Alleviation of the plastic deformation of gel ink under strong stress through an esterification of xanthan gum reinforcing its double helix structure

Abstract

As a natural organic polymer, xanthan gum (XG) can alleviate the plastic deformation of gel ink under strong stress and realize the reasonable regulation of the rheological properties of gel ink. However, as the double-helix structure connected by hydrogen bonds cannot resist the mechanical environment of strong stress, XG shows poor shear resistance. In this study, a polymer gel with interpenetrating polymer network structure was prepared by esterifying XG, taking polystyrene maleic anhydride (SMA) as the modifier. In addition to retaining the excellent rheological properties of XG, the generated polymer gel also exhibited high shear resistance. The optimal addition amount of the esterification reaction modifier was determined as mXG: mSMA = 5:3 according to the gel ink standard. With this amount, the viscosity of the modified xanthan gum (SXG) gel increased to 1578.8 mPa·s and 100.7 mPa·s at shear rates of 4 s⁻¹ and 383 s⁻¹, respectively, and the shear resistance increased more than 2 times compared to the unmodified one. It is because of the ester bond formed by esterification that the reaction strengthens the interaction between molecular segments, enabling the new gel to resist to strong mechanical stress. The new polymer gel studied in this paper and the proposed mechanism of action provide new insights for the development of high-end gel ink and also provide theoretical support for the study of rheological properties of non-Newtonian fluids.

1. Introduction

With the continuous improvement of the public use and awareness of gel pens, it has now become the mainstream product of writing instruments, and the production of gel pens will continue to rise in the future. However, due to the poor performance of the neutral ink, there are many problems with the writing of the filled gel pen, such as ink leakage, breaking, and ink accumulation, and their root cause is that the rheological properties of gel ink cannot match the strict writing requirements.

For decades, polymeric materials have shown a great potential in fluid rheological regulation, and polymers can be divided into natural organic polymer compounds and synthetic organic compounds according to the source of the material. The researchers studied the interaction between various polymers with water molecules and pigment particles in aqueous systems and realized the rheological regulation of gel ink by constructing a dynamic network structure. Souza et al. and Zhang et al. introduced acrylic-based polymers, which use their unique polar group to form hydrogen bonds with water to show excellent water absorption performance and have a three-dimensional network structure to achieve the effect of water storage. However, because acrylic acid mainly relies on an alkaline environment to function, it has the problem of pH sensitivity. Du et al. introduced polyurethane polymers; the imine group on the molecular chain and water molecules can form hydrogen bonds, and the weak polar polyester segments form physical cross-links and play the role of water storage, but the strong polarity and rigid urethane groups are easy to microscopically separate from the polyester chain segments due to their large molecular cohesion energy.

However, with the continuous improvement of the rheological properties and environmental protection requirements of gel ink, as well as the high cost of synthetic polymers, researchers have focused on natural organic polymers with excellent performance. Natural organic polymers are usually composed of thousands of atoms covalently bonded with each other to form organic compounds with particularly large relative molecular masses and repeating structural units. The polymer level is relatively large and usually has a tertiary structure. Its hydrogels have gel-like rheological properties, and it has strong plasticity, providing conditions for regulating the rheological properties of gel ink. Natural organic polymer compounds rely on the hydroxyl group on the molecular chain to hydrogen bond with water molecules adsorbing water molecules near the molecular chain, and the active groups on the molecular chain give polymer compounds a negative charge, the negatively charged side chain and the positively charged pigment particles attract each other to achieve the adsorption effect. Other than that, they rely on the intermolecular force and the winding and cross-linking.