Introduction

Nanoparticles are often incorporated in organic coatings to enhance their physical1,2,3, mechanical4,5, electrical6,7,8, thermal9,10,11 or chemical properties6,12. SiO2 nanoparticles are of the most interested reinforcements for organic coatings due to their low price, surface properties (i.e. high surface area and functionality), accessibility and high thermal/mechanical properties.

Maleki et al.13 applied fumed and precipitated silica nanoparticles to acrylic-urethane clear coat to improve its scratch and abrasion resistance. They observed that nano silica improved mechanical properties of the coatings. They also found that precipitated nano silica performed better those expensive fumed silica nanoparticles. When a 2.5 N force is applied over a 230 m sliding distance, the abrasion resistance improves by 40% and 69% for fumed clearcoats with 4% and 6% wt, respectively. For clearcoats reinforced with precipitated nano-silica, the improvements are 17% and 70%. Notably, the enhancement is more pronounced at lower weight fractions. Xu et al.14 noted that corrosion presents significant challenges across various industries. In their study, epoxy coatings modified with hydrophobic and hydrophilic nano-silica were developed for the corrosion protection of phosphatized Q235 carbon steel. The coatings were synthesized through the polymerization of trimethylolpropane triglycidyl ether and diethylenetriamine, with the modified versions incorporating nano-silica. SEM analysis revealed the microstructure of the coatings, while their wettability, thermal stability, and corrosion resistance were evaluated using static contact angle measurements, thermogravimetric analysis, and electrochemical testing. The results showed that the hydrophobic nano-silica-modified epoxy coatings exhibited enhanced corrosion resistance, with the most hydrophobic formulation demonstrating the best performance, evidenced by an impedance of 5.25 × 10⁵ Ω•cm² after 0 h of immersion. The addition of hydrophobic nano-silica significantly improved the coating’s ability to prevent corrosion by prolonging the diffusion path of water-soluble corrosive agents.

In another investigation, hydrophobic nano silica was applied to silicon rubber for fabricating a hydrophobic coating for decreasing attachment of contaminants to the glass insulator that causes flashover failure. It was observed that nano silica increased contact angle of silicon rubber from 100° to 133.3°. The hydrophobic nanocomposite coating caused decrement in the leak current of the coated sample from 3.1 to 2.4 mA15. Jena et al.16 prepared polydimethylsiloxane composite coating contained nano silica-graphene oxide as a corrosion resistant layer for marine environment. They found that hybrid nanoparticles inhibit chloride ion diffusion in to the PDMS film so decreases considerably corrosion rate of steel substrate. Wang et al.17 produced durable super hydrophobic poly methyl methacrylate (PMMA) coating contained silica nano particles. They reported that the coating showed water contact angle of 168° that demonstrated excellent abrasion resistance along with super hydrophobicity even after abrasion with sand paper (i.e. water contact angle of 159°). In another research, silica nanoparticles (i.e. hydrophobic and hydrophilic) were mixed with epoxy powder coating through ball milling. Results revealed that both types of nano SiO2, even at small amounts, decreased chemical degradation of epoxy coatings under irradiation and increased their mechanical behavior and wear resistance18. In another study, nano silica was added to PU, epoxy and polyvinyl chloride coatings. It was found that Furthermore, the addition of nano-SiO2 could decrease damages related to ultraviolet irradiation on coatings and also enhance the carbonation resistance of coated concrete19. Jiang et al.20 assessed effects of nano SiO2 on waterborne polyurethane coating. For this purpose, they firstly prepared polysiloxane/polyacrylate emulsion contained nano silica. It was found that the modified nanoparticles decreased thermal stability and water absorption of PU film. It also increased water contact angle of the PU film from 76.3° to 96.7°. It was also illustrated that nano silica caused increment in the hardness of the PU film to 4 H.

Nowadays, it is well known that nanoparticles (e.g. nano SiO2) have poor dispersion in polymer matrices due to their high surface area and hydrophilic nature21. For this purpose, they should be surface modified before application in polymers to demonstrate their required properties22. Different materials have been used to make nanoparticles compatible to polymer matrix. However, silane coupling agents have attracted many attention for surface modification of nanoparticles especially nano silica23.

