EVALUATION OF INJECTION MOLDING CONDITION ON THE MECHANICAL PROPERTIES OF IN SITU POLYPROPYLENE BLENDS Fernando Costa Almada¹, Marcelo Farah¹, Susana Liberman¹ and Raquel Mauler² 1 - Braskem S. A., Polymer Science Group, Technology and Inovation Center, Triunfo, RS, marcelo.farah@braskem.com.br 2 - Universidade Federal do Rio Grande do Sul (UFRGS) Departamento de Química mauler@iq.ufrgs.br The impact strength of isotatic polypropylene (ipp) can be improved via blending with poly(ethylene-co-propylene) (EPR). In ipp/epr blends, the dispersed-phase morphology described by parameters such as average particle size and particle size distribution plays an essential role in the mechanical properties, especially in impact strength. The Rayleigh-Taylor-Tomotika theory is often used to describe average particle size behavior. However, process variables can change the morphology in immiscible blends, and in this case, affect blend properties. In this study, the influence of injection molding conditions on morphology and mechanical properties of in reactor ipp/epr blends was studied. Three in reactor blends of ipp/epr were and content of EPR, but µ used, each one with a particular viscosity ratio (with the same ethylene content in the EPR phase. Such blends were evaluated before and after injection molding processing. Scanning Electron Microscopy (SEM) and Intrinsic Viscosity (IV) evaluation were used to validate particle average size behavior with viscosity ratio, in conformity with the Rayleigh-Taylor-Tomotika theory. Modification on Notched Izod Impact Strength was achieved with changes in processing conditions, particularly shear rates. The highest changes in this property (36%) were obtained for PP B, which showed the highest viscosity ratio (µ =1.67). Whereas the lowest variation (9%) occurred in PP A, since its viscosity ratio (µ =0.99) was lower than that for other blends. The processing conditions that lead to greater average particle size resulted in the highest levels of Notched Izod Impact Strength. Besides, it was found an effective particle area that optimizes NIIS 0.19 µm² for PPA, 0.25 um² for PP B and 0.45 um² for PP C. The coalescence process of the EPR phase was favored for in reactor ipp/epr blends with µ > 1, processed with low screw speed (10 rpm). SEM analyses reveal no morphological modification in PP A before and after the injection molding processing and more stable mechanical properties. Introduction Polypropylene blends are the aim of some scientific studies due to the blending possibilities with other polymers resulting in a significant properties increase, mainly on impact strength. The rubber phase can reach a high degree of dispersion in in-situ blends and, because of that, they are also called ipp/epr alloys(1). The EPR morphology can be modified by changes on chemical composition in in-situ polymerization. These modification in the EPR composition can also change the interfacial tension between EPR and ipp matrix. Thus, the dispersion and mechanical properties are affected (2). The influence of the dispersion of the impact modifiers in the mechanical properties have been extensively studied during the last years (3) and earlier studies have also reported the influence of viscosity ratio on average and particle size distribution (4). Processing variables such as temperature and shear rate ( γ& ) were also studied as morphological modifiers of binary systems (5) and some theories are currently accepted (6). However, not much attention has been given to the effect of shear rate on morphology with different viscosity ratios. The purpose of this study is to assess the influence of processing conditions on morphology and mechanical properties of in-situ EPR/iPP blend with different viscosity ratios. Experimental Three different polypropylenes copolymers were used in this work and their properties are shown on table 01: Table 01 Copolymer polypropylenes characteristics PPA PPB PPC PP Viscosity 3,21 1,89 2,13 EPR Viscosity 3,18 3,16 2,04 Viscosity ratio 0,99 1,67 1,18 EPR % 18,5 20,7 11,2 The samples were injected in a Sandretto Euromap 612/150 Injection molding machine, model Otto 150, using a ASTM D 638 type I cavity. The injection conditions applied are shown in table 2. The values of injection pressure are proportional to the maximum Injection pressure (1926 bar), with a maximum injection speed set. Table 2 Injection molding condition of the samples Screw Speed(rpm) Injection Pressure(%) Injection Condition 10 10 1 10 90 2 200 10 3 200 90 4
