How to Identify Constituent Fibre Percentage in a Blend-1

How to Identify Constituent Fibre Percentage in a Textile Blend

Textile fabrics are often made from blends of two or more fibres. A polyester-cotton shirt, a polyester-viscose fabric, or a cotton-viscose blend may look like a single material to the eye, but its behaviour during dyeing, finishing, washing, shrinkage, comfort, strength, and costing depends heavily on the actual fibre composition.

For this reason, textile laboratories use quantitative fibre analysis to determine how much of each fibre is present in a blend. In simple terms, one fibre component is dissolved using a suitable chemical reagent, while the undissolved residue is filtered, washed, dried, and weighed. From the change in weight, the percentage of each fibre can be calculated.

Table of Contents

• Why Fibre Percentage Identification Matters
• Basic Principle of Chemical Fibre Analysis
• General Precautions Before Testing
• Polyester/Cotton or Polyester/Viscose Blend
• Cotton/Viscose Blend
• Polyester/Cotton/Viscose Blend
• Quick Calculation Table
• Practical Points to Remember
• Related Reading
• Sources
• General Disclaimer

Visual 1: A simple schematic showing a blended textile sample, selective chemical dissolution, filtration, residue drying, and final weighing.

Why Fibre Percentage Identification Matters

Knowing the fibre composition of a textile blend is important for production, merchandising, dyeing, quality control, costing, product labeling, and customer communication. A fabric declared as polyester-cotton, for example, may behave very differently depending on whether the blend is 65:35, 80:20, or 50:50.

Polyester and cotton also require different dyeing and finishing approaches. Polyester is hydrophobic and is commonly dyed with disperse dyes, while cotton is hydrophilic and is commonly processed with dye classes suitable for cellulosic fibres. Viscose is also cellulose-based, but it has different wet strength and chemical behaviour compared with cotton.

Basic Principle of Chemical Fibre Analysis

The basic principle of quantitative chemical analysis is selective dissolution. A known weight of the blended sample is treated with a reagent that dissolves one component while leaving the other component mainly unaffected. The remaining fibre residue is then separated, washed, neutralised if required, dried, cooled, and weighed.

The percentage of the undissolved fibre can be calculated as:

\[ \text{Percentage of residue fibre} = \frac{\text{Dry weight of residue}}{\text{Original dry weight of sample}} \times 100 \]

The dissolved fibre percentage can then be calculated as:

\[ \text{Percentage of dissolved fibre} = 100 - \text{Percentage of residue fibre} \]

In actual laboratory practice, correction factors may be required because the fibre remaining as residue may still lose a small amount of weight during chemical treatment. These correction factors should be applied according to the relevant standard method or laboratory procedure.

General Precautions Before Testing

Before quantitative analysis, the fibres present in the blend should first be identified by suitable qualitative methods such as microscopic examination, burning behaviour, solubility behaviour, or other standard fibre identification methods. Quantitative analysis should not be started blindly on a completely unknown sample.

The sample should also be free from non-fibrous material such as size, oil, wax, dirt, resin, finishing chemicals, coatings, or other additives. These substances can affect the sample weight and may lead to incorrect percentage calculation. If present, they should be removed by a suitable pre-treatment method before chemical analysis.

1. Polyester/Cotton or Polyester/Viscose Blend

In a polyester/cotton or polyester/viscose blend, the cellulosic component can be dissolved using sulphuric acid under controlled conditions, while polyester remains as the residue. Cotton and viscose are cellulose-based fibres, whereas polyester is more resistant to this treatment.

Method

1. Take approximately 0.5 to 1.0 gram of the blend sample and weigh it accurately.
2. Place the sample in a clean flask and add 75% by weight sulphuric acid.
3. Maintain a material-to-liquor ratio of about 1:200 so that the sample is properly immersed.
4. Keep the flask in a water bath for about one hour at approximately \(50 \pm 5^\circ C\).
5. Filter the contents carefully. The residue left on the filter is polyester.
6. Wash the polyester residue thoroughly with water and neutralise it using dilute ammonia solution.
7. Dry the residue at approximately \(110^\circ C\), cool it in a desiccator, and weigh it accurately.

