Advances in extrusion foaming of EAs.

Foam and sponge rubber profiles are the basis for a significant usage of EPDM, polychloroprene and other thermoset rubbers in sealing and gasketing applications. Recently developed technology in foaming thermoplastic elastomers (ref. 1) has shown that elastomeric alloys (EAs), a class of TPEs, may be foamed with chemical blowing agents or with chlorofluorocarbon mechanical blowing agents. Physical and mechanical performance of foamed EAs can match or exceed that of foamed thermoset rubbers, so the economic advantages of EAs may be extended to extruded foam and sponge rubber parts.

It has now been discovered (ref. 2) that elastomeric alloys may be foamed using water as the sole mechanical blowing agent. This technology offers significant savings in material costs and eliminates the use of the environmentally suspect chlorofluorocarbons. Use of this water foaming technique will allow foamed EAs to provide low cost, high performance solutions to sealing problems in the automotive and construction industries.

Elastomeric alloys are generated from a chemical combination of two or more polymers to give an alloy having better elastomeric properties than those of the corresponding blend. EAs have penetrated a general cross-section of rubber market applications (ref. 3). An example of one of these application areas is in extruded profiles for automotive or residential weatherstripping. Sealability of such parts often depends on the use of foam or sponge rubber with good compression and load-deflection characteristics. The ability to foam EAs is a significant recent advancement (ref. 4) which has allowed these materials to be considered as replacements for thermoset rubbers in such sealing applications.

Chemical foaming of EAs

Chemical foaming of EAs is similar to technology often used with thermoset rubbers. A chemical foaming or blowing agent is typically an organic compound such as an azodicarbonamide which decomposes thermally to evolve gases, typically nitrogen and carbon dioxide. Useful azodicarbonamide blowing agents for TPEs are Unicell D-1500 (Dong Jin) and Nortech XMF 1307 (USI Chemicals). Mixtures of citric acid and bicarbonates, such as Hydrocerol-CLM 70 (Boehinger Ingelheim) generate carbon dioxide at polymer melt temperatures and have also been used to foam TPEs (ref. 1).

The solid or powdered chemical foaming agent is blended with the EA feedstock at a level of about 0.5% by weight, and loaded into the hopper of an extruder. The blend is extruded with a standard thermoplastic extruder, with a typical L/D of 24/1, just as in extrusion of dense EAs. Screw speed and heaters are used to control melt temperature so that the foaming agent decomposes and the incipient gas bubbles are thoroughly dispersed in the melt in the final zone of the extruder. The bubbles expand, forming the foam as the melt passes through the die and pressure is released. Excellent dimensional stability of hot EA foamed extrudates make these materials particularly good candidates for the chemical foaming process. The extrudate must be cooled to maintain the desired shape, often requiting a longer cooling bath than that for dense EAs because of the increased insulating characteristics of the foam. The technique gives extrudates with specific gravities in the range of 0.7 to 0.85 when foaming a dense EA with specific gravity of 0.95 to 0.98.

The foamed EAs have a closed cell structure which is maintained as the melt cools. The closed cell structure of chemically foamed EAs has been demonstrated by measuring the weight gain after immersion in water (ref. 1). Table 1 compares water absorption after 24 hours at 23[degree]C for three chemically foamed EAs with that for the dense EAs. The differences observed are largely the result of water pick-up on the cut surfaces of the foam specimens. Chemically foamed EAs also exhibit excellent mechanical properties, as shown in table 2. As expected, the tensile strength is lower from the foam, but the ultimate elongation is maintained in the range for rubbery behavior. More importantly, the resistance to permanent set under compression for foamed EAs is nearly identical to that of the dense materials.

A useful application of chemical foaming of EAs is to reduce hardness, weight and cost of an extruded part. Table 3 shows that foaming a relatively hard EA can lead to an extrudate nearly as soft as a typical EPDM thermoset rubber compound used for cable protectors, while maintaining modulus and elongation in the rubbery performance range. A polyurethane is included for comparison because it is sometimes used in this same application.

