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polysiloxane hybrid coatings
Progress in Organic Coatings 130 (2019) 44–57
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Outdoor exposure and accelerated weathering of polyurethane/polysiloxane hybrid coatings Tongzhai Gaoa, Zhouying Hea, Lloyd H. Hiharab, Hamideh Shokouhi Mehra, Mark D. Souceka, a b
T
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Department of Polymer Engineering, The University of Akron, Akron, OH, 44325, United States Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethanes Structure-property relationship Thermosets Coatings Organic-Inorganic hybrids
Two Polyurethane/polysiloxane hybrid coatings were compared using both real time exposure and accelerated weathering. The urethane coatings were differentiated by oligoesters selection with one being derived from cyclohexane diacids (CHDA) and 2-butyl-2-ethyl-1,3-propanediol (BEPD) and the other adipic acid (AA), isophthalic acid (IPA), 1,6-hexanediol (HD), and trimethylol propane (TMP). Three sites in Hawaii, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), were selected for the outdoor exposure test with the increasing severity of the environment. After weathering, the coatings were analyzed by Fourier transform infrared spectroscopy (FTIR) and solid state 13C nuclear magnetic resonance (NMR), Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), and Scanning electronic microscopy (SEM). Urethanes based on cycloaliphatic oligoesters exhibited much better appearance compared to the aromatic-based polyesters. A degradation of both the carbamate and ester functionality was observed spectroscopically which was accompanied by an increase in Tg, and erosion of the surface of the coatings. It was found that the accelerated weathering test conducted in weatherometer provides harsher conditions than the three outdoor exposure tests (marine, volcano, and industrial park), while marine is the harshest among the three outdoor exposure sites.
1. Introduction Polyurethane coatings have a wide range of applications because of their outstanding life expectancy and performance, resistance to aggressively corrosive environments, flexibility, chemical resistance, high abrasion resistance, fast application, strong adhesion, and unlimited film build [1]. The excellent properties result from the synergistic effect of the elastic soft (polyester or polyether) segments and the strengthsupplying hard (urethane groups and hydrogen bonding) segments [2]. Since polyurethane coatings have been extensively utilized as exterior coatings, it is of great importance for the coating industry to understand the weathering behavior of the polyurethane coatings. UV-radiation, moisture (humidity), temperature (temperature fluctuations) are the major factors causing the degradation and reduction in service lifetime of a coating system [3–6]. UV-light is the dominant source to cause photo-oxidation of the polymeric coatings. The photochemical processes of polyurethanes involve various types of reactions, i.e. free radical excitation by UV-light, peroxide formation, coupling reaction, etc., which finally result in complicated degradation products, such as the reduction of molecular weight, tensile strength, impact resistance,
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discoloration and loss of smoothness of the surface. Water is known to play a key role in the weathering of coatings not only by means of hydrolysis but also through the displacement of coatings from the substrate [7,8]. In addition, water is believed to act as a plasticizer so as to lower the glass transition temperature (Tg) and tensile modulus of the coatings [9]. High temperature and temperature variations also facilitate the thermal degradation of the polymeric coatings. To evaluate the weatherability of a coating, weathering studies can be conducted by outdoor exposure or accelerated weathering in chambers [10]. Outdoor weathering requires the specimens directly exposed to the natural conditions. Accelerated weathering, also called artificial weathering, is the process that the specimens are placed in a specially designed weathering chamber using artificial light source, heat and water. While outdoor exposure tests can better reflect the degradation behavior of the coatings in practical application, it is extremely time-consuming, i.e. it usually takes years to gather sufficient information for evaluation. With greatly shorter evaluation time and lower cost, accelerated weathering tests can resemble the outdoor conditions and has become a powerful alternative method to evaluate the weathering behavior of exterior coatings [11,12].
Corresponding author. E-mail address: [email protected] (M.D. Soucek).
https://doi.org/10.1016/j.porgcoat.2019.01.046 Received 13 December 2018; Received in revised form 17 January 2019; Accepted 22 January 2019 0300-9440/ © 2019 Published by Elsevier B.V.
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Soucek group had developed a “unicoat” system by introducing an inorganic polysiloxane component into the polyurethane coatings using a sol-gel process in order to replace the current multi-step coating systems. The advantages of this unicoat system include elimination of chromate pretreatment, cost reduction, increase in adhesion and corrosion inhibition, weather resistance and durability [13–18]. An alkoxysilane-functionalized isocyanurate [19] was employed as a coupling agent to provide chemical bonding between the organic and inorganic phases. The corrosion prevention has been studied and compared with “chromate pretreatment” as well as two-component epoxy-polyamide primer (Deft 02-Y-40) by Soucek and coworkers. The excellent anticorrosion performance similar to the commercial “chromate pretreatment” and Deft 02-Y-40 primer was observed based on prohesion and EIS study. An interaction mechanism between creamer coating and aluminum substrate were proposed based on XPS study to explain the improved corrosion protection [18]. Despite the extensive studies that have been done towards the hybrid unicoat systems, no experimental study on their weatherability behavior has been performed. Therefore, in the present study, we investigate the degradation behavior of the polyurethane/polysiloxane hybrid coatings under accelerated weathering (Ci4000 weatherometer) and outdoor exposure at different sites, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), all of which are located in Hawaii. The interaction of these environmental factors, either acting independently or in combination, will affect polyurethane degradation and determine their long-term service life. Hence, an understanding of degradation mechanism due to various factors would be useful for development of strategies to improve polyurethane performance. A laboratory accelerated testing protocol was utilized to evaluate the relative importance of three factors, temperature, humidity and xenon light, on the stability and degradation behavior of the hybrid coatings. Three outdoor weathering sites were selected to observe the environmental influence on the degradation of coatings. Fourier transform infrared spectroscopy (FTIR) and solid state 13C/29Si nuclear magnetic resonance (NMR) were employed to monitor the chemical structure alterations of the coatings after weathering. Scanning electron microscope (SEM) was applied to inspect the coating morphologies. Dynamic mechanical analysis (DMA) was used to determine the viscoelastic properties, differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) were applied to study the thermal behavior. A gloss meter was used to measure the gloss retention of the coatings.
byproduct, i.e. water. The reaction was performed under nitrogen purge and the maximum temperature at the post stage was kept at 210 °C. The end of the esterification reaction was determined by the acid number when it reached a value below 10 mg KOH/g resin. The ASTM standards, D 1639-89 and D 4274-94, were employed to measure the acid number and hydroxyl number of the polyesters, respectively. 2.3. Mono-functionalized isocyanurate The HDI isocyanurate was modified by reacting HDI isocyanurate (3HDI) and 3-aminopropyltriethoxysilane (APTES) with a molar ratio 1:1 [19]. The APTES (6.63 g, 0.03 mol) was dissolved in acetone (33.2 mL) to provide a 20% APTES solution. A 20% solution of 3HDI was obtained by dissolving 3HDI (5.82 g, 0.03 mol) in acetone (29.1 mL). Then the APTES solution was added dropwise into the flask containing 3HDI solution at 25 °C under magnetic stirring. After addition, the reactants were allowed to stir for another hour. The acetone was then removed at ambient temperature to afford the final product. 2.4. Preparation of TEOS oligomer Ethanol (44.16 g, 0.96 mol) was utilized as a solvent to dissolve tetraethylorthosilicate (TEOS, 50 g, 0.24 mol) in a round-bottom flask (250 mL) and distilled water (4.32 g, 0.24 mol) was then introduced into the mixture. After the mixture became homogeneous, hydrochloric acid (70 wt% in water, 0.99 mL) was added dropwise while the reactants were stirred with a magnetic bar. The reaction was allowed to proceed for 48 h at ambient temperature. Finally, the solvent was removed at 50 °C to afford TEOS oligomers (32.55 g, 65.1% yield based on TEOS). The products were characterized by 1H NMR and FTIR. 2.5. Formulation and preparation of polyurethane/polysiloxane hybrid coatings CHDA-BEPD or AA-IPA-HD-TMP polyesters (10.40 g) were completely dissolved in acetone (2.00 g). The cross-linker, 3HDI (3.82 g), the alkoxysilane modified 3HDI (4.78 g) and the TEOS oligomer (1.00 g) were mixed and diluted in acetone (2.00 g). The two components were then mixed together using a stirrer to obtain a homogeneous solution. The ratio of isocyanate group (NCO) to hydroxyl group (OH) was maintained constant at 1.1/1.0. Aluminum panels (2024 T3, QPanel) with a size of 1ʺ×4ʺ were degreased by acetone and the coatings were cast on the panels with a thickness of 203.2 μm (6 mil) using a drawdown bar for outdoor exposure test. The hybrid coatings were prepared on aluminum panels (2024 T3, Q-Panel) with a size of 3ʺ×6ʺ for accelerated weathering test. The films were kept at room temperature for one hour and then cured at 150 °C for 30 min.
2. Experimental 2.1. Materials The hexamethylene diisocyanate trimer (3HDI) was kindly provided by Bayer MaterialScience (Desmodur N-3300, unstabilized). Adipic acid (AA, 99%), isophthalic acid (IPA, 99%), hexanediol (HD, 97%), trimethylolpropane (TMP, 98%), 1,3/1,4-cyclohexanediacid (1,3/1,4CHDA, 97%), 2-Butyl-2-ethyl-1,3-propanediol (BEPD, 99%), acetone (≥99.9%), and 3-aminopropyltriethoxysilane (APTES, 99%) were purchased from Sigma-Aldrich and used as received. Aluminum panels (2024 T3, 3 × 6 in2) were acquired from Q-panel Lab Products.
2.6. Accelerated weathering test The accelerated weathering tests were carried out by exposing the coated panels in a Ci4000 Weather-Ometer (Atlas Material Testing Solutions). A Xenon Arc lamp were used in the Weather-Ometer, which contained type “S” Borosilicate inner and outer filters, a common combination for weathering tests. Six aluminum panels (2024 T3) coated with the polyurethane hybrid coatings were exposed for 60 days to the same weathering conditions. Every 20 days (denominated as cycle 1, 2 and 3) three samples were taken out from the chamber for evaluation. The test program was SAE J1960, which consisted of four segments: one dark and three light segments. The strength of this protocol was that it contained a dark cycle with water spray at low temperature to mimic the nightly condensation and the subsequently three light cycles with increasing temperature as revealing the morning and the day time.
