The Notes for
Polymer and Coatings Science-
Chapter Three- part three

PAINTS

Paint Materials A paint is composed of:
  1. a solvent, medium, or matrix in which we suspend the material
  2. a resin
  3. a pigment
For "carriers", we might use Pigments

white red yellow green
Titanium Dioxide
Calcium Carbonate
Lead Sulfate 		Iron Oxide (Fe[x]O[y]) Lead Chromate Cobalt salts
Chromium salts


Alkyds are defined as a polyol and dibasic acid derivative.

The principle materials involved in the preparation of alkyd resins are polyhydric alcohols (polyols) and dibasic acid (or corresponding anhydrides) together with the modifying oils (or corresponding acids.)

Polyols
Glycerol is the polyol most widely used. It is obtained either by chlorination or oxidation of propylene.

The following route is now used most widely in industry for the Synthesis of glycerol: 
                             OH
H   H              H   H     |      H   H     hydrogen   OH  OH  OH
|   |       O2     |   |   C-C-C    |   |     peroxide   |   |   |
C = C     ------>  C = C   ------>  C = C    -------->   C - C - C
|   |    catalyst  |   |            |   |     60-70 C
H   CH3            H   C=O          H   CH2              glycerol
                       |                |                isolated by
                       H                OH               distillation

propylene         acrolein
Glycerol has a boiling point of 290 deg C, and is a colorless liquid that can explode on contact with strong oxidizing agents such as chromium trioxide, potassium chlorate, or potassium permanganate.

 Esterification of glycerol with phthalic anhydride

The incorporation of fatty acid residues into structures such as the one above leads to highly successful surface coatings.

 Other polyols- pentaerythritol and sorbitol

Pentaerythritol and Sorbitol are important polyols: 
           OH                      OH
           |                       |
           CH2                 H - C - H
           |                       |
HO - CH2 - C - CH2 - OH        H - C - OH
           |                       |
           CH2                HO - C - H
           |                       |
           OH                  H - C - OH
                                   |
                               H - C - OH
                                   |
                               H - C - H
                                   |
                                   OH

  Pentaerythritol              Sorbitol
The previously seen reaction which converts benzene to maleic anhydride also converts naphthalene to phthalic anhydride at high temperatures.



Modifying oils and acids: saponification 
    H       O
    |       ||
H - C - O - C - R                    C - OH
    |                                |
    |       O                        |
    |       ||        excess base    |
H - C - O - C - R      -------->     C - OH
    |                    water       |
    |       O                        |
    |       ||                       |
H - C - O - C - R                    C - OH
    |
    H                               glycerol
Natural oils Oils which find commercial utilization are of animal and vegetable origin A specific example of saponification
In the preparation of soaps, oils and fats are saponified by heating with aqueous NaOH. 
    H       O
    |       ||
H - C - O - C - R                    C - OH
    |                                |
    |       O                        |                O
    |       ||          3 NaOH       |                ||   -  +
H - C - O - C - R      -------->     C - OH  +  3 R - C - O    Na
    |                    water       |
    |       O                        |
    |       ||                       |
H - C - O - C - R                    C - OH
    |
    H                                glycerol
Types of alkyd resins
The properties of the alkyd resins depend to a large extent on the nature and amount of modifying oil incorporated. There are three classes to consider with respect to drying:

drying oil resins mostly unsaturated
semi-drying oil resins- mixture
non-drying oil resins mostly saturated oil content


Alkyd resins can also be classified in terms of their oil length, that is, the amount of oil they contain:

short oil contain less than 50% oil content
medium oil contain 50%- 70& oil content
long oil contain greater than 70% oil content


It appears from the notes that take each category (drying, semi-drying, and non-drying) and divide it into short, medium, and long oil categories.

Putting alkyd type and oil length together
drying oil resins short oil resins soluble only in aromatic solvents
cured at elevated temperatures
give very hard, glossy finishes
used in finishes for appliances, signs and toys
drying oil resins medium oil resins soluble in aliphatic-aromatic blends
may be air-dried or heated
give durable, glossy finishes
used on finishes for farm implement, hardware, and metal furniture
drying oil resins long oil resins soluble in aliphatic solvents (eg. naphtha)
have good brushing characteristics
dry rapidly in air
give reasonably durable, glossy films
used in household paints
semi-drying oil resins no division based on oil length give films with improved resistance to yellowing on aging
used particularly for high gloss white finishes
non-drying oil resins short oil resins soluble only in aromatic solvents
used mainly in conjunction with amino resins
give improved adhesion and flexibility
used in stoving finishes for appliances
non-drying oil resins medium oil resins soluble in aromatic solvents
used mainly as plasticizers for cellulose nitrate for furniture finishes
non-drying oil resins long oil resins ----- does not specify if
  1. the information is not available
  2. long oil non-drying resins are not used


The drying character is associated with the portion of the molecule that deals with the unsaturation of the fatty acid derivative.

 Components of an alkyd paint
Additives 0.10 to 5%, which may include:
  1. Stabilizers (surfactants)
  2. Biocides (preservatives)
  3. Dryers (cobalt salts and crosslinking)
      ??perhaps this is to imply that if you make a film and you have a drying problem, you might try cobalt salts, or crosslinking
  4. Leveling agents
  5. Defoamers- aid in the order of blending ??
Alkyd film formation
The mechanism by which an alkyd resin is converted from a liquid to a dry film is largely determined by the nature of the fatty acid residues present. The drying process involves attack by oxygen in the unsaturated regions of the fatty acid residues followed by crosslinking and is essentially similar to the process which occurs when the corresponding glyceride oil is acted upon air. Drying consists of two steps:
  1. aerial oxidation
  2. crosslinking initiated by decomposition of the oxidation products.


The acid residues present in oils are usually straight chains containing an even number of carbon atoms, and can saturated or unsaturated.

 Synthesis of alkyd resins
Alkyd resins cannot be prepared by simply heating a mixture of oil, polyoil and dibasic acid. Because of the preferential reaction of the polyol and acid, a heterogeneous mixture of polyester and oil would be obtained which has no value as a surface coating. There are are two methods used- 1)fatty acid process and 2) alcoholysis process.

Fatty acid process
The oil is first hydrolyzed to give free fatty acids which are then heated at 200 to 400 deg C with a mixture of polyol and dibasic acid. The simultaneous condensation of the polyol, dibasic acid, and fatty acid occurs and the latter (the fatty acid??) is incorporated into the polymer structure. Fusion or solventless method
The reacts are heated in a simple kettle under an inert atmosphere. At the end of the heating period, an inert gas is blown into the resin to remove water and unreacted material.

The notes say something about alcoholysis: 
    H       O
    |       ||
H - C - O - C - R                    C - OH
    |                                |
    |       O                        |                    O
    |       ||                       |                    ||
H - C - O - C - R      -------->     C - OH    +   3 HO - C - R
    |                                |
    |       O                        |
    |       ||                       |
H - C - O - C - R                    C - OH
    |
    H                               glycerol


Termination of alcoholysis

As the reaction proceeds, the acid number decreases and the viscosity goes up. From the graph below, it appears that these changes determine at what point the reaction is terminated.





Last Update- July 2, 1995- wld