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Use of polybutadiene coagents in peroxide cured elastomers for wire and cable.

During the peroxide vulcanization of rubber, peroxides decompose to produce highly reactive free radicals which in turn react with rubber molecules to produce crosslinks. One of the strongest interchain bonds in a rubber matrix is formed by peroxide vulcanization. Its C-C bonds are stronger than sulfur cured C-S-C bonds and much stronger than C-Sx-C bonds. Peroxide vulcanization also allows rubber to be used in harsh environments; peroxide cured rubber articles have superior heat. and set resistance compared to rubber articles manufactured via other cure systems. Peroxide vulcanization not only produces stronger, better-aging rubber, but also gives the compounder a wide variety of elastomers with which to work. Saturated elastomers, like EPM, cannot be vulcanized by sulfur, due to a lack of double bonds. Use of these saturated elastomers requires the compounder to use peroxide cure agents. One of the drawbacks to peroxide vulcanization is that the same free radicals which produce crosslinks can also cause beta scission. Also, in some elastomers peroxide vulcanization is not highly efficient, so a preponderance of peroxide is required to obtain acceptable properties. Both of these difficulties may be alleviated by addition of coagents to the peroxide cure package (ref. 1).

Coagents are polyfunctional, multi-unsaturated organic compounds which readily form free radicals when exposed to the products of heat or light induced peroxide decomposition (ref. 2). These free radicals are more stable than those resulting from the decomposition of peroxide alone, thus when coagents are added to peroxide cured elastomer compounds, they often improve the efficiency of crosslink formation during vulcanization. Coagents have been categorized into two groups: Type I and Type II. During vulcanization, Type I coagents undergo hydrogen abstraction, producing radicals which lead to chain crosslinking. They also experience free-radical addition, resulting in homopolymerization. These two functions give higher cure states and faster cure rates than does peroxide vulcanization without coagents. Type I coagents include acrylates, methacrylates, bismaleimides and vinyl esters. Type II coagents include allylic compounds and low molecular weight high vinyl polymers, both of which typically increase cure state without increasing cure rate. When undergoing hydrogen abstraction, Type II coagents tend to produce more stable radicals than the small, polar radicals produced by Type I coagents. So Type I coagents tend to be more reactive than Type II coagents, but they are also more prone to beta scission and radical coupling reactions (ref. 3).

Research has proven that coagents are very effective in generic EPDM and EPM rubber formulations. They are widely used in the automotive industry in NBR, HNBR, EPDM and EPM under-the-hood applications. Coagents have also found a niche in the dynamic downhole applications of the mining and petroleum exploration industries. This article, however, focuses on the use of coagents in wire and cable (w/c) applications. It is a summary of test results obtained from EP elastomers (EPM and EPDM), EVA, EVM and CPE.

The first elastomers discussed in this study are the EP copolymers and terpolymers. They are extensively used in the w/c industry due to their excellent moisture and weather resistance, combined with very good electrical properties. EPM is a copolymer of ethylene and propylene which, due to its lack of unsaturation in the backbone, can only be peroxide cured. Incorporation of a diene (such as ethylidene norbomene or hexadiene) pendant to the backbone of EPM produces EPDM. Both of these elastomers are used in power transmission cables, portable power cables, control cables and mining cables. They are also used in many flexible cord, automotive ignition wire, and appliance wire formulations, as well as in jacketing compounds. Uncured EPM is also used as an electrical insulating material in many high voltage power cables.

The second elastomer discussed is ethylene vinyl acetate, a saturated copolymer of ethylene and vinyl acetate (VA). The saturation of this copolymer provides outstanding ozone, weather and temperature resistance. Varying the VA content of these materials changes both nomenclature and compound properties. Ethylene vinyl acetate copolymers with VA contents less than 40% or greater than 80% are thermoplastics and in this article will be called EVAs. Those with VA contents between 40% and 80% are elastomers and will be denoted as EVMS. The higher the VA content of one of these copolymers, the more resistance it has to heat, oil and solvents. Though this material has exceptional age properties, the polarity of the VA harms its electrical properties, hence it is not as widely used for insulation by the w/c industry as are the EP elastomers.

