Refrigeration Lubricants and Antioxidants

Introduction

Customers often ask us to test for antioxidant strength in refrigeration oils. We explain that antioxidants are not typically used in lubricants for refrigeration systems, but why? In this blog, we will explore the reason for this and discuss the general role of antioxidants in lubricants.

Common Antioxidants

While antioxidants are not often formulated into refrigeration system lubricants, they are widely used in other lubrication applications. Antioxidants prevent oxidation, which can lead to sludge formation, acidity buildup, and degradation of oil properties. The most common types of antioxidants found in industrial lubricants include:

1. Aromatic Amines

Aromatic amines, such as diphenylamine, are highly effective antioxidants often used in high-temperature applications. Examples of aromatic amines are shown in Figures 1 and 2. They extend oil life by reacting with and neutralizing free radicals, which propagate oxidation.

·        Radical Scavenging: Aromatic amines donate hydrogen atoms to free radicals, interrupting the chain reaction of oxidation.

·        High-Temperature Stability: They perform well even at elevated temperatures, making them ideal for engines and turbines.

·        Long-Lasting Protection: Provide durable protection under sustained elevated temperatures and heavy loads.

Figure 1

Figure 2

2.     Phenolic antioxidants

Phenolic antioxidants interrupt an oxidation chain reaction at an early stage. They are effective in environments where oil is exposed to air at moderate temperatures. Sterically hindered phenols, like butylated hydroxytoluene (BHT), are a class of antioxidants characterized by bulky substituents around the phenolic group. Figure 3 shows the chemical structure of BHT. This steric hindrance protects the reactive hydroxyl group from undergoing unwanted reactions, especially from interacting with other chemicals prematurely. These antioxidants donate hydrogen atoms to free radicals, stabilizing them, and preventing further oxidation reactions. Oil oxidation can lead to the formation of acids, sludge, and varnish, which degrade the lubricant's performance. The bulky structure of sterically hindered phenols helps prevent oxidation without significantly reacting with other components in the lubricant, ensuring long-term stability and protection against oxidation.

Figure 3

3.     Zinc dialkyldithiophosphates (ZDDPs)

ZDDPs are multifunctional additives that function as antioxidants and anti-wear agents. The structure of ZDDP is shown in Figure 4. They protect oil in more severe conditions, but their use in refrigeration systems is limited due to potential interactions with refrigerants. ZDDP will decompose hydroperoxides and protect metal surfaces from catalyzing further oxidation. Their multifunctional nature not only extends the life of lubricants but also enhances the protection of mechanical components.

Figure 4

Why are antioxidants not used in refrigeration systems?

Low temperatures: Refrigeration oils operate at low temperatures. The rate of oxidation increases with increasing temperature. The Arrhenius equation states that oxidation doubles for every 10⁰ Celsius or 18⁰ Fahrenheit. Therefore, oxidation is reduced to a slow crawl under low temperatures.

Closed system: Refrigeration systems are sealed environments, which minimizes exposure to air and oxygen. Oxidation is required for oxidation to occur so the closed nature of these systems limits the conditions under which oxidation can occur, reducing or eliminating the need for antioxidants.

Potential interactions with refrigerants: Some antioxidants could negatively react with the actual refrigerant in the system. These interactions may produce byproducts and reduce system cooling efficiency.

Let us look at a specific example where an additive can react with the refrigerant in ammonia-based refrigeration compressors, ammonia (a base) can react with phenolic antioxidants (slightly acidic) to form salts. This acid-base reaction can create deposits and system performance issues.

Summary

Antioxidants are not typically used in refrigeration lubricants because operating conditions that necessitate their use, elevated temperatures and exposure to oxygen, are absent in refrigeration systems. The low operating temperatures and sealed environments minimize oxidation, reducing the need for antioxidants. Additionally, potential negative interactions between antioxidants and refrigerants, such as ammonia, can produce undesirable byproducts that compromise system efficiency and longevity. By understanding the specific requirements and conditions of refrigeration systems, we can appreciate why antioxidants, while valuable in other applications, are not suitable here. Refrigeration oils are formulated to perform optimally under the unique conditions of these systems without the need for additional antioxidant additives.