A family of thermoplastic elastomers (TPEs) crosslinked by irradiation improve thermomechanical properties and chemical resistance at elevated temperatures. These materials can be easily injection molded and extruded into thin wall parts and tubing as well as wire and cable coatings.
This article presents an overview of these TPE families and markets and where they are used. It reviews the advantages of the irradiation crosslinking process and the improvements imparted to plastics and elastomers, particularly to the COPA family of thermoplastic.
TPEs are unique synthetic compounds that combine some of the properties of rubber with the processing advantages of thermoplastics. They consist of a number of families, the majority of which are listed in Table I.
Like rubbers, the TPE key properties are hardness and stiffness or flexibility. The families overlap each other in these characteristics. The TPUs, COPEs, and COPAs offer the hardest and stiffest grades available. Because of this, they are sometimes referred to as the engineering TPEs. This group is also the more expensive and are often used in high-end markets.
The TPO Family. Thermoplastic polyolefin (TPO) elastomers are comprised of various mechanical blends of polyolefin resins. The components for these blends are readily available and are relatively inexpensive. They have found application in less demanding large-volume areas, such as wire and cable jacketing.
More recently, a group of flexible polyolefins has been added to this group. They are sometimes called the FPOs or POEs. These flexomers are made in-situ and, unlike the physical blends, these materials come directly from the reactor ready to be extruded or molded. A number of these POEs are based on metallocene catalyst technology. They have been finding application in flexible packaging, resin modification, medical tubing, and blood bags.
The SBC Family. The SBC family, or styrenic thermoplastic elastomers, represent a class of elastomers introduced in the mid I960s. The most noted of the SBC elastomers are the SBS and the SEBS. The SIS elastomers are also a member of this family. Styrenic thermoplastic elastomers obtain their thermoplastic properties because of their structure. This is due to the multiphase composition in which the phases are chemically bonded by block polymerization. In all cases, at least one phase is a styrenic polymer that is a hard phase at room temperature. The other phase is a rubber-Iike material that is soft at room temperature.
The proportion of the styrenic phase controls the hardness and stiffness properties of this family. The greater the proportion of the styrenic phase in this block copolymer, the harder and stiffer the product will be. The choice of soft segment will control the chemical resistance and thermal properties. The polybutadiene SBS and polyisoprene SIS have unsaturation present in their structures, which leads to poorer chemical resistance and reduced thermal oxidative resistance. The poly(ethylene-butylene) SEBS being saturated exhibit better chemical resistance and are thermally more stable.
SBC thermoplastic elastomers have found a wide range of use in a number of markets, including medical.
The TPV Family. TPV elastomeric alloys are composed of mixtures of a plastic and a rubber in which the rubber phase is cured or crosslinked. The plastic phase is commonly a polyolefin; most notably polypropylene. However, it is possible to make TPVs with a variety of thermoplastics. Some that have been used are nylon, SAN, ABS, acrylates, polyesters, polycarbonates, and styrene. The rubber phase is commonly an EPR or EPDM rubber. Other rubbers that have been used are nitrite, SBR, polybutadiene, butyl, and CPE.
The end product is produced during a dynamic vulcanization mixing process. In this case the curing of the rubber phase occurs during the mastication with the thermoplastic resin. This process gives a useful elastomeric alloy with properties of a cured rubber but has the processing characteristics of a thermoplastic. It is important that the mixing be continuous throughout the masticating step, or a thermoset material could result.
The TPU Family. Thermoplastic polyurethane elastomers are made up of two types: polyester and polyether. The polyester-based TPUs are generally characterized as having better physical properties, oxidative stability, and oil resistance. While the polyether-based TPUs at a similar hardness exhibits better low-temperature properties, hydrolytic stability, and resistance to microbial attack. TPUs are noted for their inherent toughness, providing outstanding abrasion resistance and tear resistance.
Applications in medical are one of the fastest growing market segments for TPUs. Some new aliphatic specialty grades have been developed for this market as well. The applications here include cardiovascular catheters and vascular grafts. Other medical applications include blood bags, IV sets, and bioclusive dressings.
The COPE Family. COPEs are considered engineering thermoplastic elastomers because of their unusual combination of strength, elasticity, and dynamic properties. They show a high degree of heat and chemical resistance as well.
The COPA Family. The COPA family, or polyether block amide elastomers, are based on a block copolymer of nylon 12 and a polyether. Through the proper combination of polyamide and polyether blocks, a wide range of grades that offer a variety of performance characteristics is possible. The family is characterized by durometer and flexibility. The higher the durometer and stiffness the higher the level of nylon 12 in the block copolymer. COPAs have been limited to niche markets due to their cost. They are finding use in the medical market for various types of catheters.
Advantages of Irradiation of Polymers
In the early 1950s, interest began in the potential use of high-energy radiation to initiate polymerization and modify polymers by crosslinking. The ability to cause chemical reactions in the solid state by well-controlled exposure of solid polymers to high-energy radiation created a whole new industry. This industry has resulted in the creation of products such as heat-shrinkable tubing and a number of niche wire and cable products.
When considering the use of polymer irradiation, it is essential to consider the following:
What are the advantages of irradiation?
Which properties can be improved by irradiation?
What kind of radiation source is preferred?
What does irradiation cost?
There are a number of advantages of using the irradiation process for polymerization and crosslinking of polymers over the conventional chemical process, such as:
The crosslinking takes place at lower temperatures than those involved with chemical crosslinking.
In the case of polymerization, the monomers can be polymerized free of catalyst contamination.
Crosslinking and grafting can be performed in situ on finished products.
Coatings can be applied in monomeric form eliminating solvents.
