DE4328133A1 - Micro-structuring surfaces of oriented polymeric substrates - Google Patents

Abstract

Microstructures (8) are formed on surface of oriented polymeric substrate (6) by irradiating it with laser radiation (4) having an intensity that is spatially modulated in a predetermined periodic manner, the substrate absorbing the radiation with an absorbence of at least 1000 cm(-1).Pref. the radiation is applied to the surface in at least one pulse of less than 10 microseconds duration at a fluence per pulse of 20 mJ per cm2-1 J per cm2. The absorbence is at least 3000 cm(-1).

Description

Technical field

The invention relates to a method for microstructuring surfaces with Laser radiation. The preferred area of application is microstructuring of surfaces of textile products.

The surface properties of filaments, such as polymer filaments made of polyamide or polyester or of natural filaments such. B. made of silk, determines important properties of these filaments standing secondary products such as fibers, yarns, fabrics, piles etc. Synthetic polymer obtained using the melt-spun method fibers such as B. polyamide or polyester fibers typically have one smooth surface. For many, a smooth surface of such filaments is Use properties of the derived products made from them disadvantageous. The smooth surfaces lead e.g. B. unwanted specular reflections of the subsequent products. Products made from these fibers therefore have from the point of view of the consumer a less attractive appearance than products natural fibers such as cotton or sheep's wool. Namely, these have an un regular, rough surface so that it does not reflect the above comes to the reflections. In addition, consumers prefer tacti len properties of natural fibers, which may be due to the fact that  due to the irregular surface of these natural fibers Skin contact is less than with smooth surfaces. That's why a micro textured surface sought, as with some natural filaments per se.

State of the art

Different methods of creating microstructured surfaces with Laser radiation is disclosed in the prior art. Are also different process for the microstructuring of surfaces of textile products revealed.

In the following explanations, the smallest with a filament should be Spinneret generated unit can be understood. So there is a fiber from several to many filaments and a yarn from several to many companies ser.

DE-OS 35 40 411 specifies a method in which textile fibers are used be irradiated with a laser, the wavelength of which is preferably in the UV range lies and the surface by punctiform, linear or flat attachment or Melting and / or removal of fiber material is microstructured.

In the publication "Melliand Textilberichte", 4/1990, pages 251 to 256 and in the U.S. Patent 5,017,423 is a process for microstructuring polymer fibers disclosed in which the polymer fibers, which have a high absorption in the UV range have, with a pulsed excimer laser with a in the UV range lying wavelength are irradiated. This creates a microstructured Surface that has a relatively periodic mountain valley structure. In the state of the art required for microstructuring of polymer fibers, namely a very high extinction coefficient, is used for Polymer fibers in the UV range reached, so that only lasers of the type Excimer lasers can be used. However, these have the disadvantage  that they are very expensive, e.g. B. due to electrode erosion and Window pollution, and therefore for industrial use in machining tion of polymer fibers are not suitable. Furthermore, at this level Technology to create targeted surface structures solely by varying the Laser conditions not achievable ("meilland textilberichte", 4/1990, p. 255, Column 2, 3rd paragraph).

It is also known that such polymer substrates have absorption bands in the infrared spectral range, which coincide with wavelengths, as they are known from wavelength-tunable lasers, such as. B. CO ₂ - and CO lasers are emitted. The experimental use of such lasers in the microstructuring of fibers using the methods described above with the excimer laser has, however, remained unsuccessful. Even with an increasing energy density, no microstructural effects were observed; rather, this caused the fiber to melt from a certain level of energy density. The term energy density describes the energy in J / cm ² with which the fibers are irradiated and which is absorbed by these fibers when a laser pulse is directed at them.

The present invention represents a reliable and inexpensive process for microstructuring polymer surfaces in particular through the use of industrially proven CO ₂ and CO infrared lasers, special excimer lasers with a large coherence length and dye lasers. The method according to the present invention requires only about 1/10 of the energy density (J / cm ² ) of the prior art methods using standard excimer lasers.

Recent findings show that the importance of a very high extinction coefficient (ε = 2-20 µm ^-1 ) is less for the conventional method with UV excimer lasers. Polymers with weaker extinction values (ε = 0.2 µm ^-1 ) have now been restructured.

