Polymers for Advanced Technologies Volume 6, vv. 326-334

Impact Modification of Thermoplastics by Methyl Methacrylate and Styrene-grafted Natural Rubber Latexes Michael Schneider, Tha Pith and Morand Lambla* Ecole dApplication des Hauts Polymires, lnstitut Charles Sadron (CRM-EAHP), 4 rue Boussingault, Strasbourg, France

ABSTRACT

The preparation and characterization of polymer blends with structured natural rubber (NR)-based latex particles are presented. B y a semicontinuous emulsion polymerization process, a natural rubber latex (prevulca- nized or not) was coated with a shell of crosslinked polymethylmethacrylate ( P M M A ) or polystyrene (PS) . Furthermore, core-shell latexes based on a natural rubber/crosslinked PS latex semi-interpenetrating network were synthesized in a batch process. These structured particles were incorporated as impact modi- fiers into a brittle polymer matrix using a Werner 6 Pfleiderer twin screw extruder. The mechanical proper- ties of PS and PMMA blends with a series of the prepared latexes were investigated. In the case of PMMA blends, relatively simple core (NR)-shell (crosslinked PMMA) particles improved the mechanical properties of PMMA most effectively. An intermediate PS layer between the core and the shell or a natural rubber core with PS subinclusions allowed the €-modulus to be adjusted. The situation was different with the PS blends. Only core-shell particles based on NR-crosslinked PS latex semi-interpenetrating networks could effectively toughen PS. It appears that microdomains in the rubber phase allowed a modification of the crazing behavior. These inclusions were observed inside the NR particles by transmission electron microscopy. Transmission elec- tron photomicrographs of PS and PMMA blends also revealed intact and well-dispersed particles. Scanning electron microscopy of fracture surfaces allowed us to distinguish PS blends reinforced with latex semi- interpenetrating network-based particles from blends with all other types of particles.

To whom correspondence should be addressed.

KEYWORDS: natural rubber; core-shell particles; rubber toughening; polystyrene; polymethyl- methacry late

INTRODUCTION A well-established means of improving the impact strength of fragile polymers is to incorporate a dis- persed rubber phase [l-31. A possible way to obtain very fine and well-defined morphologies consists of using polymer blends with core (rubber)-shell (hard polymer) latex particles of 0.1-1 pm which are easily prepared by an emulsion polymerization process (4- 61. The latex particles act as stress concentrators and produce more extensive matrix deformation by craz- ing or shear yielding. Depending on the intrinsic brittleness of the matrix polymer, they toughen the matrix by generating multiple crazing as in the case of polystyrene (PS) [7] or can change the main fracture mechanism from crazing to shear yielding as in rubber-toughened polymethyl methacrylate (PMMA) [8]. High impact polystyrene (HIPS) is usually made by a bulk or suspension polymeriza- tion process and contains rubber particles with polystyrene inclusions. It is very difficult to deter- mine exactly the influence of factors like the rubber volume content, the morphology of the dispersed rubber particles, the adhesion of the rubber to the matrix and the dynamic mechanical properties of the rubber phase on the fracture toughness of PS. In order to study these factors separately different pre- formed core-shell particles can be incorporated into PS IS].

This work was focused on the influence of the morphology of natural rubber-based core-shell particles on the toughness of PMMA and PS. Natural

CCC 1042-7147/95/050326-09 0 1995 by John Wiley & Sons, Ltd.

