Propylene glycol is a colorless, odorless, and slightly viscous liquid commonly used in the food, pharmaceutical, and cosmetic industries as a solvent, humectant, and vehicle for active ingredients. However, propylene glycol is susceptible to oxidation, which can form acidic byproducts that can cause corrosion, fouling, and degradation of the equipment and the products. To address this issue, manufacturers may add inhibitors to propylene glycol to prevent or slow down oxidation and preserve its stability and performance. This blog post will discuss what inhibited propylene glycol means, how it differs from regular propylene glycol, and its applications, benefits, and limitations.

What is inhibited propylene glycol?

Inhibited propylene glycol is treated with one or more chemical compounds, known as inhibitors, that can protect it from oxidation and prolong its shelf life. Inhibitors work by scavenging or neutralizing free radicals, which are highly reactive species that can trigger and propagate oxidation reactions. By preventing or slowing down oxidation, inhibitors can maintain propylene glycol’s physical and chemical properties, such as pH, viscosity, density, and conductivity, and reduce the risk of corrosion, scaling, and fouling in the equipment and systems that use propylene glycol.

How is inhibited propylene glycol made?

Making inhibited propylene glycol involves blending propylene glycol with one or more inhibitors, which can be organic or inorganic compounds. The selection and concentration of inhibitors depend on the specific application and the desired level of protection against oxidation. Some common inhibitors used in inhibited propylene glycol are:

• Organic acids, such as benzoic acid, citric acid, and phosphoric acid, which can donate hydrogen ions and form stable complexes with metal ions that promote oxidation.
• Nitrites and nitrates, such as sodium nitrite and potassium nitrate, which can react with free radicals and form nitric oxide, a potent antioxidant that inhibits oxidation.
• Amines, such as monoethanolamine and diethanolamine, which can scavenge free radicals and form stable amine radicals that are less reactive than the original ones.
• Phenols, such as butylated hydroxytoluene (BHT) and propyl gallate, which can trap free radicals and form stable phenoxyl radicals that are less reactive than the original ones.

The inhibited propylene glycol mixture is then tested for its physical and chemical properties, as well as its stability and compatibility with other ingredients and materials.

What are the applications of inhibited propylene glycol?

Inhibited propylene glycol has many applications in various industries, such as:

• HVAC (Heating, Ventilation, and Air Conditioning) systems, where it is used as a heat transfer fluid and an antifreeze agent to prevent freezing and corrosion in the pipes, coils, and pumps. Inhibited propylene glycol can also improve the efficiency and lifespan of HVAC systems by reducing the buildup of mineral deposits and organic matter that can clog the system and reduce the heat transfer rate.
• Food and beverage processing, where it is used as a solvent, emulsifier, and preservative for flavors, colors, and other ingredients. Inhibited propylene glycol can also enhance food products’ texture, stability, and shelf life by preventing bacterial growth, mold formation, and dehydration.

What are the benefits of using inhibited propylene glycol?

The benefits of using inhibited propylene glycol depend on the specific application and the type and concentration of inhibitors used. Some of the general benefits of inhibited propylene glycol are:

• Enhanced stability: Inhibited propylene glycol is less likely to degrade or form acidic byproducts that can damage the equipment and products it comes into contact with. This can lead to fewer maintenance issues, longer equipment life, and higher product quality.
• Improved heat transfer: Inhibited propylene glycol has better thermal conductivity and viscosity than water, which makes it a more efficient heat transfer fluid in HVAC systems. This can result in faster heating and cooling times, lower energy consumption, and more consistent temperature control.
• Reduced corrosion and scaling: Inhibited propylene glycol contains corrosion inhibitors that can prevent the formation of rust and other metal oxides that can clog the system and reduce the flow rate. It can also reduce the buildup of mineral deposits and organic matter that can cause scaling and fouling.
• Increased safety: Inhibited propylene glycol has a lower toxicity and flammability than some other antifreeze agents, such as ethylene glycol, which makes it a safer choice for use in food, pharmaceutical, and personal care applications.

What are the limitations of using inhibited propylene glycol?

While inhibited propylene glycol has many advantages, it also has some limitations that should be considered before use. Some of these limitations are:

• Higher cost: Inhibited propylene glycol is generally more expensive than regular propylene glycol, due to the additional cost of inhibitors and the complexity of the manufacturing process.
• Limited compatibility: Inhibited propylene glycol may not be compatible with all materials, such as certain elastomers, plastics, or metals, due to the specific chemical interactions between inhibitors and these materials. It is important to check the compatibility of inhibited propylene glycol with the equipment and materials it will come into contact with.
• Reduced heat transfer efficiency: Inhibited propylene glycol has a lower heat transfer coefficient than water, which means that it may require higher flow rates or larger heat exchangers to achieve the same cooling or heating effect. This can increase the energy consumption and the equipment cost.
• Reduced effectiveness at low temperatures: Inhibited propylene glycol has a lower freezing point than water, but it may still solidify or gel at extremely low temperatures, which can impair its antifreeze performance and cause equipment damage. It is important to choose the right concentration and type of inhibited propylene glycol for the specific temperature range of the application.

What are some examples of inhibited propylene glycol products?

