PPD selection and use is highly important for ensuring high performance and prolonging the lifecycle of oil formulations by preventing equipment damages at low temperatures. With over 70 years of PPD expertise, Evonik shared elaborate information on pour point depressants.
The ability of a lubricant to flow under low temperature, low shear conditions is crucial to the operation of engines and equipment expected to run in cold climates. Without the proper selection and treat rate of a pour point depressant, a mineral oil lubricant
formulation will exhibit poor low temperature properties, leading, in the worst case, to lubrication “starvation” and equipment failure.
Virtually all paraffinic mineral oil base stocks contain small amounts of waxy materials. As the temperature of the oil is decreased, some of the waxy components come out of solution as tiny crystals, and the solution begins to appear hazy to the naked eye. The temperature at which this occurs is called the cloud point. As additional wax precipitates, the crystals grow into plates and, finally, if the temperature is decreased far enough, the plates will grow together to form a three-dimensional network that totally immobilizes the oil. This solidification process is sometimes referred to as gelation. The lowest temperature at which the oil is fluid is called
the pour point.
Emergence and development of PPDs
Prior to the 1930s, the options for dealing with cold flow problems were very limited. Heat was an obvious solution, and as foolish as it may seem today, stories of fires being built under the sumps of vehicles are indeed authentic. A more reasonable alternative, at least in some instances, was to increase the solvency power of the fluid portion of the oil by the addition of kerosene to the lubricant, but this also decreased the viscosity of the oil at high temperature. There was also the alternative of adding one of several naturally occurring materials, such as asphaltenic resins or microcrystalline waxes, which were removed at various stages of the refining process. Unfortunately, while such materials were sometimes reasonably effective, they were not broadly applicable.
Because these materials depress the pour point of oil, they were designated pour point depressants. The existence of these naturally occurring pour point depressants suggested that there could well be synthetic materials that could function at least as well and probably better. The structures of natural hydrocarbon pour point depressants, all waxy materials themselves, provided clear clues to the early synthetic efforts. In 1931, alkylated naphthalenes, where in the alkyl groups contained linear waxy paraffinic structures, were introduced. This development encouraged the examination of other waxy materials as candidates, and in 1937, Rohm and Haas patented the first polymeric pour point depressants, polyalkyl methacrylates (PAMAs), again based on waxy alkyl groups.
Over the years, a wide variety of synthetic materials has been introduced commercially as pour point depressants. Chlorinated wax is the most notable example of small molecule chemistry, but most commercial products are moderate to high molecular weight polymers, such as polymethacrylates, polyacrylates, acrylate-styrene copolymers, esterified olefin- or styrene maleic anhydride copolymers, alkylated polystyrene, and vinyl acetate-fumarate copolymers.
Chemical structure of PPDs
Even after 75 years, polyalkyl methacrylates, the first of the polymeric pour point depressants, continue to beviewed as the best chemistry available today, with a worldwide market share that far outstrips alternatives. The primary reason for this widespread preference is the molecular structure of the polymers and the tremendous flexibility in chemical structure. The basic structure of a PAMA pour point depressant is as follows:
Pour point depressing activity is only weakly dependent on polymer molecular weight, and the degree of polymerization (x+y)of PPDs may vary from about 200 to 2000. Most blenders prefer to use a single product in a full range of lubricant applications, therefore molecular weights of most of the product line thus tend to be concentrated in the low end of the range.
PPD selection and treat rate
When new pour point depressants are designed or existing ones are used, it is important to recognize that these additives are themselves waxy materials. Therefore, in the process of adding a pour point depressant to a lubricant, one is in fact adding wax to the system. Hence, the dosage must be carefully selected to get an optimum response, and PPD overtreatment should be avoided to prevent reversion of low temperature properties.
A typical concentration response curve, which can be inferred from a vertical slice of Figure 1, is shown in Figure 3. This particular example happens to be the ASTM D97 Pour Point Test, but the shape of the response curve is again generic. A properly selected pour point depressant will provide a dramatic enhancement of low-temperature performance even at low concentrations. Raising concentration may offer additional, minimal, improvement. Once wax issues arecompletely under control, additional pour point depressant offers no additional benefit and the performance response becomes flat. Further increases in pour point depressant concentration are, in effect, only adding additional wax to the system, leading eventually to a reversal of performance.
