Abstract:
A handling cable is provided, which is intended especially for electric power and/or the signal transmission with a travelling mobile machine. The invention is remarkable in that the handling cable comprises at least one sheath made by autoreticulation of a composition comprising thermoplastic polyurethane and a reticulation agent provided with at least two isocyanate functions.

Description:
RELATED APPLICATION  
       [0001]     The present invention is related to and claims the benefit of priority from French Patent Application No. 05 51243, filed on May 12, 2005, the entirety of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a handling cable, that is, an electric power and/or signal transmission cable, which is especially intended to be coupled to a travelling machine. The invention can be applied advantageously in the field of cables for coilers, which, as their name indicates, are dedicated to being wound up and unwound during shifting of the machines to which they are respectively connected.  
       BACKGROUND  
       [0003]     There is a category of supply and/or transmission cables, which is more particularly adapted to operating mobile machines moving about in severe conditions of use, such as for example cranes or mobile bridges operating in harbour zones. This category relates to the handling cables, in other terms flexible cables which exhibit flexibility to the point where they impart a certain displacement capacity when in use, and which are generally multiconductors in the sense that they are able to ensure electric power and/or signal transmission functions, whether said signal is electrical or optical in origin.  
         [0004]     Concretely, a handling cable is classically constituted by several insulated conductors, which are combined within the same protective sheath. Of course, the material comprising the sheath in question must exhibit thermomechanical properties compatible with the final conditions of use of the cable.  
         [0005]     This is why at first the sheath material should be sufficiently flexible to conserve the flexibility of the cable indispensable for being able to efficiently accompany any shift by the mobile machine to which it is attached. It is especially important to ensure that the handling cable rolls up and unrolls, when it is more specifically used in combination with one or more coilers.  
         [0006]     Ii is likewise essential that the sheath material offers excellent resistance to abrasion and tearing, given that the handling cable is going to have to tolerate incessant shifting throughout usage of the attached mobile machine, with all the implied constraints of friction. This will be precisely the case from the moment when the cable is going to be constantly rolled up and unrolled around guide wheels and other return pulleys.  
         [0007]     The sheath material should finally have very good mechanical properties, so as to be able to resist strong traction forces, which the handling cable is inevitably going to have to be subjected to during shifting of the mobile machine to which it is connected. It should be noted that this characteristic proves to be more particularly pertinent in the case of cables for coilers. Be that as it may, the sheaths of handling cables are today essentially made of reticulated synthetic rubber. This heatsetting material in fact offers excellent properties heat deformation in normal conditions of use, as well as very good flexibility.  
         [0008]     But nonetheless the fact remains that in the area of application of handling cables, reticulated synthetic rubber has certain weaknesses with respect to resistance to abrasion and mechanical behaviour under traction. Such material also proves to be excessively expensive to produce. Various known vulcanisation techniques actually require heavy equipment, which is detrimental in terms of cost, and are penalising in terms of industrial flexibility.  
         [0009]     To rectify these difficulties, it has already been proposed to make use of polyurethane as sheath material. This thermoplastic is effectively known for being exceptionally resistant to abrasion, and it also offers excellent mechanical properties, from the point of view of both resistance to traction and flexibility.  
         [0010]     This type of material all the same has the disadvantage of yielding hot, that is, progressively and irreversibly deforming from the moment the temperature exceeds a certain threshold and mechanical constraints are applied to it. In practice in fact, if there is no particular problem at ambient temperature, things are different when 80° C. is exceeded with the appearance of the creep phenomenon. Now, it is fairly common to reach 100° C. with an electrically fed handling cable. Be that as it may, the consequence of such creep is to cause significant deterioration to the mechanical properties of the sheath material, tantamount to compromising the integrity and thus the shelf life of the handling cable.  
         [0011]     Also, the technical problem to be resolved by the object of the present invention is to propose a handling cable especially for electric power and/or signal transmission with a travelling mobile machine, handling cable which would allow the problems of the prior art to be prevented, by offering substantially improved resistance to temperature under mechanical constraints, at the same time being less cumbersome to manufacture.  
       OBJECT AND SUMMARY  
       [0012]     The solution to the technical problem in question consists, according to the present invention, of the handling cable comprising at least one sheath made by autoreticulation of a composition comprising thermoplastic polyurethane and a reticulation agent provided with at least two isocyanate functions.  
         [0013]     It should be noted first of all that the notion of sheath extends here in the widest sense of the term, that is, that is can variously designate a first coating of a conductor element, an upper layer of an insulated cable, or an envelope combining several insulated cables.  
