The 20th century was gifted by a chain of events that heralded significant changes in the history of world.
In the technological front the human civilisation was blessed by three most remarkable inventions
- 1959 – Nano Technology.
- 1960 – Internet.
- 1986 – 3D Printing.
Printing is one of the important applications in the family of surface coatings, which was started in the year 220 AD with wood block printing; there after the printing technology developed to what it is now.
The age of 3D printing technology is not very old, yet this has started showing its strength in almost all the departments of science.
Coating technology is also not an exception, many puzzles can be resolved, though there are many but here the commercially viable reproduction of lotus effect can be witnessed.
It has long been known that Lotus Leaves are super-hydrophobic – it's even cited in Bhagavad Gita. Chapter 5; Karma –Yoga – Verse 10
ब्रह्मण्याधाय कर्माणि सङ्गं त्यक्त्वा करोति यः। लिप्यते न स पापेन पद्मपत्रमिवाम्भसा।।5.10।।
Brahmany adhaya karmani Sangam tyaktva karoti yah Lipyate Na SA papena Padma-patram ivambhasa
5.10 He who acts without attachment, reposing all actions on Brahman (Prakrti) is untouched by evil, as a lotus-leaf by water.
But the mechanism of super-hydrophobicity wasn't understood until the 1970s before the introduction of scanning electron microscopes. In case of lotus leaf which has nano rod-like protrusions, so tiny that they reduce its surface adhesion and encourage liquids to ball up and roll off. The underlying principle has been trademarked as the Lotus-Effect.
The self-cleaning property of ultra-hydrophobic micro-nano-structured surfaces was studied by Barthlott and Ehler in 1977, who described such self-cleaning and ultra-hydrophobic properties for the first time as the "lotus effect";
In-depth understanding of super-hydrophobicity requires the examination of the relationship between the surface energy and the surface roughness and the wet-ability of the surface. The most basic equation is the Young's equation. Derivation of the Young's equation starts with the consideration of the sessile drop on an ideal, rigid homogeneous, flat and inert surface. When a liquid is dropped on a solid substrate there exists a three phase contact line where the air, liquid and solid connects each other. Young's equation is given by
Where γSG γSL and γLG refer to the interfacial surface tension between the solid and the gas, the solid and the liquid, and the liquid and the gas, respectively. The θ is called the young's contact angle (or just contact angle). The value of the contact angle is the result of the thermodynamic equilibrium of the free energies at the solid-liquid-gas phase. If water is used as the liquid then the surfaces with < 90° are hydrophilic, while surfaces with > 90° are hydrophobic.
Most of the procedures in fabricating super-hydrophobic surfaces involve either roughening the surface followed by hydrophobization or roughening the low surface materials. Next some of the methods that have been used in fabricating super-hydrophobic coatings for anticorrosion application are presented.
It involves a reaction where the product self- assembles to coat the substrate. Other examples of chemical depositions are chemical vapour deposition, chemical bath deposition, and electro chemical deposition.
Potentiostatic electrolysis was carried out to prepare super-hydrophobic film on the surface of metallic zinc, which caused its contact angle to change from 62°±3° for bare zinc to 152.5°±3°. The resultant film was examined by Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infrared Spectroscopy (FTIR) electrochemical measurements, and contact angle test. The super-hydrophobic property of the film results from the air trapped among the sheets of zinc tetradecanoate. This air behaves as a dielectric for a pure parallel plate capacitor, thereby inhibiting electron transfer between the electrolyte and the substrate. The air can also enhance the contribution of the film itself to protection performance.
Micro and nano-particles can provide surface roughness when deposited on the metal surface. Further treatment of these particles to decrease their surface energy can tender the surface with super-hydrophobicity.
The super-hydrophobic surface (SHS) applied for corrosion protection in this study was prepared from an organic fluorinated polyacrylate incorporated with methyltriethoxysilane (MTES)-based silsesquioxanes spheres. The SHS, with a contact angle of about 153.2° from 74.1° of bare steel after coating onto the surface of cold-rolled steel (CRS) with spin-coating technology. The coating materials applied as anticorrosive coatings were tasted on a series of electrochemical corrosion-protection measurements in saline conditions. The SHS coating on CRS was found to provide superior corrosion protection to that of the hydrophobic organic coating in a series of electrochemical measurements in 3.5 wt % aqueous NaCl electrolyte. This form of coating could also provide better corrosion protection to coated CRS substrates and could serve as an effective barrier against aggressive species.
