Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 -
0 S~'~tONG DISCONTINUOUS POLYETHFIB$ES
The present invention relates to strong as-spun
discontinuous polyethylene fibres, especially strong
discontinuous plexifilamentary film-fibril strands formed
directly from fibre-forming polyethylene. Such as-spun
discontinuous fibres may be formed in a flash spinning
process.
As used herein, "discontinuous" means that the
strands have a length of not more than 100 mm.
As used herein, "plexifilamentary film-fibril
strands of polyethylene" means strands which are
characterized as a three dimensional integral network of
a multitude of thin, ribbon-like film-like elements of
random length and of a thickness in the range of about i-
20 microns, with an average thickness of less than about
10 microns, generally coextensively aligned with the
longitudinal axis of the strand. The film-fibril
elements intermittently unite and separate at irregular
intervals in various places throughout the length, width
and thickness of the strand to form the three dimensional
network. Such strands are known, being described in
further detail in Blades and White, US Patent 3 081 519
which issued 1963 March 19.
Plades and White describe a flash-spinning process
for producing continuous plexifilamentary film-fibril
strands from fibre-forming polymers. A solution of
polymer in a liquid that is a non-solvent for the polymer
at or below its normal boiling point, is extruded at a
temperature above the normal boiling point of the liquid
and at autogenous or higher pressure through a spinneret
into a medium of lower temperature and substantially
lower pressure. This flash spinning causes the liquid to
vaporize and form a continuous plexifilamentary film-
fibril strand. Commercial spunbonded products have been
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CA 02102568 2002-08-21
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made from polyethylene plexifilamentary film-fibril
strands obtained by flash-spinning using
trichlorofluoromethane as solvent, but that halocarbon
has been implicated in the depletion of the earth's
ozone.
It is known that the spinneret and tunnel used in
flash spinning a polymer solution are important with
respect to properties of flash spun continuous fibres,
e.g. tenacity and elongation to break. For instance, US
Patent 4 352 650 of Marshall, which issued 1982 October
05, discusses optimization of the tunnel for increasing
fibre tenacity e.g. from 4.2 to 5.2 grams/denier, of
flash spun continuous fibres. In general, fibre tenacity
can be increased by as much as 1.3 to 1.7 times by using
a tunnel at the exit to the spinneret. Various methods
for making discrete fibres are known e.g. as discussed in
the copending Canadian Patent Application No. 2,102,578 of
S. Cloutier, L.M. Manuel (formerly L.M. Morin) and
v.G. Zboril filed concurrently herewith.
US Patent 5 043 108 of S. Samuels, which issued 1991
August 27, discloses flash spinning of a mixture of
organic solvent, polyethylene and a non-solvent or spin
aid, especially water or an alcohol, or mixtures thereof,
in which the amount of water is less than the saturation
limit of water in the organic solvent, to produce
continuous as-spun fibres. A process for the manufacture
of a polyolefin pulp in which strands are formed and
shredded is disclosed in US Patent 5 093 060 of S.
Samuels and V.G. Zboril, issued 1992 March 03. However,
while it is possible to produce continuous filaments
using a flash spinning process and to subsequently shred
the continuous filaments by mechanical means to form
discontinuous filaments, such mechanical shredding tends
to fuse the ends of the shredded filaments, even if the
shredding is carried out under water. Fused ends make it
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difficult or impossible t~ open up the resultant web of
fibres, as well as reducing fibre orientation and
strength. For these reasons, processes that produce
discontinuous fibres without requiring use of mechanical
shredding means would be preferred.
While a variety of discrete fibres have been
produced, discrete fibres of improved properties are
still required. Improved discontinuous as-spun
plexifilamentary film-fibril strands, especially fine,
14 strong, oriented and discontinuous fibrils, and
polyethylene pulp have now been found.
Accordingly, the present invention provides fine,
strong, as-spun discontinuous fibres formed from
polyethylene, said fibres having a length of 1-25 mm, a
fibre diameter of less than 30 microns and a handsheet
zero span strength of greater than 3 kg/15 mm.
In a preferred embodiment of the fibres of the
present invention, the handsheet zero span strength is
greater than 6 kg/15 mm.
