The effect of a rubbery interfacial layer on the tensile
behaviour of polystyrene-glass bead composites
Citation for published version (APA):
Dekkers, M. E. J., Dortmans, J. P. M., & Heikens, D. (1985). The effect of a rubbery interfacial layer on the tensile behaviour of polystyrene-glass bead composites. Polymer Communications, 26(5), 145-148.
Document status and date: Published: 01/01/1985 Document Version:
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The effect of a rubbery interfacial layer on the tensile
behaviour of polystyrene--glass bead composites
M. E. J. Dekkers, J. P. M. DortmansanQ,D. Heikens
Eindhoven University of Techn%gy, Laboratory of Polymer Techn%gy, PO Box 513,5600 MB Eindhoven. The Netherlands
(Received27September 1984)
An experimental procedure is described for preparing polystyrene--glass bead composites with a thin rubbery interfaciallayer. The eftects of the interlayer on both the mechanism for craze formation at the beads and on the stiffness and toughness of the composites are reported. It is shown that besides being formed in the equatorial region of the beads, crazes are also formeel between the poles and equator as a consequence of decohesion between the bead and polystyrene matrix at the poles. It is argued that the gr.owth of these .Iatter crazes is not controlled by the dispersed particles and that therefore the composites wlth a rubbery Interlayer are not tougher than the composites without such an interlayer.
(Keywords: polystyrene-glass bead composites; rubbery interfaciallayer; craze formation; stiffness; toughness)
INTRODUCTION
For many researchersinthe field ofpolymercomposites it is achallenge to create a composite both stiller and tougher than the matrix material. The introduction of dispersed rubber particles into a 'brittle' polymer such as polystyrene (PS) results in a substantial increase in toughness but at the same time in a reduction in modulus. The mechanisms of rubber toughening have recently been reviewed1 but are still not completely understood. It is
however weIl established that the function of the rubber particles is to initiate multiple craze formation in the matrix particularly near the equators of the particles. Important requisites for optimum toughness are good interfacial adhesion and a particle diameter of ,about
1-5J-Lm. •
In a recent paper2on the tensile behaviour of PS-glass bead composites (average bead diameter 30J-Lm) it has been shown that, unlike rubber particles, the introduction of rigid glass beads results in an increase in modulus. Although the beads also act as craze initiators, the introduction of glass beads was found to have no significant toughening effect, neither in the case of excel-lent nor in the case of poor interfacial adhesion. The degree of interfacial adhesion does have a profound effect on the mechanism for craze formation at the beads3
,4.At an excellently adhering glass bead, crazes form near the poles of the bead in the regions of maximum triaxial stress concentrations. At a poorly adhering glass bead, craze formation is preceded by dewetting along the interface between bead and matrix; on dewetting a curvilinear interfacial crack is formed, starting at the pole and propagating into the direction of the equator until, at an angle of about60° from the pole, a craze originates at the tip of the interfacial crack.
In 1969 Matonis and Sma1l5 suggested that
encapsu-lation of rigid spherical inclusions within a layer of low modulus elastomer might be a method to obtain a composite tougher than the matrix material and with a modulus of the same order of magnitude as the matrix material. In thei.r theoretical study they showed that in this connection the thickness of the interfaciallayer is very critical. The layer must be thin enough in order that the matrix still 'feeis' the rigid sphere so that the sphere contributes to the stitTness of the composite. While at the same time the layer must be thick enough to create a stress field which resembles the stress field around a rubber spherical particle; so that the maximum triaxial stress concentrations are near the equator instead of near the poles in order that craze formation occurs near the equator. The optimum thickness of the interfacial layer was calculated to be about
1%
of the radius of the rigid sphere.In the present paper an experimental procedure is described for placing a thin layer of thermoplastic styrene-butadiene rubber between a glass bead and a PS matrix. The effects ofthe rubbery interfaciallayer on both the mechanism for craze formation at the beads and on the stillness and toughness of the composites are reported. EXPERIMENTAL
Ma teria Is
The PS used was Styron 634 obtained from Dow Chemical. The thermoplastic rubber used was Shell Cariflex 1102,a styrene-butadiene-styrene (SBS) block copolymer. Glass beads were used in two different bead diameter ranges: 0.5-10J.lmwith an average diameter of
2 J-Lm, and 10-53J-Lm with an average diameter of 30J-Lm.
0263--6476/85/050145--04$03.00
Tensi/e behaviour of po/ystyrene-g/ass bead composites: M. E. J. Dekkers et al. In the rest of this paper these two bead diameter ranges
will be referred to as 2 flm-glass and 30 flm-glass.
