NO155794B - HIGH-STRENGTH LAMINATED MOVIE. - Google Patents

Description

The present invention relates to a laminated strong film of thermo-plastic. Cross laminates of uniaxially oriented are known

film of crystalline polymer which exhibits a usually extremely advantageous combination of different strength properties, of which the most surprising has been the tear strength, see in this connection US patent no. 3 32 2 613, especially when you have a relatively weak bond between the layers. When tearing out from an incision, the layers will delaminate around the cut as they split or flow in different directions - the crack branches or "forks out" - thereby leveling out the cut effect.

Films of this kind are particularly useful for various applications where the foil is heavily loaded, e.g. as a replacement for tarpaulins, as covering foils, sacks and wrapping

film that is particularly strong.

The most appropriate method for the production of

film of the type described above is reproduced in British patent no. 816 607, and it consists in molecularly orienting a tubular foil strongly in the longitudinal direction, spirally cutting this

and to unfold it into a flat foil with the orientation continuing obliquely (e.g. with a 45° inclination), and then to continuously laminate this foil with a similarly produced foil, with the respective directions of orientation crossed.

It is known that the tear-off strength increases significantly when three layers with three different directions of orientation are used, e.g. obtained by laminating a longitudinally oriented foil with two foils which are obliquely oriented as described above.

A disadvantage of the method described above (and of the product produced) is that it is practically impossible to produce really thin films, and for this reason the economic advantages of high strength per weight unit limited. It has been shown that the technical/economic lower limit in connection with carrying out the spiral cutting and the lamination process is generally 30 g/m 2. This means that the lower limit for a two-layer laminate is approx. 60 g/m 2 , while for a three-layer laminate which, as mentioned earlier, is necessary for full utilization of the effect, it is approx. 90 g/m2.

Another disadvantage is the practical limitation that is placed on the width due to the rotation of heavy machine parts and cylinders in connection with the spiral cutting. In general terms, it can be said that the width is hereby limited to approx. 1.5-2 m.

The third disadvantage concerns energy absorption values for the cross-laminate produced in this way. Relatively low energy absorption has been found during rapid tearing (Elmendorf tear test), and in connection with both slow and rapid overdrawing (TEA -tearing-energy-absorption strength and Elmendorf impact strength, respectively). In this connection, it seems that the clearly anisotropic nature of the layers is disadvantageous. If e.g. a two-layer cross-laminate of this type is drawn parallel to the direction of orientation of one layer, the yield strength and elongation at break will essentially be determined in advance precisely by this layer.

A further disadvantage is the very limited possibilities for heat sealing the laminate to itself, as the impact-tear strength of the seal is very poor. This is due to loss of orientation during sealing.

The applicant's previous work to overcome the above-mentioned disadvantages and to achieve a cheaper manufacturing process for a product with similar or equivalent properties is described in British patent no. 1 261 397. In this description, a method is explained which produces a cross-laminate-like structure through a nozzle head with rotating parts, at the same time that a soft, weaker central zone is formed in the same nozzle head during co-extrusion. The method consists in the coextrusion of several concentric or almost concentric layers of crystalline polymer alternating with layers of a softer polymer and a transverse division of these layers inside the die by means of teeth which are arranged in a row and are attached to the cylindrical die walls as they point inwards from the concave and outwards from the convex wall surface. The nozzle parts rotate in opposite directions, whereby the layers are divided according to left-hand screw lines at one and right-hand screw lines at the other film surface. It is prescribed that this combing can either be carried out all the way through to the middle part of the film or can be limited to parts near the surfaces. The coextrusion of the polymers in front of the combing zone is carried out to obtain a soft, weak middle layer.

The film extruded by this method consists practically entirely of unoriented material. However, the system of alternating stiff layers of a "first polymer" and soft layers of a "second polymer" divided into fHa-men or the like in a linear pattern by means of the teeth in each half of the film produces a tendency to split or flow in one direction, and since the linear patterns of the two surfaces intersect and a tendency to delaminate is provided, a tear-stopping effect analogous to the "fork" effect in a proper cross-laminate is achieved.

The above-mentioned description further suggests stretching the thus produced laminate biaxially under such conditions that the molecular orientation becomes largely uniaxial in each layer, with the direction of orientation in the different layers crossing each other, instead of producing biaxially oriented layers. To achieve such a uniaxial orientation, the "other" material must be very prone to flow, e.g. because it is still partially or fully melted, while the "first" material is solidified, and the film centers of the "first" material must be held stretched by biaxial tension. Although the aforementioned method would in principle solve the problem of achieving smaller thicknesses and larger widths in the cross-laminate, significant difficulties were found during the later technical development.

It was confirmed that the extrusion method was commercially feasible for the production of unoriented film with high tear strength, but the impact strength was low due to lack of orientation. However, significant disadvantages were found in connection with a subsequent biaxial stretching. As also indicated in the aforementioned description, a relatively large number of rows must be used in the extrusion head to achieve the fiber fineness required for the stretching method.

However, this made maintenance of the nozzle difficult and caused a constant accumulation of lumps of polymer between the teeth. Furthermore, the interaction between the teeth in the two nozzle halves made it necessary either to use extremely large amounts of soft material in the smallest layer or to limit the combing to two relatively thin surface zones on the film. Furthermore, it was very difficult to establish the conditions for the biaxial stretching that were necessary to achieve a generally uniaxial molecular orientation as described.

A further disadvantage is that the combing takes place in a different direction from the melt drawing.

The present invention is based on the observation that even a strongly biaxially oriented film with the same orientation in two mutually perpendicular directions can exhibit a clear splitting direction, provided that the film is extruded from a mixture and melt-stretched in connection with the extrusion. Because of. during melt stretching, a directional, fibrous morphology is formed which can usually be observed in an ordinary microscope, but in all cases can be seen in a suitable electron microscope. A uniaxial orientation is also produced during melt stretching, but this orientation is generally very weak.

When the film (which, for example, can consist of 50% polypropylene and

50% polyethylene) is stretched in one direction which is different from the fiber structure direction, e.g. perpendicular to this, the fiber parts seen under a microscope will bend off and branch,

but it is still possible to follow the zigzag course in the fiber structure from branching point to branching point, and by following different courses in this way you will see that, considered in a macro course, there is always a structural direction. The microstructure explained here is very different from the filament structure described in the aforementioned British patent no. 1 261 397.

After a certain extension, e.g. of approx. 40% perpendicular to the fiber structure, an examination of the film in polarized light (or more accurately by X-ray refraction) will show that the orientation is the same in all directions.

After further pulling in the same direction, there is a clear tendency to split in the direction of the fiber structure until the splitting tendency at a certain point, e.g. at an approx. 80% total extension, equal in all directions. In case of further drawing, the main splitting direction will coincide with the main orientation direction. One can e.g. first extend the film 100% in this direction and then stretch it in the original direction until the point where it has the same orientation in all directions. At this point there is again a noticeable tendency to split in the original structural direction, and by microscopic examination it will be possible, but difficult, to follow the zig-zag course of the fiber structure and see that the fiber structure direction, viewed in macro-course, is constantly in the essentially coincide with the splitting direction.

According to this, according to the present invention, a film has been produced as stated in claim 1.

The product is a high-strength laminate consisting of predominantly weakly bonded biaxially oriented films each formed from a mixture with a distinct fibrous morphology, with the fibers formed in a distinctly directional fiber structure when viewed in macroflow, but with the fiber portions strongly deflected from this direction when viewed in microflow, and with the aforementioned directional fiber structure in at least two of the films that cross each other,

see photomicrograph fig. 10 and associated description.

It has been shown that the construction formed in this way

is particularly suitable for almost any application as a strong film where energy absorption is essential, whether this is energy absorption during tearing, puncture or impact. The tendency of the layers to split in connection with the weak bond between the layers produces a "fork effect" similar to the effect of cross-laminates of uniaxially oriented film, but an energy absorption during rapid tearing (Elmendorf tearing test) is significantly higher. Furthermore, most punches, especially Elmendorf punches, have been improved and that. will usually also be the energy absorption during rapid puncture ("Beach strength"). These improvements are considered to be a result of partly the biaxial nature of the orientation in each layer and partly the distinct and zigzag-shaped fibrous morphology. The biaxial orientation has the further advantage that the laminate according to the invention has shrink film properties, which a cross laminate of uniaxially oriented film does not have.

