Elastic bridges and elastic side panel closure systems
Elastic bridges incorporated into the closures were the first step in stretch components. They were comprised of an elastic film, ensuring stretchability and stability, laminated with a CD-extensible thermal bonded nonwoven on either side. The nonwoven material lent stability and a textile touch to the component. Figure 1 outlines the structure of an elastic bridge in diaper closures.
Elastic bridges were often applied in combination with additional elastic components, for instance the so-called neck-elastic in the back part of the diaper. These components were especially applied in the former diaper cut.
The performance of an elastic component can be evaluated using the hysteresis test. In this test, the material is stretched repeatedly, imitating the recurring elongation and contraction during movement in the finished diaper. The results demonstrate the degree of elongation the material can achieve and the necessary force. Two measurements (cycles) are taken, stretching the material up to a defined force. A value of 10 Newton is most common, representing the amount of force usually applied to the end product during use. Ideally, the curve progression should be as flat as possible, indicating that the retraction force building up during stretching is low. Moreover, the resulting curves should be as congruent as possible. A high degree of congruence indicates, that repeated stretching does not wear out the material and that perfect fit is maintained. It also shows that during the second and any additional stretching, a similar elongation is achieved at the defined force. After each cycle, the so-called permanent set is also measured. The permanent set illustrates, to what degree the material recovers to its original size. Ideally, the material should feature a low permanent set. Figure 2 illustrates the hysteresis for the first and second cycle of the hysteresis test as well as the permanent set of an elastic bridge.
The material features a high elastic recovery and a flat curve progression, yet it stretches only about 40%. In order to achieve an elongation of 10%, for example, a relatively high force of 4 Newton is necessary.
With a view to optimizing these elastic applications in diaper closures for both adult and baby diapers, the so-called diaper ear was developed. Instead of a closure system encompassing a narrow stretchable bridge that was fastened to the chassis, the diaper ear increased the elastic area, further improving fit and therefore comfort. The basic structure of an elastic bridge was retained, with usually two layers of nonwovens being applied to the elastic film during the production of the diaper ear. Figure 3 outlines the makeup of a diaper ear.
In these composites, larger quantities of nonwoven material are utilized and both the elastic film and the nonwovens are lighter. However, the demands on the material are also higher: In this application, the entire ear has to withstand a higher tensile force of 25 to 30 Newton. Very extensible yet stable nonwovens are being required. In general, the diaper manufacturer will fasten the ear to the diaper chassis himself. Thus, demands during production are elevated: In diaper production, ears are fastened and folded at high machine velocities, as the average production rate today is about 1000 units per minute. Again, high stability is needed. In order to fasten the ear to the diaper chassis and the closure tape to the ear, the rims of the ear preferably have to be non-extensible and feature high tensile strength, so-called zoned structures. Figure 4 illustrates an extensible composite with adhesive stripes.
The elastic extensibility of these nonwovens is tested by ascertaining the stress-strain-curves of the material. The nonwoven is clamped between two holders and stretched until the point of breakage. The elongation and the force necessary to achieve it are measured during this process. Ideally, the material offers high extensibility at low force. Figure 5 depicts the stress-strain-curve of a thermal bond nonwoven for stretch components.
The material can be stretched up to 160%. However, a relatively high force of up to 7 Newton is necessary to achieve this elongation. Therefore, in order to ensure optimal elastic extensibility, the utilized nonwovens are usually being activated, in other words: pre-stretched. There are various methods of activation, the so-called ring rolling being one of the most widely applied processes. During the ring rolling process, the diaper ear is conducted between two rollers that are equipped with large discs. These discs stretch the material alternately from above and from below. However, production methods for diapers are frequently patented; allowing producers little room for optimizing manufacturing processes with a view to achieving optimum extensibility.
Other methods of activating the extensible nonwoven material include incremental stretching or free stretching. During the free stretching, however, the nonwoven may tear because of its uneven structure. Therefore, predetermined stretching areas are integrated by applying the utilized adhesive in stripes.
This second step in the production of extensible components, the activation, induces additional costs and is time consuming. Thus, further developments of stretchable diaper closures strove to maximize stretchability with a view to foregoing this production step.
Advanced stretchable materials: higher extensibility, less activation
Further developed stretchable nonwovens for hygiene applications comprise thinner thermal bonded materials offering higher elastic extensibility and therefore needing less activation in order to achieve the required stretch properties. The structure of the ear encompassing an extensible film had proven itself and therefore remained the basis for these stretch components. Figure 5 shows the stress-strain-curve of an advanced thermal bond nonwoven for stretch materials.
The nonwoven features a much higher extensibility of up to 250% before tearing. At the same time, this elongation can be achieved at an even lower force of between 5 and 6 Newton. However, the high stretch of more than 200% is necessary only during activation as it increases the process capability of the nonwoven. With less activation necessary owing to the properties of the material itself, lower maximum extensibility values suffice.
