Chemorheology of Thermosetting Polymers - American Chemical

11 Curing Behavior of Segmented Polyurethane Adhesives D. M A R K HOFFMAN Downloaded by COLUMBIA UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch011

Lawrence Livermore National Laboratory, Livermore, CA 94550

Seven polyurethane adhesives, called Halthanes, were developed by the polymer group at Lawrence Livermore National Laboratory (LLNL) to meet design requirements of LLNL engineers while avoiding the use of OSHA restricted curing agent MOCA, methylene bis(2-chloroaniline). Four of these adhesives, the 73 series Halthanes, were made from 4,4' methylene bis(phenylisocyanate), MDI, terminated poly(tetramethylene oxide) prepolymers cured with a blend of butanediol and polyols. Three adhesives, the 87 and 88 series Halthanes, were made from 4,4' methylene bis(cyclohexylisocyanate), HMDI, terminated prepolymer cured with aromatic diamines. These segmented polyurethanes (1) consist of hard and soft segments whose concentration and chemical structure have been tailored for either more elastomeric character or tougher adhesive properties (2-4). Based on structure-property relationships, we have developed adhesives that bond rapidly and well, have low to intermediate modulus over a wide temperature range, and appear to be reasonably compatible with other components. Among the design requirements for these adhesives were initial v i s c o s i t i e s lower than 40 Pa.-sec., working times of about an hour, and cure times which permit removal of tools and clamping fixtures after 16 hours. LLNL engineers also wanted adhesives which would set upin4-8 hours rather than 16 for some applications. More rapid cure rates were achieved by accelerators and aromatic diamine curing agents. An adhesive curing behavior may be followed by a variety of techniques (5,6). One of the most important for engineering purposes i s the increase i n viscosity during cure. The curing behavior of these segmented polyurethane adhesives was followed by dynamic viscosity measurements with time under isothermal conditions. Viscosity measurements can be used to estimate the kinetic rate constants and the activation energy of the reactions occuring between the prepolymer and curing agent. Changing the hard segment former from MDI/BDO to HMDI/ADA s i g n i f i c a n t l y increased the cure rate. The accelerator, f e r r i c acetoacetonate, used i n 0097-6156/83/0227-0169$06.00/0 © 1983 American Chemical Society In Chemorheology of Thermosetting Polymers; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

CHEMORHEOLOGY OF THERMOSETTING POLYMERS

170

A b b r e v i a t i o n s Used i n Text A b b r e v i a t i o n s

MDI

=

4,4'

S t r u c t u r e

m e t h y l e n e

( p h e n y l

HMD I

Downloaded by COLUMBIA UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch011

BDO

=

=

0=d=N^Ô\cH -/ÔYN=C=0

b i s

2

i s o c y a n a t e )

H y l e n e

W

( s a t u r a t e d

MDI)

0 = C = N ^ S ^ - C H

b u t a n e d i o l

H O - ( C H

2

)

2

^ S ^ - N = C = 0

O H

4

OH Q

=

q u a d r o l

( C H

3

Ç H C H

2

)

2

N C H

2

C H

2

N ( C H

2

6 H C H

3

)

:

OH

Polymeg

1000

8 5

p o l y ( t e t r a m e t h y l e n e

H O - ( C H

o x i d e )

2

C H

C H

2

2

C H

2

x=14

Polymeg

2000

=

p o l y ( t e t r a m e t h y l e n e

TONOX

β

m i x t u r e 4,4'

o f

o x i d e )

x=28

a r o m a t i c

m e t h y l e n e

m - p h e n y l e n e

d i a m i n e s

d i a n a l i n e

H N ^ 0 ^ - C H - ^ £ ^ NH 2

=

m i x t u r e

o f

a r o m a t i c m e t h y l e n e

FAA

β

f e r r i c

2

d i a n a l i n e +

XU-205

2

a n d H

2

N

v ^ V

N

H

2

s u b s t i t u t e d

d i a m i n e s

o f 4,4*

H * 0^ 0 ^) -- N N H HΗ ^ ^ 0Ô ^ > - C- HC 2

d i a n a l i n e

a c e t y l a c e t o n a t e

Fe

(0-Ç=C-Ç=OX CH ~ CH ο

In Chemorheology of Thermosetting Polymers; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2

0 )

Χ

- Η

11.

c o m b i n a t i o n r a t e

o f

w i t h

t h e MDI

system.

