
Zinc stearate, or zinc soap, is a zinc salt of stearic acid with the chemical formula (C17H35COO)2Zn. It exists in the form of a white fine powder, has a characteristic odor. The substance is insoluble in polar organic liquids such as alcohol and ether, insoluble in aromatic hydrocarbons (e.g. benzene) when heated. Main areas of use are plastic and rubber industries as a lubricating and dispersing agent. The compound also acts as an "activator" to accelerate the vulcanization of rubber. It is available in various forms:
-
granules
-
powder
-
cereals
The most used is the powdered form.
The structural formula.
Production
There are two ways to produce zinc stearate:
-
the interaction of stearic acid with Zn oxide
-
metabolic decomposition of sodium stearate with Zn chloride
For the first time, zinc soap according to the first method was obtained in 1934 by reacting a solution of stearic acid with an excess of Zn oxide:
2C17H35COOH | + Zn0 | → (C17H35COO)2Zn | + H2O |
Stearic acid | Zinc Oxide | Zinc Stearate | Water |
In the same year, the substance was obtained in the form of a fine powder in the same way, but using a solvent (butanol, toluene, butyl acetate).
The reaction is usually carried out at a temperature of 120-150 ° C without solvent to increase the yield of the product. A stoichiometric amount of zinc oxide is introduced into the stearic acid melt at 85-100 ° C and stirred, heated to 120-150 ° C and maintained until the release of water stops.
Zn oxide dispersed in molten zinc stearate is used to obtain highly dispersed zinc stearate. The mixture of oxide and stearate is stirred at 120 ° C until a suspension is obtained and molten stearin is gradually added at 125-130 ° C. The reaction lasts 30 minutes. The resulting transparent melt is sprayed upon cooling to obtain a fine powder, and part of the resulting zinc stearate is returned to the cycle.
According to the second method, the zinc salt of stearic acid in industry is obtained by the reaction of the exchange decomposition of sodium stearate with Zn chloride:
2C17H35COONa | + ZnCl2 | → (C17H35COO)2Zn | + 2NaCl |
Sodium Stearate | Zinc Chloride | Zinc Stearate | Sodium chloride |
A solution of zinc chloride is mixed with an excess of sodium stearate to obtain an aqueous dispersion of zinc stearate.
Instead of chloride, other salts (for example, sulfate) can be used. NaOH and stearic acid, melted at 65-70 ° C, are added to the ZnSO4*H2O solution, then water is supplied to exclude emulsification, Zn(OH)2 solution and the resulting zinc stearate is filtered out.
Safety
The compound is flammable under drying conditions, zinc stearate mixed with dust is explosive. Zinc stearate is slightly toxic, it is allowed to stabilize polyvinyl chloride products in contact with food products that do not contain fats, and for PVC pipes in drinking water supply systems with limited dosages.
Properties and applications
Physical characteristics:
Melting point | 120-130°C |
Density | 1.1 г/см³ |
Molecular weight | 632,33 |
Flash point | 277°C |
Self-ignition temperature | 420°C |
Basic chemical characteristics:
-
Stable at high temperatures
-
Waterproof
Zinc stearate is a white amorphous powder, melts at a temperature of 120-130 ° C. It does not dissolve in water. It is soluble in the following liquids:
-
benzene
-
methanol
-
ethanol
-
toluene
-
esters
-
carbon tetrachloride
-
ketones
It has an ultra-thin structure and is well compatible with distributions. It also has heat resistance, yellowing resistance, dries quickly and is well sanded.
The main areas of use of zinc stearate:
-
Vulcanization of rubber
-
Production of polymers and plastics
-
Pharmaceutical industry
-
Manufacture of cosmetics
-
Metallurgy
-
Paint and varnish industry
-
Production of concrete
-
Lubricants for extruders
Areas of application
Among all metal soaps, zinc stearate is the most effective mold remover. It does not contain electrolyte salts and has a hydrophobic effect. It is used as a secondary stabilizer for PVC and a polymer technological additive for polymer operation at high temperatures. It is used as an acid absorber in polymer processing:
-
polyethylene
-
polyurethane
-
polystyrene
The zinc soap melt has a lower viscosity compared to other stearates. It spreads quickly over the metal surface and ensures effective separation of manufactured products from them, so the substance is used as an anti-adhesive for the production of many types of products:
-
rubbers
-
polyurethane
-
polyester processing systems
-
in powder metallurgy
In the paint industry, finely dispersed zinc stearate is used to produce a pigment suspension and to increase the stability of paint dispersions. In lacquers, it is used as a sanding agent that makes wooden surfaces smooth.
