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3.4.1 Surface energy

A particular issue is adhesion of the upper layers to the substrate. Adhesion occurs as a result of the attractive forces that exist between all atoms and which fall into three broad categories: primary (chemical); quasi-chemical (hydrogen bond); and

secondary (van der Waals) [220]. For adhesion to occur, the surface energy of the substrate should not exceed the surface tension of the fluid by more than 10-

77 15 mN/m [221]. Adhesion occurs through three components: primary chemical bonding, secondary (or polar) bonding and mechanical bonding. However, the coating must come into intimate physical contact with the substrate before adhesion can occur. This interaction is referred to as ‘wetting’, which is an important factor in the adherence of two materials and occurs when the surface energy of the substrate is able to overcome the surface tension of a liquid. Surface energy is defined as the amount of energy required to create a new unit area of surface. Polar liquids such as water have high surface energies compared to non-polar liquids such as hexane. Surface energies can be calculated based on contact angle measurements [221]. In the coating industries, Table 3.2 outlines a simple method of determining whether a liquid has wetted a surface.

Table 3.2 - Determination of wetting by measuring contact angle (From [222])

Many commonly used polymers often exhibit low surface free energy and

consequently poor adhesive properties. This makes them difficult materials to wet and is one of the many challenges to consider in the pursuit for an alternative substrate to glass. The cleaning of substrates prior to coating or printing usually involves several steps to unsure uniform deposition. These steps are important to consider for the up scaling of any process, especially the consideration of any

Contact angle Liquid interaction with substrate θ = 0 the liquid completely wets the substrate

θ < 90˚ high wetting occurs

90˚<≤ θ < 180˚ low wetting occurs

78 materials which may require extra health and safety precautions to prevent harm to workers or the environment. Cleaning procedures differ between laboratories however the basic procedure usually starts with sonication in a detergent solution, sonication in an organic solvent such as ethanol or isopropanol, followed by rinsing with deionized water and finally dried with compressed nitrogen. Some suggest starting with sonication in a dilute acid to remove oxide impurities and others also use ultraviolet ozone cleaning to reduce the surface contamination. Other methods to modify a surface and promote adhesion include corona treatment treatment [114], plasma treatment [223] and chemical etching [224]. However, each would add cost and time to the manufacturing process.

The surface energy of substrate materials was also determined using a Theta Lite optical tensiometer from Biolin Scientific, which can measure contact angle with an accuracy of ±0.1˚. The surface energy of the substrates was calculated according to

the European Standard EN 828 [219] for determining the wettability of a solid surface by measuring the contact angle and surface free energy. Drops of three different liquids (water, ethylene glycol and diiodomethane) were dispensed onto a plane test piece surface. For each drop, the left and right contact angles were

measured. From the averaged contact angles of each liquid combined with its surface tension, the surface free energy of the substrate can be calculated. The OneAttension software uses the Fowkes method [216] to calculate the total surface energy (γ) from the sum of the contributions from dispersive interactions (γd) and non-dispersive interactions (γp).

79 Figure 3.11 - Illustration of measuring contact angle

The different surface energies of each substrate material mean that droplets dry in different ways. If the surface energy of the substrate is too low, de-wetting will occur and the coating will not be homogeneously distributed over the whole surface. If the surface energy is too high then spreading will occur, resulting in poor resolution. As previously discussed, wetting occurs when the surface energy of the substrate is able to overcome the surface tension of a liquid. If the contact angle of a liquid onto a substrate is between 0 and 90˚, it is considered that high wetting occurs [222].

3.4.2 Thermal analysis

Thermogravimetric Analysis (TGA) was performed on plastic substrates and

selected components within the ink to measure the thermal stability of the materials. TGA consists of a sample pan that is supported by a precision balance located within a furnace, where nitrogen is used to control the environment. As the sample is heated (or cooled), the mass is monitored. The instrument can quantify loss of components such as water, solvent or binder material due to thermal decomposition. It can also quantify the amount of solid components remaining. The Mettler Toledo TGA2 (STARe System) used in this study could provide heating up to 1100 ºC with a weighing accuracy of 0.005 %. A heating rate of 20 ºC per minute from a

80 2 mg and 50 mg. The TGA data was used to explain how the different components within an ink decompose in relation to temperature.

Differential scanning calorimetry (DSC) measures the heat flow change of a sample (compared with a reference) due to changes in their physical and chemical

properties, where nitrogen is used to control the environment. Differences in heat flow arise when a sample absorbs (exo) or releases (endo) heat due to thermal effects such as melting, crystallization and chemical reactions. DSC was used alongside TGA to provide further results for interpretation of glass transition temperature (Tg), melt transition temperature (Tm) and degradation temperature (Td) of plastic

substrates. The glass transition temperature occurs when the bulk material ceases to be brittle and glassy and becomes more rubbery. The melting temperature defines the upper limit of stability. Investigating the thermal properties of plastic films provides important information to predict how they will behave during processing. The Mettler Toledo DSC3 (STARe System) used in this study can provide heating up to 700 °C measuring heat flow with an error of ± 200 mW. A heating and cooling rate of 25 °C per minute from a temperature of 25 ºC up to 600 ºC was used for all samples. Standard 40 µl aluminium crucibles will be used for both TGA and DSC measurements.