Beginning in the 1960s, the main driving force for the use of plastics in the medical industry was hygiene. By that time, most devices were intended for multiple operation (i.e. syringes) and needed to be sterilized after each use, but still bearing a risk of contamination. As an enabling technology, the use of plastics led to the commercially advantageous and more hygienic application of single use devices. The evolution and technical progress of modern polymers were key factors in the development of devices, which are considered a technical standard today. Examples can be as easy as the replacement of glass bottles by polyethylene pouches for packaging nutrition and infusion solutions, the development of modern catheterization systems or such complex and well researched materials for permanent implants like PEEK or silicones.

Applications in the food packaging, medical and pharmaceutical Industry
Compared to technical applications, the requirements on a medical grade plastic are much more driven by toxicological and hazard aspects. Based on these requirements Actega DS outlined a set of specific demands which were finally translated into a medical grade plastics portfolio.

  • Flexible materials, based on styrenic thermoplastic elastomers
  • Shore hardness range from A20 to A80
  • Elastic modulus in the range of 1 to 25MPa
  • Elongation at break up to 1000%
  • Transparency and/or translucency
  • Sterilizable by gamma irradiation, ethylene oxyde gassing and autoclave without major changes in optical and mechanical properties
  • No toxic or harmful effects on the human body, approved by biocompatibility standards like ISO 10993 or regulations in the United States Pharmacopoeia (USP)
  • Adhesion performance in multi-component injection molding

Biocompatibility and regulatory demands

Especially in sensitive areas like medical devices, pharmaceutical and food packaging, factors like migration or leaching of potential toxic or harmful substances are big concerns. Thus, internationally accepted norms and testing conditions were created to evaluate raw materials and extractables for their hazard potential. Namely the ISO 10993 and the evaluation according to the United States Pharmacopoeia (USP) are common demands on plastic materials. Not only do the raw materials need very thorough screening and evaluation, but also the compounding process has the potential to contaminate a material with harmful substances if not done under accurate conditions. Process conduct, production hygiene and quality control need to go hand in hand to establish constant product quality and production according to the formulation. By evaluation of the material through the above mentioned norms and certification, the end-customer can be ensured to safely use the material in his critical applications.

Sterilization is one key factor

To prevent the user and patient from infections with germs, viruses and bacteria, medical goods need to be delivered in sterile condition. To achieve the absence of these biological surface contaminations three main sterilization procedures were established as commercially interesting and technically feasible options: Gassing with Ethylene Oxyde, Irradiation with highly energetic rays (Gamma or Beta irradiation) and the sterilization via hot steam in an autoclave. All of these options have advantages and drawbacks which are not subject in this article. Nevertheless, all of these sterilization procedures do not only present a very fatal environment for biological contaminations but also have a dramatic impact on the involved materials of the medical or pharmaceutical good. Especially the irradiation with gamma rays has the potential to permanently alter the properties of plastic materials depending on the dose. These effects can range from a slight color change (yellowing) to increased stiffness or brittleness.

To overcome these issues with sterilization procedures, plastic materials need to be especially formulated and equipped with specialized stabilizers and other ingredients. Actega DS therefore developed a specialized TPE portfolio for medical applications which resists the sterilization by the above mentioned procedures. This resistance is illustrated through mechanical testing prior and after sterilization with gamma irradiation (doses 25 and 50kGy), ethylene oxyde and autoclaving. After irradiation with a dose of 25kGy the materials ProvaMed 1140 and ProvaMed 1160 show a slight increase in tensile strength.
Due to the high energetic nature of the irradiation crosslinking effects occur in and between the polymer chains and increase the strength of the material. After further irradiation (dose: 50kGy) the damaging effect of the gamma rays exceeds this effect and the polymer chains are partially broken and lose tensile strength. But still the overall tensile strength even after such a high dose as 50kGy is very close to the original values and exceeds the mechanical demands in most applications and is therefore negligible.

