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Structure, properties and corrosion resistivity of polymeric nanocomposite coatings based on layered silicates.

Abstract This paper reviews the recent research and development of polymeric nanocomposite coatings based on layered silicates. In the past few decades, extensive research activities have been conducted on clay minerals due to their unique layered structure, rich intercalation chemistry and availability at low cost, environmental stability, and good processability. One of the most important categories of layered silicates is nanoclays. The nanoclay is considered as reinforcement for polymers in the manufacture of low-cost, lightweight and high performance nanocomposite coatings. In this paper, we try to introduce the structure, properties, and surface modification of clay minerals. Different properties of polymer clay nonocomposite coatings may consist of conductive and nonconductive polymers. The corrosion resistance of each type is discussed separately. Some novel properties can be observed from the interaction of two dissimilar chemical components at the molecular level that posses enhancements in corrosion inhibition on metallic substrates. Finally, the prospective problems of industrial usage of these materials are mentioned.

Keywords Clay minerals, Montmorrilonite, Polyaniline, Conductive polymers
Abbreviations

CRS Cold-rolled Steel
CEC Cation Exchange Capacity
EPA Environmental Pollution Agency
ICPs Intrinsically Conductive Polymers
MMT Montmorillonite
PANi Polyaniline
PPO Polyphenylene Oxide
PPS Polyphenylene Sulfide
PPy Polypyrrole
PS Polystyrene
PSF Polysulfone
PCNs Polymer Clay Nanocomposites
PLS Polymer Layered Silicates
PSAN Poly(styrene-co-acrylonitrile)
SPI Soluble Polyimide
UV Ultra Violet


Introduction

Chromium-containing compounds have formed effective anticorrosive undercoatings for years, yet due to environmental and health concerns, they need to be replaced.(1-6) However, strict EPA environmental regulations require the elimination of the heavily used chromate inhibitors by the year 2007. (7) Therefore, some environmentally friendly and effective corrosion-inhibitive coatings need to be urgently considered for the shipping, aerospace, and automobile industries.

As a primary method for corrosion protection of metals, coatings make a barrier by preventing the diffusion of oxygen and water. However, there is no perfect barrier coating, and all coatings eventually fail due to existing pinhole defects. Other methods, therefore, have been developed. such as chemical inhibitors, cathodic protection, and anodic protection. (8) Moreover, a protective monolayer film can be formed on the metal surface that interferes with the cathodic or anodic reaction of the corrosion cell. (9)

Nanoscale materials have unique physical, chemical, and physicochemical properties, which may improve thecorrosion protection in comparison to bulk-size materials. It is well known that such nanoparticles create high specific surface area. This specification allows the uniform dispersion of nanoparticles into matrix materials with a low dosage so that the efficiency of nanocomposites can be significantly high in terms of material properties. (10)

Recently, organic-inorganic nanocomposites have attracted attention. Their remarkable properties and unique structure have led to the synthesis and study of a variety of hybrid composite materials. (11) The intercalation of inorganic materials by organic guest species is a way to construct an ordered organic-inorganic assembly. (12) Polymer intercalated nanocomposites, prepared by using layered materials, have a high degree of polymer ordering and exhibit advanced gas barrier, thermal stability, and enhanced mechanical properties compared with pristine polymers. (13), (14) Nanocomposites differ from traditional composites due to the complex interaction of the nanostructural heterogeneous phases. In addition, the properties of nanoscopic particles differ greatly from a macroscopic sample of the same materials. The combination of organic and inorganic materials may offer novel properties due to the synergistic effect of the two components.(15), (16) Since the commencement of the nanotechnology era, nanocomposites composed of polymers and inorganic particles have aroused much interest in the scientific community. To improve properties possessed by polymers and to generate new properties, researchers are formulating organic-inorganic hybrid materials using different polymers. (9)

Study of polymer clay nanocomposites is a functional field for many applications--not just exclusively directed towards improvement of mechanical properties of plastic materials--but also offers materials with other functions such as conductivity, membranes, corrosion protection, and fire retardant behavior.(17), (18) One of the most interesting features of polymer clay nanocomposites (PCNs) is the available variety of processing options. The variety is raised from characteristics of the polymer and mechanisms of interaction with the clay. (19)

A lot of research has been done on the integration of inorganic nanolayers like montmorillonite (MMT) clay into the organic polymeric matrices. The findings show that these materials, as a coatings layer, enhance the corrosion protection effect of steel and aluminum in comparison to pristine polymers. The primary effect of a polymeric coating is to act as a physical barrier against aggressive species such as [O.sub.2] and [H.sup.+] These inorganic materials with a plate-like shape are usually employed to effectively increase the length of the diffusion pathways for oxygen and water and decrease the permeability of the coating and lead to corrosion receptivity of coatings. (20-24)

