Enhancement of the Surface and Mechanical Properties of Polyurethane Coating Through Changing the Additive Ratio of Silane

Continuous development in the formulation and processing techniques of polyurethane (PU) coatings improves an aesthetic appearance as well as protection against environmental factors. In this study, the formulation of PU coating with polyethylene glycol 600 (PEG600), isophorone diisocyanate (IPDI), and trimethoxysilylpropylcarbamoyloxyhexane (TMSCH) silane‐based additive is optimized to obtain high scratch‐resistant coating. An addition of 43.1 wt.% TMSCH in PU formulation resulted in a significant improvement in the scratch resistance and wettability properties compared with pure PU film. The homogenously prepared micron‐thick coatings are characterized using scanning electron microscopy analysis and Fourier transform infrared spectroscopy. The results confirm the presence of silane additives in the PU coating matrix. The mechanical properties improved due to the increased crosslinking networks via the formation of urethane bonds by increasing the TMSCH in PU formulation. Prepared PU coating showed combined features of transparency, hydrophobicity, scratch resistance, and mechanical strength that make it a promising candidate for household appliances.


Introduction
The economic and ecological importance of coatings, which protect and prolong the lifetime of products is enormous.[23][24][25] The use of short-chain polyols with high functionality results in obtaining a hard and rigid surface, while introducing polyols with long chains with low functionality provides a flexible and soft surface. [26]U coatings are widely utilized in furniture coating, metal corrosion protection, electronic device protection, and other sectors due to their outstanding corrosion and wear resistance.However, the hydrophilic character and low thermal stability of PU coatings put a limit on their widespread use in waterproof applications.29] "Silanes" or "organosilanes" are hybrid organic-inorganic compounds.The organic functional group is compatible with an organic polymer matrix by carrying either a reactive group, such as epoxy, vinyl, methacrylate, or amino, or an unreactive group, such as an alkyl group.This group can diffuse into the polymer matrix, resulting in the formation of an interpenetrating polymer network at the interphase.In contrast, the inorganic functional group creates a stable polymeric siloxane network on the surface of inorganic materials such as metal or glass.Organosilanes efficiently operate as a bridge between two dissimilar materials and have thus been frequently utilized as "coupling agents" or "adhesion promoters."The strong bonding generated by silanes is very resistant to damaging conditions such as moisture and heat, which increases the endurance of the coating.[32] The chemical composition of organosilane compounds also modifies surface properties for hydrophobicity and hydrophilicity.Aliphatic hydrocarbon substituents or fluorinated hydrocarbon substituents as hydrophobic units allow silanes to enhance surface hydrophobicity. [33]he aim of this study was to investigate the role of TMSCH as a silane-based compound on scratch resistance and the hydrophobic behavior of PU-based coating.The results show that the use of 43.1 wt.% TMSCH improves the hydrophobic character and scratch resistance of the PU-based coating.In addition, the mechanical properties of household appliance coating such as an impact test, deformation test, and flexibility-conical bending test were examined.

Characterization
Attenuated total reflectance-Fourier-transform infrared spectroscopy (Bruker Tensor 27 FT-IR Spectrometer model) was used to characterize PU coating.Differential scanning calorimetry analysis was performed to measure the glass transition temperature (Tg) of PU coatings using TA Instruments Discovery 250.Thermogravimetric analysis was carried out in TA Instruments Discovery 550.The weight loss was recorded as a function of temperature.
Fischer Dualscope MPOR instrument was used to measure the thickness of the coating non-destructively.The surface images of coating samples were recorded using Olympus SZH10 Zoom Stereo Microscope.Water contact angle (WCA) was determined using Attension Theta Optical Tensiometer by Biolin Scientific.
An adhesion test was performed to assess the adhesion of the coating film to the metallic substrate by applying and removing pressure-sensitive tape over cuts made in the film by Zehntner ZCC 2087 Cross-cut tester.The gloss values of PU-coated stainless steel were measured in Multi Gloss 268 Plus Gloss Meter (Konica Minolta) according to ASTM D523 standard.The color was measured with a CM-3600 Spectrophotometer (Konica Minolta).
The morphology and thickness of the PU coatings were analyzed using a field-emission scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM/EDS) SUPRA 55VP by Carl Zeiss AG.Surface analysis was performed on Contour GT-K 3D Optical Microscope by Bruker.
An impact test, deformation test, and flexibility-conical bending test were applied to measure the mechanical strength of the coatings.Details were given in Supporting Information.

