Abstract
Wood pulp/Danufil nonwovens have been prepared by wetlaid/spunlace method as a potential and new dispersible moist wipes. To verify the potential applications of the wetlaid/spunlace nonwovens, this study investigated the wet strength, softness/smoothness, and dispersibility of wetlaid/spunlace nonwovens with different Danufil contents and length-to-diameter ratios. Meanwhile, these properties were compared with three other types of commonly-used dispersible moist wipes. The results show that an increase in Danufil content or length-to-diameter ratio results in increasing wet strength and decreasing dispersibility of wetlaid/spunlace nonwovens. However, an increase in pulp content or Danufil length-to-diameter ratio will increase real softness and weaken felt smoothness of materials. In addition, the average machine and cross directions wet strength are 9.5 N/50 mm and 6.3 N/50 mm, respectively. The real softness and corresponding felt smoothness of wetlaid/spunlace nonwovens range from 10.8 to 16.3 and from 37.1 to 22.0, respectively. The average required dispersion time of wetlaid/spunlace nonwovens are controlled within 21 min. Overall, the wood pulp/Danufil wetlaid/spunlace nonwovens have the similar wet strength but excellent softness/smoothness and dispersibility when compared with the currently used dispersible products, and possess high feasibility and potential for dispersible moist wipes industry.
Introduction
With the improvement of people's living standard, request for disposable moist wipes in daily life (incontinence care, skin care, surface care and so on) is growing rapidly [1]. In particular, dispersible moist wipes have been developed to meet consumers' pursuit of convenience and environmental protection [2]. Currently, there are three types of commonly used dispersible moist wipes, namely ion triggered air-laid, non-triggered air-laid and Hyrdraspun. Although all of them meet the current third generation of INDA/EDANA's Guidance for assessing the dispersibility of nonwoven [3], these dispersible moist wipes contain a large number of non-biodegradable chemical ingredients/binders. Meanwhile, the so-called dispersible moist wipes at market are controversial in practice due to their high clogging rate of public sewage systems [4]. Moreover, high price hampers further development of the new dispersible moist wipes [5–7].
As consumer and industrial demand for environmentally friendly products continues to grow, the use of natural based raw materials is becoming popular [8]. It is known to all that ordinary viscose fibers are produced by a series of chemical reactions, in which a large amount of harmful gas will be generated. In this study, we use Danufil fibers to prepare the wetlace nonwovens. Danufil also belongs to the viscose fiber (100% biodegradation) but is directly extracted from the natural renewable pulp and obtained by solvent spinning [9,10]. The fiber production process is environmentally friendly. At the same time, the fiber has excellent water imbibition and is suitable for sanitary products. Uusi-Tarkka [11] and Bernt et al. [12] used the Danufil to prepare wetlaid nonwoven and paper. However, dispersible moist wipes developed by wood pulp/Danufil wetlaid/spunlace nonwovens haven't been reported yet.
On the other hand, the main in-use properties of dispersible moist wipes include wet strength, hand feel (softness/smoothness) and dispersibilty [6]. The wet strength of dispersible moist wipe should be high enough to make sure the material stays intact in-use. However, after using the dispersible moist wipe needs to disintegrate as quickly and as completely as possible. At the same time, the hand feel of material determines the material comfort and quality in-use. Nevertheless, no studies could be found yet to systematically investigate these properties of wetlace nonwovens in comparison with those of the current popular dispersible products. Moreover, the effects of fiber contents or L/D ratios on the material performances also haven't been evaluated in detail.
In this work, wood pulp/Danufil nonwovens with different Danufil contents (35%, 25%) and L/D ratios (791.77, 870.51 and 1088.14) were fabricated through wetlace method. Primary properties (wet strength, softness/smoothness and dispersibility) were investigated and compared to three types of commonly dispersible moist wipes. The effects of Danufil content or L/D ratio on the wet strength, softness/smoothness and dispersibility of prepared materials were also examined. In addition, the morphology and structure of materials were evaluated through scanning electron microscope (SEM). The hand feel including real softness and felt smoothness of materials were intensively characterized through Tissue Softness Analyzer (TSA) technology. An improved laboratory set-up was chosen to monitor the material dispersibility in the flow field [13,14]. These provide guidelines for the production of high value dispersible moist wipes.
