Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-04-10
2002-10-08
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06462117
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to coating compositions and methods of preparing them. In particular, the invention relates to coatings and inks that contain solvents having negligible photochemical reactivity and favorable evaporation rates.
BACKGROUND OF THE INVENTION
Organic solvents, many of which are classified as VOCs, are widely used in traditional coating formulations (including inks). Generally, an organic compound is classified as a VOC unless it has been proven to not participate in atmospheric photochemical reactions. Ordinarily, these are compounds that undergo hydrogen atom abstraction by atmospheric hydroxyl radicals much faster than ethane. VOCS are hazardous to human health because they react with free radicals in the air and generate tropospheric ozone.
Pursuant to the Clean Air Act of 1990, the U.S. Environmental Protection Agency (EPA) recently mandated significant reductions in the amount of VOCS that may be used in coatings. Because of the mandate, the industry has an acute need for good coating solvents that have negligible photochemical reactivity and can therefore be exempt from regulation. Unfortunately, most organic solvents that have the desired low photochemical reactivity are either poor coating solvents or are subject to other regulations. For example, some solvents are classified as “ozone depleters” under the Montreal Protocol; others are regulated by the EPA as “hazardous air pollutants” (HAP solvents). Few organic solvents are valuable for coatings and have low photochemical reactivity, yet are not ozone depleters or HAP solvents.
The EPA has exempted some solvents from VOC status based on their negligible photochemical reactivity. Examples include acetone, methylene chloride, volatile methyl siloxanes, perchloroethylene, and p-chlorobenzyltrifluoride (PCBTF). Unfortunately, these solvents have drawbacks. Acetone and methylene chloride evaporate too fast. In addition, acetone is water-miscible, so coatings made with it absorb moisture too rapidly from the air. Acetone also has appreciable atmospheric photochemistry aside from its reaction with hydroxyl radicals because it strongly absorbs visible and near-UV light. Methylene chloride and perchloroethylene have toxicity concerns. The more exotic solvents, such as methyl siloxanes and PCBTF, are too expensive and are relatively poor coating solvents, as is evidenced by their relatively low Hansen solubility parameters (less than 8.0 (cal/cm
3
)
½
).
Traditional approaches to reducing the VOC content of coatings and inks have focused in developing new resins, crosslinkers, and reactive diluents that do not require as much solvent to formulate. These approaches have succeeded only marginally in spite of their considerable expense. For example, water-borne coatings, which developed as an alternative to solvent-borne systems, sometimes contain even more VOCs than comparable high-solids formulations, and often give inferior performance. As another example, powder coatings may use little or no VOCs, but they require expensive equipment to apply and cure.
In sum, the coating and ink industry needs solvents that have low toxicity, are inexpensive, are not regulated as HAP solvents or ozone depleters, do not evaporate too rapidly, are good coating solvents, and most important, have negligible photochemical reactivity and could be exempt from VOC regulations.
SUMMARY OF THE INVENTION
The invention is a method of preparing a coating composition. The method comprises using as a coating component an organic solvent having negligible photochemical reactivity. Such solvents have an oxidation rate constant of less than about 1×10
10
cm
3
/g·sec. This is a calculated or measured rate of hydrogen-atom abstraction from a compound by atmospheric hydroxyl radicals. In addition, the organic solvent has an evaporation rate of not more than about 5 times that of n-butyl acetate.
We surprisingly found that, among thousands of possible organic compounds potentially useful as coating and ink solvents, very few meet the needs of the industry, particularly when current VOC restrictions and relative evaporation rates are taken into account. By including a solvent having an oxidation rate constant of less than about 1×10
10
cm
3
/g·sec, and also having an evaporation rate of not more than about 5 times that of n-butyl acetate, formulators can significantly reduce the proportion of photochemically reactive solvent used and still make good coatings.
The invention includes thermoplastic and thermoset coating compositions which comprise an organic solvent having negligible photochemical reactivity. These compositions include a resin or a crosslinker or both. The invention benefits coatings used for wood, furniture, automotive OEM, automotive refinish, container, architectural, coil, aerosol, marine, transportation, industrial maintenance, general industrial, inks, overprint varnishes, and road-coating applications.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The invention is a method of preparing coating compositions, including inks. The method comprises using as a coating component an organic solvent having negligible photochemical reactivity, and also having an evaporation rate of not more than about 5 times that of n-butyl acetate.
By solvents having “negligible photochemical reactivity,” we mean solvents that will undergo free-radical abstraction of a hydrogen atom of the solvent by atmospheric hydroxyl radicals at a measured or calculated rate less than or about equal to the corresponding rate of abstraction of hydrogen atoms from ethane. On a weight basis, this corresponds to an oxidation rate constant of less than about 1×10
10
cm
3
/g·sec. Solvents having such low photochemical reactivities are potentially excludable as VOCs under EPA regulations. Solvents preferably used in the method of the invention have oxidation rate constants less than about 5×10
9
cm
3
/g·sec; most preferred are solvents having oxidation rate constants less than about 3×10
9
cm
3
/g·sec.
Oxidation rate constants of various coating solvents appear in Table 1 (below). As shown in the table, many traditional coating solvents such as toluene, xylenes, methyl isobutyl ketone, and n-butyl acetate have relatively high oxidation rate constants and are classified by the EPA as VOCS. In contrast, the method of the invention uses solvents having negligible photochemical reactivity such as those listed in the top half of the table (e.g., tert-butyl acetate, dimethyl carbonate).
In addition to negligible photochemical reactivity, organic solvents useful in the method of the invention are limited to those having useful evaporation rates. Some otherwise useful (and currently VOC-exempt) organic solvents have limited value in coatings because they evaporate too rapidly. Examples are acetone and methylene chloride, which evaporate (respectively) roughly 6 and 14 times faster than n-butyl acetate. Organic solvents useful in the invention have evaporation rates of not more than about 5 times that of n-butyl acetate, which is the industry standard for comparison. More preferred organic solvents have evaporation rates of not more than about 3 times that of n-butyl acetate.
Traditional coating solvents classified as VOCs can be included in the method of the invention. These solvents may be critical for maintaining satisfactory coating processability or performance. Preferably, however, at least about 5 wt. % of the total solvent content of coatings made by the method of the invention comprises one or more organic solvents that have negligible photochemical reactivity and also have an evaporation rate of not more than about 5 times that of n-butyl acetate. More preferably, the organic solvent(s) having negligible photochemical reactivity is the major solvent component, i.e., at least about 50 wt. % of the solvent used is one or more solvents that have negligible photochemical reactivity. The solvent having negligible photochemical reactivity may be the only solvent component.
Organic solvents useful in the
Guo Shao-Hua
Pourreau Daniel B.
Arco Chemical Technology L.P.
Brooks & Kushman P.C.
Michl Paul R.
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