U.S. patent application number 11/571977 was filed with the patent office on 2008-03-06 for thermoplastic, thermally bondable polyolefin fibre for production of nonwovens as well as a nonwovens obtained by thermal bonding.
This patent application is currently assigned to SAURER GMBH & CO. KG. Invention is credited to Giampaolo Guerani, Felice Polato.
Application Number | 20080057308 11/571977 |
Document ID | / |
Family ID | 35169980 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080057308 |
Kind Code |
A1 |
Polato; Felice ; et
al. |
March 6, 2008 |
Thermoplastic, Thermally Bondable Polyolefin Fibre for Production
of Nonwovens as Well as a Nonwovens Obtained by Thermal Bonding
Abstract
The invention relates to a thermoplastic, thermally bondable
polyolefin fibre for production of nonwovens as well as a nonwovens
obtained by thermal bonding of such polyolefin fibres. The
production of nonwovens for applications in hygienic end uses have
thermal bonding and softness characteristics dependent on the
fibres. For improvement the fibre of the invention shows a whole
plastic deformability under calendaring process in the
thermobonding dot and a low surface degradation during spinning.
Therefore the thermobonding dots of a nonwovens are characterized
by the whole close packing of the fibres. The thermal bonding
behavior of the fibre will be reach with a spinning process with
spinning head temperature set up suitable in order to obtain the
specified thermal degradation.
Inventors: |
Polato; Felice; (Ferrara,
IT) ; Guerani; Giampaolo; (Viterbo, IT) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
SAURER GMBH & CO. KG
Landgrafenstrasse 45
Monchengladbach
DE
41069
|
Family ID: |
35169980 |
Appl. No.: |
11/571977 |
Filed: |
July 5, 2005 |
PCT Filed: |
July 5, 2005 |
PCT NO: |
PCT/IB05/02010 |
371 Date: |
November 6, 2007 |
Current U.S.
Class: |
428/374 ;
264/103 |
Current CPC
Class: |
D01F 6/06 20130101; D04H
1/544 20130101; Y10T 428/2931 20150115; D01F 6/04 20130101; D04H
3/14 20130101; D01F 6/46 20130101; D04H 1/56 20130101; D04H 1/541
20130101; D04H 1/4291 20130101; D04H 1/54 20130101 |
Class at
Publication: |
428/374 ;
264/103 |
International
Class: |
D04H 3/14 20060101
D04H003/14; D01D 5/08 20060101 D01D005/08; D01F 6/04 20060101
D01F006/04; D01F 6/06 20060101 D01F006/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2004 |
IT |
FE2004A000012 |
Claims
1. Thermoplastic, thermally bondable polyolefin fibre, suitable for
production of nonwoven, characterized by whole plastic
deformability under calendaring process in the thermo-bonding dot
and by a low surface degradation during spinning.
2. Fibre of claim 1 characterized by the ratio of the fibre melt
flow rate (MFR fibre) to the resin melt flow rate (MFR resin) with
its value DI (DI=(MFR fibre)/(MFR resin)) in the range between 1.5
and 3.0.
3. Fibre of claim 2 characterized by a value DI of the ration (MFR
fibre)/(MFR resin) in the range between 2.0 and 2.5.
4. Fibre of claim 2 characterized by made of a first component PP
homopolymer and, at least a second component, blend compatible with
the first and composed of PP co-polymer with at least one
.alpha.-olefin co-monomer.
5. Fibre of claim 4 characterized by a weight proportion of the
blend in the range between 0% and 90% of PP homopolymer and between
100% and 10% of PP-.alpha.-olefin co-polymer.
6. Fibre of claim 4 characterized by a raw material containing
primary antioxidant in the range between 150 and 600 ppm.
7. Spinning process of polyolefin fibres made according to claim 1
characterized by a spinning head temperature set up suitable in
order to obtain a thermal degradation of fibres, so that the ratio
of fibre melt flow rate and resin melt flow rate (MFR fibre)/(MFR
resin) has a value DI in the range between 1.5 and 3.0.
8. Spinning process of claim 7 characterized by a quenching flow
temperature or a quenching flow speed or a quenching distance or a
combination of quenching variables suitable to obtain a very low
skin degradation at the fibre.
9. Nonwoven obtained by thermal bonding of polyolefin fibres made
according to claim 1 characterized by the whole close packing of
the fibres in the thermally bonded dot after calendering under
compression due of the whole plastic deformability of the
fibres.
10. Nonwoven of claim 9 characterized by a thermal degradation of
the fibres by spinning in the higher range of the value DI between
2.5 and 3.0, wherein the best softness is obtained by reducing to
the minimum allowed calender temperature.
