Making a fibrillated synthetic-resin strand

A fibrillated strand is made by a method wherein first a mass of a powder of a high polymer is compressed into a substrate having a density smaller than that of the high polymer and having a multiplicity of gas-filled voids. Then an intense heat beam is played on the substrate to melt the high-polymer powder and simultaneously heat the gas of the voids to explosively enlarge same while the beam and substrate are relatively displaced so that the substrate is melted along a path. The substrate is cooled at least at the path after irradiation by the beam to form a porous strand extending along the path in the substrate. This strand is separated from the substrate and is then at least uniaxially stretched. The beam is a CO.sub.2 TEM.sub.oo -mode laser beam and the method further comprises the step of focussing the laser beam on the substrate. The fused strand of the substrate is cooled so rapidly that it is normally not melted all the way through from top to bottom and from side to side. The rapid cooling of the strand prevents the strand from coalescing back into a smooth monofilament.

FIELD OF THE INVENTION 
The present invention relates to the making of a fibrillated textile-like 
strand of a synthetic resin. More particularly this invention concerns 
such a strand made of a high-polymer resin. 
BACKGROUND OF THE INVENTION 
It is known to make fibers, fibrils, filaments, and openwork or reticulate 
fibrillated strands of high-polymer synthetic resins and mixtures thereof. 
The various organic and/or inorganic compounds are converted by casting, 
spinning, stretching, stretch-tearing, cutting, splicing, and joining into 
the desired form. East German Pat. No. 137,951 describes a method that 
forms a textile strand out of a mass of powder of a thermoplastic 
high-polymer resin. This mass is heated and treated to form the desired 
structure. 
Fibrillated textiles, known and preferred over many synthetic-resin 
textiles for their nice look and hand, are constituted of fibrils which 
are morphologically overmolecular units that have diameters from 0.05 
micron to 10 micron and lengths from 100 microns to several millimeters 
long. The fibrils normally extend parallel to the goods. This effect is 
achieved by the tendency of melt-spun polymer mixtures of polyethylene 
terephthalate, polyamides, polystyrene, and polyolefins to split, as 
discussed in U.S. Pat. No. 3,819,769. It is also known to position the 
fiber-forming polymer in a matrix and to dissolve out this matrix before, 
during, or after treating it. Such a method is inherently discontinuous, 
and consumes considerable quantities of valuable solvents. 
More particularly, West German Pat. No. 2,040,802 describes a method of 
making compound filaments by imbedding polyethylene terephthalate fibrils 
in a polystyrene matrix. After dissolution of the matric fibers are 
obtained of a fineness of 0.1 dtex and a diameter of 4 micron. West German 
Pat. Nos. 1,949,170 and 2,063,440 describe mixtures of polyamides and 
polyethylene terephthalate including the matrix-fibril structure formed 
thereby. East German Pat. Nos. 128,965 and 84,061 describe methods of 
splicing together fibrillable foils. 
A disadvantage of all of the known processes is that the fibrillability is 
only obtained using mixtures of normally incompatible polymers that are 
forced together under pressure. In addition it is necessary to use 
subsequent treatments, such as chemical dissolving or mechanical splicing, 
to complete the procedures. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
method of and apparatus for making a fibrillated strand. 
Another object is the provision of such a method of and apparatus for 
making a fibrillated strand which overcome the above-given disadvantages. 
A further object is to make a fibrillated strand by a continuous and simple 
method having a minimum of process steps. 
SUMMARY OF THE INVENTION 
These objects are attained according to the instant invention in a method 
of making a fibrillated strand wherein first a mass of a powder of a high 
polymer is compressed into a substrate having a density smaller than that 
of the high polymer and having a multiplicity of gas-filled voids. Then an 
intense heat beam is played on the substrate to melt the high-polymer 
powder and simultaneously heat the gas of the voids to explosively enlarge 
same while the beam and substrate are relatively displaced so that the 
substrate is melted along a path. The substrate is cooled at least at the 
path immediately after irradiation by the beam to form a porous strand 
extending along the path in the substrate. This strand is separated from 
the substrate and is then at least uniaxially stretched. According to this 
invention the beam is a CO.sub.2 TEM.sub.oo -mode laser beam and the 
method further comprises the step of focussing the laser beam on the 
substrate. 
