Interlayer Cohesion and Mechanical Anisotropy in Vat-Photopolymerised Thermosets: Cure Overlap, Network Continuity and Qualification
Segurola, Juan
2026-08-27 · Report · Version 1.1
Vat photopolymerisation produces thermoset objects through sequential exposure, yet the mechanical meaning of the resulting layer boundaries remains unsettled. Interlayer adhesion, Z strength, anisotropy, delamination and fracture are often treated as synonyms even though they describe different observables and can be governed by different mechanisms. This review defines native same-material interlayer cohesion as load transfer across a boundary created during the original VPP build, before any deliberate repair operation. The evidence shows that anisotropy is formulation- and protocol-specific rather than intrinsic to the process. Some DLP and SLA studies report significant orientation dependence, while others find statistically indistinguishable properties when cure penetration, network formation and post-processing are sufficient. Strategies that reduce anisotropy include thermally activated reactions between adjacent layers, core-shell toughening, orthogonal thiol-ene/thiol-epoxy curing, interpenetrating networks, disulfide exchange and boronate-ester network rearrangement. These approaches improve different combinations of cure overlap, interface chemistry, stress relaxation and bulk toughness; none creates a universal interlayer-strength value. Direct interphase work published in 2026 further shows that diffusion width can be quantified by compositional and nanomechanical mapping and can influence tensile response. A minimum evidence set is proposed for resin state, optical overlap, interface-targeted mechanics, build and crack coordinates, failure location, conditioning, statistics and uncertainty. Standard tensile, flexural and compression methods remain useful, but no published ISO or ASTM standard directly defines interlayer cohesion for neat VPP photopolymer thermosets. The central conclusion is that interlayer performance is a realised material-process state that must be demonstrated, not inferred from layer visibility, cure depth, orientation or a product label.