         [0014]     It is then specified that the term autoreticulation conventionally signifies that reticulation of the sheath material is carried out at ambient temperature and optionally by air humidity, without subsequent processing.  
         [0015]     Be that as it may, the invention such as defined has the advantage of being able to have a sheath material combining the excellent properties of resistance to abrasion, flexibility and mechanical resistance to traction of thermoplastics, with the exceptional dimensional stability when hot and under mechanical constraints of the reticulated materials.  
         [0016]     Autoreticulation more specifically allows dispensing with costly vulcanisation processes of the prior art, which advantageously contributes to the low retail price of the handling cable forming the object of the invention.  
         [0017]     According to a particular feature of the invention, the reticulation agent is selected from the group of methane diphenyl diisocyanate (MDI) and its derivatives, isophorone diisocyanate (IPDI) and its derivatives, toluene diisocyanate (TDI) and its derivatives, hexamethylene diisocyanate (HDI) and its derivatives, or any mixture of these compounds.  
         [0018]     In a particularly advantageous manner, the composition of the sheath material comprises between 2 and 20 parts by weight of reticulation agent per 100 parts by weight of thermoplastic polyurethane, and preferably between 4 and 10 parts by weight of reticulation agent.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Other features and advantages of the invention will become apparent from the following description of embodiments of the invention given by way of non-limiting examples only, and with reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  illustrates the evolution of the rates of insolubles during reticulation of the thermoplastic polyurethanes, in accordance with one embodiment of the present invention;  
         [0021]      FIG. 2  illustrates the impact of concentration of a reticulating agent on the dimensional stability of the reticulated thermoplastic polyurethane in a situation of elongation under constraint; and  
         [0022]      FIG. 3  illustrates the impact of concentration of a reticulating agent on the dimensional stability of the reticulated thermoplastic polyurethane in a situation of permanent elongation. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Other characteristics and advantages of the present invention will emerge from the description of the following comparative example, said example being given by way of illustration and not limiting.  
         [0000]     Comparative Example  
         [0024]     It relates to six materials based on thermoplastic polyurethane, which are perfectly well suited to the manufacture of sheaths of handling cables. Samples are thus prepared from six different compositions, in view of comparing their respective performances.  
         [0025]     In this comparative example, thermoplastic polyurethane is common to the six samples. In this instance it is a polymer distributed under the trade mark Estane 58888Nat021 by the company Noveon.  
         [0026]     Only the nature and quantity of reticulation agent vary from one sample to the other. Table 1 details these differences. In this respect, it should be noted that the quantities mentioned in the different tables hereinbelow are classically expressed in parts by weight for one hundred parts by weight of thermoplastic polyurethane.  
                       TABLE 1                           Thermoplastic           Samples   polyurethane   Reticulation agent                   1   Estane 58888Nat021   none       2   Estane 58888Nat021   4 pcr MDI       3   Estane 58888Nat021   6 pcr MDI       4   Estane 58888Nat021   8 pcr MDI       5   Estane 58888Nat021   4 pcr t-IPDI       6   Estane 58888Nat021   8 pcr t-IPDI                  
 
         [0027]     It is first noticed that sample 1 constitutes an extreme case, since it is constituted solely by thermoplastic polyurethane. In other terms, its resistance in reticulation agent is zero.  
         [0028]     For these reasons, it should be considered as a reference.  
         [0029]     It is then observed that samples 2 to 4 differ from their counterparts by the fact that the reticulation agent with which they are provided is methane diphenyl diisocyanate, more currently designated by the abbreviation MDI.  
         [0030]     Samples 5 and 6 are remarkable in that the reticulating agent, which enters their compositions, is the trimer of isophorone diisocyanate, commonly identified by the abbreviation t-IPDI.  
         [0000]     Preparation of Samples  
         [0031]     Because sample 1 exclusively comprises thermoplastic polyurethane, which is not reticulated, the material is simply extruded and made into the desired shape.  
         [0032]     Since samples 2 to 6 correspond to reticulated materials, they are all prepared by following the same operating method, which consists schematically of grafting, then extrusion.  
         [0033]     The process therefore commences with a first grafting stage. Standard thermoplastic polyurethane and the reticulation agent of isocyanate type are first introduced to a dual-screw extruder by means of a feed hopper.  
         [0034]     The whole is then mixed at a temperature close to 200° C. The resulting granules of grafted thermoplastic polyurethane are dried and then stored for more than 6 months in airtight containers.  
