What is the difference of Modal from Viscose

Modal is type of Rayon, however it is different from Viscose in many aspects.

Viscose fiber, is one of the most widely used man-made fiber as it is versatile , affordable and its properties resemble to cotton .However, ordinary viscose staple fibers have low initial modulus i.e. they can be easily stretched under low load specially in wet condition . It has poor elastic recovery and is very sensitive to the effect of moisture.

Modal fiber, introduced in the year 1930 seems to be the right answer to these disadvantages. Modal as defined by the International Bureau for Standardization of Manmade Fibers (BISFA) is a distinct viscose rayon fibre genre, which has a higher wet modulus and satisfies a minimum value of tenacity in the wet stage at 5% elongation.

Modal has number of advantages over viscose fiber and mercerised Cotton.

1. Modal posses lower elongation and higher wet modulus as it has higher rate of polymerization than viscose.

2. Its exceptional similarity to cotton and silk in terms of texture (luster, sheen and gloss) results in a surface smoother than mercerized cotton .

3. It can be blended with almost all textile fibres, viz. cotton, wool, silk and other synthetic fibers.

4. Modal displays high dimensional stability, both for low shrinkage and low unrecoverable extension.

5. It is more hygroscopic in nature than cotton i.e. absorbs 50% more water than cotton, has more tensile strength with excellent wear resistance and is less likely to form pills as a result of friction.

6. Modal is also resistant to wear and strong while maintaining a soft, silky feel. Modal fibres have found a wide variety of uses in clothing, outwear and household furnishings.

7. Textiles made from modal fibres do not fibrillate, or pill, like cotton does and are resistant to shrinkage and fading .

8. It causes reduced growth of bacteria when compared to cotton, if both stored for the same period of time and it age like cotton.

Today, Modal is widely used in clothing as a replacement for cotton or mercerized cotton and viscose. (Source)

In addition to this, here is a guide to the various brand names of fibers to their generic names.

Want to go in more depth, here is an article.

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Learning about Viscose, Modal and Tencel

The production process of the three fibers has been convered elsewhere in the blog. Here I would like to discuss some of the properties useful for fabric buyers for comparison.

In dry state viscose is only slight weaker than cotton. However, in the wet state, the strength is about 38% that of cotton. That makes it a very tricky fiber to blend with cotton and subsequent dyeing with cotton. The fabric undergoes changes in shape when wet processing.

Also strength of cotton increases when wet- being 1.14 times that in dry state. However, for viscose it is about 0.5 times that in dry state. This necesssitates that the viscose should be dry cleaned rather than ordinarily washed.

Modal's strength is comparable to cotton in dry state. In wet state, it is about 78% of the cotton strength. For Tencel, it is much more than cotton both in dry and wet state.

A table comparing the properties of the three is given below:

The above table represents three fibers from Birla. VSF is the first generation viscose. Modal is second generation and Tencel is the third generation viscose.

This Link describes the precautions to be followed in viscose processing.

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Fiber Identification - Burning Test- Man-made Fibers

Viscose Rayon 

All viscose including High Wet Modulus scorch and ignite quickly when brought near the flame. Like cotton they burn quickly with yellow flame when in the flame. When removed from the flame they continue to burn. There is no afterglow unlike cotton. The smell is that of burning paper. They leave a light gray and feathery ash.

Acetate Rayon  ( And Triacetate Rayon)

When brought near the flame, it fuses away from flame turning black. When in the flame, it flames quickly. The fabric puckers, sputters and melts. It drips like burning tar. When removed from the flame, it continues to burn and melt. It smells like vinegar. It leaves a brittle hard, irregular black ash which is difficult to crush.

Nylon 

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Nylon fuses and shrinks away from the flame when brought near the flame. In flame, it burns slowly without melting. When removed from flame the flame diminishes and tends to die out. It has somewhat pungent odor. It leaves a hard, round, tough and gray bead.

Aramid  ( Nomex)

When brought near the flame, it shrinks away from the flame. When in the flames it puckers and chars. When removed from flame, it extinguishes by itself. It has no smell and it leaves a hard black bead.

