Nonwoven fabric of netting and thermoplastic polymeric microfibers

A composite nonwoven fabric-like laminate which includes an integrated mat of polymeric microfibers having a diameter of less than 30 microns and a layer of nonwoven linearly oriented thermoplastic netting, the mat and netting being either continuously bonded or spot-bonded to produce a laminate with the desired properties.

BACKGROUND OF THE INVENTION 
This invention relates generally to nonwoven fabrics and, more particularly 
to a family of nonwoven fabrics which are formed by combining a melt-blown 
polypropylene mat and directionally oriented thermoplastic netting. These 
nonwoven fabrics have a unique combination of properties including 
strength and precisely controlled porosity. 
DESCRIPTION OF THE PRIOR ART 
Nonwoven fabrics have been produced by combining an integrated mat of 
discontinuous thermoplastic polymeric microfibers with nonuniform porous 
reinforcing scrims of spunbonded fabrics. Prior art workers have sought to 
incorporate a number of critical physical properties in fabrics for use in 
surgical gowns, drapes, etc. These properties include desirable 
fabric-like characteristics, adequate strength, textile-like capability, 
water repellent capability, desirable surface abrasion characteristics, 
and high bacteria strikethrough resistance. Exemplary prior art fabrics 
are taught in U.S. Pat. No. 4,041,203 to Brock et al, issued Aug. 9, 1977. 
Other nonwoven fabrics have been produced by combining an integrated mat of 
melt-blown fibers with an apertured film, or with both apertured film and 
spunbonded fabric. Exemplary composite nonwoven fabrics are taught in U.S. 
Pat. No. 4,196,245 to Kitson et al, issued Apr. 1, 1980. This patent 
teaches a composite nonwoven fabric including at least two hydrophobic 
plies of microfine fibers and at least one nonwoven cover layer which may 
be an apertured film, a spunbonded ply or an air laid, wet laid or carded 
ply of fibers. The cover ply is used to add strength to the fabric and for 
surgical items, to provide surface stability, i.e., resistance to abrasion 
and pilling. This nonwoven fabric is particularly useful when air 
permeability and resistance to liquid and bacteria strikethrough are 
desired in a fabric having aesthetic qualities similar to a woven fabric. 
This patent teaches that it is essential that the composite fabric contain 
at least two microfine fiber plies to achieve the unique air 
permeability/liquid and bacteria strikethrough resistance relationships 
that are desired. 
The product of the instant invention includes an integrated mat of 
microfine fibers and a thermoplastic nonwoven fabric wherein the composite 
has the directional stability, uniform opening size and aesthetic 
properties of a woven fabric. This product has advantageous aesthetic 
qualities which are not obtainable in the products of the prior art which 
contain a layer of film. The instant product also has porosity control 
which is not obtainable in constructions which are reinforced by 
spunbonded scrims containing nonuniform openings. 
SUMMARY OF THE INVENTION 
This invention includes a nonwoven fabric-like material including one or 
more integrated mats of generally discontinuous thermoplastic polymeric 
microfibers and one or more layers of nonwoven continuous, linearly 
oriented thermoplastic netting having uniform network structure. The 
integrated mat includes randomly laid discontinuous filaments having an 
average filament diameter of not more than about thirty microns and having 
a basis weight of between about ten and about fifty grams per square 
meter. The netting and the mat are bonded together to form a highly useful 
multi-layer nonwoven fabric. The melt-blown microfine fiber mat 
contributes fibrous quality, excellent filtration characteristics and 
opacity to the composite while the nonwoven thermoplastic netting provides 
strength, porosity control and simulation of the directional pattern of 
woven textiles.

DESCRIPTION OF THE INVENTION 
Referring to the drawings in detail, there is illustrated in FIG. 1 a two 
ply nonwoven fabric 10 in the form of a laminate which is a product of 
this invention. The embodiment of FIG. 1 depicts a melt-blown 
polypropylene microfiber mat 12 and a directionally oriented thermoplastic 
netting layer 14. Netting layer 14 has main filaments 16 and tie filaments 
18. 
To provide a relatively soft, general purpose nonwoven fabric, especially 
suited for application as a wrapping material for surgical kits known as 
central supply room wrap, the lamination can be discontinuous. The 
discontinuous or spot bonded lamination can be provided by using an 
engraved pressure roll with a smooth backup roll. For continuous surface 
lamination, a smooth pressure roll is used with a smooth backup roll. 