The reversal of performance response, resulting from over-treatment with a pour point depressant, as shown in Figure 3, is reasonably straightforward. This phenomenon is called pour point reversion. But there is also a more subtle type of reversal. Many waxrelated phenomena are a function of time and thermal history. Hence, one sometimes finds acceptable performance in a low temperature response, only to discover that control of that property is lost during storage. The wax solution behaves like a supercooled liquid, in which wax crystallization takes place over an extended period of time, thereby converting a fluid system to a gel.
Factors that affect PPD selection and treat rate:
- Base stock
- Other waxy components
- Detergents and friction modifiers
- Viscosity modifiers
- Test methods required by different specifications
- Low-temperature performance reversion
- Aged-oil pumpability requirements
PPD selection guide
The screening of pour point depressants is best conducted on lubricants that are fully formulated but lacking pour point depressants. The base oil for a lubricant is normally a combination of two or more base stocks. A pour point depressant with one level of WIF is optimum for one base stock component, while a different level of WIF is preferred for the other base stock. The situation can become considerably more complex if a third component is used, such as a heavy stock or a bright stock.
Another critical factor in pour point depressant selection is the role of other additive components. Any additive containing a hydrocarbon structure, which is wax-like, canhave a dramatic effect on low-temperature performance. This is because waxy structures forming in a lubricant at low temperature need not be exclusively base-oil derived. These other “wax like” additives can contribute to the wax structure formation to a greater or lesser extent.
A pour point depressant simply cannot be selected based on any single test. Therefore, it is crucial to consider all of the cold temperature requirements of a lubricant, as a passing result in one low-temperature test does not guarantee success in another.
In addition to trade-offs between tests within a specific application, there must also be consideration of the requirements within the viscosity grades of a product line, as well as the requirements across product lines. While it would be possible to define an optimum pour point depressant for every product, this is obviously not practical. It is unusual that a single, “universal” pour point depressant would be optimized for all of the products in a blending plant. However, one pour point depressant will sometimes meet all of a plant’s needs with an up-treat in dosage in certain products.The blender must weigh this option against the logistical issues of handling a second pour point depressant for part of the product mix.
Another essential PPD consideration that has evolved considerably over the past decade is the low temperature performance of engine oils after oxidation. Thus, the PPD not only has to provide adequate control of wax crystallization in the fresh oil, but has to properly control low temperature properties in the aged oil. Since the oxidized oil contains polar molecules that can structure at cold temperatures, the PPD must be able to handle both the crystallization of wax species and the agglomeration of polar molecules.
Dr. Christoph Wincierz
Global OEM Relation Manager & Technical Manager, Evonik Oil Additives
When someone just relies on Pour Point as measure for low temperature fluidity of oils, it means the person underestimates the influence of the cooling conditions on the crystallization of waxes in the oil, in particular the cooling rate. While the Pour Point represents a rapid cooling, in reality the weather changes often slowly and so does the temperature. A history of broken engines and transmissions lead the industry to look not only at Pour Point, but at tests with a slow cooling procedure: MRV-TP1 for engine oils and Brookfield for transmission fluids. We support you with our experience and our laboratory to select the right PPD for your formulations.
Oil Additives Sales and Marketing Specialist
S ince base oils are not enough to perform the task expected from a lubricant, additives are used to improve the properties of the oil, and to avoid undesirable outcomes. One of the most important additivies in this scope is Pour Point Depressant. This additive consist of high molecular weight methacrylate polymer and wax-containing components. It allows the oil to continue to flow at very low temperatures, prevents wax crystallization in the oil, and depresses the pour point. There are two points to pay attention when using this additive in order to get optimum performance. The first one is to choose the right product (considering the base oil and other additives used), and the second is to determine the amount of usage correctly. This material may cause adverse reaction if not used properly. ADCO Petroleum Additives recommend Evonik’s Viscoplex series for reaching the best result.