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?
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.
Epoxy
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.
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 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.
Adhesive Colors
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.
General 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.
Tip: There are
several heated dispensers for epoxies that work very 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.
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.
UV exposure
Nothing is completely 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.
Polyester
- 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)
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
- 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)
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