Crystallinity determination is a critical quality-control test for recycled plastics, revealing how thermal history, mechanical reprocessing, and degradation have altered the morphology and performance of recyclate materials. This article explains the fundamental principles of crystallinity measurement in recycled polymers, the standard test methods such as DSC and XRD, applicable international standards including ISO 11357 and ASTM D3418, and how the results are applied across packaging, automotive, and fiber industries.
Technical Overview
Crystallinity refers to the degree of structural order in a solid polymer, expressed as the weight percentage of the material that exists in an ordered, crystalline state rather than an amorphous state. In semi-crystalline thermoplastics such as PET, PP, PE, PA, and PBT, the degree of crystallinity directly governs mechanical strength, stiffness, barrier performance, dimensional stability, optical transparency, shrinkage behavior, and melting characteristics.
For recycled materials, crystallinity determination carries particular significance. Mechanical recycling involves multiple melting, extrusion, pelletizing, and solid-state processing steps, each of which reshapes the thermal and mechanical history of the polymer. Contaminants, residual catalysts, moisture-induced hydrolysis (especially in recycled PET), chain scission, and crosslinking can all shift the crystallization kinetics and alter the achievable crystalline fraction. Measuring crystallinity therefore allows recyclers, compounders, and end users to verify whether a recyclate behaves equivalently to virgin resin and to detect degradation before it causes failures in downstream processing.
The degree of crystallinity is most commonly quantified by differential scanning calorimetry (DSC), which measures the enthalpy of melting and crystallization transitions. Complementary techniques include wide-angle X-ray diffraction (XRD), density measurement, and infrared spectroscopy. Testing is typically performed by accredited third-party laboratories in accordance with ISO, ASTM, and GB standards.
Testing Samples
Recycled PET (rPET) Flakes and Pellets
Recycled PET from bottle streams is one of the most frequently tested materials for crystallinity, because the intrinsic viscosity and solid-state polycondensation behavior of rPET are closely linked to its crystalline fraction. Both hot-washed flakes and re-pelletized rPET intended for bottle-to-bottle, sheet, or fiber applications can be evaluated.
Recycled Polyolefins (rPP, rPE)
Recycled polypropylene and polyethylene from post-consumer and post-industrial sources are tested to assess how sorting quality, filler contamination, and multiple reprocessing cycles have affected the crystalline morphology. Results guide adjustments to nucleating agents, processing temperatures, and blend formulations.
Recycled Engineering Resins (rPA, rPBT, rPC Blends)
Recycled polyamides, polybutylene terephthalate, and polycarbonate-based blends used in automotive and electrical applications require crystallinity verification to ensure that warpage, moisture uptake, and mechanical performance remain within specification after recycling.
Post-Consumer Recyclate (PCR) and Reprocessed In-House Scrap
PCR compounds, regrind, and reprocessed production scrap from injection molding or extrusion lines are tested as part of incoming material qualification, supplier audits, and recycled-content certification programs.
Testing Items
A typical crystallinity determination report for recycled materials includes the following measured and calculated parameters:
- Degree of crystallinity (Xc, %), calculated from transition enthalpies
- Melting enthalpy (ΔHm) and melting peak temperature (Tm)
- Cold crystallization enthalpy (ΔHcc) and cold crystallization temperature (Tcc)
- Crystallization temperature during cooling (Tc) and crystallization enthalpy
- Glass transition temperature (Tg), an indicator of amorphous-phase integrity
- Double melting peaks or recrystallization phenomena, often linked to reprocessing history
- Supercooling extent (Tm − Tc), reflecting nucleation efficiency in recyclate
Testing Methods
Differential Scanning Calorimetry (DSC)
DSC is the dominant method for crystallinity determination and follows ISO 11357 (Parts 1, 2, and 3), ASTM D3418, and GB/T 19466. A specimen of approximately 5 to 10 mg is heated, cooled, and reheated at a controlled rate (commonly 10 °C/min) under a nitrogen atmosphere. The first heating scan captures the as-received thermal history of the recyclate, while the second heating scan reflects the intrinsic material behavior after erasing that history.
