Characterization of High Performance Polyamides Blends for Injection Molding (November 2014)

Transcription

1 Characterization of High Performance Polyamides Blends for Injection Molding (November 2014) Ana Luísa V. D. Moreira Braga, Student of Técnico Lisboa; Prof. António J. B. Correia Diogo, Professor at Técnico Lisboa Abstract These work studied the material properties of four different polyamide components that belong to the automobile engine intercooler. The material consists of a polyamide blend, PA66+PA6, reinforced with glass fiber. Some of these polyamide components were subject to thermal cycles (that simulate the stresses during an automobile life, under 210ºC) and were also subject to a surface aspect modification. The samples of these components were characterized by SEM, DMA and DSC and their results showed an evolution of the surface aspect with increasing thermal cycles, an increase in crystallinity due to the effect of thermal cycles and a difference in the composition of the components subjected to the surface aspect modification, respectively. Key words PA66+PA6, Thermal Cycles, Surface Aspect, SEM, DMA, DSC components consist of a polyamide blend, PA66+PA6, reinforced with short glass fiber and they are the air input and output intercooler s boxes, made by injection molding. Some of these components were subject to thermal cycles and to a surface aspect modification. The thermal cycles simulate the stresses that an automobile has to withstand during its life (the stress varies between 10 and 300 kpa under 210ºC, during 5 to 6 days). The aim of this work was to study the effect of the thermal cycles on the evolution of the components material properties during their use and was also to analyze the effect that a surface aspect modification can have in the appropriate usage of the respective components. The characterization techniques selected to observe the surface morphology, to study the mechanical properties depending on temperature and to analyze the composition of the material of each component were SEM, DMA and DSC, respectively. I. Introduction These work studied the material of four different components that belong to the automobile engine intercooler. These Polyamides are semicrystalline thermoplastics and the PA66+PA6 blend is composed of PA66 and PA6 (PA66 is present in greater quantity, although the

2 exact fraction of each polyamide is unknown). PA6 is made from hydrolytic polymerization of Ɛ-caprolactam. PA66 is made from polycondensation of hexamethylenediamine and adipic acid [1], [2]. melt flux, but the fibers in the melt core stay aligned transversely to the direction of melt flux (due to the absence of shear stresses that are only present in the mold cavity walls). This dependence on melt flux direction leads to anisotropic properties [3]. The parts reinforced with glass fiber and produced by injection molding have a typical structure because the fibers at the polymer melt surface (that contacts the mold cavity walls) stay aligned parallel to the direction of II. Experimental Techniques The table below identifies the features of the four different samples of each component analyzed in this work. Table 1 - Description of the four samples analyzed. Sample A Sample B Sample C Sample D Glass Fiber Reinforcement 30% 50% 50% 50% (wt%) Thermal Cycles 0 cycles 0 cycles 70 kcycles 210 kcycles Surface Aspect Modification No Yes Yes Yes A. SEM This technique was selected to observe the differences in the morphology of the samples with and without the surface aspect modification (samples A and B) and to observe the evolution of the surface of the samples B, C and D with increasing thermal cycles. The resolution achieved by SEM was also an important requirement, since the studied material is totally black, opaque and reinforced with glass fiber. The four specimens of each component were collected from the same place. They were not polished neither etched (to not lose any feature of their surface), but it was necessary to cover them with a conductor film of a gold/palladium alloy by sputter coating. The equipment used was a SEM S 2400 by Hitachi. The operating parameters were the following: Table 2 - Operating parameters during SEM analysis. Emission Current 75 na Accelerating 20 kv Voltage Magnification 100x, 500x, 1000x B. DMA The DMA technique was selected to study the mechanical behavior and the viscoelastic properties (E, E and tanδ) dependence on temperature and was also to observe the effect of the thermal cycles on the mechanical behavior of the respective components. To obtain the viscoelastic

