Urethane Chemistry and Applications - ACS Publications - American

11 A Reaction Sequence Model for Flexible Urethane Foam F. E. BAILEY, JR. and F. E. CRITCHFIELD

Downloaded by CORNELL UNIV on September 22, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch011

Research and Development Department, Silicones and Urethane Intermediates Division, Union Carbide Corporation, South Charleston, WV 25303

In the process of making free-rise, water-blown flexible urethane foam, a mixture of relatively low molecular weight components is transformed in a matter of minutes into the controlled, highly "engineered," supramolecular architecture of foam. The foam produced meets specifications for bedding, seating, carpet padding - applications for which there are particular physical property criteria. This process is fascinating because i t is one of the few instances in industrial polymer chemistry in which rapid polymerization occurs in such a controlled way that a defined supramolecular architecture is simultaneously achieved. The very familiar, open-cell foam structure of a flexible urethane is shown in the scanning electron micrograph of a section of foam in Figure 1. The structure is that of a system of regular dodecahedra with open-faced pentagonal cell boundaries. The problem addressed in this work is the sorting out of the sequence of chemical reactions which occurs in the foaming process and defining the timing of these reactions which leads to stable foam of desired properties. In elementary descriptions of the making of urethane foam, a set of two reactions is often given: the reaction of a polyol and a diisocyanate to yield a polyurethane:

OCN-R-NCO + HO-R'-OH —

£

O-R'-O-C-NHR-NH-C

and the r e a c t i o n o f d i i s o c y a n a t e with water to y i e l d d i s u b s t i tuted ureas (polyureas) and carbon d i o x i d e :

OCN-R-NCO + H 0 2

^

0 0 I! U -f NH-C-NH-R-C ± + C 0

2

The sum o f these r e a c t i o n s i s described as leading t o block

0097-6156/81/0172-0127$05.00/0 © 1981 American Chemical Society

Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

U R E T H A N E CHEMISTRY A N D APPLICATIONS


128

Figure 1. Scanning electron micrograph of a section of water-blown, HR uretha foam.



11.

129

Flexible Urethane Foam

B A I L E Y A N D CRiTCHFiELD

urethane-urea polymer of the (AB) t y p e , blown i n t o foam by the generated carbon d i o x i d e . The f i r s t r e a c t i o n y i e l d i n g urethane from a polyol with f u n c t i o n a l i t y greater than two i s r e f e r r e d to as the " g e l l i n g r e a c t i o n " leading to the three dimensional urethane network while the second, "blowing r e a c t i o n , " i s c o n sidered a l s o to c o n t r i b u t e to " g e l " p r i m a r i l y through a s s o c i a t i o n of h i g h l y p o l a r s p e c i e s , the p o l y u r e a s , i n the polymer phase of the foam. I t has a l s o been noted that i n these p o l y m e r i z a t i o n s , some f u r t h e r degrees of c r o s s l i n k i n g can occur due to post r e a c t i o n of isocyanate with already formed ureas or urethane. Isocyanate can r e a c t with s u b s t i t u t e d ureas to form b i u r e t : 0

0

0

0

R-NCO + -(· NH-C-NH-R'-NH-C -)-—*- -(- NH-C-N-R'-NH-C -)ι C = 0 NH ι R or with urethane to form a l l o p h a n a t e : 0

0

0

0

R-NCO + £ O-R'-O-C-NHR-NH-C • ) - — * > £ 0-R*-0-C-N-R-NH-C -)C = 0 NH f R C o n c e p t u a l l y , there are severe problems i n accepting the simple view of these simultaneously o c c u r r i n g r e a c t i o n s when the regular a r c h i t e c t u r e of foam i s observed. F u r t h e r , evidence has been presented r e c e n t l y (1_) f o r the presence of other species during foaming p a r t i c u l a r l y at the e a r l y stages of r e a c t i o n (2_, 3). Infrared a n a l y s i s of foam s h o r t l y a f t e r mixing shows only very low concentrations of the polymer s p e c i e s , urethane and d i s u b s t i t u t e d urea, but s i g n i f i c a n t concentrations of carbamic a c i d and arylamine carbamates, due to h y d r o l y s i s of i s o c y a n a t e :

-R-NCO + H 0 2

*-

0 II -R-NH-C-0H carbamic a c i d


130

URETHANE

-R-NCO + H 0 - H P -

-R-NH

2

2

+ C0

+ -

-R-NH3

CHEMISTRY

AND

APPLICATIONS

2

0 »

