From micro to nano: polypropylene composites reinforced with TEMPO-oxidised cellulose of different fibre widths

Once-dried dissolving pulp derived from bagasse was kindly supplied by Guangdong Luzhou Paper Mould Packing Process Co. Ltd in the form of pressed pulp sheets (grammage = 500 g m^− 2, bulk density = 0.90 ± 0.04 g cm^− 3) and used as the cellulose source in this work. Chopped polypropylene fibres (diameter = 20 µm, length = 3 mm) were kindly supplied by Schawrzalder Textil-Werke Heinrich Kautzmann (Schenkenzell, GmbH). TEMPO (purity ≥ 98.0%), sodium bromide (purity ≥ 99.0%), sodium chloride (purity ≥ 99.0%) and sodium borohydride (purity ≥ 98.0%) were purchased from Sigma Aldrich (Dorset, UK). Sodium hydroxide (Alfa Aesar, 0.5 N), sodium hypochlorite solution (GPR RECTAPUR^®, 12% Cl₂ in aqueous solution), hydrochloric acid (AVS TITRINORM^®, 0.1 N), acetone (GPR RECTAPUR^®, purity ≥ 99.5%), 1,4-dioxane (GPR RECTAPUR^®, purity ≥ 99.0%, stabilised with 25 ppm ionol), dimethyl sulfoxide (GPR RECTAPUR^®, purity ≥ 99.0%), formamide (TECHNICAL, purity ≥ 99.7%) and ethylene glycol (Reag. Ph. Eur., purity ≥ 99.0%) were purchased from VWR International Ltd. (Lutterworth, UK). All chemicals were used as received without further purification.

The TEMPO-mediated oxidation of cellulose was conducted following a previously described protocol (Saito and Isogai 2004). Briefly, 10 g of pressed dissolving pulp sheet was cut into 10 mm × 10 mm pieces and soaked in 1 L of deionized water for 24 h. The mixture was then blended at 40,000 rpm for 3 min using a kitchen blender (Optimum 9400, Froothie Ltd., Cranleigh, UK) to produce a homogenous dissolving pulp suspension (0.1 wt%). To this suspension, TEMPO (0.16 g, 0.1 mmol) and sodium bromide (1.0 g, 1.0 mmol) were added and magnetically stirred. Once the added TEMPO and sodium bromide have fully dissolved, 65 mL of 12% Cl₂ aqueous sodium hypochlorite solution (corresponding to 10 mmol of NaClO per gram of cellulose) was pipetted into this suspension to start the cellulose oxidation reaction. The pH of the suspension was monitored (HI-2550, Hanna Instruments, UK) and maintained at a value of 10 through the titration of 0.5 N sodium hydroxide. The cellulose oxidation reaction was conducted for 5 h at room temperature. The reaction medium was then quenched with 100 mL of deionized water and the TEMPO-oxidised cellulose (TOC) was recovered through centrifugation (SIGMA 4-16S, SciQuip Ltd., UK, 3000 rpm, 3 min). It was then repeatedly washed with 1.5 L of deionised water and centrifuged until the resulting TOC suspension reached a neutral pH. The prepared TOC was then reduced with sodium borohydride following an earlier work (Takaichi et al. 2014). This step was carried out to convert the C6-aldehydes and C2/3-ketones into hydroxyl groups to increase the thermal stability and reduce heat-induced discoloration of TOC as elevated temperature was used in subsequent composite processing. The reduction reaction was carried out for 3 h at room temperature under magnetic stirring at a TOC consistency of 0.1 wt.-% and a sodium borohydride-to-TOC mass ratio of 0.1 g of sodium borohydride per gram of TOC. After the reaction, the TOC was purified following the previously described washing-centrifugation step. The resulting sodium borohydride-reduced micrometre-sized TEMPO-oxidised cellulose fibres (TOCF) are herein termed TOCF-Na as the TOC contains sodium counterions. The sample was stored in its never-dried state at 2 wt% solid content in a 4 °C fridge prior to subsequent use.

