The main component of polyurethane foam (PU foam) is polyurethane. The raw materials are mainly polyisocyanates and polyols. By adding appropriate additives—the most important of which is a series of foaming additives related to foaming— -A large amount of foam is generated in the reaction product, thereby obtaining a polyurethane foam product. This article will briefly introduce the raw materials and foaming aids for the production of PU foam.

1. Polyisocyanate
The most commonly used polyisocyanates in the production of polyurethane foams are toluene diisocyanate (TDI), polymethylene polyphenyl isocyanate (PAPI), diphenylmethane diisocyanate (MDI) and liquefied MDI (L-MDI).

TDI is mainly used in the foam plastics industry to produce flexible polyurethane foam. MDI is more reactive than TDI and less volatile. Some liquefied modified MDI can be used as a substitute for TDI in the production of flexible polyurethane foam, such as high-density flexible polyurethane foam and semi-rigid foam or microcellular polyurethane elastic materials. manufacture.

PAPI is also called crude MDI and polymeric MDI. The average molecular weight of typical PAPI products is most in the range of 30 to 400, and its NCO mass fraction is 31% to 32%. The average functionality of low viscosity PAPI is generally between 2.5 and 2.9.

In the field of foam plastics, PAPI and modified PAPI are mainly used to produce various rigid polyurethane foams, and a small amount are used to produce high resilience soft foams, whole skin foams, and semi-rigid foams. PAPI can be mixed with TDI to make cold-cured, high-resilience foam. 2. Polyether and polyester polyol

1. Polyether polyol
Polyether polyols used to produce flexible polyurethane foams are generally long-chain, low-functionality polyethers. The functionality of polyether polyol in the soft foam formulation is generally 2~3, and the average molecular weight is between 2000-6500. Polyether triol is the most commonly used in soft foam. Generally, glycerin (glycerol) is used as the starting agent, which is obtained by ring-opening polymerization of 1,2-propylene oxide or copolymerized with a small amount of ethylene oxide. The molecular weight Generally between 3000~7000.

Among them, high-reactivity polyether is mainly used for high-resilience soft foam, and can also be used for foam products such as semi-rigid foam. A small amount of polyether glycol can be used as an auxiliary material and mixed with polyether triol in soft foam formulations. Polyether polyols with low unsaturation and high molecular weight can be used to produce soft foams and reduce the amount of TDI.

Generally, polyether polyols with high functionality and high hydroxyl value are used in rigid foam formulations, so that sufficient crosslinking and rigidity can be produced. The hydroxyl value of rigid foam polyether polyol is generally 350~650mg KOH/g, and the average functionality is above 3. General hard foam formulations are mostly mixed with two polyethers, and the average hydroxyl value is about 4000mg KOH/g.

Semi-rigid foam formulations generally use part of high-molecular-weight polyethers, especially high-reactivity polyether polyols and part of high-functionality, low-molecular-weight rigid foam polyethers.

2. Polyester polyol
Ordinary low-viscosity aliphatic polyester polyols such as polydiethylene adipate diol with a hydroxyl value of about 56 mg KOH/g or slightly branched polyester polyols can be used to make polyester Polyurethane soft foam. Polyester polyol has high reactivity. At present, polyester PU block foam is only used in a small number of areas such as clothing accessories.

Aromatic polyols synthesized from dibasic acids (such as phthalic anhydride, terephthalic acid, etc.) and small molecular diols (diethylene glycol, etc.) or polyols. Among them, the high hydroxyl value can be used to produce polyurethane rigid foam and polyols. Isocyanurate rigid foam. The lower hydroxyl value of phthalic anhydride polyester alcohol can also be used for high resilience flexible foam, whole skin foam and semi-rigid foam, and non-foam polyurethane materials.

3. Polymer polyol
Polymer polyols (grafted polyether polyols) contain rigid styrene, acrylonitrile homopolymers and copolymers and graft polymers. These vinyl polymers act like organic "fillers" to improve load-bearing performance .

Polymer polyols can be used to produce high-hardness flexible foams, high-resilience foams, heat-molded soft foams, semi-rigid foams, self-skinned foams, reaction injection molding (RIM) products, etc., which can reduce product thickness and reduce The foam density reduces the cost, can also increase the open cell of the foam plastic, and give the product a certain flame retardant performance.

Polyurea polyol (PHD dispersion) is also a special polymer modified polyol, which can be used for high resilience soft foam, semi-rigid foam and soft foam. There are few products on the market at present.

There are also some special polyols used in the production of polyurethane foams, such as vegetable oil polyols, rosin polyester polyols, and polymer polyester polyols.

