Polyurethane Shoe Sole Raw Materials: What Goes Into A PU Sole

A rotary moulding line in a sole factory runs thirty moulds per carousel turn. At the start of the season the colour is new, the moulds are new, and the first soles come out with fine pinholes along the toe. Nobody changed the polyol. The cause turns out to be a mould temperature five degrees lower than in the previous collection. Work like this shows that polyurethane shoe sole raw materials cannot be judged on paper alone: the system, the mould and the machine perform together, and buying the right materials starts with knowing what each one does.

01Why Polyurethane For Soles

Polyurethane became the standard for many sole types because it combines properties that are hard to get together from rubber, PVC or EVA. A microcellular PU sole is light, because water in the formulation blows it into fine cells, yet its outer skin is dense enough to resist wear.

• Low weight: densities well below rubber reduce fatigue in shoes worn all day.
• Cushioning: the cellular core absorbs impact at the heel and returns energy with each step.
• Abrasion resistance: particularly with polyester systems, soles last longer than many lighter alternatives.
• Design freedom: the liquid system reproduces fine textures, logos and colours from the mould.
• Bonding: direct injection bonds the sole to the upper without a separate adhesive step.

02What Is In A PU Sole System

A sole system is sold as two components. The formulator balances them so that the reaction is slow enough to fill every detail of the mould and fast enough to demould in a few minutes.

The Polyol Side

• Polyols: long-chain polyester or polyether polyols that form the soft, elastic part of the polymer. The choice between the two families is the most important decision in the system (see the next section).
• Chain extender: short diols such as ethylene glycol (MEG) or 1,4-butanediol build the hard segments that give hardness and tear strength.
• Catalysts: tertiary amines set cream time, gel time and demould time. Safety footwear systems can be tuned between single- and dual-density versions largely through the catalyst package.
• Blowing agent: water, which reacts with isocyanate and releases carbon dioxide; the amount sets the density.
• Cell stabiliser: a silicone surfactant that keeps the microcells fine and even.
• Pigments and additives: colour pastes, antistatic agents for safety soles, and sometimes anti-hydrolysis or antimicrobial additives.

The Isocyanate Side

Soles are almost always made with an MDI-based prepolymer rather than raw MDI. The prepolymer is produced by reacting MDI with part of the polyol, which gives a liquid with a stable NCO content, typically in the high teens to low twenties per cent for sole systems, easier mixing ratios and lower volatility.

03Polyester Or Polyether Soles

The polyol family decides how the sole ages and where it performs best. Polyester soles are tougher; polyether soles are more resistant to water and microbes.

In practice, many manufacturers keep both families in production and choose by shoe type and market.

04Typical Densities And Hardness By Sole Type

Density and hardness are set by the system and by how much material is shot into each mould. The figures below are typical industry ranges, not product specifications.

As a supplier example, Pluskim has produced sole and insole systems since 2016 and states hardness grades from 45 to 90 Shore A for its seasonal and sports soles.

05Applications And Their Special Demands

Unit Soles And Direct Injection

Unit soles are moulded separately and cemented to the upper later; direct-injection soles are moulded straight onto the lasted upper, which saves the adhesive step but demands tight control of flow and adhesion. Both run on low-pressure rotary machines with many stations.

Slippers And Insoles

Here low density and a soft touch matter most. The challenge is to keep the surface smooth and the cells fine at very low shot weights, which needs a stable system and accurate dosing.

Safety Footwear

Safety soles have to pass the tests of EN ISO 20345 as part of the finished shoe. For the sole material, the critical properties are:

• Slip resistance: on wet and oily floors, governed by compound, hardness and tread design.
• Antistatic behaviour: electrical resistance within the specified window, usually achieved with antistatic additives.
• Energy absorption: in the heel area, delivered by the cellular midsole.
• Oil and hydrolysis resistance: for long service in workshops, kitchens and outdoor sites.

Single-density systems are simpler; dual-density soles pair a light midsole with a compact outsole and need two components shot in sequence. Suppliers commonly offer safety footwear polyurethane systems in both versions, adjusted through the catalyst package.

06Processing: Machine, Mould And Timing

Most sole defects are processing defects. Four settings deserve a daily check.

1. Component temperature: polyester polyol blends are usually kept warm so they stay liquid and dose correctly; prepolymers are kept within their data-sheet range.
2. Mix ratio and shot weight: checked at start-up and after each batch change.
3. Mould temperature: typically somewhere around 40-55 °C; too cold gives a poor skin and pinholes, too hot can scorch or shorten flow.
4. Demould time: typically a few minutes; demoulding too early causes splits and shrinkage, too late wastes cycle time.

Line trials are the reliable way to qualify a new system, because laboratory cups do not reproduce mould temperature and flow. Pluskim, for example, commissioned a dedicated footwear application laboratory in 2019, equipped with a high-pressure injection machine, to test single- and double-density sole systems under conditions that replicate series production.

07Common Defects And Their Usual Causes

• Pinholes and surface voids: moisture in the polyol or on the mould, excess or wet release agent, mould too cold.
• Poor or peeling skin: mould temperature too low, unsuitable release agent, wrong ratio.
• Shrinkage: index too low, demoulding too early, density below the system's design range.
• Splits at demould: insufficient cure at the demould time, too little chain extender or catalyst.
• Soft or tacky areas: poor mixing or off-ratio dosing, often from temperature drift in one component.

Storage prevents a share of these defects before the machine is switched on. Polyol blends pick up moisture from the air, so drums and day tanks should stay closed and, for polyester systems, be kept warm and stirred so the blend does not separate. Prepolymers react with humidity and form a skin or crust; an opened drum is best blanketed with dry nitrogen and used within the period stated on the data sheet.

08What To Ask A Sole System Supplier

• Data sheet values for density, hardness, tensile and tear strength, abrasion loss and flex resistance, stated with test methods.
• Hydrolysis data for polyester systems, so the sole's ageing in heat and humidity is known before launch.
• A certificate of analysis per batch, with OH value, NCO content and viscosity.
• Support for line trials on your own moulds and machines, including colour matching.
• Safety documentation and help with the EU training requirement for diisocyanates, in force since August 2023.
• Capacity and logistics that match seasonal peaks; footwear demand is strongly seasonal.

09Conclusion

A good PU sole starts with the right pair of components: a polyester or polyether polyol blend matched to the shoe's working life, and an MDI prepolymer that gives a stable ratio. Density and hardness follow from the system and the shot weight, while mould temperature, release agent and demould time decide whether the surface is flawless. Buying with data sheets, batch certificates and a proper line trial turns those variables into a repeatable sole. For production-ready options, the footwear systems from Pluskim cover soles, insoles, slippers and safety footwear.

10Sources And Further Reading

1. SATRA Technology, footwear testing and research: new.satra.com
2. ISOPA, REACH restriction on diisocyanates: isopa.org/reach-restriction-on-diisocyanates
3. Safe use of diisocyanates, training portal: safeusediisocyanates.eu

11Frequently Asked Questions