Chemical structure evolution of acrylic-melamine thermoset upon photo-ageing

European Polymer Journal 48 (2012) 172–182

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Chemical structure evolution of acrylic-melamine thermoset upon photo-ageing J.-F. Larché a,b,d, P.-O. Bussière b,c,⇑, P. Wong-Wah-Chung b,c, J.-L. Gardette a,b a

Clermont Université, Université Blaise Pascal, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6505, LPMM, F-63173 Aubière, France c Clermont Université, ENSCCF, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, France d PSA-Peugeot-Citroën, Direction Technique et Industrielle, Route de Gisy, 78140 Velizy Villacoublay, France b

a r t i c l e

i n f o

Article history: Received 21 March 2011 Received in revised form 25 October 2011 Accepted 28 October 2011 Available online 6 November 2011 Keywords: Photo-oxidation Hs-SPME AFM nano-indentation Mechanism Structure Acrylic-melamine

a b s t r a c t This article reports a study of the influence of UV light irradiation in the presence of oxygen on both the chemical structure evolution and mechanical properties of an acrylicmelamine (AM) thermoset. Photo-oxidation of the thermoset was investigated by several techniques, such as IR spectroscopy, DMA, headspace-SPME and atomic force microscopy (AFM). The measurements by infrared spectroscopy combined with gas analysis allowed us to elucidate details of the reaction mechanisms that are difficult to detect by only spectroscopic measurements. This approach was used to probe mechanistic aspects of the photo-oxidation of acrylic-melamine (AM) thermoset and was completed by dynamic-mechanical measurements combined with AFM and micro-hardness measurements, which revealed the effect of photo-oxidation on the mechanical properties of the thermoset. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Despite the growing use of waterborne, UV-curable or powder coatings [1–3], solvent-based acrylic-melamine coatings are intensively used in the automotive industry. Melamine technology is used alone as a hardener for acrylic or polyester resins or is mixed with urethane (isocyanate) technology [4] for a better compromise in terms of weathering and scratch/mar resistance [5,6]. Because these materials are often used as topcoats in multi-layered automotive paint systems [7], there is great interest in understanding the influence of photo-ageing on these thermosets. Numerous works [8,9] have been devoted to the weathering of clearcoats layers. These studies have concentrated on the weathering of clearcoats using ⇑ Corresponding author at: Clermont Université, ENSCCF, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, France. Tel.: +33 (4) 73 40 55 18; fax: +33 (4) 73 40 77 00. E-mail address: [email protected] (P.-O. Bussière). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.10.018

spectroscopy (IR, RMN, XPS) [10,11], mechanical analyses (DMA, hardness) [12] or more empirical techniques for quantifying the aspect properties (gloss, colorimetry, cracking) [13]. However, the literature only contains a few papers [14,15] devoted to the mechanism of degradation. Furthermore, the role played by various environmental stresses, in particular the action of water and the action of light with oxygen, is not well understood. Previous papers [16,17] have shown that the mechanical properties of acrylic-melamine clearcoat are significantly modified after environmental attack, with an important densification of the network. However, this phenomenon is currently associated with melamine hydrolysis [14,18]. The present work focuses on the photo-ageing of an acrylic-melamine thermoset used as a topcoat and work was conducted under dry UV conditions. The objective was to propose a photo-degradation mechanism. Photoageing appears to be the major cause clearcoat degradation but experiences with humidity and life prediction of coating has already been treated in another work [19,20]. The article describes the use of IR spectroscopy coupled

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with headspace-SPME-GC–MS to directly measure the photoproducts. Analysis of the changes in chemical properties was completed by mechanical analyses using AFM and micro-hardness measurements to understand how the macroscopic properties and characteristics of the network were modified by photo-ageing, in accordance with the inner mechanism of degradation that we proposed. 2. Experimental 2.1. Samples The acrylate thermoset used in this study was a solventbased acrylic-melamine commonly employed in automotive clearcoats and consisted of a mixture of an acrylatepolyol copolymer and a butyl-oxylated melamine crosslinker. Fig. 1 illustrates the idealised structure of the crosslinked network. No stabilisers (UV-absorbers, HALS, etc.) were present in the acrylate polymer formulation. Upon heating for 20 min at 140 °C, a trans-etherification occurred, which led to the formation of a three-dimensional polymer network. Bulk analyses were performed on free-standing films of isolated clearcoats obtained by curing the thermoset on TedlarÒ sheets. The cured samples were then removed from the TedlarÒ sheets. For surface analyses, the clearcoat was cured on a steel panel with a thickness of around 40 to 50 lm.