Rostami et al.24 studied influence of amino silane functionalized nano silica on PU clear coat. It was found that dispersion of nanoparticles directly depends on grafted silane content. It was also observed that amino silane grafting caused increment in the dynamic mechanical properties of PU coating. Ranjbar et al.25 modified surface of nano silica using dimethyl dichloro silane, hexamethyl disilane and methacryl silane. They were added to acrylic-melamine clear coat to improve its mechanical properties. They showed that the cured coatings contained silanized nanoparticles exhibited higher elastic modulus and indentation hardness compared to pure nano SiO2 containing one. However, the most significant results obtained in the sample contained dimethyl dichloro silane grafted nano silica. Ghanbari et al.26 studied surface modification of SiO2 nanoparticles by 3-Glycidoxypropyltrimethoxysilane (GPTMS) and their effect on anticorrosion behavior of epoxy clear coat. The epoxy coating contained modified nano SiO2 possessed excellent anticorrosion properties. The effect of fluorinated silane grafted nano silica on acrylic based coating for glass surface was investigated. It was observed that the coating contained silanized nanoparticles illustrated a water contact angle of 174 ± 2° and suitable self-cleaning properties27. In another study, surface modified nano silica by γ-methacryloxypropyltrimethoxysilane was used in polyurethane acrylate coating. The prepared coating showed water contact angle of 106° and anti-aging performances (i.e. 30% loss in gloss after 720 h UV irradiation)28.

Polyurethane materials are widely used in various industries due to their excellent mechanical properties, such as flexibility, abrasion resistance, and durability. However, they suffer from limitations such as poor thermal stability, low scratch resistance, and susceptibility to environmental degradation (e.g., UV exposure or moisture absorption). To overcome these deficiencies, researchers have explored various reinforcement strategies, with silica being one of the most common fillers. Silica, due to its high surface area, chemical stability, and ability to form strong interactions with the polymer matrix, enhances the mechanical and thermal properties of polyurethane. By adding silica, it is possible to improve the scratch resistance, enhance thermal stability, and increase the material’s durability under harsh environmental conditions29,30. This study aims to investigate the role of silica in enhancing polyurethane-based composites.

Based on the literature review, to improve the performance of polyurethane nanocomposite as a top coat, different silane coupling agents have been used for surface modification of nano silica, hitherto. The epoxy, acryl, and amine functional silanes have been mostly used to enhance the mechanical properties (i.e., wear/abrasion, scratch resistance, and hardness) of PU top coats. In contrast, fluorosilanes have been used to induce hydrophobicity for self-cleaning of the PU top coats31.

This study presents an innovative approach to improving polyurethane nanocomposite coatings by surface-modifying nanosilica with inexpensive silane coupling agents (i.e., OTES and VTES) against expensive fluorosilanes to modify the surface. By adjusting the silane concentration, the coatings’ hydrophobicity, scratch resistance, and mechanical properties are significantly enhanced. The cost-effective silane modification method provides a scalable solution for producing durable, high-performance coatings with improved surface characteristics.

Experimental

Materials

Acrylic resin 1210 N (as resin 1), acrylic 352 × 60 (as resin 2) and Desmodur N75 HDI 8000 (as curing agent) were purchased from Max-Mayer Chemical Corporation. Toluene, octyltriethoxy silane (OTES), vinyltriethoxy silane (VTES), tin catalyst (DBTDL), BYK 358 and Tinuvin 400 were provided from Merck Company. Microscope glass slides were used as substrate for specimens used in gloss, scratch and hardness tests. Steel plates, as substrate for adhesion and hardness test specimens, were sand blasted and washed by an ultrasonic cleaner with acetone for 30 min and deionized water to remove possible impurities.

