Note: Descriptions are shown in the official language in which they were submitted.
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BICOMPONENT FIBER AND NONWOVENS MADE THEREFROM
BACKGROUND OF THE INVENTION
. . ... . .
1. Field of the Invention
This invention relates to heat bondable heterofilaments
and to nonwoven fabrics made therefrom.
2. Description of the Prior Art
The use of heat bondable heterofilaments in the
manufacture of nonwovens is well known in the prior art.
Generally, such heterofilaments comprise two thermoplastic
materials which are arranged in either side-by-side or
sheath/core relationship with the two materials being coextensive
along the length of the filament. One of the thermoplastics, a
so-called latent adhesive, is selected so that its melting point
is significantly lower than that of the other in the filament,
and, by the application of heat and subsequent cooling, this
component is made to become adhesive and bond to other fibers in
the nonwoven. Such adhesion can take place either between like
heterofilaments or ~etween heterofilaments and conventional
non-bonding filaments if these are also present in the nonwoven.
The other component serves as a structural or backbone member of
the fiber.
Although heat bondable heterofilaments were developed
for use primarily in the production of light weigh~ nonwovens~
that is, nonwovens having relatively little weight per unit of
area, they have achieved a somewhat limited commercial success in
this area due to a number of deficiencies which are present in
state of the art fibers. Foremost among these deficiencies are
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excessive shrinkage during thermal bonding, which leads to
fabrics having an uneven density and non-uniformity of thickness;
insufficient fiber-to-fiber bond strength, which leads to poor
fabric tensile strength, as well as the production of nonwoven
fabrics which are relatively lacking in such traditionally
desirable textile qualities as drape, liveliness and bulk or
loft.
Admittedlyl an attempt has been made in the prior art
to deal with the above-mentioned deficiencies~ Tomioka, in an
article entitled "Thermobonding Fibers for Nonwovens", Nonwovens
Industry, May 1981, pp. ~3-31, describes the properties of ES
Fiber, a bicomponent material commercially available from Chisso
Corporation of Osaka, Japan. This fiber, which comprises
polyethylene and polypropylene in a so-called modified "side-by-
side" arrangement (actually a highly eccentric sheath/core), is
also, presumably, disclosed in UOS. Patent 4,189,338, to Ejima et
al. and assigned to Chisso Corporation. Among the attributes of
this fiber, Tomioka deals most extensively with the relatively
low thermal shrinkage which the fiber experiences during the
thermal bonding step, and goes on to note that this property
results in nonwovens which possess good uniformity of density and
thickness, as well as good bulk, hand and drape.
While it is certainly the case that the fiber described
by Tomioka represents a substantial improvement in the state of
the heat-bondable fiber art to date, this prior art fiber
nonetheless suffers from several shortcomings. For example,
while the fiber does indeed exhibit an amount of thermal
shrinkage which is less than that of earlier fibers, it can be
demonstrated that the fiber nevertheless still shrinks to a
substantial and undesirable degree. Furthermore, although the
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elimination of thermal shrinkage represents a good theoretical
approach to the improvement of heat bondable fibers, it is
believed that this approach does not go far enough.
It will be recognized that while thermal shrinkage per
se may be undesirable in a heat bondable fiber, the development
of shrinkage force in a nonwoven, brought about subsequent to the
creation of interfilamentary bonds may, in fact, be desirable.
It is reasonable to assume that shrinkage force, introduced at
this time, will not produce any substantial amount of actual
shrinkage, but will, rather, remain as a trapped tension in the
nonwoven which will enhance such fabric properties as bulk,
liveliness, drape and hand.
Accordingly, it is the general object of the present
invention to provide improved heat bondable heterofilaments which
are useful for the production of nonwoven fabrics, particularly
light and medium weight nonwovens, as well as a method for
manufacturing such fibers.
It is a more specific object of the invention to
provide heat bondable heterofilaments which may be used to
produce nonwovens which exhibit minimal thermal shrinkage during
thermal bonding but which also exhibit enhanced fabric tensile
strength, liveliness, drape, bulk and hand after bonding.
