glass bead composites

Mechanical behaviour and permeability of ABS/glass bead composites L. Nicolais*, E. Drioli and R. F. Landelt Istituto di Principi di Ingegneria Chimica, University of Naples, 80125 Naples, Italy (Received 8 May 1972; revised 4 July 1972) The stress-strain behaviour of acrylonitrile-butadiene-styrene (ABS) containing up to 33.5vol ~ of 10-40/~m glass beads as filler was measured at one strain rate at room temperature. The beads eliminate the yielding and greatly enhance the ultimate elongation and work-to-break. The change in stress-strain response is associated with the dewetting and vacuole formation around the beads and with an increase in the amount of crazing. Initial experiments using water permeability to investigate the crazes are reported.

INTRODUCTION The straining of a polymeric glass results in a volume change that is the sum of an elastic recoverable change associated with the compressibility of the material and an irreversible non-linear change associated with microcavitation within the solid 1. In many polymeric glasses, the microcavitation develops through the formation of crazes z, 3 Crazes are very similar to cracks in appearance but quite different in other respects. The distinction was first made by Sauer e t al. 4, 5, who pointed out that polystyrene is capable of bearing considerable loads even after crazes have extended across the entire crosssection of the specimen. From X-ray diffraction evidence they concluded that crazes are composed of oriented polymer interspersed by voids 9. According to KamboutS, 6 and Spurt and Niegisch 7, these crazes are structured regions analogous to that of a porous sponge in which the cell walls are highly drawn. In these zones, in fact, fibrils are drawn out of unoriented, amorphous coiled material and oriented in the direction of the tensile stress. These fibrils, then, contain oriented molecules. The porous regions may be thought of as aggregates of microscopic cavities, which concentrate stress in a manner similar to a true crack. In an unfilled polymeric glass, a unidirectional tensile load can cause crazes to nucleate and grow perpendicular to the direction of loading. While termination of the craze is related to the ability of the fibrils to form and to sustain a given load, the initiation of the crazes is considered to take place when a critical limit is reached in stress s-10, strain 1~, dilationZg, ta or distortion strain energy 1~ in the glassy polymer. The presence of a particle in the polymeric matrix, however, causes stress concentrations around the particle which enhance the rate o f craze formation s . The maximum stress concentration caused by a rigid inclusion, according to Goodier's 15 elastic analysis, is * o f the National Research Council, Italy. t Fulbright Senior Research Scholar, Italy, 1971-72, on leave from the Jet Propulsion Laboratory, California Institute of Technology, USA.

about 1.5, whereas for a rubbery inclusion it is about 212 and for a hole, about 3 s. Thus, in the case of rubbermodified polystyrene, such as acrylonitrile-butadienestyrene (ABS), the presence of the low modulus particle provides a point of stress concentration which can act as a point of initiation of a craze. However, once the craze starts it does not continue to propagate through the sample because it comes up against another rubber particle and stops. This process has been illustrated by Bucknall and Smith 9, who studied the deformation of high impact polystyrene in a thin film under the microscope. Recently a similar effect has been reported 24 in glassy polymers filled with rigid particles. The dimensions of crazes are not yet well defined but seem to be a function of the specific polymer and of the stress level. Kambour found a value for polycarbonate of 20 to 200 A 6 for the thickness of a craze*. Naturally the stress level plays a role in the craze dimension in the sense that, after crazes are nucleated, the higher the values of stress, the larger the lateral dimension or length, though the thickness remains relatively fixed. The density of this crazed material is very low, about 40-50% of the bulk material 17. However, this value is not known with certainty and may well vary with the polymer and the condition under which the craze is developed. A clearer knowledge of this point would facilitate the understanding of the mechanism of craze propagation and the properties of the crazed material. One way o f investigating such porosity would be a direct measure of fluid transport (with or without the presence of an added solute) through a crazed specimen. In the present work we report the effect of a hard filler on the mechanical properties of a material which does not craze as easily in their absence, and some preliminary results of the permeability o f such materials. The emphasis, however, is almost solely on the mechanical properties, specifically the uniaxial stress-strain behaviour of ABS/glass bead composites at different filler levels. * Zhurkov et aL 2~ report microcrack dimensions of about 90A for polycaproamide, 320A for polypropylene and 400A for polyethylene. In this case, however, it is not clear whether such microcracks contain fibrils, as in the crazes discussed here.

