Surface coating is a common method to improve the waterproof and oil-proof properties of thermal paper express labels. A layer of waterproof and oil-proof coating, such as acrylic resin, fluorocarbon resin, etc., is applied to the surface of thermal paper. These resins have low surface energy and can effectively prevent water and oil molecules from adhering to and penetrating the label surface. The thickness and uniformity of the coating are crucial to the waterproof and oil-proof effect. Generally speaking, the coating thickness is between a few microns and tens of microns. Through precise coating processes such as blade coating and gravure coating, the coating is ensured to be evenly covered on the surface of thermal paper to form a continuous and dense protective film.
The chemical treatment process mainly chemically modifies the thermal paper to make its surface waterproof and oil-proof. For example, when the thermal paper is treated with a silane coupling agent, the organic groups in the silane coupling agent molecules can react chemically with the cellulose molecules on the surface of the paper to form a chemical bond. At the same time, its siloxy groups can be hydrolyzed to form silanol groups, which are then polycondensed to form a siloxane network structure. This structure can reduce the polarity of the paper surface and improve its waterproof and oil-proof properties. In addition, thermal paper can also be treated with fluoride, which can form a fluorocarbon film on the surface of the paper, with extremely low surface energy, so that water and oil form water droplets and oil droplets on the label surface, which are difficult to penetrate.
The lamination process is to cover the surface of the thermal paper express label with a layer of plastic film, such as polyester film (PET), polypropylene film (PP), etc. The film has good waterproof and oil-proof properties and can isolate the thermal paper from the water and oil in the outside world. The lamination process usually adopts hot pressing lamination or cold pressing lamination. Hot pressing lamination is to laminate the film and thermal paper together by heating and pressurizing, so that the film and thermal paper form a strong bond. Cold pressing lamination is to paste the film on the thermal paper by applying pressure at room temperature. This method is suitable for some temperature-sensitive thermal papers. The quality of lamination depends on factors such as the quality of the film, the pressure and temperature of the lamination. These parameters need to be strictly controlled to ensure the flatness and bonding strength of the lamination.
Nanotechnology is also widely used in the waterproof and oil-proof treatment of thermal paper express labels. By introducing nanomaterials, such as nano-silicon dioxide and nano-zinc oxide, on the surface of thermal paper, the waterproof and oil-proof properties of the label can be significantly improved. Nanomaterials have a huge specific surface area and special surface properties, and can form a nano-scale rough structure on the surface of paper. According to the lotus effect, this rough structure can make water and oil form a high contact angle on the surface of the label, thereby achieving a waterproof and oil-proof effect. At the same time, nanomaterials can also interact with the chemical composition of the coating or paper surface to further enhance the waterproof and oil-proof properties. For example, nano-silicon dioxide can cross-link with organic molecules in the resin coating to improve the density and stability of the coating.
In the coating or chemical treatment process, the cross-linking curing process is a key link to improve the waterproof and oil-proof properties. Through the cross-linking reaction, a three-dimensional network structure is formed between the polymer molecules in the coating, thereby improving the hardness, wear resistance and water resistance of the coating. Cross-linking curing can be achieved by thermal curing, photocuring or chemical curing. Thermal curing is to react the cross-linking agent in the coating with the polymer molecules at a certain temperature to form a cross-linked structure. Photocuring is to use ultraviolet irradiation to make the photoinitiator in the coating produce free radicals to initiate a cross-linking reaction. Chemical curing is to make the coating undergo a cross-linking reaction at room temperature by adding a chemical curing agent. The degree of cross-linking and curing directly affects the waterproof and oil-proof properties of the coating. It is necessary to select the appropriate curing method and curing conditions according to the specific material and process requirements to achieve the best waterproof and oil-proof effect.
Post-processing technology is also very important for further improving the waterproof and oil-proof properties of thermal paper express labels. For example, after coating or laminating, the label can be heat-treated or ultraviolet-treated to make the bond between the coating or film and the thermal paper more solid and improve the waterproof and oil-proof properties. In addition, the label can also be surface-grinded or polished to reduce the roughness of the label surface and reduce the adsorption points of water and oil on the surface, thereby improving the waterproof and oil-proof effect. At the same time, during packaging and storage, care should be taken to avoid contact between the label and water and oil to prevent the waterproof and oil-proof properties from being damaged.
In order to ensure that the waterproof and oil-proof properties of thermal paper express labels meet the requirements, strict quality inspection and control are required. Commonly used detection methods include contact angle measurement, water absorption test, oil resistance test, etc. Contact angle measurement can directly reflect the waterproof and oil-proof performance of the label surface. The larger the contact angle, the better the waterproof and oil-proof performance. The water absorption test is to immerse the label in water and measure the amount of water it absorbs. The lower the water absorption, the better the waterproof performance. The oil resistance test is to contact the label with oil substances, observe whether there is penetration, discoloration, etc. on its surface, and evaluate its oil resistance. Through the detection and analysis of these indicators, the process parameters are adjusted in time to ensure the stability and reliability of the waterproof and oil-proof processing technology to meet the express industry's requirements for the waterproof and oil-proof performance of thermal paper labels.
