Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PRODUCTION OF FIREPROOF POLYACRYLATE FI-
BRE WITH A LOW EMISS-ION OF TOXIC FUMES, UNIFORMLY DYED,
AND ACRYLIC FIBRES THUS OBTAINED
The present invention relates to a process for the
production of fireproof polyacrylate fibre with a low
emission of toxic fumes, uniformly dyed.
More specifically, the present invention relates to a
process for the production of fireproof polyacrylate fi-
bre with a low emission of toxic fumes, uniformly dyed,
and to the fibre thus obtained.
Commercial acrylic fibre from a copolymer are used in
traditional textile fields such as clothing, home fur-
nishing, flooring, etc.. or in the field of outdoor awn-
ings. The acrylic fibre from a homopolymer, less dif-
fused, is used for reinforcing concrete, for the filtra-
tion of fumes, etc..
As is known, there are also various kinds of post-
treatment of acrylic fibre which fully modify it, trans-
forming it into products which are chemically very dif-
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ferent from the starting fibre and having new properties.
The most well-known type of post-treatment is that
which transforms the acrylic fibre into fireproof oxi-
dized fibre, after thermal cyclization of the CN groups,
or into carbon fibre through thermal treatment subsequent
to oxidation and at higher temperatures, used in high-
performance composite materials, such as high mechanical
and thermal resistance.
Other types of post-treatment of acrylic fibres have
been studied and used for imparting new and interesting
properties to the fibre such as fire-resistance or the
absorption of considerable quantities of water.
These latter types of treatment include the transfor-
mation of the CN groups into amide and carboxyl groups
through extremely drastic chemical reactions and under
unusual conditions. These forms of treatment can create
products with the desired characteristics without resort-
ing to the production of polymers which are difficult to
handle and expensive.
A sequence of treatment of acrylic fibres has been
known for some time, for example, first with hydrazine,
followed by an alkaline hydrolysis of the CN groups, a
subsequent neutralization with strong acids and finally a
treatment with zinc salts, calcium salts, copper salts,
etc.. After these modifications, the final product is a
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fibre with a L.O.I. (Limiting Oxygen Index) value higher
than 40%, which does not melt, and does not release
fumes.
The properties shown by the fibre after this series
of treatment are definitely of commercial interest, for
example, for protective clothing, under extreme condi-
tions of fire exposure, or for textile furnishings in en-
vironments in which fire-resistance and the absence of
toxic fumes is essential (aeroplanes, trains, passenger
ships, etc..).
In spite of these positive qualities, however, this
kind of fibre has a very limited diffusion on the market
and is restricted to the production of non-woven fabrics
to be used in multilayer materials as a fire-barrier.
The reason for this low commercial diffusion can be found
in the poor aesthetical appearance of the fibre which is
pink or, when dyed, shows strongly irregular colours
which discourage its use in the visible end-products pro-
duced, for example, in the clothes industry (working
overalls) or furnishing (seat covers).
The reason for this dyeing irregularity lies in the
irregular chemical composition and dyeing sites due to
the post-treatment.
The pressing constancy of strong acidic groups, nor-
mally of the sulphonic type, which must be guaranteed on
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the fibre if a uniform dye is to be obtained, without
striping, is known, for example, to producers of tradi-
tional acrylic fibres.
Most of the complaints a producer of acrylic fibre
must face relate to the dyeing uniformity.
To guarantee this uniformity of behaviour in the dye-
ing phase, the fibre must have a variation of dyeing
sites not higher than 5% and this amount is assured by
the constancy of the molecular weight of the polymer, if
a redox catalyst of the persulphate-bisulphite type is
used, or by the correct dosage of a sulphonated co-
monomer, of the MASS (Sodium Methallyl Sulphonated) type,
if catalysts of the AIBN azo-bisisobutyrronitrile) type
are adopted.
In the sequence of chemical reactions which, starting
from a commercial acrylic fibre lead to a polyacrylate
fibre, all the CN groups are modified: first by reaction
of an aliquot of these groups, ranging from 20 to 600,
with hydrazine for an inter- or intra-molecular
crosslinking, subsequently by alkaline hydrolysis of the
CN residues to amidic and carboxyl groups.
The subsequent neutralization with a strong acid,
sulphuric acid, for example, leads to quaternizing of
both the azide and amide groups, making the fibre suit-
able for being dyed with pre-metallized dyes 1:1.
