Effect of saturation adsorption of paper strength additives on the performance of paper

Water retention value

The water retention value (WRV) of the fibres was measured by a centrifugation method based on ISO 23714:2014 standard. 3.3 g dry weight of fibres were dispersed to a concentration of 5 g/L at ambient temperature. The fibres were filtered by 200 mesh wire under vacuum. The filtered fibre cake was collected and evenly divided into 4 parts and placed in containers for centrifugation for 30 min at 4940 rpm at ambient temperature. Afterwards, the centrifugation containers were weighed with and without the test fibres, followed by drying at 105 °C overnight. The oven dried sample were then weighed. The water retention value was calculated according to:

Where m 1 is the mass of the sample after centrifugation, and m 2 is the mass of the oven-dried sample. The WRV of the dissolving grade fibres, kraft fibres and C-kraft fibres are 1.02 g/g, 1.19 g/g, and 1.48 g/g, respectively.

Schopper-Riegler (SR) value

The SR number is a measurement of the drainability of fibre dispersions and was determined according to ISO 5267-1. In this procedure a 1000 mL fibre suspension was prepared to consistency of 2 g/L at 20 °C. The fibre dispersion was mixed by stirring with a glass stick and then poured into the measuring chamber. After 5 seconds when the fibre suspension was stabilised, the valve was opened and water drained through the wire. The water volume in the measuring flask was collected. The collected filtered cake was oven-dried at 105 °C and the solids content was calculated by weight. The SR values were determined to be 21.5 SR for dissolving grade fibres, 22.5 SR for kraft fibres and 28.5 SR for C-kraft fibres.

Fibre dimensions

Fibre dimension distributions for the dissolving grade fibres, hardwood kraft fibres, and C-kraft fibres were measured according to ISO16065-2:14. The results and explanations are demonstrated in Table S1 in supplementary information.

Zeta-potential

The zeta-potential values of the fibres in sodium form, with and without the addition of strength agents were determined by a FPA-Fiber potential analyzer (Emtec Electronic – AFG Analytic GmbH, Germany). These measurements are based on a streaming current potential method (Lindström 1989). Measurement was performed using a 0.5 % fibre dispersion in 0.01 M  NaCl (conductivity of 1 mS/cm) at a pH value 6.5. To evaluate the effect of the dry strength agents, they were added to the fibres and mixed with fibres for 10 min before the measurement. Measurements were performed in triplicates and the average values are reported.

Forming of handsheets and paper testing

Handsheets were prepared with and without the addition of cationic starch, APAM and PECs. Strength additives were added in concentrations to reach the saturation adsorption level of each type of the fibres as reported by Zhao et al. (2022). Notably it has been previously demonstrated (Zhao et al. 2022) that the saturation adsorption of cationic starch for each kind of fibre is dependent on the surface charge of the fibres, while the subsequent layers of anionic PAM and PECs were not directly influenced by the initial surface charge of the fibre. As a result, in this work we selected to add cationic starch in concentrations 2 to 4 mg/g higher than the previously detected saturation levels to ensure complete coverage by the starch, and a consistent adsorption of APAM (Zhao et al. 2022). The added amount of APAM was 1 to 2 mg/g higher than previously reported due to its sensitivity to the pre-adsorbed amount of CS. The added amount of PECs was extracted directly from the previous reported saturation adsorption study (Zhao et al. 2022). The used saturation adsorption of CS for dissolving grade fibres, kraft fibres and C-kraft fibres were 17 mg/g, 24 mg/g and 48 mg/g respectively, while the addition of APAM was 7.5 mg/g, 7.5 mg/g and 6 mg/g for the different fibres as outlined in Table 1. The used amount of PECs was varied between 50 %, 100 % and 150 % of the saturation adsorption to evaluate the effect of dosage and are named PECs 0.5x, PECs 1x and PECs 1.5x, respectively. The added amount of PECs (e. g. PEC 1x) for dissolving grade fibres, kraft fibres and C-kraft fibres are 8 mg/g, 15 mg/g and 12 mg/g, respectively.