Combining the layers 12 and 14 in a continuous manner, can provide a 
product like paperboard that is suitable for various medical packaging 
applications. FIG. 2 shows schematically one manner of continuously 
preparing the two-ply nonwoven fabric shown in FIG. 1. In FIG. 2, there is 
shown a melt-blown microfiber mat 20 and a thermoplastic netting layer 23 
being fed from supply rolls 24 and 26, respectively. Guide roll 28 is used 
to advance mat 20 and netting layer 27 into contact with heated pressure 
roll 30 and backup roll 32. If discontinuous lamination is desired, one of 
the rolls 30 and 32 will have an engraved surface and the other roll will 
have a smooth surface. Preferably, pressure roll 30 will have the engraved 
pattern. If continuous lamination is desired, rolls 30 and 32 should have 
smooth surfaces. The smooth rolls can have an elastomer covering such as 
silicone, fluorocarbon or another type with Durometer hardness of 
approximately 60 to 80. The laminated nonwoven fabric 34 is then passed 
over guide roll 36 to a take-up spool (not shown). Optionally, the 
laminate 34 can be passed over cooling rolls after leaving roll 32 to 
lower the temperature of the nonwoven fabric prior to reaching a take-up 
spool, especially if a high rate of lamination is desired. Also, radiant 
heating or additional heating rolls can be used near the position of guide 
roll 28 to raise the temperature of mat 20 and netting layer 22 to the 
required bonding temperature before reaching rolls 30 and 32. This 
optional approach is desirable for reaching higher throughput rates with 
satisfactory lamination than would be affordable with the relatively 
limited surface contact of a single heating station provided by nip rolls 
30 and 32. The operating temperature of rolls 30 and 32 should be adjusted 
to a surface temperature such that the laminate components, mat 20 and 
netting layer 22, will reach their respective softening points, but will 
not reach their crystalline melting point at the desired throughput, i.e., 
rate of production. Bonding of the layers 20 and 22 can be further 
facilitated by prior oxidative treatment of the layers, such as by high 
voltage discharge or flame applied to the surfaces to be bonded. 
FIG. 3 shows another embodiment of the subject invention where a three-ply 
structure 38 is shown. The inner layer 40 represents a melt-blown 
microfiber mat. Outer layer 42 is a layer of thermoplastic nonwoven 
netting having main filaments 44 and tie filaments 46 which are orthogonal 
to filaments 44. The other outer layer 48 also consists of thermoplastic 
nonwoven netting. Layer 48 has main filaments 50 and tie filaments 52, 
which are orthogonal to filaments 50. Outer layers 42 and 48 are 
orthogonally oriented with respect to each other wherein, for example, the 
main filaments 44 of outer layer 42 are at a ninety-degree angle to the 
main filaments 40 of outer layer 48. Since the main filaments of the 
nonwoven netting layers may have higher strength properties as compared to 
the respective tie filaments, the orientation of layers 42 and 48 in 
laminate structure 38 can provide substantially enhanced strength 
characteristics in both machine and cross-machine directions. 
FIG. 4 shows still another embodiment of the subject invention wherein 
three-ply structure 54 has outer layers of nonwoven netting 56 and 
melt-blown microfine mat 58 and an inner layer 60 of nonwoven netting. As 
shown in FIG. 4, the main filaments 62 of the layer 56 are orthogonal to 
the main filaments 64 of the inner layer of nonwoven netting 60. 
Correspondingly, the tie filaments of layer 60 are also orthogonal to the 
tie filaments of layer 56. 
Other combinations of nonwoven netting layers and microfine mat layers are 
within the scope of the subject invention. For example, it may be 
desirable to have a three-ply composite with a mat of melt-blown microfine 
fibers on each side and a layer of thermoplastic nonwoven netting between 
the mats, thereby adding the required strength and stability to the 
composite. 
FIG. 5 shows, schematically, a process which can be used for the 
manufacture of nonwoven fabrics shown in FIGS. 3 and 4. If a structure 
similar to that of FIG. 3 was desired, netting layer 66 would be supplied 
from supply roll 68. Netting layer 70, having its main and tie filaments 
at a ninety degree angle to the main and tie filaments of layer 66, 
respectively, is fed from supply roll 72. Theremoplastic melt-blown mat 74 
is fed from supply roll 76 to form the inner component of the laminate. 
