Computational and Experimental Study of Phenolic Resins: Thermal–Mechanical Properties and the Role of Hydrogen Bonding

Abstract

Molecular dynamics simulations and experimental measurements were used to investigate the thermal and mechanical properties of cross-linked phenolic resins as a function of the degree of cross-linking, the chain motif (ortho−ortho versus ortho−para), and the chain length. The chain motif influenced the type (interchain or intrachain) as well as the amount of hydrogen bonding. Ortho− ortho chains favored internal hydrogen bonding whereas ortho−para favored hydrogen bonding between chains. Un-cross-linked ortho− para systems formed percolating 3D networks of hydrogen bonds, behaving effectively as "hydrogen gels". This resulted in differing thermal and mechanical properties for these systems. As crosslinking increased, the chain motif, chain length, and hydrogen bonding networks became less important. Elastic moduli, thermal conductivity, and glass transition temperatures were characterized as a function of cross-linking and temperature. Both our own experimental data and literature values were used to validate our simulation results. ■ INTRODUCTIONThermosetting resins, such as phenolic, polycyanurates, epoxies, and polyimides, have numerous applications as adhesives, coatings, and constituents for composite materials. The irreversible three-dimensional cross-linked networks formed during cure distinguish them from their thermoplastic analogues. Cross-linked structures result in stiffer mechanical properties even at elevated temperatures. Phenolic resins in particular are an important component of ablative thermal protection materials due to their high strength, low thermal conductivity, and high char yield. Ablative composites are stateof-the-art heat shield materials that protect space vehicles from extreme atmospheric entry conditions; examples of this class of materials include PICA 1−3 and Carbon Phenolic. 4,5 Experimental characterization of phenolic resins has spanned many decades due to their versatile applications in industry, 6,7 academics, 8−11 and government. 12−16 Numerous experimental results on isolated phenolic resins as well as in composites have been published to understand their characteristics and properties as a function of cure, 17−22 processing, 23−27 and composite design. 10,28−33 In addition, experimental properties such as the coefficients of thermal expansion, 12,20,34 thermal conductivity, 18,19,32 and elastic moduli 15,35,36 have been reported. Phenolic resins also have disadvantages, however, which include void formation and shrinkage that occur during processing as well as its brittle nature when highly cured. Recent experimental work has shown that thermomechanical properties can be improved substantially by introducing additives and varying processing conditions. 2,3 Understanding the relationship between chemical structures, properties, and processing will lead to improved, high-performance resins for this important class of materials.Advanced simulation studies are expected to play an important role in complementing and guiding experimental design for these systems. Recently, Li ...

Comparison of graphene oxide and graphitic carbon nitride filled carbon–phenolic composites: Thermomechanical properties and role of the strong electronegativity of nanofillers

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Boric Acid as a Coupling Agent for Preparation of Phenolic Resin Containing Boron and Silicon with Enhanced Char Yield

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