The

v i s c o s i t y t h e

number b u t

s y s t e m

o f

a n

a v e r a g e

l i n e a r

m o l e c u l a r

i n i t i a l

v i s c o s i t y c u r e s

f o r a

v a r i e t y

t o

o f

t h e w e i g h t

m o l e c u l a r

o n

w e i g h t

( 8 ) .

t o

Under

depend

a v e r a g e

3.4

t h e

t h e

t o

p o l y m e r

k i n e t i c s ,

m o l e c u l a r

w e i g h t

c h a n g e s

power

t h e

from

dependence

a t

c o n d i t i o n s

t e m p e r a t u r e *

f o l l o w

from

r e a s o n s .

t e m p e r a t u r e .

I n

w e i g h t

i s o t h e r m a l

o n

s h o u l d

a

v i s c o s i t y

o f

w i t h

r e l a t e d

m o l e c u l a r

c u r e HMDI

d e r i v e d

i n c r e a s e

i s d i r e c t l y

v i s c o s i t y

w i l l

d e c r e a s e s

t h e t h e

from

t e c h n i q u e s

w e i g h t

t h e v i s c o s i t y

e x t r a c t e d t h a t

l i q u i d

o f

o f

w i t h

t h e r a t e

w e i g h t

m a t e r i a l

d a t a

i n c r e a s e d t h a t

c o r r e l a t e

r e a c t i o n

i s r e l a t e d

l o w

n e a r l y

u n r e a c t i n g

m o l e c u l a r

a t

a l c o h o l

t o

k i n e t i c

a l w a y s

c h e m i c a l

c u r i n g

v i s c o s i t y

h i g h

t e t r a f u n c t l o n a l

n o t

t h e d e p e n d e n c e

b e i n g


does

c o n v e n t i o n a l

D u r i n g

and

a

a d h e s i v e

U n f o r t u n a t e l y ,

measurements more

171

Segmented Polyurethane Adhesives

HOFFMAN

t h e

A s

m o l e c u l a r

t h e

w e i g h t

i n c r e a s e . T h e s e e x c e s s

p r e p o l y m e r s

t h e y

a r e warmed

t h e

MDI

t h e

e x c e s s

p r e p o l y m e r However

p r i o r

t h e

h a s

MDI

t e m p e r a t u r e

t w i l l

t e r m i n a t e d

Thus

MDI/BD0/PTMG1

T h i s

does

n o t

a d h e s i v e

f o r

s e v e r a l

a r e

d a r k

a r e

c l e a r

when

room

when

e v e n

o f

temperature*

a n d

f

t h e

h o u r s .

m i s c i b l e w i t h

t h e

room

s e p a r a t i o n , i . e . , w i t h

i n c r e a s i n g

t h e e x c e s s b y

MDI

i n t h e

t h e c u r i n g

c u r e d

a n d

i

i n

a t

60

w h i c h

t h e HMDI/ w i l l

agent*

above

s t r u c t u r e

A l lc o m p o n e n t s 60°C

t h a t

a s s o c i a t e d

phase

o f

b l o c k

a t

s o

a d h e s i v e ,

s e p a r a t i o n

time

P r e p o l y m e r

c o m p l e t e l y

p e r s i s t s

t h e h a r d - s o f t

a t

c l e a r

a r e n o t

i s c o m p a t i b i l i z e d

s y s t e m s

W i t h

f o r s e v e r a l

a p p e a r a n c e

c u r i n g

c o m p a t i b i l i z e d

h o u r s

b u t

time

m a c r o s c o p i c

t h e

p r o p e r t i e s . be

p o l y o l s

w i t h

c r y s t a l l i n e ,

r e c r y s t a l l i z e

l i q u i f i e d

m i l k y

phase

p o l y o l

a r e

s e p a r a t e *

i n d u c t i o n

a s

p r e p o l y m e r

a f f e c t

c a n

p h a s e

s e p a r a t i o n

I n

t h e

o l i g o m e r s

t h e i s o c y a n a t e *

r e m a i n

The

m a c r o s c o p i c

MDI

s y s t e m s

l o n g

i

a d h e s i v e

polymer*

t e m p e r a t u r e

a

phase

c u r e d

o f

w i l l

MDI.

r e a c t i n g

i n t h e p r e p o l y m e r

t e r m i n a t e d

e x c e s s

opaque

a d d i t i o n

i s o c y a n a t e

l i q u i d - l i q u i d

t h e

t o

b y

t h e p o l y o l

p o l y o l s

i s warmed,

t h e

made

S i n c e

t e r m i n a t e d

c r y s t a l l i z a t i o n

w i t h

a r e

i s o c y a n a t e .

