A comprehensive evaluation of the most-sold baby diapers on the market

Diapers can absorb urine and loose stools, significantly reducing the incidence of diaper rash, skin inflammation, and other conditions.^1,2 It is estimated that a child usually uses more than 6000 baby diapers. ³ With the rapid development of nonwoven material and super absorbent polymer (SAP) in the 1950s, disposable baby diapers replaced traditional cloth diapers due to convenience and efficacy.^4,5 By employing SAP in baby diapers, the incidence of severe diaper rash was decreased from 60% to 29%.^6,7 Today's diapers exhibit multiple functions, including strong air permeability, good absorption, and high leak-proof performance, which have been recognized as indispensable to improving newborns' life quality.

A typical diaper usually consists of a top sheet, an acquisition sheet (an absorbent core, and a back sheet arranged in sequence from top to bottom). It also includes other parts, such as a bottom layer, inner leg cuff, elastic, and hook, all of which are usually bonded together using melt adhesive. The structure and materials of baby diapers play a crucial role in product performance. The top sheet of a disposable baby diaper is typically made of through-air bonded nonwovens. It must be soft, breathable, and able to capture urine quickly and remain dry after urine transportation.^8,9 In recent years, many studies have been conducted to improve the properties of diaper top sheets. For example, Procter & Gamble (P&G) modified the top sheet with minerals and glycerin to reduce its hydrophilicity for better care of the baby's skin. ¹⁰ Researchers developed top sheet materials with excellent absorption capability using cellulose nanocrystal technology and surface treatment strategies.^11,12 Kimberly-Clark developed modified polylactic acid fibrous materials with good absorbency, breathability, stretchability, and mechanical properties for use as the top diaper layer. ¹²

The acquisition sheet is an essential component of a baby diaper, which spreads the excrement and transfers it into the absorbent layer. The spreading and transferring capability determine the dryness of the top sheet after excrement absorption. ¹³ Researchers designed an acquisition sheet with tiny pores to distribute urine evenly to different areas of the absorbent core and improve the overall absorption of the diaper. ¹⁴ The German company RKW Group developed Hyfol®, a three-dimensional (3D) perforated film with a wide assortment of apertured structures, which exhibited various excellent properties, including a high strike-through rate, low wetback, and light weight. The US Company WPT Nonwovens developed an acquisition sheet with hydrophilic, anti-bacterial, and anti-static treatments to prompt the passage of urine into an absorbent core.

The absorbent core directly affects the absorbency and effectiveness of diapers.^15,16 Initially, the absorbent core of diapers was made of wood pulp fibers. However, with technological advancements, SAP particles with hydrophilic networks were developed, which can absorb up to 100,000% of their own weight in urine, and thus are capable of keeping the skin dry and clean.^7,17 Absorbent cores are typically designed by wrapping SAP particles with nonwoven materials, which include single layer, double layer, and triple layer. ¹⁵ Notably, the distribution and morphology of SAP can affect the performance of absorbent cores. Common distribution patterns include the point, strip, and network. ¹⁸ Researchers have also applied ultrafine cellulose fibers to absorbent cores to improve the absorbency. ¹⁹

The back sheet of diapers is generally porous polyethylene (PE) film covered by through-air bonded nonwovens. The PE film can prevent urine seepage and keep baby clothes clean, ²⁰ while the through-air bonded nonwoven provides a soft feeling, absorbs moisture, and keeps the outside of the diaper dry when contacting with the baby’s skin. P&G developed a moisture-absorbing, breathable, laminated back sheet with inclined capillary openings, which can enhance the air circulation inside the moisture-absorbing diapers and improve comfort during usage. ²¹ Kimberly-Clark designed a back sheet that contains multiple folded perforations to promote airflow and allow water vapor to escape from the diaper. ²²

Although the performance of baby diapers has dramatically improved, diaper rash and excrement leakage have still not been eliminated. ²³ In addition, the diaper quality on the market is usually not well presented to consumers. ²⁴ In this study, we systematically evaluated the structure and performance of the six most-sold baby diapers from Pampers, Huggies, Moony, Bobdog, Babycare, and Beaba. Firstly, we disassembled the diapers and analyzed the structure and properties of the main components, including the top sheet, acquisition sheet, absorbent core, and back sheet. Next, the urine strike-through time, absorption capacity, excrement run-off, spreading properties, and wetback were evaluated. Notably, the influence of the structure and materials properties on diaper performance was comprehensively analyzed. Our study may provide scientific inspiration for researchers and enterprises to develop next-generation high-performance baby diapers and serve as a guideline for consumers to choose the right baby diapers.

