Transfer Printing Process | Advantages/Disadvantages of TransferPrinting

Transfer printing is the term used to describe textile and related printing processes in which the design is first printed on to a flexible nontextile substrate and later transferred by a separate process to a textile. It may be asked why this devious route should be chosen instead of directly printing the fabric. The reasons are largely commercial but, on occasion, technical as well and are based on the following considerations.

1. Designs may be printed and stored on a relatively cheap and nonbulky substrate such as paper, and printed on to the more expensive textile with rapid response to sales demand.

2. The production of short-run repeat orders is much easier by transfer processes than it is by direct printing.

3. The design may be applied to the textile with relatively low skill input and low reject rates.

4. Stock volume and storage costs are lower when designs are held on paper rather than on printed textiles.

5. Certain designs and effects can be produced only by the use of transfers (particularly on garments or garment panels).

6. Many complex designs can be produced more easily and accurately on paper than on textiles.

7. Most transfer-printing processes enable textile printing to be carried out using simple, relatively inexpensive equipment with modest space requirements, without effluent production or any need for washing-off.

Against these advantages may be set the relative lack of flexibility inherent in transfer printing: no single transfer-printing method is universally applicable to a wide range of textile fibres. While a printer with a conventional rotary-screen printing set-up can proceed to print cotton, polyester, blends and so forth without doing a great deal beyond changing the printing ink used, the transfer printer hoping to have the same flexibility would need to have available a range of equipment suited to the variety of systems that have to be used for different dyes and substrates using transfer technology. 

Transfer Printing Process

In addition factors such as stock costs, response time and so on do not always apply and unlike dyers, most printers are able to operate without steaming or washing by using pigment-printing methods. Thus a balance exists which not only permits but even requires the coexistence of direct and transfer printing. The relative importance of the two methods consequently varies with fluctuations of the market, fashion and fibre preference.

A great many methods of producing textile transfer prints have been described in the literature. Many of them exist only in patent specifications but several have been developed to production potential. They may be summarised most conveniently as below.

Sublimation Transfer
This method depends on the use of a volatile dye in the printed design. When the paper is heated the dye is preferentially adsorbed from the vapour phase by the textile material with which the heated paper is held in contact. This is commercially the most important of the transfer-printing methods.

Melt Transfer
This method has been used since the 19th century to transfer embroidery designs to fabric. The design is printed on paper using a waxy ink, and a hot iron applied to its reverse face presses the paper against the fabric. The ink melts on to the fabric in contact with it. This was the basis of the first commercially successful transfer process, known as Star printing, developed in Italy in the late 1940s. It is used in the so-called ‘hot-split’ transfer papers extensively used today in garment decoration.

Film Release
This method is similar to melt transfer with the difference that the design is held in an ink layer which is transferred completely to the textile from a release paper using heat and pressure. Adhesion forces are developed between the film and the textile which are stronger than those between the film and the paper. The method has been developed for the printing of both continuous web and garment panel units, but is used almost exclusively for the latter purpose. In commercial importance it is comparable with sublimation transfer printing.

Wet Transfer
Water-soluble dyes are incorporated into a printing ink which is used to produce a design on paper. The design is transferred to a moistened textile using carefully regulated contact pressure. The dye transfers by diffusion through the aqueous medium. The method is not used to any significant extent at the present time.

These different methods are considered separately in this chapter since they introduce different scientific and technical factors, and their use is best discussed in the context of the rather different commercial environments.

What is Color Model | RYB Color Model | Color Wheel |Hue/Saturation/Value

Color Model

Additive Color  

• RGB Color Model

Subtractive Color

• CMYK Color Model
• RYB Color Model

RYB Color Model
RYB (an abbreviation of red-yellow-blue) is a historical set of colors used in subtractive color mixing, and was once thought to be the set of primary colors. It is primarily used in art and design education, particularly painting. 

Color wheel

Color has there dimensions or qualities:

• Hue
• Value
• Intensity/Chroma

HUE
The name given to a color.
RED ,YELLOW ,VIOLET

VALUE
The lightness or darkness of a color.

INTENSITY
The brightness or dullness of a color.

TINT
Made by adding white to a color so that it is lighter.

HUE + WHITE = TINT

Tints are lightened colors. Always begin with white and add a bit of color to the white until the desired tint is obtained.

SHADE
Made by adding black to a color so that it is darker.

HUE + BLACK = SHADE

Shades are darkened colors. Always begin with the color and add just a bit of black at a time to get the desired shade of a color. This is an example of a value scale for the shades of blue.

