Evaluation of the Mechanical Properties of Polypropylene/calcium Carbonate Nanocomposite at Various Creep Conditions

Evaluation of the Mechanical Properties of Polypropylene/calcium Carbonate Nanocomposite at Various Creep Conditions

Chapter One

Objectives of the Study

The present work aims to:

Evaluate the Tensile properties of coated and uncoated calcium carbonate nano-filler with Polypropylene as the host polymer for different volume fractions.
Evaluate creep behavior of the coated nanocomposite, for each volume fraction of the
Examine the effects of the fillers on the mechanical behavior of the This examination can be utilized for process ability and in developing optimum morphology to maximize products performance.
This work is an attempt at nano structure fabrication and to get into the main stream of composite technology of the 21st Century.

CHAPTER TWO

LITERATURE REVIEW

There have been many studies using nano calcium carbonate to enhance the properties of polymers. Among the techniques employed to disperse the nano filler include in-situ polymerization, melt mixing using internal mixers and melt compounding using twin-screw extruders. In-situ polymerization technique has been used for PVC and PET, while melt mixing and melt compounding appeared to be the preferred method for PP (Ritchie, 1993)

Eiras and Pessan (2009) presented a paper on the effects of calcium carbonate nano particles in the crystallization of polypropylene. The experimental work includes Differential Scanning Calorimetry analysis of isothermal and non- isothermal crystallization, optical microscopy and X-ray diffraction. In their study, four compositions of PP/CaCO₃ nano composites with calcium carbonate content of 3%, 5%, 7%, and 10% by weight were prepared in a co-rotational twin screw extruder machine (Weiner & Pfleirer ZSK-30) with temperature profile of 170/190/190/190/190/195⁰C, and a screw speed of 100rpm.

DSC analysis were conducted by heating the samples from 30⁰C to 200⁰C at a heating rate of 10⁰C/min keeping the sample at this temperature for 2 minutes and then cooling down from 200⁰C to 30⁰C at a cooling rate of 10⁰C/min. The isothermal analysis and optical microscopy analysis were conducted by heating the sample at the same condition and then cooling from 200⁰C to their crystallization temperature, maintaining it at this temperature for 15min.

X-ray diffraction analysis was conducted in a Rigaku Geiger Flex equipment using Cu, Kα radiation with 2θ varying from 5 – 90⁰C. All the samples used were obtained from injection moulded tensile specimens. The results showed the presence of β phase through DSC and X-ray diffraction analysis of the nanocomposite, which is the result of the nucleation effect of CaCO₃ nano particles in PP crystallization process.

The non-isothermal analysis showed that the melting temperature is not affected by CaCO₃ nano particles but the crystallization temperature and crystallinity degree increase with the addition of CaCO₃ content.

Isothermal analysis showed that the incorporation of CaCO₃ nano particles reduces the half crystallization time and increases the kinetic constant (k) which means that the nanocomposites crystallizes faster than the neat PP due to the nucleation effect of the nano particles. Optical microscopy results show a reduction in the spherullites size with incorporation of nano particles.

From the results of their work, it is clear that CaCO₃ nano particles affect the crystallization process of PP by changing its phase formation, crystallization temperature, spherullites morphology and kinetics.

Hanim et al (2008) studied the effects of calcium carbonate nano-filler on the mechanical properties and crystallization behavior of Polypropylene.

In their study, PP/CaCO₃ nanocomposites were prepared using a co-rotating twin- screw extruder at filler loadings of 5, 10, and 15% weight. The mechanical properties of the nanocomposites were evaluated using impact, flexural and tensile test, while the crystallization behavior was analyzed using DSC and WAXD techniques.

The impact strength and Modulus of PP showed some improvement with the incorporation of nano filler while the tensile strength deteriorated. SEM photomicrographs showed evidence of calcium carbonate agglomeration within the PP matrix, indicating that the level of shear stress generated during melt compounding was far from adequate to break-up the CaCO₃ nano filler.

