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All Plastic Lumber Is Not Alike

Purified HDPE lumber, such as the Durawood product from Eaglebrook Products, is mode solely from fractional melt flow HDPE derived from milk bottles. These bottles ore separated from all other plastic types, ground into flake und processed in a wash-dry system that removes impurities such as food residual, paper labels, und adhesives.

Today many people are beginning to recognize the value of plastic lumber as a durable and effective component of construction and industrial use. Not only are these products gaining acceptance due to their performance, but they are becoming more popular for other important reasons, such as reuse of fossil fuels, landfill avoidance, and timber or ecosystem preservation. However, much confusion exists about plastic lumber, primarily because all plastic lumber is not alike.

Currently there are several categories of dimensional plastic profiles described generally as "recycled plastic lumber." Under this broad definition are products with very different compositions, quality, consistency, properties and overall performance. The high quality plastic lumber products do not absorb moisture, and are very durable as they will not rot, splinter, or peel. These products are solidly colored with pigment systems, do not require coatings, and have consistent composition and cell structure, which enables them to have specifiable mechanical properties and predictable resistance to elements.

One might become bleary-eyed from sorting through the alphabet soup of plastic materials. However, things become much clearer when you focus on some of the essential elements in the production of plastic lumber. Remember, the optimal plastic for landscape use is the "Purified HDPE Lumber."

The two important aspects of the purified HDPE processing - (1) sortation and use of only one type of plastic, and (2) the purification of this material - are the cornerstones to creating a quality plastic lumber product.

The plastic resin is combined with foaming agents to create an extruded rigid board with desirable mechanical and surface finish properties. Additionally, prime pigment systems, ultraviolet stabilizers and specific process additives are compounded into the product to provide color, polymer stability and consistent cell structure. The cleaned resin and high performance additives enable the production of reproducible lumber sections that exhibit specific physical property behavior, tight dimensional tolerance and controlled density.

Taking shortcuts, compromising on additive quality, eliminating processes, or simply using mixed plastics can create variability in product performance and can ultimately lead to application failures.

Variation from this method compromises product performance in several ways:


The cheapest form of recycled plastic lumber is commingled product, made with several types of plastic. Because these products are made by grinding mixed plastic, melting it, and compression molding the melt, the end product is not a consistent phase, but a part with dissimilar components compressed together.

At least three things are dangerous about this approach: (1) the resulting product has great variability in physical properties, and appropriate use cannot be predicted in many cases, (2) different plastics have resistance, or lack of, to materials; for example, PS is soluble in gasoline, but PE is not (in fact, gas cans are made with HDPE), and (3) durability of these products is questionable at best, as polymer stabilization cannot be accurately accomplished with mixed polymers because additives do not disperse consistently through the different plastics.

Assessing composition is not only important when evaluating lumber made completely from plastic, but when determining the suitability of using composite materials. Composite lumber products are typically a mix of approximately 50% PE (primarily LDPE) and 50?70 sawdust or other secondary fiber. These materials have a greater coefficient of friction. However, according to report from the 1992 Materials Engineering Congress of ASCE, due to the fiber component of the composite, these materials have been shown to absorb up to 8% moisture in a 24 hour period, making them unsuitable or suspect for certain applications. Additionally, these composite materials have shown poor low temperature impact strength, and can fall in certain temperature sensitive applications.

Consistent cell structure is critical for creating predictable mechanical properties. For example, Eaglebrook's plastic lumber made from washed HDPE maintains a uniform range of density and cell structure. The Durawood product has been shown to have less than 1?/o total reflectance change after exposure to over 1 million Langleys of radiant energy, or about three years of natural weathering in most regions of the United States.

Additionally, the insect resistance of composite lumber is questionable. According to a University of Florida paper published in Plastics News (June 15, 1992), tests used to screen different plastic lumber and composite lumber products for termite resistance have shown that termites consume part of the fiber portion of the composites, but do not have an appetite for the straight plastic. Also, it is questionable as to whether these products need to be coated, as a number of exterior installations of composites have exhibited a wide range of color development and discoloration. The use of these compositions is relatively new, versus the use of PE products in exterior application, and more time will allow for the proper study of durability.


Bypassing the purification step is another way to reduce the cost of recycled plastic lumber, but may lead to defects such as voids in the cell structure and concentrations of impurities that can lead to material fracture.

