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Rope finishing for braided ropes
Rope finishing for braided ropes M. Michael, C. Kern, J. Mammitzsch Technical University Chemnitz, Chemnitz, Germany
10.1
10
Introduction
Coating of textile semifinished parts is becoming increasingly important in technical applications. Whether manufacturing textile-based composite components or functionalizing textile semifinished parts for machine componentsdcoatings have become essential in the fabrication of textile elements for technical applications (Fig. 10.1). In the manufacturing of carbon fiber- and glass fiber-composite components, adhesion promoters or primers are applied (Tang and Kardos, 1997) to improve the adhesion of the polymer matrix to the textile-reinforcing structures. With the help of resins, which are cured during the process, near-net-shape semifinished products as well as tailored blanks are generated out of the textile structures as preforms. In the case of natural fiber-composite materials, eg, woodeplastic composites (WPC), the natural fibers are also coated with adhesion promoters to achieve an optimal adhesion to the polymer matrix (Sykacek et al., 2007; Bastian et al., 2005; Yang et al., 2006). Thermosetting is a process of thermomechanical treatment of textiles with multiple objectives. Beside adjustment of demanded dimensional accuracies, effects such as increase in strength, reduction of structural elongation, decrease of shrinkage during
Figure 10.1 Textile elevator rope (prototype) with coating.
Advances in Braiding Technology. http://dx.doi.org/10.1016/B978-0-08-100407-4.00010-7 Copyright © 2016 Elsevier Ltd. All rights reserved.
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heat input and others are attainable (Heinze and Mammitzsch, 2011; Heinze, 2013; Mammitzsch and Kern, 2014; Mammitzsch, 2015; Michael et al., 2013;V€olker and Br€uckner, 2006).
10.2 10.2.1
Rope finishing Coating
For manufacturing of textile mechanical components using textile-based semifinished products, a multitude of requirements must be covered by the coating materials. In particular, for fiber ropes, belts, ribbons and other textile tensional members, partly contrary requirements exist that the coatings must meet. Thus, intended high coefficients of friction are usually demanded on the textile surface to guarantee certain friction ratios between the textile and other mechanical components. At the same time, high wear resistance is required. In contrast, low coefficients of friction are preferred between the basic single-textile elements (yarns, strands), to produce as little wear as possible during relative movements between them (Mammitzsch et al., 2009a,b). Additional requirements, for instance fire protection regulations or guidelines concerning food tolerance, partly conflict with the characteristics of the coating materials, which leads to an addition of excipients or additives.
10.2.1.1 Coating procedures for technical textiles Various coating procedures have been established depending on the coating material, its aggregate phase, the desired property changes and the geometry of the textile work piece. A selection of these procedures is presented in the following.
Immersion bath/foulard For coating in an immersion bath, the textile is led through a fluid coating liquid, usually load free or under low preload, which penetrates the textile and adheres to the fibers. By controlling the viscosity of the coating liquid, the penetration of the textile can be realized similar to the application of a polymeric jacket. In general, dispersions, solutions and molten mass can be processed (Mammitzsch et al., 2009a,b). However, the different coating systems also entail different demands on the coating technology. In processing solutions, it is important to pay attention not only to optimal adjustment of the solution viscosity, but also the evaporation of solvents due to the danger of explosion. Further, solvent evaporation can change the polymer concentration and thus the solution viscosity. As a result, coating plants working with solvent-based systems need to have not only exhaust-gas escape units, but also explosion-proof drives and electronic components (Standards 94/9EG, 1994) as well as a system to measure the solution concentration, including solvent supply and circulation systems, to obtain optimal results. In the processing of polymer melts, the monitoring and regulation of the temperature of the molten mass is, with respect to the control of the viscosity, highly important
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for optimal penetration. Moreover, a cooling zone has to be provided for cooling down and, if necessary, for the calibration of the coated textile product. Due to technical challenges in the processing of polymer melt and polymer solutions, increasingly strict regulations (eg, EU regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH)) on the emission of plasticizers and additives from polymers, oils and dispersions preferred as coating agents for coating-immersion baths have been established (Standards 1907/2006, 2006). In many cases, foulard systems are connected to the coating basins for the coating of technical textiles using dispersions (cf. Fig. 10.2). These systems usually consist of rubberized pressing rollers and serve either the squeezing of surplus lowviscosity liquid or the pressing-in of high-viscosity liquid. In both cases, these foulards are used to control the quantity of coating applied during the coating process (Mammitzsch, 2010). In the processing of solutions and dispersions, a drying phase following the coating process is required. Depending on the coating material (which may influence the heat conductivity), dimensions and/or geometry of the textile and processing speed, large fabric contents might be required in the dryers for inline drying in a continuous production process to be able to guarantee the process times necessary for complete drying. As a source of energy, hot air, infrared radiation and microwave radiation are common systems to extract solvents and/or water from the textile (Mammitzsch, 2010). Recent developments are concerned with ultraviolet (UV) emitters and pulsed LASER as sources of energy for the drying process.
Laminating Laminating signifies a coating procedure in the textile industry, which is primarily applied to planar textiles and is usually used for manufacturing textile-based preforms and even composite components. For this purpose, textile webbings are merged with lamination agents between heated or cooled laminating rollers (Fig. 10.3), and are delivered mostly in the form of tapes. Powder- or foam-like lamination agents are added by means of a backing paper. Under pressure (and temperature, if necessary),
Figure 10.2 Schematic plot of a coating bath with foulard.
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Figure 10.3 Graphic presentation of the lamination procedure.
substrate and lamination agent are tightly interconnected so that a web with a structure of at least two layers emerges (Giessmann, 2010). When laminating textiles from thermoplastic fibers, the processing temperatures must not reduce the strength of the fibers and thus the textile structure. Depending on the type of textile substrate, various special machines with particular roll systems and lamination arrangements are available.
Extrusion The process of extrusion, which is already established in the production of composites (eg, WPC components; short fiber- and particle-reinforced plastic tubes, textile-reinforced hoses, etc.), should still be considered a relatively new process in the case of coating for technical textiles. A thermoplastic coating material provided as powder or granules is plasticized or melted in an extruder, and continuously pressed through a die. A distinction is made between the low-pressure method (hose extrusion) and the high-pressure method, which are primarily differentiated by the processing pressure. During the hose-extrusion process, the molten polymer is laid on the surface of the textile, and no penetration of the textile structure is achieved. In doing so, the extrusion layer can easily be peeled off the textile structure due to bending and tensile stresses (Streubel et al., 2014), and can have an additional damaging effect on the textile. In the high-pressure method, the molten polymer can penetrate into the textile structure because of a high processing pressure in the die. Proceeding this way, mechanical interlocking effects work in addition to adhesion, due to the shrinking of the extrusion layer in the event of cooling. The high-pressure method is the preferred method for applying extruded jackets on technical textiles. To enable a continuous processing of endless textiles, the polymer is fed into a crosshead die, which contains the molding tool, after escaping the extruder (cf. Fig. 10.4). Due to this system, the tools can be flexibly adapted to the geometry of the textile, which can, in addition to the jacketing of narrow fabrics (eg, car seat belts; tension
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Extruder
Fiber rope
Crosshead die
Figure 10.4 Crosshead die for the jacketing of fiber ropes.
belts, etc.), also enable the jacketing of circular braids, multilayer fabrics and spacer fabrics (Streubel et al., 2014).
Pultrusion Pultrusion is a method used for manufacturing of textile-based composite semifinished products and near-net-shape composite components. In this method, the textile structure is impregnated with a resin, cured in a heatable die and simultaneously calibrated. To enable continuous production of endless semifinished products, the textile preforms are usually withdrawn from spools. It is also possible to set up a pultrusion route directly at the end of the textile-processing machinery (eg, radial braider, cf. Fig. 10.5) to immediately process the product.
10.2.1.2 Coating materials for technical textiles For the coating of technical textiles, a variety of coating materials are available. These coatings can be classified according to their function in the textile-based component or semifinished product.
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Resin bath
Preformer
Puller
Figure 10.5 Schema of the pultrusion process.
Adhesion promoter In textiles, adhesion promoters are used for guaranteeing an optimal physical and/or chemical connection of the textile to further coating materials. They are preferably applied on nonpolar fiber materials (eg, polyethylene and polypropylene) to enable the connection of polar coating materials (eg, polyurethanes, polyester resins, polyvinyl alcohols), because a direct adhesion between polar and nonpolar polymers is unlikely. Modified silanes with organic chains, which resemble 1-propene, are well-known organic adhesion promoters for polypropylene fibers. Intermolecular bonds can be constructed between the polypropylene chains and the silane, whereas a polar coating can attach itself to the polar, functional groups (eg, methoxy and/or methylamino groups). For polyethylene materials, imines (eg, polyethylenimine) are frequently applied. They are similar to polyethylene in their chain structure, which is why dispersion forces can establish here. Imines consist of NeH groups, which are, on the one hand, moderately polar and thus enable a physical connection of polar materials. On the other hand, the aforementioned NeH groups are protonizable so that also chemical connections (eg, acidebase reactions) are possible. Organic and organosilicon adhesion promoters, as well as metaleorganic composites based on titanium or zirconium, are known as adhesion promoters on olefinic materials. They also enable dispersion forces on the fibers and therefore promote physical connections to the coating materials.