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In reality, the chemical treatment is carried out
batchwise, on traditional dyeing equipment, by position-
ing the fibre in a perforated basket, and circulating the
reagents through the fibre in sequence: reaction with hy-
drazine, alkaline hydrolysis, neutralization with a
strong acid, dyeing with pre-metallized dyes 1:1, salifi-
cation with zinc acetate.
The main drawback of this traditional process is that
the fibre obtained can have an unacceptable variability
in the chemical composition, after this treatment.
A first variable is introduced by the irregular per-
meability of the fibre inside the basket, with a conse-
quent non-homogeneous distribution of the modified CN
groups.
Another element which disturbs the homogeneity arises
during the alkaline hydrolysis phase. In this phase, the
formation of carboxyl groups leads to an enormous swell-
ing of the fibre which becomes like a gelatine, analo-
gously to what occurs with super-absorbent polymers. The
fibre, in this phase, tends to expand but is prevented by
the fact that it is blocked inside the basket.
The gelatinous mass makes the circulation of the re-
action liquid difficult, and this exalts the irregular
chemical composition of the fibre even more. During this
phase, in fact, the CN groups which have not reacted with
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hydrazine pass by alkaline hydrolysis first to amide and
subsequently to carboxyl.
The subsequent phase consists of an acidification re-
action with sulphuric acid, for example, of the carboxyl
groups. The fibre, through this operation, loses its ge-
latinous appearance and returns to a normal fibre, not
swollen by water.
Another function of the acid is to quaternize the am-
ide and azide groups responsible for the dyeing proper-
ties of the fibre. Some of the carboxyl groups are subse-
quently partly salified with metal ions, such as zinc,
for example.
From the above, it can be understood that it is dif-
ficult to guarantee a dyeing uniformity of the polyacry-
late fibre.
It is obviously not possible to use, as precursor, an
acrylic fibre dyed by means of traditional basic dyes, as
these are not capable of resisting the series of chemical
treatment envisaged.
The Applicant has now found that it is possible to
overcome all the drawbacks of the known art by using an
acrylic fibre already dyed in mass with selected organic
pigments, capable of remaining unaltered with the chemi-
cal treatment envisaged for modifying the chemical compo-
sition of the fibre into crosslinked polyacrylate.
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With the use of particular pigments, it is possible
to obtain a polyacrylate fibre also with dark colours,
which has not been possible so far, due to the low con-
centration of dyeable sites on the polyacrylate fibre.
The use of acrylic fibres dyed with pigments offers
another opportunity as, being free of dyeing problems,
the reaction can be directed so as to privilege other
fundamental properties of polyacrylate fibres, such as
the mechanical characteristics, toughness and ultimate
elongation, as well as the fire-resistance properties.
The use of pigmented fibre as precursor of polyacry-
late fibres, reduces the treatment time by more than
three hours, enhancing the productivity of the process.
An object of the present invention therefore relates
to a process for the production of fire-proof polyacry-
late fibre with a low emission of toxic fumes, uniformly
dyed, comprising:
a. dissolving an acrylonitrile copolymer, for example
acrylonitrile-vinyl acetate, in a solvent, for exam-.
ple dimethyl acetamide, in a weight ratio ranging
from 90/10 to 99/1, for example 93/7, and having a
number average molecular weight Mn ranging from
35,000 to 65,000, for example of about 50,000 atomic
units;
b. dispersing in the polymeric solution obtained, alone
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or in a combination, according to the final colour
desired, organic pigments of the type: Orange 43
(perinone), Blue 60 (indantrone), Blue 15:1 Copper
phthalocyanine), Yellow 24 (flavantrone), Yellow 109
(tetrachloroisoindolinone), Red 149 (perylene) Green
36 (phthalocyanine), Green 7 (phthalocyanine), Black
7 (carbon black), etc.. so as to have a total final
concentration of the pigment ranging from 0.5 to 3.50
by weight.
c. treating the acrylic copolymer/pigment mix to produce
a pigmented acrylic fibre as staple or tow;
d. subjecting the pigmented acrylic fibre to a
crosslinking step by means of an aqueous solution of
hydrazine hydrate;
e. subjecting the crosslinked fibre to an alkaline hy-
drolysis step;
f. subjecting the crosslinked/hydrolyzed fibre to treat-
ment with a strong acid; and
g. salifying the final fibre with an organic metal salt.
