Melt elasticity and flow properties of polypropylene impact copolymer/polyolefin elastomer blends subjected to varied capillary extrusion conditions

Abstract

In present study, rheological properties of polypropylene impact copolymer (PPcp) and polyolefin elastomer (POE) blend melts were evaluated on a capillary rheometer under shear and elongational flows. The flow and melt elastic properties (die swell and first normal stress difference) studied at varied extrusion conditions were correlated with blend morphology and elastomer content by means of image analysis and theoretical models. Dispersed particle break-up and coalescence were observed to be influenced by POE content and the viscosities of the constituent polymers which were in turn affected by the capillary extrusion conditions (shear rate in particular). The blend melts demonstrated typical pseudoplastic behavior obeying Cross model under shear flow. The elongational flow also corroborated well with the shear flow behavior. All the blends illustrated prominent dependence of melt elastic properties on POE content and the capillary extrusion conditions. The melt elastic properties were also found to critically rely on the inter-particle distance of the POE phase.

Keywords

PPcp POE morphology rheology melt elastic properties die swell

Introduction

Impact modification of polypropylene (PP) impact copolymer (PPcp) for low temperature applications (specifically below 0°C) through incorporation of elastomers has been an area of substantial research in recent times and has been discussed in our previous studies.^1,2 The studies reported that concentration of elastomeric impact modifier in the blends played a significant role in the development of distinct morphologies which ultimately dictated the impact mechanisms. These blends have the potential to be used for a vast product range suited for low temperature applications. Optimization and control of polymer processing operations, which these materials go through, require a quantitative treatment of the entire viscoelastic behavior of the polymer melt. The viscous properties of the polymer melt aid in determining the throughput, whereas the elastic properties are important for predicting dimensional stability of the end product. Among many of the processing methodologies which these polymer blends are subjected to; like extrusion, blow molding and stretch blow molding, involve application of extensional deformation of the polymers in the melt state. Thus, there is an important role of the melt elastic properties (namely the die swell ratio and the first normal stress difference) of these blends in governing the optimization of processing methodologies and consequently the final product quality.^3,4

It has been well documented that addition of elastomer phase in PP matrix results in impact strength enhancement which depend upon the concentration, size and size distribution of the elastomer phase within the PP continuous phase with trade-off in stiffness of the blend.^1,2,5,6 It has been accepted by and large that for any given blend composition, maximum toughness is restricted to a narrow range of particle sizes.⁷ Studies undertaken by Wu⁸ had pioneered widespread research on the effect of particle size on impact behavior in a wide range of polymer/rubber blends which have been detailed over the years. It was later affirmed by Wu that average particle size was not the principal factor governing the impact resistance. Instead, it was reported that the average inter-particle distance (ID) was a more elementary parameter, where ID is the smallest distance between the surfaces of neighboring rubber particles.⁹ Jiang and co-workers had later theoretically explained that properties of the matrix polymer played a critical role in the inter-particle distance directed toughening mechanism of the polymer/rubber blends.¹⁰ The effects of elastomer addition on melt elastic properties of PP blends also seem to play a pivotal role in the control and optimization of polymer processing parameters. The melt elastic properties of PP/elastomer blends have often been observed to be significantly affected by the elastomer content.^11-13 However, reports on the effect of elastomer addition on the melt elastic properties of PPcp blends are extremely rare.¹⁴ Furthermore, reports elucidating melt elastic properties of the blends composed of PPcp and polyolefinic elastomers are not available in accessible literature. It can hence be deduced that the role of volume fraction, particle size and inter-particle distance of the rubber phase will govern the eventual rheological properties of the blend in steady shear as well as extensional flow. Subsequently the quality of the finished product will accordingly be affected by the melt elastic properties of the rubber toughened polymer blends. In this regard, an ardent effort has been made in this study to correlate the flow and melt elastic properties of PP impact copolymer (PPcp) and polyolefin elastomer (POE) blends with the blend morphology. This study also strives to correlate the melt flow and melt elastic properties of PPcp/POE blends with the capillary extrusion conditions as well as the elastomer content in the blends with the intent to link the effect of various processing conditions on the final product quality of low temperature high impact resistant PPcp blends.

Experimental

Materials

The materials used in this study were PPcp (polypropylene impact copolymer, commercially available as REPOL® MI3530 with melt flow index of 3 g/10 min and density of 0.905 g/cm³) supplied by Reliance Industries Ltd. (India) and EXACT™ 5371 POE (polyolefin elastomer, copolymer of ethylene and octene, with melt flow index of 5 g/10 min and density of 0.87 g/cm³) obtained from Exxon Mobil Chemicals.

Blend preparation

POE was at first mixed with PPcp in weight percentage of 10%, 20%, 30% and 40% to prepare mixture of 1 kg batch size through tumble mixing for 10 min. The blends were subsequently labeled PPcp90, PPcp80, PPcp70 and PPcp60 respectively besides PPcp100 for pure PPcp and PPcp0 for pure POE. The blends were then melt blended in a Brabender TSE25 co-rotating, intermeshing twin screw extruder (screw diameter: 25 mm, L/D: 48). Melt blending was carried out at screw speed of 100 rpm with a temperature profile of 140°C at the feed section and 220°C at the die end with a gradual increment of 10°C over 10 heated zones. Each batch of blended compound was extruded and pelletized during the continuous melt blending process. The pellets were then dried at 100°C for 4 h in a vacuum oven prior to conduct of rheological tests.

