| 1. | Floatable cord for use especially as a head rope of a net, the interior of the cord comprising floating material (1) having a density lower than 1,0, characterised in that the floating material (1) is manufactured of a thermoplastic, crosslinked or noncrosslinked plastics material, wherein empty spaces are provided for decreasing the density and which in the longitudinal direction of the cord forms a continuous core region of a constant crosssection, or a core region widening and narrowing in a wave¬ like fashion. |
| 2. | Cord as claimed in claim 1, characterised in that the floating material (1) consists of a foamed plastics material. |
| 3. | Cord as claimed in claim 1 or 2, characterised in that the plastics material comprises one or several polyolefins, such as polyethylene (HDPE, LDPE, LLDPE) or copolymers of a polyolefin, such as ethylene vinylacetatecopolymer (EVA). |
| 4. | Cord as claimed in some of the preceding claims, characterised in that the density of the floating material (1) is lower than 0,5. |
| 5. | Cord as claimed in some of the preceding claims, characterised in that cuts (2) are provided in the core region spaced from each other in the longitudinal direction of the cord in order to increase the flexibility. |
| 6. | Cord as claimed in some of the preceding claims, characterised in that the core region comprises a core yarn (3) extending in the longitudinal direction of the cord inside the floating material (1), said yarn being preferably of heatresistant fibre, such as carbon fibre. |
| 7. | Method for manufacture of a floatable cord as claimed in claim 1, in which method floating material having a density lower than 1,0 is provided in the interior of the cord, characterised in that the interior of the cord is manufactured to form in the longitudinal direction of the cord a continuous core region of a constant crosssection, or a core region widening and narrowing in a wavelike manner, from a melted thermoplastic material, which is provided with empty spaces in order to lower the density and which optionally is crosslinked, whereafter the plastics is allowed to solidify to its final stiffness. |
| 8. | Method as claimed in claim 7, characterised in that foaming agent is added to the plastics material. |
| 9. | Method as claimed in claim 7 or 8, characterised in that the plastics material comprises one or several polyolefins, such as polyethylene (HDPE, LDPE, LLDPE) or copolymers of a polyolefin, such as ethylene vinylacetatecopolymer (EVA) . |
| 10. | Method as claimed in some of the preceding claims 7 to 9, characterised in that the interior is manufactured to an elongate core region of the cord by extrusion by extruding the plastics material through a die (5a) . |
| 11. | Method as claimed in some of the preceding claims 7 to 10, characterised in that cuts (2) spaced from each other in the longitudinal direction of the core region are made in the core region. |
| 12. | Method as claimed in claim 11, characterised in that the cuts (2) are made in the core region before the plastics material (1) has solidified to its final stiffness. |
| 13. | Method as claimed in claim 11, characterised in that the cuts (2) are made in the core region after the plastics material (1) has solidified to its final stiffness. |
| 14. | Method as claimed in some of the preceding claims 7 to 13, characterised in that the plastics material is brought around a core yarn (3), when in a melted state and it is solidified around the core yarn, which is preferably of heatresistant fibre, such as carbon fibre. APPENDIX TABLE Injection moulding tests using a spherical mould, diameter 60 mm Foaming agent: Genitron EPE = azodicarbonamide UMB 10630 (Drycolor) Composition Density of sample Structure of sample 1 LDPE + EVA 0.400.45 Gate marks on the (8% VAcontent) surface, cells 1.0% Genitron EPE greater in the centre of the sphere, becoming smaller towards the surface. Bubbles large . |
| 15. | LDPE + EVA 0.350.40 Gate marks on the (8% VAcontent) surface, cells UMB 10360 greater in the centre of the sphere, becoming smaller towards the surface. No large bubbles . |
| 16. | LDPE 0.400.45 Gate marks on the 10% Genitron EPE surface, cells larger in the centre of the sphere, becoming smaller towards the surface . Large bubbles . |
| 17. | LLDPE 0.400.50 The mould was not 10% Genitron EPE entirely filled, part of the outer surface of the sphere remained rough . Quite good cell formation. Uniform bubble size . |
The invention relates to a floatable cord disclosed in the preamble of claim 1, as well as to a method for manufacturing floatable cord, disclosed in the preamble of claim 7.
