The influence of curing systems on the properties of bromobutyl rubber: Part 2—Effect of concentration of curing resin on the dynamic mechanical properties

Polymer Degradation and Stability 311 (1990) 231-256

The Influence of Curing Systems on the Properties of Bromobutyi Rubber: Part 2 Effect of Concentration of Curing Resin on the Dynamic Mechanical Properties N a b a K. Dutta & D. K. Tripathy Rubber Technology Centre, Indian Institute of Technology, Kharagpur 72! 302, India (Received 24 November 1989; accepted 8 December 1989)

A BSTRA CT Measurements of isochronal dynamic mechanical properties of cured, uncured and thermally aged rubber vulcanizates have been carried out using a Rheovibron DD V-III-EP elasticoviscosity meter as afunction of temperature between - 1 5 0 ° C and +250°C. The effects of curing resin (phenol/ formaldehyde[resole] resin; SP-1045 ) concentration and time of heat ageing on different relaxation transition characteristics and dynamic mechanical properties have been discussed. It has been observed that the molecular mobility is not affected, but only the sensitivity of E ( in-phase modulus) in the dispersion region is affected by resin content and time of ageing. With increase in resin content in the vulcanizate, on ageing, the crosslinking reaction becomes more predominant over degradation of polymer chains and scission of crosslinks.

INTRODUCTION The butyl group of elastomers is characterized as materials having low gas and moisture permeability, high resistance to ozone, weather, abrasion, tear, flexing, heat ageing and chemical attack. Vulcanizates from this group of elastomers combine greater dynamic softness with excellent shock absorption than is usually found with other elastomers. Bromobutyl (BIIR) vulcanizates are normally acoustic loss materials having essentially the same physical and dynamic mechanical properties as regular butyl rubber. They 231 Polymer Degradation and Stability 0141-3910/90/$03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

232

Naba K. Dutta, D. K. Tripathy

have the advantage of rapid rate of cure with reduced curative levels, cure compatibility with other polymers, and good cured adhesion to themselves and other elastomers. 1-3 This combination of properties makes them potentially superior base elastomers for the manufacture of a variety of mechanical goods ranging from heavy duty shock absorbers to smaller sound and vibration damping mountings, hoses, belting, proofed goods, and many other tire and non-tire applications. Under any particular set of experimental conditions, processing, physical and dynamic mechanical properties are dependent on the type and loading of the filler, 4-13 the type and level of plasticizer, l 4- ~7 as well as on the type and amount of curatives. 15'1a'19 Type and level of a suitable chemical crosslinking system for compounding must be selected very carefully to achieve the desired combination of properties and service life, which could be well predicted from ageing behavior. Extensive studies on the ageing properties of natural rubber compounds have previously been carried out.2o-22 Comparatively little attention has been devoted to the ageing of synthetic rubber. 23 This paper is one of a series on the effect of curing systems on the processing, physical, dynamic mechanical and ageing characteristics of BIIR vulcanizates. It describes the effect of curing resin (phenol/formaldehyde [resole] resin) concentration on the dynamic mechanical properties of BIIR vulcanizate. Results over a wide range of temperature, frequency and strain amplitude have been discussed. Tawney and Little 24 first reported that vulcanizates of butyl rubber with phenol/formaldehyde (resole) resins give superior oxidation and temperature resistance characteristics compared to that of sulfur cures. Details about the curing conditions and physical properties were discussed by Tawney e t al. 2s The mechanism of phenolic resin vulcanization has been discussed in detail by Lattimer et al. 26 These resins give elastomer cure with very highly thermo-stable crosslinks.

EXPERIMENTAL Materials

Details of the compounding formulations of the mixes are given in Table 1. The bromobutyl rubber (BIIR) was Bromobutyl X2, manufactured by Polysar, Canada. Density (Mg/m 3) 0.93; Mooney viscosity ML(1 + 8) 100°C--55; Bromine (wt%)---1.9. The plasticizer was a highly saturated pharmaceutical grade paraffinic plasticizer, supplied by C. D. Pharmaceutical, Calcutta, density (Mg/m3)---0.82. Carbon black--N-550 was manufactured by Phillips Carbon Black Limited, Durgapur, India. Zinc Oxide

Influence of curing on bromobutyl rubber

233

TABLE 1 Compounding Formulations

Mix designation BIIR Carbon black (N-550) Plasticizer Zinc oxide Stearic acid SP-1045

Ro

P2

Ps

Plo

PI5

P25

100 50 5 5 1 0

100 50 5 5 1 2

100 50 5 5 1 5

100 50 5 5 1 10

100 50 5 5 1 15

100 50 5 5 1 25

had a specific gravity of 5"55 and stearic acid was manufactured by E. Merck (India) Ltd, Bombay. SP-1045 Resin [phenol/formaldehyde resin] was manufactured by Schenectady, USA. Specific gravity--l'05, melting point--63°C.

