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Synthesis of silver nanoparticles embedded novel hyperbranched urethane alkyd-based nanocomposite for high solid antimicrobial coating application.

Abstract In this study, we describe a simple method for the synthesis of a novel nanocomposite comprising hyperbranched urethane alkyd and silver nanoparticle (embedded in the resin) for a high-solid antimicrobial coating application. During the process, silver benzoate was dispersed in the hyperbranched alkyd (HBA) resin containing free hydroxyl groups and the resin is cured with an isocyanate trimer (Desmodur N3390) to make a silver nanoparticle-based nanocomposite coating. Silver benzoate is reduced by the free radicals, generated from naturally occurring oxidative curing of the fatty acid present in the alkyd resin. Unlike a conventional method, which involves the use of a toxic reducing agent or solvent, the present process does not require any toxic reducing agent for the generation of silver nanoparticles. The formation of silver nanoparticle was confirmed by spectroscopic and electron microscopic analysis. The HBA resin used in this work requires a much lesser amount of solvent for making a coating formulation and offers superior mechanical properties compared to the conventional alkyds. The surfaces coated with the nanocomposite coating showed excellent antimicrobial activity against Serratia marcescens bacteria.

Keywords Hyperbranched polymers, Nanocomposites, Antimicrobial coatings, Electron microscopy, Silver nanoparticles

Introduction

Protection of a structure against corrosion and biodegradation can be effectively provided by the application of antimicrobial coatings. (1,2) Resistance to biodegradation is important in many areas such as food industries, medical devices, water purifiers, buried pipe lines, marine environment, textile industries, and interior parts of houses. (1) The growth of algae and fungi can cause discoloration of paint films leading to the loss of integrity. (2,3) If the microorganisms are transferred to an environment that has favorable condition for growth, they multiply and eventually cause adverse effects to the environment. (3,4) The resistance to biological attack of paint is determined by the presence of an optimum concentration of a suitable antimicrobial agent. Various materials such as Ti[O.sub.2], (5) ZnO, (6) Au, (7) Si[O.sub.2] (8) Mg [(OH).sub.2], (9) CuO, (10) carbon nano tubes, (11) and chitosan (12) are used to achieve antimicrobial property of a coating. In recent years, silver nanoparticles (SNPs) have drawn considerable attention due to their superior antibacterial activity. (13-21) SNP attracts special attention, because it is a continuous reservoir for the release of silver ions. (14)

Since ancient times, silver has been used as a disinfectant and is particularly attractive because it offers a high toxicity to microorganisms and a low toxicity to human beings. (15) The processes which are used to incorporate SNP into a polymeric matrix can be divided into two categories namely an ex situ approach and in situ one. The ex situ approach involves the direct dispersion of SNPs in a polymer matrix which is extremely difficult due to the inherent tendency of SNPs to agglomerate as a result of Van der Waals interactions. (22) Therefore, research interest has been diverted more toward the generation of SNPs by an in situ method rather than an ex situ one.

Osman et al. (22) reported on the synthesis of oil-based polymer composites, in which SNPs are generated using an in situ approach by reducing silver nitrate with a radical initiator. It was demonstrated that nanocomposite samples exhibited antibacterial effect against Gram-positive and Gram-negative bacteria. Pica et al. (23) analyzed the ability of SNPs embedded coatings to release silver ions in aqueous solution by measuring its ionic conductivity. The ionic conductivity tends to increase with increasing concentration of SNPs in an acrylic polymer. Uday et al. (3) reported the process for an in situ generation of SNPs in polyester/clay matrix via reduction of silver salts by dimethylformamide at room temperature. Formation of well dispersed SNPs within the clay gallery with an average size of 15 nm shows their potential application as an antibacterial coating. Akbarian et al. (1) reported the influence of nanosilver on thermal and antibacterial properties of a waterborne polyurethane coating and observed a significant growth reduction of Escherichia coli and Staphylococcus aureus bacteria at a very low concentration of SNPs (200 ppm). Kumar et al. (24) demonstrated the synthesis of SNPs in oil using silver benzoate. They reported that silver benzoate undergoes ligand exchange with the fatty acids, which causes the metal ions to dissolve in the oil and subsequently be reduced by the free radicals to form SNPs.