It has now been found that the EA foams, like the EAs themselves, are extremely stable under conditions of air aging at elevated temperatures. This contrasts with the behavior of some thermoset EPDM rubbers and polyurethane under the same conditions. Figure 1 shows the change in hardness for extruded tubes of these materials over a 2,500 hour period at 121[degree]C. Both EA samples showed little change, while the polyurethane softened slightly and the EPDM became more brittle. The tensile strength of the EAs and the EPDM are essentially un-changed under these conditions, as shown in figure 2. The decrease in the tensile strength of the polyurethane is probably a result of hydrolysis of the polymer during the test. Figure 3 demonstrates that the EAs retain their original flexibility after aging in hot air, while the EPDM becomes more brittle and the polyurethane softens slightly. The implications of this unusually good stability of EA foams are twofold. Not only do chemically foamed EAs give performance which is equivalent to some thermoset rubber foams, but such materials may now be considered for extrusion applications where dense EAs are marginally too expensive compared to dense thermoset rubber. It is possible to achieve a higher level of performance at the same or lower cost by replacing a thermoset rubber with a chemically foamed elastomeric alloy.

Mechanical foaming of EAs

Mechanical foaming of EAs has been developed (ref. 4) to allow preparation of lower density rubber foams with elastomeric alloys than those achieved by chemically foaming. This process is also similar to technology used with thermoset rubbers. A liquid, such as a chlorofluorocarbon, with a boiling point below the polymer melt temperature, is used as the foaming agent. The liquid CFC is pumped under pressure to the second stage of a two-stage extruder where it is mixed with the molten polymer. Dispersion of the blowing agent requires intense mixing. A useful way to achieve this is to add barrel and screw extensions to a vented extruder with L/D of 24-/1, giving an L/D of about 30/1. The screw extension can be one of the economically available mixing heads, or elements, for better dispersion. The pump used for the foaming agent must be capable of metering a uniform flow of the low-boiling liquid under pressure to the extruder.

Foaming occurs when the pressure of the melt drops as the extrudate exits the die. This process is more efficient than the chemical blowing process and routinely achieves specific gravities down to the 0.2 to 0.4 range.

At these low densities, the foam is an excellent insulator and cooling times are even longer than for chemically foamed EAs. While the cooling baths are relatively long for this process, it avoids the need for equally lengthy curing equipment required for mechanically foamed thermoset rubber.

Compared to chemical foams, the low densities achievable by mechanically foaming EAs make them even more useful in demanding sealing applications (ref. 4). Table 4 shows a comparison of physical properties of a foamed EA with those measured on a sample of a commercial foam EPDM rubber automotive door seal. At about the same specific gravity and load deflection characteristics, the foamed EA has a higher tensile and tear strength and lower compression set than the EPDM foam sample.

Mechanically foaming thermoset rubber or EAs using CFCs have been a useful route to low density foam rubbers. However, the CFCs are relatively expensive and, more importantly, have been shown to contribute to depletion of the earth's ozone layer. Because of this environmental concern, the widespread use of CFCs as refrigerants, aerosol propellants and foaming agents is to be phased out by 2005. Recent discoveries (ref. 5) of greater than expected damage to the ozone layer may cause the United States Environmental Protection Agency to recommend speeding up this timing. Hydrofluorocarbons (HCFCs) have been shown to be environmentally safer substitutes, but are themselves programmed for phaseout 30 years after the CFCs are banned.

Water as a foaming agent

A recent breakthrough (ref. 2) in the technology of foaming thermoplastic elastomers promises to overcome the problems associated with CFCs, while allowing production of useful foams. It has now been found that TPEs can be mechanically foamed by using water as the foaming agent. The process is described in U.S. patent 5,070,111, Dec. 3, 1991. In this proprietary extrusion foaming process, the water is pumped under pressure to the second stage of a two-stage extruder, just as the CFC is pumped in the conventional process. Because water has a much higher boiling point than the chlorofluorocarbons used in foaming, the water may be metered into the extruder with a much simpler pump than that required for the CFCs.

