The yellowing of recycled PET (polyethylene terephthalate) bottle flakes remains a persistent challenge in the plastic recycling industry. This phenomenon not only affects the appearance of recycled products but also has a significant impact on their usability, quality, and market value—especially in high-end applications like food-grade packaging, automotive parts, and textiles.
In essence, yellowing reflects a deeper contradiction between design, production, and sustainability goals. To promote a circular economy, it is essential to consider recyclability during the initial design phase of plastic packaging and ensure compatibility throughout the product’s life cycle. Below, we explore the primary causes of yellowing in PET bottle flakes, followed by its industrial consequences and suggestions for mitigation.
PET is one of the most recycled plastic resins worldwide, valued for its strength, transparency, and barrier properties. However, the yellow discoloration of recycled PET flakes reduces consumer acceptance and limits their application in premium segments. When flakes become yellow or brown, manufacturers face additional processing costs and material limitations, resulting in downgraded applications, often in non-critical sectors such as strapping bands or dark-colored containers.
Understanding the reasons behind yellowing is the first step toward improving recycling outcomes and enhancing the commercial value of recycled PET.
Bottles exposed to prolonged UV radiation, oxygen, or high temperatures (e.g., during storage outdoors or in warm warehouses) undergo polymer chain degradation. This aging process weakens the PET molecular structure and initiates oxidation reactions that lead to yellowing.
Even before reprocessing, aged bottles already show signs of color change and physical degradation. Upon re-melting, these defects intensify, resulting in flakes with a deep yellow hue and lower mechanical strength.
Implementing a controlled pre-sorting system to remove visibly aged or discolored bottles, and using antioxidant additives during reprocessing can help mitigate this issue. Improved logistics and indoor storage of PET waste also play a preventive role.
Fluorescent whitening agents and optical brighteners are sometimes introduced during the hot washing phase to enhance the perceived cleanliness of PET flakes. While this improves visual appearance initially, these agents may degrade under heat and pressure during reprocessing, forming yellow or brown chromophores.
Although invisible at first, these additives chemically alter at extrusion temperatures (around 260°C), resulting in yellow-tinted flakes or final pellets.
Avoid or minimize the use of unnecessary chemicals in the washing process. Use pH-stable, non-reactive cleaning agents that are compatible with high-temperature environments. Quality control protocols should ensure any chemical additives used in recycling are approved for thermal stability.
To enhance shelf life, some PET bottles contain additional barrier layers or additives such as UV inhibitors, oxygen scavengers, or multilayer structures. While effective in protecting product contents, these components are not always thermally stable and can react unfavorably during reprocessing.
Color Uniformity: Barrier additives may oxidize and change color under heat, leading to yellow or brown discoloration in the resulting flakes. This is especially problematic for transparent or light-colored end products.
Increased Processing Costs: To mask yellowing, color masterbatches must be added, increasing the cost and complexity of extrusion processes.
Mechanical Degradation: Barrier materials can catalyze chain scission, leading to a decline in physical properties such as impact strength, flexibility, and durability.
Market Downgrading: Discolored and degraded recycled PET cannot meet the quality demands of premium applications. As a result, these materials are sold into low-value markets, limiting profitability.
Upstream design improvement is vital—brands and manufacturers should consider using mono-material packaging with minimal or removable barriers. In recycling facilities, near-infrared (NIR) sorting and density separation technologies can help detect and segregate barrier bottles before processing.
Final Thoughts: A Multi-Stakeholder Approach Is Essential
The yellowing of PET bottle flakes is a technical challenge, but one that can be mitigated through collaborative efforts across the value chain—from packaging design to recycling plant operation. Stakeholders including brand owners, packaging designers, recyclers, and policymakers must work together to improve the recyclability of PET bottles.
Addressing the root causes of yellowing in recycled PET flakes is critical for promoting sustainable packaging solutions and achieving circular economy targets. With growing global demand for recycled content in packaging, textiles, and automotive industries, ensuring the optical and mechanical quality of recycled PET is more important than ever.
By optimizing design, improving processing conditions, and enhancing cooperation between stakeholders, the industry can overcome these challenges and unlock the full potential of high-quality recycled PET.
RETECH is a creative company focusing on the technology R&D of Plastic recycling machinery solutions.