The dried residue gives the polyester content. The remaining percentage represents the cellulosic component, either cotton or viscose depending on the original blend.

Calculation

\[ \text{Polyester %} = \frac{\text{Dry weight of polyester residue}}{\text{Original dry weight of sample}} \times 100 \]

\[ \text{Cotton or Viscose %} = 100 - \text{Polyester %} \]

Example

If the original dry sample weight is 1.000 gram and the final dry polyester residue is 0.650 gram, then:

\[ \text{Polyester %} = \frac{0.650}{1.000} \times 100 = 65% \]

\[ \text{Cotton or Viscose %} = 100 - 65 = 35% \]

Therefore, the blend is approximately 65% polyester and 35% cotton or viscose.

Visual 2: Flow diagram for polyester/cotton or polyester/viscose analysis, showing cellulosic dissolution and polyester residue calculation.

2. Cotton/Viscose Blend

Cotton and viscose are both cellulose-based fibres, but their chemical behaviour is not identical. Viscose is regenerated cellulose and is generally more easily attacked by certain reagents than cotton. In this method, viscose dissolves and cotton remains as the residue.

Method

1. Take approximately 0.5 to 1.0 gram of accurately weighed sample.
2. Place the sample in a flask and add 60% by weight sulphuric acid.
3. Maintain a material-to-liquor ratio of about 1:100.
4. Stir the solution mechanically for about 30 minutes.
5. During this treatment, viscose dissolves and cotton remains.
6. Filter the cotton residue and wash it thoroughly with water.
7. Neutralise the residue with dilute ammonium hydroxide solution, wash again, dry, cool, and weigh.

A correction factor is important in this method because cotton may lose some weight during the treatment. A commonly used correction mentioned for this process is about 5% weight loss for cotton. Therefore, the observed cotton residue weight should be corrected before calculating the final blend percentage.

Calculation With Cotton Correction

If cotton loses approximately 5% during the process, the corrected cotton weight can be calculated as:

\[ \text{Corrected cotton weight} = \frac{\text{Observed cotton residue weight}}{0.95} \]

Then:

\[ \text{Corrected cotton %} = \frac{\text{Corrected cotton weight}}{\text{Original dry sample weight}} \times 100 \]

\[ \text{Viscose %} = 100 - \text{Corrected cotton %} \]

Example

Suppose the original dry sample weight is 1.000 gram and the observed cotton residue after drying is 0.475 gram. The corrected cotton weight is:

\[ \frac{0.475}{0.95} = 0.500 \text{ gram} \]

Therefore:

\[ \text{Cotton %} = \frac{0.500}{1.000} \times 100 = 50% \]

\[ \text{Viscose %} = 100 - 50 = 50% \]

After applying the correction factor, the blend is approximately 50% cotton and 50% viscose.

3. Polyester/Cotton/Viscose Blend

A polyester/cotton/viscose blend contains one synthetic fibre and two cellulosic fibres. The analysis is done in stages. First, viscose is dissolved using 60% sulphuric acid. The remaining fibres are cotton and polyester. Then cotton is dissolved using stronger sulphuric acid, leaving polyester as the final residue.

This step-by-step separation allows the three components to be estimated separately. The first weight loss gives viscose, the final residue gives polyester, and cotton can be calculated by difference.

Method

1. Take an accurately weighed dry sample of the polyester/cotton/viscose blend.
2. Treat the sample with 60% by weight sulphuric acid. Viscose dissolves under this treatment.
3. Filter, wash, dry, cool, and weigh the remaining cotton and polyester residue.
4. Treat this residue with 75% sulphuric acid. Cotton dissolves under this treatment.
5. Filter the final residue carefully. Wash thoroughly, neutralise if required, dry, cool, and weigh.
6. The final residue is polyester. Cotton is calculated by difference after accounting for viscose and polyester.