The present invention also provides a polyethylene
pulp formed from as-spun discontinuous fibres having a
surface area of greater than 4 m2/g, a Pulmac defect value
of less than 3~, a handsheet zero span value of at least
3 kg/15 mm and with the fibres of the pulp having a
Kajaani coarseness of less than 0.30 mg/m.
In a preferred embodiment of the pulp of the present
invention, the pulp has a Pulmac defect value of less
than 2%.
In another embodiment, the fibre length is less than
about 2 mm and with an average length in the range of
0.X0-1.20 mm.
In a further embodiment, the fibres have a fineness
such that the fibres have a Ka~aani coarseness of less
than 0.20 mg/m.
In yet another embodiment, the pulp has a surface
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CA 02102568 2002-08-21
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area in the range of 6-8 mi/ g .
In embodiments of the fibre and of the pulp, the
polyethylene has a melt index of less than 15 dg/min,
especially less than 7 dg/min and in particular less than
2 dg/min.
The measurements of handsheet zero span strength and
surface area are described below.
The fibre is in the form of plexifilamentary film
fibrils in a discontinuous form. The average length of
the fibre is in the range of 1-100 mm and especially in
the range of 1-25 mm. The fibres of the present
invention have a handsheet zero span strength of greater
than 3 kg/15 mm and especially greater than 6 kg/15 mm.
The diameter of the fibre is preferably less than 30
microns and more particularly less than 20 microns. The
fibres are referred to as "as-spun" fibres, which have
"free" ends in contrast to the fused ends that tend to
result from mechanical cutting of polyethylene fibres,
especially at commercial rates. The as-spun fibres are
characterized by an absence of fused ends. This freeness
of the fibres contributes to improved processing of the
fibres, which usually includes a step of opening up of
the fibres or separation of fibre bundles into individual
(fibres.
The fibres of the present invention are short
fibres, in comparison to the fibres produced in the
aforementioned Blades and White and Samuels processes.
The fibres of the present invention may be
manufactured in a flash spinning process, particular
examples of which are described in the aforementioned
copending Canadian Patent Application No. 2,102,578 of S. Cloutier,
L.M. Manuel and V.G. Zboril. In such an embodiment of a process of
manufacture, polyolefin is dissolved in an organic
solvent. The polyolefin may be in the form of pellets or
powder, or other forms known in the art, having been
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previously polymerized from monomers. Alternatively, the
polyolefin is already dissolved in an organic solvent
e.g. it is a solution of polymer in organic solvent from
a process for the polymerization of monomers.
The polyolefin may be a high molecular weight
homopolymer of ethylene or copolymer of ethylene and at
least one C,-C,o hydrocarbon alpha-olefin e.g. butane-1,
hexane-1 and/or octane-1. The polyolefin may also be a
homopolymer of propylene or copolymer of propylene with a
minor amount of ethylene. A wide variety of such
polymers, including by type of monomers) used, molecular
weight, molecular weight distribution and other
properties are commercially available. In preferred
embodiments in which the polyolefin is a homopolymer of
ethylene or copolymer of ethylene and at least one C4 Coo
hydrocarbon alpha-olefin, the density is in the range of
0.930 to 0.965 g/cm3, especially in the range of 0.940 to
0.960 g/cm3. The melt index of the polyolefin is
preferably less than 15 dg/min i.e. in the range of from
so-called "no-flow" e.g. less than about 0.01 dg/min, to
15 dg/min, especially in the range of 0.50 to 7.0 dg/min;
melt index is measured by the method of ASTM D-1238
(condition E).
A variety of organic solvents may be used in the
process, examples of which include pentane, hexane,
cyclohexane, heptane, octane, methyl cyclohexane and
hydrogenated naphtha, and related hydrocarbon solvents,
and mixtures of solvents.
The polyolefin may contain additives e.g.
antioxidants, ultra violet stabilizers, wetting agents,
surfactants and other additives known for use in
polyolefins, provided that the additives are capable of
passing through the orifice used in the process and not
otherwise adversely affecting the process.