Sample preparation The experimental compounds were mixed in a laboratory size (32-5cm x 15cm) two roll mixing mill according to ASTM D 3182-74. Utmost care was taken to achieve good dispersion of the filler. After mixing, the elastomer compositions were molded into a test slab (100 m m x 100 mm x 3 mm) to be used for determining dynamic mechanical properties. The slabs were moulded in an electrically heated hydraulic press using moulding conditions previously determined from the torque data obtained using a Monsanto Rheometer (R-100) at 150°C. For each composition, a long plateau cure characteristic was observed and a 30 min cure was given to all the test slabs at 150°C. The test specimens were punched out from the cured test slabs.

Test procedure A Rheovibron (DDV-III-EP) was used for dynamic mechanical analysis. The equipment has computerization and automation facilities both in operation and data processing, which provides a programmed heating rate and high measurement accuracy and sensitivity. Details of the equipment are discussed elsewhere. 27 Dynamic mechanical properties were measured from - 1 5 0 ° C to +250°C. Temperature control of the sample was restricted to a programmed increase of 1°C per minute from the lowest temperature to the highest, whenever results were scanned isochronally over a wide range of temperature. The dynamic strain amplitude was varied between 0.7 × 10-3

Naba K. Dutta, D. K. Tripathy

234

DSA (0-07% dynamic strain) to 50 x 1 0 - 3 DSA (5% dynamic strain). Double strain amplitude (DSA) refers to the ratio of peak to peak deformation (total excursion path) of the sample to the length of the sample (2AL/L). The frequency range was 3.5 to 110 Hz. For a particular sample, tests were carried out first at the lowest available strain and later the strain was increased step-wise to the maximum. This is very important because dynamic mechanical properties of filled vulcanizates are affected by prior deformation.

RESULTS AND DISCUSSION

Dynamic properties of the uncured compositions Figure 1 shows the continuous spectra of isochronal loss tangent, tan6, at 11 Hz for uncured BIIR composites containing different levels of resin SP1045, over the temperature range -150°C to +250°C. For all the resin mixed compositions four distinct peaks are clearly observed. They are designated as fl, ct, ~' and ~" respectively. For the composition without resin three peaks are seen (the ~' peak is not observed). Details about the magnitude and location of these peaks are given in Table 2. The low temperature transition (fl transition) in the vicinity of - 8 0 ° C may be attributed to the motion of the methyl groups directly attached to the backbone of BIIR. 2s It is clearly observed from Table 2 that resin concentration has no effect on the fl peak intensity and temperature. The peak is located in the transition region between the glassy state and the rubbery state. It is generally identified as the glass transition temperature. However, it is important to note that the tan6 peak temperature (T,~,,x)is not TABLE 2 R e l a x a t i o n T r a n s i t i o n T e m p e r a t u r e for U n c u r e d B I I R Resin C o m p o s i t e s - - t a n 6 Peak Location

Mix

fl Peak

~ Peak

ct' Peak

~" Peak

no.

Temperature Value Temperature Value Temperature Value Temperature Value

(oc)

R0 R2 R5 R 1o Ri5 R25

- 80 -80 -80 - 80 -79 - 80

(oc)

0"045 0-046 0"044 0"040 0'043 0'400

- 30 -31 -30 - 31 -31 - 31

(oc)

0'947 0-890 0-874 0-774 0"656 0-580

-45 45 44 46 48

(oc)

-0'315 0'415 0'480 0'500 0"630

155 155 160 156 152 150

0-85 0"89 0"90 1"06 1"20 1"35

Influence of curing on bromobutyl rubber

235

I

10

//

(g 10 0

t

z z

q -1 10

10 -2

10 .3

[

I -100

L

~ 0

I

TEMPERATURE

J + 100

I + 200

~ °C

Fig. 1. Dynamic mechanical spectra of uncured BllR-resin compositions; Effect of resin concentration on tan6. Frequency 11 Hz; strain 0-1%; rate of temperature rise, 1~C/min. , R o ; - - - , R 2 ; . . . . , Rs; . . . . . . . , R i o ; - - - - , R l s ; , R2s.

