coating formation by spontaneous polymerization on aluminum: various monomers

Coating Formation by Spontaneous Polymerization onAluminum: Various Monomers

RAJAT AGARWAL, J. P. BELL

Polymer Science Program, Institute of Materials Science, U-136, University of Connecticut,Storrs, Connecticut 06269-3136

Received 23 November 1998; accepted 15 June 1999

ABSTRACT: Spontaneous polymerization to give conformal, uniform coatings on metalshas been expanded to a series of different monomers. The monomer 4-fluoro maleimide,when copolymerized with styrene, a coupling agent, and a bis-maleimide crosslinker,imparts very low dielectric constant (2.4) to the coating while retaining high temper-ature resistance. Diethyl fumarate, with the same comonomers, enhances ductility andprovides an adjustable glass transition temperature. Addition of glycidyl acrylate to themonomer system provides reactivity of the coating to epoxy resins. Kinetic studiesusing these monomers were consistent with the free radical polymerization mechanism.The rate of reaction seemed limited by the diffusion of species to the reaction site.Extent of incorporation of the new monomers into the chain backbone was verified, andadhesion and corrosion resistance properties examined. The data illustrate the versa-tility of the conformal, chrome free spontaneous polymerization process. © 2000 JohnWiley & Sons, Inc. J Appl Polym Sci 76: 875–885, 2000

Key words: coating formation; spontaneous polymerization; aluminum; monomers;corrosion

INTRODUCTION

Spontaneous polymerization is a new process bywhich polymer coatings can be synthesized di-rectly on metal surfaces. Formation of protectivecoatings on aluminum and steel has been dis-cussed in earlier papers.1–4 The process is con-ducted at room temperature in a single tank ofmonomers, acidified water, and solvent if needed;no initiator or external driving force is required.The key difference of this process from other con-ventional coating processes such as dip coating,spray painting, or electrophoretic deposition isthat here the polymer chains “grow” at the metalsurface instead of being deposited. Significant ad-vantages of the process are that the coatings

formed are conformal and pinhole free. As a re-sult, uniform coatings can be formed on objectswith complex topographies. The process also re-sults in very good adhesion of the polymer coatingto the metal substrate, because wetting of themetal surface is easier by monomers as comparedwith wetting by polymers.

The spontaneous polymerization process onaluminum involves the use of donor and acceptormonomers, such that the electron densitiesaround the double bond, between the two mono-mers, are very different. Under suitable condi-tions such monomer pairs can polymerize rapidlyat room temperature and result in an alternatingcopolymer. Many different initiation and poly-merization mechanisms for such donor–acceptorsystems have been proposed and reviewed.5–7 Thenovel aspect of the present process is that thedonor (styrene) and acceptor (N-phenyl maleim-ide) monomers spontaneously copolymerize on

Correspondence to: J. P. Bell.Journal of Applied Polymer Science, Vol. 76, 875–885 (2000)© 2000 John Wiley & Sons, Inc.

875

aluminum surface in the absence of any exter-nally added initiator. By controlling the solventquality of the monomer solution the polymeriza-tion occurs only at the aluminum–solution inter-face.

The first coatings obtained on aluminum usingspontaneous polymerization are described in aprevious paper.1 Coatings were made using sty-rene (St) and N-phenyl maleimide (NPMI), andother comonomers added were 2(methacryloy-loxy)ethyl acetoacetate (MEA), and bis-maleimide(BMI). Addition of these monomers improved theuniformity and adhesion of these coatings to alu-minum. Spontaneous polymerization has alsobeen observed on steel by Zhang and Bell, whereprotective coatings were successfully synthe-sized.2

Other acceptor monomers such as 4-fluoro ma-leimide (4FMI), diethyl fumarate (DEF), and gly-cidyl acrylate (GA) were also used to synthesizecoatings having different properties. In all cases,the donor monomer used was styrene, and thecrosslinker used was BMI. Polymer coatings weremade using 4FMI/St/MEA/BMI, NPMI/St/DEF/BMI, and NPMI/St/GA/BMI systems. In mostcases, the ratio of the two acceptor monomers wasvaried, keeping the styrene and BMI concentra-tions constant.

Kinetic studies were performed in which theresultant coating thickness was measured as afunction of polymerization time, temperature,and monomers ratio in feed, for the 4FMI/St/MEA/BMI system. Evidence of incorporation ofDEF and GA into the NPMI/St/BMI system wasobtained using infrared spectroscopy. The effectof varying monomer ratios in feed on the Tg of theresultant polymer was also measured.

