Note: Descriptions are shown in the official language in which they were submitted.
--1--
B _ DS OF NATURAL ~ND 5YNTHETIC n~BBE~S
TECHNI_AL FIELD
The present invention provides for an improvement
in -the green strength and building tack oE natural/synthetic
rubber blends which is accomplished by partially substi-
tuting the natural rubber component with a modifled syn-
thetic emulsion polymer.
In the art of rubber compounding it is recognized
that natural rubber is processable and possesses tack and
green strength which are useEul properties for the manu-
facture oE articles such as tires. Processability, tack
and green strength are important properties of many elas-
tomers but are not usually all present in one compound.
lS Tack is the ability of two materials to resist separation
after bringing their surfaces into contact for a short time
under a light pressure. The green strength of an elastomer
is its resistance to deformation and fracture before
vulcanization. Processability is a general term embracing
processes and associated factors including mastication and
mixing time, energy consumption, scorching tendencies,
dispersion of fillers, extrusion and dimensional stability.
Rubber s-tocks that are used in tire manufacture
must be processable and have a certain minimum level of
tack and green strength. Processable stocks used for high
speed production of tire body plies can be deEined as
having a Mooney viscosity of about 45 to 55. rrhey can be
calendered at about one meter per second in tire ply
thicknesses, e.g., 1.27 mm f 0.5 and provide a smooth,
uniform surface. Tack is necessary so that the many com-
ponents of a green tire will hold together until molding.
This requires not only that the components exhibit quick
stick when building, but also that the tack bonds have long
term creep resistance, since the green tire may be hung on
a rack several days before molding and vulcanization. In
addition, an uncured tire must have good green strength so
that it will not creep and hence distort excessively before
molding or tear during the expansion that occurs upon
. .
molding, or in the second sta~e Eor a radial tire.
Styrene butadiene rubber (S~R) is widely used in
tire components to take advantage of its cost, process-
ability and physical properties, however, it~ drawbclcks o~
poor tack and green strength require the addition of
natural rubber. Natural rubber is blended with SBR because
it imparts tack and green strength which thereby facilitates
tire building.
Basically, green strength is a measure of unvul-
canized stress/strain properties of rubber which are
generally portrayed by s-tress/strain curves that can be
related to the ability of an elastomer to withstand breaking
upon being stretched or elongated. The inventor has
explained in related work, published elsewhere, that
natural rubber has greater green strength than S~R due to
its ability to strain-harden through crystallization upon
high deformation. The strain crystallization of natural
rubber provides a strengthening mechanism that is absent in
SBR. Specifically, the SBR green strength mechanism is
through chain entanglement which results in an initially
high tensile modulus upon elongation but which quickly
necks down and breaks.
BACKGROUND OF T~IE INVENTION
As explained hereinabove, it is desirable to have
a compound of SBR or other diene-containing synthetic
rubbers with high green strength and good building tack.
Through the years, increasing the green strength of SBR has
been attempted by increasing molecular weight, partial
crosslinking, modifying the polymer structure and the
addition of other polymers.
Regarding the addition of other polymers, U.S.
Pat. No. 3,798,190 discloses rubber compositions of high
green strength comprising blends of styrene-butadiene
rubber, with or without natural rubber, and a hydrogenated
random copolymer of styrene-butadiene.
A recent U.S. patent, No. 4,254,013 discloses an
~Z~1~4
attempt to improve the green strength of elastomeric blends
by the addition oE an unsaturated acid to the synthetic
component of a natural or synthetic cis~l,4-polyisoprene
and synthetic elastomer composition whereby the green
strength is increased by the formation of ionic bonds.
Despite the improvement of green strength by the addition
o~ the unsaturated acid, the patent still recommends a
range of 50 to about 90 parts of natural rubber for those
blends which exhibit improved green strength.
The preparation of a synthetic diene-containing
n~er having i~,proved green strength has been set forth in U.S. patent
no. 4 338 425, issued July 6, 1982, com~.only owned by the Assignee
of record herein. There, it has been disclosed that a
terpolymer comprising styrene and butadiene, or similar
monomers, and an N-(alkoxymethyl)acrylamide exhibits
improved green strength over conventional S~Rs.
