Using VOC-Exempt Solvents in Coatings:Performance, Productivity

Chapter 14

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Using VOC-Exempt Solvents in Coatings: Performance, Productivity, and Lower Environmental Impact Daniel B. Pourreau Lyondell Chemical Company, 3801 Westchester Pike, Newtown Square, P A 19093

Introduction For decades, regulatory agencies in most industrialized nations have enacted more and more stringent volatile organic compounds (VOC) regulations to reduce the solvent content in coatings. This effort is motivated, in part, by the desire to reduce emissions of solvents that are precursors to tropospheric ozone or smog. These V O C regulations have also spurred the growth of low-VOC technologies, such as waterborne, powder, high solids, and energy-curable coatings. While these technology advances are welcome and are excellent choices for some coating applications, they too have their limitations and environmental impacts. Despite these environmental regulations and new coating technologies, solvent-borne coatings remain the second largest coating category, after waterborne coatings, for several reasons. First, industrial solvents are inexpensive and are excellent viscosity reducers for coating resins and additives. They are used at every stage of the coating process, from resin production to gun cleanup: Resin synthesis Coating formulation Surface cleaning and degreasing © 2007 American Chemical Society

In New Developments in Coatings Technology; Zarras, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

201

202

•

Thinning Application equipment cleanup

In coating formulations, they perform several important functions, including:


Dispersing pigments • • •

Compatibilizing resins, additives and pigments Reducing viscosity for application Leveling the coating on the surface Evaporating quickly and consistently, regardless of environmental conditions

Solvents, therefore, play an important role in the formulation, storage, application, drying, appearance, performance and durability of coatings and inks. Despite their shortcomings, solvent-borne coatings are still unmatched in their economy, ease of use, versatility, and performance. Therefore, solvent producers have been developing new solvents with lower health and environmental impacts to replace the ones still in use today. Regulators are also beginning to see the benefit of encouraging this type of substitution. For example, the California A i r Resources Board (CARB) has enacted a reactivity-based policy for aerosol coatings . This approach is also being considered by the U.S. Environmental Protection Agency (EPA) as a possible future V O C compliance option instead of mass-based V O C content limits". 1

The Science Behind V O C Regulations Because most solvents evaporate and react in polluted urban atmospheres to produce tropospheric ozone, they are regulated as VOCs . Ozone is a respiratory irritant and is regulated as a criteria pollutant in the United States and Europe. Canada has also enacted strict V O C regulations patterned after U.S. regulations^. Europe also regulates VOCs, but their regulations are based on volatility instead of photochemical reactivity. This approach is less likely to produce results because it ignores the vast difference in ozone-forming 111



203 potentials between solvents and does not encourage substitution of less reactive solvents for more reactive ones. In the United States and Canada, solvents that are shown to produce less ozone than ethane are considered negligibly photochemically reactive and are exempt from V O C regulations. Not all solvents do have the same impact on ozone (see Figure 1). Some, like acetone and tertiary butyl acetate (TBAC), produce very little ozone, whereas others, like toluene and xylene, produce many times their own weight in ozone when emitted. Many halogenated solvents have very low photochemical reactivities and are VOC-exempt. However, they are also relatively persistent in the environment. Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), for example, have very long atmospheric lifetimes and have been shown to deplete stratospheric ozone . Some CFCs and their substitutes are also believed to contribute to global warming . v

vl

When comparing photochemical reactivities to that of ethane, the U.S. E P A has historically used two benchmarks. For VOCs whose reactivity is well below ethane, it is sufficient to consider their kinetic reactivities. The rate constant for hydrogen abstraction from the V O C by atmospheric O H radicals, or k H> expressed in cm /molecule*second is compared to that of ethane: 0

3

kH 0

R-H + .OH

-> Re + H 0

(1)

2

This reaction is the main initiation step for ozone formation for most VOCs. Another initiation pathway is direct photolysis to form a free radical. After the V O C radical is formed, it undergoes a series of decomposition steps, producing by-products that can also interact with ambient N O to form N 0 . Photolytic decomposition of N 0 produces singlet oxygen which then reacts with oxygen to form ozone. The V O C oxidation by-product, R O , can then undergo further reactions to produce more ozone or terminate. 2