Materials and methods
Raw material and samples preparation
Three types of the Danufil fibers (1.7 dtex/10 mm, 0.9 dtex/8 mm and 0.9 dtex/10 mm) were kindly supplied by Kelheim Fibers GmbH. As is known to all, the fiber diameter d could be calculated using following equation (1)
The sheet pulp (KAMLOOPS KRAFT) was purchased from Domtar, Quebec city, Canada and belonged to the papergrade bleached softwood kraft. Meanwhile, the wood pulp fiber was obtained from sheet pulp. The width of wood pulp fiber ranged from 27 to 42 µm and the average length was 2.2 mm. Therefore, the L/D ratio of wood pulp fiber was changed from 52.4 to 81.5. In summary, the L/D ratios of Danufil were greater than that of wood pulp fiber.
Figure 1 shows the preparation process of wetlace nonwovens in this study. Before preparing the wetlace nonwovens, the moisture regains of sheet pulp and Danufil were measured (see Figure 1(a)). As seen in Figure 1(b), the required sheet pulp and Danufil were respectively put into the pulper. Next, the pulper was to break down the materials into individual fibers or, at least, to form a suspension which could be pumped. Regarding the pulping protocol, the beating consistency was controlled at 3–5%, the pressure difference between inlet and outlet of disc mill was set at 0.1 MPa and the freeness was controlled at 700–775 ml. Basically, the formed slurry concentration needed to be controlled at 0–0.08%. Then the homogenous wetlaid nonwoven was formed in the wet machine by the distributor and pre-bonded through preliminary jet head 1 (Figure 1(c)). Additionally, the samples were bonded subsequent multiple passes of jet heads 2, 3, 4, 5, 6 and 7 (Figure 1(d)), and each waterjet pressure was listed in Table 1. The jet strips fitted into the jet heads had 12.6 jets/cm of 0.12 mm in diameter. Eventually, the wetlace nonwovens were obtained after drying (Figure 1(e)). As seen, the detail parameters of six types of wetlace nonwoven samples fabricated in this study were given in Table 2. The approximate area density of these nonwoven samples was 65 g/m2 and the production rate was kept constant at 180 m/min.
Preparation process of wetlace nonwovens: (a) Danufil and sheet pulp, (b) pulper, (c) wetlaid process, (d) hydroentangement process and (e) dry room. The setting of the process conditions. Specifications of the wetlace samples.
In order to study the advantages of wetlace nonwoven samples for dispersible moist wipes, the frequently-used dispersible moist wipes including ion triggered air-laid nonwoven (A7), non-triggered air-laid nonwoven (A8) and hydraspun nonwoven (A9) were purchased from the market. Sample A7 mainly was made up of wood pulp and salt-sensitive binder. Sample A8 consisted of wood pulp and latex binder. Sample A9 was composed of 70% wood pulp, 28% Lyocell and 2% bicomponent fibers [3].
Testing methods
All the samples were balanced in laboratory conditions for 24 h before testing (the temperature was 20℃ ± 2℃ and the relative humidity was 65 ± 2%). The ETD-2000 sputtering apparatus was used for gold plating of small samples 50 s for two times. The surface appearance and microstructure of samples were examined by SEM (M3000, Hitachi High-Technologies Co., Ltd). The determination of the wet strength (both MD and CD) of the samples had been performed on following the guidelines of ISO 9073-3:1989 strip test method [15] with an YG026MB-250 testing tester (Fangyuan Textile Ins, China). Before test, wetlace nonwovens needed to be pre-wetted by water spraying to mimic moist wipes, and the moisture content of material was three times as much as material dry weight [16]. At least five samples were tested and the average values for wet strength at break were calculated. Meanwhile, the tensile deformation of MD and CD of wetlace nonwoven were analyzed based on the uniaxial extension device and optimal microscopy [17].
The TSA from Emtec Electronic GmbH was used to simulate human finger for the measurement of softness (TS7) and smoothness (TS750) of hygiene materials [11]. It was more suitable for measuring the hand feel of moist wipes compared with KES or Phabr Ometer [18]. In addition, the measurement of TS7 and TS750 of samples were according to the standard EN ISO 12652-1 and EN ISO 4046-5-1, respectively. All the samples had to be cut in pieces of approximately 11 cm×11 cm, and the measurement area in the TSA machine was 7.5 cm×7.5 cm and the samples needed to be folded around it [11]. The TSA measures TS7-peak (smoothness) and TS750-peak (softness) in sound spectrum f [Hz]. Five replications of each sample type were chosen.