identical to Tg, determined by dilatometry or calorimetry because it is a function of the frequency of the dynamic test. The resin concentration does not affect the T~aX temperature; however, the intensity of absorption (tan6 peak height) decreases with increase in resin concentration. The observed higher temperature maximum (45°C) may be attributed to the melting point of the resin SP-1045. As expected, the ~' peak temperature is not affected but the ~' peak height increases with increasing resin concentration. No ~' peak is observed for the composition without resin. At still higher temperature the

236

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trl

=, o 0"s"

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TEMPERATURE ~ =C

Fig. 2. Dynamic mechanical spectra of uncured BIIR-resin compositions; Effect of resin concentration on E". Conditions and key as Fig. 1. composites enter into the zone o f thermal flow and further increase in the tan6 value is observed. However, as soon as the crosslinking reaction starts at higher temperature the thermal flow is restricted and with progressive increase in crosslink density the tan6 value decreases. Thus, tan6 peak ~" is obtained. The temperature and intensity of 0t" relaxations are given in Table 2. Figure 2 illustrates the out-of-phase modulus, E", for the compositions. The out-of-phase modulus curves are similar to the tan6 curves for all the compositions. Each shows a broad transition at about - 80°C which is the fl transition. The major peak in E" is also considered to

Influence of curing on bromobutyl rubber

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Naba K. Dutta, D. K. Tripathy

238 10 4

10 3

Q 0

~E

102 ', C

~J

0

101

1 00 -100

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TEMPERATURE ~ *C

Fig. 3. Dynamic mechanical spectra of uncured BIIR-resin compositions; Effect of resin concentration on E'. Conditions and key as Fig. 1. be the glass transition which occurs in the vicinity of - 5 8 ° C for all the compositions. Details of the values and locations of the various peaks are given in Table 3. It is commonly observed that E" and tan6 do not peak at the same temperature for m a n y polymer transitions. Comparing Tables 2 and 3, a difference o f ~ 27°C in the peak locations for the ct transition (glass-torubber transition) for BIIR is observed. This causes confusion in assigning precise locations to transitions. The three parameters E', E" and tan6 are related by tan6 = E"/E'. If E' were independent o f temperature, then the

Influence of curing on bromobutyl rubber

239

position of the maxima in tant5 and E" would be identical. However, E' always changes in a dispersion region, and so how nearly the locations of the tan6 and E" peaks match will depend on how much E' changes. For the fl transition, the E' drop is very small. Thus, both tan6 and E" peak at the same temperature (-80°C). For the ~ transition, E' drops by roughly a factor of 10 a and this skews the relation between tan6 and E" so their peaks occur 27°C apart. Similarly, for the ~' transition, E' drops by roughly a factor of 10 only. Thus, tan6 and E" peak only 5°C apart. Thus, E" is the more fundamental parameter and would be more indicative of shifts related to changes in molecular motions. This has also been observed and discussed by Locke and P a u l . 29 Figure 3 shows the continuous spectra of isochronal storage modulus, E', at 11 Hz for all the uncured experimental compositions. From the nature of the curve, characteristic regions may be easily seen. They are respectively A, glassy region; B, transition region; C, rubbery plateau before melting of the resin; O, the cross-over point or the melting point of the resin; D, the rubbery plateau after melting of the resin; E, high temperature region, where crosslinking of the polymer chain takes place. In the glassy and transition regions E' values of the different composites are comparable. However, as it approaches the rubbery region, marked differences in E' values between different compositions are observed. E' at the temperature at which tan6 reaches a minimum in the rubbery region (inflexion point) has been described as plateau modulus by Kraus and Rollmann. 3° Applying the same method, two plateau values, one before the melting point of the resin (E~, at ,~ 22°C) and another after the melting point of the resin (E °,,~ 80°C) are obtained. Table 4 clearly demonstrates that E ° increases with increased phenolic resin content. A resin which is incompatible with the polymer is expected to increase the modulus. However, the E ° value decreases with increase in resin content and the cross over point is observed at 55°C. At TABLE 4 Viscoelastic Properties of Uncured BIIR--Resin Composites

Mix

tan6 (rain) Temp (°C)

E' (tan6 rain), MPa

no,

tan6 (mini) Ro R2 R5 Rio R 15 REs

-22 22 22 22 23

tanf(min2) 78 80 79 81 80 79

E°

E°

5'0 12'3 12"9 16"2 25"2 45"7

3"73 3' 15 2"60 2"50 2"3 2"0

240

Naba K. Dutta, D. K. Tripathy

higher temperature an increase in the E' value is observed due to crosslinking for all the compositions.

