How to Determine Fibre Composition in Blended Fabrics
Blended fabrics are very common in textiles. A fabric may contain polyester with cotton, cotton with viscose, acrylic with wool, elastane with cotton, or many other combinations. But when a fabric is made from more than one fibre, one important question arises:
How do we know the percentage of each fibre in the fabric?
This is important for quality control, costing, labelling, performance evaluation, buyer communication, export documentation and compliance.
Why Are Fibres Blended?
No single fibre gives all the desirable properties needed in a fabric. One fibre may give strength, another may give comfort, another may improve appearance, and another may reduce cost.
For example, polyester has very good strength, but it does not absorb much moisture. Because of this, 100% polyester fabric may not feel as comfortable as cotton. When polyester is blended with cotton, the fabric can get the strength of polyester and the comfort of cotton.
Fibre blending is generally done for three major reasons:
- To obtain different properties
- To suit changing fashion requirements
- To control the cost of the fabric
Once fibres are blended, it becomes necessary to determine the actual percentage of each fibre in the fabric. This is usually done by dissolving one fibre selectively and weighing the remaining fibre.
Basic Principle of Fibre Composition Testing
Most chemical methods for fibre composition work on a simple principle:
One fibre is dissolved in a specific chemical, while the other fibre remains undissolved.
The undissolved fibre is then:
- filtered,
- washed,
- neutralised if required,
- dried,
- cooled,
- weighed.
From the weight of the remaining fibre, the percentage of each fibre in the blend can be calculated.
1. Polyester and Cellulosic Fibre Blends
This method is used for blends such as:
- Polyester + cotton
- Polyester + viscose
A small sample of the blended fabric, usually 0.5 to 1.0 gram, is weighed accurately and placed in a flask. Then 75% w/w sulphuric acid is added. The material-to-liquid ratio is kept at about 1:200.
The flask is kept in a water bath at 50 ± 5°C for about one hour.
In this process, the cellulosic fibre dissolves, while the polyester remains undissolved.
The remaining polyester fibre is then:
- filtered,
- washed properly with water,
- neutralised with dilute ammonia solution,
- dried at 110°C,
- cooled,
- weighed.
The weight of the remaining fibre gives the percentage of polyester. The percentage of cotton or viscose can be calculated by subtracting the polyester percentage from 100.
Example:
If polyester remaining after the test is 65%, then:
Cellulosic fibre percentage = 100 − 65 = 35%
So the fabric composition is:
65% polyester and 35% cotton or viscose.
2. Cotton and Viscose Blends
Cotton and viscose are both cellulosic fibres, so their separation is more delicate. The Bureau of Indian Standards has described four methods for determining cotton and viscose percentages:
- 60% w/w sulphuric acid method
- Sodium zincate method
- Formic acid and zinc chloride method
- Cadoxen solution method
Among these, the 60% w/w sulphuric acid method is commonly used.
60% w/w Sulphuric Acid Method
In this method, 0.5 to 1.0 gram of sample is weighed accurately and placed in 60% w/w sulphuric acid. The material-to-liquid ratio is kept at 1:100.
The solution is stirred properly by mechanical action for about 30 minutes.
In this process:
- Viscose dissolves
- Cotton remains undissolved
The cotton fibres are then filtered out and washed. After that, they are washed with water and treated with dilute ammonium hydroxide solution for neutralisation. Finally, they are dried and weighed.
However, in this method, the weight of cotton may also reduce by about 5%. Therefore, a correction factor is applied to calculate the actual cotton percentage accurately.
3. Polyester, Cotton and Viscose Blends
In a three-fibre blend containing polyester, cotton and viscose, separation is done step by step.
First, the sample is placed in 60% w/w sulphuric acid.
In this stage:
- Viscose dissolves first.
- Cotton and polyester remain.
The remaining fibres are washed, dried and weighed.
Then the remaining fibres are placed in 75% sulphuric acid.
In this stage:
- Cotton dissolves.
- Polyester remains.
The final remaining fibre is polyester. It is washed, dried and weighed.
In this way, the percentage of viscose, cotton and polyester can be determined separately.
4. Acrylic Blends with Wool, Silk, Cotton, Viscose, Polyester or Nylon
Acrylic fibre may be blended with many other fibres such as wool, silk, cotton, viscose, polyester or nylon.
In such blends, acrylic is first dissolved in dry dimethyl formamide, commonly known as DMF.
In this method:
- Acrylic dissolves in DMF.
- Other fibres remain undissolved.
The undissolved fibres are filtered, washed, dried and weighed. From this, the percentage of acrylic fibre in the blend can be calculated.
5. Protein Fibres with Cotton, Polyester, Nylon or Acrylic
Protein fibres include fibres such as wool and silk.