As shown in Figure 2, the improved laboratory set-up had been used to measure the dispersibility of the samples [13] and the improvements mainly focused on the image simplified treatment. The measuring system included a magnetic stirrer, a flat bottom glass beaker, flat back-light, a dark box and camera with wide angle lens. The sample size of 4 cm×4 cm was used on the disperse test. In the dispersion process, each sample was disintegrated in the beaker containing 160 ml of water and the liquid was agitated with the stirring speed of 220 rpm. Meanwhile, to avoid extra light, the measuring glass beaker containing dispersive sample was put into a box. The suspensions of samples were imaged at three minutes intervals during the dispersion period by using the camera. Then the recorded images were addressed by Photoshop software based on the limit of threshold level value. And the sample residue obtained from each processed image was recorded. Finally the relationship between dispersion time and sample residual rate could be obtained. In all measurements, each trial was replicated five times and their average values were presented.
Schematic illustration of dispersion device.
Statistical analysis
All experiments were conducted in quintuplicate independently, and data were expressed as the mean with standard deviation (SD). A p-value of less than 0.05 was considered statistically significant. The error bars in the figures were the standard deviation of the data.
Results and discussion
Morphological structures
SEM micrographs of four types of nonwovens are displayed in Figure 3. The fiber entanglements of wetlace nonwoven exhibit obvious condensed ribbon-like structures in the red dashed area (see Figure 3(a)), which is formed by high-pressure water jets generated from hydroentanglement. The relatively long fibers (Danufil) are entangled and swirled to form three-dimensional (3D) framework involving a great number of U-shape entanglements. At the same time, short fibers (wood pulp) are entangled into this structure. As can be seen in Figure 3(b) and (c), fiber assemblies are composed of fibers staggered arrangement. The former sample using the ion triggered binder for bonding doesn't show obvious adhesive particles. However, the latter sample bonded by latex exhibits binders and adhesives in the fabric surface. Figure 3 (d) also shows the fiber entanglements and cohesions. Moreover, the fabric density is higher than that of the wetlace nonwoven sample.
Scanning electron microscope images of (a) sample A1, (b) sample A7, (c) sample A8 and (d) sample A9.
Wet stretch characterization
To investigate the wet tensile deformation of wetlace nonwoven, the MD and CD stretching of wetlace nonwoven were carried out via uniaxial extension device (see Figure 4(a)). Correspondingly, deformation of sample A1 is observed simultaneously under the optical microscope. Figure 4 (b) and (c) respectively present the MD and CD microscopic images of sample A1 stretching. The progressive well-defined “ribbons” of highly entangled fibers is only observed in the MD. Meanwhile, MD and CD elongations of sample A1 are 21.9% and 42.2%, respectively. The results show that the MD elongation is obviously less than that of CD, which is similar with the previous research of carding/spunlaced nonwoven [19,20]. As expected, hydroentanglement has significant effect on the fiber orientation [21], which prompts the fibers MD arrangement and forms the condensed ribbon. Besides, when subjected to the MD stretching, the entanglements and cohesions formed by Danufil are quickly deformed along the ribbons direction once the stretching stress exceeds a certain limit. When material suffered from the CD stretching, the Danufil fibers between the ribbons are disintegrated from the ribbon structures and then straightened perpendicular to the condensed ribbons, which results in a long stretching process.
(a) Uniaxial extension device. (b) MD and (c) CD stretching processes of wetlace nonwoven sample A1 under an optical microscope.
As shown in Figure 5, the effects of Danufil content and L/D ratio on the wet strength of different wetlace nonwoven samples (A1–A6) are examined. And the MD wet strength of wetlace nonwovens is higher than that along CD. The average MD and CD wet strength are 9.5 N/50 mm and 6.3 N/50 mm, respectively. Since the material tensile strength is proportional to the degree of fiber entanglement [22]. Meanwhile, hydroentanglement impels the fiber assemblies fully entangled and fiber orientation. Regardless of the stretching directions, the difference in wet strength among the samples is significant (ρ < 0.05) and the sequence of wet strength of wetlace materials is A5 > A3 > A1 > A6 > A2 > A4. An increase in Danufil content or L/D ratio leads to the enhancement of wet strength. As we all know, the tensile failure of hydroentangled material is mainly determined by the fiber entanglements destruction, namely the disintegration of the cumulative basic “U” entanglements. Therefore, according to the previous computational derivation [23,24], the derivation of pull-out force T1 can be calculated from the following equation (2)
The MD and CD wet strength of samples.