Materials and methods

Materials

Small-size baby diapers from six brands of diapers, namely Pampers, Huggies, Moony, Bobdog, Babycare, and Beaba, were purchased from Taobao. Among them, Pampers and Huggies are American brands, Moony is a Japanese brand, while Bobdog, Babycare, and Beaba are Chinese brands. The specific diapers are Premium Care (一级帮, Hangzhou Haoyue Personal Care Co., Ltd, China), Special Delivery (小森林, Kimberly (Nanjing) Care Products Co., Ltd, China), Air Fit (皇家佑肌), Small Wave (小波浪, Unicharm Consumer Products (China) Co., Ltd, China), Royal Lion Kingdom (皇室狮子, Hangzhou Haoyue Personal Care Co., Ltd, China), and Big Fish (大鱼海棠, Zhejiang Zhiai Baby Products Co., Ltd, China).

Characterization

The area density of each layer of diapers is shown in Table S1, which was measured by weighting each disassembled layer. To quantify the ratio of pulps and SAP particles for the Moony diapers, each component was manually separated from the absorbent core and calculated by weight. NaCl (0.9%, w/v) was used as urine for the entire study, while loose stool was made with yogurt, water, and turmeric powder. The viscosity was 152 mPa·s, which is similar to natural loose stool. In general, blue food color was added to the urine for better visualization, which was purchased from Fleur Couleur Company in Shandong Province, China. All tests were repeated at least three times.

Urine strike-through time

A YG814D urine penetration machine (Fangyuan instrument, Wenzhou, China) was employed to measure the strike-through time of the diapers’ top sheet. The test was performed according to GB/T 24218.13-2010. Specifically, urine was poured onto the top sheet through a funnel three times, 5 mL each time. It should be noted that 10 pieces of filter paper with a diameter of 15 cm were placed under the top sheet. The urine strike-through time was reported by the machine, and at least 10 sets of data were collected for each diaper.

Urine absorption capacity

The urine absorption capacity was tested according to GB/T 24218.6-2010. The specific procedure was as follows: firstly, weigh the dry weight of the absorbent core as W₁ and put it into a container filled with enough urine for 2 min to achieve full absorption. Next, take out the absorbent core and weigh the wet weight as W₂ after draining it for 15 s. Three sets of data are obtained for each absorbent core. The urine absorption ratio (UAR) was calculated by the following formula:

UAR = ( W 2 – W 1 ) / W 1

where W₁ and W₂ represent the dry weight (g) and wet weight (g) of the absorbent core, respectively.

Urine absorption speed

The urine absorption speed of diapers was measured using the flat plate method according to GB/T 24218.6-2010. Specifically, diapers were laid flat and fixed on an acrylic board, with the injection point marked at the center of the diaper. A cylinder with a diameter of 6 cm and a height of 24 cm was placed in the middle of the injection point, and three 40 mL measures of urine were sequentially poured into the cylinder at 2-min intervals. The time required for the infant diaper to completely absorb the urine was measured and recorded as t₁, t₂, and t₃. The urine absorption speed (mL s⁻¹) was obtained by dividing the urine volume by the time needed for complete absorption. In addition, the maximum urine level was also obtained according to recorded movies. Five sets of data were obtained for each diaper.

Excrement run-off test

The run-off test is a quantitative method to assess the performance of baby diapers in preventing excrement leakage, which was conducted according to GB/T 28004.1-2021. The testing procedure involves securing the upper end of the diaper to an inclined table at an angle of 25° and pulling it to avoid wrinkles or distortion on the diaper's surface. To conduct the test, the urine injection point was set as 20 cm from the bottom edge and 9 cm from the left-hand edge of the sliding table, then a funnel was placed 2 cm above the injection point, and a piece of absorbent paper was next put at the bottom of the sliding table to collect any run-off urine or loose stools. After 2 min of pouring 40 mL urine or 10 mL loose stools on the diaper, the mass of the absorbent paper was weighed. Each diaper was tested at least five times. The run-off of baby diapers was determined by the following formula:

G 0 = G 2 − G 1

where G₀ refers to the run-off (g), while G₁ and G₂ refer to the dry weight (g) and wet weight (g) of the absorbent paper, respectively.