A further advantage, compared to cross-laminates of uniaxially oriented films, is a greatly improved impact-tear strength in a seam where the material is heat-sealed to itself (provided that one does not for some reason use a layer that has a negative impact on such heat sealing). One reason for this improvement is the above-mentioned good shrink film properties. When heat sealing mainly unoriented films, it is usual to relieve the pressure between the welds for cooling and thereby allow the heat-sealed part of the two films to shrink at least across the main direction of the seam, whereby the material thickness in the seam will increase and the seam will thereby become stronger.

When the present product - which has pronounced shrink film properties - is heat sealed under similar conditions, a particularly large increase in thickness can be achieved which will partially compensate for loss of orientation. Furthermore, the zigzag shape of the polymer fibers will survive the heat treatment and will, even in a state of molecular disorder, exhibit impact absorbing properties.

(Impact absorption is discussed further in connection with Fig. 10 and in example 5).

In this regard, cross laminates of uniaxially oriented films will exhibit poorer shrink film properties and in the disordered state to their microstructures will not exhibit any special impact absorption properties.

It is a significant advantage that the laminate according to the invention

in combination with the good tearing and impact properties exhibit good shrinking properties at temperatures that are significantly lower than the melting point. Ordinary shrink wrap is only melt-oriented and therefore requires heating above the melting point to shrink. Foil which is biaxially stretched below the melting point in a known manner can also shrink when heated to a temperature below the melting point, but has precisely because the biaxial orientation particularly poor tearing properties. The foil according to the invention combines these various desirable properties.

It was mentioned earlier that the method of manufacture

of cross-laminates of uniaxially oriented film under the conditions in practice gives a rather high lower limit for the laminate thickness (approx. 90 g/m for a three-layer laminate)

and a fairly high upper limit for the laminate width. In both respects, the invention is very advantageous because the fact

that the laminate can be strongly stretched in two or more directions after lamination. Therefore, it is technically and economically possible to achieve a weight of approx. 10 g/m for each film in the laminate, i.e. approx. 30 g/m 2 for a three-layer laminate. This greatly expands the application area.

While mixtures of highly compatible polymers, e.g. in some cases different polyamides will not normally form a fiber structure as described, so mixtures of fully or partially incompatible thermoplastic polymers will always do so provided that the content of one polymer is not too dominant. To be on the safe side, there should be no more than 85% of any polymer in the mix. If the polymers are totally incompatible, an "alloying agent" should preferably be added.

Very small chemical differences can be sufficient to establish the two-phase structure, and atactic polypropylene has proven to be suitable as an addition to ordinary polypropylene.

The best properties are achieved if the fiber structure consists of crystal threads "cemented" together with small amounts of surrounding elastomer. By small quantities is meant approx. 5-20% of the total amount.

To keep the content of elastomers low and constantly achieve

a distinct fibrous morphology with the elastomer tending to surround the other material, the elastomer is preferably used as "alloying agent" for two other polymers. Thus, a preferred composition is two crystalline, mutually incompatible polyolefins - e.g. iso- or syndiotactic polypropylene and low-pressure or high-pressure polyethylene with the addition of a sticky polymer that binds to both - e.g. atactic polypropylene, ethylene propylene rubber (preferably a sticky type with a relatively high content of propylene) and polyisobutylene of a molecular weight which is usually used for pressure-sensitive binders.

The melt drawing, by which the directional fiber structure is formed, can be carried out in different ways. It can e.g. happen by gradual reduction of the gap in the exit chamber in the extrusion die or by passage of the molten material through a series of closely spaced partitions or the like in the die, or by stretching in a molten state after the material has left the die head, or by a combination of such measures nings.

The splitting direction and the splitting tendency are determined by measuring the tearing strength by the "tongue tearing" method in different directions. The splitting direction is understood as the direction that exhibits the lowest tear-off strength, while the splitting tendency is understood as the ratio between the highest tear-off strength and the lowest tear-off strength. The splitting tendency in the layers according to the biaxial orientation should preferably be greater than 2:1,

however, 1.5:1 can be tolerated.

To allow a local delamination during tearing and thereby

to prevent cracks from "forking out", it is essential to ensure a largely weak bond when laminating the layers.

If the bond produced is evenly distributed and if the thickness of each layer is 20 g/m 2 , a peel strength of between 5 g/cm and 500 g/cm is generally suitable. Since there is competition between breaking forces and delamination forces during tearing, the upper limit depends on the layer thickness and is generally proportional to this.

As will be apparent from the description of different embodiments, there are different ways of establishing and controlling the strength of the bond, the most practical of which involve a co-extrusion at one stage or another of a particular layer of a binder or release component.

As regards the biaxial orientation, it has proved essential to carry it out in several stages, each of which is essentially uniaxial.

It has been shown here that simultaneous stretching in two directions maintained the rectilinear fiber structure instead of

bending it into a zigzag shape, with the result that the impact resistance was significantly worse than that of a laminate of similar composition and structure, but biaxially oriented in separate, uniaxial steps. In such simultaneously oriented cross-laminates, the fork effect only occurred during slow tearing (see example 4).

With regard to the temperature at which the orientation was to be carried out, it was found that the stretching near the melting point of the product's main components acted to spread the directions of the individual fibers, so that they did not form a regular zigzag pattern, and thereby the product's properties became worse. It is believed that the recrystallization and other phase rearrangements of a physical nature play a role in this connection. In any case, the best properties are found by stretching below the recrystallization ice ion temperature, which e.g. for polypropylene is approx. 70-80°C, and it is still an advantage to use even lower temperatures.

For stretching at such low temperatures, visible stretching methods are necessary, which will be dealt with later in this description.

The biaxial orientation produced by stretching below the melting point should preferably, in any direction, have a component at least equal to the orientation produced by the melt stretching, and it is generally preferred to make said orientation much stronger. In this context, the degree of orientation should be measured by X-ray fractionation, but for a quick and approximate examination of relative verids, an observation of interference colors between crossed polaroid filters is also applicable.

As explained above, it is essential that the bond between the layers with the fiber structure is generally weak in order to enable a local delamination during further tearing. However, this does not necessarily mean that the bond must be weak over the entire surface, but on the contrary, great advantages are generally achieved with regard to tear strength by performing the bond as a strong adhesive bond in spots or stripes, as an adhesive bond is avoided or a weak adhesive bond is produced between the other parts of the adjacent surfaces. Hereby, the necessary local delamination is easily started, but is then stopped or will continue under great resistance. At the same time, the strong bonded parts prevent delamination

of the laminate in or up to a glued or welded seam that is affected by tension, which otherwise easily happens.

By an appropriate choice of binding pattern, the different binding pieces and the type of breakage in the weakly bound or unbound area - as this binding can either be brittle or have a more fluid character - the tearing properties can be "tailored" for different purposes.

The aforementioned design of the binding is particularly applicable in connection with relatively thin layers. It has already been mentioned that there is a competition between breaking and delamination forces during further tearing, which means that the thinner the layers, the easier the start of delamination is required. As a practical rule, it will almost always be advisable to use the aforementioned bond if the layers are lighter than approx. 40-50 g/m2.

The use of strong bond/weak bond or strong bond/no bond patterns is previously known in connection with cross laminates of film that is uniaxially oriented or is rather unbalanced biaxially oriented, see here the descriptions of US patent no. 3,496,056 and 3,342,657 , British patent no. 1 316 640 and Danish patent no. 119 733. However, when tested individually, the layers in such known laminates show extremely low impact and puncture strength (except in the case of special and expensive polymers, such as nylon 6), while it has been shown that the individual layers according to the present invention exhibit clear and surprisingly higher impact and puncture strength. Therefore, you can allow yourself to perform the binding in the areas in question weaker (or to omit binding in these areas) or to give these areas a greater extent without thereby causing any significant decrease in the thickness of the punches or points.

