Rubber World Magazine — August 2009
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State Of Cure Measurements In Peroxide And Sulfur Cured EPDM
Richard J. Pazur

This article will serve as a review of the current methods used for measuring the state of cure in finished rubber products. It is well known that the level of state of cure has a profound impact on rubber properties measured on finished parts, including mechanical (hardness, modulus, tensile strength, elongation to break, tear strength), dynamic (tan delta, hysteresis, fatigue behavior) and performance properties such as resistance to compression set and to abrasion.

Flory-Rhener equilibrium volume swell, stress-strain modulus equation of state and nuclear magnetic resonance (NMR) spin echo measurements (T2 relaxation) will be employed for the measurement of state of cure. Peroxide and sulfur cured unfilled EPDM compounds will be used as model systems for this study. Crosslink densities and/or average molecular weights between crosslinks (Mc) will be compared to rheometer torque, hardness, physical property and compression set data.

A biexponential analysis of the NMR T2 relaxation data provided satisfactory curve fitting results and an average molecular weight between crosslinks was calculated using the short decay time constant. The NMR measurement of crosslink density gave higher results compared to numbers predicted by stress strain analysis. Combining NMR Mc and volume swell Mc data (in particular, from the phantom model analysis) resulted in a successful estimation of the molecular weight between chain entanglements. It was also possible by NMR T2 relaxation curve analysis on unvulcanized EPDM to estimate the molecular weight between physical entanglements.

Critical properties such as hardness, modulus and delta torque rheometry are in good correlation with crosslink density data from both NMR and stress strain techniques, while the average molecular weight between crosslinks by equilibrium volume swell follows more closely properties such as elongation to break and compression set. Finally, it was concluded that both NMR T2 data and equilibrium volume swell measurements can follow the effects of sulfur reversion, with the latter being most sensitive to the crosslink density changes taking place during reversion.

State of cure techniques The extent of the state of cure in any rubber material affects important functional part characteristics such as mechanical, dynamic and performance properties (ref. 1). The state of cure of a rubber network is itself a generic term that describes in relative terms the degree of crosslinking within the network. It is perhaps best described quantitatively in terms of a crosslink density (number of moles of crosslinks per unit volume).

Many techniques can be employed to measure state of cure of crosslinked systems, but few provide crosslink density information in quantitative terms. Perhaps the most widely used and recognized techniques for its determination include stress strain data analysis and equilibrium volume swell measurements.

NMR T2 relaxation measurement and its subsequent analysis has also proven to be a powerful analytical technique for crosslink network information.

An uncured polymer system can be simply described as a mixture of flexible coiled molecular chains in which neighboring chains are physically entangled, and held together by weak interaction forces. Chain entanglements and coupled interacting chains are often referred to as physical crosslinks. The coupled interactions are known to break down in the process of deformation since their energy of interaction is small. During the onset of cure, the linkage of adjacent chains by bonds forms chemical crosslinks which increase as a function of cure time due to the continual reaction of the vulcanization curatives.

In the event of forming these chemical crosslinks, physical entanglements become trapped between the fixed crosslink junctions. Thus, in the course of a curing reaction, the total number of physical (ne) and trapped entanglements (ne, trapped) as well as chemical crosslinks (nc) during the cure can be simply expressed as: ntotal = ne + ne,trapped + nc (1) In a practical sense, state of cure measurement techniques are not employed to measure the actual quantity of crosslinks and/or entanglements, but rather they provide a measurement of the crosslink density (íc) or an average molecular weight between crosslinks (Mc), with both being related by íc = ñ/2Mc. An average number of chain segments (backbone monomers) between crosslinks can also be estimated and is generally known to be contained between approximately 100 and 700 chain segments for crosslinked elastomer systems possessing sufficient elastic properties (ref. 1). In this treatise, state of cure results will be primarily presented in terms of íc and Mc.