11. Nonwoven of claim 9 characterized by a thermal degradation of
the fibres by spinning in the lower range of the value DI between
1.5 and 2.0, wherein the best softness is obtained by increasing
the calender temperature until the required tenacity.
12. Nonwoven of claim 9 characterized by a thermal degradation of
the fibres by spinning in the higher range of the value DI between
2.5 and 3.0, wherein the highest tenacity and the best softness are
obtained by increasing the calender temperature.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a thermoplastic, thermally bondable
polyolefin fibre for production of nonwovens as well as a nonwovens
obtained by thermal bonding of such polyolefin fibres.
BACKGROUND OF THE INVENTION
[0002] Polyolefin fibres and, more specifically, polypropylene
fibre as themselves or in blend with other fibres like wool,
cotton, polyester, are widely used for the production of several
articles with different morphology.
[0003] This usage has been remarkably improved by several
characteristics of these fibres like high chemical inertia and
absence of polar groups, no toxicity and cytoxicity, low specific
weight, low thermal conductivity and high insulating power, high
abrasion resistance, high mildews and bacteria resistance, high
colour fastness and, last but not least, easy processability and
low cost.
[0004] Textile (underwear, sportswear), floor coverings (carpets),
industrial and hygiene are some of the most important applications
which have been developed on the basis of one or more of the above
mentioned fibre behaviours.
[0005] It is well known that polyolefin fibres and, more
specifically, polypropylene fibre, are produced by the melt
spinning technology which consists in melting the polymer at high
temperature in one extruder. The melted polymer is afterwards
forced to pass through a spinneret maintained at controlled high
temperature.
[0006] In order to obtain some important additional behaviours on
the fibre, specific chemicals are added to the polymer before or
during the spinning step:
[0007] stabilizers (process stabilizers, anti-oxidant etc.)
[0008] coloured pigments
[0009] optical brighteners to improve the whiteness
[0010] matting agents to modify the transparency
[0011] After the exit from the spinneret, the hot spun filaments
are quenched by air and undergo the subsequent processing steps of
drawing, crimping, drying to reach the final cohesion and
mechanical characteristics required by the following fibre
processing.
[0012] The fibre obtained by the above mentioned steps is
afterwards cut and baled.
[0013] Specially tailored spin finish formulations are applied
during some steps of the production process to give to the fibre
the antistatic, lubricant and cohesion characteristics necessary
for the processability. Furthermore the above spin finish
formulations must impart to the fibre the additional hydrophilic or
hydrophobic behaviours required by the end use.
[0014] Several different morphologies and structural compositions
are shown by prior art polyolefin fibres for thermal bonding:
[0015] Bicomponent fibres like sheath--core o side by side
disclosed for instance in U.S. Pat. No. 4,473,677, U.S. Pat. No.
5,985,193, WO9955942 or U.S. Pat. No. 5,460,884. [0016] These
fibres are obtained by using two extruders separately feeding two
different polymers (i. e. : polypropylene/polyethylene or
polypropylene/polyolefin copolymer) to specially designed
spinnerets through separate gear pumps. [0017] Structural
bicomponent or "bicostituent" constituted by blends of polymers
directly obtained inside the spinning extruder as disclosed for
instance in U.S. Pat. No. 5,985,193, WO9955942 or U.S. Pat. No.
5,460,884. [0018] "Natural" bicomponent showing a "skin-core"
morphology and obtained from single polymers or from blends of
polymers by the use of special conditions of the spinning and
quenching steps of the production process which lead to the
formation of a degraded skin on the fibre as disclosed for instance
in U.S. Pat. No. 5,281,378, U.S. Pat. No. 5,318,735, U.S. Pat. No.
5,431,994, U.S. Pat. No. 5,705,119, U.S. Pat. No. 5,882,562, U.S.
Pat. No. 5,985,193 or U.S. Pat. No. 6,116,883.
[0019] The above mentioned patents and patent applications assert
that the achievement of the bonding behaviour of the skin-core
fibres is due to the formation of a degraded skin and claim that
such a skin is always obtained by the use of suitable processing
conditions.
[0020] On the contrary it will prove that, in absence of a
specifically tailored polymer stabilization, the thermal bonding
behaviour of these fibres may be poor because of an excessive or,
on the contrary, limited degradation of the polymer. The main
constraints of prior art are for long spinning process the
nonwovens limited softness and for short spinning process the
nonwovens low tenacity.
