US3137633A - Irradiation of polymerizable organic materials by fission products of boron-10 - Google Patents

Description

June 16, 1964 KLINE 3,137,633

D. E. IRRADIATION OF POLYMERIZABLE ORGANIC MATERIAL BY FISSION PRODUCTS OF BORON-lO Filed May 13, 1960 7 INVENTOR ATTORNEYS United States Patent 3,137,633 DIATION 0F POLYMERIZABLE ()RGANIC MATERIALS BY FESSION PRODUCTS 0F BORQN-Ifi Donald E. Kline, College Township, Centre County, Pa.,

assiguor to ERR-Singer, Inc, State (Ioilege, Pa, a corporation of Delaware Filed May 13, 1960, Ser. No. 29,093 1 Ciaim. (Cl. 176-11) This invention relates to a method of treating organic substances of low or high molecular weight and, in particular, to the utilization of nuclear fission in the treatment of polymerizable or degradable organic compounds.

It is known that one can treat organic substances by irradiation to promote polymerization or other chemical changes. In early work, use was made of radiations (alpha, beta and gamma) arising from nuclear disintegration of naturally-occurring radioactive isotopes, and of X-rays or fast electrons produced by electrical machines capable of providing high voltages. These two sources of radiation are still used and fall into two groups; those involving radiations (alpha, beta, gamma, neutrons, etc.) produced as a result of nuclear changes, such as in atomic reactors, spent uranium fuel rods, fission products, radioactive cobalt, etc., and those involving voltage accelerators, such as Van de Graalf machines, linear accelerators, resonant transformers, cyclotrons and the like.

When using the foregoing methods, the major reactions in polymers, depend primarily on the total energy absorbed and sometimes on the radiation intensity and character. For example, in the crosslinking of polyethylene or silicones, the density of crosslinking was found to depend to some extent on the energy absorbed per gram and appeared to be independent as to whether it was due to highly penetrating gamma rays absorbed at a rate of 1 mrad per day, or to mixed radiation from an atomic reactor providing radiation at an intensity of some 3 mrads per hour or to 2 mev. electrons of low penetration (about 6 mm.) at a much higher rate of about 1 mrad per second.

In the case of polymerization, the intensity of radiation is very important for the reason that the length or crosslinking of the growing polymer chain is determined somewhat by the concentration of radicals which is itself somewhat dependent on the intensity. For instance, electronic sources of radiation are particularly suitable for providing very intense beams of electrons concentrated on a small area. On the other hand, radioactive sources can be used more conveniently to expose larger areas giving lower intensity sources, the same dose being accumulated over long exposure times whereby more material can be irradiated at any one time. Where low penetrations are adequate and where the reaction is independent of intensity, electron accelerators appear to offer certain economic advantages for large scale work. However, where high penetration and/or irradiations of low intensity for long periods are required, gamma-producing fission products may be more convenient. Nuclear reactors can provide doses of high intensity over a rather wide range of volumes and thus these are increas ingly useful in irradiations.

Both of the foregoing methods have certain disadvantages. For example, in the case of the electric accelerator, it was difficult to irradiate selected portions of a material without using cumbersome masking means such as lead foil and the like. In the case of gamma and reactor radiation, gamma-producing fission products presented certain health hazards and extreme care had to be taken that the material being treated would not become radioactive and hence be dangerous to handle.

It would be desirable if a method could be provided 3,137,633 Patented June 16, 1964 having the advantages of both of the foregoing methods but be more versatile in its application. In particular, it would be desirable to provide a method which would en able the treatment of a polymerizable organic compound material or other organic material of low or high molecular weight in any selected region or portion of the material without requiring the use of cumbersome masking means or other masking devices.

I have now found a method having the foregoing advantages wherein I use nuclear fission of boron-l0 as the source of heavy, charged-particle radiation, the radiation being effected in situ, that is near, at or in the material being treated.

It is therefore the object of my invention to provide a method of treating organic substances, for example polymerizable organic compounds, by utilizing nuclear energy generated in situ, that is by nuclear fission carried out near, at or in the specific area or region of the substance where a particular change in properties is desired.

Another object is to provide a method for treating polymer compositions with the aim of polymerizing or depolymerizing the same.

It is a further object to provide a method for the surface treatment of polymers to promote crosslinking between chain molecules and inhibit or reduce crazing.

These and other objects will more clearly appear from the following disclosure and the accompanying drawing, wherein:

FIGS. 1 and 2 illustrate a polystyrene antenna insulator, the surface of which may be treated in accordance with the invention to improve its resistance to crazing.

Broadly speaking, my invention comprises associating the element boron of approximate atomic weight 10 in small but effective amounts with the substance being treated and subjecting said contained boron to neutron bombardment whereby the high energy released by the fission of 3 is advantageously used to bring about a desired change, for instance polymerization in said substance. An advantage of this process is that dangerous radioactive side effects do not occur to any extent. The nuclear reaction involved is given by the following equation:

Stable lithium-7 is produced together with an alpha particle which is represented as He that is a helium nucleus of mass number 4 and atomic number 2. The charged particles produced in this reaction are ejected in opposite directions with the release of relatively high energy. The total amount of energy released will depend upon the amount of nuclei and the neutron flux present. By controlling the amount of nuclei present where desired, the amount and/or rate at which energy is released can be determined and controlled.