It is widely known that by employing the statistical mechanical theory of rubber elasticity, one can derive an elastomer equation of state which provides a means of estimating the average molecular weight (number average) between crosslinks (ref. 1). Assuming the microscopic movement of the polymer crosslinks moves affinely with the macroscopic deformation, equation 2 may be written: ñRT 1 ó = ( )(ë2 - ) Mc,SS ë (2) where the engineering stress is defined by ó, ë is the draw ratio, R is the gas constant, T is the temperature in units of Kelvin and ñ is the density of the elastomer. An affine deformation assumes that the crosslinks are fixed in space at positions defined by the overall specimen ratio. Also commonly used is the phantom network model which permits a certain Amount of fluctuation of the crosslinks about their average affine deformation positions. The equation of state derived by this model is represented by: ñRT 1 ó = ( )(ë2 - ) 2Mc,SS ë (3) The derivation of equations 2 and 3 assumes a crosslink functionality of 4. The affine and phantom models provide upper and lower bounds to the modulus, respectively. In other words, the phantom network model predicts a modulus which is one half of the modulus calculated by the affine deformation model.

Equilibrium volume swell measurements and ensuing analysis by the Flory-Rehner relationship can be used in order to determine the average molecular weight between crosslinks (chemical crosslinks and trapped entanglements together) in the unfilled elastomer system by way of equation 4: 1 2 ñ 3 vr ln(l - vr) + vr + ÷vr = - Vs (vr - ) Mc,sw 2 (4) where Vs is the molar volume of the swelling solvent, vr is the volume fraction of rubber in the swollen state and ÷ is the Flory Huggins polymer-solvent interaction parameter. In the derivation of equation 4, an affine deformation is assumed during swelling. An equivalent form of this equation using the phantom model theory has also been derived and is expressed as: 1 2 ñ 3 ln(l - vr) + vr + ÷vr = - Vs vr 2Mc,sw (5) As in the previous stress strain modulus equations, the simplified forms of equations 4 and 5 assume a crosslink functionality of 4. It is generally accepted that the actual behavior of the swollen network lies somewhere between the limiting conditions presented by equations 4 and 5. Contrary to the molar masses calculated using stress strain analysis, it is assumed that all chain entanglements disappear during the equilibrium swelling method, leaving only the contribution of chemical crosslinks and trapped entanglements to the average value of Mc, sw.

The use of solid state nuclear magnetic resonance (NMR) techniques for measuring crosslink density is becoming more widespread in the rubber industry (refs. 2-7). The most commonly used pulse sequence for rubber systems is called the Hahn-echo spin decay. Analysis of the magnetization decay curve M(t) by way of a combined Gaussian/exponential equation (eq. 6) can provide valuable information about crosslink density and molecular mobility within the crosslinked elastomer system (ref. 6).

- t 1 -t M (t) = Aexp( - qM2t2) + Bexp( ) + Offset T2 2 T2 (6) The parameters of A, T2, qM2 and B are determined by curve fitting of the magnetization decay by using equation 6.

The coefficients A and B represent the relative amount of the rigid and mobile fractions of the rubber, respectively. The decay time T2 is related to the highly mobile part of the network and includes any solvent molecules and freely rotating molecular end groups. The quotient qM2 corresponds to the 26 remaining dipole magnetic coupling of the hydrocarbon chain protons due to the anisotropic motion of the chain segments.

In the case of natural rubber, for which the value of M2 has been calculated by way of NMR line shape calculations, one can calculate an averaged inter-crosslink chain mass using suitable equations (ref. 7). In the event where a Gaussian component is not apparent during the onset of the relaxation data, it is more appropriate to use a biexponential equation (eq. 7) to describe the magnetization decay profile:

- t -t M(t) = Aexp( ) + Bexp( ) + Offset (7) T21 T22 where T21 and T22 represent the decay times of the rigid and mobile fractions of the network, respectively. It has been pointed out, however, that the curve fitting of magnetization decay data obtained by the Hahn-Echo technique should take place with suitable knowledge of its possible drawbacks (ref.

8) .

With respect to the analysis of ethylene-propylene (EP) or ethylene-propylene vulcanizates containing a third diene based site (EPDM), Litvinov et. Al. (ref. 9) used the average high temperature plateau values of T2 calculated through extensive cure fitting models in combination with the following equation in order to estimate the number average molecular weight between chains: C T2M M C , N M R = a T r2lN U (8) In equation 8, C represents the number of backbone units per statistical Kuhn-segment, MU is the average molar mass of one monomer unit, the value of the coefficient “a” is dependent on the angle between the segment axis and the internuclear vector for the closest nuclear spins on the main chain, T2 rl relates to the strength of intrachain proton-proton interactions in the rigid lattice and N is the number of backbone bonds per monomer. Known values of these constants are tabulated in the appendix.

