Sulfur Vulcanisation IIR EPDM NR Sulfur Vulcanisation of low- vs. … · 2020. 6. 9. · der DVR bei allen Temperaturen ab. Für EPDM und IIR durchläuft der DVR bei hohen Temperaturen

ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS

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Sulfur Vulcanisation IIR EPDM NR BR crosslink density trapped entang-lements compression set tempera-ture

In this study the sulfur vulcanisation of four different, apolar rubbers, viz. IIR, EPDM, NR and BR, in gum compounds over the same range of sulfur curatives contents is compared. The four rubbers chosen differ in the unsaturation level, the entanglement density and the oc-currence of strain-induced crystallisati-on. Emphasis of this first part of our studies is on the maximum rheometer torque (MH) as a measure for the per-manent network density and the com-pression set (CS) as a function of tem-perature. It was shown that the net-work density of the vulcanised rubber samples is the sum of the chemical crosslink density and the trapped ent-anglement density.

Schwefelvulkanisation von niedrig vs. hochungesättigten Kautschuken (IIR & EPDM vs. NR & BR): Schwefelvulkanisation IIR EPDM NR BR Netzstellendichte topologisch fixierte Verschlaufungen Druckverfor-mungsrest Temperatur

Das Vulkanisationsverhalten von vier unpolaren Kautschuken wird innerhalb eines breiten Schwefelkonzentrations-bereiches verglichen. Untersucht wur-den IIR und EPDM mit einem geringen, und NR und BR mit einem hohen Ge-halt an ungesättigten Bindungen. Die Ergebnisse von Vulkametermessungen (S`max) werden als Maß für die Netz-werkdichte genutzt. Des Weiteren wird die Temperaturabhängigkeit des Druck-verformungsrestes (DVR) betrachtet. Für Kautschuke mit hohem Anteil an ungesättigten Bindungen, d.h. für BR und NR, nimmt S`max mit steigendem Schwefelgehalt zu, gleichzeitig nimmt der DVR bei allen Temperaturen ab. Für EPDM und IIR durchläuft der DVR bei hohen Temperaturen bei steigendem Schwefelgehalt ein Minimum, wobei er dennoch mit steigendem Schwefelge-halt stetig ansteigt.

Figures and Tables: By a kind approval of the authors.

IntroductionExperimental studies into crosslinking of rubbers typically zoom in one particular rubber type. Quite some studies have been dedicated to crosslinking of a single rubber with varying polymer characteris-tics (chemical composition, molecular weight, branching etc.) and/or at varying amounts of curatives (IIR [1]; EPDM [2,3]; NR [4]; BR [5,6]). These studies have not only resulted in the enhanced under-standing of the relationships between “curative content → network structure → properties” for that particular rubber, but also in the development of optimised compounds for applications of that par-ticular rubber. In contrast, the goal of the current study is to compare different rubbers and the effects of their struc-tures on sulfur vulcanisation and the properties of the corresponding vulcanis-ates with sulfur curatives contents varied in the same range, thus providing a more generic understanding of “polymer type & curative content → network structure → properties” relationships in rubber chemistry and technology. Two low-un-saturated rubbers, viz. IIR and EPDM with 0,41 and 0,65 mol/kg unsaturation, resp., and two high-unsaturated rubbers, viz. NR and BR with 14,7 and 18,5 mol/kg unsaturation, resp., were selected (Table 1). IIR and NR have a low entanglement density (Ne = 1 / 2*Me with Me is molec-ular weight between entanglements: 0,14 and 0,12 mol/kg, resp.), whereas EPDM and BR have a high Ne (0,49 and 0,46 mol/kg, resp.). In addition, NR is renowned for strain hardening as a re-sult of strain-induced crystallisation [7,8,9,10] and also IIR has been reported to show strain hardening [7,8,10], where-as the chosen EPDM and BR rubbers are

fully amorphous over the whole temper-ature and strain range. So, each of the four rubbers represents a different com-bination of i) unsaturation content and Ne and ii) unsaturation content and SIC. All four rubbers are hydrocarbon poly-mers with comparable, low polarity, as witnessed by the low solubility parame-ters (Table 1), and, thus, will have compa-rable solubility characteristics for the polar sulfur curatives.