Yuan et al demonstrated that surface energy can be effectively lowered by growing a polymer brush on the roughened surface of copper. A copper surface was roughened by etching in solution of nitric acid and hydrogen peroxide. The vinyl-terminated silane was then self- assembled on the surface and subsequently, fluorinated polymer brush was grown from it. Polymerisation for 6 h resulted in very rough surface with contact angle value of 159°. The super-hydrophobic coating shows 93% corrosion inhibition efficiency after immersion in 3.5 wt. % NaCl for 1 day and 91.3% after immersion for 21 days.
A surface is coated with black soot by holding over the flame of a wax candle, carbon nano particles forming a loose fractal-like network (a densely self-similar one) that displays super repellency of water and oil. Then this soot layer—which is inherently fragile—is covered by a silica coating applied through vapor deposition. Baking this carbon/silica combination at 594°C burns off the carbon, leaving a network of silica nano spheres. The coating's thickness is well below the wavelength of visible light; thus it's transparent. And it's super-amphiphobic (Super repellency to both water and oil). – (Science magazine (6 January 2012, Vol. 335) published by the American Association for the Advancement of Science, specialists at Germany's Max Planck Institute for Polymer Research).
Limitations of super-hydrophobic surfaces and coatings
Super-hydrophobic surfaces with high water contact angles and low contact angle hysteresis or sliding angles have received tremendous attention for both academic research and industrial applications in recent years. Though the work on advancement in academic research this part of science is highly encouraging but its commercial/industrial acceptability has yet to come because of:
The cost of super-hydrophobic materials has been relatively high due to amount of processing required to create the micro and nano-structures necessary for hydrophobic behaviour.
Nano structure stability
A high quality, high contact angle, super-hydrophobic surface requires a hydrophobic surface chemistry with a stable micro and nano topography. .
Even by using high quality super-hydrophobic particles (like super hydrophobic diatomaceous earth (SHDE) or functionalized silica nano-particles), it's still not easy to bond such particles to a substrate without significantly degrading or destroying the super hydrophobic behaviour.
While super-hydrophobic coatings and surfaces repel water, they do not repel water vapour. If the coating is in a condensation condition (i.e. the coating temperature is below the dew point) condensation will occur. When this occurs, the resulting condensate can result in substantial surface wetting.
The pinned air layer associated with super hydrophobic surfaces can be reduced or eliminated by localized high water pressure. This can be caused by a localized stream of water or by simply mechanical abrasion.
Surfactant/oil wetting issues
Super-hydrophobic behaviour is a result of amplifying (via the surface topography) the effect of water's surface tension. If the water's surface tension is greatly reduced with a surfactant or with oil, super-hydrophobic behaviour will be greatly reduced or eliminated.
Functional materials and their process of implementation are highly sophisticated and are not user friendly from the industrial point of application.
In Physics no matter how hard or how long you have pushed, if the object does not move, then work done is ZERO.
Enough work is done countless time and money is spent but the presence of super-hydrophobicity is yet to come as a reality. Achieving super-hydrophobicity is not the material itself, but also application procedure plays an important role on that.
But the answer lies in the core of the technology of 3-D printing.
3D printing or additive manufacturing is a process of making three dimensional solid objects from a digital file.
The printing process is known as “Bottom up”
The creation of a 3D printed object is achieved using additive processes. In an additive process an object is created by laying down successive layers of material until the object is created. Each of these layers can be seen as a thinly sliced horizontal cross-section of the eventual object.
3D printing is the opposite of subtractive manufacturing which is cutting out / hollowing out a piece of metal or plastic with for instance a milling machine.
3D printing enables to produce complex (functional) shapes using less material than traditional manufacturing methods.
Types of 3D Printing Technologies and Processes
There are several ways to 3D print. All these technologies are additive, differing mainly in the way layers are build to create an object.
Some methods use melting or softening material to extrude layers. Others cure a photo-reactive resin with a UV laser (or another similar power source) layer by layer.
