2 Basic Chemistry of Polyurethanes
CH 3 COOK
2.13 Several Considerations on the Polyaddition Reaction
As mentioned previously, the synthesis of PU, by the reaction of a diisocyanate (or polyisocyanate) with oligo-diols (or oligo-polyols), is a polyaddition reaction (or step- addition polymerisation), a particular type of polycondensation reaction. There is a great difference between the polycondensation and the polyaddition reactions and the classical radical polymerisation or ionic (living) polymerisation reactions. In radical polymerisations (typical chain reactions), the high-MW polymer is formed at the beginning of polymerisation. The reaction system is constituted from monomer and high-MW polymer. The radical polymerisations are characterised by strong transfer reactions, simultaneous with the polymerisation reaction.
Living ionic polymerisations are characterised by a linear increase of the MW in the resulting polymer, with the conversion. In the reaction system there are: the monomer and the polymer. In living ionic polymerisations, the termination reactions are absent.
In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high- MW polymer is formed only in the last stages of the polyaddition reaction, at high conversion rates. Chain transfer and termination reactions are absent.
In Figure 2.6 one can compare, the MW growth of polymers in radical, living anionic and step-addition polymerisation reactions.
Radical polymerisation M
Step-addition polymerisation
Living ionic polymerisation
Conversion (%)
Figure 2.6 MW growth in radical, living ionic and step-addition polymerisations
As in all polycondensation reactions, in polyaddition reactions (e.g., in PU synthesis), the molar ratio between the reactive group (in our case between [-NCO]/[hydroxyl groups]), has a very strong influence on the MW of the resulting PU polymer. The maximum-MW is obtained at an equimolecular ratio [-NCO]/[OH] = 1 [29]. A small excess of one reactant (isocyanate or hydroxyl groups), drastically reduces the MW of the resulting PU (Figure 2.7).
M
0.5 1 1.5
Molar ratio [-NCO]/[OH]
-NCO terminated PU
OH terminated PU
Figure 2.7 The effect of the molar ratio [-NCO]/[OH] on MW of the PU
References
1. M. Szycher in Szycher’s Handbook of Polyurethanes, CRC Press, Boca Raton, FL, USA, 1999.
2. Flexible Polyurethane Foams, 2nd Edition, Eds., R. Herrington and K. Hock, Dow Chemical Company, Midland, MI, USA, 1997.
Basic Chemistry of Polyurethanes 3. MDI and TDI: Safety, Health & Environment, A Source Book and
Practical Guide, Eds., D.C. Allport, D.S. Gilbert and S.M. Outterside, Wiley Publishers, Chichester, UK, 1998.
4. O. Bayer, Angewandte Chemie, 1947, A59, 257.
5. O. Bayer, Modern Plastics, 1947, 24, 149.
6. Polyurethane Technology, Ed., P.F. Bruins, Interscience Publishers, London, UK, 1969.
7. The ICI Polyurethanes Book, 2nd Edition, Ed., G. Woods, John Wiley & Sons, Chichester, UK, 1990.
8. Telechelic Polymers, Synthesis and Applications, Ed., E.J. Goethals, CRC Press, Boca Raton, FL, USA, 1989, p.203.
9. Advances in Polyurethane Technology, Eds., J.M. Buist and H.A. Gudgeon, Maclaren and Sons, London, UK, 1968.
10. S.D. Gagnon in Encyclopedia of Polymer Science and Technology,
2nd Edition, Ed., J.I. Kroschowitz., John Wiley & Sons, Inc., New York, NY, USA, 1986, p.273.
11. H. Ulrich in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Ed., J.I. Kroschowitz, John Wiley & Sons, New York, NY, USA, 1997, 24, 695.
12. J.H. Saunders and K.C. Frisch in Polyurethanes, Chemistry and Technology, High Polymers, Volume 14, Part I: Chemistry, Interscience Publishers, New York, NY, USA, 1962.
13. J.H. Saunders and K.C. Frisch in Polyurethanes, Chemistry and Technology, High Polymers, Volume 14, Part II: Technology, Interscience Publishers, New York, NY, USA, 1964.
