The polyurethane resin reaction kettle is the core equipment for gradually adding isocyanates and polyols (or other active hydrogen compounds) to generate polyurethane resin. Its working principle revolves around "precise control of reaction conditions to ensure stable synthesis process and uniform product quality", involving multiple links such as raw material pretreatment, mixing reaction, parameter regulation, and product separation, as follows:
The synthesis core of polyurethane resin isGradual addition reaction of isocyanate groups (- NCO) with active hydrogen groups (such as hydroxyl OH, amino NH ₂, etc.), mainly including:
Isocyanates react with polyols (such as polyester polyols and polyether polyols) to form polymers containing amino ester bonds (- NH-CO-O -);
If it involves chain extension/crosslinking, isocyanates will further react with small molecule chain extenders (such as ethylene glycol, ethylenediamine) to increase molecular weight or form crosslinked structures.
The reaction isexothermic reactionAnd it is sensitive to moisture, temperature, and raw material ratio (such as water reacting with - NCO to produce CO ₂, leading to product foaming); High temperature may cause gel), so the reactor needs to precisely control these conditions.
The structural design of the reaction vessel needs to be adapted to the above reaction characteristics, mainly including the following key components, whose functions directly serve the reaction requirements:
| core component |
function and role |
| Cauldron body |
Provide reaction space, mostly made of stainless steel (corrosion-resistant), with smooth inner walls to reduce material residue. |
| stirring device |
Mix the raw materials evenly to avoid local reaction overheating or uneven distribution of raw materials (such as anchor type or paddle type stirring blades, adjustable speed). |
| Heating/cooling system |
Regulating the reaction temperature (cooling is required for exothermic reactions, heating is required for initial reactions), common method: jacket with thermal oil/cold water. |
| feeding system |
Accurately control the ratio of isocyanates, polyols, and additives (catalysts, flame retardants, etc.) through metering pumps such as gear pumps and plunger pumps. |
| Vacuum/Inert Gas System |
Vacuum removal of moisture and air from raw materials (to avoid side reactions); Nitrogen (inert gas) is introduced to isolate the air and maintain a slight positive pressure. |
| control system |
Real time monitoring of temperature, pressure, and stirring speed, and automatic adjustment of heating/cooling and feed rate through PLC to ensure stable reaction. |
| Discharge and cleaning system |
After the reaction is completed, discharge the material through the bottom valve; A high-pressure cleaning device (such as a spray ball) is used to remove residual materials from the kettle. |
The working process of polyurethane resin reaction kettle can be divided intoPreprocessing → Feed mixing → Reaction regulation → Discharge cleaningThe four major stages are closely interconnected:
Dehydration treatmentPolyols (such as polyether polyols) are prone to water absorption, while water reacts with - NCO to generate CO ₂ (leading to product foaming and performance degradation). Therefore, the raw materials need to pass through the kettle (or pre-treatment tank)Vacuum heatingDehydration (such as 80-120 ℃, vacuum degree -0.09MPa or above), the vacuum system of the kettle is started at this stage to extract water in the form of steam.
Preheating treatmentSome raw materials (such as high viscosity polyols) need to be heated to reduce viscosity for subsequent transportation and mixing. At this time, the heating system (jacket with heat transfer oil) is started to preheat the raw materials to 30-60 ℃.
Feed proportionallyPump the pre treated isocyanates, polyols, catalysts (such as organotin and amines), and additives (such as antioxidants and colorants) into the kettle according to the process formula (such as - NCO/- OH molar ratio 1.0-1.2) using a metering pump. The control system provides real-time feedback on the feed rate through a flow meter to ensure that the proportioning error is ≤ 1%.
Forced mixing and blendingAfter the raw materials enter the kettle, the stirring device (usually at a speed of 50-300r/min) is started to quickly mix the materials through the blade shear force. For high viscosity systems (such as in the later stages of the reaction), variable frequency stirring may be used to reduce the rotational speed as the viscosity increases, in order to avoid local overheating.
The polyurethane reaction is an exothermic reaction, and the reaction rate is significantly affected by temperature and catalyst concentration. The reaction kettle needs to be precisely controlled through the following methods:
temperature controlIn the initial stage, the material is heated to the starting temperature of the reaction (such as 50-80 ℃) through jacket heating; When the reaction is exothermic, the jacket is switched to cold water (or low-temperature heat transfer oil) to stabilize the temperature in the process range (such as 60-100 ℃, depending on the product type: soft foam may be 60-70 ℃, and elastomer may be 80-100 ℃). If the temperature is too high, the reaction may be out of control (such as "explosive polymerization" to generate gel); If the temperature is too low, the reaction will not occur and the molecular weight of the product will be insufficient.
Pressure and Atmosphere ControlIf it is necessary to isolate the air (to avoid oxidation) or suppress volatilization, fill the kettle with nitrogen through a nitrogen system and maintain a slight positive pressure (0.01-0.05MPa); If it is necessary to remove small molecules generated by the reaction (such as trace amounts of water in the chain extension stage), the vacuum system can be opened for extraction.
Stepwise reaction controlFor the synthesis of prepolymers (such as preparing prepolymers with excess isocyanates first, and then expanding the chain), the reaction kettle can be fed in stages (adding polyols and some isocyanates to form prepolymers first, and then adding chain extenders), and the stirring rate can be adjusted to ensure uniform chain expansion and avoid local cross-linking.
dischargeAfter the reaction reaches the endpoint (determined by monitoring NCO residue, viscosity, and other indicators), stop stirring, open the bottom discharge valve (or use a pressure pump), and transport polyurethane resin (such as prepolymer, elastomer slurry) to subsequent processes (such as molding, filling).
washAfter discharge, solvent (such as acetone, DMF) or hot water is sprayed into the kettle through a spray ball, and the residual materials on the kettle wall are mixed and washed to avoid affecting the next batch reaction (such as residual NCO which may cause early cross-linking of the next batch of materials).
All operations of the reaction vessel revolve around the performance requirements of polyurethane resin:
If the product isHigh viscosity polyurethane elastomer, strictly control the reaction temperature (avoid local overheating leading to gel) and mixing uniformity (ensure narrow molecular weight distribution);
if it isPolyurethane prepolymerFor subsequent curing, precise control of NCO residue is required (by adjusting the raw material ratio and reaction time), and the metering system and time control of the reaction kettle are key.
In summary, the working principle of the polyurethane resin reaction kettle is essentially to provide a stable and controllable environment for the addition reaction of isocyanates and polyols by integrating mechanical stirring, temperature/pressure regulation, precise feeding, and other functions, ultimately achieving efficient synthesis and quality assurance of polyurethane resin