Studies on Urethane-Modified Alumina-Filled

Studies on Urethane-Modified Alumina-Filled

(Parte 1 de 4)

Studies on Urethane-Modified Alumina-Filled Polyesteramide Anticorrosive Coatings Cured at Ambient Temperature

SHARIF AHMAD, S. M. ASHRAF, ABUL HASNAT, S. YADAV, A. JAMAL Materials Research Laboratory, Department of Chemistry Jamia Millia Islamia, New Delhi 110 025, India

ABSTRACT: Coatings prepared from polyesteramide resin synthesized from linseed oil, a renewable resource, have been found to show improved physicomechanical and anticorrosive characteristics. These properties are further improved when aluminum is incorporated in the polyesteramide resin. The coatings of this resin are generally obtained by baking at elevated temperatures. With a view toward the use of linseed oil, as a precursor for the synthesis of polyesteramide resins and to cure their coatings at ambient temperature, toluylene diisocyanate (TDI) was incorporated into polyesteramide and alumina-filled polyesteramide in varying proportions to obtain urethane-modified resins. The latter resins were found to cure at room temperature. The broad structural features of the urethane-modified polyesteramide and alumina-filled polyesteramide were confirmed by FTIR and 1H–NMR spectroscopies. Scratch hardness; impact resistance; bending resistance; specular gloss; and resistance to acid, alkali, and organic solvents of the coatings of these resins were determined by standard methods. Physicomechanical and anticorrosive properties, specular gloss, and thermal stability of the urethane-modified alumina-filled polyesteramide coatings were found to be at higher levels among these resins. It was found that TDI could be incorporated in polyesteramide up to only 6 wt %, such that above this loading its properties started to deteriorate, whereas alumina-filled polyesteramide could take up to 10 wt % TDI. Explanation is provided for the increase in scratch hardness and impact resistance above 6 and 10 wt % addition of TDI in polyesteramide and alumina-filled polyesteramide, respectively, as well as for the decrease in flexibility and resistance to solvents, acid, and alkali of coatings of these resins above these limits of TDI addition. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1855–1865, 2001

Key words: coatings; polyesteramide; alumina; urethane; curing

Polyesteramide resins are amide-modified alkyds that have improved characteristics over normal alkyds in terms of hardness, ease of drying, and water vapor resistance.1–3 Generally, polyesteram- ide coatings are obtained by baking at and above 175°C.4,5 To improve the physicomechanical and anticorrosive characteristics of baked coatings, we incorporated aluminum in the backbone of the polymer.6 We found that the aforementioned properties of the coatings were enhanced appreciably in the case of alumina-filled polyesteramides.6 Curing of the polyesteramide at elevated temperature is a multistep process and is also energy consuming. It is, therefore, desirable to develop a simple curing route operative at ambient temperature to produce

Correspondence to: S. Ahmad (sharf.ch@jmi.ernet.in). Contract grant sponsor: ARDB (Ministry of Defense, India); contract grant number: Aero/RD-134/110/106/934.

coatings of improved physicomechanicaland anticorrosivepropertiescomparedto high-temperature baked coatings of polyesteramide and aluminafilled polyesteramide. It was previously reported that toluylene diisocyanate(TDI) can be used for curing aromatic polyesteramideat room temperature.7,8

The presence of a urethane linkage in the polyesteramide has been found to considerably enhance the performance of these coatings in terms of adhesion, toughness, weather resistance, and chemical/solvent resistance.9 Polyurethane resins as a class are well recognized for their excellent adhesion, ambient-temperature curing, flexibility, weather resistance, and resistance to solvent and chemical attack.10 In our earlier work we attempted to prepare alumina-filled polyesteramide from linseed oil, used as a precursor of renewable nature. In this work we have attempted to obtain urethane-modified polyesteramide using TDI with the double objective of using a precursor obtained from a renewable resource and of enhancing the physicomechanical and anticorrosive properties of the coatings of these resins through curing at room temperature. A survey of the literature reveals that modifications of polyesteramide by butylated melamine formaldehyde,1 styrene,12 and also by metals6 have been attempted. However, no work has been reported on the modification of alumina-filled polyesteramide synthesized from a vegetable oil by urethane.13–15 We report our findings on the above investigations in this communication.

