Elastic monofilm and method of making a disposable hygiene article

Abstract

The invention relates to an elastic monofilm having a film thickness between 20 μm and 100 μm, comprising an asymmetric styrene-diene triblock copolymer at a content between 35 wt % and 90 wt % as a first polymer substance A and comprising a second polymer substance B. According to the invention, polystyrene is provided at a content between 8 wt % and 40 wt % as the second polymer substance B.

Claims

1. An elastic monofilm having a film thickness between 20 μm and 100 μm, the monofilm consisting of: an asymmetric styrene-diene triblock copolymer at a content between 30 wt % and 90 wt % based on the weight of the monofilm as a first polymer and having terminal blocks with a mass ratio between 2:1 and 5:1; a second polymer of polystyrene at a content between 8 wt % and 40 wt % based on the weight of the monofilm; and remainder at least one additional constituent of polyethylene, an ethylene vinyl acetate blocking agent, a pigment, a thermal stabilizer, a UV absorber as a stabilizer, or a processing aid.

2. The elastic monofilm according to claim 1, wherein the second polymer is provided at a content between 10 wt % and 20 wt %.

3. The elastic monofilm according to claim 1, wherein the ratio of the first polymer to the second polymer lies between 3:1 and 6:1.

4. The elastic monofilm according to claim 1, wherein the elastic monofilm is formed by cast film extrusion and has a microphase-separated structure having stiffened domains oriented in a production direction.

5. The elastic monofilm according to claim 1, wherein the asymmetric styrene-diene triblock copolymer is a styrene-isoprene-styrene block copolymer.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Below, the invention is explained further with reference to a drawing in which:

(2) FIG. 1 shows a comparison of the force necessary for a stretch of 1% for different monofilms.

(3) FIGS. 2a to 3c are stretch graphs for different monofilms.

SPECIFIC DESCRIPTION OF THE INVENTION

(4) In FIG. 1, the force for 1% stretch of a monofilm in dependence on the content of polystyrene as polymer substance B in wt % is indicated in relative units. Curve I shows the curve of a monofilm not comprised by the invention that is formed from a mixture of a symmetric styrene-isoprene-styrene block copolymer and polystyrene. A certain amount of polystyrene is necessary to improve the flowability of the mixture. In addition, the raw-material costs of the mixture generally decrease with an increasing content of polystyrene. With the addition of polystyrene, the stiffness decreases at first and then remains relatively stable from a content of approximately 6 wt %.

(5) In contrast, curve II shows the stretch behavior of a monofilm formed from an asymmetric styrene-isoprene-styrene block copolymer and polystyrene. The monofilm thus formed initially has a significantly lower stiffness that, however, continuously rises as polystyrene is added. At a content of at least 10 wt % of polystyrene, a higher level than that of curve I is achieved, despite an initially lower starting stiffness.

(6) FIGS. 2a to 2c show, in relative units, the stretch behavior of a monofilm formed on the basis of asymmetric styrene-isoprene-styrene block copolymer for stretch in the machine direction (MD). FIG. 2a shows the stretch behavior for a polystyrene content of 0%, and therefore FIG. 2a concerns a comparison example.

(7) According to FIG. 2a, a monofilm comprising an asymmetric styrene-isoprene-styrene block copolymer without any polystyrene has only low stiffness in the machine direction and thus cannot be processed or can be processed only with considerable complexity. The stretch graph shows the typical curve for a first-time stretch 1 and a subsequent second stretch 2, in each case with subsequent relaxation of the monofilm.

(8) FIGS. 2b and 2c show a design of the monofilm according to the invention comprising a content of polystyrene of 10 wt % (FIG. 2b) and 20 wt % (FIG. 2c). It can be seen that the film is significantly stiffer in the machine direction (MD) and thus also can be well processed.

(9) FIGS. 3a to 3c show the stretch behavior of the monofilm transversely (CD) in the same order. For the comparison example according to FIG. 3a having a polystyrene content of 0 wt %, an asymmetric behavior of the monofilm that is only slightly pronounced overall results in comparison with FIG. 2a, the monofilm being easy to stretch in both the machine direction (MD) and the transverse direction (CD).

(10) While the stiffness in the machine direction (MD) significantly rises with an increasing content of polystyrene according to FIGS. 2b and 2c, the tensile forces required transversely (CD) change only slightly. Only a slight increase results. The monofilm according to the invention has a pronounced asymmetry of the stretch behavior, and, on the one hand, tensile forces can be adequately absorbed in the machine direction (MD) while, on the other hand, the monofilm remains easy to stretch transversely (CD).

(11) The elastic monofilm typically can be elongated by at least 100%, preferably by at least 200%, from an original length. When the film is relaxed after a first stretching, a plastic portion of the deformation remains, which is also referred to as permanent stretch or permanent set. The present invention relates in particular to elastic films that have a permanent stretch of less than 50% after a stretch of 100%. In general, the permanent stretch of the film according to the invention is significantly lower. Thus, for example, values of the permanent stretch of less than 10% and in particular less than 6% can be achieved for a stretch of 200%.