Presentation of the invention

In contrast, the object of the invention is to provide a method for microstructuring surfaces, in particular surfaces of textile products such as filaments and their secondary products, in which the surface structures are simple and inexpensive, in particular through the use of industrially proven CO or CO ₂ -Laser, who created the.

Furthermore, the invention is based on the problem of any spatial structure generate whose parameters correspond to those available in interference-forming measures are adjustable. It is also a task of present invention, in a simple and inexpensive manner periodic Mountain-valley structures on the surfaces with definable mountain-valley distances to create.

The solution to this problem consists in the characterizing part of claim 1 given characteristics. Advantageous further developments include the features of subclaims 2 to 20.

The advantages of the invention are, in particular, that no high extinction coefficient is required to produce the microstructured surface, but only spatial modulation of the power density, so that on the one hand the risk of damage to the material to be processed, such as the melting of fibers, is reduced is, and that on the other hand no special requirements must be made of the laser types used to generate the laser radiation. Therefore, e.g. B. the known from industrial material processing and proven laser types, where appropriate with suitable modifications, such as retrofitting the laser resonator with a Q-switch, and the micro-structuring can be carried out in a cost-effective manner on an industrial scale. For example, the energy to be used when producing the method according to the invention for generating the microstructuring during processing in pulse mode is only a fraction of the energy required for processing with an ArF * excimer laser at λ = 193 nm, namely only one shot with 100 mJ / cm ² instead of 20 shots with 60 mJ / cm ² each.

The advantageous embodiment of the method according to the Claims 10 ff. Also has the advantage that with known techniques in one multiple times an interference field is generated that has a periodic power has density modulation, the periodicity being the mountain-valley distance true, is adjustable with simple means.

The design of the method according to the invention according to claim 16 has the additional advantage that the degree of absorption and thus also the structure that can be achieved can be adjusted to the respective material of the surface with known wavelength-tunable lasers. For example, in addition to the universal high absorption in deep UV, polymers also have characteristic absorption bands in the IR caused by molecular groups, of which one polyester coincides with the 9P 48 line of the CO ₂ laser and an extinction coefficient of ε = 0.098 µm ^{- 1} has.

Brief description of the drawings

The invention is shown in FIGS. 1 to 10 using exemplary embodiments and is described in more detail below.

Show it:

Fig. 1 is a schematic representation of the implementation of the inventive method.

Fig. 2 is an enlarged view of the area of the radiation zone to derive the relationship for the mountain-valley distance.

Fig. 3 shows an arrangement for performing the procedure according to the invention for the processing of sheet material.

Fig. 4 shows an arrangement for performing the method according to the invention for the processing of filaments, fibers and yarns.

Fig. 5 shows a polyester filament that has been structured according to the inventive method: polyester filament, with co-ordinated on the 9P (48) line CO ₂ laser and interferential radiation. Pulse duration 600 ns, 1 × 120 mJcm ^-2 Source: FhG Inst. F. Laser technology, Aachen.

Fig. 6 polyamide-6,6 filaments which have been structured according to the prior art: polyamide (PA) -6.6 filaments, with ArF excimer laser irradiates * (10 x 114 mJ cm ^-2) Source: T. Bahners, E Schollmeyer J. Appl. Phys. 66 (4) 15. Aug 1989 p. 1884-1886.

Fig. 7 polyester filaments that have been microstructured with the inventive method, wherein a KrF excimer laser was used for irradiation.

Fig. 8 enlarged view of Fig. 7.

Fig. 9 microstructured polyester film.

Fig. 10 enlarged view of Fig. 9.

Best way to carry out the invention

In Fig. 1 the implementation of the method according to the invention is shown schematically. Two interference-capable laser beams 5 a and 5 b with the wavelength λ are superimposed at an angle R in a region 9 in which the surface to be structured is to be arranged parallel to one of the planes e _i (i = 1, 2, 3). In order to use the method according to the invention as effectively as possible, the surface to be structured must be assigned in a plane between the planes e ₁ and e ₃ , that is, where the maximum extent of the interference field is located.

The determination of the mountain-valley distance d / 2 as a function of the parameters λ and R will be explained with reference to FIG. 2, which shows the area 9 in an enlarged representation. The wave vectors k ₁ and k _{2 of} the laser beams 5 a and 5 b have the amount k = 2 π / λ and include the angle R. The surface to be structured is in the plane AB, the normal n of which is parallel to the bisector of the two wave vectors and the coordinate direction z. The direction of observation h in this plane in turn runs parallel to the plane in which both wave vectors lie.

