Why softer POE can increase shrinkage in EVA foam
How IUS-4065 aims to balance low hardness, resilience and dimensional stability during secondary pro
IUS-4065 for EVA foam applications requiring softness and control of dimensions during reheating. Conceptual illustration.
The development challenge. Low-hardness polyolefin elastomer (POE) is often added to ethylene vinyl acetate (EVA) to reduce hardness and improve resilience. If its melting range is too low, however, reheating during lamination, baking or other secondary processes may weaken crystalline support and increase shrinkage, warpage and dimensional drift. IUS-4065 is positioned to establish a different balance between very low hardness and a higher melting point.
1 Low hardness is only one part of formulation design
In chemically crosslinked EVA foam, POE is commonly used to adjust softness, elastic feel and low-temperature toughness. Developers may choose a softer POE to reach the target hardness with less material, but raw material hardness is only the first criterion. A compound that lacks sufficient crystalline support at subsequent processing temperatures can shrink again after lamination, hot pressing, oven treatment or prolonged storage at elevated temperature, even when its dimensions are correct immediately after demolding.
Selection principle. Low hardness does not necessarily mean a low melting point. Many very soft polyolefin elastomers nevertheless have lower crystallinity and a lower melting range. Evaluate hardness together with differential scanning calorimetry (DSC), crosslink density and the actual secondary processing temperature.
2 Why a low melting range can increase secondary shrinkage
A foam structure continues to evolve after molding. Cell pressure, molecular orientation, the crosslinked network, crystalline regions and residual stresses from cooling all contribute to its dimensions. When the secondary processing temperature approaches or exceeds the principal melting range of the POE, the crystalline regions that provide physical support gradually weaken. Molecular chains can relax and cell walls can rearrange more easily.
Loss of crystalline support. Reheating melts some crystalline regions, reducing the resistance of cell walls to deformation.
Release of residual stress. Orientation created during foam expansion, mold opening and cooling begins to recover.
Diffusion of cell gas. Longer exposure and higher temperature increase diffusion rates and may change the pressure difference between the inside and outside of cells.
Insufficient or uneven crosslinking. Thermal shrinkage becomes more pronounced if the dicumyl peroxide (DCP) network cannot adequately hold the dimensions.
Incomplete stabilization during cooling. Core temperature, thickness differences and stacking conditions can all contribute to later dimensional drift.
3 The design value of IUS 4065
According to the supplier comparison, DF 640 and ENGAGE 8842 have a hardness of approximately Shore A 54–56 and melting points below 50°C. IUS-4065 has a hardness of approximately Shore A 40 and a melting point of approximately 64°C. Its proposed value is therefore a lower hardness combined with a higher temperature range for crystalline support.
| Property | DF 640 / ENGAGE 8842 | IUS-4065 | Meaning for development |
|---|---|---|---|
| Raw material hardness | Shore A 54–56 | Shore A 40 | May produce a softer feel at the same loading, or reach the target with a lower loading. |
| Melting point | Below 50°C | Approximately 64°C | Potentially greater physical support when secondary processing exceeds 50°C. |
| Thermal shrinkage risk | Higher, per supplier | Lower, per supplier | Potential benefit for lamination, hot pressing and control of dimensions. |
| Dimensional stability | Lower, per supplier | Higher, per supplier | May reduce warpage, inconsistent appearance and rework. |
| Processing stability | More sensitive to formulation and temperature | Better, per supplier | Must be confirmed through the actual DCP foaming window and production trials. |
4 How a higher melting point can help retain dimensions
At the same secondary processing temperature, a soft modifier that retains more unmelted crystalline regions can provide temporary physical crosslink points. These regions help cell walls resist thermal relaxation and deformation. Compared with the reference materials whose melting points are below 50°C, the approximately 64°C melting point of IUS-4065 may place some lamination or post-processing conditions farther from its principal melting range.
A melting point is not a service temperature rating. DSC can reveal broad or multiple melting peaks. Actual dimensional stability also depends on POE loading, the vinyl acetate (VA) content of EVA, DCP crosslink density, foam expansion ratio, cell wall thickness, cooling conditions and heating time. The value of 64°C is a material selection advantage and a starting point for validation, rather than a guarantee that every finished foam can safely withstand 64°C.
5 Potential benefits in EVA and POE foam formulations
More efficient hardness reduction
Lower raw material hardness may help developers reach the desired softness with a more reasonable POE loading. It may reduce the need to add large amounts of soft POE or processing oil, which can dilute the crosslinked network and reduce physical properties. Compare hardness reduction at the same finished foam density.
