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Proceedings of the 15th Brazilian Polymer Conference (15 CBPOL), October 27-31, 2019, Bento Gonçalves, RS, Brazil
STUDY ON VULCANIZED RUBBER DEGRADATION AFTER EXPOSURE
TO ULTRAVIOLET IRRADIATION
Mostafa G. Aboelkheir1, Romildo D. Toledo Filho2 and Fernando Gomes de Souza Jr.1*
1 – Instituto de Macromoléculas, Centro de Tecnologia - Cidade Universitária, Av. Horacio Macedo, 2030, Bloco J.
Universidade Federal de Rio de Janeiro, Brazil.
2 – Programa de Engenharia Civil, COPPE, Centro de Tecnologia-Cidade Universitária, Av. Horacio Macedo, 2030,
Bloco I. Universidade Federal de Rio de Janeiro, Brazil.
fgsj@ufrj.br
Abstract - The natural degradation process of vulcanized rubber, including tires, is very slow due to the presence of
crosslinks between the rubber chains. Physical recovery processes of rubber wastes are applied with the help of external
energy; thus, the three-dimensional cross-linked rubber network is broken down and then, the resulting rubber can be
used as non-reinforcing filler. This research investigates the efficiency of Ultra-Violet (UV) irradiation on
devulcanizing/degrading vulcanized Styrene Butadiene Rubber (v-SBR) samples. The tests of crosslinking degree,
contact angle with water and FTIR were performed. The results indicate that the UV irradiation has a very strong effect
on the carbon chain configuration of v-SBR where a decrease in the crosslinks and the contact angle with water could be
observed by increasing the exposure time to the UV light.
Keywords: Rubber Residue, Degradation, Devulcanization, SBR, Tire Crumb, UV irradiation.
Resumo - O processo de degradação natural da borracha vulcanizada, incluindo pneus, é muito lento devido à presença
de ligações cruzadas entre as cadeias de borracha. Processos de recuperação física de resíduos de borracha são aplicados
com a ajuda de energia externa; assim, a rede tridimensional de borracha reticulada é quebrada e, em seguida, a borracha
resultante pode ser usada como carga ou agregado em compósitos. Esta pesquisa investiga a eficiência da irradiação de
Ultravioleta (UV) na desvulcanização/degradação da borracha vulcanizada de estireno-butadieno (v-SBR). Os testes de
grau de reticulação, de ângulo de contato com água e de FTIR foram realizados durante todo o processo de tratamento.
Os resultados indicam que a irradiação UV tem um efeito muito forte na configuração da cadeia de carbono da v-SBR,
onde uma diminuição no grau de reticulação e no ângulo de contato pode ser observada com o aumento do tempo de
exposição à luz de UV.
Palavras-chave: Resíduo de Borracha, Degradação, Desvulcanização, SBR, pó de borracha de Pneu, Irradiação UV.
Introduction
The natural degradation process of vulcanized rubber, including tires, is very slow due to the
presence of crosslinks between the rubber chains, moreover, the presence of stabilizers and other
additives. Besides wasting enormous and valuable sources of rubber with no recycling [1–3]. Physical
recovery processes of rubber wastes are applied with the help of external energy, thus, the three-
dimensional cross-linked rubber network is broken down in the presence of different energy sources
turning the network into lower molecular weight fragments so that they can be easily used as non-
reinforcing filler [2].
Elastomers undergo significant modifications by exposing them to heat, light and oxygen
(ozone), and depending on the microstructure of the diene elastomer, oxidative degradation will either
cause crosslinking (hardening) as in natural rubber (NR) and polyisoprene (IR), chain scission
(softening) as in polybutadiene (BR), isobutylene isoprene (IIR), and acrylonitrile butadiene (NBR)
Proceedings of the 15th Brazilian Polymer Conference (15 CBPOL), October 27-31, 2019, Bento Gonçalves, RS, Brazil
or both as in polychloroprene (CR) and styrene butadiene (SBR) [4,5]. Hardening is more likely to
occur because free radicals tend to arise when rubber is exposed to heat, oxygen and/or light rapidly
and form new crosslinks, while rubbers with bulky adjacent groups follow softening mechanism
because radical recombination reactions are less likely to occur due to steric hindrance, thus, softening
caused by disproportionation and hydrogen abstraction is more favorable [5–9].
