Improvement in Dispersion of Phosphate Pigments

Polymer-Plastics Technology and Engineering

ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20

Improvement in Dispersion of Phosphate Pigments Modified by Functionalized Graphite Nanoplatelets and Its Influence on Epoxy Coating Adhesion Somayeh Mohammadi, Faramarz Afshar Taromi, Homeira Shariatpanahi & Jaber Neshati To cite this article: Somayeh Mohammadi, Faramarz Afshar Taromi, Homeira Shariatpanahi & Jaber Neshati (2015) Improvement in Dispersion of Phosphate Pigments Modified by Functionalized Graphite Nanoplatelets and Its Influence on Epoxy Coating Adhesion, PolymerPlastics Technology and Engineering, 54:11, 1144-1151, DOI: 10.1080/03602559.2014.986807 To link to this article: http://dx.doi.org/10.1080/03602559.2014.986807

Accepted online: 20 Mar 2015.

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Date: 27 September 2015, At: 00:28

Polymer-Plastics Technology and Engineering, 54: 1144–1151, 2015 Copyright # Taylor & Francis Group, LLC ISSN: 0360-2559 print/1525-6111 online DOI: 10.1080/03602559.2014.986807

Improvement in Dispersion of Phosphate Pigments Modified by Functionalized Graphite Nanoplatelets and Its Influence on Epoxy Coating Adhesion Somayeh Mohammadi1 and Jaber Neshati3

, Faramarz Afshar Taromi2, Homeira Shariatpanahi3,

1

Department of Chemistry, Amirkabir University of Technology, Tehran, Iran Department of Polymer Engineering, Amirkabir University of Technology, Tehran, Iran 3 Corrosion Department, Research Institute of Petroleum Industry, Tehran, Iran

Downloaded by [Dumlupinar Universitesi] at 00:28 27 September 2015

2

GRAPHICAL ABSTRACT

Tripolyphosphate corrosion inhibitor was modified by functionalized graphite nanoplatelets to obtain a hybrid nanoparticle (functionalized graphite nanoplatelets–tripolyphosphate) with homogenous dispersion in epoxy coating. The effect of functionalized graphite nanoplatelets–tripolyphosphate dispersion on adhesion and anticorrosion behavior was discussed. Characterization analyses of the hybrid nanoparticle were performed by Fourier transform-infrared spectroscope, scanning electron microscope, and transmission electron microscope. Tripolyphosphate was linked to functionalized graphite nanoparticles by hydrogen bondings. Different epoxy coatings formulated with 1 wt% of functionalized graphite nanoplatelets, functionalized graphite nanoplatelets–tripolyphosphate, and tripolyphosphate were evaluated. Results showed, compared to traditional phosphate pigments, the adhesion of functionalized graphite nanoplatelets–tripolyphosphate epoxy coating and its anticorrosion behavior were significantly increased with the lowest loadings amounts. Keywords Adhesion; Carbon steel; Corrosion protection; Dispersion; Functionalized graphite nanoplatelets; Tripolyphosphate

INTRODUCTION Epoxy resins are extensively used as surface coatings because of their exceptional characteristics like good adhesion

Address correspondence to Faramarz Afshar Taromi, Department of Polymer Engineering, Amirkabir University of Technology, Tehran, P.O. Box 15875-4413, Iran. E-mail: [email protected] Color versions of one or more of the figures in this article can be found online on at www.tandfonline.com/lpte.

to many substrates; moisture, solvent, and chemical resistance; low shrinkage on cure and outstanding mechanical properties[1]. It is generally accepted that the coating efficiency is dependent on the intrinsic properties of the organic film (barrier properties), the substrate/coating interface in terms of adherence, the pigments used, and the degree of environment aggressiveness[2]. To enhance the barrier properties of these polymeric coatings, many researchers have used various kinds of additives such as extenders and inorganic pigments[2–4]. Corrosion