Prior art considered during prosecution of the
aforementioned patent application such as U.S. Pat. No.
3,037,963 disclosed the preparation of aldehyde-modified
carboxylic acid amide resinous materials comprising in
specific instances, styrene, butadiene and N-(alkoxymethyl)-
acrylamide. The materials were said to be useful as
coating compositions.
Thus, the art considered herein has not provided
a blend of natural and synthetic rubbers, having the green
strength, building tack and processability of natural
rubber. Merely by substituting various synthetic rubbers
to reduce the natural rubber content to about 30 percent by
weight, based upon the total weight of the rubber, it has
not been possible to provide rubber compounds suitable for
the body plies of tires.
DISCLOSURE OF INVENTION
In general, the subject invention is directed
toward a blend of natural and synthetic rubber, having
processability and improved green strength and building
tack. The blend comprises from about 5 to 55 parts by
r~
i.~
14~(~
-- 4
weight oE natural rubber, from about 30 to 50 parts by weight
of a conjugated diene-containing synthetic rubber and, from
about 5 to 50 par-ts by weight of a substituted acrylamide-
containing synthetic rubber.
More par-ticularly, the present invention proposes
a blend of natural and synthetic rubbers having processability
and improved green strength and tack, said blend comprising:
from about 5 to 55 parts by weight of na-tural
rubber;
from about 30 to 50 parts by weight of a conjugated
diene-containing synthetic rubber having an average molecular
weight of from about 50 X 103 to about 800 X 103; and
from about 5 to 50 parts by weight of a substituted
acrylarnide-containing synthetic rubber having an average
molecular weight of from about 50 X 103 to about 500 X 103;
wherein said conjugated diene-containing synthetic
rubber is selected from the group consisting of conjugated
diene homopolymers and copolymers, prepared from conjugated
diene monomers having from four to about 12 carbon atoms,
with monomers containing a vinyl group selected from the
group consisting of monovinyl aromatic compounds having from
eight to about 20 carbon atoms and acrylic compounds having
from three to about five carbon atoms including halogen-
substituted compounds;
said substituted acrylamide-containing synthetic
rubber comprises
from about 60 to 99.7 percent by weight of said
conjugated diene monomer;
from about 0 to 39.7 percent by weight of said
monomer containing a vinyl group and copolymerizable with
said conjugated diene monomer; and
from about 0.3 to 10 percent by weight of an N-(al-
koxymethyl)acrylamide monomer having the formula
- ~a -
ll2c=c-c-N-c~l2-OR
wherein R is a straight or branched alkyl chain
having from 1 to about 20 carbon atoms, the weight of each
said monomer component being based upon the total weight of
said substituted acrylamide-containing synthetic rubber; and
wherein said blend is based upon the following
relatioship:
A~B_C
C+A>B
wherein A is equal to the parts by weight of
said natural rubber, B is equal to the parts by weight of
said conjugated diene-containing synthetic rubber, and C is
equal to the parts by weight of said substituted acrylamide-
containing synthetic rubber.
The blend of the present invention is useful as a
rubber stock for tire carcasses where high green strength
and good building tack are necessary. Such rubber stocks
should also be easily processable and relatively stable.
The improvements in the natural/synthetic rubber blend of
the present invention are primarily attributable to the
second synthetic rubber component. The composition thereof
includes a conjugated diene monomer, a copolymerizable
monomer containing a vinyl group and an N-(alkoxymethyl)-
acrylamide monomer.
BRIEF DESCRIPTION OF THE DRAWI _
Figs. 1 and 2 present various stress-strain
curves for rubber blens of the present invention and for
comparison, a conventional rubber blend of SBR and natural
rubber.
',,:
4~
- 4b -
PREFERRED MODE FOR CARRYING OUT T~IE INVF.NTION
The natural/synthetic rubber blend of the present
invention contains from about 5 to 55 parts by weight of
natural rubber, with 30 parts being preferred. A second
component of the blend is a conjugated diene-containing
synthetic rubber in an amount of from about 30 to 50 parts
by weight, with 40 parts being preferred. The aEoresaid
synthetic rubber comprises homopolymers of conjugated
dienes having from about four to 20 carbon atoms, with 1,3-
polybutadiene or isoprene being preferrëd, and copolymers
thereof with plastic-forming monomers containing a vinyl
group. The conjugated dienes may have from 4 to about 12
carbon atoms.