In a comparison between the effect of Corona discharge and ultraviolet (UV) irradiation on
the surface modifications of styrene–butadiene–styrene rubber, it was found that those treatments
modified the S6 rubber surface by creating C–O, C=O and COO− moieties that improved wettability
[10–12].
This research investigates the efficiency of UV irradiation on degrading vulcanized SBR
samples. The recovered polymeric solid residue can be reutilized by its insertion as filler or aggregate
in composites since the rubber material offers good mechanical properties, allowing new applications
[13–17].
Experimental
Materials: The Styrene Butadiene Rubber, vulcanized at our laboratory (v-SBR), was used in this
work as an indicative reference for the tire crumb due to the tire complexity as it is made of different
rubber types, besides SBR which is heavily applied in tires fabrication [18]. Ultra Violet lamp from
Bravoluz - Lâmpadas Especiais, GERMICIDA 36W - GPH846T5VH/HO/4PSE - 80W - 185nm -
UV-BRAVO was used as the UV irradiation source.
Methods:
1. Exposure to UV irradiation:
A quantity of 10 g of vulcanized styrene-butadiene rubber in powder form (v-SBR) was placed
into Ultra-Violet irradiation (UV) chamber, made at our laboratory (see Fig. 1), for 1, 3, 6 and 12
hours of exposure to UV irradiation. The samples were located 7 cm distance far from the
irradiation source. After each exposure period, the samples were characterized. The rubber
particles were classified in different sizes by using different sieves of several openings and the
average particle size of the rubber samples was between 150 – 300 µm.
2. Characterization:
2.1.Cross-link Degree: Approximately 20 mg of v-SBR were submitted in a tightly closed filter paper
to a leaching system for 48 h at the boiling temperature of toluene according to the proceedings
and calculations reported by Aboelkheir et al. [19].
2.2. Fourier-transform Infrared Spectroscopy (FTIR): An amount of 1 mg of v-SBR was mixed with
300 mg of potassium bromide (KBR) to form a pastille for the test. The samples were tested by
Varian FTIR Excalibur Series spectrophotometer model 3100 (Japan) at room temperature,
applying a resolution of 4 cm-1, 64 accumulated scans and a range from 4000 to 600 cm-1 in the
transmittance mode.
2.3.Contact Angle with water: To study the change in the affinity with water and the enhancement in
wettability properties, the contact angle was measured via Data Physics contact angle goniometer
model SCA20. Measurements were carried out with water on v-SBR samples at room
temperature. The powder SBR was compacted to form a uniform horizontal layer of the
microparticles to simulate a solid rigid surface of the same rubber. The needle of the syringe was
replaced by a thinner one to produce adequate water droplet not to disturb the compacted rubber
surface. Each sample was tested at least three times to check the accuracy of the result.
Results and Discussion
1. Degree of Cross-links
Fig. 1 shows the cross-link density change as a function of time, where a descending rate of residual
cross-linking was observed by increasing the exposure time to UV irradiation. The v-SBR rubber
Proceedings of the 15th Brazilian Polymer Conference (15 CBPOL), October 27-31, 2019, Bento Gonçalves, RS, Brazil
kept 83.24, 62.65, 39.86 and 33.32 % of cross-linking after 1, 3, 6 and 12h of exposure to UV
irradiation, respectively, while the control sample registered 86.04 % of cross-linking. By applying
the linear regression function (see Fig. 1), the values of the angular coefficient (a) = - 14.88 (%/h) of
the studied v-SBR samples, while the corresponding correlation coefficient R2 = 0.9475. The results
indicate a very strong effect of UV irradiation on the carbon chain configuration of v-SBR where the
irradiation caused a scission of the existing C-S bonds. Thus, the more the exposure time is, the more
will be the devulcanization of the rubber. These findings enhance the possibility of the recovered
rubber (devulcanized rubber) be reutilized by its insertion as filler or additive to polymeric or ceramic
matrices allowing new applications [17,20–25].
