Rolling is a forming process in which metal stock is passed through a pair of rolls to produce sheet, foil, bar or rod. Chad Crocker of Clariant Industrial Lubricants explained, Rolling has two main classifications, flat rolling in which the end product is typically a sheet, and profile rolling in which the end product is typically a rod or bar. Sheet is defined as any flat product that is 0.20 to 6.3-millimeters thick while foil is less than 0.20-mm thick.

Rolling is also classified according to the metal temperature. If the temperature is above the metals recrystallization temperature, the process is termed hot rolling, said Crocker, who is based in Charlotte, North Carolina, U.S. If the temperature is below the recrystallization temperature, it is cold rolling.

Crocker noted that rolling can be done with an emulsion fluid or straight oil. Emulsions have good cooling properties and low fire risk. However, they require continuous monitoring of the formulation, have shorter lifetimes, produce lower quality surfaces and can pose higher health risks.

Rolling oils are easier to maintain because the formulations are simpler and have longer lifetimes. They also produce excellent surface quality and pose lower health risks, he said. On the negative side, they provide less cooling and are a higher fire risk.

Properties of Good Rolling Fluids

The thermal properties of rolling fluids are critical for good performance, Crocker said. Fluids for hot rolling require lubricity additives to maintain adequate film thickness at high temperatures. These additives control roll force, roll wear and surface quality. They also must burn-off cleanly in the annealing process to prevent staining, he said.

In cold rolling, high volatility is essential to minimize surface staining. Therefore, the rolling fluid needs to be volatile and burn off the metal during the rolling process. In addition, additives with insufficient volatility may be prone to drip, creating safety and cleanliness issues.

Crocker explained, One way formulators can tailor a fluid to meet the needs of a specific mill is to determine whats known as the additives inflection point from thermogravimetric analysis. TGA is a thermal analysis method that measures changes in a materials physical and chemical properties as a function of increasing temperature (with constant heating rate), or as a function of time (with constant temperature or constant mass loss).

In the analysis, the peak of the first derivative, known as the inflection point, indicates the point of greatest rate of change in weight loss. Generally, the higher the inflection point, the greater the thermal stability in the tested atmosphere, Crocker said.

Clariant researchers conducted more than 60 TGA analyses on additives commonly used in rolling oils and emulsions to determine their stability and deposit forming tendencies. We examined esters, emulsifiers, phosphate esters, ether carboxylates, amine ethoxylates and corrosion inhibitors, Crocker said.

The results showed that the type of ester had no influence on the amount of deposits. Higher molecular weight had no influence on the inflection point, Crocker said, however, the higher the number of ester groups (polyols) and the more unsaturation present, the higher the inflection point and, therefore, the higher the stability.

The type of emulsifier had no effect on the amount of deposits. However, higher ethoxylation and the presence of fatty acids increased the inflection point and, thereby, stability.

With phosphate esters, the higher the ethoxylation, the higher the inflection point and the lower the deposits, said Crocker. Alcohol size had no effect on deposits, but larger alcohols increased inflection point and stability.

For ether carboxylates, neither ethoxylation nor alcohol size had any influence on inflection point or deposits. However, Crocker said, for amine ethoxylates, higher ethoxylation and larger amine size increased the inflection point, but had no influence on deposits. Finally, for corrosion inhibitors, increasing molecular weight increased the inflection point.

Crocker concluded, In the final analysis, polyol esters produced the highest inflection points, followed by ether carboxylates, amine ethoxylates and alcohol ethoxylates. Amine ethoxylates produced the lowest deposits, followed by alcohol thoxyl-ates, ether carboxylates and phosphate esters.

Emulsions vs. Oils

Blithe, who is based in Valley Forge, Pennsylvania, U.S., explained that a primary reason for using a soluble-oil emulsion in nonferrous rolling is to transport water-insoluble components to the work surface, or roll bite, in a uniform, consistent and cost-effective manner. The water phase of the emulsion cools the mill equipment, especially the work rolls and back-up rolls. The water-insoluble components help reduce roll forces, minimize work-roll wear and produce the required finish or surface quality, Blithe said.

Over the years, nonferrous rolling products have progressed from simple mixtures of simple molecules to complex mixtures of simple molecules. The future of nonferrous rolling lubricant development is moving toward the use of simple mixtures of multifunctional molecules. This represents an innovative approach to emulsion control and the development of new or different control models.

