IL29220A - Process for preparing a strip of plastic material and a strip prepared thereby - Google Patents

Description

*¾>oH name t e naan*? T*Vnr» ft! * ?Vii:- |3ΤΠ» ¾no A proeesss for preparing a strip of plastic material and a strip prepared thereby C: -27659 This invention relates to plastics strip, and more particularly to plastics strip having longitudinal grooves therein and which is resistant to flexure about the grooves.

Attempts have been made in the past to use strips of synthetic thermoplastic material instead of metal strips for hooping or strapping trunks and cases.

Strips of synthetic thermoplastic material, when suitably drawn, have considerable mechanical strength lengthwise, but are often easily broken if folded crosswise i.e. about a longitudinal axis. The reason for this is, of course, that when the strips are drawn lengthwise, substantially all the polymer molecules are oriented in the lengthwise direction, with the result that the strip has considerable anisotropy, and thus has little resistance to folding about a longitudinal axis.

We have now discovered that if longitudinal grooves are formed under certain conditions in a plastics strip, and the strip is then stretched longitudinally, the resulting strip not only has a high longitudinal tensile strength but also has good resistance to break on flexure about the grooves.

According to the invention we provide a process for preparing a strip of plastics material having improved flexure properties, which comprises forming one or more longitudinal grooves in a strip of plastics material while at least the portion of the strip in which the grooves are formed is at a temperature from 25° to 100°C below its crystalline melting, point, and During formation of the grooves, the plastics material at the bottom of the grooves becomes oriented transversely to the direction of the grooves, and we have discovered that this orientation is not removed in the subsequent longitudinal stretching step. Thus the grooved strip has good resistance to splitting and cracking on flexure about the groove, which is due to this localised transverse orientation of the polymer molecules .

Any crystalline plastics material which can be extruded or moulded in the form of a strip can be used. Particularly preferred are polyethylene, polypropylene, and polyamides. Polypropylene of isotacticity greater than 90% is especially preferred and when this material is used, the grooves are formed at a temperature of 0 to 14-0°C, preferably between 5 and 130°C, and longitudinal drawing is effected at a temperature below 14-0°C with a drawing ratio of, for example, between 6/1 and 10/1. laminates of plastics materials can equally well be used. The laminated material comprises different plastics materials deposited in superposed layers either over the whole width of the initial strip or in thin strips of various shapes.

The laminated material is made out of at least two plastics materials which can be firmly joined together when heated, e.g. during extrusion or afterwards, by known methods. The laminae must obviously be adhered together firmly enough for the laminate to undergo mechanical processing without both lengthwise and crosswise. Preferred laminae include isotactic propylene (especially above 80% isotacticity) and another polyolefin or a crystalline copolymer of an -olefin. An especially preferred laminate consists of laminae of isotactic polypropylene and a copolymer containing at least 80% (by weight) of groups of formula: i -CH2-CH- In order that the invention may be more fully understood, certain preferred embodiments of the process will now be described, by way of example only, with reference to the accompanying drawings in which :- Figure 1 is a cross-sectional view of part of a strip with a groove therein; Figure 2 is a cross-sectional view showing the formation of a groove in one side of a strip by means of a milling wheel having teeth with an apical angle of 50°; Figure 3 shows diagrammatically the formation of five grooves on one side of a strip using a train of milling wheels and Figure 3b shows a cross-sectional view of the strip so formed; Figure 4- shows in cross-section the formation of grooves using an arrangement of milling wheels round a roller supporting the plastics strip; Figure 5 is a sectional view of one method of forming the grooves using metal cables; Figure 6 shows a cross-sectional view on the line VI-VI of Figure 5", line VI-VI of a device with a front view similar to that in Figure 5, except that a ribbed roller is used and the metal cables are replaced by a single metal band ; Figures 8, 8a and 9, 9a are diagrams of complete installations for effecting the process of this invention; Figure 10 is a cross-sectional view of a strip of laminated material suitable for use in the present invention ; Figure 11 is a cross-sectional view of a strip made up of a number of narrow strips side by side; Figure 12 is a cross-sectional view of another strip useful in this invention; Figure 13 is a cross-sectional view of the strip of Figure 10 after the formation of grooves; and Figure 14 is a cross-sectional view of the strip of Figure 11 after the formation of grooves.

