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Characterisation of crumb rubbermodifier using solid-state nuclearmagnetic resonance spectroscopyMária Kovaľakováa, Oľga Fričováa, Viktor Hronskýa, Dušan Olčáka,

Ján Mandulab & Brigita Salaiováb

a Department of Physics, Faculty of Electrical Engineering andInformatics, Technical University of Košice, Park Komenského 2,042 00 Košice, Slovakiab Department of Geotechnics and Traffic Engineering, Instituteof Structural Engineering, Faculty of Civil Engineering, TechnicalUniversity of Košice, Vysokoškolská 4, 042 00 Košice, SlovakiaPublished online: 25 Sep 2013.

To cite this article: Mária Kovaľaková, Oľga Fričová, Viktor Hronský, Dušan Olčák, Ján Mandula& Brigita Salaiová (2013) Characterisation of crumb rubber modifier using solid-state nuclearmagnetic resonance spectroscopy, Road Materials and Pavement Design, 14:4, 946-958, DOI:10.1080/14680629.2013.837835

To link to this article: http://dx.doi.org/10.1080/14680629.2013.837835

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Road Materials and Pavement Design, 2013Vol. 14, No. 4, 946–958, http://dx.doi.org/10.1080/14680629.2013.837835

Characterisation of crumb rubber modifier using solid-state nuclearmagnetic resonance spectroscopy

Mária Kovalakováa∗, Olga Fricováa, Viktor Hronskýa, Dušan Olcáka, Ján Mandulab

and Brigita Salaiováb

aDepartment of Physics, Faculty of Electrical Engineering and Informatics, Technical University of Košice,Park Komenského 2, 042 00 Košice, Slovakia; bDepartment of Geotechnics and Traffic Engineering, Instituteof Structural Engineering, Faculty of Civil Engineering, Technical University of Košice, Vysokoškolská 4,042 00 Košice, Slovakia

Crumb rubber (CR) can be used for binder modification or aggregate replacement in asphaltmixtures for road construction, when it is usually referred to as CR modifier (CRM). CRMelastic properties depend on the quality of parent material and storage conditions of finalmaterial. They deteriorate in time due to degradation processes and among other things theycan influence CRM swelling capacity during asphalt rubber preparation. The presented paperdemonstrates the sensitivity of basic nuclear magnetic resonance (NMR) parameters (protonspin-lattice relaxation time T1, proton spin–spin relaxation time T2 and the line broadeningin 13C NMR and 1H NMR spectra) for detecting changes in CRM degradation. These NMRparameters are proposed to be used to assess CRM degradation before application in asphaltmixtures. In laboratory conditions, CRMs with different degrees of degradation were preparedby thermal ageing at 85◦C for 0–36 days.

Keywords: crumb rubber modifier; waste tyre rubber degradation; 1H NMR; 13C NMR

1. IntroductionThe large number of waste tyres which are collected each year is a challenge for waste managementsystems in most countries all over the world, due to the fact that waste polymeric materials do notdecompose easily and their disposal is a serious environmental problem. Two major approachesto solving this issue are the recycling and reuse of waste tyre rubber and the reclaiming of rubberraw materials (Adhikari, De, & Maiti, 2000). One of the most environmentally friendly ways ofprocessing of this waste material is mechanical recycling, which consists in shredding, grindingand removing steel cords and textile parts from waste tyres, yielding a final product known ascrumb rubber (CR). This material can then be re-used, e.g. in production of various rubber mats,as material for playground and sports field surfaces, and also as an additive to asphalt mixtures forroad construction (Ching & Wing-gun, 2007; Chiu, 2008; Kök & Çolak, 2011; Roberts, Kandhal,Brown, & Dunning, 1989).

