Development of self-cleaning polymeric surfaces using polymer processing systems for application to high-voltage insulators

Herein, polymer processing systems are used to fabricate superhydrophobic high-temperature vulcanized (HTV) silicone rubber surfaces by direct replication. HTV silicone rubber is one of the main polymeric housing materials used in high-voltage insulators. The selected polymer processing techniques are compression molding and injection molding.The direct replication approach requires that a template or insert having the desired surface patterns be replicated onto a target polymer surface via a polymer processing. The appropriate micro-nanostructures, required for achieving ultra-water-repellency, were created on the insert materials (an aluminum alloy) using a wet-chemical etching method. As a flawless demolding is essential to acquire desirable replication quality, an antistiction coating was applied to the insert surfaces prior to the molding process to ensure the thorough removal of the silicone rubber during the demolding. The resulting silicone rubber surfaces possessed micro-nanostructures producing a water contact angle (WCA) of >160° and a contact angle hysteresis (CAH) of <3°. The surface roughness of the aluminum inserts was optimized at HCl concentrations of 15 wt.%. The self-cleaning properties of the produced ultra-water-repellent silicone rubber surfaces were rigorously investigated to ensure a self-cleaning surface at real outdoor imitated conditions. The presence of air pockets in between the surface asperities produced the Cassie-Baxter regime. The consistency of these air pockets is crucial for attaining the self-cleaning properties. A series of tests, including droplet impact, water-jet impact, trapped air layer, and severe droplet contact tests were conducted to confirm the stability of the Cassie-Baxter regime. A comprehensive series of self-cleaning experiments involving both suspended and non-suspended contaminants, e.g., kaolin, carbon black, and silica as well as contaminant-applying methods, e.g., dropwise, spraying, wet or dry contamination were performed. Self-cleaning tests were organized from less severe, i.e., non-suspended contamination tests, to severe, i.e., the wet suspended contamination test, to most severe, i.e., the dry suspended contamination test. Due to their ultra-low CAH, the produced surfaces demonstrated favorable self-cleaning properties against the various types of contaminants and the different means of contaminant application. The produced surfaces retained their water repellency following the application of the contaminants and successful cleaning of the surfaces, thereby verifying the self-cleaning performance and resistance of the fabricated superhydrophobic silicone rubber surfaces. The anti-icing properties (delayed ice formation) and de-icing properties (reduced ice adhesion strength) of the produced surfaces were evaluated. Two types of icing (atmospheric glaze and bulk ice) were considered to accumulate ice on the surfaces. The well-known ice adhesion measurement techniques, i.e., the centrifuge adhesion and push-off tests were employed to provide quantitative comparisons of the ice adhesion strength of the produced surfaces. The produced surfaces significantly delayed ice formation and reduced the ice adhesion strength. To rigorously assess the durability of the produced surfaces, a comprehensive series of experiments that covered a wide range of real-life conditions were carried out. In some cases, where the water repellency was lost, the silicone rubber surfaces demonstrated a satisfactory recovery of their anti-wetting properties. Given the importance of replication quality in the direct replication of micro-nanostructures and the role of micro-nanostructures in the formation of superhydrophobic and icephobic surfaces, the effect of processing parameters on the superhydrophobicity, icephobicity, and replication quality in the compression molding of silicone rubber surfaces were evaluated. Curing time, mold temperature, molding pressure, and part thickness were assessed via response surface methodology to determine the optimal processing parameters. Molding pressure and part thickness were revealed as two main influencing parameters in the superhydrophobic properties. The crosslink density of the fabricated silicone rubber samples, however, was found to be significantly affected by curing time and mold temperature. Replication quality was determined for various molding pressures and part thicknesses. There was an optimal molding pressure value at each part thickness level to obtain the best replication quality. Surfaces having the highest replication quality showed the longest freezing delay reflecting their potential use as anti-icing surfaces. Although all superhydrophobic surfaces offered potential icephobic properties, identifying the influential parameters controlling ice adhesion was more complicated. As this PhD project is part of an industrial-academic collaboration, the results obtained in the laboratory experiments were used for implementation in the industry (K-Line Insulators Limited). This step includes the use of aluminum and stainless-steel inserts. Using the injection molding system available at K-Line Insulators Ltd., silicone rubber insulators having superhydrophobic properties were produced successfully. The industrial partner provided facilities to modify its mold to produce superhydrophobic insulators in an industrial scale. Dans cette thèse, les systèmes de transformation des polymères sont utilisés pour fabriquer des surfaces superhydrophobes de caoutchouc de silicone vulcanisé à haute température (HTV) à partir d’une réplication directe. Le HTV est l’un des principaux matériaux polymères utilisés dans la fabrication des isolateurs à haute tension. Les systèmes considérés sont des procédés de moulage par compression et de moulage par injection. L'approche de réplication directe