U.S. patent number 3,802,429 [Application Number 05/159,935] was granted by the patent office on 1974-04-09 for surgical face mask.
This patent grant is currently assigned to Johnson & Johnson. Invention is credited to William H. Bird.
United States Patent |
3,802,429 |
Bird |
April 9, 1974 |
SURGICAL FACE MASK
Abstract
There is disclosed a surgical face mask comprising two layers of
a porous, nonwoven facing fabric and a nonwoven fabric filtration
medium between the layer of facing fabric. The nonwoven fabric
filtration medium is a spunbonded fabric made for continuous length
filaments having a diameter of from 14-20 microns. The major
proportion of the length of the filaments lie in planes which are
substantially parallel to the major surfaces of the fabric and,
therefore, perpendicular to the direction of air flow through the
mask.
Inventors: |
Bird; William H. (Lavallette,
NJ) |
Assignee: |
Johnson & Johnson (New
Brunswick, NJ)
|
Family
ID: |
22574738 |
Appl.
No.: |
05/159,935 |
Filed: |
July 6, 1971 |
Current U.S.
Class: |
128/206.19;
55/527 |
Current CPC
Class: |
B01D
39/14 (20130101); A41D 13/1115 (20130101); B01D
2239/1233 (20130101); B01D 2239/0627 (20130101); B01D
2239/0686 (20130101); B01D 2239/0668 (20130101); B01D
2239/065 (20130101) |
Current International
Class: |
A41D
13/11 (20060101); A41D 13/05 (20060101); B01D
39/08 (20060101); A62b 023/00 () |
Field of
Search: |
;128/139,14N,146.2,142.6,146.6,140 ;55/527,528,DIG.35,337,512 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Dunne; G. F.
Claims
Having thus described my invention, what I claim is:
1. A face mask having a body portion adapted to cover the nose and
mouth and having means to secure said body portion over the nose
and mouth, said body portion comprising a filtration medium
comprising a nonwoven fabric formed of continuous thermoplastic
filaments having a length of at least 2.5 inches and a diameter of
from 14 to 20 microns, substantially all of said filaments lying
generally in planes perpendicular to the direction of the flow of
air through the mask, said filtration fabric having a weight of
from 1.4 to 1.8 ounces per square yard and having a thickness of
from 0.01 to 0.02 inches and a void volume of about 85 percent and
being substantially free of binder, and a lightweight porous
nonwoven facing fabric on each major side of said filtration
medium.
2. The face mask of claim 1 in which the facing fabric has a weight
of from 200 to 400 grains per square yard.
3. The face mask of claim 1 in which the facing fabric is bonded
together with a thermoplastic binder.
4. The face mask of claim 3 in which the facing fabric and the
filtration medium are bonded together at intermittent points in
both directions over the surface of said mask.
5. The face mask of claim 4 in which the bond points constitute
less than 1 percent of the total filtration surface of the
mask.
6. The face mask of claim 1 in which the filtration medium
comprises filaments of poly(ethylene terephthalate).
Description
BACKGROUND OF THE INVENTION
This invention relates to an improved surgical face mask. Surgical
face masks have been employed in surgery for some time. The purpose
of these masks is to prevent the bacteria exhaled by the surgeon,
or other operating room personnel, from contaminating the patient
undergoing surgery, and also to protect the surgeon and other
operating room personnel from bacteria originating from the
patient. The original face masks that were used in surgery were
multiple plies or layers of gauze fabric which were positioned over
the nose and mouth of the surgeon. Multiple plies of linen fabric
were also used. These materials are not particularly good bacterial
filters and have efficiencies, in terms of filtering bacteria, of
less than 50 percent. These fabric face masks, although not
efficient bacterial filters, were extremely comfortable to wear,
that is, they were comfortable to the face of the surgeon and also
had low air resistance. Air resistance, as used herein, is a
measure of the resistance to air flow through the mask. It is
determined by measuring the pressure drop across the mask when the
mask is placed in an air stream. Air could readily flow through the
woven fabric masks because of the openness of the fabrics. That is,
the interstices formed by the weave of the fabrics were large
enough in number and individual size to provide a significant
proportion of open or fiber free area in the mask. The low air
resistance made these fabric masks quite easy to breathe through.
