Protective coatings on aluminum by spontaneous polymerization

Protective Coatings on Aluminum by Spontaneous Polymerization

RAJAT AGARWAL. and JAMES P. BELL

Polymer Program Institute of Materials Science, U- 136

University of Connecticut S t o v ~ , Connecticut 06269-3 136

A novel process to form protective polymer coatings on aluminum using spontaneous polymerization is described. When an aluminum sample is immersed in a partly aqueous monomer solution, polymerization proceeds rapidly on the metal surface, without the addition of initiator, at room temperature. The polymer coatings studied here were formed using styrene (St), N-phenyl maleimide (NPMI), 2-methacryloyloxy acetoacetate (MEA), and bis-maleimide (BMI), monomers, from soluticlns of different compositions. A possible mechanism for spontaneous initiation and propagation is proposed. The effects of process variables such as polymerization time, monomer concentration, and monomer ratios in feed on the coating thickness and properties are presented. The glass transition temperatures, adhesion strength to aluminum, thermal stability and corrosion resistance of these coatings are reported. The coatings obtained are up to 50 pm thick and conform to the shape of the substrate. They have excellent thermal stability, low dielectric constant, and show very good resistance to corrosion under the ASTM B- 1 17 accelerated salt fog test. Other monomers and metals may also be used.

INTRODUCTION

y volume, alumiinum is the most widely used B metal today. Applications range from aerospace and marine to architelhn-aJ areas. In all cases a surface treatment/finishing process is applied. Anodiza- tion and conversion coatings are the two most com- mon treatments for aluminum. Anodization is an energy intensive process and uses phosphates, while conversion coatings use chromates and other heavy metal ions. Both of these treatments generate environmentally hazardous effluents. Other coating processes currently used for prixnary or secondary surface treatment are dip coating, spray coating, and powder coating. Each of these prccesses has a limitation of either uniformity or efficiency. The process of uniformly coating an object with a complex topography is particularly challenging. Thus, there is a need to develop alternate surface treatment processes that can over- come some of the linutations of the above-mentioned conventional processes and that are also environmentally green. This paper describes a novel process for forming protective polymer coatings on aluminum and its alloys. The same process can also be used for other technologically important metals such as iron, copper, and their alloys.

Spontaneous polymerization on metals is a new process by which polymer coatings are synthesized di-

rectly on the metal surface. The process is conducted at room temperature in a single tank of monomer solution, and no external driving force is required. The key difference of this process from other conventional coatmg processes such as dip coating or electrophore- sis is that here, the polymer chains *grow” at the metal surface instead of being deposited. Significant advantages of the process are that the coatings formed are conformal and pinhole free, and uniform coatings can be formed on objects with complex top- ographies. The process also results in very good adhesion of the polymer coating to the metal substrate, because wetting of the metal surface is easier by monomers as compared with wetting by polymers.

The simplest systems studied were copolymers of styrene (St) and N-phenyl maleimide (NPMI). The St- NPMI system gives an alternating copolymer when polymerized by a free radical mechanism (1). mure 1 shows the structures of the monomers used in this research, along with their roles when present in the coating. Copolymer coatings of styrene (St) and N- phenyl maleimide (NPMX) were spontaneously polymerized on aluminum. Bis-maleimide (BMI) was incorporated in the polymer coating for crosslinking, and 2-(methacryloyloxy) ethyl acetoacetate (MEA) was incorporated as adhesion promoter. Copolymers of St with NPMI and its derivatives have been synthesized

299 POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1998, Vol. 38, No. 2

Rajat Aganoal and James P. Bell

CH3 H2C d ! H kdi ketone

I C -0 4H2-CH2-O-C -CH2'-c -CH3

0 a St: Donor, hydrophobic, II 4

ME& Acceptor, adhesion promotion, low dielectric constant crack resistance

8 e

0 0

BMI: Acceptor, cross-linker for uniformity, high 1 NPMI:Acceptor, high 6

FQ. 1. Monomer structures and their roles.

in solution and electropolymerized in the past (2). Many different initiation and polymerization mechanisms for such donor-acceptor (D-A) monomer systems have been proposed and are reviewed (3-5). The novel aspect of the present process is that under the proper conditions, a St-NPMI copolymer will spontaneously form on the aluminum surface in absence of any initiator.

Polymer coatings up to 50 pm in thickness were formed. The coatings were uniform and conformal in nature and showed excellent adhesion to the substrate. Such coatings can be used in a variety of applications ranging from coating of circuit board components to automotive components. This room temperature method of polymerization requires minimum surface pretreatment of the substrate, and has the advantage of being economical and environmentally friendly.

The focus of the current research has been three- fold: (i) to investigate the spontaneous initiation of polymerization: (ii) to study the effects of various pmc- ess parameters such as aluminum surface pretreatment, monomer feed composition, polymerization time, and drying temperatures on the properties of the resultant polymer coating: and (iii) to develop a process to synthesize useful protective coatings on aluminum for electronic applications. The desired properties for such applications are high temperature resistance and stability, good electrical breakdown resistance, low dielectric constant, corrosion protection, uniform thickness, and conformity to the substrate. The measurements of the above properties are reported.

