Note: Descriptions are shown in the official language in which they were submitted.
. 20932~~
_ 1 _
CROSSLINKING AGENT FOR ACRYLIC RUBBER
BACKGROUND OF THE INVENTION
The present invention relates to a crosslinking
agent suitable for use in curing acrylic rubbers.
It is known that acrylic rubbers are superior in
heat resistance and oil resistance to general rubbers.
Sulfur vulcanization is not applicable to the acrylic
rubbers because they have no unsaturated double bond.
Therefore, in order to make crosslinkable,
acrylic rubbers have been generally produced by
copolymerizing alkyl acrylates with comonomers to impart a
crosslinkability, e.g. halogen-containing monomers such as
2-chloroethyl vinyl ether and vinyl chloroacetate, or
epoxy-containing monomers such as allyl glycidyl ether,
glycidyl acrylate and glycidyl methacrylate, as disclosed
in Japanese Patent Publication Kokoku No. 57843/1986.
These acrylic rubbers are crosslinkable with a
crosslinking agent or curing agent, e.g. a fatty acid soap
or an amine compound such as triethylenetetramine or
hexamethylenediamine carbamate.
However, these known acrylic rubbers have some
defects. The acrylic rubbers prepared by copolymerization
with the halogen-containing monomers have the defects that
the water resistance is poor because the halogen has a
reactivity to water, and that they cause prevulcanization
of a mixed compound during roll processing or storage
thereof because the reaction of the monomer units
introduced to impart a crosslinkability with a
crosslinking agent proceeds too fast. The acrylic rubbers
prepared by copolymerization with the epoxy-containing
monomers have no particularly serious defects, but still
have problems that they are inferior in heat resistance to
the halogen-containing acrylic rubbers and also are
insufficient in water resistance and oil resistance and,
therefore, they can not be satisfactorily applied to uses
which require particularly superior properties.
There is also proposed by the present inventors
2 _
an acrylic rubber crosslinkable with a thiuram compound,
which is prepared by copolymerizing acrylic acid esters
with a cyanoacetic acid compound or a malonic acid
compound as a comonomer for imparting a crosslinkability,
as disclosed in Japanese Patent Publication Kokoku No.
3770/1979 and No. 28437/1979. This acrylic rubber has
good water and oil resistances and also has a heat
resistance on the same level as that of conventional
acrylic rubbers. However, since this acrylic rubber has
been used with a thiuram crosslinking agent which has a
relatively low activation energy, it leaves room for
improvement of heat resistance.
It is an object of the present invention to
provide a curable composition capable of providing an
acrylic rubber having excellent water resistance, oil
resistance and heat resistance.
Another object of the present invention is to
provide a crosslinking agent suitable particularly for use
in acrylic rubbers containing units of a cyanoacetic acid
monomer or a malonic acid monomer as crosslinking sites,
thus giving cured acrylic rubbers having an improved heat
resistance as well as excellent water and oil resistances.
These and other objects of the present invention
will become apparent from the description hereinafter.
SUMMARY OF THE INVENTION
It has been found that a product obtained by a
reaction of formaldehyde or a polymerized formaldehyde
with urea or thiourea in the presence of an acidic
catalyst is useful as a crosslinking agent for acrylic
rubbers, particularly acrylic rubbers containing units of
a cyanoacetic acid or malonic acid monomer, and when the
acrylic rubbers are cured using this reaction product as a
crosslinking agent, cured acrylic rubbers having an
improved heat resistance and excellent water and oil
resistances are obtained.
In accordance with the present invention, there
is provided a crosslinking agent for acrylic rubbers
~~9~14~
- 3 -
comprising a reaction product of a compound selected from
the group cosisting of formaldehyde and a polymerized
formaldehyde and a compound selected from the group
consisting of urea and thiourea in the presence of an
acidic catalyst.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a graph showing the state of
crosslinking of an acrylic rubber cured with a
crosslinking agent according to the present invention, an
acrylic rubber cured with a thiuram crosslinking agent and
an acrylic rubber cured with hexamethylenetetramine.
