Polymers for Advanced Technologies Volume 7. DU. 4 2 5 4 3 6

REVIEW ARTICLE

The Role of the Morphology of Natural Rubber and Polybutylacrylate-based Composite Latex Particles on the Toughness of Polycarbonate/Brittle Polymer Blends Michael Schneider, Tha Pith and Morand Lambla* Laboratoire d’Extrusion Rkactive Znstitut Charles Sadron (CRM-EAHP), 4, rue Boussingault Strasbourg, France

ABSTRACT

The dependence of the toughness of polycarbonate (PC)/ poly(styrene-co-acylonitrile) (PSAN) and PC/polystyrene (PS) blends on the architectural structure of incorporated natural rubber (NR) or poly(n-butylac ylate) (PBuAhbased latex particles is presented. By means of a twin screw extruder PC was melt blended with PSAN or PS blends containing different composite latex particles. Those materials that consisted of 70% PC could be effectively toughened by only 6-8% of the prepared toughening particles. Fractographic analysis by scanning electron microscopy has been used to analyze the failure mechanisms of the reinforced terna y PC blends. The principal feature of fracture surfaces of PCIPSAN blends containing pure N R particles was multiple rubber particle cavitation. A hard polyrnethylrnethacrylate shell deteriorated the effectiveness of NR-based latex particles. Rigid PS subinclusions considerably increased the performance of the NR-based core-shell particles. They shifted the failure mechanism at impact from a brittle mode to a ductile tearing mode and much more energy could be absorbed. Good impact properties were also achieved in PCIPS blends reinforced by NR-based core-shell particles containing rigid subinclusions. The morphological structure of smaller-sized PBuA-based latex particles was less crucial.

KEYWORDS: polystyrene-co-acrylonitrile; polys rene; polycarbonate; composite rubber particles; toug x ness

* To whom correspondence should be addressed.

CCC 1076-5174/96/05O425-12 0 1996 by John Wiley & Sons, Ltd.

INTRODUCTION

Blending of polymers is a very effective way to obtain a wide variety of materials with new characteristics which are controlled by the composition of the prepared polymer blend. It provides a higher performance-cost ratio and is one of the fastest growing areas of the plastics market. Some polymers are mutually miscible and the blend is characterized by a single homogeneous phase morphology 11, 21. However, most polymers are immiscible and multi- phase materials are obtained. Often poor mechanical properties result since force transfer between the polymer phases is not sufficient.

Blends of polycarbonate (PC) and ABS (acryloni- trile-butadiene-styrene) which provide an improved balance of properties at a lower cost than PC have been exploited commercially for many years D-61. Such blends consist of a two-phase matrix (PC/ poly(styrene-co-acrylonitrile), PSAN) and dispersed ABS-type rubber particles. The high impact resistance of such polymer blends is due to the excellent toughness of PC which is characterized by a low shear-yielding stress relative to its crazing resistance. Without dispersed rubber particles toughness would not be retained in a notched thick specimen since shear deformation is suppressed by a plane-strain constraint [7]. The particles cavitate at impact, relieve the unfavorable triaxial stress field in the center of the sample and produce more extensive matrix deforma- tion by shear yielding. The incorporation of rubber

Received 3 November 1995 Revised 15 December 1995

426 1 Schneider et al.

particles prepared by emulsion polymerization allows their morphology to be adjusted. Furthermore, the ratio of the components in the blend can be controlled independently. These particles whose size is in the range between 200 and 600 nm usually possess a core-shell structure [B-lOI Paul and coworkers [111 studied different ternary PC blends with brittle polymers and established that polymethylmethacry- late (PMMAbgrafted rubber particles are preferen- tially located in the PC/PS (polystyrene) and PC/ PSAN interfaces. The authors pointed out that the location of the incorporated rubber particles in PC ternary blends depends on the miscibility of their PMMA shell with the polymers of the two-phase matrix. In the case of PC/PMMA ternary blends methacrylated butadiene-styrene type core-shell particles reside entirely within the PMMA phase [ill. This result had been expected since the incorporated rubber particles were coated by a PMMA shell. Studies about PS/PSAN and high impact polystyrene (HIPS)/ABS blends also estab- lished that incorporated core-shell particles are preferentially located at the interface of the matrix polymers [12, 131.

Rubber toughening studies of such multiphase polymers ususally focus on the distribution of the incorporated rubber particles and the rubber phase content [ll, 141. The effects of interparticle distance and particle size on toughening of polymers were investigated by Wu 115,161. The purpose of this paper is to provide information on the dependence of the mechanical properties of PC/PSAN and .PC/PS blends on the morphology of incorporated composite latex particles. The thickness of the PMMA shell and the weight fraction of rigid PS subinclusions within a natural rubber (NR) or poly(n-butylacrylate) (PBuA)- based core have been varied, A typical morphology of the prepared NR-based latex particles is schematically representd in Fig. 1.

The use of very polydisperse NR particles is a definite advantage for the toughening of PSAN since a mixture of small and large-sized particles offers an impresive synergistic toughening effect in PSAN blends [17-211. It should be worthwhile to verify whether such particles toughen PC/PSAN blends. The size of the used (PBuA)-based latex particles was not fixed as in the case of an NR seed latex but could

FIGURE 1. Schematic representation of natural rubber (NR)- based coreshell particles containing 30% PS subinclusions within the NR core and 25% PMMA in the shell region.

be controlled by sequential emulsion polymerization in which seed latex particles were first formed and grown to the desired size. In this way it was possible to compare monodisperse PBuA rubber-based latex particles with polydisperse NR-based latex particles with similar morphologies.

