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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.
Polyester
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.
Epoxy
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.
Adhesive Grades
All adhesives are available in various viscosities ranging from low to high. The following are the most popular choices.
Knife Grade: Consistency is similar to spackling putty or creamy peanut butter
Flowing Grade: Consistency of a motor oil
Penetrating Grade: Consistency of water
Super Penetrating Grade: Consistency of a solvent such as mineral spirits or paint thinner
Generally knife grades are used for repairs with fills more than 1/16 in. wide. Flowing grades are used for repairs smaller than 1/16 in. and for laminations and rodding.
Penetrating and super penetrating grades are rarely used by fabricators, but can be used to resin stone and on applications where an adhesive is needed to penetrate very small pores.
Adhesive Colors
Today's adhesives are available in a number of colors; however, there may be times when you need to add coloring. Many adhesive manufacturers have tints that you can use, but if you're in a pinch I have found that artist's oil colors work well for most polyesters. Epoxies, however, need tints designed specifically for them.
General Adhesive Properties
Temperature: All adhesives are sensitive to temperature and humidity. This means that cure times will vary. Adhesives will cure faster with increasing temperatures and slower at cooler temperatures. Temperature will also affect the viscosity. Higher temperature will thin the adhesive. There are also several heated dispensers for epoxies that work well for faster cure times and easier flowing.
Cure Time: The general rule of thumb for most adhesives is the slower the cure time, the stronger the bond. An epoxy with a seven-hour cure time is going to be much stronger than an epoxy with a five-minute cure time. If too much hardener is added to polyester, though, you will increase the cure time but reduce the bond strength.
UV Exposure: Nothing is 100 percent UV proof; however, epoxies are more UV-resistant than polyesters and are the adhesive of choice for outdoor exposure.
Moisture: Polyester is very sensitive to moisture and for that reason it should be avoided in wet areas, such as showers. Epoxy has a much higher tolerance for moisture and is the adhesive of choice in wet areas. When using polyester, you must make sure that the stone is completely dry. Epoxies can tolerate some moisture.
Summary
Polyester
- The stone must be completely dry.
- If you do not thoroughly mix the resin and hardener, it will most likely still cure.
- You can use most colors for tinting.
- Many varieties are available.
- It can be easily used for patching.
- Stone surface must be abraded before sticking.
- Should not be used for exterior or in moist locations.
- Quick curing time (normally 10 minutes to one hour depending on the product and the weather).
- Works well for indoor repairs and seaming.
Epoxy
- Stone can be a little moist.
- Resin and hardener must be thoroughly mixed to cure.
- You should use only colors made for epoxy.
- Not as many varieties available.
- Not as easily used for patching.
- Stone surface should be abraded, but it will stick to a smooth surface also.
- Can be used for exterior and moist locations.
- Slow curing time (normally seven hours depending on the product and the weather).
- Works well for laminations, rodding, outdoor repair, mending and attachments.
About the author:
Frederick M. Hueston runs the National Training Center for Stone & Masonry Trades, Asheville, NC 28806; www.ntc-stone.com; and can be reached at Fhueston@aol.com.
author: By Fredrick M. Hueston