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BY LAWRENCE A. ACQUARULO, JR. AND CHARLES J. O’NEIL

INTRODUCTION : Advances in design and function of medical devices have led to an increased need for consistent, close-tolerance, high-performance thermoplastic elastomer compounds. This paper will present an overview of thermoplastic elastomers. We will list advantages and disadvantages of the different families of thermoplastic elastomers. We will then concentrate on the two families of thermoplastic elastomers that are currently being used successfully in the manufacture of a number of medical devices. We will introduce ways that these two families may be modified to produce even more high performance characteristics that could be of great benefit to the device manufacturer. TPE OVERVIEW Thermoplastic elastomers 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 .

TABLE I

TPO'S This family of thermoplastic elastomers consists of the polyolefin based elastomers. (EPDM) Ethylene Propylene Diene Modified Rubber, (EPR) Ethylene Propylene Rubber, (FPO) Flexible Polyolefin, and (POE) Polyolefin Elastomer

SBC This family of thermoplastic elastomers consist of styrenic block copolymer elastomers. SBS, SEBS and SIS TPV'S This family of thermoplastic elastomers consist of elastomers where the rubber portion is fully cured or partially cured. TPU'S This family of thermoplastic elastomers consist of polyurethane elastomers, polyester type and polyether type. COPE'S This family of thermoplastic elastomers consist of copolyester type of elastomers. COPA'S This family of thermoplastic elastomers consist of copolyamide type of elastomers. PEBA Like rubbers, the thermoplastic elastomers key properties are hardness and stiffness or flexibility. The families overlap each other in these characteristics. The TPU'S, COPE'S and the

COPA'S offer the hardest and stiffest grades available, because of this they are sometimes referred to as the engineering thermoplastic elastomers. This group is also the more expensive group of thermoplastic elastomers and finds use in high end markets. (see Figure 1) THE TPO FAMILY The TPO family or thermoplastic polyolefin 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, paintable automotive exterior trim and some automotive underhood applications. More recently a group of flexible polyolefins has been added to this group. They are sometimes called the FPO'S or POE'S. 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 the POE'S are based on metallocene catalyst technology. They have been finding application in flexible packaging, resin modification, automotive trim, medical tubing and blood bags. THE SBC FAMILY The SBC family or styrenic thermoplastic elastomers represent a class of elastomers introduced in the mid 1960's. 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-like 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 un-saturation present in their structures which will lead to poorer chemical resistance and reduced thermal oxidative resistance. The poly(ethylene-butylene) SEBS being saturated will exhibit better chemical resistance and be thermally more stable. SBC thermoplastic elastomers have found a wide range of use in a number of markets, medical, footwear, sound deadening, automotive and wire and cable. THE TPV FAMILY The TPV family or elastomeric alloys are thermoplastic elastomers 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 TPV's 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 nitrile, 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. Markets for TPV's have been found in medical, wire and cable, automotive and mechanical rubber goods. THE TPU FAMILY The TPU family or the thermoplastic polyurethane elastomers are made up of two types. The polyester type and the polyether type. The polyester based TPU'S are generally characterized as having better physical properties, oxidative stability and oil resistance. While the polyether based TPU'S at a similar hardness will exhibit better low temperature properties, hydrolytic stability and resistance to microbial attack. TPU's as a family are noted for their inherent toughness providing outstanding abrasion resistance and tear resistance. They have found application in automotive underhood, shoe sole, wire and cable jackets and more recently in medical applications. The applications in medical are one of the fastest growing market segments for TPU'S. 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, I.V. sets and bioclusive dressings. THE COPE FAMILY The COPE family or thermoplastic polyester elastomers were first developed by DuPont in the late 60's. They were marketed in 1972 under the trade name Hytrel. Since then a number of manufactures have developed products that are part of this family. COPE'S 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 COPE'S have found application in fiber optic coatings, sporting goods, transportation and industrial applications where the combination of mechanical properties and flexibility are critical. 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 co-polymer. COPA'S have been limited to niche markets do to their cost. Areas where they have been extensively used are where long flex life is required, such as air hose coils for the air brake systems on trucks. This is due, in part, to the high resilience and low hysteresis of the polyether block amide. They are finding use in the medical market for various types of catheters. Other niche use markets have been in sporting goods, pumps and wire and cable jackets.