POLYMER, 1973, Vol 14, January

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Mechanical behaviour and permeability of ABS/glass bead composites: L. Nicolais et al. EXPERIMENTAL The acrylonitrile-butadiene-styrene (ABS; Mazzucchelli Celluloide Sicoflex T 85 and Monsanto Co. Lustran I) having a glass transition temperature Tg= 100°C and a Poisson's ratio of 0.35, was used as received. The glass beads (Tradex Colori type 3000 CPO1 and Cataphote Corp. type 2740, both with a diameter range of 10-40 /zm) were cleaned by refluxing with isopropyl alcohol for 24h, after first removing the iron particles present with HCI (Tradex) or a large magnet (Cataphote). To prepare specimens the beads and polymer were mixed on a two-roll mill at 180°C. For the mechanical tests, crude sheets were cut from the mill and then moulded to about 0.25 cm thick in a 21 x 7-5 cm compression mould at 185°C under a pressure of 55kg/cm z. Specimens containing the following concentrations by volume of glass beads were prepared: 11.2, 20.5, 27.2 and 33.5 ~ . In the present paper filler content is always expressed as volume concentration assuming that the density of the matrix is 1.05 and the density of glass beads 2.5 g/cm a. Tensile specimens were cut to the shape specified by ASTM D 638-64 T with a high speed router and the filler content was determined by combustion of small pieces of broken samples. All samples were annealed at roughly 10°C below Tg for one day, to minimize moulding stresses and then conditioned at 23°C and 45-55 ~ r.h. for 14 days before testing. Samples were tested in tension at a constant rate of strain and constant temperature with an Instron universal testing machine, using a strain gauge extensometer. Fracture surfaces were examined with a Cambridge electron scanning microscope. For the permeability measurement the films were prepared in a similar manner, except that the film was created on the mill by adjusting the roller clearance TM. Permeability tests were conducted on two different types of equipment normally used in reverse osmosis laboratory tests. The first was an unstirred batch system (static cell) similar to the one described by Drioli 19, working at a maximum pressure of 6-5 atm (1 a t m - 101.33 kN/m2). With the second apparatus 20 it was possible to make permeability measurements in laminar and turbulent flow with a maximum pressure of 50 atm.

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Figure 1 Stress-strain curve for ABS filled with glass beads to the volume percentage as shown. A, ~=0%; B, ~=11.6%; C, ~=20"9%; D, ~=28"2%; E, ~=34"5%

RESULTS AND DISCUSSION Typical stress-strain curves for different concentrations of glass beads at 24°C and at a strain rate of 0.131 rain -1 are shown in Figure 1. The modulus increases and the strength decreases as filler is added. In contrast to theoretical predictions for glassy polymers, the ultimate elongation of this system increases with the addition of the filler. The curves for the filled materials exhibit an inflection and closer examination shows them to be bilinear with the discontinuity in slope occurring at about 120 kg/cm 2. The stress at this point is practically independent of the filler concentration, as previously shown for styreneacrylonitrile (SAN)/glass bead composites 9"4, while the slope of the stress-strain curve above the discontinuity is strongly dependent on the bead content. The change in slope is accompanied by stress whitening over the entire gauge length. A typical fracture surface

