I
H. 1. HEISS, F.
P. COMBS,l P. G. GEMEINHARDT, J. H. SAUNDERS, and E. E. HARDY
Mobay Chemical Co., New Martinsville, W. Va.
Influence o f Acids and Bases on
Preparation of Urethane Polymers Making the reaction mixtures acidic favors formation of a more linear product is increasing interest in the preparation of urethane prepolymers formed by the reaction of hydroxyl-terminated polyethers or polyesters with excess diisocyanate: T H E R E
+ HOW OH
20CN-R-NCO
H mv
/I 0
,
H H
I
I
OCN-R-NCO
I
I
OCN-R-hN-R-NCO
/I
I
w.wNCOw.-
II 0
+ H2O
1
c=o
H
8
(1)
Such products, most commonly prepared from tolylene diisocyanate (TDI) and poly(propy1ene glycols) (PPG) are of interest in preparing urethane flexible foams. Other prepolymers, derived from T D I or 4,4’-diphenylmethane diisocyanate (MDI) reacted with aliphatic polyesters or castor oil derivatives, are being evaluated in the preparation of urethane coatings, adhesives, and elastomers. Preparation of these urethane prepolymers on a practical basis is not simple, as complex side reactions can occur. I n addition to the principal Reaction 1, four others occur. Because most commercially available polyols contain traces of moisture, the reaction of water with isocyanates to give ureas must be considered : 20CN-R-NCO
r
HN 4
H
I
OCN-R-NCO
the most difficulty. The urethane groups obtained in Reaction 1 may react with excess -NCO to form allophanate groups:
-+
+ cog (2)
0 This reaction is not objectionable in itself, because water acts as a difunctional reactant, and only chain extension occurs. T h e presence of the very polar urea groups in the polymer, however, results in higher prepolymer viscosities and usually increased stiffness in the final elastomer or foam because of improved hydrogen bonding. The next three reactions, which often produce undesired branching, cause Present address, Southern Research Institute, Birmingham, Ala.
+~ M - N C O -NCO-...-+
I/
0
(3)
Similarly, the urea groups formed in Reaction 2 may react with excess -NCO to form biurets:
H H
I
I
w-NCNW-. II 0
+ WWNCO
+
f I
HN
c=o I
w-P$C-N-+O II HI
(4)
Reaction 4 normally occurs more readily than Reaction 3, and the resulting biuret branching is the most objectionable result of moisture in the reactive system. The fifth reaction is trimerization of terminal -NCO groups to an isocyanurate branch point: 0
b 5 Basic or acidic impurities in the reactive system have a profound effect on the extent to which these reactions occur and subsequently on the properties of the resultant product. I n general, acidic conditions are not favorable to Reactions 3, 4, or 5 with the result that more linear prepolymers of lower viscosity are
formed. I n the presence of basic impurities. branching reactions occur much more readily, and higher viscosity or even prepolymer gelation results. Reaction temperature is also a factor, and prolonged heating at elevated temperatures will cause some branching even under acidic conditions. I n contrast to reaction under basic conditions, however, branching achieved in this manner may be fully controlled to provide slightly branched prepolymers.
General Considerations Three points should be emphasized : T h e net acidity of the reactive system is the prime factor, not the acidity of any one reactant-traces of acidic or basic components in all reactants and solvents must be considered. Although this seems obvious, there is a tendency in the industry to specify and control the acidity of the isocyanate component and disregard the others. There is nothing absolute about the acidity of isocyanates with regard to their performance in the prepolymer reactions. An isocyanate which appears to be “inactive” (SURciently acidic) with one lot of polyol may be extremely “active” with another lot containing more basic impurities. Thus, activity should concern the final reaction mixture, not any one component. Here, activity refers to the extent to which branching Reactions 3, 4, and 5 occur. An active mixture is one in which unusually high viscosities or gelation of the prepolymer are observed. Other symptoms may be high exotherms and occasionally excessive gas evolution. An inactive system has a minimum of side reactions and does not necessarily involve a slow reaction rate so far as the NCO-hydroxyl reaction is concerned. Another industrial practice is the tendency to regard hydrolyzable chloride (H.C.) content of an isocyanate as a measure of acid content. Whereas acidity is determined by titration with potassium hydroxide and is reported as per cent hydrogen chloride, hydrolyzable VOL. 51, NO. 8
AUGUST 1959
929
Table I.
