Composite sheet and process for making the same

A composite sheet that comprises an elastically stretchable layer and an inelastically stretchable layer formed with inelastically stretchable continuous fibers bonded to at least one surface of the elastically stretchable layer intermittently in one direction. The continuous fibers are oriented substantially in one direction thereof so that the composite sheet may present a ratio S1/S2 of 3.0 or higher where S1 represents a tensile strength in this one direction and S2 represents a tensile strength in the direction orthogonal to this one direction.

BACKGROUND OF THE INVENTION

This invention relates to a composite sheet comprising an elastically stretchable web and an inelastically stretchable web formed with inelastically stretchable continuous fibers and also to a process for making the composite sheet.

Japanese Patent Application Publication No. 1994-184897A describes elastically stretchable composite stock material obtained by a process comprising steps of stretching inelastic material to neck this material, bonding this material as it is necked to an elastically stretchable sheet which is under a tension at three or more non-linearly arranged regions and finally relieving the stretchable sheet of its tension. So far as the necked material is of fibrous nature, such process of prior art enables the necked material to form a plurality of gathers as the elastically stretchable sheet is relieved of its tension and thereby to convert a rubber-like touch peculiar to the surface of the elastically stretchable sheet to a comfortable cloth-like touch without deterioration of a desired stretchability of the elastically stretchable sheet.

According to this technique of prior art, the necked material is obtained by stretching, for example, a spun bond nonwoven fabric comprising thermoplastic synthetic fibers fused together in one direction. Of the fibers unevenly distributed in this nonwoven fabric, some are plastically deformed in the one direction and thereby actually stretched while the others are merely reoriented in the one direction as the nonwoven fabric is stretched in the one direction. The actual stretched fibers have their diameters reduced and the merely reoriented fibers maintain their initial diameters. Consequently, the elastically stretchable composite stock material obtained in this manner is disadvantageously accompanied with a remarkable unevenness of the fiber diameters. This may lead to the unevenness in touch as well as in appearance of the product.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a composite sheet that comprises an elastically stretchable layer and an inelastically stretchable fibrous layer in which the unevenness of the fiber diameter in the inelastically stretchable fibrous layer is minimized and a process for making such a sheet.

The object set forth above is achieved, according to one aspect of this invention, by a novel composite sheet and, according to another aspect of this invention, by a novel process for making this novel composite sheet.

This invention relates to, in one aspect thereof, the composite sheet comprising an elastically stretchable layer having upper and lower surfaces and an inelastically stretchable fibrous layer formed with inelastically stretchable continuous fibers, these two layers being bonded together intermittently in first and second directions orthogonal to each other, at least, in the first direction.

In such a composite sheet, the continuous fibers are oriented substantially in the one direction so that a tensile strength S1of the composite sheet in the first direction and a tensile strength S2of the composite sheet in the second direction may define a ratio S1/S2of 3.0 or higher.

This invention relates to, in another aspect thereof, a process for making the composite sheet by bonding an elastically stretchable layer having upper and lower surfaces and an inelastically stretchable fibrous layer put on at least one of the upper and lower surfaces to each other intermittently in first and second directions orthogonal to each other, at least, in said first direction.

In such a process, the continuous fibers lie one upon another substantially without being bonded together to form the inelastically stretchable web and that the web is, in turn, bonded to the elastically stretchable web after the continuous fibers have been oriented substantially in the one direction.

According to one embodiment of the invention, the process comprises the steps of extruding the continuous fibers from a melt extruder, collecting the continuous fibers on a conveyor running in one direction to form the inelastically stretchable web, orienting the continuous fibers substantially in the one direction and at the same time placing the continuous fibers upon the elastically stretchable web and finally bonding these two webs together intermittently in the one direction to obtain the composite sheet.

According to another embodiment of the invention, the step of orienting said continuous fibers substantially in said one direction including the use of a first conveyor running at a velocity V1and a second conveyor provided downstream of the first conveyor and running at a velocity V2so that a ratio V2/V1may lie in a range of 1.05˜10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Details of the composite sheet and the process for making the same according to this invention will be more fully understood from the description given hereunder with reference to the accompanying drawings.

A composite sheet10depicted byFIG. 1in a perspective view comprises an elastically stretchable layer3formed by continuous fibers40of styrene-based elastomer and an inelastically stretchable layer2formed with an inelastically stretchable polypropylene continuous fibers6fused with upper surface of the elastically stretchable layer3at bonding regions4A. The composite sheet10has X-direction and Y-direction being orthogonal to the X-direction so that the layer3is elastically stretchable at least in Y-direction of the X- and Y-directions. The continuous fibers6of the inelastically stretchable layer2are oriented so as to extend substantially in Y-direction. In the case wherein the elastically stretchable layer3has a substantially same tensile strength in X- and Y-directions, a degree of orientation of the continuous fibers6can be expressed by a ratio S1/S2where S1represents a tensile strength as measured in Y-direction and S2represents a tensile strength as measured in X-direction. For the composite sheet10according to this invention, the continuous fibers6are preferably oriented with a ratio S1/S2of 3.0 or higher. Except at the bonding regions, the continuous fibers6are neither fused nor bonded together but substantially brought into close contact with one another. In other words, the aggregative strength among these fibers6are extremely feeble so that the continuous fibers6are easily separated from one another as the composite sheet10is slightly stretched in Y-direction.

