Toughened Polypropylene-Polyamide 6 Blends Prepared by Reactive

19 Toughened PolypropylenePolyamide 6 Blends Prepared by Reactive Blending

Downloaded by PURDUE UNIV on July 5, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch019

J. Rösch and R. Mülhaupt Freiburger Materialforschungszentrum und Institut für Makromolekulare Chemie der Albert-Ludwigs Universität, Stefan-Meier-Strasse 31, D-79104 Freiburg im Breisgau, Germany Reinforced polypropylene (PP) was prepared by blending 70 vol% PP with 30 vol% polyamide 6 (PA6) in the presence of compatibiliz ers such as maleic anhydride-grafted polypropylene and maleic an hydride-grafted rubbers. The key to this reactive blending technolo gy was the in situ formation of segmented polymers via covalent imide-coupling involving amino-terminated PA6 and succinic anhy dride functional compatibilizers. Compatibilizer volume fraction and molecular architecture gave control of PA6 dispersion and in terfacial adhesion between PP and PA6. In contrast to the simulta neous dispersion of separate PA6 and rubber microphases, the in situ formation of core-shell-type dispersed microphases, comprising a rigid PA6 core and a rubber shell, accounted for a substantially improved balance between the toughness and stiffness of the PP-PA6 (70/30) blend.

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E C E N T BREAKTHROUGHS IN CATALYST and process development have greatly simplified polypropylene (PP) production and broadened the application spectrum of PP materials, which have economic, ecological, and recycling advantages over environmentally less-friendly polymers (I, 2). Among commodity thermoplastics, only PP exhibits heat-distortion temperatures above 100 °C. In order to compete successfully with traditional materials, such as metals and engineering thermoplastics, in higher-value-in-use engineering applications, an important challenge in PP development must be met. The challenge is to overcome the limitations of the properties of PP associated with its hydrocarbon nature, namely, poor dyeability, poor adhesion, low resistance to hydrocarbon permeation, and comparatively high chain flexibility. Moreover, another important R & D objective in the development of engineering resins is to improve toughness without sacrificing stiffness and strength. One of the widely applied approaches toward reinforced PP is to incorporate stiff isotropic or 0-8412-3151-6

© 1996 American Chemical Society 291

Riew and Kinloch; Toughened Plastics II Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

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anisotropic dispersed phases, preferably inorganic fillers, into the continuous P P phase. Frequently, such stress-concentrating dispersed microphases reduce plastic deformation o f the P P matrix, giving rise to drastically lower toughness (3, 4). Therefore, b l e n d technologies are b e i n g developed to achieve property synergisms that do not reflect the properties o f the b l e n d components w h e n the mixing ratio is taken into account. Especially i n automotive applications, blends o f P P w i t h polyamides (PAs) are of particular interest for reducing polyamide water-uptake a n d i m p r o v i n g the stiffness a n d paintability o f P P (5). Because P P and P A 6 are mutually immiscible, polymeric dispersing agents, often referred to as b l e n d compatibilizers, must be added to enhance b o t h P A 6 dispersion a n d P P - P A 6 interfacial adhesion. T h e use o f compatibilizers to i m prove dispersion a n d interfacial adhesion has been demonstrated successfully for a large variety o f multiphase polymer blends (6-9). Typical P P b l e n d c o m patibilizers are maleic anhydride-grafted P P ( P P - g - M A ) and maleic anhydride-grafted rubbers, for example, ethene-propene r u b b e r ( E P R - g - M A ) or polystyrene-Z?Zocfc-poly(ethene-co-l-butene)-fefocfc-polystyrene (SEBS-g-MA). D u r i n g melt processing, succinic anhydride groups react with amino e n d groups o f P A 6 to produce segmented polymers. T h e purpose o f our research was to explore new strategies for controlled formation o f multiphase P P - P A 6 blends containing 30 v o l % P A 6 as dispersed reinforcing microphase, and to exploit the potential o f reactive processing technology a n d tailor-made b l e n d compatibilizers. P P - P A 6 (70/30) and r u b ber-modified P P - P A 6 (70/30) blends were examined to elucidate the basic p a rameters governing P A 6 microphase dispersion, interfacial adhesion, stress transfer, a n d impact-energy dissipation. Moreover, the influence o f reactive a n d unreactive compatibilizing rubbers was studied to correlate the molecular architecture and volume fraction of functionalized P P and rubbery b l e n d c o m patibilizers w i t h the mechanical and morphological properties o f rubber-modified P P - P A 6 (70/30) blends. A n o t h e r objective o f this research was to compare the b l e n d architectures o f rubber-modified P P - P A 6 (70/30) blends comprising either (I) separately dispersed P A 6 and r u b b e r microphases, or (2) i n situ f o r m e d core-shell-type microphases w i t h a P A 6 core and a covalently b o n d e d r u b b e r shell.

