BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to nonwoven fabrics which is bulky and has a soft touch
or feeling, and a method for producing the same nonwoven fabrics.
Statement of the Prior Art
[0002] Many years have elapsed since there were known in the art the side-by-side type or
sheath-core type polypropylene base heat-adhesive composite fibres, which comprised
two components having different melting points, and had a considerable portion, e.g.,
one half or more portion of their surfaces occupied by the component having a lower
melting point, and the nonwoven fabrics made thereof. In the meantime, variouos improvements
have been achieved. As disclosed in, e.g., Japanese Patent Publication No. 52-12830,
Japanese Patent Laid-Open Publication No. 58-136867 and Japanese Patent Laid-Open
Publication No. 58-180614, such improvements have primarily aimed at improving the
shrink properties of a web in processing the fibers into a nonwoven fabric by heating
and enhancing the strength, bulkiness and like factors of the resulting nonwoven fabric,
and appreciable outcomes have been attained, but, referring to the bulkiness, any
satisfactory outcome has been not yet achieved.
[0003] Hitherto, any appreciable outcome has been not attained in terms of not only the
bulkiness but also the touch or feeling of nonwoven fabrics obtained from the polypropylene
base heat-adhesive composite fibers by a heat treatment. Improvements in touch or
feeling have been attempted as by using fine deniers or increasing the proportion
of other fibers to be mixed with the composite fibers, such as rayon or wool, but
have not still resulted in any product excelling in bulkiness and softness. The situation
being like this, a strong demand for further improvements in the bulkiness and softness
of nonwoven fabrics intended for purposes such as paper diapers or sanitary materials
is not satisfied. Thus, it is strongly desired to meet such a demand.
SUMMARY OF THE INVENTION
[0004] A main object of the present invention is to provide nonwoven fabrics which is not
only bulky but has also a highly soft touch or feeling.
[0005] As a result of intensive and extensive studies made to attain the object, it has
been found that the nonwoven fabric structure is extremely stabilized and sufficiently
bulked and have soft touch or feeling, when the composite fibers to be processed into
nonwoven fabrics are constructed by a core portion which imparts bulkiness to the
nonwoven fabrics and a sheath portion which imparts heat adhesiveness to the fibers
and, furthermore, in addition to the above-mentioned construction, when a number
of nodular aggregates consisting of the sheath component are formed on the surfaces
of the fibers except for the portions of the fibers bonded together, the soft touch
or feeling is further improved.
[0006] According to one (or the first) aspect of the present invention, there is provided
a nonwoven fabric which contains at least 30 % by weight of heat-adhesive composite
fibers comprising a core portion and a sheath portion, said core portion being of
the side-by-side type composite structure comprising two core components of different
polypropylene base polymers in a composite ratio of 1:2 to 2:1, one of said core components
having a Q value, expressed in terms of the weight-average molecular weight/the number-average
molecular weight, equal to or higher than 6 and the other having a Q value equal to
or lower than 5, said sheath portion meeting at least the requirement (hereinafter
referred to as the sheath requirement) that it should comprise a sheath component
of a polyethylene base polymer having a melting point lower by at least 20°C than
the lower one of the melting points of said two core components, and said sheath portion
covering completely said core portion in a proportion of 25 to 55% by weight based
on the total weight of it and said core portion, and which is stabilized by the inter-fiber
bonds of the sheath portion of said heat-adhesive composite fibers.
[0007] According to another (or the second) aspect of the present invention, there is provided
a method for producing nonwoven fabrics which comprises the steps of:
separately subjecting to composite-spinning two polypropylene base polymers for two
core components and a polyethylene base polymer for a sheath component, which has
a melting point lower by at least 20°C than the lower one of the melting points of
said two polypropylene base polymers, thereby obtaining a composite nonstretched yarns
of the structure that a core portion of the side-by-side type composite structure
consisting of two core components in a composite ratio of 1:2 to 2:1, one of said
core components having a Q value, expressed in terms of the weight-average molecular
weight/the number-average molecular weight, equal to or higher than 6 and the other
having a Q value equal to or lower than 5, is completely covered with a sheath portion
comprising said sheath component in a weight proportion of 25 to 55 % by weight based
on the total weight of it and said core portion,
stretching said composite nonstretched yarn by an one- or more-stage stretching process
to prepare heat-adhesive composite fibers,
preparing a web containing at least 30 % by weight of said heat-adhesive composite
fibers, and
heat-treating said web at a temperature higher than the melting point of said sheath
component and lower than the lower one of the melting points of said core components.
Explanation of the First Aspect of the Invention
[0008] The 1st asepct of the present invention will now be concretely explained. First of
all, the heat-adhesive composite fibers used in the nonwoven fabrics according to
the present invention will be explained with reference to Figures 1, 2 and 3, each
being a schematical section showing the section structure of the heat-adhesive composite
fiber used in the present invention, and Figure 4 being a sketch depicting the sheath
portion on which nodular agglomerates are formed.