Dynamic mechanical (DMA) property measurements provide insight into the state of cure through the measurement of tan delta, which is the ratio of the loss modulus to the storage modulus. It has been illustrated that for measuring tan delta after 10 minutes at 100°C for a series of different elastomers (FKM, HNBR), a good correlation is obtained to compression set values (ref. 10). The technique showed good sensitivity and repeatability upon measurements, and also can be applied directly to a multitude of cured parts since the tan delta itself is independent of sample geometry. This methodology has been applied and set as an engineering standard for the determination of optimum process conditions (optimum cure times/temperatures) for new automotive production parts and can be used to distinguish between post-cured and non-post-cured parts for the majority of elastomer systems (ref. 11). State of cure measurements by this technique remain relative to samples of known states of cure.

Most NMR analyses of crosslink density have concentrated on commodity based rubbers (NR, SBR and BR), with little work accomplished on EPDM rubbers. The last major study examining chemical and chain entanglement density in EPDM Type vulcanizates using proton T2 NMR relaxation took place in the late 1990s (ref. 9). Mooney-Rivlin analysis of stress strain data along with equilibrium swelling were also used to estimate crosslink densities in both peroxide and sulfur cured vulcanizates.

In this treatise, the state of cure of both peroxide and sulfur cured EPDM unfilled compound will be evaluated. The peroxide cured EPDM will be prepared to different states of cure by varying active peroxide concentration. The sulfur model compound will be vulcanized to different cure times in order to investigate sulfur reversion effects. Analyses will subsequently take place by using a combination of Flory Rehner volume swell, NMR T2 spin echo relaxation and stress strain modulus measurements. Both compression molded tensile sheets and rheometer disks will be prepared to allow analysis between both sample preparation techniques. Values of crosslink density and Mc will be calculated for each measurement technique, and results will be compared and contrasted. Physical property and compression set data will be related to the crosslink information. Finally, an attempt will be made at estimating the physical chain entanglement density.

Experimental Table 1 presents the compounding ingredients used in preparing the peroxide cured EPDM series. EPDM was chosen as the base elastomer given its heat resistance at temperatures of sample preparation so that chain scission and oxidation effects can be minimized. A detailed description of each of the ingredients is tabulated in the appendix.

The EPDM formulation presented in table 2 was used for the sulfur based state of cure study. It is essentially an unfilled EPDM compound containing vulcanizing agents for a medium sulfur cure and an antioxidant.

The formulations presented in tables 1 and 2 were mixed in a similar manner on a 6” x 13” two-roll open mill set at 37°C.

After banding of the EPDM elastomer with the aid of the stearic acid on the mill, the vulcanization ingredient(s) and antioxidant were slowly incorporated into the compound. Total milling time was about 15 minutes.

The six peroxide cured EPDM samples were tested for 10 minutes in the rheometer (1.67 Hz, 7% strain, 180°C), removed and allowed to cool to room temperature. ASTM based tensile slabs were produced using a Lawton 50 ton compression mold set at 180°C. Tensile sheets were cured to 10 minutes and then removed and allowed to cool slowly like the rheometer disks.

In the case of the sulfur cured EPDM, rheometer specimens were prepared at different cure times (t50, t70, t90, 5, 7.5, 10,

12. 5, 15, 20 and 25 minutes) using 1.67 Hz, 7% strain, 180°C as the standard set up condition. Disk specimens were immediately quenched in cold ice water for at least 20 minutes in order to freeze in state of cures. Successful tensile sheets were prepared for 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7.5, 10, 13.33, and 16.67 minutes of cure time. Samples were immediately quenched after being taken out of the mold as in the case of the rheometer samples.

Dumbbell samples were cut from the tensile sheets for stress strain testing (25.4 mm/minute). NMR and volume swell samples were also cut from the slabs. Mc, ss determination by equations 2 and 3 were carried out using the low strain modulus data (1 < ë < 1.5). Compression set buttons were made by stacking up six died disks from the tensile slabs. Compression set aging was carried out for 22 hours at 135°C in a hot air circulating oven. The Flory Huggins interaction parameter used for swelling of EPDM in n-hexane is 0.354 (ref. 9).

A Bruker Minispec mq 20 NMR spectrometer was used to collect the T2 relaxation data. A resonance frequency of 20 Mhz was used for testing. A Hahn echo (spin echo) pulse sequence was employed (90°/t/180°/t[acquisition]) and the transverse magnetization decay was measured at 363 K in the region of the T2 plateau (i.e., where T2 is nearly independent Of temperature). The test schemes for peroxide- and sulfurcured EPDM are shown in figures 1 and 2, respectively.

