Rheological Properties of ABS at Low Shear Rates: Effects of Phase Heterogeneity

Transcription

1 Malaysian Polymer Journal, Vol 4, No, p9-36, 9 Available online at wwwfkkksautmmy/mpj Rheological Properties of ABS at Low Shear Rates: Effects of Phase Heterogeneity Asif Ali Qaiser *, Yasir Qayyum and Rehman Rafiq Department of Polymer and Process Engineering University of Engineering and Technology Lahore, Pakistan ABSTRACT: Acrylonitrile-butadiene-styrene (ABS), owing to its multiphase heterogeneity, has good service properties, especially, impact resistance Butadiene rubber phase, both its content and morphology, play vital role in the rheology of ABS resin The dependence of melt viscosity, both shear and elongational, and its pseudo-plastic character on temperature were investigated on a commercial ABS resin using a simple capillary rheometer Shear stress data were acquired at low to moderate shear rate range and power law fluid model was fit to experimental data It is concluded that ABS rheological behavior followed the power law model over a reasonable processing temperature range but this temperature dependence does not obey the Arrhenius law A saturation plateau was observed which dictates weak temperature dependence and signifies the selection of an optimal processing temperature for ABS resin Keywords: Capillary Rheometry, Power law, ABS, Rheology, Viscosity 1 INTRODUCTION Acrylonitrile-butadiene-styrene (ABS) is a heterophase terpolymer of styreneacrylonitrile (SAN) with embedded particles of butadiene rubber The rheological properties depend on rubber content, rubber particle size and interfacial morphology Aoki [1] investigated the ABS rheology and quantified the effects of rubber content and rubber particle size on rheological properties of ABS It was concluded that Cox and Merz s empirical law was generally held, well, at low rubber content of the polymer when applied to the measured rheological properties using concentric cylinder rheometer However at low shear rates the law was not followed by, owing to the strong interactions between the polymeric chains grafted on the rubber particle to that of chains on adjacent particles In another study, Dreval et al [] investigated the effects of molar mass (M W ) of styreneacrylonitrile (SAN) on various properties of ABS resins It was observed that rheology, when studied under capillary flow conditions, was depended on molecular weight (M w ) of SAN as changing M w also changed interaction with the polybutadiene (PB) rubber particles Viscosity at low frequency values was measured by rotational viscometer for varying degree of grafting on rubber particles [3] A critical degree of grafting (GDc) was recorded and related to the conformation of the grafted chains at the surface of the rubber particle Yang et al [4] investigated the effects of viscosity ratio, among other factors, on the morphology and mechanical properties of polycarbonate (PC) and ABS blends in a twin screw extruder Capillary rheometry was used as the viscosity characterization technique and various conclusions were drawn Ethylene propylene diene elastomer (EPDM) modified polypropylene vulcanized composites and ABS were studied for their comparative rheological behavior at low shear stresses using a constant shear stress creep instrument, a rotational rheometer, and a capillary extrusion rheometer [5] It was concluded that a definite yield stress existed, under shear flow conditions, below which the deformation in materials of interest *Corresponding author: A A Qaiser Asifaliqaiser@uetedupk

2 (including ABS) was finite Also it was shown that these limiting stress values were quite different from those obtained by the extrapolating to the zero shear rate Liang [6] studied and modeled rheological behavior of ABS melt under capillary flow conditions using a commercial capillary rheometer It was observed that shear stress changed nonlinearly with the change in shear rate and an abrupt change was recorded at about 1 3 s -1 Also, it was established that viscosity was an Arrhenius type function of temperature change In another study for ABS resin and ABS composites with quasi-nano CaCO 3 particles, Liang [7] established that at higher shear rate range, power law fluid (PLF) model did not hold well Also the viscosity was modeled by Arrhenius law for the temperature range of -4 o C In the present study, a simple extrusion flow meter capable of measurements at low shear rate range was used to study the effects of temperature on the rheological behavior of the commercial grade ABS resin The flow behavior at low shear rates (in the range of processes like extrusion, calendaring, compression molding etc), was modeled by PLF model and temperature dependence of model parameters (pseudoplasticity index n and consistency index K ) were studied METHODS & MATERIALS 1 Materials ABS (Chi Mei Corporation, PA-718) was purchased from commercial market The stated melt flow index value for this grade was 16 g/1min (ASTM D-138, condition G) Rheological Measurement and Methodology A moving piston melt flow tester (KARG Industrietechnik: 31) was used in this study The instrument was equipped with a two-compartment polymer heating barrel with a precise temperature control (± o C) Three capillary dies were used of nominal bore diameter 95 mm and different lengths (8, 1, and 3 mm) The instrument was pressurized by placing standard weights over the piston and shear rates were evaluated by recording the piston movement according to ASTM D-3538 Prior to each run for a single shear stress value, appropriate polymer conditioning time (ASTM D-138) was given to ensure the full melting within the barrel ABS was evaluated at,, 4, 5 and 6 o C Shear Viscosity Wall sear stress ( w ) was calculated by measuring the pressure gradient (P) along the die length (L) of a capillary die of radius R w = PR L (1) Apparent shear rate at capillary wall ( a ) was evaluated once the extrudate volumetric flow rate (Q) had been calculated as follows a = 4Q π R 3 () 3

