Applied Research -  Laboratory Testing -  Pilot Plant
Much of the hands-on work required to conduct our research is done in our Pilot Plant facilities, which contains composite panel blending and pressing equipment, a pressure treating cylinder, conditioning rooms, universal testing machines, ovens, an environmental chamber, machining equipment and tools.

Composite Manufacture:

Testing the effectiveness of experimental wood preservatives in composites often requires us to manufacture the composites and add the preservative to the product during the manufacturing process.

Pressure Treating:

Preservative pressure treating of wood is done in our pressure treating cylinder.

Wood Composite Panel Manufacture

Aspen strand board is manufactured using press settings and additive amounts typically used by large industrial plants to make oriented strand board (OSB). Experimental wood preservatives, or other desired additives may be integrated into the product. Below is a general process flow diagram showing the manufacturing process from the green log through the press where the final panel is consolidated. The process is explained in more detail below the diagram.
Aspen logs are cut into rectangular bolts at the sawmill, both to remove the bark and to reduce the wood to a managable size. The bolts are cut into 15 inch lengths in order to fit them into the strander infeed. The aspen bolt is forced by a hydraulic push arm into the rotating strander disk, which reduces the bolt into strands approximately 2-1/4 inches long x 1/2 inches wide x 0.025 inches thick. Strands collect in a temporary bin until they are raked into barrels for transport to the strand dryer. WOOD STRANDER
Brian placing an aspen bolt into the strander infeed trough

Ed raking strands from the strand collection bin into barrels
Strand dryer. The classifier is directly below the orange drying bin.

The dryer works by blowing hot air through the strands for up to two days, reducing the strand moisture content (ovendry weight basis) from approximately 100% to 3%.
The dry strands are screened, or classified by metering them slowly from the dryer unto an orbital rotating strand classifier. The classifier removes the fine particles from the strand mix.
To prevent moisture uptake the strands are sealed and stored in 55 gallon polypropylene drums until used.

The blending process involves adding an adhesive, usually polymeric di-methyl diisocyanate (pMDI) or phenol formaldehyde (PF) to the strands as a binder, as well as an emulsified wax for water repellency. These additives along with any experimental biocides may be sprayed onto the strands using spray guns mounted onto a boom inserted into the middle of the blender, which rotates to mix the strands. Alternatively, the biocide may be added in dry powder form, which can be added gradually to the blend by hand. In addition to these additives, water may be added to the blend to achieve a constant mat moisture content target. Additive canisters are carefully weighed before and after spraying to determine the exact amount added to the blend.

Brian loading aspen strands into the open blender.
Forming a strand mat by hand.

A forming box is used to help form a consistent mat for pressing. A metal caul sheet is laid in the bottom of the forming box. The amount of strands needed to make one panel is weighed, and slowly dropped by hand onto the caul sheet to form a level strand mat. A second caul sheet is then laid on top of the mat. The forming box is lifted off of the mat assembly so the mat assembly can be put into the press.

The mat assembly, including the blended strands and two caul sheets is inserted into the hot press opening. The timer is started and the press is closed under high pressure until all four thickness stops are reached. Then, the pressure is slowly reduced and stabilized to maintain the desired panel thickness. At the required time the pressure on the mat is slowly reduced, and the press is opened. The caul sheets are removed and the panel allowed to cool.

Loading a mat into the press.
Removing caul sheets from the pressed panel.

After cooling, the pressed composite panel will be edge trimmed and cut into test samples.

Quality Assurance:

Quality assurance testing verifies the integrity of each blend and ensures that the panel properties are consistent over time. This is important considering that different "batches" of raw materials, including wood strands, resin and wax will be used as the older material is used and fresh, new material comes in.

Quality assurance includes both closely monitoring the deviation from target for each blend component added as well as conducting physical and mechanical tests on the pressed panels. Physical property testing includes measurements of thickness and panel densities, determination of moisture content, and water soak properties - thickness swell and water absorption. Mechanical properties include internal bond and static bending, where the modulus of elasticity and modulus of rupture are determined.

Since the blend requirements of individual research projects may vary, a standard quality control blend is manufactured along with the protocol-specific blends for each new project. The quality control blend remains consistent with regard to type and percent raw materials and additives in order to provide a baseline for test property comparisons. The results of physical and mechanical tests performed on the quality control blend provide reassurance that blending and pressing parameters are within the "normal" range. Quality control charts are maintained for each test type to help define the "normal" range and highlight potential problems, or trends that may be developing.


