Science fair: What happens to plastic, paper and ‘compostable’ packaging over eight weeks? (ages 10–16)

The question

• What actually happens to plastic, paper and "compostable" packaging over eight weeks in soil, in water and in sunlight?
• Ages 10–16. Time: 30 minutes to set up, 10 minutes a week for eight weeks, one session to write up. Made and funded by No Plastic Today. Licence: CC BY 4.0.

Why it matters

• You will read that plastic "takes 450 years to break down". There is no measured source for that number. A major review of the research found that a high-density polyethylene (HDPE) bottle at sea takes an estimated 58 years to lose half its material (its "half-life"), while PET and polystyrene degrade so slowly at the surface that the rate is effectively unmeasurable, and "degrade" here means fragment into smaller pieces, not disappear (Chamas and others, ACS Sustainable Chemistry and Engineering, 2020: doi.org/10.1021/acssuschemeng.9b06635). This experiment lets you see the beginning of that story with your own eyes, and test the claims on "biodegradable" packets.

Hypothesis

• Example: "Paper will lose the most mass in soil; the compostable bag will change only slightly; the plastic bag will not change at all in eight weeks in any condition."

Variables

• Independent variable: material (thin HDPE bag; PET bottle strip; polystyrene tray piece; paper bag; a bag labelled "compostable" or "biodegradable"; a "bioplastic" cup if you can get one) AND condition (buried in damp soil; submerged in water; on a sunny windowsill; a dark drawer as the control).
• Dependent variable: mass change (g), visible change (photos, a 0–5 damage scale you define), and flexibility (does it crack when folded?).
• Control variables: identical 5 cm x 5 cm squares; same starting mass recorded; same soil and water; same weekly handling.

Materials

• Six materials, four conditions, two repeats = 48 squares (cut them down to three materials if you need to).
• Scale accurate to 0.01 g; ruler; scissors; permanent marker to label squares.
• Four trays of garden soil kept damp; four jars of tap water; a sunny windowsill; a drawer.
• Camera and a photo log with a ruler in every shot.

Method

• 1. Cut squares, label with a code, weigh and photograph each one. Record mass and a "damage score" of 0.
• 2. Place squares in their conditions. Bury soil samples 5 cm deep in a labelled row.
• 3. Weekly: remove, rinse gently in cold water, pat dry, wait 30 minutes, weigh, photograph, score damage, fold once to test brittleness, return.
• 4. After eight weeks, compare mass change and damage score for every material and condition. Plot mass change against time for each material.
• 5. Look at the sunlit HDPE under a hand lens for surface cracking; this is the start of fragmentation into microplastics.

Results table (columns)

• Code | Material | Condition | Week 0 mass (g) | Week 1 | … | Week 8 | % change | Damage score week 8 | Brittle? (yes/no) | Notes

Analysis questions

• Which material changed most, and in which condition? Which did not change at all?
• Did the "compostable" packet behave like paper or like plastic in your garden soil? Read its label: most need an industrial composter at 55–60 degrees. What does that mean for a packet dropped in a park?
• Why is "it breaks up" not the same as "it goes away"?
• Estimate: if the sunlit HDPE lost X% in eight weeks, and the rate stayed the same (it will not, but assume it), how long until it halves? Compare with the 58-year figure above and discuss why your estimate differs.

Safety

• Wash hands after handling soil. Do not use food containers with food residue. Do not heat samples.

Curriculum links

• England: Key Stage 2 (KS2) Science Year 5 "Properties and changes of materials" (reversible and irreversible change, over time); KS3 Chemistry (Earth and atmosphere); General Certificate of Secondary Education (GCSE) Chemistry (using materials, polymers).
• Next Generation Science Standards (NGSS): 5-PS1-3 (observe and measure properties of materials); MS-ESS3-3; HS-ESS3-4.
• Australian Curriculum v9: Science Year 6 AC9S6U04 (reversible and irreversible changes) (verify); Science Inquiry Years 7–10 (verify codes).
• International Baccalaureate (IB) Environmental Systems and Societies (ESS) internal assessment angle: degradation of labelled "biodegradable" packaging under local conditions.