TABLE II PROPERTY CHARACTERISTICS OF TPE'S PROPERTY TPO'S SBC'S TPV'S TPU'S COPE'S COPA'S Spg. 0.89-1.0 0.90-1.2 0.90-1.0 1.1-1.3 1.1-1.3 1.1-1.2 Hardness 60A-75D 30A-75D 40A-50D 70A-75D 40A-82D 75A-72D COMP. SET P P G-E F-G F F-G @ 100 C HIGH-TEMP 120 100 135 135 160 150 Deg C LOW-TEMP -60 -70 -60 -50 -65 -65 Deg C FLUID REST. P P F-E F-E G-E G-E COST ($/lB) P = POOR F = FAIR G = GOOD E = EXCELLENT HIGH PERFORMANCE TPES IN MEDICAL APPLICATIONS: Out of all the available families of TPEs, only the polyurethane (TPU) and polyether block amide (COPA) have the property balance to be successfully utilized in demanding medical applications such as catheter tubing, balloons for catheters, films for wound dressings, surgical drapes, storage bags, strain reliefs, etc. These materials offer a good balance of tensile strength, toughness, elongation, kink-resistance, hydrolysis-resistance, and thermoplastic processability. TPUs and COPAs are commonly used for short-term implants. Medical Design engineers are constantly pushing the limits of these TPEs and are always looking for ways to achieve even higher performance than what is currently available. Engineers also want to narrow the specification tolerances, enabling their use in micro-precision molding and thin wall extrusion processes. Basically, it doesn’t matter how good the material is, if it can’t be processed with the required accuracy, it is still no good. MODIFICATION OF TPES VIA FORMULATION AND SPECIALTY COMPOUNDING Many of the high performance TPEs require some form of compounding or modification prior to use in a medical device application. The addition of colors, radiopaque fillers, reinforcements, lubricants, and process aids is common. Melt filtering to reduce gels and visual impurities as well as melt homogenization and repelletizing to uniform small pellets is also quite popular. Alloying TPEs to achieve enhanced performance is also catching on.

By melt compounding one or more additional polymers into a COPA elastomer, you can effectively extend its performance range as shown in Figures 1 and 2. COPAs and TPUs can be alloyed with a number of different resins; however, achieving the exact property profile you need can be tricky business. The development of new blends is still mostly a painstaking process of trial and error, even if using advanced statistical methods such as Design of Experiments (DOE) and the like. Unless your budget and time permits, it is best to evaluate proven blends first. Another and more exciting way in which COPA elastomers have been modified to enhance design properties is through crosslinking by either gamma or E-Beam irradiation. Crosslinking of COPA addresses one of the main disadvantages of TPEs versus Vulcanized (thermoset) rubber; temperature resistance. Figure 3 shows the hot creep temperature performance of uncrosslinked COPA versus crosslinked versions. Crosslinking the COPA is shown to have a significant effect upon elevated temperature performance, allowing the design engineer to utilize a TPE in demanding high heat applications for the first time. Crosslinking also increases stiffness (Figure 4), improves hydrolysis resistance, chemical resistance, and reduces elongation. The advantage of irradiation crosslinking a COPA elastomer is that it allows a design engineer to fabricate the device using conventional thermoplastic processing methods and then adjust the material properties in a post-processing operation. Many of the crosslinkable COPAs have been designed to crosslink at radiation doses similar to sterilization doses; therefore, accomplishing both tasks simultaneously. SUMMARY: Of all the thermoplastic elastomers available, only two families, the Polyurethanes (TPUs) and the polyether block amide (COPA) demonstrate high performance design properties. Flexural modulus, hardness and tensile strength may be enhanced by alloying COPA or TPU with one or more polymers. Crosslinking COPA by gamma or E-Beam irradiation after processing is shown to have a significant effect upon elevated temperature performance. Crosslinking also imparts other characteristics of thermoset rubbers as well as increased stiffness with good kink resistance. REFERENCES: 1. Specialty Compounds for Medical Applications: An Introduction, L.A. Acquarulo, Medical

Plastics and Biomaterials, Sept. 1996. 2. High Performance Crosslinkable Thermoplastic Elastomers for Medical and Electronic

Applications, L.A. Acquarulo, C.J. O’Neil, Society of Plastics Engineers, Antec 1999, NY, NY.

3. Plastics Failure Guide, Cause, & Prevention, Myer Ezrin, Hanser/Gardner Publications, Inc., Cincinnati, OH, 1996.

COPA Alloys, Durometer Shore D

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COPA 72D 5200 Fostalon 5400 Fostalon 5600 Fostalon 5800 Fostalon 5900 Fostalon Nylon 11 Nylon 12

Figure 1

COPA AlloysFlexural Modulus

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COPA 72D 5200 Fostalon 5400 Fostalon 5600 Fostalon 5800 Fostalon 5900 Fostalon Nylon 11 Nylon 12

Figure 2

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Figure 3

Crosslinked COPA, Hot Creep Elongation@ 200° C, 29 PSI

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Figure 4

Crosslinked COPA, Flexural Modulus