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Factors affecting rewet performance of absorbent cores.

A study on determining the factors that influence absorbent pads; results show that superabsorbent location and type, pad density and fluff all play a role in the rewet performance of the product

The ability of a disposable absorbent product, such as a baby diaper, to keep the skin of the wearer dry during use is an important performance attribute. This property is commonly referred to as "rewet" or "strikeback" and has been the subject of several different testing protocols throughout the absorbent products industry[1,2]. All of these methods attempt to measure the ability of an absorbent pad or system to keep the surface dry when placed under a load after a given amount of liquid has "insulted" the pad. This type of test supposedly simulates the situation of an infant (or adult in the case of an incontinence product) sitting on a partially wet diaper or brief. While the reality of such a test can be legitimately questioned, the success of the disposables producer often depends on passing such screening test, regardless of the pertinence of it to the real world situation.

The present work is a systematic study of several factors believed to significantly influence rewet of an absorbent core system. There are many possible factors in a composite absorbent structure that can affect the measured rewet value[3]. The factors investigated for this study were those that appeared to have the greatest effect and were also easily varied. Obviously, the particular testing protocol also has a large effect on the apparent rewet performance. In the present study, the testing procedure[4] was standardized by using a synthetic urine solution as the test fluid and subjecting the pad to a 6.89 kPa (1.0 psi) load to simulate the effect of an infant's boody weight.

Although the nonwoven coverstock plays a significant role in determining rewet performance, the purpose of this study was to examine parameters of the absorbent core only. A master roll of non-woven material - Scott Nonwovens product #673 thermally-bonded poly-propylene nonwoven - was used throughout the experiments to keep this variable constant.This fabric is made with Hercules T-181 polyprolylene staple fiber at a basis weight of 20-21 gsm and has been used extensively in diaper production in the U.S. The layering convention used for all experimentation, which is intended to match the construction of a typical disposable diaper has the nonwoven layer always placed with the embossed side up (away from the pad). Tissue layers can also affect rewet and were included (16.3 gsm) since they are typically an integral part of current disposable diaper constructions.

Rewet was measured using a method developed by Weyerhaeuser by pouring an 80-gram amount of synthetic urine solution onto the test specimen via a burette and allowing it to absorb to 10 minutes (no surfactant was used to lower the surface tension of the liquid). At that time, 10 preweighed circular filter papers (VWR617, 110.0 cm circles) are placed over the point of fluid entry and a one psi weight (5 kg; 9.53 cm diameter) is applied for two minutes. when this time has elapsed, the filter papers are removed and weighed again and the net in grams is reported as the rewet value. Other rewet test used a similar in principle. but can differ in the type and amount of fluid used, time constants, applied load and type and number of filter papers.

Effect Of Superabsorbent Location

Intuitively, the placement of superabsorbent polymer (SAP) particles within an absorbent pad should have a large effect on rewet performance. Four different configurations of loose superabsorbent in a fiber pad were considered: homogeneously distributed and as a discrete layer either on the top, bottom, or in the middle of the pad. The results of these experiments are shown in Figure 1.

Rewet in grams of fluid is given for each of the four placements of superabsorbent. Each column in the bar graph represents the average of four-six replicate tests for each placement. The data shown was derived with pads made from bleached kraft Southern pine fluff pulp and a common starch-grafted superabsorbent polymer at a total basis weight of 710 gsm. Superabsobent content was held constant at 15% by weight of the pad. The pads were densified to a constant value of 0.12 g/cc just prior to rewet testing.

The results indicate that placing superabsorbent particles on top of the pad (closest to the coverstock) yields the best rewet results. Placing superabsorbent at the bottom was the worst configuration and a homogeneous distribution or a layer in the middle of the pad produced intermediate and roughly equivalent results. Given the excellent liquid retention under pressure properties of most superabsorbents as compared to pulp fiber alone, the results are not unexpected. When superabsorbent is located at the bottom of the pad, a significant layer of fluff alone is directly below the coverstock material. After liquid insult, when pressure is applied this fluff layer readily gives up absorbed liquid, much like a sponge. This results in high rewet. In contrast, when a superabsorbent layer is located next to the coverstock, it retains much more of the absorbed liquid under load. The result is significantly lower rewet values.

Effect Of Superabsorbent Polymer Type

The various commercial superabsorbent materials available today represent a range of chemistry and processes, although almost all are based on partially neutralized polycrylic acid as the main functional group. The degree of neutralization varies, but typically is in the range of 60-80% as a rough guide. Caustic soda is most commonly used, resulting in these materials being predominantly sodium polycrylates. These materials can be grouped into two main categories: pure synthetics and starch-grafted compounds. Several different commercial materials were evaluated under standardized conditions in model pads. Most were granular materials with somewhat varying particle size distributions, but two fibrous superabsorbents were also tested. Both fibrous superabsorbent materials tested had significantly different chemistry, both from each other and the granular materials.

