The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

Commonly, volumetric shrinkage occurs during polymerizations due to the shortening of the equilibrium Van der Waals distance of two molecules to the length of a (significantly shorter) covalent bond. This volumetric shrinkage can have severe influence on the materials’ properties. One strategy to overcome this volumetric shrinkage is the use of expanding monomers that show volumetric expansion during polymerization reactions. Such monomers exhibit cyclic or even oligocyclic structural motifs with a correspondingly dense atomic packing. During the ring-opening reaction of such monomers, linear structures with atomic packing of lower density are formed, which results in volumetric expansion or at least reduced volumetric shrinkage. This review provides a concise overview of expanding monomers with a focus on the elucidation of structure-property relationships. Preceded by a brief introduction of measuring techniques for the quantification of volumetric changes, the most prominent classes of expanding monomers will be presented and discussed, namely cycloalkanes and cycloalkenes, oxacycles, benzoxazines, as well as thiocyclic compounds. Spiroorthoesters, spiroorthocarbonates, cyclic carbonates, and benzoxazines are particularly highlighted.

https://www.mdpi.com/2073-4360/13/5/806/pdf

Expanding Monomers as Anti-Shrinkage Additives.

Glass fiber, GF, which was first hydroxylated and silanized, was incorporated into epoxy resin modified with amino-terminated hyperbranched polymer (ATHBP) to obtain high performance composite. The effects of GFs content on the mechanical properties of composites were investigated, discussing the results from flexural, tensile, and impact tests. The composites revealed noticeable improvement in flexural strength, tensile strength as well as impact strength but slow decrease in elongation at break, compared to the epoxy/ATHBP thermoset. FESEM morphology results indicated the good compatibility between epoxy matrix and GF in the appearance of ATHBP and showed that the toughening mechanism was mainly attributed to the stress transfer mechanism.

Investigations on mechanical characteristics of glass fiber reinforced epoxy composite modified with amino-terminated hyperbranched polymer

In this work, a novel DOPO-based spiroorthocarbonate (DSOC), 6,6ʹ-((3,9-diethyl-1,5,7,11-tetraoxaspiro[5.5]undecane-3,9-diyl)bis(hydroxymethylene))bis(6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide) was synthesized and confirmed with 1H, 31P, 13C NMR, and FT-IR spectra. It was used as anti-shrinkage additive and flame retardant in the cationic polymerization of epoxy resin. The shrinkage of the cured epoxy resin containing DSOC was investigated by measuring the density of the system before and after curing. It was found that the shrinkage of epoxy resin changed from −1.49 to 1.43% with the content of DSOC increased to 20 wt%, not only the shrinkage of epoxy resin was eliminated, but also a slight expansion was achieved. The flame retardancy and thermal stability of the cured epoxy resins were tested by limiting oxygen index (LOI), vertical burning test (UL-94), and thermogravimetric analysis (TGA). LOI values increased from 19.9% for pure E-51 to 30.9% for modified epoxy resins, UL-94 V-0 rating can be obtain...

https://www.tandfonline.com/doi/pdf/10.1080/15685551.2015.1070501

Synthesis of DOPO-based spiroorthocarbonate and its application in epoxy resin

The invention discloses a halogen-free flame-retardant prepolymer The halogen-free flame-retardant prepolymer is prepared through the following methods: heating a phosphorous compound, bismaleimide and an allyl compound in a reaction flask according to a mass ratio of m (phosphorous compound) to m (bismaleimide) to m (allyl compound) to obtain the halogen-free flame-retardant prepolymer, wherein the mass ratio of m (phosphorous compound) to m (bismaleimide) to m (allyl compound) is (1-20):(50-80):(15-50), the total mass of the phosphorous compound, the bismaleimide and the allyl compound is 100 parts, and a structural formula of the phosphorous compound contains a phosphaphenanthrene structure and at least one carboxyl group A prepreg and a laminated board made from the halogen-free flame-retardant prepolymer have integrated functions of excellent flame retardancy, dielectric property, mechanical property, high heat resistance and Tg, meet requirements on the development of a high-performance printed circuit board material, and can be applied to the fields of integrated circuit packaging, high-frequency high-speed and high-density interconnection and the like

Halogen-free flame-retardant prepolymer and prepreg and laminated board made from same

A novel copolymer containing a double spiro orthocarbonate structure, poly 3-methylene-9-methyleneoxy-3,9-Bis(hydroxymethyl)-1,5,7,11-tetraoxaspiro[5.5] undecane (SOC), was synthesized. The structure of SOC was confirmed by 1H, 13C NMR and FTIR spectra. Prepolymer (SOCP) reacting from SOC and 2,4-toluene diisocynate (TDI) was prepared and used as anti-shrinkage additives in the cationic polymerization of epoxy resin (EP-51). The volume changes of the cured epoxy resin containing SOCP were investigated by density measurements with an electronic hydrometer. The mechanical properties of the modified epoxy resin were investigated by adhesive strength, impact strength and flexural strength, while thermal properties were investigated by thermal gravimetric analysis (TGA).

Synthesis of Copolymers Containing Double Spiro Orthocarbonate and Used as Anti-shrinkage Additives in Epoxy Resin Composite

The invention discloses a preparation method of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based halogen-free flame retardant polybutylece terephthalate (PBT) resin, and relates to a preparation method of a flame retardant PBT/DOPO blend. The preparation method comprises the steps of firstly, synthesizing DDPO, namely adding the DOPO and dimethylbenzene in a four-neck flask provided with a mechanical stirrer, a condenser, a thermometer and a nitrogen conduct; under a circumstance of nitrogen protection, raising temperature and dissolving itaconic acid through dioxane, and raising temperature and implementing a reflux reaction, filtering, recrystallizing through tetrahydrofuran and drying for 8 hours; and then, preparing the PBT/DOPO blend, namely completely mixing the PBT and the DDPO according to proportions, melting and blending in a co-rotating twin-screw extruder, extruding and pelleting. According to the method disclosed by the invention, the 9,10-dihydro-10-(2,3-dicarboxyl)propyl-10-phosphaphenanthrene-10-oxide (DOPO) is synthesized, and an optimal synthesis condition is determined; the method is simple and easy to implement, high in yield and easy to realize industrial production.

Preparation method of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based halogen-free flame retardant polybutylece terephthalate (PBT) resin

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Synthesis of Copolymers Containing Double Spiro Orthocarbonate and Used as Anti-shrinkage Additives in Epoxy Resin Composite

Preparation method of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based halogen-free flame retardant polybutylece terephthalate (PBT) resin