PSI - Issue 30

E.S. Petukhova et al. / Procedia Structural Integrity 30 (2020) 105–112 Petukhova . . and Fedorov A.L. / Structural Integrity Procedia 00 (2020) 000–000

109 5

A

C O 

I



carbonyl

1368 A

(1)

A C C 

I



vinyl

1368 A

(2)

-1 ) and vinyl groups (909 cm -1 ), respectively.

where A C=O and A C=C are the peak areas of the carbonyl (1713 cm

These peaks were normalized using the peak at 1368 cm -1 . The vinylidene index was calculated similarly using the peak at 875 cm -1 . Table 1 presents the calculation results.

Table 1. Carbonyl, vinyl and vinylidene indices of 273-83/PE1296 and its composites containing stabilizers after UV-irradiation

273-83/PE1296 composite

Irradiated

Parameters

273-83/PE1296 before irradiation

Stabilizer-free

+ CO3

+ CO4

+ Stafen

Carbonyl index

negligible

7.347 0.083 0.204

8.644 0.084 1.253

7.532 0.097 0.728

14.275

Vinyl index

0.184 0.014

0.076 0.223

Vinylidene index

According to Tofa et al. (2019) and Krehula et al. (2013) change in the number of carbonyl and vinyl groups is an indicator of the degree of PE degradation. One can see that the carbonyl index of the composite containing the stabilizer Stafen is two times higher than the indices of other composites, i.e. the sample stabilized by Stafen degraded the most under the irradiation conditions. We obtained a similar result as in previous paper Petukhova and Fedorov (2019), where the degree of degradation of PE 273-83 containing the same stabilizers was estimated by changing in the elongation at rupture. According to the data in Petukhova and Fedorov (2019), after nine months of natural weathering, the relative elongation of a composite containing Stafen is two times lower than one of the other studied composites. The number of vinyl groups in the samples after UV irradiation is significantly reduced. The obtained result disagrees with the results presented in Abdelhafidi et al. (2015), Hamzah et al. (2018), Tofa et al. (2019), in which they observe an increase in the number of vinyl groups after UV irradiation or natural weathering. The most likely explanation is that there is a variety of PE photooxidation mechanisms, depending on the conditions (Fig. 4). Fig. 4 shows that decomposition of the hydroperoxide (Fig. 4A) can cause the formation of three different functional groups: hydroxyl, ketone, and aldehyde (Fig. 4B). According to the presented scheme, compounds with the ketone group under UV radiation form compounds with the vinyl group (Fig. 4D), as well as carboxylic acids, esters, and lactones (Fig. 4C). However, in the spectra of the studied composites after irradiation, there is a decrease in the number of vinyl groups, i.e. the probability of the formation of vinyl groups produced from ketones by the Norrish type II reaction (Fig. 4D) is negligible for the test conditions. The reason for the decrease in the number of vinyl groups may be the interaction of vinyl groups with alkyl radicals, resulting in vinylenes (Fig. 4E). An increase in the vinylidene index (Table 1) in composites contained stabilizers СО 3 and СО 4 indicates the accumulation of vinylidene bonds after irradiation. The obtained result indirectly indicates that such compositions are less prone to spatial crosslinking because of the interaction between vinylidene groups and alkyl radicals, according to the mechanism shown in Fig. 5, reprinted from Abdelhafidi et al. (2015). These reactions reduce the number of vinylidene groups, and the crosslinking process itself, as is shown in Hashim and Kawaeed (2011), leads to embrittlement of PE and other carbon-chain polymers.