Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~C5~
The use of maleic acid semi-esters and fumaric ac1d
semi-esters and their salts for reducing nitrosamine
formations in sulphur vulcanization
This invention relates to the use of maleic acid semi-
esters and fumaric acid semi-esters and their salts, in
particu1ar their zinc salts, for reducing the nitrosamine
content in rubber vulcanizates produced by sulphur
vulcanization.
When rubbers are mixed with vulcanizing systems containing
compounds having chemically bound nitrogen (e.g. sulphur
donors or accelerators selected from thiuramic compounds
or vulcanization accelerators selected from thiazole
compounds) there is a risk of unwanted formation of
nitrosamines, and this risk is even greater when these
rubber masses undergo vulcanisation. Owing to the
carcinogenic effect of nitrosamines, it would be desirable
to be able to produce rubber masses and vulcanizates
containing little or no nitrosamine.
Whereas sulphur donors, for example, can often easily be
replaced by elementary sulphur, no nitrogen-free vulcan-
ization accelerator has yet been found which is aseffective as the substances hitherto used~
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So long as no such highly effective vulcanization
accelerators free from nitrogen are available, it is also
of interest to find additives which either suppress
nitrosamine formation or absorb the nitrosamines by a
chemical reaction. One such compound which has been
described is ~-tocopherol (Kautschuk + Gummi-Kunststoffe
43 (1990) 95-106). The high cost of ~-tocopherol, however,
would stand against its use on a large technical scale in
the rubber industry.
It has now surprisingly been found that maleic acid semi-
esters and their salts are eminently suitable for this
purpose.
The present invention thus relates to the use of maleic
acid semi-esters and fumaric acid semi-esters corresponding
to the following formula
HC - COOHHC COOH
HC COORROOC!CH
(Ia) (Ib)
and their salts,
wherein
R denotes an organic group having 1 to 18, preferably
4 to 18 carbon atoms,
for reducing the nitrosamine content of rubber vulcaniz-
ates produced by sulphur vulcanization.
The preferred maleic acid semi-esters and fumaric acid
semi-esters include semi-esters of C1-C22, preferably
C4-C18 alcohols. The alcohol components of the semi-
esters may be aliphatic, cyclo-aliphatic or aromatic;
they may contain (cyclo)olefinic C=C double bonds and
halogen substituents, e.g. chlorine
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~c~s~
substituents, and they may be linear or branched.
Preferred alcohol components include methanol, isopropan-
ol, n-iso- and tert.-butanol, hexanols, octanols, stearyl
alcohol, allyl alcohol, cyclohexanol, benzyl alcohol and
phenol; the mono-n-butyl-, octyl-, ethylhexyl- and allyl
esters of maleic acid are particularly preferred.
The cations of the semi-ester salts to be used according
to the invention are preferably derived from alkali metals
and alkaline earth metals and zinc, the zinc salts being
particularly preferred as they also have a positive
influence on the Mooney viscosity, the compression set,
the resistance to hot air ageing and the strength of the
vulcanizates.
The maleic acid semi-ester and fumaric acid semi-ester (salt)s to be
used according to the invention may be put into the process in quantities of
from 0.5 to 8% by weight, preferably from l to 6, in
particular from 1.5 to 4% by weight, based on the rubber.
Rubbers suitable for the use according to the invention
include synthetic rubbers as well as natural rubbers.
Preferred synthetic rubbers have been described, for
example, by W.Hofmann, in Kautschuk-Technologie, Gentner
Verlag, Stuttgart 1980. They include inter alia the
following:
EPDM - ethylene/propylene/diene terpolymers
25 IIR - butyl rubber
BR - polybutadiene
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ABR - butadiene/acrylic acid-Cl-C4-alkyl ester
copolymers having acrylic ester contents
of from 5 to 60, preferably from 15 to
50% by weight
5 CR - polychloroprene
IR - polyisoprene
SBR - styrene/butadiene copolymers having
styrene contents of from 1 to 60%by
weight, preferably from 20 to 50% by
weight
NBR - butadiene/acrylonitrile copolymers
having acrvlonitrile contents of from 5
to 60, preferably from 10 to 50% by
weight, and in particular
15 HNBR - hydrogenation products of NBR.
The rubbers generally have Mooney visco6ities (according
to DIN 53 523) of from 5 to 140, preferably from 10 to
120, in particular from 20 to 80 (ML 1 + 4) 1009C and
glass temperatures below 200C, preferably below 0C,
determined by the torsion vibration test according to DIN
53 445.
