Developing Hydrophobic UV-Curable Clearcoats for Plastic Substrates Using Organic-Inorganic Hybrid Technology, Part 1

Figure 3. The FT-IR spectrum recorded at the initiation and end of the reaction
Figure 3. The FT-IR spectrum recorded at the initiation and end of the reaction
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 will be discussed. Part 2, published in Issue 2, 2025, of UV+EB Technology, will include the Results and Discussion.

There is a growing interest in coating materials tailored for non-metal substrates like plastic, particularly within industries such as automotive. Polycarbonate (PC) and other plastic variants widely are utilized owing to their distinct attributes, notably optical transparency and mechanical strength. 1 However, they grapple with limitations such as low hardness and inadequate abrasion resistance. 2-4 To tackle these challenges, a range of hard coating agents have been developed. 5-8

Despite their merits, plastics are susceptible to scratching, which compromises both product performance and longevity. 9 Achieving optimal coatings on plastic substrates faces challenges such as low melting temperatures, substrate deformation and thermal stress. Consequently, enhancing bond strength via surface modifications stands out as imperative for enhancing coating durability. 10 In a study by Wang et al., they investigated the in-situ growth of a pure amorphous coating on ethylene-propylene copolymer (EPC) plastic induced by plasma treatment. This process, involving the transformation of the top layer and epitaxial growth on the transition layer, endowed the substrate with an excellent protective surface, exhibiting strong adhesion and impressive tribological performances. 11,12

However, challenges remain in achieving strong adhesion between these coatings and plastic substrates. In another approach, waxes have been developed as constituents in fabricating corrosion-resistant plastics and rubbers. 13,14 In a review by Thomas et al., amidst the era of advanced polymers, low-temperature cure systems tailored for heat-sensitive substrates have emerged as highly sought-after materials. Initially, the introduction of two-component (2K) polyurethane coatings facilitated lower curing temperatures in plastic substrates while offering increased flexibility and abrasion resistance.

Reviewing Recent Research

Looking ahead, new opportunities arise with the introduction of novel UV-curing systems. Extensive research has focused on UV-curing systems in recent years, encompassing various systems such as free radical and cationic curing, paired with light sources like mercury, LEDs and lasers. 15 The sol-gel method stands out as an effective approach for coating colloidal silica at relatively low temperatures on a range of organic polymer substrates, such as polymethylmethacrylate (PMMA), polycarbonate (PC), polyethyleneterephthalate (PET), Nylon and others. 16-18 This method widely is favored for producing hybrid silica coatings due to several advantages, including precise control over the chemical composition of the product; suitability for coating polymers with melting points between 150° C and 300° C; ease of fabricating uniform coatings through dipping or spin coating; cost-effectiveness; and compatibility with ceramic, glass, metal and polymer substrates. 19 Sol-gel-derived coatings on plastic substrates find practical applications such as anti-reflection, abrasion resistance, super-water-repellency, electric conductivity and gas-barrier properties. 20

Numerous efforts have been directed toward enhancing the hardness and scratch resistance of sol-gel coatings through various methods, including incorporating hard nanoparticles, physical and chemical vapor deposition, and plasma-enhanced chemical vapor deposition. 21-26 The cost-effective nature and versatility of hard sol-gel coatings make them a common choice for improving PC properties, applicable to substrates of any size and shape. 27 The sol-gel technique is known for its capability to create in-situ nanoparticles with high dispersion at low temperatures, offering ease of control and operation with minimal investment. 27 Studies have demonstrated that sol-gel films can significantly enhance PC scratch resistance, with organic-inorganic hybrid composites showing promising abrasion resistance and surface characteristics. 28

Transparent, superhydrophobic coating films also have been successfully prepared on polymer substrates at low temperatures using the sol-gel method. 29 Silica coatings, commonly deposited on plastic materials, aim to improve wear resistance, hardness, light transparency and lifetime under UV and humid conditions. Additionally, they can act as effective gas permeation barriers and wettability layers, depending on their density and surface functionalization. 30,31

Various methods, including sol-gel processes, sputter deposition, electron beam deposition and plasma-enhanced chemical vapor deposition (PECVD), are employed for depositing silica coatings. 32 Aldahoudi et al. developed transparent and anti-static coatings for plastics using the aqueous sol-gel process and silica nanoparticles. 33 In another study, a simple aqueous sol-gel procedure was employed to produce hard, transparent organic-inorganic coatings on polycarbonate (PC), exhibiting good adhesion onto plasma-treated substrates. 34 Additionally, a novel water-resistant antifog coating with a special hydrophilic/hydrophobic bilayer structure was developed for plastic substrates. The bottom layer, composed of an organic-inorganic composite, acted as both a mechanical support and a hydrophobic barrier against water penetration, while the top antifog coating incorporated superhydrophilic species. 35

