Prosyma Research Ltd report to Weltonhurst Ltd.

S F Bush with J D Tonkin

Introduction

Weltonhurst supply high density polyethylene blow moulded containers to Richard Millington and Co Ltd. The containers are used to hold cleaning fluid under pressure for the purpose of cleaning out beer lines. The HDPE has come from a variety of suppliers including Dow, Atochem and Borealis, with a density around 0.95 gm/cc. The fluid has the specification:

Sodium Hydroxide: 9.2 ± 1 gm/litre

Sodium Hypochlorite: 7.6 ± 2 gm/litre

“Phosphorus butane carboxyl”: 4.8 ± 0.25 gm/litre

The precise chemical formula of the third component is unknown, but it looks like a mild detergent.

An earlier report (Ref 1) investigated the short-term mechanical properties of the polymers used, following the disintegration of a 50 litre container after six years in use. Fragments of the disintegrated container were found to be significantly less ductile than recently made containers, pointing to some long-term degradation of the polymer, exposure to sunlight being a prime possible agent.

The present investigation has been concerned with a second possible agent of long-term degradation, namely environmental stress cracking (ESC). ESC is not a well defined concept, but the essential point is that some environments reduce the mechanical properties over time. Generally these weakening effects are increased with an increase of temperature. Tests have therefore been done at both a working temperature (23 ^oC) and an elevated temperature (60 ^oC) which might be seen as the limit that the container could normally attain (by being left out in the sun for example).

References

1: S F Bush & J D Tonkin, Report on failed Bass bottle, PRL, 11.8.95.

2: S Turner, Mechanical testing of plastics, George Godwin, 1983.

Prosyma Research Ltd report to Weltonhurst Ltd

S F Bush

Introduction

In mid 1995 the extruder screw on a major blow moulding machine fractured in the feed zone, more or less under the hopper.

The machine had been run continuously over the previous 12-15 months with a single scheduled break of 14 days. The screw speed is normally 62 rpm with a maximum power available from the motor of around 65 kW. Set barrel temperatures appear to be as follows:

From inspection of the machine, zone 1 appears to apply about 3-4 turns from the feed hopper. The barrel has cooling water and electric heating for temperature control, but there is no screw heating/cooling system. The materials being processed consist of standard MDPE/HDPE blow moulding grades with varying proportions of recyclate. The recyclate usually consists of chopped product which has failed a visual inspection test in the factory.

Prosyma Research Ltd report to Weltonhurst Ltd.

S F Bush with J D Tonkin

Introduction

Weltonhurst supply high density polyethylene blow moulded containers to Richard Millington and Co Ltd. In use, the containers are filled with cleaning fluid for cleaning out beer lines. The containers are pressurized to 20-35 psi approximately, and remain in use for some years.

Depending on size of container, the safe working pressure (SWP) is given⁽¹⁾ as 40-60 psi. The cleaning fluids are specified by the brewers such as Bass. The Bass cleaning fluid specification is⁽¹⁾:

Sodium Hydroxide: 9.2 ± 1 gm/litre

Sodium Hypochlorite: 7.6 ± 2 gm/litre

“Phosphorus butane carboxyl”: 4.8 ± 0.25 gm/litre

To-date the long term effect on the bottles of these cleaning fluids has not been assessed.

Over the last few years about one bottle a year has been sent back to Millingtons⁽²⁾ having failed. It is thought that all the identified failures were five or more years old. Failure has usually been in the form of a crack, but recently a 50 litre six-year-old bottle has completely disintegrated. It is this particular failure which has prompted the present investigation.

References

1: Richard Millington Ltd, Product Specifications issued by D Turner, 8.7.92

2: D Turner to SFB Telecon, 10.8.95.

3: P Burrows to SFB, Atochem data sheet, fax 24.7.95