CPE is the final elastomer discussed in this investigation. It is produced by random chlorination of an aqueous solution of high density polyethylene. This chlorination produces a highly saturated polymer with many useful qualities such as excellent ozone, weather and heat resistance, which are desirable to the w/c industry. CPEs are used in power transmission cables, portable power and control cables, and mining cables.

Coagents from both the Type I and Type II categories were milled into these elastomers. The Type I coagents chosen for study were trimethylolpropane trimethacrylate (TMA) and a scorch retarded trimethylolpropane trimethacrylate (SRTMA). The Type H coagents were represented by triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), high 1,2-vinyl polybutadiene resin (PBD), PBD adducted with maleic anhydride functionality (PBD/MA), and solid PBD/MA. TMA, TAC and PBD were studied in the EP elastomers, while TAC, PBD and SRTMA were tested in EVM. SRTMA, TAIC and PBD/MA were studied in CPE. All of the coagents tested were shown to be useful compounding tools, but ther'e are definite toxicity, performance and cost advantages obtained by the use of PBD based coagents (table 1).

[TABULAR DATA 1 OMITTED]

Experimental

Chemicals

All ingredients used in this study are commercially available compounds the complete list is available). Three common w/c elastomers were studied in this investigation: chlorinate polyethylene, ethylene propylene rubber and ethylene vinyl acetate (table 2). Five coagents commonly utilized in both the rubber and plastics industries were evaluated. Type I coagents compounded in the test formulations were trimethylol-propane trimethacrylate (TMA) and a scorch retarded trimethylolpropane trimethacrylate (SRTMA). Type Il coagents selected were liquid high 1,2-vinyl polybutadiene (PBD), liquid and solid maleinized high 1,2-vinyl polybutadiene (PBD/MA), triallyl cyanurate (TAC) and triallyl isocyanurate (TAIC).

[TABULAR DATA 2 OMITTED]

Procedure

Lab size masterbatches were prepared off-site to provide consistent formulations for use in this study (table 3). The peroxide and coagents were mixed into these masterbatches on a two roll lab mill at a temperature of 125[degrees]F (52[degree]C), except for the EVA compounds which were milled at 212[degrees]F (100[degrees]C). Formulations were sheeted and allowed to rest 24 hours, then sampled for testing. Table 4 lists the appropriate ASTM methods and test equipment used for this study. Test plaques were cured at various temperatures with two familiar peroxides: 40% active dicumyl peroxide dispersion and 40% active t-butylperoxydiisopropyl benzene dispersion. Electrical testing was performed by independent laboratories.

[TABULAR DATA 3-4 OMITTED]

A careful log of mill time, temperatures and observations was kept on the compounding work. This allowed for consistent and reliable data to be generated. The material balances were determined for all compounds and found to be satisfactory (>99.5%).

Discussion and results

EP elastomers

The first investigations of EP elastomers were performed on the EPDM A, EPDM B and EPM A formulations shown in table 3. The three elastomers chosen varied in diene content and ethylene to propylene ratio. These compounds were cured at 160[degrees]C with 8.00 parts per hundred (phr) of 40% active dicumyl peroxide dispersion (DCP) with equivalent phr of Type I coagent TMA and of the Type 11 coagents PBD and TAC.

The study proceeded by formulation of model w/c compounds varying only in base elastomer and coagent. The base elastomers compounded were EPDM and EPM; these model formulations were denoted as WCEPDM and WCEPR respectively. The coagents utilized in these two forinulations were TMA and PBD. The following results reveal clear advantages with the use of the non-toxic PBD coagents.

Rheometry

Rheometric data for the non-w/c formulations are presented in table 5. The most noticeable difference between the coagents in these compounds was their scorch times. The scorch times of the TMA formulations were 14-25% lower than those of the three controls.

[TABULAR DATA 5 OMITTED]

Differences in scorch safety were more apparent in the w/c formulations (table 6). For example, both control formulations' scorch times were reduced more than 40% when compounded with TMA.

