Degradable film tensile strength change testing quantifies how the mechanical performance of biodegradable films, such as PBAT, PLA, PBS, and starch-based materials, declines during weathering, storage, and biodegradation. By tracking tensile strength retention against standards such as ISO 527-3, ASTM D882, and GB/T 1040.3, manufacturers can validate service life, induction periods, and compostability claims. This article explains the sample types, test items, testing methods, and typical applications involved in tensile strength change detection for degradable films.
Technical Overview
Degradable films are engineered to maintain sufficient mechanical integrity throughout their intended service life and then progressively lose strength as they fragment and biodegrade in soil, compost, or other receiving environments. Because of this designed performance profile, a single measurement of initial tensile strength is insufficient for material qualification. Tensile strength change testing instead monitors the retention of mechanical properties over time, providing a quantitative degradation profile that supports product design, quality control, and regulatory compliance.
The loss of tensile strength in biodegradable films results from several molecular mechanisms, including hydrolytic chain scission of polyester backbones, ultraviolet photodegradation, thermal oxidation, and enzymatic or microbial attack on the polymer matrix. These processes reduce molecular weight, disrupt crystallinity, and weaken intermolecular bonding, all of which manifest as declining tensile strength and elongation at break. Since blown and cast films exhibit pronounced anisotropy, testing is typically performed in both the machine direction (MD) and transverse direction (TD).
The core measurement is performed using universal testing machines in accordance with film-specific tensile standards such as ISO 527-3, ASTM D882, and GB/T 1040.3. Exposure and degradation stages are generated using complementary standards covering xenon-arc and fluorescent-UV weathering (ISO 4892 series, ASTM G154, ASTM G155), thermal aging (ISO 188, ASTM D573, GB/T 3512), soil burial (ISO 17556, ASTM D5988), and composting (ISO 16929, ISO 20200, ASTM D5338). Accredited third-party laboratories routinely combine these methods into integrated test programs covering the full life cycle of degradable films.
Testing Samples
Biodegradable Agricultural Mulch Films
Mulch films, typically 8 to 20 micrometers in thickness and based on PBAT-rich blends with PLA or starch, are the most frequently tested category. Samples include virgin rolls, films retrieved from field trials at defined crop growth stages, and laboratory-aged specimens. For these products, GB/T 35795 and related specifications require documented induction periods during which tensile strength must remain above defined thresholds before rapid degradation begins.
Compostable Packaging and Carrier Bag Films
Films intended for compost bags, organic waste liners, and flexible packaging are evaluated to demonstrate that mechanical integrity is preserved during storage and distribution yet disintegrates within composting timeframes. Testing supports conformity demonstration under EN 13432, ASTM D6400, and ISO 18606 frameworks.
Base Resin and Development Films
Blown or cast laboratory films produced from PLA, PBAT, PBS, PBSA, PHA, and their blends are tested during formulation screening. Comparative retention data allow researchers to assess stabilizer packages, UV-sensitizer loading, filler content, and processing parameters before commercial scale-up.
Aged and Degraded Specimens
Specimens retrieved at scheduled intervals from accelerated weathering devices, air ovens, controlled soil beds, or laboratory composting reactors constitute the primary input for change analysis. Each retrieval point requires sufficient replicate strips in both MD and TD orientations to generate statistically valid degradation curves.
Testing Items
The testing program for degradable film tensile strength change typically includes the following items:
- Tensile strength (MPa) in machine direction and transverse direction
- Elongation at break (%) before and after exposure
- Tensile strength retention rate (%) relative to unaged baseline
- Elongation retention rate (%) relative to unaged baseline
- Tensile modulus or secant modulus (MPa) at defined strain limits
- Yield strength and yield strain where applicable
- Strength change after thermal aging in air ovens
- Strength change after xenon-arc or fluorescent-UV weathering
- Strength change during soil burial and laboratory composting
- Break strength of welded seams and bag closures, where relevant
- Statistical evaluation including mean values, standard deviations, and degradation profiles
The central result is the tensile strength retention rate, calculated as the ratio of the mean tensile strength after a given exposure duration to the mean initial tensile strength, expressed as a percentage. Elongation retention is usually the most sensitive early indicator of degradation, often declining before measurable strength loss appears.
Testing Methods
Specimen Preparation and Conditioning
Rectangular strips are die-cut or razor-cut from flat, crease-free regions of the film. Under ISO 527-3 and GB/T 1040.3, Type 2 strips are typically 10 to 25 mm wide and at least 150 mm long; under ASTM D882, strip widths of 5.0 to 25.4 mm are permitted for films thinner than 1 mm. Thickness is measured at multiple positions using a dead-weight micrometer in accordance with ISO 4593 or ASTM D6988. Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 10% relative humidity per ISO 291, GB/T 2918, or ASTM D618 Procedure A for a minimum of 40 hours. At least five valid specimens per direction and per exposure interval are required, with specimens that fail within the grips replaced by retests.
