Increased volume synthetic fibres, procedure for producing them and their use, in particular for filters

An increased volume synthetic fiber consists of a porous central core and a large number of short porous lateral threads integral with the core and distributed along the whole length of the fiber so as to form a voluminous branching structure. The process used to produce the fiber consists of the mixing of a synthetic polymer with an expanding agent which expands when heated, then spinning by melting the mixture which results in the expansion and fringing of the threads produced, caused by the expansion agent, followed by the drawing and fixing of the threads thus produced while they are still hot.

FIELD OF THE INVENTION 
The invention relates to increased volume synthetic fibers the process used 
to produce them, and the use of the fibers, with special reference to the 
production of filters. It is known that synthetic fibers produced by means 
of spinning have a `compact` structure. 
BACKGROUND OF THE INVENTION 
For certain uses requiring fibrous masses with a certain degree of 
porosity, for example in the manufacture of filters, the porosity required 
is in reality that which can be obtained with non-woven fabrics or 
rovings, even carded rovings, i.e. a porosity that is, so to speak, 
`inter-filamentary`, created by the interstices formed between the 
individual fibres that make up the non-woven fabric. 
Now a new type of porous structure fiber has been found which is able to 
replace the porous fibrous masses so far used in all kinds of 
applications, with considerable advantages in terms of the consumption of 
materials and of cost. 
OBJECT OF THE INVENTION 
The main object of this invention is to provide a new type of porous 
synthetic fiber enabling savings to be made in terms of materials and 
costs in applications which require the use of porous fibrous masses. 
Another object of the invention is to provide a process for the production 
of the abovementioned porous fibers able to be performed using the same 
type of machinery traditionally used to produce conventional `compact` 
fibers. 
Yet another object of the invention is to provide an improved specific 
practical application of the fibers that are the subject of the invention, 
or more specifically, a filter for cigarettes utilizing the fibers. 
More specifically, the invention has the object of providing a filter for 
cigarettes which is highly selective with regard to the tar contained in 
cigarette smoke and also possesses a high condensation capacity with 
regard to the various distillates of the smoke itself. 
Still another important object of the invention is that of providing a 
cigarette filter which satisfies the requirements of the user in terms of 
rigidity and which at the same time is able to increase the absorption of 
the products of combustion of cigarette tobacco. 
A further object of the invention is to create a cigarette filter which 
does not alter the taste of the cigarette, in terms of the tobacco, and 
which is at the same time easy to breathe through and able to filter 
effectively the harmful substances generated by the combustion of the 
cigarette. 
Still another object of the invention is to obtain a high degree of 
condensate absorption for each cigarette, together with an improved 
capacity of the filter to hold a high degree of moisture, nicotine and 
tar. 
It is also an object of the invention to provide a cigarette filter, and a 
procedure for manufacturing it, which as well as considerably reducing the 
costs of producing the filter also makes it possible to considerably 
increase the quality of the absorption of the harmful substances produced 
during the inhalation of the smoke. 
SUMMARY OF THE INVENTION 
All these and other objects, which will become clearer from the description 
which follows, are achieved with a synthetic fiber consisting of a porous 
central core and a large number of porous lateral filaments integral with 
the core but shorter than it. These filaments are distributed along the 
entire length of the fiber so as to form a ramified fiber structure with 
increased voluminosity. 
According to another aspect of the invention, the process for producing 
increased volume synthetic fibers each consisting of a porous central core 
and a number of porous lateral filaments integral with the core and 
shorter than it, with the filaments being distributed along the whole 
length of the fiber so as to form a ramified structure, comprises the 
steps of: 
(a) cold mixing a fiber forming synthetic polymer with an inflating or 
expanding (blowing) agent; 
(b) spinning by melting the mixture formed in step (a) in order to obtain 
the said ramified structure through the inflation and fringing of the 
fibers caused by the inflating agent; 
(c) drawing of ramified structure fibers obtained in step (b); and 
(d) fixing the drawn fibers by heating them in a furnace. 
Finally, according to a further aspect of the invention the objects of the 
invention are achieved by a process for the manufacture of a cigarette 
filter consisting of the following phases: 
(a) cold mixing polypropylene with a blowing agent and a porogenous agent: 
(b) melting and spinning the mixture formed in step (a); 
(c) drawing of a tow of polypropylene as obtained in phase (b): 
(d) impregnating of the tow in an aqueous solution of stiffening substances 
containing, if necessary, a suspended porogenous agent; 
(e) crimping the tow; 
(f) fixing the tow by heating it in a furnace; 
(g) treating the stabilization tow with a plasticier; and 
(h) making up the tow into cylindrical shapes for cigarette filters. 
The new type of porous fibers according to the invention have a special 
ramified structure due to the presence of a porous central core and a 
large number of short lateral porous filaments, shorter than the core but 
integral with it, distributed uniformly along its entire length. 
