|Jaka Fajar Fatriansyah||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Muhammad Joshua Yuriansyah Barmaki||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Rahma Lailani||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
Impact Polypropylene Copolymer (IPC) is an material which combines properties of isotactic Polypropylene (iPP) and its own high impact toughness. However, the crystallization kinetics might be lower than iPP, which may affect the manufacturing cycles. Kenaf was used as the addition in IPC, in which kenaf acts as a nucleating agent as well as reinforcement for IPC. Kenaf was alkalized with NaOH 6 v.v% for about 8 hours to remove the dirt on the surface and to reduce lignin, which contributed polarity to kenaf. The alkalization improved the compatibility of kenaf fiber with IPC. The crystallization kinetics study was conducted by employing the Avrami model to Differential scanning calorimetry data in order to obtain half-time, Avrami index (morphology parameter) and Avrami crystallization kinetics. The addition of 5 wt.% kenaf was found to be an optimum concentration to improve crystallization kinetics. The addition of more than 5 wt.% kenaf (15 wt.% and 20 wt.%) did not improve crystallization kinetics which may be due to the agglomeration, thus preventing efficient heat transfer between nucleating seeds and the matrix. The connection between crystallization kinetics and mechanical properties was also established for the IPC+ kenaf system.
Crystallization kinetics; Impact polypropylene copolymer; Kenaf fiber; Nucleating agent; Polymer reinforcement
Impact Polypropylene Copolymer (IPC) is a unique plastic material, combining properties of isotactic Polypropylene (iPP) which has a good heat resistance (insulator), good toughness (Chen et al., 2009), good manufacturability (Karger-Kocsis et al., 1997), and most important, the superior property of high impact toughness (Michael, 1991). Because of these useful properties, IPC is widely used in many applications such as in packaging, household appliances, automotive parts and even military uses (Hongjun et al., 1999). IPC is commonly produced from homopolymerization of Propylene and followed by copolymerization of propylene and ethylene. Thermal behavior of plastics greatly affects manufacturing process and properties of the product. Thermal behavior is related to the kinetics of crystallization (Chalid et al., 2017; Chalid et al., 2018). Crystallization kinetics of IPC may be lower in comparison with iPP due to the structure of ethylene. This may affect cycle time in the manufacturing process, for example, due to the longer crystallization time. One of the methods to improve the crystallization of kinetic properties is the addition of a nucleating agent (Tolinski, 2009).
Usually natural based fiber is used as a reinforcement to improve mechanical properties of plastic composites due to its advantages such as: low density, availability, degradability as well as less abrasive and good insulator. Natural fibers can be classified into three types: fiber from plants (cellulose or lignocellulose), fiber from animals (protein) and mineral fibers (Akil et al., 2011). Examples of plant based are flax, jute, ramie, kenaf, sisal, bamboo, wheat, maize, barley and sago (Abral et al., 2012). The use of natural based fiber as a nucleating agent in plastics has increasingly attracted some researchers. However, some researchers use natural based fibers as nucleating agents as well. Fundador et al. (2012) used xylan ester from hemicellulose as a nucleating agent for polylactic acid (PLA). Guo et al. (2015) used natural protein fiber as a nucleating agent for iPP. They showed that the use of other natural fiber resources as a nucleating agent is possible. Yuanita et al. (2015) used Arenga pinnata “ijuk” fiber in PLA nucleating agent and Prabowo et al. (2017) used similar ijuk to modify crystallinity of the iPP composite.
Kenaf (Hibiscus cannabinus), a plant based natural fiber, has high tensile strength (930 MPa) as well as low density (Akil et al., 2011). This high tensile strength makes kenaf suitable as a reinforcement. However, in order to be properly used as a nucleating agent, the interface between kenaf and IPC should be compatible with each other. It is well known that the non-polar IPC will be incompatible with polar natural fibers: this is the case with kenaf (Spoljaric et al., 2009). One of the methods used to reduce the polarity of kenaf is alkaline treatment (alkalization) (Akhtar et al., 2016). The alkalization process on fiber may reduce the polarity of fiber by removing its lignin which donors the polarity property in fiber. Another study of alkali treatment (alkalization) to improve the capability of ijuk to be compatible for reinforcement and/or nucleating agent in polymer was conducted by Chalid and Prabowo (2015) by using NaOH. They found that the crystallinity, which is somehow related to the interface or compatibility with polymer, of ijuk increases after alkalization. The objective of this study is to study the use of NaOH alkalized kenaf as a nucleating agent as well as reinforcement for IPC. We expect that kenaf will improve IPC crystallization kinetics, which will be analyzed by means of the Avrami crystallization theory. In addition, in this paper, we attempt to establish the relation between crystallization kinetics and mechanical strength (the use of kenaf as reinforcement) through the experimental results.
The alkalization of kenaf by NaOH was partially successful at removing dirt on the surface of kenaf and reduced lignin, which was investigated by means of FESEM and FTIR. The reduction of lignin decreased the polarity of kenaf and made it more compatible with IPC. The addition of kenaf at concentration of 5 wt.% increased the crystallization kinetics in the following ways: rate of crystallization (crystallization time) parameter, Avrami crystallization kinetic parameter and reduced Avrami index. In general, the addition of kenaf at concentration of 5 wt. % improves crystal growth dynamics and morphology (crystal growth in one dimension). However, further addition of kenaf does not improve growth dynamics and morphology. This phenomenon could be caused by agglomeration and thus preventing effective heat transfer between crystal seeds and the matrix. In addition, the connection between crystal growth dynamics and mechanical strength (tensile strength) has been successfully established through crystallization kinetics and mechanical test results. It is shown that better crystal growth dynamics yields better mechanical strength in the IPC+ kenaf system.
This work was supported by Universitas Indonesia through HIBAH PITTA A 2019 under contract number NKB-0464/UN2.R3.1/HKP.05.00/2019.
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