Why does my parts made of Polypropylen break?

Storing PP at 0 deg C will not make permanantly it brittle, however if used at 0 deg C it will be brittle if it is homopolymer. Copolymer should be OK at 0 deg.

I am not familiar with current grade numbers, so I don't know your specific grade.

Regards
pat pprimmer@acay.com.au
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Besides patprimmers remark on embrittlement due to low temperatures on application, another cause of embrittlement is improper molding conditions. Assuming that you are using injection molding, PP Homopolymer should be processed at a melt temperature of 220 - 280°C and a die temperature of 20-80°C. If your temperatures are too low (especially if your die temperature is too cold!), you might get embrittlement due to improper crystallization. If your processing temperatures are too high, you might get embrittlement due to thermal degradation.

I'm not familiar with your grade of PP. If it an easy flow grade (MFI > 15), then its tensile strength an impact resistance might not be sufficient for your application. Try a PP grade with a lower MFI.

Add a third cause of embrittlement: environment. What is the environment you are storing these at? We've had PVC drain pipe exposed to minute traces of the wrong oil in wash water embrittle and fail just from the residual stresses of the pipe manufacturing.