WO1989009305A1

WO1989009305A1 - Open steel cord structure

Info

Publication number: WO1989009305A1
Application number: PCT/BE1988/000009
Authority: WO
Inventors: Luc Bourgois
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1988-03-25
Filing date: 1988-03-25
Publication date: 1989-10-05
Anticipated expiration: 1990-09-25

Abstract

A steel cord adapted for the reinforcement of rubber articles comprises one steel core filament (1) of diameter d0 and one layer of six steel filaments (2) of diameter d1. The tensile strength of the steel filaments (2) of the layer is greater than 2850 N/mm2, d0 is greater than or equal to 0.17 mm and smaller than or equal to 0.38 mm; d1 is greater than or equal to d0(mm) - 0.05 and smaller than or equal to d0(mm) - 0.02. The steel cord has a high breaking load and sufficient rubber penetration and presents no core migration.

Description

OPEN STEEL CORD STRUCTURE

The Invention relates to a steel cord adapted for the reinforcement of rubber articles, such as rubber tyres.

A steel cord adapted for the reinforcement of rubber articles conveniently comprises steel filaments having a carbon content of more than 0.60 per cent by weight (e.g. more than 0.65 %, 0.78 %, 0.82 % or 0.95 %) . A typical steel composition is : a minimum carbon content above 0.65 %, a manganese content between 0.40 % and 0.70 %. a silicon content between 0.15 % and 0.30 % and a maximum sulphur and maximum phosphorus content of 0.03 %, all percentages being percentages by weight. Other, more expensive elements such as chromium may also be alloyed.

The diameter of such adapted steel filament lies in the range of 0.05 mm to 0.80 mm, preferably in the range of 0.05 mm to 0.50 mm (e.g. 0.12 mm, 0.18 mm or 0.33 mm).

The elongation at rupture of a steel filament adapted for the reinforcement of rubber articles is at least 1 %. preferably at least 2.5 %.

The steel filaments are usually provided with a coating which promotes the adherence of the steel wire to rubber articles. Such a coating conveniently comprises copper, zinc, brass or ternary brass, alloy, or a combination of two or more different layers thereof. The thickness of the coating ranges from 0.05 to 0.40 micron, preferably from 0.12 to 0.30 micron. The coating can also be present 1n the form of chemical primer material for ensuring good rubber penetration and adhesion. Im this respect several steel cord constructions have already been developed. One well known construction is a (1+6)xd steel cord construction, this is a steel cord construction having one core filament which is surrounded by a layer of six steel filaments, d being the diameter of the steel fila¬ments. By a layer of steel filaments around a core is meant a plurality of filaments disposed side by side (but not necessarily in contact with each other) so as to form around s aid core a sheath of a thickness equal to the diameter of a filament.

This wel l known ( 1 +6) xd construction , however, presents some drawbacks . It has core migration and i s subjected to corrosion . U.S. patent 3.032.963 discloses a 3+6 steel cord construction, this is a steel cord construction having a core strandof three filaments, surrounded by a layer of six steel fila¬ments, e.g. 3 x 0.20 + 6 x 0.38. The diameter of the indlvidual core filaments is about half the diameter of the steel filaments in the layer. This cord construction has no coremigratlon but is still subjected to corrosion.

Im order to solve the corrosion problem other steel cord constructions have been proposed. One of these is a 3 x 0.20 * (6 x 0.35 steel cord construction. In this construction the diameter of the Individual core filaments is more than half the diameter of the steel filaments in the layer. It has the advantage that core migration is avoided and that the corrosion problem is partly solved. The six steel filaments in the layer are no longer subjected to corrosion, but corrosion still remains a problem for the three-filament-core. Another drawback with this construction (and with other 3+6 constructions) is that its manufacture needs two steps or a complicated twister to form the construction in one step. Still another drawback is that the core filaments have to be drawn to a final diameter which is significantly smaller than the diameter of the filaments of the layer. This brings about greater manufacturing costs.

It is an object of the present invention to avoid the above mentioned drawbacks. It is another object of the present invention to provide a solution for the corrosion problem whilst still preventing core migration and without a loss of tensile strength.

