Elsevier

Composites Part B: Engineering

Volume 113, 15 March 2017, Pages 218-224
Composites Part B: Engineering

High density poly(ethylene)/CaCO3 hollow spheres composites for technical applications

https://doi.org/10.1016/j.compositesb.2017.01.025Get rights and content

Highlights

  • Hollow spheres CaCO3 filler particles HDPE composites were studied.

  • Increase of the Young's modulus of elasticity with increasing filler concentration was found.

  • Dynamic mechanical testing of the studied composites was performed.

  • SEM of fracture surfaces showed typical elongation bands with cavities around filler.

  • Higher amorphous phase content was found with increasing filler concentration.

Abstract

Observed data in this study indicated creation of the combined plastic elastic mechanical behavior when spherical hollow sphere CaCO3 particles were used as a HDPE polymer matrix filler. It was found that with increasing filler concentration the corresponding increase of the Young's modulus of elasticity was accompanied with corresponding decrease of the upper yield point and elongation at break. Dynamic mechanical testing of the studied composites performed by means of the frequency dependent transfer damping function indicated lower stiffness of the filled HDPE composites in comparison to the neat polymer matrix. This founding corresponded with the strong effect of the filler particles on lowering the creation of the higher molecular conformational structures of the crystalline regions of the HDPE in favor to the amorphous parts. SEM analysis of the fracture surfaces showed typical elongation bands of high plasticity deformation regions characteristic with a typical shearing bands interpenetrated with clear cavities created around filler particles. Thermal analysis data showed minor change of the crystallinity with increasing filler particles content in the polymer composite matrix in such a way, that the higher amorphous phase content was found with increasing filler concentration. Creep behavior of the tested specimens indicated higher plasticity of the nanocomposites matrix in comparison with the virgin HDPE.

Introduction

Multifunctional metallic hollow sphere structures are used in many technical applications (so called cellular metals) in automotive and aerospace industries [1]. However, due to the economic and technical requirements, some of these are being replaced e.g. by glass hollow spheres filled synthetic polymer composites on low-density poly(ethylene) (LDPE) basis [2], [3]. With respect to the hollow spheres filler spatial distribution in the composite materials, there exists syntactic morphology and partial morphology materials. In the case of syntactic hollow sphere cellular materials, the filler hollow spheres particles are completely embedded within the adhesive matrix, while in the case of partial morphology materials, the adhesive is concentrated at the contact points of neighboring spheres. Well-known advantages of cellular materials are their excellent ability for energy absorption, good damping behavior, sound absorption, excellent heat insulation and a high specific stiffness [1]. The combination of these properties opens a wide field of potential multi-functional applications, i.e. as an ideal concept for structural design of lightweight transportation systems such as aircraft, high-speed trains and fast ships. Similarly as in the above mentioned metallic matrices, one would expect the analogy in mechanical performance of the mineral hollow spheres synthetic polymer composites. It was found, that nano/micro size non-expanded mineral fillers increase mechanical tensile and bending moduli of polyolefin composites [4], [5]. Acoustic performance of the expanded mineral fillers of macroscopic dimensions fixed by cementitious binder were modeled by Pade approximation model. There was found excellent fitting of the model data on experimental dependencies [6]. In general, a semi-crystalline polymer such as high density poly(ethylene) (HDPE) is regarded as a three-phase continuum composed of an amorphous phase, a crystalline phase, and an inter-phase. The crystalline skeleton is formed by mutually connected spherulites, each of them consists of a number of crystalline lamellae. The amorphous phase is located in between crystallites and lamellae presented in spherulites. The inter-phase is located at the borders between amorphous and crystalline domains. It consists of polymer chains whose motion is severely restricted by surrounding crystallites [7]. Flow-induced crystallization of semi-crystalline polymers was intensively studied for the past decades, since many polymer processing operations, such as extrusion, injection molding, fiber spinning and film blowing at their practical industrial application conditions strongly influence orientation of the individual polymer molecular chains, crystallization kinetics and final crystal morphology [8]. That is why, different crystalline morphologies, such as spherulites, cylindrites, shis-kebab and fibrous crystal can be obtained with variant of processing conditions [9], [10]. Of course, the type and size of the filler particles has a strong effect on the HDPE crystallization kinetics and melting behavior, as confirmed on clay and graphene nano-composites [11], [12]. For example, the kinetics of crystallization of HDPE and different graphene size nanocomposites prepared by melt mixing has shown, that the primary crystallization consists of a fast outward growth of the lamellar stacks until impingement and the secondary crystallization involves the filling of the spherulites' interstices. The latter secondary crystallization proceeds at a much slower rate than primary crystallization [11]. There was found higher crystallization temperature of HDPE after graphene incorporation into the polymer matrix, however smaller diameter graphene particles led to the increase of the number of heterogeneous crystallization nuclei which resulted in a higher crystallization rate.

