Designing with polyphenylene sulfide

PPS is a good material for replacing metal and thermosetting plastics thanks to its strong array of performance properties.

Sulfur in a plastics molecule indicates heat resistance. Polyphenylene sulfide (PPS) shares that distinction with polysulfone (PSU), which was discussed in IMM’s February 2009 issue. Both contain sulfur. Both are high-temperature materials, but PPS has a 150 deg F higher heat-deflection temperature.

Comparatively new (1973), PPS was noticed in 1888 but not pursued. In the late 1940s, A.D. Macallum experimented with PPS. A heat-resistant, moldable material was produced, but the reaction was unstable. Dow Chemical purchased the Macallum patents and, through the 1950s and 1960s, developed a new synthesis process. Dow produced a usable material, but terminated the project due to difficulties in scaling up to production quantities.

In the meantime, Phillips Petroleum Co. (now Chevron Phillips Chemical Co.) had developed a commercially viable PPS process. Pilot production was initiated in 1967 and full production started in 1973. Phillips dominated the market until its patent expired in 1984 and others entered the PPS market.

Designing characteristics
PPS is a high-performance, opaque, thermoplastic material. It is known for high resistance to heat, burning, and chemicals, and has good electrical properties. Most commercial grades are glass-fiber reinforced. Others contain mineral filler, or carbon, or PTFE. The following values are based on 40% glass-reinforced PPS, which is the most frequently used.

This is a strong material with a tensile strength of 17,500 psi and a flexural modulus of 1.7 million psi. PPS is notch sensitive with a notched Izod impact of 1.6 ft-lb/in. Unnotched specimens have an impact of 5.5 ft-lb/in. These are good values for a material with such a high flexural modulus.

These impressive properties and a good surface appearance can only be achieved with a mold temperature of at least 275ºF. Lower temperatures do not allow PPS to develop its full crystallinity.

With a heat deflection temperature of 500ºF at a 264-psi loading, PPS is comfortably positioned above glass-reinforced PSU and polyetherimide, but below liquid crystal polymers. This high operating temperature coupled with good overall physical properties has allowed PPS to replace metals and thermosetting materials in many applications.

With a dielectric strength of 450V, PPS is a good insulator. A good balance of other electrical properties accounts for its wide use in electrical applications.

The air we breathe is 21%-22% oxygen. With an oxygen index of 47%, PPS will not burn in the atmosphere. With enough oxygen, PPS will burn if heated to 1004ºF. At this temperature PPS chars, creating a carbon-rich crust that protects the interior from heat and from the oxygen necessary to support further combustion. PPS exceeds the UL 94 V-0 and 5V requirements without fire-retarding additives. This is a material that avoids Europe’s restrictive fire-retarding additives requirements.
The overall chemical resistance of PPS is only surpassed by fluorocarbons, such as Teflon.

PPS absorbs ultraviolet energy with a reduction in physical properties. In one 10,000-hour weatherometer test, PPS retained 63% of its original tensile strength. Retention increased to 97% with the addition of 2% of carbon black.

The current list price for 40% glass-reinforced PPS averages $4.85/lb and $0.24/in³/truckload. These prices seem high until compared to glass-reinforced PSU at $7.37 and liquid crystal polymers at $8.95/lb.

Part design tips
Wall thickness. PPS has been successfully molded with 0.015-inch-thick walls. A better range of thickness would be a minimum of 0.030 and a maximum of 0.375 inch. PPS is an easy-flow material. On average, a 0.030-inch-thick part can have a flow length of 5.0 inches. A 20.0-inch flow length is possible with a 0.125-inch-thick wall. Wall thickness variations should be smoothly blended and limited to 40% of the thickness of the thinner wall.

Corner radiuses. Rounding corners improves melt flow while reducing stress. PPS is a notch-sensitive material that benefits from rounded corners. A minimum inside corner radius must be at least 25% to preferably 60% of the part’s thickness.

Draft angles. PPS is abrasive and very stiff. Undercuts should be avoided. A 1⁄4º/side draft angle is required on cores and cavities with a length or depth of 0.187 inch. Draft must be increased to 2º/side for a 2-inch depth of draw.

Projections. Ribs, gussets, and solid bosses are projections off of a part’s nominal wall. PPS is a low-mold-shrinkage material. The thickness of projections at their juncture with the nominal wall can be equal to the part’s wall thickness. A thickness of 75% of the nominal wall will minimize the possibility of sink marks and internal voids. Radiuses should be provided at the tip of a rib and where that rib meets the nominal wall.

Depressions and holes. The cores that form holes produce weldlines. Melt flowing around these cores reunites on the far side, creating weldlines. As with most other glass-reinforced plastics, there is a loss of strength at weldlines. With PPS, the loss can be 50%-70% of the material’s original tensile strength. Where possible, cavities should be gated to locate weldlines in non-load-bearing areas on a part. PPS is a strong, abrasive material. Holes must be designed with rounded corners and the maximum allowable draft angle.

Tolerances. The dimensional stability of PPS accounts for its use in intricate precision parts. A commercial tolerance for a 1-inch-long, 0.125-inch-thick part is ±0.003 inch. A more costly fine tolerance on the same part is ±0.0015 inch. This fine tolerance should only be specified when that dimension is more important than part cost.

PPS is worth considering for applications requiring a dimensionally stable, strong, non-burning, chemically resistant material with a service temperature higher than that provided by polycarbonate, polysulfone, and polyetherimide, but less costly than liquid crystal polymers.

Typical applications for PPS
Electrical: Connectors, capacitor headings, bobbins, sockets, printed circuits, electric motor brush holders, hair dryers, and other heat-generating household appliances; encapsulation of electrical circuitry. In many of these applications PPS replaces thermosetting plastics and provides a shorter molding cycle and the ability to reuse sprues and runners.

Automotive: PPS was the first thermoplastic with enough heat resistance to be used on automobile engines. The first successful plastic intake manifold used PPS to replace 22 diecast parts with a 56% weight reduction. Other applications include components for emission controls, cooling systems, generators, electrical sensors, light sockets, hydraulics, and thermostats. High service temperature and resistance to gas and oil account for many of these applications.

Healthcare and food handling: At 428ºF an electrically heated surgical scalpel relies on the heat resistance and insulating properties of PPS. Food-handling applications include microwave cookware and oven parts, coffee pot bases, and heat-insulated appliance handles.

Other applications: Chemical-resistant pump housings, impellers, pistons, bearings, and valves; oilfield sucker-rod guides. A PPS condenser baffle exhibited improved corrosion resistance at one-fifth the weight of stainless steel. Neat PPS is also used as coatings for a wide range of applications.