Most compounding engineers know their efficiency ratios by heart. DINP at 1.04, TOTM at 1.11, DOTP somewhere in between. Yet when tasked with reducing formulation cost by 5%, many struggle to translate these numbers into actual savings. The problem isn’t the data—it’s the metric.
Efficiency ratios compare relative softening power, which helps you rank plasticizers. But for cost optimization, you need substitution factors combined with volume cost calculations. This framework tells you exactly how much to use and what it will actually cost. I’ve found that formulators who switch from efficiency-ratio thinking to substitution-factor thinking consistently find more cost reduction opportunities.
What Efficiency Ratio Actually Tells You
Plasticizer efficiency measures how effectively a compound softens PVC relative to dioctyl phthalate (DOP), the industry reference standard. Testing typically occurs at Shore A hardness of 80 or at 50 phr loading—both methods yield comparable rankings.
The calculation works like this: if 100 phr of DOP yields a hardness of 50, and 110 phr of another plasticizer achieves the same result, that plasticizer has an efficiency of 91% (100/110). Published efficiency ratios include DINP (1.04 vs DOP), DIDP (1.11), TOTM (1.11), and DTDP (1.26).
These numbers answer one question well: which plasticizer provides more softening per unit weight? DINP at 1.04 means it provides slightly less softening than DOP per phr. That’s useful for quick comparisons.
But efficiency ratio stops short of telling you what you need for cost work. It doesn’t account for density differences. It doesn’t incorporate price. And it requires additional calculation to determine actual PHR needed for your target hardness. For optimization projects, I reach for a different metric.
Substitution Factor: The Missing Link
Substitution factor (SF) directly answers the formulator’s real question: how much plasticizer do I need to replace DOP and maintain the same hardness?
The definition is straightforward: SF equals the phr of plasticizer needed to achieve Shore A 80 divided by the phr of DOP needed for the same hardness. According to research published in Polymers, SF values for common plasticizers are: BBP (0.94), DOP (1.0), DINP (1.06), DIDP (1.1), TOTM (1.17), and DOA (0.93).
The difference from efficiency ratio is subtle but critical. If DINP has an SF of 1.06, you need 1.06 phr of DINP to replace 1 phr of DOP. Industry expert Yashodhan Kanade puts it directly: “For the same Shore A hardness, requirement of DINP is 1.04 phr to replace 1 phr of DOP.”
Both metrics describe the same underlying relationship, but SF gives you the multiplier you can apply immediately. When reformulating 50 phr DOP to DINP, multiply by 1.06 to get 53 phr. No intermediate calculation needed.
This direct actionability is why I prefer SF for cost optimization work. Efficiency ratio tells you DINP is 96% as efficient as DOP. SF tells you to use 1.06x as much. The second number plugs straight into your formulation spreadsheet.
Volume Cost: Turning SF Into Dollars
SF determines dosage. Volume cost determines true expense.
Teknor Apex articulates the formula: Volume Cost = Specific Gravity x Cost Per Pound. This adjustment matters because plasticizers with identical per-pound pricing can have wildly different per-volume costs—and you’re filling a mold cavity, not purchasing weight.
Consider two plasticizers both priced at $1.50/lb. Plasticizer A has SG 0.92; Plasticizer B has SG 1.08. Per gallon, A costs $11.50 while B costs $13.48. The material with lower specific gravity produces more parts per dollar.
This connects to a principle that transformed how I approach cost reduction: evaluate total cost, not compound cost alone. Yashodhan Kanade demonstrates this with a case where increasing additive cost by Rs.1/kg reduced production cost by Rs.4/kg—net savings of Rs.3/kg. Cheaper compound ingredients sometimes create processing problems that cost more than the savings.
For plasticizer optimization, the complete calculation combines SF-adjusted dosage with volume cost:
True Cost per Shore A Point = (SF x Base PHR x Volume Cost) / Target Hardness Reduction
This formula lets you compare plasticizers on what actually matters: cost to achieve your specification.
Practical Optimization Workflow
Step 1: Establish Baseline
Document your current formulation’s plasticizer loading, Shore A hardness, and cost. If using 50 phr DOP at $1.40/lb with SG 0.98, your baseline volume cost per phr is $1.37.
Step 2: Calculate SF-Adjusted Dosage
For candidate replacement DINP at SF 1.06: required loading = 50 x 1.06 = 53 phr. For DOTP at SF 1.03: required loading = 51.5 phr.
Step 3: Apply Volume Cost
DINP at $1.35/lb with SG 0.97: Volume cost = $1.31/phr. For 53 phr, total = $69.43.
DOP baseline at $1.40/lb with SG 0.98: Volume cost = $1.37/phr. For 50 phr, total = $68.60.
In this example, despite DINP’s lower per-pound price, the higher SF requirement makes it $0.83 more expensive per 100 parts resin—a conclusion you’d miss looking only at efficiency ratio or raw pricing.
Step 4: Verify Results
Run a trial batch at calculated loadings. Confirm Shore A hardness meets specification. GEON Performance Solutions demonstrated this approach during a wire and cable supply disruption: their L9TM plasticizer substitution achieved results “under budget” by precisely matching SF requirements, “positioned customer at a better cost position than the original 810TM plasticizer.”
For blends, calculate SF-weighted averages. A 70:30 DOP/chlorinated paraffin oil blend achieves equivalent hardness to 100% DOP at reduced cost—but only if you account for each component’s contribution.
Your Optimization Checklist
Start your next cost reduction project with this sequence:
- Document baseline: Current phr, Shore A, plasticizer cost and SG
- Look up SF values: Not efficiency ratios—you need the multiplier
- Calculate volume cost: SG x price, not just price
- Compare total cost per specification: SF-adjusted phr x volume cost
- Verify with trial batch: Shore A must meet spec, or savings are meaningless
Efficiency ratios remain valuable for quick plasticizer rankings. But when the goal is cost optimization, substitution factor plus volume cost gives you numbers you can act on immediately.