SUMMARY OF THE INVENTION
[0021] It is therefore an object of the invention to create a
thermal bonding fibre for high nonwovens tenacity, which allows a
wider process operating window and better control of the quality
consistency of the fibre and the nonwovens.
[0022] An other object of the present invention is to solve some
main constraints in the thermobonding process versatility and
nonwovens quality when standard homo-PP fibres are used, from both
long spinning or short spinning process.
[0023] In accordance with the invention, this object is
accomplished by the thermoplastic, thermally bondable polyolefin
fibre with the features of claim 1, the spinning process of such a
polyolefin fibre with feature of claim 7 and the nonwovens obtained
by thermal bonding of such polyolefin fibres with feature of claim
9.
[0024] The present invention wants to combine the welding effect
due to the enhanced plastic behaviour of the fibre together with
the effect of the minimal useful thickness of the welding skin. The
invented fibre shows a very low surface degradation during spinning
and a whole plastic deformabilility after calandering under
pressure. Therefore the thermobonding dots in a thermobonding
nonwovens are like a thin and homogeneous polymer foil. All the
fibres are loosing their single identity and are welded completely
together to the thermobonding dot. In the area of forced contact
under the calendar compression the fibres show a complete melting
and molecular interpenetration of the surfaces.
[0025] For production of such a fibre a spinning process is
proposed wherein the spinning head temperature set up suitable in
order to obtain the specified thermal degradation of the fibre.
[0026] The invented nonwovens obtained by thermal bonding of said
fibres show a higher tenacity in comparison to prior art due of
whole close packing of the fibers in the thermally bonded dot.
[0027] By such a technique the following advantages of the
invention are realized: [0028] the operating window of the spinning
process becomes remarkably wider thus improving the quality
evenness of the fibre [0029] a tailor made optimization of the non
woven characteristics becomes feasible by favouring the tenacity or
the softness or intermediate combinations
[0030] To realize the above targets it's necessary to utilize
polymeric systems in which it might be possible to enhance the
plastic deformability phenomenon during the calendering stage.
[0031] In the second instance the additive formulation of the
polymer must allow the formation of the minimal useful thickness of
skin.
[0032] With reference to the analytical problems concerning the
determination of the depth of the skin really usable in the welding
process, the field of the values of the Degradation Index (DI) lays
between 1.50 and 3.0 depending on the characteristics which are
selected as targets on the nonwovens. The degradation index DI is
the value of the ration between the fibre melt flow rate and the
resin melt flow rate as will be described in detail later on.
Especially good effects during thermobonding good be reach with a
degradation index DI in the range of 2.0 and 2.5.
[0033] As previously mentioned, in order to achieve the plastic
deformability, it's necessary to add structural disorder to the
crystalline phase to allow an easy trigger of the molecular sliding
under stress.
[0034] Among the possible polymeric system combinations, the
followings may be mentioned as no limiting example:
[0035] PP+PP/PE+PP/PB
[0036] PP+PP/PB
[0037] PP+PP/PB/PE
[0038] and, also, all the other combinations containing an high
crystallinity homo or copolymer as base, one or more components
constituted by homopolymer PP or copolymer PP/PE and an additional
component constituted by copolymers of PP or PE with
.alpha.-olefins characterized by a structure with limited
crystallinity. The weight proportion of the blend could be in the
range between 0% and 90% homopolymer and between 100% and 10% of
PP-.varies.-olefin copolymer.
[0039] All the above components must show a total miscibility among
them in order to assure a good processability during the spinning
stage.
[0040] An important characteristic of the above copolymers is the
melting temperature of their polypropylene crystalline phase which,
generally, is inversely proportional to the content of comonomer
("Polypropylene Handbook", edited by Edward P. Moore, Jr., 1996.
Chapt. 6.3.2, FIG. 6.6).
[0041] For better clarity, furthermore, it can be outlined that a
lower melting temperature of crystalline PP corresponds to a lower
binding energy of the crystallite itself and this fits perfectly
with the previously mentioned concept of easier plastic
deformability.
[0042] In fact, the invention is concerned with the spinning
process of PP fibre by doling out the skin degradation and by using
the plastic behaviour of some blends of polymers. The above target
is achieved by using specific set up solutions for:
[0043] additive formula
[0044] raw material blend
[0045] process conditions
[0046] More in particular, the dosing of the fibre skin degradation
is controlled by the additive formula and by suitable process
conditions. A raw material containing primary antioxidant in the
range between 150 ppm and 600 ppm leads to good degradation
control.
[0047] By using specific raw material blend, the thermoplastic
behaviour of the fibre in the calendering plant is optimized in
order to achieve the top of tenacity by also controlling
temperature and pressure of the rolls.