2.2. Synthesis of polyesters Two polyesters, CHDA-BEPD and AA-IPA-HD-TMP, were prepared using a typical step polymerization technique as described in the literature [14] CHDA-BEPD was synthesized via the condensation of cyclohexane diacids (CHDA) and 2-butyl-2-ethyl-1,3-propanediol (BEPD), while AA-IPA-HD-TMP was synthesized via the condensation of adipic acid (AA), isophthalic acid (IPA), 1,6-hexanediol (HD), and trimethylol propane (TMP). A 500 mL reactor was equipped with mechanical stirrer and nitrogen purge, a Dean-Stark trap was used to collect the 45
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Scheme 1. Formulation of polyurethane/polysiloxane hybrid coatings.
is located in an industrial region with oil refineries and power plants, which has chloride ion deposition. The yearly average temperature was 25.5 °C and relative humidity was 64%. Kilauea Volcano is located on the Big Island of Hawaii. Test racks are located in the path of the volcanic plume and near the ocean. This site is characterized by severe acid rain with measurements as low as pH 3. MCBH represents a severe marine environment. The specimens were prepared on 1ʺ×4ʺ aluminum panels (2024 T3) with the same formulation as used for accelerated weathering. The materials were exposed with 45° inclination at all the sites. Each specimen had nine replicates and three replicates were retrieved every four months (i.e., retrieval at 4 months, 8 months and one year intervals) for evaluation. It is worthy of noting that the coatings were remained intact before weathering in order to observe the actual service life. Fig. 1. Images of 12-month outdoor exposure of the polyurethane/polysiloxane hybrid coatings at different sites; (a) CIP, CHDA-BEPD based Hybrid; (b) CIP, AA-IPA-HD-TMP Hybrid; (c) MCBH, CHDA-BEPD based Hybrid; (d) MCBH, AAIPA-HD-TMP Hybrid; (e) Kilauea Volcano, CHDA-BEPD based Hybrid; (f) Kilauea Volcano, AA-IPA-HD-TMP Hybrid.
2.8. Attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) Fourier Transform Infrared Spectroscopy was obtained on a Nicolet 380 FTIR instrument (Thermo Electron Corp.) equipped with monolithic design diamond Attenuated Total Reflectance (ATR) accessory. Spectra were collected in the range of 4000–600 cm−1 with a resolution of 4 cm−1. The OMNIC software package was employed to acquire and analyze the data. Spectra were collected in the range of 4000–600 cm−1. Background collections and corrections were completed before each sampling. Number of scans were adjusted as 64 and
2.7. Outdoor exposure Three exposure sites, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), all of which are located in Hawaii, were selected to run the natural weathering tests. CIP
Fig. 2. Images of three cycles accelerated weathering of the polyurethane/ polysiloxane hybrid coatings in the weatherometer; (a) CHDA-BEPD Hybrid; (b) AA-IPA-HD-TMP Hybrid.
46
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Fig. 3. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after accelerated weathering test in the weatherometer. (Left: PU based on CHDABEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
2.9. Solid state
Table 1 Major IR band assignments of the polyurethane/polysiloxane hybrid coatings. Wavenumber / cm−1
Assignments
References
3367(L), 3377(R) 2933(L), 2931(R) 2857(L), 2861(R) 1725(L), 1718(R) 1681(L), 1681(R) 1527(L), 1528(R) 1460(L), 1462(R) 1378(L), 1375(R) 1238(L), 1229(R) 1137(L), 1133(R) 764(L), 761(R)
ν(OH) + ν(NH) H-bonded ν as(CH2) ν sym(CH2) ν (C = O) ester ν (C = O) urethane (H-bonded) ν (C-N)+δ(N-H) (amide II) δ(CH2) δ(CH3) ν (C = O)+ ν (O-CH2) ester ν (C = O)+ ν (O-CH2) isocyanurate ring
[21,22] [21,22,23] [21,22,23] [21,22,24] [23,24] [23,24,25] [24] [21] [21,22,23,24] [26] [27]
13
C/29Si NMR
Solid state NMR spectra were recorded on a Varian Direct-Drive 500 MHz (11.7 T) spectrometer equipped with VnmrJ 3.2 A software, five broad-band rf channels, and a Varian narrow-bore, triple-resonance T3HXY NMR probe at room temperature. The samples were packed into 4 mm zirconia rotors and collected with magic-angle scanning (MAS) at a spinning speed of 15 kHz. 13C CP MAS NMR spectra were collected with a spectral width of 50 kHz, a 0.1 s acquisition time, a 4 s relaxation delay, a 4 ms contact time, and 50 kHz TPPM 1H decoupling. 29Si CP MAS NMR spectra were collected with a spectral width of 50 kHz, a 0.01536s acquisition time, a 5 s relaxation delay, a 4 ms contact time, and 50 kHz TPPM 1H decoupling. 2.10. Dynamic mechanical thermal analysis (DMTA)
resolution was 4 cm−1. To have a trustful quantitative study all spectra were normalized against CH3 intensities appear as discrete narrow bands at around 1378 cm−1. Methyl signal does not change during the study and can be considered as an internal standard. The transmission spectrum of each PU sample before the weathering was considered as a reference. Accurate −CH3 intensities were achieved after standard baseline corrections. All spectra of weathered samples (WS) were normalized by dividing the transmittance values to their −CH3 value; then, multiplied by the corresponding PU’s methyl reference intensity.
The viscoelastic properties were measured on a dynamic mechanical thermal analyzer (DMTA, Q800, TA Instruments) with a frequency of 1 Hz and a heating rate of 3.0 °C/min over a range of −50 to 150 °C. The gap distance was set up at 4 mm for rectangular specimens (10 mm × 6 mm×(0.05–0.10) mm). Tg of the crosslinked polyesterurethane-ureas were determined from the tan δ vs temperature plots. 2.11. Thermal analysis The Tg was measured using differential scanning calorimeter (DSC Q2000, TA Instruments) with a heating rate of 10 °C/min in a nitrogen
Fig. 4. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the industrial site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester). 47
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Fig. 5. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the volcanic site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
Fig. 6. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the marine site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
Scheme 2. Degradation of Polyurethane Coatings [23].
Nanoscope III from Digital Instruments.
environment. The thermal stability was observed by thermogravimetric analysis (TGA Q50, TA Instruments) with a heating rate of 10 °C/min under nitrogen purge. The temperature of 5% weight loss was recorded as initial thermal degradation.
2.13. Gloss measurement The gloss of the coatings before and after weathering was measured by glossmeter 406 (Elcometer® Inspection Equipment) with a specular angle of 20° according to ASTM D523-89. Five measurements were performed and the average was used.
2.12. Scanning electron microscopy Field Emission Scanning Electron Microscope (SEM, JEOL JSM7401 F) was used to study the morphology. The surfaces were sputter coated with silver prior to analysis. The operating accelerating voltage is 5 kV. The surface morphologies of the samples were recorded with a 48
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Fig. 7. Solid state
13
C NMR spectra of polyurethane/polysiloxane hybrid coating based on CHDA-BEPD polyester before and after weathering.
deterioration of the hybrid coatings. The difference in the two oligoesters was meant to compare a high performance oligoester to more industrially relevant oligoester. Since exposure time and space in Hawaii was limited the samples had 100x more chloride than normal to quicken the corrosion process at the film substrate.
Table 2 The intensity of the chemical shifts at δ 8, 156 and 175 ppm.
Before weathering Three cycles after accelerated weathering 12 months exposure at the volcanic site
8 ppm (-CH3)
156 ppm (-NHCOO-, urethane)
175 ppm (-COO-, ester)
1 1
0.96 0.31
0.29 2.54
1
0.80
1.34
3.1. Visual observation of the weathered specimens The polyurethane/polysiloxane hybrid coatings based on different polyesters were pictured after 12 months of outdoor exposure and three cycles of accelerated weathering, as shown in Figs. 1 and 2, respectively. Fig. 1(a), (c) and (e) show the outdoor exposed samples of polyurethane hybrid coatings based on CHDA-BEPD polyester, and (b), (d) and (f) represent the ones based on AA-IPA-HD-TMP polyester. There appear to be two key factors affecting the degradation behavior, namely the environment and the chemical structure of the coating. It is obvious that the most severe corrosion in the form of blisters took place at the marine site, which is featured by the high corrosive environment near the sea. In contrast, the blisters for the weathered samples from industrial, volcanic, and accelerated weatherometer are significantly
3. Results and discussion This paper is focused on the degradation behavior of the organic and the inorganic phases in the polyurethane/polysiloxane hybrid coatings under different conditions. The hybrid coatings were formulated using the following technique as depicted in Scheme 1. The common degradation factors presented in all conditions were air (oxygen), moisture (water), and UV-light, which caused the
Fig. 8. Solid state 29Si NMR spectra of polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD (a) and AA-IPA-HD-TMP (b) polyesters before and after (accelerated and volcanic) weathering. 49
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Table 3 29 Si Chemical shifts (-δ ppm) for the siloxane structures. CHDA-BEPD polyester based PU Hybrid 2
3
3
Siloxane structures
T
T
Q
Unexposed Accelerated Weathering Volcanic
−58.87 −58.00 −57.02
−66.25 −67.36 −65.39
−102.21 −101.34 −100.85
AA-IPA-HD-TMP polyester based PU Hybrid Q
4
−110.09 −110.21 −106.76
AA-IPA-HD-TMP polyester based PU Hybrid
Area Ratio
T2/T3
Q3/Q4
T2/T3
Q3/Q4
Unexposed Accelerated Weathering Volcanic
0.41 0.06 0.08
1.47 5.56 10.0
0.86 0.21 0.09
1.61 2.44 7.69
T3
Q3
Q4
−57.73 −56.03 −57.51
−65.61 −65.88 −66.87
−100.07 −100.85 −100.85
−106.96 −109.72 −106.27
coating [28]. The hybrid coatings exposed in three different weathering sites: industrial, volcanic and marine, were also evaluated by surface FTIR and the spectra are shown in Figs. 4–6. Similar trends were found in the variations of the specific IR bands as described for those specimens from the weatherometer, i.e., increasing peak intensity in the band of NH and OH functionalities and reduction in the peak intensity of urethane groups and ester groups. However, the extent of variation was not as significant as that shown in the weatherometer, which could be attributed to the different corroding species in the three sites. The abovementioned findings demonstrated that UV light and moisture (water) played an important role in degrading the hybrid coatings. The FTIR spectra indicates that UV light tends to extract a hydrogen from the amide II group and break the bond to form an amine and a carboxylic group. The water hydrolyzes the urethane group as well as the ester group which is a component of formulating polyurethane coatings. The photo-degradation behavior and ultimate failure of the polyurethane has been extensively studied in the past half century. It was found that the methylene groups in the α-position of nitrogen atoms were susceptible to hydrogen abstraction under long wavelength UVirradiation (λ > 300 nm), while a dual mechanism, induced oxidation and direct photolysis of the urethane groups, was observed for short wavelength irradiation. In the present study, a combined degradation behavior was found according to FTIR and 13C NMR results (see below). In all the IR spectra, special attention has been drawn to the bands at 3100–3600 cm−1 (peak at 3367 cm−1), 1527 cm−1, and 1238 cm−1. It is evident that the peaks at 1527 and 1238 cm−1 reduce significantly in intensity with the evolution of time, which are assigned to the amide II band and the ester group of the soft segment in polyurethane, respectively. The reduction in the amide II band is attributed to the loss of urethane functionality due to photo-oxidation. The functionality at 1238 cm−1 is found to decrease, which contradicts the findings by Wilhelm [23] who observed no photo-activity in the soft segments, but complies with the results obtained by Irusta [24] and Kenee [29], both of whom showed that the photo-oxidation of the ester groups accounted for the band reduction. Other than this reason, hydrolysis of the ester group is believed to play a role in the reduction of the ester functionality band. The band at 3367 cm−1 broadens which indicates the formation of hydroxyl and amine photo-products. The degradation mechanism is proposed as follows (Scheme 2): Scrutiny of the spectra reveals that the intensity of the peak at 1527 cm−1 of the specimens decreases more in magnitude when exposed in the weatherometer than those exposed at the sites of different conditions. This may be ascribed to the fact that the weatherometer presents more intense UV irradiance in a shorter time. It is also noticed that the specimens exposed at the marine site has an abnormal increase in the band of 3367 cm−1, which can be explained by the high humidity at the site, rendering the coatings absorb a certain amount of water.