nécessite un modèle ou un insert ayant les structures de surface souhaitée à répliquer sur la surface du polymère. Les micronanostructures appropriées pour obtenir la non-mouillabilité de la surface ont été créées sur les matériaux d'insert (alliage d'aluminium) en utilisant un procédé de gravure chimique. Comme un démoulage sans défaut est essentiel pour obtenir la qualité de réplication souhaitable, un revêtement antiadhésif est appliqué sur les surfaces de l'insert avant le processus de moulage afin d’assurer l'élimination complète du caoutchouc de silicone lors du démoulage. Les surfaces de caoutchouc de silicone développées possédaient des micronanostructures produisant un angle de contact eau (WCA) de > 160 ° et une hystérésis angle de contact (CAH) de < 3 °. La rugosité optimale de surface des inserts en aluminium est obtenue à une concentration massique de HCl de 15%. Les propriétés autonettoyantes des surfaces produites ont été rigoureusement étudiées pour assurer que ces propriétés autonettoyantes demeuraient efficaces dans des conditions extérieures réelles. La présence de poches d'air entre les aspérités de surface est responsable de la formation du régime de Cassie-Baxter. La consistance de ces poches d’air est cruciale pour obtenir des propriétés autonettoyantes. Par conséquent, une série d’essais ont été effectués pour confirmer la stabilité du régime Cassie-Baxter. Ensuite, une série complète d'expériences de propriétés autonettoyantes a été réalisée en impliquant des contaminants en suspension et non suspendus (non dispersés) utilisant divers matériaux (par exemple, le kaolin, le noir de carbone, la silice, etc.) et des méthodes d'application de contaminants (par exemple, goutte à goutte, pulvérisation, contaminants humides ou secs) ont été effectuées. Les tests d’autonettoyage ont été organisés, du test le moins sévère, c’est-à-dire de la contamination non suspendue (non dispersée), au test plus sévère, c’est-à-dire de la contamination en suspension humide, et se terminant par le test le plus sévère, à savoir la contamination en suspension sèche. En raison du CAH ultra-bas, les surfaces produites ont montré des propriétés autonettoyantes favorables contre les différents types de contaminants et de différents moyens d'application de contaminants. Les surfaces produites ont conservé leurs propriétés répulsives après l'application des contaminants et après le nettoyage des surfaces, permettant ainsi de vérifier les performances d'autonettoyage et la résistance des surfaces en silicone superhydrophobe fabriquées. Les propriétés anti-givrantes (la formation retardée de la glace) et les propriétés glaciophobes (la force d'adhérence réduite de la glace) des surfaces produites ont été évaluées. Les surfaces produites sont exposées à la formation de deux types de givrage. Les techniques bien connues de mesure de l'adhésion sur la glace, à savoir le test d'adhérence par centrifugation et le test de poussée, ont été utilisées pour obtenir une comparaison précise des résultats. Les surfaces superhydrophobes produites ont considérablement retardé la formation de glace et réduit la force d'adhérence de la glace. Afin d’évaluer de manière rigoureuse les propriétés de durabilité, une série complète d’expériences a été réalisée sur les surfaces. Les expériences de durabilité ont été menées pour couvrir un large éventail d'applications réelles. En ce qui concerne la capacité attractive du caoutchouc de silicone dans la récupération des propriétés anti-mouillantes, la perte de la propriété de répulsion de l’eau a été régénérée jusqu’à un niveau satisfaisant dans certains cas. Compte tenu de l’importance de la qualité de la réplication dans la réplication directe des micronanostructures d’une part, et d’autre part du rôle des micronanostructures dans la formation de surfaces superhydrophobes et glaciophobes, les effets des paramètres de moulage par compression des surfaces en caoutchouc de silicone sur la superhydrophobicité, la glaciophobicité et la qualité de la réplication ont été évaluées. Le temps de durcissement, la température de moulage, la pression de moulage et l'épaisseur de la pièce ont été choisis comme paramètres de traitement à évaluer. La méthodologie de surface de réponse a été utilisée pour déterminer les paramètres de traitement optimaux. Bénéficiant des résultats, la pression et l'épaisseur ont été révélées comme les deux paramètres d'influence principaux des propriétés superhydrophobes. La densité de réticulation des échantillons de caoutchouc de silicone fabriqués s'est toutefois révélée être significativement affectée par le temps et la température. Les valeurs de qualité de réplication ont été déterminées en fonction de diverses pressions et épaisseurs. Il y avait une valeur de pression optimale à chaque niveau d'épaisseur pour obtenir la meilleure qualité de réplication. Il a également été observé que les surfaces présentant la meilleure qualité de réplication affichaient le plus long retard de gel de la gouttelette d’eau, ce qui représentait leur potentiel élevé d'utilisation en tant que surfaces antigivrantes. Bien que toutes les surfaces superhydrophobes aient présenté des propriétés potentiellement glaciophobes, il a été constaté que le scénario d’adhérence sur la glace était plus compliqué en termes de paramètres influents. Ce projet de doctorat fait partie d'une collaboration industrielle-académique. Les résultats obtenus en laboratoire ont été utilisés pour la mise en œuvre dans l'industrie (K-Line Insulators Limited). À cette étape, des inserts en aluminium et en acier inoxydable ont été utilisés. En utilisant le système de moulage par injection disponible chez K-Line Insulators Ltd., des isolateurs en caoutchouc de silicone ayant des propriétés superhydrophobes ont été produits avec succès. Par conséquent, le partenaire industriel fournit des installations pour modifier son moule afin de produire des isolateurs superhydrophobes à l'échelle industrielle