As discussed above, antimicrobial coatings based on SNPs reported so far have utilized linear alkyds and other resin systems, which require a considerable amount of volatile organic compounds (VOCs) for their processing. After application of the coating, the VOCs evaporate out and cause environmental pollution. The risk of environmental hazards associated with VOCs has led to the government restriction on VOCs. Flence, the recent trend in the field of organic coating is to develop coating with lower and lower amounts of VOCs. (25-31) One of the most effective ways to reduce VOCs in an organic coating is to the develop coatings by using dendritic/hyperbranched polymers. (32,33) Due to the compact three-dimensional structure of dendritic polymers, these molecules mimic the hydrodynamic volume of a sphere in solution and also show low viscosity in the melt, even at high molecular weight, due to the lack of restrictive interchain entanglements. (34,36) Hyperbranched alkyds (HBAs) have been reported as promising resins for the development of low VOC or high-solid coatings. (37,38) Recently, we have reported a hyperbranched polyol-based urethane alkyd resin which not only shows better performance but can be processed using a much lower amount of solvent (xylene) compared to that required for the processing of a conventional alkyd resin.

In this work, the HBA resin is used as a matrix for the synthesis of silver nanoparticle-based composites. Silver benzoate was used as a source of nanosilver. The free radicals generated during oxidative drying of HBA-urethane resin will reduce the silver benzoate leading to the in situ formation of SNPs. (39,40) Thus, the process is comparatively greener as it involves use of minimum quantity of solvent and it does not use any toxic reducing agents like dimethyl sulfoxide or hydrazine for the synthesis of SNP-based nanocomposites. To our knowledge, no such study has been reported in the literature so far.

The synthesis and characterization of nanosilver-embedded hyperbranched urethane alkyd nanocomposites will be discussed in the present paper.

Experimental

Materials

Linseed oil (M/s Jayant Oil Mill, India), sodium hydroxide, sodium chloride, anhydrous sodium sulfate, and hydrochloric acid (S.D. Fine Chem., India) were used to produce the linseed fatty acid. 2, 2-bis methylol propanoic acid, dipentaerythritol, and silver benzoate were procured from Aldrich, Germany. P-Toluene sulfonic acid (Merck, India) was used as a catalyst. Xylene and methanol (High Purity Laboratory Chemicals Pvt. Ltd, India) were used as the solvents. Potassium hydrogen phthalate (Aldrich, India) was used for the determination of acid value. Cobalt octoate and lead naphthenate (Globe Products, India) were used as the driers. Di-butyl tin di-laurate (Aldrich, USA) was used as a catalyst. HDI trimer (Desmodur N 3390) was obtained from Bayer Chemicals, India. All the chemicals were used as received without any further purification.

Preparation of silver nanocomposites

A second generation hyperbranched polyol was synthesized using dipentaerythritol as a core material and 2, 2-bis (methylol) propionic acid as a chain extender. This was reacted with linseed oil fatty acid (LOFA) to make a HBA resin having free hydroxyl groups. The HBA resin having free hydroxyl groups was used for in situ generation of SNP. The detailed method for synthesis of HBA resin is reported elsewhere. (41) During the process, silver benzoate was dispersed in the HBA resin using a mechanical stirrer and ultrasonic processor. Hyperbranched urethane alkyd-nanosilver (HBUANS)-based coating was prepared by mixing the silver benzoate (1 g) dispersed HBA resin having free hydroxyl groups with the isocyanate trimers (Desmodur N 3390) in an appropriate ratio along with catalysts such as of lead naphthenate (1.20 g), cobalt octoate (0.5 g) for oxidative curing and DBTL (0.025 g) for isocyanate curing for 100 g of the coating. Figure 1 illustrates the flow chart of nanocomposite preparation by using HBA resin. The composition of raw materials used for formulating nanosilver-based antimicrobial coating is presented in Table 1.

Characterization

UV Visible spectroscopy

The formation of SNPs in solution was monitored by The UV-Vis measurements of the HBA and Hyper-branched alkyd-nanosilver (HBANS) composites by diluting the samples with xylene. The UV-Vis measurements in solution were made on a Varian Carry 500 scan UV-Vis-NIR spectrophotometer at a resolution of 1 nm.