Results and discussion P-EPDM rheometer disks The rheological behavior of the six peroxide cured EPDMs is displayed in figure 3. As a function of the active concentration of dicumyl peroxide, a good separation is observed between the maximum torque values, indicating the desired progression in crosslinking density. Samples are showing a plateau behavior of the torque after about four minutes of cure time, indicating that samples are assured to be completely cured and the peroxide completely consumed after 10 minutes in the rheometer.

Flory-Rehner equilibrium volume swell measurements were conducted on the rheometer disks, and crosslink density values were calculated using both the affine and phantom models. These values, along with the maximum and delta torque values from figure 3, were plotted against the peroxide concentration in figure 4. As expected, both crosslink density and torque values increase in a relatively linear fashion as a function of DCP concentration. The phantom model predicts a higher value and a steeper rise in crosslink density versus the affine model with increasing peroxide concentration. In this comparison, the affine model for volume swell measurements seems to better parallel the torque behavior as it follows along the same slope as the torque data. Error effects in measuring the crosslink density by volume swell and torque values were estimated to be no higher than 5%.

Crosslink densities by volume swell using the affine model for the rheometer disks were compared to those obtained on the tensile slabs in figure 5.

An excellent correlation was obtained between both crosslink densities, indicating that the state of cures obtained in the disks were for all intents and purposes the same as those in the tensile slabs prepared by compression molding. This will allow for direct comparison of rheometer to tensile data in a later analysis.

P-EPDM tensile sheets Stress strain and hardness testing on the P-EPDM test sheets confirm the trends expected between crosslink density and macroscopic properties (figure 6). Hardness and modulus increase linearly, while elongation decreases as a function of peroxide concentration. The elongation drops significantly from 0.4 to 0.8 phr of active peroxide, then linearly decreases to 2.4 phr. Tensile strength remains relatively constant, but shows a slight trend downward as a function of peroxide concentration.

NMR T2 relaxation curves were obtained on all samples of P-EPDM and an unvulcanized sample (figure 7). Relaxation of the echo response during a T2 measurement is rather rapid in the beginning and then slows going to longer times. The fast decaying component reflects constrained segments near crosslinks and physical entanglements, whereas the slow decaying component originates from the less constrained remote chains.

The relaxation curves exhibit good separation in order going from the unvulcanized sample through to the highest crosslinked one (2.4 phr active peroxide).

A curve fitting analysis of the relaxation data presented in figure 7 was carried out by using equations 6 and 7. Due to the non-Gaussian nature of the relaxation curve, the best curve fitting was provided by the biexponential equation (R2 values greater than 0.999), and T21 values decreased steadily in value as a function of increasing peroxide concentration (figure 8).

Relative maximum error estimated by repeats for the relaxation times was estimated to about 5%. The T21 values were immediately used to calculate a crosslink density by way of equation 8.

Crosslink density values were also evaluated from the stress strain data and compared with those from both equilibrium volume swell and NMR. These results are plotted in figure 9.

In all cases, the crosslink density increases as a function of peroxide concentration. In the case of the NMR and stress strain equation of state data, the increase takes place in a linear fashion, while for the swelling data, it takes place with some initial curvature in the data for low peroxide concentrations.

This graph illustrates that both the NMR and stress strain techniques are measuring the same type of crosslinking, a crossLink density comprised of temporary and trapped chain entanglements, as well as chemical crosslinks. Volume swell measurements, on the other hand, neglect the chain entanglement density contribution to the total average density, hence the low values observed particularly at low peroxide concentrations.

Also, note that the phantom model analysis of stress strain data and the NMR data are closest in actual values, but that the NMR results predict the highest crosslinking numbers.

Figure 10 takes the Mc data calculated from figure 9 and combines it with the property data of figure 6, including the delta rheometer torque. It can be observed that relative linear trends of both MH-ML and modulus can relate to the Mc values obtained by both stress strain and NMR measurements. On the other hand, the dependence of Mc, sw on peroxide concentration shows a similar behavior as the stress strain elongation to break results, indicating that from the microscopic to macroscopic level, they are interrelated. During stretching of the samples, the molecular chains will slowly take on the macroscopic orientation imposed by the applied stress in the uniaxial direction. At very high elongations, the chains that will be breaking first will be predominantly those constrained by the chemical crosslinks, while any entanglements are able to slip and slide to new positions without causing chain breakage. It follows, as well, that the temporary and trapped chain entanglements along with the chemical crosslinks will contribute to the macroscopic measurements of modulus and the rheometer delta torque.

