Many dairy farms use fiber from digested dairy manure as bedding for their cows, one way to add value to this material. A study conducted by researchers at Michigan State University examined its use as an ingredient in fiberboard and composites.
Fiber used in the study came from anaerobic digesters located on three dairy farms, and is referred to as digester fiber. The 0.425 mm (40 mesh) maple and pine wood flours obtained from American Wood Fibers of Schofield, Wisconsin were also used as fibers to compare performance of wood-plastic composite (WPC) made from hardwood and softwood species with that of digester fiber counterparts. The wood flour was ovendried at 105 °C for approximately 48 hours before processing to remove moisture. On the other hand, various drying schedules were used to dry the digester fiber.
Due to the popularity of polyethylene resins in the wood-plastic composite market, high-density polyethylene (HDPE) was used as the polymeric matrix. The polymer was in flake form with a melt flow index (MFI) of 0.49 g/10 minutes, and density of 0.9 g/cm3. A lubricant was used to reduce friction between components and equipment, which eased processing. A liquid urea formaldehyde (UF) resin was used for medium density fiberboard (MDF) manufacturing. Solid content, viscosity and pH of the resin were 65.9 percent, 185cPs at 20°C and 8.56, respectively.
A 10 liter high-intensity mixer was used for room temperature dry blending of the HDPE matrix, digester fiber and lubricant. The formulation of the composites was maintained at 50 percent digester fiber, 44 percent HDPE and 6 percent lubricant by total weight of the composites. The compounded materials were fed into a 32 mm conical, counter-rotating, twin-screw extruder with a length to diameter ratio of 13:1. An unpressurized vent allowed residual moisture to escape.
The heating profile, from the hopper to the horizontal die, was 190, 170, 140, 140°C with a rotational speed of the screws maintained at 40 rpm throughout processing. Extrusion online processing torque and pressure were recorded to evaluate the processing ease of the composites. The rectangular die created samples with a nominal width and thickness of 2.54 cm (1 inch) and 0.95 cm (3/8 inch), respectively. The samples were cooled using a recirculated water bath to maintain the sample size and geometry after exiting the die (prevents the swell). These samples were used for flexural property testing.
For comparisons, composites made with pine and maple flour were also manufactured at the same formulations as those used to make composites with digester fibers, but using different processing conditions.
MDF panels were manufactured as follows. Oven-dried digester fibers were placed in a high intensity mixer and sprayed with 15 percent liquid UF resin (based on oven dry weight of wood furnish). After blending, resinated fibers were manually formed in a 406 by 406 mm forming box and hot pressed at 149°C in a laboratory press using the following pressing conditions: The initial pressure of 6.9 MPa (1000 psi) was first applied for 15 seconds, followed by 4.5 MPa (650 psi), which continued for four minutes and then decreased to 0 MPa over 45 seconds before press opening. The panel thickness was 9.53 mm (3/8 inches) and the targeted density of the panel manufactured in this study was 720.8 kg/m3, to produce medium density panels.
Property testing and statistical Analysis
Property Testing: Three-point flexural tests of digester fiber- HDPE composites were carried out in a walk-in conditioning room at 23°C +- 2°C and 65 percent +- 4 percent relative humidity. The crosshead rate was 4.5 mm/minute, in conformance with ASTM standard D6109-97. At least 10 replicates were tested and data was collected on modulus of rupture (MOR, or flexural strength) and modulus of elasticity (MOE, or flexural stiffness).
The modulus of rupture, modulus of elasticity and internal bond (IB) strength of fiberboards were evaluated according to the procedure outlined in ASTM 1037-99 standard. Specimens were produced from three replicated panels, and the tests were carried out in the walk-in conditioning room. The crosshead rates were 4.5 mm/minute for the bending test, and 5.4 mm/minute for the IB test. The bending and IB test results were compared with values listed in the standard ANSI A208.1-1999 Particleboard.
Statistical Analysis: A two-sample t test (single-factor) was employed to determine the statistical differences among the three sources of digester fibers investigated at a 95 percent significance level.
Digester Fibers-HDPE Composites: The online processing conditions (torque and pressure) were recorded during experiments. Overall, the results clearly demonstrate that digester fibers can be processed with plastic using existing machinery and processing conditions similar to those used to process conventional woodplastic composites. Both the torque and pressure generated during the manufacture of digester fiber-HDPE composites are similar to those generated during pine flour-HDPE composites.
The bending properties of digester fibersHDPE composites are compared with the Data for wood flour-HDPE composites made with maple and pine flours. It should be mentioned that wood flour-HDPE composites also contained 50 percent wood flour (mesh size 40), but different processing conditions were used in their manufacture.
Generally, digester fiber-HDPE composites outperformed their wood- flour/HDPE counterparts in flexural strength, regardless of digester fiber source. Digester fiber from Stencil Farm produced the highest strength, followed by Gordondale Farms and Vir-Clar Farms. Although composites based on Stencil Farms’ digester fibers appeared to have greater strength than maple-HDPE counterparts, this difference was not statistically significant at the alpha = 0.03 level. Composites made with digester fiber from Gordondale Farms had the greatest stiffness, followed by those made with Stencil Farm and Vir-Clar Farms digester fibers.
Digester fiber-based composites had greater flexural modulus (stiffness) than those of maple flour counterparts, regardless of digester fiber source. By contrast, the stiffness of pineflour/HDPE composites was greater than that of digester fiber-based composites.
Fiberboard Composites: The MOR and MOE values of digester fiber fiberboard panels met the property requirements for particleboard listed in the ANSI standard for both medium density panels. By contrast, the IB strength values of fiberboard panels made with digester fibers exceeded the property requirements for particleboard listed in the ANSI standard.
The results show that value-added products such as fiberboard and fiber-plastic composites can be manufactured successfully from digester fibers. Overall, the bending properties of the fiber- plastic composites made with digester fibers compared favorably to or exceeded those containing pine or maple flour, regardless of the source of the digester fibers. The fiberboard panels made with digester fibers performed very well in mechanical tests, in many cases meeting or exceeding the standard requirements for particleboard of medium density in bending strength (MOR), stiffness (MOE) and internal strength (IB).
Tests are ongoing under the direction of Laurent Matuana to ascertain the overall suitability of digestate fiber in construction materials. This includes evaluating the durability, strength, stiffness and mechanical properties of these materials. Some work has been done in terms of moisture absorption, but more work is needed to see how the materials will respond to sunlight and UV rays, and how well it will resist mold and fungi. Charles Gould is seeking to conduct a market assessment to determine acceptance and demand for decking and fiberboard made from digestate fiber. The results should be completed by the spring of 2009 at the latest, about the same time as Matuana concludes his durability research.
Laurant Matuana is Associate Professor in the Department of Forestry at Michigan State University in East Lansing. His email is: firstname.lastname@example.org. M. Charles Gould is an Extension Educator in Nutrient Management at Michigan State University in Grand Haven. His email is: email@example.com. The authors acknowledge the cooperation of the three dairy farms in Wisconsin that provided digester fiber for the project.