The injected samples were analyzed on a cryogenically fractured surface and analyzed on a JEOL-JSM 6060 SEM. The morphological dispersion was analyzed by Image Tool 3 software. Mechanical properties were tested on Instron 4301 and ASTM D-790 standard and CEAST Resil Impactor following ASTM D-256. Rheological data was obtained from compressed samples (raw materials). A Rheometrics SR200, with parallel plates, with 25 mm diameter and 1 mm gap between fixtures, was used at 200 C. Results and Discussion Processing At each condition, the Injection time for different PP s was collected from injection machine and the apparent shear rate was calculated. Since the injections were controlled by pressure, the most viscous resin showed a higher injection time, consequently a lower shear rate (Figure 01). Resins B and C, that show almost the same viscosity curve, show the same injection time at lower injection speed. This time could have been predicted observing the complex viscosity in figure 01-b. At higher speed, a significant difference was observed when the screw speed was low. Since the PPC resin presents smaller amounts of rubber, the lower screw speed apparently did not promote a complete melt of the material, which was indicated by the lower shear rate in injection condition 2. 1000 1,E+05 Nominal Shear Rate (s-1) 800 600 400 200 0 PPA PPB PPC 1 2 3 4 η* (Pa.s) PP A PP B PP C 1,E+04 1,E+03 1,E+02 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 Injection condition a) b) Figure 01 a) Apparent viscosity on Injection conditions and b) complex viscosity PP s. ω (Hz) Shear rate increased at higher screw rotation speed (conditions 1-3 and 2-4). Probably, an additional melt temperature has been reached, and at the same injection pressure, the part has been filled faster by the polymer mass. Morphology The original morphologies of the cpp s are shown in figure 02: PP B PP B PP C Figure 02 Copolymers morphologies as synthesized from reactor.
Figure 03 - Domain size and cumulative 90% particles area for original samples. It is known that samples have the same molar rubber composition; the bright points (PE inside EPR drops) are also similar in figure 2. This is an important fact because these samples were produced by compression molding, therefore in the absence of flow action. This way, the phase distribution has been considered the initial morphology of pellets. These results confirm the theory that, as close to 1 the viscosity ratio, smaller the particles size are because the pellets came from an extrusion processing. Applying the processing conditions, the following morphologies were obtained: PP A Screw rotation 3 4 1 2 Injection pressure ratio Figure 04 PPA changes over injection molding conditions. For A system, no significant changes on morphologies were observed, except on low screw rotation and high injection velocity, where the samples seem to be larger. The accumulative drop areas were measured and the results are shown in figure 05.
Figure 05 - Size distribution of particles on different injection molding conditions to PPA. It was observed that, to lower injections speeds, a higher number of small particles were present in the samples. It could be explained by an increase on shear rate in more inner regions in the sample or by some coalescence phenomenon in high injection condition. The increase of frozen layer near the cavity wall increases the shear rate, which can be illustrated in figure 06. In lower injection speed, higher heat transfer mold wall/polymer happens, decreasing the flow area and increasing the shear level and position of its maximum. Figure 06 - Increase on shear rate in the thicker frozen(crystalline) layer. To PPB morphologies: P P B Screw rotation 3 4 1 2 Injection pressure ratio Figure 07 PPB changes over injection molding conditions.
The accumulative curve can be seen on figure 08: Figure 08- Size distribution of particles on different injection molding conditions to PPB. On this polymer sample, the morphology of original pellets was not maintained. This sample shows the higher evidence of dependence of morphologies to the viscosity ratio (1,67), as the theory predicted, and it can be proved by accumulative curves, where a higher difference of particles area (size) distribution can be observed. To PPC morphologies: PP C Screw rotation 3 4 1 2 Figure 09 - PPC changes over injection molding conditions. Injection pressure ratio As in PPB samples, the injected samples did not maintain the original morphology. Not so circular particles can be observed, probably due to the viscosity ratio close to 1, and lower EPR molecular weight. Figure 10 - Size distribution of particles on different injection molding conditions to PPC.
