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Absorbent cores in baby care, adult incontinence, and certain period care articles share many of the same needs, from efficiently absorbing the insult to protecting against odor, leaks, and rewet. Others have specific requirements for ensuring function and a positive user experience.

The absorbent core is a primary area of focus in the hygiene industry today. From baby diapers to the products created for adult incontinence and period care, many of the design and performance concerns overlap. Make it thinner. Make it more absorbent. Give it channels. Keep it from cracking, because leaks are bad for business. 

At the same time, the list of consumer expectations of products and their producers continues to grow. As a result, manufacturers are researching and experimenting with new core designs to stay competitive. Understanding the key functions of the core system is the first step in meeting user needs and exceeding expectations.

The Complex Duties of an Absorbent Hygiene Core

Growing Complexity Helps Meet Consumer Needs

While there are many types of diapers and pads on the market, they have the same central purpose of absorbing and containing insults. Over the years, other qualities have become expected. Today, a top performing core should:

  • Attract and absorb liquid away from skin
  • Disperse fluid quickly throughout the product
  • Prevent the feeling of a wet sensation after insult
  • Promote skin health by avoiding rewet
  • Hold up to the stresses of daily life without cracking and leaking
  • Ensure comfort by not shifting, bunching, or sagging during use
  • Offer protection against noticeable odors
  • When appropriate, provide even the most self-conscious adult users with the discretion they desire

How hygiene products accomplish these tasks varies by core type … in many cases supported by absorbent hygiene adhesives.

Traditional Fluff/SAP Cores for Baby and Adult Diapers

The Drive Toward Thinner Products

A mainstay of diapers and pads since superabsorbent polymer (SAP) was first introduced, the traditional core as we know it today appears in products for baby care, adult incontinence, and certain articles for period care. They typically consist of a topsheet, acquisition distribution layer (ADL), backsheet, and core wrap, enclosing a matrix of fluff pulp and SAP.

Over the decades, there has been a shift toward higher SAP-to-fluff ratios. We have seen traditional cores ranging from 30% to 70% SAP. As the ratios rise, the extent to which the fluff supports various core functions becomes clear. When less fluff is present, other components are required to take up aspects of its previous role.

  • In cores with up to 50% SAP. Fluff fibers entangle to create a pad that keeps the SAP in place, supporting core integrity. To further improve its reliability, a core wrap can be used. This material surrounds the fluff/SAP matrix and is sealed with a core adhesive. 
  • At 70% SAP/30% fluff. A core wrap and a core adhesive are both needed to prevent the loss of SAP powder. But there is another issue: The loose SAP within it needs further support. Using Bostik’s Conditioned Core Integrity Test, we have seen that a core at 70% SAP will immediately fail if a core integrity adhesive is not used. 

Best Practices for Adhesive Application

All components of the core add value, and adhesives are no exception. Knowing how and where to use the right adhesives can impact overall core integrity and performance. Particular points to consider when building a traditional core include:

  • Adhesive placement. Hot melts applied to the top of the core can slow down liquid absorption into the matrix. Applying adhesive to the bottom of the core instead can improve durability and enhance absorption.  
  • Add-on level. Different amounts of adhesive may be required based on application method and substrates. The function of the hot melt in the product and product expectations must also be considered. Testing can be invaluable, though the specific protocols used may impact results.  

Contemporary Fluff-Free Core Designs

Compensating for the Loss of Fluff

To create ever-thinner cores, some manufacturers have opted to remove fluff entirely. Doing so can also simplify supply chains and create sourcing efficiencies. However, this requires all tasks usually performed by the fluff to be met by other means. Examples include: 

  • SAP stabilization in use. When the polymer shifts out of position during normal wear, it can cause uneven areas of absorption. In certain core designs, this can lead to core cracking and leaks. There is also a greater risk of uncomfortable clumps of SAP which increase in size when the shifted powder swells after insult.
  • Absorption and wicking. To ensure proper functioning, the adhesives used should support—or at least not interfere with—the diaper’s ability to bring fluid into the core. They should also allow the SAP to expand and fully absorb the insult, up to its intended capacity.
  • In addition to these primary needs, newer core designs have their own specific requirements and expectations.

Compound and Pre-compound Cores

One configuration that is gaining popularity, especially in Asia, addresses the need for core integrity and discretion in a different way. Called pre-compound or compound cores (depending on whether they are constructed offline or inline), they consist of SAP within layers of high-loft synthetic nonwoven fabric. This fluffy material in part replaces the task of fluff in wicking the insult to and throughout the absorbent core.

While core cracking is not an issue with compound and pre-compound cores, there is a different challenge. They cannot be easily formed at speeds typically used for the diaper manufacturing process. Even when produced at lower line speeds, they require:

  • SAP stabilization in construction. A special core adhesive helps to prevent loss of polymer powder during core forming and shipping. This is particularly true in the case of the pre-compound version, which can suffer additional loss or displacement in transit to the production line.


Channel Cores

Unlike a more traditional configuration, the channel core has longitudinal zones (channels) free of any absorbent material. These zones are created by bonding the upper and lower core wrap materials together with adhesive. The absorbent materials are contained in the tubular areas formed by these channels, and may contain SAP alone, or SAP with fluff.

Because these cores have such a distinctive structure, they require a channel adhesive to support:

  • Channel integrity. In some designs, the channels are intended to remain intact after insult. In others they are not. Each requires that the channel adhesive has the appropriate peel and other attributes to perform as intended, especially when subjected to the added pressure of the swelling SAP after insult.

Bostik: Supporting Your Absorbent Hygiene Core

Ask the Adhesive Experts

No one adhesive works in every core application or design. That’s why choosing the right hot melt for each intended use is key to delivering optimal performance and core integrity to your consumers. As your company looks to enhance or advance the core systems in your absorbent hygiene products, select an adhesive partner who can help you achieve your product’s performance goals. Bostik is here to help.


©2024 Bostik, Inc. All Rights Reserved

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