Film, compounding get new twist with breakthrough mixer

A Toronto-area company is out to change the way industry mixes polymers. Extensional Flow Mixer Inc. (Mississauga, Ont.) manufactures and sells an extensional flow mixer (EFM) which is based on the principle of elongation mixing, rather than the mixing via shear forces typically used in single or twin-screw extruders. According to managing director Wayne Song, elongation mixing works by stretching and breaking polymer particles. Elongation mixing raises the temperature of material by only a few degrees, in comparison to a 10 to 70 C temperature increase from shear mixing, thus making it possible to effectively mix heat-sensitive materials. It also uses thousands of times less energy.

The mixer was developed by Leszek A. Utracki and his staff at the National Research Council’s Industrial Materials Institute (Boucherville, Que.). Extensional Flow Mixer Inc. has signed an agreement with the NRC to manufacture and market the mixer.

EFM is designed for use with any existing pumping system, for example a melt pump or a single- or twin-screw extruder. Attached to a single-screw extruder, EFM is a significantly cheaper alternative to a twin-screw extruder, and, perhaps more importantly, Song notes, it provides improved mixing properties not obtainable with conventional extruder technology. For manufacturers of film, the net result is a film product with enhanced or new performance properties at no additional materials cost.

Song says one of the main benefits of the EFM is its ability to effectively mix plastics in which the viscosity of the disperse phase is at least four times higher than that of the matrices, for example elastomers and thermoplastics, something not easily accomplished by shear mixing. Song notes the results of one study which compared the mixing characteristics of a variety of blends made with and without EFM shows that EFM improves mixing morphology. For instance a blend of polystyrene with 10 percent HDPE showed tremendously improved dispersion using EFM. The blend is used for medical blister packaging; adding HDPE increases impact strength.

Mixing resin with a combination of a single screw extruder and EFM can also reduce the number and size of gel particles in certain types of film, in comparison to film made from blends mixed with a twin-screw extruder, says Song. He says an NRC study reports making blends of LLDPE using EFM in which the ultimate elongation of the resulting film was improved by a factor of two, in comparison to conventional LLDPE. This, says Song, may allow film extruders to make film with performance comparable to film made from metallocene-catalyzed resins, at a much cheaper cost.

The key to EFM is the design of the mixer’s converging/diverging plate system, which is critical to maximizing elongation without large temperature or pressure changes. The company has a partnering arrangement with Cronus Tool & Die to manufacture the plates. Different plastics require different plate design, notes vice president of manufacturing, Mike Mandic. The cost for an EFM unit designed for use on a six in. single-screw extruder is about US$15,000.

“It’s time for the film industry to reach a higher technical level,” says Song. “Our technology can help reach that goal.”