Gel Dyeing: Properties, Composition, Mechanism and Advantages
Md. Rubel Miah
Department of Textile engineering
World University of Bangladesh
There are three types of online coloration such as tow dyeing, dope dyeing and gel dyeing. Gel dyeing, sometimes also called mass dyeing, is a continuous tow dyeing method where soluble dyes are applied to wet-spun synthetic fibers such as acrylic or modacrylic fibers in the gel state. The gel state means that the material is not yet at full crystallinity or orientation. The gel dyeing method is applied after extrusion and coagulation, but it happens before drawing and drying. Gel dyeing can be applied only for fibers manufactured by the wet spinning process.
Passing a wet-spun fiber that is in the gel state (not yet at full crystallinity or orientation) through a dye bath containing dye with affinity for the fiber. This process provides good accessibility of the dye sites.
We can express Gel Dyeing like:
- In the wet spinning of Acrylic fiber, dope is prepared first for extrusion into a solvent solution, then the drawing, rinsing, oiling and drying stages follow for eventual densification. The process is completed with subsequent crimping, crimp-setting and cutting.
- Before being dried for densification, Acrylic fiber remains gel-swollen or in other words, spongy, a state which allows the absorption of cationic dye liquor. In short, gel dyeing is the inclusion of the dyeing and fixing processes after the rinsing stage of spinning.
Structure of Gel Dyes:
Application of Gel Dyeing in Fiber:
This dyes are used for Hydrophobic Fibre such as:
- Regenerated fibers……..cellulose acetate
- Synthetic fibers……… polyamides, polyester, polyacrylonitrile
Properties of Gel Dyeing:
SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis, describes a technique widely used in biochemistry, forensics, genetics and molecular biology to separate proteins according to their electrophoretic mobility (a function of the length of a polypeptide chain and its charge) and no other physical feature. SDS is an anionic detergent applied to protein sample to linearize proteins and to impart a negative charge to linearized proteins. In most proteins, the binding of SDS to the polypeptide chain imparts an even distribution of charge per unit mass, thereby resulting in a fractionation by approximate size during electrophoresis.
Preparing Acrylamide Gels:
The gels typically consist of acrylamide, bisacrylamide, SDS, and a buffer with an adjusted pH. The solution may be degassed under a vacuum to prevent the formation of air bubbles during polymerization. Alternatively, butanol may be added to the resolving gel after it is poured, as butanol removes bubbles and makes the surface smooth. A source of free radicals and a stabilizer such as ammonium per sulfate and TEMED are added to initiate polymerization.The polymerization reaction results in a gel because of the added bisacrylamide, generally about 1 part in 35 relative to acrylamide, which can form cross-links between two polyacrylamide molecules. The ratio of acrylamide to bisacrylamide can be varied for special purposes. The acrylamide concentration of the gel can also be varied, generally in the range from 5% to 25%. Lower percentage gels are better for resolving very high molecular weight proteins, while much higher percentages are needed to resolve smaller proteins. Determining how much of the various solutions to mix together to make gels of particular acrylamide concentration is possible.
Gels are usually polymerized between two glass plates in a gel caster, with a comb inserted at the top to create the sample wells. After the gel is polymerized the comb can be removed and the gel is ready for electrophoresis.
1. Chemical buffer Stabilizes the pH value to the desired value within the gel itself and in the electrophoresis buffer. The choice of buffer also affects the electrophoretic mobility of the buffer counterions and thereby the resolution of the gel. The buffer should also be unreactive and not modify or react with most proteins. Different buffers may be used as cathode and anode buffers, respectively, depending on the application. Multiple pH values may be used within a single gel, for example in DISC electrophoresis. Common buffers in SDS-PAGE include Tris, Bis-Tris, or imidazole.
2. Counterion balance the intrinsic charge of the buffer ion and also affect the electric field strength during electrophoresis. Highly charged and mobile ions are often avoided in SDS-PAGE cathode buffers, but may be included in the gel itself, where it migrates ahead of the protein. In applications such as DISC SDS-PAGE the pH values within the gel may vary to change the average charge of the counterions during the run to improve resolution. Popular counterions are glycine and tricine. Glycine has been used as the source of trailing ion or slow ion because its pKa is 9.69 and mobility of glycinate are such that the effective mobility can be set at a value below that of the slowest known proteins of net negative charge in the pH range. The minimum pH of this range is approximately 8.0.
3. Acrylamide (C3H5NO; mW: 71.08). When dissolved in water, slow, spontaneous autopolymerization of acrylamide takes place, joining molecules together by head on tail fashion to form long single-chain polymers. The presence of a free radical-generating system greatly accelerates polymerization. This kind of reaction is known as Vinyl addition polymerisation. A solution of these polymer chains becomes viscous but does not form a gel, because the chains simply slide over one another. Gel formation requires linking various chains together. Acrylamide is a neurotoxin. It is also essential to store acrylamide in a cool dark and dry place to reduce autopolymerisation and hydrolysis.