ExPERnuEmAL

Matdab. Styrene from Aldrich Chemical Co. was vacuum distilled at 4OOC. "MI and BMI were purchased from Mitsui Toatsuo Chemical (20.. Japan. ~ p m was recrystallized from cycloh-e and BMI

was used as obtained. The compound 2(methacryloy- 1oxy)ethyl acetoacetate (MEA) from Aldrich Chemical Co. was purified by passing through DHR-4 inhibitor removal column from Scientific Polymer Products. N- methyl pyrrolidinone (NMP) was purchased from Fisher Scientific Co. The aluminum used was 2024, 6061, or 7071 alloy. Al 2024 (4.5% Cu, 1.5% Mg, 0.6% Mn) has a high shear strength and is used as a structural aircraft alloy, but is highly prone to corrosion. A6061 (1.0% Mg, 0.6% Si, 0.2% Cr, 0.27% Cu) is used in heavy duty structures where corrosion resistance is needed (6). Polymerization was conducted in a Pyrex beaker or custom-designed 1 cm X 8 cm X 8 cm polypropylene tanks.

Aluminum Retrerrtmeas. Aluminum was given to two types of pretreatments: (i) Samples were de- greased with 5% aqueous Micro (a laboratory general- purpose alkaline soap) solution, rinsed with distilled water and treated with 5 O h hydrofluoric acid for 15-30 sec, followed by a distilled water rinse. (ii) Samples were grit blasted using 170 mesh alumina and rinsed with distilled water. Sometimes, grit blasted samples were also etched by hydrofluoric acid as above.

solved in NMP. Dilute aqueous sulfuric acid (0.025M) was then added slowly to the solution during stirring until a 57/43 volume ratio of NMP/water was reached. MEA was added at the end. A clear solution was obtained by this method. At these compositions the solution is at its limiting solubility for the monomers. Polymerization. The monomer solution was first

purged with nitrogen: the dissolved oxygen content was kept to less than 2 ppm. When the pretreated aluminum was immersed in the monomer bath, a white, swollen polymer coating Started forming. The presence of a small amount of oxygen pmented the polymerization process from spreading into the entire

M.runnnr teed "MI, BMI and StWere dis-



solution, after being initiated at the aluminum surface. Polymerization times for samples in the bath were varied from 5; min to 120 min. Following the polymerization, the coated sample was immersed in a gently stirred 10Yo NMP aqueous solution to remove excess NMP and trapped monomers. The coated coupons were oven dried at 150°C for 1 hour to remove most of the water and NMP, and then at 225 to 250°C for 4-6 hours to remove the last traces of NMP.

characterization. Fourier transform infrared (FTIR) spectra for the polymer coatings were taken using a Nicolet 60X FI'IR spectrometer, by pressing KBr pel- lets from mixtures of about 150 mg KBr powder and 1-2 mg polymer. Spectra of resultant polymer coatings were recorded for varying monomer feed compositions. One hundred twenty-eight scans at 4 cm-' resolution were recorded for all spectra.

A Perkin-Elmer DSC-7 was used to measure the glass transition temperature (TJ of the polymer coatings made from v q n g monomer feed compositions. Thermal stability was measured using a Perkin-Elmer TGA-7. A heating rate of 20"C/min was used in the DSC, while the TGA was run at 2"C/min in nitrogen. Thermal mechanical analysis (TMA) was done on a PE-TMA 7 instrument using a 0.035 inch diameter flat head quartz probe to measure the thermal expansion coefficients (10 rnN force) and softening temperatures (500 mN force) of the polymers.

Molecular weights were measured using a Waters 550C- 150 gel permeation chromatograph using a calibration curve from polystyrene standards. For NMP/ St polymers, THF was used as the solvent and the mobile phase. The NPMI/St/MEA polymers present a solubility problem if precipitated. They were kept in solution in N M P and either NMP or THF was used as the mobile phase.

Adhesion of the polymer coatings to the aluminum substrate was measured using the torsional testing method of Lin and Bell (7). Using this test, the polymer metal joints can be broken in a pure shear mode, and the joint shear strerlgth can be measured precisely. This method proved particularly suitable in our case where the coatings were only 25 pm thick. The co-

polymer coatings were first formed spontaneously on the grit blasted aluminum joints and dried. The joint pairs were then adhered together by applying 0.045 gms of an equimolar mixture of epoxy (Epon 828 from Shell Chemical) with methylene dianiline (MDA) cur- ing agent, uniformly on the annular ring, and cured at 150°C for 3 hours. The setup of the joints is shown in Q. 2.

Dynamic mechanical analysis was done with the aid of a Polymer Labs DMTA analyzer at 2"C/min and 1 Hz, in single cantilever mode. Specimens of 2 cm x 1 cm size with 1-2 mm thickness were used. Dielectric constant measurements of the polymer coatings were performed on a Time Domain Dielectric Spectrometer (TDDS) at room temperature. Corrosion protection properties of the polymer coatings were studied by ex- posing samples to a 5% NaCl salt fog, following the ASTM B- 117 test method. The samples tested were grit-blasted Al 6061 and 2024 alloy coupons and had a 25 pm thick polymer coating.