ADA/

r e m a i n

T h e r e f o r e ,

° C .

c o n t r o l s PTMG2

d i s s o l v e d

t h e s e

s y s t e m s

c u r e d *

E x p e r i m e n t a l

The and

c o m p o s i t i o n s

I I *

F u r t h e r

p r e p o l y m e r s , depth the

c u r i n g

e l s e w h e r e

r a t i o s

e n t r a p p e d c a s e s ,

g i v e n a i r

f r e s h l y

p r e h e a t e d

cone

a n d

R h e o m e t r i c s

a s

a d h e s i v e s o n

a g e n t s ,

f i n a l

(4)· i n

t h e s e

a n d

P r e p o l y m e r t h e

t a b l e s

r a p i d l y

p r e p a r e d

a s

a r e g i v e n

p r e p a r a t i v e a d h e s i v e s

a n d

c u r i n g

a n d

t h e n

p o s s i b l e

a d h e s i v e

was

i n

t a b l e s

p r o c e d u r e s

a r e d e s c r i b e d

a g e n t s

were

d e g a s s e d

i n

m i x e d

t o

( 3 - 8 m i n u t e s ) *

i m m e d i a t e l y

I

f o r t h e

i n

remove I n

most

t r a n s f e r r e d

t o

f i x t u r e s .

Dynamic t e m p e r a t u r e

o f

i n f o r m a t i o n

v i s c o s i t y

was

f o l l o w e d

( 2 3 - 2 5 ° C ) ,

40,

60,

p l a t e

f i x t u r e s

M e c h a n i c a l

80,

w i t h

a s a n d

0.1

S p e c t r o m e t e r .

a

f u n c t i o n

100°C r a d i a n T h e

o f

u s i n g cone

25

time

a t

mm a n g l e

o s c i l l a t i n g

room

d i a m e t e r o n

a

f r e q u e n c y


172


Table I .

73 s e r i e s prepolymer and c u r i n g agent

formulations.


Curing Agents Component

73-14

73-15

73-18

73-19

Polymeg 1000 1,4-butanediol quadrol FAA

90 10 -

90 10 0.0156

85 10 5 -

85 10 5 0.0107

Prepolymer Polymeg 1000 Polymeg 2000 MDI

47.6 7.4 45.0

47.6 74 45.0

47.6 7.4 45.0

47.6 7.4 45.0

Prepolymer/ c u r i n g agent

62/38

62/38

65/35

65/35

Table I I .

Prepolymer and c u r i n g agent forumuations f o r polyurea hard segment adhesives. Curing Agents

Component

87-1

Tonox 60/40 XU-205

100

Polymeg 2000 HMD I Prepolymer/ Curing agent

87-2

88-2

100

100

Prepolymer 77.6 22.4

77.6 22.4

74.0 26.0

93/7

92/8

88/12


11.

HOFFMAN

173


was h e l d constant a t 1.0 Hz. o r 1.59 Hz. (10 r a d / sec) i n some instances. F u r t h e r d i s c u s s i o n of the equations and use of these f i x t u r e s i s g i v e n elsewhere (7,8).