The strip may be grooved on one or both sides, and in the latter case the grooves may be made on both sides of the strip simultaneously.

A strip which has been grooved on one side only is illustrated in Figure 1. The angle A is the aperture angle of the groove ( 5° in this example) and the round parts Ar on the edges of the groove result automatically from creep of the material.

Spontaneous contraction of the grooved material results in a depression, S, on the other surface, opposite the groove . This depression, in conjunction wi the groove, gives the material a cross-sectional pivot for flexure about the longitudinal axis of the groove .

The grooves can be made, for example, by milling, and this is illustrated in Figure 2. The teeth of the milling wheel are of apical angle less than 120°, and preferably between 30° and 90°, and the length of arc of the round part at the top of the groove (Ar in Figure 1) will be between P/4- and P, where P is the depth of the groove.

The depth of the groove should preferably be such that the thickness of material from the bottom of the groove to the opposite surface (herein called the "residual thickness") will be between 15 and 50 of the initial thickness of the strip; the proportion should decrease as the initial thickness increases. The residual thickness should not be greater than 0.8 mm or less than 0.2 mm. After lengthwise stretching, the residual thiclaiess will be reduced to between 0.05 and 0.3 mm, The number of grooves formed in the strip will be chosen according to the width of the strip. Each groove may be formed by for example one milling wheel or a number of wheels in succession.

If a number of grooves are being made in one strip, fixed milling wheels can be used, in which case the material which is displaced in the formation of the grooves goes to thicken the strip between the grooves, with the result that the molecular orientation in these areas is somewhat changed without any observable loss of toughness in the finished strip after drawing. milling wheels which can each "be displaced sideways on their axis, each wheel being automatically placed in the groove hollowed by the preceding wheel. In this case, the wheels are at a fixed distance longitudinally from one another. This method is illustrated in Figure 3a. As grooves are formed by the wheels, the displaced material serves to widen the strip. Figure 3b shows a strip before and after the formation of grooves .

The wheels can be arranged in a plane or round a roller over which the plastics strip is passed. The latter arrangement is shown in Figure .

The invention can be used for preparing several strips from a single band of plastics material of suitable width. The grooves made by the wheels form precutting lines and the transverse orientation of the material beneath the groove enables a clean cut to be made. The band is preferably cut before being drawn, either by sharp-edged wheels or circular knives.

Instead of using milling wheels to form the grooves, metal cables may be used by pressing them onto the plastics strip as it turns on a smooth roller.

Alternatively the grooves may be formed by pressing a smooth steel strip on one side of a plastics strip while passing it o\^er a driving roller with a suitably ribbed surface.

These methods are shown diagrammatically in Figures 5, 6 and 7.

In Figure 5 one or more cables are placed round two driving rollers 1 and 2, the distance between which cables at a suitable value, and this can also be adjusted. A plastics strip moves over a roller 5 which has a smooth surface. The relative positions of rollers 1, 2, 5 and the tension on the cables (due to roller 3) are adjusted so that the cables press onto the plastics strip as it turns round, forming grooves therein.

Figure 6 shows the roller 5, on which the plastics strip 4 rests, with three cables 6 pressing into its upper surface.

The pressure exerted by the metal cables on the plastics strip is a maximum when the strip moves between rollers 1 and 5 and rollers 2 and 5. The main purpose of roller 3, the position of which is adjustable, is to prevent the cable from sliding with respect to the strip by varying the tension in the cable .

