Mastic adhesive is a very strong bonding agent used in many commercial and industrial settings, but is perhaps most popular for setting tiles and sealing windows, walls, and ceilings in building construction. It is traditionally derived from the resin of the mastic tree, which is where it gets its name, though it is commonly manufactured synthetically as well. Depending on the application it is generally available in thin liquid, thick glue, or paste form. It can quickly and permanently bind many different materials together, though in most cases it works best on hard, non-porous surfaces. Over time it can and sometimes will seep into cracks and crevices, which can lead to discoloration and general weakening.
Where it Comes From
Originally, this sort of adhesive was made from the resin droplets or “tears” of the mastic tree. This tree, known scientifically as Pistacia lentiscus, grows most prolifically in the southern Mediterranean region, particularly in Greece and Turkey. In most cases the resin comes out of the tree the same way that sap comes out of many pine trees. Harvesting it is easiest if the whole tree is cut down, but killing the plant limits the supply dramatically. Traditional harvesters construct a number of shunts and filtration systems to capture the resin as it is produced, which saves the trees — but can make the end product very costly, both in terms of labor involved and natural limitations on supply.
A number of modern manufacturers create their own mastic in labs. In most cases it has the same intense adhesive properties and is chemically almost identical, but is a lot less expensive to make. Synthetic mastic is often a better choice for large projects and is more widely available in many places as well.
When used in construction, mastic adhesive is typically in liquid form and applied with a caulking gun. It is squeezed out by hand in a thin line along wall or ceiling joints, and the strength of the adhesive helps hold load-bearing walls in place. The quick-setting adhesive eliminates the need to support heavy drywall for extended periods of time.
Construction adhesive is also used as a temporary hold for fixtures so they can be nailed or screwed in place by one person — that is, without anyone else there to help hold things up or double-check measurements. In these cases the adhesive can serve as a reinforcement, but it’s not usually intended as a final sealant. It’s often difficult to remove, though, so people in this situation usually have to be very careful to set things properly the first time.
Popularity in Industry
Industrial uses for this adhesive include repairing heating and air duct work in buildings and joining panels of concrete and asphalt in warehouses and storage facilities. Its ability to bind permanently to metal makes it really useful in certain auto and mechanical repair settings, too, thanks in part to its heat resistance and general durablility. Most of the time mastic adhesive comes in a finely ground powder that is mixed to form a paste in these sorts of settings; it is smeared onto the repair area and allowed to dry, usually for about 24 hours.
Even though it will bind to almost any sort of surface from tile to leather to rugged metal, mastic isn’t always the best choice. A lot depends on the intended use, as well as the likelihood of the adhesive coming into contact with excessive or prolonged moisture. It’s very common in tiling for instance, but most builders recommend it only for porcelain tiles, especially when used outdoors. Other tile types, such as ceramic, are less dense and more porous. Over time the adhesive can seep into these pores, causing discoloration.
There is sometimes also a risk of water damage if moisture is able to seep into spaces between the surface of the wall, joint, or tile and the adhesive strip. This is most common when porous materials are used in places like steam rooms and pool houses where there is a relatively constant amount of moisture and high humidity. Moisture doesn’t usually impact the integrity of the adhesive since it's generally waterproof, but it can lead to mold growth in the crevices of the building material which can be harmful to health, unsightly to look at, and difficult to remove thanks to the bonding agent's strength.