Overcoming medical polymer compounding problems
Peter M Galland and Ross Van Royen, Teknor Apex Company, discuss the challenges in compounding medical polymers
In compounding plastics for medical devices, use of dedicated processing lines to ensure product purity is just a starting point. Equally fundamental is the need for sophisticated process controls and production monitoring systems to meet stringent codes and critical performance requirements, as well as to provide device manufacturers with lot to lot documentation. Beyond these basic considerations, medical compounders face new challenges with every formulation that they develop.
The challenges vary with the type of polymer. Teknor Apex specialises in medical compounds based on flexible and rigid PVC and on thermoplastic elastomers (TPEs). While having extensive involvement in two major medical product ranges generates valuable technical and marketing synergies, each of these polymer families also poses its own unique compounding challenges.
PVC compounds: Evolving plasticiser formulations
A major challenge confronting PVC device manufacturers today is that of issues involving ortho-phthalate plasticisers, particularly DEHP, which is widely used in medical-grade PVC and has decades of application history. Selecting alternative plasticisers for use in producing medical compounds is not a simple matter, since most alternatives now available entail some compromise in cost or performance in comparison with DEHP, and since different device applications pose different plasticising requirements.
Key considerations for evaluating alternative plasticisers include chemical structure, toxicology, solubility, migration, crazing, sterilisation, mechanical properties, processing, design, economics, and current market penetration. As a producer of both plasticisers and PVC compounds, Teknor Apex has prepared comprehensive data on plasticiser alternatives as a way to help device manufacturers make informed decisions in addressing marketplace issues concerning phthalates. The company is prepared to work with designers and processors no matter which alternatives they select.
One example of the complications that arise when selecting plasticisers has to do with applications where a flexible PVC device, such as tubing, comes into contact with non-PVC rigid components, such as ABS, acrylic, polycarbonate (PC), or polystyrene (PS). Certain plasticisers can cause softening, cracking, or other defects in these components as they migrate across the interface from the component made of PVC.
In some cases the problem can be avoided by switching to rigid PVC for the component to be used in contact with the flexible PVC device. In fact, Teknor Apex has developed a rigid PVC to replace PC in clear medical components such as connectors and check valves, obtaining comparable strength and excellent clarity while reducing or eliminating the stress cracking that often occurs in PC at the interface with flexible PVC components such as tubing.
TPEs: Optimising performance vs incumbent materials
TPEs bring unique capabilities to the medical device industry, and in some cases these benefits represent valuable cost/performance advantages over conventional incumbent materials such as PVC and thermoset rubber. Two of the challenges facing compounders of medical-grade TPEs are:
1 developing a range of compounds broad enough to match the diversity of requirements in the many well-established applications dominated by conventional materials
2 perfecting the TPE compounds not just in terms of properties to be found on data sheets but in terms of real-world requirements.
To meet the first challenge, Teknor Apex has developed a broad family of medical compounds, called Medalist, for applications as diverse as tubing, solution bags, syringe stoppers, dropper bulbs, face masks, films, drip chambers, gaskets, valves and even medical device cable insulation and jacketing. This has involved making use of a range of TPE polymer types, including styrenic, olefinic, vulcanizate, and alloy formulations, with hardness offerings ranging from ultra-soft gels at 25 Shore OO to hard yet ductile compounds at 60 Shore D. (Customer options can be further broadened by customising the surface aesthetics, haptics, clarity and colour.) Medalist compounds are produced in facilities certified under ISO-13485, an international standard for quality management in medical manufacturing.
The second challenge, anticipating practical, real-world requirements, means that new formulations must be tested not only for essential physical properties and pre-compliance with regulations, but also for characteristics that are critical for sterilization, chemical resistance, fabrication, assembly, and everyday handling by healthcare workers in clinical settings. To develop truly practical TPE alternatives to PVC in tubing, for example, Teknor Apex worked with medical industry experts, processors, and equipment suppliers to ensure that downstream processes like hole cutting, tipping, and printing would not be an issue. Medalist tubing compounds meet such functional requirements as gamma stability, autoclavability, bondability to connectors, clarity, flexibility, kink resistance, clamp resilience, and resistance to “necking” when subjected to longitudinal force.
It was determined that clinical workers would not find acceptable any TPE tubing that did not duplicate the look and feel, or haptics, of the PVC tubing to which they were long accustomed. For a haptics test, Teknor Apex sought the opinion of experienced individuals in the medical industry, who participated in blind tests with one PVC tube mixed with several produced from the new TPE compounds. They couldn’t tell the difference.
Probably no other major market for plastics is as demanding as that of medical devices. One of the many challenges in manufacturing medical compounds is to develop formulation and processing technologies in order to meet or even anticipate regulations, market trends, and end use practices. Successful compounders are those that have invested in the capabilities needed to meet this challenge.