NL190319C - A method for manufacturing a mesh structure from flat or substantially flat plastic material by biaxial stretching, and using a mesh structure manufactured in this way for stabilizing soil. - Google Patents

Description

11 190319

The invention relates to a method for manufacturing a mesh structure from a flat or substantially flat plastic material, by means of biaxial stretching, and the use of a mesh structure manufactured in this way for stabilizing soil. plastic flat sheet, patterned with recesses centered at the intersections of an imaginary rectangular grid of rows and columns, which sheet is first stretched in a first direction parallel to the columns to form longitudinal strands, the orientation of which the longitudinal strands on both sides of the plastic sheet penetrate into the joints during the stretching of the first direction, and the sheet is then stretched in a second direction parallel to the rows to form oriented cross-linked cross-ties, each joint branching with edge zones which, during stretching, are oriented in the second direction to oriented edge zones that adjoin the longitudinal and transverse strands coupled together in the joint and wherein the orientation of each strand penetrates the joint.

Such a method is known from US Pat. No. 4,013,752, in which a starting plastic sheet with a thickness of about 0.2 mm is formed by biaxial stretching into a relatively flat mesh structure, the connections of which are not clearly thicker than the strands. As a result, the product without raised points and lumps in the joints is only suitable for reinforcing light materials, for example as reinforcement in non-woven fabrics or paper.

The compounds are relatively strongly oriented in this known method, the strands dividing into separate filaments enclosing thin film regions. Figure 5 even shows an oriented filament extending through the connection.

A drawback of these mesh structures is that they are extremely weak. The reason for this weakness is that the foil areas in the joints easily split and act as starting points for cracking. The cleavage or tear thus created continues in the strands which divide as they enter the joint, so that the entire structure is easily torn apart.

Such a delicate mesh structure is not suitable for stabilizing and strengthening ground masses where the joints are subjected to forces exerted by the strands. Since the connections of integral mesh structures must be sufficiently strong without containing too much plastic material, the object of the invention is therefore to carry out a method of the type mentioned in the beginning such that a mesh structure obtained thereby with a reduced amount of plastic has an increased strength per unit area.

This object is achieved according to the invention in that the mesh structure is produced by stretching a plastic sheet with a thickness of at least 1.5 mm, such that, measured on the axis of each of the 35 strands which merge into the joint, no zone in the connection is formed with a thickness of less than 75% of the thickness of the center of each of the strands which merge into the connection, and the maximum thickness of the connection is greater than the thickness of the center of each of the strands which merge into the connection .

The first characteristic feature of the present invention is based on the recognition that there is a tendency to form the prior art highly oriented joints with thin film regions below a thickness of 1.5 mm and that this tendency increases as the thickness of the sheet or plate decreases below one mm and in particular when the plate thicknesses fall in the range of 0.75-0.5 mm, especially when the ratio of the distance between the cavities or depressions of adjacent columns or rows in the output plate until the thickness of the plate is too great.

45 By applying the said thickness of the starting sheet of at least 1.5 mm, a mesh structure is obtained which proves to lend itself well to reinforcement and reinforcement of ground masses. Namely, with such a starting material one obtains compounds which as a whole are resistant to forces exerted by the strands. Namely, since the centers of the compounds in the mesh structure according to the invention are not oriented as strongly, or even at all, as the strands, it is achieved that in the connection there is a zone thicker than the center of each the connection continuous strand. The joints will be wider than the strands and will therefore have a reasonable cross-section with respect to the strands. As a result, the mesh structure has quite a lot of "lumpy" connections, but this is an advantage in stabilizing or strengthening ground masses. By ensuring that on the axis of each strand passing through the joint, there is no zone 55 in the joint having a thickness less than 75% of the thickness of the center of a strand entering the joint, ensuring that the any thinner zones of the joint present have sufficient strength to withstand the forces exerted by the strands.

190319 2

The joints retain a shape that provides good stress transfer paths. The oriented branches allow the structure to withstand large forces between two strands initially perpendicular to each other.

In the mesh structure obtained by the method of the invention, the elongation and molecular orientation also include the compounds. This makes the connections lighter than with a mesh structure in which the connections are completely unaffected. Due to the orientation, i.e. the elongation of the joints, plastic material is also saved.

