Introduction
When wearing diapers, a damp surface layer can increase humidity and heat between the diaper and the skin, exacerbating skin hydration. This condition can damage the skin barrier and, in severe cases, lead to eczema. A dry diaper, on the other hand, effectively transfers urine to the absorbent core, locking it in place, while channeling the heat from the urine and body to the diaper’s exterior. Thus, dryness and breathability are critical to ensuring a comfortable experience with diapers.
Factors Affecting Nonwoven Fabric Diaper Dryness and Breathability
Diapers are made of various materials, including a hydrophilic top sheet, standing leak guards, an absorbent core, hot-melt adhesives, a breathable film, and spandex, as shown in Figure 1.
From the cross-sectional structure of a diaper, heat and moisture generated during use are transferred from the hydrophilic top sheet in contact with the skin through the acquisition layer, the absorbent core, and finally dissipated through the breathable film and nonwoven backsheet to the environment. This process involves a series of sequential conduction pathways, where every material layer influences the transmission of heat and moisture.
The hydrophilic top sheet, acquisition layer, and absorbent core have the greatest impact on dryness, while the absorbent core and breathable film primarily determine breathability.
Typically, diapers with an acquisition layer offer better dryness. However, many mid-to-low-end baby diapers on the market have eliminated the acquisition layer. In such cases, the top sheet directly transfers urine to the absorbent core, and the functions of directional flow and liquid diffusion, previously performed by the acquisition layer, are shared between the top sheet and absorbent core. This design places higher demands on the material and design of both the top sheet and the core.
To improve breathability, there is a trend toward thinner absorbent cores. However, thinner is not always better. Absorbent cores composed solely of SAP (superabsorbent polymer) tend to develop gel blockages as SAP swells upon absorbing liquid, hindering further liquid penetration. Currently, a mix of SAP and fluff pulp fibers is widely used. Fluff pulp fibers dispersed among SAP particles enable continued rapid liquid diffusion and penetration, ensuring a dry top sheet.
The breathable film beneath the absorbent core acts as a barrier against liquid leakage while allowing air and moisture to pass through. Given its low weight and mechanical fragility, it is often laminated with a layer of nonwoven fabric for reinforcement.
The breathable film’s primary function is to block liquid while allowing heat and moisture to escape through its micropores. However, because the relationship between barrier performance and breathability is inversely proportional, a high barrier can compromise breathability, leading to discomfort.
Strategies to Improve Nonwoven Fabric Diaper Dryness and Breathability
(1) Hydrophilic Top Sheet
Until 2014, the main material for hydrophilic top sheets was PP spunbond/thermal-bonded nonwoven fabric. As of 2024, PE/PET short-fiber carded thermal-bonded nonwoven fabric has become the dominant choice due to its soft texture and superior loftiness compared to spunbond nonwovens.
To enhance the dryness of top sheet materials:
First, Optimize Hydrophilic Agents: Use hydrophilic agents with good multi-cycle hydrophilicity to ensure quick liquid penetration, even after repeated soiling.
Second, Control Application Levels: Excessive application of hydrophilic agents improves penetration but increases rewetting, reducing dryness. Insufficient application results in slower penetration and potential water repellency. Balancing hydrophilic agent levels is essential.
Structural modifications like embossing or perforation can enhance the material’s three-dimensionality, reducing skin contact area, increasing instantaneous liquid contact area, and speeding up liquid penetration.
(2) Acquisition Layer
Situated between the hydrophilic top sheet and the absorbent core, the acquisition layer guides and diffuses liquid to prevent uneven absorption and core bunching. Common materials include coarse-denier thermal-bonded nonwovens, chemically bonded nonwovens, and coarse-denier PP spunbond/thermal-bonded nonwovens.
Advanced design and chemical treatments, such as using layered acquisition layers with varying capillary sizes or hydrophilicity, can further enhance directional liquid flow and reduce rewetting.
(3) Absorbent Core
The absorbent core is the heaviest component of a diaper and is responsible for rapid liquid absorption, retention, and locking. Traditional cores consist of SAP and fluff pulp wrapped in hydrophilic materials like SMS nonwoven fabric.
- To improve performance:
- Use thinner SMS layers with reduced meltblown (M) content to expedite liquid transfer.
- Add small amounts of fluff pulp to balance SAP distribution and avoid gel blockages.
- Employ emerging technologies like MPM (Meltblown/Pulp/Meltblown) to create hybrid cores that combine superior absorption, locking performance, and thinness.
(4) Breathable Film
Breathable films in diapers are typically PE microporous films containing calcium carbonate (CaCO3), with pore sizes between 2–3 µm. These films allow moisture vapor to pass while blocking liquid.
The most straightforward way to enhance breathability is to reduce film weight. Alternative approaches, such as using high-static-pressure SMS materials, have faced challenges due to liquid leakage under pressure. Developing alternative microporous membranes, such as ultra-light PTFE films, could offer a promising solution.
Emerging Technologies and Materials for Improved Dryness and Breathability
A diaper’s perceived dryness depends on the top sheet, but achieving optimal performance requires the top sheet, acquisition layer, and absorbent core to work in harmony. The ideal diaper should combine lightness and breathability, allowing liquid to penetrate while maintaining a dry top sheet.
Directional flow and diffusion technologies, advanced core designs, and precise SAP-pulp ratios have significantly improved performance in premium diaper products. For breathable films, while SMS replacements for PE films remain commercially impractical, exploring innovative microporous materials like PTFE shows potential for breakthroughs in breathability.
By adopting new materials and techniques, the hygiene industry continues to make strides in delivering products with superior comfort and performance.