Stearic Acid Uses
Stearic acid is a versatile fatty acid used in a vast array of applications. Predominantly used in rubber processing, stearic acid serves numerous functions in rubber manufacturing, such as internal lubrication and adhesion.
Stearic acid also has functional value in other industries, including textiles, construction, and personal care. How manufacturers use stearic acid depends on the product's concentration and form. The innumerable uses for this naturally derived fatty acid prove its usefulness as an industrial ingredient.
In this article you will learn:
What Is Stearic Acid
Stearic acid, also known as octadecanoic acid, occurs naturally in plants and animals, though it appears more abundantly in the latter than the former. Animal fats can contain up to 30% of their composition of stearic acid and its close constituents, whereas plant sources typically have less than 5%. Cocoa butter and shea butter stand out as two exceptions of plant fats with high levels of stearic acid in it, up to 45%. Stearic acid, considered a non-toxic saturated fat, does not affect human cholesterol levels, making it a possible substitute for trans fats and other saturated fats used in food.
The chemical makeup of octadecanoic acid comes from its name, which alludes to the 18 carbons that make up the backbone of the molecule. Like other saturated fats, stearic acid is solid at room temperature and floats on water. It appears white and has a mild aroma. Some vendors will sell stearic acid in crystalline or powder form. The structure of stearic acid, which includes both polar and non-polar components, allows it to function in the creation of soap or detergent. The polar end links to water, while the non-polar end surrounds organic dirt or fats. The non-polar end encapsulates these particles, which wash away with rinsing to remove the water.
Though edible, stearic acid has many other uses outside the human body. One of the best-known applications in industries is its use in vulcanizing rubber. However, octadecanoic (stearic) acid does much more than help in the creation of tires and industrial grade rubber. Its uses extend to several other industries, aided by stearic acid's softening abilities, inherent lubricity and non-toxic nature.
Applications of Stearic Acid
Stearic acid's attributes of activating and lubricating allow this fatty acid to play a key role in several sectors. Its most frequent industrial use is for the creation of rubber. Without this or similar fatty acids, modern vulcanization methods would look quite different.
Activators and accelerators do different things chemically. Stearic acid appears as an activator triggering the reactions of the accelerators. Understanding the differences helps to know why both components are crucial to success for certain processes.
An accelerator is a chemical added to a process to speed the reaction. Accelerators are critical in industrial applications during which waiting for sluggish reactions wastes time and money. The type of accelerator used depends on the reaction that requires hastening. Sometimes, chemists use multiple accelerators for the best results.
Activators start a process, whether that process uses accelerators or not. Stearic acid acts as an activator for catalyzing process chemicals. Often, stearic acid reacts with zinc oxide to serve as an activator, while having secondary lubricating properties as a functional metallic stearate.
When activators work in conjunction with accelerators, chemical processes occur more predictably and quickly.
Internal lubricants enable molecules to move easier between each other. When this happens, the product flows easier due to lower viscosity. Fatty acids, with their polar and non-polar components, function as internal lubricants due to their differing polar parts.
With polar polymers, a fatty acid coats fillers and aids in the movement of stabilizers in the polymer. By these means, fatty acids can make products they are part of disperse and flow more smoothly by reducing the viscosity.
Just as stearic acid improves flow and viscosity as an internal lubricant, it also acts as an external lubricant.
Lubrication occurs when a substance, usually fat or oil, coats a pair of surfaces, allowing them to move smoothly over one another. As a fatty acid, stearic acid can function as a topical lubricant for metals, though it requires a high number of layers to remain active through multiple uses and not wear off.
Though this particular single use as an external lubricant proves useful in some situations, stearic acid can also be a dual-purpose agent in facilitating movement. If a polymer already has a plasticizer added, fatty acids in the mixture become external lubricants. To ensure proper use as a surface treatment for aiding in releasing products from molds, engineers add stearic acid to a polymer in higher amounts, compared to when it only has a use as an internal lubricant.
Why it is Used in Rubber Products
Attributes of stearic acid rubber grade that make it useful across multiple industries also make it an essential component of rubber production. The actions of stearic acid as an activator and lubricant yield benefits for the rubber industry of aiding in vulcanization and improving the texture of the finished products.
When mixing in other ingredients, the ability of those components to disperse evenly is vital for the quality of the results. Adding stearic acid as a secondary dispersant will disperse the filler ingredients in the rubber. While this activator component helps keep the molecules spread throughout the rubber, too much will cause problems.
If a rubber has too high of a dose of zinc stearate created during activation, the resulting rubber will have an excessive breakdown that does not allow for even distribution of carbon black to the mix. Without the protective effects imparted by stearic acid, rubber will have a shorter lifespan due to the ease of physical deterioration.
As a molecule with polar and non-polar parts, stearic acid assists in evenly spreading molecules through rubber and improving the material's viscosity. Too much is never good, though, because excessive amounts of stearic acid in the mixture will lead to the opposite of the desired physical traits. Not exceeding recommended levels of zinc oxide and stearic acid will prevent these severe problems that negatively impact the finished rubber.
Stearic acid for rubber applications provide lubrication and improved viscosity, as well as a softer texture for the finished product. The amount of stearic acid determines the final thickness of the rubber. While a little bit aids in lubrication and activation, indexing too high can have the opposite effect.
