Acuerdo Relacional como Medidas de Similitud Molecular Novedosas

The DP600 sandwich (DPSW) was also produced using heating lamination, the process is described in Chapter 3.

Initially, to ascertain whether the bond strength of MO–RAD M801 adhesive to the galvanised DP600 steel was sufficient, panels were made and press brake bent (process is described in Chapter 3) to determine if the material was able to form a crash structure. The steel was first prepared by degreasing with acetone and MEK. The level of mill scale on the steel was significant, and MEK alone was not able to remove it. The steel was found to peel away from the adhesive during pressing and thus a greater bond strength was sought. Therefore, methods to improve the adhesive bond are considered in the following section.

4.3.1 Surface preparation

In order to determine the influence of the steel’s (DP600) zinc surface on the adhesive bond strength, the surface of the steel was prepared using a range of methods, shown in Table 4.2. The PMT was fixed to 241 °C as it produced the highest bond strength in the ECCS sandwich.

In the first two methods, the surface was first etched using sulphuric acid to remove the zinc coating of the steel and then cleaned with either xylene or acetone and propan–2–ol (IPA). In methods 3–5, the surface was abraded and cleaned to remove the zinc oxide layer and then degreased. In methods 6–9, the steel surface was treated with an iron phosphate solution; this is known as conversion coating or passivation. The phosphate solution reacts with the zinc to produce zinc

phosphate, to increase the adhesive bond strength. Method 6 has a passivated zinc surface, in method 7 the passivation was performed after the surface was abraded. After passivation, a polymer coating was applied to the top surface in method 8. In method 9, the passivated surface was degreased using xylene (the main solvent in the adhesive).

Table 4.2 – Methods attempted to improve the adhesive bond strength.

Method Description of action taken

1 Etch using sulphuric acid to remove zinc, abrade surface with 180 grit sand paper, clean and degrease with xylene 2 Etch using sulphuric acid to remove zinc, abrade surface with 180 grit sand paper, clean and degrease with acetone and IPA 3 Abrade zinc surface with 180 grit sand paper, clean and degrease using Xylene 4 Clean and degrease zinc surface with acetone and IPA 5 Abrade zinc surface with 180 grit sand paper, clean and degrease with acetone and IPA 6 Phosphate conversion of surface using iron phosphate solution 7 Abrade zinc surface and then apply phosphate conversion of surface using iron phosphate solution 8 Phosphate conversion of zinc surface using iron phosphate solution and then addition of polymeric coating 9 Phosphate conversion of zinc surface using iron phosphate solution, addition of polymeric coating, clean and degrease with xylene Even after surface treatment, the peel force was low compared to the ECCS and Steelite sandwich materials (over 25 N.mm–1 _{and around 5 N.mm}–1 _{respectively),} see Figure 4.5. The removal of zinc from the surface (method 1 and 2) had a beneficial effect on adhesive bonding compared to merely cleaning or abrading the steel (methods 3, 4 and 5). The phosphate conversion combined with the addition of a polymeric coat on the steel surface showed the highest peel force (method 8). Degreasing of the passivated surface is not recommended (method 9). It is noteworthy that the peel force is still low at 3.57 N.mm–1_.

Figure 4.5 – Average peel force for differing methods of steel preparation, errors are the standard deviation of the result.

4.3.2 Optimisation of adhesive curing and sandwich lamination processes

After identifying the best method for surface preparation, the PMT and lamination time were then optimised. No significant pattern with respect to PMT was observed, Figure 4.6. Thus 232 °C was chosen as the optimal PMT, since a high bend strength and lowest variability was obtained. Additionally, the low value at 224 °C would also be avoided.

Figure 4.6 – Peel strength of the DPSW material when changing the adhesive curing PMT.

Next, the lamination time was altered to further improve the bond strength. The lamination time was found to have a significant effect on the bond strength, as demonstrated by the resulting peel force, Figure 4.7. The dashed line in Figure 4.7

2.42 2.80 2.20 1.79 1.77 3.01 2.36 3.57 3.30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Av er ag e Pe el Fo rc e /N.m m -1

Method 1 Method 2 Method 3 Method 4 Method 5 Method 6 Method 7 Method 8 Method 9

0 1 2 3 4 5 6 216 224 232 241 254 Av er ag e Pe el Fo rc e / N.m m -1

shows a guide to the eye of the trend in the results. 90 seconds again showed the highest average peel force, but also the greatest variability. It was decided that between 60–180 seconds, the greatest bond strength was achieved.

Figure 4.7 – Lamination time vs. peel strength for the DPSW material. The dashed line shows the suggested pattern in the results.

In conclusion, by carrying out a range of experiments, it was determined that the greatest adhesive bonding strength was achieved by first treating the zinc coated DP600 with a phosphate conversion coating and then applying a polymeric layer on top. Adhesive curing at a PMT of 232 °C then followed. Lamination of an assembled sandwich was performed at 210 °C. This temperature showed the most consistency in the case of Steelite sandwich and was therefore used for the DP600 sandwich. The optimal lamination time was determined to be 90 seconds, resulting in an increase in bond strength from 1.8 N.mm–1 _{to 5.5 N.mm}–1_.