Coating Techniques and Process Parameters

There are different techniques for coating a polymer on to a specific substrate, such as metal or textile. For bulk coating on textiles mainly three types of coating techniques are used: melt coating, solution coating, and transfer coating. Melt coating is mainly of two types: extrusion-melt coating and powder coating. Extrusion-based hot- melt Zimmer coating is very popular for making coated textiles [35]. Solution coating on textiles can be done by methods such as knife coating (knife-over roll and floating knife), roll coating (direct roll, gravure roll, and kiss roll), and rotary screen coating [36]. Among different solution coating techniques, the knife coating technique is preferred for the application of a uniform coating on textiles.

Polymer nanocomposite coatings can be applied on textile substrates by the bulk coating techniques described above. However, for a lab-scale trial or for coating on a metallic or polymeric substrate, some other techniques are used, such as hand coating, spin coating, dip coating, in situ polymerization, layer-by-layer (LBL) deposition, sol-gel process, chemical vapor deposition, and electrophoretic deposition, followed by the evaporation of the solvent and drying or curing [37]. The following aspects are very important in any polymer nanocomposite coating [11]:

Effect of the sheet orientation on the relative permeability in exfoliated nanocomposites at a volume fraction

Figure 19.3 Effect of the sheet orientation on the relative permeability in exfoliated nanocomposites at a volume fraction (0S) = 0.05, width (W] = 1 nm, and various sheet lengths (L). The illustrations show the definition of the direction of the preferred orientation (n) of the silicate sheet normals (p) with respect to the film plane. Illustrations for three values of the order parameter (5) —1/2, 0, and 1 are also shown. Reprinted with permission from Ref. [34]. Copyright (2001) American Chemical Society.

The nanofiller should be uniformly dispersed or exfoliated in the polymer matrix.

  • • The whole mixture should be microscopically homogenous.
  • • In a solvent-based coating, the solvent have to be removed completely by evaporation after drying or curing of the resin.

• The solvent evaporation rate should be controlled in such a way that the nanomaterials do not collapse or aggregate.

Gas Barrier Property of Polymer Nanocomposite Coatings

PMMA Nanocomposite Coatings

For a long time, polymeric or organic coatings have been utilized to protect metals from corrosion, where the main requirement is a barrier against aggressive species, such as oxygen and water vapor. Yeh et al. synthesized a series of PMMA/MMT nanocomposites via in situ thermal polymerization [38]. Methyl methacrylate monomers were first intercalated into the interlayer spaces of clay, which was followed by typical free radical polymerization. The synthesized PMMA/clay nanocomposites were coated on cold-rolled steel (CRS), where superior anticorrosion property was obtained with only 1 wt% clay loading compared to neat PMMA coating. The significant reduction of oxygen and water vapor permeability of PMMA in the presence of clay contributed to the enhancement of the anticorrosion property of the coated CRS [38].

Chang et al. created a unique nanocasting technique for developing biomimetic hydrophobic structures on the surface of a PMMA/graphene nanocomposite (PGN) and applied this in corrosion protection coatings [39]. Initially, a transparent soft template was produced with negative patterns of a Xanthosoma sagittifolium leaf by thermal curing of polydimethylsiloxane (PDMS) prepolymer at 60°C for 4 h (Fig. 19.4). The PDMS negative template was detached from the surface of the natural leaf, and subsequently it was used to generate a superhydrophobic biomimetic surface on the PGN through casting onto a CRS electrode. The SEM image of the PGN surface shows a lot of microscaled mastoids, where each mastoid is decorated with many nanoscaled wrinkles. As a consequence, the contact angle increased from 80° (for neat PMMA) to 150° (for the PGN surface) and the sliding angle decreased from 60° (for neat PMMA) to 5° (for the PGN surface). Moreover, a veiy good dispersion of graphene nanosheets was observed

Preparation of a superhydrophobic gas barrier and an anticorrosion PMMA/graphene nanocomposite surface by the nanocasting technique

Figure 19.4 Preparation of a superhydrophobic gas barrier and an anticorrosion PMMA/graphene nanocomposite surface by the nanocasting technique. Reproduced from Ref. [39], with permission of the Royal Society of Chemistry.

under a transmission electron microscope [ТЕМ), which resulted in a significant improvement in the oxygen barrier property. The synergistic effect of the superhydrophobic surface and the lower oxygen permeability was enhancement in the corrosion protection of PGN coatings on the CRS electrode [39].

