EP0712418A1

EP0712418A1 - Polychloroprene copolymer composition

Info

Publication number: EP0712418A1
Application number: EP94923913A
Authority: EP
Inventors: Furman Eugene Glenn, Sr.; Fred Kenneth Jones, Jr.; Steven Joseph Klaas; William Kenneth Witsiepe
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1993-08-04
Filing date: 1994-07-29
Publication date: 1996-05-22
Also published as: CN1128540A; JPH09501459A; WO1995004766A1

Abstract

Highly resilient xanthogen disulfide-modified chloroprene copolymers and blends thereof with sulfur-modified chloroprene polymers are provided which are particularly adapted for use in high temperature dynamic applications such as in engine mounts.

Description

TITLE

POLYCHLOROPRENE COPOLYMER COMPOSITION BACKGROUND OF THE INVENTION

This patent relates to novel highly resilient polychloroprene compositions which are resistant to crystallization and creep.

Polychloroprene rubber has replaced natural rubber in a wide variety of automotive applications because it offers superior heat resistance coupled with comparable elastomeric properties. There are certain automotive applications, however, particularly those in which dynamic properties are critical, which require an elastomer having a combination of high resilience, low hysteresis, creep resistance, and durability (fatigue resistance) not attainable using prior art polychloroprene compositions. Consequently, natural rubber has remained the elastomer of choice for vibration isolation applications, especially for engine mount applications. Recent automobile design trends have led to size reductions of the engine compartment and, as a result, engine compartment temperatures are higher than in the past. At these higher temperatures natural rubber compositions degrade rapidly. Thus, there is a need in the art for elastomeric compositions having enhanced heat resistance which exhibit dynamic elastomeric properties comparable to or which exceed those of natural rubber. The present invention is directed to a class of novel chloroprene copolymers which meet this need. The compositions, which have copolymerized chloroprene units of a specific cis.trans ratio, exhibit the performance properties and heat resistance necessary for use in applications un<"' zτ dynamic high temperature conditions wherein a low degree of damping is required.

SUMMARY OF THE INVENTION In particular, this invention is directed to highly resilient dialkyl xanthogen disulfide-modified or dialkoxy xanthogen disulfide- modified 2-chloro-l,3-butadiene copolymers comprising 92-99 percent by weight copolymerized units of 2-chloro-l,3-butadiene, up to 91%, of said 2- chloro-l,3-butadiene units having a 1,4-trans configuration, and 1-8 percent by weight copolymerized units of at least one comonomer selected from the group consisting of 2,3-dichloro-l,3-butadiene, styrene, methyl methacrylate, acrylonitrile, isoprene, and butadiene, said copolymers when cured having a tan delta of less than 0.20 at 5 Hertz and 23°C.

The invention is also directed to a process for preparation of the highly resilient 2-chloro-l,3-butadiene copolymer compositions, which comprises copolymerizing a mixture of 2-chloro-l,3-butadiene and comonomer described above to a conversion of up to 75% in an aqueous alkaline emulsion in the presence of a free radical initiator at a temperature of at least 42°C and in the presence of no greater than 0.8 parts of a dialkyl xanthogen disulfide chain transfer agent or a dialkoxy xanthogen disiilfide chain transfer agent per 100 parts monomer, thereby forming a 2-chloro-l,3- butadiene copolymer having 92-99 percent by weight copolymerized 2- chloro-l,3-butadiene units and 1-8 percent by weight copolymerized units of the comonomer or comonomers.

The invention is further directed to blends of the above- described copolymers with sulfur-modified 2-chloro-l,3-butadiene homopolymers or sulfur-modified copolymers of 2-chloro-l,3-butadiene and 2,3-dichloro-l,3-butadiene containing up to 5 percent by weight copolymerized units of 2,3-dichloro-l,3-butadiene, said sulfur-modified homopolymers or sulfur-modified copolymers containing less than 0.6 percent by weight combined sulfur.

In addition, the invention is directed to curable compositions of the novel copolymers and copolymer blends and to engine mounts made from the copolymer and copolymer blend compositions.

For the engine mount utility, the xanthogen disulfide-modified copolymer described above exhibits an unusual combination of properties. The small amount of comonomer copolymerizable with the chloroprene monomer is effective to increase resistance to low temperature crystallization, and xanthogen disulfide modification is effective to increase resilience (low vibration damping), characterized by tan delta of less than 0.2 at 5 Hertz and 23°C, and increase durability. The modified copolymer also exhibits high heat resistance and low creep. This copolymer can be further modified through compounding to develop properties desired for particular applications. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1, 2, and 3 are, respectively, a perspective view, a front elevational view, and a cross-sectional view, all schematically presented, of an A-mount test specimen. In the figures, each reference numeral refers to the same structural element in each figure.

