The Notes for
Polymer and Coatings Science-
Chapter Three

Polyurethanes
A report on polyurethane by Darren Schilberg of Penn State , provides a lot of interesting information.

"Chemical Reactions of Polymers", 1964-Wiley, edited by E.M. Fettes, contains an article by K.C. Frisch and H.C.Vogt (p. 927) entitled "Polyurethanes and Related Isocyanate Polymers"

Polyurethanes are defined as polymers containing urethane linkages in the main polymer chain: 
         : :
          O
      :   ||  :
    - N - C - O -     
      |       :
      H

      urethane
Polyurethanes can be classified into the following major groups:
  1. flexible foams
  2. rigid foams
  3. elastomers
  4. fibers and molding compositions
  5. surface coatings
  6. adhesives
Isocyanates
Isocyanates have the extremely reactive group - N = C = O, which will react with the -OH groups of your lung tissue if you inhale a volatile isocyanate.

Diisocyanates will react with a diol to form a polyurethane: 
 
                                        O               O        
                                        ||             ||
O=C=N - R - N=C=O + HO - R - OH --> -[- C - N - R - N - C - O - R - O -]-
                                            |
                                            H

  diisocyanate       diol                      polyurethane
Isocyanates will react with an amine to form a polyurea: 
                      H       H           O               O
                      |       |           ||             ||
O=C=N - R - N=C=O + H-N - R - N-H --> -[- C - N - R - N - C - N - R - N -]-
                                              |       |       |       |
                                              H       H       H       H

  diisocyanate      diamine                    polyurea
Synthesis of isocyanates
Isocyanates are prepared by the phosphogenation of amines: 
                       O                     O
                       ||                    ||
  R - N - H   +   Cl - C - Cl  --->  R - N - C - Cl  +    HCl 
      |                                  |
      H                                  H

    amine          phosgene           isocyanate      hydrochloric
                                      precursor           acid 


        O
        ||
R - N - C - Cl  --->   R - N = C = O  +     HCl
    |
    H

 isocyanate             isocyanate
 precursor

Phosgene

Diisocyanates
  1. Toluene diisocyanate (TDI)
  2. Naphthalene diisocyanate (NDI)
  3. Hexamethylene diisocyanate (HDI)


Hexamethylene diisocyanate
Hexamethylene diisocyanate is prepared by the phosgenation of hexamethylenediamine: 
                            O
        H                   ||                 H
        |              Cl - C - Cl             |
H N -[- C -]- NH   +   ---------->   O=C=N -[- C -]- N=C=O
 2      |   6   2                              |
        H                                      H

hexamethylenediamine                 hexamethylene diisocyanate
HDI sees limited use in the preparation of moulding materials.

The difference between polyureas and polyurethanes is that the latter are not as stable as the former.

 Diols
Low molecular weight polyethers can be prepared by polymerizing epichlorohydrin onto a diol. 
              O
C - OH       / \         C - O -[- C - C - O -]-m H
            C - C
|      epichlorohydrin   |                            n,m range from 1 to 10
        ------------>
C - OH                   C - O -[- C - C - O -]-n H

diol                     low molecular weight polyether
The same trick can be done with glycerol (1,2,3-trihydroxypropane), which would have three polyether chains off the glycerol unit. The molecular weight of this is 1000 to 3000.

The use of monomeric diols is now limited to moulding materials and cast elastomers.

Polyester prepared by the reaction of dibasic acid (such as adipic and sebacic acid) with glycols (such as ethylene glycol and diethylene glycol) or polyhydric alcohols (glycerol, thrimethylalpropane) are -OH capped if a stoichiometric excess of diol is used.

 Isocyanate reactions
As mentioned previously, isocyanates react with alcohols to form polyurethanes, and with amides to for ureas.


Reaction of isocyanate with a carboxylic acid 
                      O                         O       O                     O
                      ||                       ||       ||     -CO2           ||
R - N = C = O  +  R - C - O - H  ---->  R - N - C - O - C - R  ---->  R - N - C - R
                                            |                             |
                                            H                             H
 isocyanate      carboxylic acid           unstable product              amide

An amide can react with isocyanate, as seen further on.

Reaction of isocyanate with water 
                                            O
                                            ||
R - N = C = O  +  H - O - H  ---->  R - N - C - O - H  ---->  R - N - H
                                        |                         |
                                        H                         H
 isocyanate         water             unstable product          amine

An amine can react with isocyanate, as seen further on.

Reaction of isocyanate with urethane 
                          O                         O       O
                          ||                        ||      ||
R - N = C = O  +  R - N - C - O - R ----->  R - N - C - N - C - O - R
                      |                         |       |
                      H                         H       H
isocyanate             urethane                 allophanate

Reaction of isocyanate with urea 
                          O
                          ||
R - N = C = O  +  R - N - C - N - R  --->   What is drawn in the notes does
                      |       |           not make sense.  The word "biuret"
                      H       H           is used.

Reaction of isocyanate with amide 
                          O                     O       O
                          ||                    ||      ||
R - N = C = O  +  R - N - C - R  ---->  R - N - C - N - C - R
                      |                     |       |
                      H                     H       R

Reaction of isocyanate with phenol 
                                               O
                                              ||
R - N = C = O  +  HO - benzene  ---->  H - N - C - O - benzene 
                                           |
                                           R
Dimerization of isocyanate 
                               O
                               ||
                               C
                             /   \
2 R - N = C = O  --->  R - N       N - R
                             \   /
                               C
                               ||
                               O

                          uretidione
Trimerization of isocyanate



Aromatic isocyanates show a faster reaction rate than aliphatic isocyanates.