TDI
TDI Lot ND 37 ND 37 ND 38 ND 38
Purification Caused the Prepolymer Reaction Mixture to Become Active Exotherm, Prepolymer Treatment H.C., % c. Type No 0.29 62 Liquid Yes 0.007 127 Solid No 0.029 67 Liquid Yes 0.004 121 Solid
chloride is determined as the amount of silver chloride precipitated by the addition of silver nitrate and is reported as per cent chloride or hydrolyzable chloride (7). Thus, nonacidic chlorides with no discernible effect on activity are included in the hydrolyzable chloride determination, and its use as a measure of activity is only approximately correct. T h e exact nature of nonacidic hydrolyzable chloride content in isocyanates has not been reported. Many lots of TDI high in hydrolyzable chloride are low in acidity. Although there is a partial correlation in that low hydrolyzable chloride indicates low acidity, the reverse is not true, and for many applications it is not a sufficiently quantitative measure of acidity.
Commercial TDI
H.C.,
HCl, 0.012 0.002 0.010 0.001 0.004 0.005 0.005 0.007 0.001 0.005
Lot
%
A
1 2
0.021 0.003 0.010
3
1
C
2 3 1 2
3 4
0.001 0.010 0.012 0.015 0.020 0.006 0.007
yo
% of
H.C., Acidic 57 67 100 100 40 42
33 35 17 71
Acidic and basic impurities alter the over-all course of the reactions as well as the rate. As the final effect involves governing the ratio of chain extension Reactions 1 and 2 to branching Reactions 3, 4, and 5, entirely different prepolymers can be prepared from the same components, depending upon the level of acidity.
Table II. Effect of Reaction Temperature W a s Not Sufficient to Account for the Polymers Obtained Reactiv- Reaction ity of Temp., System C. Prepolymer Type Inactive
30
150 Active
30
150
930
Effect of Strong Acids and Bases. This work was done in 1952 before the current interest in higher molecular weight poly(propy1ene glycols). At that time materials of interest were 2,4-TDI and a poly(ethy1ene glycol) of average molecular weight 400 (PEG 400) ( 3 ) : and these were usually reacted at an NCO-hydroxyl ratio of 2 :
Liquid Liquid (more than above)
viscous
Vis. liquid, solidified on standing Solid
+ HO-.-.OH
~ C H , ~ - N C O
-
/ NCO
H
H C H 3 - - a - I K C I0
/
NCO
. %
II
0
OCN-
I1
0
0 - m \ “co
(80/20 Isomer)
Supplier
B
Prepolymer Preparations
(6)
Such a prepolymer would have an idealized molecular weight of 2 X 174 400 or 748 and an equivalent weight of 374. I n determining activity, small charges totaling 25 grams were reacted in dry, 2-ounce bottles insulated in expanded vermiculite. T h e reactants were well mixed at room temperature for 1 minute, after which a thermometer was inserted and the exotherm determined. I n addition to the temperature rise observed, the degree of activity of the mixture was also indicated by the type of prepolymer obtained. the more active charges being of higher viscosity and sometimes setting u p to hard resins. Still another method involved following the disappearance of the terminal -NCO by the amine equivalent method ( I ) , the calculation being as amine equivalent, or the weight of sample required to react with 1 mole of di-nbutylamine-Le., the weight of sample associated with each -NCO group. I n earlier work, before it was discovered that hydrolyzable chloride and acidity did not always correlate, analysis of T D I was for hydrolyzable chloride only. Later, the following method was developed to determine the actual acidity. Slowly and with vigorous stirring 10 grams of T D I were added to 75 ml. of ethanol in a 250-ml. beaker, covered loosely, and allowed to stand until cooled to room temperature (about 1 hour). Then 10 ml. of water were added, and the mixture was titrated with 0.01N potassium hydroxide in ethanol,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
+
using a p H meter to determine the end point (pH 7.0). Some of the first experiments revealed that one lot of PEG 400 yielded both liquid and solid prepolymers when reacted with different lots of T D I under identical starting conditions. As it was known that hydrolyzable chloride content of T D I varied over a considerable range, efforts were made to remove impurities by absorbing them with calcium oxide and activated carbon: filtering off absorbents, and redistilling the T D I . Two lots were treated in this manner, then reacted with PEG 400 (Table I). The treatment decreased hydrolyzable chloride content and caused the reaction mixture to become active. Subsequent work demonstrated that other inorganic alkaline materials also reduced hydrolyzable chloride content. I n each case, the decrease in hydrolyzable chloride resulted in a more exothermic reaction and formation of a solid prepolymer. To determine whether solidification of the prepolymer was