Polytetrafluoroethylene Overview: History, Producing, Application And Environmental Issues
Structure
Polytetrafluoroethylene (PTFE) is a vinyl polymer made of a carbon chain with two fluorine atoms attached to each carbon. A polymer is an extremely large polymer that is made of series of repeated subunits. Fluorine is an element with an atomic number of 9 and thus it is readily looking for an electron but it also has a very high electronegativity (the highest of all the elements) this makes it very reactive. The carbon fluorine creates a polar covalent bond which is extremely strong. The fluorine’s ‘hunger’ for another electron has now been satisfied by sharing one with the carbon, thus now making all the bonds satisfied and creates a very inert polymer.
When fluorine is in a molecule it likes to be by itself and not next to other molecules, even molecules with fluorine in them and this causes it to repel nearly any other atoms and molecules and nothing sticks to the polymer. It is also hydrophobic, which means that it repels water. It has the lowest coefficient of friction of any known solid (0. 05 - 0. 1) and is extremely stick resistance. It is a white solid at room temperature and is a thermoplastic polymer, meaning it becomes mouldable above a specific temperature and solidifies upon cooling. Specifically it has a melting point of 327 °C. Even at very low temperatures (up to −268. 15 °C) it maintains good strength, high toughness and its anti-stick properties. PTFL also is flexible down to a temperature of −79 °C. PTFL is also has extremely good dielectric properties. Meaning it inhibits electrical charge from flowing through it, thus making it very good at being an insulator. Also due to it’s not reactivity and it’s resistance to aggressive chemicals, it is extremely wear resistant.
A brief history on PTFE
PTFE was accidentally discovered by Roy Plunkett, an American chemist, in 1938. He was attempting to make a new chlorofluorocarbon refrigerant when he noticed that a pressurised gas bottle of tetrafluoroethylene gas stopped flowing before the weight of the bottle singled it to be empty. This led him to cut the bottle open to discover a white powder coating the inside. By 1948 DuPont, the company Plunkett was working for, had started mass producing PTFL. Producing over 900 tons per year. It is now more commonly known as its trademarks such as Teflon, Fluon and Hostaflon.
Processing
There are two main ways of producing PTFL. The one method is called suspension polymerization and this method water is used as the bas in which TFE is polymerized, this method produces grains of PTFE which can be later processed into pellets. The other method is the dispersion method and this method produces a white paste that can also be further processed in to a powder and into pellets.
Both methods start with the same first step, which is the production of TFE, the monomer tetrafluoroethylene, tetrafluoroethylene (C₂F₄) is family of fluorocarbons and consist of four fluorine atoms bonded to two carbon atoms, and the carbon atoms have a double bond and are single bonded each to two fluorine atoms. TFE is an odourless, colourless and nontoxic gas. It is made by heating three ingredients (fluorspar, hydrofluoric acid and chloroform) to a temperature between 590 and 900 degrees Celsius. They are heated in a reaction chamber where they all combine and the resultant gas is cooled down and then distilled several times to remove any unwanted gases to be left with pure TFE. The gas is then pressurized into liquid form. Due to TFE being highly flammable and extremely dangerous to transport, it is normally manufactured by the PTFL manufacturers.
The Suspension Polymerization
A reaction chamber is prepped by filling it with purified water and an initiator agent that is a chemical that helps kick-start the polymer forming. The TFE from the previous process is pumped into the reaction chamber where it begins to polymerize by free radical polarization where the TFE monomers are easily split and when they split two electrons become free and are the free radicals, these electrons quickly try find something to bond to and thus bonding to another molecule that splits and has free radicals propagating to form long polymers with now single bonded carbon. These propagations carry on until the free radical on the end of the chain either couples to another chain or bonds to another fluorine atom. The resultant PTFE floats to the top of the reaction chamber as small solid white grains. During this whole process the reaction chamber is shaken by a mechanical mechanism and cold purified is circulated through the chamber to keep the temperature low and counteract the heat the chemical reactions give off. When enough PTFE has been made the flow of TFE is shut off and the purified water is drained leaving all the white flakes of PTFE behind. The PTFE is dried in an oven and then sent through w mill which pulverized it into a fine white powder. It is now either left as this fine powder or is made into small PTFE pellets.
Dispersion Polymerization
The dispersion method is very similar to the polymerization. It also starts with purified water and an initiator agent pumped into a reaction chamber. The TFE liquid is also pumped into the chamber and the polarization process accurse exactly the same was as in the polymerization. But now instead of the chamber being shaken mechanically, it is gently agitated and this causes tiny beads of PTFE to form and fall to the bottom of the chamber. During the process water is filtered out and the end result is a milky white paste called PTFE dispersion. This paste is extremely useful when application is needed to layer a material or coat something in PTFE. The PTFE dispersion can also be dried and milled into a fine powder.