[TABULAR DATA 6 OMITTED]

The most effective indicator of increased crosslink density among rheometric properties is maximum torque. The base elastomer had the largest effect on the maximum torque of these compounds. The maximum torque values for the non-w/c compound's controls (no coagent) were 74.1, 62.8 and 62.1 dNm for EPDM A, EPDM B and EPR A, respectively. EPDM A contains the highest ethylidene norbomene (ENB) content, causing the highest maximum torque value. EPDM B and EPR A both contain low amounts of ENB, 2.9% and 0.0% respectively, and their control compounds' maximum torques were approximately equivalent. Although EPM does not contain any diene in its backbone, the high ethylene content of this elastomer helped its maximum torque value to be comparable to the maximum torque value of EPDM B.

The only other factor affecting this property was the selection of coagent. All the coagents increased the maximum torque value of the control compounds, indicating an increase in crosslink density. The largest changes in maximum torque values included an 8.3% increase in EPDM B with the addition of the PBD and TAC coagents and a 12.7% increase in EPR A compounded with the TAC. The compounds modified with the PBD coagent recorded the largest increases in maximum torque figures in both w/c fonnulations. A dramatic 48% increase in maximum torque occurred in WCEPR due to the addition of PBD. Higher maximum torque values were also reported in the WCEPDM formulations - 17% for the PBD formulation and 10% with the TNU addition. All rheometric data indicated increased scorch safety and crosslink density were obtained with the utilization of the Type II coagents.

Unaged and aged physical properties

The physical properties indicate that all the coagents increased crosslink density since they increased tensile moduli and hardness while decreasing ultimate elongations (Eb) and tear strength. In the WCEPR formulation, PBD produced slightly lower Eb and higher moduli values than those of the other coagent modified formulations. This is a clear indication that the PBD increased peroxide efficiency in comparison to the TMA.

As the demand for higher temperature performance of elastomers increases, so does the importance of accelerated age testing. Table 4 displays the heat aged results of all the EP elastomers. Most of the EP formulations aged very well, which was not surprising because heat aging is one property where EP elastomers excel.

Comparatively, the non-w/c TAC formulations did not age as well as the other coagent modified compounds. For instance in the EPM A compound, the TAC formulation posted changes of -75.2% and -63.0% for tensile strength and Eb respectively. Similar results occurred with these two properties in EPDM A and B, with the exception of the Eb in EPDM B, which only changed -9.3%. All the other coagent altered compounds had changes less than 50% in both tensile strength and Eb. Also, all the percent changes for Eb were negative, and positive for the moduli, indicating additional crosslinking. Curing the test samples beyond their T90s and/or a post cure of these parts would probably increase the heat aged performance of these compounds.

Electricals

Electrical properties are obviously very important to w/c compounds. Volume resistivities and dielectric strengths of the w/c formulations were tested by an independent laboratory (table 6). The results suggest that volume resistivity and dielectric strength were inconsistent with respect to each other. For example, in the WCEPDM compounds the PBD formulation posted a higher volume resistivity than did the TMA compound. In contrast, the PBD reported lower dielectric strength than did the TMA material. Therefore, it was difficult to draw a concise conclusion; however, both coagents did improve the electrical performance compared to the control formulation. In the WCEPR compound, the PBD compound reported higher values than did the TMA compound, for both electrical properties tested.

Fluid resistance

Because water resistance is very important to electrical properties, testing of the w/c compounds proceeded by measurements of water resistance. Test samples were immersed in water at 82[degrees]C for 70 and 166 hours. The data showed that both coagent modified w/c masterbatches resisted water penetration more than did the formulations without coagents (table 6). In WCEPDM, the 70 hour results indicate that the PBD modified elastomers had higher resistance to water molecules compared to the resistance of the control. However, after 166 hours, the TMA compound reported the highest resistance to water at elevated temperatures. In the WCEPR formulation the opposite trend was noticed. The TMA samples posted a 0.0% mass change after a 70 hour test period compared to a 0.02% mass change for the PBD compound. After 166 hours, the PBD and TMA samples averaged 0. 14% and 0. 18% mass change respectively. It is very interesting to note that the lowest reported mass percent changes for the 70 and 166 hour testing were those of the compounds based on the EPM and not the EPDM. PBD modified EPMS have been reported by Drake et al., to perform as well as or in many cases better than EPDMS formulated with or without coagents (ref. 2).