[0009] Referring to the drawings, reference numeral 1 is a core portion of the side-by-side
type composite structure comprising core-dividing zones 1a and 1b each consisting
of a core component of a different polypropylene base polymer. The side-by-side type
composite structure of the core 1 may take on various forms. For instance, the core
1 may be of the sectional structure which is diametrically divided into two identical
semi-circles, as illustrated in Figure 1. Alternatively, the core 1 may be of the
sectional structure in which one core-dividing zone 1a is mostly surrounded with the
other core-dividing zone 1b, except for its slight peripheral portion, as illustrated
in Figure 2. In most cases, the core actually assumes a structure lying between the
aforesaid extreme structures. Still alternatively, the core 1 may be located off the
center in section of the fibers, as illustrated in Figure 3.
[0010] Polypropylene base polymers, which are represented by crystalline polypropylene,
may include copolymers of propylene with a small amount of other alpha-olefins save
propylene, such as ethylene, butene-1 or pentene-1. In this case, it is preferred
that the comonomer component content is up to 40 % by weight.
[0011] Such polypropylene base polymers are used as the core components of the respective
core-dividing zones 1a and 1b, and are different from each other in the Q value that
is a numerical value expressing the molecular weight distribution of polymers and
calculated from the following equation:
Q = Mw/Mn
wherein Mw stands for the weight-average molecular weight, and Mn indicates the number-average
molecular weight.
The core component of one core-dividing zone 1a (which may hereinafter be simply referred
to as the component 1a) has a Q value of at least 6 and corresponds to the general-purpose
polypropylene, while that of the other core-dividing zone 1b (which may hereinafter
be simply referred to as the component 1b) has a Q value of up to 5, preferably 3
to 5.
[0012] The composite ratio of the core components 1a and 1b forming the core 1 is in a range
of 1:2 to 2:1.
[0013] Thus, the side-by-side type composite structure of the core 1 comprising the components
1a and 1b having different Q values assures that revealed crimps and latent crimps
to be developed by a heat treatment are imparted to the composite fibers to thereby
make the nonwoven fabrics bulky.
[0014] Reference numeral 2 is a sheath portion which is formed of a sheath component of
a polyethylene base polymer, the melting point of which is lower by at least 20°C
than the lower one of the melting points of the two core components of the core 1,
viz., the components 1a and 1b (or the melting point common to the components 1a and
1b, if there is no difference in the melting point therebetween). Such polyethylene
base polymer may include polyethylene or a copolymer of ethylene/vinyl acetate, having
an ethylene content of 98 to 60 % by weight. That polyethylene is exemplified by a
low-, intermediate- or high-density polyethylene.
[0015] The sheath-core type composite fibers of the present invention are constituted by
covering the core 1 with the sheath 2 in such a manner that the proportion of the
sheath 2 is in a range of 25 to 55 % by weight based on the total weight of it and
the core 1. When the proportion of the sheath 2 is below 25 % by weight, the strength
of the resulting nonwoven fabric decreases to such a low level that some problems
arise practically. In a proportion of the sheath 2 exceeding 55 % by weight, on the
other hand, the development of the crimps due to the core 1 is inhibited so that the
composite fibers are insufficiently crimped and, hence, the resulting nonwoven fabrics
become inferior in bulkiness.
[0016] As described above, since the sheath 2 is formed of a polyethylene base polymer of
a low melting point, the inter-fiber bonds can be formed by a heat treatment as in
the case of the conventional heat-adhesive composite fibers.
[0017] As long as the sheath 2 meets the aforesaid sheath requirement that it be of the
above-mentioned structure, a nonwoven fabric product obtained by using as the raw
material the heat-adhesive composite fibers constituted by it together with the core
1 may have a sufficient bulkiness and shown excellent touch or feeling. Moreover,
the following structure may impart a much softer touch or feeling to the nonwoven
facbric product. More specifically, the structure is such that the sheath 2 has on
a number of its portions nodular aggregates 3 consisting of the sheath component,
as illustrated in Figure 4. In most cases, a diameter (D₂) of the greatest portion
of the nodular aggregate 3 is about two times the diameter (D₁) of the thinnest portion
adjacent thereto. Per one centimeter of the actual length of fiber, there are formed
0.1 to 0.5 nodular aggregates 3 having such a diameter (D₂). When the proportion of
the sheath 2 exceeds 55 % by weight of the total weight of it and the core, the number
of the aggregates 3 formed is not sufficient and, hence, makes no contribution to
improvements in the touch or feeling of nonwoven fabrics.
[0018] Although no special limitation is imposed upon the fineness of the heat-adhesive
composite fibers, 1.5 to 7 deniers are suitable in applications in which weight is
given to the touch or feeling of nonwoven fabrics. More suitable is a range of a finer
value of 0.7 to 7 deniers.
