Next-Generation Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) are a versatile family of materials that are finding many uses in new medical device designs. TPEs offer a range of desirable properties, including flexibility, soft touch, colorability and the potential to be recycled – unlike thermoset rubbers that are permanently crosslinked. Further, TPEs enable techniques, such as overmolding on a variety of substrates, which cannot be done with conventional elastomers. For system cost savings, TPEs can be processed using the same injection-molding and blow-molding equipment as traditional materials, allowing them to be integrated into existing manufacturing rather than requiring a separate operation for creating and assembling thermoset parts.

Thermoplastic elastomers are generally low-modulus, flexible materials that can be stretched repeatedly to at least twice their original length at room temperature and return to their approximate original length when stress is released. Traditional materials (e.g., styrene butadiene rubber [SBR], latex, polyisoprene) with this characteristic are thermoset rubbers; however, many families of injection-moldable TPEs are replacing traditional rubbers. The softness and suppleness of TPEs appeal to consumers in particular, making them popular for a wide range of healthcare products used in a home setting.

TPEs have been used for a number of years in the medical industry. With technology advancements, TPEs have gained improved performance and increasing acceptance in medical applications. Specifically, these materials are often considered for overmold designs to enhance the feel, ergonomics and aesthetics of the final part. Overmolding, combined with further technological advances, provides many opportunities to use TPEs in medical applications such as orthopedics, enhanced-feel surgical equipment, syringe plungers, needle shields, face masks, resuscitator and breathable bags, and home-use medical devices.

Next-Generation TPEs

Continuous innovation in TPE technologies is delivering new materials that address today's key challenges, including more stringent health and environmental regulatory requirements, cost containment and competitive pressures.

One of the newest uses for TPEs is in fluid delivery systems including dynamic-seal gaskets and static-seal stoppers for the healthcare industry. A key focus area for the Food and Drug Administration (FDA) is reducing leachables and extractables from rubber and plastic packaging components that can affect patient health when introduced into drugs or other formulations. The FDA is concerned about polynuclear aromatics (PNA), nitrosamine and 2-MBT, among other leachables and extractables. For example, health issues around 2-MBT – a vulcanization accelerator for rubber – have led to reformulation of EDPM, polyisoprene and butyl rubber stoppers for parenteral drugs.

New TPE grades from GLS Corp., which avoid the use of halogens, polynuclear aromatics (PNAs), nitrosamines, 2-MBT and other controversial additives, help to enhance patient safety and meet evolving regulations while delivering highly-effective sealing performance to safeguard the integrity of medical fluids.

These TPEs can also be overmolded onto polyolefins to avoid points of failure from discrete components. Overmolding is an injection-molding process in which one material (usually a TPE) is molded onto a second material (typically a rigid plastic). If properly selected with the help of TPE suppliers, the overmolded TPE will form a strong bond with the plastic substrate that is maintained in the end-use environment. With overmolding, the use of primers or adhesives is no longer required to achieve an optimum bond between the two materials.

Phthalate-Free TPEs

Similarly, new TPE grades offer plasticizer-free and phthalate-free formulations to address current concerns about these additives in various products and markets. These TPEs can help companies comply with existing and future regulations restricting phthalate use. This becomes increasingly beneficial as more states and regulatory agencies look to adopt similar policies.

Phthalate-free, plasticizer-free TPEs from GLS offer water-clarity, sterilization capability and low extractables. They are well-suited for a variety of extruded tubing and films as well as blow-molded bags, containers and infant care items. Finally, there is a growing trend in the marketplace to reduce the use of phthalates in global products.

TPEs vs. PVC and Silicone

Another key application area for TPEs is replacing polyvinyl chloride (PVC) and silicone rubber in tubing, bags and films. The industry is moving away from PVC due to its health and environmental concerns. It is virtually the only material that requires phthalate plasticizers (developmental toxins linked to bronchial irritation and asthma) and can include heavy metal stabilizers (neurotoxins and carcinogens). In addition, during manufacturing, PVC produces a large number of highly toxic chemicals including dioxins (potent human carcinogens), vinyl chloride (carcinogen), ethylene dichloride (probable carcinogen), and PCBs (carcinogens and reproductive and developmental toxins). When incinerated at end of life, PVC releases more dioxins – even before it ignites, PVC releases hydrogen chloride gas that forms hydrochloric acid upon contact with moisture, including moisture in the lungs of those who inhale it.

