Methodology

In this subchapter, the descriptions and parameters for each of the tests done are mentioned.

Figure 23 Eftec - Efbond DV 993

6.2.1 Surface Free Energy of Substrates - Analysis

The surface energy is an important criterion for wettability of the substrates, thereby this was one of the preliminary tests done on the substrate to calculate the surface energy. The critical surface energy to achieve was approximately 38 mN/m for effective adhesion. The most common way to determine the value of surface tension is by the ‘Drop Test Method’. In this test, a drop of liquid (usually water) is dropped on the substrate and the contact angles between the surface and the edge of the droplet of liquids are computed using an optical microscope. After a series of trials, equation of states is used to calculate the surface energy of the substrate. Another method is to use the ‘Dyne Pens’ to test the surface tension. Different colours denoting different viscosity and flow of ink representing wettability of the substrates are used.

Straight lines are drawn using these markers. The marker or colour which does not form droplets on the surface denotes the surface free energy of the substrate.

In the frame of this thesis, ‘Drop Shape Analysis (DSA)’ was used.

The contact angles were measured using the image of a sessile drop at the point of intersection (three-phase contact point) between the drop contour and the projection of the surface (baseline). The instrument used was from KRUSS, GmbH – DSA100 (see figure 24). This equipment used a liquid needle technique for dosing of the drop onto the substrate. After dosing and drop of liquid onto the substrate, an image was taken of the view from the optical microscope for further analysis. Contact angles were measured using this image. Using ‘Equation of states’

law, surface free energy was computed.

Figure 24 DSA 100 - Drop test analyser

6.2.2 Differential Scanning Calorimetry (DSC) Analysis The principles of this method are described in chapter 5.3.1.

For this thesis, difference in heat flux is measured, which results in endothermic and exothermic peaks in the graphs obtained. The temperature profile was set so that it was a linear with a constant rate of heating and cooling at 10 deg. / min. The standard for DSC analysis is ASTM D3417 and EN ISO 11357-1.

The equipment used for this test was from Mettler Toledo – DSC 3. The specimen being the adhesive is scooped into a pot made of aluminium and sealed with a hole on top. This pot is placed on the automated DSC disc. The temperature profile (10 deg. / min. and 5 deg. / min) and the inert atmosphere is set. Nitrogen gas at a constant flow rate was in the inert atmosphere used for the test.

Ten cycles of heating and cooling were carried on to observe the changes and peak characteristics. This test was done to understand the chemical reactions which would explain the ‘Fast

fixing Effect’. The samples were tested in nitrogen and oxygen atmospheres to understand the changes in characteristics.

6.2.3 Temperature Profile Analysis

For this thesis, temperature profiles both in the core of the cartridge and the core of the adhesive joint was measured. This test was done to understand the heat removing capacity of the adhesives. Both the adhesives were heated in separate ovens for the recommended temperatures for approximately 3 hours before application.

About thermocouples used in the experiment:

Type K thermocouples made of Chromel and Alumel were used during the tests. Chromel with a composition of 90% nickel and 10% chromium; and Alumel with a composition of 95% nickel, 2%

manganese, 2% aluminium and 1% silicon were used³. These thermocouples were used as they provide a wide operating temperature range from -270^oC to 1260^oC. Chromel and Alumel wires were separately measured and cut into individual pieces with a length of approximately 50 cm. The end cover of the wires was removed and fused together with high voltage to form a pin-head shape. These pin-heads were introduced in the substrates, which were drilled with a diameter of ~ 0.1mm. The holes were in the mid region of the overlap of adhesive bond (see figures 25, 26).

Figure 26 Arrangement of thermocouples (type K)

Test – 1: Core temperature in the cartridge analysis

To achieve the required temperature for application of adhesive, the ovens were set with a profile (linear increase in temperature and maintenance of this constant temperature) with respect to the respective application temperatures. For Teroson PU 8599, if the application temperature was mentioned as 65^oC as the higher limit of application temperature. For Efbond DA 174, the recommended application temperature was 70^oC. Holes were drilled on the cartridges at various points and thermocouples (Type K) were inserted. The changes in temperature were recorded and a plot of temperature versus time defines these changes.

Test 2 – Temperature profile in adhesive bond analysis

Adhesives were tested both at the minimum and maximum limits of the recommended temperatures. Substrates after surface treatments were prepared on the base for application of adhesive*.

The tests were conducted for three different thicknesses; 1mm, 3mm and 5mm respectively. The base for application of adhesive was made of three components. The base, seddle and support were made out of PC (polycarbonate) / ABS (acrylo-nitrile butadiene styrene) material, which was 3D printed (more information about dimensions in the appendix). The support, being the movable part of the assembly with the seddle, was to maintain the uniform thickness of adhesive on the substrate during application. The thickness of adhesive was maintained by the addition or removal of 1mm thickness discs which supported the support on the base.

Small holes to allow the wires of the thermocouples to pass through, were drilled on the base. Substrates with thermocouples in place were set on the base. Adhesive was applied according to the required thickness, after which another substrate was pressed against it to maintain the thickness. The seddle ensured the required overlap of the substrates with the adhesive and a weight of 1,5 kilograms was placed over the seddle to ensure no movement of the joint (see figures 27, 28 for experimental set-up).

The thermocouples recorded the changes in temperature which were further plotted to understand the heat removal properties of the adhesives.

Figure 28 (Left) Application of load on the adhesive bond; (Right) Thickness rings of 0.5mm each

6.2.4 Shear test (Gangel Test)

This test was conducted to understand the relationship amongst adhesive strength, viscosity, application temperature and waiting time. The test followed similar methodology of application of adhesive onto the substrate without the thermocouples. The

Figure 27 (Left) Base for application of adhesive; (Right) temperature profile analysis experimental set-up

before testing was specified as 1 minute, 3 minutes and 5 minutes respectively. All the possible combinations with thickness and waiting time were carried out with defined parameters.

Experimental Set-up: The support for the Gangel test was made out of stainless steel board, which was bent to achieve a perpendicular angle to the vertical board. The edge of this perpendicular side of the board was further bent in right angle to the horizontal (upwards) to support the clips, which in-turn held the substrates during the tests. The weight, which attached to the substrate during the test was optimized by a series of trial and error tests on substrates with adhesive joints of thickness 1mm and waiting period of 1 minute. The optimized weight was accurately 252.80 grams, this was achieved by adding thickness plates on the bottom of a 200 grams’ standard weight (see figure 29 for principle and figure 30 for experimental set-up).

Each of the substrates were weighed individually before the application of adhesive in order to use it in the calculation of dynamic viscosity. After the application of adhesive (the length of overlap was 15 mm and width 25 mm) and waiting period, the substrates were clipped to the board and the weight was put on the other end of the substrate. Time from the application of weight until the failure of the joint was recorded. These recorded times were used in the calculation of apparent viscosity from the equation 7 discussed in chapter 5.2.

Figure 29 (Left) Principle of modified shear test; (Right) Overlap dimensions of the adhesive joint

Figure 30 Experimental set-up for modified shear test (Gangel test)