Analysis of Functional Treatments for Specialty Papers
During the production of specialty papers, various processes are employed-along with the incorporation of specific chemical additives-to fulfill their distinct functional requirements. The functions of these specialty papers are diverse, encompassing properties such as oil resistance, water resistance, flame retardancy, and heat resistance. In the following sections, we will delve into the underlying principles and methodologies behind the realization of these functions.
▲ Oil-Resistant Treatment
First is the oil-resistant treatment. The key to this process lies in the use of fluorine-containing oil repellents. The primary constituents of these repellents are fluoroalkyl compounds-such as perfluoroalkyl-substituted amines, quaternary ammonium salts, and acrylates. These cationic dispersions are introduced into the paper structure via a "wet-end" addition process during papermaking. After the paper is formed and subsequently dried under heat, these additives arrange themselves on the paper fibers to form a monomolecular layer with the perfluoroalkyl chains oriented outward, thereby effectively preventing the penetration of oils and greases. Additionally, anionic dispersions can be applied to the paper through surface impregnation or surface sizing techniques; these methods similarly create a monomolecular layer on the fiber surfaces, achieving the desired oil-resistant effect.
▲ Water-Resistant Treatment
Next is the water-resistant treatment. Paraffin wax emulsion is an early-stage and relatively low-cost water-repellent agent, typically applied to the paper via surface impregnation. Furthermore, cationic water repellents can be utilized through wet-end addition during the papermaking process. However, silicone-based agents are the most widely used water repellents. Their aqueous emulsion products are formulated by incorporating emulsifiers to create polysiloxane emulsions, to which nano-scale silicon dioxide is then added. This formulation induces a "lotus leaf effect" on the paper's surface, allowing water droplets to roll off easily and thereby achieving effective water resistance.
▲ Flame-Retardant Treatment
Let us now examine the flame-retardant treatment. There are two primary methods for incorporating flame retardants into paper: one involves adding water-insoluble solid flame-retardant powders to the pulp as fillers during papermaking; the other involves introducing water-soluble flame retardants into the paper structure through an impregnation process. These flame retardants include nitrogen-based and boron-based compounds, as well as water-insoluble solid powders such as aluminum hydroxide and magnesium hydroxide.
▲ Heat-Resistant Treatment and Anti-Stick/Release Properties
Finally, we address heat-resistant treatments and anti-stick/release properties. To protect fibers and enhance the heat resistance of paper, chemically modified natural polymers-such as mixtures of glucomannan and nitrogen-containing compounds-are applied to the paper via coating, impregnation, or spraying methods. Furthermore, synthetic polymer resins-such as silicone resins and higher-carbon-chain polyolefins-are utilized to further improve the paper's thermal resistance. Heat-dissipating chemicals, such as metal hydroxides containing crystal water, are primarily employed to protect paper fibers against high temperatures. Silicone resins, in particular, stand out as superior high-temperature protective agents due to the high bond energy, stability, and thermal resistance of their Si–O bonds, as well as their inherent non-stick and barrier properties.
The process of applying non-stick and barrier treatments to paper is critical, impacting production across various sectors, including self-adhesive labels and pressure-sensitive adhesive papers. The chemicals used in this process-known as release agents (also referred to as peel agents, anti-stick agents, or mold release agents)-are designed to form a dense layer on the paper's surface. This layer acts as a barrier, preventing the penetration of adhesive polymer resins and thereby facilitating easy detachment. Release agents come in a wide variety of forms; among them, silicone-based release agents are currently the most widely used due to their low toxicity, excellent surface energy, and superior wettability with paper substrates. Available in solvent-based, emulsion-based, and solvent-free formulations, these agents undergo cross-linking reactions classified as addition-cure, condensation-cure, or radiation-cure types. Addition-cure release agents demonstrate particularly outstanding performance in terms of curing completeness, release properties, anti-adhesion capabilities, and the flexibility afforded in coating formulation design.
▲ Impregnation Coating Processing
Furthermore, impregnation coating is a vital technique for endowing paper with specialized properties, such as water resistance and fire resistance. A wide array of impregnating agents-including synthetic resins and synthetic latexes-are utilized for this purpose, and the types of base paper used for impregnation coating vary significantly. Regardless of the specific type of paper employed, however, it must satisfy a set of specific performance criteria:
① The base paper must possess sufficient wet strength to prevent tearing or rupturing during the impregnation process; consequently, long-fiber wood pulp is typically selected for this application.
② The base paper must also exhibit excellent absorbency to facilitate the effective penetration of the impregnating agent. Concurrently, to preserve the inherent properties of the base paper, sizing treatment is typically omitted; furthermore, the degrees of pulping, pressing, and calendering are all maintained at relatively low levels.
Next, let us examine in detail the various types of impregnating agents and their respective applications. First are resin-based impregnating agents-including phenolic resins and melamine-formaldehyde resins-which endow the impregnated paper with a multitude of properties such as water resistance, oil resistance, abrasion resistance, and electrical insulation; these are widely utilized in fields such as decorative papers and laminated composites. Second are latex-based impregnating agents-such as styrene-butadiene latex and neoprene latex-which enhance the internal strength and abrasion resistance of the paper sheet, making them suitable for specialized applications like security papers and wood veneer backing papers. Additionally, there are oil- and paraffin-based impregnating agents-such as drying oils and paraffin wax-which impart excellent water resistance, moisture resistance, high tensile strength, and transparency to the paper; these are frequently employed in packaging papers designed to be oil-proof, water-resistant, and moisture-proof.
Regarding the impregnation process itself, a shallow-tray immersion tank is typically employed, wherein both the immersion roller and the tank are constructed from stainless steel to prevent corrosion. The nip rollers consist of a rubber roller and a chrome-plated roller, featuring adjustable pressure settings; moreover, the rubber roller undergoes a release treatment to prevent the latex from adhering to its surface. Prior to entering the drying section, the impregnated paper is preheated using far-infrared radiation to prevent it from sticking to the drying cylinders. Simultaneously, the first cylinder within the drying section is also subjected to a release treatment to ensure that the paper can be smoothly peeled away.
