Making bioplastic at high school level chemistry using PLA/Starch blends

Question

I'm currently working on a high school chemistry essay about biodegradable plastics. I know that there is an easy way to synthesize Thermoplastic Starch (TPS), a starch-based bioplastic using glycerol and other materials at home (https://www.wikihow.com/Make-Bioplastic). But, I was wondering if there would also be a way to make Polylactic acid (PLA)-based bioplastic using the same process. I know that PLA bioplastic synthesis usually goes through extrusion or melt blending process, but since conducting these processes are not possible at my school due to equipment limitation, I just wonder if doing the same process described for starch-based could produce PLA-bioplastics. I got PLA pellets in hand, so would crushing them into powders and repeating the same process, or maybe mixing it with starch, be a valid procedure?

Answer 1

I think you are in correct track in preparing Polylactide (PLA)-starch bioplastic, in a short. The question is actually an opinion-based but interesting one. So, even though there is high possibility that I may get down-voted, I'd like to give few of my opinions.

I think you may have to tryout few steps. I agree with the crushing PLA pellets into powder would be advantages because it increases the surface area. I think it is also important to prepare Thermoplastic Starch (TPS) as one of your reagents, rather than having PLA, starch, and glycerol together and heat. I'm telling this because it has been done in literature (peer-review) before with polystyrene (Ref.1):

Conventional plastics has a large impact in increasing the environment’s pollution. That’s why the interest has turned towards novel partially and completely biodegradable polymers. In this work, blends of polystyrene and thermoplastic starch with glycerol and Buriti (Mauritia flexuosa L.) oil as plasticizers were prepared. Samples were analyzed using TG/DTG and DSC techniques. The TG results indicated that the blends with Buriti oil are thermally more stable than those with glycerol. The DSC analysis that Buriti oil provides a higher degree of plasticization of PS, compared to the blends plasticized using glycerol under the studied conditions.

According to the proceedure in Ref.1:

Preparation of thermoplastic starch: TPS was obtained by mixing starch powder, water and glycerol or Buriti oil in $50:15:35 \ (mass/vol/vol)$ ratio according to Ramsay et al. (Ref.2) but after some modifications. The constituents were mixed for $\pu{30 min}$ to obtain a paste which was transformed to TPS by heating at $\pu{95 ^\circ C}$ in water bath with continuous stirring for $\pu{30 min}$.
Preparation of the blends: PS and TPS were mixed in different ratios $(0.9:0.1, 0.7:0.3, 0.5:0.5$ and $0.3:0.7,$ $(w/w\%)$ and then stirred $(\pu{3 h}, \ \pu{50 ^\circ C})$ with hot ethyl acetate. The blends were prepared by conventional-casting technique using Teflon$^\mathrm{TM}$ dishes as casting surfaces for at least $\pu{24 h}$ at room temperature and vacuum (between $6.6 \times 10^2$ – $\pu{13.2 \times 10^2 Pa}$). The obtained material, with $200$ to $\pu{300 \mu m}$ thickness was stored far from light to avoid the decomposition of the oil.

The Thermogravimetric analysis (TG) and DTG analysis were depicted in following image:

The authors mentioned that to convert starch to thermoplastic starch (TPS) and allow its proper blending with PS, it is necessary to use a plasticizer agent like water, glycerin, sorbitol, etc., at high temperature $(\pu{90–180 ^\circ C})$ and shear (Ref.3).

As you can see from the image we have different blends behaving differently according to the ratio. Thus, it is safe to say that PLA may act similar to PS. However, you may need to try few ratios and blend in different temperatures before you conclude the results in your review. Good Luck!

References:

1. Daniela Schlemmer, E. R. de Oliveira, and M. J. Araújo Sales, "Polystyrene/thermoplastic starch blends with different plasticizers: Preparation and thermal characterization," Journal of Thermal Analysis and Calorimetry 2007, 87(3), 635–638 (DOI: https://doi.org/10.1007/s10973-006-7776-y).
2. Bruce A. Ramsay, Vincent Langlade, Pierre J. Carreau, and Julianna A. Ramsay, "Biodegradability and mechanical properties of poly-(beta-hydroxybutyrate-co-beta-hydroxyvalerate)-starch blends," Appl. Environ. Microbiol. 1993, 59(4), 1242-1246 (DOI: https://doi.org/10.1128/aem.59.4.1242-1246.1993).
3. P. Aggarwal, D. Dollimore, and K. Heon, "Comparative thermal analysis study of two biopolymers, starch and cellulose," Journal of Thermal Analysis 1997, 50, 7-17 (DOI: https://doi.org/10.1007/bf01979545).