Disposable diaper

A disposable diaper comprises a liquid-permeable surface sheet, a liquid-impermeable backing sheet, and an absorbent pad sandwiched between the two sheets, characterized in that water-absorptive, shrinkable fiber strings, which contract lengthwise on contact with water and are rendered stretchable, are secured to the two sheets, extending midway longitudinally of the diaper in parallel along, but without overlapping, the both edges of the pad, the edge portions of the surface sheet outside the water-absorptive, shrinkable fiber strings and the surface sheet portion on the absorbent pad being partly bonded together at two spots about halfway of the length, the distance between the two bonded spots being at least 50 mm and the bonded spots themselves being 10 to 50 mm long each, and elastic, stretch members, at least 70 mm long each and having a combined elastic force or tensile strength (the weight equivalent to the force required to stretch the diaper including any shrank portions thereof up to the total diaper length) of 20 to 600 g, are provided midway and longitudinally of the both edge portions of the diaper at least 5 mm outside the both bonded spots.

This invention relates to a disposable diaper having a leakproof seal 
means. 
Various leakproof designs for disposable diapers have hitherto been 
proposed. A typical group of them, as disclosed in Japanese Patent 
Publication No. 40267/1977 and Japanese Patent Laid-Open Nos. 115939/1979 
and 120045/1977, form gathers along the both edges of a disposable diaper 
by elastic members fitted to the edges longitudinally of the diaper so as 
to keep the diaper in intimate contact with the thighs of the wearer and 
minimize the leakage of excretions. Another seal design taught in Japanese 
Patent Laid-Open No. 35002/1982, uses water-absorptive, shrinkable fiber 
strings which contract on wetting along the edges longitudinally of a 
disposable diaper so as to fit the diaper tightly to the wearer and 
minimize the leakage. 
These leakproof designs of the prior art have disadvantages, however. The 
seal means of the former group, which utilize the elastic shrinking forces 
of elastic material such as rubber, have to be pressed against the thighs 
of the wearer tightly enough to avoid urine leakage. Fastening which is 
too light often results in reddish inflammation, irritation, or injury of 
the thighs. In addition, the close contact of the diaper tends to make the 
skin hot and moist. 
The latter design also has the disadvantage of inviting frequent leakage of 
feces when the wearer evacuates the bowels, discharging especially watery 
or loose feces, before urination. This is because the water-absorptive, 
shrinkable fiber strings cannot gather the edges of the diaper unless they 
are wet with urine. 
With the view to overcoming these disadvantages of the two prior art 
concepts, the present inventors have made intense studies and have now 
arrived at this invention. 
The present invention provides a disposable diaper comprising a 
liquid-permeable surface sheet, a liquid-impermeable backing sheet, and an 
absorbent pad sandwiched between the two sheets, characterized in that 
water-absorptive, shrinkable fiber strings, which contract lengthwise on 
contact with water and are rendered stretchable, are secured to the two 
sheets, extending midway longitudinally of the diaper in parallel along, 
but without overlapping, the both edges of the pad, the edge portions of 
the surface sheet outside the water-absorptive, shrinkable fiber strings 
and the surface sheet portion on the absorbent pad being partly bonded 
together at two spots about halfway of the length, the distance between 
the two bonded spots being at least 50 mm and the bonded spots themselves 
being 10 to 50 mm long each, and elastic, stretch members, at least 70 mm 
long each and having a combined elastic force or tensile strength (the 
weight equivalent to the force required to stretch the diaper including 
any shrunken portions thereof up to the total diaper length) of 20 to 600 
g, are provided midway and longitudinally of the both edge portions of the 
diaper at least 5 mm outside the both bonded spots.