EPDMs include rubbers in which the ratio by weight of
ethylene to propylene groups is in the range of from 40:60
to 65:35 and which may have from 1 to 20 C=C double bonds
per 1000 carbon atoms. The following are examples of
suitable diene monomers in the EPDM: Conjugated dienes,
e.g. isoprene and butadiene-(1,3), and non-conjugated
dienes having 5 to 25 carbon atoms, e.g. 1,4-pentadiene,
Le A 28 006 5
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1,4-hexadiene, 1,5-hexadiene, 2,5-dimethyl-1,5-hexadiene
and 1,4-octadiene; cyclic dienes, e.g. cyclopentadiene,
cyclohexadiene, cyclooctadiene and dicyclopentadiene;
alkylidene and alkenyl norbornenes, e.g. s-ethylidene-2-
norbornene, 5-butylidene-2-norbornene, 2-methallyl-5-
norbornene, 2-isopropenyl-5-norbornene and tricyclodienes.
The non-conjugated dienes, hexadiene-(1,5), ethylidene
norbornene and dicyclopentadiene are preferred. ~he diene
content in EPDM is preferably from 0.5 to lO~ by weight,
based on the EPDM.
Such EPDM rubbers are described, for example, in DE-OS
2 808 709.
~he term "butyl rubber" used in the context of this
invention includes isobutene copolymers of from 95 to
99.5~ by weight, pre~erably from 97.5 to 99.5% by weight,
of isobutene and from 0.5 to 5% by weight, preferably from
0.5 to 2.5% by weight, of copolymerisable diene, e.g.
butadiene, dimethylbutadiene, pentadiene-(1,3) and in
parti¢ular i~oprene. Butyl rubber ls produced on a large
technical scale almost exclusively as the isobutene/iso-
prene copolymer by cationic solution polymerisation at a
low temperature; see e.g. Kir~-Othmer, Encyclopedia of
Chemical Technology, 2nd Edition, Volume 7, page 688,
Interscience Publishers, New York-London-Sydney, 1965, and
Winnacker-Xuchler, Chemische Technologie, 4th Edition,
Volume 6, pages 550-555, Carl Hanser Verlag, Munich-
Vienna, 1962.
The polybutadienes include polybutadiene rubbers contain-
ing from 20 to 100%, preferably from 30 to 100%, o~ the
cis-1,4-structure, which may be obtained e.g. by butadiene
polymerisation with the aid of catalysts based on lithium,
Le A 28 006 6
nickel, titanium, cobalt, rare earths or uranium.
Polychloroprenes are chloroprene polymers which in
addition to containing polymerised 2-chloroprene units may
contain up to 30~ by weight, preferably up to 20% by
weight, based on the chloroprene polymer, of copolymerised
units of other ethylenically unsaturated monomers, in
other words polychloroprenes such as those described, for
example, in "Methoden der Organischen Chemie" (Houben-
Weyl), Volume E20/2, 842-859, Georg Thieme Verlag,
Stuttgart - New York 1987.
Preferred ethylenically unsaturated "other monomers" which
can be copolymerised with chloroprene include compounds
having 3 to 12 carbon atoms and 1 or 2 copolymerisable C=c
double bonds per molecule. The following are examples of
preferred "other monomers": Styrene 2,3-dichlorobuta-
diene, 1-chlorobutadiene butadiene, isoprene, acrylic
acid, methacrylic acid, acrylonitrile and methacrylo-
nltrlle. The most important comonomers are 2,3-dichloro-
butadiene and 1-chlorobutadiene.
Preferred styrene/butadiene copolymers are those contain-
ing from 18 to 60% by weight, pre~erably ~rom 20 to 50% by
weight of styrene incorporated by polymerisation. Solution
and emulsion polymers are particularly preferred.
Nitrile rubbers and hydrogenated nitrile rubbers include
nitrile rubbers such as those described e.g. in Ullmanns
Encyclopadie der technischen Chemie, Volume 13, Verlag
Chemie, Weinheim-New York 1977, pages 611-614, and fully
or partially hydrogenated nitrile rubbers such as those
described e.g. in Die Angewandte ~akromolekulare Chemie
145/146 (1986) 161-179.
