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An MD&DI June 1999 Column

SPECIAL SECTION — THERMOPLASTIC ELASTOMERS

Cross-Linking Thermoplastic Elastomers for Improved Product Performance

A study examines the effect of different TPE formulations and radiation dosages on changes in material physical properties following cross-linking.

Lawrence A. Acquarulo Jr. and Charles J. O'Neil

The process of cross-linking—the setting up of chemical links between the molecular chains in a polymer—has been used for many years in the wire-and-cable industry to obtain improved performance from commodity materials. This article describes the use of cross-linking to achieve improvements in the thermomechanical properties and the chemical resistance of a nylon-based material. A high-performance resin from the copolyamide (COPA) family of thermoplastic elastomers (TPEs) was selected and used to develop a cross-linkable material that would retain the positive qualities of TPEs such as strength, elasticity, and processing versatility. A major objective was to examine how cross-linking would affect the high-temperature limitations normally associated with these melt-processable resins.

A manipulation of cross-linking parameters can result in specific property improvements for selected TPE materials. Photo courtesy of Foster Corp. (Dayville, CT).

Production of the cross-linkable formulation involved compounding the resin with a cross-link promoter, or a cross-linker. The material can be injection molded to close tolerances, extruded, blow molded, rotational molded, or thermoformed. After processing, cross-linking is achieved by exposing the finished, molded product to high-energy irradiation, which modifies the physical characteristics of the thermoplastic material, making it behave more like a thermoset.

The development of this cross-linkable TPE involved several challenges:

• Selecting the best TPE to produce the desired physical properties.

• Determining the optimum amount of cross-linker for the formulation.

• Choosing the most practical method of irradiating the material.

• Optimizing the amount of irradiation exposure for the best results.

• Testing the irradiated material to confirm the existence of cross-links.

• Demonstrating that performance characteristics had in fact improved.

MATERIAL SELECTION

Selecting the best material for cross-linking involved consideration of the complete range of polymers available in the alphabet soup of TPE types. These include styrenic thermoplastic elastomers, thermoplastic polyolefins (TPOs), thermoplastic vulcanizate elastomeric alloys (TPVs), thermoplastic polyurethanes (TPUs), copolyesters (COPEs), and copolyamides (COPAs) (Table I). Figure 1 shows how the various TPEs compare on the cost/performance scale, with styrenics at the bottom (low cost/low performance) and copolyamides at the top (high cost/high performance).

Figure 1. The cost/performance scale for comparing TPEs.

Styrenic Thermoplastic Elastomers. Styrenic block copolymers (SBCs)—which include styrene-butadiene-styrene (SBS), styrene-ethylene butylene-styrene (SEBS), and styrene-isoprene-styrene (SIS)—are widely used in a number of markets, such as medical equipment, footwear, automotive, and wire and cable. Low-cost, low-performance elastomers, styrenics are the most widely used TPE and are popular replacements for thermoset rubber materials.

Thermoplastic Polyolefins. Relatively inexpensive, TPOs are employed in less-demanding, high-volume applications such as wire-and-cable jacketing and in certain automotive parts. Flexible polyolefins (FPOs) and polyolefin elastomers (POEs) are used for resin modification and for producing flexible packaging, medical tubing, and blood bags. Other polyolefin-based TPEs include ethylene propylene rubber (EPR) and ethylene propylene diene modified rubber (EPDM).

Thermoplastic Vulcanizate Elastomeric Alloys. Combinations of plastic and cross-linked rubber mixtures, TPVs are produced through a vulcanization mixing process. The resultant materials have the properties of a cured rubber and the processing characteristics of a thermoplastic. TPVs are used in medical, wire-and-cable, and automotive applications.

Thermoplastic Polyurethanes. Based on either a polyester or a polyether formulation, TPUs provide outstanding abrasion and tear resistance. They are used in the medical equipment industry to produce catheters, vascular grafts, blood bags, IV sets, and bioclusive dressings. TPUs are also widely used in underhood automotive applications and in shoe soles and wire-and-cable jacketing.

Copolyesters. Considered engineering TPEs, COPEs are less flexible than other types but maintain high strength and elasticity under dynamic loading. Resistant to high heat and chemicals, they are commonly used in fiber-optic coatings and sporting goods and in transportation, automotive, and aerospace applications.

Copolyamides. COPAs—or polyether block amide elastomers—are based on a block copolymer of nylon 12 and a polyether. A wide range of grades and performance characteristics can be achieved by varying the polyamide and polyether blocks. These high-performance TPEs can withstand high heat and offer good heat-aging characteristics, long flex life, and excellent chemical resistance. COPAs are used in demanding medical applications such as catheters and in wire-and-cable jackets, automotive parts, and sporting goods.

