Recycled Polymer Quality Control with Thermal Analysis

Plastics manufacturers are faced with the ongoing transition toward a circular plastics economy. Countries such as the United Kingdom and Spain are charging taxes on non-recycled plastics, with more regulations planned in countries around the world.^1,2 While these laws encourage the use of post-consumer recycled (PCR) resins, they also create new challenges for manufacturers.

Unlike virgin polymers, recycled materials can exhibit significant variability in composition and contamination, which may affect their processing and end-use properties. Assuming that recycled polymers possess the same properties as their raw counterparts creates a landmine for inconsistencies and failed products.

Variability in recycled polymers can be quantified with thermal analysis. Methods such as DSC, TGA, and SDT provide critical insight into the purity, composition, quality, stability, and processability of recycled plastics, helping manufacturers confidently produce reliable and safe products while adhering to new sustainability requirements. The following examples illustrate the benefits of thermal analysis in quality control for recycled plastics.

Before designing a manufacturing process or product prototype, manufacturers must first determine the primary components and additives in their incoming PCR. Since each polymer has a unique melting point, analyzing melting temperature with differential scanning calorimetry (DSC) is a straightforward method for narrowing down the possible material components. In our analysis of unidentified recycled polyolefins, we compared reference curves for two common polymers, low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), with our three unknown recycled polyolefin samples.³

The melting point of LDPE is 110.7 °C. LLDPE has a higher melting point of 121.4 °C and shows a wider main melting point peak with a characteristic shoulder. Our recycled polyolefin samples’ curves showed melting point peaks between 114.2 °C – 126.5 °C as well as a shoulder, indicating that the main component is LLDPE.

Our compositional analysis was furthered by thermogravimetric analysis (TGA), which measures weight change as a function of temperature to reveal decomposition onset temperature and filler content. We used TGA to determine additive content in each recycled polyolefin. Sample P1 had the highest amount of additives, which explains its shifted DSC curve and slightly higher melting point.

Compositional and thermal analysis from DSC and TGA are vital for determining the main constituents and additives in recycled polymers. Manufacturers need this information to design compatible manufacturing methods that result in quality products without unknown behaviors emerging later in fabrication or use.

After determining the constituents and additives of PCR, DSC and TGA also help manufacturers assess recycled materials’ thermal stability and degradation to predict lifetime and end-use performance.

DSC reveals differences in thermal transitions when incorporating significant levels of recycled content.⁴ For example, researchers used Waters TA Instruments DSC 250 to test whether their additives improve recycled polymers’ final properties. Their DSC analysis confirmed the additive improved “thermal stability, crystallization rate, and phase compatibility of rPA6/rPP blends, promoting a more homogeneous structure with improved long-term performance.”⁵

TGA also supports formulation selection and quality testing by showing the changes in stability and degradation temperature between samples. We tested samples with varying amounts of recycled content to determine recycled amount’s effect on degradation.⁴

Using the temperature where the maximum mass loss occurs as a measure of thermal stability (figure 2 above), we see a minor correlation between added recycled material and stability at low to medium recycled material contents, with a sharp decline at 100% recycled material. This result indicates that a significant amount of recycled content can be added with only a minor effect on stability. TGA is a useful tool in determining recycled polymers’ thermal stability and lifetime performance.⁶

Finally, thermal analysis helps determine optimal processing windows and confirm that recycled polymers are suitable substitutes for virgin materials.

TGA degradation temperatures give manufacturers an absolute upper limit on possible processing temperatures, as well as showing that the material will retain its properties in end-use conditions up to that limit.³

TGA of three recycled polyolefin samples showed degradation onset between 437.0 °C – 468.5 °C, showing that all the resins require processing below 400 °C.

For higher temperature testing that combines heat flow and degradation analysis, simultaneous DSC and TGA (SDT) is a useful method for determining recycled polymer quality in a single experiment. For example, researchers used Waters TA Instruments SDT 650 to compare repolymerized PET to virgin material.⁷ In their DSC scan, the repolymerized PET showed melting temperatures and crystallinity values in agreement with virgin PET. TGA results showed the absence of other degradation stages, confirming the sample had no significant impurities. Both tests show the recycled PET can be a suitable substitute for virgin PET without major inconsistencies or quality concerns.

The studies above demonstrate how DSC, TGA, and SDT support recycled polymer quality control, from material selection and process design to final sample analysis. The table below summarizes what these techniques measure and their benefits:

See more experiments and analysis in our Biopolymer eBook, or learn about the different thermal analysis techniques and how to choose the right one for your lab in the Thermal Analysis Selection Guide.