Zinc Oxide Applications
World Wide Use
World-wide use of zinc oxide is in excess of 1.2 million tonnes annually. On a contained zinc basis the oxide accounts for about 9% of metallic zinc. Probably 60% of zinc oxide uses secondary zinc, primarily top dross from continuous galvanising, as the zinc source. China is by far the dominant supplier and also largest user, followed by the U.S.. Although rubber products and in particular tires are the major use for rubber, there are considerable variations around the world in use patterns. For example the ceramic market is equally as important as rubber in some areas. Oil additive compounds are dominated by manufacture within the U.S..
World-wide Zinc Oxide Applications
Over the years zinc oxide applications have changed. Some major uses such as photocopy paper and linoleum have disappeared. Paint, once the dominant use is important now only in niche applications.
The most visible applications for zinc oxide are in skin care for example sun screens, ointments, creams and powders. In product volume these are not so important. The most recent major new use for zinc oxide is in varistors, ceramic zinc oxide components that protect circuits from voltage spikes.
Zinc oxide has typically been able to generate new uses to take the place of older applications. There is an unprecedented level of research work being directed toward electronic and photonic uses. These include LED’s, transparent transistors, solar cells and memory devices. Many of these applications, for example LED’s for solid state lighting, even if widely adopted, would not be a high volume zinc oxide application, due to the minimal quantities used in such devices.
Rubber is the most important application of zinc oxide. It has been used in this capacity for over a century. Zinc oxide along with stearic acid activates sulfur crosslinking of rubber. In addition it provides pigmentary properties which enhance the ability to absorb frictional heat an important property in tire performance. The level of zinc oxide use is expressed in terms of ‘parts per hundred of rubber, phr’. The usual level is 3-5 phr. Essentially all rubber goods contain zinc oxide.
French process zinc oxide, made from metallic zinc, is preferred for rubber uses. The major reason for this is that the purity and physical characteristics can be controlled within close limits. The quality requirements the automobile industry imposed on the tire industry were passed on to the tire raw material suppliers. Automobiles are a significant market for rubber, in addition to tires there are belts, hoses, oil seals, trim and mountings.
The ease of incorporation of the zinc oxide into rubber compound can be improved by coating the particles with propionic acid. The acid binds to the zinc oxide surface and the resulting hydrocarbon chain is more compatible with the rubber compound.
The important properties of zinc oxide that are relevant to rubber are:
- Low oversize – to prevent point defects in the compound.
- Particle size – to provide the required level of reactivity. Note particle size is usually expressed as the inverse measurement, surface area.
- Purity – Some elements, for example manganese, are detrimental to rubber curing at very low levels.
- Soluble salts – Soluble salts would be expected to reduce the resistance of rubber to degradation.
The major use test for zinc oxide is to compare the compounding characteristics with a known sample. Rheometer equipment measures the build up in the viscosity of rubber compound as the cross linking reaction occurs. It is the sulfur crosslinking of rubber polymer that gives it the combination of properties of flexibility, strength and resilience that we expect from products such as tires.
Latex rubber products are often formulated with very fine zinc oxide known as Active grades which have low levels of opacity.
Ceramic and Glass Compositions
Zinc oxide is a standard ingredient of ceramic glazes, for example in tiles. The zinc oxide is part of the frit formulation from which the glaze is derived. Zinc oxide has two important properties, it reduces the melt temperature of the glaze, thereby reducing the energy and equipment requirements. It also allows other pigments to develop stronger intrinsic colors.
Zinc oxide is also present in heat resistant glass and cookwear as well as speciality glass applications such as photochromic lenses.
Animal feed is a major use of zinc oxide. All living things need zinc to function because zinc compounds form major pathways in the body’s metabolism. Given that animals are either farmed or cared for by humans, the intake of vital ingredients, including zinc is controlled. Our animals usually have better diets than we do. The product requirements for animal feed applications are significantly different to most other uses. As the zinc oxide is formulated with many other ingredients into custom mixes, it is essential that it can be accurately metered. Regular zinc oxides do not flow very readily and also tend to stick to surfaces. Consequently the zinc oxides used in this application are the product of kiln operations and typically have a very high bulk density and flow very readily.
Zinc oxide is used to catalyse organic reactions such as dehydrogenation. Very fine (Active) zinc oxides derived from zinc hydroxide or carbonate are used in these applications.