The four rubbers have been vulcanised with variations on a simple, accelerated sulfur recipes as used previously in more academic studies on sulfur vulcanisation of EPDM [2,3], with the elemental sulfur content ranging from 0,75 to 3,0 phr. It is known that the unsaturation content of a rubber strongly determines the interac-tion with carbon black [11] and, thus, the reinforcing strength of carbon black for that particular rubber. In addition, the presence of fillers and plasticizer affects

Sulfur Vulcanisation of low- vs. high-unsaturated Rubbers (IIR & EPDM vs. NR & BR): Part I: Network Structure and Compression Set at elevated Temperatures*

AuthorsMartin van Duin, Geleen, The Netherlands, Christoph Gögelein, Leverkusen, Germany Corresponding Author:Martin van DuinHPE InnovationArlanxeo Elastomers B.V.P.O. Box 1856160AD GeleenThe NetherlandsE-Mail: [email protected]. +31(0) 467020853

* The content of this manuscript was presented during Kautschuk Herbst Kol-loquium in November 2018 in Hannover, Germany.


25KGK · 06 2020www.kgk-rubberpoint.de

the “structure → properties” relation-ships of a vulcanised rubber compound. Thus, for the sake of simplicity this study has been performed on gum rubber sam-ples in the absence of any fillers and plasticiser. Because of the large amount of experimental results, the various ways of correlating these results and the vari-ous view points to present and discuss the results, we will publish the results of our study in a series of sub-studies. In this first part of this series we focus on the maximum rheometer torque (MH) as a measure for the permanent network density and the compression set (CS) as a function of the temperature. In a next part to be published later, we will zoom in on the vulcanisate properties as a function of the network density and the occurrence of strain hardening.

ExperimentalTable 1 provides an overview of the rele-vant polymer characteristics of the sam-ples of the four rubbers used. Note that commercial polymers have been used, which obviously limits the final choice of the rubber samples. For example, the four rubbers have similar, low Mooney viscosity in the range of ML 1 + 4 @ 125°C from 28 to 38 MU. Note that four rubber samples with identical Mn, Mw and Mz, which may be preferred from a purely academic point of view, is virtually impossible. The entanglement density Ne is obtained from Dynamic Mechani-cal Analysis (DMA) measurements from -100° to +100°C using a Mettler Toledo DMA 861e rheometer, equipped with a double-sandwich simple shear sample holder. The gum rubber test specimens are 8 mm in diameter and 1 mm in thick-

ness. Applying the affine network model the Ne is calculated from the plateau storage modulus, which is found within the van Gurp-Palmen plot where the loss factor is at a minimum [12,13,14].

The rubbers were vulcanised with five simple, accelerated sulfur recipes, con-sisting of sulfur (Rhenogran S-80), active

zinc oxide (Zinkoxyd Aktiv), stearic acid (Edenor C18-98 MY), 2-mercaptobenzo-thiazole (Rhenogran MBT-80) and te-tramethylthiuram disulfide (Rhenogran TMTD-70) with the sulfur content rang-ing from 0,75 to 3,0 phr (Table 2), de-rived from a previously used curative formulation for more academic studies

Fig. 1: Rheometer curves for EPDM vulcanised at 160°C with variation of the amount of sulfur curatives.