tubes. The tubes were sealed under very low pressure (103 Pa) using a diffusion vacuum line. The tubes were then placed in a SEPAP 12/24 device. 2.3. Analysis Infrared (IR) spectra were recorded in transmission mode with a Nicolet 760-FTIR spectrophotometer (nominal resolution of 4 cm1, 32 scans summations) and in reflection mode with a Nicolet 380-FTIR spectrophotometer equipped with a thunderdome-ATR (4 cm1, 32 scans) accessory. Analyses were performed in transmission mode on 20 lm films and in reflection mode on clearcoat layers cured on steel plates with thicknesses of around 45 lm. The thunderdome was a single reflection ATR accessory with a diamond crystal. Measurements of photoproduct profiles in the irradiated films were conducted by a technique described in the literature [22,23]. Measurements were performed on a Nicolet 6700-FTIR equipped with a CONTINUlM microscope (liquid nitrogen-cooled MCTA detector, 128 scans summations). The films were fixed in an epoxy resin and then sliced with a Leica RM2165 microtome. Slices with a thickness of 10 lm were examined under the FTIR microscope. The spectra were recorded every 1 lm with a width of 10 lm. Vickers micro-hardness (Hv) measurements were performed on a Shimadzu hardness tester. Hv can be expressed by equation (1).

Hv ¼ 1:8544  106 2.2. Thermo-oxidation, photolysis and photo-oxidation Irradiation was performed under artificial ageing conditions. The ageing device used was a SEPAP 12/24 unit [21] from Atlas equipped with four medium pressure mercury lamps (Novalamp RVC 400 W) located in a vertical position at each corner of the chamber. Wavelengths below 295 nm were filtered by the glass envelopes of the sources. The temperature at the surface of the samples was fixed at 60 °C. Thermo-oxidation experiments were performed in a ventilated oven at 60 °C. Photolysis experiments were performed on samples that were introduced in borosilicate

OH

P d

ð1Þ

2

where P is the load applied in N, d is the diagonal of the indentation in mm and Hv is expressed in MPa. In the present study, P was fixed at 0.343 N, and the application time was 10 s. The glass transition temperature of the clearcoat was measured with a Mettler Toledo DSC 822 programmed from 10 to 140 °C at a heating rate of 15 °C/min. Analyses were performed on cut films with a thickness of 40 lm that had been put in a 40 lL crucible. Dynamic mechanical analysis measurements were performed with a DMA Q800 from TA instruments. The polymeric film was tested in tensile mode (temperature range: 100 through 150 °C;

ROCH2 OCH2

N CH2O

N OCH2

CH2OR ROCH2

OCH2

N CH2

HO

N N

CH2O

N

N N

N

N N CH2

N CH2O

O HO

CH2OR

O

Fig. 1. Idealised structure of the crosslinked acrylic-melamine network.