DETAILED DESCRIPTION OF THE INVENTION The compositions of the present invention are copolymers of 2-chloro-l,3-butadiene (i.e. chloroprene) which are especially adapted for use in high temperature dynamic applica^tions in which high resilience is required. Under such conditions it is desirable to minimize polymer internal heat buildup by minimizing hysteresis or mechanical energy dissipation.

In rubber technology, hysteresis losses are the losses of energy irreversibly converted to heat when an elastomer is subjected to dynamic stress. Hysteresis losses for a given material are dependent on the relationship between stress and strain and can be measured as tan delta. When the strain is alternating, tan delta is defined as the ratio of loss modulus, i.e. the component of shear stress that is out of phase with strain, to storage modulus, i.e. the component of stress that is in phase with the strain. For a given material stiffness, tan delta is large when energy dissipation is large. It is small for a highly resilient polymer.

The copolymers of the present invention are characterized by the type of copolymerized monomer units present and their ratio. This selection plays a major part in insuring that the tan delta of the polymers is low without compromising other important polymer properties, such as crystallization resistance. That is, the copolymers contain 1-8 weight percent, preferably 3-6 weight percent, of at least one copolymerizable monomer selected from the group consisting of 2,3-dichloro-l,3-butadiene, styrene, methyl methacrylate, acrylonitrile, isoprene, and butadiene, and 92- 99 weight percent chloroprene units. Although dipolymers are preferred, higher copolymers are also within the scope of the present invention, for example terpolymers wherein more than one of the above-described copolymerizable monomers is present or copolymers which additionally contain other copolymerizable monomers such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, lower alkyl acrylamides, lower alkyl methacrylamides, methacrylonitrile, lower alkyl acrylates, other lower alkyl methacrylates, lower alkyl maleates, and lower alkyl fumarates. In all cases, at least 92 weight percent of the copolymerized units are chloroprene units and 1-8 weight percent of the remaining units are 2,3-dichloro-l,3-butadiene, 5 styrene, methyl methacrylate, acrylonitrile, isoprene, or butadiene units. 2,3- Dichloro-l,3-butadiene is the preferred copolymerizable comonomer because it has a higher reactivity with chloroprene than do styrene, methyl methacrylate, acrylonitrile, isoprene, or butadiene. However, use of any of these comonomers will produce compositions which have low tan delta. If 0 less than about 1 weight percent of the copolymerizable monomer is present the copolymer has poor low temperature crystallization resistance (high compression set e.g. 22 hrs. at -10°C), whereas if greater than about 8 weight percent is present the tan delta of the copolymer is not acceptable for many applications. Tan delta for cured compositions of the present invention is 5 less than 0.20, and preferably no greater than 0.10, at 5 Hertz and 23°C as determined by resilience testing using the dynamic testing procedure and A- mount test apparatus used therein as described hereinafter. The A-mount is a generic engine mount and consequently closely approximates actual use conditions. Referring to Figure 1, 10 is a 1008-1010 steel cylinder having an 0 approximate inner diameter of 76.3mm and a length of 48.9mm. The inner cylinder, 12, also of 1008-1010 steel, has an approximate inner diameter of 72.9mm, an outer diameter of 76.3mm, and a length of 48.9mm. A thin layer 14 of compounded composition described hereinafter, having an approximate thickness of 1.5mm is coated on the inner surface of inner 5 cylinder 12. As illustrated in Figure 2, two legs 22 of compounded composition, are symmetrically situated 31.5 degrees from the vertical center line 4. Each leg is 15mm wide and 41.6mm thick. The height of each leg is determined by the distance between the centerline 5 of the insert and the horizontal centerline 8 of cylinder 12; this distance is 7.03mm. Referring to o figure 3, an adhesive primer layer 16, composed of Thixon^®P15, available from Morton International, Inc., approximately 0.06mm thick, is applied to the surface of inner cylinder 12 and the outer surface of aluminum center insert 30. Two adhesive layers, composed of Thixon^®511T, 18 and 20, each 0.18mm in thickness, are applied to adhesive primer layer 16 as shown in Figure 3. The adhesive primer layer 16 and the two adhesive layers 18 and 20 serve to bond the legs 22 and layer 14 to the inner surface of cylinder 12 and the legs 22 to the outer surface of insert 30. Again referring to Figure 2, the distance between points a and b of insert 30 along line 4 is 18.95mm and the distance between points c and d along line 5 is 31.75mm, which is the horizontal centerline of the insert 30. The angle formed by lines 5 and 6 is 31.36°. The length of aluminum insert 30 is 77.32mm. Insert 30 has a hole 32 extending along its length for receiving a rod (not shown), which is attached to a fixture (not shown), which, in turn is attached to the test equipment (not shown).