The effect of the mainly weak bond between adjacent layers can be improved by providing one or both of the adjacent surfaces of at least two layers with a release or adhesive agent in points, spots or stripes.

By release agent is meant here an agent, preferably a polymer material with either low cohesive strength in itself or with low adhesion to the adjacent polymeric layer.

Whether a release agent or a binder should be chosen depends on whether or to what extent the polymer mixtures are mutually compatible, and on the method used, e.g. the temperature step used during the lamination.

Under all circumstances, it is thereby possible to better control the size of the binding force.

If the strips or points of the release or adhesive agent are displaced relative to each other on the two sides of a middle layer, the advantage is achieved that the free or weakly bound areas of the middle layer during tearing will be elongated by the applied tensile forces and will absorb some energy , and thus further stop the release effect.

Although there are several operational advantages to uniting the layers within an extrusion head, as will be described later, it is advantageous to extrude and melt draw the layers independently of each other before uniting them into a film, as this enables the formation of the most distinct uniaxial fiber structure in each layer. The solidified prefabricated film can be placed around a mandrel extending through the center of the extrusion head. The solidified film can be advanced along the mandrel and, as it passes the exit gap from the extrusion head, can catch and lead itself and thus melt stretch the rotating, constantly flowing polymer which in this way is wound around the solidified film with a fiber structure running in a helical line.

As already mentioned, the tear strength for a given total thickness is clearly better in a three-layer laminate than in a two-layer laminate. Therefore, it is also preferred to extrude two or more films immediately one after another from two or more counter-rotating output slits onto one and the same solidified film in the same manner as mentioned above. An advantage is that a completely directional fiber structure can be produced during the individually performed oblique drawing. Because of. the supporting and advancing effect of the film on the mandrel, which can lie very close to the circular exit gap, it is also possible to use more difficult polymers, e.g. polymers with a particularly high molecular weight.

In the above description, the rotating liquid tubular layer is drawn inward from the rotating extrusion head

the solidified film that is being fed. This measure will generally be advantageous because the elastic stresses induced by the rotation tend to reduce the diameter of the tubular film, thus promoting the capture of the liquid film and its sticking to the advanced, solidified film.

However, it is also possible to place the solidified advanced film in a largely tubular form with a diameter larger than the rotating exit gap, and to blow or otherwise extrude the liquid, rotating film outwards onto the solidified film.

It should be mentioned that the rotating part can be the advanced solidified film, while in that case the extrusion head is stationary.

It should also be noted that by machine direction is meant here the direction of advance of the solidified prefabricated film.

Advantageously, the film can consist of three layers with the fiber structure

in the middle layer running lengthwise,! as will later appear from the drawing description.

By allowing each layer containing a fiber structure to solidify

before they are combined into a film, simpler and more conventional equipment can be used.

In order to simplify the lamination of individually extruded and solidified films and thereby also enable the lamination of thinner films, at least the first phase of the lamination process can be combined with at least the first phase of the cross section. This significantly helps to prevent the appearance of wrinkles during lamination. If the desired stretching temperature is lower than the temperature necessary to make the films stick together, the first 10-20% of the stretching can take place at a higher temperature without any significant damage.

To facilitate joining of the layers, a lower melting adhesive polymer may be coextruded onto at least one surface of at least one layer.

This can be carried out by the union of the layers including co-extrusion, for example in strips of a lower melting adhesive polymer on at least one surface of at least one layer, the co-extruded adhesive polymer being selected as a material which is sufficiently adhesive so that adjacent layers can be joined exclusively by the stretching process under pressure. This gives the practical advantage that controlled heating means can be avoided, whereby the equipment becomes very much cheaper.

In this regard, it has been found that the simultaneous stretching of two films while being pressed together has a surprisingly high tendency to cold weld the films to each other so that only a slight degree of tackiness is required. E.g. it has been shown that surface layers of polyethylene with 16% vinyl acetate copolymerically weld at around 20°C in this way,

and gives a "peel" strength of 10 g/cm. Bond strength can then be improved by passing over or between heated rollers.

Another convenient way of achieving a binding pattern with spots or stripes consists in coextruding a continuous coating of an adhesive polymer on one of the two adjacent surfaces of two layers and further providing the other of these surfaces with a release layer.

It can be advantageous to carry out the biaxial stretching so that the transverse stretching forces are exerted by and large evenly distributed over the plane of the film. As mentioned, the stretching process is preferably carried out at a relatively low temperature, e.g. room temperature,

and if, under such circumstances, you want to use a stretch frame, as you normally do, this would almost inevitably

lig give an uneven "necking-down" with a tear strength that varies across.

It is known to carry out transverse stretching by uniform application of the stretching forces, see the descriptions in French patent no. 1 331 095 and British patent no. 1 07 8 7 32. Both make use of two rubber conveyor belts which expand transversely at the same time as they are pressed firmly together, whereby they retain and stretch the film. However, it has been shown that when manufacturing a product according to the invention, it is more appropriate to carry out the biaxial stretching, in the form of repeated transverse stretching operations by linear depression to deform the film into a temporarily evenly folded cross-sectional shape. In this connection, it should be noted that an exact uniform distribution of the tensile forces is not necessary, but that, on the contrary, a certain unevenness on a small scale will be advantageous with regard to the tear strength.

It has been particularly advantageous for the tear strength to allow the degree of orientation to vary according to a strip-shaped pattern, see fig. 8 and 9, in such a way that in one set of stripe-shaped zones the biaxial orientation is unbalanced and most strongly parallel to the stripes, while therein in the intermediate stripe-shaped zones it is likewise unbalanced, but most strongly perpendicular or almost perpendicular to the stripes.

If particularly high tear strength and puncture strength are desired and if a relatively low yield strength can be allowed, the stretching can be carried out by the temporary folded form being induced by repeated transverse stretching productions between grooved rollers, the folds forming stripes which are parallel to or form a small angle to the longitudinal direction of the film , as the longitudinal stretching is carried out continuously, preferably over a short stretching zone.

A greater elongation at break is thus achieved. In order to allow contraction at the same time that the longitudinal stretching is carried out over a short zone, the laminate can be provided with very fine, longitudinal folds similar to what appears in the description in US patent no. 3,233,029. In this connection, a suitable result is usually achieved if the fine folds formed in the last step of the transverse stretching process just mentioned are retained in the film when it is led into the longitudinal stretching zone.

Although it has been established here that there are advantages in producing a stripe pattern of orientation and thickness variations, particularly with regard to the tear strength, such small

this effect is not normally exaggerated as this will have an adverse effect on both the suitability for graphic printing as well as on the strength against penetration by sharp objects and the cold-temperature properties.

The purpose of what has just been stated is therefore to avoid an excessive tendency towards a sharp design of the linear stretching zones which would usually occur if the film was longitudinally oriented to a distinct degree after transverse stretching.

Due to the fibrous morphology, the stretching process will normally form internal, but usually not through, cavities in the layers with fiber structure. This effect is particularly evident when the stretching takes place at a relatively low temperature, and a greater or lesser degree of opacity can thus be achieved. This effect can be used to replace white pigmentation, but can also be canceled by subsequent press rolling.

The drawings show:

Fig. 1 a section through an extrusion head for production

of a film material according to the invention.

Fig. 2 shows, in perspective, with offset cross-sections, the principle of an extrusion head with two counter-rotating exit slits and means for extruding two layers through each slit. Fig. 3 similarly shows the principle in an extrusion head with counter-rotating and a stationary exit slit and with intermediate exit slits for air flows. Fig. 4 shows, in perspective, and partly in section, the principle of a rotating, ring-shaped extrusion head with a mandrel extending through the center of the head. Fig. 5 shows the course of a process for the production of a

product according to the invention.

Fig. 6 is a process diagram of a cold-stretching method.

