6 Polyamidation in the Solid Phase R. J. G A Y M A N S and J. S C H U I J E R
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Twente University of Technology, Department of Chemical Technology, P.O. Box 217, Enschede, The Netherlands
Polyamides can be polymerized i n the solid-phase i n an oxygenf r e e atmosphere a t a temperature range of 20 - 160 C below t h e i r f i n a l melting p o i n t (1-9). The r e s u l t s from the l i t e r a t u r e are not easy to i n t e r p r e t due to the l i m i t e d temperature ranges, small v a r i a t i o n s i n p a r t i c l e s i z e s and the occurrence of s i d e r e a c t i o n s with chain branching. We s t u d i e d the polyamidation i n the s o l i d phase process of nylon 4,6, which has a high melting t r a n s i t i o n (264 - 320 C) and does not show any tendency to g e l (10). The r a t e of the s o l i d phase p o l y m e r i z a t i o n (SPP) depends on -
the the the the the
k i n e t i c s o f the chemical r e a c t i o n d i f f u s i o n of the r e a c t i v e groups d i f f u s i o n o f the condensate out of the p a r t i c l e d i f f u s i o n a t the p a r t i c l e - gas i n t e r f a c e heat t r a n s f e r
The p o l y m e r i z a t i o n r a t e i s c o n t r o l l e d by the slowest process. Thus i t i s important to e s t a b l i s h the r a t e c o n t r o l l i n g steps. The s t a r t i n g m a t e r i a l f o r the (SPP) can be the dry n y l o n s a l t (3,4) but mostly a low or middle molecular weight polymer i s used. The polyamide-salts have the disadvantage of high amine l o s s e s (3^4). Griskey (5) and Chen (6) s t u d i e d the r e a c t i o n of nylon 6,6 and 6,10 i n a SPP i n a stream of dry n i t r o g e n i n the temperature range of 90° - 180°C. They found that the r e a c t i o n l i m i t i n g step was n o t the d i f f u s i o n of water but the chemical r e a c t i o n . The k i n e t i c r e l a t i o n s h i p they observed was Mn = k t
n
(1)
n = 0,5 and 1.0 f o r nylon 6,6 and nylon 6,10, r e s p e c t i v e l y . The a c t i v a t i o n energies of the rateconstant k are r e s p e c t i v e l y 10.5- 12.96 and
0-8412-0506-x/79/47-104-137$05.00/0 © 1979 American Chemical Society Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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POLYMERIZATION REACTORS AND PROCESSES
13.2 kcal.mol." . Monroe ( 7 ) , r e p o r t i n g on nylon 66, found a dramatic e f f e c t o f the s t a r t i n g molecular weight on the r e a c t i o n r a t e . I n c r e a s i n g the s t a r t i n g molecular weight by a f a c t o r o f two decreased the r e a c t i o n time to reach M = 15,000 by a f a c t o r two. Zimmerman (8), comparing SPP and m e l t - p o l y m e r i z a t i o n , showed that i n the presence of water the SPP leads to much higher molecular weights a t a g i v e n pressure than the m e l t - p o l y m e r i z a t i o n . A t the same time he n o t i c e d a broadening i n the molecular weight d i s t r i b u t i o n (m.w.d.) o f nylon 6,6. Ramsey and D u n n i l l (9) reported the formation of h i g h l y branched s t r u c t u r e s i n nylon 6,6 by r e a c t i n g under anhydrous c o n d i t i o n s . According to them t h i s could be prevented by r e a c t i n g under a blanket o f super-heated steam. A t h e o r e t i c a l study o f the m.w.d. broadening during the SPP of a s e m i - c r y s t a l i n e polymer showed that f o r l i n e a r s t r u c t u r e s , according to the S c h u l z - F l o r y r e l a t i o n s h i p , no narrowing o r broadening o f the m.w.d. i s to be expected (11). The k i n e t i c s o f the m e l t - p o l y m e r i z a t i o n o f nylon 6,6 i s t h i r d order (1) "
d [
"S? dt
Q H ]
"
([-COOH] [-NHJ - [-C00H] C-NHj ) (2) \ 2 eq 2 eq/
[-C00HI
The r a t e constante k has an a c t i v a t i o n energy of 16.8 kcal.mol"" In water f r e e c o n d i t i o n s equation 2 can be s i m p l i f i e d to
.d^opa
=
k
^
>
( 3 )
I f the polymer i s balanced w i t h equation i s
([-C00H]J ~([-C00H]j
=
2
k
(12).
1
E-COOH ] = [-NH„]the i n t e g r a t e d
t
(
4
)
For an unbalanced polymer w i t h [-C00H] - [-NH^] = D and the assumption that D has a constant value r e s u l t s i n the i n t e g r a t e d equation 1 D
n l
n
[-C00H] [- NH ] 2
"
1 [-C00H]
=
D
k
t+
C O n S t
(
-
5
)
The SPP r e a c t i o n i s not n e c e s s a r i l y t h i r d order. I f the endgroups are unbalanced a good approximation o f the SPP r e a c t i o n order can be obtained by expressing i t as f u n c t i o n of /[-COOH] [-NE ] = /P 2
The r a t e o f r e a c t i o n i s then d [-C00H] dt
k.
(/[-COOH] [-NH^ )
n
=k.(/p)
n
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
(6)
6.
