TECHNICAL FIELD

The present invention relates to a method of manufacturing a tube, a tube and a twin tube.

BACKGROUND OF THE INVENTION

With particular reference to Figures 1 to 3, in the field of plastic tubes it is known to provide twin tubes A (namely, a pair of tubes facing one another and joined) , wherein each tube B comprises an inner layer C in polymer material, a reinforcement layer D arranged around the inner layer C and an outer layer E in polymeric material arranged around the reinforcement layer D.

When this type of tubes A is subjected to vulcanization, the inner layer C, by crosslinking, exerts a strong centrifugal pressure that can damage (and often damages) the outer layer E.

To overcome this drawback, in some cases (Figure 1) it is provided a particularly thick outer layer E. However, also this expedient is not free from drawbacks. In this regard, it is important to emphasize that the tubes A provided with an outer layer E often need to be assembled with metal fittings at their ends. To this aim, a section of the outer coating ("outer peel") must be removed, resulting in higher costs and rubber material to be disposed of. In other cases (again to reduce the risk of damaging the outer layer E - Figures 2 and 3), a plastic protection element F, which must be removed after the vulcanization, is arranged around the tube. Even this approach, however, has the following disadvantages.

• The vulcanization step is inefficient (it must therefore be performed for longer times and/or at higher temperatures) because of the insulating effect of the element F.

• The process is particularly complex, since it requires a coating step for applying the element F and a removal step for removing the element F.

The use of the element F requires the presence of a connecting portion G that determines greater overall dimensions of the main tube A and that, once the tubes are separated, creates a bulge on the surface of each tube B. This bulge must be removed, or it can give problems (worsened seal) when the tube B is connected with bushings to be used.

The object of the present invention is to provide a method of manufacturing a tube, a tube and a twin tube which overcome, at least partially, the drawbacks of the prior art and are, at the same time, cheap and easy to manufacture and/or to use.

SUMMARY According to the present invention it is provided a method for the production of a tube, a tube and a twin tube, as claimed in the independent claims below, and, preferably, in any one of the claims depending directly or indirectly on the independent claims.

The tube may have inner and outer cross sections of different shapes (e.g. circular, square, triangular, etc.) . Usually, the tube has a substantially circular (inner and outer) section.

In particular, tube means an elongated element with a longitudinal through cavity (lumen) . Typically, the tubes are used for the transport of fluids (liquids or gases) . BRIEF DESCRIPTION OF THE FIGURES

The invention is described below with reference to the accompanying figures, which show a non-limiting example of an embodiment, wherein:

- Figures 1 and 2 are cross-sections of twin tubes of the prior art;

- Figure 3 is a cross section of the tube of Figure 2 during a processing step (more precisely, during vulcanization) ;

- Figure 4 shows a cross section of a tube during a step of a method in accordance with the present invention;

- Figure 5 is a cross section of a twin tube in accordance with the present invention; - Figure 6 is a cross section of a tube in accordance with the present invention;

- Figure 7 is a graph, which is experimentally obtained for a given polymeric material, and shows the tensile strength of the polymeric material (in the ordinate - measuring unit: N/mm2) depending on how long (on the abscissa - measuring unit: minute) the polymeric material has been subjected to vulcanization;

- Figure 8 is an experimentally obtained graph which shows for a given polymeric material the percentage elongation

(compared to the original length of the material) at break of the material (in the ordinate) depending on how long (on the abscissa - measuring unit: minute) the polymeric material has been subjected to vulcanization;

- Figure 9 is a graph of a known type, which is experimentally obtained for a given polymeric material and shows the load (ordinate - measuring unit: MPa) to which the material is subjected depending on the relative elongation (abscissa - changed length/original length) of the material for different times (in minutes) of vulcanization; and

- Figure 10 shows a rheometric curve experimentally obtained for a polymeric material sample following the procedure according to the provisions of ASTM D6204; the ordinate shows the torque (measuring unit: dNm) and the abscissa shows the time (measuring unit: minute) .

DETAILED DESCRIPTION

A first aspect of the present invention provides a method for the production of a tube 1 (Figure 6) .

The method comprises a first vulcanization step of an intermediate tube 2 (Figure 4) having a layer 3, which comprises (in particular, is made up of) a first polymeric material; at least one reinforcement layer 4, which is arranged around (and in contact of) the layer 3; and a layer 5, which comprises (in particular, is made up of) a second polymeric material and is arranged around (and in contact of) the reinforcement layer 4.

In particular, the layer 4 comprises (in particular, is made up of) a thread material.

According to some embodiments, the layer 4 comprises (consists of) a material selected from the group consisting of: metal, aramid fibre, polyester, polyamide, polypropylene, natural fibres (and combinations thereof) .