[0019] The nonwoven fabrics according to the present invention may consist of the aforesaid
heat-adhesive composite fibers alone, or may comprise at least 30 % by weight thereof
and other fibers such as, for instance, rayon, wool, hemp, polyamide fibers, polyester
fibers and acryl fibers, and are allowed to be of the nonwoven structure by the inter-fiber
bonds of the sheath 2 of the aforesaid heat-adhesive composite fibers.
Explanation of the 2nd Aspect of the Invention
[0020] In manufacturing the nonwoven fabrics according to the present invention, the heat-adhesive
composite fibers are first prepared in the following manner. Provided are three polymers,
i.e., two polypropylene base polymers for the core components and one polyethylene
base polymer for the sheath component, as already mentioned in connection with the
1st aspect of the present invention. With regard to the polypropylene base polymers
for the core components, the polypropylene base polymer for the component 1a having
a Q value of at least 6 should preferably show a melt flow rate (hereinafter sometimes
abbreviated as MFR and measured according to Table 1, Condition 14 provided by JIS
K 7210) of 4 to 40, and the polypropylene base polymer for the component 1b having
a Q value of 5 or less should preferably show a melt flow rate of 4 to 60. Polypropylene
base polymers having a Q value of 5 or less may be prepared by the following methods,
using polypropylene base polymers having a Q value of more than 5 as the starting
material. According to the one method, added to and mixed with the starting polymer
is an organic peroxide compound in an amount of 0.01 to 1.0 % by weight based on the
starting polymer, said organic peroxide compound releasing oxygen by heating at a
temperature equal to or higher than the melting point of the starting polymer, such
as t-butyl hydroperoxide, cumene hydroperoxide or 2,5-dimethylhexane-2,5-dihydroperoxide,
etc., and the resulting mixture is subjected to melting extrusion from an extruder
for granulation. According to another method, the starting polymer may be subjected
to melting extrusion several times at elevated temperatures, with no addition of the
aforesaid organic perioxide compound, for repeated granulation. Since the Q value
is decreased a little by melting extrusion, the polymer for the component 1a before
melt spinning should preferably have a Q value of slightly higher than 6, while the
polymer for the component 1b may have a Q value of slightly higher than 5. The polyethylene
base polymer should preferably have a melt index (hereinafter sometimes abbreviated
as MI and measured according to Table 1, Condition 4 provided by JIS K 7210) of 2
to 50.
[0021] After the aforesaid three polymers have been provided, they are separately supplied
to the respective three extruders for melting extrusion, and the obtained molten polymers
are guided to a known appropriate composite spinning nozzle by way of the respective
gear pumps. For instance, such a spinning nozzle as disclosed in Japanese Patent Publication
No. 44-29522 may be used as the known compoiste spinning nozzle capable of spinning
out three polymer components into a sectional structure similar to that of the heat-adhesive
composite fibers according to the present invention. When the aforesaid three polymers
are guided to such a spinning nozzle, the outputs of the respective gear pumps are
regulated in such a manner that the ratio of the amounts of the polymers for the core
components 1a and 1b is a given composite ratio within the range of 2:1 to 1:2, and
the amount of the polymer for the sheath component is a given one within the range
of 25 to 55 % by weight based on the total amount of it and the core components.
[0022] The thus obtained nonstretched composite yarns of the given sectional structure are
stretched in a single or multi-stage manner. To increase the latent crimping properties
of the obtained composite yarns, it is generally preferred that the multi-stage stretching
is carried out under the condition that the first-stage stretching temperature be
lower than the second-stage stretching temperture, and that the single-stage stretching
is effected at normal temperature (15 to 40°C) or a relatively low temperature close
thereto. Since stretching is usually accompanied by the generation of heat, the single-stage
stretching or the first-stage stretching of the multi-stage stretching is preferably
carried out while passing the yarns through the water maintained at normal temperature,
or in a room maintained at normal temperature by cooling water.
[0023] The stretching conditions vary somewhat depending upon the heat-adhesive composite
fibers to be produced.
[0024] If it is intended to produce the heat-adhesive composite fibers meeting only the
aforesaid sheath requirement imposed upon the sheath 2, the stretching temperature
may then be within a range of normal temperature (15 to 40°C) to 130°C. The draw ratio
is within a range of 1.3 to 9, preferably 1.5 to 6, as expressed in terms of the overall
draw ratio. Especially, the following stretching conditions are very preferable, viz.,
the stretching temperature being normal temperature with the draw ratio being within
a range of 4 to 5 at the first-stage stretching, and the stretching temperature being
within a range of 70 to 90°C with the draw ratio being within a range of 0.8 to 0.9
at the second-statge stretching.