P O L Y M E R , 1973, Vol 14, January

Figure 2 Electron scanning photomicrograph of the fracture

surface of ABS/glass beads (440×) (~=0.20)

for a 20 vol ~o bead filled composite is shown in Figure 2. The denuded beads indicate that the adhesion of the ABS to the glass is poor, as would be expected with these materials. At higher stresses the unfilled material shows a welldefined yielding, but the filled materials pass through a broad maximum with little change in the stress level. After this region of zero slope is attained, these samples continued to elongate with no further change in appearance and no necking. The initial elastic modulus of the particulate composite Ee increases with filler content and can be expressed

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Mechanical behaviour and permeabiOty of ABS/glass bead composites: L. Nicolais et al. 50

:

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Figure 4 Strength of ABS/glass bead composites versus ~, compared with equation (2) 0.9

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Figure 3 Initial modulus of ABS/glass bead composites versus (~ compared with equation (1)

as a function of its volume fraction, ¢, by the well known Kerner equation 25 as: Ec

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ttv is the Poisson's ratio of ABS and El, E v are the Young's moduli o f the filler and polymer respectively. In Figure 3 the elastic moduli calculated from equation (1) are compared with the experimental values. The strength of these composites decreases as the volume content of beads increases. This decrease is simply a reflection of the decreased cross-sectional area o f the polymer bearing the load and can be expressed as a function of concentration by the following equation24: oc = av(1 - 1"21¢ 2/3)

(2)

were ac and a v are the strengths of the composite and the polymer respectively. In Figure 4 the experimental values of the strength are compared with those calculated. Figure 5 shows the ultimate elongation of the ABS/glass bead composites as a function of filler content. It can

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Figure5 Ultimate elongation of ABS/glass bead composites as a function of filler content

be seen that, in contrast to theoretical predictions 2z, 22, the ultimate elongation of these composites increases sharply when the filler is added, up to about 10vol%, * Smith e3 arrives at a similar prediction for a rubbery matrix.

POLYMER, 1973, Vol 14, January

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Mechanical behaviour and permeability of ABS/glass bead composites: L. Nicolais et aL and then decreases slowly. However, it should be recalled that the theories are only applicable to simple two-phase systems, not to those which are dilatable and can form a third phase. The local strain at the fractured surface in the unfilled ABS is quite high. While this is very revealing in terms of the mechanism of rupture, the fact remains that the ultimate elongation of the unfilled polymer is small for typical specimens by conventional standards. This high elongation of the filled polymer can be explained if one can assume that the growth of crazes can be terminated by the glass beads or vacuoles around them. It is known 13, ~ that the rubber phase can act as a stress concentrator to initiate crazes and then assist in the terminating process. A similar process also occurs in glassy polymers filled with rigid particles. In fact if the propagating craze encounters a glass sphere to which the matrix is not strongly adherent, interfacial debonding can effectively blunt the tip of the craze and prevent further propagation. A similar but even more effective blunting occurs when a growing craze encounters a vacuole around a bead. In the case of ABS/glass bead composites these two processes act together to give a more extensive crazing which, when coupled with the added elongation associated with the vacuoles, permits the much greater specimen elongation. Moreover, the effect of glass beads on the behaviour o f the ABS is also that of equalizing the stress concentrations all along the specimen (throughout it, really) by causing crazes to initiate over the whole volume rather than just in a few localized regions. Thus Figure 6 shows a picture of specimens with and without beads after fracture. It can be seen that the unfilled sample

Figure 6 Appearance of two specimens after a tensile test, showing difference in stress-whitening. Unfilled, left, and filled with 90% glass beads, right