The relatively open arrangement of the fibers in the fabric mask
which allow the ready passage of air through the mask also allowed
bacteria to penetrate through the mask.
Because of the poor bacteria filtration efficiency of the fabric
masks, efforts were directed to the production of a single use
disposable mask which would have greater bacterial filtration
efficiency than the fabric masks previously used. The single use
masks have substantially increased bacterial efficiency as compared
to the fabric masks, attaining efficiencies in the range of 90
percent or greater, but have created another problem. The new
problem of the single use mask is that with the high bacterial
efficiency, air resistance has substantially increased. These
single use masks have air resistances so great that the mask will
literally move away from the mouth of the wearer when the wearer
exhales. This indicates that air is not flowing through the mask
but building up pressure between the inner surface of the mask and
the mouth and nose of the wearer. This pressure not only makes
these masks uncomfortable to wear but also forces the masks away
from the face when exhaling and allows the bacteria to escape out
of the edges of the mask thus defeating the purpose of wearing the
mask. Additionally, a number of these single use masks have been
made with very short fiberglass, asbestos or other fibers which
tend to become detached from the filter medium of the mask and are
inhaled by the wearer. These small particles have a tendency to
irritate the wearer of the mask. The high air resistance of these
masks also makes the masks uncomfortably warm when worn. This is
especially true when the masks are being worn for any length of
time. The passages within the mask that allow air to flow through
tend to eventually clog from dust or bacteria or other extraneous
small particles found in the atmosphere of the operating room and
eventually, over a period of two or three hours, it becomes
extremely difficult to breathe through the mask. This condition
creates considerable discomfort to the wearer of the mask.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a surgical face mask that has high
bacteria filtration efficiency and yet has very low air resistance.
It would normally be expected that as the bacteria filtration
efficiency is increased, the air resistance would increase or at
least remain at a relatively constant level. The air resistance can
be considered to be related to the number of openings in the
effective filtration medium of the face mask or the total open area
in the mask. As the total open area of the mask increases, the air
resistance would decrease but so would the bacteria filtration
efficiency. In the present mask, I have obtained a low air
resistance but have maintained the level of bacteria filtration
efficiency that is equivalent to the previously used single use
surgical masks. This seemingly contradictory result has been
obtained by employing as the filtration medium a continuous length
synthetic thermoplastic filament rather than short fibers. The
term, continuous length filament, as used herein is a filament
which is greater in length than two and one half inches and
preferably, of an indeterminate length. These filaments are
arranged in the form of a nonwoven fabric by bonding the filaments
immediately after extrusion into a self-supporting web by the
application of minimal amounts of pressure to the web but using no
binder. The particular nonwoven fabric that has been found to be
useful is made from poly ethylene terephthalate filaments which are
deposited to form a fabric immediately after extrusion in the
manner taught by Canadian Pat. No. 775,807. Generally this process
consists of spinning the filaments, applying an electrostatic
charge to the filaments, permitting the filaments to separate due
to the applied electrostatic charge, orienting the filaments, and
laying the filaments down in a random nonwoven web which is
essentially free of filament aggregates. The process may be
operated to give a high level of crimp to the filaments. The
filaments in the fabric are arranged in a substantially parallel
relationship in planes perpendicular to the direction which air
will move through the fabric in the mask. The reason why this
arrangement of the filaments results in a high bacterial filtration
efficiency mask with a low air resistance is not clear. It would
appear that the arrangement of the filaments of a particular
diameter in this manner provides for a maximum number of open areas
and yet maintains a relatively small diameter of these open areas
so as to effectively trap bacteria and prevent them from moving
through the filter medium while at the same time providing a high
volume of air to move through the mask. It is also possible that
the high bacterial filtration efficiency of the mask is related to
the electrostatic charge that is placed on the filaments in the
processing of forming the web. The electrostatic charge could
either attract and hold the bacteria in the mask or even exert a
lethal electrostatic charge to bacteria passing through the
mask.
