I/
Effect
o f Catalysts
I
on
Urethane Foam Properties 158' F. They were compressed to about 107, of their original height as soon as possible (usually 1 5 to 30 minutes after foaming): then returned to the curing oven for a final cure a t 158' F. for 24 hours or at 2.50' F. for 4 hours.
I s THE fast-groiving field of polyu-ethane foams relatively little has been published on the improvement of foam properties by the selective choice of type and amount of catalyst. Baker and others (4-7) were first to show that isocyanate reactions lvith active hydrogen compounds are base-catalyzed. More recently. Bailey and others (7-3) applied infrared absorption data to study the activities of a number of basic catalysts. T h e present article deals with the effect of six tertiary amine catalysts on the foaming characteristics and physical properties of flexible urethane foams. T h e prepolymer used was prepared from a mixture of polyoxyalkylene glycol available commercially as nonclarified, urethane-grade Pluronic polyol L-61 and a polyoxyalkylene polyol derivative of ethylenediamine, available as Tetronic polyol 701 and tolylene diisocyanate, following the procedure described by Frisch and Davis ( 8 ) .
Foam Formulation Prepolymer (9.570 free NCO) Dimethylsilicone fluid (50 csts.) Water Catalyst
Effect of Catalyst Concentration
With the exception of a slight increase in tensile strength for catalyst D. the tensile strength, modulus, compression defiection values. and density all decrease, xvhereas the compression set values increase tvith increasing catalyst concentration (Table I). No particular trend appeared to be indicated for the elongation in the fully cured foams. Foams made with moderate amounts of catalyst showed slightly improved tensile strength and compression set \\-hen aged for 1 week a t 70' C. (158' F.) and 95 to 1OOYc relative humidity. Exceptions were triethylamine and .V,LV'diethyl - 2 - methylpiperazine-catalyzed foams, which showed deterioration of foam properties on aging. Foams made \vith an excess of any of the other catalysts also had poor aging properties. Foams made with moderate amounts of .V>.2"-bis- (2 - hydroxypropyl) - 2 -methylpiperazine and A'-methylmorpholine had exceptionally good properties after 4 weeks'humid agingat 7 0 ° C . (158'F.).
Parts by Weight 100
0.4 2.2
As shown ~~
T h e water solution of the catalyst !vas stirred rapidly into the prepolymer containing the silicone fluid. T h e foams were permitted to rise to full height and then placed in a n air-circulating oven at
Comparison of Catalyst Activity
T h e activities of the six catalysts investigated were compared on the basis of time of foam rise, exotherm, pH, and
Amine Catalyst S,Nf-Bis(2-hydroxypropyl)-2-methylpiperazine
A7,~V'-Diethyl-2-methy1piperazine N-2-Hydroxypropyldimethylmorpholine N-Methylmorpholine Dimethylethanolamine Triethylamine
Catalyst Triethylamine Dimethylethanolamine X,\r'-Diethyl-2-methylpiperazine A',L\r'-Bis(2-hydroxypropyl) -2-methylpiperazine .V-Methylmorpholine N-(2-Hydroxypropyl) -dimethylmorpholine Determined on 1% solution of catalyst in
PH"
Odor Practically odorless Slight amine odor Practically odorless Strong amine odor Strong amine odor Strong amine odor Foaming Exotherm Initial Max. temp. temp., rise, ' F. F./min.
11.6 11.0 10.7
170 165 160
10.25 10.00 9.95
150 150 150
distilled water.
'
43 31 30.5 25.5 26 18
Boiling Point, C. 145/3 mm. 186 122/20 mm. 115 135 90
Order of Increasing Time of Foam Rise 1
2 3 6 4 6
B. G. ALZNER
and K. C. FRISCH
Research Laboratories, Wyandotte Chemicals Corp., Wyandotte, Mich.
base rate constants. As the p H increases, the activity increases as measured by the time of foam rise and exotherm. These results are in general agreement \vith the data of Bailey and others ( 7 , 3 ) for .\--methylmorpholine> dimethylethanolamine, and triethylamine. .4 kinetic study of the activity of the six catalysts in which the disappearance of the isocyanate was traced by an infrared absorption technique (2) revealed the same order of activity. For this investigation sufficient excess of Pluronic polyether L-61 \vas used in the reaction with phenyl isocyanate so that the relative concentration changes of polyether diol could be ignored. If the reaction bet\veen diol and diisocyanate is considered second order, the kinetic equation approximates that of a first-order reaction.
r k[NCO] Plotting reaction rate constants against catalyst concentration (Figure 1) resulted in straight lines, which can be represented by the general equation: k = kb C k,,, in which k is the reaction rate constant. C is the catalyst concmtration, kh is the base rate constant, and k , is the uncatalyzed reaction rate constant. The base rate constants can be considered the most significant measure of the activity of the catalysts. On this basis the folloxving order of decreasing catalyst activity- was established : triethylamine > dimethylethanolamine > .Y..V'-diethyl-2-methylpiperazine > .V;.I-'- bis(2 - hydroxypropyl) - 2 methylpiperazine > iV-methylmorpholine > .Y - (2 - hydroxypropyl) - dimethylmorpholi ne. -4 plot of the base rate constants against p H (Figure 2) resulted in a straight line. Therefore, this relationship could be utilized to estimate the activity of candidate catalysts for urethane foams. T h e foam properties depend upon the concentration of catalyst; the best balance of properties is obtained with a particular concentration for each catalyst. I t is generally accepted that Reactions 1 and 2 are largely responsible for foaming and cross-linking processes. T h e latter affect the physical properties of the foam and the rate of cross linking is also affected by the catalyst.
+
-
VOL. 5 1 , NO. 5
M A Y 1959
715
I
,
I
I
4
$
/ //
I
Figure 1. Catalyst concentration has a linear relation to reaction rate constants
N,N'. DIETHYL 2-METHYLPlPER4ZlNE kb=lxlO~'
F
2-METHYLPIPER4ZINE N-METHYLMORPWLINE k b i G 4 x 10-1 Y e
N-l2-HYDROXYPRGPYLlDIMETMYLMORPHOLINE
L
I
I
D
KI
20
Figure 2. The relation between pH and base rate constants can b e used to estimate activity o f catalysts for urethane foams
/
cTRIETHYLAHINE
a
E
2 f
N-METMYLYORPHOLINE
C4TALYST BISICITY. pM
MOLE %CATALYST B4SED ON PHENYL lSGCY4NATE
+ HzO + R-N=C=O
R-N;=C=O
while the formation of allophanate by reaction of carbamate with isocyanate is relatively slow. T h e base-catalyzed Reactions 1 and 2 can be represented by the following equations:
+
0
+
0
1I
R-NH-C-NH-R Substituted urea
+ R'--T\;=C=O
R-NH-CO-NR-CO-NH-R ' Biuret (2) T o determine the catalyst concentration necessary to bring about a balance of the two major chemical reactions involved in foaming. it is proposed that the best balance of foam properties, particularly with respect to tensile strength and compression set. is obtained when Reactions 1 and 2 proceed a t the same rate. Reaction 1 schematically represents the foaming step with the liberation of carbon dioxide. Reaction 2 is assumed to be largely responsible for crosslinking and polymer growth. Morton and Deisz (9) have experimentally determined that in the reaction of phenylisocyanate with a series of active hydrogen compounds, Reactions 1 and 2 are of prime importance as measured by their relative reaction rate, +
Table
I.
k(')
k,(')
jp)
k,(Z)
literature Cited (1) Bailey, M. E., Khawam, A., Toone, G. C., Division of Paint, Plastics and
+ kb(l)C + kb