Finally for the P-EPDM samples, it is possible to estimate an entanglement density by assuming additivity of the inverse average molecular weights between crosslinks calculated by the NMR and volume swell techniques. In this special case, equation 9 can be used to estimate by extrapolation an average molecular weight between entanglements (Me) (ref. 11).

1 1 M c , N M R . M e + Mc,1sw (9) Figure 11 shows the inverse molecular weight data from NMR plotted against the same data obtained from swelling data assuming either the phantom or the affine model. ExcelLent linearity is observed in fitting this data. Average molecular weight between entanglements is estimated as 2,400 ± 240 g/mol and 2,250 ± 225 g/mol for the affine and phantom models, respectively. These numbers, in particular the phantom model, are a prediction of entanglement density for EPDM polymers and are in good agreement with reported values in the literature for EPDMs in the 50 to 60 wt. % by ethylene range (ref. 9). Such concordance of values provides confidence in the constants used in calculating both Mc, NMR and Mc, sw, and also validates the approach of using equation 8 in spite of the uncertainty for some of the constants in the equation. The T21 time constant for the unvulcanized P-EPDM sample was used to calculate Me and it was found that Me had a value of 2,040 ± 100 g/mol, which is in excellent Agreement with values from the literature (ref. 9). The measurement of the contribution of physical entanglement density by transverse NMR on unvulcanized samples has been accomplished using the remaining dipolar magnetic coupling of the chain protons (qoM2) (refs. 2 and 7). The measured fast decay T21 time constant used in conjunction with equation 8 appears to provide a good estimate for the molecular weight between entanglements by NMR.

S-EPDM rheometer disks Figure 12 summarizes the rheological data (elastic and loss torque) obtained during the curing of the disk specimens up to 30 minutes. The solid lines represent the 30 minute cure sample, while the discrete symbols represent the individual samples taken out of the rheometer at specific cure times. A relatively good correspondence is noticed between the data, in particular for the elastic torque. The maximum torque reaches a maximum value at about 6 minutes (. 17 dNm) and then slowly decreases to a value of 15 dNm at 30 minutes, indicatIng that a loss of crosslink density is taking place between 6 and 30 minutes (about 15%). It is known that the reversion effect of the sulfur crosslinks comprises two processes: 1) sulfur crosslink lengths decrease (S8 towards S2 and S1); and 2) some of the initially formed S-S chain linkages are destroyed due to the effect of heat (ref.

1) . The loss torque slowly rises as a function of cure time during this time, corroborating with the loss of crosslink density in the chain network.

Figure 13 displays a plot of maximum torque and crosslink density by volume swell measurements as a function of cure time. The crosslink density measurements by volume swell follow the behavior of the maximum torque and also reveal the loss of crosslink density at the longer cure times due to reversion of the sulfur cure system. Again, both techniques predict a maximum state of cure in the six minute range at 180°C.

The NMR T2 relaxation curves of the SEPDM disk samples are presented in figure 14. Similar to the observations seen in figure 7, a quicker relax-Ation time (steeper initial slope) is observed in going from the unvulcanized sample through to tc90, after which all the curves appear to overlap with little apparent separation between them. The T2 relaxation curves of figure 14 were successfully fit with the biexponential function, and subsequently, T21 values were calculated (figure 15). The T21 values decrease during the initial part of the cure, then stabilize and slightly increase before the end of the cure time. Plotting the inverse of T21 actually provides a data set which resembles the maximum torque and volume swell data in figure 13. The sulfur reversion is being picked up by NMR, but it does not seem to be as sensitive as the volume swell and/or rheometer techniques for measuring this effect. Mc NMR values were calculated and plotted as a function of cure time along with the Mc sw data in figure 16. The Mc values calculated by NMR are lower than those given by the equilibrium volume swell method. High Mc, sw values are obtained for the initial tc50, tc70 and tc90 samples of the series. The phantom model method in analyzing volume swell data is closer in value to the NMR data.

