PVC is an unusual plastic. Compared to the other majors, it's the only one that is not primarily fossil-fuel-based (it's 56% chlorine) and, therefore, is flame-resistant; has good chemical resistance; gives the most rigidity per unit cost; can also be flexible or rigid, as desired; and takes less energy per pound to make and process. And, despite its foes, PVC is one of the most extruded plastics, with products ranging from electrical conduits and building wire to garden hoses and gummy-worm fish baits.
What's the difference between vinyl and PVC? In common usage, none. PVC stands for poly vinyl chloride, and, thus, is one of the vinyl polymers. This is a large family, which even includes polyethylene and polystyrene, but only a few of the materials retain "vinyl" in their names; of these, PVC is by far the most important. As the acronym PVC has received so much negative publicity, some users prefer the term vinyl, but it usually refers to good old PVC. Other vinyls include PVOH (polyvinyl alcohol), PVDC (polyvinylidene chloride) and PVAc (polyvinyl acetate).
All plastics contain additives, even PE, PP and PET, which are usually formulated by their producers. And it's the additives that cause most of the anxiety in the anti-PVC camp, where they may be (incorrectly) considered as part of the PVC itself.
PVC comes in two types: Soft-floppy and hard-rigid. The PVC resin itself is hard and rigid, and its hard-rigid applications have stabilizers to prevent molecule breakdown, lubricants to make extrusion easier (lower viscosity) and other additives such as colorants, as needed. The soft-floppies, on the other hand, have these additives but also need 20 to 50% plasticizers to get the floppiness. There's very little in between: The properties of a lightly plasticized PVC fit very few big applications. Plasticizers are typically high-boiling liquids and may be plant-based, but most of them are manufactured compounds, especially phthalates (esters of phthalic acid). The Greek-based spelling with phth makes them sound "chemical" and contributes to the popular aversion to PVC. Never mind that the rigids don't contain any plasticizer at all, nor that there are many FDA-OK non-phthalate alternatives, nor that the harm attributed to phthalates is still questionable and, in any case, must depend on how much gets into whom and how. The public image is still that PVC, and by extension, all plastic, is bad for you.
There is an important distinction between orthophthalates and terephthalates that matters in any study of toxicity or legal activity, and is well known to chemists, serious environmentalists and even lawyers in this area. All phthalates have two usually identical groups attached to the phthalic acid ring, but the orthos have their two groups next to each other, where they may interact and cause the behavior that has raised concern. The teres have their groups opposite each other, which makes them far less likely to interact. The most important use of terephthalates is in PET polyester (that's what the T stands for), our #1 in the recycle code, used for food and beverages for more than 40 years, and unlikely to receive significant opposition. So much for the phth. Out of sight, out of mind, out of danger.
All extruders of flexible product have to deal with this mess. Do they pay more or sacrifice properties to be able to say "phthalate-free?" That will depend on the market, of course; makers of dam seals and sewer pipe gaskets are less likely to worry than makers of tubing used in food processing plants. The medical area has been the main arena of conflict so far, where users of tubing and blood bags are under pressure to "get rid of the vinyl."
In the rigid world, the stabilizers are the main worry, and there are some bad ones (e.g., those with lead) but it's easy to find stabilizers that are FDA-OK for food contact. The PVC-phobes will say that vinyl siding and pipe somehow contaminate the air or water, no matter what additives are used, and if you can pin them down, they will usually point to chlorine as the villain. No matter that the chlorine is tied up as chloride (like salt = sodium chloride), nor that the resin is very stable in service and not likely to leach chlorine anywhere. As for chlorine-containing dioxin, its evil is well known, but conditions to transform PVC to dioxin are not seen in its manufacture or fabrication, and certainly not in service. The ‘phobes have even brought up the BPA issue in attacking PVC, which is meaningless, as BPA isn't involved in PVC at all, nor does it contain any chlorine.
By the way, if you ever get attacked by someone who says chlorine is bad, ask them what makes plants green, and if they know it's chlorophyll, stop there. They may not know that there is no chlorine in chlorophyll.
A word on additives: They usually cost more than the PVC resin, so we want to use as little as possible, which means good mixing to get uniform compound, and low processing temperatures to reduce the need for stabilizers. This is the real justification for the higher cost of twin-screw extruders, which are common but not necessary to run PVC. If they can mix well at lower temperatures, they use less stabilizer and the savings can pay for the machinery cost differential.
In conclusion, if you're already using PVC or considering its use, maybe this article can help make you less afraid and more able to defend your decision. Know the properties of what you're using, know the ingredients and their alternatives, know how to test for stability and viscosity and whatever else is important in your use, and work safely, make useful product and keep reading PlasticsToday!
Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or email him at [email protected].