Received 20 May 1994 Revised 25 September 1994

Impact Modification of Thermoplastics I 327

rubber (NR) as the core polymer in latex particles has an overall balance of properties which is unmatched by synthetic elastomers. Besides, NR is a cheaper alternative to synthetic impact modifiers because of its botanical and, hence, renewable origin. However, because of its low glass transition temperature T g , and lightly tacky nature, NR cannot be used directly in polymer blends. First, it has to be adapted in order to facilitate the melt mixing and to be effective for rubber toughening. NR-based latex particles with a sufficiently thick shell of a hard polymer give a free- flowing powder, which is usually employed in con- tinuous extrusion processes for the preparation of polymer blends. Last but not least, emulsion poly- merization allows the adjustment of the microstruc- ture of the latex particles, their size and their inter- facial properties which determine primarily the mechanical properties of blends with such particles. In order to be suitable for the reinforcement of PS or PMMA matrixes, NR-based particles were coated with a shell of PMMA or PS. Redox systems allowed to control the site of the polymerization and thereby determined the morphology of the prepared compo- site NR-based particles. The first part of this article will be devoted to the description of specific poly- merization recipes for different types of structured particles. Later on, the mechanical properties of PMMA and PS blends with the prepared latexes will be evaluated.

The z-average mean value of the seed latex particle size was 490 nm. The glass transition tem- perature of the vulcanized NR lies at -60°C. Styrene (S, Fluka AG) was purified by passing through an aluminum oxide column, and methylmethacrylate (MMA, Atochem) was distilled. Ethylene glycol dimethacrylate (EGDMA, Merck), tert-butyl hydro- peroxide (t-BuHP, 70% aqueous solution, Merck), tetraethylene pentamine (TEPA, Fluka AG), dimeth- ylaniline (DMA, Fluka) and the surfactant AD-33 (37% aqueous solution of an ammonium nonylphe- nylether sulphate, Seppic) were used as received.

Emulsion Polymerization The NR seed latex (crosslinked or not) was charged into a stirred stainless steel reactor (5 1) and, if necessary, ammonia was added in order to keep the pH of the latex above 9. For at least 30 min the system was purged by nitrogen before the styrene or methylmethacrylate (66% based on NR) emulsions were fed continuously into the main reactor which was thermostated at 50°C. Water had been added to the seed latex and the monomer pre-emulsion in order to maintain the latex solid content at 4090. The monomers always contained 0.25 wt% of ethylene glycol dimethacrylate as the crosslinking agent and 1.50 wt% of the surfactant Ad-33 based on the final latex solids content. A bipolar redox initiating system [lo, 111 of tert-butyl hydroperoxide and tetra- ethylene pentamine was employed in the ratio 1 : 1. In the case of the MMA polymerization 1.0 wt% of the initiator system based on NR was used. The quantity of the initiator needed had to be raised to 1.8 wt% for the S polymerization. The activator TEPA was added into the main reactor together with the NR seed latex, whereas the initiator t-BuHP was fed together with the pre-emulsion during 6 hr (MMA) or 12 hr (S). At the end of the monomer addition, the temperature was maintained for 2 hr at 50°C in order to finish the reaction. The degree of conversion was determined gravimetrically. Table 1 summarizes a typical recipe for a grafted NR latex.

As indicated previously, three-layer particles were also prepared by sequential emulsion polymeri- zation of first S and then MMA in equal amounts (2X33% based on NR). The feeding pump was stopped for 2 hr between the two polymerization steps in order to avoid MMA being copolymerized with residual S of the first addition.

COMPOSITE NATURAL RUBBER-BASED LATEXES Materials A centrifuged prevulcanized NR latex (Revultex MR) and an un-crosslinked NR latex (Revertex AR) with a solids content of 60% supplied by Revertex Ltd were used as seed latexes in a sequential emulsion poly- merization. The extreme polydispersity in size of natural rubber latexes is well known. The particle size distribution is plotted in Fig. 1 as obtained by photon correlation spectroscopy performed on a Malvern Autosize 11.

TABLE 1. with 40 wt% crosslinked PMMA in the Particle

Synthesis of a Composite NR-based Latex

partlcle size [nm]

FIGURE 1. Particle size distribution by number of the NR seed latexes.

