Polymeric Wet-Strength Agents in the Paper Industry: An Overview of Mechanisms and Current Challenges

Abstract

Polymeric wet-strength agents are important additives used in the paper industry to improve the mechanical properties of paper products, especially when they come into contact with water. These agents play a crucial role in enhancing the durability, strength, and dimensional stability of paper products. The aim of this review is to provide an overview of the different types of wet-strength agents available and their mechanisms of action. We will also discuss the challenges associated with the use of wet-strength agents and the recent advances in the development of more sustainable and environmentally friendly agents. As the demand for more sustainable and durable paper products continues to grow, the use of wet-strength agents is expected to increase in the coming years.

Keywords: bio-based wet-strength agents; chitosan; paper packaging; polyamideamine-epichlorydrin resin.

Figure 1

Simplified scheme of the process of paper making. The additives, including the wet-strength agents, are added to the pulp in the paper making step, before the formation of the mat of fibers.

Figure 2

Scheme of polymerization of PA and its functionalization with epichlorohydrin to give PAE. The cationic azetidinium group is essential for additive adsorption on the cellulose fibers and for the crosslinking of PAE.

Figure 3

Reactive sites of PAE for co-crosslinking with oxidized cellulose (covalent bond between PAE azetidinium groups and cellulose carboxylic groups) and for homo-crosslinking (covalent bond between PAE free primary/secondary amines and azetidinium groups). Adapted from [15].

Figure 4

Effect of PAE dosage (%) on (A) the increase in wet and dry strength of softwood bleached Kraft pulp and (B) the increase in wet tensile strength of TEMPO-oxidized cellulose nanofibril.

Figure 5

Schematic representation of polycondensation of formaldehyde and melamine and subsequent resin homo- and co-crosslinking with cellulose.

Figure 6

GPAM resin formation per reaction between a PAM cationic copolymer and glyoxal (A); GPAM H-bond and covalent interactions with cellulose (B).

Figure 7

Linear and branched PEI (A); PEI-cellulose cross-linking by glutaraldehyde (B) or epichlorohydrin (C).

Figure 8

Structure of poly(N-vinylformamide)-polyvinylamine PNVF-PVAm copolymer obtained by basic hydrolysis of PNVF.

Figure 9

Main polycarboxylic acids investigated as crosslinkers or wet-strength agents for cellulose fibers and mechanism of action.

Figure 10

Strengthening of cellulose fibers by formation of polyelectrolyte complexes (PECs) onto the fiber surface by adsorption of cationic (A) and anionic (B) polymers.

Figure 11

Examples of techniques used to investigate the effect of wet-strength agents on paper’s physico-chemical properties. (a,b) SEM micrographs of paper fibers (a) and paper fiber reinforcement by CMC-aGO (b) (red circles represent interfibrillar voids) (reproduced from [81]); (c) tensile strength–strain behavior of cellulose acetate butyrate (CAB)/acetone-treated cellulose nanofibers (A-CNF) with different amounts of the bio-derived crosslinker polyisocyanurate D376N. (A) A-CNF; (B) CAB500-5; (C) CAB/D376N (0 wt%); (D) A-CNF/D376N (3.9 wt%); (E) CAB/A-CNF/D376N (7.7 wt%); (F) CAB/A-CNF/D376N (14.3 wt%); (G) CAB/A-CNF/D376N (24.5 wt%). (Reproduced from [82]); (d) FTIR spectra of cottonseed protein (CSP) isolate and paper samples treated with CSP or CSF and nanocellulose (CNC), expansion of the cellulosic region of the amide bonds (1750 cm⁻¹ to 1400 cm⁻¹) (reproduced from [83]).