Valve Magazine - Summer 2009

UNDERSTANDING
PLASTICS

In the last 25 years, the use of thermoplastics in the valve industry has grown steadily. Early applications were limited to check balls, seals and decorative caps. The timing of when the first plastic valve was introduced is an issue of debate, but here is my own personal experience, and it illustrates the strengths and weaknesses of certain plastics:

When I was seven, my parents gave me a Coca-Cola dispensing machine. The machine consisted of a red plastic base with a mount to hold a bottle, which, because it was the 1960s, was made of glass.

The top of the unit had a white plastic cover with the trademark logo, a plastic handle and a small plastic valve. After opening the cola bottle, you slid the valve over the top and laid that bottle on its side in the base. The valve slipped through an opening that provided push-button function. Of course, this plastic valve suffered the effects of long-term use and chemical exposure, and it still retains the masking tape I placed on it in the 60s to stop a leak. More than likely, a hoop stress failure was the culprit.

Although I didn’t evaluate these aforementioned valves to determine their material composition, it’s a safe bet they were made from widely available commodity plastics including polystyrene and polyolefin-based

HIGH-PERFORMING ENGINEERING
THERMOPLASTICS HAVE MADE STRONG
INROADS INTO THE VALVE MARKET AND
WILL CONTINUE TO DO SO BECAUSE OF
THEIR HIGH STRENGTH, LIGHT WEIGHT
AND CORROSION RESISTANCE.
BY PATRICK NEEL

materials. While functional, these materials are more limited by temperature, mechanical performance and pressure than engineering-grade plastics.

While this is only my own experience, it brings up a point about plastics grades. Specifically, the limited property profile of commodity plastics in the valve market has left a sizeable window of opportunity for higher-performing plastics, namely engineering materials. These products afford higher temperature performance, greater chemical resistance and more strength (see Figure 1).

Because of this broader performance portfolio, the use of engineering plastics has grown over the years. For example, one of the first polymers in the valve industry was polytetrafluoroethylene (PTFE), which was used for soft-seated ball valves. Today, engineering plastics such as nylon (PA), polysulfone (PSU) and polyetheretherketone (PEEK) can be found in many check valves, medical valves, beverage dispenser valves, ball valves, quarter-turn valves, diaphragm valves, seals, check rings and bushings.

What’s more, valve suppliers are increasingly considering plastics as a replacement for brass in quarter-turn valves. For example, a Midwest manufacturer of valves and components for the plumbing industry uses unreinforced polyphenylsulfone (PPSU) because of its light-weight strength, corrosion resistance and ability to withstand prolonged exposure to hot, chlorinated water. This lead-free plastic alternative eliminates mineral build-up and is about 75% lighter than brass. Another key advantage is the ease of installation of PPSU compared to brass. The injection-molded quarter-turn valves offer quick connect or crimp-style connectivity, which speeds up installation and eliminates the need for brazing.