Advances in materials lead to more plastics under the hood
Diesel industry and automotive manufacturers have a history of using plastic materials in an effort to realize lower production costs and better performance. Applications vary but can include fan blades, radiator end tanks, surge tanks and connectors. Unfortunately, plastic materials, such as reinforced polypropylene and Nylon 66, are limited and cannot be used to any large extent in cooling, fuel and emissions systems. Due to these limitations, the chemical industry worked to create a plastic material that performs better, weighs less and is more cost-effective. The automotive and diesel industry is now beginning to realize the fruits of working with performance plastic materials.
The challenges of the two popular plastic materials used in the automotive and diesel industries, polypropylene and Nylon 66, are well documented. Polypropylene is limited to a continuous use temperature of about 212[degrees]F, and is brittle when used with the high filler content required for these temperatures. Nylon 66 can be used at slightly higher temperatures, approximately 257[degrees]F, but is vulnerable to attack by various coolants, steam, zinc chloride and many "environmentally friendly" fuels or biofuels, which contain alcohol-based chemicals.
In addition to these drawbacks, new emission control requirements have led to increased underhood temperatures on diesel-powered trucks. Diesel truck applications require part performance measured in hundreds of thousands of hours, as opposed to tens of thousands for automotive components. This makes thermal stability and creep resistance even more critical.
Materials that have been considered for the next level of applications are Nylon 46, polyphthalamide (PPA) and polyphenylene sulfide (PPS). Automotive manufacturers have used these materials for several years, as they offer various combinations of strength, temperature and chemical resistance. But Nylon 46 and PPA have some drawbacks.
Nylon 46 offers slightly higher temperature capability than Nylon 66 (347[degrees]F), but is also more susceptible to moisture absorption--resulting in greater property variation with changes in moisture level. Nylon 46 is also susceptible to attack by zinc chloride and many coolants.
PPA offers slightly better chemical resistance than Nylon 66, but is still susceptible to attack by coolants. It offers good short-term thermal properties (up to 392[degrees]F), but can lose modulus and strength after extended exposure to high temperatures--and is subject to creep above 302[degrees]F.
PPS demonstrates superior high temperature thermal properties and is resistant to all common automotive and diesel fluids, as well as byproducts of combustion that can contain low levels of sulfuric, nitric and hydrochloric acids.
Ryton R-4-220 PPS has been qualified for use in cooling systems that involve testing at 284[degrees]F for 2000 hours. Short term, PPS can function over 437[degrees]F and it has good property retention up to 392[degrees]F. Several grades of Ryton PPS have been exposed to a 50% solution of zinc chloride for 100 hours at 185[degrees]F with no loss of tensile strength. Fig. 1 shows long-term temperature limits, based on several materials' ability to retain 50% of its original strength for 100,000 hours at the indicated temperature.
[FIGURE 1 OMITTED]
One grade of Ryton PPS has been formulated especially for enhanced electrolytic stability in steam and in long-life coolants. Data comparing this compound (Ryton R-4-220 BL PPS) to another PPS compound and other engineering plastics is shown in Table 2.
In addition, in testing of Ryton R-4-220NA PPS, PPA, HTN and a standard PPS material, the materials were exposed to a Sierra long-life coolant at 280[degrees]F for 76 days. The stabilized Ryton PPS material retained more than 95% of its tensile strength. The PPA material retained less than half of original strength over the duration of the test.
Table 1 shows results for Ryton R-4-02XT PPS tested in several other automotive fluids. Ryton PPS is highly stable when exposed to various chemicals and elevated temperatures, making it an attractive choice to replace metal. Many value engineering projects rely on the economics of part consolidation and elimination of metal fabrication. Ryton PPS components offer the cost advantages of injection molding, which allow many parts of an assembly to be combined into a few. Machining costs can be eliminated and assembly can be simplified.
Where earlier materials were limited to components away from the engine systems, Ryton PPS can be used in emissions such as EGR systems and vent tubes, cooling applications that include thermostat housings, crossover tubes, end tanks, electrical systems that have connectors and engine sensors, and fuel components such as fuel rails, fuel pump impellers and fuel pressure sensors.
In demanding environments, each of these applications requires that the material be thermally, chemically and dimensionally stable throughout the operating range.
Ryton PPS compounds that are typically glass or glass/mineral reinforced offer high flexural modulus in the range of 2 to 3 mm psi. They also offer excellent chemical resistance and thermal stability needed for metal replacement applications. However, their use can be limited by their lack of elongation and impact performance. Most high temperature reinforced plastics exhibit elongations of only 1.5 to 2% before breaking. In some applications, this challenge could limit use in housings, engine covers, snap fits, air ducts and connector overmolds. This limitation can be addressed by alloying the PPS polymer material with an elastomer.
Even with 35% glass reinforcement, an alloy material (XTEL XE3035) will provide 3% elongation at break. Even with significantly improved impact strength, the alloy has nearly the same temperature capability as glass-filled Ryton PPS. Yet, the tensile strength and flex modulus are reduced by 30 to 40%. Table 2 shows a comparison of typical physical properties of Ryton R-4-200NA and XTEL XE3035.
This newer blend of physical properties with improved impact allow for enclosures, air ducts, engine covers, overmolded connectors and brake components to be produced at less cost and reduced weight than traditional materials. In addition, the chemical resistance is maintained when exposed to automotive and diesel fluids since the XTEL XE3035 etastomeric alloy contains PPS.
Critical parts must meet specified loads and deflections, often at high temperatures and in various chemical environments. Plastic parts also must be designed so that they can be made quickly and in mass quantities. This includes molding, ultrasonic welding and other processes. Extensive FEA modeling can verify the performance of these materials in each application.
New plastic materials offer exciting advantages to traditional materials and can be used for metal replacement or part consolidation in cooling, fuel, engine sensor and electrical applications. Combined with an accurate FEA, Ryton PPS and XTEL XE alloys pave the way for significant value engineering opportunities.
Bryan Fox is trucking market manager for Ryton PPS, Chevron Phillips Chemical Co. LP, The Woodlands, Texas.
24 MONTH ELEVATED TEMPERATURE PROPERTY RETENTION DATA FOR Ryton[R]
R-4-02XT PPS IN VARIOUS AUTOMOTIVE FLUIDS
Fluid Temperature Strength
DOT 3 Brake Fluid 250[degrees]F 104%
Motor Oil 300[degrees]F 91%
Type F Transmission Fluid 300[degrees]F 98%
Diesel Fuel (12 months) 200[degrees]F 99%
Fluid Flex Modulus
DOT 3 Brake Fluid 110% 103%
Motor Oil 101% 94%
Type F Transmission Fluid 105% 97%
Diesel Fuel (12 months)
In addition, no samples exhibited more than minimal (0.9%)
swell during the test.
COMPARISON OF RYTON[R] PPS, R-4-200NA AND XTEL[R], XE3035
PHYSICAL PROPERTY XTEL[R] XE3035 Ryton[R]
ALLOY R-4-200NA PPS
Tensile Strength, Ksi 16.0 28.0
Elongation, % 3.0 1.6
Flexural Strength, Ksi 24.0 40.0
Flexural Modulus, Msi 1.1 2.1
Notched Izod, lb.ft./in. 3.5 1.6
Unnotched Izod, lb.ft./in. 20.0 12.0
HDT at 264 psi, [degrees]C 264 265
CLTE, -50[degrees]C to
50[degrees]C, [micro]m/m/[degrees]C 30/150 20/45
CLTE, 100[degrees]C to
200[degrees]C, [micro]m/m/[degrees]C 30/150 15/95
Density, g/cc 1.45 1.65