Electrochemical and mechanical properties of the PEO coatings containing halloysite nanotubes

A newly designed smart self-healing epoxy coating system comprised of modified halloysite nanotubes (HNTs) having capping is proposed for corrosion protection of steel. In the first step, HNTs were loaded with 8-hydroxyquinoline (8HQ), used as a corrosion inhibitor. Then the HNTs were sealed/capped using cobalt (II), aiming for an efficient and controlled release of the loaded inhibitor. The smart coatings were developed by reinforcing loaded HNTs into the epoxy matrix. The structural, thermal, mechanical, and electrochemical properties of capped modified HNTs and smart coatings were studied using various techniques. UV–Vis analysis depicted that the capping of the metal-inhibitor complex was decomposed at acidic pH resulting in a controlled release of the loaded inhibitor into HNTs. Electrochemical impedance spectroscopic (EIS) analysis of blank and smart coatings demonstrated that the low-frequency impedance modulus of smart coatings is 109 Ω.cm2 for 20 days compared to blank coatings (105 Ω.cm2), reflecting their excellent corrosion inhibition performance. The superior corrosion protection properties of these smart coatings can be ascribed to the controlled and efficient release of the loaded inhibitor from the capped HNTs. Finally, X-ray photoelectron spectroscopy (XPS) analysis of the steel substrate after the corrosion analysis revealed the adsorption of 8HQ on the steel surface, confirming the formation of iron complex due to the release of loaded inhibitor. This work demonstrated the adeptness of 8HQ in mitigating the corrosion due to the controlled and effective release of the inhibitor from capped HNTs because of dissociation of the metal-inhibitor complex (Co-8HQ).Graphical abstract

At present, petrochemical-based plastics are commonly used in many industries. Yet, environmental concerns over these materials have led to the search for biodegradable alternatives in several industries including packaging. In this regard, the renewable resource-based polymer, poly (lactic acid) (PLA), holds a greater demand due to its unique properties (such as biodegradability and biocompatibility). Despite the merits of PLA, limitations in mechanical, thermal and gas barrier properties have restricted the usage of PLA in a wider range of applications in the packaging industry. Therefore, it has become a necessity to reinforce PLA to overcome these drawbacks. The main objective of this research is to investigate the feasibility of halloysite nanotubes (HNTs) as a nanofiller to reinforce PLA. PLA/HNTs nanocomposite films were prepared using the solution casting method by varying the HNTs loading (from 2.5 – 10 (w/w %)). Evaluation of the physico-chemical properties of the films revealed that the mechanical and thermal properties of PLA films were enhanced with the addition of HNTs. The interfacial interaction was further investigated with Fourier transform infrared (FTIR) spectroscopy and the end-hydroxyl groups of PLA were found to chemically interact with outer surface siloxane groups of HNTs. Furthermore, a comparison study was conducted to investigate the influence of the nanocomposite processing methods, namely melt compounding and solution casting, on the thermo-mechanical properties of PLA/HNTs nanocomposite films. Moreover, the ductility of PLA/HNTs films was improved by incorporating a plasticiser, poly (ethylene) glycol (PEG), while the dispersion of HNTs in PLA at high filler loadings was further improved by modifying the outer surface of HNTs with a silane modifier, γ-aminopropyltriethoxysilane (APTES). In addition, after evaluating the effect of three different types of HNTs (which are structurally different) on the tensile properties, this study suggests the best type of HNTs that can be incorporated into the PLA matrix in order to obtain the optimum tensile properties. In this dissertation, an alternative finite element (FE) approach to accurately predict the elastic modulus of polymer/HNTs nanocomposites is proposed. A real-structure-based 3-D computational model with randomly oriented HNTs was developed and compared with the conventional, idealized modelling approach. The developed idealized model consists of nanotubes with fixed aspect ratio and the proposed alternative real-structure-based model takes the experimentally observed variations in HNTs sizes, impurities and aspect ratios into account. According to the parametric studies, a unit cell model with cylindrical reinforcements (representing HNTs) and at least 30 inclusions gave promising results, provided the model included actual information about HNT's size ranges and aspect ratios. Numerical studies were validated with experimental findings and the developed real-structure-based model gave more accurate results than idealized and analytical models. Furthermore, the reinforcing mechanism was carefully studied in terms of the stress distribution. As the final stage of this research, the PLA/HNTs nanocomposite films were further developed for better end user application by creating high performance, multifunctional, active packaging films. ZnO nanoparticles have been successfully investigated to remarkably enhance the antimicrobial properties of poly (lactic acid) (PLA) composite films for active packaging, where they can enhance the shelf life of goods, but the addition of ZnO into PLA decreases its thermo-mechanical properties. In this study, ZnO nanoparticles were deposited on the outer and inner surfaces of halloysite nanotubes (HNTs) using a novel solvothermal method and these ZnO deposited HNTs (ZnO-HNTs) were incorporated into the PLA matrix as a reinforcing filler. PLA nanocomposite films with ZnO had inferior mechanical properties compared to PLA films with ZnO-HNTs which showed significant improvements, with an increase in the tensile strength and modulus by 33% and 74%, respectively, with the addition of 5 (w/w %). Antimicrobial tests revealed that ZnO-HNTs can act as a promising antimicrobial agent against bacteria such as Escherichia coli and Staphylococcus aureus where the bacteria count reduced by more than 99%.

