How Can CPP Resin Overcome Migration Testing Challenges for Food Contact Materials?
The National Food Safety Standard GB 4806.15-2024: Adhesives Used in Food Contact Materials and Articles was officially implemented on February 8, 2025. This is China’s first dedicated standard specifically regulating adhesives for food contact applications. It clearly defines requirements for key indicators, including overall migration limit (≤10 mg/dm²), potassium permanganate consumption (≤10 mg/kg), and heavy metal migration (calculated as Pb, ≤1 mg/kg) for adhesives that directly contact food.
For companies using chlorinated polypropylene (CPP) resin in inks and coatings, migration testing has become the “entry ticket” for entering the food packaging market. Without meeting compliance requirements, products cannot be approved for food packaging applications.
So, what challenges does CPP resin face during migration testing? How can the gas-solid phase chlorination process reduce migration risks from the source? And how should ink manufacturers proactively control potential migrants through formulation design?
1. What Does Migration Testing Measure? Understanding the Key Indicators
The core principle of food contact material migration testing is to evaluate whether the total amount and specific levels of substances migrating from packaging materials into food exceed safety limits under simulated real-use conditions.
Testing conditions, including temperature, time, and food simulants, are determined based on the actual packaging, storage, and application scenarios of the food product.
For example:
Ambient-temperature food packaging commonly uses migration conditions such as 20°C for 10 days.
High-temperature retort food packaging may require more stringent conditions, such as 121°C for 30 minutes or 100°C for 2 hours.
For non-food-contact layers such as ink coatings, migration test results mainly depend on the migration rate and total amount of potentially migratable substances passing through the barrier layer from the ink system.
Overall migration refers to the total amount of all non-volatile substances migrating from food contact materials and articles into food simulants.
GB 4806.15-2024 specifies that the overall migration limit must be:
This is the most fundamental “total migration control” indicator and reflects the overall migration risk level of the material.
Potassium permanganate consumption measures the total amount of oxidizable organic substances present in migrated materials. It indirectly reflects the level of organic soluble substances migrating from the material.
The standard requirement is:
This indicator is particularly important for ink binders. Incomplete reactions of organic oligomers remaining in the resin are one of the main sources contributing to potassium permanganate consumption.
Heavy metal migration is calculated based on lead (Pb), with the standard requirement:
CPP resin itself does not contain heavy metals. However, trace metal elements may be introduced through catalyst residues or equipment wear during production. Therefore, strict control throughout the manufacturing process is essential.
During migration testing, the main challenges faced by cpp resin are concentrated in two areas:
Overall migration
Potassium permanganate consumption
The first relates to the total amount of soluble low-molecular-weight substances released from the resin, while the second relates to the oxidizable characteristics of these substances.
2. Three Major Sources of Migration Risks in CPP Resin
The migration performance of CPP resin mainly depends on three key factors.
2.1 First: Solvent Residues
Traditional solvent-based CPP resin production processes use organic solvents such as carbon tetrachloride and toluene during manufacturing.
Even after post-treatment and solvent removal processes, residual solvent levels in the final product may still remain at tens to hundreds of mg/kg.
During migration testing, these residual solvents can slowly release from the ink coating, penetrate through barrier layers, and enter food simulants, directly contributing to the overall migration value.
2.2 Second: Low Molecular Weight Polymer Components
CPP resin is a polymer material with a polydisperse molecular weight distribution.
Lower molecular weight fractions (typically components with a number-average molecular weight below 5,000) have certain solubility in oil-based food simulants. These components may dissolve out from the coating layer and migrate into food, becoming part of the overall migrants.
The migration risk of low molecular weight components becomes significantly higher under:
- High-temperature conditions, such as 121°C retort sterilization
- Contact with high-fat food products
2.3 Third: Chlorination By-products
During the chlorination process of CPP production, small amounts of chlorinated by-products may be generated.
If post-treatment is insufficient, these by-products may remain in the resin and can also be detected during migration testing.
3.Gas-Solid Phase Chlorination Process: Technical Advantages for Reducing Migration Risks at the Source
The gas-solid phase chlorination process offers inherent technological advantages in reducing migration risks of CPP resin.
Extremely Low Solvent Residue
The gas-solid phase chlorination process directly chlorinates powdered polypropylene in a fluidized bed, without using any organic solvents throughout the entire reaction process.
As a result, the content of volatile organic residues in the final product can be reduced to an extremely low level. When tested according to the GB 31604.60-2024 standard, the residual solvent level can typically reach a “not detected” level.
This means that solvent residues, one of the major sources of migration risk, can be significantly reduced from the production stage.
This is also one of the key differences between gas-solid phase CPP resin and traditional solvent-based CPP products.
A Cleaner Molecular Structure
The reaction system of the gas-solid phase chlorination process is clean and simple, with minimal interference from external impurities.