Also treatment in an autoclave is a common procedure not only for multi-use medical goods. Materials are exposed to high humidity, pressure and temperatures of up to 134°C. These conditions can severely damage thermoplastic materials which do not exhibit sufficient resistance. Regularly observed effects are shrinkage, deformation or even melting of the plastic. The TPE materials ProvaMed 1060, 1070 and 1080 were especially designed, formulated and tested to resist the challenging conditions in an autoclave. As demonstrated in Diagram 2 by measuring the tensile strength of the materials, the resistance to the autoclave process is proven. The temperature has also the beneficial effect of inducing relaxation and crystallization processes in some of the polymers which are part of the TPE formulation. This can cause a noticeable increase in tensile strength and elongation at break of the plastic. A similar effect appears during the sterilization with Ethylene oxide gas.
Overall, specific ProvaMed TPE compounds were designed and tested for suitability in medical applications, which demand the sterilization of the final products. The change in mechanical properties is minimal and can be considered negligible in the design process.

New Developments in food contact applications

The majority of metal-vacuum closures worldwide is equipped with PVC based sealants and lacquers today. PVC, additives such as lubricants and stabilizers are dispersed in a suitable plasticizer to form plastisoles. These paste-like sealants can exhibit a wide viscosity range and are coagulated by thermal influence. The PVC paste is injected into the rim of the vacuum closure and cured in a temperated feed oven up to 220°C. During curing cycles between 60 and 80 seconds, the PVC paste solidifies and forms a circular sealant that helps maintaining the vacuum in glass jars. The completed closures are boxed and sold to fillers of food stuffs. PVC as well as the additives and plasticizers are under immense pressure from ecology groups and therefore also under legislative observation. Since 2004 a variety of EU-wide amendments regarding the approval of food contact materials and chemicals were adopted, causing certain additives and especially plasticizers based on phthalates to be limited or even prohibited.

Confronted with these regulatory adjustments the industry was ever looking for alternatives to be used as a replacement for the PVC-based sealant materials. But for a long time, the mechanical and processing conditions of PVC based sealants for vacuum closures were not met. Based on its tradition and technical experience with thermoplastic elastomers, the supplier invented a non-PVC solution for use in vacuum closures and brought it to market in 2011 in first commercial applications. Under the trade name Provalin, a wide product range for use in conventional closures and applications as well as for specialized closures (i.e. baby food) was brought to market. These materials also fulfill the demanding regulatory requirements for long term food contact, even with fatty foods. Besides the necessary fulfillment of regulatory aspects, the absence of PVC in the glass packaging also contributes to positive environmental effects. Since Provalin is free of heavy metals, plasticizers or other critical chemicals and additives, it can also be thermally recovered and the overall footprint is minimized compared to PVC. Provalin grades have been developed for use in all kind of common applications like hot-fill, pasteurization and sterilization and are legally conforming with the 4th amendment to the Plastics Directive 2011/10/EC including the global migration limit of 10mg/dm². Since the material is a solid thermoplastic material no oven is needed for curing. The cap is equipped with the sealant in a specialized compression moulding process. Today the commercialization of Provalin made good efforts with notable partners trusting in the material. The company Feinkost Dittmann for example equipped their food stuffs with high fat content with the novel blue Provalin sealing and promoting the innovation with a designated logo on the cap provided by closure manufacturer Pano.

TPE als flexible Werkstoff-Alternative in pharmazeutischen und lebensmitteltechnischen Anwendungen

Aufgrund ihrer enormen Flexibilität, hohen thermischen und Medienbeständigkeit, großen Vorteile bei Verarbeitung und Recycling stellen die heute anspruchsvollen und ausgereiften Rezepturen der thermoplastischen Elastomere in fast allen Bereichen die perfekte Alternative zu vulkanisierten Kautschuken, PVC und Silikon dar. TPE können aufgeschmolzen, extrudiert und spritzgegossen werden und bieten eine Reihe verarbeitungstechnischer Vorteile. Insbesondere in sensiblen und hoch regulierten Märkten, wie zum Beispiel bei Lebensmittelverpackungen, in Pharmazie und Medizintechnik, in denen Minimierung des Migrationspotentials, Biokompatibilität, Verbraucher- und Patientenschutz, Hygiene und Sterilisationsfähigkeit wichtige Anforderungen sind, hat sich der Einsatz von TPE als unverzichtbar erwiesen.

Über den Autor

Dennis Siepmann, Business Development Manager, Actega DS, Bremen