Pour point depressants – better known as PPDs – have kept fluids flowing in cold weather since the 1930s. With changing needs in the marketplace, a recent study confirms that selection of the right pour point depressant has never been more important than it is today.

Equipment manufacturers and users are demanding fluids that deliver greater efficiency and greater durability. At the same time, new specifications for engine and driveline oils place tougher limits on low-temperature fluidity. And base oil slates are continually changing, with growing use of severely hydrotreated API Group II and III oils.

Because they must be tailored to specific base oils, there is no universal pour point depressant. Moreover, different PPDs may be necessary for the same applications as product specifications change.

Despite a common misperception, even so-called synthetic API Group III base stocks require the use of PPDs. Also, certain additive components in new oil formulations often act as PPDs – for a while, at least. As a result, new oil occasionally can meet low-temperature viscosity specifications without adding a PPD. As the additives break down or are consumed in use, however, the desired effect is lost.

With an inadequate or incorrect PPD, low-temperature viscosity can increase rapidly, and an oil could fail to protect the engine in critical conditions, such as during cold start-up. Recognizing this issue, many OEMs are beginning to add used oil low-temperature viscosity limits in their engine oil performance specifications (see table, above).

PPDs: The Basics

A pour point is the lowest temperature at which a fluid will flow under standard conditions, as measured by the ASTM D 97 Pour Point Test; this test is widely used today as a quality-control tool. A pour point depressant lowers that temperature. It also lowers lubricant viscosity in other ASTM tests which are known predictors of low-temperature lubricant flow in engines, axles and transmissions.

At low temperatures, the wax contained in the base oil crystallizes to form fragile, interlocking crystal structures that can prevent the lubricant from flowing. PPD molecules contain segments that co-crystallize with the wax, modifying the shape of the crystals and preventing the interlocking structure from forming. Instead, they promote the formation of small, free-flowing wax particles.

The ability of a pour point depressant to improve the low-temperature flow characteristics of a lubricating oil is largely determined by how well the alkyl side-chains within these compounds can co-crystallize with the wax components of the base oil. To be most efficient, the PPD chemistry must be matched to the base oil wax structure.

Wax crystals arent the only problem. The viscosity of any fluid increases considerably at low temperatures, reducing its flowability. A PPD can ameliorate only the waxs contribution to viscosity. Thus, there is a limit to which any PPD can improve low-temperature viscosity.

Group III, a New Challenge

Group III base stocks are coming into greater use for a number of reasons.

For one, many OEMs are specifying the use of more fuel-efficient fluids, such as SAE 0W-30 and 5W-20 engine oils, lower-viscosity automatic transmission fluids, and SAE 75W-90, 75W-110 and 75W-140 wide-span multigrade gear oils.

Additionally, ILSAC (an international lubricants standards committee), the American

Petroleum Institute, and ACEA (the European automakers trade group) continue to lower the maximum allowable volatility limits. Lower-volatility oils protect exhaust aftertreatment systems and help comply with global air quality standards.

Third, passenger car engines and drive-trains are becoming smaller and more powerful, placing a higher thermal load on the lubricants. Finally, low-temperature viscosity requirements for driveline oils continue to become more challenging.

Group III oils have considerable appeal in addressing each of these factors. Compared with Group I and Group II oils of equal kinematic viscosity measured at 100 degrees Celsius, Group III base stocks have lower volatility, improved thermal/oxidative stability and often improved low-temperature fluidity. These attributes are shared by API Group IV synthetics (polyalphaolefins), but Group III oils are considerably less expensive than PAOs. As a result, Group III base stocks are gaining market share in Asia, the Americas and Europe.

While lubricants based on Group III oils perform similar to PAO-based fluids in many ways, the low-temperature flow characteristics of Group III oils simply are not as good as those of PAO fluids. A number of feedstocks are used to make Group III oils, including highly paraffinic slack wax as well as hydrocracker bottoms and raffinates. Manufacturers often employ catalytic isomerization techniques to convert linear paraffins to branched paraffins, thus improving low-temperature properties and maximizing production yield.

Catalytic isomerization converts the base stocks paraffins, linear hydrocarbons that tend to crystallize and form wax, into branched hydrocarbons with short branches. These branches break up the crystallinity and make a fluid more amenable to be used as base oil for lubricants. Catalytic isomerization lowers the pour point – but not to the levels characteristic of PAO fluids. In most cases, PPD additives are still required to be used with Group III oils.

Figure 1 illustrates this, by comparing the pour point of several Group III base oils with PAO fluids of the same starting viscosity. All these base oils did fine down to -10 C, but as the temperature continued to drop, the differences in pour point become more pronouced.

Performance Tests

Several tests under the auspices of ASTM International help to provide assurance that lubricating oils will flow properly at low temperatures. Most significant among these industry-accepted measures of low-temperature fluidity are the Mini-Rotary Viscometer Test, or MRV TP-1 (ASTM D 4684), and the Brookfield Viscosity Test (ASTM D 2983). Another method, the Cold Cranking Simulator Test (ASTM D5293), measures lubricant viscosity under cold start conditions; since it measures viscosity at high shear rates, fragile wax structures are broken down mechanically and do not adversely affect CCS viscosity. Therefore, the PPD is not effective in lowering CCS viscosity.

The MRV Test simulates the pumpability of engine lubricants under cold start-up conditions. MRV viscosity is measured at a temperature that depends upon the viscosity grade. Thus, an SAE 0W-30 oil is tested at -40 C, while a 20W-50 oil is tested at -20 C.

Similarly, the Brookfield Viscosity Test is designed to ensure good low-temperature lubricant flow properties in vehicle transmission, axle or hydraulic system applications. The Brookfield viscosity at – 40 C is generally specified by OEMs for automatic and commercial vehicle transmission lubricants. The Brookfield viscosity measurement temperature for automotive gear oils depends upon viscosity grade. For example, an SAE 75W-90 gear oil is tested at -40 C, and an SAE 85W-140 oil is tested at -12 C.

Choosing Right

Choosing the right PPD depends on the base stocks being used, the performance package and the viscosity modifier in the formulation. Suppliers can offer recommendations based on this information, but there is no substitute for testing to assure that all of the complex interactions among base oils and additives that can affect low-temperature performance are under control.

The broader the range of lubricants that are made at a particular blending plant, the greater the challenge in choosing one PPD to cover all bases. Thats because passenger car engine oils, heavy-duty diesel oils, gear oils, automotive transmission fluids and hydraulic fluids all use different base oils and additives. In many cases, however, one PPD can be utilized throughout a plant, although one or two additional PPDs may be needed for special hard-to-treat, low volume products.

Even Mother Nature can affect PPD selection. After last years hurricanes disrupted oil supply in the United States, many oil marketers had to scramble to find alternate sources of base oils to keep production going. Chances are, those new base oils required different PPDs.

In choosing the right one, start by evaluating PPDs at a relatively low concentration, and make sure to use low-temperature tests that are appropriate for the application. Select the most effective PPD or combination of PPDs, and then optimize the treat level.

Keep in mind, too, that the characteristics of an oil change as it is used, affecting its ability to perform in cold temperatures. For example the used-oil MRV performance of two SAE 15W-40 lubricants was measured in the Mack T-10A test. The oil with the lowest MRV viscosity when new had the higher T-10A MRV viscosity when it was tested again after use (see Figure 2).

Waxy buildup may not trouble todays modern homeowners like it did their mothers. And as long as oil formulators select the right pour point depressant to do the job, automotive OEMs need not be concerned either.