Assumed that the composite sheet10is stretched in Y-direction with tensile force such that breaking extensions of the two layers2,3are not exceeded and an elasticity limit of the elastically stretchable layer3is not exceeded, the layer3is elastically stretched while the layer2is inelastically stretched. As a result, the continuous fibers6of the layer2are plastically deformed so as to be thinned and lengthened. Relieved of the stretching force, the composite sheet10contracts substantially to its initial dimension under contractible force provided by the layer3. Thereupon, the continuous fibers6having been inelastically stretched form a plurality of gathers and the layer2of these continuous fibers6become more bulky than before stretched. In this way, the layer2offers a comfortably soft touch.

FIG. 2is a graphic diagram plotting a load/extension percentage curve observed as the composite sheet10is stretched anew after the sheet10has been once stretched until the extension percentage reaches 150%. Specific construction of the composite sheet10will be described below. The extension percentage of the composite sheet10for the first time of stretching is defined as the initial extension percentage EI.

In the inelastically stretchable layer:

As will be apparent fromFIG. 2, the curve gently rises and then reaches a inflection point P corresponding to an extension percentage of 100% under a substantially uniform load. From this inflection point, the curve sharply rises. Specifically, the gathers formed with the continuous fibers6of the inelastically stretchable layer2are flattened and the layer3is elastically stretched to the inflection point P as the composite sheet10is stretched. After the inflection point P, the continuous fibers6are plastically deformed so as to be thinned and lengthened while the layer3continues to be elastically stretched. The load/extension percentage curve thus indicates that, in the composite sheet10having been initially stretched until the extension percentage reaches 150%, the layer3can be elastically stretched for the second time with a relatively low stress until the extension percentage reaches 100% substantially without being affected by the presence of the layer2. The extension percentage at which the composite sheet10can be stretched with a relatively low load for the second time until the inflection point P is reached is defined as the secondary extension percentage ES. ES/EI, the ratio of this secondary extension percentage ESto the initial extension percentage EI, is defined as the stretch efficiency SE. In the case illustrated by the graphic diagram ofFIG. 2, the stretch efficiency SE is given by 100(%)/150(%)×100=67(%). In the composite sheet10, the continuous fibers6are oriented substantially in Y-direction. Therefore, most of the continuous fibers6are stretched in Y-direction and lengthened as the composite sheet10is stretched in Y-direction and a stretch efficiency SE, as high as in the order of 60˜90% is achieved. In the contrast with this composite sheet10, the composite sheet of prior art in which the continuous fibers6are distributed at random presents a stretch efficiency SEless than 60%.

FIG. 3is a diagram schematically illustrating the process for making the composite sheet10. At the left hand in the diagram, a first extruder31adapted to discharge the continuous fibers6, a first conveyor belt32and a suction box33are illustrated. The continuous fibers6discharged from the first extruder31are collected on the first conveyor belt32and form inelastically stretchable web52under an effect of hot air blast (not shown) ejected sideward with respect to nozzle arrays of the first extruder31in combination with an effect of the suction box33. An extruding condition of the first extruder31, a cooperating condition of hot air and suction, a cooling condition for the continuous fibers6and a velocity V1of the first conveyor belt32are appropriately adjusted to prevent the continuous fibers6from being fused together or to minimize such fusion.

The inelastically stretchable web52is then transferred onto a second conveyor belt36. The second conveyor belt36runs at a velocity V2and two pairs of pressure rolls37,38rotate at a peripheral velocity V2. A ratio V2/V1is in a range of 1.05˜10. The inelastically stretchable web52is stretched in the machine direction with the continuous fibers6being reoriented in the machine direction as the web52is transferred from the first conveyor belt32onto the second conveyor belt36. The continuous fibers6, even if they have been fused together to some extent in the precedent step, are reliably separated apart from one another on the second conveyor belt36.

Now the inelastically stretchable web52is transferred from the second conveyor belt36onto a third conveyor belt41. Elastomeric continuous fibers40are discharged from a second extruder42and accumulated on the inelastically stretchable web52to form elastically stretchable web53under a suction effect of a suction box43. These two webs52,53are bonded together at the binding regions4A which are formed as the two webs52,53pass a pair of heat-embossing rolls44,44. In this manner, the composite sheet10is obtained.

According to the process as has been described above, the relationship between the velocity V1of the first conveyor belt32and the velocity V2of the second conveyor belt36is preferably adjusted to V2/V1=1.05˜10. The velocity ratio V2/V1=1.05˜10 ensures an orientation degree of the continuous fibers6sufficient to achieve a stretch efficiency SEof the composite sheet10as high as 60˜90%. The composite sheet10with the continuous fibers sufficiently oriented in this manner presents a ratio S1/S2of 3.0 or higher where S1represents a tensile strength S1in the machine direction and S2represents a tensile strength in the direction orthogonal to the machine direction.

The composite sheet10is not limited to the illustrated embodiment and it is possible without departing from the scope of the invention to place the inelastically stretchable layers2upon both surfaces of the elastically stretchable layer3. To this end, the process illustrated inFIG. 3may be correspondingly added with a third extruder and a plurality of conveyor belts.

The composite sheet according to this invention comprises a plurality of inelastically stretchable continuous fibers that are evenly stretched as these continuous fibers are stretched in one direction since they are oriented substantially in the one direction. Consequently, a possible unevenness in the fiber diameter after stretched and therefore in touch as well as in appearance can be minimized.

The process according to this invention comprises the steps adapted to orient the inelastically stretchable continuous fibers substantially in one direction. In this way, the process of this invention facilitates the composite sheet to be made.