The effects of silane coupling agents on the properties of PHBV/halloysite nanocomposites

Rubber composites based on halloysite nanotubes (HNT) have attracted tremendous attention during the last decades owing to the improved mechanical, dynamic mechanical, and thermal properties [1–3]. Before exploring the various aspects of HNT/rubber composite systems, we first briefly describe the HNT. It was first introduced by Berthier long back in the year of 1826 [4]. HNTs are a type of naturally occurring silicates with rolled nanotubular or spiral morphology, which have an analogous chemical structure of kaolinite [5]. They are naturally occurring and economically viable, and many countries such as China, France, Belgium, and New Zealand have deposits of these clay minerals. The major origin of HNT is a clay mineral obtained from a weathering product of granitic and rhyolitic volcanic rocks. The chemical composition of HNT is reported as Al2(OH)4Si2O5∙nH2O with n 1⁄4 0 or 2 for the anhydrous and the fully hydrated halloysite, respectively. The nanotubular structure was evolved from kaolinite by rolling up the layers under natural conditions. Most of the HNTs are multiwall or double-wall nanotubes [6]. Usually, the inner diameter of HNT is in the range of 50–80 nm and length of about 1,000 nm. Based on the state of hydration, HNTs are classified in two categories: hydrated HNTs with a crystalline structure of 10 A (d001) spacing and dehydrated ones with 7 A (d001) spacing [7]. HNTcontains two different types of hydroxyl groups, inner and outer hydroxyl groups, which are positioned in between the layers and on the surface, respectively. Attributed to the multilayer structure, most of the hydroxyl groups are inner groups, and only a few hydroxyl groups are located on the surface of HNT. The surface of HNT is mainly composed of outer O–Si–O (silanol) groups and O–Al–O (aluminol) groups situated inside the lumen. It is interesting to note that the density of surface hydroxyl groups in HNT is rather lower compared to other silicates clay minerals, for instance, kaolinite and montmorillonites (Fig. 1).

Comparison of the effect of carbon, halloysite and titania nanotubes on the mechanical and thermal properties of LDPE based nanocomposite films

