Transparent composite structural elements and methods for producing same

The present invention relates to transparent composite structural elements or members capable of withstanding mechanical stresses and having for this purpose properties such as compressive, tensile and flexural strength, rigidity 10 and, absence of brimeness.

Such structural members can be employed, inter alia, to replace some conventional structures, made of metal or reinforced concrete, with structures which have identical mechanical characteristics but which are transparent, this making it possible in particular to give them an aesthetic character, or else to ensure better transmission of light especially in constructions, hangars, swimming pools, greenhouses, store windows, storage vessels and some com- 2Q ponents of automobile bodies which could advantageously be transparent, for example the uprights surrounding the windows and the windshield.

The invention therefore finds its application in many fields such as those of building, public works, urban furni- 2s ture, naval or automobile construction and the like. In fact, the structural members according to the invention can take on various shapes ranging from simple shapes to more complex shapes.

By way of examples, there may be mentioned elongate 30 members of the pillar, pylon, post, beam, arch or similar type, members of sheet type, both planar and curved, as well as objects of much more complex shape. Particular mention may be made of vertical structures of the post or pylon type used as supports for constructions or for the conductors of 35 electric or telephone aerial transmission lines, aeration shafts or those for extracting gases, fumes and steam, road signposts, posts for grid fencing, for glazing buildings, building facade panels, cladding panels, bottoms of boats and other craft, water towers, gas tanks, silos, transmitting or 40 receiving antennas, cranes and lifting equipment and the like.

PRIOR ART 45

The present invention makes it possible in particular to replace conventional posts or pylons made of metal or reinforced concrete, or at least a proportion of these, with transparent posts or pylons that will be capable of blending more harmoniously into the scenery. 50

Transparent materials known hitherto do not satisfy the combination of the characteristics required for the above applications:

glass does not withstand, for example, high flexural 55 forces; moreover, it is brittle;

less brittle, transparent plastics do not withstand high tensile or compressive forces; as a general rule they are also very costly, their cost greatly exceeding that of glass; 60

fibers, especially of glass, have remarkable tensile properties, but not those of compression or rigidity, and they are costly.

It is well known that mechanical properties of plastics can be modified and greatly improved by incorporating long or 65 short fibers. However, the products obtained are translucent or even opaque in great thicknesses.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide structural members such as referred to above, in the case of which the best possible compromise can be obtained between the mechanical properties sought after and transparency.

To meet this objective, there are proposed, in accordance with the present invention, transparent composite structural members based on three categories of different components, namely massive glass components, transparent fibers and a transparent matrix and/or coating resin, the refractive indices of these three components having furthermore to satisfy particular conditions and, with regard to the structure, the regions of maximum tensile stress and/or to the regions of impact strength of the member to be produced, those of the massive glass members, to the regions where the maximum compressive strength and the greatest rigidity is called for, the resin being employed for filling the gaps between the glass members and the fibers and to ensure the cohesion of the whole.

The subject of the present invention is therefore a transparent composite structural member capable of withstanding mechanical stresses, which comprises: at least one glass member (Al) of the stem, reed, tube, strip, sheet or similar type and/or glass members (A2) of the bead, particle or fragment type; transparent fibers (B); and

a cured transparent resin (C) in which the fibers (B) and the glass members (A2) are embedded, the fibers (B) and the resin (C) with which they are coated being bonded to the glass member (or members) (Al) and being arranged in order to protect the latter against impacts and/or to make it possible to obtain the required mechanical properties, said resin (C) being furthermore chosen so as to exhibit, in the cured state, a refractive index which, at a wavelength of 510 to 520 nm, does not differ by more than 0.001 from that of the fibers (B) and from that of the glass members (Al) and (A2), it being nevertheless possible for the difference in index between the resin (C) and the glass members (Al) and/or (A2) to be up to 0.01 when these members are large in size in relation to the overall structural member in question. It may be considered that such members of large size are, for example, massive glass fragments of dimensions greater than Vio of the smallest dimension of said structural member. The glass (Al) and (A2)

There is no particular limitation as to the type of massive glass which can be employed according to the invention, provided that the abovementioned conditions relating to the differences in refractive index with the other constituents of the final structural member are adhered to.

