Substrate Secrets

In a reply to the post of orange peel or melt disturbance a reader asks:

“I am producing BOPET film and some of our customers are complaining for ORANGE PEEL defect but we are unable to understand that this defect can be in BOPET film also.  Please suggest possible causes and ways to rectify this defect.”

Answer:   First without seeing the defect or watching the process run I cannot be sure of its true nature and therefor a true cause in your situation.  But in regards to the presence of orange peel on BOPET film the answer would be yes it can be present.  In fact I saw my first orange peel in a BOPET operation when I worked at ICI.  Generally speaking the orange peel is a surface defect which causes an optical defect in the film surface, a fine grainy appearance which has the character of an orange peel.  In BOPET one cause is a chill roll deposit under the cast sheet which interferes with the film laydown on the chill roll surface.  In fact, a roll surface deposit under the film would be a general cause of a film surface defect typically called orange peel in most cast and oriented films.

In BOPET there will form a thin layer of oligomer on the chill roll surface (usually a cyclic trimer of the degradation product of PET).  This can be seen as a slight haziness on the chill roll surface which develops under the cast sheet as the line runs.  It will increase in thickness and the chill roll will develop a bluish white hazy deposit over all or a portion of the roll surface.   The deposit will increase in thickness and coverage ultimately causing the orange peel defect in the film, and periodically it must be cleaned from the roll surface to get the best contact between the roll and the cast sheet surface.  The chill roll deposit can be caused by too cold of a chill roll temperature, excessive low molecular weight material in the melt, perhaps too high a melt temperature, poor quality reclaim or insufficient drying (anything in the process causing oligomer formation). 

There are other possible causes to the orange peel defect (or other defects which are called orange peel) which might be occurring in the stretching section of the line which can cause a film surface defect, which some might be calling orange peel.  You will have to do a process analysis of the problem to find the true cause of your defect “orange peel” in your operation.