The construction industry uses zinc soap as a hydrophobizer for plasters.
In cosmetics, it is used as a lubricant and thickener to improve texture.
Being lipophilic, it acts as an interphase catalyst for saponification of fats.
The compound for stabilizing PVC in its pure form is rarely used, since it individually has low efficiency, but the substance is widely used in combination with
-
calcium carboxylates
-
magnesium carboxylates
-
epoxidized soybean oil
In the production of synthetic rubbers, it can also be used as an antiagglomerator (prevents particles from sticking together) in the process of water degassing.
In the production of polymers
The zinc salt of stearic acid is used in the polymer industry as a heat stabilizer and lubricant. Thus, PVC stabilizers control the thermal decomposition of polyvinyl chloride (PVC) compounds, which are later used for packaging food and pharmaceutical products. The substance is also used in the manufacture of crystalline polystyrene or other transparent polymers and as a separating agent in polyurethane.
Lubricants for extruders
Zinc stearate 0.3-1.0% of the total amount of the mixture is added to increase the productivity of the process during extrusion production and lamination of plastics and metals. It acts as a solid additive as well as a lubricant. The compound remains stable at high temperatures and gives the product uniformity and saves energy and raw material consumption in industrial synthesis.
Getting rubber
Zinc stearate acts as an anti-adhesive agent and is especially useful for reducing adhesion. It can also work as an anti-adhesive for molds, a rubber powder and a lubricant that prevents rubber from sticking to the mold.
Zinc stearate is an "activator" of vulcanization, since Zn has a beneficial effect on the reaction of sulfur with polyolefin.
Zinc stearate facilitates dispersion because it dissolves well in the nonpolar part of polyolefins.
It acts as an antacid with polyolefins, thereby contributing to color stability and preventing corrosion.
Pharmaceutical industry
The zinc salt of stearic acid, in accordance with GOST, is used in the pharmaceutical industry in the manufacture of tablets and capsules as a lubricant. Sometimes it can be part of:
astringent solutions
mild antiseptics
dental cement
Getting concrete
The use of zinc stearate in the construction industry is effective, as it acts as a waterproofing agent to improve the hydrophobicity of concrete and cement.
Metal processing
Zinc soap is an excellent lubricant. It is a high-quality lubricant additive for processing
-
metals
-
wires
-
stainless steel
-
aluminum and its alloys
At the pressing stage, lubricants improve the flow and compressibility characteristics, while binders have the opposite effect. Different amounts of the additive affect the porosity and final properties of the material.
The advantage of this compound over other lubricants is that it provides an alloy with Zn, which is useful in some processes.
Cosmetic industry
Zinc stearate can be used as an adhesive agent to create sealants and anti-slip adhesives for the manufacture of cosmetics. It is used as a lubricant, as well as as a thickener and for bonding the liquid and oily parts of a cosmetic product, improving the final appearance of the product. It does not harden, flows very well and gives the mixture a silky feeling. Zinc stearate is usually included in the composition:
-
eye shadow
-
mascara
-
lipsticks
-
powders
It can also act as a cosmetic dye.
Paint and varnish industry
The zinc salt of stearic acid is one of the most common coating materials with many useful functions. In its pure form, it is transparent, which is why it is used in the production of paint and varnish materials. In the production of paints and varnishes, the compound is used as a matting and grinding agent, making the coatings smooth and pleasant to the touch, as well as to prevent softening of the film caused by an increase in temperature during mechanical polishing. As a grinding agent, this substance improves the ability to mold wood coatings. In the manufacture of floor and wall coverings, in combination with other additives, zinc soap is used as an adhesive. It disperses pigment well, acts as a sealant for surface defects and as a thickener. Protects the paint from moisture due to its waterproofing ability.
Other areas of use
The compound can also be used as a dietary supplement in the form of a very small percentage (less than 1%).
Also, zinc stearate is sometimes used as a defoamer, an abrasive material, as a waterproofing agent for
-
papers
-
fabrics
-
concrete
-
asbestos