Micro-nanostructured polymer surfaces using injection molding : a review

Micro injection molding is in great demand due to its efficiency and applicability for industry. Polymer surfaces having micro-nanostructures can be produced using injection molding. However, it is not as straightforward as scaling-up of conventional injection molding. The paper is organized based on three main technical areas: mold inserts, processing parameters, and demolding. An accurate set of processing parameters is required to achieve precise micro injection molding. This review provides a comparative description of the influence of processing parameters on the quality of final parts and the precision of final product dimensions in both thermoplastic polymers and rubber materials. It also highlights the key parameters to attain a high quality micro-nanostructured polymer and addresses the contradictory effects of these parameters on the final result. Moreover, since the produced part should be properly demolded to possess a high quality textured polymer, various demolding techniques are assessed in this review as well

The thermodynamic stability of the Cassie–Baxter regime determined by the geometric parameters of hierarchical superhydrophobic surfaces

Understanding the geometric parameters of hierarchical superhydrophobic surfaces and their impact on the thermodynamic stability of the Cassie-Baxter regime are invaluable for surface wettability–related applications. Herein, we fabricated hierarchical micro-microstructured silicone rubber surfaces having superhydrophobic properties via an industrially applicable direct replication method. The mold inserts were fabricated using photochemical milling, laser ablation, and wet chemical etching to create different hierarchical levels. We calculated surface roughness ratios and equilibrium contact angles and considered the contribution of sub-microstructures in the wettability properties via physical and statistical analyses. Comparing the calculated theoretical wettability properties and experimental measurements revealed a good agreement among all samples because of the accurate predictions of the governing wetting regime. It was worthwhile insights into the design and fabrication of superhydrophobic structured surfaces. The presence of superimposed sub-microstructures produced desirable water-repellency properties because of the reduced solid–liquid contact area as low as 0.086. Given the primary importance of the Cassie–Wenzel wetting transition on the design and fabrication of superhydrophobic surfaces, we evaluated the thermodynamic persistence of the Cassie–Baxter regime by analyzing the energy barrier to be overcome by droplets with various volumes. Finally, we discuss the contribution of the dimensional parameters of microstructures on the stability of the Cassie–Baxter regime