A further characteristic of the copolymers of the invention is that no greater than 91% of the polymerized chloroprene units are in the 1,4-trans configuration. Such copolymers are produced at polymerization temperatures which exceed 42°C. As a result of the high polymerization temperature the amount of copolymerized chloroprene cure-site monomer units which result from 1,2 polymerization is increased compared to copolymers prepared at lower temperatures. It is postulated that the increased crosslink density which is obtained on curing such compositions contributes to a lower tan delta in the cured copolymers. Another important structural feature of the copolymer compositions is that they are modified by dialkyl or dialkoxy xanthogen disulfides. It has been found that cured xanthogen disulfide-modified copolymers exhibit lower values of tan delta than comparable compositions which have been modified by other chain transfer agents such as mercaptans.

The chloroprene copolymer compositions of the invention are solids, that is they are neither fluids nor latexes. They have Mooney viscosities, MLχ+4 @ 100°C, of at least 25.

An effective method for preparation of the chloroprene copolymer compositions is aqueous emulsion polymerization of chloroprene and comonomer or comonomers in the presence of a free radie-1 catalyst. Any of the conventional free radical polymerization catalysts r y be utilized including alkali metal or ammonium ferricyanides and peroxy compounds such as alkali metal or ammonium persulfates, hydrogen peroxide, cumene hydroperoxide, and dibenzoyl peroxide. The amount of free radical catalyst which is employed is between the ranges of 0.001-0.2% by weight, based on the total amount of monomers present. In order to provide an acceptably high rate of polymerization it is usually desirable to employ reducing agents such as sodium formaldehyde sulfoxylates or sodium hydrosulfite in combination with the free radical catalyst.

Any of the conventional emulsifying agents may be used in preparing the monomer emulsion. These include the water-soluble salts, particularly the sodium, potassium, or ammonium salts, of compounds of the following types: long-chain fatty acids; rosins or rosin derivatives, such as wood rosin, tall oil rosin, disproportionated rosins, or partially polymerized rosin; higher alcohol sulfates; and arylsulfonic acids such as alkylbenzenesulfonic acids and the condensation product of formaldehyde with a naphthalenesulfonic acid. The dialkyl xanthogen disulfide chain transfer agents used for modifying the chloroprene copolymer can be represented by the formula

s s

II I I

RO — C — S — S — C — OR '

wherein R and R' are alkyl radicals having one to eight carbon atoms.

Examples of suitable alkyl radicals are methyl, ethyl, propyl, isopropyl, and the various isomeric butyl, amyl, hexyl, heptyl, and octyl radicals. Preferred dialkyl xanthogen disulfides are those in which each alkyl radical has 1 to 4 carbon atoms. Diisopropyl xanthogen disulfide is particularly preferred because it has a lower toxicity than other dialkyl xanthogen disulfides while maintaining good efficiency. Dialkoxy xanthogen disulfide modifiers may also be employed. These compounds are compositions of the formula

RO — ( CH , ) m - O — ( CH₂ ) OR m wherein R and R' are independently hydrogen, alkyl radicals having 1-8 carbon atoms, or an oxygen-containing heterocyclic ring system having from 5 to 6 ring members and m is 2 to 6. Examples of suitable alkyl radicals are methyl, ethyl, butyl, and hexyl. A preferred heterocyclic R group is 5-ethyl- l,3-dioxan-5-yl-methyl. Generally the modifier content of the polymer is from 0.4-0.9 weight percent. The preferred range for modifier content of the polymer is 0.76-0.84 . Generally, at least 0.2 parts dialkyl or dialkoxy xanthogen disulfide chain transfer agent per 100 parts monomer is used during the polymerization in order to control Mooney viscosity and to control gel content. Preferably 0.57-0.63 parts per 100 parts of total monomer is used. The maximum amount of chain transfer agent that may be used to produce polymers of the desired Mooney viscosity is 0.9 parts per 100 parts of monomer. In the present invention the amount of units derived from comonomers other than chloroprene in the copolymer is 1-8 percent by weight, preferably 3-6 percent by weight. 2,3-Dichloro-l,3-butadiene, the preferred comonomer, is substantially more reactive than chloroprene. Therefore, the dichlorobutadiene is incorporated into the copolymer more rapidly than is chloroprene. It is usually necessary to limit conversion to 65-

