Food-Grade Plastic Raw Materials: Virgin, Recycled, and Reclaimed Materials

Identification of Food-Grade Plastic Raw Materials: The 3 Core Differentiation Methods for Virgin, Recycled, and Reclaimed Materials. In the quality control of food-grade PP plastic raw materials, accurately distinguishing between virgin, recycled, and reclaimed materials is a crucial step in ensuring product safety and quality stability. Although these three types of materials are similar in appearance, they have significant differences in molecular structure, chemical composition, and physical properties. Based on the latest national standards and industry practices, the following will detail three core identification methods and their operating procedures.

1.1 Definitions and Differences of Virgin, Recycled, and Reclaimed Materials

Virgin material refers to PP material directly polymerized from petrochemical raw materials, characterized by a regular molecular structure and high purity. This type of material has never been used, has a complete molecular chain, a single chemical composition, and all performance indicators meet food-grade standards. Virgin PP has a highly ordered isotactic structure, with all methyl side groups located on the same side of the main chain, forming a helical shape, with a crystallinity of 50%-80% and a melting point range of 160-176℃.

Recycled material refers to PP waste that has been simply crushed and cleaned after use, mainly from scraps, defective products, or post-consumer plastic products from the production process. Although this type of material retains the basic structure of PP, it may contain residual additives, pigments, impurities, and degradation products generated during use. The molecular chains of recycled materials may be partially broken, and the molecular weight distribution is wider, leading to changes in performance parameters.

1.2 Performance Parameter Comparative Analysis

In terms of physical properties, there are significant differences among the three types of materials. Density is the most intuitive distinguishing indicator. The density of virgin PP is usually in the range of 0.90-0.915 g/cm³, while the density of recycled PP is generally in the range of 0.9-0.91 g/cm³. The difference between the two is small but can still be distinguished using precision instruments. Tensile strength is another important parameter. The tensile strength of virgin PP can reach 30-40 MPa, while the tensile strength of recycled material is only 20-30 MPa, 20-30% lower than that of virgin material.

In terms of thermal properties, virgin PP exhibits a single, clean, and smooth melting peak in its melting curve, with a peak temperature between 165-169℃. The melting curve of recycled material usually shows multiple melting peaks, around 132℃ and 165℃, due to the different melting points of PP from different sources. In addition, due to multiple processing steps, the melt flow rate (MFR) of recycled material increases significantly, which is a result of molecular chain breakage leading to a decrease in molecular weight.

The differences in chemical composition are more complex. The chemical composition of virgin PP is relatively simple, mainly containing PP polymer and a small amount of additives such as antioxidants. Recycled and reclaimed materials may contain various pollutants, including heavy metals (the content of which may be more than two orders of magnitude higher than that of virgin material), pesticide residues, toughening agents, adhesives, bacteria, viruses, and other harmful substances. The presence of these pollutants not only affects the performance of the material but, more importantly, may pose a potential threat to food safety.

2.1 Physical Performance Testing Method

Physical performance testing is the most basic and commonly used identification method, mainly including density measurement, melt flow index testing, and thermal analysis.
Density measurement is the first step in identifying PP materials. According to national standards GB/T 1033.1-2008 and ISO 1183-1:2019, the density requirement for food-grade PP is 0.90-0.91 g/cm³. Specific methods include the immersion method, the liquid pycnometer method, and the density gradient column method. Among these methods, the density gradient column method is the most accurate. It involves placing the sample in a precisely prepared n-heptane-ethanol gradient solution and determining the density value based on its suspension position. The test must be conducted in a constant temperature environment of 23±0.5℃ to eliminate thermal expansion errors. Modern laboratories widely use automatic densimeters, which combine Archimedes' principle with vibration frequency measurement technology, improving the testing accuracy to ±0.0001 g/cm³.

In practice, the density of virgin PP is usually stable within the range of 0.905-0.910 g/cm³, while recycled materials may show larger deviations due to the possible inclusion of other plastics or impurities. The density variation of recycled materials is more complex, depending on their source and processing technology. It should be noted that density testing alone cannot completely distinguish between the three types of materials; other methods must be combined for comprehensive judgment.

The melt flow rate (MFR) test is a core indicator for evaluating the processing fluidity of materials. According to the GB/T 3682 standard, a melt flow indexer is used to measure the amount of material extruded through a standard die in 10 minutes at a specific temperature (usually 230℃) and load (2.16 kg), with the unit being g/10min. The melt flow rate of food-grade PP is generally controlled within the range of 2-10 g/10min, while the range for general-purpose PP is 0.5-30 g/10min.