GAYMANS AND SCHUIJER
Polyamidation
in the
Solid
Phase
139
The i n t e g r a t e d form with the unknown order n i s then (7)
I f the endgroups are balanced. = > "7?
the equation i s (8)
For e v a l u a t i o n purposes
i t i s changed to
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(9) where a i s chosen as near as p o s s i b l e to (n-1). We studied the polyamidation of nylon 4,6, and v a r i e d the r e a c t i o n time, r e a c t i o n temperature, p a r t i c a l s i z e , s t a r t i n g molecular weight, and type of r e a c t o r gas. At the same time we looked at the molecular weight broadening and the degradation with colour formation. In order to have good heat and mass t r a n s f e r the r e a c t i o n s were mainly conducted on f i n e powder i n a f l u i d i z e d bed r e a c t o r and with dry n i t r o g e n as c a r r i e r gas. EXPERIMENTAL The s t a r t i n g m a t e r i a l s were low molecular weight polymers prepared by r e a c t i n g 1,4 diaminobutane and a d i p i c a c i d f o r two hours at 220°C i n a capsule i n an autoclave (10). The low molec u l a r weight m a t e r i a l was powdered by crushing and b a l l m i l l i n g . The f l u i d i z e d bed r e a c t i o n s were c a r r i e d out i n a g l a s s r e a c t o r ( f i g . 1) 2.5 cm diameter and 50 cm long. The r e a c t o r was heated i n an oven i n which the temperature could be c o n t r o l l e d w i t h i n 0.2°C. As c a r r i e r gases dry n i t r o g e n and super-heated steam, both at a pressure of 1 bar, were used. The gas v e l o c i t y was 4,0 cm. sec f o r both gases. The samples were f l u s h e d from the sample-holder i n t o the pre-heated r e a c t o r and reached temperature w i t h i n a minute . The r e a c t i o n s were stopped by removing the r e a c t o r from the oven. For comparison, we c a r r i e d out some r e a c t i o n s under vacuum i n a r o t a t i n g 50 ml f l a s k . The f l a s k was attached to a rotavap apparatus and heated i n a s i l i c o n o i l bath, the vacuum a p p l i e d was *3 mbar. Endgroup analyses were c a r r i e d out with an automatic p o t e n t i o meter. The [-NH ] and [-C00H] were determined simultaneously. The polymer was d i s s o l v e d i n o - c r e s o l / c h l o r o f orm mixture (70/30), excess a l c o h o l i c KOH added and t i t r a t e d with a l c o h o l i c HC1 (0.1 N). The inherent v i s c o s i t i e s C ^ ^ ) were determined i n 0.5% s o l u t i o n s i n 90% formic a c i d . We observed the f o l l o w i n g r e l a t i o n s h i p : 2
1
log M
n
= 1.122
l o g n. u + 4.182 inn
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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140
POLYMERIZATION REACTORS AND PROCESSES
Table I
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Temp (°C)
Reaction time (hours)
Prepolyiner A
-
-
220
.83 3.0 6.0 21.5
Prepolymer B
190
-
205
235
250
265
280
Reaction Data Endgroup a n a l y s i s
Viscosimetry
1
inh
M
n
[COOHl Oral , eq/10 gr eq/TO^ gr (VP)-'
0,177 2,175 0.377 8,200 0,823 12,200 0,917 13,800 1,444 23,000
43.50 9.78 6,45 4,32 4,00
58.90 13.56 11.92 10.17 6.12
1,960 8,700 11,400 15,100 20,200
0,165 2,010 0,374 5,000 0,442 6,100 0.438 6,000 0.575 8,200 0,728 10,600
48,20 19,36
57,80 18.49
1,890 5,300
-
-
13.85
1 2 4 6.75
0.429 6,800 0*564 8,000 0.661 8,600 0.731 10,700
13,73
15.48
-
-
1 2,25 4 8
0,732 0.866 1.024 1.218
1 4 7
0.900 13,500 1.408 22,300 1.601 25,800
1 2 4 8
1.220 1.420 1.658 2.461
1 2 4
1.508 24,100 1.769 28,900 2,717 46,800
8
2.728 46,000
1 2 4 8 24
12.22
6,80
5,19 5,26
6,900
11,300
-
-
19,000 22,500 26,700 41,800
7,700
-
11,58
10,700 13,000 15,600 19,000
-
-
4,09 2,52
_
_
-
-
-
-
21,700 27,500
--
— From u.c. M = 49 ,000 n ^ = 61,000 M
= 74,000
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
-
-
—
6.
GAYMANS AND scHuijER
Polyamidation
in the
Solid
141
Phase
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With t h i s r e l a t i o n s h i p f o r a l l samples was c a l c u l a t e d from ninh* This M i s used f o r e v a l u a t i n g the r e a c t i o n data. The u l t r a c e n ? r i f u g e (u.c^ measurements were c a r r i e d out i n a Spinco model E a n a l y t i c a l u l t r a c e n t r i f u g e , with 0.4% s o l u t i o n s i n 90% formic a c i d c o n t a i n i n g 2.3 M KC1. By means of the sedimentat i o n ^ d i f f u s i o n e q u i l i b r i u m method of Scholte (13) we determine M , M and M . The buoyancy f a c t o r (1- vd = -0.086) necessary f o r tSe c a l c u l a t i o n of these molecular weights from u l t r a c e n t r i f u g a t i o n data was measured by means of a PEER DMA/50 d i g i t a l d e n s i t y meter. U.V. absorptions were measured on 0.5% s o l u t i o n s i n 90% formic a c i d at 290 mn. RESULTS AND
DISCUSSION
Reaction k i n e t i c s . The r e s u l t s of the r e a c t i o n of f i n e l y devided powdered polymer (0.1
