Effects of additives on non-isothermal crystallization kinetics and morphology of isotactic polypropylene
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- Durmus, A. & Yalçınyuva, T. J Polym Res (2009) 16: 489. doi:10.1007/s10965-008-9252-9
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In this study, effects of commercial additives such as antioxidant and stabilizer on the non-isothermal crystallization kinetics of isotactic polypropylene without nucleating agents were investigated by differential scanning calorimetry (DSC) method. Kinetic parameters by Osawa, Avrami and Liu-Mo models and apparent activation energy of the crystallization by Kissinger model were calculated. A polarized optical microscope was also used to observe crystalline morphology of the polypropylene samples crystallized at different cooling rates. On the contrary rate inducing effects of the nucleating agents on the crystallization kinetics of the polypropylene, interestingly, it was found that such types of commercial additives reduced the overall crystallization rate of the polypropylene. Based on the crystallization kinetics and morphology of the samples, it was observed that commercial additives inhibit the chain diffusion toward the growing crystal faces thus slow the crystal growth rate. Furthermore, calculated nucleation activity (ϕ) for the additives showed that they do not act as effective nucleating agents. It was found that the crystallization activation energy of additive-free sample was higher than that of the sample which has commercial additives. Activation energies were found to be 233.6 and 276.7 kJ mol−1 for the PP-1 and PP-2, respectively. Kinetic results also show importance of using of nucleating agents to increase the crystallization rate of polypropylene by increasing the nucleation and thus overall crystallization rate during polypropylene processing operations (esp. for a fast processing cycle in injection molding).
KeywordsPoly(propylene) (PP)CrystallizationAdditivesDifferential scanning calorimetry (DSC)
Polypropylene (PP) is one of the most widely used semi-crystalline thermoplastics in many industrial applications such as extruded pipes, films and fibers, injection molded and thermoformed parts due to its physical, thermal and chemical properties including low density, high melting point, stiffness, good impact properties, chemical inertness, good clarity, excellent barrier properties and low prices. Overall crystallization process of semi-crystalline polymers consists of the nucleation and crystal growth steps. Crystallization of polypropylene can start through homogenous, heterogeneous or self nucleation . It has been known that small amount of heterogeneous material in polypropylene may considerably change the crystallization temperature, crystal size, density, clarity and physical performance of the polymer. Recently, more attention has been focused on the crystallization behavior of nucleated polypropylene [2–11]. Isotactic PP (i-PP) has three different crystalline forms, namely the monoclinic α form, hexagonal β form and triclinic γ form . Many studies have been published on the crystallization kinetics, morphology and mechanical properties of polypropylene having various types of organic or inorganic chemicals or additives called nucleating agent such as organic phosphorus salts , sorbitol derivatives [14–16], sodium benzoate , alkaline dehydroabiatate [18, 19], organic pigments , rosin type additives [21, 22] and nanofillers [23, 24]. Generally, it was found that the nucleating agents or impurities accelerated the nucleation rate of PP and caused the higher crystallization temperature for nucleated PP than that for non-nucleated one, and sorbitol derivatives and organic phosphates are the most effective nucleating agents in PP. It has been reported that these compounds accelerate the cystallization rate of PP by reducing fold surface energy of the growing crystals. Several authors studied on effects of specific nucleating agents on the crystallization of β-PP and the effects of β crystalline phase on the mechanical properties of i-PP products [25–29]. They showed that the β-iPP has better mechanical performance than that of other crystal types of i-PP. Nucleating additives are routinely used in the processing of PP to shorten injection molding cycles, improve the mechanical and optical properties by reducing spherulite size. To the best of our knowledge, there is no study yet on effects of commercial additives on the crystallization rate of polypropylene although many research efforts have been focused on the effects of nucleating agents on the crystallization behavior of i-PP in recent years.
In this study, effects of commercial additives such as antioxidant, UV stabilizer etc. added to the polymer in granulation step on the crystallization rate and morphology of i-PP were investigated by Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM) methods under non-isothermal conditions. Since the solidification generally occurs under non-isothermal conditions in the actual polymer processing operations, non-isothermal crystallization often provides useful complement to understand crystallization behavior of a semi-crystalline polymer in industrial processes. Therefore, non-isothermal crystallization was chosen as the experimental method in this study.
Isotactic polypropylenes (PP-1 and PP-2, fiber grade MH-418) were kindly supplied from the Petkim Petrochemicals, Turkey. Melt Flow rates (MFI) and density of the samples used in this study were reported as 4–6 g 10 min−1 (ASTM D1238, 230°C and 2.16 kg) and 0.89 g cm−1, respectively. PP-1 sample is in granular form and has some commercial additives; stablizer and process aid such as calcium stearate; primary and secondary antioxidants such as aryl phosphite and hindered phenols. PP-2 sample is in powder form and contains no additives. PP-2 was produced in a pilot-plant reactor in the research laboratory of the company by the same Ziegler/Natta catalyst, process and equipment design with the commercial scale system.
Non-isothermal crystallization behaviors of the samples were examined with a Setaram (DSC 131) differential scanning calorimeter. Temperature and heat flow calibrations of the instrument were achieved with high purity indium (In), tin (Sn) and lead (Pb) metals. PP samples weighing about 10–12 mg in an aluminum crucible were heated from room temperature to 220°C at a heating rate of 10°C min−1. Samples were kept at this temperature for 3 min to eliminate thermal history and unmelted crystals that may cause the self nucleation. Then they were cooled to 20°C by using liquid nitrogen cooling device at a given constant cooling rate and the crystallization exotherms were recorded. Cooling rates employed in this study were 1.5, 2.5, 5, 10 and 20°C min−1 for investigating the crystallization kinetics of PP samples. All melting and crystallization experiments were carried out under nitrogen (N2) atmosphere at a flow rate of 100 ml min−1 to prevent oxidative degradation of the samples, esp. additive-free PP (PP-2).
Spherulitic crystal images of each PP sample that crystallized at different cooling rates were photographed with an Olympus polarized optical microscope (POM).
Results and discussion
Non-isothermal crystallization kinetics
Crystallization peak temperatures (Tc) of the PP samples
Cooling rate (°C min−1)
Half-width (w1/2) of the crystallization exotherms of the samples
Cooling rate (°C min−1)
Crystallization rate parameters of the PP-1 and PP-2
Cooling rates (°C min−1)
Osawa kinetic constants of the PP-2
Avrami kinetic constants of the samples
Cooling rate (°C min−1)
Kinetic parameters by the Liu model
Crystallization activation energy
In this paper, effects of very small amount of commercial additives such as antioxidant, stabilizer etc. having chemical structure different from the nucleating agents, on the crystallization rate of i-PP were investigated by DSC method. It was found that these type of additives do not behave as an effective nucleator and do not cause an increase in the overall crystallization rate of PP. Contrarily, we observed that these additives reduced the crystallization rate of PP. Based on the kinetic parameters and morphological observation, we conclude that they limit the crystal growth rate by slowing the chain transfer toward the developing crystal face. Our results also indicate the importance of nucleating agents for PP processing in order to accelerate overall crystallization rate.
This study was supported by the Research Fund of Istanbul University (Project number: T-835/07032000). The authors thank to Petkim Petrochemicals for supplying samples used in this work and Professor Dr. Serhat Pabuccuoglu, Istanbul University, for his helps in POM study.