2

e

R* + 0 R0 «

+

2

N0

2

+

NO 0

2

2

R0 -

(2)

2

—> RO« + N 0 hu —• N O + 0

2

3


(3) (4)


Figure L Ozone Forming Potential of Common Coating Solvents


2


205 Hence, depending on the V O C decomposition mechanism and environmental conditions, each mole of V O C might form less than one mole of ozone, exactly one mole, or several moles of ozone. To account for this vast difference in ozone forming potential, VOCs whose kinetic reactivities are close to that of ethane are compared based on their "mechanistic" reactivities. Mechanistic reactivities are most often expressed as grams ozone/grams V O C to reflect the nonstoichiometric nature of ozone formation. Several state-of-the-art models are available for estimating mechanistic reactivity, but the most commonly used by the E P A is the S A P R C 99 model, developed by Dr. William Carter . The mechanistic reactivity conditions historically used for regulatory purposes is the one measured by adding a small amount of a V O C to a standard V O C mixture under optimal N O and photolytic conditions to form the maximum amount of ozone, which is called the maximum incremental reactivity (MIR). The MIRs of common coating solvents are compared in Figure 1. vn

x

Comparing VOC-Exempt Solvents Acetone was the first compound exempted from V O C regulations based on MIR data. It is an excellent viscosity reducer for a variety of coating resins, but has several properties that make it less than ideal for formulating coatings. First, its low flash point makes it a severe fire hazard. Second, it has a high evaporation rate and is hygroscopic, which can result in the appearance of defects known as "blushing" and "solvent popping." Blushing is the appearance of a whitish haze in a clearcoat when moisture condenses in the coating due to evaporative cooling. Solvent popping appears as visible pinholes in the coating caused by the solvent escaping after the viscosity has increased to a point where the coating no longer flows (see Figure 2). Finally, acetone is a very strong solvent that can redissolve undercoats, causing the color to "bleed" into clear topcoats or even lift from the substrate. Since acetone was exempted, solvent producers have petitioned the E P A to exempt several other solvents based on MJR. O f those, only volatile methyl siloxanes (VMS), parachlorobenzotrifluoride (PCBTF), methyl acetate, and T B A C have been exempted. V M S and methyl acetate have found limited use in coatings. V M S is relatively expensive and is not a very effective viscosity



206

Figure 2. Solvent Popping in a 2K Urethane Clearcoat Using Surface Magnification (15x) and Coaxial Lighting

reducer for common coating resins. Methyl acetate has properties similar to acetone, but is more expensive. Despite its added cost, acetone's higher electrical resistivity and flash point make it a safer solvent more suitable for electrostatic spray applications. P C B T F is currently used in a variety of compliant coatings, but is likely to be replaced by newly exempted T B A C in applications for which flammability is not an issue. The chief complaints from coating formulators regarding P C B T F are its high cost, unpleasant odor, and high density (11.2 lb/gallon). The high density of P C B T F compounds its high cost because coating solvents are usually purchased by the pound but coatings are sold by the gallon. The properties of these exempt solvents are listed in Table 1. A l l VOC-exempt solvents have low ozone forming potentials (MIRs). Methyl acetate is the least reactive of the four, and acetone is the most reactive. P C B T F is about half as reactive as T B A C on a weight basis but almost the same on a volume basis. Therefore, substituting P C B T F for T B A C is not expected to



207

Table I. Physical Properties of Exempt Solvents for Coatings PCBTF

79-20-9

540-88-5

98-56-6

6.3

6.2

2.8

0.9

56

56

98

139

Acetone

CAS number

67-64-1

Evaporation rate, n-BuAc = 1 Boiling point, °C Flash point, °C

Methyl

TBAC

VOC Exempt Solvent

Acetate

-4

9

42

109

6.55

7.78

7.24

11.2

23.3

25.8

22.4

25

3.0

10%

0.8 to 3.0

80%

Using VOC-Exempt Solvents in Coatings:Performance, Productivity

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