Compression set

Although compression set testing is not as important for many w/c compounds as it is for hose or bridge bearing compounds, there are w/c applications where compression set properties are important. In addition to being directly related to crosslink density, compression set testing at elevated temperatures helps predict heat aged characteristics of the material.

The results in table 5 indicate that in all the EP formulations, the PBD coagent produced lower compression set figures, with the exception of EPDM A. In the w/c formulations the PBD coagent outperformed the TMA coagent during compression set testing For example, in the PBD WCEPDM compound the compression set was 50.7% compared to 55.9% for the TMA compound. Also in the WCEPM tested samples, the PBD and TMA compression set values were 56.9% and 78.9%, respectively. This indicates that the PBD coagent incr-eased the crosslink density of these systems.

EVA and EVM elastomers

The test formulations compounded were based on an EVA elastomer containing 18% vinyl acetate (VA) and an EVM compound with 50% VA content (table 3). The coagents employed were a scorch retardant version of TMA (SRTMA), TAC and PBD. These compounds were cured at 180[degrees]C for two additional minutes beyond their 90% cure times (T90).

Rheometry

Rheometric data show considerable changes in the maximum torque values with utilization of these different coagents (table 7). For the coagent modified EVAS and EVMS, the ascending order of maximum torque values was TAC, PBD, then SRTMA. The cure times revealed that the TAC material was the fastest compound to reach its T90 in both the EVA and EVM compounds. The PBD and SRTMA reached their T90s at approximately the same time. These faster cure times demonstrated by the compound formulated with the TAC contradicted the traditional classification of Type I and Type II coagents. By definition, Type I coagents cure faster and scorchier than Type II coagents. The Type II coagent, TAC, cured faster than the Type I coagent SRTMA. Also in the EVM formulation, the TAC and PBD modified compounds had lower scorch times than those of the control compounds, while the Type I coagent, SRTMA, had an increased scorch time compared to the scorch time of the control compound.

[TABULAR DATA 7 OMITTED]

Unaged physical properties

The unaged physical property data are shown in table 8. In the EVA formulations, the addition of TAC increased peroxide efficiency more than the other coagents increased it. For example, the EVA formulation compounded with the TAC produced the largest decreases in tear strength and Eb compared to the control. The PBD coagents clearly outperformed the SRTMA, for the same reasons stated above. The order of performance was not as clear for the EVM compounds (figure 1).

[TABULAR DATA 8 OMITTED]

In the EVM compounds, data indicated that the SRTMA did not increase the crosslink density to the same extent as did the PBD and TAC coagents. In addition, the data suggested that the TAC and PBD compounds reached approximately equivalent cure states.

All the physical properties of these two compounds were equivalent; the largest difference was in the 100% modulus. The TAC formulation produced a 100% modulus of 5.8 Mpa, compared to a 5.4 MPa 100% modulus for the PBD compound. Since tensile strength has a bell-shaped relationship with crosslink density, this property is not considered to be an accurate measure of crosslink density.

It was noticed that the PBD coagent performed more efficiently in the EVM formulation than in the EVA compound. A possible theory for this phenomenon pertains to the polarities of the EVM and PBD. As the VA content increases, so does the compatibility of the PBD with the EVM, due to the inherent polarity contained in the unsaturation of the PBD. With increased compatibility between PBD and EVM, the high 1,2-vinyl polybutadiene became more efficient in increasing crosslink density by suppressing unwanted chain terminating reactions such as beta scission and disproportionation.

Fluid resistance

The EVA and EVM compounds were tested for water resistance by immersing all compounds in distilled water for 166 hours at 82[degrees]C (table 11). The most distinguishable differences were the small percent changes produced by the lower VA materials (EVAS) relative to the higher VA materials (EVMs). For example, the EVA control compound reported a 0.64% volume change compared to the EVM control compound's 7.21% volume change. Although these base elastomers' formulations varied in additives, VA content was believed to have the greatest influence on water absorption.

[TABULAR DATA 11 OMITTED]

In the EVA materials, all coagents tested relatively equivalent by decreasing water absorption approximately 50% from the control's values. In the EVM compounds, Type II coagent modified formulations (PBD and TAC) outperformed the SRTMA altered formulation (figure 4). For instance, Type II coagent systems decreased water absorption 23% compared to a 10% decrease produced by the SRTMA additive from the control's absorption.

