US3683179A

US3683179A - Means for irradiating materials

Info

Publication number: US3683179A
Application number: US23524A
Authority: US
Inventors: John R Norman
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-03-11
Filing date: 1970-03-11
Publication date: 1972-08-08
Anticipated expiration: 1989-08-08

Abstract

In electron irradiation of organic materials to cause chemical changes therein the material is subjected to at least five successive doses of electron radiation of substantially equal intensity, the material preferably being moved successively past at least five beams of electrons spaced apart along the direction of movement of the material.

Description

United States Patent Norman Aug. 8, 1972 541 MEANS FOR mnmumuc MATERIALS [72] inventor: John I. Norman, Hinxton Hall, Safv fron Walden, Essex, England [22] Filed: March 11, 1970 [2]] Appl. No.: 23,524

[52] u.s.c| ..2so/49.s 11:,250/52 [51 int. Cl. .1101; 31/00 [58] Field of Search ..2so/49.s re, 52;:13/14 [5 6] References Cited UNITED STATES PATENTS Knowlton ..250I49.5 TE

Colvin ..250/49.5 TE Marker.... ..3l3/74 Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney-Scrivener, Parker, Scrivener and Clarke 1 [57] ABSTRACT in electron irradiation of organic materials to cause chemical changes therein the material is subjected to at least five successive doses of electron radiation of substantially equal intensity, the material preferably being moved successively past at least five beams of electrons spaced apart along the direction of movement of the material.

1 Claim, 3 Drawing Figures PATENTED AUG 8 1972 saw 1 or 2 cgckcmuceu Bpfi m to: om

MEANS FOR IRRADIATING MATERIALS This invention relates to an improved method of, and apparatus for, continuous treatment of certain organic materials to cause beneficial changes in them by the action of electron irradiation.

Many of the organic materials and mixtures of organic materials (including mixtures with inorganic substances) which are known to undergo beneficial changes on irradiation exhibit an overall dose rate effect such that the properties of the irradiated material are dependent not only on the total irradiation dose received (in megarads) but also on the rateat which it is delivered. This phenomenon has been widely studied using substantially continuous and uniform sources of irradiation such as gamma rays and lowintensity electron beams, and it has been shown that for classical systems which have a simple bimolecular termination the dose required to achieve a given degree of reaction is proportional to the square root of the steady doserate.

This general phenomenon is exhibited by most freeradical polymerizations of monomers including those that take place in the presence of polymers, to give graft polymerization. Specifically it is observed for systems of unsaturated polymers (such as unsaturated polyesters) blended with unsaturated monomers (such as styrene). These systems can be applied as liquid films to manufactured materials and then polymerized in situ by electron radiation to give hard and durable coatings, and the radiation-curing process has many advantages over conventional thermal curing processes, particularly for high linear processing speeds. However for high linear speeds, conventional electron beam sources operate at very high dose rates, such that the dose required for dose rate dependent systems may be uneconomical or so high as to generate unacceptable temperatures.

However many systems and materials which do not exhibit a true or classical type of dose-rate, do exhibit an overall dose-rate effect for a variety of reasons. We are only here concerned with those for which a reduction in dose-rate is desirable and in most such cases, the cause of the overall dose-rate dependence is the heat produced by the adsorption of energy from the electrons (together in some cases with the heat of the reaction initiated by the irradiation).

The heat produced is directly proportional to the dose absorbed, but the temperature to which the material is raised also depends upon the rate of loss of heat and so is dependent upon the dose-rate, or overall time of the irradiation process. As it is the temperature (or product of temperature and time) which determines the reactions or processes, such systems have a definite dose-rate dependence. The form of such a dependence is usually such as effectively to set a limit on the steady dose-rate which can be used, rather than a relatively continuous relationship between dose and dose-rate.

It is thus seen that it is desirable to spread the exposure over a considerable distance in the direction of travel. However, to produce a beam of electrons uniformly covering an appreciable area (i.e. the width of the material by the length) which may be several square feet, involves serious difficulties in electron optics and in the provision of -a suitable window for the electrons. Particularly at lower voltages (e.g. below 300 kilovolts) large windows which are thin enough to transmit the electrons efficiently would be very vulnerable and so would require a large pumping capacity to handle leaks.

