Formulation design and curing characteristics of NBR mixes for seals.

This article will be concerned with a detailed account of attempts at developing nitrile rubber vulcanizates suitable for applications such as seals and other fittings for use in distribution systems conveying natural gas. Rubbers used in such applications need to be able to satisfy certain property requirements which are directly relevant to in-service conditions. Moreover, the integrity of seals (and alike) must be maintained over an acceptable lifetime under various environmental changes. In awareness of such requirements, British Gas has proposed three types of tests in order to assess the potential usefulness of rubber vulcanizates for their applications. These three types are: mechanical properties, environmental resistance and aging behavior.

Probably the two most important mechanical properties for this type of application are compression stress-relaxation and compression set at low and elevated temperatures. Compression stress-relaxation measures the rate of decay of stress under a fixed compression. It is related to the loss of sealing force in a compression seal. It is obviously desirable for a seal to maintain a high outward force with the surfaces with which it is in contact in order to prevent profuse leaks during service. Compression set, on the other hand, is the amount of unrecovered deformation, after a deforming stress is removed. This property measures the ability of a rubber vulcanizate to accommodate displacement of seal faces (e.g., joint movement). At low temperatures, compression-set measurements are an indication of low-temperature flexibility. At elevated temperatures compression-set measurements can be used as an indication of the degree of cure attained in rubber vulcanizates.

From the foregoing remarks it is apparent that in order for a compression seal to perform satisfactorily there is a need to minimize compression stress-relaxation and compression set. In order to attempt this it is necessary at first to review the compounding techniques available for minimizing these two properties. After an extensive literature search carried out by the author (ref. 1 ), it was found that in order to produce rubber vulcanizates with low rates of stress-relaxation and low compression set at low and high temperatures it was desirable to choose rubbers with:

* Good low-temperature flexibility, therefore reflecting a low glass-transition temperature;

* high primary molecular mass;

* high concentration of crosslinks;

* thermally stable crosslinks, e.g., those which are produced by peroxide vulcanization systems and efficiently-vulcanized (EV) curing systems;

* vulcanization systems which remain soluble in the bulk of the rubber vulcanizate;

* ideally no fillers, but, for commercial reasons and/or specific property requirements the incorporation of fillers sometimes becomes essential in which case it is desirable to minimize the concentration of filler;

* fillers of large particle size, i.e., those with low surface areas and, having a high degree of structure (three dimensional chain4ike structures);

* no plasticizers;

* good resistance to moisture; and,

* a good antioxidant system.

The composition of natural gas is predominantly hydrocarbon by nature. Therefore, the choice of rubbers suitable for this type of application are restricted to a class of rubbers generally termed oil-resistant rubbers. For this work NBR was chosen. This rubber is a copolymer of butadiene and acrylonitrile. NBR was chosen in the initial stages of the investigation because it was considered to be the most facile in terms of cost, compounding, availability and processing; it also allowed the possibility of assessing various curing systems which had come to light as a consequence of the extensive literature searches on compression stress-relaxation and compression set. Preliminary work on nitrile rubber indicated that an acrylonitrile level of 34% (by weight) was adequate to meet the stipulated requirement with respect to swelling in a hydrocarbon environment (ref. 2).

Formulation design and experimental

Base formulation

The base formulation or masterbatch was based upon the considerations outlined above. It comprised: NBR with a bound acrylonitrile level of 34% by weight, having a Mooney viscosity of about 79; fast extrusion furnace (FEF, N550) carbon black; polymerized-1,2-dihydro-2,2,4 trimethyl-quinoline (Flectol H). FEF and Flectol H were added at levels of 60.0 phr and 2.0 phr respectively. Six masterbatches were prepared (each of batch weight ca. 2958g) in an internal mixer with a capacity of 2,500 [cm.sup.3] operating at a rotor sp eed of 44 rpm and under a ram pressure of 0.69 MPa. A single-stage mixing technique was used. NBR, Flectol H and half of carbon black were added first. The remainder of carbon black was added during the second minute. The throat of the mixer was swept during the fourth minute. Finally, the mix was discharged during the seventh minute. Discharge temperatures ranged between 80-90 degrees C. Each masterbatch was cooled and refined on a 0.46-m two-roll mill operating at an even speed of 15 rpm. Cooling and refining times ranged from 7-10 minutes.

