Caulks

DEAR TIM: I need some help with caulks. There are many different types available. How are the silicone caulks different from the acrylic latex caulks. Are they both paintable? Also, I purchased a clear acrylic latex caulk, but it came out of the tube white. Was the caulk in the tube out of date, or was it defective? One last thing, is there a caulk available to fill a driveway crack? M. W.

DEAR M. W.: Caulks have experienced a significant transformation during the past 45 years. Linseed and oil based caulks were used on a widespread basis up until the 1950's. During the 60's and 70's acrylic latex and silicone caulks began to appear. Because caulks need to stick to things, they are closely related, chemically speaking, to adhesives. In fact, I'll bet that you have seen adhesive caulks at your local stores.

In as much as caulks are used to fill cracks between things that frequently move, it is important for them to have a high degree of flexibility. The older oil based caulks almost always became brittle with age. Acrylic latex and silicone caulks are formulated so that they will remain flexible for many years.

100 percent silicone caulk is made by reducing silica sand into a basic silicone oil polymer. In order to give the caulk body, fillers such as mica (a mineral) and clay are added. These caulks work best when used on non-porous objects such as metals and glass. The silicone oil makes it virtually impossible for paints to adhere to 100 percent silicone caulk. This oil tends to bleed slowly out of the caulk for many years.

100 percent silicone caulks require moisture from the air in order to cure. If you live in a dry climate, you will notice that these caulks take a longer time to dry. As the silicone caulk cures it emits acetic acid, one of the primary ingredients of vinegar. These fumes can irritate your eyes and nose.

Acrylic latex caulks are comprised of acrylic polymers, latex, water, and fillers. They bond very well to porous materials such as wood, masonry, plaster, and drywall. These caulks cure, or dry, as the water in them evaporates. Acrylic polymers and latex have excellent flexibility. They also retain this flexibility for long periods of time. Also, paints adhere well to the fillers that are present in these caulks.

Some of your confusion may arise from acrylic latex caulks that contain silicone or 'paintable' silicones. Some acrylic latex caulks contain small amounts of silicone. The silicone is added to improve their flexibility. The paintable water based silicone caulks often contain high amounts of fillers which allow paint to adhere to them. However, the high filler content tends to have an adverse affect on the overall performance of the caulk.

You and I have shared a similar experience. When I first used a clear acrylic latex caulk, I thought the manufacturer had made a mistake. I never believed that the white caulk would dry clear. I was wrong. The white color in the caulk is caused by the presence of latex. The latex in the caulk is derived chemically. It is similar in nature to the milky colored fluid produced from plants belonging to the milkweed family. As the water evaporates from the latex the structure of the caulk changes so that the caulk becomes transparent. Light waves travel directly through the caulk.

There are caulks available to suit just about any purpose. You can purchase caulks that look like brick mortar, blacktop, and concrete. Special low temperature caulks are available that adhere to cold, damp surfaces. Caulks are made that seal aluminum gutter joints which commonly leak due to expansion and contraction caused by temperature changes. There is a caulk for just about any need.