In the case where glass beads are employed it is advantageous, in order to obtain better mechanical properties, to provide not a homogeneous particle size but a distribution comprising a number of particle sizes. Thus, if D is the diameter of the large particles, very good results have been obtained by tamping very energetically a mixture comprising, by volume, approximately 90% of beads of diameter D, 15

20

30

2.5% of beads of diameter 0.225 D, 1 % of beads of diameter 0.1 D and very fine dust (for example ground glass fiber). When the tamping cannot be so energetic, which may be the case in some manufacturing processes employing viscous polymerizable compounds for forming the resin matrix (C), 5 the best particle size distribution is different. Thus, in another case where there was less tamping, approximately 55% by volume of beads of diameter D, 20% of beads of diameter 0.7 D, 5% of beads of diameter 0.25 D and very fine dust were required. To put things simply, it is advantageous to employ, by volume, 70 to 90% of large particles, 3 to 5% of intermediate particles (Vio to Vs of the diameter of the latter) and very fine dust. The latter has the advantage of being in the form of microbeads and of being mixed directly with the viscous liquid, the rheological properties of which it may even improve, thus facilitating some conversion processes (extrusion, mold filling) as will be described later.

The diameter of the largest particles will be a few centimeters, but in any event smaller by at least an order of magnitude than the dimensions of the structural member to be produced. The fibers (B)

There is no particular theoretical limitation either as to the type of transparent fibers (B) that can be employed, provided that the abovementioned conditions relating to the differences in refractive index with the other constituents of the 25 final structural member are adhered to.

The fibers (B) are preferably industrial glass fibers; there may be mentioned, inter alia, E, D or R glass fibers marketed by the company "Vetrotex International", which have the following refractive indices:

E glass=l ,550-1.557;

D glass =approximately 1.47;

R glass =approximately 1.54. There is nothing against employing a glass quality for the 35 members (Al) and (A2) which is different from that employed for the glass fibers (B), which are generally more costly, as long as the conditions laid down for the refractive indices are adhered to.

Nevertheless, natural or artificial silica fibers as well as 40 organic fibers such as aromatic polyamide fibers may also be mentioned. However, in this latter case it will be made certain that the manufacture of the structural members does not require the resin (C) composition to be heated to temperatures that are too high. 45

Depending on the applications, the fibers (B) may be used in the form of long fibers (several meters or more'in length), optionally woven or assembled as sheets or ravings or in the form of staple fibers (a few millimeters in length). These fibers have preferably been subjected to a sizing operation or 50 to a surface treatment to facilitate coupling and bonding with the resin (C). The resin (C)

The resin (C) is made from a polymer, of a copolymer, or of a mixture of curable transparent polymers and/or copoly- 55 mers, such as chiefly unsaturated polyesters, polyurethanes, polyalkyl methacrylates crosslinked by the addition of a functionalized (meth)acrylate, partially crosslinked polybutadienes, interpenetrating or semiinterpenetrating networks of styrene or alkyl methacrylate and urethane, and the like. 60 It can also be made from at least one transparent polymer or copolymer such as those based on styrene, alkyl methacrylate, vinyl chloride, vinyl acetate, vinyl alcohol, acrylonitrile, polycarbonates, polyaromatics, polyamides and polycellulosics. These latter polymers and copolymers must 65 therefore be considered as also included within the definition of the resin (C) according to the invention.

Use is made of at least one monomer, one polymer, one prepolymer, one polycondensation product or one polymerizable composition, in the liquid state, capable of producing, by polymerization, copolymerization, condensation or crosslinking, if need be under the effect of heat or of at least one chemical agent such as a curing catalyst, crosslinking agent and the like, a transparent hard matrix. The initial liquid materials or compositions must have a viscosity which is sufficiently low to make it possible to obtain, in the processing conditions which are described below, an intimate mixture with the fibers (B) and, if appropriate, with the members of type (A2), while avoiding the presence of gas bubbles which would introduce optical defects in the final product.

In some cases the resin (C) may be obtained from a single monomer giving a transparent polymer directly by itself, with a refractive index which is substantially identical with that of the glass components.

However, in most cases the resin (C) is obtained from at least two components (monomers, prepolymers, polymerizable compositions) of different refractive indices in order to obtain a transparent resin (C) whose refractive index can be varied, by adjusting the proportions of these components, from a value lower than that of the refractive index of the fibers (B) and of the members (Al) and/or (A2) to a higher value. The proportions adopted for these components are those that, after polymerization, make it possible to obtain a polymer with a refractive index which is substantially identical with that of the glass components.

