Epoxy vs. Polyester. Are You Using The Right Adhesive?
Frederick M. Hueston
I was sitting in my office when I received a 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 where failing and falling off. I examined the laminations closely and discovered that Polyester was used in the laminations.
Several weeks later I got a similar call from a home owner who had a similar problem but this time the laminations where failing on her new granite countertop. Again, upon inspection, the adhesive used was polyester.
How many of you use polyester for your laminations and other applications where you are bonding 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 what type:
Epoxy or Polyester?
Polyesters use a hardening agent to catalyze the curing reaction. Most polyesters use a peroxide hardening agent.
Of all the adhesives available polyester is the least inexpensive but also has the weakest bond strength. It also has a very high shrinkage rate and the highest water absorption amongst all the adhesives. It is also very 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 area 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 parts 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 2000 PSI. It has a lower shrinkage rate than polyester and is more UV stable with a lower water absorption rate.
It is clear that if you want great adhesion, epoxy is the stronger adhesive. I always recommend using epoxy for laminations and rodding and anywhere else you want the stone to stick and not fall apart.
A little experiment.
Here is a crude test that I perform when testing adhesives for 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
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 for fills that are over 1/16 of an inch wide. Flowing grades are used for repairs smaller than 1/16th and for laminations and rodding.
Penetrating and super penetrating grades are rarely used by fabricators but are used for resining stone and for applications where an adhesive is need to penetrate very small pores.
Today’s adhesives are available in a number of colors however there may times when you need to add coloring. Many of the adhesive manufacturers have tints that you can use. If you in a pinch I have found that artist oil colors work well for most polyesters, however for epoxies you will need to use tints designed specifically for them.
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.
Tip: There are several heated dispensers for epoxies that work very well for faster cure times and easier flowing.
The general rule of thumb for most adhesives is the slower the cure time the stronger the bond. Epoxies with a 7 hour cure time are going to be much stronger than an epoxy with a five minute cure time.
If too much hardener is added to polyester you will increase the cure time but you will reduce the bond strength.
Nothing is completely UV proof; however epoxies are more UV resistant than polyesters and are the adhesive of choice for outdoor exposure.
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.
The stone must be completely dry
If you do not thoroughly mix the resin and hardener it will probably cure anyway
You can use most colors to tint
Many varieties available
Good adhesion when prepared properly
Can easily be 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 1 hour depending on the product and the weather)
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
Great adhesions when prepared properly
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 7 hours depending on the product and the weather)
Indoor repair- Polyester
Outdoor Repair- Epoxy
Mr. Hueston is the founder of The National Training Center for Stone and Masonry Trades (www.ntc-stone.com) and Stone University (stoneuniversity.org).