The invention will now be described in more detail in connection with 
embodiments thereof shown in the accompanying drawings. 
The disposable diaper embodying the invention comprises a liquid-permeable 
surface sheet 1, a liquid-impermeable backing sheet 2, and an absorbent 
pad 7, with two strings 5 of a fiber, which contracts lengthwise on 
contact with water and is rendered stretchable, secured to the two sheets 
in parallel along, but without overlapping, the both edges of the 
absorbent pad 7. The edge portions of the surface sheet 1 outside the 
water-absorptive, shrinkable fiber strings 5 and portions of the surface 
sheet that overlie the absorbent pad 7 are partly bonded together at two 
spots halfway of the length. These bonded areas 4 are referred to herein 
as leg spots. Between the superposed sheet portions farther outside the 
leg spots, there are secured two elastic, stretch members 3' designed to 
provide elastic gathers 3 outside these spots. 
This disposable diaper is aimed at absorbing the urine and feces of the 
wearer without any leakage outside or excessive pressure upon the skin. 
The functions of the diaper will now be explained. The leg spots 4 combine 
with the elastic gathers 3 to form pockets 8 in the diaper between the 
buttocks and thighs of the wearer. The pockets prevent leakage of feces 
out of the diaper while the latter is not wet with urine. In case of 
urination, the water-absorptive, shrinkable fiber strings 5 prevent the 
leakage out of the diaper edges facing the thighs of the wearer. The 
elastic gathers 3 have to be just tight enough to form the pockets. Since 
the diaper uses no highly elastic stretch member, the pressure to be 
exerted upon the skin of the wearer is negligible, and leakage of urine 
and feces out of the diaper is avoided. 
It is important here to form effective pockets by means of the gathers 3 
and the leg spots 4. In attaining this end the location and length of the 
elastic, stretch members and the length of the leg spots are critical. In 
order to provide the useful pockets it is necessary that the two leg spots 
be spaced apart a distance of at least 50 mm and the leg spots be 10 to 50 
mm long each. Also, the distance (a) between each leg spot 4 and the 
associated elastic, stretch member 3' must be at least 5 mm. If the 
distance is less than 5 mm, the resistance of the leg spot will render it 
difficult to form effective gathers in that area. Moreover, the resulting 
pocket will be so shallow and small that frequent leakage will result. 
As regards the length of the elastic, stretch members, too short members 
will fail to form pockets which effectively prevent leakage of feces. The 
members should, therefore, have a length in a stretched state of at least 
70 mm, preferably not shorter than 100 mm. 
Another important factor is the elasticity of the elastic, stretch members. 
Closely related to the pressure upon the skin, the elasticity should be 
the lowest possible for the diapered person. Thus, it should be at a 
minimum required for forming effective gathers. In this respect the diaper 
of the invention, which utilizes water-absorptive, shrinkable fiber 
strings, need no elastic gathers for preventing urine leakage but gathers 
just enough for forming pockets. Only slightly elastic, stretch members 
are needed. In connection with the pressure upon the skin, a diaper having 
a tensile strength of more than 600 g when stretched will make the wearer 
uncomfortable. On the other hand, a strength of at least 20 g is required 
for the purpose of gathering. Any material which has such a tensile 
strength may be used for the elastic, stretch member. 
It is preferable that the strength of the elastic, stretch member be in the 
range of 20 to 100 g. In this preferable embodiment, it may be made of 
ordinary natural rubber, urethane rubber, or any other elastic material. 
Among the useful members, the most desirable is one formed of fine 
urethane filaments matted into nonwoven cloth and elasticized by the spun 
bond method. Such a member is the best as it is soft to the touch and yet 
forms effective gathers. 
Another preferable range of the tensile strenth of the elastic, stretch 
member is from 50 to 600 g when stretched. More preferably, it is from 20 
to 150 g when it is elongated by 20 percent. A material suitable for this 
preferable embodiment is polyurethane, polybutadiene, a copolymer of 
ethylene and vinyl acetate, modified polyethylene and a composition 
containing any of the beforegoing, for example in the form of a sheet or 
foamed sheet. 
The elastic, stretch member is preferred to have a width of 4 mm or larger, 
especially 5 mm or larger. For example the width of 4 to 40 mm is 
practicable to lessen the pressure on the skin of the user. 
If the water-absorptive, shrinkable fiber strings are located close to the 
absorbent pad, they can be kept from shrinking by the pad. In order to 
attain more effective shrinkage, the strings must be kept at a certain 
distance away from the pad. For the intimate contact of the diaper with 
the skin of the wearer by dint of the shrinkage of the water-absorptive, 
shrinkable fiber strings, the distance (b) between each edge of the 
absorbent pad and the associated string should be at least 10 mm. This 
spaced relation is helpful in avoiding the leakage of urine out of the 
diaper edge. 
The water-absorptive, shrinkable fiber strings so far described are 
effective if they are made up of twisted yarns of a 
carboxymethylcellulose, hydrolyzed acrylic fiber or the like as disclosed 
in Japanese Patent Laid-Open No. 35002/1982. Desirably, the 
water-absorptive, shrinkable fiber strings are provided with means for 
conducting urine out of the absorbent pad into the strings. 
While preferred embodiments of the disposable diaper of the invention have 
been described, it is to be understood that the invention is not limited 
thereto but various modifications may be made within the scope of the 
invention. 
EXAMPLES 1 TO 4 AND COMATIVE EXAMPLE 1 
As shown in FIG. 4, a diaper was made by placing an absorbent layer 11 
having 150 mm width and 380 mm length at the center of a polyethylene 
sheet 10 having 300 mm width and 430 mm length; placing an elastic, 
stretch member 12 each 22 mm inside of the edge of the polyethylene sheet; 
placing a water-absorptive, shrinkable fiber string 13 each 40 mm inside 
of the member; covering the resulting assembly with a non-woven sheet 14 
having the same size as the polyethylene sheet, folding both non-woven 
sheet and polyethylene sheet each at 68 mm inside of either edge thereof 
and making the adhesion between each other at two positions each at 25 mm 
inside of the folding position. The elastic, stretch member is listed in 
Table 1 by material. Each has an elastic length of 100 mm. 
TABLE 1 
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when 
width when stretched 
stretched 
mate- of the at the maximum 
at 20% 
rial the elongation 
elastic 
elastic 
No. material member percent force force 
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1 polyethylene 
15 mm 44% 310 g 90 g 
composite film 
2 polyethylene 
15 mm 45% 440 g 180 g 
composite film 
3 polyethylene 
10 mm 42% 130 g 45 g 
composite film 
4 the same as 3 
5 mm 35% 65 g 25 g 
5 modified 20 mm 62% 640 g 120 g 
polyethylene 
film 
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The materials 1 to 3 are each available from Right Co with grade number of 
15040, 82035 and 15030, respectively. The material 5 is Tafmer P, a 
tradename of Mitsui Petroleum Chemical Co., Ltd. 
Each obtained diaper was put on babies of various sizes 10 babies of the 
body weight of 5 to 7 kg, 10 babies of 7 to 9 kg and 10 babies of 9 to 10 
kg. The skin where the diaper was attached to the babies was observed 
after 3 hours had passed. Results are shown in Table 2 giving the number 
of babies who had red marks in red of the diaper. 
TABLE 2 
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influence on the skin 
material the babies of 
that of that of 
No. 5-7 kg 7-9 kg 9-10 kg 
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Example 1 
1 0 0 0 
Example 2 
2 1 0 1 
Example 3 
3 0 0 0 
Example 4 
4 0 0 0 
Comparative 
Example 1 
5 0 2 5 
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