The stretching in the first direction takes place in such a way that a point, applied to a line tangent to the recesses in a non-stretched state, after stretching in the direction of the rows is at a relatively small distance from the tangent line to the existing recesses located, with the orientation of the longitudinal strands penetrating into the joints. It has been found that with excessive stretch and orientation in the direction of the columns, the joints are split or split. lead to breakage, especially when stretching towards the rows afterwards. The stretch in the direction of the columns is therefore limited by the risk of splitting. It is noted that from U.S. Pat. No. 4,140,826 it is known to biaxially orient a flat plastic film having a thickness of 0.076 mm to 0.635 mm with holes punched therein according to a rectangular grid by stretching, to form a flat product with flat connections between the strands. This creates a region of biaxial molecular orientation in the center of the junction that is no thicker than the oriented strands. This area is less resistant to tearing than the strands, which have greater uniaxial strength than the center, causing these areas to act as a cracker and cause cracks to continue. The small thickness of the starting material is necessary to obtain a completely flat soft-grip material that can be used as reinforcement material in paper sheets. These fragile thin products are not suitable for strengthening or stabilizing ground masses for which much thicker starting products must be used, whereby the joints can be appropriately profiled during stretching. There is no indication in this document 25 that the use of raw materials with a thickness of at least 1.5 mm would give mesh structures which would be effective for the latter purpose.

Furthermore, it is known from British Patent 982,036 to produce a mesh structure starting from a flat plastic sheet with a recess pattern with an imaginary, rectangular grid constructed of columns and rows coupled together by biaxial stretching. Although a sheet thickness of 3 mm has been mentioned, a thickness of 0.3 mm is used according to the examples. When stretching, the strands are oriented but not the joints, making them thick and heavy. Although the strands have greater tensile strength due to orientation, the overall strength is also determined by the compounds that are not stretched. As a result, in order to obtain a certain strength, a mesh structure must be manufactured from a relatively large amount of plastic per unit of surface.

In the method according to the invention, mesh structures with a much greater tensile strength per meter width and even per meter width per unit weight are obtained.

Another drawback of this known mesh structure is that the non-oriented joints give a lot of creep to the material, so that this structure is not suitable for stabilizing or strengthening 40 ground masses.

With this known mesh structure, the orientation of the branches also ends approximately in the middle of the branch, that is to say that it does not extend around the branch as in the invention.

Finally, it is known from British Patent 969,205 to stretch a diamond mesh structure in a first diagonal direction and then in a second diagonal direction to form fully 45 oriented joints. The aim is to obtain a strong connection so that the product obtained can be used as a fishing net. However, the joints are flat and have a poor orientation against forces applied to the opposing strands, making these mesh structures unsuitable for stabilizing ground masses where forces are applied transversely to opposing strands.

According to an effective embodiment of the invention, the structure is released to shorten in the first direction during the second stretching operation, which leads to a particularly good strength.

The invention also relates to a method for holding or stabilizing particulate material such as soil, in which a mesh structure of plastic material is buried in the particulate material, characterized in that a mesh structure obtained using the method of the invention is used. In this way a very good stabilization is obtained.

In the present description, by "substantially flat" is meant that the plastic sheet may have only insignificant deviations from a completely flat shape and which deviations must not affect the stretching process symmetrical with respect to the sheet center plane.

The term "oriented" means molecularly oriented and the terms "rows" and "columns" are used to indicate the axes of the rectangular grid.

The term "recess" includes both through-holes and recesses with the exception of recesses or recesses asymmetrical with respect to the center plane of the stock material, because the required orientation symmetrical to the sheet center plane in the strands and joints is not achieved. Any membrane present in a recess should be so thin that the symmetrical orientation is not disturbed.

The terms "thick", "thin", "" thickness "," "deep", "" depth ", and" "shallow" refer to dimension 10 perpendicular to the plane of the mesh or mesh structure and the terms "" wide ”,” “narrow” and “” width ”refer to the respective dimension in the plane of the starting material or the mesh structure.

The thickness of the stock material or mesh structure is the distance between the outer surfaces of the stock material or mesh structure.

The thickness or depth of a strand is the thickness of the cross-section, not including the raised edges.

The invention will be further elucidated with reference to the drawing. Herein resp. Figures 1 and 2 show two stages during the manufacture of a mesh structure according to the method of the invention; Figures 3a, 3b and 3c show sections along the corresponding lines shown in Figure 2; figure 4 shows a section according to that of figure 3c but with a variant; Figures 5 and 6 show the connections in two different structures obtained according to the method; Figure 7 shows various shapes of cavities or recesses that can be used in the starting material, and Figure 8 shows a schematic representation of a device for the production of biaxially stretched structures according to the invention.