solely the result of the high exotherm, an inactive system was reacted for 1 hour at a high and a low temperature, as was an active mixture (initially cooled to absorb the heat of reaction) (Table 11). Although reaction temperature did have an effect, it \vas not sufficient to account for the liquid and solid prepolymers obtained. Cooling the active system to absorb the exotherm slowed the reaction so that a liquid product was obtained, but the reaction continued at room temperature so that solidification to the expected solid prepolvmer occurred overnight. As temperature differences did not account for the different prepolymers obtained, it appeared that hydrolyzable chloride content influenced not only the rate but the course of the reaction. O n the assumption that the hydrolyzable chloride content was caused by traces of hydrogen chloride and phosgene in the T D I and was entirely acidic, other acidic materials were added to determine their behavior. One lot, previously distilled from lime to provide active reaction mixtures, was treated with aluminum chloride, iron(II1) chloride, hydrogen chloride, and sulfur dioxide. I n each case the reactivity of the system was reduced. and liquid prepolymers were obtained, indicating that acidity, not hydrolyzable chloride, was the controlling factor. Treatment of purified T D I with nonacidic chlorides such as sodium chloride, and calcium chloride did not reduce the activity of the reaction mixture. Two possible sources of hydrolyzable chloride were hydrogen chloride in the form of the carbamyl chloride: H I R--Pi-C=O -+ R-N=C=O 4- HC1 A1
URETHANE POLYMERS
I30
REACTION 1-
F
&" m %
H.C ADDED
AS'HCI
-
OR PHOSQENE
30
.
Figure 1. Hydrogen chloride in small amounts decreased reaction mixture activity regardless of the form in which it was added
or traces of dissolved phosgene which liberates hydrogen chlorine in the presence of hydroxyl groups : COClz
+ ROH
--L
R-OCOC1
+ HCI
Accordingly, T D I purified by distillation from lime was treated with hydrogen chloride and with phosgene, analysis for hydrolyzable chloride being used to monitor the addition of each. These portions were then reacted with portions of a single lot of PEG 400 in the usual activity determining procedure (Figure 1). This conclusively demonstrated that hydrogen chloride in small amounts could decrease the activity of the reaction mixture regardless of the form in which it was added. In later work acidification was simplified by replacing hydrogen chloride with liquid dimethyl carbamyl chloride, which liberates hydrogen chloride in the presence of hydroxyl groups : C1
(CHI)&-&=O
+ ROH
0
2.0
30 MICROEO /G
10
&J
04
7.0
OF PREWLYMER
Figure 2. Maximum temperature, signifying active mixtures, occurred when added base was sufficient to neutralize acidity Basic and acidic contents expressed in microequivalents of each per gram of mixture
son. In each case the increase in maximum reaction temperature signifying attainment of an active reaction mixture occurred when potassium hydroxide dissolved in the PEG 400 was sufficient to neutralize hydrogen chloride present in the TDI. The transition from active to inactive reaction mixtures is quite sudden and involves differences on the order of 1 peq. of acid or base per gram of reactive mixture. Outside the narrow transition zones, the mixtures are only slightly sensitive to further changes in concentrations of acidic or basic contaminants. The inactive is the normal state existing when no impurities are present. Reagents such as cyclohexanol and ethylene glycol, which may be purified by distillation, never provided active mixtures when reacted with TDI, even
t - NEUTRAL INCREASINQ BASICITY
INCREASIHS ____.) ACIDITY
Figure 3. General effect of strong acids and bases on rate of isocyanate ieactions Reaction 2 with water omitted because of anamalous results
that which was free of acidic impurities. Thus, inactive systems are the result not of the presence of acids but of the absence of basic impurities. Although acid in T D I is capable of rendering a system inactive, the initial creation of active reaction conditions can be attributed only to basic impurities in the polyol. Later, an analytical method for determining T D I acidity was developed, and it was no longer necessary to rely on hydrolyzable chloride measurements. Table I11 indicates how acidity correlates more closely with activity than does hydrolyzable chloride. Whereas small amounts of acid inhibit the reaction because of neutralization of basic impurities, larger amounts are catalytic. Acid catalysis, however, seems to involve only the NCO-hydroxyl Reaction 1 and not branching Reactions
+
OR
I
REACTIONS 3,4,05 LO
(CHB)zN-C=O
Table 111.
