The rubber industry, in its development over the past one hundred years, has utilized an increasing number of the many optical, physical and chemical properties of Zinc Oxide. Zinc Oxide proved the most effective activator to speed up the rate of cure with the new accelerators. Heavy-duty pneumatic tires carry high loadings of Zinc Oxide for heat conductivity as well as reinforcement since heat-buildup is critical at their higher operating speeds compared with their solid-rubber counterparts.
In the curing process for natural rubber and most types of synthetic rubbers, the chemical reactivity of Zinc Oxide is utilized to activate the organic accelerator. The unreacted portion of the Zinc Oxide remains available as a basic reserve to neutralize the sulfur-bearing acidic decomposition products formed during vulcanization. Adequate levels of Zinc Oxide contribute markedly to chemical reinforcement, scorch control, and resistance to heat-aging and compression fatigue.
Zinc Oxide serves as the accelerator with some types of elastomer and the crosslinking, which it induces, takes several forms. With some systems, Zinc Oxide is an effective co-accelerator in the vulcanization process.
Biochemical Activity
Zinc Oxide is useful in the preservation of plantation latex as it reacts with the enzyme responsible for the decomposition. The oxide is also a fungistat, inhibiting the growth of such fungi as mold and mildew.
Dielectric Strength
In high-voltage wire and cable insulation, Zinc Oxide improves the resistance to corona effects by its dielectric strength, and at elevated operating temperatures it contributes to maintenance of the physical properties of the rubber compound by neutralization of Acidic decomposition product.
Heat Stabilization
Zinc Oxide similarly retards devulcanization of many types of rubber compounds operating at elevated temperatures.
Latex Gelation
In the production of latex foam rubber products, Zinc Oxide is particularly effective in gelation of the foam with sufficient stability.
Light Stabilization
Zinc Oxide is outstanding among white pigments and extenders for its absorption of ultraviolet rays. Thus, it serves as an effective stabilizer of white and tinted rubber compounds under prolonged exposure to the destructive rays of the sun.
Through its high brightness, refractive index, and optimum particle size, Zinc Oxide provides a high degree of whiteness and tinting strength for such rubber products as tire sidewalls, sheeting and surgical gloves.
Zinc Oxide provides reinforcement in natural rubber, and in some synthetic elastomers such as polysulfides and chloroprenes. The degree of reinforcement appears to depend upon a combination of the particle size of the oxide, the finest size being the most effective, and the reactivity of the oxide with the rubber. Under such service condition involving rapid flexing or compression, Zinc Oxide also provides heat conduction to more rapidly dissipate the heat and thereby provides lower operating temperatures. In addition, it imparts heat stabilization by reacting with acidic decomposition products.
Rubber - Metal Bonding
In the bonding of rubber to brass, Zinc Oxide reacts with copper oxide on the brass surface to form a tightly adhering Zinc-copper salt.
Tack Retention
One of the unique properties of Zinc Oxide is its ability to retain over many months of shelf –storage the tack of uncured rubber compounds for adhesive tapes. French process Zinc Oxides impart heat-aging resistance superior to that of American-process Zinc Oxides. The former type, being sulfur-free, has a higher pH and, thus, can neutralize more effectively the acidic decomposition products formed during aging. Moreover, the finer French-process Zinc Oxides prove superior to coarser grades in heat-aging resistance.
Zinc compounds can provide a variety of properties in the plastics field. Heat resistance and mechanical strength are imparted to acrylic composites by Zinc Oxide. Zinc Oxide contributes to the formation and cure of epoxide resin. Addition of Zinc Oxide to epoxy resins cured with aliphatic polyamines imparts higher tensile strength and water resistance. Zinc Oxide imparts fire-resistant properties to nylon fibers and moldings. Zinc Oxide is also useful in the preparation of nylon polymers and in increasing their resistance. The formation of polyesters in the presence of Zinc Oxide imparts higher viscosity and other improvements. ZnO reacts with unsaturated polyesters to form higher viscosity and a thixotropic body. The dyeability of polyester fibers is improved by Zinc Oxide. Zinc Oxide mixtures stabilize polyethylene against aging and ultraviolet radiation. Zinc Oxide increases the transparency of poly(chlorofluoroethylene) molding resin. Polyolefin's are improved in color, tensile strength, and vulcanization properties by addition of Zinc Oxide. Thermal stabilization of PVC is effected by Zinc Oxide. Antistatic, fungistatic and emulsion stability are additional properties imparted to vinyl polymers by Zinc Oxide. Applications in development for Zinc Oxide-stabilized polypropylene and high-density polyethylene include safety helmets, stadium seating, insulation, pallets, bags, fiber and filament, agricultural and recreational equipment.
