Crystallinity Testing Experiments for Recycled Materials: DSC, XRD, and Density Methods Explained

Crystallinity testing is a key quality-control experiment for recycled thermoplastics, quantifying how reprocessing, degradation, and contamination alter the crystalline phase that governs stiffness, shrinkage, barrier properties, and optical clarity. This article explains crystallinity testing experiments for recycled materials using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and density methods in accordance with ISO, ASTM, JIS, and GB standards, covering sample preparation, test items, data analysis, and typical industrial applications.

Technical Overview

Most commodity thermoplastics, including PET, PP, PE, PA, and PLA, are semi-crystalline polymers whose solid state consists of ordered crystalline lamellae dispersed within an amorphous matrix. The weight fraction of the crystalline phase, known as the degree of crystallinity, directly governs stiffness, yield strength, hardness, permeability, transparency, dimensional stability, and molding shrinkage. Because these properties define whether a recycled material can replace virgin resin, crystallinity is one of the most informative single measurements in recyclate characterization.

Mechanical recycling exposes polymers to repeated melt processing, thermo-oxidative stress, hydrolysis (particularly severe in PET), and contamination by foreign resins, pigments, and additives. These factors alter molecular weight and chain architecture, which in turn shift crystallization kinetics, melting behavior, and the crystallinity ultimately achieved in extruded or molded products. A crystallinity testing experiment therefore acts as a sensitive fingerprint of recyclate quality, degradation level, and batch-to-batch consistency.

• Chain scission from thermal or hydrolytic degradation shortens chains, which accelerates nucleation but limits achievable lamellar perfection and enthalpy of fusion.
• Cross-contamination, for example PP or PVC carried into a PET stream, introduces foreign thermal events and acts as either heterogeneous nuclei or obstacles to crystal growth.
• Thermal history from hot washing, drying, regranulation, and solid-state polycondensation leaves measurable traces in the first DSC heating scan.
• Additives, mineral fillers, and color masterbatches may nucleate or inhibit crystallization, shifting crystallization temperature by several degrees.

Accredited third-party laboratories quantify crystallinity using complementary, standards-based techniques so that recyclers, converters, and buyers can share a common basis for material acceptance and specification writing.

Testing Samples

Crystallinity experiments can be performed on a wide range of recycled-material forms, provided the specimen is representative, dry, and properly conditioned. Common sample categories are described below.

Recycled PET (rPET)

Typical specimens include hot-washed bottle flake, food-grade pellets, amorphous regrind, and solid-state-polycondensed chip. Because PET crystallizes slowly, flake crystallinity depends strongly on hot-wash temperature and thermal history: amorphous flake tends to agglomerate and stick in extruder feed sections, so converters frequently verify that flake has been thermally crystallized before processing. Crystallinity data also correlate with intrinsic viscosity behavior and drying requirements.

Recycled Polyolefins (rPP, rHDPE, rLDPE)

Samples include post-consumer bottle flake from HDPE dairy and detergent containers, PP crates and battery cases, retail-ready regranulate, and post-industrial regrind. Polyolefins crystallize rapidly, so the test focuses on comparing crystallization temperature, melting enthalpy, and peak shape against virgin references to reveal degradation and resin cross-contamination, for example PE traces in a PP stream.

Recycled Engineering and Specialty Resins

Recycled PA6 and PA66 from fibers, carpets, or fishing nets, recycled PLA from industrial scrap, and other semi-crystalline engineering resins can be evaluated by the same methods. For amorphous recycled resins such as rPC and rABS, crystallinity is not applicable, but the DSC thermal fingerprint (glass transition and foreign-melting peaks) remains valuable for screening contamination.

Sample Preparation and Conditioning

• Dry hygroscopic resins before testing, for example desiccant-air or vacuum drying of rPET at approximately 150 to 160 degrees Celsius and rPA at approximately 80 degrees Celsius, to suppress hydrolysis during melting and eliminate moisture artifacts.
• Cut pellets or flakes into 5 to 10 mg specimens with a clean blade, ensuring flat, uniform contact with the pan base without pre-stressing the specimen.
• Weigh specimens to within 0.01 mg on an analytical balance and use matched aluminum reference pans.
• Prepare compression-molded plaques or cast films of controlled thickness for XRD analysis to avoid orientation and thickness effects.
• Document source stream, grade, color, and prior processing steps so that results remain traceable and comparable between lots.