Resilience and a flexible feel
POE can contribute flexibility, low-temperature performance and energy recovery as well as softness. If IUS-4065 maintains an appropriate molecular network at low hardness, it may be suitable for insoles, midsoles, protective pads and cushioning materials that require comfort and resilience.
Less dimensional drift after lamination
For foams that undergo fabric lamination, hot pressing, baking or other secondary processing, a higher melting point may improve process tolerance and reduce the risk of secondary shrinkage, warpage, curled edges and inconsistent appearance.
Higher yield and more consistent batches
If existing defects mainly result from sensitivity to thermal history and dimensional recovery, a wider material processing window may improve yield and reduce reshaping and rework. Confirm the benefit through A/B trials using the same mold, expansion ratio and post-processing conditions.
6 Recalibrate the DCP formulation when changing POE
Replacing the current POE with an equal weight of IUS-4065 while keeping DCP, the blowing agent and molding conditions unchanged may not deliver the best result. Different polymer structures can differ in their response to peroxide radicals, melt viscosity and crystallization rate. These differences may change the balance between crosslink formation and gas generation.
| Variable to recalibrate | Possible effect | Suggested method |
|---|---|---|
| Effective DCP dosage | Insufficient crosslinking increases shrinkage; excessive crosslinking limits expansion and elongation. | Screen three dosage levels using rheological or cure curves. |
| Blowing agent and activation system | Gas generation may not match the rate at which the new material forms a network. | Keep the density target fixed and examine the cells and expansion after mold opening. |
| Compound temperature during mixing | Earlier softening of low-melting materials changes dispersion and scorch risk. | Record the actual compound temperature as well as equipment settings. |
| Molding and mold-opening time | These affect cure completion, expansion and residual stress. | Establish a processing window using core temperature and dimensional stability. |
| Cooling and conditioning | These affect crystallization and subsequent shrinkage. | Standardize cooling, stacking and measurements after 24 and 72 hours. |
7 Validate performance beyond hardness at demolding
To demonstrate the value of IUS-4065, trials must reproduce the customer’s actual secondary processing and use conditions. Include at least three groups: the existing formulation as a control, an equal-loading replacement, and a formulation optimized for the target hardness. Control the finished foam density.
| Stage | Tests | Main purpose |
|---|---|---|
| Raw material | Shore A hardness; DSC melting peaks and enthalpy; melt flow rate (MFR); density. | Confirm differences in softness, melting range and flow. |
| 24 hours after foaming | Density, hardness, dimensions, cells and resilience. | Exclude apparent advantages caused by different expansion ratios. |
| 72 hours of conditioning | Changes in length, width, thickness and hardness. | Observe natural shrinkage and crystalline reorganization. |
| Simulated lamination | 50, 60 and 70°C at different exposure times. | Build temperature–time–shrinkage curves. |
| Mechanical performance | Tensile strength, elongation, tear strength and compression set. | Confirm that lower hardness does not compromise durability. |
| Production validation | Cavity position, batch variation, yield and rework rate. | Translate material differences into manufacturing results. |
Core measurement
Dimensional shrinkage (%) = Dimension before heating - dimension after heatingDimension before heating × 100%
Measure length, width and thickness separately. Clearly define the sample conditioning time and measurement temperature.
8 Applications to evaluate first
EVA/POE foam insoles and midsoles requiring low hardness and high resilience.
Foam sheets that will undergo fabric lamination, hot pressing or oven treatment.
Molded parts sensitive to dimensional tolerances, surface flatness and warpage.
Sports protective pads, cushioning pads and products requiring a soft, conforming feel.
Products already showing secondary shrinkage with DF 640, ENGAGE 8842 or another soft POE with a low melting point.
Conclusion
Adding POE to EVA is commonly intended to reduce hardness and improve resilience. If the POE melting range is too low, however, crystalline support may weaken during lamination or secondary heating, increasing the effects of residual stress, cell contraction and dimensional deformation. This is one reason a foam can look satisfactory at demolding yet become unstable during later processing.
IUS-4065 is positioned to combine approximately Shore A 40 hardness with an approximately 64°C melting point, aiming to support softness, resilience and dimensional stability in one material. Its most useful development test is whether it can reduce secondary shrinkage, widen the processing window and improve production yield at the same density and required performance.
Development decision. Assess which material can retain cell structure and dimensions under the customer’s lamination temperature and use conditions, as well as which material gives the lowest hardness.
Data and technical statement. The hardness, melting points and qualitative comparisons for IUS-4065, DF 640 and ENGAGE 8842 are taken from the supplied INNOARK image. Before external publication, verify them against the supplier’s latest TDS, DSC reports and test results from the actual formulation.