Experimental Details M a t e r i a l s . A l l polymers were commercially available and used without further purification. P P was purchased from Hoechst A G (Hostalen P P N 1060, number-average molecular weight (M ) = 63,000 g/mol, weight-average molecular weight ( M ) = 182,700 g/mol, as determined by size exclusion chromatography ( S E C ) i n 1,2,4-trichlorobenzene at 135 °C using a polystyrene standard, melt flow index ( M F I ) (230/2,16) = 2 dg/min, and melting temperature ( T J = 165 °C). N o n functionalized E P R (Exxelor V M 2 2 , M F I (230/2,16) = 5 dg/min, T = 56 °C), maleic anhydride-grafted P P (Exxelor PO2011, 0.031 mol anhydride/kg polymer, M F I (230/2,16) = 125 dg/min), and maleic anhydride-grafted E P R (Exxelor V A 1803, n

w

g


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RÔSCH AND MULHAUPT.

Polypropylene-Polyamide

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60 mol anhydride/kg polymer, M F I (230/2,16) = 3 dg/min, glass-transition temperature (T ) = -52 °C) were products of Exxon Chemicals. Maleic anhydride-grafted S E B S was supplied by Shell (Kraton G1901 X 2 , 208 mol anhydride/kg S E B S , M F I (230/2,16) = 3.2 dg/min). PA6 was obtained from Snia, Milano (Sniamid A S N 27, T = 222 °C, 0.031 m o l amine end groups/kg PA6). g


m

R e a c t i v e B l e n d i n g a n d C h a r a c t e r i z a t i o n . PA6 was dried for 6 h at 80 °C under oil-pump vacuum prior to use. M e l t blending was performed using a Haake Rheomix 90 twin-screw kneader equipped with a 60-ml mixing chamber and on-line temperature and torque recording. In a typical run, after preheating the mixing chamber at 240 °C for 10 min, 40 g of the blend composed of PP, PA6, P P - g - M A , and 0.2 g stabilizer mixture (80 wt% Irganox 1010 and 20 wt% Irgafos 168) was charged. Blending was performed for 4 m i n at 240 °C; this time included 2 min required for melting the components. Afterwards, the blend was quickly recovered and quenched to room temperature between water-cooled brass plates. F o r testing, sheets 1.5 m m i n thickness were prepared by compression molding as follows: the samples were annealed for 10 m i n at 260 °C i n a heated press (Schwabenthan Polystat 100) and then quenched between water-cooled metal plates. The cooling rate was monitored by means of a thermocouple and was 50 K/min between 230 and 110 °C and 20 K/min below 110 °C. F o r tensile testing, dumbbell-shaped tensile bars 18 m m i n length were cut and machined as described i n D I N 53544. After sample conditioning (3 days at 23 °C and 5 0 % h u m i d ity), stress-strain measurements to determine Youngs modulus (from the initial slope of the stress-strain curve) and yield stress were recorded at a 10-mm/min crosshead speed on a tensile tester (Instron 4204) at 23 °C. The average deviation of the modulus measurement was less than 15%. Notched Charpy impact strength was determined on five test specimens according to standard procedures ( D I N 53453) using a Zwick 5102 pendulum impact tester equipped with a 2 J pendulum. Morphological studies were performed using a Zeiss C E M 902 transmission electron microscope ( T E M ) . T h i n sections suitable for T E M analysis were cut after staining and hardening the samples i n ruthenium tetroxide vapors for 6 h. M i c r o toming of the samples into sections 80 to 100 n m thick was performed using a Reichert Jung Ultracut Ε device equipped with diamond knives.

Results and Discussion P P - g - M A - C o m p a t i b i l i z e d P P - P A 6 (70/30) B l e n d s . O n e o f the traditional concepts for compatibilizing P P a n d P A , pioneered by Ide a n d Hasegawa (10) d u r i n g the early 1970s, involves using P P - g - M A as a b l e n d compatibilizer. D u r i n g melt processing, the reaction o f P A 6 amine e n d groups w i t h succinic anhydride-functional P P affords i m i d e - c o u p l e d P P - g m / i - P A 6 , w h i c h represents efficient dispersing agents and interfacial-adhesion promot ers for P P - P A 6 blends. This basic principle is illustrated i n F i g u r e 1. Several research groups have investigated the morphological, rheological, and m e chanical properties of such blends (10-16), emphasizing the influence of b l e n d compatibilizers. However, it is important to note that most b l e n d compatibiliz ers p r o d u c e d by reactive extrusion processes are ill-defined. F o r instance, free-radical grafting o f P P w i t h M A is accompanied by extensive P P degrada tion. T h e r m a l P P degradation i n the presence of maleic anhydride results i n a



