Design of polymer blend rheology

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Polymer Bulletin 33, 221-228 (1994)

Polymer Bullelin

9 Springer-Verlag 1994

III. Effect of maleic anhydride containing copolymers on the melt viscosity of polyamides

R. Vankan, P. Degee, R. Jer6me, and P. Teyssie

Center for Education and Research on Macromolecules (CI=RM), University of Liege,

Sart-Tilman, B6, B-4000 Liege, Belgium

S u m m a r y

Addition of small amounts of maleic anhydride containing copolymers remarkably increases the melt viscosity of the m x D,6 polyamides (PA). Random copolymers of styrene and maleic anhydride, P(S-co:MA), have been melt blended with PA in a ratio ranging from 0.5 to 5 wt%. A linear dependence has been observed for the blend viscosity on the molar ratio of the maleic anhydride subunits in the copolymer and the amino end-groups of PA. This linear relationship holds even for blends containing a molar excess of anhydride compared to amine. These rheological effects are of prime importance for the highly desirable control of the PA processing.

I n t r o d u c t i o n

Nowadays a great research effort is devoted to polymer blends and alloys (PBA). This very fast shift in the plastics industry towards multicomponent and usually multiphase systems is due to the prohibitively high cost for the development of new polymers. Blending is a more straightforward and less expensive technology, whereas PBA properties can be designed to meet the large spectrum of customer demands. In this respect, processing plays a decisive role on the ultimate properties of PBA (1,2). Furthermore, in the range of extreme compositions for multiphase PBA, size and shape of the minor phase (3,4) are controlled by a complex interplay of blend composition, melt viscosity ratio, interfacial tension, shear stress, elasticity ratio and the traditional processing parameters (5o15). As an example, when the viscosity ratio between dispersed and continuous phases, Tlr = 11d/~lc , highly exceeds 2, it is quite a problem to disperse finely the minor phase. There is rather formation of a cocontinuous two-phase morphology, which might be detrimental to the ultimate mechanical properties, because of delamination problems (7). The use of graft copolymers, P(A-g-B), hopefully formed by a reactive processing, is a convenient way to cope with this problem. Numerous examples are now available which support that block and graft copolymers can significantly enhance the interfacial adhesion. This strategy is particularly suitable for the melt blending of a polyamide terminated with primary amino groups and a maleic anhydride containing polyolefin used as a compatibilizer in ternary blends (16-24).

This paper aims at reporting on the deep change in theology of m x D,6 polyamides (PA) when melt blended with small amounts (0.5-5 wt%) of styrene-

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maleic a n h y d r i d e r a n d o m copolymers, P(S-co-MA), of v a r i o u s compositions. The s e m i - c r y s t a l l i n e P A u n d e r c o n s i d e r a t i o n r e s u l t s from the step-growth p o l y m e r i z a t i o n of m-xylene d i a m i n e a n d adipic acid. They display high m e c h a n i c a l p r o p e r t i e s a n d a low N e w t o n i a n melt viscosity (~1260oc = 170 Pa.s). However, the high fluidity of PA h a s a d v e r s e effects on polymer molding. A previous s t u d y h a s shown t h a t small a m o u n t s of m e t h y l m e t h a c r y l a t e - m e t h a c r y l i c acid r a n d o m copolymers, P(MMA-co-MAA), can efficiently i n c r e a s e the m e l t flow r e s i s t a n c e of PA (25). This beneficial effect h a s been accounted for by a c o n d e n s a t i o n r e a c t i o n of neighbouring carboxylic acid groups of the a d d i t i v e into a n h y d r i d e s able to r e a c t with the p r i m a r y amino end-groups of PA a n d f o r m a t i o n of g r a f t copolymers. Based on the evidence t h a t carboxylic acid groups a r e less r e a c t i v e t o w a r d s a m i n o end-groups t h a n maleic a n h y d r i d e units (26), a f a s t e r a n d m o r e q u a n t i t a t i v e r e a c t i o n is expected from the a d d i t i o n of maleic a n h y d r i d e c o n t a i n i n g copolymers to polyamide chains. Accordingly, within the time of blending, a b e t t e r i m p r o v e m e n t of t h e flow r e s i s t a n c e of PA might be predicted.