An ion exchange treatment was also carried out on TOCF-Na to convert the sodium carboxylate groups to free carboxyl groups. This ion exchange treatment was performed at a TOCF-Na consistency of 0.1 wt%. To this suspension, 0.1 N hydrochloric acid was added until the suspension reached a pH of 4. The suspension was then left to stir for 1 h at room temperature. After the ion exchange treatment, the sample was purified following the previously described washing-centrifugation step until a neutral pH was attained. The resulting TOCF containing free carboxyl groups are herein termed TOCF-H. The sample was also stored in its never-dried state at 2 wt% solid content in a 4 °C fridge prior to subsequent use.

The previously prepared TOCF-Na was subjected to mechanical disintegration to produce TOCN. The mechanical disintegration process was carried out at a consistency of 0.1 wt% in batches of 500 mL using a blender (Optimum 9400, Froothie Ltd., Cranleigh, UK) operating at 40,000 rpm for 3 min. The TOCN suspension was stored at a consistency of 0.1 wt% in a 4 °C fridge prior to subsequent use.

Majority of the TOCN-reinforced polymer composite fabrication methods reported in the literature are based on solution casting, whereby TOCN is first dispersed in a polymer solution, followed by solvent evaporation (Johnson et al. 2009; Fujisawa et al. 2012; Bulota et al. 2013; Wang et al. 2018) and an optional compression moulding step (Wang et al. 2015; Noguchi et al. 2020). Melt compounding of TOCN-reinforced thermoplastic starch based on liquid feeding has also been reported (Hietala et al. 2014). In this work, PP composites reinforced with 5 wt% (TEMPO-oxidised) cellulose were fabricated using polymer melt extrusion but based on a dry feeding process. Prior to melt extrusion, the chopped PP fibres were pre-mixed with (TEMPO-oxidised) cellulose at a mass ratio of 95:5 (dry basis) in water at a consistency of 0.1 wt% (relative to cellulose) using an overhead stirrer (Hei-Torque 100, Heidolph Instruments GmbH & Co. KG, 6 mm diameter three-blade impeller, 300 rpm) for 3 min. The wet pre-mix was then freeze dried (Alpha 1-2 LDplus, Martin Christ, Osterode, Germany) to produce dry (TEMPO-oxidised) cellulose-PP pre-mix prior to feeding it into a co-rotating twin-screw extruder (Eurolab XL, screw diameter = 16 mm, L/D = 25, Thermofischer Scientific, Karlsruhe, Germany). Melt extrusion was performed at 175 °C with a screw speed of 300 rpm and a throughput of 2.3 ± 0.1 kg h⁻¹. The extrudate was then pelletised (VariCut Thermofischer Scientific, Karlsruhe, Germany) and injection moulded (Haake Minijet Pro, Thermofischer Scientific, Karlsruhe, Germany) into dog bone and rectangular shaped test specimens. The barrel and mould temperatures of the injection moulder were kept at 185 °C and 40 °C, respectively. The sample was injection moulded at an injection pressure and time of 650 bar and 30 s, respectively, followed by a post-pressure of 650 bar for a further 90 s. The injection moulded tensile dog bone test specimens had an overall length of 65 mm, a gauge length of 10 mm, a thickness of 3 mm and the narrowest part of the dog bone specimen was also 3 mm. The injection moulded rectangular test specimens, used in subsequent single-edge notched fracture toughness test and dynamic mechanical thermal analysis, had an overall length of 80 mm, a width of 13 mm and a thickness of 3 mm.