Adhesion of the coatings to aluminum wasmeasured using a torsional testing method,where the shear strengths of the joints were mea-sured. The protective properties of the coatingswere analyzed using accelerated salt fog corrosiontesting and electrochemical impedance spectros-copy (EIS). Dielectric constants and electricalbreakdown potentials were also measured to testfor suitability of these coatings for electronic ap-plications.

EXPERIMENTAL

Materials

MonomersStyrene from Aldrich Chemical Co. was vacuumdistilled at 40°C to remove inhibitor. NPMI and

BMI were purchased from Mitsui Toatsuo Chem-ical Co. NPMI was purified by dissolving it inboiling cyclohexane and subsequent crystalliza-tion on cooling. Bright yellow needle-like crystalswere obtained after drying at 60°C in vacuum.BMI was used as obtained. 4FMI was synthesizedfrom 4-fluoro aniline and maleic anhydride byadapting the method of Rao.8 MEA from AldrichChemical Co. and GA from Lancaster Chemicalswere purified by passing through a DHR-4 inhib-itor removal column from Scientific PolymerProducts. The monomers were refrigerated afterpurification. DEF was used as received. Thestructures of these monomers are shown in Fig-ure 1. N-methyl pyrrolidinone (NMP) was pur-chased from Fisher Scientific Co. and used asreceived.

Aluminum Alloys and Pretreatment

The aluminum used was 2024, 6061, or 7075 al-loy. Al 2024 (4.5% Cu, 1.5% Mg, 0.6% Mn) has ahigh shear strength and is used as a structuralaircraft alloy, but is highly prone to corrosion. Al6061 (1.0% Mg, 0.6% Si, 0.2% Cr, 0.27% Cu) isused in heavy duty structures where corrosionresistance is needed.9

The aluminum samples were given one of thefollowing two pretreatments: (1) Samples weredegreased with 5% aqueous Micro (a laboratorygeneral purpose alkaline soap) solution, rinsedwith distilled water, and treated with 5% hy-drofluoric acid for 30 s, followed by a distilled

Figure 1 Monomers used for spontaneous polymer-ization and their roles.

876 AGARWAL AND BELL

water rinse. (2) Samples were grit blasted using170 mesh alumina and rinsed with distilled wa-ter. The aluminum samples were coated immedi-ately after the cleaning process.

Monomer Feed Solution Preparation

The monomers to be used were dissolved in NMP.Dilute aqueous sulfuric acid (0.025M) was thenadded slowly to the solution while stirring. TheNMP/water ratio was such that the monomerswere close to their solubility limit. This ratio de-pends on the monomer feed composition and con-centration. A higher NMP ratio improves the sol-vent quality, but if too high the formed polymerwill dissolve. A 63/37 volume ratio of NMP/waterwas used for making monomer solutions contain-ing NPMI, St, BMI, GA, and DEF. For solutionscontaining 4FMI, an NMP/water ratio of 70/30was used. A clear solution was obtained by thismethod. Solutions containing NPMI were yellowand those with 4FMI were light brown but clear.

Polymerization Procedure

Polymerization Cell

Polymerization was conducted in Pyrex beakersof various sizes, depending on the sample size, orin custom-designed 1 3 8 3 8 cm polypropylenetank compartments. The polypropylene tanks al-lowed polymerization of large surface areas withsmall solution volumes. Typically, 30 3 30 alumi-num coupons could be polymerized using 50 mLsolution. The post polymerization rinsing bathwas a 12 cm diameter bath, gently stirred.

Process

The monomer solution was first purged with ni-trogen to reduce the dissolved oxygen concentra-tion to less than 2 ppm, since oxygen quenchesfree radicals. When the cleaned aluminum wasimmersed in the monomer bath, a white, swollenpolymer coating began to form. Polymerizationtime for samples in the bath was varied from 3 to120 min.

Following the polymerization, the coated samplewas immersed in a gently stirred 10–15% aqueousNMP solution for about 15 min to remove excessNMP and trapped residual monomers. The coatedcoupons were then oven dried at 150°C for 1 h toremove most of the water and NMP, and then at225–250°C for 4–6 h to remove the last traces ofNMP. Cooling was done slowly over 3–4 h.