The melt flow rate test is particularly effective in distinguishing between virgin and recycled materials. Studies have shown that after multiple processing cycles, PP undergoes chain scission due to shear forces, leading to a decrease in molecular weight and a significant increase in MFR value. The MFR value of virgin PP is relatively stable, while the MFR value of recycled material may be several times higher than that of virgin material. For example, a batch of virgin PP may have an MFR of 5 g/10min, while recycled material processed 5 times may have an MFR of 15-20 g/10min. It should be noted that the change pattern of PE-LD is the opposite; its MFR decreases with increasing processing cycles, because PE-LD mainly undergoes cross-linking reactions rather than chain scission reactions. Thermal analysis, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), is one of the most effective methods for identifying PP materials. DSC accurately determines the melting point, crystallization temperature, crystallinity, and oxidation induction time (OIT) of a material by measuring the heat flow difference between the sample and a reference. TGA analyzes the thermal stability and decomposition behavior of a material by measuring the change in sample mass with temperature or time.

In DSC testing, virgin PP typically exhibits a single, sharp melting peak with a peak temperature between 165-169°C and high crystallinity. Recycled PP, due to molecular chain scission and a broader molecular weight distribution, shows a broader melting peak in its DSC curve, and may exhibit multiple melting peaks. For example, recycled PP may show a small peak around 132°C (possibly due to low molecular weight components or other plastics) and a main peak around 165°C. Furthermore, the crystallinity of recycled PP is usually lower than that of virgin PP, due to molecular chain structure damage caused by multiple processing cycles.

TGA analysis can reveal differences in the thermal stability of the materials. Virgin PP typically has a thermal decomposition temperature above 300°C, and the decomposition process is relatively simple. Recycled PP, due to the presence of various additives and impurities, exhibits more complex thermal decomposition behavior, potentially starting to decompose at lower temperatures and showing multiple weight loss stages during decomposition. It is particularly noteworthy that the residual mass of recycled PP varies greatly, ranging from 0.2% to 66%, while the residual mass of virgin PP is usually between 0.2% and 0.5%.

2.2 Chemical Composition Analysis Methods

Chemical composition analysis is the most accurate method for identifying PP materials, mainly including techniques such as infrared spectroscopy, elemental analysis, and chromatography.
Infrared spectroscopy (FTIR) is the most commonly used chemical analysis method. FTIR can precisely analyze the functional groups and molecular structure characteristics of a material, and quickly identify the type of PP base material (homopolymer/copolymer) and the type of additives by comparing characteristic absorption peaks. The typical infrared spectrum of PP shows four sharp peaks at 2960-2800 cm⁻¹, corresponding to the C-H stretching vibrations of CH, CH₂, and CH₃; peaks at 1460 cm⁻¹ and 1376 cm⁻¹ correspond to the C-H bending vibrations; the peak at 1165 cm⁻¹ represents the out-of-plane rocking bending vibration of the methyl group; and the 998 cm⁻¹ band is related to 11-13 repeating units and can be used as a crystalline band to calculate crystallinity.

In distinguishing between virgin and recycled materials, the key to FTIR is observing the C=O absorption peak in the 1600-1750 cm⁻¹ region. Studies have shown that PP samples all exhibit weak C=O absorption peaks in this region, which may be due to oxidation of recycled materials or the presence of additives containing carbonyl functional groups. The C=O peak intensity of virgin PP is weak and stable, while the C=O peak intensity of recycled material is significantly stronger due to the oxidation process. In addition, ATR-FTIR can also detect recycled PE-LD. Recycled material processed 6 times shows a new methyl characteristic peak (2950.7 cm⁻¹), but the methyl characteristic peak is not obvious in recycled material processed only once, indicating certain limitations of this method.

The operating procedure for FTIR analysis is relatively simple. First, the sample is cut into an appropriate size and placed on the ATR (Attenuated Total Reflectance) accessory of the Fourier transform infrared spectrometer. The scanning range is set to 4000-400 cm⁻¹, the resolution is 4 cm⁻¹, and the number of scans is usually 32. By comparing with a standard spectral library, the basic composition of the material can be quickly determined. For complex samples, two-dimensional infrared spectroscopy can also be used to identify different components by analyzing changes in the spectrum.

