Sol-Gel Approach for Fabricating Silica/Epoxy Nanocomposites

Abstract

This review focuses on the opportunities provided by sol-gel chemistry for the production of silica/epoxy nanocomposites, with significant representative examples of the "extra situ" approach and an updated description of the "in situ" strategy. The "extra situ" strategy enables the creation of nanocomposites containing highly engineered nanoparticles. The "in situ" approach is a very promising synthesis route that allows us to produce, in a much easier and eco-friendly manner, properly flame-retarded silica/epoxy nanocomposites endowed with very interesting properties. The review highlights the recently proposed mechanism of nanoparticles formation, which is expected to help to design the synthesis strategies of nanocomposites, changing their composition (both for the nanoparticle and matrix nature) and with in situ-generated nanoparticles possibly more complex than the ones obtained, until today, through this route.

Keywords: epoxy; flame retardancy; highly engineered nanoparticles; nanocomposites; silica in situ formation mechanism; sol–gel.

Figure 1

Effects of pH on particle morphology in sol–gel reactions. At low pH, a polymer gel forms, while a particulate gel forms at high pH. Reprinted with permission from [39]. Copyright 2023 Swiss Chemical Society.

Scheme 1

Functionalization through coupling agent. APTES anchors to the silica surface thanks to condensation reaction with silicatic surface silanols. The reaction of amino group with oxirane of epoxy allows, finally, a strong covalent bond matrix/silcatic surface to be set up.

Figure 2

Surface modification mechanism of silica particles by PEI, showing H-bonds formed between silanol groups of silica and N–H groups of polyethyleneimine. Reprinted with permission from [56]. Copyright 2023 Elsevier.

Figure 3

TEM micrographs of SiO₂ and SEPA clearly showing the formation of a layer 6–14 nm thick. Reprinted with permission from [57]. Copyright 2023 Elsevier.

Figure 4

Decoration process of nanoparticle depicting the formation of a PANI shell around the mesoporous silica nanoparticle (MSN). Reprinted with permission from [58]. Copyright 2023 Elsevier.

Figure 5

(A) Silica/titania core/shell nanoparticles scheme; (B) photo of nanocomposites as a function of the shell weight percentage. Reprinted with permission from [59]. Copyright 2023 American Chemical Society.

Scheme 2

Scheme of (a) one-step procedure, (b) simultaneous, (c) sequential, and (d) chronological two-step procedure. The difference is in the order of addition of reagents. Reprinted with permission from [23]. Copyright 2023 MDPI.

Figure 6

Structure of ionic liquids.

Figure 7

Representation of interaction between polymer network and filler surface beyond physical interactions; covalent bonds form thanks to reaction of carboxylic-ILs at the epoxy interface. Reprinted with permission from [86]. Copyright 2023 Royal Society of Chemistry.

Figure 8

Chemical structure of 6H-dibenz [c, e] [1,2] oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo [2.2.2] oct-4-yl) methoxy]-, 6-oxide (DP).

Figure 9

3-(6-oxidodibenzo [c, e] [1,2] oxaphosphinin-6-yl) propenamide (DA).

Figure 10

(A) HRTEM micrograph of EPO_6%Si_1.25, (B) zoom of A, showing how the sheet thickness was evaluated. Reprinted with permission from [31]. Copyright 2023 American Chemical Society.

Figure 11

HRTEM micrograph of EPO_6%_Si_2.32. Reprinted with permission from [32]. Copyright 2023 American Chemical Society.

Figure 12

Scheme of the formation mechanism of multisheet silica nanoparticles in the epoxy matrix. Reprinted with permission from [31]. Copyright 2023 American Chemical Society.