Influence of curing systems on the properties of bromobutyl rubber: Part III—Effect of different types of curing systems on the cure characteristics, physical properties and thermo-oxidative degradation characteristics

Polymer Degradation and Stability 36 (1992) 73-80

Influence of curing systems on the properties of bromobutyl rubber. Part lll--Effect of different types of curing systems on the cure characteristics, physical properties and thermo-oxidative degradation characteristics Naba K. Dutta & D. K. Tripathy* Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721 302, India (Received 17 January 1991; accepted 3 February 1991)

The effect of various curing systems on the properties of bromobutyl rubber vulcanizates was studied. Various curatives, e.g. conventional sulphur, efficient vulcanization (EV), semi-EV, maleimide/promoter (both mercaptobenzothiazyl disulphide and dicumyl peroxide used as promoter), resin (resin concentrations between 2 and 25 parts per hundred resin (phr)) and metal oxide systems were employed. Curing characteristics, physical properties and thermo-oxidative degradation characteristics were evaluated. Crosslink density (before and after ageing for various periods of time) was assessed by swelling measurements in cyclohexane. The results indicate that scorch safety of resin-cured systems is higher than that of metal oxide, sulphur, semi-EV and EV systems. Vulcanization with dimaleimide provides a very safe processing stock and a very high final state of cure. The strength properties before ageing are comparable among the various curing systems. Significant differences in heat resistance characteristics among the various cure systems are observed. Conventional sulphur cure, semi-EV and EV systems offer poor heat resistance characteristics at high temperature (150°C). However, phenolic resins in conjunction with zinc oxide, and bismaleimide/promoter systems, give excellent heat resistance characteristics. It is also observed that increase in phenolic resin concentration in the vulcanizate imparts better heat ageing behaviour.

group of rubbers is comparable with that of other low-unsaturated rubbers such as ethylenepropylene-diene rubber (EPDM). Curing systems with higher etticiencies than those previously known, and crosslink structures that are more resistant to ozone or oxidative attack, have been developed. Thus, to achieve a desirable combination of properties and service life, judicious selection of chemical crosslinking system is essential. Service life can be satisfactorily predicted from accelerated ageing behaviour. 1-5 The chemical crosslinking system used has a profound influence on the degradation characteristics of the elastomer. Bromobutyl rubber has considerable cure

INTRODUCTION The combination of properties that distinguishes the butyl group of rubbers from other elastomers includes exceptionally low permeability to gases and vapours and outstanding ability to absorb mechanical energy. Compared with highly unsaturated elastomers they have excellent stability on exposure to ozone, wet and dry heat, all climatic conditions and aqueous corrosive chemicals. The inherent heat resistance of this * To whom correspondence should be addressed

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 73

N. K. Dutta, D. K. Tripathy

74

EXPERIMENTAL

versatility. It produces excellent vulcanizates when cured with zinc oxide alone. However, in industrial practice, most systems contain sulphur, sulphur donor, accelerators, or phenolic resins in two-, three- or four-component combination with zinc oxide. 6 Recent studies have shown that chlorinated butyl rubber (CIIR) can be crosslinked with bis-maleimide. 7 The use of bismaleimides in the crosslinking of polymers, under either irradiation 8-1° or heat, 11,12 has become increasingly attractive because it gives heat-resistant crosslinks without reversion. Timer and Edwards 11 reported, in terms of per cent retention of tensile properties, that outstanding dry heat resistance could be achieved by curing bromobutyl rubber with zinc oxide in conjunction with sulphur donors or with dithiocarbamates together with active magnesium oxide. Recently, Edwards 13 reviewed developments in the chemistry and technology of butyl/halobutyl vulcanization systems. However, no systematic scientific investigation to optimize the technically important properties and to maximize resistance to thermo-oxidative degradation through judicious choice of chemically crosslinked systems has been undertaken. In our previous papers we have discussed the effect of concentration of curing resin on network structure, kinetics of vulcanization 14 and dynamic mechanical properties 15 of bromobutyl rubber. In this paper, the influence of various curing systems on the properties of bromobutyl rubber has been reported. Various curatives, e.g. sulphur, efficient vulcanization (EV), semi-EV, maleimide, resin and metal oxide systems, have been used.