Polyaniline Nanocomposite–Based Coatings

An electroactive PANI coating has huge importance for corrosion- resistant coatings on metals due to its redox catalytic capability, which results in the formation of a passive metal-oxide layer [40- 42]. PANI/clay or PANI/graphene nanocomposites could increase the diffusion path lengths of gases such as oxygen and water vapor, which are mainly responsible for the generation of corrosion [43]. Therefore, recently the synergistic effect of the gas barrier property and redox catalytic capability of PANI nanocomposites has been utilized to obtain metal or steel with effective corrosion resistance [44].

Chang et al. studied the anticorrosion capability of coatings based on PANI/graphene composites (PAGCs) and PANI/clay composites

Oxygen and vapor permeability rates of PANI and PANI/ graphene composites

Figure 19.5 Oxygen and vapor permeability rates of PANI and PANI/ graphene composites (PAGCs) with varying amounts of functionalized graphene (0.1, 0.25, and 0.5 wt%) and PANI/clay composites (PACCs) with 0.5 wt% clay. Reprinted from Ref. [42], Copyright (2012), with permission from Elsevier.

(PACCs) [42]. PAGCs showed outstanding barrier properties against water vapor and oxygen as compared with neat PANI and PACCs (Fig. 19.5), a consequence of the relatively higher aspect ratio of conductive 4-aminobenzoyl group-functionalized graphene (ABF- G) than that of organophilic clay (nonconductive filler), which resulted in a better dispersion of graphene within the polymer matrix and also lengthened the effective pathway of gas diffusion [42].

Rubber Nanocomposite Coatings

Goldberg and coworkers have patented a water-based approach for forming elastomeric (preferably butyl containing) nanocomposite coatings with high loadings of high-aspect-ratio platelets (water- based vermiculite dispersion), which were specifically designed for gas barrier applications [45,46].

Takahashi et al. applied coatings of butyl rubber/vermiculite clay nanocomposites on a poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)-coated anapore ceramic disk [47]. The gas permeability of butyl rubber decreased with the incorporation of clay, and a remarkable decrease (higher than 97%) was observed with 30 wt% loading of vermiculite clay against all five gases (H2, He, N2, 02, and C02). Vermiculite is a layered silicate that can disperse in water, like MMT, but has a much higher aspect ratio compared to MMT [48]. Moreover, proper dispersion of vermiculite clay in the aqueous butyl rubber matrix and good alignment of layered silicates even at very high concentrations results in good barrier performance of the coated material [47].

Polyurethane Nanocomposite–Based Gas Barrier Coatings

PU- or polyurea-based one-pack or two-pack coating systems of American Society for Testing and Materials (ASTM) are extensively used for improving the chemical or corrosion resistance of substances [49, 50]. More recently, gas barrier PU nanocomposites have become more popular because of the better performance of coatings in terms of additional functionality. For coating purpose, different solvent-based PU solutions or a waterborne polyurethane (WBPU) solution may be used.

Joshi et al. observed a significant improvement in the hydrogen gas barrier property of PU/clay (3 wt%) nanocomposite-coated nylon fabric compared with that of neat PU-coated fabric [8]. In another study, 41% and 32% reduction in helium gas permeability was observed for PU/clay nanocomposites reinforced with two different organoclays, Cloisite 30B and Claytone APA, respectively [15]. Here, the increase in the tortuosity of gas diffusion played the main role in improving the barrier property.

