Do You Know the Secret of Disposable Diapers?

From birth until the baby learns to take care of himself, diapers have always been an indispensable daily necessities. In the current diaper market, disposable diapers occupy a considerable market share. Compared with traditional reusable cloth diapers, disposable diapers do not require cleaning and are easy to use, so they are widely popular in young parents. But do you really know the secret about disposable diapers?

Disposable diapers have been around since the 1940s. They have developed better and better performance today. In fact, this is mainly due to a unique compound - sodium polyacrylate.

1. What is sodium polyacrylate?

Sodium polyacrylate is a kind of polymer prepared from sodium acrylate. Sodium acrylate looks just like table salt (sodium chloride) and baking soda (sodium bicarbonate) commonly found in the kitchen. They are all white powders, and in fact they also belong to the same ionic compound, which is what we usually call salt. But sodium acrylate is unique in that it has a less stable structure in its molecule - a carbon-carbon double bond. So sodium acrylate molecules always want to get rid of this carbon-carbon double bond to make themselves more stable, so what is the best way?

Just as different people can pull hands with each other to form a long queue, under appropriate conditions, hundreds of sodium acrylate molecules can react with each other to obtain a very long molecule, which is sodium polyacrylate.

However, the sodium polyacrylate obtained in this way cannot be directly used in the production of disposable mats like dry diaper overnight, because there is still a lack of a key process - cross-linking.

As a company dealing with bulk sanitary disposables, we have just seen that the sodium polyacrylate obtained by the reaction of sodium acrylate with each other is a linear molecule. If we add a cross-linking agent to the reaction, the two ends of these molecules can each react with a sodium polyacrylate molecule, then the two sodium polyacrylate molecules are linked together.

If there are enough cross-linking agents, many sodium polyacrylate molecules will be connected to each other to get a network structure, just like many yarns are woven into sweaters. We call this process cross-linking. We will see later why cross-linking is such a critical process.

2. The role of sodium polyacrylate in disposable nappies

So how does sodium polyacrylate play a key role in disposable diapers including disposable adult diaper liners? Let's do a simple experiment: add a teaspoon of powdered sodium polyacrylate to a cup, then add water and stir. It didn't take long before we were surprised to find that the sodium polyacrylate powder and the water in the cup were gone, replaced by a solid jelly-like mass. If the cup is turned upside down, not only will no water flow down, but the whole jelly-like solid will stick firmly to the bottom of the cup and will not move.

It turns out that the role of sodium polyacrylate in disposable diapers is to firmly absorb the moisture in the urine so that the baby's skin and clothes can be kept clean and dry. In the current production process of newborn disposable nappies, sodium polyacrylate is generally mixed with fibrous substances such as wood pulp to form a sandwich. The function of wood pulp is to help sodium polyacrylate maintain a fixed shape.

The interlayer containing sodium polyacrylate is then wrapped with a suitable fabric. The side facing the baby's skin is usually made of porous fabric, while the other side of the fabric is made of impermeable material. In this way, after the baby's urine flows onto the diaper, it will penetrate through the holes in the fabric and then be absorbed by the sodium polyacrylate.