Mercerization is used for both pretreatment process and finishing process of fabric. Commercial applications of mercerization are carried out to enhance the value of the finished fabric for the consumer. Therefore it is mostly considered to be a finishing process rather than a preparatory process, which is mainly carried out to facilitate coloration processes. Mercerisation refers to the treatment of cotton yarns or fabrics with caustic soda solution. It is not a fiber purification process, but it does induce desirable changes in cotton yarn and fabric properties. The objective of mercerising is to swell the cotton fibre, increasing its lustre, tensile strength (as well as retain it after an easy-care finish), dimensional stability and dyeability (in terms of uniformity and colour yield).
Determination of Degree of Mercerization
The effect of mercerization depends on the conditions of mercerization. A quantitative assessment of the degree of mercerization is carried out mainly in three different ways i.e. variation in the mercerized product, external appearance (luster) and internal appearance (x-ray diagram) etc.
Determination of deconvolution count
In this method cotton hairs are cut in large number of hair fragments 0.2 mm long. They are then mounted in liquid paraffin on a microscopic slide, and then counted the proportion of fragments free from twist on convolution during mercerization. The result is expressed as percentage and is called “Deconvolution Count”. If the figure is above 20, the fabric is mercerized. The ratio of the two sets of data may be used to estimate the degree of mercerization. The disadvantage of this method is that the extent of deconvolution is influenced by the maturity of cotton and by the structure of material that is twist of the yam or weave of cloth.
Determination of swelling index
Untwisting number of single yam can give reliable means for estimating degree of swelling of cellulose in non-polar liquids such as carbon tetrachloride and benzene. Strong alkali solution and cadoxen (cadmium ethylene diamine complex of 3.6%) also causes large amount of untwisting. In this method, the yarn together with the weight (0.8 g) is hanged into the measuring cylinder containing sufficient amount of solvent as to dip the upper end of the yam. The yam is then allowed to untwist and the number of revolutions made by weight are measured for 3 min.
Swelling index = (Untwisting number in solvent) / Untwisting number in water)
Swelling index increases with degree of mercerization. In this method there is no necessity of untreated sample.
Benzopurpurine test
In this method mercerized and unmercerized cotton samples are treated in a 0.5% solution of Benzopurpurine for 30 min at boiling temperature. The treated samples are then washed, dried and compared visually or spectrophotometrically. The mercerized sample is always more deeply dyed than the unmercerized. However, it is recommended that a standard swatch be calibrated so that the actual degree of mercerization can be assured.
Sodium hydroxide spotting test
In this method the undyed fabric is spotted with 30% solution of caustic soda and then both mercerized and undyed spotted fabric samples are dyed by using Benzopurpurine. If the fabric is fully mercerized, the spots will not be evident after dyeing. On the other hand, if the fabric is not mercerized or semi-mercerized, dark spots will be evident and the degree of mercerization can be evaluated on comparing the spots.
Goldthwaite Red-Green test
In this test a mixture of red and green direct dyes is used to compare the maturity of cotton fiber samples. Immature fibers dye red, and mature fibers green. Mercerization increases the fibre’s affinity for green compound and” causticization number” can be assessed related to the strength of the green hue. Fabric treated with liquid ammonia “under industrial mill condition” dyes red.
Staining test
The fiber is immersed in iodine solution (20 g iodine in 100 ml of standard KI solution) for 3 min and rinsed thoroughly. Mercerized cotton is stained bluish black and unmercerized cotton remains white. Cotton fibers in the yarn bundle can be counted using a microscope and the ratio of dyed to undyed fibers can be used to determine the degree of mercerization.
Barium activity number
Mercerized sample absorbs barium hydroxide (alkali) to a greater degree than sodium hydroxide and from practical point of view, barium hydroxide is more easy to estimate. The ratio of uptake for this reagent has been referred to barium activity number.
Barium activity number = [(b-s) / (b-u)]*100
Where,
b = ml required for blank test,
s = ml required for mercerized cotton,
u – ml required for unmercerized cotton.
Determination of luster
Pulfrich photometer, the Gorez Glarimeter comparative glass method and microscopic examination of cross-section of fiber are the qualitative methods for the assessment of degree of mercerization. Mercerization increase the luster and reduces the axial ratio (Fig).
Microscopic examination shows that the corss-section of the cotton fiber changes from elliptical to circular form due to mercerization. This can be measured as a ratio of two axes ‘a’ and ‘b’. The axial ratio (a/b) of unmercerized cotton is about 2.2 to 2.6, whereas that of mercerized cotton is about 1.5 to 1.6. Caustic mercerized samples appear to give superior luster than ammonia treated fiber.
X – ray analysis
X-ray photograph of native cellulose (unmercerized) reveals the presence of two arcs close together and inside the prominent 002 arc. In case of completely mercerized or regenerated cellulose, X-ray photograph shows a change in position of two shorter arcs. On the basis of this estimation it is estimated that mercerizing efficiency seldom exceeds 60-70% for yam and 35-40% for cloth.
Infra-red analysis
Determination of infra-red crystallinity index at different wavelengths (1429 cm-~/893 cm -~) by the usual base line technique can be used. When cellulose- II samples are considered, the 1372 cm-~/2900 cm -~ ratio is more reliable index. A rapid method for estimation of the degree of mercerization has been developed using a near IR diffuse reflection technique.
References:
- Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
- Principles of Textile Finishing by Asim Kumar Roy Choudhury
- Textile Engineering – An Introduction Edited by Yasir Nawab
- Textile Finishing Edited by Derek Heywood
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