Rahman et al. used a series of WBPU/clay nanocomposite- based coating formulations to coat a nylon fabric. The water vapor permeability (WVR) values of the coated nylon fabric increased with increasing temperature [20]. However, both the gas permeation rate and the WVR of the coated fabric reduced significantly with increasing clay concentration. This improvement in the gas barrier property was a combined effect of three factors: (i) increase in the tortuous path length for gas diffusion, (ii) increase in Ts of PU, and (iii) increase in the storage modulus of PU due to tighter chain packing. Stratigaki et al. developed economic and ecofriendly gas barrier coatings based on waterborne acrylic and PU emulsions by incorporating different nanoclay suspensions [11]. The waterborne resins formed colloids having a core-shell structure with a hydrophobic core and a hydrophilic shell. The C02 gas barrier property of the coated substrate was enhanced significantly with the incorporation of both modified and unmodified clay due to the proper dispersion of clay platelets in resins.

In an interesting study, Osman et al. investigated the permeation behavior of water vapor and oxygen through a PU/organoclay nanocomposite adhesive [51]. They reported that the transmission of water vapor through the nanocomposite adhesive was much less than the transmission of oxygen, while the same amount of layered silicates was used for both cases. Clustering of water vapor during diffusion by the formation of hydrogen bonding with a PU matrix resulted in better barrier properties against water vapor. Moreover, the transmission rate was also affected by the differences in the hydrophobicity of the organomodifier [51].

More recently, different UV-cured and moisture-cured PU or PU nanocomposite coatings have been much in demand for anticorrosion applications [52, 53]. Moller et al. developed a simple and economical process for the preparation of UV-curable cationic PU (ccPU)-based oxygen gas barrier coating that is flexible and transparent [54]. With the addition of a ccPU dispersion in aqueous suspensions of two different types of clay (natural MMT and Li-HEC synthetic clay), hybrid materials were formed (O-MMT and O-HEC) and coated on a substrate (Fig. 19.6). Subsequently, they were placed under UV radiation to obtain a transparent, highly flexible oxygen gas barrier film. The gas barrier performance of the O-HEC hybrid was better compared to that of the O-MMT hybrid by almost 1 order of magnitude [54].

Steps for the preparation of a UV-curable PU/clay nanocomposite-based barrier coating [54]

Figure 19.6 Steps for the preparation of a UV-curable PU/clay nanocomposite-based barrier coating [54].

Epoxy Nanocomposite–Based Coatings

Thermosetting epoxy resins are commonly used for corrosion- resistant coatings due to their ability to strongly adhere to metallic substrates and their excellent chemical resistance. Use of clay- or graphene-reinforced epoxy nanocomposite is reported to provide better corrosion resistance.

Yeh et al. synthesized a series of siloxane-modified epoxy/MMT nanocomposite coating formulations through a thermal ringopening polymerization using l,3-bis(3-aminopropyl)-l,l,3,3-tetra- methyldisiloxane as a curing agent [55]. The epoxy/MMT nanocomposite showed better corrosion resistance over CRS compared to a pure epoxy coating. It was a consequence of a significant improvement in the oxygen and water vapor barrier property. Additionally, the epoxy/MMT nanocomposite materials showed a much higher glass transition temperature (Tg), lower water absorption, lower cure shrinkage, and higher tensile strength [55].

Li et al. adopted a facile spray-coating of epoxy/modified graphene (MG) nanocomposite on a polyimide film to produce high-gas-barrier films [56]. The preparation method allowed the MG to disperse properly in an epoxy matrix even at a larger volume fraction, which resulted in a high effective aspect ratio of highly exfoliated MG nanosheets showing liquid-crystalline order (Fig. 19.7a,b). After curing of the epoxy/MG nanocomposite, it showed significant improvement in the oxygen gas barrier property (Fig. 19.7c,d), along with good thermal stability and good electrical conductivity [56].

ТЕМ images of cross-sectional epoxy/MG films at different loadings of MG

Figure 19.7 ТЕМ images of cross-sectional epoxy/MG films at different loadings of MG: (a) 2.4 vol%, (b) 3.6 vol%; (c) oxygen transmission rate (OTR) and (d) oxygen permeability of pure polyimide film, epoxy/MG (3.6 vol%) coating on a polyimide substrate, and epoxy/MG (3.6 vol%) coating alone at two different humidity conditions. Reproduced from Ref. [56], with permission of the Royal Society of Chemistry.

