Lazzari et al. studied the drying and oxidative degradation of linseed oil by
Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), differential
scanning calorimetry (DSC) and size exclusion chromatography (SEC).[45] Linseed oil
was spread on selected supports, obtaining a film thickness of 80 µm, and was then
exposed to different conditions. Different films were naturally aged, thermo-oxidised at
80 °C, or irradiated with wavelengths > 295 nm. Analysing the film dried at 80 °C after 6
hours with FTIR, the same observations were made as reported by van de Voort et al. in a
study concerning the oxidation of edible oils.[46] After an induction time of 4 hours, the
peak due to hydroxyl groups rapidly increased and reached a constant value after 8 hours,
while isolated double bonds disappeared.[45] An insoluble polymer was formed after
extended oxidation times at 80 °C (up to 150 hours). The soluble part of this polymer was
extracted and analysed by SEC and IR. The SEC chromatogram of the soluble part of the
aged sample shows a peak due to the original component, as well as a peak assigned to
dimers and a continuous distribution of higher molecular weight fractions. The IR
spectrum of the soluble (extracted) sample was compared with the IR spectrum of the
sample before extraction. The spectrum of the soluble sample shows signals of lower
intensity for the ester groups relative to the methylene signals in the sample, which would
suggest that a process of fragmentation takes place in which aliphatic chains are
preferentially released from the insoluble network.[45]
Analysis of a sample film kept at room temperature exposed to the atmosphere
(natural aging/drying) by FTIR yields exactly the same spectral changes as seen for
samples treated at 80 °C, albeit over a much longer time period. It was thus supposed that
treatment of linseed oil at moderately higher temperatures only accelerated the natural
drying and degradation processes, not altering the type and extension of the reactions.[45]
Mallégol et al. sought to gain insight into the long-term behaviour of oil-based
paints by studying the thermo- and photo-oxidation of several different drying oils.[47-49]
FTIR was again used as the preferred analytical method, as well as Fourier transform
Raman spectroscopy, to study the structure of linseed oil and poppyseed oil films
oxidised at 60 °C.[47] Samples were spread out as a thin film on a either a KBr or a glass
window, after which they were oxidised in a ventilated oven at 60 °C in the dark. After
oxidation for 30 hours a very weak peak at 885 cm-1 evidenced the formation of trans
epoxides.[47] Oxidised samples were treated with NO after increasing reaction times to
differentiate between alcohol and hydroperoxide formation. NO reacts with alcohols to
form a nitrite ester (R-O-N=O), which has a specific band at 779 cm-1.
[47] Studying the
time course of hydroperoxide formation and alcohol formation, it was concluded that
alcohols are secondary products formed only after hydroperoxides are created. To assess
the amount of carboxylic acid present in the oxidised oils, samples were treated with SF4
to form the acid fluorides. Two new peaks were observed after treatment, i.e. at
1843 cm-1, attributed to non-conjugated acid fluorides, and at 1810 cm-1 attributed to
conjugated acid fluorides. The formation of carboxylic acids in the curing step evidences
chain scission reactions that can weaken the dried film. Carboxylic acid formation was
proposed to occur by oxidation of aldehydes or via hydrogen abstraction from the tertiary
carbon with a hydroperoxide function and subsequent β-scission after hydroperoxide
decomposition, as shown in Fig. 1.5.