Materials used

Details of the formulations of the mixes are given in Table 1.

BllR--Bromobutyl X2, manufactured by Polysar, Canada. Density 0.93 g c m -3, Mooney viscosity ML,+8) 100°C = 55, bromine 1.9 wt.%. ZnO--Zinc oxide was of chemically pure grade. Specific gravity 5-4. Stearic acid--Stearic acid was of chemically pure grade. FEF Black--The carbon black, fine extrusion furnace was N-550, manufactured by Phillips Carbon Black Ltd., Durgapur, India. Process oil--Paraffinic oil was a pharmaceutical grade highly saturated oil manufactured by C.D. Pharmaceutical Works, India. S - - S u l p h u r was of chemically pure grade and had a specific gravity of 1.9. MBTS--Dibenzthiazyl disulphide was supplied by ICI Ltd., Rishra, Hooghly, India. Specific gravity 1.54, melting point 167°C. TMTD--Tetramethyl thiuram disulphide was supplied by ICI Ltd., Rishra, Hooghly, India. Specific gravity 1.42, melting point 140°C. DTDM----4,4'-Dithiodimorpholine was obtained from R. T. Vanderbilt, USA. Specific gravity 1.34, melting point 123°C. DCP--Dicumyl peroxide was obtained from R. T. Vanderbilt., USA. MPD--N,N'-l,3-Phenylene bis-malemide (HVA-2) was obtained from D u P o n t , USA. Specific gravity 1.30, melting point 201°C.

Table 1. Formulations of the mixes: composition in I00 parts by weight of rubber

Mix no.

A

B

C

D

E

F

BIIR ZnO Stearic acid FEF Black Paraffin oil Sulphur MBTS TMTD DTDM DCP HVA-2 SP-1045

100 5

100 5

100 5

100 5

100 --

100 --

100 5

1 50 5 1 1.2 0.7 -. . .

1 50 5 0.5 1.2 2.0 --

1 50 5 0.5 1-2 0.7 1-5 .

1 50 5

-50 5 . 1-5 ---

-50 5

50 5

. . .

. .

.

.

.

. 1.2 0.7 2-0

.

. .

.

2.0

.

---0-2 2.0

Ro/R2/RJR10/R1JR25

1

----0/2/5/10/15/25

75

Curing systems and bromobutyl rubber properties: Part III SP-lO45---SP-1045 is a cure active phenolic formaldehyde resin obtained from Schenectady, USA. Specific gravity 1.05, melting point 54°C.

Sample preparation technique The compounds were mixed in a laboratory size (325 mm x 150 mm) mixing mill at a friction ratio of 1:1-19 according to ASTM D 3182,16 with careful control of temperature, nip gap, time of mixing and uniform cutting operation. After mixing, the elastomer compositions were moulded into ASTM test slabs of 152-4mm × 152.4 mm x 1.83 mm, to be used for determining original and aged properties. The slabs were moulded in an electrically heated hydraulic press to the optimum cure (90% of the maximum cure) using moulding conditions previously determined from the torque data obtained by means of a Monsanto rheometer (R-100). The test specimens were punched from the test slabs. Specimens for the Goodrich flexometer were vulcanized to 10 min excess of the optimum cure time.

Test procedure Physical test methods

Processing and curing characteristics of the compounded stocks were determined using a Mooney viscometer (ASTM D 1646-81) 1~ and an oscillating disc rheometer (R-100) (ASTM D 2084-81). TM Modulus, tensile strength and elongation at break were determined according to ASTM D 412-80 at 25°C, using dumb-bell specimens punched out using a type D die.19 Tear strength was determined according to ASTM D 624-81 (type C). 2° Both tensile and tear tests were carried out using a Zwick tensile UTM1445, at a crosshead speed of 500mm min -1. Hardness was measured according to ASTM D 2240-81. 21 A Goodrich flexometer conforming to ASTM designation D623-78 (method A) 22 was used to measure heat build-up and dynamic set after a specified dynamic flexing time. Heat resistance characteristics are expressed in terms of ASTM D 2000-80 criteria, z3 Fatigue-to-failure experiments were performed with a Monsanto fatigue-to-failure tester. Chemical test methods

