PVC has long been the material of choice for tubing. Increasingly, however, issues such as those focusing on the phthalate plasticizers used to make PVC flexible are causing some tubing producers or their device manufacturer customers to look for alternative materials. Until now, the proposed replacements have failed to duplicate the advantages that made PVC the preferred material for so many tubing applications over a period of decades.
Tests and full-scale production trials conducted by Teknor Apex Company (Pawtucket, RI) over a three-year period have established the Medalist® MD-500 range of medical-grade thermoplastic elastomers (TPEs) as the first practical alternative to PVC in tubing. The compounds are now in commercial use by medical device manufacturers. This article will present information on how Medalist MD-500 elastomers compare with PVC in every phase of manufacture and end use typical for tubing, from extrusion through everyday handling by healthcare workers.
To demonstrate the viability of these compounds as alternatives to PVC, researchers in the TPE Division of Teknor Apex worked with medical application experts in the Vinyl Division, established tubing manufacturers, and medical device OEMs. The researchers took advantage of the unusual versatility of Teknor Apex as a supplier to medical manufacturers. The company produces materials representing the entire technology spectrum for medical compounds, including DEHP, non- DEHP, and non-phthalate plasticizer, standard PVC compounds, new BioVinyl™ PVC compounds with bio-based plasticizer, and TPEs. Drawing on this broad involvement, the developers of Medalist MD-500 compounds placed great emphasis on meeting “real-world” requirements that are not typically considered in a product data sheet.
Basics: Flexibility, Clarity, Sterilization Resistance
Flexibility is one of three functional properties that are fundamental in medical tubing. In PVC compounds, this property is the result of high levels of plasticizer — as much as one-third of the formulation by weight. The inherent elasticity of TPEs, on the other hand, renders the use of plasticizer unnecessary.
The degree of flexibility varies with the level of hardness as measured on the Shore scale. Because compounds with Shore A hardness levels of 75 to 85 are most common in tubing, Teknor Apex has established a standard portfolio of MD-565, MD-575, and MD-585 grades covering this range. In addition, the company supplies custom formulations of MD-500 compounds that span the range from 45 to 90 Shore A (or wider as needed). Even at the highest end of the hardness spectrum, the Medalist compounds exhibit greater values of elongation at break than comparable PVC materials, indicating their greater inherent flexibility. Medalist MD-500 TPEs are also more flexible than other elastomeric materials that have been proposed as alternatives to PVC.
Another basic functional property is clarity, which is essential for medical staff to assess flow, bubble appearance, and the condition of a fluid as evidenced by its color. Medalist MD-500 compounds provide the same crystal clarity as PVC and in this respect also surpass other alternative materials.
The capacity to withstand even the most aggressive methods of sterilization is a third basic requirement for tubing. In tests for color shift following heat aging of gamma irradiated samples, Medalist compounds proved markedly superior to PVC. Gamma exposure causes degradation of the PVC polymer, resulting in discoloration, embrittlement, and a falloff in mechanical properties. Compounders like Teknor Apex have developed gammastable grades of PVC that exhibit reduced levels of such deterioration, but the Medalist compounds surpass even these in gamma resistance, with heat-aged color shift values less than a third of those of the PVC (Table 1).
Medalist MD-500 compounds provide these basic functional properties while exhibiting values comparable to PVC for mechanical properties critical in tubing, such as tensile strength and tensile stress. Their specific gravity is 25 percent lower, indicating that a pound of Medalist compound yields more linear feet of tubing than a pound of PVC.
Everyday Clinical Considerations
Thus far, the greatest failings of PVC alternatives have been specific to tubing as employed daily by healthcare workers. Teknor Apex researchers screened candidate Medalist compounds in terms of their success in delivering such properties in comparison with PVC.
Kink resistance: In many applications, tubing is used for infusion of fluids into the body. If the tube is bent through a radius that is too tight, it collapses, resulting in potentially lifethreatening consequences. This kinking occurs when the compressive forces on the tubing surface inside of the bend exceed the tensile forces on the external bend surface. Changes in wall thickness and hardness or modulus will impact the kink resistance.