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heating rate: 3 °C/min; thickness of the film: 40 lm; strain frequency: 1 Hz; deformation: 0.2%). For SPME experiments, the free films were aged in SEPAP 12/24 and then introduced and sealed in a 20 mL headspace vial with an aluminium-coated silicone rubber septum. The low-molecular-mass products formed during the exposure were extracted from the film after 10 min at 60 °C. The septum of the vial was then pierced with the needle of the SPME device, and the fibre was exposed for 1 min above the film sample at the same temperature. Afterwards, the SPME fibre was retracted into the protective sheath, removed from the vial and manually transferred into the injector of the gas chromatograph for thermal desorption at 280 °C for 2 s before the GC–MS run was started. The SPME holder for manual use and the 75 lm carboxen-polydimlethylsiloxane (CAR/PDMS) fibres were supplied by Supelco (BelleFonte, PA, USA). The fibre was conditioned at 280 °C for 30 min before use. GC–MS analyses were performed on an Agilent Technologies gas chromatograph GC Network System 6890 N interfaced to an electron impact mass spectrometer (Network Mass Selective Detector MSD 5973). The fused silica capillary column used was 30 m long, with a 0.25 mm internal diameter, a bonded and crosslinked polar Carbowax 20 M phase and a film thickness of 0.25 lm (Supelco, Bellefonte, PA, USA). The temperature was programmed from 35 °C (10 min hold) to 60 °C at 5 °C/min and then at 10 °C/min to 200 °C (15 min hold). Helium was used as a carrier gas with a constant pressure of 38 kPa. The temperature of the splitless injector was 280 °C, and the flow was 50 mL/ min. The transfer line temperature was 280 °C, and the ion source temperature was kept at 230 °C. The ionisation occurred with a kinetic energy of the impacting electrons of 70 eV. The detector voltage was 70 eV. Mass spectra and reconstructed chromatograms (total ion current, TIC) were obtained by automatic scanning in the mass range of m/z 20–400. Identification of the compounds was realised by comparison of the retention times and mass spectra of standards, study of the mass spectra and comparison with data in the spectral library. A Nanoscope IIIa atomic force microscope (AFM) from Veeco Instruments was used for nanoindentation measurements. The evolution of the mechanical properties was followed versus irradiation time using AFM nanoindentation. The force-distance curves were monitored under a constant deflection (or constant load). Nanoindentations were performed using a diamond tip (64.05 kHz). The given characteristics of the diamond tip corresponded to a spring constant of 234.5 N/m and a curvature radius of 25 nm. To determine the relative stiffness of the polymer, three tests of nanoindentation in different areas of the polymer were performed. Each test corresponded to sixteen indentations; thus, 3  16 force curves were used with a load variation from 3 to 10 lN. Several models [24–26] are able to correctly measure the mechanical properties using the nanoindentation testing method. The Oliver and Pharr [27] procedure, first used with commercial nanoindenters, has been applied in previous works [28] to measure the hardness and Young’s modulus of polymers analysed by AFM nanoindentation. Thus, the data extracted from the force–displacement

curve obtained with AFM nanoindentation measurements have been used with the model proposed by Oliver and Pharr (O&P). The O&P procedure is based on Sneddon’s [29] approach, in which the force F is given as a power law of displacement, F(h) = a hm, where a and m are constants that depend on the mechanical properties of the polymer and indenter geometry, respectively. The model assumes that the sample deformation is elastic and plastic, the deformation is time-independent, the indenter is a rigid punch and there is no sink-in or pile-up. Fig. 2 gives an illustration of the O&P procedure. The hardness is defined by equation (2), and the reduced elastic modulus is given in equation (3).

H¼

F max ðhmax Þ Aðhc Þ

Er ¼

S 2b

ð2Þ

rffiffiffiffiffiffiffiffiffiffiffiffi

p

Aðhc Þ

ð3Þ

where S is the slope of the unloading curve and b = 1.034 for a Berkovitch indenter. Modulus was obtained from the unloading curve of indentation because during the loading of a sample elastic–plastic deformation occurs, while the initial unloading is an elastic event. The initial slope of the unloading curve (called stiffness) can therefore be used to calculate the reduced Young’s modulus of the sample [30–33]. If hc is known, then the area function can be calculated. 2

Aðhc Þ ¼ 24:56hc

ð4Þ

It should be noted that AFM nanoindentation is different from conventional nanoindentation because it implies a cantilever deflection.

FðdÞ ¼ kd

ð5Þ

where k is the cantilever stiffness and d is the cantilever deflection. Assuming this, the raw data, which is the force–displacement (F–z) curve, are converted to the

Fig. 2. Schematic illustration of the force–displacement curve. The meanings of the parameters are described in the text.

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force–indentation (F–h) curve. The indentation depth is given by equation (6).

h¼zd

resin and a melamine binder. The end of curing can be followed [34] by IR spectroscopy using the ratio R given in Fig. 4. As shown in Fig. 4, this ratio remained constant during thermo-oxidation. Therefore, we can state that no postcuring reaction occurred at 60 °C. These results indicate that the chemical properties of the acrylic-melamine clearcoat were not modified, as described in other publications [35], and no solvent loss was detected. This result clearly shows that no thermooxidation occurred at 60 °C within the period required to produce efficient photo-oxidation (see below). The UV–visible light absorption of the acrylic-melamine clearcoat extended up to 340 nm, which makes this material directly accessible to UV light present in terrestrial solar radiation. To monitor the influence of the UV light being directly absorbed by the chromophoric species, irradiation in the absence of O2 (photolysis, as described in the experimental part) was performed. Once again, no significant modification of the IR, UV–visible or Hv data could be observed, even after 2000 h of irradiation.