[0025] If it is intended to produce the heat-adhesive composite fibers meeting the aforesaid
sheath requirement and further having many aggregatable portions, as defined later,
on the sheath 2, stretching has to be effected by somewhat complicated steps as mentioned
below. Prior to stretching, the composite nonstretched yarns are first by heat-treated
under no tension at a temperature ranging from 80°C to below the melting point of
the sheath component for 10 seconds or longer, preferably for 12 to 180 seconds. This
heat treatment promotes the crystallization of the two core components 1a and 1b,
and decreases the interface affinity of the sheath 2 with respect to the core 1. For
the heat treatment, for instance, the yarns may be continuously passed through a dry
heat oven or hot water, or batchwise treated in a large dryer. The heat-treated nonstretched
yarns are cooled down to normal temperature (15 to 40°C), and the first-stage stretching
is then carried out at that normal temperature in a draw ratio of 1.3 to 2, preferably
1.5 to 1.8. Synergistically combined with the said heat treatment occurring prior
to stretching, the first-stage stretching promotes a reduction in the interface affinity
between the sheath 2 and the core 1. In consequence, the sheath 2 is actually or latently
released from the core 1 at their interface to produce many portions on which the
aggregates 3 are to be formed by the heat treatment as described later (the portions
are defined as the aggregatable portions). A draw ratio exceeding 2 at the first-stretching
stage offers problems such as fuzzing, a drop in fiber strength and an increase in
the degree of shrinkage of the resulting nonwoven fabrics, whilst a draw ratio of
less than 1.3 renders it difficult to obtain the effects as contemplated in the present
invention. Subsequently following the first-stage stretching, the second-stage stretching
is carried out, without relaxing the yarns between the first-stage stretching and
the second-stage stretching, at a temperature of 80°C or higher and below the melting
point of the sheath component. In this case, the draw ratio should be equal to or
higher than 90 % of the maximum draw ratio (at which the yarns drawn at the first-stage
stretching begin to snap off by a gradual increase in the draw ratio at the second-stage
stretching). As the fibers are stretched at the second stage without letting the fibers
loose after the first-stage stretching, as mentioned above, it is possible to prevent
the fibers from being entangled together due to the crimps to be developed by fiber
releasing and snapping off by the second-stage stretching. The second-stage stretching
carried out at the temperature and draw ratio, as mentioned above, gives rise to three-dimensional
crimping, whereby the fiber strength is increased, the degree of shrinkage and bulkiness
of the resulting nonwoven fabric are decreased and increased, respectively, and the
formation of the aforesaid aggregatable portions are further promoted. The heat-adhesive
composite fibers obtainable in this manner are noticeably characterized by having
many aggregatable portions formed on the sheath 2 which form a number of nodular aggregates
3 consisting of the sheath component by the heat treatment at a temperature higher
than the melting point of the sheath component and lower than the lower one of the
melting points of the two core components 1a and 1b. In the aggregatable portions,
the sheath 2 is released from the core 1, or is not released but may latently be released
from the core 1 due to their feeble interface affininity. The aggregatable portions
are distinguishable from the other portions, depending upon whether or not the nodular
aggregates 3 are formed by the heat treatment at the aforesaid temperature, as illustrated
in Figure 4.
[0026] When it is desired to produce the heat-adhesive composite fibers meeting the aforesaid
sheath requirement and further having the aggregatable portions, the touch or feeling
of the resulting nonwoven fabrics is then made by far softer, if the nonstretched
yarns prepared in the following manner are used. That is, when composite spinning
is carried out with three polymers, a chemical agent for reducing the interface affinity
(which may hereinafter be called the affinity-reducing agent) is added to these polymers.
More exactly, the affinity-reducing agent is added to both polypropylene base polymers
for the two core components, or to the sole polyethylene base polymer for the sheath
component, or to both the polymers for the two core components and the sole sheath
component. As such affinity-reducing agents, effective use is made of polysiloxanes
such as polydimethylsiloxane, phenyl-modified polysiloxane, amino-modified polysiloxane,
olefin-modified polysiloxane, hydroxide-modified polysiloxane and epoxy-modified polysiloxane,
and fluorine compounds such as perfluoroalkyl group-containing polymers, perfluoroalkylene
group-containing polymers and modified products of these polymers. The affinity-reducing
agent is added to each pertinent polymer in an amount of 0.05 to 1.0 % by weight based
thereon. Thus, if stretching is applied to nonstretched yarns obtained by composite
spinning with the addition of the affinity-reducing agent to at least either one of
the polymers for the core and sheath components, the heat-adhesive composite fibers
can then be made, while further promoting the formation of the aggregatable portions.
[0027] After the composite nonstretched yarns have been stretched by the single- or multi-stage
stretching, the stretched yarns are dried, as the occasion may be, and may immediately
be used, or may be cut to a given length for the purpose intended.
[0028] In view of efficiency, the treatments of nonstretched yarns such as heating, cooling
and stretching after spinning should preferably be carried out usually with nonstretched
yarn bundles formed into a tow of several ten thousand to several million deniers.
