Developing Hydrophobic UV-Curable Clearcoats for Plastic Substrates Using Organic-Inorganic Hybrid Technology, Part 2

By Tahereh Hayeri (Neda), Coating Research Institute, Eastern Michigan University

Editor’s Note: Coatings for plastic substrates are subject to challenges that can result in reduced gloss appearance and weak adhesion, among other issues. Researchers at Eastern Michigan University developed UV-curable Organic-Inorganic Hybrid (OIH) coatings that are said to offer superior hydrophobicity on plastic surfaces, resulting in decreased permeability and enhanced weatherability. In Part 1, the Materials and Methods were discussed, as published in Issue 1, 2025, of UV+EB Technology. Part 2 includes the Results and Discussion.

Results and Discussion

Characterization of Synthesized Precursors: Physical Characteristics

The glass transition temperature (Tg) of the oligomers was determined through differential scanning calorimetry (DSC) using a NETZSCH DSC 214 Polyma instrument under a nitrogen atmosphere. Each sample, weighing 5-7 mg, was encapsulated in a T-zero aluminum pan with a corresponding standard aluminum lid (NETZSCH Co.). The scanning range extended from -150° C to 200° C at a rate of 10° C/min. The corresponding results are shown in Table 1.

The weight average molecular weight (MW), number-average molecular weight (Mn), and polydispersity index (PDI) are determined by size exclusion chromatography (SEC) using a Thermo Ultimate 3000 liquid chromatography system and an Optimax refractive index detector. For analysis, all oligomers were dissolved in tetrahydrofuran (THF) to create an approximately 1% solution. Subsequent to thorough mixing, the samples underwent filtration using a 0.2 µm syringe filter and were analyzed. The SEC system was calibrated across a molecular weight range of 580-364,000 Da and the column set consisted of two mixed bed 8*300 mm Agilent/PSS SDV Linear M SEC columns running 100% tetrahydrofuran (THF) with a flow rate of 1 ml/min. Column pressure was approximately 58 bar throughout the run at a temperature of 30° C. The results are summarized in Table 1.

Characterization of Synthesized Precursors: FT-IR Characterization

The synthesized oligomers were characterized using a Bruker-Tensor 27 FT-IR spectrometer to collect the spectra at 64 scans and a resolution of 2 cm^-1 within the wavelength range of 4000 to 500 cm^-1, utilizing the standard KBr disks.

Figure 5 illustrates the FT-IR spectra for the three multi-functional silane precursors. The FT-IR spectrum reveals distinctive peaks associated with specific molecular vibrations for each precursor. The evident peaks corroborate the multi-functional silane precursors, including the peak at 3,326 cm^-1 corresponds to the N-H stretching vibration, the peak at 1,705-1,710 cm^-1 attributed to C=O stretching vibration, N-H bend vibration peak at 1,530 cm^-1, and C-O stretch vibration peak at 1,222 cm^-1. Peaks in the range of 2,800-2,900 cm^-1 and 2,957 cm^-1 are indicative of C-H stretching vibrations from methylene groups. An intense peak at 1,754 cm^-1 corresponds to C=O stretching vibrations associated with the C=ONR structure. Furthermore, persistent peaks at 1,087 cm^-1 and 950 cm^-1 confirm the presence of alkoxysilane (Si-O-R) groups, which remain unchanged throughout the precursor synthesis process.  

Characterization of Synthesized Precursors: Viscosity

Viscosity stands out as a critical flow parameter for any thermosetting resin, exerting a substantial influence on its application and substrate-wetting properties. The significance of lower viscosity in resin lies in its capacity to enhance application characteristics and facilitate substrate wetting. Moreover, reduced viscosity contributes to the formulation of compositions featuring low VOC and high solid content.

Viscosity measurements of the oligomers were conducted using a Brookfield high-shear CAP 2000 cone/plate viscometer with Spindle #1, applying shear rates ranging from 20 to 500 s⁻¹ at 25° C. In Figure 6, the viscosity of multi-functional silane precursors is presented as a function of shear rate. Notably, both petroleum-based and bio-based polyurethane precursors exhibit shear-thinning behavior, a phenomenon attributed to the hydrogen bonding potential inherent in these oligomers ⁴⁶. Conversely, the acrylic precursor, synthesized in xylene, does not display shear-thinning behavior, but its viscosity is inherently lower. It is important to highlight that the overall flow rates of all three precursors are sufficiently low, facilitating practical application through the draw-down method.