0-C-NH-R-


arylamine carbamate Carbamic a c i d , an intermediate i n the h y d r o l y s i s , i s an app a r e n t l y p e r s i s t e n t species i n the r e a c t i o n forming measurable concentrations of carbamate s a l t s . Later i n the r e a c t i o n s e quence these carbamic a c i d species are converted to polyureas: 0 II -R-NH-C-OH + - R ' - N H

2

0 II —-R-NH-C-NH-R - + H 0 1

2

F u r t h e r , water i n concentrations measurable by such c l a s s i c t e c h niques as Karl F i s c h e r t i t r a t i o n can be found i n the foaming system, Figure 2. During the f i r s t minute o f foaming, the r e a c t i o n of water appears to follow f i r s t order k i n e t i c s . In t h i s paper, instrumental means are used to obtain a p h y s i c a l d e s c r i p t i o n of the r i s e of urethane foam and c e l l opening. Infrared a n a l y s i s i s used to i d e n t i f y the chemical species present i n r e a c t i n g foam and to determine the order and r e l a t i v e rates of r e a c t i o n of these s p e c i e s . From these d a t a , a r e a c t i o n sequence model i s deduced f o r the process of making s t a b l e , water-blown urethane foam. Experimental In the work d e s c r i b e d , three urethane formulations have been p r i n c i p a l l y used. These formulations which produce "good," r e p r e s e n t a t i v e foam were s e l e c t e d f o r convenience i n laboratory manipulation. With water l e v e l s of 2.5 phr, these formulations given i n Table I permit a convenient q u a n t i t y of polyol to be handled to produce one or two-gallon volumes of 2.0 to 2.6 lb per cu f t foam. The s t u d i e s have covered the range of water contents from 1.5 to 4.5 phr water while centering on the mid-range without loss of g e n e r a l i t y i n developing the r e a c t i o n model. A standard mixing procedure (_2, 3) f o r laboratory formul a t i o n s has been used. This procedure i s one which has evolved in the l a b o r a t o r i e s e m p i r i c a l l y over the years to permit making or small s c a l e , l a b o r a t o r y foams which c l o s e l y approximate foams from the same formulations made on foam machines. The procedure involves i n t e n s i v e mixing i n a b a f f l e d one-quart container f o r 60 seconds p r i o r to pouring i n t o an open topped one or two-gallon container. An e l e c t r i c timer which records t o t a l mixing time s i g n a l s end-of-mixing by t u r n i n g on the instrumented measuring system. T h i s end-of-mixing i s taken as time " z e r o " for k i n e t i c and r i s e p r o f i l e measurements.





3.0 r

2.0 μ

1.5 \

V ζ ο ο

tr

\

\

1.0 0.8

Lu

0.5

0 20 40 60 80 TIME, SECONDS, AFTER THE END-OF-MIXING

Figure 2. Disappearance of water as a function of time during the first minu after the end-of-mixing: HR Formulation at 105 index. Key: , 2.0 phr wate - - - -,2.5 phr water; - — -, 3.0 phr water.



"A"

?

"Trio!" w i t h a Hydroxyl

J

(5) S i l i c o n e S u r f a c t a n t , L - 5 3 0 3 .

9

60 phr 40 1.,5 2.,5 0.,1 0.,02 0.,3 33.,03

2 . 5 Ib/cu f t 54.5 l b 7 m i n / f r 15.5 p s i 160%

"HR"

E t h y l e n e Oxide Capped P o l y e t h e r P o l y o l w i t h a H y d r o x y l

(4) Polymer P o l y o l , 20% S o l i d s , w i t h a H y d r o x y l Number o f 2 8 .

(3) A High M o l e c u l a r W e i g h t , Number o f 34.

Number o f 4 7 .

2 . 6 Ib/cu f t « 52.2 f r / m i n / f r 22.5 psi 315%

Foam P r o p e r t i e s

P o l y e t h e r P o l y o l (3) 100 phr S u r f a c t a n t (2) 1.0 Water 2.5 Amine C a t a l y s t , A - l 0.1 Stannons O c t o a t e 0.05 TDI (105 Index) 31.11

F o r m u l a t i o n "Β" P o l y e t h e r P o l y o l (3) Polymer P o l y o l (4) S u r f a c t a n t (5) Water Amine C a t a l y s t , A - l Dibutyl t i n Dilaurate T r i e t h y l e n e d i a m i ne SF58 I s o c y a n a t e (105 Index)

URETHANE FOAM FORMULATIONS

F o r m u l a t i o n "B"

2.1 Ib/cu f t 63.2 f t / m i n / f t 12.2 p s i 230%

"A"

100 phr 1.0 2.5 0.12 0.2 34.28

(2) S i l i c o n e S u r f a c t a n t , L - 6 2 0 2 .