A still more specific object is to provide a heat
bondable heterofilament which does not experience substantial
shrinkage force, and hence shrinkage, prior to or during thermal
bonding, but which does develop `substantial shrinkage force
subsequent to the formation of interEilamentary bonds in a
nonwoven.
It is a further object to provide a method for
manufacturing heat bondable heterofilaments whereby the thermal
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characteristics of said fiber can be adjusted or altered to meet
specified requirements.
Finally, it is an object of the invention to provide
nonwovens manufactured from these novel heterofilaments, with
said nonwovens being producible at high rates and with modest
energy consumption and having enhanced properties.
SUMM~.RY OF THE INVENTION
In furtherance of the aforementioned objects, it has
now been discovered that improved heat bondable heterofilaments,
for use in either staple or filament form in the manufacture of
nonwovens, may be produced from polyester and another suitable
thermoplastic polymer having a melting point which is at least
about 15C below that of polyester, wherein polyester is the
backbone polymer and the other thermoplastic component serves as
the latent adhesive. The two components may be arranged in side-
by-side relationship, i.e., collinearly, but preferably they are
arranged in sheath/core relationship with polyester occupying the
core. Subsequent to the usual steps of spinning, drawing and
winding, a heterofilament prepared in accordance with the
invention is subjected to a thermal conditioning step. This step
involves heating the fiber at a preselected temperature for at
least a preselected time so that a change is brought about in the
thermal response of the fiber such that the fiber becomes
characterized by the fact that upon the application of heat
sufficient to melt the latently adhesive component and subsequent
cooling, a substantial shrinkage force appears in the polyester
component only after the resolidification of the latently
adhesive component. The temperatue at which shrinkage Eorce does
appear is termed the "conditioned response temperature". The
precise parameters of temperature and time which must be employed
to properly condition a fiber in the manner described above
cannot be given as a general matter. As will be seen from the
further description to follow, the parameters required for
thermal conditioning will be governed by such things as the prior
thermal history of the particular fiber being used and the
temperature at which the nonwoven is to be thermally bonded,
which, in turn, will be determined by the particular latently
adhesive component being employed~
It has been observed that with fibers prepared in
accordance with the invention, interfilamentary b~nds are enabled
to form between fibers before the development of shrinkage forces
therein. As explained in greater detail hereinafter, this
property is believed to enhance the stren~th of the
interfilamentary bonds in a nonwoven fabric, and, further, to
contribute to a superior drape, hand, bulk and liveliness in the
fabric.
As a still further attainment of the objects, it has
been found that nonwoven fabrics may be prepared from the
heterofilaments of the invention, and that this can be done using
relatively uncritical processing conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
.
Figure 1 is a photomicrograph of a nonwoven prepared
from the heterofilaments of the invention illustrating the
interfilamentary point bonding present in the fabric.
Figure 2 is a graph depicting shrinkage force in the
polyester component of a heterofilament of the invention as a
function of temperature, as compared to the shrinkage force in
the polypropylene component of a prior art fiber.
Figure 3 is a graph depicting the effect of thermal
conditionin~ upon the shrinkage force response of a hetero-
filament fiber having a polyester component.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A photomicrograph of a non-woven ~abric manufactured
from the fibers of this invention is shown in Figure 1. In the
production of such a fabric, the fibers are formed into a web and
subjected to heating sufficient to activate the la~ent adhesive
element, and then cooling to solidify the bonds (2) that have
been formed by the molten adhesive at the intersections of the
individual filaments.
Because it may be assumed that the thermo-mechanical
behavior of bicomponent fibers subjected to such a hea~ing and
cooling cycle will affect both the nature of the bonds formed as
well as the general character of the resulting fabric, it is
appropriate to characterize these fibers by some type of
thermo-mechanical analysis. Accordingly, the ~ibers of the
present invention were investigated using a technique known as
Thermal Stress Analysis (TSA). In this technique, a sample is
held at a constant length while its temperature is changed, and
the resulting tensile forces developed in the sample are recorded
as a function of temperature. This TSA method is discussed in an
article by Buchanan and Hardegree in the Textile ~esearch
Journal, November 1977, p. 732. However, as far as is known,
these authors, as well as others who have published results from
this technique, have concentrated solely on the reactions of the
sample to increasing temperatures only. In contrast, the studies
of this invention have put equal emphasis on the sample reaction
during the cooling portion o~ the test, since it seemed
appropriate for the proper simulation of the complete thermal
treatment given to a nonwoven material in its fabrication, as
described above.
