How Corona Treatment Enhances Adhesion on PP Materials: A Detailed Principle

PP (Polypropylene) has a symmetrical molecular structure with no polar groups, resulting in extremely low surface energy (about 29-30 mN/m). This is far lower than the surface energy required by inks, adhesives, and other bonding substances (usually ≥38 mN/m), making it hard for these substances to spread and bond on PP surfaces. Corona treatment solves this problem through dual physical and chemical effects, significantly improving PP’s adhesion. Its specific principle has three core steps:

1. Surface Cleaning: Remove Low-Molecular Contaminants to Eliminate Bonding Barriers

During production and storage, PP surfaces easily retain low-molecular wax, oil, dust, and other contaminants. These form an “isolation film” that directly blocks contact between inks/adhesives and the PP base material.

When corona treatment is applied, “corona discharge” (triggered by a high-voltage electric field) produces high-speed active particles like electrons and ions. Acting like “micro-cleaners,” these particles hit the PP surface with high kinetic energy. They break down low-molecular wax, oil, and other contaminants into small-molecule gases (such as CO₂ and H₂O) or strip them away. This exposes the PP’s “clean surface,” clearing obstacles for subsequent bonding.

2. Surface Etching: Create Micro-Rough Structures to Boost Mechanical Bonding

Raw PP surfaces are smooth, so bonding substances have limited contact with PP. They only stick via weak intermolecular forces, which easily peel off.

High-energy particles (e.g., electrons, oxygen ions) from corona discharge cause “micro-etching” on the PP surface:

• First, particles hit PP’s molecular chains, breaking some C-C bonds. This forms tiny pits and grooves (micro-scale, invisible to the naked eye but observable under an electron microscope) on the surface.
• Second, etching increases PP’s specific surface area. The once-smooth surface becomes “uneven,” greatly expanding contact with inks or adhesives.

When bonding substances cover the etched PP surface, they fill these micro-pits and grooves, forming a “mechanical interlock” like “anchors.” This “mechanical bonding force” significantly improves bonding strength and prevents the bonded layer from falling off easily.

3. Surface Modification: Add Polar Groups to Strengthen Chemical Bonding

PP molecules consist of C and H, with non-polar molecular chains (no groups with charge imbalance). However, inks and adhesives often contain polar groups like hydroxyl (-OH) and carboxyl (-COOH). Based on the “like dissolves like” principle, the weak van der Waals forces between non-polar PP surfaces and polar bonding substances cannot form stable bonds.

A key role of corona treatment is to introduce polar groups via “surface oxidation”:

• During corona discharge, oxygen in the air is excited into active oxygen species like oxygen free radicals (·O) and ozone (O₃).
• These active species react with the broken C-C bonds on the PP surface, adding polar groups (e.g., -OH, -COOH, C=O) to the PP molecular chain.

These new polar groups form stronger chemical forces (e.g., hydrogen bonds, dipole-dipole interactions) with polar groups in inks/adhesives. This turns the bond from “weak physical adsorption” to “stable chemical bonding,” further enhancing adhesion durability.

Summary: The Core Value of Corona Treatment – Synergy of “Cleaning + Etching + Modification”

Corona treatment is not a single effect but a synergistic process: “surface cleaning (removing barriers) → micro-etching (increasing area, enhancing mechanical force) → polar modification (strengthening chemical force).” It fundamentally solves PP’s low surface energy and non-polarity issues.

Finally, PP’s bonding conditions are optimized: it gains high surface energy for bonding substances to spread, and micro-structures plus polar groups boost mechanical and chemical bonding forces respectively. This achieves a significant adhesion improvement.

Note: Corona treatment effects are “time-sensitive.” Polar groups on modified PP surfaces may gradually revert to non-polar due to storage conditions (e.g., humidity, temperature). Thus, it’s recommended to complete ink printing or bonding within 24 hours after treatment for the best results.