Structural Characterization
The conversion of ─NCO groups was monitored to evaluate the reaction process by Attenuated total reflectance-Fouriertransform infrared spectroscopy (ATR-FTIR).The FTIR spectra of precursors IPDI, PEG600, and PU0 coating are shown in Figure 1.The successful synthesis of PU0 (Scheme 1(1)) coating was verified by the disappearance of a strong peak at 2244 cm −1 (─NCO) indicating that NCO groups of IPDI reacted with ─OH of PEG600 and PU0 prepolymer had been formed.The peaks at 3323, 2922, and 1694 cm −1 are attributed to the stretching vibration of the ─NH─, ─CH 2 ─, and ─CO─ groups, respectively.The peak at 1531 cm −1 is due to the deformation vibration of the ─NH─ group.The other characteristic band is 1707 cm −1 due to the stretching vibration mode of the carbonyl (C═O) group.The peak at 1108 cm −1 is attributed to the stretching vibration of the C─O─C group.
As seen in Figure 2, a similar FTIR pattern was found for a series of PU coatings prepared with TMSCH.PU0 stands for PU coating without silane additive and PU1 to PU6 stands for PU coatings with different silane additive content.The chemical structure of the prepolymer (2) obtained via a reaction between the silane-based agent TMSCH and the isocyanate group of IPDI is given in Scheme 1.The peak at 1532 cm −1 on the spectra of all PU coatings confirmed the formation of urethane linkage.The disappearance of an isocyanate peak at 2244 cm −1 (─NCO) indicates a complete usage of IPDI.As seen in Figure 2, the three bands at 2931, 1085, and 787 cm −1 correspond to vibrations of Si─OCH 3 .The intensity of the peak at 1035 cm −1 rises which indicates the formation of the siloxane single bond ( as Si─O─Si).
On the spectrum of PU6 coating with the highest TMSCH content, the intensity of the CNH bond at 1532 cm −1 has increased drastically (Figure 1; Figure S1, Supporting Information).This result could be attributed to the formation of urea linkage.The peaks at 3307, 2950, and 1697 cm −1 correspond to the stretching vibration of ─NH─, ─CH 2 ─, and ─CO─ groups, respectively.

SEM/EDS Analysis of PU-Coated Samples
The surface morphology and elemental composition of PU coatings on SS304 are given in Figure 3 (High-resolution copies of SEM/EDS analysis are given in Figure S3, Supporting Information).While microcracking on the surface of PU0 coating was observed, PU2 and PU6 coatings revealed a smooth surface topography without recognizable microcracking and macroscale defects.
The EDS analysis was used to obtain the elemental composition of the coated steel.The EDS spectra of the coatings showed an increase in Si content as TMSCH content increased from 0.0 to 2.9 and 43.1 wt.%.The observed significant increase in Si content was attributed to the deposition of silane-based coating in the presence of TMSCH.Increasing the TMSCH content in the coating composition also changed the oxygen content in the coatings.The corresponding increase in the oxygen content is an additional confirmation of the formation of the siloxane bond (Si─O) in the coatings PU2 and PU6.These results show consistency with FTIR results that indicated the formation of the siloxane bonds.

Physical Properties of PU Coatings
The coating thickness affects the physical and mechanical properties of the coating.Therefore, the optimization of the thickness is critical according to the desired application area and usage conditions.In the present work, the formulation was applied by a spin coating method to achieve a 2-3 μm dry coating thickness after curing at 120 °C for 3 h.The measured thickness was found to be 2 μm for PU0-PU3 and PU5 whereas the thickness of PU4 and PU6 was 3 μm for PU4 and PU6.The thickness of the coating is listed in Table 1.
Specular gloss and surface roughness are important factors for coatings as they influence the visual perception of a surface.In this study, the roughness of the substrate (SS304) and PU coatings was evaluated based on the arithmetic average deviation from the mean line (Ra).
As seen in Figure 4, the introduction of TMSCH affected the surface roughness.The main effect of TMSCH on the surface roughness was obtained by increasing the TMSCH content up to 43.1 wt.%.The surface roughness of PU6 was found to be 1.53.As it was expected, the increased surface roughness resulted in a decrease in the surface gloss level (Figure 4).The lowest gloss value is found to be 130.6 GU for PU6 while the gloss value of uncoated 304 stainless steel is measured as 155.0 GU at a 60°a ngle of incidence.The desired gloss intensity varies according to the application area.High-gloss finish coatings are demanded mostly due to the high aesthetic perception, while surface imperfections become more noticeable on these coatings.Lowering the gloss value of SS 304 after the application of PU coatings did not cause any problem on the perceptional quality.The obtained value is high enough to classify the PU-coated surface as a gloss surface.
In addition to the measurement of gloss properties, changes in the optical properties of the substrate are important to be defined.The colorimetry analysis was performed to define and compare color in a quantitative scale where L* indicates lightness; the chromatic color (a*: green-red field; b*: blue-yellow field based on the Commission Internationale de l'Eclairge L*a*b* color space.As seen in Table 1, the L * , a * , and b * values showed that the coating did not cause any significant effect on the color perception of the coated substrate.