The Color Wheel
If the ends of the spectrum are bent around a color wheel is formed:
 

Colors on the wheel can be described using three parameters:
1.Hue: degrees from 0˚ to 360˚
2.Saturation: brightness or dullness
3.Value: lightness or darkness
(As suggested by Henry Albert Munsell in A Colour Notation, 1905)
 

Hue  

Hue or Spectral Color is represented as an angle. 

Primary Colors:
•0˚ = Red
•120˚ = Green
•240˚ = Blue 

Secondary Colors:
•60˚ = Yellow
•180˚ = Cyan
•300˚ = Magenta
 

Saturation  

• Saturation or Chroma is the intensity of a color.
• A highly saturated color is bright and appears closer to the edge of the wheel.
• A more unsaturated color is dull.
• A color with no saturation is achromatic or in the grey scale.

Value 

“The quality by which we distinguish a light color from a dark one.” Value represents the luminescent contrast value between black and white.

Theory of Color | Color Theory | Additive Theory | Subtractive Theory

Theory of Color
Color theory is a body of practical guidance to color mixing and the visual impacts of specific color combinations

Two Types of Color (Based on Application)

• Electronic Color
• Pigment-Based Color

Theory of color

Electronic Color
Electronic color is used in video cameras and displayed on television and computer screens through the use of cathode-ray tubes (CRT). Color monitors use three different types of phosphors that appear red, green, and blue when activated. These phosphors are placed close together, and when combined in differing intensities can produce any color.

Pigment-based Color
Pigment-based color is used in color printers, color filters, and color paints. Color printers use cyan, magenta, and yellow pigments. These pigments when combined in differing intensities can produce any color.

Theories of Color 

1. Additive Theory (Based on Electronic Color)
2. Subtractive Theory (Based on Pigment Color)

Additive Theory  

• The Additive, or light theory deals with radiated and filtered light.
• Additive Theory
• Black radiates no light
• White (sun) radiates all light
• Video is the process of capturing and radiating light, therefore it uses Additive (Light) theory not Subtractive (Pigment) theory.

The primary colors in Additive theory are:

–Red ( R )
–Green ( G )
–Blue ( B )

• The primary colors add together to make white
• Light Theory is also called Additive Theory.
• Light Theory is used in Television, theater lighting, computer monitors, and video production.

Subtractive Theory 

• The subtractive, or pigment theory deals with how white light is absorbed and reflected off of colored surfaces. A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting (that is, absorbing) some wavelengths of light and reflecting the others.
• Black absorbs most light
• White reflects most light
• Colored Pigments absorb light and reflect only the frequency of the pigment color.
• All colors other than the pigment colors are absorbed so this is called subtractive color theory. 
• Subtractive or Pigment Theory is used in printing and painting. 
  

The primary colors in Subtractive Theory are:
–Cyan ( C )
–Magenta ( M )
–Yellow ( Y )
–Black ( K )

Introduction of Under Lap | Closed Lap | Open Lap

The Under Lap

The underlap shog occurs across the side of the needles remote from the hooks on the front of single-needle bar, and in the centre of double-needle bar, warp knitting machines. It supplies the warp yarn between one overlap and the next (Fig.A).The underlap shog generally ranges from 0 to 3 needle spaces, but it might be 14 needle spaces or more depending upon the design of the machine and the fabric structure (although efficiency and production speed will be correspondingly reduced with long underlaps).

A.Under lap shog

Underlaps as well as overlaps are essential in warp knitted structures in order to join the wales of loops together but they may be contributed by different guide bars.

The Closed Lap
A closed lap is produced when a subsequent underlap shogs in the opposite direction to the preceding overlap, thus lapping the same yarn around the back as well as around the front of the needle (Fig.B).

B.Closed Lap

The Open Lap
An open lap is produced either when a subsequent underlap is in the same direction as the preceding overlap (Fig.C) or an underlap is omitted so that the overlap of the next knitting cycle commences in the needle space where the previous overlap finished. Closed laps are heavier, more compact, more opaque, and less extensible than open laps produced from the same yarn at a comparable knitting quality. 

C.Open Lap

Main Parts of the Bearded Needle

The Main Parts of the Bearded Needle

There are five main parts of the bearded needle (Fig:A):

1 The stem, around which the needle loop is formed.

2 The head, where the stem is turned into a hook to draw the new loop through the old loop.

3 The beard, which is the curved downwards continuation of the hook that is used to separate the trapped new loop inside from the old loop as it slides off the needle beard.

4 The eye, or groove, cut in the stem to receive the pointed tip of the beard when it is pressed, thus enclosing the new loop.

5 The shank, which may be bent for individual location in the machine or cast with others in a metal ‘lead’.

  Fig. A:  Main parts of the bearded needle.