ð6Þ

where z is the piezo displacement and d is the cantilever deflection. The conversion of the F–z curve to the F–h curve is depicted in detail in Fig. 2. The elastic modulus (E) can be directly calculated from the reduced modulus given previously. Furthermore, we used the linear law given in previous works [28] between the hardness and Young’s modulus obtained by the O&P procedure for a set of 10 polymers. Thus, by calculating the hardness (Eq. (2)), we deduced the Young’s modulus value using the cited linear law. A quantitative determination (i.e., elastic modulus or hardness) required a wellcharacterised tip (spring constant and curvature radius) associated with a judiciously selected mechanical model. The AFM displacements also needed to be calibrated, and the piezo hysteresis had to be accounted for. When all of these requirements were met, the data obtained still contained a rather large uncertainty [28]. As a consequence, the stiffness variation corresponding to an elastic modulus (expressed in MPa) that will be proposed in the next part should be considered as a versatile tool to measure the mechanical properties of the polymers. This means that the absolute value of calculated modulus as to be taken with caution and that is the reason why we will concentrate in comparing modulus evolutions during ageing.

3.2. Photooxidation of acrylic-melamine (AM) clearcoat Exposure of the acrylic-melamine clearcoat under accelerated photoageing conditions led to several modifications in the IR, UV–visible and Hv data. Under irradiation, a dramatic increase of the Vickers micro-hardness was observed, as presented in Fig. 5. It was anticipated that the observed modifications of the micro-hardness implied modification of the clearcoat chemical structure. The main modifications of the chemical structure were followed by IR, as shown in Fig. 6. Most of the modifications were observed in three parts of the spectrum: the hydroxyl region (2700–3700 cm1), carbonyl region (1400–1900 cm1) and fingerprint region (600–1300 cm1). In the region of 2700–3700 cm1 (Fig. 7-left), the formation of a broad band between 3100 and 3600 cm1 was observed. Three absorption maxima were observed after subtraction of the initial spectrum before irradiation, at 3500, 3345 and 3230 cm1; these maxima were ascribed to the formation of OH, NH and COOH end groups,

3. Results and discussion 3.1. Preliminary studies At first, we focused on the eventual oxidation that could result from thermal effects during exposure in the accelerated photoageing device. The samples were exposed in a ventilated oven at 60 °C without light. After 4200 h at 60 °C, we observed no significant modifications in the infrared (IR) spectrum, UV–visible spectrum or Vickers micro-hardness (Hv), as shown in Fig. 3. The second aspect concerned post-curing of the samples. The formation of an acrylic-melamine clearcoat is characterised by crosslinking reactions between an acrylic

140 120

Micro-hardness [MPa]

Abs = 0.2

100 80 60 40 20 0

3000

1000 -1

Wavenumbers [cm ]

0

1000

2000

3000

4000

Thermo-oxidation time [h]

Fig. 3. Left: ATR-IR spectra of the AM clearcoat at a thermo-oxidation time of 0 h (line) and 4200 h (dot). Right: Micro-hardness as a function of thermooxidation time.

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O

R =

( (

−1

Abs 1085cm Abs 1165cm − 1

R'

CH2

) )

O CH2

ν(COC) = 1165 cm

-1

N N

N CH2 N

O CH2

R -1

ν(COC) = 1085 cm

Fig. 4. Ratio R and representation of the ether functions present in the clearcoat.

Micro-hardness Hv [MPa]

240 220 200 180 160

In the region of 600–1300 cm1 (Fig. 7-centred), several modifications were noticed. The main feature was the loss of ether (COC) groups at 1090, 1020 and 875 cm1. The COC band disappearances were correlated with an absorbance increase at 1250 cm1, which corresponds to a CN band. The IR band at 815 cm1 was not modified under irradiation, but there was a decrease of the two bands characteristic of the styrene groups (700 and 760 cm1). With an acrylic-melamine clearcoat, it is important to discriminate between the degradation arising from the acrylic resin and that from the melamine ethers. Previous results [37] have shown that degradation of the acrylic resin can be followed by an increase in the anhydride band at 1780 cm1. Plotting the absorbance at 1780 cm1 related to anhydride photo-products as a function of irradiation time gave a linear variation, as shown in Fig. 8. The degradation of melamine ethers could be followed [38–40] by a decrease in the ether band at 1085 cm1. Fig. 9, which represents the conversion rate a as a function of irradiation time, gives a qualitative description of the loss of the COC groups. The relative conversion a used for the 1085 cm1 band is defined by equation (7).

