High-density polyethylene (HDPE), due to its high crystallinity, low surface energy, and non-polarity, often faces the problem of insufficient surface printing adhesion when used as packaging materials such as high-density polyethylene medicinal ointment boxes. Inks or coatings struggle to form a durable and robust coating on its surface, easily resulting in peeling and fading, affecting product appearance and information transmission. Improving the printing adhesion of HDPE medicinal ointment boxes requires a comprehensive approach encompassing surface treatment, material selection, and process optimization.
Surface cleaning is a fundamental step in improving adhesion. During HDPE production and processing, the surface easily becomes contaminated with oil, mold release agents, dust, and other impurities. These contaminants hinder direct contact between ink and the substrate, forming a physical barrier layer. Solvent cleaning and ultrasonic cleaning methods are necessary to thoroughly remove surface dirt. For example, wiping the surface with isopropanol or ethanol can effectively dissolve oily contaminants; ultrasonic cleaning uses high-frequency vibration to peel away tiny particles, ensuring surface cleanliness and creating favorable conditions for subsequent processing.
Physical modification enhances mechanical interlocking by increasing surface roughness. Abrasion uses mechanical friction to create a micro-uneven structure on the HDPE surface, increasing the contact area between the ink and the substrate. For example, sanding or sandblasting can generate a uniform rough texture on the surface, enhancing ink penetration and anchoring. Flame treatment alters the surface chemical composition through high-temperature oxidation, forming polar groups on the HDPE surface and increasing surface energy. Strict control of flame temperature and treatment time is crucial to avoid over-oxidation that could lead to material deformation or performance degradation.
Chemical treatment introduces active groups through surface corrosion or grafting reactions. Solvent vapor treatment places HDPE in a hot solvent atmosphere (such as toluene or trichloroethylene), dissolving the amorphous regions of the surface and forming a micro-rough structure, while simultaneously oxidizing and introducing hydrophilic functional groups. Chemical oxidation uses oxidizing agents such as ammonium persulfate or silver sulfate to treat the HDPE surface at specific temperatures, generating polar groups such as carbonyl and ether groups, significantly increasing surface energy. For example, immersing HDPE products in a prepared oxidizing agent solution at room temperature for more than 20 minutes can significantly increase surface tension and enhance ink adhesion.
Corona treatment and plasma technology activate surface molecules through high-energy particles. Corona discharge uses high-frequency, high-voltage equipment to generate a corona phenomenon on the HDPE surface, ionizing air to produce ozone and nitrogen oxides, which react with surface molecules to form polar groups. This method is simple to operate and convenient, but the treatment effect decays over time, requiring timely completion of printing operations. Plasma treatment generates highly active plasma through glow discharge or arc discharge to etch and activate the HDPE surface, introducing oxygen- or nitrogen-containing functional groups and significantly increasing surface energy. For example, low-temperature plasma jetting can form a uniform active layer on the HDPE surface, enhancing the chemical bond between the ink and the substrate.
Adhesion treatment agents, acting as an intermediate layer, enhance interlayer bonding through intermolecular forces and mechanical interlocking. Targeting the non-polar nature of HDPE, specialized treatment agents (such as E-2 type substances) have been developed. Their molecular structure contains polyethylene segments similar to HDPE, enabling bottom-layer anchoring through the principle of similar compatibility. Simultaneously, the carboxyl and amino groups at the end groups can form chemical bonds with the resin or solvent in the ink, enhancing the upper-layer bonding. For example, applying E-2 treatment agent to the HDPE surface and baking it plasticizes it forms a transition layer that combines flexibility and adhesion, significantly improving print fastness.
Ink and process matching are crucial for ensuring adhesion. Specialized inks should be selected based on the surface characteristics of HDPE, such as inks containing chlorine-based solvents or highly polar resins, whose solubility parameters are closer to HDPE, enhancing wetting and dissolving effects. Printing process parameters (such as spraying pressure, drying temperature, and coating thickness) must be compatible with the surface condition of the treated HDPE. For example, a low-temperature, slow-drying process should be used to avoid excessively rapid ink curing, which could lead to internal stress concentration; the coating thickness should be controlled within an appropriate range to prevent cracking or peeling due to excessive thickness.
Improving the print adhesion of high-density polyethylene medicinal ointment boxes requires a comprehensive approach, utilizing surface cleaning, physical modification, chemical treatment, plasma activation, adhesion promoters, and ink process optimization. Through systematic surface treatment and process control, the surface polarity and wettability of HDPE can be significantly improved, enhancing the chemical bonding and mechanical anchoring between ink and substrate, thereby meeting the stringent requirements of pharmaceutical packaging for printing quality and durability.