In the preparation of samples for this study, a
sufficient number of individual fibers were mounted together to
make a bundle with an equivalent denier in the range of 100 to
500. The mounting system used was that prescribed by the
Perkin-Elmer Co., of Norwalk, Conn., for use with their thermo-
mechanical analyzer, designated TMS-1. A standard pre-tension of
tO 0.02 gmfden was selected to improve uniformity of testing. The
temperature was increased at a rate of approximately 15C/min in
all tests.
Figure 2 gives representative results of a TSA test of
fibers of the invention, in this case designated as high density
polyethylene/polyester, in comparison with the prior art fibers,
designated as high density polyethylene~polypropylene. The
tension in the sample is plotted on the vertical, or "Y" axis,
and the temperature of the sample is plotted on the horizontal,
or "~" axis. The arrows on the curves show how the changes
progressed with time. Starting with test samples of 200 denier,
a pretension of 4 grams is applied at room temperature, and the
sample length is held constant for the rest of the test. As the
temperture is increased, this pre-tension is seen to decay to
essentially zero in both cases, resulting from the normal
relaxation and thermal expansion shown by materials in general.
After this relaxation of the initial pre-tension, however, the
two samples tested show quite different thermal-stress behavior.
The prior art sample high density polyethylene/propylene shows an
increase in tension as the temperature increases to 150C, and,
more significantly, a rapid increase in tension as the sample is
cooled. It shoulcl be emphasiæed that this tension build-up on
heating, and the subsequent rapid tension increase on cooling is
regarded as undesirable in the production of non-woven
structures. In contrast to this pattern, the sample of the
subject invention shows no increase in tension, either in the
heating stage or in the cooling stage, until the assembly of
fibers forming the bundle has cooled sufficiently to ensure that,
were the fibers in a web, inter-fiber bonds would have solidified
without shrinkage forces being applied to these newly-formed
bonds.
It is apparent that the temperature at which tension
begins to develop as the sample is cooled is of primary
importar,ce in distinguishing the fibers of this invention. As a
means of determining this temperature, we have defined the onset
of tension build-up as that temperature at which the tension
exceeds a threshold value of OOO1 gms/den, based on the denier of
the backbone component. This value was selected as being as low
as practical but still clearly distinguishable over the
instrumental background variations in the recorded tension. As
an example, a 50/50 composition sample formed into a 300 denier
bundle for testing, as described above, will comprise only a 150
denier backbone fiber, and its threshold tension value will be
1.5 gms.
As a means of showing how fibers having this desired
cooling curve are produced, Figure 3 shows the results obtained
when several conditioning treatments are applied to fibers
produced by one common spinning and drawing scheme. Table 1
below lists the different conditioning treatments used, and
Figure 3 is a composite of the TSA curves of these several
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different samples, each with its own thermal ~onditioning
treatment:
Table 1
Sample Conditioning Treatment
A None
B 3 minutes at 90C
C 3 minutes at 100~C
3 minutes at 110C
E 3 minutes at 120C
F 3 minutes at 130C
As in Figure 2, the tension is plotted on the vertical
scale, but in this case, the scale is different from that of
Figure 2, and the initial pre-tension of 6 gms is seen in the
lower left portion of the diagram. All samples show the same
decrease in this tension as the temperature increases at the
beginning of the test. Beyond this initial heating phase, the
different samples are easily distinguished from one another.
It will be noted that Sample A, which received no heat
treatment subsequent to drawing, shows a build up of tension as
the temperature increases to about 100C, reaching a peak at
approximately 120C and decreasing to a minimum (but not zero-
level) at 140C. This is typical of a polyester, and is
described by Buchanan and Hardegree in the reference cited. On
cooling, this sample shows an increase in tension at the
beginning of cooling, with a rapid increase below 130C.
Actually, this sample exhibits a tension exceeding th~ threshold
value of 0.01 gm/denier throughout its high temperature
residence; consequently, it cannot be given a value for the start
of tension build-up.