Characterization of Synthesized Precursors: Thermal Stability

The thermal stability of the oligomer was determined by thermogravimetric analysis (TGA) using a NETZSCH TGA 209 F1 Libra instrument under a nitrogen atmosphere. The scanning temperature range extended from 25° C to 900° C at a rate of 10° C/min.

Figures 7 to 9 illustrate the thermograms derived from TGA, while the decomposition temperatures at 5%, 50% and 95% results are summarized in Table 2. These findings suggest that all three multi-functional silane precursors demonstrate robust thermal stability.

Film Properties

Various test panels, encompassing hard TPO, soft TPO, PC, ABS, PC/ABS blend and Vinyl, underwent coating deposition using a draw-down applicator under controlled conditions to achieve a precise wet film thickness of 2 mils (equivalent to 50 µm). The curing procedure for each coating series (designated as Series P, B and A) adhered rigorously to specific procedures dictated by the catalyst employed. A methodical assessment was conducted to evaluate the coatings systematically, according to the relevant ASTM standards or industrial test methodologies, and compared against a reference product. Drawing from previous research ^47,48 and adhering to established standard protocols, the evaluation transpired following a 16-hour exposure of the coated panels to ambient conditions. The film properties for all coating formulations within series P, B and A, compared to the benchmark product or reference, are delineated in Table 3. It is noteworthy that, while industrial norms prescribe surface preparation such as flaming or the application of adhesion promoters on plastic substrates, especially for TPO (both hard and soft), no specific preparatory steps or adhesion promoters were employed on the test panels prior to the application of the developed coatings in this study.

Film Properties: Hydrophobicity

Static Water Contact Angle

The contact angle technique stands as a cornerstone in the examination of coating surface properties, offering precise measurements of the angle (θ) formed at the interface of liquid, gas and solid phases. This method provides direct insights into surface wettability, crucial for understanding coating behavior. ⁴⁹ Consistent with existing literature, the study confirms that the enhancement of surface hydrophobicity in cured films is achieved through the establishment of a siloxane crosslinked network. ⁵⁰ These siloxane linkages create a robust barrier against oxygen, chemicals and water, thereby improving optical, thermal, sealability, weatherability and mechanical properties. ^{51, 49} The results presented in Table 3 reveal significantly increased contact angle values for all examined coatings compared to the reference thermally curable coating, indicating a notable improvement in surface hydrophobicity attributed to the formation of siloxane (Si-O-Si) linkages during the curing process. However, variations in water contact angle values observed on different substrates highlight the influence of substrate properties such as surface roughness, heterogeneity, and wettability. ^{52, 53}

Film Properties: Mechanical Performance

The pencil hardness (ASTM D522) and cross-cut method (ASTM D3359) results, as depicted in Table 3, underscore an optimal equilibrium between scratch resistance and adhesion across all coating compositions compared to the reference product. The outcomes demonstrate that the developed coatings consistently achieve a rating of 5B on the ASTM adhesion scale in most cases, indicating excellent interaction between the coatings and substrates. Notably, these results are achieved without the need for surface preparation or the application of any adhesion promoter on the plastic substrates, contrasting with the reference product. This highlights the robust adhesion achieved by the developed coatings, enhancing their suitability for practical applications.

Film Properties: Specular Gloss at 60°

The visual appeal of the different coating formulations on various substrates was evaluated through the measurement of specular gloss at a 60° angle using a BKY gloss meter. This test was incorporated to examine the impact of the curing method, coating composition and substrate wettability, particularly the formation of alcohol during the sol-gel process, on surface roughness and film leveling, which subsequently influence specular gloss. The findings revealed that all coatings exhibited high gloss levels, surpassing 80 at the 60° angle, meeting the specified requirement for clearcoats on automotive interior plastics. This suggests that the coatings not only offer excellent surface aesthetics but also meet industry standards for visual quality and appearance.

