In the formulation of printing inks, the components are often polymers or condensation products of organic compounds. Today, we will examine some key aspects of resins used in printing inks:
　　(1) Solubility and release properties of petroleum resins. Solubility refers to a resin’s ability to dissolve in various solvents. In ink formulation, resins are typically used in liquid form; therefore, solid resins must exhibit a certain degree of solubility to be applicable. Even some resins that are already liquid may have excessively high viscosities and thus require solvent dilution to reduce viscosity before use. Consequently, solubility is an important performance characteristic of resins.
　　Release refers to the ability of a solvent to be liberated from a resin that has been dissolved by it. After ink printing, the solvent separates from the resin, causing the resin to gel and dry. The release properties of petroleum resins are closely related to their solubility: petroleum resins with excellent solubility hinder the release of solvent, resulting in poorer release performance. Since petroleum resins transition from a liquid state upon dissolution to a solid state upon release—this transformation being exploited in printing—the resin’s solubility must be appropriately balanced to ensure an optimal equilibrium between its solubility and release characteristics.
　　(2) Softening point of petroleum resins. Solid resins gradually soften with increasing temperature, becoming more pliable before eventually turning into a liquid state. The softening point is the temperature at which the resin, during its softening process, transitions into a viscous liquid; this temperature is also referred to as the flow temperature. The softening point rises as the resin’s relative molecular weight increases. In general, a higher softening point corresponds to greater solvent release. This trend is particularly pronounced for resins with identical chemical compositions. Therefore, resins used in inks are preferably those with relatively high softening points.
　　(3) Viscosity of petroleum resin. Viscosity refers to the solution viscosity of the resin under specified conditions, also known as dissolution viscosity. The viscosity of the resin is influenced by its relative molecular mass, solubility, solution temperature, and solvent content.
　　(4) Acid value of petroleum resins. Many resins used in inks are synthesized via condensation reactions between organic acids—such as resin acids, fatty acids, and aromatic acids—and other compounds. Consequently, these resins invariably contain unreacted carboxyl groups, imparting them a certain acid value.
　　(5) Color and transparency of petroleum resin. The color of the resin is attributable to various pigment impurities present, and it depends on the properties and purity of the raw materials as well as the synthesis process. Resins typically exhibit a pale yellow hue.
　　Transparency is related to the moisture and other impurities present in petroleum resin. Transparency affects the resin’s solubility; resins with low transparency exhibit poor solubility.

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