Elemental analysis is mainly used to detect heavy metals and other harmful elements in materials. Food-grade PP has strict requirements for heavy metal content, with cadmium content ≤0.005 mg/kg, mercury content ≤0.01 mg/kg, and lead content ≤0.01 mg/kg. Detection methods typically employ inductively coupled plasma mass spectrometry (ICP-MS), with a detection limit of 0.001 mg/kg, or atomic absorption spectrometry (AAS).

Elemental analysis is an important method for distinguishing between virgin and recycled materials. Studies have shown that the heavy metal content in virgin PP materials is very similar, with a relative deviation of no more than 57%, while the heavy metal content in recycled materials is often more than two orders of magnitude higher than that of virgin materials. This is because recycled materials may come into contact with various sources of pollution during the recycling process, including industrial waste and household waste, leading to heavy metal accumulation. In actual testing, if the heavy metal content of a sample is found to be abnormally high, it can generally be determined to be recycled material or a mixture containing recycled material.

Chromatographic analysis includes gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC), mainly used to detect volatile organic compounds, residual monomers, and additives in materials. GC-MS can be used to analyze volatile organic compounds and residual monomers, while HPLC is used for the analysis of non-volatile additive migration. In particular, headspace gas chromatography-mass spectrometry (HS-GC-MS) technology has been included in the national standard GB/T 46019.2-2025, specifically for the identification of recycled polypropylene.

The HS-GC-MS method involves the following procedure: Weigh 1.5 g of the sample (accurate to 0.1 mg) and place it in a 20 mL headspace vial. Add 20 μL of D8-naphthalene working solution (0.3 μg/mL) as an internal standard. After equilibrating at 150°C for 30 minutes, perform the analysis. The retention index of each volatile component is calculated by extracting the retention time of n-alkanes, and the relative peak area is calculated by internal standard normalization of the peak area. Researchers analyzed 170 virgin PP samples and 119 recycled PP samples, screened out 25 characteristic volatile components, and established an identification model based on the random forest algorithm, with an accuracy of over 95%.

2.3 Microstructure and Morphology Observation Methods

Microstructure and morphology observation is a method for identifying PP materials from the molecular level and microscopic morphology perspective, mainly including differential scanning calorimetry, polarized light microscopy, and scanning electron microscopy.

Differential scanning calorimetry (DSC) can not only measure the thermal performance parameters of the material but also identify the material type by analyzing its melting and crystallization behavior. DSC can provide characteristic thermal performance parameters of the material, such as glass transition temperature, melting point, and crystallinity. These parameters are of great significance for distinguishing between virgin and recycled materials. In practice, 5-10 mg of the sample is weighed and placed in an aluminum sample pan, and the temperature is increased from room temperature to 20°C above the melting point at a heating rate of 10°C/min, and the DSC curve is recorded.

The DSC curve of virgin PP usually shows a single, sharp melting peak with a symmetrical shape, and the melting temperature is between 165-169°C. The DSC curve of recycled material, however, shows significantly different characteristics: the melting peak widens, multiple melting peaks may appear (e.g., at 132°C and 165°C), the peak shape is asymmetrical, and the melting temperature decreases. For example, in one study, the melting points of samples from #4 to #1 decreased sequentially, and all were below 170°C, and the crystallinity also decreased sequentially. Sample #5 also showed a cold crystallization peak during the heating process, indicating that the molecular chains' mobility increased with increasing temperature, and chain segments rearranged to form crystals.

The calculation of crystallinity is also important for identification. According to the formula Xc = ΔHm/ΔH0 × 100% (where ΔHm is the melting enthalpy of the sample, and ΔH0 is the melting enthalpy of 100% crystalline PP, 240.5 J/g), the crystallinity of the material can be calculated. The crystallinity of virgin PP is usually between 60-80%, while the crystallinity of recycled material may decrease to 40-60% due to the destruction of the molecular chain structure. By comparing the changes in crystallinity, it is possible to determine whether the material has undergone multiple processing steps. Polarizing microscopy allows for direct observation of the spherulite morphology and size of PP, thus determining the crystallization characteristics of the material. Virgin PP, due to its high molecular chain regularity, forms uniform spherulites with complete morphology. Recycled PP, however, has a broader molecular weight distribution, resulting in spherulites of varying sizes and irregular shapes. Particularly when observing the birefringence phenomenon of spherulites, virgin PP exhibits a clear Maltese cross extinction pattern, while the extinction pattern of recycled PP may be blurred or incomplete.

