Determination of Tensile Properties of Degradable Agricultural Mulch Films

The determination of tensile properties of degradable agricultural mulch film is a fundamental laboratory procedure used to quantify the mechanical strength, ductility, and degradation retention of biodegradable covering films used in crop production. By evaluating specimens before and after environmental or laboratory-induced degradation in accordance with standards such as ISO 527-3, ASTM D882, and GB/T 1040.3, manufacturers, researchers, and regulators can verify product quality, monitor the degradation process, and predict field service life.

Technical Overview

Degradable agricultural mulch film is a thin polymeric covering laid over cultivated soil to conserve moisture, suppress weeds, stabilize soil temperature, and improve crop yield. Unlike conventional polyethylene mulch, which must be recovered and disposed of after the growing season, degradable mulch film is engineered to lose its mechanical integrity and fragment under the combined action of sunlight, heat, water, and soil microorganisms, allowing it to be incorporated into the soil or left to biodegrade in situ.

As degradation proceeds, chain scission, hydrolysis, and oxidation progressively reduce the molecular weight of the material, which manifests as a measurable decline in tensile strength, elongation at break, and toughness. Tensile property determination is therefore the principal quantitative method for tracking the degradation behavior of mulch film. The film must retain sufficient strength throughout the crop season to withstand laying, wind loading, and mechanical stress, while ultimately losing strength rapidly enough to satisfy degradation and environmental requirements.

Testing is performed on a universal testing machine using strip or dumbbell specimens cut from the film in both the machine direction (MD) and the transverse direction (TD). The measurement scheme is defined by internationally and nationally recognized standards, including ISO 527-1 and ISO 527-3, ASTM D882, and GB/T 1040.1 and GB/T 1040.3, with product-level requirements specified in documents such as GB/T 35795 and EN 17033 for biodegradable mulch films.

Testing Samples

Virgin (Unexposed) Production Film

Samples are taken from finished production rolls at defined intervals across the width and length of the web. Biodegradable mulch films based on PBAT, PLA, PBS, PHA, or starch blends typically have nominal thicknesses of approximately 8 to 20 micrometers. Virgin samples establish the initial, zero-time mechanical property baseline against which all degradation data are compared.

Field-Exposed Film

Specimens are recovered from trial plots after defined exposure periods, for example 30, 60, 90, and 120 days of crop coverage. Field-exposed samples reflect real service conditions, including solar radiation, rainfall, temperature cycling, agrochemical contact, and soil microbial activity. Recovered film is carefully cleaned, dried, and flattened before specimens are die-cut from visually representative areas.

Laboratory-Degraded Film

When field exposure is impractical or a controlled comparison is required, degradation is simulated in the laboratory using accelerated weathering in xenon-arc or fluorescent UV chambers operated according to the ISO 4892 series, controlled soil burial, or laboratory composting. Laboratory-degraded samples allow degradation kinetics to be studied under reproducible temperature, humidity, and irradiation conditions.

Testing Items

The tensile property determination typically covers the following measurable characteristics, each evaluated in both the machine direction and the transverse direction:

• Tensile strength at break, expressed in megapascals (MPa)
• Elongation at break, expressed as a percentage of the initial gauge length
• Tensile stress and strain at yield, where a yield point is discernible
• Secant modulus, commonly the 1% secant modulus, in MPa
• Tensile energy to break, in joules or joules per unit volume
• Maximum breaking force and breaking force per unit width
• Tensile strength retention rate after degradation, expressed as a percentage
• Elongation at break retention rate after degradation, expressed as a percentage

Testing Methods

Specimen Preparation and Conditioning

Rectangular strip specimens are cut from the film using a sharp razor die or calibrated cutting template to avoid notch defects that would cause premature failure. Typical specimen widths are 10 to 15 millimeters, with gauge lengths selected according to the expected elongation and the governing standard. Because thin films are highly sensitive to ambient conditions, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity, per ISO 291 or GB/T 2918, for at least 40 hours before testing. Thickness is measured at several points along each specimen using a low-pressure micrometer, since thickness variation directly affects the calculated stress.