RESULTS AND DISCUSSION

Polymerization Rocesa. The solvent quality of the 57/43 NMP/water solution is close to the solubility limit of the monomers, The polymer formed on the aluminum surface is insoluble in the solution but is in a swollen state. This condition is essential, as it al- lows the initiation and formation of the polymer at the aluminum surface without dissolution. The swollen nature of the polymer coating permits the diffusion of monomers through the polymer to reach the aluminum surface for initiation and also to react with the propagating chain ends.

Reaction Mechanism. Requirements for spontaneous polymerization to initiate on aluminum are that at least one electron donor monomer (styrene) and one electron acceptor monomer (NPMI, MEA) be present together in a solution of pH less than 3.3. Homo- polymers of any of the monomers used in this study could not be obtained by applying the above conditions. The electron donating character of styrene comes from the presence of the phenyl ring next to the C=C bond. The electron withdrawing character of

Face View Section View Fig. 2. Schematic of the torsional testing joints. Dark ring is the contact face.

POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1998, Vol. 38, No. 2 301

Rajat Agarwal and James P. Bell

the NPMI comes from the presence of the two carbonyl groups attached to the C=C bond. Spontaneous copolymerization between the two monomers can take place only if there is a substantial polarity difference between the two reaction partners (3, 8). In the styrene-NPMI system under investigation, such spontaneous copolymerization was not seen in the free standing monomer solution from which the polymer coatings on aluminum were formed. The fact that the polymerization was initiated only in the presence of the aluminum surface indicates that the metal plays an active role in the initiation process.

This indicates that the polarity difference between St-NPMI or St-MEA monomer pairs is not strong enough to cause spontaneous polymerization in solution, and a further driving force is needed. Also, polymerization is initiated only in acidic solutions. Solu- tions at neutral or basic pH showed no spontaneous polymerization. Different mechanisms have been proposed in the literature on the nature of initiation and propagation on the polymerization of donor-acceptor monomer systems (4). Hall has arranged the monomers in their donor and acceptor strengths to prepare an "Organic Chemist's Periodic Table" (3). According to such a table, the initiating specie is a tetramethylene diradical for weak donor-acceptor pairs. Polymeri- zation then proceeds by free radical mechanism and a high degree of alternation is achieved as a result of cross propagation. With increasing polarity difference between the donor-acceptor monomers, the diradical assumes an increasingly polar character and the polymerization mechanism shifts from free radical to zwit- terionic (8). Other Diels-Alder cycloadducts or low molecular weight products may also be obtained. The role of Lewis acids in such systems has also been in- vestigated (4, 9. 10). Lewis acids increase the electrophilicity of the acceptor monomers by complexing with them, and in some cases cause spontaneous polymerization of otherwise stable donor-acceptor systems. In almost all cases the polymerization rates show a large increase in the presence of Lewis acids.

Other mechanisms proposed are formation of a donor-acceptor complex, which then may add to a free radical, to give an alternating copolymer (1 1). Forma- tion of donor-acceptor monomer complexes has been detected by measuring proton shift using NMR and UV-Vis spectroscopy (12). It has also been proposed that even though donor-acceptor complexes are formed, they might not take part in the polymerization reactions. Studies on NPMI-St systems have shown that addition of NPMI to styrene takes place by conventional free radical methods (13, 14). In an exten- sive review of the styrene-maleic anhydride system, Ebdon et aL (1 5) have concluded that 'most, if not all, alternation arises as a consequence of relatively rapid cross-propagation reactions rather than from incorporation of styrene-maleic anhydride complexes." These observations can also be applied to the styrene-NPMI copolymer system.

m e driving force for spontaneous polymerization in the system under study comes from the alwnirmm

4 ' 1760 ' 1720 ' 1680 ' ' "&i ' '&a0'

Wavenumber (cm-l)

W

C=O band shift in presence of SnCI, Lewis Acid in Chloroform

Fig. 3. FIW spectra showing C=O band shi in h P M l on addition of SnCl, in chloroform.

surface, where Lewis acids such as A13+ are generated when a sample of aluminum is immersed in an acidic solution. The A13+ ion is classified as a hard Lewis acid and therefore has a strong affinity for an electron pair (16). In other studies, Lewis acids such as ZnCl, and SnCl, have been shown to cause or promote spontaneous reactions of acrylonitrile with substituted 1,3-dienes or MMA-styrene systems (10). Interactions of A13+ Lewis acids with NPMI cannot be observed di- rectly by FI'IR or NMFt spectroscopy techniques since the solutions we are working with are partly aqueous.

To understand the role of Lewis acids, a solution of SnCl, and NPMI in chloroform was studied using liq- uid cell FTIR In Rg. 3 we can see that as the amount of SnC14 in solution is increased, the C=O peak posi- tion shifts to lower wavenumber. indicating the formation of a complex. NMR experiments show that the protons adjacent to the C=C in NPMI shift downfield from 6.769 ppm to 7.029 ppm upon addition of equimolar amount of SnCl, in a NPMI solution in CDC1,. It is proposed here that these aluminum ion Lewis acids interact with NPMI and increase the electrophilicity of the monomer, consequently resulting in the formation of an NPMI-St tetramethylene diradical and subse- quent polymerization. Such Lewis acid sites can be formed only when aluminum is immersed in an acidic solution. This is probably why no polymer is formed when the solution is at a neutral pH. The polymerization then proceeds by alternate addition of the donor [styrene) and acceptor (NPMI) monomers to the grow- ing radical. Figure 4 shows the proposed mechanism for spontaneous initiation and polymerization. Similar schemes for spontaneous polymerization in non-aqueous solvents have been proposed by Hall and Padias (8).