The volume fraction (lit) in the vulcanizate was determined by equilibrium swelling in cyclo-

hexane, using the method reported by Ellis and Welding. 24 The relationship used for calculating Vr is represented by eqn (1): Vr --

(D-FT)pr

1

(D - F T ) p r 1 + AopU 1

(1)

where T is the weight of the test specimen, F is the weight fraction of the insoluble components in the sample, D is the deswollen weight of the test specimen, Ao is the weight of absorbed solvent, corrected for swelling increment, Pr is the density of rubber and Ps is the density of the solvent. If the number of effective network chains per unit volume of rubber is v then this quantity is related to V~ by the well-known Flory-Rehner equation :2s'26

-1

I n ( l - Vr) + V~+ #V~

Vs

V~/3- Vr/2

v = -- x

(2)

where v is the number of effective network chains per unit volume of rubber, V~ is the molar volume of the solvent and # is the polymersolvent interaction parameter (Flory-Huggin's interaction parameter). The quantity v is inversely proportional to the average molecular weight Mc between fixed points in the network and is taken to be a measure of the total contributions of chemical crosslinks, entanglements and filler to rubber links. The value of # changes with change in recipe and V~ according to the following relationship: # = #o + flVr

(3)

where #0 is taken as 0-443 and fl as 0-18 for the BIIR-cyclohexane system.

RESULTS AND DISCUSSION Curing characteristics The curing characteristics of the systems studied are given in Table 2. For all the systems except dimaleimide systems (E and F) results obtained from the rheograph at 150°C are reported. For mixes E and F, cure properties observed at 170°C are given. Among sulphur and sulphur donor systems ( A - D ) , the efficient vulcanization (EV) system (D) shows the highest scorch safety. Vulcanization by dimaleimide (E and F) provides

N. K. Dutta, D. K. Tripathy

76

Table 2. Curing characteristics of the mixes

A B, @150°C C, @150°C D, @150°C E, @170°C F, @170°C Ro, @150°C R2, @150°C Rs, @150°C Rio, @150°C Rls, @150°C Rzs, @150°C

Initial viscosity (dN m -1)

Minimum viscosity (dN m -1)

Scorch time (min)

Optimum cure time (min)

Cure rate (min -t)

Maximum torque (dN m -t)

22 22 22 22 19 19 22 21 20 18 16 13

17 17 16 16 13 15 18 15 14 12-5 10.5 9

2.25 2 3 4 4.5 2.5 5 7 6 6 5.5 7

20 14 19 20 32.5 31 15.5 22 21.5 20 19.5 27

7.0 8.33 6.25 6.25 3.63 3.5 9-52 6.66 6.45 7.14 7.14 5.

42 43 46 44 56 64 33 38 54 57 57 45

a very safe processing stock and a very high final state o f cure. It is important to note that dimaleimide reacts very slowly by itself, and requires the addition of a p r o m o t e r and high cure t e m p e r a t u r e to p r o d u c e a practical curing rate. Both dicumyl peroxide (DCP) and M B T S have been used as initiators for the maleimide cure systems. H o w e v e r , it is observed that dicumyl peroxide initiated maleimide cure gives scorchy stock with a higher m a x i m u m r h e o m e t r i c t o r q u e (higher crosslink density) as c o m p a r e d with that initiated by MBTS. For resin-cured systems (R2-Rz0, initial viscosity and m i n i m u m viscosity decrease with increase in resin content. T h e m a x i m u m r h e o m e t r i c t o r q u e increases progressively with resin concentration up to 10 parts per h u n d r e d resin (phr), and decreases with further increase in resin concentration. Scorch safety of resin-cured systems is higher than that of metal oxide (Ro), sulphur or semi-EV and E V vulcanization systems.

Physical properties

The physical properties of the vulcanizates are summarized in Table 3. T h e physical crosslink density obtained using the F l o r y - R e h n e r 2s,26 equation is also r e p o r t e d in this table. It is interesting to note that the tensile strength is similar for all the compositions with the various curing systems. The conventional, (A) semi-EV (B and C) and E V (D) systems exhibit comparable hardness, tear strength, elongation at break, heat build-up and dynamic set characteristics. T h e observed differences a m o n g t h e m are very small and m a y be accounted for by the differences in the crosslink density. H o w e v e r , semi-EV systems (B and C) show the longest fatigue life ( c o m p a r e d at 100% elongation). Comparison of the resin-cured systems (Ro-R2s) reveals that, with increasing resin concentration, the hardness, tensile strength and tear strength pass through a m a x i m u m , and heat build-up,

Table 3. Physical properties of BIlR vulcani~ates

Mix no.

