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DOI: 10.1016/j.jmrt.2018.12.004
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Available online 19 January 2019
Innovative polyetherimide and diatomite based composites: influence of the diatomite kind and treatment
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Ilaria Cacciottia,b,
Corresponding author
ilaria.cacciotti@unicusano.it

Corresponding author.
, Marianna Rinaldib,c, Josè Fabbrizic, Francesca Nannib,c
a University of Rome “Niccolò Cusano”, Engineering Department, Via Don Carlo Gnocchi 3, 00166 Rome, Italy
b Italian Interuniversity Consortium on Materials Science and Technology (INSTM), Italy
c University of Rome “Tor Vergata”, Enterprise Engineering Department, Via del Politecnico 1, 00133 Rome, Italy
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Table 1. Tensile modulus (E), ultimate tensile stress (σmax), elongation at break (ɛmax) and Young's modulus variation with respect to neat sample of all tested films (all values are expressed as mean values±standard deviation (SD).
Abstract

In this work innovative composites were developed, using as polymeric matrix polyetherimide (PEI), a high performance thermoplastic, and as natural filler two different kinds of diatomaceous earth, i.e. uncalcined and calcined. The uncalcined powder was amino-functionalised to improve its chemical compatibility with the matrix, whereas the calcined one was submitted to a purification treatment with HCl to remove possible contaminants.

Mechanical tests and microstructural characterisation were carried out to evaluate the influence of the filler kind, of its content (i.e. 1–10wt.%) and of its amino-functionalisation or purification in the case of uncalcined and calcined diatomite, respectively, on the composites performance. The collected results show an increase of Young's modulus with the filler content, particularly in the case of amino-functionalised diatomite, suggesting a good chemical compatibility between filler and matrix. This is substantiated by the scanning electron microscopy (SEM) observation of the fracture surface of samples after tensile test.

Keywords:
Polymer-matrix composites (PMCs)
Polyetherimide (PEI)
Diatomite
Amino-functionalisation
Purification
Mechanical properties
Full Text
1Introduction

Several efforts have been and are devoted to the development of innovative composites and nano-composites [1], based on polymeric matrices and raw materials of natural origin and then easily disposable to limit and confine pollution [2]. Among them, polymeric matrix composites reinforced by natural fillers and characterised by an optimal filler/matrix interaction could be designed for prospective applications in automotive, aerospace and packaging industries [3–5]. In this manner, they would present an improvement from both performance and environmental points of view [6].

In particular, engineering polymers (e.g., polyetherimide (PEI), polyamides (PAI), polyetheretherketone (PEEK), etc.) are gaining great attention in view of many engineering applications. Furthermore, there is a great attention on the availability of bio-based fillers, such as banana, hemp, cellulose [7], coconut fibres [8], nanoclays [9], diatomite [3], to insure some degree of recycle. Among them, bioderived fibres usually offer limited mechanical properties and thermal stability, whereas inorganic ceramic natural particles are able to provide good thermal and mechanical properties [7,10].

In this framework, the purpose of the present work was to develop an innovative polymeric matrix composite, composed of a conventional thermoplastic polymer commonly and widely used in several sectors, i.e. polyetherimide (PEI), and a natural inorganic filler, i.e. the diatomaceous earth (DE).

Indeed, PEI is a high-performance thermoplastic polymer with good heat resistance, excellent mechanical properties, inherent flame and solvent resistance [11]. Recent NASA studies [12] have demonstrated that it is possible to obtain PEI without the use of any hazardous solvent, thus making this polymer a greener choice with respect to other biobased polymers (e.g. polylactic acid, polyhydroxyalkanoates, poly(ɛ-caprolactone), etc.), which, on the contrary, exhibit rather low mechanical properties [13–16] and, in some cases, are difficultly machined with usual production techniques.

For these reasons, in the present work, in order to develop new green composite materials based on polymers with a good Life Cycle Assessment (LCA) and appropriate fillers, PEI was chosen as polymeric matrix and diatomaceous earth is proposed as filler, since it is a natural inorganic powder, already proven to be effective in reinforcing thermoplastic polymers [17,18].