Maghrebi et al. prepared polyimide-silica hybrid films with nanostructure using the sol-gel technique by hydrolysis-polycondensation of tetraethoxysilane (TEOS) in a polyamic acid solution. 36 Wang et al. obtained a superhydrophobic transparent film on a glass substrate by hydrolyzing TEOS in an acidic environment and reacting it with hexamethyldisilazane (HMDS). 37 Islam et al. demonstrated the preparation of nanoparticle optical coatings of silica and titania on PMMA substrates via a sol-gel method at room temperature. 38 Ortelli et al. reported the preparation of organic-inorganic hybrid compositions (ceramers) through the sol-gel process using different alkoxysilane precursors, exhibiting thermal stability and water-repellency properties. 39 Fasce et al. prepared poly (ethylene oxide)/silica hybrid coatings deposited onto a PVC substrate, achieving coatings with uniform thickness. 40 Al-Bataineh et al. showed that PVC/polystyrene hybrid films doped with silica nanoparticles with high transmittance (>80%) could be achieved using the dip-coating method. 41 Abdel-Baset et al. demonstrated that silica nanoparticles synthesized via the sol-gel method could be well dispersed on PVC film surfaces to obtain transparent PVC-SiO2 nanocomposite films. 42 Sutar et al. reported the preparation of superhydrophobic coatings by applying multiple layers of PVC/SiO2 nanoparticles on glass substrates. 43 Purcar et al. prepared transparent and antireflective coatings by depositing silica materials on PVC substrates via a sol-gel process, achieving hydrophobic coatings without post-growth treatment or chemical functionalization. 44

Despite significant efforts, achieving promising adhesion of coating systems on plastic substrates without pretreatment remains a major challenge. Furthermore, previously developed organic-inorganic hybrid systems generated through aqueous sol-gel processes pose challenges and generate hazardous waste. 45 Therefore, there is a need to develop a new generation of UV-curable organic-inorganic hybrid coatings that offer excellent adhesion on plastic substrates without requiring pretreatment or adhesion promotion.

Study Parameters

In this study, a series of high-solid organosilane oligomers utilizing both petroleum-derived and bio-based polyols was developed. These oligomers were used to deposit Organic-Inorganic Hybrid (OIH) films via a non-aqueous sol-gel process. Thin wet films containing organosilane oligomer, silane functional reactive diluent and a catalyst were exposed to a UV source to initiate a sol-gel reaction at ambient temperature, utilizing atmospheric moisture to form OIH coatings on various plastic substrates. The synthesized oligomers were characterized by FT-IR spectroscopy to confirm the structure, and physical properties, such as glass transition temperature (Tg) (DSC), viscosity (Plate and Cone viscometer), molecular weight and distribution (SEC), and thermal stability (TGA) were measured.

Subsequently, the chemical and stain resistance, mechanical strength, hydrophobicity and weatherability properties of the coatings were evaluated on standard plastic substrates. Figure 1 illustrates the schematic representation of the Organic-Inorganic Hybrid network generated under a UV-mercury lamp and a combination of UV and ambient temperature conditions.

Figure 1. Schematic representation of Organic-Inorganic Hybrid network generated under UV mercury lamp and ambient conditions
Figure 1. Schematic representation of Organic-Inorganic Hybrid network generated under UV mercury lamp and ambient conditions

Materials and Method

Materials

Thermoplastic Polyolefins (TPO)-hard and soft, Acrylonitrile Butadiene Styrene (ABS), polycarbonate (PC), PC/ABS blend and vinyl sheets (automotive grade) were provided by United Paint, USA. Desmophen® VPLS-2328, polyester polyol with OH content = 8.0%, Uralac® 7130 X-65, acrylic polyol with OH content= 4.2% and Desmodur® I (Isophorone diisocyanate (IPDI) monomer) were provided by Covestro, USA. Kuraray® P-1050, a polyester polyol derived from Sebacic acid featuring an OH content of 3.4%, was supplied by Kuraray Co., Japan. Dynasylan® AMEO (3-Aminopropyltriethoxy Silane) and Dynasylan® MTES (Methylthriethoxysilane) were procured from Evonik Industries, USA. 3-isocyanatopropyltriethoxysilane (IPES) with a purity of 95% was purchased from Gelest Co. USA. A photo-latent superacid, a blend of diphenyl(4-phenylthio) phenylsulfonium hexafloroatimonate and (thiodi-4,1-phenylene) bis(diphenylsulfonium) dihexafluoroatimonate. (Chivacure® 1176) was provided by Chitec Technology Co., Taiwan. King Industries, USA supplied K-Kat® 670 as a Tin-free catalyst for ambient-curing of silane-functional oligomers used to formulate the dual-curable coatings. Xylene and Dibutyltin dilaurate (DBTL) were purchased from Sigma-Aldrich. GENIOSIL® PTM (Phenyl trimethoxy silane) as a moisture-scavenger additive was provided by Wacker Chemical Corporation, USA.