Table 1 summarises the carboxylate contents of dissolving pulp and the various TOC prepared in this work. The carboxylate content increased from 0.06 mmol/g for dissolving pulp to 1.5 mmol/g for TOC. No differences were observed in the carboxylate content of TOCF-Na, TOCF-H and TOCN, as all the samples were prepared under the same oxidation condition. Figure 1 shows the visual appearance of the various (TEMPO-oxidised) cellulose in water suspensions at 0.1 wt% consistency. Clear sedimentation at the bottom of the vial can be observed for the dissolving pulp suspension. However, both the TOCF-Na and TOCF-H suspensions remained stable with no observable sedimentation even after 48 h. This is due to the introduction of anionically-charged moieties onto the surface of these TOC samples, which increases the thickness of the electrochemical double layer. Streaming potential measurements showed that TOC possessed a ζ-potential plateau of ca. -40 mV whilst the ζ-potential plateau of neat and unmodified cellulose was only − 10 mV (both measured in 1 mM KCl supporting electrolyte) (Mautner et al. 2014, 2015). The high magnitude of ζ-potential plateau led to the formation of a stable TOC-in-water suspension.

The translucency of the TOCF-Na and TOCF-H suspensions can be attributed to the presence of large TOC fibres. Without mechanical disintegration, the width of both TOCF-Na and TOCF-H was found to be ~ 17 µm (see fibre width distribution on Fig. 1, right column), similar to that of dissolving pulp fibres despite their high carboxylate content. However, the presence of surface microfibrillation (see arrow in Fig. 1, middle column) on TOCF-Na and TOCF-H can be observed, which is due to the onset of cellulose fibril individualisation. By comparison, TOCN suspension was transparent, consistent with the observation by Saito et al. (2007) for TOCN at similar carboxylate content. This also indicates the successful individualisation of the micrometre-sized TOCF-Na (~ 17 µm in fibre width) to nano-sized TOCN with a fibre width of ~ 9 nm.

The wetting behaviour of (TEMPO-oxidised) cellulose was quantified in this work to delineate any improvements in the mechanical properties of the resulting model PP composites from their surface chemistry. The \({\gamma }_{\text{c}}\) of a solid, defined as the surface tension at which an imaginary liquid just completely wets the surface (Zisman 1964), was determined from the wicking of various test liquids into a packed bed of (TEMPO-oxidised) cellulose. A typical wetting curve, i.e. \({\Delta m}^{2}=f\left(t\right)\), is shown in Fig. 2a. The initial slope of the wetting curve corresponds to the wicking of the test liquid into the sample due to capillarity. This is then followed by a plateau, which is caused by an equilibrium between capillary and gravitational forces (Tröger et al. 1998). By plotting the normalised wetting rate (right hand side of Eq. 2) based on the initial slope of the wetting curve as a function of the surface tension of the various test liquids, \({\gamma }_{\text{c}}\) of the sample can be determined from the maxima of this plot (Fig. 2b). The data in Fig. 2b were fitted with a Gaussian curve. This maxima is analogous to the Zisman’s critical solid-vapor surface tension (Tröger et al. 1998). Liquids with surface tension lower than \({\gamma }_{\text{c}}\) (i.e. the maxima of the plot) will fully wet the sample whilst only partial wetting of the sample can be achieved with liquids with surface tension higher than \({\gamma }_{\text{c}}\).

The \({\gamma }_{\text{c}}\) of the (TEMPO-oxidised) cellulose are summarised in Table 1. Dissolving pulp was found to possess a \({\gamma }_{\text{c}}\) of 45.5 mN m⁻¹ and this agrees well with the \({\gamma }_{\text{c}}\) reported by Luner and Sandell (1969) for regenerated cellulose film derived from cotton, which is also pure cellulose without the presence of hemicellulose. It can also be seen from Table 1 that all TOC samples were found to possess similar \({\gamma }_{\text{c}}\), independent of the fibre width of the TOC, indicating that the TEMPO-mediated oxidation reaction does not alter the surface energetics of cellulose. This could be attributed to the fact that substituting the C6 hydroxyl groups with carboxyl groups does not significantly affect the polar component of the surface energy of cellulose (Sacui et al. 2014). Henceforth, any improvements observed in the measured mechanical properties of the resulting (TEMPO-oxidised) cellulose-reinforced PP composites would not be a direct result of any improvements in the cellulose (nano)fibre-matrix interface due to the introduction of carboxylate/carboxyl groups.