Excrement spreading and wetback of diapers

Excrement spreading and wetback is tested to reflect the amount of excrement remaining on the baby's skin after absorption according to GB/T 24218.14-2010. The test was conducted as follows: firstly, spread the baby diaper flat and fix it on an acrylic board. Secondly, mark the excrement injection point at the center of the diaper and place a funnel 2 mm above the injection point. Thirdly, inject 40 mL of urine and 10 mL of loose stool through the funnel. Fourthly, cover a piece of filter paper on the diaper right at the injection point after 2 min of injection, then press the filter paper with a 10 cm diameter, 1.2 kg weight for 30 s. It should be noted that urine was delivered onto the diaper three times at 2-min intervals before placing the filter paper, 40 mL each time, while loose stool was only injected once (10 mL). Excrement spreading was quantified according to the video. Five sets of data were obtained for each diaper. The wetback of baby diapers was calculated by the following formula:

W 0 ( g ) = W 2 – W 1

where W₀, W₂, and W₁ represent the wetback (g), dry weight (g), and wet weight (g) of the filter paper, respectively.

Air permeability

The air permeability of baby diapers was tested using a YG (B) 461E computerized air permeability tester (Wenzhou Jigao Testing Instrument Co. Ltd, Wenzhou, China) according to GB/T 24218.15-2018. The principle of this test is to measure the airflow passing through the same diaper area vertically under a fixed pressure difference for a certain time. The sample area is 20 cm², the pressure difference is 200 Pa, 10 sets of data were collected for different parts of each diaper, and the average was calculated. The overall absorbency of the baby diaper is calculated using the following formula:

R = Q v / A

where R represents air permeability (mm s⁻¹), while Q_v and A represent the average airflow (L/s) and the test area (cm²), respectively.

Softness, contact angle, and scanning electron microscopy

Softness was measured using an LLY-01 electronic stiffness tester (Laizhou City Electronic Instruments, Laizhou, China) with the following parameters: the speed of advancing the platen, the test angle, and the size of the top sheet specimen were 4 g/cm²/s, 41.5 degrees, and 250 mm × 25 mm, respectively. The linearity compression (cm) was recorded as the softness. Each sample was repeated three times.

The contact angle of the top sheets was measured using an OCA15EC optical contact angle meter (Dataphysics, Filderstadt, Germany) using the suspension drop method. Scanning electron microscopy (SEM) images were obtained using a TM-3000 electron microscope (Hitachi Ltd, Tokyo, Japan).

Results and discussion

The baby diaper is a complex absorbent system made of multiple-layer materials, including a top sheet, acquisition sheet, absorbent core, back sheet, and others (Figure 1(a)). To systematically evaluate the selected diapers from Pampers, Huggies, Moony, Bobdog, Babycare, and Beaba, detailed diaper structures were outlined through a disassembling process (Figure S1). Then the urine absorption capability, excrement spreading capability, run-off, and wetback of the diapers were investigated using urine and loose stool to reflect the functions of diapers in practical usage. Notably, the critical factors to designing diapers with enhanced quality were highlighted by correlating the diaper performance to the structure and properties of the main diaper components. In particular, the absorption capability includes absorption capacity and speed, which are highly correlated to run-off and wetback performance. In addition, the excrement spreading capability was systematically investigated as the excrement spreading area will determine the surface area of the absorbent core where excrement will be absorbed by SAP particles. If the excrement spreading area is too small, then the absorption capacity of diapers will not be fully effective, which may result in enormous excrement wetback. It should be noted that run-off was quantified to reflect the ability of diapers to prevent excrement leakage, while wetback was measured under pressure after diapers absorbed excrement to reflect the dryness of the diaper surface, which is highly correlated to the incidence of diaper rash. Besides, the softness and air permeability were also measured, which determine the comfort when wearing a diaper. OPEN IN VIEWER

Figure 1.

The structure and properties of diaper top sheets and acquisition sheets: (a) a schematic diagram showing the main structure of a typical diaper; (b) diaper urine absorption process; (c)–(t) images showing the structures of the top sheets for the selected diapers; (u) diameter and number of large holes for Pampers and Bobdog top sheets; (v) contact angle of diaper top sheets; (w) urine strike-through time of diaper top sheets and (x) softness of diaper top sheets. SAPs: super absorbent polymers.

The structure and properties of diaper top sheets and acquisition sheets

The urine absorption of baby diapers is a top-down process (Figure 1(b)). When a baby urinates, the top sheet quickly captures urine and transports it to the acquisition sheet. The acquisition sheet then allows the seeped urine to rapidly spread along the length, width, and thickness directions. Next, the absorbent core absorbs and locks the urine. Finally, the back sheet serves as a protective barrier and prevents urine leakage and soiling of baby clothes. In general, the main functions of diapers include (1) quickly absorbing excrement, (2) keeping the skin dry and clean, (3) preventing leakage of excrement, and (4) maintaining good air permeability.

