Self-Stratifying Particulate Coating for Robust Superhydrophobic and Latex-Repellent Surface

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

https://doi.org/10.47277/JEET/8(3)1111

Self-Stratifying Particulate Coating for Robust

Superhydrophobic and Latex-Repellent Surface

1,3,*

Sulaiman Hajeesaeh , Sobiroh Kariyo , Nantakan Muensit , Chalongrat Daengngam

,2,3

1,3

¹Department of Physics, Faculty of Science, Prince of Songkla University, Songkhla 90112, Thailand

Department of Research and Development, Faculty of Science and Technology, Fatoni University, Pattani 94160, Thailand

Center of Excellence in Nanotechnology for Energy, Prince of Songkla University, Songkhla 90112, Thailand

Received: 19/05/2020

Accepted: 08/07/2020

Published: 20/09/2020

Abstract

A technique for preparing superhydrophobic and natural latex-repellent surface requires at least two fabrication components: surface

roughness, and surface layer with low free energy. Here, multiscale surface roughness in micro-/nanoscales with low surface energy can be

simultaneously achieved through the deposition of fluoroalkyl-functionalized silica aggregates. However, the mechanical durability of such

film remains problematic. Therefore, third component such as polymer binder was incorporated carefully to improve adhesion between film-

substrate interface without deteriorating surface roughness and surface energy. In this work, we employed self-stratifying coating technique

to induce vertical phase separation between particles and polymer during film drying, such that the silica aggregates densely accumulated on

the top surface, while polymer binder concentrated near the film bottom. The governing transports during film stratification process involve

diffusion and convection driven by evaporation. Thus, this research focused on the effect of drying temperature and evaporation rate on the

anti-wetting performance of the coating. The results showed that the liquid-repellent properties of the surface improve with increasing drying

temperature, indicating the convection-dominated transport that induced substantial particle trap at the film surface. With polymer binder

added, the coatings still showed decent superhydrophobic and natural latex-repellent properties with maximum contact angles 166.4°±0.6°

and 157.5°±0.5°, as well as minimum sliding angles 2.7°±0.3° and 2.9°±0.2° for water and natural latex respectively. Also, AFM result

revealed that significant surface roughness of 581 ± 18 nm was still achievable even at high blending mass ratio of polymer binder up to half

of the silica weight.

Keywords: Superhydrophobic, Natural latex-repellent surface, Multiscale roughness, Self-stratifying coating

mechanical durability remains a major drawback for real uses.

Introduction

Also, it is quite

a contradiction to expect non-sticky

It is well known that the extreme liquid-repellent properties of

functionalized particles to adhere tightly on a substrate. The

strategy to improve the adhesion between superhydrophobic film

and substrate requires addition of fluoro-containing polymer

binder, which allows good dispersion of fluoroalkyl

functionalized silica nanoparticles into the polymer matrix.

Nonetheless, it remains quite complicated to maintain the surface

roughness and the topmost functional groups of final film, as

polymer binder tends to swamp the surface.

Therefore, an asymmetric particle distribution induced by self-

stratification during film drying is introduced, in order to produce

spatially controlled polymer blending, which does not destroy

superhydrophobic features (4). As particulate coating dries,

diffusion and convection transports determine the final particle

distribution inside the film. Particles tend to accumulate more on

coating surface when the convection dominates, i.e. high

evaporation rate, so higher surface roughness can be expected. On

the other hand, if the diffusion dominates, particles and polymer

surface are the synergistic effect of surface morphology and

chemical compositions (1). Surface free energy of coating films

can be lowered by mean of surface functionalization with

hydrophobic molecules, and the hydrophobicity can be further

enhanced by surface asperities to reach superhydrophobic state.

Therefore, both surface topographical roughness and the outmost

functional groups play a crucial role in producing such superior

liquid-repellent properties. Furthermore, surface roughness in

multiple length scale has been proven as the key to achieve more

stable Cassie-Baxter non-wetting state (2). In our previous

research, we also demonstrated that tipple-scale surface

roughness, obtained from silica aggregates functionalized with

fluoroalkylsilane molecules, were superiorto from stableextreme

anti-wetting surface that can repel water or even highly adhesive

liquid like concentrated natural latex (3). As the film was mainly

composed of functionalized nanoparticles, however, its poor

Corresponding author: Chalongrat Daengngam, (a) Department of Physics, Faculty of Science, Prince of Songkla University, Songkhla

0112, Thailand. (b) Center of Excellence in Nanotechnology for Energy, Prince of Songkla University, Songkhla 90112, Thailand. E-mail:

chalongrat.d@psu.ac.th.