[0048] Process conditions in the fiber production and in the
following calendering.thermalbonding step are driven according to
the raw material formula and characteristics. In such way, the
tenacity-softness can be taylored according to the final
applicative need.
[0049] Taking into account the poor thermal conductivity of
polyolefins, together with the very short residence time of the
fibre into the calendering treatment, the thermoplastic behaviour
of semi-crystalline polyolefins can assume the dominant role during
thermal bonding step in calendering machine.
BRIEF DESCRIPTION OF THE TABLES AND FIGURES
[0050] FIG. 1: Thermal bonding model for skin-core fibres prior
art
[0051] FIG. 2: Nonwovens bonding dot after calendaring prior art
fibres
[0052] FIG. 3: Thermal bonding model for fibres according
invention
[0053] FIG. 4: Nonwovens bonding dot after calendaring fibres
according invention
[0054] Tab. 1: Influence of the spinning temperature on fibre
degradation (MFR) and nonwovens
[0055] Tab. 2: Influence of the stabilization formula on PP thermal
stability and thermal oxidative stability of polypropylene
[0056] Tab. 3: Influence of the polymer blend composition and
degradations index (DI) on fibre thermal bondability
DETAILED DESCRIPTION OF THE INVENTION
[0057] The plastic behaviour of the polymer is the capability to
withstand large deformations (until 600-700% in some cases) and to
retain the deformed shape after removing the deforming stress. In
such deformation process, two different steps are recognized.
[0058] In the first step, below 1%, the deformation is elastic and
reversible with the applied stress. During the elastic deformation,
some temperature decrease can be observed in the body.
[0059] In the second step, over the elastic limit, the deformation
become plastic or irreversible and the relative flow of material in
the body is observed. The molecular friction due to the above flow
can produce increase of the body temperature if the deformation
process is fast enough in reference to the heat dispersion effect
due to the thermal conductivity of the material.
[0060] In the calendering process of the nonwoven web, the material
plastic behaviour can play active role in the thermal bonding
result if a wide plastic deformation of the fibre section is
carried out in the suitable way. To this purpose, the following
main actions are required: [0061] in the spinning plant, use of the
suitable polyolefin raw material (containing molecular disorder in
the crystalline phase) [0062] in the calender plant, increase
pressure and, if required, decrease temperature of the rolls.
[0063] Concerning the molecular disorder, it has to be considered
that such areas can be the starting point for the molecular plastic
flow under external stress. In fact, they are areas where the
bonding energy of the crystalline building is lower.
[0064] PP homopolymer can be disordered in different ways when
crystallinity is high. One of the more straight ways is by blending
to PP homopolymer some quantity of compatible polyolefin copolymer
between PP and (.alpha.-olefin) co-monomer, where the
(.alpha.-olefin) co-monomer is below 10%. The effectiveness of the
above solution is explained by the disorder effect of the
(.alpha.-olefin) chain segment during crystallization of the PP
chain.
[0065] It follows from the above description that the thermal
bonding mechanism of polyolefin fibres is the result of the
presence of the degraded skin and of the plastic behaviour of the
fibre section under mechanical stress.
[0066] The fibre bonding mechanisms like prior art is using
skin-core PP-fibres. Such skin-core PP fibre is widely used in
thermal bonding as known. The main feature of the above fibre is
the difference in melting point between skin and core. More in
particular, being the skin degraded in molecular weight, its
melting point is lower in comparison to the high molecular weight
core section. In more detail, during the calendering action, when
the skin layer is quite in molten state, the core of the fibre is
still solid. Following the above considerations, the thermal
bonding model with skin fibres according prior art can be outlined
as in FIG. 1, where it is shown: [0067] under the hot roll
compression, the single fibre aims to keep its original circular
section [0068] the roll compression is putting close together all
the fibre and the skin layer is molten firstly, so flowing into the
residual free volume between the fibre and like a glue. [0069]
after very short time (10 milliseconds about) the compression
effect is ended and the fibre assembly aims to re-arrange its
position under the residual elastic effect, until the
solidification of the molten skin layer. During such re-arrangement
the "glue" is stretched and tends to form bridges of membrane
and/or filaments between neighbouring fibre, as shown in FIG. 2. Of
course, quantity and size of the bridges are depending from many
process variables (thickness and quality of the skin, temperature,
pressure, speed, etc). [0070] as for confirmation of the individual
core keeping by the single fibre, in spite of the compression
stress applied on the dot during calendering, it can be seen (FIG.