To unveil the design of the selected diapers, we started by investigating the structure of materials used for each layer of diapers (Figure S1, Table S2), which determines the functions of diapers during usage. In general, most components of diapers are made of fibrous nonwovens, except for the PE film in the back sheet, SAP particles in the absorbent core, and the adhesive that connects each layer. Among the six diapers, through-air bonded nonwovens were used for all top sheets, which exhibit different structures and patterns (Figures 1(c)–(t)). Specifically, the top sheet of the Pampers and Bobdog diapers have large holes of diameters of 1.33 and 0.33 mm, respectively (Figures 1(c)–(e) and (l)–(n)), which should be fabricated by the perforation process. In addition, the large holes on the Bobdog diaper were densely distributed in a wavy shape, with a total of 9029 large holes on the top sheet, while the Pampers diaper had a uniform distribution with 2488 mesh holes on the top sheet (Figure 1(u)). For Moony diapers, plain through-air bonded nonwovens were used as the top sheet (Figures 1(i)–(k)). For the Huggies, Babycare, and Beaba diapers, the top sheets exhibited a 3D convex–concave structure surrounded by 12, 6, and 6 hot-rolling points, respectively. It should be noted that the hot-rolling points were circular for the top sheet of the Huggies diapers and elliptical for the top sheets of the Babycare and Beaba diapers (Figures 1(f)–(h) and (o)–(t)). All of these hot-rolling points did not penetrate through the top sheet and, therefore, are not referred to as large holes.

Unlike the one-layer top sheets of the other diapers, the top sheets of the Huggies, Babycare, and Beaba diapers consisted of two-layer through-air bonded nonwovens, which should be designed to maintain a stable 3D convex–concave structure (Figures 1(f)–(h) and (o)–(t)). Notably, the bottom layer of the Huggies and Babycare top sheets exhibited an aligned fibrous structure parallel to the length direction of diapers, which may also play the role of the conventional acquisition sheet (Figure S2A and B). In contrast, a random fibrous structure was found on the bottom layer of the Beaba top sheet (Figure S2C).

With the unique structure of Pampers diapers, a conventional acquisition sheet was placed between the top sheet and the absorbent core (Figure S2D). In addition, there was one more layer of spunlaced nonwoven below the acquisition sheet Figure S2E). Furthermore, a through-air bonded nonwoven was found between the absorbent core and the back sheet (Figure S2F). For the Huggies diapers, a layer of spunbonded nonwoven existed between the top sheet and the absorbent core (Figure S2G), which was used to fix the absorbent core stably. It should be noted that the abovementioned structures were not found in the other diapers.

We next quantified the properties of the top sheet that may influence the diaper performance. Results indicated that all top sheets of the selected diapers exhibited similar hydrophobicity, with a contact angle ranging from 129.8° ± 5.8° to 136.5° ± 4.6° (Figure 1(v)). Nevertheless, urine was able to penetrate through the top sheet due to the highly porous and fluffy structure of through-air nonwovens. The urine strike-through time of the top sheets varied significantly among the selected diapers (Figure 1(w)). Specifically, the Babycare and Huggies top sheets showed the best urine strike-through performance, which took 4.65 ± 0.57 and 7.08 ± 0.55 s, respectively, for urine to penetrate the top sheets, probably attributing to the design of the 3D convex–concave structure. In addition, the Pampers and Beaba top sheets also had relatively short urine strike-through times of 10.19 ± 1.29 and 13.22 ± 0.94 s. As mentioned, the Pampers top sheets had densely distributed large holes (Figures 1(c)–(e)), which may facilitate rapid urine penetration. In comparison, the Moony and Bobdog top sheets took 17.38 ± 1.65 and 21.42 ± 1.03 s, respectively, which were relatively long compared to the others. It should be noted that fast urine strike-through performance is not always pursued as it may result in poor urine spreading performance, especially for diapers without the acquisition sheet. When the top sheet is assembled on diapers, faster urine transport will be achieved with the help of an absorbent core, while the hydrophobic top sheet also plays a crucial role in preventing urine wetback. Furthermore, we quantified the comfort by measuring the softness of the top sheet. Results indicated that Pampers (2.5 cm), Bobdog (3.1 cm) and Babycare (3.1 cm) top sheets were relatively soft (Figure 1X), which could provide a better contact feeling during usage.