The degree of crystallinity is calculated using the enthalpy balance equation:
Xc (%) = [(ΔHm − ΔHcc) / ΔHm°] × 100, where ΔHm is the measured melting enthalpy, ΔHcc is the cold crystallization enthalpy, and ΔHm° is the reference melting enthalpy of a 100% crystalline polymer. Commonly used reference values include approximately 140 J/g for PET, 207 J/g for isotactic PP, 293 J/g for polyethylene, and 230 J/g for PA6, based on published literature.
Wide-Angle X-Ray Diffraction (XRD)
XRD quantifies crystallinity by comparing the integrated intensity of crystalline diffraction peaks with the amorphous halo in the scattering pattern. The method is non-thermal and particularly useful for confirming crystal structure changes, crystallite size, and polymorphic transitions in recycled materials without erasing the original morphology.
Density Measurement
Because crystalline and amorphous regions of the same polymer have different densities, crystallinity can be estimated from density gradient column measurements per ASTM D1505 or ISO 1183. This method is fast and inexpensive but requires accurate two-phase density values and can be biased by voids, fillers, or contamination in recyclate.
Infrared Spectroscopy (FTIR)
FTIR crystallinity indices, based on the ratio of crystalline-sensitive bands to amorphous bands (for example the 972/898 cm⁻¹ ratio in PET or the 998/973 cm⁻¹ ratio in PP), provide a surface-sensitive, micro-scale assessment useful for mapping crystallinity distribution across recycled parts and films.
Applications
Crystallinity data for recycled materials is applied throughout the value chain:
- Incoming recyclate inspection and supplier qualification for compounders and molders
- Verification of rPET solid-state polycondensation and drying efficiency before injection stretch blow molding
- Formulation of PCR blends with virgin resin, nucleating agents, and clarifiers
- Prediction of shrinkage, warpage, and cycle time in injection molding of recycled polyolefins
- Assessment of sterilization and hot-fill suitability for recycled packaging
- Support for recycled-content declarations, eco-label applications, and regulatory compliance dossiers
- Root-cause analysis of brittleness, haze, poor barrier properties, or dimensional instability in recycled products
FAQ
How is the crystallinity of a recycled polymer calculated?
By DSC, the net crystallization enthalpy (melting enthalpy minus cold crystallization enthalpy) is divided by the melting enthalpy of the fully crystalline polymer and multiplied by 100. This yields the weight fraction of crystalline material in the specimen.
Which standards govern crystallinity testing of recycled plastics?
The principal standards are ISO 11357-1, ISO 11357-2, and ISO 11357-3, ASTM D3418, ASTM E1356, and GB/T 19466 for DSC, together with ASTM D1505 and ISO 1183 for the density method and relevant XRD procedures for crystallographic analysis.
Why do first and second DSC heating scans give different crystallinity values for recyclate?
The first heating scan reflects the processing and storage history of the recycled material, including orientation and thermal treatment from its previous life. The second heating scan, recorded after controlled cooling from the melt, represents the intrinsic crystallization capability of the material and is often used to detect degradation-induced changes.
What does unusually low or high crystallinity indicate in recycled material?
Reduced crystallinity may indicate chain scission, contamination with amorphous polymers, or incomplete crystallization during reprocessing, while unexpectedly high crystallinity or sharpened peaks can reflect annealing, extensive thermal exposure, or the presence of nucleating contaminants. Both situations warrant further investigation of recyclate quality.
How much sample is required and what is the typical turnaround?
DSC crystallinity testing generally requires only a few grams of material, with individual specimens of 5 to 10 mg. Routine analysis, including calibration, scanning, and reporting, is commonly completed within three to five working days at accredited third-party laboratories.
Can crystallinity testing detect the recycled content fraction in a blend?
Crystallinity alone cannot quantify recycled content percentage, but it provides strong evidence of thermal reprocessing history and degradation level. It is typically combined with other analyses, such as oxidation onset temperature, melt flow rate, colorimetry, and contaminant characterization, for a complete recyclate quality profile.