technical coagent choice can boost compound...

Rubber & Plastics News • November 4, 2002 www.rubbernews.com

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Technical

Coagent choice can boost

compound properties By Clay B. McElwee

Sartomer Co. Inc.

Executive summary

Liquid acrylate coagents are low viscosity, low molecular weight functionalized materials that enhance

crosslinking in peroxide-based cure systems. Because of their low viscosity, these coagents also dramatically

reduce the Mooney viscosity during the mixing stages. Polybutadiene coagents, although higher in viscosity at

room temperature, also have a significant plasticicizing effect. Solid acrylates can be used as crosslinkers and to

improve dynamic and flex properties. This paper describes the various types of coagents in use today and shows

the effect of each type on the properties of commonly used elastomers.

TECHNICAL NOTEBOOK

Edited by Harold Herzlich

Crosslinking with peroxide alone results in the formation of a covalent bond. This carbon-carbon bond is quite

rigid and stable (343.2 kJ bond energy), compared to the carbon-sulfur and sulfur- sulfur bonds in sulfur-cured

systems (C-S, 276.2 kJ bond energy; S-S, 205.1 kJ bond energy). The excellent heat stability of this carbon-

carbon covalent bond explains the superior heat aging characteristics of peroxide-cured systems. 1

Coagents function with the peroxidecured system to modify this C-C network. It has been stated that the

coagent homopolymerizes and grafts onto the polymer backbone.2 This results in several property

improvements such as higher modulus, tensile and tear strength, and lower compression set than curing with

peroxide alone. These property enhancements will be discussed in greater detail later in this paper.

Like sulfur-cure systems, concentration level and ratio of the peroxide-coagent system should be optimized.

When converting the peroxide-only system to a peroxide-coagent cure, the peroxide level usually is reduced

such that the total concentration of curatives rarely exceeds the original peroxide level.

The peroxide-coagent cure system offers greater flexibility in cure over the sulfur-accelerator system. Many

peroxides can be selected that have cure ranges from room temperature to 200°C. A wide range of coagents can

be selected to complement the peroxide. Coagents that serve as adhesion-promoters also are available. The

coagents focused on in this paper are listed by trade name for reference purpose in Table I.


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The Saret-brand coagents contain a non-nitroso scorch retarder to prevent premature curing during processing

and/or vulcanization.

Experimental

Materials

A masterbatch containing 100 phr Natsyn 2200, 50 phr N660 carbon black, 5 phr zinc oxide, 1 phr stearic acid

and 1 agerite resin D was used in all experiments related to natural rubber (synthetic polyisoprene). Natsyn

2200 was obtained from Goodyear Chemical.

Unless otherwise noted, a masterbatch containing 100 phr Nordel IP4640, 100 phr N660 carbon black, 50 phr

Sunpar 2280, 5 phr zinc oxide and 1 phr stearic acid was used in all experiments related to EPDM rubbers.

Nordel IP4640 was obtained from DuPont Co.

All Saret- and Ricon-brand coagents were supplied by Sartomer Co. Inc. TAC was supplied by Akzo-Nobel.

HVA-2 was supplied by DuPont. Peroxides were obtained from Atochem and Hercules.

Formulations

Rubber compounds were mixed in a Brabender prep mixer model R.E.E.-6. In all cases, the order of addition

was as follows: masterbatch, antioxidant, coagent and peroxide. All samples were compression molded for 40

minutes at 320°F unless otherwise indicated.

Measurement

Processability was determined by measuring Mooney viscosity at 100°C for four minutes according to ASTM

standard D-1646-64. An Alpha Technologies MV-2000 was used for all viscosity measurements.

Cure characteristics which include scorch time, cure rate and torque values were measured over a 60-minute

period at 160°C and 3° arc using a Tech Pro oscillating- disk rheometer according to ASTM method D-2084-

95.

Original physical properties were measured using die D dumbbells tested at 20 in/min. All samples were tested

according to ASTM standard D-412-97, method A; 2240-97.