Use may also be made of a slightly different device with a front view similar to that in Figure 5. In this, a smooth steel strip 6 is used, rolled round two rollers 1 and 2. The steel strip can be put under suitable tension by adjusting the position of an auxiliary roller 3; the strip under tension presses the plastics strip 4 against a roller 3, the surface of which has continuous ribs perpendicular to the axis of rotation of the roller. Figure 7 shows the steel strip 6 pressing the plastics strip 4 against the roller 5 which has ribs 7· In this case also, the greatest pressure exerted on the plastics strip is when the band passes In the process of the present invention, it is not of course necessary for the grooves to be made by a single device of the type shown in Figures 5 - 6 or 5 - 7· A number of similar devices can be used in series. If a number of devices are used in series, each can operate under different conditions (e.g. temperature of band) , and simple trials will reveal whether type 5 - 6 or 5 - 7 is the more suitable to the particular circumstances. The devices in series can of course be either of the same or of different types, and the plastics strip may also undergo suitable thermal and/or mechanical treatment in its passage between each device „ The metal cable or the ribs on roller may be triangular, cylindrical or any other shape. The apical angle of the groove may be greater, less or equal to 120°. Figures 6 and 7 show semicircular cross-section grooves but the shape could equally well be elliptical or approximately elliptical. When a number of 5 - 6 or 5 - 7 devices are used in series as described above, the devices can form differently shaped grooves.

Figure 8 shows an installation for extruding, milling (forming the grooves) and drawing thermoplastics strips by the method of the invention. The extruded strip or band, after being cooled by immersion and dried, is wound on a roller train in an oven 1 and brought to the required temperature in the principal roller 2 where milling takes place. The strip is then carried along by the rollers into oven 5, where it is peripheral velocity of which is n times the peripheral velocity of the first set of rollers, where n is the drawing ratio.

Figure 8a shows an installation very similar to that of Figure 8, except that the extruded strip or band is cooled by contact on a thermostatically controlled calender 1 and then wound on thermostatically controlled heating rollers 2 which raise it to the required temperature before it is milled on a large-diameter ribbed roller 3.

The pre-heating rollers are either free or driven, except for the driving inlet roller. They are hooded or enclosed in an oven. The principal roller, where milling occurs, is a driving roller, like all the other rollers downstream.

In particular, the strip upstream of the oven should be wound so that the tension is not too great round the milling roller, to prevent any premature lengthwise drawing, which would make it impossible to produce regular grooves.

Figures 9 and 9a show a slightly different arrangement in which the extruded band or strip is received on a calender 1, with or without rolling, and preferably in a bath to allow the amount of cooling to be more easily controlled. The strip then passes over the milling roller 2. The two types of milling device in Figures 8 and 8a are combined here. Finally, the strip is taken up by a conventional drawing installation. While passing along the roller train upstream, the strip becomes considerably cooler and heated in the oven to a suitable temperature for drawing.

The residual thickness of material beneath the grooves should preferably be between 0.15 and 0.5 times the thickness of the original strip which will preferably be less than 5 mm thick. Various widths of grooves may be formed. It is often an advantage in practice to make the grooves relatively wide with a flat surface at the bottom. The resulting "pivot" is relatively wide with respect to its thickness before drawing, and considering the part of the bottom of the groove which has a relatively constant thickness, it is preferable that the ratio of the width of this part to the residual thickness be at least 2.

During formation of the grooves, the temperature of the plastics material is important; it should preferably be between 25° and 100°C below the crystalline melting point of the material used. If the temperature is too low, a large force is required to form the grooves, which means that the apparatus has to be heavy and expensive. If, on the other hand, the material is at too high a temperature, the orientation produced by the mechanical treatment is much less pronounced and the strip will break more easily between the grooves after being folded several times.

The strip need not be at the same temperature at all points through its thickness while the grooves are made. If, for example, the surface temperature is relatively low while the internal temperature is fairly high, the orientation can be restricted to can "be obtained with strips made of different materials as will be illustrated below.

