Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to an apparatus for preparing
irradiated polyolefin film with substantially uniform dosage throughout the
width of the film.
This application is a divisional of our copending application
serial No. 296879, filed February 15, 1978.
Polyolefins and particularly polyethylene have been widely used
for various functions. A particular use of such material has been in the
form of a film or sheet of material used in packaging. Such material is
frequently oriented so as to be heat shrinkable around articles packaged
therein.
In order to enhance the properties of polyolefins and particularly
polyethylene, such materials are crosslinked by irradiation. An example
of such an irradiation process is disclosed in U.S. Patent No. 3,144,399
to Rainer et al. Such an irradiation process comprises subjecting the
polymer tb an irradiation dosage of about 1 to about 20 megarads.
In the process of producing oriented polyethylene film a tape
of extruded material is subjected to such irradiation and is then heated
and oriented by known techniques. The technique for so doing is
described in U.S. Patent No. 3,741,253 as it relates to the irradiation and
orientation of a copolymer of ethylene and vinyl acetate.
A problem, however, of producing irradiated film is that for
some reason the edge area of a film tends to be significantly less
crosslinked than the central area of the film, i.e., for some reason the
effective irradiation dosage at the film edge is less than the effective
dosage within the central portion of the film. This obviously results in
a film having non-uniform properties across the width thereo. A technique
is disclosed in U.S. Patent No. 2,993,120 for magnetically concentrating
electrons around a thin film during the irradiation process. This process,
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however, while increasing the efficiency of electron irradiation, does not
solve the problem of nonuniform dosages across the width of the film.
Accordingly, the invention provides in an apparatus for irradiating
a film of thermoplastic material comprising a source of accelerated electrons,
a space in the path of said accelerated electrons for passing a film to be
irradiated, the improvement comprising:
a deflection block located adjacent said space for deflecting
electrons; and
a deflection surface for receiving electrons deflected from said
deflection block for thus further deflecting said electrons toward a film
edge within said space.
The film produced using the apparatus of the invention may be
generally defined as a film of electron irradiated polymer with a
substantially uniform electron irradiation dosage across the entire width
thereof. This film has an electron dosage variation of less than plus or
minus 15 percent of the mean dosage and is produced by passing a film
through an electron irradiation zone having disposed therein, adjacent the
film edges, deflection blocks for deflecting electrons toward a deflection
bottom for further deflecting said electrons toward the bottom edges of
the film being irradiated.
The process involved in using the apparatus of the invention may
be generally defined as comprising the following steps: (1) passing
polymer film through said electron irradiation space; and (2) disposing
within the irradiating zone adjacent said film near the edges thereof said
deflection block for deflecting said electrons toward the deflector surface
disposed beneath said film to further deflect said electrons toward the
bottom edges of said film whereby said film receives a substantially
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uniform electron irradiation dosage across the entire width thereof. The
irradiation dosage has a variation of less than plus or minus 15 percent
of the mean dosage.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically illustrates film being irradiated by prior
art techniques.
Figures 2 and 3 schematically illustrate film being irradiated
by the technique of this invention.
Figure 4 is graphic profile representation of film dosages
produced using the prior art technique and the apparatus of this invention.
DETAILED DESCRIPTION
It has been found that film may be irradiated within a trough
having disposed adjacent the edges thereof elec~ron deflectors for the
purpose of deflecting electrons toward the bottom of the trough and thus
back toward the film edge. This invention may be more readily understood
by referring to the various figures of drawing. Figure 1 of the drawings
illustrates a conventional prior art process for irradiating film. As is
shown in Figure 1, a section of a film 1 passes beneath an electron
generator generally designated at 3. The electron paths are shown as the
lines 5, 7 and 9. In the conventional technique the electrons pass through
the sheet 1 and are absorbed or dissipated by the surrounding atmosphere.
For reasons which are not totally understood this arrangement does not ha~e
the efficiency in the edge portion of the film as it has in the central
portion of the film. This is perhaps attributal to the angle of the
electrons as they impringe upon the edges of the film as well as the
geometry of the film edge. Regardless of the reason, however, the prior
art process produces a film which generally has a 20 to 50 percent lower
dose in the edge portions than the body of the film~
In accordance with this invention it has been found that if the
film is irradiated within a trough, the film dosage is made substantially
more uniform across the width of the film. Figure 2 of the drawing
illustrates the technique utilized in carrying out this invention. As is
shown in Figure 2 of the drawings, the trough in which the film 1 is
irradiated possesses deflection boxes 13 which deflect the incident electron
beam at the edges of the irradiation zone toward the bottom of the trough
where the electrons are further deflected back toward the edge of the film.