In general, two processes for introducing CR into hot-mix asphalts have been developed –dry and wet processes. In the dry process, CR modifier (CRM) replaces part of the aggregatesand is added directly to the hot-mix asphalt mixture. In the wet process, CRM is well blendedwith bitumen and then added to the asphalt mixture (Ching & Wing-gun, 2007). Polymers havebeen reported as improving some properties of the asphalt binder (Kanitpong & Bahia, 2005),although in the case of CRM more tests still have to be done to find out which physical and

∗Corresponding author. Email: maria.kovalakova@tuke.sk

© 2013 Taylor & Francis

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Road Materials and Pavement Design 947

chemical properties of CRM are decisive in the preparation of asphalt binder with propertiesrequired by standards (Chiu, 2008). Generally, the CRM in hot-mix asphalt has been reported asimproving the resistance to rutting, and produces pavements with better durability by minimisingthe distresses caused in hot-mix asphalt pavement (Mashaan, Ali, Karim, & Abdelaziz, 2012).Asphalt road pavements prepared using the dry process exhibit unstable performance possiblydue to poor control in the gradation of the aggregate and CR and lack of understanding of theswelling process of CRM particles which takes place during preparation of hot-mix asphalt (Ching& Wing-gun, 2007). The properties of CRM can therefore be decisive for the quality of final asphaltroad pavements prepared using either the dry or the wet process, and the influence of CRM onthe properties of asphalt mixtures have been the subject of several studies (Cao, 2007; Ching& Wing-gun, 2007; Dong, Huang, Li, & Zhang, 2012, Kök & Çolak, 2011; Liu, Cao, Fang, &Shang, 2009; Moreno, Rubio, & Martinez-Echevarria, 2012; Xiang, Cheng, & Que, 2009).

The aim of the study presented in this article was to show the potential of using solid-statenuclear magnetic resonance (NMR) for fast characterisation of CRM, i.e. to identify the chemicalcompounds in CR which give signals in 13C NMR spectra and to obtain information on the degreeof degradation, from the values of proton spin-lattice relaxation time T1(

1H), which characterisesthe local mobility of rubber chains, and proton spin–spin relaxation time T2(

1H), which providesinformation on the restriction of segmental motion and hence cross-link density in the rubbernetwork (Alexandre, Feio, Marvão, & Figueiredo, 2004; Dutta, Roy Choudhury, Haidar, Vidal, &Donnet, 1994; Garbarczyk, Kuhn, Klinowski, & Jurga, 2002; Kuhn, Barth, Denner, & Miller,1996; Litvinov & van Duin, 2002; Pazur & Walker, 2011; Somers, Bastow, Burgar, Forsyth, &Hill, 2000; Stapf & Kariyo, 2005; Whittaker, 2006, Zhao, Zhao, Weina, Kuhn, & Jian, 2007).The sensitivity of the NMR technique for detecting changes in CRM degradation and hencecross-link density was demonstrated on a set of samples in which the degradation was acceleratedby thermal ageing. Both factors, the composition (the ratio of natural and synthetic rubber) andthe degree of CRM degradation (cross-link density) of starting material, can influence amongother things the swelling processes during hot-mix asphalt preparation (Cao & Zhang, 2011), andconsequently the quality of the resulting hot-mix asphalt mixtures. Our preliminary results on thecharacterisation of degradation of ground rubber from waste tyres using NMR techniques werereported by Kovalaková et al. (2011).

2. Experimental2.1. NMR measurementsThe NMR experiments were performed on a Varian 400 NMR spectrometer (Palo Alto, CA,USA). The studied sample was placed in a 4 mm rotor with volume of 50 μl.

Standard single-pulse excitation 13C NMR spectra were obtained using a 90◦ pulse with aduration of 1.9 μs, high-power proton decoupling of 92.6 kHz, recycle delay of 6 s, magic-anglespinning (MAS) rate of 6 kHz, and averaging over 4000–6000 scans.

