Chemistry Reference
In-Depth Information
Photo-initiator
Reactive species
Cross-linked polymer
UV radiation
Multifunctional
monomer
Figure 1.11 A typical process for the formation of cross-linked networks through UV
curing.
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1.3.1 Photo-oxidation of Vegetable Oils for Direct
Cross-linking
Drying oils are vegetable oils that are composed of mixtures of triglycerides.
The high rate of unsaturation of these vegetable oils makes them sensitive to
auto-oxidation under air. Drying oils are wildly applied as binders and film
formers in paint and coating formulations because they can form polymer
networks by auto-oxidation, peroxide formation and subsequent radical
polymerization.
8,62-64
Linseed oil is the most successful example of a drying oil, and the superior
performance of linseed oil compared to other vegetable oils is mainly due to
its faster drying.
65
Linseed oil is extensively used as a medium for paintings
and elaborate linoleum, owing to its capacity to form a continuous thin layer
easily, with good optical and mechanical properties within a reasonable
time.
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The cross-linking mechanism (Figure 1.12)
9,62,69
was investigated in de-
tail, and the formation of the lipidic network was attributed to the successive
formation of radical species, isomerization, hydroperoxidation and cross-
linking. The oxidation process, accelerated by UV irradiation using metal-
based catalysts has also been studied.
63,66-69
1.3.2 Photo-cured Polymer Networks based on Acrylated
Vegetable Oils
Acrylated resins are the most widely used photo-curing systems, because of
their high reactivity and the variety of available monomers and telechelic
oligomers. A typical photo-curing formulation contains three basic com-
ponents: (1) a photo-initiator which can generate free radicals by photolysis;
(2) the acrylated functionalized oligomer which constitutes the backbone of
the polymer network; and (3) the acrylated monomer which acts as a reactive
diluent.
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As shown in Figure 1.13, the photo-initiator plays a key role in the
polymerization, and it governs both the rate of polymerization and the cure
depth. The final degree of the polymerization and the physical and chemical
properties of the photo-curing polymers are determined by the chemical
structure and functionality of both the monomer and the oligomer.
Wuzella et al.
48
have studied the kinetic properties of acrylated epoxidized
linseed oil monomers by UV-curing. They found that the photo-initiator