1

1 Polymer characteristics of (the samples of the) IIR, EPDM, NR and BR rubbers (used).rubber product density unsaturation

contentML 1 + 4 @ 125°C

Mw Mw/ Mn

Tg strain-in-duced crys-tallisation

entangle-ment

density **

solubility parameter***

comments

(kg/l) (mol/kg) (MU) (kg/mol)

(°C) (mol/kg) ([J/cm3] 0.5)

IIR X_Butyl® RB 402

0,92 0,41 36 480 3,7 -67 + 0,14 14,7 2,25 mol% isoprene; linear

EPDM Keltan® 2750

0,88 0,65 28 180 3,7 -52 - 0,49 15,8 48 wt% ethylene; 7,8 wt% ENB;

branchedNR SVR CV

600,92 14,7 35 1555 5,7 -66 ++ 0,12 16,6 linear

BR Buna® CB 530 T

0,90 18,5 37 560 2,0 -95 - 0,46 16,6 51% trans-1,4, 38% cis-1,4 and 11% vinyl-1,2;

linear* the glass transition temperature has been measured with Differential Scanning Calorimetry at 10 K/min. (second heating curve);** the entanglement density has been calculated from the rubber plateau modulus as measured with Dynamic Mechanical Analysis (see experimental);*** the solubility parameter has been calculated via a group contribution method.

2 Sulfur curative packages (phr for pure chemicals; corrected for masterbatch).sulfur ZnO stearic acid MBT TMTD(phr) (phr) (phr) (phr) (phr)0,75 5 1 0,25 0,501,13 5 1 0,38 0,751,50 5 1 0,50 1,002,25 5 1 0,76 1,503,00 5 1 1,00 2,00



on sulfur vulcanisation of EPDM [2,3]. The w/w ratio of the sulfur crosslinker over the MBT and TMTD accelerators is kept constant in an effort to vary only the density, but not the length of the sulfur crosslinks. Compounds were mixed on a two-roll mill from Troester GmbH & Co. KG with a 20 cm roll diameter, 1,2 fric-tion, 40°C roll temperature, and 20 rpm roll speed. First, a narrow gap width was used to form a revolving rubber sheet and, thereafter, all ingredients were add-ed. The total milling time was constant at 10 min for all compounds.

Rheometry was used to assess the vulcanisation kinetics and the final state of cure using a Monsanto MDR 2000E Rheometer at 130°, 140°, 150°, 160°,

170°, and 180°C (DIN 53529 part 3). For compression set (CS) tests, 6,3 mm thick rubber plates were compression mould-ed for t’c90 plus 25% at 120 bar and 160°C. CS measurements were undertak-en for 72 hours at 23°, 70°, 100°, and 125°C using mica powder to avoid stick/slip effects (DIN ISO 815).

Results and discussion

Rheometer maximum torqueRheometry was used to assess the vul-canisation kinetics, including scorch time ts2, half-way cure time t50, optimum cure time t’c90, the final state of cure MH and the occurrence of reversion at tem-peratures between 130° and 180°C. Be-

cause the vulcanisation kinetics will be discussed in detail in another part of this series, it suffices here to comment that ts2, t50 and t’c90 decrease with both sulfur content (Figure 1: EPDM at 160°C shown as example) and temperature (not shown), as expected. The vulcanisa-tion rate, defined as 1/t50 or 1/(t’c90-ts2), increases in the sequence: IIR < EPDM ≤ BR ≤ NR as illustrated by Figure 2 with the note that EPDM vulcanises rela-tively fast, considering its low level of unsaturation. In general, reversion, as measured by the decrease of the rheom-eter torque during prolonged vulcanisa-tion, is observed especially at the highest temperatures and the highest sulfur con-tents, again as expected. Reversion in-creases in the sequence: IIR < BR ≤ EPDM < NR, showing that reversion is not relat-ed to the level of rubber unsaturation in a simple fashion. In the remainder of this study the rheometer maximum torque MH will be used as a measure for the network density [15].