Fig. 7 is a detail of the grooved rollers which carry out transverse stretching in uneven zones called stripes. Fig. 8 is an enlarged schematic sketch of the stripe pattern and its orientation in a film which is biaxially stretched according to the process diagram in fig. 6. Fig. 9 shows, on an enlarged scale, a section through the film 8 as it really appears by microscopy, but

for the sake of clarity, the thickness is shown on a double scale in relation to the width, and

Fig. 10 shows a scanning electron micrograph photograph in 30,000 x magnification and shows the zigzag polymer fiber structure in a fiber layer according to the invention.

In the extrusion head shown in fig. 1, two polymer-in-polymer dispersions are extruded into a common collection chamber through two rows of partitions rotating in opposite directions. The two dispersion streams 1 and 2 are fed through input channels in the lower part of the extrusion head to annular channels 4 or 5 in the two walls of the groove 6, in which the two rings 7, 8 are moved in opposite directions with driving means, e.g. using teeth and gears not shown. The two rings 7 and 8 are provided with rows of partitions, 9 and 10 respectively, by means of which two rows of openings II and 12 are formed and through which the two dispersions are extruded into the collecting chamber 15 which is formed by the two parts 13 and 14, and which ends in output column 16.

For the sake of clarity, the partitions 9 and 10 are shown lying radially, but in reality they are arranged at an angle to the radius to prevent the formation of lines of weakness or "cast lines" in the extruding film. During the extrusion through the two rotating rings 7 and 8, the two dispersions will be hot-drawn and will thereby acquire a fibrous morphology. The two sets of heat-drawn streams will then unite in the collection chamber 15 and form a laminate with intersecting fibrous morphology. The thickness of this laminate is reduced by the passage through the exit gap 16 and further by the normal withdrawal and inflation process. The film is then stretched both longitudinally and transversely at a relatively low temperature.

Because of. the two different fiber directions, two halves of the film will exhibit tendencies to split in different directions during tearing. The materials from which the two halves are formed are chosen so that they stick poorly to each other. The workpiece will therefore delaminate in the small area around the tear from where the tear originates and this will equalize the cutting effect.

The nozzle shown in fig. 2 consists of the main parts, namely a fixed input part 17 for circular distribution of the polymers as explained below, a fixed bearing part 18 and carried by this, two rotating parts 19 and 20 which together form an exit slot 21. The polymers A and B are fed into the entrance part 17 where they are distributed in concentric circular streams. A is extruded through annular conduits 22

and 23, for which one or two extruders may be used.

B is extruded through the annular conduit 24. In order to obtain an even distribution, 22, 23 and 24 are provided with distribution plates or other distribution means which are not shown.

For clarity, no bearings or seals are shown between the bearing part 18, the rotating part 19 and the rotating part 20, nor is the drive mechanism for the parts 19 and 20 shown.

From the three annular conduits 22, 23 and 24, the polymer flows pass the supporting part 18 through three circular rows of channels 25, 26 and 27 each of which is connected to a circular chamber 28, 29 and 30.

The two rotating parts 19 and 20 are preferably rotated

at the same speed but in opposite directions as indicated by arrows 31 and 32. Each rotating part is itself a co-extrusion head for two layers, one consisting of A and one of B.

For the sake of clarity, the reference numbers for the explanation of the process are only shown on section 20, while the process through section 19 is similar. From the chamber 29, the polymer A passes into the rotating part through channels 33, while the polymer

B from the chamber 30 passes into the rotating part through channels 34. Inside the rotating part there are two circular lines 35 and 36 which are in connection with the channels 33 resp. 34, and are separated from each other by a thin circular wall 37.

After passing the edge of the wall 37, A and B slide together in a circular collection chamber 38 which ends in the exit gap 21.

When passing through the collecting chamber 38 and into the exit gap 21, the thickness of the liquid film is greatly reduced, whereby the material is melt-stretched.

The partitions between the channels 33 or 34 should be streamlined as shown. For clarity they extend radially in the drawing, but in reality they should form an angle with the radial direction to reduce the tendency to form lines of weakness or "cast lines".

The polymer A is a mixture of two completely or partially incompatible polymers, while the polymer B is intended to give the film a suitable delamination tendency. It can therefore consist of an elastomer that sticks poorly to the two layers of the polymer A, and it can optionally be extruded in strips. If, however, the wires 22 and 23 are fed with two different polymeric mixtures which are mutually incompatible, the polymer B can be an adhesive with a relatively strong bond to the two polymeric mixtures and must in this case be extruded in strips or otherwise interrupted.

The apparatus shown in fig. 3 essentially consists of the components 39, 40, 41 and 42 which correspond to those previously described, but there is an output gap 53 or 54 in each of the rotating parts 41 and 42, and there is a further stationary outlet gap 43 formed by the supporting part 40. From the three circular conduits 44, 45 and 46, the polymer flows C and D pass the supporting part 40 through channels 47, 48, respectively, and enter the three circular chambers 49 and 50, the latter of which continues in the stationary outlet slot 43. Each of the cannons 49 is formed by a stationary part 40 and a rotating part 41 or 42. Through channels 51 in the parts 41 and 42, each of the chambers 49 is in connection with the corresponding of the two circular chambers 52 in the rotating parts, and each chamber 52 ends in an exit slot 53 resp. 54.

Arrows 55 and 56 show the directions of rotation.

After leaving the extrusion head, the three tubular foils run together at the same time as the row tubular foils formed by the polymer C are subject to twisting due to the rotation of parts 41 and 42.

From the outside and inside of the part 40, air is led through the channel 55 which ends in the opening 56. For the sake of clarity, the channels 55 from the outside of the part 40 are not shown. Via other channels 57 in the parts 41 and 42, air is led through the outlet gap 58 and 59 between the stationary part 40 and the rotating parts 41 and 42. The channels 57 in the part 42 are not shown for the sake of clarity. The annular pockets of air thus formed between the neighborhoods prevent the rotating outer and inner polymer films from puckering against the middle layer immediately outside the exit slits.

Air is preferably blown onto the extruded film and the air is supplied to both internal and external air cooling.

The apparatus as shown in fig. 2, is usually easier to work with than what is shown in fig. 3, while the latter presents some special possibilities. One of them is to use expanded polymer in the middle layer and another is to produce a longitudinally extending fiber structure in this layer so that you get three structural directions in this laminate. Having three directions instead of two in connection with the preferred tendency to delaminate will significantly improve the tear strength. Furthermore, the melt stretching process is more clearly terminated before the lamination, which, as previously mentioned, is advantageous.

In fig. 4, 60 is a rotating annular extrusion head. Through a not shown stationary part of the extrusion head,

which is connected by drawing with the rotating part 60,

the polymer mixture is fed into a circular chute 61 and led to the exit gap 62 through the channel 63, which is separated by thin, plate-shaped partitions 64. For the sake of clarity, the partitions 64 are shown standing radially, but in reality they form an angle with the radial plane to avoid lines of weakness or casting lines in the finished film. 65 is a mandrel which is held in place by means of devices not shown, and 66 is a longitudinally melt drawn, prefabricated flat film which is laid in tubular form around the mandrel 65.

For the sake of clarity, a space is shown between the mandrel 65 and the folded film 66, but the film naturally lies close to the mandrel. The film 66 is fed through the extrusion head,

over the mandrel as indicated by the arrow 67. When the polymer film 70 continuously floating leaves the rotating exit slot 62 it is caught by the folded film 66 due to the elastic contraction in the melt-drawn polymer mixture and it is thus wrapped around the folded film and advanced together with it, thereby achieving a spiral-shaped progressive splitting direction which is indicated by the arrows 68.

The splitting direction in the prefabricated, folded film 66 is indicated by the arrows 69.

The bond strength can be controlled, e.g. by co-extruding an adhesive layer together with the film 66. With a sufficiently high temperature in the mandrel, the lamination of the film can be carried out on

this. However, the film often remains preferably shock-cooled on the mandrel, in which case the temperature of the mandrel may be insufficient to weld the two films 66 and 70 together. The lamination can therefore be completed with hot or cold welding after the film has left the mandrel 65.

With the expression "machine direction", which is used earlier in the description, is meant the direction of advance of the film 66.