Thermal Properties
Changes in the thermal stability and decomposition pattern of PU coatings with increasing the TMSCH content were examined through thermal analysis.The initial degradation phase with high weight loss was observed for PU2-PU6 at the temperature range of 200-300 °C.This could be attributed to the decomposition of hard segments which content increased in the coating by the introduction of TMSCH.PU1 coating with the lowest TMSCH content showed a similarity in onset degradation temperature and profile compared to PU0 coating which does not contain TMSCH.This indicated that the thermal stability of the coatings depends on the concentration of used TMSCH content in the coating.TGA analysis of PU0 and PU6 were explained in detail to show differences in the degradation pattern of the coating prepared without using TMSCH and the use of the highest content of TMSCH.TGA analysis of PU0 is shown  in Figure 5 which shows a two-step degradation pattern.The initial degradation phase with 16.2% weight loss at 286.3 °C was observed due to the decomposition of hard segments in the coating.
The lowest weight loss at the temperature range of 200-300 °C was observed for PU0.9.6% weight loss at 286-500 °C corresponded to the decomposition of the soft segments (polyol moieties) of the coating.At 500 °C char residue of PU0 was obtained 74.3%.
TGA analysis of PU6 shows that 0.8% weight loss occurred at 138.8 °C corresponding to the evaporation of moisture and entrapped solvent from the curing process (Figure 5).The initial degradation phase with 54.3% weight loss at 343.3 °C was observed due to the decomposition of hard segments in the coating.18.2% weight loss at 343-500 °C corresponded to the decomposition of the soft segments (polyol moieties) of the coating.At 582.3 °C char residue of PU6 was obtained 25.5%.
As mentioned before, the initial thermal decomposition took place in the hard segment and the subsequent stage of degradation occurred in the soft segment of the coating.Overall thermal stability of PU coating showed a decreasing trend as the TMSCH content increased in the rigid segment.
The thermal behavior of PU coatings was analyzed based on their glass transition temperature (Tg) (Figure S4, Supporting Information).The glass transition temperature which is a measure of the mobility of the chain segments present in the polymer depends on various factors such as functionality, structure of the polyols and isocyanates, and crosslink density.In the present study, cycloaliphatic isocyanate (IPDI) and polyethylene glycol were used as isocyanate and polyol sources for the main polymer backbone, respectively.TMSCH is used as a silane-based agent.The Tg value for cured PU0 coating was found to be 20.4 °C based on the endothermic peak of the second heating DSC curve.The Tg of PU coatings in which TMSCH content increased from 1.5 to 7.9 wt.% ranged from 21.7 to 27.3 °C (Table 2).The highest Tg obtained for PU6 could be based on an extended chain with urea linkages and the higher crosslink density in the presence of the highest TMSCH content.

Adhesion
The resistance of the coatings to separate from the metal substrate was evaluated by performing a lattice pattern cut into the coating and performing a peel test to evaluate delamination.Adhesion between stainless steel 304 and PU0 coatings results from secondary bonding (hydrogen bonding and van der Waals forces) via polar interactions.PU coatings are polar due to the presence of urethane groups, while stainless steel 304 has also a polar nature due to its oxidized surface.Polar-polar interactions between the hydroxyl groups on the substrate and the functional groups in PU0 coating resulted in high adhesion strength (Figure 6).
With the TMSCH content increased from 1.5 to 43.1 wt.%, the presence of a covalent bond between the coating and substrate also could be taken into account for adhesion.Prepared coatings showed good adhesion according to the ASTM D3359 standard.Non-detrimental effect (peeling or removal of the coating) on the adherence was observed on the sample surfaces (Figure 7).It means that all the PU coatings with different TMSCH content have 100% adhesion to the surface.