3 properties over a wide temperature range, first the four specimens were tested under - 60ºC to 150ºC and after under 35ºC to 250ºC. The equipment used was a DMA Q800 by TA Instruments. The operating modes were single cantilever (10 mm), multi-frequency mode and temperature ramp. The operating parameters are in the table below: Table 3 - Operating parameters during DMA analysis. Isothermal (Initial 5 min Temperature) Heating Rate 2ºC/min Frequency 1, 3 and 10 Hz Deformation 10 µm Amplitude C. DSC This technique was selected to study the effect of the thermal cycles on the microstructure of each component and also to compare the composition of each component without the thermal history (second heat after the first melting). The equipment used was a DSC 2920 by TA Instruments and the operating parameters for the two heating cycles are in the following table: Table 4 - Operating parameters during DSC analysis. Initial Temperature 30ºC Final Temperature 280ºC Heating Rate 10ºC/min Purge Gas Helium Each specimen were a rectangular prism and the desirable thickness to obtain a uniform heating through the entire specimen is 2 mm (especially concerning specimens with low thermal conductivity). The average mass of the specimens is 5,6 mg. III. Results A. SEM Figure 1 - SEM images for the samples A (left) and B (right) obtained by the secondary electrons mode (100x).

4 Figure 2 - SEM images for the samples C (left) and D (right) obtained by the secondary electrons mode (100x). B. DMA Figure 4 - DMA results of E', E'' and tanδ for sample A (1 Hz). Figure Figure 6 - DMA 5 - DMA results results of E', of E'' E', and E'' and tanδ tanδ for sample for sample B (1 B. Hz). Figure 3 - DMA results of E', E'' and tanδ for sample C (1 Hz).

5 Figure 7 - DMA results of E', E'' and tanδ for sample D (1 Hz). Figure 8 - DMA results of E (log) dependence on temperature for each sample (1 Hz). Table 5 - E' values for each sample at initial run temperature (-60ºC). Sample A Sample B Sample C Sample D E (GPa) Table 6 - Tg determined by E'' and tanδ peaks for each sample [4]. Tg Tg Sample A Sample B Sample C Sample D (literature) PA ºC E Peak 40.20ºC 48.06ºC 55.40ºC 48.66ºC PA ºC Tanδ Peak 55.02ºC 63.84ºC 68.15ºC 69.15ºC

6 C. DSC Figure 9 - DSC results of each sample for the first heating cycle. Figure 10 - DSC results of each sample for the second heating cycle. Table 7 - Tm, ΔHf and %Cx values for the fusion peak of each sample (first heating cycle). Sample A Sample B Sample C Sample D Peak 1 Peak 2 Peak 1 Peak 2 ΔHf (J/g) Normalized ΔHf (J/g) Tm (ºC) %Cx

7 Table 8 - Tm, ΔHf and %Cx values for the fusion peak of each sample (second heating cycle). Sample A Sample B Sample C Sample D ΔHf (J/g) Normalized ΔHf (J/g) Tm (ºC) %Cx Since the samples are reinforced with glass fiber is necessary to divide the value of ΔHf obtained from the fusion peak integral by the weight fraction of PA66+PA6 present to get the normalized value of ΔHf for the polyamide component [5]. %Cx (crystallinity percent) is calculated by the ratio of ΔHf (obtained by the peak integral) and ΔHf100, which is the value of ΔHf for the same material with 100% of crystallinity, according the following equation: %Cx = H f 100 (1) H f100 (1 W f ) where Wf is the weight fraction of glass fiber present in the PA66+PA6 blend [6]. The ΔHf100 value of PA66 (the polyamide in greater quantity) from literature is 255 J/g [7]. IV. Discussion and Conclusion The SEM image for the sample A (the sample that doesn t have the surface aspect modification) is an image of a typical surface of an injection molded part. The melt flux during the mold cavity filling leads to the deposition of the polymer melt on the mold cavity walls with the same direction of the mold filling. Once the polymer melt quickly solidifies on the cavity mold walls, the mold filling direction is maintained in the polymer surface after solidification occurs [3], [8]. From the SEM images of the samples B, C and D is not possible to notice a typical surface of an injection molded part because of an increase in their surface roughness and irregularity. It s observed a surface evolution in the samples that were subjected to the thermal cycles (samples C and D), indicated by an increase in surface smoothing with increasing thermal cycles. The DMA results show a typical E dependence on temperature for a semicrystalline material, with the first drop due to the glass transition, followed by a reasonable stable elastic plateau (due to the crystalline structure) and finally by a second drop due to the approximation of the melt temperature of the crystalline structure [9]. The samples that were subjected to thermal cycles have the elastic plateau extended for higher temperatures. This can be justified by an increase in crystallinity caused by the thermal cycles. This increase in crystallinity explains the higher E value of the sample D with respect to sample B, when their structure enters in the elastic plateau. The DMA results also show a difference in the material characteristics of sample C compared to the other samples because its E value is almost 2 GPa higher than others. Its Tg value is also higher (from E peak)