140

a ¼ ðC 0  C t Þ=C 0

120 100 0

500

1000

1500

2000

Irradiation time [h] Fig. 5. Micro-hardness evolution of the AM thermoset versus irradiation time.

respectively, which reflected oxidation of the polymer, in agreement with a decrease in the CH bands at 2955 and 2875 cm1. In the region of 1400–1900 cm1 (Fig. 7-right), a broadening of the initial CO band was noticed, with maxima at 1780 cm1 and 1705 cm1; these bands were attributed by a derivation method [36] to the formation of anhydrides and COOH groups, respectively. A maximum was also observed at 1610 cm1, which confirmed the formation of NH groups during oxidation.

abs = 0.2

1080 1550

B 1730

1160

2930 3370

ν (cm -1 )

Assignments

3500

OH stretching of acrylic resin

3370

OH and NH stretching of melamine resin

2960, 2930, 2870

CH3 symmetrical, asymmetrical stretching and CH2 asymmetrical stretching

1730

C=O stretching

1550

triazine ring in-plane deformation and CH bending

1160

COC stretching of ester group

2870

3500 750

A

3500

A comparison of Figs. 5 and 9 clearly indicates a relationship between the degradation of the melamine ethers at 1085 cm1 and an increase in micro-hardness. A comparison of variations of the IR absorption bands showed that the formation of a band at 1250 cm1, corresponding to a CN bond, has the same shape as the COC and Hv curves (Fig. 10). These results indicated that degradation of the melamine ethers, the increase of the CN band and the increase in the micro-hardness were linked. Based on these results and those of previous work [41], one can propose a degradation mechanism that explains the close relationship between the modifications of the chemical structure and the evolution of the mechanical properties of the clearcoat upon ageing. An important point is that 50% of melamine butoxy groups are unreacted after the curing reaction because of steric hindrance [42]. The mechanism is depicted in Fig. 11.

1020 815 700

ð7Þ

3000 2500 1500 -1 Wavenumbers [cm ]

1000

1080, 1020

COC stretching

815

triazine ring out of plane deformation

750, 700

CH bending of styrene

Fig. 6. Left: ATR-IR spectra of the clearcoat in the wavenumber range of 650–3800 cm1 at different times of irradiation. Trace A, t = 0 h; Trace B, t = 750 h. Right: Assignments of the main IR features of the initial clearcoat.

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3.0

3.0 1705

2955

2.0 1.5 1.0 0.5

1610

2.5

Absorbance

Absorbance

2.5

2875

3345

3500

3230

D C B A

1.5

1780

D C B A

1.0 0.5 0.0 1900

0.0 3600

2.0

3400

3200

3000

2800

1800

-1

1700

1600

1500

1400

-1

Wavenumber [cm ]

Wavenumber [cm ] 3.0 1165

Absorbance

2.5

1250

1090

2.0

1040

D C B A

1.5 1.0

815 700 875

760

0.5 0.0 1300 1200 1100 1000 900 800

700

600

-1

Wavenumber [cm ] Fig. 7. Transmission FTIR spectra of the clearcoat at different irradiation times. Trace A, t = 0 h; Trace B, t = 300 h; Trace C, t = 750 h; Trace D, t = 1300 h. Left: In the wavenumber range of 2700–3700 cm1. Right: In the wavenumber range of 1400–1900 cm1. Centred: In the wavenumber range of 600–1300 cm1.

1.0

0.7 0.8

0.5

conversion rate - α

Δ Absorbance

0.6

0.4 0.3 0.2 1780 cm

0.1

-1

0.6

0.4

0.2

1085 cm

0.0 0

500

1000

1500

2000

Irradiation time [h] Fig. 8. DAbsorbance at 1780 cm1 of the AM thermoset versus irradiation time.