The invention relates especially to a floatable cord, which can be used as the head rope of a net. The demands on the head rope are great, as far as the buoyancy, weather resistance and easiness in handling are concerned.
Up to now, a cord around which there are disposed floating pieces of e.g. cellular polystyrene at regular intervals has been used. These pieces cause difficulties in using the cord, because said pieces tend to get entangled in the cord, with each other and with the net, especially when the net is gathered.
Further, a cord having floating pieces strung on a yarn at short intervals and a plastics braiding braided around the yarn and the floating pieces is known on the market. It has been capable of eliminating the above-mentioned drawbacks partly. The manufacture of the cord in question is, however, laborious and moreover it has not sufficient buoyancy. Due to its construction the cord is also prone to damages.
It is an object of the invention to eliminate the above-mentioned drawbacks and to provide a floatable cord, combining easiness in the manufacture, good floating properties, strong construction as well as simple handling. For achieving this, the cord in accordance with the invention is mainly characterised by what is disclosed in the characterising portion of claim 1. When the floating material of the interior of the cord is cross-linked or non-cross-linked
thermoplastic plastics material and in the longitudinal direction of the cord it forms a continuous core region of constant cross section or a core region widening and narrowing in a wave-like manner, the cord has strong construction and good buoyancy. For decreasing the density, empty spaces are provided in the thermoplastic plastics material. Due to the material used the cord can also easily be manufactured e.g. by extrusion.
The empty spaces decreasing the density are most preferably made by forming the core region of the cord of foamed plastics material. The plastics material used in the cord contains preferably one or several polyolefins, such as polyethylene (HDPE, LDPE, LLDPE) or copolymers of a polyolefin, such as ethylene- vinylacetate-copolymer (EVA) . The aforementioned plastics have advantageous properties for the use in question, which properties include good plasticity, flexibility, toughness, weather resistance as well as inherent low density.
The invention is described more closely in the following description with references to accompanying drawings, wherein
Fig. 1 shows a cord of the invention in longitudinal section,
Fig. 2 shows an alternative embodiment of the cord, also as a longitudinal section,
Fig. 3 shows another alternative embodiment of the cord as a longitudinal section,
Fig. 4 is a schematic representation of an apparatus used for the manufacture of the cord, the method steps being also shown schematically and
Fig. 5 is a sectional view of a die used for the manufacture of the cord.
The interior of the cord shown in Figs. 1-3 comprises floating material 1 having a density lower than 1,0. The floating material forms a core region in the longitudinal direction of the cord. In the case shown by Fig. 1 the core region has a cross-section of constant magnitude and of substantially circular shape in a direction perpendicular to the longitudinal direction of the cord. The core region further comprises a core yarn 3, around which the floating material 1 is disposed and which most preferably consists of heat-resistant fibre, such as carbon fibre. For increasing the flexibility, cuts 2 spaced from each other in the longitudinal direction of the cord and extending over a part of the cross-sectional area of the floating material are made in the core region, but the cuts do not cut off the floating material 1 into entirely separate parts in the longitudinal direction of the cord.
Fig. 2 shows an alternative shape of the cord not having a cross-section of constant magnitude in the core region, but the cross-section varies in the longitudinal direction of the cord by forming at regular intervals thicker portions la in a wave-like manner. By this arrangement, an appearance more resembling a traditional head rope of a net is provided for the cord. The above-described cuts 2 are formed between the thicker portions la.
Fig. 3 shows an alternative where regions having only the core yarn 3 in the core region without a surrounding floating material 1 are left in the longitudinal direction of the cord between the thicker portions la of Fig. 2. In this case the thicker portions also have a wave-like or a "shuttle-like" configuration, i.e. their cross-sections vary by widening and narrowing smoothly in the longitudinal direction of the cord.
Figs. 1-3 also show a surface braiding 4 of the cord braided around the core region. The braiding can consist of any common braiding material, e.g. of a suitable plastics tape. The surface braiding lies against the interior of the cord along the whole length of the cord, that is, also against the exposed core yarn 3 between the thicker portions la as shown in Fig. 3.
