A process for the preparation of a block of fiber-reinforced polyurethane and/or polyisocyanurate foam of a thermal insulation slab of a sealed and thermally insulating tank, the block of fiber-reinforced polyurethane and/or polyisocyanurate foam being composed of cells storing a gas and exhibiting a density of less than 50 kg/m3 with a content of fibers Cf representing at least 4% by weight of the block of fiber-reinforced foam, the preparation process comprising:
- a) mixing chemical components necessary for obtaining a polyurethane and/or polyisocyanurate foam, said components comprising reactants for obtaining polyurethane and/or polyisocyanurate, optionally at least one reaction catalyst, optionally at least one emulsifier, and at least one blowing agent,
- b) impregnating, by gravitational flow of the mixture of chemical components obtained in a), of a plurality of fiber reinforcements chosen from fabrics of fibers and mats of fibers, said fabrics of fibers and said mats of fibers having a length of at least five centimeters (cm), arranged in superimposed layers, in which the plurality of fiber reinforcements extend essentially along a direction perpendicular to the direction of said gravitational flow, the plurality of fiber reinforcements exhibiting a permeability Kc to the mixture of chemical components obtained in a), expressed in m2, equal to:
- Kc=(rf2×p3)/(k×τ2×4×Vf2),with
- rf=mean radius of the fibers in the fabrics of fibers or mats of fibers, expressed in meters (m),
- p=porosity of the fibers in the plurality of fiber reinforcements which is dimensionless with a value between 0 and 1,
- k=form factor of the plurality of fiber reinforcements, which is dimensionless, wherein k=1 for a fabric of fibers and k=6 for a mat of fibers,
- τ=tortuosity of the fabrics of fibers or mats of fibers constituting the plurality of fiber reinforcements, which is dimensionless,
- Vf=volume fraction of the fabrics of fibers or mats of fibers in the plurality of fiber reinforcements, value between 0 and 1, and
- c) forming and expanding of the fiber-reinforced polyurethane and/or polyisocyanurate foam,
- characterized in that the mixture of chemical components obtained in a) exhibits a dynamic viscosity η, during the impregnation stage b), such that an impregnation time of the fabrics of fibers or mats of fibers ti is less than the cream time tc of the polyurethane and/or polyisocyanurate foam, the impregnation time of the fabrics of fibers and mats of fibers ti being equal to:
- ti=(η×em2)/(Kc×ΔP),
- ΔP=(Msd×gt×kp),with
- η=the dynamic viscosity, expressed in pascal·seconds (Pa·s),
- em=sum of the mean thicknesses of the plurality of fiber reinforcements, expressed in meters (m), each fiber reinforcement exhibiting a mean thickness corresponding to the mean of the distances between a plurality of pairs of local extremums of said fiber reinforcement spaced out from one another along a direction of thickness of said fiber reinforcement,
- ΔP=pressure gradient, expressed in pascals (Pa),
- Msd=surface density of the mixture of chemical components obtained in a), expressed in kilogram per square meter (kg/m2),
- gt=9.8 N/kg1, and
- kp=0.5.