The thermal degradation behaviour of dissolving pulp, TOCF-Na, TOCF-H and TOCN in N₂ atmosphere are shown in Fig. 3a. Their respective onset thermal degradation temperatures (\({T}_{\text{d,onset}}\)) are tabulated in Table 1. It can be seen from Fig. 3a that all samples underwent multiple thermal degradation steps. The initial thermal decomposition of dissolving pulp between 300 and 350 °C can be attributed to the cleavage of the glycosidic linkages of cellulose (Dietenberger and Hasburgh 2016; Santmartí and Lee 2018). During this degradation step, two reactions occurred: (i) the partial cross-linking of cellulose molecules, resulting in the formation of char and (ii) the depolymerisation of the cellulose chains, converting cellulose into tar (Mamleev et al. 2007). The initial thermal degradation of the TOC samples occurring between 150 and 250 °C can be attributed to the decarboxylation of TOC (Fukuzumi et al. 2010; Lichtenstein and Lavoine 2017). This is then followed by the cleavage of glycosidic linkages of cellulose at 300–350 °C. It can also be observed from Fig. 3a that TOCF-H exhibited slower thermal degradation rate compared to TOCF-Na. This is because the rate of thermal degradation of the TOCF is governed by the decarboxylation of the thermally unstable sodium anhydroglucuronate unit, which is present in TOCF-Na (Fukuzumi et al. 2009; Lichtenstein and Lavoine 2017). However, the \({T}_{\text{d,onset}}\) of TOCF-Na and TOCF-H were found to be similar (see Table 1), indicating that the structural difference between the sodium carboxylate groups in TOCF-Na and the free carboxyl groups in TOCF-H had little influence on \({T}_{\text{d,onset}}\).

TOCN was found to possess the lowest \({T}_{\text{d,onset}}\) and degrade at a much faster rate than TOCF-Na. This is rather surprising as TOCN and TOCF-Na are essentially the same TOC chemically; both samples contain the thermally unstable sodium anhydroglucuronate. Since TOCN and TOCF-Na possessed the same carboxylate content, this difference in the thermal degradation behaviour must be due to the difference in the fibre width between TOCN and TOCF-Na. The smaller fibre width and hence, the higher exposed area of TOCN could accelerate its rate of thermal degradation compared to TOCF-Na, which possessed a larger fibre width. Increased reactivity due to increased accessibility is a well-recognized fact in liquid/solid systems upon cellulose modification (Ye and Farriol 2005). Figure 3b shows the thermal degradation behaviour of neat PP and model PP composites reinforced with (TEMPO-oxidised) cellulose. PP underwent single thermal degradation step whilst a two-step thermal degradation process was observed for (TEMPO-oxidised) cellulose-reinforced PP composites, with a lower \({T}_{\text{d,onset}}\) (Table 2). This is consistent with the lower thermal stability of the (TEMPO-oxidised) cellulose reinforcement.

Trans-crystallisation of PP is known to occur from the surface of cellulose fibres (Gray 2008). This could lead to a significant increase in the crystallinity of the PP matrix in the various model composites prepared. Consequently, this would complicate the delineation of the effects of PP crystallinity and the reinforcing ability of TOC of different fibre widths when analysing the mechanical properties of the model composites. Therefore, DSC was conducted to investigate the crystallisation and melt behaviour of PP and the various model PP composites prepared (Fig. 4). Their characteristic crystallisation and melting temperatures are tabulated in Table 2, along with the degree of crystallinity of the PP matrix. A single melting peak at ~ 166 °C and a single crystallisation peak at ~ 106 °C were observed. The crystallinity of all the samples was found to be similar at ~ 45%, suggesting that cellulose-induced trans-crystallisation of PP did not occur in the composite samples. This could be due to the low loading fraction of (TEMPO-oxidised) cellulose in the PP composites. Nevertheless, the similarity in the crystallinity of neat PP and PP composites reinforced with (TEMPO-oxidised) cellulose of different fibre widths calculated based on the 1st heating DSC thermograms implies that a direct comparison in the mechanical properties of the samples can be made. It is also worth mentioning that the \({T}_{\text{g}}\) of PP was not detected by DSC as this transition is typically weak. Therefore, DMTA was used to detect the \({T}_{\text{g}}\) of PP (see the viscoelastic behaviour of model PP composites section). The 2nd heating DSC thermograms also showed the presence of double melting peaks. This can be attributed to the transition between different modifications of the alpha crystal form due to the fast cooling rate employed in our DSC measurement (50 °C min⁻¹), which may have promoted primary and secondary crystallisations (Paukkeri and Lehtinen 1993).