Many early nonferrous hot rolling emulsions were based on simple soluble-oil chemistry that produced very stable emulsions. These emulsions tended to contain a high percentage of emulsifying agents, and they were usually alkali-soap based systems, Blithe said. Oil droplet particle size distribution for these emulsions tended to fall below 1 micron and usually was below 0.5 micron. But while many of these products produced stable emulsions, they often did not provide good lubrication, he added.

The next generation of nonferrous hot rolling products used anionic chemistry with alkanolamine soaps as the principal emulsifier. These products tended to rely upon natural fats, oils and fatty carboxylic acids for both the emulsifying components and the lubrication agents, Blithe said. Lubrication properties were controlled by shifting the emulsion particle size distribution.

Over time, synthetic lubrication components replaced natural fats and oils, which provided better emulsion control and more consistent lubrication. But the basic technology was still an anionic alkanolamine soap that required control of the emulsion particle size distribution to maintain the required lubrication properties, Blithe said.

One of the biggest issues with anionic alkanolamine soaps is that aluminum debris from the rolling process reacts with the carboxylic acids in the emulsion to form metallic soaps. As these soaps build up, they alter emulsion stability, shift the emulsion particle size distribution, change viscosity in the roll-bite and modify lubrication properties, Blithe explained. This creates an out of control emulsion that requires significant or frequent partial dumps and additions of fresh oil to regain control.

Anionic chemistries with minimal amounts of free carboxylic acid were developed to combat this issue. However, said Blithe, the technology still relies on particle size for emulsion control.

Importance of Stability

Patrick Deneuville of Constelliums Research Center in Voreppe, France, explained that a rolling oil emulsion must meet two seemingly contradictory requirements. It must be unstable enough to provide good friction reduction by reacting with the metal, yet stable enough to survive long-term storage in tanks and long idle times in the reservoir.

Deneuville said, The stability of an emulsion depends on the chemical nature and concentration of the emulsifier, other chemical components and contaminants as well as physical conditions such as temperature, duration of shearing and type of shearing. The best way to manage emulsion stability on a rolling mill is a matter of great discussion. On a typical mill, the emulsion varies between static and dynamic states, depending on where it is.

The main question is where to measure stability: at the spray bars, in the tanks, etc. Unfortunately, he said, stability is not constant throughout the system, and there are several ways to measure it.

The traditional way is to measure particle size distribution, which provides an overall value and is easy to obtain. However, Blithe said, Particle size analysis captures information for a specific operating condition or point in time, and particle size changes the longer the emulsion sample sits in a static environment without shear. Therefore, particle size distribution does not always provide information relevant to emulsion stability.

Simple forms of emulsion stability index testing have been used to track changes in emulsion characteristics, Blithe explained. But emulsion stability index tests do not tell the whole story for many emulsion systems used in nonferrous rolling.

New analysis techniques that measure changes in optical density of an emulsion over time in a static sample provide better information about emulsion stability. For example, a Turbiscan instrument from Formulaction can provide a more detailed understanding of the changes that occur in a sample and can be used in combination with particle size distribution analysis, Deneuville said.

Another approach is to measure emulsifier concentration; however, this value does not show the influence of physical conditions such as shear, energy, temperatures, aging, etc. A third method is the so-called plate-out test, which is delicate and does not provide a direct measure of stability. Fourth, Deneuville added, measuring the change in emulsion concentration vs. settling time gives an indication of stability over time, but it is hard to perform.

New Rolling Oil Technology

Many new lubricating agents for nonferrous rolling products have been developed under green technology initiatives, Houghtons Blithe said. These agents use natural or renewable components. But, as with previous technology, many of the emulsions are built on anionic alkanolamine soaps and rely on particle size shifts to control both stability and lubrication. New vegetable oil derived or modified chemistries represent an improvement over natural fats and oils, but the emulsion systems can be difficult to control, he said.

New rolling oil technology has been developed that is not based on carboxylic acids or anionic soap. It minimizes the formation of metallic soap in a rolling emulsion, greatly reducing the negative effects.

Removing anionic alkanolamine soaps required the development of new emulsification packages as part of an innovative rolling oil development program, Blithe said. This new technology delivers consistent lubrication while maintaining the required metal characteristics. It also provides a clean and uniform surface finish at both high and low temperatures.

In the new emulsification system, performance is independent of particle size, and particle size distribution is a secondary factor in emulsion control. The new emulsion control mechanism presents a challenge for many formulators who have worked with traditional anionic chemistry for many years, Blithe concluded. They will have to learn new particle size behavior and emulsion control mechanisms because lubrication properties can now be controlled independent of particle size distribution.