Boron found in nature has an average atomic weight of 10.82 in which the 13-10 content amounts to about 18.8% by weight with the balance substantially B-ll. It is the 13-10 portion of the natural boron which is the effective ingredient in carrying out the reaction.

Boron is particularly advantageous in that it has a high capture neutron cross section. The capture cross section decreases with increase in velocity of the bombarding neutron and therefore tends to maximum values with slow (thermal) neutrons. The microscopic thermal-neutron cross section for natural isotopic boron is about 750 barns (about 4,020 barns for boron-l0) compared to the much lower value of 0.0035 barn for natural isotopic carbon. Thermal neutrons, which have a distribution of energies near 0.025 ev. at ordinary temperatures, are particularly favorable for the fission of boron-10, the reaction being more efliciently initiated with thermal neutrons. The most probable speed of thermal neutrons at ordinary temperatures (about 20 C.) is of the order of about 2.2 l cm./sec.

Neutrons for carrying out the reaction may be produced in any ordinary manner. A well known source having rather high fiuxes is the atomic pile- Thermal neutrons are always available in largedensities in any thermal reactor (the common type) or can be made available by building into the face of a pile alayer of graphite of appropriate thickness, known as a thermal column, into which high energy neutrons diffuse and are moderated down to the thermal level, that is to a fraction of an electron volt.

' Or as disclosed in the Fermi et al. US. Patent No. 2,206,634, neutrons may be produced by the action of radon on beryllium or of polonium on beryllium, the neu-' trons emitted thereafter being moderated down to speeds of slow neutrons and even down to thermal levels by passing them through a screen or moderator, such as a hydrocarbon (e.g. wax), graphite, Water, etc. By embedding the material containing B- in or near the modera tor, the thermal neutrons therein can be utilized to initiate the fission of B-l0.

The rate at which any particular nuclear reaction occurs depends upon the number of neutrons impinging a particular target material, their velocity, and the number and nature of the target nuclei (i.e. boron atoms) in the ning longitudinally therethrough (note FIG. 1) and a' reduced end portion 3 for fitting within opening 4 of a metallic surface shown in cross section in FIG. 2, such as a portion of an aircraft element 5. Theinsulatoris held by mounting bolt 6 which looks the insulator" in place via nut 7, the nut and bolt being insulated from the aircraft element via insulating. washer 8. Because surface 9 of the insulator is-subjected in use to aggra vated weather conditions which may cause crazing or other detrimental surface effects, I propose to improve the surface characteristics of the insulator in accordance I with my invention as described inthe following example: Example 1 A polystyrene insulator having the configuration shown" in FIGS. 1 and 2 is coated with a thin layer of boron conmaterial. As has been stated, the microscopic cross section of a target nucleus for any given reaction is a property of the nucleus and of the energy of the incident neutron. This is expressed as follows:

taining 18.8% byweight of B-10 by vacuum deposition in anapparatus adapted for depositing a coating of desired material upon a surface by thermal evaporation as,

for example, is shown in U.S. Patent No. 2,079,784. The' boron is placed in a carbon boat in a vacuum bell and the boat heated electrically to a temperature sufficient to cause the boron to evaporate and deposit upon the surface of the insulator which is mounted in the bell within the vicinity of the boat but not-so close as to be afiected by Where C is the number of neutron captures per cm. I is the number of neutrons/cm. of a uniform parallel beam of neutrons impinging on a thin layer of dx centimeter thick containing N atoms of boron or nuclei/cm. and Ndx the number of target nuclei/cmf Since the micro-v scopic cross section applies to a single nucleus, then multiplying o by N (nuclei/cm?) gives a macroscopic cross section 2 of the nuclei/cm. referred to as 2=Nr Cmf The number of atomic nuclei N per cm. of material is easily calculated from the density p, atomic mass A and Avogadros number Na by N Na/A.

It becomes apparent from the foregoing that the rates of neutron reactions in the fission of boron can be determined. Assume a neutron beam to havea neutron density n (neutrons/cmfi); if v is the neutron velocity then generally speaking, nv would be the neutrons per cm. in the target material per sec. The use of the product nv gives a rate of neutron interaction per (cm?) (sec.). Or referring to nv as the neutron flux (neutrons/cm.

pressed by the formula 21.

As has been stated hereinbefore, the polymer being treated need only contain a small but effective amount of 13-10. The amount of boron contained in the material as B-10 may be as little as 0.001% by Weight and may range broadly to as high as 10% by weight of the ma terral to be treated. Where the boron is to be used at the surface of a polymer material for the purpose of inducing a surface change during neutron bombardment (e.g. molecular crosslinking), the amount of contained B-10 may be as low as 0.01 mg./cm. of surface to be treated and may range up to about 10 mg./ cm. For surface treatments, I prefer to use B-l0 alone as it permits more sec.), then the rate of neutron interaction would be exintense effects.