Lubricants Lubricants fulfill multiple tasks in textile structures. Thus, so-called fiber sizings are applied on the fibers to prevent agglomeration of the filaments in the yarn production process and to protect the fiber against wear during further textile processing. As a result, metallic and ceramic loops in the processing machines are also protected. Many of those sizings work as lubricants in the textile to reduce fiberefiber friction and the resulting wear. In specific cases, additional lubricants are applied to textiles. Primarily waxes (mostly paraffin, olefin or polyamide) or short-chain oils (eg, silicone oils, paraffin oils and others) are used.
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The lubrication effect of the waxes adhering to the fibers is primarily due to low shear strength, which leads to decomposition of the lubricant films in the shear zone, thus enabling sliding. Only when the melting temperature is exceeded (eg, because of high shear velocities or high application temperatures), the waxes start working, similar to an oil, by the production of a hydrodynamic lubricant film, which separates the contact surfaces. Similar to the case of short-chain oils, it can happen that, due to high compression in the application, the hydrodynamic film is broken, and the fibers, which are actually supposed to be separated, come into direct contact with each other. This is why oils are usually applied when high-frequency load changes and low compressions are expected, whereas waxes are applied when low-frequency load changes with high local compressions are supposed to occur. For the optimization or adjustment of the lubricants to certain application requirements, chemically consimilar oils and waxes are often mixed and, as the circumstances require, soaps might be added in addition. Moreover, polytetrafluorethylene powders (particle diameter approximately 500 mm) can be admixed.
10.2.1.3 Functional coatings Functional coatings are applied for adjustment of certain properties in technical textiles and for improving their efficiency in particular applications. Thermoplastic polyurethane elastomers (TPUs) are a common coating material. Due to their chemical structure, TPUs are variably adjustable so that they can not only coat films with hardebrittle properties, but coating of viscoplastic or soft-resilient properties are also possible. Further, several resin systems, eg, for the production of composites, are applied as functional coatings using pultrusion.
Thermoplastic polyurethane elastomers TPUs, when applied on technical textiles, are used either as dispersions for application from the aqueous phase or in the form of powders or granulate materials for thermoplastic processing. Polyurethane dispersions offer the advantage that they are not only applicable for full penetration of the textile, but also for surface treatment because of the simple procedure of dip coating (with and without foulard). For this purpose, only the viscosity of the supplied dispersions needs to be adapted. With the addition of adhesion promoters and additives, polyurethanes can be adjusted in such a way that they also adhere to nonpolar fiber materials or are even suited for chemically different fibers. In combination with other additives, eg, waxes, oils or silicones, it is possible to generate abrasion-resistant, wear-resistant protective films from polyurethane dispersions on technical textiles. A disadvantage is that the drying parameters in the subsequent drying process need to adapt to the melting temperature of the respective polyurethane and, as circumstances require, also to accessory agents/additives. Powders and granulate materials are primarily applied in the production of jackets or abrasion-resistant cover layers. A full penetration of the textile work piece cannot be achieved, but very high coefficients of friction for good wear-protection properties can be generated on the textile surfaces. Various systems are available, which feature the
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property profiles from hard and brittle to soft and resilient. Thus, a polyurethane type can be found for almost any desired surface property. By additional compounding with additives or short fibers, TPU granulated materials or powders can be further adapted to the requirements of the final application. Two disadvantages are very cost-intensive processing technology with precise temperature control and the necessity to cut process times to a minimum to avoid damaging the fibers. The necessary cooling causes additional technical efforts.
Resins for composites and products pre-impregnated with resin Numerous resin systems, selected according to the desired characteristics of the product, are known for the manufacturing of composites. Unsaturated polyester resins (also UP resins) are used to produce hard matrix. Hardness can be controlled to a certain extent by adding a curing agent, which connects the resins. Undercured UP resins display higher elasticity and longer pot lives but might remain sticky on the surface in some cases. Fast-curing systems (eg, with a cobalt accelerator) are available, but these display short pot lives (approximately 15 min) and are already completely cured after a few hours without the need of supplying thermal energy. Saturated polyester resins (eg, from aliphatic dicarboxylic acids and diols or diethylene glycol) are only used in the rare case of an elastic matrix. Epoxy resins also provide a hard and stiff resin matrix. In contrast to UP resins, an exact dosage of resin and curing agent is necessary. Cold-curing and heat-curing systems are known. The curing is exothermic so that additional heat is generated in the resin. This can cause thermal damage or, in the worst case, lead to fire outbreaks. For that reason, the processing temperatures need careful monitoring during the curing process. Phenolic resins (also phenoleformaldehyde resins) are primarily used for manufacturing heat-resistant molding batches. Storable fluid or meltable precursors are available, which can only be cured at higher temperatures while adding formaldehyde. Formaldehyde presents a health risk, which is why attention has to be paid to reliable respiratory protection and exhaust ventilation. Beside the aforementioned coatings, further coatings in the form of UV absorbers, antistatic agents and flame retardants are known. Applied as additives, they can be provided for almost every functional coating or directly admixed to the fiber base material during fiber production.
10.2.2
Thermosetting of fiber ropes
For fiber ropes, thermosetting is an integral production stage, primarily to increase the strength of the resulting fiber ropes while reducing operating elongation (Heinze, 2013; Mammitzsch and Kern, 2014; Michael et al., 2013). The ropes are loaded with a tensile load and heated to a material-dependent temperature. Heating the ropes through to the core is ensured via a warm-up period. To improve optimal strength, dwell times after the total heat through of approximately 3 min are specified in the literature (Heinze and Mammitzsch, 2011; Michael et al., 2013).
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10.2.2.1 Process of thermosetting Fig. 10.6 demonstrates the schematic structure of a continuously working stretching unit in combination with a heating section to heat the free-rope segment, that can be used for continuous thermosetting of fiber ropes. Comparable systems established for the finishing of ropes are distributed by various machinery manufacturers as series machines, especially developed for finishing and thermalemechanical treating of fiber ropes. The load, required for thermosetting, is generated via a speed difference between the hold mangle and the pull mangle. The embracement, due to wrap friction, ensures that the incoming and outgoing rope just needs to be applied with a low preload to generate the necessary thermosetting tension between hold mangle and pull mangle. Sufficiently large fabric content in the heating section can guarantee that, regardless of high processing speeds and large rope diameters, the rope (after heat-through) can remain exposed for approximately 3 min to load and thermal energy. Hot air, as well as oil and water baths, has prevailed as a suitable medium for heat transmission in the processing of textiles. In fiber ropes, the subsequent removal of oil or the drying of the ropes is necessary, which is why procedures with hot air are widely established. During thermosetting of fiber ropes, several effects are superimposed, which lead to an increase of the tensile strength of the ropes and a simultaneous decrease in elongation. The first effect is due to the adjustment of irregularities in the fiber rope, caused by manufacturing tolerances. Scientific studies (eg, Heinze, 2013) have ascertained that the load cycles before testing until rupture of the rope set the rope structure. These studies followed the test standards for determination of the break load of fiber ropes (cf. Standards DIN EN ISO 2307:2011, 2011). This provided for a more consistent load distribution in the rope, leading to an increase of the measured break load and simultaneously to a decrease of the standard deviation of the measured data. Comparable effects are achieved when loading ropes during the thermosetting process.
Grooved deflection sheaves
Fiber rope
Hold mangle
Pull mangle
Figure 10.6 Scheme of a stretching unit for the continuous thermosetting of fiber ropes.
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This effect is superimposed by the heating to a material-specific thermosetting temperature. In published investigations of twisted yarns, the thesis was put forward that a change of the fiber morphology takes place (Heinze and Mammitzsch, 2011). In other published investigations, treated twisted yarns unraveled after thermosetting, and the filaments were loaded along the fiber axis in the course of a dynamicemechanical analysis (Heinze, 2013). In doing so, an increase of the material damping and a decrease in Young’s modulus have been determined (cf. Heinze, 2013). These effects can be justified in that the orientation of the molecules no longer conformed with the fiber longitudinal axis after the thermosetting of the twisted yarns, which substantiates the thesis of the modified-fiber morphology. Because the temperatures during thermosetting are lower than the melting temperature and thus far beneath the decomposition temperature, processes of a physical nature can take place during thermosetting. Chemical reactions can be avoided, because no reagents are added and because polyethylene is a material with good chemical resistance (Bargel and Schulze, 2000; Domininghaus, 2005). As test facility for the execution of the thermosetting experiments, the plant “Rope Liner” of the MAGEBA Textilmaschinen GmbH & Co.KG (cf. Fig. 10.7) available at Chemnitz University of Technology (Technische Universit€at Chemnitz) was used. This plant is meant for the finishing, coating and thermosetting of fiber ropes. The coating plant is divided into individual aggregates, which can be controlled separately. This enables, eg, the bridging of the dryer if the thermosetting is carried out without coating. The plant is a custom-built model optimized for laboratory and small-series operations, which offers the full-service spectrum of a large-scale plant. Solely with regard to the maximum operating speed, the plant is limited to a maximum processing speed of 5 m/min. Infrared-dryer Microwave dryer
Pull-off unit Coating unit
Pull mangle Hold mangle
Hot-air oven
Cooling zone with load cell
Figure 10.7 Thermosetting plant rope liner.