The three layers pass over guide roll 78, into the nip of heated pressure 
roll 80 and backup roll 82 wherein the three layers are bonded together, 
either continuously, using smooth rolls 80 and 82 or discontinuously, 
where spot-bonding is provided by the embossing on preferably engraved 
roll 80. The laminated fabric then is fed to a take-up spool (not shown). 
The melt-blown mat, as used in this invention, consists of randomly laid 
discontinuous filaments ranging from less than 1 micron to about 30 
microns in diameter. The integrated mat can be prepared by known 
techniques such as is taught in the article entitled SUPERFINE 
THERMOPLASTIC FIBERS by Van A. Wente, appearing in Inudstral Engineering 
Chemistry, Volume 48, Number 8, August, 1956, pp. 3142-3146, or disclosed 
in U.S. Pat. No. 3,849,241 to Buntin et al. 
The melt-blown microfiber mats disclosed herein may be made from a wide 
variety of thermoplastic polymers. In addition to polypropylene, 
polyethylene, polyamides, polycarbonates, polyesters, acrylic polymers, 
fluorocarbon polymers or other thermoplastic materials which have a 
suitable viscosity for melt-blowing may be used. Modacrylic polymers, 
which are fire resistant, may also be used for special applications where 
fire retardancy is required. 
The oriented netting or network structures used in this invention may be of 
the types disclosed in the following patents: Mercer (U.S. Pat. Nos. 
4,020,208 and 4,059,713); Larsen (U.S. Pat. No. 4,152,479); Kim et al 
(U.S. Pat. Nos. 3,914,365 and 4,144,368); and Liu (U.S. Pat. 4,140,826). 
This netting may be either a polypropylene homopolymer, a 
propylene-ethylene (2 to 50% by weight) copolymer, or other polymers of 
choice, and may be either natural or pigmented. Preferably, the netting 
should have uniform network structure. 
One type of thermoplastic netting useful in this invention is disclosed in 
U.S. Pat. No. 4,207,375 to Kim et al, incorporated herein by reference. 
This patent discloses single layer network structures having oriented 
parallel continuous main filaments extending in one direction, with 
uniform cross-sections and discontinuous parallel tie filaments extending 
in another direction, wherein the tie filaments interconnect the main 
filaments without any substantial portion of the tie filaments crossing 
over the main filaments. Also, each of the tie filaments between each pair 
of adjacent main filaments has its longitudinal axis in axial alignment 
with the longitudinal axis of the adjacent tie filament. 
Another type of thermoplastic netting useful in this invention consists of 
two sets of parallel continuous filaments in two different planes and in 
two different directions. The two sets of filaments are an example of 
nonwoven fabrics consisting of thermoplastic netting bonded together. The 
two sets of continuous filaments can be of the same weight and strength, 
or one set of filaments may provide most of the weight and strength, while 
the other set of filaments act as cross or tie filaments. The filaments 
can range in size from 5 to 400 microns in diameter and the pore openings 
therebetween can range from 1 to 6,000 microns in largest dimension. 
Where discontinuous lamination is desired, one of the pressure rolls used 
in the lamination process can be engraved. The embossing roll pattern 
should be designed to provide a spot-bonded effect which will in turn 
result in softness, foldability and aesthetically pleasing surface 
formations. 
Having set forth the general nature of the invention, the following 
examples illustrate some specific embodiments of the invention. It is to 
be understood however, that this invention is not limited to these 
examples since the invention may be practiced by the use of various 
modifications. 
EXAMPLE 1 
A fibrous mat of melt blown polypropylene microfibers was placed between 
two layers of directionally oriented thermoplastic netting having uniform 
network structure similar to the structure shown in FIG. 3. The mat of 
melt-blown thermoplastic microfibers, made by Riegel Products Corporation, 
Milford, New Jersey, weighed 30 grams per square meter and contained 
fibers ranging from approximately 1 to approximately 30 microns in 
diameter. The thermoplastic netting used, sold by Hercules Incorporated, 
Wilmington, Delaware, was made from a copolymer of propylene with 25% 
ethylene. One of the polypropylene netting layers had its heavier, 
stronger filaments in the machine direction while the other netting layer 
had the same size filaments but had its heavier, stronger filaments 
oriented in the cross machine direction as shown in FIG. 5. The heavier 
filaments in each of these types of netting were of approximately 100 
micron diameter while the lighter, tie filaments in each of the netting 
layers were approximately 10 microns in diameter. 