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Figure 1. In situ formation of PP-bloek-PA6 derived from amino-terminated PA6 and mono(succinic anhydride)-terminated PP.

mixture o f m o n o - a n d bis(succinic anhydride)-terminated propene oligomers via ene-type addition of maleic anhydride to olefin-terminated P P i n t e r m e d i ates resulting from β-chain scission of the P P chain. I n o u r research, we used well-defined mono(succinic anhydride)-terminated P P - g - M A b l e n d compatibilizers w i t h independently varied molecular weights a n d stereoregularities. I n the melt PP-fc/ocfc-PA6, diblock copolymers are f o r m e d at the P P - P A 6 interface and compatibilize P P w i t h P A 6 . A s report e d i n more detail i n a previous communication (17,18), P P - g - M A m o d e l c o m patibilizers w i t h one terminal succinic anhydride e n d group are readily avail able via ene-type addition o f maleic anhydride to vinyUdene-terminated oligopropenes, w h i c h are p r o d u c e d i n h i g h yields using metallocene-catalyzed propene oligomerization (J 8-21). I n contrast to conventional melt-grafted P P g - M A , this process yields completely atactic as w e l l as highly isotactic mono(succinic anhydride)-terminated P P s w i t h narrow molecular-weight dis tributions (1.5 < M / M < 2.5), and M varying between 400 a n d 30,000 g/mol. T h e morphological a n d mechanical properties of P P - P A 6 (70/30), pre p a r e d i n the presence of atactic P P - g - M A w i t h M = 5000 a n d isotactic P P - g M A w i t h M = 10,000 g/mol as b l e n d compatibilizers, are displayed i n F i g u r e 2 as a function o f the P P - g - M A volume fraction. P r o v i d e d M was greater than 3000 g/mol, both atactic a n d isotactic P P - g M A enhanced the dispersion o f P A 6 i n the continuous P P matrix. Because o f the miscibility o f atactic a n d isotactic P P i n the melt phase, stereoregularity d i d not influence P A 6 dispersion, w h i c h was controlled primarily b y the P P - g w

n

n

n

n

n



Figure 2. Influence of atactic (aPM) and isotactic (iPM) PP-g-MA blend compatibilizers on the morphological and mechanical properties of PP-PA6 (70/30): PA6 domain size (a), Young's modulus (b), yield stress (c), and notched Charpy impact strength (d).


^ §j


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M A volume fraction. I n terms o f solid state properties, as expressed b y Youngs modulus, y i e l d stress, and notched C h a r p y impact strength, high-molecularweight highly isotactic P P - g - M A compatibilizers were m u c h more efficient than atactic P P - g - M A . I n fact, as is apparent from F i g u r e 2, y i e l d stress and n o t c h e d C h a r p y impact strength increase substantially w i t h increasing stereoregularities, molecular weights, and volume fractions. This behavior is closely associated w i t h cocrystallization of the isotactic P P segment of the i n situ f o r m e d PP-Moefc-PA6, w h i c h is likely to accumulate at the P P - P A 6 interface, although complete coupling is unlikely (17). I n spite o f a doubling i n impact strength w i t h respect to PP, this balance between the toughness, stiffness, and strength of P P - P A 6 must be i m p r o v e d further to meet the demands o f engi neering applications. Therefore, ternary blends o f PP, P A 6 , and r u b b e r (i.e., E P R a n d S E B S ) were investigated. In principle, rubber can be dispersed i n the P P matrix as a separate microphase or as a shell e m b e d d i n g a P A 6 core. E P R - M o d i f i e d P P - P A - P P - g - M A . I n the first approach to improv i n g the toughness o f P P - P A 6 (70/30), discrete E P R microphases were dis persed simultaneously w i t h P A 6 . As a rule, w h e n P P and E P R exhibited simi lar M F I s , it was possible to control E P R and P A domain sizes independently (21, 22). A l t h o u g h the P A 6 d o m a i n size was affected exclusively by the volume fraction o f P P - g - M A b l e n d compatibilizer, the average E P R domain sizes i n creased p r i m a r i l y w i t h increasing E P R volume fraction. T h e average E P R do m a i n size varied between 0.1 and 10 μηι. A c c o r d i n g to T E M studies, i n c o n trast to the spherical P A 6 microphases, E P R microphases exhibited irregularly shaped structures w i t h small P P subinclusions. As is apparent i n F i g u r e 3, de creasing the size o f P A 6 domains by increasing the P P - g - M A volume fraction

Figure 3. Impact strength of EPR-toughened PP-PA6 (70/30) blends containing 2.5, 5.0, and 10 vol% PP-g-MA compared with EPR-toughened PP, as a function of EPR volume fraction.