4. Sodium Dodecyl Sulfate (SDS) (C12H25NaO4S; mW: 288.38). SDS is a strong detergent agent used to denature native proteins to unfolded, individual polypeptides. When a protein mixture is heated to 100°C in presence of SDS, the detergent wraps around the polypeptide backbone. It binds to polypeptides in a constant weight ratio of 1.4 g SDS/g of polypeptide. In this process, the intrinsic charges of polypeptides becomes negligible when compared to the negative charges contributed by SDS. Thus polypeptides after treatment become rod-like structures possessing a uniform charge density, that is same net negative charge per unit length. The electrophoretic mobilities of these proteins will be a linear function of the logarithms of their molecular weights.
Without SDS, different proteins with similar molecular weights would migrate differently due to differences in mass-charge ratio, as each protein has an isoelectric point and molecular weight particular to its primary structure. This is known as Native PAGE. Adding SDS solves this problem, as it binds to and unfolds the protein, giving a near uniform negative charge along the length of the polypeptide.
5. Ammonium persulfate (APS) (N2H8S2O8; mW: 228.2). APS is a source of free radicals and is often used as an initiator for gel formation. An alternative source of free radicals is riboflavin, which generated free radicals in a photochemical reaction.
6. TEMED (N, N, N’, N’-tetramethylethylenediamine) (C6H16N2; mW: 116.21). TEMED stabilizes free radicals and improves polymerization. The rate of polymerisation and the properties of the resulting gel depend on the concentrations of free radicals. Increasing the amount of free radicals results in a decrease in the average polymer chain length, an increase in gel turbidity and a decrease in gel elasticity. Decreasing the amount shows the reverse effect. The lowest catalytic concentrations that will allow polymerisation in a reasonable period of time should be used. APS and TEMED are typically used at approximately equimolar concentrations in the range of 1 to 10 mM.
7. Bisacrylamide (N,N’-Methylenebisacrylamide) (C7H10N2O2; mW: 154.17). Bisacrylamide is the most frequently used cross linking agent for polyacrylamide gels. Chemically it can be thought of as two acrylamide molecules coupled head to head at their non-reactive ends. Bisacrylamide can crosslink two polyacrylamide chains to one another, thereby resulting in a gel.
Mechanism of Gel Dyeing:
The first step after performing denaturing polyacrylamide gel electrophoresis (SDS-PAGE) is to disassemble the gel cassette and place the thin (1 mm thick) polyacrylamide gel in a tray filled with water or buffer. The electrophoresed proteins exist as concentrated “bands” embedded within each lane of the porous polyacrylamide gel matrix. Typically, the proteins are still bound to anionic SDS detergent, and the entire gel matrix is saturated in a particular buffer.
To make the proteins visible, a protein-specific, dye-binding or color-producing chemical reaction must be performed on the proteins within the gel. Depending on the particular chemistry of the stain, various steps are necessary to hold the proteins in the matrix and to facilitate the necessary chemical reaction. All steps are done in solution, i.e., with the gel suspended in a tray filled with one liquid reagent or another.
Given the common constraints of this format, most staining methods involve some version of the same general incubation steps:
- A water-wash to remove electrophoresis buffers from the gel matrix.
- An acid- or alcohol-wash to condition or fix the gel to limit diffusion of protein bands from the matrix.
- Treatment with the stain reagent to allow the dye or chemical to diffuse into the gel and bind (or react with) the proteins.
- De-staining to remove excess dye from the background gel matrix.
Depending on the particular staining method, two or more of these functions can be accomplished with one step. For example, a dye reagent that is formulated in an acidic buffer can effectively fix and stain in one step. Conversely, certain functions require several steps. For example, silver staining requires both a stain-reagent step and a developer step to produce the colored reaction product.
Advantages of Gel Dyeing:
- Dyeing Economy: Lower Energy, Water and Dye requirements and material Handling.
- Better fastness levels: Light, Rubbing & Hot processing.
- Availability of Shrinkable Dyed acrylic in Box-cut permits various innovative blends on the cotton-spinning system.
- Best for Uniform Dyeing & Big Lots. (Lot size >150 MT even is possible).
- Tow Breaking & Spinning performance of Gel Dyed Tow & Fiber is the best and waste generation is lower compared to acrylic dyed by any other method.
- Most Environment friendly process amongst all acrylic dyeing methods.
- Water requirement is five times less compared to conventional tow dyeing;
- No extra Energy requirement and negligible dyestuff losses.
- Spinners do not have to follow strict pollution norms.
- Consistency in shade reproduction.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.