Biodegradable Mulch Film Tensile Load Testing: Standards, Methods and Technical Requirements

Biodegradable mulch film tensile load testing measures the breaking force of bio-based agricultural films in the machine and transverse directions, verifying that they can withstand mechanical laying and season-long field stress before degrading in soil. This article explains the applicable standards, including GB/T 35795, ISO 527-3, GB/T 1040.3 and ASTM D882, together with sampling, specimen preparation, test procedures, calculation methods and typical applications for manufacturers and accredited third-party laboratories.

Technical Overview

Biodegradable mulch films are thin polymer films, commonly 8 to 20 micrometers in nominal thickness, produced from materials such as PBAT (polybutylene adipate-co-terephthalate), PLA (polylactic acid) and thermoplastic starch blends. They are laid over cultivated soil to suppress weeds, conserve moisture and elevate soil temperature, and are designed to fragment and biodegrade in the field after the growing season, eliminating the recovery and disposal burden associated with conventional polyethylene mulch.

Because mulch film is unrolled and stretched by laying machinery at planting and must then resist wind loading, hail, soil abrasion and contact stresses for an entire cropping cycle, its tensile load capacity is a decisive quality attribute. Tensile load testing determines the maximum force, expressed in newtons (N), that a specimen of specified width can sustain before fracture in the machine direction (MD, longitudinal) and the transverse direction (TD, lateral). Blown-film extrusion induces strong molecular orientation, so MD and TD results frequently differ considerably, and both directions must always be evaluated and reported.

The test method is executed in accordance with GB/T 1040.3 / ISO 527-3 for thin plastic films, or ASTM D882 for tensile properties of thin plastic sheeting. Product-level classification and minimum mechanical requirements for fully biodegradable mulching film are specified in GB/T 35795-2017. Specimen conditioning is carried out under GB/T 2918 / ISO 291 at 23 ± 2 °C and 50 ± 10 % relative humidity. Manufacturers, distributors and certification bodies routinely commission these tests from accredited third-party laboratories operating under ISO/IEC 17025 quality systems.

Testing Samples

Finished Film Rolls

Samples are drawn from production lots or from full-width rolls submitted for inspection. At least one metre from the outer end of the roll is discarded, since surface layers are often elongated, nicked or contaminated. A section of full-width film, typically 1 to 2 metres in length, is then removed, laid flat on a clean surface and marked to identify the machine direction before specimens are die-cut.

Rectangular Strip Specimens

Rectangular strips are the default specimen type for thin mulch film under GB/T 1040.3 / ISO 527-3. Typical strips are 10 mm in width with an overall length of at least 150 mm and an initial grip separation of 100 mm. Specimens are cut cleanly with a sharp die or razor template so that edges are parallel and free of notches, tears or bubbles, because edge defects artificially depress the measured breaking load. Separate sets of strips are prepared along the machine direction and across the transverse direction.

Dumbbell Specimens

Where the film is comparatively thick or the invoked specification requires it, small dumbbell specimens such as types 1B, 5A or 5B defined in GB/T 1040.2 / ISO 527-2 are die-cut. The dumbbell geometry confines fracture to a narrow gauge section and improves the reproducibility of tensile strength and modulus measurements, although rectangular strips remain the norm for very thin mulch film.

Aged and Weathered Samples

Retained samples taken after artificial weathering (for example xenon-arc exposure under GB/T 16422.2 / ISO 4892-2), natural outdoor exposure or soil burial trials are tested using exactly the same procedure. Results are expressed as percentage retention of tensile load and elongation relative to unaged control specimens, which is central to verifying the designed service lifetime of degradation-tailored films.

Testing Items

The core scope of a biodegradable mulch film tensile testing program typically includes the following parameters, each reported separately for machine and transverse directions:

• Tensile load at break (N) for a defined specimen width — the primary acceptance item for mulch film
• Tensile strength at break (MPa), calculated from the breaking force divided by the original cross-sectional area
• Elongation at break (%), indicating ductility and lay-down robustness
• Yield stress and strain, where a distinct yield point is exhibited
• Tensile modulus or 1 % secant modulus, for stiffness characterization
• Retention of tensile load and elongation after aging, expressed as a percentage of the unaged values

Testing Methods

Specimen Conditioning

Before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity for at least 24 hours in accordance with GB/T 2918 / ISO 291. Biodegradable polyesters such as PBAT are moderately moisture-sensitive, so conditioning stabilizes the polymer state and ensures that differences between laboratories reflect material quality rather than ambient history.