Plasticity tests were canned out on each masterbatch using a plastimeter (ref. 3) to assess the degree of uniformity that had been achieved during mixing and refining. Fifteen test specimens were cut along the central axis (this axis being parallel to the direction of haul-off from the mill) of each masterbatch. The plasticity values for each of the 15 test specimens for the six

masterbatches are shown in table 1. An analysis of variance was carried out to see whether or not the plasticity data were statistically significant. The results of this analysis are summarized in table 2. The high value of F-ratio obtained (35.54) indicates a significant variation in the plasticities among the masterbatches. This calculated F-ratio was greater than the tabulated value of F at O. 1% probability level (i.e., 4.76) thus confirming that differences in plasticities among masterbatches were highly significant. Furthermore, 95% confidence intervals shown in table 3 for the various masterbatches clearly show variations in plasticity measurements. These variations were minimized by cross-blending techniques, as will be described subsequently.

Curing agents

Zinc oxide, stearic acid, tetramethylthiuram disulfide (TMTD), 4,4'-dithiodimorpholine (DTDM), and tetrabutylthiuram disulfide (TBTD) were used. Benzothiazyl disulfide (MBTS), hexamethylene-l, 6-bisthiosulfate disodium salt dihydrate (ERP 390), 2-(morpholinothio) benzothiazole (MBS or MOR), and a blend comprising 90% MOR and 10% MBTS (MOR 90) were also used, along with N-oxydiethylene thiocarbamyl-N-oxydiethylene sulfenamide (OTOS) zinc 2-ethyl hexanoate (ZEH); and MC sulfur. These curing agents were used as received without additional purification.

Curing systems used

To facilitate presentation of results the following abbreviations were used to specify various vulcanization systems used in this work: Conv. - conventional system; Conv (PVS) - conventional system with post vulcanization stabilizer; EV EV system; SD - sulfur-donor system; Semi EV - semi EV system; SEV (MOR) - soluble EV system with MOR; SEV (MOR 90) - soluble EV system with MOR 90; Sol. semi EV - soluble semi EV system; and Hof- Holmann system.

* Conventional system - Polysulfidic crosslinks produced by conventional sulfur systems are relatively unstable at high temperatures. Nevertheless, crosslinks of this type impart flexibility to rubber networks. Such flexibility leads to vulcanizates having low compression set at ambient and low temperatares. It was for this reason that a conventional system was included in this work.

* Conventional system with a post vulcanization stabilizer - polysulfidic crosslinks could re-arrange chemically under anaerobic aging to generate crosslinks of lower sulfur rank (e.g., monosulfidic crosslinks). In addition, pendant groups containing sulfur could also be generated. Such chemical modifications in the network can lead to changes in the mechanical properties of a vulcanizate. For example, these changes might lead to significant reductions in tensile strength, stress at a given elongation, elongation at break, fatigue life, resilience and tear strength. Heat build-up under dynamic flexing may also increase. To counteract these changes in vulcanizates containing predominantly polysulfidic crosslinks, a post vulcanization stabilizer (PVS) was developed. This material is the dihydrate of the sodium salt of hexamethylene - 1,6-bis-thiosulfate-s-ester:

NaO.sub.3SS(CH.sub.2)6SSO.sub.3Na.2H.sub.2O.