Rheological analysis

Capillary rheometer is arguably the most common instrument used to determine deformation of polymeric melts under shear flow mainly owing to the precise replication of the actual processing conditions which the polymeric melt is likely to encounter. It is furthermore used to evaluate the melt elastic properties such as die swell behavior. A number of studies have been reported on die swell as a function of the length-to-diameter (L/D) ratio of the die, the pressure drop across the die, entry angle of the die, shear stress, shear rate, etc.^15-18 In present study, rheological properties of the blends were assessed using a Rosand RH7 twin bore capillary rheometer (50 kN maximum load capacity). Capillary dies having diameter (D) of 2 mm with three different length-to-diameter ratios (L/D) of 8, 12 and 16 were fitted individually in the bore to measure the melt elastic properties. Shear stress at the capillary wall (τ_w), apparent shear rate ( ${\dot{γ}}_{a}$ ) and shear viscosity (η_a) data were collected for the base polymers as well as the blends in the shear rate range of 10 to 1000 s⁻¹. Additionally, τ_w, ${\dot{γ}}_{a}$ and η_a data were also collected for the base polymers as well as the blends at three different temperatures (190, 200 and 210°C) in the shear rate range of 10 to 1000 s⁻¹ using die with L/D = 16. The extrudate diameter (D_e) was measured using a Keyence-VG 301 single axis laser scanning instrument attached with the capillary rheometer.

Die swell ratio (χ) of the extrudate coming out of the die was calculated from the extrudate diameter and the die diameter by means of equation (1):

χ = \frac{D_{e}}{D_{}}

The first normal stress difference (τ_xx – τ_yy) was calculated from the die swell (χ) and shear stress at the wall (τ_w) using Tanner’s equation¹⁹ expressed as equation (2):

τ_{x x} - τ_{y y} = 2 τ_{w} \sqrt{2 (χ^{6} - 1)}

Additionally, in order to measure the entry pressure losses and elongational properties of the blends, two different length dies (L/D = 16 and L/D ∼ 0) were fitted in each bore of the capillary rheometer. Both the dies had capillary diameter of 1 mm. Elongational stress (σ_e), elongational strain rate (´∊) and elongational viscosity (η_e) data were collected for the base polymers as well as PPcp/POE blends in the shear rate range of 100 to 1000 s⁻¹ at 200°C.

Morphological studies

Samples for morphological studies were taken from the extrudates obtained after the capillary extrusion of PPcp/POE blends at three distinct apparent shear rates of 10, 100 and 1000 s⁻¹. The extrudates of different blends were cut and then cryogenically fractured in liquid nitrogen. The cryo-fractured samples were then etched in heptane at 50°C for 1 h to preferentially extract the POE phase from the fractured surface. Morphological features of the blends were studied using a Carl Zeiss EVO 50 scanning electron microscope. Image analysis of the scanning electron micrographs was carried out on ImageJ software in which the micrographs were first converted into binary images and then analyzed for statistical distribution of dispersed particle size as per reference scale.

Analysis of POE particle size and its distribution in the blends

It was reported by Liu et al.²⁰ that in case of rubber toughened polymer blends, where the rubber particles are dispersed as distinct domains, the particle size distribution can be considered to be a probability size distribution density function which is found to follow a log-normal distribution curve. The size distribution function f (d_i) can be defined as

f (d_{i}) = \frac{1}{\sqrt{2 π} ln σ} exp [\frac{- {(ln d_{i} - ln d)}^{2}}{2 {ln}^{2} σ}]

where d and σ are evaluated by means of equations (4) and (5) respectively:

ln d = \frac{\sum_{i = 1}^{N} n_{i} ln d_{i}}{\sum_{i = 1}^{N} n_{i}}

ln σ = \sqrt{\frac{\sum_{i = 1}^{N} n_{i} {(ln d_{i} - ln d)}^{2}}{\sum_{i = 1}^{N} n_{i}}}

Here, n_i and d_i are the number and apparent particle diameter of the ith domain.

The size distribution function, σ, represents the amount of heterogeneity in the particle size for any blend composition. It is often assumed to be one for a monodispersed system and more than one for a polydispersed system.

Inter-particle distance

The surface to surface inter-particle distance (ID), also recognized as the matrix ligament thickness, was evaluated using the theories promulgated by Wu.^8,9,21 ID as proposed by Wu⁸ was calculated from the particle size (d) and volume fraction of the POE phase ( ϕ _POE ) as per equation (6).

I D_{W u} = d_{} [κ {(\frac{π}{6 ϕ_{P O E}})}^{1 / 3} - 1]

The volume fraction of the POE phase in the blends was obtained using equation (7).

ϕ_{P O E} = \frac{w_{P O E} ρ_{P P c p}}{w_{P O E} ρ_{P P c p} + w_{P P c p} ρ_{P O E}}

where, w_PPcp and ρ_PPcp are the weight fraction and density of PPcp whereas w_POE and ρ_POE are the weight fraction and density of POE respectively. Wu in his work had propounded two basic assumptions to simplify his theory; firstly, all particles have the same diameter, d, and secondly, the particles form a regular cubic array (or simple cubic lattice, SCC) so that ‘κ = 1’ can be effectively substituted in equation (6).

