Plastic can be made without petroleum from milk protein, plant cellulose, starch, coal tar and natural rubber latex, and the easiest by far is starch plastic, a glycerin-softened gel that dries to a stiff, translucent, non-waterproof material. Casein from milk is the most useful bio-based plastic for small hard parts, and none of them replaces Bakelite where heat and electrical insulation matter.
Part of Plastics and Polymers
Bio-based plastics are polymers derived from renewable sources such as milk protein, plant cellulose, starch and plant oils instead of petroleum.
A Settlement Has Six Non-Petroleum Routes
| Source | Plastic | Difficulty | Uses |
|---|---|---|---|
| Milk | Casein plastic | Easy | Buttons, beads, small items |
| Cotton or wood pulp | Celluloid | Moderate, hazardous | Film, combs, handles |
| Coal tar | Bakelite | Moderate | Electrical parts, structural |
| Starch (corn, potato) | Starch-based bioplastic | Easy | Packaging, disposable items |
| Natural rubber latex | Vulcanised rubber | Moderate | Seals, gaskets, tires |
| Linseed oil | Linoleum | Easy | Flooring, sheet |
The table is from Plastics Manufacturing. For the wider set including shellac and cellulose acetate see Bioplastics.
Starch Plastic Is Simple And Weak
The carrier’s recipe is the simplest in this section:
- Mix corn starch with water at 1 part starch to 4 parts water, and add a small amount of glycerin (the carrier gives 1 tablespoon per cup of starch) or vegetable oil.
- Heat while stirring until the mixture thickens into a gel.
- Pour into moulds or spread into sheets.
- Dry for 24-48 hours.
The result is a stiff, translucent material that suits packaging and lightweight containers. It is not waterproof, and it will take up moisture, soften and in damp conditions grow mould. Glycerin comes as a by-product of soap and biodiesel making; see Glycerin By-Product. Starch crops are covered in Starch Crops.
Choose By The Job, Not By The Source
- Needs to stay dry, hard and small: casein, hardened with formaldehyde. See Casein Plastic.
- Needs to insulate electrically or tolerate heat: Bakelite. See Bakelite.
- Needs to be a coating: shellac or a lacquer. See Lacquers and Coatings.
- Needs to be a one-use wrapper: starch plastic is adequate.
“Bio-based” does not mean “harmless” or “biodegradable”. Casein hardened with formaldehyde lasts, and celluloid is made from plant cellulose and is among the most dangerous plastics in this section.
Worked Example: How Much Starch Gel From One Batch
A batch uses 200 g of starch. Taking the carrier’s 1:4 as parts by mass (it does not say), the water is 4 × 200 g = 800 g, so the wet mass before glycerin is 200 + 800 = 1,000 g. The starch is one fifth of that mass, so after the water dries away the sheet weighs at most about 200 g plus glycerin, and drying removes most of the 800 g of water. That shrinkage is why starch sheets curl and crack when dried too fast.
Common Mistakes
- Expecting starch plastic to hold water — it dissolves and softens.
- Drying fast in the sun or on a hot surface — the sheet curls and cracks.
- Calling celluloid a green plastic — its source is a plant, its hazard is not.
Formaldehyde and solvents
Formaldehyde used to harden casein is a respiratory irritant and carcinogen; use it dilute, outdoors or well ventilated. Alcohol and other solvents used for coatings are flammable. Starch gel itself is harmless but sticks hot to skin and burns.
What’s Next
- Learn next: Casein: Applications and Limits — the most useful bio-based plastic
- Combine with: Early Plastics — where bio-based fits among the first three