EXPERIMENTAL Materials

All the polymers used for the blend preparation are commercially available. The PS matrix lacqritne 1240 was supplied by Elf Atochem. The polystyrene-co- acrylonitrile copolymer Luran 368R with an acryloni- trile content of 25% was kindly donated by BASF. The bisphenol-A polycarbonate Makrolon 31 00 is a commercial product of Bayer AG. An uncrosslinked natural rubber latex (Revertex AR) supplied by Revertex Ltd was used as a seed latex in a sequential emulsion polymerization. Several initiation systems allowed to control the morphology of the prepared composite NR-based particles [221. In order to constitute a crosslinked PMMA shell around a NR core the bipolar redox initiation system tert-butyl hydroperoxide/tetraethylene pentamine was used [23-241. On the other hand, the formation of cross- linked PS subinclusions within the NR seed latex particles resulted from an initiation induced by azobisisobutyronitrile (AIBN). All secondary poly- mers contained 0.25% ethylene glycol dimethacrylate as crosslinking agent. The detailed emulsion poly- merization technique for the preparation of the composite natural rubber latexes has been described elsewhere 1251. Polymerizing 40% styrene in a NR seed latex produced 50-250 sized microdomains within the rubber particles [251. A perfect PMMA shell could not be achieved but the PMMA mean- dered into the rubber core. A 100-200 nm large intenneshed interface region was developed [25]. All latexes were prepared in a 5 1 stainless steel reactor. The extremely polydisperse particle size distribution curves of the used NR latex is situated between 0.2 and 2.5 pm as obtained by photon correlation spectroscopy performed on a Malvern Autosizer I1 1251. The used types of composite NR particles comprise pure NR latex particles, NR-based latex particles containing 15 or 25% crosslinked PMMA in the shell region and NR-based coreshell particles containing 0, 15, 30 or 45% crosslinked PS subinclu- sions within the rubber core and 25% crosslinked PMMA in the shell region. The actually obtained morphology of the particles was verified by transmis- sion electron microscopy (TEM) observations [251.

Furthermore, crosslinked PBuA-based compo- site latexes were synthesized by ammonium persul- fate initiation at 72°C. Analogous to composite NR particles, PS subinclusions were introduced into a PBuA seed latex by AIBN initiation during a batch process. If not specified otherwise, the PBuA-based latexes contained 25% crosslinked PMMA in the shell region. The detailed emulsion polymerization recipes are described elsewhere [26]. Photon correla- tion spectroscopy gave 350 nm for the z-average

Natural Rubber and Polybutylacrylate-based Composite Latex Particles / 427

FIGURE 2. Scannin electron photomicrograph of crosslinked poly(n-bu$ac late) based latex particles containing 50 wt% crossliXed PMMA in the shell.

size of a simple core-shell (75/25) latex. Core-shell (75/25) latexes containing 15 or 30% crosslinked PS subinclusions within the rubber core were 370 and 340 nm in size. Another PBuA-based latex which contained 50% PMMA in the shell was synthesized in a 50 1 pilot plant reactor (271. Its z- average mean particle size, which was obtained by photon correlation spectroscoy, was 180 nm. The PBuA and PMMA polymers always contained 0.25% ethylene glycol dimethacrylate as crosslinking agent.

Figure 2 is a typical scanning electron photo- micrograph of crosslinked PBuA-based core-shell particles containing 50% crosslinked PMMA in the shell. Monodisperse and isolated latex particles are clearly visible.

Blend Preparation

A set-up for the addition of wet latexes directly into a co-rotating ZSK 30 Werner & Pfleiderer twin screw extruder ( L I D = 42) was used for the incorporation of composite latex particles into the PSAN or PS matrix [28, 291. Subsequently, dried PC and PS or PSAN polymer pellets were combined in the ratio 70/30 and melt blended in the same twin screw extruder using four kneading zones. The PS or PSAN polymers already contained the composite rubber toughening particles. The used extruder has a modular barrel and screw design for a flexible set-up. A vacuum section was placed near the die exit in order to remove any residual moisture or volatile products. The blend was extruded through a two-hole die plate, pulled through a water bath and palletized. The heating zones of the extruder were set in the range of 200- 220°C in the case of PSAN and PS blends and increased to 240-250°C for the preparation of ternary PC blends. The extrusion process was performed at a throughput of 5 kg/hr and a screw speed of 150 rpm in the case of all prepared blends.

Mechanical Tests

ASTM test samples were moulded on a Billon 150/ 150 injection moulding machine at 270°C and left at 23°C and 50% relative humidity for one week. The blend pellets had been dried before injection moulding.

The Izod impact resistance of V-notched samples (based on ASTM D256) was obtained by using a standardized Zwick pendulum impact testing ma- chine. A minimum of five samples was meausured.

Tensile testing on dumbbell samples (based on ASTM D638) was performed on a hydraulic Instron 8031 machine at room temperature. The elongation (strain rate 50 mm/min) was measured directly on the sample by an extensometer.

Electron Microscopy

A Joel JSM 840 scanning electron microscope (WEM) was used for the examination of Izod fracture surfaces of the prepared blends which had been coated with gold vapor deposition before viewing. The gold layer was deposited on the sample with a Cambridge Instruments E 5200 Auto Sputter Coater. If not specified otherwise, the presented SEM photomicro- graphs were taken from the core area of the fractured injection-moulded Izod bars. A Cambridge Instru- ments Stereoscan 120 SEM was used in the case of Fig. 16.