Characterization

Sample Preparation

The polymers to be characterized were all spon-taneously polymerized on aluminum unlessstated otherwise. The swollen polymer wasscraped from the coated aluminum prior to thedrying step. This polymer mass was then washedwith ethanol in order to remove NMP, water, andany residual monomers. The polymer precipitatedto the bottom and the clear solution was decanted.This washing with ethanol was repeated 3–4times. The polymer precipitate was then driedin vacuum at 100°C for 24 h, yielding a whitepowder.

Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectra for thepolymer coatings were taken using a Nicolet 60SX FTIR spectrometer with a deuterated trigly-cine sulfate detector and a germanium/KBr beamsplitter. Samples were prepared by pressing KBrpellets from mixtures of about 1–2 mg polymerand 150 mg KBr powder. FTIR was used to detectthe incorporation of the acrylic monomers into theSt–NPMI copolymer.

Thermal Analysis

A Perkin Elmer DSC-7 instrument was used toestimate the glass transition temperature (Tg) ofthe polymer coatings made from varying mono-mer feed compositions. The instrument was cali-brated using indium and tin standards. A 10–12mg sample was placed in aluminum pans andscanned over 50–300°C at a heating rate of 20°C/min. A mid point method was employed to esti-mate the Tg.

Adhesion Measurements

Adhesion of the polymer coatings to the alumi-num substrate was measured using the torsionaltesting method of Bell and Lin.10 The joint assem-bly is shown in Figure 2. Using this test, thepolymer–metal joints can be broken in a pureshear mode and the joint shear strength can bemeasured accurately. For water immersion tests,the joints were soaked in 60°C water for 3 daysand broken. Adhesion of polymer coatings formedfrom various monomer feeds was measured usingthis method under both dry and wet conditions.

Electrical Properties

Dielectric constant measurements of the polymercoatings were performed on a Time Domain Di-

COATING FORMATION 877

electric Spectrometer at room temperature. Vari-ous polymer compositions were tested in order toexamine the effect of different monomers on theelectrical properties. Frequencies applied rangedfrom 0.001 to 1000 Hz. Coatings of thickness25–35 mm were used.

Corrosion Testing

Salt Fog Testing. Corrosion protection propertiesof the polymer coatings were studied by exposingsamples to a 5% NaCl salt fog, following theAmerican Society for Testing and Materials(ASTM B-117) test method. Samples tested weregrit blasted Al 6061 and 2024 alloy coupons andhad a 25 mm thick polymer coating.

Electrochemical Impedance Spectroscopy. Elec-trochemical impedance studies were performedon coated samples exposed to 0.5M NaCl solution.The impedance cell was a 20 diameter, 40 highPyrex cylinder sealed to a 30 3 30 specimen via aViton O-Ring, which provided an exposed speci-men area of 18.3 cm2. The cells were open toatmosphere, permitting natural aeration. Thealuminum specimen was the working electrode;the counter electrode was graphite and a satu-rated calomel electrode was used as a referenceelectrode. Impedance spectra were obtained witha Solatron Model 1250 Frequency Response Ana-lyzer, coupled with a Solatron Model 1286 Poten-tiostat and an online computer. Z-plot softwarewas used to control the experiments, and equiva-lent circuit modeling was performed with Bou-kamp’s EQUICRT software.11 Measurements wereperformed using an EG&G Princeton Applied Re-search model 263 potentiostat/galvanostat drivenby an IBM compatible PC. Impedance spectrawere recorded for various immersion times.

RESULTS AND DISCUSSION

The mechanism of spontaneous polymerization onaluminum has been discussed in detail in earlier

works.1,3 Various coatings made from using thenew monomers mentioned above were evaluatedfor their kinetics and properties.

Kinetic Studies of the 4FMI/MEA/St/BMI System

In this series of experiments, the 4FMI/MEA/St/BMI system was used in an effort to make coat-

Figure 3 Dependence of polymerization kinetics on4FMI/MEA ratio in feed 4FMI/MEA/St/BMI 5 x/x/0.2/0.005M. (Error bars indicate standard deviation.)

Figure 4 Dependence of polymerization kinetics of4FMI/MEA/St/BMI system on polymerization temper-ature. 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M.

