TPR, EVA and TPU are three of the most widely used materials for outsoles, midsoles and shoe components. All three can be foamed to cut weight and improve cushioning, but they do not behave the same way in the barrel, the mold or the cooling stage. Choosing a foaming agent for footwear applications based only on a target expansion ratio is a common mistake, and it often leads to uneven cells, shrinkage or poor surface quality.
A chemical foaming agent (CFA) is a solid that decomposes in a defined temperature range and releases gas into the polymer melt. How well that works depends on five things at once: decomposition temperature, gas yield, cell uniformity, dimensional stability and surface quality. This article compares the three materials and shows how to match each one to a suitable grade.
TPR (thermoplastic rubber) soles are valued for flexibility, grip and stable injection molding. The foaming challenge is uniformity: the gas has to be released in step with melt temperature and mold filling, or the sole shows coarse cells and inconsistent hardness.
The decomposition behavior of the foaming agent must fit the formulation and processing temperature. When you evaluate a TPR grade, validate at least these parameters:
EVA is the standard choice for lightweight midsoles and outsoles. Here the foaming agent directly drives density: gas yield, addition level and target density are linked. A higher-yield grade can reach the same density at a lower dose, but only if the cells stay uniform and the part holds its size after demolding.
To balance weight reduction, cell uniformity and shrinkage, adjust the grade, dosage, mold conditions and cooling together rather than one at a time. Shrinkage that appears after the part leaves the mold is often a sign that the gas pressure, cell structure and cooling were not matched.
TPU footwear foaming is usually judged on appearance and structure as much as on weight. Melt flow, cell size and the finished surface all interact, so a grade that foams well but leaves a rough or discolored surface may still fail.
Whiteness, low dust and good dispersion can become real selection factors for certain TPU processes. A dust-free granular grade avoids airborne powder at the feeding stage, and a high-whiteness grade helps in light-colored parts. Joysun lists TP204NS, a dust-free granular grade, for TPU one-shot footwear molding with high whiteness, low density and good adhesion.
The table below gives buyers and engineers a quick comparison of where to focus during selection.
| Selection factor | TPR | EVA | TPU |
|---|---|---|---|
| Primary focus | Uniform foaming and flexibility | Lightweighting and shrinkage control | Cell structure and surface quality |
| Foaming agent selection | Match decomposition behavior to the formulation | Balance gas yield and dimensional stability | Match the grade to the molding process and appearance requirements |
| Key quality checks | Cell uniformity, flexibility, dimensional consistency | Density, shrinkage, cell structure | Surface finish, whiteness, cell structure |
| Process validation | Temperature and decomposition timing | Dosage, mold conditions and cooling | Dispersion, molding conditions and cooling |
These are focus areas for selection, not an absolute performance ranking of the three materials. Real requirements depend on the resin grade, the formulation and the molding process.
Decomposition temperature controls when gas is released. If it is too low, gas escapes before the mold is closed or pressure builds. If it is too high, the melt may solidify before foaming, giving weak expansion or collapsed cells. Activators such as zinc compounds can also shift the effective decomposition point of ADC-based systems.
As a practical rule, the decomposition onset should sit above the barrel temperature to prevent premature foaming during feeding and conveying, and the decomposition range should match the mold temperature and dwell time. Values measured by DSC (see ISO 11357 in the references) are a starting point; the effective temperature in a polymer melt under pressure can differ, so confirm with trials.
Gas yield, in mL/g, shows how much gas a gram of foaming agent releases. In general, a higher yield lets you use less agent for the same density reduction, which can lower residue and sometimes cost. For example, Joysun lists ADC at 220 mL/g and OBSH at 140 mL/g on its synthesis and custom masterbatch page, so the two need different loading levels.
Yield alone should not decide the choice. Assess dosage, target density and finished-part performance together, because a high-yield grade with the wrong decomposition range can still give poor cells, shrinkage or surface defects.
Uneven cells, excess shrinkage and surface flaws usually come from the interaction of several factors: foaming agent dispersion, temperature control, mold design and cooling. Fixing only one of them rarely solves the problem. Confirm your choice with trial molding, then measure the finished parts (for example density by ISO 845 and dimensional change after a set conditioning time).
Joysun's technical information likewise stresses that decomposition temperature, gas yield, cell structure and processing conditions should be assessed together, not compared one parameter at a time.
Joysun's chemical foaming agent range is organized by application. For footwear, the page lists the following grades:
For compression-molded EVA midsoles with residue limits, see the low-VOC, low-formamide, low-ammonia ADC foaming agents. FWHJ05 (190°C, 180 mL/g) lists formamide below 200 ppm, and FWHJ201 (195°C, 180 mL/g) lists both formamide and ammonia below 200 ppm. Formamide is on the ECHA REACH Candidate List of substances of very high concern, which is one reason footwear brands set residue limits.
If you need a different chemistry or a pre-compounded form, Joysun also offers OBSH, whose decomposition products contain no ammonia or formamide, and a custom pre-dispersed masterbatch service. Products from the same family are also used for IXPP/IXPE foam and PE foam, and Joysun's high-performance TPEE is available for elastomer applications.
Parameters and uses above come from Joysun product information. Always rely on the latest technical data sheet and your own formulation trials.
Need help choosing a grade? Joysun has more than 20 years of foaming agent production experience, its own synthesis capability and an R&D center that supports non-standard applications. Learn more about us, browse the full product list, or contact the sales team at sale@joysunsh.com for data sheets and samples.