In complex notation and decomposition of k _1,2 into h and z components, the following applies to the field strengths of the waves (the time dependence exp (-iωt) as a common factor is eliminated):

E ₁ = exp (ik ₁ r) = exp (ik ₀ z cos (R / 2) + ik ₀ h sin (R / 2))
E ₂ = exp (ik ₂ r) = exp (ik ₀ z cos (R / 2) - ik ₀ h sin (R / 2)).

By adding you get:

E _tot = E ₁ + E ₂ = 2 exp (ik ₀ z cos (R / 2)) cos k ₀ sin (R / 2) h)
and for intensity I:

I = E ² _tot = 4 cos ² (k ₀ sin (R / 2) h) exp (2ik ₀ z cos (R / 2)).

In the direction of travel h (z remains constant), the intensity I is periodically (cos ² period = π) modulated with the period length d = λ ₀ / (2sin (R / 2)) (k _{0 is} again substituted).

In the AB plane - and each parallel plane - the distance d between two interference maxima is d = λ ₀ / (2 (sin R / 2)), so that the structure produced has a mountain-valley distance of d / 2.

Since this distance d is the same in every plane parallel to plane A-B, the is Arrangement of the surface to be structured is not ge on a fixed plane bound, so that none with regard to the positioning of the parts to be machined special requirements for the accuracy of this positioning are.

The formation of such an interference field results in a periodi spatial power density modulation. Will one, preferably stretched or pre-oriented fiber exposed to such a radiation field, a microstructuring is formed on their surface. The term "Microstructuring" here means that transverse furrows form (Mountain / valley structure). The targeted setting of a spatial power density Modulation is the random fluctuations that occur with the prior art Excimer lasers may be preferable.

Mainly suitable for a microstructuring according to the invention oriented synthetic polymer fibers and yarns or their derivatives. Preferred fibers are nylon or spunbond Polyester fibers, which are used in a variety of ways in the textile sector, e.g. B. Be clothing, furnishings and carpets. Oriented films are as well suitable. Microstructuring these films improves their adhesion.  

The term "oriented" refers to substrates that have a tendency to Have shrinkage when heated to the melting point. All commercial textile products and films tested had sufficient Orientation so that a microstructuring is formed on the surface could. Polyester filaments that run at a speed of 1000 m / min. were spun and had a draw ratio of 1.5, showed Irradiation a microstructured surface. Yarns with a speed speed of more than 5000 m / min. be spun, one for the Microstructuring sufficient orientation without additional ver need to be stretched. In this respect, depending on the degree of orientation of the filaments additional drawing may be required. Furthermore there is a possibility of stretching the fibers during drawing or under tension to irradiate.

If an oriented polymer substrate, such as. B. a drawn fiber, with a Laser radiation is applied, which has a spatial power density Has modulation and is sufficiently absorbed by the surface of the fiber , the surface of the substrate melts and it alternately forms hotter and colder zones in the melt. By stretching one Fiber, e.g. B. by a factor of 4, the average polymer molecule becomes four times longer, so that for the preservation of the volume the molecule in transverse Direction becomes thinner twice. It is believed that this becomes a Significant reduction in entropy leads to a frozen Tension condition in the drawn fiber or film is caused. When the surface of such a fiber melts, there is a large negative pressure in the melt. With a uniform Temperature distribution then the molecules slide past each other and take a more or less spherical shape. This can be a loss of Birefringence seen in the top micron layer of the fiber become. However, if there are temperature gradients, there are also according to viscosity gradients. It is believed that due to the entanglement of the polymer molecules the molecules from the colder areas  the molecules melt from the hotter, less viscous areas pull themselves into the colder areas so that it forms the mountain / valley Structure is coming. The "mountains" correspond to the colder areas while the "valleys" correspond to the hotter areas of the fiber surface.

In a fiber, the tension in the melt always runs along the fiber axis, so that the structures are always orthogonal to the fiber axis, even if the spatially power density-modulated radiation field is not orthogonal to the fiber axis. In a biaxially oriented film there is no preferred direction and nub structures are formed (see FIGS . 9, 10).