There are many brands and types of inhibited propylene glycol products available on the market, each with its own formulation, concentration, and specifications. Some examples of inhibited propylene glycol products are:

• DOWFROST: DOWFROST is a trademarked inhibited propylene glycol product manufactured by Dow Chemical Company. It contains a blend of organic and inorganic inhibitors that provide corrosion protection and freeze protection down to -50°F (-46°C). It is commonly used in HVAC systems, food and beverage processing, and pharmaceutical applications.
• Dynalene PG: Dynalene PG is an inhibited propylene glycol product manufactured by Dynalene Inc. It contains a blend of organic acids and nitrites that provide corrosion protection and freeze protection down to -60°F (-51°C). It is commonly used in HVAC systems, geothermal heating and cooling, and solar thermal applications.
• Solarol: Solarol is an inhibited propylene glycol product manufactured by The Dow Chemical Company. It contains a blend of inhibitors that provide corrosion protection and freeze protection down to -50°F (-46°C). It is specifically designed for use in solar thermal systems, where it can prevent freezing and overheating of the collector and fluid.
• SIERRA: SIERRA is an inhibited propylene glycol product manufactured by SIERRACHEM. It contains a blend of organic and inorganic inhibitors that provide corrosion protection and freeze protection down to -50°F (-46°C). It is commonly used in HVAC systems, food and beverage processing, and pharmaceutical applications.

How can I determine the appropriate concentration of inhibited propylene glycol for my application?

The appropriate concentration of inhibited propylene glycol for your application depends on several factors, such as the desired freeze point, the specific heat transfer requirements, and compatibility with the equipment and materials. Generally, the higher the concentration of inhibited propylene glycol, the lower the freeze point and the better the corrosion protection, but also the higher the viscosity and the lower the heat transfer efficiency. A typical range of inhibited propylene glycol concentration is between 30% and 60% by weight. It is recommended to consult the manufacturer’s recommendations and technical data sheets and to perform compatibility and performance tests before using inhibited propylene glycol.

How can I maintain and dispose of inhibited propylene glycol?

The maintenance and disposal of inhibited propylene glycol depend on the specific application and the local regulations. In general, it is recommended to periodically test the physical and chemical properties of inhibited propylene glycol, such as pH, color, and inhibitor concentration, and to replace it when necessary. It is also important to monitor and control the pH and alkalinity levels of the system to avoid acidic conditions that can promote oxidation and corrosion. Disposal of inhibited propylene glycol should follow the guidelines and regulations of the local authorities and may involve treatment, recycling, or disposal in a designated hazardous waste facility.

Conclusion

Inhibited propylene glycol is a versatile and effective compound that can provide corrosion protection, freeze protection, and thermal stability in various applications. Its benefits include improved equipment performance, product quality, and safety, but it also has some limitations, such as higher cost and limited compatibility. It is important to choose the right concentration and type of inhibited propylene glycol for the specific application and to perform maintenance and disposal according to the guidelines and regulations

FAQ

What is propylene glycol?

Propylene glycol is a colorless, odorless, and slightly viscous liquid commonly used in the food, pharmaceutical, and cosmetic industries as a solvent, humectant, and vehicle for active ingredients.

How is propylene glycol different from ethylene glycol?

Propylene glycol is less toxic and safer for use in food, pharmaceutical, and personal care applications than ethylene glycol, which is a poisonous and flammable antifreeze agent that can cause severe health effects when ingested or inhaled.

What is oxidation and why is it a problem for propylene glycol?

Oxidation is a chemical reaction that involves the loss of electrons and the formation of free radicals, which can cause degradation, corrosion, and fouling of the equipment and products that come into contact with propylene glycol. Oxidation can also cause the formation of acidic byproducts that can lower the pH and damage the system.

What are inhibitors and how do they work?

Inhibitors are chemical compounds that can prevent or slow down oxidation by scavenging or neutralizing free radicals, which are highly reactive species that can trigger and propagate oxidation reactions. Inhibitors can maintain the physical and chemical properties of propylene glycol, such as pH, viscosity, density, and conductivity, and reduce the risk of corrosion, scaling, and fouling in the equipment and systems that use propylene glycol.

What are the benefits of using inhibited propylene glycol?

Inhibited propylene glycol has several benefits, including enhanced stability, improved heat transfer, reduced corrosion and scaling, and increased safety. These benefits can lead to fewer maintenance issues, longer equipment life, higher product quality, and safer use in various industries.

What are the limitations of using inhibited propylene glycol?

Inhibited propylene glycol has some limitations, including higher cost, limited compatibility, reduced heat transfer efficiency, and reduced effectiveness at low temperatures. These limitations should be considered before use and the appropriate concentration and type of inhibited propylene glycol should be chosen for the specific application.

What are some examples of inhibited propylene glycol products?

There are many brands and types of inhibited propylene glycol products available on the market, including DOWFROST, Dynalene PG, Solarol, and SIERRA. Each product has its own formulation, concentration, and specifications, and should be chosen based on the specific requirements and conditions of the application.

How can I determine the appropriate concentration of inhibited propylene glycol for my application?

The appropriate concentration of inhibited propylene glycol for your application depends on several factors, such as the desired freeze point, the specific heat transfer requirements, and the compatibility with the equipment and materials. Generally, the higher the concentration of inhibited propylene glycol, the lower the freeze point and the better the corrosion protection, but also the higher the viscosity and the lower the heat transfer efficiency.

How can I maintain inhibited propylene glycol?

Inhibited propylene glycol should be periodically tested for physical and chemical properties, such as pH, color, and inhibitor concentration, and replaced when necessary. The pH and alkalinity levels of the system should also be monitored and controlled to avoid acidic conditions that can promote oxidation and corrosion.

How can I dispose of inhibited propylene glycol?

The disposal of inhibited propylene glycol should follow the guidelines and regulations of the local authorities and may involve treatment, recycling, or disposal in a designated hazardous waste facility.