When protein fibres are blended with cotton, polyester, nylon or acrylic, they can be separated using alkali.
The accurately weighed sample is placed in a conical flask. Then 5% w/w sodium hydroxide or potassium hydroxide solution is added. The mixture is boiled for about 10 minutes.
In this process:
- Protein fibres dissolve.
- Other fibres remain undissolved.
The remaining fibres are filtered and washed thoroughly with water. Then they are washed with dilute acetic acid to neutralise the alkali.
Finally, the sample is dried, cooled and weighed. From this, the percentage of protein fibre and the other fibre can be calculated.
6. Polyester with Cotton or Viscose
Polyester can also be determined by using meta-cresol.
In this method, the blended fibres are weighed accurately and heated with meta-cresol.
In this process:
- Polyester dissolves.
- Cotton or viscose remains undissolved.
The remaining insoluble fibres are washed, dried and weighed. From this, the percentage of polyester is calculated.
7. Elastane, Spandex or Lycra with Cotton or Viscose
Elastane is also known by names such as spandex and Lycra.
When elastane is blended with cotton or viscose, it can be separated using DMF.
In this method, the mixed fibres are treated with DMF.
In this process:
- Elastane dissolves in DMF.
- Cotton or viscose remains undissolved.
The remaining fibres are filtered, washed, dried and weighed. From this, the percentage of elastane is calculated.
Summary Table: Fibre Blend Testing Methods
| Fibre Blend | Chemical Used | Fibre Dissolved | Fibre Remaining |
|---|---|---|---|
| Polyester + cotton/viscose | 75% sulphuric acid | Cotton/viscose | Polyester |
| Cotton + viscose | 60% sulphuric acid | Viscose | Cotton |
| Polyester + cotton + viscose | 60% and 75% sulphuric acid | Viscose first, then cotton | Polyester |
| Acrylic + other fibres | DMF | Acrylic | Other fibres |
| Wool/silk + cotton/polyester/nylon/acrylic | Sodium hydroxide or potassium hydroxide | Wool/silk | Other fibres |
| Polyester + cotton/viscose | Meta-cresol | Polyester | Cotton/viscose |
| Elastane/spandex/Lycra + cotton/viscose | DMF | Elastane | Cotton/viscose |
Why Fibre Composition Testing Matters
Fibre composition testing is very important in the textile industry because it helps in:
- correct fabric labelling,
- buyer compliance,
- export documentation,
- quality control,
- cost verification,
- performance evaluation,
- identifying wrong claims in fabric composition.
For example, if a fabric is sold as 80% cotton and 20% polyester, a laboratory can verify whether the actual fibre content matches the claim.
Similarly, in stretch fabrics, the elastane percentage may be small but very important. Even 2% to 5% elastane can change the stretch, recovery and comfort of the fabric.
Important Precautions
While carrying out fibre composition testing, the following precautions are important:
- The sample should be weighed accurately.
- The correct chemical concentration should be used.
- The material-to-liquid ratio should be maintained.
- Temperature and time should be controlled.
- The residue should be washed completely.
- Neutralisation should be done properly.
- The sample should be dried and cooled before final weighing.
- Correction factors should be applied wherever required.
Small errors in weighing, washing or drying can affect the final fibre percentage.
Conclusion
Fibre blending is done to improve fabric properties, reduce cost and meet fashion requirements. But once fibres are blended, it becomes necessary to know their exact proportion.
The basic method of fibre composition analysis is selective dissolution. One fibre is dissolved in a suitable chemical, while the other fibre remains. The remaining fibre is then washed, dried and weighed.
Different fibres require different chemicals. Polyester, cotton, viscose, acrylic, wool, silk and elastane all behave differently in different solvents. Therefore, correct identification of the fibre blend is necessary before selecting the test method.
For merchandisers, textile students, quality professionals and buyers, understanding these methods is very useful. It helps them read laboratory reports better and understand how fibre composition claims are verified scientifically.
Understanding 75% (w/w) Sulphuric Acid and M:L Ratio
In textile testing instructions, we often come across statements such as:
At first glance, this looks like a short laboratory instruction, but it contains two important pieces of information. The first is the concentration of sulphuric acid, and the second is the amount of acid solution to be used in relation to the weight of the textile material.
What is meant by 75% (w/w)?
The term w/w means weight by weight. Therefore, 75% (w/w) sulphuric acid means that 75 parts by weight of pure sulphuric acid are present in 100 parts by weight of the final solution.
In simple terms:
\[ 75\% \; (w/w) = \frac{75 \text{ g pure } H_2SO_4}{100 \text{ g final solution}} \]
So, if we prepare 100 g of 75% (w/w) sulphuric acid solution, it should contain 75 g of pure sulphuric acid and 25 g of water.