--St copobmer Coating*. The simplest monomer system capable of spontaneous polymerization is the St-NPMI system. In order to study the effect of monomer feed composition on the composition of the



Ph 4 Alternating +

Ph

resulting polymer coating, the St-NPMI monomer ratio in feed was varied, keeping the total monomer concentration constant at 0.2M. The polymer obtained was scraped off the aluminum while wet, rinsed with ethanol and dried. The relative amounts of NPMI and St in the copolymer were calculated by measuring the 11183/11453 peak intensity ratios obtained from FI'IR spectra and by using the calibration curve equation: 1,,,,/1,4,3 = 0.038 PMI (wt?!) (17). Figure 5 shows that the copolymer composition is close to 1: 1, almost in- dependent of the feed composition. The monomer re- activity ratios were calculated at r,,, = 0.025 and rst = 0.0878 using the copolymerization equation. Other studies have reported rm = 0.052 and rst = 0.116 in THF (18). and rm = 0.36 and rst = 0.01 in DMF at 50°C using the Kelen'kdos method (1). Drying. The drying of the coating was done in a

forced air convection oven. Drying was accomplished in two stages. The first stage involved drymg at 130°C - 150°C for 30 min under forced convection. During this time, most of the water evaporates and the polymer becomes relatively rich in the higher boiling NMP (B.P. = 205°C). This leads to flow of the polymer, since NMP is a good solvent. The drying temperature in the first stage was critical in obtaining uniform coatings. If the temperature was greater than 150°C. water showed a tendency to boil and create blister de- fects in the coatings. The second stage drying at 200°C resulted in evaiporation of all the remaining solvent. In the early experiments, the final dried coating was nonuniform in thickness because of flow of the polymer while drymg. In order to eliminate this prob-

Plot Showing Forniation Of 1:l NPMI/St Copolymer

0 20 40 60 80 100 % NPMI in Feed

Fig. 5. Copoh~mer composition in NPMl/St system as a m - tion of the feed compos&on

copolymerization Flg. 4. Proposed mechanism for spontaneous initiation and propagation

lem, the copolymer coating was lightly crosslinked by adding BMI in the solution. The concentration of BMI was kept at 2.5% of the styrene concentration in solution. The coatings containing BMI were very uniform after drying. In all cases, however, coatings thicker than 5 p.m showed a tendency to crack upon cooling. The above conditions are not necessarily optimum, and further research is under way in this area.

Polymers containing P-diketone groups have been shown to increase corrosion resistance and adhesion of coatings formulations applied to metals ( 19-2 1). For this purpose, MEA was incorporated into the NPMI/St/BMI polymer system. MEA is a weak acceptor monomer and it can be expected that MEA will copolymerize with styrene. In order to test the inclu- sion of MEA, a series of polymers were made with varying concentration of MEA in the feed. While the St and BMI concentration was kept constant, the "MI/ MEA ratio was varied such that the total acceptor concentration was equal to the total donor concentration. Thus, as MEA in feed increased, NPMI decreased. ETIR spectra for the series are shown in Fig. 6. As the concentration of MEA in feed was increased, a car- bony1 peak in the 1625-1655 cm-' region appeared, and increased in intensity, relative to the 1774 cm-' imide C=O peak. The low wave number from the MEA peak can be assigned to the en01 form of the 1-3 beta- diketone moiety (22). Also, a general broadening of the peak in the 1720 cm-' region occurred, due to the overlapping from MEA and NPMI carbonyl stretching bands. The 1598 cm-' peak is due to the phenyl rings from NPMI and BMI. It also decreased in intensity as the MEA content relative to NPMI increased in the resulting polymer.

Addition of MEA had several effects on the coating properties. When NPMI/St/BMI coatings were cooled after drying, a large number of cracks developed. This tendency of the coatings to crack decreased as MEA content in feed increased, and cracking was eliminat- ed when equimolar molar amounts of MEA and NPMI were present in the feed. The reduction and elimination of cracks is evident in the optical micrographs of coated aluminum samples shown in Fig. 7. For all these samples, styrene was kept at 0.1M and BMI at 0.025M. This effect is probably due to plasticization of the polymer by the long side group chain on the MEA and increased hydrogen bonding in the polymer re-

I u ~ o n o f M E A i n I I I p I w I s t I B I M ~ .

Ezleect of Addition of MEA on coatAqg Rqmrtie8.


Rqjat Agarwal and James P. Bell

Rg. 6. FITR spectra showing incorporation of MEA in the NPMl/St copolymer.