More precisely, these threads are twisted and/or helically (spirally) arranged.

Advantageously, the second polymeric material is different from the first polymeric material.

During the first vulcanization step, the first and the second polymeric material are partially vulcanized.

The method further comprises a coating step, which follows the first vulcanization step and during which a layer 6 is arranged (in particular, by extrusion) around (and in contact of) the intermediate tube 2 (more precisely, the layer 4) . In particular, the layer 6 comprises (more specifically, is made up of) a third polymeric material. Advantageously, the third polymeric material is different from the second (and the first) polymeric material.

It is also provided a second vulcanization step (which follows the coating step) , during which the first and the second polymeric material are further vulcanized and the third polymeric material around (and in contact of) the layer 5 is at least partially (in particular, completely) vulcanized. In particular, during the second vulcanization step, the vulcanization of the first, second and third polymeric material is substantially completed (obtaining a vulcanization of at least 98%) .

It has been experimentally observed that, during the second vulcanization step, the obtained centrifugal thrust is relatively low, thus consequently reducing the risk of damaging the layer 6. Presumably, this is at least partially due to the fact that the layer 3 is partially vulcanized during the first vulcanization step, and therefore during the second vulcanization step it has a reduced tendency to change its volume and/or its geometry. In this regard, during the second vulcanization step nothing coats the layer 6. More precisely, a protection element F (or something similar) is absent.

According to some embodiments, during the first vulcanization step, a helically (spirally) wound tissue (of known type and not shown) is arranged around the layer 3. After the first vulcanization step (and before the second vulcanization step), the tissue is removed.

There are several standard methodologies to verify whether a polymer material has been partially vulcanized (or to measure the vulcanization percentage) .

In the present application, the following steps are carried out to verify whether a polymeric material has been partially vulcanized. An experimental curve (Figure 7) may be drawn by measuring the tensile strength of a material subjected to vulcanization for different time periods (while maintaining the other conditions, in particular temperature) ; and an experimental curve (Figure 8) may be drawn by measuring the percentage elongation at break of the same material submitted to vulcanization for different time periods (while maintaining the other conditions, in particular temperature) . In particular, measurements are carried out according to the provisions of the standard ISO 37 (specimen type 2) . Once drawn the two graphs, to know whether a sample of that material is partially vulcanized (not knowing its vulcanization time) , both its elongation at break and tensile strength are measured. If the tensile strength is more than the minimum and less than the maximum of the curve of Figure 7, it means that the sample is partially vulcanized or over-vulcanized. Now, if the measured elongation at break is, along the curve of Figure 8, to the left of the vulcanization time where the curve of Figure 7 reaches its peak, the sample is partially vulcanized; vice versa, if the measured elongation at break is, along the curve of Figure 8, to the right of the vulcanization time where the curve of Figure 7 reaches its peak, the sample is over-vulcanized.

Analogously, in the present application, vulcanization degree means what is determined by measuring the tensile strength of the sample. The percentage of the measured tensile strength with respect to the maximum tensile strength indicates the vulcanization percentage.

Obviously, once the graphs of Figures 7 and 8 have been drawn and the vulcanization time of the sample is known, if this time is shorter than the vulcanization time where the curve of Figure 7 reaches its peak, the sample is partially vulcani zed .

A further example of knowing whether a polymeric material has been partially vulcanized involves the use of experimentally obtained graphs, of the type shown in Figure 9 and shown in textbooks . According to some embodiments, after the first vulcanization step (and just before the second vulcanization step) the first polymeric material has a vulcanization degree up to 60%. In particular, the first polymeric material has a vulcanization degree of at least 30%.

According to some embodiments, after the first vulcanization step (and just before the second vulcanization step) the second polymeric material has a vulcanization degree up to 50%. In particular, the first polymeric material has a vulcanization degree of at least 20%.

In the following description, the vulcanization temperature will be reported in °C, and the relative pressure of the steam in mBar will be reported between the brackets, in case the steam is used as a heating means in the vulcanization process, which is a method normally used for this type of product.

In some cases, the first vulcanization step occurs at a temperature ranging from 130°C (1688 mBar) to 160°C (5167 mBar) (in particular, from 140°C (2601 mBar) to 150°C (3747 mBar) ; more specifically, about 145°C for a total time ranging from 15 to 50 minutes (in particular, from 30 to 40 minutes, more specifically, about 35 minutes) . During the first vulcanization step, it is maintained a temperature of at least 140°C (2.601 mbar) (in particular, up to 155°C (4420 mBar) ) for a time of at least 10 minutes (in particular, up to 30 minutes) .