A number of properties are also improved that yield additional advantages:
Improved thermomechanical properties.
Increased service use temperature.
Improved dimensional stability.
Heat memory induced in crystalline polymers.
Lower permeability and improved chemical resistance.
Reduced stress cracking.
Generally improved physical toughness.
The irradiation source considered for the process is dependent on the depth of treatment required. If considerable depth of treatment is necessary, radioactive isotopes such as cobalt 60, sometimes referred to as gamma radiation, is preferred. Electron beam accelerators are used for thinner products and are dependent on the voltage ratting of the equipment. For thin coatings, electron beam of low voltage or ultraviolet light can be used.
The cost of the irradiation process is dependent on the type of polymer, formulation shape, and thickness of the fabricated product. It is also dependent on the required irradiation dosage needed to achieve the desired properties. The cobalt 60 process is more expensive and much slower. It is used for thick products and materials that are dose rate sensitive. The electron beam process is for thinner products and can be run at very high rates. It is also less costly. The use of specifically designed formulations can reduce cost, speed up the process, and create even greater overall advantages using the irradiation process.
Trial 1. For the first trial, a 72 shore D durometer polyamide thermoplastic elastomer was selected. This polyamide thermoplastic elastomer was compounded with a crosslink promoter on a corotating twin screw extruder. The crosslink promoter chosen was capable of being processed at the temperatures needed to mix and mold this polyamide thermoplastic elastomer. The 72 D durometer was chosen because it is the most widely used and also has the highest temperature rating.
After compounding tensile bars were molded for testing. The molded samples were then exposed to cobalt 60 as the irradiation source. The irradiation dosage was 5, I0, 15, and 20 megarads. Following the irradiation, the samples were conditioned at room temperature for 24 hours. Samples were tested for tensile strength and elongation. Samples were also tested to determine whether they crosslinked. The ICEA Publication T-28-562 test used to determine whether the samples crosslinked. This publication provides a procedure that is suited for determining the relative degree of crosslinking of polymeric electrical cable insulation. The test method was modified to accommodate this material. The melting point of this material is approximately 175 °C; therefore, it was tested at 200 °C to be above the melting point of the material. It is important to use a temperature that is between 20° and 30 °C above the melt point so that the true measure of crosslinking can be made. The results of this first trial appear in Table II.
Trial 2. The second trial also used the 72 D durometer polyamide thermoplastic elastomer. The crosslink promoter was the same, but the level of crosslink promoter was increased by 1/2 percent. This was done because the first trial showed a good response at lower dosage and a rather large loss of elongation at the higher dosage. The goal was to obtain maximum crosslinking with a low irradiation dose. The reason for this was because this material is used in the manufacture of catheters, and a low dose would simulate the type of dose used to sterilize the catheter. The material was compounded and molded the same way as done in the first trial. The irradiation dosage used was refined and done at 2, 4, 6, 8 and I0 megarads. The results are shown in Table III.
Trial 3. The third trial consisted of using the 72 D durometer polyamide thermoplastic elastomer with the same crosslinker and level used in the second trial. In this trial, samples were molded for tensile and elongation, flex modulus, and heat deformation. The samples were irradiated at a dose of 5 megarads.
The heat deformation test used a Randal & Stickney gauge with a 2,000 gram weight. The temperature of the test was 175 °C. This type of test is commonly used to measure deformation resistance of wire and cable insulations and jackets. The results are shown in Table IV.
Trial 4. The fourth trial evaluated the crosslinking of some lower durometer grades of polyamide thermoplastic elastomer. The durometers chosen were 35 D, 55 D, and 70 D. The samples were irradiated at I0 megarads. The samples were tested for crosslinking only. The results are shown in Table V.
Discussion and Conclusion
The work done in the first trial was done just to determine whether a COPA was capable of being crosslinked using a crosslink promoter. The crosslink promoter used was one that was known to work for some high-temperature polymers used in wire and cable compounds.
The results in Table II indicate that the 72 D grade of a COPA did indeed crosslink and is also sensitive to high dose levels of irradiation. It appears that the useful crosslinking takes place when the material is exposed to an irradiation dose of less than 10 megarads. The physical property gains are minimal as can be seen by the small increase in tensile strength at 5 megarads. The tensile strength begins to decline at 10 megarads. The elongation drops off at 5 megarads and steadily decreases with a dramatic loss at 20 megarads. The hot creep done above the melting point is the true indicator of crosslinking. If the material did not crosslink, the samples would melt and break as can be seen in the zero megarad sample.
The second trial was done with a 1/2 percent increase in crosslinker level in order to gain the maximum crosslink level with a minimal exposure to irradiation dose. The results in Table IV indicate that the best results occur somewhere between 3 and 7 megarads. Again, tensile properties increased slightly while a reasonable level of elongation was maintained. The hot creep indicates a good level of crosslinking as can be seen by the lower elongation. From this data, it is estimated that a good dosage for this material will be around 5 megarads.
The third trial reflects the refinement taken from the first two trials with an expansion of the test evaluation to measure the change in flexibility and thermomechanical properties. The flex modulus appears to have increased by about 35 percent. The thermomechanical property measured using a modified wire and cable test indicates that the crosslinking gives this material good deformation resistance at a temperature just below the melting point. The data for this trial is shown in Table V.
The fourth trial was done using three softer grades of a COPA to see whether they too would crosslink. The data in Table V indicates hot creep values that show crosslinking. The crosslinking does not appear to be as great as in the 72 D grade and indicates a need to do some refinement. COPAs are capable of being crosslinked by irradiation and formulations are being refined to obtain maximum crosslink levels with a good balance of physical properties.
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