Dynamic mechanical properties of cured compositions Figure 4 illustrates the E' spectra for the compositions after vulcanization. The characteristic glassy, transition and rubbery plateau regions are observed. As expected, unlike uncured compositions, the cured compositions show only one plateau in the rubbery region. Table 5 compares E' at 10 4

__

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100

I -100

I

I

0

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I

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÷200

TEMPERATURE ~ °C

Fig. 4.

Dynamic mechanical spectra of cured BIIR-resin compositions; Effect of resin concentration on E'. Conditions and key as Fig. 1.

241

Influence of curing on bromobutyl rubber TABLE 5 Comparison of Mechanical Behaviour at Different Characteristic Zones Composition

E' (MPa) - 130°C (MPa x 10 -3)

-30°C (Mpa)

25°C (Mpa)

IO0°C (Mpa)

9"4 9'25 9'46 9"43 9'22 9"0

95 108 160 205 234 319

8"3 8"9 9'8 10"8 13"2 14"0

3"55 3"80 4'53 5"15 4"24 3-61

Ro R2

R~ Rio Rts R25

four temperatures, representative of the characteristic glassy, transition, room temperature and high temperature plateau regions. At low temperature, E' values among different vulcanizates are quite comparable whereas in the transition region they are different and E' increases considerably as the resin content is raised. At 25°C the E' value increases with resin content but the difference among the vulcanizates is quite small. However, in the high temperature plateau region the E' value increases with resin content up to the 10 phr level while above that level the plateau 2O

*~

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1 18

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f

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20

22

24

26

28

30

32

34

1

36

4

T } RECIPROCAL TEMPERATURE (xlO ) Fig. 5.

Complex modulus versus reciprocal absolute temperature. C), Ro; 0 , R2; A, R~; A, Rio; IS], Rls; II, R2s.

242

Naba K. Dutta, D. K. Tripathy

2xlO 3 103 -

102

/

g 14J

3 0 0

I 101 _

B

100 4 xl0 -1 ~

~ " " 0 +100 +200 TEMPERATURE~°C Dynamic mechanical spectra of cured BllR-resin compositions; Effect of resin Fig. 6. concentration on E". Conditions and key as Fig. 1. -100

modulus value decreases with resin content. This may be explained by the fact that initially, with increase in resin content, the crosslink density increases, and above the 10 phr level the excess resin or its self condensation products increase the thermal sensitivity of the vulcanizate.31 To observe the temperature sensitivity and to measure the apparent heat of formation of hard zone, the logarithm of the complex modulus (E*) is plotted against 1/T as described by Payne 4 (Fig. 5). It is observed that the modulus is temperature dependent and follows a linear relation up to 100°C. Above that level E* becomes progressively less temperature dependent and eventually levels off. The E* value at which it levels off is dependent upon the resin content of the particular composition. The apparent heat of formation

Influence of curing on bromobutyl rubber

243

101

10 o o

Z V~

o. -1 10

-2

10

1 0 "3

I - 100

0

I

1

+ 100

I

1

+ 200

TEMPERATURE 7 °C

Fig. 7.

Dynamic mechanical spectra of cured BllR-resin compositions; Effect of resin concentration on tant$. Conditions and key as Fig. 1.

of the hard zone from soft zone 4 has been calculated by using Vant Hoff's isochore, din k/d t = - H / R T 2, where H is the heat of formation and R and T have the usual significance. The corresponding heats of formation, calculated from the slope of the line in Fig. 5, are given in Table 6. Figures 6 and 7 depict the temperature dependence of E" and tan6 for the cured compositions. The loss modulus and tan6 curves are similar for all the compositions. Each shows a broad transition at about - 80°C (as observed in uncured compositions) which is the fl transition. The intensity and location of the fl peak are independent of resin content. The major peak (~ transition)

Naba K. Dutta, D. K. Tripathy

244

TABLE 6 Apparent Heat of Formation of Hard Zone Mix no.

