Granular gas-phase EPDM rubber.

Significant advances over the last 25 years in gas-phase technology by Union Carbide have led to the development of low cost, environmentally advanced commercial processes for the polymerization of olefins. The Unipol polyethylene (PE) gas-phase process was commercialized in 1968 and since then has become the most widely used and licensed polyethylene process in the world (ref. 1). In 1985, this proprietary gas-phase technology was applied successfully to the production of polypropylene in conjunction with Shell USA. Flexomer polyolefins, introduced in 1988, are a further advance and the first commercial products in the very low density ethylene copolymers region (ref. 2). Worldwide use of these low cost, flexible, environmentally advanced and safe performing processes has enabled the achievement of over 600 reactor years of excellent operations.

We have been focusing on the extension of gas-phase technology to higher comonomer content polyolefins. The latest significant development and the topic of this article is production of EPDM and EPM rubber (ref. 3). The U-shaped curve in figure 1 also traces the earlier advances of gas-phase technology from high density polyethylene (HDPE) to linear low density polyethylene (LLDPE) through Flexomer polyolefins as well as from polypropylene (PP) homopolymer through the impact grades.

Experimental

The EPDM terpolymers designated UCC-A,-B and -C were produced using a gas-phase fluidized bed reactor and polymerization technology developed by Union Carbide. Commercially available EPDM products designated Control-A,-B and -C from several producers were used for comparisons. Polymer compositions were measured by [C.sup.13] and proton NMR. Mooney viscosities were measured using ASTM D1646 methods. Crystalline content and peak melting points were measured with a Du Pont Instruments model 1090 differential scanning calorimeter (DSC) heating at 20 degrees C/min.

Polymer mixing and properties were compared in ASTM D3568 formula no. 1 or modifications thereof shown in table 1. Test formulations were compounded with a Brabender PL 2000 torque rheometer with a PreMixer miniature internal mixer or a 'B' internal mixer using standardized mix procedures. Mixing was conducted at 50 rpm. Bale samples were cut into chunks for loading in the small mixers. Comparative observations were made of mixing behavior, melt mix torques and mix temperatures, as well as observations of mill processability. Capillary extrusion measurements were made with a piston rheometer through a 60 degrees tapered entry 5:1 L/D die at 90 degrees C.

Vulcanization behavior was measured with a Monsanto Instruments model R-100 oscillating disc rheometer (ODR) at 160 degrees C, 1 degrees arc and 100 cpm following ASTM D2084. Test samples were vulcanized by compression molding 20 min. at 160 degrees C following ASTM D3182. Tensile strength, tensile stress (modulus) and ultimate elongation were measured following ASTM D412 at 50 cm/min. Dispersion index was measured using a Federal Products dispersion analyzer model EMD-4000-W7 following ASTM D2663 Method C and verified by light microscope measurements on microtomed samples following Method B.

Results and discussion

Previous work discussed the characteristics of gas-phase ethylene-propylene-diene rubber (EPDM) and described their processing and properties in end-use applications (ref. 3). This article discusses the advantages in product handling, mixing and dispersion of a unique free-flowing granular form.

Gas-phase EPR process overview

A brief description of this process is valuable in discussing granular EPDM. Monomers and catalyst are fed to the reactor and solid, granular product is discharged. In our development efforts we have concentrated on ethylidene norbornene (ENB) as the diene of choice for EPDM, but 1,4-hexadiene, dicyclopentadiene and other comonomers can be used.

Heat of reaction is removed through use of circulating gas which also serves to fluidize the polymer bed. Different levels of a "fluidization aid" (FA) can be incorporated into the reactor to prevent agglomeration. These fluidization aids can be selected from a number of ingredients, including several particulate fillers extensively used in EPR formulations.

Solvents are not used in the gas-phase process unlike the current solution- or slurry-processes used in the industry for producing EPDM and EPM (ref. 4). As a result, investment and operating costs are lower, there is no potential for solvent spills and fire hazard is significantly reduced.

EPDM rubber physical form

The majority of EPDM products using current processes are produced as solid bales. Some are friable, though still largely solid. Only a small fraction of products are available in pelletized or crumb form, these largely at high ethylene contents or containing high levels of a thermoplastic additive.