The elimination of the binder in the fabric substantially
contributes to the conformability of the finished face mask. The
binder content in nonwoven fabrics has a tendency to stiffen the
fabrics. In the present face mask, the filtration medium is
substantially free of binder and is readily conformable to the
contours of the face of the wearer. The presence of the binder does
not aid in increasing the filtration efficiency of the filtration
medium in the mask.
DETAILED DESCRIPTION OF THE INVENTION
The present invention may best be understood with reference to the
following drawings in which:
FIG. 1 is an isometric view of the mask of the present invention
folded in the form it would appear on the wearer's face.
FIG. 2 is a plan view of the fabric of the present mask showing the
folds in the mask and a magnified exploded section of the mask.
FIG. 3 is a schematic cross sectional view taken along the lines
3--3 in FIG. 2 showing the folds in the fabric of FIG. 3.
FIG. 4 is an enlarged cross sectional view of the mask taken along
the lines 4--4 of FIG. 2.
FIG. 5 is a photomicrograph of the filtration medium of the present
mask at a magnification of 100 X.
FIG. 6 is another photomicrograph of the filtration medium of the
present mask at a magnification of 150 X.
As shown in FIG. 1, the face mask, as it appears in use, consists
of a main body 10 to which is attached at the upper edge, a metal
nose clip 11. The metal nose clip is made from a metal which can be
easily bent to conform to the nose of the wearer, such as aluminum.
The mask has attached to the main body, by sewing or heat sealing
or other suitable means, a seam binding 13 on the upper and low
edges of the mask. There is also attached to the main body of the
mask on the side edges of the mask, an additional seam binding 12
which is of sufficient length to extend beyond the top and bottom
edges of the mask and is employed as the tie string to affix the
mask to the head of the wearer. There also appears in FIG. 1,
bonding points 14 whose function will be later explained.
FIG. 2 shows the configuration of the folds of the mask. The
laminate which constitutes the main body of the mask is folded on
itself in an accordion shape from the top edge of the mask 15 to
the bottom edge of the mask 16. The mask of FIG. 2 is shown to be
folded in three places at 17, 18 and 19, but the particular folding
arrangement or number of folds is not part of the present
invention. This particular configuration of the mask has previously
been found to provide excellent conformability to the face of the
user.
The magnified break away portion 20 of FIG. 2 shows the various
layers of the mask. The outside surface of the mask is a highly
porous nonwoven facing fabric 21. The center portion containing the
spun bonded fabric filtration medium 22 of the present invention
lies between the facing fabric 21 and another layer of facing
fabric 23. Both layers of facing fabric may be constructed of the
same highly porous nonwoven fabric. The purpose of the facing
fabric portions of the mask is to contain and retain the filtration
medium. The outside layers are a highly porous nonwoven fabric of
relatively light weight and contribute very little if any
filtration properties to the mask construction. Being highly
porous, these materials also offer very little resistance to the
flow of air through the mask. The weight of the facing layer of the
mask may be between about 200 and 400 grains per square yard. It is
advantageous to use extremely light weight facing webs so as not to
increase the stiffness and thereby reduce the conformability of the
face mask. The facing material can be a carded nonwoven fabric
bonded with a thermoplastic binder. A suitable thermoplastic binder
is an emulsion polymerized self-curing acrylic binder.
