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your home-made bioplastic project!

I'm glad you've been trying the recipe in the video!

First check the ingredients. Use pure cornstarch (available in grocery stores, labeled as cornstarch). Tapioca starch is starch from the cassava plant, so "tapioca cornstarch" doesn't really exist. If the "tapioca" contained ingredients other than starch (e.g., sugar, as in tapioca pudding mix) that would have caused problems.

Use pure glycerin (chemical name glycerol), available in pharmacies. The vinegar is not really necessary; try it without the vinegar. Tap water is OK.

If you're using good ingredients, the problem is likely to be the proportions. In hot, humid weather (as in summer), the moisture absorbed also acts as a plasticizer, making the material sticky, hard to dry, and then cracking when it does dry. Just reduce the amount of glycerol to a fraction of what you were using.