Case Study - Poly Carbonate Mixing Bowls

A manufacturer eventually asked Smithers Rapra to report on product failure after a hygiene scare.

Introduction

The manufacturer of domestic food mixer/blenders had reacted to a low frequency of mild complaints by simply supplying replacements. Customers complained about deterioration in the appearance of injection moulded polycarbonate mixing bowls & jugs. The manufacturer knew that the problem was surface crazing but did not attempt to identify its cause.

Following an outbreak of food poisoning, health officials traced the source to a small café. One of the items that attracted the attention of the investigating team was a transparent mixing bowl containing "multiple surface fissures that could be the location for colonisation by micro-organisms". On receipt of this very serious observation, the manufacturer was finally stimulated to investigate the extent & cause of the crazing problem.

Inspection & Analysis

100 owners of the mixer/blender were traced & persuaded to return the mixing bowls for inspection. 15% were found to be visibly crazed, compared with an historical complaint rate of only 0.2%. Evidently for most customers crazing was either not observed or not considered to be serious enough to justify a complaint.
A typical example of crazing is shown in the photograph. Crazes are invariably oriented in an axial direction. The polycarbonate supplier suggested that the problem was due to the use of aggressive surfactants combined with the high washing temperatures. This suggestion was soon eliminated by the observation that all crazes had initiated on the inside surface of the bowls, if aggressive washing was the cause then all surfaces would be equally effected. The evidence pointed to a reasonably common food/ingredient that was acting as a powerful ESC agent for polycarbonate.

In an attempt to identify the culprit(s), letters of complaint accumulated over several years were studied. Fortunately the letters did provide some valuable clues. Two correspondents observed that the problem occurred soon after fragmentation of pepper corns. Another noted crazing immediately after the bowl was filled with the zest from lemon skins. The common abundant type of constituent in these two foodstuffs are 'essential oils' classified chemically as terpenes. Terpenes are known to be powerful solvents for many polymers & indeed are in common use as tackifiers.
The craze pattern was replicated by exposing new bowls to freshly crushed pepper corns & the zest from orange & lemon peel.

Failure Diagnosis

The crazing was due to the combination of residual moulded-in stress & contact with food products that contain high concentrations of essential oils.

Lessons & Consequences

1. Customer complaints should be treated seriously as a potential source of valuable information.
2. The frequency of complaints often underrepresents the frequency of the problem.
3. Specific foods & ingredients for human consumption include powerful ESC agents. Polycarbonate & other amorphous thermoplastics are prone to crazing & embrittlement in contact with these including highly flavoured ingredients & spices.
4. The manufacturer was advised to reduce the moulded-in residual stress by increasing cycle time &/or annealing.
5. Eventually the crazed bowl was cleared by investigators as the source of the outbreak of food poisoning. However, the bowl manufacturer could no longer be complacent in the face of potential litigation (past & future) concerning the unhygienic qualities of a crazed product intended for food contact.