The solution of polyolefin in organic solvent is at
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~~.~~~~8
-6_
an elevated temperature and pressure, the solution being
at a pressure that is at least the autogenous pressure
and at a temperature sufficient to maintain the
polyolefin in solution. In preferred embodiments, the
solution also contains a non-solvent e.g. water, as a
spinning aid, as described in the aforementioned patent
of Samuels. The spinning aid may contain wetting agents,
surfactants or the like. The temperature and pressure
used, and the composition of the solution especially the
percent of polymer in the solution, affect the properties
of the film-fibril strands obtained on spinning and
consequently the fibrous material subsequently formed in
the process. For instance, the temperature, pressure and
solution composition may be selected so that highly
oriented fibres are obtained, such fibres being
preferred.
The solution is fed through a feed section to a
spinneret exit to form plexifilamentary film-fibril
strands, the strands being formed as the polymer solution
passes from the spinneret exit. A mixture of steam and
water is contacted with the solution passing from
spinneret exit substantially simultaneously with the
passage of the solution from the spinneret exit. The
mixture of steam and water may be fed as a stream to the
tunnel or, preferably, a stream of hot high pressure
water is flashed through an orifice into the tunnel,
where the mixture of steam and water is formed. The
temperature and pressure of the stream are selected so as
to produce the required ratio of steam to water in the
tunnel.
In a preferred embodiment, the ratio of steam to
water is in the range of 20:80 to 80x20 on a weight
basis, especially in the range of 35:65 to 65:35. fihe
temperature of the inert fluid is 2-~0~C lower than the
melting point of the polymer.
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-~_
It is to be understood that the surfaces of pipes,
vessels and the like used in the process of the invention
should be free of snag points or other obstructions that
might prevent or retard the passage of the film-fibrils
or fibrous material.
The fibre may be converted to a polyethylene pulp,
which has a variety of uses. For instance, the pulp may
be used as part of blends with cellulose for use in e.g.
diapers, disposable wipes, feminine hygiene products and
incontinence products, as a filler e.g. in polymers,
cement and the like, thixotropic agent in paints and as
synthetic paper. In some end-uses, fibres especially
longer fibres with lengths in the range of about 5-25 mm,
may be used, without refining, to produce sheet
structures using either wet-lay or dry-lay sheet forming
technologies.
pulp may be obtained by feeding the fibres to a
refining process that reduces the length of the fibres to
less than about 2 mm and with an average length in the
range of about 0.80-1.20 mm as well as opening up the
fibre structure. The fibres are fed to the refiner in
the form of a slurry e.g. about 2% by weight, with
polyvinyl alcohol added as surfactant; other surfactants
may be used in combination with or instead of polyvinyl
alcohol. The fibres must be of a length of not more than
about 10 mm, preferably with an average length of about 6
mm in order to produce an acceptable slurry. The
refining process may be carried out in a pulp and paper-
type refiner. Suitable refiners include single disk,
twin-flow and conical refiners.
Synthetic pulp is synthetic fibre having a very
short length. ~,s used herein, the pulp preferably has an
average length of less than about 2 mm, especially in the
range of 0.8-1.2 mm, arid preferably about 1 mm. In
addition, the pulp preferably has a surface area of
-
r..
greater than 4 m2/g, especially than 6 m2/g and in
particular in the range of 6~8 mZ/g. Pulp has a handsheet
zero span value of at least 3 kg/15 mm, especially at
least 5 kg/15 mm. In use, it is preferred that the pulp
have a low percentage of long fibres and a low percentage
of agglomerates. Long fibres are measured by Clark 14
mesh, and acceptable values are less than 12% and
especially less than 7%. Agglomerates are measured by
Pulmac defects, and acceptable values are less than 3%
and especially less than 2%. The fineness of the fibres
may be characterized using, a coarseness test viz. the
Kajaani test. As used herein "fine fibres" have a
coarseness of less than 0.3 mg/m and preferably less than
0.2 mg/m.
In the examples, the fallowing test methods were
used:
Handsheet zero span is a measurement of fibre strength,
and is obtained using the procedures of TAPPI T205 om and
a Pulmac Troubleshooter, using the method suggested by
2o the manufacturer. Handsheets with a basis weight of 60
g/m2 are made in a handsheet mould, by forming a slurry of
fibres in water and then depositing the fibres on a
screen using vacuum. Handsheet zero span is the force
required to break a strip measuring 2.54 cm x 10 cm,
using a jaw width of 15 mm and a jaw separation of 0 mm.