When working without targeted spatial modulation of the power density and there are only small stochastic fluctuations in the laser beam, such as this when processing with excimer lasers according to the prior art is assumed, a relatively large number of pulses is required for one forms certain microstructuring on the substrate surface. If that However, according to the invention, the substrate has a spatially modulated power density te and the periodicity of the modulation in the range the stochastic structures produced according to the prior art, then a single radiation pulse is sufficient to close the microstructuring produce.

Irradiation with the method according to the invention results in practically no loss of material in the fiber. Also the physical properties of the fibers such. B. breaking strength and tensile strength not significantly changed. The only remarkable physical change is the wavy structure, which takes the fiber surface in a layer of about 1 micron (see Fig. 5 to 10). All or almost all of the orientation in this upper micrometer layer is lost. This layer is initially amorphous immediately after the irradiation, but the polymer then recrystallizes again, either spontaneously or after a slight heating and, apart from the difference in orientation, is practically indistinguishable from the rest of the fiber. Because apparel fibers are typically 15 µm in diameter, the irradiated material makes up only a fraction of the entire fiber.

Suitable lasers for the method according to the invention include infrared lasers such as CO and CO ₂ lasers which have a good cooling system and which have been modified to have diffraction gratings and Q switches. The use of diffraction gratings has the advantage that these lasers can be operated at different wavelengths. A Q switch (a Q-switch that relates to the possible amplification of the laser medium) is required in order to be able to generate the shortest possible laser pulses. The possibility of using these large, reliable infrared gas lasers results in an economical application of the present invention in the expensive textile industry. A significant difference between CO and CO ₂ infrared lasers on the one hand and conventional excimer lasers on the other is the quality of the laser beam. Excimer lasers show large stochastic spatial and temporal variations in their rays, which is related to inhomogeneities in the plasma, especially in ArF excimer lasers. In contrast, CO ₂ and CO lasers have an almost Gaussian intensity distribution in the laser beam. In addition, there are some special excimer lasers that have a sufficiently large coherence length to be able to generate a sufficiently large interference field for the treatment of fibers or products thereof. The coherence length of the laser beam should be at least 5 mm, preferably 20 mm in length. Conventional excimer lasers are not suitable for generating sufficient interference because their coherence length is only in the order of µm. Microstructuring can also be carried out with laser radiation in the visible spectral range. For example, dye lasers and titanium sapphire lasers have an excellent coherence length and they can be used to generate good interference patterns.

In all cases, however, it is necessary for the absorption coefficient of the fiber to exceed about 1000 cm ^-1 at the respective wavelength with which this fiber is exposed, so that most of the radiation is absorbed in the surface of the fiber. The suitable laser type must be selected accordingly. Since at least 50% of the radiation should be absorbed in the upper 3 μm of the substrate surface, preferably in the uppermost micrometer layer. This corresponds to an absorption factor of at least 1000 cm ^-1 , preferably greater than 3000 cm ^-1 . A lower absorption of about 500 cm ^{-1 also} results in a, albeit lower, microstructuring. In comparison, with conventional methods with standard excimer lasers, absorptions in the range from 8600 to 230 000 cm ^{-1 are} achieved.

For example, an absorption band of polyester at 9,814 µm has an absorption of about 1000 cm ^-1 and coincides with the 9P 48 line of the CO ₂ laser. Further absorption bands of polyester are 9.091 µm with an absorption factor of about 2000 cm ^-1 and coincide with the 9 R 46 line of the CO ₂ laser; the absorption band of polyester at 5,817 µm with an absorption factor of about 3000 cm ^-1 is covered by about eight emission lines of the CO ₂ laser; the nylon 6.6 absorption band at 6.106 µm is covered by five lines of the CO ₂ laser and has an absorption factor of approximately 3000 cm ^-1 .

