Core issues for diapers: Thin is in

The introduction of SAPs set the hygienic disposables industry on a course of being able to gradually reduce the weight of its products, since a gram of SAP could replace four or more grams of fluff pulp.Indeed, Life Cycle Analysis (LCA) studies by industry body EDANA have shown that the weight of an average baby diaper has subsequently been reduced by nearly 50% – from 65g in 1985 to just 33g in 2011.

Meanwhile, the superabsorbent polymer industry has grown to become a market worth $7.2 billion in 2014, with the 2.2 million tonnes produced last year used almost entirely in diapers, adult incontinence and other related products.

Trade-off

Initially, however, there was quite a trade-off between the desired capacity and the absorbency under-load of such products. As a result, the first of these new absorbent cores contained just five grammes of SAP to 40g of fluff pulp.
A big improvement was made at the beginning of the 1990’s with the introduction of core-shell structured SAP particles, allowing the SAP content to be increased to around 50% of the absorbent core, with a 25-30g core containing between 10-14g of SAP.

Diaper production machines however, put such products under extreme pressure, with the result that the shells of the SAP particles in the early heavily-loaded cores would crack – with a disastrous effect on the diaper’s performance.

But the industry has continued to increase its call for a greater proportion of SAP to be included in products, and such obstacles have been overcome – to the extent that some products are now even available with an absence of fluff altogether. At the same time, the SAP is now required to not only absorb and retain fluid, but also to transport it after swelling.

Many regular exhibitors at INDEX – the leading nonwovens show which next takes place at Palexpo from April 4th-7th, 2017 – have designed and patented material systems and components for fluffless, or near-fluffless cores.

New structures

At the 2014 Insight Conference held in Indianapolis, USA, Barbara Bulleri, Sales and Marketing Director for Italy’s Texsus – a manufacturer of highly-engineered acquisition and distribution layers (ADLs) for hygiene products – detailed a number the core structures that have recently been introduced to the market.

They include:

• Procter and Gamble’s DryMax, which is produced by SAP being deposited from reservoirs in a printing roll onto a substrate on a grid. Thermoplastic adhesive is used to form a fibrous network applied to the SAP to immobilize it in both dry and wet states. The diaper employs a combination of a chemically-bonded ADL and curled cellulose fibres as its acquisition and distribution system.
• Drylock, by Drylock Technologies, which comprises a carrier layer, an auxiliary layer and an absorbent material between them. Layers are bonded together ultrasonically, creating permanent primary attachments and temporary secondary attachments. During absorption, SAP swelling opens some temporary secondary attachments, creating new void spaces in which to store additional liquid. Application in the diaper includes a 140-150 gsm airthrough-bonded ADL layer.
• Fameccanica’s Hexacore – a layered structure comprising two elements connected to one another by hot melt adhesives. A honeycomb conformation optimises the ratio between the area of the cells compared to the total area. During absorption, SAP swelling opens some weak bonds to form channels and create void space for further swelling and distribution of fluid. Application in the diaper includes a high grammage airthrough-bonded ADL.
• Helixbond by C4S and Hermann, which is manufactured by ultrasonic SAP fixation, with no fluff, glue or curly fibres. The helical anvil design on the ultrasonic unit ensures a minimal contact area per bond point and SAP granules are displaced from the welding area continuously and embedded in the ADL, resulting in a material which offers both good wet and dry core integrity.
• A core, made jointly by BASF and Bostik, in which no pulp is employed and zones of increased absorbency hold higher amounts of SAP in place.

Pre-made cores

Both Texsus and Glatfelter are now manufacturing pre-made cores with low fluff pulp content.

The Texsus Absortex Core is made of a special compound of SAP particles and additional fillers fixed between two carrier layers of tissue or nonwovens. The core contains no adhesive and is secured by a hydrogen bonding technology. Its main advantages are a high SAP concentration – a 1mm-thick core includes 700 grammes of SAP per square metre of material – no gel blocking, high core flexibility and softness. Absortex is laminated with an airthrough bonded ADL.

E-Core by Glatfelter features an intermediate storage layer to take care of rapid liquid intake, distribution and intermediate containment before it is taken up by an absorbent layer comprising over 80% SAP. This is contained within a sandwich of cellulose nonwovens.

In conventional absorbent cores, Barbara Bulleri notes, the ADL layer needs to absorb and transport liquid efficiently to move it quickly into the absorbent core, distribute it evenly and provide an additional barrier between the stored liquid and the skin. In thinner diapers it is necessary to increase the weight of the ADL to ensure the core functions properly.

While this will provide a better acquisition time, it can also lead to less effective rewet, which is a problem that Texsus has solved with its Acquitex ADL – characterised by a multi-layered asymmetric structure designed with different upper and lower sides. The gradient structure has resilient and bulky fibres in the upper layer and capillary, thinner fibres in the bottom layer.

Integrity

Consideration of the adhesive solution in the early stages of the design can also make a significant difference to the size, performance and competitiveness of such diaper cores.

“In modern baby diapers the core is wrapped in nonwovens, or sometimes tissue, and this is where the adhesive is applied to fix the core,” says Dr Stefan Eller, Technical Service Manager for Nonwovens Hygiene at H.B. Fuller.  “In many diapers, the core is enclosed by overlaps at the sides, which are laminated together, or manufacturers use the ‘C-fold’ core wrap where the nonwoven is wrapped around the entire core. Most adult incontinence diapers have a more simple structure, with the fluff core placed directly between the top-sheet and back-sheet substrate, with the adhesive solution fixing the structure together. Whatever the design, the core fixation adhesive is fundamental.”

“The ongoing movement towards thinner disposable hygiene products means producers must take a completely different approach to core design,” adds Diane Toonen, Global Director of Strategic Marketing at Bostik. “As core fluff is decreased or eliminated, the role of the materials, including adhesives, becomes more essential to core integrity. The selection of the right materials is paramount to achieving a custom design with the specific desired performance.

“An approach that takes 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 use of adhesive in a thin or fluff-free core enables diaper producers to thoroughly customise their core to provide differentiated performance either in terms of absorption or cost.”

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