Nanoclays are the form of mineral silicates present in the form of nanoparticles which make them so small that they cannot be seen with a naked eye and instead a microscope is used. However, the characteristics and properties of these nanoclays make them exceptional products to be used in the industry. Nanoclays have a lot of sustainable applications and eight out of them are explained in this article.
All these applications are promoting the industry towards success as they not only help in difficulties but also enhance the economy of the industries and country. These nanomaterials are playing a key role in maintaining and building the economy of our country and constantly uplifting it.
The nanoparticles of the layered mineral silicates are known as nanoclays. Nanoclays are classified into various classes like halloysite, hectorite, kaolinite, bentonite, and montmorillonite, based on their Nanoparticle’s morphology and chemical composition. Organoclays (Organically-enhanced nanoclays) can be utilized as drug delivery carriers, gas absorbents, and rheological modifiers because they have potential usages in polymer nanocomposites as they are attractive class of hybrid organic-inorganic nanomaterials.
Structure of clay particles
Clay’s physical properties are more significant in defining clay’s various groups, relative to the known practice of using the chemical behavior of the material to assess the performance of the material. If there are changes regarding the simple humidity content in the environment around them, the clay particles respond by losing or absorbing water. The water, when absorbed, starts filling the spaces that are present in between the stacked silicates layers. The gain or loss of water is what determines any type of clay's specific gravity, making it highly variable.
You can rarely find most of the known types of clay individually in nature and usually, they are mixed with microscopic crystals of other types like quartz, micas, feldspars, and carbonates. Clay particles have such a structure that it is perceived in the layer. For instance, structural sheets of two types i.e. tetrahedral and octahedral make up each layer. Three corners are shared by silicon-oxygen tetrahedra for linking to neighboring tetrahedra and they make up the tetrahedral, leading to a hexagonal network.
Corner of a tetrahedron
Each tetrahedron's remaining fourth corner makes a part of the adjacent octahedral sheet. Magnesium or aluminum coordinates in a six-fold with hydroxyl and with oxygen from the tetrahedral sheet to make up the octahedral sheet. A layer is formed when two sheets come together, and through hydrogen bonding, electrostatic force, Van Der Waals force, and interlayer cations, several layers are joined in a clay crystallite.
Variety of clay minerals
The arrangement of octahedral and tetrahedral sheets can describe various clay minerals. Per clay layer, one octahedral and one tetrahedral sheet is possessed by 1:1 clay mineral. One octahedral sheet mixed between the two tetrahedral sheets and two tetrahedral sheets is possessed by 2:1 clay mineral. For instance, an example of a clay mineral, montmorillonite has a 2:1 sheet structure. An octahedral sheet adjacent to a 2:1 layer makes up 2:1:1 clay minerals. Thus, clay mineral's structure can explain the arrangement of the octahedral and tetrahedral sheets. The layered structure is influenced by the charge's presence in the octahedral and tetrahedral sheets.
In clay minerals, the isomorphous substitution significantly leads to the development of the charge. Replacing the element’s isomorphous substitution with another element in the mineral crystal with no new modifications in its chemical structure. For instance, in octahedral conditions, Fe3+, Fe2+, and Mg2+ can possibly replace Al3+ whereas, in tetrahedral conditions, Si+4 can be replaced by Al3+.
Recently, there has been an emergence of novel applications of the advanced polymer/nanoclay composites because of their remarkable engineering characteristics like improved thermal behavior, high fatigue endurance, stiffness, high specific strength, high damping, and low density. There is an implementation of almost 75-80 percent of the polymer/nanoclay composites in the packaging, aeronautical, and automotive industries. There have been investments of billions of dollars by companies in the development of novel polymer/nanoclay composite materials. In this article, there is an overview of the latest applications and enhancements of polymer/nanoclay composites and the main focus is on the applications in wastewater pretreatment, food packaging, rheology modification, and the biomedical industry.
1. Rheological Control Agent
In developing advanced products with usages in numerous fields like pharmacy and petroleum, there is great relevance of the rheology of the polymer/nanoclay composites. Because of the differences in their surface energies, there is not much compatibility of most of the petroleum-based polymers with nanoclay materials, therefore the clay layer's surface energy is reduced by using the surfactants.