For these experiments a middle layer superabsorbent location was chosen for two reasons:

1. This distribution was consistently the easiest to make on the laboratory pad formers.

2. Results from the location experiments indicated that a layered distribution was the most reproducible in terms of rewet values and a middle layer produced values in the middle for the range of the rewet test.

Table 1 list that various superabsorbents used in model pads and the measured rewet values for each. Specific manufacturers or product names are not given in the interest of maintaining scientific objectivity.
 Table 1
Sample Rewet (g) No. Pads
Granular SAP:
SG 1 6.9 6
SG 2 5.5 8
SG 3 1.4 8
SG 4 1.4 16
SG 5 12.1(*) 6
S 1 3.7 7
S 2 8.6 8
S 3 3.0 8
S 4 10.0 5
Fibrous SAP
 (homogenous distribution)
F 1 23.2 3
F 2 25.2 3
(*) excessively small particle size

As can be seen from the table, a wide range of rewet performance is obtainable, varying from a low of 1.4 grams to a high of 25.2. Two of the starch-grafted products are the best performers. The highest rewet values exhibited were those from the fibrous superabsorbent materials. The results for SG5 are suspect due to the fact that this materials exhibited a much smaller average particle size than the other granular materials. Because of this, it was difficult to assure that the particles remained in a middle layer in the pad. Shifting towards the bottom of the pad would cause the rewet to increase. From these results it can be seen that the choice of polymer can make a large difference in the rewet properties of the absorbent core.

Effect Of Pad Density

Superabsorbent containing pads. it has been recognized[5,6,7,8] that pad density is an important variable to control in the production of superabsorbent-containing absorbent products. Density not only impacts rewet, but other important absorbent properties as well. If this is studied on a full-scale diaper line, however, control of other pad variables is marginal at best; it is important to be able to control pad density to less that 0.01 g/cc to achieve consistent rewet results. With the availability of laboratory pad formers, important variables such as basis weight, superabsorbent content and placement, pad formation and density can be closely controlled. A standard set of conditions representative of superabsorbent baby diapers were chosen and the pad density systematically varied. The conditions chosen for this part of the study were as follows:

total basis weight: 706 gsm

superabsorbent type: SG2

superabsorbent content 15% by weight

superabsorbent location: middle layer

The results of these experiments are given in Table 2, with the rewet values being averages of six data points per density value.
 Table 2
 Target Actual Rewet
Density Density (g)
 (g/cc) (g/cc)
 0.06 0.061 22.2
 0.08 0.080 15.1
 0.10 0.098 10.6
 0.12 0.119 5.5
 0.14 0.142 1.4

It can be seen that rewet is strongly affected by pad density. As density is increased, rewet values decrease. This effect begins to level off when the pad density approaches 0.14-0.16g/cc.

Fluff-only pads. To investigate the effect dispersed superabsorbent has on densification of absorbent pads, a variable density study on fluff-only pads was also conducted. In this way it could be determined whether or not any changes to the superabsorbent were occurring upon compression and densification. Basis weight was reduced to 609 gsm in order to keep the fiber basis weight the same as for the superabsorbent-containing pad described above.

Due to the lack of any superabsorbent in the pad, the absolute values of rewet are much higher. Importantly, however, approximately the same relationship between rewet and density is observed.

In considering why rewet is improved with increasing pad density, it is useful to consider the concept of capillary pressure. This is the pressure that allows a liquid to spontaneously penetrate a porous medium (such as a fiber network) that is wettable to that liquid. As an absorbent pad is densified, the effective pore radius is decreased, which increases the capillary pressure. This results in lower rewet values (up to the point where other factors, such as liquid holding capacity, begin to have an effect).

Another way in which capillary pressure can effect rewet is via the rate of spreading in a fibrous structure. Increased capillary pressure also tends to increase radial spreading in the pad, which in turn utilizes more of the pad structure (especially more superabsorbent). This would also tend to improve rewet performance.

Other properties, most notably total capacity of a composite absorbent core, can be adversely affected by increasing density. Pads containing high amounts of superabsorbent would be less affected by overall density that fluff-only pads. However, a compromise must be struck between optimizing rewet performance and producing a workable product when choosing design specifications.