Scanning electron microscopy (SEM) analysis can observe the surface morphology and cross-sectional structure of the material. The cross-section of virgin PP shows uniform ductile fracture characteristics, a smooth surface, and no obvious defects. The cross-section of recycled PP may exhibit brittle fracture characteristics, a rough surface, and various defects such as voids, cracks, and impurities. SEM can also be used for energy dispersive spectroscopy (EDS) analysis to detect the elemental composition of the material, which is particularly effective for identifying contaminants.

Researchers used a combination of field emission scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to analyze the morphology and elemental composition of the samples, providing a precise analysis of the samples' microscopic composition and morphology. This method can reveal subtle differences invisible to the naked eye, such as tiny impurity particles, surface oxide layers, and processing marks. Especially for samples containing a small amount of recycled material, macroscopic methods may not be able to identify them, but SEM-EDS analysis can reveal abnormal elemental distribution.

3.1 Systematic Identification Process Design

Based on the three core methods described above, we can design a systematic identification process to ensure accurate differentiation between virgin, recycled, and reclaimed materials. This process employs a three-level identification system: "preliminary screening - in-depth analysis - comprehensive determination".

First level: Preliminary screening. First, perform visual inspection and density testing. High-quality virgin material should have a uniform matte texture, pure color (mostly off-white or translucent), no impurities, black spots, or granular feel, and no pungent odor. Density testing uses the density gradient column method or an automatic densimeter to compare the sample density with the standard value (0.90-0.91 g/cm³). If the density value deviates from the standard range by more than ±0.005 g/cm³, it can generally be determined as non-virgin material.

Simultaneously, a melt flow rate (MFR) test is performed. The MFR value of virgin PP should be within the standard range and relatively stable. If the MFR value is abnormally high (more than twice the standard value), it may be recycled material.

Second level: In-depth analysis. More detailed analysis is performed on the samples after preliminary screening. First, FTIR analysis is conducted, focusing on the intensity of the C=O absorption peak in the 1600-1750 cm⁻¹ region. If the C=O peak is significantly enhanced, it indicates that the material may have undergone oxidation and is likely recycled material. Then, DSC analysis is performed to observe the shape, number, and temperature of the melting peaks. If multiple melting peaks appear or the melting temperature is significantly lower, combined with changes in crystallinity, it can further confirm whether it is recycled material.

Third level: Comprehensive judgment. For samples that still cannot be determined, the HS-GC-MS method is used for final confirmation. According to the national standard GB/T 46019.2-2025, the judgment is made by analyzing 25 characteristic volatile components combined with a random forest algorithm model. This method has an accuracy of over 95% and can effectively distinguish between virgin PP and recycled PP. Simultaneously, elemental analysis is performed to detect heavy metal content. If the heavy metal content is more than two orders of magnitude higher than the normal range, it can be determined as recycled material.
In practical operation, it is recommended to use multiple methods for mutual verification. For example, first use density and melt flow index for preliminary screening, then use FTIR and DSC for confirmation, and finally use HS-GC-MS for arbitration. This combination of methods can avoid the limitations of a single method and improve the accuracy of identification.

3.2 Result Judgment Standard System

Establishing a scientific result judgment standard is key to ensuring the accuracy of identification. Based on national standards and industry practices, we can establish the following judgment standard system.

Physical Properties Judgment Criteria:

• Density: Virgin PP is 0.905-0.910 g/cm³, recycled material may fluctuate within the range of 0.900-0.915 g/cm³, and recycled material has a greater density variation due to its complex composition.
• Melt Flow Rate (MFR): The MFR value of virgin PP should be within the standard specifications (usually 2-10 g/10min), the MFR value of recycled material may be slightly higher, and the MFR value of recycled material may be 2-5 times higher than that of virgin material.
• Melting Point: The melting point of virgin PP is 165-169℃, the melting point of recycled material remains basically unchanged, and the melting point of recycled material may decrease by 5-10℃, and multiple melting peaks may appear.
• Crystallinity: The crystallinity of virgin PP is 60-80%, and the crystallinity of recycled material is 40-60%.