Testing Items

A complete crystallinity testing experiment on recycled material typically reports the following parameters:

• Degree of crystallinity (Xc, percent), the primary quantitative result.
• Melting temperature (Tm, onset and peak) and enthalpy of fusion (delta Hm).
• Crystallization temperature on controlled cooling (Tc) and enthalpy of crystallization (delta Hc).
• Cold crystallization temperature and enthalpy (Tcc, delta Hcc) observed during heating of partially amorphous recyclates.
• Glass transition temperature (Tg) as supporting evidence of degradation or plasticization.
• Supercooling interval (Tm minus Tc), an indicator of nucleation efficiency.
• Comparison of first and second heating scans, which separates as-received thermal history from intrinsic polymer behavior.
• Detection of foreign-polymer thermal events as a contamination screening tool.

For recycled materials this dual-scan comparison is essential. The first heating scan reflects the actual drying, regranulation, and service history of the as-received specimen, while the second heating scan, measured after erasing thermal history, characterizes the intrinsic crystallization capability of the current molecular structure.

Testing Methods

Differential Scanning Calorimetry (DSC), Primary Method

DSC measures the heat-flow difference between a specimen and an inert reference as they are scanned through thermal transitions. Melting and crystallization transitions are integrated to obtain enthalpies, from which crystallinity is calculated. The experiment is conducted in accordance with ISO 11357 (Parts 1, 2, and 3), ASTM D3418, ASTM E793, JIS K 7121, K 7122, and K 7123, or the corresponding GB/T 19466 series.

• Equilibrate the DSC at 25 degrees Celsius under nitrogen purge of 20 to 50 mL/min.
• First heating at 10 degrees Celsius per minute to about 30 degrees above complete melting; record Tm, delta Hm, and any cold crystallization.
• Hold isothermally for 2 to 3 minutes to erase thermal history, keeping the hold short for degradation-sensitive resins.
• Cool at 10 degrees Celsius per minute to well below crystallization range; record Tc and delta Hc.
• Second heating at 10 degrees Celsius per minute; record Tg, Tcc, Tm, and delta Hm for the standardized material fingerprint.

The degree of crystallinity is calculated as: Xc (%) = [(delta Hm minus delta Hcc) / (delta Hm0 multiplied by w)] multiplied by 100, where delta Hm0 is the enthalpy of fusion of the 100 percent crystalline polymer and w is the mass fraction of the target polymer when fillers or blends are present. Commonly cited reference enthalpies include approximately 140 J/g for PET, 207 J/g for isotactic PP, 293 J/g for PE, 230 J/g for PA6, and 93 J/g for PLA; the specific literature value used should always be stated because reported figures vary slightly between sources.

Interpretation for recyclates is well established: melting-point depression, broadened or multiple melting peaks, upward shift of crystallization temperature caused by shortened degraded chains nucleating more readily, and reduced melting enthalpy all indicate thermo-mechanical history or contamination. Extra endotherms or exotherms reveal foreign polymers at levels as low as a few percent.

Wide-Angle X-Ray Diffraction (XRD)

XRD measures long-range molecular order directly and non-destructively. Crystalline reflections are superimposed on a broad amorphous halo; after background subtraction and peak deconvolution, crystallinity is calculated from the ratio of crystalline peak area to total scattered area, Xc = sum of Ac / (sum of Ac plus k multiplied by sum of Aa), where k is a scattering correction factor often taken as unity. Typical conditions employ Cu K-alpha radiation, approximately 40 kV and 40 mA, scanning 3 to 40 degrees two-theta with a 0.02 degree step. For example, recycled PE shows strong (110) and (200) reflections near 21.5 and 23.9 degrees two-theta, while rPET displays characteristic triclinic reflections between 16 and 26 degrees. XRD requires no melting of the specimen, so it captures the true as-received structure, but results are model-dependent and demand careful amorphous-halo fitting.

Density Method (Gradient Column)

Because crystalline and amorphous phases of the same polymer have different densities, measured density can be converted to crystallinity using: Xc = [rho c multiplied by (rho s minus rho a)] / [rho s multiplied by (rho c minus rho a)]. For PET, typical values are rho c of approximately 1.455 g/cm3 and rho a of approximately 1.335 g/cm3. Testing follows ASTM D1505 or ISO 1183-1 and ISO 1183-2. The method is fast and inexpensive but is easily biased by voids, moisture, fillers, and contamination, so it is best applied to clean, unfilled, single-resin recyclates and cross-checked against DSC.