Wu had later modified his assumptions for the inter-particle distance to account for the effect of polydispersity with regard to the size distribution.²¹ Thus equation (6) can suitably be modified into equation (8).

I D_{P O E} = d_{} [{(\frac{π}{6 ϕ_{P O E}})}^{1 / 3} - 1] exp ({ln}^{2} σ)

Results and discussion

Blend morphology, micro-structural domain size distribution and inter-particle distance

It has been widely reported that there are five principal factors that control the evolution of blend morphology during melt mixing. These are, blend composition; rheological behavior of the constituent polymers; processing temperature; duration of mixing; and mixing speed. Of these, blend composition is the key factor to envisage which of the constituents will form the continuous phase and the dispersed phase. It has been elaborately reported by many researchers that minor component always forms the dispersed phase whereas major component forms the continuous phase.⁴ Development of morphology in immiscible polymer blends processed in a twin screw extruder and their stabilization leading to diverse end use properties had been rationally illustrated by Macosko.²² The SEM images of the blend extrudates collected at varied capillary extrusion shear rates of 10, 100 and 1000 s⁻¹ are depicted in Figures 1(a) to (d), 2(a) to (d) and 3(a) to (d), respectively. All of these investigated SEM images portray the biphasic morphology which symbolizes immiscible polymer blends. The SEM images reveal dissolution of POE phase with cavities obtained through preferential etching of the blend samples.

Figure 1.

Scanning electron micrographs of (a) PPcp90; (b) PPcp80; (c) PPcp70; and (d) PPcp60 blends analyzed from the extrudates obtained at apparent shear rate of 10 s⁻¹.

Figure 2.

Scanning electron micrographs of (a) PPcp90; (b) PPcp80; (c) PPcp70; and (d) PPcp60 blends analyzed from the extrudates obtained at apparent shear rate of 100 s⁻¹.

Figure 3.

Scanning electron micrographs of (a) PPcp90; (b) PPcp80; (c) PPcp70; and (d) PPcp60 blends analyzed from the extrudates obtained at apparent shear rate of 1000 s⁻¹.

Table 1 illustrates significant blend parameters like w_POE, ϕ_POE, d, σ and ID_POE for the blends. It is apparent from Table 1 and the histograms of POE particles shown in Figures 4(a) to (d), 5(a) and (b) and 6(a) to (d) that increasing the weight fraction of POE particles in the blends results in a concomitant increase in POE particle size. Figures 4(a) to (d), 5(a) and (b) and 6(a) to (d) also indicate that breadth of the distribution and the frequency of occurrence of larger size particles increases with increasing ϕ_POE and increasing ${\dot{γ}}_{a}$ . However, one important observation was seen in this case which points that the particle size distribution function, which is an indication of the heterogeneity in particle sizes for any blend composition, remains stable (in between 1.48 and 1.73) for all concentrations of POE particles studied except at intermediate ${\dot{γ}}_{a}$ of 100 s⁻¹. The SEM images of PPcp70 and PPcp60 blends obtained from samples collected at ${\dot{γ}}_{a}$ of 100 s⁻¹ reveal highly elongated POE particles due to the dominance of particle coalescence over break-up. These morphological features agree well with the findings presented in reported literature.⁴

Table 1.

Weight fraction (w_POE), volume fraction (ϕ_POE), apparent particle diameter (d), size distribution function (σ) and inter-particle distance (ID_POE) for the POE phase in PPcp/POE blends analyzed from the extrudates obtained at varied shear rates.

		Blends
		PPcp100	PPcp90	PPcp80	PPcp70	PPcp60	PPcp0
w_POE		0	0.1	0.2	0.3	0.4	1
ϕ_POE		0	0.104	0.206	0.308	0.409	1
	10 s⁻¹		0.74	0.82	0.91	0.97
d (μm)	100 s⁻¹		0.73	0.83
	1000 s⁻¹		0.78	0.87	0.97	1.16
	10 s⁻¹		1.48	1.54	1.73	1.65
σ	100 s⁻¹		1.65	1.66
	1000 s⁻¹		1.57	1.59	1.66	1.59
	10 s⁻¹		0.61	0.36	0.24	0.11
ID_POE (μm)	100 s⁻¹		0.67	0.39
	1000 s⁻¹		0.68	0.39	0.25	0.12

Figure 4.

POE particle size distribution in (a) PPcp90; (b) PPcp80; (c) PPcp70; and (d) PPcp60 blends analyzed from the extrudates obtained at apparent shear rate of 10 s⁻¹.

Figure 5.

POE particle size distribution in (a) PPcp90; and (b) PPcp80 blends analyzed from the extrudates obtained at apparent shear rate of 100 s⁻¹.

Figure 6.

POE particle size distribution in (a) PPcp90; (b) PPcp80; (c) PPcp70; and (d) PPcp60 blends analyzed from the extrudates obtained at apparent shear rate of 1000 s⁻¹.