Kim et al. coated epoxy-ZrP (a-zirconium phosphate) nanocomposites on various inorganic layers that were deposited on a plastic substrate by atomic layer deposition or sputter coating (Fig. 19.8a) [57]. The epoxy-ZrP nanocomposite solution was bar- coated on the inorganic layers and cured thermally. The water vapor transmission rate (WVTR) of uncoated inorganic layers varied from 10“1 g/m2/day to 10-4 g/m2/day. In the coating solution, the ZrP nanoplatelets were well dispersed in the epoxy matrix, which resulted in a dramatic decrease (up to 92%) in the WVTR through the epoxy-ZrP-coated inorganic layers (Fig. 19.8b,c) [57].

Gas Barrier Layer-by-Layer Assembly of Polymer Nanocomposites

Transparent polymeric coatings with very low oxygen transmission rates are needed for food packaging and flexible organic

(a) A schematic representation of different layered structures

Figure 19.8 (a) A schematic representation of different layered structures

of coated samples; IS (substrate + inorganic layer), EIS (substrate + inorganic layer + epoxy coating), and EZIS (substrate + inorganic layer + epoxy-ZrP nanocomposite coating); (b) cross-sectional ТЕМ image of the epoxy-ZrP nanocomposite coating layer; (c) WVTR values of IS, EIS, and EZIS samples [57]. Reprinted from Ref. [57], Copyright (2017), with permission from Elsevier.

light emitting diodes to achieve the requirements of sufficient performance and lifetime [58]. Though vapor-deposited thin films, such as А120з or SiOx, perform well in terms of gas barrier property, they are prone to cracking when flexed or require special processing conditions (e.g., vacuum). Their adhesion property is poor with plastic substrates, and also the fabrication process is very complex when they are layered with polymers [59, 60]. Therefore, polymeric LBL assembly is getting more attention for many gas barrier applications that show a potential to overcome some of the limitations of metallic thin film or coating. LBL assembly is a method of building multilayered and multifunctional thin films via exposure of a substrate to cationic and anionic liquid (generally aqueous) mixtures alternately [61].

(a) Schematic of a layer-by-layer assembly of a polymer

Figure 19.9 (a) Schematic of a layer-by-layer assembly of a polymer

(positively charged) and clay (negatively charged) to form "brick-wall" multilayer films with ultrahigh oxygen barriers, (b) Oxygen permeability values (with actual permeability values on top of each bar) for brick- wall films of various bilayer numbers formed from branched PEI and sodium MMT at pH 10. (c) The inset shows a schematic the 10-bilayer film and highlights the excellent exfoliation achievable through this fabrication technique. Reprinted from Ref. [62], Copyright (2011), with permission from Elsevier.

LBL-assembled polymer nanocomposite films or coatings show a good potential to improve the gas barrier property of a substrate [62]. Grunlan and coworkers are pioneers in fabricating LBL assemblies of different polymer/clay nanocomposite films with high structural order and tailorable oxygen permeability. In two studies [63,64], they reported LBL-assembled films of poly(ethylene imine)/MMT and poly(acrylamide)/MMT where 02 permeability and transmission rate were below the instrument detection limit while the thickness of the LBL assembly was only a fraction of 1 micron. An oxygen permeability of <2 x 10-9 cc m-1 day-1 atm-1 was obtained with a 70-bilayer film of a thickness of only 230.75 nm (Fig. 19.9) [63]. Moreover, oxygen permeability was dependent both on the pH of the polymer solution and on the number of bilayers in the film, which could tailor the oxygen transport property of the films to any desired value within a broad range.

Followed by previous studies, the Grunlan group developed a four-quadlayer-flexible-LBL-assembled film (thickness of merely 51 nm), where each quadlayer contained polyethylenimine (PEI)/ poly(acrylic acid)/PEI/MMT that exhibited virtually undetectable oxygen transmission when coated on a polyethylene terephthalate (PET) film substrate. Then oxygen permeability of this thin LBL- assembled nanocomposite film was several orders of magnitude lower than a 25,400 nm (1 mil) thick film of EVOH, which is generally considered as the best food packaging polymers when high oxygen barriers are required [65].

 
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