News Article

Evaluation of the Physical Properties of Epoxy Resins modified with blocked TDI prepolymer

Karl Prescott
Isothane Ltd,
Newhouse Road,
Huncoat Business Park,
Accrington, Lancashire.
BB5 6NT.
United Kingdom.


Coatings based on epoxy resins have a number of good characteristics but are prone to brittleness, which limits their use in areas where good impact resistance and flexibility are required. Incorporating a blocked prepolymer into the epoxy increases the flexibility of the hybrid product. Blocked TDI prepolymers which are chemically incorporated into the cured product are known to increase the flexibility of the hybrid product. TDI prepolymers blocked with nonyl phenol are known to be a suitable product however they have a high viscosity and there is increasing concern about the health and environmental effects of nonyl phenol.



Isothane BP42-241 is a branched TDI prepolymer with crosslinking blocked isocyanate groups. It is used as a raw material for the flexibilisation of epoxy resins and in the formulation of flexible coatings and sealants. The product contains an alternative blocking agent, which is bio based, to nonyl phenol. The viscosity achieved with this product is significantly lower than the traditional nonyl phenol blocked prepolymer product. The curing of this product is achieved by reaction of the blocked isocyanate groups with aliphatic or cycloaliphatic amines. This product differs from standard blocked isocyanates as it cures at ambient temperature rather than elevated temperatures The main application is to improve the flexibility and impact resistance of 2K epoxy coatings.

Isothane BP42-241 blocked TDI prepolymer can be used to formulate liquid, solvent free polyurethane systems. The blocking agent is not volatile and remains in the cured product. Isothane BP42-241 reacts readily with standard amine hardeners which are used in 2 pack epoxy systems, such as aliphatic and cycloaliphatic amines.

The viscosity of BP42-241 is significantly lower than nonyl phenol blocked prepolymers but it can be further reduced if necessary by adding common plasticisers such as diisononyl phthalate (DINP).

As supplied, the product has the following typical characteristics:

Viscosity @ 25°C, cPs 30,000 – 45,000
Appearance Clear viscous liquid
Calculated NCO content, % 2.1 – 2.4
Calculated NCO Equivalent Weight, % 1750 – 2000
Solids content, %  >99.5
Specific Gravity, g/cm3 1.03


Curing agents
The final properties are dependent on the choice of curing agent. For ambient cure, the best results are achieved with aliphatic or cycloaliphatic amines.

The following data was obtained by curing Isothane BP42-241 with Aradur 40 (cycloaliphatic amine, Huntsman) and Araldite GY250 (epoxy resin, Huntsman).

Starting Formulations

Formulation A B C D
Isothane BP42-241 (pbw) 100 80 60 40
Araldite GY250 (pbw) 0 20 40 60
Aradur 40 (pbw) 6.8 11.8 16.9 20.9


A film of each formulation was cast at approximately 2 mm thickness and allowed to cure for a least 48 hours. Tensile dumbbell specimens were cut from each sheet, from which an average tensile strength, elongation and hardness was determined.

Experimental results

BP42-241 (pbw) Epoxy resin (pbw) Amine (pbw) Tensile Strength (Mpa) Elongation (%) Hardness (oA) Hardness (oD)
100 0 6.8 1.9 560 53 13
80 20 11.8 4.9 180 88 34
60 40 16.9 13.0 160 58
40 60 21.9 24.7 140 71

The results above are typical values and do not constitute a specification.


Isothane BP42-241 showed excellent compatibility with a standard two pack epoxy system. Increasing the BP42-241 content increases the flexibility and elongation of the epoxy coating.




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Huncoat Industrial Estate,

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