It is also preferred that such a tow is subjected to the given treatments such as
heating, cooling and stretching, while passing the tow continuously therethrough or
moving the tow therethrough at a low speed in an assembled state, without cutting
the tow into short fibers, if possible. The treatments such as heating may be carried
out in a batchwise manner, as already mentioned.
[0029] Prepared is a web consisting of the thus obtained heat-adhesive composite fibers
alone or comprising at least 30 % by weight thereof and other fibers, which is then
heat-treated at a temperature higher than the melting point of the sheath component
and lower than the lower one of the melting points of the core components to produce
the nonwoven fabric according to the present invention.
Effects
[0030] The heat-adhesive composite fibers used for the nonwoven fabrics according to the
present invention are of the side-by-side composite structure that the core 1 of the
side-by-side type composite structure is composed of the polypropylene base polymers
having different Q values and is covered with the sheath 2 of the polyethylene base
polymer having a melting point lower than those of the polymers forming the core components.
Accordingly, the nonwoven fabrics obtained by heat-treating webs containing such heat-adhesive
composite fibers are made sufficiently bulky and extremely stabilized. The reasons
are that although the heat-adhesive composite fibers forming the nonwoven fabrics
are of the sheath-core structure which is generally recognized to show reduced or
limited development of crimps, the crimps revealed prior to the heat treatment and
the crimps developed by the heat treatment are sufficiently large and take on a moderate
three-dimensional shape due to the core being of the side-by-side structure, whereby
the nonwoven fabrics are made sufficiently bulky, and that since the composite fibers
are of the sheath-core structure in the whole section, the sheath 2 assures sufficient
heat adhesiveness and the structure of the nonwoven fabrics is extremely stabilized
by the inter-fiber bonds. In addition, when many aggregatable portions formed on and
at least latently releasable from the sheath due to a reduction in the interfacial
affinity of the sheath 2 and the core 1 are molten and solidified by the heat treatment
to give a number of nodular aggregates 3 consisting of the sheath component, improved
softness is afforded to the touch or feeling of the nonwoven fabrics. The reasons
are considered to be that the area of contact of the fiber surfaces is reduced to
a remarkable degree, since the nodular aggregates 3 come into point contact with the
surfaces of the adjacent fibers.
[0031] Accordingly, the nonwoven fabrics according to the present invention are markedly
improved in terms of the bulkiness and touch or feeling which were problems in the
prior art.
Examples and Comparative Examples
[0032] In what follows, the present invention will be explained in further detail with reference
to the examples and comparative examples.
I. Nonwoven fabrics comprising the composite fibers having no aggregate
Examples 1 to 12 & Comparative Examples 1 to 5
(A) Preparation of heat-adhesive composite fibers
[0033] Eight polypropylenes a, b, c, d, e, f, g and h and two polyethylene base polymers
i and j set forth in Table 1 were used in the combinations set forth in Table 2. The
composite fibers of the structure, in which the cores of the side-by-side type composite
structure constructed from the core components 1a and 1b of two polypropylenes were
covered with the sheaths formed of one polyethylene base polymer were prepared by
the following composite-spinning, heating and stretching treatments.
[0034] The spinning nozzle used had 120 holes each of 1.0 mm in diameter. The components
1a and 1b forming the core were used in a composite ratio of 1:1, whilst the proportion
of the sheath to the total amount of the core plus sheath was varied in a range of
33.3 to 66.7 % by weight. Referring to the spinning temperature (the polymer temperature
just prior to spinning out), the polypropylenes for both components1a and 1b and the
polyethylene base polymer were spinned at 260°C and 220°C, respectively. In this manner,
composite nonstretched yarns of 11 d/f (deniers per filament) were obtained. The composite
nonstretched yarns were bundled into a tow of about 90,000 deniers, and were stretched.
For stretching, three-stage rolls were used. The single-stage stretching was carried
out by passing the tow through the first and second stretching rolls, whilst the double-stage
stretching was done by passing the tow through the third stretching roll following
the same first-stage stretching as the above-mentioned single stage stretching. Referring
to the stretching temperatures, the first-stage stretching temperature (identical
with the stretching temperature in the case of the single-stage stretching) is defined
as being identical with the temperature of the first stretching roll, whilst the second-stage
stretching temperature is defined as being identical with the temperature of the second
stretching roll. In this manner, the tow was passed through a bath containing 0.2
% of a surface finishing agent at 21°C, and was successively passed through the first
stretching roll of 26°C, the second stretching roll of 80°C, and the third stretching
roll of 28°C for double-stage stretching (Examples 1 to 9, Comparative Examples 1
to 5), or was passed through the second strethcing roll of 70°C after the first stretching
roll of 26°C for single-stage stretching (Examples 10 to 12) without using the third
stretching roll. Afterwards, the products of a temperature higher than room temperature
were cooled down to room temperature. The strength and elongation of the thus obtained
respective heat-adhesive composite fibers was measured, whilst the shape of crimps
thereof was observed.