An object of the invention is to provide a novel method of irradiating materials in an economical manner with electrons of relatively low energy and at low dose-rates.

A further object of the invention is to provide apparatus suitable for carrying out this method.

We have now found that the total radiation dose required to achieve a given desired property (e.g. degree of polymerization, or cross-linking) for a given chemical system which exhibits adose-rate dependance" may be significantly reduced by a new method of processing and/or a new design of electron source. While the main advantages in reducing the total dose are economic (lower power consumption and capital cost) there are also other advantages, i.e. less shielding is required and less secondary heat is developed in the material being irradiated.

According to the invention we now propose to provide electron irradiation equipment consisting of a number of electron beams spaced apart along the direction of movement of the material to be processed. The dose required to achieve a given degree of change in the material being treated depends upon the number of separate beams, their width, (i.e. the dimension of individual beams along the direction of movement of the material being treated) and their average spacing, and also upon the linear speed and reaction characteristics of the system being treated. For a free-radical reaction with kinetics corresponding approximately to bimolecular termination the main reaction characteristic involved is the radical halfllife although this time also depends upon the irradiation dose-rate.

Such equipment may consist of a number (preferably greater than four) of separate electron sources suitably spaced in accordance with the principles stated below, but a preferred equipment consists of one or more special irradiation sources each comprising four or more electron beams preferably having in common one or more of the following: anode plate, vacuum chamber and/or pumping system, high voltage power supply, filament supply and/or biassing supply, control instrumentation, interlocks etc. The several electron beams may each have its own associated source of electrons and beam forming system or several beams may originate from a single source of electrons with suitable beam splitting or scanning systems. The accelerating voltage should be as low as possible for reasons of economy, but must be such as to give reasonably uniform penetration within the material to be treated.

We are aware that it has been proposed to subject materials to electron radiation from a main generator of substantial voltage and current, and then to subject it later to the actions of one or more auxiliary sources of lower electron energy and lower density but this has been solely to achieve economy by approaching greater uniformity of dosage measured perpendicularly in from the surface of the material, as compared with a single source, and this prior proposal was wholly dependent on the penetration of the electrons from the auxiliary generator being less than the penetration of those from the main generator. It has also been proposed, again for reasons of economy, to irradiate strip material as it wound up onto a reel, which is rotated under the electron beam, so that each particle is irradiated repeatedly, but again the energy and penetration of the electrons are different each time.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a graph to illustrate the thinking behind the invention;

FIG. 2 is a diagrammatic side elevation of the irradiation apparatus, shown in section; and

FIG. 3 is a transverse section to a larger scale through one of the windows in the anode, showing the details of the window construction.

Referring first to FIG. 1, this is a graph showing the intensity I of radiation impinging on an element of material being irradiated, and the radical concentration produced, both against time. An efi'icient form of irradiation for certain reactions, especially polymerization and grafting reactions, would be a relatively low dose-rate maintained for a long time, as indicated by the curves A. The total dosage received by the material is given by the area under the intensity curve. Now if the same total dosage were given in a very much shorter time (but with a consequently higher intensity) as indicated by the curve B, the efi'ectiveness of the reaction obtained would be very much less. The reason for this is as follows: The impingement of the electrons on the molecules of the material serves to create radicals, which initiate active (i.e. free radical) polymerized chains, until tenninated by reaction with another active chain. At higher free-radical concentrations, the polymerization chains are shorter and therefore a lower degree of conversion or cure is achieved for a given amount of energy.

Therefore it is desirable to spread the radiation over a period of time longer than the radical half-life. As indicated earlier, it is difficult and expensive to produce a uniform dosage over a long period, but the same result, with the same advantages, is approached asymptotically by the use of several sources irradiating the material in turn. The curves C show radiation produced by six separate sources each having a width only a fraction of the spacing between the sources, and the resulting radical concentration which is lower than for case B and approximates to the more efficient case A.

Referring now to FIG. 2, the apparatus comprises an evacuated chamber C containing a number (five in the example illustrated) of individual electron sources spaced at substantially uniform intervals along its length and shown as filaments F with biassing grids C. Beam-forming systems (not shown) associated with each filament form vertically downwardly directed beams which pass through five respective windows W in a single final anode A which is common to all five electron sources. The anode A forms the bottom wall of the chamber C and is preferably at earth potential, the filaments F being therefore at a high negative potential, to simplify external insulation problems.