Table 4 Relative intensity of T2/T3 and Q4/Q3 structures in the polyurethane hybrid coatings. CHDA-BEPD polyester based PU Hybrid
T2
reduced. This may attribute to the severe corrosive environment and high humidity in Marine site. Chemical structure of the coatings is another key factor determining the degradation behavior and therefore the protection of the substrate. For the outdoor exposed samples, no obvious blisters are found in the specimens based on CHDA-BEPD polyester weathered at the industrial and volcanic sites, whereas a few show up for the ones based on AA-IPA-HD-TMP polyester. This difference is attributed to the chemical structures of the polyesters used in the formulation. The saturated cyclic ring and the steric hindered side chain in the CHDA-BEPD structure provides the coatings with excellent resistance to hydrolysis, which impeded the degradation of the coatings and consequently improved the resistance to corrosion [14]. However, the aromatic ring presented in the AA-IPA-HD-TMP polyester readily absorbs UV-light resulting in the photo-oxidation of the coatings [20]. Also, the difference in the steric hinderance and flexibility of esters are manifested in the appearance of the blistering of the AA-IPA-HD-TMP based hybrid coatings. This was also reflected on the accelerated weathering test. The substrate covered by the coating based on CHDABEPD appeared almost intact while the one based on AA-IPA-HD-TMP showed coarsened surface. 3.2. Surface FTIR The surface FTIR measurements of the coatings after weathering are indicative of the degradation mechanisms, in general photo-degradation and hydrolysis. Fig. 3 shows the FTIR-ATR spectra of the polyurethane/polysiloxane hybrid coatings before and after three cycles of accelerated weathering test. The major IR band attributions are summarized in Table 1. The range of 2500–1900 cm−1 has been omitted due to the lack of characteristic bands. It is noteworthy that the spectra were normalized based on the peak intensity of the IR band with maxima at 1378 cm−1, which is attributed to δ(CH3) and known to be relatively stable towards the degradation. The broad band 3100–3600 cm−1, which is attributed to OH species and primary/secondary amine functionalities, broadened and increased in intensity, which indicated the increase in the concentration of NH and OH groups in the coatings. The intensity of the amide II band with maxima at 1527 cm−1 decreased significantly which was ascribed to the degradation of the urethane groups under UV light. It is also obvious that the ester functionality at the IR band with maxima at 1238 cm−1 diminished, which suggested the hydrolysis of ester groups by water in the weatherometer. These values and interpretations are consistent with previously researched literature on the polyurethane
3.3. Solid state
13
C NMR
Solid state 13C NMR spectroscopy has been carried out to study the carbonaceous species in the chemical structures of the hybrid coatings based on CHDA-BEPD polyester before and after weathering as shown in Fig. 7. As identified in the figure, the chemical shift at δ 8 ppm was 50
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Fig. 9. SEM images of polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD polyester under different weathering conditions for 12 months.
resonance at δ 175 ppm (ester group) showed a significant increase from 0.29 to 2.54 and 1.34 for accelerated weathering and outdoor exposure, respectively. This could be attributed to the decomposition of the urethane groups and leading to the formation of ester groups after weathering. The increase in the intensity of the ester group for the specimens that have undergone both accelerated weathering and outdoor exposure suggests that the amount of ester groups generated from photo-degradation of urethane (Equation 1 in Scheme 2) is more than the disappearance of ester due to hydrolysis (Equation 3 in Scheme 2).
assigned to the carbon in the methyl groups from butyl ethyl propanediol which constituted of the polyester; the resonances at δ 156 ppm and 175 ppm were from the carbons in the urethane groups and the ester groups, respectively. As stated in the previous section, methyl groups are stable against degradation. Therefore, the intensity of (-CH3) was taken as an internal reference to integrate the resonances at δ 156 ppm and 175 ppm and the results were summarized in Table 2. It was found that the relative intensity of the resonance at δ 156 ppm (urethane groups) decreased from 0.96 to 0.31 after three cycles of accelerated weathering and from 0.96 to 0.80 after 12 months of exposure at the volcanic site. The magnitude of the chemical shift at δ 156 ppm decreases more for the specimens in the weatherometer than those exposed at the volcanic site, which is correspondent with the observation by FTIR study. On the contrary, the relative strength of the
3.4. Solid state
29
Si NMR
One of the research goals was to study the enhancement of introducing the inorganic component, polysiloxane, via sol-gel process to 51
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Fig. 10. SEM images of polyurethane/polysiloxane hybrid coatings based on AA-IPA-HD-TMP polyester under different weathering conditions.
Q3 was calculated and summarized in Table 4. The decrease in the intensity of chemical shifts for T2 and Q4 suggested the Si-O-Si bonds in the polysiloxane were broken or reorganized after weathering. The degradation and reorganization of the inorganic phase after accelerated weathering and outdoor exposure is similar for both oligoesters.
the polymeric coating system in terms of weathering properties. In the above studies, the change of the polymeric components was investigated. To probe the alteration of the inorganic component during both accelerated weathering and outdoor exposure, solid state 29Si NMR was employed to observe the silicon species in the chemical structure in the hybrid coatings. Fig. 8 showed the solid state 29Si NMR spectra of the polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD (Fig. 8a) and AA-IPA-HD-TMP (Fig. 8b) polyesters. According to notation of 29Si NMR as described in Reference [30], the assignments of the chemical shifts of the siloxane structures were summarized in Table 3. Close scrutiny of the spectra revealed that the intensity of T2 structures diminished as well as the Q4 structures after three cycles of accelerated weathering and 12 months of natural weathering at the volcanic site. The relative intensity of T2/T3 and Q4/
3.5. Morphology by SEM Figs. 9 and 10, showed the SEM images of the two types of polyurethane/polysiloxane hybrid coatings based on different polyesters studied in this research. All the images were obtained with same magnification (×6000) for comparison. Figs. 9(a) and 10 (a) are two hybrid coatings which were kept in ambient environment for the same period of time as those under outdoor exposure for 12 months. In 52
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Fig. 11. Storage modulus and tanδ of the polyurethane/polysiloxane hybrid coatings based on AA-IPA-HD-TMP polyester before and after three cycles of accelerated weathering test.
converting it into heat. Before weathering, the coating presented a single glass transition temperature at 30 °C. It was surprising that the tan δ plot of the coating after exposure did not have a well-defined maximum so that it was not possible to specify Tg. In fact, it appeared that there were three transition regions peaked at the temperatures of 62 °C, 85 °C and 135 °C. It is very interesting to observe that the tan δ show multiple peaks after three cycles of accelerated weathering. This is indicative of the degradation of the polyurethane coatings to different extent from the top layer to the bottom. The lower values may come from the lower layers adjacent to the substrate that has not been altered as much by weathering. The high value may be characteristic of the upper layers which have been decomposed by UV light. It is also hypothesized that the urethane groups decomposed under UV light and recombined to form a more complex crosslinked system resulting in the increasing Tg values.
general, all the coating surfaces coarsened after weathering. Figs. 9(b) and 10 (b) are the images of the specimens under accelerated weathering test for three cycles. It is obvious that the hybrid coating based on AA-IPA-HD-TMP underwent much more severe decomposition as compared to the one based on CHDA-BEPD. Considering the chemical structure of the polyester resins, it is not unexpected since the AA-IPAHD-TMP one contains aromatic ring, which is susceptible to the UV photo-degradation. The coatings exposed in three different weathering sites showed degraded surfaces to a certain extent. In comparison, both the hybrid coatings exhibited good corrosion resistance to the volcanic environment. Consistent with the macroscopic observation shown in Figs. 1 and 2, the marine site caused the most severe damage to the coatings due to the harsh conditions presented in the environment. As the degradation proceeds, the oxidation of the coatings generates gaseous or soluble products due to the scission of the backbone of the binder, which causes the loss of organic components and thus the roughness of the surface. The degradation of the polyurethane coatings initiates due to the moisture, which might correspond to the hydrolysis of the ester groups or even the urethane groups. It is also obvious that under different weathering conditions, the surface display different morphologies, which is ascribed to the variation of the corrosive species presented in the sites. In order to observe the interface between the coatings and the metal substrate, the coatings were lifted by pull-off adhesion test. It is worthy of noting that the sample was selected in the center of the panel where no blisters were found. The bottom of the organic coatings were examined at the same magnification. As shown in Fig. 9(f), it is interesting to find that the interface of the coatings is almost intact even for the harshest exposure conditions (marine), which suggests the coating is a good barrier of the corroding species like oxygen and water.