75%, preferably 70%, to eliminate undesirable quantities of gel in the polymer. Because only up to 75% by weight of the total dichlorobutadiene and chloroprene monomers charged is converted to polymer, the proportion of dichlorobutadiene in the isolated polymer is higher than the proportion of dichlorobutadiene in the diene monomers charged. For these reasons, the isolated polymer will contain more units derived from 2,3-dichloro-l,3- butadiene than the amount charged to the polymerization vessel. Conversely, styrene, methyl methacrylate, acrylonitrile, isoprene, and butadiene are less reactive than chloroprene and the proportion of these comonomers in the isolated polymer will be less than their proportion in the monomer mixture charged. The presence of units derived from 2,3-dichloro- 1,3-butadiene and the other copolymerizable monomers can be shown by nuclear magnetic resonance spectroscopy. The amount of comonomer in the copolymer can be determined by chlorine analysis, infrared spectroscopy, and nuclear magnetic resonance spectroscopy of a refined polymer sample.

The concentration of organic monomers present in the aqueous emulsion is not critical. In general 30 to 60 percent, by weight, based on the total weight of the emulsion, is the range of concentration of organic monomers used in the preparation of the copolymers.

The exact proportion of comonomer used will depend on the reactivity of the particular comonomer or comonomers and the amount of the comonomer desired in the resultant copolymer. For example, the monomer ratio in the polymerization emulsion wherein the comonomer is 2,3-dichloro-l,3-butadiene will generally be 0.6-5.6 parts per 100 parts by weight of total monomer.

The proportion of the units derived from chloroprene units in the copolymer which iave a 1,4-trans configuration is a further important feature of the present invention. This proportion is a function of the temperature used during polymerization. Polymers prepared at temperatures above 42°C will have 1,4-trans content of monomer units derived from chloroprene of less than 91% based on the total number of units derived from chloroprene present since this is strictly a function of polymerization temperature when using a free radical polymerization system. Although polymerization temperatures greater than 55°C will produce copolymers having lower amounts of trans 1,4-chloroprene units, such compositions are deficient because compounded polymers tend to be scorchy and because popcorn polymer formation and other side reactions make polymerization more difficult. A polymerization temperature of 42- 52°C is most preferred because it balances the crystallization resistance and processibility of the copolymer. The 1,4-trans content can be determined by carbon-13 nuclear magnetic resonance spectroscopy, by the methods of Coleman, Tabb, and Brame, Rubber Chemistry and Technology. Vol. 50, (1977), pages 49-62, and Coleman and Brame, Rubber Chemistry and

Technology. Vol. 51, No. 4, (1978), pages 668-676.

Polymerization is carried out in an inert atmosphere free of oxygen, such as an atmosphere of nitrogen or other inert gas. It is preferred to operate at a pH in the alkaline range as is customary in chloroprene polymerization processes. Polymerization may be stopped at any desired point by the use of conventional short-stopping agents, such as thiodiphenylamine, p-tertiarybutylcatechol, hydroquinone, and sodium diethyldithiocarbamate. Unreacted monomer is removed by known methods, such as by steam stripping.

The present invention is further directed to blends of the above-described dialkyl or dialkoxy xanthogen disulfide-modified 2-chloro- 1,3-butadiene copolymer compositions with sulfur-modified chloroprene polymers containing to 5 percent by weight 2,3-dichloro-l,3-butadiene. The sulfur-modified polymer contains less than 0.60 wt.% combined sulfur, preferably less than 0.45 wt.% combined sulfur. These blends are composed of 15-85 parts of the sulfur-modified polymer and 85-15 parts of the dialkyl or dialkoxy xanthogen disulfide-modified copolymer. Preferably 40-60 parts of the sulfur-modified copolymer is present, most preferably 45-55 parts. If less than 15 parts of the sulfur-modified polymer is present fatigue resistance is low. If greater than 85 parts is present, heat resistance and creep resistance are adversely affected.

The sulfur-modified homopolymers and sulfur-modified copolymers may be prepared by polymerization techniques which are well- known in the art. Polymerization is conducted in an aqueous en ion and may be, for example, carried out as follows: an aqueous solution or the polymerization initiator, e.g. an alkali metal or ammonium ferricyanide or a peroxy compound, is added to an aqueous emulsion containing chloroprene, sulfur, emulsifiers, and optionally 2,3-dichloro-l,3-butadiene. The pH of the polymerization medium is maintained at about 11-13. Generally, about 0.2- 0.4 parts sulfur per 100 parts chloroprene monomer is used in the polymerization process. It is important that the amount of sulfur incorporated in the polymer be below 0.60 wt.% , preferably below 0.45 wt.%, to insure that the polymer blends have adequate heat resistance. The polymerization temperature range is quite broad, 0-80°C being suitable, although a temperature of 35-50°C is preferred. Polymerization is carried out to the desired conversion, normally from about 70% to about 90%. When the desired conversion is reached, polymerization is short-stopped by the addition of a convenient short-stopping agent, such as, for example, a mixture of p-t-butyl catechol and phenothiazine. The polymer, as made, contains sulfur atoms in its chain, and at high conversion it is appreciably crosslinked. Because such polymers are intractable, the molecular weight must be reduced. This step, known as peptization, is normally accomplished by treating the polymer latex with a tetraalkylthiuram disulfide in combination with a sodium dialkyl dithiocarbamate or other sulfur- containing nucleophile. Unchanged monomer is then steam-stripped from the polymer latex and the stripped latex is acidified. The polymer is conveniently isolated on a freeze-roll or by other well known isolation means.