At the end of the process object of the present in-
vention, a uniformly dyed polyacrylate fibre is obtained
with a L.O.I. (Limiting Oxygen Index) higher than 37%
According to the present invention, the pigmented
acrylic fibre can be used in the form of a staple or tow,
bearing in mind the variation in length which will occur
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on the staple and the doubling of the titre on the tow at
the end of the treatment.
In particular, a solution with a concentration rang-
ing from 23 to 28o by weight of solids, at 25.5% for ex-
ample, of an acrylonitrile/vinyl acetate copolymer in a
93/7 ratio with the dispersion of the desired amounts of
pigments, described above, is fed with a gear pump and
with a volume flow-rate of 23.5 cm3/min., to a 1,000 hole
die, each hole having a diameter of 52 microns. The die
is immersed in a coagulation bath containing a wa-
ter/dimethyl acetamide solution in a 1:1 ratio at a tem-
perature of 500C.
The cord of coagulated tows is extracted from the co-
agulation bath by a pair of rolls at a rate of 6.15
m/min.
The fibre is contemporaneously washed with demineral-
ised water to remove the solvent and passed through a
tank containing boiling water, to be stretched in a ratio
of 6.5:1 by a second pair of rolls having a peripheral
rate of 40 m/min.
After stretching, the fibre is immersed in a tub in
which a finishing mix, having lubricant and antistatic
properties, is fed in continuous. At the outlet of the
finishing tank, the rope of fibres is collected on a pair
of rolls having a peripheral rate of 40 m/min in differ-
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ent coils and heated with vapour up to a temperature of
160 C.The fibre rope thus obtained has a titre for each
filament of 1.5 dtex and is collected on bobbins.
Skeins are prepared from the bobbins obtained accord-
ing to the procedures described above, in various coils
having a diameter of a meter, and are treated in an auto-
clave with saturated vapour, at a relative pressure of
1.7 bar. The fibre undergoes a shrinkage of 30o with re-
spect to the initial length and the titre of each fila-
ment becomes 2.0 dtex.
The fibre treated in the autoclave, uniformly dyed
with the desired colours, has the following characteris-
tics:
titre 2.0 dtex
toughness 32.5 CN/Tex
Ultimate elongation 3506
With respect to the colours which can be used for the
acrylic fibre dyed with solid pigments, examples of prod-
ucts can be those described in the colour chart of Leac-
ril OD of Montefibre, among which:
Black N034 - N039
Blue B069 - B109 - B112 - B115
Red R035 - R042 - R089
Green V005 - V006 - V034
Brown M113 - M074 - M560
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Yellow G028 - G043 - G033
Once the pigmented acrylic fibre has been obtained,
the same can be treated to make it fire-proof. This step
of the process, object of the present invention, starts
with step (d), according to which the pigmented fibre is
crosslinked with an aqueous solution of hydrazine hydrate
at a concentration ranging from 5 to 25%, preferably from
7 to 20% (weight of hydrazine hydrate with respect to the
volume of water) . The crosslinking is carried out at at-
mospheric pressure, or slightly higher, at a temperature
ranging from 70 to 1500C, preferably from 80 to 120 C.
After a period of time sufficiently long to have a
crosslinking, inter- and/or intra-molecular, higher than
20%, for example from 25 to 60%, with respect to the to-
tal number of CN groups contained in the polymeric chain
of the polyacrylonitrile homopolymer, the fibre is
treated with an alkaline aqueous solution, step (e), for
example with a solution of soda (NaOH) and/or potassium
hydroxide, having a concentration of 1 to 80, preferably
from 3 to 6% (weight/volume). The treatment with the al-
kaline solution takes place at atmospheric pressure, or
slightly higher, and at a temperature ranging from 70 to
120 C, preferably from 80 to 110 C. The treatment with
the alkaline solution is carried out for times ranging
from 90 to 150 minutes, preferably between 100 and 130
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minutes.
The third fireproofing treatment, step (f), comprises
wetting the fibres with a strong inorganic acid, for ex-
ample sulphuric, nitric, phosphoric or hydrochloric acid,
diluted in water. Preferred concentrations range from 2
to 10%, for example from 3 to 8% (weight/volume). The fi-
bres remain in contact with the acid for a period of time
ranging from 40 to 90 minutes, preferably between 50 and
70 minutes. This step is also carried out at atmospheric
pressure, or slightly higher, but at temperatures ranging
from 40 to 80 C, preferably from 50 to 70 C.
