Degradation of high-density polyethylene (HDPE) can be a significant problem for recyclers. Although HDPE is one of the most commonly recycled plastics, and can be separated using a range of methods, recycling of this polymer is not completely efficient in practice. Following are ways to adjust processing to address performance and quality degradation and meet the strong demand for this material, which had a global market value of $68 billion USD in 20191
Heat and shear from the HDPE recycling process can exacerbate any existing wear and tear from previous usage. This degradation may change the resin structure by introducing gels and specks, kick off side reactions that alter the color of the polymer, or cause variations in melt flow—all of which can negatively impact the performance and quality of the recyclate.
Contaminants from the recycle stream such as acidic species can also initiate degradation, making them “pro-degradants.” They can be present due to catalyst residues from polymer production, causing side reactions that create acids, or they can be introduced from the recycling stream. Acids degrade the structure of the HDPE polymer chains and diminish the effectiveness of antioxidants unless an antacid is also present.
There is strong global demand for post-consumer recycled (PCR) content, driven by sustainability commitments from major brands and new regulations encouraging the reuse of plastics to support circularity. In particular, the market for natural-color recycled HDPE has expanded in 2020,2 due primarily to growing demand from consumer-packaged goods (CPG) companies.
To supply the market, it is important for recyclers to maintain the properties of post-consumer HDPE, including color, as it goes through the recycling process. One proven strategy is to add stabilizers to the PCR content before it is subjected to heat and shear stresses.
Chemical stabilization of HDPE PCR content can:
- Preserve the physical properties of the virgin resin, such as impact strength
- Maintain the color of the original resin and prevent yellowing
- Ensure homogeneity of the resin structure by preventing the formation of gels and other artifacts
- Retain melt flow characteristics needed for efficient processing
How Stabilizers Work with HDPE
Resin producers tightly control the structure of their virgin materials to provide excellent consistency in physical and visual properties. They often add a base level of stabilizer blends to help prevent these properties from being affected by harsh process conditions during conversion to the final product.
These stabilizer blends are typically comprised of several components, including:
- An antacid (acid scavenger), which neutralizes catalyst residuals left over from polymer production. These residuals can undergo side reactions with contaminants and some antioxidants, negatively affecting color and physical properties. Common antacids include metallic soaps, mineral agents and metal oxides.
- A primary antioxidant (radical scavenger), which is active in the processed resin and functions during final product use. Antioxidants prevent auto-oxidation by capturing oxygen-centered free radicals that are created when polymer chains break under high processing temperatures. Free radicals propagate the degradation chain reaction. Primary antioxidants convert free radicals into more-stable, non-radical products. However, once these antioxidants are consumed as they protect the product from oxygen during use, the polymer begins to degrade. The most common primary antioxidants are phenolics, which include Irganox 1010, Irganox 1076, BHT and Vitamin E.
- A secondary antioxidant, which works during the melt phase. It scavenges organic hydroperoxides formed by the actions of primary antioxidants. Although hydroperoxides are less reactive than free radicals, they can initiate new reactions. Therefore, their removal requires more-reactive antioxidants such as phosphite esters. Secondary antioxidants also help to delay the formation of quinones, a color-changing biproduct that forms when primary antioxidants are consumed.
If the base level of stabilizer in HDPE is only sufficient for limited use (resin conversion) and has been depleted by the time the end product is collected for recycling, the PCR content will lack protection against degradation from harsh recycling conditions. This is why some recyclers add stabilizers to the melt when converting scrap to PCR content. They have found stabilization offers an attractive benefit-cost ratio. Recycled HDPE that retains its original properties, color and processability often can command higher prices.
Quantifying HDPE Stabilization
The effect of stabilizers on recycled HDPE can be determined by testing for oxidative induction time (OIT), melt flow and color retention.
Oxidative induction time is a measure of the resistance of a polymer to oxidative decomposition. It indicates how well parts made from the material can resist aging (such as cracking, crazing, weakening or failure) under exposure to environmental elements including heat, oxygen, light and radiation.