Table 3 summarises the tensile properties of neat PP and model (TEMPO-oxidised) cellulose-reinforced PP composites. The tensile modulus of PP was measured to be 1.8 GPa and the introduction of only 5 wt% dissolving pulp as reinforcement increased the tensile modulus to 2.2 GPa. When PP was reinforced with 5 wt% of TOCF-Na and TOCF-H, a tensile modulus of up to 2.5 GPa was achieved. This represents a ~ 36% increase over neat PP. As the crystallinity of neat PP and model (TEMPO-oxidised) cellulose-reinforced PP composites was found to be similar (Table 2), the observed improvements must be due to the introduction of (TEMPO-oxidised) cellulose as reinforcement. Since dissolving pulp and the various TOC samples possessed similar \({\gamma }_{\text{c}}\) (Table 1), the observed higher tensile modulus of PP/TOCF-Na and PP/TOCF-H compared to PP/dissolving pulp and PP/TOCN can be attributed to the differences in fibre morphology. We postulate that the presence of surface microfibrillation on TOCF-Na and TOCF-H (see Fig. 1, middle column, yellow arrows) led to improved mechanical interlocking and the local stiffening of the PP matrix around TOCF-Na/H, improving the quality of the fibre-matrix interface. Similar effects have been observed in hierarchically structured composites reinforced with micrometre-sized sisal fibres coated with nano-sized bacterial cellulose (Lee et al. 2012). X-ray µCT images (Fig. 5) also showed that TOCN is highly agglomerated in the PP matrix, which further reduced the reinforcing efficiency of TOCN and led to the observed  lower tensile modulus of PP/TOCN composite (Geng et al. 2018) compared to PP composites reinforced with TOCFs.

The tensile strength of PP, as well as model PP/dissolving pulp, PP/TOCF-Na and PP/TOCF-H composites was found to be similar at ~ 36 MPa, indicating that the incorporation dissolving pulp and TOCFs did not have any significant influence. The failure of a composite is a complex process, which has not been successfully elucidated for cellulose (nano)composites (Lee et al. 2014). Nevertheless, the lack of improvements in tensile strength could be due to the small amount (5 wt%) of reinforcement introduced, which is insufficient to impose any obvious improvements. However, there is a noticeable decrease in the tensile strength of PP composite reinforced with TOCN. The model PP/TOCN composite possessed the lowest tensile strength (33.8 MPa); 6% decrease over neat PP and model PP/TOCFs. This can also be attributed to the agglomeration of TOCN in the PP matrix (Fig. 5).

Figure 6 presents the representative force-displacement curves of PP and model (TEMPO-oxidised) cellulose-reinforced PP composites. The initial slope of the force-displacement curve corresponds to the linear elastic response of the material, followed by yielding and crack propagation. Neat PP failed catastrophically, characterised by a sharp drop in the force-displacement curve once maximum force was reached. Progressive failure, characterised by a continuous and progressive decrease in load after maximum load was reached, was observed for all model (TEMPO-oxidised) cellulose-reinforced PP composites. To elucidate this further, fractographic analysis was conducted on the fractured SENB test specimens.

