18 th International Conference on composite materials CO-INJECTION OF SANDWICH STRUCTURE XPP/XHDPE/XPP T. Norraprateep* 1, 2 and U. Meekum 1, 2 1 School of Polymer Engineering, Institute of Engineering, Suranaree University of Technology, Nakorn Ratchasima, Thailand; 2 Center of Excellent for Petroleum, Petrochemical and Advanced materials, Chulalongkorn University, Bangkok, Thailand *Thnatiwat@hotmail.com Keywords : Sandwich Composite, Crosslinked PP, Crosslink HDPE, Sauna Treatment 160 o C, 165 o C, 165 o C, 170 o C, and 170 C from feed 1 Introduction It is known that the fusion of properties achieved zone to pelletized die, respectively. DCP and by co-injection molding technique range from VTMS were used at 0.3 and 1.0 phr, respectively. environment friendly production and cost saving to They were used to promote crosslink reaction via aesthetics and combinations of engineering free radical addition and then in situ condensation properties[1]. In the case of engineering properties reactions. The sandwich specimens were fabricated improvement, the surface adhesion should be using dual barrel co-injection molding machine considered for the combination of two different (TEDERIC, model TRX-60C) at the identical materials. The rule of mixture approaches have temperature profile, 160 o C, 165 o C, 170 o C, 175 o C been used successfully to predict strength, modulus, and 180 o C from feed to nozzle, respectively. The and flexural stiffness of sandwich composites that sandwich specimen with volume fraction of core has a good surface adhesion[2]. In this study, varied from 0.5, 0.7 and 0.9 were controlled by shot sandwich structure obtained by co-injection volume and finally calculated from the surface ratio molding of the crosslinked system of between core area and total cross section area of the polypropylene(PP) and high density sample at the middle position of the sandwich polyethylene(HDPE), namely xPP/xHDPE/xPP, specimen. Sample incubation in the oven saturated were prepared and investigated by mean of service with moisture at 105 o C, called post cured or sauna temperature, mechanical properties, and treatment, for at least 12 hours was performed to morphology. The PP skin and HDPE core were accelerate the completion of crosslink reaction via crosslinked using dicumyl peroxide(DCP) and vinyl silane/water condensation. trimethoxysilane(VTMS) system to improve the 2.3 Testing thermal properties and perhaps the surface adhesion of the sandwich composites[3]. The complete Heat distortion temperature(HDT) of all specimens crosslink reaction via silane/water condensation were tested in accordance with ASTM D648 using was achieved by sauna treatment after injection the Atlas HDT testing machine (model HDV 1) molding. at the heating rate of 2ºC/min and standard load of 455 kPa. The mechanical properties by mean of tensile, flexural, and notched Izod impact testing 2 Experimental Procedures were performed according to ASTM D638, ASTM 2.1 Materials D5943, and, ASTM D256, respectively. The HDPE and PP used were H5814J and PP700J, both tension and flexure were performed using an are injection molding grads, obtained from SCG Instron universal testing machine (UTM, model Chemical, respectively. VTMS and DCP are 5565) with a load cell of 5 kN. The scanning standard laboratory reagents and used as received. electron microscope(SEM) was used to examine the impact fractured surface and trace of surface 2.2 Sample Preparation adhesion of the composites. The xPP and xHDPE were prepared in the identical manner. Polymer pellet; PP or HDPE, with all 2.4 Calculation ingredients were melt blended in co-rotation closely In this work, the rule of mixture(ROM) model was intermeshing twin screw extruder (Brabender, employed for prediction the tensile and flexural model PL2100) at the temperature profile of
18 th International Conference on composite materials strengths and Young’s modulus of the injected of both tensile and flexural strengths, they are again sandwich composites. The formula of ROM is found that the post cured samples are more closely given by equation (1). obey the rule than the original. The outcomes indicate that sauna treatment can superiorly enhance σ t =ν c σ c +ν s σ s (1) the mechanical properties by mean of impact, tensile and flexural strength of the composites when where σ t σ c and σ s are strength(or modulus) for the compare with the sample without treatment. The constituents, core, and skin, of the composite, values calculated from ROM show better agreement respectively. ν c and ν s are denoted as volume for the sauna cured systems than the original ones. fraction of the core and skin. The well known Fox’s These results strengthen that crosslink reaction via equation is also gathered for prediction the HDT of peroxide and silane can improve the surface the sandwich samples. adhesion between skin and core of the sandwich composite and hence enhance the properties of sandwich structure. Tensile modulus and flexural 3 Results and discussions modulus as shown in Table 3 and Fig. 3 presents 3.1 Thermal properties the decreasing trend when volume fraction of xHDPE core is increased. However, the post cure As expected, the HDT of the sandwich composite specimens show higher value than the original one. specimen as shown Table 1 and plotted in Fig. 1, They are indicated that ductility of the samples is were decreased with increasing the volume fraction improved by the tough xHDPE. of xHDPE for both original and post cured systems. The sauna cured process also obviously enhanced 3.3 Morphological properties the thermal properties of the specimen. As seen, the The fractured surface SEM micrographs of xPP, HDT of xPP and xHDPE before sauna incubation xHDPE, 50% xHDPE core(original) and 50% are approx. 100 o C and 75 o C and they are increased xHDPE core(cured) are given in Fig. 4, to 123 o C and 113 o C after curing, respectively. It is respectively. There is no evidence of crosslink probably due to the forming of network chains and occurred at the fractured surface of xPP. However, also crystallinity during annealing at 105 o C. By there is clearly observed the crosslink webs on the applying the Fox’s equation, the calculated results interfacial surface of xPP/xHDPE. Moreover, it is are reviewed that the post cured samples are more obviously seen that interfacial adhesion between closely relied on Fox’s than the original ones. This xPP and xHDPE is improved after silane/water probably indicates that enhancing in surfaces crosslinking process. The SEM study confirms that adhesion of the samples via silane/water crosslink surface bonding of the sandwich structure can be reaction would be more homogenous in nature. achieved by silane/water crosslink reaction via sauna treatment. 3.2 Mechanical properties Table 1 and Fig. 1 also illustrate the impact strength of the sandwich. Yet again, it is shown that the 4 Conclusions measured values are increased with increasing the Thermal property by mean of HDT and mechanical volume fraction of the core. It is also obviously properties of the sandwich structure between xPP seen that degree of elevating in strength is and xHDPE are the combined properties between significant for the post cured specimen. This result xHDPE core and xPP skin. The crosslink process strongly reinforces the statement that crosslinking via silane/water condensation reaction can enhance via silane/water reaction and can enhance the the interfacial adhesion and hence the mechanical surfaces bonding and then superior in toughness. properties of the composite structure and the ROM Table 2 and Fig. 2 show the tensile strength and can be closely applied for approximation. flexural strength of the sandwich structure. As expected, it is seen that the properties are graduately decreased with increasing the volume fraction of xHDPE core. However, the completed crosslink composite, sauna cured, show higher test values than the original ones. Calculation by ROM
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