Sulfur vulcanisation can only applied to unsaturated elastomers. It proceeds via the substitution of allylic hydrogen atoms by sulfur, i.e. the unsaturation ac-tivates sulfur vulcanisation but it is not consumed [16,17,18]. The observation that for a particular rubber a higher sul-fur content results in a larger MH (Figure 1) is as expected. It might also be expect-ed that BR and NR with much more un-saturation than EPDM and IIR have a much higher MH. It is somewhat surpris-ing to observe that the rheometer torque of EPDM and BR is quite similar and much larger than that of NR and IIR (Fig-ure 2: samples with 1,5 phr sulfur system at 160°C as example). Figure 3 shows the rheometer maximum torque MH at 160°C for the four rubbers as a function of the sulfur content. This temperature was chosen, because the rheometer torque reaches a stable plateau for all four rubbers, indicating complete vul-canisation and the absence of any signif-icant reversion. The MH values at zero sulfur were obtained by simply perform-ing the rheometer experiments without sulfur curatives, i.e. for just the gum rubber. Obviously, there is no vulcanisa-tion and just a small decrease of the torque is observed, because of heating of the cold rubber sample when it is intro-duced in the hot rheometer chamber, to a small, but non-zero plateau value. As expected, MH increases for all four rub-bers with the sulfur content (Figure 3). At lower sulfur contents the increase of MH

Fig. 3: Rheometer maximum torque MH for EPDM, IIR, BR and NR as a function of sulfur content at 160°C. Lines are to guide the eye.

3

Fig. 2: Rheometer curves for BR, EPDM, NR, and IIR vulcanised with 1,5 phr sulfur curatives at 160°C.

2



is rather pronounced, whereas at higher sulfur contents MH levels off. For NR and IIR at higher sulfur contents, a linear cor-relation between MH and sulfur is ob-served. However, for EPDM and BR such a linear correlation is not found for sulfur contents up to 3 phr. Figure 3 clearly shows that MH increases in the se-quence: IIR ≤ NR << BR = EPDM over the whole range of sulfur contents with for some reason the NR data separating from the IIR data at around 1 phr sulfur. It is somewhat surprising that the low-unsaturated EPDM shows a very high network density comparable with that of the high-unsaturated BR, where-as the high-unsaturated NR shows a low network density.

These seemingly surprising findings can be rationalised by considering a crosslinked gum rubber network consist-ing of chemical crosslinks plus transient and trapped entanglements. Upon chemical crosslinking of a rubber the transient entanglements become trapped [19, 20]. The contribution of the residual, transient entanglements to the network elasticity is time-dependent and for experimental techniques, such as rheometry, hardly contributes (see be-low). Only, the chemical crosslinks and the trapped entanglements are elastical-

ly active. The permanent network densi-ty is, thus, composed of two contribu-tions, i.e. the chemical crosslink density and the trapped entanglement density. Figure 4 shows a schematic representa-tion of the evolution of the trapped en-tanglement density and the permanent network density as a function of the chemical crosslink density. The chemical crosslink density typically increases with the amount of rubber curatives. The den-sity of trapped entanglements increases with chemical crosslinking in a convex fashion [19]. Commercial rubbers are characterised by very high molecular weights (Table 1) and relatively high crosslink densities are applied in the thermoset rubber industry. As a result, almost all entanglements may become trapped in the end. The trapped entan-glement density reaches a plateau value and becomes equal to the original entan-glement density of the non-cured rubber (Ne). The permanent network density then just increases linearly with the chemical crosslink density.

Let us now return to experimental rheometry. The very small but non-zero MH values for the non-crosslinked rub-bers (Figure 3: sulfur content = 0) shows that rheometry at a temperature of 160°C, a deformation of 0,5° and a fre-quency of 1,7 Hz does not probe tran-sient entanglements. Thus, the rheome-ter torque is just a measure for the den-sity of active junction points, which cor-responds to the sum of the chemical crosslink density and trapped entangle-ment density. Table 1 lists Ne of the four rubbers studied, as calculated from the

rubber plateau in a DMA measurement. Ne decreases in the sequence: NR ~ IIR << BR ~ EPDM. Ne reflects the flexibility of the polymer chain, which is limited by i) the presence of unsaturation in the main chain, which simply prevents rota-tion around the C-C=C-C torsion angle, as for BR, NR and to a very limited extend IIR, and ii) alk(en)yl substituents at-tached to the main chain, which hinder rotation around torsion angles of the main chain, as for all four rubbers. The rheometer MH values correlate very well with Ne (Figure 5: plot of MH values at 1,5 phr sulfur from Figure 3 versus Ne values from Table 1; R2 = 0,98), i.e. the rheometer torque is dominated by the trapped entanglement density with only a minor contribution of the chemical crosslink density. The observation that the rheometer MH in Figure 3 increases with the sulfur content is in accordance with Figure 4.