The five beams emerging from the final anode A at substantially equally spaced intervals along a straight line each have a width (measured along that line) approximately defined by the associated window. They all impinge on a strip of (flat) material M, the object to be irradiated, which has its plane substantially perpendicular to the axes of the beams and is moving in a direction parallel to the line along which the five beams are arrayed.

To maintain the vacuum inside the chamber C each of the windows W must have a membrane or foil T across it, as shown in FIG. 3. The foil is as thin as possible commensurate with the necessary strength, and in one construction is clamped by means of a clamping ring R with inner and outer O-ring seals S and with the annular space between the seals pumped by connection to an auxiliary pump (not shown). The heat generated in the foil by the impingement of the electrons is carried away by cooling water circulating in a duct D.

The following table gives a list of electron energies (measured as the accelerating voltage between filament and final anode) necessary to irradiate materials of different thicknesses, and of the thickness of the foil membrane that is suitable with this voltage, the membrance being of aluminum foil;

Approximate Maximum Instead of aluminum, the foil could be of any other suitable material, such as beryllium or an aluminum/magnesium alloy, or titanium. Where the electron energy is low, say below 200 KV, and where consequently it is necessary to use very thin foils to avoid unacceptably high losses in the window, the clamping ring or frame R helps to ensure good thermal contact with the anode A but still allows the foil to be replaced when necessary without undue difficulty.

The main parameters of the apparatus which determine the total dose required for a given material are the overall length L of the irradiation zone and the number of beams N. The ratio of the dose required using the apparatus of the invention to the dose that would be required from an orthodox single-source narrow-beam apparatus also depends on the speed of travel V of the material and on the characteristics of the material, namely its radical life-time t and the dependence of the dose R on the dose-rate l. The overall relationships are complex, but the following two examples illustrate the factors influencing the design of the apparatus and of the sources used in it. Both the examples refer to reaction systems in which R is proportional to the square root of 1.

EXAMPLE A Where the overall length L of the irradiation zone can be varied and the spacing between the individual beams is large compared with, or at least much greater than, the distance Vt moved by the material during the radical life-time, then the radiation dose R varies approximately inversely with the number of beams; thus if we double the number of beams without decreasing their spacing we halve the total dosage that is required to carry out the given reaction. Secondly the dosage required is approximately proportional to the velocity V of movement of the material. Thus doubling the speed of movement will mean that double the total dosage is required, so that the intensity of radiation must be multiplied by four.

EXAMPLE B Where the overall length of the irradiation zone is fixed, then increasing the number of beams while reducing the spacing between them will reduce the total dosage required. The dosage will fall asymptotically to that dosage which would be required if the source were a uniform one of length L. Where the overall time of irradiation is approximately greater than the radical life-time, the dose required usually falls within about percent of the dose required for uniform irradiation after some 10 to sources. Where the number of sources is of this order, i.e. is such that the dosage required is not substantially different from that which would be required from a uniform source of length L, the dosage R is approximately proportional to the velocity V and inversely proportional to the length L.

Although the apparatus according to the invention allows reactions that require a low dose-rate to be performed at appreciably lower doses than with normal single-source electron radiation apparatus, it may also be usefully employed in reactions that do not necessarily require a low dose rate, for it has other advantages such as low secondary heating and (particularly at low voltages) simplified electron windows.

lclaim:

1. Apparatus for the continuous treatment of polymeric materials with electron radiation to cause chemical changes therein comprising at least five electron beam sources spaced apart along a straight line, said sources producing beams which are mutually parallel and are directed transverse to said line, a single common anode plate extending parallel to said line and placed such that said beams are directed substantially perpendicular to said anode plate, a number of windows in said anode plate corresponding to the number of said sources, each one of said windows being associated with one of said beams and permitting said associated beam to pass through said anode plate, the width of each of said windows in a direction parallel to said line being less than the spacing between adjacent ones of said windows, and means for moving material past said anode plate through the paths of said beams in succession in a direction parallel to said line whereby any given element of said material receives successive part-doses of electron radiation from each of said beams in turn.