The Effects of Cure Time and O2 Plasma Exposure

on Epoxy Based Primers for Superhydrophobic

Contamination Control Coating (LOTUS)

Code 546

Mark Hasegawa EngineeringStephen Lebair

Stephen.Lebair@gmail.com

The Lotus coating mimics the superhydrophobicity of the Lotus Flower after which it is

named, and can be used to prevent particles from settling on the surface of coating substrates. Lotus

consists of three main compounds. An aluminum substrate, which is surface prepped to remove any

oxides; a binder, which is applied with a small weight percentage of silica nano-particles that vary in size

from 15 nanometers to 80 nanometers; and a self-assembly monolayer (SAM) that creates the low surface

energy of a superhydrophobic system. The SAM is made using 1H,1H,2H,2H-perflorooctyltrichlorosilane.

The fluorinated carbon tail allows for the superhydrophobic traits that are observed. A major problem that

has emerged with Superhydrophobic surface coatings is durability and the Lotus coating is not yet resilient

enough for consistent handling. Production process changes can alter the adhesion characteristics of

Lotus. This caused a change in the cure time of its primer, and/or the addition of oxygen plasma etching.

Therefor, several studies were conducted to investigate the durability effects of shortening cure time, and

O2 plasma exposure.

To be considered “superhydrophobic” the surface contact angle of a water droplet must be

greater than 150 degrees. A surface is considered hydrophobic if the contact angle falls between 130 to 150

degrees. In contrast, a surface contact angle of 90 degrees or less is “hydrophilic” (having an affinity to

water). Surface contact angles are measured with an instrument considered called a goniometer, wherein a

drop of deionized water is placed onto the surface, and its contact angle is measured. Lotus could potentially

provide, a space-stable coating that removes any minute particulate contamination with a quick tip, shake, or

rinse of deionized water. The final goal for Lotus is to be able to apply it to flight hardware, and prevent loss

of superhydrophobic, anti-contamination properties under harsh space environments.

First and foremost, I’d like to thank my mentor, Mark Hasegawa, for supporting this project

and providing the lab equipment to make this research possible. Additionally, I’d like to thank three

colleagues for their groundbreaking contributions to Lotus in the past years: Kenneth O’Connor, Tori

Stotzer, and Alexson Harris-Kirksey. ReferencesGnanappa, A. K., O’Murchu, C., Slattery, O., Peters, F., O’Hara, T., Aszalós-Kiss, B., & Tofail, S. A. M. (2008). Effect of Annealing on Improved

Hydrophobicity of Vapor Phase Deposited Self-Assembled Monolayers. The Journal of Physical Chemistry C, 112(38), 14934–14942. doi:10.1021/jp804745t

Li, J., Xu, J., Fan, L., & Wong, C. P. (2004). Lotus effect coating and its application for microelectromechanical systems stiction prevention. In Electronic

Components and Technology Conference, 2004. Proceedings. 54th (Vol. 1, pp. 943–947). IEEE. Retrieved from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1319451

Samuel, B., Zhao, H., & Law, K.-Y. (2011a). Study of Wetting and Adhesion Interactions between Water and Various Polymer and Superhydrophobic

Surfaces. The Journal of Physical Chemistry C, 115(30), 14852–14861. doi:10.1021/jp2032466

Samuel, B., Zhao, H., & Law, K.-Y. (2011b). Study of Wetting and Adhesion Interactions between Water and Various Polymer and Superhydrophobic

Surfaces. The Journal of Physical Chemistry C, 115(30), 14852–14861. doi:10.1021/jp2032466

Yildirim, A., Khudiyev, T., Daglar, B., Budunoglu, H., Okyay, A. K., & Bayindir, M. (2013). Superhydrophobic and Omnidirectional Antireflective Surfaces

from Nanostructured Ormosil Colloids. ACS Applied Materials & Interfaces, 5(3), 853–860. doi:10.1021/am3024417

Zhu, L., Xu, J., Zhang, Z., Hess, D. W., & Wong, C. P. (2005). Lotus effect surface for prevention of microelectromechanical system (MEMS) stiction. In

Electronic Components and Technology Conference, 2005. Proceedings. 55th (pp. 1798–1801). IEEE.

The foundation of epoxies, from a

chemistry standpoint, are epoxide rings which open

to connect with other epoxy molecules. Since

epoxies are polymers, they undergo cross linking

between the individual mers. Heat treatment

(commonly referred to as a “cure”) promotes the

rate at which these mers react.