This special fiber structure is achieved by means of the procedure which is 
one of the subjects of the invention. 
Thanks to the inclusion in the fiber-forming polymer, of the inflating or 
blowing agent and the subsequent heating during the phase of melting the 
mixture for the spinning of the fiber, the inflating agent first forms 
gaseous compounds trapped in the fiber in the form of micro-bubbles which, 
following further expansion due to the heating, "explode" and cause the 
fiber to fringe, at least on the surface, with the consequent formation of 
the previously described porous ramified structure. The subsequent phase 
of drawing the fiber completes the "fringing" effect of any remaining 
micro-bubbles giving rise to the finished ramified structure of the fiber 
in question which is then fixed by means of heat treatment, as happens 
with traditional fibers. 
It has been discovered that the best results are achieved by preparing the 
fibers of the invention from polypropylene or from copolymers of propylene 
with ethylene in various proportions, such as, for example, the 
commercially available copolymers which contain up to 50% of ethylene in 
the copolymer. 
Thanks to the intrinsic properties of these polymers and in particular to 
their visco-elastic properties and their consequent high resistance to 
elongation and high tensile strength, the process of "fringing" and 
ramification does not lead to the breaking of the central core of the 
fiber which in practice acts as the carrying frame for the structure 
obtained. 
The fibers according to the invention are therefore more voluminous than 
traditional fibers and also (bulky) than mechanically carded fibers, for 
example, with the result that it is possible to achieve the same degree of 
porosity by using a smaller quantity of polymer to produce the fiber (for 
example, a quantity at least 10% smaller by weight) or to obtain a 
considerably greater amount of porous fiber with an equal amount of 
polymer. 
In the spinning process required to obtain the fiber as per the invention, 
a large number of fibers is obviously obtained, as with all spinning 
processes. 
Thanks to the special nature of the process which includes the blowing 
agent in the fiber forming polymer, and as a result of the fringing effect 
of the blowing agent, the ramified structure of each fiber will 
interpenetrate that of the adjacent fibers, so that rovings are obtained 
which are directly suitable for many kinds of application involving the 
use of porous fibers. 
The blowing agent to be used according to the invention may be one of a 
series of compounds acting chiefly as expanding agents, in particular, 
azobicarbonamide, 4-4-hydroxybis((benzenesulphonyl)hydrazide, ammonium 
carbonates and bicarbonates and/or alkaline metals. 
Particular preference is given to azobicarbonamide since it gives rise to 
extended ramification of the fiber. 
The process of the invention is preferably performed by mixing the polymer 
and the blowing agent in a weight ratio of 0.05 to 1.0%. The spinning 
through melting is preferably performed by using special "X" or "Y" 
profile dies at a temperature which varies according to the specific 
polymer in question, but which for polypropylene and its copolymers is 
generally from 260.degree. to 310.degree. C. 
The drawing of the fibers thus obtained is generally carried out with a 
drawing ratio from 1:2 to 1:3, while fixing is performed in the 
traditional way (for example in a furnace at a temperature of 
approximately 105.degree.-130.degree. C.). 
The fibers or tow obtained by means of the procedure as per the invention 
can be used in all applications that until now have required the use of 
porous fibrous masses, in particulr filters, padding, etc. 
In this type of application of fibrous mass can be combined with additives, 
adjuvants, auxiliaries, etc., selected according to the specific use 
required. Thus, one particular application of the fibers that are the 
subject of the invention is, for example, that of the preparation of 
filters for cigarettes, as is described in more detail in the paragraphs 
below. 
As mentioned previously, the procedure for the preparation of such a filter 
implies the addition during the mixing phase (a) of a porogenous agent as 
well as the inflating agent. 
The porogenous agent, as per the invention, can be mixed with the polymer, 
for example to the polypropylene before spinning, or it can be applied to 
the fiber at a subsequent phase, after spinning. 
Porogenous substances that are especially suited for the invention are: 
calcium carbonate, talc and amorphous silica. 
The particle size of the amorphous silica is preferably less than 1 micron. 
One of the porogenous agents that is particularly active in holdng back the 
harmful substances contained in cigarette smoke is calcium carbonate. 
After the spinning phase, the porogenous agent is distributed 
statistically on the threads of both the core and the lateral filaments. 
This makes each thread highly absorbent and at the same time extremely 
rigid. 
Moreover, the filaments tend to join together, by means of the lateral 
threads, thus giving rise to a tow of polypropylene which acts as a 
support, inasmuch as it has a large number of interstices inside it, for 
the other absorbent and sizing substances used in the impregnation phase 
during the "foulard" bath. 