Introduction of Wool | Bleaching Process of Wool | Bleaching Method ofWool Fiber/Fabrics

Introduction of Wool

Wool exhibits by nature a pronounced yellow color and also on exposure to light , alkali or by microbial degradation. Commercially, wool bleaching is carried out using either an oxidative or a reductive system, or a combined oxidation/reduction process. Oxidative bleaching in the dyebath is also possible .

In general, oxidative bleaching with hydrogen peroxide gives superior whiteness over reductive methods..Recent research also revealed the use enzymes to enhance the whiteness of bleached wool.

Wool cannot be bleached with sodium hypochlorite solutions, as for cotton, since it is extensively damaged to the point at which it even dissolves in the solution.

Oxidative Bleaching Method
A batch treatment with hydrogen peroxide is used for most bleaching applications.An activator (eg an alkali) is normally added to increase the rate of bleaching. Typically, wool is bleached at pH 8–9 for 1 h at 60◦C with a stabilized solution of hydrogen peroxide (0.75% w/w). It is generally accepted that, under alkaline conditions, the active bleaching species is the perhydroxy anion (OOH− ), the formation of which is encouraged by higher pH .

Peroxide bleaching of wool under mild acidic conditions (pH 5–6) can also be carried out using a peracid activator such as Prestogen W (BASF) or citric acid . As wool sustains some damage in the presence of alkali, this method is useful for bleaching delicate fabrics.

An undesirable side effect is the rapid decomposition of hydrogen peroxide to water and oxygen, a reaction catalyzed by transition-metal ions. A stabilizer, which sequesters these ions, is used to prevent this side reaction occurring. The most common stabilizers for alkaline wool bleaching are phosphates, particularly tetrasodium pyrophosphate. However, recent concerns over phosphates in effluents from textile treatment have led to the development of alternative stabilizers based on silicates .

Heavily pigmented fibers, such as Karakul wools, require a more severe approach known as mordant bleaching. In this method, the wool is treated with a metal salt and then with hydrogen peroxide. In the first step, the melanin pigment in the wool preferentially absorbs the metal cations; and in the second step, the cations catalytically decompose the peroxide to produce highly aggressive hydroxyl free radicals, which selectively attack and bleach the melanin.

Reductive Bleaching
The two most popular chemicals used for reductive bleaching of wool are stabilized sodium dithionite and thiourea dioxide. Most reductive bleaching of wool is carried out using stabilized dithionite (2–5g/L) at pH 5.5–6 and 45–65◦C for 1 h. Thiourea dioxide is more expensive than sodium dithionite, but is an effective bleach when applied (1–3 g/L) at 80◦C and pH 7 for 1 h. Whiter fabrics are produced when oxidative bleaching is followed by a reductive process—this is often referred to as “full bleaching.”

Reductive Bleaching with Sulfur Dioxide
In the early days sulfur dioxide was used to bleach wool , but disadvantage of this method of bleaching wool was that the white was not permanent when exposed to sun and air.

Disadvantage of reductive bleaching with hydro
1.It tends to part harsh handle to wool.
2.Reducing agents tend to break cystine cross links in protien fibers.

Full Bleaching Of Wool
However, with even the best oxidative bleaching processes, there is a limit to the whiteness that may be achieved on wool, within the limits of acceptable fibre damage.Whiter fabrics are produced when oxidative bleaching is followed by a reductive process(combined oxidative/reductive bleaching treatments)—this is often referred to as “full bleaching.”

Shrink Resistance Treatment of Wool
Felting in garments and fabrics that leads to excessive shrinkage is, however, undesirable. It occurs when the wet material is subjected to severe mechanical action, for example, in laundering or tumble drying . Shrink-resist treatments are directed at preventing felting shrinkage, whereas minimization of relaxation shrinkage requires careful control during fabric finishing.

The term shrink-resistant is preferred to “shrinkproofed,Shrink resist treatment of wool adds consumer desired properties to this superior natural fibre, the improved processing technology allows processors and retailers to offer fabrics and garments with added value and performance. The improved performance offered by the Total Easy Care range of processes includes machine washability and tumble dry qualities.

Principle of SR Treatment
A variety of methods produce wool fabrics that withstand repeated washing without shrinkage and felting. They are particularly important for knitted woollens and worsted fabrics. 

Two main principles are used:
(1) modification of the scale structure of the fibre cuticle to decrease the directional friction effect – this can be achieved by chemical treatments that either partially remove the scales or cover them with a smooth film of polymer;

(2) reduction of fibre mobility by adhesion of fibres and yarns at their points of contact and by decreasing fibre elasticity by means of intermolecular crosslinking.