Figure 2 Schematic of the torsional testing joints.10

Dark ring is the contact face.


ings with high corrosion resistance associatedwith its hydrophobic properties and low surfaceenergy. For the 4FMI–St copolymers, molecularweight was in the range of 70,000 g/mol, based ongel permeation chromatography, using polysty-rene standards. The kinetics of this system areshown in Figure 3. The coating thickness in-creased linearly as a function of the square root of

time, which indicates the rate of polymerizationwas diffusion controlled. The rates for the threecompositions showed dependence on the maleim-ide–MEA ratio, and decreased with decreasing4FMI in the system; a similar effect was reportedfor the N-phenyl maleimide/MEA/styrene/BMIsystem.3 Kinetic studies were also performed as afunction of temperature and are shown in Figure

Figure 5 FTIR showing incorporation of DEF in increasing amounts in the coating, asthe DEF/NPMI ratio is increased, by the appearance of the 1733 cm21 peak.


4. The polymerization rate increased significantlywith increasing temperature.

The kinetic data show that polymerizationrates are dependent upon monomer concentra-tions and temperature. Alternatively, the desiredcoating thickness can easily be controlled by vary-ing polymerization time, as illustrated on Figure4. In absence of MEA, the 4FMI/St/BMI coatingsalone are brittle, as one might expect from thehigh glass transition temperature. Adhesion mea-surements and protective properties of these coat-ings are discussed in a later section.

Incorporation of Other Monomers into the NPMI/St/BMI System

In order to modify the properties of the polymercoatings, other acceptor monomers were intro-duced into the feed. Incorporation of such othertypes of monomers allows coating properties to betailored to meet specific applications.

Incorporation of Diethyl Fumarate

DEF is a good acceptor monomer due to the pres-ence of CAO groups on each of the carbons (Fig.1); it does not homopolymerize under normal con-

ditions. Copolymerization of various substitutedfumarates with donor monomers such as styreneand N-vinyl carbazole has been reported.12–14

DEF has a high reactivity toward styrene andalternating copolymers have been reported.13

Polymer coatings were formed by varying theNPMI/DEF ratio while keeping the styrene con-centration constant. The incorporation of DEFwas visible clearly through FTIR, as shown inFigure 5. As the amount of DEF in polymer in-creased, a shoulder at 1733 cm21 from the car-bonyl ester appeared, and also a distinctive bandat 1029 cm21 from the {OCOOO(CAO)O} groupin DEF increased in intensity. The Tg of the poly-mer also decreased to as low as 125°C. The DSCresults are shown in Figure 6. The molecularweight of the NPMI/DEF/St polymer showed aslight decrease with increasing amount of DEF.Incorporation of DEF offers the possibility of syn-thesizing flexible coatings with good adhesion.

Incorporation of GA

GA is a weak acceptor monomer and was used fortwo reasons: (1) adding epoxide groups to thepolymer, and (2) reducing the Tg of the resultantcoatings. The presence of the epoxide groups can

Figure 6 Differential scanning calorimeter thermograms for NPMI/DEF/St polymersfrom various NPMI/DEF feed ratios.


make the coatings top coatable by providing goodintercoat adhesion with epoxy based powder coatsystems, where the spontaneously formed coatingserves the function of a primer. The homopolymerof GA, poly(glycidyl acrylate), has a low Tg ofabout 10°C. Thus its incorporation should makethe coatings more flexible and crack resistant.Coatings were made using varying NPMI/GA ra-tios and keeping the St and BMI amounts con-stant. The incorporation of GA into the polymercoating was again confirmed using FTIR; thespectra are shown in Figure 7. As the concentra-

Table I Dependence of Tg on the NPMI/GAFeed Ratio

Monomer FeedNPMI/GA/St/BMI

(M)Glass Transition

Temp Tg (°C)

0.16/0.04/0.20/0.0025 2180.12/0.08/0.20/0.0025 2080.08/0.12/0.20/0.0025 1910.04/0.16/0.20/0.0025 176

Figure 7 FTIR spectra showing incorporation of GA in increasing amounts intocoating, as the GA/NPMI ratio in feed was increased, from the increasing intensity ofthe 908 cm21 peak from the oxirane group.

Figure 8 Coating thickness obtained for differentNPMI/GA ratios in feed. NPMI/St/GA/BMI 5 x/0.1M/(1 2 x)/0.0025M. Ninety minute polymeriza-tion.


tion of GA in feed increased, the peak at 910 cm21

due to the epoxide groups increased in intensityalong with the increase in the weak 1030 cm21

band {OCOOO(CAO)O}. The glass transitiontemperatures of the polymers formed also de-creased with increasing concentration of GA inpolymer as shown in Table I, from 212 to 125°C.This decrease in Tg indicated that more flexiblecoatings can be made by incorporating low Tgmonomers in larger amounts into the coating.Again, the rate of coating formation decreasedwith decreasing NPMI concentration in feed. Thecoating thickness obtained for various NPMI/GAfeed ratios, shown in Figure 8, decreases linearlywith decreasing NPMI concentration in feed for agiven polymerization time. This again is an indi-cation that only the NPM–St diradical is the ini-tiating intermediate in this monomer system.