There are various novel applications of the nanoclay composites, for instance, they enhance the fatigue and rutting resistance of the asphalt mixtures, and improve the aging resistance along with storage stability of the polymer-modified asphalt mixtures. Nanoclay assisted by surfactants was applied by Guo et al. in CO2 foam as a stabilizer. The results showed that the CO2 foam’s formability and stability was improved by nanoclay, resulting in enhanced oil recovery in a microfluidic device from a homogeneous porous medium
Surfactant/nanoclay composites are successfully implemented as a foam stabilizer which refers to the improved oil recovery and petroleum industry being the most suitable areas of application for such composites. Pan et al. used the nanoclay composites to enhance the resistivity and rheological characteristics of the synthetic-based drilling fluids for high-temperature applications. In addition of1 percent nanoclay composites, there was an almost 30% decrease in the drilling fluid’s electrical resistivity and there was a major increase in the shear stress and yield point of the fluids.
Dong et al. applied bentonite nanoclays to asphalt mixtures for enhancing the cracking resistance and initial strength of the asphalt mixtures and lessened their susceptibility towards moisture.
2. Food Packaging
Polymer/nanoclay composites function as a barrier against the permeation of many gases like volatile compounds (taints and flavors), water vapors, carbon dioxide, and oxygen. These composites have a remarkable potential to be food packaging materials because of this gas barrier characteristic, and their basic thermal, optical, and mechanical characteristics. In these latest decades, there has been the development of various polymer/nanoclay materials with enhanced mechanical and thermal characteristics and lessened permeability to gases, all because of their benefits over the other conventional materials, like plastics.
Innovative nanoclay composites
The innovative polymer/nanoclay composites are smart packing materials as they control their permeability towards selected gases like CH4 and CO2, thus increasing the preserved food's shelf life. In collaboration with NASA, the US military implemented nanoclays for ethylene vinyl alcohol (EVOH) as barrier enhancers for developing long-shelf-life packaging material (non-refrigerated food). With no refrigeration, EVOH/nanoclay composites are used by the developed material to have 3 to 5 years of shelf life. Raine et al. used polyamide 11/graphene composites to develop a laminate with an exceptional gas barrier characteristic.
Permeability to H2S was lessened to an undetectable level by the laminate film whereas the permeability to CO2 was lessened to more than one order of magnitude by the laminate film. Irganox® 1076 antioxidant (BASF, Ludwigshafen, Germany) behavior was studied by Cherif et al. in a novel food contact packaging film that consists of high-density polyethylene nanoclay composites. Fick’s law was used to determine the diffusion coefficients, showing that with 3 wt% nanoclay loading, the antioxidant's maximum diffusion rate reduction reached 78%. The great potential was seen in the composite material for the plastic packaging industry.
3. Drug Delivery and Biomedical Applications
Due to remarkable characteristics like thermal, barrier, and mechanical characteristics, and flame retardancy, innovative polymer materials, and advanced nanotechnologies have helped in creating advanced materials at an increasingly fast rate.
Recent investigations were regarding the cellular interactions with polymer/nanoclay composites, with the main focus on their potential usages in biomedical applications, like drug delivery, bioimaging, biosensing, food preservation, gene therapy, and tissue engineering. The reason for this being the main purpose is polymer/nanoclay composite’s excellent characteristics, for instance, their rheological and swelling characteristics, a large affinity for interacting with biopolymers, large surface-area-to-volume ratio, and high retention capacities.
Due to the increase in chronic diseases and the growth of the aging population, tissue engineering has become a very significant field. Damaged organs/tissue can be replaced and/or repaired with the help of polymer/nanoclay composites. For instance, there has been successful implementation of the polymer/nanoclay-based scaffolds in cell-transplantation applications in neural tissue engineering made with various advanced methods and display high biodegradability, biocompatibility, and a high degree of porosity.
Fundamental insights were discussed by Sheikhi et al. into polymer/nanoclay hydrogels in biomedical applications, making the way to design clay-based hydrogel scaffold materials. The effect of the salts like CaCl2 and NaCl, on the characteristics and structure of the composites, were investigated by them. Li et al. used poly(N-isopropyl acrylamide)/nanoclay composites to make a thermoresponsive and ultra-stiff hydrogel material. The high strength of almost 1700 kPa and a high tensile modulus of almost 3500 kPa was displayed by the developed hydrogel, along with well-defined, thermo-responsive deswelling/swelling behavior. This hydrogel can also be utilized as a scaffold material.