+ HCl
Apparently this reaction does not proceed as readily as in the case of phosgene, for on the basis of the theoretically liberated amount of hydrogen chloride, dimethyl carbamyl chloride was only one tenth as effective as phosgene. Various lots of PEG 400 reacted with a single lot of T D I produced different types of prepolymers, indicating that these also contained varying amounts of acids and/or bases. T o establish a possible relationship between impurities in the two reactants, varying amounts of potassium hydroxide were dissolved in portions of a lot of PEG 400, and these were allowed to react (according to the activity determination procedure) with three portions of a lot of T D I containing three levels of hydrogen chloride content (Figure 2). Potassium hydroxide concentrations of PEG 400 and hydrogen chloride contents of T D I are calculated as microequivalents of each per gram of reaction mixture to facilitate compari-
Various Lots of TDI Reacted with 1 Lot of PEG 400
Comparison of H.C. and acid contenfs of TDI acidity correlates more closely with activity % of H.C. Prepolymer T D I Lot Aciditya H.C.a Acidic Type 8.4 15.1% Liquid ND 38 1.27 17.7 52.5 Liquid 1167 9.30 16.9 70.5 Liquid 1219 11.9 13.8 103 Liquid 14.2 1225 13.0 3.4 Solid 1140 0.40 6.8 0.7 Solid 88320F 0.05 1.1 0.0 Solid 88323 0.00 5.4 0.0 Solid 88454C 0.00 a
Expressed as microequivalents per gram of prepolymer.
Table IV.
Effect of Acid Catalysis on Prepolymer Preparation Branching was minimized
PEG 400
Lot 969518 96953 A 96955 92698D
a
Acid Content5
Exotherm,
C. 58 69 82 59 59 67 80
O
0 216 HCI 580 HCI 0 28 &PO4 B A 54 HsPO4 92699AA 167 Hap04 Expressed as microequivalents of acid per gram of prepolymer.
VOL. 51, NO. 8
Prepolymer Type Liquid Liquid Liquid Liquid Liquid Liquid Liquid
AUGUST 1959
931
3, 4: or 5, for high acid concentrations never caused formation of solid prepolymers. Data in Table IV were obtained when PEG 400 was acidified to various levels with hydrogen chloride or with 8570 phosphoric acid. T h e acids were added to the polyol in this case.