A Phillips EM 300 transmission electron micro- scope served for the observation of the morphology of the prepared PSAN blend in Fig. 3. Osmium tetroxide vapors had been applied in order to stain the natural rubber phase [301.

RESULTS AND DISCUSSION The main purpose of this research project is to provide new insights into the role of the morphology and the physical properties of composite rubber particles on the mechanical properties of PC/PS and PC/PSAN blends. NR and PBuA-based core-shell particles which contained different weight fractions of

FIGURE 3. Transmission electron photomicrograph of a PSAN blend reinforced by 27% composite natural rubber particles containing 15% PS subinclusions within the NR core and 25% crosslinked PMMA in the shell region. (Osmium tetroxide stained ultramicrotome cut.)

428 / Schneider et al.

PMMA in the shell region were tested. Furthermore, PS subinclusions were introduced into the rubber phase. First, different weight fractions of composite latex particles had been incorporated into PS or PSAN matrices which were melt blended with PC in a second extrusion step. A blend composition of 70% PC and 30% PS or PSAN which contained the impact modifiers was always maintained. The mechanical properties and the morphology of the prepared PS and PSAN binary blends are reported elsewhere [28, 29/31]. Figure 3 shows an example of the morphology of a PSAN blend reinforced by 27% NR-based core- shell containing 15% PS subinclusions. The PS subinclusions within the NR core are clearly visible.

It is known that the morphology of ternary PC blends depends on composition, processing condi- tions and mixing sequence [ll, 12,32,331. A two-step mixing sequence was applied in this study since wet latexes can be directly introduced into the melt of a co-rotating twin screw extruder. This process is much more flexible than the feeding of dried latex powders which are used for the weight feeders of conventional blend preparation machines. Furthermore, blends of PC, a brittle polymer and an impact modifier exhibit worse mechanical properties when all components are mixed simultaneously 1111. It has been shown that premixing PS or PSAN with the impact modifier prior to blending with PC maximizes toughness since the particle content in the brittle polymer phase and the PC/PS or PSAN interface is increased 1111.

All tested samples had been notched since PC ternary blends have a very high impact strength on unnotched specimens. The incorporated rubber particles are to improve the relatively low toughness of notched samples. Toughening of such pseudoduc- tile polymers is achieved by shifting the failure mechanism from a localized shear yield brittle mode to a mass shear yield ductile mode [341. PC can absorb much more energy at impact than the second thermoplastic matrix polymer and rubber toughen- ing of the prepared ternary blends has to be achieved via the PC matrix. However, the induction of premature fracture by the brittle polymer because of insufficient stress transfer between the two matrix polymers must be avoided. Thus, the natural and PBuA rubber particles with a shell of PMMA should reside in the brittle polymer phase or in the interface of the PC/PSAN or PS blends.

Blends of PC, PSAN and Composite Latex Particles

Effect of the Shell Thickness. In general, rubber particles need to adhere to the matrix polymer. Satisfactory adhesion can be obtained by grafting of the rubber phase with polymer chains which are identical or similar to the matrix polymer. A core- shell impact modifier with a shell of crosslinked PMMA was chosen because it is compatible with both the PC major matrix component [35, 361 and also the PSAN minor matrix copolymer with an acrylonitrile content of 25%. PMMA is miscible with PSAN copolymers for acrylonitrile contents between 9 and 33% [19, 37-391. The used PSAN copolymer was chosen because the adhesion between PC and PSAN

Notched hod impact resistance

1'

0 1 , . , . , . , . , . , . , 0 2 4 6 8 10 12

wt-% particles in the blends

FIGURE 4. Impact energy of PC/PSAN blends reinforced by different amounts of NR-based latex articles containing 0, 15 or 25% crosslinked PMMA in the she! region (70% C content).

is greatest at about 25% AN [401. Figure 4 summarizes the impact resistance of ternary blends of PC, PSAN and different weight fractions of PMMA-grafted NR- based latex particles. The incorporated composite particles are represented schematically beside the corresponding graph of the impact resistance.

The toughness of the prepared ternary blends which contained natural rubber particles with different mass fractions of shell-forming polymer was somewhat surprising. The material that ab- sorbed most energy at impact had been toughened by pure natural rubber particles. Coating the NR- based latex particles with a shell of crosslinked PMMA deteriorated the efficiency of the toughening agent. The effect is more pronounced when the mass fraction of the shell-forming polymer is increased. Plotting the impact energy against the rubber weight fraction in the blends did not change the rating of the different rubber-toughening agents as the latex particles contained only 15 or 25% PMMA in the shell region. It has to be noted that only a very few (<lo%) particles were necessary to toughen the prepared PC/PSAN blends. It is expected that the few large-sized NR particles act as very effective initiation sites for shear deformations since they form cavities a lower stress than small-sized rubber particles 1411. Hence, detrimental crazing which requires higher stresses can be avoided.

SEM examination of the morphology of fracture surfaces of the prepared ternary PC/PSAN blends containing different latex particles can elucidate the failure mechanism at impact. In order to provide comparison, all toughened ternary blends contained 70% PC and 6% rubber phase. The PMMA shell was treated as being part of the matrix. First, the morphology of a PC/PSAN binary blend without any rubber particles was analyzed. Figure 5 clearly shows the PC matrix and embedded spherical PSAN domains in the range between 0.5 and 3pm. The fracture propagated through the dispersed PSAN domains because of the good adhesion of the two matrix polymers.