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

Materia]

Polyamides m x D,6 (PA) were supplied by Solvay (IXEF PARA 0000 : Mn = 17,000; 75 10 .6 mol-NH2/g; 55 10 .6 mol-COOH/g a n d I X E F PARA XOOO 9 Mn = 16,500; 43 10 -6 mol-NH2/g; 93 10 -6 mol-COOH/g, d e s i g n a t e d as PA-1 and PA-2, respectively). Because of hygroscopicity, PA were d r i e d a t 100~ for a t l e a s t 18 h o u r s u n d e r r e d u c e d p r e s s u r e p r i o r to melt processing. Styrene, maleic a n h y d r i d e a n d e t h y l a c e t a t e were p u r c h a s e d from J a n s s e n a n d used without f u r t h e r purification. Azo-bis-isobutyronitrile (AIBN) was p u r c h a s e d from Merck. Toluene was d r i e d by refluxing over C a l l 2 for 48 hours.

Syn thesis of_P(S-co-MA)

S t y r e n e - m a l e i c a n h y d r i d e r a n d o m copolymers, P(S-co-MA), with a MA content r a n g i n g from 0 to 22 10 .4 mol/g, were synthesized in e t h y l a c e t a t e using A/IBN (5 g/mol, of monomers) as an i n i t i a t o r . A f t e r the careful r e m o v a l of oxygen from the r e a c t i o n medium, polymerization was i n i t i a t e d a t 60~ u n d e r reduced p r e s s u r e for 24 hours. The copolymer was recovered by p r e c i p i t a t i o n in h e p t a n e a n d d r i e d up to c o n s t a n t weight. The m o l a r p e r c e n t a g e of maleic a n h y d r i d e was d e t e r m i n e d by t i t r a t i o n of the previously hydrolyzed copolymers with t e t r a m e t h y l a m m o n i u m hydroxide in a t o l u e n e - m e t h a n o l mixture. A n h y d r i d e s were h y d r o l y z e d in a 5/1 t e t r a h y d r o f u r a n - w a t e r solution, for 72 h o u r s a t 60~ Hydrolysis p r o g r e s s was followed by i n f r a r e d spectroscopy (vCOanhydr = 1856 a n d 1780 cm 1, vCOO-H = 3450 cm 1 a n d vCO acid = 1710 c m l ) .

Polymer blending

PA were mixed with P(S-co-MA) in a l a b o r a t o r y B r a b e n d e r P l a s t i c o r d e r of 60 cm 3 a t 260~ a n d 50 r p m u n t i l torque was stabilized, i.e. for a p p r o x i m a t e l y 10 minutes. F i n a l b l e n d s were compression-molded as 2 m m t h i c k sheets a t 260~ for 3 m i n u t e s u n d e r a p r e s s u r e of 15 bars.

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Measurements

IR spectra were recorded by using a Perkin-Elmer 1600 FTIR. Size exclusion c h r o m a t o g r a p h y (SEC) was performed in tetrahydrofuran at 45~ using a Hewlett-Packard 1090 liquid chromatography equipped with a HP1037A refractometer index detector and four PL GEL columns of various pore sizes (105, 104, 500 and 100 .~). Molecular weight distribution was calculated in reference to a polystyrene calibration curve. Melt viscosity of previously molded and then finely grinded blends was measured by using a Gottfert Rheograph 2001 capillary rheometer at 260~ Polymers and blends were carefully dried at 100~ for at least 18 hours before measurement. The capillary was of a 1 mm diameter, a length over diameter ratio of 20 and 180 ~ entrance angle. The force of the plunger was measured at constant crosshead speeds.

R e s u l t s a n d D i s c u s s i o n

Random copolymers P(S-co-MA) of an increasing maleic anhydride content have been synthesized by a free-radical polymerization process (Table I). The maleic anhydride, MA, composition of the previously hydrolyzed copolymers has been measured by potentiometric non-aqueous titration of the carboxylic acid groups. It must be noted that only one carboxylic acid group per MA is neutralized with t e t r a m e t h y l a m m o n i u m hydroxide (27,28).

