The plastic fuel tanks investigated are produced from Lupolen 4261A, supplied by BASF AG, Ludwigshafen. In Stage 1 of the trials the virgin HDPE was formed into injection-moulded test pieces using an 'Engel CC 80' injection moulding machine, and tested in a variety of mechanical tests. The tested samples were then successively reground and remoulded. The following mechanical properties were assessed, and the change in each property reported after 15 cycles:
The HDPE of fuel tanks contains small amounts of fluorine, determined as 0.3-1 g per 10 kg tank, incorporated as a sealant and present as polytetraethylene (PTFE). In Stage 2, to determine the effect of this fluorine, samples offluorinated and unfluorinated tanks were sawn into small pieces and reground before being formed into injection-moulded test specimens for determination oftorsion modulus, and oftensile, impact and ballistic properties. The thermal properties investigated were the degree of crystallinity, melting temperature, and flow characteristics. The measured properties were compared with those provided for in the relevant quality order for new material.
Torsion properties satisfied the quality order, with no significant difference between fluorinated and unfluorinated materials.
The tensile properties of the recycled material lay within the 10% tolerance band of the values of new material, and hence satisfied the provisions of the quality order.
The quality order requires that the test samples must not break. Some of the test specimens fractured and, for those, the results are therefore invalid in the context of the quality order.
Modulus of elasticity Yield stress
Elongation at yield stress Charpy impact test
Unnotched Notched MVI (melt volume index)
All test samples showed a ductile fracture. It is, however, difficult to compare the values determined for injection-moulded test specimens with those exhibited by the original tank material and those specified by the quality order since the test specimens were only 3 mm thick, compared with the average thickness of a tank, which is between 5 and 8 mm.
The degree ofcrystallinity ofboth the fluorinated and unfluorinated materials, both after one regrinding and one remoulding, lay within the provisions of the quality order.
The remelting temperature of fluorinated and unfluorinated materials, both after regrinding and after remoulding, lay below the temperature range stipulated by the quality order.
No difference could be detected between fluorinated and unfluorinated samples evaluated by a spiral flow mould test.
Parallel investigations were carried out to establish how much fuel the tank material absorbs, and the time involved for the fuel to migrate from the material when no longer exposed to fuel.
At a room temperature, samples punched from the original (fluorinated) tank achieved constant weight, with weight increases in the range 6.6-7.3%. Since material recycling cannot take place without elimination of the fuel, drying tests were carried out under a range of conditions. It was established that temperatures exceeding 100oC were necessary to eliminate all fuel.
The material used for the investigation was obtained from Volkswagen Golf and Passat vehicles, which have employed plastic fuel tanks for some 10 years. The time required for tank removal, cleaning, and elimination of tank components, made either of metal or a plastic other than HDPE, averaged some 5 min 30 sec. At a labour cost of DM 45/h this implies some DM 4.2 for recovery of a fuel tank yielding 3.8 kg of HDPE with, at a price of about DM 1.8/kg for secondary HDPE, a nominal value of DM 6.9.
Before the tank was reground and reprocessed the fuel was extracted for 24 h at 120oC, to avoid the risk that reprocessing might reach the fuel ignition temperature of 230-260oC. Subsequent material reprocessing caused no problems other than the unpleasant odour, which was absent from samples made from virgin material.
Since all finished fuel tanks are in fact fluorinated, the mechanical, rheological and thermal properties of the recovered material were compared only with the fluorinated samples.
The recovered material met the requirements of the quality order in respect of torsional properties.
The moduli of elasticity of samples made from recovered material were much higher than those made from unused tanks. Though perhaps partially attributable to cross-linking, it is more likely to reflect a change of specification of the Lupolen 4261A, whose modulus of elasticity was in 1978 given as 1200 N/mm2 and in 1989 as 850 N/mm2. The higher value of the modulus of elasticity does not necessarily constitute a poorer quality of material so far as the requirements of plastic fuel tanks are concerned.
As before, none of the test specimens broke, so satisfying the criteria of the quality order. It was noted that the impact properties did not deteriorate on recycling, as usually occurs.
Since all specimens exhibited ductile fracture it was possible to compare used and unused polymers. It was noted that, although the materials had been in service for ten years before recycling, the low-temperature impact properties had not deteriorated. As before, it is difficult to compare the values determined for injection-moulded test specimens with those exhibited by the original tank material and those specified by the quality order since the test specimens were only 3 mm thick, compared with the average thickness of a tank, which is between 5 and 8 mm.
Degree of crystallinity
The results fell within the ±10% tolerance band of the quality order. Melting temperature
The melting temperature, evaluated by differential scanning calorimetry, was lower than that specified by the quality order.
Was this article helpful?