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ADHESION OF FGNP–TPP/EPOXY NANO-COATING

inhibitors are also incorporated in organic coatings to provide active corrosion protection. Chromate, phosphate, nitrite, tungstate, and molybdate ions have been investigated in terms of their abilities to inhibit the corrosion of steel[5]. Among these, phosphate salts have some advantages compared to the other inhibiting agents such as lower cost, lower toxicity, and ready availability that led to their wide use in practice[5]. Great deals of studies have been devoted to the corrosion inhibition effects of phosphate compounds[6,7]. During the recent decades, classical toxic anticorrosive pigments like chromate and lead have been gradually replaced by zinc phosphate and related compounds[8]. Studies of the anticorrosion performance of zinc phosphate have led to contradictory results that are not yet conclusive[9,10]. The low solubility of zinc phosphate in water and its extremely coarse crystalline precipitates hinder the growth of effective protective films[11]. Reduction of particle size, chemical modification, and changing orthophosphate anion to tripolyphosphate (TPP) enhance the corrosion inhibition properties because of its improved water solubility[9,11]. One key feature of the most polyphosphate grade is the enhancement of phosphate content which enables excellent long-term protective behavior[12]. Polyphosphates have shown efficient corrosion inhibition. Moreover, inorganic TPPs constitute a promising novel group of nontoxic phosphate inhibitors for paints[8,11]. It is believed that TPP anions P3 O5 10 react with iron ions at anodic sites of metal substrate and constitute an insoluble layer containing mainly, ferric TPP[11]. This iron phosphate passive film is very hard and exhibits great adhesion to the steel substrate to prevent corrosion product formation (Fig. 1). However, since all of these anticorrosive phosphate pigments are micro-sized, they should be used in high values of 10–30% volume fraction or 40–60 wt% in organic coatings to be effective, and they lead to adhesion loss and poor mechanical properties[13]. In addition, leveling and dispersing

FIG. 1. Effect of STPP on preventing of carbon steel corrosion process, (a) Water with 250 ppm STPP after 15 days of immersion and (b) water without STPP after 15 days of immersion. Note: STPP, sodium tripolyphosphate.

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agents are needed for these pigments dispersions[8,9,13]. Nanoparticles build a recent class of fillers, and polymeric nanocomposites have gained much interest in the last 10 years[14–16]. Indeed, compared to conventional fillers, addition of nanoparticles into polymeric coatings offers a more economical method because of low loading and more durable protection against corrosion[17]. However, direct addition of nanoparticles in polymers does not necessarily lead to improvement of properties because of incompatibility between nanoparticles and polymers[1]. The performances of nanocomposites are greatly affected by the dispersion of nanoparticles in polymers and polymer– nanoparticle interfaces[1]. In recent years, graphene and graphite derivatives such as graphite nanoplatelets (GNPs) have garnered the most interest for nanomaterials[18]. Graphite nanoplatelets are not individual graphene sheets but comprise of multiple stacks of graphene layers[19]. While graphene and graphene oxide (GO) contain a monolayer of carbon atoms arranged in a two-dimensional lattice, GNPs have several graphene layers with high aspect ratio and are prepared using cost-effective method[19]. They can initially be prepared from graphite that is naturally abundant and widely available[20]. Electrochemical and anticorrosion behaviors of functionalized graphite nanoplatelets (FGNP) in epoxy coating were investigated in our previous published report in detail[21]. FGNP was used as an efficient and compatible nanoparticle to produce homogenous epoxy nanocoating with impressive anticorrosion behavior for carbon steel[21]. The results showed that FGNP– epoxy coating had excellent adhesion to the metal substrate and protected it by physical barrier and passivation (iron oxide passive film) mechanisms[21]. In this study, TPP was modified with FGNP nanoparticles to achieve a hybrid FGNP–TPP nanoparticle. FGNP was used as a substrate for linking TPP corrosion inhibitor anions by hydrogen bondings. It was also used as a carrier for uniform distribution of single TPP anions in epoxy coating. The hybrid nanoparticle was capable of being thoroughly distributed in epoxy coating without requiring any dispersing and leveling agents. The adhesion of the coating containing FGNP–TPP was increased compared to traditional phosphate epoxy coating because of the presence of FGNP. This hybrid nanoparticle was used in lower amounts than micro-sized zinc phosphate and related compounds in paint formulations that is preferred from economic point of view. The characterization of the hybrid nanoparticle was performed by Fourier transform-infrared spectroscopy (FT-IR) and scanning electron microscope (SEM), and its dispersion in epoxy coating was investigated by transmission electron microscope (TEM). Different epoxy coatings formulated with 1 wt% of FGNP, FGNP–TPP, and sodium tripolyphosphate (STPP) were prepared and evaluated.