The polymer blends may be prepared by latex blending prior to isolation or the already isolated polymers may be mixed, for example on a rubber mill or in a Banbury mixer. Latex blending provides a more homogeneous blend of the polymeric blend components. The blends are particularly suitable for use in applications wherein they are subjected to high flex, such as roll mounts, because such compositions are optimized with respect to flex cracking when compared with the xanthogen disulfide-modified copolymers themselves.

The dialkyl or dialkoxy xanthogen disulfide-modified copolymers and copolymer blends of the present invention can be cured and compounded in the same manner as other polychloroprene copolymers. During the compounding operation the copolymer compositions are mixed uniformly with the other compounding ingredients using any of the usual mixing devices such as a roll mill, extruder, Banbury mixer, or other internal mixing device which is capable of mixing the materials without undue heating of the mixture. In addition, the compounding step and polymer blending step may be combined to produce a compounded copolymer blend composition in one step. A variety of well-known compounding ingredients useful in the compounding of polychloroprene elastomers may be added to the copolymer compositions. These include solid additives such as reinforcing agents, fillers, pigments, and resins. Examples of such materials are carbon black, silica, calcium carbonate, and titanium dioxide, hydrocarbon resins, and phenolic resins. Liquid additives include processing oils and plasticizers, both monomeric and polymeric, and liquid polychloroprene rubber. Other materials such as stabilizers, antioxidants, antiozonants, lubricants, release agents, and additives that improve certain properties of the compound can be added in minor proportions.

Vulcanization of the blends is accomplished by known means. The vulcanizing agents can be added to the copolymer compositions using a Banbury mixer, a roll mill, or other mixing device under conditions which avoid premature curing of the compound. Chemical agents usually employed for vulcanization of chloroprene polymers are satisfactory for curing the compositions of the present invention and typically include combinations of zinc and magnesium oxides, alone or with organic accelerators; sulfur donor or sulfur/accelerator systems; and peroxides. A preferred curing system is a combination of zinc oxide, magnesium oxide, and ethylene thiourea. Alternatively, 3-methyl-thiazolidine-thione-2 may be used in place of ethylene thiourea. Generally 5 parts of zinc oxide and 4 parts of magnesium oxide are used per 10^r t>arts polymer.

To improve processing safe mercaptobenzothiazyl sulfide, tetraethylthiuram disulfide, or poly(ethyleneoxide) glycol can be used as retarder-activators. The amounts of accelerator and retarder-activator can be varied over a wide range depending on the particular chemical composition, the accelerator, the retarder-activator, and the intended use of the vulcanizate. In most cases, however, it is preferable not to use retarder- activators because the presence of these additives adversely affects various physical properties, especially creep resistance.

It has been found that certain physical properties of the vulcanized compositions can be optimized by varying particular additive parameters. For example, creep resistance of the copolymers and copolymer blends is improved by maintaining a low filler level, generally 10-60 parts per 100 parts polymer, preferably 15-25 parts. In prior art compositions a combination of low tan delta, creep resistance, and durability was not achievable with such low filler concentrations. In addition, in compositions wherein the xanthogen disulfide-modified copolymer is the only polymer component present, it has been found that durability is improved by addition of 0.1-1.0 and preferably 0.1-0.3 parts of sulfur per 100 parts of polymer to the compounding recipe. It has also been found that it is not necessary to use standard flex additives such as zinc mercaptotoluimidazole or mercaptobenzothiazole in the compositions of the present invention. Vulcanized compositions having excellent flex resistance under dynamic high temperature conditions are obtained in the absence of such additives. This is true for compositions wherein the xanthogen disulfide-modified copolymer is the only polymer present as well as in the copolymer blend compositions. Also, it has been observed that addition of a small amount, generally 5-25 parts by weight, preferably 5-15 parts by weight, per 100 parts of polymer of an ester plasticizer such as dibutoxyethoxy adipate, dibutoxyethyl adipate, or dioctyl sebacate enhances tan delta and low temperature flexibility of the compositions of the present invention. The preferred amount of such additives as filler and plasticizer is dependent on the dynamic stiffness requirements of the final compounded elastomer. The compositions of the present invention are useful for engine mounts, power transmission belts, bushings, and vibration isolators. However, they are particularly adapted for use as engine mounts because of the combination of resilience, low tan delta, and flex resistance, which they exhibit. Compounded compositions containing the xanthogen disulfide- modified copolymers of the invention as the single polymeric component are best adapted for use in applications wherein the composition is subjected to compressive loads. Compounded compositions containing the polymer blend compositions of the present invention are most suitable for use in applications wherein the compositions are subjected to extensive flexing.

