Vinyl Usage in Medical Plastics: New Technologies

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published July 1998

Vinyls were first employed to fabricate disposable medical devices for use in the field during World War II. As the need for flexible, biocompatible plastics for devices evolved over the ensuing half century, vinyl became by far the most commonly used polymer in the medical plastics industry. By 1995, it was estimated that vinyls represented 37% (800 million pounds) of all medical plastics used in the United States, with worldwide percentages believed to be even higher.

As one of the oldest commodity polymers, polyvinyl chloride (PVC) continues to confound numerous "experts" who have regularly predicted its demise as a medical material. This enduring polymer has borne more than its share of well-meaning but misplaced indictments on supposed health, safety, or environmental grounds, even as competing polymers—particularly metallocene polyolefins—are said to be poised to depose flexible vinyl in medical applications. Nevertheless, use of PVC continues to grow at substantially higher rates than the gross national product. In addition to improvements in resin quality, consistency, and economics, this steady growth has been spurred by important new technology: for example, new PVC resins, enhanced compounding techniques, or novel alloys of PVC and other polymers. The result is that—rather than vinyl being superseded by other polymers—advanced vinyl compositions are in many instances replacing higher-priced plastics such as TPEs, polyurethanes, and silicones.

Materials used in a resuscitation kit include clear, semirigid PVC (mask lens) and soft UHMW PVC (mask cushion and bag). Photos: Teknor Apex Co.

This article will review recent vinyl technologies that are currently available to the medical device design engineer and look at the near-term future to discern additional advances that are on the horizon.

The precursor to PVC is vinyl chloride monomer (VCM), which is made by oxychlorination from ethylene and chlorine (see Figure 1). Typically, PVC is made by polymerizing VCM in aqueous suspension, separating the water, and drying. The polymer precipitates in the form of a white powder, and has the generalized structure shown in Figure 2. There are many varieties of PVC, but the degree of polymerization (number of repeating monomer units) typically varies between 500 and 1500, with number-average molecular weights (Mn) from 30,000 to 75,000.

Figure 1. Structure of vinyl chloride monomer (VCM).

Figure 2. Generalized structure of polyvinyl chloride (PVC).

As a neat polymer, PVC is rigid, brittle, and thermally unstable. For medical use, the polymer is plasticized with high-boiling-point phthalate esters, and stabilized with the metal soaps of zinc and calcium. Other ingredients—colorants, lubricants, etc.—are added as required.

Vinyls have achieved their prominent role in the medical plastics industry by virtue of a unique combination of desirable properties. These include clarity; ease of processing; a range in flexibility from rigid to highly rubbery; the ability to be sterilized via EtO, gamma, or autoclave; and the ability to be sealed by all conventional techniques (RF, heat, or solvent welding). To these beneficial physical characteristics must be added a long history of safe medical use and attractive economics, with wide global availability.

Ultra-High-Molecular-Weight (UHMW) Polymers. As indicated previously, typical PVC resins have a range of number-average molecular weight between 30,000 and 75,000. In recent years, polymer manufacturers (e.g., Geon Co., Oxychem, Shin Etsu Chemical) have made available to the industry UHMW resin with Mn as high as 150,000 (K value > 100).^1,2 Compared with traditional polymers, these materials are more linear and have a higher degree of crystallinity. They are most useful for applications requiring very soft, rubbery components.

Unlike conventional vinyls, compounds formulated with UHMW resins—even those with very low durometers (35 Shore A and less)—offer dry, satin-like matte surfaces and have compression-set characteristics similar to many TPEs. Such products are well suited for use in peristaltic pump tubing and various types of drainage tubing that must be pinched off and then have the ability to recover. Compositions based on these resins have successfully replaced silicone rubber and TPE in a number of applications.

PVCs Cross-Linked During Polymerization. Resins produced by this technique contain a small fraction of microscopically sized particles that, by virtue of their being cross-linked, are no longer thermoplastic and are insoluble in plasticizers. The tiny particles form microscopic irregularities on the surface of the article into which the polymer is fabricated. This results in the creation of a matte surface that will not be destroyed by elevated processing temperatures.