The fused strip of the substrate is cooled so rapidly that it is normally 
not melted all the way through from top to bottom and from side to side, 
so that this fused strip is resting completely in a bed of unfused 
particles that utterly prevent it from sticking to the conveyor surface it 
is on. The rapid cooling of the strand is fairly critical, as it prevents 
it from coalescing back into a smooth monofilament. The intense heat of 
the beam melts the resin and causes the inclusions of air and water to 
extend explosively, thereby making the strand quite porous, which porosity 
is then frozen in. The voids, holes, and craters thus formed are elongated 
on subsequent longitudinal stretching of the strand to form the desired 
network of fibrils. During such stretching the longitudinal edges of the 
strand will curl or turn in, imparting a tubular shape to the strand, and 
the strand will, or course, be axially oriented. 
The powder can be admixed with various additives, such as short inorganic 
fibers. It normally has a particle size of at most 200 micron, normally 
less than 100 micron. 
It is also possible according to this invention to play a plurality of such 
beams on the substrate to form a plurality of respective strands that are 
cooled and subsequently separated from the substrate. These beams may be 
generated by fixed lasers, with the substrate moving relative to them. It 
is also possible to move the beams, either by moving the laser guns 
themselves or by reflecting their beams by means of a moving mirror or the 
like, or even to move the beams and the substrate both. The plural strands 
thus formed can be parallel or even cross each other When they cross it is 
possible to biaxially orient the web formed by the interlinked strands. 
The powder is compressed according to this invention at a pressure of 
between 4.multidot.10.sup.3 kPa and 10.sup.5 kPa, preferably between 
7.28.multidot.10.sup.3 kPa and 5.89.multidot.10.sup.4 kPa. 
The beam has, in accordance with this invention, a residence time t.sub.r 
on the substrate of between 10.sup.-1 sec and 10.sup.-3 sec. The beam 
strength is in turn adjusted depending on vertically how thick the fused 
strand is to be. The optimal beam energy level E.sub.opt is a function of 
the laser power N.sub.Leff, the residence time t.sub.r, and the diameter 
d.sub.f of the spot irradiated by the beam. The relationship is: 
EQU E.sub.opt =4.multidot.N.sub.Leff .multidot.t.sub.r /d.sub.f.sup.2 
.multidot.pi, 
in which it is noted that d.sub.f.sup.2 .multidot.pi/4 is the area of the 
irradiated spot. 
According to another feature of this invention the substrate is cooled at 
least at the strand by flowing a coolant gas over it.. This gas is an 
inert gas. Thus the method further comprises the step of preventing 
oxidation of the molten high-polymer powder until same has cooled. 
During stretching of the strand according to the invention it is heated. 
This can most simply be done by stretching it between pairs of heated 
rolls, with the upstream rolls cooler than the downstream ones. Normally 
the strand is stretched enough to pull some fibrils free at one end, 
thereby giving the strand a soft texture or hand. 
If the powder carries some water, it is vaporized by the beam along the 
path. Thus with the system of this invention there is no need to maintain 
special low-humidity conditions. In fact the meticulous resin purity and 
moisture-free conditions that must be maintained in the systems of the 
prior art are unnecessary. A resin like polyester can be recirculated from 
the downstream end of the system to the upstream end, simply by breaking 
up the unfused part of the substrate and feeding it back upstream. 
The process of the instant invention can also operate discontinuously. In 
this arrangement blocks or bodies of the substrate shaped like disks or 
cylinders and stable enough to be mounted in a support are rotated, 
reciprocated, or spun in front of an appropriate intense heat beam. In 
fact the strand can be peeled off such a body like a thread from a spool, 
thereby using the body most efficiently. 