Layered silicates, structure, and modeling

Layered silicate clays are an interesting class of filler materials. These minerals include both natural clays(e.g., MMT, hectorite, and saponite) and synthesized clays (e.g., fluorohectorite, laponite, and magadiite) while MMT and hectorite are the most widely used. (25)

There are different articles in the literature on organoclays, clay-polymer interactions, or the processing aspects of clay-polymer nanocomposites. (17), (25-35) The layered structure of clay minerals used for polymer nanocomposites can be classified into three main groups that include: (a) 2:1 type, (b) 1:1 type, and (c) layered silicic acids. (25)

2:1 Type

The clays belonging to the smectite family have a crystal structure consisting of nanometer thick layers (platelets) of aluminum octahedron sheet sandwiched in between two silicon tetrahedron sheets. Stacking of the layers leads to a van der Waals gap between the layers. Isomorphic substitution of Al with Mg, Fe, and Li in the octahedron sheets, and/or Si with Al in tetrahedron sheets, gives each three-sheet layer an overall negative charge. This charge is counterbalanced by exchangeable metal cations residing in the interlayer space, such as Na, Ca, Mg, Fe, and Li.

1:1 Type

Clays consists of layers made up of one aluminum octahedron sheet and one silicon tetrahedron sheet. Each layer bares no charge due to the absence of isomorphic substitution in either the octahedron or tetrahedron sheet. Thus, except for water molecules, neither cations nor anions occupy the space between the layers, and the layers are held together by hydrogen bonding between hydroxyl groups in the octahedral sheets, and oxygen in the tetrahedral sheets of the adjacent layers.

Layered silicic acids

Clays consist mainly of silicon tetrahedron sheets with different layer thicknesses. The basic structure is composed of layered silicate networks and interlayer hydrated alkali metal cations. (25).

Among the many layered materials, clay minerals (e.g., MMT) have been extensively investigated because they are natural, abundant, and inexpensive.(36) MMT--also known as 2:1 phyllosilicates--is usually used as a sorbent of organic compounds mainly because of the large surface area (760 X 10 e3 m[sup.2]/kg), the high cation exchange capacity (CEC) (~1 mol k[g.sup.-1] molar monovalent cations), and relative ease of forming interlayer complexes with a wide variety of organic molecules.(37-40) Due to the above advantages, MMT is a favorable candidate for reinforcing organic polymer materials. (41) As shown in Fig. 1, the layers of clay align themselves in a parallel manner, form stacks, and areattracted to each other by a weak van der Waals force. The gap between the layers is called the gallery and the distance is called [d.sub.001] spacing, which can vary over a wide range depending on the size of the cations adsorbed.

[FIGURE 1 OMITTED]

The nature of pristine clay is hydrophilic and the increase in interlayer spacing that occurs with swelling of the Na+-MMT clay in water causes the particles to be penetrated by relatively large molecules. (42) The normally hydrophilic silicate surface must be converted into an organophilic one to render layered silicates miscible with other polymer matrices and make the intercalation of many engineering polymers possible. Generally, this can be done by ion-exchange interactions with cationic onium surfactants, including primary, secondary, tertiary, and quaternary alky1 ammonium or alkylphosphonium cations. (43), (44). These organic clays are more compatible with polymers and swell in a polar solvent and easily exfoliate and disperse in polymers. (45)

Two particular characteristics of layered silicates are generally considered for nanocomposites. The first is the ability of the silicate particles to disperse into individual layers. The second characteristic is the ability to fine-tune their surface chemistry through ion exchange reactions with organic and inorganic cations. These two characteristics are, of course, interrelated, because the degree of dispersion of nanolayered silicate in a particular polymer matrix depends on the interlayer cation. (34)

Most of the literature about the structure and modeling of layered silicates in the nanocompositeshas focused on the type of the intercalant and the compatibility of the clay with different polymers. The processing of the clay with different functional groups, however, has attracted the most attention.