In order to produce a polyurethane foam, a foaming or blowing agent is used. The presence of water produces carbon dioxide, not enough, so freon 11 or additional carbon dioxide is added.

Rigid foams are highly crosslinked. A continuous belt process is used. A reacting isocyanate/polyol mixture is expanded by a generated gas.

The difference between rigid and flexible foams is the degree of crosslinking. A high degree of crosslinking is achieved by using relatively low molecular weight polyols for coupling with the isocyanate. Flexible foams are mainly based on polyethers. Flexible foams use carbon dioxide gas. Rigid foams can be blown by carbon dioxide, but usually a low boiling point halogenated alkane such as carbon trichlorofluoride, which is "volatized" by the exothermic isocyanate-polyol reaction, and the gas expands the foam.

Diphenylmethane diisocyanate is the preferred isocyanate for rigid foams on account of its low volatility. Other blowing agents besides carbon dioxide include freon and pentane.

Diazabicyclo[2.2.2]octane, manufactured by Air Products under the trade name "DABCO", is a tertiary amine used to catalyze the reaction between an isocyanate and an alcohol. 
  C-C 
 /   \
N-C-C-N
 \   /
  C-C
Both polyesters and polyethers can be used. However, polyethers are hydrolytically susceptible.??

 One package urethanes
The paint absorbs moisture from the water. ??

Epoxies are crosslinked polymers in which the crosslinking is derived from reactions of the epoxy group. 
      O
     / \
 -- C - C  

    epoxy
Applications of Epoxides include adhesives, paints, coatings, plastics, and extremely strong glues used for woodwork.

At the present time (date not listed), most (95%) commercial epoxy resins are prepared by the reaction of Bisphenol A (2,2-bis(4 hydroxyphenyl) propane) and epichlorohydrin.

 Synthesis of Bisphenol A


The optimum molar ratio is 4:1. Hydrogen chloride (catalyst) is passed into the mixture for about 8 hours, during which period the temperature is kept to less than 90 deg C to suppress the formation of isomeric products.

Bisphenol A precipitate is filtered off and washed with toluene to remove unreacted phenol. The phenol is recovered. The product is then recrystallized from an aqueous ethanol solution.

Since epoxy resins are of low molecular weight, the color and purity of bisphenol A is not critical. The larg impurities are Op' and O,O' isomers. ??

 Synthesis of epichlorohydrin
 
                                                   base
 C = C - C   +  Cl   --->  C = C - C - Cl  +  HCl  ---->
                  2                                HOCl

         O
        / \
---->  C - C - C - Cl
        epichlorohydrin        (reacts with bugs and humans)
Resin Preparation:

In a typical process a mixture of bisphenol A and epichlorohydrin (Cl:4 molar)?? is heated to about 60 deg with stirring. The reaction is exothermic and cooling is applied.

The product (residue) consists of epoxy resin mixed with NaCl. The residue is filtered off. Toluene is added to facilitate the filtration.

The toluene is removed by distillation under reduced pressure and the resin is heated at 150 deg C/5 mm Hg to remove traces of volatile matter. This last step is important since the presence of volatiles may lead to bubble formation when the resin is used.



?? ask Dr. Stoffer to check page 124 where the notes show a divalent hydrogen.

For the mechanism, a base is used to abstract the two phenolic hydrogens of the Bisphenol A. The results are shown for the base NaOH: 
                    CH3
  +  -              |              -  +
Na    O - benzene - C - benzene - O    Na
                    |
                    CH3
And as you would deduce, the reaction of two epichlorohydrins with the above would produce a NaCl byproduct.

Epichlorohydrin can react with the phenolic hydrogen to place a hydroxy group on the middle carbon, and attach the epichlorohydrin to the Bisphenol A.

The notes below state that n can vary from 0.1 to 12. I'd venture to say the Diglycidic ether Bisphenol A (DGEBA) might be made by mixing epichlorohydrin, Bisphenol A, and calculating the mole fraction of base needed to make -O(-) (+)Na for epichlorohydrin attack of Bishpenol A, and the remaining -OH groups are available for attack by the 
  O
 / \
C - C.
We've discussed exact stoichiometric match and step polymerization before. It should make sense that if the Bisphenol A to epichlorohydrin to base stoichiometry is exactly 1:1:1, then the whole system would form one molecule.

Notice that the variable 'n' corresponds to the number of OH groups on the DGEBA molecule.

The variable 'n' is determined by the molar ratio of the reactants, and as this ratio approaches 1, a higher molecular weight product is produced. (this repeats what I inserted above about the 1:1:1.)

Lower molecular weight DGEBA resins find use in adhesives, castings, incapsulations and laminates.

Higher molecular weight DGEBA resins are used in surface coatings. They can be modified through reactions with the pendant hydroxy groups.

Data of how the reactant ratio effects the molecular weight of the epoxy resins, and this effects the softening point:

Molar ratio-
epichlorohydrin/Bisphenol A 		Molecular Weight Softening Point
10 : 1 370 9 deg C
2 : 1 451 43 deg C
1.4 : 1 791 84 deg C
1.33 : 1 802 90 deg C
1.25 : 1 1133 100 deg C
1.2 : 1 1420 112 deg C






Last Update- July 9, 1995- wld