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Sample B, which was treated for 3 minutes a~ 90C,
shows a substantial reduction in the tension peak during the
heating portion of the curve, and a tension build-up curve on
cooling just a little below that of sample A.
Samples C, D, and E show no tension on heating, and
only develop an appreciable tension when they have cooled well
below the temperature of re-solidification of the sheath
material. These samples are representative of fibers prepared in
accordance with the invention.
Sample F illustrates the fact that a heat treatment
that is too severe can completely eliminate any tendency to
develop a tension on cGoling.
Without wishing to be bound by any particular theory,
it is believed that in nonwovens made from fibers produced in
accordance with the invention, the strength of interfilamentary
bonds, such as the fillets 2 of Figure 1, is enhanced by the fact
that when the fibers are heated, in order to melt the sheaths
thereof, and, subsequently, cooled to solidify the bonds there-
between, little or no tensile force develops in the fibers until
the temperature has dropped below the solidification range of the
sheath and such bonds have already formed. That is, to say, it
is believed that with the fiber of the invention, bonds are
formed in an unstressed state, a condition which enhances
interfilamentary bond strength. In comparison, tensile forces do
develop in the prior art fiber prior to the formation of
relatively weak bonds.
Again, without wishing to be bound by any theories
expressed, it is further believed that the development of tension
or shrinkage force in fibers according to the invention, after
the formation of interfilamentary bonds, serves to enhance
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various textile qualities in nonwovens made from the fiber.
Thus, it is theorized that the unique thermomechanical behavior
of the novel fiber functions to trap tension in the nonwoven
fabric and it is believed that this tension is, at least in part,
responsible for the pleasing liveliness, drape, bulk and hand
possessed by nonwovens made from fibers produced in accordance
with the invention.
Turning now to a more detailed description of the com-
position and preparation of the fibers which are the subject of
the present invention, reference is made to the several examples
which follow which describe the preparation of a number of such
fibers. In each case, a heat bondable bicomponent filament was
produced wherein the structure or backbone polymer was polyester.
The latently adhesive components used were, in each case,
selected from the group comprising polyethylene and polypropylene
of fiber forming grade, although it is to be assumed that other
polymers having melting points at least about 15C below that of
polyester would serve equally well for this purpose.
The fibers in each of the exa~ples are of a sheath/core
configuration wherein the polyester component occupies the core
location. Both eccentric and concentric sheath/core arrangements
were utilized. It is to be understood, however, that bicomponent
fibers having side-by-side configurations are also considered to
be within the scope of the invention.
Par~icular note will be paid to the fact that, while
very dif~erent thermal conditioning parameters were utilized with
respect to each of the fibers in the various examplesf with
proper thermal conditioning, it was possible in each case to
produce a fiber which exhibited a thermomechanical response
characteristic of fibers according to the invention; that is to
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say, it was possible in each case to produce, with the proper
thermal conditioning, a fiber which was characterized by the fact
that upon the application of heat sufficient to melt the latently
adhesive component and subsequent cooling, substantial shrin~age
force appeared in the polyester component only after the
resolidification of the latently adhesive component. A precise
description of the parameters required for proper thermal
conditioning cannot be given, and it will be noted from the
examples that these parameters are governed by such things as the
prior thermal history of the particular fiber being used; the
temperature at which the nonwoven is to be bonded, which, in
turn, will be determined by the particular latently adhesive
component being employed; and, also by the amount of shrinkage
force desired in the fiber. As a general rule, it can be stated
that there appears to be a direct relationship between the
melting point of the latently adhesive component and the thermal
conditioning temperature which is required, fibers with high
melting point adhesives requiring higher thermal conditioning
temperatures. It is believed, although exact guidelines cannot
be given, that the precise parameters for conditioning any given
fiber can be determined with the aid of this disclosure with
minimal experimentation.
Finally, it will be noted that there is considerable
scatter in the "conditioned response temperatures" given for the
various samples of the fibers in each example. Such variation
should be considered as typical for staple fiber samples.