The term "nitrile rubber" is used to denote butadiene/-
Le A 28 006 7
~3 J31
acrylonitrile copolymers having a copolymerised
acrylonitrile content of from 5 to 60% by weight,
preferably from 10 to 50~ by weight. "Hydrogenated" means
in this context that from 90 to 98.5%, preferably from 94
to 98%, of the C=C double bonds capable of being hydrogen-
ated are hydrogenated while the C--N triple bonds of the
nitrile groups are not hydrogenated. The hydrogenation of
nitrile rubber is known: US-PS 3 700 637, DE-OS
25 39 132, 30 46 008, 30 46 251, 32 27 650, 33 29 974,
EP-A 111 412 and FR-PS 2 540 503.
Detailed descriptions of sulphur vulcanization systems are
found in "Vulkanisation und Vulkanisationshilfsmittel" by
W.Hofmann, Verlag Berliner Union GmbH, Stuttgart 1965, and
in "Vulcanization of Elastomers" by Alliger and Sjothun,
Reinhold Pub. Corp. New York 1964. Examples of suitable
sulphur donors include thiuramic polysulphides such as
dipentamethylene thiuramic tetra- and -hexasulphide and
tetramethyl thiuramic disulphide; amine disulphides such
a~ dimorpholyl disulphide; sodium polysulphides and
thioplasts.
Pre~erred sulphur vulcanization systems contain
a) sulphur or sulphur donors,
b) optionally vulcanization accelerators and
c) optionally one or more activators.
Component a) is generally used in a quantity corresponding
to from 0.2 to 3.0% by weight of sulphur (calculated as
the amount of sulphur liberated in the case o~ sulphur
donors), based on the rubber. Sulphur modified polychlo-
roprene may also function as sulphur donor.
Le A 28 006 8
;~53~
The vulcaniztion accelerator b) is generally used in
quantities of from 1 to 3.5% by weight, based on the
rubber. Preferred vulcanization accelerators b) include
thiazole accelerators, e.g.
2-mercaptobenzothiazole,
dibenzothiazyl disulphide,
benzothiazyl-2-cyclohexylsulphenamide ( CBS ),
benzothiazyl-2-tert.-butylsulphenamide (TBBS),
N-morpholinothio-2-benzothiazole (MBS),
benzothiazyl-2-diisopropylsulphenamide ~DIBS),
benzothiazyl-2-tert.-amylsulphenamide (AMZ),
benzothiazyl-dicyclohexylsulphenamide (DCBS~ and
morpholino-thiocarbonyl-sulphenemorpholide (OTOS).
Further examples of-preferred vulcanization accelerators
b) include diphenylguanidine ( DPG) and di-o-tolylguanidine
(DOTG); thiurams such as thiuramic mono- and disulphides;
dith~ocarbamates and thiophosphates and their derivatives
and salts, e.g. their zinc salts.
The most important activators c) are metal oxides, in
particular zinc oxide. Magnesium oxide or calcium
hydroxide is also used in some cases.
Fillers such as carbon black, plasticizers, age resistors
and/or processing auxiliaries may be added to the rubbers
in the usual guantities before vulcanization.
The processing auxiliaries used may be, for example, fatty
acids, e.g. stearic acid.
Mixing of the components may be carried out in convention-
al mixing apparatus.
Le A 28 006 9
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The preferred mixing apparatus are those conventionally
used in the rubber industry, such as kneaders, rollers,
internal mixers and mixing extruders, which generally
operate at shear rates of from 1 to 1000 sec~l, preferably
from 1 to 200 sec~l.
Vulcanization may be carried out at temperatures of from
100 to 200QC, preferably from 130 to 180~C, optionally
under a pressure of from 10 to 200 bar. The vulcanizates
may be tempered by storage at elevated temperatures after
they have been vulcanized.
"Vulcanized" in the context of this invention means that
less than 10% by weight, preferably less than 5% by
weight, based on the rubber, can be extracted in the
course of 10 hours' extraction with toluene in a Soxhlet
apparatus.