Wood-Thermoplastic Composite Manufacture

(Note: Extrusion has almost entirely replaced the below described method as a means of manufacturing wood-thermoplastic composites here at Michigan Tech.)

Wood-thermoplastic composite panels are manufactured using a process developed here at Michigan Tech. Below is a general process flow diagram showing the manufacturing process from the acquisition of wood furnish and plastic through the pressing and cooling processes where the final panel is consolidated. The process is explained in more detail below the diagram.

Wood furnish is acquired in the form of fine particles, for example 20-mesh ponderosa pine. The wood furnish is dried in a forced-air oven until no moisture remains. Plastic, typically polypropylene is also obtained in the form of a fine powder.

The predetermined amount of wood and plastic is weighed along with any additional additives that may be desired and put into a high-speed mixer. The blend components are mixed for approximately five minutes.

Steve emptying the high-speed mixer.
Pouring the mixed wood-plastic blend unto the forming sheet.
Stainless steel caul sheets are sprayed with a release agent, and a forming mold frame is placed on one of the sheets. The wood-plastic mixture is poured into the frame and spread to fill the frame evenly.
Spreading the mixed wood-plastic blend on the forming sheet.
A 1/16 inch spacer is taped to each side of the forming frame. This prevents the hot press from closing completely. Later, before transferring the assembly from to the hot press to the cold press the spacers will be removed, allowing the molten wpc material to be compressed further, filling all air voids. FORMING WPC
Adding thickness spacers to the forming mold assembly.
Placing the top caul sheet on the assembly.
When the forming is complete the top caul sheet is placed on top of the formed wood-plastic mat, and the entire assembly is placed into the hot-press. The press is closed to contact and heat the top caul sheet, but not closed completely. This allows the wood-plastic mixture to soften, so it will flow more freely when pressed to the stops. After five minutes of preheating the press is closed completely for an additional 7 minutes.
Closing the hot-press to contact the top caul sheet for preheating.
After the required time in the hot press the press is opened and the thickness spacers are removed from the assembly. The caul sheet assembly with molten wood-plastic still contained in the metal frame is placed in the cold-press, which is closed completely. Excess wood-plastic is squeezed out around the edges of the frame, and a void-free composite is formed. COLD PRESS
Final consolidation takes place in the cold press
WPC Completed
Solid WPC
After cooling to form a solid panel, the cold press is opened and the WPC panel is removed from the frame. The panel may be cooled further under a heavy, heat-sinking weight to avoid warping of the panel surface. The edges are trimmed and the required test specimens may be cut from the panel.

Wood Pressure Treating

Test Method: AWPA Standard, E7-93. Standard Method Of Evaluating Wood Preservatives By Field Tests With Stakes, 1999.

When testing a wood preservative system a standardized wood treating procedure must be followed to ensure the desired preservative levels are retained in the wood. Wood stake specimens are cut to the required size from clear southern pine sapwood containing 6-10 growth rings per inch. They are then sorted by weight to ensure similar densities between treatment groups. The wood stakes are pressure treated in separate vats for each retention level, with the solution concentration calculated to achieve the desired level of the preservative active ingredient in the wood. This concentration is based on the gross retention, determined by first treating a test batch of stakes with plain water, or treating solvent.
To pressure treat a batch of wood stakes, the stakes are first laid in a treating pan up to three layers high. A space is maintained around all stakes to ensure full exposure to the treating solution. The stakes are weighed down to prevent floating and enough treating solution is poured into the pan to ensure the stakes are fully covered throughout the pressure treating cycle.
The treating pan containing the wood stakes in treating solution is slid into the pressure cylinder and the door is tightly closed. A vacuum is pulled inside the treating cylinder for the required time to draw air from the wood voids, replacing the air with treating solution. Compressed air is then pumped into the cylinder and maintained for the required time to force the treating liquid to fully saturate the wood stakes. Following the pressure cycle the stakes remain in solution for a short time to allow for "spring back", and are then removed to allow the free liquid to drain from the wood surfaces. The stakes are weighed to determine the retention of treating solution and allowed to air dry. Each of the treated stakes will be cut into two field stakes plus a center, analytical specimen.
Pressure Chamber