Final Formulation. The material selected as the formulation for the cross-linking trials was Pebax resin (Atochem North America; Philadelphia), a copolyamide thermoplastic elastomer from the COPA family. The material is at the top end of both the cost and performance scales and is used primarily in high-end applications. A grade with a durometer of 72 Shore D was chosen because it is widely available and it has the highest temperature rating of the copolyamides. The resin was compounded with a cross-linker that could be processed at the melting point of the Pebax—approximately 175°C. Trials were conducted with samples of the material to determine the optimal level of the cross-linker in the formulation and the radiation dosage required to achieve the best results.

CROSS-LINKING

Modifications in polymeric structure can be induced either by exposing a material to high-energy radiation or through a chemical process using peroxide. Both methods promote molecular bonding, or cross-links, within the polymer. The reactions produced by cross-linking depend on the particular polymer, the presence of modifying agents, and variables in processing, such as the level of irradiation. Cross-linking can produce significant property benefits in a polymer, among them:

• Improved thermomechanical properties.

• Higher service-use temperature.

• Improved dimensional stability.

• Inducement of heat memory in crystalline polymers.

• Lower permeability and improved chemical resistance.

• Reduced stress cracking.

• Overall improvement in physical toughness.

Radiation was selected rather than chemical processing to induce the cross-linking, for several reasons. The primary one is that radiation permits cross-linking and grafting in situ on finished products. In addition, cross-linking occurs at lower temperatures with radiation than with chemical processing.

Several types of radiation can be employed in cross-linking. Gamma radiation, using radioactive cobalt atoms, is preferred when considerable depth of treatment is necessary. For thinner products and coatings, electron-beam accelerators or UV light sources can be used. The cost of irradiating a polymer can be reduced with specifically designed formulations that accelerate the cross-linking process and produce additional performance advantages in finished products.

MATERIAL DEVELOPMENT AND TESTING

The Pebax resin was compounded first with 2% and then with 2½% cross-linker. Tensile bars (ASTM type-4 dog bones) were molded from the material, and some were irradiated to induce cross-linking. Using cobalt 60 as the radiation source, the irradiated samples were exposed to dosages ranging from 2 to 20 Mrd to determine the optimum combination of cross-linker and level of radiation. After irradiation, the samples were conditioned at room temperature for 24 hours.

The tensile bars were tested to verify that cross-linking had occurred in the irradiated samples. In addition, both the irradiated and the nonirradiated samples were tested for tensile strength and elongation, then compared. A hot creep test and a heat-deformation test were used. Standard test procedures developed by the Insulated Cable Engineers Association (ICEA) to determine the relative degree of cross-linking in polymeric materials were followed. The tests were modified to accommodate this material by raising the first temperature from an industry standard 150°C to 200°C (above the melting point of the Pebax material).

Hot Creep Test. For the hot creep test, a 29-psi load was hung from two suspended samples—one irradiated and one nonirradiated—for comparison. A ½-in. benchmark was inscribed on the surface of the samples, which were placed in an oven at 200°C for 15 minutes. It is important to conduct this test at 20° to 30°C above the polymer's melting point so as to determine the true measure of cross-linking.

In the first trial, samples were molded with a compound containing 2% cross-linker. They were irradiated at dosages of 5, 10, 15, and 20 Mrd. After conditioning, the samples were placed in the oven. As expected, the nonirradiated samples were unable to stand up to the high heat and they melted, whereas the samples that had been irradiated exhibited only various degrees of elongation. The results proved that the material had cross-linked.

Table II shows the results of the first trial. Data for the nonirradiated sample are shown in the 0-Mrd column, and the other columns show the samples that were irradiated at various dosages. Samples irradiated at 20 Mrd became brittle and broke in the jaws of the test jig, indicating that the material degrades at higher levels of radiation. The best response was seen at lower levels. Tensile strength begins to decline at 10 Mrd; elongation drops off at 5 Mrd and steadily decreases with increasing dosage.

A second trial employed samples that had been molded with the cross-linker level increased to 2½%. They were irradiated at 2, 4, 6, 8, and 10 Mrd. The objective was to achieve maximum cross-linking with low levels of radiation—similar to the dosages used in the sterilization of catheters.

The best results in this second trial occurred with dosages of 4 and 6 Mrd (Table III). Tensile properties increased slightly, and a reasonable level of elongation was maintained. The lower elongation in the hot creep results indicates that a significant level of cross-linking had occurred.