According to specific embodiments, the first vulcanization step includes an ascent portion, during which the temperature is increased from room temperature (0 mBar) up to at least 140°C (2601 mBar) (in particular, up to below 155°C (4420 mBar); more precisely, up to 145°C (3147 mBar)) and which has a duration of about 10-30 minutes (in particular, about 15 minutes); a maintenance portion, which follows the ascent portion and during which a temperature of at least 140°C (2.601 mbar) is maintained (in particular, up to 155°C (4420 mBar); more precisely, 145°C (3147 mBar)) for at least 10 minutes (in particular, up to 30 minutes, more specifically, about 15 minutes) ; and a descent portion, which follows the maintenance portion, during which the pressure is brought to 0 mBar and which has a duration of at least 3 minutes (in particular, up to 10 minutes, more specifically, about 5 minutes) .

In other words, during the first vulcanization step the intermediate tube 2 is subjected to the above conditions. In some cases, the second vulcanization step occurs at a temperature ranging from 130°C (1688 mBar) to 160°C (5167 mBar) (in particular, from 140°C (2601 mBar) to 150°C (3747 mBar) ; more specifically, about 145°C) for a total time ranging from 60 to 95 minutes (in particular, from 75 to 85 minutes, more specifically, about 80 minutes) . During the first vulcanization step, it is maintained a temperature of at least 140°C (2.601 mbar) (in particular, up to 155°C (4420 mBar) ) for a time of at least 55 minutes (in particular, up to 75 minutes) .

According to specific embodiments, the second vulcanization step includes an ascent portion, during which the temperature is increased from room temperature (0 mBar) up to at least 140°C (2601 mBar) (in particular, up to below 155°C (4420 mBar); more precisely, 145°C (3147 mBar)) and which has a duration of about 10-30 minutes (in particular, about 15 minutes) ; a maintenance portion, which follows the ascent portion and during which a temperature of at least 140°C (2.601 mbar) (in particular, up to 155°C (4420 mBar), more precisely, 145°C (3147 mBar)) is maintained for at least 40 minutes (in particular, up to 80 minutes, more specifically about 60 minutes); and a descent portion, which follows the maintenance portion, during which the pressure is brought to 0 mBar and which has a duration of at least 3 minutes (in particular, up to 10 minutes, more specifically, about 5 minutes) .

There are several standard methodologies to measure the curve trend and the vulcanization rate of certain materials. For example, by carrying out the aforesaid measurements described in relation to Figure 7; the material showing the minimum vulcanization time to reach the maximum tensile strength is the material with the higher vulcanization rate.

Advantageously, but not necessarily, the third polymeric material has a vulcanization rate higher than the vulcanization rate of the second polymeric material.

This allows a relatively homogeneous vulcanization for the layers 5 and 6. In this regard, the layer 5 is subjected to two polymerization steps, whereas the layer 6 is subjected to a single vulcanization step.

In some cases (but not always), the third polymeric material has a scorch time Ts2 shorter than the scorch time Ts2 of the second polymeric material.

According to some embodiments, the third polymeric material has T30, T60 and T90 respectively shorter than the T30, T60 and T90 of the second polymeric material.

In particular, the Ts2, T30, T60 and T90 are measured according to the provisions of ASTM D6204 (where several measurements of a material are taken to draw the graph of Figure 10) . The parameters used during the procedures specified in ASTM D6204 are: duration time 3 minutes; swing arc degrees 0.5; temperature 190°C (MDR-type equipment) . More precisely, the scorch time Ts2 is the time during which the torque grows by 2 dNm with respect to the minimum obtained torque (actually, this is considered the vulcanization starting point) .

More precisely, T30 is the time where the torque reaches a value equal to ML (minimum obtained torque of the curve) plus 30% of the difference between MH (maximum obtained torque of the curve) and ML (like, e.g. the one of Figure 10) ; T60 and T90 are identified by following the same procedure described for T30 but by calculating 60% and, respectively, 90% of the difference between the maximum obtained torque of the curve (like, e.g. the one of Figure 10) and the minimum obtained torque of the curve.

Advantageously, but not necessarily, the first polymeric material has a vulcanization rate higher than the vulcanization rate of the second polymeric material. In addition or alternatively, the second polymeric material generally has a vulcanizing rate lower than the vulcanization rate of the third polymeric material.

It has been experimentally observed that a high-quality tube 1 is obtained under these conditions. In this regard, although it is subjected to two vulcanization steps, during these vulcanization steps the first polymeric material receives a smaller amount of thermal energy (being partially thermally isolated) if compared to the outside of the layers 4, 5 and optionally 6.