Adhesion Studies

The shear strengths of systems containing NPMI/MEA, NPMI/DEF, and 4FMI/MEA, along withstyrene and BMI, are shown in Figure 9. The drystrength was highest for the system containingNPMI/MEA, at 35 MPa, and lowest for the systemcontaining 4FMI/MEA, at 28 MPa. The wetstrength for the 4FMI/MEA system was mea-sured at 21 MPa, however, which was consider-ably higher than wet results for the other twosystems. This points to the more hydrophobic na-ture of coatings containing 4FMI.

Dielectric Constant

The dielectric constant value was lower than 3.0for coatings made from the three different mono-

Table II Dielectric Constants Measured for Various Polymers

Frequency(Hz)

Dielectric Constant

NPMI/MEA/St/BMI0.05/0.05/0.10/0.0025

NPMI/DEF/St/BMI0.05/0.05/0.10/0.0025

4FMI/MEA/St/BMI0.05/0.05/0.10/0.0025

0.001 2.80 2.73 2.490.01 2.73 2.69 2.420.1 2.68 2.66 2.39

60 2.63 2.62 2.341000 2.62 2.60 2.33

10000 2.62 2.60 2.32

Figure 9 Adhesion measurements of coating compo-sitions under dry conditions and after 3 days immer-sion in 60°C water (error bars indicate 95% confidenceinterval). Styrene and BMI were also present in sameamounts of 0.2 and 0.005M, respectively, in all threesystems.

Figure 10 Al 2024 sample coated from 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M coating bath after 2000 hexposure to salt fog at 35°C (ASTM B-117 method).


mer systems, in the frequency range from 1023 to104 Hz (Table II). Values for commercial polyim-ides such as Pyralin are higher at about 2.8.15 Thelow dielectric constant for these polymers is prob-ably due to the presence of relatively nonpolarstyrene units in an alternating sequence alongthe main chain, and the high Tg of the polymer.The dielectric constant was highest for the NPMI/MEA/St/BMI system, which has a relativelyhigher concentration of polar groups. For theNPMI/DEF/St/BMI system the dielectric constantwas lower, and fluorinated maleimide furtherlowered the dielectric constant to the range of 2.4.The low dielectric constants of these coatings,especially at low frequencies, make them suitablefor use as dielectrics in electronic applicationssuch as circuit boards, where high resistance toheat and potential are required.

Corrosion Studies

Salt Fog Corrosion Testing

The protective qualities of the polymer coatingswere evaluated using the ASTM B-117 salt fogaccelerated corrosion test.16 Both 6061 and 2024alloys were tested. The 2024 alloy is highly proneto corrosion due to presence of 4–5% copper. Fig-ure 10 shows results from an Al 2024 samplecoated from a 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M solution exposed for 2000 h. A 0–1 mmcreep was seen around the scribed “X,” and nofield blisters were seen. Samples with the 4FMIcoating thickness of 10 mm also did not show anyvisible corrosion after 3000 h exposure. Theseresults are consistent with those previously re-ported for the NMPI/St/MEA/BMI system.1

Electrochemical Impedance Spectroscopy

EIS is a very widely used technique to studycorrosion behavior of systems such as polymercoated metals.17,18 By applying the AC signal overa wide range of frequencies and recording thecurrent, the resistive and capacitative compo-nents of system can be calculated using a simpleequivalent circuit such as one shown in Figure 11.

Figure 12 shows a Bode plot of a 4 mm thickcoating on Al 2024 alloy. The impedance of thecoating decreased with exposure time. This indi-cates the permeation of salt and water through

Figure 11 Equivalent circuit diagram used to fit theelectrochemical impedance spectra.

Figure 12 Bode plots at various immersion times of a4 mm thick coating on Al 2024 made from 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M coating bath.

