HEXAMETHYLENE-N,N’BIS(TERT-BUTYL PEROXYCARBAMATE) AS A
CROSSLINKING AGENT FOR NATURAL RUBER AND NATURAL
RUBBER/POLYBUTADIENE BLEND
D.S. OGUNNIYI*
DEPARTMENT OF CHEMISTRY,
UNIVERSITY OF ILORIN,
ILORIN, NIGERIA
And
C. HEPBURN
INSTITUTE OF POLYMER TECHNOLOGY AND MATERIAL ENGINEERING
UNIVERSITY OF TECHNOLOGY,
LOUGHBOROUGH, LE11 3TU. UNITED KINGDOM.
ABSTRACT
The results of technological evaluation of hexamethylene-N,N’bis(tert-butyl
peroxycarbamate) (HBTBP) as a crosslinking agent for natural rubber and natural
rubber/polybutadiene rubber blend are presented. Similar studies were carried out
using sulphur and peroxide systems as controls. The cure characteristics were
evaluated with the Monsanto Oscillating Disc Rheometer while tensile sheets were
prepared and stress-strain measurements in simple extension were used to
characterize the mechanical properties of the vulcanized elastomers. It was found that
the properties obtained with the various systems decrease in the following order:
sulphur > peroxide > HBTBP. The cure mechanism of HBTBP in natural rubber and
the blend was explained as occurring thorough the free radical process.
Key Words: natural rubber, polybutadiene, blend, crosslinking/vulcanization, HBTBP.
*CORRESPONDING AUTHOR
INTRODUCTION
A major advancement to the use of rubber was the discovery of vulcanization by
Charles Goodyear in the U.S. in 1839 and, by Thomas Hancock in the U.K. in 1844 (1).
Since the time of Goodyear and Hancock, vulcanization has remained an important
area of rubber compounding technology.
From a historical perspective, vulcanization is the process of heating rubber, sulphur
and white lead (2). Crosslinking, which is the process by which a network polymer is
formed, is also referred to as vulcanization and/or curing. It must be noted however,
that curing is also applied to processes which may not involve crosslinking since curing
is defined as changing the properties of resin or rubber formulations by chemical
reaction usually under the action of heat and varying conditions of pressure (3,4).
Sulphur remains the most widely used vulcanizing agent for polydienes including natural
rubber (4). For example, in accelerated sulphur vulcanization, there is greater control
over the process especially by minimizing modulus and tensile strength reversion so
that vulcanizates with specified physical properties are readily obtained. Also, the
physical appearance and physical properties are good in accelerated systems. In
addition to sulphur vulcanization systems, rubbers can be crosslinked by peroxides.
Peroxide vulcanization of rubber has been reviewed recently in the literature (5,6).
Some obvious advantages of peroxide vulcanization are non-reversion during cure and
good ageing resistance when long cure times are employed. Another characteristic of
peroxide system is that saturated rubbers (e.g. silicone rubber) can only be crosslinked
by peroxides. When diene-containing rubbers are crosslinked by peroxides, the
properties of vulcanizates obtained are inferior to those of vulcanizates cured by
sulphur. Nevertheless, peroxide vulcanizates of these diene rubbers, and indeed of
other rubbers may be desirable in applications where creep resistance is required.
Crosslinking is also effected by radiation, the use of resins, metal oxides and other
miscellaneous vulcanizing agents. Among the miscellaneous vulcanizing or
crosslinking systems are the bisperoxycarbamates. Hepburn (7) reported the use of
bisperoxycarbamates as crosslinking agents in gum natural rubber. Also, the use of
HBTBP as crosslinking agents for fluoroelastomers has been reported in the literature
(8,9). Furthermore, the results of technological evaluation of HBTBP in many
elastomers have been reported (10). Therefore, it was decided that HBTBP should be
evaluated in natural rubber (NR) natural rubber/polybutadiene (NR/BR) practical
compounds. NR/BR blend was used because it is customary to use BR in many blends
to enable it improve abrasion resistance, crack resistance, to improve processing and to
confer many of its desirable properties to the blend. In particular, sidewalls of
passenger tyres are commonly based on blends of nearly equal proportions of natural
rubber and polybutadiene (11).