The structure and properties of absorbent cores and back sheets

We next analyzed the structure and properties of the absorbent core responsible for absorbing excrement. By separating each component of the absorbent core, detailed structures were obtained (Figures 2(a)–(f), S3A, and S3B). In general, the wrapping materials were organized in two ways. One is that SAP particles were covered by a piece of wrapping material and interface material, forming a D-type absorbent core. Examples include the Pampers, Bobdog, and Beaba absorbent cores (Figures 2(a), (d), and (f)). The other is that SAP particles were covered by a single piece of wrapping material, forming an O-type absorbent core. Examples include the Huggies, Moony, and Babycare absorbent cores (Figures 2(b), (c), and (e)). In brief, the main difference between D-type and O-type absorbent cores is whether there is an interface material at the overlapping ends of the wrapping materials. When there is an interface material, the cross-view of the wrapping material and interface material looks like the letter “D”; otherwise, it looks like the letter “O.” In addition, most wrapping materials were spunbonded nonwovens, including Pampers, Bobdog, Babycare, and Beaba (Figures 2(a), 2(d)–(f), and S3C), which had uniformly distributed hot-rolling points to enable the desired mechanical properties (Figures 2(g) and (h)). It is worth mentioning that an absorption channel was found in the Pampers absorbent core (Figures 2(i) and (j)), which did not contain SAP particles. The absorption channel was formed by hot-rolling bonding of the top and back spunbonded nonwoven wrapping materials. Differently, spunlaced nonwovens and airlaid paper were used as wrapping material for Huggies and Moony, respectively (Figures 2(b) and (c)). The spunlaced nonwoven had an aligned structure that was generated during the hydroentanglement process (Figures 2(k), 2(l), and S3C). In comparison, the airlaid paper had some wrinkles on the surface (Figures 2(m), 2(n), and S3C).

OPEN IN VIEWER

Figure 2.

The structure and properties of absorbent cores and back sheets: (a)–(f) the absorption core structures of the selected diapers; (g), (h) spunbonded nonwovens are used as the wrapping materials; (i), (j) the absorption channel for pampers diapers; (k), (l) spunlaced nonwovens are used as the wrapping materials; (m), (n) airlaid paper is used as the wrapping material for Moony diapers; (o) weight of diapers; (p) thickness of diapers; (q) weight of absorbent cores and (r) air permeability and the number of breathable pores. SAP: super absorbent polymer.

For the interface materials, spunlaced nonwovens were typically used, including the Bobdog and Beaba absorbent cores (Figures 2(d), (f), and S3D). The exception is Pampers, which used spunbonded nonwoven as the interface materials (Figures 2(a) and S3D). An extra layer of spunlaced materials as an interface material was also found on the O-type absorbent core of the Huggies and Babycare diapers (Figures 2(b), 2(e), and S3D). For absorbent materials, SAP particles were generally separated by a layer of through-air bonded nonwoven, including Huggies, Bobdog, Babycare, and Beaba (Figures 2(b) and (d)–(f)). However, no separation layer was found for the Pampers and Moony diapers (Figures 2(a) and S3E). To the best of our knowledge, the separation layer is used to maintain a stable distribution of SAP particles, which is particularly useful in preventing aggregation of SAP particles after absorbing urine. It should be noted that a mixture of fluff pulps and SAP particles roughly in a 1:1 ratio was used as absorbent material for the Moony diapers (Figures 2(c), 2(q), S3H, and S3I).

We next quantified the weight and thickness of the diapers. It was found that the absorbent core contributed to the largest proportion (∼50%) for all diapers (Figures 2(o) and (p)). Among the selected diapers, the weight and thickness of the Moony diaper were 26.34 g and 2.78 mm, respectively, which were relatively larger than those of other diapers as fluff pulps were used as a major component of the absorbent core (Figure 2(q)). In contrast, the Pampers diapers exhibited the lowest weight (21.03 g) and thickness (1.96 mm), while the weight and thickness of the Huggies, Bobdog, Babycare, Beaba, and Moony diapers were similar.