Spunlace for higher elongation and stability
A further step in the development of nonwovens for stretchable hygiene applications was the introduction of spunlaced materials into these products. Owing to the unique production technology, these nonwovens feature high extensibility while simultaneously providing higher tensile strength. Consequently, a high elongation can be achieved at a very low force, making utilization even easier and more comfortable. Figure 5 outlines the stress-strain-curve of a spunlaced material for stretch applications.
Up to an elongation of 100%, which represents the characteristic extent of elongation a diaper ear is subjected to during use, the spunlaced nonwovens can be stretched at a comparatively low force of less than 2 Newton. The very flat curve progression is indicative of an equally flat hysteresis curve and therefore a low build-up of retraction force during stretching. After this point, the force needed to achieve further elongation increases exponentially, thus indicating that extensibility is decreasing. This curve is characteristic for spunlaced materials and supports the so-called stop function in the ear. Thermally bonded extensible nonwovens do not feature this pattern, allowing elongation at evenly increasing force and breaking at a certain point without any indication that maximum extension has been reached.
Figure 5 outlines the extensive range of stress-strain behavior that can be covered with nonwovens. Different materials provide different combinations of maximum elongation and necessary force: Thermal bond materials offering high elongation at a comparatively low force, advanced thermal bond nonwovens combining even higher extensibility at lower force, and spunlaced nonwovens, providing high elongation at very low force and the additional benefit of the stop function. The right nonwoven for every application.
Notwithstanding, spunlaced materials do not feature real elasticity as required in stretch applications for diapers. A comparison of the hysteresis curve of the spunlaced material to the test results of a finished diaper ear applied in premium quality diapers illustrates the material’s properties:
The diaper ear is extensible to about 130% using a comparatively low force. After stretching, it contracts almost to its original size—featuring a low permanent set. During the second stretching, a similar elongation and a similarly low permanent set are achieved. This stress-strain-behaviour can serve as a benchmark for the performance of the nonwoven stretch components.
The stress-strain-behavior of the spunlace nonwoven is very close to that of the finished product during the first stretching, while the nonwoven is not yet activated. Very high elongation at a very low force can be achieved, without having to activate the material first.
Moreover, owing to the higher tensile strength (stop function) of the spunlaced material, the weight of the elastic film, which usually provides the stretch properties, can be reduced. Thereby, raw material utilisation and the quantity of waste after use are lowered in turn. The nonwoven also lends a more voluminous, more textile-like feel to the diaper ear and provides the manufacturer with a wider choice of raw materials. Not only the widely used polypropylene is applicable. Among others, polyester as well as blends of this raw material and polypropylene can also be applied. Taking sustainability considerations into account, the polymer PLA (polylactide), which is based on lactic acid and made from renewable resources, may be deployed as well.
Another issue to the end user is the clinging of the closure hook to the fibres of the diaper ear. In many cases, fibers are torn from the nonwoven by the adhesive film of the tape hook. Spunlaced materials feature higher fibre integrity and thereby prevent this effect; the tape hook does not damage the diaper ear. All in all, spunlaced nonwovens for stretch applications offer higher performance than previously applied materials. Moreover, their high extensibility at a low force allows manufacturers to reduce the size of the diaper ear, saving costs as well as preserving resources.
In some specific applications for diaper ears, the use of spunlaced nonwovens for stretch applications yields advantages: In these components the layer of extensible film is shortened along the edges – both on the outer rim and where the ear is attached to the diaper chassis – because extensibility is not desired in these parts. In this case, the stability that is needed during use has to be provided solely by the nonwoven. Thermal bond materials do not offer this kind of stability whereas spunlaced materials, due to their particular structure, are well-suited for this application.
In latest developments, extensible spunlace materials are combined with another type of nonwoven, which may just take elastic applications to a new level.
Spunlace-meltblown composites – added value at lower expenditure
Making use of state-of-the-art production technology, meltblown nonwovens can be utilized as the central layer of an elastic component, combined with two outer layers of spunlaced nonwovens.
As opposed to composites containing an elastic film, the resulting textile stretch materials are air-permeable, further increasing the wearing comfort. Additionally, the layers are bonded during the production process without the application of adhesives. These nonwovens are ideal materials for use in adult incontinence products: They are air-permeable and may even let the wearer forget about them.
Figure 7 depicts the results for the second cycle in the hysteresis test of a spunlace-meltblown-spunlace extensible nonwoven composite in comparison with the second cycle of the benchmark hysteresis curve. During use, the elasticity of the nonwoven composite is activated and the material demonstrates high performance regarding elasticity: High elongation at a low force, almost similar to that of the benchmark product.
These materials are in development for other stretch applications, but already represent a further expansion of the product portfolio for extensible nonwovens. Their permanent set, however, is still too high to be utilized in diaper ears for baby diapers. Figure 7 outlines the gap between the permanent set of the benchmark product and that of the nonwoven composite. Nevertheless, they hold great potential and are further prove, that the evolution of materials for stretch applications in hygiene products has all but reached its climax. In a sector as fast-moving and as innovative as the nonwovens industry, the next improvement might just be one step ahead.