Why Is the Root Cause of Shoe Material Mold Often Not in the Upper but in the Glue Layer?
Quality control personnel in shoe factories often encounter a strange phenomenon: with the same batch of shoe upper leather and the same warehouse, some shoe styles develop severe mold while others remain completely unaffected. Upon disassembling moldy shoes, we found that mold spots are concentrated at the bonding interface between the midsole cardboard and the upper, as well as in corner areas where glue accumulates. This indicates that the problem lies in the adhesive layer, not the shoe upper material itself.
Two key factors make the adhesive layer a breeding ground for mold: first, the glue itself contains a large amount of water or hydrophilic groups; second, the film formed after the glue dries adsorbs moisture from the environment. When the moisture content of the glue layer exceeds 12%, the germination cycle of mold spores can be shortened to within 48 hours. In other words, as long as the glue layer is not completely dry, the shoe material is essentially soaked in a mold culture medium.
Quantitative Impact of Adhesive Moisture Content on Mold Germination
Our laboratory conducted a comparative test: the same batch of midsole cardboard was bonded with PVA glue at moisture contents of 8%, 12%, and 16%, and placed in an environment at 30°C and 85% relative humidity for 7 days. The results showed that the sample with 8% moisture content had no mold spots, the 12% sample developed sporadic mold spots on day 4, and the 16% sample experienced a full outbreak of mold spots on day 2.
This data corresponds to actual production lines, meaning that if too much water is added during glue mixing or if drying after bonding is insufficient, the residual moisture in the glue layer becomes the “startup capital” for mold. A more insidious issue is that many shoe factories use water-based adhesives (such as white glue and powder glue), which themselves have a neutral pH and contain cellulose-based thickeners, both of which are preferred nutrient sources for mold.
Limitations of Different Glue Types on Fungicide Selection
The most direct way to solve mold in the glue layer is to add a fungicide to the glue. However, different glues have completely different tolerances to fungicides:
- Water-based adhesives (PVA, white glue, powder glue): pH range is wide (2-9). It is recommended to use iHeir-JSTC glue fungicide at an addition rate of 1-2%. The active ingredient of iHeir-JSTC is benzimidazole compounds, which can be uniformly dispersed in water-based systems and have MIC values as low as 5-10 mg/kg against Aspergillus niger and Penicillium funiculosum. Note that it should not be mixed directly with anionic thickeners; it must be diluted first before adding.
- Solvent-based adhesives (chloroprene rubber, PU glue): Solvent-based systems. It is recommended to use iHeir-G glue fungicide at an addition rate of 1-2%. iHeir-G is an oil-soluble formulation that can be completely dissolved in toluene and ethyl acetate without affecting the viscosity or open time of the glue.
- Hot melt adhesives (EVA, polyolefins): High-temperature processing (150-200°C) requires a heat-resistant fungicide. It is recommended to use iHeir-907 plastic antibacterial powder at an addition rate of 0.5-1%. iHeir-907 is an organic zinc ion type with a thermal decomposition temperature exceeding 280°C, ensuring it does not fail due to high temperatures in hot melt adhesives.
A common misconception must be emphasized here: some factories add powdered fungicides directly into the glue for convenience. However, ordinary fungicide powders have large particle sizes (tens of microns) and tend to settle in the glue, leading to uneven distribution of the fungicide. Both iHeir-JSTC and iHeir-G are liquid forms that can be directly stirred and dispersed without settling issues.
Midsole Cardboard Pretreatment: Cutting Off the Second Nutrient Source for Mold
While the glue layer is addressed, the midsole cardboard itself is also a risk. Cardboard absorbs moisture from the air during manufacturing, especially recycled cardboard with loose fiber structures, which typically has an equilibrium moisture content of 10-12%. If the cardboard is not dried to a moisture content of ≤8% before bonding, even if the glue is fine, the moisture inside the cardboard will slowly release to the glue layer interface during storage, providing secondary nutrition for mold.
Solution: Before the cardboard enters the production line, treat it with iHeir-3 packaging paper fungicide through impregnation. iHeir-3 is a non-release fungicide that forms an antibacterial layer on the fiber surface through bonding, which cannot be washed off by water or migrate. Impregnation parameters: soak the cardboard in iHeir-3 working solution (dilute the original solution 5 times, i.e., 20% concentration) for 15-30 seconds, then dry to a moisture content of ≤8%. Even if the treated cardboard later contacts moisture from the glue, the antibacterial layer can continuously inhibit mold germination.
The synergistic relationship between iHeir-3 and iHeir-JSTC must be clarified: iHeir-3 is responsible for long-term protection of the cardboard fiber surface, while iHeir-JSTC is responsible for short-term antibacterial action inside the glue layer—they are complementary, targeting different material matrices and cannot replace each other. If only iHeir-3 is used to treat the cardboard, once the glue layer molds, mold spores can still spread to the cardboard surface through the glue; if only iHeir-JSTC is used to treat the glue, moisture inside the cardboard can still migrate to the interface and cause mold.
Overlooked Process Detail: Cooling Time After Bonding and Condensation Risk
Many shoe factories, in order to meet deadlines, directly stack shoe materials together after the bonding process and move to the next step. However, hot melt or water-based adhesives are at a high temperature (60-80°C) during bonding. When stacked, internal heat cannot dissipate, creating a temperature difference between the cardboard and the glue layer, leading to water vapor condensation on the glue layer surface—this is known as “process condensation.”
Measured data: When the stack thickness exceeds 20 pairs of shoes, the temperature in the central area drops only 5°C within 30 minutes, while the edge area has already cooled to room temperature. The relative humidity on the glue layer surface in the central area can reach over 95%, lasting 2-3 hours. This time window is sufficient for residual mold spores to complete germination.
Countermeasure: Set up a forced cooling station after the bonding process, using axial fans to blow for 30 minutes to ensure the internal temperature of the shoe materials drops to room temperature before stacking. If the production line cannot accommodate an additional cooling station, insert moisture-proof spacers between each pair of shoes before stacking, or use iHeir-SP quick-drying fungicide for a secondary spray on the bonding surface. iHeir-SP has a fast solvent evaporation rate, forming a dry antibacterial film within 5 minutes.
Summary: Complete Technical Chain for Shoe Material Mold Prevention
iHeir-3 locks down the midsole cardboard as a hidden carrier, iHeir-JSTC cuts off the nutrient chain of the glue layer, and the cooling process eliminates the physical risk of process condensation—if any of these three links is missing, the entire mold prevention system may collapse from the weakest point. When formulating mold prevention plans, shoe factories should not only focus on mold treatment of the upper material but should also take the moisture content of the adhesive as a key control point, incorporating it into incoming material inspection and production line SOPs.