Determination of Crystallinity and Aging in Plastics: Testing Methods, Standards, and Applications

Crystallinity determination and aging evaluation are two of the most critical analyses for characterizing the long-term performance of semi-crystalline plastics. By combining techniques such as Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and accelerated weathering in accordance with ISO and ASTM standards, accredited third-party laboratories can quantify crystalline content, track thermal and photo-oxidative degradation, and predict product service life.

Technical Overview

Plastics can be broadly classified into amorphous and semi-crystalline polymers. In semi-crystalline materials such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polyoxymethylene (POM), and polyetheretherketone (PEEK), the degree of crystallinity directly governs mechanical strength, stiffness, dimensional stability, barrier performance, chemical resistance, and optical transparency. Quantifying crystallinity is therefore essential for material qualification, process optimization, and failure analysis.

Polymer aging refers to the irreversible changes in chemical composition, molecular weight, and morphology that occur during processing, storage, and service. Thermal aging, UV exposure, humidity, and mechanical stress can all induce degradation mechanisms such as chain scission, oxidation, crosslinking, post-crystallization (physical aging), and chemicrystallization. Because these mechanisms often alter the crystalline phase, crystallinity measurement before and after aging serves as a powerful indicator of material durability.

Determination of crystallinity and aging behavior is performed in accredited third-party laboratories using internationally recognized methods, including ISO 11357 series, ASTM D3418, ASTM E1356, ISO 4892 series, ISO 188, and ASTM D3045, ensuring traceable and reproducible results for R&D, quality control, and regulatory compliance.

Testing Samples

Resins, Pellets, and Powders

Virgin polymer granules and powders are tested to establish baseline crystallinity, melting behavior, and thermal stability prior to processing. Small sample masses, typically 5–10 mg for DSC analysis, are sufficient for accurate characterization.

Molded Parts and Finished Components

Injection-molded, extruded, and blow-molded products such as automotive trim, pipe fittings, housings, and closures are sampled directly from the part. Crystallinity may vary with mold temperature, cooling rate, wall thickness, and distance from the gate, so sampling location is documented carefully.

Films, Sheets, and Fibers

Biaxially oriented films, cast films, and textile fibers possess orientation-dependent crystallinity. Thin sections are prepared to enable consistent thermal analysis and X-ray diffraction measurements.

Aged and Weathered Specimens

Samples exposed to natural weathering, oven aging, or accelerated laboratory aging are submitted for comparative analysis against unaged control specimens to quantify changes in crystallinity, oxidation level, and mechanical property retention.

Testing Items

Typical test parameters for crystallinity and aging determination include:

• Degree of crystallinity (Xc, %)
• Melting temperature (Tm) and melting enthalpy (ΔHm)
• Crystallization temperature (Tc) and crystallization enthalpy (ΔHc)
• Cold crystallization enthalpy (ΔHcc) during heating
• Glass transition temperature (Tg)
• Crystallization kinetics and half-crystallization time (ISO 11357-7)
• Oxidation induction time and oxidation onset temperature (OIT /OOT)
• Carbonyl index and hydroxyl index by FTIR for oxidation assessment
• Heat distortion temperature and Vicat softening point after aging
• Tensile strength and elongation retention after aging
• Melt flow rate (MFR) changes indicative of chain scission or crosslinking
• Visual assessment: discoloration, cracking, chalking, and embrittlement

Testing Methods

Differential Scanning Calorimetry (DSC)

DSC is the most widely used method for crystallinity determination. The sample is heated and cooled under nitrogen at controlled rates while heat flow is recorded. The degree of crystallinity is calculated as Xc (%) = [(ΔHm − ΔHcc) / ΔH°m] × 100, where ΔH°m is the melting enthalpy of a 100% crystalline reference polymer. Measurements follow ISO 11357-1, ISO 11357-3, and ASTM D3418. First and second heating scans are evaluated to distinguish as-processed crystallinity from intrinsic material behavior.