Surface treatments of the beads
Before being dispersed in PS by melt-mixing, the glass beads were given different surface treatments to obtain (a) excellent interfacial adhesion, (b) poor interfacial adhesion, (c) a thin rubbery interfacial layer. Excellent interfacial adhesion was obtained by coating the beads with a cationic vinylbenzyl trimethoxysilane [(CH30hSi(CH2hNH(CH2hNHCH2-e6H4-eH =CH2· HCI] (Dow Coming Z-6032). Poor interfacial adhesion was obtained by coating with vinyltriethoxy-silane (Fluka). Both coating procedures are described elsewhere3. The procedure for coating the beads with a rubbery layer is based on the work ofPlueddemann6.Itis essential that the rubbery layer remains on the beads during melt-mixing and that it adheres weIl to both glass and PS. The principle of the coating procedure described below is that the SBS rubber and the silane coupling agent that adheres SBS to glass are applied simultaneously from a mutual solvent. Adhesion between the rubber coating and PS is then to be achieved as a result of the compatibility of PS with SBS during melt-mixing.
Coating procedure. First the glass beads were cleaned by refluxing isopropyl alcohol for 2 hand vacuum dried for 1 h at 130°e. Then 50 g ofcleaned glass was stirred in a refluxing solution of 200 mI toluene containing 7 g SBS, 7 mI Z-6032 and 1 g dicumylperoxide..After 2 h, 2 mI concentrated hydrochloric acid was added dropwise and stirring was continued for 1/2 h.After evaporation of the solvent, the remaining material was cured for 2 h at 11
ooe
under vacuum and then thoroughly washed with toluene to remove redundant SBS. The remaining coated beads were vacuum dried for 1 h at 70°C. Before meIt-mixing with PS, the larger agglomerates of beads were removed by sieving: for 2 flm-glass the agglomerates larger than40flm, for 30 flm-glass the aggIomerates larger than
200flm were removed.
Specimen preparation and tensile testing
The PS-glass bead {90/10 vol%) composites were pre-pared by melt-mixing on a laboratory mill at 190°e. The total mixing time was 8 min. Tensile specimens were machined in accordance with ASTM D 638 III from
Figure 1 Two examples of the typical craze pattern around glass beads embedded within a thin rubbery layer. The arrow indicates the direction of the applied tension
compression moulded sheets. To reduce thermal stresses the specimens were annealed at 800
e
for 24 hand then conditioned at 200
e
and 55% relative humidity for at least 48 h before testing. The tensile tests were performed on an Instron tensile tester at 20°e. The strain rate was 0.04 min-1.
The mechanism for craze formation at the beads was investigated by straining small dumbbell-shaped speci-mens uniaxially on a small tensile apparatus which was fitted to the stage of a Zeiss light microscope. The specimens used contain a very low percentage (about 0.5 vol%) of 30 flm-glass beads. As these specimens are transparent the crazes, formed at the beads during straining, are weIl visible.Itshould be noted that this kind of microscopie investigation cannot be done with" 2
flm-glass beads because these are too small to be well-visible with a light microscope.
RESULTS AND DISCUSSION
M echanism for craze formation
Two typical examples of the craze pattern around gIass beads embedded within a rubbery interfacial layer are shown inFigure1. Besides being formed in the equatorial region, crazes are also formed between the poIes and equator at an angle of 20-40° from the pole. With nearly all the investigated beads the equator crazes were initiated first and then the crazes between pole and equator. The opposite occurred with only a very few beads.
The occurrence of equator crazes proves the presence of a rubbery interfaciallayer that is thick enough to induce maximum triaxial stress concentrations near the equator. The shape of the crazes between pole and equator resembles the shape of crazes formed at poorly adhering gIass beads3,4.The cause of their occurrenee must
there-fore be decohesion between the bead and PS at the poIe, resulting in the formation of a curvilinear crack that propagates into the direction of the equator until a craze originates at its tip. This decohesion at the poles of the beads is quite logical. The thin rubbery interlayer has a Young's modulus that is hundreds of times smaller than that of glass and PS. Thus already at low overall strains near the poIes of the beads the rubbery Iayer is highly extended. As a result not onIy the interlayer itseIfbut aIso the adhesive bonding at both the glass-rubber interface
Tensile behaviaur of palystyrene-glass bead composites: M. E. J. Dekkers et al.
Figure 3 Tensile stress--strain curves at 20"C for unfilled PS (curve A) and for PS-2 jlm-glass 90/10 (vol%) composites with excellent interfacial adhesion (curveBl,poor interfacial adhesion (curve C) and a rubbery interfacial layer (curve0)
Figure 4 Tensile stress-strain curves at 20·C for unfilled PS (curve A) and for PS-30 jlm-glass 90/10 (vol%) composites with excellent interfacial adhesion (curve B). poor interfacial adhesion (curve C) and a rubbery interfacial layer (curve 0)
and the rubber-PS interface is highly loaded, which eventually leads to premature decohesion. Failure at the glass-rubber interface as the cause of decohesion is unlikely as can be seen from the fracture surface shown in
Figure 2. Clearly a lot of material has remained on the beads indicating that even during fracture the adhesive bonding between glass and rubber has lasted. Thus failure of either the rubbery interlayer itself or failure at the rubber-PS interface remains as the cause of decohesion. Without further evidence a definite choice between these two possibilities cannot be made because neither the tear resistance of the interlayer material nor the adhesion strength at the rubber-PS interface is precisely known.
