Method of making a stretchable orthopaedic fiberglass casting tape

A method of making a non raveling stretchable fiberglass fabric by knitting an elastic yarn under tension into the fabric in the length direction, releasing the tension from the elastic yarn to compact the fabric and removing the elastic yarn from the fabric.

FIELD OF THE INVENTION 
The present invention relates to an improved fiberglass casting tape. The 
casting tapes of the present invention have little tendency to fray or 
ravel and have substantial extensibility in their length direction which 
results in improved conformability, and thus allows better application of 
the casting tapes to the patient. The improved conformability results in a 
cast which better fits or conforms to the patient's limb. 
BACKGROUND OF THE INVENTION 
Plaster of Paris casts have been in use to immobilize body members or limbs 
for some time. In recent years, the plaster of Paris bandages have been 
supplemented and, to some extent, superseded by synthetic casting tapes or 
bandages which employ polymeric materials on a substrate. The polymeric 
materials are of the type that have been cured by exposure to ultra violet 
light or which would cure when reacted with water. Examples of the ultra 
violet light cured cast can be found in U.S. Pat. No. 3,881,473. More 
recently, water-cured or water-reactive polyurethane compositions have 
been used in forming orthopedic casts and the polyurethane compositions 
have largely supplanted other polymeric synthetic casting materials. The 
polyurethane casting materials are of the type which are disclosed in U.S. 
Pat. Nos. 4,376,438 and 4,411,262. 
The fibrous substrate used in the synthetic casting tapes may be made from 
any natural or synthetic fiber including a fiberglass material. The 
fiberglass materials offer advantages in terms of strength of the finished 
cast when compared to other fibers and various constructions of fiberglass 
fabrics have been used for the substrates for synthetic casting tapes. The 
patents mentioned above disclose the use of different fiberglass materials 
as the substrate for casting tapes. In addition, U.S. Pat. Nos. 3,686,725, 
3,787,272 and 3,882,857 disclose specific fiberglass materials, or the 
treatment of fiberglass materials, to produce fiberglass substrates which 
are particularly suitable for use in orthopedic casts. 
U.S. Pat. No. 4,323,061 discloses a cast substrate made from a combination 
of glass fibers and a second fiber such as cotton, flax, rayon, wool, 
acrylic resin, nylon, polytetrafluoroethylene or polyester. The purpose of 
the second fiber in the substrate is to hold the curable resin on the 
substrate. 
U.S. Pat. No. 3,332,416 discloses a plaster of Paris cast bandage with a 
woven substrate made with a combination of elastic and inelastic fibers. 
U.S. Pat. No. 4,609,578 discloses a fiberglass substrate for casting tapes 
which has an extensibility of at least 20% and up to 25% to 35%. The 
fiberglass substrate is heat-set to prevent fraying. Care must be taken 
when handling of the fabric after knitting and before and after heat 
treatment to avoid applying undue tension to the fabric which would 
distort the knots and loops, i.e. stretch the fabric, and permanently lose 
some of the stretch of the fabric. 
U.S. Pat. No. 4,668,563 discloses a cast substrate made from a combination 
of glass fibers and an elastomeric highly extensible fiber. The substrate 
has a stretch of from 40 to 200%. The elastomeric fiber is selected or is 
treated to insure that it is compatible with the water curable 
polyurethane prepolymer employed in the casting tape. Although the 
conformability of the tape disclosed is excellent, the presence of the 
elastomeric fiber in the substrate can cause storage stability or shelf 
life problems or add cost to the product because of the treatment of the 
elastomeric fiber that is required to insure longer shelf life. The 
presence of the elastomeric fiber in the finished product also 
necessitates a secondary process step, i.e. coating the fabric with a 
binder, to reduce fraying or ravel. 
SUMMARY OF THE PRESENT INVENTION 
The present invention provides an orthopedic casting tape which has a 
substrate which is all fiberglass does not fray or ravel but which has a 
very high degree of extensibility. The casting tape of the present 
invention is knitted with an elastic fiber in the length direction of the 
fabric which acts to compact or gather the fabric in the length direction 
when the fabric is removed from the knitting machine. The elastic fiber is 
subsequently removed from the fabric by a heat treatment process which 
removes the elastic yarn and heat sets the fabric in a contracted 
configuration with little or no fraying or raveling. The removal of the 
elastic fiber avoids the problems of storage stability which may be caused 
by the presence of the elastomeric fiber in the finished casting tape. 