Addition of natural inorganic nanotubes to epoxy acrylate resin can significantly improve their mechanical properties. Halloysite nanotubes are one kind of novel reinforcing and toughening materials for polymers. In the present work, halloysite nanotubes were compounded to epoxy acrylate resin to improve the wear resistance and toughness of the composites. To improve the embedding of halloysite nanotubes within the epoxy acrylate resin matrix, the halloysite nanotubes surface was grafted by poly (methyl methacrylate). The morphology of the grafted halloysite nanotubes particles demonstrates a core-shell structure with halloysite nanotubes as the core and the grafted polymer as the shell. Poly (methyl methacrylate) was chemically attached to the halloysite nanotubes particles as indicated by the Fourier transform infrared and X-ray photoelectron spectroscopy results. The poly (methyl methacrylate) grafted halloysite nanotubes (poly (methyl methacrylate)- g-halloysite nanotubes) then compounded with epoxy acrylate resin and composites are formed via ultraviolet-curing method. The hardness, flexibility, impact resistance and wear resistance of epoxy acrylate/poly (methyl methacrylate)- g-halloysite nanotubes composites were markedly improved in comparison with that of neat epoxy acrylate. The improvement was correlated to the well-dispersed halloysite nanotubes and the interfacial bonding between epoxy acrylate resin and poly (methyl methacrylate)- g-halloysite nanotubes. The well dispersion of halloysite nanotubes in the matrix is attributed to the modification on the surface of halloysite nanotubes.

Novel hybrid halloysite nanotubes (HHNTs) were developed and used as smart carriers for corrosion protection of steel. For this purpose, as-received halloysite nanotubes (HNTs) were loaded with a corrosion inhibitor, imidazole (IM), by vacuum encapsulation. In the next step, a layer by layer technique was employed to intercalate another inhibitor, dodecylamine (DDA), in the polyelectrolyte multilayers of polyethylenimine and sulfonated polyether ether ketone, leading to the formation of HHNTs. During this process, IM (5 wt %) was successfully encapsulated into the lumen of HNTs, while DDA (0.4 wt %) was effectively intercalated into the polyelectrolyte layers. Later, the HHNTs (3 wt %) were thoroughly dispersed into the epoxy matrix to develop smart hybrid self-healing polymeric coatings designated as hybrid coatings. For a precise evaluation, epoxy coatings containing as-received HNTs (3 wt %) without any loading denoted to as reference coatings and modified coatings containing HNTs loaded with IM-loaded HNTs (3 wt %) were also developed. A comparative analysis elucidates that the hybrid coatings demonstrate decent thermal stability, improved mechanical properties, and promising anticorrosion properties compared to the reference and modified coatings. The calculated corrosion inhibition efficiencies of the modified and hybrid coatings are 92 and 99.8%, respectively, when compared to the reference coatings. Noticeably, the superior anticorrosion properties of hybrid coatings can be attributed to the synergetic effect of both the inhibitors loaded into HHNTs and their efficient release in response to the localized pH change of the corrosive medium. Moreover, IM shows an active release in both acidic and basic media, which makes it suitable for the protection of steel at the early stages of damage, while DDA being efficiently released in the acidic medium may contribute to impeding the corrosion activity at the later stages of deterioration. The tempting properties of hybrid coatings demonstrate the beneficial role of the development of novel HHNTs and their use as smart carriers in the polymeric matrix for corrosion protection of steel.

The incorporation of unmodified halloysite nanotube (HNT) in a thermoplastic sago starch (TPSS) film to form a nanocomposite material was investigated. The TPSS/HNT nanocomposite was fabricated through solvent casting method at varying HNT loading of 0, 0.25, 0.5, 1.0, 3.0, and 5.0 wt.%. Evaluation on mechanical and physical properties (tensile test, water absorption, thickness and density) was made to study the effect of HNT loading on the TPSS properties. Tensile strength achieved an optimum value at 0.25 wt.% of HNT loading and decreased with higher addition of HNT. Meanwhile higher amount of HNT in the nanocomposite film exhibited brittleness with the reduced tensile strain. Water absorption decreased with the addition of HNT due to the difficulty of water molecules to pass through the tortuous path of HNT structure. Thickness and density of the nanocomposite film, however, increased at higher HNT contents. FESEM (field emission scanning electron microscope) which examined the surface morphology of the TPSS/HNT nanocomposite displayed uniformly dispersed HNT in the plasticized starch matrix.