A promising potential is shown by the polymer/nanoclay composites in other fields of the biomedical industry like drug delivery agents, bio-sensors, wound healing, and bioimaging. Recently, several polymer/nanoclay composites have been developed as contrast agents for various bioimaging techniques like X-ray computed tomography (CT) scan, fluorescence imaging (FI), photoacoustic (PA) imaging, and magnetic resonance imaging (MRI), because of their effect on pharmacokinetics and bio-distribution.
Polydopamine/Fe3O4 composites with remarkable characteristics like near-infrared absorption and high fluorescence quenching efficiency were used by Lin et al. to develop a novel theranostic agent. It is also utilized in various bioimaging systems, for instance, MRI, PA, and mRNA detection imaging.
A huge amount of interest is gained by the development of polymer/nanoclay composite biosensors for various applications because of their hybrid characteristics and synergistic effects. Those various applications include environmental monitoring, detecting animal and human pathogens, and analysis of the food product. Although, there still are some compatibility issues in implementing polymer/nanoclay composites as a biosensor.
A fast and convenient biosensor was developed by Kong et al. for detecting glucose using a graphene/PANI/AuNPs/ glucose oxidase enzyme (GOx) modified screen-printed carbon electrode (SPCE) in blood. Sample-impregnated paper disks cover the sensor.
4. Wastewater Treatment
Due to potential health concerns, contaminated water is now a significant problem because of the presence of various toxicity chances like dyes, aromatic molecules, and heavy metals. Another broadly utilized technique for removing pollutants from the water is absorption. Over the recent years, there has been the development of various novel adsorbents. Due to their comparatively low toxicity and cost, large surface area, effective cation exchange, and easy processability, Polymer/nanoclay composites are capable of delivering a remarkable life cycle for remediation/water treatment and a high adsorption capacity.
Various factors affect the adsorption performance (Reusability, selectivity, and capacity), for instance, the existence of the molecules of water between the clay layers, methods employed to modify them, nanoclay structure in the matrix, and the exchangeable cations, all of these substantially affect the adsorption performance.
Advanced polymer composites
There has been development and usage of numerous advanced polymer/Nanoclay composites in emerging adsorbents. Moreover, some of the recent studies have displayed numerous synthesized polymer/nanoclay composite's remediation potential and effectiveness as promising adsorbents. An in-situ polymerization method was used by Kara et al. to make poly (vinyl imidazole) (PVI)/sepiolite nanoclay composites.
It was seen that in wastewater, this synthesized composite is an effective absorbent for Hg(II). Nanoclay’s uniform dispersion in the polymer matrix and a smooth, dense surface were displayed by PVI/sepiolite composites, delivering a higher adsorption capacity than sepiolite alone. According to observations, at a pH of 6, the optimum pH value occurred, however, there was an increase in its adsorption capacity values with an increase in the temperature.
PVI/bentonite nanoclay composites were used in wastewater treatment by Yildiz et al. for removing anionic dye remazol black B (RB) from the water. Numerous pollutants can be removed from the aqueous solutions through various polymer–nanoclay composites and they have proved to be effective and efficient in water treatment processes.
There are some concerns regarding the safety of the nanoparticle’s potential to be released into the environment and cause harm through the interactions between natural organic matter and particles or biomolecules, physical aggregation or transformation, biological, and chemical transformation. If we are going to develop such materials further, then we should consider human and environmental health implications, regeneration potential, modes of application, and strategies of synthesis.
5. Polymer-clay nanocomposites
Synthesis and formulation of polymer-clay Nanocomposites is the most significant application of organoclays (hydrophobic). According to the name of these materials, these hybrid materials have a dispersed (organoclay) phase and a continuous (polymer) phase or matrix. For a long time, there have been practices of the incorporation into polymers of fine-grained solids (‘microfillers'), like silica, metal oxides, coal, calcium carbonate, and cellulose, as reinforcing agents. A nylon-clay nanocomposite was made by scientists at Toyota Central Research Laboratories in pioneering work by intercalating e-caprolactam into an organoclay and then heating the monomer to polymerize it.
As compared to the pristine polymer, the resultant nylon-6 nanocomposite possesses excellent and better thermal and mechanical characteristics. Polymer-clay nanoparticles don't only show excellent impact, flexural, and tensile strengths, and heat distortion temperature, but it also exhibits major enhancements in solvent resistance, flame retardancy, storage stability, breaks elongation, and gas barrier characteristics over the corresponding neat polymers.
