Characterization of EPDMs produced by constrained geometry catalysts.

Since the introduction of EPDM elastomers in the 1960s, there has been a tremendous growth in the number and variety of applications using these polymers. This growth is due, in part, to the low cost of these polymers, and also due to the range of compounds and processing requirements which can be met with these materials. In the general rubber industry, compounds based on EPDM must be capable of meeting performance requirements while also processing well. The major fabrication methods include calendering, extrusion and molding.

In recent years, a new technology for the production of polyolefins has been developed by Dow Chemical. This Insite technology, including a new catalyst system, has been used to produce a variety of polyolefins, including EPDM (ref. 1). By the use of this constrained geometry catalyst (CGC), several EPDM grades have been produced with compositions targeted for specific end uses by rubber fabricators (ref. 2).

Traditional Ziegler-Natta catalysts, while capable of producing quality products, can show a tendency to incorporate ethylene in a "blocky" manner. It has been shown that CGC catalyst systems provide a significantly more random incorporation of ethylene into the polymer (ref. 3). CGCs also offer a high degree of control and reproducibility, which in turn produces polymer having consistent rheological and performance properties. Since the catalyst operates with a single active site, a more homogenous polymer composition should be produced.

In this study, three CGC produced polymers were prepared with compositions suitable for specific processes (molding, extrusion and calendering). These EPDMs were compared to very similar commercial EPDMs which are currently used successfully in these applications.

Polymer descriptions

A series of three developmental polymers was produced in pilot plant quantities for this evaluation (table 1). The compositions of these polymers were such as to target each for a specific fabrication method. All of these polymers are considered amorphous and contain ethylidene norbornene (ENB) cure sites. The product designed for calendering has approximately 2% ENB, while the molding and extrusion polymers contain approximately 5% ENB. To avoid the use of trade names, a simple naming convention is used for this article. The name of each CGC produced polymer consists of characteristic viscosity and crystallinity information. For example, 20M/A is used to describe a 20 Mooney, amorphous polymer.

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The commercial EPDMs were chosen to provide a valid comparison in each application and are shown in table 1. Some of these polymers contain ENB as a cure site, while others contain 1,4, hexadiene (HD) as a cure site. Again, to avoid commercial trade names a naming convention was used. All commercial polymer names are preceded by the letter "c." After a characteristic alphanumeric designation, the type of cure system is noted (hd for 1,4 hexadiene, enb for ethylidene norbornene). For example, c-1470hd refers to a commercial, hexadiene based EPDM.

Experimental

All compounds were mixed in lab scale internal mixers, either BR or OOC depending on the sample size needed. A single pass, upside down procedure was used on all compounds with curing agents added at 80[degrees]C and the batch discharged at 105[degrees]C. The stock was transferred to a 2-roll mill for cooling and sheeting for further sample preparation. Test slabs were cured by compression molding at time and temperatures noted on data sheets. The testing procedures were based on ASTM standards where possible, but some special procedures were necessary. Table 2 shows a list of the appropriate test procedures.

Table 2 - test methods used to evaluate
compounds

Test                                          Method

Mooney viscosity                          ASTM D1646
Scorch                                    ASTM D1646
Cure properties                           ASTM D2084
Tensile properties                         ASTM D412
Shore A                                   ASTM D2240
Heat aging                                 ASTM D573
Tear strength                              ASTM D624
Compression set                            ASTM D395
Spider mold        Du Pont Dow Akron Lab test method
                                               TP-29
Green strength     Du Pont Dow Akron Lab test method
                               1/2" strips, 5mm/sec.

Processing tests were conducted in industry standard equipment. Calendering trials were conducted on a 4-roll, inverted " L" laboratory calender having 8" x 16" rolls. The extrusion trials were conducted on a 2-1/2" cold feed extruder using a generic profile shown in figure 1. The extrudate was cured in a microwave/hot air oven which was inline with the extruder. The molding trials were conducted in steam heated presses using a transfer mold. A standard mold designed to evaluate flow characteristics, known as a spider mold, was used. Conditions were modified to provide for reasonable comparisons and are noted on the appropriate data sheets.

Discussion

Each of the three developmental polymers was designed with a specific application in mind and the evaluation process was conducted accordingly. Typical calendering, extrusion and molding compounds were used to provide a framework to compare these CGC produced polymers to the commercially available polymers.

Calendering polymer comparison

The EPDMs used in compounds for calendered products tend to be medium to low ethylene content (amorphous) and somewhat broad in molecular weight distribution. By volume, EPDM single ply roofing is the largest use for calendered EPDM. The CGC produced polymer in this evaluation was 60M/A; an approximately 60 Mooney, 56% ethylene composition. For large calendered sheeting, a long, controlled cure cycle is used, so a low level (2%) of ENB cure site was included.

The commercial products used as benchmarks were the c-1070hd and c-6874hd. Both of these polymers are high Mooney, amorphous polymers with a broad molecular weight distribution and successfully meet the requirements for single ply roofing.

