An Extensive Technical Guide
Cotton fiber possesses a variety of distinct properties, and we know there are plenty of people who want to dig a little deeper.
That’s why we’ve packed as much technical information onto this page as humanly possible. If you have a technical mind, then this is probably the page you’ve been searching for, and dreaming about.
- Definitions
- Properties
- Cellulose
- Polymerization
- Fiber Strength
- Purification
- Biodegradability
- OSHA Cotton Dust Standard
Key Cotton Terms Defined
- Micronaire
- A unique cotton term related to fiber maturity and fineness (diameter). Micronaire, however, is a unit-less value. It’s the measurement of airflow resistance through a 2.34 gram fiber specimen that is compressed to a specific volume. Micronaire can be converted to approximate denier value by dividing micronaire value by 2.82.
- Cotton Fiber Length
- Fiber length varies. Being a natural fiber, there are always going to be fibers of different lengths present (length distribution or fiber array). Cotton fiber length is measured and reported as the upper half mean length (average length of the longest 50% of fibers) to an accuracy of one hundredth of an inch.
- Cotton Fiber Strength
- Fiber strength is measured by breaking the fibers held between clamp jaws. It’s reported as grams per tex, which is the force in grams required to break a bundle of fibers one tex unit in size. A tex unit is equal to the weight in grams of 1000 meters of fiber.
- Nep
- A nep is small tangled knot of cotton fibers. They are produced from mechanical processing of cotton, starting with the equipment used to pick the cotton from the plant.
Purified Cotton Fiber Properties: A Definitive Outline
1. Upper Half Mean Length (in inches)
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2. Fiber Diameter
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3. Elastic Recovery (by percent)
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| 4. Breaking Elongation (dry) | 3-9.5 | ||||||||
5. Tensile Strength (g per tex/g per denier)
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| 6. Moisture Regain at Standard Conditions | 7% | ||||||||
| 7. Water Absorbing Capacity (USP method) | >24 grams of water per gram of fiber | ||||||||
| 8. Density (g/cm3) | 1.54 | ||||||||
| 9. Degree of Polymerization | 9,000 – 15,000 | ||||||||
| 10. Crystallinity by X-ray Diffraction (average) | 73% | ||||||||
| 11. Color (Whiteness Index) | 90 – 100 | ||||||||
12. Thermal Resistance
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13. Acid Resistance
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14. Alkali Resistance
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15. Organic Solvent Resistance
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| 16. General Properties: See list to right (or below if viewing on mobile device) | |||||||||
Note: Purified cotton is exempt from OSHA cotton dust standards.
The Full Cellulose Lowdown
Cotton, like rayon and wood pulp fibers, is made of cellulose. Cellulose is a macromolecule made up of anhydroglucose unit connected by 1, 4 oxygen bridges with the polymer repeating unit being anhydro-beta-cellulose.
Cotton cellulose differs from wood and rayon cellulose by having higher degrees of polymerization and crystallinity. The degree of polymerization is the number of repeating units that are linked together to form a molecule of cellulose. Crystallinity indicates that the fiber’s cellulose molecules are closely packed and parallel to one another. Higher degrees of polymerization and crystallinity of polymers are associated with higher strengths.
Polymerization for Cellulosic Fibers
| Fiber | Average Degree of Polymerization | Average Degree of Crystallinity |
|---|---|---|
| Cotton | 9,000 – 15,000 | 73 |
| Rayon (regular) | 250 – 450 | 60 |
| Wood Pulp | 600 – 1500 | 35 |
Due to the increased degree of crystallinity and hydrogen bonding between the molecules in the crystalline areas in cotton, moisture can’t penetrate the molecules. Moisture penetrating molecules can act as a lubricant and result in loss of strength. Therefore, cotton cellulose does not lose strength when wet like rayon does; in fact, it gets stronger. Synthetic fibers are hydrophobic; therefore their strength is not affected at all by moisture.
Fiber Strength (grams/tex)
| Fiber | Dry Strength | Wet Strength |
|---|---|---|
| Cotton | 27 – 45 | 30 – 54 |
| Rayon (regular) | 22 – 27 | 10 – 14 |
| Polyester | 27 – 54 | 27 – 54 |
Mold And Yeast Spore Removal
With cotton as the customer-preferred choice, the purification process is critical. The data shown below is from testing conducted by an independent laboratory. It shows how effective the Barnhardt purification process is at eliminating mold and yeast spores from the cotton fibers (colony forming units per gram = CFM/g).
| Total Plate Count (CFU/g) | Mold/Yeast (CFM/g) | |
|---|---|---|
| Before Purification | 40,000 – 450,000 | 210 – 550 |
| After Purification | < 30 | < 10 |
A Cotton Biodegradability Breakdown
Cotton fibers and fabrics, being natural cellulose polymers, are biodegradable under aerobic conditions. Independent lab results have also shown that cotton is compostable. In the case shown below, 100% cotton wet wipe hydroentangled fabrics were tested for compostability using ASTM method 6400. The test procedure calls for at least 90% weight loss to constitute complete biodegradability.
Average Biodegradability (Percent Weight Lost)
| Sample | After 2 weeks | After 4 weeks |
|---|---|---|
| Cotton Wet Wipe | 40 | 90 |
OSHA Cotton Dust Standard
Only washed and purified cotton are exempt from the OSHA cotton dust standard.
In the early 1970’s byssinosis, commonly known as “brown lung” disease, was found to be caused by cotton dust exposure. In 1978 approximately 12,000 textile workers suffered from the disease. That same year, OSHA introduced the cotton dust standard (29 CFR # 1910.1043) to protect workers. In 1999 the number of textile workers suffering from the disease had dropped to 700.
OSHA Standard Requirements:
- Employee exposure limits, also known as permissible exposure limits (PEL), are average exposure as measured over an 8 hour-hour workday. There are different PEL exposure limit levels (ranging from 200 to 1000 micrograms per cubic meter) depending on production area. Employers are required to measure the cotton dust levels at least every six months for each shift.
- If the cotton dust levels are above the PEL, the employer must select suitable Personal Protective Equipment (PPE) to protect their employees. The employees are to be fitted and receive training on using, cleaning and maintaining the equipment before using their PPE.
- Free annual medical exams, including breathing tests, must be provided by the employer for all employees in dust areas (even if the area is below PEL). Results must be provided to the employee. Records of the test results must be maintained for 20 years and be available to OSHA upon request.
The standard, as it was originally released, included all types of cotton – even if the fiber had been wet processed. The standard was later amended based on work done by a joint task force that included industry, union and government officials. The work of this joint task force showed that the batch kier method of purifying cotton (which is used by Barnhardt employees) eliminates the bioactivity of cotton dust and fiber and is exempted from the standard.