Layer breakup in micro or nanolayer coextrusion
Earlier I discussed the need for outer lubricating layers when forming micro or nanolayer coextrusions. This is discussed in US Patent 5,269,995 where the causes of the layer breakup are said to be the small layer thickness (less than 10 micron), shear stress, interfacial tension between polymer layers, interfacial adhesion between polymer melt and the channel walls and combinations of these factors. In general the layer break up is found close to the channel wall indicating that it is likely related to the rapidly changing velocity profile found near the flow channel walls as well as the maximum shear stress being at the channel walls. This is common in multilayer coextrusion and a common source of melt disturbance. There are several ways to lower the interfacial stress at the wall or at the interfaces near the wall. One is to control the viscosity ratio with the lower viscosity material being against the wall, a second is to simply move the interface away from the wall. This is the rational for adding an outer lubricating layer to the nanolayer stack flowing through the layer generators and down stream in the die. In the 995 patent this is termed a “protective boundary layer “or PBL.
The patent gives a means of calculating the percentage of PBL needed based on the viscosity ratios of the PBL and the average core viscosity (average of the two core layer polymer)
%PBL = m(VISCPBL/VISCcore) – b
Where m and b are coefficients calculated at a given shear stress and which are dependent on the mass flow rates and channel dimensions.
In the case of feeding polymers A and B to form a multilayer stack of A/B/A/B/…./A/B/…A/B it is suggested to choose the PBL to be a lower viscosity version of either A or B. This is important in optical films as the viscosity of a polymer should have little impact on its refractive density, an important factor in reflective optical films. The example given in the patent is for 165 layer film comprised of a 10 MF polycarbonate and a 2 MF PMMA. The lubricating PBL was a blend of a 10MF and an 80 MF polycarbonate. The result given is
%PBL = 39.5 (VISCPBL/VISCcore) – 26.71
The PBL can be added at all steps in the formation of the nanolayer stack such as in the feedblock, the layer multipliers and in the die.
Other approaches discussed oar to have “slip” additives added to the polymer streams, perhaps what we today call process aids, the modification of the flow channel surface with Titanium nitride (TiN) etc. The TiN surface coating teaches the importance of materials of construction in polymer flows, a greatly overlooked factor which along with lowering melt stress can improve cleanliness of the flow system minimizing die build up, die lines etc. More about this later.
