Odor Analysis Methods for Printed Films: Standards, Techniques, and Evaluation Procedures

Odor analysis of printed films is a critical quality control step in flexible packaging, labeling, and converting industries, where volatile substances from inks, adhesives, and substrate resins can compromise product acceptability. This article provides a comprehensive technical overview of printed film odor analysis methods, covering sensory olfactory evaluation, residual solvent testing by headspace gas chromatography, and odorant identification by GC-MS, in accordance with recognized ISO, EN, ASTM, and GB standards.

Technical Overview

Printed films used in flexible packaging, labels, and decorative applications can release volatile organic compounds (VOCs) that produce perceptible odor. The primary sources of odor include residual processing solvents from gravure and flexographic inks (ethyl acetate, ethanol, isopropanol, methyl ethyl ketone, toluene, and xylenes), residual solvents and reaction byproducts from polyurethane lamination adhesives, oxidative degradation products from polyolefin substrates and slip agents (such as hexanal and heptanal), and photoinitiator decomposition products from UV-cured inks and varnishes (such as benzaldehyde). Because some of these compounds have extremely low odor thresholds, even trace-level concentrations can cause significant sensory impact.

Odor analysis of printed films therefore combines two complementary approaches: sensory evaluation, in which trained assessors rate odor intensity under controlled conditions, and instrumental chemical analysis, in which headspace gas chromatography quantifies residual solvents and GC-MS identifies specific odor-active compounds. Relevant methodological frameworks include ISO 6658 (general guidance on sensory analysis methodology), ISO 8586 (selection and training of sensory assessors), EN 1230-1 (sensory odor testing of materials intended for food contact), ASTM E679 (determination of odor thresholds), and GB/T 10004-2008, which defines residual solvent limits for plastic laminated films and bags for food packaging in China.

Testing Samples

Solvent-Based Printed Films

Gravure- and flexographic-printed films on BOPP, PET, PE, CPP, and PA substrates constitute the largest sample category. Testing focuses on residual solvents retained by ink layers and on the interaction between solvents and the substrate film. Representative samples should be taken from different positions across the web width to account for dryer temperature gradients.

Laminated Multilayer Films

Films laminated with solvent-borne or solvent-free polyurethane adhesives are evaluated for retained solvent trapped between layers and for adhesive-related odorants such as amines and low-molecular-weight reaction products. Insufficient curing and inadequate drying are common root causes identified through this testing.

UV-Cured and Coated Films

UV-printed films, over-varnished films, and films with functional coatings such as cold seal or heat seal layers are analyzed for photoinitiator fragments, acrylate monomers, and coating-specific volatiles that are not captured by conventional residual solvent methods.

Finished Pouches and Bags

Converted packaging in its final form can be tested to assess combined odor contribution from printing, lamination, coating, and sealing processes, as well as odor developed during storage and transportation.

Sample handling requirements apply to all categories: specimens are typically cut to a defined area (commonly 0.5 to 1 square decimeter or greater), conditioned at 23 °C and 50% relative humidity in accordance with ISO 291 or GB/T 2918, and stored in clean, inert glass containers with PTFE-lined closures to prevent cross-contamination. Samples must not be sealed in the original plastic packaging, since plastic bags themselves can contribute odor.

Testing Items

A complete odor characterization of printed films typically includes the following testing items:

• Sensory odor intensity rating on a calibrated 5-point scale, performed by a trained panel
• Total residual solvent content, expressed in mg/m² of film area
• Individual residual solvent quantification, including ethyl acetate, ethanol, isopropanol, methyl ethyl ketone, toluene, and xylenes
• Total volatile organic compound (TVOC) emission screening
• Identification and quantification of specific odor-active compounds by GC-MS, such as aldehydes, ketones, organic acids, amines, and residual monomers
• Odor threshold determination and odor activity value (OAV) calculation for key odorants
• Taint transfer screening, in which the film is exposed to a fatty or sensitive food model medium to evaluate odor and flavor transfer potential

Testing Methods

Sensory Odor Evaluation

In the sensory chamber method, a defined area of film is placed in a clean borosilicate glass vessel, sealed, and thermally equilibrated to drive volatiles into the headspace. Typical conditioning protocols range from milder treatments, such as 40 °C for two hours or more, to accelerated treatments at 60 to 80 °C for 30 to 60 minutes, depending on the product specification and the referencing framework. A panel of at least three to six assessors trained in accordance with ISO 8586 then evaluates the headspace odor immediately after opening. Odor intensity is rated on a five-point scale: 1 indicates no perceptible odor, 2 a faint odor detectable only on careful sniffing, 3 a moderate and clearly noticeable odor, 4 a strong odor, and 5 a very strong, objectionable odor. Most food packaging specifications require an intensity of 2 or lower, while less sensitive applications may accept level 3. Blanks, reference materials, and randomized sample presentation are used to control panel bias, and discrimination tests such as the triangle test under ISO 4120 or ranking tests under ISO 8587 may be applied when comparing material variants.

