ABSTRACT

The functionality of modern disposable absorbent goods gains more and more interest in the hygienic products market. In the present work, both the acquisition surface geometry and the nature of top sheet constituents were studied. Their effects on hygienic pads performance were then determined in terms of acquisition rate of excreted body fluids, rewet, and diffusion length.

A closure ratio was defined to characterize the rewet efficiency of a hygienic pad and could be used as a rating criterion of disposable absorbent goods.

Keywords: Hygienic pads, top sheet, acquisition rate, rewet, diffusion length, closure ratio.

Introduction

Because of their product diversity and different fabrication processes, nonwovens have a multitude of application fields. The annual growth rates in these fields are conspicuously different. The market of hygienic products is the most important. In terms of production volume, it covered in Europe 36.2% of the total nonwoven products in 1999.

The modern hygienic pads are domestic goods of daily use. Most users are generally unaware of the sophisticated and elegant technologies involved in this product. In the present work, we studied the influence of a set of constitution parameters on the properties of hygienic pads.

Constituents of hygienic pads

The absorbent core (figure 1) based on wood pulp is covered with a thermally bonded nonwoven top sheet, noted N.W, having an average basis weight of 23 g/m2 and an average porosity of 0.8. Some hygienic pad top sheets consist of perforated and laminated nonwoven structures, noted P.L.N.W This kind of topsheets has a good dry level, a property highly appreciated. The P.L.N.W is formed with a very fine laminate of polypropylene (PP) over a fine layer of polyester nonwoven (PET) thermally bonded. Then the laminate goes through a perforator where a dented calender creates semi cylindrical shaped openings on the deposited film (figure 3). The P.L.N.W has an average basis weight of 44 g/m2. The absorbent core is located between the N.W or P.L.N.W top sheet and a film of polyethylene (PE) preventing leakage.


Studied proprieties

In order to respond to women exigencies in terms of performance of hygienic pads available in the market, the following properties were studied:

The acquisition rate of the liquid, expressed in equation (1) in terms of volume of liquid divided by the area of the surface of acquisition, the penetration time and the weight of the absorbent core.

The length of diffusion defined as the maximum length by which a liquid of a given volume has spread through the pad during a given laps of time.

The dry level evaluated by the amount of liquid released back from the pad under a given test pressure. It is considered as a highly important parameter with regard to pad performance [1].

The top sheet constituents determined by the nature of material used and the surface geometry.

The test conditions used to measure the properties mentioned above are:

Sample weight: 11.20  0.2 g
Volume of miction: 20 ml / pad
Surface of miction: 7.5 cm
Pressure applied during miction: 4.2 kPa
Time after miction: 2 min
Time during which sponge papers are pressed against the wet sample: 2 min
Sample thickness: 6.7 0.2mm

Comparison of the properties of pads covered with N.W and P.L.N.W

To compare the performance of N.W and P.L.N.W used as top sheets, the acquisition rate and dry level were measured.

The P.L.N.W enhanced remarkably the dry level, consequently the comfort [2], of the hygienic pads from 0.0165 g/g (case of N.W) to 0.0089g/g, hence a ratio of 0.54. However s shown in figure 2, the acquisition rate of the tested liquid decreases from 1.35 mm/s/g (N.W) to 1.08 mm/s/g (P.L.N.W.), hence a ratio of 0.8.

These variations could be due to the modification of the geometry of the contact surface between the liquid and the pad (figure 3). We suppose that the openings in the P.L.N.W structure are half-circles with a radius equal to the depth, d. All openings are considered to be identical. The initial ratio of porosity zInitial of the openings area in P.L.N.W to that of the corresponding openings in NW is presented in equation (2).

The capillaries present in the top sheet, particularly in the nonwoven structure, allow the liquid to migrate from the surface to the absorbent core due to the capillary pressure [2-4]. The higher the number of capillaries in the contact surface, the higher the amount of liquid absorbed at a given time, hence a higher acquisition rate. Knowing that only the geometry of the acquisition surface was changed, we suggest that the ratio of the acquisition rates is close to the ratio of the porosities of P.L.N.W and N.W defined in equation (2). In deed the experimental value of the ratio of acquisition rates is 0.8 and the theoretical ratio of porosity, zInitial, is 0.76 (π/4).

In terms of dry level, the ratio of the weight of liquid released back over the weight of tested absorbent core characterize the final ratio of porosities, zFinal, during the dry level test and is 0,54. The difference between zInitial and zFinal is mainly due to a valve like behavior of the P.L.N.W structure. Indeed, during the dry level test, because of the applied pressure, the portion of liquid released back exercise an upward pressure on the engraved part of the perforations. Therefore, the depth d decreases and so the openings surface does.

This phenomenon explains the improvement of the dry level of hygienic pads and leads to an estimate, as stated in equation (3), of the percentage of closure, zClosure characteristic of P.L.N.W. The tested samples of P.L.N.W exhibited an experimental closure percentage of 27.6 %.

Effect of paper layers on studied parameters

The paper layers (P.L) are cellulose paper sheets made by papery techniques with an average basis weight of 22 g/m2. Figure 4 shows the effect of the addition of P.L on the performance of hygienic pads on the top sheet composed of N.T or N.T.L.P.

The rate of acquisition decreased when paper layers are added, whereas the length of liquid diffusion through the pad increased with 10 mm for both N.W and P.L.N.W. The dry level is unchanged after the addition of one layer, and then it is ameliorated after the addition of the second layer.

The decrease in the values of acquisition rates (figure 4.a) could be a result of the change in the top sheet structure specifically the pore size and distribution. These parameters affect the dragging force applied on the penetrating liquid, hence influencing the acquisition rate.

As for the amelioration of the dry level (figure 4.b), it could be due partly to the increase of storing capacity of the tested pad due to the addition of cellulose present in P.L, and to the density of fibers in these paper layers. Finally, the increase of the length of diffusion (figure 4.c) is probably due to the contribution of inter-layers area adding extra-capillary capacity and to the additional trans-planar movement [5].

Conclusion

Through the present work, we tried to identify the influence of the composition of the top sheet (N.W, P.L.N.W, and P.L) on the performance of one model of hygienic pads. To conclude with, we showed that the laminated and perforated nonwoven structure studied as a top sheet, the P.L.N.W, enhance the dry level of hygienic pads compared to common thermally bonded nonwovens, N.W. The addition of paper layers, P.L, increased the length of diffusion and dry level, whereas the acquisition rate was lowered.

References

[1] Ratnapandian, S., and Warner, S. B.; Modern Diaper Technology; Tappi J.; 79 (6);173(1996).
[2] Zhong, W., Ding, X., and Tang, Z. L.; Analysis of fluid flow through fibrous structures; Textile Res. J.; 72 (9); 751(2002).
[3] Lucas, R.; Kolloid Z.; Ueber das Zeitgesetz des Kapillaren Aufstiegs von Flssigkeiten 23; 15 (1918).
[4] Washburn, E. W.; The Dynamics of Capillary Flow; Phys. Rev.17(3); 273 (1921).
[5] Miller, B.; Friedman, L.H.; Absorption rates for materials under compression; Tappi J.; 163 (1992).