Laser Welding of Fabrics Using Infrared Absorbing Dyes

by P A Hilton and I A Jones, TWI, Cambridge, UK

R Sallavanti, Gentex Corporation, Carbondale, USA

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International Conference on Joining of Advanced and Speciality Materials III ASM 2000, 9-12 October 2000, St.Louis, Missouri, USA

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Abstract A technique has been developed for laser welding plastics with infrared dye, creating a joint almost invisible to the human eye. Typically carbon black would be used as the absorbing medium for the laser light, however, this new approach enables two similar clear (or coloured) plastics to be joined with a minimal mark weld line. The process can also be applied to polymer based fabrics, where heat absorption and melting can be restricted to just the surfaces in contact, resulting in strong and leak tight seams. The selection of dyes is discussed in terms of strength of light absorption at 1064nm wavelength (Nd:YAG laser), as well as at 808nm and 940nm (wavelengths available from the relatively new diode lasers).

Introduction The development of the laser as an industrial heat source has resulted in a range of applications that utilise the precise, controllable energy it delivers. Cutting and drilling of a range of metallic and non-metallic materials, and welding of metals (in particular Fe, Ni and Ti based alloys) have become established industrial processes. Lasers have also had an impact on the processing of polymers, with applications being developed initially in cutting and drilling. In spite of there being more than fifteen techniques for welding thermoplastics,(1) the unique properties of laser beams have also led to the development of their use for welding plastics, where they offer advantages such as speed, precision, and flexibility. Early developments in welding plastics with lasers showed that thin films could be joined.(2) However, at this stage in the 1960s and 1970s, CO2 lasers were the principle power source. The nature of the interaction between the 10.6μm beam from the CO2 laser and thermoplastic materials, meant an analogue to the deep penetration process used to weld metals could not be developed. The CO2 laser beam is absorbed at the surface of the plastic, relying on conduction to heat through the thickness of the material, which results in decomposition,

vaporisation and charring, before any significant depth of material is melted. However, thin polyolefin films, of the order 0.1mm thick, have been successfully welded with a CO2 laser at speeds up to 500m/min.(3) The increasing use of Nd:YAG solid state lasers, and the advent of diode lasers (both producing beams with a near- infrared wavelength), meant lasers became available that had a different beam/material interaction with thermoplastics, from that of the far infrared CO2 laser. The shorter wavelength beam available from both these near-infrared types of laser will transmit through several millimetres of unpigmented polymer, but can be absorbed by the polymer if an appropriate filler is added. Based on this filled polymer technique, a transmission laser welding process was developed for thermoplastics where the transmitting plastic overlays an absorbing plastic. The laser beam passes through the top (transmitting) layer and is absorbed by the filler in the lower layer, producing sufficient heat to make a weld at the interface between the two parts. This process was first described for welding automotive components in 1985.(4) Typically, carbon black is used as the absorbing pigment or filler, resulting in one half of the assembly being coloured black. A refinement of this process is currently being used by Mercedes Benz, to manufacture a sealed electronic key fob, where a black dye that is absorbing in the visible region, but transmissive in the infrared, is used in the overlaying component, giving a welded assembly that appears completely black, top and bottom, to the human eye.(5) An example of the transmission welding technique, utilising a visually transmissive plastic material for the upper section and a carbon black loaded plastic for the lower layer, can be seen in Figure 1. Although infrared absorbing pigments are available which offer distinct stability advantages over infrared dyes, their drawbacks are considerable. As mentioned earlier, a classical example is carbon black, which absorbs visible and near infrared light. In pigments, the particulate nature means that light scattering occurs and light absorption efficiency is reduced. In addition, the low molar absorption coefficients of pigments means that high concentrations have to be used to

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Fig.1 Laser transmission weld in 4mm thick polypropylene using a 100W Nd:YAG laser at a speed of 1.6m/min. The weld is at the interface between the light and dark materials. produce a given heating effect, and, apart from the cost disadvantages, this can lead to undesirable changes in the physical properties of the host, including the appearance of unwanted colour. The residual colour problem is particularly true in the case of carbon black. An extension of the transmission laser welding process which allows completely clear or similarly coloured components to be welded by using a dye, clear in the visible range of the spectrum, but tailored to absorb heavily the specific wavelength of the laser beam being used, has also been described.(6) The nature of the dye means the laser wavelength is absorbed with high efficiency, thus requiring relatively small amounts of the dye at the interface between the two components to be welded. The dye can be introduced as a discrete layer at the interface (in a film, as a coating, or in a dye rich surface layer of the component), or incorporated into the bulk of the polymer of the underlying component. Development work on the process was mainly carried out using polymethylmethacrylate (PMMA) test specimens, and an example of an overlap weld made by applying a painted layer of dye to the joint region between two transparent sheets of 3mm thick PMMA can be seen in Figure 2.

