Heat Reactive Polyurethane Adhesive, commonly known as Reactive PU Hot Melt (HMPUR), is a high-performance adhesive technology used where strong, flexible and durable bonding is required. Unlike conventional hot-melt adhesives, reactive polyurethane hot melts can continue curing after application when they come into contact with atmospheric moisture.

This makes HMPUR useful for applications such as footwear, furniture, automotive interiors, textile lamination, mattresses and selected plastic bonding applications.

What Is Heat Reactive Polyurethane Adhesive?

Heat Reactive Polyurethane Adhesive is a polyurethane-based adhesive that is heated to become fluid enough for application. After application, it cools and provides initial bonding, while the remaining reactive groups can subsequently react with moisture to develop stronger final bonding.

The basic bonding sequence is:

Heat → Melt/Flow → Apply → Cool → Initial Bond → Moisture Reaction → Final Bond

The combination of hot-melt processing and reactive curing provides a useful balance between processing speed and final adhesive performance.

Main Advantages

Some important characteristics of reactive PU hot-melt adhesives include:

• Strong bonding
• Flexible adhesive film
• Good durability
• Heat resistance after curing
• Moisture resistance after proper curing
• Good adhesion to various substrates
• Fast initial setting after cooling
• Suitable for automated adhesive application systems
• Potentially higher final performance than conventional non-reactive hot melts

Actual performance depends heavily on the polyurethane chemistry, substrate, application temperature, coating weight and curing conditions.

1 KG Starting Formulation

The following is a starting formulation, not a universally fixed commercial formulation.

Raw Material	Quantity
Polyester Polyol, OH value approximately 50–60 mg KOH/g	570 g
Polyether Polyol, OH value approximately 50–60 mg KOH/g	100 g
4,4′-MDI, approximately 31–33% NCO	225 g
TPU / PU Modifier	40 g
Tackifying Resin	35 g
Antioxidant	1.5 g
PU Catalyst	0.5 g
Subtotal	972 g

Important Formulation Note

The above quantities should not be treated as a final production formula.

Reactive PU hot-melt formulation requires calculation based on the actual:

• OH value of each polyol
• Molecular weight/functionality of the polyols
• %NCO of the selected MDI
• Desired NCO/OH ratio
• Target residual NCO
• Desired viscosity
• Application temperature
• Required curing speed
• Required open time and final bond strength

Therefore, the remaining quantity required to make exactly 1 kg should be determined after the actual raw-material specifications and NCO/OH calculation are available.

Function of Each Raw Material

1. Polyester Polyol

Polyester polyol forms an important part of the polyurethane backbone. It can contribute to:

• Adhesive strength
• Cohesion
• Flexibility
• Substrate adhesion
• Chemical resistance

The choice of polyester polyol can significantly change the final viscosity, flexibility and resistance profile.

2. Polyether Polyol

Polyether polyol can be incorporated to modify flexibility and low-temperature performance.

It may also influence:

• Elasticity
• Moisture interaction
• Flexibility
• Adhesive film properties

The correct ratio between polyester and polyether polyols depends on the target application.

3. 4,4′-MDI

4,4′-MDI is the principal isocyanate component used to create the polyurethane structure.

It reacts with hydroxyl groups from the polyols and creates polyurethane linkages.

In a reactive PU hot melt, controlled residual isocyanate functionality can subsequently react with moisture during curing.

4. TPU / PU Modifier

A TPU or compatible polyurethane modifier can be used to adjust:

• Melt strength
• Flexibility
• Toughness
• Initial bonding
• Rheology

Compatibility with the selected polyurethane system must be evaluated before commercial production.

5. Tackifying Resin

Tackifying resin can improve initial tack and substrate wetting.

The correct resin depends on the chemistry of the adhesive and the substrates being bonded.

6. Antioxidant

An antioxidant helps protect the adhesive from thermal and oxidative degradation during processing and storage.

7. PU Catalyst

The catalyst controls the rate of polyurethane-related reactions.

Too much catalyst can accelerate reaction excessively, while too little may produce slow curing.

The catalyst should therefore be selected according to the required processing and curing profile.

Manufacturing Process

Step 1: Dry the Polyols

Polyols should be heated under controlled conditions and vacuum-dried to remove moisture.

Moisture is particularly important in reactive PU systems because it can react with isocyanate groups.

Step 2: Add Polyester and Polyether Polyols

Transfer the dried polyols into a suitable reaction vessel equipped with controlled heating and agitation.

Mix until the system becomes homogeneous.

Step 3: Add TPU / PU Modifier

Add the polyurethane modifier gradually while maintaining controlled temperature and agitation.

Continue mixing until a uniform melt is obtained.

Step 4: Add Tackifying Resin

Add the selected tackifying resin and continue mixing until completely incorporated.

The mixture should remain homogeneous without visible undissolved particles.

Step 5: Controlled MDI Addition

MDI should be added under dry/inert and moisture-controlled conditions.

The addition rate and reaction temperature should be controlled to prevent unwanted side reactions and excessive viscosity development.

Step 6: Add Antioxidant and Catalyst

Once the main polyurethane system has reached the desired reaction stage, add the required additives under controlled conditions.

Mix thoroughly.

Step 7: Vacuum Degassing

Apply suitable vacuum conditions to remove entrapped air and volatile components.

