High-solids coatings for general indus- trial spray finishing are typically
defined as coatings with a volume solids content greater than 60%. Many of today’s
high-solids products are 80% or higher, thanks to concerted efforts by coating
chemists and suppliers to the industry.
These coatings have evolved steadily with the advent of the Clean Air Act of
the 1970s. Since that time, terms like VOC, TSCA and SARA have become common
watchwords for coating users and formulators. The 1990 amendments to the Clean
Air Act regulated many more chemicals, creating an increased urgency to develop
coatings with a lower ozone depleting solvent content. New terms such as HAPs
and Title V became buzzwords.
Before one can appreciate the technology of high-solids spray coatings, a look
at conventional coatings is in order. These conventional or “low-solids”
products have the advantage of being very fluid at room temperature.
Thermoplastic coatings are made using high-molecular-weight non-functional polymers
dissolved in organic solvents. They are easily sprayed and dry relatively fast
as the organic carrier solvent evaporates. They wet the substrate rapidly and
flow into a smooth, defect-free finish that decorates and/or protects the surface
to which it is applied.
Thermoset coatings typically require baking the finish to convert them into
a crosslinked material of a high molecular weight with associated high performance
properties. The polymers used are functional and reactive unlike their thermoplastic
counterparts.
High-Solids Coatings
High-solids coatings are one of the principal coating types used today to comply
with the Clean Air Act.
A key challenge in high-solids technology is to formulate coatings with reduced
or no solvent levels that can still be applied using conventional equipment.
This would insure that the cost of equipment modifications is less of an issue
for the applicator. The use of existing spray guns, pretreatment systems and
line configurations would be the “ideal” and most economical approach.
The resultant coating should look as good as its low-solids counterpart and
perform as well...or better!
With a high-solids coating there is less solvent for thinning purposes. To reach
spray viscosities, the use of lower molecular weight polymers, lower viscosity
crosslinkers and more efficient solvents are required. The oxygenated solvents,
such as ketones and glycol ethers, have replaced many of the traditional aromatic
and aliphatic solvents. The ketone solvents in particular have a more favorable
viscosity reduction profile for some of the newer functional low-molecular-weight
polymers and crosslinkers. However, a few of these very efficient solvents,
besides being labeled VOCs, are now labeled as hazardous air pollutants (HAPs).
The formulating chemist continues to prevail by developing alternate ways to
achieve high solids with environmentally acceptable solvents. The EPA has excluded
acetone, methyl siloxanes and parachlorobenzotrifluoride from the definition
of a VOC. Other solvents are being assessed for exemption. This action has opened
new avenues to formulating very high solids with no or very low VOC and HAP
contribution.
The greatest durability and performance in high-solids coatings are achieved
by baking the finish in the same manner as conventional solids thermoset coatings.
Since high-solids coatings start out with lower-molecular weight polymers, it
takes different approaches to rapidly build molecular weight and end up with
the required performance. The required performance and cure method dictates
the solids content achievable. Thermoset coatings with minimal chemical resistance
can reach 100% solids while air dry systems have difficulty reaching 60% solids.
Polymer Advancements
The area of greatest advancement for the high-solids formulator has been in
polymer technology. New polymers are constantly being developed to reduce molecular
weight and/or viscosity while improving upon physical property performance.
Various functional groups are being added onto the backbone of high-solids polymers
to enhance characteristics such as reactivity, adhesion, wetting abilities,
flow properties and gloss stability. These engineered polymers have been essential
to high-solids coating performance since the polymer must act as a solvent as
well as a binder.
Although lower in molecular weight, the new generation of high-solids polymers
react very fast and efficiently to develop performance that rivals the best
conventional products.
A great deal of synthesis work continues by the major polymer suppliers. Various
chemistries are available as high-solids, such as polyesters, alkyds, epoxies,
urethanes, silicones and acrylics. The benefits normally associated with these
chemical types are readily found in high-solids products so that coating formulators
can meet traditional performance expectations.
How have the polymer chemists been able to meet the rigorous demands from an
industry needing higher solids materials? One way is in the creation of new
reactive diluents. Reactive diluents are very-low-molecular-weight polymers
that can be highly reactive. These diluents are used to thin high-solids coatings.
Since they react completely into the coating matrix, they do not evaporate as
a solvent would. However, a material with too low of a molecular weight must
be avoided to be sure no volatilization takes place at the required curing temperature;
otherwise, the reactive diluent may become part of the VOCs.
Another major advancement in polymer technology is the dramatic improvement
of reactor process control. The outcome of this process enhancement has been
purer polymers with lower viscosities. This is due to the elimination of high
molecular weight tails that add to viscosity but not to the performance of the
base polymer.
Crosslinker Advancements
In conventional low-solids polymers, molecular weights are typically around
20,000, whereas in high-solids polymers it can range from 500-2,000. At these
low-molecular weights, an increase in the amount of crosslinker is needed in
the coating to build up the molecular weight. It is the high molecular weight
of polymeric materials that develops the hardness, chemical resistance and general
durability of a coating. However, with improved process control and new functional
high-solids polymers, the amount of crosslinker can potentially remain at lower
levels. Keep in mind that advancements in high-solids crosslinker chemistry
have also been developed!
Because high-solids coatings must perform under many cure conditions and perform
many functions, various crosslinker technologies have been developed to be compatible
with the new polymers and reduce viscosity. As an example, several melamine
crosslinkers used in baked finishes are now available at 98% solids. The combination
of a high-solids polyester with a high-solids melamine crosslinker is one of
the widely used coatings for applications such as appliances, office furniture,
shelving, lighting fixtures, recreational equipment and other similar products.