Diatomaceous earth was selected as suitable filler, due to its large availability in many areas of the world (and thus low cost), chemical resistance, non-toxicity, high porosity and bulk volume, large specific surface area (up to 200m2/g) [19]. Indeed, it consists in a fossil flour of sedimentary origin, composed of fragments of diatom siliceous skeletons (“frustules”) with different sizes (ranging from 2μm to 2mm) and shapes (e.g. rounded, elongated, etc.) [20,21]. The main constituent of diatomite is amorphous silica, although it can contain impurities, such as organic components and metallic oxides (MgO, Al2O3, Fe2O3) coming from environment [22]. For all the reported properties, these fossil structures have been widely used in a lot of industrial applications, such as food production, water extracting agent, production of cosmetics and pharmaceutics [23–26]. Thus, the present research is aimed to set up the optimal composition of this innovative PEI/diatomite composite material. In order to achieve this objective, films loaded with different diatomite contents, ranging between 1% (w/w) and 10% (w/w) with respect to the polymer, were prepared by solvent casting technique. Both uncalcined and calcined diatomites were used and compared, to evaluate the influence of different morphologies, compositions and crystalline phases of the employed fillers on the microstructural and mechanical properties of the composites. To improve the compatibility between the polymeric matrix and the filler, the uncalcined diatomite was also organosilane-modified, using aminopropyldiethoxymethylsilane (APDEMS) as silane coupling agent [27,28] and the calcined diatomite purified by means of HCl treatment.

To the best of our knowledge, up to now PEI has never been reinforced with natural fillers, but only with conventional artificial and synthetic fillers, such as glass or carbon microfibers, nanosilica [29–34].

2Materials and methods2.1Materials

Diatomaceous earth powders, both calcined (SCF, SuperCellFine, average size 14μm, composition: Al2O3 4.0%, CaO 0.5%, Fe2O3 1.3%, Na2+K2O 1.1%, SiO2 91.1%, all in wt.%) and uncalcined (Celite® S, composition: Al2O3 4.1%, CaO 0.4%, Fe2O3 1.6%, MgO 0.2%, Na2O+K2O 1.4%, P2O5 0.3%, SiO2 90.2%, TiO2 0.2%, all in wt.%) were purchased from Sigma Aldrich.

Polyetherimide (PEI, melt index: 9g/10min (337°C/6.6kg), density: 1.27g/cm3 at 25°C, glass transition temperature 219°C (Vicat, rate B, ASTM D1525)) was obtained from Sigma Aldrich.

All the other reagents, e.g. aminopropyldiethoxymethylsilane (APDEMS), toluene, chloroform, were furnished by Sigma Aldrich and used as received.

2.2Amino-functionalisation of the natural diatomite

1g of diatomaceous earth untreated powder, i.e. CeliteS, was added to a mixture composed of 8ml of APDEMS, used as silane coupling agent, and 200ml of toluene, and magnetically stirred for 24h at room temperature [35] (Fig. 1). Then the suspension was vacuum filtered and the obtained powder was washed several times with toluene to remove the unreacted silane and, finally, dried in oven at 110°C (NH2-CeliteS).

Fig. 1.

Schematic of amino-functionalisation of uncalcined diatomite (CeliteS) using a silane coupling agent (APDEMS).

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2.3Purification of diatomite powder

Calcined diatomite powder, i.e. SCF, was dried at 100°C for 12h, and then leached in 5M HCl solution (powder/solvent ratio of 1g/20ml) at 75°C for 72h under continuous magnetic stirring at 500rpm, following the procedure reported elsewhere [36,37]. After 72h, the powder was filtered, washed with distilled water and dried in an oven at 60°C for 12h. The purified powder was designed as SCFpur.

2.4Preparation of PEI/diatomite films by solvent casting

Neat and hybrid PEI based films were prepared by solvent casting technique. In the case of hybrid samples containing diatomite, dispersions of diatomite powder were prepared suspending the particles in chloroform (CHCl3) and sonicating them for 30–60min in an ultrasound bath (Elmasonic S30H). Then pellets of PEI were added to the prepared suspensions to obtain a PEI concentration of 5% (w/w) with respect to the solvent. The obtained suspensions were maintained under continuous magnetic stirring up to complete dissolution of polymer and, finally cast on a teflon Petri and kept under fumehood for around 2–3 days in order to completely evaporate the solvent. Different weight percentages of diatomite (i.e. 1, 3, 5, and 10wt.% with respect to PEI) were investigated and the samples were designed as PEI/xDE where x=filler weight percentage, DE=CeliteS, NH2-CeliteS, SCF and SCFpur on the basis of the used filler), investigating only the higher amounts (i.e. 5 and 10wt.%) in the case of NH2-CeliteS and SCFpur.