What is meant by M:L :: 1:200?
The term M:L means Material to Liquor ratio. In textile processing and testing, “material” usually refers to the fabric, fibre, yarn, or textile sample. “Liquor” refers to the solution in which the textile material is treated.
Therefore:
\[ M:L = 1:200 \]
means that for every 1 g of textile material, 200 mL of acid solution should be used.
| Fabric Weight | M:L Ratio | Required Acid Liquor |
|---|---|---|
| 1 g | 1:200 | 200 mL |
| 2 g | 1:200 | 400 mL |
| 5 g | 1:200 | 1000 mL |
| 10 g | 1:200 | 2000 mL |
The general formula is:
\[ \text{Liquor required in mL} = \text{Weight of material in g} \times 200 \]
Example: If the fabric sample is 5 g
If the fabric sample weighs 5 g and the required M:L ratio is 1:200, then:
\[ 5 \times 200 = 1000 \text{ mL} \]
So, 5 g of textile material will require 1000 mL of 75% (w/w) sulphuric acid solution.
How to Prepare 75% (w/w) Sulphuric Acid Solution
Since the concentration is given as w/w, the correct method is to prepare the solution by weight, not simply by volume. Laboratory concentrated sulphuric acid is commonly about 98% (w/w), not 100% pure. Therefore, we must account for this while calculating the amount of concentrated acid required.
Suppose we want to prepare 100 g of 75% (w/w) sulphuric acid solution.
Required pure sulphuric acid:
\[ 75 \text{ g} \]
If concentrated sulphuric acid is 98% (w/w), then the amount of concentrated acid required is:
\[ \frac{75}{0.98} = 76.53 \text{ g} \]
Therefore, water required will be:
\[ 100 - 76.53 = 23.47 \text{ g} \]
Take approximately 23.5 g water and slowly add 76.5 g concentrated sulphuric acid.
Preparation for 1000 g of Final Solution
If a larger amount is required, the same calculation can be scaled up. For example, to prepare 1000 g of 75% (w/w) sulphuric acid solution:
Required pure sulphuric acid:
\[ 75\% \text{ of } 1000 = 750 \text{ g} \]
Amount of 98% concentrated sulphuric acid required:
\[ \frac{750}{0.98} = 765.3 \text{ g} \]
Amount of water required:
\[ 1000 - 765.3 = 234.7 \text{ g} \]
Take 234.7 g water first, then slowly add 765.3 g concentrated sulphuric acid with stirring and cooling.
Important Safety Precaution
Dilution of sulphuric acid releases a large amount of heat. If water is added directly to concentrated acid, the mixture can heat suddenly, splash, or even boil violently. Therefore, the safe method is to take the required quantity of water first and then add concentrated sulphuric acid slowly, with continuous stirring.
The preparation should be done using proper laboratory safety equipment such as chemical-resistant gloves, safety goggles, apron or lab coat, and acid-resistant glassware. Cooling should be provided if necessary, especially when preparing larger quantities.
Summary
| Term | Meaning |
|---|---|
| 75% (w/w) | 75 g pure sulphuric acid in 100 g final solution |
| M:L | Material to Liquor ratio |
| M:L :: 1:200 | 1 g textile material requires 200 mL liquor |
| For 5 g sample | Required liquor = \(5 \times 200 = 1000\) mL |
| For 100 g of 75% solution | Use 23.5 g water + 76.5 g of 98% sulphuric acid |
General Formula
If concentrated sulphuric acid strength is known, the required weight of concentrated acid can be calculated as:
\[ \text{Weight of concentrated acid} = \frac{\text{Required pure acid}}{\text{Strength of concentrated acid as decimal}} \]
For 98% sulphuric acid:
\[ \text{Weight of concentrated acid} = \frac{\text{Required pure acid}}{0.98} \]
Water required:
\[ \text{Water required} = \text{Final solution weight} - \text{Weight of concentrated acid} \]
Practical Note for Textile Testing
When a test method says 75% (w/w) sulphuric acid at M:L :: 1:200, it is not merely asking for “some strong acid.” It is specifying both the exact concentration of the acid solution and the amount of solution to be used per gram of textile material. Both are important because fibre dissolution, reaction rate, and test reproducibility depend strongly on acid concentration and liquor ratio.
General Disclaimer
This article is intended for educational and general textile knowledge purposes only. Actual fibre composition testing should be carried out only by trained laboratory personnel using recognised test standards, calibrated equipment, proper safety procedures and appropriate chemical handling protocols. Chemicals such as sulphuric acid, sodium hydroxide, potassium hydroxide, DMF and meta-cresol can be hazardous and should not be handled casually. Always refer to the relevant national or international testing standard before conducting any laboratory procedure.
