Reinforced absorbent material: a cellulosic composite of TEMPO-oxidized MFC and CTMP fibres

The pulp was disintegrated in hot water at 3000 rpm for 30 000 revolutions (cf. SCAN-M 10:76), dewatered in a Büchner funnel, solvent-exchanged with acetone and air-dried.

The softwood kraft pulp fibres were dried in the form of sheets (approx. 600 g/m²) and dry defibrated in a hammer mill. A detailed procedure of the sheet forming has been described elsewhere (Lund et al. 2012).

The fibres were mixed with the TEMPO-oxidized MFC in a glass beaker (inner diameter = 56 mm) with an impeller (1450 rpm, impeller diameter = 50 mm, four pitched blades). A fixed amount of TEMPO-oxidized MFC was used for each sample and the amount of fibres was varied. This gave materials with different dry content, and consequently, different test pad weights. Stirring was continued until a homogenous suspension was attained; which in this context means that no fibre bundles could be detected with ocular inspection.

After the mixing step, the suspensions were freeze-dried (from dry contents of 0.6–5.0 % depending on fibre content). 20 g suspension was frozen in liquid nitrogen in a glass beaker (inner diameter = 47 mm). During freezing, the glass beaker was surrounded by liquid nitrogen. The freeze drying was carried out using a Hetosicc CD 2.5 instrument operated at 0.3 mbar for 48 h. The dried test pads were stored in plastics bags at room conditions. The stored test pads had a dry content of around 95 %. The dried material with the addition of fibre will be referred to as composite material and the material without the addition of fibre will be referred to as foam material.

Fibre properties were analyzed with a Kajaani FS300. Duplicates were made for all samples. The pooled standard deviation was 0.10 mm for the fibre length, 1.6 μm for the fibre width, 8 μg/m for the coarseness and 0.4 % for the fines content. The fibre length, fibre width and fines content are reported as length-weighted averages.

The Free Swelling Capacity (FSC) and Centrifuge Retention Capacity (CRC) were measured according to Edana 440.1–99 and Edana 441.1–99. The FSC analysis involved soaking the samples in saline solution (0.9 % NaCl) at specified intervals and measuring their liquid uptake (by weight). CRC is the amount of liquid retained in a material after centrifugation. The methods deviated from the standard in two ways: the bags were allowed to drip off for 2 min before FSC weight measurements and the sample amount was 25 % of a test pad (0.03–0.25 g). The results from these measurements are reported as mean values based on four samples. The relative standard errors were ± 10 % for FSC and ± 6 % for CRC.

The wet bulk is an indirect measure of how much liquid can be retained in the material; a high wet bulk at a certain load indicates greater liquid retention. In the dry state, the bulk is the inverse of density. Wet bulk under load was measured in a compression tester. The equipment used for this analysis has been described previously (Lund et al. 2011). The freeze-dried samples were put in a compression tester and the height was measured at 0.6 kPa. The pressure was then released and an open liquid container was put under the sample. The sample absorbed liquid (water with 0.9 weight-% NaCl) for 10 min followed by 2 min drainage. The pressure was then increased stepwise (as seen in Table 1) from 0.1 to 19.4 kPa and after 2 min of equilibration at each pressure the height was measured. The wet bulk was calculated by dividing the sample volume by the dried weight of the test pad. The relative standard error of the wet bulk measurements was less than ± 5 % at all pressures.

The CRC and wet bulk tests are two different ways to measure liquid retention, but the force acting on the samples are different. In the CRC test both the liquid and solid phases are affected by the forces during centrifugation, while in the wet bulk test the external load mainly affects the solid phase. Thus, the CRC value gives an indication of the amount of liquid stored in the capillary structure and the wet bulk is a test of the mechanical stability of the solid network.

Samples were prepared by cutting the dried material with a sharp scalpel. Prior to analysis, the samples were coated with a thin layer (approx. 10 nm) of gold (JEOL JFC-1100E ion sputter). The microscope was a JEOL JSM-820 scanning microscope (voltage 20 kV).