The options of physical form, as well as EPDM properties and processing are significantly affected by crystallinity. Crystalline contents are relatively low and decrease exponentially with decreasing C2 mole %. This is illustrated in figure 2 from measurements of the heat of fusion for several gas-phase EPDM polymers. This behavior and melting points are essentially identical to comparable commercial EPDM rubbers made by the solution process using homogeneous catalysts and indicates random monomer sequence distributions.

Because of their high amorphous fractions, EPDM rubbers have a tendency to self-agglomerate or cold flow at room or elevated temperatures. Producing free-flowing forms is therefore not generally achievable across the full range of polymer compositions. Thus, most products are ordinarily produced only in solid bales.

The granular gas-phase rubber introduced in this article is not only different from bale forms, but is also different from previous powder, crumb or pelletized rubber forms of EPDM. The introduction of a fluidization aid in the gas-phase reactor enables the production of granular products regardless of polymer composition. Also, no further work or heat history is needed for the polymer to remain granular.

Examples

Some examples of free-flowing granular gas-phase EPDMs are listed in table 1. ENB contents ranging from 1.8 to 7.3 wt. % and ethylene contents from 66 to 71 wt. % are shown with Mooney viscosities from 56 to 70 ML 1+4 @ 125 degrees C. The compositions, processing and sulfur cured properties of these granular EPDMs are comparable to commercial products. Cure behavior is demonstrated with cure rheometer and vulcanizate physical properties and processability is demonstrated by favorable internal mixing and mill behavior and capillary die extrusions. The granular particle morphology is largely uniform. Average particle sizes discussed here are about 1 mm.

Advantages of granular rubber

The granular EPDM forms provide many options and advantages in handling, mixing and dispersion. Many of the desirable features of powdered and particulate rubbers are already well-known (ref. 5) including reduced mix times, lower power consumption, lower dump temperatures, packaging options and advantages in compounding such as faster and better ingredient dispersion. Other advantages include elimination of rubber bale cutting or mastication.

Handling and packaging

Gas-phase EPDM products have a unique free-flowing granular form which can be packaged into bags or bulk systems and do not require baling. A packaging system which is unique in the industry for EPR is therefore being developed. This will be an environmentally "green" system which will employ the use of batch inclusion bags. These bags are intended to be added to the mixer, so the packaging will be totally consumed in the customer's process.

Current practice and the requirement of non-gas-phase processes to make baled EPDM demands sturdy and expensive packaging to contain the material. The granular packaging system will eliminate the chimney or ladder boxes now in use in the industry and will eliminate a potential recycling and disposal problem.

In addition, work has shown the granular material has the potential to be bulk handled. Bulk handling can provide added handling benefits, including feeding and compound pre-blending in both batch and continuous processing equipment.

Mixing and dispersion

The granular form promotes compounding ingredient incorporation and desirable mixing. High surface area provides for rapid oil absorption and for pre-distribution of ingredients, like reinforcing fillers such as carbon black or mineral fillers. Dispersion and rapid heat transfer are also facilitated by the granular form. In contrast, solid and friable bales must first be broken down to physically incorporate fillers, plasticizers and other additives.

Some advantages of granular gas-phase EPDM compared to solid bale rubber are illustrated in comparisons of compounding in internal mixers. For example, UCC-B and UCC-C were compared to comparable commercial bale products, Control-B and -C, that have similar Mooney viscosity and rheological behavior. As shown in figure 3, their shear stresses in capillary extrusions are nearly identical over a wide range of shear rates in ASTM D3568 formula no. 1 at 90 degrees C.

Mixing was also compared in different masterbatches with a range of carbon black and plasticizer oil levels in an internal mixer using the same rpm. For example, torque and temperature responses for masterbatch mixes using 80 phr reinforcing N-650 carbon black are illustrated in figure 4. The torque, energy and temperature required to mix the granular product are lower compared to a comparable solid rubber form of the same Mooney viscosity. In particular, mixing is smoother and torque lower in the early stages compared to the wide torque and ram fluctuations of solid bale products. The onset of peak power which we associate with the black incorporation time is also faster. Consistent and desirable mixing is achieved with the gas-phase granular forms.

It was also found that carbon black dispersion was better with granular EPDM products. For example, dispersion index values are compared for the granular UCC-C and the Control-B bale products in figure 5 for different mix times from 1 to 15 min. using the same mix procedures and rpm. Carbon black dispersion is consistently higher and achieved at shorter mix times in the granular product.