Water Contact Angle
The water contact angle measurement was performed to find out the wettability of the PU-coated surfaces.Compared to the uncoated stainless steel 304 which has a contact angle of 58.4°, PU coating with TMSCH content ≥ 2.9 wt.% showed hydrophobic character.As seen in Figure 8, the water contact angle of the coating increased from 78.9°to 110°as the TMSCH content was increased from 1.5 to 43.1 wt.%.The increase in the contact angle confirmed the deposition of silane-based PU coating on the steel substrate.PU6 has a hydrophobic character as indicated by its water contact angle value that is above 90°.A hydrophobic nature which prohibits permeation of water droplet and increasing the coating's hydrophobicity can be assigned to the high crosslink density of PU6 coating.
PU coating prepared without using TMSCH showed the lowest water contact angle.This could be explained by the presence of hydroxyl group on the surface.
As shown in Figure 9, PU6 coated stainless steel was water repellent, where water droplets could easily slide off over the surface.Water drop showed a noticeable tendency to shrink on the PU6 coating while a wider spreading area on an uncoated stainless steel surface was observed.The sliding speed of the water droplet on the coated stainless steel was higher than that of showing adhesion between coating and surface. [34,35]ncoated stainless steel.The use of 43.1 wt.% of TMSCH increased both the contact angle and the sliding acceleration of the coating.Also, the use of silicone derivative TEGO Glide 410 as a surface additive to enhance the film homogeneity with surface flow control contributed to improving slipping performance.

Scratch Resistance
The progressive load test that provides quantitative information about coating resistance against scratch was performed to compare the scratch resistance of PU coatings and uncoated stain-less steel 304.This test is performed by applying a constantly increasing load from 0.1 to 1.0 N to the coating surface with a hard metal tip.The critical normal load at which damage processes start, has been identified.A load of 0.1 N produced scratches on the uncoated stainless steel 304.Although the TMSCH content increased from 1.5 to 7.9 wt.% PU0-PU5 coatings showed similar scratch resistance behavior.The delamination was seen at 0.1 N the coating PU0-PU5 with the naked eye due to the reflection of light caused by the gap between the delaminated coating and the stainless steel substrate.As seen in Figure 10, the maximum scratch depth was found for the uncoated stainless steel 304 at 0.1 N. The scratch depth of PU coatings decreased as the TMSCH   content increased in the PU coating composition.This trend is additional evidence to show the formation of the additional urethane bond and inorganic crosslinking network that has resulted in increasing the resistance of PU coatings.
As seen in Figure 11, the scratch was seen at 0.9 N for PU6 that prepared with the introduction of TMSCH up to 43.1 wt.%.The differences in the amount of load indicated that PU6 coating has higher scratch resistance than that of the uncoated substrate and the PU coating with TMSCH content varied from 1.5 to 7.9 wt.%.This could be explained by the formation of the additional urethane bond and inorganic crosslinking network in the presence of high TMSCH content in PU composition.Images of scratch deformation on PU6 and PU1 after scratch testing at 0.9 and 0.1 N, respectively were given in Figure S5 (Supporting Information).

Mechanical Properties
In this study, an impact test, deformation test, and flexibilityconical bending test were applied to measure the mechanical strength of PU0, PU2, and PU6 coatings.The ability of the coat-  ing to absorb the applied external energy and dissipate this to adjacent networks is determined by applying the impact test.The PU coatings showed excellent resistance under direct and indirect loads of 40 and 100 kg.As seen in Figure 12, no whitening or cracking was observed in the PU0, PU2, and PU6 coating.Impact resistance could be attributed to PEG units which reduce  polymer rigidity and improve polymer flexibility through molecular movement.
The elongation properties of the coatings are examined by the deformation resistance (cupping) test following DIN EN ISO 1520.The sample is subjected to bilateral bending and stretching for a given drawing distance and speed during the cupping process.For coated stainless steel panels, the "forming process" was done at a 6 mm deformation depth.There was no cracking and peeling at the tested area, which means that PU0, PU2 and PU6 coatings withstand elongation stresses (Figure 13).This result is consistent with the test at which the coated SS panels were subjected to direct and indirect loads of 40 and 100 kg.
The flexibility/conical bending test was performed to determine the elastic properties and crack resistance of PU coated SS panel.While PU0 and PU2 coatings with 0.0 and 2.9 wt.% of TMSCH content showed poor deformation resistance, PU6 coating with 43.1 wt.% of TMSCH had good deformation resistance.Whitening was observed in the tested area with the naked eye for both PU0 and PU2 coatings.As the TMSCH content increased up to 43.1 wt.%, PU coating had sufficient flexibility and could be bent on a conical mandrel of diameter 3-38 mm (Figure 14).No whitening or cracking was observed in the tested area of PU6 with the naked eye.This result indicates that the introduction of TMSCH up to 43.1 wt.% is good enough to resist fracture by improving the stress transferring and distribution between rigid and flexible unit of the coatings.