Tensile Testing Procedure
Testing is performed on a universal testing machine equipped with a low-capacity load cell, typically 50 N to 500 N and calibrated to ISO 7500-1 or equivalent, together with pneumatic or lever-acting grips faced with elastomer or emery inserts to prevent slippage and stress concentration. Under ISO 527-3, the initial grip separation is normally 100 mm with crosshead speeds selected from 1 to 500 mm/min according to the material behavior of the film; under ASTM D882, grip separations of 100 mm or 250 mm are used with crosshead speeds of 50 to 500 mm/min depending on the elongation characteristics of the specimen. Tensile strength is calculated as the maximum force divided by the original cross-sectional area, and elongation at break is determined from grip separation or, where greater precision is required, from an extensometer. Machine direction and transverse direction results are reported separately because film anisotropy can produce substantially different degradation rates in each orientation.
Accelerated Aging and Degradation Exposure
To generate measurable tensile strength changes, specimens are subjected to controlled exposure regimes before tensile testing:
- Xenon-arc weathering per ISO 4892-2 or ASTM G155, and fluorescent-UV condensation weathering per ISO 4892-3 or ASTM G154, using cycles such as 8 hours of UV irradiation at 60 °C alternating with 4 hours of condensation at 50 °C
- Air-oven thermal aging per ISO 188, ASTM D573, or GB/T 3512 at selected temperatures such as 70 °C or 80 °C for predetermined durations
- Soil burial exposure per ISO 17556 or ASTM D5988 in controlled soil beds with defined moisture, temperature, and microbial activity
- Laboratory-scale composting per ISO 20200 at 58 ± 2 °C, or pilot-scale composting per ISO 16929 and ASTM D5338
- Natural outdoor weathering per ISO 877 or ASTM D1435 for correlation with accelerated results
Retrieval points are established at intervals such as 0, 7, 14, 28, 45, 60, and 90 days so that the decline in strength can be plotted as a time-dependent degradation curve.
Retention Calculation and Data Analysis
Tensile strength retention is calculated by dividing the mean tensile strength of aged specimens by the mean tensile strength of the unaged baseline and multiplying by 100. Results are reported as mean values with standard deviations for each orientation, exposure condition, and interval. The resulting retention curves are used to identify the induction period, defined as the duration over which strength remains above a specified threshold, and the subsequent active degradation phase. This analysis allows product designers to match the performance window of a mulch film to a crop season or the storage window of a compostable bag to its distribution cycle.
Applications
Tensile strength change detection for degradable films serves a wide range of practical purposes:
- Validating the induction period and service life of biodegradable agricultural mulch films prior to soil incorporation
- Establishing shelf life and storage stability limits for compostable packaging and carrier bags
- Supporting conformity demonstration under compostability frameworks such as EN 13432, ASTM D6400, and ISO 18606, and under mulch film specifications such as GB/T 35795
- Comparing resin grades, blend ratios, stabilizer systems, and photodegradation promoters during research and development
- Verifying incoming raw material consistency and monitoring production quality by batch
- Correlating laboratory accelerated aging with real-world field degradation for predictive modeling
- Providing evidence for regulatory submissions and customer documentation through accredited third-party laboratories
FAQ
Why is the change in tensile strength more important than the initial tensile strength for degradable films?
Degradable films are designed to lose strength at a controlled rate after fulfilling their function. A high initial strength with an unpredictable or overly rapid decline can cause premature field failure, while an overly slow decline can prevent timely soil incorporation or compost disintegration. Tracking the retention profile over time is therefore essential for matching material behavior to the intended application window.
Which tensile testing standard should be selected?
ISO 527-3 and its Chinese counterpart GB/T 1040.3 are widely used for films in ISO-oriented markets, while ASTM D882 is the reference method in North America. The choice depends on the target market and the specification invoked by the customer or regulator. Because specimen geometry and test speeds differ between the standards, results should not be compared directly across methods without documented correlation.
How much film material is required for a full test program?
Each exposure condition and retrieval interval requires at least five valid strips in each of the machine and transverse directions, plus reserve specimens for retests after invalid breaks. In practice, a flat, defect-free film area of several hundred square centimeters per condition is recommended to accommodate cutting, conditioning, and multiple intervals.
How long does a typical testing program take?
Tensile measurement itself is completed within one working day per batch once specimens are conditioned. The overall timeline is governed by the exposure phase: UV or thermal aging programs commonly run from several days to eight weeks, soil burial trials from one to six months, and composting trials up to 90 days or longer depending on the standard and the retention profile required.
How are field degradation conditions simulated in the laboratory?
Accelerated weathering reproduces sunlight, temperature, and moisture using xenon-arc or fluorescent-UV devices; soil burial and composting standards reproduce moisture, temperature, and microbial activity in controlled beds and reactors. Periodic retrieval and tensile testing of specimens from these environments yields degradation curves that can be correlated with outdoor field data from natural weathering trials.
What retention level indicates that a mulch film has completed its functional life?
Thresholds are defined by the product specification and the crop calendar rather than by the tensile standards themselves. In practice, a retention rate approaching or falling below approximately 50 percent, together with substantial loss of elongation, generally signals the transition from the induction period to the active fragmentation phase, at which point the film can be incorporated into the soil. Specific acceptance values should always be agreed against the applicable specification.