Atomic force microscopy (AFM)

AFM was used to characterize the roughness and surface topology of the HBUA and HBUANS coating. The samples were scanned using Nanosurf Flexy Scan 2 AFM (Switzerland) in contact mode. Rectangular cantilever of silicon nitride (length: 200 [micro]m and width: 40 [micro]m having force constant of 303 N/m) was used. The software used for processing and analyzing the data is Nanosurf easyscan 2 control software version V.3.0.2. The average roughness ([R.sub.a]) of the film was obtained using the software provided with the system.

Scanning electron microscopy (SEM)

A high-resolution scanning electron microscope (Zeissa Supra 40 VP, Germany) was used to study the morphology of the HBUANS composites. The composite samples were quenched in liquid nitrogen and cryogenically fractured to obtain the cross sections, which were sputter coated with carbon to avoid the charging before the SEM observation.

Transmission electron microscopy (TEM)

High-resolution transmission electron microscope analysis was conducted on a JEOL, (JEM-2100, Japan) electron microscope at 200 kV. A drop of dilute solution was cast on copper grid (300 meshes) and the solvent is allowed to evaporate out.

Thermal analysis

Thermogravimetric analyses (TGA) of HBUA and HBUANS composite coatings were carried out on a thermo gravimetric analyzer, TA Instrument (TGA Q500, USA). About 6-8 mg of sample was placed in an aluminum pan and heated from room temperature to 600[degrees]C at a heating rate of 20[degrees]C [min.sup.-1] under nitrogen atmospheres.

Mechanical properties

Tensile properties of HBUA and HBUANS coatings were measured as per the method given in ASTM D882-97. Test specimens of size 100 mm x 15 mm (having approx film thickness 115 [+ or -] 5 [micro]m) were cut from the films and conditioned at 50% relative humidity for a period of 24 h before examination using a Universal Testing Machine (UTM, Lloyd, Model-LR30 K). The gap between the grips was maintained at 25 mm and specimens were strained at a rate of 20 mm/min. Ten specimens of each sample were used to conduct tensile testing, and the average of the five highest readings at peak load was reported as the tensile strength.

The HBUA and HBUANS coatings were applied onto the burnished mild steel panels, which had been degreased with xylene and burnished with an emery paper. The average film thickness of 115 [+ or -] 10 [micro]m was maintained in all the compositions for the determination of mechanical properties by various tests such as (i) cross hatch adhesion (ASTM D3359, ASTM D4541), (ii) scratch resistance (BS 3900), (iii) Taber abrasion (ASTM D 4060), (iv) impact resistance (ASTM D 2794) and flexibility-bend test (ASTM-D 522).

Testing of antimicrobial activity

Disk diffusion method

Antibacterial activity of silver nanoparticle-based antimicrobial coating was tested by a disk diffusion method as described by Khan et al. (42) Bacterial inoculums were prepared in a normal saline (0.9% (w/v)) by inoculating the isolated colonies of the test organism (Serratia marcescens, NMRL isolate) for 18-24 h. The bacterial inoculum was adjusted to match the 0.5 McFarland turbidity standards. This suspension will contain approximately 1-4 x [10.sup.6] (CFU)/ml colony-forming units. Using a sterile cotton swab (Hi-Media Lab, Mumbai) the bacterial inoculums were inoculated in Mueller-Hinton agar. The test compounds for various concentrations 5, 10, 15, 20, 25, 30, 35, and 40 [micro]g (w/v) were loaded on sterile Whatman paper disk (10 mm dia, Hi-Media Lab, Mumbai) and were placed in the center of the Mueller Hinton agar and inoculated with the test organism at 28[degrees]C. The zone of inhibition was measured after 24 h.