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The plant includes a coating unit with foulard, which is bypassed for the thermosetting tests. The connected infrared dryer was deactivated, but could not be bridged to maintain a certain pretension in the ropes. As already described above, this plant generates the load transmission to the fiber rope via a speed difference between hold mangle and pull mangle. The plant allows for two modes of operation: In the position-controlled operational mode, a specified speed difference is adjusted (eg, hold mangle is 10% faster than the pull mangle). This principally allows a constant stretch ratio of the rope that is to be treated. A problem that appears in this operating mode is that ropes composed of polyethylene fibers exhibit a low coefficient of friction on steel (mH z 0.08; Michael, 2010), which can lead to the emergence of slippage between the deflection sheaves in hold and pull mangle and the rope. This slippage can cause a decrease in the applied load, which leads to varying strength in the treated areas of the rope, because an optimal result of thermosetting always requires an optimal combination of thermosetting load and temperature (Heinze and Mammitzsch, 2011; Mammitzsch and Kern, 2014). To be able to adjust such traction-related load variations during operation, the thermosetting plant is additionally equipped with a load-controlled operational mode. In this mode, the rope load is measured via an integrated load cell. The operator sets a target value for the thermosetting load and the control system of the machine automatically readjusts the speed difference between hold and put mangle. The advantage of such an operational mode is that transient slippage effects are compensated by deliberate intrusion of the control system.
10.2.2.2 Test parameters For the conducted experiments, the load-controlled operational mode was chosen. Because only short rope sections with a length of approximately 35 m have been treated, it was essential to obtain as consistent thermosetting results as possible on these short lengths. The thermosetting load was set to 12% of the break load of the ropes. The thermosetting temperature varied between 130 and 150 C in steps of 5 K. In the heating tunnel, fabric contents of 12.5 m were installed, which, under consideration of a dwell time of 3 min and heating time of approximately 2 min (to a temperature of 130 C), led to a machine speed of 2.5 m/min. For temperatures above 140 C, the machine speed was reduced to 2.4 m/min, which increased the retention time in the heating tunnel by 12.5 s. The summarized test parameters are found in Table 10.1.
10.2.2.3 Findings, results and comparison The following results are the findings in the thermosetting of fiber ropes made from selected ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers. Because the increase in break load caused by the rope diameter decreasing during thermosetting is not meaningful, the results of the tensile tests have been related to the linear density to receive a specific strength, as presented in Figs. 10.8e10.11. During thermosetting, an interesting effect appeared in all rope types, in which the strength did
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Table 10.1
Test parameters during thermosetting Value
Relative thermosetting load
12% of breaking load (F index B)
Thermosetting temperatures
130, 135, 140, 145, 150 C
Machine speed
2.5 m/min (up to 140 C) 2.4 m/min (up to 140 C)
1000
Specific strength (N/ktex) 1500 2000
2500
Parameter
Untreated
130
135
140
145
150
Thermosetting temperature (°C)
2000 1500 1000
Specific strength (N/ktex)
2500
Figure 10.8 Specific strength of the ropes made of Dyneema® SK75 after thermosetting.
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
Figure 10.9 Specific strength of the ropes made of Spectra® S1000 after thermosetting.
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2000 1500 1000
Specific strength (N/ktex)
2500
257
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
1000
Specific strength (N/ktex) 1500 2000
2500
Figure 10.10 Specific strength of the ropes made of TNX® after thermosetting.
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
Figure 10.11 Specific strength of the ropes made of Eosten® FT093 after thermosetting.
not continuously increase with the rise of the thermosetting temperature. All UHMW-PE fibers show a drop in specific strength at a thermosetting temperature of 135 C. However, the strength is still clearly higher after the thermosetting at 135 C than without thermosetting. If the thermosetting temperature was raised further up to 145 C, a further increase in specific strength was found for all fibers. The ropes made from Spectra® S1000 und TNX® showed a further increase in specific strength as the thermosetting temperature was raised, whereas the specific strength of the fibers Dyneema® SK75 und Eosten® FT093 turned out slightly lower in comparison to the values achieved at 145 C.
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The samples showing the highest increase in specific strength in N/ktex have been taken for a following density determination, as described previously in Section 3.3.2 of chapter Three-dimensional (3D)-maypole braiding, for calculating the area of cross section according to equations in Chapter 10. For this purpose, the linear density of the ropes was determined after thermosetting again. For the determination of the density, individual strands have been extracted from the treated ropes and prepared accordingly. All fibers showed a decrease in density during thermosetting processes with an increasing thermosetting temperature (cf. Table 10.2). In Heinze and Mammitzsch (2011), Heinze raises the theory that thermosetting of fiber ropes made of UHMW-PE fibers leads to a change of the fiber morphology in the filaments. In further investigations, this thesis is extended by Heinze in that the orientation of the molecules along the axis decreases. Heinze supports his thesis with the results of a dynamicemechanical analysis (Heinze, 2013). The density of a material is primarily determined through the bonding energy in the material. In the case of polymers which contain neither multiple bonds nor functional
Selected samples and corresponding thermosetting temperatures Table 10.2
Material
Thermosetting temperature (8C)
Density (g/cm3)
Dyneema® SK75
Without
0.975
130
0.943
145
0.931
150
0.935
Without
0.9671
130
0.929
145
0.936
150
0.932
Without
0.984
130
0.902
145
0.917
150
0.917
Without
0.9345
130
0.912
140
0.918
150
0.929
®
Spectra S1000
TNX
®
®
Eosten FT093
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2000 1500 1000
Tensile strength (N/mm2)
2500
groups, similar to the present case of PE fibers, the bonds between the molecule chains, which are based on the van der Waals forces, are dependent on the distance between the molecule chains and thus are also dependent on the morphology (Bargel and Schulze, 2000; Domininghaus, 2005). From the decreasing density, it can be concluded that a change of the orientation in the polymer fibers occurs because higher bond lengths indicate that the distance between the molecule chains increases, which again means larger space requirements of a certain number of molecules, thus describing a lower density of material. On the basis of the density values shown in Table 10.2, the break loads from the tensile tests have been converted into break strengths of the treated ropes. Figs. 10.12e10.15 show the break strengths of the ropes, which have been gained by thermosetting, in comparison to the values without thermomechanical treatment. The results of thermosetting at 130 C confirm the data shown in Heinze (2013) for the fiber Dyneema® SK75. After thermosetting at temperatures above 130 C, it was not possible to achieve any higher strengths for the fiber Dyneema® SK75. Further, the thermosetting temperature to gain the highest possible strength increases is not the same for all fibers made of UHMW-PE. Generally, increases in strength have been obtained for all fibers at a thermosetting temperature of 130 C. Some fibers, however, show an additional significantly increasing strength at higher thermosetting temperatures. It is not possible without further considerations, to derive any statement from the break strengths presented in the aforementioned figures regarding achievable increases in strength, because, eg, the pitch length of braided fiber ropes influences the initial strength of the rope before thermosetting (McKenna et al., 2004), but also changes significantly during the thermosetting process. Table 10.3 shows the change of the
Untreated
130
145
150
Thermosetting temperature (°C)
Figure 10.12 Break strength of the ropes made of Dyneema® SK75 after thermosetting at temperatures above 130 C.
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2000 1500 1000
Tensile strength (N/mm2)
2500
260
Untreated
145 130 Thermosetting temperature (°C)
150
2000 1500 1000
Tensile strength (N/mm2)
2500
Figure 10.13 Break strength of the ropes made of Spectra® S1000 after thermosetting at temperatures above 130 C.
Untreated
130
145
150
Thermosetting temperature (°C)
Figure 10.14 Break strength of the ropes made of TNX® after thermosetting at temperatures above 130 C.
pitch length due to the variation of the thermosetting temperature for the examined materials. In this regard, the percentage increase of the pitch lengths is related to the truly existing pitch lengths of the ropes in their initial states (cf. Section 4.2.1 of chapter Cartesian 3D braiding) and not to the predefined rope parameters. As can be seen, the pitch length increases up to 14.7% with raised thermosetting temperature. In this context, according to Heinze (2013), an increase of the pitch length
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2000 1500 1000
Tensile strength (N/mm2)
2500
261
Untreated
150
140
130
Thermosetting temperature (°C)
Figure 10.15 Break strength of the ropes made of Eosten® FT093 after thermosetting at temperatures above 130 C.