Hardness Shore (A)

Tensile strength (MPa)

Elongation at break (%)

A B C D E F Ro R2 R5 R,o Rls R2s

55 56 56 56 61 64 50 54 58 61 60 56

12.5 13-0 13.2 13-25 12-5 11.5 10.6 12-5 12.3 11.9 11-9 10.9

620 650 625 630 320 230 490 480 380 330 380 440

Tear Heat Dynamic Strain energy density strength build-up set (%) (J m -3 × 10 6) (kN m -~) 27.3 26-4 27.3 27.5 25-9 25-3 16.7 18-3 25-7 30-6 31.0 24.2

24 24 25 25 30 32 25 26 23 21 23 25

1-9 1.6 1-5 1-30 0.50 0.45 2-22 1-26 0.55 0.50 1.39 2.57

40.25 49.92 40 38.4 17.20 11.20 28.08 31 25.4 21.12 27.36 26.5

Crosslink density xl0 s (mol cm-3)

Monsanto fatigue to failure (KC)

6.60 7.20 8.47 8.25 12.72 14.60 5-3 9.79 11.9 13.1 13.0 7.48

130.00 248.87 252.90 162.28 927.00 17.60 783.2 677-3 339.9 265.06 211.9 549.9

77

Curing systems and bromobutyl rubber properties: Part III

dynamic set and elongation pass through a minimum. The observed maximum in tear strength and minimum in elongation occur at the 10 phr level of resin (Rio), and this composition (Rio) also exhibits the highest crosslink density. An excess of resin (above 10phr), acts as a diluent and reduces the crosslink density. However, fatigue life (100% elongation) decreases monotonically with increase in resin concentration. Among the vulcanizates, maleimide cure systems (E and F) provide the greatest hardness and lowest level of elongation at break. The crosslinking efficiency is also found to be very high. However, peroxide is more efficient as a promoter than MBTS for initiating maleimide cure, and this composition (F) offers the highest level of crosslink density. It is interesting to note that composition E gives the longest fatigue life, whereas composition F gives the lowest value. In Table 3, the fatigue life of the various vulcanizates has been compared at the same strain level (A=2, 100% elongation). The elongation characteristics of the vulcanizates are found to be widely different. The fatigue life in terms of strain energy input level is represented in Fig 1. The fatigue life has been tested with dumb-bell test specimens without any deliberately inserted cuts over a range of strain 80-140% (;t = 1-8-2.4). The results are plotted as N (fatigue life in kilocycles) vs W (energy density at the maximum extension ratio, 3., which occurs during each cycle). From the fracture mechanics point of view (failure being essentially a cut growth process taking place from small flaws present in the test piece), and considering the tearing energy theory of Rivlin and Thomas, 27 Lindley and coworkers 28,29 developed a mathematical relationship between N and W. For the tensile fatigue test, it may be represented by eqn (4): G N = (n - 1 ) ( 2 k W ) " x c~ -1

'°°° I

t~

'°° F

V A

\

• El x o

B C D E

•

F

\

u 2g z

$ Q;

t~ LL

10

10

1

0.1 Strain

energy

density, J cm -3

2000 o RO •

1000

R2

A R5 • R10 n R15 • R25

tn Q; £) u

100 Z

C~ t~ LL

(4)

where k is a numerical constant and slowly varying function of strain, 3., Co is the concentration of small inherent flaws present in the specimen, G is the dynamic cut growth constant and n is a constant known as the strain exponent. The value of n is obtained from the slope of the straight line when N and W are plotted on a double-logarithmic scale. From Fig. 1, n has been calculated for all the vulcanizates,

10

2 0-1

~ Strain

1 e n e r g y d e n s i t y , J cm - 3

Fig. 1. Fatigue life as a function of strain energy.

10

78

N. K. Dutta, D. K. Tripathy Table 4. Strain exponent for various BIIR vulcanizates

Mix no.

