By Jan H. Schut
The world’s first coextruded microcapillary film was reported by Dow Chemical Co., Midland, MI (www.dow.com) at the Society of Plastics Engineers’ recent ANTEC conference in Orlando, FL (www.4spe.org) in March. The films are made by coextruding microscopic parallel threads in the machine direction in a film matrix.
Dow associate research scientist Wenyi Huang presented the “Effect of Rheology on the Morphology of Coextruded Microcapillary Films,” showing how different viscosities and different processing conditions affect microcapillary structure and describing some of the unique films that can be made.
Huang’s paper doesn’t say what the benefits of microcapillary film might be, but two recent Dow patent applications give clues. Depending on processing parameters and what resin or other material is pumped into the microcapillaries, they could be used to heat, cool, insulate, or even strengthen film.
The idea first surfaced in a 2005 PhD dissertation by Bart Hallmark at the University of Cambridge in the U.K. (www.cam.ac.uk), who put hollow microcapillary channels in the machine direction in monolayer extrudate. Dow licensed the microcapillary concept from Cambridge, but upped the ante by creating its own patent-applied-for film die and coextruding a second polymer into the microchannels.
The Cambridge and Dow dies are conceptually similar—both use hollow tubes to create microchannels. The Cambridge Hallmark patent (U.S. Pat. # 8641946) describes a die that forces monolayer polymer around needles, which inject air to make film or profile with hollow channels in the machine direction. Potential applications, described in subsequent Cambridge patent applications, include food products, tear guides in packaging, and medical devices.
Dow, which is the primary plastics licensee, has industrially scaled the die and added a second polymer, coextruded through hollow conduits, which open into the die land for the matrix polymer. A Dow patent application (U.S. Pat. Applic. # 20140113112) says the microcapillaries are at least five microns thick with at least five microns between them.
Dow made microcapillary test films with a 38-mm single-screw extruder with a gear pump for the matrix and a 19-mm single-screw extruder for the microcapillaries, testing different combinations of five commercial Dow polyolefins with different viscosities. Huang’s paper gives relative viscosities and processing temperatures of the five polymers. Polymers 1, 2, and 3 process at 200 degrees C; Polymers 4 and 5 process at 130 degrees C. Polymer 1 is higher viscosity than Polymer 2; Polymer 3 is higher viscosity than Polymer 1; and Polymer 4 is 100 times higher viscosity than Polymer 5, which is very, very low molecular weight – nearly Newtonian.
First, Dow tested film made with Polymer 1 in both the capillaries and matrix, coloring the capillary resin black and leaving the matrix natural, so the capillaries are visible. Dow tested different microcapillary extruder speeds. Not surprisingly, increasing microcapillary extruder speed increased the size of the microcapillaries. With the matrix extruder at 15 RPM and the capillary extruder at 25 RPM, capillaries account for 11.1% of the film. At 50 RPM, capillary percentage goes up to 23.1%. Increasing winding speed makes the film thinner and flattens the microcapillaries.
Dow then tested Polymer 1 in the matrix and lower viscosity Polymer 2 in the capillaries and with the matrix extruder at 15 RPM ran the capillary extruder at 25 RPM and 50 RPM. Microcapillary content went from 11.0% at 25 RPM up to 19.4% at 50 RPM. With the microcapillary screw at 50 RPM, winding speed was increased from 3 meters/min to 18 meters/min, which flattened the microcapillaries in much the same way as when capillaries and matrix were the same polymer.
Next Dow tested film made with Polymer 1 matrix and higher viscosity Polymer 3 in the microcapillaries and found that microcapillary content dropped to only 2.3% with the capillary screw at 25 RPM and to 18.6% at 50 RPM. Increasing winding speed didn’t flatten higher viscosity capillaries the way it did lower viscosity ones in a higher viscosity matrix or capillaries of the same polymer as the matrix.
The three different polymer combinations also produced films with different surfaces. With Polymer 1 in both matrix and capillaries at winding speed of 18 meters/min, the film surface is smooth. With different viscosity polymers, the film surface is wavy. When capillaries are lower viscosity than the matrix, the film is thinner over capillaries and thicker between them. When capillaries are higher viscosity than the matrix, the film is thicker over capillaries and thinner between them. Waviness is only visible under a microscope, but the wavy surface feels different to the touch, Huang says.
SQUARE PEGS FROM ROUND HOLES
The most unusual finding is that extremely low viscosity capillaries in an extremely high viscosity matrix produce square or rectangular microcapillaries. Dow tested this extreme viscosity mismatch using watery Polymer 5 in the capillaries and Polymer 4 with 100 times higher viscosity in the matrix. The extreme viscosity pair was tested at four capillary screw speeds – 10, 20, 30 and 40 RPM – with the matrix extruder going very slowly at only 5 RPM.
The microcapillaries were rectangular with the capillary screw at 20 RPM and square with the capillary screw at 40 RPM. As capillary screw speed increased, capillaries also took up a much larger percentage of the film. With the capillary screw at 20 RPM, capillaries are 20% of the film. At 40 RPM they’re 42% – almost half!
Dow then wound films with rectangular and square microcapillaries at line speeds of 1.5 and 3 meters/min and found surprisingly that the rectangular and square microcapillaries only flatten slightly at higher winding speeds. Despite being much lower viscosity than the matrix, they retain squarish shapes.
What might Dow do with these unusual films? Two recent Dow patent applications give an idea of potential applications, depending on the size and shape of microcapillaries. Patent application (WO2013009538) for “Microcapillary films containing phase change materials” describes filling microcapillaries with “phase change” materials including carnowax in an LDPE matrix. The list of possible phase change materials in the patent application is long, but the patent suggests that they’re used to add or remove heat to or from the matrix polymer. A second patent application describes “Reinforced microcapillary films and foams” (U.S. Pat. Applic. # 20140072776) and suggests that microcapillary coextrusion could create reinforcing fibers in a film or foam in the machine direction, much like pultrusion.