Sample preparation

Silane grafting

The stoichiometric concentration of silanes (X) needed to react with surface OH groups of nano silica was determined based on Mrkoci32 method. The silane content was calculated by the following Eq. (1):

$$\:{m}_{modifier}=3\frac{{M}_{modifier\:}.{m}_{{sio}_{2}}.\:{n}_{OH}.\:{S}_{{sio}_{2}}.{10}^{18}\:}{{N}_{A}}$$
(1)

Where mmofifier, Ssio2, msio2, Mmodifier, nOH and NA are the mass of the modifier (gr), the specific surface area of the nanoparticle, the amount of the silica nanoparticles (gr), the molecular mass of the modifier, the number of hydroxyl groups per nm2 and Avogadro number, respectively.

Number 3 in the equation implies that each silane molecule could react to three surface OH group. The number of hydroxyl groups was determined using thermogravimetric analysis (TGA). Silanization was performed at concentrations of 1, 5, 10 and 20X.

To silanize nanoparticles, firstly 1 gr nano silica was added to 20 ml toluene (as silane solvent) and homogenized using sonication for 30 min. The suspension was pour in a reactor equipped to condenser and stirred at speed of 1000 rpm at 80 °C. At the same time, the silane was added to toluene at ratios of 0.8:20 and 0.43:20, respectively. The silane solution was added to the nanoparticle suspension dropwise and stirred for 4 h to perform silane hydrolysis on nanoparticles surface. Thereafter, the suspension was centrifuged for 15 min at speed of 3000 rpm. The silanized nanoparticles were filtered and washed three times using acetone to remove physically adsorbed silane molecules. The nanoparticles then dried in vacuum oven at 80 °C for 2 h to complete silane concentration step33 (see Fig. 1).

Fig. 1
figure 1

Scheme of surface modification and dispersion of nanomaterials in PU.

Full size image

PU samples

Table 1 shows the ingredients of the pristine PU sample. The codes of the PU nanocomposites are shown in Table 2.

Table 1 The formulation of PU clear coat.
Full size table
Table 2 The codes and ingredients of the prepared nanocomposite samples.
Full size table

The nanoparticles, at various concentrations, were added to Part A of the resin and mixed using a high-shear homogenizer to achieve a uniform dispersion. The mixtures was then sonicated for 10 min to further break up any agglomerates and ensure a finer, more consistent dispersion. After sonication, the mixture was combined with Part B and mixed thoroughly with a mechanical stirrer to facilitate the curing reaction. The coating was then applied to substrates using an applicator to achieve a consistent wet film thickness of 100 μm. Finally, the samples were cured at room temperature for 8 h to allow full polymerization and crosslinking, resulting in a durable, scratch-resistant, and hydrophobic coating (see Fig. 1).

Characterization

The silanized and pure nano silica were characterized using Fourier Transform Infrared Spectroscopy (FTIR) (i.e., by SPECTRUM ONE, PerkinElmer) with a spectral range of 4000–400 cm⁻¹ and a resolution of 4 cm⁻¹, thermogravimetric analysis (TGA) (i.e., by TGA 209 F3 Tarsus, Netzsch) at a heating rate of 10 °C/min under nitrogen atmosphere, and X-ray diffraction (XRD) (i.e., by Philips PW1730) with Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 10°–80°, with a step size of 0.02°.

The pull-off test was performed based on ASTM D4541. The gloss of different samples was performed based on ASTM D 523. The nano-scratch test was performed by Nano Indenter BS 3900-E2 with a diamond tip. The test was performed based on ASTM D4366. Scratch load increased linearly from 0 N to 10 N at constant scratch velocity of 30 μm/s over a distance of 500 μm. The maximum sustained loads (per grams) were reported as scratch resistance.

Results and discussions

Characterization of nanoparticles

TGA analysis

Figure 2 shows TGA curves of pure and silanized nano particles. The weight loss related to each temperature range are reported in Table 3.