The three-layer composite was heated by contact on the surface of an 
engraved steel roll for approximately two (2) seconds at a surface 
temperature of approximately 270.degree. F. The composite was then nipped 
between the engraved roll and a silicone-rubber covered steel backup roll, 
heated to surface temperature of 150.degree. F., at 48 lbs per linear inch 
of nip pressure. 
The embossing pattern of the engraved roll used for this lamination 
contained 178 dots per square inch, with about half of the dots of 40 mil 
diameter, and the remaining half having a diameter of about 25 mils. The 
dots were uniformly spaced in a geometric pattern with a minimum distance 
of 10 mils between dots. 
The laminated product had physical properties as listed for Example 1 in 
Table I. This product had sufficient permeability for gas sterilization 
and sufficient thermal stability for autoclave sterilization at 
270.degree. F. 
The laminated product also met the requirements for bacteria hold-out for a 
hospital central supply room wrap for routine wrapping of surgical 
implement packs or kits. 
EXAMPLE 2 
A laminate similar to that made in Example 1 was produced with the 
following exceptions: The two layers of thermoplastic netting were 
pigmented white; in the laminating process the two layers of thermoplastic 
netting were placed together as shown in FIG. 4 and against the hot roll 
surface; and the melt blown thermoplastic microfiber mat was pigmented 
blue. 
The composite resulting in this case had the mat of thermoplastic 
microfibers bonded to one side of two layers of the thermoplastic netting, 
instead of being placed between the thermoplastic netting as it was done 
in Example 1. This construction had the unique advantage of a 
"tamper-proof" central supply room wrap. When surgical instruments are 
wrapped with this laminate and the wrapping is sealed with adhesive tape 
applied to the colored microfine fiber mat, the package cannot be opened 
without lifting the tape together with the tell-tale pigmented layer of 
melt-blown mat which will be under and adhering to the adhesive of the 
tape. 
EXAMPLE 3 
In this example a mat with white pigmented thermoplastic microfibers 
similar to the mat used in Example 1 was placed between two layers of 
thermoplastic netting which were made from a polypropylene homopolymer. 
The three-layer composite was heated and laminated as in Example 1, except 
that in this example both pressure rolls had a smooth surface. A 
continuously bonded laminate resulted which had properties as shown in 
Table 1. Although the melt-blown thermoplastic microfiber mat was of the 
same type as used in Example 1, in this example, the thermoplastic netting 
was of homopolymer polypropylene. Also the main filaments in the two 
layers of thermoplastic netting had a diameter of approximately 150 
microns and the tie filaments had a diameter of approximately 10 microns. 
The results of the tests performed on the laminate of this example are 
also shown in Table I. The laminated product was dimensionally stable in 
steam sterilization at 270.degree. F. and was satisfactory as a barrier to 
bacteria penetration for useage as a lid material for surgical instrument 
packaging trays and as a component for pouch packaging. 
EXAMPLE 4 
A two-ply nonwoven fabric was made similar to the structure shown in FIG. 
1, having a mat of polypropylene microfibers and a layer of polypropylene 
homopolymer netting. The netting layer had a rectangular pattern, with the 
main and tie filaments having cross-sectional diameters of approximately 
150 microns. This example illustrates a simple two-ply, lightweight, 
economical wrapping material. 
EXAMPLE 5 
This five-ply laminate illustrates a construction which combines a high 
strength reinforcing scrim with polypropylene netting on both exterior 
surfaces, in part to provide protection against linting. The laminate 
includes an inner layer of polypropylene homopolymer reinforcing scrim 
having an original basis weight of approximately 0.53 ounces per square 
yard. On each side of the inner scrim layer is placed a layer of 
thermoplastic microfibers similar to those used in Example 1. The cover 
layers of the laminate are polypropylene homopolymer netting having 
uniform size openings. The expected properties for such a netting are 
shown in Table 1. 