304

NPMl1

a) V c 0 f % n a

NPMVMENSt = ~ M 0 . 1 M 1

Increasing MEA in feed L

2

Wavenumbers (cm-1)

Fig. 7. cm&ng behavior for &@rent coating wmpositions. NPMl/MEA rnol/mol (a) 1 O/O; b) 8/2; (c) 6/4: (4 5/5. (Styrene= 0 . l M i n a l l c a s e s . j

POLYMER ENGINEERING AND SCIENCE, FEBRUARY ISSe, Vol. 38, No. 2


50 -.

40 - n m n Z 30 s B

I- ------7

NPMVMEA O.lO/O.OO NPMVMEA 0.08/0.02

W NPMI/MEA0.05/0.05 t-. NPMVMEA 0.02/0.08

-1

40 { - BMI =0.0025M 6 1 - - - BMI=O.OM

\--

50 100 150 200 300 350

'Temperature (OC) Fg. 8. T M A in penetratiim comparing softening behavior for two feed compositions (Probe force = 500 miV,l.

sulting from the presence of the enol groups. The coefficients of expansion measured for two compositions showed that linear expansion coefficient decreased significantly, from 1.725X 10-4/"C for NPMI/St to 7.539XlO 5/0C for the NPMI/St/MEA 0.05/0.10/ 0.05M feed composition, with the incorporation if MEA in the coating. The lower value of expansion coefficient also results in lower stress buildup in the coating when it is cooled after drymg, thus reducing the tendency to crack. It may be possible to eliminate cracking by introducing other monomers apart from MEA. This is an additional area where further research is being conducted.

EEect of BMI on Coating Properties. Addition of BMI gave coatings of uniform thickness since flow of the coating during drying was arrested. Thermo- mechanical analysis of the coatings showed that in presence of BMI, the coatings showed a thermoset behavior. N o penetration of the probe could be seen above the Tg of the coating. w e 8 shows the probe penetration for two different compositions. In absence of BMI, the coating can flow above Tg

I NPMIIMEA 0.10/0.00 1

t-l NPMUMEA 0.08/0.02 NPMIIMEA 0.0510.05

c---( NFMIIMEA 0.02/0.08 n 1 I

40

3 30 5

2 20 z h 2 10

n m a

TJ, t

cn

n Increasing MEA v

- Rg. 9. Joint shear strengths for diikrent NpIw/MEA ratios in feed styrene = 0 . l M .

Adhedon Measurements. The addition of MEA also increases the adhesion of the polymer to aluminum. Joints were made from solutions with different NPMI/MEA ratios in feed, keeping St and BMI concentrations constant a t O.1M and 0.0025M, respectively. Figure 9 shows the joint shear strength measured as a function of increasing MEA/NPMI ratio in feed. In all cases, the failure occurred at the aluminum-polymer interface. The adhesion of the polymer coating increased from a value of 28 MPa for no MJ3A to 42 MPa for MEA/NPMI 0.08/0.02M. This es- tablishes the role of MJ3A as an adhesion promoter in the system. The wet strengths (3 days in 60°C water) for all compositions were measured in the range of 18-20 MPa as shown in Rg. 10. This indicated con- siderable strength retention in the aqueous environ- ment. The water uptake by the polymer reduces the adhesion. The equilibrium water uptake at 25°C was less than 2.5% for all compositions, but did show an increase with increasing MEA content in feed. Polymerization Kinetics. Figure 11 shows a plot of

coating thickness obtained as a function of polymerization time for various monomer compositions in the feed. The ordinate axis is plotted as the square root of time, which results in linear fits of the data. This indicates that such a polymerization process is limited by rate of diihsion of monomers to the propagating radi- cals. These observations are consistent with the equations obtained using a flat plate surface reaction model (23), where the rate limiting step is the process of diffusion of monomers in the swollen polymer coating being formed on the aluminum surface. In such a model the amount of the reaction product is proportional to the square root of the reaction time. If the process was reaction rate limited, then the coating thickness would have increased linearly with time.

As the ratio of MEA to NPMI in the feed was increased, the rate of polymerization decreased. This suggests two possibilities: (i) a lower activity of the MEA monomer as compared to the NPMI monomer;

i! 20 5 2 6 10

0 Increasing MEA

Fig. 10. Joint shear strengths ajkr 3 days' eqwsure to 60°C distilled water.


Rajat Aganual and James P. Bell

N P M I/M E A/ST/B M I xMO. 1 M/O .0025 M

NPMIIMEA 0.09M/0.01 M

h NPMIIMEA 0.02M10.08M

v

5 a 20 251 1 5 1

lo I,, , , e;;, __-- , , , 1 rp9----- 5

0 0 1 2 3 4 5 6 7 8 9 10

Time (min"') Rg. 1 1 . Coating thickness for di@ment feed co~sitions as a jimction of polymerization time.

(ii) the initiation occurs only from the NPMI-St tetramethylene diradical and the MEA monomer is incorporated in the propagation step.

Qure 12 shows the coating thickness vs. time for three concentrations of monomers in feed. As expected, the rate of polymerization decreases with decreasing concentration. In Fig. 13 the same data were trans- posed and the coating thickness was plotted as a function of monomer concentration for various polymerization times. From this curve, monomer concentration dependence was calculated for the rate of polymerization at different polymerization times. For 10 min polymerization time, a value of [M]2.048 was obtained and for 120 min, a value of [M11.523 was obtained. These exponents are higher than the value of one that can be expected in the classical case. To understand the above results, we will have derived the polymerization kinetics relevant to our system.