3.7. Differential scanning calorimeter (DSC) Fig. 12 showed the DSC thermograms of the hybrid coatings at different cycles of accelerated weathering and outdoor exposure at the volcanic site, and Table 5 summarized the glass transition temperatures. It was found that the glass transition temperatures of the hybrid coatings based on CHDA-BEPD dropped slightly from 41 °C to 36 °C after one cycle of accelerated weathering, and then hiked up to 80 °C after the third cycle. The coatings based on AA-IPA-HD-TMP polyester showed increasing glass transition temperature from 21 °C to 67 °C with the evolution of weathering. The increase in the glass transition temperature is consistent with the DMA results, which is likely due to the high-branched system or even crosslinked structure during the photooxidative process. Contrarily, the glass transition temperatures of the hybrid coatings exposed at the volcanic site revealed a reverse trend, decreasing from 41 °C to 26 °C for those based on CHDA-BEPD polyester and from 21 °C to 9 °C for the ones based on AA-IPA-HD-TMP polyester after twelve months of exposure. This may be due to the different mechanism for the degradation in weatherometer and the volcanic site. It was also observed that the glass transition region (from the onset to the end of glass transition) broadened after weathering, which implied the system became more heterogeneous after exposure. This is expected since the layers of the coatings from the bottom to the top underwent different levels of degradation, which is correspondent with the DMA results.
3.6. Dynamic mechanical analysis (DMA) DMA was used to characterize the thermomechanical property of the coatings before and after weathering. However, it is very difficult to obtain a good film for DMA tests for all the samples after outdoor exposure. Therefore, only the samples from accelerated weathering were studied. Fig. 11 showed the storage modulus and tan δ of the polyurethane/polysiloxane hybrid coatings evaluated by dynamic mechanical analysis (DMA) before and after accelerated weathering. It was found that the storage modulus of the coatings decreased by approximately three times at a given temperature below Tg attributed to the photo-degradation of the backbone under UV-light exposure in the weathering chamber. The sharp reduction in tan δ maximum indicated the lowered ability of the coatings to dissipate mechanical energy by
3.8. Thermogravitational analysis (TGA) Fig. 13 presents the TGA thermograms after accelerated weathering 53
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Fig. 12. DSC thermograms of the polyurethane hybrid coatings based on AA-IPA-HD-TMP polyester before and after accelerated weathering and outdoor exposure. (a) PU based on CHDA-BEPD polyester in weatherometer; (b) PU based on AA-IPA-HD-TMP polyester in weatherometer; (c) PU based on CHDA-BEPD polyester in Kilauea volcanic site; (d) PU based on AA-IPA-HD-TMP polyester at the volcanic site.
Table 5 Glass transition temperatures of the hybrid coatings based on AA-IPA-HD-TMP polyester after accelerated weathering test. Weathering Condition Weatherometer Volcanic
CHDA based AA based CHDA based AA based
Before weathering
Cycle 1/4M
Cycle 2/8M
Cycle 3/12M
41 21 41 21
36 31 35 16
68 56 36 20
80 67 26 9
coatings after accelerated weathering, it was observed that the onset degradation temperatures of the hybrid coatings based on CHDA-BEPD polyester decreased from 259 °C to 188 °C and then slightly raised to 197 °C. The degradation temperatures of the ones based on AA-IPA-HD-
and outdoor exposure. Table 6 summarizes the onset degradation temperature with 5% weight loss and the temperature at which Table 7 shows the temperature with the maximum weight loss (calculated by taking first derivative of the weight loss curve). With respect to the 54
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Fig. 13. TGA thermograms of the polyurethane hybrid coatings after different cycles of accelerated weathering and outdoor exposure. (a) PU based on CHDA-BEPD polyester in weatherometer; (b) PU based on AA-IPA-HD-TMP polyester in weatherometer; (c) PU based on CHDA-BEPD polyester at the volcanic site; (d) PU based on AA-IPA-HD-TMP polyester at the volcanic site. Table 6 Onset degradation temperature with 5% weight loss of the coatings undergone different period of accelerated weathering (PU-CHDA refers to polyurethane based on CHDA-BEPD, PU-AA refers to polyurethane based on AA-IPA-HD-TMP).
Weatherometer Volcanic
CHDA based AA based CHDA based AA based
Before weathering
Cycle 1/4M
Cycle 2/8M
Cycle 3/12M
259 275 259 275
198 235 225 255
188 216 202 243
197 211 192 202
TMP polyester decreased constantly from 275 °C to 211 °C. Similarly, the temperatures with 5% weight loss for the coatings undergone outdoor exposure at the volcanic site reduced constantly with time, i.e.,
from 259 °C to 192 °C for CHDA-BEPD based coatings, and from 275 °C to 202 °C for the ones based on AA-IPA-HD-TMP polyester. It is not difficult to understand that the thermal stability of the coatings 55
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Table 7 The temperature with maximum weight loss of the coatings undergone different period of accelerated weathering (PU-CHDA refers to polyurethane based on CHDABEPD, PU-AA refers to polyurethane based on AA-IPA-HD-TMP).
Weatherometer
CHDA based AA based CHDA based AA based
Volcanic
Before weathering
Cycle 1
Cycle 2
Cycle 3
454 460 454 460
424 396 443 455
420 395 442 463
427 393 447 454
measured at 20° of the coatings after weathering is summarized in Fig. 14 (CHDA-BEPD based PU) and Fig. 15 (AA-IPA-HD-TMP based PU). It revealed that the gloss retention reduced to approximately 60% at all outdoor exposure sites and after accelerated weathering. The coatings exposed at MCBH site showed the greatest gloss loss compared to those at other sites. 4. Conclusion This work has demonstrated the degradation behavior of the polyurethane/polysiloxane hybrid coatings based on two types of polyester. Based on the available data, the accelerated weathering test conducted in weatherometer is more intensive than the three outdoor exposure tests (marine, volcano, and industrial park), while marine is the harshest among the three outdoor exposure sites. Urethane coatings based on cycloaliphatic diacid exhibited much better appearance compared to the aromatic-based polyesters. It is postulated that the aromatic rings to absorb more UV-light and therefore offer a direct degradation pathway. The reduction of the band intensity at 1527 cm−1 and 1238 cm−1 as shown by FTIR spectra proved the degradation took place by the scission of the urethane and ester functionalities. This hypothesis was also corroborated by solid state 13C NMR, where chemical shift of the carbons of the urethane and ester groups decreased over the weathering period. The tan δ curves broadened and showed multiple peaks indicating the different degree of degradation across the layers. Both DMA and DSC data showed an increase in glass transition temperature after weathering, which might be ascribed to the coupling reactions of the photo-oxidative products. The SEM images presented the coarsened surface of the coatings due to the fact that the degraded species were washed away by water or rain during weathering test. This was also manifested by the reduction of the gloss for all the weathered specimens.
Fig. 14. 20° gloss retention of the polyurethane hybrid coatings based on CHDA-BEPD polyester under different weathering conditions.
Fig. 15. 20° gloss retention of the polyurethane hybrid coatings based on AAIPA-HD-TMP polyester under different weathering conditions.
Acknowledgements The sponsorship from the U.S. Department of Defense Office of Corrosion Policy and Oversight and the US Air Force Academy: FA7000-2-0013, FA7000-13-2-0023, and W9132T-11-C-0035.
deteriorates with the evolution of time due to the breakage of the backbone of the coatings under photo-degradation. It was also found that after accelerated weathering, the degradation temperatures with maximum weight loss dropped from 454 °C to 420 °C for CHDA-BEPD based hybrid polyurethanes and from 460 °C to 393 °C for AA-IPA-HDTMP based ones. However, the coatings exposed at the volcanic site did not show significant change in the degradation temperature with maximum weight loss. Moreover, the coatings based on CHDA-BEPD polyester showed superior thermal stability in comparison to the ones based on AA-IPA-HD-TMP polyester, owing to the presence of the aromatic rings in the backbone of the AA-IPA-HD-TMP resins.
References [1] X.F. Yang, C. Vang, D.E. Tallman, G.P. Bierwagen, S.G. Croll, S. Rohlika, Polym. Degrad. Stabil. 74 (2001) 341–351. [2] Z.S. Petrović, J. Ferguson, Prog. Polym. Sci. 16 (1991) 695–836. [3] S.P. Pappas, Prog. Org Coat. 17 (1989) 107–114. [4] R.R. Blakey, Prog. Org Coat. 13 (1985) 279–296. [5] V. Rek, M. Braver, T. Jocić, E. Govorćin, Angew. Makromol. Chem. 158 (1988) 247–263. [6] L.F.E. Jacques, Prog. Polym. Sci. 25 (2000) 1337–1362. [7] G.Z. Xiao, M.E.R. Shanahan, J. Polym. Sci. Part B: Polym. Phys. 35 (1997) 2659–2670. [8] R.A. Dickie, Prog. Org Coat. 25 (1994) 3–22. [9] D.Y. Perera, D.V. Eynde, J. Coat. Technol. 59 (1987) 55–63. [10] G. Wypych, Handbook of Material Weathering, 4th ed., ChemTec Publishing, Toronto, 2008. [11] V. Baukha, H.P. Huininka, O.C.G. Adan, L.G.J. van der Ven, Prog. Org Coat. 76 (2013) 1197–1202. [12] R.M. Santos, G.L. Botelho, C. Cramez, A.V. Machado, Polym. Degrad. Stabil. 98 (2013) 2111–2115. [13] H. Ni, A.D. Skaja, M.D. Soucek, Prog. Org Coat. 40 (2000) 175–184.
