One day meeting on "Abhesion", held at the Society of the Chemical Industry, 15 Belgrave Square, London, on 23 April 2009.
This was our first meeting on this topic. Many situations arise when we wish things would not stick although most of our time is spent trying to make things that stick and stay stuck for long periods of time.
The first paper, entitled ‘Easy release and “non-sticking”: An overview of fundamental principles’, was given by David Packham from the University of Bath. David is a regular attendee and contributor to these meetings. He began with a table of surface energies for a range of materials, which illustrated the range of variation from Nickel to PTFE. He then considered release agents and rough surfaces. The paper contains several useful equations covering the theory and it is worth studying in detail. He concluded that easy release and low adhesion can be obtained where substrate surfaces are of low energy or where some cohesively weak release layer can be introduced between the materials to be separated. Many surfaces have been developed with features of roughness designed to resist intimate contact with a liquid and thus provide low adhesion.
The second paper was given by Glen Mc Hale from Nottingham Trent University and was entitled ‘Wetting and Adhesion: Manipulating Topography and Surface Free Energy’.
He began by saying that the wetting of a solid surface by a liquid depends on the surface free energy, but surface free energy does not depend on the chemistry of the surface alone. Topography can enhance hydrophobicity so that an almost complete roll-up of droplets to create superhydrophobicity occurs or it can enhance hydrophilicity so that water spreads into a film; the same is true of other liquids. He then showed examples of superhydrophobic surfaces and described how surface free energy considerations can explain their wetting behaviour. Glen then described four examples of how a combination of topography and surface chemistry can be used to promote “slippy” and drag reducing properties of surfaces. The first of these put forward the idea that hydrophobic solids can adhere to the water-air interface and developed this idea to suggest that he had a method to convert a water-solid contact into a solid-solid contact thereby creating feely rolling liquid marbles. His fourth example described a classic experiment in which a spherical object falls through a column of water under the action of gravity.
He then presented data suggesting that when objects possess a superhydrophobic surface and retain a layer of air when immersed, they fall faster rather than slower and so show a significant reduction in drag. It is easy to think of uses for such surfaces.
The third paper entitled ‘Development of a novel fibronectin based biomaterial: A slick solution to a sticky problem’ was given by Vivek Mudera from the Tissue Repair and Engineering Centre (TREC) at University College, London.
Vivek described some interesting work in the medical field. He said that TREC had previously developed shear aggregated orientated fibronectin (Fn) materials that were biocompatible and biodegradable. They now a have a version that can be produced to custom shape and size. This novel Fn material actively prevented cell attachment but is not cytotoxic. Prevention of cell attachment and migration across tissue interfaces has significant clinical applications for surfaces where gliding is critical to function. Post–surgical adhesions form across gliding interfaces (tendon and bowel are common examples) and affect functional outcomes. They are made up of dense collagenous bridges, across these gliding interfaces, of disorganised extracellular matrix.
He concluded that TREC has developed a novel Fn biomaterial which inhibits cellular attachment and migration across interfaces with potential use in therapeutic anti-adhesive applications. All in all, this was a very interesting paper.
The fourth paper four by Sasha Heriot of Chamelic Ltd was entitled ‘Smart Design for Easy-Clean Surfaces’.
She said that Chamelic Ltd develops novel block co-polymers for both industrial and domestic applications using technology based on stimuli responsive polymers.
These polymers can assemble into micelles which are able to adsorb onto a broad range of substrates including metal, glass, plastic and painted surfaces. They have a wide range of uses where controlled changes of properties such as adhesion, lubrication and wetting are required. By carefully tuning the polymer properties using controlled/living radical polymerisation (CLRP) techniques, they have developed surface treatments that exhibit beneficial properties. These are:
a. Prevention/decrease in the build up of dirt/grime. The treatment repels dirt and dust particulates and if they do stick, they are rinsed away easily with water.
b. Spot free finish: water wets the surface and forms a thin layer, resulting in faster drying and the absence of water marks on the surface.
c. Prevention of fogging by the formation of a thin transparent layer of water rather than small droplets which refract light.
Sasha said that tests have demonstrated that the coatings can tolerate extreme and prolonged weathering as well as mechanical endurance. Look out for these useful materials.
Paper five, entitled ‘Siliconised Release Liners-Born to Separate’, was given by Dr Roland Wilberger from B. Laufenberg GmbH.
Roland gave us a useful insight into the complexities of developing siliconised release liners. He started by saying that double sided release liners with differential release rely on exact release values and the highest possible reproducibility of release performance for each production batch.
He said that he would concentrate on solventless silicones and that these primarily consisted of five components. In general, the platinum catalysed reaction takes place in the drying oven of the coating machine within a few seconds. An inhibitor is used to avoid premature reaction/solidification of the silicone/crosslinker mix already in the coating head. To adjust the release force a silicone resin (controlled release additive [CRA] can be added. The release force increases with the concentration of CRA that is added.
Roland also said that polymers can be differentiated by their molecular chain length and their structure. The vinyl content and the formation of vinyl groups is also important.
The chemical formulation of the silicone is important and also the process parameters like oven temperatures, coating head adjustments, machine speed and re-humidification have considerable influence on release force. The selection and qualification of optimal release liners requires intensive work from all participants in the value chain.
Next time we use one of these liners maybe we shall think of the amount of science required to produce it.
Paper six, ‘Anisotropic Abhesion and Oleophobicity: Lessons from Nature’, was given by Stuart Brewer of DSTL.
Unfortunately no extended abstract was presented to the meeting.
The seventh and last paper of the meeting, was given by Gary Critchlow; another regular contributor. His paper was entitled, ‘Generation of Durable Ultrahydrophobic Surfaces’.
He began by saying that of particular interest is the application of abhesion-promoting or non-adhering surfaces in the aerospace sector. Low drag has always been important in aviation. Such surfaces can be useful in areas such as, resin transfer moulding (RTM) of large parts, coating of airframes to aid fluid flow or for ease of de-icing and the coating of filters to prevent premature blockage from the micrometer scale upwards; clearly a wide range of applications.
Early studies were focussed on the attainment of smooth, low energy surfaces for ease of release for RTM purposes. Improvements over silicone–based systems were demonstrated using fluoroalkylsilane [FAS] coatings plus a range of PTFE-based coatings. He has studied these coatings using XPS, SEM, contact angle analysis and SPM. These gave information relating the physicochemical properties of a surface to its release performance against common moulded matrix materials such as epoxides. When pressure is applied and in a static situation there appears to be a requirement for a smooth mould surface and sacrificial coatings may also be important. The requirement for a smooth surface is to reduce the area of interaction or to reduce the potential for mechanical interlocking of the epoxide with the mould surface.
In contrast, in the more dynamic case, the ultimate ‘abhesion-promoting’ surfaces, at least in terms of their apparent free energy, exist when some degree of micro- or nano-scale roughness is present. Gary presented a number of surface modification methods which provide superhydrophobic surfaces generally regarded as those with contact angles greater than 150o. He then said that such surfaces can be produced fairly easily using FAS methods but their durability is doubtful. He then mentioned more complex approaches giving nanometre scale texture and better durability.
This all sounds very interesting and important so please contact Gary for further details.
The whole meeting was greatly appreciated by those who attended and has raised both awareness and interest in this very useful topic.