Understanding Plastics: Science, Material Reality, and Local Innovation
Plastics are everywhere in modern life, yet they are often misunderstood. To make informed decisions as consumers and community members, we don't need to fear or villainize plastic—we need to understand the material science behind how it is made, how it behaves, and how local communities are finding innovative ways to re-enter it into the circular economy.
Check out the presentation recapping this webpage from our Materials Education Council board liaison Adam Farmer ➡️
How Plastic Is Made: The Molecular Lifecycle
At its core, plastic is composed of polymers, long chains of repeating chemical units. Producing plastic from raw or virgin natural resources involves a multi-step industrial manufacturing process:
- Extract: Crude oil and natural gas are extracted from the ground. In the United States, most polyethylene plastic starts as ethane gas.
- Separate: Distillation columns at refineries and gas processing plants split raw hydrocarbons into lighter fractions like naphtha, ethane, and propane.
- Crack: High-heat steam crackers break those molecular fractions down into small reactive molecules called monomers (such as ethylene and propylene).
- Polymerize: Chemical reactors link thousands of monomers together into long polymer chains, forming tiny solid resin pellets.
- Convert: Manufacturers melt these resin pellets in molders or extruders to shape them into final products, such as bottles, pipes, container tubs, and flexible films.
Where Recycling Re-Enters the Loop
- Mechanical Recycling melts, filters, and re-pelletizes collected plastic products directly at the polymer stage (Step 4).
- Chemical Recycling breaks plastic waste back down to its raw monomers or chemical feedstocks (Step 3).
Polymers 101: Decoding the Resin Identification Codes
‼️ A Resin Code Does Not Automatically Mean Recyclable ‼️
The resin stamp only identifies what material the item is made from, not whether your community can process it. A plastic item is only recycled if four conditions align:
- Collection: A local curbside program or drop-off facility accepts it.
- Sorting: Sorting machinery at Material Recovery Facilities (MRFs) can physically catch its shape and size.
- Condition: The item is clean and free of contamination.
- Market: End-market buyers exist to purchase that specific sorted resin
You have likely seen the small numbers (1 through 7) inside solid triangles on plastic containers. Standardized by ASTM D7611, these Resin Identification Codes identify the specific polymer type used to make the item.
Thermoplastics vs. Thermosets
- Thermoplastics (Codes 1–6) can be repeatedly softened with heat and hardened by cooling, similar to candle wax. Mechanical recycling relies entirely on this property.
- Thermosets (such as tires, epoxy, or fiberglass) undergo a permanent chemical reaction when formed—like baking a cake. They cannot be remelted; reheating them burns the material.
PET
Polymer Name: Polyethylene terephthalate
Common Household Uses: Water & soda bottles, clamshell containers
Molecular Structure Characteristics: Strong, clear, easily recyclable
HDPE
Polymer Name: High-density polyethylene
Common Household Uses: Milk jugs, detergent bottles, rigid pipes
Molecular Structure Characteristics: Straight polymer chains that pack tightly (rigid)
PVC
Polymer Name: Polyvinyl chloride
Common Household Uses: Plumbing pipes, siding, blister packaging
Molecular Structure Characteristics: Contains chlorine atoms
LDPE
Polymer Name: Low-density polyethylene
Common Household Uses: Grocery bags, squeeze bottles, plastic film
Molecular Structure Characteristics: Branched polymer chains that pack loosely (flexible)
PP
Polymer Name: Polypropylene
Common Household Uses: Yogurt tubs, bottle caps, pill bottles
Molecular Structure Characteristics: Heat-resistant, durable
PS
Polymer Name: Polystyrene
Common Household Uses: Foam cups, takeout containers, plastic cutlery
Molecular Structure Characteristics: Rigid or expanded foam structure
OTHER
Polymer Name: Other
Common Household Uses: Polycarbonate, compostable PLA, multi-layer pouches
Molecular Structure Characteristics: Varies by resin blend
The Physical Realities of Recycled Plastic
Recycling plastic involves physical and chemical trade-offs that dictate how materials can be reused:
- Density Separation (Float/Sink): Recycling facilities separate mixed plastic resins using water density (1.00 g/cm³).
- Light polymers like Polypropylene (0.90 g/cm³), LDPE (0.92 g/cm³), and HDPE (0.95 g/cm³) float to the top.
- Heavier polymers like Polystyrene (1.05 g/cm³), PET (1.38 g/cm³), and PVC (1.40 g/cm³) sink to the bottom.
- Degradation Across Melt Cycles: Every time a thermoplastic is remelted, its polymer chain length shortens. This reduces structural strength, requiring processors to blend recycled resin with virgin material or additives.
- Color Limitations: Dyes cannot be removed during mechanical melting. Mixed-color plastic scraps melt into gray or black resin. Consequently, clear and natural plastics carry the highest market value.
- Food-Grade Approval: Recycled resins require specialized deep-cleaning processes and regulatory review before being re-approved for direct food contact
Innovative Community Programs & End Markets in Colorado
While statewide waste diversion in Colorado currently sits at 15.7%, local communities across the state are pioneering small-scale and industrial innovations to collect and process hard-to-recycle materials:
- Recycled Plastic Roads in Pueblo County: Hard-to-recycle flexible film and grocery bags cause equipment tangles at standard sorting facilities. Through local partnership with Driven Plastics in Pueblo, 13.5 tons of recycled polyethylene (~2 million grocery bags) were blended into an asphalt mixture to pave 1.75 miles of Siloam Road, keeping plastic out of landfills while creating durable road infrastructure.
- Community-Based Facility Mapping: Infrastructure varies widely between the Front Range and mountain or rural communities. Colorado tracks an interactive network of 54 regional recycling hubs and processors to help residents locate specialized drop-off points for items that cannot go into curbside bins.
- Precious Plastics: Helps people meet, share and collaborate to tackle plastic waste. Check out their map of local spots diverting plastic.
- Breck Create is a Precious Plastic partner where community members and visitors to Breckenridge can transform plastic waste into art.