2) that the single fibre is visible also in the fibre intersection
zone, in spite of the compressive stress applied.
[0071] With the invented fibre, object of the present invention,
apart the possible presence of degraded skin, the thermal bonding
model is outlined as in FIG. 3, where it is shown: [0072] the
single fibre, under the roll compression in calender, is loosing
quite completely the original circular section and id deformed in
order to allow the fibre close-packing. In such volume arrangement,
all the fibre are loosing also their single identity and the welded
dot becomes like a thin and homogeneous polymer foil. [0073] as
shown in FIG. 3, fibres are closely packed and, even if degraded
skin is present, number and size of "glue" bridges between
neighbouring fibres is very low. [0074] it is crucial to note that,
as first result of the high plastic deformability of the fibre
section, the strong thermal bonding effect is obtained with the
minimum thickness of degraded skin. [0075] as shown in FIG. 4, the
welding dot is well homogeneous. With the naked eye, the welding
dot appears to be transparent due to the optical homogeneity in the
polymer bulk.
[0076] The presence of crystalline disorder in polymers can be
observed by Differential Scanning Calorimetry (DSC) analysis, where
it is measured the enthalpy of fusion and the melting
temperature.
[0077] In this analysis, a blend made by PP homopolymer and PP-PE
random copolymer shows its melting temperature in between the two
components and more close to PP, not just in the middle according
to a linear low of just blending.
[0078] This effect is well explained by assuming that, in the
solidification process of the blend, the two components are
included by a unique crystalline phase having a unique melting
process. The lower value of the melting temperature of the blend in
comparison to the pure homopolymer means a lower binding energy of
the crystalline phase, according to the known theories of the
polymer physics. Of course, the inclusion of the copolymer into the
homopolymer crystalline building, because of the different
molecular stereo-regularity, causes the disorder effect during the
blend solidification.
[0079] In a different technique, X-ray diffraction (XRD), the
crystalline disorder of polymers can be observed in terms of:
[0080] variation of crystal planes distance
[0081] crystal plane completeness
[0082] On molecular scale, crystalline disorder means
"displacement/insertion of atoms/chain segment in the crystalline
lamella of PP. As a matter of fact, for example, the PP-PE random
copolymer with low content of PE can be considered as imperfect PP
where the chain segments of PE are forced to stay inside the PP
crystalline building during solidification, so creating disorder
and reducing number and energy of the molecular bonds in the solid.
This is the reason why also pure polyolefin random co-polymers are
suitable resins for the plastic thermal bonding effect. On the
other hand, polyolefin blend can be more suitable than pure
copolymers for the flexibility of the fibre bulk
characteristics.
[0083] The production of calendered nonwovens from fibre staple is
carried out several days after the fibre spinning. It is a good
cost saving tool to test the staple thermal bondability just after
the spinning, before packaging.
[0084] To this purpose, it has been developed the lab test W.I.
(Weldability Index, by F. Polato, private com, Nov. 30, 1998)
[0085] In the method, few grams of staple are carded. The small web
is submitted to compression load at high temperature for a short
time. The tenacity of the thermally bonded web is measured.
[0086] By using controlled conditions for all the steps, the test
results are closely related with the tenacity of the industrial
nonwovens.
[0087] Different spinning technologies can be used for industrial
production of polyolefin staple fibres. Today, the most widely used
are usually known as "long spinning" and "short spinning".
[0088] The two technologies are different for both technical and
economical factors. The usual trend for plant set up is looking for
the skin-core fibre with the following characteristics: [0089] the
skin is the external layer of polymer degraded by thermal-oxidation
(chain scission) where: [0090] the average MW is very much lower
than in the starting resin [0091] the MFR is much higher than in
the core of the fibre [0092] the melting temperature is clearly
lower than in the starting resin [0093] the core of the fibre is
the internal remaining section, and is quite unchanged in
comparison to the starting polymer.
[0094] In fact, after the hole spinneret, the fibre at high
temperature is immersed into air and the oxidation process starts
immediately from the fibre surface and penetrate the fibre in
radial direction. The oxidative degradation of PP, as known, is a
chain scission process in which the polymer molecular weight is
reduced.
[0095] The target is to achieve the lower melting temperature and
the suitable thickness of the skin, in order to obtain the highest
tenacity in calender plant with the minor roll temperature.
[0096] As matter of plant experience, the degraded skin having the
right quality for the high tenacity of the thermally bonded
nonwovens is obtained only in a narrow range of spinning
temperature (see Tab. 1). The most important process conditions for
quality and thickness of the skin are: [0097] polymer temperature
out of the hole spinneret (high temperature inside the spinning
line are ineffective [0098] air quenching flow, in terms of thermal
capacity flow, for the freezing effect of the thermal-oxidative
degradation by decreasing the fiber temperature.
