Determination of Degree of Mercerization

Determination of Degree of Mercerization

The effect ofmercerization 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 (lustre) 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.

Goldthwait Red-Green test
In this test a mixture of red and green direct dyes is used to compare the maturity of cotton fibre samples. Immature fibres dye red, and mature fibres 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 fibre 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 fibres in the yarn bundle can be counted using a microscope and the ratio of dyed to undyed fibres can be used to determine the degree ofmercerization.

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 lustre
Pulfrich photometer, the Gorez Glarimeter comparative glass method and microscopic examination of cross-section of fibre are the qualitative methods for the assessment of degree of mercerization. Mercerization increase the lustre and reduces the axial ratio (Fig).

Figure. Cross-section of cotton fibre before and after mercerization.

Microscopic examination shows that the corss-section of the cotton fibre changes from eliptical 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 lustre than ammonia treated fibre.

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 estimations 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.

Properties Changes of Cellulose Due to Mercerization

Introduction:
Cellulose undergoes chemical, physico-chemical and structural modifications on treatment with caustic soda solution of mercerizing strength. Chemical reactions lead to the formation of alkali cellulose, physical reactions, to a change in arrangement of units of cellulose. The optimum modifications in the properties of cellulose can be manipulated with the selection of proper concentration of alkali, time, tension and temperature during the mercerizing process.

Swelling and shrinkage of cellulose:
When cellulose is immersed in a solution of caustic soda of mercerizing strength, water and alkali diffuses in and the material swells. The fibre hair quickly commences to untwist from its twisted ribbon like form and tends to become cylindrical rodlike surface due to deconvolution. The cross -section of the fibre diminishes, the diameter of the fibre becomes more round. The surface of the nearly cylindrical cotton fibre after mercerizing reflects light more evenly to all sides than the kidney shaped cotton fibre and the fibre surface becomes more lustrous. As the fibre swells, the fibre shrinks in length. Swelling and shrinkage are more when there is no tension in the fibre, but the alteration in cross-section caused by swelling is more when mercerization is carried out under tension. Under optimum conditions each cotton fibre may contract nearly 9% in length and swell nearly 150%. Swelling of the cotton fibre also has a disadvantage. The fibre becomes more compact in its swollen condition. This compacting diminishes the further penetration of caustic soda into the fibre i.e. penetration slows down and mercerization in the fiber’s core is lower than on its surface.

Changes of Cellulose Due to Mercerization

Structural modification:
Due to swelling of cellulose in caustic soda solution of mercerization strength, many hydrogen bonds are broken, the plane of molecular chains have been moved apart, molecular structure tends to become decrystallised, the chains or spaces within the cellulose structure become more uniform and the chains of glucose residues have been given a slight twist. Because of the distortion of polymer network and changes in crystalline structure, the process of mercerization is irreversible. Mercerization also affects the size of the crystallites and orientation of the crystalline region and the extent of orientation depends on the tension during mercerization.

Increased luster:
Unmercerized cotton has a general appearance of a flat ribbon with spiral twists, its surface is rough and non-uniform, its cross-section is irregular and ear-shaped while the lumen, the central canal, is broad, irregular and resembles a collapsed tube. All these factors result in less lustre. When a cotton hair is brought into an aqueous solution of sodium hydroxide of 18% (40~ cellulose begins to swell immediately, the hair is elliptical in section in a few seconds, and on further swelling becomes circular and the lumen is practically eliminated. The untwisting of the fibre takes place under effect of swelling and increased alignment and packing of the fibres in the yam also take place.

Lustre also depends on other factors. Mercerization reduces the axial ratio and increase the light scattering within the fibre (transparency) and thus increases the lustre.

Gain in strength:
Mercerization, both slack and with tension, increases the strength uniformity along the fibre length, but mercerized fibre with tension shows greater gain in strength than that of without tension. In practice, the improvements in strength are noticed mostly upon yarn treatments, with fabric the major effect is on the surface only. Mercerization increases the tensile strength of cotton fibres by eliminating the weakest points in the fibre. Mercerization increases the cohesion between individual cotton hairs and this closer embedding of the hairs in the yam not only increases the strength but makes it more uniform in strength and less in diameter.

Increased moisture absorption:
Mercerized celluloses absorb more water, have higher regains and more easily wet out than unmercerized fibres. Due to caustic soda penetration, many hydrogen bonds are broken and it is estimated that the number of available, hydroxyl groups are increased by about 25%. Mercerization, thus decreases the amount of crystalline part or increases the amorphous content of the fibre. This increase in the proportion of amorphous part is directly related to the moisture sorption. Moisture is assumed to be absorbed by suitable groups in the amorphous region and on the surface of the crystallites. When mercerization is carried out under tension, the changes in crystalline portion is comparatively lower than that without tension and hence also the moisture sorption. Standard cotton has moisture content of about 7%, mercerized cotton with tension has about 9% and that of without tension about 11%.

Increased dye adsorption:
Mercerized cotton shows increased depth of shade, increased rate of dyeing and the irregularities due to neps and unripe cotton are less prominent. Generally immature cotton with large lumen responds particularly well to increased light scattering and hence decreased dye uptake.

Mercerization thus lowers the dye costs, savings at 2% depth averaged 40%, while at 6% with the same dyes the average saving is about 60%. Cotton can be causticized or half-mercerized to increase the dye uptake and economics up to 25% may be realised.

Increased reactivity:
The reactivity of mercerized cotton is increased by about 1 I/2 times at lower temperature in comparison to that of unmercerized cotton. The increased reactivity is not so marked when mercerization is carried out under tension. The reactivity ratio is generally referred to as the ratio of copper number of the mercerized to unmercerized samples. The increased reactivity increases dye absorption, moisture absorption and chemical reaction, but at the same time it also accelerates the reaction with acids and oxidising agents and is susceptible to degradation.

Removal of immature cotton:
Mercerization has been recognized as a method for removing immature (dead) fibres to obtain level dyeing effect on cotton fabrics. The dead fibres are underdeveloped and appear as flat or slightly twisted tapes. They are non-crystalline, convolutions are sometimes absent, cell walls are extremely thin and the lumen is collapsed and hence do not contain dye to same extent as matured fibres.

Physical compactness:
Mercerization improves dimensional stability of cotton woven fabrics. When knitted fabrics are compared with respect to their relative openness, temperature increases can be said to improve mercerization because when the goods are bleached and then mercerized, the fabric becomes more dense. However, when unbleached fabrics are mercerized, the fabrics become more open. Mercerization also gives moderate improvement in crease recovery of cotton fabrics as well as some protection against the decrease in tensile strength caused by easy-care finishing.