Abstract:
The invention relates to a process and a device for manufacturing a composite strand formed by combining continuous glass filaments with continuous high-shrinkage organic thermoplastic filaments.

Description:
BACKGROUND 
     The invention relates to a process and a device for manufacturing a composite strand formed by combining a multiplicity of continuous glass filaments with continuous high-shrinkage organic thermoplastic filaments. 
     The manufacture of composite strands is disclosed especially in EP-A-0 367 661 which describes a process employing a first installation comprising a bushing that contains molten glass, from which continuous glass filaments are drawn, and a second installation comprising a spinning head, supplied under pressure with an organic thermoplastic that delivers continuous filaments. 
     The two types of filaments are assembled into at least one composite strand and during the assembly the filaments may be in the form of webs, or in web and strand form. In the composite strand, the glass filaments or strand are surrounded by thermoplastic filaments that protect the glass from rubbing on the solid surfaces with which the strand is in contact. 
     Although the incorporation of thermoplastic filaments enables the abrasion resistance of the strand to be improved, it also introduces tensions in the strand due to a shrinkage phenomenon of said filaments, which causes waviness of the glass filaments. The presence of waviness is particularly visible when the composite strand is wound in the form of a bobbin as this is deformed over its entire periphery. 
     The shrinkage phenomenon has several drawbacks: it requires resorting to thick spools for producing the bobbins so that they can withstand the shrinking exerted by the composite strand and it disrupts the unwinding of the strand from the bobbin due to the fact that it does not have the ideal geometric characteristics that are required for the desired application. Furthermore, such a strand is not advantageous for producing a fabric that can be used as a reinforcing material for large-size flat parts since, because of the waviness, the filaments are not perfectly aligned in the final composite. The reinforceability of the strands in a given direction is found to be reduced. 
     To solve the problem of shrinkage of the thermoplastic filaments, various solutions have been proposed. 
     In EP-A-0 505 275, a process for manufacturing a composite strand similar to that described previously in EP-A-0 367 661 is proposed, which plans to form the thermoplastic filaments using a spinning head that is normally used in the field of the synthetic fibre industry. In this way, it is possible to obtain a composite strand formed from one or more glass strands surrounded by organic filaments, which is independent of the configuration of the spinning head used for extruding the organic filaments. 
     In EP-A-0 599 695, it is proposed to mingle the thermoplastic filaments with glass filaments at a speed during the commingling that is greater than the drawing speed of the glass filaments. The speed difference is determined so that the shrinkage phenomenon compensates for the excess initial length of the thermoplastic filaments relative to the glass filaments. 
     In one embodiment, the thermoplastic filaments pass onto a variable speed drawing unit of the type comprising drums, which accentuates the excess length, which makes it possible to obtain a composite strand of which the glass filaments are linear and the thermoplastic filaments are wavy. 
     In EP-A-0 616 055, a process for producing a glass/thermoplastic composite strand is also proposed, which consists in mingling a web of thermoplastic filaments with a bundle or a web of glass filaments, the thermoplastic filaments being, upstream of the point of convergence, heated to a temperature above their relaxation temperature, drawn then cooled. The composite strand obtained has no waviness and is stable over time. 
     The direct manufacture of rovings, without passing through an intermediate step of unwinding the tape and winding the strand, is carried out continuously by drawing the composite strand under the bushing at a speed compatible with the drawing of the glass filaments. This already high speed (of the order of a few meters to about ten meters per second) is associated with a drawing speed of the thermoplastic filaments upstream of the convergence points that is even higher. 
     The production of a composite strand without waviness under such conditions occurs via a precise synchronization of the relative speeds of the rotating elements of the drawing unit and by maintaining the initial difference between the drawing speeds of the glass filaments and the thermoplastic filaments. 
     These conditions are limited to thermoplastic materials that undergo a limited shrinkage. When the shrinkage is larger, the drawing unit becomes inoperable due to the fact that its speed can no longer be increased so as to sufficiently increase the length of the thermoplastic filaments so that the composite strand does not have any waviness. 
     SUMMARY 
     The object of the present invention is to provide a process enabling the manufacture of a composite strand comprising commingled continuous high-shrinkage thermoplastic filaments and continuous glass filaments that does not have any waviness during its manufacture and that remains stable over time. 
