Comprehensive Guide to Creating and Optimizing Biodegradable Plastics

Educational Objectives and Curricular Standards

• Target Audience and Content Areas: This curriculum is designed for grades 6 through 12. It covers the core scientific disciplines of Chemistry, Engineering, and Physical Science.

• Next Generation Science Standards (NGSS):

  • Disciplinary Core Ideas:

    • PS1.A: Structure and Properties of Matter.

    • ETS1.B: Developing Possible Solutions.

  • Performance Expectations:

    • MS-PS1-2: Analyzing and interpreting data on substance properties before and after interaction to determine if a chemical reaction occurred.

    • MS-ETS1-4: Developing a model for iterative testing and modification to achieve an optimal design.

    • HS-PS1-5: Applying scientific principles to explain the effects of temperature/concentration on reaction rates.

    • HS-ETS1-3: Evaluating solutions to complex problems based on constraints like cost, safety, and environmental impact.

  • Practices: Planning and carrying out investigations; constructing explanations and designing solutions.

  • Crosscutting Concepts: Structure and function.

• Intended Outcomes:

  • Development of biodegradable plastic types and material optimization for product viability.

  • Engagement in the engineering design and testing process.

  • Understanding the factors that affect the characteristics of bioplastics.

Lesson Timeline and Structure

• Making the Utensil: Approximately $45 - 60$ minutes.

• Curing the Bioplastic: Requires a duration of $3 - 5$ days.

• Testing and Evaluating: Approximately $45$ minutes.

• Improving the Design: Approximately $45 - 60$ minutes.

• Sequencing: The experiment is conducted in small groups and broken into three parts over the course of two weeks, depending on drying time.

Fundamental Concepts of Bioplastics

• Definition of Bioplastic: Plastic derived from renewable sources, such as animal, alga, or plant origins.

• Carbon Footprint: The total amount of greenhouse gases produced during a product's manufacture, use, and breakdown. The environmental impact is determines by the source, type, and chemical composition of the plastic.

• Environmental Challenges: Most modern plastics originate from nonrenewable oil and reside indefinitely in landfills because they are not biodegradable. A primary modern challenge is developing inexpensive, durable, and biodegradable alternatives from renewable sources.

• Carbon Neutrality: Bioplastics are considered carbon neutral because the plants used to create them remove as much carbon dioxide from the atmosphere during growth as is released during manufacturing and breakdown. This results in no net increase in greenhouse gases.

Classification of Plastic Degradation

• Degradable Plastic: Refers to any plastic that can be broken down physically or chemically. This category includes all plastics.

  • Photodegradable: Plastics containing additives that cause them to become brittle and crumble when exposed to sunlight.

  • Oxo-degradable: Plastics with additives that facilitate breakdown through oxidation (reaction with oxygen in the air).

  • Note: These processes often only reduce the plastic to smaller pieces (size of sand) that remain as plastic until microorganisms can eventually assimilate them over many years.

• Biodegradable Plastic: Materials that can be broken down by the metabolism of bacteria and fungi. Microorganisms digest these plastics, allowing carbon atoms in the polymer chains to separate and be reused in other organic molecules.

  • Constraint: Breakdown in landfills is often slowed or stopped due to the lack of oxygen and water required for the metabolic process.

• Compostable Plastic: A specific subset of biodegradable materials that degrade within a reasonable timeframe in a compost pile.

  • Performance Standard: At least $60 \%$ of the material must degrade within $180$ days.

  • End Products: Degradation yields water, $CO_2$, biomass, and inorganic compounds at a rate consistent with paper, leaving no toxic residue.

Materials and Equipment

• Construction Materials:

  • $2 \text{ feet}$ of Aluminum foil.

  • Non-stick spray (e.g., Pam).

  • Sharpie marker for numbering molds.

• Substrates and Plasticizers:

  • Tap water.

  • Corn starch: $9 \, g$ (approx. $2 \text{ tsp.}$).

  • Unflavored gelatin: $12 \, g$ (approx. $3 \text{ tsp.}$).

  • Agar agar: $3 \, g$ (approx. $1 \text{ tsp.}$).

  • Glycerin (Plasticizer): Approximately $1 \text{ tablespoon}$.

• Laboratory Tools:

  • Heat-resistant, disposable cups (Standard plastic cups should be avoided as they melt in the microwave).

  • Plastic straws for mixing.

  • Medicine dropper for plasticizer measurement.

  • Teaspoon and $1/4 \text{ cup}$ measure.

Experimental Procedure

Part 1: Building the Molds

• Construction: Use aluminum foil to create molds in the shape of a fork handle. Each group creates nine molds total (three per substrate type).

• Dimensions: Molds should be approximately $1 \, cm \text{ wide} \times 2 \, cm \text{ deep} \times 10 \, cm \text{ long}$.

• Preparation: The molds must be designed to prevent leaking. Number them with a Sharpie and spray the interiors with non-stick spray to ensure the samples can be removed after drying.

Part 2: Making the Bioplastics

• Mixing: For each source type (Corn starch, Gelatin, Agar agar), mix the substrate with tap water and glycerin in a heat-resistant cup. Stir thoroughly until no clumps remain.

• Heating: Heat each cup separately in a microwave until the mixture begins to froth (typically less than $1$ minute). It is vital to watch the mixture through the window to prevent boiling over. Stir again after heating.

