Beyond the Wood: Material Selection and Performance Metrics for Pallet Tier Sheets

A pallet interlayer is a thin sheet placed between stacked layers on a pallet to protect products, improve stability, and control slip. Material choice balances tensile strength, moisture resistance, and coefficient of friction for the intended transport and storage environment.

A pallet interlayer (also called a tier sheet or slip sheet) is a thin material placed between loads or layers on a pallet to protect product surfaces, increase stack stability, and control interlayer slippage. Interlayers can be single-use or reusable and are selected based on mechanical performance (e.g., tensile strength), environmental resilience (e.g., moisture resistance), and surface properties (e.g., coefficient of friction, COF). Choosing the right interlayer is especially important for global logistics, where temperature swings, humidity, vibration, long dwell times, and regulatory constraints impose diverse demands.

Primary functions of pallet interlayers include:

• Load separation and surface protection to prevent abrasion, crushing, or contamination.
• Slip control — increasing static and dynamic friction between layers to reduce sliding during handling and transport.
• Load distribution — spreading point loads to avoid product deformation, particularly for fragile goods.
• Moisture and contamination barrier when coated or constructed of non-porous materials.

Common interlayer materials and comparative performance — the following summarizes typical classes of interlayer materials and their relative performance on the three metrics of primary concern: tensile strength, moisture resistance, and COF.

• Corrugated and solid fiberboard (paperboard)
• Tensile strength: low to medium. Performance varies widely by grade and ply; suitable for lighter loads and where cushioning matters.
• Moisture resistance: poor in plain papers; can be improved with wax, poly-lamination, or water-resistant coatings.
• COF: moderate. Natural paper surfaces provide some friction; coatings can raise or lower COF intentionally.
• Use cases: single-use retail shipments, stack protection for non-hygroscopic goods, low-cost light loads.
• Polyethylene and polypropylene sheets (HDPE/LDPE/PP)
• Tensile strength: medium to high (plastics like HDPE have appreciable tensile strength and yield resistance).
• Moisture resistance: excellent — inherently waterproof and resistant to condensation and salt spray.
• COF: variable — smooth films have low COF (may promote slip), while textured or embossed versions provide high COF.
• Use cases: export containers, refrigerated/frozen transport, wet environments, reusable interlayers for heavy loads.
• Laminated composites (paper laminated to film or foil)
• Tensile strength: medium — improved by lamination and multi-ply construction.
• Moisture resistance: good if film/foil layer is continuous; offers product protection and printing capability.
• COF: controllable depending on outer layer texture.
• Use cases: situations requiring printed handling instructions, moderate moisture protection and limited reuse cycles.
• Foam and nonwoven fabrics
• Tensile strength: low to medium; primarily used for cushioning rather than structural support.
• Moisture resistance: varies — closed-cell foams resist moisture, open-cell foams absorb it.
• COF: generally high, reducing slip between layers.
• Use cases: fragile items, cosmetic and electronics shipments needing cushioning and anti-slip.
• Metal or composite sheets (aluminum, thin plywood composites)
• Tensile strength: high; excellent load distribution for heavy, point-loaded pallets.
• Moisture resistance: good for metals; wood-based composites require treatment to resist moisture and meet quarantine rules.
• COF: variable — metal can be low COF unless textured or coated.
• Use cases: heavy machinery parts, long-term storage stacks in controlled facilities.
• Slip sheets (very thin plastic or laminated sheets used with special push/pull attachments)
• Tensile strength: low thickness but engineered to transfer shear using forklift adapters.
• Moisture resistance: good if plastic based.
• COF: low on smooth films — these are intended to slide against the transport platform when using slip-sheet handling.
• Use cases: when weight and pallet reduction are priorities and compatible handling equipment is available.

Key performance metrics and testing

• Tensile strength — indicates resistance to tearing and deformation under load. Laboratories measure tensile strength in units such as megapascals (MPa) or newtons per millimeter, using standardized test methods appropriate for the material type. Higher tensile strength is advantageous for reusable interlayers and for heavy or high-stacking loads.
• Moisture resistance — assessed by water absorption, wet tensile strength, or accelerated aging tests. In humid or marine environments, choose non-porous materials or those with effective coatings; saturated paperboard will lose strength and increase slip risk.
• Coefficient of friction (COF) — both static (initiation of slip) and dynamic/kinetic (once slipping starts) COF are relevant. COF for plastics is commonly measured by ASTM D1894; corrugated materials use similar friction testing rigs. For warehouse and transport stability, aim for interlayer/pack surface combinations that produce sufficiently high static COF to resist vibrational and inertial forces during handling and transit.

Design targets for global logistics — while targets depend on product fragility, transport mode and stacking, practical guidance is:

• Specify a minimum static COF based on risk: a common practical threshold is a static COF of at least 0.35–0.45 for standard palletized loads; increase to 0.55–0.65 for high-center-of-gravity or high-vibration shipments (ocean or long-haul trucking).
• Require materials to retain a specified percentage of their dry tensile strength after moisture exposure (e.g., a pass/fail threshold based on your product’s tolerance).
• Consider real-world aging: repeated cycles of cold/hot or wet/dry can embrittle some plastics and delaminate composites; select materials tested to survive projected cycles.

Selection guidance by scenario

• Cold chain/refrigerated transport: use plastics or moisture-resistant laminated materials that remain flexible at low temperatures and resist condensation.
• Ocean freight and high-humidity routes: favor non-porous plastics or foil-laminated papers with tested salt-spray resistance.
• Export subject to phytosanitary rules: avoid untreated wood interlayers or ensure wood is heat-treated/ISPM 15 compliant; plastics and laminates simplify compliance.
• Retail and short domestic runs: corrugated or coated paperboard often balances cost and protection; add anti-slip coatings where needed.
• Heavy industrial loads: engineered plastic or thin metal/composite sheets distribute loads and resist crushing.

Lifecycle, cost and sustainability — reusable HDPE/PP interlayers can be justified when they survive multiple cycles; single-use fiberboard is lower cost upfront but generates waste. Consider total cost per trip, recyclability in destination markets, and contamination risks for food/pharma. Recycled-content plastics and fully recyclable fiberboard reduce environmental impact but must meet performance specs.

Common mistakes and practical best practices

• Choosing materials by upfront price alone without testing under expected humidity, temperature, and vibration conditions.
• Neglecting COF testing for the actual product/interlayer surface pair — underestimating slip risk during real transport conditions.
• Using smooth low-COF plastics where anti-slip is required without adding texture or adhesive dots.
• Ignoring regulatory constraints (food-contact approvals, ISPM 15 for wood) in export chains.
• Failing to pilot candidate interlayers on a representative route and revise specs before full roll-out.

Implementation checklist

• Define mechanical (stack height, load weight), environmental (humidity, temperature), and regulatory requirements for the route.
• Shortlist materials that meet core requirements (moisture, tensile, COF) and request technical data sheets.
• Conduct laboratory testing for tensile strength, wet strength, and static/dynamic COF using representative product samples.
• Pilot in an operational environment to validate real-world performance and handling impacts.
• Review lifecycle costs, reuse potential, and end-of-life recycling before selecting standardized SKUs.

Selecting the right pallet interlayer requires balancing mechanical performance, environmental resilience, handling characteristics and cost. By specifying measurable acceptance criteria (tensile retention after moisture exposure, minimum static/dynamic COF values, and durability targets) and validating choices through lab and field testing, logistics teams can reduce damage, improve pallet stability and optimize total cost across global supply chains.