The plan in fig. 5 schematically mentions the various steps i

a process that avoids the use of rotating nozzle parts. The last two steps can be carried out using the cold stretching method indicated by the process diagram in fig. 6, where "Q" is the transverse tensile section and "R" is the longitudinal tensile section.

The system of rollers in section "Q" consists of driven press rollers 71, driven groove rollers, running rollers 73 and banana rollers 74. The banana rollers after each step serve to smooth out the folds that have appeared during the transverse stretching. Above the running rollers 75, the film 79 enters the section "R", namely the longitudinal stretching section, where it is stretched through a water bath 76 which serves to eliminate the stretching heat and maintain a suitable stretching temperature of e.g. 2^-40°C, and onto a coil 77. The arrow 78 indicates the direction of the machine.

In fig. 7 shows in detail a pair of driven grooved rollers 72 with the film 79 pressed and stretched between the teeth 80 of the rollers 72.

In fig. 8, the relative lengths of the arrows in "stripe sets" I and II on the film 79 suggest the relative magnitudes of orientation achieved by the biaxial stretching node as shown in FIG. 6 and 7.

In fig. 8 as well as in fig. 9 suggests the I and II "stripe" zones which are generally of varying width and uneven character. It should further be noted that the outer layers 81 and 82 of the film 79 are not always symmetrical about the thin middle layer 83. This asymmetry further serves to cause a tear to "fork out".

Fig. 10 shows in 30,000 x magnification the structure of the two main layers in the two-film laminated sample according to the invention, which is described in example 5. These main layers have a composition of 85% polypropylene, which from the polymerization contains approx. 20% particularly high molecular atactic component, plus in intimate mixture with 15% ethylene vinyl acetate copolymer (EVA), on both sides of the main layers is coextruded pure EVA. The laminate is produced from unoriented tube foil, which is first spiral-cut at 45° in relation to the longitudinal direction, which is also the melt-stretching direction, then brought together with corresponding cut foil in the cross arrangement, so that the angle between the two melt-stretching directions becomes 90°, and finally laminated as well as biaxially oriented at 35°. This lamination and orientation is carried out by passing 5 times between interlocking rifled rollers, and then by continuous longitudinal stretching. The stretching conditions are chosen so that the resulting biaxial stretching is in the ratio approx. li 6 both in the longitudinal and transverse direction, which is measured using inscribed circles.

Laminates with the same compositions, but respectively in other thicknesses and in a three-film laminated structure, are otherwise described in examples 1 and 3.

The preparations for electron microscopy are clamped in frames and washed out for 24 thaws with xylene at 80°C. Although the copolymer is soluble at a much lower temperature, it has proved necessary to choose this temperature, which is very close to polypropylene's dissolution point. This relationship will be further commented on below.

During the treatment with solvent, the surface layers and at least the main part of the copolymer in the main layers are washed out. In return, fibrils or micelles become essentially pure polypropylene, and a surface topography arises that can be described as "mountain ranges" and "valleys", which mainly follow the original melting and stretching direction.

Because of. the limitations of the electron microscope can be overcome by the preparation itself

only used for studying the structure of the "mountain ranges",

while the "valleys" need replica technique, fig. 10 is a replica recording. The photomicrograph of the preparation itself, which is not reproduced here, shows that the fibrils on the tops of the mountain ranges are essentially parallel to them, while the fibrils in the "valleys" as can be seen from fig. 10 is strongly deflected from the original melt stretching direction. However, the course of the fibrils here is not random, but represents a well-ordered zigzag pattern.

The cross-sectional dimension of the fibrils is of the order of magnitude

0.06 micron. Corresponding investigations have been made of laminates, produced from lower compositions, but always according to the invention and in all cases, fibrils of approximately the same cross-sectional dimension and in a similar course have been found. However, the fibrils are usually gathered in bundles, which bundles have a cross-sectional dimension of the order of 1 or a couple of microns because the structure can thus be easily observed with a scanning electron microscope without the use of a replica technique, in most cases also with an optical microscope.

When, however, to fig. 10, a sample has been chosen where the fibrils are not gathered in bundles, so this is because this particular composition has been particularly thoroughly investigated in the examples. The zigzag sequence is inextricably linked to the fact that, on the one hand, the transverse orientation and, on the other, the longitudinal orientation are carried out as separate process steps (although, of course, in practice, if desired, they can be carried out on the same machine unit) and each for are carried out as essentially uniaxial stretches. In this connection, a corresponding electron microscopic examination has also been carried out of a laminate which was produced from the same film, which was used in connection with fig. 10, but which was stretched simultaneously in the longitudinal and machine direction using a stretching frame (see example 5). In this case too, a distinct fibril structure was found with fibrils of similar cross-sectional dimension as in fig. 10, but these fibrils ran straight out along the melt stretching direction without showing any zigzag structure. As illustrated in example 5, this results in much poorer shock properties in the laminate.

When choosing components for practicing the invention, preferably polymer is used which is only to a small extent incompatible - such as e.g. isotactic and atactic polypropylene or as in the present case polypropylene and EVA - or which if they are completely incompatible are united through an alloying agent. When the mixing process is carried out efficiently, a polymer-in-polymer dispersion is then formed, where the particles are already drawn out by the mixing process into micelles with a cross-sectional dimension of approx. 0.1 micron or finer, but with a somewhat diffuse demarcation. In the predominantly uniaxial melt stretching, which according to the invention is to be carried out before solidification, these diffuse micelles arrange themselves so that molten material parallels each other, and immediately before or during solidification, the polymer with the highest melting point stands out as the dezincified fibrils, which can be seen from fig. 10, encapsulated in the material, which solidifies at a lower temperature.

It appears from fig. 10 and corresponding cross-sectional views

(not reproduced here) that it is essentially polypropylene that is fibril-shaped, while the additive (such as EVA)

despite the relatively small amount, has formed basic substance. Correspondingly, e.g. by electron microscopic examinations of laminates according to the invention produced on the basis of a mixture of 85% isotactic polypropylene without the content of a tactical component and 15% ethylene-propylene rubber. In this way, a structure consisting of stiff fibers encapsulated in relatively small amounts of softer material can be produced.

The product's strength properties are significantly increased as a result of constantly regular courses of the fibrils, which is clearly evident from fig. lo, and is described below. As can be seen from example 4, these deflections into a zigzag course have a shock-absorbing effect, so that impacts etc. can be triggered in a calm stretching process.

Although in this connection it is a great advantage that the rigid crystalline fibrils are encapsulated in softer mater-

In general, however, it can be established that the effect regarding the zigzag course is also achieved when starting from a mixture of two rigid components such as e.g. polypropylene and high density polyethylene, see the comparison in example 5.

For economic reasons, the present invention is particularly applicable in connection with mixtures which mainly contain crystalline olefins. Best for the most general applications are blends of polypropylene and low-pressure or high-pressure polyethylene.

Which mixing ratio should be used and whether it should be used

whether low-pressure or high-pressure polyethylene is used depends on the desired stiffness, low-temperature strength and, in general, on which strength properties are particularly desired. To achieve sufficient cohesive strength in each layer, the polypropylene should either be a copolymer that is partially compatible with polyethylene,

e.g. polypropylene with 2-5% ethylene content, or a suitable "alloying agent" should be used. In this connection, it is sufficient to maintain the content of the atactic modification high in the iso-(syndio-)tactic polypropylene during the production of this polymer instead of removing this "impurity" as is normally done. It is a special purpose of the invention that polypropylene with a large content of the atactic material can be made extremely usable. Other "alloying agents" are mentioned earlier in the description.

Also of economic interest are mixtures of polypropylene and an elastomer, e.g. ethylene-propylene rubber, ethylene-vinyl acetate copolymer, polyisobutylene or a "thermoplastic rubber" based on butadiene/styrene.

When high low-temperature resistance and/or high flexibility are particularly desired, mixtures of high-pressure polyethylene and a semi-compatible rubber are preferred. It should be pointed out that the mixtures do not necessarily have to be formed by mechanical mixing, but can be formed already in the polymerization process. Thus, polypropylene with an extremely high content of the atactic component should be usable without any further addition,

and the known polymerization processes which aim to make mixtures of polypropylene, polyethylene and block polymers between these may also be suitable.