Calculation

Let:

\[ W_0 = \text{Original dry sample weight} \]

\[ W_1 = \text{Dry weight after dissolving viscose} \]

\[ W_2 = \text{Final dry polyester residue} \]

Then:

\[ \text{Viscose %} = \frac{W_0 - W_1}{W_0} \times 100 \]

\[ \text{Polyester %} = \frac{W_2}{W_0} \times 100 \]

\[ \text{Cotton %} = 100 - \text{Viscose %} - \text{Polyester %} \]

Example

Suppose the original dry sample weight is 1.000 gram, the dry weight after dissolving viscose is 0.700 gram, and the final polyester residue is 0.400 gram. The blend percentages are calculated as follows:

\[ \text{Viscose %} = \frac{1.000 - 0.700}{1.000} \times 100 = 30% \]

\[ \text{Polyester %} = \frac{0.400}{1.000} \times 100 = 40% \]

\[ \text{Cotton %} = 100 - 30 - 40 = 30% \]

Therefore, the blend is approximately 40% polyester, 30% cotton, and 30% viscose.

Visual 3: Three-stage analysis chart for polyester/cotton/viscose blend, showing viscose removal, cotton removal, and polyester residue.

Quick Calculation Table

Blend	First Dissolved Component	Residue Obtained	Main Calculation
Polyester/Cotton	Cotton	Polyester	Polyester % = residue weight ÷ original weight × 100
Polyester/Viscose	Viscose	Polyester	Polyester % = residue weight ÷ original weight × 100
Cotton/Viscose	Viscose	Cotton	Corrected cotton weight may be required before percentage calculation
Polyester/Cotton/Viscose	Viscose first, cotton second	Polyester final residue	Viscose by first loss, polyester by final residue, cotton by difference

Practical Points to Remember

The accuracy of fibre percentage analysis depends on careful sampling, accurate weighing, complete dissolution, complete washing, correct neutralisation, and proper drying. Even a small error in drying or weighing can affect the final percentage, especially when the sample weight is small.

Chemical analysis is different from a simple burning test. A burning test can help identify the probable fibre type, but it cannot reliably provide the percentage of each fibre in a blend. For blend percentage, controlled quantitative analysis is required.

Correction factors should not be ignored. The fibre left as residue may still lose a small amount of weight during the chemical process. If a standard method prescribes a correction factor, it should be applied before reporting the final blend composition.

Related Reading on Fibre Identification and Textile Testing

• How to Identify Constituent Fibre Percentage in a Blend-2
• Fiber Identification - Burning Test- Man-made Fibers
• Distinguishing Linen from Cotton
• Regenerated Cellulose Fibres: Understanding Rayon, Viscose, Modal, Lyocell, Cupro, Acetate and Triacetate

Sources

1. ISO 1833-1:2020. Textiles — Quantitative chemical analysis — Part 1: General principles of testing. International Organization for Standardization. https://www.iso.org/standard/74881.html
2. Bureau of Indian Standards. IS 3416:1988 / IS 3416 Part 1: Method for quantitative chemical analysis of binary mixtures of polyester fibres with cotton or regenerated cellulose. https://law.resource.org/pub/in/bis/S12/is.3416.1988.pdf
3. Bureau of Indian Standards. IS 3416-1:1988 Amendment: Sulphuric acid method for polyester with cotton or regenerated cellulose. https://law.resource.org/pub/in/bis/S12/is.3416.1.1988.pdf
4. Japan Customs. Textile Analysis: Natural or Chemical Fiber? https://www.customs.go.jp/ccl/e_etc/3.htm
5. AATCC. Textile Research, Test Methods, and Standards Development. https://www.aatcc.org/

General Disclaimer

This article is intended for textile learning and general technical understanding. It is not a substitute for an accredited laboratory test report, official standard method, or professional chemical safety training. The procedures mentioned involve strong chemicals such as sulphuric acid and should be performed only by trained personnel in a properly equipped laboratory with suitable safety precautions, ventilation, neutralising arrangements, and personal protective equipment.

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How to cite this article:
Goyal, P. How to Identify Constituent Fibre Percentage in a Blend-1. My Textile Notes. Available at: https://mytextilenotes.blogspot.com/2009/06/how-to-identify-constituent-fibres.html

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