Le A 28 006 10
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Examples
List and abbreviations of the test methods:
Vulcameter Frank-Vulkameter System Bayer, heating
for l minute, measuring range 3/20 mV,
operating time at 150~C: 30 min, feed
rate 600 mm/h
ts/120~C or Prevulcanization time from vulcameter
measurement at 120~C and 130sC,
130~C (min) Time until the shear modulus curve rises
by 15 mm above the minimum
tgo (min) Heating time, time required for 90% of
the shear modulus (Bayer-Vulkameter) to
be reached at the end of the operating
time
15 tgo - tS Reaction time, difference between the
(min) heating time tgo and the prevulcanization
time ts measured at the same temperature,
e.g. 150~C
Shear modulus Final value-initial value o~ the shear
20 ~F ~N) modulus from the vulcameter test
Stepwise 4 mm claps, pres6 heating in several time
heating stage6
M300 (MPa) Tension at 300% elongation, DIN 53 504
F (MPa) Tear resistance, DIN 53 504, Standard
ring R I
D (%) Elongation at break, DIN 53 504, Standard
ring R I
H (Shore A) Shore A hardness, DIN 53 505
E (%) Recoil elasticity DIN 53 512
30 W (N) Ring structure according to Pchle (in-
house method)
Compression Based on DIN 53 517, constant de~orma-
set ~%] tion, cylinder 10 mm in height, 10 mm in
diameter
Le A 28 006 ll
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Determination of the nitrosamine content was carried out
according to Franck, Kunststoffe-33 Lfg. August 1984,
pages 37 et seq.
.
A. HNBR Rubber mass
The rubber used was a hydrogenated acrylonitrile/butadiene
copolymer having an acrylonitrile content of 33.7~ by
weight, a degree of hydrogenation of 96.4%, based on the
C-C double bonds originally present, and a Mooney
viscosity of 67 (ML 1+4) lOO~C ( (R)Therban 1707 S of
Bayer AG).
The rubber was masticated for 0.5 minutes in a laboratory
kneader at 50sC and sulphur, stearic acid, zinc oxide
~(R) zink oxide aktiv of Bayer AG), styrenised diphenyl-
amine (R)Vulkanox DDA of Bayer AG), zinc-methylmercapto-
benzimidazole ((R)Vulkanox ZMB2 of Bayer AG), carbon black(Cor~x N550 o~ Degussa/Wesseling) and zinc-ethyl hexyl
maleate (~or quantities, see Table 1) were then added and
the components were kneaded until homogenised (4.5
minute~) .
After the rubber mass had cooled to about lOO~C on a
roller, an accelerator system consisting of tetramethyl
thiuramic disulphide (~R)Vulkacit Thiuram C of Bayer AG)
and benzothiazyl-2-cyclohexyl sulphenamide ((R)Vulkacit
CZ/MG of Bayer AG) was added.
The properties of the mixtures obtained and their
vulcanizates are listed in Table 2.
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Table 1: Composition
1 2 3
.
HNBR loO loO lOo
Sulphur 0.51 0.51 0.51
Stearic acid
Zinc oxide 2 2 2
Vulkanox
DDA
Vulkanox 0.4 0.4 0.4
ZMB 2
20 Carbon black 45 45 45
Vulkacit 2 2 2
Thiuram C
25 Vulkacit 0.5 0.5 0.5
CZ/MG
Zinc-ethyl _ 1.5 3
hexylmaleate
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XC~3 J~l.
Table 2: Properties
1 2 3
Mooney viscosity
MLl+4/1209C 76 74 72
Mooney Scorch/
130sC (min) 18.3 21.7 20.9
Vulcameter 1609C
ts (min) 5.2 4.7 4.7
t80 (min) 8.9 7.6 7.7
Fmax (N) 50 4 52 9 521:9
Press vulcanization: 30 minutes/160sC (S2 rods
according to DIN 53 502 and
53 504)
20 Tensile strength (MPa) 29.4 28.6 27
Elongation at break (%) 500 500 490
Tenslon S10O (MPa) 3.9 4.1 3.9
Shore hardness (A) 73 72 73
Recoil elasticity (%) 42 43 43
Tear propagation
resi~tance according
to DIN 53 515 15.1 19.8 20.6
Nitrosamine content
(ppb)
be~ore vulcanization 28 29 32
a~ter vulcanization 185 45 35
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B. NBR rubber mass
The rubber used was an acrylonitrile/butadiene copolymer
having an acrylonitrile content of 28% by weight and a
Mooney viscosity of 45 (ML 4) 1009C ((R)Perbunan N 2807 NS
of Bayer AG).
The rubber was masticated for 0.5 minutes in a laboratory
kneader at 50UC and a 50% by weight sulphur paste
((R)Struktol SU 105 of Schill & Seilacher/Hamburg), carbon
black ((R)Corax N 326 or (R)Durex O of Degussa/Wesseling),
styrenised diphenylamine ((R)Vulkanox DDA of Bayer AG),
polymeric 2,2,4-trimethyl-1,2-dihydroquinoline
((R)Vulkanox HS of Bayer AG), plasticizer (ether thio-
ether; (R)Vulkanol OT of Bayer AG), processing auxiliary
based on highly active silica ((R)Aflux S of Rheinchemie
Rheinau GmbH, Mannheim), zinc oxide (zinc white RS of
Zinkweiss-Forschungsgesellschaft, Oberhausen), stearic
acid and zinc-n-octylmaleate were then added and the
components were kneaded until homogenised (4.5 minutes).
