Critical volume of binder is calculated from the oil absorptions of pigments which are expressed in terms of gm. linseed oil per 100 gm pigment
Critical Vol. Binder = g oil absorbed/ density of oil
Usually PVC is well below the CPVC but sometimes eg zinc silicates PVC can approach or exceed CPVC
Please calculate your PVC and CPVC by hand before add (片层防腐材料) and after, then you will know the differences.
Maybe PVC is not the problem!
More experiments need to be done if you want to find out the real reason.