14. Handbook of Polymeric Foams and Foam Technology, Eds., D. Klempner and K.C. Frisch, Hanser, Munich, Germany, 1991.
15. Plastic Foams, Eds., K.C. Frisch and J.H. Saunders, Marcel Dekker, New York, NY, USA, 1972.
16. Polyurethane Handbook, Ed., G. Oertel, Hanser Verlag, Munich, Germany, 1985.
17. Developments in Polyurethanes, I, Ed., J.M. Buist, Applied Science Publishers, London, UK, 1978.
18. H. Ulrich in Reaction Polymers, Hanser Publishers, New York, NY, USA, 1992.
19. A.A.R. Sayigh, H. Ulrich and W.J. Farrisey in Condensation Monomers, Eds., J.K. Stille and T.W. Campbell, Wiley Interscience, New York, NY, USA, 1972, p.231.
20. Analytical Chemistry of the Polyurethanes, Volume 16, Part III: High Polymers, Eds., D.J. David and H.B. Staley, Wiley Interscience, New York, NY, USA, 1969.
21. G.W. Woods in Flexible Polyurethane Foams, Chemistry and Technology, Applied Science Publishers, Englwood, NJ, USA, 1982.
22. H. Ulrich in Encyclopedia of Polymer Science and Engineering,
Ed., J.I. Kroschowitz, John Wiley & Sons Inc., New York, NY, USA, 1987, 8, 448.
23. Kunststoffe Plast Europe, 1997, 87, 9, 6.
24. S.G. Entelis, V.V. Evreinov and A.I. Kuzaev in Reactive Oligomers, Brill Publishers, Moscow, Russia, 1988.
25. W.D. Vilar, Chemistry and Technology of Polyurethanes, 3rd Edition, Vilar Poliuretanos Ltd., Lugoa, Rio de Janeiro, 2002.
http://www.polyuretanos.com.br/.
26. A.F. Johnson, P.D. Coates and M.W.R. Brown in Reactive Processing of Polymers, Rapra Review Reports No.73, Volume 7, No. 1, Rapra Technology, Shrewsbury, UK, 1994.
27. G. Wegener, M. Brandt, L. Duda, J. Hofmann, B. Klesczewski, D. Koch, R-J. Kumpf, H. Orzesek, H-G. Pirkl, C. Six, C. Steinlein and M. Weisbeck, Applied Catalysis, 2001, 221, 1–3, 303.
28. Houben Weyl: Methoden der Organische Chemie, Eds., E. Müller, O. Bayer, H. Meerwein and K. Ziegler, Georg Thieme Verlag, Stuttgart, Germany, 1963, Volume 14, Part 2, p.57.
29. C.V. Oprea and V. Bulacovschi in Polimeri, Teoria Proceselor de Sinteza (Polymers: Theory of Synthesis Processes), Editura Tehnica, Bucharest,
Polyols used in the manufacture of polyurethane(s) (PU) are divided, from a structural viewpoint, into two groups. The first group is low-molecular-weight (MW) polyols.
They are very well described in organic chemistry, and include propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, triethanolamine, and glycerol. These polyols are used in PU fabrication as chain extenders (polyols with two hydroxyl groups/mol called diols) or as crosslinkers (polyols with more than two hydroxyl groups/mole such as triols, tetraols, etc.).
These polyols are very well characterised and the chemistry and technology of these compounds is well-known [1, 2], so this chapter does not cover this group.
The second group of polyols for PU contains low-MW polymers [oligomers with a maximum MW of 10,000 daltons (Da)] with terminal hydroxyl groups (hydroxy telechelic oligomers) called oligo-polyols. They are characterised by an average MW and molecular weight distribution (MWD) of homologous species. This chapter is dedicated exclusively to this second group of oligo-polyols for PU which, together with isocyanates, are the most important raw materials to build the complex architecture of a PU polymer [3–12].
The general formula of an oligo-polyol for PU is shown in Figure 3.1.
OH n HO
HO
Oligo-polyol
n = 0,1,2,3,4,5,6
= A chemical organic structure, aliphatic, cycloaliphatic, aromatic, heterocyclic and so on
n = The number of chains derived from one hydroxyl group f = n + 2 (the total number of hydroxyl groups/mol=functionality)
OH = Terminal hydroxyl group
= Oligomeric chain (polyether chain, polyester chain, polyhydrocarbon chain, polysiloxane chain and so on)
Figure 3.1 General formula of oligo-polyols for PU
An oligo-polyol for PU may have two, three, four, five, six, seven or a maximum of eight hydroxyl groups/mol. Polyols with a higher number of hydroxyl groups/