[0099] The "thickness" of the degraded skin is the result of
interaction between the temperature of the fibre leaving the hole
spinneret and the time at high temperature available to oxygen for
its central diffusion in the fibre itself.
[0100] In other words, the thermal-oxidative process for the
formation of the skin is controlled by two minimum threshold:
temperature and time
[0101] Concerning time, the two technologies above mentioned allow
similar residence time of the fibre at high temperature (10
milliseconds is the time magnitude order). On the other hand, it is
well known that the short spinning technology don't allow the skin
degradation of PP in easy way. For this, it must be taken into
account that short spinning technology must use high speed
quenching flow and very close to the spinneret holes. The final
effect is the lower temperature of the fibre in output of the
spinneret and the degradation kinetics lower speed.
[0102] In addition, commercial grades of PP for fibres are
containing heavier additive formulas, optimized in long spinning
technology, where the thermal-oxidation reaction is easier.
[0103] Further on, it must be related the thermal-degradation
process for the skin with the final characteristics of the
nonwovens.
[0104] In Tab. 1 it is shown the "fibre MFR" and "nonwovens
tenacity TBI" versus the spinning head temperature, all the others
process conditions kept constant.
[0105] Firstly, polymer degradation (MFR) is growing slowly with
the temperature increase, until the "threshold" value of
280.degree. C. Over the threshold, the degradation process is
accelerated more and more. At the same time, the nonwovens tenacity
starts to improve at 280.degree. C., reach the peak value at
290.degree. and after decreases in spite of the increase of
degradation above mentioned. Of course, the relationship is
depending quantitatively from plant type and additive formula.
[0106] From Tab. 1 the standard process dynamics can be explained
as follows.
[0107] Until 280.degree. C. of spinning head temperature, skin
degradation does not take place on the fibre.
[0108] Over this threshold, the degraded skin layer is growing in
thickness with exponential law versus temperature. Of course, the
increase of skin thickness means that degradation is proceeding
versus the middle, so reducing the size of the residual unchanged
core and, at the same time, the tenacity of the fibre. For very
high spinning temperatures, the fiber thermal bondability would be
excellent but, because of the very poor mechanical characteristics
of the degraded fibre, the nonwovens tenacity is worst.
[0109] From all the above points, it can be concluded: [0110] the
skin-core structure can be obtained only over the temperature
threshold [0111] the spinning temperature operating window for the
highest nonwovens tenacity and by using PP homopolymer and standard
spinning technology is narrow (only few degrees)
[0112] Moreover, taking into account the interactions of the
several variables, some compensating effect can be used for plant
set up among: [0113] spinning head temperature [0114] quenching
flow temperature [0115] quenching flow speed [0116] distance
between spinneret surface and upper surface of quenching flow
(=quenching distance)
[0117] In fact, the above variable are inter-dependent for the skin
formation. In particular, for the same additive formula, the set up
of the above variables allows the control over the amount of skin
quantity and quality.
[0118] Other useful comments are: [0119] over its minimum
threshold, the spinning head temperature is dominant for the skin
control [0120] below, the skin is undetectable [0121] far over the
threshold, the nonwovens tenacity is worst [0122] the amount of
antioxidant additives in the polymer recipe is dominant for the
skin degradation. More in particular, for skin degradation in short
spinning lines, the antioxidant level must be low. [0123] optimal
thickness and low melting temperature of the skin are required for
the high tenacity of the thermally bonded nonwovens obtained from
skin-core PP fibre (see model of FIG. 1) [0124] for high tenacity
of the thermally bonded nonwovens obtained from plastic PP fibre,
the skin thickness required is much lower than with skin-core fibre
(see model of FIG. 3)
[0125] For the detection of the skin in PP fibres, some test
methods have been considered: [0126] optical microscope analysis of
the silicon oil ultrasonic extract of the fiber at high temperature
(Takeuchi et al. U.S. Pat. No. 5,705,119; Jan. 6, 1998) [0127] TEM
analysis of the fibre section previously stained by RuO4 (Trent et
al, Ruthenium tetra-oxide staining of polymers for electron
microscopy, Macromolecules, vol 16, Nov. 4, 1983).
[0128] Unfortunately it was found that the two test methods are
unreliable for analytical use because none close relationship was
shown among test results and thermal bondability of the PP.
[0129] On the other hand, it is well accepted that the welding skin
is formed on the fibre surface during spinning and because of
degradation by chain scission.