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Preparation of Hybrid FGNP–TPP Nanoparticles For the preparation of hybrid FGNP–TPP nanoparticles, after sonication of EG in water/ethanol solution for 8 h, STPP was added to the mixture and gently stirred at 300 rpm at 40°C to achieve a viscous homogeneous paste followed by drying in an oven at 40°C for 48 h. The weight ratio between FGNP and STPP was 1:1. The process of preparing FGNP–TPP is shown schematically in Figure 3.


FIG. 2. Schematic process of preparing FGNP from GICs. Note: FGNP, functionalized graphite nanoplatelets; GICs, graphite-intercalated compounds.

EXPERIMENTAL Materials The purchased graphite-intercalated compounds (GIC) was an expandable sulfuric acid-intercalated graphite Spec: 9950250 (Boading Action Carbon Co. Ltd), STPP technical grade of 85% was prepared from Sigma-Aldrich and 96% ethanol was prepared from Merck. Epoxy resin EPIRAN6 (EEW 185-196), H46 amine hardener (H active:100), and epoxy thinner T51 were prepared from Khuzestan Petrochemical Co., Pars Gohar Co., and Rangin Zereh Co., respectively. Carbon steel (ST37) plates were used as base metal for the coatings. Sodium chloride was obtained from Merck Company and aqueous solutions were prepared using double distilled water. Preparation of FGNP Nanoparticles Functionalized graphite nanoplatelet was prepared by the rapid thermal expansion (for 1 min) of GICs at 900°C under inert argon atmosphere to form expanded graphite (EG). Sonication of EG was carried out within [water/ethanol 25/75 (v/v%)] solution according to[22], in an ice bath for 8 h. The process of preparing FGNP from GICs is shown schematically in Figure 2.

Preparation of Coating Systems FGNP, FGNP–TPP, and STPP pigments at concentration of 1 wt% were incorporated into epoxy coatings hereafter named as FGNP epoxy, FGNP–TPP epoxy, and STPP epoxy, respectively. FGNP–TPP powder was first dispersed in epoxy thinner (T51) by sonication for 30 min, subsequently added into the epoxy resin and mixed for 2 h to ensure a homogeneous dispersion. FGNP epoxy was prepared by the same method. For preparing STPP epoxy, a mixture of the resin and STPP pigments were mixed by Perl Mills for 8 h to get homogenous distribution. To achieve the final coatings, amine hardener H46 curing agent was added to the mixtures. The coatings were applied by air spraying over sand blasted carbon steel panels (SSPC-SP3) of 15 cm × 10 cm and were evaluated after 14 days of curing at room temperature. The thicknesses of the dry films after subtracting the sand blast profile (ASTM D4417) (20 µm) were obtained 60 5 µm. Characterization The morphology of FGNP and FGNP–TPP nanoparticles was investigated using AIS-2100 SERON Co. SEM. For investigation of the interactions between FGNP and TPP, FT-IR was utilized with KBr pellets using a Bruker alpha FT-IR spectrometer in the range of 4000–500 cm1. Distribution homogeneity and the particle size of FGNP and FGNP–TPP pigments in the coatings were investigated using a CM 30, Philips Co. TEM instrument.

FIG. 3. Schematic process of preparing FGNP–TPP. Note: FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.