In Figure 1, the starting material consists of a plastic sheet 11 with flat sides, in which circular cavities or recesses 12 are formed. The 'cavities' 12 need not run entirely through the sheet or plate, but may also be depressions in one or both sides of the plate, leaving an unbroken membrane, preferably in the center plane of the plate. Figure 1 shows a junction or connecting zone 13. This zone 13 forms the intersection of the column zone 14, which lies between and touches two columns of cavities or floors 12 and the row zone 15, which lies between and touches two rows of 35 cavities or floors 12 In Figures 1 and 2, lines 14 ', 15' are drawn to show what happens during stretching.

When the sheet 11 is stretched in a vertical direction (looking at Figure 1) in a first stretching operation, the column zones 14 (Figure 1) are stretched to the longitudinal strands 17 shown in Figure 2. The material is stretched such that the outer portions of the joining zones 13 are stretched and oriented 40 (which occurs if the distance between the ends of an opening after stretching is, for example, 7 times greater than before stretching) to form the oriented end portions of the strands 17, which flow smoothly into the rest of the oriented strand (see figure 3c). The orientation can run wholly or almost entirely through the center of the connecting zone 13. A point 18 (figure 1), which lies at the exit plate 11 on the tangent line 19 to the rows of cavities or floors 12, has moved along with the strand 45 (figure 2) to a distance x (figures 2 and 3c) of the tangent line 19 '. This is illustrated by the lines 15 'shown in Figure 2. The distance x is preferably not less than 25% of the thickness of the center of the strands 17.

As a result of the first stretching operation, the row zones 15 form cross bars, as shown in Figure 2, which alternately connect first zones 20, which connect the ends of aligned strands 17, and 50 second zones 21, which are between the first zones 20 lie. The second zones 21 are not substantially oriented and still have the original thickness of the plate 11. They have flat outer sides, as shown in Figures 3c and 4. However, the first zones 20 are oriented in the direction of the strands 17 to form a trough in the crossbar, as shown in Figures 3a and 3b. The center of each first zone 20, corresponding to the central area of the first 55 junction zone, is thicker and less oriented than the strands 17 (Figure 3c). The thickness of this center can vary from just slightly greater than that of the strands 17 to the thickness of the starting material 11. When the orientation passes entirely through the first zone 20, the central part of the first zone 190319 4 20 may be stretched up to 1.5 times its original length. In the structure shown in Figure 3c, the thickness gradually increases from the point 18 to the center of the first zone 20.

Figure 4 shows a variant in which so little is stretched that practically only orientation has taken place along the edge of the first zone 20. There is only slight thinning in the center of zone 20 5, and a sharp transition from this zone to strands 17 at point 23 (Figure 2) has occurred.

If the orientation extends through the first zone 20, the flow there will be low under strain on the tensile stress. Since the center of each connecting zone 13 is oriented considerably less than at the strands, there is a smaller risk of cracks in that connecting zone when bending the cross bars.

The structure of Figure 2 is then subjected to a second stretching operation in the horizontal direction according to the view of Figure 2. By this second stretching operation, the zones 24 of Figure 1, which correspond to the second zones 21 of Figure 2, are stretched into oriented transverse strands 25 which are partially visible in Figures 5 and 6. During the second stretching operation, when the structure is released to shorten in the first direction, the length of the openings in the direction of the first stretching operation becomes smaller, possibly up to 33%, and the end portions of the strands 17 become partially or completely the joining zones are drawn and these end portions are even extended in the direction of the second stretching operation to form end portions of the strands 25. The outer portions of the original joining zones 13, which at the end of the first stretching operation have an orientation in the direction of the first stretch, at the end of the second stretch, have a predominant orientation toward the second stretch or about the same orientation in both directions. The result depends on the stretching ratios in the two stretching operations.

When the orientation runs entirely through or almost completely through the connecting zone 13, a better connection in the end product is obtained. However, it has been found that the orientation need not even go almost entirely through the connecting zone 13.

Figures 5 and 6 each show an example of connections 26 formed between strands 17 and 25. Each connection 26 is generally diamond or lens shaped with the major axis or maximum dimension aligned with the strands 25 which the second stretching operation is formed and is larger or much larger than the minor axis or minimum dimension aligned with the strands 17. The sides of the joint 26 are bounded by branches. The branches between neighboring strands 17, 25 have edge zones oriented in the second direction during stretching into oriented edge zones connecting to the longitudinal and transverse strands 17, 25 merging into the joint. The edge zones transition very gradually into the sides of the strands 25 but (especially in the embodiment of Figure 6) relatively abruptly in the sides of the strands 17. The size of the joint 26 is much larger than that of the intersection zone 26 'formed by the extension of the strands 17 and 25 (see Figure 5).

