What are gels in extruded film and extrusion laminations and coatings? Are they particulate and do they affect the sterile barrier?
Polymeric Gels are a common occurrence in the extrusion process and are primarily visual defects. Gels can originate either during the resin polymerization or the film extrusion process and can be caused by a number of factors.
Some gels are present in the incoming resin received from a supplier. These “polymerization” gels can be formed in stagnating regions of reactors where resin overheats, or on the reactor walls. These gels are high molecular weight version of the same resin, beginning to cross-link by exposure to excess heat over time. Reactor design, operation and maintenance contribute to formation of such gels. During the film extrusion process these high molecular weight or cross-linked gels due to their soft nature can be difficult to trap via screen packs. They sometimes can elongate and pass through fine screen mesh. They will manifest themselves in the extrudate as a small blemish, differentiated from the film around it because of the increased haze of the higher molecular weight material.
Gels can also be formed during the film extrusion process. Unmelted and degraded resins are typical of extrusion gel formation. Unmelted resin is material that has not been adequately melted due to heat and shear in the extruders. Degradation occurs due to exposure to high temperatures for an extended period of time. Degraded polymer can be slightly hazy dots, yellowish or actually black, which indicates highly degraded gel that is also referred to as carbon. Resin that does not follow a normal melt stream path through the extruder and die, is exposed to prolonged residence time and thereby is degraded and or oxidized. Eventually the degraded residues break off of the stagnated areas and enter the melt stream. Contributing factors to creation of degraded resin are equipment design, equipment surface condition, equipment cleaning, and operational parameters.
Gels are made of the parent material of the polymer extrusion. There are other defects that can occur in films that can have similar appearance to gels but are due to other causes such as foreign matter and anomalies. There can be outside contamination of the parent polymer, or undispersed additive or modifier.
It is possible to reduce the size and frequency of gels in the extrudate, but it is not possible to totally eliminate their occurrence. Since gels cannot be entirely eliminated, specifications have been set up that define limits on the size and frequency of film gels. Size is frequently estimated using the Dirt Estimation chart in TAPPI T437 Dirt in Paper and Paper Board. Ideally, the supplier and customer collaborate to develop a specification. These limits will vary based on polymer, specific structure and applications. For further reference on writing specifications on sterile packaging see The ASTM F2559 Guide for Writing a Specification for Sterilizable Peel Pouches and F99 Standard Guide for Writing a Specification for Flexible Barrier Rollstock Materials.
Gels are encapsulated in the film and cannot easily be removed from the film matrix. They will not ‘fall out’ of the film and be a source of particulate or leave a hole in the film. During the thermal stress of sealing and forming the gels will remain embedded.