A new simple agar overlay method

S. marcescens produces a red pigment called Prodigiosin at 28[degrees]C when grown in Mueller-Hinton agar without an antibacterial agent. Hence, the growth of the bacteria will be indicated by the appearance of red color due to formation of the red pigment. The inhibition of the bacteria can also be understood by visual inspection i.e., absence of red color. Based on this principle, we have designed a new simple agar overlay method for the determination of antibacterial activity of SNPs embedded in hyperbranched urethane alkyd. A 10 ml of hyperbranched urethane alkyd without SNPs and with SNPs was poured into two separate sterile glass petriplates in Class II Biosafety Cabinet (Nuaier, USA). These two plates were kept opened for 12 h in Class II Biosafety Cabinet for the evaporation of solvent followed by the film formation. Bacterial inoculums were prepared in 0.9% (w/v) normal saline by inoculating well-isolated colonies of test organism (.S', marcescens, NMRL isolate) and grown nutrient agar plate for 18-24 h. 100 [micro]l of S. marcescens inoculums prepared in normal saline was added to 10 ml of sterile Mueller-Hinton agar briefly vortexed for 10 s and poured into two separate petriplates containing hyperbranched urethane alkyd resin without SNPs and with SNPs. After the solidification of Mueller-Hinton agar, the plates were incubated at 28[degrees]C for 24 h. The growth of S. marcescens was observed after 24 h by visual inspection.

Accelerated Weathering Test

Weathering resistance of HBUA coatings was evaluated as per the method described in ASTM G-53 by exposing the resin coated aluminum panels (150 x 75 x 1 mm) to UV-radiation ([lambda] = 285-315 nm, UV-B bulbs) and high humidity condition in a QUV-accelerated weatherometer. A weathering cycle, comprising 2 h of condensation (at 45 [+ or -] 5[degrees]C) and 4 h of UV exposure (at 60 [+ or -] 5[degrees]C), was maintained during the study. Gloss measurements (before and after exposure) were carried out at periodic intervals as per method described in BS No. 3900 (Part D5) using a multihead glossmeter (Novogloss, Rhopoint) at 27[degrees]C.

Results and discussion

Formation of SNP

The dispersion and stabilization of nanoparticles in a polymer matrix is useful from the viewpoint of film casting. (3) However, the strong tendency to form agglomerates makes dispersion of nanoparticles difficult by an ex situ approach and homogeneous dispersion of nanoparticles still remains as a great challenge. The issue has been addressed by adopting an in situ approach, in which the nanoparticle is coated with the resin as soon as it is generated. This reduces the surface energy of SNPs and prevents the agglomeration. Therefore, SNPs are generated in situ into the HBA resin by the reduction of silver benzoate. In order to study the formation of SNP, silver benzoate and alkyd driers were mixed with 10 wt% of HBA solution made with xylene. The formation of silver nanoparticle is clearly seen from the visual identification of HBA resin solution as shown in Fig. 2. The silver benzoatedispersed HBA solution became dark brown within 48 h, which indicates the formation of nanosilver in the HBA resin. (24) It may be noted that the HBA solution without silver benzoate does not show any color change. The free radicals formed during the auto-oxidation of linseed oil fatty acid (present in the resin) reduce the silver benzoate into SNPs. (24-26) This involves the ligand transfer reactions, which are depicted in Fig. 3.

Hyperbranched urethane alkyd composition comprises HBA and isocyanate curing agent. Thus, the resin undergoes curing with simultaneously occurring two mechanisms namely the reaction of free hydroxyl groups with isocyanate and oxidative drying of linseed oil via free radical mechanism. To investigate the formation SNPs in the HBUA coating, we mixed this silver benzoate-dispersed HBA with an isocyanate trimer and coated a glass surface. After curing for about 4-6 h at ambient temperature, the coating turned brown (Fig. 4), indicating the formation of SNPs in the HBUA coatings. (24)

UV-Visible spectroscopy

The formation of SNPs in HBUA matrix is characterized by using UV-Vis absorption spectra. Figure 5 shows the UV-Vis spectra of FIBA (control) without SNPs and FIBANS with SNPs, respectively. The UV-Vis spectrum of FIBANS shows an absorbance peak at 430 nm. The peak at 430 nm appears due to the presence of nanosilver in the polymer matrix and which is known to exhibit a characteristic property of surface plasmon resonance (SPR) effect originating from the quantum size of SNPs. Note that no absorption peak was observed in the UVVis spectrum of the HBA (control) resin due to the absence of silver nanoparticle. Furthermore, in order to get additional evidence of the presence of nanosilver within FIBANS, an elemental analysis was carried out using ED AX techniques and these findings are further supported by AFM and TEM analysis.