Increased exotherms as acid content was increased indicated the catalytic action of the acid. but since gelation never occurred only the chain extension reactions were affected and branching was held to a minimum. Recent work has demonstrated that the same general relationships hold for the reactions of TDI and other hydroxyl compounds such as poly(propy1ene glycols), polyesters, or castor oil derivatives. The situation in the two latter cases is complicated by the presence o weaker carboxylic acids, and some buffering effects are encountered. Figure 3 represents the effect of strong acids and bases on the various reactions possible in a prepolymer system. Thus, large amounts of acid preferentially catalyze the NCO-hydroxyl reaction, in more linear Prepolymers than are obtained in the presence of
I10
0
g 90
w
t
70
50 0
EOUlV OF NaOH ADDEO/EQUIV OF H,PO,
Figure 4. Effect of sodium phosphate buffers on prepolymer preparation Sodium salts of acids with K,, a b o v e l o - 7 can buffer in the active region
i P
5. Effect of sodium phenolates on exotherm
Figure --&-----a-
I-
o
x 70
Results agree with sodium phosphate results
W
50
1
1
0.1
0
I
0.3
I
I
I
05 SODIUM SALT IN PEG 400 0.4
0.6
0.7
5,d 1 O/o
0 $
I
0.2
FREE FILM
180
/
2000
LJ-4 ADHESIVE
1000 l 500 So0l I
300
O
b
Figure 6. Formation of linear prepolymer is f a v o r e d b y increased acidity
lesser amounts of acid. I n the abscnce of catalyst, the reaction occurs so much more rapidly than branching Reactions 3, 4: and 5 that essentially linear prepolymers are produced unless extended heating at temperatures above 100" C. is employed. In the presence of bases, however, the reaction rate for branching reactions approaches that of the NCOhydroxyl reaction so that highly branched prepolymers or gelation result. I t can thus be postulated that active systems (in which considerable branching occurs) are those in which traces of strongly alkaline materials are present. Inactive systems result when they are absent or neutralized. Certain poly(ether glycols) and polyesters may contain traces of metals and catalyst neutralization products which also act as catalysts for the isocyanate branching reactions. Addition of strong acids, usually in the form of hydrogen chloride present in the isocyanate, neutralize impurities and facilitate more controlled reaction in the prepolymer process. Use of Weak Acids and Bases. Attempts to control activity with weaker acids and bases do not provide comparable results. Buffering effects are apparently created which limit the ultimate acidity or basicity of the reaction system. The PEG 400 used in these experiments contained 0.1% ash resulting from neutralization of the basic ethylene oxide condensation catalyst with phosphoric acid. Because PEG 400 lots were normally pH 4 to 7, the sodium hydroxide catalyst was evidently neutralized to the monosodium phosphate stage. T o substantiate this, sodium hydroxide and phosphoric acid were dissolved in portions of one lot of PEG 400 so that each gram contained 120 peq. of base or acid. These were then blended to provide varying degrees of neutralization at a salt content greatly in excess of any already present in the PEG 400. These blends were then rcacted with 2,4-TDI in the usual method for activity determination (Figure 4). These data suggested that monosodium phosphate buffers in the inactive region, disodium phosphate in the transition region, and trisodium phosphate in the active zone. Dissociation constants for phosphoric acid at 25" C. are (3): H3PO4 F? H +
K , 7.5 X
MlCROE9 HCI /G. OF PREPOLYMER
932
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
H2P04-
, i HPO4=
6.2 X 1 0 - 8
e
Hf+- PO,';
10 X
Thus, sodium salts of acids with K , values above 10-7 are capable of buffering the reaction mixture in the inactive region. Presumably a large concentration of the proper buffer in the reaction mixture would compensate for trace amounts of strongly acidic or basic impurities present in the reactants and
URETHANE POLYMERS Table V. With NEM, a Higher p H Still Produced Liquid Polymers
Polyol Lot 1
Base None
Polyol PH
6.19 8.81 9.25 11.62 8.89 9.49 None 6.39 0.0024% K O H 7.29 7.75 0.004 9.0% NEM 9.75 O.OlO%KOH 0.016 0.050 1.9% NEM 9.5
2
Prepolymer Type Liquid Liquid Solid Solid Liquid Liquid Liquid Liquid Solid Liquid
obviate any concern for lot-to-lot variations. To find more readily soluble buffers for this purpose, sodium salts of phenol and its chlorinated derivatives were evaluated. When dissolved in PEG 400 prior to reaction with T D I in the activity determination, their effect on the exotherm is shown in Figure 5. Dissociation constants for the pure phenols are (4): K , at 25' C.
Phenol 2-Phenolphenol 2,4,5-TrichlorophenoI 2,3,4,6-Tetrachlorophenol 2,3,4,5,6-Pentachlorophenol
1.3 X 10-lO
...