Polyester  

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Polyester fuses and shrinks away from flame. When in flame, it burns slowly with melting. When removed from the flame, it burns with difficulty. It has slightly sweetish smell. It leaves a hard round brittle, black bead.

Acrylics 

Orlon, Acrilan and Creslan and Zefran fuse and melt away from Flame when brought near the flame. When in flame Orlon flames rapidly. The fiber puckers, sputters and melts. Acrilan flames rapidly and melts. Creslan flames and melts and Zefran sputters slightly and flames. When removed flame all of acrylics continue to burn and melt. Orlon has a slightly burning meat-like smell. Acrilan has a buring steak smell. Creslan has sharp sweet smell and Zefran has a turmeric like smell. Orlon, Acrilan and Cresla have hard, brittle and irregular black bead. Zefran has irregular black ash that can be crushed easily.

Modacrylics

Verel and SEF fuse and shrink away from the flame when approached near a flame. When in flame, Verel burns very slowly with melting. SEF shrinks, melts and smolders. When removed from flames, all modacrylics are self extinguishing. Verel has a gunpower smell whereas SEF has a sharp sweet smell. Verel leaves a hard and irregular black bead whereas SEF leaves a hard and irregular black bead.

Spandax 

Fuses but doesn’t shrinks away from the flame when approached near the flame. When in flame, it burns with melting. It has an acrid smell. It leaves a soft, fluffy black bead.

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Properties of Viscose Rayon

Properties of Viscose Rayon

Moisture Absorption

It absorbs more moisture than cotton. Moisture Content of Coton is 6% at 70 deg F and 65% RH, and for Viscose Rayon it is 13% under the same conditions.

Tensile Strength

The Tensile Strength of the fibre is less when the fibre is wet than when dry. It is 1.5-2.4 gpd in the dry state and 0.7-1.2 gpd in the wet state. For high tenacity variety the values are 3-4.6 gpd and 1.9 to 3.0 gpd.

Elasticity

The elasticity of Viscose Rayon is less than 2-3%. This is very important in handling viscose yarns during weaving, stentering etc when sudden tensions are applied.

Elongation at Break

Ordinary Viscose rayon has 15-30% elongation at break, whule high tenacity rayon has only 9-17% elongation at break.

Density

The density of Viscose rayon is 1.53 g/cc. Rayon filaments are available in three densities: 1.5, 3.0 and 4.5

Action of Heat and Light

At 300 deg F or more, VR loses its strength and begins to decompose at 350-400 deg F. Prolonged exposure to sunlight also weakens the fibre due to moisture and ultraviolet light of the sunlight.

Chemical Properties

Viscose rayon consists of cellulose of lower DP than cotton cellulose. Also amorphous region of Viscose rayon is present to a greater extent, therefore, Viscose rayon reacts faster than cotton with chemicals. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form oxycellulose. Cold acid solutions for a short time do not attack viscose rayon.

Action of Solvents

Textile solvents can be used on Viscose rayon without any deteriorating effect. Viscose rayon dissolves in cuprammonium hydroxide solution.

Effect of Iron

Contact with iron in the form of ferrous hydroxide weakens viscose rayon yarns. Therefore staining, marking or touching of rayon to iron or iron surface should be avoided.

Action of Microorganisms

Microorganisms ( moulds, mildew, fungus, bacteria) affect the colour, strength, dyeing properties and lustre of rayon. Clean and dry viscose rayon is rarely attacked by moulds and mildew.

Longitudinal View

The longitudinal view of these fibres show many striations running parallel to the long axis of the fibre. The cross section of viscose has striated periphery, having many sharp indentations, and cross sectional contours vary from circular and oval to ribbon-like forms.

Manufacturing Process of Viscose Rayon

Viscose Rayon

It is a regenerated cellulosic fibre and cellulose is the raw material for producing this man made fibre.

The raw material is obtained from a special variety of wood called spruce.