As the back sheet of diapers plays a crucial role in preventing urine leakage and soiling of baby clothes, it usually has the lowest air permeability. However, air permeability that is too low also induces discomfort, especially in hot weather. Results suggested that the air permeability of baby diapers is highly dependent on the number of breathable pores in the back sheet. For example, the Beaba diapers exhibited the best overall air permeability of 42.511 mm s⁻¹ due to the existence of more breathable holes (1 × 10⁷) in the entire back sheet. In comparison, the Moony diapers had 5 × 10⁶ breathable pores in the entire back sheet, which resulted in the worst air permeability of 19.087 mm s⁻¹ (Figure 2(r)). It should be noted that all the selected diaper back sheets consisted of a porous PE membrane and a through-air bonded nonwoven (Figures S3F and G). The through-air bonded nonwoven layer could provide a soft feeling when contacting with the skin. In comparison, the porous PE membrane was a barrier to prevent excrement leakage.

Urine absorption capability of the selected diapers

To quantify the urine absorption ability of the absorbent core, the UAR was measured, which is defined as the gram of absorbed urine per gram of absorbent core. Results indicated that the Babycare, Beaba, and Huggies absorbent cores exhibited relatively high UAR, which were 25.5, 24.6, and 24.2, respectively (Figure 3(a)). Notably, the absorbent core UAR is highly dependent on the content of SAP particles in the absorbent core. For example, the Moony absorbent core exhibited the smallest UAR of 16.6 as it had the lowest content of SAP particles after combining fluff pulps. Besides the UAR, the urine absorption capacity is also essential, which determines the amount of urine that can be absorbed by a baby diaper. The urine absorption is dependent on the UAR, the weight percentage of SAP particles in the absorbent core, and the weight of the absorbent core. Notably, the Moony absorbent core showed the largest urine absorption capacity of 359.1 ± 3.21 g mainly because the mass of its absorbent core is the highest (Figure 2(o)), while the Pampers absorbent core had the lowest urine absorption capacity of 210.6 ± 0.72 g (Figure 3(b)).

OPEN IN VIEWER

Figure 3.

Urine absorption capability of the selected diapers: (a) urine absorption ratio and sap weight of absorbent cores; (b) urine absorption capacity of absorbent cores; (c) thickness of dry and wet diapers; (d)–(u) images showing the maximum urine height during the absorption speed test; (v) the absorption speed of the selected diapers and (w) the maximum urine height during the absorption speed test. SAP: super absorbent polymer.

Since baby diapers tend to bulge after absorbing urine, excessive bulging height will result in discomfort during wearing. Therefore, we further measured the thickness of the diapers after absorbing 40 mL of urine (Figure 3(c)). Similar to the trend of dry diapers, the Moony absorbent core made of fluff pulp and SAP particles showed the most bulging with a wet thickness of 6.29 mm. In comparison, the Pampers absorbent core exhibited the least bulging with a wet thickness of 2.05 mm, which should be related to the low dry weight and thin absorbent core.

We next conducted the urine absorption speed test, which reflects how fast urine could be absorbed. It should be noted that fast urine absorption could lower the possibility of urine leakage. To simulate the situation of a baby urinating multiple times, three injections of urine at 2-min intervals (40 mL each injection) were made during the test (Figures 3(d)–(u)). For the first injection, urine absorption speed was fast for all diapers except for the Moony diaper, indicating that the urine absorption speed of fluffy pulp was lower than that of SAP particles. Logically, the urine absorption speed was reduced when the second and third injections were conducted (Figure 3(v)). Contrary to the other diapers, the urine absorption speed increased and the maximum urine height in the cylinder for the Pampers diaper decreased as the injection was conducted from the first to the third time (Figures 3(v) and (w)), which should result from the transport effect of the absorption channel and the large pore on the top sheets (Figures 2(i) and (j)). In addition, the urine absorption speed and the maximum urine height in the cylinder for the Babycare diaper were relatively stable from the first to the third injection (Figures 3(v) and (w)), which should be related to the excellent strike-through performance of the top sheet and the large urine absorption capacity.

Excrement run-off of the selected diapers

To quantify the capability of diapers in preventing excrement leakage, excrement run-off was measured on an inclined table using both 40 mL urine and 10 mL loose stool (Figures 4(a)–(c)). The urine run-off of the selected diapers ranged from 0 to 19.8 ± 0.9 g (Figure 4(d)). Notably, the run-off of the Bobdog and Babycare diapers was 0 g, indicating that urine could be rapidly captured by the diapers, which should be correlated to the relatively fast urine absorption speed. However, the time to reach the length edge for the Bobdog and Babycare diapers was 7.06 and 8.49 s, respectively (Figure 4(e)), much longer than that of the other diapers, indicating that the urine spreading capability along the length direction was poor.

OPEN IN VIEWER

Figure 4.