Materials

Oil was extracted from linseed (procured from a local market) through Soxhlet apparatus. Petro- leum ether was used as a solvent. The fatty acid composition of the oil was determined by gas chromatography (GC; 1/8 s.s. column, FID detector) (Table I). Phthalic acid, sodium methoxide, aluminum hydroxide, xylene, toluylene-2,4 diisocyanate (Merck, India and Germany), and diethanolamine (S.D. Fine Chemicals, India) were of analytical grade.

Synthesis

Synthesis of Polyesteramide (PEA) and Alumina- Filled Polyesteramide (APEA)

PEA and APEA were prepared according to a previously reported method.6

Synthesis of Urethane-Modified Polyesteramide (PEAU) and Alumina-Filled Polyesteramide (APEAU)

Polyesteramide and alumina-filled polyesteramide were dissolved in xylene and treated with varying amounts of toluylene-2,4 diisocyanate in a four-neck round-bottom flask fitted with a nitrogen inlet, a thermometer, and a stirrer. The extents of loading of TDI (in wt %) in PEA and APEA are provided in Table I. The reaction was carried out under stirring at 145 6 5°C. The progress of the reaction was monitored by thin layer chromatography (TLC) as well as by hydroxyl value determination. The solvent was removed from PEAU and APEAU in a rotatory vacuum evaporator.

Characterization

PEAU and APEAU were characterized by FTIR and 1H–NMR spectroscopies. FTIR spectra of these polymers were taken on Perkin–Elmer 1750 FTIR spectrophotometer (Perkin Elmer Cetus In-

Table I Characterization of Oil, HELA, PEA, and APEA

Characteristic Linseed Oil HELA PEA APEA

1856 AHMAD ET AL.

struments, Norwalk, CT) using a NaCl cell. 1H– NMR spectra were recorded on a JEOL 300 MHZ FX-1000 spectrometer (JEOL, Peabody, MA) using deuterated chloroform and tetramethyl silane (TMS) as an internal standard. The solubility of these polymers was tested in various organic solvents. The intrinsic viscosity of PEA, APEA, PEAU, and APEAU in N,N-dimethyl pyrollidone (stock solution: 5 g/100 mL solvent) at 25°C was determined with the help of a Ubbelhode viscometer. Iodine value and hydroxyl value were determined by ASTM method D555-6. Specific gravity and refractive index were determined by standard laboratory methods.

Preparation and Testing of Coatings

Coatings of PEAU and APEAU, cured at ambient temperature, were prepared by brush technique using a solution containing 40 wt % urethanemodified polymer in xylene on commercially available mild steel strips 30 3 10 3 1 m, for chemical resistance test, and 70 3 25 3 1m m strips for the determination of specular gloss at 60° [by gloss meter (model RSPT-20; Digital Instruments, Santa Barbara, CA)], scratch hardness (BS 3900), bending test (ASTMD 3281-84), and impact resistance (IS:101 part 5/sec. 3, 1988). Coating thickness was measured by Elcometer (model 345; Elcometer Instruments, Manchester, UK). The thickness of these coatings was found to be between 130 and 160 m. Dry-to-touch and dry-to-hard times were also determined (Table I). Corrosion tests were performed in water, ac- ids (5 wt % HCl, 5 wt % HNO3), alkali (5 wt % NaOH), and xylene by placing them in 30-diame- ter porcelain dishes and dipping the coated sam- ples in the aforementioned media. Periodic examination was conducted until the coatings showed evidence of softening or deterioration (Table I). Salt-spray tests (ASTM B 117-94) were also carried out for a period of 10 days in a mist salt chamber.

Figure 1 represents the FTIR spectra of PEA and APEA and Figure 2 represents the FTIR spectra of PEAU and APEAU. The 1H–NMR spectra of PEAU and APEAU are presented in Figures 3 and 4, respectively. Figure 5(a) shows the reaction scheme of HELA [N,N-bis(2-hydroxy ethyl) linseed fatty amide] with phthalic acid to form polyesteramide; Figure 5(b) shows the formation of alumina-filled polyesteramide. Figure 5(c) and (d) represent the scheme of formation of urethane-modified PEAU and APEAU.