(1) A P o l y e t h e r

Density A i r Porosity Tensile strength Elongation

P o l y e t h e r P o l y o l (1) S u r f a c t a n t (2) Water Amine C a t a l y s t , A - l Stannons O c t o a t e TDI (105 Index)

Formulation

WATER-BLOWN, FLEXIBLE

TABLE I


m

δ

> Η

Ο

Ε

>

> Ό

H

Χ m g

M Ο

>

S3

H

11.



133


Foam Rise P r o f i l e . At the e n d - o f - m i x i n g , formulations are poured i n t o a one or two-gallon open-topped c o n t a i n e r and free r i s e foaming i s measured. The r i s e p r o f i l e and r a t e of foam r i s e has been measured with a Fluidyne System (4-, S). Foam r i s e and r a t e of r i s e as functions of time a f t e r the end-of-mixing are received as g r a p h i c a l output from the Fluidyne System. Foaming Pressure. Using the same foam measuring system (4, 5), a "foaming pressure" can a l s o be determined. It i s p o s s i b l e to measure "foaming pressure" with a small transducer placed at the s i d e , near the bottom, of the c o n t a i n e r i n which foam i s poured and allowed to expand i n f r e e - r i s e . This "foaming p r e s sure" i n f r e e - r i s e can be recorded as a f u n c t i o n of time i f desired. F o r m a l l y , i t i s p o s s i b l e then to c a l c u l a t e a "foam v i s c o s i t y " knowing "foam pressure" and r a t e - o f - r i s e . While t h i s c a l c u l a t i o n leads to a q u a n t i t y with the u n i t s of v i s c o s i t y , i t must be remembered that the foam i s a multiphase system, one phase of which i s an expanding gas; and, that the r i s i n g foam i s not flowing as a l i q u i d but i s a system i n a n i s o t r o p i c expansion. In any c a s e , the "foaming pressure" i n f r e e - r i s e foams i s of the order of 0.02 p s i g i n a system with a r i s e - r a t e of about four inches per minute (.3, 6). "Gel P r o f i l e . " During foam r i s e , the i n i t i a l l y f r o t h - l i k e foam develops some mechanical i n t e g r i t y . This mechanical i n t e g r i t y or " g e l " has been measured i n a number of ways and described in terms of a "gel p r o f i l e " i n p a r a l l e l with " r i s e p r o f i l e " (7_). One method of determining a "gel p r o f i l e " of a r i s i n g foam i s using a "BB" drop t e s t i n which " B B ' s " are dropped, from a height of one inch above a r i s i n g foam, i n a l i n e across the foam s u r face. L a t e r , the p o s i t i o n of " B B ' s " i n the foam i s determined. T h i s method generates an i n t i t u i v e l y a p p e a l i n g , u s u a l l y symmetric curve r e f l e c t i n g the p o s i t i o n of the " B B ' s " i n the foam ( l o c a t i o n of the "BB" from the bottom of the foam as a percent of the foam height p l o t t e d as a f u n c t i o n of the time at which the "BB" was dropped). While on c l o s e i n s p e c t i o n such a "gel p r o f i l e " i s d i f f i c u l t to i n t e r p r e t s i n c e only the f i n a l p o s i t i o n of the "BB" i s measured ( i t i s not known whether the "BB" reached t h i s p o s i t i o n by descending ever more slowly i n t o a " g e l l i n g " s t r u c t u r e or by being buoyed upward by an expanding foam), "gel p r o f i l e " does correspond to the very r e a l circumstance of the development of a measurable mechanical strength i n the foam and provides some measure of the development of s t r u c t u r e w i t h i n the r e a c t i n g foam. Cell-Opening. An important parameter i n the formation o f f l e x i b l e foam i s the time of c e l l - o p e n i n g . In the formation o f foam, Figure 1, the r e g u l a r dodecahedra with open-faced pentagonal c e l l boundaries a r i s e from gas c e l l s expanding as spheres i n the r e a c t i n g foam (8). These c e l l s begin to achieve a c l o s e s t packing geometry with t h i n n i n g w a l l s developing i n areas of



