Introduction
Polythene (polyethylene – PE) is one of the most widely used plastics in the world due to its excellent chemical resistance, flexibility, toughness, and low cost. It is extensively used in packaging films, carry bags, containers, coatings, pipes, and molded products. However, despite its numerous advantages, polythene suffers from a critical limitation: its inert surface nature in virgin form.
Virgin polyethylene does not readily accept printing inks, adhesives, lacquers, or coatings. This makes direct printing, lamination, or bonding extremely difficult. To overcome this limitation, the surface of polythene must be altered through controlled treatment methods that oxidize an extremely thin surface layer without affecting the bulk properties.
This article explains why PE is inert, why surface treatment is necessary, and how different surface treatment methods work, including electronic and chemical processes. Economic and practical considerations behind each method are also discussed.
Why Virgin Polythene Is Chemically Inert
Non-Polar Molecular Structure
Polyethylene is made of long chains of repeating –CH₂– units. This structure results in:
- No polar functional groups
- No active chemical sites on the surface
- Very low surface energy
Because of this, inks and adhesives cannot wet the surface properly.
Low Surface Energy of PE
Surface energy plays a crucial role in adhesion. For effective bonding:
- Surface energy of substrate must be higher than the surface tension of ink or adhesive
- Virgin PE has surface energy of ~30–31 dynes/cm
- Most printing inks require ≥38 dynes/cm
As a result, inks bead up instead of spreading, leading to poor adhesion.
Problems Faced Without Surface Treatment
When untreated polythene is printed or glued, several failures occur:
- Ink smudging or peeling
- Poor lamination bond strength
- Delamination during handling
- Print fading after rubbing
- Failure under moisture or heat
These issues make untreated PE unsuitable for commercial printing and packaging applications.
Need for Surface Modification of Polythene
To enable printing, coating, or gluing, the PE surface must be chemically activated. This is achieved by:
- Introducing polar functional groups
- Increasing surface roughness at molecular level
- Raising surface energy
- Improving wettability
Importantly, only an extremely thin surface layer (a few nanometers thick) is modified, while the core material remains unchanged.
Principle of Surface Oxidation
Surface treatment works on a simple principle:
Controlled oxidation of the PE surface creates polar groups such as hydroxyl, carbonyl, and carboxyl groups.
These groups improve:
- Ink anchorage
- Adhesive bonding
- Coating adhesion
The oxidation must be precise, uniform, and shallow to avoid damaging the polymer.
Electronic Surface Treatment Methods
Electronic or electrical treatment methods are the most widely used for PE films, sheets, and coatings because they are economical, fast, and easy to integrate into production lines.
Corona Discharge Treatment
What Is Corona Treatment?
Corona treatment involves passing PE film over a high-voltage electrode that generates a corona discharge in air.
- Voltage: 5–20 kV
- Medium: Atmospheric air
- Process time: Milliseconds
How Corona Treatment Works
The high-energy discharge:
- Breaks molecular bonds on PE surface
- Generates ozone and reactive oxygen species
- Oxidizes the topmost molecular layer
This results in increased surface energy.
Advantages of Corona Treatment
- Continuous in-line process
- Low operating cost
- Suitable for films, sheets, and coatings
- Widely used in packaging industry
Limitations of Corona Treatment
- Treatment effect may decay over time (aging effect)
- Less effective on thick molded parts
- Surface energy may reduce if stored improperly
Plasma Treatment
Overview of Plasma Treatment
Plasma treatment uses ionized gas under controlled conditions to modify PE surfaces.
- Gases used: Oxygen, nitrogen, argon
- Environment: Atmospheric or vacuum plasma
Mechanism
High-energy ions and radicals:
- Etch the surface at nano-scale
- Introduce polar functional groups
- Improve surface roughness
Advantages
- Very uniform treatment
- High adhesion strength
- Long-lasting effect
Disadvantages
- Higher equipment cost
- Slower compared to corona
- More complex operation
Flame Treatment
What Is Flame Treatment?