Paper sheets were formed at a set grammage of 60 g/m 2 using a KCL type sheet former according to the ISO 5269-1 standard. 2.5 g/L fibre dispersions in 0.01 M NaCl (conductivity of 1 mS/cm) were stabilized overnight before use. Cationic starch was added and mixed into the fibre dispersions for 60 s, followed by addition of APAM or PECs and mixed further for 60 s. Following mixing, the fibre dispersions were dewatered to form a paper. Once the paper sheets were formed, they were pressed between blotter papers at 400 kPa for five minutes to remove excess water, followed by an exchange of blotter papers and further pressing at the same pressure for additional two minutes. This was performed to remove water phase before drying the papers. Rapid Köthen drying was then employed to dry the papers at 92 °C for eight minutes at a reduced pressure of 96 kPa. The final paper sheets were conditioned at 50 % RH and 23 °C for at least 24 h before characterization. The grammage of paper sheets was measured by a L&W device according to ISO 536. Paper tensile strength and strain at break were determined by a L&W tensile tester according to ISO 1924-3 and the Z-directional tensile strength was measured following ISO 15754. The light scattering coefficient was evaluated according to ISO 9416 and are plotted together with tensile index, density in Figure S1 to S3 in supplementary information.

Determination of paper formation by using beta-radiography

Paper formation was evaluated by using an AMBERTEC Beta Formation Tester, Finland. This technique measures beta radiation absorbency at 400 points over an area of 68.4 × 68.4 mm 2 with a distance between the points of 3.6 mm and a measuring aperture of 1 mm in diameter. The parameters used were recommended by the instrument supplier to evaluate the specific formation of paper.

Scanning electron microscopy (SEM) of paper sheets

Selected paper sheets were collected for SEM-evaluation before and after Z-directional tensile testing. Samples were imaged using a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM, Hitachi, Japan). The paper sheets were placed on conductive tape and coated by a thin layer of Pt/Pd before the characterization. 1 kV accelerating voltage was used as the typical operating voltages with working distance of 15 mm. Characterization was carried out under variable pressure at low vacuum mode to avoid burning the measured samples.

Figure 1

a) Zeta-potential of fibres with and without the addition of dry strength agents. The chemical addition corresponds to the saturation adsorption of each additive. This means that different amounts (in g/g) have been added for the different fibres; b) Specific formation of the sheets prepared with and without strength additives (the dashed lines with arrows are added only as a guide to the eyes); c) Specific formation of the sheets as a function of the zeta potential of the fibres; d) Tensile index of the sheets as a function of the zeta potential of the fibres. Error bars correspond to 95 % confident limits.

Zeta-potential of fibres and its effect on the formation and mechanical properties of handsheets

The zeta-potential of fibres with and without the addition of strength additives was measured to assess whether the surface properties of fibres change with the addition of polyelectrolytes (see Figure 1a). Without strength additives the zeta-potential was measured to be, −25 mV for dissolving grade fibres, −28 mV for kraft fibres and −66 mV for C-kraft fibres, in agreement with the previously determined total and surface charge of the fibres (Zhao et al. 2022). Addition of cationic starch increased the zeta-potential to values close to neutral for all fibres. Following the addition of APAM to the CS-saturated fibres reduced the zeta-potential to more negative values, as expected. Addition of the same amount of PECs as APAM (7.5 mg/g) resulted in a similar zeta-potential values for kraft and C-kraft fibres, when compared to APAM. Considering the charge density of the components (−0.65 meq/g for the PECs and −1.1 meq/g for the APAM), these results indicate that the PECs more efficiently decrease the zeta-potential of the fibres. However, it is also clear that the magnitude of the potentials are all rather low, and in fact lower than the values of the reference fibres without added chemicals (Zhao et al. 2022).

In practice it has been suggested that the zeta-potential might be of importance for paper formation and the final consolidation of the paper during drying, and therefore it is of interest to quantify these effects. Figure 1b shows the specific formation of the papers and Figure 1c shows the specific formation as a function of the zeta potential. Generally, there is no dramatic change in the formation of the sheets following the addition of strength additives. As a result, the changes of the mechanical properties (Figure 1d) can not be ascribed to changes in the formation of the sheets. Nonetheless, there are some interesting trends shown in Figure 1b. First of all, the dissolving grade fibres yielded papers with the best formation (lowest specific formation values). Additionally, the addition of CMC to the kraft fibres improved the formation of the sheets. On the other hand, this also seems to make the fibres more sensitive to the addition of CS which decreased the formation of the sheets. Upon addition of the APAM and the PECs the formation is improved again. As shown in Figures 1c and 1d the zeta potential had no major effect either on the formation or the mechanical properties of the papers. As such, when interpreting the influence of the additives on the mechanical properties of the papers the influence of the formation changes can largely be neglected.

To further analyse the influence of the dry strength additives on paper structure, the handsheets were analysed via SEM to characterize the morphology of the surface. Figure 2 reveals that paper sheets were formed evenly, without detectable flocculation, or the presence of macroscopic flocs, regardless of the charge of the fibre or the addition of dry strength additives, in accordance with the formation measurements.

