The pallet industry is constantly searching for cost effective alternative materials to manufacture pallets. Could lumber made out of recycled plastic be the answer? Researchers at Virginia Tech’s Center for Unit Load Design have studied the potential of recycled plastic lumber (RPL) and have come up with some potentially surprising results.
For all industrial use pallets, material and design translate specifically to function, efficiency, and effectiveness. Because pallet cost, product damage, and safety are primary concerns, there is always a trade-off between the materials used to manufacture pallets and how well and how safely they will perform in terms of strength, stiffness, durability, and service life. In light of these relationships pallet manufacturers, and their users, incorporate many alternative materials to manufacture pallets — solid wood, wood composites, corrugated fiberboard, plastic, and metal to name a few.
At present wood pallets dominate the pallet market, and they will continue to do so into the foreseeable future due to their high performance versus low cost ratio. In the U.S., about 90%-95% of new pallets and pallets already in service are made from solid wood materials.
Nevertheless, pallets made from alternative materials are necessary in certain niche markets due to the fact that some characteristics of wood prohibit their use or are simply undesirable.
While wood is relatively available, stiff, strong and inexpensive, as a biological material, it is also prone to contamination caused by insects and mold and moisture related problems. Wood also is known to splinter and has lower impact resistance compared to certain plastic materials.
Markets that use some non-wood pallets include automotive and supplier industries, the U.S. Postal Service, certain dry grocery and produce industries, meat processors, beverage and beverage container industries, pharmaceutical industries and warehouses that use captive pallets.
While a variety of alternative materials exist for non-wood pallet manufacturing and end use applications, pallets made from plastic materials are the most common. The plastic pallet industry uses a variety of different plastic resins, manufacturing processes and designs. Plastic resins can be molded into pallets and extruded plastic lumber can be handled and assembled into pallets much the same as wood is used.
The target of this column pertains to a specific type of plastic pallet that is manufactured using RPL. Figure 1 shows a typical RPL manufactured pallet. For specific markets, such as mentioned above, RPL is a viable alternative material that is well suited to many palletization applications.
Recycled Plastic Lumber
Recycled plastic lumber is manufactured by forcing post-consumer plastic resins through a rectangular extrusion die to create a solid, durable, waterproof building material. Extrusions are made to match standard lumber sizes (i.e. 1×6, 2×4, 2×6) and then cut, routed, and nailed or screwed together to create a rugged, sturdy shipping platform.
RPL pallets, which typically resemble standard stringer-style wood manufactured pallets, consist of RPL deck boards and stringers. Some recycled plastics can be extruded into profiles that retain strength and stiffness while at the same time reducing resin requirements. Figure 2 shows an example of profiled plastic lumber.
A primary benefit of RPL is that it consumes waste plastics that may otherwise end up in landfills and converts the waste plastic into useful, durable products — such as park benches, railroad ties, patio decking…and pallets.
The adoption of an identification code by manufacturers of plastic products and packaging has greatly enhanced the recovery, characterization, and sorting of plastics in the waste stream. Six polymer codes constitute the majority of post-consumer plastics. These polymer codes are identified numerically as follows:
1. Polyethyleneterephthalate (PET)
2. High-Density Polyethylene (HDPE)
3. Polyvinyl Chloride (PVC)
4. Low-Density Polyethylene (LDPE)
5. Polypropylene (PP)
6. Polystyrene (PS)
Recycled plastic lumber is typically manufactured from several types or combinations of post-consumer/industrial resins (Figure 3). There are three general RPL categories: HDPE, composite, and commingled.
HDPE consists of up to 95% HDPE, primarily recovered from recycled plastic milk jugs. RPL manufactured from HDPE is available in many colors but is also quite expensive since material sorting increases costs. Like most plastic products, HDPE has much lower stiffness than wood, although its physical and mechanical properties are consistent (due to its homogenous composition). Because of its cost, RPL made from HDPE is rarely used to manufacture pallets.
Composite RPL is manufactured with a mixture of plastics and wood or other recycled fiber (fiberglass, rubber, peanut shells, etc.). This product has the little interior void space and good surface roughness, which allows for a relatively high coefficient of friction and good paintability. Although less expensive than HDPE grade, it is still more expensive than wood and less strong and stiff. Steel reinforcing rods can improve this characteristic, but at much higher cost. If wood is used in the composite, it can absorb moisture and may not be completely insect resistant.