     This object is achieved via a process for manufacturing a composite strand formed by commingling continuous glass filaments mechanically drawn from the holes in a bushing filled with molten glass and continuous organic thermoplastic filaments emanating from a spinning head, said thermoplastic filaments being mingled in the form of a web with a bundle or a web of glass filaments, in which, before their commingling with the glass filaments, the thermoplastic filaments are drawn, heated then projected onto a moving support with a speed during their projection onto the support that is greater than the running speed of said support. The combined effect of the drawing and the projection of the heated thermoplastic filaments gives them a high level of crimping that consequently makes it possible to compensate for the shrinkage of the thermoplastic in the composite strand. 
     Advantageously, the heating and protection of the thermoplastic filaments are carried out simultaneously. 
     According to a first embodiment of the invention, the thermoplastic filaments are guided in the form of a web up to the glass filaments, also in the form of a web, and are combined with the latter at identical speeds between the coating roll and the point of gathering all the filaments into a composite strand. 
     According to another embodiment, the thermoplastic filaments are projected onto the glass filaments deposited onto the moving support, in the running direction of said support. Thus a web formed by the entanglement of the crimped thermoplastic filaments with the linear glass filaments is obtained, this web consequently being assembled to form the composite strand. 
     The process according to the invention makes it possible to obtain a composite strand without any waviness: the glass filaments that are incorporated into the composition of the composite strand are linear immediately after their assembly with the thermoplastic filaments, and they retain their linearity after the collection in the form of a package. In the end, the thermoplastic filaments in the composite strand may be linear or wavy depending on the level of crimping that has been conferred on them at the beginning. 
     Thanks to the invention, it is possible to form bobbins under the normal conditions for producing glass strands, especially using spools of conventional thickness given the absence of shrinkage of the composite strand, these spools possibly being removed in order to obtain balls and reused if necessary. This has the advantage of being able to extract the composite strand according to the method of unwinding (from the outside) or unravelling (from the inside). 
     Besides the fact that it makes it possible to obtain a composite strand without waviness using a high-shrinkage thermoplastic, the process according to the invention ensures a homogeneous distribution and a high commingling of the filaments within the composite strand. 
     The invention also provides a device for carrying out this process. 
     According to the invention, in order to enable the manufacture of a composite strand formed from continuous glass filaments and from continuous high-shrinkage thermoplastic filaments, this device comprises, on the one hand, an installation comprising at least one bushing supplied with molten glass, the lower face of which has a very large number of holes, this bushing being associated with a coating device, and on the other hand, an installation comprising at least one spinning head supplied under pressure with molten organic thermoplastic, the lower face of which is equipped with a very large number of holes, this spinning head being associated with a drawing unit of the type comprising drums, with a device for projecting thermoplastic filaments that is provided with heating means, with a drum-type moving support and with a means enabling the thermoplastic filaments to be mingled with the glass filaments, finally means common to the two installations enabling the assembly and winding of the composite strand. 
     The drum drawing unit has at least two rolls operating at variable speeds, preferably ensuring an increasing linear speed of the thermoplastic filaments. When the drawing unit comprises more than two rolls, the latter advantageously operate in pairs. The drawing unit may be provided with heating means, for example electric or infrared heating means, preferably placed in the first drum encountered by the thermoplastic filaments with the objective of preheating them and thus promoting their drawing. 
     Preferably, the means enabling the thermoplastic filaments to be projected onto the moving support is a device using the properties of fluids that may be liquids or gases, such as pulsed or compressed air. Advantageously, it is a Venturi system, the role of which is solely to project the thermoplastic filaments by giving them an adequate spatial distribution and orientation, without giving them any additional speed. 
     According to a preferred embodiment of the invention, the heating means, especially electrical, are associated with the device ensuring the projection of the thermoplastic filaments. In this way, the heating of the thermoplastic filaments at a temperature close to their softening point is carried out homogeneously and rapidly, which makes it possible to obtain a satisfactory crimping state during the projection onto the moving support. 
     The moving support may be made from a drum, the surface of which consists of perforations, comprising an element for separating the internal volume into at least two compartments, one connected to means enabling it to be maintained under vacuum, the other associated with means enabling it to be put under excess pressure. The size and placement of the compartments are chosen so as to maintain the thermoplastic filaments in their initial crimping state, in the form of a web at the surface of the drum situated above the first compartment, and to obtain the separation of the web when it passes above the second compartment. 