To be more precise: since 2010, the American Society for Testing and Materials (ASTM) group “ASTM F42 – Additive Manufacturing”, developed a set of standards that classify the Additive Manufacturing processes into 7 categories according to Standard Terminology for Additive Manufacturing Technologies. These seven processes are:
- Vat Photopolymerisation
- Stereolithography (SLA)
- Digital Light Processing (DLP)
- Continuous Liquid Interface Production (CLIP)
- Material Jetting
- Binder Jetting
- Material Extrusion
- Fused Deposition Modelling (FDM)
- Fused Filament Fabrication (FFF)
- Contour Crafting
- Powder Bed Fusion
- Selective Laser Sintering (SLS)
- Direct Metal Laser Sintering (DMLS)
- Sheet Lamination
- Directed Energy Deposition
Below are the brief definitions of all the seven processes for 3D printing:
This technology employs a vat of liquid ultraviolet curable photopolymer resin and an ultraviolet laser to build the object's layers one at a time. For each layer, the laser beam traces a cross-section of the part pattern on the surface of the liquid resin. Exposure to the ultraviolet laser light cures and solidifies the pattern traced on the resin and joins it to the layer below.
This technique was invented in 1986 by Charles Hull, who also at the time founded the company, 3D Systems.
Digital Light Processing (DLP)
DLP or Digital Light Processing refers to a method of printing that makes use of light and photosensitive polymers. While it is very similar to Stereolithography, the key difference is the light-source. DLP utilises traditional light-sources like arc lamps.
Continuous Liquid Interface Production (CLIP)
Other technologies using Vat Photopolymerisation are the new ultrafast Continuous Liquid Interface Production or CLIP and marginally used older Film Transfer Imaging and Solid Ground Curing.
In this process, material is applied in droplets through a small diameter nozzle, similar to the way a common inkjet paper printer works, but it is applied layer-by-layer to a build platform making a 3D object and then hardened by UV light.
With binder jetting two materials are used: powder base material and a liquid binder. In the build chamber, powder is spread in equal layers and binder is applied through jet nozzles that “glue” the powder particles in the shape of a programmed 3D object. The finished object is “glued together” by binder remains in the container with the powder base material. After the print is finished, the remaining powder is cleaned off and used for 3D printing the next object. This technology was first developed at the Massachusetts Institute of Technology in 1993 and in 1995 Z Corporation obtained an exclusive license.
Fused Deposition Modelling (FDM)
The FDM technology works using a plastic filament or metal wire which is unwound from a coil and supplying material to an extrusion nozzle which can turn the flow on and off. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism, directly controlled by a computer-aided manufacturing (CAM) software package. The object is produced by extruding melted material to form layers as the material hardens immediately after extrusion from the nozzle. FDM was invented by Scott Crump in the late 80's. After patenting this technology he started the company Stratasys in 1988. FDM are trademarked by Stratasys Inc.
Fused Filament Fabrication (FFF)
The exactly equivalent term, Fused Filament Fabrication (FFF), was coined by the members of the RepRap project to give a phrase that would be legally unconstrained in its use.
Pioneer of Contour Crafting, Dr. Behrokh Khoshnevis of USC, developed a method which leverages the power of additive manufacturing to build homes. Contour crafting essentially uses a robotic device to automate the construction of large structures such as homes. This device prints walls layer-by-layer by extruding concrete. The walls are smoothed as they are built, thanks to a robotic trowel.
The most commonly used technology in these processes is Selective Laser Sintering (SLS).
Selective Laser Sintering (SLS)
SLS uses a high power laser to fuse small particles of plastic, ceramic or glass powders into a mass that has the desired three dimensional shapes. The laser selectively fuses the powdered material by scanning the cross-sections (or layers) generated by the 3D modelling program on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness. Then a new layer of material is applied on top and the process is repeated until the object is completed.
Direct Metal Laser Sintering (DMLS)
DMLS is basically the same as SLS, but uses metal instead of plastic, ceramic or glass. All untouched powder remains as it is and becomes a support structure for the object. Therefore there is no need for any support structure which is an advantage over SLS and SLA. All unused powder can be used for the next print. SLS was developed and patented by Dr. Carl Deckard at the University of Texas in the mid-1980s, under sponsorship of DARPA.
