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Solvents for High-Solids Coatings in the 21st Century
By Jimmy G. Basset, Eastman Chemical
Modern Paint and Coatings Issue Sept. 1998

The 21st Century is a popular topic with everything from cars to coatings being touted as meeting consumer needs for the new century. But will the 21st Century be that much different? Will society, on Jan. 1, 2000, be thrust into a new age with vast changes and super wonders?

The ‘60s

Before tackling that question, go back 30 years to see how the 21st Century was viewed then. In 1968 the space program was in full swing, plans were made to put a man on the moon within the next year. Calculators had replaced the slide rule, and computers the size of a house could store up to 512 kilobytes of memory using vacuum tubes. The starting salary for a paint chemist was $8,000 a year, a dozen eggs cost $.53, a loaf of bread $.22, and a hot dog $.25. The first electronic watch, the Accutron, was introduced at a price of $200, a satellite navigation unit could be installed to large ships for plotting their location in the ocean, which was accurate within two miles.

The University of Southern Mississippi’s Polymer Science School was just a dream. The 21st Century was depicted by movies such as "2001, A Space Odyssey." Science fiction writers projected such far-fetched things as laser surgery, small wireless telephones, global satellite communications, television cameras as small as a deck of cards, computer-guided airplanes, and computers that could fit inside a briefcase. They even went so far as to suggest that we would be able to transplant a human heart.

Terms like bits, bytes, compact disk, camcorder, electrostatic spray, hazardous air pollutants (HAPs), volatile organic compounds (VOCs), powder coatings and frequent flyer were not even dreamed of in 1968.
As for the coatings industry, prior to this time Ford and Chrysler used crosslinking enamels on their automobiles while GM used acrylic lacquers. The furniture industry used nitrocellulose lacquers, while the appliance industry used oil-based alkyds. House paints were still mostly oil- based, while maintenance paints were often epoxies.

A typical lacquer would be sprayed at 15-17 percent solids while enamels would be applied at 35-40 percent solids. A solvent system for a lacquer would contain 25 percent esters/ketones/glycol ethers; 15 percent alcohol; and 60 percent aromatic hydrocarbon (to lower the cost).

Solvent systems for enamels were often 90 percent hydrocarbon and 10 percent alcohol with a typical formulation being 90 percent xylene and 10 percent butyl alcohol. If one were to calculate the VOC of an automotive topcoat, it would run between 5-8 pounds/gallon.

Air pollution regulations were taking shape on the West Coast with Rule 66 limiting solvents such as xylene, toluene and methyl isobutyl ketone (MIBK).
The first Clean Air Act came in 1970, and among other things, it provided a definition for clean air. By 1972 Rule 66 in some form or another had swept all of the United States. Coatings formulators were busy reducing solvents restricted by the new rule.

A formula had been derived to show how solvents reacted to form photochemical smog or ground level ozone.

The ‘70s

The By the end of the ‘70s, VOC limits had replaced Rule 66 in most coating areas. Now all solvents were classed as VOCs and restricted, except for the chlorinated solvents. Thus, the formulator could meet VOC limits with active oxygenated solvents and then dilute with the less active chlorinated solvents.

That Practice is still relevant today as many formulators search for a replacement for 1,1,1,trichloroethane, one that is not listed as a VOC. In the ‘70s, VOCs on that automotive top coat had now dropped to 2.8 pounds of solvent per gallon of coating.

By the mid-80s, water-based coatings had grown significantly in certain markets. Latex house paints had replaced oil-based paints and the architectural market now composed 50 percent of the coatings market.

By the end of the ‘80s, we had learned about holes in the ozone layer, and a conference was held in Montreal which led to limiting ozone-depleting products. Among these were solvents like 1, 1,1,trichloroethane.

Also, new terms, such as RCRA, MACT and SARA, were introduced.

The ‘90s ushered in a new Clean Air Act that expanded VOC regulations and introduced the air toxics list, better known as the HAPs list. The Industry Toxic Project, or what be came known as the 33/50 Program of Solvent Reduction, was also introduced.

However, all regulatory changes were not so demanding for our industry. In 1995, for example, acetone was removed from the SARA list and de-listed as a VOC. Things were looking up for the high-solids formulator.

In 1997, petitions were filed to remove methyl ethyl ketone (MEK), MIBK, methanol and ethylene glycol butyl ether (EB) from the HAPs list. Methyl acetate was delisted as a VOC in June 1998. Other solvents like t-butyl acetate have also been requested to be removed from the VOC list.

When considering technology growth from 1987-1997 for US industrial coatings, it is evident that water-based coatings grew significantly in the early years, until finding their niche markets (Table 1). In 1996, demand began tapering off as the result of acetone’s de-listing.

Changes also occurred within individual coating markets, starting with the beer and beverage container market (Table 2).

Much of the beer and beverage containers market is water- based. One reason for this is that the coating is a short-life coating. For example, the beer industry tells us that a container of beer is consumed within three months from the time it is canned. Thus, the coating needs to last just a little longer than three months.