Wide-Angle X-Ray Diffraction (WAXD/XRD)

XRD measures the ratio of crystalline diffraction peak area to total scattered intensity. After baseline correction and amorphous halo subtraction, the crystalline fraction is obtained. XRD is particularly valuable for highly oriented samples such as films and fibers, and it can identify crystal polymorphs, for example the alpha, beta, and gamma phases of polypropylene.

Density Method

Based on the two-phase model, crystallinity is derived from the measured density of the sample compared with the densities of fully amorphous and fully crystalline polymer. Density gradient columns are performed per ASTM D1505. This method is simple and precise but sensitive to voids and additives.

Infrared Spectroscopy (FTIR)

FTIR quantifies crystalline versus amorphous absorption bands and measures aging-induced oxidation products. The carbonyl index (ratio of carbonyl band near 1715 cm⁻¹ to a reference band) is a standard marker of photo-oxidative and thermo-oxidative degradation in polyolefins.

Oxidation Induction Time (OIT)

OIT measures the time to oxidative degradation at elevated temperature under oxygen and reflects the stabilizer package effectiveness and remaining antioxidant capacity of aged materials. Testing is conducted per ISO 11357-6 and ASTM D3895, commonly for polyethylene pipes and cable insulation.

Accelerated Aging and Weathering

Aging exposure is conducted using air oven aging per ISO 188 and ASTM D3045, xenon-arc light exposure per ISO 4892-2 and ASTM G155, and fluorescent UV lamp exposure per ISO 4892-3 and ASTM G154. Specimens are exposed for defined durations or radiant exposures, then re-tested for crystallinity, mechanical retention, and oxidation indicators to construct aging curves and estimate service life.

Applications

Crystallinity and aging determination supports numerous industries and product categories:

• Automotive: under-hood components, interior trim, fuel systems, and battery housings exposed to heat and fluids over long service lives
• Packaging: PET bottles and barrier films where crystallinity controls gas permeability, clarity, and sterilization tolerance
• Pipes and fittings: PE and PVC pressure pipes evaluated by OIT and slow crack growth resistance after thermal aging
• Medical devices: sterilizable components requiring validated morphological stability after gamma or steam exposure
• Electrical and electronics: wire insulation, connectors, and housings assessed for thermal endurance and flame-retardant compatibility
• Renewable energy: photovoltaic backsheets, cable jackets, and composite matrices subjected to outdoor weathering qualification
• Failure analysis: comparing crystallinity and oxidation levels between failed and reference parts to identify processing or aging root causes

FAQ

How is the degree of crystallinity calculated from DSC data?

The crystallinity is obtained by dividing the net melting enthalpy (melting enthalpy minus cold crystallization enthalpy) by the melting enthalpy of the 100% crystalline polymer, then multiplying by 100%. For example, polyethylene uses a reference enthalpy of approximately 293 J/g, while polypropylene uses approximately 207 J/g.

Why do DSC and XRD sometimes give different crystallinity values?

The two techniques measure different physical phenomena. DSC senses all crystalline order that melts, including very small or imperfect crystallites, while XRD detects only sufficiently ordered structures producing diffraction peaks. Sample preparation, thermal history, and orientation can also influence results, so both methods are complementary.

How does aging change polymer crystallinity?

Aging can increase crystallinity through post-crystallization or chemicrystallization following chain scission, or decrease it through crystal perfection loss and oxidative attack. Tracking crystallinity, OIT, and carbonyl index together reveals the dominant degradation mechanism.

How long does an aging evaluation take?

Accelerated weathering tests typically run from several hundred to several thousand hours depending on the target market and standard, while oven aging may range from days to months. Time-temperature extrapolation and high-rate exposure protocols can shorten development timelines.

What sample preparation is required?

For DSC, approximately 5–10 mg is cut from the specimen and sealed in an aluminum pan. XRD requires flat, parallel surfaces; density testing requires defect-free specimens free of voids. Conditioned and unaged control samples should always be submitted together with aged samples for valid comparison.