0 1Omin 40

[MIA1 523

[ MIA2.048

15-

0 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Monomer Concentration (M) Rg. 13. Dependence of coating thickness on the monomer concentration at d i i m t polymerization times.

C = NPMI/St/MENBMI 0.05/0.10/0.05/0.0025M

0 2 4 6 8 1 0 1 2

Time (min'") Rg. 12. Coating thickness for three monomer feed concentrations as afunction of polymerization time.

The polymerization equation can be written as:

q = kp [MI[M'] (1)

where kp is the polymerization rate constant, [1M] is monomer concentration and [W] is radical monomer concentration. If initiation rate is linked to the Lewis acid sites on the aluminum and a diradical is formed, then the rate of initiation can be written as:

R, = 2fk, [MI[Ll (2) wherefis initiation efficiency, = initiation rate constant, [L] is the concentration of Lewis acid sites on the aluminum surface. If termination is by coupling or disproportionation then the rate of termination can be written as:

Using the steady state assumption, we can say that R, = Rt, in which case we get the expression:

Substituting this expression for [W] in the polymerization Equation 1 we can write

or,

In the case of termination by chain transfer to monomer:

Rt, = [W][SHI (7)

where [SH] is the concentration of chain transfer agent. Again, at steady state 4 = 4 so:



and

(9)

If chain transfer to mcmomer dominates,

From Equations 6 and 9 it can be seen that a monomer exponent greater than 1 can be expected if rate of polymerization is plotted for different concentrations. At longer polymerization times, the diffusion of monomers through the swollen coating becomes an increasingly important factor and the exponent value for [MI can be expected to fall. Thus the observations in Fig. 13 can be explained by the above kinetic equations. How- ever, there were only three concentrations for all times available for the fit to [MI, so more experiments were done where solutions with seven different concentrations ranging from O.EIC to C were prepared and coating thickness was measured for two polymerization times of 20 min and 40 min. The results are shown in

C = NPMI/St/MEA/BMI 0.05/0.10/0.05/0.0025M 12

c

2 I I . . , l . . . . l l . . . I

0.5C 0.6C 0.7C 0.8C 0.9C 1.Oc 1.1c Concentration

Fig. 14. Dependence oj' coaiing thickness on the monomer concentration at 20 min and 40 minpolymerization times.

NPMI/St/lUIEA 0.05M/0.10M/O.O5M 0 0

A

600k 400k

- -- l o i

1 9 2

'ILJ A , ' , I , I , I , t 5 4

1.0 1.5 2.0 2.5 3.0 3.5 4.0 Solution pH

Rg. 15. EJkt of solution pH on the molecuIar weight of the po@ner obtained

Flg. 14 where exponents of 1.73 and 1.31 were obtained for polymerization times of 20 min and 40 min respectively. A second order dependence on [MI has been observed in other systems that undergo primary termination and certain redox polymerizations, where- as a lower than first order dependence has been seen under conditions where diffusion of monomers is slower than the normal propagation rate (24, p. 219).

Molecular Weight Measurements

Effect of pH on Molecular Weight. GPC measurements were performed on NPMI-St polymers spontaneously polymerized at various solution pHs. At a low pH, one would expect a greater number of Lewis acid sites generated at the surface, resulting in a high rate of initiation. This would give a relatively low molecular weight polymer in keeping with classical free radical polymerization equations (24). As the pH is increased, the number of Lewis acids formed should be less, and consequently the molecular weght of the polymer obtained should be higher. Flgure 15 shows the molecular weight (MA of the spontaneously polymerized coating plotted as a function of solution pH. As expected, higher pH gave higher molecular weight. The weight average molecular weight increased from about 6 X lo4 g/mol to about 6 X lo5 g/mol as the pH was increased from 1.25 to 3.2. Effect of Polymerization Time. Figure 16 shows

the molecular weight of the polymer obtained at different polymerization times, ranging from 10 to 120 min. Both the average molecular weights and the polydis- persity (PDI) remained rather constant with time. an =4X1@ g/mol and PDI=3. Thus, the polymerization can be terminated at any time to obtain coating of a desired thickness without influencing its properties.

EBect of Concentration. The molecular weight of the NPMI/St copolymer also increases with increasing monomer concentration as expected. From the free radical polymerization theory, kinetic chain length v is defined as the average number of monomer molecules consumed per each radical that initiates a polymer chain.

160kT

,140k'

g120k-

g100k- - 80k

Q, - \

Ul

60k

40k

NPMI/St 0.1 M/0.1 M

A PDI

A A A A

8 . I

8 8

1 0 20 40 60 80 100120140

Time (min) Q. 16. Effect of polymerization time on molecular weight of thepdymerobtained


Rajat Aganoal and James P. Bell

if termination is by couplin or disproportionation, then we can substitute [Mj from Equation 4 into Equation11,andweget:

or

or, we can say that:

- ~ ~ 1 0 . 5 (14)

In case of termination by chain transfer, by substituting the expression for [W] from Equation 8 into Equa- tion l l, we can say:

v - [MI1 (15)

Generally then, the kinetic chain length can be ex- pressed as:

v = m[Mn (16)

where rn and n are constants. Also, v is proportional to the number average molec-

ular weight a,,. A nonlinear regression done on a,, vs. C gives a value of n = 0.61, which seems to be in good agreement with theory in the light of all the assump- tions made.