It has been found to be advantageous to spot bond, by heat sealing
or other bonding procedures, the three layers of the face mask
together. As each of the individual layers of the laminate are
relatively light weight, the layers are bonded together so they may
be pleated and the seam binding applied with the minimum
possibility of tearing or otherwise disintegrating the webs during
the fabrication of the mask. Additionally, the bonding of the inner
layer of the face mask to the fabric filtration medium at certain
points prevents the inner layer of the mask from moving over the
nose or mouth of the wearer as the wearer inhales. If the inner
layer is not bonded, it is possible that it could move against the
nose or mouth of the wearer when the wearer inhales. Although this
does not reduce the efficiency of the mask, it has been found to
cause some wearer discomfort. The bonding may be carried out by
heat sealing the laminate together with sufficient force to
effectuate the bond through all layers of the mask. It has been
found that a heat sealer heated to a temperature of approximately
400.degree.F and using 80 psi and a 1 second dwell time is
sufficient pressure to bond the three layers of the mask together.
As shown in 14 in FIG. 4, the bonded spot may extend through the
total depth of the mask. As the filtration medium fabric filaments
are thermoplastic, the application of heat and pressure causes the
filaments to melt upon cooling and harden and form a permanent bond
between the three layers of the mask.
It is also possible to employ hot melt adhesives, or other adhesive
materials to effectuate the spot bonding of the layers of the mask
together. It is desirable to make the total bond areas as small as
possible. It has been found that six to nine bonded areas located
beneath the folds of the mask, each area approximately one-eighth
inch square, are sufficient to bond the three layers of the mask
into a unitary laminate. The total area of the bond points is not
critical although it should be recognized that the bond area must
be large enough to accomplish the desired result but not so large
that it could materially reduce the filtration efficiency or
increase the air resistance of the mask. The bond points tend to be
impervious areas which prevent passage of air through the mask. A
total bonded area covering less than 1 percent of the total surface
area of the mask has been found to be adequate to provide the
desired bonded properties. A total bonded area of this order will
not measurably reduce the bacterial filtration efficiency or
increase the air resistance of the mask.
Although the particular placement of the bond areas on the mask is
not critical to the functioning of the mask, I prefer to place the
bond area beneath the folds of the mask as shown schematically in
FIG. 3.
As previously stated, the filtration medium itself is a spunbonded
fabric. A spunbonded fabric of a weight of between about 1.4 to
about 1.6 ounces per square yard has been found to offer the
desirable combinations of high bacterial efficiency and low air
resistance. The filaments in the spunbonded fabric are from about
14 to about 20 microns in diameter. The filaments are not perfectly
circular in diameter as can readily be seen from the
photomicrographs FIG. 5 and FIG. 6. In a spunbonded fabric of the
type used in the present invention, the greater majority if not all
of the filaments in the fabric are of indeterminate length. When
the spunbonded fabric is made, these filaments are extruded as a
continuous filament and are laid down on a moving belt in a random
fashion, but the major portion of the length of the filaments lie
in planes that are substantially parallel to the conveying
direction of the moving belt. This can be seen in FIG. 4. The
facing fabrics 21 and 23 contain short fibers some of which are
oriented perpendicular to the major faces 24 and 25 of the mask.
The filaments in the filtration medium fabric 22 are oriented so
that the major portion of the filament length lies in planes that
are parallel to the major faces of the mask or perpendicular to the
direction of air flow through the mask. These filaments are shown
as 26 and 27 in FIG. 4. The fabric is utilized in the face mask in
such a manner that the air flow through the mask is in the
direction perpendicular to the parallel planes in which the fibers
are arranged. This results in a maximum utilization of the
filaments in the fabric to form fiber free openings in the web and
these openings are of a sufficiently small size to prevent passage
of bacteria through the web.
