Maurice E. Bailey Pikeville College Pikeville, Kentucky 41501
I
Polyurethanes A study in chemical principles
The chemist's first serious encounter with urethanes probably came as a student during the college course on qualitative organic analysis, viz., in the identification of alcohols through formation of the traditional urethane derivative by reaction with an isocyanate (I). Over the past fifteen years, several large industries have been formed having their basis in this reaction. Industrially, diisocyanates and polyhydric alcohols are normally used, leading to polyurethanes. Commercial products of greatest significance include flexible foams for cushioning and rigid foams for insulation and flotation. Coatings and elastomers which exhibit unusually high abrasion resistance are also important segments of the polyurethane industry. Table 1.
Sand Abrasion-Resistance of Floor Finishes (4) Polymer Type
Liter of Sand per Mil
37
Urethane S ~ a Varnish r
22
It is always worthwhile to view any science or technology from the standpoint of the fundamental principles which are involved. In the case of polyurethanes applications are found for the principles of hydrogen bonding, steric hindrance, inductive effects, chemical kinetics, catalysis, simultaneous and consecutive reactions, equilibrium, and, of course, polymerization. I n several of these respects, the parallel with the use of urethane in organic analysis is particularly noteworthy. Hydrogen Bonding
In the identification of alcohols the following reaction is used RNCO R'OH -.RNH CO .OR' 25 Kcal (1)
+
isocyanate
+
alcohol
urethane
This derivative is chosen because the reaction usually proceeds spontaneously and the product is easily crystallized in a high state of purity, facilitating identification by use of the melting point. By way of comparison, esters have proven less suitable than urethanes as characterizing derivatives of alcohols, not only because they are usually more difficult to prepare, but in addition, are less likely to provide crystalline derivatives. The following melting points of esters and urethanes of nearly equal molecular weights derived from benzoic acid and phenyl isocyanate, illustrate this point Methyl alcohol derivative Ethyl alcohol derivstive n-Propyl alcohol derivative
Meltzng Po&, ' C Ester Urethane - 12 +47 -35 +52 -99 +58
Obviously, the energy of fusion of the urethane is higher than that of the ester. It is appropriate to describe this higher energy to secondary bonds or hydrogen bonds (2). ----H-N
/ H A ' --\ -O -=c
Weisfeld (3) hm studied hydrogen bonding in polyurethane elastomers produced by the reaction of a diisocyanate, polyols and diols, or diamines. For example, eight moles of adipic acid are allowed to react with nine moles of a glycol, such as ethylene glycol to form a linear polyester of about two thousand molecular weight and terminating in hydroxyl groups. One mole of this product is allowed to react with two moles of a diisocyanate to form a somewhat larger but still small polymer terminating in isocyanate groups. This product is then allowed to react with equal molar quantities of a dial or a diamine to form a high molecular weight polymer. (With the diamine, obviously urea groups are formed rather than urethanes.) Three generalizations are fundamental to understandmg Weisfeld's work: (a) as temperature is increased, hydrogen bonding tends to disappear at significantly lower temperatures than primary bond decomposition, (b) as temperature is increased, stiffness in an elastomer increases, and (c) as crosslink density (either primary or secondary) in a polymer is increased, stiffness of an elastomer increases. Accordmgly, at low temperatures in urethane elastomers where abundant hydrogen bonding is available stiffness should be high. As temperature is increased, a softening should occur as these bonds disappear, offset by the tendency of elastomers to stiffen on heating. Weisfeld did indeed observe this result as shown in Figure 1. Urethane technologists have generally been successful in utilizing this hydrogen bonding effect to achieve high performance through the increase in effective molecular weight over what is obtained from primary chemical bonds alone. (The analogy with water which might be expected to be a gas is useful to consider.) As an example of the value to he gained from hydrogen bonds, polyurethane coatings are widely used as floor finishes because of their outstanding toughness or abrasion resistance (Table 1). Polymer and Properties
As in the example above and accordmg to polymer principles, simple molecules which are reactive with Volume 48, Number 12, December 1971