The results are expressed in kg/15 mm;
Surface area is a measure of the degree of fineness and
fibrillation of the product, and is measured by the
Strohlein nitrogen adsorption method. Nitrogen is
adsorbed on the fibres at liquid nitrogen temperature.
The amount adsorbed is measured as a pressure difference
between sample and reference flask. Because of the small
size of nitrogen molecules, small differences in surfaces
can be detected. In effect, the method is a one-point
measurement using the principles of the BET nitrogen
_
~~.~q
9
adsorption of S. Brunauer, P.H. Emmett and E. Teller, J.
Amer. Chem. Soc., vol 60, p 309-319 (1938). The results
axe reported in m~/g;
Linear Shrinkage is measured by immersion of fibre
bundles in ethylene glycol at 155°C for 5 seconds, and is
expressed as the ratio of the initial length to the
shrunken length. Linear shrinkage is an indication of
the orientation imparted to the fibres during the
spinning process;
Average fibre length and coarseness was meas~zred using
Kajaani apparatus in which a very dilute slurry of fibres
in distilled water is drawn through a small orifice using
a vacuum. The length and size of the fibres are detected
by a diode array detector as the fibres pass through the
orifice. A distribution of fibre lengths and sizes is
obtained. For unrefined samples, where the fiber length
is greater than 2 mm, a very dilute slurry of 0.0078g of
fibre in water is prepared, deposited onto a screen and
then pressed into a plastic slide measuring 12.7 x 12.7
cm; a visual estimate of the length distribution and
average length of the sample is then made;
A Clark classifier is used to measure the proportion of
long fibres (mostly longer than 2 mm) in a sample;
samples that have not been refined completely will have
unacceptably high Clark values e.g. greater than 12%.
The procedure used is TAPPT T233 os and TAPPI T2E>1 pm.
Fibres are dispersed in water, and then circulated
through a series of different mesh screens. For refined
fibres, only the amount collected on the 14 Mesh screen
(1.19 mm openings) is reported; and
Pulmac defects is a measure of agglomeration in which a
slurry is passed through a slot opening that is 0.1 mm or
0.15 mm wide at a given flow rate. The fibres or
agglomerates which do not make it through the slot after
two passes are considered defects.
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CA 02102568 2002-08-21
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The present invention is illustrated by the
following examples.
E7~L~?~e I
Fibrous material was manufactured using semi-works
scale apparatus equipped with a spinneret and die having
a venturi tunnel, such a tunnel being shown in Fig. 1 of
the copending Canadian Patent Application No. 2,102,578 of S. Cloutier,
L.M. Manuel and V.G. Zboril filed concurrently herewith. The solution of
polymer fed to the spinneret was a solution of
ethylene/butene-1 copolymer dissolved in cyclohexane and
containing 7% (w/w) of water as a spin aid. Water was
introduced at high temperature and pressure into the zone
immediately adjacent the spinneret so that a mixture of
steam and water contacted the fibres exiting from the
spinneret.
In the spinning vessel, the product was in the form
of a slurry of fibres in water at a 0.5% consistency
(w/w). The fibre slurry was conveyed, using a water-
driven venturi, through a large smooth pipe to a vessel
where live steam was injected to boil off residual traces
of cyclohexane. The slurry, free of solvent, was further
conveyed, using a water-driven venturi, through a large
smooth pipe to a belt filter press where water was
removed. The product was collected in the form of a
loose cake with an approximately 50% solids content.
Further details of the polymer used and the
conditions in the flash spinning process are given in
Table I.
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_ 11 _
Table I
Run No. 1 2 3
POLYMER PROPERTIES:
Melt Index (dglmin) 0.28 0.33 0.43
Density (g/cm') 0.937 0.942 0.958
SP1NNINC3 CONDITIONS:
Solution Tomperaturo (C) 249 232 237
Let-down Chamber Presauae 6690 6310 6345
(kPa)
Solution flow (kg/br) 291 315 225
Poly~r in Solution (%) 14.8 14.9 15.6
SPIN SHATTERINtr CONDTfIONS:
Water flow rate (kg/hr) 220 210 240
Wad ~tu,e (off 300 298 302
Water Pressure (kPa) 10760 13730 10030
2 Water flsahing to steam ( 42 42 43
0 %)*
Water to polymer ratio (kg/kg)5.1 4.5 6.9
FIBRE PROPERTIES:
L~~ ag~ 10.5 10.5 10.1
2 Handsheet zero span (kg/15 b.3 8.1 9.0
5 mm)
Fibre lengths (mm) 18-25 18-25 4-10
Note: Solution flow = polymer
plus solvent
* _ assumes that pressure
in tunnel is 104 IcPa and
the temperature is 100oC
30
In this example, polymers of low melt index I.e.