The properties imparted by Zinc Oxide to some of the newer applications are as electronic glass, low-melting glass for metal-to-glass seals, thermistors for use as lighting arresters and devitrified glasses of low thermal expansion. Zinc Oxide imparts a unique combination of properties when used in glass. Zinc Oxide reduces the coefficient of thermal expansion, imparts high brilliance and luster and high stability against deformation under stress. As a replacement flux for the more soluble constituents, it provides a viscosity curve of lower slope. The specific heat is decreased and the conductivity increased by the substitution of Zinc Oxide for BaO and PbO.
Pharmaceutical Industry
Zinc Oxide is mainly used in Zinc soap, ointment, dental inlays, food powders etc. Zinc Oxide forms an indispensable element of the production process of this industry.
The optical and biochemical properties of Zinc Oxide and its derivatives impart special features to a variety of cosmetic preparations for care of the hair and skin. In powders and creams it protects the skin by absorbing the ultraviolet sunburn rays; in burn ointments it aids healing. Simple salts of Zinc provide astringent and skin-conditioning properties to creams, while more complex salts furnish fungistatic properties which contribute to the effectiveness of deodorants, soaps, and antidandruff preparations.
Adhesives, Mastics, Sealants
Zinc Oxide has long been a major constituent of surgical and industrial tapes based on natural or synthetic rubber as it is outstanding in retention of tack during shelf–aging. Neoprene adhesives are improved by the addition of both Phenolic resins and Zinc compounds (including Zinc Oxide), the reaction products imparting special properties such as high heat resistance, high bond strength, improved peel and shear – stress resistance, and stability to settling of Neoprene solution adhesives.
Some of the unique electronic properties of Zinc Oxide are distinctively utilized in the photocopying process. For use in that process, Zinc Oxide is increased in photoconductivity and semiconductor properties by special heat and/or doping treatments (addition of foreign elements). Also, Zinc Oxide is greatly modified in optical properties to increase its absorption of light rays in the visible region. This process known as sensitization, is generally carried out by addition to certain dyes, which are absorbed on the surface of the Zinc Oxide. Commercial Zinc Oxide for photocopying is generally produced from metallic zinc, rather than ore, to obtain a product of higher purity.
In the research and development of improved lubricants over the past two decades, Zinc Oxide and its derivatives have been intently studied. Zinc Dithiophosphates which are prepared by reacting Zinc Oxide with organic Phosphates, are used in substantial quantities as additives to lubricating oils for automotive engines, to reduce oxidation corrosion and wear. Zinc Oxide has been found to contribute special properties in many types of lubricants, such as extreme-pressure lubricants, seizure-resistant lubricants, and greases. Improved adhesion is another feature which Zinc Oxide contributes. Zinc Oxide imparts special properties to greases and other variety of lubricants. Such greases are useful in the lubrication of food–processing equipment.
Zinc Oxide in organic coatings provides a broad spectrum of properties: optical, chemical, biochemical and physical. Over the past century the paint industry, in its constant development of improved products, has utilized various aspects of those properties to high degree. Manufacturers discovered that they could produce coatings of brushing consistency and good suspension properties by incorporation of Zinc Oxide into their pastes. Painters noted that they furnished better hiding power, whiteness, cleaner tints, tint retention, and durability as well as nondarkening in the presence of sulfur fumes. French-process Zinc Oxide has been proved superior to American-process type in fungus resistance and less sulfide staining.
Metal – Protective Coatings
Zinc metal powder (zinc dust) and zinc compounds have long been utilized for their anticorrosive properties in metal-protective coatings, and today they are the basis of such important specially metal primers as Zinc Chromate primers. Zinc dust-Zinc Oxide paints are especially useful as primers for new or weathered galvanised Iron. Such surfaces are difficult to protect because their reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc dust-Zinc Oxide paints however, retain their flexibility and adherence on such surfaces for many years. Zinc dust-Zinc Oxide paints also provide excellent protection to steel structures under normal atmospheric conditions, as well as to steel surfaces in such under-water conditions as dam faces and the interior of fresh water tanks.