3.9. 20º gloss retention The gloss retention is calculated using the following equation:
Gloss Retention =
Gi × 100% G0
where G0 is the gloss of the coatings without weathering test and Gi the gloss after the ith cycle of accelerated weathering. The gloss retention 56
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Diego, 1991. [22] D. Pavia, G. Lampman, G. Kriz, Introduction to Spectroscopy, Cengage Learning, 2008. [23] C. Wilhelm, J.-L. Gardette, Polymer 38 (1997) 4019–4031. [24] L. Irusta, M.J. Fernandez-Berridi, Polym. Degrad. Stabil. 63 (1999) 113–119. [25] T. Miyazawa, T. Shimanouchi, S.I. Mizushima, J. Chem. Phys. 24 (1956) 408–418. [26] J.R. Schoonover, D.G. Thompson, J.C. Osborn, E.B. Orler, D.A. Wrobleski, A.L. Marsh, H. Wang, R.A. Palmer, Polym. Degrad. Stabil. 74 (2001) 87–96. [27] Y. Zhang, A Spectroscopic Study of the Degradation of Polyurethane Coil Coatings, (2012) Queen Mary, University of London. [28] Y. Zhang, J. Maxted, A. Barber, C. Lowe, R. Smith, Polym. Degrad. Stabil. 98 (2013) 527–534. [29] L.T. Keene, G.P. Halada, C.R. Clayton, Prog. Org Coat. 52 (2005) 173–186. [30] R.H. Glaser, G.L. Wilkes, C.E. Bronnimann, J. Non-Cryst. Solids 113 (1989) 73–87.
[14] H. Ni, J.L. Daum, M.D. Soucek, Prog. Org Coat. 45 (2002) 49–58. [15] H. Ni, A.D. Skaja, R.A. Sailer, M.D. Soucek, Macromol. Chem. Phys. 201 (2000) 722–732. [16] M.D. Soucek, H. Ni, J. Coat. Technol. 74 (2002) 125–134. [17] H. Ni, W.J. Simonsick Jr, A.D. Skaja, J.P. Williams, Prog. Org Coat. 38 (2000) 97–110. [18] H. Ni, A.H. Johnson, M.D. Soucek, J.T. Grant, A.J. Vreugdenhil, Macromol. Mater. Eng. 287 (2002) 470–479. [19] H. Ni, D.J. Aaserud, W.J. Simonsick Jr, M.D. Soucek, Polymer 41 (2000) 57–71. [20] F. Pilati, M. Toselli, M. Messori, H. van Dijk, S.G. Yeates, B. Petterson, N. Tuck, B. Storer, D.M. Howell, G. Rayner, Waterborne and Solvent Based, Saturated Polyesters and Their End User Applications, Wiley, New York, 2000. [21] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characterstic Rrequencies of Organic Molecules, Academic Press, San
57
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Outdoor exposure and accelerated weathering of polyurethane/polysiloxane hybrid coatings Tongzhai Gaoa, Zhouying Hea, Lloyd H. Hiharab, Hamideh Shokouhi Mehra, Mark D. Souceka, a b
T
⁎
Department of Polymer Engineering, The University of Akron, Akron, OH, 44325, United States Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyurethanes Structure-property relationship Thermosets Coatings Organic-Inorganic hybrids
Two Polyurethane/polysiloxane hybrid coatings were compared using both real time exposure and accelerated weathering. The urethane coatings were differentiated by oligoesters selection with one being derived from cyclohexane diacids (CHDA) and 2-butyl-2-ethyl-1,3-propanediol (BEPD) and the other adipic acid (AA), isophthalic acid (IPA), 1,6-hexanediol (HD), and trimethylol propane (TMP). Three sites in Hawaii, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), were selected for the outdoor exposure test with the increasing severity of the environment. After weathering, the coatings were analyzed by Fourier transform infrared spectroscopy (FTIR) and solid state 13C nuclear magnetic resonance (NMR), Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), and Scanning electronic microscopy (SEM). Urethanes based on cycloaliphatic oligoesters exhibited much better appearance compared to the aromatic-based polyesters. A degradation of both the carbamate and ester functionality was observed spectroscopically which was accompanied by an increase in Tg, and erosion of the surface of the coatings. It was found that the accelerated weathering test conducted in weatherometer provides harsher conditions than the three outdoor exposure tests (marine, volcano, and industrial park), while marine is the harshest among the three outdoor exposure sites.
1. Introduction Polyurethane coatings have a wide range of applications because of their outstanding life expectancy and performance, resistance to aggressively corrosive environments, flexibility, chemical resistance, high abrasion resistance, fast application, strong adhesion, and unlimited film build [1]. The excellent properties result from the synergistic effect of the elastic soft (polyester or polyether) segments and the strengthsupplying hard (urethane groups and hydrogen bonding) segments [2]. Since polyurethane coatings have been extensively utilized as exterior coatings, it is of great importance for the coating industry to understand the weathering behavior of the polyurethane coatings. UV-radiation, moisture (humidity), temperature (temperature fluctuations) are the major factors causing the degradation and reduction in service lifetime of a coating system [3–6]. UV-light is the dominant source to cause photo-oxidation of the polymeric coatings. The photochemical processes of polyurethanes involve various types of reactions, i.e. free radical excitation by UV-light, peroxide formation, coupling reaction, etc., which finally result in complicated degradation products, such as the reduction of molecular weight, tensile strength, impact resistance,
⁎
discoloration and loss of smoothness of the surface. Water is known to play a key role in the weathering of coatings not only by means of hydrolysis but also through the displacement of coatings from the substrate [7,8]. In addition, water is believed to act as a plasticizer so as to lower the glass transition temperature (Tg) and tensile modulus of the coatings [9]. High temperature and temperature variations also facilitate the thermal degradation of the polymeric coatings. To evaluate the weatherability of a coating, weathering studies can be conducted by outdoor exposure or accelerated weathering in chambers [10]. Outdoor weathering requires the specimens directly exposed to the natural conditions. Accelerated weathering, also called artificial weathering, is the process that the specimens are placed in a specially designed weathering chamber using artificial light source, heat and water. While outdoor exposure tests can better reflect the degradation behavior of the coatings in practical application, it is extremely time-consuming, i.e. it usually takes years to gather sufficient information for evaluation. With greatly shorter evaluation time and lower cost, accelerated weathering tests can resemble the outdoor conditions and has become a powerful alternative method to evaluate the weathering behavior of exterior coatings [11,12].
Corresponding author. E-mail address: [email protected] (M.D. Soucek).
https://doi.org/10.1016/j.porgcoat.2019.01.046 Received 13 December 2018; Received in revised form 17 January 2019; Accepted 22 January 2019 0300-9440/ © 2019 Published by Elsevier B.V.
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Soucek group had developed a “unicoat” system by introducing an inorganic polysiloxane component into the polyurethane coatings using a sol-gel process in order to replace the current multi-step coating systems. The advantages of this unicoat system include elimination of chromate pretreatment, cost reduction, increase in adhesion and corrosion inhibition, weather resistance and durability [13–18]. An alkoxysilane-functionalized isocyanurate [19] was employed as a coupling agent to provide chemical bonding between the organic and inorganic phases. The corrosion prevention has been studied and compared with “chromate pretreatment” as well as two-component epoxy-polyamide primer (Deft 02-Y-40) by Soucek and coworkers. The excellent anticorrosion performance similar to the commercial “chromate pretreatment” and Deft 02-Y-40 primer was observed based on prohesion and EIS study. An interaction mechanism between creamer coating and aluminum substrate were proposed based on XPS study to explain the improved corrosion protection [18]. Despite the extensive studies that have been done towards the hybrid unicoat systems, no experimental study on their weatherability behavior has been performed. Therefore, in the present study, we investigate the degradation behavior of the polyurethane/polysiloxane hybrid coatings under accelerated weathering (Ci4000 weatherometer) and outdoor exposure at different sites, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), all of which are located in Hawaii. The interaction of these environmental factors, either acting independently or in combination, will affect polyurethane degradation and determine their long-term service life. Hence, an understanding of degradation mechanism due to various factors would be useful for development of strategies to improve polyurethane performance. A laboratory accelerated testing protocol was utilized to evaluate the relative importance of three factors, temperature, humidity and xenon light, on the stability and degradation behavior of the hybrid coatings. Three outdoor weathering sites were selected to observe the environmental influence on the degradation of coatings. Fourier transform infrared spectroscopy (FTIR) and solid state 13C/29Si nuclear magnetic resonance (NMR) were employed to monitor the chemical structure alterations of the coatings after weathering. Scanning electron microscope (SEM) was applied to inspect the coating morphologies. Dynamic mechanical analysis (DMA) was used to determine the viscoelastic properties, differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) were applied to study the thermal behavior. A gloss meter was used to measure the gloss retention of the coatings.
byproduct, i.e. water. The reaction was performed under nitrogen purge and the maximum temperature at the post stage was kept at 210 °C. The end of the esterification reaction was determined by the acid number when it reached a value below 10 mg KOH/g resin. The ASTM standards, D 1639-89 and D 4274-94, were employed to measure the acid number and hydroxyl number of the polyesters, respectively. 2.3. Mono-functionalized isocyanurate The HDI isocyanurate was modified by reacting HDI isocyanurate (3HDI) and 3-aminopropyltriethoxysilane (APTES) with a molar ratio 1:1 [19]. The APTES (6.63 g, 0.03 mol) was dissolved in acetone (33.2 mL) to provide a 20% APTES solution. A 20% solution of 3HDI was obtained by dissolving 3HDI (5.82 g, 0.03 mol) in acetone (29.1 mL). Then the APTES solution was added dropwise into the flask containing 3HDI solution at 25 °C under magnetic stirring. After addition, the reactants were allowed to stir for another hour. The acetone was then removed at ambient temperature to afford the final product. 2.4. Preparation of TEOS oligomer Ethanol (44.16 g, 0.96 mol) was utilized as a solvent to dissolve tetraethylorthosilicate (TEOS, 50 g, 0.24 mol) in a round-bottom flask (250 mL) and distilled water (4.32 g, 0.24 mol) was then introduced into the mixture. After the mixture became homogeneous, hydrochloric acid (70 wt% in water, 0.99 mL) was added dropwise while the reactants were stirred with a magnetic bar. The reaction was allowed to proceed for 48 h at ambient temperature. Finally, the solvent was removed at 50 °C to afford TEOS oligomers (32.55 g, 65.1% yield based on TEOS). The products were characterized by 1H NMR and FTIR. 2.5. Formulation and preparation of polyurethane/polysiloxane hybrid coatings CHDA-BEPD or AA-IPA-HD-TMP polyesters (10.40 g) were completely dissolved in acetone (2.00 g). The cross-linker, 3HDI (3.82 g), the alkoxysilane modified 3HDI (4.78 g) and the TEOS oligomer (1.00 g) were mixed and diluted in acetone (2.00 g). The two components were then mixed together using a stirrer to obtain a homogeneous solution. The ratio of isocyanate group (NCO) to hydroxyl group (OH) was maintained constant at 1.1/1.0. Aluminum panels (2024 T3, QPanel) with a size of 1ʺ×4ʺ were degreased by acetone and the coatings were cast on the panels with a thickness of 203.2 μm (6 mil) using a drawdown bar for outdoor exposure test. The hybrid coatings were prepared on aluminum panels (2024 T3, Q-Panel) with a size of 3ʺ×6ʺ for accelerated weathering test. The films were kept at room temperature for one hour and then cured at 150 °C for 30 min.