Microscopic analysis

Atomic force microscope (AFM) spectra for HBUA and the related nanocomposites having SNP are presented in Fig. 6. A change in surface roughness between neat HBUA (5 nm) and HBUANS (7 nm) was observed due to the incorporation of SNPs. This indicates that SNPs are uniformly dispersed and embedded in the polymeric matrix. No agglomerated silver particle was detected. In conventional microcomposites, the surface roughness changes significantly. After detailed AFM analysis, the phase images of the HBUA coating did not show regions with different mechanical stiffnesses, hence there is no evidence of phase separation in HBUANS matrix.

Figure 7 shows SEM image and ED AX (energy dispersive analysis of X-rays) spectrum recorded in the spot-profile mode from one of the densely populated silver nanoparticle regions on a fractured surface of the HBUANS sample. From the ED AX spectrum, we can clearly observe a peak at 3 keV that confirms the presence of SNPs within the HBUA coating. The rest of the peaks of the ED AX spectrum correspond to the other elements present in HBUANS coating. The EDAX analysis coupled with the SPR absorption bands of the UV-Vis spectra clearly indicates the formation of elemental silver nanoparticle inside the HBUA coatings.

The formation of SNPs in HBA resin is also confirmed from TEM studies of HBANS. The TEM photograph of HBANS is shown in Fig. 8. From the figure, it is clear that SNPs are well dispersed with no significant agglomerations in the polymer matrix. It should also be noted that the generated SNPs are almost spherical in shape. The average particle sizes of the SNPs are in the range of 20-40 nm. The reason for this good dispersion is the secondary interactions between the polar groups (-OH) present in the HBA resin and the SNPs. This leads to a good adsorption of HBA resins on surface of silver nanoparticle. It clearly indicates that SNPs are completely embedded in the HBA resins preventing the growth of agglomeration.

Thermal analysis

Thermal properties of HBUA and HBUANS coatings were determined as per method described in "Thermal analysis" section and the results are presented in Fig. 9. A two-step degradation behavior was observed for both the HBUA and HBUANS coatings. Interestingly, HBUA exhibits a weight loss of 100% at the highest applied temperature (460[degrees]C), whereas the nanocomposite (HBUANS) shows 95% weight loss of its original weight. This implies that the presence of nanosilver even in very little amount (0.5 wt%) gives a considerable resistance to thermal degradation of HBUA coatings.

Mechanical properties

The tensile strength of neat HBUA and HBUANS coatings is shown in Table 2. From the results, it is clear that the HBUANS composite exhibits higher tensile strength compared to the neat HBUA. This can be attributed to the good reinforcing effect of nanosil ver with HBUA matrix. The abrasion, scratch resistance, impact resistance of the neat HBUA, and HBUANS composite coatings were also investigated and the results are summarized in Table 2. The abrasion, scratch resistance, and impact resistance of the HBUA coatings were found to be increased with the addition of SNPs.

The HBUANS composite showed better abrasion resistance. Similarly, the result of the scratch resistance tests also clearly indicates (Table 2) that the tolerance of HBUANS coating is better than that of the neat HBUA coating. Both the coatings HBUA and HBUANS show adequate flexibility as they can be bent in a mandrel with diameter of 14" inch as evident from Table 2. This can be attributed to the presence of long flexible hydrocarbon chain of the fatty acid. Good flexibility of the HBUA coating is also reflected in their impact properties. The cross hatch adhesion test was performed as per ASTM D3389 on mild steel substrates (Table 2). The cross hatch adhesion was found to be increased with the addition of SNPs into the HBUA coatings. No detachment of the coating at the edges and within the square lattice was observed for the HBUANS composites. This can be attributed to the strong interfacial adhesion between HBUA and SNPs via H-bonding.