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809
Obviously, however, 25% aromatic seems much less effective for altering melting point than 8% addition in urethane content, and this seems to be a general principle in the science of polyurethanes. Greater stiffness is usually obtained in polyurethanes when ester groups are present than with ether groups. It is helpful to compare cohesive energies of these groups. Cohesive Energy Kcal/mole 8.7 3.9
-4CONH- (urethane) --Car (sromstie) -COO(ester) 4- (ether)
each other and which possess two or more reactive groups, yield large molecules or polymers, which often are useful in themselves. In polyurethanes, the variety in choice of simple molecules is great and consequently, the spectrum of product properties is wide. An early example is the following (n
+ 1) HO(CH&OH + 1,4 butanediol
(n
+ 1) OCN(CHn)sNCO
-
[",4 Itl II
Ho(cH,),o h-N-(cH2)mo(cn,),00
RNCO
.cN(cH,),mo H
Effect of Aromatic and Urethane Content on Polymer Melting Point ( 5 )
% Diisocyanate Diol OCN(CHn)8NCO HO(CHn)aOH
%
-
+ RNH-CO-NHR'
(3)
Characteristic of the general case, this polyurethane is considered to be reinforced through hydrogen bonding between the urethane groups. It resembles nylon not only in its molecular structure (replace the urethane groups with amide groups) but in toughness as well. Also like nylon and other linear polymers, this particular polyurethane is thermoplastic and is processed like its nylon counterpart. However, in the wide selection of reactants and accordingly in the wide range of resultant properties, polyurethanes contrast with nylons, and for that matter, most other polymer classes. As one example, substituting 1,10 decanediol should and does provide greater elasticity than 1,4 butanediol. I n addition to this modification of the urethane content of the polymer, properties may be altered in a number of other ways, such as, for example, by introducing aromatic rings into the polymer chain (Table 2). Table 2.
+ R'NH, amlne
followed by the slower
0
%' ?d'
matic thane Ure0 38 153 . . .
1.0
In some cases, the technologist wants to avoid the themoplastic property shown generally for products of diisocyanate and diols, particularly when it is desired to retain the shape of an object a t elevated temperatures. He can do this by using a hydroxyl component having a functionality greater than two, i.e., by use of a "crosslinking" agent. Popular examples of such components include polyols such as trimethylolpropane, castor oil, and pentaerytbritol. Going one step further than in the case of Weisfeld's work, diamines can function as crosslinking agents by exerting under forced conditions functionality greater than two. I n this respect each amine group is capable of reacting with a t least two isocyanate groups in the following consecutive reactions RNCO isocyanate
hexamethylene diisocyanate
2.9
isocyanate
urea
RNH4O-NHR' urea
-
RN-40-NHR'
/ co
h
(4) (5)
biuret
The biuret hydrogens in eqn. (5), as well as the urethane hydrogens in eqn. (1) are also capable of reaction, however, at insignificant rates under most conditions. Kinetics
In the selection of reactants, the technologist can control the elasticity, hardness, thermoplasticity, and other properties of the polyurethane. The choice of reactants and, accordingly, the property differences are great. The expertise with which he accomplished the desired result is dependent also on his understanding and abiity to control the rate a t which the various processes occur. Kinetic measurements in urethane processes are made using two methods. One is a titrametric method based on the fact that isocyanates react with amines (particularly aliphatic types) with almost ionic speed. Dibutylamine is normally used. The other is based on the strong absorption shown by the NCO group at 4 . 5 ~in the infrared, a part of the spectra not occupied by other species significant in urethane technology. By conducting reactions in or on the infrared cell, immediate readings of rates of change are obtained through the application of Beer's Law (6). Steric and Inductive Effects
The infrared method has been used to ascertain steric and inductive effects of various substituent 810
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Journal of Chemical Education
Table 3. Approximate Relative Effects of Different Catalyst Types on Reactivity of Various Functional Groups with Isocyanate
No. Catalyst
Amine Water Organic Hydroxyl Urea Urethane
1,000 1 1 1 0.001
Amine
Tin
1,000 2,000 5 1,000 10 50,000 3 1,000