In the case of polymers outside the polyolefin group, the following combinations will e.g. be applicable for special purposes:

a) Polyester/polyamide or polyurethane

b) polyester or polyamide/polycarbonate

c) vinylidene copolymers in various combinations.

In addition to the layers with the particular morphology described can

there may also be teams with special characteristics. Thus it is

almost always advantageous to co-extrude thin surface layers of a suitable adhesive component to enable bonding of the laminate without destroying the orientation. As another example, it will also often be necessary or advantageous, particularly for food packaging, to add one or more special layers to improve the barrier properties.

Extra strong laminate according to the invention is considered to be advantageous for use in the following areas:

1. Packaging of foodstuffs:

Food bags in general, 100% plastics or combined with paper, packaging of frozen goods.

2. Packaging of non-food items:

Fertilizer bags, cement bags, bags for valuable chemicals, e.g. plastic granules, bags for coarse-grained chemicals, e.g. natural salt and for other sharp objects, wrapping steel sheets, wrapping carpets and wrapping bales, e.g. cotton and wool, timber protection, carrier bags, individual wrapping of machine parts, weapons etc., sterilization bags for heavy or sharp objects and various other purposes, e.g. for textiles, cloth, paper, medicine, toiletries, tobacco.

3. Film in connection with containers:

Shrink wrap, stretch wrap for pallets, waste bags, especially compact packs, industrial shipping packages.

4. Non-wrapping:

Disinfection foils, foils for soil cover for erosion control, demarcation in the construction of ponds, water reservoirs and canals, road surfaces, wind screens,

greenhouse foils, plant protection foils (also for agriculture and forestry), covering of piles of agricultural and forestry products, salt etc., weather protection for animals ("clothing" for animals),

rain gear, tents, inflatable architectural structures, water-borne structures, "lighter-than-air" floating structures, rib structures (architectural, low-cost containers), pillow-shaped cargo containers, railway wagons, truck covers, weather protection over buildings under construction, vapor barrier layers over concrete structures for to delay drying out, insulation of roofs under the roof, insulation of cold rooms, "membrane foils" in house constructions, ceiling boards, various types of construction cardboard (laminated with cardboard), cheap swimming pools and adhesive tapes for the industry.

It should be pointed out that the above-described new extrusion method and apparatus for carrying out the method in which rotating nozzle parts are used will also have useful application possibilities outside the area defined in claim 1. Thus, it can be used for extrusion and lamination of film without "fiber structure" as described, which is still useful for various purposes. Furthermore, the general extrusion system shown in fig. 4 and stated in claim 6, e.g. is used to spin a melt-oriented film (with or without fiber structure as described) around a cold-stretched film.

Example 1

A three-layer tubular film is extruded with the following composition:

The middle layer (70% of the total)

85% isotactic polypropylene of the gas phase type ("Novolen") with a high atactic content,

15% ethylene-vinyl acetate copolymer (16% vinyl-(acetate).

Both surface layers (one 10% of the total, the other 20% of the total): Ethylene-vinyl acetate copolymer (16% vinyl acetate) which will serve

as an adhesive layer.

The polypropylene has a melt index of 0.3-0.6 according to ASTM D 12 38, under condition L, while the ethylene-vinyl acetate copolymer has a melt index of 2.5 according to the same ASTM number, but under condition E. The tubular film is extruded from a 1 mm wide slit at 180°O230°C and drawn to 0.130 mm in the molten state. The degree of inflation is kept very low, namely 1.2:1.

It is then cut along a helical line into a flat film with a fiber structure with a 45° inclination. Two films cut in this way with the fiber structure running perpendicular to each other and with the thinnest surface layers facing each other are fed at 20°C through seven pairs of grooved rollers, see fig. 6 and 7.

The width of each groove is 1 mm and the width of each ridge is 0.5 mm. The overlap between the combs (the difference in height between the peaks) is 1 mm. Between each pass through a pair of grooved rollers, the wrinkles that form in the laminate are stretched out.

During the mechanical treatment between the grooved rollers and due to the copolymer layers acting as adhesives, the two films are cold-welded together with a relatively low bond strength - the peel strength measured at 10 g/cm - and are simultaneously stretched across•

After the seven passes at 20°C, the film is passed once more between a corresponding pair of grooved rollers with the same dimensions and engagement, but heated to 120°C, whereby strips with a strong bond are formed.

Finally, the laminate is oriented lengthwise in three steps with approx. 1 cm stretch zone (to minimize transverse contraction). The last stretching step is adapted so that the total cross-cold stretching ratio and the total longitudinal stretching ratio are equal, whereby the product of this, i.e. the area stretching ratio, is 2.4:1.

Test results compared to a high-pressure polyethylene film of high tensile quality (heavy-duty-bag-quality) and with 85% higher weight per m o and with a melt index of 0.3 according to ASTM, condition E. Gram weight 100 g/m 2 for the laminate and 185 g/m 2 for the polyethylene film. Impact strength measured by "falling ball" (diameter 61 mm, weight 329 g) is for the laminated film of 100 g/m 2:5.5.m.

For the polyethylene film of 180 g/m 2: 2 m.

Tongue tear strength:

Tearing at a speed of 100 mm/min., the total width of the sample

5 cm, average length 10 cm.

For the laminated film:

5.9 kg in the machine direction and 6.8 kg in the transverse direction. For the polyethylene film of 180 g/m 2: 1.3 kg.

Elmendorf tear strength (shock tear):

The test is a modification of the standard tests and covers

a more symmetrical tearing. Results: For the laminated film of 100 g/m 2 in the longitudinal direction 441 kg-cm/cm 2 , in the transverse direction 334 kg cm/cm<2>.

For the polypropylene film of 180 g/m 2: In the longitudinal direction

167 kg cm/cm 2 , in the transverse direction 172 kg cm/cm 2.

A piece of the film is delaminated by peeling and the structure is examined under a microscope after washing out ethylene-vinyl acetate with exylene. The main layers have a clear fibrous morphology with zigzag fiber structure directions.

Example 2

The procedure in example 1 is repeated with the following change:

The three-layer coextruded film has the following composition:

The middle layer (70% of the total)

85% isotactic polypropylene (same type as in example 1.

15% ethylene-propylene rubber (approx. the same melting index as the polypropylene).

Both surface layers (each 15% of the total): Ethylene-vinyl acetate copolymer (same type as in example 1). The film was more strongly melt drawn after it had left the extrusion head, namely by being drawn from 1 mm thickness to 0.065 mm (60 g/m 2 ). Investigations in polarized light showed that the melt orientation produced by this corresponded to approx. 35% uniaxial cold drawing.

After the helical cut, it was produced

a three-layer laminate. The third layer placed in the middle had a longitudinally continuous fiber structure obtained by cutting the same film lengthwise. The lamination and stretching was carried out on the same machinery as in Example 1, but all steps were carried out at 20°C, and the apparatus was adjusted to thereby produce a total area stretch ratio of 2.5:1, whereby the thickness of the final laminate was 72 g/m <2>. The peel strength of the bond between the layers was measured to be 10 g/cm. Examinations under a microscope showed a similar structure as in example 1

The following test results were obtained:

Example 3

A variety of polyolefin based films have been produced with the extrusion head shown in fig. 2. The diameter of the exit gap 21 on the head was 130 mm due to the width 1 mm. The largest width of the collecting chamber 38 was 4 mm, which means that the degree of melt drawing during the passage through the collecting chamber towards the exit gap has been less than preferred. The extrusion temperature was approx. 240°C. After longitudinal cutting of the tubular film, stretching was first carried out transversely between 4 and 8 steps and then lengthwise between 2 and 4 steps in the same machinery as used in examples 1 and 2.

The composition, the width of the flat tube (expression for the degree of inflation), the stretching temperature, the stretching ratio and the results are shown in the table below. "NOV" stands for

Novolen, a gas-phase polymerized polypropylene with relatively

high content of the atactic modification, "PE" stands for high-pressure polyethylene, "ERP" for ethylene-propylene rubber, "SA

872", "7823" and "8623" are different types of polypropylene with less content of polymerized ethylene.

