How Can CPP Resin Be Optimized for Benzene-Free Ink Systems as the Ink Industry Accelerates Its Transition Toward Solvent-Free Solutions?
On April 17, 2026, five Chinese government ministries, including the Ministry of Industry and Information Technology (MIIT), jointly issued the Industrial Product Green Design Guidelines (2026 Edition). The guidelines identify 11 key directions for green product design, including long-life design, harmless design, lightweight design, energy-saving design, water-saving design, material-saving design, noise reduction design, space-saving design, easy recycling and regeneration design, reusable design, and zero-carbon design.
For ink manufacturers and printing companies using chlorinated polypropylene (CPP) resin, the transition from traditional toluene/xylene-based systems to benzene-free solvent systems is no longer a question of whether to make the change, but rather how to achieve the transition successfully.
1. Solvent System Comparison: Key Differences Between Toluene-Based Systems and Methylcyclohexane-Based Systems
From the perspective of solvent systems, toluene and xylene have traditionally been regarded as excellent solvents for CPP resin. They offer fast dissolution rates, high solubility limits (which can exceed 40%), and stable adhesion performance.
However, benzene-based solvents have been increasingly restricted due to environmental and safety concerns, making the development of alternative solvent systems an urgent requirement for the ink industry.
Methylcyclohexane (MCH): The Most Widely Used Alternative Solution
Methylcyclohexane (MCH) is currently the most common alternative solvent used in benzene-free ink systems. As a cycloalkane-based organic solvent, MCH has a boiling point of approximately 101°C and can partially replace toluene and xylene in environmentally friendly formulations.
In addition to traditional dissolution systems based on toluene and xylene, CPP resin can also be dissolved in mixed solvent systems containing ester solvents and MEK, making it suitable for the production of eco-friendly inks and coatings.
2. Resin Selection: Adaptation Strategies for Different Chlorine Contents of CPP Resin in Benzene-Free Systems
In benzene-free solvent systems, chlorine content is one of the most critical factors when selecting cpp resin.
According to industry data, chlorinated polypropylene resin can generally be divided into two categories based on chlorine content:
- Low chlorination level CPP resin: approximately 20%–40% chlorine content
- High chlorination level CPP resin: above 60% chlorine content
CPP grades used for printing inks and coatings are mainly concentrated in the 20%–40% chlorine content range.
CPP with 20%–25% Chlorine Content
(Example: H-2A grade, chlorine content 20%)
CPP resin within this range is relatively easy to dissolve in benzene-free solvent systems, especially when using MCH/ethyl acetate mixed solvents, where dissolution conditions are relatively mild.
Typical applications include:
- PP surface treatment agents
- Adhesion promoters for modified PP substrates
- (such as PP+EPDM and PP reinforced with glass fiber)
In industrial applications, CPP with approximately 26%–27% chlorine content is commonly used as a surface treatment resin to improve adhesion between non-polar substrates such as PP and EPDM.
CPP with 25%–30% Chlorine Content
(Example: 602P grade, chlorine content 27%)
This is the most widely used CPP specification for BOPP film printing inks and provides the best overall balance in benzene-free systems.
For BOPP film printing applications, this chlorine content range provides:
- Excellent adhesion to PP substrates
- Balanced dissolution performance
- Stable printing performance
Therefore, CPP resin with 25%–30% chlorine content is usually considered the preferred base resin for conventional benzene-free ink formulations.
CPP with 30%–35% Chlorine Content
(Example: 601P grade, chlorine content 33%)
CPP resin in this range performs exceptionally well in benzene-based solvent systems, offering:
- High gloss
- Excellent compatibility
- Strong overall coating performance
However, in benzene-free systems, dissolution becomes significantly more difficult. It may require:
- Increasing the proportion of ester solvents
- Additional heating assistance during dissolution
Ester-Soluble CPP Resin: A Specialized Solution for Benzene-Free Systems
Ester-soluble CPP resin is specifically developed for benzene-free applications.
Unlike conventional CPP grades, ester-soluble CPP does not require methylcyclohexane. It can dissolve directly in ester solvents such as:
- Ethyl acetate
- n-Propyl acetate
This makes it particularly suitable for producing:
- Benzene-free gravure composite inks
- Benzene-free and ketone-free environmentally friendly printing inks
Ester-soluble CPP products typically feature:
- Low viscosity
- Excellent dissolution performance
- High transfer efficiency in fine printing areas
- Good redissolution properties
- Reduced risk of plate blocking
However, it should be noted that ester-soluble CPP generally has a lower chlorine content (usually below 20%). Therefore, its adhesion performance may be slightly weaker compared with conventional CPP products containing 25%–30% chlorine.
When selecting CPP resin, manufacturers need to balance dissolution performance and adhesion requirements according to the specific application.
Relationship Between Chlorine Content and Solubility
The relationship between chlorine content and solubility deserves special attention.
Some studies indicate that when CPP chlorine content reaches 50.1%, its solubility can reach 90.0%, allowing almost complete dissolution in xylene. However, adhesion performance becomes significantly poorer.
This demonstrates that when selecting CPP resin for benzene-free systems, manufacturers should not simply pursue higher solubility at the expense of adhesion performance.