[0130] Following this concept, it can be shown the close
relationship between nonwoven tenacity (TBI) and the Degradation
Index (DI) of the polymer during spinning.
[0131] Definitions TBI=SQRT(CD*MD))*20/W (1) DI=(MFR fibre)/(MFR
resin) (2) [0132] where: CD=cross direction tenacity of the
non-woven [0133] MD=machine direction tenacity of the non-woven
[0134] W=weight of the non-woven [0135] MFR=polymer fluidity
according to ASTM D-1238-L
[0136] Of course, the above close relationship can be obtained by
keeping constant the calendering process set up and the resin
spinning process, being the spinning temperature variable. In such
configuration, the degradation effect (DI) is the straight effect
of the spinning temperature.
[0137] The following are first references:
[0138] DI=1.0 is the lower limit (theoretical) with lack of any
degradation
[0139] 1.5<DI<3 is for intermediate degradation and partial
skin formation
[0140] 3<DI<4 is the range of typical skin core commercial
fibres
[0141] DI>4 is for excessive degradation, fragile fibre and
worst non-woven tenacity
[0142] With reference to the thermal bonding mechanism (FIGS. 3,4),
if the fibre plastic behaviour in calender is suitable, it is
found: [0143] higher tenacity of the nonwovens for the same DI
value in comparison to skin-core homo PP fibre [0144] high tenacity
of the nonwovens also for low DI values, corresponding to low skin
presence.
[0145] The additive formulation of the polymer is an essential
feature as it controls, by definition, the polymer degradation
mechanism. Such a control becomes particularly effective on the
outer layers of the fibre at the exit of the die when the hot
polymer gets in touch with the oxygen of the atmosphere.
[0146] The additive formulation of the polypropylene fibre for non
wovens in the hygiene applications is generally studied on the
basis of the main degradation mechanisms deriving from:
[0147] a) oxygen at high temperature
[0148] b) high processing temperature in absence of oxygen
[0149] c) long storage time (shelf life)
[0150] The protection to oxygen at high temperature is generally
carried out by primary anti oxidants like sterically hindered
phenols (C.A.S. Nos. 6683-19-8, 27676-62-6, 2082-79-3 and others),
afterwards reported as AO1 or by more recently developed additives
like lactones (C.A.S. No.181314-48-7 and others) afterwards
reported as AO2.
[0151] The protection to the high processing temperature in absence
of oxygen is generally carried out by secondary anti oxidants like
organic phosphites (CAS Nos. 31570-04-4, 119345-01-6 and others) or
organic phosphonites (CAS No. 119345-01-6 and others) in
combination with AO1 or AO2.
[0152] The protection to long storage time (shelf life) is assured
by both AO1 and sterically hindered amines (polymeric HALS; CAS
Nos. 71878-19-8,106990-43-6 and others).
[0153] Among the above mentioned mechanisms, the most important one
is that which controls the thermal oxidative degradation of the
polymer at high temperature.
[0154] More specifically, the thermal oxidative mechanism must be
quantitatively controlled to obtain the required thickness of
degraded skin.
[0155] In other terms, as the degraded and low melting point
polymer has insufficient mechanical characteristics, it is
necessary to dose the thermal oxidative degradation to reach the
minimal useful thickness of degraded skin. An excessive degradation
leads to an increase of the bonding skin but the mechanical
characteristics of the non woven become worse as also the core of
the fibre undergoes degradation (see table 1).
[0156] In order to get the properly dosed thermal oxidative
degradation, according to the present invention, the concentration
of primary anti oxidants must be between 150 ppm (highest
degradation) and 600 ppm (lowest degradation).
[0157] T.O.S.I. (Thermal Oxidation Stability Index, "F. Polato:
comunicazione privata Nov. 30, 1998") represents a very effective
testing method to separately and jointly evaluate the stability of
polypropylene to oxygen at high temperature and to the high
processing temperature in absence of oxygen.
[0158] This method assumes that the MFR, as it is well known, is a
good indicator of the average Mw and it's based on the evaluation
of the molecular degradation of the polymer as a consequence of:
[0159] exposure to a constant temperature and for a defined time in
a closed cell, in absence of oxygen [0160] exposure to a thermal
oxidative action by extruding the polymer at high temperature in
presence of oxygen
[0161] A common instrument for the measurement of MFR is used for
the above trials.
[0162] As it is shown in Table 2, different additive formulations
of the polymer lead to a remarkable difference of the degradation
at high processing temperature in absence of oxygen and of the
thermal oxidative degradation (Formulations 1, 2).