SEM allowed us to distinguish between rubber

Natural Rubber and Polybutylacrylate-based Composite Latex Particles / 429

FIGURE 5. Scanning electron photomicrograph of the notched lzod fracture surface of a PC/PSAN (70/30) binary blend.

particles which contained different amounts of shell- forming polymer. Figures 6-8 show SEM photomicro- graphs of notched Izod fracture surfaces of PC/PSAN blends reinforced by pure NR particles or NR-based latex particles containing 15 or 25% crosslinked PMMA in the shell region. Impact testing, summar- ized in Fig. 4, indicated that pure rubber particles were most effective to confer toughness to such PC/ PSAN/NR particle ternary blends.

The principle feature of the fracture surface of a PC/PSAN blend containing pure natural rubber particles is multiple rubber particle cavitation as clearly shown by Fig. 6. The internal rubber particle cavitation provides many sites that permit the relaxation of the unfavorable triaxial tension and, thus, more extensive shear yielding of the ternary PC blend. The fractured Izod test bars of the PC/PSAN blends which had been toughened by the different NR-based latex particles exhibited stress whitening. The most extensive stress whitening occurred when pure NR particles had been incorporated and the toughness of these materials depends on the size of the stress whitened zone that forms prior to critical

FIGURE 7. Scanning electron photomicrograph of the notched Izod fracture surface of a PC/PSAN ternary blend reinforced by 7% NR-based latex particles containing 15% crosslinked PMMA in the shell region (6% NR, 70% PC content).

crack propagation. The stress whitening corresponds to cavitation of the natural rubber phase. Further- more, SEM indicates that most of the NR particles are located within PSAN domains or in the PC/PSAN interface. The cavities in the PSAN domains and many of the imprints in the PC matrix correspond to the NR particle sizes. It is evident that the cavitated rubber particles in the PC/PSAN interface induced the PC matrix to yield. The markings in the PC matrix are generally larger than the cavitated rubber particles which can be distinguished within the PSAN domains. This observation can be explained by coalescence of voids in the PC matrix. In fact, such micrometer-sized markings were also observed in binary PC blends reinforced by core-shell rubber particles as shown by Fig. 14. Another important point is the changed arrangement of the two matrix polymers when rubber particles are incorporated into a binary PC/PSAN blend. The PSAN inclusions within the PC matrix are no longer spherical (Fig. 5) but elliptical and very elongated; 2-15 pm sized

FIGURE 8. Scanning electron photomicrograph of the notched lzod fracture surface of a PC/PSAN ternary blend reinforced by 8% NR-based latex particles containing 25% crosslinked PMMA in the shell region (6% NR, 70% PC content).

FIGURE 6. Scanning electron photomicrograph of the notched Izod fracture surface of a PC/PSAN ternary blend reinforced by 6% pure NR particles (70% PC content).

430 / Schneider et at.

domains can be distinguished in Fig. 6. The morphology tends to approach a co-continuous state. Co-continuous morphologies involving PC/ PSAN (60/30) blends have also been observed when 10% of 180 nm PMMA graftd core-shell particles has been incorporated ill].

A hard PMMA shell around the incorporated natural rubber particles changed the failure mechan- ism at impact and a totally different fracture morphology resulted. Figures 7 and 8 indicate that multiple rubber cavitation is suppressed when the rubber particles had been coated by PMMA. Increas- ing the PMMA mass fraction in the latex particles intensifies this effect. Rubber cavitation is retarded and shifted to higher stresses since more PMMA increased the particle modulus. Besides, the redox initiation system used for the shell synthesis grafts the natural rubber chains which further reduces the ease of the incorporated rubber particles to form cavities. Grafting of the NR phase is thought to be especially detrimental since PBuA-based latex particles with 50% PMMA in the shell region were an effective toughening agent. Excessive grafting can be excluded in this case. Models of Bucknall and coworkers [41- 431 and Dompas et al. [44, 451 explain the cavitation behavior of rubber particles in polymer blends. It has to be taken into account, though, that cavitation in itself cannot absorb much energy. However, it relieves triaxial stresses in the matrix which can be deformed more extensively. In particular Fig. 7 indicates that the incorporated rubber particles are preferentially located within the PSAN phase and the PC/PSAN interface. The location of the PMMA-grafted rubber particles in the prepared PC ternary blends is in accordance with TEM studies of literature 1111. The imprints in the PC matrix correspond to the size of the NR-based core-shell particles which can be distin- guished in the surface of the PSAN domains. SEM clearly indicates that the PC/PSAN blends containing core-shell particles have been deformed less inten- sively at impact than PC/PSAN blends containing pure NR particles. Very few of the latex particles which contained 15% PMMA in the shell region cavitated. Figure 8 indicates that the latex particles which contained 25% PMMA did not cavitate internally, no markings in the PC matrix can be distinguished and a brittle fracture surface is visible. These observations explain why the impact resistance of PC/PSAN blends was decreased when the incorporated rubber particles had been coated by crosslinked PMMA.