The key is to achieve an optimal balance between:
- Solubility
- Adhesion strength
- Printing performance
3. Four Key Control Points in Formulation Design
The MCH/ethyl acetate ratio of 1:1 is one of the most widely tested and applied solvent combinations in the industry.
Depending on the chlorine content of CPP resin and printing speed requirements, the proportion of ester solvents can be adjusted accordingly.
For CPP resin with higher chlorine content (above 30%), the ethyl acetate ratio can be increased appropriately to 60%–70%.
Aliphatic hydrocarbon solvent 120# solvent oil (white oil) can also be used as a supplementary solvent to help achieve a better evaporation balance during the drying process.
The boiling point of MCH (101°C) is lower than that of toluene (110°C), which means MCH evaporates faster.
Therefore, compared with traditional toluene-based systems, it is recommended to reduce the hot air drying temperature by 5–10°C to prevent problems caused by excessive solvent evaporation, such as:
- Poor leveling
- Ink layer whitening
- Uneven coating appearance
At the same time, manufacturers may need to:
- Increase oven length
- Reduce printing speed
to ensure sufficient solvent release and complete drying of the ink layer.
The recommended CPP resin content is generally 5%–15% of the total ink formulation.
If the addition level is too low (below 5%), the improvement in adhesion performance will be insufficient.
If the addition level is too high (above 20%), the internal cohesion strength of the ink film may decrease, potentially affecting:
- Lamination strength
- Overall printing performance
Whenever changing CPP resin grades or adjusting solvent ratios, small-scale testing should always be conducted before full-scale production.
4. Three Frequently Asked Questions About CPP Resin in Benzene-Free Systems
When CPP resin is fully dissolved and its molecular chains are sufficiently extended, there is usually little difference in adhesion performance between benzene-free systems and traditional toluene-based systems.
The adhesion of CPP resin to PP substrates mainly comes from the shared polypropylene backbone structure between CPP and PP.
Once CPP molecular chains are fully expanded, the “swelling and anchoring” effect on PP substrates depends primarily on molecular structure rather than the solvent itself.
However, it is important to note that the dissolution capability of MCH for CPP resin is indeed lower than that of toluene.
If dissolution is incomplete and the polymer chains cannot fully extend, adhesion performance will be negatively affected.
To ensure stable adhesion performance, three key factors should be controlled:
- Select an appropriate chlorine content range It is recommended to start with CPP grades containing 25%–30% chlorine.
- Use an MCH/ethyl acetate mixed solvent system at approximately 1:1 ratio
- Apply heating assistance when necessary A temperature range of 40–50°C can help improve dissolution efficiency.
In addition, selecting specially designed ester-soluble CPP resin can fundamentally reduce the risk of incomplete dissolution.
The difference is significant.
CPP with 20%–25% Chlorine Content
CPP resin in this range dissolves relatively easily in an MCH/ethyl acetate mixed solvent system.
Under room temperature stirring conditions, complete dissolution can usually be achieved within 30–60 minutes.
CPP with 25%–30% Chlorine Content
CPP resin in this range requires:
- Longer dissolution time
- Stronger mixing conditions
It is recommended to combine stirring with heating assistance at 40–50°C.
CPP with More Than 30% Chlorine Content
CPP resin with chlorine content above 30% shows significantly reduced solubility in MCH.
In this case, manufacturers may need to:
- Increase the proportion of ester solvents such as ethyl acetate
- Or directly select ester-soluble CPP resin
Ester-soluble CPP can dissolve directly in ester solvents without adding MCH.
If the formulation requires higher chlorine content, manufacturers may consider using specially optimized high-chlorine CPP grades designed for benzene-free systems.
For example, CPP 833S with approximately 32% chlorine content provides broader compatibility and improved dissolution performance compared with conventional products.
There are three practical evaluation methods:
- 1. Visual Inspection Method
A fully dissolved CPP solution should appear as:
- Uniform
- Transparent
- Free from visible particles or sediment
Any cloudiness, particles, or precipitation may indicate incomplete dissolution.
- 2. Drawdown Film Test Method
Use a scraper or applicator to coat a small amount of CPP solution onto a glass plate.
After drying, observe the coating appearance:
- A uniform transparent film indicates complete dissolution
- Particles or white haze indicate incomplete dissolution
- 3. Printing Verification Method
Conduct a small-batch printing trial and perform a cross-cut adhesion test.
This is the most reliable final evaluation method.
Before switching completely to a benzene-free system, manufacturers are strongly advised to conduct:
- Small-scale dissolution testing
- Adhesion verification
- Printing performance evaluation
Only after confirming satisfactory results should full-scale production conversion be carried out.
Final Thoughts
The transition toward benzene-free ink systems is not simply a replacement of one solvent with another. It requires a systematic adjustment involving:
- CPP resin selection
- Solvent formulation design
- Drying process optimization
- Printing performance verification
Among these factors, CPP resin selection is the key factor determining the success of the transition.
CPP grades with 25%–30% chlorine content provide the best balance between dissolution performance and adhesion strength and are recommended as the preferred choice for benzene-free ink systems.
Meanwhile, ester-soluble CPP resin offers a more convenient technical pathway, allowing direct dissolution in ester solvents without the need for MCH.
Regardless of which technical route is selected, small-scale testing and validation remain essential steps before large-scale implementation.
