Adhesion of New Thermoplastic Materials Printed on Textile Fabrics

Combining 3D printing, especially fused deposition modelling (FDM) as a material extrusion technique, with textile fabrics can lead to full-layer composites as well as partly reinforced textiles with different mechanical properties at different positions. While the combination of both techniques enables the production of new kinds of objects different from common fibre-reinforced matrices, the adhesion between both materials is still challenging and the subject of intense research activities. Besides well-known setup and printing parameters, such as the distance between nozzle and fabric or the extrusion temperature, material combinations, in particular, strongly influence the adhesion between 3D printed polymer and textile fabric. In this study, we investigate composites of woven fabrics from cotton (CO), polyester (PES) and a material blend (CO/PES) with newly developed thermoplastic materials for FDM printing, and show that depending on the FDM polymer, the adhesion can differ by a factor of more than four for different blends, comparing highest and lowest adhesion.


Introduction
In recent years, 3D printing has emerged from a technique that necessitated expensive equipment and was mostly used for rapid prototyping towards a technology that is easily available at low costs, and that is also used for the rapid printing of single objects, from spare parts to gimmicks.Amongst the various additive manufacturing techniques, material extrusion, in particular FDM printing, has found its way into companies and laboratories, schools and private households due to its ease of use, non-toxic or low-toxic materials and inexpensive printers.Nevertheless, the mechanical properties of such FDM printed parts are usually weaker than those of injection moulded objects, since the layer-wise production process leads to a strong anisotropy and often induces air voids, further reducing stability [1−3].Commonly, two different approaches are reported in literature to improve the mechanical properties of FDM printed parts: on the one hand, new filaments with improved mechanical properties can be used, e.g.fibre-reinforced polymers [4][5][6]; on the other hand, macroscopic combinations with mechanically stronger materials are possible, such as textile fabrics.In the latter case, the adhesion between both materials is important to avoid the detaching of the polymer from the fabric under mechanical forces.When combining stereolithography (SLA) printed areas on textile fabrics, low-viscous resin can easily penetrate into even fine fabrics [7], while FDM printed objects often adhere better to thicker fabrics with relatively large pores [8,9].Besides the fabric structure, primarily the nozzle-fabric distance influences the adhesion due to form-locking connections [10,11], while the printing bed temperature [12,13], chemical pretreatments [14,15] or thermal post-treatments [16,17] can also influence adhesion.It has also been shown that soft thermoplastic materials printed with FDM often have a higher adhesion on textile fabrics than rigid ones and, amongst the latter, that poly(lactic acid) (PLA) demonstrates higher adhesion on examined textile fabrics than acrylonitrile butadiene styrene (ABS), while nylon has higher or lower adhesion than PLA, depending on the substrate [18,19].That is why this study investigates six newly developed FDM filaments, partly glass or carbon fibre-reinforced, invented by Grauts GmbH [20].We describe the filaments, having different elastic properties, and show their strongly varying adhesion on three different textile fabrics.

Materials and methods
The textile fabrics used in this study are depicted in Table 1.The pure CO and PES fabrics have a similar structure, while the CO/PES woven fabric is thicker and has a higher mass per unit area.The latter value was measured using an SE-202 analytical balance (VWR International GmbH, Darmstadt, Germany), while the fabric thickness values were measured using a J-40-T digital thickness gauge (Wolf-Messtechnik GmbH, Freiberg, Germany).
Printing was performed using an Orcabot XXL Pro 2 FDM printer (Prodim, The Netherlands), enabling printing with an extrusion temperature of up to 285 °C.The printing parameters for all filaments were as follows: nozzle diameter: 0.4 mm; layer height: 0.2 mm; unheated printing bed; 100% linear infill in ± 45° orientation; two perimeters; printing speed: 30 mm/s; and z-distance between nozzle and printing bed: 0.5 mm.A relatively high z-distance was chosen because it is well-known that near the optimum z-distance, even the smallest deviations have a large impact, while around the setting where the nozzle just slightly touches the fabric, this impact is almost negligible [10].Thus, the chosen z-distance facilitated the comparison of the slightly thicker CO/ PES fabric with thinner fabrics.The extrusion temperatures were optimized in pre-tests and were set in the range 230 °C to 250 °C, as depicted in Table 2.