1H MAS NMR spectra of sufficient resolution were obtained using a 90◦ pulse duration of4.75 μs, recycle delay of 4 s and averaging over 20 scans. An inversion recovery pulse sequenceand MAS rate of 6 kHz were used for T1(

1H) measurements in the temperature range 22–100◦C(the results are discussed in Section 3.2.2). T2(

1H) was obtained from the NMR experiments usingMAS rate of 10 kHz and 90-τ -180 sequence in which spin-echo was detected after delay time τ .The spectra measured for τ from the range of 0.1–25 ms were deconvoluted using MestReNovasoftware, and the areas under the peak given by methylene protons were plotted versus the delaytime td = 2τ (the results are discussed in Section 3.2.1).The curve fitting was performed with aSciDavis programme using the scaled Levenberg–Marquardt algorithm.

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948 M. Kovalaková et al.

2.2. MaterialThe samples of CRM were supplied by V.O.D.S, Kechnec, Slovakia. The material had undergonestandard recycling procedure. Prior to NMR measurements, the traces of metallic parts in thesamples were removed using a permanent magnet. The measurements were carried out on theas-supplied sample (CR-0) with grain size of 0–2 mm, and on the samples CR-6, CR-12, CR-16,CR-20, CR-28 and CR-36, which were thermally aged at 85◦C for 6, 12, 16, 20, 28 and 36 days,respectively, in an oven with natural convection. Thermal ageing at this temperature was chosendue to the fact that above 80◦C the oxidation rate of tyre rubber decreases, which is consistentwith the diffusion-limited oxidation responsible for tyre rubber degradation (Bauer, Baldwin, &Ellwood, 2007), and hence such thermal ageing can simulate the degradation of CRM, e.g. duringstorage of this material in plastic bags without access to oxygen.

3. Results and discussion3.1. 13C NMR experimentsTyre rubber is a complex elastomeric material consisting of natural and synthetic elastomers,carbon black, zinc oxide, stearic acid, processing oils and other curatives (Zhu, Zhang, Liang, &Lu, 2011). The dominant components of the tyre rubber are cis-1,4polyisoprene chains of naturalrubber (NR), cis-1,4styrenebutadiene rubber (SBR), and poly-butadiene rubber (BR) (Williams& Besler, 1995) (Figure 1).

The average composition of tyre rubber varies with the type of tyre and there are also slightdifferences between tyres of the same type supplied by different manufacturers (Roberts et al.,1989; Williams & Besler, 1995). Vulcanisation of rubber involves the formation of chemicalcross-links between linear chains of rubber which restrict the motion of the chains, so the materialbecomes fully elastic (Zhao et al., 2007). During its service life the structure of rubber changes,and depending on the type of rubber this process involves chain scission and/or increased cross-linking, which results in a weaker brittle polymer (Somers et al., 2000). Waste tyre rubber ishighly cross-linked, solid-like material.

The cross-polarisation technique is frequently used to detect 13C NMR spectra for solid-statepolymers to shorten recycle delay in experiments. However, extensive reorientational motion ofmolecules in elastomers reduces dipolar interactions and shortens 13C relaxation times (Buzaré,

Figure 1. The chemical structure of main components of the tyre rubber: (a) cis-1,4 polyisoprene, (b)styrene-BR, (c) cis-1,4 and trans-1,4 polybutadiene and (d) 1,2 polybutadiene.

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Road Materials and Pavement Design 949

Silly, Emery, Boccaccio, & Rouault, 2001), which makes it possible to obtain quantitative infor-mation on their structure from the spectra recorded in single pulse excitation 13C MAS NMRexperiments with high power proton decoupling during data acquisition.

Tyre rubber gives signals in two regions of 13C MAS NMR spectra – 150–100 ppm and50–0 ppm. The spectra recorded for as-supplied crumb rubber (CR-0) can be seen from Figure 2.The spectra were deconvoluted, and based on the knowledge from the literature (Buzaré et al.,2001; Mori & Koenig, 1998; Nielsen, Bildsoe, & Jakobsen, 1992), the peaks were assigned to thecarbons of the rubber network. The peaks with chemical shift of 30.6 and 15 ppm can be assignedto methylene and methyl groups of the fatty acid ester group (Buzaré et al., 2001). The peaks withchemical shifts of 38 and 44 ppm which are present in all spectra and are of very low intensitycan be assigned to the carbons involved in polysulfidic cross-links (Mori & Koenig, 1998). Thedetection of carbon–carbon cross-links and monosulfidic cross-links in this kind of experiment isnot possible due to the fact that these carbons can have long spin-lattice relaxation times (Buzaréet al., 2001).