The invention is further illustrated below by certain preferred embodiments wherein all parts, proportions, and percentages are by weight unless otherwise indicated.

EXAMPLES TEST PROCEDURES The following test methods were used to measure physical properties of the cured copolymer compositions described in the examples.

Dynamic testing, i.e. testing for complex dynamic stiffness (K*) and tan delta, was performed on an MTS 831 Dynamic Characterization Machine. A-mount test specimens, as shown in Figures 1, 2, and 3, were prepared from the polymer compositions by a transfer molding process. Tests were conducted at least 24 hours after molding or performance testing. The A-mount test specimen is assembled in two steps. First, a compounded composition containing curing agents is introduced to a mold cavity which is defined by the metal cylinder 12 having an outer diameter of 78.2mm, as shown in Figures 1-3, the aluminum insert 30, as shown in Figures 1-3, and core inserts to give the shape and orientation of legs 22 shown in Figures 1-3. The transfer molding of heated compounded composition to form the A-mount test specimen (legs 22) also forms the layer 14 (Figure 2) by virtue of the composition flowing between the core inserts and the inner wall of cylinder 12. The part is than cured for 30 minutes at 160°C using conventional transfer molding techniques. Once the molding process is complete the part is removed from the mold and swaged into the outer steel cylinder 10, thus resulting in an A-mount test specimen wherein the inner cylinder 12 has a final outside diameter of 76.3mm, thereby placing the test legs 22 of compounded composition under compression for the conduct of the dynamic testing. When tested the A- mount specimens were loaded in the test apparatus in such a way that the legs pointed symmetrically downward. Dynamic testing was performed in the vertical direction using the following conditions to capture and report K* and tan delta at constant load control.

Preload -444N

Precycles 10

Temperature 23 2°C

Dynamic Amplitude 2mm peak to peak sine Test Frequency 5 hz

Durability testing was performed using a servohydraulic fatigue test machine. The test specimen is placed in the fatigue test apparatus and oriented in such a way that the legs of the A-mount are pointed symmetrically downward. The following test conditions using load control were applied. Frequency 5.0 Hz sinusoidal

Amplitudes . 34.0 kg tension,

-113.4kg compression

Temperature 100 _+ 2°C

Parts are visually inspected for any cracks, tears or failures after intervals of 100,000 cycles but continuous monitoring of the dynamic properties enables 5 the point of failure to be more exactly determined. Failure is defined as any crack that exceeds 5mm in length, wherein the crack involves solely rubber to rubber separation within a leg of the A-mount test specimen.

The creep resistance test is a static test which measures deflection under compressive load at elevated temperature. A pre-test free o height of the A-mount test specimen is recorded by measuring the free position of diamond-shaped aluminum insert 30, as shown in Figure 1, relative to the outer cylinder. The test specimen is placed in the creep test apparatus and oriented so that the legs are pointed symmetrically downward. The test chamber is heated to a temperature of 100°C. The part is 5 maintained at a temperature of 100°C for 30 minutes prior to application of a 56.75kg compressive load. Once the load is applied deflection at zero minutes is recorded. Deflections are recorded at 24 and 144 hours. Example 1

A mixture of 1.13 parts 50% aqueous sodium hydroxide, 0.40 0 parts formaldehyde-naphthalene sulfonic acid condensate, 0.15 sodium sulfite, and 1.65 x 10"6 parts p-tertiarybutyl catechol was dissolved in 105.8 parts deionized water and added to a solution of 97 parts of chloroprene, 3 parts of 2,3-dichloro-l,3-butadiene, 3 parts of disproportionated tall oil resin, 0.6 parts of diisopropyl xanthogen disulfide, and 0.75 parts mixed oleic and 5 elaidic acids. The resultant mixture was emulsified in a high-speed mixer for 5 minutes in a nitrogen atmosphere. The emulsion was then heated to 48°C and polymerization was initiated by addition of a solution composed of 5 parts potassium persulfate and 0.125 parts sodium anthraquinone-beta sulfonate dissolved in 2000 parts water. The rate of addition of the initiator solution was 2ml per minute initially. A suitable rate of polymerization was maintained by control of initiator addition rate, external cooling, and agitator speed. Polymerization was continued to a conversion of 70% as determined by attainment of a specific gravity of 1.053g/cc of the emulsion. The polymerization was stopped by addition of 1.40 parts per hundred monomer of an aqueous emulsion containing 1.76% phenothiazine, 2.08% p-tertiarybutyl catechol, 5% sodium dodecylbenzene sulfonate, and 67.6% toluene. Unreacted monomer was steam-stripped from the latex and the stripped latex was acidified to a pH of 5.5-5.6 with a 10% acetic acid solution containing 2% of the sodium salt of a condensate of formaldehyde and naphthalenesulfonic acid. The polymer was isolated on a freeze roll and - ir dried at 100°C. The resultant xanthogen disulfide-modified polymer haα a Mooney viscosity, MLj + 4 @100°C, of 50 and contained 4.23% copolymerized units of 2,3-dichloro- 1,3-butadiene.