During vulcanization it reacts by interposing a hexamethylene 1,6-dithiyl group within the polysulphidic chain:

[FORMULA OMITTED]

Conventional curing systems containing PVS are said to generate polysulfidic and hybrid crosslinks; the latter containing the hexamethylene-1,6-dithiyl group. Furthermore, it has been shown that the number of short polysulfidic crosslinks is increased as compared to the lengths of similar crosslinks in normal conventional curing systems (ref. 4). Hybrid crosslinks are said to have good flexibility and stability. Obviously, this stability is desirable, because it implies longevity at low and elevated temperatures.

* EV system - These systems contain a low concentration of sulfur and a high concentration of an organic accelerator; consequently monosulfidic crosslinks become the most abundant type. Such crosslinks are very stable, particularly at high temperatures. Therefore, changes in the network are minimized on prolonged exposure to high temperatures and mechanical properties remain relatively constant on overcure or high temperature aging. Compression sets of vulcanizates cured by these systems are very low at elevated temperatures.

* Sulfurless or sulfur-donor systems - These may also be classed as EV systems. Cashion (ref. 5) has shown that NBR vulcanizates cured by TMTD, without sulfur, give very low compression-set values at high temperatures. It is common with most EV systems that reductions in free sulfur together with increasing level of accelerator can produce vulcanizates having predominantly mono- and disulfidic crosslinks. However, progressive reductions in free sulfur must be accompanied with commensurate increases in the loading of accelerator if modulus and other properties are to be mainmined at a reasonable level. When concentrations of sulfur are reduced to levels below 0.5 phr, it is then not possible to match the modulus by simply increasing accelerator concentrations unless the accelerator is of the type which can donate further sulfur for crosslinking. One example of an accelerator which can donate sulfur in this way is TMTD. Some vulcanizing agents are able to donate sulfur but cannot act as accelerators, for example DTDM. Such curing agents must always be used in conjunction with an accelerator.

* Semi-EV - In many applications where EV systems would be an advantage, they are nevertheless not often used. The main reason for this is that they are too expensive to be of commercial use. Furthermore, the improvement in productivity or performance associated with using EV systems may not justify the extra cost. In these situations there is a tendency to use an intermediate curing system between the two systems, namely EV and conventional. Such a system is called semi-EV. This curing system was included in this work in an attempt to achieve a useful compromise between the high mechanical properties associated with conventional curing systems (e.g., tensile strength) and low compression set and stress-relaxation mainly associated with EV systems.

* Soluble EV and soluble semi-EV systems - Soluble curing systems were included in this work because it is believed they produce vulcanizates with low rates of physical and chemical stress-relaxation. A further advantage of using soluble systems is that they produce vulcanizates with homogeneous crosslink concentrations leading to mechanical properties that are highly reproducible.

* EV system of Holmann - Holmann (ref. 6) has shown that NBR vulcanizates having low compression set and high modulus can be produced by using an appropriate combination of sulfur donors (TMTD and DTDM) in conjunction with a sulfenamide (OTOS).

Preparation of NBR mixes containing various curing systems It is noteworthy that for comparison two soluble systems were used in this work, namely MOR and MOR 90. A crossblending technique was used to minimize variations in plasticities among the six masterbatches prepared earlier. Crossblending was carried out by first mill-mixing 162 g of each masterbatch to obtain a series of nine base mixes each of mass 972 g. To each of these base mixes, the appropriate amounts of curatives shown in table 4 were added on a laboratory-sized two-roll mill operating on cools (ca. 19 degrees C) at a friction ratio of 1:1.27. Mixing times ranged between 20 and 23 minutes. After mixing, each rubber mix was cross-cut six times from each end of the coherent sheet of rubber stock revolving about the front roll. Thus, in total, each rubber mix was subjected to twelve cross-cuts at the end of each mixing cycle. Finally, each rubber mix was end-passed six times on a tight nip to ensure adequate dispersion of the curatives, Each mix was sheeted off at approximately 2.5 mm and left to cool at room temperature on a clean dust-free surface, in the dark.

