Why doesn’t your PVC formulation match the calculated hardness? You followed the published tables, used the recommended phr, yet the Shore A reading is 5 points off target. This frustration is common because even identical formulations can produce different results under different processing conditions.
Published phr tables give you a starting point, not a final answer. Every mixer, every extruder, every batch of resin behaves slightly differently. The formulation ratio I recommend is always a starting point that requires verification through testing and adjustment based on what you actually measure.
PHR Ranges by Target Hardness
Plasticizer dosage in PVC is measured in phr (parts per hundred resin). To make PVC flexible, you need at least 15 phr. Below this threshold, you may encounter antiplasticization, where adding plasticizer actually increases stiffness rather than reducing it. This counterintuitive effect occurs at 1-20 phr depending on the plasticizer type.
Shore A 90-95 (Semi-Rigid)
For semi-rigid applications requiring Shore A 90-95, use 25-40 phr of general-purpose plasticizer like DOP or DOTP. At 40 phr, expect approximately Shore A 95. These low-plasticizer formulations maintain good mechanical strength and heat resistance while providing limited flexibility.
Shore A 70-85 (Standard Flexible)
Standard flexible PVC targets Shore A 70-85, the most common range for general applications. This requires 45-65 phr. At 50 phr, expect Shore A around 88. At 60 phr, hardness drops to approximately Shore A 80. DEHP achieves Shore A 70 at roughly 30 phr, while less efficient plasticizers like TOTM may need 45 phr for the same result.
Shore A 50-70 (Highly Flexible)
Highly flexible products like medical tubing and soft gaskets need Shore A 50-70, requiring 70-100+ phr. Very soft products may use 140-150 phr for maximum flexibility. At these high levels, each additional phr unit reduces heat resistance by 2-3C, so balance flexibility against thermal requirements.
Plasticizer Efficiency Differences
Not all plasticizers deliver the same softening effect at equal phr levels. After selecting the right plasticizer type for your application, the next step is determining how much you need. Plasticizer efficiency describes the ability of a plasticizer to soften PVC relative to DOP as the baseline reference. Efficiency is typically compared at Shore A 80 or at 50 phr concentration.
The substitution factor tells you how to convert between plasticizer types:
| Plasticizer | Efficiency Factor | PHR Needed for Shore A 80 |
|---|---|---|
| DBP | 0.86 | ~43 phr |
| DOA | 0.93 | ~47 phr |
| DOP (baseline) | 1.00 | 50 phr |
| DOTP | 1.03 | ~52 phr |
| DINP | 1.04 | ~52 phr |
| TOTM | 1.11 | ~56 phr |
| DTDP | 1.26 | ~63 phr |
A lower efficiency factor means fewer phr needed for the same hardness. DOP is approximately 5-10% more efficient than DOTP. If your formulation uses 50 phr DOP and you switch to DOTP, increase to approximately 53-55 phr to maintain the same Shore A.
This efficiency difference explains why copying a competitor’s phr number without knowing their plasticizer type leads to inconsistent results. The same 45 phr produces vastly different hardness depending on whether it’s DBP or TOTM.
Application-Specific Starting Points
Different applications have established phr ranges based on performance requirements. Use these as starting points, then verify through testing.
Wire and Cable
Cable insulation typically uses 40-50 phr for general grades. Cable sheathing, which requires more flexibility, uses 50-60 phr. High-temperature cables (90C, 105C ratings) require heat-resistant plasticizers like TOTM at slightly higher levels to compensate for lower efficiency.
Films and Sheets
Calendered films use approximately 50 phr total plasticizer. Blown film applications typically require 45-50 phr. These levels balance flexibility with the processing requirements of film-forming equipment.
Medical Tubing
Medical-grade PVC tubing often uses 50 phr plasticizer with 2.5 phr stabilizer and 0.25 phr lubricant as a baseline formulation. For improved low-temperature flexibility, increase to 70 phr. Medical applications are available in grades from Shore A 50-90 using non-phthalate plasticizers like ATBC, TOTM, or DOTP.
General Flexible Products
Plastisol formulations for general flexible products target Shore A 75-80 at approximately 57 phr. Gaskets, seals, and flexible profiles fall within the 50-70 phr range depending on required flexibility and environmental exposure.
From Starting PHR to Final Formulation
Calculation tools produce reliable results within the 40-80 phr range. Outside that range, you are extrapolating rather than interpolating, and results become less predictable. Treat all calculated values as estimates requiring verification.
Verification Testing
After mixing your trial batch, verify properties before production runs:
- Shore A hardness – The primary specification. Test at multiple points on the sample.
- Processing behavior – If torque or motor load is unusually high, the compound may not be plasticizing smoothly, indicating formulation imbalance.
- Surface quality – Oily or greasy surfaces indicate excess plasticizer or compatibility problems. A white bloom suggests plasticizer migration.
- Flexibility at temperature – Test at both ambient and service temperature extremes.
Adjustment Guidelines
When test results miss the target:
Too hard (Shore A higher than target):
- Increase phr by 3-5 units
- Verify plasticizer is fully incorporated (check mixing time and temperature)
- Consider a more efficient plasticizer if at equipment limits
Too soft (Shore A lower than target):
- Reduce phr by 3-5 units
- Check for measurement errors in phr calculation
- Verify resin K-value matches formulation assumptions
Surface issues appearing within weeks:
- Reduce total plasticizer to prevent migration
- Consider higher molecular weight plasticizer for better permanence
- Verify plasticizer-PVC compatibility
Document each adjustment and its result. Building this data for your specific equipment and resin sources is more valuable than any published table.
Conclusion
Start with the phr ranges from this guide, but expect to adjust based on your verification testing. The tables give you a reasonable starting point. Processing conditions, equipment characteristics, and resin batch variations determine whether that starting point needs 5 phr more or less.
For existing formulations where you need to verify or reverse-engineer the plasticizer content, extraction testing using the concentration calculation method reveals actual plasticizer levels. New formulations start with target hardness, select the appropriate plasticizer type, calculate starting phr using efficiency factors, then verify and adjust through testing.