The successive reactions can be described as follows:(1) ? (2): The first radical formed in the acrylic-melamine clearcoat can be obtained by direct photochemistry, as described in previous work [43]. It can also be obtained by abstraction of the labile H (carbon between the oxygen and nitrogen atoms) by an intrinsic or extrinsic radical coming from chromophoric impurities [44,45] contained in the clearcoat.(2) ? (4): Formation of a hydroperoxide by fixation of atmospheric oxygen [46].(4) ? (5): Hydroperoxide decomposition under light and temperature to form an alkoxy radical. This radical is then discomposed through two possible b-scissions.

-1

0.0 0

400

800

1200

1600

Irradiation time [h] Fig. 9. Conversion rate of the COC groups at 1085 cm1 of the AM thermoset versus irradiation time.

Alkoxy radical decomposition leads to the formation of various photo-products. Low molecular weight products can be formed (butanol and butyl formate) along with the formation of a N radical, which can react with another radical to form a new crosslink through a CN bond, which is indicated by an increase of absorbance at 1250 cm1. To confirm the formation of volatile products during photo-oxidation and to validate the proposed mechanism, we performed headspace-SPME-GC–MS experiments. The GC–MS chromatogram (TIC), shown in Fig. 12, showed that

J.-F. Larché et al. / European Polymer Journal 48 (2012) 172–182

1.0

-1

α 1085cm [%]

220

0.30

200

0.25

180

0.20

160

0.15

140

0.10

120

0.05

200 0.6

180

0.4

160 140

0.2

120

0.0 0

500

1000

100 2000

1500

-1

0.35

220

C-N at 1250 cm [Abs]

240

Micro-hardness [MPa]

0.8

240

Micro-hardness [MPa]

178

0.00 2000

100 0

500

1000

1500

Irradiation time [h]

Irradiation time [h]

Fig. 10. Left: Micro-hardness and COC conversion rate versus irradiation time. Right: Micro-hardness and CN band at 1250 cm1 versus irradiation time.

N N

N N

N

N

N

O2

CH

X

O

N

N

hν

CH2

N

N N

N

N

PH

CH OO

N N

CH2

CH2

CH2

CH2

C3H7

C3H7

C3H7

C3H7

(2)

N

hν Δ

CH OOH

O

O

CH O O CH2 C3H7 (5)

(4)

(3)

N + HO

N

O

(1)

N

+P

(5)

N

N

N

N N

N

N

PH N

NH 3345, 1610 cm N N

N N

crosslinking

N β-scission (1)

-1

N N

N (1) CH O (2) O

CH O

CH2

O

C3H7

CH2 C3H7

N

β-scission (2)

N N CH O -1 1610 cm

O CH2 C3H7

OH PH

CH2 C3H7 SPME

SPME

Fig. 11. Photo-oxidation mechanism of acrylic-melamine clearcoat.

low molecular weight products were formed during the photo-oxidation. The retention times (tret), the main observed fragments, the molecular weights of the compounds and identification of the products are given in Table 1. According to the total ion abundance, the results given in Table 1 indicated that the products with retention times of 9.3 and 16.0 min were main ones formed during the photo-oxidation process. These two volatile products were, respectively, butyl formate and butanol. The formation of these two products is in complete agreement with the mechanism given in Fig. 11, with butanol and butyl formate being formed by photo-oxidation of the melamine moiety. The other volatile compounds cited in Table 1 are

consistent with degradation of the acrylic part of the polymer, as described in the literature, with the formation of acetic acid, acetone and ethanol [37]. These products were not responsible for the mechanical property modifications. Cross-sectional analysis by IR micro-spectroscopy was performed to characterise the distribution of the oxidation photo-products within the exposed samples. The results are given in Fig. 13. The initial absorption at 3370 cm1 was plotted on trace B. Trace A shows the oxidation profile of the film after 1000 h under irradiation. It can be observed that the oxidation photo-products were heterogeneously distributed. Most of the oxidation appeared at the surface, but oxidation could also be monitored in the bulk polymer. In fact,

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5000000 4500000

7

4000000

Relative Abundance

6 3500000

9

3000000 3

2500000

10

2000000

8 5

1500000

12

4

1000000 500000

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Fig. 12. HS-SPME-GC–MS total ion chromatogram of a headspace of a film sample before irradiation (in black) and after irradiation for 500 h.