Shore A hardness values were determined for samples after molding using a hand-held Shore A durometer

according to ASTM standard D-2240-91.


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Tear strength was determined according to ASTM method D-624 using a die- C template. All hot tear

measurements were taken after preheating each die-C specimen for 10 minutes at 150°C.

Compression set was determined by compressing samples at 25-percent deflection for 70 hours at 212°F. Each

sample was then removed and the permanent set measured as a percentage of original height according to

ASTM standard D-395B.

Flexibility testing was measured according to deMattia flexibility ASTM D- 813-95. In this test, each specimen

was pierced 0.08 inches and tested at 300 cycles per minute. Cycles to failure were reported when the 0.08

piercing grew to 0.5 inches. Samples were cured 30-32 minutes at 320°F for the peroxide-based samples and 20

minutes for the sulfurbased samples.

Results and discussion

General coagent description

The Saret salts of acrylic and methacrylic acid as peroxide-cure coagents vary in effectiveness in controlling

cured rubber properties. These coagents also vary in speed of cure. Acrylate coagents generally are faster curing

than methacrylate coagents. Also, curing speed generally increases with increasing functionality. The solid

coagents offer certain unique properties due to both ionic and covalent bonding during crosslinking. The Ricon


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coagents function by state addition only, and normally do not offer cure speeds as fast as acrylates. This can be

an advantage or a disadvantage depending on the desired physical properties. No single coagent has been found

to produce optimum properties for all applications. Hence, it is necessary to pick the right coagent for the

application.3


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Scorch

As mentioned earlier, the chemical nature of methacrylate and acrylate coagents results in an increase in cure

rate to peroxide alone. Even though the Saret line of coagents includes a scorch retardant package, scorch times

still are shorter than peroxide alone. This is evidenced in Fig. 1.

By following the graph in Fig. 1, it becomes apparent that by careful coagent selection, potential scorch

problems (particularly during high-temperature curing) can be eliminated. Alteration of cure temperature and

coagent level also can be used to overcome scorch.

Processability

Liquid acrylate and polybutadiene coagents can be classified using the term “reactive plasticizers.” Their

function is twofold:

1. reduce the viscosity in the mixing/ processing stages; and

2. promote crosslinking upon vulcanization.

A previous publication4 has used the general graph in Fig. 2 to explain this trend.

In addition to the benefits listed above, these materials act as non-extractable processing aids since they

crosslink into the elastomer matrix. Many sulfur-cured systems require the use of a plasticizer that does not cure

into the elastomer matrix. This could cause migration in some cases, as well as volitalization and embrittlement

of the elastomer upon heat aging.

To show the plasticizing effects of liquid acrylate and polybutadiene coagents in peroxide-cured systems, Fig. 3

focuses on a black-filled natural rubber system. In this case, coagents are substituted in place of traditional

extractable plasticizers.

Modulus

Modulus at 100-percent elongation was measured for samples containing 2, 5 and 10 phr additions of

methacrylate, acrylate or polybutadiene coagents. Modulus response graphs are shown in Figs. 4, 5 and 6.

These graphs indicate that in most cases, using acrylate and methacrylate coagents over polybutadiene coagents

(on a part-by-part basis) results in a greater modulus response in EPDM.

Hot tear strength

Tear strength usually is quoted as a deficiency for peroxide cure vs. Sulfur cure. Fig. 7 illustrates 150°C hot

tear values in EPDM from several coagents as well as a sulfur-accelerator control.

Fig. 7 indicates that methacrylates as well as polybutadiene coagents are the most suitable choices for

improving hot tear strength in peroxide-cured formulations. SR 634 is an interesting coagent in that it increases

tear resistance of peroxide-cured rubbers, even though it gives a relatively high modulus response as shown in

Figs. 4-6. A similar study was performed in a natural rubber formulation that also included TAC and HVA-2

coagents. The results are shown in Fig. 8.

Fast cure

Table II shows a comparison in performance properties between SR 519 and HVA-2. SR 519 is generally the

fastest curing liquid acrylate among the coagents listed in Table I. The solid maleimide HVA-2 reacts similar

to acrylate coagents by increasing both the state and rate of reaction.