In some cases (but not always), the first polymeric material has a scorch time Ts2 lower than the scorch time Ts2 of the second polymeric material. In addition or alternatively, the second polymeric material generally has a scorch time Ts2 longer than the scorch time Ts2 of the third polymeric material.

According to some embodiments, the first polymeric material has T30, T60 and T90 respectively shorter than T30, T60 and T90 of the second polymeric material. In addition or alternatively, the second polymeric material generally has T30, T60 and T90 respectively longer than T30, T60 and T90 of the third polymeric material.

In some cases, the first polymeric material has a Ts2 ranging from 0.50 to 0.80 minutes, a T30 ranging from 0.80 to 1.20 minutes, a T60 ranging from 1.30 to 1.70 minutes and a T90 ranging from 2.10 to 2.50 minutes. In addition or alternatively, the second polymeric material has a Ts2 ranging from 1.20 to 1.80 minutes, a T30 ranging from 1.45 to 1.85 minutes, a T60 ranging from 1.90 to 2.30 minutes and a T90 ranging from 2.50 to 2.90 minutes. In addition or alternatively, the third polymeric material has a Ts2 ranging from 1.10 to 1.70 minutes, a T30 ranging from 1.30 to 1.70 minutes, a T60 ranging from 1.70 to 2.10 minutes and a T90 ranging from 2.40 to 2.80 minutes.

The above characteristics relative to the vulcanization rate, the scorch time, T30, T60 and T90 etc. are advantageous (i.e. involve some advantages and improvements), but are not essential to obtain a basic technical effect of reducing the risk of damages to the layer 6 due to a remarkable change in volume and/or geometry of the layer 3.

According to some embodiments, the method comprises a first coating step (before the first vulcanization step) , during which the reinforcement layer 4 is arranged (in particular, by braiding and/or coiling) around the layer 3.

In some cases, during the first coating step, more reinforcement layers 4 are applied. In such cases, among the different reinforcement layers 4 it is arranged a thin layer (typically called innerliner, in particular with a thickness smaller than 0.8 mm) of a fourth polymeric material .

Advantageously, the first polymeric material has a vulcanization rate higher than the vulcanization rate of the fourth polymeric material.

In particular, the first polymeric material has a scorch time Ts2 shorter than the scorch time Ts2 of the fourth polymeric material.

According to some embodiments, the first polymeric material has T30, T60 and T90 respectively shorter than the T30, T60 and T90 of the fourth polymeric material.

Advantageously, the fourth polymeric material has a vulcanization rate higher than the vulcanization rate of the second polymeric material.

It has been experimentally observed that a high-quality tube 1 is obtained under these conditions. In this regard, what has been said relative to the layer 3 applies, mutatis mutandis, also to the innerliner or innerliners.

In particular, the fourth polymeric material has a scorch time Ts2 shorter than the scorch time (Ts2) of the second polymeric material.

According to some embodiments, the fourth polymeric material has T30, T60 and T90 respectively shorter than the T30, T60 and T90 of the second polymeric material.

In some cases, the fourth polymeric material has a Ts2 ranging from 0.70 to 1.10 minutes, a T30 ranging from 1.00 to 1.40 minutes, a T60 ranging from 1.60 to 2.00 minutes and a T90 ranging from 2.30 to 2.70 minutes.

Advantageously, the method also comprises a second coating step (which at least partially follows the first coating step), during which the layer 5 is arranged (in particular, by extrusion) around the reinforcement layer 4. Thus, it is obtained the intermediate tube 2 (Figure 4) .

In particular, the layer 3 has an elongated shape and a longitudinal through cavity (lumen) 7. Typically, during the aforesaid steps, the cavity 7 is engaged by a mandrel (of known type and not shown - a flexible plastic round element - in particular, possibly treated with gliding agents) .

According to some embodiments (Figure 5), during the coating step, two intermediate tubes 2, arranged side by side, are externally coated (wrapped) , each with a respective layer 6 so that the two third layers 6 are mutually in contact and joined to obtain a twin tube 8. The twin tube 8 is then subjected to the second vulcanization step. Subsequently, in order to be used, the twin tube 8 needs to be assembled with the metal parts at the two ends. To this aim, it is longitudinally cut for a limited length to obtain two tubes 1 at the two ends.

Although, as already stated, the method is usable with particular and further advantages to obtain twin tubes, it may be (as it is clear from what is described above) used for manufacturing a single tube. Also in this case (namely, for manufacturing the single tube) it is obtained the basic technical effect of reducing the risk of damages to the layer 6 due to a remarkable change in volume and/or geometry of the layer 3.