Method: Synthesis of Polyurethane Precursors

Bio-Based Isocyanate Prepolymer: Kuraray P-1050 was accurately weighed and placed into a three-neck flask equipped with a mechanical stirrer, nitrogen inlet, temperature-controller probe and water-condenser set-up. The flask was heated to 80° C and held for 20 min to homogenize and to flush off any moisture present. IPDI then was added dropwise from an additional funnel connected to the top of the water condenser. The starting time of the reaction was recorded. The NCO content of the reaction periodically was monitored following the ASTM D2572 method. After three hours of reaction time, the targeted NCO content was achieved. Subsequently, heating was discontinued, and the reaction mixture gradually was cooled to ~20-30° C, while GENIOSIL PTM and Dynasylan MTES, 0.1 wt.% and 20 wt.% based on the total weight of the reaction mixture, were added slowly with continuous stirring. GENOSIL PTM was added as a moisture scavenger to stabilize the reaction mixture, whereas Dynalsylan MTES was added as a reactive diluent to reduce the viscosity of the reaction mixture for the next step.

Petroleum-Based Isocyanate Prepolymer: The procedure described in Bio-Based Isocyanate Prepolymer was replicated with Desmophen VPLS-2328 as a conventional polyester polyol. The target NCO for this reaction was achieved after four hours.

Synthesis of the Polyurethane Multi-Functional Silane Precursors: The temperature of the prepolymers prepared as described above was allowed to cool to ~20-30° C. Subsequently, Dynasylan AMEO was added slowly from an additional funnel while maintaining gentle stirring. After a stirring duration of 30 minutes, the extent of the reaction was monitored by measuring the NCO content of the mixture, following the aforementioned ASTM method. The NCO content exhibited a decrease of 98.8% from its initial value, indicating the completion of the reaction between NCO and amine. At this point, agitation was stopped, and the resulting products, the 100% solid multifunctional silane precursors, were transferred to a plastic container and stored in a desiccator for further use. The same procedure was done for both bio-based and petroleum-based polyurethane precursors. Figure 2 shows the schematic of the chemical route to obtain the Polyurethane Multi-Functional Silane Precursors.

Figure 2. The chemical route to obtain the Polyurethane Multi-Functional Silane Precursors
Figure 2. The chemical route to obtain the Polyurethane Multi-Functional Silane Precursors

Method: Synthesis of Acrylic Precursors

Figure 3. The FT-IR spectrum recorded at the initiation and end of the reaction
Figure 3. The FT-IR spectrum recorded at the initiation and end of the reaction

Uralac 7130 X-65 and IPES were weighed accurately and placed into a three-neck flask equipped with a mechanical stirrer, nitrogen inlet, temperature-controller probe and water-condenser set-up. Xylene slowly was added to the system under gentle agitation to dissolve the reactants. After 30 minutes, when the mixture was homogenized uniformly, DBTL was added as a catalyst and then the mixture in the flask was heated to 60° C and maintained at this temperature throughout the reaction. The FT-IR spectrum was recorded continuously from the initiation of the reaction to monitor the transmission peak around 2260 cm-1, which is associated with the stretching vibration of NCO groups. After 72 hours of reaction time, the NCO peak disappeared, indicating the completion of the reaction. Figure 3 displays the FT-IR spectrum collected at the initiation and end of the reaction, while Figure 4 illustrates the chemical route to obtain the Acrylic Multi-Functional Precursor.

Figure 4. The chemical route to obtain the Acrylic Multi-Functional Silane Precursors
Figure 4. The chemical route to obtain the Acrylic Multi-Functional Silane Precursors

Method: Preparation of coating compositions

All three precursors, Petroleum-based polyurethane (series P), Bio-based polyurethane (series B) and Acrylic (series A) multi-functional silane oligomers, were formulated in two distinct coating compositions varied in catalyst type and curing techniques. For the UV-cure coating formulation, 3 wt.% of Chivacure 1176 was added and, in the case of the dual-cure system, both Chivacure 1176 and K-Kat 670 catalysts were used. The amount of Chivacure 1176 was calculated based on the solid contents of the system, while the amount of K-Kat 670 was calculated based on the total weight of the coating.

Method: Curing conditions

The curing processes employed for the different coating compositions within series P, B and A are summarized briefly as follows:

UV-Curable Coating: The UV-curable coatings underwent three passes at the conveyor belt speed of 13 ft/min. under a medium-pressure UV mercury lamp (H-bulb, Fusion UV-curing system), with a total energy density of 700 mJ/cm2, as calculated by a radiometer (UVPS). The UV exposure facilitated the generation of an active super-acid catalyst that initiated the moisture-curing mechanism of silane functional groups leading to crosslinking via sol-gel reactions and the formation of the OIH film network.

Dual-Curable Coating: The dual-curable coatings followed a two-step curing process. First, the coatings were exposed to a UV source to generate a super-acid catalyst that initiates the sol-gel process. Subsequently, the partially cured coatings were allowed to further cure and crosslink under ambient conditions by moisture-cure mechanism. This dual-curing approach was intended to afford the advantages of rapid initial curing with UV exposure, followed by extended curing to achieve optimal crosslinking and coating properties. The dual-cure mode was explored to investigate the effectiveness for curing sustainable coatings on 3D objects with shadow areas.

Part 2, published in Issue 2, 2025, of UV+EB Technology, will conclude with a discussion of the experiment’s results. Data presented will include the physical characteristics of synthesized multi-functional silane precursors, film properties of the cured coatings and a review of their hydrophobicity, mechanical performance, chemical and stain resistance, and weatherability.

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