Next, we will quantify the trends in Figure 3. Starting with IIR rubber, a poly-mer consisting of a main chain with two methyl substituents connected to every second carbon-atom, resulting in high steric hindrance plus a small amount of unsaturation from the build-in isoprene monomer. As a result, the IIR polymer chain is not that flexible and has a rela-tively large molecular weight between entanglements (Me) and, thus, a low en-tanglement density: Ne = 1 / 2xMe = 0,14 mol/kg (Table 1). The 1,5 phr sulfur system used in this study has been shown to correspond to a chemical crosslink density of 0,18 mol/kg rubber [2,3] and, as a result, most IIR entangle-

Fig. 4: Schematic representation of evo-lution of the density of trapped entang-lements upon chemical crosslinking with transient entanglements being conver-ted to trapped entanglements. The per-manent network density is the sum of the chemical crosslink density and the trapped entanglement density.

4

Fig. 5: Correlation between rheometer maximum torque of EPDM, IIR, NR and BR vulca-nized with 1,5 phr sulfur curatives at 160°C with corresponding entanglement density of non-crosslinked rubbers.

5



ments will be trapped. The 1,5 phr ele-mental sulfur plus the sulfur from the 1,0 phr TMTD sulfur donor corresponds with 0,51 mol sulfur atoms per kg rubber available for crosslinking. Assuming full conversion of the sulfur available yields an average length of the sulfur crosslinks of 0,51 / 0,18 = 2,8 sulfur atom per crosslink. The permanent IIR network density for the 1,5 phr sulfur system now corresponds to 0,18 mol/kg (chemical crosslinks) plus 0,14 mol/kg (trapped entanglements) is 0,32 mol/kg with the contributions of the chemical crosslinks and the trapped entanglements to the permanent network density being in same order of magnitude. The IIR sample used in this study has an unsaturation content of 0,41 mol/kg (Table 1). One sulfur crosslink requires two unsatura-tions from two rubber chains, so the maximum crosslink density for this par-ticular IIR corresponds to 50% of its un-saturation content, i.e. 0,41 / 2 = 0,21 mol/kg. Therefore, there is just sufficient unsaturation in IIR to satisfy the 0,18 mol/kg sulfur crosslinks for the 1,5 phr sulfur system. At sulfur contents above

1,5 phr, all IIR unsaturation will be con-verted and there is simply no residual unsaturation in IIR to further react to sulfur crosslinks. As a result, for IIR MH levels off at higher sulfur contents in Figure 3.

NR is characterised by one unsatura-tion plus one methyl substituent for every four C-atoms in the main chain. As a result, NR has the smallest Ne of the four rubbers studied: 0,12 mol/kg (Table 1). As for IIR, the NR entanglements will be fully trapped in the 1,5 phr sulfur sys-tem. However, NR has a much higher unsaturation content than IIR (14,7 ver-sus 0,41 mol/kg) and, thus, crosslinking will continue at higher sulfur contents. This explains why for NR MH shows a marked change in the slope at around 1,25 phr sulfur (all entanglements have been trapped), but continues to increase at a higher sulfur content (Figure 3). EPDM rubber is characterised by a fully saturated polyethylene chain with pro-pylene-derived methyl side groups along the chain and some cyclic norbornene units in the chain. BR rubber has a main chain with one unsaturation for every