EXAMPLE 6 
This two-ply laminate, having a structure similar to that shown in FIG. 1, 
illustrates the use of 0.3 ounce per square yard polypropylene netting, 
known as Conwed SX-2086 (available from the Conwed Corporation of 
Minneapolis, Minnesota). This netting has 12 and 14 strands per inch, in 
the machine and transverse direction, respectively, and has quadrangular 
shaped openings. 
EXAMPLE 7 
This two-ply composite has netting similar to that used in Example 6, 
except that the openings are diamond-shaped and the reinforcing strands 
are diagonal to the main filaments. 
The capability of steam sterilization of the laminates of this invention is 
a unique feature. Many of the prior art materials which would otherwise be 
very useful for sterile wrapping applications cannot withstand 
temperatures approaching 270.degree. F. Nonwoven fabric laminates, made of 
polypropylene in accordance with this invention, can withstand the 
required temperatures for steam sterilization. This property renders 
materials made in accordance with this invention very useful for the 
wrapping and subsequent high temperature sterilization of surgical 
utensils. 
The thermoplastic netting materials used as components of the instant 
invention contribute to another significant advantage over the prior art. 
Some prior art products are reinforced by means of spun-bonded fabrics 
which are nonuniform in sizes of openings. The variation for a dimension 
of opening size can range from negligible up to several hundred mils in 
the space of a square inch. The preferred nonwoven linearly oriented 
thermoplastic netting materials used in the instant invention have a 
variation in dimensions of size opening of less than 5% over hundreds of 
square yards. The uniform opening size, contributed prior to lamination by 
the plastic netting, gives assurance of a certain minimum coverage 
capability for the combination with a layer of microfine fibers. In prior 
art products consisting of two inherently nonuniform components, e.g., 
spun-bonded and melt-blown mat, low coverage segments in each component 
can occasionally coincide to give areas of poor barrier properties. 
The plastic netting of uniform, small opening size, when used on the 
surface of the fabrics as produced in this invention, provides the 
capability to substantially prevent the release of lint from the fabric. 
The net type facings immobilize the microscopic fiber particles which 
generally migrate from the surface of a fibrous material such as a 
melt-blown microfiber mat. This lint proofing effect is not attainable 
from a nonuniform opening size type scrim such as a spun-bonded material. 
The netting faced fabrics of this invention, when tested by the 
Gravimetric Dry Lint Method (Parker et al, INDA Tech. Sympos., March 1978) 
showed less than one-sixteenth of the weight of particulate matter which 
is released from comparable melt-blown prior art fabrics. 
Uniformity of opening size, with its corresponding assurance of a definite 
minimum opening size, is also important in filtration related applications 
of the products of this invention. The preferred embodiments of the 
invention for this application, consisting of polyolefin components, are 
especially useful as chemically inert filter media. 
In addition, the ability to provide a reinforcing scrim which has both high 
strength and controlled porosity is advantageous in applications which 
utilize the well known excellent thermal insulating properties of 
melt-blown, microfibrous mats. The products of this invention can function 
as nonwettable, inert, lightweight thermal insulating liners for garments, 
gloves, boots and the like. 
It is to be understood that the above description and drawings are 
illustrative of this invention and not in limitation thereof. As will be 
evident to those skilled in the art, various modifications can be made in 
light of the foregoing disclosure and discussion without departure from 
the spirit or scope of the disclosure or from the scope of the claims. 
TABLE I 
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Basis Strip Tensile 
Edge Tear 
Mullen 
Frazier Air 
Example 
Weight 
Thickness 
Strength, Lbs./in. 
Strength, Lbs. 
Burst 
Permeability 
No. Oz./Yd..sup.2 
Mils MD/TD MD/TD psi Ft..sup.3 /Min./Ft..sup.2 
__________________________________________________________________________ 
(a) (b) (c) (d) 
1 1.7 13 5.6/8.9 9.5/11.5 
22 38 
2 1.7 14 6.1/7.6 9.5/8.9 22 56 
3 2.2 8 15.9/16.7 
16.5/18.0 
38 13 
4 1.5 11 5/6 8/9 18 60 
5 2.3 19 23/26 30/33 45 10 
6 1.5 8 5/6 7/9 21 45 
7 1.5 9 5/7 8/9 23 56 
__________________________________________________________________________ 
(a) ASTM D1682, Part 24 
(b) ASTM 82767 
(c) ASTM D75168, par. 32, proc. 1 
(d) ASTM D73675 
MD = Machine direction 
TD = Transverse direction