Main reactor NR seed latex (prevulcanized or not) 2000 g Activator (TEPA) 129 Water 1000 g

MMA 798 EGDMA 2.0 g Surfadant (AD-33,37% in water) 8’ 9

1200 g

Feeding tank (6 h feeding time)

Initiator (t-BuHP, 70% in water) Water

17.1 g

328 1 Schneider et al.

TABLE 2. Recipe for the Latex Semi-IPN Synthesis

NR seed latex 2000 g Monomers Initiator (1-BuHP) 8.6 g Activator (DMA) 6.0 g Surfactant (AD-33) 20.0 g (37%) Water 1435 g

299.3 g styrene + 0.75 g EGDMA

Latex semi-interpenetrating networks (IPNs) consisting of uncrosslinked NR (80 wt%) and cross- linked PS (20 wto/o) were synthesized in a stirred stainless steel batch reactor (5 1) at 50°C and subse- quently coated with crosslinked PMMA (33 wt% based on the semi-IPN) as before. A hydrophobic redox system of tert-butyl hydroperoxide and dimethylaniline (ratio 1 : 1, 0.5 wt% based on NR) served for the semi-IPN preparation. First, the NR rubber seed latex Revertex AR is diluted with water so that the final latex solid content is 40%, and 0.50%, based on the final solid content of the surfac- tant Ad-33, was added. Then a solution of styrene containing the activator DMA and the crosslinking agent EGDMA (0.25 wt%) was stirred into the seed latex. After a period of 20 min, the initiator t-BuHP was added and the temperature raised to 50°C for 12 hr. Details of the quantities of reagents used for the latex semi-IPN synthesis are indicated in Table 2.

In order to provide comparison, all composite latex particles contained 40 wt% of secondary polymers. The antioxidant Ralox-LC of Raschig AG (0.50 wt% based on NR) and the antibacterial agent Preventol DG of Bayer preserved the prepared latexes.

Electron Microscopy A Cambridge Instruments Stereoscan 120 scanning electron microscope was used for the examination of latex samples which were prepared in the following way. A diluted drop of latex was put onto an aluminum support and subsequently immersed into liquid nitrogen. Then a gold layer was deposited on the still-frozen support with a Cambridge Instruments sputter coater.

A Phillips CM12 transmission electron micro- scope was used to observe latex particles, which were deposited onto a collodion film. The PMMA shell was stained with a solution of 2% phospho- tungstic acid (PTA) for 2 hr. PTA has already been used to contrast PS/PMMA core-shell latexes [12]. Ultramicrotome cuts of the prepared blends were studied too. In this case, osmium tetroxide vapors stained the NR phase in order to enhance the con- trast [13].

Results and Discussion Natural rubber-based core-shell latexes have never purposely been prepared. Although particles may be synthesized in two consecutive stages, a core-shell structure does not necessarily result. It is well known from the literature (141 that many other arrangements are possible, such as, for example, raspberry-like,

acorn-like, sandwich-like and inverted particles. The nature of the initiating system determines primarily the morphological structure of composite NR particles (151.

For instance, in order to promote a core-shell structure, we used the redox initiator couple tert- butyl hydroperoxide/ tetraethylene pentamine. In this case, most of the free radicals are produced at the monomer swollen particle/water interface, taking into account the fact that the peroxide is soluble in the organic phase, whereas the activator TEPA is water-soluble. It is reasonable to assume that the distribution of the secondary polymer (11) within the NR particle is non-uniform with a polymer 11-rich phase at the surface of the latex particle. On the other hand, the formation of NR/crosslinked PS interpene- trating networks resulted from the initiation induced by the lipophilic redox system tert-butyl hydroperoxide/dimethylaniline, since the polymerization takes place entirely within the monomer-swollen NR seed latex particles.