four C-atoms and some vinyl substitu-ents along the chain, resulting from 1,2-butadiene incorporation. As a result, EPDM and BR have higher Ne values compared to IIR and NR (Table 1: 0.49 and 0.46 mol/kg, respectively vs. 0.14 and 0,12 mol/kg, respectively). Upon chemical crosslinking much more entan-glements will become trapped in EPDM and BR compared to IIR and NR, explain-ing the stronger increase of MH with sulfur content in Figure 3 for the former two rubbers. Even for the 3.0 phr sulfur systems with an estimated chemical crosslink density of 0.36 mol/kg, not all the entanglements will be trapped and, therefore, hardly any levelling off of MH is observed in Figure 3. The EPDM poly-mer has an unsaturation content of 0,65 mol/kg (Table 1), which corresponds with a maximum crosslink density of 0.65/2 = 0.33 mol/kg. Thus, there is almost suffi-cient unsaturation in EPDM to satisfy the available sulfur in the 3.0 phr sulfur sys-tem (0.33 vs. 0.36 mol/kg, respectively). For BR as for NR the unsaturation con-tent is very high and, thus, not limiting sulfur vulcanisation.

Fig. 6: Compression sets (CS) for EPDM, IIR, BR and NR versus sulfur content at 23, 70, 100 and 125°C; note that scales of y-axes vary; the li-nes are to guide the eye.

6



Fig. 7: Compression sets (CS) for EPDM, IIR, BR and NR versus rheometer maximum torque at 23, 70, 100 and 125°C; note that scales of y-axes vary; the lines are to guide the eye.

7

Compression set vs. temperatureFirst, the compression set (CS) will be discussed in an empirical fashion in terms of its dependency of the sulfur curatives content (Figure 6; note that scales of y-axes vary). CS at 23°C decreas-es with sulfur content for EPDM, NR and BR, as expected [21]. For IIR CS at 23°C does not depend on MH. CS at 70°C de-creases with sulfur content for BR and NR, but for EPDM it goes through a mini-mum whereas for IIR it even increases. CS at 100°C decreases with sulfur content for BR and NR, but for EPDM and IIR it actually increases. Finally, CS at 125°C decreases only for BR and NR and shows shallow minima for EPDM and IIR. As ex-pected, CS increases with temperature for all rubbers at all sulfur contents.

Before providing a possible explana-tion for the somewhat surprising CS re-sults for EPDM and IIR at 70, 100 and 125°C, we will first discuss the behavior of CS as a function of rheometer MH as a measure for network density (Figure 7; note that scales of y-axes vary), which is the more correct approach. CS at 23°C decreases with MH for EPDM, NR and BR,

as expected. A higher network density will result in less viscous relaxation in the compressed stage and more elastic resil-ience in the non-compressed stage of the cs test. For IIR this trend is not observed, may be simply because the CS data for IIR show a relatively large scatter around a very low value: 3 ±1%. Note that for all rubbers CS at 23°C has a lowest value of ~3%. For a given MH CS at 23°C increases in the sequence IIR < NR < EPDM ≤ BR, which parallels the increase of the loss factor tan δ of the non-crosslinked rub-bers at 23°C. CS at 70°C decreases with MH for BR and NR, again as expected, but in contrast for EPDM it goes through a minimum whereas for IIR it even in-creases. CS at 100°C decreases with ML for BR and NR, but for EPDM and IIR it increases. CS at 125°C decreases for BR and NR, but for EPDM and IIR it shows shallow minima. Note that CS does not show single master curves vs. MH for any of the four rubbers in contrast as to what is observed for the mechanical proper-ties, such as hardness, modulus, tensile strength and elongation at break (cf. part II of this series). There is a clear trend

with CS increasing with temperature for a given rubber. During the compression stage at more elevated temperatures the thermally labile sulfur crosslinks shorten with sulfur being split off, which is then used for the formation of new crosslinks. As a result, the sample is fixated in the compressed state and CS increases.