Effect Of Basis Weight:

SAP And Fluff Reduction

In order to assess the potential for cost savings in a absorbent core system, the effect of basis weight reduction on rewet performance was investigated. This can be examined in three different ways for a composite superabsorbent/fluff pad: reduced SAP content; reduced fiber content and reduced superabsorbent and fiber content. For all of these scenarios, pas parameters that had emerged as the "best conditions" from the work previously described were chosen. These included superabsorbent located in the top of the pad, superabsorbent type SG3 or SG4 and a pad density of 0.12g/cc.

Reduced Superabsorbent Content. For this scenario the total pad basis weight was kept constant at 705-710 gsm by reducing the superabsorbent percentage and increasing the fiber content accordingly. Superabsorbent percentage was varied starting at a concentration of 15% down to 7.5% in 2.5% increments. The results are listed in Figure 2.

As can be seen from the data, rewet for these pads does not change appreciably until the superabsorbent content falls below 12.5%. Thus, if these pad conditions were used there is an opportunity to lower the superabsorbent content to some degree and still maintain the same level of rewet performance. Since superabsorbent at this writing is approximately three times the cost of wood fluff, reducing superabsorbent and increasing fluff should result in a sizable net savings. Again, these data points generally represent averages of six replicates per point.

Reduction Of Fluff Content. The second phase of this study was to keep the absolute superabsorbent content the same and lower the fiber basis weight, thereby lowering the total basis weight of the pad. Since the total weight of the pad decreases, the percentage of superabsorbent in the pad increases. These results are listed in Figure 3.

A basis weight reduction of about 30% can be tolerated (720-500 gsm) before the rewet value rises above one gram. Not surprisingly, the pad is less sensitive to a loss in fluff fiber than superabsorbent with regard to rewet performance. This was expected since the main advantage a superabsorbent adds to an absorbent core is superior retention under pressure. This characteristic is the essence of any rewet test and explains why rewet is typically poor for non-superabsorbent products.

Reduction Of Fluff And Superabsorbent. The total basis weight of the pad was lowered by reducing both superabsorbent and fluff, maintaining a constant percentage of superabsorbent at 15%. This should have the most effect on rewet, but the greatest cost-saving potential. The results are given in Figure 4.

With the superabsorbent percentage kept constant at 15%, it appears that rewet begins to climb very quickly as the total basis weight is reduced. This net reduction in both fiber and superabsorbent results in the greatest detrimental effect on rewet performance.

The above empirical studies on reducing the basis weight of the composite pad are intended to provide guidelines regarding rewet performance for those concerned with optimizing and/or changing pad structure. Depending on how much importance is placed on achieving certain results on rewet tests, a significant cost savings potential exists in changing pad specifications.


Several key properties of absorbent core construction have been shown to have a significant effect on the dryness or rewet performance of a composite system. Locating superabsorbent particles in a layer next to the facing material yields the lowest rewets. Current commercial superabsorbents vary greatly in rewet performance and a careful choice should be made by the absorbent product maker to achieve the correct balance of properties.

Pad density has a large effect on rewet through capillary pressure phenomena. Gains in rewet can be made by increasing pad density up to a point. Reducing the pad basis weight negatively effects rewet, with reductions in superabsorbent content having the most severe effect. However, it appears possible to reduce basis weight somewhat from that typically found in disposable baby diapers and still maintain good rewet performance if other parameters are optimized. Finally, three alternative fluff pulps to virgin bleached Southern pine were tested and found to all give poorer rewet values. It may be possible to incorporate one or more of these fibers as a blend with Southern without catastrophic effects on rewet if the pad design is carefully chosen.


[1.] K.T. Hodgson, Weyerhaeuser Company

Report, "Rewet Testing of Disposable

Absorbent Products," January 1990. [2.] L.H. Sanford, U.S. Patent #4,041,951 (Aug.

16, 1977). [3.] K. T. Hodgson to C. Elston, et al.,

Weyerhaeuser Company Interoffice

Communication, December 9, 1988. [4.] Weyerhaeuser Company Test Method D-3004-D:

"Superabsorbent Diaper Rewet

Test (High Capacity)," July 17, 1987. [5.] W.C. Sigl and P.A. LaBorde, U.S. Patent

#4,213,459 (July 22, 1980). [6.] P.T. Weisman and S.A. Goldman, U.S.

Patent #4,610,678 (Sept 9, 1986). [7.] L.J. Bernardin, U.S. Patent #4,699,619 (Oct.

13, 1987). [8.] M.H. Hasse and M.J. Steinhardt, U.S.

Patent #4,685,915 (Aug. 11, 1987).
COPYRIGHT 1992 Rodman Publications, Inc.
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Title Annotation:includes bibliography
Author:Hodgson, Kevin
Publication:Nonwovens Industry
Date:Apr 1, 1992
Previous Article:The North American adult incontinence market.
Next Article:An historical look at the technology of spunbonding.

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