Table 10.3
Increase in pitch length through thermosetting Relative increase in pitch length (%) after thermosetting
Material ®
Dyneema SK75 ®
Spectra S1000 ®
TNX
®
Eosten FT093
1308C
1358C
1408C
1458C
1508C
4.0e4.8
8.8e9.4
9.0e9.9
9.8e10.4
11.3e12.4
4.0e5.1
8.3e8.6
8.8e9.6
9.5e10.1
12.5e13.2
5.5e6.3
7.5e8.1
8.5e9.4
9.5e10.4
11.5e12.2
5.8e6.3
8.5e9.4
8.8e9.6
9.0e10.1
13.8e14.7
of 5% entails an increase of the initial strength of an untreated, braided rope made of Dyneema® SK75 of approximately 2%, whereas an increase of the pitch length of 15% implies an increase of the initial strength of the rope made of Dyneema® SK75 of approximately 6%. Moreover, the load-bearing cross section of the ropes changes. As a result, statements on the absolute increase of the strength are not possible. To generate this information, ropes would have to be braided again conforming to the treated ropes with regard to twist per meter, pitch length and linear density. In the case of such ropes, the break strength has to be determined in an untreated condition and compared to the break strengths obtained here during thermosetting. Nevertheless, some insights can be derived from the available data. It is, for example, recognizable that strength increases of approximately 25% can be achieved for the fiber Dyneema® SK75, whereas for the fiber Spectra® S1000 strength only increases by approximately 12%. However, the break strength of the ropes made of
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Dyneema® SK75 and Spectra® S1000 displays almost identical values after thermosetting, because the ropes made of Spectra® S1000 provided a higher strength before thermosetting. In the case of the TNX® fibers, strength increases of 10% and for fibers made of Eosten® FT093, strength increases of 15% have been reached. Because literature on this subject (eg, Heinze and Mammitzsch, 2011) shows that the results during thermosetting not only depend on the temperature, but also on the thermosetting tension. The 12% relative thermosetting tension, which has been chosen for the tests, is only known to be optimum for the fiber Dyneema® SK75. Further investigations need to be carried out for the present UHMW-PE fibers. Initial tests have already showed that for ropes made of the fiber Eosten® FT093, better results could be obtained at a temperature of 150 C and a relative thermosetting tension of 18%. Further rope finishing procedures are, eg, steaming and assembly, which should be mentioned here without any claim to completeness. Steaming is usually only a common procedure for nylon fiber ropes and shall not be further considered here (McKenna et al., 2004). The process step of making-up concerns all textile products.
10.3
Future trends in rope finishing
Due to the textile structure of fiber ropes, present conventional technologies for coating and finishing of ropes meet their limits. The complete penetration of fiber ropes with coating liquid at diameters larger than 20 mm is insufficient because coating, in the case of mass production, is always a compromise between the highest possible processing speed and optimal penetration. By increasing the processing speed of a fiber rope through an immersion bath, which has established itself on the market as state of the art, critical flow effects increasingly emerge at the surface of the rope, which impede or even prevent a penetration of the rope core. A further problem arises during the total time needed for heating ropes with large diameters, which are to be treated through to the core. During this process, hot-air ovens are used because heating in water requires postdrying and thus an additional heating process, the effects of which on the results of thermosetting have not yet been fully clarified. Heating in oil baths requires an additional cleaning process including postdrying, which, on the one hand, is economically nonfeasible and, on the other hand, raises the issue of unexplained influences of repeated heating during drying on the results of thermosetting. Based on coating in an immersing bath as a state-of-the-art process, modified and new procedures for the coating and drying of ropes have been developed. New approaches are, for instance, the integration of ultrasonic technology in the coating tank to facilitate the penetration of the coating into the core of the textile. One problem to be considered is the thermal energy input into the water due to ultrasonic waves, which leads to the vaporization of the water and thus to a change in the concentration of the polymer in the coating liquid. Due to these circumstances, a complex system for the monitoring and regulation of the temperature and concentration of the coating liquid is inevitable.
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A further promising approach is the use of annular-gap nozzles for injecting the required amount of coating material via forced dosage into the rope. At this point, the rope structure characterized by surface irregularities poses a challenge, because forced dosage with small overpressures requires a certain density between the ring-shaped nozzle and the fiber rope. Another option would be to apply the coating liquid in a modified vacuume infiltration process. The challenge here would be the realization of a continuous process with the needed sealing against ambient air. For drying ropes that are coated with a more effective procedure, an optimized drying technology also needs to be developed, because the coating liquids are mostly water based. In this way, not only is there a higher amount of coating material, but also significantly more water penetrates the core of the rope, which needs to be fully evaporated in the drying process. A special challenge for future technical developments is the thermosetting of ropes with large diameters d (d is the rope diameter). Due to the textile structure, the air-filled cavities for which delay heating to the core because of insulation effects, very long heating times are required, which only enable a high processing speed if the fabric content in the dryer is sufficiently high. Such high fabric contents, which can easily be 350 m or more, in combination with critical bending diameters D (D is the diameter of the deflection sheave) that preferably should not drop below a ratio of D/d¼10, require, require thermosetting plants with an enormous installation space in the case of ropes with diameters larger than 40 mm. However, this method is not only economically nonfeasible because of the required production area and the energy costs that arise from the large ovens, but it also restricts the flexibility of the usage of the present operating areas (eg, for internal logistics) and thus has counterproductive effects on the demanded maximum flexibility of the companies in a competitive global environment. Ultrasonic technology offers the possibility to realize an energetically meaningful heating in a short time via the direct coupling of ultrasonic waves into the rope. The high processing speeds combined with the high efficiency that is generated by ultrasound technology can positively influence economic viability, so that cost advantages over conventional technologies can be achieved. The developers of finishing technologies will face major challenges during the next few years, when increasing the efficiency of coating systems becomes more important. A successful implementation of a more effective procedure will not only positively influence rope making, but can, at the same time, bring along a dominant market position for the machine developer.
10.4
Conclusion
With regard to rope finishing, thermosetting has become an indispensable part of the process chain of manufacturing of high-strength textile ropes for mechanical engineering applications. Further, the materials of one and the same material group do not necessarily react in the same way to the processing parameters. As could be shown, the processing
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parameters and the obtainable improvements in strength vary considerably within the group of UHMW-PE fibers. The same also applies to the aramid fibers. Here, fibers made of aramid copolymer show a different behavior compared to fibers made of poly-paraphenylene terephthalamide (PPTA), and it can be anticipated that such big differences also occur between PPTA fibers of different manufacturers, as it was shown for the fibers made of UHMW-PE. If thermosetting is supposed to be applied on a textile product to benefit from the advantages of the procedure, it is advisable to conduct sufficient preliminary tests regarding optimal temperatures, dwell times and thermosetting tensions on the particular textile. Existing publications on similar textiles made of similar materials can only serve as a working basis for the clarification of the field of investigation, because not only the material, but also the textile basic structure has an influence on the result.
References Bargel, H.-J., Schulze, G., 2000. Werkstoffkunde, 7. Auflage. Axel Springer Verlag. Bastian, M., Radovanovic, I., Kurda, K., 2005. Einfluss der Haftvermittler. In: Kunststoffe, vol. 8/2005. Hanser Verlag. Domininghaus, H., 2005. Die Kunststoffe und ihre Eigenschaften. In: Auflage, 6. Springer Verlag, Berlin. Giessmann, A., 2010. Substrat- und Textilbeschichtung, second ed. Springer Verlag Heidelberg. Heinze, T., 2013. Zug- und biegewechselbeanspruchte Seilgeflechte aus hochfesten Polymerfasern (Dissertation TU Chemnitz). Heinze, T., Mammitzsch, J., 2011. Thermofixieren von Seilen aus hochfesten Synthesefasern. Logist. e-J. Mammitzsch, J., 2010. Coating of synthetic fibre ropes. In: 4. Fachkolloquium Innozug, Proceedings. Technische Universit€at Chemnitz, pp. 160e161. Mammitzsch, J., 2015. Untersuchungen zum Einsatz von ultrahochmolekularen Polyethylenfasern in Seilen f€ur die F€ordertechnik (Dissertation TU Chemnitz). Mammitzsch, J., Kern, C., 2014. Potentials of thermosetting for increasing the strength of HMPE fibre ropes. In: 4th International Fiber Applications Conference. Antwerpen/Belgien. Mammitzsch, J., Kunz, M., Michael, M., Nendel, K., 2009a. Anwendungsspezifische Beschichtungen f€ur Faserseile im Maschinenbau. In: 12. Chemnitzer Textiltechnik-Tagung. Technische Universit€at Chemnitz. Mammitzsch, J., Michael, M., Heinze, T., Nendel, K., 2009b. Beschichtungen zur Funktionalisierung von Kunststoff-Faserseilen f€ur die F€ ordertechnik. Technomer 2009. Technische Universit€at Chemnitz. McKenna, H.A., Hearle, J.W.S., O’Hear, N., 2004. Handbook of Fibre Rope Technology. Woodhead publishing limited. Michael, M., 2010. Beitrag zur Treibf€ahigkeit von hochfesten, synthetischen Faserseilen (Dissertation; Technische Universit€at Chemnitz). Michael, M., Kurtz, P., Hillig, R., Mammitzsch, J., 2013. Optimization of Mechanical Properties of HMPE Fiber Ropes by Thermosetting. TexComp11; Leuven/Belgien. Standards DIN EN ISO 2307:2011, 2011. Faserseile - Bestimmung einiger physikalischer und mechanischer Eigenschaften. Beuth Verlag Berlin.