CPE

The primary motivations for the use of coagents in CPE are to improve physical and aged properties. They improve physical properties through increased crosslink density caused by increased peroxide efficiency (ref. 4). Coagents improve aged properties by redirecting harmful radicals produced by ozone attack, into cross-linking reactions rather than backbone unsaturation reactions or by side chain radical scavenging (ref. 5). The physical properties which are influenced by coagents and discussed in this article includeultimate elongation (Eb), tensile strength, tensile modulus, maximum torque and fluid swell properties. The coagents examined in this study were: TAIC, SRTMA, PBD/MA and solid PBD/MA. TAIC was tested at 1 phr, 2 phr and 3 phr. SRTMA was tested at 3 phr and 5 phr. PBD/MA was tested at 2.5 phr and 3 phr. These levels reflect both the usage ranges found in industry and the product manufacturers' recommended levels. The coagents were milled into a toll manufactured masterbatch.

Rheometry

Cure schedules must be considered along with physical properties when compounding with coagents. A quick glance at the rheometer data for these compounds indicate that all of the tested coagents decreased both scorch and cure times from the times of the control (table 9). The SRTMA formulations were much scorchier than the other two coagent formulations. The rheometer data also show the maximum torque values of the TAIC and SRTMA formulations to be higher than the PBD/MA formulations' torque values. Maximum torque values are usually good indicators of coagency, but as other properties will demonstrate, they are deceptive indicators of coagency in this instance.

[TABULAR DATA 9 OMITTED]

Unaged physical properties

The effects of these coagents on CPE's physical properties are best compared by the properties of Eb, modulus and fluid swell resistance. The better a coagent performs, the more it will decrease Eb, increase modulus and improve fluid resistance. Tensile strength is also affected by coagents, but due to its bell-shaped curve in relation to crosslink density it is not the best indicator of coagency. However, it is still important for a compounder to know what influence an additive will have on tensile strength.

All three of the tested materials increased tensile strength. There was a clear trend for these formulations - the higher the coagent level, the more the formulation's tensile strength was increased over the control's. TAIC had the biggest impact on tensile strength. At 2 phr, it gave a larger increase over the control's tensile strength than did any of die other tested coagents. Even at I phr it had more influence on tensile strength than did SRTMA at both 3 phr and 5 phr. The PBD/MA coagent formulations at 2.5 phr and 3 phr also had a greater influence on tensile strength than did both of the SRTMA formulations. So TAIC had more influence on tensile strength than did PBD/MA which was followed by SRTMA.

An increase in crosslink density usually results in a decrease in Eb. This physical property is one of the primary indicators of how much a coagent has increased peroxide efficiency (ref. 6). The PBD/MA coagent at 2.5 phr decreased the Eb from that of the control more than the other tested coagents decreased Eb. It reduced Eb to 250% from the control's 640%. Solid PBD/MA was also very active in lowering Eb. Both SRTMA formulations gave Ebs of 470%. TAIC at 2 phr resulted in lower Eb than both of the SRTMA formulations, but at 1 phr it was outperformed by the SRTMA formulations. The PBD/MA materials clearly had the greatest effect on Eb, followed by TAIC, then SRTMA.

A decrease in Eb, especially when accompanied by an increase in tensile strength, will be evidenced by increased modulus strength. Therefore, the coagents which decreased elongation the most also gave the most increase in modulus. The PBD/MA formulations had the highest 100% and 200% moduli. The solid PBD/MA formulation broke at 280% elongation, and the liquid PBD/MA modified compound at 2.5 phr broke at 250% elongation, so they had no 300% modulus, but the 300% modulus of the liquid PBD/MA formulation at 3 phr was the highest obtained by the tested coagents. TAIC gave the next highest moduli and, following the elongation trend, SRTMA had the lowest moduli.