Investigations [16] about the film-forming ability of the different modified NR latexes helped to dis- tinguish the two initiating systems. Poor filmifica- tion (cracked film) is a sign of a high concentration of the secondary polymer in the outer region of the particles and good filmification (coherent film) char- acterizes NR composite particles with an even distri- bution of the polymer I1 inside the NR seed particles. In fact, all prepared composite NR-based particles led to cracked films by coalescence, except of course the NR/crosslinked PS latex semi-IPN which was syn- thesized with the fully oil-soluble redox system. In order to examine the formation of a shell around the NR core more closely, electron microscopic studies were performed. Figures 2 and 3 are typical scanning and transmission electron photomicrographs of core (crosslinked NR)-shell (crosslinked PMMA) particles which contain 40 wt% of PMMA.

The scanning electron photomicrograph in Fig. 2 confirms the extreme polydispersity of the synthe- sized NR-based particles. Intact and isolated particles are clearly visible. Forty wt% of crosslinked

FIGURE 2. SEM photomicrograph of core (crosslinked NR)-shell (crosslinked PMMA) particles.

Impact Modification of Thermoplastics / 329

FIGURE 3. TEM photomicrograph of core (crosslinked NR)-shell (crosslinked PMMA) particles.

PMMA in the particles seem to be sufficient to form a closed shell around the vulcanized NR core in order to prevent the NR with a glass transition temperature of -60°C from film-forming.

The PMMA shell which has been stained with phosphotungstic acid is clearly visible in Fig. 3. Of course only the external region of the shell is formed of pure crosslinked PMMA followed by an interme- diate layer of an interpenetrating network of cross- linked NR and crosslinked PMMA between the NR core and the PMMA shell.

If we reduce the amount of the crosslinked PMMA to 209'0, the latex partially recovers filmoge- neity. Isolated composite NR particles with a shell of 40 wt% of crosslinked PS could be observed too. NR (80%)/crosslinked PS (20%) semi-IPN latex particles formed a coherent film.

The scanning electron photomicrograph in Fig. 4 shows that a crosslinked PMMA shell of 33 wt% was sufficient to prevent any film formation. The second- ary polymers PS and PMMA of these particles repre- sent 40 wt% of the total particle mass as in the other synthesized core-shell particles.

The morphology of the latex semi-IPN particles which were synthesized with the fully oil-soluble

FIGURE 4. SEM photo of core (NR/crosslinked PS semi-IPN)-shell (crosslinked PMMA) particles.

redox couple will be confirmed later on (see Fig. 13). The transmission electron photomicrograph of a PS blend containing these particles shows clearly that the styrene monomer polymerized in microdomains inside the uncrosslinked NR seed latex.

Thermodynamic considerations [ 17, 181 applied to composite latex particles indicate that a polar PMMA shell around the hydrophobic NR core can be realized easily. It is the minimizing of the interfacial tension of each phase that primarily controls the arrangement of the polymer phases in latex particles. The morphology with the lowest free energy G will be taken up. That is to say, the hydrophilic PMMA forms the shell. The low temperature (50°C) and the semicontinuous feeding process which ensures a small monomer concentration in the system reduce the chain mobility during the shell synthesis. These two factors allow even an unpolar PS shell to be realized.

TOUGHENED THERMOPLASTICS - Materials

The PMMA matrix polymer Altulite GPM 2652 (MN = 55,000; M w = 114,000; M,= 192,000) and the PS matrix Lacqrtne 1260 (& = 118,000; Mw = 248,000; M, = 416,000) were kindly donated by Elf Atochem. All prepared NR-based latexes were dried under vacuum in order to obtain free-flowing powders for the computer-controlled weight feeders of the extruder.

Blend Preparation A ZSK 30 Werner & Pfleiderer twin-screw extruder was used for the incorporation of the dried latex particles into the PS or PMMA matrix. Figure 5 shows the chosen screw configuration. Three mixing zones consisting of first dispersive and then distribu- tive mixing elements helped to distribute the particles uniformly in the matrix. Any traces of residual water of the latex powders were taken out of the blend by a vacuum pump just before the last mixing zone. The extrusion process was performed at a throughput of 5 kg/hr, 150 rpm and 215°C.