Fig.2. Transmission laser overlap weld in clear PMMA made with infrared dye impregnated film at the interface. Although the example in Figure 2 is shown with two visibly clear sheets of PMMA, a dye applied in this way can

be used to join several other materials, coloured or otherwise. In this context the technique has also been used to perform transmission laser welding of thermoplastic fabrics, in an overlap joint configuration, which is the subject of this paper.

Light Absorption and Energy Conversion in Organic Dyes

Conventional dyes, by definition, absorb visible electromagnetic radiation, the process involving excitation of an electron from the highest occupied molecular orbital of the chromophore into the first vacant antibonding orbital. The process occurs without electron spin change and the dye is promoted from the singlet ground state to the first excited singlet state. The energy increase is quite considerable (ca. 150-300kJ.mol-1), and the dye molecule must lose the energy rapidly by various processes, returning to the ground state, where it is again available to absorb a photon of light. The most common deactivation process is internal conversion, followed by vibrational relaxation, in which the molecule passes from the zero vibrational level of the excited state into a high vibrational level of the ground state, and then undergoes rapid vibrational relaxation (ca.10-13 seconds) to the lowest vibrational level of the ground state, with the excess thermal energy being transferred to the surrounding host molecules. Other deactivating processes, such as fluorescence, or vibrational relaxation via the triplet state (intersystem crossing) are generally less important for most dyes. The overall effect is for the host matrix to become heated, and the local temperature rise will be determined by the wavelength and intensity of the light, and the thermal conductivity of the host. This phenomenon has been used extensively in heat mode optical data recording, where a finely focussed laser beam is absorbed by a dye and the heating effect produces a change in the polymer substrate, by melting or ablation. It has been shown, for example, that writing energies as low as 0.1 nanojoule per square micron can produce local temperatures up to ca, 300°C, sufficient to melt most plastics and dyes, if a dye layer absorbing some 99% of the incident energy is employed.(7) For maximum conversion of light energy to thermal energy by a dye, the dye should absorb as much of the light as possible, and, if broad band radiation is employed, then the naturally broad bands of most dyes are an advantage. However, if laser sources are employed, it is important to match the λmax of the dye (the wavelength at which maximum absorption occurs) as closely as possible to the laser wavelength. In addition, a narrow band dye is likely to have a much higher molar absorption coefficient at its λmax than a structurally similar dye with a broad absorption band, and consequently less dye will be needed to achieve maximum absorption of the incident light.

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Selection of Near-Infrared Dyes for Laser Welding

The ideal near-infrared dye for laser welding, whilst producing minimal marking in the visible region, would have the following attributes:

• A narrow absorption band near the laser wavelength with a high molar absorption coefficient.

• Little if any absorption in the region 400-700nm. • Good solubility in the host material. • Good stability towards the incorporation method

used. • Should not degrade to coloured by-products.

If all the known near-infrared dye systems are considered, the vast majority can be discounted on the grounds that they have pronounced visual colour. Others can be disregarded because of their instability or their low absorption intensity. Selection of the most appropriate candidates from the remainder then becomes a matter of trial and error. Examples of three dye types which can satisfy all of the above requirements are the cyanine dyes, e.g. (Fig.3), the squarylium dyes (Fig.4) and the croconium dyes (Fig.5). Fig.3. Example of cyanine dye [λmax = 785nm; εmax ≈ 360,000 1.mol-1.cm-1 in dichloromethane] Fig.4. Example of squarylium dye [λmax ca.800nm; εmax ≈ 150,000 1.mol-1.cm-1]

Fig.5. Example of croconium dye [λmaxca. 820nm; εmax ≈ 200,000 1.mol-1.cm-1]