Results 73 (MDI/ B u t a n e d i o l / P o l y o l ) Systems. Halthane 73 s e r i e s adhesives c o n t a i n MDI - b u t a n e d i o l hard segments and p o l y e t h e r s o f t segments* The I n c o r p o r a t i o n of s m a l l amounts of h i g h e r molecular weight s o f t segment reduces the tendency o f the s o f t segment t o c r y s t a l l i z e and improves e l o n g a t i o n t o break* 73-14 and 73-15 a r e e s s e n t i a l l y l i n e a r segmented polyuretbanes. The addition of the tetrafunctional alcohol Ν,Ν,Ν',Ν' t e t r a k i s ( 2 - h y d r o x y p r o p y l ) e t h y l e n e d i a m i n e o r Quadrol improves the modulus of the 73-18 and 73-19 adhesives above the hard segment g l a s s t r a n s i t i o n temperature (^70°C) by c r o s s l i n k i n g the system to prevent the onset of v i s c o u s f l o w . Our r e s u l t s (2-4) a r e c o n s i s t e n t w i t h a number of other s t u d i e s on MDI - b u t a n e d i o l hard segment polyurethane systems (9-12). In t h i s paper we examine the e f f e c t of temperature on the dynamic v i s c o s i t y of these adhesives* F i g u r e s 1 and 2 a r e i s o t h e r m a l v i s c o s i t y - time curves f o r 73-18 and 73-19. As expected, the r a t e of change of v i s c o s i t y w i t h time i n c r e a s e s w i t h temperature s i n c e t h i s depends on cure k i n e t i c s * The initial viscosity decreases w i t h temperature because the temperature dependence of v i s c o s i t y a f f e c t s the r e a c t i n g mixture more r a p i d l y that k i n e t i c s * On c l o s e examination the a d d i t i o n of f e r r i c a c e t y l a c e t o n a t e i s seen t o i n c r e a s e the l o g a r i t h m i c r a t e of change of v i s c o s i t y i n the 73-19 system by about a f a c t o r of 2* T h i s i s not an e x c e p t i o n a l l y f a s t c a t a l y s t , but i s s u f f i c i e n t to reduce the h a n d l i n g time from 16 t o 4 hours ( 4 ) . 73-18 mixes may be r e f r i g e r a t e d f o r short p e r i o d s and reused, but a f t e r 24 hours a t -10°C 73-19 v i s c o s i t y has been i n c r e a s e d beyond a u s e f u l l i m i t as shown i n f i g u r e 2* The v i s c o s i t y change w i t h time a f t e r r e f r i g e r a t i o n i s I d e n t i c a l t o the 40°C f r e s h l y mixed adhesive except that i t i s s h i f t e d upward about an order of magnitude* Note that the dynamic v i s c o s i t y curves of f u l l y cured 73-19 a t 100°C and 80°C a r e lower than the curve a t f u l l cure f o r 60 °C. T h i s i s probably caused by s i d e r e a c t i o n s of the isocyanate o c c u r i n g a t the higher cure temperatures which have a d i l a t o r i o u s e f f e c t on the mechanical p r o p e r t i e s * The initial viscosity τ?(0,Τ) of the unreacted prepolymer/curing agent mixture w i l l vary i n v e r s e l y w i t h temperature a c c o r d i n g t o equation ( 1 ) (13,14). η(0,Τ) - A exp (ΔΕ/RT)

(1)

where A and R a r e constants, Δ Ε i s the a c t i v a t i o n energy f o r v i s c o u s f l o w , and Τ i s the temperature* Semilog p l o t s of the i n i t i a l v i s c o s i t i e s of the 73 s e r i e s polyurethanes versus



174


40 Time

80 (minutes)

F i g u r e 1. The dynamic v i s c o s i t y (G"/w i n p a s c a l s e c o n d s ) o f H a l t h a n e 73-18 segmented p o l y u r e t h a n e a d h e s i v e i n c r e a s e s w i t h time u n t i l the chemorheology of cure i s complete. Cure c u r v e s f o r f o u r d i f f e r e n t t e m p e r a t u r e s show a p o s i t i v e t e m p e r a t u r e c o e f f i c i e n t w h i c h s h o u l d be p r o p o r t i o n a l t o t h e o v e r a l l r e a c t i o n rate.



11.

HOFFMAN


40 Time

80

175

120

(minutes)

F i g u r e 2. Examining the cure chemorheology of Halthane 73-19 p o l y u r e t h a n e s r e v e a l s t h e e f f e c t o f t e m p e r a t u r e on i n i t i a l v i s c o s i t y , c o l d s t o r a g e on s h e l f l i f e , and s i d e r e a c t i o n s on f i n a l cured p r o p e r t i e s .


176

CHEMORHEOLOGY OF THERMOSETTING P O L Y M E R S

r e c i p r o c a l temperature are g i v e n I n f i g u r e 3* The a d d i t i o n of 5% quadrol^ a v i s c o u s t e t r a f u n c t i o n a l alcohol» Increases the i n i t i a l v i s c o s i t y of 73-18 and 73-19 above that of 73-15. As expected, the i r o n c a t a l y s t does not a f f e c t the i n i t i a l v i s c o s i t i e s * The a c t i v a t i o n energies f o r v i s c o u s f l o w are 10 ± 2 Kcal/mole f o r these adhesives* S c a t t e r I n the data i s h i g h s i n c e a t h i g h e r temperatures the v i s c o s i t y i n c r e a s e s very rapidly and e x t r a p o l a t i o n t o zero time i s d i f f i c u l t * Assuming the v i s c o s i t y , r?(t,T), change i s directly p r o p o r t i o n a l t o p o l y m e r i z a t i o n under steady s t a t e c o n d i t i o n s , equation (2) i s expected to h o l d (13,14)*


