High-solids coatings for general industrial spray finishing are typically defined as coatings with a volume solids content greater than 60 pct. Many of today's high-solids products are 80 pct or higher, thanks to concerted efforts by coatings chemists and raw material suppliers to the industry. These coatings have evolved steadily with the advent of the Clean Air Act of the 1970's. Since that time, terms like VOC, HAP's, TSCA
and SARA have become common watchwords for coatings users and formulators. The 1990 amendments to the Clean Air Act regulated many more chemicals, thus creating an increased urgency to develop coatings with a lower ozone depleting solvent content.
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 polymers dissolved in organic solvents. They are easily sprayed and dry relatively quickly as this 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 was applied. Thermoset coatings require baking the finish to convert it into
a crosslinked material of a high molecular weight with associated high performance properties.
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 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 and more efficient solvents have been 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. However, a few of these very efficient solvents, besides contributing to the VOC, are now labeled as HAP's (Hazardous Air Pollutants.) The formulating chemist continues to prevail by developing alternate ways to achieve high solids with environmentally acceptable solvents. Recently, the EPA has excluded acetone, methyl siloxanes and parachlorobenzotrifluoride from the definition of a VOC. This action has opened new avenues to formulating very high solids with no VOC 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.
Here are some of the technological advancements pertaining to high-solids coatings.
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 coatings' performance since the polymer must act as a solvent as well as the 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 coatings 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 oligomers. Oligomers are very-low-molecular-weight polymers that can be highly reactive. Some oligomers are used as diluents 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 VOC's.
These new oligomers are also used as raw materials in the manufacture of modern functional high-solids polymers. They are one of the new "tools" that polymer chemists have to work with in the pursuit of higher-solids compositions.
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 to 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 newer polymers. As an example, several melamine crosslinkers used in
baked finishes are now available at 98 pct 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 in the marketplace. Other crosslinker/polymer systems have been developed to cure at room temperature 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 crosslinkers play a critical role. They must be neither too reactive to prevent premature crosslinking nor must they be too sluggish and 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.
The high-solids coatings 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. Now let's look at some of the application factors that these high-solids coatings must face.
Application Methods. High-solids coatings usually atomize easily due to their lower-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 higher-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 coatings 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 very 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 pct solids is
a reality.
Coating Defects. High-solids coatings systems are not without their own special problems. Properly managed, however, they can become minor issues for the
applicator. Several of the areas that coatings chemists have to deal with include the following: 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. Here are some of the more common problems and how they might be addressed.
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 uniformity or a textured look of the cured coating. It typically occurs due to the higher or non-uniform surface tensions within these coatings. 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. They work by reducing the surface tension of the coating to enhance uniform flowout during cure.
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 coatings 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; again 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 pct solids and solventless. 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 20 years. Commercial products are routinely being sold now with solids in the 80-90 pct range. The ultimate goal of achieving 100 pct solids or 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.
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