During the synthesis and preparation of polyurethane, the acidity and alkalinity of the system have a direct impact on the reactive. In particular, the presence of some impurities in the raw materials can significantly change the reaction speed, which may cause the system to react faster and gel; it may also make the reaction speed difficult to control, which directly affects the stability of the product and is not conducive to production control. The purpose of this paper is for the majority of polyurethane researchers to pay attention to the acidity and alkalinity of the system and avoid unnecessary troubles caused by it.
1. Factors affecting the pH of the system
(a) Presence of ionic impurities. For example, potassium and sodium ions are easily present in Polyether Polyols, which will make the system alkaline and cause the reaction speed to be too fast and cause the system to gel.
(b) The presence of acidic impurities. Polyester polyols synthesized from diols and diacids usually have a certain amount of diacids remaining. The presence of these acids will make the system weakly acidic. If the acidity is too large, the reaction speed will be too slow or the system reaction will be incomplete. Affect the physical and chemical properties of the final product.
2. Adjustment and application of pH of the system
The pH of the polyurethane reaction system is related to the reaction speed of the system, so in some specific cases, the pH of the system needs to be adjusted. Here are some typical examples so that clients can better use the pH for the development and production of polyurethane products.
Problem: The presence of a large number of small molecular chain extenders in the system leads to too fast reaction speed and severe system exotherm. It is easy to cause local overheating to cause gelation or difficult to control reaction.
Solution: Add a small amount of acidic substances such as phosphoric acid, benzoyl chloride, oleic acid, azelaic acid, and adipic acid to the system to control the reaction speed of the reaction system.
Problem: Polyurethane prepolymers containing NCO have low stability and are difficult to store.
Solution: Add a small amount of acidic substances such as phosphoric acid, benzoyl chloride, oleic acid, azelaic acid, and adipic acid to the system to reduce the NCO reactivity.
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