[dlnT?(t,T)/dt] - A' exp (-ΔΕ /RT)

(2)

where t i s the time, Τ i s temperature, A' and R a r e constants and ΔΕ i s the energy of a c t i v a t i o n of the o v e r a l l c u r i n g process* The s l o p e of the e a r l y time p o r t i o n of the dynamic v i s c o s i t y versus time curves of the 73 s e r i e s urethanes i s p l o t t e d i n f i g u r e 3* E s t i m a t i o n of the r a t e constants from the i n i t i a l slopes of the v i s c o s i t y - t i m e curves r e s u l t s i n a c t i v a t i o n energies of 8-10 Kcal/mole* These r e s u l t s are too low compared to values from k i n e t i c measurements on model systems (15) and IR and thermal measurements on segmented polyurethanes (16)* The reason f o r t h i s appears to be the i n f l u e n c e o f molecular weight on the v i s c o s i t y (17)* The e f f e c t of the f e r r i c a c e t y l a c e t o n a t e c a t a l y s t i s t o i n c r e a s e dlnrç/dt o r the r a t e constant by about a f a c t o r of 2. Whereas the i n i t i a l v i s c o s i t y i s a f f e c t e d by the a d d i t i o n of 5% v i s c o u s quadrol making 73-18 and 73-19 comparable, the k i n e t i c s of cure are a f f e c t e d by c a t a l y s t and thus 73-15 and 73-19 f a l l on the same l i n e w h i l e the uncatalyzed 73-18 has lower a c t i v a t i o n energy and r a t e constants (see f i g u r e 3)· Another s u r p r i s i n g r e s u l t i s that no maximum occurs i n e i t h e r the l o s s modulus o r tangent d e l t a d u r i n g cure of these systems a t temperatures above 40°C (see f i g u r e 4)· One would expect that the onset of g e l a t i o n would produce maxima as has been observed f o r epoxy systems (18) and peroxide cured polysiloxanee (19)* V i t r i f i c a t i o n of the hard segments should a l s o produce peaks i n the l o s s modulus and/or t a n d e l t a ( 2 0 ) . The v i t r i f i c a t i o n of MDI - b u t a n e d l o l hard segments has been reported (21) i n systems s i m i l a r t o ours d u r i n g p o l y m e r i z a t i o n as i n d i c a t e d by DSC. I n the 73 s e r i e s adhesives d i f f e r e n c e s i n G" and t a n d e l t a would be expected above and below the hard segment g l a s s t r a n s i t i o n temperature. Above the hard segment t r a n s i t i o n (60°C) v i t r i f i c a t i o n would not be observed. We a t t r i b u t e the broad maximum s i x t y minutes i n t o the 40°C l o s s curve (shown i n f i g u r e 4) t o hard segment v i t r i f i c a t i o n . There i s e x c e s s i v e s c a t t e r i n the data i n t h i s r e g i o n probably due t o poor s e g r e g a t i o n of the MDI-BDO hard segments. Above the hard segment g l a s s t r a n s i t i o n temperature no v i t r i f i c a t i o n peak would be expected but the g e l a t i o n peak should be observed and i s n o t .


HOFFMAN


177


11.

2.6

2.8

3.0 1/T

X 10

3

3.2

3.4

1

(V )

F i g u r e 3. P l o t s of the l o g of the i n i t i a l v i s c o s i t y v e r s u s i n v e r s e temperature e x h i b i t A r r h e n i u s behavior w i t h a c t i v a t i o n e n e r g i e s f o r v i s c o u s f l o w s o f a b o u t 10 K c a l / m o l e . P l o t s of the change i n v i s c o s i t y w i t h t e m p e r a t u r e v e r s u s i n v e r s e t e m p e r a t u r e have n e g a t i v e s l o p e and a c t i v a t i o n e n e r g i e s f o r o v e r a l l c u r i n g k i n e t i c s o f 8-10 Kcal/mole.