Silicone rubber is very expensive and is reported to have some absorption of proteins and antioxidants, which could reduce the efficacy of drugs. Specifically, silicone can absorb protein from biopharmaceuticals, potentially affecting these organisms and upsetting the delicate balance of the media in which they are carried.

Soft and clear, temperature-resistant TPEs can replace PVC and silicone rubber while adding performance and design advantages. These TPEs can match or exceed the physical properties of silicone and PVC, such as clarity and flexibility/low modulus, while offer ing the additional benefits of inertness and the ability to undergo various types of sterilization.

Benefits of TPEs

TPEs deliver benefits across many different aspects of medical device design and manufacturing.

TPEs address the FDA concern over leachables and extractables from rubber and plastic packaging components.

For designers and manufacturers of medical devices, TPEs provide alternative approaches to address deficiencies with current thermoset elastomers. For example, TPEs ease concerns about potentially leachable heavy metals used in the curing process for thermoset elastomers. As thermoplastics, TPEs can utilize a wider range of raw materials to tailor performance properties such as low oxygen (OTR) and water vapor (MVTR) permeation for medical packaging that requires such attributes to provide longer shelf life.

TPEs can even supply alternatives to latex, polyisoprene and butyl rubber used in various medical products while eliminating concerns about nitrosamine, mercapto benzo thiazole (MBT) and other byproducts of thermoset elastomers, which are known carcinogens and irritants. They can provide a broad hardness/flexibility range, as well as clarity, which is not possible using the abovementioned thermoset elastomers.

Molding TPEs instead of fabricating thermosets can improve the safety and consistency of the final product. Injection molding is a one-step process, whereas producing thermosets requires three or more steps: compression molding, trimming, washing, siliconizing and sometimes manual assembly. Each step introduces the possibility of compromised quality, which can add up to wide variations in the finished product. Medical device manufacturers and drug makers want zero contamination in incoming products, and thermosets – no matter how many precautions are put into place – often generate complaints about contamination found in the end product.

Expanded Design Freedom

Design innovation is another benefit of TPEs: medical device makers can leverage overmolding capabilities and the precision of injection molding, instead of compression molding used in the thermoset process, to achieve greater design freedom. Specifically, injection molding – thanks to its high pressure and processing speed – makes it possible to mold complex geometries that are not achievable using low-pressure compression molding.

Also, TPEs enable part consolidation to eliminate potential points of failure. Instead of having multiple components mechanically joined together with fittings that could leak or break, TPE overmolding can create a single, seamless assembly. This approach can even speed up regulatory approvals; by significantly consolidating the number of parts, there is less potential-failure testing that must be demonstrated to regulatory bodies, since the risk of leaks that could cause cross-contamination has been reduced. The design flexibility of TPEs also helps manufacturers achieve device miniaturization and improved usability.

Reducing System Costs

In most medical devices, raw materials make up only a portion of the final product cost. There are costs associated with assembly, quality assurance, validation and the overall manufacturing process. Although the material price of TPEs on average could be 50% higher than thermosets, they help reduce overall costs by reducing or even eliminating scrap, shortening regulatory cycles, enabling part consolidation and improving quality. In comparison with thermosets, TPEs can reduce finished part cost by up to 30% through processing efficiency, elimination of secondary processes and their associated materials, and fewer rejects from contamination.

The ability to substitute TPEs for conventional materials in the same tool offers the opportunity for reduction of cycle time and secondary operations. For example, TPEs can replace low-density polyethylene (LDPE) in the blow-fill-seal process for making bottles containing eye drops and nasal drops. TPEs enhance these applications by providing transparency (vs. translucent PE) and flexibility, and give the option of terminal sterilization including autoclaving at 121oC, a method favored by the FDA and one which is not possible using PE.

TPEs can also help to consolidate a multi-step process. Traditionally, rubber tips for needle shield applications were fabricated from thermosets and polyolefins, which required different manufacturing processes followed by sterilization and assembly. Because TPEs can be injection-molded on the same equipment as the polyethylene and polypropylene used for the needle shield, these components can be molded, pre-sterilized and assembled in a clean room environment, saving valuable time and cost.

Results

TPEs offer the formulation freedom to match or exceed the properties of many thermoset offerings for healthcare devices and equipment. Advancements in TPE compounding science have helped these versatile materials make significant inroads into a variety of healthcare sectors. The desirable combination of design freedom, system cost benefits, regulatory compliance and improved performance are fueling strong growth of TPEs in a wide range of medical applications.