  We present a non-equilibrium quantum field theory approach to the
initial-state dynamics of spin models based on two-particle irreducible (2PI)
functional integral techniques. It employs a mapping of spins to Schwinger
bosons for arbitrary spin interactions and spin lengths. At next-to-leading
order (NLO) in an expansion in the number of field components, a wide range of
non-perturbative dynamical phenomena are shown to be captured, including
relaxation of magnetizations in a 3D long-range interacting system with
quenched disorder, different relaxation behaviour on both sides of a quantum
phase transition and the crossover from relaxation to arrest of dynamics in a
disordered spin chain previously shown to exhibit many-body-localization. Where
applicable, we employ alternative state-of-the-art techniques and find rather
good agreement with our 2PI NLO results. As our method can handle large system
sizes and converges relatively quickly to its thermodynamic limit, it opens the
possibility to study those phenomena in higher dimensions in regimes in which
no other efficient methods exist. Furthermore, the approach to classical
dynamics can be investigated as the spin length is increased.