Softness and smoothness of the nonwovens
In Figure 6(a), the softness and smoothness of samples have been described in detail. Also, the peaks of smoothness and softness tests are presented in Figure 6(b). According to the measuring principle, the nonwoven has excellent real softness when TS7 peak is low and has a high smoothness value when TS750 peak is low [18]. Thus, the wetlace nonwoven samples (A1–A6) have better softness/smoothness compared with the traditional dispersible moist wipe samples (A7–A9). On the basis of the Uusi-Tarkka and Grüner G's researches [11,18], the real Softness of material relates to the fiber stiffness, strength and micro/macro compressibility, and is also affected by chemicals used. Meanwhile, the smoothness of material relates to the surface structure and geometry. Although the sample A9 was also developed by wetlaid and hydroentangled process, a large number of obvious waterjet holes in the surface and a small amount of blended binder seriously affect its hand feel performance. Moreover, samples A7 and A8 both are air-laid and binder reinforcement. Although the materials contain a lot of wood pulp promoting the material softness property, the adhesive have significant negative effect on the softness/smoothness of materials.
(a) Relationship between real softness (TS7) and felt smoothness (TS750). (b) The peaks of felt smoothness and real softness tests.
According to the measured softness or smoothness values of wetlace nonwoven samples presented in Figure 6(a), the felt smoothness of them can be plotted as a negatively linear function against real softness based on the different Danufil contents and L/D ratios. In addition, the fitting curve shows a high correlation index, that is, an increase in real softness from 10.8 to 16.3 results in the decrease of felt smoothness from 37.1 to 22.0. Moreover, the results also illustrate that the increase in wood pulp content or Danufil L/D ratio leads to the increase of sample softness and decrease of sample smoothness. At the same time, the best results in smoothness and softness come from the sample A6 and sample A1, respectively. This trend may be due to the wetlace nonwoven structure and constituent fibers. As is mentioned above, the fabricated wetlace nonwovens in this paper are only composed of fiber entanglements. The real softness of wetlace material is mainly attributed to the fiber flexural properties based on the Grüner G [18]. Wood pulp fiber has more flexibility than Danufil fiber. Also, increase in fiber L/D ratio expected to increase the fiber flexural property [25]. Therefore, the increase in wood pulp content or Danufil L/D ratio will increase the softness property of material. As expected, with the same spunlace condition, an increase in the formation of condensed ribbon will increase the felt smoothness to a high peak. Wood pulp has short length and is easily entangled into the ribbon-like structure. The increase in wood pulp content results in enlarging condensed ribbon as well as the smoothness 750 peak of material. Additionally, the increase in Danufil L/D ratio will enhance the Danufil entanglements and generate more obvious condensed ribbon, which also improve the smoothness 750 peak. Thus, with increasing wood pulp content or Danufil L/D ratio, the smoothness property of material is deteriorated.
Dispersibility characterization
As can be seen in Figure 7, the effect of dispersion process on the sample residue is observed by the step by step images. Meanwhile, the dispersion time of each sample is recorded in detail. According to the dispersion process of samples A1 and A9 (Figure 7(a) and (d)), wood pulp is initially separated from the fiber aggregation and subsequently both of the samples are divided into several pieces, which are connected with a few long fibers. However, with the constant action of shear force, the former sample A1 finally forms the fiber rope and the latter sample A9 appears the fiber agglomeration. The required dispersion time of samples A1 and A9 are 21 min and 45 min, respectively. As seen in Figure 7(b) and (c), sample A7 changes into the evenly small parts and then the small parts also become smaller with the dispersion time. The required dispersion time of sample A7 is only 18 min. However, the sample A8 is initially divided into several parts with unequal-sizes and then gradually diminished with the dispersion time. Eventually, the required dispersion time is 21 min.
Step by step images in dispersion process of (a) sample A1, (b) sample A7, (c) sample A8 and (d) sample A9.
