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Awareness of the environment has grown rapidly over recent years - and has resulted not just in international protocols and targets for politicians to argue over, but in legislation with real bite. In many parts of the world, such as Europe, Japan and particularly California, the regulations involving hydrocarbon emissions are becoming ever more stringent.
Today over 30% of the weight of a new car comes from polymeric components, including much of the fuel containment and delivery system. Although these materials offer considerable price, strength, weight and other advantages, they are unfortunately not the almost perfect barrier to hydrocarbons that metals are. So the new regulations mean that extensive materials testing is becoming the norm.
The most important part of this relates to vapour Permeability as embodied, for example, the SHED test (Sealed House for Evaporative Determination). SHED sets a European level of 20g of hydrocarbon emissions from a whole (new) car over a 24 hour period, though this is set to reduce to 5g in the foreseeable future. In the US a part of the test also involves emissions from a car while running.
The main source of hydrocarbon emissions is the fuel circuit, which is mainly produced from polymeric components - often polyamide for the fuel pipes and polyethylene for the fuel tank. The use of fuels and fuel blends add an extra layer of complexity to automotive testing as these are often more hostile to the polymers than straight unleaded fuel. Adding Methanol (e.g. grain alcohol) to the fuel swells the polymer increasing its permeability - as can temperature and a variety of lesser factors.
Emissions from the multitude of different components, materials and joints each add to the overall result, and all of these need testing, optimizing and to be made from the most suitable materials using highly reliable mechanical designs and seals.
Testing the hydrocarbon permeability of a component has traditionally not been an easy task and only the recent advent of accurate and reliable instrumental techniques to the area has made it a practical desktop reality.
The few Instrumental VTR (vapour transmission rate) meters there are vary in design, appearance, mechanism, components and even in the type of measurement sensor used. However, in most instruments one side of a sample is exposed to a gaseous hydrocarbon environment, and the vapour that passes through the sample is swept to a sensor on the other side by an inert gas. Typically the instrument allows the sample to be fixed between two halves of a stainless steel chamber housed in an accurate thermostatically controlled oven. In the lower half of the chamber, the desired hydrocarbon vapour pressure conditions are generated, and dry gas is passed through the upper half where an extremely accurate measurement is made. When dealing with polymers, equilibrium is usually reached with a few hours, although in some cases, results are obtained in as little as 45 minutes. This compares to days, weeks and even months for traditional gravimetric measurements of the same materials.
There are various ways to test the permeability of containers such as fuel tanks and piping. If it is a tank, the most reliable technique is to incorporate the fuel in the container, and seal it as normal. The tank is then placed inside a special chamber, through which dry gas is channeled. Similarly, a fuel pipe can be fed through the container, carefully sealing it. In this way, any hydrocarbon vapour that diffuses out through the walls of the pipe or tank can be detected easily, and a reading can be obtained as soon as the diffusion rate has become steady. Accuracy is typically in the parts per million range. By running the test with a number of samples at once, the average permeability can be more accurately measured.
Variations of this technique can be used to check the flow of hydrocarbons inward through the walls of a pipe or container where this is required. While this provides a measure of the vapour entering the container, it demands careful sealing. In most, though not all, cases, the permeation rates in one direction match those in the other, and the more reliable former method can be used.
Any part of a pipe or container can be tested for permeability, as can samples of any materials – either by using a specially manufactured jig to hold the sample or component in the testing environment, or by sealing off the other parts of the container using a non-permeable material.
The automotive industry is currently researching new compound materials with enhanced permeability properties, so it is worthwhile to review a few of the commonly used polymers and compounds.
High-Density Polyethylene, due to its high molecular mass this material is widely used for fuel tanks, especially after fluorinating (sulfonating) as this reduces the permeability of the material by around 90%. Other coatings are being currently being tested as are
dispersions, films, different compounds and co-extrusion into composite fuel tanks. Polyethylene’s performance is improved by adding polyamide, modified polyamides, polyvinyl alcohol and compatibilizer precursor.
Polyamide, (Nylon 12), is conventionally used for fuel (and air) pipes, though this could change as it may not meet SHED requirements. More expensive, improved barrier products such as multilayer tubes may need to be used in its place. The inner layer will provide the required vapour barrier and the outer layer (possibly still polyamide) will provide mechanical strength and impact resistance, with a tie layer to prevent delamination.
Fluoropolymers resins, permeate 80-90% less hydrocarbons than Polyamide and thus provide low permeability to automotive fuels.
Acetal, is a copolymer which provides excellent mechanical and permeability properties for fuels, including reformulated fuels such as petrol and grain alcohol mixtures (gasohols).
Current and future legislation is driving up standards across the entire automotive industry and nowhere is this truer than with fuel. The technical performance of materials and systems will need to rise, and rise quickly to keep pace, not just in the developed world, but increasingly in the third world as well.
Testing is set to become the byword of the age with the word “permeability” increasingly (if often slightly inaccurately) on the lips of politicians, manufacturers and materials scientists alike. But it is a vastly complex subject and I hope that this article has “shed” a little light on the matter!
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