Icephobicity and durability assessment of superhydrophobic surfaces: the role of surface roughness and the ice adhesion measurement technique

The durability of anti-wetting properties is of great importance for ensuring long-lasting superhydrophobic and icephobic surfaces that require minimal maintenance and resurfacing. Herein, we fabricated superhydrophobic silicone rubber surfaces having ultra-water repellency and icephobic properties via two industrially applicable methods: micro compression molding (μCM) and atmospheric pressure plasma (APP) treatment. We produced surfaces covered by micro-nanostructures of differing sizes. We evaluated the anti-icing properties (delayed ice formation) and de-icing properties (reduced ice adhesion strength) of the produced surfaces that were subjected to two forms of icing conditions. The well-known ice adhesion measurement techniques, i.e., the centrifuge adhesion and push-off tests, provided quantitative comparisons of the ice adhesion strength of the produced surfaces. We observed two different mechanical deformations during the ice detachment from the surfaces. Although both superhydrophobic surfaces reduced ice adhesion strength, the smaller surface micro-nanostructures produced a greater reduction in ice adhesion by favoring less ice interlocking with the surface asperities. To rigorously assess the durability of the produced surfaces, we carried out a comprehensive series of experiments that covered a wide range of real-life conditions. Under harsh environmental conditions, the surfaces maintained a water contact angle and contact angle hysteresis of >150° and <10°, respectively, thereby confirming the resistance of the superhydrophobic silicone surfaces to severe chemical and mechanical damage. In some cases where water repellency was lost, the silicone rubber surfaces demonstrated a satisfactory recovery of their anti-wetting properties

Fabrication of Polymer Micro-Nanostructured Surfaces: Mold Inserts, Processing Parameters and Demolding

Polymer surfaces having micro-nanostructures can be produced using injection molding and hot embossing, high efficiency techniques able to meet the needs of the industry for the mass production of polymer parts. Micro-nanosurfaces are in great demand for multiple applications that include antipollution and self-cleaning surfaces, microlenses, dry adhesion surfaces, antireflection coatings, cell culturing and differentiation as well as superhydrophobic surfaces

Evaluating the effect of processing parameters on the replication quality in the micro compression molding of silicone rubber

Given the role of micro-nanostructures in producing superhydrophobic and icephobic surfaces and the importance of high-quality replication of these micro-nanostructures in direct replication processes, we evaluated the effect of processing parameters on the superhydrophobicity, icephobicity, and replication quality of silicone rubber surfaces created via micro-compression molding. Molding pressure, mold temperature, curing time, and part thickness were selected as the processing parameters to be assessed. We used a response surface methodology to illustrate the optimal values of the selected processing parameters. Molding pressure and part thickness were the main influencing parameters to attain the superhydrophobicity. In a second set of experiments, we assessed the replication quality of silicone rubber surfaces of variable thickness subjected to different molding pressures. Each part thickness had an optimal molding pressure for obtaining the best replication quality. Surfaces having the highest replication quality also demonstrated the longest freezing delay and confirmed their potential use as anti-icing surfaces. Although all developed superhydrophobic surfaces showed icephobicity, the influence of processing parameters affecting ice adhesion was complex