     The means enabling the two types of filaments to be mingled may be constituted by a Venturi system as described previously that enables the thermoplastic filaments to be projected into a web or a bundle of glass filaments. Preferably, this system projects the thermoplastic filaments at an identical speed to the drawing speed of the glass filaments. 
     The means ensuring the commingling of the filaments may also be constituted by the drum-moving support. In this case, the drum is used to support the web of glass filaments, which winds around it, and the crimped thermoplastic filaments in web form are mingled with the glass filaments along a generatrix of the drum. 
     The devices described previously enable the production of composite strands, from precrimped high-shrinkage thermoplastic filaments and from glass filaments, which do not have any subsequent deformation, that is to say that remain stable over time. 
     Such devices can be applied to any type of known glass, for example E-glass, R-glass, S-glass, AR-glass or C-glass, E-glass being preferred. 
     In the same way, it is possible to use any thermoplastic capable of having a high shrinkage, for example a polymer belonging to the group of polyurethanes, polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), and polyamides such as nylon-6, nylon-6,6, nylon-11 and nylon-12. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other details and advantageous features of the invention will become apparent on reading the description of the examples of devices for carrying out the invention described with reference to the appended figures that represent: 
         FIG. 1 : a schematic representation of an installation according to the invention; and 
         FIG. 2 : a schematic representation of a second embodiment of the invention. 
     
    
    
     DESCRIPTION 
     Represented in  FIG. 1  is a schematic view of a complete installation according to the invention. It comprises a bushing  1  supplied with molten glass either via a hopper containing cold glass, for example in the form of beads that drop simply by gravity, or from the forehearth of a furnace that feeds glass directly to its top. 
     Whatever the type of feed, the bushing  1  is usually made of a platinum-rhodium alloy and it is heated by resistance heating so as to remelt the glass or keep it at a high temperature. A multitude of streams of molten glass flow from the bushing  1 , these streams are drawn in the form of a bundle  2  of filaments by a device, not shown, also allowing the bobbin  3  to be formed. Placed in the path of the bundle  2  is a coating roll  4 , for example made of graphite, which deposits a size onto the glass filaments that is intended to prevent or limit the rubbing of the filaments on the members with which they come into contact. The size may be aqueous or anhydrous (that is to say comprising less than 5% by weight of water) and contain compounds, or derivatives of these compounds, which are incorporated into the composition of the thermoplastic filaments  5  that will combine with the glass filaments to form the composite strand  6 . 
     Also represented schematically in  FIG. 1  is a spinning head  7  from which the thermoplastic filaments  5  are extruded. The spinning head  7  is supplied with a molten, high-shrinkage thermoplastic, for example coming from an extruder, not shown, supplied with granules which flows under pressure through a large number of holes positioned under the spinning head  7 , to form the filaments  5  by drawing and cooling. Cooling of the filaments is carried out by forced convention, by means of a conditioning device  8  having a suitable shape for the spinning head  7  and that generates a laminar air flow perpendicular to the filaments. The cooling air has a flow rate, a temperature and a humidity that are kept constant. The filaments  5  then pass over a roll  9  that makes it possible to assemble them in the form of a web  10 , on the one hand, and to deflect their path, on the other hand. 
     After passing over the roll  9 , the web  10  of thermoplastic filaments passes over a drawing unit  11  formed, for example, from rolls  12 ,  13  that may turn at the same speed or have different speeds so that the acceleration is carried out in the run direction of the thermoplastic filaments. The drawing unit  11  has the role of drawing the filaments  5  and of giving a set speed to the web  10 . It is possible to vary the rotational speed of rolls  12  and  13  so as to precisely adjust the projection speed of the thermoplastic filaments onto the drum  17 . Rolls  12  and  13  may be associated, where appropriate, with a heating system, for example an electric heating system, which makes it possible to ensure a homogeneous and rapid preheating of the thermoplastic filaments by contact with the surface of the rolls. The drawing unit  11  may be formed from a higher number of rolls, preferably functioning in pairs, for example four or six rolls. 
     The web  10  of thermoplastic filaments, optionally preheated, is then directed towards the deflecting roll  14 , which may be heated and optionally be motor-driven, then it passes into a crimping device  15  formed, for example from a Venturi system  16  and a drum  17 . 