In general, as ϕ_POE increases, the propensity of occurrence of larger particles increases due to the dominance of coalescence. The log-normal average diameter, d, is observed to increase from 0.74 µm to 0.97 µm with an increment of ca. 31% when ϕ_POE was increased from 0.104 to 0.409 at ${\dot{γ}}_{a}$ of 10 s⁻¹. At a significantly higher shear rate ( ${\dot{γ}}_{a}$ of 1000 s⁻¹), d is observed to increase from 0.78 µm to 1.16 µm with an increment of ca. 48% when ϕ_POE was increased from 0.104 to 0.409. The inter-particle distance values (ID_POE) obtained from the SEM image analyses of the blends and calculated through equation (8) were based on the assumption that the POE particles dispersed in PPcp matrix occupies a simple cubic lattice structure. It is further manifested from Table 1 that inter-particle distance reduces with increasing ϕ_POE owing to an increase in the domain size of POE phase due to coalescence, thus shrinking the PPcp matrix in between the POE particles. This assessment has also been exemplified by Macosko, where a higher weight fraction of the dispersed phase results into larger drop size as the particles are more susceptible to coalescence.²² This observation is also in agreement with the findings reported by Bitinis and co-workers.²³ Droplet break-up and coalescence in polymer blends were also studied by Souza and Demarquette²⁴ and later by Joseph and Thomas²⁵ which validate the findings in this study whereby low volume fraction of POE (ϕ_POE < 0.2) facilitated droplet break-up while high volume fraction of POE promoted coalescence. Moreover, the biphasic structures illustrated in Figures 1(a) to (d), 2(a) to (d) and 3(a) to (d) are considered important for impact modification. There is also an increase in the magnitude of domains with increasing POE content with majority of POE particles having size less than 1 µm which is critical to impact toughening of the blends.^26,27 This is in good concurrence with the domain sizes that are considered necessary for impact modification which have been reported earlier by various researchers.^1,2,26,27

Melt flow behavior of PPcp/POE blends

The melt flow characteristics of PPcp/POE blends under capillary extrusion at 200°C are exemplified in Figures 7 and 8 which show typical pseudoplastic behavior reminiscent of thermoplastic polymer melts. Representation of the melt flow curves (apparent shear viscosity as the function of apparent shear rate) of the blends illustrated in Figure 7 established good fit with the Cross model exemplified through equation (9).

η_{a} = \frac{η_{0}}{1 + {(λ {\dot{γ}}_{a})}^{n - 1}}

where, η_o is the ‘zero-shear viscosity’, λ is the ‘characteristic relaxation time’ and n is the ‘viscosity index’ which is also the measure of shear-thinning nature of the polymer melt. The fitted parameters of the Cross model for the blends are compiled in Table 2. By means of Figure 7 and Table 2, it can be envisaged that PPcp60 blend shows the least sensitivity to applied shear rates. This can be ascribed to the relatively higher volume fraction of highly branched POE⁵ dispersed in PPcp matrix. The other blends show comparatively higher sensitivity to the applied shear rates. This can be attributed to some interaction between POE and PPcp phases at these compositions whereby upon application of shear, PPcp chains are initially restricted to flow past each other at low apparent shear rates giving rise to increase in viscosity. Subsequently when the apparent shear rates are increased, which results into increased shear stresses (see Figure 8), the two constituent polymeric phases separate out and orientation of the particle segments in the direction of flow yields overall viscosity reduction to the extent that the blend melt behaves more or less like pure PPcp. An interesting feature of the flow curves visualized in Figure 7 is that the viscosity ratio (η_PPcp/η_POE) of PPcp and POE are different at three distinct ${\dot{γ}}_{a}$ of 10, 100 and 1000 s⁻¹ resulting into distinct morphological features illustrated in Figures 1(a) to (d), 2(a) to (d) and Figures 3(a) to (d) respectively.

Figure 7.

Apparent shear viscosity (η_a) of PPcp/POE blends as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ) at extrusion temperature of 200°C. The flow curves obey the Cross model.

Figure 8.

Shear stress at the wall (τ_w) for PPcp/POE blends as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ) at extrusion temperature of 200°C.

Table 2.

Fitted parameters of PPcp/POE blend melts obeying cross model.

Blend	η_o (Pa.s)	λ (s)	n	R ²
PPcp100	1677	0.042	0.33	0.9997
PPcp90	2033	0.046	0.28	0.9998
PPcp80	1279	0.021	0.23	0.9991
PPcp70	1327	0.024	0.26	0.9995
PPcp60	1498	0.040	0.34	0.9996
PPcp0	639	0.003	0.12	0.9986

The variety of immiscible polymer blends often demonstrate wide array of properties which are governed primarily by the blend composition, distribution and dispersion of the minor phase, and the viscosity difference between the constituent polymers. Empirical equations are often applied in order to assess the melt flow data as polymer blending is not an extremely standardized operation.²⁸ For this purpose, in the present study, three types of models were utilized to compare theoretical and experimental viscosity variations with respect to apparent shear rate and blend composition, namely, Additivity model;

η_{b l e n d_{_{}}} = \sum_{i} ϕ_{i} η_{i}

Log additivity model reported by Plochocki²⁹;

log η_{b l e n d} = \sum_{i} ϕ_{i} log η_{i}

and Fluidity model used by Heitmiller and co-workers.³⁰

\frac{1}{η_{b l e n d}} = \sum_{i} \frac{ϕ_{i}}{η_{i}}

where, ϕ_i and η_i are the volume fraction and viscosity of the ith component respectively.