A~ter the rubber mas~ had cooled to about 100C on a
roller, tetramethylthiuramic disulphide ((R)Vulkacit
Thiuram/C of Bayer AG) was added~ Composition and
experimental findings are shown in the following Tables 3
and 4.
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Table 3: Composition
Composition [Parts by weight]
5 NBR 100
Sulphur paste 0.6
Carbon black Corax N 326 30
Caron black Durex O 65
Vulkanox DDA 1.5
Vulkanox HS
Plasticizer 6
Aflux S 3
Zinc oxide 5
Stearic acid
25 Vulkacit Thiuram/C 2.5
Zinc-n-octyl maleate see Table 4
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2g;
Table 4: Properties
Vulkameter without zinc with 4% zinc-n-
n-octyl- octyl maleate
maleate
ts 1209C [min] 18.0 14.7
ts 150 9 C [min] 2.8 2.3
tgo 150sC tmin] 10.0 11.5
tgo-tS 1509C [min] 7.2 9.2
Fmax tN~ 53 46
Nitrosamine content 47 36
before vulcanization
t ppb ~
after vulcanization 153 89
~ppb]
C. EPDM ~ubber mass
The rubber used was an EPDM (~R)Buna AP 331 containing
ethylidene norbornene a~ termonomer, Mooney-Viscosi~y: 70
~ML 1+4) 1009C, product of Huls AG, Marl).
The rubber was masticated for- 0.5 minutes in a laboratory
kneader at 509C, and sulphur, stearic acid, zinc oxide
((R)Zinkoxyd aktiv of Bayer AG), styrenized diphenylamine
((R)Vulkanox DDA of Bayer AG), zinc-methylmercaptobenz-
imidazole ((R)Vulkanox ZMB 2 of Bayer AG), carbon black
((R)Corax N 550 of Degussa, Wesseling) and zinc-mono-n-
octylmaleate were then added and the components were
kneaded until homogenized.
After the rubber mass had cooled to about 1009C on a
roller, an accelerator system consisting of tetra-
~e A 28 006 17
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methylthiuramic disulphide ((R)Vulkacit Thiuram C ofBayer AG) and benzothiazyl-2-cyclohexyl-sulphenamide
((R)Vulkacit CZ/MG of Bayer AG) and optionally N-tri-
chloromethylsulphenyl-benzene sulphanilide ((R)Vulkalent E
of Bayer AG) was added.
Compositions and properties of the vulcanizate are shown
in Tables 5 and 6 below.
Table 5:. Composition
1 2 3
EPDM 100 loo lOo
Sulphur 0.51 0.51 0.51
Carbon black 45 45 45
Zlnc oxide 5 5 5
Stearic acid 0.5 0.5 0.5
Vulkanox DD~ 1 1 1
Vulkanox ZMB 2 0.4 0.4 0.4
Vulkacit Thiura~l C 2 2 2
Vulkacit CZ/MG 0.5 0.5 0.5
20 Zn-n-octyl ~aleate - 3 3
Vulkalent E
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Table 6: Properties
1 2 3
Mooney viscosity 77 75 74
(ML 1+4) 1209C
Vulcameter 1609C
tS [Min.] 3.4 2.4 2.2
t80 tMin-] 8.6 8.1 8.0
tgo tMin.] 13.5 12.8 12.1
tmax tMin.] ,2 9 425.3 2.3
min tN3 50 7 44.1 40.9
Fmax-Fmin [N] 47.8 41.8 38.6
Press vulcanisation: 30 min/160sC
F ~MPa] 15.3 12.1 12.6
D (%) 495 545 605
S 100 tMPa~ 2.6 2.2 2.1
25 S 200 tMPa] 5.7 4.2 3.9
S 300 tMPa] 9.1 6.6 6.1
H 239C (Shore A) 66 67 63
E 239C ~%) 59 58 56
WW* DIN 53 515
[N/mm] 13.2 16.1 16.8
Compression set (Sample l ~ody II, according to DIN 53 517)
70 h/1009C 43.0 47.1 46.5
70 h/1259C 66.4 70.8 69.1
Nitro~amine [ppb]
before vulcanization 18 14 15
45 after vulcanization 24 9 8
*WW = tear propagation resistance
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