Depending on the amount of material to be. treated, the amount of contained Bl0, and the time of treatment, the neutron flux (neutrons/ cm. sec.) may be as little as 10 or range broadly from about 10 to 10 Usual levels used in reactors for carrying out the invention range from about 10 to 10 While, as stated above, thermal neutrons are more effective, slow neutrons of higheremlayer.

the temperature thereof. About 1 mg./cm.2 is deposited on the surface of the insulator, of which about one-fifth mg./ cm. is attributed to B-10. I

The coated insulator is then subjected to neutron bombardment with thermal neutrons having a flux of about 10 neutrons/,cmfi-sec. and the bombardment continued 'for a total time of about 10,000 seconds. The boron on the surface is thereafter removed by washing.

The improvedeffect of the treatment on the surface of the insulator is determined by testing the surface hardness in ,the usual manner and by noting the resistance of the test specimen similarly treated to stress crazing by subjecting it to .a long time tensile test for instance as described in an article by Sauer and Hsiao1(ASME,

Trans, Vol.75, 1953,'p. 895).

The insulator treated in the foregoing manner exhibits improved resistance to crazing caused by weather-' ing and other environmental factors.

7 Example 2 The invention is particularly applicable to the" produc-f tion'of laminated plastic products, for example the type in which one layer might be hardened or further hardenedif already polymerized by polymerization while the otherflayer is maintained in the elastic or resilient state; Assuming a layer of resilient plastic of one-eighth inch thickof polyethylene, a polymerizable or pre-polymerized coating of about one-thirty-second inch thick of a polymer it also comprising polyethylene and containing about 1.0%-

by ,weight of B-lO dispersed uniformly therethr o ugh as a fine powder is applied to the surface of said plastic The laminate is'then subjected to neutron bombardment witha beam of thermal neutronshaving a neutron fluxof about 10 produced by thermal neutron reactor The. foregoing treatment refor about 20,000 seconds. suits in a laminate characterized by increased hardness,

modulus strength and rigidity while retaining the resilient properties of the other part of the laminate. Thus the material is preferentially'treated and changed.

d The invention is also applicable .to the production of a plastic rod or other shape characterized by a hardcore and a resilient outer layer. For example, assuming a rod is desired having a hard core, I would have B-lO localized in the center portion of the rod so that by neutron bombardment of the rod with thermal neutrons to initiate fission of the localized B-lO, the center portion is caused selectively to harden while the outer portion remains relatively unchanged.

Conversely, the invention could be utilized to treat a plastic rod so that a sort of case hardening effect is achieved at the surface. In this case, the B-10 would be localized at the surface and the surface then subjected to bombardment by thermal neutrons to efiect hardening through polymerization.

In carrying out the various embodiments of the invention, B-lO may be present in any form. For example, boron may be present as an inorganic or organic compound. The term boron substance used in some of the claims is meant to cover boron present in either the elemental form or as a compound or other useful form.

As examples of the various types of polymerizable organic materials that can be treated with my invention, the following are given: polyethylene, polystyrene, polyisobutylene, polymethyl methacrylate, polytetrafiuoroethylene, epoxy resins, and the like.

Summarizing the invention, a method is provided for treating an organic substance, e.g. a polymerizable organic material, wherein at least a portion of said substance is provided With a small but eifective amount of B which is thereafter subjected to neutron bombardment whereby the high energy released by the fission of 3 is used to effect a change in said substance.

In addition the invention also provides a method for the differential treatment of a polymerizable organic material wherein a selected region of the material is provided with a small but efiective amount of B in which fission is thereafter induced by neutron bombardment, whereby the high energy released and deposited in the material causes said selected region of the material to undergo polymerization.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. For example, the boron may be applied to the plastic by vacuum deposition or controlled settling. In the vacuum deposition process, the boron would be placed in a carbon boat which also serves as a conductor. In the controlled settling process, powdered boron is temporarily suspended in liquid and allowed to settle on the surface. The liquid is then removed or evaporated, leaving only the boron on the surface of the material being treated. Such modifications and variations are considered to be within the purview and scope of the invention and appended claim.

What is claimed is:

A method of promoting crosslinking between chain molecules in a selected region of an article formed of an organic polymer having crosslinking characteristics which comprises, providing a first portion of an organic polymer selected from the group consisting of polystyrene and polyethylene containing at least a small but effective amount of 13 sufficient to yield high energy through nuclear fission, providing a second portion of said organic polymer substantially free of said 13 integrating said first portion with said second portion and forming an article from said integrated portions whereby a selected region of said article contains B and subjecting said article to bombardment by slow neutrons to effect fission of said 3 whereby the high energy released by fission of B in said selected region is utilized to promote crosslinkage between the chain molecules in said selected region.

References Cited in the file of this patent UNITED STATES PATENTS 2,743,226 Newson Apr. 14, 1956 2,793,970 Jeppson May 28, 1957 2,904,484 Houston et a1 Sept. 15, 1959 FOREIGN PATENTS 741,826 Great Britain Dec. 4, 1955 OTHER REFERENCES Proc. of the Royal Society (London), vol. 215A, Nov. 25, 1952,1 11 187-212.

Nucleonics, vol. 12, No. 6, June 1954, pp. 18-25.

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