REINFORCED COMPOSITE STRUCTURE

Abstract: One or more plies of reinforced elastomer, particularly two plies (10), (12) of rubber reinforced with twisted filament metallic cords (14) and more particularly a tire belt made up of the two plies (10), (12) is disclosed having cords (14) of 2x.30HT construction with opposed 23° angles to the direction of reinforcement of the belt.

Description: The present application relates to a composite laminate structure of cord reinforced elastomer and more particularly to a cord reinforced composite having rubber where preferably the cord is metallic. Even more particularly, the structure is for tires and preferably a tire belt wherein at least one of two plies in the belt has the cords therein biased with respect to the direction of rotation of the tire. as described in patent application DE-A-34.42. 799 Reinforced elastomeric articles are well known in the art for example for conveyor or like type belts, tires etc., with cords of textile and/or fine steel wire, particularly belts for pneumatic tires with up to four plies with the cord reinforcement between adjacent plies being opposingly biased with respect to the direction of movement of the tire where it is desired to reinforce in the lateral direction in addition to the direction of rotation of the tire. Further, cords made of multi twisted filaments of fine wire with two or more filaments in a single strand construction having a wrap filament therearound to reinforce the above structure have also long been known. More recently multi strand cords such as 2+7x.22+1 have been found necessary to meet the higher demand of fatigue life for composites in tire belts but are more expensive to make. Most recently, there has been use of single strand cords of multi filaments which are not twisted about each other but rather twisted altogether as a bundle or bunch to simplify the cord construction. Higher fatigue life requirements for composites in tires have resulted in cords with smaller filament diameter requiring more filaments in the cord to obtain the necessary strength. Many problems have arisen particularly with respect to bonding a ply with an opposing angle of reinforcement to an adjacent ply to form a composite laminate. For example when the above composite laminate has a flexible rubber matrix as in a tire belt and is loaded by a uniform tensile stress resulting from both tire inflation and footprint load, the oppositely directed in-plane shear stresses in each ply result in a large strain gradiant near each edge of the laminate as indicated in Figure 1. Consequently under tension the cord rubber composite belt is highly susceptible to initiation of interply shear fracture near the edge of the belt which is known as belt edge separation. Under both static and cyclic loading, the belt edge separation is initiated by cracking around the individual cord ends at the cord-rubber interface of the plies. The load for initiation of belt edge separation constitutes a threshold level for semi-infinite fatigue life of cord-rubber composites. When the maximum stress during cyclic loading does not exceed the initiating load for belt edge separation, the cord rubber composites exhibit no damage and therefore virtually infinite fatigue life. The load at which the composite fails in delamination measures the load, referred to below as gross load, carrying capability of the composite structure which is dependent upon three factors assuming adequate cord to rubber adhesion. The factors are cord modulus, the ratio of cord volume to rubber volume which is often expressed as the number of cord ends per inch, and the angle of cord reinforcement. As the angle of cord reinforcement approaches the direction of rotation of the tire the support from the reinforcement in the lateral direction moves toward zero. An increase in the above-mentioned two other cord related factors generally results in an increase of weight for the belt. Added weight means added cost and higher rolling resistance of a tire. Lighter cords with a lower modulus do not solve the problem because even though they have lower weight they also have a lower cord modulus which must be offset by increasing the ratio of cord to rubber volume. This increase in cord volume is limited by the physical size of the cord and the resulting spacing between the cords which governs the ability of the rubber to penetrate between the cords for good cord to rubber adhesion. As indicated below the present invention will be shown to have substantially improved the critical load for preventing belt edge separation without decrease of the load carrying capability of the overall tire belt over conventional belt constructions typically reinforced by 2+2x.25 and 2+7x.22+1 cord construction. The invention as claimed is intended to remedy the above mentioned problems. The composites according to the invention, have the advantages of a 20 % increase in critical load over a comparable composite reinforced with 2+2 x .25 cord. A smaller