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from 2.5 to 10 v o l % gave i m p r o v e d notched C h a r p y impact resistance. H o w e v er, impact performance o f E P R - t o u g h e n e d P P - P A 6 (70/30) was m u c h poorer than that o f E P R - t o u g h e n e d PP, especially at an E P R rubber volume fraction exceeding 10 v o l % . This difference i n impact performance c o u l d result from mutually overlapping stress fields o f interconnected P A 6 microparticles, depressing the efficiency o f energy dissipation involving E P R microphases. Moreover, a small E P R rubber volume fraction is sufficient to reduce the stiffness o f the P P - P A 6 matrix and eliminate PA6-microparticle reinforcement. I n conclusion, the simultaneous dispersion o f separate, discrete E P R a n d P A 6 microparticles i n the continuous P P accounted for antisynergistic b l e n d properties c o m b i n i n g poor stiffness w i t h inadequate impact strength. Slightly i m p r o v e d impact strength was achieved at the expense of unacceptable losses o f stiffness. P P - P A 6 Blends Containing Dispersed Core-Shell Microparticles. I n the t h i r d type o f P P - P A 6 b l e n d system, the P P - g - M A b l e n d c o m patibilizer a n d r u b b e r was completely substituted b y maleic anhydride-grafted rubbers such as E P R - g - M A and S E B S - g - M A . A s reported previously (22, 23) and schematically represented i n F i g u r e 4, imide-coupling at the P P - P A 6 i n terface, a n d surface-tension gradient and immiscibility between PP, P A 6 , a n d r u b b e r are responsible for the accumulation o f the rubber at the P A 6 m i croparticle surface, w h i c h results i n microparticles w i t h a P A 6 core and a r u b ber shell. L i k e P P - g - M A b l e n d compatibilizers, maleic anhydride-grafted r u b -

Figure 4. In situ formation of core-shell-type microparticles, in which a PA6 core is surrounded by a SEBS-g-MA shell


298


bers are polymeric dispersing agents, i m p r o v i n g P A 6 dispersion i n the P P m a trix. T h i s fact is reflected b y small average P A 6 domain sizes w i t h increasing volume fractions o f functionalized rubbers. F i g u r e 5 shows T E M images o f R u 0 - s t a i n e d t h i n cuts, w h i c h indicate the absence o f separate r u b b e r m i crophases a n d the formation o f microparticles w i t h a P A 6 core and a r u b b e r shell. T h e dispersing-agent performance of S E B S - g - M A was m u c h better than that o f P P - g - M A . Less than 5 v o l % S E B S - g - M A was sufficient to achieve P A 6 d o m a i n sizes o f 0.1 μιχι, whereas more than 15 v o l % P P - g - M A w o u l d be re q u i r e d to achieve similar P A 6 dispersion. Clearly, S E B S - g - M A yielded m u c h smaller P A 6 microparticles, w h i c h were encapsulated i n an S E B S - g - M A shell.


4

I n F i g u r e 5 it is apparent that the m u c h smaller P A 6 core-shell micropar ticles, obtained i n the presence o f S E B S - g - M A , form large agglomerates. As reported previously (23), w i t h increasing S E B S concentration a second cocontinuous phase consisting o f large clusters of P A 6 core-shell-type microparticles appears. Interestingly, even at small S E B S volume fractions, such cocontinuous phases o f P P and S E B S - P A 6 were detected i n the P P matrix. M o s t likely, this unusual morphological feature o f P P - P A 6 (70/30) blends compatibilized w i t h S E B S - g - M A c o u l d account for the unexpected c o m b i n a tion o f h i g h toughness, stiffness, and strength (see Figures 6 and 7). A t an S E B S - g - M A volume fraction greater than 15 v o l % , crack propagation d u r i n g p e n d u l u m impact was stopped, and intense stress-whitening was observed near the crack tip. As reflected b y high y i e l d stresses, interfacial adhesion be tween the S E B S shell a n d P P matrix was excellent i n spite o f the i m m i s c i b i l ity o f both components. T h e S E B S - m o d i f i e d P P - P A 6 gave higher stiffness than the E P R - m o d i f i e d P P - P A 6 . This difference c o u l d result from the m u c h higher Youngs modulus of the S E B S compared w i t h that o f the rather soft EPR.