Suitable vinyl monomers include monovinyl aromatic
compounds, having from eight -to about 20 carbon atoms and
optionally one or more halogen substituents, and acrylic
compounds having from three to about five carbon atoms.
,~
;~,
(L84
Examples of the monovinyl aromatics include styrene and
alpha-methylstyrene, and examples of the acrylic monomers
include methyl methacrylate and acrylonitrile. Most
suitable as the synthetic rubber component is polybutadiene,
styrene-butadiene rubber (SBR) or acrylonitrile-butadiene
copolymer (nitrile rubber). Average molecular weight (Mw)
of the synthetic rubber ranges from about 50x103 to
800x103 with ~00x103 being preferred.
The third and last component of the blend com-
prises a second synthetic rubber, differing from the first,
in that it also contains an N-(alkoxymethyl)acrylamide
monomer. More specifically, the second synthetic rubber,
sometimes referred to herein as the substituted acrylamide-
containing rubber, comprises a conjugated diene monomer as
described hereinabove; optionally a copolymerizable monomer
containing a vinyl group, as described hereinabove; and an
N-(alkoxymethyl)acrylamide monomer having the formula
o
H2C=C--C-N-CH2-OR
H H
wherein R is a straight or branched alkyl chain having ~rom
1 to about 20 carbon atoms with 3 to 8 carbon atoms being
preferred. One particularly useful acrylamide employed for
the work reported herein is N-(isobutoxymethyl)acrylamide
(hereinafter IBMA) the R group being isobutyl The amount
of substituted acrylamide-containing synthetic rubber
ranges from about 5 to 50 parts ~y weight, with 30 parts
being preferred.
The composition of the substituted acrylamide-
containing rubber includes from about 60 to 99.7 percent by
weight of the conjugated diene monomer; from about 0 to
39.7 percent by weight of the monomer containing a vinyl
group and, from about 0.3 to about 10 percent by weight of
the N-(alkoxymethyl)acrylamide, the weight of each said
monomer component being based upon the total weight of said
substituted acrylamide-containing synthetic rubber. Average
~1 4~9~
molecular weight (Mw) of the substituted acrylamide-
containing rubber is from about 50 x 10 with 20G x 103
being preferred.
Amounts of the three rubber components employed
,~
8(~
in the blend are based upon the followin~ relationship:
C -~ ~ > B
A + B > C
where A is natural rubber; B is S~R or other conjugated
diene-containing rubber and C is IBMA. Natural rubber has
the three desired properties while SBR and IBMA have only
processability and high green strength, respectively. By
the proper combination of the two synthetic rubbers with
natural rubber, the amount of natural rubber employed can
be significantly reduced to the point of constituting only a
minor component of the blend.
Preparation of the copolymer is via emulsion
polymerization with an emulsifier such as sodium lauryl
sulfate. A water soluble initiator is employed such as
diisopropylbenzene hydroperoxide and, a molecular weight
modifier, n-dodecyl mercaptan, can be added. An activator
such as tetraethylenepentamine is added to the reaction
vessel immediately after the diene monomer is charged.
Other emulsifiers suitable for the present
invention include long chain metal sulfonates, rosin acid
and fatty acid salts. Amounts added range from about two
parts per hundred of monomer (phm) to about five phm.
Initiators suitable for the present invention other than
diisopropylbenzene hydroperoxide include other free radical
types such as peroxides and peroxydicarbonates r benzoyl
peroxide, cumene hydroperoxide and tert-butyl peroxide, and
the amount employed ranges from about 0.1 phm to about 0.6
phm depending upon the desired molecular weight of the
` polymer product. Similarly, the activators can include
FeSO4 7H2O, alkyl amines having primary and secondary
amines and sodium formaldehyde sulfoxylate in an amount of
from about 0.1 phm to 0.6 phm. Other molecular weight
modifiers suitable for this invention include tertiary
alkyl mercaptans which are employed in amounts of from
about 0.1 phm to about 0.3 phm. Antioxidants can also be
employed such as di-tert-butyl para-cresol (DBPC) and
Santoflex which are added to the polymer latex in amounts
134
of from ahout 0.1 to about l.0 percent by weight of the
solid polymer.