4.3 x 10-10 7.2 X 10-6 5.5 X 10-0
Sodium phenolates which do not carry the reaction into the active zone are salts of tetra- and pentachlorophenols which have dissociation constants of 10-6, in fair agreement with sodium phosphate results. Quaternary ammonium compounds were also evaluated as a type of basic compound. Benzyltrimethylammonium hydroxide was capable of providing active reaction mixtures, but, as with sodium hydroxide, its inherent basicity was too high, and the transition from active to inactive occurred over an impractically narrow concentration range. Determination of alkaline impurities in the hydroxyl component by pH measurement has little value, especially with experimental materials containing contaminants of unknown types. For example, two lots of a poly(propy1ene glycol) of molecular weight 2000 were treated with varying amounts of a strong base (potassium hydroxide) and a weak base, N-ethylmorpholine (NEM), to provide a series of polyols of varying pH. These were reacted with the same lot of T D I (80 to 20 isomer) at NCOhydroxyl ratio of 2.5 by mixing the reactants at 70' C. and allowing the mixture to react in an insulated reactor (Table V). I n each lot of poly(propy1ene glycol) (PPG) it was possible, when using NEM, to increase the p H of the PPG to higher values than when potas-
Table VI.
HC1 in TDI, peq./Gram 0
Increasing the Acidity Favors Formation of Linear Polymers
0.105 0.158 0.210 0.262 0.324 0.525 16.40 a
Adhesive Adhesive Bond Prepolymer Drying Time, Strength,b AEG Min. P.S.I. Gelled t o solid prepolymer Gelled t o solid prepolymer Gelled to solid prepolymer
Exotherm,
Amine equivalent.
O
c.
90 87 77 72 73 64 49 46
820 544 515 405 413
33 44 62 194 300
M.P.;
c.
1970
225 227 218 175 160
2000
1940 504 669
Tensile rupture in aluminum t o aluminum bonds.
groups because of branching reactions), results in Table V I were obtained. Because the idealized amine equivalent (AE) of the PEG 400-TDI prepolymer is 374, increasing the acidity of the system favors formation of the ideal, liquid prepolymer, accompanied by increasing adhesive drying time and decreasing adhesive bond strength and film melting point. Figure 6 shows the general trends in greater clarity. Further clarification is provided by calculating the various types of prepolymer which may be obtained. Representing the basic molecule obtained from 2 moles of T D I (indicated by T) and 1 mole of PEG 400 (indicated by P) as T-P-T and assuming that the principal side reaction invokes allophanate formation by reaction of the terminal -NCO group on one prepolymer molecule
sium hydroxide was used and still obtain liquid prepolymers. This discussion has been limited to the gross effects of activity in formulating solid or liquid prepolymers. The more subtle differences are of greater practical interest as they concern formation of liquid prepolymers of varying degrees of branching. In many applications the partially branched, more viscous prepolymers are preferred to the entirely linear types, and these can be obtained by control of reaction time and temperature while maintaining acidic conditions. In previous examples the PEG 400TDI prepolymers were usually evaluated as adhesives or in the form of free films by allowing them to react with atmospheric moisture (2). When reaction conditions were varied to provide prepolymers of increasing amine equivalent (indicating decrease in terminal -NCO 700
I
p
Figure 7. Heating always increases molecuIa r complexity and decreases amine equivalent
400
I
0
1
!2
I
I
I
I
I
I
I
I
24 36 48 HEATINQ PERIOD, HRS.
60
24
so
I
72
Figure 8. Increased temperatures cause more rapid increases in prepolymer viscosity
I
0
I2
3s 48 HEATINQ PERIOD, HRS.
VOL. 51,
NO. 8
AUGUST 1 9 5 9
1
72
933
H i g h Reaction Temperatures under Acidic Conditions. Because attaining the proper degree of activity involves extremely delicate adjustment of reaction system acidity, this method is not practical as a production device. Fortunately, much better control of the prepolymer process can be obtained by adjusting the acidity of the mixture all the way to the inactive area and forcing the branching reactions to occur a t a slow, controllable rate by increasing the reaction temperature after the initial XCO-hydroxyl reaction has essentially subsided. A large batch of the usual PEG 400TDI prepolymer was reacted under acidic conditions at an NCO-hydroxyl ratio of 2.0, keeping the reaction temperature below 7'0' C. The resulting product had an amine equivalent of 414,
1
3000
2000
-
0 HRS-
X ELOHOATION RlEWLYMER HEATED FOR INDICATED TIME AT l l O * C
Figure 9. Prolonged prepolyrner heating alters the tensile properties of resulting free films
with the urethane group on another, the following series can be visualized : Degree of Prepolymer
Terminal
1
Molecular Structure T-P-T
2
,Y
Asaocia-
tion, 11.