Table I : Molecular characteristics of P(S-co-MA) random copolymers Copolymers

SMA-1

MA content (xl04 mol/g) Mn (xl0 3 g/mol)

7.4 5.0

Mw/Mn 2.5

SMA-2 16.8 4.6 2.2

SMA-3 22.0 3.0 2.5

Figure 1 shows how theology of PA-1 is modified at 260~ upon the addition of 5 wt% of P(S-co-MA) of various MA contents. In contrast to the Newtonian behavior of the neat PA-1, the addition of P(S-co-MA) promotes a sharp increase in the melt viscosity at low shear rates, which however seems to level off when the MA content exceeds ca. 20 10 .4 mol/g. Indeed, the viscosity ratio, ]~b]end/~]pA, at 10 s 1 is 24, 37 and finally 34 when the MA content of the additive is 7.4 10 4, 16.8 10 .4 and 22.0 10 .4 mol/g, respectively. For the sake of comparison, a homopolystyrene prepared by an anionic process (PS : Mn = 16,000; Mw/Mn = 1.1) has been melt blended with PA-1. It is clear that the addition of 5 wt% of PS does not significantly affect the melt viscosity of PA-1 (]]blend/T]pA = 0.9 at 10 sl). As a second effect of blending, a rheothinning behavior is observed to occur, i.e. a decrease in the PA-1/P(S-co-MA) blend viscosity upon increasing shear rates. Figure 1 : Viscosity versus shear rate for PA-1 melt blended with 5 wt% o f

P(S-co-MA) of various MA contents at 260~ (@ PA-1;: [] (PA-I-b-PS); 9 (PA-I-b-SMA-1); 9 (PA-I-b-SMA-2); O (PA-I-b-SMA-3))

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1 0 4 1000 .~. 100 <c lO 10. i 1 T I , , 1 s f i r T ' r l o o Shear ~te

(s~)

i i 1ooo

This general feature is of the utmost importance since it nicely fits the target of this research : a substantial increase in the melt viscosity at low shears

( r l b l e n d / T l p A ---- 37 at 10 s "1) while preserving a high injectability at high shear rates

(~b]end/~l'A = 2.7 at 1000 sl). This non-Newtonian power-law behavior at low frequency is usually attributed to long branching components in multiphase PBA (29,30). Accordingly, the high reactivity of PA amino end-groups towards the anhydride subunits of P(S-co-MA) more likely results in graft copolymers during the melt mixing (equ. 1). In this respect, it is worth examining whether the melt flow resistance is related to the extent of the amine-maleic anhydride reaction. The maleic anhydride to amino end-group molar ratio, (MAsMA/NH2PA.1) , is listed in table I I along with the viscosity ratio of blends of a c o n s t a n t composition (5 wt% of P(S-co-MA) additive). 260~ PA-NH 2 + " v ~ v ' C H - C H ~ l= 9 ~J~V'CH-CH,,'~,v~" + H20

o5-o)-o

' (equ. 1 ) PA

Table II : Molecular and rheological characteristics of PA-1/P(S-co-MA) blends (95:5) Blends (MAsMA/NH2PA.1) (PA-I-b-PS) 0.00 (PA-I-b-SMA- 1) (PA- 1-b-SMA-2) (PA- 1-b-SMA-3)

(T]blend/~PA.l~

at 10 s "z 0.9 0.49 24.0 1.12 37.1 1.47 34.0

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From these results, it is clear that the melt viscosity of blends increases ~dth the MA content of the additive up to a (MAsMA/NH2PA) molar ratio close to 1. When the MA content is higher than the number of available amino end-groups, the a p p a r e n t viscosity does not change anymore. This implies t h a t the reaction between amino groups and maleic anhydride subunits to form N-alkylmaleimide, is quantitative within the time devoted to melt mixing, i.e. approximately 10 minutes at 260~

In order to have a better insight into the dependence of the melt viscosity on the

(M.As~viA/NH2PA.1)

molar ratio, PA-1 and PA-2 have been melt blended with various amounts of SMA-2. Combination of two different PA and various amounts of SMA-2 (from 0 to 5 wt% with respect to PA) is indeed an easy way to modulate the maleic anhydride to primary amine molar ratio for the PA]additive pair under consideration (Table III). For the sake of a straightforward comparison, the rheological data have been plotted versus the (MAsMA/NH2p A) molar ratio in figure 2.

Table I I I : Effect of maleic anhydride to amine molar ratio on the viscosity ratio and the solubility of the extrudate in formic acid (S : soluble; I : insoluble) P A PA-I SMA-2 content (wt%) 0.0

(MAs,~A/NH2PA)

(~blend/YlpA)

at 10 s 1 0.9 0.00 Solubility S 0.5 0.11 3.2 S 0.9 0.20 7.3 S 1.2 0.27 9.4 S 3.0 0.67 23.4 I 5.0 1.12 37.1 I PA-2 0.0 0.00 1.0 S 0.6 0.23 1.9 S 1.3 0.51 3.9 S 2.5 0.98 14.1 I 5.0 1.96 27.0 I

