Abstract

Cellulose-based materials are becoming more of a value-added commodity and can be extracted from any plant material across the globe. Unlike lignin, cellulose is consistent within and between plant tissue types because of its linear and repeatable β(1→4)-linked D-glucose unit chain. Such a uniform material from a biological material makes cellulose unique compared with lignin, which is a complex three-dimensional polymer made up of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) phenylpropanoid units that vary in polymerization, concentration, and distribution across plants (Ghaffar and Fan 2013). The cellulose microfibers are protected by a lignin sheath where the cellulose dictates stiffness while the lignin provides plastic deformation, particularly at high microfibril angles (Via et al. 2009). Other plants, such as flax, exhibit a complex relationship between pectic acid and microfibril angle to yield a negative correlation to tissue rupture (Via et al. 2009, Bourmaud et al. 2013). Although the lignin sheath provides additional matrix hardening and degradation resistance to fungi, its removal during industrial processing is more tedious and requires higher temperatures and solvent extraction. Thus, the thermochemical process necessary to yield pure cellulose is costly, so it is important to find profitable value-added uses for cellulose-based bioproducts for which cost is not prohibitive.

Cellulose is receiving increased attention for dispersion into various forest and paper products, wood composites, films, and wood plastic composites (Kojima et al. 2018, Murayama et al. 2018, Solikhin et al. 2018, Iglesias et al. 2020). Cellulose fibers are becoming a more viable option for polymer and wood composites. The modulus of the fiber can be as high as 100 to 130 GPa when measured at the nanoscale and 40 GPa when tested in films (Cheng and Wang 2008, Dai et al. 2013, Dufresne 2013). Cellulose can be processed to microcrystalline or nanoscale size and be placed in various polymer and wood composites. Poly(vinyl alcohol) was reinforced with ultrasonically treated cellulose to make biodegradable nanocomposites through film casting (Cheng et al. 2009). Others placed cellulose and nanocellulose into phenol-formaldehyde (Veigel et al. 2012) and wood-based composites (Arta-Obeng et al. 2012) with significant gains in modulus. However, when larger microcrystalline cellulose was placed in particleboard, gains in modulus were not observed. In fact, the addition of cellulose resulted in a reduction in modulus. It was postulated that the rupture at the cellulose–adhesive interface was due to differences in springback between cellulose, wood, and the adhesive matrix on press release (Arta-Obeng et al. 2012). It was also seen that cellulose dispersion during blending was difficult because of cellulose-to-cellulose affinity. Modification for better hydrophobicity has been a common method to assist in dispersion (Missoum et al. 2013), but other ways could also be helpful.

Encapsulation of cellulose with wax is a physical way to restrict cellulose-to-cellulose attraction (Pan et al. 2014). Because wax is often used in the wood composite process at around 1 percent (wt/wt) loadings, industrial manufacturers would not have to upgrade or add expensive equipment to handle cellulose addition. The encapsulated cellulose particles could easily be transported to composite and polymer facilities and blended in a homogeneous fashion for a multitude of products and composites. However, because wax coverage would restrict bonding between the cellulose and polymer or adhesive, thin layers that can melt and flow during processing may be important. Incomplete coverage may also be beneficial in that the encapsulated portion of cellulose would provide a bulking effect and make it difficult for groups of cellulose fibers to agglomerate. Yet the exposed portion of cellulose could be available for bonding to the polymer or adhesive matrix and used for polymer reinforcement (Pan et al. 2016a, 2016b). Ways to microscopically characterize such coverage and morphology are important steps toward the development and application of encapsulated cellulose.

