smockers83 wrote:What's the environmental problem with oil-based paints that caused the switch?
Not sure of the specific chemicals, but the target of the legislation that significantly changed the paint compounds were VOCs (volatile organic compounds).
"The two basic types of paint are oil-based and latex, which is water-based. In the past, oil-based paints were used for most exterior surfaces as well as woodwork, trim[6] plus bathrooms and other rooms where moisture was an issue.[7] That is not the case anymore, as concern over VOC emissions has lead to national and state regulations that increasingly restrict the amount of VOCs in paint so that over 85 percent of paint is water-based.[8] Latex paint is now the top choice in most situations.[6] Oil-based paints are also more difficult to apply, take longer to dry and are harder to clean up and dispose of than latex paints.[9]
According to a 2002 US Environmental Protection Agency (EPA) study, oil-based paint produces far more VOCs than latex paint, with 32-42 percent VOC content in oil-based paints vs. 2 to 5 percent VOC content in latex paints.[10] Even so, emissions from latex paint is a major source of indoor air pollution.[11] The EPA states that 9 percent of the airborne pollutants causing ground level ozone come from the VOCs in paint.[12] Some manufacturers produce low or zero-VOC latex paint. One ton of VOCs will be eliminated for every 1,000 gallons of low-VOC paint used.[13"
Source:
http://www.greenyour.com/home/...=1073
Looks like wikipedia has some explanation of the actual chemicals, but I'm not sure if it applies directly to automotive paints. I would suspect at least some of it might:
"Unlike water-based paints, oils do not dry by evaporation. The drying of oils is the result of an oxidative reaction, chemically equivalent to slow, flameless combustion. In this process, a form of autoxidation, oxygen attacks the hydrocarbon chain, touching off a series of addition reactions. As a result, the oil polymerizes, forming long, chain-like molecules. Following the autoxidation stage, the oil polymers cross-link: bonds form between neighboring molecules, resulting in a vast polymer network. Over time, this network may undergo further change. Certain functional groups in the networks become ionized, and the network transitions from a system held together by nonpolar covalent bonds to one governed by the ionic forces between these functional groups and the metal ions present in the pigment.
Vegetable oils consist of glycerol esters of fatty acids, long hydrocarbon chains with a terminal carboxyl group. In oil autoxidation, oxygen attacks a hydrocarbon chain, often at the site of an allylic hydrogen (a hydrogen on a carbon atom adjacent to a double bond). This produces a free radical, a substance with an unpaired electron which makes it highly reactive. A series of addition reactions ensues. Each step produces additional free radicals, which then engage in further polymerization. The process finally terminates when free radicals collide, combining their unpaired electrons to form a new bond. The polymerization stage occurs over a period of days to weeks, and renders the film dry to the touch. However, chemical changes in the paint film continue.
As time passes, the polymer chains begin to cross-link. Adjacent molecules form covalent bonds, forming a molecular network that extends throughout the painting. In this network, known as the stationary phase, molecules are no longer free to slide past each other, or to move apart. The result is a stable film which, while somewhat elastic, does not flow or deform under the pull of gravity.
During the drying process, a number of compounds are produced that do not contribute to the polymer network. These include unstable hydroperoxides (ROOH), the major by-product of the reaction of oxygen with unsaturated fatty acids. The hydroperoxides quickly decompose, forming carbon dioxide and water, as well as a variety of aldehydes, acids, and hydrocarbons. Many of these compounds are volatile, and in an unpigmented oil, they would be quickly lost to the environment. However, in paints, such volatiles may react with lead, zinc, copper or iron compounds in the pigment, and remain in the paint film as coordination complexes or salts. A large number of free fatty acids are also produced during autoxidation, as most of the original ester bonds in the triglycerides undergo hydrolysis. Some portion of the free fatty acids react with metals in the pigment, producing metal carboxylates. Together, the various non-cross-linking substances associated with the polymer network constitute the mobile phases. Unlike the molecules that are part of the network itself, they are capable of moving and diffusing within the film, and can be removed using heat or a solvent. The mobile phase may play a role in plasticizing the paint film, preventing it from becoming too brittle.
One simple technique for monitoring the early stages of the drying process is to measure weight change in an oil film over time. Initially, the film becomes heavier, as it absorbs large amounts of oxygen. Then oxygen uptake ceases, and the weight of the film declines as volatile compounds are lost to the environment.
As the paint film ages, a further transition occurs. Carboxyl groups in the polymers of the stationary phase lose a hydrogen ion, becoming negatively charged, and form complexes with metal cations present in the pigment. The original network, with its nonpolar, covalent bonds is replaced by an ionomeric structure, held together by ionic interactions. At present, the structure of these ionomeric networks is not well understood."Source:
http://en.wikipedia.org/wiki/Oil_paint