Of the three types of RPL, commingled may be the most applicable for pallet manufacture. Commingled RPL is made from a mixture of recovered thermoplastics that can be re-melted and re-molded or extruded into plastic lumber. Only earth tone colors are available, and its physical and mechanical properties can vary due to the random input of its recycled plastic composition. Because little or no plastic sorting is required before extruding the mixture into a solid lumber product, it costs less.
What makes commingled RPL attractive for pallet manufacturers is its reduced cost in combination with specialized performance capabilities. Like all plastic pallets, commingled RLP offers pallet manufacturers and users the benefits of an insect-free, durable, washable, and weather-moisture resistant pallet option for specialized markets where wood is not well suited.
In the past, an understanding of the performance characteristics of pallets manufactured out of recycled plastic lumber has been limited due to a lack of performance-based standards and product specifications that would ensure the proper and successful use of this material—particularly in structural type applications.
Over the last 10 years the American Society of Testing and Materials (ASTM) has been developing standards and specifications for RPL, and these have resulted in increased sales. Moreover, previous research conducted at Virginia Tech’s Center for Unit Load Design has examined and evaluated the performance of both individual pallet components and assembled pallets fabricated from recycled plastic lumber.
Plastic lumber can be used by wood pallet manufacturers as it is compatible with some of their existing manufacturing procedures. Plastic lumber provides a material option for wood pallet manufacture.
Virginia Tech Studies
Two studies performed at Virginia Tech investigated the physical properties and mechanical performance characteristics of pallets manufactured from commingled RPL. Tests were based on the standard procedures outlined in ASTM D-1185 – Standard Test Methods for Pallets and Related Structures in Material Handling and Shipping. The objectives of the Virginia Tech studies were:
1. To evaluate and compare the physical and mechanical properties of commingled RPL plastic pallet parts with wood pallet components.
2. To evaluate and compare commingled RPL pallet performance with wood pallet performance.
3. To evaluate and compare the mechanical fastener connection performance of commingled RPL with wood pallet components.
The results from these studies showed that commingled RPL pallet parts are approximately 1.3 and 1.8 times more dense than their hardwood and softwood counterparts, respectively. Consequently, pallets manufactured from RPL are approximately 50% heavier than wood pallets. Since heavy pallets are more prone to changes in diagonal length resulting from impacts, the test for diagonal rigidity showed commingled RPL pallets to be 15 percent less rigid than similarly designed hardwood pallets. Plastic pallets made from profile extruded lumber will weigh less (Figure 2).
Although more dense, commingled RPL pallets and pallet parts are considerably less stiff and strong than wood pallets and pallet parts. The Virginia Tech studies reported that flexural strength and stiffness values for commingled RPL pallet parts were only 10%-20% of the values measured for wood. Table 1 shows comparative test results for similarly designed 48×40, three-stringer, commingled RPL pallets and hardwood pallets in the racked across stringers and racked across deck boards load and support conditions.
Another important performance-based test when evaluating pallets is the inclined impact test of top deck end boards. The purpose of this test is to determine the resistance of the top deck end board to impacts by fork tine heels. A 700 pound static load is placed on each pallet, and a travel distance to impact on the fork tine heels varies from 12 to 36 (or more) inches.
The results of this test showed that the commingled RPL pallets were more resistant to lead edge impacts than the solid wood pallets. Because RPL deck boards are more flexible than wood, they could more easily absorb and transfer the energy of the impact. Nail withdrawal from the stringers was the primary mode of failure for lead edge end boards of RPL pallets.
Finally, the research at Virginia Tech showed that commingled RPL pallet components exhibit lower nail withdrawal resistance than their oak counterparts but are comparable to southern yellow pine and spruce. Comparative test of helically threaded and annularly threaded nails indicate that the annularly threaded fastener will result in better connections when fastening into plastic stringers such as those that were used in these tests.
Although pallets manufactured with commingled RPL components are considerably less stiff and strong than similarly designed wood pallets, the research shows that this material is still a viable alternative for limited (lighter load) pallet uses where pallet loads are not supported in free span warehouse racks.
Recycled plastic lumber currently has a higher purchase price than wood, but RPL pallets will likely be more durable than wood. When maintenance, replacement, and life cycle analysis are included in the cost analysis, RPL pallets can actually end up costing less than similarly designed wood pallets with much shorter life cycles.
(Peter Hamner and Marshall White are, respectively, Research Associate and Professor, Center for Unit Load Design, Department of Wood Science and Forest Products, at Virginia Tech in Blacksburg, Va. For more information regarding recycled plastic lumber or other plastic type pallets, contact Peter or Marshall at the Virginia Tech Center for Unit Load Design. Peter can be reached by calling (540) 231-3043 or e-mail at phamner@vt.edu.)