In terms of effect on the process, the adhesion of the wet swollen polymer coating depends on the concentration of the polymerization solution. In Rg. 17, C is a concentration of NPMI/St = 0.2M/0.2M, below a concentration of 0.08M each of NPMI and St, the wet adhesion is very poor and it is difficult to obtain uniform polymer coatings. If the wet adhesion is poor, the polymer slides off the aluminum when the sample is

NPMI/St 0.2W0.2M 5e+5 4

0 t 3

I 0 I

r u I

0.04 0.08 0.12 0.16 0.20

Concentration (moleshit)

Rg. 17. mkct on monomer feed conceniration on the molecu- larweigNof theaxting.

Table 1. Glass Transition Temperatures for Polymers Made From Diffennt NPMUMEAISt Monomer Ratios.

~

Monomer Feed Composition (Mole Ratios) NPMUMEA/St

Monomer Feed Composition (Mole Ratios)

NPMUMEA/St/BMI ('8) 1 OlOl10 21 6.2 6/2/10 - 7/3/10 207.7 61411 0 - 515110 205.5 41611 0 197.2 3/7/10 200.4 2/8/10 200.6 11911 0 198.6

1 OIOl1 Ol0.25 8l2l10l0.25 7l3llOl0.25 61411 010.25 51511 010.25 41611 010.25 31'711 010.25 2/8llOI0.25 1l911010.25

21 9.8 21 8.4 21 5.8 21 5.5

21 5.1 210.3

201.8

-

-

taken out of the monomer feed solution. Threshold concentrations for wet adhesion also depend on the type of monomer feed composition used.

Thermpl Pmpertiem. Glass transition temperatures were measured using differential scanning calorime- tery (DSC). A single Tg was observed for all NPMI/St copolymers and all other compositions of NPMI/St/ MEA and NPMI/St/MEA/BMI. This indicates that MEA-St acceptor-donor monomers were included in the NPMI/St copolymer in random units. The Tg decreased only slightly with increasing MEA content in the polymer coatings, both in presence and absence of BMI (Table I). All transition temperatures were greater than 200°C. Glass transition temperatures of the coating also increased with addition of BMI in feed. This is a significant outcome. It demonstrates that the spontaneous polymerization process is suitable for making high temperature resistant coatings by polymerization at room temperature. Thermal stability of the coatings was found to be very good. Onset of degradation took place at more than 350°C under nitrogen atmosphere and was not affected by incorporation of MEA, as shown in Rg. 18.

Mochaniud hoper tb. Dh4TA was done on two different coating compositions shown in F'ig. 19. The elastic modulus obtained in the glassy region was about 1 GPa, indicating that the coatings were very

308 POLYMER ENGINEERING AND SCIENCE, FEBRUARY lesS, Vol. 38, No. 2


Modulus 01 2 Coating Compositions of the edges of the sample, illustrating the uniformity of the polymer coating. Corrosion studies were also performed on the Al 2024 alloy since this alloy is highly prone to corrosion. As seen in Fig. 21, after 2000 hours' exposure, very little corrosion was seen around the scribed 'X' on the sample.

0 50 100 150 200 250 300

Temperature ("C) Rg. 19. E' and E" of p!ymer coatings made from two diim- ent monomer feeds as a j i n of temperature.

stiff. The addition of MEA did not result in loss of modulus. Again, only one transitition was seen in both E' and E curves. These results are in agreement with the single transitions observed in all the samples in DSC. Electrical Properties. The coatings exhibited good

resistance to DC potential. Samples coated with 20 pm thick polymer, spontaneously polymerized from a feed solution of NPMI,'St/MEA/BMI 0.05/0.10/0.05/ 0.0025M. did not show any sigdicant current leak- age up to an applied potential of 1800 V, where the experiment was stopped. The dielectric constant value was well below 3 for the measured frequency range from Hz to lo4 I& (Table 21. The low dielectric constant is probably clue to the presence of relatively non-polar styrene units in an alternating sequence along the main chain, ,and the high Tg of the polymer.

Corroaion Studies,. Figure 20a shows an unex- posed 6061 aluminuin sample coated with 20 km thick NPMI/St/MEA polymer coating on the right half. Flgure 20b shows the sample after 3000 hours of exposure to salt fog (ASTM B- 1 17). The coated side is unaffected, while the uncoated side is corroded. No propagation of corrosian is seen occurring at the polymer-metal interface. No corrosion is seen around any

Fig. 20b. sample after 2000 hours of 8-117 salt SP- expo SUre.

Table 2. Dielectric Constant of the Polymer Coating for Various Frequencies.