(B) Preparation of nonwoven fabrics consisting of the respective heat-adhesive composite
fibers alone
[0035] The respective heat-adhesive composite fibers obtained in (A) were passed twice through
a carding machine to make webs, each of 100 g/m². Each web was placed in a hot-air
circulation type dryer of 145°C for 5 minutes to make a nonwoven fabric, which was
in turn cooled at room temperature. The bulkiness of each nonwoven fabric was tested.
[0036] The results were set forth in Table 2.
Examples 13 to 17 & Comparative Examples 6-7
Preparation of nonwoven fabrics from mixed fibers of varied proportions of the heat-adhesive
compsite fibers and other fibers
[0037] The heat-adhesive composite fibers (2.9 d/f) obtained in Example 3 were cut to a
length of 64 mm, and were mixed with rayon of 2d X 51 mm in the proportions specified
in Table 3. Substantially according to the procedures of Examples 1 to 12 (B), prepared
were nonwoven fabrics of about 100 g/m², the bulkiness and touch or feeling of which
were tested and the strength and elongation of which were measured.
[0038] The results are set forth in Table 3. In Example 17, the results of which are also
shown in Table 3, a nonwoven fabric was prepared in the same manner as mentioned above,
except that 100 % of the composite fibers obtained in Example 3 were used in the absence
of any other fiber.
[0039] The procedures of the tests as mentioned above are as follows.
Fiber Strength and Elongation:
Crimp Shape:
[0041] After heating at 145°C for 5 minutes, visual estimation was made of whether the fibers
were three-dimensionally or two-dimensionally crimped.
Bulkiness of Nonwoven Fabric:
[0042] Each nonwoven fabric was cut into 20 cm X 20 cm pieces. Such five pieces were formed
into a stack on which a cardboard sheet was placed, and the thickness of one nonwoven
fabric was calculated from the overall thickness of the stack to find the value in
mm for bulkiness.
Strength and Elongation of Nonwoven Fabric:
[0044] From Table 2, the following are understood with respect to the relationship between
the nonwoven fabrics and the structure of the heat-adhesive composite fibers forming
them. More exactly, the comparison of Examples 1 to 12 with Comparative Examples 1
to 4 indicates that in the case where the two core components forming a part of the
heat-adhesive composite fiber have their Q values coming under the range defined by
the present invention, the development of three-dimensional crimps are so considerably
noticeable that the bulkiness of the obtained nonwoven fabrics are very excellent,
if other requirements satisfy the present invention. The comparison of Examples 6
to 12 with Comparative Example 5 also indicates that the nonwoven fabrics obtained
by the method of the present invention are excellent in all the properties including
bulkiness and the development of three-dimensional crimps; however, when use is made
of the composite fibers obtained under the condition that the proportion of the sheath
component departs from the presently defined range, the resulting nonwoven fabric
are poor in the aforesaid properties irrespective of whether the starting polymers
are identical with or different from those used in the composite fiber constituting
the nonwoven fabrics obtained by the method of the present invention.
[0045] From the comparison of Comparative Examples 6 and 7 with Examples 13 to 17 in Table
3, it is also noted that when the heat-adhesive composite fibers used in the present
invention are employed in an amount of at least 30 % by weight in the form of fibers
mixed with other fibers such as rayon, it is then possible to obtain the nonwoven
fabrics excelling in bulkiness, touch or feeling and strength.
(II) Nonwoven fabrics comprising the composite fibers having aggregates
Examples 18-26 & Comparative Examples 8-19
(A) Preparation of heat-adhesive composite fibers
[0046] Same polymers as those used in Examples 1-12(A) were used except that in Example
20 polymer i (high-density polyethylene) was used for the sheath component after being
mixed with 0.10% by weight of dimethylpolysiloxane, and were processed in a similar
manner to obtain the nonstretched yarns of composite fibers comprising various combinations
set forth in Table 4. The composite nonstretched yarns were bundled into a tow of
about 90,000 deniers, which was first heat-treated by passing it under no tension
through a dry heat chamber of 105°C for 30 seconds. (However, any heat treatment was
not applied in Comparative Examples 8-10, 17 and 18). Thereafter, the tow was allowed
to stand in a tow can to completely cool it down to room temperature (22°C). Then,
the tow was passed through a bath of 21°C containing 0.2 % of a surface finishing
agent, and was subjected to the first-stage stretching between a pair of cold stretching
rolls of 26°C (but of 60°C in Comparative Example 14 and of 90°C in Comparative Examples
16 and 17 at a draw ratio of 1.6). This tow was transferred successively to the subsequent
second-stage stretching porcess, in which it was stretched, without letting it loose,
between a pair of stretching rolls heated at 90°C (but at different temperatures in
Comparative Examples 12 to 14) at the draw ratios corresponding to various per cents
of various maximum draw ratios in the second-stage stretching, as specified in Table
4, and was thereafter cooled down to room temperature. The strength and elongation
of each of the thus obtained heat-adhesive composite fibers were measured, whilss
the shape of crimps was examined.