Each connection 26 is substantially symmetrical with respect to the center plane of the mesh structure, however, each connection is not flat but has a certain contour. The minimum thickness of each joint is not less than 75% of the thickness in the center of each of the strands 17, 25. When the minimum thickness decreases to less than the thickness of the center of the thickest strand 17 or 25, up to 90 % or 40 80% of the value or less, the strength of the connection decreases. The maximum thickness of the joint is considerably greater than the thickness of the center of each of the strands 17, 25.

It is common to measure the thickness of a strand of a mesh structure in the center of the strand. However, it has been observed that especially when the original cavities or depressions were circular, the center of a strand need not be the thinnest.

45 Each joint has a central zone 27, which is thicker than the oriented zones 28, 28 'on at least two opposite sides thereof, and usually also thicker than the centers of at least two of the strands 17, 25. As a result a significant increase in thickness occurs at the transition from a strand 17 via the connection 26 to a similarly aligned strand 17. When the orientation has not taken place entirely through the first zones 20, the central zone 27 may become even 50 thicker to be. In general, the central zone 27 will be considerably less oriented than the side zones 28, 28 'and the central part of the central zone 27 may not even be oriented. Most of the connection, however, must be oriented. At worst, only 70% of the surface of the joint would be oriented.

In the embodiment of Figure 5, the second zones 21 (see Figure 2) are stretched after the first zones 55 20. The first zones 20 are not fully stretched; there may even be a narrow central zone of non-oriented material left in each zone 20. As a result, the central zone 27 of the joint 26 has the

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190319 is greater than that of each of the strands 17 or 25, where they merge into the joint 26 and which thickness is greater than the thickness of the center of the strands 17 and 25. The structure may have approximately the same strength along each axis if the cross section and the spacing between the strands 17 and 25 are equal. The formation of the type of joint 26 shown in Figure 5 is facilitated by preventing the material from shortening in the second stretching direction when performing the first stretching operation and allowing the orientation to pass well through but not entirely through the first zones 20.

Figure 6 shows a connection obtained with further stretching in the second direction. The central zone 27 is more rectangular in appearance. The branches are still gradually curved with an orientation in the marginal zones, but are bent outward at the corners of the central zone 27.

The manufacture of the described joints 26 depends on the shapes and distances of the cavities or depressions, the stretching conditions such as the temperature and the plastic material. Some details are given as a guide below.

Preferably, stretching temperatures below the stretching temperatures recommended by the manufacturer are used, for example 97 ° C for high density polyethylene instead of slightly below 126 ° C.

During the first stretching operation, it is possible that the orientation does not pass through the connecting zones 13 (figure 1) or even does not reach sufficiently far into the zones 13. This tendency can be avoided or reduced by reducing the distance between the cavities or depressions in the first stretching direction, reducing the distance between the cavities or depressions in the second stretching direction, or reducing the radius of the corners of the cavities or depressions.

Generally, by reducing the ratio w: d, the tear resistance is increased.

The starting material may have any suitable thickness of 1.5 mm and higher and be plate or tubular. It is believed that the behavior of the material changes at smaller thicknesses, as the dimensions of the molecules themselves will play a larger role. The material used is flat or substantially flat, meaning that apart from a membrane (which need not be on the center plane 25), all zones of the stock material are symmetrical with respect to the center plane of the stock material. However, minor deviations from this are not excluded. Said membranes and deviations must not influence the stretching process which is symmetrical with respect to the sheet center plane. The cavities or depressions can be formed by punching or during the formation of the starting material itself by closing a slit die, for example as described in French patent 368,393. It is preferable to avoid any significant protrusion around the perimeter of the cavities or depressions. Thus, zones 21 preferably have planar tops and bottoms as shown in Figures 3c and 4, thereby avoiding any tendency to form foil patches in the joints of the biaxially stretched structures. In the formation of depressions, the membrane sealing the depressions can be broken during stretching and the remaining film-like material can be removed.

The starting material is practically not oriented, although orientation by melt flow may be present.

The starting material may be any suitable thermoplastic material, such as, for example, high density polyethylene, low density polyethylene, polypropylene, copolymers of high density polyethylene and polypropylene, and polyamides. The starting material may have a skin on either side containing an ultraviolet-40 stabilizer.