The sheet, strip and coil coatings market is another good example of a niche market that has undergone changes
(Table 3).

When we buy a refrigerator we expect the coating to las the life of the refrigerator-10, 20 or 30 years. Solvent-based coatings are used for the long-life product.

Therefore, by looking at various markets, we see that some manufacturers are staying with solvents and others are switching to other technologies. In looking at the selection of solvents for high-solid coatings for the 21st Century, four areas regulations, air dry coatings, baked coatings and electrostatic coatings—need to be studied.

Regulations

Regulatory challenges abound and for solvents, while several are included on the HAPs list, it is hoped that MEK, MIBK, methanol and glycol ether EB will be removed by the 21st Century. Currently, no new solvents have been added to the list

Another regulatory challenge expected to be achieved by the 21st Century is the elimination of most of the ozone-depleting solvents from coating formulations.

VOC guidelines will continue to tighten so that the formulations of the 21st Century will use only strong active solvents along with VOC- exempt solvents. Acetone and methyl acetate have already been delisted and there is a good probability that the VOC- exempt list will be expanded to include t-butyl acetate. There is a trend to standardize some of the regulations. For instance, solvents for the consumer products area are not VOCs if they have a vapor pressure below 0.1 mm Hg. (At present, a product such as glycol ether DP solvent is a VOC for a coating, but not a VOC for a floor polish.)

By the 21st Century we most likely will be calculating VOCs by the pound/pound method rather than pound/gallon This is already taking place in the wood furniture sector. A look at the two calculations shows why this change will assist the high-solid formulators.

MIR List

Since individual solvents are not equal in their potential to form ozone, look for a push to have levels or tiers set up on the solvents Measured Index of Reactivity (MIR) list. Thus, a formulator may be allowed to add more of a solvent that is lower on the list.

Air-Dry Systems

Many of these systems are non-crosslinking lacquers, thus, it is difficult to increase the solids above 35-40 percent. However, with exempt solvents like acetone and methyl acetate, and the pound/pound VOC calculation, the formulator could still achieve low VOC coatings. Examples of active oxygenated solvents that could be blended with acetone or methyl acetate to achieve a desired evaporation rate are shown in Tables 5-7.

Remember that the last evaporating solvent in any system must be an active solvent for the resin user

Baking Systems

Don’t expect a great deal of change in baking systems. Coatings companies have for years had four basic choices:

1. Incinerate the solvents
2. Capture the solvents
3. Change technologies
4. Reformulate

Incineration is the best choice because it allows the formulator to continue with conventional solvent systems. However, cost and regulations sometimes prevent this option. Most formulators have already reformulated to lower VOCs. In the past this often meant not only changing the solvent system, but the resin system as well. Many formulators have formulated out the HAP solvents as well. Some of the systems currently in use that are formulated without HAP solvents are worth mentioning. Most of the baking systems tend to fall into one of three major areas: polyester/melamine, alkyd/melamine and acrylic/melamine. Without HAP solvents, these systems are each formulated in much the same manner. The first solvent in the system is usually n-butyl alcohol for stability.Tlie mos; often used working solvent tends to be a ketone, such as methyl n-amyl ketone (MAK. Since both the alco- hol and ihe ketone have low electrical resistance, an ester such as ethyl 3-epoxypropionate (EEP) is added to adjust the electrical resistance. If the coating system will allow, an aliph~ itic could be used instead 0 an ester.

Electrostatic Coatings
Electrestatic coatings will continue to grow in the 21st Century This will be due to an increase in this coating’s transfer efficiency. Such an increase would positively affect cost—as transfer efficiency goes up, cost goes down.

In fact, in the 21st Century, perhaps it would not be surprising to see an aerosol can of paint with a power pack arid an e electrostatic spray head. A system like this would reduce over-spray and con-serve paint.

In electrostatic paints, if the electrostatic charge is not correct, there are a number of things that can be done to alter tho charge. The most common and easiest is to adjust or change the solvent system.

Ketone’s, for example, have a very low electrostatic charge, while esters have a high charge. So replacing solvent EEP with MAK would lower the electrical resistance of a system, or the reverse would raise the electrical resistance.

The type of equipment will also have a bearing on your solvent selection. A conventional electrostatic spray system may use a fast-evaporating solvent system based on methyl n-propyl ketone (MPK). On the other hand, a spining bell electrostatic system that greatly atomizes the coating will need a slow-evaporating highly active solvent such as MAK and/or C-11 ketone to give it solvency yet allow flow and leveling.

Summary
The 21st Century is almost here and there will continue to be advances in all types of technologies. Solvents will still play a vital role in coatings far into the 21st Century. While we have seen a reduction from 13 billion pounds of solvent to 10 billion in the last 30 years, the solvents that the formulator uses today are vital working solvents. No longer are non-working diluent solvents added just to lower costs.

Solvent-based coatings provide many performance and environmental advantages over water-based and powder. Until the time when something new shows a great improvement, solvent-based coatings will remain in use.

This article appeared in Modern Paint and Coatings Issue Sept. 1998

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