Vinyl compounds with these self-frosting characteristics are finding increased use in fluid-transfer tubing applications. It has been reported that the low-gloss, matte finish created on the inner diameter of the tubing allows the fluid to pass with reduced drag along the tube wall as compared with smooth, high-gloss PVC tubing. This flow characteristic is a definite advantage for blood-handling applications, where a high degree of friction can cause cell separation and hemolysis.

Alloys of PVC with Other Polymers. Alloys, in this context, are defined as mixtures of polymers that may or may not be initially compatible with each other but that when combined produce a product that offers properties superior to those of either individual polymer. PVC, as a slightly polar material, has wide compatibility with many other polymers. In addition, researchers have developed materials known as compatibilizers that allow some of the nonpolar polymers, normally not miscible with PVC—for example, polyethylene, polypropylene, or butyl rubber—to form useful alloys.

The creation of alloys enables flexible PVCs to be formulated without liquid extractable plasticizers, with enhancements in gas-transition properties, or with alterations in high- and low-temperature characteristics.³ Some of the more interesting current alloys include PVC/nylon, for enhanced physical and high-temperature properties; PVC/urethane, for high abrasion resistance, reduced extractables, and high moisture-vapor transmission; PVC/chlorosulfonated polyethylene, which is soft, cross-linkable PVC with rubber-like properties; and PVC/polyolefin, especially suitable for applications requiring soft, oxygen-barrier materials.

Dynamically Vulcanized Vinyl Compositions. Dynamic vulcanization is a technology associated primarily with thermoplastic rubbers, which are partly cross-linked (vulcanized) during their manufacture. The resultant compositions exhibit thermoplastic behavior during processing as well as many properties of typical vulcanizates. The same concept has been applied to vinyls, wherein vulcanizable polymers such as nitrile rubbers or SBS/SBES block polymers are mixed with the PVC and dynamically cross-linked during manufacture using only a portion—less than 50%—of the available reactive sites. The end products have shown to be useful alternatives to TPEs, TPUs, and silicone rubbers.

All of the technologies described above have resulted in products that are available in today's market.

A number of promising new vinyl technologies, though not yet available in the marketplace, are currently in the development stage. Among the more promising are formulations that reduce the thermoplasticity of flexible vinyls, structurally ordered vinyls, and functionalized PVCs for specific purposes.

Reducing the Thermoplasticity of Flexible Vinyls. For flexible vinyls, a reduction in thermoplasticity allows the compositions to be subjected to higher temperatures than are conventional vinyls. Testing of these formulations at elevated temperatures shows that they are able to retain their shape and avoid the loss of any significant physical properties.

Moisture Curing. There are specific silane monomers that may be copolymerizable with or graftable to vinyl chloride or PVC and that when mixed with a tin catalyst in an extruder will cross-link upon exposure to moisture. If such compositions are molded or extruded into devices or tubing and then subjected to controlled high humidity for short periods of time, they will cross-link, retain most of their flexibility, and yet demonstrate high-temperature and chemical resistance far superior to most thermoplastics. The required curing process is simple, economical, and consistent.

In Situ Polymerization. Although the process is not new, compounding PVC with reactive monomers can be used to create very hard, nonthermoplastic compositions. If mono- or multifunctional oligomers are copolymerized or compounded with PVC and then exposed to free radicals either through chemical decomposition or by ionizing radiation, one can achieve three-dimensional polymers that remain flexible and have a broad potential spectrum of properties dependent on the chemistry of the oligomers.

This type of technology has enormous possibilities. For example, flexible polymers with no liquid extractables could be fabricated, as could flexible grades offering high-temperature resistance and excellent compression-set properties.

Structurally Ordered Vinyls. At the present time, all commercially available PVC is produced by traditional free-radical polymerization, which typically results in polymers that are highly branched, low in crystallinity, and inclusive of numerous structural defects. (The UHMW PVCs discussed earlier—though also produced by free-radical polymerization—are more linear and slightly crystalline only because they are manufactured at low temperature, which allows for more orderly structures.)