Instrumentation and Setup

Testing is conducted on a calibrated universal testing machine fitted with a suitably sensitive load cell, typically in the 50 N to 500 N range for thin films. Pneumatic or mechanical grips with rubber or cork faces are used to prevent slippage without crushing or cutting the specimen. Elongation is determined from crosshead displacement or, where greater accuracy is required, from a non-contact extensometer.

Test Procedure

• Verify machine calibration and select the load cell appropriate to the expected breaking force.
• Measure and record specimen width and thickness at multiple positions and compute the average cross-sectional area.
• Mount the specimen in the grips with its longitudinal axis aligned to the direction of loading, and set the initial grip separation.
• Select the crosshead speed prescribed by the standard, typically within the range of 100 to 500 mm/min for high-elongation films.
• Run the test to specimen rupture and record the maximum force, force at break, and extension at break.
• Repeat for a statistically valid number of specimens in each direction and for each degradation interval.
• Reject and replace specimens that slip in the grips or fail at an obvious flaw, and document the rejection.

Calculation and Evaluation

Tensile strength at break is calculated as the maximum force divided by the original minimum cross-sectional area of the specimen. Elongation at break is the extension at rupture divided by the initial gauge length, expressed as a percentage. The degradation retention rate, the key indicator for degradable mulch film, is calculated as the property value after a given degradation period divided by the initial value, multiplied by 100. Results are reported as the mean or median of valid replicates, together with the standard deviation and the number of specimens tested, in accordance with the statistical rules of the applied standard.

Applications

• Factory quality control of biodegradable mulch film production, verifying that each batch meets the minimum tensile strength and elongation requirements before release.
• Support for biodegradability and eco-label certification programs, where demonstrated loss of mechanical properties complements biodegradation testing under standards such as ISO 17556 and ASTM D5988.
• Prediction of field service life, correlating laboratory retention data with crop-season durability requirements for different crops, climates, and laying methods.
• Research and development of new resin formulations, plasticizers, UV stabilizer packages, and blend ratios for PBAT-, PLA-, PBS-, and starch-based films.
• Comparative procurement evaluation, enabling agricultural authorities and large growers to compare candidate films on a consistent technical basis.
• Academic study of degradation kinetics, linking changes in molecular weight, crystallinity, and carbonyl index to the decline of tensile performance.

FAQ

Question: Which standards govern the tensile testing of degradable agricultural mulch film?

Answer: Specimen preparation, conditioning, and tensile measurement are governed by ISO 527-3 and GB/T 1040.3 for plastics films and by ASTM D882 for thin plastic sheeting. Product-level mechanical requirements for biodegradable mulch film are defined in documents such as GB/T 35795 and EN 17033, which reference the tensile test methods above.

Question: Why must samples be tested in both the machine direction and the transverse direction?

Answer: Mulch film is produced by blown-film or cast-film extrusion, which induces molecular orientation along the machine direction. As a result, strength and elongation can differ significantly between the MD and TD, and both directions must be characterized to describe true mechanical performance and anisotropic degradation.

Question: How is the degradation retention rate calculated?

Answer: Retention equals the property value measured after a defined degradation period divided by the initial, zero-time value, multiplied by 100. For example, a film with an initial MD tensile strength of 25 MPa that measures 10 MPa after 90 days of exposure shows a retention rate of 40%.

Question: How many specimens are required for a valid determination?

Answer: A minimum of five valid specimens per direction per condition is customary, and many protocols use ten to improve statistical confidence. Specimens that slip in the grips, fail at a grip edge, or break at an obvious artifact are rejected and replaced.

Question: Can severely embrittled, heavily degraded film still be tested?

Answer: Yes, with adaptations. Testing very fragile specimens requires a low-capacity load cell, gentle gripping, and minimal preload, and results may exhibit high scatter. When specimens can no longer be handled intact, the material is conventionally reported as having lost measurable tensile integrity, which is itself a meaningful degradation endpoint.

Question: What type of laboratory should perform this determination?

Answer: Tensile determination of thin degradable films requires calibrated universal testing machines, controlled conditioning facilities, and experienced operators. Accredited third-party laboratories with an established polymer and film testing scope are the appropriate choice for certification, dispute resolution, and regulatory submissions.