It was observed that the stress-strain profiles of the PBD/MA formulations were very different in shape from the curves of the other formulations. The control, SRTMA and TAIC samples had stress-strain curves which were steep for the first 100% elongation (slope about 2), but tapered off to more gentle slopes after reaching 100% elongation (slopes from 3/5 to 1). The PBD/MA compound's stress-strain profiles did not taper off until the materials reached 200% elongation. They maintained slopes of about five up to this point and then tapered to slopes of about two. This indicates that PBD/MA modification of CPE formulations results in mechanical properties which are not obtained by other coagent modified CPE compounds.

Aged physical properties

All of the physical properties were measured after aging the test parts in air at 121[degrees]C for 70 hours (tables 10 and table 11). Two of the most common indicators of age resistance used in industry are retention of tensile strength and retention of elongation. The TAIC formulations had the smallest magnitude changes in both properties, with changes in tensile strength under 6% and changes in Eb under 15%. PBDIMA and SRTMA had comparable aging properties. The formulations containing SRTMA had tensile strengths that were 4%-5% higher after aging than they were prior to aging. The tensile strengths of the PBD/MA formulations were 6%-14% higher after aging. Both the PBD/MA and SRTMA formulations had Eb changes in the 14%-20% range.

[TABULAR DATA 10 OMITTED]

Electrical properties

If a coagent enhances a compound's physical properties but destroys its electrical properties, it should not be used in the w/c industry. The volume resistivities (VR) and dielectric strengths (DS) of the control compound and the three modified compounds which contained 3 phr of coagent were determined.

Only one of the coagent modified formulations, TAICs, had a VR lower than that of the control. Both of the other tested coagents produced compounds with VRs greater than the control's. 3 phr of PBD/MA yielded the most resistive compound. Its VR was 9.4 x [10.sup.12] [ohms]*cm, more than triple the control's 3.1 x [10.sup.12] [ohm]*cm. The SRTMA formulation's VR (4.7 x [10.sup.12] [ohm]*cm) was also higher than that of the control. TAIC at 3 phr had the lowest VR of the group, 2.3 x [10.sup.12] [ohm]*cm.

All three of the coagent modified materials increased DS from the control. The highest DS value belonged to the PBD/MA modified vulcanizate. It was 270 V/mil, 100 V/mil higher than the 170 V/mil DS of the control. The SRTMA and TAIC compounds tested at 250 V/mil and 240 V/mil respectively for DS. This information shows PBD/MA to be the best coagent for use in compounds which must have good electrical insulation properties.

Fluid resistance

Perhaps the best indicator of how well a coagent performs is its affect on fluid resistance. As crosslink density increases, the rubber matrix becomes more tightly knit, promoting greater fluid resistance. Since an increase in peroxide efficiency yields a greater crosslink density, and since coagents added to peroxide cure packages increase peroxide efficiency, low fluid swell values are evidence of effective coagency (ref. 6). CPE is used in many w/c applications, so it is vital that compound ingredients decrease water swell to ensure maintenance of electrical properties. At 70 hours in 82[degrees]C water, the PBD/MA coagents reduced mass swell from 23.4% for the control to 6.5%-7.0% for the PBD/MA formulations. At 166 hours, PBD/MA reduced mass swell from 35.4% for the control to 11.0%-12.5% for the PBD/MA formulations. The SRTMA coagents were the next best protection against water, reducing the 70 hour mass swell to 10.9% at the 3 phr level, and 11.5% at the 5 phr level. At 168 hours, the SRTMA modified compounds gave mass swells of 15.2% and 19.2% (3 phr and 5 phr, respectively). TAIC formulations were the least water resistant of the coagent formulations. At the 70 hour milestone, I phr TAIC actually increased mass swell to 23.5% while at 2 phr and 3 phr it reduced mass swell to only 19.9% and 17.4%, respectively. All of these numbers either nearly tripled, or more than tripled, the PBD/MA swell numbers. The TAIC mass swells at 166 hours ranged from 26.6% to 34.6%, again roughly tripling the PBD/MA swell values. The volume swell, as expected, followed the same trend as the mass swell. Since the study's experimental CPE formulation was not compounded for a specific industrial application, the water swell numbers are higher than expected for industrial use. However, this is not a poor reflection on the use of coagents in CPE. Because the coagents decreased the swell from that observed in the control, it can be deduced that they will have a similar impact in formulations compounded for swell reduction. The PBDFMA materials can be expected to decrease swell more than either SRTMA or TAIC will, and SRTMA can be expected to give more improvement than will TATC.