On this sample, there is a more evident effect of smaller particles generation at low injection speed. Although a smaller amount of EPR exists in this sample, a correlation among particles size, viscosity and elasticity can explain the behavior of this sample. Mechanical Properties The mechanical properties of these samples are show in figures 11-13. PPA - Mechanical Properties 1400 1300 1200 1100 1000 900 800 No break Impact 23 C [J/m] Flexure modulus [MPa] Impact -20 C [J/m] 140 10/10 10/90 200/10 200/90 120 100 80 60 40 20 Figure 11 - Mechanical properties measured for PPA samples in different injection condition. For sample PPA, higher injection speed generates higher flexural modulus. Comparing size distribution, we can considered that the increase of smaller particles slickly decreases the final property of the material. None influence or standard characteristics in impact properties could be observed. 140 120 100 80 60 40 PPB - Mechanical Properties Impact 23 C [J/m] Impact -20 C [J/m] Flexure modulus [MPa] 1400 1300 1200 1100 1000 900 20 10/10 10/90 200/10 200/90 800 Figure 12 - Mechanical properties measured for PPB samples in different injection condition. For PPB samples, higher injection speed generates higher flexural modulus at lower screw speed. This effect could be generated by lower melt temperature in the absence of shear on the injection machine screw, increasing the effect of frozen layer formation. The behavior of PPA sample for flexure modulus was observed in the impact at 23 C for PPB, where the higher injection speed, the greater the Impact at 23 C at the same screw rotation. In the forming process of small particles, it was observed a decrease in the effectiveness of toughness mechanisms. This is a strong evidence of a particle critical size in these samples.
140 120 100 80 60 40 PPC - Mechanical Properties Impact 23 C [J/m] Impact -20 C [J/m] Flexure modulus [MPa] 1300 1100 900 700 20 10/10 10/90 200/10 200/90 500 Figure 13 - Mechanical properties measured for PPC samples in different injection condition. In PPC samples, no changes on flexure modulus or impact at low temperature were observed. However, in the impact at room temperature, the increase of the injection speed reduced the property. Comparing PPB and PPC samples a competition between blend and matrix effects, which present opposite behavior, can be suggested. Comparing the level of changes on properties due to the processing conditions, it can be seen a strong influence of viscosity ratio, as show in figure 14, where the higher the distance to ideal value of 1 to viscosity ratio, the greater the variation on properties depending of the processing conditions (to impact at 23 C) is. Figure 14 Properties changing on Impact properties of different PP. In the analyzed system, the PPA sample (µ close to 1) shows a stable morphology, very important to industrial resins because much more stable properties can be maintained in different machines or injection molding conditions. In other samples (PPB and PPC), the final properties can be different, for different costumers. Specific analyses in each one must be done to get the best resins performance. Different ideal particles area are obtained to each sample, being 0,19µm² to PPA, 0,25µm² to PPB and 0,45µm² to PPC. It is evident how complex the system PP/EPR behavior is on commercial resins. Conclusion The Rayleigh Taylor Tomotika theory was validated for PP/EPR system, where as close to 1 the viscosity ratio of phases, smaller particles size were obtained. The final properties variation was higher as the viscosity ratio was higher, being much more sensitive to changing on processing conditions. Very small size particles do not increase the impact properties because they could be smaller than best particles size. The particles must be bigger or equal to the critical size to be more effective. Effects of matrix morphology (shear induced crystallized layer) and rubber viscosity could be conclusive on results interpretation and mechanism description. The final properties were sensitive to morphologies, but no simple correlations were observed. Each system shows different behavior, probably due to different critical particle sizes.
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