According to one aspect of the present invention there is provided a steel cord adapted for the reinforcement of rubber articles comprising one steel core filament of diameter d₀, d₀ being greater than 0.17 mm and smaller than or equal to 0.38 mm, and one layer of six steel filaments of diameter d₁, characterized in that d₁ (mm) is greater than or equal to [d₀ (mm) - 0.05] and smaller than or equal to [d₀ (mm) - 0.02] and in that the tensile strength of the steel filaments of the layer is greater than 2850 N/mm².

The minimum difference of 0.02 mm between d₀ and d₁ is chosen taking into account the requirement of openness in order to provide sufficient rubber penetration. The maximum deviations on the specified diameters d₀ and d₁ and the lay length L must be such that even in the worst case, i.e. d₀ + maximum deviation, d₁ - maximum deviation for all the filaments in the layer, there must be sufficient rubber penetration. The maximum deviations on the specified diameters d₀ and d₁ conveniently lie between 0.03 and 0.04 mm and one may easily understand that the smaller the lay length the smaller the openness. The maximum difference of 0.05 mm between d₀ and d₁ is chosen for stability reasons. Maximum differences above 0.04 mm often cause a non-uniform distribution of the six filaments in the layer. A preferable maximum difference is 0.03 mm.

In a preferable embodiment of the invention the core filament of the steel core has an elongation at rupture which is greater than 4 per cent. This prevents the core filament of being broken before fracture of the steel filaments in the layer.

This elongation of the core filament can be obtained by a core filament with a carbon equivalent which is lower than 0.70 per cent by weight.

The carbon equivalent Ce is determined by the following equation : Ce = % C + 0.3 x (% Mn - 0.40) where % C is equal to the carbon content in per cent by weight and % Mn is equal to the manganese content in per cent by weight.

The carbon equivalent is more correct in determining the reinforcing action than the mere carbon content.

As an example steel filaments with a carbon equivalent of 0.80 per cent by weight usually have an elongation at rupture of about 2 per cent, whereas a steel filament with a carbon equivalent of 0.70 per cent by weight may have an elongation at rupture of more than 4 per cent.

The elongation at rupture of at least 4 per cent can be obtained by giving to the core filament a spatial or plane wave form. The necessary elongation at rupture can also be obtained by a proper thermal treatment : The core filament can be heated electrically, this reduces the breaking strength and increases in some circumstances the elongation at rupture. In example, for a 0.3 mm steel filament of 0.85 per cent carbon with an initial breaking strength of 3000 N/mm and an elongation at rupture of 2.0 %, it has been found that thermal treatment at a certain temperature and for a certain period of time brings about a reduction of breaking strength of about 10 % without any increase in elongation at rupture. However, by increasing the temperature slightly over a similar treatment period we note that breaking strength decreases as a linear function of the temperature applied and that elongation at rupture rapidly increases to reach a maximum level of almost 6 % once breaking strength has been reduced to about 2500 N/mm². If the filament is heated at an even higher temperature for the same period of time, the above mentioned breaking strength decreases linearly to approximately 2100 N/mm², whilst the resulting elongation at rupture diminlshes almost linearly to about 4.5 %.

The necessary elongation at rupture may also be obtained by a combination of a lower carbon equivalent, a proper mechanical treatment and a proper thermal treatment.

The steel cord according to the invention may or may not be surrounded by a wrapping filament. This wrapping filament is not necessarily round but may be of any shape. The pitch of this wrapping filament preferably lies between 2.5 and 5.0 mm. According to a second aspect of the present invention there is provided a rubber article comprising a steel cord according to the first aspect of the present invention. The steel cord according to the first aspect of the present invention is preferably used as belt reinforcement for a truck tyre because of its stiffness and as carcass reinforcement for a truck tyre because of its tensile strength.

The invention will now be described more in detail with reference to the accompanying drawing wherein

- figure 1 is a cross-section of a steel cord according to the invention ;

- figure 2 is a side view of a steel cord according to the invention ; - figure 3 shows a double twister adapted to manufacture the steel cord according to the invention ;

- figure 4 shows a guiding plate of the double twister ;

- figure 5 shows a possible use of the steel cord according to the invention in a truck tire.

Figure 1 represents a steel cord according to the present invention. The core filament 1 has a diameter d₀ and is surrounded bij a layer of 6 filaments having a diameter d₁ and a tensile strength greater than 2850 N/mm². The average values of d₀ and d₁ must fulfil the following conditions : d₀(mm) - 0.05 ≦ d₁(mm) ≦ d₀(mm) - 0.02 This has as a consequence that there is always sufficient openness to allow the necessary rubber penetration. The openness is defined as that proportion of the circumference of the circle containing the axes of the filaments in the layer which is not occupied by the filaments of the layer. Table 1 gives a list of possible steel cord constructions according to the present invention, their respective values of openness corresponding to the average values of d₀ and d₁, and the minimum tensile strengths of the layer filaments (HT= high tensile).