diameter of the cord of the reinforcement of the present invention results in less rubber gauge being used where a comparable thickness of rubber is laid on each side of the reinforcing cord upon calendering. Where two filament cord is used the result is an open cord having no core resulting in better rubber penetration. The two filament cord results in a cost savings over cords of three filaments or more in the manufacture of the cord. A smaller diameter cord results in less weight in the reinforcement resulting in lower rolling resistance for a tire thereby reinforced. A tire belt reinforced with a two filament cord having filament diameters of 0.30 mm results in 1 to 2% better rolling resistance compared to the same system reinforced with 2+2x.25 cord. Similarly, the above two filament cord results in a 15% or better plunger energy for tires over 2+2x.25 cord for comparable reinforcements. Use of a two filament cord having filament diameters of 0.30 mm and made of high tensile steel results in a 16% increase in the composite modulus without effecting the composite strain which remains the same in comparison to 2+2x.25 conventional steel cord at 7.87 ends per centimeter where the end count for the two filament cord is increased to 9.45 ends per centimeter. Finally, the above two filament cord results in a 2 to 6% greater composite stiffness over 2+2x.25 cord reinforced composites in tire belts even though the diameter of the two filament cord is smaller and its weight has been reduced over 2+2x.25 cord. The above advantages of the invention will become readily apparent to one skilled in the art from reading the following detailed description of an embodiment of the invention when considered in the light of the accompanying drawings in which Figure 1 illustrates schematically a composite of cord and rubber in a loaded and unloaded condition; Figure 2 illustrated a perspective of a portion of a tire having parts cut away to illustrate a belt package having composite structures according to the present invention; and Figures 3 and 4 are cross sections through cords in accordance with an embodiment of the present invention at points one half lay length apart for each other in the cords. Referring to Figures 1 and 2 of the drawings a single ply is illustrated in an unloaded condition, shown dotted, and in a loaded condition under tension indicated by the arrows F. This ply is shown in Figure 2 within a pneumatic tire with a radial carcass and a second ply 12 forming the belt package for the tire. Both plies are reinforced with cords 14 spaced laterally to the direction of reinforcement indicated by arrows F and preferably at an angle of 23° but with the cord angles of the two plies 10 and 12 opposing each other. Angles of 18 to 28° for reinforcing cords are found useful. The cords are surrounded by an elastomer preferably rubber and the cord-rubber composite structure forms the plies 10 and 12. The plies 10 and 12 in turn form a laminate structure such as the belt reinforcement for the tire illustrated in Figure 2. It will be appreciated that other laminates can be formed using principals of the present invention for reinforcing other articles such as industrial belts and that a single ply of the present invention can be used with known or conventional plies to also form new useful reinforced composite structures. Preferably the cords 14 are comprised of two filaments of finely drawn high tensile steel wire twisted about each other. Preferably the filament diameter is 0.30 mm and its tensile elastic modulus is 190 GPa or greater. The cord has a uniform lay length of 10 to 16 mm and in the preferred embodiment 14 mm. The cord of the preferred embodiment will be designated as 2x.30HT designating a two filament twisted cord having filaments of 0.30 mm diameter of a high tensile steel wire wherein high tensile is steel made with a carbon content by weight of 0.7 to 0.9% and preferably 0.82%. Referring to Figures 3 and 4 if a cross section is taken through a ply at right angles to the cords 14 they would appear as illustrated in Figure 3 wherein d is the diameter of the cord defined by the circle inscribing the two filaments, being 0.60 mm in the preferred embodiment. Conventionally the inscribed diameter of the cord is used to define the rivet illustrated by the space designated in Figure 3 which is a function of the spacing of the cords laterally across the ply. While this definition of the rivet, being a nominal one, is correct for the cross section of the cord at one point as is illustrated in Figure 4, the actual rivet h at a point a half a lay length down the length of the card from that point illustrated in Figure 3 is quite different. For a tightly twisted two filament cord as illustrated in the preferred embodiment the ratio of the actual to nominal rivet h,r approaches 2 to 1 between the point in Figure 4 and that in Figure 3 as nominal rivet approaches zero. This high ratio of actual to nominal rivet over the length of the two filament