Interaction between layered silicate and polymers

The interaction among the organic and inorganic components is important because it controls the ultimate properties of the nanocomposite materials. (9) A main group of nanocomposite materials is constituted by those in which the organic polymer is confined in the inorganic layers. The inorganic layered materials exist in great variety with ordered intra-lamellar space that is potentially accessible by foreign species. They can act as matrices or hosts for polymers, yielding interesting lamellar organic-in-inorganic nanocomposite materials. (46)

The physical mixing of a polymer and layered silicate may not form a nanocomposite. This situation is comparable to polymer blends, where in most cases phase separation occurs. One of the differences between the nanocomposites and traditional composite is that no polymer chain penetrates into the gallery of the layered silicate, and the polymer and layers are immiscible. (13), (17), (30) In immiscible systems that typically correspond to the more conventionally filled polymers, the poor physical interaction between the organic and the inorganic components leads to poor mechanical and thermal properties. In contrast, strong interactions between the polymer and the layered silicate in nanocomposites lead to the organic and inorganic phase dispersion at the nanometer level. As a result, nanocomposites exhibit unique properties not shared by their micro counterparts or conventionally filled polymers. (28), (47-50)

The low-weight percentage of properly dispersed layered silicates throughout the polymer matrix can create a much higher surface area for polymer/filler interaction in comparison to the conventional composites. Depending on the strength of interfacial interactions between polymer matrices and layered silicates (modified or not), the following three different types of polymer layered silicate nanocomposites are thermo-dynamically achievable (see Fig. 2).

[FIGURE 2 OMITTED]

(a) Intercalated nanocomposite: The insertion of a polymer matrix into the layered silicate structure occurs in a crystallographically regular fashion, regardless of the clay-to-polymer ratio. Intercalated nano-composites are normally interlayered by a few molecular layers of polymer. Properties of these nanocomposites typically resemble those of ceramic materials.

(b) Flocculated nanocomposites: Conceptually, these are similar to intercalated nanocomposites. However, silicate layers are sometimes flocculated due to hydroxylated edge-edge interaction of the silicate layers.

(c) Exfoliated nanocomposite: Individual clay layers are separated in a continuous polymer matrix by an average distance that depends on the clay loading. Usually, the clay content of an exfoliated nanocomposite is much lower than that of an intercalated nanocomposite. (51) It has been claimed that a completely exfoliated morphology is required to achieve the highest properties, such as mechanical and conductivity. (46), (52)

Enhancement of material properties has been linked to the interfacial interaction between the polymer matrix and the organically modified layered silicate structure. (46) The factors that control the interaction between a particular organoclay and polymer include the cation exchange capacity (CEC) of the clay, the polarity of the reaction medium, and the chemical nature of the interlayer cations (e.g., onium ions). Proper selection of the modified clay is essential to ensure effective penetration of the polymer/monomer into the interlayer spacing of the clay, which results in the desired exfoliated or intercalated product. Further development of the compatibilizer chemistry is undoubtedly the key to the expansion of this nanocomposite technology. (53)

It is well-known that nanocomposites usually exhibit superior physical properties such as solvent resistance, ionic conductivity, (19) optical properties, (54) heat resistance, (55) decreased gas permeability, (56) and flammability, (57) better process ability, (58) thermal stability and UV susceptibility compared with various commercial polymers, (50), (59), (60) or conventional polymer composites. They also show better mechanical properties such as tensile modulus and strength which may be due to the synergy caused by host-guest interactions. (17)

Most of the works in this field focus on the morphology of the nanocomposites and different processing of the polymer and the layered silicates (i.e., in situ polymerization, melt interaction, and solution interaction). The other interesting subjects in this field are the characterization techniques and evaluation of properties.

Corrosion resistivity of nanocomposite coatings based on conductive polymers and layered silicates

There are many references about the corrosion resistance of electroactive polymers such as polyaniline (PANi) (61-72) poly-n-ethylaniline, (73) polyphenylene oxide, (74) and poly (2,5-bis (N-methy1-N-hexylamine) -p-phenylene vinylene (75) as pigment in paint or coatings. Because the mechanisms of corrosion protection in conductive polymers are complex and affected by many factors, several mechanisms have been proposed to explain the nature of protection with these materials. Some researchers believe that the ICP coatings protect aluminum and steel simply by creating a barrier mechanism, but others say that ICP coatings aid in the formation of a passive oxide film on the metal surface through an oxidation-reduction process. (67), (68) Another mechanism for corrosion protection is suggested to be anodic (i.e., the polyaniline film withdraws change from the metal and passivates its surfaces against corrosion). (76) Based on this mechanism, the role of polyaniline in active corrosion protection is due to its ability to intercept electrons on the metal surface and to transport them to the outside of the coating. (77) It is believed that the presence of layered silicates improves the corrosion resistance of the ICP coatings, not only with decreasing the coating porosity and decreasing permeability of gases and liquids, but also with stabilizing their electronic structure. (9) Recently, researchers have found that incorporation of theinorganic nanolayers of MMT clay into the polymeric matrix effectively enhance the corrosion protection of pristine conducting polymers (e.g., PANi (20), (78) poly (0-methoxyaniline), (79) poly (0-ethoxyaniline), (21) poly (3-alkylthiophene), (80) polypyrrole, (81) and pani-thiokol rubber. (82) These findings have resulted from a series of electrochemical corrosion measurements in different corrosive environments.