The electron paths are illustrated by the lines 15 in Figure 2. It is, of
course, understood that this is not exactly the path that electrons follow
but merely an examplification for purposes of illustration of the function of
this invention. The actual electron probability distribution as has been
empirically determined from the utilization of the trough of this invention
is schematically illustrated in Figure 3 of the drawings. Figure 3 of the
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drawings illustrates a lobe of high electron probability density which is
substantially and desirably impinged by the edges of the film being
irradiated. The lobes 17 are large enough such that the film may actually
be sinuously looped through the lobes so as to have more than one section
of film irradiated by the back scattered electrons produced by the deflection
boxes 13.
The location of the side boxes 13 with reference to the incident
electron beam 15 is such as to produce an impingement upon the bottom 19 of
the trough at an angle which is as close to a normal angle as is possible.
By impinging at close to normal angle, the lobes 17 are produced. Utilizing
electrons accelerated at a voltage of 500 KV, it has been found that ideally
the deflection box 13 is located about 2 inches from the film edge and that
the film edge be located 1 inch from the trough bottom 19. The deflection
surface defined by trough bottom 19 need not be coextensive from one side
of the trough to the other, but needs only to be extended beneath the edge
to be irradiated by the back scattered electrons.
It is preferred that the angle between the deflection box 13 and
trough bottom 19 be substantially normal. However, this angle may be
deviated from so long as the electrons are deflected from the box to impinge
upon the film edge after being deflected from the trough bottom.
Figure 4 of the drawings illustrate the irradiation dosage profile
produced both by the prior art technique and by the technique of this
invention. As is observable in Figure 4, the prior art technique produces
substantially less dosage at the film edges marked by the lines E than in
the central portion of the film. The lower part of Figure 4 illustrates
the dosage produced by utilizing the deflector box of this invention. By
properly locating the deflector boxes the dosage profile can be made very
close to a substantially flat line, i.e., uniform dosage across the width
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of the Eilm. Utilizing the technique of this invention, irradiated film
having a dosage which deviates from the mean dosage of the film by plus
or minus 15 percent is produced. In contrast thereto, prior art techniques
without the deflection blocks of this invention produce film which has a
deviation across its width of from 30 to 50 percent of the mean dosage of
the film section. It is thus seen that the technique of this invention
produces irradiated film with a substantial improvement in the uniformity
of dosage across a profile thereof. As an illustration of the uniformity
produced utilizing the deflection technique of this invention, polyethylene
tubing having a single wall thickness within the range of about 10 to 40
mils has been irradiated to a mean dosage of about 8 megarads with a
deviation across the profile thereof of only 1 megarad. This represents
a substantial and surprising improvement over the prior art processes which
produce similar film having variations of up to 4 megarads across the profile.
The deflector blocks are of high molecular weight materials which
are normally used for electron deflection purposes. Generally a material
selected from tungsten, gold, lead or uranium may be utilized with equal
efficiency. While low molecular weight materials may be utilized, the
deflection efficiency is decreased with the lower molecular weight material.
Virtually any of the heretofore irradiated materials may be
irradiated utilizing the technique of this invention. Most suitable for
this invention are films which comprise polymers of ethylene as well as
the other olefins. Particularly applicable to the process of this invention
are copolymers of ethylene and vinyl acetate. The invention, however, is
not limited to such materials and may be used on materials such as silicone
rubber, ethylene propylene rubber and polyvinyl chloride. Generally the
materials and dosages described in "Radiation Processing" by Thomas G.
Mysiewicz, Plastics Technology, Vol, 23, No. 3, March 1977, may be utilized
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with this invention. This article is herewith incorporated by reference.
While this invention has been described generally with reference
to the irradiation of film generally, its application is most advantageous
when irradiating a flattened tubing of ext~uded material. A complete
descrîption of a process for extruding a cylindrical tube for irradiation
by prior art techniques is described in United States Patent No. 3,741,253.
As is described therein, the cylindrical tube is flattened to form a tape
for passage through an irradiation vault. The prior art problems of
non-uniform dosage ar0 magnified when such a tape is irradiated. The
process of this invention, however, substantially eliminates the prior art
problem associated with dosage non-uniformity. The process of this invention
produces a tape as described above having a dosage profile which is within
the range of plus or minus 15 percent of the mean dosage.
While the above description emphasizes particular aspects of this
invention, various changes may be resorted to without departing from the
spirit and scope of this invention as is defined by the following appended
claims.
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