3 Shear viscosity was calculated as the ratio of shear stress to shear rate taken from eqn (1) and eqn (), respectively Rabinowitsch and Begley corrections For shear stress and shear rate, various corrections have been recommended in the literature but only most significant ones, Rabinowitsch and Begley corrections, were incorporated Rabinowitsch correction accounts for the non-newtonian behavior of the melt where apparent shear rate ( a ) was converted into true shear rate ( w ) by the following formula w = 4 (3n n 1) a (3) Bagley or end pressure losses account for a significant fraction of the total pressure drop across the capillary die It is only skin frictional pressure loss in eqn (1) which yields wall shear stress under Poiseulli flow The pressure losses at the entrance and at the exit of a capillary are not only significant but also non-linear So to get the pressure drop only at the capillary wall, the pressure drops at the entrance and at the exit of an orifice die (P ) were subtracted from the total pressure drop (P) across the capillary w = R(P - P) L (4) For the present case of constant shear stress mode of capillary rheometry, first, a polynomial type of relationship was fitted to the pressure drop versus shear rate data at various temperatures Pressure drops for various shear rates were evaluated from these expressions, based on the selected shear rate sub-ranges covering the whole experimental range These values of pressure drops were plotted versus capillary L/D values with shear rate and temperature as the parameters Orifice pressure drop values were calculated by extrapolating the linear fit equation of pressure drop versus L/D for L/D= Afterwards, these values have been used for calculating the corrected shear stress w values (eqn (4)) in each shear rate range Elongational Viscosity Elongational viscosity (η e ) is a function of end pressure loss (P ) and it was calculated using following relationship [8] η e = 9 (n 1) (P) 3( )( Where n is power law index (section 3), η e is elongational viscosity and w is true shear rate w ) (5) 3 RESULTS & DISCUSSION 31 Flow curves Flow curves (pressure drop vs apparent shear rate), at each temperature are presented as Figure 1 These are typical flow curves for a pseudoplastic fluid where pressure drop (P) exponentially rises and then tends to flatten at high shear rates 31

4 35 3 o C o C 4 o C 5 o C 6 o C 5 P,KPa Apparent shear rate,s -1 (a) 35 3 o C o C 5 o C 4 o C 6 o C 5 P, KPa Apparent shear rate,s -1 (b) 35 3 o C o C 5 o C 4 o C 6 o C 5 P, KPa Apparent shear rate,s -1 (c) Figure 1: Flow curves of ABS using capillary dies of various lengths (a) 8mm (b) 1 mm and (c) 3 mm 3

5 3 Viscosity Plots Figure shows the plots of elongational viscosity at different temperatures as calculated using equation (5) and values can be compared with the shear viscosity values shown in Figure 3 As shear rates increase, the viscosities decrease nonlinearly Also the decrease in viscosity values by increasing temperatures is evident from these plots At a given shear rate, elongational viscosities always assume higher values as compared to the shear viscosities at the same shear rate This gap widens as we move to higher shear rates Elongational viscosities maintained the similar temperature dependence to that of shear viscosities ie, decreasing by increasing the temperature but this trend got intensified at high shear rates 18 Elongational Viscosity, KPas T= C T= C T=4 C Shear rate,s -1 Figure : Elongational viscosity vs shear rate at different temperatures (Note: Lines are only for guiding the eyes) 33 Power Law Fluid(PLF) model parameters Power law fluid (PLF) model is given by the relationship: w = K ( w ) n (6) Replacing from equation (3) n n w = K ( 3 1) ( n 4 a ) n (7) Where n is the power law (Pseudoplasticity) index and K is called Consistency coefficient Power law parameters ie n and K were calculated by fitting the PLF model equation (Eqn 7) n shear stress vs shear rate data in the following form 4n K +n log a (8) 3n 1 log w = log Linear fitted curves for each temperature on these data points satisfy the t- distribution criterion for 95% confidence interval (α = 5), indicating reasonable linear 33