The presence of the elastic fiber in the knitted fabric allows the fabric 
to be handled normally, i.e. without undue regard to tension, until the 
elastic fiber is removed from the fabric. After the elastic fiber is 
removed from the fabric, some care should be taken to avoid applying 
tension to the fabric. However since the fabric is heat set by the process 
which eliminates the elastic yarn and therefore has some memory, normal 
handling during processing will not cause a major loss of stretch. Also, 
since the elastic fiber is removed from the fabric, the concerns of the 
reactivity of the water curable resin with the elastic fiber and the 
resultant storage stability or shelf life problems are not present in 
casting tapes made from the fabric of the present invention. 
The purpose of the use of the elastic fiber is to compact or gather the 
fabric when the fabric is removed from the knitting machine. When the 
fabric is knitted the elastic yarn is stretched by being fed under 
predetermined tension to the knitting machine. The degree of tension or 
stretch of the elastic fiber or yarn will depend on the percent stretch 
desired in the final fabric. Increasing the tension in the elastic fiber 
results in more stretch in the fabric. When the knitted fabric is removed 
from the knitting needles on the machine, the stretched elastic fibers 
exert a force on the fabric which pulls the fabric in the length 
direction. The elastic fibers pull the courses of the fabric into closer 
proximity to each other thereby causing the fabric to contract. For 
example, a fabric of the present invention may be knitted with about 12 
courses per inch and when the fabric is removed from the knitting machine 
and gathered by the elastic yarn the fabric may have 20 courses per inch. 
The fabric retains its compacted or gathered states to a substantial 
degree even after the elastic fibers are removed from the fabric by the 
heat treatment process because the fiberglass yarns are heat set in the 
contracted state which imparts a "spring like" memory to them. 
The compacted or gathered fabric has a greater extensibility than a similar 
fabric made not using an elastic fiber.

DETAILED DESCRIPTION OF THE INVENTION 
The substrate of the casting tape of the present invention is knitted with 
a combination of continuous filament fiberglass and elastic filaments or 
yarns. Fiberglass substrates are generally characterized as made from 
filaments which are formed into yarn, sized and knitted into a desired 
construction. In the present invention the substrates are knitted on a 
Raschel Knitting Machine having 6 to 28 needles per inch. The cast 
substrate fabrics of the present invention are knitted fabrics which 
combine high modulus fiberglass with an elastomeric highly extensible 
fiber when the fabric is knitted. The elastomeric extensible fiber may be 
a natural rubber or a synthetic elastomer such as polyisoprene, 
polybutadiene, copolymers of a diene and styrene, copolymers of 
acrylonitrile and a diene or polychloroprene, copolymers of chloroprene 
and other monomers, ethylene propylene elastomers including ethylene 
propylene copolymers and ethylene propylene diene terpolymers, and 
thermoplastic elastomers which are block copolymers of styrene and 
butadiene or isoprene. The elastomeric extensible fiber may also be a 
spandex (polyurethane) fiber. The most common commercially available 
elastic yarns are natural rubber and spandex. Natural rubber has been used 
as the elastic yarn in the process of forming the substrate of the present 
invention. 
The extensible fiber is present in the knit fabric in the warp or wale 
fibers, i.e., machine direction, but not in the fill fibers. The elastic 
or extensible fiber cannot be the fiber used to form the loop or chain 
stitch as the elastic fiber will eventually be removed from the fabric. 
Preferably about 0.25 to 25% of the fibers based on the total volume of 
fibers in the fabric are extensible. The knitted fabric, prior to the 
removal of the elastic fiber, should have a stretch in the length 
direction of at least 70% and up to 200%. The process of removing the 
extensible fiber by a heat treatment process could reduce the 
stretchability by some amount, 50% or more, of the stretch of the fabric 
prior to heating. Therefore, the preferred extensibility of the fabric, 
before heating to remove the elastic extensible fiber, is between 70% and 
200%. 
Since the elastic fiber used in the present invention will be removed from 
the substrate before the substrate is coated with the polyurethane 
prepolymer, i.e. a water curable isocyanate terminated resin, there is no 
concern of the possible reaction between the elastic fiber and the 
polyurethane prepolymer. 
The knit patterns that may be used in the manufacture of the substrates of 
the present invention are numerous. Generally, the fabrics are knitted 
using a three bar knitting machine, one bar for the elastic thread and two 
bars for the fiberglass. In the knitted substrates of the present 
invention the elastic yarn must give the fabric stretch in the length 
direction of the fabric. The elastic yarn may be in bar 3, of a 3-bar or 
possibly bar 4 of a 4 bar Raschel knit construction fabric. As previously 
indicated the elastic yarn in the present invention should not be in the 
chain stitch. The elastic yarn may be intermittently spaced in the fabric. 