The mixtures used during the finishing phase contain porogenous substances, 
such as, for example, CaCo.sub.3 prepared in particular with anti-static 
and lubricating substances, such as stearic acid, bathed in an antistatic 
lubricating substance, such as polyethyleneglycol, and with the addition 
of absorbent sizing substances such as starch. 
These mixtures enable the product to undergo crimping to increase the 
voluminosity of the polypropylene tow, without problems of processability. 
Moreover, during the filter making phase the CaCo.sub.3 does not become 
powdery, thanks to the stearic acid coating the individual particles. 
Another advantage is that as well as bonding the filaments together these 
substances also absorb the products contained in the tobacco smoke and the 
product thus obtained effectively condenses the distillates of the smoke 
so that they can be cooled as a result of the large number of interstices 
that exist between the various threads that make up the filter itself. The 
polypropylene tow is then inserted into a filter-making machine which 
advantageously has the rollers in a closed position at 1-1.8 ate; in 
addition, polyvinyl pyrrolidone is used as a plasticizer during the 
filter-making phase in order to achieve greater cohesion.

SPECIFIC DESCRIPTION 
In FIG. 1 of the drawing, I have shown a filament spun as a Y-shaped 
structure 10 which has been subjected to fringing so as to form threads 
11, 12, 13, 14, 15 and 16 which branch from the core and have lengths less 
than the lengths of the core. The threads or fringes are formed by the 
explosion of the gaseous compounds trapped in the form of the 
micro-bubbles in the filament as it emerges from the melt spinning stage. 
The fringes 11-16 can be seen to be porous as is the core 10 itself. A 
X-section filament can also be spun as described previously. 
In FIG. 2, I have represented diagrammatically the method of process of the 
invention. 
The blowing agent introduced at 21, the porogenic agent introduced at 22 
and the polypropylene introduced at 23 are cold-mixed in the initial stage 
20 and the composition 24 is then heated in stage 25 to form a melt which 
is subjected at 26 to spinning through X-shaped or Y-shaped orifices to 
effect filament formation and the formation of the threads or fringes on 
the filaments. 
The filaments are then gathered at 27 into a tow and the tow is subjected 
to drawing at 28. The drawn tow may be crimped at stage 29 and the crimped 
drawn tow can be subjected to heat fixing by passing it through an oven 
represented at 30 at a temperature of 105.degree. to 130.degree. C. at a 
speed of 2 to 5 meters per minute. The heat fixed tow can be form at 32 
into cigarette filters. 
It is possible to subject the spun fibers to drawing without tow formation 
as represented by arrow 33 and then to subject the drawn fibers to heat 
fixing as represented by arrow 34, thereby yielding a product at 35 which 
is useful for other applications than the formation of cigarette filters. 
In FIG. 3 I have shown an orifice 40 of Y configuration while in FIG. 4 the 
orifice 41 has an X configuration for spinning the filament. 
The procedure for the manufacture of the cigarette filter, which is the 
subject of the invention, consists of the following phases. 
Cold mixing of the various components in the form of flakes and highly 
stereospecific base polypropylene with MI=12 in a slow mixer at a 
temperature of approximately 20.degree. C. for a period of about 30 g/min. 
Then spinning by melting is performed at a temperature of between 
260.degree. C. and 310.degree. C. with the use of the previously described 
"Y" or "X" section dies in order to create the lateral threads on the 
individual filaments which are charged with porogenous agents such as, for 
example, calcium carbonate. 
After spinning the filaments are treated with antistatic substances and 
then bathed in water to eliminate as much of the anti-static substances as 
possible in order to create a product that is non-toxic. 
After the water bath, the polypropylene tow is passed over the first roller 
(a slow roller) at a temperature of 60.degree. C.-80.degree. C. and is 
then drawn in a steam furnace at 120.degree. C. with a drawing ratio of 
between 1:2 and 1:3, before being fed onto a second roller (a fast roller) 
at a temperature of approximately 120.degree. C. 
After the tow has passed over the fast roller at a temperature of 
120.degree. C., it is subjected to a "foulard" bath in an aqueous 
solution of lubricants, for example polyethyleneglycol, which also 
consists of porogenous inorganic charges, CaCo.sub.3 prepared with 
lubricating, anti-static stearic acid, and sizing substances such as 
starch; the concentrations of the abovementioned substances varies 
according to the charge required to obtain filters of the compactness 
desired. 
The subsequent phases of the procedure consist chiefly of phases of the 
mechanical type, such as for example the wringing of the tow and the 
crimping of the same to increase its voluminosity, forming on it about 5-8 
waves cm. 
Finally the tow is stabilized (annealed) in a furnace at a temperature of 
105.degree.-130.degree. C. at a speed of approximately 2-5 mts. a minute, 
followed by packing with presses and the unwinding of the polypropylene 
tow in order to be fed into an opener on the filter making machine which 
will have rollers closed at a pressure of 1 to 1.8 ate; cellulose paper is 
also used for the making of the filters. 