The subject matter of this paper is the technological evaluation of NR and NR/BR blend
in different crosslinking systems.
EXPERIMENTAL
Materials
The elastomers used, natural rubber (NR, SMR 5 CV ) produced by Malaysian Natural
Rubber and cis-polybutadiene (BR Intene 50) produced by Enichem Corporation, were
obtained from commercial sources. Also, other compounding ingredients were standard
materials used in rubber formulations and were used in compounding as received.
HBTBP was prepared as described in the literature (12).
Mill Mixing
The mixing of the various rubber batches was on a two-roll mill of 850cm3 capacity. The
mixing procedures adopted were along the guidelines set in British Standards (BS)
1674; 1976. After the elastomer was first masticated on the mill, activators and other
ingredients were added and blended thoroughly.
Mooney Test
The principles laid down in BS 1673 Part 3, 1969 were followed in the determination of
Mooney scorch i.e. use of a large rotor, 1 minute preheat time, test temperature of
120oC and scorch time was taken as 5 Mooney units above the minimum reading.
Cure Characteristics
The cure characteristics of the compounds were studied with the aid of a Monsanto
Oscillating Disc Rheometer TM 100, using the test procedure specified in BS 1673; Part
10, Method B, 1977.
Moulding
The compression moulding techniques specified in BS 1674; 1976, were used to obtain
vulcanized rubber sheets of 2mm thickness.
Tensile Stress-Strain Tests
The tensile stress-strain properties of the resulting vulcanizates were determined
according to BS 903; Part A2, 1971, using Type 2, dumb-bell specimens.
Other tests
Other tests included tear strength, which was determined according to BS 903 part A3,
1982; rebound resilience, which was determined according to DIN 53512; hardness
test, which was determined according to BS 903 Part A26, 1969 and compression set
measurement which was determined according to the guidelines of BS 903 Part A6,
1969.
RESULTS AND DISCUSSION
Curing Characteristics
The Oscillating Disc Rheometer (ODR) traces obtained when HBTBP cure is compared
with those of conventional systems (used as controls) in NR and NR/BR are shown in
Figures 1 and 2. The ODR cure traces obtained shows that HBTBP is capable of
crosslinking compounds of NR and NR/BR blend. The two elastomer systems
investigated are diene-based and in both systems, sulphur vulcanization system gave
the best cure response while HBTBP gave the least cure response; peroxide
vulcanization systems were intermediate. The rate of cure of HBTBP cure is
intermediate between those of sulphur and peroxides. Interestingly, there is no
tendency to reversion in HBTBP systems on overcure.
Our observations of the cure systems in this work were similar to what was observed
previously in other elastomers (10). The Mooney scorch for HBTBP cures were very
short and this observation is further supported in the HBTBP cures shown in Figures 1
and 2. Some of the HBTBP compounds produced a rather unpleasant odour during
curing. As it is usual with HBTBP cures, the compounds must be cured longer than t95
cure times to ensure that the carbon dioxide evolved during cure is sufficiently absorbed
by the calcium hydroxide system. It was further observed that the maximum torque
attained in the NR/BR blend was greater than when NR was used alone in all the curing
systems. This may be due to a peculiarity of BR, which undergoes spontaneous
crosslinking during vulcanization. In sulphur vulcanization systems, BR requires less
sulphur than other diene rubbers to obtain the optimum crosslink density (13).
Physical Properties of vulcanizates
The results of physical properties of vulcanizates prepared from HBTPB compounds,
sulphur compounds and peroxide compounds are shown in Tables 1 & 2. The physical
properties of vulcanizates cured with conventional crosslinking agents were better than
those obtained from vulcanizates cured with HBTBP. Further compounding studies with
HBTBP would be needed to make its vulcanizates comparable to those obtained from
conventional crosslinking systems.