It is to be understood that the method set forth
herein is not the suhject of the present invention which
invention is not to be limited by the recitation of par-
ticular emulsifiers, initiators, activators or modlfiers.
All of these and others are well known to those skilled in
the art of emulsion polymerization and therefore, the
present invention does not encompass any selection of such
components or polymerization techniques.
Synthesis generally involves the steps o~ charg-
ing a reaction vessel with -the water, detergent, a modifier
if desired, the initiator, and the acrylamide and vinyl
monomers; purging the vessel with an inert gas such as
nitrogen; charging the diene monomer and activator to the
vessel and polymerizing the monomers therein for a period
of time of from about 12 to about 16 hours at a temperature
of from about 0 C to about 30 C. Following polymeriæa-
tion, the polymer product is obtained by coagulating or
precipitating into isopropanol.
Coagulation can be conducted by any of the known
techniques for coagulation of a polymer latex with an
electrolyte such as by mixing the latex and electrolyte
together at a temperature above the freezing temperature
and below the boiling temperature of the latex, the amount
of electrolyte employed varying with several factors such
as the solids content of the latex, the particle size of
the latex, the amount of emulsifying agent in the latex,
the particular electrolyte used, and so forth. The latex
and electrolyte are usually mixed by adding the latex to an
aqueous solution of the electrolyte, adding the electro~
lyte, usually as a dilute aqueous solution, to the latex,
or simultaneously feeding the latex and an aqueous solution
of electrolyte to a mixing chamber. Temperatures of about
5 to 30 C are preferred and agitation is normally con-
tinued throughout the coagulation. Among the electrolytes
usually used for coagulation are alcohols or aqueous metal
salt solutions.
Coagulation with alcohol or aqueous metal salt
solutions will provide the solid polymer which is then
washed and dried Eor subsequent usage such as compression
or injection molding. While either system will coagulate
the latex when metal salt solutions are selected, the latex
blend also undergoes coordination which is believed to
occur between the metal ions and the substituted acryl-
amides. Where alcohol is selected, coordination does not
occur, however, upon heating, the polymer crosslinks
through the substituted acrylamides.
The metal ions that coagulate the copolymer latex
are all divalent, trivalent and polyvalent metal ions such
as aluminum, barium, cadmium, calcium, chromium, cobalt,
iron, magnesium, manganese, nickel, tin, zinc and the like
which are supplied as metal salts having the formula MX
wherein M is one of the foregoing elements, X is an anion
such as an organic carboxylate, halide, hydroxide, nitrate,
sulfate, sulfonate and the like and n is from two to six.
The amount of the metal salt employed is that amount which
will provide at least one equivalent mole of the metal ion
to two equivalent moles of IBMA bonded to the polymer latex
in the final product.
In the experimental work set forth hereinbelow
exemplifying the blend of the present invention, a sub-
stituted acrylamide-containing synthetic rubber was pre-
pared which was thereafter blended in various amounts with
natural and synthetic rubbers also in various amounts.
Preparation of a substituted acrylamide rubber has been
presented, with all parts being given on a weight percent
basis based upon 100 parts of monomer. Subsequent pre-
paration of the blend has been reported, presenting parts
of each rubber component on a weight percent basis based
upon 100 parts of the three rubber components, unless
otherwise specified.
A typical synthesis of the acrylamide copolymer
of the present invention was conducted as follows: A 295
. .
34
ml bottle was charged with 100.00 g of water, 2.50 g of
sodium lauryl sulfate, 0.20 g oE diisopropylbenzene hydro-
peroxide, 0.05 g of n-dodecyl mercaptan, 10.00 y oE styrene
and 2.00 y of N-(isobutoxymethyl)acrylamide. The bottle
was sealed with a rubber lined, three-hole crown cap and
was puryed with ni-troyen for 15 minutes. 36.00 g oE
butadiene was charyed followed by 0.15 g of tetraethylene-
pentamine. The polymerization was conducted at 5 C for 13
hours. At the end of this time, a 60% conversion of
polymer latex was obtained from the bottle after precipi-
tatiny with isopropanol.
