Plastics : Recycled plastics versus virgin plastics

Recycled plastics and virgin plastics behave differently because recycling changes the material’s molecular structure and purity. 

Heat, moisture, and processing conditions also affect the quality of recycled plastic.

1. Effect of the heat:

1.1- Virgin plastic:

• Has long, uniform polymer chains
• Withstands processing heat within its designed range
• Melts and re-solidifies with minimal damage (if not overheated)

1.2- Recycled plastic:

• Has already been melted and reprocessed before
• Its polymer chains (plastics are made of long chains of polymers) are often shorter and weaker due to previous heating
• Mechanical stress during shredding and extrusion weakens polymer chains

When heated again:

• It degrades faster
• Becomes more brittle
• May lose strength and flexibility

Each heating cycle slightly weakens the plastic structure more.

2. Effect of moisture (water absorption):

2.1- Virgin plastic:

Many plastics (like polyethylene, polypropylene) absorb very little water, and their properties remain stable.

2.2- Recycled plastic:

More likely to contain:

• Impurities
• Mixed polymer types
• Micro-cracks in structure

Recycled plastic allows slightly more moisture interaction (depending on polymer type)

Moisture can cause:

• Hydrolysis (in plastics like PET) 
• Weakening of polymer chains
• Reduced strength during later heating (steam formation, bubbles)

Hydrolysis: is the breaking down of a substance using water.

Moisture can ruin recycled plastic more easily than virgin plastic.

3. Effect of contamination:

3.1- Virgin plastic:

Pure resin with controlled additives

3.2- Recycled plastic:

May contain:

• mixed plastic types (PET, PVC. ...)
• Dyes, labels, adhesives
• Dirt or organic residue

Results in:

• Uneven melting
• Weak spots in final product
• Lower mechanical strength
• Discoloration

Virgin plastic is a clean uniform material,

Recycled plastic is made up of mixed ingredients.

4. Effect of mechanical processing (shredding, remelting):

4.1- Virgin plastic:

Processed once into final shape

4.2- Recycled plastic:

• Undergoes shredding
• Undergoes re-melting
• Undergoes re-extrusion
• Repeated heating
• Repeated cooling

These repeated thermal and mechanical processes gradually shorten the polymer chains, reducing the plastic's strength and quality.

4.3- Each cycle causes:

• Chain scission (breaking of long molecules)
• Reduced elasticity
• Lower impact resistance

Repeated processing gradually produces a weaker material.

5. Overall property changes:

Compared to virgin plastic, recycled plastic usually has:

• Lower tensile strength (ability to resist pulling forces without breaking)
• Lower impact resistance
• Reduced heat resistance
• Higher brittleness in many cases
• More variability in quality

6. Summary:

6.1- Heat: 

• Causes recycled plastic to degrade faster and lose strength.

6.2- Moisture: 

• Speeds up chemical breakdown and can cause defects.

6.3- Contamination: 

• Reduces the quality, consistency, and strength of recycled plastic.

6.4- Repeated reprocessing: 

• Breaks down polymer chains, making the plastic weaker.