Figure 5. TEM images of thin sections of PP-PA6 (70/30) blends compatibilized with 5 vol% SEBS-g-MA (A) and 5 υοΙΨο PP-g-MA (B),



35

| — Δ — E P R

15 20

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—A—EPR

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n

—•—SEBS-g-MA |

— • — E P R

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- Δ — E P R - g - M A

—O—SEBS-g-MA

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—•—SEBS-g-MA

Rubber content in vol-%

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—O—EPR-g-MA


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|

Figure 6. Morphological and mechanical properties of PP-PA6 (70/30) blends compatibilized with EPR, EPR-gMA, and SEBS-g-MA as a function of the volume fraction blend compatibilizer: PA6 domain size (a), Youngs modulus (b), yield stress (c), and notched Charpy impact strength (d). In the case of nonfunctionalized EPR, 10 vol% isotactic PP-g-MA (M = 30,000 g/mol) was added.

—O—EPR-g-MA


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fi

If

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—O—SEBS-g-MA |

40

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—Δ—EPR-g-MA

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15

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25

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I —D—EPR

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Rubber content In vol-%

β



Figure 7. Relationship between morphology and mechanical properties of PP-PA6 (70/30) blends compatibilized with PP-g-MA (a), EPR (b), and SEBS-g-MA (c), as compared with the mechanical properties of PP.


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Conclusions T h e morphologies o f PA6-reinforced P P - P A 6 (70/30) can be tailored to e n hance y i e l d stress and impact strength without drastic losses of stiffness. I n F i g u r e 7, the basic correlations between P P - P A 6 (70/30) b l e n d morphologies and mechanical properties are summarized. T h r e e different types o f b l e n d morphologies have been realized. T h e first type is characterized b y discrete, spherical P A 6 microparticles dispersed i n the P P matrix. In situ formation o f PP-Mocfc-PA6 at P P - P A 6 interfaces accounts for both steric stabilization o f the P A 6 dispersion and interfacial adhesion via cocrystallization o f P P w i t h the isotactic P P segments o f PV-block-PA6, provided the average P P segment length exceeds the entanglement molecular weight. I n spite of significantly increased y i e l d stresses w i t h increasing P P - g - M A stereoregularities a n d molecular weights, improvement i n impact strength is rather poor, and impact strength does not change w i t h the P P - g - M A volume fraction. T h e second type o f m o r phology seen i n F i g u r e 7 is characterized by simultaneously dispersed, discrete P A 6 a n d E P R microphases, where the average domain sizes are controlled i n dependently by E P R or P P - g - M A volume fractions. F o r example, at a constant r u b b e r content, P A 6 domain size can be reduced by increasing the P P - g - M A volume fraction. Obviously, dispersed, discrete E P R microphases fail to i m prove toughness, as expected from the behavior o f P P - E P R blends. T h e best results i n terms o f the stiffness-toughness balance were obtained w i t h the t h i r d type of P P - P A 6 (70/30) b l e n d morphology, w h i c h is characterized b y colloidal, dispersed P A 6 microparticles e m b e d d e d i n thin rubber shells. T h e key to morphologic control is the succinic anhydride rubber b l e n d compatibilizers, especially S E B S - g - M A . W h e n core-shell-type microparticles agglomerate to form a cocontinuous phase, unusual blend-property synergisms are achieved, especially substantially i m p r o v e d impact strength c o m b i n e d w i t h high y i e l d stress without drastic losses o f stiffness, as expressed by Youngs modulus. T h i s result indicates that fine-tuning the molecular architectures o f b l e n d compatibilizers, and the controlled formation of structured microparticles and cocontinuous b l e n d morphologies, plays a key role i n developing novel P P materials exhibiting unusual combinations o f properties.

Acknowledgments W e thank B u n d e s m i n i s t e r i u m fur F o r s c h u n g u n d Technologie and Badische A u i l i n u n d Soda F a b r i k AG/Ludwigshafen for supporting the research o n metallocene-catalyzed propene oligomerization and functionalization, a n d the Bundesminister fur Wirtschaft a n d Arbeitsgemeinschaft Industrie-Forschung for supporting research on adhesion promoters utilizing the potential o f functionalized olefin polymers.


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Toughened Polypropylene-Polyamide 6 Blends Prepared by Reactive

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