Radiat. Phys. Chem. Vol. 42, Nos 1-3, pp. 535-538, 1993 Printed in Great Britain. All rights reserved

0146-5724/93 $6.00 + 0.00

Copyright 0 1993 Pergamon Press Ltd

COST OF VARIOUS INDUSTRIAL APPLICATIONS OF EB

W. H. Scheerer

SCHEERER + PARTNER CONSULTING

JOHANNESSI‘RASSE 1 I FR(i - 6780 I’IRMASENS G t: R M A N Y

ABSTRACT

The article consists of a brief summary which covers direct ionizing radiation for various industrial applications.

Transparent processing parameters make the electron beam treatment a sure, safe, efficient way to make products stronger, tougher, heat resistant, or even impart special properties to the products.

From the field of crosslinking two typical applications are selected for close process investigations

Furthermore, it reflects the view from the standpoint of an equipment supplier, since electron beam treatment gained acceptance as a reliable and economical production technique, machinery and associated equipment grew into a new generation.

APPLICATION

Electron beam irradiation is used in a variety of diverse applications and industries. The subject of this paper is a brief cost analysis, which specifically deals with electron beam vulcanization of tire components. Basically, radiation curing of rubber, whether it be tire components or industrial products, looks good because:

. It allows partial-cure of calendered fabric.

. It allows for reduction of rubber coating gauge.

. It prevents from ply movement during final curing.

. It is competitive with heat curing.

. It decreases operating costs.

. It increases production rates.

. It reduces thermal and chemical pollution.

Before examining an example of radiation vulcanization, it is worthwhile to review the previously stated advantages, which apply to almost all radiation processing applications.

In contrast to heat, which produces ionization as a secondary consequence through mechanical vibration, electrons have sufficient kinetic energy to induce ionization as the primary interaction within materials. At the same time the electron energy can be transferred into matter on an explicit geometric and / or spatial basis and can influence a defined area.

Today these effects are feasible and offer the most potential for a speedy return on investment by the partial curing of calendered fabric. By virtue of this partial curing you not only impart green strength to the calendered fabric which prevents cord shadowing or pull through, but this same green strength, with its good gauge retention, allows you to reduce the original thickness of the calendered fabric in most instances. It is this gauge reduction, or material savings if you will, that provides the economic incentive for this process.

The work with calendered fabric is concerned as well with partially curing only one side of the calendered fabric and leaving the other side virtually uncured. It is this type of specialized or selective curing that can be accomplished most easily with an electron beam accelerator, since it is a highly directional, controllable source of energy. Generally speaking, by proper sizing or regulation of the machine voltage, which controls the degree of electron beam penetration, it is possible to either partially or fully cure only one side of a tire component while leaving the other side virtually uncured. One of the reasons this might be desirable is to avoid affecting the degree of tack on one side of a component. This would obviously be beneficial at some later stage in the tire building process where adhesion is important.

Major tire manufacturers have done considerable work with the partial curing of calendered fabric and based on their studies they have found they have been able to reduce their calendered fabric thickness from 1.2 mm down to

535

536 W. R. SCHEERER

1 .O mm for passenger car tires and from 2.9 mm down to 2.2 mm for truck tires. Using their studies, let us examine the economics, the through-put capability, the capital and return on investment numbers. In Figure 1, we see a breakdown of an individual calendered fabric with the material and cost savings that can be expected. In this figure we see the materials savings to be about 0.3 kg, res. 2 kg per tire, and the cost savings, based on a calendered fabric cost of 1.2 $ kg-‘, to be about 35 c, res. 2.63 $ per tire.

The reduction in cost per kilowatt hour is another part of the story. As a general rule of thumb, for every ten kilo Gray (kGy) of dose imparted to a product, you get a temperature rise of approximately 4OC. On this basis it would be just as wrong to use radiation to boil water as it would be to use heat to create chemical cross links.

If one assumes a dose of 10 kGy for vulcanization of rubber, then we have an energy input of 10 kJ kg-l. The contrasting situation for heat, based upon a requirement of 140°C (with the specific heat for rubber of 0.5) is 29.3 kJ kg-l. Thus we can see that theoretically heat is not the most efficient method of imparting energy to this type of system. The penalty one pays with heat gets still worse in a “real life” situation, for it is very difficult to conduct heat energy into rubbers and thermo-plastics at any reasonable rate of efficiency.

As a point of reference, synthetic rubber when properly compounded will fully vulcanize with electron beam radiation at from 100 - 150 kGy. Natural rubber, when properly compounded will fully vulcanize at from 200 to 300 kGy.

To achieve a partial cure (green strength) in synthetic rubber it would take from 30 to 70 kGy. To achreve this same state of partial cure in natural rubber it would take from 50 to 100 kGy. The variations in dose requirement are subject to the degree of cure desired and the manner in which the material has been compounded.

The economics of radiation processing hinge to a large extent upon product through-put. To get some understanding as to the production capacity of the typical accelerator, it might be well to dwell for a moment on some of the basic machine parameters.

In examining an DC accelerator you will note it has two primary specifications, voltage and beam current. Today DC accelerators are available in voltages from 300 keV to 5.0 MeV and up to 200 kW beam power.

In selecting the proper machine voltage, the nominal guide-line is:

The first 500 keV will penetrate 1 mm of material with a specific gravity of 1 .O and each additional 100 keV will penetrate an additional 0.4 mm of material. To illustrate this, let’s look at a calendered rubber sheet 1.2 mm thick, 1500 mm wide, with a specific gravity of 1.4. The parameters then require a minimum voltage of 670 keV. A 800 keV DC accelerator has an effective penetration of approximately 1 6 mm. This match of product thickness to machine voltage is excellent and bears the possibility of multiple product passes, too.

The required beam current for a given process depends on the desired radiation dosage, the process efficiency (38 % for cable, wire, pipes and tubes, 75 % for sheet material, 90 % for liquids) and the material through-put rate, which can be expressed as:

Beam Current [mA] = T [kg s-l] x D [kGy] I E x V [MeV]

E = Efficiency as a fraction of one D = Dosage V = Voltage (determined by product thickness) T = Product through-put

Using the above relationship, if one, for example, is processing a product requiring 670 kV electrons, with a required dose of 50 kGy, and product through-put of approximately 0.375 kg s-l, then one would have a beam current requirement of 50 mA.

For this study I chose a 800 kV 50 mA accelerator (Frgure 2). This choice IS based on a calendered fabric of 1 0 mm thickness with a specific gravity of 1.35 giving us an effective voltage of 600 keV. With the accelerator operating at full beam power (40 kW) and multiple passes one coulcl process 26 m2 min-’ of calendered fabric with single passes, while a dosage of 100 kGy is applied.

With the assumption of a two shift operation result these numbers in a production rate of at least 20 000 passenger car tires per day.

In Figure 1 we see a capital and operating cost breakdown for a 800 kV 50 mA accelerator, as might be used in a typical tire application. The capital cost is about $2.6 mio. with an operating cost of $ 0.059 per tire, which gives a net saving of $0.29 per tire, res. an annual saving of $ 2.5 mio.

Economically seen, electron beam radiation is certainly competitive in the rubber industry. The success lies in using the enormous radiation capacities offered by modern facilities. In comparison to alternative methods, higher investment costs must be taken into account, because the electron beam treatment offers far greater processing speed, the process costs, however, in relation to the product through put are lower.

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