As an example, we give below an example of the manufacture of a filter: 
(1) Mixing for 15 minutes in a Battaggion type slow mixer at 30 rpm at a 
temperature of 20.degree. C. of the following components: 
(1a) 97.8% of polypropylene, fiber type, highly stereospecific, melt 
index=12, containing: 
0.2% of calcium stearate (anti-acid) 
0.15% of heat stabilizer (anti-oxidant) 
(1b) 2% of white flake containing: 
1% of TiO.sub.2 
0.5% of CaCo.sub.3 
0.5% of low density polyethylene, MI=20 
(1c) 0.2 azobicarbonamide flake, containing: 
0.08% of azobicarbonamide 
0.12% of low density polyethylene MI=20 
(2) Melting and spinning of the tow under the following conditions: 
(2a) Use of a temperature profile of: 
260.degree. C. in the feed zone 
290.degree. C. in the body of the extruder 
300.degree. C. in the filter zone 
290.degree. C. in the die zone 
(2b) Use of a filter before the die composed of three 10,000 mesh/sq.cm. 
mesh filters: 
(2c) Use of "Y" section dies 
Tow on output from die with 20.4 denier per hole. 
(3) Treatment with an anti-static product 
(4) Hot drawing in steam at 120.degree. C. with a temperature of 80.degree. 
C. before the furnace and a roller temperature after the furnace of 
120.degree. C. Drawing ratio 1:3. The characteristics of the tow thus 
obtained are as follows: 
count of each filament: 6.8 denier 
total count: 35,360 denier 
(5) "Foulard" bath treatment of tow in an aqueous solution containing: 
(5a) Starch solution in water with traces of diluted acetic acid to assist 
hydrolysis into: 
maltose C.sub.12 H.sub.22 O.sub.11 and 
Dextrin (C6H1005) n=50-60. 
(5b) 50% solution of polyethyleneglycol (steeping) and 50% CaCo.sub.3 with 
stearic acid on the outside of each particle (this is necessary for 
lubrication purposes during the subsequent crimping phase and to avoid the 
powdering of the CaCo.sub.3 when the tow is put into tubes of cellulose 
paper to make the filter). 
(5c) Solution for `foulard` bath after mixing the two solutions, the 
solution for the `foulard` bath must contain, in total: 
8.3% starch 
8.3% CaCo.sub.3 
25% polyethyleneglycol, n=600 
58.4% water 
(6) Wringing 
(7) Crimping to create 8 waves/cm. of undulation on the filaments. 
(8) Passage through furnace at 105.degree.-110.degree. C. to stabilize the 
tow at a furnace belt speed of 3 mts/min. 
(9) Packing of the tow in order to avoid folds and/or twists. 
(10) The total count of the tow, with the addition of starch and CaCO3, 
becomes: 43,600 denier. The table below gives comparative data between the 
analysis of a filter as per the invention as prepared in the way described 
above and a traditional cellulose acetate filter. 
______________________________________ 
FUNCTIONAL ANALYSIS OF A P.P. FILTER AS 
COMED WITH A CELLULOSE ACETATE FILTER 
CELL. 
P.P. FILTER 
AC. FILT. 
______________________________________ 
Length of cigarette (mm) 
84 84 
Weight of cigarette - filter (gr) 
1080 1080 
Weight of tobacco cylinder (gr) 
0.925 0.9235 
P inhalation of cig. (mm/water) 
100-110 100-110 
Relative humidity of environment 
of smoke test (%) 12.46 13.16 
P inhalation only of 20 mm. 
filter (mm/water) 41 49 
no. of inhalations by smoke 
machine needed to finish cig- 
arette (no. of inhalations) 
13.7 13.5 
Residue of filter and cigarette 
after smoke test (mm) 
28 28 
Condensate + Humidity + Nico- 
tine (mg/cigarette) 
38.27 34.77 
Humidity held in filter (mg/ 
cig.) 4.37 4.26 
Nicotine held in filter (mg/ 
cig.) 1.39 1.26 
Tar held in filter (mg/cig.) 
32.5 29.25 
Weight of one filter (mg) 
177.5 156.5 
Total denier count (den.) 
43.620 36,000 
______________________________________ 
In practice it was shown how the cigarette filter and the procedure for 
producing it are particularly advantageous with regard to the reduction of 
the a-mount of tar contained in the tobacco smoke and to the high 
condensation of the distillates of the smoke to enable cooling in the 
large number of interstices between the various filaments that make up the 
thread itself. 
As the invention has been conceived it can be modified in many ways and 
still remain within the sphere of the concept of the invention; moreover, 
all the details can be replaced by technically equivalent elements. In 
practice any materials can be used and any dimensions adopted according to 
the requirements of the state of the art.