Topochemical Engineering of Cellulose Fibres using Organic–Inorganic Hybrids

Organic-inorganic hybrid materials are usually a composite of carbon containing compound and metal hydroxides. The carbon precursors can be long aliphatic chain compounds having alcohol, acidic, or ester groups, whereas metal compounds can be layered double hydroxides synthesized from metal nitrates and precipitating hydroxide and carbonate agents. Layered double hydroxide (LDH) is an inorganic clay material written in an empirical way: [M|__x^IIM_x^III(OH)₂]^x+ [A_x/nⁿ"yH,0]^x~, w'here M¹¹ and M¹¹¹ denote the divalent (Ca²⁺, Mn²⁺, Ni²⁺, Mg²⁺, Fe²⁺, Co²⁺, Cu²⁺, Zn²⁺ etc.) and trivalent metal cations (Ni³⁺, Cu³⁺, Ti-^,+, Cr-^,+, Fe-^,+. Al³⁺ etc.). A"- is the intercalated anion (CL, Br, CO,^2-, S0₄²", NO," etc.) located in the cationic inter layer hydrated galleries (45^47). The metal cationic layers are interconnected through anionic layers by the compensation of electric charge. The hybridization of LDH with carbon moieties is mainly driven by the charged sites in the mineral structures as well as on the carbon chain. These hybrids are formed through w'eak electrostatic interactions arising from hydrogen bonding or polarization of covalent bonds. Pulp fibres can also be composed w'ith those hybrids (48) to form novel composites with improved properties like hydrophobicity, tensile strength, etc. This produces a new kind of sandwich material.

Cellulose-layered Double Hydroxides Composite for Pulp Upgrading

Haartman et at. have reported an LDH catalysed oxygen delignification process in eucalyptus Kraft pulp and peroxide bleaching in thermomechanical pulp (TMP). LDH was found to be improving the quality parameters of pulps. They modified a commercially available Mg-Al LDH material by heat treatment (525°C, 4 h) and then dispersed this LDH in anion modifier solutions (benzoate, terephthalate, etc.) in separate experiments. These LDHs were used to modify pulp fibre surfaces as metal cations in LDH have natural affinity to anionic cellulose groups. The presence of LDH on fibres can be confirmed by XPS and water-contact angle analyses.

These LDH particles served as binding sites for applied bleaching and brightening agents in the pulp upgrading process. The results showed that LDH has increased the ISO brightness up to 10% and decreased the kappa number to 2 units in Kraft pulp oxygen delignification process. LDH has also managed to selectively remove the undesirable lignin which contributes brown colour to the pulp. LDH with carbonate anions replaced by terephthalate anions made the hydrogen peroxide (H₂0₂) bleaching process economical as the consumption of expensive and toxic H,0₂ was brought down considerably. LDH enhanced the opacity of TMP to 3 units. Further, the dosage of optical brightening agent (OBA) can be increased during pulp process as LDH- modified pulp fibres showed high retention of OBA. Thus the waste stream from pulp bleaching plants, contaminating water bodies can be avoided (49). Therefore, LDH- cellulose fibres composite pulp can be utilized in the sustainable production of paper and cardboard which requires high ISO brightness of pulp feedstock.

LDH Bridged Stearic Acid–Cellulose Composite

Layered double hydroxides (LDH) can also be used to compose functional organic compounds with cellulose by utilizing LDH’s interlayer hydroxyl groups. Cellulose, the natural polymer containing -OH groups is known to have hydrophilicity, due to hydrogen bonds it forms witli water. This makes cellulose products hydrophilic and liphophobic in nature. There will be always research interest to change a naturally occurring property associated with particular material. Pulp and papermaking industries always wanted to bring hydrophobic property to cellulose, so that modified cellulose can be used in water repellent coating, packaging, thin films applications. Liji Sobhana and co-workers have found this scope in cellulose materials chemistry and attempted to make cellulose into hydrophobic. They used an external reagent (stearic acid) on cellulose, not directly but through LDH linker material. They carefully noted down the bond making sites in cellulose, stearic acid, and LDH and designed a synthesis to produce LDH sandwiched stearic acid-cellulose composite. Stearic acid is a fatty acid that is highly renewable which contains long aliphatic carbon chain and -COOH group (50). These carboxyl groups have tendency to form hydrogen bonding or electrostatic attraction witli LDH’s interlayer -OH groups and metal cations. Similarly, cellulosic -OH groups also have affinity towards LDH’s - OH groups. Therefore, they used LDH as bridging entity between stearic acid (SA) and cellulose (CEL). There are many reported methods where hydrophobic moieties

FIGURE 5.7 (a) Schematic representation showing the formation of hybrid fibres (b) XRD patterns the pristine and modified fibres.*Peaks from the copper sample holder (to be ignored), (c) Influence of pH on water contact angle as demonstrated with SA-LDH-CEL hybrid fibres. Reprinted with permission from Elsevier (Ref. 51: Liji Sobliana, S. S. : Zlmng, Xue.: Kesavan, L; Liias, P.: Fardim, P. Layered double hydroxide interfaced stearic acid - Cellulose fibres: A new class of super-hydrophobic hybrid materials. Colloids Suif A Physicochem. Eng. Asp. 2017, 522, 416-424)

are directly applied on cellulose to make it hydrophobic, but that can be only done with regenerated cellulose only. Whereas LDH-mediated SA-LDH-CEL composite can be produced even from wet pulp fibres (Figure 5.7a). Here LDH-CEL hybrid fibres w'ere formed first, then stearic acid was introduced on this hybrid material.

The characterization experiments like XRD revealed that the hydrophobic moiety, stearic acid was anchored only on the cationic brucite layers of LDH and not on the interlayer galleries of LDH containing carbonate anion (Figure 5.7b).

This suggested that stearic acid and cellulose self-assembled around LDH on either side, forming SA-LDH-CEL hybrid composite material. The self polarized cellu- losic - OH and Stearic -COOH groups were electrostatically attracted towards the positively charged brucite layers of LDH particles and interlayer -OH groups via electrostatic interaction. This led to the non-polar aliphatic carbon chain of stearic acid facing out from the surface of LDH, which ultimately accounted for hydro- phobicity in pulp fibres. Now, how to check the hydrophobic behavior of this composite material? They used a simple water contact angle (CA) measurement study to prove the synthesized materials were hydrophobic. This sandwich material exhibited water contact angle of 150° (directly proportional to water-repellence), which is a desirable property in water-proof materials (51). Also, water solutions of different pH were drop casted on SA-LDH-CEL material and contact angle values were measured (Figure 5.7c)

The introduction of LDH between SA and CEL made the process highly efficient as the material showed high order of hydrophobicity for even minimal loading of SA. Also this process reduced the operation time from 5 days to just 1 day by maintaining the same properties reported elsewhere (52). This synthesis strategy can be extended to incorporate other long chain fatty acids such as palmitic, arachidic, lignoceric acids (having number of carbons 16, 20, 24 respectively) in pulp fibres. These composite materials can be used in water-proof packaging materials, films, and oil adsorbents.