Compression set


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Fig. 9 shows the effects of both types of coagents on compression set in an EPDM formulation. Acrylates

generally give lower compression set values than methacrylates and polybutadienes.

Rubber-to-metal adhesion

SR 633 is a coagent that produces extremely strong rubber-to-metal adhesion without the use of external

adhesives. Previous papers5, 6

have shown that strong bonding can be obtained with a variety of rubbers such as

neoprene, SBR, NBR and HNBR. Ricobond 1756 also is used as an adhesion promoter in peroxide-cured

systems. Since Ricobond 1756 is a liquid, it works more as a plasticizer than the solid SR 633. Tables III and

IV show adhesion values for SR 633 and Ricobond 1756 in both EPDM and synthetic natural rubbers.

These tables also show that interesting results are obtained by using both coagents in a single system (far right

column in both tables). In this case, the flexibility of the polybutadiene (as evidenced by high t-peel adhesion)

is combined with the high hardness bond of the solid acrylate (evidenced by high lap shear adhesion). This is a

key example of how coagents sometimes can be combined in a single system to create the “best of both

worlds.”

SR 634 also can be used as an adhesion promoter in applications where higher elongation properties are

required. This coagent normally offers 50- 60 percent of the adhesive strength compared to SR 633.

As shown in Fig. 10, Ricobond 1756 also can be used in applications where adhesion is desired as well as

relatively low compression set. Fig. 10 illustrates this effect vs. SR 633 in synthetic natural rubber.

Flex fatigue

DeMattia flexibility testing in natural rubber shows that the solid coagents SR 633 and SR 634 give outstanding

flex fatigue resistance vs. peroxide alone, sulfur, trifunctional methacrylates and several polybutadiene

coagents. Fig. 11 highlights these results.

Studies also have been duplicated in EPDM rubber with similar results.7 In these tests, SR 633 and 634 both

reached 1 million cycles without failure. The sulfur-based cure system failed at 5,000 cycles.

Summary

It has been shown that liquid (meth)- acrylate and polybutadiene coagents are non-extractable materials that

provide additional crosslinking in a peroxidebased cure system. This can lead to an improvement in tear

strength, compression set and other performance properties. Liquid coagents can be used to replace extractable

plasticizers and still have the same effect in process viscosity reduction.

Solid acrylate coagents, although usually not as effective in reducing process viscosity of a formulation,

deserve special attention because of their high modulus, adhesive and flex properties. Because no single


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coagent can produce optimum properties for all applications, dual-coagent systems also can be used in some

cases to combine the positive performance properties of each coagent.

References

1. C.B. McElwee and J.E. Lohr, “Comparison of Rubber Curing Systems: Peroxide-Coagent Versus Sulfur-

Accelerator in Polyisoprene,” April 24-27, 2001, ACS Spring Rubber Show, Paper No. 34.

2. J.B. Class, “Fundamentals of Crosslinking With Peroxides,” Oct. 11-14, 1994, ACS Fall Rubber Show,

Paper No. 3.

3. R.C. Costin, W. Nagel and C.B. McElwee, “Coagent Selection for Peroxide-Cured Elastomers,” Oct. 17-20,

2000, ACS Fall Rubber Show, Paper No. 77.





6. R.C. Costin and W. Nagel, “Metallic Coagents for Rubber-to Metal Adhesion,” Oct. 11-14, 1994, ACS Fall

Rubber Show, Pittsburgh.



The author

Clay McElwee joined Sartomer Co. Inc. in 1998 as a member of the UV/EB technical group.

Promoted to rubber development chemist in 1999, his responsibilities include research of existing Sartomer

coagents in a variety of potential rubber applications, development of new peroxide curatives for elastomer

vulcanization, and providing technical support to the entire elastomers industry.

McElwee graduated from West Chester University of Pennsylvania with a bachelor’s degree in chemistry.

Presented at a meeting of the American Chemical Society Rubber Division, held Oct. 16-19, 2001, in

Cleveland.

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