Effect of PS Subinclusions within NR-based Core- 'Shell Particles It is possible to modify the mechan- ical behavior of the incorporated rubber particles by the introduction of rigid subinclusions within the natural rubber core. The impact resistance of PC/ PSAN blends reinforced by different weight fractions of pure natural rubber particles or composite core- shell particles containing 0 or 15% PS subinclusions within the NR core and an equal amount of shell- forming polymer (25%) is compared in Fig. 9.

It is known that PS subinclusions induce high- impact polystyrene type rubber particles to fibrillate

Notched Izod impact resistance

15% Ps indusioru

40

/

. , ' I ' " A ' 4 . i ' i ' ;I 10 12 wt.-% particles in the blends

FIGURE 9. Comparison of the effectiveness of pure NR particles and coreshell particles containing O or 15% crosslinked PS subinclusions in the rubber core and 25% crosslinked PMMA in the shell region to toughen PC/PSAN blends (70% PC content).

extensively when a sufficiently high stress is externally applied [46, 471. Furthermore, the impact resistance of PS blends containing similar NR-based core-shell latex particles could be doubled when many small-sized subinclusions had been introduced within the soft rubber phase 128, 291. TEM studies showed that the rigid subinclusions induce multiple cavitation of the incorporated composite NR-based latex particles at impact 1481. Small-angle X-ray scattering studies performed by Bubeck et al. 1491 indicate that PC/ABS blends fail by shearing in the PC phase and associated rubber particle cavitation in the ABS pase. Figure 9 clearly indicates that rigid subinclusions considerably improved the perfor- mance of the incorporated rubber particles. They constrain the rubber phase which cavitates internally at a lower externally applied stress. Shear yielding is promoted and detrimental crazing of the PC matrix which requires higher stresses is largely avoided. Hence, the PC matrix can be deformed to a higher degree. and more energy is absorbed at impact. Crazing usually amounts to only a few percent of the plastic strain 1491. However, it is the precursor event to fracture of PC blends. The highest impact strength was achieved when about 7% NR-based latex particles had been incorporated into the prepared PC/PSAN blends. Increasing the rubber particle content gave rise to decreasing toughness. According to Michler this effect may be due to a variation of stress fields around the rubber particles [501. At a higher particle content a critical minimum interpar- ticle distance is reached where highly localized stress concentration will be "smeared" over a larger volume. The result is a lower stress concentration and toughness will be reduced. This effect is due to the coupled movement of highly entangled and interconnected macromolecules which impose a minimum size for the stress relaxation zone in polymers. Ternary PC blends containing less than 4% toughening particles were not equally tough since less particles relieved hydrostatic tension in the

Natural Rubber and Polybutylacrylate-based Composite Latex Particles / 43 1

Notched Izod 8o 7 impact resistance T

0 1 2 3 4 5 6 7 8 wt.-YO NR content in the blends

FIGURE 10. Impact resistance of PC/PSAN blends reinforced by coreshell particles containing 0, 15, 30 and 45% crosslinked PS subinclusions within the NR core and 25% crosslinked P M M A in the shell region (70% PC content).

mutiphase blend. Furthermore, cooperative cavitation of rubber particles which can impart higher impact strength is less likely to occur when the attained interparticle distance increases at a lower particle content in the ternary PC blends [511.

In order to test whether more NR can be substituted by rigid subinclusions, core-shell parti- cles containing different weight fractions of rigid PS subinclusions within the NR core and 25% PMMA in the shell region were incorporated into PSAN blends. The impact energy of these blends is summarized in Fig. 10. The effectiveness of the impact modifier could be further improved by increasing the PS mass fraction in the composite latex particles from 15 to 30 or 45%. The plots of the impact resistance versus natural rubber content in PC/PSAN blends reinforced by core-shell particles containing high weight fractions of PS subinclusions were superimposed.

This means that the rubber content controlled the impact resistance of the prepared PSAN blends in the case of highly occluded core-shell particles. In fact, TEM studies of the latex particle morphology showed that a higher PS mass fraction within the NR core increased the subinclusion size but did not affect the number [251. It has been demonstrated that the efficiency of larger-sized occlusions to modify the fibrillation behavior of the natural rubber core at impact did not change in the case of PS blends (291. It appears that PC/PSAN blends which were toughened by composite NR particles behaved similarly. The rubber particles should contain an optimal amount of 30% PS subinclusions. Figure 10 shows that most effective use is made of natural rubber in this case. The PS weight fraction in the particles can be further increased. However, more toughening particles have to be incorporated in order to achieve the same toughness since part of the NR phase had been substituted by the glassy occlusions.

The impact data of Fig. 10 indicates that rigid subinclusions within the NR core improved the impact resistance of ternary blends of PC, PSAN

and NR particles by a factor of four when compared with composite NR-based latex particles with an equal amount of shell-forming polymer. SEM demon- strated that the failure mechanism of these ternary PC blends had been altered.

The significant change of the notched Izod fracture surfaces of PC/pSAN blends containing occluded NR-based core-shell particles, compared with the PC ternary blends discussed previously, is a much greater plastic deformation of the PC matrix. Figure 11 reveals cavitated NR particles and elongated PC lamellae at the boundary between the PC and PSAN domains. The triaxial stress state in the matrix was relieved by rubber cavitation at the PC/ PSAN interfaces and the ternary blend could be deformed more extensively. The ductile stretching and tearing can be regarded as the main toughness contributors to resist crack initiation. A very similar morphology of the fracture surfaces was observed when the incorporated coreshell particles contained less (e.g. 15%) or more (e.g. 45%) rigid PS subinclu- sions within the NR core. It has to be mentioned that the fracture surface consists of two distinct zones, a highly fibrillated zone in the center of the fracture surface and a shear zone in the outer skin area. The

(bl FIGURE 1 1. Scannin electron photomicrograph of the notched Izod fracture surEce of a PC/PSAN blend reinforced by 8% NR-based latex particles containing 30% PS subinclusions within the NR core and 25% crosslinked PMMA in the shell region: (a) outer skin area and (b) middle core area (6% NR+PS, 70% PC content).