The system according to this invention produces a soft fibrillated strand 
suitable for use either alone or piled with other such strands to make a 
soft but strong yarn. Such a yarn will have the feel of a more expensive 
natural-fiber yarn, but is much cheaper, and normally more durable. It is 
also possible simply to tear up the strand made according to this 
invention to obtain fibrils usable in the manufacture of textiles.

SPECIFIC DESCRIPTION 
As seen in FIG. 1 a high-polymer powder 1 is dosed by a feed arrangement 2 
having an upstream feed plate 2a, a downstream feeler roller 2c, a hopper 
2d, and a controller 2b onto a conveyor or transport surface formed by a 
belt 7' spanned over two rollers 7, one of which is connected to a motor 
7" operated in turn by a controller 25 linked to the controller 2b. 
Upstream relatively widely vertically spaced compacting rolls 3 and 3', 
middle less widely spaced rolls 4 and 4', and downstream closely spaced 
rolls 5 and 5' compress the powder 1 to form a substrate 6 that is 
slightly less dense than the powder itself, due to the inclusion of a 
multiplicity of microscopic air-filled voids. 
A CO.sub.2 laser 8 operating in the TEM.sub.oo mode is directed down 
through an adjustable focussing system 9 at a spot 10 on the substrate 6. 
This focusing system 9 and the laser beam downstream of it are surrounded 
by a nozzle 13 to which cool nitrogen is fed under pressure through an 
inlet pipe 11 so that the spot 10 is flooded with this relatively inert 
and cool gas. 
Thus the powder of the substrate 6 will be fused along a path having a 
width equal to the dimension of the spot perpendicular to the transport 
direction D of the substrate and a depth determined by the power of the 
laser 8. This will form in the compacted powder substrate a fused 
thermoplastic strand 13. The tiny air-filled voids in this region will be 
explosively heated and at least some of them will burst, leaving the 
molten strand with a multiplicity of gas inclusions and surface craters 
that will give it an openwork structure, since the strand 13 is cooled 
before the craters and voids can flow together and close. In addition any 
moisture in the powder 1 will be vaporized, increasing this cratering. The 
melting will be limited by the cooling effect of the nitrogen, which 
simultaneously will prevent the hot resin from oxidizing. 
Subsequently the thus formed openwork strand 13 is separated from the 
substrate 1 by a blade 14 having a cutting edge 15 positioned under a 
hold-down roller 22 above the upper stretch of the conveyor belt 7'. The 
strand passes through an upstream pair of heated rolls 16 and 16' and then 
between a downstream pair of heated rolls 17 and 17' that rotate faster 
than the rolls 16 and 16'. This produces a stretched strand 18 which is 
wound up on a take-up roller or spool 21. As the strand stretches its 
longitudinal edges curl in or under, thereby giving it an oval and tubular 
shape. 
The substrate 6 that has been separated from the strand is meanwhile broken 
up by a wheel 10 to fall as powder 24 into a hopper 19. The powder from 
this hopper 19 as well as the powder picked up by vacuums 23 provided 
between the roller 2c and spot 10, over the roll 22, and underneath the 
stretching rolls 16-17' is returned to the supply hopper 2d at the 
upstream end of the arrangement. Any moisture picked up from the heating 
and cooling of the substrate 6 will merely add to the desirable 
vaporizable water inclusions. In addition there is no need to shield the 
arrangement from ambient moisture and the like. 
FIGS. 2 and 3 show an arrangement wherein three fixed lasers 8' are used 
which are directed upstream against the direction D parallel to but 
laterally offset from each other in a common horizontal plane at a mirror 
9' that is arranged at 45.degree. to their beams. This mirror 9' is 
oscillated back and forth about a vertical axis A by a motor 26 operated 
by the controller 25 to reflect the beams of the lasers 8' onto the 
substrate 6 in sine waves extending in the direction D and forming 
sinusoidal strands 13' as best shown by FIG. 3. 