Strain exponent

A B C D E F Ro

5-35 4.62 4-36 3-58 6.84 2-38 3-50

R2

2"81

R5 Rio R15 R25

4"37 7-17 6"01 3"44

and values are given in Table 4. It is clear that n has a strong dependence on the chemical crosslinking systems. The physical significance of n is that the higher the value of n, the longer the low strain fatigue life and vice versa. The compositions R,0 and E give the longest fatigue life at low strain level. However, the superiority of RE and F under higher strain test conditions may be predicted. Among the sulphur, EV and semi-EV systems ( A - D ) , the conventional system (A) gives longer fatigue life at low strain, and the beneficial effect of EV systems is obtained under higher strain conditions.

Resistance to heat and long-term ageing

In Fig. 2, the per cent retention of tensile strength of the various vulcanizates is shown as a function of ageing time at 150°C. The formula-

tions A - D offer lower heat resistance. However, better heat resistance is obtained with Z n O alone (Ro). Phenolic resins in conjunction with zinc oxide, and bismaleimide/promoter systems, give excellent heat resistance characteristics. It is also observed that increased phenolic resin concentration imparts better ageing behaviour. Figure 3 depicts the retention of percentage elongation at break with ageing time. For sulphur/sulphur donor systems (compositions A - D ) there is a decrease in elongation initially. However, after 72 h ageing, elongation at break increases. For resin-cured systems (RE-Rz0 a monotonic decrease in retained elongation has been observed. The slope of the line, however, decreases with time of ageing. For maleimide/promoter systems (E and F) there is a decrease in elongation to 24 h ageing, after which it remains practically constant. The retained cohesive energy density is plotted against ageing time in Fig. 4. Composition F clearly gives the best retention characteristics. Even after ageing for 7 days at 150°C, the retention is 55% of the initial value, whereas R,o and E exhibit 45 and 40% of the original values respectively. Sulphur and sulphur donor systems show very poor results for similar conditions of ageing (5% of the original value is retained). The loss of the mechanical properties during oven ageing of vulcanizates is mainly due to oxygen attack at the reactive sites on the surfaces of the rubber vulcanizates (which may cause both crosslinking and scission of the polymer chains). The initial crosslink density and the subsequent ratio of oxidative crosslinking to oxidative chain scission are the main factors which control the

100

"3100~

,:8o

=

.

I

40-

c-

•

-~ 20 0

24

48

72

96

120

1~4

168

Time of ageing at 150°C, h

Fig. 2. Per cent retention of tensile strength of various vuleanizates aged at 150°C with time of ageing. Key as for Figs. la and lb.

0

I 24

I 48

I 72

I 96

I 120

I 144

I 168

Time of ageing at 150°C, h

Fig. 3. Retention in per cent elongation for various vulcanizates with ageing time. Key as for Figs. la and lb.

79

Curing systems and bromobutyl rubber properties: Part III 100,

8o

-~

60

4o

f, ~ 2o c

nO

I

I

I

I

I

I

I

24

48

72

96

120

144

168

T i m e of ageing at 1 5 0 " C , h

Fig. 4. Plot of cohesive energy density retained for various vulcanizates as a function of ageing time. Key as for Figs. l a and lb.

oven-ageing characteristics. In Fig. 5 the crosslink density (derived from swelling measurements) of various vulcanizates has been shown as a function of ageing time at 150°C. In the early stages, the network structure is found to increase because of post-vulcanization. Thereafter, oxidative degradation predominates and the effective network density declines. It is clear that resin cure and maleimide cure systems show the best retention in crosslink density. A severe drop in crosslinking within 4 0 h of ageing has been observed in the case of sulphur or sulphur donor vulcanization systems. This result corroborates that obtained for retention of physical properties on ageing (as observed from Figs 2 - 4 and Table

5). REFERENCES

m, 2 o E u

(.)

0

24

48

72

96

120

144

168

T i m e o f a g e i n g at 1 5 0 ° C , h

Fig. 5. Crosslink density of various vulcanizates as a function of ageing time. Key as for Figs. l a and lb.