Headspace Gas Chromatography for Residual Solvents

Static headspace gas chromatography with flame ionization detection (GC-FID) is the standard instrumental method for residual solvent quantification. Film discs of known area are placed in headspace vials, equilibrated at an elevated temperature, typically 80 to 100 °C for around 30 minutes, and the headspace vapor is injected into a capillary GC column. Individual solvents are quantified against external calibration standards and results are normalized to film area. Under GB/T 10004-2008, plastic laminated films and bags for food packaging must meet a total residual solvent limit of 5 mg/m², and benzene-series solvents must not be detected. Individual specification limits are frequently imposed by brand owners for sensitive products such as chocolate, confectionery, and tobacco packaging.

GC-MS Identification of Key Odorants

When the sensory result cannot be explained by conventional residual solvents, or when an unexplained odor complaint must be investigated, headspace GC-MS analysis is performed in full-scan mode to identify unknown volatiles through mass spectral library matching. Frequently identified odorants in printed films include benzaldehyde and other aldehydes from UV-cured systems, hexanal and heptanal from polyolefin oxidation, acetic acid, low-boiling amines from polyurethane systems, styrene and acrylate monomers, and plasticizer-derived alcohols. Targeted quantification using selected ion monitoring improves sensitivity to the sub-mg/m² level.

GC-Olfactometry and Odor Activity Evaluation

GC-olfactometry (GC-O) couples chromatographic separation with human sniffing at an olfactory detection port, allowing assessors to record the retention time and descriptive character of each odor-active zone. When combined with MS identification and odor threshold data obtained using methods consistent with ASTM E679, odor activity values can be calculated as the ratio of the actual concentration to the odor threshold. This prioritization identifies which compounds, sometimes present at concentrations far below instrumental specification limits, actually dominate the perceived odor.

Emission and Migration Screening

For materials destined for food contact, emission screening using microchamber-type thermal desorption cells, TVOC headspace profiling, and taint transfer tests with fatty or sweet model media are used to simulate storage conditions. Thermal aging studies, in which specimens are held at elevated temperature for defined periods before analysis, help predict odor development over the shelf life of the packaged product. Food contact compliance frameworks such as the GB 4806 series additionally require that materials be free of off-odor as a baseline sensory requirement.

Applications

Odor analysis of printed films supports a wide range of industrial applications:

• Release quality control for flexible packaging converters before lamination, slitting, and shipment
• Qualification and incoming inspection of low-odor inks, adhesives, coatings, and substrate films
• Root cause investigation of customer odor complaints, distinguishing between ink, adhesive, substrate, and process-related sources
• Verification of compliance with food contact sensory requirements under GB 4806 series regulations and customer specifications
• Validation of process improvements such as extended dryer residence time, increased curing, or conversion to solvent-free lamination
• Assessment of printed decorative films for automotive interiors, where odor and fogging characteristics are jointly evaluated using frameworks such as ISO 6452
• Support for export shipments, where accredited third-party laboratories provide test reports aligned with ISO, EN, ASTM, and GB standards required by international customers

FAQ

What standards govern odor testing of printed films?

Sensory evaluation is generally performed under ISO 6658 and ISO 8586, with EN 1230-1 providing a food-contact material odor protocol. Residual solvent limits and test requirements for laminated food packaging films are defined in GB/T 10004-2008, and odor threshold determination follows ASTM E679.

What residual solvent level is considered acceptable?

The widely applied acceptance criterion is a total residual solvent content of no more than 5 mg/m², with benzene-series solvents not detected. Many sensitive applications, such as confectionery and tobacco packaging, impose stricter internal limits well below this value.

Why can sensory results and GC results disagree?

Instrumental methods quantify concentration, while the nose responds to odor threshold. Compounds such as aldehydes and amines can produce a strong odor at concentrations that appear negligible in a residual solvent chromatogram, which is why GC-MS and GC-O identification is essential when sensory odor persists despite compliant solvent results.

Can solvent-free laminated films still exhibit odor?

Yes. Even without lamination solvents, odor can originate from oxidative degradation of the substrate, adhesive reaction byproducts, ink components, and storage-related aging, so full odor analysis remains necessary.

How much sample is required for testing?

Typically 0.5 to 1 square meter of representative film is sufficient for combined sensory and instrumental testing, conditioned under standard atmosphere and delivered in inert packaging to avoid contamination before analysis.

How can odor in printed films be reduced?

Effective measures include optimizing dryer temperature and residence time, ensuring complete adhesive curing, selecting low-odor inks and raw materials, controlling substrate oxidation during extrusion, and improving ventilation during storage and winding of freshly printed rolls.