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How Paint Dries
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 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 un
pigmented 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.
Linear Mode