The ageing of vulcanizates was carried out under relatively mild conditions (24 hours at
100oC in air cell oven) and the vulcanizates used as controls were protected with
antioxidant while HBTBP-cured systems were unprotected. It was found that the
retention of tensile properties in most HBTBP systems was better than those of the
controls after ageing. This may be an indication that air-oven ageing may be useful in
improving physical properties in this instance. The compression set for HBTBP-cured
vulcanizates was extremely poor, as samples did not show any recovery. This
suggests that the crosslinks originally formed were thermally unstable. However, the
compression set of HBTBP-cured blend was better than HBTBP-cured natural rubber
vulcanizates. The improvement in compression set resistance may be due to the
spontaneous crosslinking of polybutadiene rubber. Also, further compounding studies
would be required to improve the compression set resistance of HBTBP-cured
vulcanizates.
Cure Chemistry
Detailed studies on cure chemistry have not been carried out in this work. However, NR
and NR/BR vulcanizates cured with HBTBP were extracted with hot acetone and after
removal of solvent, these vulcanizates were tested for nitrogen. The extracted
vulcanizates were found to contain nitrogen. This suggests that nitrogen, probably from
amino groups, is attached to the polymer main chain of the elastomers. This finding is
similar to that of Hepburn (7) who proposed a free radical mechanism for the
crosslinking of gum natural rubber by bisperoxycarbamates. In this mechanism, it was
proposed that the bisperoxycarbamate decomposes homolytically; as the initiator
decomposes, the free radicals formed abstract hydrogen from the methylene groups
along the polymer chain, and the resultant polymer radicals interact, directly or through
the intermediary of radical traps to form crosslinks. It was also suggested that both
flexible crosslinks and carbon-carbon crosslinks are formed from the interaction of
polymer radicals. The same mechanism as proposed by Hepburn (7), is thought to
apply in the present work where HBTBP is used as crosslinking agent for filled NR and
NR/BR compounds.
Conclusion
This work has established that HBTBP is capable of curing filled natural rubber and
natural rubber/polybutadiene compounds. Although the physical properties of
vulcanizates cured by conventional crosslinking systems were found to be better than
vulcanizates cured by HBTBP, HBTBP-cured vulcanizates tend to have good retention
of properties on heat ageing. The mechanism of crosslinking of NR and NR/BR is
proposed to occur through homolytic decomposition of HBTBP, interaction of radicals
with rubber, interaction of the resultant rubber radicals, directly or through the
intermediary of radical traps and subsequent formation of crosslinks.
References
1. Hoffmann, W., “Vulcanization and Vulcanizing Agents” Maclaren, London, England.
1967.
2. Sjothun, I.J. and Alliger, G. in “Vulcanization of Elastomers”, I.J. Sjothun and G.
Alliger (Eds.) Robert E. Frieger Publishing Company, Huntington, New York. 1978.
p3.
3. Gillam, J.K. in “Encyclopedia of Polymer Science and Engineering” J.I Kroschwitz
(Ed.), Wiley, New York. Vol.4 1986. p519.
4. Quirk, R.P.,”Overview of Curing and Crosslinking of Elastomers” Prog. Rubber
Plast. Technol. 4(1) 31, 1988
5. Ogunniyi, D.S., “Peroxide Vulcanization of Rubber” Prog. Rubb. Plast. Technol.
15(2) 95, 1999.
6. Dluzneski, P.R., “Peroxide Vulcanization of Elastomers” Rubb. Chem. Technol.
74(3) 451, 2001
7. Hepburn, C., “Bisperoxycarbamates: vulcanizsing agents that possess bound
antioxidant properties” Rubb. World 190 49, 1984
8. Ogunniyi, D.S. and Hepburn C., “Hexamethylene-N,N’ bis(tert-butyl
peroxycarbamate as a curing agent for fluoroelastomers” Plast. Rubb. Process.