Figure 2. 13C MAS NMR spectrum of as-supplied CR in the region 35–10 ppm (top) and 150–110 ppm(bottom).

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950 M. Kovalaková et al.

The spectra were also recorded for thermally aged samples (Figure 3) and deconvoluted. Thepeak areas resulting from deconvolutions are listed in Table 1. The line broadening of methylene,methyl and methine lines due to thermal ageing can be seen from Figure 4. The most sensitive tothe duration of thermal ageing is the line width of methylene carbons (27.3 ppm).

Since the number of −CH2 carbons in the monomer units of cis-1,4 polyisoprene and 1,4polybutadiene chains is the same, the portion B/(B + I ) of BR in CR can be calculated from thearea A of the peaks with chemical shifts of 33.1, 28.3 and 27.3 ppm using the following formula:

BB + I

= A(33.1) + A(28.3) − A(27.3)

A(33.1) + A(28.3).

Figure 3. 13C MAS NMR spectra of as-supplied and thermally treated samples of CR.

Table 1. The values of peak areas obtained from deconvolutions of the spectra in Figure 3.

Cα Cβ Cγ (I ) Cδ Cε Cβ-cis Cα-cis Cα-trans C2−6 B/(B + I )Carbon (I )a (I ) Cβ-trans(B)b (I ) (I ) (B) −CH2− (B) (B) (S)c (%)

Shift 135.4 125.9 33.1 27.3 24.2 28.3 30.6 130.2 130.8 128.6(ppm)CR-0 19 21 29 20 21 8 7 14 4 7 46CR-6 21 21 31 20 22 8 7 10 5 7 49CR-12 19 20 31 20 21 7 8 9 5 6 47CR-16 19 20 29 20 21 8 8 13 4 8 46CR-20 19 19 31 19 20 8 8 11 6 8 51CR-28 18 20 29 19 21 8 8 13 4 6 49CR-36 17 16 28 19 20 6 10 12 4 6 44

aI -isoprene.bB-butadiene.cS-styrene.

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Road Materials and Pavement Design 951

Figure 4. The line widths of the lines at 24.2 ppm (Cε(I )), 27.3 ppm (Cδ(I )) and 135.4 ppm (Cα(I )) in 13CMAS NMR spectra of as-supplied and thermally aged samples. The dashed line is a guide for eyes.

In spite of the fact that waste tyre rubber can be expected to be rather inhomogeneous material,since its composition varies depending on the tyre grade and manufacturer (Roberts et al., 1989;Williams & Besler, 1995), the results listed in Table 1 show that the percentage of BR in therubber network varies in a narrow range 44–51%.

Thermal ageing of the samples resulted in slightly broadened lines and successive decreasein the total area of the spectra with the duration of thermal treatment. This can be explainedby the increasing number of cross-links and more extensive interaction of the rubber networkwith additives, e.g. carbon black, in the thermally treated samples. Consequently, the number ofcarbons in the rigid parts of the rubber network with long spin-lattice relaxation times increasesand hence these carbons cannot be detected in this NMR experiment. The increase in cross-linkscan also be deduced from slightly lower intensities of Cα(I ) and Cβ(I ) carbons for CR-28 andCR-36 samples, since cross-linking could involve the loss of double bonds, as has been observedfor cross-linking of polybutadiene due to gamma irradiation (Whittaker, 2006).