3. Foaming aids
Foaming aids are indispensable in the manufacture of polyurethane foams, including catalysts, foam stabilizers (foam stabilizers), foaming agents, etc. There are also some additives that are optional and can be used when needed, such as barriers. Burning agent, chain extender/crosslinking agent, antioxidant, light stabilizer, foam softener, cell opener, filler, color paste, antistatic agent, hydrolysis stabilizer, foam composition storage stabilizer, etc.

1. Foaming agent
Water is an important foaming agent in the production of polyurethane materials. It is a chemical foaming agent. The carbon dioxide gas generated by the reaction with isocyanate expands, foams, and solidifies the viscoelastic foam material to obtain various polyurethane foams.

Because carbon dioxide has high thermal conductivity and strong permeability, physical foaming agents must be used for rigid polyurethane foam formulations that require high thermal insulation properties. Because the initial stage of material mixing in rigid foam production generates a lot of heat within tens of seconds, it needs the foaming agent to absorb part of the heat, and at the same time, the gasification of the foaming agent makes the foam expand and foam.

In the production of flexible polyurethane foams, in order to obtain low-density flexible foams without too much water causing the foam to become rigid, it is generally necessary to control the amount of water and add an appropriate amount of physical foaming agent as an auxiliary foaming agent.

CFC-11 (Trichlorofluoromethane) realized industrial production in the late 1920s. Because CFC-11 has the characteristics of non-combustibility, suitable boiling point, easy gasification, low gas thermal conductivity, low toxicity, good compatibility with polyurethane raw materials, non-corrosive, low price, simple foaming process, etc., it is a polyurethane foam production Very ideal blowing agent in the medium.

From the 1960s to the early 1990s, CFC-11 was widely used as a blowing agent for polyurethane foams. However, in the 1970s, scientists discovered that CFC-11 emitted in the atmosphere could slowly destroy the ozone layer, which attracted the attention of environmentalists worldwide. The main types of blowing agents currently used as alternatives to CFC-11 are HCFC (hydrochlorofluorocarbon), HFC (hydrofluorocarbon), HC (alkane), liquid CO2, and water.

2. Foam stabilizer
When producing polyurethane foam, foam stabilizer (or foam stabilizer) is an indispensable component. It can increase the mutual solubility of each component, and play the role of emulsifying foam materials, stabilizing foam and adjusting cells. Foam stabilizers are surfactants, and there are two types of non-silicon compounds and organosilicon compounds.

Most of the foam stabilizers currently in use are polysiloxane oxyalkylene block copolymers, which are polyether modified silicone surfactants, sometimes commonly referred to as "silicone oil" in the industry. Due to the wide range of structural composition of this type of surfactant and good use effect, polyether-modified silicone surfactants have been widely used as foam stabilizers in the polyurethane foam industry.

3. Pore opener
Method of obtaining open-cell polyurethane foam:

Use a suitable catalyst to achieve the required balance between the gel reaction and the foaming reaction. When the foam material rises to the highest point, the cell wall film strength is not enough to seal the bubbles inside, and the gas breaks out of the wall to form open cells. The foam structure;
Use suitable polyether polyol raw materials to form open-cell foam;
When the catalyst and the main raw materials are not enough to solve the problem, a small amount of cell opener is used to disperse the urea formed by water foaming to obtain a foam with a certain open cell ratio.

Cell opener is a special type of surfactant, generally containing hydrophobic and hydrophilic segments or groups. Its function is to reduce the surface tension of the foam, promote cell rupture, and increase the open cell rate of polyurethane foam. Improve the shrinkage of soft, semi-rigid and rigid foam plastic products caused by closed cells.

Due to the high cross-linking density of ordinary rigid polyurethane foam, the cell wall film strength in the foaming is high, and it is generally a closed-cell structure. However, by adding a cell-opening agent, an open-cell rigid polyurethane foam can be manufactured for noise reduction, Purposes such as filtering.

Early hydrophobic liquid paraffin, polybutadiene, dimethyl polysiloxane, etc. can be used as foam stabilizers and cell openers. Paraffin dispersions and polyethylene oxide can also be used as cell openers. At present, special Chemical composition of polyoxypropylene-ethylene oxide copolyether, polyoxyalkylene-polysiloxane copolymer, etc. are used as pore-opening agents.

4. Softener
The use of softeners in the production of flexible polyurethane foams formulated with high water content can inhibit the stiffness of the foam caused by excessive urea groups. The foam softening modifier has a softening effect. The use of softeners can reduce the amount of isocyanate and thus the hardness of the foam. It is used in the production of flexible polyurethane foam. Commercial softeners generally contain special polyethers, special polyols and water.