cord permits closer lateral spacing of the cord in the ply over conventional and larger diameter cords such as 2+7x.22+1 and 2+2x.25. The limit of how many cords can be placed in a rubber body to reinforce it is determined by the minimum rivet allowable for proper adhesion of rubber to cord. For 2+2x.25 cord this limit is 7.87 ends per centimeter while for 2+7x.22 wire it is 6.30 ends per centimeter the end count dropping as the diameter of the wire increases as illustrated in Table 1 below. Table 1 values are based on the numerical average for 4 to 7 specimens wherein the values of cord volume content are calculated from cross sectional areas of reinforced cords, neglecting the space between the filaments. The Table's values are further based on the preferred embodiment illustrated herein of a 2 ply tire belt for radial passenger tires with cords at opposing 23° angles to each other. Table 1 illustrates a substantial improvement in the resistance to belt edge separation initiation load which is the critical load for the composite reinforced by 2x.30HT cord. For the 2x.30HT cord of the present invention the critical load was 19.8 MPa while for 2+2x.25 and 2+7x.22 cords the same load was well below 17 MPa. Note that the smaller diameter of the 2x.30HT cord permitted a higher end count of 9.45 ends per centimeter within the limitation of good adhesion. It was further found that the end count for 2x.30HT could be raised as high as 11.02 ends per centimeter without loss of required rubber to cord adhesion due to the reduction in rivet. Initially cords of two twisted filaments of diameters of up to 0.38 mm using conventional tire cord steel were tried which had large enough diameters to give sufficient tensile strength of the cords but were found to fail for lack of fatigue life of the composite. The new reinforcing cords constructed from two high tensile steel wire filaments were found to have an increase in fatigue life of the composite in addition to the necessary tensile strength of the cord as illustrated in Table 2 and filament diameters of less than 0.34 mm were found satisfactory for meeting fatigue requirements. Further for the same diameter and construction reinforcing cords made of high tensile steel wire exhibited lower weight for equivalent modulus values compared with those made of conventional steel wire for tire cord of a lower carbon content. For example, the weight of 2+2x.25 high tensile cord is 4% lower than that of 2+2x.25 cord of conventional steel for tire cord. For a given wire system two twisted filaments provide the lightest and simplest cord construction which has the flexibility of a multi filament cord. It has been determined that the use of a single filament of sufficient diameter to provide the necessary tensile strength for a tire belt reinforcement lacks the flexibility to give the necessary fatigue life required for the belt composite. It was experimentally observed that when the filament diameter of the preferred embodiment cord, two filament cord, exceeded 0.34 mm Tabor bending stiffness of the cord departs sharply from that of conventional wire reinforcement with a similar cord diameter. The above results suggest that a new metallic cord constructed from two high tensile steel wire filaments with individual filament diameters of less than 0.34 mm would be the preferred choice for belt reinforcement in terms of bending stiffness and weight of the cord. More particularly, 2x.30HT cord becomes a direct substitute for 4x.25 or 2+2x.25 conventional steel tire cord. While the 2x.30HT cord is a direct substitute it should be noted that it has a lower weight and smaller cross sectional area compared with 2x.30 conventional steel cord and 2+2x.25 cord respectively (see Table 3). It was also found that compared to conventional construction of similar cord diameter for example 2+2x.25 the cut ends of two filament cord such as 2x.30HT of the present invention are found to be more likely to separate where they are cut often referred to as flare (see Table 4). This increased tendency of filament separation at the cut ends of the cords can result in an increased constraint over a local region of the rubber matrix at a belt edge. Some of the more unexpected results of the belt package of the preferred embodiment include a greater belt stiffness of 2 to 6% over the comparable 2+2x.25 cord reinforced belt package wherein usually a smaller diameter cord in a belt of lighter weight would give less stiffness in a tire belt reinforcement. Initial experimental data also indicates that the belt of the preferred embodiment also gives better ride characteristics of the tire while at the same time improving handling characteristics of the tire. Ride is a quality associated with hoop stiffness and handling is associated with the belt stiffness and while a softer ride usually is obtained at the sacrifice of better handling, i.e., a stiffer belt, in the present belt package both a softer ride and improved handling were achieved. Also