In FIG. 4 a cylindrical body 6' formed of a compacted mass of an 
appropriate synthetic-resin powder is simultaneously rotated about its 
axis A' and reciprocated slowly along it as indicated by arrows 27. A 
fixed laser gun 8" is directed radially at the moving surface of the 
cylinder 6' to form a helical strand 13" on it. A blade 14' moving with 
the body 6' strips this strand 13" from the body 6' and feeds it to a 
stretcher such as shown in FIG. 1. As the diameter of the body 6' 
decreases, the blade 14' moves radially inwardly. This arrangement 
therefore works discontinuously, stopping each time the precompacted body 
6' is exhausted. 
EXAMPLE I 
With the system of FIG. 1 the powder 1 is a polyester reduced by grinding 
to a particle size of less than 30 micron. The rollers 3-5' exert a 
pressure of 5.89.multidot.10.sup.4 kPa to give the substrate 6 a density 
of 1.3 g/cm.sup.3, which is equal to 94% the density of high-polymer 
polyester being used. The substrate 6 thus formed has a vertical thickness 
or height of 1 mm and a horizontal width perpendicular to the direction D 
of 2 mm. 
The substrate 6 is advanced by the belt 7' at a speed of 49 m/sec. The 
laser 8 is a CO.sub.2 TEM.sub.oo -mode laser energized at 45 W and 
focussed by the adjustable focussing system 9 to form a round spot 10 
having a diameter d.sub.f equal to 0.35 mm. The residence time t.sub.r of 
the laser beam on any given location on the substrate 6 therefore is 
4.31.multidot.10.sup.-4 sec, and the powder 1 will be melted to a depth of 
about 0.05 mm. The strand 13 is therefore a strip about 0.35 mm wide and 
0.05 mm thick, some seven times wider than it is thick. The edge 15 of the 
blade 14 is positioned about 0.05 mm below the upper surface of the 
substrate 6, so that it just lifts off the strand 13 and the top portion 
of the compacted substrate 6. 
The upstream stretch rolls 16 and 16' are heated to a temperature of 
150.degree. C. and the downstream rolls 17 and 17' to 180.degree. C. The 
peripheral speed of the upstream rolls 16 and 16' is 49 m/sec, the same as 
that of the substrate, and the peripheral speed of the rolls 17 and 17' is 
about 200 m/sec, stretching the strand 13 by a factor of 4.1. This is 
enough to pull some of the fibrils that form loose at one end, creating a 
fuzzy effect that is extremely desirable in, for instance, a sweater 
knitted of a yarn formed by such strands 18. 
This stretching causes, due to the biaxial gradient, the longitudinal edges 
of the strand 13 to turn in or roll so that the strand 18 is of oval 
section, U-shaped or tubular, with a thickness of about 0.018 mm and a 
width of about 0.141 mm, some eight times wider than it is thick. The 
strand 13 has a fineness of 7.6 tex. It is wound up at 200 m/sec on the 
spool 21. 
EXAMPLE II 
With the arrangement of FIGS. 2 and 3 the transport speed is approximately 
halved to 25 m/sec. The beams of the laser guns 8' are deflected laterally 
through a trough-to-peak distance of 3 mm measured horizontally 
perpendicular to the direction D by deflection of the mirror 9' through an 
appropriate arc about the axis A. The mirror 9' is oscillated through this 
arc at a rate of 70 Hz. This moves the spot 10 at a rate of 75 m/sec. 
Thus the system according to the present invention allows a fibrillated 
strand to be produced in one smooth and continuous operation. No messy 
solvents or complex stop-and-go procedures need be employed. Virtually 
none of the powder is wasted, and the method can easily be altered for 
different kinds of resins. The scrupulous care needed in the prior-art 
systems to keep the resin dry and pure is unnecessary, indeed a controlled 
level of contamination is a desirable thing, increasing the desired 
cratering and blistering effect that creates the voids that are eventually 
elongated to form the spaces between the fibrils. Otherwise the process is 
relatively easy to control to produce a high-quality product.