DETAILED DESCRIPTION
I5 The crosslinking agent of the present invention
is prepared by reacting formaldehyde and/or a
polyformaldehyde with urea and/or thiourea in the presence
of an acidic catalyst. When these compounds are reacted
in the presence of an acidic catalyst, a condensation
polymerization occurs. Since the reaction rate is high,
the reaction product is obtained as a derivative of
methyleneurea or methylenethiourea.
Various polyformaldehydes (polymerized
formaldehyes having various degrees of polymerization) as
generally shown by the formula: HO(CHZO)nH wherein n is an
integer of not less than 3, can be used in the present
invention. Usually, paraformaldehyde is used as the
polyformaldehyde.
Inorganic acids and organic acids can be used as
the acidic catalysts. Representative examples of the
acidic catalyst used in the present invention are, for
instance, hydrochloric acid, phosphoric acid, formic acid
and acetic acid.
The reaction is usually carried out in an
aqueous medium, typically water. The reaction temperature
is usually from 5° to 90°C , and the reaction time is
usually from IO to 60 minutes.
Theoretically, it is sufficient to use the
2Q9~2~9
formaldehyde compound and the urea compound in equimolar
ratio. However, in consideration of evaporation of
formaldehyde during the reaction, purity of available
formalin and the like, it is preferred to use the
formaldehyde and/or polyformaldehyde in excess. In
general, formaldehyde, polyformaldehyde or a mixture
thereof is used in an amount of 1 to 10 moles, preferably
1.1 to 2.5 moles, per mole of urea, thiourea or a mixture
thereof.
The thus obtained reaction product is used as
the crossliking agent for acrylic rubbers. The reaction
product is a water-insoluble white powder and is obtained
as a high molecular weight compound and, therefore, it is
easy to handle. Moreover, it has no formalin odor.
In the reaction product, the portion derived
from the formaldehyde compound functions to crosslink
acrylic rubbers, and the portion derived from the urea
compound functions to control the rate of crosslinking
reaction caused by the formaldehyde portion. Since the
crosslinking agent of the present invention contains the
portion derived from the urea compound, it has the
features that the crosslinking reaction does not proceed
at a too fast rate and consequently the prevulcanization
of a mixed compound does not occur.
The crosslinking agent of the present invention
also has the advantage that acrylic rubbers cured
therewith do not cause a decomposition reaction of sulfur
molecule, which has a relatively low activation energy, at
high temperatures as seen in acrylic rubbers cured by a
sulfur type crosslinking agent such as thiuram ' and,
therefore, the cured rubbers have an improved excellent
heat resistance. Also, the water resistance and oil
resistance that acrylic rubbers possess in itself are not
impaired.
A reaction of urea and formalin has been made in
the preparation of a urea-formaldehyde resin. In that
case, the reaction is generally carried out in the
presence of a basic catalyst, and a methylolurea compound
- 5 -
is produced. The reaction product of methyleneurea
derivative type according to the present invention
prepared in the presence of an acidic catalyst and used as
the crosslinking agent is distinct from the methylolurea
products in the preparation of urea resin.
The crosslinking agent of the present invention
is applicable to various known acrylic rubbers curable
with thiuram crosslinking agents to improve the heat
resistance. It is particularly useful for acrylic rubbers
containing units derived from a cyanoacetic acid or
malonic acid comonomer, preferably those prepared by
copolymerization of an acrylic acid ester and 2 to 10 % by
weight, especially 2 to 6 % by weight, based on the
acrylic acid ester, of a cyanoacetic acid or malonic acid
comonomer, as disclosed in U.S. Patent No. 4,154,914.
Representative examples of the acrylic acid
esters are, for instance, an alkyl acrylate such as methyl
acrylate, ethyl acrylate, butyl acrylate or 2-ethylhexyl
acrylate, an alkoxyalkyl acrylate such as methoxyethyl
acrylate or ethoxyethyl acrylate, and the like. Alkyl
acrylates having a C1 to Cx alkyl group and alkoxyalkyl
acrylates having a CZ to C8 alkoxyalkyl group wherein the
alkoxy group has 1 to 4 carbon atoms are generally
used. These acrylic acid esters may be used alone or in
admixture thereof.