The copolymer was compounded with the components shown in Table I. An A-mount specimen was prepared and tested to determine complex dynamic stiffness, tan delta, durability, and creep resistance of the cured copolymer composition. Results are shown in Table I.

TABLE I

Component Parts

Copolymer of Example 1 100

N-774 Carbon Black 20

Rapeseed Oil 3.0

Dibutoxyethoxyethyl Adipate 7.0

Octylated Diphenylamine 4.0

Microcrystalline Wax 1.5

Mixed Ditolyl p-Phenylenediamine 1.5

Magnesium Oxide 4.0

Zinc Oxide 5.0

Ethylene Thiourea 1.0

(75% Dispersion) Sulfur 0.2

Physical Properties

Complex Dynamic Stiffness, K*, 137 tan delta 0.083

Durability @ 100°C 10⁶ (No Failure)

(Cycles to Failure)

Creep Resistance (mm Displacement @ 100°C)

24 hours 0.75

144 hours 1.66

Change 0.91

Example 2

Two xanthogen disulfide-modified copolymers were prepared in substantially the same manner as described in Example 1 using the same components and conditions except that the amounts of diisopropyl xanthogen disulfide added and conversion were varied as shown in Table II. Mooney viscosities, ML1+4 @ 100°C, of the products are also shown in Table II.

TABLE II

Example Modifier Level Conversion Mooney (phm) Viscosity

2A 0.60 70.5 50.9

2B 0.66 68.3 39.5

Example 3

A sulfur-modified chloroprene polymer was prepared essentially in accordance with the procedure of Example 1 of U.S. Patent 4,124,754 using 0.36 parts sulfur per 100 parts chloroprene and 70% polymerization conversion. The resultant sulfur-modified polymer had a Mooney viscosity MLj ₊ 4 @100 °C of 50.

A compounded polymer blend was prepared by mixing the components shown in Table III in a Banbury internal mixer. A sample of the compounded polymer blend was formed into an A-mount test specimen. Physical properties of the cured copolymer blend are reported in Table III.

TABLE m

Component Paris

Example 3 Polymer 75.0

Example 1 Copolymer 25.0

N-774 Carbon Black 40.0

Rapeseed Oil 5.0

Dibutoxyethoxyethyl Adipate 15.0

ZnO 5.0

MgO 4.0

Mixed Ditolyl p-Phenylenediamine 1.5

Octylated Diphenylamine 4.0

Microcrystalline Wax 1.5

Ethylene Thiourea 0.7 (75% Dispersion)

2-Mercaptobenzothiazole 0.7

Phvsical Properties

Complex Dynamic Stiffness, K* 144

(N/mm) tan delta 0.125

Durability @ 100°C 7.5 x 10 (Cycles to Failure)

Creep Resistance (mm Displacement @ 100°C)

24 hours 1.63

144 hours 3-20

Change 1.57

Example 4

A latex blend of the xanthogen disulfide-modified copolymer of Example 1 and the sulfur-modified polymer of Example 3 was prepared as follows. Fifty-three parts of steam-stripped Example 1 emulsion (37.98% solids) was blended with 47 parts of steam-stripped Example 3 emulsion (41.83% solids). The resultant emulsion blend was acidified to a pH of 5.6 with a 10% aqueous acetic acid solution containing 2% of a condensation product of formaldehyde and naphthalene sodium sulfonate. The acidified emulsion was then isolated on a freeze roll. The polymer blend was air- dried at 100°C. The blend had a Mooney viscosity, MLι + 4 @ 100°C, of 50.7. This polymer is designated Polymer Blend A. It was compounded on a mill with the components shown in Table IV and an A-mount test specimen was prepared from the compounded composition. Physical properties of the cured copolymer blend composition are also shown in Table IV.