Applications (what are the typical applications and products made from this material)PTFE can be very expensive and difficult to use as it has a high melting point and is a very viscous liquid when melted so it doesn’t flow nicely. Some PTFE parts are made by cold moulding in which powdered PTFE is placed in a mould under high pressure and then exposed to heat, fusing all the particles into one. It can also be sprayed or rolled onto objects.
In the early years just after PTFE was accidentally discovered, it was used exclusively in the coating of the insides of pipes and valves, of radioactive material in the Manhattan project, to help prevent against corrosion.
Today it is still widely used in pipes and machine parts, specifically in the chemical and steel industry. The PTFL is lined inside the pipes reducing the deterioration. Due to PTFE’s inherently high coefficient of friction in is used in application of bearings, seals, gaskets, bushings and other applications with sliding parts.
Then in 1954 a wife of a French engineer asked her to try out the material, he had been applying to his fishing tackle, on one of her pans. Subsequently the non-stick pan was created. Nowadays this is one of the main uses of PTFE. Due to it having a very high melting point and its non-stick properties. But the question is, how do you get it to stick the pan when it that repels everything and nothing sticks to it?
Well this is done by taking an aluminium pan and specially preparing the surface to accept the coating of PTFE. Firstly the pan is washed to remove any foreign dirt. The pan then takes a bath in hydrochloric acid to etch the surface. This basically roughens the surface of the pan. The pan is then rinsed and etched again. Now the pan is primed with a special primer which is a secret kept by the manufacturers. This primer helps the PTFE stick. The PTFE is now either rolled or sprayed on and placed into a convection oven to dry. A few more coats of PTFE are applied and now the pan is ready for use.
PTFE is used very often in building, for a wide range of things. For one it is used to coat walls to prevent insects from being able to climb the walls (not even geckos can walk on Teflon). It is also widely used in medicine. It is coated to surgical tools to help with hygiene as because PTFE is extremely non-stick, not even bacteria can get onto it. It is also used in catheters for this reason. In the dentistry industry it is used for teeth fillings to prevent adjacent teeth from sticking to the filling.
A major application of PTFE is as an insulator, it is used frequently in wiring for aerospace and computers. This is due to its good dielectric properties. The smooth surface of nail polish is sometimes also achieved with the addition of the polymer. This also helps to prevent the nail polish from cracking. Windshield wiper blades are coated so as to glide smoothly across the windshield and to help prevent weathering of the blades.
Another big use of PTFE is of the coating of carpets and fabrics, mainly for protection but also because of the PTFLs ability to prevent bacteria build up. The PTFE coating also helps prevent stains and makes for easy cleaning of the coated material.
Environmental impact and sustainability
Teflon pans are said to be extremely healthy as now you don’t need oil or butter to lubricate your pans. But is it really healthier? Some studies have shown that Teflon and PTFE is somewhat toxic. Once a pan gets a scratch the Teflon ends up getting into your food and then into your body.
Birds are very susceptible to the toxicity of PTFE. It is known as Teflon toxiosis, exposed lungs begin to haemorrhage and fill up with fluid, drowning the bird from the inside. But is Teflon actually toxic to humans? Well Teflon toxicity causes polymer fume fever, which is similar to the flu. The illness comes from fumes that come from overheated PTFE coated pans. Only very few cases of people going to hospital for Teflon toxicity have been recorded.
The biggest worry in the talk of toxicity and PTFE is the presence of PFOA which is used in the production of PTFE. PFOA also does not break down and contaminates everything too. Scientists who conducted a report on PFOA found it present in the blood of 96% of all the hundreds of children tested. Fluorinated polymers such as PTFE release TFA in large amounts at high temperatures. The TFA enters the atmosphere and makes its way back down to earth in rainwater.
PTFE is very widely used and is produced in thousands of tons per year, so what is happening to all of the material once it is dumped and ends up not being used anymore. PTFE does not breakdown and this means it will continue to build up in the environment, causing bioaccumulation in food chains. It contaminates everything and in hundreds of years will still not have broken down. PTFE can be recycled by is too specialized and too expensive to be recycled, meaning it will all eventually end up in dumps and landfills. When PTFE is burnt it releases very toxic gases into the atmosphere, so the dumps are the worst possible places for the PTFE to end up.