Good dispersion and physical properties can be obtained at shorter mixing times and lower temperatures in gas-phase granular EPDM compared to conventional bale products. In addition, mixing energies and power consumption are lower.

Mixing and dispersion behavior may vary with specific equipment and conditions, but we expect these advantages to translate into most commercial scale mixing and compounding operations. Our experience in commercial operations with granular EPDM products has been very favorable, producing improved dispersion, lower mix temperatures and faster cycle times.

Future directions

There are other opportunities to explore for granular EPDM and EPM rubber in mixing and fabrication operations for reducing operating and energy costs and for improved product uniformity and process control. This article primarily discusses batch internal mixing. The granular forms can also provide potential advantages in continuous mixing operations such as bulk handling, uniform compound pre-blending and various options in feeding, blending, mixing and fabricating in rubber processing equipment.

Conclusions

The new fluid-bed gas-phase reactor process is a revolutionary manufacturing process for EPM and EPDM rubber that addresses many of the cost and environmental challenges of the current and coming decades. No solvents are involved in the process which results in environmental benefits and energy savings over current commercial processes that require intensive solvent stripping and washing.

The granular rubber produced by this process is a significant advance toward providing free-flowing product over the wide range of compositions and Mooney viscosities desired in the market today. Products can be produced in unique, unvulcanized granular forms that have a number of major advantages and options in handling, mixing and dispersion compared to bale rubber.

Gas-phase EPDM polymers have competitive properties compared to commercial products. They are comparable in cure, processing and physical properties and are suitable for typical rubber applications using extrusion, milling, calendering and other fabrication methods.

Previous powdered or particulate EPDM rubber technology has been limited by a lack of availability of products in the composition, quantity, form, packaging and cost most desirable to the industry. The current gas-phase EPR manufacturing process offers a revolutionary and effective approach to producing a family of new EPDM and ethylenepropylene products in highly desirable and advantageous forms and in environmentally friendly packaging.

References

1. D.E. James, "Linear low density polyethylene," Encyclopedia of Polymer Science and Engineering, H. Mark and C. Overberger [Ed.], John Wiley & Sons, New York, Vol. 6, 429-454 (1988).

2. M.R. Riff et al, "Flexomer polvolefins: Bridging the gap between rubber and plastics," presented at the ACS Rubber Division Meeting, Las Vegas, June 1990.

3. F.G. Stakem, A.U. Paeglis and J.D. Collins, a) "Gas-phase EPDM and EPM rubber, "paper 1195, presented at the ACS Rubber Division Meeting, Nashville, TN, Oct. 1992; b) "Gas-phase techniques benefit EPM and EPDM," Rubber & Plastics News, vol. 22, no. 28, pp. 17-19, August 2, 1993.

4. G. Ver Strate, "Ethylene-propylene elastomers," Encyclopedia of Polymer Science and Engineering, H. Mark and C. Overberger [Ed.], John Wiley & Sons, New York, Vol. 6, 522-564 (1988).

5. C.W. Evans, Powdered and Particulate Rubber Technology, Applied Science Publishers Ltd., Essex, England (1978.).

Acknowledgements

"Developments in fuel hoses to meet changing environmental needs" is based on a paper given at the October, 1993 meeting of the Rubber Division, ACS.

"Advancements in new tire sidewalls with a new isobutylene based copolymer" is based on a paper given at the October, 1993 meeting of the Rubber Division, ACS.

"Granular gas-phase EPDM rubber" is based on a paper given at the October, 1993 meeting of the Rubber Division, ACS.

  Table 2 - EPDM test compounds


Ingredients                     ASTM D-3568        Masterbatches
                                formula no.
                                   1&3
EPDM rubber                     100.0 + X*              100
ASTM oil type 103                50.0 - X*
ASTM oil type 104A                                      0-100
Reinforcing black                 80.0
Semi-reinforcing black                                 40-120
Zinc oxide                         5.0
Stearic acid                       1.0
Tetramethylthiuram disulfide       1.5
(TMTD)
2-Mercaptobenzothiazole            0.5
(MBT)
Sulfur                             1.0
Total                         237.0 phr               140-320 phr

[Tabular Data Omitted]