Although PA-1 and PA-2 have the same theology, thus independent of the content of the amino and carboxylic acid end-groups, this similarity disappears when they are melt blended with the same amount of SMA-2. In the whole range of shear rates, the melt flow resistance is expectedly higher for blends of PA-1 compared to FA-2. This is in a direct connection with ~he amino functionality of PA, which is higher for PA-1 (75 10 .6 tool/g) t h a n for PA-2 (43 10 .6 mol/g). More surprisingly, figure 2 shows a linear dependence for the blend viscosity versus the amine to maleic anhydride molar ratio. This linear relationship hohLs even at (hIAs.~A/NH2PA) ratios higher than 1, which might be in contradiction ~dth the observations reported in table II. There is however a marked difference in the two series of measurements. Indeed, the (MAs~A/NH2pA) ratio has been changed while keeping constant the additive content (5 wt% of P(S-co-MA)) in table II, although data in table I I I and figure 2 have been reported for an increasing content of SM_A-2. In the latter case, the additive m a y form a dispersed phase,

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the r e l a t i v e volume of which changes p a r a l l e l to the e x t e n t of the grafting reaction.

F i g u r e 2 : Dependence of the viscosity r a t i o a t 10 s 1 on t h e maleie a n h y d r i d e to amino end-group molar r a t i o for blends of PA-1 (O) or PA-2 (A) w i t h SMA-2 a t 260~ 40 30 10 0 0 . 4 0 . 8 1 . 2 1 . 6 2 MA / NH SMA 2 PA

It m u s t also be emphasized t h a t the e x t r u d a t e s of blends c o n t a i n i n g a m o l a r excess of a n h y d r i d e s compared to a m i n e a r e insoluble in formic acid which is a good solvent for PA (Table III). This clearly i n d i c a t e s a cross-linking r e a c t i o n of PA, which is more likely due to the r e a c t i o n of some a , o - d i a m i n e PA chains with the r a n d o m s t y r e n e - m a l e i c a n h y d r i d e copolymer. This h y p o t h e s i s is feasible a t l e a s t for PA-1, t h a t contains a molar excess of a m i n e with r e s p e c t to carboxylic acid end-groups. Accordingly, p a r t of PA-1 chains a r e end-capped a t both e x t r e m i t i e s with an amino function. Nevertheless, the m a i n origin for a,o- d i a m i n o PA h a s to be found in the hydrolysis of PA, a n d p a r t i c u l a r l y of PA branches, d u r i n g the melt blending process (equ. 2). Since cross-linking is only effective when t h e extent of the a m i n e / a n h y d r i d e r e a c t i o n is close to completion (MAsMA/NH2pA m o l a r r a t i o close to 1), thus when the m a x i m u m a m o u n t of w a t e r is formed, h y d r o l y s i s is essentially promoted by the g r a f t i n g r e a c t i o n a n d w a t e r which is r e l e a s e d (equ. 1). H20 SMA - , ~ / v v - C H - C H , . . v v v . ~ ~ A , . v , CH - C H . , v v V , },, - x ~ , , ~ , C H - C H . , / v v , ~ I I a NH NH2 [ I N

oo,

oy .To

PA + PA-COOH + H20 C H - C H - ~ v v " (equ.2)

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C o n c l u s i o n s

Addition of small amounts of maleic anhydride containing copolymers is a convenient way for tailoring rheology of polyamide m x D,6 (PA). This has been shown by melt blending copolymers of styrene and maleic anhydride, P(S-co- MA), with PA in a ratio ranging from 0.5 to 5 wt%. At low shear rates, the rheofluidity of PA at 260~ is greatly reduced in relation to the molar ratio of maleic anhydride subunits in the copolymer and PA amino end-groups. Grafting of PA onto P(S-co-MA) is responsible for this increase in melt flow resistance, as a result of condensation of the primary amino end-groups of PA with the anhydride functions of the additive. Interestingly enough, blend viscosity is linearly dependent on this maleic anhydride over amine molar ratio. Hydrolysis of PA branches during melt blending is responsible for cross-linking reaction. A c k n o w l e d g m e n t

The authors are very grateful to Solvay and to the ~ Services de la Programmation de la Politique Scientifique, in the frame of the (( Poles d'attraction interuniversitaire : Polym~res ~ for financial support. They also thank Mrs Constant and Mahy for skillful technical assistance.

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Design of polymer blend rheology - III. Effect of maleic anhydride containing copolymers on the melt viscosity of polyamides

Polymer Bullelin