Solutions Current Issues > Jan.Feb.Mar_2009 > Tech Session
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CARB CERTIFIED PRODUCTS
As regulations grow, so do RL Hudson's material capabilities.
In sports, entertainment, business, or politics, everyone knows the major players. They’re the teams or individuals with the greatest influence. The California Air Resources Board, also known as CARB, has long been a major player in air pollution control. By the 1950s, the Golden State had earned a reputation for having air that was sometimes unfit to breathe. “Smog” (coined from “smoke” and “fog”) often blanketed the Los Angeles basin, the result of atmospheric conditions, a dense population, and large numbers of motor vehicles.
In 1960 California became the first state to enact clean air regulations. California was (and remains) the most important market in the United States for motor vehicles. Any company wanting to sell vehicles in California had to comply, whatever the cost, or stop selling its products. This later became true for motorcycles, boats, off-road vehicles, and outdoor power equipment as well.
CARB soon became the model for national air quality regulations, although California’s standards have traditionally outpaced the rest of the nation. As the technology to meet today’s requirements has been developed, new and stiffer requirements have been introduced by CARB and subsequently adopted by the US Environmental Protection Agency.
That’s why meeting CARB requirements is so important — and there’s another, even more important reason: products that meet CARB requirements are state-of-the-art when it comes to protecting the environment. RL Hudson’s design engineers and materials specialists are fully committed to meeting, and exceeding, the standards established by CARB.
After tailpipe emissions, a major secondary source of air pollution is unburned hydrocarbons, mostly gasoline vapor, that enters the air by passing through plastic tanks, plastic fuel caps, and rubber hoses, diaphragms, and seals. We say that rubber and plastics are permeable by hydrocarbons.
For now, let’s define permeability as a measure of the rate at which gases, vapors, and liquids pass through a solid material.
Permeation does not rely on microscopic (or larger) pores in the material, although these will usually increase permeability. For a gas, vapor, or liquid to permeate through a polymer, two different processes must occur. First, the penetrant must dissolve into the polymer surface, and second, the penetrant must diffuse through the polymer bulk. This is the solution-diffusion mechanism of permeation. These processes occur together, but must be analyzed separately.
This dual mechanism suggests why certain polymeric materials are less permeable to specific penetrants than others are. Permeability is thus material- and penetrant-specific, which presents some interesting challenges because motor fuels contain a blend of ingredients, many of which vary in proportion in different parts of the country.
Reducing the permeability of polymers via the application of a coating is one solution, but it is complicated by the high elongation that many polymers are subjected to. Many low-permeability coatings, such as fluoroplastics or vapordeposited glass or metal, will fracture under elongation. For this reason, at RL Hudson we recommend coatings only for static applications.
The permeability of many polymers can be reduced by the incorporation of platy fillers, such as clay, ground oyster shells, or mica. Such materials effectively block the movement of the molecules, forcing them to take a longer path (a quantity called tortuosity). In addition, most fillers are themselves impermeable, so their presence reduces the percentage of the mass that is composed of polymer. Such fillers alter the physical properties of the polymer, normally increasing hardness, reducing elongation, and increasing compression set. They may also alter rubber curing behavior. These limitations have led to increasing interest in nanoparticle fillers, which pose formidable technical challenges but can be effective at relatively low concentrations.
One of the concerns for designers of rubber and plastic products that are exposed to gasoline is that the upper limit for permissible transmission is continually being lowered. Today, the maximum allowable permeation rate for motor vehicle fuel hose to be CARB certified is 15 g/m2/day, as tested per SAE J1737. That’s an extremely difficult requirement for flexible materials to meet, usually calling for costly multiplelayer products, although the advantages of rubber and plastics will continue to make these materials attractive as a replacement for impermeable metal piping.
There are two phases to the test: a 42-day pre-conditioning soak phase and a 21-day equilibrium (weight loss) phase. The test is conducted at 40° C or higher (which greatly increases the mobility of hydrocarbon molecules) and at ambient pressure. The hose samples are filled with test fuel: Phase II California Reformulated Certification (CERT) fuel, CE10, CM15, or Indolene. Once the samples have reached equilibrium for five consecutive days, the permeation rate is determined: Permeation Rate = Weight Loss per Day / Area of Hose Surface at ID.
For any “hose product family” using a given combination of type and thickness of barrier material(s), CARB requires that samples with the smallest ID for the product family pass the permeation test. Five samples must pass.
And it’s not just fuel hoses for motor vehicles. In addition to fuel hoses, RL Hudson sells fuel tanks, fuel tank caps, primer bulbs, and primer bulb assemblies. These products have their own permeation and leakage requirements; for example, CARB Small Off-Road Engine and Equipment Evaporative Emission Control Test Procedure TP-901 defines the test procedure for small off-road engines and equipment fuel tanks. The performance standard defined by CARB states that fuel tank permeation shall not exceed 2.0 g/m2/day. An optional, more stringent performance standard states that fuel tank permeation shall not exceed 1.0 g/m2/day. As shown in the table, a number of materials may be used to meet CARB requirements. Each of these materials must be considered in light of its permeability, cost, low temperature flexibility, poor resistance to certain types of fluids (chief among which are ethanol and methanol), flex life, ease of fabrication, and other relevant characteristics. Many times, only a combination of materials will meet all of a product’s multiple requirements.
To meet CARB requirements, our material development lab has added in-house testing capability that allows for the efficient screening of candidate materials. This capability, coupled with our state-of-the-art rubber mixing and curing equipment, has allowed RL Hudson to develop several successful candidate materials.
Are you looking for cost-effective low-permeation system solutions that meet or exceed today’s — and tomorrow’s — strict requirements? Put us to the test. Whether it’s part design, system design, material development, or certification, new product or upgrade, at RL Hudson we stand ready to help.