Figure 9 illustrates the experimental and theoretical values of η_a as the function of ϕ_POE at three different apparent shear rates ( ${\dot{γ}}_{a}$ ) of 10, 100 and 1000 s⁻¹ at 200°C. By and large, multi-component polymer blends do not firmly follow the aforesaid three models and regularly there are cases of either a positive deviation shown by the positive deviation blends (PDB) or a negative deviation shown by the negative deviation blends (NDB) between the experimentally observed and theoretical shear viscosities.³¹ PDB blends are reported to show some interfacial interactions between the constituent polymers whereas in the case of NDB blends, these interfacial interactions are reported to be absent and interlayer slip is the dominating factor.³² In the present study, an interesting feature is observed wherein the blends show positive deviation at a low shear rate ( ${\dot{γ}}_{a}$ = 10 s⁻¹), close approximation from the three theoretical viscosity predictions at intermediate shear rate ( ${\dot{γ}}_{a}$ = 100 s⁻¹) and negative deviation at a high shear rate ( ${\dot{γ}}_{a}$ = 1000 s⁻¹) which can be visualized from Figure 9. The PDB feature of the blends observed at ${\dot{γ}}_{a}$ = 10 s⁻¹ can be drawn from Figures 1(a) to (d), 4(a) to (d) and 7 which show well dispersed POE particles in PPcp matrix suggesting the presence of some interfacial interactions between the components which has been attained at a significantly low value of the viscosity ratio. On the other hand, the NDB behavior can be inferred from Figures 3(a) to (d), 6(a) to (d) and 7 which illustrate different morphological features as a result of a much higher value of the viscosity ratio at ${\dot{γ}}_{a}$ = 1000 s⁻¹ signifying lack of interfacial interaction between the component phases. At an intermediate shear rate ( ${\dot{γ}}_{a}$ = 100 s⁻¹) the shear viscosities of both PPcp and POE are quite similar (as illustrated in Figure 7) due to which the PPcp/POE blends show close approximation to the three theoretical viscosity predictions.

Figure 9.

Experimental and theoretical values of apparent shear viscosity (η_a) as the function of polyolefin elastomer volume fraction (ϕ_POE) at apparent shear rates ( ${\dot{γ}}_{a}$ ) of 10, 100 and 1000 s⁻¹ when extruded at 200°C.

The effects of POE volume fraction and capillary extrusion conditions on flow behavior of PPcp/POE blends

To study the effects of POE volume fraction, temperature, length-to-diameter (L/D) ratio of the capillary die and apparent shear rate on apparent shear viscosities of the blends, η_a values of different blend melts and the pure polymers were analyzed at three different temperatures (190°C, 200°C and 210°C) utilizing three dies of same outlet diameter (2 mm) but with discrete L/D ratio (8, 12 and 16) at three distinct values of ${\dot{γ}}_{a}$ (10, 100 and 1000 s⁻¹), the cumulative results of which have been depicted in Figures 10 and 11, respectively.

Figure 10.

Apparent shear viscosity (η_a) as the function of POE volume fraction (ϕ_POE), the apparent shear rate ( ${\dot{γ}}_{a}$ ) and the extrusion temperature.

Figure 11.

Apparent shear viscosity (η_a) as the function of POE volume fraction (ϕ_POE), the apparent shear rate ( ${\dot{γ}}_{a}$ ) and length-to-diameter (L/D) ratio of the capillary die.

To study the role of extrusion temperature on the flow behavior of PPcp/POE blends, Figure 10 may be examined. η_a of different blend melts are seen to be most affected by ${\dot{γ}}_{a}$ followed by temperature (illustrated in Figure 10). POE content (ϕ_POE) does not seem to influence any noteworthy variation in η_a of different blend melts except at very low shear rates. This can be ascribed to the fact that with an increase in the apparent shear rate, PPcp and POE molecules deviate from equilibrium conformations and orient along the flow direction accordingly making the molecular movements relatively easier. Therefore, the apparent shear viscosities of blend melts reduce in accordance with the behavior of pseudoplastic polymeric melts. An additional attribute about Figure 10 is the fact that at the higher shear rates the flow behavior shows minimal difference at different temperatures than at the lower shear rates. This observation can be ascribed to the fact that the polymer structures are quite similar at the higher shear rates due to the dominant effect of the response times of the molecules by the higher velocity gradient induced due to the deformation rate. Furthermore, the energy of thermal molecular motions increases with a rise in temperature reducing the interaction forces between molecules which ultimately result into reduction in the flow resistance. Thus, the apparent shear viscosity is observed to be decreasing with increasing temperature in this study which is an accepted phenomenon found in case of polymer melts. Nevertheless, the lowest temperature still provides the highest viscosity as anticipated.

