Ce travail a été effectué dans le cadre des recherches mis en place pour répondre à l’axe 1 du projet de doctorat. Il s’avère que les information obtenues n’ont pas permis de répondre à l’objectif de recherche qui était de caractériser les différences et particularités des méthodes d’expositions. Cette partie n’a donc pas été intégrer à la thèse. Cependant, les résultats sont pourrait être d’intérêt à qui qu’onques voulant connaitre le mécanisme de pyrolyse du polymère acrylique p(BA/MMA). Les résultats sont présentés ici en anglais car il avait en premier lieu vocation à être intégré à l’article « Evaluation of the impacts of four weathering methods on two acrylic paints: showcasing distinctions and particularities ».
Py-GC/MS characterisation
In order to determine the nature of the acrylic paint used in the chapter 4, the pure acrylic binding medium was first characterized. A co-emulsion of poly(butyl acrylate/ methyl methacrylate), p(BA/MMA) has been identified. These paints are especially used for outdoor utilisations due to their good weathering resistance. The use of this specific binder with longer carbon chains rather than other copolymers such as p(EA/MMA) or p(MA/MMA) allows a better hydrophobicity.
The protocol developed in this experiment has allowed to describe the copolymer thermal properties. However, no significant information on the particularities of the different methods have could be extract from this experiment. The pyrolysis leads to the generation of several monomers and oligomers as dimers and trimers but also sesquimers due to the thermal decomposition. Learner describes this phenomenon as consequence to the presence of tertiary hydrogen atoms on the polymer acrylic chain (Learner, 2001). If the main chain is not totally linear, those kinds of compounds are prone to make chain abstraction during the depolymerization process. These additional compounds cannot be easily identified due to their "hybrid character" and the lack of standard materials. They all appear after 15 minutes of retention on the pyrogram. The first ten minutes of retention time for the pure acrylic binder are presented Figure 1. The identifications have been completed from literature data, the retention times and the mass spectra. Table 1 summarizes the main mass fragments from the mass spectrum of each contribution.
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Figure 2 : Pyrogram of the ten first minutes of the pure acrylic medium
Table 1 : Peak identification and main ion fragments linked
Peak Retention time
(minutes) (m/z) Compound 1 1.63 39, 41, 55, 69 Methacrylate 2 2.42 39, 41, 55, 56, 57 Butyl 3 2.83 39, 69, 101 Methyl methacrylate 4 6.23 55 Butyl methacrylate 5 8.45 39, 41, 69, 87 Methyl Butylacrylate
Even if methyl butylacrylate (MBA) is not present in the pure acrylic medium, it can be formed as satellite with both MMA and BA units from the thermal decomposition of copolymers (Chiantore et al., 2003). Some of the ion fragments are found several times in the pyrograms. The whole of them can be summarized in 8 differents m/z: 39, 41, 55, 56, 57, 69, 87 and 101. In order to uncover the mechanisms of thermal decomposition of the acrylic medium, a hypothetical representation of the p(BA/MMA) copolymer is presented Figure 2. In absence of further information on the paint copolymer’s nature, it is delicate to know its exact shape at the molecular scale. However, a hypothetical molecule allows to suggest degradation mechanisms. The arrows describe the two main impacts of the thermal decomposition on the chain copolymer (Figure 2). As indicated, cleavages
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take place in two ways: depolymerization on the main chain C-C linkage (Cβ) and cleavage of the side chain. Three mechanisms (a, b and c) have been identified from the literature and the study of the whole mass spectra.
Cleavage of the side chain (mechanism a)
The accumulation of side chain cleavages produces a polyene backbone that by isomerization and cyclisation, generate benzenes. This rearrangement is observed and well known in the case of polyvinyl emulsion during the pyrolysis (Kern et al., 1995; Learner, 2001; Tang et al., 2006). This phenomenon is called gasification. There are plenty of researches on this degradation process due to the interest for plastic recycling. Even if in the present case, the polymer is an acrylic base and the gasification is less known, a similar mechanism could take place. Then, the thermal decomposition of the benzene leads to the generation of two C3H3 compounds (m/z = 39) called propargyls which are resonantly stabilized radical (Tang et al., 2006). These fragments are found in almost every pyrogram of the peaks between 0 and 10 minutes proving the importance of this mechanism. The side group elimination generates molecular ions with m/z = 101 leading to the production of both CO2 and butyl monomer (m/z = 57) identified in the peak 2.
Depolymerization (mechanism b and c)
The depolymerization mechanism leads to the production of either methyl methacrylate (mechanism b) or butyl acrylate (mechanism c). The latter leads to a fragment ion with m/z = 55 due to its loss of a butoxy. The protonation of this fragment gives a new fragment with m/z = 56.
The depolymerization of methyl methacrylate follows the mechanism b in two steps. The first one is the production of a fragment ion with m/z = 69 as a consequence of the loss of a methoxy group. The second step corresponds to a decarbonylation in order to produce a compound with m/z = 41. The ion fragment with a m/z = 78 corresponds basically to the protonation of methacrylate acids (Irwin, 1979; Learner, 2001).
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Figure 2 : Production of the different fragment ions from the copolymer p(BA/MMA). Mechanism a) cleavage of the side chains. Mechanism b) production of ion fragments m/z = 41 and c) production of ion fragments m/z = 55
The analyses of paints 1 and 2 have given similar results than the pure acrylic binder (Figure 3.). Pyrograms were not affected by the presence of pigments, dyes or other additives. Py-GCMS analyses have only allowed the study of the polymer stability rather than the other materials inside. Concerning the paints after degradations, only the oligomer parts of the pyrograms (groups 6, 7 and 8) have slightly changed after the exposures. Because there are no new contributions, phenomena leading to oligomers formation are still the same as well as those describe in Figure 2. Even if a complete characterization of the copolymer thermal properties has been performed, no significant information on the particularities of the different methods and their impact on the paints have could be determined.
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