Differential interference contrast (DIC) microscopy has been available for some time, but recent improvements in performance and cost make it a new analytical tool for the characterization of cellulose-based fibers (Peter et al. 2003). DIC utilizes polarized light beams that concurrently illuminate a thin tissue or particle with two perpendicular light beams, resulting in a three-dimensional appearance on a two-dimensional plane (Rojas et al. 2009). Visually, there is a shadow effect in which the topography of the cellulose particle can be enhanced through shadow contrasting. This allows for better estimation of wax coverage during cellulose encapsulation, while general image analysis tools can be employed to understand the shape morphology of the cellulose–wax composite. Compared with scanning electron microscopy (SEM) and atomic force microscopy, the advantages of DIC microscopy include easier operation, lower cost, and that it can be used for larger samples (up to hundreds of microns) and is suitable for wax–microcrystalline cellulose (MCC) encapsulation characterization.

The objective of this article was to utilize DIC microscopy to characterize the wax encapsulation morphology of the wax–cellulose composite. A new encapsulation method was demonstrated that is simple and cheap for industrial processing (Pan et al. 2014, 2016a, 2016b). Relationships between recipe formulation and morphology will be explored to determine the different recipes that would be necessary to achieve a given morphology. This work can help yield optimal recipes for a desired coverage and encapsulation shape of wax–cellulose composites that incorporate wax-covered cellulose into polymer- and wood-based composites.

Materials and Methods

Cellulose capsulation

The cellulose particles were capsulated with wax and mixtures of surfactants and wax, respectively, in this study. The components of the three encapsulated cellulose with wax and mixtures of surfactants and wax are shown in Table 1. The ratio of the two surfactants (span:Tween = 7:3) was chosen because it was the best ratio to make small wax spheres in order to improve cellulose capsulation, since smaller wax spheres can make thinner layers on the cellulose surfaces, and the ratio of surfactants and wax of 1:20 was used for the mixture (Pan et al. 2014). The process to capsulate the MCC particles using wax and mixtures was a modified microcapsule synthesis method (Pan et al. 2014). Briefly, the wax and mixtures were mixed with the MCC particles while maintaining the temperature at 70°C with continuous mechanical stirring. The wax or mixtures were placed in a beaker and then stirred at 1,000 rpm for 5 minutes and melted completely at 70°C. Then cellulose particles were added with different ratios (Table 1) to the above mixtures with stirring at 1,000 rpm for 10 minutes. Finally, the temperature was decreased to room temperature with stirring at 300 rpm.

Table 1 Components of the three encapsulated cellulose with wax and mixtures of surfactants and wax.

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Morphology measurement and data analysis

All the final products were observed by an optical microscope (BX 53; Olympus, USA) under regular light mode and DIC mode. The Olympus BX53 provides advanced modularity to suit a variety of observation styles, such as bright field, phase contrast, fluorescence, DIC, and polarized light, and features four objectives: ×4, ×10, ×40, and ×100. SEM (EVO 50VP; Zeiss, Germany) was also used to confirm the images obtained by DIC. ImageJ software (National Institutes of Health 2019) was used to measure the length and width of the uncoated MCC and encapsulated MCC particles. One hundred particles were selected and measured from each group. The particles were from hundreds of images that were similar, as shown in Figure 1. The coverage of wax or wax and surfactant (WS) mixtures was observed from the DIC images and divided into four groups: nonencapsulated (no coating materials on single MCC particles), partly encapsulated (only part of the single MCC particles covered by coating materials), fully encapsulated (single MCC particles wholly covered by coating materials), and bundle encapsulated (two or more MCC particles partly or wholly covered by coating materials). The coverage percentages of the untreated MCC and encapsulated MCC particles were calculated and compared. Analysis of variance (α = 0.05) was used to examine differences between the samples (t tests).

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Results and Discussion

Image comparisons using DIC, regular light microscopy, and SEM

Figure 1 shows images of uncoated MCC, wax-coated MCC, and WS-coated MCC from regular light microscopy (a, c, and e) and DIC light microscopy (b, d, and f), provided that DIC images had better visible features than those from light microscopy. Figure 2 shows SEM images of wax-coated MCC and WS-coated MCC, which showed the differences in wax coverage and particle size and indicated that wax-coated MCC (Fig. 2A) had more particle bundles than WS-coated MCC (Fig. 2B).