After the grooves have been formed, the material is preferably kept at the same temperature or slightly heated before being drawn lengthwise.

The following examples are given by way of illustration only:-EXAMPLE 1 A strip was formed of crystalline isotactic polypropylene homopolymer of fluidity index 1.5 when hot (measured according to French standard "T. 1.016") at a temperature 230°G and under a load of kg, with 95% isotacticit . The strip of width 30 mm and thickness 2 mm, weighing 4- g per metre, was first grooved by milling at a temperature of 110°C. Three equidistant grooves were made, leaving a residual thickness of 0.45 mm. The band was then drawn at 120°C with a drawing ratio of 8/1, i.e. the weight per metre before and after drawing was in the ratio of 8/1. The speed of grooving was 8 m/rnin, the grooved strip after stretching was thus formed at 64 m/min.

The resulting strip was 0.7 nun thick in the ungrooved areas and had a residual thickness of 0.15 ^ The width between adjacent grooves was 13 mm and the weight per metre of the strip was 6.8 g. The breaking strength was 98 kg, or 43.8 kg/g/m (average of 10 measurements). (The unit kg/g/rn gives the strength referred to an imaginary sheet weighing 1 g per metre; it is related to the "metric strength" which gives the length of sheet having a weight related to breaking stren th b the ex ression 1 k m = 1000 m .

The sheet was undamaged after being folded 100 times in alternate directions along the central groove, EXAMPLE 2 (Comparison) The conditions were the same as in Example 1, except that no grooves were formed in the strip. The band had a width of 11.5 mm, a uniform thickness of 0.65 -mm, and weighed 6. 8 g/m. Its breaking strength was 302 kg (average of 10 measuremen s), or 44.4 kg/g/m. The band broke at the first attempt to fold it along a longitudinal axis.

EXAMPLE 5 A polypropylene homopolymer was used with a higher molecular weight than that of Example 1 (melt index 0. 5 ; isotacticity 96%) . The strip was 50 mm wide and 3 mm thick and weighed 1.35 g/m. Three equidistant grooves were made in the strip by milling at 100°C, leaving a residual thickness of 0.6 mm.

After being drawn at 120°C in a ratio of . 1/1 , a strip was obtained with a width of 22 mm, a maximum thickness of 1. 1 mm, a residual thickness of 0.22 mm, and a weight of 19 g/m; the speed of milling was 5 m/min. The breaking strength was 770 kg of 40.5 kg/g/m. The strips could be folded more than 50 times in alternate directions along a groove without breaking.

EXAMPLE 4 The conditions were the same as in Example 3, except that a propylene copolymer was used containing 4% ethylene. The resulting strips had a breaking strength of 700 kg or 36. 8 kg/g/m. It was unbroken after being folded 100 times in alternate directions alon one of the rooves.

EXAMPLE A strip of laminated material was obtained from two extruders, feeding material through the same die. It consisted of a layer of polypropylene 3e thick and a layer of propylene-ethylene copolymer of thickness e_. After cooling to about 90°C, longitudinal grooves were made, using wheels (milling) (and in further experiments wires and ribs cut on a roller), on the polypropylene side of the laminate to form a groove with an approximate depth of 3e_. An examination of a cross-section of the resulting strip showed that most of the grooved polypropylene had undergone creep, leaving a very thin polypropylene layer firmly bound to the copolymer layer beneath. After drawing, the grooves were found to be particularly strong and did not tear after being folded many times along their length.

EXAMPLE 6 A laminated material was obtained by the same method as in Example 5. The material was made of a central layer of polypropylene with a layer of propylene/ ethylene copolymer on each side. This strip is shown in cross-section in Figure 10, which depicts the central polypropylene layer (1) and the adjacent copolymer layers (2).