[0163] In the mean time, certain additive formulations may show
very similar levels of thermal oxidative degradation and a
noticeable difference of the stability to the high processing
temperature in absence of oxygen (Formulations 1, 3).
[0164] Polyolefin homopolymers and copolymers like PP and PE are
widely used for the production of thermally bondable fibres for non
wovens in the hygiene applications.
[0165] PE homopolymer, nevertheless, shows some important
limitations as far as price and tenacity of non woven are
concerned, even if its relevant contribution to the softness of the
non woven is well known
[0166] The use in low concentration of other polymers as ethylene
copolymers containing polar monomers like vinyl acetate,
methyl-metacrylate and others, blended with polyolefin homopolymers
and copolymers, is reported several times in the existing patent
documentation.
[0167] The use of such polymers in the real industrial practice is,
nevertheless, very limited due to several factors like:
[0168] price of the raw material
[0169] compatibility limits with polyolefins leading to troubles
during the spinning process
[0170] PP homopolymer shows, therefore, the major interest for the
production of staple fibres for non wovens in the hygiene
applications due to the following reasons:
[0171] lower cost of the raw material
[0172] good processability
[0173] satisfactory tenacity behaviour of the non woven
[0174] On the other hand, the thermal weldability of the PP
homopolymer fibre is due to the degraded skin which is formed
during the spinning according to the process stages previously
reported.
[0175] Polymers different from homopolymer PP (with the exclusion
of bicomponent sheath--core fibres obtained by feeding the
spinneret with two different polymers) are used only in the cases
in which there is the will to improve the softness.
[0176] Even in such cases, as well as in the case of the use of
homopolymer PP, the spinning process is performed in a way to
optimize the formation of the skin to reach the highest tenacity of
the non woven.
[0177] The above mentioned limits of this technology are still
existing in any case.
[0178] In Table 3 the results obtained by experimental spinning
trials done on a NEUMAG spinning line.
[0179] 2.2 dtex/40 mm. cut length PP fibres have been produced by
adopting several polymeric compositions and by keeping constant all
the process parameters with the exception of the spinning head
temperatures.
[0180] These temperatures have been specially tailored to reach
well defined levels of DI value on the spun fibres.
[0181] The weldability of the fibres has been afterwards measured
by the W. I. testing method Results may be summarized as follows:
[0182] in absence of both the welding mechanisms (fibre plastic
behaviour and presence of degraded skin), a 100% homo PP fibre with
a DI<1.50 shows a very low value of W I. (test nr. 1) [0183] in
presence of the sole plastic behaviour mechanism (obtained by the
use of increasing quantities of raco PP in the polymeric
formulation), PP fibres with a DI<1.50 show W. I. values which
increase accordingly to the concentration of raco PP till reaching
high levels of weldability (tests 2-8)
[0184] when both welding mechanisms are present in the fibres
(plastic behaviour and presence of degraded skin with a
DI>1.50), the fibres themselves reach very high values in the W.
I. test (tests 9,10). TABLE-US-00001 TABLE 1 Influence of the
spinning temperature on polymer degradation (MFR) and non-woven
Tenacity (TBI) spinning temp. fibre MFR non-woven TBI .degree. C.
g/10 min N/5 cm 270 10.1 11.5 275 11.1 12.4 280 14.3 14.3 285 23.4
19.6 290 36.0 24.8 295 49.5 19.6 300 64.0 11.7 305 68.0 10.8 310
73.0 9.8
[0185] TABLE-US-00002 TABLE 2 Influence of the stabilization
formula on PP thermal stability and thermal oxidative stability of
polypropylene AO1 AO2 total additives MFR formula ppm ppm ppm g/10
min TSI OSI 1 150 1150 10.2 1.40 12.5 2 250 1700 10.2 1.14 10.2 3
150 1250 10.2 1.05 12.2 where: MFR = starting fluidity of the
polymer TSI = thermal stability index OSI: = oxygen stability
index
[0186] TABLE-US-00003 TABLE 3 Influence of the polymer blend
composition and degradability (DI) on fibre thermal bondability
(WI) blend composition (%) n. test PP homo PP/PE raco DI WI 1 100 0
1.30 370 2 90 10 1.31 510 3 80 20 1.32 780 4 70 30 1.32 900 5 60 40
1.33 1150 6 50 50 1.36 2600 7 40 60 1.37 3900 8 20 80 1.41 7800 9
60 40 1.9 2100 10 60 40 2.3 4050 11 60 40 3.1 13000 where: DI =
degradation index WI = weldability index
* * * * *