Film Properties: Chemical and Stain Resistance

Chemical Wipe Resistance

Following industrial standard requirements, clearcoats applied on plastic substrates are expected to demonstrate high resistance to a variety of solvents and chemicals. To assess this property, a cheesecloth pad soaked with various chemicals, including cleaning fluid, MEK, ethanol and hand sanitizer, was used to rub the surface of the coatings with moderate pressure. Any alterations in the appearance of the tested coatings carefully were examined. The results indicated that all coating compositions exhibited high chemical wipe resistance on various substrates, comparable to the performance of the reference product.

Spot Test

To assess the stain resistance of the clearcoats, a spot test was conducted in accordance with industrial standard requirements. In this test, an X-shaped incision was made on the surface of the coating, and a drop of the chemical was applied at the center of the X-cut. After specific time intervals (5 hours, 16 hours and 24 hours), the chemical is wiped away, and the coating is examined for any changes in appearance. Spot tests were performed using tap water and windshield washer fluid as the test chemicals. The results indicated that all coating compositions exhibited high stain resistance, comparable to the reference product, due to their inherent high hydrophobicity. This demonstrates the effectiveness of the coatings in repelling stains and maintaining their visual appeal over time, meeting the stringent requirements of industrial standards.

Suntan Lotion Test

Suntan lotions represent one category of chemicals that frequently come into contact with interior automotive plastic coatings, making it imperative to assess the resistance of clearcoats to such substances. To evaluate the lotion resistance of coatings in accordance with industrial standards, a drop of suntan lotion is applied to the coating surface, followed by the application of a specific weight. The coated panels then are subjected to elevated temperatures, and any resulting changes in appearance and adhesion are assessed. Ratings for appearance range from C-0 (indicating no change) to C-4 (denoting complete coating destruction), while adhesion ratings span from P-0 (no change) to P-4 (complete removal of the coating from the surface). Any rating beyond C-3 or P-3 is considered a failure. The outcomes of the suntan lotion resistance test are summarized in Table 4. When Vinyl served as the substrate, all coatings demonstrated promising results compared to the reference product, indicating their effectiveness. For PC substrates, there was a notable improvement in adhesion across all coatings post-test, surpassing the performance of the reference product. Similarly, with ABS substrates, except for the dual-curable series B (B-D), all coatings showed promising results compared to the reference product. Moreover, using PC/ABS blend substrates, coatings labeled P-U, P-D and B-U exhibited significant enhancements in adhesion relative to the reference product. On the other hand, with Hard TPO as the substrate, all coatings displayed either promising or improved results except for the coating labeled B-U. Conversely, when soft TPO was used as the substrate, only coatings P-U and A-U demonstrated promising outcomes compared to the reference product. Overall, the developed coatings showcased promising performance attributed to their high hydrophobicity, which minimized interaction between lotion chemicals and the substrates.

Air Freshener Resistance Test

Air fresheners commonly are used in cars and may release chemicals when exposed to high temperatures, potentially affecting the coatings on interior automotive plastics. To assess the resistance of clearcoats against these chemicals, a specialized test has been devised according to industrial standards. Following this standard procedure, the coatings were exposed to an air freshener under a specific weight load. Subsequently, the panels were subjected to elevated temperatures in an oven and the surface of the coatings was examined for any signs of wrinkling, deformation, staining or other physical or color changes. The findings illustrating the extent of surface affected by the air freshener chemicals are depicted in Table 5. It is evident that all coatings exhibited either promising or improved results compared to the reference product, underscoring their efficacy, which can be attributed to their high hydrophobicity.