     The Venturi system  16  makes it possible to keep the thermoplastic filaments separate and to project them as a regular web of suitable size onto the drum  17 . The Venturi system  16  operates by an injection of compressed air and imparts no additional speed to the web  10 . This system is associated with a heating device (not shown), for example using a fluid such as hot air or steam, and has the role of bringing the thermoplastic filaments to a temperature close to the softening point of the thermoplastic in order to improve their crimpability. 
     At the outlet of the Venturi system  16 , the web  10  of thermoplastic filaments is projected onto the drum  17 . The rotational speed of the drum  17  is lower than the speed of the web  10  during its projection so that the filaments crimp when they come into contact with the surface of said drum. 
     The drum  17  is equipped with a central groove  18 , having a width slightly less than that of the drum, which is pierced by multiple holes (not shown). It also comprises an element  19 , that is coaxial and immobile relative to the drum, which is used to separate the interior of the drum into two compartments  20 ,  21 . Compartment  20  is connected to a device, not shown, which enables it to be put under vacuum, for example a suction pump, and compartment  21  is connected to a device, not shown, enabling it to be put under an excess pressure, for example an air injection device. 
     After its projection onto the drum  17 , the web  10  of crimped filaments is held in the groove  18  level with the compartment  20  under vacuum and it is cooled, by simple contact with the perforated surface or via a fluid, for example water or a sizing composition sprayed onto the filaments. Next, the web  10  is separated from the surface of the drum  17  level with the compartment  21  under the effect of the pressurized air passing through the perforations. 
     The web  10  then passes onto a deflecting roll  22 , then into a Venturi device  23  that keeps the crimped thermoplastic filaments in individual form until they are mingled with the glass filaments of the web  24 . 
     Joining of the web  10  of thermoplastic filaments and the web  24  of glass filaments takes place between the coating roll  4  and the element  25  being used to assemble the filaments into a composite strand. During the commingling of the filaments, the thermoplastic filaments arrive with a speed equal to that of the glass filaments. 
     A deflector  26  equipped with a notch keeps all the filaments in place, in particular along the edges, and helps to reduce the disturbance undergone by the web  24  of glass filaments at the moment when the web  10  of crimped thermoplastic filaments is projected onto it. 
     The web  27  of intermingled crimped thermoplastic filaments and glass filaments then passes onto the device  25  that enables assembly of the filaments into a composite strand  6 , which is immediately wound in the form of a bobbin  3  thanks to a drawing device, not shown, that operates at a given linear speed kept constant to guarantee the desired linear density. 
     This linear speed that enables the drawing of the glass filaments is in general equal to that imparted by the drum  17  to the web  10  of crimped thermoplastic filaments. Nevertheless, it is possible to mingle the thermoplastic filaments with the glass filaments at a speed, during their projection, which may be lower in order to given an extra tension to the thermoplastic filaments to improve the ability to keep them in web form until the point of commingling with the glass filaments. Under these conditions, the difference between the projection speed of the thermoplastic filaments and the drawing speed of the glass filaments does not exceed 10%. 
       FIG. 2  represents an installation according to a second embodiment of the invention. In this figure, the common devices and means bear the same numbers as in  FIG. 1 . 
     The bundle  2  of glass filaments flowing from the bushing is drawn by a device (not shown) that forms the bobbin  3 . The bundle  2  passes over the coating roll  4  that deposits a size on the glass filaments and the web  24  formed is wound over the drum  17 . 
     The thermoplastic filaments  5  extruded from the spinning head  7 , cooled by the conditioning device  8  are assembled into a web  10  level with the roll  9 . The web  10  then passes onto the drawing unit  11  having rolls  12 ,  13  and is drawn under the same conditions as in  FIG. 1 . After roll  13 , the web  10  is directed towards the roll  14 , that is optionally heated and/or motor-driven, and into the crimping device  15  formed from the Venturi system  16  and the drum  17 . 
     In the Venturi system, the thermoplastic filaments of the web  10  are kept in their individual state and are heated at a temperature close to the softening point in order to help to obtain a high level of crimping. 
     The heated web  10  is projected onto the drum  17  that rotates at a lower speed than the projection speed of the filaments, which crimps them. Joining of the web  10  of crimped thermoplastic filaments and the web  24  of glass filaments is carried out along a generatrix of the drum  17 . The projection of the web  10  takes place while the filaments of the web  24  are contained within the groove  18  of the drum  17 ; this way of proceeding avoids disturbing the web of glass filaments and thus makes it possible to reduce the risk of said filaments breaking. 
