Grafting Starch with Acrylic Acid and Fenton's Initiator: The Selectivity Challenge

Abstract

Through the graft polymerization of acrylic monomers onto starch, materials with interesting new properties can be synthesized. Fenton's chemistry, Fe²⁺/H₂O₂, is considered to be attractive for the initiation of graft polymerization with the monomer acrylic acid since it is cheap and reacts quickly at ambient conditions and should therefore be easy to scale up. However, the selectivity of the grafting versus the homopolymerization reaction poses a challenge with this monomer and this type of initiator. In the present review paper, we investigate why data from the literature on grafting systems with other monomers and initiation systems tend to show higher graft selectivity. A scheme is presented, based on reaction engineering principles, that supports an explanation for these observed differences. It is found that more selective activation of starch is a factor, but perhaps even more important is a low monomer-to-starch ratio at the starting sites of graft reactions. Since water is the most common solvent, monomers that are less water-soluble have an advantage in this respect. Based on the proposed scheme, methods to improve the graft selectivity with Fenton's initiator and acrylic acid are evaluated. Most promising appears to be a method of gradual monomer dosage. With gelatinized cassava starch in a batch reactor, both the grafting percentage (17 => 29%) and graft selectivity (18 => 31%) could be improved. This can be considered a principal breakthrough. Still, more research and development would be needed to refine the method and to implement the idea in a continuous reactor at a larger scale.

Keywords: Fenton’s initiator; acrylic acid monomer; dedicated monomer dosage; graft selectivity; graphical representation of different grafting systems; methods to improve selectivity; starch graft copolymerization; water solubility of the monomer as a key factor.

Figure 1

Drawing of the batch reactor for graft polymerizations.

Figure 2

A picture for a visual impression of the gelly mass obtained from a standard reaction run (left) and of the grafted starch after separation and drying, before grinding for the instrumental analysis (right).

Figure 3

Reaction scheme of grafting versus homopolymerization with Fenton’s reagent and the important result parameters. St = starch, AA = monomer acrylic acid, • = radical state, PAA = polyacrylic acid.

Figure 4

The course of the graft selectivity during the reaction, at two moleculare ratios of monomer to starch (M/S).

Figure 5

The three principal methods (class I–III) to create starch macroradicals. In = Initiation agent, St = Starch, hv represents high energy radiation, ● = radical state.

Figure 6

Maximum in GP and GE with initiator concentration (data from Khalil et al. [23]).

Figure 7

The opposite effect of M/S on GP and GE.

Figure 8

Schematic representation of different grafting reaction environments: (A) Monomer is poorly water soluble; (B) Monomer is well soluble; (C) Direct activation of starch only In = Initiation agent, M = monomer, ● = radical state.

Figure 9

Effect of crosslinker content on GE in synthesized superabsorbent materials in our laboratory. (A) Data from Witono et al. [37]; (B) Data from more recent lab work; AA-Starch 3:1 (molar), Ammonium Persulfate (Class-I) initiator, more details are in the Supplementary Material.

Figure 10

Disappointing results from pre-initiation with Fenton’s and cassava starch.

Figure 11

The principle of dedicated dosage, versus the more conventional method, all monomer directly at the start of the reaction. In = Initiation agent, M = monomer, -M-M-M- = growing polymer chain, ● = radical state.

Figure 12

A sketch-projection of a tube reactor to perform gradual monomer dosage.