FR Add-On Durability & Initial Loading Required

A constant fractional loss is the simplest defensible model and it is not exactly what happens: real finishes lose a disproportionate amount in the first few washes as unbound and loosely held material leaves, then settle to a slower rate, so a rate fitted on a five-wash result will be pessimistic at fifty and one fitted on the tail will be optimistic early. Fit the rate across the range the claim covers. Add-on is a proxy for performance rather than performance itself; the specification is a flame test, and char length depends on fabric construction and areal weight as much as on the quantity of retained phosphorus, which is why two fabrics at the same residual add-on can behave differently. Laundering conditions dominate the loss rate and are part of the claim: temperature, alkalinity, chlorine and the presence of a tumble dry step all move it, and a domestic-wash figure does not transfer to industrial laundering, which is far more aggressive. The required loading this returns should be checked against what the fabric can physically carry and still meet its hand and tensile specification - above roughly a quarter of fabric weight most durable systems begin to stiffen the cloth beyond what the end use will accept, and the honest conclusion is often that the claim needs a different chemistry rather than more of this one.