The Construction of Composite Plastic Lumber

Not all plastics are considered composites. When additional strength is required, many types of polymers (plastics) can be reinforced with structural materials to meet higher performance demands. Only when a polymer is reinforced, is it considered a fiber reinforced polymer (FRP) composite. The definition of an FRP composite is a combination of a polymer (plastic) matrix and a reinforcing agent, such as glass, carbon or aramid, that provides a sufficient aspect ratio (length to thickness) of a discernable reinforcing function in one or more directions. FRP composites may also contain fillers, additives or core materials that enhance the final product. The constituent elements in a composite retain their identities (they do not dissolve or merge completely into each other) while acting in concert to provide a host of benefits including:

  • High strength-to-weight ratio
  • Dimensional stability
  • High impact strength/resistance
  • Long-term durability
  • Low maintenance
  • Weather resistance 
  • Corrosion resistance

Each of the constituent materials or ingredients in the polymer matrix plays an important role in the processing and final performance of the end product. The resin or polymer is the "glue" that holds the composite together. The reinforcement provides the mechanical strength. The additives are used as process or performance aids to impart special properties to the end product.


ACT Composite Timbers - The Constituent Materials


The primary functions of the resin are:

  • to transfer stress between the reinforcing fibers
  • act as a glue to hold the fibers together
  • protect the fibers from mechanical and environmental damage

Resins are divided into two major groups known as thermoset and thermoplastic. Thermoplastic resins become soft when heated, may be shaped or molded while in a heated semifluid state, and become rigid when cooled. Thermoset resins, on the other hand, are usually liquids or low-melting-point solids in their initial form. When used to produce finished goods, these thermosetting resins are "cured" by the use of a catalyst, heat, or a combination of the two. Once cured, solid thermoset resins cannot be converted back to their original liquid form.

ACT Timbers are manufactured from recycled thermoplastic resins that are produced by utilizing a mixture of post-consumer and post-commercial plastics. The primary thermoplastic used in ACT is a high-density polyethylene (HDPE). Thermoplastic resins are inherently tough and provide excellent impact resistance. Plus, these versatile resins are naturally impervious to harsh chemicals and environmental conditions. An additional benefit is that composites manufactured from thermoplastic resins are readily recyclable so future replacement does not mean an addition to a landfill. HDPE is a commonly recycled good.



The primary function of fibers or reinforcements is to carry load along the length of the fiber to provide strength and stiffness in the direction of the fiber. While there are many different types of reinforcing agents possible, ACT Timbers utilizes fiberglass filaments evenly dispersed in the three-dimensional resin matrix. Glass is the composites industry workhorse because of its high mechanical properties, low susceptibility to moisture and low cost.


ACT Timber Additives

Additives are a key ingredient to any resin. ACT Timbers use additives that specifically enhance the performance of timber, extend its durability and increase its aesthetic appeal:

  • UV stabilizers prevent crazing (development of ultra-fine cracks), embrittlement and disintegration due to UV absorption
  • Colorants are additives that are mixed into the resin to provide uniform color throughout the timber. ACT is available in 14 standard colors. Special colors may be formulated


The Next Step

Unique to the composites industry is the ability to create a product from many different manufacturing processes. There are a wide variety of processes available to the composites manufacturer to produce cost-efficient products. ACT Timbers use an extrusion process for its products. To learn more about how a composite timber is manufactured, click here.



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  • Murphy, John. Reinforced Plastics Handbook. Oxford, England: Elsevier Science,1998. 
  • Richardson, Terry. Composites: A Design Guide. New York: Industrial Press, 1987. 
  • Rosato, Dominick V. Designing with Reinforced Plastics. Cincinnati, OH: Hanser/Gardner, 1997