In flame treatment, a controlled flame is passed over the PE surface.
How It Works
The flame produces:
- Heat
- Ionized species
- Oxidative environment
This modifies the surface chemistry instantly.
Applications
- Bottles and molded PE containers
- Automotive plastic parts
- Thick sections where corona is ineffective
Limitations
- Risk of overheating
- Requires precise flame control
- Not ideal for thin films
Electron Beam Treatment (Advanced Method)
Electron beam treatment uses high-energy electrons to activate polymer surfaces.
Features
- Deep and uniform activation
- Highly effective for industrial coatings
- Used in specialty films and coatings
Drawbacks
- Extremely expensive
- High safety requirements
- Limited commercial use
Chemical Surface Treatment of Polythene
Overview
Chemical treatment involves exposing PE surface to strong oxidizing chemicals such as:
- Chromic acid
- Potassium permanganate
- Nitric acid
How Chemical Treatment Works
These chemicals:
- Attack the PE surface
- Introduce polar functional groups
- Increase surface roughness
Why Chemical Treatment Is Rarely Used
Although effective, chemical treatment is seldom used due to several practical challenges.
Economic Limitations
- High cost of chemicals
- Need for chemical recovery systems
- High waste treatment expenses
Handling and Safety Issues
- Corrosive and hazardous chemicals
- Strict safety protocols required
- Environmental compliance issues
Process Control Difficulties
- Difficult to achieve uniform treatment
- Risk of over-etching or surface damage
- Batch-wise processing only
Because of these limitations, chemical treatment is largely restricted to laboratory use or specialized applications.
Comparison: Electronic vs Chemical Surface Treatment
| Parameter | Electronic Treatment | Chemical Treatment |
|---|---|---|
| Cost | Low to moderate | High |
| Safety | Relatively safe | Hazardous |
| Speed | Very fast | Slow |
| Uniformity | High | Difficult |
| Environmental impact | Minimal | High |
| Industrial usage | Very common | Rare |
Applications Requiring PE Surface Treatment
Surface-treated PE is essential in:
- Printed packaging films
- Laminated flexible packaging
- Labels and stickers
- Medical packaging
- Extrusion coatings
- Adhesive tapes
- Automotive plastic parts
Without treatment, adhesion failures are unavoidable.
Surface Energy Testing After Treatment
To ensure effectiveness, surface treatment is measured using:
- Dyne pens
- Contact angle measurement
- Ink wetting tests
A minimum of 38–42 dynes/cm is typically required for good printability.
Aging Effect of Treated Polythene
What Is Surface Aging?
Over time, treated PE surfaces may:
- Reorient polar groups inward
- Absorb contaminants
- Lose surface energy
How to Reduce Aging
- Print or laminate soon after treatment
- Store films in clean, dry conditions
- Use higher treatment levels when needed
Future Developments in PE Surface Modification
Research is focused on:
- Nano-coatings for permanent activation
- Eco-friendly plasma systems
- Hybrid electronic-chemical treatments
- Long-lasting surface functionalization
These advancements aim to reduce energy use while improving adhesion stability.
Conclusion
Polythene in its virgin form is an inert, low-energy material that does not accept printing inks or adhesives. To make it suitable for commercial applications, the surface must be altered by oxidizing an extremely thin surface layer.
Among available methods, electronic surface treatments such as corona, plasma, and flame treatment dominate industrial use due to their efficiency, safety, and economics. Chemical treatment, although effective, is rarely adopted because of high costs, safety risks, and handling difficulties.
Understanding PE surface treatment is essential for manufacturers, converters, and designers who seek reliable printing, bonding, and coating performance without compromising the polymer’s bulk properties.
Author
Sujith Shanmugam
Artist, Mold & Idol Maker | Material Processing Enthusiast
Experience: 5+ Years
Website: https://gift-for-you.online/
Email: sujith11me100@gmail.com
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