Excrement run-off of the selected diapers: (a) schematic diagram showing the setup of the run-off test; (b), (c) images showing (b) urine and (c) stool reaching the length edge during the run-off test; (d) urine run-off; (e) time for urine to reach the length edge; (f) urine spreading area when reaching the length edge; (g) urine run-off area as a function of time; (h) stool run-off; (i) time for stool to reach the length edge; (j) stool width and (k) spreading area when reaching the length edge.

For the Pampers and Huggies diapers, the urine run-off was 19.8 ± 0.9 and 9.2 ± 1.1 g, respectively, much higher than that of the other diapers as urine flowed rapidly on the top sheet surface along the length direction. It only took 0.34 and 0.38 s for the urine to reach the length edge for the Pampers and Huggies diapers, respectively. The corresponding spreading area was 56.3 cm² and 69.1 cm², respectively (Figure 4(f)). It should be noted that the urine run-off of the Moony and Beaba diapers was intermedium, and the run-off mechanism was different. Specifically, the run-off of the Moony diapers happened in the initial pouring. In contrast, the run-off of the Babycare diapers occurred after the urine spread to the length edge (Figure 4(b)), indicating that the Babycare through-air bonded nonwoven top sheet with a 3D convex–concave structure is better than the Moony plain through-air bonded nonwoven top sheet. It is worth noting that the spreading area of the Pampers and Huggies diapers during the run-off test was mostly larger than that of the other diapers (Figure 4(g)), which may be beneficial to urine absorption during practical usage, although the urine run-off performance was high. To reduce the possibility of urine leakage, Huggies diapers were designed with a cuff on the length edge closed to the buttocks (Figure S3J).

The stool run-off of the selected diapers ranged from 2.5 ± 0.2 to 6.1 ± 0.5 g (Figure 4(h)). Similar to the urine run-off results, the Pampers (6.1 ± 0.5 g) and Huggies (4.8 ± 0.1 g) diapers had the most extensive stool run-off. The corresponding time for the stool to reach the length edge was 0.49 and 0.81 s, respectively (Figures 4(h) and (i)). In addition, the Babycare diapers showed the lowest stool run-off of 2.5 ± 0.2 g. In contrast to the excellent urine run-off performance, the Bobdog diaper had relatively high stool run-off of 4.8 ± 0.4 g, although the spreading width and spreading area were relatively high (Figures 4(j) and (k)), suggesting that the Bobdog diaper had a weaker capability in transporting loose stool through the top sheet. It should be noted that stool rarely spreads along the width direction, while urine could spread to the entire width for all diapers.

Excrement spreading and wetback of the selected diapers

To evaluate the capability of maintaining dryness after a baby urinates, we measured the spreading capability and wetback of diapers using urine and loose stool. It should be noted that urine was injected three times, 40 mL each injection, while 10 mL loose stool was injected once. Notably, the spreading capability determines the surface area of the absorbent core that could be used for urine absorption, which will influence the amount of wetback excrement. To be quantitative, the spreading area, spreading length, and spreading width were measured. Results indicated that the spreading width was similar among the six representative diapers, while the spreading area and spreading length were significantly different (Figures 5(a), (e), and (f)). Logically, the spreading length and spreading area of all diapers increased from the first to the third urine injection (Figures 5(e) and (f)). OPEN IN VIEWER

Figure 5.

Excrement spreading and wetback of the selected diapers: (a), (b) images showing (a) the urine spreading and (b) wetback performance; (c), (d) images showing (c) the stool spreading and (d) wetback performance; (e) urine spreading length; (f) urine spreading area; (g) urine wetback; (h) stool spreading area; (i) stool spreading length and (j) stool wetback. MD and CD represent the machine direction and cross direction of diapers, respectively. It should be noted that MD is parallel to the length direction of the diaper.