Conclusion
Transparent and hydrophobic silane-based PU coatings with high flexibility and excellent scratch resistance performance have been prepared successfully in the presence of TMSCH silane based additive while PEG600 used as polyol source, IPDI used as isocyanate source, DBTDL used as a catalyst.The change in the properties of the coatings was examined by changing the TM-SCH content in the coating formula from 1.5 to 43.1 wt.%.The PU coating without silane additive showed hydrophilic properties and fractured surface morphology, while the surface properties of PU coatings improved with increasing silane additive.Although all the transparent silane-based PU coatings showed good adhesion to the surface, PU6 gave the best results, especially in terms of hydrophobicity, smoothness, flexibility, and scratch resistance.

Experimental Section
Materials: Isophorone diisocyanate (IPDI) (cis-and trans-isomer mixture), polyethylene glycol 600 (PEG600), dibutyltin dilaurate (DBTDL, ≥ 95%) were sourced from Sigma-Aldrich.Slip and flow additive TEGO Glide 410 was obtained from EVONIK.1-methoxy-2-propanol was obtained from Sigma-Aldrich.Trimethoxysilylpropylcarbamoyloxyhexane (TMSCH), silane-based additive was obtained from Worlee (Figure S2, Supporting Information).PEG600 was subjected to drying for 2 h at 80 °C under vacuum in an oven to remove water content.All other chemicals were used as received without further treatment.Stainless steel panels were first washed with detergent and then washed with acetone and ethyl alcohol, respectively.Preparation of PU Coating: Polyethylene glycol 600 (3.36 g, 5.6 mmol), DBTDL catalyst (59 μL), and TEGO Glide 410 (0.04 were added to a three-neck round-bottom flask equipped with a magnetic stirrer and then sealed with a rubber septum and reaction maintained at 40 °C for 0.5 h under Ar atmosphere in an oil bath.Then, IPDI (5.49 g, 24 mmol) was added to the reaction mixture and then heated in an oil bath at 60 °C to initiate the polymerization for 2 h.After 2 h, preheated 40 mL TMSCH was added to prepare the coating solution for PU6.The solution was left to at 60 °C for 1 h to remove bubbles, then the coating was obtained via a spin coating application.For PU0 coating, the solution was prepared as described above except addition of TMSCH.At this step, 40 mL of 1-methoxy-2-propanol was added.For PU1, PU2, PU3, PU4, and PU5, during the addition of IPDI (5.49 g, 24 mmol) to the reaction flask, TMSCH additive, which was mixed with 40 mL of 1-methoxy-2-propanol by heating for 1 h, was added to the flask at the concentrations given in the table.PU coating was prepared by the sol-gel method as seen in Scheme 2. In this reaction, IPDI was chosen to form the hard segments in the general PU structure, and PEG was selected to form the soft segments.The formulation was cast onto preheated 10 × 10 cm 2 304 stainless steel panels using a Laurell WS-650-23 B spin coater.The coated metal panels were cured in an oven at 120 °C for 3 h (PU6), for 12 h (PU0, PU1, PU2, PU3, PU4, PU5).The chemical compositions of the coatings are given in the Table 3.

Figure 2 .
Figure 2. FTIR spectrum of all PU coating samples.

Figure 4 .
Figure 4. Gloss and roughness values of coating samples.

Figure 8 .
Figure 8. Water contact angles of PU coating samples.

Figure 9 .
Figure 9. Drop flow rate image of coated and uncoated surface.

Figure 10 .
Figure 10.Scratch depth at 0.1 N of coating samples.

Figure 12 .
Figure 12.Direct/indirect impact test results of a)PU0 and b)PU6.

Table 1 .
Physical properties of coating samples.

Table 2 .
DSC data of PU coatings.

Table 3 .
Chemical composition of PU coating samples.