EOW May 2014

Technical article

2.4 COF Measurement The coefficient of friction (COF) is measured according to the ASTM procedure D1894 using a tribometer. HDPE balls made by Precision Plastic Ball Co were used for the measurements. For each sample, the friction force was measured at two points for each normal force of 100N, 200N and 300N. The slope of normal force vs friction force was used to calculate the COF. Each point of measurement was done with a new HDPE ball and repeated for 40 cycles to demonstrate the effect of surface wear on COF. The data reported here are the COF values obtained on the 40 th cycle. The coefficient of friction measured on plaques is shown in Figure 1 . Sample A which has 1.25 per cent of SA2 shows ~30 per cent reduction in COF and sample B which has 1.25 per cent of SA1 shows ~40% reduction of COF over the control. If the COF reduction of the mixture of SA1 and SA2 went as a weighted average, then the COF of sample C should be a weighted average of the sample A and sample B. However, when both the additives are added so that the total additive content is 1.25 per cent, a synergy in the slip behaviour is seen, resulting in a ~50% reduction of COF over the control. To understand the origin of the synergy between the slip additives, Atomic Force Microscopy (AFM) was used to image the surface of the plaques used for COF measurements. The reason for this is because AFM is a surface technique and is least affected by the depth of the plaque and would give the best understanding of COF, which is a surface phenomenon. AFM images of plaques made from samples A, B and C are shown in Figure 2 . The figure shows distinct differences between the surface morphologies of the three samples. In Figure 2a , containing just SA2, the HDPE banded spherulites of the base resin are still visible in the topology. The corresponding phase image ( Figure 2d ) shows no phase domains which indicate a surface with homogeneous viscoelastic behaviour. This suggests that the surface is covered by a thin superficial layer of migrated SA2 on top of the HDPE resin. Tapping mode AFM will typically probe sample to a depth of ~20nm. This tapping depth allows the bottom layer spherulite structure of the HDPE to be captured in the image, but is slightly blurry due to the surface layer of SA2. The surface roughness of sample A plaque is 4.2nm, which is ~50 per cent less than the surface roughness of the neat HDPE plaque. 3 Results and Discussion

Jacket Material Formulation

Samples Description Resin

SA1

SA2

Total SA%

A B

Resin + SA2 98.75%

1.25%

1.25 1.25

Resin + SA1 97.50% 1.25%

Resin + SA1 + SA2

C

97.75%

1%

0.25

1.25

Control

Resin

100%

▲ ▲ Table 1 : Sample description

Coefficient of Friction

Control

▲ ▲ Figure 1 : Coefficient of friction measured on plaques with equal additive content showing synergy between the two additives

were added while the blades were rotating at 20rpm. Note that since slip additive 1 (SA1) is a master batch with 50 per cent slip agent, 2.5 per cent of the master batch needs to be added to the formulation to get 1.25 per cent SA1 content. SA2 (if present) was then added last at 10rpm after all materials were in the flux stage. The rotating blades were then increased to 20rpm after the SA additives were fully incorporated into the polymer melt. Mixing was continued for ten minutes, and the sample was taken out by reversing the blades at 10rpm. The rest of the sample was removed by disassembling the front plate and removing by hand using a Brabender knife. Compounded materials were then placed between two mylar sheets and pressed flat in a press for further processing. 2.3 Plaque Preparation Compounded material was first pre- weighed to the desired amount and placed in between two mylar sheets. Outside the mylar sheets were two aluminium sheets and stainless steel mould plates. The mylar was in contact with the material to prevent sticking to the metal plates. The filled mould was then placed into the press at 180°C (+5°C or – 5°C). The press was closed and pressed at 500psi for five minutes followed by 2,500psi for five minutes. The cooling system was set to cool the moulded plaques at the rate of 10°C per minute. The plaque was taken out when the temperature reached 35°C.

Two slip additives (SA) are used for the study. These slip agents are designed to be used as additives in a resin compatible system to modify surface characteristics including COF reduction. To evaluate the synergistic effect of the slip additives, three samples were made as shown in Table 1 , with each formulation having a total additive content of 1.25 per cent by weight. Sample A and B are made with 1.25 per cent single SA content and sample C has both the slip agents, with a total SA content of 1.25 per cent. The neat resin DGDA-6318 BK is utilised as the control. 2.2 Brabender Batch Mixing Blending of the formulations was con- ducted in a Brabender model Prep-Mixer. This is a three-piece mixer with a 420ml mixer chamber volume equipped with Cam mixing blades. The total volume used was approximately 294ml, in line with the guideline for best mixing with the Bradender mixer, which is 70 per cent. There are three thermocouples which measure three separate temperature zones of the mixer. The first thermocouple measures front plate, the second thermo- couple measures centre of the chamber and the third thermocouple extends into the centre of the mixer bowl chamber, which measures the actual temperature of the sample. The mixing bowl was pre-heated at 180°C, and then the resin and SA1 (if present)

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May 2014