Figure 13 Bode plots at various immersion times of a45 mm thick coating on Al 2024 made from 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M coating bath.


some pores and possible initiation of localized cor-rosion. After 50 days of exposure, the impedancewas still much higher than that for bare alumi-num. A drop in the impedance of a coating canalso occur if only a few defects are present in thecoating. No corrosion was seen upon visual in-spection. Thus EIS analysis reveals corrosion pro-cesses occurring at the microscopic level, whichcannot be simply correlated to visual observa-tions. Figure 13 shows the Bode plot for a 45 mmthick coating, made from 4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M solution, for exposure up to 106days. No significant change was observed in theimpedance of the coating, indicating excellent re-sistance to salt and water diffusion. Further, nocorrosion could be detected upon visual inspec-tion.

In order to gain some insight into the corrosionprocess, the impedance behavior of the 4 mm thickcoating was modeled using the simple equivalentcircuit described earlier. This circuit can be usedto model behavior of polymer coated metal sys-tems. The values of the pore resistance Rpo andcoating capacitance Cc after fitting the data areshown in Table III.

A decrease in the pore resistance indicates theformation of conductive paths and possible dam-age to the coating.18 An increase in the coatingcapacitance occurs due to the salt water uptakeby the polymer. Since the dielectric constant ofthe salt solution is much greater than that of thepolymer, even a small amount of water uptake isdetected as an increase in the coating capaci-tance. A negative value of the solution resistanceRsol is because of a relatively large error in mea-suring a small resistance. An overlay of modeleddata with the experimental data for 3 h immer-sion time is shown in Figure 14. In the Nyquistplots shown in the figures, the phase angle u isdefined as tan(u) 5 Zimag/Zreal (Z0/Z9). The simpleequivalent circuit model fits the data well in the

initial exposure times. Some deviations betweenthe experimental data and the model were seenfor 50 days of exposure time because the entirecorrosion process is much more complex than theone represented by the equivalent circuit model.

CONCLUSIONS

The spontaneous polymerization process was ap-plied successfully to form uniform conformal coat-ings on aluminum using three different monomersystems. Polymerization occurred only on the sur-face of the metal and not in the solution. In allsystems discussed here, the rate of polymeriza-tion seemed to be limited by diffusion of the mono-mers to the reaction site, based upon the linearincrease in the thickness of the coatings formedas a function of the square root of polymerizationtime. The rate constants for different coatingswere dependent on the specific monomers; how-ever, a coating of desired thickness could be ob-tained by varying either polymerization time ortemperature. Varying monomer concentration in-

Figure 14 Nyquist and Bode plots along with equiv-alent circuit model fits for sample from Figure 12: 3 himmersion.

Table III Values of the Various Elements of theEquivalent Circuit Obtained by Fitting the Datafor an Al 2024 with a 4 mm Coating Made from4FMI/MEA/St/BMI 0.1/0.1/0.2/0.005M Solution

Immersion TimeRpo

(V-cm2)Cc

(mF/cm2)Rsol

(V-cm2)

1 h 2.26e7 1.87e-2 2203 h 1.82e7 1.84e-2 220

18 h 8.78e6 3.3e-2 26050 days 5.05e5 39.7e-2 220


fluences both the polymerization rate and the mo-lecular weight of the coating.1,2

The coating compositions investigated had ex-cellent adhesion to the aluminum substrate, andafter soaking in 60°C water the 4FMI test jointsretained to up to 70% of their dry strength value.These results correlated well with impedance val-ues on the same coating system, where a verylittle drop was seen as a function of immersiontime in salt water. Both these observations indi-cate low water uptake in the coatings. Coatingsless than 10 mm thick also exhibited excellentprotective behavior. Salt spray results were alsoexcellent in light of the fact that no passivatingpretreatment was provided to the aluminum.

High glass transition temperatures and lowdielectric constant data for the tested coatingsmake them suitable for use as conformal coatingsin electronic applications.

Final properties of the coatings made by spon-taneous polymerization can be tailored to specificrequirements by using different monomers in dif-ferent ratios. Increasing amounts of GA and DEFwere found in the polymer, from their infraredspectra, as their ratio relative to NPMI was in-creased. Increasing amounts of GA or DEF alsolowered the glass transition temperatures of thecoatings and made them more flexible. Other ac-rylate and methacrylate monomers can be incor-porated and more complex polymer systems canbe formed in order to obtain a wider range ofphysical and chemical properties. Thus the spon-taneous polymerization process provides a novelmethod to form protective coatings using an en-vironmentally benign, chrome free process.

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coating formation by spontaneous polymerization on aluminum: various monomers

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