2. Experimental 2.1. Materials The hexamethylene diisocyanate trimer (3HDI) was kindly provided by Bayer MaterialScience (Desmodur N-3300, unstabilized). Adipic acid (AA, 99%), isophthalic acid (IPA, 99%), hexanediol (HD, 97%), trimethylolpropane (TMP, 98%), 1,3/1,4-cyclohexanediacid (1,3/1,4CHDA, 97%), 2-Butyl-2-ethyl-1,3-propanediol (BEPD, 99%), acetone (≥99.9%), and 3-aminopropyltriethoxysilane (APTES, 99%) were purchased from Sigma-Aldrich and used as received. Aluminum panels (2024 T3, 3 × 6 in2) were acquired from Q-panel Lab Products.
2.6. Accelerated weathering test The accelerated weathering tests were carried out by exposing the coated panels in a Ci4000 Weather-Ometer (Atlas Material Testing Solutions). A Xenon Arc lamp were used in the Weather-Ometer, which contained type “S” Borosilicate inner and outer filters, a common combination for weathering tests. Six aluminum panels (2024 T3) coated with the polyurethane hybrid coatings were exposed for 60 days to the same weathering conditions. Every 20 days (denominated as cycle 1, 2 and 3) three samples were taken out from the chamber for evaluation. The test program was SAE J1960, which consisted of four segments: one dark and three light segments. The strength of this protocol was that it contained a dark cycle with water spray at low temperature to mimic the nightly condensation and the subsequently three light cycles with increasing temperature as revealing the morning and the day time.
2.2. Synthesis of polyesters Two polyesters, CHDA-BEPD and AA-IPA-HD-TMP, were prepared using a typical step polymerization technique as described in the literature [14] CHDA-BEPD was synthesized via the condensation of cyclohexane diacids (CHDA) and 2-butyl-2-ethyl-1,3-propanediol (BEPD), while AA-IPA-HD-TMP was synthesized via the condensation of adipic acid (AA), isophthalic acid (IPA), 1,6-hexanediol (HD), and trimethylol propane (TMP). A 500 mL reactor was equipped with mechanical stirrer and nitrogen purge, a Dean-Stark trap was used to collect the 45
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Scheme 1. Formulation of polyurethane/polysiloxane hybrid coatings.
is located in an industrial region with oil refineries and power plants, which has chloride ion deposition. The yearly average temperature was 25.5 °C and relative humidity was 64%. Kilauea Volcano is located on the Big Island of Hawaii. Test racks are located in the path of the volcanic plume and near the ocean. This site is characterized by severe acid rain with measurements as low as pH 3. MCBH represents a severe marine environment. The specimens were prepared on 1ʺ×4ʺ aluminum panels (2024 T3) with the same formulation as used for accelerated weathering. The materials were exposed with 45° inclination at all the sites. Each specimen had nine replicates and three replicates were retrieved every four months (i.e., retrieval at 4 months, 8 months and one year intervals) for evaluation. It is worthy of noting that the coatings were remained intact before weathering in order to observe the actual service life. Fig. 1. Images of 12-month outdoor exposure of the polyurethane/polysiloxane hybrid coatings at different sites; (a) CIP, CHDA-BEPD based Hybrid; (b) CIP, AA-IPA-HD-TMP Hybrid; (c) MCBH, CHDA-BEPD based Hybrid; (d) MCBH, AAIPA-HD-TMP Hybrid; (e) Kilauea Volcano, CHDA-BEPD based Hybrid; (f) Kilauea Volcano, AA-IPA-HD-TMP Hybrid.
2.8. Attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) Fourier Transform Infrared Spectroscopy was obtained on a Nicolet 380 FTIR instrument (Thermo Electron Corp.) equipped with monolithic design diamond Attenuated Total Reflectance (ATR) accessory. Spectra were collected in the range of 4000–600 cm−1 with a resolution of 4 cm−1. The OMNIC software package was employed to acquire and analyze the data. Spectra were collected in the range of 4000–600 cm−1. Background collections and corrections were completed before each sampling. Number of scans were adjusted as 64 and
2.7. Outdoor exposure Three exposure sites, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), all of which are located in Hawaii, were selected to run the natural weathering tests. CIP
Fig. 2. Images of three cycles accelerated weathering of the polyurethane/ polysiloxane hybrid coatings in the weatherometer; (a) CHDA-BEPD Hybrid; (b) AA-IPA-HD-TMP Hybrid.
46
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Fig. 3. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after accelerated weathering test in the weatherometer. (Left: PU based on CHDABEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
2.9. Solid state
Table 1 Major IR band assignments of the polyurethane/polysiloxane hybrid coatings. Wavenumber / cm−1
Assignments
References
3367(L), 3377(R) 2933(L), 2931(R) 2857(L), 2861(R) 1725(L), 1718(R) 1681(L), 1681(R) 1527(L), 1528(R) 1460(L), 1462(R) 1378(L), 1375(R) 1238(L), 1229(R) 1137(L), 1133(R) 764(L), 761(R)
ν(OH) + ν(NH) H-bonded ν as(CH2) ν sym(CH2) ν (C = O) ester ν (C = O) urethane (H-bonded) ν (C-N)+δ(N-H) (amide II) δ(CH2) δ(CH3) ν (C = O)+ ν (O-CH2) ester ν (C = O)+ ν (O-CH2) isocyanurate ring
[21,22] [21,22,23] [21,22,23] [21,22,24] [23,24] [23,24,25] [24] [21] [21,22,23,24] [26] [27]
13
C/29Si NMR
Solid state NMR spectra were recorded on a Varian Direct-Drive 500 MHz (11.7 T) spectrometer equipped with VnmrJ 3.2 A software, five broad-band rf channels, and a Varian narrow-bore, triple-resonance T3HXY NMR probe at room temperature. The samples were packed into 4 mm zirconia rotors and collected with magic-angle scanning (MAS) at a spinning speed of 15 kHz. 13C CP MAS NMR spectra were collected with a spectral width of 50 kHz, a 0.1 s acquisition time, a 4 s relaxation delay, a 4 ms contact time, and 50 kHz TPPM 1H decoupling. 29Si CP MAS NMR spectra were collected with a spectral width of 50 kHz, a 0.01536s acquisition time, a 5 s relaxation delay, a 4 ms contact time, and 50 kHz TPPM 1H decoupling. 2.10. Dynamic mechanical thermal analysis (DMTA)
resolution was 4 cm−1. To have a trustful quantitative study all spectra were normalized against CH3 intensities appear as discrete narrow bands at around 1378 cm−1. Methyl signal does not change during the study and can be considered as an internal standard. The transmission spectrum of each PU sample before the weathering was considered as a reference. Accurate −CH3 intensities were achieved after standard baseline corrections. All spectra of weathered samples (WS) were normalized by dividing the transmittance values to their −CH3 value; then, multiplied by the corresponding PU’s methyl reference intensity.
The viscoelastic properties were measured on a dynamic mechanical thermal analyzer (DMTA, Q800, TA Instruments) with a frequency of 1 Hz and a heating rate of 3.0 °C/min over a range of −50 to 150 °C. The gap distance was set up at 4 mm for rectangular specimens (10 mm × 6 mm×(0.05–0.10) mm). Tg of the crosslinked polyesterurethane-ureas were determined from the tan δ vs temperature plots. 2.11. Thermal analysis The Tg was measured using differential scanning calorimeter (DSC Q2000, TA Instruments) with a heating rate of 10 °C/min in a nitrogen
Fig. 4. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the industrial site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester). 47
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Fig. 5. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the volcanic site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
Fig. 6. FTIR spectra of the polyurethane/polysiloxane hybrid coatings before and after outdoor exposure at the marine site. (Left: PU based on CHDA-BEPD polyester; Right: PU based on AA-IPA-HD-TMP polyester).
Scheme 2. Degradation of Polyurethane Coatings [23].
Nanoscope III from Digital Instruments.
environment. The thermal stability was observed by thermogravimetric analysis (TGA Q50, TA Instruments) with a heating rate of 10 °C/min under nitrogen purge. The temperature of 5% weight loss was recorded as initial thermal degradation.
2.13. Gloss measurement The gloss of the coatings before and after weathering was measured by glossmeter 406 (Elcometer® Inspection Equipment) with a specular angle of 20° according to ASTM D523-89. Five measurements were performed and the average was used.
2.12. Scanning electron microscopy Field Emission Scanning Electron Microscope (SEM, JEOL JSM7401 F) was used to study the morphology. The surfaces were sputter coated with silver prior to analysis. The operating accelerating voltage is 5 kV. The surface morphologies of the samples were recorded with a 48
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Fig. 7. Solid state
13
C NMR spectra of polyurethane/polysiloxane hybrid coating based on CHDA-BEPD polyester before and after weathering.
deterioration of the hybrid coatings. The difference in the two oligoesters was meant to compare a high performance oligoester to more industrially relevant oligoester. Since exposure time and space in Hawaii was limited the samples had 100x more chloride than normal to quicken the corrosion process at the film substrate.
Table 2 The intensity of the chemical shifts at δ 8, 156 and 175 ppm.
