Dyes & Pigments

Question 1

Define Pigments

Answer 1

• insoluble powders of very fine particle size (0.01 micrometers to 1 micrometers)
• may be natural or synthetic in origin
• may be organic or inorganic

Question 2

What are some Pigment Functions?

Answer 2

• provide color
• hide previous colors
• improve strength of paint film
• improve adhesion of paint film
• improve durability of paint film
• reduce gloss
• modify flow and application properties

Question 3

Extender Pigments

Answer 3

• solids added to paints to control properties other than color and hiding power
  
  • fine silica particles might be added to a paint to reduce gloss

Question 4

Pigment Properties

Answer 4

• tinting strength
• lightfastness
• bleeding characteristics
• hiding power
• refractive index
• particle shape
• specific gravity (density)
• chemical reactivity
• thermal stability

Question 5

What is Tinting Strength?

Answer 5

• type of pigment property
• the amount of colored pigment required to obtain a specific shade of color
• large amounts of pigments with low tinting strengths are required to produce a given shade
• small amounts of pigments with high tinting strengths are required to produce the same shade

Question 6

What is Lightfastness?

Answer 6

• type of pigment property
• says that the color of a paint film should last as long as the paint film itself
• photochemical degradation due to ultraviolet rays in sunlight can cause fading or darkening of pigments
• fading can be controlled by protectice top coats
• pigments able to absorb ultraviolet rays without degradation can protect the film former

Question 7

What are Bleeding Characteristics?

Answer 7

• type of pigment property
• caused by solvents in a newly applied paint layer dissolving pigments in the paint layer underneath
• organic red pigments are particularly prone to bleeding

Question 8

What is Hiding Power?

Answer 8

• type of pigment property
• ideally a single application of a paint layer would completely hide the color of the layer underneath
• expressed as the number of square meters of paint covered by one liter of paint with complete hiding
• determined by
  
  • total amount of light that a pigment will absorb
  • its refractive index
  • particle size and shape

Question 9

What is Refractive Index?

Answer 9

• type of pigment property
• white pigments are transparent
• when finely ground and dispersed in the paint’s film former they appear white b/c they scatter light
• light scattering occurs b/c of the difference in refractive indices of the pigment particles (2.0 - 2.7) & the film former (1.4 - 1.6)
• extender pigment typically have indices of refraction similar to film formers (& therefore do not contribute to the color of the paint film)

Question 10

What is Particle Size?

Answer 10

• type of pigment property
• the ideal particle diameter for maximum light scattering is approx. the wavelength of light in the particle
• as a rougn rule of thumb the optimum particle diameter is approx. half the wavelength of light in air (0.2 - 0.4 micrometers)
• below the lower size limits particles lose their scattering power
• above the upper size limits the number of scattering interfaces per unit weight of pigment is reduced
• typical pigment particle sizes ranges from 0.01 micrometers (carbon black) to 50 micrometers (some extenders)

Question 11

What is Particle Shapes?

Answer 11

• type of pigment property
• pigment granules may be spherical, cubic, nodular, acicular or lamellar
• particle shape affects pigment packing and hiding power
• acicular particles can reinforce paint films, reduce gloss, pr promote adhesion of subsequent layers
• lamellar particles reduce water penetration

Question 12

What is Chemical Reactivity?

Answer 12

• type of pigment property
• reactions with film formers
  
  • ZnO reacts with resins containing acidic groups to form soaps; divalent Zn will cross-link resin chains through formation of -O-Zn-O- bridges
• reactions with the atmosphere
  
  • paints containing lead pigments should not be used in industrial areas

Question 13

What is Thermal Stability?

Answer 13

• type of pigment property
• important if the paint is to be baked (stoved) or if it is to be used in a high temperature environment
• most organic compounds decompose below 300 degrees C

Question 14

Inorganic Pigments

Answer 14

• White pigments
  
  • nonreactive (do not react with vehicle)
    
    • titanium dioxide
  • reactive
    
    • basic carbonate white lead
• white extender pigments (control gloss, texture, suspension and viscosity)
• colored inorganic pigments
  
  • iron oxides

Question 15

What is the difference between Dyes and Pigments?

Answer 15

• dyes are soluble at the molecular level
• pigments are insoluble
  
  • stay as particles
• “toner pigments” are insoluble in their pure forms
• “lakes” are precipitated by a metal or other inorganic base
  
  • need inorganic substrate

Question 16

What are the 11 types of dyes?

Answer 16

• acid dyes
• basic or cationic dyes
• direct dyes
• sulfur dyes
• vat dyes
• reactive dyes
• disperse dyes
• mordant dyes
• azoic dyes
• oxidation dyes
• ingrain dyes

Question 17

What does Pigment Analysis involve?

Answer 17

• chemical tests
  
  • 3 reagents are used
    
    • concentrated nitric acid
    • alcoholic potassium hydroxide
    • concentrated sulfuric acid
  • the colors of the reaction products are noted
• IR analysis of organic pigments
  
  • must first extract the paint flake with chloroform

Question 18

What is an Acid Dye?

Answer 18

• a type of dye
• depend on one or more acid groups (usually sulfonic acid groups) to form salt linkages to basic groups in substrate

Question 19

What is a Basic or Cationic Dye?

Answer 19

• a type of dye
• form salt linkages with acidic groups in substrate

Question 20

What are Direct Dyes?

Answer 20

• colorants that are strongly absorbed on cellulose