The objective of potential new technologies would be to mimic the successes of metallocene catalytic chemistry in polyolefins, and thus develop the ability to precisely control polymer structure. For example, PVC composed of blocks of syndiotactic, atactic, or isotactic resin could be designed to offer polymers with varying levels of crystallinity. Another result could be materials with tailored glass-transition temperatures and gas-transmission characteristics for medical packaging applications. Totally syndiotactic, highly crystalline PVC could be produced for use as chemical-resistant fibers, high temperature—tolerant filter media, or heat-shrinkable fabrics.

Clarity, ease of processing, and a range of durometers make PVC well suited for extruded products such as endotracheal tubes.

Another possibility is the fabrication of highly ordered copolymers or block polymers with comonomers (for example, olefins) that currently cannot be copolymerized with vinyl chloride. Among the many interesting developments in this line could be internally plasticized, flexible PVC with no liquid extractables and with properties that combine those of polyolefins and vinyls. Finally, the ability to produce vinyl polymers without common structural defects such as those induced by tertiary chlorines or initiator fragments could result in products with improved resistance to heat degradation and ionizing radiation.

These technologies represent only a few of the possibilities that could be realized when non-free-radical-polymerized PVC becomes commercially available. Although the science is still in the developmental stage, significant financial resources have been committed to achieving this goal, and there is considerable activity in both commercial and academic laboratories.

Functionalized PVCs for Specific Purposes. Advances in vinyl-polymer chemistry will soon bring about a new generation of PVCs formulated to function in specific medical applications. For instance, certain nonthrombogenic plastics are currently made using heparin coatings, which can be expensive and have a limited shelf life. Medical PVCs will soon be compounded to contain various additives with long-term nonthrombogenic properties, or will be copolymerized with specific monomers to enhance hemocompatibility. Also under study are oxygen-scavenging vinyls, which will be synthesized using additives or comonomers that react with oxygen and will be employed to store drugs or solutions that are oxygen sensitive.

These and other types of specifically functionalized PVCs are definitely on the horizon, and the technology to produce many of them already exists.

Polyvinyl chloride can be easily formulated to produce a broad variety of physical properties that make it suitable for myriad end uses in the medical market. Developments of particular interest include the possibility of extremely soft, rubbery compounds based on UHMW resins and the use of reactive cross-linking to achieve a consistent, controlled surface finish. Combining PVC with other polymers to create alloys that offer the best attributes of each component has resulted in new, low-cost medical plastics. Similarly, dynamically vulcanized vinyl polymer compositions have further augmented material choices available to device designers.

Looking forward, we may expect to see structurally ordered PVC homo- and copolymers that will maintain the desirable characteristics of conventional vinyls while improving physical properties to levels not attainable with current products. Moisture-cured vinyls with reduced thermoplasticity for elevated-temperature applications; flexible PVCs, formed via in situ polymerization that contain no liquid extractables; oxygen-scavenging PVC-based compounds; and nonthrombogenic vinyl formulations are all promising technologies that should come to fruition in the near future.

As always, safety, cost, and performance will dictate which materials become the polymers of choice for tomorrow's medical device designers. It seems certain that PVC—one of the oldest, most widely used, and most versatile medical plastics—will continue to offer fresh and exciting alternatives.

1. Brookman RS, "PVC Thermoplastic Elastomers," in Society of Plastics Engineers, Inc., Technical Papers, vol XXXIII (ANTEC 1987), Brookfield, CT, SPE, 1987.

2. Brookman RS, Schmeyea D, and Mazeg P, "Compounds Based on Ultrahigh Molecular Weight Resins," J Vinyl Tech, 15(1), 1993.

3. Wickson EJ (ed), Handbook of PVC Formulating, New York City, Wiley, 1993.

Robert S. Brookman (PhD), is currently PVC business manager at Teknor Apex Co., Plastics Div. (Pawtucket, RI). Trained in polymer chemistry, he has more than 35 years' experience in PVC technology and applications, including previous positions with Firestone Plastics Co., Pantasote, Inc., and Colorite Polymers. A fellow of the Society of Plastics Engineers, he frequently serves as technical spokesperson for the Vinyl Institute.

Copyright ©1998 Medical Plastics and Biomaterials