The substrate should be irradiated with an energy density per pulse of approximately 20 mJ / cm ² to 1 J / cm ² , preferably from 20 mJ / cm ² to 200 mJ / cm ² , the pulse length being less than 10 μsec. should be. In practice, 1 to 8 pulses, usually 2 to 4 pulses, are sufficient to obtain the desired microstructured surface. In contrast, a pulse number of 40 or more is required when microstructuring with standard excimer lasers. Higher energy densities usually lead to an unwanted melting of the fiber. Pulse durations of more than 10 µsec. Lengths, on the other hand, are ineffective because the heat is dissipated into the interior of the fiber, just as quickly as it is applied to the fiber surface. As a result, the fiber is generally heated and no longer just the surface in the specifically desired modulated manner. The pulse duration is preferably a few µsec .; however, it is also possible to work with pulses of shorter pulse duration, e.g. B. nanoseconds, picoseconds or even femtoseconds. In all cases, care must be taken to ensure that a suitable energy density is applied to the surface and that the time within which the surface absorbs this energy is shorter than the time for the heat transport into the interior of the fiber. These times for heat transport are on the order of several µsec.

In Fig. 3, an embodiment is illustrated for performing the method according to the invention. The laser beam 4 generated by a laser 1 is separated by a beam splitter 2 into several partial beams, in this case two partial beams, 5 a and 5 b. By means of a deflecting mirror 3 , a partial beam 5 b is deflected in one plane and crosses with the second partial beam 5 a at an angle R ge. In the area of overlap of the partial beams 5 a and 5 b (irradiation zone 9 ) is the material 6 to be irradiated, which is transported, for example, by means of a suitable conveyor 7 a and 7 b through the laser radiation field. A flat radiation can z. B. can be achieved by a line-wise transport movement. An oscillating beam deflection perpendicular to the direction of transport is also possible, the oscillation advantageously taking place by jointly tilting the optics 2 and 3 influencing the beam at an angle perpendicular to the direction of transport. The laser beam 4 brought into interference with itself in the above-mentioned manner leaves a trace 8 on the material which has the desired microstructuring.

The depth of the microstructuring results - in addition to the dependence on the extinction coefficient - on the one hand from the laser power introduced into the material 6 and on the other hand from the duration of the interaction of the material 6 with the power density-modulated laser radiation field. While the first aspect concerns the selection of a suitable laser and the setting of the laser parameters, the duration of the interaction can e.g. B. can be adjusted in a suitable manner by varying the transport speed.

Without limiting the general inventive concept, in addition to the arrangement shown and described in FIG. 3, any other arrangements are possible, provided that it is only ensured that the surface to be structured is located in the region of the radiation zone. For example (see FIG. 4), individual filaments, fibers or yarns 10 can be moved over a rotating cylinder 11 , the filament, the fiber or the yarn being placed several times around the cylindrical surface, and the overlapping laser beams 5 a, 5 b oscillate parallel to the cylinder axis 12 . Due to the rotation of the cylinder, the filament, the fiber or the yarn is rolled up in the area of one base area of the cylinder and unrolled in the area of the opposite base area of the cylinder.

The microstructuring of the fiber surface is most effective when the periodicity of the external radiation field comes as close as possible to the "natural" periodicity as occurs when irradiating with the stochastically fluctuating excimer lasers. This natural periodicity is in the order of a few microns for most apparel fibers including polyester and nylon 6,6. An interference pattern with a periodicity of about 1 to 10 microns, preferably 2 to 6 microns, and most preferably between 3 and 5 microns is most effective. The periodicity D of the radiation field can be set by varying the interference angle R. About 5 µm can be achieved with CO ₂ lasers and even 3 µm with CO lasers.

Fig. 5 shows the microstructured surface of a polyester-fiber clothing, using a TEA CO ₂ laser -Infrared. The laser (Uranit, model ML204) was tuned to the 9P 48 line (wavelength 9,817 µm) with a grating that replaces the rear mirror. The wavelength of the 9P 48 line is close to the absorption peak of polyester at 9,814 µm. The laser beam was split into two beams using a beam splitter, which were recombined using mirrors at an angle of 136 ° and interfered. The interference pattern generated in this way had a periodicity of 5.3 µm. A short section of a polyester carpet yarn (68 filaments, 17 dtex / filament) was stretched over a hole in a metal body. The body was placed in the interference field so that the yarn spanning the hole was irradiated by this interference field. The yarn was irradiated with a single pulse of 200 mJ / cm ² (absorption factor about 1000 cm ^-1 ) with a pulse length of about 600 nsec.