Chemical Composition Judgment Criteria:

• FTIR Characteristic Peaks: C=O peak intensity (1600-1750cm⁻¹), weaker in virgin material, significantly stronger in recycled material; methyl characteristic peak (2950cm⁻¹), appears after multiple processing steps.
• Heavy Metal Content: The heavy metal content of virgin material is extremely low (relative deviation < 57%), and the heavy metal content of recycled material may be more than two orders of magnitude higher than that of virgin material.
• Volatile Components: 25 characteristic components are detected by HS-GC-MS, and there are significant differences in the types and content of components between virgin and recycled materials.

Microstructure Judgment Criteria:

• DSC Melting Peak: Virgin material shows a single sharp peak, while recycled material shows a broader peak shape and may have multiple peaks.
• Spherulite Morphology: Virgin material has uniform spherulite size and complete morphology, while recycled material has varying spherulite sizes and irregular morphology.
• Surface Morphology: The cross-section of virgin material is smooth and uniform, while the cross-section of recycled material is rough and may have defects.

In actual judgment, multiple indicators need to be considered comprehensively. For example, if a sample simultaneously meets the following conditions: density within the standard range, normal MFR value, single melting peak in DSC, weak C=O peak in FTIR, and low heavy metal content, then it is judged as virgin material. If the sample shows a significant increase in MFR value, multiple peaks in DSC, enhanced C=O peaks in FTIR, and high heavy metal content, it is determined to be recycled material. For samples falling between these two extremes, HS-GC-MS analysis is required, combined with a random forest model for final determination.

3.3 Method Limitations and Quality Control Points

Although the above methods have high accuracy, each method has its limitations, which need to be considered in practical applications.

• Limitations of density testing: Although density testing is simple and fast, it only provides limited information. The density of different types of PP (such as homopolymer and copolymer) may vary slightly, and some additives (such as fillers) can significantly affect the density value. Therefore, density testing can only be used as a preliminary screening method and cannot be used as the final basis for determination.

• Limitations of melt flow rate testing: MFR testing is greatly affected by temperature and shear history, and small changes in test conditions can lead to deviations in results. In addition, some modifiers (such as plasticizers) will also affect the MFR value. Therefore, when conducting MFR testing, the test conditions must be strictly controlled, and multiple parallel tests should be performed.
• Limitations of FTIR analysis: The ATR-FTIR method works well for identifying PE-LD recycled materials, but it has limitations in identifying PP recycled materials, especially recycled materials that have undergone one processing cycle, which may not show significant differences. In addition, FTIR can only provide functional group information and cannot determine the specific chemical structure.

Requirements for the HS-GC-MS method: Although this method has high accuracy, it requires sophisticated equipment and highly skilled operators. It requires a headspace gas chromatograph-mass spectrometer with an EI source, a headspace sampler operating at a temperature of at least 150℃, and professional analytical software and well-trained operators. 

 

To ensure the accuracy of the identification results, a comprehensive quality control system must be established:

Sample representativeness control: Strictly adhere to sampling standards (such as ISO 2859) to ensure that the samples taken accurately reflect the characteristics of the entire batch of material. For granular materials, samples should be taken from multiple points in different locations, mixed evenly, and then tested.

Instrument calibration and maintenance: All testing equipment must be calibrated regularly. Electronic balances, universal testing machines, and other measuring equipment require annual calibration by a legally recognized metrology institution. Melt flow rate testers and heat distortion temperature testers should be calibrated in-house or by a third party every six months. Calibration items include temperature accuracy, force value accuracy, and rate stability. Calibration reports must be archived for future reference to ensure the traceability of test data.

Environmental condition control: The testing environment should meet standard requirements, as temperature, humidity, and cleanliness can all affect test results. For example, density testing requires a constant temperature environment of 23±0.5℃; FTIR analysis should be conducted in a dry environment to avoid water vapor interference; and microbiological testing needs to be performed in a clean room.

Recording and traceability: All testing processes and results should be recorded in detail, including sample information, testing conditions, raw data, calculation process, and final results. Records should be clear, accurate, traceable, and retained for a specified period.

By establishing a comprehensive quality control system, the advantages of various identification methods can be maximized, ensuring accurate differentiation of virgin, recycled, and reclaimed food-grade PP plastic raw materials, providing reliable technical support for product quality control. In practical applications, an appropriate combination of methods should be selected based on specific circumstances, ensuring both accuracy and considering testing costs and efficiency. For food-grade PP, a material with extremely high safety requirements, it is recommended to use multiple methods for comprehensive identification to ensure product quality and food safety.