CHEMORHEOLOGY OF THERMOSETTING

178

POLYMERS


10^

o.i ι

, 20

40

60

80

100

Time (min) F i g u r e 4. The l a r g e b r o a d "maximum" i n t h e l o s s t a n g e n t i s o t h e r m o f 73-19 p o l y u r e t h a n e a d h e s i v e a t 40*C i s a t t r i b u t e d t o h a r d segment d e v e l o p m e n t . T h i s peak i s n o t o b s e r v e d above 50*C b e c a u s e t h e h a r d segment g l a s s t r a n s i t i o n t e m p e r a t u r e has been e x c e e d e d .



11.

HOFFMAN


179

C u r r e n t l y the reason f o r the absence of maxima i n the dynamic mechanical p r o p e r t i e s i s not known* One f u r t h e r phenomena observed i n a l l 73 systems i s the decrease i n o p a c i t y on c u r i n g a t e l e v a t e d temperatures* Above about 60°C the poly(tetramethylene g l y c o l ) and excess isocyanate become m i s c i b l e * T h i s m i s c i b l l i t y may be a s s i s t e d by the f a c t that the MDI-BDO hard segments are above t h e i r g l a s s t r a n s i t i o n temperature* To an extent which has not been q u a n t i f i e d as y e t , l i q u i d - l i q u i d phase s e p a r a t i o n of MDI and MDI terminated p o l y o l i n the prepolymer a t low temperatures p r e s i s t s i n t o the f i n a l adhesive* The dynamic mechanical behavior of transparent or opaque adhesive >i*e*, cured a t 60-100 °C compared to room temperature are v i r t u a l l y i d e n t i c a l * S i m i l a r i m m i s c i b i l l t y has been observed i n other prepolymers (20)* T h i s does not appear t o a d v e r s e l y a f f e c t the adhesive p r o p e r t i e s of these Halthanes* 87 and 88 (HMDI/ Aromatic diamine/ P o l y o l ) Systems. The 87 and 88 s e r i e s adhesives c o n t a i n aromatic - c y c l o a l i p h a t i c polyurea hard segments which have a c o n s i d e r a b l y higher g l a s s t r a n s i t i o n temperature ( 190 C ) * The s o f t segments are 2000 molecular weight macro g l y c o l s which reduce the s o f t segment g l a s s t r a n s i t i o n temperature to -77°C ( 2 ) . The h i g h polyurea hard segment g l a s s t r a n s i t i o n temperatures extend the e l a s t o m e r i c behavior of these adhesives to f a i r l y h i g h temperatures f o r polyurethanes. The Increased i n c o m p a t i b i l i t y of the polyurea hard segment w i t h the p o l y o l s o f t segments tends to i n c r e a s e the s t i f f n e s s and s t r e n g t h of these adhesives compared to the 73 systems* Our r e s u l t s are s i m i l a r to other l i t e r a t u r e r e s u l t s (22-25). In f a c t , a l i p h a t i c diamines are e q u a l l y e f f e c t i v e f o r o b t a i n i n g h i g h g l a s s temperature hard segments according to P a i k Sung, e t . a l . (25) . F i g u r e s 5 and 6 are i s o t h e r m a l v i s c o s i t y time curves f o r Halthanes 87-1 and 88-2* Again the I n i t i a l v i s c o s i t y depends i n v e r s e l y on temperature* The h i g h e r c o n c e n t r a t i o n of hard segment formers i n 88-2 causes more r a p i d cure compared to 87-1. S u b s t i t u t i o n of e t h y l groups on the 2 p o s i t i o n s of 4,4' methylene dianiline (XU-205) in 87-2 for 4,4' methylene dianiline/m-phenylene diamine (tonox) i n 87-1 has very l i t t l e i n f l u e n c e on the v i s c o s i t y isotherms and so the 87-2 curves are not shown. From equation (1) the i n i t i a l v i s c o s i t y decreases with increasing temperature according to an Arrhenius r e l a t i o n s h i p w i t h a c t i v a t i o n energies f o r v i s c o u s f l o w of 9-12 Kcal/mole (see f i g u r e 7)· Because of the r a p i d r e a c t i o n r a t e a t h i g h temperatures accurate viscosities are difficult to determine. Apparent a c t i v a t i o n energies of 6.6 - 7*6 Kcal/mole from the i n i t i a l slopes of the v i s c o s i t y curves w i t h i n v e r s e temperature, as per equation ( 2 ) , are below the values expected f o r these systems (15,20) and f u r t h e r study i s needed i n t h i s area. e



180


40 Time

80 (minutes)

POLYMERS

120

F i g u r e 5. The change i n dynamic v i s c o s i t y o f H a l t h a n e 87-1 segmented p o l y ( u r e a - u r e t h a n e ) a d h e s i v e w i t h t i m e i n c r e a s e s d r a m a t i c a l l y w i t h temperature. Since the i n i t i a l v i s c o s i t y decreases w i t h temperature, the p l o t c r o s s through each o t h e r .