Manufacturing Process

a. Purification of Cellulose:

The manufacture of viscose rayon starts with the purification of cellulose. Spruce trees are cut into timber. Their barks are removed and cut into pieces measuring 7/8" x 1/2" x 1/4". These pieces are treated with a solution of calcium bisulphite and cooked with steam under pressure for about 14 hours.
The cellulosic component of the wood is unaffected by this treatment, but the cementing material called lignin, which is present in the wood, is converted into its sulphonated compound which is soluble in water. This can be washed off, thereby purifying the remaining cellulose. This cellulose is treated with excess of water. After this it is treated with a bleaching agent (sod hypochlorite) and finally converted into paper boards or sheets. This is called wood pulp, which is normally purchased by the manufacturers of viscose rayon.

b. Conditioning of Wood Pulp:
The pulp sheets are cut by a guillotine to the required dimension and are kept in a special room. Air moves freely among the divisors by means of ventilatorys, the temperature is maintained at 30 deg celcius. In this way the desired moisture content can be had.

c. Steeping Process:
The conditioned wood pulp sheets are treated with caustic soda solution ( about 17.5%). It is called mercerising or steeping. The high DP cellulose (1000) is converted into soda cellulose. The sheets are allowed to soak (steep0 until they become dark brown in colour. This takes about 1-14 hours. The caustic soda solution is drained off and sheets are pressed to squeeze out excess caustic soda solution. 100 kg of sulphite pulp gives about 310 kg of soda cellulose.

4. Shredding or cutting process:
The wet, soft sheets of soda cellulose are passed through a shredding machine which cuts them into small bits. In 2-3 hours the sheets are broken into fine crumbs.

5. Ageing Process:
To obtain almost ideal solution of cellulose, the soda cellulose is stored in small galvanised drums for about 48 hours at 28 deg C. This process is called ageing process.The ageing process is essential. During This process, the DP od soda cellulose is decreased from 1000 to about 300 by oxygen present in the air, contained in the drum.

6. Churning Process or Xanthation:
After ageing, the crumbs of soda cellulose are transferred to rotating, air tight, hexagonal churners or mixers. Carbon disulphide ( 10% of the weight of the crumbs) is added to the mixer and churned together for 3 hours by rotating the mixers at a slow speed of 2 rev per minutes. Sodium cellulose xanthate is formed during this process and the colors of the product changes from white to reddish orange.

7. Mixing or dissolving Process:
The orange product i.e. sod.cell.xanthate is in the form of small balls. These fall into a mixer called dissover which is provided with a stirrer. A dilute solution of caustic soda is added, and the contents are stirred for 4-5 hours and at the same time, the dissovler is cooled. The sod.cell.xan. dissovles to give a clear brown thick liquor, similar to honey. This is called 'viscose' and it contains about 6.5% caustic soda and 7.5% cellulose.

8. Ripening Process:
This viscose solution requires to be ripened to give a solution having best spinning qualities. Ripening is carried by storing the viscose solution for 4-5 days at 10 to 18 deg. The viscosity of the solution first decreases and then rises to its original value. The ripened solutoin is filtered carefully and is now ready for spinning to produce viscose rayon filaments.

9. Spinning Process:

The viscose solution is forced through a spinnerette, having many fine holes ( 0.05-0.1mm) diameter. The spinnerette is submerged into a solution containing the following chemicals.
10% --> sulphuric acid, 18%- Sod sulphate, 1% - Zinc sulphate, 2% glucose, 69% water.

The spinning solution is kept at 40-45 deg celcius.

Sodium sulphate precipitates the dissoved sod. cell.xanthate. Sulphuric Acid converts xanthate into cellulose, carbon disulphide and sod. sulphate. the glucose is supposed to give softness and pliability to the filaments whereas zinc sulphate gives added strength.

The quality of viscose rayon filament formed depends upon:

1. The temperature of the spinning bath
2. The composition of the spinning bath.
3. The speed of coagulation
4. The period of immersion of the filament in the spinning bath.
5. The speed of spinning.
6. The stretch imparted to the filaments.

As a number of filaments emerge from the spinnerette, they are taken together to an eye at the surface of the spinning bath and then guided to two rollers from where they are wound on to a spindle.