Avoid unnecessary exposure of the reactive adhesive to atmospheric moisture.

Step 8: Hot Filling

The finished adhesive can be filled while hot into moisture-barrier packaging.

The packaging should be sealed immediately to prevent moisture contamination.

How Does HMPUR Work?

Reactive PU hot melt has two important stages of bonding.

Stage 1 — Physical Bonding

When heated, the adhesive becomes fluid and can wet the substrate.

After application, cooling causes the adhesive to solidify and provide initial bond strength.

Stage 2 — Chemical Curing

Remaining reactive isocyanate groups can react with moisture from the surrounding environment.

This produces additional chemical crosslinking and increases final bond performance.

Therefore, the adhesive can provide both:

Fast initial bonding + subsequent reactive curing

Applications

Heat Reactive Polyurethane Adhesives are used in several industrial sectors.

Footwear

Used for selected bonding operations involving:

• Shoe soles
• Uppers
• Insoles
• Synthetic materials
• Textile components

Furniture & Woodworking

Potential applications include:

• Edge bonding
• Panel lamination
• Decorative surface bonding
• Furniture component assembly

Automotive

Used in selected interior applications such as:

• Door panels
• Interior fabrics
• Headliner components
• Trim materials

Textile & Lamination

Reactive PU hot melt can be used for selected:

• Textile lamination
• Fabric bonding
• Composite structures
• Functional textile assemblies

Mattress & Upholstery

Applications can include selected combinations of:

• Foam
• Fabric
• Textile
• Upholstery materials

Plastic Bonding

Depending on the adhesive grade and surface preparation, HMPUR can be used with selected plastics such as PVC, ABS and other compatible substrates.

Important Quality Control Parameters

Before commercial production, the adhesive should be evaluated for:

• Appearance
• Melt viscosity
• Softening/melting behavior
• Application temperature
• Open time
• Initial tack
• Final bond strength
• Peel strength
• Shear strength
• Heat resistance
• Moisture resistance
• Residual NCO
• Curing time
• Storage stability

Testing should always be performed using the actual intended substrates.

NCO/OH Ratio: The Most Important Calculation

One of the biggest differences between ordinary hot-melt formulation and reactive PU hot-melt formulation is the importance of NCO/OH stoichiometry.

The MDI quantity cannot reliably be selected simply by saying:

“Use X grams of MDI per kilogram.”

Instead, the OH equivalents contributed by the polyols must first be determined and then matched against the NCO equivalents supplied by the isocyanate.

The desired NCO/OH ratio depends on the adhesive design and required residual NCO.

Therefore, the exact commercial formula should be calculated from the supplier’s Certificate of Analysis (COA) for the selected polyols and MDI.

Safety Precautions

MDI and reactive polyurethane systems require appropriate industrial safety controls.

Important precautions include:

• Use appropriate chemical-resistant gloves.
• Use suitable eye and face protection.
• Provide adequate ventilation.
• Avoid inhalation of vapors/aerosols.
• Keep raw materials dry.
• Prevent uncontrolled moisture exposure.
• Use temperature-controlled equipment.
• Follow the supplier’s SDS for MDI and other raw materials.
• Do not use an open vessel for reactive PU production.
• Conduct small laboratory trials before scaling up.

Storage

Reactive PU hot-melt adhesive is highly sensitive to moisture.

Recommended practices include:

• Use airtight moisture-barrier packaging.
• Keep containers tightly sealed.
• Store in a cool, dry environment.
• Avoid unnecessary opening of containers.
• Protect the product from atmospheric humidity.

Actual shelf life should be established through stability testing and the raw-material supplier’s recommendations.

Frequently Asked Questions

What is HMPUR adhesive?

HMPUR stands for Hot Melt Polyurethane Reactive adhesive. It is a polyurethane adhesive that is applied in molten form and subsequently undergoes moisture-reactive curing.

Is reactive PU hot melt waterproof?

Properly formulated and cured HMPUR can provide good moisture and water resistance, but the actual resistance depends on the formulation, curing level and substrate.

What is the difference between normal hot melt and reactive PU hot melt?

Conventional hot melt generally bonds primarily through cooling and solidification. Reactive PU hot melt additionally undergoes chemical curing after application.

Can HMPUR be used for footwear?

Yes. Reactive PU hot melt technology is used in various footwear bonding applications, although the exact adhesive grade must be selected according to the materials and required bond performance.

Why is moisture control important?

Moisture reacts with isocyanate groups. Uncontrolled moisture can cause premature reaction, viscosity changes, foaming and reduced product stability.

Can the same formula be used for every application?

No. A formulation optimized for footwear may not be suitable for automotive, textile, furniture or plastic applications. Polyol selection, NCO level, viscosity, open time and curing behavior should be adjusted according to the application.

Conclusion

Heat Reactive Polyurethane Adhesive is an advanced adhesive system that combines the fast processing characteristics of hot-melt adhesives with the subsequent curing capability of polyurethane chemistry.

For successful commercial production, the most important factors are correct NCO/OH calculation, moisture control, controlled processing temperature, suitable raw-material selection and application-specific testing.

The formulation provided above should be treated as a development starting point, and the final composition should be established using the actual specifications of the selected polyols and MDI.