134

URETHANE

CHEMISTRY

AND

APPLICATIONS

c l o s e s t contact as l i q u i d phase drains i n t o the i n t e r s t i c e s . Rupture of the t h i n c e l l w a l l s or "windows" produces the o p e n - c e l l foam. Since c e l l - o p e n i n g w i l l lead to a path for the e v o l u t i o n of carbon d i o x i d e from the r i s i n g foam, carbon d i o x i d e e v o l u t i o n as a f u n c t i o n of time has been used to follow the c e l l - o p e n i n g p r o cess. I n i t i a l l y , to measure carbon d i o x i d e e v o l u t i o n , a mixed f o r mulation was poured i n t o a container which was covered and through which a c a r r i e r gas, n i t r o g e n , was passed over the r i s i n g foam. The c a r r i e r gas was then sparged i n t o lime water. The time was noted when a cloud point was observed due to carbon dioxide. It was found that t h i s cloud point was r e p r o d u c i b l e . In order to measure c e l l opening more a c c u r a t e l y , a simple i n f r a red technique was adopted. The c a r r i e r gas was passed through an i n f r a r e d gas c e l l i n a Perkin-Elmer Model 281B Infrared Spectrophotometer set at 2320 cm" and absorbance measured as a function of time a f t e r the e n d - o f - m i x i n g . Infrared A n a l y s i s . Infrared a n a l y s i s has been used to i d e n t i f y the chemical species present i n r e a c t i n g foam and to d e t e r mine the order and r e l a t i v e rates of r e a c t i o n of these s p e c i e s . For these measurements, a Foxboro/Wilks Model 80 Computing I n f r a red Analyzer and a Perkin-Elmer Model 281B Infrared Spectrophotometer have been used. Infrared absorption band assignments i n the carbonyl region are summarized i n Table II (1_, 2, 3», 9-17). In Figure 3, a p o r t i o n of the i n f r a r e d spectrum o f a water-blown urethane foam i s shown. Absorbances can be i d e n t i f i e d due to isocyanate at 2270 cm" , urethane.at 1730 c m , b i u r e t at 1670 c m " . , d i a r y l u r e a at 1645 cm" and aromatic carbon-carbon at 1605 cm" . Independently, i n f r a r e d absorbances due to carbamic a c i d and arylamine carbamates have been e s t a b l i s h e d i n biochemi c a l systems (10). R e c e n t l y , information concerning competing absorbances due to non-hydrogen bonded ( s o l u b l e ) polyureas has been developed (Tji, 16^, 17_). General agreement, however, i s that p r e c i p i t a t e d d i s u b s t i t u t e d urea shows an i n f r a r e d absorbance at 1645 cm" . For i n f r a r e d a n a l y s i s , a sample of foam i s taken with a small spatula very s h o r t l y a f t e r pouring from the r i s i n g foam, spread on the s a l t p l a t e of a thermostated i n f r a r e d c e l l and covered. The sample can e i t h e r be quenched on a c o l d s a l t p l a t e or held at a r e a c t i o n temperature. The chemical r e a c t i o n s o c c u r r i n g (3) can be followed i s o t h e r m a l l y . For these measurements, a r e a c t i o n temperature (18) must be s e l e c t e d which w i l l allow the measurements to be r e l a t e d to the r e a c t i o n sequences o c c u r r i n g i n the f r e e - r i s e foam under approximately a d i a b a t i c conditions. T h i s temperature has been determined by i n s e r t i n g a f i n e - s i z e , h i g h l y responsive thermocouple through the s i d e of the r e a c t i o n c o n t a i n e r to a p o i n t i n the c e n t e r , about three inches from the bottom of the r e a c t i n g foam. Omega subminiature thermocouples (SC PSS-020G-6, copper-constantan) and BLH HT Microminiature thermocouples (TCC-ES 200, copper-constantan) have been used. Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.



TABLE II


INFRARED ABSORPTION BAND ASSIGNMENTS Isocyanate

2270 cm"

Urethane

1730

Carbamic A c i d

1710

Allophanate

1710

Arylamine Carbamate

1670

Biuret

1660-1680

Diarylurea

1645

Aromatic Carbon-Carbon

1605

χ

I

2400

Figure 3.

1

1

80 %

TRANSMISSION

I ι I I 2000 1800 WAVENUMBERS, C M .