Stiffness and toughness
Before discussing the tensile properties of the PS-glass bead composites it must be noted that with the coating procedure described in the experimental section it is, of course, not possible to coat the beads with a rubbery layer of completely uniform thickness and equally thick for each bead. The latter appeared for instanee from light microscopie investigation: at few beads the interlayer was so thin that no equator crazes but only crazes between the poles and equator were formed. From electron micros-copie investigation of the fracture surfaces it appeared that the prepared composites hardly contain any agglo-merates of beads. Apparently the aggloagglo-merates split up because of the high shear involved in melt-mixing.
Figures3 and 4 show typical tensile stress-strain (a-e)
curves for the PS-glass bead 90/10 (vol%) composites containing 2 ,um-glass and 30 ,um-glass, respectively. For comparison thea-ecurve for unfilled PS is also given. On comparingFigures3 and 4 it immediately appears that for the two investigated bead size ranges the bead size does not significantly affect the tensile behaviour of the com-P9sites. Upon addition of 10 vol% of either excellently or paorly adhering beads the Young's modulus increases
Figure 2 Scanning electron micrograph of the fracture surface of a PS-30 jlm-glass (90/10 vol%) composite with a rubbery interfacial layer 40 30
t
&
:L b 20 10 40 30t
&
6
b 20 10 0.5 0.5 B 1.0 € (Ofo) _ A Dc
A 1.5 1.5Tensile behaviour of polystyrene-glass bead composites: M. E. J. Dekkerset al. from about 3250MPa (for unfilled PS) to about
4000MPa. As to be expected, the Young's modulus of the composites with a rubbery interfacial layer is smaller (about3600MPa) but stilliarger than that of unfilled PS. None of the composites has an elongation at break that is larger than that of unfilled PS and also the work to break is not increased on adding beads. Thus neither excellently adhering beads nor poorly adhering beads nor the rubbery interfaciallayer has a significant toughening effect, not even when2 j.Lm-glass beads are used which are of similar size as the rubber particles in typical high-impact polystyrenes (HIPS).
A prerequisite for toughness is that the dispersed particles initiate multiple craze formation in the matrix. In this rt"spect rigid particles are equally effective as rubber particles. However, to achieve significant toughening the growth of crazes must be controlled by the dispersed particles and in this respect adhering rubber particles are by far the most effective. Over the past two decades several reasons for the rubber particles' ability to stabilize craze growth have been suggested. One suggestion has been that the rubber particles act as craze terminators7.
Recently8 the rubber particles' ability to accommodate the displacements due to craze formation at their equators has been advanced as the main reason; an adhering rubber particle links the two faces ofthe equator craze and in doing so the particle prevents premature craze break-down9.Itis obvious that when a dispersed particle has to
perform this function, the first requirement is that the craze formation process starts near the equator and not near the poles as is the case with both adhering and non-adhering glass beads. Therefore already beforehand glass beads are unsuitable for stabilizing craze growth, still apart from the fact that they would therefore be unable to deform to a significant degree. In the case of glass beads embedded within a thin rubbery layer craze formation
does start near the equator. However, in a second step also at those beads unstabilized crazes are formed in con-sequence of decohesion at the poles, and this must be the cause of the observed fact that with a rubbery interfacial layer no increase in toughness is achieved either. CONCLUSION
The rubbery interfacial layer as applied in this study appears to have no significant toughening effect. This has been attributed to decohesion at the poles, resulting in the formation of unstabilized crazes between the poles and equator. The prevention ofthis decohesion is the obvious prerequisite for obtaining a tough composite with the encapsulation method. However, we believe that this will be very difficult to achieve. The fact is that the thin low modulus rubbery interlayer (which must be thin, other-wise the rigid inclusions do not contribute to the stiffness) is highly extended at the poles and to accommodate the large displacements in these regions without decohesion, the tear resistance of the rubbery interlayer as well as the adhesive bonding at both interfaces must be of a probably unachievably high quality.
REFERENCES
Kinloch, A. 1. and Young, R.J.'Fracture Behaviour of Polymers', Applied Science Publishers, London, 1983
2 Dekkers, M.E.J. and Heikens, D.J.Appl. Polym. Sci.1983,28,3809 3 Dekkers, M.E.J. and Heikens, D.J. Mater. Sci. 1983,18,3281 4 Dekkers, M. E. J. and Heikens, D.J.Mater. Sci. Lett. 1984,3,307 5 Matonis, V.A.and SmalI, N. C. Polym. Eng. Sci. 1969,9,90 6 Plueddemann, E. P. SPI, 29th Ann. Tech. Conf. Reinf. Plast. 24;A
(1974)
7 Bucknall, C.B. 'Toughened Plastics', Applied Science Publishers, London, 1977
8 DonaId,A. M. and Kramer,E.J.J.Mater. Sci.1982, 17,2351 9 Heikens, D., Hoen, N., Barentsen, W., Piet, P. and Ladan, H. J.
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