Coatings adhesions were measured using a direct pull-off adhesion test method in accordance with ASTM D4541 type III by a Posi Test-AT-N digital adhesion tester. The corrosion resistances of the coatings were studied in a salt spray test cabin ERICHSEN according to ASTM B117. The coated samples were exposed to a direct spray (0.7 bars) of NaCl 5 wt% solution at 35°C for 650 h. RESULTS AND DISCUSSION Fourier Transform-Infrared Spectroscopy Fourier transform-infrared spectroscopy contains some of oxygenated functional groups, which means that it is not made of pure graphene sheets (monolayers of sp2-hybridized carbon atoms arranged in a two-dimensional lattice). There are still some oxygenated groups on its surface and this is related to the production process of GIC with strong acid (H2SO4 and HNO3) treatments. Rapid heating (in argon) is believed to cause various small molecule species (CO, CO2, water) to evolve and increase internal pressure, forcing the sheets apart, and yielding EG[19]. Hence, thermal expansion of GIC in an inert gas can remove a large numbers of oxygenated groups. By this method, just some of the oxygenated groups primarily as carboxylic moieties, hydroxyl, and epoxide, retain on EG and specially FGNP[21]. To check the way that TPP anions bind to FGNP, FT-IR spectra of FGNP, STPP, and FGNP– TPP were recorded (Fig. 4).

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A major difference between unmodified FGNP and FGNP–TPP is the stretching of O–H peak at 3436.43 cm1 in FGNP shifted to a lower wavenumber (3371 cm1) and flattened in FGNP–TPP nanoparticles. This indicated that the interactions between FGNP and TPP were because of hydrogen bondings[23]. In addition to P–O bonds, polyphosphate compounds contain bridged P–O–P bonds that limit bond movement. The frequencies of the TPP anion are assigned on the basis of the characteristic vibrations of the P–O–P bridge, PO2, PO3 groups, and P¼O stretching. Bands in the 1300– 1200 cm1 range correspond to P¼O stretching mode. The related bands to the symmetric and antisymmetric stretching frequencies of PO2 and PO3 in TPP are generally observed in the region 1190–1010 cm1, and the observed bands in the domain 970–840 cm1 are attributed to the antisymmetric and symmetric P–O–P stretching modes[24]. In STPP spectrum (Fig. 4b), these characteristic bands can be observed: in 1214 cm1 (P¼O stretching), 1168–1010 cm1 the range for symmetric and antisymmetric stretching vibrations in PO2 and PO3 groups and then 895 and 952 cm1 for antisymmetric and symmetric stretching of the P–O–P bridge. On the other hand, FGNP–TPP nanoparticles spectra (Fig. 4c), showed PO2 and PO3 in 1159–1129 cm1 range and P–O–P bridge in 894 cm1, respectively, which were absent in unmodified FGNP. These are the characteristic peaks

FIG. 4. FT-IR spectra of (a) FGNP, (b) STPP, and (c) FGNP–TPP. Note: FGNP, functionalized graphite nanoplatelets; STPP, sodium tripolyphosphate; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

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for polyphosphates, indicating the presence of TPP in FGNP–TPP[24]. The band in 1214 cm1 corresponds to P¼O stretching mode shifted to lower wavenumbers (1189 cm1) in FGNP–TPP which indicated a decrease in the electron density of P¼O bond. Thus, demonstrated that the interactions with FGNP took place through the lone electron pairs of the oxygen atoms in TPP anion. Therefore, it can be concluded that TPP anions interacted strongly with the hydroxyl groups of the FGNP through hydrogen bondings[25]. Schematic hydrogen bonding interactions between FGNP functional groups and TPP anion is shown in Figure 3.

no any STPP particles aggregations on FGNP surfaces after drying. This indicated that single TPP anions were linked to the functional groups of FGNP. During the preparation process of hybrid FGNP–TPP, micro-sized STPP particles were completely dissolved in water/ethanol mixture leading to linkages of single TPP anions to FGNP nanoparticles using hydrogen bondings.