I 1600

l

Infrared spectrum of a water-blown polyether polyol polyurethane


136

URETHANE

CHEMISTRY

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APPLICATIONS


Results and D i s c u s s i o n Physical D e s c r i p t i o n of Foam R i s e . In Figures 4, 5 and 6, the r i s e p r o f i l e , rate of foam r i s e and "gel p r o f i l e s " for the three formulations " A , " "B" and "HR" are summarized. The r e s u l t s for formulations "A" and "B" are s i m i l a r . The maximum r a t e of foam r i s e occurs between 20 and 40 seconds a f t e r the end-ofmixing and f u l l r i s e i s achieved i n about three minutes. Structure which develops w i t h i n the r i s i n g foam and i s measured by "gel p r o f i l e " occurs a f t e r the maximum r a t e - o f - r i s e and i s genera l l y complete before f u l l r i s e height i s achieved. The major d i f f e r e n c e observed i n the case of formulation "HR", Figure 6, i s i n the shape and timing of the "gel p r o f i l e , " i n comparison, the "gel p r o f i l e " for "HR" begins to r i s e l a t e r , r i s e s more abruptly but i s complete at about the same time. For these f o r m u l a t i o n s , the maximum "foam pressure" (_3, 6) of 0.02-0.03 psig i s observed at about the same time as the maximum foam r a t e - o f - r i s e , c a . four inches per minute. It i s formally p o s s i b l e to c a l c u l a t e a "foam v i s c o s i t y " which, a f t e r appropriate f a c t o r s are m u l t i p l i e d to adjust dimensions, w i l l be of the order of 10 cps at the time of 30 to 60 seconds a f t e r the end-of-mixing. During t h i s t i m e , the foams w i l l have reached 30 to 60 percent of f i n a l r i s e height. Without f u r t h e r a n a l y s i s of the c o m p l e x i t i e s , i t i s p o s s i b l e to say that the very low foaming pressure i n f r e e - r i s e , f l e x i b l e foam coupled with the r a p i d r i s e rate i n t u i t i v e l y supports the c a l c u l a t i o n of a r e l a t i v e l y low l i q u i d phase v i s c o s i t y , about the same as that of the s t a r t i n g polyol at t h i s stage of the foaming process. Cell-Opening. In the e a r l i e r experiments using a cloud point d e t e c t o r , a surge i n carbon d i o x i d e e v o l u t i o n was observed r e p r o d u c i b l y at a p o i n t i n the foaming process a t which there was a change i n shape, an i n f l e x i o n , i n the r a t e - o f - r i s e curve. In Figure 7, the rate of foam r i s e i s shown as a f u n c t i o n of time for Formulation " B . " A f t e r the maximum r i s e r a t e at about 50 seconds, the rate of foam r i s e decreases r a p i d l y . At 90 seconds, there i s an i n f l e x i o n a f t e r which there i s a l e s s r a p i d d e c l i n e in r i s e - r a t e . T h i s p o i n t of i n f l e x i o n corresponds c l o s e l y to the time the cloud p o i n t would be observed. I t should be noted that the shape of the r a t e - o f - r i s e curve a f t e r the maximum r i s e - r a t e can be a s s o c i a t e d with f a m i l i a r but unwanted s i t u a t i o n s a l s o r e l a t e d to c e l l - o p e n i n g . I f a f t e r the maximum r a t e - o f - r i s e , the r i s e - r a t e f a l l s sharply to zero as a r e s u l t of c e l l - o p e n i n g , foam c o l l a p s e i s c a t a s t r o p h i c . I f the d e c l i n e i n r i s e - r a t e to zero i s sharp without i n f l e x i o n and without a p p r e c i a b l e c e l l - o p e n i n g , foam shrinkage i s severe. In Figure 8, the e v o l u t i o n of carbon d i o x i d e from formulation "B" during foaming i s shown measured by i n f r a r e d absorbance at


BAILEY AND

CRITCHFIELD


TIME, SECONDS, AFTER THE

Figure 4.

0

60

120

TIME, SECONDS, AFTER THE

Figure 5.

END-OF-MIXING

Rise and ''gel" profiles, Formulation A. Key: rise profile; - · - ·, "gel" profile.

, rise rate;

180 END-OF-MIXING

Rise and "gel" profiles, Formulation B. Key: -, rise rate; rise profile; - · - ·, "gel" profile.

ION

ο 100 \— C\J _l LU 0_

X

U_ ζ Ο

Ο Lu Ο χ. Li. Ο OC Lt-f ω or

NT OR


1.

Lu Ο α: Lu CL

Figure 6.

α

>

Η

m g

Ο

M

M H

11.

BAILEY AND

CRITCHFIELD


147

with growth of polymer molecular weight. This sequence of chem ical reactions permits polymerization and molecular architecture to be achieved simultaneously to produce useful structures from these very fast reactions. Literature Cited 1. 2. Downloaded by CORNELL UNIV on September 22, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch011

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

17.

18. 19.

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