high molecular weight, were
spun into discontinuous
fibres. The fibres were all
strongly oriented, with
linear shrinkages slightly
above 10. Fibre length was
in
35 the range of 3.8-25 mm for 1 and 2, in which the
Runs melt
index was 0.28 and 0.33 dg/minrespectively. However,
in
Run 3 in which the polymer
melt index was higher viz.
0.43 dg/min, the fibre length
was shorter, generally
below 10 mm.
40 The largest effect of increasing
polymer density was
an increase in fibre strength,as measured by handsheet
zero span. Fibre strength 6.3 kg/15 mm at a polymer
was
density of 0.937 g/cm3 (Run :3.1 kg/15 mm at a polymer
1),
density of 0.942 g/cm3 (Run and 9.0 kg/15 mm at
2) a
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_....!'::.;..
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polymer density of 0.956 g/cm' (Run 3).
The fibres of this example were strong and
discontinuous, with combinations of the unique properties
described herein.
The procedure of Example I was repeated using
polymers of differing malt index. Polymers having a melt
indices in the range of 0.79 to 7.6 dg/min were spun into
fibres"
la
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.r . ~ Q~ :'l~ ~~
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Further results are given in Table II.
Table II
Run No. 4 5 b 7 8
POLYMER PROPERTIES:
Melt lades (dg/min) 0.79 1.161.90 3.8 7.6
Density (g/cm') 0.9550.9560.9380.9470.959
SPINNING CONDITIONS:
Solution Temperature (~C) 2b2 260 237 229 234
Let-down Cleamber Pressure7165 66?58550 8080 8240
(kPa)
Solution flow (kg/hr) 235 220 284 264 224
Polymer in Solution ('%) 15.4 15.414.9 16.9 20.1
SPIN SHATTERING CONDTTIONS:
Water rate (kg/Iv) 280 265 250 250 2b8
Water temperature (~C) 299 300 301 302 299
2 Water Pressure (kPa) - 10070100 9890 10510
0
Water flashing to steam 42 42 42 43 42
(~O)*
Water to polymer ratio 7.8 7.8 6.0 5.6 6.0
(kg/kg)
FIBRE PROPERTIES:
2 Linear shrinkage - 9.9 - _ _
5
Handsheet zero span (lcg/158.5 8.4 5.0 4.3 3.8
mm)
Fibre lengths (mm) 2-8 2-6 1-4 1-~1 1-3
Note: Solution flow = polymer
plus solvent
30 ~ - assumes that pressure 100aC
in tunnel is 104 kPa and
the temperature is
Polymer melt index.(or polymer molecular weight) had
an effect on both the fibre length and fibre strength.
All the fibres were strong and discontinuous, with the
35 unique combination of properties described herein.
For the polymer with the lowest melt index (0.70
dg/min, in Itun 4), the individual fibres lengths ranged
from 2 to 8 mm. In contrast, fox the polymer of highest
melt index (7.C dg/min, in Run 8), the indisridual fibre
40 lengths ranged from 1 to 3 mm. Fibres with the lengths
reported in this Example are short enough to be dispersed
into a slurry in a well agitated vessel, preferably a
pulper, and refined to pulp length. In contrast, some of
the fibres obtained in Example 1 had lengths of up to 25
45 mm, which would be expected to cause entanglement
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.~ ~ ~ ~ .~ J
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problems in a pulping and refining process.
Polymers with lower melt index (higher molecular
weight) give fibres with higher strength, as measured by
handsheet zero span, than fibres with higher melt index.
The polymer with a melt index of 0.79 dg/min had a
handsheet zero span of 8.5 kg/min (Run 4), whereas the
polymer with a melt index of 7.6 dg/min had a handsheet
zero span of less than half that value (Run 8). In Run 3
of Example I, the polymer had the same density and a melt
index of 0.42 dg/min; the handsheet zero span was 9.0
kg/15 mm.