In order to study the role of length of the capillary die on the flow behavior of PPcp/POE blends, Figure 11 may be scrutinized. Here, η_a of different blend melts are again seen to be most affected by ${\dot{γ}}_{a}$ followed by L/D ratio of the capillary die which has been illustrated in Figure 11. POE content (ϕ_POE) again does not seem to significantly influence any discernible variation in η_a of different blend melts apart from capillary extrusions carried out at very low shear rates. The effect of melt residence time inside the capillary die on the flow behavior of studied polymer blends has a similar role as that of temperature. That is, with an increase in the effective length of the capillary die at a particular apparent shear rate, PPcp and POE molecules get more time to orient along the flow direction after leaving their equilibrium conformations, thus making the molecular movements comparatively easier owing to reduction in the flow resistance. Therefore, the apparent shear viscosities of blend melts is observed to be reduced with the increase in L/D ratio of the capillary die at a particular value of the apparent shear rate.

Effect of POE content and extrusion conditions on the elongational properties of PPcp/POE blends

Polymeric melt flow at the die entrance during the extrusion process is a resultant of both shear and elongational flows. Elongational stress (σ_e), elongational strain rate (´∊) and elongational viscosity (η_e) are therefore important parameters for characterizing melt elastic properties of polymers as well. The log-log plots of η_e and σ_e against ´∊ for PPcp, POE and their blends at 200°C are portrayed in Figures 12 and 13, respectively. The basic trend of these figures representing elongational properties of PPcp/POE blends are quite similar to those observed in Figures 7 and 8. Based on these similarities it may be appraised that the rheological properties dictated by elongational flows are likely to corroborate well with the rheological properties dictated by shear flows. An important feature of the flow behavior of the melts under elongation depicted in Figure 13 is the striking resemblance with the flow behavior of polymer melts under shear depicted in Figure 8.

Figure 12.

Elongational viscosity (η_e) of the PPcp/POE blends as the function of elongational strain rate (´∊) at 200°C.

Figure 13.

Elongational stress (σ_e) in PPcp/POE blends as the function of elongational strain rate (´∊) at 200°C.

To further exemplify the behavior of the polymer melts studied in the present work, plots of Trouton ratio for various blends at varied elongational strain rates (´∊) are provided in Figure 14. It was theoretically and experimentally proven by Trouton that the ratio of elongational viscosity (η_e) to that of shear viscosity (η_a) is approximately 3 for a Newtonian liquid.³³ This ratio is famously termed as the Trouton ratio and was derived by Jones et al.³⁴ for non-Newtonian fluids considering the viscoelastic effects and can be represented through equation (13).

Figure 14.

Trouton Ratio (T_R) for the PPcp/POE blends as the function of elongational strain rate (´∊) at 200°C.

T_{R} (\dot{ε}) = \frac{η_{e} (\dot{ε})}{η_{a} (\sqrt{3} \dot{ε})}

Variable shear viscosity effects have been worked out in the analysis of Jones et al.³⁴ which illustrates convincingly that a fluid which is shear-thinning must also be anticipated to be tension-thinning in extension. Elastic fluids are generally renowned for having high Trouton ratios and the evidences observed in Figure 14 reveal that the blend melts have Trouton ratios which are more than one order of magnitude higher than those of the Newtonian fluids.

Relationship between the die swell ratio (χ), apparent shear rate ( ${\dot{γ}}_{a}$ ) and POE content (ϕ_POE) in the blends

Die swell ratio is often considered as the most important parameter among the melt elastic properties of the polymer melt which can be directly measured and quantified. Numerous noteworthy experimental data of die swell ratio focusing on different aspects affecting extrudate swell have been published by several researchers.^14-18 Figure 15 illustrates the relationship between χ and ${\dot{γ}}_{a}$ for different blends when extruded through the capillary die (L/D=16) at 200°C and shows that die swell ratio is enhanced noticeably when the apparent shear rate is increased (an increase of ca. 25% in case of PPcp60 blend). This can be attributed to the fact that when polymer melt is forced to flow inside the capillary die, polymer molecules are subjected to deformation due to shear as well as extension. The energy supplied to the polymer molecules for deformation and flow through the capillary die, through disentanglement and change of molecular structure, is thereby either lost due to viscous dissipation or stored as elastic deformation energy. When polymer melt comes out of the die, this stored elastic deformation energy is released and the polymer tries to regain its entangled molecular structure resulting in the extrudate or die swell. Thus, at a particular temperature, the shear deformation energy stored in the melt increases with increasing ${\dot{γ}}_{a}$ , giving rise to increased die swell. The die swell ratio of the studied blends can thus be assumed to follow a power law type of relationship with the apparent shear rate.

Figure 15.

Die swell ratio (χ) as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ) at extrusion temperature of 200°C using capillary die with L/D = 16.

χ = α {\dot{γ}}_{a}^{β}

where α is a constant and β is the power law index in equation (14). The fitted parameters of this power law relationship have been presented in Table 3.

Table 3.

Power law parameters of PPcp/POE blend melts representing relationship between die swell and shear rate.