Other rubbers were prepared followiny this same
synthesis. The amount of IBMA and styrene content for
seven such polymers is presented in Table I. Amount of
water was approximately 200 percent, based upon the total
weight of the monomer charge.
Table I
IBMA-Containing SBR
Ex. No.IBMA Wt% Styrene Wt_
1 2.24 12
2 1.12 21
3 1.79 12
4 1.46 20
1.68 21
6 3.02 12
7 4.26 21a
a) ~liyh conversion
Following preparation of the substituted acrylamide-
containing synthetic rubber, that is, the IBMA-containing
SBRs, blends of examples 1-7 were each compounded with
natural rubber and conventional SBRs. The amounts of the
various rubbers are presented in Table II for Stocks 1-12.
4B~
--10--
Stock 1 appears as a Control with no IBMA-containing SB~.
Stocks 2-4 contain one type of IBMA rubber, example 7, in
varying amounts as a substitute for equivalent amounts o~
natural rubber in Stock 1. Stocks 5-10 contain different
IBMA rubbersr examples 4-6 and 1-3, respectively, again as
a substitute for an equivalent amount of natural rubber in
Stock 1, in a fixed amount. Lastly, for comparative
purposes, Stocks 11 and 12 were prepared as controls com-
prising 30/70 blends of natural/SBR rubber without any
IBMA rubber. For Stock 12, 30 parts of a second SB~ were
substituted for an equivalent amount of the SBR in Stock 11.
The properties of the natural rubber, Hartex 20,
and the commercial SsRs, S1502 and HX567 employed in the
blends are as follows: Hartex 20 is a tire grade blend of
natural rubbers. S1502 is a commercially available SBR
containing 23.5 percent bound styrene nonstaining polymer;
having a Mooney viscosity (ML/4/100 C) from 46 to 58 and a
specific gravity of 0.94. HX567 is an oil extended co-
polymer of styrene and butadiene containing 20 parts of
nonstaining oll; 30 percent bound styrene; having a Mooney
viscosity (ML/4/100 C) from 65 to 75 and a specific
gravity of 0.94.
* Trademark
,~a.
3lZ~1~134
Table II
Stock Compositions
Stock Nos: 1 2 3 4 5 6 7_ 8 9 10 11 12
Component
Hartex 20 60 40 30 20 30 30 30 30 30 30 30 30
S1502 40 40 40 40 40 40 40 40 40 40 70 40
HX567 -- -- -- -- -- -- -- -- -- -- -- 36
Ex. 7 -- 20 30 40 -- -- -- -- -- -- -- --
Ex. 4 -- -- -- -- 30 -- -- -- -- -- -- --
Ex. 5 -- -- -- -- -- 30 -- -- -- --
Ex. 6 -- -- -- -- -- -~ 30 -- ~ -- -- --
Ex. 1 -- -- -- __ __ __ __ 30 _ __ __ __
Ex. 2 -- -- -- -- -~ -- 30 -- -~ --
Ex. 3 -- -- -- -- -- -- -- -- -- 30 -- --
a) Contains processing oil, 30 parts of HX567 present
In addition to the rubber components listed in
Table II, several ingredients conventionally employed in
the compounding of rubber stocks, suitable for the manu-
facture of tires, were added as follows: To Stocks 1-12
were added 50 parts carbon black; 7.5 parts zinc oxide and
3.4 parts stearic acid. Stock 1 also contained 8.2 parts
of an aliphatic resin and 2,2 parts of processing oil.
Stocks 2-12 contained 8.0 parts of the aliphatic resin and
5.0 parts of a phenolic resin. Stocks 2-11 also contained
5.0 parts of processing oil. All amounts of the components
are given in a parts per hundred rubber (phr) basis and it
is to be understood that the components are disclosed only
to provide one skilled in the art at least one typical
rubber stock with which to work. The specific formulation
is, therefore, not deemed to be part of the present in-
vention which is the blend of the three rubbers disclosed.
