SAP and fluff pulp cellulose in ultra-thin diapers

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SAP and fluff pulp cellulose in ultra-thin diapers

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Changes in diapers

Before using SAP in the 1980s, the single diaper quality was more than 100 grams, of which cellulose fibers (wood pulp, fluff pulp) accounted for 60-80 grams. It was a thick and heavy big guy. The advantage was that the child wrestled his butt If it doesn’t hurt, there are many shortcomings, such as urine leakage, red buttocks, high prices and so on. Since the beginning of using polymer water-absorbing resin (SAP), the amount of fluff pulp fiber has been reduced by multiples. Now the mass of fluff pulp in the corresponding products is less than 20 grams, so the absorbent core is getting thinner and its performance is getting better and better .
This trend brings comfort to consumers, efficiency to retailers and supply chains, and opportunities and challenges for product manufacturers and raw material suppliers. Fully understanding the technical principles in between will allow us to design products that are more competitive in the market and lead the market.

Therefore, this article attempts to make a summary of many factors that affect the product performance of the core’s absorption capacity, liquid absorption rate, core utilization, and re-infiltration under pressure.

One. Urine suction volume requirement

The design of large-size products generally absorbs 400-500 ml of urine. Before using SAP, people need to use more than 60 grams of cellulose fibers (fluff pulp) to achieve this absorption capacity (the liquid absorption capacity of the fluff pulp is about 8 g/g in the free absorption state, but the centrifugal water retention rate is only about 2 g /g). After the introduction of SAP, designers have gradually increased the amount of SAP (from 3g to 12-13g per tablet) due to the ability of SAP to absorb liquid (saline or urine) (centrifugal water retention rate) above 30 g/g. ), and gradually reduce the amount of fluff pulp, while meeting the design of liquid absorption capacity requirements. The current ultra-thin products mostly control the total weight of the absorbent core at 25-28 g (the ratio of SAP and fluff pulp is 10/15 or 13/15), on the one hand, it guarantees the liquid absorption capacity and prevents leakage, on the other hand, it is effective Control costs.

The complete replacement of cellulose fluff pulp with SAP is the direction that many companies have been working on in recent years, but it is also very difficult. Why can’t you use SAP alone? There is a very important physical phenomenon in the process of SAP liquid absorption, that is, “gel blocking” phenomenon. “Gel clogging” refers to the water-absorbing structure that when the SAP particles are wetted and swelled by the absorbed liquid, due to the softness of the hydrogel and the pressure of the product in the use environment, it deforms and conforms to each other, inhibiting the further transmission of liquid to other parts of the product .

After the SAP particles absorb water and swell, there is not enough gel strength to maintain the liquid diffusion capacity of the absorbent core. These swollen water bags require a hydrophilic fiber environment, that is, the use of fluff pulp or hydrophilic synthetic fibers (ES, PET fiber Etc.) Let SAP particles not stick together as much as possible, but to maintain a certain void structure. In addition to having a certain water absorption capacity, these fibers are also very good liquid conduction media. Their fibers and surface structure constitute a capillary structure for diversion, which is very conducive to the diffusion of liquid in the horizontal direction or even the vertical direction (counter gravity direction).
SAP particles have a capillary structure if they are not completely pressed together, but the orientation of the relative fibers is much worse than that. In the early days, the amount of SAP in the whole core was very low, mainly due to the limitation of low gel strength. The new generation of SAP has a higher gel strength after expansion through the optimization of the chemical cross-linking degree and the secondary surface cross-linking technology. Even after expansion, it can maintain a certain capillary structure, helping the liquid in the second and third times. Evacuation and absorption after reaching the product. With the flow and diffusion of liquid, it is only possible for a long absorbent core to deviate from the liquid entry point at both ends to participate in the absorption of urine and contribute to the product capacity. Even so, their capacity contribution is discounted. When we observe the diaper products after use, we will find that the liquid distribution is mainly in the center of the product, which is very uncomfortable for the baby to wear. Good product design is to distribute the liquid as evenly as possible and improve the utilization of the entire core.

two. Urinary absorption rate requirements

Product absorption capacity is not only a capacity issue, but also a matching problem of absorption speed. A core structure parameter very related to the absorption rate is called porosity (also called void volume fraction). Porosity refers to the volume fraction of the core structure (which can be the original state before use or the state after water absorption and expansion) that has not been occupied by solid (aqueous gel) material and is available for liquid flow. Another structural parameter that affects the absorption rate is called permeability, which is a parameter that characterizes the size of the voids in the structure and the connectivity between the voids. The more voids in a structure, the smoother the connection, the higher its permeability and the faster the liquid absorption rate.

Generally, the initial structure density of the mixed core structure product is about 0.3 g/cc, and the void ratio is close to 80%, that is, 80% of the volume in the structure can be occupied by liquid. When the liquid is injected, the void space near the injection point is quickly filled, and the liquid that is followed up at the injection point must wait for these liquids to be emptied before being accepted by the product without side leakage. There are two mechanisms for emptying the liquid, one is to continue to diffuse along the connected void space to the peripheral structure (mechanism A), and the other is to be absorbed by the water-absorbing particles into the particle body and become a hydrogel (mechanism B). These two mechanisms are also competing with each other. In an ideal state, we always hope that mechanism A has the upper hand, so that the liquid has more opportunities to diffuse to both ends of the product, but mechanism B should not be too slow, otherwise it will spread to the core boundary. Liquid cannot be absorbed in time and side leakage may occur.