• Pouring: Pour each mixture into three molds at a consistent thickness of approximately $0.5 \, cm - 0.75 \, cm$.

• Curing: Allow the mixtures to dry in a warm area for $3 - 5$ days. Repeatable results are best achieved in controlled environments like a food dehydrator or an oven set to $150^{\circ} \text{F}$.

Part 3: Design Improvements

• Basic Optimization: Choose the substrate that provided the best initial strength and repeat the experiment, adjusting only the amount of substrate used.

• Advanced Optimization: Adjust both the substrate amount and the plasticizer (glycerin) amount. The plasticizer determines flexibility; adding more makes the material flexible, while using less makes it solid/brittle.

Evaluation Criteria for Material Characteristics

• Color and Opacity: Determine if light can pass through the material.

• Flexibility Rating: Use a scale from $1$ to $4$ ($1 = \text{cracks}$, $2 = \text{stiff}$, $3 = \text{somewhat flexible}$, $4 = \text{very flexible}$).

• Environmental Simulation:

  • Freezing: Freeze a sample to simulate winter and rate flexibility.

  • Heat: Heat samples to $120^{\circ} \text{F}$ under a lamp or in an oven to simulate summer and rate flexibility.

• Stain Resistance: Apply mustard or coffee and check if the stain remains after wiping.

• Tensile Strength: Hold the sample by one end and tape pennies to the other end one by one; record the number of pennies held before the material breaks.

Historical Evolution of Plastic

• Natural Polymers: Humans have utilized natural plastics for thousands of years. Early materials included shellac, egg proteins, and blood proteins.

• Ancient History ($1600 \, \text{BC}$): Mesoamericans used natural rubber to manufacture balls and figurines.

• Middle Ages: Cattle horns were treated to create transparent windows for lanterns. Later, materials mimicking horn were made by treating milk proteins (casein) with lye.

• First Man-Made Plastic ($1860$): Parkesine, a bioplastic made from wood cellulose treated with nitric acid. It was elastic and transparent, becoming moldable when heated.

• Fully Synthetic Plastic (Early 1900s): Bakelite was created using phenol and formaldehyde.

• Mid-20th Century Expansion ($1940\text{s} - 1950\text{s}$): Development of Polystyrene, Polyvinyl Chloride (PVC), Styrofoam, and Polyethylene Terephthalate (PET). PET eventualy replaced glass in drink bottle manufacturing.

Chemical Bonding and Properties

• Terminology:

  • Polymer: A large, chain-like molecule made of many small molecules.

  • Monomer: The small molecules that bond together to form polymers.

• Bonding in Non-Biodegradable Plastic: Primarily comprised of Carbon-Carbon ($C-C$) bonds. Microbes find it difficult to break these bonds down.

• Bonding in Biodegradable Plastic: These contains Carbon-Nitrogen ($C-N$) and Carbon-Oxygen ($C-O$) bonds. These specific bonds allow microbes to break the larger molecular structure down.

• Role of Plasticizers: Plasticizers like glycerin keep chemical bonding from occurring between molecular strands. This interference increases the material's flexibility.

Expected Experimental Outcomes

• Substrate Performance: Gelatin generally produces the strongest material, though results vary by brand.

• Plasticizer Effects: No plasticizer results in brittle material. Increasing plasticizer increases flexibility by preventing complete bonding in the substrate.

• Success Factors: Non-stick spray is essential; without it, aluminum molds cannot be removed. Controlled drying ($150^{\circ} \text{F}$) is superior to open-air drying for repeatable results.

Questions & Discussion

• Basic Level Questions:

  • Q: Would you recommend making fork handles out of the bioplastic you designed? What would be the advantages and disadvantages?

  • A: Answers vary by student result.

  • Q: Why are some types of the bioplastic flexible and others brittle?

  • A: The amount of plasticizer affects flexibility. Higher amounts prevent complete bonding in the substrate, allowing the material to flex.

  • Q: What type of degradation would be ideal for plastic bottles?

  • A: Ideally, they would completely degrade. However, there is a trade-off with the ability to hold liquid. Photodegradability for bottles thrown on roadsides could be a reasonable compromise.

  • Q: What type of degradation would be ideal for plates and plastic utensils?

  • A: A combination of bio- and hydro-degradation is optimum for quick breakdown in moist compost environments.

• Advanced Level Questions:

  • Q: What elements are bonded in non-biodegradable plastic versus biodegradable plastic?

  • A: Non-biodegradable plastics use primarily carbon-carbon bonds. Biodegradable plastics add carbon-nitrogen and carbon-oxygen bonds.

  • Q: What chemical bonds can microbes degrade?

  • A: Microbes can break carbon-nitrogen and carbon-oxygen bonds.

  • Q: What is the difference between flexible and brittle bioplastic bonding?

  • A: Plasticizers prevent strands from bonding together; the more present, the more flexible the result.

  • Q: To reduce greenhouse gases, should plastic be biodegradable? What is the difference between biodegradable plastic and bioplastic?

  • A: Bioplastic refers specifically to the renewable source, whereas biodegradable refers to the ability for microorganisms to break the material down. Bioplastics help reduce greenhouse gases by being carbon neutral.