I was sitting in my office one day, when I received a phone call from a hotel manager about a swimming pool coping that was falling apart. I asked him what was falling apart and he told me that the stone was falling into the pool. I got in my truck and made my way over to the hotel and discovered that the swimming pool had a travertine coping and all the laminations were failing and falling off. I examined the laminations closely and discovered that the adhesive used was polyester.
Several weeks later I got a phone call from a homeowner who had a similar problem, but this time the laminations were failing on her new granite countertop. Again, upon inspection, polyester had been used.
How many people use polyester for laminations and other applications to bond two pieces of stone together? If you do, your laminations are prone to failure.
There are many choices when it comes to the selection of adhesives and it can get very confusing. The following is a brief explanation of the two most popular adhesives in the stone industry and a guide as to where to use those types of adhesives.
In my experience, all the adhesives available polyester is the costliest with the weakest bond strength. Polyester uses a hardening agent to catalyze the curing reaction. Most polyesters use a peroxide hardening agent. It also has a high shrinkage rate and the highest water absorption amongst all adhesives. Polyester is prone to UV degradation and will crack and become brittle.
For this reason polyester should never be used for laminations or for repairs in wet areas or used outdoors. Polyesters are good for small repairs for indoor stone applications, such as filling and seams, or in applications where bond strength is of no concern. The bond strength of polyesters is less than 500 psi.
Epoxies are two-part adhesives, labeled part A and part B. Most epoxies are mixed at a ratio of 2:1, but some are 1:1. Of all the adhesives used in the stone industry, epoxies have the strongest bond. Bond strengths can exceed 2,000 psi. It has a lower shrinkage rate than polyester and is more UV-stable with a lower water absorption rate. I always recommend using epoxy for laminations and rodding, and anywhere else you want the stone to stick.
A Little Experiment
Here is a test I perform on adhesives when determining bond strength: Take two pieces of stone with the polished sides facing each other. Place some adhesive on the polish faces and clamp them together. Let the clamped pieces set overnight. The next morning take a hammer and try to break them apart. A weak glue, such as polyester, will break clean, leaving the face of the stone intact. A strong adhesive, such as an epoxy, will be so strong that the face of the stone will spall off. This indicates that the adhesive bond is stronger than the natural bond of the stone.
All adhesives are available in various viscosities ranging from low to high. The following are the most popular choices.