Thinking Outside the Tube
The evolution of plastic tubing in the water industry
Fluoropolymers have a long history in our society. Polytetrafluoroethylene (PTFE) was discovered in April 1938 at DuPont’s Jackson Laboratory in New Jersey by Roy J. Plunkett. During World War II, PTFE was used in the Manhattan Project in the development of the atomic bomb. It was also used in Oakridge, Tenn., as a gasket and packing material in a gaseous diffusion piping system handling uranium hexafluoride, a very toxic and corrosive gas. Only Teflon could do the job. Other uses of the resin were in naval and airborne radar systems as a coax cable dielectric and nose cones for a very effective weapon in World War II – the Proximity Fuse.
The applications of plastic in our society have become household commodities. Almost every household has products that contain plastic polymers, ranging from appliances and toys to computers and tools. Hardened polymers have replaced metals in many applications and are often as strong and even more durable than metal. The automobile industry uses these plastics for car parts because they make cars lighter and more fuel-efficient.
The water industry has been using these plastics to replace traditional metal piping. Plastic PVC water pipes for cold water use and CPVC piping for hot water use often replace older galvanized and copper pipes. Flexible tubing that does not leach contaminants into the water is being used for RO and other drinking water systems. With FDA, EPA and NSF approval, this tubing is almost a standard for these types of installations.
Depending on their use, most polymers have distinctive roles in the marketplace, but PTFE is considered the “all-purpose” polymer. With an extensive chemical resistance (except elemental fluorine and molten alkali metals), PTFE brings more to the table than any of the other polymers. The following characteristics contribute to PTFE’s versatility:
- Insoluble in all solvents below 300°C;
- High service temperature stability up to 250°C;
- Very low adhesiveness;
- Lowest friction coefficient of any solid;
- Extremely hydrophobic;
- Physiologically inert;
- Contaminant free;
- Good electrical (high resistance) and dielectric (very low dielectric constant and dissipation factor) properties; and
- Nonflammable, weather resistant (UV resistant) and tough elastic properties.
PTFE is used to make porous tubing. Fine powder is compacted with a lubricant, and tubing is extruded cold. Length is determined by the size of the extrusion cylinder. It is then sintered in a long oven. Stretching in the sintering process yields the porous tubing that is used to inject ozone into water and air. PTFE hollow fiber porous membrane tubing is also used for filter media. The porous tubing is being developed in membrane technology, where applications demand the inert characteristics of PTFE.
Perfluoroalkoxy-polymer (PFA) has similar characteristics to PTFE, including an operating temperature of 250°C. PFA is generally used in high purity applications where contamination cannot be compromised, such as the production of integrated circuits. PFA is more expensive, and the uses are limited to these types of applications because the benefits outweigh the cost.
Fluorinated ethylene-propylene (FEP) has similar properties to PFA, except it has a higher density, which makes it heavier. FEP is translucent and flexible. A good example of using FEP would be in coiled or straight tubing replacement of quartz sleeves in UV water treatment products or devices. FEP is cost effective in most applications, and a thin wall can be lined inside a polyethylene jacket for durability in groundwater sampling. FEP is acceptable to deliver deionized water and for sampling chemicals and air.
Polyvinylidine difluoride (PVDF) is also known as Kynar. It is very durable and resistant to chemicals. It is reserved for specialty applications where the highest purity is required, such as the pharmaceutical and electronics industries. PVDF is replacing stainless steel in many applications where rouging, a phenomenon of corrosion, is compromising the integrity of water purification. A relatively new co-polymer of PVDF makes flexible tubing possible. This newer tubing is suitable for use in ozone delivery, especially in areas where space is limited and kinks must be avoided.
As one can see, by understanding fluoropolymers, we can project where the uses of these plastics are going to be a standard in the water industry. Tubing and hard pipes are replacing metal pipes at a very rapid rate because of flexibility and ease of use. Coils of tubing can be carried on service trucks much easier than long 10-ft sections of copper or steel pipe. Flexibility allows for easy maneuvering around ductwork, floor joists and hard-to-reach areas. The tubing is also very forgiving when it comes to measurements and cutting proper lengths. It is always better to measure twice and cut once, but the ease of use with these plastics can make the installer’s life easier.
Sizes & Diameters
Tubing can come in many sizes and diameters. Quarter-inch and 3/8-in. tubing is the most common, but tubing can come in 1/2- and 5/8-in sizes. Make sure the measurement of the tubing is inside diameter or outside diameter. This makes a big difference when ordering fittings and tubing, and having them work properly. The thickness of the tubing varies depending on the type of tubing and application. Each type of tubing, whether it is PTFE, FEP, PFA or PVDF, has various tensile strengths, densities and pressure ratings. Check specifications with the manufacturer or supplier for each type of tubing, and make sure they match application requirements.
Connectors & Fittings
In addition, new slip-on connectors and fittings are making installations very simple—just cut the tubing, insert into the fitting, apply the lock ring, and you are done. RO systems are a snap to install. This cuts down on installation time and reduces costs. Now the installer can be in and out of a home in less time. Crawl spaces and hard-to-access areas can be done quicker and more efficiently as well.
Overall, plastic tubing uses vary, but the water industry has seen some newer applications evolve with more products using these fluoropolymers. Ozone delivery is one application that comes to mind in which PTFE and PVDF are being utilized in product and system design. Ozone is highly oxidative, and the tubing is very resistant to ozone. Porous PTFE tubing has been developed and used for bubbling ozone into water. The tubing has also been used to deliver ozone into air with a more directional flow pattern and streamlined delivery for areas that could not be reached in a conventional manner.
RO has used this type of tubing for years as well. Its non-leaching aspects make it very acceptable for RO water because of the aggressiveness of the water with the minerals removed.
The list of applications for fluoropolymers is virtually endless. These polymers can be developed for specific designs, and linings can be used to enhance the look, feel and durability of a particular combination of resins. Whether it is PTFE, FEP, PFA, PVDF or even a porous PTFE, the manufacturer or supplier can help with any design and compatibility issues. Water is an easy fluid to work with, and designs are pretty standard, but other fluids and gases have been tested for each of these polymers. Chemical resistance and other parameters have been documented as well. Standard applications are always available, but for truly innovative solutions, you have to think outside the tube.