The surfaces of the strip were then cooled so that the copolymer solidified while the polypropylene inside was sufficiently hot to remain plastic and easily deformable. Grooves were then formed in the band as it passed between one or more pairs of rollers, each pair containing at least one roller with upstanding - section of this strip was examined, it was found that the two copolymer surface layers had approached and sometimes touched one another at the place where the grooves had been formed. The grooves were found to hav a particularly high tensile strength and resistance to folding and tearing,.

A cross-section of the resulting strip is shown in Figure 15. This depicts the polypropylene layer (1) with thickness considerably reduced where it was grooved; (2) denotes the copolymer layers.

EXAMPLE 7 A special die fed by two extruders, one extrudin polypropylene and the other extruding an ethylene-propylene copolymer, was used to produce a strip made of a number of narrow strips joined together, alternate strips being formed of the same substance. A cross-section of the strip is shown in Figure 11, in which 1 denotes the narrow polypropylene strips and 2 denotes the copolymer strips. After cooling to approximately 60°C, grooves were made and the strip was drawn lengthwise. This gave a grooved strip with excellent properties: a cross-section is shown in Figure 1 .

Grooving was carried out on both sides of the strip in the copolymer.

Similar use can be made of for example strips consisting of a polypropylene core to which strips of other thermoplastics materials have been attached during extrusion. A band of this type is shown in Figure 12. The grooves could then be formed on one or both sides of the band where it is covered by strips of other materials the method bein the same as in Example 6, i.e. with a temperature distribution suc that the polypropylene inside the band remains plastic.

The laminated strip can be made by pressing the copolymer film on one or both sides of a strip of polypropylene. The copolymer film is pre-heated and then pressed on the polypropylene by rollers, at least one of which should be heated to 120°G. As soon as the laminated material has been made, it can be processed to form a strip as described in Example 6.

Claims (1)

HAVING NOV/ particularly described and ascertained the nature of our said invention and in what roanner the same is to be performed, we declare that what ie claim isi 1 . A process for preparing a strip of plastics material having improved flexure properties, which comprises forming one or more longitudinal grooves in a strip of plastics material while at least the portion .of the strip in which the grooves are formed is at a temperature from 25° to 100°C below its crystalline melting point, and subsequently stretching the strip longitudinally. 2. A process according to claim 1 wherein the grooves are formed by applying one or more milling wheels, metal wires or ribbed rollers under pressure to a surface of the strip. 3 « A process according to claim 1 or 2 wherein the plastics material is polypropylene of isotacticity above 90% and the grooves are formed whilst the polypropylene is at a temperature of from 0° to 1 0°C. „ A process according to claim 3 wherein the temperature is from 95 ° to 130°C. 5, A process according to claim 3 or wherein the grooved polypropylene strip is stretched in a ratio of from 6: 1 to 10 : 1 . 6. A process according to claim 1 or 2 wherein the strip of plastics material is a laminate formed from strips of two or more different plastics materials. 7 c A process according to claim 6 wherein the strip is a laminate formed from one or more strips of polypropylene" of isotacticity at least 80% and one or more strips of a crystalline propylene copolymer, 8, A process according to claim 7 wherein the propylene copolymer is an ethylene/propylene copolymer. 9. process according to claim 6, 7 or 8 wherein the laminated plastics strip comprises at least one crystalline propylene copolymer surface layer extending over all or part of the width of the strip, and a polypropylene core, and the grooves are formed in the propylene copolymer whilst the propylene core is at a temperature such that it retains its plastic properties. 10, A process according to any preceding claim wherein the grooves have a depth of from 50% to 85% of the thickness of the strip before stretching. 11. A process according to any preceding claim wherein one or more grooves are formed in both surfaces of the strip, 12 A process according to any of claims 1 - 10 wherein when one or more grooves are formed in only one surface of the strip, the ratio of the width of the bottom of the groove to the residual thickness (as herein defined) of the strip before stretching is at least 2. 15. A process for preparing a grooved strip of plastics material substantially as herein described in the Examples.

1. . Grooved plastics strips when prepared by the