Thus, while methyl methacrylate, considered alone, gives a polymer with a refractive index of 1.491, the polymerization of a mixture of approximately 70% by weight of acrylonitrile and approximately 30% by weight of methyl methacrylate results in a polymer with an index of 1.510, compatible with a glass with an index of 1.509, for example (index difference not exceeding 0.001).

Similarly, polymerization of pure styrene gives a "crystal" polymer with an index of 1.590; the addition of a little styrene in methyl methacrylate makes it possible to obtain a polymer with an index which is higher than that of polymethyl methacrylate, and therefore higher than 1.491.

Other reasons for proceeding by starting with a mixture of at least two components can also be mentioned: this mixture may make it possible to obtain properties other than transparency and the required mechanical properties (for example, to improve the impact strength or weather resistance); since commercial products do not have a perfectly uniform composition it will thus be possible, using a slight adjustment in the proportions, to obtain the required refractive index with a high precision; the achievement of this condition is of great importance for obtaining a good transparency; the processing conditions may be facilitated thereby; for example, the viscosity of the mixture is frequently an important characteristic for the conversion of plastics; now, this conversion can be affected by employing as components of the mixture a prepolymer and a liquid in which it is dissolved. The transparent resin (C) is preferably an unsaturated polyester resin. Such resins are well known to a person skilled in the art. They are obtained by polycondensation of one or more diols with one or a number of saturated, unsaturated and optionally aromatic diacids (or acid dianhydrides). The unsaturated anhydride most widely employed is maleic anhydride; the aromatic units are generally introduced using phthalic, for example ortho-phthalic, anhydride. Immediately after its synthesis the unsaturated polyester is stabilized and then mixed with at least one vinylaromatic monomer such as styrene. A catalyst such as a peroxide is generally added at the time of use and this, when heated at the time of the forming, initiates the polymerization of the 5 whole. Acetylacetone peroxide and cyclohexanone peroxide may be mentioned as particular peroxides.

The unsaturated polyester resin compositions may also include additives such as quaternary ammonium compounds or ethoxylated tertiary amines which facilitate the wetting of the fibers (B), compounds such as methyl methacrylate, which improve transparency, inhibitors and accelerators. Unsaturated polyester resin compositions which have improved transparencies are, for example, those comprising a saturated (ortho-phthalic) polyester and a maleic unsaturated polyester. 15

The refractive indices of the cured unsaturated polyester resins are, in the wavelength range from 510 to 520 run, generally approximately between 1.52 and 1.56. The refractive index can be adjusted by using a comonomer, such as methyl methacrylate or an itaconic acid ester, by mixing 20 with another resin until the desired value is obtained, or by mixing two unsaturated polyesters of different indices. The bonding between the three components of the structural member

The bonding between the fibers (B) embedded in the resin 25 (C) and the glass member(s) (Al) and/or (A2) may be effected by direct adhesion of the resin (C) to the said member(s) (Al) and/or (A2).

Provision may also be made for facilitating this bonding—which is important for obtaining transparency and 30 maintaining it—by a sizing operation or a surface treatment of the massive glass, of the type of sizing operation or surface treatment indicated with regard to the fibers (B).

The sizing agent is advantageously chosen as a function of the nature of the resin (C) employed. Thus, a silane-based 35 mixture constitutes an agent for bridging between the glass members and resin (C). In the case where the resin (C) is a polyester, such a sizing agent may, for example, consist of a water solution of approximately 15% by weight of polyvinyl acetate), of approximately 0.5% by weight of meth- 40 acrylic silane and of approximately 20% by weight of a fatty acid amide. The silanes employed may correspond to the formula:

in which
n is between 0 and 3;
X is a hydrolyzable group; and

Y is an organic group selected as a function of its 50 reactivity with the organic matrix. Similarly, there are sizes which can be applied to organic fibers and which are capable of forming bridges between the polymer constituting the fiber and that constituting the matrix, by virtue of polyfunctional molecules. These sizing 55 agents are very closely related to those employed in adhesives for plastics.

Provision may also be made for applying to the massive glass an intermediate bonding layer (D) made of a transparent plastic advantageously chosen to make it possible to 60 absorb the differential expansion effects which could bring about a rupture of the bonding between the massive glass and the resin. Polyvinylbutyral may be mentioned as an example of these bonding and absorbing plastics, applied as a thin layer (of the order of 1 to 10 um) to the massive glass, 65 for example by spraying or coating and, if appropriate, curing.