Heat of formation (kCal/mole; Minus)

R0 R2 Rs Rio Rl5 R25

0-84 0"90 0.98 1.08 1'30 1'40

in E" occurs in the vicinity of - 5 7 ° C and is independent of the concentration of the resin as was obtained in the case of uncured compositions. However, the tan6 peak location shifts towards higher temperature concomitant with a decrease in peak intensity with increased resin level. Oswald and K u b u 32 arbitrarily defined the glass-transition temperature as the temperature where E' = 100 MPa. Transition temperatures determined by applying these various criteria to Figs 4, 6 and 7 are summarized in Table 7. As discussed above, E" may be considered as a more fundamental criterion to molecular mobility and it may be concluded that changes in resin content and crosslink density do not affect the molecular mobility of the polymer. Only the sensitivity of E' in the dispersion region changes. A b o v e 200°C rapid increases in E" and tanfi are observed for composition R o. F o r all other compositions tang and E" values decrease 15

no ~r ~LI.I

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-

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-

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Fig. 8.

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10-3

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I 10-2

STRAIN

AMPLITUDE

I

I

I J Ill 10-1

~

2AL L

Storage modulus as a function of strain amplitude. Frequency 11 Hz; temperature 25°C. Key as Fig. 5.

Influence of curing on bromobutyl rubber

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Naba K. Dutta, D. K. Tripathy 0.5

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o, 0"2

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I

,

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~ t I=[

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10-3

J

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I

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10-2

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10-1 2,~L L

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14

no Z ~w

12

10 3 Q 0 Ig

8 IIC u~

6

4

0

I

I

I

L

20

40

60

80

, I

100

120

FREOUENCY~ HZ

Fig. I0.

Storage modulus as a function of frequency. Strain 5 %; temperature 25°C. Key as Fig. 5.

Influence of curing on bromobutyl rubber

247

0.8

'~ 0.6 o

z

~0"4 Z q o.~ 0

I

I

1

1

1

20

40

60

80

100

120

FREOUENCY~ HZ

Fig. I 1.

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monotonously. This may be attributed to the poorer thermal stability of conventional metal oxide cure systems than resin cured systems. The change of E' with strain for the vulcanizates is shown in Fig. 8. The typical sigmoidal change of E' with strain is apparent for all the vulcanizates. It appears from the figure that the strain dependence of E' increases with increasing concentration of resin. Figure 9 shows the change of tan6 with strain. The curves for all the vulcanizates are similar in nature. Initially, there is a decrease in the tan6 value as resin concentration increases between 0 phr and 5 phr. However, above that level tan6 increases with increase in resin content. Figure 10 shows the effect of resin level on the frequency dependence of E'. The sensitivity of E' of the compositions towards frequency increases with increasing resin content. However, frequency dependency of tan6 is not affected by the resin concentration, Fig. 11, and the curves for different vulcanizates appear to be approximately parallel over the experimental range of frequency.

Effect of ageing on dynamic mechanical properties Figures 12-17 illustrate the effect of ageing on the dynamic properties of the cured compositions. Ageing of the samples was carried out in a multicell ageing oven at 150°C for different time periods. Ageing does not affect the fl peak location and magnitude (true for both tan6 and E"). The effects of ageing time on the temperature position of the peaks are as follows: (i) The E" peak occurs at the same temperature ( ,-~57°C) irrespective of the time of ageing and resin concentration. (ii) With time of ageing, the tan6 peak location shifts regularly to higher temperature concomitant with increase in

248

Naba K. Dutta, D. K. Tripathy

#,

0

6 6 6 6 o 6

[-

Y. ~0

.=. t~

<

~d

Y.

~o

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Influence of curing on bromobutyl rubber 10

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/•

a(

1

/

103

10 0

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g_

.../

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~W Ul

=, 0

249

l/

c,o

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102

10-1

101

10-2

LU iD a:


0

\ 100

I - 100

I 0

I

TEMPERATURE

I ~ + 100

]

I + 200

lo-3

~ °C

Fig. 12. Effect o f heat ageing on the dynamic mechanical properties of the R 0 composition. Ageing temperature 150°C; frequency 11 H z ; s t r a i n 0 . 1 % ; - - , 0 d a y ; - - - - - , 24 h; - - , 72 h; . . . . . . , 168h.

peak width and decrease in peak height. Table 8 shows in detail how the tan6 peak location, tan6 peak height and temperature at which E' = 100 MPa change with time of ageing. Table 9 compares the storage moduli at three representative temperatures. In the glassy region ( - 130°C) ageing has no significant effect on E'. However, in the transition region E' increases with time of ageing for all the compositions. In the high temperature rubbery region the magnitude of change is dependent on the resin concentration. For the R o composition, E'

104

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Fig. 13.

Effect of heat ageing on the dynamic mechanical properties of the R 2 composition. Conditions and key as Fig. 12. TABLE 9 Effect of A g e i n g on Mechanical Behaviour

Mix

E'× I O - 3 M P a a t - 1 3 0 ° C

E' M P a a t - 3 0 ° C

E' MPa, at lO0°C

nO.