Table 1 Volatile organic compounds released from irradiated film sample as identified by headspace-SPME-GC–MS analysis. Number

tret (min)

Main fragment, m/z

Molecular weight

Identification

1 2 3 4 5 6 7 8 9 10

2.5 2.8 3.2 5.4 5.6 9.3 16.0 19.0 22.6 25.2

100.2, 71.2, 57.2, 43, 29 60, 31, 15 58.1, 43.1, 27 45, 43, 31, 29, 27 78, 52, 39 73, 56, 41 56, 41, 31 70, 55, 42 60, 43, 29 88, 73, 60, 42

100.2 60.0 58.1 46.1 78.1 102.1 74.1 88.1 60.0 88.1

Heptane Methyl formate Acetone Ethanol Benzene Butyl formate Butanol Pentanol Acetic acid Butanoic acid

0.45

-1

Abs [3370 cm ]

0.40 0.35 0.30

A

0.25 0.20

B

0.15 0.10 0

10

20

30

40

50

60

Distance from the exposed surface [μm]

Absorbed light at 300 nm [%]

25 20 15 B A

10 5 0

0-10 10-20 20-30 30-40 40-50 50-60

Every 10 μm

Fig. 13. Left: Photo-oxidation profile measured by micro-FTIR spectroscopy of an AM film (60 lm). Trace B, t = 0 h; Trace A, t = 1000 h. Right: % of absorbed light at 300 nm versus distance from the exposed surface.

the oxidation profile shown in Fig. 13 followed the light absorption profile at 300 nm, as calculated with the Beer– Lambert law. This result indicated that the profile was not dependent on the oxygen concentration. This observation is consistent with previous work [47], and under these irradiation conditions, the rate of oxidation is not limited by oxygen diffusion up to 200 lm.

3.3. Evolution of the physical properties as measured by DMA, DSC and AFM Measurement of the Vickers micro-hardness (Hv) versus exposure time (Fig. 5) showed that there was a net increase of the clearcoat micro-hardness and one could observe a heterogeneous distribution of the photo-products in the

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clearcoat profile (Fig. 13). Based on these results, we investigated the network densification in the polymer bulk by DSC and DMA measurements and at the polymer surface by AFM nanoindentation. The consequences of photo-oxidation were first monitored by DMA with respect to the storage modulus (E0 ) and loss factor (Tan d), as reported in Figs. 14 and 15. Fig. 14 shows a distinct relaxation that can be attributed to an a-transition commonly associated with the glass transition temperature (Tg). Table 2 reports the values associated with the Tan d peak. We observed an offset of the Tan d peak maximum to a higher temperature, and the peak became larger and higher. For highly crosslinked polymers, the peak position is related to the crosslink density and the peak height and width are related to chain mobility [48,49]. The predominant phenomenon is the crosslink density increase upon ageing, which is highlighted by the offset of the peak to higher temperatures and modification of the peak width. However, there are also some chain scissions highlighted by modification of the peak height. These observations are in complete agreement with previous results. 0.50

Table 2 Main parameters of the Tan d peak versus irradiation time. Irradiation time (h)

Area (a.u.)

Centre (°C)

Width (°C)

Height (a.u.)

0 250 500 750

16.3 19.6 24.6 27.7

70 99 114 120

45.4 48.1 54.8 54.5

0.29 0.32 0.36 0.41

Quantitative data could be extracted from Fig. 15 (E0 = f(tirrad)). The crosslink density as a function of time exposure was estimated using the rubber elasticity equation (8) [50,51].

Mc ¼

m¼

Tan δ

0.35 0.30 0.25 D C B A

0.20 0.15 0.10 0.05

q

ð9Þ

Mc

The results are given in Table 3. It was observed that Mc decreased and m increased versus irradiation time. This increase of m confirmed that crosslinking reactions take place in the clearcoat bulk during photo-oxidation. This phenomenon is described by well-established equation (10) [52,53], which indicates that if Mc decreases (crosslinking), Tg or Tan d increase.

T g ðnetworkÞ ¼ T g ðlinearÞ þ K=Mc

0.00 0

40

80

120

160

Fig. 14. Loss factor – Tan d of AM clearcoat after different irradiation times. Trace A, t = 0 h; Trace B, t = 250 h; Trace C, t = 500 h; Trace D, t = 750 h.

10000 A B C

1000

ð10Þ

200

Temperature [°C]

Storage modulus [MPa]

ð8Þ

where Mc (g mol1) is the average molecular weight between two crosslink points, q (g cm3) is the density of the network at the absolute temperature T (K), R (J mol1 K1) is the gas constant and E’r (MPa) is the rubbery modulus (E’r was taken from the rubbery state above Tg at Td + 35 °C). From equation (8), we can also calculate the crosslink density m.

