The structure and physical properties of polypropylene and thermoplastic olefin nanocomposite containing nanosilica
Abstract
The morphology and physical properties of thermoplastic olefin blend (TPO) based nanocomposites containing nanosilica are reported. Addition of maleated PP resulted in improved filler dispersion within the PP matrix, where the filler resided exclusively. This separated morphology resulted in selective reinforcement of the PP matrix without compromising ductility, as demonstrated by mechanical property characterization. The tensile moduli, impact and flexural properties of TPO/nanosilica composites showed improvements at low loadings of nanosilica, indicating a good balance of stiffness and toughness. The addition of nanosilica into the TPOs decreased the size of the dispersed elastomer phase, which was a factor in the observed improvements of impact strength. Silane-modified nanosilica dispersed more efficiently in the polymer matrix, giving rise to improved impact properties of the TPO composites,compared to the unmodified filler. 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Thermoplastic olefin (TPO) blends consist of polypropylene (PP) and olefinic elastomer components. Most commonly PP comprises the major phase and thus forms the matrix of the TPOs [1e4]. The dispersed elastomer phase serves to improve the toughness and low-temperature impact resistance of PP.However, presence of the elastomeric component inevitably has a detrimental effect on the stiffness of the material [5]. Inorganic fillers are commonly added in TPO formulations to increase their stiffness, improve their dimensional stability, and lower the cost of the compounds. Various PP/elastomer/filler ternary systems containing fillers such as CaCO3, talc and silica have been investigated in the past [6e11]. When fillers are added in PP/elastomer blends, and depending on the location of the filler, three types of microstructures may form: ‘‘separate’’ dispersion structure, where the filler resides in the matrix, encapsulation/coreeshell structure, where the filler partitions preferentially in the dispersed phase, and mixtures of the former two. A separated microstructure, wherein the filler partitions favourably in the PP matrix, without affecting the elastomer phase, is desirable for optimum reinforcement of TPO blends [6e8]. Recent reports have highlighted the effects of addition of organoclay-type nanofillers into TPOs [12e17]. Although introduction of clay into the TPOs improves their flexural and tensile moduli, this is generally counteracted by decreases in elongation [17]. Lee and Goettler [14] used different addition sequences during compounding to control the localization of the clay, thus significantly affecting the mechanical properties of the compounds. Lee et al. [12] studied systematically the morphology and mechanical properties of a PP/ethylene octene elastomer/clay system, and found that the reduction in size of elastomer particles taking place in the presence of clay particles affected favourably the impact properties of the material.
Besides organoclay, other nanofillers such as nanosilica can also serve as reinforcing agents. Silica has been widely used as reinforcing filler in the rubber industry. Several research studies pertaining to its application in PP reinforcement have been published in recent years [18e21]. Due to the non-polar nature of PP and the large surface area of polar nanosilica particles, it is challenging to achieve good dispersion of nanosilica into a PP matrix. Efforts have centered on improving the dispersion of nanosilica in PP by modifying the filler through irradiation grafting [18,19], using surface treated nanosilica [21], or adding maleated PP as a compatibilizer [20]. These studies revealed that reinforcement can be achieved at relatively low filler loadings, between 2.5 and 5 wt%. In our previous work [22,23], we have reported that when added to polyolefin blends, high energy fillers such as organoclay and nanosilica migrate into the maleated phase. In this work we aim at taking advantage of this trait to preferentially localize the filler inside the PP matrix, thus selectively reinforcing it, without affecting the toughening ability of the dispersed ethyleneeoctene copolymer elastomer phase. The structure and properties of the resulting TPO nanocomposites are presented. The properties of the PP matrix/nanosilica filled composites were also studied to provide a suitable |
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