In a typical foam extrusion, Santoprene thermoplastic rubber (Advanced Elastomer Systems) having a hardness of 64 Shore A and specific gravity of 0.97 was continuously fed at a rate of 5.6 Kg/hr to a vented single-screw thermoplastic extruder having an L/D ratio of 30/1. Liquid water was pumped at a pressure of 22 MPa and a flow rate of 91 g/hr to the vent port between zones 2 and 3 of the five-zone two-stage extruder. The die temperature was 174[degree]C, and the pressure at the die was 5.5 MPa. The resulting EA foam had a specific gravity of 0.29. The foam was largely closed cell in nature, having a water absorption of only 2.4% after 24 hours at ambient temperature.

Water may be used as a foaming agent for TPEs based on blends or alloys of a variety of rubbers with crystalline polyolefins, such as polypropylene. While the rubber portion of these TPEs can range from uncrosslinked to fully crosslinked, the preferred starting materials are fully crosslinked TPEs. Table 5 compares foams prepared from several TPE types using both water and CFC as foaming agents. These data show that closed cell foams with useful properties can be prepared form several types of TPEs using water as the only foaming agent. Future work will undoubtedly broaden the scope and applicability of this new technology.

Conclusion

Foamed elastomeric alloys have been produced by both chemical and mechanical processes, making available EA foams with specific gravities over the range from dense starting material down to about 0.25. The mechanical properties of EA foams make them suitable replacements for thermoset rubber foams. Foamed EAs have been shown to have unusually good stability in hot air aging tests, making them candidates for some very demanding uses, such as automotive sealing applications. The ability to foam EAs using water as the foaming agent is a new development with much promise in cost savings and environmental safety.

  Table 1 - water absorption by chemically
            foamed
  % Weight gain in water,
after 24 hrs. at23[degrees]C
                             Dense               Foamed
55 Shore A elastomeric alloy 0.13                0.36
64 Shore A elastomeric alloy 0.08                0.17
73 Shore A elastomeric alloy 0.10                0.20
  Table 2 - physical properties of chemically
            foamed EAs
                   Specific  Ult.     %          Comp.
                   gravity   tensile  Ult.       set
                             strength elong.     22 hr.
                             MPa                 770[degrees]
55 Shore A dense   0.98      3.6      322        23
         foamed    0.85      2.2      240        25
64 Shore A dense   0.98      5.4      330        29
         foamed    0.77      3.3      318        32
73 Shore A dense   0.97      6.3      365        36
         foamed    0.73      3.6      286        38
  Table 3 - comparison of chemically foamed EA with
            EPDM and urethane
                       Dense Foamed   Dense  Dense
                       EA     EA      EPDM   urethane
Specific gravity       0.95   0.66    1.16   1.12
Hardness, Shore A      94     78      63     92
Tensile strength, MPa  12.6   4.7     11.8   32.3
100% modulus, MPa      6.4    3.8     3.4    6.7
Ult. elongation, %     525    24.8    457    532

[TABULAR DATA OMITTED]

References

1. P.D. Agrawal and K.E. Kear, 1989 International Plastics and Rubber Conference, Dusseldorf, Geranty, Oct. 31-Nov. 1, 1989.

2. G.L. Dumbauld, United States Patent No. 5,070, 111, December 3, 1991.

3. Dr. G.E. O'Connor, "TPEs challenge the thermosets." Machine Design, September 11, 1986.

4. D.E. Peterson and P.D. Agrawal, "Foam extrusion of elastomeric alloys," International Journal of Cellular Polymers, p. 475, 1989.

5. "New discoveries could accelerate CFC phaseout," Oonstruction Specifier, p. 18, December 1991.

Acknowledgements "Rheological test to characterize injection molding" is based on a paper presented at the ACS Rubber Division meeting May 1992.

"Miscibility and phase behavior of IR/BR and BR/BR blends" is based on a paper presented at the ACS Rubber Division mccting May 1992.

"Advances in extrusion foaming of EAs" is based on a paper presented at the American Chemical Society meeting August 1992.

"NR breaker compound containing specific chlorotriazines as adhesion promoter; is based on a paper presented at the ACS Rubber Division meeting May 1991.