How Do I Know When to Use Epoxy vs Polyester Glue?

In the stone fabrication world, I have been asked often what is the best product to glue together granite or marble.

In the US any glue is commonly referred to as “an epoxy” regardless of the chemical composition and properties. But epoxy is a very specific term to refer to a very specific glue.

Epoxy vs. Polyester

Epoxy are by nature polymers that contain epoxide groups. They react with a specific hardeners and form a cross link (referred as curing). These kind of cross-link usually offer high mechanical properties, temperature and chemical resistance. Usually the part A (resin) has to be mixed with a specific ratio of part B (Hardener) to obtain a complete curing. Typical ratios are 1:1. 2:1, 4:1 5:1.

The glue that instead is usually referred to as an epoxy (in the stone industry) is usually a unsaturated polyester resin, that in conjunction with 3% of a catalyst (usually BPO) creates a cross link chain. These glues have usually a solvent (up to 4-5%), are faster in the curing process and usually have a weaker strength compared to epoxy systems due to the chemical reaction.

Characteristics of Epoxy

The main characteristics of an epoxy are: very good mechanical and physical strength, high resistance to thermal variation (freeze /thaw cycles) , good resistance to sub-freezing temperatures. Epoxy can also be engineered to have very different curing times so to have different application and use.

Although their colors tend to darken up in presence of UV rays, They are typically a higher grade and more expensive glues than the polyester resin.

Characteristics of Polyester

Polyester resins are a fabricator first choice because of the intrinsic easiness of use, speed of operation and relatively low cost. Having solvents, they do tend to retract in the joints and so having a weaker strength then a true epoxy glue. They tend to be a little bit more UV stable in terms of color change but since they are a very rigid system they tend to delaminate when subjected to quick temperature change.

When to Choose Epoxy

Epoxy is usually a better choice when choosing the right glue to have durable and strong bonding on different kinds of stone. Its chemical property makes epoxy a glue of choice on granite where the surface is very hard and not very porous. It is also one of the few glues suggested for indoor and outdoor application.

When to Select Polyester

Polyester glues can be used in all fabrication job for gluing or seaming when the material will only be indoor and strength is not an issue (i.e. the glue will not be indicated for strong structural job).

If cost is an issue then polyester can be cheaper then epoxy. For lamination or seams the cost of the glue is only a fraction of the entire cost of the project, making the choice based on cost a very poor one.