Thin is in, even when it comes to disposable hygiene products. Producers must take a completely different approach in the core design of sanitary napkins. If the core fluff is reduced or eliminated, the role of the existing materials i.e. cotton fluff, super absorbent polymers and adhesives becomes more prominent in core integrity and in achieving the desired specific properties.
To a majority of people in the disposable hygiene industry, the goal to produce a thinner diaper that would require a non-bulky or thin core, means reduced core tuff. To others, it means a fluff-free core. However if the content of fluff is reduced in the core, then super absorbent polymers (SAP) are increased, but it's very important to note that a key function of fluff is to keep the SAP in place. Hence, removing or eliminating fluff will minimise the ability to control SAP positioning in the core and also the ability to control SAP gel blocking that will basically change the entire absorbent system of a diaper. It might also change the overall shape and dimensions of the diaper, which could impact fit. An integrated approach, taking into consideration all of the issues involved in a product's make-up or system is necessary to create a core design solution that will provide optimised core performance.
The core of a diaper is a complex, dynamic composite of many individual components that must work together as one system to function properly. The following are the basic components of the core of the napkin:
1. Top sheet: It is the first layer against the skin and facilitates liquid acquisition to the core.
2. Acquisition Distribution Layer (ADL): It creates a low density void space beneath the top sheet to keep skin dry and enables rapid fluid absorption to help minimise leaks.
3. Tissue/Non-woven core wrap: It maintains the core integrity during processing by containing the tuff/SAP matrix and also provides stability in use by containing core materials.
4. Fluff: Distributes the fluid and assists with core integrity. Improves core efficiency through rapid absorption and distribution via capillary action. Improves core matrix stability through hydrogen bonding of fibres to prevent cracking and slumping.
5. SAP: Absorbs liquids slowly but can retain large volume relative to weight under pressure.
6. Adhesive: Bonds core components together, may also provide structure to core, when core specific adhesive is not in use.
7. Back sheet: Polyolefin-based barrier film which might be breathable or non-breathable.
8. Wetness indicator: Functional adhesive that indicates when a pad is wet.
Each component has a specific task to fulfil in order for the core to enable the napkin to achieve its main goal i.e. keep the wearer dry and comfortable. As in the case of any dynamic system, if one component is altered, it can impact the performance of the other components. For an optimal overall performance of napkins, producers must consider the core system in a holistic manner. The absorbent core which is a blend of SAP and fluff in the middle of the napkin is just one aspect of what performs the absorption function in the napkin.
To produce thinner cores, the manufacturer has to first think of altering the ratio of SAP and fluff and then reducing the latter. With a holistic approach, the next step is to assess how that change will affect all the other components that contribute to the absorption distribution layer the core wrap, ADL, top sheet material, adhesives etc. All of the materials and applications from top sheet, through to leg cuff and back sheet play an important role in the performance of the diaper core.
Adhesives have to perform a number of functions in the core area. Adhesives may be added to seal the wrap and avert SAP migration, if a core wrap is used. They are also being used to bond the layers of the core: The ADL to core; core to back sheet. Also they are used to bond the fluff fibers to the back sheet.
Although the supplier of the films, layer materials and non-woven for back sheets and top sheets are making strides in adapting their products to meet with the demand for thinner core component, adhesives also need to undergo innovative changes that would enable disposable hygiene products manufacturer to more effectively use these emerging material innovations.
While some of the adhesives like wetness indicators and elastic attachment adhesives do not contribute directly to core absorption, they do have a significant impact on and contribution to the entire system. The system cannot perform its task with fine fit and liquid containment, without the steadfast performance of adhesives throughout the product. Overall, using adhesives in the core design provides the napkins manufacturers with the flexibility to opt for the best materials in each category to meet the requirement of a specific core design.
Testing core performance
No single industry standard lab test method is defined to gauge core integrity or the stability of the fluff/SAP distribution within the napkin core. Test methods that are being employed today range from using simple tools to complex equipment. A majority of the tests focus on testing core integrity by direct exertion of force on the napkin to monitor core cracking as a measure of how well the core stays in place within the napkin. Normally, the force is a kind of mechanical spinning, swinging or shaking after the napkin is made wet for a specified length of time.
As the trend across the global market is a drift towards a thin core, it is vital that producers and suppliers adapt a holistic approach in responding to demand. Key to that is mainly played by the role of adhesive selected. The use of adhesives in fluff-free or thin core presents the maximum flexibility in design and implementation. Also, when there is a consideration of changes in the core, it is most important to select the right adhesive partner who can support in the holistic evaluation of goals around core design.