The possibility of introducing a wet latex directly into the melt was established by our colleagues from the German Plastics Institute in Darmstadt. However, only the pure NR latex was fed in this way, as it is not possible to obtain a powdered product of pure NR without high levels of partition agents such as, for example, zinc oxide or clay.

ASTM test samples were molded on a Billon 150/ 150 injection-molding machine.

Mechanical Tests The IZOD impact resistance of notched samples (ASTM D256) was obtained using a standardized Zwick pendulum impact testing machine at room temperature. Tensile testing on dumbbell samples (ASTM D638) was performed on a hydraulic Instron 8031 machine at room temperature. The strain rate

330 I Schneider et al.

FIGURE 5. Machine set-up for the blend preparation.

was 50 mmlmin for the PMMA samples and 5 mml min for the PS samples. The elongation was mea- sured directly on the sample by a tensometer.

Results and Discussion Mechanical Properties of Rubber-toughened PMMA Blends. In order to provide comparison to subse- quent tests, all composite NR latexes contained 40 wt% of secondary polymers which were all cross- linked with 0.25 wt% of EGDMA. Results of the notched Izod impact test for the PMMA blends with either crosslinked NR or uncrosslinked NR-based core-shell particles are presented in Figs 6 and 7. The incorporated composite particles are represented schematically on top of each bargraph. Under each graph, the Young's modulus ( E ) and the elongation at break of the tensile tests are given.

Figure 6 shows that there was a substantial increase of the notched impact energy of PMMA only when core-shell particles were incorporated. The intermediate IPN layer between the shell and the NR core (Fig. 3) has created a strong interface between the NR and the PMMA matrix. Natural rubber is able to play a role in modifying the deformation mode of PMMA only in this way. Unmodified vulcanized NR lowered only the Young's modulus of the blend without an increase of the impact resistance. The hard shell was helpful to make the NR compatible with the matrix polymer. A good dispersion and stability of the particles were ensured by it. A PS shell was less effective than a PMMA shell, because PMMA and PS are not compatible. Besides, PMMA

Notched lzod impact resistance

Emodulus 3700 2190 2260 2230 m8 break strain 1.9 3.7 13 16 %

FIGURE 6. lzod impact energy of injected PMMA blends with different prevulcanized natural rubber based core-shell particles (20 wt% of NR in the blends).

protects the NR more effectively against oxidation, since oxygen permeates PS 25 times more than PMMA [19].

Figure 7 indicates that uncrosslinked NR was better suited than prevulcanized for the impact modification of PMMA. Complex structures with an intermediate layer of PS or a NR/crosslinked PS latex semi-IPN lowered the impact resistance. However, these structures conferred a significantly higher E-modulus to the blend compared with a blend with simple core (NR)-shell (crosslinked PMMA) particles. The superior impact performance of simple core-shell particles was also reflected by the high elongation at the break of the tensile test samples. This type of particle acted as a very effective stress concentrator and produced more extensive matrix deformation by shear yielding. Blends with less NR rubber content (e.g. 15% by weight) confirmed these findings.

The transmission electron photomicrograph of a PMMA blend with core (NR)-shell (crosslinked PMMA) particles in Fig. 8 shows well-distributed and intact latex particles. The polydispersity of the incorporated NR particles between 0.1 and 1.5 pm is clearly illustrated .

Figure 9 shows that the impact resistance of PMMA blends with NR-based latex particles rises linearly with the natural rubber mass fraction in the blend. The incorporated impact modifier is based on uncrosslinked NR particles containing 40 wt% of

E-modulus 3700 2040 1960 1650 bnrk strain 1.9 16 16 25

PMMA

NR

FIGURE 7. lzod impact energy of injected PMMA blends with different uncrosslinked NR based core-shell particles (20 wt% of NR in the blend).