Heat-Deformation Test. To measure the changes in flexibility and in the thermomechanical properties of the material, 1/8-in.-thick disks molded from the same formulation (72 Shore D resin compounded with 2½% cross-linker) were irradiated at 5 Mrd. This standard ICEA test is commonly used to measure deformation resistance of wire-and-cable jacketing.

The test apparatus for this trial was a Randal & Stickney (Waltham, MA) micrometer gauge weighted with a 2000-g load. A sample disk was placed between the foot and the base of the dial gauge, and a reading was taken at room temperature. Next, the sample was placed in a 175°C oven for 15 minutes, and a second reading was taken to calculate the percentage of change in the thickness of the sample.

Results are shown in Table IV. The flex modulus increased by approximately 35% as a result of cross-linking. Heat deformation was less than 3% with the cross-linked sample, whereas the non-cross-linked sample lost more than 55% of its thickness. Hot creep was only 2.3% for the cross-linked sample, but the non-cross-linked sample melted in the oven. The testing shows that the material exhibits good deformation resistance at a temperature just below the melting point.

A final trial was carried out in order to evaluate the cross-linking of softer grades of polyamide thermoplastic elastomer, using resins with durometers of 35, 55, and 70 Shore D in the formulation. Samples were molded and irradiated at 10 Mrd. The hot creep results show that cross-linking occurred but not in as great a degree as with the semirigid, 72 Shore D grade (Table V).

APPLICATIONS

The easy melt processability of thermoplastic elastomers is the same characteristic that limits their use in elevated temperatures. Cross-linking overcomes this primary limitation so that the material can be used at and even above its melting point without changing dimensionally. In the medical industry, applications such as catheters can benefit from the availability of a cross-linkable material because it allows for greater stiffness to be achieved in thin-wall extrusions while retaining elastomeric properties.

A prototype device was fabricated using the cross-linked formulation described earlier. The prototype is a single-lumen, 6 French catheter created from two compounds with different durometers in one continuous extrusion, which eliminates the need to bond the two together. A 72 Shore D and a 55 Shore D material were used for the shaft, with the stiffer compound at the proximal end and the softer compound at the distal end. The 72 Shore D compound contained 40% bismuth oxychloride, and the 55 Shore D compound contained a blend of bismuthoxychloride and tungsten for higher radiopacity. Initial feedback indicates that the material's burst strength appears to increase with the cross-linking. Because of the material's high strength, catheters can be designed with the thinner outside diameters that are most appropriate for less-invasive clinical procedures.

The heat-resistant material is also proving useful for devices that are steam sterilized, since it does not degrade or undergo dimensional changes during sterilization. There has been some indication that lubricity is also improved, as cross-linking tends to produce a harder surface. Biocompatibility testing on both irradiated and nonirradiated materials showed them to be nontoxic.

The thermal memory of cross-linked materials can potentially benefit another application—that of heat-shrink tubing. In this process, tubing is extruded in the desired diameter, then irradiated. Next, the tubing is heated to soften it, then expanded to the working diameter. Subsequent cooling freezes the cross-links in the stretched position, allowing the tubing to be easily slipped over smaller-diameter parts. When the shrink tubing is reheated, it contracts to its original size, forming a tight bond.

CONCLUSION

A cross-linkable TPE formulation was developed by taking a high-performance COPA resin and modifying it to perform in even more demanding applications. The material was shown to offer elevated heat resistance, improved chemical resistance, and increased tensile strength and toughness, among other improvements. High-temperature performance was especially notable: in fact, once cross-linked, the material can be used in temperatures at and even above the melting point of the non-cross-linked resin. Available in a range of durometer ratings from soft to semirigid, the material can be used to produce thin-wall parts, tubing, wire-and-cable coatings, and other products that can benefit from these improved physical properties.

This study demonstrates that it is possible to modify formulations and radiation dosages so as to tailor cross-linked materials to the properties most desired in finished products. Refinement—including ongoing work with alloys—continues to optimize the level of cross-linking and provide an improved balance of physical properties. The further development of cross-linked TPE compounds is certain to lead to the creation of new molded products possessing unique physical properties.

Lawrence A. Acquarulo Jr. is president of Foster Corp. (Dayville, CT) and Foster West Corp. (North Las Vegas, NV). The companies provide design, development, and manufacturing of plastic compounds for the medical and electronics industries. A specialist in radiation cross-linking, Charles J. O'Neil is Foster Corp.'s technical director, with primary responsibilities in the development of compounds for medical applications.