Methods of Introducing Dyes for Laser Welding of Polymer Fabric Materials

When the incident laser light is absorbed, the dye molecules dissipate the absorbed energy principally as heat to the dye molecules and their local environment. In the case where the local environment is a thermoplastic polymer, melting occurs at the surface between the dye and the polymer. If a polymer that does not absorb near-infrared radiation but which may be clear or coloured, is adjacent to this surface, the melting will cause a weld to occur. Thus for overlap joining of polymer fabric material, the dye can either be incorporated between the two layers of fabric as a thin film, or incorporated into the bulk of the lower (with respect to the incident laser radiation) piece of material. In the latter case, only the dye at the upper surface of the material will be used in the joining process, as most dyes completely absorb the incident laser light in a depth of less than about 40 μm. In its simplest form, the dye can be dissolved in a suitable host and painted onto the joint line. More sophisticated versions of this technique would employ the use of stick-on films containing the dye. Dye concentrations of approximately 0.02% on a film weight basis are typically adequate but are a function of the particular dye used as well as the plastics being welded. A film thickness of 10-40μm is also typical. One advantage of using a film/tape containing the absorber dye, is that the film is only needed where a joint is required and, in solid form, such a tape can be easily handled and applied. If the dye is introduced into the bulk of the polymer fabric, clearly this has the advantage that any area subject to the incident laser light will be affected, however the cost of applying dye to the whole fabric could be uneconomic in some applications. The method employed in any production process would depend on the particular application and the restriction of added manufacturing steps and cost. Description of the Laser Welding Process The dye at the interface between the materials acts as the site where the light from the laser is absorbed and converted into heat in a well defined area. The materials to be joined are placed one on top of the other, and lightly clamped together. The area of heating and hence joining, may be defined by either the size of the laser beam or the extent of the dye containing region. In the experiments reported here, both Nd:YAG and diode laser light have been used. Both these laser wavelengths are easily transmitted via optical fibres, which enhances the flexibility of the process in industrial terms. Nd:YAG lasers are usually employed in a de-focus position to produce a spot of laser energy some few mm in diameter. The current methods of beam forming used with diode lasers, tend to produce a natural focus of the diode light which is rectangular in shape, a few mm by a few mm in size. This energy profile is almost ideal for the fabric welding process. The welding occurs as the heat generated in the dye is

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sufficient to melt of the order ≈ 0.1mm of the polymer fabric. The heat generation at the interface is controlled by the absorption coefficient of the dye layer and the processing parameters. The main parameters, for a given width of weld, are laser power, energy distribution in the focus, and the welding speed. A schematic of the process can be seen in Figure 6. In this case, an Nd:YAG laser is being used. Fig.6. Schematic representation of the transmission laser welding process using infrared dye.

Results Figure 7 shows continuous and hermetic overlap welds made in a waterproof fabric using this technique and an Nd:YAG laser beam of approximately 100W in power. The welding speed was 500mm/min. Fig.7. Continuous overlap welds made using infrared absorbing dye in a waterproof fabric. Figure 8 shows a cross-section through a similar overlap joint between two pieces of TactelTM, a nylon based fabric employing a polyurethane top layer, welded at 1500mm/min. In this case, both yellow and blue fabric have been used and the joint has been made between the uncoated surfaces. In these particular sections the upper polyurethane layer on the blue fabric is visible, as are some unmelted blue fibres. An homogenised melt region can be seen in the centre of the picture and lower down, unmelted fibres from the yellow material are visible. The presence of unmelted fibres means that a degree of the flexibility of the original fabric can be maintained after welding.

Fig.8. Cross section of a joint made utilising infrared absorbing dye. in the fabric TactelTM. Table 1 lists the results of mechanical testing of a range of lap joints made on a selection of woven fabrics manufactured from nylon 66.

Material Colour

Thickness (mm)

Peel Strength (N/mm)

Lap/Shear Strength (N/mm)

Parent Strength (N/mm)

