Fluoroelastomers for Extrusion of High Quality Tubing and Thin Walled Fuel Line Hose Veneers*
DuPont Performance Elastomers has recently developed innovative polymerisation technology to produce fluoroelastomers designed for extruded automotive components such as thin walled fuel line hose veneer. These Viton® fluoroelastomers offer excellent surface quality with barrier properties sufficient to help automotive fuel system OEMs meet stringent evaporative emissions regulations.
The ultimate function of any fuel system is to manage the delivery of precisely metered quantities of fuel from the tank to the engine combustion chamber. Critical to achieving these objectives are the integrity and durability of the elastomeric sealing devices like O-rings, seals, tubing and hose that connect the fuel system components (see Fig. 1).
Figure 1 - Passenger Vehicle Fuel System
Fluoroelastomers (FKM) are frequently specified for their excellent physical properties and compression set resistance coupled with a high degree of fuel, additive and heat resistance. They provide a highly effective barrier to fuel permeation.
Fuel hose is differentiated from tubing in that it contains a reinforcement layer to provide functionality at higher pressures. Fuel hoses tend to be multilayer constructions where the materials for each layer are selected according to the specification requirements. The layers generally combine fluid and heat resistance properties and contribute to low temperature bending and burst pressure resistance in the finished article.
Cost effectiveness is a very important consideration of designers. Multilayer fuel line hose often comprises four layers including an inner layer of FKM for fuel containment, typically less than 1mm thick. The barrier properties of an FKM lined multi-layer hose are defined largely by the type of FKM selected for the inner layer.
Figure 2 shows a five-layered F-200 hose construction used to achieve very low permeation of =<7 g/m2/day. It takes advantage of the barrier properties of FEP but still retains the FKM inner veneer to provide sealing at the hose coupling points. F-200 low permeation barrier hose is proprietary technology.
Fuel hoses are manufactured by extrusion. Thin veneers of 0.2 to 0.3 mm provide a very cost effective hose construction having a sufficient barrier to fuel permeation. The initial extrudate must have a good surface finish and sufficient strength to be drawn down (stretched) to its target wall thickness without breaking. These are the characteristics of the specially designed extrusion polymer from DuPont Performance Elastomers, VTR-7551.
VTR-7551 and its two precompounds, VTR-9209 and VTR-9217, are now commercially available from DuPont Performance Elastomers. VTR-7551, designed specifically for extrusion of thin walled tubing and hose veneers, provides excellent extrudate surface quality and can be drawn down to wall thicknesses below 0.1 mm, without surface defects.
Compounds based on these products can be formulated to give physical properties suitable for automotive fuel line or other applications. The base polymer, VTR-7551, provides a unique balance of excellent extrusion performance with fuel permeation significantly lower than usual. VTR-7551, or VTR-9209, provides a roughly 25% reduction in the permeation of CE-10 fuel at 40°C when compared to a reference B type compound based on Viton® B-202 and B-600.
*Excerpt from a full technical paper, entitled “New Fluoroelastomers Having a Molecular Structure Specially Designed for Extrusion of High Quality Tubing and Thin Walled Fuel Line Hose Veneers”, by Stephen Bowers, Edward Ferber, Ronald Stevens, Christopher Grant of DuPont Performance Elastomers, given at the High Perfomance & Speciality Elastomers 2005 conference (RAPRA), Geneva, Switzerland, April 20-21, 2005.