TGA was employed to investigate the thermal stability and surface functionalization of the VTES-modified nanosilica samples (NS-V1, NS-V5, NS-V10, and NS-V20) in comparison with unmodified nanosilica (NS) (see Fig. 2a). The TGA curves are divided into two main regions based on weight loss behavior: the first region (up to ~ 120 °C) and the second region (~ 120–600 °C). In the first region, a minor weight loss is observed in all samples, primarily attributed to the evaporation of physisorbed water and volatile impurities. The NS sample exhibits the lowest weight loss in this range, reflecting the absence of hydrophobic surface groups that typically result from silane grafting. In contrast, the functionalized samples display slightly higher weight loss, indicating increased moisture retention due to surface silanol or partially hydrolyzed silane groups. The second region (120–600 °C) corresponds to the decomposition of organic moieties grafted onto the silica surface. The extent of weight loss in this region increases progressively from NS-V1 to NS-V10, confirming the successful incorporation of higher silane content. NS-V10 exhibits the highest thermal degradation among the series, indicating a greater amount of vinylsilane loading. Interestingly, NS-V20 shows a lower overall weight loss compared to NS-V10, which may be attributed to the saturation of surface silanol groups, leading to reduced grafting efficiency at higher silane concentrations. Unmodified nanosilica (NS) shows minimal weight loss over the entire temperature range, as expected due to the absence of any organic modification. This stark contrast further confirms the thermal decomposition of vinyl silane functionalities in the modified samples.

TGA was conducted to evaluate the thermal stability and degree of surface functionalization of NS, NS-O1, NS-O5, NS-O10, and NS-O20 samples in Fig. 2b. The first region (up to ~ 120 °C) corresponds to the desorption of physically adsorbed water and residual solvents. All silanized samples show slightly higher weight loss than pristine NS in this range, suggesting the presence of surface groups that enhance moisture adsorption or retention. Among them, NS-O5 shows the highest mass loss, potentially due to its larger surface area or more loosely bound water. The second region (~ 120–600 °C) reflects the decomposition of grafted organic moieties. A clear trend of increasing weight loss is observed from NS-O1 to NS-O10, confirming the progressive incorporation of octyl silane onto the silica surface. NS-O5 exhibits the most substantial thermal degradation, indicating a higher level of organic content. Surprisingly, NS-O20 shows a lower weight loss compared to NS-O10 and NS-O5, suggesting a decline in grafting efficiency at high silane concentrations. This behavior may result from steric hindrance or the saturation of accessible silanol groups, which limits further silane attachment. The pure nanosilica (NS) remains thermally stable with minimal weight loss throughout the heating range, confirming the absence of organic functional groups. Compared to the vinyl-modified series, the octyl-functionalized samples show a more pronounced degradation profile, consistent with the higher molecular weight and greater hydrophobicity of octyl chains. These TGA results validate the successful grafting of octyltriethoxysilane onto the nanosilica surface, with an optimum functionalization efficiency occurring around 5–10% silane content.

Fig. 2
figure 2

TGA curves of nanoparticles silanized by; (a) VTES and (b) OTES.

Full size image
Table 3 TGA data for nanoparticles.
Full size table

The extent of silane grafting onto the surface of nanosilica was quantitatively evaluated using TGA (see Table 3). Based on the weight loss data in the 120–600 °C temperature range, which corresponds to the thermal decomposition of surface-grafted organic moieties, the grafting ratio (%) was calculated by subtracting the weight loss of pure nanosilica (1.78%) from that of each silane-modified sample. Among the OTES-functionalized samples, NS-O5 exhibited the highest grafting ratio (7.45%), indicating optimal silanization efficiency at 5% silane concentration. NS-O1 and NS-O10 showed moderate grafting ratios of 4.29% and 3.65%, respectively, while a significant decrease was observed for NS-O20 (0.66%). This reduction at higher concentration may be attributed to steric hindrance and limited availability of reactive silanol groups, leading to incomplete grafting and possible silane self-condensation. In the case of VTES-modified nanosilica, the grafting ratio reached a maximum for NS-V1 (5.59%) and NS-V5 (5.16%), followed by a gradual decrease for NS-V10 (3.75%) and NS-V20 (1.64%). This trend is consistent with the saturation behavior observed in the octyl-functionalized series, suggesting that excessive silane dosage does not necessarily lead to higher surface coverage. Overall, these findings highlight that both silane type and concentration significantly influence the grafting efficiency. Moderate silane content (typically 1–5%) resulted in optimal surface functionalization, while further increases caused diminishing returns due to surface saturation and steric effects.