Novel polyvinyl alcohol (PVA)/styrene butadiene rubber (SBR) latex/carboxymethyl cellulose (CMC)/halloysite nanotubes (HNTs) nanocomposites were successfully prepared through physical blending. The as‐obtained PVA/SBR/CMC/HNTs nanocomposites were coated on the surface of old corrugated container (OCC)‐based paper in an effort to improve the mechanical properties of paper. To improve the dispersion of HNTs and enhance the compatibility between HNTs and polymer matrix, HNTs were modified with titanate coupling agent (TCA). FT‐IR, together with TGA, confirmed that TCA was grafted onto the surface of HNTs successfully. XRD demonstrated that the crystal structures of HNTs remained almost unchanged. TEM showed that modified HNTs exhibited good dispersion and possessed nanotubular structures with an outer diameter of around 50 nm and an inner diameter of about 20 nm. SEM gave an indication that modified HNTs were dispersed more uniformly than unmodified HNTs within PVA/SBR/CMC matrix. Rheological measurement exhibited that surface modification process enhanced the compatibility between HNTs and polymer matrix, thus resulting in the decreased viscosity of nanocomposites. In comparison with unmodified HNTs, modified HNTs were found to contribute more to the enhancement in mechanical properties, which might be attributed to the better dispersion and compatibility of modified HNTs evidenced by TEM, SEM, and rheological measurement.

Influence of surface treated halloysite nanotubes on mechanical and thermal properties of polypropylene nanocomposites

Bone pathology entails an important average of physical disability, being bone tissue regeneration one of the most actively researched fields in Tissue Engineering. Accordingly, large efforts are focused on the research of novel bioabsorbable materials as a prosthesis with stiffness values similar to that of the host tissue capable of fulfilling the requirements for bone fracture remodelling. This work is focused on studying mechanical properties and cell interaction as a function of the chemical structure and hydrophobicity of different bioabsorbable polymers compared with non-absorbable polymers. Hydroxyapatite (HA) and Halloysite nanotube (HNTs) were used as fillers in order to grant better cell attachment, proliferation and differentiation along with hydrophobicity behaviour. For this end, it was aimed to firstly monitor the effects of bioactive fillers on the structural properties of bioabsorbable Polycaprolactone (PCL). Thus, mechanical and thermal properties of PCL were studied by modifying the additivation percentage of the bioactive fillers HA and HNTs. This preliminary study allowed to understand the synergic effect among the polymeric matrix and the functional groups present on the additives chemical structure by establishing the additivation threshold and optimizing the additivation rate. In general terms, a noticeable improvement of mechanical properties was achieved with the simultaneous addition of the two fillers. Additionally, taking advantages of the HNTs nano-tubular shape, those were studied as drug carrier structures and their loading and release ability with curcumin was monitored. Knowing in one side that HA promotes the formation of a layer of new bone composed of biological apatite and collagen; and in the other side, that HA and HNTs alter hydrophobicity behaviour; morphological properties supplied by both fillers were studied and compared among different pairs of polymers with similar chemical natures but different hydrophobicity. Accordingly, the hydrophobic polyester PCL was modified by its blending with Polylactic acid (PLA) and combined with HA nanoparticles and HNTs. On the other hand, the hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) was copolymerized with ethyl methacrylate (EMA) and also combined with HA and HNTs. Thus, the effect of both fillers was studied on Hydroxyapatite nucleation, distribution of the fillers into the polymeric matrices and PCL/PLA degradation rate. Therefore, it was observed that introduction of HA in polymers with moderately hydrophilic character induce a higher rate of hydroxyapatite nucleation and a faster degradation rate. However, HNTs tends to form big aggregates when the hydrophilic character increases, driving to crack initiation sites and failure of the material. The completion of this study was accomplished by means of monitoring cell viability, proliferation, and morphology on the two pairs of polymers varying the polymer's chemical surface by blending hydrophilic and hydrophobic polymers, copolymerizing monomers of opposite natures, and/or loading the polymer matrix with nanoparticles such as HA or HNTs. Polymer surface wettability is known to affect cell attachment and can be enhanced by modifying the polymer with HA and HNT. In this way, improvement in cell viability with the addition of HA and HNTs was observed due to the generation of new reactive sites with Ca2+ and PO4 3? groups present in HA, and silanol groups (Si-OH) located at the surfaces of HNTs. Thus, it was concluded that on hydrophobic materials, due to a faster arrival rate of proteins, those compete for surface absorption driving to low interaction sites between cells and polymer surface showing a round shape. However, on hydrophilic materials, the highly solvated surface at initial stage limits protein arrival and allowed protein rearrangement and spreading over the surface promoting cell adhesion and proliferation with better cytoskeleton spreading.