Conclusion

Through this investigation, the authors verified that properties of EP elastomers, EVAs, EVMs and CPEs are greatly improved with the addition of coagents in peroxide cured compounds. The reason for this improvement is that more C-C bonds are formed during peroxide vulcanization when coagents are added than when they are absent. The peroxide cured compounds with coagents are superior to sulfur cured compounds in many room temperature and heat aged physical properties, fluid resistance and electrical properties.

This research compared commonly utilized coagents primarily in wire and cable formulations. In the EP, EVA and EVM elastomers, liquid high 1,2-vinyl polybutadiene (PBD) coagents outperformed and often exceeded the performance of the Type I coagents, trimethylolpropane trimethacrylates, and the Type II coagent, triallyl cyanurate. The PBD modified compounds particularly excelled in heat aged, water resistance and electrical properties. The reason to use any coagent in a peroxide cured formulation is to increase peroxide efficiency, hence increasing crosslink density. The main advantage of the polymeric PBD systems is the stability of the PBD radical. With increased radical stability, PBD coagents give better properties due to the suppression of competing reactions like beta scission, disproportionation and homopolymerization (refs. 2, 3, 4 and 7).

Solid and liquid maleinized PBD (PBD/MA) were developed as coagents for CPE elastomers during the course of this study. PBD/MA resins have an effect on the vulcanization of chloroprene rubber. This is believed to be due to the chlorine on the rubber backbone interacting with the MA sites on the PBD/MA chain (ref. 8). There is also an increased amount of conjugation on the PBD chain, due to maleinization, which can increase peroxide efficiency. These chemistries are most likely responsible for the outstanding coagent properties obtained from PBD/MA materials in CPE. The stress-strain profiles for the PBD/MA formulations were completely different in shape from the curves for the control, TAIC and SRTMA formulations, indicating that unique mechanical properties are imparted to CPE by PBD/MA. Further study is underway to determine the mechanisms for this phenomenon which is a benefit to the CPE compounder.

Liquid PBD resins are currently used in geothermal and oil well cable compounds because they are among the few coagents which can adequately perform in these harsh environments. With the utilization of non-toxic PBD coagents, compounders will be able to produce improved wire and cable jackets, insulators and connectors by improving cost effectiveness and physical and electrical properties of peroxide cured formulations.

Figure 1 - ultimate elongation of EVA/EM

[ILLUSTRATION OMITTED]

Figure 2 - fluid resistance of EVA compounds

[ILLUSTRATION OMITTED]

References

[1.] Michael Fath, "Vulcanization of elastomers," Course Notes of Compounding, Processing and Testing of Elastomers, American Chemical Society, 1994. [2.] R.E. Drake, J.M. Labriola and J.J. Holliday, "Improving properties of EPM and EPDM with coagents," American Chemical Society, Chicago, IL, April 19-22, 1994. [3.] R.E. Drake, Coagent Bulletin: Introduction, April 27, 1992. Ricon Resins, Inc., Grand Junction, Colorado. [4.] R.E. Drake, J.M. Labriola and J.J. Holliday, "1,2 polybutadiene coagents for improved elastomeric properties," American Chemical Society, Nashville, TN, November 3-6, 1992. [5.] William H. Davis, Jr., Raymond L Laukso, Jr., Ldyd B. Hutchinson and Sandra L Watson, "Peroxide-cured chlorinated polyethylene compounds having enhanced resistance to ozone-induced cracking, " American Chemical Society, May 29-June 1, 1990. [6.] J. W. Martin, "1, 2-polybutadiene resin co-agents for peroxide cure of rubber compounds, " Rubber Chemistry and Technology, Vol. 47, No. 1, American Chemical Society, October 3-6, 1972. [7.] Robert C Keller, "Peroxide curing of ethylene-propylene elastomers, " American Chemical Society, October 6-9, 1987 [8.] R.E. Drake and J.M. Labriola, "New polymeric curative for polychloroprene," American Chemical Society, Atlanta, GA, October 7-10,1986.
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Author:Costello, Michael S.
Publication:Rubber World
Date:Dec 1, 1995
Words:5307
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