0.45 0.40

3qRT E0r

where K is a factor depending on the free volume evolution and Mc is the molecular weight average between two crosslinking points. Fig. 16 gives a qualitative description of the Tg, as measured by DSC, and Tan d (maximum of the peak) obtained by DMA versus irradiation time. Glass transition temperature (Tg) measurements using DSC confirmed the offset of Tg to higher temperatures, and a good agreement was found with Tan d evolution upon ageing. Evolution of the mechanical properties in the clearcoat bulk upon ageing was consistent with densification of the network, as observed by micro-hardness measurements. AFM measurements in nanoindentation mode were per-

100 Table 3 Rubbery modulus (E’r), average molecular weight between two crosslinking points (Mc) and crosslink density (m) versus photo-oxidation duration.

10 -100

-50

0

50

100

150

Temperature [°C] Fig. 15. Storage modulus – E0 of AM clearcoat after different irradiation times. Trace A, t = 0 h; Trace B, t = 500 h; Trace C, t = 750 h.

Irradiation time (h)

Tan d (°C)

E’r (MPa)

Mc (g/ mol)

m (mol/ cm3)

0 500 750

70 114 120

41.3 49.1 56.7

240.3 226.2 197.7

4.4  103 4.6  103 5.3  103

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3.5

69 126

3.0

120

66

Young modulus [GPa]

114 102 96

60

90

Tan δ [°C]

Tg [°C]

108 63

84 78

57

72

2.5

A B

2.0 1.5 1.0 0.5

66

54 0

200

400

600

800

0.0

1000

0

Irradiation time [h]

250

500

750

1000

1250

1500

Irradiation time [h] Fig. 16. Glass transition – Tg (K) and loss factor – Tan d (K) versus irradiation time.

Fig. 18. Increase in the Young’s modulus versus irradiation time. Trace A = Young’s modulus obtained from the O&P procedure; Trace B = Young’s modulus obtained by DMA.

formed. Fig. 17 shows the loading/unloading indentation curves for three irradiation times. Fig. 17 shows that the lowest slope was obtained with the non-irradiated sample, while the highest was obtained with the one irradiated for the longest duration. These data are characteristic of an increase in the surface micro-hardness and Young’s modulus, which is consistent with the conclusions given above. Quantitative values of these properties can be estimated by various mathematical models. We used the Oliver and Pharr procedure to calculate the theoretical values of H and E, which are the micro-hardness and Young’s modulus, respectively. Fig. 18 gives the obtained values of the Young’s modulus as a function of irradiation time, and we compared these calculated values with those obtained by DMA measurements. We observed that the Young’s modulus increased by a factor of 2 after 500 h under irradiation. A good agreement was obtained between the values given by DMA and those calculated from the AFM indentation measurements. The differences between the two sets of data can be explained by the shape of the oxidation profile presented above and the inherent error of the mathematical model. These sets of data confirmed the network densification, even if we considered the extreme surface of the clearcoat. Whatever

the studied scale, the crosslink density increased as a function of irradiation duration. We have seen in the Fig. 13 that even if the polymer is more degraded at the surface, the bulk was also well degraded because of the light penetration. Close value of modulus between the surface (AFM) and the whole bulk (DMA) confirms that the degradation takes place thought the 50 lm of the clearcoat film. 4. Conclusions This paper deals with the photo-oxidation of an acrylicmelamine clearcoat. It was shown that light and O2 lead to significant material property changes. From a proposed mechanism of degradation, and because of the dramatic increase in all of the mechanical properties of the polymeric film, we demonstrated that chain scission processes are not the predominant pathway of degradation. This paper showed that the mechanical property increases can be attributed to degradation of the residual unreacted melamine ethers, which induces a release of volatile products and the formation of new crosslinks. These new crosslinks lead to a global increase in the polymer network density

C B A

7 6

Load [μN]

5 4 3 2 1 0 0

20

40

60

80

100

120

Displacement [nm] Fig. 17. Left: Force–displacement curves of AM clearcoat at different irradiation times. Trace A, t = 0 h; Trace B, t = 500 h; Trace C, t = 1500 h. Right: Image of the AFM nanoindentations on the clearcoat surface.

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