Hi Nicolas,
Well, changing it to "integer" seemed the easiest solution, but this wasn't the case. I found out that if you duplicate a product the product_code gets some text attached, xxx-copyxxx. Because this field is an integer now, the data can not be stored, which results in removal of both products! Auch... 
Is there a way to get around this?
(If I use trailing zero's then I need to remove all products before importing, because the poduct_code is needed to match the import with the product. I'd rather not delete all products).
Thanks.