432 / Schneider et al.

Notched Izod 50 nm, PBuA, 25 ./r PMMA

80 nm, PBuA, 50 Ye PMMA

500 IUD. NR, 25 ?4 PMMA 10

0 2 4 6 8 10 12 14 16 18 20 22

% particles in the blend

FIGURE 12. Comparison of the impact resistance of PC/ PSAN blends reinforced by monodisperse pBuA rubber-based latex particles containing 25% or 50% crosslinked PMMA in the shell and polydisperse NR-based latex particles containing 25% PMMA in the shell (70%) PC content).

highly deformed plastic zone is due to a triaxial stress field when the sample was loaded prior to crack propagation. The same distinction could be made for P C / E ternary blends, described later.

PCIPSAN Blends Containing NR or PBuA-based Latex Particles The toughness of PC/PSAN blends containing different amounts of small PBuA-based coreshell particles was also studied. Figure 12 summarizes the impact resistance of the prepared ternary blends. The PC content in the blends was fixed at 70%.

PBuA-based coreshell latex particles are clearly more effective in conferring toughness to the prepared PC/PSAN blends. This finding can be explained by their smaller particle size in relation to the NR-based latex particles. Natural rubber latexes contain particles with a wide range of diameters from 0.01 to 5 pm. Most of the particles are less than 0.5 pm, but most of the mass of the rubber resides in

0 5 10 15 20

% later particles

FIGURE 13. Correlation of the €-modulus of PC/PSAN blends with the wei ht fraction of the incorporated crosslinked PBuA rubber-basedLex particles containing 50% PMMA in the shell region (70% PC content).

modulus of PC/PSAN blends containing different weight fractions of PBuA-based core-shell particles. A linear dependence of the modulus and the incorporated particle weight fraction could be established. However, the influence of the weight fraction of the incorporated particles on the blend modulus was less pronounced than their influence on the impact properties.

Twelve percent of 180 mm PBuA-based latex particles containing 50% PMMA in the shell region were also incorporated into a pure PC matrix in order to compare the morphology of the notched Izod fracture surfaces of binary and ternary blends of PC. It has been shown that this amount of latex particles which corresponds to 6% rubber phase confers the highest impact resistance to binary PC blends [531. A wet latex could not be introduced directly into the used twin screw extruder since PC is sensitive to hydrolysis and the latex particles had been dried and fed as a powdered product.

Figure 14 shows many signs of voiding and ductile tearing. Particle cavitation was the principal characteristic feature in enhancing the impact tough-

particles greater than 0.6 pm 1521. Hence, fewer particles are available in order to cover the interfacial area of the two matrix polymers. It has also to be considered that the optimal diameter of rubber particles for the impact modification of polycarbo- nate is in the range between 200 and 300 nm 1531. The larger-sized natural rubber particles with a z-average mean size of 500 nm should be less effective. The location of the PMMA-grafted rubber particles should not be affected by kinetic issues since a residence time of 3 4 min in the extruder leaves enough time to reach the equilibrium position with the lowest surface energies. The limited number of PC/PSAN ternary blends containing PBuA-based latex particles indi- cates that increasing the amount of the shell-forming polymer from 25 to 50% had little effect on the toughness of the prepared blends.

rubber Particles not only determines toughness but also affects the modulus of the rubber-modified polymer. Figure 13 shows a typical example of the dependence of the E-

The amount of the FIGURE 14. Scannin electron photomicrograph of the notched Izod fracture su&e of a PC binary blend reinforced by 12% PBuA-based latex particles containing 50% crosslinked PMMA in the shell region (6% PBuA).

Natural Rubber and Polybutylacrylate-based Composite Latex Particles / 433

FIGURE 1 5. Comparison of the impact energ of PC/PSAN blends reinforced b NR-based core-shell partic r es or 350 nm

subinclusions within the rubber core (coreshell ratio: 75/25, 8% particles, 70% PC content).

PBuA-based cores I ell latexes containing 0, 15 or 30% PS

ness of the prepared PC blend since it relieves triaxiality. Most of the voids in the fracture surface are 200400 nm in size and correspond to the particle size of the incorporated latex particles. However, the SEM photomicrograph provides evidence of limited coalescence of voids which explains that some micrometer-sized cavities can be discerned too. Cavities that had not completely coalesced can be seen in the center of Fig. 14. This failure mechanism is more pronounced at sites where the rubber particles are very close to each other. Secondary crack initiation at large-sized cavities (20-40 pm) is possible and a low impact resistance of the ductile PC matrix would result [54]. However, the size of the coalesced voids is limited below 2 pm and it is reasonable to assume that detrimental secondary crack initiation had been avoided.

The performance of NR-based core-shell particles containing rigid PS subinclusions within the rubber phase was clearly superior compared with simple core-shell particles. It was also verified whether PS subinclusions can improve the effectiveness of smaller-sized PBuA-based core-shell particles. Fig- ure 15 compares different NR and PBuA-based core- shell particles containing different amounts of PS subinclusions. All latex particles contained 25% PMMA in the shell region.