Furthermore, in the case where a number of glass members (Al) are employed for a single final structural member, said glass members (Al) may be bonded together using resin (C).

Relative proportions and arrangements of the three constituents of the structural member, and examples of use

The proportions and the arrangements of the three main constituents of the structural members according to the invention depend on the application being considered and the required performance, and may vary within wide limits. In general, staple fibers are employed for improving impact strength, long fibers for obtaining good rigidity and good tensile strength, and glass members (Al) and/or (A2) for increasing the rigidity and the compressive strength and to reduce the cost of manufacture. Since stresses are not homogeneous inside the structural members to be produced, this will be taken into account in the relative arrangement of the constituents, the optimized material then exhibiting a heterogeneous macrostructure, combining microstructure regions which are homogeneous but different. The regions which are subject to the highest tensile stresses will be enriched chiefly in long fibers, the regions situated at the periphery receiving staple fibers, the regions under compressive loads, or else not subject to high stresses, being filled with massive glass members.

The overall quantity of the glass members may vary within wide limits; it is commonly from 30 to 50% by volume in the fiber-filled regions.

Thus, if the intention is to produce a structure which bears only relatively moderate forces (a transparent massive member working chiefly in compression), a random bulk mixture of staple fibers (a few millimeters in length), of glass particles and of resin may be produced, the fibers representing approximately 2to 3% by volume of the total and the particles between 30 and 80%. An embedded beam working in flexure may be taken as an example of a high-performance article. It will then be appropriate to arrange a high percentage of fibers in the regions subject to high tensile stresses, that is to say at the periphery, and a high percentage of particles in the regions where the stresses are low (cost economy) or in those where the compressive stresses are high, that is to say in the center. In the former regions, the percentage of fibers may reach 50 to 55% by volume, in the latter regions the percentage of particles may even approach 75 to 80% by volume.

In another particular embodiment of the present invention, the structural member is intended to constitute an elongate member such as a pillar, a pylon, a post, a beam, an arch or the like. It then comprises a backbone consisting of at least one glass member (Al) or of glass members (A2) bonded together using resin (C), the fibers (B) embedded in the resin (C) being arranged so as to surround the backbone while being bonded to it.

In particular, the backbone consists of a glass tube or stem (Al), a core formed by glass fragments and/or beads (A2) embedded in the resin (C), a bundle of glass reeds or stems optionally assembled by adhesive bonding with the resin (C) or glass sheets (Al) assembled using resin (B) in order to form an elongate hollow core of polygonal section, the backbone being surrounded by a sheath which is integrally attached to it and which consists of at least one sheathing layer applied around the backbone by filamentary winding of a glass fiber roving (B) impregnated with curable transparent resin (C), or by wrapping with a glass fiber fabric (B) impregnated with curable transparent resin (C), the whole having been subjected to curing of the resin (C) for impregnating the fibers (B), with optional interposition, between 8

said backbone and the first sheathing layer, of a layer of size for the backbone or of a layer (D) of bonding plastic, making it possible in particular to absorb the differential expansion effects which could bring about a rupture as a result of the variations in temperature between the backbone and the 5 sheath.

In the case where the backbone consists of a glass tube, it is advantageously possible to obtain a transparent post or pylon whose length may reach 15 m, which can be employed as a support for aerial lines. The tube employed may have a 10 diameter ranging up to 1 m and a thickness ranging up to 10 cm.

In this case, the fibers (B) may be advantageously glass fibers which have been used in the form of a roving impregnated with curable transparent resin and wound 15 around the glass tube (Al) as adjoining turns of roving in order to form at least one layer for sheathing the tube (Al).

The sheath may comprise from 2 to 20 layers and, in particular, from 4 to 10 layers.

If such a structural member is intended to be subjected to 20 tensile stresses, which is frequently the case, it may be found advantageous to make the glass fibers absorb a proportion of the tensile stress. Thus, for each of the sheathing layers which are obtained by helical winding of the roving, the pitch of the helix can be adjusted as a function of the 25 required properties. In this case, too, and if at least two layers of sheathing are envisaged, the helices of two successive layers may be oriented in an opposite manner. In the case where the structural member is intended to be subjected to compressive stresses, which is the case with some pylon 30 members, a quasi-circumferential winding of the roving may be envisaged for each of the sheathing layers. Any combination of helical and circumferential windings may obviously be envisaged for the different layers of the same structural member of this type. Lengthwise sheets of roving 35 may also be arranged between two layers.