The openness has been calculated according to SHITKOW and POSTECHOW, Drahtseile, Chapter 3.

Figure 2 represents a side view of another steel cord according to the invention. A wrapping filament 3 surrounds the steel cord.

The steel cord may be made by a process using a conventional double-twister 10 as shown in figure 3. The seven filaments are unwound from a creel which is not shown in the figure, pass through a guiding plate 11 and converge towards a twisting-die 12 into a bundle. Then the bundle enters axially through the rotation axis 13 of the twister, over the rotating flyer 14 back to the rotation axis on the other side, where it enters axially into the stationary cradle 15 inside the twister over capstan 16 for winding up on the bobbin or creel 17. If a wrapping steel filament 3 must be provided, this may be done by a conventional wrapping machine 19 situated between pulley 18 and capstan 16.

A cross-section of guiding plate 11 along the line IV-IV is shown in figure 4. Holes 111 and 112 guide resp. core filament 1 and layer filaments 2.

A position aspect of the steel cord according to the invention is due to the fact that it has a core filament that is thicker than the layer filaments. This means that the back-pull tension in the core filament during the twisting step may be lowered and that the core filament can easily be maintained in a central position during the twisting step.

The rubber articles according to the second aspect of the invention are then obtained by introducing such steel cords in an unvulcanized rubber composition and then vulcanizing the whole. In general, the steel cord is firstly impregnated In an adhesion rubber composition. Such adhesion rubber will conveniently comprise 40 to 70 parts of carbon black per 100 parts of rubber, 2 to 6 parts of coumarone resin, 4 to 12 parts of zinc oxide and 1 to 5 parts of sulphur, and further no more than 10 parts 1n total of antioxidant or accelerator or other agents, all parts being parts by weight.

A preferable use of the steel cord of the invention is as a reinforcement of a belt or ply of a radial truck tyre. In such a case, the steel cords are laid side by side to form a foil of one or more superposed layers of cords and this foil is covered on either side with a foil of unvulcanized adhesion rubber which enters between and into the cords. The whole composite is cut into strips and the result is a strip of cord pieces, lying side by side in one or more superposed layers, and impregnated with unvulcanized adhesion rubber.

Figure 5 represents a cross-section of a radial truck tyre. There are two belt or breaker plies 21 and 22. The cords in these plies form an angle α to the direction of movement of the tyre which is indicated by an arrow, α usually lies between 18 and 70 degrees. A tread 23 covers the belt plies 21 and 22 and a carcass ply 24 is covered by the belt plies 21 and 23.

Table 2 summarizes applications of the steel cord according to the invention.

Comparison between 1+6 and 3+6

A series of tests and calculations has been carried out on two 3+6-constructions and on one 1+6 steel cord construction according to the present invention. The average openness corresponds to the average diameters d₀ and d₁. The maximum openness corresponds to a situation where d is 0.004

m greater than specified and where d₁ is 0.004 mm smaller than specified for all the filaments of the layer. The minimum openness corresponds to the opposite situation, i.e. where d is 0.004 mm smaller than specified and where d is 0.004 mm greater than specified for all the filaments of the layer. The compactness is a measure for the reinforcing action of steel cord per unit of length of the rubber ply, provided that the distances between the steel cords are equal for the compared plies. The stiffnesses have been measured with a three-point bending tester.

The cabling loss is the loss in breaking load due to the cord formation in the last manufacturing step.

The fatigue limit has been determined by the Hunter test.

The relative cost is the relative cost of the cord formation, I.e. of the last manufacturing step. Drawing costs are not taken into account.

More details about test methods for breaking load, stiffness, rubber penetration and fatigue limit can be found in the paper by Bourgols, L., "Survey of Mechanical Properties of Steel Cord and Related Test Methods", Tire Reinforcement and Tire Performance, ASTM STP 694, R.A. Fleming and D.J. Livingston, Eds., American Society for Testing and Materials, 1979, pp 19-46.

