Enabling a better balance of strength, stiffness, toughness, and moisture resistance in rLLDPE–wood systems.

Wood–plastic composites (WPC) are gaining ground in outdoor building products for their durability and circularity potential. Our latest work explores how CirKular+ additives can enhance performance of WPC compositions based on recycled linear low-density polyethylene (rLLDPE) paired with wood sawdust. The study targets evaluation of decking-relevant properties:

• Flexural modulus (modulus of elasticity, MOE)
• Flexural strength (modulus of rupture, MOR)
• Tensile behavior, impact, processability
• Short-term water uptake.

Using recycled polyolefins and wood sawdust can enable sustainable design. The practical challenge is to address the polarity mismatch between an apolar polymer and polar wood fibers that can negatively affect the strength, toughness, moisture performance, and extrusion processability for the end product.

Kraton’s R&D experts developed a solution to address this challenge by using CirKular+ modifiers to improve the performance of rLLDPE–sawdust composites used in the core layer of outdoor decking boards. The R&D goal was to enable high levels of wood content, which increase stiffness and sustainability, while improving toughness, strength, and water resistance.

Approach to Solution Development

WPC formulations were examined which contain recycled LLDPE, sawdust at 50–70%, CirKular+™ modifiers at 3–5%, and optional process aids. C1000 and C1010 compatibilizers were key due to their ability to form chemical bonds with the wood surface while remaining compatible with the polyolefin matrix. C3000 modifier was also tested alone and in combination with C1000 and C1010 to evaluate synergistic improvements. Test specimens were compression molded and measured for flexural, tensile, impact, moisture absorption, and rheological behavior. SEM imaging provided insight into fracture morphology.

Key Findings

Stronger, tougher, and less brittle

• Adding C1000 or C1010 significantly increased tensile strength and elongation compared to the unmodified rLLDPE–sawdust blend, which failed in a brittle manner.
• The modified systems showed a better balance of stiffness, strength and water resistance, which is critical for decking. Better flexural performance at high wood loadings.
• Most formulations contained 60% sawdust, with some at 50 and 70%.
• Higher sawdust content raised stiffness but usually reduces ductility and increases water uptake. This effect could be mitigated by increasing the level of C1010 and C1000.
• With 3% C1000 or C1010, the composites maintained good mechanical integrity, even at 70% sawdust.
• The combination of C3000 and C1000 at an 80/20 ratio gave a notable improvement in flexural modulus (MOE) and flexural strength (MOR) versus the unmodified control.

Improved moisture resistance

• The unmodified composite showed the highest thickness and weight increase due to moisture absorption after 72 hours in water.
• Increasing C1000 or C1010 content to 3% significantly reduced moisture uptake.
• Better fiber–matrix bonding and fewer voids limited pathways for water to penetrate, which is crucial for outdoor applications exposed to rain and humidity.

Improved rheology & processability

• C1000 and C1010 decreases viscosity, enabling improved wet-out of wood particles and easier extrusion.
• Lower yield stress with C1010 and C1000 indicates better filler dispersion which combined with lower viscosity, could translate in higher productivity and lower warpage.

For WPC core layers, CirKular+ C1000 and C1010 can open processing and performance windows with recycled polyolefin matrices, helping achieve stiffness targets without sacrificing tensile toughness, impact, or moisture resistance. Tunable combinations can further refine the MOR/MOE and ductility balance to fit application needs.

Conclusions

CirKular+ modifiers enable high-performance, high-wood loading (up to 70%) WPC formulations using recycled LLDPE. Their ability to chemically bond with wood fibers results in:

• Improved tensile and flexural properties
• Enhanced elongation and impact resistance
• Superior moisture resistance
• Lower viscosity and better processability
• Stronger interfacial adhesion and better dispersion

Together, these benefits support high-durability WPC decking with a reduced environmental footprint. For decking producers and compounders, CirKular+ helps differentiate products in the fast-growing WPC market by providing greater durability, longer lifespan, higher sustainable content, improved aesthetics and lower production costs.

Read the official paper in the SPE ANTEC 2026 proceedings (link will be added once available).