(B) Preparation of nonwoven fabrics consisting of the respective heat-adhesive composite
fibers alone
[0047] The respetive heat-adhesive composite fibers obtained in (A) were used in a manner
similar to that applied in Examples 1-12(B) of the aforesaid (I) to obtain various
nonwoven fabrics. Formation of the aggregates, bulkiness and touch or feeling of these
nonwoven fabrics were tested.
[0048] It is to be noted that the reference nonwoven fabric for the estimation of touch
or feeling was obtained from 100 % of the composite fibers of Comparative Example
17 wherein the nonstretched yarn was heat-treated and stretched substantially according
to the prior art.
[0049] The results were set forth in Table 4.
Examples 27 to 31 & Comparative Examples 20 to 21
Preparation of nonwoven fabrics comprising mixed fibers of varied proportions of heat-adhesive
composite fibers and other fibers
[0050] The heat-adhesive composite fibers (2.7 d/f) obtained in Example 21 were cut to a
length of 64 mm, and were mixed with rayon of 2 d X 51 mm in the proportions specified
in Table 5. Then, nonwoven fabrics having a weight of about 100 g/m² were obtained
by a manner similar to that applied in Example 12(B), and were tested in terms of
its bulkiness and touch or feeling, while measured in terms of its strength and elongation.
It is to be noted that the reference nonwoven fabric for the estimation of touch or
feeling was obtained by the same manner as above from 30 % by weight of the composite
fibers obtained in Example 17 and 70 % by weight of rayon.
[0051] The results are set forth in Table 5. In Example 31, a nonwoven fabric was obtained
from 100 % by weight of the heat-adhesive composite fibers obtained in Example 21
in a similar manner to that applied in Example 17.
[0052] Reference is here made to the testing procedures, unexplained in the foregoing.
Formation of Aggregates
[0053] The respective heat-adhesive composite fibers prior to nonwoven fabric making were
heated at 145°C for 5 minutes, and 100 pieces of the fibers of about 3 to 12 cm in
length are subjected to observation under an optical microscope. The evaluation is
made according to the classification given below for the average number of the nodular
aggregates per actual fiber length of 1 cm, which have a maximum diameter two times
or more larger than the minimum diameter of the thinner portion adjacent thereto.
1 .......... more than 0.30
2 .......... 0.10 to 0.29
3 .......... 0.01 to 0.09
4 .......... less than 0.01
[0054] This heating condition is identical with that for nonwoven fabric making. The formation
of the aggregates of such fibers was substantially identical with that of the fibers
processed into a nonwoven fabric, and working of the evaluation is very difficult
after nonwoven fabric making.
Touch or Feeling of Nonwoven Fabric
[0055] The touch or feeling of the nonwoven fabrics was examined by a five-man panel test,
while comparing with that of the reference nonwoven fabric. Estimation was made by
majority in terms of the following numerals.
1: Softness was very good
2: Softness was considerably good
3: Softness was substantially identical
4: Softness was poor
[0056] The aforesaid reference nonwoven fabric for the estimation of touch or feeling was
obtained from the composite fibers of Comparative Example 17 wherein the nonstretched
yarn was stretched substantially according to the prior art.
[0057] The results are shown in Table 4.
[0058] From Table 4, the following are understood with regard to the relationship between
the nonwoven fabrics and the structure of the hea-adhesive composite fibers forming
them. More exactly, the comparison of Examples 18 to 27 with Comparative Examples
8 to 11 indicates that when the two core components have their Q values within the
range defined by the present invention, the development of three-dimensional crimps
are so considerably noticeable that the obtained nonwoven fabrics excel in bulkiness,
as in Examples 1 to 12, if other requirements satisfy the present invention. From
the comparison of Examples 24 to 26 with Comparative Examples 12 to 18, it is further
noted that the nonwoven fabrics obtained by the method of the present invention are
excellent in all the properties including bulkiness and the development of three-dimensional
crimps; however, the nonwoven fabrics obtained using the composite fibers prepared
under conditions in which the proportion of the sheath, the stretching temperature,
the draw ratio, etc. departed from the presently defined range are poor in the aforesaid
properties, even though the same starting polymer is used. From the comparison of
Examples 25 and 26 with Example 24 in particular, it is still further noted that the
nonwoven fabrics obtained by the method of the present invention using the composite
fibers prepared by applying the heat treatment prior to stretching of the composite
nonstretched yarns are more excellent in the formation of the aggregates and hence
touch or feeling than those obtained using the composite fibers obtained without any
heat treatment. Accordingly, it is found that the heat treatment of the compoiste
nonstretched yarn takes great part in the formation of the aggregates. From Examples
18 and 19, it is also noted that a much larger number of the aggregates are formed
in the nonwoven fabric in which the affinity-reducing agent such as polysiloxane is
added to the raw polymer than in the nonwoven fabric in which such a agent is not
added to the raw polymer.
