Petrol permeation

By a combination of tank redesign and surface fluorination the problems were overcome and today a number of local motor car manufacturers are using surface-fluorinated LLDPE petrol tanks. These tanks performed exceptionally well in petrol permeation tests in which control tanks lost about 16g/day, while fluorinated tanks lost less than 0.5g/day at 40°C. With advanced fluorination techniques mass loss values of less than 0.1 g/day have already been achieved for some of these fluorinated tanks. 

Figure 16.1. Petrol permeation through HDPE pipe at 50°C Pipe = Type 4 Class 10 petrol grade = 90 octane high altitude.

As far as can be ascertained, no performance standards exist for this product. In the absence of such standards, the existing standards for automotive vehicles were used as guidelines. By using the most stringent standard, the SHED test, a petrol permeation rate of approximately 3.3 g/m2 for 24 h at 40°C can be estimated. With a single-fluorination treatment a pipe already exceeds this standard with a steady state permeation rate of 1.7 g/m2 per 24 h at 50°C. Since it is a known fact that permeability increases drastically with a rise in temperature, a permeability ofless than 0.17 g/m2 per 24h at 30°C is expected for a single fluorination treatment. 

Petrol permeation through HOPE pipe at 50°C Pipe=Type 4 Class 10 petrol 93 octane high altitude. 

Diesel, as well as petrol, was poured separately into bottles of neat HOPE (control) and the selected blend bottles, at least three bottles for each case, covered with aluminum foil using a silicone sealant that cured at room temperature. It was further covered with molded plastic lid and was sealed with the same sealant. After the adhesive is cured, diesel/petrol permeation is tested and recorded at regular intervals by measuring the weight of each bottle. 

Fig. 6. Fuel permeation. Petrol TFl lUUU TF2 S TF3. Note Based on European standard fuel Halterman (CEC-RF-08-A-85). TF-1 = 90% + 10% EtOH, TF-2 = 92.5% -F 7.5% EtOH, TF-3 = 95% + 5% EtOH. HDPE Fluor-1 and HDPE Fluor-2 refer to fluorinated HDPE fuel tanks from different suppliers. HDPE 5% EVOH is an HDPE, ethylene vinyl alcohol multilayer system.

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