The plastics material 1 forming the core region comprises one or several polyolefins, such as polyethylene or copolymers of a polyolefin, such as ethylene-vinylacetate-copolymer. The former grades are commonly known by abbreviations HDPE, LDPE and LLDPE according to the properties, especially density of the material used. The latter is commonly known by abbreviation EVA. The aforementioned materials can be used for the floating material 1 of the core region of the cord either as such or blended with each other. The density of said materials can be still lowered considerably by forming empty spaces, such as cavities and/or bubbles in the core region during the manufacture. In practice this can be accomplished by foaming the plastics material used, in other words, foaming agent or expanding agent (blowing agent) is introduced into the material when manufacturing the core region of the cord. Various such agents are known in plastic technology. An agent which at the
moulding temperature of the plastics is gaseous or generates gas, which creates bubbles and/or cavities in the melted plastic, may act as the foaming agent. Azodicarbonamide or 1, 1-azobisformamide can be mentioned as an example of such foaming agent.
The thermoplastic plastics material may also be cross- linked before its solidification. This alternative is advantageous especially when using polyethylene. A suitable peroxide can be used as the cross-linking agent and the cross-linking can be induced e.g. by means of UHF-radiation.
Experimental runs have been conducted with different thermoplastic plastics and different foaming agents for finding suitable floating material. A blend of LDPE and EVA, wherein the portion of the vinylacetate monomer is ca. 8% of the total weight of the polymer blend, has allowed to obtain cord core regions having a density within the range of 0,35 to 0,5. Same densities are obtained by using solely LDPE, LLDPE or EVA as the raw material. If EVA or a mixture of polyethylene and EVA is used, the portion of the vinylacetate monomer is preferably 3 to 40 w-%. In all cases foaming agent can be used, depending on its grade in the proportion of 0,5 to 5 w-% of the total amount of the plastics raw-material. When azodicarbonamide is used as the foaming agent, its amount is usually 0,5 to 1,5%. Some of the results are shown in the appended table, where properties of samples made by injection moulding are described.
Fig. 4 illustrates a practical arrangement for manufacturing the cord of the invention. The cord is manufactured by extrusion in an extruder designated 5 in Fig. 3. Melted plastics material is extruded to a continuous core region of the cord through a die 5a so that it surrounds the core yarn 3, which is pulled
as a continuous yarn out of the die 5a by means of the aftertreatment devices for the cord, which are known as such and are therefore not described in more detail. The granular plastics rawmaterial and the powder-like foaming agent are supplied to the extruder at the location denoted by arrow A, and the plastic material melts inside the extruder, whereafter the foaming takes place at the exit of the melted plastic out of the die 5a or at the location of the die. The foamed plastic material emerging from the die 5a becomes solidified around the core yarn 3 and the solidification can be accelerated by providing cooling in the line, whereafter at location C cuts 2 are accomplished by means of a suitable device, e.g. a knife performing a regular cutting movement, while the core yarn 3 and the surrounding floating material 1 continuously passes the location C. In the direction of travel of the cord after the location C the surface layer 4 is braided around the core region at the location D and this step can be carried out by means of automatic well-known braiding machines. In general, devices generally known in the manufacture of cords can be used as aftertreatment devices for the cord. Further, well-known methods can be utilized in the control of temperature in the extrusion.
The cuts 2 can be formed in the cord also using a method, where opposite moving plates or rotating discs indent the core material as the plastic material is still in a deformable state.
The temperature in the extrusion is adjusted preferably to a range of 180° to 145° by decreasing it from 180° at the supply to 140° at the die.
When using polyolefins or copolymers thereof, such as EVA, their polymeric chains can be cross-linked by using a suitable peroxide as the cross-linking agent,
acting in suitable conditions as the initiator of the cross-linking reaction. The cross-linking agent is supplied to the extruder and the cross-linking is induced by high-activity radiation, such as UHF- radiation, for instance at once at the location where the plastics material exits out of the die (point B) . The activation of the peroxides can be accomplished also thermally, and on the other hand solely radiation may effect the cross-linking of the polymeric chain.
