Hand Batch Processing Steps

Following are guidelines for hand batching polyurethane prepolymer systems. The steps can be applied to Airthane and Versathane TDI prepolymers.

1. Weigh the desired amount of prepolymer into a suitable container that is 2-3 times larger than the volume of the prepolymer.
2. Heat the prepolymer in a convection, forced air or microwave oven. A hot plate may also be used. To prevent overheating when using a hot plate, continuously stir the prepolymer. Temperatures will vary based on the type of prepolymer. Generally, polyether-based materials should be heated to 160 to 185°F (70-85°C), while polyester-based materials should be heated to 160 to 203°F (70-95°C). Take special precautions to minimize the heat exposure of the prepolymer. At no time should it be heated to above 230°F (110°C), as excessive heating can cause the prepolymer to lose NCO content, which will dramatically affect the elastomer physical properties. Refer to your prepolymer product bulletin for specific heating guidelines.  

   Hours Required to Complete Melting Without Rolling Container
   
   

   	Polyester Type Prepolymers		Polyether Prepolymers
   	Regular	Quick Melts
   Recommended Temperature	160 °F (70 °C)	160 °F (70 °C)	160 °F (70 °C)
   1-Quart Can	4	2	2
   5-Gallon Pail	32	16	16
   55-Gallon Drum	72	24	24

   
   

3. Incorporate additives, plasticizers, fillers, pigments and degassing aides into the prepolymer.
4. Degas the prepolymer in a vacuum chamber until vigorous foaming stops. The addition of a drop of silicone surfactant will aide in degassing. The vacuum chamber should reach 1mm to 5mm of Hg vacuum.
5. Weigh the correct amount of curative. Heat the curative to the recommended temperature. Liquid curatives like Dabco® BDO, Voranol® 234-630 and Ethacure® 300 can be used at room temperature. Solid curatives, on the other hand, like MBOCA, Versalink® 740M and Lonzacure® MCDEA should be heated to the temperature indicated in the product literature.
6. Reheat the degassed prepolymer to the desired mixing temperature. Note that the mixing temperature will vary based on the type of backbone, and it will affect the length of the pot life. The higher the temperature, the shorter the pot life.
7. When the prepolymer has reached the mixing temperature, add the curative and mix the material thoroughly. Scrape the walls of the container during mixing to ensure a homogeneous mix. Upon completion of mixing, swirls should not be apparent.
8. To obtain the best results, return the mixture to the vacuum chamber to remove air entrapped during mixing. This second degassing step can be done in 1-2 minutes. Systems with pot lifes of less than 5 minutes can not be degassed after mixing.
9. Pour the mixture into a preheated mold that has been treated with a release agent. The mold temperature will vary based on the type of system. Generally, TDI systems using diamine curatives (MBOCA, Versalink 740M, Lonzacure MCDEA or Ethacure 300 curatives) will require mold temperatures of 212°F (100°C), while polyol-cured systems (Voranol 234-630 curative) will require mold temperatures of 248°F (120°C).
10. Mold temperature should be maintained until the cured elastomer has obtained sufficient strength to be demolded. Based on the system, this may occur in as quickly as 15 minutes or in as long as several hours. Generally, the shorter the pot life, the shorter the demold time.
11. After demolding, the finished part should be subjected to appropriate post-cure. TDI systems can be post-cured at 212°F (100°C) for a period of 4 to 16 hours.

Trademarks and Suppliers

Ethacure...................................... Albemarle Corporation

Lonzacure........................................... Lonza Group Ltd.

Voranol............................................. Dow Chemical Co.

Please refer to the appropriate Air Products product literature to obtain additional information on the processing of a particular polyurethane prepolymer system.