Rapidly increasing automation in global food production is leading to higher output, higher loads and higher performance requirements for food processing, packaging and preservation equipment. In this fast-paced industry with demanding operating conditions, there is a growing appetite for H1 food machinery greases that meet elevated quality and productivity needs.
One solution is the addition of polytetrafluoroethylene powder, originally branded by DuPont as Teflon, to these greases, said K. Babu Prasad, deputy general manager for research and development at Mumbai-based lubricants manufacturer Apar Industries. Automation requires a high level of machinery and operations, he said. From tractors harvesting crops to production lines in a factory, food-grade greases need to function in environments where metal-to-metal surface contact, extreme temperatures, high loads, water ingress, corrosion and dry or wet climate conditions are possible.
In addition, they must withstand contamination from sources including process water, steam and high-pressure water during cleaning, as well as chemicals, acids and substances such as sugar present in the manufacturing process, Prasad added.
All of these conditions have led manufacturers to not only focus on choosing the appropriate food-grade grease, but also turn their attention toward additives that provide durability and increase performance, Prasad explained during the annual conference of the National Lubricating Grease Institutes India Chapter in Varanasi, India.
Thickeners that can be used in the production of H1 food-grade greases are limited to polyurea, aluminum complex and calcium sulfonate complex, Prasad said. Anhydrous calcium and sodium complex soaps have lower drop points and less structural stability compared to other thickening agents, and lithium is an irritant chemical and is not useful for food-grade products, Prasad later told LubesnGreases.
Polyurea grease is the first choice for these applications, he asserted at the conference held in February. Despite higher cost of production compared to other greases, polyureas manufactured using H1 base oils such as esters and polyalphaolefin are ashless, inert, have high thermal stability and offer longer life to industrial ball and roller bearings, generator and electric motor bearings, harvester machines, high-speed spindles and automization equipment for food such as meat, fish, vegetables and animal feeds, Prasad highlighted.
Polyurea is an organic polymer resulting from the reaction of isocyanates with an amine-terminated polyether resin, forming a rubber-like compound. Its properties make it ideal for applications where water resistance, wear protection and durability are fundamental, he noted.
At present, polyurea greases are produced in-situ by a step-growth polymerization reaction of different isocyanates and amines in mineral or synthetic base oils. However, the toxicity of these raw materials can cause problems for grease makers during production.
Prasad noted that grease manufacturers have simplified the process by using commercially-available polyurea, which is prepared separately, pulverized and dispersed in the lubricating oil to produce the grease. Regardless of the production method, the finished greases can then be safely used in food processing applications.
But the properties of polyurea grease alone wont suffice to meet the diverse requirements of the dynamic and fast-paced food industry, said Prasad. The addition of an extreme pressure additive helps the grease decrease wear in parts exposed to very high pressures by depositing a protective barrier on surfaces with metal-to-metal contact.
There are various EP additives that can be used in food-grade polyurea grease, such as active and inactive sulfurized fat, active sulfur hydrocarbon, sulfur-phosphorus, poly-sulfides, certain molybdenum compounds and micronized PTFE powder, Prasad stated. He cautioned that sulfur-phosphorus and molybdenum additives may not be ideal in food processing applications if their usage goes beyond certain levels.
These are reactive chemicals, and molybdenum falls under the heavy metal category, Prasad clarified. Hence, they are not suitable for food grade applications. He pointed out that both additives have a tendency to emit odor and cause discoloration.
Micronized PTFE is commonly used for H1 lubricants and grease. PTFE is a synthetic, nontoxic fluoropolymer of tetrafluoroethylene. Better known by brand names like Teflon, Microflon, Algoflon and Syncolon, PTFE is distinguished by its slippery surface due to a low friction coefficient, high melting point, and for being inert and non-reactive with most chemicals, Prasad noted.
These properties have made it a suitable choice as the coating on nonstick cookware and many industrial products, including bearings, pipe liners and parts for valves and pumps, Prasad said in his presentation. PTFE also has good electric insulation properties and is used to insulate cables and connector assemblies.
Micronized PTFE, also known as PTFE micro powder, is prepared by molecular weight reduction through electron beam radiation and thermal cracking. The E-beam radiation technique also sterilizes the substance without eliminating its inherent properties, said Prasad. After irradiation and high pressure air micronization, the high-molecular weight PTFE becomes a fine micronized powder, which can have different grades based on particle size. A number of PTFE micronized powders are registered with NSF International as HX1 additives for use where incidental contact with food may occur.
Apar tested several EP additives in three synthetic oil based polyurea grease samples with 4.5 percent micronized PTFE powder, 3 percent sulfur-phosphorus and 4.5 percent molybdenum disulfide. All were compared against a sample of plain polyurea grease to measure load carrying, antiwear, oil separation, heat stability and corrosion resistance properties.
Using the Institute of Petroleums IP 239 method for load carrying ability revealed the weld load capacity of the plain polyurea grease was 200 kilograms. With PTFE, the samples load capacity increased to 620 kg, compared with 500 kg recorded by the molybdenum disulfide grease and 420 kg from the sulfur-phosphorus sample, said Prasad.
The company also measured antiwear properties of the additives using ASTM Internationals D2266 method. Tests revealed that the plain polyurea grease had a wear scar profile of 0.6027 millimeters in diameter, compared to a 0.588 mm scar in the PTFE sample.
The addition of PTFE has not deteriorated antiwear properties, but increased its value, Prasad noted. Similarly, the scar diameter was 0.6027 mm for the molybdenum disulfide and 0.6011 mm for the sulfur-phosphorus sample.
The oil separation tendency of the grease samples was tested using two different methods. Storage stability of the greases was measured using ASTM D1742 at 40 degrees Celsius. The test revealed oil separation for plain polyurea grease was 1.04 percent. For PTFE-added grease, it was 0.64 percent; for molybdenum disulfide it was 0.80 percent and for sulfur-phosphorus, 0.88 percent.
When lubricating grease separates oil, the remaining composition changes its consistency. This can affect the ability of the product to function as designed, Prasad elaborated. The addition of PTFE to polyurea grease also showed improvement in preventing oil separation in elevated temperatures.
Heat stability of the greases was measured using the ASTM D6184 method at 100 C. Oil separation for plain polyurea grease was at 3.37 percent. For PTFE, it was 2.36 percent. The molybdenum disulfide and sulfur-phosphorus grease samples showed oil separation of 2.76 percent and 2.80 percent, respectively.
Further, Apar exposed the grease samples to 200 C in a muffle furnace for three hours to test their extreme pressure characteristics, using the IP 239 method. Prasad stated there was no remarkable change in the weld load parameter of any of the samples except for sulfur-phosphorus, which indicated some drop in performance variables under prolonged exposure to high temperature.
The company also conducted a salt spray test using ASTM B117 to measure the greases reactiveness to metals and resistance to corrosion. Researchers applied 1.5 grams of plain polyurea grease and PTFE-added grease uniformly over steel plates of fixed dimensions. The plates were exposed to salt water solution in a salt spray chamber for 10 days and evaluated for rust and uniformity of the grease. Prasad said the test showed the particle distribution and coating was uniform for both samples.
Prasad concluded that polyurea grease with micronized PTFE additive can have higher temperature performance, inherent antioxidative properties and higher load-bearing capacity. It also improved the structural stability, which enhanced the storage and heat resistance properties of polyurea grease.