Systematic characterization and enhanced corrosion resistance of novel β-type Ti-30Zr-5Mo biomedical alloys with halloysite nanotubes (HNTs) and zirconia (ZrO2)-reinforced polylactic acid (PLA) matrix coatings

Polypropylene (PP) and polylactic acid were blended in the ratio 80:20 by weight and compatibilized with 3 wt% of maleic anhydride-grafted-PP. The compatibilized blend was chosen as the base matrix for reinforcement with halloysite nanotubes (HNTs). The nanotube content varied from 0 to 10 wt%. Blend and the nanocomposites were prepared by melt mixing technique. Dielectric analysis of the base matrix and the nanocomposites was carried out using interdigitated electrode sensor in a DEA 288 Epsilon-dielectric analyser. The dielectric properties of the composites were measured at temperatures from 30 to 120°C at various frequencies ranging from 1 Hz to 1 kHz. Permittivity values slightly decreased as the HNT content increased from 0 to 2 wt%. It increased at 4 wt% of HNT and again slightly decreased at 6 wt% of HNT, and with further increase in HNT (HNT 8 and HNT 10) led to increase in permittivity values. Loss factor values decreased slightly as the HNT content in the composites increased from 0 to 4 wt%; but with further increase in HNT, the loss factor showed a sharp increase. Loss tangent (tan δ) values decreased up to 4 wt% of HNT (HNT 4) and then increased up to 8 wt% (HNT 8) of HNT and then decreased slightly (for HNT 10). Analysing the different dielectric properties, consistent properties were shown by 6 wt% of HNT similar to static and dynamic mechanical properties. The analysis showed that the composites can be utilized in microelectronic devices or in microelectronic packaging applications.

The aim of this study was to evaluate the chemomechanical properties, antibacterial activity, and cytotoxicity of an experimental adhesive resin containing halloysite nanotubes (HNT), doped with alkyl trimethyl ammonium bromide (ATAB). A filler of HNT doped with ATAB was obtained (ATAB:HNT) and incorporated (5 wt%) into a resin blend made of bisphenol A glycerolate dimethacrylate, 2-hydroxyethyl methacrylate and a photoinitiator/co-initiator system (GATAB:HNT). The same resin blend without ATAB:HNT was used as control (Ctrl). The ATAB:HNT filler was assessed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The two tested adhesives were evaluated for degree of conversion (DC) in vitro and in situ, softening in alcohol, dentin microtensile bond strength (µTBS), antibacterial activity, and cytotoxicity (n = 5). SEM showed that the nanotubes had a characteristic tubular-needle morphology, while the TEM analysis confirmed the presence of ATAB inside the lumens of HNT. The incorporation of ATAB:HNT induced no reduction (p > 0.05) of the DC either in situ or in vitro. No difference was encountered after the softening challenge test (p > 0.05) and no difference was found in µTBS between the two adhesives, both at 24 h (p > 0.05) and after 6 months of storage in distilled water (p > 0.05). However, ATAB:HNT reduced Streptococcus mutans viability (p < 0.05) without a cytotoxic effect on pulp cells (p > 0.05). GATAB:HNT adhesive demonstrated appropriate polymerization without significant differences in softening after solvent immersion, while concomitantly maintaining reliable bond strength after 6 months of water aging. Moreover, the ATAB:HNT filler can provide antibacterial activity to the adhesive resin without affecting pulp cell viability.

Improvement of mechanical and thermal properties of poly(3-hydroxybutyrate) (PHB) blends with surface-modified halloysite nanotubes (HNT)