An assessment of non-purified lumber products determined that cavities of 5.0mm to 8.0mm have been common, with occasional cavity diameters exceeding 10.0mm. These cavities cause some loss of rigidity, as well as unpredictable tensile and compressive strengths. A report from the 1993 Society of Plastic Engineers ANTEC Proceedings showed that eliminating these regions would improve material integrity and reduce fracturing. In compression tests, it has been determined that contaminants such as oils may be affecting bonding in the plastic matrix, and creating sites for fracturing to occur.

In the case of composites, impurities of metal are often found, as the wood fiber component is derived from ground pallets in many cases. This could lead to material fracturing, and could create a safety hazard when fabricating.

Cell Structure:

As noted previously, a consistent cell structure is important for maintaining density and creating predictable tensile and compressive properties. Additionally, a tight cell structure is important when mechanical fasteners are used for attachment, as voids or inconsistencies can affect the pull out resistance of the fasteners. Also, compressive tests have shown that phase changes can lead to the separation of the exterior wall of the lumber and porous foam cores when subjected to severe load.


Several types of pigments may be used in plastic lumber, and as with most things, you get what you pay for. For exterior applications, it is important to incorporate specific types of pigments that have chemical bonds that are stable under significant radiant energy load. Additionally, special W grades of some pigments are available with alumina or silica coating to decrease the pigment's photoreactive surface characteristics.

It is important to note that pigment systems are used in the form of concentrates when manufacturing plastic lumber. These concentrates come in carriers that are designed to disperse in specific polymers at targeted temperature ranges. Here again, the use of dissimilar plastic types can have a detrimental effect on quality, in this case with color match and retention.


All plastics are prone to aging by weathering and can lose their visual and mechanical properties. In addition, recycled materials may have suffered some degradation in their first use, and may have been stabilized for an interior, household use, rather than an exterior building product application. For these reasons, it is essential that plastic lumber products are compounded with appropriate stabilizers.

Effective stabilizers are relatively expensive as compared to many raw material components found in some plastic lumber products. In fact, in the best plastic lumber, additives can account for approximately half of the total raw material cost.

Also, specific additives are designed for use with specific polymers. For example, hinder amine light stabilizers are exceptionally effective W stabilizers for PE products. Hindered amines work like antioxidants, trapping the free radicals generated by photodegradation; the best plastic lumber products use these compounds for optimum stability.

With the great variety of plastic lumber materials being manufactured today and confusing nomenclature of plastics, it is easy to become confused about which of these materials may be suitable for an application. It is important to remember that only specific composition can yield specific material properties, and that the removal of impurities is a critical step in providing consistency. Also, the use of appropriate pigments and stabilizer additives is essential for durability and effective service life of plastic lumber products.

Confusing as the subject may be, a common rule of thumb probably applies to determining whether a particular plastic lumber product is appropriate for an application.

You get what you pay for.


To understand plastic lumber better, It Is essential to decipher the language of plastics. Common plastics found in packaging and industrial waste that are use to make lumber include:

Polyethylene: also referred to as PE, it comes in the form of high density PE (HDPE) or low density PE (LDPE). Examples of HDPE are milk and detergent bottles: LDPE may be found in the form of bags, film or wrapping.

Polyethylene Terephalate: or PET, is commonly found in one and two liter soda bottles and many other household containers.

Polypropylene: also referred to as PP, is often found in bottles that have special filling requirements, such as ketchup containers, or plastic parts that require considerable memory.

Polyvinylchloride: also known as PVC. Examples of PVC packing include cooking oil bottles, some mineral water bottles and blister packs.

Polystyrene: also PS, is commonly found in expanded foam coffee cups and serviceware such as knives, forks and spoons.

In some cases, all of these plastics are found at the same time in plastic lumber parts. However, the high quality lumber products are made with specific type of plastic. At present, several types of plastic lumber, in order of descending value, are:

  1. Structurally reinforced (with metal) purified fractional melt HDPE
  2. Purified fractional melt HDPE
  3. Non-purified fractional melt HDPE
  4. Multiple melt flow PE (HDPE & LDPE; not necessarily purified)
  5. Composite; PE with secondary fiber from sawdust or other
  6. Commingled; several types of plastic, composition is variable
** Note: fractional melt HDPE means that the melt flow index (MFI) of this plastic is between, 0 and 1; dairy bottles are made with 0.5 - 0.7 MFI material. Examples of other HDPE packing waste MFI include butter tub (5 MFI) and base cup from PET soda bottles (30 MFI).

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October 17, 2019, 9:41 am PDT

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