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MCC length and width changes after coating with wax and WS

Figure 3 shows the length and width changes of MCC particles after coating with wax and WS. All the lengths of coated MCC were significantly longer than those of uncoated MCC (P < 0.05), except WS:M = 1:4, which has a P value of 0.046. This difference indicated that the coatings extended the MCC particles and/or that the coated MCC particles were MCC bundles, as shown in Figures 1 and 2. Similar to the lengths, the widths of coated MCC were significantly larger than those of uncoated MCC (P < 0.05). For both wax-coated and WS-coated MCC particles, the lengths and widths were nearing the uncoated MCC lengths and widths along with the wax and WS loading decreasing, respectively. WS-coated MCC particles had lower lengths and widths compared with those of the wax-coated MCC with corresponding wax or WS ratios, which indicated that the surfactant helped the wax disperse on the MCC surfaces, and WS-coated MCC (Fig. 2B) had fewer bundles than wax-coated MCC (Fig. 2A; Pan et al. 2014). Therefore, the lengths and widths of the particles were significantly changed after encapsulation due to the wax or the mixture covered on the particle surfaces and particle bundles.

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Length and width distributions of wax-coated and WS-coated MCC particles

The length (Figs. 4A and 4B) and width (Figs. 4C and 4D) distributions of MCC particles before and after coating with wax and WS are shown in Figure 4. All the lengths of coated MCC were significantly longer than those of uncoated MCC. The more wax (Fig. 4A) or WS (Fig. 4B) was used, the longer the particles/bundles obtained, and the median values (100 to 149) were closer to those of uncoated MCC. However, the length distribution of WS-coated particles had smaller median length values and narrower ranges (Fig. 4B) compared with those of wax-coated MCC (Fig. 4A). This illustrates again that the surfactant helped the wax disperse on the MCC surfaces and WS-coated MCC since surfactant makes smaller wax spheres, which can make thinner layers on the cellulose surfaces and improve cellulose capsulation. The width distributions of coated MCC (Figs. 4C and 4D) had similar changing trends to the length distributions, and WS-coated MCC particles had smaller median width values and narrower ranges (Fig. 4D) compared with those of wax-coated MCC (Fig. 4C).

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Coverages of wax and WS on MCC particles

Figure 5 shows the coverages of wax (Fig. 5A) and WS (Fig. 5B) on the coated MCC particles. When more wax was used (W:M or WS:M = 1:2), there were fewer nonencapsulated and more bundle-encapsulated MCC particles. When less wax was used (W:M or WS:M = 1:4), there were more nonencapsulated MCC particles and many more fully-encapsulated, but fewer bundle-encapsulated particles, which could be a benefit for the particle dispersion in the composite materials. According to these ratios and MCC and wax additions to the designed composites, an optimized W:M or WS ratio can be calculated before the encapsulated MCC particles are made.

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Conclusions

MCC particles can be successfully encapsulated with paraffin wax and mixtures of wax and surfactant. Surfactants are helpful for the dispersion of the wax on the surfaces of MCC particles. DIC microscopy is better than regular microscopy to characterize the coverage and encapsulation morphology of the MCC particles coated with wax and the mixtures of wax and surfactant. The lengths and widths of the particles were significantly changed after encapsulation. The coverages with wax and the mixtures of wax and surfactant were also changed with the changing ratios of wax or the mixtures of wax and surfactant to MCC. When less wax was used, there were more nonencapsulated and many more fully-encapsulated particles, but fewer bundle-encapsulated particles, which could be a benefit for the particle dispersion in the composite materials. An optimized ratio of wax to MCC or WS to MCC can be calculated according these ratios and MCC and wax additions to the designed composites because wax is often used in the wood composite process. The encapsulated cellulose particles could easily be transported to composite and polymer facilities and blended in a homogeneous fashion for a multitude of products and composites.