How to Wash Polycarbonate Bottles Without Stress

I have written about micro-cracks in polycarbonate bottles previously, but it is worth going into more detail here because this phenomenon can be avoided.  The consequences of not being aware of micro-cracks can be leaking bottles and contamination of water with unfiltered air while the bottle is on the cooler.

Polycarbonate bottles are produced using the extrusion blow moulding process in which a tube of hot plastic is extruded between two open halves of a mould.  When the tube, or parison, reaches the proper length, the mould halves clamp together and air is injected to force the parison to the shape of the mould.

Cooling water is then circulated inside the mould halves to cool the plastic until it is rigid.  The mould opens, the part is ejected, any excess flash is removed, and the bottle is almost ready for use.

The need for a high production rate dictates a rapid cool-down following the blowing operation.  Unfortunately this creates areas of unequal stress in the plastic, particularly at the clamping points where the mould comes together.

Bottle producers relieve the stress by re-heating the bottle to just below the softening point of the plastic and then allowing it to cool slowly.  If done properly, this succeeds in removing most of this induced stress.  If hurried, some of the stress remains.

The bottles then can easily form micro-cracks when subject to further stress.  This may be from the use of incorrect bottle-wash chemicals, the bouncing movement of bottles in the delivery truck, stress of pressurising during the filling operation or use of badly designed bottle racks.  The older the bottle, the more likely micro-cracks will appear.

These cracks are sometimes difficult to see (see the microscope view).  However, when the bottle is inverted on the cooler, after drawing water, a vacuum exists briefly and air can enter through the micro-cracks as well as through the filter.  Bacteria and algal spores also enjoy the haven of the micro-cracks during bottle washing.

Stress cracking with the wrong kind of detergent can can occur rapidly and reduce the number of viable wash cycles of the bottle while encouraging algal growth ("green bottles").  Test rigs can easily demonstrate the results of chosing the wrong bottle-wash detergent (see photo showing broken polycarbonate strips).

 Various bottle-wash detergents are available on the market that are specially formulated to avoid stress cracking of polycarbonate while ensuring a clean bottle.