Scanning Electron Microscope Figure 5a and 5b shows SEM micrographs of FGNP and FGNP–TPP, respectively. A droplet of epoxy thinner dispersions of FGNP and FGNP–TPP was poured into a specific support separately and allowed to evaporate to obtain the micrographs. The micrographs revealed that there was no change in FGNP morphology in the presence of TPP and there were

Transmission Electron Microscope Transmission electron microscope analysis was carried out for further investigations of FGNP–TPP particle size and distribution in the epoxy coating. TEM images of the cured 1 wt% FGNP–TPP epoxy coating, shown in Figure 6a and 6b, confirmed the uniform dispersion of the hybrid nanoparticles in the epoxy coating. Indeed, they showed that the presence of TPP on the surface of FGNP did not have any negative effect on FGNP–TPP distribution. In the TEM images, the light background is referred to the epoxy and the dark spots inside are related to the FGNP–TPP nanoparticles. The average particles sizes were about 20–40 nm with a smooth and flat overlapping structure.

FIG. 5. SEM micrograph of (a) FGNP and (b) FGNP–TPP. Note: SEM, scanning electron microscopy; FGNP, functionalized graphite nanoplatelets; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

FIG. 6. TEM image of (a) 1% FGNP epoxy coating and (b) 1% FGNP–TPP epoxy coating. Note: TEM, transition electron microscopy; FGNP, functionalized graphite nanoplatelets; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

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TABLE 1 Adhesion loss (%) values of the samples exposed to 3.5 wt% of NaCl solution after 180 days of immersion Sample


Dry pull-off strength (MPa) Wet pull-off strength (MPa) Pull-off strength loss (%)

Neat epoxy

STPP

FGNP

FGNP–TPP

5.88 1.94 67.2

5.72 2.93 48.7

11.62 9.36 19.4

10.67 9.05 15

Adhesion Strength Measurements A digital pull-off test was used to test possible changes in adhesion as well as quantifying coating/metal substrate adhesion. Dollies with 0.5 cm2 in area were bonded to the coating using an appropriate adhesive. The glued dollies were then allowed to dry for 48 h in room temperature. A digital adhesion tester was used with a maximum applied load of 20 MPa. All measurements were carried out in triplicate. High adhesion strength of the coatings is a basic requirement for good corrosion protection[26]. Adhesion is quantified in terms of forces used to detach the test aluminum dollies glued to the film from the underlying metal. The pull-off strength values of the coatings were measured after 180 days of immersion in NaCl 3.5%. Table 1 shows the results of the adhesion tests for the coatings. The pull-off strength loss values were calculated by Eq. (1): Pull off strength lossð%Þ ¼

ab 100 a

ð1Þ

where a and b are the dry pull-off strength and wet pull-off strength, respectively. As can be seen in Table 1, the highest and the lowest values of pull-off strength loss were observed for epoxy and FGNP–TPP coatings, respectively. From these results, it is obvious that FGNP epoxy coating adheres more strongly to the metal substrate than the pristine epoxy. This excellent adhesion can be described by the presence of oxygenated functionalized groups on the FGNP surface. They make a homogeneous distribution of FGNP in epoxy resin (polar resin) by hydrogen bondings and polar–polar interactions. This means that nano-sized FGNP with ideal distribution in epoxy resin increases contact areas of the

nanocoating to the metal substrate. Excellent adhesion occurs because of the interactions of lone electron pairs of oxygen atoms on FGNP surface and empty d-orbitals of the metal atoms. In the case of FGNP–TPP epoxy coating, because of the presence of FGNPs as substrates and carriers for TPP anions and their excellent distributions in epoxy coating, ideal adhesion of the nanocoating to the carbon steel substrate could be achieved in comparison with traditional STPP epoxy coating with micro-sized phosphate pigments. The pull-off strength loss value for FGNP–TPP epoxy coating was lower than FGNP epoxy coating. This illustrated that disbonded area in the presence of FGNP–TPP was less than FGNP. In fact, formation of the intact iron phosphate passive layer on the metal surface prevents hydroxide ions creation and cathodic delamination. The photographs of the coatings morphologies after 180 days of immersion in 3.5 wt% of NaCl solution are shown in Figure 7. As can be seen, there are many blisters and some corrosion products on neat epoxy coating surface that where created by the low barrier property. In the case of STPP epoxy coating, some blisters can also be seen after 180 days of immersion, those have been created by high water solubility of STPP pigments. FGNP and FGNP–TPP showed no sign of any blisters or corrosion products after 180 days of immersion. After immersion for 180 days, the adhesion strength loss percentages were 67.2 and 48.7% for epoxy and STPP epoxy coatings, respectively, which result in significant loss of the protective property. However, FGNP epoxy and FGNP–TPP epoxy coatings had relatively low adhesion strength losses (19.4 and 15%, respectively). The surface morphologies after adhesion measurements are summarized in Figure 8. After immersion for 180 days, the