There was found that the addition of clay nanoparticles into the PE matrix have strongly increased the PE nanocomposite decomposition temperature [12]. However, a mild polymer decomposition reaction was found as catalyzed by the filler.

It is well known, that by injection molding a skin-core structure of prepared polymeric testing articles is formed. They are consisted of the thick core layer and the thin skin (shear) layer. For the core layer, the macromolecular chain orientation degree is low and only spherulites are created. Many experimental studies were focused on improving mechanical properties of products by means of increasing the thickness of the oriented layer and several new technologies were developed, such as oscillation shear injection molding, gas-assisted injection molding, periodical vibration injection molding etc. [8]. There were performed several experimental studies on determination of mechanical properties of HDPE nanocomposites filled with different size and shape CaCO3 fillers [3], [13], [14], [15]. There was found a significant increase of the impact strength and of the tensile yield strength of the nanocomposites with decreasing filler particle size accompanied with the increase of the polymer crystallization degree. Sphere-like CaCO3 filled nanocomposites were exhibiting improved thermal stability whereas cubic CaCO3 was decreasing composite material mechanical toughness [13]. The creep behavior of poly(ethylene) (PE) nanocomposites reinforced with different nano-sized CaCO3 depends strongly on the filler content and filler surface modification [16], [17], [18]. There was found the creep strain and creep rate increase for PE specimens filled with untreated glass spheres. This increase was attributed to a severe debonding at the filler-matrix interface as confirmed by SEM analysis [3], where clean surfaces without the presence of PE layer were found at the fracture surface. During tensile loading, the interface between PE and CaCO3 was pulled apart. It was ascribed as the result of the tensile yielding composed of interface and matrix yielding.

With respect to the impact fracture surface morphology, distinct five zones were recognized, the pre-crack zone, the crack braking zone, the slow crack propagation zone, the transition zone and the rapid crack propagation zone [14]. A typical fibrillation occurs in the crack breaking zone, where the signs of crack slow movement were found. After transition zone between ductile and brittle behavior, the crack rapidly propagates in the fifth zone and the rapture occurs. A typical fibrillation of neat HDPE matrix in slow propagation zone is clearly demonstrating occurrence of the severe plastic deformation. The rapid propagation zone has a brittle fracture morphology due to crazing traces.

As discussed above, stress transfer in the complex nanocomposite matrix is affected by many structural, morphological and surface characteristics parameters. That is why, paper presented is focused on study of the effect of the spherical hollow sphere CaCO3 nano-fillers in HDPE polymer matrix. There are studied thermal, mechanical and impact resistance properties combined with SEM analysis.

Section snippets

Materials

High density poly(ethylene) (HDPE) type 25055E (The Dow Chemical Company, USA) was purchased in the form of white pellets (lot. No. 1I19091333). Nano-particular calcium carbonate hollow spheres [19] were used as the filler material (The University of Birmingham, UK). Filler moisture content was 0.1 w. %.

Composite samples were made using the injection molding technology on the injection molding machine Arburg Allrounder 420C (Germany). Processing temperature was ranging from 190 to 220 °C, mold

Scanning electron microscopy

Scanning electron microscopy (SEM) was used to determine the shape and size of the studied mineral composite filler particles. SEM images were captured using a Hitachi 6600 FEG microscope (Japan) operating in the secondary electron mode using an accelerating voltage of 1 kV.

Thermal analysis

Thermogravimetry (TG) and differential thermal analysis (DTA) experiments were performed on simultaneous DTA-TG apparatus (Shimadzu DTG 60, Japan) to determine the moisture content of the samples, and to determine whether

Results and discussion

For evaluation of dynamic mechanical load damping capacity of the tested hollow spheres HDPE composites, the forced oscillations were generated and subsequently transferred through the investigated specimens, allowing thus measurement of the frequency dependent transfer damping function D (as given in the equation (3)). This physical quantity allows mutual comparison of the dynamic stiffness of the tested materials as modulated by different weight fractions of the filler particles in the

Conclusions

Plastically deformed fortified material is a natural barrier against growth of the crack. Observed data indicated creation of the combined plastic elastic mechanical behavior when spherical hollow sphere CaCO3 particles were used as a HDPE polymer matrix filler. It was found in this study that with increasing filler concentration the corresponding increase of the Young's modulus of elasticity was found accompanied with corresponding decrease of the upper yield point and elongation at break.

Acknowledgements

Financial support from the grant no. LO1305 of the Ministry of Education, Youth and Sports the Czech Republic (LO1305) is gratefully acknowledged. Authors would like to express their gratitude to Ing. P. Bazgerova (Palacky University in Olomouc) for SEM analysis and to M.J.A. Ruszala (The University of Birmingham) for hollow spheres filler particles supply.

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