ISO 11721 Determination of the Microbial Resistance of Cellulose-Containing Textile Products - Soil Burial Test

The “ISO 11721 Textiles - Determination of the resistance of cellulose-containing textiles to microorganisms - Soil burial test” standard, published by the International Organization for Standardization (ISO), consists of the following sub-sections published under the same title:

• The standard “ISO 11721-1 Part 1: Evaluation of decay-retardant finishes” describes a method for determining the resistance of chemically pretreated textiles to the effects of microorganisms found in soil, compared to untreated textiles. This method is applicable to plain textiles made from cellulosic-containing yarns (tentages, tarpaulins, woven fabrics, and tapes) that will generally come into contact with soil during use.

Due to the natural resistance of most synthetic fibers to microbial attack, fabrics containing a high percentage of synthetic fibers can only be evaluated in terms of changes in structure and appearance using these methods. Although this method ensures good reproducibility of results, it aims to show comparative performance rather than providing absolute values.

Heavy tarpaulin fabrics and weaves may possess a structure that makes untreated samples resistant to burial for up to 14 days. Furthermore, in such cases, the rate of degradation of the untreated sample determines the length of the burial period.

In the complex field of textile science, where natural fibers face the harsh realities of environmental exposure, the ISO 11721-1 standard emerges as a fundamental standard for durability. This standard provides a rigorous methodology for evaluating how well chemical treatments protect fabrics from soil-dwelling microbes. As industries grapple with the demands of outdoor applications and global supply chains, this standard ensures that textile products such as tents, tarpaulins, and agricultural nets can withstand decay.

Cellulose fibers such as cotton, linen, and viscose dominate the textile market due to their softness, absorbency, and affordability. However, their organic nature makes them prime targets for biodegradation. In soil-rich environments (moist, nutrient-rich, and teeming with fungi and bacteria), these fibers can rapidly decompose, leading to loss of strength, discoloration, and structural degradation. Untreated fabrics buried in soil can lose up to 80 percent of their tensile strength in just two weeks under optimal microbial conditions.

The ISO 11721-1 standard addresses this issue by establishing a comparative soil burial test, allowing for the comparison of treated textiles with untreated controls. Unlike absolute measurements, it emphasizes relative performance by acknowledging the biological unpredictability of soil ecosystems.

The basic steps of the ISO 11721-1 standard test method are as follows:

• Sample preparation: Test samples are cut into standard strips from both treated and untreated fabrics (e.g., 25 mm x 300 mm for tensile testing). To equalize moisture, samples are conditioned at 20 degrees Celsius and 65 percent relative humidity. At least five replicates are prepared for each set to account for variability.
• Immersion (optional but recommended): Samples are immersed in water or a mild detergent solution to simulate environmental washing. This removes unbound chemicals, allowing the test to measure natural surface durability rather than surface residues.
• Soil assessment and setup: Microbial viability of the soil is verified using a control fabric strip; it should be significantly degraded (e.g., more than 50% loss of strength) to confirm activity. Loamy garden soil containing 20-30% organic matter and adjusted to a water-holding capacity of 50-60% is used. Containers such as plastic trays prevent cross-contamination.
• Burial and incubation: Samples are placed horizontally in soil to a depth of 5-10 cm, avoiding overlapping. They are incubated at 28 degrees plus/minus 2 degrees for 2-4 weeks, maintaining moisture to promote microbial growth without waterlogging.
• Recovery and evaluation: Samples are gently scraped, rinsed with water, and air-dried. Degradation is assessed as follows:
• Tensile strength: Using a universal testing machine, the breaking load is measured and the retention percentage is calculated (machined and unmachined).
• Weight loss: Weighed before and after to measure the decrease in mass.
• Visual inspection: Its appearance is graded on a scale from no change to severe decay.
• Reporting results: Results are expressed as relative retention (e.g., “treated samples retain 80%, controls 20% retention”). Detailed information about soil type, incubation period, and possible deviations is included.
• The standard “ISO 11721-2 Part 2: Determination of the long-term resistance of an anti-decay finish” describes a test procedure for determining the long-term resistance of an anti-decay finish to attack by microorganisms in the soil. It allows for differentiation between anti-decay finishes with no long-term resistance, normal long-term resistance, and enhanced long-term resistance, for the purpose of evaluating their suitability for use in tropical regions.

Because the soil burial test is a biological process and the test soil is not accurately defined, this standard only covers the comparison of finished and unfinished samples.

In the world of textile engineering, where durability meets environmental resilience, this standard plays a crucial role. It addresses a critical but often overlooked aspect of fabric performance: resistance to microbial degradation. Specifically, the ISO 11721-2 standard focuses on soil burial testing to assess the long-term effectiveness of decay-retardant finishes in cellulose-containing textiles. As global trade expands and textiles find applications across a spectrum from outdoor equipment to tropical clothing, this standard ensures that products can withstand the relentless onslaught of soil-borne microorganisms without compromising their integrity.

The basic steps of the ISO 11721-2 standard test method are as follows:

• Sample preparation: Representative cellulose-containing textile samples, both with and without decay-retardant treatment, are selected. The samples are cut to standard sizes (e.g., strips for tensile testing) and adjusted according to equilibrium moisture content.
• Soil preparation: A natural, microbially active soil is prepared: usually garden soil enriched with organic matter to promote fungal and bacterial growth. The soil should have a neutral pH, sufficient moisture content (approximately 50-60% of its water-holding capacity), but not be waterlogged.
• Embedding and incubation: Samples are embedded horizontally to a depth of approximately 5-10 cm, ensuring that no air pockets remain. They are incubated at 28-30 degrees Celsius (a temperature that supports microbial activity) for periods ranging from 2 to 16 weeks, depending on the desired long-term evaluation.
• Collection and evaluation: To avoid mechanical damage, samples are carefully scraped, rinsed, dried, and examined for signs of decay (e.g., brittleness, mold spots). Degradation is measured using the following metrics:
• Tensile strength retention: Percentage of the original breaking load retained.
• Weight loss: Mass reduction due to fiber breakdown.
• Visual assessment: A qualitative scale for surface changes.
• Classification: Based on the results, the performance of the finish is classified as follows:
• No long-term resistance: Treated samples deteriorate similarly to controls.
• Normal long-term resistance: Moderate protection, suitable for use in temperate climates.
• Enhanced long-term resistance: Superior durability, ideal for tropical or high-exposure applications.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services compliant with ISO 11721-1 and ISO 11721-2 standards.