A process and an apparatus suitable for providing, directly by means of calendering, an excellent anchorage between a fabric and its plastic coating material

Abstract: A process for bonding, without any adhesive, a plastic material layer onto a backing fabric of any kind, by stretching said fabric lengthwise and crosswise to widen its meshes, which fabric is then heated by the coating cylinder to a temperature depending on the type of fabric, to maintain the plastic coating material to be applied in the semimolten state, whereupon it is calendered. The apparatus providing the biaxial widening of the meshes of the fabric also provides for its preheating in a preheating station and guides said fabric onto the coating cylinder, simultaneously controlling its contact time with the latter.

Description: The present invention relates to a process and an apparatus suitable to provide a strong anchorage between a backing fabric and its coating, said coating consisting of a plastic material layer, by means of calendering, without the interposition of an adhesive layer. For various industrial applications, fabrics of different kinds are required which are composed of natural, artificial or synthetic fibres, coated on at least one side with a layer of plastic material. While the fabric constitutes the strength-providing backing, the plastic material coating imparts properties of impermeability, hygiene, or resistance to chemicals of any kind. For example, said composite fabrics can be used for the manufacture of conveyor belts for food or pharmaceutical industries, rubber boats, wet suits for scuba divers, life jackets or emergency slides for planes, waterproof tarpaulins for commercial vehicles, blowable safety cushions for cars (the so-called "airbags"), antistatic fabrics, artificial leather, table cloth with filted back-coating, flooring elements, and the like. In order to realize a product of this kind, the adhesion between the backing and the plastic material coating layer of the desired thickness must in any case satisfy certain breakaway resistance values in accordance with the final use of the product. Calendering processes for coating a fabric with a layer of plastic material are known in the art. An article by Dieter Zickler, which appeared in "Textiltechnik",vol.25, No.11, November 1975,pp.704-706, teaches in general a calendering process for coating a fabric with a layer of plastic material, employing the latter in a plastic state, the calender being provided with a preheating cylinder, cooling cylinders, and so forth. When describing this process, no mention is made of maintaining the fabric to be coated at the maximum temperature possible so as to avoid the cooling of the plastic coating material which only in the semi-molten state provides an excellent adhesion to the fabric, nor are there foreseen any particular devices for this purpose. US 4,052,521 to S.Ferrari teaches to tension the fabric both longitudinally and transversally, the aim of the invention being solely that of attaining a smooth surface of the coated fabric, by avoiding the transversal shrinkage of the latter. According to this patent, the transversal tensionally is performed after a first spreading process. After the transversal tensioning, a second coating is performed and the coated material is passed through a curing oven. Such process is complex and costly because it involves numerous series of subsequent devices and a double coating instead of performing the coating in one step in a calendering system. According to the present state of the art, satisfactory anchorage values can only be achieved with a complicated process wherein, before calendering, an adhesive primer has to be applied onto the fabric, said primer having been dissolved in suitable solvents, to a semiliquid state. The necessary equipment for this treatment demands a considerable amount of space, implies additional working time, and requires a tunnel drying oven and a recovery plant for the flammable solvents to reduce fire hazard and air pollution, all of which is very expensive. It is an object of the present invention to provide a relatively simple, on-line process for applying a plastic material coating onto a backing fabric, said coating exhibiting excellent anchorage characteristics, which allows to use already existing calendering means, said process not needing the coating of a primer as required by the prior art methods. According to the invention, the backing fabric is unwound, under adjustable tension, from a braked beam in order to stretch the fabric meshes lengthwise, and said fabric is then passed onto a stave widening cylinder to widen the meshes, i.e. to attain a deformation in the orthogonal direction. The so biaxially stretched fabric is then heated in a preheating station, then contacted with a heated coating cylinder for a variable length depending on the needs, thereby adjusting the final heating time of the fabric on that cylinder according to the nature of the backing fabric. Said fabric, which has been heated to a suitable temperature according to its nature, receives a layer of plastic material from a pair of upper melting cylinders of the calender, said plastic mass being also subjected to a further heating between said coating cylinder and one of said upper melting cylinders to bring and keep it at a temperature close to the melting point in order to apply to said fabric a plastic material layer of desired thickness. The so obtained layer of plastic material penetrates excellently the widened and heated meshes of the fabric, thereby providing a surprisingly strong adhesion that, from laboratory tests, has proved to be higher than that obtained with the prior art primer coating method, thus confirming the possiblity of eliminating said additional coating step which adds to the manufacturing costs and is not without drawbacks, and which demands bulky working stations. The inventive manufacturing process comprises a: pressing step of the coated fabric which is effected on the same coating cylinder by means of a further heated cylinder, while, from this point on, it proceeds according to the conventional steps viz.: detachment of the fabric from the coating cylinder; a possible passage over an embossing cylinder (if the plastified fabric to be obtained so demands); cooling on one or several cooling cylinders; winding-up of the finished fabrics. The process according to the invention, which is suited to eliminate the coating step of an adhesive primer onto the backing fabric to be coated with a layer of plastic material, will be better understood upon reading the description of the accessory equipment that can be fitted on-line to a conventional calender for fabric coating, which description is provided as a non-limiting example, with reference