Blend	α (s^β)	β	R ²
PPcp100	1.238	0.037	0.976
PPcp90	1.307	0.025	0.981
PPcp80	1.220	0.038	0.950
PPcp70	1.186	0.039	0.975
PPcp60	1.096	0.054	0.978
PPcp0	1.018	0.068	0.970

Effect of capillary extrusion conditions (shear rate, temperature and length-to-diameter ratio of capillary die) on die swell ratio (χ) of the blends

To understand the effect of varied capillary extrusion conditions like temperature and capillary die configurations, die swell ratio values of each blend material at three ${\dot{γ}}_{a}$ (10, 100 and 1000 s⁻¹) were evaluated at three different temperatures (190, 200 and 210°C) using capillary die of L/D = 16 and subsequently for two other lengths of capillary dies (L/D = 8 and 12) at 200°C. Figure 16 highlights the effect of temperature on die swell ratio measured at varied apparent shear rates for each blend and reveals that with an increase in temperature the die swell values tend to reduce. However, the die swell ratio of different blend melts are seen to be most affected by ${\dot{γ}}_{a}$ followed by temperature in the manner similar to that of the flow behavior of the PPcp/POE melts. POE content (ϕ_POE) yet again does not seem to affect any noticeable variation in χ of different blend melts except at very low shear rates. This observation can be ascribed to the state of already more oriented molecular structures at higher temperature which are subjected to comparatively less deformation upon entry into the die thereby resulting into release of a comparatively lower melt elastic energy upon exit from the die consequently leading into a lesser value of the die swell. Figure 17 portrays the effect of capillary die length (die L/D), or in other words the time which the polymeric melt gets to stay inside the die at a particular value of ${\dot{γ}}_{a}$ , on the die swell ratio for varied PPcp/POE blends at 200°C. The graph noticeably indicates that more time the blend melt gets to deform inside the capillary die, the lesser is the stored elastic energy of the polymer melt, which is later released in the form of die swell to come back to its original molecular structure upon exit from the die. Similar findings have also been reported by Dangtungee and co-workers for low-density polyethylene/ethylene-octene copolymer blends.³⁵

Figure 16.

Die swell ratio (χ) as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ), extrusion temperature and POE volume fraction (ϕ_POE).

Figure 17.

Die swell ratio (χ) of the PPcp/POE blends as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ), L/D ratio of capillary dies and POE volume fraction (ϕ_POE).

Comparative effects of capillary extrusion conditions (apparent shear rate, temperature, L/D ratio of capillary die) and POE content (ϕ_POE) on the first normal stress difference (τ_xx – τ_yy)

The first normal stress difference (τ_xx – τ_yy) which is an estimate of the elastic recovery of the restrained molecular chains in a plane perpendicular to the extrusion direction after leaving the capillary die exit, gives valuable information about the behavior of polymer melts during processing operations. Tanner, in his published article had reported the existence of a direct proportionality between the first normal stress difference and the shear stress at the capillary wall along with the die swell ratio.¹⁹ Thus, one would anticipate obtaining a similar dependence of τ_xx – τ_yy on ϕ_POE and ${\dot{γ}}_{a}$ as that of χ on these variables. However, the first normal stress difference values of the blends give a relatively better portrayal of the melt elastic properties at varied extrusion conditions since the first normal stress difference is a function of both τ_w and χ. Plots of τ_xx – τ_yy and ϕ_POE at varied extrusion temperatures (190°C, 200°C and 210°C) measured at different values of ${\dot{γ}}_{a}$ (10, 100 and 1000 s⁻¹) have been illustrated in Figure 18 which seem to follow the identical pattern (decrease in τ_xx – τ_yy with increasing temperature) as observed in the case of die swell ratio. However, the change in extrusion temperature has a more pronounced effect on the first normal stress differences of the blends at high shear rates which leads to the inference that the effect of temperature on τ_xx – τ_yy is perceptible only at an apparent shear rate sufficient to cancel the effect of melt residence time inside the capillary die. Similarly, the effect of length-to-diameter ratio of the capillary die on first normal stress differences of the blends measured at different ${\dot{γ}}_{a}$ of 10, 100 and 1000 s⁻¹ and capillary extrusion temperature of 200°C has been depicted in Figure 19 and resembles a similar trend (decrease in τ_xx – τ_yy with increasing length of the capillary die) as seen in the case of die swell. In this case also, the effect of length-to-diameter ratio of the capillary die on τ_xx – τ_yy is evident only at higher values of ${\dot{γ}}_{a}$ . This behavior can be attributed to the increased time period which the polymer molecules get to completely orient and disorient when extruded through the capillary dies at low values of ${\dot{γ}}_{a}$ resulting into reduction in melt elastic nature of the polymeric material. These findings further substantiate that melt elastic properties of the polymer blends depend more on the extrusion variables ( ${\dot{γ}}_{a}$ , extrusion temperature and length of the capillary die) than on the elastomeric phase content in the blends (ϕ_POE).

Figure 18.

First normal stress difference (τ_xx – τ_yy) as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ), extrusion temperature and POE volume fraction (ϕ_POE).

Figure 19.

First normal stress difference (τ_xx – τ_yy) as the function of apparent shear rate ( ${\dot{γ}}_{a}$ ), L/D ratio of capillary dies and POE volume fraction (ϕ_POE).