3.2. 1H NMR experiments3.2.1. Measurements of proton spin–spin relaxation time T2

The proton spin–spin relaxation time T2 is mainly determined by the mobility of polymer chainsas a whole, and is thus strongly affected by cross-linking (Litvinov & van Duin, 2002). Proton T2values have been successfully correlated with macroscopic viscoelastic properties of vulcanisedindustrial NR elastomers with different cross-link densities, and it has been proved that parameter1/T2 is directly related to the network structure, that it depends on the cross-link density and canbe used to quantify the cross-link density in network systems (Alexandre et al., 2004). Thermaltreatment of CR is expected to accelerate cross-linking processes and in this way to deteriorateits elastic properties, which can result in changes of the proton spin–spin relaxation time T2.

The networks of carbon black-filled elastomers have been reported as consisting of three kindsof regions – a tightly bound region, i.e. a highly immobilised rubber layer around active filler, anouter annular of loosely bound rubber and free materials with properties of unfilled rubber (Duttaet al., 1994). In the case of CR which is produced from highly cross-linked waste tyre rubber, onecan expect the presence of only two regions in the rubber network – tightly and loosely boundrubber. Since the reactions between the rubber network and fillers and cross-linking additives arepart of the ageing processes, it can be expected that the ratio of these regions can change withtime and also after thermal treatment, which can to some extent simulate the ageing process.

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952 M. Kovalaková et al.

The 1H MAS NMR spectra of CR samples display three dominant lines with chemical shifts of1.6, 2.0 and 5.1 ppm (Figure 5), assigned to the methyl, methylene and unsaturated methine protonsof cis-1,4-isoprene units of NR (Kawahara, Chaikumpollert, Sakurai, Yamamoto, & Akabori,2009). The 1H lines of aromatic and methine protons in SBR are visible as a broad line at 7 ppm,and a shoulder at 5.3 ppm respectively. The line of methylene protons has the shift of 2.0 ppmand overlaps with the line of NR methylene protons. The peak with chemical shift of 1.2 ppmcould arise from the methylene protons in 1,2 BR (Dutta et al., 1994) and methylene protons offatty acids. The peak with chemical shift of 0.8 ppm can be assigned to methyl protons in fattyacids (Knothe & Kenar, 2004), as illustrated in Figure 5 with the 1H MAS NMR spectrum ofsample CR-0.

The T2 values were determined for methylene protons (the peak with chemical shift of 2.0 ppm)from the deconvoluted 1H MAS NMR spectra recorded with increasing delay time. Curve fittingrequired the sum of two exponential decay functions:

M (t)M (0)

= A exp(

− tdT2A

)+ B exp

(− td

T2B

).

The plots of the areas under the peaks and fitting functions for clarity only for CR-0 and CR-36samples versus delay time td can be seen from Figure 6. The values of parameters A and B and spin–spin relaxation times T2A and T2B versus the duration of thermal treatment are plotted in Figure 7.The parameters A and B are the network fractions with different mobility. The number of segmentsfar away from cross-links and not interacting with filler is proportional to parameter A, which isrelated to longer spin–spin relaxation time (T2A), i.e. slower decay of transverse magnetisation, inthis part of the network. Faster decay of magnetisation M (t) with spin–spin relaxation time T2Bis expected for segments close to the cross-links; the amplitude of the fastest decay (parameterB multiplied by M (0)) does not give a direct measure of the number of cross-links, but ratherthe number of segments adjacent to the cross-links, and hence it is proportional to the cross-linkdensity (Whittaker, 2006). As can be seen from Figure 7, the values of parameter B successively

Figure 5. 1H MAS NMR spectrum of CR-0 sample recorded at room temperature.

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Road Materials and Pavement Design 953

Figure 6. Plots of normalised transverse magnetisation versus delay time td for all studied samples andfitting functions for sample CR-0 and CR-36.

Figure 7. (a) The dependence of A and B parameters and (b) spin-spin relaxation time T2 on the durationof thermal ageing. The dashed lines are a guide for eyes.

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954 M. Kovalaková et al.