[0059] From the comparison of Comparative Examples 19 to 20 with Examples 28 to 32 in Table
5, it is still further noted that when at least 30 % by weight of the heat-adhesive
composite fibers used in the present invention are used in the form of fibers mixed
with other fibers such as rayon, it is then possible to obtain the nonwoven fabrics
excelling in bulkiness, touch or feeling and strength.
A nonwoven fabric which contains at least 30 % by weight of heat-adhesive composite
fibers comprising a core portion and a sheath portion, said core portion being of
the side-by-side type composite structure comprising two core components of different
polypropylene base polymers in a composite ratio of 1:2 to 2:1, one of said core components
having a Q value, expressed in terms of the weight-average molecular weight/the number-average
molecular weight, equal to or higher than 6 and the other having a Q value equal to
or lower than 5, said sheath portion meeting at least the requirement that it should
compirse a sheath component of a polyethylene base polymer having a melting point
lower by at least 20 °C than the lower one of the melting points of said two core
components, and said sheath portion covering completely said core portion in a proportion
of 25 to 55 % by weight based on the total weight of it and said core portion, and
which is stabilized by the inter-fiber bonds of the sheath portion of said heat-adhesive
composite fibers.
2. A nonwoven fabric as defined in Claim 1, in which said sheath portion of said heat-adhesive
composite fibers satisfies said requirement alone.
3. A nonwoven fabric as defined in Claim 1, in which said sheath portion of said heat-adhesive
composite fibers satisfies said requirement, and include thereon a number of nodular
aggregates formed of said sheath component.
4. A nonwoven fabric as defined in any of Claims 1 to 3, in which at least one polypropylene
base polymer of said two core components of said heat-adhesive composite fibers is
polypropylene.
5. A nonwoven fabric as defined in any on Claims 1 to 3, in which at least one polypropylene
base polymer of said two core components of said heat-adhesive composite fibers is
a copolymer of propylene with a small amount of an alpha-olefin other than propylene.
6. A nonwoven fabric as defined in any one of Claims 1 to 5, in which the polyethylene
base polymer of said sheath component of said heat-adhesive fibers is polyethylene.
7. A nonwoven fabric as defined in any one of Claims 1 to 5, in which the polyetylene
base polymer of said sheath component of said heat-adhesive composie fibers is a copolymer
of ethylene with vinyl acetate having an ethylene content of 98 to 60 % by weight.
8. A method for producing nonwoven fabrics which comprises the steps of:
separately subjecting to composite-spinning two polypropylene base polymers for two
core components and a polyethylene base polymer for a sheath component, which has
a melting point lower by at least 20 °C than the lower one of the melting points of
said two polypropylene base polymers, thereby obtaining composite nonstretched yarns
of the structure that a core portion of the side-by-side type composite structure
consisting of two core components in a composite ratio of 1:2 to 2:1, one of said
core components having a Q value, expressed in terms of the weight-average molecular
weight/the number-average molecular weight, equal to or higher than 6 and the other
having a Q value equal to or lower than 5, is completely covered with a sheath portion
comprising said sheath component in a weight proportion of 25 to 55 % by weight based
on the total weight of it and said core portion,
stretching said composite nonstretched yarn by an one- or more-stage stretching process
to prepare heat-adhesive composite fibers,
preparing a web containing at least 30 % by weight of said heat-adhesive composite
fibers, and
heat-treating said web at a temperature higher than the melting point of said sheath
component and lower than the lower one of the melting points of said core components.
9. A method for producing nonwoven fabrics as defined in Claim 8, in which, at the
stretching step, said composite nonstretched yarns are stretched at a temperature
of normal temperature to 130°C in an overall draw ratio of 1.3 to 9.
10. A method for producing nonwoven fabrics as defined in Claim 8, in which, prior
to the stretching of said composite nonstretched yarns, said composite nonstretched
yarns are heated under no tension at a temperature of 80 °C to below the melting point
of said sheath component for 10 seconds or longer and cooled down to normal temperature,
and is then subjected to the first-stage stretching at normal temperature and a draw
ratio of 1.3 to 2, and without letting the yarns loose they are subsequently subjected
to the second-stage stretching at a temperature of 80 °C to below the melting point
of said sheath component and a draw ratio of at least 90 % of the maximum draw ratio
of the second-stage stretching, then make the heat-adhesive composite fibers have
many aggregatable portions on their sheath portions.
11. A method for producing nonwoven fabrics as defined in Claim 10, in which, at the
composite spinning step, 0.05 to 1.0 % by weight of at least one selected from the
group consisting of polysilosxanes and fluorine compounds is added to at least one
of polypropylene base polymers for said core components and the polyethylene base
polymer for said sheath component, which are then subjected to the composite-spinning
to obtain composite nonstretched yarns.