it is observed that even at the preferred belt embodiment end count of 9.45 ends per centimeter in the smaller diameter cord there is a greater plunger energy achieved over a comparable 2+2x.25 cord reinforced belt which is a measure of resistance to penetration of a tire by foreign objects. Finally, a series of belt composites were prepared to obtain the data in Table 1 below with the same 'nominal intercord spacing' but different 'cord diameter' and 'cord volume content'. The following variation of 'cord construction' and 'cord end count' was made to maintain roughly identical 'nominal intercord spacing' of 0.457-0.508 mm. Compared with the 4X.25HT cord composite, 2X.30HT cord composite has a much lower 'cord volume content' in spite of the same 'nominal cord diameter' and 'nominal intercord spacing'. The following conclusions are drawn from the initial results of cyclic testing. All 'belt' composites with roughly identical 'nominal intercord spacing' (0.347-0.408 mm) exhibit a very similar profile of 'fatigue lifetime' when the maximum cyclic stress exceeds a threshold; i.e., the critical load. With the same 'intercord spacing', the increase of 'cord volume content' resulting from the use of larger diameter cord shows a slight marginal improvement of fatigue resistance of belt composites. However, the difference may not be significant because of the large range of the data. The belt composites reinforced by 2X.30HT cord are found to have distinctly higher 'threshold cyclic stress for semi-infinite fatigue life', i.e., critical load, than the composites reinforced by 2-filament cord of either larger or smaller 'nominal cord diameter' with the same 'intercord spacing' (Table 1). The belt composites reinforced by 2X.30HT wire (9.45 ends/cm) are found to have higher 'threshold cyclic stress for semi-infinite fatigue life', i.e. critical load, than the composites reinforced by 4X.25HT wire (9.45 ends/cm) having the same 'nominal cord diameter' and 'nominal intercord spacing' but much lower 'rubber matrix volume' (Table 1). The above fact reaffirms the advantage of 2-filament cord as belt reinforcement in achieving superior 'belt edge durability' over conventional 4-filament cord. A higher threshold for a critical load for 2X.30HT reinforced belt composites could be attributed to a greater tendency of 2-filament cord to form 'flare of cut ends'. While the preferred embodiment has been described herein as a two ply belt, the present invention would also apply to belts for tires having more than two plies as well as to other articles of multiple plies. As pointed out above, even a single ply in accordance with the present invention would be useful with conventional plies and further a single ply could well be useful in tires or other articles as a single ply reinforcement. The reinforcement of this invention is not limited to the particular embodiment illustrated nor to necessarily the preferred two filament embodiment, rather multi filament cords of a smaller diameter using high tensile steel filaments having the characteristic high tensile modulus coupled with the necessary flexibility for fatigue life and the stiffness for lateral and planar support would also fall within the scope of this invention.

Claims: Tire belt having at least two plies (10,12) comprising an elastomeric body and reinforcing cords (14), said cords having a plurality of individual twisted steel filaments with a diameter ranging from 0.25 to 0.34 mm, characterized in that the cord modulus is greater than 190 GPa, in that each cord is comprised of two steel filaments, in that each ply has a cord spacing ranging between 8.66 to 11.02 cords per centimeter, spaced in a direction lateral to the direction of reinforcement of the ply and in that the cords are oriented at angles from 18° to 28° to the direction of movement of the tire, the cord angles of said two plies being opposed to each other. Tire belt as defined in claim 1 further characterized by said filaments having a carbon content by weight of 0.82 %. Tire belt as defined in claim 1 further characterized by said cord having a lay length of 20 to 100 times the filament diameter. Tire belt as defined in claim 1 further characterized by said cord having a lay length ranging between 10 to 16 millimeters. Tire belt as defined in claim 1 further characterized by the diameter of said filaments being 0.30 millimeters. Tire belt as defined in claim 1 further characterized by the diameter of said cord being 0.60 millimeters. Tire belt as defined in claim 1 further characterized by the cords being oriented at opposing angles of 23° to the direction of movement of the tire. Tire belt as defined in claim 1 further characterized by each ply having a modulus greater than 120 MPa. A tire incorporating a belt as defined in claims 1 to 8.

Inventors: KOT, KENNETH MICHAEL, LEE, BYUNG-LIP
Applicants: THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1987
Main classification: B60C9
IPC codes: C08J5, B29D30, B29C70, B60C9, D07B1