The surface of such a struatural member may be preferably subjected to an adjustment, and may comprise a layer of a varnish (E) protecting against the action of oxygen and of ultraviolet radiation. Varnishes of polyurethane type, to 40 which an activator is generally added at the time of use, may be mentioned as varnishes that can be employed for this purpose.

This elongate structural member, as just defined, may be manufactured by a process comprising the operations of: 45

performing a filamentary winding of a glass fiber roving (B) impregnated with curable resin (C), around the glass tube (Al) intended to form the backbone of the structural member, in order to form at least one, preferably, at least two layers of sheathing of the tube (Al); 50

immediately after the gelling of the resin (C) begins, winding around the tube (Al), thus endowed with its sheath, a film (F) which is neutral towards the resin and capable of protecting the surface of the member from the effect of oxygen and of correcting any surface 55 microcorrugations;

leaving the member wrapped in neutral film (F) at a temperature of 15° to 40° C. for a period of 1 to 10 hours to allow the resin (C) to cure;

unreeling the neutral film (F);

if appropriate, adjusting the surface of the tube obtained so as to eliminate the surface defects due to the reeling of the film (F) without reaching the glass fibers (B); and; 65

if appropriate, applying to the surface of the resulting structural member, for example by spraying, a varnish

60

(E) which protects against the action of the UV and of oxygen.

Before performing the filament winding, the tube is advantageously coated with a layer (D)—as defined above— for bonding to the first layer of sheathing to be applied.

The protective neutral film employed is advantageously a polyester film, for example a film marketed under the name of "Mylar" by the DuPont company, which is in the form of a ribbon or sheet.

In accordance with another embodiment of the invention, the structural member is in the form of a transparent sheet or of a transparent tube, rigid and nonbrittle, made from a layer of glass fragments and/or beads (A2), which is applied onto a glass fiber fabric (B) or sandwiched between two glass fiber fabrics (B) in a coating of cured resin (C). To obtain such a sandwichtype sheet, a layer of glass fibers which is impregnated with curable resin (C) may be arranged in the bottom of a mold, the curable resin (C) mixed with the members (A2) may be poured to form the intermediate layer, a second sheet of impregnated fibers may be placed, and curing may then be performed. To obtain a two-layer or three-layer tube of this type, a rotating cylindrical mold may be employed, and there may be thrown successively against its inner wall a layer of glass fiber fabric, followed by a mixture of resin + members (A2) and then, if appropriate, by another layer of glass fabric.

In the simplest case, fibers and members (A2) which are intimately mixed in the resin (C) before it is cured are dispersed, and the polymerization is next carried out. A homogeneous and isotropic material is thus obtained, unless the device allows an orienting force to remain (lamination, extrusion or molding without sufficient viscosity or agitation).

In general, however, the aim is to produce a heterogeneous and anisotropic composition so as better to absorb and/or to distribute mechanical stresses. One technique consists in prearranging the fibers and the particles according to the desired scheme (for example in a mold for heat-curable resin) and, while taking care to avoid allowing air bubbles to remain, embedding the whole, thus arranged, in the viscous mixture which is delivered into the mold and which subsequentiy cures under the effect of temperature and of the polymerization agents added to the mixture (peroxides or the like).

For some applications, the production of the material may be effected by progressively mixing the different components. Thus, the manufacture of a section by pultrusion may be effected by first preparing the mixture of the glass particles and the prepolymer, the sheet of fibers then being charged into this mixture before passing through the pultrusion die.

The manufacture of structural members according to the invention is therefore performed by making use of known techniques such as molding using contact, in vacuum, by spraying, or in a press, injection molding, centrifuging, reeling, calendering, pultrusion, extrusion, filament winding and the like. The choice of the method depends on the properties of the resin (C) which is chosen and on the desired arrangement of the glass members within the structural member. The production of the latter may also take place in a number of stages. Thus, to produce a cylindrical post it is possible, for example, to mold the center of the article by employing the mixture of glass beads (A2) and resin (C) and then to perform the laying of a glass fabric impregnated with the organic component around the core thus formed, or else to prepare by pultrusion, sticks of fibers embedded in the resin, these sticks being then incorporated in a molding or 10

extrusion operation so as to be finally incorporated into the remainder of the mixture of matrix + beads and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate further the objective of the present invention and the numerous embodiments which are possible, a description thereof will be given below, by way of examples which are purely illustrative and non-limiting, of a number of embodiments shown in the attached drawing. In the examples, the percentages are given by weight, unless otherwise indicated.

