CALCIUM OXIDE IN THE RUBBER INDUSTRY

Luis Tormento2017/February

INTRODUCTION• Calcium oxide plays an important role in the rubber industry, since any raw material used in

the rubber compounding has some moisture, which is the largest source of volatiles during vulcanization.

• Mineral fillers like calcium carbonate containing up to 2% of water, polymers, plasticizers, carbon blacks and others contains water.

• Economically it is impossible to completely eliminate traces of water from the raw material. Vulcanization at atmospheric pressure as in the manufacture of profiles requires the addition of an efficient desiccant, and it is at this point that calcium oxide functions through its reaction with water to form stable calcium hydroxide at the vulcanization temperatures.

• Some water is eliminated during the mixing process; For example in the mixer.• Vacuum extrusion also eliminates some volatile ingredients, yet 0.2% of water will create

porosity and the use of calcium oxide is absolutely necessary to get the correct product.

CHARACTERISTICS OF DEHYDRATING AGENTS

• Calcium oxide absorbs approximately 32% of its weight of water and forms calcium hydroxide, which is stable at the temperatures used in the vulcanization process.

• Other dehydrating agent such as silica, chalk, etc. Release combined water at temperatures around 200 ° C.

• The use of powdered calcium oxide is extremely difficult due to its strong characteristic of absorbing water, especially during storage, difficulty of dispersion in the compounds and its dangerousness of handling by having corrosive properties.

• Alternatively the suppliers sell it in the form of pastes or granules using various substrates to cover the calcium oxide particle, allowing it to be accessible to water only when in the compost.

• The various types of substrates used are, for example, aromatic or paraffinic oil, paraffin wax or polyethylene.

• The type of substrate to be chosen will depend on the polymer and artifact that we will use.

INFLUENCES IN PHYSICAL PROPERTIES

• Especially with natural rubber the vulcanization rate is influenced and the formulation needs to be adjusted to maintain the physical properties.

• This change must be made, because calcium oxide reacts as an accelerator in the composition of rubbers, combined with the existing vulcanization system, will slightly modify the curing curve. The addition of accelerators such as dithiocarbamates or another type of ultra-accelerator will correct the vulcanization curve.

• Expanded compounds using calcium oxide will have the following improved characteristics:– The rate of expansion increases– The pore structure will be better– The surface of the artifact is better– Resistance to permanent deformation will be improved

INFLUENCES IN PHYSICAL PROPERTIES

• Aging• The addition of calcium oxide has no significant influence on the aging of the elastomers.

Tests performed between 50 and 100ºC of up to 14 days did not show any significant variation.

• Electrical Properties• Calcium oxide does not affect the dielectric properties of the elastomers used in wire and

cable industry based on rubber, its use for several years has not shown a compromise of these properties.

STABILITY• The most important factor in using a desiccant is its proper storage and

use.• Calcium oxide is highly hygroscopic and hydrates in 2 days when exposed

to ambient conditions very humid, such as above 60% of relative humidity. Its dispersion in the substrates already mentioned will delay this absorption, but it is noted that they will not impede it, so it is important to use calcium oxide as soon as possible after its manufacture;

USE OF CALCIUM OXIDE• The use of a desiccant is unavoidable when vulcanizing the rubber

artifact occurs at atmospheric pressure or low pressure. The main applications are in vulcani9zation on continuous processes, as profiles in salt baths, cure by microwave, and hot air tunnels.

• The rubber compounds may be based on both natural rubber and synthetic rubbers, such as EPDM.

• The profiles may be solid or expanded; • The calcium oxide concentration of 8 phr is the most common and

sufficient in industrial applications.

USE OF CALCIUM OXIDE• The profiles produced are used to:• Auto Industry

– Door seals– Fittings– Gaskets

• Construction– Window fittings– Gaskets– Bridge joints

USE OF CALCIUM OXIDE• Calcium oxide is also used in other applications such as:

– V-belts– Conveyor belts– Construction Materials– Power Cables– Stickers– Mastiques

HOW TO USE CALCIUM OXIDE• It is imperative for optimum efficiency to obtain optimal dispersion of

the calcium oxide in the polymer during the compounding process.• The best time to introduce calcium oxide must be tested for each

application, thus obtaining the best dispersion and porosity control.• The temperature and the reaction time are the main factors that

influence the efficiency of the calcium oxide. In general, it is recommended to add the calcium oxide at the initial times of the mixing process, for example in the internal mixer at the highest temperature.

HOW TO USE CALCIUM OXIDE• When we add the desiccant in the mixer mill, the temperature and the

time may be insufficient to obtain the total absorption of water. In this case the compound should be allowed to rest for a longer period before use, for calcium oxide to perform its function.

• After preparation of the compounds it can be stored for up to 1 week without absorbing water, after this period there is the possibility of water absorption and the compound must be reprocessed with the addition of more calcium oxide.

Contact

LT QuímicosAv. Pedro Severino Jr., 366 Cjto 35

04310-060 – São Paulo – SP – BrasilLuis TormentoNPD Director

Luis.tormento@ltquimicos.com.brTel: +55 (11) 5581-0708