Decoded Science received the following question from a reader:
“Credit cards are made from layers of PVC (polyvinyl chloride). Are credit cards flammable? If so, do they easily catch fire?”
To answer this question, we’ll describe the structure and the properties of PVC and discuss the features which may affect the flammability of a material.
Polyvinyl Chloride Structure
The majority of credit cards used today are made of polyvinyl chloride (PVC), a plastic polymer.
Polymers are large molecules made by the repetition of one or more small basic unit(s); this unit is called a monomer.
Monomers are bonded together to form long chains; the process to form a polymer from its monomer(s) is called polymerization.
In the case of PVC, the monomer is vinyl chloride (see picture (a) on the side), a small molecule with two carbon atoms linked by a double bond and a chlorine atom bonded to one of the carbons.
With vinyl chloride polymerization, the double bond between the carbon is broken; each carbon atom will form a bond with a carbon atom belonging to a different monomer. In this way, a chain will be formed; the repeating unit is shown in the picture (b) on the side.
The properties of PVC can be affected by many parameters. One important element, for instance, is its molecular weight, which depends on the length of the polymeric chains – i.e. how many monomeric units each chain contains. Another factor can be the presence of various additives added to the PVC itself, such as stabilizers, plasticizers, etc.
Generally, PVC materials are divided into two classes: rigid and flexible PVC. Credit cards are made of rigid PVC.
Flammability and the Oxygen Index
Flammability is the property that describes the tendency of a substance to catch fire.
Several tests and classifications are available to measure and indicate the flammability of a particular material. In the case of polymers, one of the most common scales used is the oxygen index.
The principle of this method is that different compounds require different amounts of oxygen to burn; a compound that is easily flammable will burn with a smaller amount of oxygen, in comparison to a less flammable one. More flammable materials will have, therefore, a relatively low oxygen index; materials which catch fire less easily, on the other hand, will have a higher oxygen index.
Credit Cards: Oxygen Index of PVC
Rigid PVC has an oxygen index of 45; this indicates that it is less flammable than many other materials. The oxygen index for wood, for instance, is around 20-25; other polymeric species such as polyethylene or polypropylene also have lower oxygen indexes (around 20). This higher value is due to the presence of a chemical bond between carbon and chlorine; this requires more energy to be broken, and hence makes combustion more difficult.
Burning Credit Cards: Possible Dangers
Although PVC has a higher oxygen index, this does not mean it cannot catch fire.
In the case of combustion, moreover, the gases formed when burning the PVC used to make credit cards can be quite dangerous. In fact, due to the chlorine presence, hazardous molecules such as hydrochloric acid (HCl) and dioxins can be formed. HCl, being an acid, is corrosive and toxic for humans and the environment. Dioxins are a very dangerous class of chlorine-containing organic compounds, which can cause cancer. In short – if you’d like to get rid of your credit cards, you’re better off cutting them up, than burning them.
Environmental Protection Agency. Dioxins. Accessed December 2, 2012.
E.M. Pearce. Flame retardants for polymer systems. (1986). Pure and Applied Chemistry, 58(6), 925-930. Accessed December 2, 2012.
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