Yes, PVC and CPVC resins can be blended, but they are only marginally compatible, and the higher the CPVC’s chlorine content, the worse they mix. Mixing here means blending the two resins in one compound; joining PVC pipe to CPVC pipe is a different question.
Which PVC and CPVC Grades Blend
The pairs that blend best use a CPVC from the lower end of the chlorine range and, when PVC is the minor resin, a PVC of higher molecular weight than the PVC the CPVC was chlorinated from. The CPVC’s chlorination process matters as well.
CPVC also goes into plasticized PVC compounds: one CPVC producer’s blending guidance has covered flexible profiles.
CPVC Chlorine Content
The higher the CPVC’s chlorine content, the less compatible it is with PVC. More chlorine also brings more heat resistance into the blend, so choosing the CPVC grade means trading heat resistance against compatibility.
Lower compatibility does not rule a CPVC out, though; patented CPVC/PVC blends with high impact strength have been made with a 68% chlorine CPVC. Nor does the higher processing temperature that comes with more chlorine prevent a blend, because in one CPVC compound producer’s injection-molding guide, PVC’s barrel range of 160–185 °C sits inside CPVC’s 160–200 °C.
PVC Molecular Weight
The PVC should have the higher molecular weight, with an inherent viscosity (I.V.) 0.2 to 1.0 units above that of the PVC the CPVC was chlorinated from, and the best results at 0.35 to 0.65 units above.
Ask the CPVC supplier for the I.V. or K value of the PVC the CPVC was chlorinated from. PVC resin grades are usually specified by K value, which rises with I.V., so in practice choose a PVC grade of higher K value than that PVC. The 0.2 to 1.0 window is stated in I.V. units only.
The I.V. window was worked out for PVC as the smaller share in a CPVC compound; the patent’s description extends it to blends of up to 70 parts PVC per 100 parts of combined resin, but no example tests CPVC as the minor resin.
Resin Source
A CPVC from a different producer or chlorination process can blend differently even at the same chlorine content. Treat a blend qualified on one CPVC grade as qualified on that grade only, and repeat the compatibility check when the grade or the producer changes.
How Much CPVC or PVC Goes Into a Blend
A BF Goodrich patent on CPVC blends claims 1 to 30 parts PVC per 100 parts CPVC for blends where CPVC is the main resin, and in the other direction CPVC goes into PVC at about 1% to 50% by weight of the total resin. Watch the basis: 30 parts per 100 parts CPVC is about 23% of the resin, while the patent’s preferred ranges are stated per 100 parts of combined resin.
In a CPVC compound, the PVC is there to reduce the impact modifier and flow aid the compound would otherwise need, particularly for injection molding. The patent’s most preferred range is 5 to 15 parts PVC per 100 parts of combined resin.
In a PVC compound, CPVC resin is added to lift the softening temperature to a point between the two resins’. Where in the 1–50% range a compound sits depends on how high a softening temperature the part needs, and the loading is found by trial against that target.
How to Tell Whether a Blend Is Compatible
A processor tells by running DSC or DMA on the blend and comparing its glass transitions with those of the two neat resins; the part itself shows only severe incompatibility.
Defects in the Molded or Extruded Part
A badly incompatible PVC/CPVC blend shows streaks or striations on the surface and a drop in impact strength. These are the signs of any two-phase blend with a weak interface, so no defect is specific to a PVC/CPVC blend. Unfused specks or gels point elsewhere, to a melt too cold for the CPVC, which is a processing fault rather than a compatibility one.
DSC and DMA Testing
A miscible blend shows one glass transition, between those of the two resins; a phase-separated blend shows two, each close to one resin’s own. Two transitions that have moved toward each other from the neat-resin values mean the resins have partly mixed.
Run the neat PVC and CPVC on the same instrument first. In a low-chlorine pair the two transitions sit close together and can merge into one broad step on a DSC trace.
DMA is the more sensitive of the two methods. In a blend of PVC with high-chlorine CPVC, the modulus-versus-temperature curve drops sharply at PVC’s own Tg.