There need not be an elastic fiber for every needle employed in knitting 
the fabric. The elastic fiber need only be present in the fabric in a 
sufficient amount to give the desired compaction to the fabric when the 
fabric is removed from the knitting machine. 
The third bar, and the fourth bar if used, could be used to lay in 
fiberglass in a zig-zag or a sinusoidal pattern which would increase the 
crush strength of the final cast by comparison to transversely laid in 
yarns in these positions. The elastic yarn may be in bar 3 in 3-bar knit 
or in bars 3 and/or 4 in a 4-bar knit fabric. The tension in the elastic 
yarn during the knitting process is important. The tension in the elastic 
yarn should be controlled to cause the fabric to gather or bunch uniformly 
to the desired degree when it is released from the knitting machine. When 
the finished heat treated fabric is stretched, the extensibility is 
achieved as the gathers are pulled out and any further stretch or 
extensibility of the fabric is limited by the loops in the chain stitch. 
The preferred fabric is a 3 bar knit with the elastic yarn in bar 3. 
Typical bar patterns for the knit fabric substrates of the present 
invention are shown in the drawings. 
FIG. 1 is a three bar pattern with the elastic thread on bar 3 and 
fiberglass on bars 1 and 2. 
FIG. 2 is a three bar pattern in which the elastic thread is on bar 3 and 
fiberglass is on bars 1 and 2. This fabric would be heavier than the 
fabric of FIG. 1 as more fiberglass would be added to the fabric by bar 2. 
FIG. 3 is a four bar pattern in which the elastic thread is on bar 3 or 4. 
It should be understood that the above bar patterns may be modified. For 
example, the pattern of FIG. 3 may be employed with an elastic thread in 
bars 3 and 4 and fiberglass yarn in bars 1 and 2. 
Also, the patterns of FIGS. 1 and 2 could be modified by employing a 
zig-zag pattern on bar 3 similar to that shown in bar 3 or bar 4 of FIG. 
3. The particular knit pattern is not important as long as the fabric is 
compacted by the tension of the elastic yarn when the fabric is removed 
from the knitting machine. 
The elastic fiber is removed from the fabric by heating the fabric in an 
oven at a temperature sufficiently high to burn off the elastic yarn and 
set the fabric in the contracted state. This can be accomplished by 
heating the fabric in an oven at a temperature between 400.degree. F. and 
about 850.degree. F. Heating the fabric to temperatures above about 
1000.degree. F. should be avoided as subjecting the fiberglass to 
temperatures of about 1000.degree. F. can weaken the fiberglass yarns in 
the fabric which may result in reduced strength of casts made from such 
fabrics. Rapid burning of the elastic yarn should be avoided as this can 
increase the temperature of the fiberglass high enough to cause breakage 
of the fiberglass yarns. 
The burning conditions can be varied depending on the particular 
elastomeric fiber used but the heat treatment must be sufficient to set 
the fiberglass yarns in the contracted configuration. 
After the fabric is treated to remove the elastic yarn, the fabric should 
be carefully handled so that it is not stretched sufficiently to pull the 
gathers completely out of the fabric during the winding, coating and other 
casting tape manufacturing procedures. 
EXAMPLE I 
A fabric was knitted on a 24-gauge Raschel knitting machine using a 3-bar 
configuration as shown in FIG. 1. The first bar contained DE75 1/0 
fiberglass yarn, the second bar contained the same fiberglass, and the 
third bar contained a natural rubber thread designated L83 from J. P. 
Stevens. The rubber thread was fed to the knitting machine under 
sufficient tension so that when the fabric was removed from the knitting 
machine and was allowed to contract by the action of the elastic yarn, the 
fabric had a machine direction stretch of 75%. The contracted fabric was 
then formed into a loose skein and placed in an oven at a room 
temperature. The temperature of the oven was raised to 450.degree. F. 
during a period of two hours and held at 450.degree. F. for 90 minutes. 
The temperature of the oven was then raised to 600.degree. F. over a 
period of one hour and held at that temperature for one hour. The oven was 
then heated to a temperature of 695.degree. F. over a period of one hour 
and held at that temperature for six hours. The oven was then allowed to 
cool to room temperature and the fabric removed for subsequent use as a 
substrate for a water reactive polyurethane casting tape. When the elastic 
fiber had been removed from the fabric samples the fabric had an 
elongation of about 35% (between 32% and 37%). The fabric was coated at 
42% add on, with a water reactive with a polyurethane prepolymer of the 
type disclosed in U.S. Pat. No. 4,433,680 the disclosure of which is 
incorporated herein by reference and used in commercially available 
polyurethane casting tapes. The resulting casting tape was evaluated and 
was found to have improved conformability when compared to casting tapes 
made with heat set fiberglass coated with the same resin.