Figure 8. The dependences of the observed line width and the line width calculated using spin–spinrelaxation time on the duration of thermal ageing.

increase with the duration of thermal treatment at the expense of the values of parameter A. Thiscan be explained by the ongoing cross-linking process and interactions between rubber chainsand fillers, which result in the increase in cross-links in thermally treated samples. The T2B valuesdo not change much since they reflect mobility of segments adjacent to the cross-links, whichshould not depend on the cross-link density (Figure 7). The T2A values slightly decrease, whichreflects restricted mobility of the segments far away from cross-links, proportional to parameterA, due to higher cross-link density in the whole sample. The cross-link density, as can be inferredfrom the values of parameter B, increases linearly with the duration of thermal ageing.

The average spin–spin relaxation time T2, which can be calculated using the formula:

1T2

= AT2A

+ BT2B

can be used for the calculation of the “true” line width value given by the expression 1/πT2,which is related to the observed line width ν1/2 by the equation:

ν1/2 = 1πT2

+ γB0

π

(Becker, 2000). The 1/πT2 values are plotted together with the observed line width values of themethylene protons peaks versus the time of thermal treatment in Figure 8.

As expected the observed line width values are greater due to imperfection in the homogeneityof the magnetic field. They increase with duration of thermal treatment and qualitatively theygive the same information as 1/πT2, i.e. they indicate increase in cross-link density in the rub-ber network, which results in more severe restrictions of chain motion and shorter spin–spinrelaxation time.

3.2.2. Measurements of proton spin-lattice relaxation time T1

The spin-lattice relaxation time T1 is associated with short-range motions in the polymer chain(Andreis, Veksli, Ranogajec, & Hedvig, 1989) and therefore it is largely determined by local chainmobility and only slightly affected by the degree of cross-linking usually obtained for elastomers(Litvinov & van Duin, 2002). The dominant mechanism that governs the spin-lattice relaxationprocess of the protons attached to the chain is the dipolar interaction between nuclear spins locatedwithin one given segment (Tajouri & Kassab, 2008).

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Road Materials and Pavement Design 955

The relaxation rate 1/T1 can be described by the general expression

1T1

= K[

τc

1 + ω20τ

2c

+ 4τc

1 + 4ω20τ

2c

], (1)

where parameter K = Cγ 4�

2(μ0/4π)2 includes C, which depends on the specifics of the motion,interproton distance (r), and the number of protons being relaxed (via spin diffusion) by the protonsundergoing the motion, whereby ω0 is the Larmor frequency and τc is the correlation time for themotion (Ratcliffe, 2009). If we assume that two different stable conformations of the polymericchain are separated by a potential energy barrier of height Ea, then the temperature dependenceof the correlation time can be expressed using the Arrhenius theory as

τc = τ0 exp(

Ea

RT

),

where pre-exponential factor τ0 is the limiting value for correlation time when temperatureincreases in infinity. The temperature dependence of the spin-lattice relaxation time reachesminimum when the condition ω0τc = 0.616 is fulfilled, where τc is the average (local) cor-relation (segment reorientation) time. Then, the parameter K for our experimental conditions(ω0 = 2π × 400 MHz) can be expressed as

K = 17.75 × 108

T1 min. (2)

The experimental values of relaxation rate 1/T1 were fitted to the function (1) in which the initialK value was calculated by using Equation (2), the order of the activation energy was calculatedfrom the slope of the plot ln T1 versus 1/T in the high temperature region, and pre-exponentialfactor τ0 was obtained by fitting using the scaled Levenberg–Marquardt algorithm in the SciDavisprogramme. The experimental T1 values measured at room temperature and 30◦C were omittedfrom the fitting procedure. The results of fitting are listed in Table 2, and the plots of spin-latticerelaxation time T1 for protons in methylene groups together with fitting functions versus 1/T canbe seen from Figure 9.

In general, the T1 values increase with the duration of thermal ageing, as also observed for nitrilerubber elastomers aged at temperatures above 100◦C (Garbarczyk et al., 2002). The minimum ofT1 values has a slight tendency to shift to higher temperatures (Table 2) with duration of thermalageing due to the increasing degree of cross-linking, which is reflected in the shift of glass transitiontemperature of elastomers (Stapf & Kariyo, 2005). The T1 min values increase from 0.451 s forsample CR-0 to 0.536 s for sample CR-36 (Table 2). If the protons of the methylene groupsare considered as an isolated two-spin system, then the theoretical value of T1 min for methylene

Table 2. The values of K , Ea , τ0, T1 min and TT min parameters obtained from fittingprocedure.