In the drawing:

FIG. 1 is a perspective view of a tubular structural member in accordance with a first embodiment of the invention, in the course of manufacture;

FIG. 2 is a view in cross-section of the member of FIG. 1, after finishing;

FIGS. 3 and 4 are perspective views, similar to FIG. 1, of a second and of a third embodiment of the invention, respectively;

FIG. 5 is a view in section of a flat structural member in accordance with a fourth embodiment of the present invention;

FIG. 6 illustrates diagrammatically the continuous manufacture of a flat structural member similar to that shown in FIG. 5;

FIGS. 7a to 7c illustrate diagrammatically the manufacture of a tubular structural member in accordance with a fifth embodiment;

FIGS. 8a to 8d are views in cross-section of elongate structural members in accordance with four other embodiments of the invention; and

FIG. 9a is an elevation view of another elongate structural member according to the invention and FIG. 9b is a view in cross-section of this member, on a larger scale.

EXAMPLE 1

Manufacture of transparent sheets which have excellent mechanical properties

Members (A2) consisting of glass microbeads which have the following composition:

Si02: 53-54%;

A1203: 14-15%;

CaO+MgO: 20-24%;

B203: 6.5-9%;

F: 0-0.7%

and glass fibers (B) in the form of fabric, of the same composition, are employed; to form the resin (C) a polyester prepolymer is employed to which a few percent of styrene are added to adjust the refractive index to that of the glass.

A mixture of the prepolymer and of the microbeads is prepared. Two tapes of glass fiber fabric are passed continuously through the prepolymer to which 1-2% of methyl ethyl ketone peroxide have been added, in order to preimpregnate them with the prepolymer, which gels during their travel.

A layer of a resin-microbead mix is deposited continuously on the first tape of preimpregnated fabric, before proceeding to the lamination of the two tapes, and to passing the whole through a polymerization oven where the composite cures while passing between rolls which control its thickness and its surface quality. The diagram in FIG. 6 illustrates this embodiment.

After postcuring, transparent sheets which have excellent mechanical properties are obtained.

EXAMPLE 2

Manufacture of hollow transparent elongate members (glass content: approximately 70%

If reference is made to FIGS. 1 and 2, it can be seen that (Al) has been used to denote a glass tube surrounded by a sheath obtained by circumferential filamentary winding, in a number of successive layers, of a glass fiber roving (B) impregnated with a curable resin (C), and by curing this resin. An intermediate layer (D) promoting the bonding of the sheath (B-C) to the glass tube (Al) is placed between the tube (Al) and the first layer of sheathing. An outer coat of varnish (E) protects the tube from the effects of oxygen and UV radiation.

This structural member is manufactured by performing the following successive stages:

(a) the surface of a glass tube 500 mm in length, 60 mm in outer diameter and 4 mm in thickness is degreased with solvent;

(b) the outer surface of the glass tube is treated by applying with a pad a solution of silane A174 (marketed by Union Carbide) and drying for 15 minutes at 40° C;

(c) using filamentary winding on a Baer machine, a plurality of layers (for example 4 to 10 layers) of a glass fiber roving with a linear density of the order of 50 tex are applied around the tube, the roving being marketed by Vetrotex International under the name "VETROTEX 5136", impregnated with an unsaturated polyester resin composition formulated as follows:

This filamentary winding is performed quasi-circumferentially, with adjoining turns, the thickness of one layer in the finished structural member being on the order of 0.1-0.2 mm.

(d) after the winding stage the beginning of the gelling of the polyester resin is observed and, at this precise time, a tape of 50 mm wide MYLAR film is wound helically around the coated tube, leaving free spaces between the turns on the order of approximately 1 mm in order to control the exudation of the resin during the crosslinking stage, to entrain the air bubbles included between the film and the surface of the coated tube;

(e) after the resin has cured cold, the MYLAR tape is unwound and an adjustment of the surface is then