The Effect of Stoichiometric Ratio in Cast Polyurethanes

Effects of Stoichiometric Ratio in Cast Polyurethanes In cast polyurethanes, the quantity of hardener to be added to the raw material (prepolymer) and the correct calculation and determination of this quantity are critical to the quality of the resulting elastomer product. Generally, for prepolymers produced with NCO terminology and containing free NCO (isocyanate) radicals, the amount of hardener to be added is calculated using the following formula: This formula serves to determine how many units of hardener should be added per 100 units of raw material (prepolymer) by weight. In simpler terms, it tells users how many grams of hardener should be added to 100 grams of prepolymer. Equivalent molecular weights of hardeners are calculated based on their molecular weights and number of functions. Below are the equivalent molecular weights of some commonly used hardeners: Although the %NCO value of the prepolymer and the equivalent molecular weight of the hardener are relatively constant values, the stoichiometric ratio value in the formula should be adjusted according to requirements and expectations. The stoichiometric ratio, in its simplest definition, refers to what percentage of the free NCO radicals in the prepolymer will be bonded with hardener. For example, a stoichiometric ratio of 0.95 (95%) means that theoretically 95% of the free NCO radicals in the prepolymer will be bonded with hardener, leaving theoretically 5% of free, unbonded radicals. Since MOCA is the most commonly used hardener type in our country, the use of MOCA and how the preferred stoichiometric ratio affects the final product quality and durability is summarized below. In our country, the formula for calculating MOCA quantity has generally been simplified to the following form: Conducted experiments and tests have shown that when a stoichiometric ratio of 0.95 (95%) is selected, the resistance to all effects such as tear, abrasion, flexibility, and permanent set is at optimum level. For this reason, a stoichiometric ratio of 0.95 is most commonly preferred in cast polyurethane manufacturing. Although the preference for a stoichiometric ratio of 0.95 is a common practice, the most correct method would be to determine the stoichiometric ratio based on the working environment and the effects that polyurethane elastomer products will be intensely exposed to in that environment. A stoichiometric ratio between 0.90 (90%) and 1.05 (105%) enables ideal results to be achieved, while staying outside this range—that is, going below 0.90 or above 1.05—will cause serious quality and durability problems in the final product. When the stoichiometric ratio falls below 0.90, demolding time will increase, and a product with lower hardness than expected will be obtained. At the same time, even if the product is subjected to postcure treatment and baked in a 100°C oven for 16 hours, the elastomer part will be so fragile that pieces can be torn off by hand or fingernail. Exceeding a stoichiometric ratio of 1.05 will lead to problems such as reduced abrasion resistance, elongation tendency under pressure or increased elongation, and easy breakage of the elastomer part under impact. For these reasons, it is very important that calculations be made and MOCA be added to the prepolymer such that the stoichiometric ratio remains between 0.90 and 1.05. While the ideal range is between 0.90 and 1.05, the durability values these ratios will provide can be summarized as follows:

0.90 Stoichiometric Ratio

If low permanent set and good dynamics under load are desired, work should be conducted with a 0.90 stoichiometric ratio. A 0.90 stoichiometric ratio is ideal, particularly in the production of parts such as heavy load wheels, hydraulic breaker striking wedges, and mold springs. If a 0.90 stoichiometric ratio is used and consequently less MOCA is used, the excess isocyanate in the system reacts with a substituted urea group and, being trifunctional, creates a biurea structure that is a chain branching point. The resulting biurea groups improve load-bearing performance. A 0.90 stoichiometric ratio is recommended to users, particularly when producing elastomer parts at hardnesses above 93 ShA-95 ShA and 55 ShD.

0.95 Stoichiometric Ratio

If the elastomer part is exposed to many different effects, the 0.95 stoichiometric ratio is the most ideal. When working environments that subject the elastomer part to abrasion and tearing, while also questioning its flexibility properties, are involved, the 0.95 ratio will give correct results. Since mining screens operate under the multiple effects mentioned above, the appropriate stoichiometric ratio for such products is 0.95.

1.00 Stoichiometric Ratio

If the elastomer part is particularly exposed to abrasion and tear effects and at the same time is desired to be somewhat flexible, the 1.00 stoichiometric ratio is the most ideal. This ratio should be preferred for parts such as conveyor belt scrapers.

1.05 Stoichiometric Ratio

If the elastomer part is intensely exposed to abrasion and tear effects, the 1.05 stoichiometric ratio is the most ideal. At a 1.05 ratio, while abrasion and tear resistance improves, it should not be forgotten that resistance to permanent set and flexibility properties deteriorate, and a choice should be made accordingly. Serkan İltan Rubber & PU Sales Manager Veser Kimyevi Maddeler A.Ş.