After stretching, the structures can be tempered in a known manner.

Figure 7 shows different shapes for the cavities or depressions. For the manufacture of the bidirectional stretched structures, the grid on which the centers lie may be square or rectangular.

Somewhat depending on the shape of the cavities, generally the surface of the cavities or 45 wells is preferably less than 50% of the planar surface of the starting material and in particular less than 25%.

A suitable device for carrying out the method is schematically shown in Figure 8, but the units of the device themselves are conventional units. An unwinding unit 41 supports a roll 42 of non-perforated stock material, which runs through the device along the path indicated by the dotted lines and arrows. The stock material passes through a flattening unit 43, a perforator 44, a transverse orienting (stretching) machine 45, a longitudinal orienting (stretching) machine 46 and is wound on a winding unit 47. In the second orientation machine 46, one must be too short avoid distance between the clamps to allow any lateral contraction of the mesh structure.

Claims (4)

190319 6 Examples Tables A and B list two examples. The dimensions are in mm and a dash means that the value is not recorded. The rack ratios are overall. Table B lists thicknesses in all columns. Only in Example II was some limitation, although not complete, in the direction perpendicular to the stretching direction during the second stretching operation, while the first stretching operation was without limitation. In Example II, the ratio w: d was less than 1, and although the stretch ratios were relatively low, the entire compound 26 was oriented. The two-way stretched mesh structure can be used, for example, for fencing up stocks, in horticultural areas, in construction, harvesting olives and as reinforcement between layered plates. The described mesh structure can be advantageously used for holding and stabilizing particulate material of any suitable shape, such as earth, sand, clay or gravel, and starting in any suitable location, such as on the side of an excavation or embankment, under a road surface runways or railways, under a building or quay. The mesh structure may in particular be suitable for preventing prop walls from being forced out of place by the pressure of particulate material behind it. TABLE A 20 Example Mate- Start- Size- w / d Distance Distance 1st 2nd Tem- No. rial thickness tings cavities between between elongation-stretching cavities cavities cavities ver ° C (1st (different posture posture direction) direction) 25 - I HDPE 4.5 6.35 cir- 1.41 12.7 12, 7 4: 1 3.5 1 97 boulder shaped II HDPE 4.5 3.18 Cir- 0.71 6.35 6.35 3.5: 1 3.75: 1 97 30 boulder shaped TABLE B 35 Example Midpoint Midpoint - Midpoint Zone 28 Zone 28 'No. strand 17, strand 25 zone 27 two-way stretched 40 _ I 1.37 1.61 4.33 1.63 2.43 II 1.85 1.7 3.22 2.2 45 1. A method of manufacturing a mesh structure from a flat or substantially flat plastic sheet, which is provided with a pattern of recesses, the centers of which lie at the intersections of an imaginary 50 rectangular grid of rows and columns, which sheet is first stretched in a first direction parallel to the columns to form longitudinal strands, the orientation of the longitudinal strands on both sides of the plastic sheet penetrating in the joints in the first direction during stretching, and the sheet then being stretched in a second direction parallel to the rows to form of oriented, interconnected transverse strands, each interconnection comprising branches with edge zones, which are oriented in the second direction during stretching into oriented edge zones which connect to the longitudinal and transverse strands coupled together in the connection and - ----- u :: HAnrHrinnf maf Kat l / anmarU Hat Ha ma? onctniptiπi r 17 190319 is manufactured by stretching a plastic sheet (11) with a thickness of at least 1.5 mm, such that measured on the axis of each of the strands (17, 25) merging into the joint (26), no zone (28, 28 ') in the joint (26) is formed with a thickness less than 75% of the thickness of the center of each of the strands (17, 25) merging into the joint (26), and the maximum thickness of the joint is greater than the thickness of the center of each of the strands (17, 25) merging into the joint (26). A method according to claim 1, characterized in that the structure is released to shorten in the first direction during the second stretching operation. Method according to claim 1 or 2, characterized in that it is stretched such that no zone in each connection (26), measured on the axis of each of the strands (17, 25) which merge into the connection, is formed with a thickness less than 100% of the thickness of the center of the strands (17, 25) that merge into the joint. 4. Method for holding or stabilizing particulate material such as soil, in which a mesh structure of plastic material is buried in the particulate material, characterized in that a mesh structure is obtained obtained using the method according to one or more of the preceding claims 1- 3. Hereby 3 sheets drawing