11.

HOFFMAN

181


40 Time

80

120

(minutes)

F i g u r e 6. B e c a u s e o f t h e r a p i d i n c r e a s e i n dynamic v i s c o s i t y o f H a l t h a n e 88-2 p o l y ( u r e a - u r e t h a n e ) a d h e s i v e c a u s e d by a h i g h e r c o n c e n t r a t i o n o f HMDI- a r o m a t i c d i a m i n e c h a i n e x t e n d e r , t h e i n i t i a l v i s c o s i t i e s are d i f f i c u l t to determine a c c u r a t e l y .



182


V

88-2

•

87-2

Π 2.8

3.0 1/T X 10

3.2 3

3.A

CK" ) 1

F i g u r e 7. The i n v e r s e t e m p e r a t u r e dependence o f i n i t i a l v i s c o s i t y and d i r e c t dependence o f c u r e c h e m o r h e o l o g y f o r poly(urea-urethane) adhesives y i e l d a c t i v a t i o n energies of 9-12 K c a l / m o l e f o r v i s c o u s f l o w and 6-8 K c a l / m o l e f o r o v e r a l l cure, respectively.



11.

HOFFMAN


183

As pointed out previously, dynamic mechanical loss measurements should show G" and tan d e l t a peaks d u r i n g g e l a t i o n and hard segment v i t r i f i c a t i o n * We have a r b i t r a r i l y i n t r o d u c e d peaks i n the s c a t t e r of the tan d e l t a data shown i n f i g u r e 8 f o r the 87-1 system* U n f o r t u n a t e l y , the s c a t t e r i n g behavior i s not c o n s i s t e n t appearing to give peaks i n the 80°C and 40°C t a n d e l t a curves but not i n the 60°C or 100°C (not shown) curves. Neither can the room temperature l o s s curve data be r e s o l v e d i n t o a peak* I t would be e q u a l l y accurate to draw the l o s s curves f o r t h i s system without maxima* Perhaps w i t h a more s e n s i t i v e transducer the e a r l y time l o s s behavior of these r a p i d c u r i n g amine systems could be resolved* I n the 87 and 88 polyurea-urethanes macrophase s e p a r a t i o n does not occur, i . e . , the components are m i s c i b l e , and the broad maxima found i n the MDI-BDO systems cured below the hard segment g l a s s t r a n s i t i o n are not observed* Again i n these systems no c l e a r evidence of g e l a t i o n or v i t r i f i c a t i o n i s found from our dynamic mechanical measurements* Conclusions I n v e s t i g a t i o n i n t o the dynamic v i s c o s i t y change on c u r i n g of seven segmented polyurethane polymers has shown t h a t i n i t i a l v i s c o s i t i e s decrease w i t h temperature w h i l e the r a t e of cure and change i n v i s c o s i t y depend d i r e c t l y on temperature* The polyurea hard segment (87 and 88) systems cured more r a p i d l y than polyurethane hard segment (73) systems* By u s i n g i r o n c a t a l y s t s the cure r a t e of the polyurethane hard segment systems begins to approach that of the 87 and 88 systems. For an as yet undetermined reason no d e f i n i t e evidence of gelation or v i t r i f i c a t i o n was found i n the dynamic mechanical measurements on these systems d u r i n g cure even though the hard segment g l a s s t r a n s i t i o n s f o r the polyurea systems are w e l l above the cure temperature. The observed l i q u i d l i q u i d phase s e p a r a t i o n i n 73 systems a t low c u r i n g temperatures i m p l i e s the presence of an upper critical solution temperature and/or chemical c o m p a t i b i l i z a t i o n of the MDI terminated p o l y e t h e r d u r i n g cure a t the h i g h e r temperatures. A l l of these cured adhesives have low to i n t e r m e d i a t e moduli over a wide temperature range, bond r a p i d l y and w e l l to most s u b s t r a t e s , and can be cured a t room temperature or accelerated with heat in modestly dry environments. At cure temperatures above 60-80 °C some s i d e reactions may occur reducing the p r o p e r t i e s of the cured adhesives.