         [0035]     The process is followed by a second extrusion stage. For this, the previously grafted thermoplastic polyurethane is processed in a conventional single-screw extruder, similarly to standard thermoplastic polyurethane. Autoreticulation takes place classically over a period of 4 to 7 days at ambient temperature and humidity.  
         [0000]     Kinetics of Reticulation  
         [0036]     The evolution of the rate of insolubles over time is a good indicator of the speed of reticulation reaction, considering that the more a material is reticulated, the more its rate of insolubles is raised close to 100%.  
         [0037]     It was thus decided to compare the reticulations of different thermoplastic polyurethanes, in this instance those of samples 2, 4 and 5. For each sample, a series of measurements is made aimed at determining, at regular intervals, the rate of insolubles of the material throughout reticulation.  
         [0038]     The operating method is identical for each measurement. Concretely, 1 g of the material studied (M1) is placed in an Erlenmeyer flask containing 100 g tetrahydrofurane (THF), and the whole is brought to reflux (67° C.) with magnetic agitation over 24 h. The contents of the Erlenmeyer flask is then filtered hot on a metallic grille whereof the mesh size is 120 μm×120 μm. The solid residue obtained is then dried in a kiln at 80° C. for 24 h, then weighed (M2). The rate of insolubles expressed in % is then calculated by making the ratio of masses M2×100/M1.  
         [0039]     Above all it can be specified that sample 1 gives a rate of insolubles of 0%, which proves to be quite logical, given that this reference sample corresponds to the non-grafted polymer composition, and thus to a non-reticulated material.  
         [0040]     Be that as it may,  FIG. 1  illustrates the evolution of the rates of insolubles during reticulation of the thermoplastic polyurethanes of samples 2, 4 and 5. It especially reveals the influence of the nature and concentration of the reticulation agent within the compositions of the materials tested.  
         [0041]     It is evident first of all that complete reticulation is obtained in 7 days with MDI (samples 2 and 4), whereas it requires more than 3 weeks in the case of t-IPDI (sample 5).  
         [0042]     It is evident that the quantity of polymer matrix participating in the reticulated network, which is proportional to the rate of insolubles, augments with the concentration of the reticulation agent (samples 2 and 4), to reach values greater than 95% in the case of MDI (sample 4).  
         [0000]     Mechanical Properties  
         [0043]     Assays are conducted at ambient temperature, with the aim of determining the principal mechanical properties of samples 1, 3 and 4, namely the breaking stress and breaking elongation.  
         [0044]     The Shore A hardness as well as the friction coefficients of each material on itself are likewise measures, pursuant to the ISO 8295 protocol with respect more particularly to said friction coefficients.  
         [0045]     The objective is to compare the properties of two reticulated thermoplastic polyurethanes (samples 3 and 4) to those of a simple, non-reticulated thermoplastic polyurethane (sample 1), but also to evaluate the impact of concentration in the reticulation agent on said properties.  
         [0046]     Table 2 lists the different results.  
                                                     TABLE 2                                   Sample 1   Sample 3   Sample 4                                        Breaking stress   45.8   44.3   42.6           (MPa)           Breaking   455   368   341           elongation (%)           Shore A   88-89   90-91   89-90           hardness           Static friction   1.74   1.13   0.93           coefficient           Ks           Dynamic   1.50   1.05   0.84           friction           coefficient           Ks                      
 
         [0047]     Comparison of the mechanical properties clearly shows that even though breaking stress remains relatively unchanged after reticulation, the breaking elongation decreases from 19% (sample 3) and 25% (sample 4) when the composition of the material integrates respectively 6 and 8 pcr MDI. This result materialises due to cohesion of the polymer network generated during reticulation.  
         [0048]     Analysis of the friction coefficients confirms that reticulation generates a considerable decrease in the phenomenon of chatter observed during friction tests conducted with standard thermoplastic polyurethane (sample 1), and which manifests schematically in a succession of slipping and adherence restart. The static and dynamic coefficients are in effect both reduced from 30% (sample 3) and 45% (sample 4) since the composition of the material comprises respectively 6 and 8 pcr MDI.  
         [0049]     Finally, it is evident that the hardness of the material remains unchanged after reticulation. It is evident that significant attenuation of the phenomenon of chatter constitutes a major advantage in the filed of interest to us, given that the winding and/or unwinding of a handling cable will inevitably massively engender frictions, especially between different portions of the protective sheath.  
