Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02048346 2000-08-31
1
TITLE
Electric Arc Resistant Lightweight Fabrics
BACKGROUND OF THE INVENTION
Clothing made from flame resistant fibers provide
electrical workers protection from the intense radiation
given off by powerful electric arcs which may pass near them
in accidental discharge in high voltage equipment. However,
such garments when made from flame retarded cotton (FR
cotton) are uncomfortable in warm environments because of
the heavyweight fabric required for adequate protection.
The garments can be lighter and still offer adequate
protection if made from certain flame resistant synthetic
fibers but such garments are also uncomfortable because of
reduced water absorption as compared with FR cotton.
Clearly lightweight fabrics with improved shielding from
electric arcs are needed for electrical workers to provide
comfort and protection.
Summarv of the Invention
This invention provides woven fabrics having a
basis weight of 135-203 g./m2 and which are suitable for use
in clothing having high resistance to radiant energy from
high voltage electric arcs and yet offer a high degree of
comfort to the wearer comprising warp yarns of 15-50% heat
resistant staple fibers having a Limiting Oxygen Index (LOI)
of at least 25, and 50-85% of flame retarded cotton and fill
yarns of 0-50% heat resistant staple fibers and 50-100% of
flame retarded cotton, the said yarns having a linear
density of 215-550 dtex.
Detailed Description of the Invention
The stable fibers used herein are textile fibers
having a linear density suitable for wearing apparel, i.e.
less than 10 decitex per fiber, preferably less than 5
decitex per fiber. Still more preferred are fibers that
have a linear density of from about 1 to about 3 decitex per
fiber and length from about 1.9 to 6.3 cm (0.75 to 2.5 in).
HT-3000-A
1
2
Crimped fibers are preferred for textile aesthetics and
processibility.
By °'heat resistant°' is meant fibers which heave a
heat resistance time measured as described herein of at
least 0.018 sec/g/m2 (0.6 sec/o~/yd2). For comparison,
flame retarded cotton has an LOI of 30 but a heat resistance
time of only 0.01 sec/g/m2 and is considered flame resistant
(ZOI > 25) but not heat resistant.
A process for making the fabrics of the invention
involves the steps of first preparing a blend comprising 15-
500 heat resistant staple fibers and 50-85% cotton. Single
ply yarns of from 195 to 500 dtex (nominal 12 to 30 cotton
count ~cc] are spun from the blend and 118-187 gm/m2 (3.5-
5.5 oz/yd2) basis weight fabric is woven using these yarns
as the warp and a fill produced using a blend of 0-500 heat
resistant fibers and 50-100% cotton. Yarns of lower linear
density can be plied to achieve the same linear density.
The fabrics are then treated with commercially
available flame retardants such as "Proban CC" from Abright
& Wilson Inc., P. 0. Box 2229, Richmond, VA or "Pyrovatex
CP" from Ciba-Geigy. Both treatments are described in
3a~anese Textile News, No. 394, September, 1987. Basis
weight after flame retarding is 135 to 203 gm/m2 (4-6.0
oz/yd2) and yarn linear densities are 215 to 550 dtex.
The amount of heat resistant fibers required in
the fill direction in fabric of the invention depends upon
the fabric construction. In plain weave fabrics, at least
15% heat resistant fibers and up to 850 cotton is needed in
the fill whereas in 2X1 and 3X1 twill fabrics, the fill can
be all FR cotton. Too little heat resistant fiber in the
warp can result in fabric break open upon exposure to an
electric arc caused by discharge of high voltage equipment.
On the other hand, an excess of heat resistant fiber results
in a loss of desirable cotton aesthetics and higher costs.
It has been found that with 2X1 and 3X1 twills,
heat resistant fibers need be present only in the warp
yarns, that is, the fill yarn may be all cotton. Severe
break open will be avoided provided that the warp faces the
2
arc, i.e., is at the surface of the garment away from the
t,,earer. In the reverse condition, with the warp face away
yrom the arc and 100% FR cotton fill exposed, fabrics will
have severe break open even though there is an adequate
amount of heat resistant fibers in 'the warp. With~adequate
amounts of heat resistant fiber in both warp and fill,
fabrics will resist break open from either direction. It is
believed that the ability of 2X1 and 3X1 twills having 100%
FR cotton fill yarn to survive is due to the longer warp
float which shields the fill yarn and absorbs the radiation
preferentially in the surface exposed to the arc. While 2X1
twills are superior to plain weave in that they meet the
criteria for minimal fabric break open, 3X1 left hand twills
are even more preferred because they experience no break
open even with fill yarn of 100% cotton. This is thought to
be due to the longer float of the 3X1 versus 2 X1 twill and
the elasticity imparted by the "z" twist yarns in the left
hand construction.