Figure 1 – Linear regression modeling of crosslinking degree of v-SBR rubber as a function of exposure
time to UV irradiation.
2. FTIR:
Fig. 2 shows the FTIR spectra of the v-SBR UV-modified samples. The characteristic band 1,
assigned as the O-H stretching, appeared increasingly for all the UV-treated samples, while this band
was absent in the v-SBR sample with no exposure to UV irradiation (ABOELKHEIR et al., 2019).
The characteristic band 4, assigned as the C=O conjugated with C=C stretching, the characteristic
band 8, assigned as the S=O and SO2 conjugated stretching, have been increased by increasing the
exposure time to UV irradiation. Those bands appear as a result of a chain scission free radical
terminating mechanism, where the sulfur tends to form sulfur compounds with oxygen, meanwhile,
the vulcanization degree tends to decrease as shown in Fig. 1. It was observed also a decrease in the
intensities of the doublet band 2 and 3, related to the stretching of C-H bond in CH2 and CH3 groups
of the main rubber chains. This decrease calls the attention to a degradative effect caused by the UV
irradiation on the polymeric main chains, which may limit the overall mechanic properties of the
recovered rubber. Though, an optimal use of the UV irradiation should be limited to a specific
exposure time between 1h and 3h.
Proceedings of the 15th Brazilian Polymer Conference (15 CBPOL), October 27-31, 2019, Bento Gonçalves, RS, Brazil
Figure 2 – FTIR absorption bands of v-SBR after the exposure to UV irradiation.
3. Contact Angle
The contact angle has been decreased drastically by increasing the UV treatment time indicating an
increment of water affinity and an enhancement in wettability properties for all the rubber samples
which may facilitate later its insertion in polar matrices to guarantee a good dispersion. UV irradiation
caused a declination in the contact angles values of v-SBR samples after 1, 3, 6 and 12h by forming
polar functional groups (OH-, S=O, C=O) increasingly within the rubber structure, where the angles
registered 83.70°, 52.45°, 34.75°, 6.0°, respectively, while the control sample registered 118° (see
Fig. 3). By applying the linear regression modeling, the corresponding correlation coefficient (R2)
registered 0.9911 for v-SBR treated samples.
Figure 3 – Linear regression modeling of contact angle variations of v-SBR rubber as a function of
exposure time to UV irradiation.
Conclusions
Decreasing the cross-linking degree indicates a scission of the cross-linked chains network that also
indicates a devulcanization/desulfurization and/or a degradative effect of UV irradiation on the
vulcanized rubber. The FTIR spectra reveals the scission of some chemical bonds and the formation
of new ones during the exposure to UV irradiation. From the findings, one can conclude that UV
irradiation shows a clear devulcanization effect on v-SBR rubber samples by degrading and
Proceedings of the 15th Brazilian Polymer Conference (15 CBPOL), October 27-31, 2019, Bento Gonçalves, RS, Brazil
decomposing C-C and C-S chemical bonds in the rubber chain system. The appearance of new polar
functional groups, like O-H- and S=O, enhances the wettability properties. Future investigations
should focus on finding out an optimal exposure time of the UV irradiation between 1h and 3h to
keep good mechanic properties of the devulcanized rubber. The results are promising and highlighting
UV as a strong recovery tool of vulcanized rubber residues.
Acknowledgements
This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES - -Finance Code
001), Financiadora de Estudos e Projetos (FINEP PRESAL Ref.1889/10) and Fundação Carlos
Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).
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