The fracture surface of neat PP (Fig. 7a) showed a ‘mirror/mist/hackle’ morphology, which is typical of polymers exhibiting catastrophic failure (Greenhalgh 2009). The ‘mirror’ region observed near the notch region is relatively smooth and linked to slow propagation as the incipient crack develops. As the fracture accelerates, ‘mist’ region is produced, resulting in a smooth, matt surface in which scarps and riverlines begin to form (see Fig. 7a at 200 × magnification). Further acceleration of the crack to its terminal velocity produced ‘hackle’ morphology consisting of distinct scarps and riverlines. The SENB fracture surface of the model PP composites showed drawing of the PP matrix (Fig. 7b–d). It is hypothesised that during fracture, voids nucleated and developed at the interface between the PP matrix and the reinforcing (TEMPO-oxidised) cellulose. Consequently, the voids grew through plastic deformation until they ultimately coalesce, causing the observed ductile drawing and fibrillation in the SENB fracture surface (Hull 1999). These energy-absorbing mechanisms are responsible for the progressive failure exhibited by the model PP composites.

Table 3 shows the \({K}_{\text{I}\text{C}}\) of neat PP and model PP composites reinforced with (TEMPO-oxidised) cellulose. Neat PP was found to possess a \({K}_{\text{I}\text{C}}\) of 2.87 MPa m^1/2. The introduction of dissolving pulp fibre as reinforcement led to a 5% decrease in \({K}_{\text{I}\text{C}}\) to 2.73 MPa m^1/2. A 12% decrease in \({K}_{\text{I}\text{C}}\) value over neat PP to 2.52 MPa m^1/2 was observed when TOCN was used as reinforcement. This is consistent with the low tensile strength of TOCN-reinforced PP composite (see Table 3). However, it is worth mentioning that the \({K}_{\text{I}\text{C}}\) of PP reinforced with TOCF-Na and TOCF-H were found to be ~ 2.96 MPa m^1/2; a slight improvement over neat PP. This can be attributed to the presence of surface microfibrillation, which increased the energy required to propagate the crack due to improved adhesion through mechanical interlocking and local PP matrix stiffening.

The temperature dependence of the storage modulus (\(E^{\prime}\)) and tan \(\delta\) of neat PP and model PP composites obtained from DMTA are presented in Fig. 8. The respective \(E^{\prime}\) at 20 °C is summarised in Table 3. It can be seen from this table that \({E}^{\prime}\) of all model (TEMPO-oxidised) cellulose-reinforced PP composites are distinctly higher than that of neat PP. Neat PP possessed a \({E}^{\prime}\) at 20 °C of 1.68 GPa and the model TOCF-Na/H reinforced PP composites possessed \({E}^{\prime}\) at 20 °C of 2.28 GPa; ~36% improvement over neat PP. However, the model TOCN-reinforced PP composite only showed 29% improvement (\({E}^{\prime}\) = 2.17 GPa) over the neat PP. These results corroborated with the tensile modulus of neat PP and model PP composites.

In general, the increase in \(E^{\prime}\) of the model PP composites is due to the ability of the cellulose reinforcement to restrict the motion of the PP molecular chains (Amash and Zugenmaier 1998; Petersson et al. 2007). As previously discussed, the presence of surface microfibrillation (Fig. 1) on TOCFs led to the local stiffening of the PP matrix. This resulted in higher restriction in the motion of the PP chains and subsequently, higher \(E^{\prime}\) compared to the model PP/TOCN composite. This is also supported by the shift in mechanical \({T}_{\text{g}}\) towards higher value (see also Table 3) and the decrease in tan \(\delta\) exhibited by the model PP composites. The model TOCF-reinforced composites exhibited the largest shift in mechanical \({T}_{\text{g}}\); ~ 4 °C increment over the neat PP, as well as the lowest tan \(\delta\) value. Model PP composite containing TOCN exhibited only a ~ 2 °C increase in mechanical \({T}_{\text{g}}\) over neat PP and higher tan \(\delta\) value compared to TOCF-reinforced PP composites.