Micro-nanostructured polymer surfaces using injection molding: A review

Micro injection molding is in great demand due to its efficiency and applicability for industry. Polymer surfaces having micro-nanostructures can be produced using injection molding. However, it is not as straightforward as scaling-up of conventional injection molding. The paper is organized based on three main technical areas: mold inserts, processing parameters, and demolding. An accurate set of processing parameters is required to achieve precise micro injection molding. This review provides a comparative description of the influence of processing parameters on the quality of final parts and the precision of final product dimensions in both thermoplastic polymers and rubber materials. It also highlights the key parameters to attain a high quality micro-nanostructured polymer and addresses the contradictory effects of these parameters on the final result. Moreover, since the produced part should be properly demolded to possess a high quality textured polymer, various demolding techniques are assessed in this review as well

On the icephobicity of damage-tolerant superhydrophobic bulk nanocomposites

To address the increase in demand for superhydrophobic and icephobic surfaces with greater mechanical robustness, we fabricated damage-tolerant, abrasion-insensitive, and icephobic superhydrophobic bulk nanocomposites using a facile, cost-effective, industrially applicable, and environmentally benign strategy. We prepared nanocomposites composed of high-temperature vulcanized silicone rubber through the highly controlled incorporation of nanosized fumed silica and microsized aluminum trihydrate particles. The produced nanocomposites did not require additional processing, such as sand abrasion or plasma treatment, to acquire their superhydrophobic properties. The extended roughness throughout the whole bulk of the nanocomposites imparted the volumetric superhydrophobicity and resistance to mechanical damage. The presence of micro-nanoparticles also enhanced the thermal stability and icephobic properties of the silicone rubber. The icephobic behavior of the developed nanocomposites was assessed based on freezing delay and push-off tests both of which denoted improved icephobic properties, i.e., high freezing delay time and low ice adhesion strength. We verified the extended duration of superhydrophobicity within the bulk nanocomposite using sandpaper abrasion, severe cutter scratching, tape peeling, and water-jet impacts. This study represents the first evaluation, to the best of our knowledge, of the icephobic properties of both the surface and bulk of the produced nanocomposite subjected to several cycles of sandpaper abrasion. Interestingly, even after multiple abrasion cycles, the samples demonstrated considerably low ice adhesion strength confirming their bulk icephobicity. In a nutshell, our findings are very promising for the fabrication of mechanically robust icephobic materials

Advances in the fabrication of superhydrophobic polymeric surfaces by polymer molding processes

Superhydrophobic materials are found in a suite of scientific and industrial applications, and given their broad potential use, there is great interest in facilitating their mass production. Although numerous methods have been used to produce superhydrophobic materials, only a few are capable of fabricating superhydrophobic surfaces and materials at an industrial scale. Techniques such as injection molding, compression molding, hot embossing, and polymer casting play an important role in the mass production of superhydrophobic polymer surfaces. This technical literature review summarizes recent advances in the polymer molding processes used to fabricate superhydrophobic materials. Here, we review replication methods and the materials that can be used by these approaches. We also evaluate the advantages and disadvantages of these methods and discuss the challenges of molding and demolding single-level structures (e.g., microstructures and nanostructures) and multilevel structures (e.g., micro-nanostructures, micro-microstructures, and micromicro-nanostructures), with a focus on superhydrophobic surfaces. We evaluate the relationship between structure geometry and the wettability of a surface, highlighting the effect of structure type and size in achieving the desired wettability. We then offer perspectives, discuss current limitations, and suggest required studies. This review aims to assist researchers in understanding the fundamentals related to the fabrication of patterned surfaces via polymer molding processes and offer avenues for the successful creation of superhydrophobic polymeric surfaces

Micro-nanostructured Silicone Rubber Surfaces Using Compression Molding

A facile method is introduced for production of micro-nanostructured silicone rubber surfaces by means of direct replication using a compression molding system. The fabricated samples possessing surface roughness display water contact angle of more than 160o and contact angle hysteresis (CAH) and sliding angle of less than 5o. Such low surface wettability of silicone specimens verifies the induced superhydrophobic property. Chemically etched aluminum surfaces could work excellently as templates whose patterns were replicated on the rubber surfaces successfully. Various etching conditions were examined. Surface characterization techniques revealed the presence of micro-nanostructures on the produced silicone surfaces