As can be derived from table 3 and table 4 the steel cord according to the invention has sufficient rubber penetration in spite of a smaller openness. This is due to the fact that the necessary volume of rubber which must penetrate through the layer in order to obtain a complete rubber penetration is smaller with the steel cord according to the invention than with a steel cord according to the prior art.

Another advantage of the steel cord according to the inven tion is a greater reinforcing action per unit off ply length (compactness).

Still another advantage is the smaller manufacturing cost :: less 10 per cent !

Comparison between 1+6 and 1+5

Table 5 compares the properties of a number of 1+5-construc tions with the properties of a number of steel cord construc tions according to the invention. Table 5 must be read in that way that a 1+5-construction is compared with a steel cord construction according to the Invention of comparable diameter.

According to table 5 no 1+5-construction provides the same breaking load and compactness of a comparable steel cord construction according to the invention.

Table 6 gives values of bending tensions in a 1+5-construction and in a steel cord construction according to the invention.

The bending load is not symmetrically distributed between core and layer filaments for 1+5-constructlons. This is harmful to fatigue behaviour.

Claims

CLAIMS :

1. A steel cord adapted for the reinforcement of rubber articles comprising one steel core filament (1) of diameter d₀, d₀ being greater than 0.17 mm and smaller than or equal to 0.38 mm, and one layer of six steel filaments

(2) of diameter d₁, characterized in that d₁ (mm) is greater than or equal to d₀ (mm) - 0.05 and smaller than or equal to d₀ (mm) - 0.02 and in that the tensile strength of the steel filaments of the layer is greater than 2850 N/mm².

2. A steel cord according to claim 1, characterized in that the core filament has an elongation at rupture which is greater than 4 %.

3. A steel cord according to claim 2, characterized in that the core filament has a carbon equivalent which is lower than 0.70 %.

4. A steel cord accσrding to claim 2, characterized in that the core filament has a spatial wave form.

5. A steel cord according to claim 2, characterized in that the core filament has a plane wave form.

6. A steel cord according to claims 1, 2, 4 or 5, characterized in that both core filament and layer filaments have a tensile strength greater than 3200 N/mm².

7. A steel cord according to any one of the preceding claims, characterized in that the steel cord is surrounded by a wrapping filament (3).

8. A rubber product, characterized in that said rubber product comprises a steel cord according to any one of claims 1 to 7.

9. A rubber tyre, characterized in that said rubber tire comprises a steel cord according to any one of claims 1 to 7.

10. A rubber tyre according to claim 9 said rubber tire having a width between 9 and 18 inch (22.86 - 45.72 cm), an aspect ratio between 55 and 90 per cent, a rim between 17 and 24 inch (43.18 - 60.96 cm), and comprising two to four belt plies, said belt plies being reinforced with a steel cord according to any one of claims 1 to 7, t!ne diameter d₀ of the cere filament of said steel lying between 0.30 and 0.38 mm, the angle of reinforcement of said steel cord lying between 18 and 70 degrees to the direction of movement of the tyre.

11. A rubber tyre according to claim 9 said rubber tire having a width between 7 and 10 inch

(17.78 - 25.4 cm), an aspect ratio between 55 and 90 per cent, a rim between 14 and 19.5 inch (35.56 - 49.53 cm), and comprising two to four belt plies, said belt plies being reinforced with a steel cord according to any one of claims 1 to 7, the diameter d₀ of the core filament of said steel cord lying between 0.265 and 0.30 mm, the angle of reinforcement of said steel cord lying between 18 and 70 degrees to the direction of movement of the tyre.

12. A rubber tyre according to claim 9 said rubber tire having a width between 9 and 18 Inch (22.86 - 45.72 cm), an aspect ratio between 55 and 90 per cent, a rim between 17 and 24 inch

(43.18 - 60.96 cm), and comprising a carcass ply, said carcass ply being reinforced with a steel cord according to any one of claims 1 to 7, the diameter d₀ of the core filament of said steel cord lying between 0.22 and 0.25 mm.

13. A rubber tyre according to claim 9 said rubber tire having a width between 7 and 10 inch

(17.78 - 25.4 cm), an aspect ratio between 55 and 90 per cent, a rim between 14 and 19.5 inch

(35.56 - 49.53 cm), and comprising a carcass ply, said carcass ply being reinforced with a steel cord according to any one of claims 1 to 7, the diameter d₀ of the core filament of said steel cord lying between 0.175 and 0.22 mm.