The fact that even the best specimens in this example are generally inferior to the product of example 1

and 2 is explained by a smaller uniaxial total melt stretching. Some biaxial melt stretching is unavoidable in this embodiment, as the streams are first united inside the extrusion head, crossing each other and then further melt drawn through the passage through the exit slit and immediately thereafter. On the other hand, this method is particularly easy to handle.

Example 4

The purpose of this example is to show the significance of the zig-zag structure that emerges by orienting cross-laminated film with a "grain of polymer" lengthwise and crosswise in separate, individually uniaxial processes.

A two-layer cross laminate is produced based on exactly those

the same foils as in example 2, although with slightly different thicknesses and stretch ratios, as appears from the form. The same stretching procedure as in example 2 is used, however, before the transverse and longitudinal stretching, the two webs are laminated by passing over a 90°C hot roller under a light tension. This fuses the two EVA layers together.

For comparison, it is produced from corresponding foils

a cross laminate which is similarly first laminated at 90°C, but then stretched simultaneously in both directions at 90°C in a laboratory stretching frame.

In both cases, the cross-lamination takes place so that the melt stretch directions (i.e. the original longitudinal direction of the foil) are perpendicular to each other.

Despite the fact that the EVA layers are fused together, it turns out that the bond between the two cross-laminated films is suitably low, namely approx. 1 kg/2.5 cm, measured as peel strength, as the co-extrusion die has not been completely streamlined,

and the EVA layers therefore only stick weakly to the middle layers. The bond between the films is the same in the two cases being compared.

Both laminates have been examined electron microscopically, and it is

the sample stretched in separate steps which is used for the one in fig. 10 reproduced discovery that clearly shows a regular zigzag pattern. As also mentioned in the description of

fig. 10, the sample of simultaneously stretched laminate, on the other hand, did not show such a zigzag pattern, but a completely similar course of the fibrils.

The two different series of samples are tested for shock tear strength. For this purpose, the Elmendorf tear strength test used in the previous examples is not considered to be suitable, especially because it starts at almost zero speed. Instead, a combination of the so-called trapezoidal tear propagation test (ASTM D 3-2203) and tensile impact strength test (ASTM

D 1822-68). The former is a slow tear test, where the clamped part of the test piece is trapezoidal. The latter is a pendulum impact test, where the over-pulling starts when the pendulum is in its lower position, and the breaking energy, i.e. the integral of pulling forces and extensions at the present speeds, is measured as a height loss.

The two test methods are combined into a shock tear strength test

by using a pendulum apparatus which is basically set up as an apparatus for determining tensile impact strength, however modified for the purpose, while the test piece is as for trapezoidal strength determination, albeit in smaller dimensions.

Explained in more detail, it has the modifying pendulum apparatus

2 pendulum arms that swing about the same horizontal axis, and the test piece is clamped between two arms. These are in a lightweight aluminum design. During the stroke, the rear pendulum is stopped by a stop block which acts in the pendulum's lowest position. A small container of lead shot is placed on the pendulum, preventing it from swinging back. The front pendulum continues its movement, tears the sample over and produces a deflection, from which the tear-off energy can be determined by comparison with the deflection during a blind test.

It should be noted that the two pendulums are tuned to have the same oscillation time.

The distance from the axis of rotation to the center of the test piece is

24.0 cm and the moment gravity affects the pendulum with, when it is horizontal, is 8.88 kp x cm. The center of gravity is almost in the middle of the pendulum.

The clamped part of the test piece has the shape of a 60° equilateral trapezoid, the base line of which is 7.0 cm and the height is 4.1 cm. The incision to start the tear starts in the middle of the parallel edges and is 1.5 cm long. It is perpendicular to these edges.

The slanted trapezoidal lines are drawn with a stencil and the tensioning between clamps on the two pendulums is done according to these drawn lines.

During initial investigations, it is established that the simultaneously stretched product has such a dominant tendency to tear along one of the two melt stretching directions that the tear will always propagate along one of these directions, regardless of how they lie in relation to the cutting direction. If they lie below 4 5° with this, the tear runs towards one clamp,

and stops here. All comparative tears are therefore made so that the incision is parallel to one of the melt-stretching directions.

In the forms, the columns have the following meaning:

"ni" and n2" mean, in the case of the series which is stretched in separate steps, the stretching ratio in the longitudinal and transverse direction, respectively.

In the series where the stretching is simultaneous (i.e. at the same time in

both directions) "n2" means the largest stretch ratio, and "ni" the smallest stretch ratio. The conditions are in all cases measured by drawing circles on the test pieces within the stretch and measuring deformation after the stretch.

"Percent tearing" indicates whether the sample is completely torn ("100%") or if the pendulum's energy has not been sufficient for tearing, how much of it is torn.

In the event that the sample is not completely torn, the tear energy is calculated as the location energy of the pendulum at full swing (i.e. of the blank) divided by the tear percentage and multiplied by 100.

"are v" indicates the swing of the pendulum in purely angular terms. In cases where full tearing has not occurred, this value is set as (0).

"1 - cos.v" indicates the ratio between the swing height and the corresponding arm length, while A (1 - cos.v) indicates the difference between the swing height of the pendulum during the blind test and during the test in question.

"E" denotes the tearing energy and is calculated by multiplying the above-mentioned relative height difference by an apparatus constant of 8.88, which is the torque acting on the pendulum arm in a horizontal position.

"E 100 g/m 2 " indicates the tearing energy referred to a 100 g/m 2 sample, since for the sake of comparison and approximately proportionality between square meter weight and tearing energy is calculated.

"K 100 g/m 2" indicates the average tear force and is calculated by dividing the corresponding energy by the tear length, i.e. the distance they have moved apart at the moment the tear is finished. This length is calculated theoretically in a geometric way, as it is assumed that the test piece is a completely rigid material and is then determined to be 2.60 m.

The single samples marked with an asterisk, i.e. on one side a, d, f and on the other side h, i, j and k are selected for comparison because roughly the same stretch ratio.

The results of the determination are then that the product, which is biaxially oriented by separate, mainly uniaxial stretching processes, gives approx. 4^ times as high shock tear strength as that which is simultaneously biaxially stretched. This must be explained by the shock-absorbing effect of the zigzag fibril structure.

Example l 5

The purpose of this example is to show the difference between a cross-laminate which is made from pure polymer and which is therefore essentially without any "grain of polymer" and a cross-laminate which is made according to the present invention from a polymer mixture, which shows a clear " grain of polymer" with zigzag courses. In the first case, pure homopolypropylene is used for the main layers, essentially without any content of atactic component and with a melt index of 0.4 according to the ASTM test mentioned in example 1.

In the second case, the same polypropylene is used, but

with the addition of high density polyethylene with the same melting index as well as 20% of a polypropylene type, which is produced by successive polymerization of propylene and ethylene (trade name Hostalen 1022). It also has the same melting index, and according to an analysis, it consists of approx. 80% homopolyethylene and approx. 10% ethylene propylene rubber. It has, however, proven to be suitable as a dispersing and alloying agent and it is therefore assumed that, despite the analysis result, there is a formation of sand block copolymer that is significant for the purpose. A low addition percentage and an addition polymer which has almost the same stiffness as the main component have been deliberately used, thereby avoiding introducing errors into the basis of comparison. Such would occur if a significantly softer additive component were used. You thus have, one might say, an extreme point of what may be appropriate to use in connection with the invention.

The individual films are extruded as a two-layer tube foil, in order to establish a suitable bond during the subsequent cold lamination process, a thin layer (10% of the foil) is co-extruded consisting of equal parts of the aforementioned successively polymerizing propylene (Hostalen 1022) and ethylene propylene rubber with the same melting index.