Impact Modification of Thermoplastics / 331

FIGURE 8. Transmission electron photomicrograph of a PMMA blend with core (NR)-shell (crosslink PMMA) particles (20 wt% of NR in the blend, NR stained with

crosslinked PMMA in the shell. This type of particle reinforced PMMA most effectively.

The values of the Young's modulus (E) and the elongation at break obtained from tensile testing are plotted in Fig. 10 versus the NR mass fraction in the blends. Both values were controlled by the NR content in the blend.

A similar dependence of the mechanical proper- ties of PMMA blends with core (polybutyl acry1ate)- shell (crosslinked PMMA) particles on the rubber volume fraction was established by Lovell and co- workers [20, 211. He found that the Young's modulus was controlled only by the rubber volume fraction, whereas the impact resistance depended also on particle size and morphology.

As a natural rubber latex with a very polydis-

OSO.).

Notched Izod im actresistance

141 pkJ/rnll =/

Notched Izod im actresistance

14- pkJ/rnll

12-

10-

8 -

incorporated partklm

. I . , . I . I . I *

0 5 ?O 15 M 25 natural rubber mass fracton in PMMA

FIGURE 9. Dependence of the lzod impact strength on the NR content of injected PMMA blends with core (NR)- shell (crosslinked PMMA) particles.

perse particle size distribution (Fig. 1) had been used, it could not be determined how the impact resistance depended on particle size. However, the optimum particle size of toughened PMMA has been established in the literature (221 to be in the range of 200-250 nm for acrylic core-shell particles. Bigger particles between 300 and 600, which is the scope of the used NR latexes, were only slightly less efficient. Furthermore, a minor synergy effect was observed [23]. Consequently the utilization of polydisperse NR latexes is efficient for the impact modification of PMMA. It has to be taken into account, though, that the majority of the NR rubberr particles are less than 0.5 pm, but most of the mass of rubber resides in particles greater than 0.6 pm. Hence the NR is more efficiently exploited for the rubber toughening of PS or especially poly(styrene-co-acrylonitrile) (PSAN) which requires bigger particle sizes of 2.5 pm [24] and 0.75 pm [25] respectively for optimum rubber toughening.

The scanning electron photomicrograph in Fig. 11 of a PMMA blend with core (NR)-shell (cross- linked PMMA) particles shows an Izod fracture sur- face. All but a few toughening particles which stayed in one of the two fracture surfaces have debonded from the matrix. The size of the holes is approxima- tely equivalent to the particle size distribution observed in the transmission electron photomicro- graph of Fig. 8. A similar picture has been observed for all other composite NR particles with a cross- linked PMMA shell. Pure NR particles debonded from the fracture surface too. In the case of a PS shell, which is incompatible with the PMMA matrix, fewer particles have been tom out of the fracture surface but stayed embedded in the matrix polymer.

Based on scanning electron microscopy, one has to assume that a strong rubber-matrix adhesion did not change the fracture mechanism of PMMA. However, the impact resistance and the elongation at break of core (NR)-shell (crosslinked PMMA) and unmodified NR blends indicated clearly that a strong rubber/matrix adhesion is essential in order to induce extensive matrix deformation by shear yield- ing at high (impact testing) and slow (tensile testing) speeds.

Mechanical Properties of Rubber-toughened PS Blends. Figure 12 summarizes the results of the notched Izod impact strength of PS blends with different NR-based core-shell particles. The circles on top of each bar graph schematically represented composite latex particles are identical to the latexes which were incorporated in the PMMA blends. So it was possible to compare the effectiveness of these particles for the rubber-toughening of two polymers which deform by either crazing (PS) or by shear yielding (PMMA).