PBuA-based latex particles were slightly less effective in confering toughness to PC/PSAN blends. Subinclusions within PBuA-based latex particles. The reason why no greater difference in fracture toughness was found when using different composite PBuA-based latex particles could be related to their smaller particle size. The presence of latex particles in such ternary blends is known to promote compatibility between PC and PSAN [55]. More and smaller-sized latex particles should be more effective. Furthermore, a particle size of about 250 nm is optimal for the toughening of PC (531. A change of the particle morphology could not further improve their performance. The incorporated PBuA-based composite latex particles were ineffective to toughen PSAN binary blends [311. However, they proved to be a very effective toughening agent for ternary PC/ PSAN blends. This finding indicates that an impact

FIGURE 16. Scannin electron photomicrograph of the notched lzod fracture sujace of a PC/PSAN ternary blend reinforced by 8% 350 nm PBuA-based latex particles containing 25% crosslinked PMMA in the shell region (6% PBuA, 70% PC content).

modifier must be adapted to the PC major matrix component which can absorb more energy than PSAN. On the other hand, NR-based core-shell particles containing 0, 15 or 30% PS subinclusions toughened PSAN blends [311. However, their mor- phology had to be optimized by the introduction of rigid subinclusions in order to reinforce ternary PC/ PSAN blends. This finding reflects the reasoning that rubber particles have to be tailor-made to toughen the PC matrix component. The toughening of the PSAN matrix is less crucial.

SEM reveals a ductile fracture surface in the case of a ternary PS//PSAN blend containing 350 nm sized PBuA based core-shell particles. The PC and PSAN phases cannot be discerned in Fig. 16. This finding may be due to a better adhesion of the two matrix polymers in PC/PSAN blends containing many small-sized PBuA-based latex particles. The improved stress transfer at impact induced the PSAN domains to be deformed to the same degree as the PC matrix and the two matrix polymer phases could not be distinguished by SEM. Similar fracture surface morphologies were observed when PBuA-based latex particles containing 15 or 30% rigid subinclusions within the core had been incorporated into PC/PSAN blends.

Blends of PC, PS and Composite Latex Particles

Polystyrene as the second matrix component was chosen since it is not miscible with either PC, the major matrix component, or the PMMA shell of the incorporated impact modifiers [39, 561. It has been reported in the literature ill1 that PC blends with PSAN or PMMA could be effectively toughened by a commercial core-shell impact modifier. However, PC/PS blends containing the same core-shell particles were not equally tough. These core-shell particles were 180 nm in size and did not toughen PS to any degree [ll]. On the other hand, polydisperse

434 / Schneider et al

Notched Izod 6o 1 ;;pact resistance p

50 -

40 - 30 - 20 -

/ SO LJ

O ' , . l . , . , . , . l . l . l . , l 0 1 2 3 4 5 6 7 8

wt.-% NR content in the blends

FIGURE 17. Impact resistance of PUPS blends reinforced by pure NR articles or coreshell particles containin

and 25% crosslinked PMMA in the shell region (70% PC content).

%O! l 5 Or 30% crosslin t: ed PS subinclusions within the natural ru ber core

NR-based core-shell particles with a z-average mean particle size of 500 nm are very effective in conferring toughness to PS [9, 28, 291. Toughening synergisms for PS blends containing a minimum number of large- sized particles and a maximum number of small-sized particles have been reported [57, 581. It should be worthwhile testing whether it is possible to reinforce PC/PS blends by NR particles. All presented PC/PS ternary blends had been prepared and tested analogous to the studied PC/PSAN ternary blends and a direct comparison of all ternary blends is possible.

Effect of PS Subinclusions within NR-based Core- Shell Particles Figure 17 compares the effectiveness of different composite NR-based latex particles to increase the impact resistance of PC/PS blends containing various amounts of composite rubber particles. Both pure NR particles or simple core- shell particles only improved the impact resistance of PC/PS blends by a factor of three. On the other hand, the impact energy of such blends could be increased by a factor of ten when 6 or 8% NR-based coreshell particles containing 15 or 30% PS subinclusions had been incorporated. Maximal impact resistance corre- sponds to a NR weight fraction of 3.6 in the blends. Figure 17 shows that the plots of the impact resistance against the NR weight fraction in PC/PS blends reinforced by core-shell particles containing different weight fractions of rigid PS subinclusions within the rubber core are superimposed. This means that the NR weight fraction is controlling the toughness of the prepared PC/PS blends. Only 15% PS subinclusions within the rubber particles were sufficient to considerably increase the toughening effectiveness of NR-based core-shell particles.

In order to gain insights into the failure mechanism, SEM investigations were performed. Figure 18 shows the notched Izod fracture surface of a PC/PS blend containing 6% pure natural rubber

FIGURE 18. Scannin electron photomicrogra h of the

reinforced by 6% pure NR particles (70% PC content). notched Izod fracture su 4 ace of a PC/PS ternary E lend

latex particles. It indicates a brittle fracture mode for the PC/PS blend containing 6% pure natural rubber particles. Elongated PS domains, 1-15 pm, within a PC matrix can be seen. The morphology tends to become co-continuous as observed in the case of PC/ PSAN ternary blends. Holes of debonded natural rubber particles are visible in the PSAN phase. The prepared blend was not very tough since multiple rubber cavitation was not effective as in the case of the PC/PSAN blends containing pure NR particles. It shoald be noted that voids or markings were not observed in the PC major matrix component. This finding is an indication that the PC matrix had not been intensively deformed by shear yielding as shown by Fig. 14. It is assumed that the inferior adhesion between the PC and PS matrix polymers did not ensure a sufficient stress transfer between the two matrix polymers at impact and the incorporated pure rubber particles did not cavitate.