Results and discussion
The results of the adhesion force measurements are depicted in Figure 1.Since the z-distance is not optimized, the values are generally smaller than possible with these material combinations.The largely small error bars, however, indicate the reliability of the measured values.Of all fabrics included in this study, the HPP 1444 filament shows the lowest adhesion.Much larger values are visible for HPP 1476, in particular, but also for the Mid GF 1461 and Mid GF 1470 glass-fibre reinforced filaments.It should be mentioned that Mid GF 1461 could not be printed properly on the PES fabric, so these values are omitted.Unexpectedly, the CO/PES woven fabric, although thicker than the pure CO and PES fabrics, mostly shows smaller adhesion values than the others, while no large differences between CO and PES are visible.However, most of these differences are insignificant.
To evaluate these results, they can be compared with other 3D printing filaments reported in literature, using results that were measured for the nozzle just touching the textile surface.Some results found in literature (approximated for a z-distance identical with the fabric thickness) are given in Table 3.As this comparison shows, the adhesion reached with the recently tested filaments is higher than some of the other results, but there are also adhesion forces more than twice the values measured in this study.One possible reason is that the new filaments tested in this study are high-temperature filaments, which should possibly be printed at even higher temperatures than in this study to reduce their viscosity during printing and enable deeper penetration into the textile fabrics under examination.Previous   results from literature show that the shift in optimum z-distance is strongly correlated with the extrusion temperature [21], meaning future tests with increased printing temperatures are necessary.
On the other hand, previous studies showed that the textile fabrics used also played an important role, not only due to their pore dimensions, but also with respect to the fibre lengths in the fabrics, where hairy fabrics with long -and thus well-fixed -fibres in the yarn resulted in higher adhesion than fabrics from short-staple yarns, where the fibres are more easily pulled out of the yarn and thus cannot fix the imprinted polymer layers properly.
To investigate this possibility, Figure 2 shows microscopic images of the detached back of different polymers printed on the cotton fabric under investigation here.On the back of the black filaments, white cotton fibres are clearly visible, especially on the sample from the Mid GF 1470 filament.This matches the results of the adhesion tests where both Mid GF filaments gave high values on the cotton fabric.For HPP 1444, only very few fibres are visible, while a more in-depth look at HPP 1476 shows several fibres, but with optically reduced contrast on the blue filament.None of the images on HPP 1444 reveals more than a few fibres, which is in line with the finding that this filament has the lowest adhesion of all three textile fabrics.It should be mentioned, however, that here only small parts of the printed samples are depicted and that variations of the amount of fibres stuck on the polymer are visible for all samples.While the filaments investigated here do not reach the maximum adhesion forces reported in previous studies, most of them (especially HPP 1476) show reliable adhesion on all three tested textile fabrics.Due to their good mechanical properties and high heat distortion temperature, compared with PLA, further experiments in combination with other fabrics, at higher extrusion temperatures and with optimized z-distance will be performed to enable the use of these filaments in composites for high-temperature applications or improved mechanical properties.

Conclusion and outlook
Six new filaments were FDM printed on different woven fabrics.Most of them showed reliable adhesion forces on PES, CO and CO/PES fabrics and fibre bundles attached to their back after separation during adhesion tests.The largest differences were found between two high-performance polyolefin materials, unexpectedly with the softest material (HPP 1444) having the lowest adhesion amongst the tested samples.The chosen substrates did not have a significant influence on adhesion to the thermoplastic materials printed on them.
Future tests will concentrate on optimizing the z-distance between fabric and printing nozzle, as well as the extrusion temperature, and on the investigation of more fabrics with different woven structures to further improve adhesion, so that applications, especially in high-temperature surroundings where PLA cannot be used, are enabled.

Figure 1 :
Figure 1: Adhesion forces between different textile fabrics (coloured) and novel FDM printing filaments

Figure 2 :
Figure 2: Back of the printed polymer, detached from cotton fabrics by adhesion tests according to EN 53530

Table 1 :
Textiles used in this study

Table 2 :
Thermoplastic materials for FDM processing and corresponding printing temperatures.HPP = highperformance polyolefin

Table 3 :
Values found in literature for adhesion forces, measured at or approximated for a z-distance identical to the fabric thickness.TPU = thermoplastic polyurethane (here with Shore hardness 86A), TPS = thermoplastic styrene (here with Shore hardness 67A and 79A)