At the end of their useful life, most plastic products can be recycled into post-consumer material. Mechanical recycling is one commonly used approach to process the recovered plastics where a series of sorting, washing, shredding and extrusion steps produce Post Consumer Recycled (PCR) flakes or pellets that can be converted into new products. Alternative approaches include chemical or advanced recycling in which techniques like gasification, pyrolysis or methanolysis are applied to break down the recovered material into raw material feedstock, providing a circular pathway to virgin-equivalent plastics.
Regardless of the approach used to process PCR, recycled resins are inherently complex and challenging compared to processing virgin materials. Analytical technologies help polymer researchers and process engineers identify the implications of feedstock variability and contamination on process conditions and product performance, enabling them to reformulate products to mitigate any adverse effects.
Instruments and Test Parameters
Material strength with PCR
- Young’s modulus, yield strength, ultimate strength, elongation at break
- Fatigue and durability, S-N curves
- Strength vs. temperature
Final assembly strength with PCR
- Flexural, bending or crush failure points
- Fatigue and durability, S-N curves
- Strength vs temperature
Understanding the temperatures at which polymers soften and melt is a fundamental material property relevant to polymer processing. As one of the first steps in extrusion, injection molding and film blow molding processes, resin pellets are routinely heated past the melting point; for thermoforming and blow molding, the resin is heated above its glass transition temperature to soften it, but without completely melting it. This transformation from a solid resin pellet (lower energy state) to a softened or completely melted pellet (higher energy state) requires the input of energy and can be measured using Differential Scanning Calorimetry (DSC).
In a DSC test, the heat flow of the sample is monitored as the temperature is increased at a constant rate. Thermal transitions such as melting and glass transition show up as endothermic events, where the material absorbs heat as it moves into the higher energy state. The results also reveal information about the polymer morphology, with clear differences between amorphous and semi-crystalline states. During a DSC test’s first heat cycle, amorphous materials display a broad glass transition without melting, while semi-crystalline polymers have a sharp and well-defined melting peak. Since the melting and glass transition temperatures are unique to each polymer, this information can be used to quickly evaluate the quality of the incoming feedstock prior to processing.
Answer the following questions with results from your DSC:
- Feedstock evaluation: Is this a neat polymer, or is it a blend? Can vendor A’s resin be replaced with lower cost resin from vendor B?
- Processing: How much thermal energy is needed to completely melt the resin pellets?
- After processing: Is there a thermal history after processing vs. as-received? (1st vs. 2nd heat)
- End-of-life recycling: Does this batch of PCR (post-consumer resin) have significant contamination from other polymers?
Common thermoplastic processing techniques, like extrusion, injection molding and blow molding, require the resin to be heated above the melting point for easy processing. However, it is important to carefully control the processing temperatures to avoid resin degradation that can occur at elevated temperatures. For polymers, the onset of degradation can be identified as the temperature at which significant weight loss (typically >5%) starts to take place and can measured using a Thermogravimetric Analyzer (TGA).
During thermal analysis of polymers, TGA tests are routinely performed before DSC testing since the TGA results help establish the upper temperature limits for subsequent testing. Apart from identifying the degradation window for processing, TGA results also quantitatively reveal the composition of the major ingredients in the resin, such as the amount of base polymer, plasticizer, and filler present. The off-gas generated during a TGA experiment can be further analyzed to gain insights into the chemical identity of the decomposition products. This type of Evolved Gas Analysis (EGA) is especially powerful since it combines real-time TGA data with results from FTIR and GC-MS.
Answer the following questions with results from your TGA:
- Feedstock evaluation: At what T does this resin decompose? What is the decomposition profile?
- Processing: Are there volatile materials in this batch of resin? Will there be off-gassing after processing?
- Failure Analysis: Is there a difference in the filler content or the decomposition profiles of the good vs. bad parts?
- End-of-life recycling: During pyrolysis, at what temperature does the maximum weight loss occur? What contaminants are presents in this batch of recycled resin?
Related Application Notes:
Stabilizers and other additives are often added to resins to prevent degradation from environmental effects encountered during processing and end-use conditions. These additives include antioxidants, oxygen scavengers, heat and UV light stabilizers, or flame retardants, to ensure the polymer’s intended properties are maintained during processing and the product’s lifetime. Stabilizers are inherently sacrificial and are gradually consumed when exposed to high temperature or UV light; once the stabilizer is completely exhausted, the polymer properties start to degrade rapidly.
The performance of stabilizers can be evaluated through Oxidative Induction Time (OIT) analysis on the DSC. In this isothermal test, the purge gas in the DSC is switched from Nitrogen to Oxygen, providing an environment where the stabilizer is consumed. At the onset of polymer degradation, the heat flow signal starts to increase and the time is noted as OIT.
Temperature ramps on the DSC can also be used to measure the Oxidative Onset Time (OOT), a related measure of polymer stability. Both OIT and OOT tests can also be performed using a high-pressure DSC, which reduces test time by accelerating stabilizer consumption.
Answer the following questions with OIT & OOT results from your DSC:
- Feedstock evaluation: Can this resin be processed as-is? Are antioxidants needed for additional stability?
- Failure Analysis: Did this part antioxidant at sufficient levels suitable for the end-use conditions?
- End-of-Life recycling: How much antioxidant is needed to stabilize and process this batch of PCR?
Related Application Notes:
The viscosity and viscoelastic behavior of polymer melts plays an important role when processing polymers using injection molding and extrusion techniques. At a basic level, the viscosity represents the material’s internal resistance to flow – resins with higher viscosity flow slower and take more time to fill the mold, increasing the cycle time and introducing the possibility of defects like short shots. As a result, it is critical to measure and carefully control the viscosity of the resin to ensure process stability and eliminate batch-to-batch variations.
For polymer melts, the viscosity profile depends on the rate of deformation, also known as the shear rate. At the high shear rates encountered in the extrusion and injection molding processes, the viscosity curve displays a shear-thinning behavior in the power law region – as the shear rate increases, the viscosity decreases. This shear rate dependence is influenced by the polymer’s molecular weight distribution and degree of branching.
While high shear rates are relevant to processing conditions, viscosity measurements at low shear rates are essential for revealing the resin’s molecular structure. The zero shear viscosity in the first Newtonian plateau directly correlates with resin’s molecular weight, and can be measured using rotational rheometers.
How is viscoelasticity related to Molecular Weight/ Molecular Weight distribution?
Oscillatory tests on rotational rheometers provide valuable insights into a polymer’s viscoelastic properties by probing the polymer’s structure through small deformations over a range of time scales. The results provide the polymer’s Storage Modulus (G’), Loss Modulus (G”) and complex viscosity (η*) as a function of the oscillation frequency and can be used to better understand the dynamics of polymer relaxation. These parameters are strongly influenced by resin’s molecular weight, molecular weight distribution, and long chain branching structure. Compared to melt flow indexers or capillary rheology, the viscoelastic profile from rotational rheology testing is particularly sensitive to the presence of high molecular weight contamination that can cause processing issues.
Answer the following questions with viscosity and viscoelastic measurements from your rotational rheometer:
- Feedstock evaluation: How is the viscoelastic profile affected by batch-to-batch changes in the resin’s Molecular Weight/ Molecular Weight Distribution?
- Processability: Does the resin have the right viscosity at all shear rates relevant to the manufacturing process?
- End-of-life: End-of-life recycling: How do contamination and MW variation in the recycled resin impact processing?