to the accompanying drawings, wherein: Fig. 1 schematically shows the arrangement of a state of the art calendering plant for coating a backing fabric; Fig. 2 schematically shows the arrangement of a plant for the same purpose provided with an apparatus according to the invention. In the figures, like parts are identified by like symbols. With reference to Fig. 1, according to the known art, a backing fabric 10, coming from a feeding beam (not shown), passes through a preliminary treatment device, identified as A in the left corner of the figure, wherein it is subjected to a coating operation with an adhesive primer in a semiliquid form suited to the nature of the backing fabric and of the coating layer which can consist, e.g., of PVC (polyvinyl chloride), PU (polyurethane) or the like. Device A implies therefore that the backing fabric 10 passes through a coating machine 11, followed by drying in an oven 12, and not to be overlooked, a plant (not shown) for the removal and recovery of the solvent, which plant is expensive and involves fire hazard and environmental pollution hazard. In said state of the art process the fabric treated with the primer, identified as 10', can be separately collected in B, or it can be directly sent to calendering with the interposition of a suitable recovery festooner (also not shown), of known construction. The primed fabric 10' is then coated in a calendering unit identified as C with a layer of variable thickness, depending on the needs, ranging of from 0.02 mm to 1 mm. The main cylinders 20, 22, 24, 26 and auxiliary cylinders 30, 32, 34, 36, 38 of said unit are shown schematically. Cylinders 20, 22 and 24, implementing the heating and plastification step for the plastic material, are driven and heated, while driven cylinder 26 serves for detaching the coated fabric from cylinder 24. The plastic material mass 29, which is preprocessed between cylinders 20 and 22 and further processed and gauged to the desired thickness between cylinders 20 and 24, forms, along arch 25 of the surface of cylinder 24, a layer that is applied and pressed onto fabric 10' by means of a pressing cylinder 30, which coated fabric 10' has been previously heated by means of an infrared device 40. Whereupon the fabric, coated with the plastic material layer on its primed side is detached from cylinder 24 by means of a rubber-coated cylinder 26 and can be embossed, if so required, by means of an embossing cylinder 32. The fabric is made to advance by the driven cylinder 38, through the idle cooling cylinders 34 and 36, and it is finally wound-up. Fig. 2 schematically shows one of the possible positions where the apparatus according to the invention can be fitted onto the framework of a calendering unit as conventionally used in industry and illustrated in Fig. 1, thereby allowing to eliminate the burdensome priming step. The apparatus, identified as D, comprising an unwinding beam 50 mounted on an axle 52 provided with an adjustable brake 54. Along its path, in the direction shown by arrow F, from unwinding beam 50, the untreated, i.e. unprimed, backing fabric 10 is subjected to a first stretching in the zone a of its meshes in the lengthwise direction, said stretching being proportional to the pulling force imparted by the adjustable braking action of brake 54; it then passes over a widening cylinder 60, for example of the kind with elastic staves, which effects a transversal stretching of the backing fabric 10, thereby causing a widening of the meshes in the orthogonal direction. The backing fabric with the so stretched meshes, identified as 13, is driven through a rocking device 70, containing a heating device 76, having at the front and rear end freely rotating rolls 72 and 74 respectively, which guide said fabric 13 through said heating element 76 arranged between said rolls. The rocking device 70 is pivoted on the same axis as the freely rotating roll 72, so that the rear roll 74 can take up various positions that, as shown in the figure, are concentric with the axis of said roll 72. According to these positions, so varies the tangent point in which backing fabric 13, stretched biaxially and preheated, touches the coating cylinder 24, thereby attaining a longer or shorter dwelling time proportional to the total length of arch 25', during which the fabric remains on the heated coating cylinder 24 in order to precisely set the temperature, according to the nature of the fabric 13, at which said fabric 13 enters beween cylinders 20 and 24. The position of roll 74 is adjusted by means of a hydraulic piston 80 pivoted at 78 on the rocking device 70 and, at the opposite end, on the fixed frame of the calender at point 79. The heated fabric 13 is coated, as it passes between cylinders 20 and 24, with a layer of the desired thickness of plastic material fed in a semimolten state, as shown in Fig. 1, between the upper melting cylinders 20 and 22 and between said cylinders 20 and 24. Maintaining the plastic material in this physical state during contact with the backing fabric is a decisive condition for the process according to the invention; this because the plastic material can best penetrate the widened meshes of the heated fabric 13, in the semimolten state, which state can be maintained thanks to the heating of the fabric, thereby providing an excellent adhesion and penetration even without a primer. The so coated fabric, passed between cylinders 20 and 24 of the calendering unit C, is pressed between cylinders 24 and 30, the latter cylinder also being heated, for example by means of an infrared device 31, whereafter the processing follows along the conventional steps already described in relation to Fig. 1 as for a primed fabric. This process implemented with the inventive apparatus is much more economical than the prior art process due to the fact that the cumbersome, expensive and also dangerous priming step has been eliminated. The relative apparatus, by means of which said inventive process is carried out, can easily be fitted onto the conventional calendering units already being used by those performing this type of process. The process and apparatus described and illustrated are suitable for any kind of backing fabric by selcting case by case the appropriate preheating temperature povided by heating device 76 and the stretching degree in the biaxial directions of the fabric. The preferred arrangement of the inventive apparatus as described and illustrated in Fig. 2, can obviously be modified according to, e.g. the type and position of calender C, on the understanding that the same object exploiting the same inventive idea is achieved. In particular, the rocking device 70 with its heating device 76 can be variously positioned about cylinder 24 using driving means different from hydraulic piston 80.