Testing for OIT characterizes the thermo-oxidative stability of polyolefins, particularly polyethylene (PE). It is a sensitive measure of the level of antioxidant additives within the polymer. The ASTM D3895 standard specifies a test method for OIT using differential scanning calorimetry (DSC). Briefly, it is an accelerated thermal aging test.
During this test, the resin undergoes controlled heating above its melting point in a nitrogen atmosphere. Once molten, the resin is exposed to pure oxygen, and a timer starts as the primary antioxidant is consumed over time. When the primary antioxidant is fully consumed, it is signified by a change in heat flow, and the induction time is determined. In general, longer OIT times suggest a higher level of stabilization. In practice, OIT is a good option for a fast test when heat aging is not practical. It’s best to use it in conjunction with heat aging, since different applications may have identical OIT results but different heat aging results. In recycling, OIT results near zero are often seen.
Some specifications require minimum OIT numbers. According to the American Association of State Highway and Transportation Officials (AASHTO), “Pipes manufactured from recycled PE materials (PCR or PIR content, or both) shall have an Oxidation Induction Time (OIT) of 20 minutes.3”
In one example, Baerlocher USA demonstrated through OIT testing how its Baeropol® T-Blend stabilizers could increase stability of HDPE PCR flake. After multiple extrusions of the material, the OIT levels stayed elevated above the initial reading of the un-stabilized flake, providing a longer lifetime for the polymer and extending its recyclability.
- Melt Flow
One argument against using PCR content is the potential impact on high-speed molding, blow molding and extrusion processes. Post-consumer recycled material with inconsistent melt flow can reduce throughput and lead to incomplete mold filling and high scrap rates.
Melt flow can be impacted by the stresses of the mechanical recycling process. During oxidative degradation, which can occur in recycling, polymer chains change in different, negative ways. Polymers like HDPE can crosslink, raising melt viscosity, which slows down melt flow. The addition of a stabilizer can help prevent this issue.
The standard for measuring melt flow rate is ASTM D1238. The resin is melted in a chute and extruded at a controlled rate by applying constant pressure via a weight and piston. The rate at which the resin is extruded is measured. Crosslinking occurs particularly during molten processes (re-pelletizing and processing.) The main issue is that when melt flow rates change, it makes it difficult for processors to maintain quality because they have to alter process conditions.
- Color Retention
Color stability of HDPE recycle streams is often considered more important in natural grades than in mixed streams, since users of natural recycled HDPE often want greater control. Control of color in mixed streams with different pigments is not practical. The natural grades, on the other hand, can potentially allow customers to produce more color options. They need natural grades to be consistent so they can consistently achieve desired color results. In natural grades, yellowing can occur due to degradative mechanisms that create color-change bodies. Incorporating acid scavengers in a stabilizer blend can prevent these color-change bodies.
Color changes can be measured using a colorimeter. Data is often reported using several values, like L* (a measure of lightness/darkness), a* (a measure of redness/greenness) and b* (a measure of yellowness/blueness).
Advantages of Stabilizer Blends
There are several benefits of working with preblended additive packages. Synergistic components are more efficient when they are blended prior to compounding. The option of low-dust/no-dust packages may be necessary for customers that cannot use powder blends for safety or cleanliness reasons. Blends can also cut down on user error and provide cost benefits, since they help simplify the supply chain compared to using individual additives.
Traditional stabilizer binary blends (primary plus secondary antioxidants) come in limited product forms.
Recycled HDPE can be used for a range of applications,4 including non-food bottles, pipe, pallets and decking. While HDPE is reasonably easy to recycle, its color and performance can be negatively affected by heat, shear and contamination during the recycling process. The addition of chemical stabilizers, particularly stabilizer blends, to the melt during recycling can help prevent degradation of the end-product, enhance consistency of melt flow and retain desirable natural color. Baerlocher’s T-Blends, which are preformulated for optimal synergies and performance, can improve melt stability, minimize yellowing and enhance final product performance to extend the useful life of the part.
Ian Query is a technical specialist in the Special Additives Division for Baerlocher USA.
- American Association of State Highway and Transportation Officials (AASHTO). (2018) AASHTO M 294 2018 Edition: Standard Specification for Corrugated Polyethylene Pipe, 300- to 1500-mm (12- to 60-in.) Diameter