The Know How in Film for Vulcanization

Vulcanization is a chemical process invented by Mr. Charles Goodyear in the first half of the 18 hundreds. The inventor wanted to find out an use at natural rubber. Natural rubber is obtained cutting the bark of the gum tree and collecting the latex which springs out. At that time this raw material was of little (if any) use because it is sticky and easily deformable (it is very soft and ductile). After many experiments Mr. Goodyear realized that adding small quantities of sulphur to the latex of the gum tree and submitting the whole to a cooking cycle rubber would harden permanently and would got elastic properties.

This is a thermosetting reaction, that is to say a process that through an exposition to heat changes the physical state of the substance, from viscous to solid. In addition, differently from thermoplastics (where the transition solid-fluid is reversible and can be done many times) in thermosetting processes the transformation is usually irreversible.

Since then the process has basically remained unchanged. Technological progress has introduced many chemical substances generically called elastomers, natural and synthetics, as complement or substitues to the natural gum latex (nowadays there are many substances, we list only the more common: polyisoprene – natural rubber, polycloroprene – neoprene, SBR – styrene butadiene rubber, silicon based rubbers, poyurethanic rubbers) chemical catalists have been added (sulphur, peroxides, acetoxylane, urethane crosslinkers, metallic oxides), accelerators and retardants have been invented, and much more. In any case, still today temperature and the presence of the proper catalyst transform uncured rubber and makes it permanently solid.

Rubber can thus be found in two different forms: uncured rubber, which is usually unknown to the majority of people, and cured or vulcanized rubber, which is on the opposite in the everyday life of all human beeings. At the extrusion uncured rubber likes very much to chewing gum, quite soft and ductile, hot and sticky. With vulcanization it hardens, becomes elastic and does not deteriorate with the time. In other terms it improves considerably mechanical properties, in particular load resistance, and increases its elasticity.

The first step to carry out the vulcanization is the preparation of the blend, which consists of mixing together in the proper quantities uncured rubber and the catalyst. To transform it in the shapes we usually know it must then be “cooked”, that is to say undergo vulcanization.

In all termosetting processes it is necessary that the raw material, which is at the beginning in a viscous state, and thus with an unstable shape and volume, during the process is restricted, is contained inside proper moulds or forms. This basically for 3 reasons:

1. first of all it is possible that the viscous substance during the cooking cycle before hardening permanently passes through a phase in which it further increases its fluidity (becomes much more liquid of the starting status). In this moment if it is not constrained inside a mould it can pour out and end to harden in another place of the oven (where it wants, not where it is necessary to go);
2. than because the mould often gives the product a form very similar to what later will be the final shape of the finished product. This is the case, for example of tyres, gaskets, o-rings and others;
3. finally because the use of moulds permits to reduce dramatically the consumption of raw materials, in particular whereas the shape has important volumes of empty space (for example it would be possible to obtain a tyre or a gasket starting from a full body, but this would mean the use of much more raw materials and very long processing times).

During the vulcanization is thus necessary that the raw material is constrained inside proper containers. According to the shape of the finished product, moulds or other means are used to contain the uncured rubber. Here too many different approaches are possible.

For some products one or more moulds are used (very common is the use of 2 moulds a “male mould” and a “female mould”).

In other case an inner mould (a male one) is used and the external containment is made with other means, the most common, practical and economical is to make an external bandage, a wrapping, with a flexible tape.

In other cases the mould is external (a female mould) and inside a flexible bag is introduced and inflated.

Our films for vulcanization arrive at this point of the history and can be grouped into 3 classes:

• tapes of narrow width (5 to 20 centimetres) for the external wrapping in the coverings with rubber of rolls and cylinders;
• separator films, much wider, for the manufacturing of rubber in flat sheet (flat sheets, conveyor belts);
• release sheets for the protection of the moulds.

We are supplying the rubber tube manufacturers since 1996. At that time we have developed with a major Italian company involved in the vulcanization a specific film tailor made on their needs.

Our proposal for this application is a specially designed polypropylene film: PP AM per VULCANIZZAZIONE which has an excellent performance when used in the production of tubolar covered rolls.

This film overcomes the drawbacks of traditional polypropylene films and nylon films. Unlike traditional polypropylene films it has the capacity, during the curing cycle, to shrink at a rate similar to the rate of shrinkage of cured articles.  It does not tear or split during the vulcanization process, which is a very common defect of single ply nylon films, especially in wet steam and at higher temperatures ranges (150 C).

In addition it is much cheaper than laminated 2 plies nylon films, and it does not infringe existing patents on this field.

Nowadays PP AM per VULCANIZZAZIONE is sold in many countries, and it is used daily for the rubber covering of cylinders.

Recently a new film has been launched in the market for the industry of vulcanization. POLIPROPILENE SPECIALE per VULCANIZZAZIONE is a film specifically designed for the manufacture of rubber sheets. It has a limited heat shrinkage and can thus be used immediately after the extrusion at medium temperatures. It acts as a release film and separates the different plies, in particular when they are wound in reels.

For any further information on films for vulcanization, please don’t hesitate to contact us. Our technical department is at your disposal you may need for any explanation, in-depth analysis, tests, new product or application development both for films and for vulcanization processes.

Write, phone, fax or email at:
A. Peruzza srl – via Portelle 1 – 31010 Mareno di Piave – Tv – Italy
Telephone: int+ 39 0438 492335. Telefax: int+ 39 0438 492365. Email: info@aperuzza.com

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