The following examples will illustrate the invention:
EXAMPLE 1
A staple fiber consisting of a sheath composed of a 42
Melt Index high density polyethylene (Fortiflex F-381 obtained
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from Soltex Polymer Corp.) having a molecular weight of 46,000
and a narrow molecular weight distribution (dispersity) of about
3.6 (high density polyethylene), and a core consisting of a
standard fiber grade of semi-dull polyester was spun in an
eccentric sheath/core arrangement into a fiber of about 50% by
weight of high density polyethylene and 50% by weight of
polyester. The high density polyethylene used had a density of
0.96 gr/cc, the polyester had a density of 1.38 gr/cc, and the
conjugate fiber had a density of 1.12 gr/cc. The melting point
of the high density polyethylene was 132C. The melting point of
the polyester was about 260C.
The two polymers were melted in separate screw
extruders, and fed through separate polymer lines and pumpblocks
into the spinneret. The high density polyethylene was brought to
a temperature of ~65-270C in the extruder, conducted through a
pump and into the spinneret. The polyester was brought to a
temperature of 285C in its extruder and conducted through a pump
and into the spinneret. Inside the spinneret, the polymers were
initially introduced to each other just prior to entering the
capillary opening for extruding the filaments. Once the poly-
ethylene melt contacted the polyester melt, its temperature
jumped to about 285C for a short time period before being cooled
and solidified in the blow box. In spinning for a 3.0 dpf
staple, each component was metered to the spinneret at 0.583
grams/minute/hole, or a total throughput for both polymers of
1.166 grams/minute/hole. Each of the spinneret holes had
diameter of 400~1. The filaments, still in tow form, were cooled,
a spin finish, conventional for polyolefins, was applied by a
water wheel, and the tow wound at 1752 meters/min. The filaments
were drawn in t-~o stages in order to develop maximum orientation
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and fiber properties; the draw ratio in the first stage being
1~05, with draw being conducted at room temperatue and the draw
ratio in the second stage being 2.50, with draw being conducted
in steam so ~hat the tow temperature was 80C, the total draw
ratio thus being 2.62. During drawing, the tow developed a
spontaneous, curl~ crimp, when tension is released, because of a
difference in tensions between the two polymer phases, which is
not permanent. The tow was crimped, for aid in processing as a
staple fiber by conventional stuffer box crimping. After
crimping, the fiber was subjected to a thermal conditioning
treatment by heating, under no tension~ in a forced air oven at
230F (110C) for 240 seconds. The fiber was then cut into 1
1/2-inch staple fiber having the following properties:
Denier 3.00 dpf
Tenacity 3.2~ gpd
Elongation 55.9%
Crimp/Inch 24
Seven examples of this fiber were prepared, and, following the
testing procedure outlined hereinbefore, each subjected to a
thermal stress analysis. The peak temperature reached during the
test procedures was 150C, a temperature around which the
particular fiber might typically be bonded into a nonwoven. In
the case of each sample, no significant increase in shrinkage
force was noted in the fiber bundle during the heating phase of
the test. Each sample did, however, experience a marked
development of tensile force during the cooling phase. The
conditioned response temperature, that is, the temperature at
which a shrinkage force equal to the threshold force of 0.01
grams per denier was first o~served upon cooling of the fiber, is
given below or each of the samples.
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Sam~le Conditioned Response Temperature
1 80~
2 70C
3 52C
4 33C
48~C
6 86C
7 88C
EXAMPLE 2
A sample of the staple fiber of Example 1 was hand
formed into a matt and passed through a lab carding machine. The
re~ulting web was rolled to give 4 plies. The sample was then
compressed into a batt at 2000 psi on a 6" ram and after 5
minutes was removed and trimmed. The batt was thermally bonded
by placing the sample into a forced draft oven at 145C for 90
seconds. Other samples were prepared in the same way at 60 and
120 seconds. The samples all had considerable structural
integrity as evidenced by their recovery from a small
elongational stress placed by hand on the battO The samples also
exhibited a high degree of resilience and liveliness, which is
demonstrated by observing the recovery to the original volume
after being squeezed by small compressive forces, e.gO, by hand
pressure. The handle of these fabrics was soft and lofty.