Theoret. Mol. Wt., n X 748
Groups, w 1
+
Theoret. Equiv. Wt., SF,
748 1496
2 3
348 499
1496
2244
4
561
1122
NCO
Theoret. SVt./Branch Pt , -Mol. Wt./n - 1 LY
,Y.??'*
3
4 5
etc.
2992
5
598
997
3740 4488
6
623 641
935 898
Optimum adhesive behavior, including bond strengths, viscosity, and stability (shelf life), was obtained at prepolymer amine equivalent values around 500, indicating a degree of association value of about 2. More highly branched prepolymers tended to have solubility and shelf life difficulties, while more linear prepolymers did not provide necessary bond strengths. Prepolymer suitability often goes through a maximum, then declines as molecular complexity is increased via these side reactions.
7
indicating, when compared with the theoretical value of 37'4, that only a small amount of side reactions had occurred. This batch was next divided into four portions which were further l l O o : and processed a t 90', loo', 120" C. to produce branching reactions (Figures 7' and 8). The resultant prepolymers were again evaluated as adhesives and as free films by allowing them to react with atmospheric moisture. Increased branching (leading to increased cross linking in the cured adhesive or film) suggested by the
A
'"V'
2 ' 1
400
1
1
1
1
1
1
1
1
500
1 1 600
PREPOLYMER A €
934
INDUSTRIAL AND ENGINEERING CHEMISTRY
A
1
_i 1
Figure 10. Prepolymer heating affects adhesive and film characteristics even though amine equivalent values are the same
increasing prepolymer viscosity is in agreement with the shapes of the stressstrain curves obtained on the cured films (Figure 9). Although heating the prepolymer always results in increasing molecular complexity and diminished amine equivalent. temperature has an effect beyond that expected from a rate standpoint. Prepolymers taken to the same amine equivalent values at different temperatures do not provide cured adhesives and films with exactly the same characteristics (Figure 10). At an amine equivalent of 500, the prepolymer heat-treated a t 120' C. provides a higher modulus than that treated at 100" C., indicating a tighter or more cross-linked structure. At higher temperatures molecular rearrangements occur, possibly because of urethane dissociation. which create molecular structures other than those indicated above. Taking into account dissociations in the prepolymer molecule, the ideal molecule representing a degree of association of 2 could have other forms, each having 8 moles of TDI, 4 moles of PEG 400, and 6 free NCO groups :
2(TvP-T--P-T) T T 'I-PAT-P-T-P~T--P--'T
T
+T
Still other forms are possible, but those above demonstrate that dissociation a t elevated temperatures may create species which possess different structures, although all prepolymers have the same ratio of moles of TDI to PEG 400 and the same number of terminal groups. Modification of prepolymers by adjustment of hTCO-hydroxyl ratios, reaction conditions, or catalysts is too complex to be considered here. Once the proper degree of acidity has been obtained, however, branching reactions a t temperatures below 70 O C. are sufficiently suppressed so that a wide variety of prepolymers can be prcpared in a consistent manner. Literature Cited
(1) Am. Sac. Testing Materials, Philadelphia, Pa., Committee D-20, tentative method, D 1638-59T. (2) Heiss, H. L., Saunders, J. H., Morris, M. R., Davis, B. R., Hardy, E. E., I N D . ENG.CHEkf. 46, 1498 (1954). (3) Moeller, T., "Inorganic Chemistry," p. 646, Wiley, New York, 1952. (41 Tiessens, G. J.: Rec. trau. c h m . 48, 1066-8 (1929). RECEIVED for review January 22, 1759 A C C E P T E D April 10, 1959