Varistors were developed around 30 years ago, and still represent the most recent significant new use for zinc oxide. Varistors are a zinc oxide composition that has the ability to change its electrical resistance depending on the applied voltage. Commercially this is of great value because it is the basis of protection devices for electrical components. Varistors consists of ceramic blocks of zinc oxide that have been sintered along with small quantities of other metals such that there are defined grain boundaries between crystals. Depending on the formulation, the varistor’s electrical characteristics can be controlled for certain applications. The varistor is connected from the circuit to an earth connection. If a voltage spike occurs, the varistor conducts the excess voltage to earth. This device has an extremely rapid response time. Voltage spikes are very common for example through switching and induced currents by lightning. Small varistors are used extensively in electrical circuitry, much larger ones are used to protect the high power grid systems from both terrestrial and solar storms.
Although zinc oxide is not magnetic, it is an integral part of iron oxide/nickel oxide ferrite compositions. A range of ferrite materials for different applications can be made, depending on the formulation.
Zinc oxide is mildly antiseptic. It is sufficiently active toprotect skin from infections without inducing an adverse reaction. Consequently it is used in powders, creams and ointments, particularly for babies. This application of zinc oxide is one of the more well known uses of zinc and its value in this respect has been known for millennia.
Zinc oxide is also a very effective sunscreen due to its ability to strongly absorb ultra violet light. Ultra violet light is responsible for sunburn and skin disorders. In recent years, sunscreens have been formulated with very fine zinc oxide which is transparent.
Lubricating Oil Additive
Engine lubricating oils contain a package of ingredients that are essential in providing theperformance, protection and serviceability that we all take for granted. One of these additives is zinc dialkyl dithio phosphate, ZDDP. This group of additives were developed with the growth of the Automobile Industry. These products are formed by the reaction of zinc oxide with the dialkyl dithio phosphoric acid intermediate. Properties can be modified by changing of the alkyl groups. There has been some pressure to change away from these materials because phosphorus compounds poison the catalyst in catalytic converters. However, there do not appear to be any alternate products available. It also appears that the ZDDP industry will be able to react with improved products in which phosphorus transfer from the lubrication to the emission system is inhibited. As an organophosphorus compound, ZDDP also has fungistatic properties and this is a minor use compared to auto requirements.
Historically paint was the major outlet for zinc oxide. It was superceded by the development of titanium dioxide pigment which has a higher opacity. Zinc oxide is still used in some niche applications. Direct Process zinc oxide is preferred due to its lower reactivity, the primary function of the zinc oxide is as a fungistat. Zinc oxide is also used in ship’s antifouling paint compositions.
Zinc oxide and carbonate compositions are made into absorbent pellets which are used to desulfurize liquid and gas streams. Zinc oxide is able to absorb SOx and H2S.
Zinc oxide is an ingredient of fire retardant compounds. Zinc borate, formed by the reaction of zinc oxide and boric acid, is used extensively in plastics.
Phosphate Coatings & Pigments
Zinc phosphate coatings, formed by reaction of zinc oxide and phosphoric acid, are an essential component of anti corrosive paint systems. Zinc phosphate prepaint coatings are applied to bare metal or galvanized surfaces, prior to paint application.
Zinc phosphate pigment is also used in anticorrosive paints.
Inorganic Zinc Salts
Zinc salts are used in a variety of applications. Zinc sulfate as the monohydrate or heptahydrate, is a significant compound of commerce. However it is usually made from a variety of secondary zinc materials, as is the chloride. Salts that are made from zinc oxide include the bromide and chromate. Zinc sulfate is used mainly in agriculture for both crops and animal nutrition. Elecrtolytic zinc is plated from zinc sulfate solution as an in-process material.
Organic Zinc Salts
Zinc salts of fatty acids are usually made by direct reaction of zinc oxide with the acid. Zinc stearate is used in the Rubber, PVC and Pharmaceutical Industries. Zinc naphthenate is used as a wood preservative. Acrylate compounds are also used in elastomers, for example the diacrylate is used in golf balls. Zinc monomethacrylate also activates rubber curing. Zinc oxide is also used in pharmaceutical care products such as zinc bacitracin and zinc pyrithione.
It is also used medicines such as zinc gluconate.