Controlling Viscosity in High-Solids Paints

January 1997

Traditional, solvent-based paints are usually blended with a solids content (by volume) from 20%-30% solids content. High-solids paints are generally blended in the 40% plus range for solids content. A good rule of thumb (which paint technicians use) is that one gallon of 100% solids (100% is used for simplicity) will coat 1600 square feet at one thousandth of an inch thick (.001 [.001 inch = 1 mil.]), and at 100% transfer efficiency--100% also used for simplicity. The application of this rule of thumb results in the following (with the noted assumptions):


Low-solids paint (by volume) - 20%	High-solids paint (by volume) 40%

Transfer efficiency - 25%	Transfer efficiency - 25%

Dry paint film thickness - 2 mils.	Dry paint film thickness - 2 mils.

Calculation of 1 gallon coverage rate:

Low-solids paint: (20%)

          20% solids	   25% transfer	         1 mil.
1600 ft x------------	x --------------	x--------------	=	40 square feet/gallon
	  100% solids	   100% transfer	 2 mils.

High-solids paint: (40%)

	 40% solids	          25% transfer	         1 mil.
1600 ft x------------------x ---------------------	x------------	=	80 square feet/gallon
	 100% solids	          100% transfer	         2 mils.

Thus, a 40%-solids paint will coat twice the surface area as will a 20%-solids paint. This, generally, means that a 40%-solids paint will coat twice the number of parts as will a 20%-solids paint.


Generally, 40%-solids paints are, approximately, one-third higher in price, per gallon, than their 20%-solids counterparts. However, they coat twice the surface. Thus, on an applied cost/basis, the high-solids coating yields (Y) an overall cost savings of, approximately, one-third.

20%-solids paint:

$1.00 Y/gallon x 1 gallon used = $1.00 Y

40%-solids paint:

[$1.00 Y + 1/3 ($1.00 Y/gallon)] x gallon used* = $0.66Y

$1.00 Y - $0.66Y = $0.33Y = 1/3 savings

*40%-solids use is approximately one-half that of 20% solids coating.

VOC Reductions

VOC use is reduced because

Thus, the VOC overall reduction potential which may be realized by switching to a 40%-solids paint from a 20%-solids paint is approximately 62%.

20%-solids paint:

1 lb. used x 80% volatility = .8 lbs. emitted

40%-solids paint:

lb. used* x 60% volatility = .3 lbs. emitted

*40%-solids use is approximately one-half of that of 20% solids coating.

VOC reduction:

.8 - .3 = .5 lbs. reduction
.5 .8 = 62% reduction in VOCs

Processing Issues


Typically, higher-solids paints are thicker (more viscous) than lower-solids coatings. Therefore, higher-solids paints may not atomize as readily during the spray process as lower-solids coatings. Many companies add thinning solvents to the high-solids paints so that they will spray more easily. Unfortunately, this defeats the purpose (to reduce VOCs) of using high-solids in the first place.

Spray gun tip size:

Because a 40%-solids coating applies twice the solids (coating) per unit time as a 20%-solids coating, less wet mil thickness must be applied per unit time. This is, typically, achieved by substituting a smaller .013 fluid tip size (for example) for a .019 inch tip. The smaller spray tip size restricts the orifice of the spray gun so that less material flows out of the gun. If a 2 mil-thick dry coating is desired, using a 20%-solid coating, the sprayed, wet film must be 10 mils wet (8 mils of the coating flashes off as volatile organic compounds); the 40%-solids coating can be sprayed at 5 mils wet to achieve the 2 mils of dry coating (3 mils of the coating flashes off as volatile organic compounds). Painting a substrate to a 10 mil wet film thickness can be difficult to achieve without incurring sags. Often, thick coats are accomplished by applying the material in two, separate steps.

Thinning solvent alternatives:

An alternative to reducing the viscosity by adding solvent is to add heat.

Several companies offer heated paint lines which heat the paint in the paint hose as it moves toward the spray gun tip. There are two types:

The closed-loop recycle category is the most satisfactory with regard to performance. This type heats the paint material in a continuously-looped cycle. Thus, when the spray gun is not operating, a valve switches the flow from the gun back through the heated paint line system. This reversal provides the paint with a constant (steady state) temperature characteristic. Single-pass paint line heat systems cannot reach steady state conditions (therefore, the viscosity of the paint varies). However, single-pass systems are lighter and, thus, more maneuverable during the spraying process.

Two-part (catalyzed) paint systems can present challenging, processing issues for line heater systems. High heat reduces the pot life (spray working time) of two-part paints, and one can risk ruining the spray equipment if the mixed, two-part paint "pre-kicks" (prematurely cures) in the paint line. This issue greatly discourages the use of heat as an alternative to solvent-based thinners in sprayed, two-part paint systems. However, there are high-tech valve switching systems which prevent "pre-kick." New mix-at-the-head spray paint systems allow the bulk part "A" of a two-part paint system to be heated in a closed-loop recycle process without mixing with the part "B" catalyst. Computer-controlled valves open and shut to allow precise mixing of "A" and "B" internally in the gun. The mixed material exits the spray gun immediately after it mixes within the gun. The heat conveyed to part "A" reduces the paint system's viscosity so that it can be properly sprayed without thinning solvents. The computer-controlled valves insure proper mix ratios of parts "A" and "B" and also prevent any of the mixed paint from being recycled. This mix at the head scenario has become quite reliable; however, early attempts at such systems failed, and their failure still unjustly haunts the new reliable versions.

The Institute has found that while these mix-at-the-head and closed-loop recycle systems have become quite reliable and provide dramatic, VOC reduction potential, only a handful of companies adopt them. Most companies are leery of the technology because of past history and/or receive little (or negative) support from their paint suppliers regarding the new, closed-loop, heating system. When a paint company provides input and assistance to the design of a customized, on-line system which meets their specifications, the technology nearly always succeeds.