6. Bone Cement
Bones are complex materials. They are made up of inorganic calcium phosphates, and they provide strength and organic collagen. Organic collagen provides flexibility, therefore they can be known as composite materials.
The classification of bones is under cortical bones or cancellous bones. Almost 80% of the total skeleton is represented by the cortical bones. These 2 types of bones have different mechanical characteristics of flexibility and strength: the modulus and strength of elasticity range between 200 MPa and 70 MPa and 30 GPa and 3 GPa for the cortical bones, however, the values are lower for cancellous bone: elastic modulus is 0.02-0.5 GPa and tensile strength is about 0.1–30 MPa.
Polymethyl methacrylate (PMMA) is broadly utilized as a bone cement with the main aim to fix knee and hip replacement implants into the adjacent bones. Inadequate mechanical characteristics and poor fatigue strength are shown by these materials for applications of load-bearing. However, there is another disadvantage which is that during polymerization, PMMA has a high exothermic temperature, which is capable of causing necrosis, leading to implants being lost in the body.
There have been studies on the nanoclay materials as reinforcements for PMMA composites for bone implants or bone cement applications with enhanced bioactivity and mechanical characteristics.
7. Tissue Engineering:
Tissue engineering's main aim is improving, maintaining, and restoring the functions of the tissue. There have been incorporations of nanoclays to hydrogels like polysaccharides (gellan gum, chitosan) because of the capability of the polymer to support adhesion and proliferation of the cells. Adding the nanoclay fillers helps them in getting excellent mechanical and physical characteristics.
A Hydrogel made up of HNTs, glycerol, and gellan gum (GG) was proposed by Bonifacio et al. for applications of soft tissue engineering, for instance, regeneration of skin, liver, and pancreas. The material viscosity improved on glycerol's addition to GG, whereas the water uptake was decreased by HNTs to 30-35 percent. De Silva et al. made the membranes of chitosan/HNTs through solution casting and it displayed enhanced mechanical characteristics by adding 5% HNTs, along with improvement of its thermal stability.
Nitya et al. used electrospinning to prepare a fibrous polycaprolactone/HNT composite scaffold for bone tissue engineering and performed its in vitro evaluation. Katti et al. made a biopolymer consisting of chitosan mixed with HAP and MMT, and it displayed an intercalated structure that enhanced nanomechanical characteristics and its thermal stability.
Ambre et al. made scaffolds based on chitosan/polygalacturonic acid (ChiPgA) complex for applications of bone tissue engineering. According to the results, there was growth and proliferation of the human osteoblasts. These composite porosity also increased by 90 percent, which facilitates the transportation of the nutrient throughout the scaffold. Incorporation on MMT was specifically found to be highly responsible for moderating these characteristics.
8. Wound Healing:
There have been wide explorations going on currently on nanoclay’s wound healing applications for lessening pain and preventing scarring, and infection, making the characteristics like swelling ability and flexibility very significant. Sabaa et al. made biodegradable polyvinyl alcohol (PVA) composite with carboxymethyl chitosan (CMC) and MMT, and as compared to standard drugs like Penicillin G, they displayed good antimicrobial potency and increased swelling behavior. In order to adjust the stimuli response of collagen/N isopropylamide hydrogels as a scaffold with enhanced regenerative and healing characteristics, MMT nanoparticles were incorporated by Nistor et al. into collagen/N isopropylamide hydrogels.
MMT nanoparticles made a 3-dimensional network of interconnected pores as it allowed the formation of new bonds. Electrospinning was done to make another PVA composite with Iranian gum tragacanth (IGT) and it was improved with a kaolinite-based nano clay. In addition, nano clay enhanced its chemical stability and mechanical characteristics, which made it appropriate and showed its potential for wound healing applications.
Cross-linked nano clays
Yang et al. explored cross-linked nano clays, like semi-IPN sericin/poly (NIPAm/LMSH) (HSP) nanocomposite hydrogels, as a wound dressing too. After 6 days, on treatment with nanocomposites, the wound healing area increased threefold over the area covered by gauze, and by the 13th day, they almost displayed total recovery. Here, HNTs increased long-term water retention, drug loading, and porosity. Good biocompatibility and cytotoxicity were showed by hybrid hydrogels which were utilized for the healing of the wound.
Nanoclays are an essential product in the market that is why their production has been increasing constantly over the course of past years. These materials are providing the industries with all the essentials that are required to run the industries. The success has come through because of all the excellent characteristics and properties.