Film Properties: Weatherability

Accelerated weathering tests serve as essential tools for assessing coating durability and weather resistance, based on established theories of coating degradation. ⁵⁴ Among these methodologies, the QUV chamber stands out as a widely adopted apparatus in both industrial and academic contexts. Within the controlled environment of the QUV chamber, alternating cycles of UV-A radiation and water condensation are employed to replicate real-world environmental conditions, thereby enabling precise simulation of environmental stressors on coatings. The coating formulations under investigation in this study underwent rigorous examination within the QUV chamber, adhering to ISO 4892-3 standards. The exposure protocol involved two distinct cycles: an initial 8-hour period of UV irradiation at 0.76 J/cm³ and a temperature of 60° C, followed by a subsequent 4-hour phase of condensation at 100% relative humidity and 50° C. Subsequent evaluations encompassed measurement of yellowing (quantified by assessing the b* parameter within the L*a*b* color space using a 45/D65 BYK colorimeter), gloss variation (determined by measuring specular gloss at a 60° angle [G60°] using a 3-angle BYK gloss meter), and observation of any blistering or film degradation in the coatings after 300 hours of exposure. Table 6 illustrates the results, and it is evident that all coatings demonstrate promising outcomes when compared to the reference product. This suggests a significant advancement in XLD development, resulting in enhanced hydrophobicity and decreased permeability ultimately leading to high durability for developed multi-functional silane-based coatings. It should be noted that all the results were obtained without adding any light stabilizer or UV absorber to the coating formulations that might exist in the commercial benchmark products.

Conclusion

The synthesis of multi-functional silane oligomers with varied chemical compositions successfully was accomplished. OIH plastic coatings then were formulated and cured via exposure to a UV mercury lamp at ambient temperature. Remarkably, all OIH coatings exhibited superior hydrophobicity on plastic surfaces compared to a commercial reference sample. This heightened hydrophobicity contributed to a significant reduction in plastic substrate interaction with moisture or other chemicals, resulting in decreased permeability and enhanced weatherability, various chemicals and stain resistance. Moreover, the coatings demonstrated exceptional adhesion to plastic substrates without requiring surface preparation or the application of adhesion promoters, streamlining industrial processes by eliminating one major step. The observed dual curability of the OIH system broadens its potential for commercial applications, particularly for objects with complex geometries. The scientific advancements presented in this study establish the OIH system as a sustainable option for plastic coatings, particularly for interior automotive grades.

References

46. Lewis, C. L.; Stewart, K.; Anthamatten, M. The Influence of Hydrogen Bonding Side-Groups on Viscoelastic Behavior of Linear and Network Polymers. Macromolecules 2014, 47 (2), 729–740. https://doi.org/10.1021/ma402368s.
47. Hayeri, T. N.; Mannari, V. Bio-based Oligomer for Hydrophobic Organic-Inorganic Hybrid Clearcoats with Multiple Cure Capabilities. Prog. Org. Coat. 2024, 190, 108358. https://doi.org/10.1016/j.porgcoat.2024.108358.
48. Hayeri, T. N., & Mannari, V. Eco-friendly topcoats: VOCs/HAPs-free solutions with high bio-renewable content, grounded in organic-inorganic hybrid technology, 52 (2024) Waterborne Symposium Annual Book.
49. Hussain, A.; Calabria-Holley, J.; Schorr, D.; Jiang, Y.; Lawrence, M.; Blanchet, P. Hydrophobicity of Hemp Shiv Treated with Sol-Gel Coatings. Appl. Surf. Sci. 2018, 434, 850–860. DOI: 10.1016/j.apsusc.2017.10.210.
50. Tshabalala, Mandla & Gangstad, John. (2003). Accelerated weathering of wood surfaces coated with multifunctional alkoxysilanes by sol-gel deposition. Journal of Coatings Technology. 75. 37-43. 10.1007/BF02730098.
51. Farris, S., Introzzi, L., Biagioni, P., Holz, T., Schiraldi, A., & Piergiovanni, L. (2011). Wetting of biopolymer coatings: Contact angle kinetics and image analysis investigation. Langmuir, 27(12), 7563-7574.
52. Chau, T. T., Bruckard, W. J., Koh, P. T. L., & Nguyen, A. V. (2009). A review of factors that affect contact angle and implications for flotation practice. Advances in colloid and interface science, 150(2), 106-115.
53. Ahmad, D., van den Boogaert, I., Miller, J., Presswell, R., & Jouhara, H. (2018). Hydrophilic and hydrophobic materials and their applications. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 40(22), 2686–2725. https://doi.org/10.1080/15567036.2018.1511642
54. Blakey, R. R. Evaluation of Paint Durability-Natural and Accelerated. Prog. Org. Coat. 1985, 13 (5), 279-296.