The pallet industry has long considered the notion of utilizing splicing technologies to increase the value of residual “short” material left over from pallet component manufacturing. A major obstacle to industry adoption of these practices has been the lack of research data that would direct the industry in terms of proven splicing procedures. Because pallets are load bearing structures, the applicability of finger jointing pallet parts must assure the safe end use of products. A recent pilot study by Virginia Tech’s Center for Unit Load Design has concluded that finger jointed green and dry hardwood pallet components can perform at strength and stiffness levels approaching those of solid wood.
This discovery could benefit recyclers who continue to look for new sources of material as the market continues to stay fairly tight. Typically, wood material left over from pallet part operations is chipped or shredded for sale as pulp chips, fuel, or mulch. The potential exists to add value to this material by means of splicing short sound segments to create full size and longer pallet parts.
The Potential For Finger Jointing Pallet Parts
Over the last couple of years, the production of new pallets in the United States has slightly declined from the 433 million (6.6 billion BF) manufactured in 2001. In the process of manufacturing new pallets, nearly four percent (approximately 264 million BF) of raw material is left over as salvage material—sound segments of pallet material considered too short for pallet parts. This material is typically shredded and sold as mulch or used as fuel.
The recovery, repair, and recycling of wood pallets, however, has continued to grow. It is estimated that in 2001 US recyclers recovered approximately 299 million pallets (4.5 billion BF). Roughly 12% (540 million BF) of recovered pallet wood volume is typically shredded and disposed of annually.
Approximately 67% of all new and recovered pallets are manufactured from hardwoods, and 33% from softwoods.
Together, recovered but unusable recycled pallet material and new salvage pallet material combine for nearly 804 million board feet of wood fiber that either enters the waste stream or is sold as lower value alternatives. There is tremendous potential to add value to these short sound segments by utilizing splicing technologies to create full size and longer components.
Pallet recyclers receiving pallet parts are limited by the part lengths they receive—typically 48-inches or less. Longer used parts are less attainable. Splicing may permit recyclers to produce longer parts, and ultimately, longer and wider remanufactured pallets. This would not only increase the value of the wood fiber, but would also help control pallet repair and manufacturing costs and improve utilization of timber supplies.
Previous research
The use of finger jointing technology to produce wood pallet components has been studied. However, the adoption of these procedures in the industry has been limited due to concerns over pallet performance especially when joining green hardwood sections.
Several projects at West Virginia University have shown limited success in finger jointing green pallet parts. A previous project at Virginia Tech studied pallets assembled from green finger jointed components using a soy/resorcinol adhesive. While the finger-jointed deckboard components performed well, the finger jointed stringers failed at the finger joints due to improper adhesive application.
Virginia Tech Finger Jointing Pilot Study
The latest research at Virginia Tech’s Center for Unit Load Design has demonstrated a new breakthrough in finger jointing technology for pallet parts. This pilot study is the beginning of a larger study designed to examine the strength and stiffness of pallet parts (deckboards and unnotched stringers) assembled using both finger jointing and metal connector plate splicing technologies. While this study focuses exclusively on green and dry hardwood pallet parts, it is logical splicing will apply to softwoods as well. This study was funded by the USDA Forest Service Wood Education Resource Center (WERC) in Princeton, WV.
The objectives of this pilot study were:
1. To compare the flexural strength and stiffness of both green and dry finger jointed hardwood deckboards and unnotched stringers with non-spliced hardwood pallet parts (control).
2. To determine the influence of moisture content, species, and the orientation of the finger joint (vertical vs. horizontal) on the performance of finger jointed hardwood deckboards and stringers.
3. To analyze the applicability of two Georgia Pacific patented Greenweld™ adhesive systems for gluing green/wet wood.
Among the objectives listed above, two key variables of particular interest were the type and application of the two Greenweld wet wood resins used, as well as the orientation of the fingers that join the components relative to direction of stress applied to pallets during use. Finger jointed parts were manufactured from both oak and yellow poplar to represent dense and less dense hardwoods, respectfully. Finally test samples were divided into two moisture content groups: finger jointed green and tested green (MC > 28%) to represent new manufacture salvage material, and finger jointed dry and tested dry (MC < 15%) to represent salvage material during recycling.
Georgia Pacific’s Greenweld System is a relatively new two part (resin + hardener) cold-set adhesive and gluing process that finger joints and laminates wood that is wet, dry, or frozen. It can successfully glue wet wood to dry wood, is not temperature sensitive, and works on all major commercial softwood and hardwood species. The Greenweld glue system can be used with conventional finger-jointing equipment, and it does not need costly heating or RF curing equipment. The glue is environmentally friendly (no formaldehyde), and the Greenweld process has been certified for structural materials.
For performance comparisons, two versions of the Greenweld adhesive were used in the pilot study: one utilizing a liquid resorcinol (RF) resin plus hardener and the other a liquid phenol-resorcinol (PRF) resin plus hardener. The RF resin is approximately twice as expensive as the PRF resin. An activated polyvinyl acetate (PVA) adhesive that is used in many dry wood structural finger jointing operations was used to finger joint all dry pallet hardwood components.
All finger jointed components were manufactured using a Grecon ProfiJoint finger jointer supplied by the Weinig Group. All finger joints were located at the center of each deckboard and stringer test samples. To determine the performance effect of the orientation of the finger joint, samples were prepared with the finger joints oriented both vertically and horizontally in both the deckboards and the stringers.
Horizontal or vertical finger joint orientation refers to the direction of the finger joint in relation to the wide face of a board. They are opposite for deckboards and stringers. Figure 1 (page 18) depicts the finger joint orientations for both stringers and deckboards. The continuation of this study will focus on finger jointed pallet parts with the joints oriented horizontally for deckboards and vertically for stringers. The fingers
were 10mm long. There is some indication that fingers of 16mm length will perform better than the 10mm used in this pilot study.
All finger jointed pallet part test samples were stressed to failure by means of a three point bending test using a 36-inch span for the 40-inch deckboard test samples, and a 44-inch span for the 48-inch stringer samples. All deckboards were tested as they realistically bear loads in pallets—laying flat wise. All unnotched stringers were tested resting on the narrow face. Figure 2 (page 20) shows a deckboard bending test in progress.
Results
A summary of the results is presented in Table 1 on page 22. The important finding in this pilot study is that finger jointed green and dry hardwood pallet components can perform at strength (MOR) and stiffness (MOE) levels approaching those of their non-spliced control specimens. This is particularly relevant to the pallet recycling industry.
The Georgia Pacific Greenweld system is an effective adhesive for gluing finger jointed green hardwood pallet parts. There was virtually no difference between the PRF and RF resins. Because the PRF is effective and significantly less expensive, research will continue using this resin exclusively. The activated PVA resin used for finger jointing dry pallet parts was extremely effective.
The orientation of the finger joints significantly affects the strength and stiffness of finger jointed components under load. For deckboards, finger joints with a horizontal orientation were approximately 30% stronger than deckboards with vertical finger joints.
For stringers, it was less clear, but the consensus is that finger joints oriented vertically perform best. For stringers and deckboards the best orientation of the fingers is actually the same since deckboards on pallets resist bending loads across the wide face, and stringers resist bending across the narrow face.
For more information or to request copies of Virginia Tech reports, contact Peter Hamner at the Virginia Tech – Center for Unit Load Design (phone: 540-231-3043; email: phamner@vt.edu).