Advantageously, during the first and the second vulcanization step, the water vapour is used to heat the polymeric materials and, therefore, to obtain the vulcanization .

According to some embodiments, the layer 3 has a thickness ranging from 0.3 mm to 3.0 mm; the layer 5 has a thickness ranging from 0.1 mm to 1.5 mm; the layer 6 has a thickness ranging from 0.2 mm to 2.5 mm.

In accordance with a second aspect of the present invention (Figure 6) , it is provided a tube 1 comprising a layer 3, which comprises (in particular, is made up of) a first polymeric material and has an elongated shape and a longitudinal through cavity (lumen); at least one reinforcement layer 4, which is arranged around (and in contact of) the layer 3; a layer 5, which comprises (in particular, is made up of) a second polymeric material and is arranged around (and in contact of) the reinforcement layer 4; and a layer 6 which comprises (in particular, is made up of) a third polymeric material and is arranged around (and in contact of) the layer 5.

According to some embodiments, the reinforcement layer 4 includes (in particular, is made up of) a thread material (for example, selected from group consisting of: metal, aramid fibre, polyester, polyamide, polypropylene, natural fibres (and a combination thereof) ) .

In some cases, the tube 1 comprises several reinforcement layers 4. In such cases, a layer (typically called innerliner, in particular with a thickness smaller than 0.8 mm) of a fourth polymeric material is arranged among the different reinforcement layers 4. In particular, the layer 3 has a thickness ranging from 0.3 mm to 3 mm; the layer 5 has a thickness ranging from 0.1 mm to 1.5 mm; the layer 6 has a thickness ranging from 0.2 mm to 2.5 mm .

Advantageously, the tube 1 is obtained by the method described in the first aspect of the present invention.

In accordance with a further aspect of the present invention, it is provided a twin tube 8 comprising two tubes 1, each of which is defined in accordance with the second aspect of the present invention. In particular, the tubes 1 are arranged side by side, in contact and joined. More precisely, the respective layers 6 (having an annular shape with a circular cross section) are in contact (welded together) .

Further characteristics of the present invention will become apparent from the following description of a merely illustrative and non-limitative example.

Example 1

This example shows the method of manufacturing a twin tube. A polymer (rubber) is extruded on a mandrel (using an extrusion line) so as to obtain a layer 3; the whole is wound on metal reels.

The extruded material is unwound from the reels and is processed in braiding or coiling lines, where a reinforcement layer 4 is applied on the layer 3; the whole is wound on metal reels.

The semi-finished product is brought back on the extrusion line, and a polymer (rubber) is extruded on it to obtain a thick layer 5 of about 0.3 mm; the whole is wound on metal reels.

A spiral-wound fabric is applied on the tube so far made, using a bandaging line in order to obtain a bandaged tube, which is then wound on the stainless steel reels, and then inserted in a suitably designed vessel to withstand an inner pressure. A first partial vulcanization is carried out (hereinafter called vulcanization cycle 1) thanks to the heat supplied by the water vapour. After this vulcanization, the fabric is removed from the tube, using an unbandaging line and obtaining an unbandaged tube. The whole is wound on metal reels.

At this point, two unbandaged tubes are brought to an extrusion line, and by means of particularly shaped tools (to make an extrusion having an 8-like shape) , a polymer is extruded (rubber) on them to obtain a layer 6 with a thickness of about 0.7 mm. This extrusion joins the two tubes 2 and produces the twin tube 8. The whole is wound on the stainless steel reels and inserted in a suitably designed vessel to resist to an inner pressure, and a second vulcanization is carried out (hereinafter called vulcanization cycle 2) thanks to the heat supplied by the water vapour. In this step, the vulcanization of the various layers 3, 5 and 6 is completed and the obtained final twin tube 8 is absolutely free of damage in the different layers.

An extraction plant is used to remove the spindle. By pumping pressurized water on the spindle section, the plant pushes the spindles out of the twin tube 8. The water from the two tubes is removed by entering pressurized air inside them. The whole is wrapped in rolls ready to be packaged and marketed.

The twin tube 8 shows no damage in any of its parts.

The characteristics of the used polymeric materials are reported in the following Table 1 (calculated according to the provisions of ASTM D6204 and as set out above in this regard - also the measured parameters are shown in the table) .

Table 1

The conditions used for the vulcanization cycles are shown Table 2 below.

Table 2

VULCANIZATION

STEP PRESSURE CYCLE 1 CYCLE 2 min . min . from room T (0 mBar) to

ASCENT 15 15

145°C (3.142 mBar)

MAINTAINANCE at 145°C (3.142 mBar) 15 60 from 145°C (3.142 mBar)

DESCENT 5 5 to 0 mBar