Lightweight Natural Rubber Foam: Formula, PHR Ratios and Cure Data

How much blowing agent goes into a lightweight natural rubber compound, and what does the cure schedule actually mean? This technical note brings the recipe, weighing calculations and cure-data interpretation into one place for footwear buyers and development teams.

The formulation below is a published laboratory example. The batch calculations and worked purchasing examples are our editorial analysis. Neither should be read as DSHT production-test results or approval of this foam for a ground-contact outsole.

The published recipe: natural rubber with 6 phr ADC

The following data come from Baru, Saiwari and Hayeemasae, Polímeros (2022), Tables 2–4 and experimental methods. Amounts are phr; mixing times follow the reported two-roll-mill sequence:

STR 5L natural rubber — 100 phr; 5 min
Stearic acid — 1 phr; 1 min
Zinc oxide — 5 phr; 1 min
TMQ — 1 phr; 1 min
Calcium carbonate — 30 phr; 3 min
TDAE processing oil — 10 phr; 2 min
CBS — 1 phr; 1 min
ADC (azodicarbonamide) — 6 phr; 1.5 min
Sulfur — 2.5 phr; 1 min

The study used free expansion in compression-moulding equipment at 150°C for 12.5 mm specimens, with duration based on MDR cure measurements. For this formulation, MDR t90 was 15.48 min and scorch time ts1 was 2.35 min at 150°C. Reported relative density was 0.58 ± 0.02; hardness was 60 ± 1.23 Shore OO. Relative density is a ratio, not g/cm³. Shore OO is not Shore A.

Weighing a trial: 1 kg of rubber is not 1 kg of compound

Phr means parts by mass per hundred parts of rubber. The recipe totals 156.5 parts. Calculated quantities for 1,000 g rubber are:

Rubber 1,000 g; stearic acid 10 g; zinc oxide 50 g; TMQ 10 g; calcium carbonate 300 g; TDAE oil 100 g; CBS 10 g; ADC 60 g; sulfur 25 g.

The total charge is therefore 1,565 g. These are arithmetic conversions, not a second experimental batch. If the laboratory wants exactly 1,000 g of total compound instead, multiply every phr number by 1,000 / 156.5. Check the mixer’s working capacity before choosing either batch size.

Keep supplier grades on the batch sheet. A chemical name alone is not a complete purchasing specification. Ask the technical team to record the ADC grade, filler grade, oil specification and whether each additive is neat powder or supplied in a carrier. If a supplied product is diluted, its gross weight cannot simply replace the stated active ingredient weight.

A 10 kg compound batch: a worked weighing sheet

For a total compound charge of 10,000 g, the scale factor is 10,000 / 156.5 = 63.8978 g per phr. Applying that factor gives the following calculated weights, rounded to 0.1 g:

Natural rubber — 6,389.8 g
Stearic acid — 63.9 g
Zinc oxide — 319.5 g
TMQ — 63.9 g
Calcium carbonate — 1,916.9 g
TDAE oil — 639.0 g
CBS — 63.9 g
ADC — 383.4 g
Sulfur — 159.7 g

The rounded entries total 10,000.0 g. These figures describe material charged before processing, not the guaranteed mass of finished foam. Keep them on a weighing sheet with separate columns for target mass, actual mass and material lot. Do not silently round every minor ingredient to the nearest handful or scoop.

Why 6 phr does not mean 6% of the finished mixture

In this recipe, the calculated ADC share of the total charge is 6 / 156.5 × 100 = 3.83% by mass. The denominator for phr is rubber; the denominator for a whole-mixture percentage is everything in the charge. Mixing up the two changes the recipe.

Consider a purchasing error in the 10 kg example. Someone reads “6 phr” as “6%” and weighs 600 g ADC. The calculated target above is 383.4 g. The discrepancy is about 216.6 g, or 56.5% above the intended ADC charge. That arithmetic does not predict the resulting foam properties; it shows why the weighing instruction must identify its denominator.

Read the cure result without inventing a production cycle

The cited 150°C result belongs to the paper’s formulation and experimental setup. Keep the measured rheometer t90 in one field and the proposed mould residence time in another. A production worksheet should not replace the second field automatically with the first.

Before a trial, ask the laboratory to establish the actual batch’s cure behaviour and document the specimen geometry, stock temperature, charge mass and release procedure. The cited preparation description does not supply a complete factory specification for roll temperature, nip, mould pressure or demoulding. We have deliberately left those settings unspecified rather than adding plausible-looking numbers.

The supplier grade also belongs beside the ingredient name. If an additive arrives in a carrier, obtain its active-content specification before calculating a substitution; the carrier itself changes the formulation. Review the current grade-specific safety data and required powder-handling and heated-process ventilation controls before processing.

What to ask when a supplier quotes “lightweight rubber”

Ask for a record that distinguishes measured results from targets. A useful reply would identify the compound revision, the component being proposed, the density measurement and its units, the hardness scale, and the conditions used for curing. A density ratio should never appear in the same column as an absolute density without a label explaining the difference.

Then match the question to the component. A proposal for an internal cushioning layer needs a different acceptance discussion from a proposal for the walking surface. Our separate compound-change sample approval guide covers the finished-boot review; this page is the reference for recipe and calculation details.

Discuss a rubber footwear development project with Dongshi Huateng

Send your selected model, intended component and the property you want to change through our contact page. Include an existing material specification or test report if available, and identify whether your weight target concerns one component or a complete pair.

The hunting boot collection can help identify a starting style. Its product photographs do not demonstrate use of this experimental formula. For construction choices rather than a weighing recipe, read our guide to reducing rubber-boot weight.