FIGS. 7 and 8 microstructured Sontara polyester fibers which have been exposed to an interference field generated by a KrF excimer laser with a coherence length of a few mm. The wavelength was 248 nm. An interference field with a periodicity of 3.0 µm was generated. The Sontara polyester fibers were irradiated with eight pulses, each of which had an energy density of 100 mJ / cm ² (absorption factor about 150,000 cm ^-1 ) and a pulse with a length of 20 nsec. The experiment was carried out in accordance with the processing of the fiber of FIG. 5. The examination of fiber samples which were exposed to a different number of pulses showed that the structures appeared after the first pulse and were fully developed after about four to eight pulses . Even low energy densities of around 30 mJ / cm ^{2 were} sufficient to quickly create the desired structures. In contrast, the prior art methods with standard excimer lasers require approximately 5 to 10 times higher energy densities, either by using multiple pulses and / or higher energy densities per pulse. These methods only produce a finely corrugated surface after the first pulse and typically 10 or more pulses are required until the desired structure is achieved.

Without restricting the general inventive concept, he can In principle, all surfaces are processed according to the invention and be provided with the desired microstructuring, provided that  Can be processed with laser radiation. Therefore, in addition to the Mi crostructuring of textile fabrics and the filaments building them up their secondary products also other surfaces with a microstructuring are provided, such as plate-shaped products.

Reference list

1 laser
2 beam splitters
3 deflecting mirrors
4 laser beam
5 partial beam
6 material
7 conveyor
8 lane
9 radiation zone
10 filaments, fiber or yarn
11 cylinders
12 cylinder axis

Claims (20)

1. A method for microstructuring surfaces with laser radiation which is absorbed by the surface, characterized in that the surface is acted upon with a spatially power-density-modulated laser radiation, that a spatially modulated temperature distribution is formed on the surface, and that this temperature distribution is a triggering the melt structure that determines the microstructure. 2. The method according to claim 1, characterized, that the surface is an oriented (oriented) polymer substrate acts. 3. The method according to claim 1 or 2, characterized, that the surface is the surface of textile products acts. 4. The method according to claim 3, characterized, that the textile products are artificially manufactured or natural filaments or fibers, yarns, fabrics made from them or piles. 5. The method according to claim 4, characterized, that the artificially produced filaments are plastic elements such as B. polymer filaments made of polyamide or polyester.   6. The method according to claim 5, characterized, that the plastic filaments are stretched. 7. The method according to claim 6, characterized, that the plastic filament is made of nylon or polyester. 8. The method according to any one of claims 1 to 3, characterized, that the surfaces are foils, especially plastic lien acts. 9. The method according to claim 8, characterized, that the films are made of nylon or polyester. 10. The method according to any one of claims 1-9, characterized, that the spatial power density modulation with interference-forming Measures taken. 11. The method according to any one of claims 1-10, characterized, that the power density modulation by superimposing at least two interference-capable laser beams is formed. 12. The method according to claim 11, characterized, that the interference-capable laser beams from a La be trained.   13. The method according to any one of claims 1 to 12, characterized, that the spatial power density modulation is periodic. 14. The method according to any one of claims 1 to 13, characterized in that for the formation of mountain-valley structures with a predetermined mountain-valley distance d / 2 (d = distance of the interference maxima) two interferable laser beams at an angle R cross, which at the specified wavelength λ of the laser radiation fulfills the following condition:
d = λ / [2 * sin (R / 2)]. 15. The method according to claim 14, characterized, that the mountain-valley distance d / 2 is at least 0.5 µm and preferred wise does not exceed 10 µm. 16. The method according to any one of claims 1 to 15, characterized, that the laser radiation can be tuned with one or more wavelengths Ren lasers is generated and the setting of the emitted wavelength or wavelengths depending on the absorption lines or ranges chen of the material to be processed. 17. The method according to any one of claims 1 to 16, characterized in that the surface is irradiated with at least one laser pulse, that the laser pulse has a pulse length of less than 10 µs, and that the energy density per pulse between 20 mJ / cm ² and 1 J / cm ² , preferably less than 200 mJ / cm ² . 18. The method according to any one of claims 1 to 17, characterized in that the absorption coefficient is greater than 1000 cm ^-1 , in particular greater than 3000 cm ^-1 . 19. The method according to any one of claims 1 to 18, characterized, that a pulsed laser, for. B. a TEA Laser, is provided. 20. The method according to any one of claims 1 to 19, characterized, that with a Q-switched laser to generate the laser radiation continuous or pulsed excitation of the laser medium is provided is.