The Benefits and Challenges of Manufacturing the Pre-Compound and In-line Compound Core

For years, most absorbent cores in disposable hygiene products were a combination of fluff and SAP (super-absorbent polymer). But more recently, companies have re-imagined the core in a variety of ways to address weaknesses in core integrity. One problem: core cracking. This split in the core matrix allowed liquid to leak through. Cracked cores could also clump and become uncomfortable for the wearer.

Manufacturers in China created an entirely new type of baby diaper core called the pre-compound core, or 复合芯体 in Chinese. Instead of fluff, the SAP is sprinkled on layers of a high-loft nonwoven, commonly PET (polyethylene terephthalate). This structure ensures the core would not crack, but it presented other difficulties.

Offline production: the pre-compound core

Early versions of the pre-compound core tended to lose SAP. Without fluff to hold it in place, SAP could shake loose during production. It could also be challenging to spray the SAP accurately at higher speeds. For these and other reasons, production lines for the new core had to run much slower than the line speeds typically used for absorbent hygiene products. This led to the cores being produced offline. The finished cores could then be shipped and incorporated into the product as it was constructed.

It was not uncommon for pre-compound cores to be produced by a separate company and sold to the diaper manufacturer. The pre-assembled cores could easily be slotted into existing space in manufacturing lines and added as one might any substrate. This allowed for easy adoption by a number of manufacturers in rapid succession.

We have seen manufacturers retain the equipment for the traditional SAP/fluff core in their lines. The benefit here is the ability to swap between the two core types as their production needs require.

Addressing cost and comfort

Being fluff-free, pre-compound cores cannot rely on fluff fibres to pull in liquid and help with absorption. Instead, the amount of SAP is increased—which adds cost. This can make pre-compound cores more expensive to produce, though they are thinner than fluff cores. Many consumers see ultra-thin as a positive selling point, leading some manufacturers to reduce the thickness even further.

While pre-compound cores have an increased chance of SAP touching the outer layer, adhesive can help immobilise the SAP for better performance. Conversely, too much contact with the adhesive can hinder the SAP’s ability to absorb. Care must be taken to find the correct balance.

Increased absorbency and wicking

To assist with absorption, an outer layer can be added. However, this outer layer should not be confused with the traditional ADL (acquisition distribution layer). Instead, the outer layer of the pre-compound core should be more hydrophilic material than typical in the ADL. This ‘absorption prep layer’ allows it to absorb more liquid and hold it for absorption by the inner core. In early pre-compound versions, this material was made of fluff heated and compressed with ES fibres.  ES fibres are composed of polypropylene (PP) inside and polyethylene (PE) outside, like a wrapped wire structure. This serves to position and connect the fluff, due to the PE and PP respectively. Unfortunately, the result was not soft and could be expensive as well.

To minimise these issues, other substrate options were explored. One that we have seen is a spun lace viscose material. The exact nonwoven chosen will impact both the core’s softness and its ability to absorb and wick away moisture. Adhesive selection, add-on, and application methods can also affect these attributes positively or negatively. Expert advice is recommended.

Improving line speeds and the compound core

With time, manufacturers found methods to increase production line speeds for the pre-compound core. This eventually led to the core being constructed in-line with the rest of the diaper. When produced in-line, they are called compound cores. In-line production also allowed for thinner cores, which was a selling point to consumers. Another benefit: They could be produced more economically.

However, there are some drawbacks. Shifting to the compound core may require an initial investment for line reconfiguration. (In-line production typically requires additional space.) Storage for the component materials is another factor to consider. Lastly, it can be challenging to spray the SAP accurately under a higher line speed.

To date, the pre-compound and in-line compound cores have been used predominantly in baby care products. Period products more commonly use a compressed fluff/SAP core. For adult incontinence articles, the larger size needed tends to make the pre-compound/compound core too expensive—especially for institutional use.

If you are considering switching to pre-compound or compound cores for your products, talk to us.

CODE: CO-22A22