Among the conducting polymers, incorporation of layered silicates into the PANi and polypyrrole (PPy) has been of particular interest. These two polymers have a wide application and they have been evaluated as anticorrosive coatings for about two decades. (83-89)

Chang and coworkers (90) presented the first evaluation of the corrosion protection effects of a series of PANi/Na+-MMT clay nanocomposites by in situ emulsion polymerization through standard electrochemical corrosion measurements. They reported significant improvement in corrosion resistance of the PANi coating on cold rolled steel (CRS) with the incorporation of clay.

As shown in the schematic view of the nanocomposite coating based on PANi and clay (see Fig. 3), the presence of clay in nanoscale dispersion lengthens the diffusion path of corrosive media and improves the barrier properties. (9)

[FIGURE 3 OMITTED]

As shown in Fig. 3, clay also has a possible positive effect on protecting PANi from UV damage. In addition, clay platelets will improve the thermal stability of the ICPs. Consequently, the service life of the conducting polymer coatings can be significantly improved by the incorporation of clay.

In spite of the numerous studies about the corrosion resistance properties of PCN coatings based on ICP and layered silicates, the unsolved problem is the issue of electrical conductivity. Electrical conductivity of ICPs has always been of great interest to scientists; however, the effect of clay incorporation (a dielectric material) on the conductivity of the compound is unclear. Although the clay layers can be regarded as insulators, (25) these minerals in the conductive polymer matrix exhibit unique electrical properties due to their ionic conductivity. It is an interesting phenomenon inthe PCN coatings that the incorporation of clay did not decrease the conductivity of ICP--on the contrary, the conductivity is enhanced. (9), (29), (91) It is proposed that, to a certain degree, nano-sized clay platelets carrying a negative charge serve as a dopant in the conducting polymer, and facilitate the delocalization of electrons, thus improving the conductivity. (9) The relationship between ionic and electrical properties with corrosion protection needs to be additionally investigated in future research projects.

Corrosion resistant properties of nanocomposites based on nonconductive polymer-layered silicates

There are also some references about the corrosion resistance of nanocomposite coatings based on layered silicates and conventional polymers like polysulfone (92) and other thermoplastic polymers such as poly methylmethacrylate, (23) polystyrene, and thermosetting polymers such as polyimide, (22), (93) epoxy, (94) and polyurethane. (95), (96)

Chen et al. (97) described the processing and different properties of epoxy layered-silicate nanocomposites as corrosion resistant coatings on Al (AA2024-T3) surfaces. They concluded that there is small improvement in the anticorrosion properties for the exfoliated nanocomposite coatings, and no improvement for the intercalated nanocomposites. It seems that these criteria are better related to the dispersion of silicate nanosheet in some epoxy matrix than the other grades of resin matrixes.

Some researchers have studied the corrosion resistance of polystyrene (PS)-clay and poly (styrene-co-acrylonitrile) (PSAN) nanocomposites with quaternary alkyl ammonium salt as an intercalating agent. They concluded that these types of nanocomposite coatings on CRS were found to be a much superior anticorrosion over those of bulk PS and PSAN. Their conclusion was based on a series of electrochemical measurements of corrosion potential, polarization resistance, and corrosion current in 5 wt% aqueous NaCl electrolyte. Enhancement of anticorrosion properties of PS-claynanocomposites incorporated with low clay loading (e.g., 1 wt%) compared to bulk PS and PSAN might be comprised from dispersion of silicate nanolayers of clay in a polymer matrix. This enhancement is created by increasing the tortuosity of diffusion pathway for oxygen and water. (98), (99)