The relationship between dispersion time and sample residue ratios is presented in Figure 8. Generally, an increase in dispersion time of wetlace nonwovens will decrease sample residue ratio as shown in Figure 8(a). Samples need to be separated through the shear exerted on them by the water flow field. As mentioned previously, Danufil fibers form the 3D skeleton structure and wood pulp entangle into it. With the constant shear stress, wood pulp disintegrates firstly from the fiber assemblies because of it is relatively short and flexible. Then the unstable parts are gradually disentangled and separated into several small pieces. Moreover, the residual ratios of all wetlace nonwoven samples reach a limit about 21% as a result of the Danufil fibers re-entanglement. That is, the Danufil fiber tends to be entangled to form the fiber rope suffering from the shear stress of agitation field and finally the fibers achieve the balance of the disintegration and re-entanglement. It is evident that the average required dispersion time of wetlace nonwoven samples are around 21 min, which suggests that there is an asymptotic limit for dispersion time of wetlace nonwovens. And the detailed trend of required dispersion time of the wetlace nonwoven samples is A2 > A4 > A6 > A1 > A3 > A5. Increasing the Danufil content or L/D ratio leads to the increase of dispersion time. This may be due to the fact that the increase in Danufil content or L/D ratio leads to increasing wrapped areas among fibers. Meanwhile, Figure 8(a) also indicates that there is a high linear fitting relationship between dispersion time and residue ratio of wetlace nonwoven samples, because the correlation degrees of all wetlace nonwoven samples are very close to unity. The slopes of fitted linear equations all are close to −3.9. At any rate, they seem to be in relatively narrow ranges.
Relationship between dispersion time and sample residue ratios: (a) samples A1–A6 and (b) samples A7–A9.
As shown in Figure 8 (b), samples A7 and A8 also illustrate highly correlated between dispersion time and sample residual rate. Also, the dispersion rate of sample A7 is slightly faster than that of sample A8 according to the slope of linear fitting. When sample A7 is exposed to the water flow field, the binder swells which assists the sample rapid dispersion. In the dispersion process of sample A8, pulp fibers are bonded by latex binders and the bonded areas are gradually disintegrated as the shearing stress from constant water flow field. However, the dispersion process of sample A9 can be divided into two parts which both also can be linearly fitted and highly correlated. Obviously, the 18th min is the dividing point of dispersion curve and the 45th min is the final point of dispersion process. This is expected because sample A9 is mainly consist of fiber entanglements structure which is similar to wetlace nonwoven samples. However, the 2% bicomponent fibers are used as the reinforced phase, leading to the formation of the second dispersion interval. Additionally, the residue limit of samples (A7, A8) both are kept near 25%, but sample A9 has the large residual rate limit around 26%.
In brief, wetlace nonwovens have the similar dispersion rate with ion triggered air-laid nonwoven and non-triggered air-laid nonwoven. However, the average dispersion residue ratio of wetlace nonwovens is lower than the current frequently-used dispersible moist wipe samples (A7, A8 and A9). This may be due to the fact that wetlace nonwovens are physical reinforcement without binders.
Conclusions
In this study, wood pulp/Danufil wetlace nonwovens with different Danufil contents and L/D ratios were prepared for dispersible moist wipes. Danufil fibers provide wetlace nonwovens with high wet strength and dimensional stability, while wood pulp adds other attributes like softness and disintegration. Meanwhile, an increase in Danufil content or L/D ratio results in the increase of material wet strength and decrease of material dispersibility. The increase in pulp content or Danufil L/D ratio will enhance the softness and weaken the smoothness of materials. Moreover, the wet strength of wood pulp/Danufil wetlace nonwoven exceeds the lowest limit value of moist wipe market. Average dispersion time of materials can be controlled within 21 min and there is a high linear fitting relationship between dispersion time and material residue ratio. The real softness and corresponding felt smoothness respectively range from 10.8 to 16.3 and from 37.1 to 22.0, which are both better than those of the current products. In short, wood pulp/Danufil wetlace nonwovens possess reasonable balance of wet strength and dispersion, excellent softness/smoothness, and have great potential in replacing the commercial dispersible moist wipes.
Footnotes
Acknowledgement
The author(s) wish to express their sincere gratitude to Zhejiang Hezhong Nonwoven Co., Ltd for their support in providing the means for conducting experiments.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (grant number CUSF-DH-D-2016017) and the “Chen Guang” Project from Shanghai Municipal Education Commission and Shanghai Education Development Foundation (grant number 14CG34).