Using too much stearic acid can make rubber too hard. This result likely happens because this stearic acid creates crosslinks of the macromolecules. When it creates too many links, the rubber becomes harder. This increase in hardness corresponds to a rise in tensile strength. However, while hardness continues to increase linearly with the amount of stearic acid added, strength rises, then drops, with amounts of stearic acid more significant than 4 phr.
Stearic acid, though, can increase the ability of rubber to stretch or elongate. Elongation effects a negatively trending linear relation to the amount of stearic acid used. Without stearic acid, rubber would not be as stretchy or soft as needed, but too much stearic acid decreases the amount of stretch of the material.
Vulcanization describes the process of transforming rubber materials into more useful materials for specific purposes. Three chemical pillars exist for this process — activators, vulcanizers, and accelerators. In this series, stearic acid combined with zinc oxide creates an activator.
Generally, in rubber production, the proportions to use for stearic acid and zinc oxide are two and five parts per hundred rubber (phr), respectively. The resulting creation of zinc stearate kicks off the accelerators used to make the vulcanization process happen faster.
Zinc stearate also plays a role in aiding in the final product of the vulcanization. The use of zinc stearate assists in promoting the creation of crosslinking between macromolecules of the rubber by reacting with the accelerator and sulfur in the rubber.
Properties of stearic acid that make it an ideal activator for vulcanization are its molecular weight, melting point, and its solubility in the rubber. Other forms of fatty acid, such as oleic acid, do not perform as well as stearic acid when used as activators for vulcanization. Rubbers activated with zinc stearate show higher crosslinking density and rubber strength compared to oleic acid-activated products.
For activating a sulfur-based vulcanization process, accelerators dithiocarbamate and thiazole typically require stearic acid and zinc oxide to enable them. The latter two also function as accelerators by helping hasten vulcanization.
While its primary use is as an activator, stearic acid also contributes to the texture and mixing of rubber.
Other Uses of Stearic Acid
While rubber production is one of the most common uses of this acid, many other industries use stearic acid for other applications. As a non-toxic substance, it frequently plays a role in personal care product manufacturing. Its fatty nature makes it an ideal lubricant and softener for plastics and textiles.
Industries using stearic acid include
Personal and household cleaning products, including many soaps, frequently use stearic acid as an ingredient. With its chemical design of a non-polar chain and polar head, each molecule of stearic acid can link oil and water that do not normally blend, acting as a bridge between oil and water phases. It also can trap dirt and other organic materials for later cleaning off or physical removal.
In construction, cement grinding often uses stearic acid to ensure the ideal particle size. During the grinding phase of cement powder, adding small amounts of stearic acid provides for a finer grind and improved compression strength of the finished concrete. In a study on the amounts of stearic acid needed to optimize compression strength, the results showed a curve with peak strength occurring when using 0.1% stearic acid and dropping at higher and lower amounts.
In oilfield applications, stearic acid serves as one of the ingredients in the salt stearate. Like similar fats, stearic acid combines with other elements to create lubricants, drilling fluid additives, wetting agents, and emulsifiers. Stearic acid, as well as other fatty acids, are valuable for their versatility in oilfield applications
The personal care industry uses stearic acid as a component of skin lotions, mascaras, cosmetics, and shaving creams. Multiple studies have shown stearic acid does not cause skin irritation or skin sensitivity, even at percentages as high as 10% by solid weight of the formula.
Stearic acid is not a carcinogen in humans. Stearic acid side effects are minimal enough to not be significant statistics reported in human studies.
As a fatty acid, similar to those humans must consume daily, stearic acid is non-toxic. Dangers from ingesting extreme amounts of this fat could only lead to intestinal obstruction rather than toxicity. The probable fatal dose for this fat is more than one quart for a 150-pound human.
Soaps and body washes take advantage of this stearic acid’s inherent surfactant property. This attribute gives it the ability to grab dirt for easier removal. In addition to binding dirt, stearic acid also acts as an emulsifier, uniting the oil and water parts of personal care products and allowing them to bridge and blend when they would otherwise separate. This quality preserves the texture of many cosmetics and cleansers.
Just as in other sectors, stearic acid acts as a lubricant and softener during processing in the plastics industry. It also serves as a heat stabilizer for the production of polyvinyl chloride.
In the plastics industry, a novel use of this fatty acid is in reducing damage from degenerative forms of water in wood-plastic composites. One study showed treatment of a surface of a wood-plastic composite with stearic acid greatly improved the material's resistance to acid rain and saltwater. The strengthening effects of the fatty acid's addition help overcome one of the industry's concerns about fully embracing wood-plastic composites as an eco-friendly alternative to other construction materials.
The textile industry uses stearic acid to improve the water-repelling quality of fabric. The acid functionality mixes with formaldehyde and melamine to create components that make the material last through more launderings. Stearic acid also improves the softness of the material, also known as the fabric's hand. However, using stearic acid as a method of extending the life of clothing has a handful of disadvantages, such as increasing the likelihood of tearing or fading fabric dyes.
Brenntag North America can fulfill your stearic acid needs
With jobs of activating, softening, and dispersing ingredients in rubber, stearic acid is a critical component in the production of rubber. Contact us today if you want information on obtaining stearic acid for your manufacturing use.
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