Other crosslinker/polymer systems have been developed to cure at room temperature
or with UV/EB energy for heat sensitive substrates such as wood, plastics and
paper products.
Both single- and two-component high-solids products are available to meet a
variety of application needs. In all of these products, the high-solids crosslinker
plays a critical role. They must be neither so reactive as to prevent premature
crosslinking nor so sluggish as to allow sagging during cure. The evolution
of modern crosslinkers has taken these critical performance factors into account.
Catalysts have also been improved to produce better storage stability and moderate
the cure profile of high-solids systems. Advancement of catalyst technology
will be a key to another leap in low-temperature cure performance of high-solids
coatings.
The high-solids coating chemist must judiciously select the proper blend of
polymer, crosslinker, catalyst, solvent, pigment, surfactant and other additives
to deliver the desired characteristics. No one material can stand alone to resolve
all of the complex issues pertaining to high-solids products.
Application Methods
High-solids coatings usually atomize easily due to their low- molecular-weight
constituents. However, the use of anti-sag additives, flow control agents, flatting
agents and a host of other coating additives may require that high-solids coatings
be applied from high-efficiency spray equipment.
High-speed rotary atomizers, such as turbo bells or turbo discs, are very effective
for high-solids coatings. By using electrostatics with these atomizers, improved
transfer efficiency is possible. This will reduce waste of the high-solids coatings.
In fact, many high-solids coatings will wrap better than conventional solids
coatings. This is in part due to the use of oxygenated solvents, which are typically
more polar in nature than their aliphatic or aromatic counterparts used in low-solids
coatings. Here again, the coating chemist can formulate the proper resistivity
into the high-solids coating by selecting the appropriate solvent, catalyst,
polymer and additive package. Varying the amount of electrostatic charge to
complement the inherent electrical properties of the coating is another factor
to be managed properly.
Hot-spray techniques have been used for many years to lower the viscosity of
various coatings. High-solids coatings also work well with this technique due
to the rapid viscosity reduction that heat has on many high-solids products.
Caution must be used to insure that paint heaters do not cause some very reactive
high-solids systems to prematurely react in the line or paint reservoir. Hot
spray offers the advantage of using heat instead of solvents to reduce the viscosity
to a level low enough for effective atomization. This opens up the possibilities
for very-high-solids coatings to be spray applied...95% solids is a reality.
Coating Defects
High-solids coatings have their own special problems. Properly managed, however,
they can become minor issues for the applicator. Several of the areas that coating
chemists have to deal with include sagging, streaking, uniformity, solvent popping,
tackiness, telegraphing, orange peel and edge coverage.
Some of these application issues have to do with the fact that high-solids coatings
don’t contain as much or the same type of solvents that low-solids products
contain. Solvents are excellent wetting agents for poorly cleaned surfaces.
With high-solids products, surface preparation becomes very important since
the amount of solvent used is reduced. Line contaminants such as greases, lubricating
fluids, dust and dirty filters must be isolated so that they do not cause appearance
or adhesion problems on the coated parts. Proper pretreatment and rinsing are
also critical to high-solids to insure uniform appearance and the elimination
of streaking.
Orange peeling is a lack of smoothness or a textured look of the cured coating.
It typically occurs due to the higher or non-uniform surface tensions within
these coatings. In some applications, orange peel is desirable to hide defects.
For other applications, surface active materials (surfactants) can be used in
the formula to improve the appearance due to poor flow and leveling caused by
high surface tension.
Solvent popping usually occurs at the edges where there is a tendency to have
a higher film build than elsewhere on the part. Uniform application of high-solids
coatings is a key factor in the elimination of this phenomenon. Automated spray
equipment goes a long way in eliminating popping because of the precise application
control. The coating chemist may also be able to control this problem by choosing
solvents that evaporate more suitably for the ap-plication and cure conditions
encountered.
Sagging or thermal flow generally occurs during the baking of high-solids coatings.
It is a result of the temperature rapidly reducing the viscosity of the coating
before it has had a chance to begin crosslinking and reach a gel point. Thicker
films aggravate this condition, pointing to the need for controlled and uniform
application. Sagging can be effectively eliminated by use of special anti-sag
agents developed specifically for high-solids coatings. Faster reacting crosslinkers
will also help to solve this problem since they can cause the coating to build
viscosity faster during the curing process.
Edge coverage can be made uniform by controlling flow properties, sag resistance
and prevention of excessive wrap during electrostatic spraying. High-solids
coatings can be designed to have the proper resistivity by the selection of
solvent, catalyst, resin and additives. Varying the electrostatic charge during
application is another way to control edge coverage.
Future High Solids
What lies ahead for high-solids coatings? These coatings will continue to be
a major factor for the foreseeable future. Simply stated, high-solids coatings
will eventually be 100% solids and contain no traditional “solvent.”
The technology is here now, but it is not perfected for all end uses. We have
seen continuous movement towards “higher” high-solids technologies
in the past 25 years. Commercial products are routinely being sold now with
solids in the 80-90% range. The ultimate goal of achieving 100% solids with
zero volatile emissions in a liquid non-aqueous coating is a very logical resolution
to the need for environmentally friendly products. It has been achieved using
UV cure techniques and with some two-component air-dry systems. Achieving this
goal with a baking system will eventually be possible through continued technological
advancements.