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Standards 94/9EG, 1994. Richtlinie 94/9EG e Atmospheres Explosibles. Europ€aisches Parlament und Europ€aischer Rat, Br€ussel. Standards 1907/2006, 2006. Verordnung 1907/2006 e REACH. Europ€aisches Parlament und Europ€aischer Rat, Br€ussel. Streubel, P., Mammitzsch, J., Michael, M., 2014. Functionalization of high-strength fibre ropes by extrusion. In: 8th Aachen-Dresden International Textile Conference. Dresden. Sykacek, E., Frech, H., Mundigler, N., 2007. Eigenschaften hochgef€ ullter Holz-Polypropylen€ Composites mit unterschiedlichen Haftvermittlern. In: Osterreichische Kunststoff-Zeitschrift, vol. 38. Fachverlag Wien. Tang, L.-G., Kardos, J.L., 1997. A review of methods for improving the interfacial adhesion between carbon fiber and polymer matrix. Polym. Compos. 18 (1), 100e113 (John Wiley & Sons). V€ olker, U., Br€uckner, K., 2006. Von der Faser zum Stoff: Textile Werkstoff- und Warenkunde, 33. Auflage. Verlag Handwerk und Technik, Hamburg. Yang, H.-S., Wolcott, M.P., Kim, H.-S., Kim, S., Kim, H.-J., 2006. Effect of Different Compatibilizing Agents on the Mechanical Properties of Lignocellulosic Material Filled Polyethylene Bio-composites. Composite Structures/Science Direct. Elsevier.
10.1
10
Introduction
Coating of textile semifinished parts is becoming increasingly important in technical applications. Whether manufacturing textile-based composite components or functionalizing textile semifinished parts for machine componentsdcoatings have become essential in the fabrication of textile elements for technical applications (Fig. 10.1). In the manufacturing of carbon fiber- and glass fiber-composite components, adhesion promoters or primers are applied (Tang and Kardos, 1997) to improve the adhesion of the polymer matrix to the textile-reinforcing structures. With the help of resins, which are cured during the process, near-net-shape semifinished products as well as tailored blanks are generated out of the textile structures as preforms. In the case of natural fiber-composite materials, eg, woodeplastic composites (WPC), the natural fibers are also coated with adhesion promoters to achieve an optimal adhesion to the polymer matrix (Sykacek et al., 2007; Bastian et al., 2005; Yang et al., 2006). Thermosetting is a process of thermomechanical treatment of textiles with multiple objectives. Beside adjustment of demanded dimensional accuracies, effects such as increase in strength, reduction of structural elongation, decrease of shrinkage during
Figure 10.1 Textile elevator rope (prototype) with coating.
Advances in Braiding Technology. http://dx.doi.org/10.1016/B978-0-08-100407-4.00010-7 Copyright © 2016 Elsevier Ltd. All rights reserved.
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heat input and others are attainable (Heinze and Mammitzsch, 2011; Heinze, 2013; Mammitzsch and Kern, 2014; Mammitzsch, 2015; Michael et al., 2013;V€olker and Br€uckner, 2006).
10.2 10.2.1
Rope finishing Coating
For manufacturing of textile mechanical components using textile-based semifinished products, a multitude of requirements must be covered by the coating materials. In particular, for fiber ropes, belts, ribbons and other textile tensional members, partly contrary requirements exist that the coatings must meet. Thus, intended high coefficients of friction are usually demanded on the textile surface to guarantee certain friction ratios between the textile and other mechanical components. At the same time, high wear resistance is required. In contrast, low coefficients of friction are preferred between the basic single-textile elements (yarns, strands), to produce as little wear as possible during relative movements between them (Mammitzsch et al., 2009a,b). Additional requirements, for instance fire protection regulations or guidelines concerning food tolerance, partly conflict with the characteristics of the coating materials, which leads to an addition of excipients or additives.
10.2.1.1 Coating procedures for technical textiles Various coating procedures have been established depending on the coating material, its aggregate phase, the desired property changes and the geometry of the textile work piece. A selection of these procedures is presented in the following.
Immersion bath/foulard For coating in an immersion bath, the textile is led through a fluid coating liquid, usually load free or under low preload, which penetrates the textile and adheres to the fibers. By controlling the viscosity of the coating liquid, the penetration of the textile can be realized similar to the application of a polymeric jacket. In general, dispersions, solutions and molten mass can be processed (Mammitzsch et al., 2009a,b). However, the different coating systems also entail different demands on the coating technology. In processing solutions, it is important to pay attention not only to optimal adjustment of the solution viscosity, but also the evaporation of solvents due to the danger of explosion. Further, solvent evaporation can change the polymer concentration and thus the solution viscosity. As a result, coating plants working with solvent-based systems need to have not only exhaust-gas escape units, but also explosion-proof drives and electronic components (Standards 94/9EG, 1994) as well as a system to measure the solution concentration, including solvent supply and circulation systems, to obtain optimal results. In the processing of polymer melts, the monitoring and regulation of the temperature of the molten mass is, with respect to the control of the viscosity, highly important
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for optimal penetration. Moreover, a cooling zone has to be provided for cooling down and, if necessary, for the calibration of the coated textile product. Due to technical challenges in the processing of polymer melt and polymer solutions, increasingly strict regulations (eg, EU regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH)) on the emission of plasticizers and additives from polymers, oils and dispersions preferred as coating agents for coating-immersion baths have been established (Standards 1907/2006, 2006). In many cases, foulard systems are connected to the coating basins for the coating of technical textiles using dispersions (cf. Fig. 10.2). These systems usually consist of rubberized pressing rollers and serve either the squeezing of surplus lowviscosity liquid or the pressing-in of high-viscosity liquid. In both cases, these foulards are used to control the quantity of coating applied during the coating process (Mammitzsch, 2010). In the processing of solutions and dispersions, a drying phase following the coating process is required. Depending on the coating material (which may influence the heat conductivity), dimensions and/or geometry of the textile and processing speed, large fabric contents might be required in the dryers for inline drying in a continuous production process to be able to guarantee the process times necessary for complete drying. As a source of energy, hot air, infrared radiation and microwave radiation are common systems to extract solvents and/or water from the textile (Mammitzsch, 2010). Recent developments are concerned with ultraviolet (UV) emitters and pulsed LASER as sources of energy for the drying process.
Laminating Laminating signifies a coating procedure in the textile industry, which is primarily applied to planar textiles and is usually used for manufacturing textile-based preforms and even composite components. For this purpose, textile webbings are merged with lamination agents between heated or cooled laminating rollers (Fig. 10.3), and are delivered mostly in the form of tapes. Powder- or foam-like lamination agents are added by means of a backing paper. Under pressure (and temperature, if necessary),
Figure 10.2 Schematic plot of a coating bath with foulard.
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Figure 10.3 Graphic presentation of the lamination procedure.
substrate and lamination agent are tightly interconnected so that a web with a structure of at least two layers emerges (Giessmann, 2010). When laminating textiles from thermoplastic fibers, the processing temperatures must not reduce the strength of the fibers and thus the textile structure. Depending on the type of textile substrate, various special machines with particular roll systems and lamination arrangements are available.
Extrusion The process of extrusion, which is already established in the production of composites (eg, WPC components; short fiber- and particle-reinforced plastic tubes, textile-reinforced hoses, etc.), should still be considered a relatively new process in the case of coating for technical textiles. A thermoplastic coating material provided as powder or granules is plasticized or melted in an extruder, and continuously pressed through a die. A distinction is made between the low-pressure method (hose extrusion) and the high-pressure method, which are primarily differentiated by the processing pressure. During the hose-extrusion process, the molten polymer is laid on the surface of the textile, and no penetration of the textile structure is achieved. In doing so, the extrusion layer can easily be peeled off the textile structure due to bending and tensile stresses (Streubel et al., 2014), and can have an additional damaging effect on the textile. In the high-pressure method, the molten polymer can penetrate into the textile structure because of a high processing pressure in the die. Proceeding this way, mechanical interlocking effects work in addition to adhesion, due to the shrinking of the extrusion layer in the event of cooling. The high-pressure method is the preferred method for applying extruded jackets on technical textiles. To enable a continuous processing of endless textiles, the polymer is fed into a crosshead die, which contains the molding tool, after escaping the extruder (cf. Fig. 10.4). Due to this system, the tools can be flexibly adapted to the geometry of the textile, which can, in addition to the jacketing of narrow fabrics (eg, car seat belts; tension
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Extruder
Fiber rope
Crosshead die
Figure 10.4 Crosshead die for the jacketing of fiber ropes.
belts, etc.), also enable the jacketing of circular braids, multilayer fabrics and spacer fabrics (Streubel et al., 2014).
Pultrusion Pultrusion is a method used for manufacturing of textile-based composite semifinished products and near-net-shape composite components. In this method, the textile structure is impregnated with a resin, cured in a heatable die and simultaneously calibrated. To enable continuous production of endless semifinished products, the textile preforms are usually withdrawn from spools. It is also possible to set up a pultrusion route directly at the end of the textile-processing machinery (eg, radial braider, cf. Fig. 10.5) to immediately process the product.
10.2.1.2 Coating materials for technical textiles For the coating of technical textiles, a variety of coating materials are available. These coatings can be classified according to their function in the textile-based component or semifinished product.
