Polymer Basics for Polyvinyl Chloride
Polyvinyl chloride is the third most produced plastic worldwide, just behind polyethylene and polypropylene, and it is the most widely used plastic in medical applications. Also known as PVC or vinyl, the thermoplastic synthetic polymer allows manufacturers to tailor it to yield specific properties for each application.
Characteristics and Properties of PVC
In its pure form, the versatile solid is white, rigid, and brittle. It's available in both powder and granule form. It is low-cost, easy to process, and lightweight. Its dielectric strength makes it a good insulation material, and its high chlorine content makes it flame-retardant and self-extinguishing, making it ideal for medical applications.
Throughout its long lifecycle, PVC is relatively low maintenance. Its durability makes it resistant to weathering, corrosion, shock, and abrasion. PVC is also resistant to inorganic chemicals as well as chemical rot, diluted acids, and alkalis.
PVC is formed from vinyl chloride (H2C = CHCl). Polyvinyl Chloride is produced in three steps using the following raw materials: salt, oil, or natural gas in a ratio of 57 percent chlorine to 43 percent carbon. Chlorine is then combined with ethylene to produce ethylene dichloride (C2H4 + Cl2 = C2H4Cl2.)
The second step in the PVC process takes the ethylene dichloride and heats it to a very high temperature in a furnace or reactor to form a monomer (VCM) after oxychlorination. The final second step reaction: Ethylene + Chlorine + oxygen = VCM + water or 2C2H4 + Cl2 + ½ O2 = 2C2H3Cl + H2O.
The third step involves additive polymerization. This opens the double bonds to allow for neighboring molecules to form long-chain molecules. The final reaction (vinyl chloride monomer = polyvinyl chloride) looks like this: nC2H3Cl = (C2H3Cl)n. Various additives will create variants to the initial 57:43 chlorine-carbon ratio for the needed application.
The three main manufacturing processes are suspension, bulk, or emulsion. Suspension polymerization accounts for 80 percent of PVC manufacturing worldwide. Although PVC begins as a rigid material, it is plasticized (flexible PVC) for medical applications. PVC can also be extruded, thermoformed, or blow-molded.
Polyvinyl chloride is also recyclable when not identified as hazardous waste.
Common Uses in Medical Devices and Life Sciences
Polyvinyl chloride has been used for over half a century within the healthcare field's demanding applications. It's both chemically stable and highly biocompatible. Both single-use and reusable products are precisely manufactured and engineered for medical-grade use. PVC is used not only in surgery but also in pharmaceuticals, drug delivery, and medical packaging.
Other medical applications using flexible PVC include oxygen masks, catheters, ostomy bags, surgical and exam gloves. Oxygen tents, blood bags, IV tubing, drips, and dialysis components are also medical products that depend upon PVC's clarity, kink and scratch resistance, bonding ease, and affordability.
Polyvinyl chloride can also be used for containers and protective sheeting, in blister packaging, as artificial skin in emergency burn units, and as blood vessels for artificial kidneys.
Sterilization destroys or removes living organisms or contaminants on medical products without performance loss. The most common PVC sterilization method is ethylene-oxide (EtO). This method is recommended because it doesn't involve the use of heat, affecting the stability of the PVC.
Autoclave sterilization can be utilized in some instances, applying steam at the lowest possible end of the temperature range. Still, it is not recommended for rigid PVC as it can cause warping and deformities.
As mentioned above, polyvinyl chloride has poor heat stability, so the chosen use and sterilization method must take that into account. PVC properties can also change over time due to plasticizer migration, and hard materials connected to flexible PVC can be damaged during the migration. Other limitations include flexible PVC's lower chemical resistance and rigid PVC's low continuous service temperature of 50 degrees Celsius.