The tube foils are cut spirally under the 45° and laminated and stretched as in example 1. However, the stretching is carried out crosswise between riffle rolls in 12 steps, which is a larger number than one would think of using under practical conditions. The purpose herewith is to reduce the profiling of the foil as much as possible, as the tendency to form a profiled foil is particularly great with such rigid materials. It has been shown that this tendency is greatest with the pure polypropylene, and significantly less with the mixture shown here in the example, and the large number of stretching steps is therefore carried out to avoid sources of error as a result of different degrees of profiling. The test was carried out as in example 4, however, in this case the cutting and thus the tear direction was parallel to the longitudinal direction of the laminate, i.e. formed an angle of 45° with each of the melt stretching directions. During preparatory tests, it turned out that the longitudinal direction of the laminate in these cases was the most critical in terms of tearing.

The explanations below further illustrate the advantages of the cross-laminates according to the invention in comparison with cross-laminates of uniaxially oriented foils. A high-strength foil, e.g. for heavy-duty bags, must on the one hand have a high yield point during static testing or long-term testing, so e.g. a filled sack can lie under constant pressure in a stack for a long time without being significantly deformed, and on the other hand possess dynamic strength properties, e.g. expressed by shock tear tests and shock penetration tests. For foils, the static and dynamic strength properties will generally be in opposition to each other, so that particularly high static strength values will correspond to relatively poor dynamic properties and vice versa, but with the distinctive structure produced by the present invention, effects are achieved which serve to to reconcile these opposites.

A key factor in characterizing the dynamic strength properties has been shown to be the elongation at break, determined in dependence on the strain rate.

In what follows, a principle will be made on this basis

comparison between three different types of cross-laminates, all of the same composition in their main layers, e.g. as in example 1, 85% gas phase polymerized polypropylene and 15% ethylene vinyl acetate. The three types are the following:

a) cross-laminates of two foils which have only been stretched in the molten state in connection with the extrusion under normal cooling conditions and with inflation conditions approx. 1 : 1. As a result, each foil in the laminate has been given a weak uniaxial molecular orientation and in the case of cross-lamination, the strength properties are approximately the same in all directions. b) cross-laminates of foils which, after extrusion, but during cross-lamination, are uniaxially stretched at room temperature in a stretching ratio of 2.5:1. c) cross-laminates according to the invention which, after or in connection with lamination, are stretched in a ratio of 2.5:1 both

in the machine and transverse direction.

The areal stretch ratio between the layers in the cross-laminates b) and c) is therefore chosen to be the same, namely 2.5 : 1. Thereby it has been shown that the flow force during slow pulling is also (approximately) the same, in other words the foils generally have the same static strength. The individual layers in b) have, in their main direction, approx. twice as high yield strength as the individual layers in c), but in return they have practically no strength perpendicular to this. The cross-laminate a) has, on the other hand, a significantly lower yield strength due to the very weak orientation.

As far as the dynamic properties are concerned, this shall be done

a principled comparison of the elongations at break for the

three types of cross laminates at different speeds. When narrow strips of the cross-laminate a) are stretched at room temperature slowly enough to avoid a temperature rise, the strip's dependence on the melt orientation is extended in a ratio between 4.5:1 and 5.5:1 before it breaks. In the following, the MD average is calculated as 5.0 : 1, i.e. elongation at break 400%.

Under similar conditions, the cross-laminate will b) if simple

layers have already been stretched in the ratio 2.5 : 1, when stretched in a main direction to the breaking limit is extended in the ratio (5.0 - 2.5) : 1 = 2.5 : 1, i.e. the breaking elongation is only 150%.

If for the cross-laminate c) you want to make the assumption that the stretching in two mutually perpendicular directions will proceed independently of each other, you will similarly calculate that the extension in the main direction can be carried out up to the limit ratio (5 - 2.5) : 1 = 3, 5 : 1. Thus, under the aforementioned assumption, an elongation at break of 240% is found.

Now, however, it actually turns out to be very surprising that the elongation at break is significantly greater, namely approx. 350%, i.e. more than double for the cross-laminate b). This is an expression of the fact that the special structure produced partly by mixing and melt stretching, partly by biaxial orientation in separate steps, makes the foil particularly suitable for further stretching.

The ultimate breaking strength along one of the main directions is for the cross-laminate b) largely determined by one layer alone,

while that for the cross-laminate c) is determined more or less equally by both layers, and therefore becomes approximately twice as high.

The above applies to the conditions for slow pulling. If the stretching of the cross-laminate a) occurs more quickly, the stretching heat is wholly or partly in the foil during the stretching process. In the event of a moderate increase in the stretching speed, an easing of the stretching is thereby achieved, so that the elongation at break e.g. can be increased from 400% to 600%. When the stretching speed is successively increased, however, a point is quickly reached where the stretching process becomes completely unstable and the foil practically breaks without any lasting elongation at break. This is presumably due to the fact that the internal friction has now become so high that a start of the stretching process in a random place produces such a heat generation that the further process is concentrated at this place, whereby the foil presumably almost melts locally.

In principle, something similar takes place when the cross-laminate

c) is stretched shock-like, but the zigzag-progressing fibril structure has a shock-absorbing effect, so that the speed limit value, where the smooth stretching course suddenly changes to an unstable course, is at the very high level. The shock-absorbing (shock-absorbing) effect of the structure is dealt with in more detail in example 5 and in the description of fig. 10.

In order to avoid misunderstanding, it must be emphasized that the high shock tear-off strength achieved in connection with the invention is not only due to the high elongation at break due to rapid tensile influences, but that in combination with this it is also very important that the previously mentioned fork- effect that equalizes the impact during demolition.

Claims (14)

1. High-strength laminated film, characterized in that it mainly includes weakly bonded biaxially oriented layers each of which is formed from a mixture of at least two polymers and each exhibits a fibrillar morphology where the fibers form a fiber structure that gives a splitting direction in the layer, which fiber structure in detailed observation is deflected and has a zig-zag course, and in that the layers are arranged in such a way in the film that at least two of the aforementioned splitting directions cross each other. 2. Laminated film according to claim 1, characterized by at least three biaxially oriented layers. 3. Laminated film according to claim 1 or 2, characterized in that the bond between at least two layers is strong in spots or lines and weak or absent in the remaining part of the adjacent surfaces. 4. Laminated film according to claim 3, characterized by at least three layers where the strips or points with strong bonding or weak bonding are mutually offset on the two sides of a middle layer. 5. Laminated film according to claim 1, characterized by two layers with obliquely running fiber structure and with the fiber structure in these layers arranged crossing each other. 6. Laminated film according to claim 5, characterized by a layer with a longitudinal fiber structure between the aforementioned two layers. 7. Laminated film according to claim 1, characterized by at least one layer with a transverse fiber structure. 8. Laminated film according to claim 1, characterized in that each layer has varying orientation and thickness in accordance with a pattern of essentially parallel narrow strips. 9. Laminated film according to claim 8, characterized in that the strips are formed by two sets a) and b) which are mixed with each other, each strip including parts of each of the oriented layers, of which sets the strips a) are relatively thick and are biaxial cold drawn in a distinctly unbalanced manner, with the main axis of cold drawing essentially parallel to the direction of the strip and with the direction of tear propagation at a small angle to the direction of orientation, while the strips b) are relatively thin and are biaxially cold drawn mainly in a balanced manner or in an unbalanced manner with the main axis of the cold drawing crossing the strip direction. 10. Laminated film according to claim 1, characterized in that the mixture which forms at least one of the aforementioned layers mainly contains crystalline polyolefins. 11. Laminated film according to claim 10, characterized in that the mixture which forms at least one of the aforementioned layers is essentially a mixture of polypropylene and high- and/or low-density polyethylene. 12. Laminated film according to claim 10, characterized in that the mixtures contain atactic polypropylene or ethylene-propylene rubber as an alloying agent. 13. Laminated film according to claim 1, characterized in that two or more extruded layers, which co-extruded layers have a mixture with fibrillar morphology and an adhesive layer on each side, the layers being connected to each other by means of the aforementioned adhesive layer. 14. Laminated film according to claim 13, characterized in that said layer with said mixture mainly consists of polypropylene and that said adhesive layer consists of ethylene-vinyl acetate.