I had a question today about blocking in a film and thought I would talk a bit about what blocking is and how it is generally prevented. Blocking is when two film surfaces in a roll weld together and form a bond which is stronger than the film. The surface bond generally occurs when the two film surfaces physically contact each other in a roll of film. Sometime the layer to layer bond can be “broken” and the film unwound at low speed but if the roll is unwound at high speed then the bond causes the film to break and it may cost too much to recover the film with the slow unwinding speeds in a subsequent process.
When a roll of film is formed, the top and bottom film surfaces are in contact with each other in the roll. How close the layers are to each other will determine if they can bond together and block. In unformulated films the layer to layer separation is determined by the amount of air wound into the roll, so tight winds would tend to cause blocking in single layer films. If the film was treated it could increase the blocking tendency of a film. The surface to surface bond can be so good that the film essentially becomes a solid roll of polymer with inseparable layers.
The blocking can be across the full width of the film, occur near the core (where the roll is in compression), occur at points (pin point blocking) where dirt is in the film or between tow layers causing a raised bump in the film, at wrinkles or corrugation etc. When ever two film surface contact each other blocking is possible.
To insure that the film layers can be separated it is customary to add migratory additives to the film which diffuse to the surface and create a weak boundary between the film layers. If this sounds like slip additives to you that would be correct. You can also add particulate additives (anti block) to separate the film layers and prevent the surface to surface contact. Now typically I would add a migratory slip additive to decrease blocking and particulates to improve winding and perhaps impact blocking, but in general the particles for blocking improvement are too small in diameter (0.7 to 1 micron) to improve winding so I keep them separate in my mind. So for me the slip additive are good antiblocking additives. But they will forma weak boundary layer for all adhesion related processes such as printing and laminating.
So the goal to prevent blocking is to prevent two film surfaces from welding together. This can be accomplished in many ways; the best is to have two surfaces which cannot bond together because they are incompatible, but this is not always possible. Next you try and keep the layers apart (low winding tensions, or antiblock particles) by conditions or formulation and third you try and put a surface coating on the surfaces to keep them from bonding together or which form a weak boundary layer between the film surfaces such as a migratory slip additive.