Fabrics of the invention containing blends of FR
cotton and heat resistant fibers provide better protection
from the blast and heat from an electric arc than presently
available commercial fabrics of equal basis weight made
entirely of synthetic flame resistant fibers.
Table 1 shows that under severe and moderate
exposure conditions, fabrics of the invention performed as
well as heavier poly(m-phenylene isophthalamide), (MPD-
T)/poly(p-phenylene terephthalamide) (PPD-T) 95/5% fiber
blend fabrics, and better than flame retarded cotton fabrics
used in garments commonly worn by electrical workers.
It is important that the yarns employed in fabric
of the invention not exceed 550 dtex since the use of such
heavy yarns in lightweight fabrics results in undesirably
open fabric and inadequate protection to the wearer. If the
yarn size is less 'than 215 dtex, fabric thickness of the
lightweight fabric will be inadequate to protect against
damage from absorbed radiation, and the fabric will break
open.
3
4
The fibers can be spun into yarns by a number of
different spinning methods, including but not limited to
ring spinning, air-jet spinning and friction spinning and
can be intimate blends or sheath-core.
An exemplary heat resistant fiber for use in the
present invention is polyp-phenylene terephthalamide) (PPD-
T) (LOI 28, heat resistance time of 0.04 sec/g/m) staple
fiber. This fiber can be prepared as described in u.s.
Patent 3,767,756 and is commercially available.
Other heat resistant organic staple fibers may be
used including, but not limited to, the following: fiber of
a copolymer of terephthalic acid with a mixture of diamines
comprising 3,4'-diaminophenyl ether and p-phenylenediamine
as disclosed in U.S. Patent 4,075,172 (LOI 25, heat
resistance time 0.024 sec/g/m). Polybenzimidazole is also
suitable (LOI 41, heat resistance time 0.04 sec/g/m).
Test Measurements
Arc Resistance Test
The test for measuring resistance to an arc
consists of exposing fabrics in air to an electric arc which
is generated by applying 15,000 volts to two electrodes
spaced one foot apart. A small copper wire connecting the
electrodes is employed for arc initiation. Once the arc is
initiated, voltage is decreased to an average of 500 volt
RMS (root mean square) and a current flow of 8,000 amps RMS
using 60 cycle alternating current is applied for one-sixth
second.
Two levels of exposure were used. In the more
severe test, samples (30 x 30 cm) are held in a frame at a
distance of 15 cm from the arc. Only 20 x 20 cm of the
sample is exposed to the arc by virtue of a 0.08 cm thick
stainless steel plate 30 x30 cm with a 20 x 20 cm opening in
the middle being mounted on the frame facing the arc. The
test specimen, is clamped between the stainless steel plate,
a 0.63 cm phenlic spacer (constructed like the stainless
plate) and a 0.08 cm which copper plate. This provides a
0.63 cm air space between the test speciment and the copper
plate. For testing under moderate exposure, shirts made
4
5
from the fabrics are placed over a mannequin clothed in a
1000 cotton tee-shirt and spaced at a distance of 20 cm.
from the arc.
To pass the arc resistance test, the fabric or
shirt must not form a split of more than 7.5 cm in~length or
0.75 cm wide. If more than two splits occur or if either
the tee-shirt or the outer shirt ignites, the sample has
failed the test.
Heat Resistance Time
Heat Resistance Time is measured using a device
described in U.S. Patent 4,198,494 for measurement of Fabric
Break Open. The same heating conditions are used but as in
the aforementtioned patent, the sample holder was modified
to expose 2.5 x 6.3 cm area of the test sample (a strip 2.5
x 2.5 cm) to the heat flux. The test sample is placed under
a tensile load of 1.8 kg by holding one end fixed and
attaching the other to a 1.8 kg weight suspended with a
string over a pulley. Measurements are made with the fabric
loaded in the warp direction only, and with the fabric face
down against the flame. The time recorded is the time
required for the sample to break. Time in seconds before
the sample breaks divided by the basis weight of the fabric
ing/m is reported as Heat Resistance Time. This type of
heating device is available as moiled CS-206 from Custom
Scientific Instruments, Inc., 13 wing Drive, Cedar Knolls,
NJ 07927.
Fox the determination of heat resistance time
fabrics from staple or continuous filament yarn may be used.
Plain weave fabric with substantially equal numers of ends
and picks of the same yarns should be used. the fabric
basis weight should be between 170 and 340 g/m (5-10 oz/yd).