Figure 2

SEM images of paper sheets made from dissolving grade fibres (low-charged fibres), kraft fibres (medium-charged fibres), C-kraft fibres (high-charged fibres), without and with saturation adsorption of CS, APAM or PECs.

Tensile strength and strain at break of paper sheets

Figure 3a shows the tensile index as a function of the density of papers made from dissolving grade fibres, kraft fibres and C-kraft fibres with and without strength additives, and Figure S4 illustrates the details for each fibre and the corresponding strength agents. From the results it is clear that the tensile index increased with the addition of strength additives by 33–84 %. The reference papers (without strength additives) made of kraft fibres and C-kraft fibres displayed similar tensile index (49 Nm/g and 50 Nm/g respectively), indicating that CMC modification of the kraft fibre did not change the strength properties of the fibres or the joints between the fibres within fibre network to any large extent. Laine et al. (2002) demonstrated that the tensile index of sheets made of kraft softwood increased when at least 5 mg/g CMC was adsorbed onto the fibres. However, since the tensile index of reference C-kraft fibres, containing 1.7 mg/g CMC (Zhao et al. 2022), is similar to non-CMC modified kraft fibres, the improved tensile index can be attributed to an increased adsorption of strength additives for the highly charged fibres. The papers made from C-kraft fibres saturated with strength additives showed the highest tensile index values, closely followed by the kraft fibres with adsorbed strength agents. In terms of strength agents, CS, a traditional cationic strength agent, indeed improved the tensile index of handsheets (e. g. 64 Nm/g for kraft fibres), however, the addition of the anionic strength agents (APAM and PECs) further increased the tensile index (e. g. 80 Nm/g for CS/APAM adsorbed onto kraft fibres). Papers made from dissolving grade fibres showed the lowest starting tensile index (29 Nm/g), and even upon the addition of strength additives, they showed similar performance as reference kraft and C-kraft fibres. However, this was achieved at a much lower density ( 541 kg/m 3 ), which indicates that there is a bulk advantage for the dissolving grade fibres. Notably the tensile index of kraft fibres and dissolving grade fibres increased with no or only minor increases in density, whereas the C-kraft fibres showed a considerable increase in density following addition of strength agents. For board qualities this is a drawback for C-kraft fibres, since the bending stiffness index (i. e. S b = Et 3 / 12, where S_b is the bending stiffness, E is the Young’s modulus and t is the thickness of paper), is significantly reduced by a decrease in thickness, and since additives are known to have only a minor effect on the Young’s modulus (Lindström et al. 2005) of the papers this densification will have negative effects on the bending stiffness.

Figure 3

a) Tensile index and density properties (the dashed lines are added only as a guide to the eyes), b) bending stiffness and tensile index of papers made of three levels of charged fibres and saturated strength additives. Error bars correspond to 95 % confident limits.

To clarify the influence of the additives on the bending stiffness, this entity was plotted as a function of the tensile index in Figure 3b. The results show that the bending stiffness increased following the addition of strength additives by 2.6–25 %. While CS increased the bending stiffness of the paper, the combination of CS and APAM or PECs significantly improved the bending stiffness when compared to the fibres with only CS. For example, sheets made of dissolving grade fibres increased in bending stiffness from 0.37 mNm to 0.38 mNm (≈2.6 % increase) following CS adsorption, however upon the addition of CS/PECs x1.5 the bending stiffness increased to 0.47 mNm (≈25 %). Considering the fact that bending stiffness is an important property in packaging board, the addition of a second layer of strength additives and the subsequent improvement is a very positive trend for board properties. The highest bending stiffness values for papers from dissolving grade fibres and kraft fibres are similar, and both were achieved by the combination of CS and PECs x1.5. Reference papers from C-kraft fibres show the lowest bending stiffness compared to dissolving grade fibres and kraft fibres. It can be speculated that the CMC modification densified the sheets and reduced the bending stiffness.

Figure 3a and Figure S4 show that CS improved the tensile index, but with the further addition of APAM or PECs, significant high improvements in kraft fibres and C-kraft fibres were observed. Furthermore, the CMC-modification of the kraft fibres created fibres that respond better to the addition of other additives, (i. e., higher adsorption of CS imparted higher strength properties of the papers). In order to establish the exact reason for the strength improvement further investigations are required, but it can be speculated that for the dissolving grade fibres and the kraft fibres that the improvement is largely due to increased joint strength between the fibres. In contrast, the improvement for the papers from the CMC-treated fibres is due to a combination of joint strength improvement and an increase in the total number of active joints per sheet volume. This is also to some extent supported by the light scattering data shown in Figure S3, S4 and S5 in the supporting information.