As a reference, neat PEI film was also prepared following the same procedure. Fig. 2 shows the mechanism behind the chemical bond formation between NH2-CeliteS and PEI.

Fig. 2.

Schematic of chemical bond formation between amino-functionalised CeliteS diatomite and PEI.

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2.5Characterisation techniques

The morphology of both fillers, before and after the applied treatment (i.e. amino-functionalisation and purification in the case of CeliteS and SCF powders, respectively) and the elemental compositions were analysed by means of scanning electron microscopy (FE-SEM, Cambridge Leo Supra 35), coupled with Energy dispersive X-ray spectroscopy (EDS, INCA Energy 300, detector Oxford ELXII), after gold coating (25mA, 1×10−4bar, 120s).

Phase analysis of the used fillers, i.e. CeliteS and SCF, and of selected composite samples was performed by X-ray diffraction (XRD, Philips X’Pert PRO 1710, Cu Kα radiation λ=1.5405600Å, 2θ=10–80°, step size=0.020°, time per step=2s, scan speed=0.020°/s).

The characteristic chemical groups of the uncalcined and calcined diatomite powder, before and after the related treatment, were investigated by Fourier Transform Infrared Spectroscopy (FT-IR, Spectrum 100 Optica, Perkin Elmer) in the following conditions: wavenumber range 400–4000cm−1, spectral resolution 4cm−1, scans number 32. The mechanical properties of the prepared films were determined by means of uniaxial tensile tests on dog-bone specimens (width 4.8mm, length 22.25mm), at 1.2mm/min up to rupture. These tests were carried out through an electromechanical machine equipped with a 500N load cell (Lloyd LRX), following the ASTM D1708 standards. Finally, the fracture surface of the samples after the tensile test was investigated by means of scanning electron microscopy (FE-SEM, Cambridge Leo Supra 35), to investigate the distribution of the fillers within the polymeric matrix and the compatibility between the inorganic and the organic components.

3Results and discussion3.1Morphological and spectroscopic characterisation of diatomite powder

In Fig. 3 the SEM micrographs of both CeliteS and SCF diatomite powders, before and after the amino-functionalisation and the purification, respectively, are compared. From the comparison between the uncalcined and calcined powders, it is evident that SCF is composed of particles of different shapes (i.e. rounded, elongated, etc.) and dimensions (between 2 and 70–80μm), whereas CeliteS shows a more uniform and homogeneous distribution in size and shape, with similar hierarchical porosity characteristics. It is interesting to note that all particles present a hierarchical porosity, with pore dimension ranging from few micrometres down to few tens nanometres. Moreover, comparing the CeliteS particles, before and after the functionalisation, similar morphology can be observed, allowing to conclude that the amino-functionalisation process did not significantly affect the microstructure of uncalcined diatomite. Similarly, for SCF powder the HCl treatment did not alter the diatomite morphology, as expected, since this treatment is only aimed to remove the present impurities [38]. The related EDX spectra showed the presence of Si and O, being both powders mainly composed of silica (bottom inset, Fig. 3). The extra peak at was ascribed to Au, due to the gold sputter coating, whereas in the case of NH2-CeliteS the C characteristic peak probably come from the carbon tape and/or the used organosilane. This result was also confirmed by XRD analysis: the XRD patterns of CeliteS and SCF powders evidenced the presence of the typical diffraction peaks of cristobalite low phase (JCPDS #77-3317) in both cases (Fig. 4). It is interesting to highlight the remarkably higher intensity of cristobalite low diffraction peaks and the presence of quartz phase (JCPDS #76-1906) in the case of SCF powder, due to its higher crystallinity as a consequence of the thermal treatment at 900°C, whereas, on the other hand, CeliteS powder presented an amorphous nature, as expected and testified by the presence of a very broad band around 2Theta 22°, accompanied by the main diffraction peaks related to cristobalite low phase. These experimental evidences were also confirmed by FT-IR spectroscopy. FT-IR spectra of the uncalcined diatomite before and after the amino-functionalisation and FT-IR spectra of calcined diatomite before and after HCl treatment are displayed in Fig. 5a, b and c, respectively.

Fig. 3.

SEM micrographs of CeliteS and SCF before and after the amino-functionalisation and the purification, respectively (insets: (top) higher magnification SEM micrograph, (bottom) EDX spectrum).