8 which can be explained by a change in Mw. An increase in Mw leads to a more resistant structure because the polymer chains strength also increases [9], [10]. The DSC results show a typical DSC curve for a semicrystalline material, with the glass transition and the crystalline melting peak. The first heating cycle demonstrates an additional melting peak that can be related to the melting of the secondary crystalline structure caused by the thermal cycles [11]. The second heating cycle only shows the melting peak of PA66, which suggest that PA66 has a much higher weight fraction than PA6 and, in this case, it is a PA66 copolymer [12], [13]. The DSC results also show a small endothermic peak around 150ºC for the samples with the surface aspect modification (B, C and D), that matches with the melting temperature of adipic acid. This adipic acid can be present in PA66 or else in a lubricant used for polyamides processing [14], [7], [15]. References [1] ISO : Polyamides Designation System and Basis for Specification. [2] D. K. Platt, "Polyamide (PA)," in Engineering and High Performance Plastics, Shawbury, Rapra Technology Ltd., 2003, pp [3] S. Shaharuddin, M. Salit and E. Zainudin, "A Review of the Effect of Moulding Parameters on the Performance of Polymeric Composite Injection Moulding," Tübitak, Selangor, [4] R. J. Mehta, "Physical Constants of Various Polyamides," in Polymer Handbook 4th Edition, J. Brandrup, E. H. Immergut, E. A. Grulke, 1998, pp. V/121-V/135. [5] E. Espigulé, X. Puiguert, F. Vilaseca, J. A. Mendez, P. Mutjé and J. Girones, "Thermoplastic Starch-Based Composites with Rape Fibers: Water Uptake and Thermomechanical Properties," Bioresources.com, vol. 8, no. 2, pp , [6] S. Mutlur, Thermal Analysis of Composites Using DSC, [7] Polymer DSC, Mettler Toledo, Inc.. [8] L. C. Sawyer, D. T. Grubb and G. F. Meyers, "Polymer Morphology," in Polymer Microscopy 3rd Edition, Nova Iorque, Springer, 2008, pp [9] J. Ducan, "Principles and Applications of Mechanical Thermal Analysis," in Principles and Applications of Thermal Analysis, Blackwell Publishing Ltd., 2008, pp [10] B. Sivasankar, "Structure and Properties of Polymers, Size of the Polymer Molecule Chain," in Engineering Chemistry, Nova Deli, Tata McGraw-Hill Publishing Company Ltd., 2008, p [11] J. C. Salamone, Polymeric Materials Encyclopedia, Florida: CRC Press, Inc., [12] F. Rybnikar and P. H. Geil, Melting and Recrystallization of PA-6/PA-66 Blends, Illinois: Air Conditioning &

9 Refrigeration Center, Mechanical & Industrial Engineering Dept., University of Illinois, [13] M. Evstatiev, S. Petrovich and B. Krasteva, "Calorimetric Studies on PET/PA6 and PA66/PA6 Drawn Blends with Various Thermal Histories," Bulg. J. Phys., vol. 30, pp , [14] Adipic Acid CAS Nº: , Paris: UNEP Publications, [15] G. Wypych, "Polyamide," in Handbook of Plasticizers, Toronto, Elsevier, Inc., 2012, pp