In order to maintain a high degree of conformability that is
desirable in the surgical face mask, I employ a spunbonded fabric
which has relatively low tensile strength. Tensile strengths of
less than 1 pound per square inch width are suitable for the
spunbonded fabric filtration medium. These realtively low tensile
strengths are present in the spunbonded fabrics which contain no
binder and relatively few self-bonded areas. The addition of binder
has a tendency to increase the tensile strength of the spunbonded
fabric filtration medium, thereby reducing the conformability but
adding no particular advantage in filtration efficiency.
I have found that the filtration medium fabric should be one which
contains no significant amounts of added binder. The number of
self-bonded areas should be only those that are required to process
the fabric filtration medium web through the remainder of the
process equipment. Additional bond points add no particular
advantage in bacterial filtration efficiency and can reduce the
conformability of the web and thus the conformability of the face
mask.
EXAMPLE I
Table I shows the result of a comparison of the average filtration
efficiency and air resistance of six surgical face masks with a
construction containing poly ethylene terephthalate (reported as A)
filtration medium of the present invention with six masks
containing a glass fiber filtration medium of the prior art
(reported as B).
TABLE I
A B Bacterial Filtration Efficiency 92% 92% Air Resistance 0.020"
0.220"
The bacterial filtration efficiency was determined in the following
manner: Staphylococcus aureus bacteria was nebulized into a spray
mist and forced through an aperature in a closed conduit. The
bacteria passing through the aperature were trapped on a Milipore
filter and then innoculated on agar plates. After a period of 24
hours, the bacteria colonies were counted. The same procedure was
repeated with a face mask blocking the aperature of the conduit.
The efficiency of the face mask is determined by comparing the
colony count of the plates with and without the face mask in the
aperature.
The air resistance, reported in Tables I and II in inches of water
differential, was determined by passing air at a predetermined flow
rate (85 liters per minute in Table I) through 17.8 square inches
of a face mask of the indicated construction. The pressure drop
between the upstream and downstream sides of the mask is a measure
of the air resistance of the mask. Table II shows the effect of air
resistance at different flow rates of masks of the construction of
A and B above and on both facing layers of the masks which were
identical in both the A and the B masks.
TABLE II
Flow Rate A B Facing Layers 85 Liters/min 0.0183" 0.2033" 0.008" 50
Liters/min 0.0115" 0.1175" 0.006" 30 Liters/min 0.0075" 0.0675"
0.0035"
Tables I and II clearly show the low air resistance of the face
masks of the present invention. The air resistance of the mask of
the present invention is less than the air resistance of the prior
art mask by a factor of about 10.
The spunbonded fabric filtration medium when formed by extruding
the polymeric filaments onto a moving conveyor and subsequently
subjecting the web to light pressure to self-bond the filaments
tends to keep the filaments in planes which are parallel to the
surface of the conveyor on which the web is formed. FIG. 5 and FIG.
6 are photomicrographs of the spunbonded fabric employed in the
face mask. It can readily be seen from these photomicrographs that
the majority or substantially all of the fibers lay in planes which
are parallel to the surface of the sheet of the drawing. As
previously stated, this parallel arrangement of fibers in which the
fiber diameter is from 14 to 20 microns results in a filtration
medium containing a large number of open areas but with a
relatively small size of any individual open area. There is thus
provided a relatively large total cross sectional area in the mask
for air to pass through, but each individual oepning is small
enough to prevent the passage of bacteria. The filtration fabric is
formed in relatively thin webs of from about 0.01 to about 0.02
inches in thickness and preferably between 0.014 and 0.018 inches.
This thickness is sufficient to prevent the passage of more than 90
percent of the bacteria that could pass through the mask as
determined by invitro tests. The void volume, that is, the total
volume which contains no filaments is greater than 85 percent. The
high void volume and the low air resistance would indicate that
most of the fibers lie in planes which are parallel to the major
surfaces of the filtration medium, and that there is a relatively
small percentage of filaments which extend from one of the major
surfaces of the filtration medium to the other. Both FIG. 5 and
FIG. 6 which show a relatively small number of filament ends also
would tend to support this conclusion.
* * * * *