Claims: A process for directly applying, by calendering, a plastic material coating onto a natural, artificial or synthetic backing fabric without any pretreatment with an adhesive comprising the following processing steps: a) biaxial stretching of the backing fabric in order to widen the meshes thereof; b) heating of the backing fabric with such widened meshes; c) coating by means of a heated coating cylinder of the backing fabric with a layer of plastic material in the semimolten state, said material penetrating into the widened meshes of said backing fabric; d) pressing of the so coated backing fabric onto the said coating cylinder by means of a heated auxiliary pressing cylinder, in order to achieve further penetration of the plastic material layer into the meshes of the fabric and its adhesion thereto; e) detaching the fabric from the coating cylinder, followed possibly by an embossing step, then cooling and picking up of the so treated fabric, wherein in the processing step b) the heating of the backing fabric, depending on the nature of the latter, is performed by varying the contact time of the fabric in its widened meshes condition on the heated cylinder provided for coating said fabric with a layer of plastic material, in order to bring said fabric to the highest temperature it can withstand, which may vary from one fabric to the other yet which is the closest possible to the temperature of the plastic coating material, so as to prevent the mass of coating plastic material from cooling, said backing fabric being preheated by means of a rocking device which regulates said contact time of the backing fabric on said heated cylinder and hence the temperature of the fabric. A process according to claim 1, characterized in that the heating of the backing fabric, immediately before being coated with a layer of plastic material near to its melting temperature, is maintained and completed on the heated coating cylinder by varying the length of the contact arch and thus the dwelling time of the fabric on said heated cylinder, this by adjusting the first contact point of said backing fabric on the heated coating cylinder providing the final heating. A process according to claim 2, characterized in that said adjustment is implemented by means of an angular displacement of said rocking device. A process according to any of claims 1 to 3, characterized in that in the step c)inwhich the backing fabric is coated with a gauged layer of plastic material, the latter, coming from a first preprocessing and heating step carried out by an upper pair of melting cylinders of the calender, is heated and/or maintained at a temperature near to the melting point between the coating cylinder and one of the cylinders of said upper pair of melting cylinders immediately before depositing the layer of plastic material onto the heated and stretched backing fabric. An apparatus for carrying out the process according to any of claims 1 to 4, comprising: a beam (50) for feeding a web of untreated fabric (10), said beam (50) being mounted on an axle (52) provided with an adjustable brake (54) in order to cause the fabric to be stretched length wise; a device (60), arranged in line, for orthogonally stretching the fabric, a three cylinder, preferably a four cylinder (20, 22, 24, 26) calender (C) comprising two melting cylinders (20,22) and a heated coating cylinder (24); an embossing cylinder (32) and cooling cylinders (34, 36), on which the fabric (13) runs after its coating with plastic material; and a final wind up system of the coated fabric, said apparatus further comprising: a rocking device (70) for guiding and depositing the biaxially stretched fabric (13) onto said heated cylinder (24) having the function of coating said stretched fabric (13) with a layer of plastic material, said rocking device (70) being provided with a preliminary heating element (76) interposed between a front roll (72) and a rear roll (74) which guide the fabric (13) on said rocking device (70). An apparatus according to claim 5 characterized in that the rocking device (70) is pivoted on the same axle as the front roll (72), which guides the already stretched backing fabric (13), so that the opposite end of the device, provided with the rear guide roll (74), is capable of taking up various vertical positions, said positions being concentrical with the axis of said front roll (72), in order to vary the first contact point of the fabric (13) with the heated cylinder (24) whereon the fabric (13) has to be coated with a layer of plastic material, and consequently to vary the dwelling time of said fabric (13) on said heated cylinder (24), thus controlling the final temperature of the fabric (13) itself, while being coated with the plastic material in a semimolten state, said dwelling time depending on the characteristics of the backing fabric. An apparatus according to claims 5 and 6, characterized in that the rocking device (70) is rotated angularly by means of a piston (80), preferably a hydraulic one, an end (78) of which is hinged at an intermediate point of the frame of the rocking device (70), the opposite end (79) being pivoted at a fixed position of the frame of a calender unit (C), in order to adjust the position of the rear guide roll (74) of the device (70) in relation to the optimal contact point of fabric (13) on the heated coating cylinder (24).

Inventors: FUX CHARLY, FUX, CHARLY
Applicants: INTERPLASTICA SA
Application year: 1995
Main classification: D06N3
IPC codes: D06N3