MaterialsGuru

Using recycled Polyethylene (HDPE) from milk jugs and cellulose from wheat straw, a Wyoming company has developed a process to make picket fences that look like wood but have the durability and a maintenance-free benefit of plastic. The latest issue of Plastics Technology reports that Heartland Bioplastics LLC is now marketing its PrairiePicket(R) privacy fence using this process hoping to capture a share of the fast growing residential fence market.

Total US fencing market in 2007 is estimated at $3.3 billion growing at 5% each year, of which 59% is for residential use (2003 note from Dartmouth). Wood and metal are predominant and account for nearly 90% of the sales. Vinyl plastic and wood-plastic composites (WPC) are gaining in popularity although cost remains a major issue. Utilization of recycled plastic and wheat straw should result in lower cost, although I do not know the exact cost of the PrairiePicket fence.

An interesting fact is that one picket (6 ft long by 5 1/8″ wide by 3/8″ thick) takes 12 jugs and that Heartland uses upto 40,000 lbs of plastic a week (See Tom Lacock’s post on Wyospace). Based on this information, I estimate a production rate of about 23000 pickets/week.

Wheat straw is a good source of reinforcement for plastics that melt at low temperatures (less than 200 °C). Good thing is that it is makes up about half of the yield of a cereal crop such as barley, oats, rice, rye or wheat, which means it is available in abundant quantities. Sure, there is a lot of interest in converting the straw cellulose into ethanol as a biofuel, however that technology is in its infancy and only a small amount is produced on a pilot scale. So, there should be plenty of wheat straw available for these applications.

At a 30% loading by weight in polypropylene, wheat straw as a reinforcing filler delivers a modulus of 3.3 GPa, nearly 2.5 times more than virgin polypropylene, and 3-16% incresae in tensile strength and 13 - 48% increae in flexural strength (see paper from the 4th International Conference on Woodfiber-Plastic Composites). However, similar to other filled composites, there is a significant drop in impact strength.

Wheat Straw fibers have an interesting microstructure as shown in this picture. Dr. Vik Malhotra at Southern Illinois University, Carbondale, has been using wheat straw to develop biocomposites for wheat byproduct wallboards. In order to get good reinforcement, it is important to convert wheat straw into fibers of uniform length without causing excessive damage. Heartland, it appears, has developed its own proprietary “dry process” to prepare wheat straw for mixing with recycled HDPE. At 50 - 60% loading, the wheat straw-HDPE composite picket has a flex modulus of 3.3 GPa, similar to the Polypropylene-wheat straw composite described above.

A few more things to keep in mind - it is critical to dry the wheat straw prior to mixing with plastic in a hot extruder. Typically, wheat straw at ambient conditions have about 10 -15% moisture by weight, which must be reduced to less than 1% prior to extrusion. Otherwise, this moisture will steam up and create voids in the final product.

Since wheat straw is a natural material, it can lead to mildew growth. Reportedly, Heartland adds Zinc Borate to prevent mildew. Zinc Borate is also commonly used as a flame retardant in polymers.

Overall, a good use of recycled plastic and a natural biomaterial. Who knows, if you save up enough milk jugs, you might be able to get a free fence!