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Fig. 4.

XRD patterns of CeliteS powder, SCF powder, PEI/10CeliteS and PEI/10SCF.

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Fig. 5.

FT-IR spectra of the uncalcined diatomite before and after the amino-functionalisation: (a) between 400 and 1800cm−1; (b) between 2500 and 4000cm−1; (c) FT-IR spectra of calcined diatomite before and after HCl treatment (wavenumber range 400–2000cm−1).

(0.17MB).

In both spectra of untreated and amino-functionalised CeliteS the typical vibrational modes of silica were detected (Fig. 5a), such as the peaks at 470, 800 and 1100cm−1 that correspond to the Si–O–Si bending mode, Si–O stretching mode of (Si–O–Si), and asymmetric stretching mode of Si–O–Si bonds, respectively [39]. The broad band around 3400cm−1 and the peak at 1636cm−1 are associated to OH vibration mode of the physically adsorbed H2O and bending vibration of water molecules retained in the diatomite silica matrix [40].

In the case of the pristine uncalcined powder, the peak at 917cm−1 corresponds to (Si–O) stretching of silanol group (Si–OH). This peak is not detectable in the case of organosilane-modified powder, showing that functionalisation successfully occurred. This observation is further supported by the presence of additional peaks at 1470, 1542 and 1575, 2917 and 2850cm−1.

In details, the peaks at 2917 and 2850cm−1 (Fig. 5b) are ascribed to the asymmetric and symmetric stretching modes of the –CH2– moiety, respectively, which is directly related to the carbon chain of organosilane molecules [41,42].

The peaks at 1575cm−1 and 1470cm−1 are due to the –NH2 terminal group and the –NH3+ group, respectively (Fig. 5a). Both these modes are associated with the –NH2 groups from APDMES [43]. In conclusion, these FT-IR results clearly demonstrate the presence of –NH2 and –CH3 functional groups on the diatom surface originating from the used silane and the effectiveness of the silanization procedure.

On the other hand, FT-IR spectra of untreated and purified SCF are comparable, presenting the typical vibrational modes of silica (peaks at 470, 800 and 1100cm−1 that correspond to the Si–O–Si bending mode, Si–O stretching mode of (Si–O–Si), and asymmetric stretching mode of Si–O–Si bonds, respectively [39,44,45]) (Fig. 5c), in agreement with Chaisena and Rangsriwatananon [38].

3.2Microstructural and mechanical properties of the PEI based films

The filler addition did not affect the crystallisation of the prepared composite films. Indeed, the XRD patterns showed the co-presence of a broad band, ascribed to PEI which is amorphous, and the main typical peaks of the considered fillers, with very low intensities. As an example, in Fig. 4 the XRD patterns of PEI/10SCF and PEI/10CeliteS are reported and compared with those of the used fillers.

The mechanical properties of prepared films, in terms of ultimate tensile strength (σmax), Young's modulus (E), Young's modulus variation with respect to neat sample (ΔE [%]), and elongation at break (ɛmax), are collected in Table 1. The σɛ curves of neat and hybrid films are compared in Figs. 6 and 7, evidencing a more brittle mechanical behaviour in the case of higher filler content (5 and 10wt.%), independently of the used powders. From the comparison of data reported in Table 1, it is evident that the Young's modulus remarkably and progressively increased with the filler content, especially for films loaded with functionalised CeliteS and purified SCF, whereas the diatomite addition did not seem to significantly affect the ultimate tensile strength value, probably due to the employed process technology (i.e. solvent casting). Furthermore, higher E values were recorded in the case of films loaded with untreated CeliteS powder with respect to untreated SCF, due to its more homogeneous microstructure (Fig. 3) and more uniform distribution within the film thickness.

Table 1.

Tensile modulus (E), ultimate tensile stress (σmax), elongation at break (ɛmax) and Young's modulus variation with respect to neat sample of all tested films (all values are expressed as mean values±standard deviation (SD).