Several cords can be manufactured using the same extruder 5 by providing it with several dies, as illustrated in Fig. 5. In this Figure there are three dies 5a, through which their respective core yarns 3 pass. The yarns exit each from their respective openings 5b into a chamber 5d located between the end of the extruder screw 5c and the dies 5a, and they pass through the melted plastics material within the chamber to the dies 5a. Any device used in plastics technology can be used as an extruder. In the device shown by Fig. 5 the cross-sectional area of the flow chamber 5c narrows down in the direction of flow of the plastics material, the narrowing being represented by broken lines. In this fashion the maintenance of a sufficient high pressure is ensured so that the foaming will take place only at the die 5a or immediately thereafter.
The thicker portions la shown in Figs. 2 and 3 may also be manufactured by extrusion, either by changing periodically the extrusion rate of the plastics material through the die 5a of a constant size, or by mechanically altering the magnitude of the diameter of the die 5a in a periodic fashion. The surface layer 4 can be braided around such kind of core region as well, using automatic braiding devices. It is to be noted, that if one wishes to increase the strength of the cord, two or several parallel core yarns can
be led through the same die and the plastics material is formed around the yarns.
The advantage of an extrusion method is the production rate, for instance in comparison with methods, where a surface braiding is wrapped about separate particles in order to manufacture a continuous cord. It must also be noted, that the core region can be manufactured using the method of the invention without any braiding, whereafter the half-fabricate so obtained can be stored in an intermediate storage or it can be transported, and the braiding of the surface layer can be carried out thereafter.
The cord of the invention is applicable especially to fishing as the head rope of a surface net or a submerged net, and in this context by the term "floatable cord" is meant a cord floating on the surface of water when it has no weights. The cross- section of the core yarn 3 is in the aforementioned fishing net application lower than 1 mm and the "effective" diameter of the floating material of the cord is 3 to 10 mm depending on the desired buoyancy. By effective diameter is meant the diameter of a circle having an area equal to an area obtained by dividing the total volume of the floating material 1 by the length of the cord. The degree of foaming, being controllable in the terms of the amount of the foaming agent, is determined by the immersion depth used. Cords which will be immersed down to considerable depths must have smaller portion of bubbles and/or cavities in the floating material so as to better endure the pressure of water.
The cord according to the invention has the following beneficial characteristics
frost-resistance, resistance to sun light, resistance to oils, solvents etc., resistance to pressure, good tensile strength, - bending without breaking
Some experiments have been conducted with the cord of the invention in order to test the compatibility of the cord with severe circumstances at sea. The experiments have been conducted in 7°C sea water.
A cord manufactured from polyethylene of LLDPE grade using the method of the invention (weight 12,0 g/m, total diameter ca. 7,0 mm and the diameter of the core region ca. 5,5 mm, tensile strength 148 kg, i.e. 38 N/m 2 when dry) required a mass of 11,3 g/m in order to be submerged in the water. The resistance to pressure was tested in a test chamber, where a water pressure corresponding to a depth of 1000 m was generated. The cord was found to possess surprisingly good pressure resistance characteristics, which is illustrated by the fact, that the cord retained its buoyancy at this pressure (in other words, the position of a section of cord 1,75 m in length was straight up when it was fastened at its one end onto the bottom of the test chamber) .
The cord was also subjected to long-term testing at a pressure corresponding to a depth of 1000 m in the same circumstances as above. The weight increase after two days was ca. 40% due to the intruded water, but it was, however, low enough to retain the buoyancy of the cord. Moreover, it was observed that when the cord was allowed to dry in the air after the experiment, the weight ingrease dropped to 6% and after 5 days it was only 0,5%. Hence, the intrusion of the water into the cord is reversible.
The good stability of the cord is still illustrated by the fact, that when the cord was charged with a mass of 75 kg (load ca. 740 N) , the elongation was 20%, but when the charging ceased the cord returned to its original lenght.
The invention is by no means restricted solely to the examples disclosed in the description, but it can be modified within the scope of the invention presented in the claims. Hence, the selection of various thermoplastic plastics materials and blend ratios as well as the amount of foaming agents can be chosen according to the desired final density, the selection being of course effected by cost factors. Various polyethylene grades, especially LDPE and LLDPE proved to be tough, as well as ethylene-vinylacetate- copolymers, are preferred due to their low costs. Further the advantage of polyolefins and copolymers thereof is inherent low density and good resistance to environmental circumstances, such as sea water.