Normal diapers will receive 2-4 times of urine from the baby during the time of use. How to maintain the highest possible permeability in a partially liquid-swelling core structure is of utmost importance. Different SAPs have different water absorption speeds. Some have fast initial speeds, and further water absorption speeds become slower after water absorption; some SAPs have slow initial speeds, and further water absorption speeds gradually increase after water absorption. Every time the product accepts the baby’s urination, if the mechanism B prevails, the liquid will collect in the local area and it will be difficult to spread. The soft gel particles will cause the gel to block under ordinary pressure and hinder the flow of the liquid. Of course, the higher the SAP gel strength, even after swelling, it can still maintain a certain structural porosity between the gel particles, which helps the capillary flow of the liquid.

Another component in diapers that helps fluid flow and diffusion is the deflector (called ADL or Surge in English literature). This is a kind of fiber structure with rapid inhalation, diffusion, drainage and release of liquid in addition to the core. Its functional purpose is to quickly incorporate and transport liquid from the product entry point to the front and rear body of the product as far as possible and release For core absorption. The deflector layer was invented after SAP entered the product to make up for the difficulty of liquid diffusion. With a good flow guide layer, the liquid absorption speed of the product will be greatly improved, and because the utilization rate of the entire product is increased, the total absorption capacity of the core design can be reduced. In a composite core with no fluff pulp at all, the requirements for liquid diffusion are entirely borne by the synthetic fiber network, and its dosage and cost are not low, but the advantages are a thinner core and a stable structure under wet conditions.

Third, the requirements of liquid re-infiltration

The dry surface of the product can keep the baby’s butt dry. It is scientifically proven that this is the best way to ensure the health of the baby’s skin and prevent diaper rash, so this is one of the most important considerations in product design. The dryness of the product first requires the core to have sufficient liquid absorption capacity. Even when the product contains more urine, the liquid can be locked under the weight and pressure of the child’s body outside, so that the liquid does not return and the moisture does not remain in the product. Surface layer. Early pure fluff pulp diaper products are difficult to achieve a dry surface, because we mentioned earlier that the centrifugal water absorption of cellulose fibers is only 2g/g, and most urine is stored in the gaps between the fibers. Once pressure is applied, this moisture will diffuse out of the core and remain on the product surface. After using SAP, this situation has been greatly improved, because the urine inhaled into the SAP hydrogel particles is actually locked and will not flow backwards under pressure. Of course, this does not guarantee that there will be no moisture between the voids of the particles or fibers, and today the requirements for product re-infiltration are much higher than they were 20 years ago. Not only moisture, but also moisture should be removed from the product surface as much as possible. This requires that our cores do not have local liquid saturation, that is, urine is distributed as evenly as possible in the product, and there is no liquid collection saturation or gel blocking area at the intermediate liquid injection point.

Liquid seepage is the overall performance of the product, so all components in the product must cooperate to achieve the design purpose. SAP must have high CRC and AUL for urine. There must be no performance degradation and dissolution during the period of use (overnight is 12 hours). The rate of water absorption and product diversion must match. The surface layer of the product is in direct contact with the baby’s skin, so good liquid permeability and the dryness of the fiber surface are crucial, which is related to the type of fiber and the choice of surface hydrophilic oil. The guide layer not only helps the liquid distribution in the product to be more reasonable, but also through the choice of fiber type and size (structural density, ratio, capillary connectivity), constructs favorable structural characteristics, and guides the liquid and moisture to the product core in one direction. More and more high-end products use multi-layer composite fiber materials as surface layer or deflector layer structure to build a favorable capillary structure to lead the flow, diffusion, absorption, and reverse osmosis of liquid in the product.


Fourth, SAP surface treatment improves the comprehensive performance of the core
The new generation of SAP can better control the initial absorption speed of the SAP particles and the gel strength after swelling through surface cross-linking and hydrophilicity treatment, so as to achieve a better balance of liquid absorption and diffusion in the core. Further increase the liquid absorption speed of the core, reduce surface re-infiltration. The following experiment is based on two SAPs of a company, one without surface cross-linking and the other with surface treatment. The other core settings and test conditions are completely the same. The multiple liquid absorption time and resorption data are shown in the table below.

SAP 1 (without surface treatment) SAP 2 (with surface treatment)
CRC (g/g) 37.9 34.5
AUL (g/g @ 0.3 psi) 12.5 31.2
AUL (g/g @ 0.9 psi) 9.8 17.6
Aspiration time (first time, seconds) 303 91
Aspiration time (second time, seconds) 397 106
Aspiration time (third time, seconds) 454 129
Aspiration time (fourth time, seconds) 629 142
Liquid back osmosis (first time, grams) 0.1 0.1
Liquid oozing (second time, grams) 0.9 0.3
Liquid oozing (third time, grams) 7.4 3.5
Liquid back osmosis (fourth time, grams) 18.2 4.7


Here, I want to show how inappropriate it is if we look at SAP water absorption capacity (CRC) alone. SAP on the market today has a variety of specifications, suitable for different core structure design. SAP manufacturers and users must have a deeper communication and understanding of product design before they can choose the most suitable and ideal raw material combination and ratio to meet product performance requirements.

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