The cyanoacetic acid or malonic acid comonomers
are malonic acid derivatives represented by the formula:
COOR'
C$a
I
g
wherein Rl is vinyl, allyl and X is
or methallyl CN
group,
or COORZ in which R2 methyl, ethyl propyl group.
is or
Representative examples the malonic
of acid derivatives
are, for instance, methylallyl malonate,ethyl allyl
malonate, allyl cyanoacetate,
methallyl cyanoacetate,
an
ester of cyanoacetic with hydroxyethylacrylate
acid or
20~3~4~
methacrylate, and other cyanoacetic acid esters and
malonic acid esters having a copolymerizable double bond.
The acrylic rubbers containing units of a
cyanoacetic acid monomer or a malonic acid monomer are
crosslinkable with hexamethylenetetramine or
paraformaldehyde which decomposes to produce formaldehyde
by heating for crosslinking. They are also crosslinkable
with N, N'-dimethylolurea as obtained by the above-
mentioned reaction of urea and formalin in the presence of
a basic catalyst. In these cases, however, the
crosslinking reaction is too fast and consequently the
prevulcanization occurs. Moreover, bad formalin smell is
given out during the working for crosslinking. The
crosslinking with hexamethylenetetramine also has the
fatal defect that the dispersibility in roll processing is
bad.
In contrast, the crosslinking agent according to
the present invention does not decompose to produce
formaldehyde at the time of crosslinking acrylic rubbers
because formaldehyde or a polyformaldehyde is
intramolecularly fixed with urea or thiourea.
Consequently, it has a good rising in crosslinking rate
and a good plateau effect as shown by curve A in Fig. 1
which shows the state of crosslinking of an acrylic
rubber. The crosslinking agent according to the present
invention does not cause prevulcanization and has a good
storage stability. Moreover, it does not generate a
formalin odor during the crosslinking operation. Further,
it has a good dispersibility in roll processing.
Fig. 1 is a graph showing the state of
crosslinking of an acrylic rubber containing cyanoacetic
acid comonomer units when the rubber is cured with the
crosslinking agent according to the present invention, a
thiuram crosslinking agent or hexamethylenetetramine,
wherein the abscissa indicates the crossliking time
(minute) and the ordinate indicates the torque of the
rubber (kg~ cm). In Fig. 1, curve A is a curing curve
obtained when using the crosslinking agent of the present
2~~3~~~
_ 7 _
invention, curve B is a curing curve obtained when using
the thiuram crosslinking agent, and curve C is a curing
curve obtained when using hexamethylenetetramine as a
crosslinking agent. The detail of the acrylic rubber used
in the experiments is shown in Example 1, paragraph (B),
described after. The detail of the crosslinking agent of
the present invention and the formulation of a rubber
composition fox curve A is shown in Example 1, paragraphs
(A) and (C), described after. The detail of the
crosslinking agents and the formulation of rubber
compositions for curves B and C is also shown in
Comparative Examples 1 and 2 described after,
respectively.
In case of the thiuram crosslinking agent, since
there is an induction period until the torque begins to
rise as shown by curve B, the storage stability and the
safety in processing are good. However, curve B shows
that the rise of torque is not so large and, therefore,
the crosslinking density is not high. These results show
that the physical properties of the cured product such as
compression set are not good. In order to increase the
crosslinking density, the amount of the crosslinking
monomer to be copolymerized in the acrylic rubber must be
increased as well as the amount of the crosslinking agent
to be used.
In case of using hexamethylenetetramine as a
crosslinking agent, the crosslinking density is high and
physical properties such as compression set are good, as
understood from curve C. However, the storage stability
and the safety in processing are poor because of 'abrupt
rising of the torque-time curve in a short period of time
after starting the crosslinking.