Stocks were mixed in a laboratory Brabender after
first premixing the natural rubber and substituted acryl-
amide rubber. All stocks milled smoothly without melt
* Trademark
,,
-12-
fracture. Properties, obtained using standard laboratory
procedures, are presented in Table III and Figs. 1 and 2.
Compared to the control, Stock 1., all stocks exhibited about
the same scorch time and maximum torque although cure time
was slightly greater in all cases. Stocks 5-10 in par-
ticular had acceptable Mooney viscosities, indicative of
processability. All stocks of the present invention, viz.,
2-10 exhibited good tensile strengths and elonyations,
several exceeding the values reported for Stock 1. With
respect to tack, Stocks 4-10 were significantly better than
the control, Stock 1.
413~L
--13--
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~r ~ ~ ~ ~ ~ ~r
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~r co
~ ~ o ~ ~ ~ coco a~ u~
coco u~ co o ~D ~i
~r ~ ~ 1-- ~ Ll~ r~
o
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X ~co ~ I~ ~ r~ er o ~ ~ ~
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1~7
~ ~ ~J ~ ~ ~ O O ~ O
H ~ ~ ~~1 ~I CO r~
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r~ c~ ~ LO ~ O ~ ~r
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o o o ~ O E
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With respect to the graphs, Fiy. 1 compares the
green stress-strain curves of Stock 1 and Stocks 2-4.
After the standard Brabender mix conditions, Stock 1 did
not exhibit an upturn in its green stress-strain curve.
It is believed that mix conditions were suf-
ficiently severe to cause drastic chain scission of the
natural rubber and hence decrease green strength. This is
also shown by the low Mooney viscosity (ML/4/100 C = 22)
of this stock. Lab Banbury-mixed Stock 1 exhibited a
Mooney in the range of ~5-50 with a corresponding improve-
ment in green strength (Fig. 2). Fig. 1 also shows that
the higher the level of IBMA-containing SBR the greater the
green strength. Referring back to Table III, it can be
seen that wind-up tack values were similar Eor Stock 1 and
Stocks 2-4. Again, the Brabender-mixed Stock 1 had low
tack compared to Banbury-mixed Stock 1 because of the poor
green strength of the former. Thus, green strength has
been improved without loss oE tack.
The green strength of Stocks 5-10 is depicted in
Fig. 2. All the stocks containing 30 phr of the various
IBMA-containing SBR exhibited an upturn in their green
stress-strain response. This is in sharp contrast to
Stocks 11 and 12 in which 30 phr of S1502 or HX567 respec-
tively, were substituted for an equivalent amount of
natural rubber in Stock 1. These two stocks were found to
neck down and break at short elongation when tested. As
noted hereinabove, wind-up tack values ~Table III) were
excellent for several of the stocks containing the various
IBMA-containing SBR. Elends of this type thus show ex-
cellent promise with respect to tack and green strength for
replacing Stock 1.
As the foregoing data demonstrates, the sub-
stitution of a portion of the natural rubber in a natural
rubber/SBR blend with a substituted acrylamide-containing
synthetic rubber provides a useful blend exhibiting
processability as well as good green strength and good
building tack. The blends of the present invention also
--15--
mill smoothly without melt ~ractur~. It has previously
been customary to employ at least 50 ~hr oE natural rubber
in a blend with a diene-containing synthetic rubber to
provide sufficient building tack and green strength for
tire carcasses.
While 60/40 blends of natural/synthetic rubber are
useful for building tire carcasses and a blend of 30/40/30
natural/diene-containing synthetic/substituted acrylamide-
containing synthetic is comparable, the blends oE the
present invention should not be so limited. Similarly, the
diene-containing synthetic rubber SBR, and substituted
acrylamide rubber disclosed, a terpolymer of IBMA, styrene
and butadiene have been provided herein merely for purposes
of exemplification and to demonstrate operability and,
therefore, the selection of specific synthetic rubbers can
be determined without departlng from the spirit of the
invention herein disclosed and described. Moreover, the
scope of the invention shall include all modifications and
variations that may fall within the scope of the attached
claims.
