Given that the printing process could be a topical dyeing, it’s vital to have a specific boundary within the print design. For this reason, they use thickeners that are the main component the printing paste.
The printing paste contains auxiliary and pigmented materials, and as the matter of fact, the thickener existing in this paste reduces its fluidity and prevents colorant (dye) effluence out of the border lines.
The viscosity of the printing paste is another vital point during this process. As a result of the paste effluence and spreading on the surface and within the fabric texture is effective within the final print quality. For example, once high-speed roller printing is performed; If the paste viscosity is high (due to the limited time in the transfer of the paste from the roller to the fabric), it is possible that only the surface spots of the fabric be coated by the paste and the paste doesn’t get transferred to lower surfaces.
This situation causes several issues, for example the beauty within the desired design, the mandatory stability and reduction of other printing standards.
Also, after the printing and through the cleaning process, whereas removing the thickener should be utterly occurring, high viscosity of the paste will cause trouble in the process, and not only increases the price of the finished good, but also will make more problems for the environmental wastewaters.
On the opposite hand, the exaggerated pressure between the printing roller and the fabric at the same time, the increased contact time between fabric and the printing roller increases the amount of paste transfer to the fabric surface, which the viscosity is also important in this case.
Low viscosity within the paste separately from the considered factors such as the speed of the printing machine, the type of consumed fabric and… greatly affects the paste spreading and eventually reduces the delicateness of the border lines in the print design.
Therefore, as to manage the flow and transfer of the printing paste, the paste must have the specified viscosity so that additionally to optimum spreading on the surface and depth of the fabric, preventing it from being transmitted outside the design.
Considering these points are fully essential in selecting a thickener, however aside from viscosity, different points are necessary in thickener usage, including:
- Stability and sustainability
- Easy cleaning
- Ease of paste preparation
Today, protecting earth and environment may be a major international concern, particularly the textile and clothing industry that is sadly one amongst the foremost harmful industries in this field.
Therefore, the utilization of natural base chemicals, for instance, natural thickeners that are environmentally friendly, is effective in this regard.
These thickeners are sodium salts, potassium or calcium alginic acid that are kind of linear polysaccharide. Alginate, derived from seaweed, is definitely dissolved in cold and warm water and creates a thick soluble.
The viscosity of the alginate soluble decreases with the rise in temperature and if it’s not kept at high temperatures for an extended time, returns to the initial state after cooling.
The viscosity of this substance hadn’t changed in 5-10 pH, however precipitates in extreme acidic pH and becomes gel in high alkaline pH.
In general, the viscosity of alginate salts is based on their molecular mass. 2 percent solubles with high molecular mass and 6 percent solubles with low molecular mass are sufficiently viscosity.
It is fascinating to know that alginate salts are capable of absorbing water at 300 times their weight and may be dissolved in water even after stabilization at high temperatures.
Alginates are called the most effective alternative for printing with reactive dye, as a result of anionic chromatins emit the ionized carboxyl groups within the alkaline environment and are extremely appropriate for creating pastes with reactive dye.
The main source of starch is potato, wheat, corn and rice. Hemopolymer starch is units of glucose.
Ten to twenty percent of the starch is Amylose, that is a linear polysaccharide and the opposite half is Amylopectin, which constitutes ninety to eighty percent of starch.
Because of the various solubility of Amylose and Amylopectin, these two can be separated. Amylose is less complicated to make a gel comparing with Amylopectin, which is not suitable for using starch as a thickener, whereas the Amylopectin branch chains forestall the formation and molecular arrangement necessary for the formation of the gel.
Wheat starch has been used in limited cases as a thickener for textile printing, whereas other starches aren’t used.
To change and modify the properties of starch, heating can be used to change some of its physical structure.
English glue is one of these items that is well stable against alkaline and is employed to create printing paste in vat dyes. It’s capable of restoring Azo dyes, therefore Anti-regenerative materials need to be added to the print paste.
Starch solubility can be increased by chemical correction or etherizing. As a result of etherizing, molecular mass and viscosity gets reduced, that has a negative result on dye absorption. Starch etherizing is done in an alkaline environment using monochloro acetic acid, oxide ethylene and Dimethyl sulfate. A little amount of alginate can be added to the current recipient to increase the dye absorption.
Starch ether is usually used in fabric printing with high speed printing machines, because of the viscosity of the print paste is reduced at the moment, leading to a more complete transmission of the paste from the gravure to the fabric.
Guar is a kind of tree cultivated in India, Pakistan and also America and its flour is used as thickener. 85 percent of guar flour is guaran, 63 percent is mannose, 35 percent is lactose and 5 to 7 percent is protein. The thickening ability of the guar is 5 to 8 times more than starch. It’s well hydrated in hot and cold water and makes a thick soluble. Its complete hydration depends on the temperature, for example, complete hydration in cold water could take up to 1 day, however it takes solely 15 minutes at 80°C.
This seed comes from the Caro tree, which is an anionic polysaccharide. It doesn’t simply dissolve in cold water and requires heat to utterly dissolve. 3 to 11 pH has very little impact on its thickness and using only 2 percent of its solid material can obtain an efficient viscosity.
Acacia seeds makes complex with borates, which the created gel is used in the two-staged printing with vat dyes.
It gets back to its original state by adding acids.
These types of thickeners are obtained as syrup from the trunk of trees or the plant, composed of complex polysaccharides and have totally different uronic acid groups.
Tragacanth is one of the plant gums found in southwestern Europe, Greece, Turkey and Syria; however the most high quality kind belongs to Iran. It has a good solubility within the water and produces a thick soluble with pH scale of 5 to 6. It’s stable till pH 2, but its viscosity decreases with heat and salt increasing. The 4 to 5% gum soluble has a good viscosity as a thickener.
Arabic gum comes from the Senegalese Acacia tree, which grows in Sudan, Nigeria and West Africa. This thickener is a mixture of calcium salt, magnesium and… that has a good solubility in water.
Crystal gum is a more specific kind of Karaya gum, which India is the main manufacturer of it and is marketed in powder. Because of the high molecule weight of the gum, the gum doesn’t fully dissolve in water, however it shows good inflation. The viscosity of this thickener decreases with temperature increasing.
Synthetic and semi-synthetic thickeners are used today because of affordable costs and easy preparation. Wood cellulose is the main supply of semi-synthetic thickeners after recovery implementing necessary chemical reactions; and it manufactures substances like CMC (Carboxymethyl cellulose) that’s used as stiffness and in some cases thickener.
PVA or Polyvinyl alcohol is one of the synthetic thickeners and is consumed in a restricted amount. Since low amount of synthetic thickeners also produce viscosity, they’re very appropriate to be used in printing processes such as pigmenting.
Types of thickeners
Printing in textile industry, Compiled by Dr. Hossein Tavanaei
An introduction to Textile printing W.clarke, 4th edition, Newnes-Bulferworlh، ISBN : 0408001402
Textile PrintingT L.W.C Miels, Society of Dyers and colourists, 1994، ISBN: 0901956570
Encylopedia of chemical Technology KRIK-OTHMER, john Wiley and sons1978
Bayer Farben Revue Special Edition No. 10
An introduction to Textile Printing Second Edition, Butterwoths