Compared with solvent-based processes, which require additional steps to remove solvents and by-products, gas-solid phase CPP resin contains fewer impurities and has a more regular molecular structure.
This advantage directly contributes to lower overall migration levels.
Under the same formulation conditions, inks formulated with gas-solid phase CPP resin generally demonstrate significantly lower overall migration values compared with formulations using traditional solvent-based CPP products.
Narrower Molecular Weight Distribution
By precisely controlling reaction parameters, the gas-solid phase chlorination process enables more accurate control of the chlorination reaction degree.
Compared with traditional solvent-based methods, CPP resin produced through the gas-solid phase process features a narrower molecular weight distribution.
This means:
- A lower proportion of low molecular weight components
- Fewer soluble substances migrating into oil-based food simulants
- Easier compliance with potassium permanganate consumption requirements
4. Compliance Recommendations for Ink Manufacturers
Selecting gas-solid phase CPP resin as the basic raw material is one of the most direct and effective ways to reduce migration risks.
Due to its technical advantages, including:
Extremely low solvent residue
Cleaner molecular structure
gas-solid phase CPP resin provides natural advantages when meeting migration testing requirements.
For ink products intended for food packaging applications, manufacturers are recommended to prioritize CPP resin produced by the gas-solid phase chlorination process.
Reserve Compliance Margin During Formulation Design
Migration testing results are determined by multiple factors, including:
- Resin quality
- Ink formulation design
- Printing and drying processes
- Lamination structure selection
All of these factors can influence the final migration level.
Therefore, during formulation development, it is recommended to set the target value for overall migration at 50%–70% of the regulatory limit.
This provides sufficient compliance margin to accommodate:
- Process fluctuations
- Production batch variations
- Raw material differences
Conduct Migration Pre-Testing for Finished Ink Products
During product development, manufacturers should conduct migration pre-tests according to:
GB 31604.1 — General Rules for Migration Tests of Food Contact Materials and Articles
Early testing can help identify potential compliance risks before commercial production.
The recommended testing environment should follow standard conditions:
- Temperature: 23 ± 2°C
- Relative humidity: 50% ± 10%
5. Three Frequently Asked Questions
Under laboratory testing conditions, ink coatings formulated with gas-solid phase CPP resin can generally maintain overall migration values within the range of:
(The actual value may vary depending on ink formulation and coating thickness.)
This is significantly lower than the limit of:
10 mg/dm² specified by GB 4806.15-2024.
By comparison, traditional solvent-based CPP resin under the same formulation conditions often shows overall migration values in the range of:
Although these products may still meet regulatory requirements, their compliance margin is significantly smaller.
For production lines with larger process fluctuations or stricter batch-to-batch quality control requirements, the wider compliance margin provided by gas-solid phase CPP resin is particularly valuable.
Not necessarily.
Migration testing results are influenced by multiple factors, including:
- Resin quality
- Ink formulation design
- Printing and drying processes
- Lamination structure
Gas-solid phase CPP resin significantly reduces migration risks from the raw material side, but successfully passing migration tests still requires systematic compliance management throughout:
- Formulation design
- Process control
- Finished product testing
It is recommended to conduct migration pre-testing according to GB 31604.1 during the product development stage to identify and resolve potential issues in advance.
Migration test failures usually occur in two situations.
Situation 1: Overall Migration Exceeds the Limit
Possible solutions include:
- Replace the CPP resin with a lower-migration grade (For example, selecting gas-solid phase CPP resin with lower solvent residue and fewer low molecular weight components.)
- Adjust the solvent system (Avoid using high-boiling-point solvents, as they are difficult to completely evaporate and may remain in the coating, increasing migration risks.)
- Check other potentially migratable additives (Identify whether the formulation contains other low-molecular-weight additives that may migrate, evaluate their migration risks, and replace or remove them when necessary.)
Situation 2: Potassium Permanganate Consumption Exceeds the Limit
This usually indicates that the resin contains a relatively high level of organic soluble substances.
Recommended solutions include:
- Select CPP resin with a narrower molecular weight distribution and lower oligomer content (For example, gas-solid phase CPP resin.)
- Optimize the solvent system (Use solvent combinations with better evaporation performance to reduce the contribution of residual solvents to potassium permanganate consumption.)
Final Thoughts
Food contact material migration testing has become the “entry ticket” for ink products entering the food packaging market.
Through:
- Extremely low solvent residue
- Cleaner molecular structure
- Narrower molecular weight distribution
gas-solid phase CPP resin significantly reduces the risks of excessive overall migration and solvent residue from the raw material stage, providing ink manufacturers with a source-control approach for regulatory compliance.
However, whether a product can ultimately pass migration testing still depends on systematic management throughout the entire process, including:
- Formulation design
- Manufacturing process control
- Finished product inspection
By combining high-quality CPP resin selection with scientific formulation optimization and strict process management, ink manufacturers can achieve safer, more reliable, and more compliant solutions for food packaging applications.