Before weathering Three cycles after accelerated weathering 12 months exposure at the volcanic site
8 ppm (-CH3)
156 ppm (-NHCOO-, urethane)
175 ppm (-COO-, ester)
1 1
0.96 0.31
0.29 2.54
1
0.80
1.34
3.1. Visual observation of the weathered specimens The polyurethane/polysiloxane hybrid coatings based on different polyesters were pictured after 12 months of outdoor exposure and three cycles of accelerated weathering, as shown in Figs. 1 and 2, respectively. Fig. 1(a), (c) and (e) show the outdoor exposed samples of polyurethane hybrid coatings based on CHDA-BEPD polyester, and (b), (d) and (f) represent the ones based on AA-IPA-HD-TMP polyester. There appear to be two key factors affecting the degradation behavior, namely the environment and the chemical structure of the coating. It is obvious that the most severe corrosion in the form of blisters took place at the marine site, which is featured by the high corrosive environment near the sea. In contrast, the blisters for the weathered samples from industrial, volcanic, and accelerated weatherometer are significantly
3. Results and discussion This paper is focused on the degradation behavior of the organic and the inorganic phases in the polyurethane/polysiloxane hybrid coatings under different conditions. The hybrid coatings were formulated using the following technique as depicted in Scheme 1. The common degradation factors presented in all conditions were air (oxygen), moisture (water), and UV-light, which caused the
Fig. 8. Solid state 29Si NMR spectra of polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD (a) and AA-IPA-HD-TMP (b) polyesters before and after (accelerated and volcanic) weathering. 49
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Table 3 29 Si Chemical shifts (-δ ppm) for the siloxane structures. CHDA-BEPD polyester based PU Hybrid 2
3
3
Siloxane structures
T
T
Q
Unexposed Accelerated Weathering Volcanic
−58.87 −58.00 −57.02
−66.25 −67.36 −65.39
−102.21 −101.34 −100.85
AA-IPA-HD-TMP polyester based PU Hybrid Q
4
−110.09 −110.21 −106.76
AA-IPA-HD-TMP polyester based PU Hybrid
Area Ratio
T2/T3
Q3/Q4
T2/T3
Q3/Q4
Unexposed Accelerated Weathering Volcanic
0.41 0.06 0.08
1.47 5.56 10.0
0.86 0.21 0.09
1.61 2.44 7.69
T3
Q3
Q4
−57.73 −56.03 −57.51
−65.61 −65.88 −66.87
−100.07 −100.85 −100.85
−106.96 −109.72 −106.27
coating [28]. The hybrid coatings exposed in three different weathering sites: industrial, volcanic and marine, were also evaluated by surface FTIR and the spectra are shown in Figs. 4–6. Similar trends were found in the variations of the specific IR bands as described for those specimens from the weatherometer, i.e., increasing peak intensity in the band of NH and OH functionalities and reduction in the peak intensity of urethane groups and ester groups. However, the extent of variation was not as significant as that shown in the weatherometer, which could be attributed to the different corroding species in the three sites. The abovementioned findings demonstrated that UV light and moisture (water) played an important role in degrading the hybrid coatings. The FTIR spectra indicates that UV light tends to extract a hydrogen from the amide II group and break the bond to form an amine and a carboxylic group. The water hydrolyzes the urethane group as well as the ester group which is a component of formulating polyurethane coatings. The photo-degradation behavior and ultimate failure of the polyurethane has been extensively studied in the past half century. It was found that the methylene groups in the α-position of nitrogen atoms were susceptible to hydrogen abstraction under long wavelength UVirradiation (λ > 300 nm), while a dual mechanism, induced oxidation and direct photolysis of the urethane groups, was observed for short wavelength irradiation. In the present study, a combined degradation behavior was found according to FTIR and 13C NMR results (see below). In all the IR spectra, special attention has been drawn to the bands at 3100–3600 cm−1 (peak at 3367 cm−1), 1527 cm−1, and 1238 cm−1. It is evident that the peaks at 1527 and 1238 cm−1 reduce significantly in intensity with the evolution of time, which are assigned to the amide II band and the ester group of the soft segment in polyurethane, respectively. The reduction in the amide II band is attributed to the loss of urethane functionality due to photo-oxidation. The functionality at 1238 cm−1 is found to decrease, which contradicts the findings by Wilhelm [23] who observed no photo-activity in the soft segments, but complies with the results obtained by Irusta [24] and Kenee [29], both of whom showed that the photo-oxidation of the ester groups accounted for the band reduction. Other than this reason, hydrolysis of the ester group is believed to play a role in the reduction of the ester functionality band. The band at 3367 cm−1 broadens which indicates the formation of hydroxyl and amine photo-products. The degradation mechanism is proposed as follows (Scheme 2): Scrutiny of the spectra reveals that the intensity of the peak at 1527 cm−1 of the specimens decreases more in magnitude when exposed in the weatherometer than those exposed at the sites of different conditions. This may be ascribed to the fact that the weatherometer presents more intense UV irradiance in a shorter time. It is also noticed that the specimens exposed at the marine site has an abnormal increase in the band of 3367 cm−1, which can be explained by the high humidity at the site, rendering the coatings absorb a certain amount of water.
Table 4 Relative intensity of T2/T3 and Q4/Q3 structures in the polyurethane hybrid coatings. CHDA-BEPD polyester based PU Hybrid
T2
reduced. This may attribute to the severe corrosive environment and high humidity in Marine site. Chemical structure of the coatings is another key factor determining the degradation behavior and therefore the protection of the substrate. For the outdoor exposed samples, no obvious blisters are found in the specimens based on CHDA-BEPD polyester weathered at the industrial and volcanic sites, whereas a few show up for the ones based on AA-IPA-HD-TMP polyester. This difference is attributed to the chemical structures of the polyesters used in the formulation. The saturated cyclic ring and the steric hindered side chain in the CHDA-BEPD structure provides the coatings with excellent resistance to hydrolysis, which impeded the degradation of the coatings and consequently improved the resistance to corrosion [14]. However, the aromatic ring presented in the AA-IPA-HD-TMP polyester readily absorbs UV-light resulting in the photo-oxidation of the coatings [20]. Also, the difference in the steric hinderance and flexibility of esters are manifested in the appearance of the blistering of the AA-IPA-HD-TMP based hybrid coatings. This was also reflected on the accelerated weathering test. The substrate covered by the coating based on CHDABEPD appeared almost intact while the one based on AA-IPA-HD-TMP showed coarsened surface. 3.2. Surface FTIR The surface FTIR measurements of the coatings after weathering are indicative of the degradation mechanisms, in general photo-degradation and hydrolysis. Fig. 3 shows the FTIR-ATR spectra of the polyurethane/polysiloxane hybrid coatings before and after three cycles of accelerated weathering test. The major IR band attributions are summarized in Table 1. The range of 2500–1900 cm−1 has been omitted due to the lack of characteristic bands. It is noteworthy that the spectra were normalized based on the peak intensity of the IR band with maxima at 1378 cm−1, which is attributed to δ(CH3) and known to be relatively stable towards the degradation. The broad band 3100–3600 cm−1, which is attributed to OH species and primary/secondary amine functionalities, broadened and increased in intensity, which indicated the increase in the concentration of NH and OH groups in the coatings. The intensity of the amide II band with maxima at 1527 cm−1 decreased significantly which was ascribed to the degradation of the urethane groups under UV light. It is also obvious that the ester functionality at the IR band with maxima at 1238 cm−1 diminished, which suggested the hydrolysis of ester groups by water in the weatherometer. These values and interpretations are consistent with previously researched literature on the polyurethane
3.3. Solid state
13
C NMR
Solid state 13C NMR spectroscopy has been carried out to study the carbonaceous species in the chemical structures of the hybrid coatings based on CHDA-BEPD polyester before and after weathering as shown in Fig. 7. As identified in the figure, the chemical shift at δ 8 ppm was 50
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Fig. 9. SEM images of polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD polyester under different weathering conditions for 12 months.
resonance at δ 175 ppm (ester group) showed a significant increase from 0.29 to 2.54 and 1.34 for accelerated weathering and outdoor exposure, respectively. This could be attributed to the decomposition of the urethane groups and leading to the formation of ester groups after weathering. The increase in the intensity of the ester group for the specimens that have undergone both accelerated weathering and outdoor exposure suggests that the amount of ester groups generated from photo-degradation of urethane (Equation 1 in Scheme 2) is more than the disappearance of ester due to hydrolysis (Equation 3 in Scheme 2).
assigned to the carbon in the methyl groups from butyl ethyl propanediol which constituted of the polyester; the resonances at δ 156 ppm and 175 ppm were from the carbons in the urethane groups and the ester groups, respectively. As stated in the previous section, methyl groups are stable against degradation. Therefore, the intensity of (-CH3) was taken as an internal reference to integrate the resonances at δ 156 ppm and 175 ppm and the results were summarized in Table 2. It was found that the relative intensity of the resonance at δ 156 ppm (urethane groups) decreased from 0.96 to 0.31 after three cycles of accelerated weathering and from 0.96 to 0.80 after 12 months of exposure at the volcanic site. The magnitude of the chemical shift at δ 156 ppm decreases more for the specimens in the weatherometer than those exposed at the volcanic site, which is correspondent with the observation by FTIR study. On the contrary, the relative strength of the
3.4. Solid state
29
Si NMR
One of the research goals was to study the enhancement of introducing the inorganic component, polysiloxane, via sol-gel process to 51
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Fig. 10. SEM images of polyurethane/polysiloxane hybrid coatings based on AA-IPA-HD-TMP polyester under different weathering conditions.
Q3 was calculated and summarized in Table 4. The decrease in the intensity of chemical shifts for T2 and Q4 suggested the Si-O-Si bonds in the polysiloxane were broken or reorganized after weathering. The degradation and reorganization of the inorganic phase after accelerated weathering and outdoor exposure is similar for both oligoesters.
the polymeric coating system in terms of weathering properties. In the above studies, the change of the polymeric components was investigated. To probe the alteration of the inorganic component during both accelerated weathering and outdoor exposure, solid state 29Si NMR was employed to observe the silicon species in the chemical structure in the hybrid coatings. Fig. 8 showed the solid state 29Si NMR spectra of the polyurethane/polysiloxane hybrid coatings based on CHDA-BEPD (Fig. 8a) and AA-IPA-HD-TMP (Fig. 8b) polyesters. According to notation of 29Si NMR as described in Reference [30], the assignments of the chemical shifts of the siloxane structures were summarized in Table 3. Close scrutiny of the spectra revealed that the intensity of T2 structures diminished as well as the Q4 structures after three cycles of accelerated weathering and 12 months of natural weathering at the volcanic site. The relative intensity of T2/T3 and Q4/
3.5. Morphology by SEM Figs. 9 and 10, showed the SEM images of the two types of polyurethane/polysiloxane hybrid coatings based on different polyesters studied in this research. All the images were obtained with same magnification (×6000) for comparison. Figs. 9(a) and 10 (a) are two hybrid coatings which were kept in ambient environment for the same period of time as those under outdoor exposure for 12 months. In 52
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Fig. 11. Storage modulus and tanδ of the polyurethane/polysiloxane hybrid coatings based on AA-IPA-HD-TMP polyester before and after three cycles of accelerated weathering test.
