The Materials Analyst, Part 86: Telling sulfone polymers apart

Traditional means of distinguishing similar polymer families don’t apply in sulfone-based materials. The key is in how they decompose.

Sulfone chemistry represents the upper end of the performance pyramid. It is a fascinating chemistry that was first commercialized by Union Carbide in 1965 in the middle of the golden age of new polymer development. The sulfone group produced an amorphous polymer that was more heat resistant, chemically resistant, and hydrolysis resistant than polycarbonate. Variations on the polysulfone chemistry that came along in later years produced enhancements to some, but not necessarily all, of these properties.

Both polyethersulfone (PES) and polyphenylsulfone (PPSU) exhibit higher glass transition temperatures (Tg) than that of polysulfone. Glass transition temperatures are rarely mentioned in published data sheets; however, an approximation of the Tg in amorphous resins can be obtained by referring to the heat deflection temperature (HDT). The HDT of both PES and PPSU is 30 deg C higher than that of the original polysulfone. In addition, PES can sustain performance when exposed to high current flow rates while polysulfone can only be used at lower amperages. On the mechanical property side, PES is somewhat stronger and slightly stiffer than polysulfone. Finally, PES is inherently self-extinguishing, rated V-0 without the use of additives at wall thicknesses as low as .75 mm. Polysulfone is borderline in this department until the wall thickness reaches 3.0 mm.

However, the changes are not always positive. PES is more than 10% denser, resulting in molded parts that weigh more. And while all sulfone polymers are hygroscopic, PES absorbs almost 2.5 times as much moisture as polysulfone in a 24-hour period. Of much greater importance is the effect that prolonged exposure to hot water has on PES.

One of the distinguishing strengths of polysulfone is its ability to retain properties after prolonged exposure to water at elevated temperatures and even to steam. This leads to a general expectation that any material with the “sulfone” label shares this capability. But as it turns out, PES has limited hydrolysis resistance and utility for steam sterilization. In addition, hot water is a stress crack agent for PES.

Enter PPSU. This variation on the sulfone theme displays the same upper-end temperature limits as PES. However, it has the same hydrolysis resistance and ability to withstand steam sterilization as polysulfone.

There is more. Morpholine is a common additive used in many steam systems at very low levels to adjust the pH of the water. As it turns out, the addition of the morpholine causes significant problems for both polysulfone and PES during steam sterilization, but PPSU does very well in this environment. And while all sulfone polymers exhibit good practical toughness, polysulfone and PES are both notch sensitive, a property not shared by PPSU, which has a notched Izod impact of 13 ft-lb/in.

TGA to the rescue

Given these subtle but important differences in performance, it can be important to be able to distinguish between these various sulfone-based materials. When analysts think of techniques for fingerprinting materials, the method of choice that is typically considered is infrared spectroscopy. Unfortunately, this method identifies materials according to the chemical bonds that are in the compound.

Figure 1 shows the chemical structure for these three sulfone-based polymers. While there are some subtle differences in structure, the essential chemical bonds are common to all three materials. Consequently, infrared spectroscopy does not do a good job of delineating between these polymers. Figure 2 provides a comparison of the spectra for polyethersulfone and polyphenylsulfone, and illustrates the difficulty in telling these materials apart.

This same limitation exists for high-density polyethylene and low-density polyethylene, for polypropylene homopolymers and copolymers, for acetal homopolymers and copolymers, for PET and PBT polyesters, and for the various types of aliphatic nylons such as nylon 6 and 6/6. However, in all of these cases the materials are semicrystalline. Therefore, we can differentiate these materials based on their melting points, which can be determined by differential scanning calorimetry (DSC).

But sulfone polymers are amorphous. By DSC we can measure the glass transition temperatures. Polysulfone has a Tg of approximately 190°C, but PES and PPSU both have Tgs near 220°C, so this is only a satisfactory technique for separating polysulfone from the other two polymers.

At this point we are at an apparent impasse. The softening temperature and molecular structure of PES and PPSU are essentially identical. So what is left? Fortunately, there is something, and it relates to details in the structures shown in Figure 1. The distinguishing method is thermogravimetric analysis (TGA). This technique examines composition according to the manner in which materials decompose.

When a polymer is decomposed in an inert atmosphere, a process known as pyrolysis, a certain portion of the material will decompose. However, in some materials a carbon char will form in this inert atmosphere that cannot be decomposed until oxygen is introduced. By observing the ratio between the weight loss during pyrolysis and the weight loss in air, a significant amount of information can be determined about the chemical structure of the polymer.

The amount of char that forms during pyrolysis is related to a feature in the chemical structure known as an aromatic ring. This ring is a six-sided constituent with a carbon atom at each vertex. Polymers that contain no aromatic rings, such as polyethylene, polypropylene, and acetals, decompose completely in an inert atmosphere and form no char. A material like PBT polyester produces approximately 7% char while PET forms 13% char because of different degrees of aromatic character in the polymer backbone. Polycarbonate forms 25% because of an even higher degree of aromatic content.

If we examine the structures in Figure 1 we can see that the concentration of aromatic character increases as we progress from polysulfone to polyethersulfone and then to polyphenylsulfone. Consequently, we would expect that the amount of char formed by each of these polymers would be sufficiently different to make a distinction between the materials.

Figures 3 and 4 show the TGA results for an unfilled PES and PPSU, respectively. As expected, the PES produces a smaller amount of char than the PPSU, approximately 42% vs. 56%. This provides a distinctive result that penetrates the identical results obtained by infrared spectroscopy and DSC. The ability to tell these two materials apart can be the difference between success and failure in hot, wet, chemically active environments.