Two optimal samples (i.e., NS-V1 and NS-O5) were selected in TGA analysis as the optimal samples with the highest silane grafting, based on their thermal stability and mass loss patterns. The TGA analysis was conducted under a nitrogen atmosphere at a heating rate of 10 °C/min, with both samples demonstrating the most significant weight retention at elevated temperatures, indicating the successful and stable grafting of the silane groups. These samples exhibited the highest silane grafting percentages, as evidenced by the minimal mass loss in the temperature range associated with the thermal decomposition of organic groups. The results were compared with those of other samples, and NS-V1 and NS-O5 were identified as the most efficient in terms of silane surface modification.

FTIR analysis

Figure shows the FTIR spectrums of pure and silanized nano silica particles (see Fig. 3). The peak appeared at 3500 cm− 1 was related to stretching vibration of O-H groups. It confirms presence of OH groups on nano silica surface. The peak at 1620 cm− 1 was corresponded to bending vibration of OH groups in H2O (as absorbed humidity on the hydrophilic nano silica surface). The peak appeared at 1100 cm− 1 was related Si-O-Si (silanol) groups in chemical structure of nanoparticles. The peaks at 470 and 810 cm− 1 was attributed to Si-O and Si-C bonds, respectively. In the silanized nanoparticles, presence of peaks at 2800–3000 cm− 1 was related to stretching vibration of CH and CH2 groups that confirms successful grafting of silanes. Furthermore, the intensified peaks of Si-O and Si-O-Si groups approves the silanization of nanoparticles34.

Fig. 3
figure 3

FTIR specturms of pure and silanized SiO2 nanoparticles.

Full size image

XRD analysis

XRD patterns of pristine nanosilica (NS) and its surface-modified counterparts using vinyltriethoxysilane (VTES, NS-V1) and octyltriethoxysilane (OTES, NS-O5) are presented in Fig. X. All samples exhibit a broad diffraction band centered around 2θ ≈ 22°, which is characteristic of the amorphous structure of silica (see Fig. 4). The increased intensity of the broad peak after VTES modification can suggest improved short-range order or local structuring within the silica network due to the formation of siloxane (Si–O–Si) bonds or densification of the silica shell. This increased intensity might be interpreted as enhanced electron density or better-defined local arrangements of the Si–O–Si network, rather than crystallization or long-range ordering. The sharpness of the peak slightly improves as well, indicating a modest improvement in structural uniformity at the nanoscale. In contrast, the OTES-modified sample (NS-O5) also shows an increased intensity compared to pristine NS, though to a lesser extent than NS-V1. The bulky and hydrophobic C8 octyl chains in OTES likely form a thicker organic layer on the nanosilica surface, which contributes to partial attenuation of the X-ray signal due to the amorphous nature of the organic moiety. The lower intensity relative to NS-V1 reflects the dominance of the disordered organic phase in NS-O5. Overall, the XRD results confirm successful surface functionalization of nanosilica by both VTES and OTES, with distinct effects on the local structure and X-ray scattering behavior. The enhancement in the amorphous band intensity for NS-V1 implies a more rigid and potentially reactive siloxane network, while the more moderate response in NS-O5 is consistent with the formation of a hydrophobic and structurally disordered organic coating.

Fig. 4
figure 4

XRD patterns of the pure and silanized SiO2 particles.

Full size image

XPS

The graph presents XPS data for three different pure silica (NS) and surface-modified nanosilica (i.e., NS-O5 and NS-V1) (see Fig. 5). The spectra show distinct peaks for elements such as Si 2p, Si 2s, C 1s, and O 1s. The NS sample shows peaks mainly for silicon, with no significant surface modifications or organic groups. In contrast, NS-O5 displays prominent O 1s and C 1s peaks, indicating the introduction of oxygen and carbon-containing groups on the surface due to the treatment with OTES. Similarly, NS-V1 also shows O 1s, C 1s, and Si 2p peaks, but the intensities may vary, suggesting that VTES has been incorporated, likely altering the surface properties compared to NS-O5. These XPS results confirm that surface modification with silane reagents successfully introduces oxygen and carbon groups, which likely affect the surface chemistry and properties, such as hydrophobicity.