EXAMPLE 3
A sample was spun substantially as in Example 1, except
that a draw ratio of 1.10 was utilized in the first drawing stage
and a draw ratio of 2.136 was utilized in the second drawing
stage. The staple fiber thus prepared had the following
properties:
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Denier 2,97 dpf
Tenacity 3.43 gpd
Elongation 54
Crimp/inch 23
Four samples of this fiber were prepared and subjected
to thermal stress analysis, as in Example 1. Again, no
significant shrinkage force was noted in any of the samples
during the heating phase of the tests, but shrinkage force was
noted during the cooling phase. The conditioned response
temperatures for the four samples are given below.
SampleCondîtioned Response Temperature
1 98C
2 111C
3 78C
4 62C
EXAMPLE 4
Another sample was spun exactly as in Example 1, except
that a diferent spinneret was used to make symmetrical sheath/
core filaments. The total draw ratio was 2.28. The staple fiber
values were:
3~03 dpf
3.28 gpd tenacity
43.2% elongation
1~ crimps/inch
2~ 1.5" fiber length
Two samples of this fiber were prepared and subjected
to thermal stress analysis, as in Example 1. Againl no
significant shrinkage force was noted in either of the samples
during the heating phase of the tests, but shr~nkage force was
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noted during the cooling phase. The conditioned response
temperatures for the two samples are given below.
Sample Conditioned Response Temperature
1 87C
2 6~C
A nonwoven fabric from the fiber obtained above was
made in substantially the same manner as Example 2. The fabric
had substantially the salne properties as the nowoven of Example
2.
_AMPLE 5
A staple fiber consisting of the high density
polyethylene and the polyester of Example 1, arranged in an
eccentric sheath/core relationship at 50% by weight of sheath and
50% weight of core was spun from separate screw~pressure melters
for each polymer~ In spinning, each polymer was metered to the
spinneret at the rate of 0.501 grams/minute/hole, or a total
throughput for hoth polymers of 1.002 gram~/minute/hole. Each
spinneret hole had a diameter of 250~. The filaments, in tow
form, were cooled, a spin finish conventional for polyolefins was
applied by water wheel, and the tow wound at 1000 m/min. The
filaments were drawn through a water bath at 50C to a ratio of
4.53 to develop maximum orientation and fiber properties. The
tow was crimped by conventional stuffer box crimping, then
treated under no tension in forced air heat for 200 seconds at
~5 100C to develop the conditioned response desired, in addition to
stabilizing the crimp. The fiber, cut into 1 1/2 inch staple,
has typical values as follows:
Denier 3.06 dpf
Tenacity 3.~2 gpd
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Elongation 49%
Crimp/Inch 18
Six samples of the fiber thus prepared were subjected
to thermal stress analysis, as in Example 1~ No substantial
tensile forces developed in any of the samples during the heating
phase of the tests, but tension did develop in all cases during
the cooling phase after the fiber sampled had cooled below the
melting (resolidification) point of the polyethylene sheath. The
conditioned response temperatures for each sample are given
below.
SampleConditioned Response Temperature
1 96C
2 93C
3 69C
4 60C
64C
6 114C
EXAMPLE 6
A staple fiber consisting of the high density
polyethylene and the polyester of Example 1, arranged in an
eccentric sheath/core relationship at 50% by weight of sheath and
50~ by weight of core was spun from separate screw extruders for
each polymer. In spinning, each polymer is metered to the
spinneret at the rate of 0.50 grams/minute/hole, or a total
throughput for both polymers of 1.000 gram/minute/hole. Each
spinneret hole had a diameter of 400~ The filaments so extruded
are cooled, a spin finish conventional for polyolefins applied by
water wheel, and the tow would at 1000 m/min. The filaments are
drawn through a water bath at 70C to a ratio of 2.50, and then
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through another water bath at 85C to a ratio of 1.3. The total
ratio of drawing was thus 3~25. The tow was crimped by
conventional stuffer box crimping, then treated under no tension
in forced air heat for 300 seconds at 90C to develop the
conditioned response desired. The fiber, cut into 1 1/2 inch
staple, had typical values as follows:
Denier 5.37 dpf
Tenacity 1.95 gpd
Elongation 81.8%
Crimp/Inch 31
A single sample of the fiber thus prepared was
subjected to thermal stress analysis, as in Example 1. No
substantial tensile forces developed during the heating phase of
the test, but tension did develop during the cooling phase after
the fiber sample had cooled below the melting (resolidification)
point of the polyethylene sheath. The conditioned response
temperature for the sample was 98C.