Apparatus

Testing is performed on a constant-rate-of-extension universal testing machine fitted with a calibrated load cell of appropriate capacity, commonly in the range of 50 N to 500 N, verified in accordance with ISO 7500-1 / GB/T 16825.1. Pneumatic side-action grips with rubber or line-contact faces are preferred to prevent slippage and stress concentration at the jaw faces. Elongation is usually derived from grip separation for routine breaking-load determinations, while a non-contact video extensometer or contact extensometer is used when modulus measurement at small strains is required.

Test Procedure

Specimen width is measured to the nearest 0.1 mm and thickness to the nearest 1 micrometer at several positions along the gauge region in accordance with GB/T 20220 / ISO 4593, and the mean values are used to compute the cross-sectional area. The specimen is mounted in the grips at the specified gauge length, a small preload is applied to remove slack, and the crosshead is driven at the specified speed. A crosshead speed of 500 mm/min is customary for mulch-film strip tests conducted under GB/T 1040.3, whereas ASTM D882 prescribes lower speeds selected from its tables according to the expected elongation at break. The test proceeds to rupture, and the maximum force, force at break and extension at break are recorded. At least five valid specimens are tested per direction; specimens that fail inside the grips, slip or fracture at an edge flaw are discarded and replaced, and the arithmetic mean of valid results is calculated.

Data Processing and Reporting

Tensile load at break is reported directly in newtons for the stated specimen width, or normalized per unit width where required. Tensile strength is calculated as the breaking force divided by the original width-times-thickness cross-section, and elongation at break as the increase in gauge length relative to the initial gauge length, expressed as a percentage. Retention after aging is computed as the aged value divided by the unaged value, multiplied by one hundred. The test report identifies the standard used, specimen type and dimensions, conditioning, test speed, individual and mean results for each direction, and any discarded specimens together with the reason for rejection.

Applications

Tensile load testing of biodegradable mulch film supports the following activities:

• Factory quality control and lot release testing during blown-film production, including verification of resin blends and process settings
• Demonstration of compliance with GB/T 35795-2017 for market access, agricultural-input registration and government procurement of biodegradable mulching film
• Support of biodegradability certification dossiers, in which mechanical fitness for purpose must accompany biodegradation performance data
• Research and development of PBAT, PLA and starch-based formulations, thickness downgauging studies and optimization of blow-up ratio and draw-down parameters
• Weathering and storage stability programs, using retained tensile properties after xenon-arc exposure, outdoor exposure or soil burial as the key degradation indicator
• Import and export trade verification, supplier qualification and dispute resolution through accredited third-party laboratories

FAQ

Q: Which standards govern biodegradable mulch film tensile load testing?

A: The method standards are GB/T 1040.3 (equivalent to ISO 527-3) for thin films and ASTM D882 for thin plastic sheeting. The product standard GB/T 35795-2017 defines classification and the minimum mechanical requirements that the test results must satisfy, while conditioning follows GB/T 2918 / ISO 291.

Q: Why must both machine direction and transverse direction be tested?

A: Blown-film extrusion orients polymer chains preferentially along the machine direction, so MD strength and elongation usually differ markedly from TD values. Mulch film experiences loads in both directions during machine laying, anchoring and wind exposure, so a single-direction result cannot represent fitness for use.

Q: What is the difference between tensile load and tensile strength?

A: Tensile load at break is the force in newtons sustained by a specimen of specified width, which is the form in which mulch film requirements are commonly written. Tensile strength divides that force by the original cross-sectional area and is expressed in megapascals, allowing comparison independent of specimen dimensions.

Q: What acceptance criteria apply?

A: GB/T 35795-2017 assigns minimum tensile load values according to nominal thickness class, with heavier films required to carry proportionally higher loads. Because requirements and classification limits are periodically revised, the current edition of the standard and the contractual product specification should always be consulted when setting acceptance limits.

Q: How many specimens are required, and how should samples be submitted?

A: At least five valid specimens per direction, per condition, are recommended, with additional specimens prepared to allow for grip failures. For submission to a laboratory, a full-width roll section of 1 to 2 metres should be supplied, protected from heat, ultraviolet light, moisture and creasing, and clearly marked to identify the machine direction.

Q: Can testing be performed after field exposure?

A: Yes. Testing exposed or buried film and reporting percentage retention of tensile load and elongation relative to unaged controls is a standard method for confirming that the film maintains integrity for its designed induction period and then degrades on schedule.