Fig. 5
figure 5

Wide scan XPS of NS, NS-V1 and NS-O5.

Full size image

Water contact angle

Nanosilica (NS) is widely used in various industries due to its high surface area, thermal stability, and biocompatibility; however, its inherent hydrophilicity can limit certain applications. To address this, chemical surface modification with organic agents such as VTES and OTES was employed to tailor its surface properties. Water contact angle measurements revealed distinct wettability behaviors (see Fig. 6). Unmodified NS exhibited a hydrophilic nature (52°), while VTES-modified NS (NS-V1) showed strong hydrophobicity (135°) due to vinyl groups, and OTES-modified NS (NS-O5) displayed intermediate hydrophobicity (107°) attributed to its shorter alkyl chain. These results demonstrate that surface functionalization enables precise control over nanosilica’s wettability, making it adaptable for hydrophobic coatings or polymer composites35.

Fig. 6
figure 6

WCA of NS, NS-V1 and NS-O5.

Full size image

Properties of PU coatings

Hardness test

Table 4 presents results of the pencil hardness test for the pristine polyurethane sample and polyurethane nanocomposites contained 1, 3 and 5 wt% of nanoparticles. It is clearly seen that silanizations caused decrement in the hardness of the nanocomoposite films at some nanopartilces concentrations. This was attributed to the oily (hydrophobic) nature of the grafted silanes. However, at 5 and 1wt.% loading of octyl silane and vinyl silane grafted nanoparticles, respectivey, the hardness remained unchanged compared to the pristine PU film.

Table 4 The hardness test results for pristine and nanocomposite PU films.
Full size table

Pull-Off test

The pull-off adhesion data (see Fig. 7) reveal a complex interfacial behavior where VTES-modified nanocomposites demonstrate superior bonding strength (4.63 MPa at 1 wt%) due to optimal chemical interactions between vinyl groups and the polyurethane matrix, while OTES-modified systems show loading-dependent performance with an unexpected peak at 1 wt% (3.95 MPa) suggesting potential octyl chain reorientation or substrate bonding advantages that overcome typical hydrophobicity limitations. The pristine PU shows baseline adhesion of 1.8 MPa, which increases significantly with unmodified SiO₂ up to 3 wt% (3.0 MPa) due to mechanical interlocking. However, beyond this concentration, nanoparticle agglomeration starts to occur, reducing the effectiveness of the adhesion. These results highlight three critical interfacial phenomena: (1) chemical bonding potential of vinyl groups, (2) loading threshold for nanoparticle effectiveness, and (3) the counterintuitive adhesion capability of properly dispersed hydrophobic surfaces. The inverse relationship between VTES concentration and performance suggests self-limiting bonding mechanisms that warrant further XPS analysis of interfacial chemistry.

Although vinyl silane-modified silica at 1 wt% is hydrophobic (contact angle 135°), at this low concentration, the uniform dispersion of nanoparticles and the chemical interactions of the vinyl groups (C = C) with polyurethane chains through weak but effective bonds like polar interactions and van der Waals forces help maintain the hardness at 4 H. At higher concentrations (3–5 wt%), nanoparticle aggregation and the cumulative hydrophobic effect disrupt the matrix homogeneity, reducing hardness to 3 H. In contrast, octyl silane, even at low loadings, significantly reduces hardness due to its strong hydrophobicity and lack of effective interfacial interactions with the matrix36.

Fig. 7
figure 7

Pull-Off test results for pristine and nanocomposite PU coatings.

Full size image

Gloss measurement

The gloss of the samples were evaluated that results are shown in Fig. 8. Usually, by incorporating fillers to the polymers their gloss decreases due to the nano/micro scale roughnesses that occur on the film surface.

It is observed that the incorporation of nano-fillers into the PU film led to a decrease in gloss, particularly at higher nanoparticle loadings, due to the increased surface roughness. However, the modified nanocomposite coatings contained 1wt.% of silanized nano silica, the gloss illustrated the same gloss as pure PU film. This was attributed to the well dispersion of the silanized nanoparticles in PU matrix due to improved interfacial interactions between silane grafted nanoparticles and polyurethane chains that prevented them to immigrate to the film surface. Besides, at higher nanoparticle contents, they were agglomerated that increased the roughness of the films and decreased the gloss.