Ro R2 R5 R10 Rls R25

Oh

24h

72h

168h

Oh

24h

72h

168h

Oh

24h

72h

168h

9"40 9"25 9"46 9'43 9"22 9"20

9"5 9"30 9'59 9"04 8"85 8"90

9"4 9"5 9"41 9"1 9"23 8"87

8"8 8'8 9"49 8"9 9"02 8'93

95 108 160 205 234 329

132 152 215 255 324 477

152 176 220 252 361 420

179 170 234 289 379 572

3-6 3'8 4'5 5"1 4"2 3"6

3'4 4"1 5"5 5"5 6'6 6-0

2"6 3"6 4"7 6"3 7'5 6-3

2'3 2'6 4"2 7"8 8"0 8-4

Influence of curing on bromobutyl rubber 104

~

251

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10 0

10 3

w

.

E

. ~ . . . .

.,-

v~ z hi O

.J r~ 0

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10 2

z

h,

o

rr 0

-2

10 1

10

1o 0 ~

io-3 -100

0

÷I00

+ 200

TEMPERATURE , °C

Fig. 14. Effect of heat ageing on the dynamicmechanicalproperties of the Rs composition.

Conditions and key as Fig. 12. decreases with ageing, initially slowly and then rapidly with time of ageing. In the case of the g 2 composition, the E' value in the rubbery plateau region initially increases, then decreases with time of ageing. The increase in E' indicates increase in crosslink density. Compositions with 10phr or more of SP-1045 show only increase in E' value with ageing. Above the 10phr level the principal effect of increasing the resin content is to increase the rate of increase of E' with time of ageing. Thus, as the resin concentration increases on ageing, crosslinking becomes more prominent

Naba K. Dutta, D. K. Tripathy

252 10 4

10 1

10 3

10 0

6O C= O

~LU f

a 0

I'-Z t/d

....

-1 10

10 2

Z

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<

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',,, "'..

lO-2

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~o o

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I

I

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TEMPERATURE ~ °C

Fig. 15.

Effect of heat ageing on the dynamic mechanical properties of the R1 o composition. Conditions and key as Fig. 12.

than degradation. This is also observed from tang curves. The tang value in the rubbery zone increases with time of ageing (Fig. 12-17). The temperature at which tang = 1 (E' crosses E" and drops below it) at the end of the rubbery plateau is used as an indication of entry to the zone of terminal flow. 33 It is observed from Fig. 12 that for Ro the location of the inflexion point O is unchanged; however, the temperature of terminal flow shifts toward lower temperature with time of ageing. This indicates that with

Influence of curing on bromobutyl rubber

253 101

10 l'

10 3

10 0

o~

o

~r E . f

..:

3 Q 0

.....

z

. ...... _ . _ -11

10 2

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z

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101

1o 0

I -100

I

I 0

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I

I

+100

[

1o-3

+ 200

TEMPERATURE ~ °C

Fig. 16. Effect of heat ageing on the dynamic mechanical properties of the R t s composition. Conditions and key as Fig. 12.

time of ageing, degradation of the polymer chain and scission of crosslinks take place, which cause lowering of the terminal flow temperature (because fewer entanglement and crosslinks will provide less resistance to flow). The R 2 composition also shows the same characteristic feature. For compositions with resin contents of 5 phr or more, no such terminal flow is observed and the rate of increase in tan6 with time of ageing decreases as resin content is increased.

254

Naba K. Dutta, D. K. Tripathy 10 4

10 1

10 3

10 0

g. tn ._1 a o

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Fig. 17. Effectof heat ageing on the dynamic mechanical propertiesof the R 2s composition. Conditions and key as Fig. 12. Figure 18 shows the details of how E' and tan6 change with time o f ageing for different compositions, when the experiment is carried out at higher strain, 5% DSA. The higher amplitude test is more important as far as practical utility is concerned. It is clearly observed that the effect is similar to that for low amplitude tests. The rate o f increase of E' with ageing increases with increase in resin content. Rate of increase o f tan6 decreases with resin content. It is observed that for compositions having resin contents o f more than 10 phr, the tan6 value remains practically unaltered with time of ageing.

Influence of curing on bromobutyl rubber

255

12

~o

"-I

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AGEING

Effect of heat ageing on the dynamic mechanical properties of B11R compositions. Frequency 11 Hz; strain 5%. Key as Fig. 5.

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