The average molecular weight between chain entanglements was estimated on the S-EPDM disk samples by using the Mc, NMR and Mc, sw data according to equation 9. Both the phantom and affine models used for equilibrium volume swells were analyzed. The results are plotted in figure 17. Sufficient separation was observed in the data and linear regression lines were drawn between the data points. In this case, the phantom model provided a better fit to the NMR Mc data than the affine model. Calculated values of Me for the phantom and affine models were 1,980 ± 200 g/mol and 3,030 ± 300 g/mol, respectively. The phantom model for calculation of crosslink density by equilibrium volume swell measurements is providing a more accurate depiction of the real value of Me. This is in line with some recent data looking at the swelling behavior of natural rubber (ref. 13). As in the prior case with P-EPDM, by using the T21 value measured for the unvulcanized sample, a value of 2,350 ± 120 g/mol was calculated for Me.

S-EPDM tensile sheet evaluation Figure 18 provides a summary of the effect of mold cure time On the hardness, stress strain properties and compression set.

The hardness increases and plateaus at values close to the mid- 50s after six minutes of cure. Elongation to break decreases in value until about five or six minutes of cure, thereafter holding relatively constant around 200% to 16.7 minutes. Tensile strength trends slightly downward. The resistance to compression set displays a similar behavior as elongation data, except that the lower plateau is reached at times after 7.5 minutes.

Very good separation between NMR T2 relaxation curves is observed in figure 19 in going from the unvulcanized through to the longest cured sample, indicating that a crosslink density variation exists in the tensile slabs.

Figure 20 summarizes the crosslink density data from stress strain, equilibrium volume swell and NMR data. It is noticed that the NMR technique is providing a higher estimate of the crosslink density, especially in comparison to the stress strain determined Mc. The phantom model derivation of the stress strain equation of state is providing the closest Mc values to NMR. The Mc calculated by using swelling data displays more variation in crosslink density at low cure times due to the elimination of entanglement density to the overall average molecular weight values. The crosslink data from these three techniques is not showing any noticeable reversion (loss of crosslinks in particular) for the duration of mold curing.

The crosslink density data of figure 20 was converted to Mc values and then plotted together on figure 21 with the physical property, hardness and compression set data of figure 18. The hardness values as a function of cure time show the same trend as the Mc NMR and Mc SS data. The behavior of the compression set and elongation data seems to line up best with the Mc sw data, although it is noticed that the transition to the linear region (five to eight minutes) seems to be longer than for the Mc sw data. It is expected that the heat aging of the samples (24 hours at 135°C) for compression set resistance would have the additional effect of shortening crosslink length and lowering the total amount of crosslinks.

Chain entanglement values (Me) were estimated from figure 22 using linear regression analysis. The phantom and affine models for equilibrium swelling gave values of 1,990 ± 200 g/mol and 1,960 ± 200 g/mol, respectively. From the T21 measurement on the unvulcanized compound, Me was estimated to be 2,060 ± 100 g/mol, which is again in good agreement with the extrapolated and reported values (ref. 9).

Conclusions In this work we have successfully measured the crosslink density by using stress/strain modulus, equilibrium volume swell and NMR T2 data on both unfilled peroxide and sulfur cured EPDMs.

Crosslink densities and/or average molecular weights between crosslinks were compared to rheometer torque, hardness, physical property and compression set data. The following generalized conclusions came out of this study:

• Biexponential analysis of NMR T2 relaxation obtained by the Hahn-Echo spin sequence was the preferred curve fitting technique for sulfur and peroxide cured unfilled EPDM;

• Mc calculations by using the short component of the NMR T21 relaxation decay generally predicted a higher crosslink density;

• The phantom model Mc analysis of the stress strain equation of state data was closest to the NMR crosslink data;

• Successful estimations of the molecular weight between chain entanglements was possible by combining NMR Mc and volume swell Mc data (in particular, by the phantom model analysis);

• NMR T2 relaxation curve analysis on unvulcanized EPDM samples provided good estimates of the molecular weight between physical entanglements;

• Properties such as hardness, modulus and delta torque rheometry are in good correlation with crosslink density data from both NMR and stress strain techniques;

• The average molecular weight by equilibrium volume swell is particularly sensitive to properties such as elongation to break and compression set; and

• NMR T2 data and equilibrium volume swell measurements can follow the effects of sulfur reversion, with the latter being most sensitive to the changes taking place during reversion.

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