Decorative Concrete Resource Center

Understanding Technical Data in Coatings

by Ron Fitzkee

In order to use coatings properly, it is imperative to understand each product’s technical data both when selecting a coating and when using it.  The following is a description of the most important product details and what effect they have on performance.

Solids:

There are two ways to report solids or solid content of a coating.  One is by weight.  This method is primarily a measurement used for establishing VOC level, and for manufacturing products.  It is most commonly the units of measure in the product “recipe”.

Solids by volume is a more practical measurement for the installer.  It reflects how much of the material will remain on the floor after it is dried or cured.  Simply illustrated, it will tell you how much of the wet film will remain as dry film after cure.  Use this data to establish how thick to install the product in order to achieve the dry film thickness you intend.   A 100% solids coating will cure to the same thickness as it is applied.  No mass (thickness) is lost to evaporation.  A 50% solids material will lose 50% of its mass during cure.  So for a 50% solids coating, the final result (dry film) will be half as thick as when you apply it (wet film).

The loss of thickness is due to evaporation of solvents, which can be either hydrocarbons (VOC’s) or water.

So to calculate how many wet mils to apply, divide the desired dry mil thickness by the % solids.  For instance, a typical water based epoxy has a solid content of 41% by volume.  So if you want 4 mils of dry film, divide 4 mils by .41 (41%), and the wet mil thickness you want to install is 9.7 mils.  (Since a mil is so thin, 1/1000^th”, figure you want to install about 10 mils wet.  Coverage rate calculations will be addressed below.)

Volatile Organic Compounds (VOC):

VOC’s report how much hydrocarbon (solvent) material is released into the atmosphere through evaporation.  It is reported as a value of weight per a selected volume of product.  Some manufacturers report this in pounds per gallon (lbs./gal.).  But it is most commonly reported in metric values of grams per liter (g/L).

There is a national limit for VOC emission for floor coatings (ie.: 400 g/L), and there are also state (250 g/L) and regional limitation statutes (100 g/L).

Recommended Film Thickness:

Manufacturers’ film thickness is a value you must follow.  It is less important for 100% solids materials, but very important when using a lower solids product.  The reason for this is that the film will usually skin over at the surface before the lower mass of the coating layer.  If this happens before the solvents have adequately evaporated, the solvent (whether that is water or hydrocarbons) will be trapped under the skinned over surface (solvent entrapment) and the film will not cure throughout its entire depth.  That solvent will be trapped under the surface and cause the film to be soft.  This can lead to deformation and failure (peeling).

Recommended Film Thickness:

Proper film thickness of an applied coating is a mathematical function of the desired dry film thickness divided by the % solids.  It takes into account how thickly the coating can be applied without experiencing solvent entrapment.  It is imperative to follow the range of film thickness to apply as recommended by the manufacturer. 

Coverage Rate:

Coverage rate is how many square feet is to be coated per one gallon of material.  It is a secondary check to assure you are applying it within the proper film thickness range.  Once you know how thick to apply the coating (how many wet mils), take the number of mils a gallon of product contains when covering one square foot.  A gallon of liquid poured onto a square foot builds up to 1604 mils.  To calculate how many mils to apply, divide 1600 (1604 if you prefer) by the coverage rate in square feet per gallon (say 100 square feet per gallon), 1600 divided by 100 sf/gal is 16 mils.  Conversely, if you know how many wet mils you are going to apply and want to figure out the coverage, divide the 1600 by the wet mil thickness (1600 divided by 16 mils is 100 sf/gal).

Mix Ratio:

Most high performance coatings are two component products.  That means there are two packages that need to be mixed.  Improper mixing will lead to the coating either not curing to provide the intended physical properties or not curing adequately.  Most manufacturers state the mix ratio as how many parts of resin to how many parts of hardener.  An common example is 2 parts resin to 1 part hardener (2:1).  However, sometimes it may be reported in terms of weight (8.55 lbs. of resin to 1.75 lbs. of hardener).  The second of these is more difficult to measure out on site.  If this is the case, ask the manufacturer to relate this in a more practical volume measurement of what the ratio is, or how many volume ounces of resin to mix with how many volume ounces of hardener.  It is imperative to make all mixes exactly as the manufacturer describes.

Gloss:

Gloss is a function of how uniformly light reflects off the coating surface.  If the surface is truly smooth (on a microscopic level), light waves will reflect off the surface in a uniform manner.  That way, reflections are seen uniformly and will look shiny.  If the microscopic reflection is not uniform, the surface will appear less shiny or glossy.  Gloss is measured by use of an Erichsen glossmeter.  The meter throws a light beam onto the coating at a 60° angle and reads how much light is collected coming off the floor at the same angle.  It is typically reported in a range of 40 to 90 or so.  The higher the number, the higher the reflection or gloss is.

Abrasion Resistance:

Coatings are tested as to how well they resist abrasion.  Abrasion is an indicator of how much scratching will occur at the surface of the coating.  Abrasion or scratching is how the shine is lost or the coating wears through.  It is measured by an ASTM procedure using a Taber Abrasor.  The higher value in this test reports that the coating is more “scratchable”.  A value of 16 (mg of mass lost in the test) is a very good value.  A value of 70 or higher is not so good.

Impact Resistance:

Impact resistance is a function on whether a film (coating) will deform enough, when a weight is dropped on it, to cause the coating to disbond from the concrete.  Typically, it is reported as a pass or fail result when impacted by a 50 or 150 lb. weight.

Adhesion:

A well mixed and installed polymer coating will generally adhere to a properly prepared concrete substrate.  A successfully installed coating will stick to concrete stronger than concrete sticks to itself.  Testing is done using an elcometer, which measures how much force is required to pull the coating off of the concrete.  Usually, the concrete fails in this test before the coating pulls off the concrete.  That is why, often, the data will report “concrete failure, no delamination”.  That means the concrete separated from itself but the coating still has concrete stuck to it.  The report lists how much force (psi) was required to cause the failure.  As a generality, the failure will occur when the force applied approaches 10% of the compressive strength of the concrete.

Viscosity:

In non-technical terms, viscosity is how “thick” a fluid is.  It is a measurement of how rapidly the liquid flows through a standard opening.  It can also be measured with a viscometer.  In practical terms, it represents how well it is expected to flow out and level on the concrete surface.

Others:

To avoid problems or delays, follow any published cure schedule with attention to temperature and humidity levels.

Contributed by Ron Fitzkee.  Ron is associated with Koester American moisture mitigation coatings through the R.L. Bryant Associates, and Lythic Solutions Colloidal Silica Concrete Densifiers through FloorAmerica.  He can be contacted through Decorative Concrete Resource Center.