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Resin bath
Preformer
Puller
Figure 10.5 Schema of the pultrusion process.
Adhesion promoter In textiles, adhesion promoters are used for guaranteeing an optimal physical and/or chemical connection of the textile to further coating materials. They are preferably applied on nonpolar fiber materials (eg, polyethylene and polypropylene) to enable the connection of polar coating materials (eg, polyurethanes, polyester resins, polyvinyl alcohols), because a direct adhesion between polar and nonpolar polymers is unlikely. Modified silanes with organic chains, which resemble 1-propene, are well-known organic adhesion promoters for polypropylene fibers. Intermolecular bonds can be constructed between the polypropylene chains and the silane, whereas a polar coating can attach itself to the polar, functional groups (eg, methoxy and/or methylamino groups). For polyethylene materials, imines (eg, polyethylenimine) are frequently applied. They are similar to polyethylene in their chain structure, which is why dispersion forces can establish here. Imines consist of NeH groups, which are, on the one hand, moderately polar and thus enable a physical connection of polar materials. On the other hand, the aforementioned NeH groups are protonizable so that also chemical connections (eg, acidebase reactions) are possible. Organic and organosilicon adhesion promoters, as well as metaleorganic composites based on titanium or zirconium, are known as adhesion promoters on olefinic materials. They also enable dispersion forces on the fibers and therefore promote physical connections to the coating materials.
Lubricants Lubricants fulfill multiple tasks in textile structures. Thus, so-called fiber sizings are applied on the fibers to prevent agglomeration of the filaments in the yarn production process and to protect the fiber against wear during further textile processing. As a result, metallic and ceramic loops in the processing machines are also protected. Many of those sizings work as lubricants in the textile to reduce fiberefiber friction and the resulting wear. In specific cases, additional lubricants are applied to textiles. Primarily waxes (mostly paraffin, olefin or polyamide) or short-chain oils (eg, silicone oils, paraffin oils and others) are used.
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The lubrication effect of the waxes adhering to the fibers is primarily due to low shear strength, which leads to decomposition of the lubricant films in the shear zone, thus enabling sliding. Only when the melting temperature is exceeded (eg, because of high shear velocities or high application temperatures), the waxes start working, similar to an oil, by the production of a hydrodynamic lubricant film, which separates the contact surfaces. Similar to the case of short-chain oils, it can happen that, due to high compression in the application, the hydrodynamic film is broken, and the fibers, which are actually supposed to be separated, come into direct contact with each other. This is why oils are usually applied when high-frequency load changes and low compressions are expected, whereas waxes are applied when low-frequency load changes with high local compressions are supposed to occur. For the optimization or adjustment of the lubricants to certain application requirements, chemically consimilar oils and waxes are often mixed and, as the circumstances require, soaps might be added in addition. Moreover, polytetrafluorethylene powders (particle diameter approximately 500 mm) can be admixed.
10.2.1.3 Functional coatings Functional coatings are applied for adjustment of certain properties in technical textiles and for improving their efficiency in particular applications. Thermoplastic polyurethane elastomers (TPUs) are a common coating material. Due to their chemical structure, TPUs are variably adjustable so that they can not only coat films with hardebrittle properties, but coating of viscoplastic or soft-resilient properties are also possible. Further, several resin systems, eg, for the production of composites, are applied as functional coatings using pultrusion.
Thermoplastic polyurethane elastomers TPUs, when applied on technical textiles, are used either as dispersions for application from the aqueous phase or in the form of powders or granulate materials for thermoplastic processing. Polyurethane dispersions offer the advantage that they are not only applicable for full penetration of the textile, but also for surface treatment because of the simple procedure of dip coating (with and without foulard). For this purpose, only the viscosity of the supplied dispersions needs to be adapted. With the addition of adhesion promoters and additives, polyurethanes can be adjusted in such a way that they also adhere to nonpolar fiber materials or are even suited for chemically different fibers. In combination with other additives, eg, waxes, oils or silicones, it is possible to generate abrasion-resistant, wear-resistant protective films from polyurethane dispersions on technical textiles. A disadvantage is that the drying parameters in the subsequent drying process need to adapt to the melting temperature of the respective polyurethane and, as circumstances require, also to accessory agents/additives. Powders and granulate materials are primarily applied in the production of jackets or abrasion-resistant cover layers. A full penetration of the textile work piece cannot be achieved, but very high coefficients of friction for good wear-protection properties can be generated on the textile surfaces. Various systems are available, which feature the
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property profiles from hard and brittle to soft and resilient. Thus, a polyurethane type can be found for almost any desired surface property. By additional compounding with additives or short fibers, TPU granulated materials or powders can be further adapted to the requirements of the final application. Two disadvantages are very cost-intensive processing technology with precise temperature control and the necessity to cut process times to a minimum to avoid damaging the fibers. The necessary cooling causes additional technical efforts.
Resins for composites and products pre-impregnated with resin Numerous resin systems, selected according to the desired characteristics of the product, are known for the manufacturing of composites. Unsaturated polyester resins (also UP resins) are used to produce hard matrix. Hardness can be controlled to a certain extent by adding a curing agent, which connects the resins. Undercured UP resins display higher elasticity and longer pot lives but might remain sticky on the surface in some cases. Fast-curing systems (eg, with a cobalt accelerator) are available, but these display short pot lives (approximately 15 min) and are already completely cured after a few hours without the need of supplying thermal energy. Saturated polyester resins (eg, from aliphatic dicarboxylic acids and diols or diethylene glycol) are only used in the rare case of an elastic matrix. Epoxy resins also provide a hard and stiff resin matrix. In contrast to UP resins, an exact dosage of resin and curing agent is necessary. Cold-curing and heat-curing systems are known. The curing is exothermic so that additional heat is generated in the resin. This can cause thermal damage or, in the worst case, lead to fire outbreaks. For that reason, the processing temperatures need careful monitoring during the curing process. Phenolic resins (also phenoleformaldehyde resins) are primarily used for manufacturing heat-resistant molding batches. Storable fluid or meltable precursors are available, which can only be cured at higher temperatures while adding formaldehyde. Formaldehyde presents a health risk, which is why attention has to be paid to reliable respiratory protection and exhaust ventilation. Beside the aforementioned coatings, further coatings in the form of UV absorbers, antistatic agents and flame retardants are known. Applied as additives, they can be provided for almost every functional coating or directly admixed to the fiber base material during fiber production.
10.2.2
Thermosetting of fiber ropes
For fiber ropes, thermosetting is an integral production stage, primarily to increase the strength of the resulting fiber ropes while reducing operating elongation (Heinze, 2013; Mammitzsch and Kern, 2014; Michael et al., 2013). The ropes are loaded with a tensile load and heated to a material-dependent temperature. Heating the ropes through to the core is ensured via a warm-up period. To improve optimal strength, dwell times after the total heat through of approximately 3 min are specified in the literature (Heinze and Mammitzsch, 2011; Michael et al., 2013).
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10.2.2.1 Process of thermosetting Fig. 10.6 demonstrates the schematic structure of a continuously working stretching unit in combination with a heating section to heat the free-rope segment, that can be used for continuous thermosetting of fiber ropes. Comparable systems established for the finishing of ropes are distributed by various machinery manufacturers as series machines, especially developed for finishing and thermalemechanical treating of fiber ropes. The load, required for thermosetting, is generated via a speed difference between the hold mangle and the pull mangle. The embracement, due to wrap friction, ensures that the incoming and outgoing rope just needs to be applied with a low preload to generate the necessary thermosetting tension between hold mangle and pull mangle. Sufficiently large fabric content in the heating section can guarantee that, regardless of high processing speeds and large rope diameters, the rope (after heat-through) can remain exposed for approximately 3 min to load and thermal energy. Hot air, as well as oil and water baths, has prevailed as a suitable medium for heat transmission in the processing of textiles. In fiber ropes, the subsequent removal of oil or the drying of the ropes is necessary, which is why procedures with hot air are widely established. During thermosetting of fiber ropes, several effects are superimposed, which lead to an increase of the tensile strength of the ropes and a simultaneous decrease in elongation. The first effect is due to the adjustment of irregularities in the fiber rope, caused by manufacturing tolerances. Scientific studies (eg, Heinze, 2013) have ascertained that the load cycles before testing until rupture of the rope set the rope structure. These studies followed the test standards for determination of the break load of fiber ropes (cf. Standards DIN EN ISO 2307:2011, 2011). This provided for a more consistent load distribution in the rope, leading to an increase of the measured break load and simultaneously to a decrease of the standard deviation of the measured data. Comparable effects are achieved when loading ropes during the thermosetting process.
Grooved deflection sheaves
Fiber rope
Hold mangle
Pull mangle
Figure 10.6 Scheme of a stretching unit for the continuous thermosetting of fiber ropes.