TABLE IV

Component

Polymer Blend A 100

N-774 Carbon Black 20.0

Rapeseed Oil 3.0

Dibutoxyethoxyethyl Adipate 7.0

ZnO 5.0

MgO 4.0

Mixed Ditolyl p-Phenylenediamine l.f

Octylated Diphenylamine 4.0

Ethylene Thiourea 0.8 (75% Dispersion)

Microcrystalline Wax 1.5

Phvsical Properties

Complex dynamic Stiffness-K* 133 (N/mm) tan delta 0.071

Durability @ 100°C 10⁶ (No FΪ (Cycles to Failure)

Creep Resistance (mm Displacement @ 100°C)

24 hours 0.50

144 hours 1.36

Change 0.86

Claims

CLAIMS:

1. A highly resilient dialkyl xanthogen disulfide-modified or dialkoxy xanthogen disulfide-modified 2-chloro-l,3-butadiene copolymer comprising 92-99 percent by weight copolymerized units of 2-chloro-l,3- butadiene, up to 91% of said 2-chloro-l,3-butadiene units having a 1,4-trans configuration, and 1-8 percent by weight copolymerized units of at least one comonomer selected from the group consisting of 2,3-dichloro-l,3- butadiene, styrene, methyl methacrylate, acrylonitrile, isoprene, and butadiene, said copolymer when cured having a tan delta of less than 0.20 at 5 Hertz and 23°C.

2. The composition of Claim 1 wherein the comonomer is 2,3- dichloro-l,3-butadiene.

3. The composition of Claim 2 wherein 3-6 percent by weight copolymerized units of 2,3-dichloro-l,3-butadiene are present in the copolymer.

4. A process for preparation of the modified copolymer of Claim 1, which comprises polymerizing a mixture of said 2-chloro-l,3- butadiene and said comonomer to a conversion of up to 75% in an aqueous alkaline emulsion in the presence of a free radical initiator at a temperature of at least 42°C and in the presence of no greater than 0.8 parts of said dialkyl xanthogen disulfide chain transfer agent or a dialkoxy xanthogen disulfide chain transfer agent per 100 parts monomer, thereby forming a 2- chloro-l,3-butadiene copolymer having 92-99 percent by weight copolymerized 2-chloro-l,3-butadiene units and 1-8 percent by weight copolymerized units of said comonomer.

5. A polymer blend composition which comprises a) 85-15 parts by weight of a dialkyl xanthogen disulfide-modified or dialkoxy xanthogen disulfide-modified 2-chloro-l,3-butadiene copolymer comprising 92-99 percent by weight copolymerized umts of 2-chloro- 1,3-butadiene, up to 91 % of said 2-chloro- 1,3-butadiene units having a 1,4-trans configuration, and 1-8 percent by weight copolymerized units of at least one comonomer selected from the group consisting of 2,3-dichloro-l,3-butadiene, styrene, methyl methacrylate, acrylonitrile, isoprene, and butadiene, said copolymer when cured having a tan delta of less than 0.20 at 5 Hertz and 23°C, and b) 15-85 parts by weight of a sulfur-modified 2-chloro-l,3-butadiene homopolymer or a sulfur-modified copolymer of 2-chloro-l,3- butadiene and 2,3-dichloro-l,3-butadiene containing up to 5 percent by weight copolymerized units of 2,3-dichloro-l,3-butadiene, said sulfur-modified homopolymer or sulfur-modified copolymer containing less than 0.6 percent by weight combined sulfur.

6. The composition of Claim 5 wherein component a) is present in an amount of 55-45 parts by weight per 100 parts polymer blend composition and component b) is present in an amount of 45-55 parts by weight per 100 parts polymer blend composition.

7. The composition of Claim 5 wherein component b) is a sulfur-modified homopolymer of 2-chloro-l,3-butadiene.

8. The composition of Claim 5 wherein the component b) polymer contains less than 0.45 percent by weight combined sulfur.

9. The composition of Claim 1 which additionally contains 5- 25 parts of an ester plasticizer per hundred parts copolymer.

10. The composition of Claim 5 which additionally contains 5- 25 parts of an ester plasticizer per hundred parts polymer blend composition.

11. The composition of Claim 1 which additionally contains 10-60 parts filler per 100 parts of copolymer.

12. The composition of Claim 5 which additionally contains 10-60 parts filler per 100 parts polymer blend composition.

13. The composition of Claim 1 which additionally contains 0.1-1.0 parts elemental sulfur.

14. The process of Claim 4 wherein the chain transfer agent is diisopropyl xanthogen disulfide.

15. An engine mount comprising the composition of Claim 1.

16. An engine mount comprising the composition of Claim 5.