PS could only be toughened effectively by core- shell latex particles which consist of a NR/crosslinked PS semi-IPN in the core and a cross- linked PMMA shell. Simpler core-shell particles which are based on vulcanized or uncrosslinked NR reinforced PS to a much lesser degree. The chemical composition of the shell of vulcanized NR-based

332 I Schneider et al.

0500

25w

m

1500

NR

6r, . I . I . 8 . 1 . ; . ; 10 1s 20 28

natural robber mass fraction in the PMMA bksd

FIGURE 10. Plots of Young’s modulus and elongation at break against NR mass fraction for the injected PMMA blends with core (NR)-shell (crosslinked PMMA) particles.

core-shell particles had little effect on the impact toughness of PS. A PMMA shell which is incompa- tible with PS should render the rubber toughener ineffective compared with a PS shell. In fact Rudin and coworkers [9] found that poly (n-butyl acrylate) rubber-based core-shell particles must have a PS shell in order to provide a strong bond between the rubber core and the matrix. However, the results in Fig. 12 suggest that the physical properties of the core play the dominant role for the impact modifica- tion of PS. Core (NRIcrosslinked PS semi-1PN)-shell (crosslinked PMMA) particles improved the tough- ness of PS by a factor of 20 without interfacial bonding between the PMMA shell of the modifier and the PS matrix.

The transmission electron micrograph in Fig. 13 shows clearly the morphology of the NR core with PS microdomains. Only NR particles which contain PS occlusions were able to nucleate a large number of crazes, permitting a large volume of plastic deforma- tion before failure. It has been demonstrated [26, 271 that rigid occlusions in rubber particles, as in a typical HIPS, constrain the rubber that separates

them. As an occluded particle is extended in the tensile direction, the rubber is broken up into fibrils and no large voids are formed. A homogeneous rubber particle behaves differently. It extends in the tensile direction and contracts along a direction normal to the tensile axis till the particle debonds from the matrix polymer. Large voids are formed between the rubber core and the outer grafted layer, resulting in a premature failure of the craze.

Examination of the Izod fracture surface in Fig. 14 of a PS blend with NR/crosslinked PS semi-IPN based core-shell particles confirms these findings. Every single latex particle was well integrated into the PS matrix.

Figure 15 shows a typical fracture surface of a PS blend with core (crosslinked NR)-shell (crosslinked PMMA) particles. Here nearly all incorporated particles have debonded from the matrix polymer. The picture is very similar to the fracture surfaces of PMMA blends.

A PS shell led to the fracture surface in Fig. 16, where only about half of the incorporated particles have been tom out. The better adhesion of the

Notched lzod impact NWcr.PS mistance

I B t a d W N :

PUMA

\

4 3 . r 2.4 breakstrain 2 2 33 2.5 2.2 2.6 ?”

FIGURE 11. Fracture surface of a PMMA blend with core (NR)-shell (crosslinked PMMA) particles (20 wt% NR in the blend).

FIGURE 12. lzod impact energy of injected PS blends with different NR-based core-shell particles (20 wt% NR in the blend).

Impact Modification of Thermoplastics / 333

FIGURE 13. TEM photo of a PS blend with core (NR/crosslin ked PS semi-IPN)-s hell (cross tin ked PMMA) particles (20 wt% NR in the blend, NR stained with

PS-coated particles to the matrix explains this beha- vior. Some PS inclusions in the NR core were formed during the shell synthesis. These few PS domains were not sufficient to fibrillate the NR during impact.

The polydisperse particle size distribution of the incorporated impact modifier (Fig. 1) is beneficial for the rubber-toughening of PS. Optimum mechanical properties for a bimodal HIPS with a minimum number of large particles and a maximum number of small particles have been reported in the literature (28, 291. Okamoto et LIZ. [28] found that the improve- ment of bimodal HIPS is due to longer crazes initiated primarily at large particles and not to an increased number of crazes compared to monomodal HIPS. Their model, which is based on a finite element method, suggested that crazes are first nuc-

oso, ) .