24 POLYMER,1973, Vol 14, January

Figure 7 Electron scanning photomicrograph of the fracture surface of ABS containing 20vo1% glass beads (300Ox)

shows a stress-whitening only in proximity to the fracture surface while the filled one is white over the whole gauge length. However, it is not clear to what extent the stresswhitening in the filled material is due to void formation around the particles and how much is due to crazing per se. In the unfilled material, it must clearly be the latter. In the filled material, as indicated by Figure 2, and more clearly seen in Figure 7, there is a strong suggestion of the formation of elongated voids, such as are observed in filled elastomers 1, 2a, 88. Certainly it is evident that a strain of 80 ~o must lead to elongated vacuoles if the beads dewet--the moot point is then whether the major part of the dewetting occurs first and causes crazing or subsequently, after the general initiation of crazing. In any event, the picture emerges, to explain this high elongation, of the beads initiating crazes, either as rigid inclusions if they remain bonded until crazing starts or, if the dewetting occurs first, as sites for holes which are even more effective as stress raisers and hence initiators. Subsequently the beads or holes act as craze and crack stoppers, permitting other nuclei to grow and propagate. Eventually the resulting strain is large enough to dewet other beads in the matrix, with concurrent enhancement of further nucleation and further crack prevention capability. The result of this combined dewetting and crazing leads to greatly improved toughness by the simple fact that a large volume of the specimen is involved in the deformation and hence increases the work-to-break. Here we take toughness to mean simply the work-tobreak in these uniaxial tensile tests. Notched impact or tensile tests, which localize the fracture plane, would be expected to show a lesser enhancement of toughness, thus measured, by the beads. These results are very important practically in the fact that the addition of these rigid inclusions to the ABS leads to a new material with better mechanical properties.

Mechanical behaviour and permeability of ABS/glass bead composites: L. Nicolais et al. The interesting point that only a small amount of glass beads increases the work to break very sharply is shown in Figure 8. The rapid fall-off at higher bead content indicates that the beads are more effective in stopping craze propagation than they are in initiating it. Even if the work is assumed to have been done only on the polymer and the results corrected for the decrease in polymer content, i.e. Wpol= W(1 _ ~ ) - z the work to break is still decreased by the higher bead contents (though it always remains greater than that for the unfilled polymer). To investigate the resultant porosity in this material, sheet specimens were formed as previously indicated and are being tested for permeability to water. The unstrained material shows no permeability under a pressure of 36atm for 24h in a stirred cell, while a pre-strained sheet tested under the same dynamic conditions shows a flow rate as indicated in Figure 9. The initial high rate of l lml/min decreases with time towards an asymptotic value of roughly half this magnitude. This 60 /

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Flow rate Vagainst time for an ABS/12% bead composite in a stirred cell under conditions as described in text

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Figure 10 Flow rate against time for an ABS/16% bead composite during repetitive tests in a static cell under 6.5atm pressure. ©, Test immediately after mounting in cell; ×, test after 576 hours of discontinuous operation for various periods of time

decrease, analogous to compaction in reverse osmosis membranes, is partly reversible. Figure 10 indicates the course of the permeability with time of another membrane in repeat experiments at 6.5 atm in an unstirred cell. The strain in these sheets is unknown. To obtain a known value, the planar metal support plate in the static cell was replaced with a concave Teflon support plate. An unstrained sheet containing 127o beads was damped in the cell and the applied pressure, 6-5 atm, was sufficient to stretch the sheet out against the plate. The hemispherical concavity in the plate was designed to give a maximum (biaxial) strain of 2.5 ~o, i.e. above the strain at the discontinuity observed in the uniaxial stress-strain curve. The unstretched sheet showed a permeability of 0-11 ml/min, while the stretched sheet had an initial permeability of 0-17ml/min. This permeability could result from any of three sources, pinholes or other large defects, contiguous voids, or the crazes themselves. In the third case, the pore dimensions are such that a large polymer molecule would pass through with difficulty, if at all. Therefore 0.2 7o solutions of poly(oxypropylene glycol) (mol.wt~600 000) were used in both the static and dynamic test cells under the same conditions as used with the water. The effluent had a polymer concentration roughly half that of the original solution, which indicates that significant water permeation must have occurred through the crazes. Further experiments to assess the utility of permeation as an investigative tool for such microporous materials are currently in progress. CONCLUSIONS