Chlorine SR Treatment
Chlorine-Based Shrink-Resist Treatments. The principal oxidizing agent used in degradative shrink-resist treatments is chlorine. Free chlorine reacts very rapidly with wool; hence, it is difficult to treat a mass of wool fibers evenly. Two different types of chlorination methods are used commercially: continuous treatment and batch treatment. In the continuous method, top or loose wool is reacted with an aqueous solution of chlorine gas for a short time (
Chlorine Free SR treatment
Chlorine-Free Shrink-Resist Treatments. Commercially, the only other oxidizing agent used to any extent is permonosulfuric acid (PMS; HOOSO3H). This is used in the form of its potassium triple salt, containing potassium sulfate and bisulfate. It is employed in batch processes at elevated temperatures, because it reacts more slowly with wool than chlorine. The process sequence is similar to that used for chlorine-based treatments. It involves degradative oxidation with PMS, followed by neutralization with sodium sulfite and then application of a resin. Unlike chlorine, however, PMS does not remove the bound lipid or oxidize cystine to cysteic acid. The main product of the reaction is cystine sulfonic acid or Bunte salt groups.

Plasma Treatment in SR wool
There is an enormous potential in the plasma treatment of natural fibre fabrics. Plasma treatment has proved to be successful in the shrink-resist treatment of wool with a simultaneously positive effect on the dyeing and printing.

Additive Shrink-Resist Treatments

The principal additive shrink-resist treatment for wool fabrics uses the polymer Synthappret BAP (Bayer AG). This is a poly(propylene oxide) polyurethane, containing reactive carbamoyl sulfonate (bisulfite adducts of isocyanate) groups, ie NHCOSO3 − Na+. An aqueous solution of this polymer is padded onto woven fabrics. The polymer cross-links on drying to form flexible linkages between fibers and yarns . Other polymers may be applied at the same time to modify the handle. 

What is Float Stitch/Tuck Stitch? | Successive Tucks and Floats on theSame Rib Needle

Float Stitch

A float stitch or welt stitch (Fig.A) is composed of a held loop, one or more float loops and knitted loops. It is produced when a needle (M) holding its old loop fails to receive the new yarn that passes, as a float loop, to the back of the needle and to the reverse side of the resultant stitch, joining together the two nearest needle loops knitted from it.

Fig.A

In Fig.B, the float stitch shows the missed yarn floating freely on the reverse side of the held loop. (This is the technical back of single-jersey structures but is the inside of rib and interlock structures.) The float extends from the base of one knitted or tucked loop to the next, and is notated either as an empty square or as a bypassed point. It is assumed that the held loop extends into the courses above until a knitted loop is indicated in that wale.

Fig.B

A single float stitch has the appearance of a U-shape on the reverse of the stitch. Structures incorporating float stitches tend to exhibit faint horizontal lines. Float stitch fabrics are narrower than equivalent all-knit fabrics because the wales are drawn closer together by the floats, thus reducing width-wise elasticity and improving fabric stability.

The Tuck Stitch
A tuck stitch is composed of a held loop, one or more tuck loops and knitted loops. It is produced when a needle holding its loop also receives the new loop, which becomes a tuck loop because it is not intermeshed through the old loop but is tucked in behind it on the reverse side of the stitch(Fig.C). Its side limbs are therefore not restricted at their feet by the head of an old loop, so they can open outwards towards the two adjoining needle loops formed in the same course. The tuck loop thus assumes an inverted V or U-shaped configuration. The yarn passes from the sinker loops to the head that is intermeshed with the new loop of a course above it, so that the head of the tuck is on the reverse of the stitch.

Fig.C

The tuck loop configuration can be produced by two different knitting sequences:

1 By commencing knitting on a previously empty needle. As the needle was previously empty, there will be no loop in the wale to restrict the feet of the first loop to be knitted and, in fact, even the second loop tends to be wider than normal. The effect is clearly visible in the starting course of a welt. By introducing rib needles on a selective basis, an open-work pattern may be produced on a plain knit base.

2 By holding the old loop and then accumulating one or more new loops in the needle hook. Each new loop becomes a tuck loop as it and the held loop are knocked-over together at a later knitting cycle and a new loop is intermeshed with them. This is the standard method of producing a tuck stitch in weft knitting.

Successive Tucks and Floats on the Same Rib Needle

Successive tucks on the same needle are placed on top of each other at the back of the head of the held loop and each, in turn, assumes a straighter and more horizontal appearance and theoretically requires less yarn. Under normal conditions, up to four successive tucks can be accumulated before tension causes yarn rupture or needle damage. The limit is affected by machine design, needle hook size, yarn count, elasticity and fabric take-down tension (Fig.D). 

Fig.D :Successive tucks and floats on the same rib needle.