Infrared Spectroscopy (FTIR) Method

FTIR estimates crystallinity from the ratio of a crystallinity-sensitive conformer band to an internal-thickness reference band, for example trans versus gauche conformer bands in PET. It enables rapid surface screening of flakes and films and is useful for sorting line verification, but it requires calibration against DSC or XRD and is affected by surface orientation and additives.

Method Selection and Cross-Validation

• DSC is the most standardized and information-rich technique, delivering crystallinity plus Tm, Tc, Tg, and degradation fingerprints, but it consumes the specimen thermally.
• XRD provides a direct structural measurement without heating, making it ideal for as-received flakes and finished parts, at the cost of more complex data reduction.
• Density measurement is the fastest and cheapest option but is only reliable for clean, void-free, unfilled materials.
• FTIR offers high-throughput screening but must be calibrated against absolute methods.

Absolute crystallinity values differ slightly between techniques because DSC does not detect the rigid amorphous fraction in the same way as XRD. Reports should therefore always state the method, scan rates, and reference enthalpy used.

Applications

Crystallinity testing experiments on recycled materials support decisions across the entire recycling and converting value chain:

• Incoming quality control and lot acceptance of recyclate pellets, verifying supplier consistency and screening for contamination before production commitments are made.
• rPET bottle-to-bottle and sheet extrusion programs, confirming that flake has been adequately thermally crystallized for free-flowing extrusion and that intrinsic crystallization capacity supports target bottle properties.
• Injection molding with rPP and rHDPE, where crystallinity and crystallization temperature data feed shrinkage and warpage prediction and guide mold-temperature settings.
• Fiber spinning and film production from recyclates, where crystallinity targets control tenacity, dimensional stability, and optical clarity.
• Development of virgin and recycled blend systems, tracking how crystallinity, melting point, and peak shape change with blend ratio.
• Multiple-reprocessing research studies, quantifying degradation through the drift of Tc, Tm, and melting enthalpy across successive cycles.
• Recycled-content claim support, providing standardized technical data packages generated by accredited third-party laboratories for eco-labeling and customer audits.

In practice, crystallinity results are interpreted alongside melt flow rate, intrinsic viscosity, color measurement, and mechanical testing to form a complete recyclate qualification profile.

FAQ

Why does crystallinity change after recycling?

Reprocessing causes chain scission, branching, and oxidation, while contaminants and additives introduce or block nucleation sites. Shortened chains generally crystallize faster and at higher temperatures during cooling, but severe degradation, cross-contamination, and impurities reduce achievable crystallinity, melting point, and melting enthalpy. The direction and magnitude of the change therefore reveal both the type and severity of the recycling history.

Should crystallinity be reported from the first or second heating scan?

Both are meaningful. The first heating scan reflects the as-received material with its real thermal history, which is what a converter will actually process. The second heating scan, recorded after erasing that history, characterizes the intrinsic polymer. For recyclates, best practice is to report both scans together with the cooling scan.

How much sample is required and what is the typical turnaround?

DSC requires only 5 to 10 mg per specimen, usually tested in duplicate or triplicate, while XRD and density methods need a few grams. Including mandatory drying of hygroscopic resins, typical turnaround at accredited third-party laboratories is three to seven working days for standard programs.

Which standards govern the experiment?

Core references include the ISO 11357 series for DSC of plastics, ASTM D3418 and ASTM E1356 for transition temperatures, ASTM E793 for enthalpy determination, JIS K 7121, K 7122, and K 7123 for Japanese-market data, and the GB/T 19466 series as the Chinese national equivalent. Density-based crystallinity follows ASTM D1505 and ISO 1183.

What are typical crystallinity ranges for common recyclates?

• Thermally crystallized rPET flake and pellets: approximately 25 to 40 percent, with amorphous flake near zero.
• Recycled HDPE: approximately 55 to 75 percent.
• Recycled PP: approximately 40 to 60 percent.
• Recycled PA6 and PLA: strongly dependent on drying and cooling history, generally 20 to 45 percent.

Exact values depend on the measurement method and processing conditions, so comparisons should always reference the same technique and scan parameters.

Can a crystallinity test alone prove recycled content?

No single parameter can. Crystallinity testing provides strong evidence of thermal history, degradation, and contamination, but recycled-content verification is normally built from a combined data set including intrinsic viscosity or melt flow rate, color measurement, FTIR identification, oxidative induction time, and trace analysis, assembled by accredited third-party laboratories under recognized schemes.