Simple core-shell particles could not induce the PC matrix polymer chains to be drawn out. However, core-shell particles containing subinclusions can instigate large-scale plastic deformation of the

FIGURE 19. Scannin electron photomicr raph of the notched Izod fracture sujace of a PC/PS b l e g reinforced by 8% NR-based latex particles.

Natural Rubber and Polybutylacrylate-based Composite Latex Particles / 435

FIGURE 20. Comparison of the im act ener y of PC/PS

350 nm PBuA-based coreshell latexes containing 0, 15 or 30% PS subinclusions in the rubber core (coreshell ratio: 75/25, 8% particles, 70% PC content).

blends reinforced by NR-based cores E 9 ell partic es or about

ternary PC/PS blend as revealed by the SEM photo of Fig. 19.

A co-continuous blend morphology is indicated when the fracture surface is observed in less intensively deformed zones which are located further away from the middle core area of the test sample. Decreasing the amount of the rigid subinclusions within the NR core did not further change the morphology of the notched Izod fracture surface.

PUPS Blends Containing NR or PBuA-based Rubber Particles. Toughening PC/PS blends by small PBuA-based core-shell particles was also attempted. Figure 20 compares the impact resistance of PC/PS blends containing different NR or PBuA-based latex particles. NR-based latex particles with subinclusions were more effective in conferring toughness to PC/PS ternary blends than all types of PBuA-based latex particles. As in the case of ternary PC/PSAN blends, the performance of small-sized PBuA-based latex particles could not be further improved when different amounts of subinclusions had been intro- duced.

Figure 21 shows a SEM photomicrograph of PC/ PS blends containing 350 nm sized PBuA-based latex particles. The toughness of the ternary blend can be attributed to the large amount of surface area which has been created. The PC and PS phases cannot be clearly distinguished. This finding could be due to the smaller particle size of the PBuA-based latex particles which cover the PC/PS interface to a higher degree. Hence, both matrix polymers had been extensively deformed.

The morphologies of the fracture surfaces of PC/ PS blends with the other types of PBuA-based latex particles do not show significant differences.

CONCLUSIONS In this study the influence of the morphology of NR and PBuA-based latex particles on the toughness of PC and PSAN- or PS ternary blends was examined. Seventy per cent of PC and 30% of PSAN or PS blends which contained the composite rubber particles were melt blended in a co-rotating twin screw extruder.

FIGURE 2 1. Scannin electron microphot raph of the notched Izod fracture suface of a PC/PS b l e g reinforced by 8% PBuA-based latex particles containing 30% PS subinclusions within the PBuA core and 25% crosslinked P M M A in the shell region (6% PBuA+PS, 70% PC content).

The thickness of the PMMA shell and the mass fraction of rigid PS subinclusions within the rubber particles were varied.

1.

2.

3.

4.

SEM indicates that the incorporated rubber particles were distributed preferentially in the brittle polymer phase or at the interface of the two matrix polymers. Multiple cavitation within the rubber phase was the predominant mechanism for improving the toughness of PC/PSAN blends containing pure NR particles. A hard PMMA shell reduced this effect and the toughening effectiveness of the incorpor- ated rubber particles was considerably decreased. PC/PSAN blends reinforced by core-shell particles containing rigid PS subinclusions were deformed more intensively by a ductile tearing mode and the impact resistance of such blends was considerably increased. It is especially noteworthy that PC/PS blends which have a low interfacial adhesion could be toughened as well. The highest fracture toughness was also achieved with core-shell particles con- taining rigid subinclusions within the natural rubber phase. Simple NR-based core-shell par- ticles were considerably less effective. Pure natural rubber particles did not form cavities at impact and consequently did not significantly improve the toughness of PC/PS blends. PBuA rubber-based core-shell particles have been demonstrated to be also very effective in toughen- ing both PC/PSAN and PC/PS blends. However, the morphology of about 350 nm synthetic PBuA- based core-shell particles could not be improved since they cover a larger area of the PC/PS or PSAN interface and are already well adapted to toughen the PC major matrix component.

The results presented in this paper demonstrate the importance of the morphology of rubber particles on the toughness of ternary blends with a PC major

A36 / Schneider et al.

matrix component. Polydisperse NR-based core-shell particles, 500 nm in size, containing rigid subinclu- sions within the rubber core were most effective in conferring toughness to the prepared ternary PC blends. The NR weight fraction determines the toughness of such blends. This approach to reinfor- cing polymer blends seems to be transferable to industrial polybutadiene-based latex particles.

ACKNOWLEDGMENTS The authors would like to take this opportunity to express their appreciation for support of this research project by the European Union (BRITE-EURAM Project BE-4260). Bayer, Elf Atochen and BASF are thanked for supplying the PC, PS and E A N polymers used in this study. Valuable discussions concerning the results presented in this paper were held at at the Deutsches Kunststoff-Institut in Darmstadt, Germany, and the Foundation for Research and Technology in Heraklion, Greece.

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