The effect of elastomeric inter-particle distance (ID_POE) on the melt elastic properties of PPcp/POE blends

Figure 20 demonstrates the effect of ID_POE on χ and τ_xx – τ_yy of PPcp/POE blends at three different values of ${\dot{γ}}_{a}$ (100, 100 and 1000 s⁻¹). The graph advocates that there is a non-linear relationship observable at high ${\dot{γ}}_{a}$ between χ and ID_POE and also between τ_xx – τ_yy and ID_POE with a critical inter-particle distance at 0.24–0.25 µm below which there is a sharp increase in the melt elastic properties of PPcp/POE blends. Thus it can be manifested that there exists a critical value of inter-particle distance below which there is an abrupt increase in the stored elastic energy. Since the toughening mechanism of PPcp blends is restricted to a narrow range of particle sizes (or the inter-particle distance to be more precise), it is anticipated that the stored elastic energy in the melt form will also follow a similar trend. Thus it is notably observed that there exists a critical inter-particle distance for the melt elastic properties of the PPcp blends as well. However, these observations are more prominently observed at higher values of ${\dot{γ}}_{a}$ signifying existence of competing effect of blend microstructure and processing variables inside the capillary die at lower values of ${\dot{γ}}_{a}$ .

Figure 20.

Die swell ratio (χ) and first normal stress difference (τ_xx – τ_yy) as the function of POE inter-particle distance (ID_POE) at different shear rates ( ${\dot{γ}}_{a}$ ).

Conclusions

Consequent to the study of melt flow and melt elastic properties of PPcp/POE blends at varied capillary extrusion conditions, this study revealed following noteworthy aspects:

Morphology development depended sturdily on POE content where high POE content (w_POE ≥ 0.3) promoted coalescence as a result of which the inter-particle distance (ID_POE) decreased significantly with increasing ϕ_POE. The evolution of distinct morphological features also depended upon the viscosity ratios of the constituent polymers which were in turn governed by the capillary extrusion conditions (shear rate in particular).

The blend melts demonstrated typical pseudoplastic behavior obeying Cross model under shear flow. η_a of the blends were seen to be most affected by ${\dot{γ}}_{a}$ followed by temperature and then followed by ϕ_POE. In the same way, η_a of the blends were seen to be most affected by ${\dot{γ}}_{a}$ followed by L/D ratio of the capillary die and then followed by ϕ_POE. PPcp60 blend was found to show the least sensitivity to the change in ${\dot{γ}}_{a}$ owing to probable interaction between the PPcp and POE phases at a significantly low value of ID_POE. In the correlation between theoretical predictions of viscosities with the experimentally observed values, the blends behaved like both PDB and NDB blends depending on the values of ${\dot{γ}}_{a}$ .

The elongational flow correlated well with the shear flow behavior and thus indicated that processes involving extensional deformation can substantially be affected by the tension-thinning PPcp/POE blend melts.

All the blends demonstrated pronounced dependence of melt elastic properties (χ and τ_xx – τ_yy) on ${\dot{γ}}_{a}$ which can be expressed in the form of power law type of function. Furthermore, the melt elastic properties were seen to be reduced with the increase in capillary extrusion temperature and capillary die length owing to the reduction in stored elastic energy in the blend melts. However, these reductions were more prominently observed at higher values of ${\dot{γ}}_{a}$ signifying existence of competing effect of blend microstructure and processing variables inside the capillary die at lower values of ${\dot{γ}}_{a}$ .

It was observed that there is a critical value of inter-particle distance in PPcp/POE blends below which there is an abrupt increase in the stored melt elastic energy due to the blend microstructure which could be elucidated vividly only at high shear rates.

These observations portray the strong relationship between polymer composition, blend morphology, rheological properties under shear, melt elastic properties and processing conditions which can judiciously be applied to control and optimize the processing window for various polymer processing operations.

Footnotes

Acknowledgements

The authors are grateful to all the officers and staff of Polymer Science Division of DMSRDE for providing necessary support and help to carry out this work. The authors also acknowledge the support rendered by Dr. Kavita Agarwal and Rakesh Kumar for SEM analysis in this research work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

AK Pandey

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Melt elasticity and flow properties of polypropylene impact copolymer/polyolefin elastomer blends subjected to varied capillary extrusion conditions

Abstract

Keywords

Introduction

Experimental

Materials

Blend preparation

Rheological analysis

Morphological studies

Analysis of POE particle size and its distribution in the blends

Inter-particle distance

Results and discussion

Blend morphology, micro-structural domain size distribution and inter-particle distance

Melt flow behavior of PPcp/POE blends

The effects of POE volume fraction and capillary extrusion conditions on flow behavior of PPcp/POE blends

Effect of POE content and extrusion conditions on the elongational properties of PPcp/POE blends

Relationship between the die swell ratio (χ), apparent shear rate ( γ ˙ a ) and POE content (ϕPOE) in the blends

Effect of capillary extrusion conditions (shear rate, temperature and length-to-diameter ratio of capillary die) on die swell ratio (χ) of the blends

Comparative effects of capillary extrusion conditions (apparent shear rate, temperature, L/D ratio of capillary die) and POE content (ϕPOE) on the first normal stress difference (τxx – τyy)

The effect of elastomeric inter-particle distance (IDPOE) on the melt elastic properties of PPcp/POE blends

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References

Relationship between the die swell ratio (χ), apparent shear rate ( ${\dot{γ}}_{a}$ ) and POE content (ϕ_POE) in the blends

Comparative effects of capillary extrusion conditions (apparent shear rate, temperature, L/D ratio of capillary die) and POE content (ϕ_POE) on the first normal stress difference (τ_xx – τ_yy)

The effect of elastomeric inter-particle distance (ID_POE) on the melt elastic properties of PPcp/POE blends