This research explores the epoxy

layer, and its resistance to physical wear. Our

current formulation can withstand the pressure of a

finger stroke. When ramped up to two pound of

pressure while sliding across the surface, it is

evident that the coating particulates and loses its

superhydrophobicity.Figure 2: SEM image of half-cured MLP-300 w/

silica nano-particles, provided by M. Grossman

Figure 1: Scanning Electron Microscope

(SEM) image of uncured MLP-300 w/ silica

nano-particles, provided by M. Grossman

Figure 9: Cure study of an Huntsman Hybridized

Epoxy, this chart isolates different cure times and

illustrates wear characteristics of the samples

Figure 8: Plasma study of an Huntsman

Hybridized Epoxy, comparisons are made

between O2 plasma exposure and controls on

samples of varying cure lengths

Figure 10: Imaging software

calculates contact angle, volume

S-06242014-2 (Uncured)

Finger Test:

# Left Angle Right Angle Avg Angle Vol (µL)

0 156 156 156 10.219

1 158 158 158 9.335

2 152 153 152.5 12.17

3 156 156 156 7.586

4 154 154 154 8.963

5 154 155 154.5 7.994

6 154 155 154.5 8.632

7 153 154 153.5 8.205

8 153 154 153.5 8.907

Figure 11: Sample spreadsheet of data

used to create finger test graphs

Discussion:

• Cure Study

• The SEM shows that uncured epoxy has a greater surface area than cured epoxy

• Theoretically, an increased surface area should lead to more areas for FOTS to deposit, in

turn creating a lower surface energy

• Experimentally, the cured epoxy was more resistant to physical wear than the half cured

sample, and subsequently the uncured sample

• Unable to conclude why the SEM and the experimental phenomenon regarding initial

contact angle contradict

• Plasma Study

• The epoxies that I’ve tested this summer all have fumed silica and silica nanoparticles

incorporated to increase adhesion to the FOTS and create a nanostructure, respectively

• O2 plasma should etch any surface epoxy down to silica, providing more

adhesion to the FOTS, and a more defined nanostructure, while adding a

penalty to durability

• In every case, regardless of cure time, O2 plasma caused the Lotus coating to degrade in

contact angle retention, while not substantially affecting initial contact angle

• An adhesion study should follow, to see if O2 plasma was beneficial at all

Future Direction with Lotus:

The Lotus coating is currently being further developed for non-space environment applications

including solar panels. We currently are changing our formulation to take advantage of relief from specific

limitations which accrue when working in the space environment. Specifically for solar panels, Lotus will be

focused more on optical clarity and durability, without the hindrance outgassing requirements.

• Contamination Study

• Verify the assumed relationship between contact angle and contamination

• Provide more data on the dust mitigation aspect of Lotus

• Materials Study

• Now that this coating isn’t limited by outgassing requirements, use of more adhesive binder

layers should be explored to increase wear resistance

• The solar panels themselves pose an addition problem: our substrate will change from

aluminum to glass

New Processing Techniques

o Apply FOTS on uncured samples, as well as half-cured samples

o Some samples allowed to cure after FOTS

application

o Run oxygen plasma on uncured, half-cured, and cured samples

o Once FOTS applied, some samples allowed to cure

Figure 5: Thermal vacuum

chamber where plasma is run

Contact Angle Measuring (Sessile Drop Method)

1. A sample is labeled into thirds for finger, 2lb, and 5lb testing

2. The sample is place onto the stage of the goniometer

3. Approximately eight microliters of deionized water is dropped

onto the sample

4. Using the camera in figure 4, a picture is taken of the water

droplet resting on the surface of the sample

5. Using imaging software, one can determine the contact angle

of the droplet as well it’s volume

Wear Testing Method

o An initial “zero wear” contact angle measurement is taken

o The droplet is removed by a techwipe without contaminating the

coating

o The coating is then wore down by one of three set forces

o Finger (Figure 6), Two Pound (Figure 7), Five Pound

o Contact angle is measured after each pass, eight times in total, to

determine hydrophobic degradation

Figure 6: Finger testing for

wear properties

Figure 7: Two pound weight

testing for wear characteristics

Figure 4: Goniometer

Θ

Figure 3: Chemical

structure of a simple epoxy

Methods

Background

Abstract Results

Conclusions

References and Acknowledgements

Science, Discovery, and the Universe