Testing of antimicrobial activity

The antibacterial activity of the HBUA and HBUANS coatings was characterized against S. marcescens bacterial strain using agar overlay and disk diffusion methods as described in "Testing of antimicrobial activity" section. S. marcescens is a Gram-negative rod-shaped bacteria in the family of Enterobacteriaceae. It produces a nondiffusible red pigment called prodigiosin. It is widely distributed in the environments like air, water, and soil. Therefore, we have used this bacterium for our study. Results of the agar overlay method are shown in Fig. 10. The plate containing HBUA (control, without SNPs) supported the growth of S. marcescens as evidenced by red color formation. The second sample containing HBUANS having 0.5 wt% SNPs inhibited the growth of S. marcescens as indicated by the absence of red color formation.

This indicated that HBUANS completely inhibited the growth of S. marcescens. The disk diffusion method results of HBUANS are shown in Fig. 11. The results indicated an excellent antibacterial activity of the HBUANS coating films against the bacterial stains from 5 pg (wt/v) onwards. The antimicrobial efficacy increases with increasing silver concentration in the HBUANS. These results suggest that SNPs embedded in hyperbranched urethane alkyd coatings have the potential to be considered as an effective antimicrobial coating material. To the best of our knowledge, this is the first report showing the effect of hyperbranched urethane alkyd with 0.5 wt% SNPs which completely prevents the growth of S. marcescense.

Accelerated weathering test

During the service of a coating, gloss retention is of great importance for maintaining the esthetic appearance of the painted surface for longer periods. The HBUA and HBUANS coatings were examined for their gloss and nonleaching property of SNPs from HBUA coatings by exposing the coated aluminum panels to QUV-accelerated weatherometer. The test was carried as per the method described in "Accelerated Weathering Test" section. It was observed that the HBUA and HBUANS coatings retained 75 and 70% gloss, respectively, after an exposure for a period of 300 h (Fig. 12). The gloss retention and nonleaching characteristic property of the coatings (Fig. 12) are due to the presence of UV resistant urethane linkage in the polymer backbone.

Conclusion

A simple process for the in situ generation of SNPs by the reduction of silver benzoate during oxidative curing of fatty acid of hyperbranched urethane alkyd has been demonstrated. The advantage of the process is that it does not require any toxic solvent or reducing agent for the generation of SNPs. The formation of silver nanocomposites was thoroughly characterized by UV-Vis, AFM, TEM, XRD, SEM-EDAX, and TGA analysis. The nanocomposite coating requires a much lower amount of solvent for processing and also offers superior mechanical properties compared to conventional alkyd resin-based coatings. The nanocomposite coating containing 0.5 wt% SNP offers complete inhibition of S. marcescens. At the same time, the nanocomposite coating shows marginally better mechanical properties compared to neat HBUA coating. Hence, the developed SNPs-embedded hyperbranched urethane alkyd will have potential application as an antimicrobial coating in marine and moist environment.

DOI 10.1007/sl 1998-015-9702-3

R. B. Naik (13), D. Ratna

Naval Materials Research Laboratory, Anandnagar MIDC, Thane-Dist, Ambarnath, Maharashtra, India e-mail: balukala.drdo@gmail.com

Acknowledgments The authors would like to thank Dr. R. S. Hastak, Director NMRL for providing guidance and encouragement during the work.

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Table 1: Composition of the antimicrobial coating

S. No.   Raw materials                      Weight (g)

1        Dipentaerythritol                  2.5
2        2, 2-bis methylol propanoic acid   23
3        Linseed oil                        51
4        p-Toluene sulfonic acid            0.2
5        Silver benzoate                    1.0
6        HDI trimer (Desmodur N 3390)       23.3

Table 2: Physical and mechanical properties of HBUA and HBUANS coating

S. No.   Tests/ HBUA coatings                   HBUA        HBUANS

1        Abrasion resistance [Weight loss in    140         127
           mg/1000 cycles] ASTM D 4060
2        Scratch resistance [Tolerance weight   1.0         1.5
           in kg] BS 3900
3        Flexibility [Conical mandrel bend      Pass up     Pass up
           test] ASTM D522                        to 1/4"     to 1/4"
4        Impact resistance ASTM D 2794          Pass        Pass
5        Cross hatch adhesion ASTM D 3389       3           4
6        Tensile strength, Mpa ASTM D 882-97    7           9


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Author:Naik, R. Baloji; Ratna, D.
Publication:Journal of Coatings Technology and Research
Article Type:Report
Geographic Code:9INDI
Date:Nov 1, 2015
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