Vulcanization characteristics

Vulcanization characteristics were determined on the shearing disc Mooney viscometer (SDV) and the oscillating disc rheometer (ODR) for all the various NBR mixes. Operating ments were in accordance with the appropriate British Standard documents (refs. 7 and 8). Temperatures used for the SDV and the ODR were 120 degrees + or - 0.5 degrees C and 165 degrees + or - 0.3 degrees C, respectively. A warming-up period of one minute was given to test specimens used on the SDV, just prior to the initiation of the test. On the other hand, no warming-up period was given to test specimens used on the ODR. Additional settings for the ODR are given in table 5.

Three ODR tests were performed on each NBR mix to assess the degree of dispersion of curing agents. The test specimens were cut with a circular die having a diameter of ca. 45 mm. The positions from which these test specimens were cut were front, central and end portions of each sheet of NBR mix.

Results and discussion

Cure profiles obtained from SDV and ODR tests are not shown here; but the various parameters obtained from them are given in tables 6 and 7, respectively. Seemingly, the reproducibility of the data obtained from ODR cure profiles are excellent, indicating good dispersion of curing agents.

On masticating NBR containing low levels of acrylonitrile, most of the mechanical work is expended in breaking down high molecular mass fraction (ref. 9). This breakdown of NBR (can decrease from 20 to 0 Mooney units as the Mooney viscosity increases from 40 to 80 (ref. 9). Mooney viscosities obtained for NBR mixes used in this work were between 67 and 79. However, these values probably overestimate the viscosity of the rubber phase because all NBR mixes used in this work contained carbon black filler. Presence of filler can lead to very high shear stresses during mixing and therefore considerably reduce the viscosity of the polymer phase. The Mooney viscosity of raw NBR used in this work is quoted as being 79. Comparing this value with those in table 6 clearly indicates that considerable reductions have taken place in the Mooney viscosities during mixing. Normally, higher shear stresses are imposed on test specimens in the SDV as compared to the ODR because lower test temperatures and higher shear rates are generally used for the former instrument. Disruption of various filler structures (figure 1) is therefore quite severe in the SDV. In contrast, the ODR may only be capable of disrupting secondary or weak carbon-black structures. Moreover, the Mooney test is sensitive enough to detect the effects of minute concentrations of accelerators, activators, retarders, moisture and short-stopping agents which are added in the manufacture of some synthetic rubbers. Surprisingly, in view of the above considerations, the correlation between Mr values (minimum torque values on torque-time cure profiles) and Mooney viscosity values is better than expected (figure 2). A correlation coefficient was calculated to assess the significance of the co.elation between ML and Mooney viscosity values. A value of 0.72 was obtained. Also, a statistical t value was determined to see whether there was statistical evidence of an association between Mr and Mooney viscosity; a t-value of 2.74 was obtained. Consultation of t-tables showed that the calculated t-value (for seven degrees of freedom) exceeded the value of t at the 5% probability level; thus concluding that the correlation between ML and Mooney viscosity is probably significant.

Mooney scorch time is the time required for the viscosity to rise five units above the minimum point on the Mooney torque time cure profile. This scorch time assesses the time at which crosslinking reactions begin to affect the processing behavior of rubber mixes. Hence, Mooney scorch time is an indication to the compounder as to the sensitivity and safety of a particular curing system. Results of scorch times for the various NBR mixes are given in table 6. The lowest scorch time was obtained for the mix designated sulfur donor. This observation was unexpected because the sulfur donor system contains DTDM which is said to be a scorch-delaying curing agent. Apparently, DTDM only becomes effective at temperatures of above ca. 130 degrees C (ref. 10). The temperature at which Mooney measurements were carried out was about 120 degrees C. This temperature is considerably below that at which DTDM becomes of practical use. Therefore, it is assumed that the Mooney scorch time obtained for the mix cured by the sulfur donor system was not affected by the presence of DTDM. NBR mixes containing MOR gave the longest induction periods as compared to other curing systems used in this work; the mix containing MOR 90 gave a slightly longer scorch-delay time than did that containing MOR. These observations were consistent with the claims made by manufacturer's of MOR and MOR 90 (ref. 11).