FIG. 7. Photographs of the coatings morphologies after 180 days of immersion in 3.5 wt% NaCl solution, (a) neat epoxy, (b) STPP epoxy, (c) FGNP epoxy, and (d) FGNP–TPP epoxy coating. Note: FGNP, functionalized graphite nanoplatelets; STPP, sodium tripolyphosphate; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

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FIG. 8. Surface morphologies after adhesion measurements, (a) neat epoxy, (b) STPP epoxy, (c) FGNP epoxy, and (d) FGNP–TPP epoxy coating. Note: FGNP, functionalized graphite nanoplatelets; STPP, sodium tripolyphosphate; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

epoxy coating peeled off almost entirely because of the low adhesion strength. A large number of corrosion pits and corrosion products appeared. In the case of STPP epoxy, the coating peeled off entirely because of low adhesion strength but no corrosion products appeared beneath STPP epoxy coating. The surface of FGNP and FGNP–TPP epoxy coating were only partially peeled off. It is clear that adding FGNP and FGNP–TPP nanoparticles to epoxy coating improve the adhesion of the coatings.

Salt Spray Test More severe conditions were used for the coatings. Figure 9a–d shows the visual performance of the samples exposed to salt spray test condition up to 650 h. After 650 h, disbonded area with a large number of blisters and also corrosion products were observed on the neat epoxy coating. Figure 9b shows the growth of blisters on the whole surface of STPP epoxy coating. However, the number of blisters and also corrosion products, produced near scratches, significantly reduced when the coating was loaded with FGNP. The best corrosion inhibition performance was observed in FGNP–TPP epoxy coating. As can be seen in Figure 9b and 9d, a white color film was produced at the coating/metal interface of STPP epoxy coating and also in the scratch areas of FGNP–TPP epoxy coating. This may be attributed to the passive layer consisting of metal complexes precipitated on the metal surface, which may be produced in the presence of TPP inhibitor. In scratch areas, in the presence of this protective film, the corrosion was minimal. CONCLUSION In this work, TPP was combined with FGNP to gain a hybrid nanoparticle with homogenous distribution in epoxy coating. Different epoxy coatings containing of 1 wt% of FGNP, FGNP–TPP, and STPP were prepared. Dispersion of nanoparticles in the coatings and coatings adhesion to the metal substrate were investigated. Results showed in comparison with the traditional micro-sized phosphate pigments that are used in high amounts to be effective, the hybrid nanoparticle results in excellent adhesion of the nanocoating and anticorrosion property for carbon steel in very small quantities. These properties are related to the presence of FGNP as a substrate and carrier for linkages of TPP anions.

FIG. 9. Surface appearance of the coated plates after exposing to salt spray for 650 h for (a) neat epoxy, (b) STPP epoxy, (c) FGNP epoxy coating, and (d) FGNP–TPP epoxy coating. Note: FGNP, functionalized graphite nanoplatelets; STPP, sodium tripolyphosphate; FGNP–TPP, functionalized graphite nanoplatelets–tripolyphosphate.

FUNDING The authors would like to acknowledge Research Institute of Petroleum Industry and Iran Nano Technology Initiative Council for the financial support during this research. The authors also gratefully acknowledge of Vahid Ashayeri from Aryogen company for helpful suggestions.


ORCID Somayeh Mohammadi 0493

http://orcid.org/0000-0002-7006-


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