Notably, the Pampers diapers exhibited the best spreading capability, which had the longest spreading length of 31.57 ± 0.83, 33.67 ± 0.81, and 36.40 ± 2.00 cm, as well as the largest spreading area of 278.92 ± 8.96, 328.02 ± 5.47, and 373.31 ± 10.07 cm² for the corresponding three urine injections (Figures 5(e) and (f)). Attributed to the excellent urine-spreading properties, the Pampers diapers exhibited a minimum urine wetback of 3.9 ± 0.4 g (Figure 5(g)). Second to the Pampers diapers, the Moony diapers also had excellent spreading performance with diffusion areas of 286.58 ± 21.67 and 353.13 ± 8.63 cm² for the second and third injections, respectively, although the spreading area for the first injection was similar to that of the other diapers, and was as small as 187.40 ± 25.84 cm² (Figure 5(f)). As a result, the Moony diapers also showed relatively less urine wetback of 5.2 ± 0.6 g, which should result from the excellent urine spreading properties and the largest urine absorption capacity for the absorbent core (Figure 5(b)). In general, the other diapers showed weaker urine-spreading properties. For example, the spreading area of the third injection was 303.19 ± 14.63, 288.36 ± 16.29, 276.53 ± 15.34, and 257.82 ± 11.68 cm² for the Bobdog, Babycare, Beaba, and Huggies diapers, respectively. The corresponding urine wetback was 6.4 ± 0.4, 6.7 ± 0.1, 5.8 ± 0.5, and 7.2 ± 0.6 g, respectively, which is not too far away from that of the Pampers and Moony diapers.

To unveil the reason for the different spreading properties, we sequentially removed each component on the top of the absorbent core and further investigated the diaper spreading properties by pouring on 40 mL of urine. Results indicated that the spreading behavior was greatly influenced by the materials on the top of the absorbent core for the Pampers, Huggies, and Moony diapers. In contrast, the other diapers were rarely influenced (Figure S4–S9). For the Pampers diapers, urine could spread in the width and length direction when the acquisition sheet stayed on the top of the absorbent core. However, urine tended to spread on the edge along the length direction when the acquisition sheet of the Pampers diapers was removed. The spreading area was reduced from 283.4 to 261.4 cm², suggesting that the acquisition sheet with an aligned fibrous structure along the length direction enabled the rapid urine spreading after penetrating through the top sheet (Figure S4).

For the Huggies diapers, there was no conventional acquisition sheet. However, the spreading area decreased from 183.2 to 160.5 and 148.4 cm² when the top layer and the bottom layer of the top sheet were removed, respectively, indicating that the top sheet also served as an acquisition sheet (Figure S5). For the Moony diapers, the one-layer top sheet had a huge influence on the diaper-spreading properties. Specifically, the spreading area was decreased from 187.4 to 108.9 cm², indicating that the airlaid paper wrapping material of the absorbent core can absorb urine rapidly, leaving no time for urine to spread on the surface (Figure S6).

For the loose stool test, the spreading area ranged from 33.9 ± 4.6 to 47.1 ± 2.2 cm². Among the selected diapers, the Pampers and Bobdog diapers exhibited the minimum and maximum spreading area, respectively (Figure 5(h)). Unlike urine, loose stool rarely spreads on the diaper surface due to its much larger viscosity (Figure 5(c)-(d) and (i)). Although the Pampers diaper exhibited the minimum spreading area, the stool wetback (0.8 ± 0.1 g) was also the least (Figure 5(j)), which should be attributed to the large pores on the top sheet, allowing for the rapid penetration of viscous stool through the large pores. In addition, the stool wetback of the Moony diapers (1.5 ± 0.2 g) is the worst, probably because the plain through-air bonded nonwoven top sheet lacks space structures such as mesh or 3D convex points that can absorb loose stools. The stool wetback for the other diapers was similar, suggesting that a unique design is needed to absorb loose stool.

Conclusions

We systematically investigated the performance of the most-sold diapers from Pampers, Huggies, Moony, Bobdog, Babycare, and Beaba. Generally, through-air bonded, spunbonded, and spunlaced nonwovens were mainly used in diapers. Our results indicated that the performance of baby diapers is highly dependent on the structure and properties of each diaper component. Specifically, excrement run-off and wetback were highly related to the excrement absorption speed and excrement spreading properties, respectively, which are dependent on the complex interactions among the top sheet, acquisition sheet, and absorbent core. In general, designing top sheets with a 3D convex–concave structure or large holes was beneficial to fast excrement penetration, while the employment of an acquisition sheet could improve the excrement spreading properties. In addition, an absorbent core with high SAP content and absorption channel also contributed to a better excrement absorption property. Notably, the Pampers and Moony diapers exhibited relatively low urine wetback. The Bobdog and Babycare diapers showed 0 g urine run-off. Besides, the Huggies and Moony top sheets were relatively soft, which could provide a better contact feeling during usage. Furthermore, the Beaba diapers exhibited the best air permeability of 42.511 mm s⁻¹ due to the existence of more breathable holes. We foresee that the performance of diapers could be further improved by designing better diaper materials. For example, the back sheets may be replaced by more air-permeable materials while still being able to prevent urine leakage. The wetback could be potentially eliminated by generating materials with directional water transport properties.