A generic, carbon black filled, roofing formulation was used for the evaluation of these polymers and is shown in table 3. This formulation is a medium quality compound recommendation found in Du Pont Dow literature. It represents a simplified version of a typical calendering formulation, its basic cure system and straightforward use of carbon black and paraffinic oil serve to accentuate polymer effects.

Table 3 - roofing formulation comparison

     Formulation     phr
     Polymer         100
     N650 black      130
     Paraffinic oil  70
     Zinc oxide       5
     Stearic acid     1
     Sulfur           1
     TMTM            1.5
     MBT             0.5
     Total           309

Polymer                c-6874hd   60M/A   c-107hd
Mooney viscosity

ML1+4, 100[degrees]C   69.6       71.6    81

ODR, 3[degrees] arc at 160[degrees]C

M-L (N * m)            12.4       11.7    14.5
M-H (N * m)            40.6       59.9    41.4
Ts-2 (minutes)         5.58       4.43    5.75
Tc-90 (minutes)        18.13      16.4    19.18

Calendering trial
Rating: 1=poor, 5=good

Calenderability        5          5       5
Surface appearance     4.5        4.5     4

Green strength

10% modulus (MPa)      0.23       0.32    0.33
20% modulus (MPa)      0.28       0.39    0.38

Physical properties cured 23 min. @ 160[degrees]C

100% modulus (MPa)     1.80       3.02    1.72
200% modulus (MPa)     4.59       7.05    4.36
Tensile strength (MPa) 10.84      12.33   11.02
Elongation (%)         676        502     704
Hardness, Shore A pts. 55         66      57

After aging 670 hrs. at 115[degrees]C

100% modulus (MPa)     4.1        5.5     3
% change               125.7      81.1    74.3
Tensile strength (MPa) 11.70      13.90   11.30
% change               8.00       12.60   2.40
Elongation (%)         310        283     408
% change               -54.1      -43.6   -42
Hardness, Shore A pts. 68         75      69
change                 13         9       12

QUV Weatherometer (80[degrees]C/20h light-4h cond./UVB-313)
4,000 hrs.                    No cracks No cracks No cracks

When comparing the three polymers, the effect of the ENB cure site can be seen in the ODR properties, figure 2. The CGC produced 60M/A shows a higher state of cure and slightly faster cure than do the HD containing commercial polymers.

Since a certain level of green strength is needed for successful calendering and downstream handling of the produced membrane, it is very encouraging to note that the CGC produced 60M/A is the equal in green strength to c-1070hd, shown in figure 3. This is especially notable given the difference in viscosity between the two base polymers.

While the capillary rheometer indicates a slightly higher shear viscosity for 60M/A, the calendering trial showed excellent processing characteristics for this polymer. The compound based on the 60M/A polymer exhibited excellent flow characteristics when calendering down to a thickness of 0.5 mm. The stock released easily from the rolls and had effective dimensional stability. The surface of the calendered sheet was smooth without incidence of blisters, tom stock or teardrop imperfections. When comparing the cured stress/strain properties of these three polymers, figure 6 through figure 8, the effect of the ENB cure sites can be noticed. The overall state of cure measured by modulus at 100% elongation is higher both before and after aging for the 60M/A polymer. The tensile strength of the CGC produced polymer is also higher than the commercial grades. The effect of aging for 670 hours at 115[degrees]C shows that the CGC polymer ages very similarly to the two commercial HD based grades.

Extrusion/hose polymer comparison

The EPDMs used in compounds for extrusion and hose applications tend to be high viscosity polymers with % ethylene contents which range from the low 50s to the low 70s. In this study we evaluated polymers in the amorphous region of less than 60% ethylene, but future evaluations will include the semicrystalline grades. The major applications for these extrusion grades of EPDMs include both dense profiles, such as weather-stripping, and hose.

The CGC produced polymer designed for these applications was 70M/A, an approximately 70 Mooney, amorphous polymer with a medium level (5%) of ENB cure site. The commercial polymers used for comparison were all high viscosity, amorphous grades. They included both HD and ENB products listed in table 1.

In this study, a coolant hose formulation was chosen for general property comparisons and a dense extrusion formulation was chosen for the extrusion trial.

Coolant hose formulation

The coolant hose formulation chosen for this comparison is shown in table 4. This is a medium quality, carbon black filled compound found in Du Pont Dow literature.

[TABULAR DATA OMITTED]

As expected, when comparing the various polymers, the 70M/A shows properties which are similar to the other ENB-containing commercial polymers. In ODR properties, shown in figure 4, the HD-containing c-1470hd presents an expected lower state of cure and a slower rate of cure. All of the ENB polymers (including 70M/A) show differences in cure rates and states of cure which are probably related to differences in the ENB content. The capillary rheometer measured shear viscosity shows all polymers to behave similarly.