The total cumulative volume (TCV) describes the amount of liquid that can be stored in a material in pores that are smaller than or equal to the pore radius of the measurement point, and can be used to measure pore volume distributions in fibrous networks. This is done by a step-wise removal of liquid from the sample pores. The equipment is described in e.g. Miller and Tyomkin (1994). In this investigation, the receding mode was used where the sample was pre-saturated prior to the measurements. The liquid used was water with the addition of a wetting agent (Triton X-100, 0.1 % tenside in water solution). By using a wetting agent the material was considered to be totally wetted. The samples were tested with and without a load of 5.2 kPa.

The foam material and two composite materials, with 85 weight- % fibres of either CTMP or softwood kraft pulp, were examined with SEM. In the three samples, the amount of TEMPO-oxidized MFC was equal. The low magnification images (Figs. 2a, 3a and 4a) show that the materials were homogenous and the fibres in the composite materials were evenly distributed. At higher magnification (Figs. 2b, 3b and 4b), the pores in the materials can be seen. In the foam material, the pores were divided by sheet-like layers connected to a fibrillar network. In the composite materials, the TEMPO-oxidized MFC was found between the fibres creating a narrow pore structure. Consequently, the relatively large capillaries between the fibres in a fibre network without TEMPO-oxidized MFC had been replaced by TEMPO-oxidized MFC with small capillaries. In Figs. 3b and 4b, TEMPO-oxidized MFC can also be seen connected to fibre surfaces and may possibly provide a more stable fibre network.

The composite materials were tested for their absorption capacity (FSC). Futhermore, two different types of tests were used to investigate the liquid retention properties of the composite materials; CRC and wet bulk under load. It should also be noted that freeze drying of the materials resulted in a shrinkage of 10–15 %. Thus, the densities were between 7 mg/cm³ (at 0 % fibre content) and 60 mg/cm³ (at 88 % fibre content).

Table 4 shows a comparison between using CTMP and softwood kraft pulp in the composite material. The foam material, which does not include any fibres, and the two fibre types (CTMP and softwood kraft pulp) are also included in the table as references. It can be seen that when CTMP was used in the composite material, greater absorption and retention capacities were generally obtained than when softwood kraft pulp was used. The only exception is the CRC at fibre contents of 25 and 50 % where a larger retention capacity was seen in the softwood kraft pulp. It is likely that the stiffer CTMP fibres preserve the porous structure of the network to a larger extent, especially under pressure.

The data in Table 4 shows that composite materials with CTMP fibres as reinforcing elements have better mechanical stability than the composite materials with softwood kraft pulp fibres. Therefore, a more comprehensive characterization of the composite materials with CTMP fibres was done. Additional fibre contents were evaluated to find the optimal mixture of TEMPO-oxidized MFC and CTMP fibres. Comparisons were also made between foam, fibres and composite materials.

The Total Cumulative Volume (TCV) and the Relative Total Cumulative Volume (RTCV) for the foam and the composite materials (with 85 % CTMP fibres and 15 % foam material) were analyzed with and without load (5.2 kPa).

The initial TCV of the uncompressed foam material was 62.2 cm³/g (see Fig. 5a), which can be regarded as a measure of the maximum absorption capacity. The shape of the curve shows that the foam material has a relatively high content of large pores. This is in contrast with the uncompressed composite material, which mainly consists of smaller pores (small increase in TCV at large pore radii). The liquid removed from a sample at small pore radii was tightly bound in the material by strong capillary forces.

When these two materials were compressed under load, the initial TCV was less than in uncompressed materials. It was found that neither of the compressed materials contained a significant amount of large pores, and that mostly large pores in the foam material were compressed by the load (cf. Fig. 5a, b). In contrast to this, a rather similar RTCV was obtained for the composite material from both the compressed and uncompressed sample (cf. Fig. 5b), which means that the material had not collapsed according to pore size. This may be explained by effects of the fibre network in the composite material when it is compressed. Parts of the foam material attached to the fibres are compressed, while some parts are left intact (see Fig. 6). Consequently, the RTCV result suggests that the composite material has an improved mechanical stability in comparison with the foam material.