Finally, the last fireproofing treatment step, step
(g), comprises salification of the acidic groups present
on the fibre with a metal salt, preferably an organic
metal salt. This treatment is effected with an aqueous
solution containing from 1 to 8% of the organic salt,
preferably a metallic organic salt. This treatment is
carried out with an aqueous solution containing from 1 to
8% of the organic salt, preferably from 2 to 5%
(weight/volume), operating at a temperature ranging from
80 to 120 C, preferably from 90 to 110 C, with a contact
time ranging from 30 to 90 minutes, preferably from 45 to
70 minutes.
Examples of said organic salts are zinc formate
and/or acetate.
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At the end of the fire-proofing treatment, the fibres
are abundantly washed with hot water, centrifuged and
possibly treated with a finishing product to eliminate
the electrostatic charges and to impart feel softness.
An illustrative and non-limiting example is provided
hereunder for a better understanding of the present in-
vention and for its embodiment.
EXAMPLE
The tow of pigmented acrylic fibre, which can be
transformed into a staple on a cutting machine, is pro-
duced on industrial equipment according to the procedures
described above and by pre-dispersing the above-mentioned
pigments, individually or in a mixture, to reproduce the
desired colour.
For illustrative purposes, the above-mentioned pig-
ments are dispersed in the solution of the acryloni-
trile/vinyl acetate copolymer at 25.5o in dimethyl
acetamide, in such a concentration as to cover the
pink/orange-coloured background that the raw fibre would
acquire due to its new chemical composition, as a conse-
quence of the crosslinking - hydrolysis - salification
treatment.
Lots of fibre were produced, for example, in differ-
ent colours, mostly dark, to cover the pink-coloured base
deriving from the chemical treatment, and using a pre-
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dominant pigment in the concentrations shown in table I:
TABLE 1
Pigment Pigment concentra- Fibre colour
(Colour Index) tion on the fibre (%)
Blue 15:1 1.78 Turquoise blue
Blue 60 1.42 Sea blue
Yellow 24 0.92 yellow
Red 149 1.39 bright red
Orange 43 1.5 orange
Green 7 2.3 forest green
Black 7 2.3 dark black
300 kg of a tow of pigmented fibre, with a filament
titre of 2.0 dtex and a tow titre of 100 ktex, are
charged into a 1,000 litre punched basket, with contempo-
raneous wetting and pressing so as to obtain a packing
density of the fibre of 0.30 kg/l.
The basket is placed in a 2,000 litre reaction tank
equipped with a circulation pump, in which the flow-rate
and the functioning frequency can be programmed, and with
a heating/cooling coil for the reaction liquid.
The tank is filled with water and with a quantity of
hydrazine hydrate so as to have a concentration in the
liquid phase of 150 (weight/volume).
The solution is brought to 105 C and the circulation
pump is kept in operation for 5 hours.
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At the end of the reaction, the reacted hydrazine hy-
drate solution is discharged (concentration 9.7% equal to
0.30 kg of hydrazine hydrate per 1 kg of fibre) through a
cool exchanger to bring the temperature of the solution
to about 50 C.
A soda solution is then charged in such a quantity as
to have a concentration of 5% (weight/volume) in the liq-
uid phase. The solution is heated to 100 C and the recy-
cling pump is run for 120 minutes.
At the end of the reaction, a 60 (weight/volume) so-
lution of sulphuric acid is charged, the mixture is
heated to 60 C and is reacted with circulation of the so-
lution for 60 minutes.
Finally, the sulphuric acid solution is discharged
and the mixture is washed with deionised water, for three
cycles, at a temperature of 50 C in order to wash out the
ammonium sulphate and hydrazine sulphate salts formed.
127 Kg of dihydrated zinc acetate are charged together
with 16.8 litres of glacial acetic acid, the whole solu-
tion is heated to 100 C and is kept under circulation for
60 minutes.
At the end of the reaction, the solution is dis-
charged and washed with a cycle of water at 50 C.
The washing water is discharged, the basket is ex-
tracted from the reaction tank, it is then transferred to
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a centrifuge to eliminate the excess water.
After drainage of most of the liquid in excess, a
finishing solution is fed, containing a cationic finish
in such an amount that the concentration of this product
in the final fibre is over 0.5% (weight/weight).
After centrifugation, the fibre is extracted and
dried in a hot air oven.
The characteristics of the final fibre are indicated
in the following table II
Table II
Parameter Measuring unit Value
Titre dtex 4.0
Toughness CN/tex 18.0
Ultimate elongation % 25.5
L.O.I. % 37.5
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