The cured stress/strain properties also reflect the effect of the cure site differences and can be seen in figure 5 through figure 8. The HD-containing polymer (c-1470hd) shows lower modulus, higher elongation and lower compression set than the ENB polymers. The CGC produced 70M/A demonstrates a blend of good tensile and elongation both before and after aging for 70 hours at 125[degrees]C.

Extrusion trial

In an attempt to compare the processibility of the CGC produced 70M/A to some commercial EPDMs, an extrusion trial was performed on a 2-1/2" cold feed extruder with a microwave/hot air oven inline for crosslinking. The commercial polymers used for comparison, described in table 1, were the c-451enb and the c-70Aenb. The formulation chosen was a generic dense extrusion compound found in Du Pont Dow literature and can be seen in table 5.

[TABULAR DATA OMITTED]

The extruder was operated at three different screw speeds and the appearance and throughput of the compound was noted at each speed. The throughput of the 70M/A measured during the extrusion trial was higher than that of the commercial grades, and was especially noticeable at higher screw speeds. From an admittedly subjective point of view, the 70M/A gave the smoothest and glossiest appearance at all screw speeds.

Molding polymer comparison

The uses of EPDM in molding applications is probably the most varied of all markets. The tendency is for polymers to be amorphous and low to medium in viscosity. This is also an area where peroxide cured formulations are common. The principle processing characteristic in transfer and injection molding is the need for excellent flow at high temperatures. This need can be addressed by low viscosity polymers; however, tensile strengths can suffer when viscosity is too low.

The CGC produced polymer designed for the molding market is the 20M/A, which is an approximately 20 Mooney, amorphous polymer with ENB (5%) as a cure site. The commercial grades of polymers used for comparison are c-132hd, a low viscosity, amorphous polymer with HD as a cure site; and c-2504enb, a low viscosity, amorphous polymer with ENB as the cure site. These grades are described in table I.

Since many molding applications, such as brake seals, generally require a polymer rich peroxide cured compound, such a formulation was used for the polymer comparison. The brake cup formulation, shown in table 6, is very polymer rich with no plasticizer, since plasticizers will extract in brake fluid. Extraction of plasticizers by brake fluid will result in a shrinkage in part dimensions.

[TABULAR DATA OMITTED]

When the ODR properties of the three polymers are compared, it is clear that the type of cure site is not as important in this peroxide cured compound. While the sulfur cure system used in the previous applications indicated faster crosslinking with ENB, the peroxide cure system used in this application is less preferential.

As was mentioned earlier, the ability to flow is a key performance characteristic for EPDM in molding applications. One way to measure this flow property is with the spider mold. A specified amount of EPDM compound is loaded into the transfer pot of the mold assembly and the material is transferred to the cure cavity under specified pressure and temperature. The tendency to flow is measured by weighing the spider produced and/or counting the number of divisions on the thin spider legs. Of course, there are times when a visual inspection of the parts shows an obvious difference.

When the three polymers were compared in the spider mold, the resulting specimens showed a significant difference. The CGC produced 20M/A was the only polymer to produce a complete spider. Of course, some of the difference between c-2504enb and 20M/A can be attributed to viscosity differences between the two polymers. The same statement cannot be made about c-1320hd and 20M/A, since the viscosities of these two are very similar. When the sprue diameter was increased to twice the original, new spiders were produced. In this case, 20M/A still produced the more complete molded specimen.

It was mentioned previously that the polymer viscosity (and its effect on flow) also tended to affect tensile strength. Inspection of the stress/strain properties before and after aging 22 hours at 175[degrees]C show that the 20M/A has the highest tensile strength of the three polymers. When closer attention is placed on the effect of aging, it is clear that the 20M/A polymer shows the smallest changes in properties after exposure to 175[degrees]C.

Summary

When a new technology is announced for the production of established polymers, it must be shown that the new products are truly similar to commercial materials and fit for use in existing processes. In this case, we have proven that constrained geometry catalysts can produce terpolymers of ethylene which can truly be called EPDM. In addition, there seem to be several obvious processing advantages which are imparted by the CGC system.

* In calendering, the appearance of the formed sheet was better than any of the commercial grades.

* In extrusion, the extrusion rates which were achieved with the CGC polymer were higher than with the commercial grades.

* In molding, the combination of excellent flow and high tensile strength was more than the commercial grades could achieve.

In summary, it appears that the new CGC technology can produce EPDM polymers capable of delivering both physical property and processing advantages over existing technology.

References

[1.] M.S. Edmondson, T.A. Plumley, D.J. Mangold, "New hydrocarbon elaslomers via constrained geometrv catalyst technology," ACS-Rubber Division 148th Technical Meeting, Paper #8 Oct. 1995. [2.] Janct L. Laird, John A Riedel, "Evaluation of EPDM materials produced by constrained geometry catalyst chemistry against current commercial EPDM products and performance requirements," ACS Rubber Division 148th Technical Meeting, Paper #9, Oct. 1995. [3.] Spaleck et al, Agnew. Chem. Int. Ed. Engl., 1992, 31 No. 11, 1347-1350.