Mooney scorch results for the conventional system, and the same system with additional PVS were in agreement with those reported in literature (ref. 12). It is noteworthy that the addition of a PVS to a conventional system increases the risk of scorching at ca. 120 degrees C.

Figure 3 shows a plot of SDV cure times, t35, against ODR cure times, t (90) (estimate of the time required to insert 90% crosslinks in the ODR test specimen; assuming the difference between maximum and minimum torque represents 100% insertion of crosslinks). With the exception of NBR mixes cured by the following systems: sulfur donor, Holmann and conventional with additional PVS, a good corelation was found between t35 and t(90) values. NBR mixes containing sulfur donor and Holmann systems failed to conform to the general correlation owing to the presence of DTDM. The vulcanization-delaying nature of DTDM is apparent only when temperatures of 130 degrees C and higher are used. This is consistent with the observation that times for t(90) are much longer than that of (t.sub.35). The same reasoning can also be applied to the system containing PVS. This shows a marked deviation from the correlation shown in figure 3.

Test specimens subjected to the SDV showed no signs of rupture at the end of the test. This observation might imply that the extent of cure in these test specimens was not too high. Realizing that relatively high shear stresses are imposed on SDV test specimens, it is reasonable to expect rupture or tearing of these test specimens on attaining a high extent of cure. Moreover, a general criticism of the Mooney instrument is that the test ends at a point in time when the test specimen is just beginning to develop an appreciable modulus due to the onset of vulcanization.

In contrast, t(90) values obtained from ODR give a reasonable indication of times requited to attain a high extent of cure. One reason for this is that generally higher temperatures are used for the ODR than are for the SDV. Also, the relatively gentle low-amplitude oscillatory action of the rotor in the ODR is not as severe as the rotary action of the rotor in the SDV.

Conclusions

The results presented and discussed in this article are concerned with the curing characteristics of selected vulcanization systems used in NBR mixes. These curing systems where chosen with the intention of minimizing stress-relaxation and compression set. Both of these properties are relevant to sealing applications for which NBR is very popular. Work described in this article shows a significant correlation between Mooney viscosity and the minimum torque (determined from cure traces produced by a rheometer). The practical effectiveness with respect to vulcanization using selected novel accelerators is also discussed.

References

1. Ahmed Saeed Farid, "Development ofelastomeric materials for seals to be used in gas-distribution systems," Ph.D Thesis, Chapter III, School of Polymer Technology of the University of North London, 1989.

2. Ahmed Saeed Farid, "Development of elastomeric materials for seals to be used in gas-distribution systems," Ph.D Thesis, Chapter VII, School of Polvmer Technology of the University of North London, 1989.

3. British Standard, BS 903, Part A59 (1990).

4. U.S. Patent, 4,417,012 ('November 22, 1983).

5. CG. Cashion, Rubber Age, 65, 307 (1949).

6. W. Holmann, "Crosslinking and network formation in polymers: Materials methods and applications," Annual National Conference, Plastics and Rubber Institute, London Section (1982).

7. British Standard, BS 903, Part A58 (]990).

8. British Standard, BS 903, Part 1673. Part 10 (1977).

9. Technical Bulletin, "Krynac oil resistant rubbers," Polysar, Ontario, Canada.

10. Technical bulletin, "Cure-Rite 18 accelerator," BF Goodrich, Ohio (1985).

11. Technical bulletin, "Santocure MOR 90," Monsanto, Brussels (1980).

12. Technical bulletin, A new concept to stabilize NR properties during thermal aging and improve adhesion to brass," no. 51, Monsanto, Brussels (1984).