,


10

POLYMERS

Time (min) 20

40

60

80

100

Figure 8. The s c a t t e r i n t h e l o s s t a n g e n t dai.a f o r 8 7 - 1 p o l y ( u r e a - u r e t h a n e ) makes i d e n t i f i c a t i o n o f t h e o n s e t o f g e l a t i o n o r h a r d segment v i t r i f i c a t i o n i m p o s s i b l e . A l t h o u g h we have drawn p e a k s i n some o f t h e i s o t h e r m s , S t a t i s t i c a l f i t s do n o t j u s t i f y them.


11.

HOFFMAN

185


Acknowledgments

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W-7405-Eng-48.

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Literature Cited 1. Estes, G.M.; Cooper S.L.; Tobolsky, A.V. J. Macromol. Sci., Rev. Macromol. Chem. 1970, C4, 167. 2. Hoffman, D.M. in "Urethane Chemistry and Applications," Edwards, K.N. Ed. ACS SYMPOSIUM SERIES ACS: Washington, D.C., 1981; p. 343. 3. Hoffman, D.M.; Hammon, H.G.; Althouse, L.P. SPE Ann. Tech. Papers 1981, 39, 314. 4. Hammon, H.G.; Althouse, L.P.; Hoffman, D.M. "Development of Halthane Adhesives for Phase 3 Weapons: Summary Report"; UCRL- 52943, December, 1980. 5. Prime, R.B. in "Thermal Characterization of Polymeric Materials" Turi, E.A., Ed.; Academic : New York, 1981, Chap. 3. 6. Collins, E.A.; Bares, J.; Billmeyer, F.W., Jr., "Experiments in Polymer Science" John Wiley: New York, 1973, Chap. 5. 7. Rheometrics Mechanical Spectrometer Operating Manual, Rheometrics Inc., 1974. 8. Middleman, S. "The Flow of High Polymers"; John Wiley: New York, 1968.



186

CHEMORHEOLOGY

OF THERMOSETTING

POLYMERS

9. Kajiyama T.; MacKnight, W.J. Trans. Soc. Rheol. 1979, 13, 527. 10. Huk D.S.; Cooper,S.L. Polym. Eng. S c i . 1971, 11, 369. 11. Schollenberger C.S.;. Dinbergs, K. J. Polym. S c i . Symposium, 1978, 64, 315. C.S. Schollenerg, i n "Multiphase Polymers", Cooper, S.L.; Estes, G.M., Eds.; ADVANCES i n CHEMISTRY SERIES No. 176, ACS: Washington, D.C., 1979; p. 83. 12. Dzierza, W. J. Appl. Polym. S c i . , 1978, 22, 1331. 13. Roller, M.B.; Polym. Eng. S c i . , 1975, 15, 406. 14. White, R.B., Jr., Polym Eng. S c i . 1974, 14, 50. 15. Reegen, S.L.; Frisch, K.C.; Adv. i n Urethane S c i . and Techno1., 1971, 1, 1. 16. Richter, E.B.; Macosko, C.W. Polym. Eng. S c i . , 1978, 18, 1012. 17. L i p s h i t z , S.D.; Macosko, C.W. Polym. Eng. S c i . , 1976, 16, 803. 20. Gillham, J.K. Polym. Eng. S c i . , 1979, 19, 676. Enns, J.B.; Gillham, J.K. ACS Org. Coatings Appl. Polym. S c i . Prepr., 1982, 46, 592. 21. Hager S.L.; MacRury, T.B.; Gerkin, R.M.; C r i t c h f i e l d , F.E. i n "Urethane Chemistry and Applications", Edwards, K.C., Ed. ACS SYMPOSIUM SERIES No. 172, ACS: Washington, D.C., 1982; p. 149. 22. Work, J.L. Macromol., 1976, 9, 759. 23. I l l i n g e r , J.L.; Schneider, N.S.; Karasz, F.E. Polym. Eng. s c i . , 1972, 12, 25. 24. Paik Sung, C.S.; Hu,H.B.; Wu, C.S. Macromol. 1980, 13, 117, 111. 25. Van Bogart, J.W.C.; Lipaonitkal, Α.; Cooper, S.L. i n "Multiphase Polymers", Cooper, S.L.; Estes, G.M., Eds.; ACS Advances i n Chemistry Series No. 176, ACS: Washington, D.C., 1979, p. 1. RECEIVED

March 31, 1983


Chemorheology of Thermosetting Polymers - American Chemical

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