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Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

will blend uniformly causing smoother surface, unfavorable for

superhydrophobic properties.

The coating solution was applied on a glass slide substrate by

simple drop-casting technique and allowed to dry at different

evaporation temperatures. During film drying, self-stratification

of the silica particles and fluorinated polymer binder are

investigated. The effect of evaporation temperature on stratified

particulate coating and the resulting liquid-repellent properties

were examined.

To accomplish the aforesaid idea of stratified drying, a 1D

vertical model of particle transport in film during drying

formation, developed by Routh and Russel, was considered (5). It

can be used to predict skin formation occurring for non-uniform

vertical drying combined with wet sintering (6, 7). This model

approach can be employed to determine whether the particle

transport is controlled by diffusion or convection through the

dimensionless Peclet number , which is defined by the following

expression

Experimental

.1 Formulation of latex-repellent coating

Fumed silica nanoparticles with primary particle size in a

range of 5-25 nm were chemically modified to alter their surface

from hydrophilic silanol groups (–SiOH) to hydrophobic

fluoroalkyl groups (–CF ) by stirring them with 0.2 ml of

퐸퐻₀

푃_푒=

(1)

퐷₀

perfluorooctyltriethoxysilane molecules in a mixture of xylenes

for 70-75 h at room temperature. The silica content in the mixed

solvent was 5 %w/v. Then, polymer binder poly(vinylidene

fluoride-co-hexafluoro propylene) (PVDF-HFP) was dissolved

in acetone for 0.5 h at room temperatureand added into the silica

solution for 2.5 %w/v amount. The coating solution was stirred

further for 0.5 h to attain homogeneous formulation.

where 푃 is the Peclet number. 퐸 is the film evaporation rate

푒

measured from the speed of the descendingsurface. 퐻 is the film

initial thickness, and 퐷 is the diffusion coefficient of particle,

which is related to the particle size described by the Stokes-

Einstein equation,

푘푇

.2 Measurement of solvent evaporation rates

퐷 =

(2)

휋휂푅

The evaporation rate of the solvent was measured by mean

of film surface descending rate in unit of mm/s by using an

ultrasonic sensor for thin film measurement system. The

evolution of drying film thickness can be tracked in real time

inside a temperature-controlled chamber, in order to study the

effect of drying temperature as shown in Figure 2.

where 푘푇 the thermal energy, 휂 is the viscosity of solvent, and 푅

is the particle effective radius. Equation (1) and (2) infer that

stratified drying may be achieved for evaporation-dominated

coating process (high 푃 ). The particles consolidation front forms

푒

at the air–solutioninterface and grows maximumpacking fraction

near the surface top. On the other hand, when diffusion dominates

LinkControl adapter

(

low 푃 ) particles concentration are predicted to remain dispersed

푒

uniformly in the film throughout the coating thickness as it dries

10).

As described in Figure 1, smaller particles with 푃 < 1

undergo faster Brownian motion as “kicked” by the solvent

molecules, and thus they are able to escape from the descending

surface into the bulk film solution during evaporation. On the

(

Ultrasonic

sensor

푒

Computer

other hand, the larger particles with 푃 > 1 experience much

푒

slower Brownian motion unable to escape away from the

descending surface. Thus, comparatively heavy particles tend to

accumulate and form skin layer at the film surface.

Thermostat

Figure 2: Measurement of film evaporation rates using ultrasonic sensor

.3. Contact angles and sliding measurements

An optical contact angle (CA) measurement system

Figure.1: Schematic diagram shows drying steps of particulate coating,

beginning with uniform distribution of particles at the initial time (a).

Then evaporation starts and brings particles toward, where larger silica

particles are trapped at the surface, while the smaller polymer binder

molecules can diffuse away (b). This results in high concentration of silica

particles at the film surface, and the polymer binder submerges down to

bottom (c)

(Dataphysics OCA-15EC) was employed to measure static

contact angles and droplet sliding angles (SA). CA measurement

was performed using probe liquids of water, and 35% natural

latex with droplet size ∼2 µL placed at multiple locations on a

film surface.

In this study, low surface energy silica particles with

푃_푒(silica) > 1 was obtained by particle functionalization with

fluoroalkylsilane compound, and they was blended with smaller

.4. Morphology characterizations

The surface roughness of the film was studied using an atomic

force microscope (AFM) and scanning electron microscope

fluorinated polymer binder nanoparticles with 푃 (polymer) < 1.

푒

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