,~"xample III
The procedure Of Example I was repeated, with the
fibres obtained being refined to a pulp length viz.
approximately 1 mm.
Refining was conducted by first dispersing the
fibres in water in an agitated tank, at a fibre
consistency of 1.5-2%. 1~ surfactant (polyvinyl alcohol,
2% by weight of fibres) was used in order to make the
fibres more wettable. Two different single disk refiners
were used, with a 30 cm plate diameter in Runs 9 and 10
and with a one metre plate diameter in Run 1~.. The plate
gap setting was between 0.05 and 0.15 mm, with the larger
gap settings being used on the smaller refiner. The
samples were refined until the average fibre length was 1
mm.
Further details and the results obtained are given
in Table III.
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~~.~~~6~
° 15
Table III
Run No. 9 10 11
POLYMER PROPERTIES:
Melt Index (dg/min) 0.78 1 .04 7.5
Density (g/cm3) 0.962 0.956 0.958
SPINNING CONDITIONS:
Solution Temperature (~C) 237 249 245
Lot-down Chamber Pressure 7115 ?320 8255
(kPa)
Solution flow (kg/6r) 250 219 231
Polymer in Solution (96) 15.4 15.1 19.0
SPIN SHATTERING CONDITIONS:
Water rate (kg/hr) 280 245 283
Water temperature (~C) 300 301 302
Water pressure (kPa) 10855 9510 -
Water flashing to steam 42 42 43
(96)*
Water to polymer ratio 7.2 7.4 6.4
(kg/kg)
UNREFINED Fibre PROPERTIES:
Linear shrinkage 9.7 6.9 -
2 Handsheet zero span (kg1157.5 7.4 3.6
5 mm)
Fibre lengths (mm) 6-12 4-8 1-3
REFINED Fibre PROPERTIES:
Average fibre length (mm) 0.97 1.02 0.98
3 Average fibre coarseness 0.171 0.219 0.26
0 (mglm)
Surface area (m2/g) 6.22 6.61 5.97
Pulmac defects (Wo) 0.2 0.9 1.2
Clark 14 mesh (~) 1.1 1.8 1.4
Canadian standard freeness402 432 525
(ml)
35 Handsheet zero span (1g/155.7 5.7 3.1
mm)
Note: Solution flow = polymer
plus solvent
* - assumes that pressure is
in tunnel is 104 kPa and 100oC
the temperature
This Example compares the properties of refined
fibres (pulp) to the unrefined fibres. Most applications
of the fibres are expected to be in the form of pulp. In
addition, certain fibre properties cannot be measured on
unrefined fibres.
The fibre samples were obtained by spinning high
density polymers with varying melt indices viz. from 0.7E
to 7.5 dg/min. P'or Run 9, in which the melt index was
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0.78 dg/min, the handsheet zero span for unrefined fibres
was 7.5 kg/15 mm and the individual fibre lengths ranged
between 6 and 12 mm. Fox Run 11, in which the melt index
was 7.5 dg/min, the handsheet zero span was 3.6 kg/15 mm
and the individual fibres lengths ranged between 1 and 3
mm.
The fibres were refined to an average length of 1 mm
viz. 0.97 mm for Run 9, 1.02 mm for Run 10 and 0.98 mm
for Run 11. The Pulmac defect test, which measures long
fibres and agglomerates, gave results of less than 2% for
the fibres of all three runs. The Clark 14 mesh test,
which measures the proportion of long fibres, was also
less than 2% for all three runs.
Fibre strength as measured by the handsheet zero
span test was lower for the refined fibres than for the
unrefined fibres. For the lower melt index polymer of
Run 9, the handsheet zero span decreased from 7.5 kg/15
mm before refining to 5.7 kg/15 mm after refining. In
Run 11, the decrease was from 3.6 to 3.1 kg/15 mm.
Nonetheless, fibre strength was stall acceptable.
Fibre size and surface area are typically measured
only an refined fibres, because refining opens up the
fibre structure. ~rverage fibre coarseness as measured by
the Kajaani method increased with increasing melt index,
from 0.171 mg/m in Run 9 to 0.260 in Run 11. This shows
that finer, more oriented fibres may be obtained with
lower melt index polymers. Surface area measured by
nitrogen adsorption was between 6 and 7 m2/g for all three
runs.
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