Cigarette Filters
Zinc Oxide as a constituent of cigarette filters is effective in removal of selected ingredients from tobacco smoke. A filter consisting of charcoal impregnated with Zinc Oxide and Iron Oxide removes significant amounts of HCN and H2S from tobacco smoke without affecting its flavour.
Sulfur Removal
Zinc Oxide is effective in removal of Sulfur and Sulfur compounds from a variety of fluids and gases, particularly industrial flue gases. Zinc is also effective in removal of H2S from hydrocarbon gases and for desulfurization of H2S and certain other sulfur-containing compounds.
Foods and Food-Packaging Materials
Zinc Oxide and its derivatives contributing special fungi-static and chemical properties to the processing and packaging of various animal and vegetable products. In the packaging of meat, fish, corn and peas, for examples, Zinc Oxide has long been incorporated into the varnish linings of the metal containers to prevent formation of black Sulfides which discolor the food.
Fire Retardants
Zinc Oxide and its derivatives were used extensively in fire retardants for the military in World War II and those Zinc compounds have since been the subject of extensive research and development for preparation of fire-retardant compositions for a variety of substances. Solutions for fireproofing textiles contain ZnO, Boric Acid, and Ammonia in various proportions. It deposits water-insoluble Zn Borate on the fibers.
Zinc Oxide continues as an essential ingredient in the "soft" type of ferromagnetic materials for television, radio, and tele-communication applications. In these fields ferrites based on Magnetite, Nickel Oxide and Zinc Oxide are used as elements in many types of electronic devices. Numerous electronic instruments for the consumer market utilize ferrites to impart specific functions. In portable and car radios, the antenna cores are ferrites designed to provide highly selective tuning. Television picture tubes constitute a major market for ferrites, particularly for use in flyback transformers and deflection yokes. In the communications area, ferrites are extensively used in the filter inductors of telephone circuits to permit precise inductance adjustment for the purpose of separating channels. Magnetic tape for recorders is improved by use of a Magnetite precipitated in the presence of ZnO.
Batteries, Fuel Cells, Photocells
Zinc Oxide is used in Zinc-Carbon dry cells, Zinc-Silver Oxide batteries, Nickel Oxide-Cadmium batteries and even in secondary batteries. In Fuel cells, Zinc Oxide is used as electrode material, cathodic material and as a fuel element. And in Solar Energy Cells, it can act as a photocatalyst. Purification of motorcar exhaust gases is currently the subject of extensive research, and Zinc Oxide is already demonstrating its catalytic properties in some of those programs.
Zinc Oxide plays an important role in semiconductor ceramic elements for operation at elevated temperatures or High Voltages. Such Thermoelements can be produced to cover a broad range of thermal and electrical properties. Varistors are composed of semiconductor ZnO modified by other Oxides. Developed for use as lightning arrestors and high-voltage surge arrestors in electric transmission lines, they are based on a unique electronic property of semiconductor ZnO, nonlinear resistance. ZnO varistors have high-temperature stability and resistance to humidity, electrical load, and current shocks.
Silicate Compositions
Zinc Oxide reacts with aqueous solutions of Silicates, such as Sodium Silicate solutions (commonly known as water glass) to form Zinc Silicate, a waterproof, fireproof refractory material, which is useful as a binder in paints. Such refractory adhesives are used in bonding Asbestos-cement molded products to building structures.
Zinc Oxide and its derivatives contribute effectively to the control of fungi in many different types of applications. Zinc Oxide per se is not a fungicide; rather it is a fungistat, that is, it inhibits the growth of fungi, such as mildew on the surface of exterior house paints. This property is also used to particular advantage in the fortifying of fungicides; Zinc Oxide is added to fungicides to increase their effectiveness in specific applications.
Portland Cement
The beneficial effects of Zinc Oxide additions to Portland cement have long been known -retardation of setting and hardening (to reduce the rate of heat evolution), improvement in whiteness and final strength.