Sheet lamination involves material in sheets which is bound together with external force. Sheets can be metal, paper or a form of polymer. Metal sheets are welded together by ultrasonic welding in layers and then CNC milled into a proper shape. Paper sheets can be used also, but they are glued by adhesive glue and cut in shape by precise blades. A leading company in this field is Mcor Technologies.
Directed Energy Deposition
This process is mostly used in the high-tech metal industry and in rapid manufacturing applications. The 3D printing apparatus is usually attached to a multi-axis robotic arm and consists of a nozzle that deposits metal powder or wire on a surface and an energy source (laser, electron beam or plasma arc) that melts it, forming a solid object.
Six types of materials can be used in additive manufacturing: polymers, metals, concrete, ceramics, paper and certain edibles (e.g. chocolate). Materials are often produced in wire feedstock (filament), powder form or liquid resin. All seven previously described 3D printing techniques cover the use of these materials, although polymers are most commonly used and some additive techniques lend themselves towards the use of certain materials over others.
3-D printing and intelligent coatings
When it was discovered that the self-cleaning qualities of ultra hydrophobic surfaces come from physical-chemical properties at the microscopic to nanoscopic scale rather than from the specific chemical properties of the leaf surface, the discovery opened up the possibility of using this effect in manmade surfaces, by mimicking nature in a general way rather than a specific one.
The complete set of work on developing a manmade copy of the lotus leaf surface was on the attempt of matching the physical properties of the leaf.
The 3-D printing technology can print the complicated three dimensional structure of the lotus leaf on any substrate to create the exact super-hydrophobic surface as lotus leaf.
Now let us examine why lotus leaf is special which is drawing our interest from time immemorial to now.
The specification of Lotus Leaves
- Papillae density (per mm2) – 3431
- Contact angle (static) -163°
- Drop adhesion force (µN) - 8–18
- Wax type tubules
- Wax melting point(°C) - 90 - 95
- Papillae - extraordinary shape and density, extremely reduced contact area between surface and water drops, mechanical robustness
- Epicuticular wax tubules - very small, exceptional dense layer, unique chemical composition, mechanical robustness
- Stomata are located in the upper epidermis.
It was summarised that to create lotus leaf like super-hydrophobicity. The required task is to prepare a 1) blend of waxes as per above specification (melting point, chemical structure and contact angle) 2) Papillae shaped material to be implanted densely on the surface of the substrate.
3-D printing is software driven; the well-known software is Autodesk Inventer, Onshape.
Netfabb, 3D-Tool Free, Viewer, Make Printable, MeshLab.
The (b) of Figure 1 was drawn by using Autodesk Invention; the product is successfully created as below:
The above product was printed by Fused Deposition Modelling (FDM), this will show the super-hydrophobicity as we find in Lotus Leaf.
The application processes of paints are equally important to that of the material. By replacing the normal painting process by 3-D printing, imagination of different intelligent and smart coatings will come alive.
As far as we understand that no industry other than paint have access/knowledge to the endless types of resins, their application and curing technique, Such knowledge can enrich 3D printing technology to be fully fit to work for paint industry. Apart from material the various processes can be clubbed with the equipment of 3D printing to yield a new platform in the history of coating technology.
- Intelligent coatings for corrosion control. – edited by Atul Tiwari, James Rawlins, Lloyd H. Hihara.
- Superhydrophobicity in perfection: the outstanding properties of the lotus leaf Hans J. Ensikat*1, Petra Ditsche-Kuru1, Christoph Neinhuis2 and Wilhelm Barthlott1 Beilstein Journal of Nanotechnology.
- https://www.google.co.in/search?q=SEM+IMAGE+OF +LOTUS+LEAF
- Advanced Materials Research, vol.832, 2014, pages 773-777 Surface tension analysis of cost-effective paraffin wax and water flow simulation for micro fluidic device.
- https://en.wikipedia.org/wiki/Ultrahydrophobicity#Examples_ in_natureen.wikipedia.org
- https://en.wikipedia.org/wiki/Ultrahydrophobicity l https://www.google.co.in/search?q=image+of+lotus+leaf
Dr K K Sengupta
Director, Centre for Fundamental Studies in Coatings Technology (NABL accredited Chemical Testing Laboratory), Sivasagar, Assam, India
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