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This effect is superimposed by the heating to a material-specific thermosetting temperature. In published investigations of twisted yarns, the thesis was put forward that a change of the fiber morphology takes place (Heinze and Mammitzsch, 2011). In other published investigations, treated twisted yarns unraveled after thermosetting, and the filaments were loaded along the fiber axis in the course of a dynamicemechanical analysis (Heinze, 2013). In doing so, an increase of the material damping and a decrease in Young’s modulus have been determined (cf. Heinze, 2013). These effects can be justified in that the orientation of the molecules no longer conformed with the fiber longitudinal axis after the thermosetting of the twisted yarns, which substantiates the thesis of the modified-fiber morphology. Because the temperatures during thermosetting are lower than the melting temperature and thus far beneath the decomposition temperature, processes of a physical nature can take place during thermosetting. Chemical reactions can be avoided, because no reagents are added and because polyethylene is a material with good chemical resistance (Bargel and Schulze, 2000; Domininghaus, 2005). As test facility for the execution of the thermosetting experiments, the plant “Rope Liner” of the MAGEBA Textilmaschinen GmbH & Co.KG (cf. Fig. 10.7) available at Chemnitz University of Technology (Technische Universit€at Chemnitz) was used. This plant is meant for the finishing, coating and thermosetting of fiber ropes. The coating plant is divided into individual aggregates, which can be controlled separately. This enables, eg, the bridging of the dryer if the thermosetting is carried out without coating. The plant is a custom-built model optimized for laboratory and small-series operations, which offers the full-service spectrum of a large-scale plant. Solely with regard to the maximum operating speed, the plant is limited to a maximum processing speed of 5 m/min. Infrared-dryer Microwave dryer
Pull-off unit Coating unit
Pull mangle Hold mangle
Hot-air oven
Cooling zone with load cell
Figure 10.7 Thermosetting plant rope liner.
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The plant includes a coating unit with foulard, which is bypassed for the thermosetting tests. The connected infrared dryer was deactivated, but could not be bridged to maintain a certain pretension in the ropes. As already described above, this plant generates the load transmission to the fiber rope via a speed difference between hold mangle and pull mangle. The plant allows for two modes of operation: In the position-controlled operational mode, a specified speed difference is adjusted (eg, hold mangle is 10% faster than the pull mangle). This principally allows a constant stretch ratio of the rope that is to be treated. A problem that appears in this operating mode is that ropes composed of polyethylene fibers exhibit a low coefficient of friction on steel (mH z 0.08; Michael, 2010), which can lead to the emergence of slippage between the deflection sheaves in hold and pull mangle and the rope. This slippage can cause a decrease in the applied load, which leads to varying strength in the treated areas of the rope, because an optimal result of thermosetting always requires an optimal combination of thermosetting load and temperature (Heinze and Mammitzsch, 2011; Mammitzsch and Kern, 2014). To be able to adjust such traction-related load variations during operation, the thermosetting plant is additionally equipped with a load-controlled operational mode. In this mode, the rope load is measured via an integrated load cell. The operator sets a target value for the thermosetting load and the control system of the machine automatically readjusts the speed difference between hold and put mangle. The advantage of such an operational mode is that transient slippage effects are compensated by deliberate intrusion of the control system.
10.2.2.2 Test parameters For the conducted experiments, the load-controlled operational mode was chosen. Because only short rope sections with a length of approximately 35 m have been treated, it was essential to obtain as consistent thermosetting results as possible on these short lengths. The thermosetting load was set to 12% of the break load of the ropes. The thermosetting temperature varied between 130 and 150 C in steps of 5 K. In the heating tunnel, fabric contents of 12.5 m were installed, which, under consideration of a dwell time of 3 min and heating time of approximately 2 min (to a temperature of 130 C), led to a machine speed of 2.5 m/min. For temperatures above 140 C, the machine speed was reduced to 2.4 m/min, which increased the retention time in the heating tunnel by 12.5 s. The summarized test parameters are found in Table 10.1.
10.2.2.3 Findings, results and comparison The following results are the findings in the thermosetting of fiber ropes made from selected ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers. Because the increase in break load caused by the rope diameter decreasing during thermosetting is not meaningful, the results of the tensile tests have been related to the linear density to receive a specific strength, as presented in Figs. 10.8e10.11. During thermosetting, an interesting effect appeared in all rope types, in which the strength did
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Table 10.1
Test parameters during thermosetting Value
Relative thermosetting load
12% of breaking load (F index B)
Thermosetting temperatures
130, 135, 140, 145, 150 C
Machine speed
2.5 m/min (up to 140 C) 2.4 m/min (up to 140 C)
1000
Specific strength (N/ktex) 1500 2000
2500
Parameter
Untreated
130
135
140
145
150
Thermosetting temperature (°C)
2000 1500 1000
Specific strength (N/ktex)
2500
Figure 10.8 Specific strength of the ropes made of Dyneema® SK75 after thermosetting.
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
Figure 10.9 Specific strength of the ropes made of Spectra® S1000 after thermosetting.
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2000 1500 1000
Specific strength (N/ktex)
2500
257
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
1000
Specific strength (N/ktex) 1500 2000
2500
Figure 10.10 Specific strength of the ropes made of TNX® after thermosetting.
Untreated
130 135 140 Thermosetting temperature (°C)
145
150
Figure 10.11 Specific strength of the ropes made of Eosten® FT093 after thermosetting.
not continuously increase with the rise of the thermosetting temperature. All UHMW-PE fibers show a drop in specific strength at a thermosetting temperature of 135 C. However, the strength is still clearly higher after the thermosetting at 135 C than without thermosetting. If the thermosetting temperature was raised further up to 145 C, a further increase in specific strength was found for all fibers. The ropes made from Spectra® S1000 und TNX® showed a further increase in specific strength as the thermosetting temperature was raised, whereas the specific strength of the fibers Dyneema® SK75 und Eosten® FT093 turned out slightly lower in comparison to the values achieved at 145 C.
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The samples showing the highest increase in specific strength in N/ktex have been taken for a following density determination, as described previously in Section 3.3.2 of chapter Three-dimensional (3D)-maypole braiding, for calculating the area of cross section according to equations in Chapter 10. For this purpose, the linear density of the ropes was determined after thermosetting again. For the determination of the density, individual strands have been extracted from the treated ropes and prepared accordingly. All fibers showed a decrease in density during thermosetting processes with an increasing thermosetting temperature (cf. Table 10.2). In Heinze and Mammitzsch (2011), Heinze raises the theory that thermosetting of fiber ropes made of UHMW-PE fibers leads to a change of the fiber morphology in the filaments. In further investigations, this thesis is extended by Heinze in that the orientation of the molecules along the axis decreases. Heinze supports his thesis with the results of a dynamicemechanical analysis (Heinze, 2013). The density of a material is primarily determined through the bonding energy in the material. In the case of polymers which contain neither multiple bonds nor functional
Selected samples and corresponding thermosetting temperatures Table 10.2
Material
Thermosetting temperature (8C)
Density (g/cm3)
Dyneema® SK75
Without
0.975
130
0.943
145
0.931
150
0.935
Without
0.9671
130
0.929
145
0.936
150
0.932
Without
0.984
130
0.902
145
0.917
150
0.917
Without
0.9345
130
0.912
140
0.918
150
0.929
®
Spectra S1000
TNX
®
®
Eosten FT093
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2000 1500 1000
Tensile strength (N/mm2)
2500
groups, similar to the present case of PE fibers, the bonds between the molecule chains, which are based on the van der Waals forces, are dependent on the distance between the molecule chains and thus are also dependent on the morphology (Bargel and Schulze, 2000; Domininghaus, 2005). From the decreasing density, it can be concluded that a change of the orientation in the polymer fibers occurs because higher bond lengths indicate that the distance between the molecule chains increases, which again means larger space requirements of a certain number of molecules, thus describing a lower density of material. On the basis of the density values shown in Table 10.2, the break loads from the tensile tests have been converted into break strengths of the treated ropes. Figs. 10.12e10.15 show the break strengths of the ropes, which have been gained by thermosetting, in comparison to the values without thermomechanical treatment. The results of thermosetting at 130 C confirm the data shown in Heinze (2013) for the fiber Dyneema® SK75. After thermosetting at temperatures above 130 C, it was not possible to achieve any higher strengths for the fiber Dyneema® SK75. Further, the thermosetting temperature to gain the highest possible strength increases is not the same for all fibers made of UHMW-PE. Generally, increases in strength have been obtained for all fibers at a thermosetting temperature of 130 C. Some fibers, however, show an additional significantly increasing strength at higher thermosetting temperatures. It is not possible without further considerations, to derive any statement from the break strengths presented in the aforementioned figures regarding achievable increases in strength, because, eg, the pitch length of braided fiber ropes influences the initial strength of the rope before thermosetting (McKenna et al., 2004), but also changes significantly during the thermosetting process. Table 10.3 shows the change of the
Untreated
130
145
150
Thermosetting temperature (°C)
Figure 10.12 Break strength of the ropes made of Dyneema® SK75 after thermosetting at temperatures above 130 C.
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2000 1500 1000
Tensile strength (N/mm2)
2500
260
Untreated
145 130 Thermosetting temperature (°C)
150
2000 1500 1000
Tensile strength (N/mm2)
2500
Figure 10.13 Break strength of the ropes made of Spectra® S1000 after thermosetting at temperatures above 130 C.