Sample K(108 K−1 s−1) Ea (kJ mol−1) τ0 (10−13 s) T1 min (s) TT1 min (K)

CR-0 39.2 21.6 1.12 0.451 338CR-12 38.2 20.8 1.62 0.465 341CR-16 37.9 18.7 3.30 0.468 340CR-20 36.4 17.9 4.46 0.488 341CR-28 35.4 17.5 5.55 0.502 345CR-36 33.1 16.6 7.08 0.536 342

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956 M. Kovalaková et al.

Figure 9. Temperature dependence of spin-lattice relaxation time of methylene protons in as-supplied andthermally treated CR samples.

protons is 0.31 s. The experimental values of T1 min are longer for all samples under study, whichindicates that averaging the dipole–dipole interactions in thermally aged samples is less effectivewith ongoing rubber network degradation.

The observed changes indicate that thermal ageing makes the structure of thermally treated sam-ples more rigid. The increasing restriction of local chain mobility is also reflected in the limiting(minimum) values of correlation time τ0, which increase from 1.12 × 10−13 s for sample CR-0 to7.08 × 10−13 s for sample CR-36 (Table 2). The apparent activation energy of the conformationaltransition of segments decreases with the ongoing degradation of the rubber network.

4. ConclusionsThe 13C and 1H NMR experiments performed on as-supplied and thermally aged CRM samplesprovide information on the hydrocarbons present in the CRM structure, as well as on the changein cross-link density of the rubber network due to thermally induced degradation.

The following are particularly important for characterisation of CRM samples:

• the 13C MAS NMR spectra, which make it possible to estimate the ratio of natural andsynthetic rubber in the CRM network and identify other hydrocarbons in CRM structure,

• the spin–spin relaxation time T2A and T2B for methylene protons obtained from 1H MASNMR experiments, which decrease with increasing CRM degradation,

• the parameter B associated with the shorter spin–spin relaxation time T2B, which isproportional to the cross-link density of the rubber network and increases with CRMdegradation,

• the observed line widths of the methylene protons peaks in 1H MAS spectra, which increasewith CRM degradation and can be used for fast characterisation of CRM samples withdifferent cross-link densities.

The spin-lattice relaxation times T1 for methylene protons at the minimum of the temperaturedependence of spin-lattice relaxation time also indicate restricted chain mobility in CRM sampleswith increasing degradation, but this parameter is not as sensitive to increase in cross-link densityas parameter B and spin–spin relaxation times.

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Basic parameters obtained from 1H MAS NMR spectra (line widths of methylene proton peaksand proton spin–spin relaxation times T2A and T2B of methylene protons, together with networkfractions A and B associated with these relaxation times) have been shown to detect increasingdegradation of CRM samples which have undergone thermal ageing at 85◦C for 0–36 days. Forthis reason, they could be used for characterisation of CRMs produced by different manufacturersand stored for different periods and in different storage conditions from the point of view of theirdegradation.

AcknowledgementsThe “We support research activities in Slovakia” project is co-financed from EU funds. This paper was devel-oped as part of the project named “Centre of Excellence for Integrated Research & Exploitation of AdvancedMaterials and Technologies in Automotive Electronics,” ITMS 26220120055. This study was supportedby funding from the Project of the State Program of Research and Development No. 2003SP200280203,Slovakia which is acknowledged by two authors (J.M. and B.S.). The paper presents results of the researchactivities of the “Centre for Progressive Constructions and Technologies in Transportation Engineering.” TheCentre was supported by the Slovak Research and Development Agency under contract No. SUSPP-0013-09and the companies Inžinierske stavby and EUROVIA SK.

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