Comparison of PEA and APEA Structures

On perusal of FTIR and 1H–NMR peaks data (Table IV) we notice that the OH stretching peak in PEA appears as a shoulder at 3400 cm21 in the IR spectra. In the case of APEA the peak is broad and is spread over 3120–3650 cm21 because of the presence of the OH group attached to aluminum in APEA. The CAO peak in PEA appears at 1725 cm21, whereas in APEA it appears at 1760 cm21, attributed to the presence of aluminum in the chain. 1H–NMR spectra of PEA show the peak of OH of carboxyl at d 7.92 ppm. This peak is absent in APEA, thus verifying elimination of the

Table I Characterization of Polyesteramide Urethane and Alumina Filled Polyesteramide Urethane

Resin Codea

Acid Value

(mg KOH) Hydroxyl Value (%) SaponificationValue IodineValue SpecificGravity RefractiveIndex

Viscosityb (dL/g) a Last digit of resin code indicates the wt % of TDI. b Inherent viscosity.

POLYESTERAMIDE ANTICORROSIVE COATINGS 1857

OH group of carboxyl on interaction with

Al(OH)3. The above observation confirms the incorporation of alumina in APEA. The 1H–NMR peak of the CH2 group adjacent to ester in PEA is sharp and pronounced (d 2.0 ppm). This peak is suppressed and shifted slightly downfield (d 2.29–2.30 ppm) in APEA because of the presence of aluminum in the chain. The peak for CH2 attached to amide is observed at d 1.58 ppm in PEA as well as in APEA. The peak for the aliphatic chain is observed at d 1.29 ppm for PEA. In the case of APEA it is present at d 1.25 ppm. These observations broadly confirm the structures of PEA and APEA, as shown in Figure 5(a) and (b).

Comparison of PEA and PEAU Structures

Spectra for PEAU are more spread than those for PEA in the rangeof 3500–3150cm21, showingthe overlapof OH and NH groups.In the case of PEA thepeakissuppressedandappearsasashoulderat 3460cm21, indicatingthepresenceoftheOHgroup only.In PEAU,the NH deformationmode appears at 1557 cm21. In addition to the terminal methyl group peak of aliphatic chain at d 0.9 ppm, 1H– NMR spectra of PEAU show the presence of the methyl group of TDI at d 2.1–2.24ppm. The presence of CAO of ester, CON, benzene ring, and aliphaticchain are confirmedby the appearanceof their peaks in the spectra (Table IV). These observationsbroadlyconfirm the structuresof PEA and PEAU,proposedinFigure5(a)and(c),respectively. The peakof carboxylOH at d 7.9 ppm in PEA does not appearin PEAU. This also verifiesthe interaction of TDI with carboxyl-terminatedPEA and disappearanceof the carboxylgroup.16

Comparison of APEA and APEAU Structures

Perusal of Table IV reveals that APEA shows a broad peak spread between 3650 and 3120 cm21 (centered at 3455 cm21). APEAU shows well-resolved peaks in this range at 3600, 3460, 30, and 3150 cm21. The broadness in the APEA peak may be attributed to the presence of alcoholic OH and OH attached to aluminum. NH stretching peak of the urethane group in APEAU appears at 3460 and 30 cm21. The CAO peak in APEA appears at 1760 cm21, whereas in APEAU this peak shows at 1727 cm21. The CAO peaks in PEAU (1724 cm21) and in APEAU (1727 cm21) could be related to the presence of the urethane group in the chain. We also notice that the 1H–

NMR peak of CAO in APEAU is shifted upfield Table

I Film

Properties of Ambient

Cured Polyesteramide

Urethane and Alumina-Filled

Polyesteramide

Urethane

Resin Code

Drying

Time (min)

Scratch Hardness (kg)

Impact Resistance [lbs/in. (passes)]

(Parte 1 de 4)

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