Chemistry of Dyes Lecture 6 2023 PDF

Chemistry of dyes By ▪ Dr. Mohamed Ramadan (Ph.D. organic Chemistry , Faculty of Science, MU) Dr. Mohamed R. Elmorsy  Color Fastness ❖ Color fastness is the resistance of the colour of textiles to the different agents to which these mater...

Chemistry of dyes By ▪ Dr. Mohamed Ramadan (Ph.D. organic Chemistry , Faculty of Science, MU) Dr. Mohamed R. Elmorsy  Color Fastness ❖ Color fastness is the resistance of the colour of textiles to the different agents to which these materials may be exposed during manufacture and their subsequent use. ❖ Color fastness of textile products is an important quality feature. As an actual fact, poor color fastness stops the list of consumer complaints.. ❖ The degree to which a dyed material can withstand such treatments and wearing is called colour fastness. ⚫Common color fastness: 1- Fastness to washing https://youtu.be/A2O-FbLmYJQ 2- Fastness to light https://youtu.be/k3fCKHsu_1U 3- Fastness to sublimation https://youtu.be/ocpqNINIvfc 4- Fastness to perspiration https://youtu.be/NhkfWPUuZjY 5- Fastness to rubbing https://youtu.be/z7_Ho6J1cVY  ⚫ Assessment of color fastness: ▪ Color fastness is usually assessed separately with respect to change in color and staining. ▪ Grey Scale is used for assessing changes in color in color fastness tests. ▪ The scale consists of nine pairs) 1 / 1. 5 / 2 / 2. 5 / 3 / 3. 5 / 4 / 4. 5 / 5 ) of grey color chips each representing a visual difference and contrast. ⚫ The result of a color fastness test is rated by visually comparing the difference in color or the contrast between the un-treated & treated specimens with the differences represented by the Scale. ⚫ The color fastness grade is equal to the gray scale step which is judged to have the same color or contrast difference: 1-poor, 1.5-2-fair, 2.5-3-moderate, 3.5-4-good, 4.5-5- excellent  Gray Scale: A “gray scale” version of an image uses a mix of black and white to represent the “value” of the colors. Gray scale is used for visually evaluating changes in color of textiles resulting from colorfastness tests. There are two types of gray scale – one for assessing color change and another for staining. 1. The color change gray scale consists of nine pairs of grey- colored chips, in grades ranging from 1 to 5. 2. The staining scale consists of nine pairs of grey and white-colored chips, in grades ranging from 1 to 5. Gray Scale for Color Change: In colorfastness tests, it is necessary to determine the magnitude of the change in color between the original sample and the sample after testing. Slight changes may be acceptable to the consumer, whereas large changes would certainly be unacceptable. The correct use of the gray scale is important, and the angle of viewing, and the quality and intensity of radiation used to view the samples and the gray scales are specified in the standards, in order to minimize operator bias. 4.5 Gray Scale for Color Staining: There is also a second set of gray scales, known as the gray scale for staining. The result of staining from a colorfastness test is rated by visually comparing the difference in color or the contrast between the stained and unstained specimens with the differences represented by the Scale. Here, the first pair rated 5 are white, whereas the contrast again increases with the second shade becoming grayer as the rating decreases. This scale is used to assess the wet and dry rubbing fastness test as well as several others that measure the transfer of color to other substrates, including washing fastness. A chromatic transference scale for assessment of rubfastness tests, with a range of different shades, can also be used. A different set of gray scales is used for measuring staining. Fastness rating 5 is shown by two identical white samples (that is no staining) and rating 1 shows a white and a grey sample. The other numbers show geometrical steps of contrast between white and a series of greys. A piece of untreated, unstained, undyed cloth is compared with the treated sample that has been in contact with the test specimen during the staining test and a numerical assessment of staining is given. A rating of 5 means that there is no difference between the treated and untreated material.  Dye classes’ color fastness properties Color Fastness to Dye Class Washing Light Dry cleaning Perspiration Rubbing Direct Moderate Moderate Good Good Good Disperse Good Good Good Good Good Reactive Good Good Excellent Excellent Good Vat (except indigo) Excellent Excellent Good Excellent Good Azoic Good Good Moderate Good Moderate  Textile Fibers Classification Fibers Protein fibers Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy  Wool is a protein—a naturally occurring polymer made up of amino acid repeating units. Many of the amino acid units have acidic or basic side chains that are ionized (charged). The presence of many charged groups in the structure of wool provides excellent binding sites for dye molecules, most of which are also charged. Cotton is a polysaccharide composed of glucose units attached to one other in a very rigid structure. The presence of three polar hydroxyl (–OH) groups per glucose repeating unit provides multiple sites for hydrogen bonding to ionic and polar groups in dye molecules. Cotton is hydrophilic, and resistance to alkali Dr. Mohamed R. Elmorsy  Acetate is cellulose in which some of the –OH groups have been replaced by acetate groups (–OCOCH3). The presence of acetate side chains makes acetate softer and easier to work with than cotton but also provides fewer binding sites for dye molecules. Nylon was the first completely synthetic fiber. It is a polyamide, made up of hydrocarbon repeating units joined together by highly polar amide (–CONH–) functional groups. The amide groups provide sites for hydrogen bonding to dye molecules. The repeating units in polyester are joined together by ester (– COO–) functional groups. Dr. Mohamed R. Elmorsy In dyeing there are four different types of forces of attraction or bond at work: ionic bonds, covalent bonds, hydrogen bonding and Van Der Waal’s forces. 1. Ionic bonds It is interaction between negative center of dye with positive center of fiber and vice versa. Wool fiber is bound with ionic bond with sodium salt of dye. The ionic bond is formed when dyeing process occur in diluted acidic medium as follow: Dr. Mohamed R. Elmorsy 2. Covalent bond It is actual chemical bond between fiber and dye through crosslinkage agent such as 2,4,6-trichloro triazine (reactive dye). Example of covalent bond of cotton (R-OH) and Dyes containing NH2 or OH. Dr. Mohamed R. Elmorsy 3. Hydrogen bonding A hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative atom or group, and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor (Ac). Such an interacting system is generally denoted Dn–H···Ac, where the solid line denotes a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are the second-row elements nitrogen (N), oxygen (O), and fluorine (F). Dr. Mohamed R. Elmorsy 4- Van der Waals forces Van der Waal forces are due to interaction between π-orbitals of dye and fiber. Van der Waal forces are very weak forces. Dr. Mohamed R. Elmorsy Experiment! Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy Dr. Mohamed R. Elmorsy