Poor Physical Properties
Inferior elastomer physical properties will generally show up in field performance, though testing is possible to confirm abnormal behavior before the part gets to a customer. Repetitive errors, such as incorrect stoichiometry and incomplete cure, are easy to determine through testing. Random errors, such as an overheated drum of prepolymer, are more difficult to spot.
Potential Causes
Incorrect Stoichiometry
Maintaining the correct ratio of prepolymer to curative, which is called stoichiometry, is critical in producing consistent, high-quality elastomers. The typical ratio used to obtain the best combination of properties is 95%. The table below outlines the effect of stoichiometry on elastomer physical properties.
| Hardness |
Stable; minor change over the range of 85-100% |
| Modulus |
Stable; minor change over the range of 85-100%; decrease outside this range |
| Break Tensile |
Maximizes at 90-95%; slight decrease outside this range |
| Tear Strength |
Maximizes at 100-105%; significant decrease below this range |
| Elongation |
Maximizes at 100-105%; minor decrease below this range |
| Compression Set |
Low percent stoichiometry results in low compression set |
| Abrasion Resistance |
High percent stoichiometry favors good abrasion resistance; 100-105% is optimum range |
| Hysteresis, Dynamic Mechanical |
Low percent stoichiometry is preferred; 90-95% is optimum |
| Flex Life |
High percent stoichiometry favors good flex performance; 100-105% is optimum range |
| Resilience |
Maximizes at 85-90%; slight decrease above this range |
Overheated Prepolymer
Use of an overheated prepolymer will have the same ultimate effect on the elastomer as processing at an incorrect stoichiometry. You will be able to tell that a prepolymer has become overheated because it will have a lower percent NCO than what is indicated on the prepolymer container.
Incomplete Cure
Post-curing is used to relieve internal stresses and to allow proper alignment of the polymer chains.
Solutions
Calibrate Balances Regularly
Balances must be calibrated regularly. In a production environment, balances can become covered with prepolymer, curative and other materials. If these materials are on the weighing pan, they can force the balance out of calibration.
Calculate the Amount of Curative Based on the NCO of Each Lot of Prepolymer
Many processors will use an average percent NCO for determining the amount of curative to use with a prepolymer. However, with some prepolymers, this can amount to a stoichiometry error of up to 5%, which can affect properties significantly.
Each drum of prepolymer has a specific percent NCO. It is important to use the percent NCO of the particular prepolymer lot that you are using. The amount of curative to use can then be determined with the following equation:
Minimize the System Heat History
Follow the manufacturer's guidelines for heat exposure of both prepolymer and curative.
Follow Supplier Curing Guidelines
Prepolymer suppliers have standard recommendations for the curing of their materials. A typical guideline is to cure the prepolymer for 16 hours at 100°C, though please verify this recommendation for each system. Some prepolymer/curative combinations require different conditions.
Use an Elemental Analyzer During Production
For Mboca, Lonzacure® MCDEA curative or Ethacure® 300 curative-based elastomers, an analyzer with an X-ray source can be used to determine the stoichiometry during production. |
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