1. Bevilacqua, E. M., In Thermal Stability of Polymers, ed. R. J. Conley. Marcel Dekker, New York, 1970. 2. Caps, R. N., Rubber Chem. Technol., 59 (1986) 103. 3. Shelton, J. R., In Stabilization and Degradation of Polymers, eds D. L. Allara & W. L. Hawkins, Adv. Chem. Ser. (1978) p. 169. 4. Saha Deuri, A. & Bhowmick, A. K., Mater. Chem. Phys., 18 (1987) 35. 5. Saha Deuri, A. & Bhowmick, A. K., Polym. Deg. and Stab., 16 (1986) 221. 6. Gunter, W. D., In Development of Rubber Technology, Vol. 2, ed. A. Whelan & K. S. Lee, Applied Science Publishers, Barking, Essex, 1981. 7. Ho, K. & Steevensz, R., Rubber Chem. Technol., 62 (1989) 43. 8. Miller, S. M., Roberts, R. & Vale, R. L., J. Polym. Sci., 58 (1962) 737. 9. Heinzl, J. & Heusinger, H., Angew, Makromol. Chem., 1117 (1982) 191. 10. Krashennikov, N. A., Prashchikina, A. S., Fel'dshtein,

Table 5. Variation of hardness and modulus at 100% elongation with ageing for various composites Modulus at 100% elongation (MPa)

Hardness

A B C D E F Ro R2 R5 Rio Rt5 R25

0h

24 h

72 h

120 h

168 h

0h

24 h

72 h

120 h

168 h

55 56 56 56 51 64 51 54 58 61 60 56

45 46 46 47 60 62 53 59 63 69 70 66

40 41 41 41 60 62 45 60 64 68 70 70

38 39 39 40 57 60 42 57 63 69 71 73

35 37 37 38 55 59 41 54 62 69 73 77

1.5 1.40 1-42 1.35 2-60 3.20 1.22 1.71 2.27 2.46 2.45 1.86

1-0 0-92 0.92 0-85 2.80 3.20 1.48 2-25 2-95 3-92 3-91 3.98

0"50 0-50 0-55 0.53 2.50 2.9 1-06 2.97 2.87 3.99 4.20 4.67

0.35 0'35 0.40 0.40 2.20 2-7 0.9 1.7 2.9 4.1 4.7 4.9

0-25 0.30 0.35 0.35 1.90 2.63 0.74 1"60 2-99 4-43 5-65 5.89

80

11. 12. 13. 14. 15. 16. 17. 18.

N. K. Durra, D. K. Tripathy M. S. & Kaplunov, M. Ya., Int. Polym. Sci. Technol., 1 (1974) 57. Krashennikov, N. A., Prashchikina, A. S. & Fel'dshtein, M. S., Int. Polym. Sci. Technol., 2 (1975) 68. Timer, J. & Edwards, W. S., Rubber Chem. Technol., 52 (1979) 319. Edwards, D. C., Elastomerics, March (1990) 19. Dutta, N. K. & Tripathy, D. K., Kautsch. Gummi Kunstst., 43 (1990) 880. Dutta, N. K. & Tripathy, D. K., Polym. Deg. Stab., 30 (1990) 231. ASTM D 3182-74, Rubber materials, equipments and procedure for mixing standard compounds and preparing standard vulcanized sheet. ASTM D 1646-81, Rubber viscosity and vulcanization characteristics using mooney viscometer. ASTM D 2084-81, Rubber properties-vulcanization characteristics using oscillatory disk couremeter.

19. ASTM D 412-80, Rubber properties in tension. 20. ASTM D 624-81, Rubber property: tear resistance. 21. ASTM D 2240-81, Rubber property: Durometer hardness. 22. ASTM D623-78, Rubber property, heat generation and flexing fatigue in compression. 23. ASTM D 2000-80, Rubber products in automative applications. 24. Ellis, B. & Welding, G. N., Rubber Chem. Technol., 37 (1964) 571. 25. Flory, P. J. & Rehner, J. Jr, J. Chem. Phys., 11 (1943) 512. 26. Flory, P. J., J. Chem. Phys., 18 (1950) 108. 27. Rivlin, R. S. & Thomas, A. G., J. Polym. Sci., 10 (1953) 291. 28. Gent, A. N., Lindley, P. B. & Thomas, A. G., J. Appl. Polym. Sci., 8 (1964) 455. 29. Lake, G. J. & Lindley, P. B., J. Appl. Polym. Sci 8 (1964) 707.