FIGURE 15. Fracture surface of a PS blend with core (crosslinked NR)-shell (crosslinked PMMA) particles (20 wt% NR in the blend).

leated at large particles and then at small particles closest to a large particle, because the stress is highly concentrated there.

CONCLUSIONS In this study the applicability of NR-based composite latex particles as a possible impact modifier for thermoplastics has been evaluated. It was shown how the morphology of synthesized NR-based latex particles affects the impact resistance of PMMA and PS. These two polymers require entirely different particles sizes for optimal rubber toughening, as large particles are more efficient in initiating crazes and small particles are more efficient in inducing shear yielding. In fact, the extreme polydispersity of the employed natural rubber latex allowed to toughen the two polymers which deform either by shear yielding (PMMA) or crazing (PS).

1. Conventional emulsion polymerization pro- cesses have been adapted for the preparation of NR-based composite latex particles. Both vulcanized and uncrosslinked NR seed latexes

FIGURE 14. Fracture surface of a PS blend with core (latex-IPN NRIcrosslinked PS)-shell(cross1inked PMMA) particles (20 wt% NR in the blend).

FIGURE 16. Fracture surface of a PS blend with core (NR)-shell (crosslinked PS) particles (20 wt% NR in the blend).

334 I Schneider et al.

were used. A bipolar redox initiator (tert-butyl hydroperoxide/tetraethylene pentamine), low temperature and a semicontinuous feeding process promoted the formation of crosslinked PS or PMMA shells around a NR core. The core-shell morphology has been verified by transmission and scanning electron micros- copy. A fully oil-soluble redox system (tert- butyl hydroperoxide/dimethylaniline) served for the preparation of NR/crosslinked PS semi-interpenetrating networks. A twin-screw extruder with three kneading zones has been adapted for the incorporation of the modified NR particles as a powder into PS or PMMA matrixes.

Transmission electron photomicrographs of the prepared blends which show well- distributed and intact particles proved that the employed extrusion procedure was well adapted for the incorporation of the NR-based impact modifiers. NR-based core-shell particles toughened PMMA effectively. A crosslinked PMMA shell made the natural rubber compatible with the PMMA matrix. Unmodified NR particles did not toughen PMMA. Prevulcanized rubber proved to be less effective than uncrosslinked natural rubber. The Young’s modulus could be raised with only a minor loss of the impact strength by complex latex particles, such as, for example, a latex semi-interpenetrating network of NR/crosslinked PS as the core polymer or with an intermediate PS layer between the PMMA shell and the NR core. The impact resistance and the Young’s modu- lus of PMMA blends with core (NR)-shell (crosslinked PMMA) was controlled by the NR mass fraction.

Scanning electron pictures of the Izod fracture surface of the prepared PMMA blends containing latex particles with a crosslinked PMMA shell revealed that nearly all the incor- porated particles have been torn out of the matrix. A crosslinked PS shell which is incom- patible with the PMMA matrix led to a fracture surface where less particles have debonded. No effective stress transfer between the NR core and the PMMA matrix was possible. PS which requires at least three times bigger particles for optimal reinforcement can be effectively toughened too. However, NR- based core-shell particles toughened PS only when PS microdomains were present inside the NR core. This is in accordance with the literature about HIPS. The subinclusions have been observed by transmission electron mic- roscopy. Scanning electron microscopy showed debonded particles for all types of composite natural rubber particles except the particles with PS micmdomains inside the core. Interfacial bonding between the impact modifier and the PS matrix seemed not to be very important, as the most effective system

was based on natural rubber/crosslinked PS semi-IPN particles covered with a PMMA shell.

ACKNOWLEDGMENTS The authors would like to take this opportunity to thank the European Economic Community for financing this research project (BRITE-EURAM Project BE-4260). The as- sistance of the German Plastics Institute in Darmstadt for the transmission electron studies and the optimized wet latex feeding on the extruder is also gratefully acknow- ledged.

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