converting it into heat. Before weathering, the coating presented a single glass transition temperature at 30 °C. It was surprising that the tan δ plot of the coating after exposure did not have a well-defined maximum so that it was not possible to specify Tg. In fact, it appeared that there were three transition regions peaked at the temperatures of 62 °C, 85 °C and 135 °C. It is very interesting to observe that the tan δ show multiple peaks after three cycles of accelerated weathering. This is indicative of the degradation of the polyurethane coatings to different extent from the top layer to the bottom. The lower values may come from the lower layers adjacent to the substrate that has not been altered as much by weathering. The high value may be characteristic of the upper layers which have been decomposed by UV light. It is also hypothesized that the urethane groups decomposed under UV light and recombined to form a more complex crosslinked system resulting in the increasing Tg values.
general, all the coating surfaces coarsened after weathering. Figs. 9(b) and 10 (b) are the images of the specimens under accelerated weathering test for three cycles. It is obvious that the hybrid coating based on AA-IPA-HD-TMP underwent much more severe decomposition as compared to the one based on CHDA-BEPD. Considering the chemical structure of the polyester resins, it is not unexpected since the AA-IPAHD-TMP one contains aromatic ring, which is susceptible to the UV photo-degradation. The coatings exposed in three different weathering sites showed degraded surfaces to a certain extent. In comparison, both the hybrid coatings exhibited good corrosion resistance to the volcanic environment. Consistent with the macroscopic observation shown in Figs. 1 and 2, the marine site caused the most severe damage to the coatings due to the harsh conditions presented in the environment. As the degradation proceeds, the oxidation of the coatings generates gaseous or soluble products due to the scission of the backbone of the binder, which causes the loss of organic components and thus the roughness of the surface. The degradation of the polyurethane coatings initiates due to the moisture, which might correspond to the hydrolysis of the ester groups or even the urethane groups. It is also obvious that under different weathering conditions, the surface display different morphologies, which is ascribed to the variation of the corrosive species presented in the sites. In order to observe the interface between the coatings and the metal substrate, the coatings were lifted by pull-off adhesion test. It is worthy of noting that the sample was selected in the center of the panel where no blisters were found. The bottom of the organic coatings were examined at the same magnification. As shown in Fig. 9(f), it is interesting to find that the interface of the coatings is almost intact even for the harshest exposure conditions (marine), which suggests the coating is a good barrier of the corroding species like oxygen and water.
3.7. Differential scanning calorimeter (DSC) Fig. 12 showed the DSC thermograms of the hybrid coatings at different cycles of accelerated weathering and outdoor exposure at the volcanic site, and Table 5 summarized the glass transition temperatures. It was found that the glass transition temperatures of the hybrid coatings based on CHDA-BEPD dropped slightly from 41 °C to 36 °C after one cycle of accelerated weathering, and then hiked up to 80 °C after the third cycle. The coatings based on AA-IPA-HD-TMP polyester showed increasing glass transition temperature from 21 °C to 67 °C with the evolution of weathering. The increase in the glass transition temperature is consistent with the DMA results, which is likely due to the high-branched system or even crosslinked structure during the photooxidative process. Contrarily, the glass transition temperatures of the hybrid coatings exposed at the volcanic site revealed a reverse trend, decreasing from 41 °C to 26 °C for those based on CHDA-BEPD polyester and from 21 °C to 9 °C for the ones based on AA-IPA-HD-TMP polyester after twelve months of exposure. This may be due to the different mechanism for the degradation in weatherometer and the volcanic site. It was also observed that the glass transition region (from the onset to the end of glass transition) broadened after weathering, which implied the system became more heterogeneous after exposure. This is expected since the layers of the coatings from the bottom to the top underwent different levels of degradation, which is correspondent with the DMA results.
3.6. Dynamic mechanical analysis (DMA) DMA was used to characterize the thermomechanical property of the coatings before and after weathering. However, it is very difficult to obtain a good film for DMA tests for all the samples after outdoor exposure. Therefore, only the samples from accelerated weathering were studied. Fig. 11 showed the storage modulus and tan δ of the polyurethane/polysiloxane hybrid coatings evaluated by dynamic mechanical analysis (DMA) before and after accelerated weathering. It was found that the storage modulus of the coatings decreased by approximately three times at a given temperature below Tg attributed to the photo-degradation of the backbone under UV-light exposure in the weathering chamber. The sharp reduction in tan δ maximum indicated the lowered ability of the coatings to dissipate mechanical energy by
3.8. Thermogravitational analysis (TGA) Fig. 13 presents the TGA thermograms after accelerated weathering 53
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Fig. 12. DSC thermograms of the polyurethane hybrid coatings based on AA-IPA-HD-TMP polyester before and after accelerated weathering and outdoor exposure. (a) PU based on CHDA-BEPD polyester in weatherometer; (b) PU based on AA-IPA-HD-TMP polyester in weatherometer; (c) PU based on CHDA-BEPD polyester in Kilauea volcanic site; (d) PU based on AA-IPA-HD-TMP polyester at the volcanic site.
Table 5 Glass transition temperatures of the hybrid coatings based on AA-IPA-HD-TMP polyester after accelerated weathering test. Weathering Condition Weatherometer Volcanic
CHDA based AA based CHDA based AA based
Before weathering
Cycle 1/4M
Cycle 2/8M
Cycle 3/12M
41 21 41 21
36 31 35 16
68 56 36 20
80 67 26 9
coatings after accelerated weathering, it was observed that the onset degradation temperatures of the hybrid coatings based on CHDA-BEPD polyester decreased from 259 °C to 188 °C and then slightly raised to 197 °C. The degradation temperatures of the ones based on AA-IPA-HD-
and outdoor exposure. Table 6 summarizes the onset degradation temperature with 5% weight loss and the temperature at which Table 7 shows the temperature with the maximum weight loss (calculated by taking first derivative of the weight loss curve). With respect to the 54
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Fig. 13. TGA thermograms of the polyurethane hybrid coatings after different cycles of accelerated weathering and outdoor exposure. (a) PU based on CHDA-BEPD polyester in weatherometer; (b) PU based on AA-IPA-HD-TMP polyester in weatherometer; (c) PU based on CHDA-BEPD polyester at the volcanic site; (d) PU based on AA-IPA-HD-TMP polyester at the volcanic site. Table 6 Onset degradation temperature with 5% weight loss of the coatings undergone different period of accelerated weathering (PU-CHDA refers to polyurethane based on CHDA-BEPD, PU-AA refers to polyurethane based on AA-IPA-HD-TMP).
Weatherometer Volcanic
CHDA based AA based CHDA based AA based
Before weathering
Cycle 1/4M
Cycle 2/8M
Cycle 3/12M
259 275 259 275
198 235 225 255
188 216 202 243
197 211 192 202
TMP polyester decreased constantly from 275 °C to 211 °C. Similarly, the temperatures with 5% weight loss for the coatings undergone outdoor exposure at the volcanic site reduced constantly with time, i.e.,
from 259 °C to 192 °C for CHDA-BEPD based coatings, and from 275 °C to 202 °C for the ones based on AA-IPA-HD-TMP polyester. It is not difficult to understand that the thermal stability of the coatings 55
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Table 7 The temperature with maximum weight loss of the coatings undergone different period of accelerated weathering (PU-CHDA refers to polyurethane based on CHDABEPD, PU-AA refers to polyurethane based on AA-IPA-HD-TMP).
Weatherometer
CHDA based AA based CHDA based AA based
Volcanic
Before weathering
Cycle 1
Cycle 2
Cycle 3
454 460 454 460
424 396 443 455
420 395 442 463
427 393 447 454
measured at 20° of the coatings after weathering is summarized in Fig. 14 (CHDA-BEPD based PU) and Fig. 15 (AA-IPA-HD-TMP based PU). It revealed that the gloss retention reduced to approximately 60% at all outdoor exposure sites and after accelerated weathering. The coatings exposed at MCBH site showed the greatest gloss loss compared to those at other sites. 4. Conclusion This work has demonstrated the degradation behavior of the polyurethane/polysiloxane hybrid coatings based on two types of polyester. Based on the available data, the accelerated weathering test conducted in weatherometer is more intensive than the three outdoor exposure tests (marine, volcano, and industrial park), while marine is the harshest among the three outdoor exposure sites. Urethane coatings based on cycloaliphatic diacid exhibited much better appearance compared to the aromatic-based polyesters. It is postulated that the aromatic rings to absorb more UV-light and therefore offer a direct degradation pathway. The reduction of the band intensity at 1527 cm−1 and 1238 cm−1 as shown by FTIR spectra proved the degradation took place by the scission of the urethane and ester functionalities. This hypothesis was also corroborated by solid state 13C NMR, where chemical shift of the carbons of the urethane and ester groups decreased over the weathering period. The tan δ curves broadened and showed multiple peaks indicating the different degree of degradation across the layers. Both DMA and DSC data showed an increase in glass transition temperature after weathering, which might be ascribed to the coupling reactions of the photo-oxidative products. The SEM images presented the coarsened surface of the coatings due to the fact that the degraded species were washed away by water or rain during weathering test. This was also manifested by the reduction of the gloss for all the weathered specimens.
Fig. 14. 20° gloss retention of the polyurethane hybrid coatings based on CHDA-BEPD polyester under different weathering conditions.
Fig. 15. 20° gloss retention of the polyurethane hybrid coatings based on AAIPA-HD-TMP polyester under different weathering conditions.
Acknowledgements The sponsorship from the U.S. Department of Defense Office of Corrosion Policy and Oversight and the US Air Force Academy: FA7000-2-0013, FA7000-13-2-0023, and W9132T-11-C-0035.
deteriorates with the evolution of time due to the breakage of the backbone of the coatings under photo-degradation. It was also found that after accelerated weathering, the degradation temperatures with maximum weight loss dropped from 454 °C to 420 °C for CHDA-BEPD based hybrid polyurethanes and from 460 °C to 393 °C for AA-IPA-HDTMP based ones. However, the coatings exposed at the volcanic site did not show significant change in the degradation temperature with maximum weight loss. Moreover, the coatings based on CHDA-BEPD polyester showed superior thermal stability in comparison to the ones based on AA-IPA-HD-TMP polyester, owing to the presence of the aromatic rings in the backbone of the AA-IPA-HD-TMP resins.
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Gi × 100% G0
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