EXAMPLE 7
A staple fiber consisting of a sheath composed of a 33
MFI Polypropylene (Fortilene HY-602A obtained from Soltex
Polymers Corp.), and a core consisting of a standard fiber grade
of semi-dull polyester was spun in an eccentric sheath/core
arrangement into a fiber of about 50% by weight of polypropylene
and 50~ by weight of polyester. The melting point of the
~5 polypropylene was 162~C. The melting point of the polyester was
260C
The polypropylene and the polyester were melted in
separate screw extruders, and spun and wound as specified in
Example 1. The filaments were drawn at a ratio of 2.6 to develop
maximum orientation, and, thusly, textile fiber properties.
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After conventional stuffer-box crimping to aid in textile
processing as a staple ~iber, the ~ow was thermally conditioned
under no tension for 240 seconds at 230F ~110C). Fiber
prepared thusly had typical values as follows:
Denier 2.60 dpf
Tenacity 4O04 gpd
Elongation 20.2
~rimp/Inch 12
In order to highlight the effect of a fiber's thermal
history upon the temperature and time parameters which must be
observed in order to achieve the desired thermal response
according to the invention, a thermal stress analysis was run on
two samples of the above fiber in the manner prescribed in
Example 1. While the thermal conditioning used to this point in
the present Example was the same as in Example 1, the results of
the thermal stress analysis were not~ As in the desired thermal
response, no shrinkage force was seen during the heating phase of
the two tests. Very notably, however, significant tension build
up was seen to occur in both samples during the cooling phase at
conditioned response temperatures which were much above those
recorded in Example 1. Specifically, the conditioned response
temperatures were about 136~C and 137C. Without wishing to be
bound by any particular theory, it is clear that the polyester
component of the fiber of the present Example experienced a
thermomechanical history which was different from that of the
fiber produced in Example 1, due mostly to the use of a higher
melting sheath material, which was introduced to the core at a
higher temperature, and that this different thermo-mechanical
history produced a conditioned response in the fiber which was
unlike that seen in Example 1.
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In order to highlight the importance of the melting
point of the sheath material, which in the case of polypropylene
was 162C, a thermal stress analysis was carried out on four
samples of the fiber as in the manner prescribed in Example 1,
except that the peak temperature reached was 200C, a temperature
at which the fiber of the present Example might typically be
bonded into a nonwoven. One effect of the high peak temperature
utilized was the buildu~ of shrinkage force in each of the
samples during the heating phase of the tests, something not seen
when the fiber was only heated to 150C. In addition, it was
noted that the buildup of tension during the cooling phase of the
tests occurred at a much higher temperature in all but one
instance. - -
To produce fibers of the present Example having a
conditioned thermal response in keeping with the requirements of
the invention, the fiber prepared thus far was subjected to an
additional thermal conditioning step which involved heating the
fiber at 140C for 300 seconds. Two samples of the thus treated
fiber were then subjected to thermomechanical analysis as in
Example 1, except that a peak temperature of 200C was reached.
The fibers exhibited the thermal response which characterizes
fibers in accordance with the present invention. This is to sa~,
no substantial shrinkage force was observed in either sample
during heating, while both samples did exhibit a substantial
shrin~age force upon cooling, but only after cooling well below
162C, the resolidification point of the sheath material. The
conditioned response temperatures for the two samples were
approximately 110C and 135C.
Lastly, to show the effect of an excessive thermal
conditioning treatment, a second batch of fiber was subjected to
an additional thermal conditioning step, as described above
- 21 -
'7~
which this time involved heating the fiber at 145C for 300
seconds. Two samples of this fiber were prepared and subjected
to thermal mechanical analysis, with the peak temperature reached
once again being 200Co In the case of both samples, no tension
S buildup was seen upon either heating or cooling.
While the invention has been described with reference
to certain specific examples and illustrative embodiments, it is,
of course, not intended to be so limited except insofar as
appears in the accompanying claims.