CHAPTER THREE

EXPERIMENTAL WORK

Materials

The grade of Polypropylene used in this work was SEETEC Homo polymer PP by LG Chem Korea. This acts as the matrix. The homo polymer PP has a density of 0.90g/cm³ and a melt flow rate of 14g/10 minutes (2.16kg at 230^oC). The nano filler used in this work was Calcium Carbonate purchased from the local market in Aba. The Calcium Carbonate used was CALCO brand by Freedom Group Nigeria. The mould release agent used was Petroleum jelly (Vaseline).

The nanocomposites were prepared by melting the PP in a mixer and melt compounding it with a coated Calcium Carbonate and uncoated Calcium Carbonate respectively at filler loadings of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35% to 60% volume fractions of the coated and uncoated Calcium Carbonate respectively. The tensile samples were cast in an aluminum mould in accordance with ASTM standard D638 for tensile tests.

Method of Preparation
Manual Mixing and Compounding

The PP was melted from its pelletized form at a temperature exceeding 180⁰C in a mixing chamber. Measured amounts of Calcium Carbonate were added to the melted PP by volume fractions and stirred continuously for 10 minutes to ensure a uniform dispersion of the mixture. The compounded mixture was cast in an aluminum mould that has been treated with a mould releasing agent and dried. The composite was allowed to cure and was later de-moulded after 24 hours. The above procedure was repeated for the stearic acid coated Calcium Carbonate previously done (i.e. at 5%, 10%, 15%, 20%, and 30% volume fractions). The fabricated material has a dimension of 80mm x 30mm x 10mm.

CHAPTER FOUR

ANALYSIS

Results

The test values presented below were averages. For each volume fraction, six samples were tested. Plots were made to show the relationship between the Tensile strength, Modulus of elasticity, elongation. Also the results obtained from the creep tests were equally shown. Plot of the graphs i) Elongation vs. Time and ii) ε vs. t were made. Tests were conducted at different loads and stresses.

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

Conclusion

The results of the study showed that the addition of CaCO₃ nano-filler have resulted in some improvement in the tensile mechanical properties of the Homo Polypropylene. The tensile strength and elongation at break of PP were slightly reduced with the incorporation of the nano-filler at different volumes of 5%, 10%, 15%, 20%, 25%, 35%, 40%, 45%, and 50%.

The Young’s modulus showed some improvement on addition of nano-fillers. Stearic acid coated fillers showed the highest improvement in the above tensile properties at low volume fractions .It is important to note that as the volume fractions increased both the tensile strength and elongation decreased.

According to (Liang, 2002) the strength of particulate-filled polymer composites depends, to a great extent, on the interfacial adhesion between the matrix and the filler which will facilitate the transfer of a small section of stress to the filler particle during deformation. Tensile mechanical properties seem to be affected by the dispersion of the nanoparticles. The poor mechanical behavior exhibited by the nanocomposites at high volume fractions may be due to agglomeration of the nano-fillers.

Creep and Creep fracture represents one of the major problems associated with the selection and use of engineering materials for high temperature applications.

Poor creep resistance and dimensional stability of thermoplastics is generally a deficiency, impairing the service life and safety which is a barrier for further expansion in engineering. Resistance to creep is very high for nanocomposites compared to neat Polypropylene matrix because calcium carbonate resists the slippage, re-orientation and motion of the polymer chain in the nanocomposite. From the foregoing, it is clear that the strength of a nano-filled composite depends to a great extent on the interfacial adhesion between the polymer matrix and the nano-filler which aids the transfer of a small section of stress to the filler particle during deformation. The creep rupture is a function of Temperature

Recommendations

Further work is recommended in the following areas:

The research should be performed with a Copolymer PP as the polymer matrix to compare
The Crystallization behaviour of the nanocomposites should be evaluated with a Transmission Electron Microscope and Scanning Electron Micrographs.
Thermal characterization using differential scanning calorimetry (DSC) of the different samples would assist in analyzing the interaction between the filler and
The damage level calculations of the fractured materials be computed using Finite Element Analysis. Performing failure analysis on the fracture sites of samples of nanocomposites and pure samples would help explain the mechanics of failure in

REFERENCES

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