Fig. 8
figure 8

Results of gloss test for pristine and nanocomposite PU coatings.

Full size image

Scratch test

Table 5 shows the scratch test results for PU films. The results showed that even pure nano silica has considerable influence on scratch resistance of PU film. Besides, the scratch resistance improved by increment in the nanoparticles content up to 3 wt%.

It was also seen that incorporating OTES grafted nano SiO2 to PU matrix, caused increment in the scratch resistance compared to pristine PU film. However, there were slight decrements in the scratch resistances of these samples compared to the pure nano silica containing samples (except at 5 wt% nanoparticle loading).

Table 5 The scratch test results for pristine and nanocomposite PU films.
Full size table

The highest scratch resistance was obtained by the nanocomposite contained 1wt% of VTES grafted nano silica. This was attributed to the improved interfacial interactions between PU matrix and surface modified nanoparticles that reinforced effectively the matrix and increased considerably scratch resistance. In contrast, a reverse trend in the scratch resistance was seen in the PU nanocomposites that contained VTES grafted nano silica. It is seen that by increment in nanoparticle content, the scratch resistance decreased considerably. This was attributed to the fact that at higher nanoparticle loadings, the binder is insufficient to wet al.l the silanized nanoparticles, leading to aggregation and reduced scratch resistance. This forced the silanized nanoparticle to coagulate in the matrix that caused decline in the scratch resistant37.

WCA of coatings

The WCA measurements presented in the image highlight the effect of surface modifications on the hydrophobicity of polyurethane (PU) substrates (see Fig. 9). The pure PU surface exhibits a contact angle of 34°, indicating a moderately hydrophilic nature. Upon incorporating nanosilica (PU-NS), the surface becomes hydrophilic, as seen with a contact angle of 5°, suggesting enhanced water absorption. Adding OTES (PU-NS-O5-1) results in a significant increase in the contact angle to 91°, indicating improved hydrophobicity due to the silane treatment, which likely forms a water-repellent layer on the surface. Further modification with VTES (PU-NS-V1-1) enhances the hydrophobicity even more, with a contact angle of 111°, demonstrating the most effective water-repelling properties. This progression demonstrates the impact of nanosilica and silane-based treatments on tailoring the surface wetting behavior of polyurethane, with potential applications in creating coatings with enhanced water resistance. Table 6 compares the work of others with this work.

Fig. 9
figure 9

WCA of PU, PU-NS, PU-NS-V1-1 and PU-NS-O5-1.

Full size image
Table 6 Comparing the work of others with this work.
Full size table

Conclusions

This study demonstrated the effective surface functionalization of nano-silica with hydrophobic silane agents, VTES and OTES, and evaluated their effects on the mechanical and surface properties of polyurethane (PU) nanocomposite coatings. TGA and FTIR analyses confirmed effective grafting of silanes, particularly at moderate concentrations (1–5X stoichiometric), while XRD patterns indicated preservation of silica’s amorphous nature with slight structural modifications. Incorporation of silanized nano-silica into PU matrices influenced key coating attributes. Notably, 1 wt% of VTES-modified silica improved scratch resistance by up to 37.5%, highlighting enhanced nanoparticle matrix interaction. Similarly, 5 wt% OTES-modified silica achieved a 25% improvement in scratch performance. However, excessive filler loading or silane concentration adversely affected mechanical strength, gloss, and interfacial adhesion, primarily due to agglomeration and reduced binder interaction. Despite a slight reduction in hardness in certain formulations, optimized nanocomposite coatings preserved or even surpassed the mechanical integrity of pristine PU. Also, adding hydrophobic nanoparticles made the PU hydrophobic. Overall, the silanization strategy adopted here offers a scalable and cost-effective route to develop hydrophobic, mechanically reinforced PU top coats without relying on expensive fluorosilanes. This approach holds potential for large-area applications such as clear coats, offering a balance between durability and surface protection.