Creating conductive compounds

from the March 2009 issue of Compounding World
article by Peter Mapleston

In a world increasingly dependent on electrical and electronic devices, surrounded by an atmosphere increasingly full of electromagnetic radiation, the conductive properties of a growing number of plastics products are becoming more and more significant.

Therefore, it should come as no surprise that the options available to compounders to provide the conductivity required by their customers are increasing too.

Additives are available for applications ranging from low-level conductivity providing anti-static properties, through dissipative (ESD) and on to shielding against electromagnetic interference (EMI). They start with relatively simple hydroscopic surfactants, which migrate to the surface of the finished product and pick up moisture; and they end with the results of the latest nano technology, taking in more conventional carbon and metal fibres on the way. Here is a review of some of the latest developments.

Vorbeck Materials Group in Jessup, MD, USA, is one of the most recent entries into the conductive additives market. It was founded in 2006 to commercialise graphene products developed at the University of Princeton. Graphene belongs to the same family of carbon nano-products as nanotubes and buckyballs, but has a two-dimensional structure that looks like chicken wire. A sheet of graphene is effectively an unrolled carbon nanotube (CNT).

According to John Crain, vice-president of strategy and business development at Vorbeck, the material’s surface area can be theoretically as high as around 2,600 m2/g and in practice is as high as 1,700 m2/g. As a result, graphene can affect the bulk properties of the matrix at very low loadings.

Vorbeck makes a proprietary form of graphene called Vor-x, containing functional groups, so it can “tune its compatibility to a specific matrix or enhance specific material properties.” It has carried out compounding trials in standard compounding extruders and has also created liquid dispersions. It is currently supplying masterbatch and dispersions for trials at various key customers.

Masterbatches are suitable for adding to plastics at rates of up to around 20% by weight, depending on the application. Many applications can be fulfilled with a total weight of Vor-x of well under 1%. Crain says that compounding the masterbatches into plastics is far less difficult than compounding nanotubes. “Vor-x is a robust filler that can take the high shear forces of a twin-screw extruder or mixer,” he says.

Vorbeck has worked on masterbatches in a wide range of thermoplastics, from polyolefins to PEEK. Vor-x can yield conductivities “well beyond anti-static and into the conductive regime,” Crain claims.

A pilot plant with a rated capacity of 4 tonnes/yr has been running for around 18 months. The company plans to have a commercial plant ready some time next year, with an annual capacity of several hundred tonnes.

Late last year, Vorbeck established a joint research program with BASF to develop graphene-based formulations and composite materials. They are developing graphene dispersions for conductive coatings and compounds, especially for the electronics industry. The newly established joint research program will lead to commercial applications in the near future, the two companies say.

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