Limitinq_Oxygfen Index
This was determined using ASTM Method 42863-77.
Example 1
An arc resistant fabric of the present invention
was prepared from ring-spun yarns of intimate blends of PPD-
T staple fibers and cotton.
5
~'~~:~ ~~t~~
6
A picker blend sliver of 30% of PPD-T fibers
having a linear density of 1.65 decitex (1.5 dpf) of a cut
length of 3.8 cm (1.5 in), and 70% carded cotton was
processed by the conventional cotton system into a spun yarn
having 7.3 turns per cm of '°z" twist (18.5 tpi) using a ring
spinning frame. The yarn sa made was a 272 dtex (nominal
21.5 cotton county 247 denier) singles spun yarn which was
used as the warp on a shuttle loom in a 3 x 1 left hand
twill construction with a singles ring spun fill yarn made
from 100% cotton having the same twist and linerar density
as the warp yarn. The twill fabric had a construction of 30
ends per cm x 19 picks per cm (76 ends per in. x 47 picks
per in.), a basis weight of 162 g/m (4.8 oz/yd ). The
fabric was dyed blue and then treated with and aqueous
solution of a 2:1 mole ratio tetrakis (hydroxymethyl)
phosphonium chloride (THPC)/urea condensate, a flame
retardant available as "Proban CC" from Abright F. Wilson.
The fabric was made into a shirt and placed on a mannequin
cm from the electric arc with the warp facing the arc.
20 The shirt did not break open or ignite and the tee--shir.~t did
not ignite when given the moderate wxposure arc resistance
test. When the shirt was turned inside-out, with the cotton
fill facing the arc, and given the same test, it split
vertically along the entire length of one side, opening up
to about 1.25 cm.
Example 2
A 3X1 right hand twill fabric was constructed in
which the warp yarn of Example 1 was used in both the warp
and fill directions. After treatment with flame retardant,
this fabric also passed the arc resistance test (moderate
exposure) when tested as a shirt on a mannequin 20 cm from
the arc.
Example 3
A 2X1 right hand twill was constructed using the
warp yarn of Example 1 and a 100% cotton fill yarn having a
linear density of 354 dtex (nominal cotton count 16.5 cc,
322 denier). The fabric had a construction of 30 ends per
cm, 14 pinks per cm (76 ends per in. x 36 picks per in.) and
6
7
a basis weight of 162 g/m (4.8 oz/yd ). When a shirt of
this fabric (after flame rea~tarding) was exposed with the
wrap face out on a mannequin 20 cm from the arc and
subjected to the arc resistance test, there were only two
small splits, no after flame and no tee-shirt ignition.
When turned inside-out, the shirt fabric by excessive break
open.
Example 4
A 3X1 right hand twill fabric was made in a manner
similar to the fabric of Example 2. Yarns with 50% PPD-T
and 50o cotton were used for both the warp and fill. The
fabric tested as a shirt (warp face out) on a mannequin 20
cm from the arc passed the arc resistance test.
Example 5
A fabric similar to that of Example 1 was prepared
except that the fill yarn linear density was 354 dtex
(nominal cotton count 16.5, 322 denier). The fabric had a
construction of 30 ends per cm, 16 picks per cm (76 ends per
in. z 41 picks per in.) and a basis weight of 179 g/m (5.3
oz/yd ) . The fabric passed the arc resistance test when
tested as a shirt on a mannequin 20 cm from the arc.
Example 6
A Plain weave fabric was constructed in which both
the warp and fill yarns were blends of 15% PPD-T/85% cotton
and the linear density of the warp and fill yarns was 390
dtex (15 cc, 354 denier). The fabric was dyed green and had
a construction of 21 ends per cm x 20 picks per cm (54 ends
per in. x 50 picks per in.) and a basis weight of 203 g/m
(6.0 oz/yd). The .fabric passed the more severe arc
resistance test when held in a frame 15 cm from the arc.
7
8
Table 1
Arc Test Comparison of
Examples of the Invention Controls
and
Moderate Exposure - Mannequin cm From
20 Arc
Test
basis Wt. Result
gm/m
MPD-I/PPD-T (95/50) 203 PASSED
100% FR Cotton 203 FAILED
Examples 1-4 162 PASSED
Example 5 179 PASSED
Severe Exposure - Frame 15 From Arc
Cri
100% FR Cotton 203 FAILED
Plain Weave 291 FAILED
PPD-T/FR Cotton
50/50% Warp
100% FR Cotton Fill
Example 6 203 PASSED
8