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The addition of a hard filler to a glassy polymer enhances its toughness if the latter can craze, and the easier the craze formation the greatest the effect. Thus glass beads have essentially no effect on polystyrene 16 but raise the elongation and work to break progressively in styrene-acrylonitrile, poly(phenylene oxide) (PPO) and acrylonitrile-butadiene-styrene. In the ABS the breaking strength depends on the available

POLYMER,

1973, V o l 14, J a n u a r y

Mechanical behaviour and permeability of ABS/glass bead composites: L. Nicolais et al. polymer cross-section, according to equation (2), just as it does for S A N and PPO. I n this m o r e readily crazed material, however, the elongation at break is very high. There is no adequate theory for this behaviour, t h o u g h a reasonable explanation can be advanced based on craze initiation and propagation, coupled with the growth o f voids a r o u n d the beads. The resulting porous materials are permeable to water with a pore size such that large flexible molecules seem to be excluded. REFERENCES 1 Farris, R. J. Trans. Soc. RheoL 1968, 12, 2, 303 2 Kambour, R. P. J'. Polym. Sci. (:t-2) 1965, 3, 1713; 1966, 4, 17; 1966, 4, 349 3 Kambour, R. P. Polymer 1964, 5, 143 4 Sauer, J. A., Marin, J. and Hsiao, C. C. J. AppL Phys. 1949, 20, 5O7 5 Hsiao, C. C. and Saner, J. A. J. ,4ppl. Phys. 1950, 21, 1071 6 Kambour, R. P. Conf. 'Yield, Deformation and Fracture of Polymers', 1970, Churchill College, Cambridge, Paper 4.1 7 Spurt, O. K., Jr and Niegisch, W. D. J. Appl. Polym. Sci. 1962, 6, 585

POLYMER, 1973, Vol 14, January

8 Sternstein, S. S., Ongchin, L. and Silverman, A. AppL Polym. Syrup. 1968, 7, 175 9 Bucknall, C. B. and Smith, R. R. Polymer 1965, 6, 437 10 Nicolais, L. and Di Benedetto, A. T. J. AppL Polym. Sci. 1971, 15, 1585 11 Maxwell, B. and Rahm, L. F. Ind. Eng. Chem. 1948, 41, 1988 12 Newman, S. and Strella,S. J. AppL Polym. Sci. 1965, 9, 2297 13 Strella,S. J. Polym. Sci. (.4-2)1966, 4, 527 14 Matsuoka, S., Daane, J. H., Kwei, T. K. and Huseby, T. W. Polym. Preprints, 1969, 10, I198 15 Goodier, J. N. Trans. A S M E 1933, 55, A-39 16 Nicolais,L., Lavengood, R. E. and NarkJs, M. Ing. Chim. ItaL 1972, 8, 51 17 Kambour, R. P. Nature 1962, 195, 1299 18 Drioli,E., Landel, R. F. and Nicolais,L. Ital.Pat. AppL 48505 A/72 (22 Feb. 1972) 19 Drioli, E. Ing. Chim. Ital. 1969, 5, 151 20 Drioli, E., Alfani, F. and Iorio, G. Ing. Chim. ltal. 1972, in press 21 Nielsen, L. E. J. Appl. Polym. Sci. 1966, 10, 97 22 Kenyon, A. S. and Duffey, H. J. Polym. Eng. Sci. 1967, 7, 1 23 Smith, T. L. Trans. Soc. Rheol. 1959, 3, 113 24 Nicolais, L. and Narkis, M. Polym. Eng. Sci. 1971, 11, 194 25 Kerner, E. H. Proe. Phys. Soe. (B) 1956, 69, 808 26 Haward, R. N. in 'Advances in Polymer Blends and Reinforcement', Institution of the Rubber Industry, London, 1969 27 Zhurkov, S. N., Kuksenko, V. S. and Slutsker, A. I. Paper 46, Proc. 2nd Int. Conf. Fracture, Brighton, April 1969 28 Oberth, A. E. and Bruenner, R. S. Trans. Soc. RheoL 1964, 9, 165