion silicones are not the best choice for RV exteriors. Solvent-based products are well represented in the RV store. Brands such as Dicor and Geocell have a long history in the RV industry though we typically go with the newer reactive technologies.

A good source for RV sealants is