So, in summary, depending on the temperature CS for EPDM and IIR de-creases, goes through a minimum or in-creases as a function of MH, which is quite different from the “expected” de-crease with MH as indeed is observed for BR and NR. The quantitative description of the sulfur-vulcanised networks as pre-sented in the previous section on the maximum rheometer torque MH as a function of the sulfur content will be used to explain these surprising findings. Upon sulfur vulcanisation hydrogen at-oms at the allylic positions of the unsatu-rated rubbers are substituted by sulfur and converted into di-allylic sulfide crosslinks. At higher sulfur contents all unsaturation may be converted and, thus, no more additional sulfur crosslinks can be formed. It is speculated that still



all sulfur atoms from the elemental sul-fur and the TMTD sulfur donor will be built in the sulfur crosslinks [22], which will then result in longer sulfur crosslinks which are known to be more thermally labile. As a result, these longer sulfur crosslinks will degrade during the CS tests at elevated temperatures, resulting in sulfur being split off which forms new crosslinks and thereby fixates the sample in the compressed sate, resulting in high-er CS values. For IIR with 0,42 mol un-saturation per kg rubber, the “stoichiom-etry” point is at 0,21 mol sulfur crosslinks per kg, corresponding with a ~1,9 phr sulfur system. Thus the IIR vulcanisates for the 2,25 and 3,0 phr sulfur systems will have longer sulfide crosslinks and thus inferior CS at elevated tempera-tures. For EPDM with 0,65 mol unsatura-tion per kg, this stoichiometric point is at 0,33 mol sulfur crosslinks per kg, corre-sponding to a ~2,8 phr sulfur system. Thus, for EPDM the 3,0 phr sulfur cura-tive system will result in increased CS at elevated temperatures. For BR and NR with 18,5 and 14,7 mol unsaturation per kg, respectively, the stoichiometry points are at 9,3 and 7,4 mol sulfur crosslinks per kg, respectively. Obviously, the amount of sulfur curatives added to these two polydiene rubbers is far from sufficient to fully convert all unsatura-tion, even for the 3,0 phr sulfur system, and thus normal CS versus MH behav-iour is observed. The CS results for NR do suggest some leveling off, which may be related to sulfur-vulcanised NR being more prone to desulfuration similar to its higher sensitivity towards reversion (cf. previous section). A result of these phe-nomena is that CS at 125°C for a given MH increases in the sequence BR ~ NR < EPDM < IIR, which is quite different from that observed for CS at 23°C.

ConclusionsIn this first part of our studies on the sulfur vulcanisation of IIR, EPDM, NR and BR, we have focused on the rheometer torque maximum (MH) and the com-pression set (CS) as a function of temper-ature. Our findings indicate that MH is mainly determined by the trapped en-tanglements and, to a smaller extent, by the chemical crosslink density. The in-crease of MH with increasing sulfur cura-tives content depends on both the poly-mer entanglement density and its de-gree of unsaturation with MH increasing in the series: IIR ≤ NR << BR = EPDM. With increasing amounts of sulfur curatives,

more sulfur crosslinks will be formed and more entanglements will be trapped. In general, increasing the amount of sulfur curatives results in a higher MH, but the response depends on the type of rubber. First of all, for the rubbers with a low entanglement density, viz. IIR and NR, all entanglements become trapped. Sec-ondly, for the rubbers with a low unsatu-ration content, viz. EPDM but especially IIR, the unsaturation is fully converted to sulfur crosslinks. For BR and NR with a high unsaturation content, MH contin-ues to increases with sulfur curatives content and, thus, CS decreases with sulfur content at all temperatures. How-ever, for EPDM and IIR with a low unsatu-ration content, CS at elevated tempera-tures goes through a minimum or even increases with sulfur content. This sug-gests that for the latter two low-unsatu-rated rubbers a higher sulfur curative content not only results in a higher MH but also, when (sulfur crosslink/rubber unsaturation) stoichiometry is reached, in longer sulfur crosslinks, which have a detrimental effect on the high-tempera-ture CS. In part II of this series the results of the mechanical testing of these sam-ples, such as hardness, tensile and tear testing, will be presented and explained in terms of total network density.

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[22] M. van Duin, non-reported results: thin-layer chromatography analysis of acetone extracts of gum EPDM vulcanized with similar sulfur curative systems showed that elemental sul-fur is consumed for more than 99%.

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Sulfur Vulcanisation IIR EPDM NR Sulfur Vulcanisation of low- vs. … · 2020. 6. 9. · der DVR bei allen Temperaturen ab. Für EPDM und IIR durchläuft der DVR bei hohen Temperaturen