Yu et al. (22) have evaluated the anticorrosive properties of organo-soluble polyimide (TBAPP-PDA)/clay nanocomposite coatings derived from the solution dispersion technique. Based on a series of electrochemical measurements in aqueous NaCl electrolyte, they found that these soluble nanocomposite materials with low clay loading have much superior anticorrosion properties that those with soluble polyimide (SPI) coatings. The effects of the material composition on barrier properties and optical clarity have also been studied. Gas permeability analyses show that the incorporation of clay platelets into an SPI membrane causes an enhancement of [0.sub.2] and [H.sub.2]O molecular barrier properties. Due to the better dispersion of nanoclay and more smooth surface morphology, the insoluble polyimide clay nanocomposite membranes have better barrier properties than the soluble polyimide clay nanocomposite membranes. Higher clay loading in SPI membranes leads to a significant decrease of optical clarity, as obtained from the UV-visible transmission spectra studies. (22)

Diffusion of moisture through vinyl ester nanocomposite coatings composed of MMT clay has been studied. It was found that water diffusivity is decreased by increasing clay content. Regardless of the nature of clay surface treatment, decreasing water diffusivity to half of its value in the neat resin will occur when the clay content is only 1 wt%. (100)

Siloxane-modified epoxy resin clay nanocomposite coatings with advanced anticorrosive properties have been prepared by a solution dispersion method. Better protection against corrosion on CRS coupons has been observed in comparison to that of bulk epoxy resin. Molecular permeability (e.g., [O.sub.2], [N.sub.2], and [H.sub.2]O), through epoxy resin-clay nanocomposite membranes, was lower than the bulk epoxy resin, along with the loading of nanoclay. These findings resulted from the gas and vapor permeability analysis. (101)

Yeh et al. (92) studied the corrosion prevention properties of polysulfone-clay (PSF-Clay) nanocomposite materials prepared by a solution dispersion method. They found that the prepared PCN coatings with low clay loading (1 wt%) on CRS were superior in corrosion prevention compared with bulk PSF using different chemical and electrochemical measurements in a 5 wt% aqueous NaCl electrolyte. (92)

The corrosion resistances of polyphenylenesulfide (PPS)/MMT clay nanocomposite coatings have also been studied. It was found that MMt-PPS nanocomposite coating was effective in mitigating corrosion compared with MMT-free coating and, in fact, the uptake of corrosive ionic electrolyte by unmodified coating increased with an extended exposure time. (102)

Recently, the effects of using the organophilic MMT additives on the properties of epoxy coatings have been evaluated. It was concluded that the addition of organophilic layered silicates in epoxy systems results in improved coating properties such as adhesion, Persoz hardness, durability, gloss, water resistance, elasticity, and corrosion resistance. (103) In nonconductive polymeric nanocomposites, the addition of a small amount of suitable grade of clay increases the corrosion resistance and better protection of substrates in different conditions.

However, it seems that the research on the corrosion resistance of nonconductive PCN coatings is not complete and needs further efforts. Most of the previous work focused on the electrochemical and permeation properties without any attention to other specifications required for coatings, such as mechanical properties, adhesion, compatibility with other layers of coating systems, or type of surface pretreatment.

Conclusion

Recent studies on the use of conductive and nonconductive polymeric nanocomposites based on layered silicates have been reviewed. The use of clays within a composition of conductive polymer nanocomposites is a new area of research for obtaining coatings with tailored properties. The presence of the clay in nanoscale dispersion lengthens the diffusion path of corrosive media and improves barrier properties. Consequently, the lifetime of the conductive polymer coatings can be significantly improved by incorporation of the clay.

Thus, layered silicates play an important role in terms of providing barrier properties and high impedance for the coating systems, which lead to improvement in corrosion protection. It is believed that, due to strong homogeneous impervious layers, the good corrosion protection properties of PCN coatings prevent corrosive media from seeping through it. As a result, clay-based nanocomposite coatings may be considered energy-saving and environment-friendly materials. The future of these special coating markets will further expand in different industries such as marine, building, construction, and defense. Finally, it must be considered that for industrial and commercial application of PCN coatings, they must be robust systems available in sufficient quantities at an acceptable price. It also must provide an equivalent or better performance in the field. The ancillary equipment used to apply the coatings must be also available. The challenge that still remains is a better understanding of the process and their reorganization of characteristics, and finding a protection mechanism for these materials. By solving these problems, an environmentally friendly coating material at a reasonable cost will be achieved.

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D.Zaarei, A. A. Sarabi , F. Sharif, S.M.Kassiriha Polymer Faculty, Amirkabir University of Technology, Tehran, Iran

e-mail: sarabi@aut.ac.ir
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Title Annotation:REVIEW PAPER
Author:Zaarei, Davood; Sarabi, Ali Asghar; Sharif, Farhad; Kassiriha, Seid Mahmood
Publication:JCT Research
Article Type:Report
Date:Jun 1, 2008
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