In contrast, curve A shows that the crosslinking
agent of the present invention provides cured products
having a high crosslinking density and good physical
properties such as compression set by the use of a slight
amount thereof, and also has goad storage stability and
processing safety because of no abrupt rising of torque.
-
The amount of the crosslinking agent according
to the present invention can be selected from a wide range
in accordance with purposes. Although it also varies
depending on the content of a crosslinking monomer
incorporated in acrylic rubbers, the crosslinking agent
according to the present invention is usually employed in
an amount of 1 to 15 parts by weight, especially 1 to 8
parts by weight, per 100 parts by weight of an acrylic
rubber. This range is preferred from the viewpoints of
rate of crosslinking reaction, storage stability,
processing safety, and physical properties of cured
products such as mechanical properties, heat resistance
and compression set. When the amount of the crosslinking
agent is less than the above range, the rate of
crosslinking reaction is lowered and it is hard to obtain
cured products having satisfactory physical properties.
When the amount is more than . the above range, the rate of
crosslinking reaction generally increases, but ~ the storage
stability, processing safety and general physical
properties of cured rubbers are often impaired.
The crosslinking agent according to the present
invention may be used in combination with known
vulcanizing agents or vulcanization accelerators used for
general rubbers, such as 2-(morpholinothio) benzothiazole,
morpholine disulfide and 2-(4'-morpholinodithio)
benzothiazole in order to adjust the rate of crosslinking
reaction. These compounds are used in an amount effective
to control the rate of crosslinking reaction, usually in
an amount of 0 to 10 parts by weight, especially 0.1 to 2
parts by weight, per I00 parts by weight of the acrylic
rubber.
The crosslinking agent is added to an acrylic
rubber together with usual additives such as reinforcing
agent and filler and other additives optionally employed
such as antioxidant, plasticizer, lubricant and processing
aid. After mixing them through a usual means such as a
roll mill or a Banbury mixer, the resulting mixed compound
is cured under heat and pressure to crosslink the rubber.
20~32~0
The curing temperature is on the same level as that
adopted to usual crosslinking agents, and it is usually
selected from about 120° to about 250 C , especially about
150° to about 200°C .
Acrylic rubbers cured with the crosslinking
agent according to the present invention have excellent
characteristics such as mechanical properties, compression
set, weathering resistance, ozone resistance and electric
properties as well as heat resistance, oil resistance and
water resistance. Accordingly, they can be effectively
utilized for various uses, e.g. various kinds of sealing
materials such as gasket, packing, 0-ring and oil seal,
various kinds of hoses, covering materials, various kinds
of belts and various kinds of rolls.
Also, since the crosslinking agent according to
the present invention is a white powder, it is applicable
to not only black compounds, . but also colored compounds.
The crosslinking agent according to the present invention
has the advantage in this respect.
The present invention is more specifically
described and explained by means of the following
Examples, in which all parts and % are by weight unless
otherwise noted. It is to be understood that the present
invention is not limited to the Examples.
Example 1
(A) In 200 parts of water was dissolved 100
parts of urea, and thereto were added 1.7 parts of sodium
laurylsulfonate as an emulsifier, 167 parts of 37
formalin and 1 part of 35 % hydrochloric acid 'as a
catalyst in that order. Exothermic reaction was started
at 25°C and continued for 20 minutes. The obtained
reaction product was filtered and dried to give a white
powder to be used as a crosslinking agent according to the
present invention.
(B) A reactor was charged with 200 parts of
water, and thereto were added 0.5 part of sodium
laurylsulfonate and 2 parts of polyoxyethylene lauryl
- 10 -
ether as an emulsifier. To the reactor were further added
parts of allyl cyanoacetate, 0.05 part of potassium
persulfate and 0.05 part of sodium hydrogensulfite, and
the temperature was elevated with introducing nitrogen gas
5 into the reactor. The emulsion polymerization was carried
out at 60°C with intermittently adding 95 parts of ethyl
acrylate over 30 minutes to give an acrylic rubber.