Appl. 6(1) 3, 1986
9. Ogunniyi, D.S., “A Novel System for Crosslinking Fluoroelastomers. Rubb. Chem.
Technol. 61(5) 735, 1988.
10. Ogunniyi, D.S. and Hepburn C., “Hexamethylene-N,N’ bis(tert-butyl
peroxycarbamate as a crosslinking agent in some elastomers” Iran J. Polym. Sci.
Tech. 3(1) 48, 1994
11. Kim, H.J. and Hamed, G.R., “On the reason that passenger tyre sidealls are based
on blends of NR and cis polybutadiene” Rubb. Chem. Technol. 73 743, 2000
12. Pedersen, C.J., “Preparation and Properties of esters of N-substituted
peroxycarbamic acids” J.Org. Chem. 23 252, 1958
13. Blow, C.M. and Hepburn, C., “Rubber Technology and Manufacture” 2nd edition.
Newnes-Butterworths, London. 1982
Table 1: Comparison of physical properties of NR vulcanizates obtained using HBTBP
crosslinking agent and controls
NR SMR 5 CV
ZnO
CaO
Ca(OH)2
Stearic Acid
SRF Black (N762)
Coumarone Resin
CBS
Flectol H
Sulphur
Dicup 40C
HBTBP
Mooney Scorch at 120oC
Cure Conditions (mins/ oC)
Unaged Properties
Tensile Strength (Mpa)
300% Modulus (Mpa)
Elongation-at-break (%)
Tear Strength (kN/m)
Hardness IRHD
Rebound Resilience (%)
Aged Properties (24 hours at 100 oC
Tensile Strength (Mpa)
300% Modulus (Mpa)
Elongation-at-break (%)
Tear Strenth (kN/m)
Compression Set %(24 hours at 100 oC;
25% strain)
% Retention of Tensile Strength
Sulphur (phr) Peroxide (phr) HBTBP (phr)
100.0
100.0
100.0
5.0
5.0
4.0
6.0
2.0
30.0
30
30
5.0
2.0
1.0
0.5
1.5
1.5
6.0
40 mins
> 40 mins
2 mins
9/160
14/170
10/150
23.54
5.12
600
89.43
36
63
17.37
10.68
525
27.93
36
57
4.97
4.07
375
8.75
< 36
-
21.55
5.99
525
63.37
46
14.81
9.88
550
23.42
32
5.17
3.11
400
16.89
-
92
85
104
Table 2: Comparison of physical properties of NR/BR vulcanizates obtained using
HBTBP crosslinking agent and controls
NR SMR 5 CV
BR Intene 50
ZnO
CaO
Ca(OH)2
Stearic Acid
SRF Black (N762)
Coumarone Resin
CBS
Flectol H
Sulphur
Dicup 40C
HBTBP
Mooney Scorch at 120oC
Cure Conditions (mins/ oC)
Unaged Properties
Tensile Strength (Mpa)
300% Modulus (Mpa)
Elongation-at-break (%)
Tear Strength (kN/m)
Hardness IRHD
Rebound Resilience (%)
Aged Properties (24 hours at 100 oC
Tensile Strength (Mpa)
300% Modulus (Mpa)
Elongation-at-break (%)
Tear Strength (kN/m)
Compression Set %(24 hours at 100 oC;
25% strain)
% Retention of Tensile Strength
Sulphur (phr) Peroxide (phr) HBTBP (phr)
50.0
50.0
50.0
50.0
50.0
50.0
5.0
5.0
4.0
6.0
2.0
30.0
30
30
5.0
2.0
1.0
0.5
1.5
1.3
6.0
45 mins
> 45 mins
2 mins
12/160
12/170
10/150
14.31
6.08
525
25.74
39
67
6.35
3.97
400
8.67
40
68
4.39
3.18
400
10.95
35
58
9.9
7.31
375
18.05
36
5.48
3.70
375
9.77
16
4.26
2.71
425
10.49
53
69
86
97