The FSC measurements showed absorption capacities after 30 min of 56 g liquid/g foam and 37 g liquid/g composite (15 % TEMPO-oxidized MFC/85 % CTMP fibres), cf. Fig. 7. The absorption capacity of the foam is on about the same level as conventional SAPs (cf. e.g. Mudiyanselage and Neckers 2007; Shimomura and Namba 1994). Given that the FSC after 1 min for the foam was already at 48.4 g liquid/g foam, the rate of absorption was rapid. This enables the material to quickly absorb liquid, which may be a desirable property in absorption applications. Similar trends were seen in the CTMP sample and the composite material. The absorption capacity results are consistent with the findings regarding pore volume distribution (cf. Fig. 5a). The foam material has a higher porosity and is thus able to absorb more liquid than the composite material in a free swelling test (FSC).

Results related to the liquid retention properties of the composite materials with different amounts of fibres can be seen in Fig. 8 (wet bulk under load) and Fig. 9 (CRC). Results obtained at three different pressures are shown for the wet bulk under load. The theoretical bulk is shown as a dotted line and represents the bulk obtained if the contributions from the foam and the CTMP fibres are considered separately. It is calculated by:where x is the mass fraction of each material. It can be seen that the composite material performs better than the theoretical bulk at low pressure (0.5 kPa), especially at fibre contents around 50 %. This effect can also be seen at higher pressures. Nevertheless, at the higher pressures the results indicate a maximum gain in wet bulk (in comparison to the theoretical wet bulk) at approx. 85 % fibre content. Thus, at low loads only a small number of fibres are required to stabilize the network in the composite material, while at higher pressures more fibres are needed to achieve the same result. It is likely that this is an effect of fibres reducing the load on the foam material. This effect thereby enables the TEMPO-oxidized MFC to retain more of its liquid retention properties when subjected to a load. An enhanced wet bulk was also obtained when softwood kraft pulp fibres were used, but as seen in Table 2, not to the same extent.

The results of the CRC measurements showed results similar to the ones for the wet bulk (cf. Figs. 8 and 9. The theoretical CRC was calculated in the same way as the theoretical bulk (cf. Eq. 1). CTMP fibres have a poor ability to retain liquid with a CRC of 2.2 g saline solution/g dried material, and consequently there is a lower CRC in the composite materials than in the foam (see Fig. 9). However, the values are consistently greater than the theoretical CRC and the effect is, similar to the wet bulk at higher pressures, more pronounced when fibre contents exceed 50 %.

The additional liquid (the difference between CRC and the theoretical CRC) that is retained in the composite material can be considered to be stored in the TEMPO-oxidized MFC, and in between the TEMPO-oxidized MFC and the fibre surfaces. If the additional liquid is entirely contributed to the TEMPO-oxidized MFC, a new CRC value can be obtained with a mass balance equation according to:Fibre addition shows a positive effect on CRC_MFC and the trend is that increasing the fibre content results in greater retention capacities of the TEMPO-oxidized MFC. Consequently, these findings indicate that more liquid was retained in the TEMPO-oxidized MFC when it was combined with fibres. However, it should be noted that the standard deviations are greater for the CRC_MFC than for the CRC of the composite material. The CRC_MFC is on about the same level as the SAPs, which have a CRC of about 30 g/g (cf. e.g. Nagorski 1994).

The absorption core of disposable diapers contains a mixture of fluff pulp fibres and SAPs. The mixture is different depending on product requirements. Fluff pulp fibres give integrity to the network and distribute liquid throughout the product, while the main function of SAPs is to retain liquid. The composite material absorbs liquid rapidly and it has an enhanced ability to retain liquid in comparison to a network containing only fluff pulp fibres. Thus, it may be possible to, at least partly, replace the oil-based materials in the absorbent core of a disposable diaper with cellulosic material and maintain product performance.