6 relationship (Table 1) Table 1: Linear regression of the data (eqn8) corrected for 8mm die Temperature ( o C) Statistical correlation y=463x + 19, r=91,n=6 y=675x + 133, r=9,n=6 4 y=74x + 13, r=97,n=6 5 y=671x + 115, r=99,n=6 6 y=774x + 68, r=91,n=6 Instead of correcting shear rate ( a ) to true shear rate ( w ), first, non ideality was incorporated into the consistency parameter and K was calculated from the intercept value The evaluated power law parameters are plotted as the temperature function in Figure 4 The power law index increases by increasing temperature but tends to level off at higher temperatures Converse but similar plateau trend is also shown by the consistency index (K) values Power Law Index (n) Temperature, K (a) log K log temperature(k) Figure 4: (a) Power law index (n) vs temperature and (a) consistency index (K) vs temperature (b) 34

7 Since ABS is a two-phase system (SAN as continuous phase with embedded butadiene rubber particles), the rheology depends on the morphology and mutual interaction of the phases during the flow Two different morphological models have been proposed to explain the rheological behavior of ABS flow behavior [6] At lower shear rates, see-island conception prevails which states more pronounced interfacial control by the deforming rubber particles embedded in SAN continuous phase At higher shear rates, laminar deformational behavior dominates where both phases deform simultaneously and viscosity decreases rapidly, comparatively For PLF parameters, the former interfacial behavior was observed when studied as the function of temperature since we operated at low shear rates In ABS system, rubber particles deform easily so various phenomena were observed SAN component behaved more Newtonian ( n approaching 1) by increasing the temperature, which is usual for homogenous single phase thermoplastics Rubber deformational behavior showed transition at higher temperatures and appeared to control the overall rheological behavior of terpolymer (Figure 4) At relatively higher temperatures (T > 4 o C), it is most probable that rubber phase deformed significantly and contributed towards flow more as a pseudoplastically deforming phase It is also realized that under mild to moderate shearing conditions, ABS reached a saturation deformational morphology, the plateau (Figure 4) and after which the shear thinning behavior becomes a weak function of temperature To establish whether power law parameters followed the Arrhenius trend or not, Pseudoplasticity index (n) and consistency index (K) were plotted against (1/T), (Figure 5) Although the viscosity follows the Arrhenius dependence on temperature [6, 7], the Pseudoplasticity does not obey exactly the Arrhenius law, signifying the more pronounced interfacial interactions between the deforming phases Ln 'n' r = (1/T) K (a) ln K 6 4 r = (1/T) K (b) 35

8 Figure 5: Arrhenius plots for (a) power law index (n) and (b) consistency index (K) 4 CONCLUSIONS It has been shown that commercial grade ABS resin can be characterized for their rheological behavior in a simple rheometer setup comprising various L/D capillary dies ABS was observed to behave as a power law fluid under mild to moderate shearing conditions for to 6 o C temperature range However, the PLF pseudoplasticity parameters did not obey the Arrhenius relationship, perfectly, and a saturation plateau was observed at higher temperatures This behavior was attributed to the strong interfacial character of ABS phases deforming at low shear rates REFERENCES [1] Aoki, Y 1986 Rheological properties of ABS polymer melts having a good dispersion of rubber particles, Journal of Non-Newtonian Fluid Mechanics :91-99 [] Dreval, V, G Vasil ev, E Borisenkova and V Kulichikhin 6 Rheological and Mechanical Properties of ABS Plastics Prepared by Bulk Polymerization, Polymer Science Series A 48(3): [3] Bertin, M, G Marin and J Montfort 1995 Viscoelatic properties of Acrylonitrile- Butadiene-Styrene (ABS) Polymers in the molten state, Polymer Engineering and Science 77: [4] Yang, K, S Lee and J Oh 1999 Effects of Viscosity Ratio and Compatibilizers on the Morphology and Mechanical Properties of Polycarbonate/Acrylonitrile- Butadiene-Styrene Blends, Polymer Engineering and Science 39(9): [5] Araki, T and J White 1998 Shear Viscosity of Rubber Modified Thermoplastics: Dynamically Vulcanized Thermoplastic Elastomers and ABS Resins at very Low Stress, Polymer Engineering and Science 38(4): [6] Liang, J Effects of Extrusion Conditions on Rheological Behavior of Acrylonitrile Butadiene Styrene Terpolymer Melt, Journal of Applied Polymer Science 85: [7] Liang, J Melt Rheology of Nanometer-Calcium-Carbonate filled Acrylonitrile Butadiene Styrene (ABS) copolymer Composites during Capillary Extrusion, Polymer International 51: [8] Cogswell, FN 1996 Polymer Melt Rheology, Woodhead Publishing Cambridge 36