Untreated
130
145
150
Thermosetting temperature (°C)
Figure 10.14 Break strength of the ropes made of TNX® after thermosetting at temperatures above 130 C.
pitch length due to the variation of the thermosetting temperature for the examined materials. In this regard, the percentage increase of the pitch lengths is related to the truly existing pitch lengths of the ropes in their initial states (cf. Section 4.2.1 of chapter Cartesian 3D braiding) and not to the predefined rope parameters. As can be seen, the pitch length increases up to 14.7% with raised thermosetting temperature. In this context, according to Heinze (2013), an increase of the pitch length
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2000 1500 1000
Tensile strength (N/mm2)
2500
261
Untreated
150
140
130
Thermosetting temperature (°C)
Figure 10.15 Break strength of the ropes made of Eosten® FT093 after thermosetting at temperatures above 130 C.
Table 10.3
Increase in pitch length through thermosetting Relative increase in pitch length (%) after thermosetting
Material ®
Dyneema SK75 ®
Spectra S1000 ®
TNX
®
Eosten FT093
1308C
1358C
1408C
1458C
1508C
4.0e4.8
8.8e9.4
9.0e9.9
9.8e10.4
11.3e12.4
4.0e5.1
8.3e8.6
8.8e9.6
9.5e10.1
12.5e13.2
5.5e6.3
7.5e8.1
8.5e9.4
9.5e10.4
11.5e12.2
5.8e6.3
8.5e9.4
8.8e9.6
9.0e10.1
13.8e14.7
of 5% entails an increase of the initial strength of an untreated, braided rope made of Dyneema® SK75 of approximately 2%, whereas an increase of the pitch length of 15% implies an increase of the initial strength of the rope made of Dyneema® SK75 of approximately 6%. Moreover, the load-bearing cross section of the ropes changes. As a result, statements on the absolute increase of the strength are not possible. To generate this information, ropes would have to be braided again conforming to the treated ropes with regard to twist per meter, pitch length and linear density. In the case of such ropes, the break strength has to be determined in an untreated condition and compared to the break strengths obtained here during thermosetting. Nevertheless, some insights can be derived from the available data. It is, for example, recognizable that strength increases of approximately 25% can be achieved for the fiber Dyneema® SK75, whereas for the fiber Spectra® S1000 strength only increases by approximately 12%. However, the break strength of the ropes made of
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Dyneema® SK75 and Spectra® S1000 displays almost identical values after thermosetting, because the ropes made of Spectra® S1000 provided a higher strength before thermosetting. In the case of the TNX® fibers, strength increases of 10% and for fibers made of Eosten® FT093, strength increases of 15% have been reached. Because literature on this subject (eg, Heinze and Mammitzsch, 2011) shows that the results during thermosetting not only depend on the temperature, but also on the thermosetting tension. The 12% relative thermosetting tension, which has been chosen for the tests, is only known to be optimum for the fiber Dyneema® SK75. Further investigations need to be carried out for the present UHMW-PE fibers. Initial tests have already showed that for ropes made of the fiber Eosten® FT093, better results could be obtained at a temperature of 150 C and a relative thermosetting tension of 18%. Further rope finishing procedures are, eg, steaming and assembly, which should be mentioned here without any claim to completeness. Steaming is usually only a common procedure for nylon fiber ropes and shall not be further considered here (McKenna et al., 2004). The process step of making-up concerns all textile products.
10.3
Future trends in rope finishing
Due to the textile structure of fiber ropes, present conventional technologies for coating and finishing of ropes meet their limits. The complete penetration of fiber ropes with coating liquid at diameters larger than 20 mm is insufficient because coating, in the case of mass production, is always a compromise between the highest possible processing speed and optimal penetration. By increasing the processing speed of a fiber rope through an immersion bath, which has established itself on the market as state of the art, critical flow effects increasingly emerge at the surface of the rope, which impede or even prevent a penetration of the rope core. A further problem arises during the total time needed for heating ropes with large diameters, which are to be treated through to the core. During this process, hot-air ovens are used because heating in water requires postdrying and thus an additional heating process, the effects of which on the results of thermosetting have not yet been fully clarified. Heating in oil baths requires an additional cleaning process including postdrying, which, on the one hand, is economically nonfeasible and, on the other hand, raises the issue of unexplained influences of repeated heating during drying on the results of thermosetting. Based on coating in an immersing bath as a state-of-the-art process, modified and new procedures for the coating and drying of ropes have been developed. New approaches are, for instance, the integration of ultrasonic technology in the coating tank to facilitate the penetration of the coating into the core of the textile. One problem to be considered is the thermal energy input into the water due to ultrasonic waves, which leads to the vaporization of the water and thus to a change in the concentration of the polymer in the coating liquid. Due to these circumstances, a complex system for the monitoring and regulation of the temperature and concentration of the coating liquid is inevitable.
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A further promising approach is the use of annular-gap nozzles for injecting the required amount of coating material via forced dosage into the rope. At this point, the rope structure characterized by surface irregularities poses a challenge, because forced dosage with small overpressures requires a certain density between the ring-shaped nozzle and the fiber rope. Another option would be to apply the coating liquid in a modified vacuume infiltration process. The challenge here would be the realization of a continuous process with the needed sealing against ambient air. For drying ropes that are coated with a more effective procedure, an optimized drying technology also needs to be developed, because the coating liquids are mostly water based. In this way, not only is there a higher amount of coating material, but also significantly more water penetrates the core of the rope, which needs to be fully evaporated in the drying process. A special challenge for future technical developments is the thermosetting of ropes with large diameters d (d is the rope diameter). Due to the textile structure, the air-filled cavities for which delay heating to the core because of insulation effects, very long heating times are required, which only enable a high processing speed if the fabric content in the dryer is sufficiently high. Such high fabric contents, which can easily be 350 m or more, in combination with critical bending diameters D (D is the diameter of the deflection sheave) that preferably should not drop below a ratio of D/d¼10, require, require thermosetting plants with an enormous installation space in the case of ropes with diameters larger than 40 mm. However, this method is not only economically nonfeasible because of the required production area and the energy costs that arise from the large ovens, but it also restricts the flexibility of the usage of the present operating areas (eg, for internal logistics) and thus has counterproductive effects on the demanded maximum flexibility of the companies in a competitive global environment. Ultrasonic technology offers the possibility to realize an energetically meaningful heating in a short time via the direct coupling of ultrasonic waves into the rope. The high processing speeds combined with the high efficiency that is generated by ultrasound technology can positively influence economic viability, so that cost advantages over conventional technologies can be achieved. The developers of finishing technologies will face major challenges during the next few years, when increasing the efficiency of coating systems becomes more important. A successful implementation of a more effective procedure will not only positively influence rope making, but can, at the same time, bring along a dominant market position for the machine developer.
10.4
Conclusion
With regard to rope finishing, thermosetting has become an indispensable part of the process chain of manufacturing of high-strength textile ropes for mechanical engineering applications. Further, the materials of one and the same material group do not necessarily react in the same way to the processing parameters. As could be shown, the processing
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parameters and the obtainable improvements in strength vary considerably within the group of UHMW-PE fibers. The same also applies to the aramid fibers. Here, fibers made of aramid copolymer show a different behavior compared to fibers made of poly-paraphenylene terephthalamide (PPTA), and it can be anticipated that such big differences also occur between PPTA fibers of different manufacturers, as it was shown for the fibers made of UHMW-PE. If thermosetting is supposed to be applied on a textile product to benefit from the advantages of the procedure, it is advisable to conduct sufficient preliminary tests regarding optimal temperatures, dwell times and thermosetting tensions on the particular textile. Existing publications on similar textiles made of similar materials can only serve as a working basis for the clarification of the field of investigation, because not only the material, but also the textile basic structure has an influence on the result.
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Standards 94/9EG, 1994. Richtlinie 94/9EG e Atmospheres Explosibles. Europ€aisches Parlament und Europ€aischer Rat, Br€ussel. Standards 1907/2006, 2006. Verordnung 1907/2006 e REACH. Europ€aisches Parlament und Europ€aischer Rat, Br€ussel. Streubel, P., Mammitzsch, J., Michael, M., 2014. Functionalization of high-strength fibre ropes by extrusion. In: 8th Aachen-Dresden International Textile Conference. Dresden. Sykacek, E., Frech, H., Mundigler, N., 2007. Eigenschaften hochgef€ ullter Holz-Polypropylen€ Composites mit unterschiedlichen Haftvermittlern. In: Osterreichische Kunststoff-Zeitschrift, vol. 38. Fachverlag Wien. Tang, L.-G., Kardos, J.L., 1997. A review of methods for improving the interfacial adhesion between carbon fiber and polymer matrix. Polym. Compos. 18 (1), 100e113 (John Wiley & Sons). V€ olker, U., Br€uckner, K., 2006. Von der Faser zum Stoff: Textile Werkstoff- und Warenkunde, 33. Auflage. Verlag Handwerk und Technik, Hamburg. Yang, H.-S., Wolcott, M.P., Kim, H.-S., Kim, S., Kim, H.-J., 2006. Effect of Different Compatibilizing Agents on the Mechanical Properties of Lignocellulosic Material Filled Polyethylene Bio-composites. Composite Structures/Science Direct. Elsevier.