         [0000]     Thermomechanical Properties  
         [0050]     New assays are being conducted so as to now evaluate the thermomechanical properties of samples 1 to 6 when subjected simultaneously to mechanical constraints and high temperatures. This aspect proves essential in the field of handling cables, given that it is representative of future conditions of use.  
         [0051]     Three types of tests are thus carried out to study respectively hot creep under mechanical constraint, accelerated ageing, as well as long-term performance under permanent mechanical constraint.  
         [0000]     Hot Creep Under Mechanical Constraint  
         [0052]     The NF EN 60811-2-1 standard relates to the measuring of hot creep of a material under mechanical constraint.  
         [0053]     The corresponding test is commonly designated by English expression Hot Set Test.  
         [0054]     It basically consists of ballasting one end of a test specimen of dumbbell H2 type with a mass corresponding to the application of a constraint equivalent to 0.2 MPa, and placing the whole in a kiln heated to a temperature of given variable at +/−1° C. for a period of 15 minutes. On completion of this time, hot elongation under constraint of the test specimen, expressed in %, is revealed. The suspended mass is then removed, and the test specimen is left in the kiln for 5 minutes longer. The remaining permanent elongation, likewise called remanence, is then measured prior to being expressed in %.  
         [0055]     It is obvious that the more a material is reticulated, the lower the elongation and remanence values will be. It is further specified that in the event where a test specimen would be broken during an assay, under the conjugated action of mechanical constraint and temperature, the test result would logically be considered as a failure.  
         [0056]     Be that as it may, Table 3 lists the results of the tests conducted on samples 1, 3 and 4, by taking into consideration permanent elongation only.  
                           TABLE 3                                   Sample   Maximum Hot Set Test temperature (° C.)                           1   160           3   175           4   180                      
 
         [0057]     First of all, it is evident that sample 1, which corresponds to non-reticulated thermoplastic polyurethane, can only resist a maximum Hot Set Test temperature of 160° C.  
         [0058]     A significant increase in thermomechanical properties is noted when the material is reticulated (samples 3 and 4). The maximum Hot Set Test temperature actually moves up to 175, and even to 180° C., from the moment when the composition of the base material comprises respectively 6 pcr (sample 3) and 8 pcr (sample 4) of reticulation agent of type MDI.  
         [0000]     Accelerated Ageing  
         [0059]     This hot creep test takes up the principle of the Hot Set Test, but the samples are earlier subjected to accelerated ageing of 15 h at a temperature of 120° C.  
         [0060]     The aim is to compare the thermomechanical properties of samples 4 and 6, that is, of thermoplastic polyurethanes reticulated due to the presence of two different reticulation agents, namely respectively MDI and t-IPDI.  
         [0061]     Table 4 lists the different results of the accelerated ageing test.  
                                                                                             TABLE 4                                       Sample                4   6                            Hot set test 175° C.                Elongation under   20    10           constraint (%)           Permanent elongation (%)   20    0                Hot set test 200° C.                Elongation under   Failure   400           constraint (%)           Permanent elongation (%)   Failure   115                      
 
         [0062]     Even if this does not appear in Table 4, it should be noted that a thermoplastic polyurethane modified with 6 pcr or plus of t-IPDI reticulant requires more than 3 weeks at ambient temperature and humidity to carry into effect the Hot Set Test at 175° C. This is very much in keeping with the evolution of the rate of insolubles discussed earlier.  
         [0063]     However, in spite of its apparently lower reactivity, t-IPDI seems to be a more efficacious reticulant during accelerated ageing by heat, since its presence allows sample 6 to pass the Hot Set Test at 200° C.  
         [0064]     Long-term performance under permanent mechanical constraint There is a test, which is more directly specific to handling cables. It is commonly designated by the English expression creep test, and it is intended to evaluate hot creep of materials over the long term and under permanent mechanical constraint.  
         [0065]     There is no particular standard associated with this assay, but its principle is identical to that of the Hot Set Test. Only the operating conditions are different.  
         [0066]     The temperature is thus fixed at 80° C., the application time considered is 24 h, and the intensity of the mechanical constraint can be 2, 3 or 4 MPa.  
         [0067]     In the present case, the creeping test is conducted on samples 1 to 4 to determine the impact of the concentration of reticulating agent on the dimensional stability of the reticulated thermoplastic polyurethane.  
         [0068]      FIGS. 2 and 3  show how growth of this concentration improves the dimensional stability of the reticulated thermoplastic polyurethane. It should be noted that this characteristic is valid just as much for the elongation under constraint shown in  FIG. 2 , as for the permanent elongation, the object of  FIG. 3 .