Frequency NPMVSthIEAIIBMI (W 0.0!VO.1/0.05/0.0025M

0.001 2.80 0 01 2.73 01 2.68 60 2.63

1000 2.62 2.62 Flg. 21. A1 6061 sample with X scribe, exposed to 2500

hours' salt spray. 10000


Rajat Aganual and James P. Bell

CONCLUSIONS

From this work, the first conclusion is that the spontaneous polymerization process can be used success- fully to form uniform conformal coatings on aluminum. Spontaneous initiation of polymerization takes place, possibly owing to the formation of NPMI-St tetramethylene diradicals at the metal surface. The tetramethylene diradical concept has been proposed before by Hall et al. to explain the phenomenon of spontaneous polymerization in other donor-acceptor monomer systems. l b o distinct features of this process are (i) polymerization is carried out in the presence of water and (ii) polymerization process occurs only around the metal surface. All the previous studies on spontaneous initiation and polymerization were done in anhydrous solutions. The presence of water leads to a heterophase polymerization, since the polymer formed is insoluble in the monomer solution. The propagation takes place by a free radical mechanism. The molecular weights of the coatings were high, in the range of Rn = 50,000 and an = 125,000. The properties of the coatings could be modified and tai- lored by varying the ratio of MEA/NPMI in feed. Incor- poration of MElA resulted in elimination of cracks and improvement of adhesion of the polymer to aluminum. The kinetics of the process seem to be limited by the W s i o n of the monomers to the reaction site. The desired coating thickness could be controlled by varying Merent variables such as polymerization time, monomer concentration. and solution pH. It is possible that the process could be accelemted by gently agitating the polymer solution to promote mass transfer. The polymer coatings obtained were noncrystalline and clear in appearance. The electrical, thermal, and corrosion resistance properties of the coatings obtained were excellent. Such coatings seem suitable for high temperature and electronic applications. Aluminum samples with complex shapes were also coated d o r m l y . The method is economical since the process occurs at room temperature, requires a simple surface pretreatment, and polymers are formed in partly aqueous solutions. In the future, other donor-acceptor monomers will be incorporated into the polymer to obtain coatings with desired specific thermal or electrical properties. More work needs to be done on the exact nature of the initiation and polymerization mechanisms.

REFERENCES S. Iwatsuki, et al., Macromlecules, 24,5009 (1991). X Zhang and J. P. Bell, J. Appf. Pdym Sci, 62. 1303 (1996). H. K. Hall Jr., Angmadie Chemie, Internationnl Edition English, 2a, 440 (1983). J. M. G. Cowie. Altenurting Copolymers, Plenum Press ( 1985). A. Matsumoto, T. Kubota, and T. Otsu, Marromolecules, 23, 4508 (1990). W. F. Smith, Structure and Properties of Engineering AUoys, McGraw-Hill Book Co.. New York (1981). J. P. Bell and C. J. Lin, J. AppL Po@m Sci, 16, 1721 ( 1972). H. K. Hall Jr. and A. B. Padias, Accounts of Chemical Research 23.3 (19901. T. Ikegami and H. Hirai, J. Polym Sci: Part A-1, 8, 163 (1970). M. G. Mikhael, A. B. Padias, and H. K. Hall Jr., Macro- molecules, 17. 7409 (1994). J. R Ebdon, MaJcromL Chem. MacromoL Symp., 10111, 4 4 1 (1987). E. R. Garrett and R. L. Guile, J. A m Chem Soc., 75, 3958 (1953). S. A. Jones and D. A. Tirrel. J. Polym Sci: Part A: Polym Chem, 2s. 3177 (1987). A. Saito and D. A. Tirrel, European P d y m J., a@, 343 (1993). J. R Ebdon and C. €2. Towns, J. MacromL Sci-Macro- mf. Chem Phys., -6, 523 (1986). W. B. Jensen. The Lewis Acid-Base Concepts An

Y. Yuan, A. Seigmann, M. Narkis , and J. P. Bell, J. AppL pdym Sci, 61. 1049 (1996). H. Cheng, G. Zhao, and D. Yan. J. Po@m Sci: Part A: P o r n Chem, SO, 2181 (1992). V. P. Wystrach, et aL, Chemical T~eabnent of Metal; V. P. Wystrach, etaL;ed., US. Patent3,615,888 (1971). N. T. Castellucd, et aL, Betaililcetoneep~y Resin Reac- tion products Useful for Providing Corrosion Resistance: N. T. Castellucci, et aL; ed.: PPG Industries. Inc.: US. Patent 4,321,305 (1982).

lating Compounds; H. S. Gilbert, ed., The Dow Chemical Company: US. Patent 4,324,676 (1982). R. M. Silverstein et aL, Spectrometric Ident i iat ion of Organic Compounds, 5th Ed.: John Wiley & Sons. Inc. (1991). 0. Levenspiel. Chemical Reaction Engineering, 2nd Ed., John Wiley & Sons, Inc. (1989). G. Odian, Principles of Polymerization. 3rd Ed., John Wiley & Sons, Inc. (1991).

overview, Wiley-Interscit~~~ (19791.

H. S. Gilbert, C o r n p ~ ~ i t i o n ~ Containing beta-diketo Che-

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3.

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8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

Receiued February 5,1997 Revised April 1997

31 0 POLYMER ENGINEERING AND SCIENCE, FEBRUARY 19919, Vol. 38, No. 2

Documents

Protective coatings on aluminum by spontaneous polymerization