     Immediately after their joining with the web  24 , the crimped thermoplastic filaments intermingle with the glass filaments and are flattened to the bottom of the groove  18  level with the compartment  20  under vacuum. When the web of thermoplastic filaments and glass filaments wound up onto the drum  17  arrives level with the compartment  21  that is under the action of pressurized air, it is detached from the surface under the effect of the air pressure coming from the inside of said compartment. 
     The web  27  passes onto the roll  22  and onto the device  25  for gathering the filaments into a composite strand  6 , which is wound in the form of the bobbin  3 . A second device  25  may be placed between the exit of the drum  17  and the roll  22  in order to help to obtain a better assembly of the composite strand. 
     The bobbins obtained using the process according to the invention are composed of a composite strand, of which the glass filaments are linear and the thermoplastic filaments are crimped (or wavy) in a way that is permanent and stable over time. The level of crimping or waviness of the thermoplastic filaments in the composite strand depends on the size of the crimping that was given to them during projection onto the moving support. 
     Moreover, the distribution of the glass filaments and the thermoplastic filaments within the composite strand is homogeneous, which translates into good commingling of the filaments. 
     It is possible to apply some modifications to the process and device that have just been described. Firstly, it is possible to use a size made up of several solutions, whether aqueous or not, comprising compounds that are capable of copolymerizing over a relatively short time when they are brought into contact with each other. In this case, the coating device comprises separate rolls, each of them depositing one of the sizing solutions on the glass filaments. It is also possible to anticipate a drying device that enables water to be removed from the glass filaments, or at least for the water content to be substantially reduced, before winding. 
     It is also possible to combine the invention with the production of complex composite strands, that is to say composite strands comprising organic thermoplastics that have different shrinkages. For this, it is possible to form different types of filaments, for example from one or more spinning heads, and to project them, in individual form or after having been assembled, onto the glass filaments. 
     EXAMPLE 1 
     A composite strand was manufactured in the installation described in  FIG. 1  under the following conditions:
         thermoplastic filaments:
           thermoplastic: polyethylene terephthalate (PET);   number of filaments: 1200 filaments;   linear density: 359 tex;   flow rate of the device  8 : 500 m 3 /h;   speed of the drawing unit: 1500 m/min; temperature of rolls  12  and  13 : 240° C.; draw ratio in the melt phase: 1560;   air temperature in the Venturi device  16 : 260° C.;   rotational speed of the drum  17 : 990 m/min; cooling by water-spraying; and   degree of crimping: 8%.
               The degree of crimping was measured according to the formula 100×(L−L o )/L o , in which L o  is the length of a crimped filament and L is the length of the same filament after a sufficient drawing to make it linear.   
               
           glass filaments:
           number of filaments: 1600;   
           composite strand:
           glass/thermoplastic weight ratio: 75/25;   linear density: 1491 tex; and   linear speed (winding): 1000 m/min.   
               

     The bobbin  3  was dried in an oven at 118° C. for 32 hours. The shrinkage of the thermoplastic filaments was around 6%. The geometry of the bobbin was not changed after drying. 
     EXAMPLE 2 
     A composite strand was manufactured in the installation described in  FIG. 2  under the following conditions:
         thermoplastic filaments:
           thermoplastic: polyamide (PA);   number of filaments: 1200 filaments;   linear density: 466 tex;   flow rate of the device  8 : 400 m 3 /h;   speed of the drawing unit: 1800 m/min; temperature of rolls  12  and  13 : 180° C.; draw ratio in the melt phase: 3640;   air temperature in the Venturi device  16 : 200° C.;   rotational speed of the drum  17 : 1008 m/min; cooling by water-spraying; and   degree of crimping: 10%.
               The degree of crimping was measured according to the formula 100×(L−L o )/L o , in which L o  is the length of a crimped filament and L is the length of the same filament after a sufficient drawing to make it linear.   
               
           glass filaments:
           number of filaments: 1600;   
           composite strand:
           glass/thermoplastic weight ratio: 70/30;   linear density: 1597 tex; and   linear speed (winding): 1008 m/min.   
               

     The bobbin  3  was dried in an oven at 118° C. for 32 hours. The shrinkage of the thermoplastic filaments was around 7%. The geometry of the bobbin was not changed after drying.