(C) To 100 parts of the obtained acrylic rubber
were added 50 parts of MAF carbon (commercially available
under the trade mark " Seast" #116 from Tokai Denkyoku
Kabushiki Kaisha), 1 part of stearic acid, 2 parts of a
substituted diphenylamine, 4 parts of the crosslinking
agent obtained in the above (A), and 0.5 part of
morpholine disulfide as crosslinking accelerator. The
mixture was kneaded sufficiently in an open roll. It was
then press-cured at 170 C for 15 minutes to produce a
rubber sheet, and subjected to postcure at 150 C for 8
hours. Morpholinedisufide itself has no crosslinking
action, but serves to control the crosslinking reaction
rate.
The physical properties of the thus cured
acrylic rubber under the ordinary state were measured
according to JIS K 6301. The results are shown in Table
1.
The air-oven aging test at 177°C for 70 hours,
oil resistance test at 150°C for 70 hours, water
resistance test at 100 C for 70 hours and measurement of
compression set of the cured acrylic rubber were also made
according to JIS IC 6301. The results are also shown in
Table 1.
Example 2
The procedure of Example 1 was repeated except
that 127 parts of thiourea was employed instead of 100
parts of urea to give a crosslinking agent.
The cured acrylic rubber was estimated in the
same manner as in Example 1. The results are shown in
Table 1.
20~93~~9
- 11 -
Comparative Example 1
The same acrylic rubber as used in Example 1 was
cured and tested in the same manner as in Example 1 except
that 2 parts of tetramethylthiuramdisulfide and 2 parts of
dibenzothiazyl disulfide were used instead of 4 parts of
the crasslinking agent obtained in (A) of Example 1.
The results are shown in Table 1.
Comparative Example 2
An acrylic rubber containing units derived from
an epoxy monomer (commercially available under the trade
mark "Nipol AR-51" from Nippon Zeon Co., Ltd.) was
compounded and cured in the same manner as in Example 1
except that 3 parts of ferric dimethyldithiocarbamate was
used as the crosslinking agent instead of the crosslinking
agent obtained in (A) of Example 1.
The results are shown- in Table 1.
Comparative Example 3
An acrylic rubber containing units derived from
a halogen-containing monomer (commercially available under
the trade mark " Nipol AR-~71" from Nippon Zeon Co., Ltd. )
was cured in the same manner as in Example 1 except that
0.3 part of sulfur, 0.5 part of potassium stearate and 3.2
parts of sodium stearate were used as the crosslinking
agent instead of the crosslinking agent obtained in (A) of
Example 1.
The results are shown in Table 1.
2~1~~~4~
- 12 -
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2~193~~9
- 14 -
As apparent from the results shown in Table 1,
the acrylic rubbers cured with the crosslinking agent
according to the present invention in Examples 1 and 2
have a high tensile strength even after the air-oven aging
test under such severe conditions as 177 C X 70 hours, and
is superior in heat resistance to the cured rubbers
obtained in Comparative Examples 1 to 3.
Also, in the oil resistance tests, the cured
rubbers obtained in Examples 1 and 2 show a volume
changing rate on the same level as those of Comparative
Examples 1 to 3, and no decrease in physical properties
due to oil is seen. Thus, it is understood that the cured
rubbers of Examples l and 2 have an excellent oil
resistance.
In the water resistance test, the cured rubbers
obtained in Examples 1 and 2 show a volume changing rate
on the same level as that of Comparative Example 1 and
show a less volume changing rate than those of Comparative
Examples 2 and 3. Thus, it is understood that the cured
rubbers of Examples 1 and 2 have an excellent water
resistance. In particular, the halogen-containing acrylic
rubber in Comparative Example 3 shows a large volume
changing rate in water resistance test, and is poor in
water resistance.
The cured rubber obtained in Examples l and 2
have at' compression set on the same level as that of the
halogen-containing . acrylic rubber which is known to have
the lowest compression set among known acrylic rubbers,
and accordingly are superior in this respect.
In addition to the ingredients used iri the
Examples, other ingredients can be used in the Examples as
set forth in the specification to obtain substantially the
same results.