Sample  σmax [MPa]  E [GPa]  ΔE [%]  ɛmax [%] 
PEI  46±1272±13  –  7.9±1.6 
PEI/1CeliteS  44±1491±54  17  4.8±1.0 
PEI/5CeliteS  49±1571±98  24  4.95±0.8 
PEI/10CeliteS  44±0.4  2066±48  62  3.4±0.6 
PEI/5NH2-CeliteS  51±1773±51  39  5.3±0.7 
PEI/10NH2-CeliteS  44±2278±50  79  2.9±0.4 
PEI/1SCF  40±1427±105  12  14±
PEI/5SCF  45±0.5  1488±27  17  10±
PEI/10SCF  48±1558±121  23  6±0.1 
PEI/5SCFpur  41±1660±69  30  4±0.5 
PEI/10SCFpur  40±1951±15  53  4±

PEI: polyetherimide

SCF: SuperCellFine

Fig. 6.

Stress–strain curves of PEI films loaded with calcined diatomite (i.e. SCF) in different amounts (5 and 10wt.%) before and after purification.

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Fig. 7.

Stress–strain curves of PEI films loaded with uncalcined diatomite (i.e. CeliteS) in different amounts (5 and 10wt.%) before and after amino-functionalisation.

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The improved E values detected in the case of PEI films loaded with NH2-fucntionalised and purified diatomites, with respect to those loaded with pristine particles, can be ascribed to a more uniform and homogeneous distribution of the filler throughout all the thickness of the film, as supported by the SEM micrographs of the stress-strained fracture surfaces (Figs. 8 and 9). Indeed, in the case of pristine diatomite particles, it is possible to observe a tendency of the fillers to sediment on the bottom of the Petri dish, generating a biphasic material, composed of a thicker polymeric layer and a thinner biosilica based layer with a low content of polymer that acts as a binder. Thus, the ultimate tensile strength seems to be mainly influenced by the polymeric layer, increasing the inorganic layer thickness with the filler amount with consequent Young's modulus increment.

Fig. 8.

SEM micrographs of the surface fracture of neat PEI film and of PEI films loaded with 10wt.% of CeliteS before and after amino-functionalisation.

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Fig. 9.

SEM micrographs of the surface fracture of PEI films loaded with 10wt.% of SCF before and after purification.

(0.55MB).

Specifically, in the case of films containing NH2-CeliteS, this experimental evidence suggests that the functionalisation provided for a better bonding and improved interface and chemical compatibility between functionalised powder and the polymeric matrix, as expected (Fig. 2).

The progressive increment of E value with filler amount, in the case of hybrid films, testifies a good compatibility/wettability between the used fillers and the polymeric matrix, as supported by the SEM observation of the stress-strained fracture surfaces. Indeed, higher magnification SEM micrographs of the fracture surfaces of stress-strained samples, after tensile test, highlight that diatomite frustules were fully covered by the polymer that was able to penetrate within the diatomite structure micro-/nano-pores, thus providing mechanical interlocking (Fig. 10).

Fig. 10.

Higher magnification SEM micrographs of the surface fracture of PEI films loaded with 10wt.% of CeliteS before and after amino-functionalisation, and 10 wt.% of SCF before and after purification.

(0.46MB).
4Conclusions

Innovative composite materials based on polyetherimide as matrix and natural diatomite as filler were successfully produced by solvent casting technique. The effect of different typologies of diatomite, i.e. uncalcined (CeliteS) and calcined (SuperCellFine), of the filler concentration (i.e. 1–10wt.%), of the surface functionalisation in the case of CeliteS powder (using APDEMS in order to obtain an amino-functionalisation to improve the chemical compatibility with PEI) and of the HCl treatment (in order to remove the present impurities) in the case of SuperCellFine, was investigated. The efficacy of the procedure followed for the CeliteS surface functionalisation was demonstrated by FT-IR analysis, whereas the enhanced chemical compatibility between the polymeric matrix and the organosilane-modified filler was confirmed by the mechanical test results. However, a good interfacial adhesion between diatomite and the matrix was achieved in all cases, as evidenced by SEM observations of the fracture surface of stress strained films. Indeed, mechanical testing shows a progressive increment of the Young's modulus with the filler content, particularly evident for the films loaded with amino-functionalised CeliteS powder and with purified SCF powder, due to a more homogenous distribution of the functionalised and purified structures throughout the film thickness. On the other hand, the maximum tensile strength was comparable to neat PEI matrix and did not seem significantly influenced by the filler addition, being mainly dependent on the generation of a bilayer structure, composed of a thicker polymeric layer and a thinner biosilica based one, due to the tendency of the diatomite to sediment on the Petri dish bottom.

Conflicts of interest

The authors declare no conflicts of interest.

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