Brown Orange Bronze Yellow

0.19 0.23 0.16 0.41

0.70 2.16 2.07 4.40

2.08 5.22 2.76 6.79

8.47 13.95 9.38 16.12

Table 1. Results of mechanical testing on a range of woven fabrics. For these experiments an Nd:YAG laser, with a 7mm diameter focal spot was used at powers between 50 and 100 watts. The welding speeds were in the range 500-1000mm/min. The peel and lap/shear tests were performed on 25mm wide samples at a test rate of 5mm/min. The test results are quoted as the maximum applied force per mm of seam. Each result is the average of three tests. It can be seen that, as a percentage of the strength of the parent materials, values between 25 and 40 percent were obtained for the welded joints in a simple lap configuration. Figure 9, shows the absorption coefficient of two particular infrared dyes (A and B) plotted against laser wavelength. The 808nm and 940nm wavelengths were obtained using absorption results from a diode laser and the 1064nm wavelength was obtained using an Nd:YAG laser. The figure shows clearly how wavelength sensitive the dyes can be and this sensitivity can be put to some interesting uses. The nature in which a diode laser is constructed makes it relatively easy to assemble a single laser, capable of operating at two separate or combined wavelengths, simply by switching the power to different stacks of diodes. TWI has such a diode laser capable of producing 150W of power at either 808nm or 940nm. 300W of power is available if both wavelengths are used simultaneously. Conveniently, both wavelengths are focussed with the same set of optics. Using this laser and two selected

translucent plastic sheets

incident defocused laser beam

clamping

almost colourless infrared absorbing film at interface between sheets

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dyes, it is possible to simultaneously weld three sections of plastic material or fabric together, as depicted in Figure 10.

Fig.9. Absorption coefficient of two infrared dyes as a function of laser wavelength. Dye B, highly absorbing at the shorter (808nm) wavelength, is applied to the top surface of the intermediate layer, whereas dye A, strongly absorbing at the longer (940nm) wavelength, but also able to transmit a significant fraction of laser power at the shorter (808mm) wavelength, is applied to the lower side of the intermediate piece. This assembly is then subjected to a single pass of a laser beam containing both wavelengths resulting in a triple layer weld. The technique has been applied to both solid plastics and fabrics, as can be seen in the weld section shown in Figure 11, made using 300W of laser power at a travel speed of about 500mm/minute. Fig.10. Schematic representation of a triple element weld, made using two infrared dyes and two laser wavelengths.

The material used was PMMA. Similar joint have also been made in a woven nylon based fabric covered with two layers of polyurethane. The triple weld was obtained by selectively melting the polyurethane layers.

Fig.11. Section through a triple layer joint made using infrared absorbing dye in 2mm thick sheets of clear PMMA.

Potential Applications The textile industry has the problem of producing hermetic seams in man-made fabrics. There are a wide range of applications which require such seams ranging from sports goods, industrial and safety clothing, sails, tents, inflatable goods and even balloon envelopes, which laser welding using an absorbing dye could address. The laser process also offers the possibility of automation in what is largely a manual labour based industry.

Conclusions Polymer fabric materials can now be laser welded using near infrared absorbing dye as a mechanism to produce heat and localised melting. The success of the technique has been demonstrated on a wide range of materials. The welds produced are cosmetically appealing and the upper and lower surfaces of the material are unaffected by the process. In mechanical testing, joint strengths of between 20 and 40 percent of the parent material strength have been achieved, in a simple lap joint. The welding process is efficiently achieved using the very compact diode laser sources now commercially available, and lends itself easily to high levels of automation. Potential applications of this technology exist in a wide variety of industry sectors.

---- -- ---- The process of laser welding using an infrared absorbing dye has been given the trademark ClearWeldTM. In addition, patent protection has been initiated by TWI on this process. Gentex Corporation, who manufacture suitable dyes for the technique, is licensed by TWI to exploit the ClearWeldTM process.

Laser absorption for IR absorbing dyes in acrylic film, based on mass of dye per unit area

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References 1. Wise R J: 'Thermal Welding of Polymers', Woodhead

Publishing Ltd, Cambridge, Oct 99. 2. Silvers H J Jr and Wachtell S: 'Perforating, welding

and cutting plastic films with a continuous CO2 laser'. PA State University, Eng. Proc, pp.88-97, August 1970.

3. Jones I A and Taylor N S : 'High speed welding of plastics using lasers', ANTEC '94 conference proceedings, 1-5 May 1994, San Francisco, USA.

4. Toyota Jidosha K K: 'Laser beam welding of plastic plates', Patent Application JP85213304, 26 September 1985.

5. Puetz H et al, 'Laser welding offers array of assembly advantages', Modern Plastics International, September 1997.

6. Jones I A, Hilton P A, Sallavanti R, Griffiths J: 'Use of infrared dyes for transmission laser welding of plastics', ICALEO 1999 Vol 87, Section B 1999, p 71-79.

7. Hurditch R: Colour Science '98, ed J Griffiths, University of Leeds Print Services, Leeds, 1999, p.202.