The combination of tensile index and strain at break show how the paper toughness, (i. e., the tensile energy absorption) is affected by the adsorption of strength additives. Figure 4a and Figure S5 show that there is a strong correlation between the improvement in tensile index and strain at break. Moreover, the tensile energy absorption of sheets from the kraft fibres and C-kraft fibres is significantly greater than the dissolving grade fibres. Figure 4b, shows the Young’s modulus of the fibres with and without the addition of strength agents, with dissolving grade fibres and C-kraft fibres showing the lowest and the highest values, respectively. The fibre dimension measurements (Table S1) show that the kraft fibres are longer than dissolving grade fibres (0.80 mm compared to 0.69 mm), indicating that the shorter fibre length may be the reason for the lower Young’s modulus of the dissolving grade fibres. Comparatively, because kraft fibres and C-kraft firbes have the same dimensions the increase modulus is presumed to be the result of strength agents addition.

Figure 4

a) Strain at break and tensile index (the dashed lines are added only as a guide to the eyes, error bars correspond to 95 % confident limits.) and, b) Young’s modulus of papers made from the different fibres with or without different additives (Continuous error bars are added for guiding eyes).

Figure 5

Z-directional tensile strength and density of handsheets made from three types of charged fibres and different strength agents (the dashed lines are added only as a guide to the eyes). Error bars correspond to 95 % confident limits.

Z-directional tensile strength of paper sheets

Z-directional (ZD) strength properties are used to determine the out-of-plane strength properties of sheets from differently treated fibres. Figure 5 and Figure S6 show that addition of strength additives increased the Z-directional tensile by 139 % (from 345 kPa up to 823 kPa) for papers made of dissolving grade fibres. Papers from kraft and C-kraft fibres without additives show a similar ZD tensile strength at 577 kPa and 620 kPa, respectively. This similarity further demonstrates that the CMC modification did not significantly change the strength properties of kraft fibres when no further strength additives were added. The strength agents CS, APAM and PECs all improved the ZD tensile strength of sheets made of kraft fibres, with CS/APAM resulting in 1154 kPa, which is two times of the properties of reference kraft paper (577 kPa). The adsorption of strength additives onto C-kraft fibres similarly increased the ZD tensile strength from 620 kPa to 1141 kPa. Importantly, with CS alone, C-kraft papers reached the highest ZD strength of kraft papers which were saturated with both CS and APAM. This reveals that an increasing surface charge of the fibres can be a viable route for adsorbing higher amounts of strength agents to produce stronger papers. Interestingly, there is no significant difference in ZD tensile strength between the papers made from kraft fibres with CS/APAM and C-kraft fibres with different strength additives, whereas C-kraft fibres with strength additives show significantly higher tensile index, as shown in Figure 3a. As demonstrated in Table 1, the addition of cationic starch onto C-kraft fibres was 48 mg/g while it was only 24 mg/g to kraft fibres. The lack of increase in ZD tensile strength with APAM or PECs compared to CS alone may be due to different reasons. One explanation could be that the mechanism of failure was different for the differently treated fibres. For the sheets from kraft fibres, treated with CS/APAM, it was noticed that the sheets delaminated during the ZD-tensile testing while this was not obviously detected for the C-kraft fibres as demonstrated in Figure 6. This indicated that there is an adhesive failure for the sheets from kraft fibres whereas the sheets from the C-kraft fibres failure due to a fibre breakage. The other hypothesis is that adsorption of CS on C-kraft fibres during the sheet forming process was not enough for the fibres to be recharged and not completely saturated to the same extent with APAM or PECs as shown previously (Zhao et al. 2022).

The surface morphology of paper sheets after Z-directional tensile test was evaluated by SEM and images are presented in Figure 6. Reference sheets made from each type of fibres, and fibres modified with CS/APAM were imaged. During the SEM sample preparation, it was realised that the sheets prepared with CS/APM modified C-kraft fibres did not delaminate during the z-directional tensile test. As a result, this sample was not evaluated with SEM. Breakage of individual fibres and fibre-fibre joints were detected in all other analysed paper sheets. However, more fibre breakage occurred for the dissolving grade fibres compared to the kraft fibres even with the addition of CS and APAM. This can be explained by the low preserved amount of hemicelluloses on dissolving grade fibres that makes the fibres more fragile.

Figure 6

SEM images of handsheets after Z-directional tensile test.