Production process flow - polyurethane elastomer

1. Polyurethane elastomer

Polyurethane elastomer, also known as polyurethane rubber, is a special category of elastomers. It has a wide variety of raw materials, various formulas, and a wide range of adjustments. The hardness range of polyurethane elastomer is very wide, ranging from low modulus rubber below Shore A10 to high impact rubber elastic material of Shore D85. Therefore, the performance range of polyurethane elastomer is very wide, and it is a kind of polymer material between rubber and plastic.

The so-called elastomer refers to a polymer material with a glass transition temperature lower than room temperature, a tear elongation of >50%, and good recovery after the external force is removed, while a polymer material with a glass transition temperature higher than room temperature is called plastic. Among elastomers, those with a large tear elongation (>200%), a small 100% elongation stress (such as <30Mpa), and good elasticity can be called rubber. Therefore, elastomers are a more extensive class of polymer materials than rubber.

2. Main raw materials of polyurethane elastomers

The raw materials used for polyurethane elastomers are mainly divided into three categories, namely oligomer polyols, polyisocyanates and chain extenders (cross-linking agents). In addition, in order to increase the reaction speed, improve the processing performance and product performance, some compounding agents need to be added. The following is a specific description of the raw materials used in the production of polyurethane saddles.

Reaction process: Polyol reacts with diisocyanate to produce low molecular weight prepolymer; after chain extension reaction, high molecular weight polymer is produced; then appropriate cross-linking agent is added to produce polyurethane elastomer. The process flow is as follows:

2.1 Polyester polyols

Polyester polyols, referred to as polyesters, are one of the most important raw materials for polyurethane elastomers. It is formed by the polycondensation of dicarboxylic acids and polyols. The most commonly used dicarboxylic acid is adipic acid, and the most commonly used polyols are ethylene glycol, propylene glycol, butanediol, and diethylene glycol. In addition, some special polyesters also use polyols such as pentanediol, ethylene glycol, trimethylolpropane, and glycerol. Due to the wide variety of available polyols, the molecular structure of polyesters is diverse, and there are many varieties and brands. In order to obtain hydroxyl-terminated polyesters, an excess of polyols must be reacted with dicarboxylic acids. Generally, polyesters are produced by an intermittent method. The reaction process is divided into two stages: esterification reaction and transesterification reaction. The production equipment is similar, and the general process flow, the main equipment includes condensation kettle, fractionation condenser, condenser, metering tank, vacuum system, heating and cooling system, and control system. The airtightness requirements of the entire system are very strict. The stirring shaft of the condensation kettle can be sealed with an end face mechanical seal. The order of adding materials is to add polyols and compounding agents first, then adipic acid, and then nitrogen filling. The esterification reaction is basically completed about 1 hour after heating to 220~250℃. This stage is mainly a dehydration process at normal pressure, generating low molecular polyester and condensation water. When the temperature rises to about 135℃, the esterification reaction is most intense, generating a large amount of condensation water. Due to the evaporation of condensation water, a large number of bubbles will rise, and the bubbles are particularly intense when 1,4-butanediol and 1,6-ethylene glycol are used as raw materials. At this time, the heating power should be adjusted in time to control the water outlet speed of the condenser to prevent a large amount of water vapor from bringing a large amount of molecular polyols out of the fractionation condenser. After the intense reaction, maintain an appropriate water outlet speed and gradually raise the reaction temperature to 220~250℃. When the acid value drops to about 30mgKOH/g or the water outlet is approximately equal to the theoretical water volume, due to the small amount of hydroxy acid in the mixture, esterification is difficult to continue, water outlet basically stops, and the esterification reaction stage is basically over.

The oligomer polyol used in this polyurethane saddle is polyester polyol, brand ODX-218, molecular weight 2000.

2.1.1 Oligomeric polyols

The average functionality of oligomer polyols used in polyurethane is low, usually 2 or 2~3. The relative molecular mass is 400~6000, but the commonly used is 1000~2000. The main categories are polyester polyols, polyether polyols, poly-ε-caprolactone diols, polybutadiene polyols, polycarbonate polyols and polymer polyols. They play a very important role in the synthesis of polyurethane resins. Generally, the physical and chemical properties of polyurethane can be adjusted by changing the type, molecular weight, functionality and molecular structure of polyol compounds.

2.2 Polyisocyanate

There are many varieties of polyisocyanates, but only two of them have the largest output, namely diphenylmethane diisocyanate (MDI) and its polymer polyphenyl polymethylene polyisocyanate (PAPI) and toluene diisocyanate (TDI). For polyisocyanate, we use TDI-100 from Bayer of Germany.

TDI is made of toluene as the basic raw material. It is nitrated with a mixture of nitric acid and sulfuric acid to produce dinitrotoluene, which is then dissolved in methanol and hydrogenated to toluenediamine (TDA) in the presence of a Raney catalyst and a hydrogen pressure of 15~20Mpa, and then phosgenated.

The first stage of toluene nitration produces a mixture of three nitrotoluene isomers, namely ortho, para and meta, with contents of 55%~60%, 35%~40% and 2%~5% respectively. The content of isomers is hardly affected by reaction conditions. The above mononitrotoluene mixture is not nitrated to produce 2,4-dinitrotoluene and 2,6-dinitrotoluene in a ratio of 80/20, which are then reduced and photochemically produced to obtain 80/20 TDI (T-80). If the mononitrotoluene mixture is now crystallized to separate pure ortho- and para-mononitrobenzene, and then subjected to a second nitration and reduction, photochemically produced, 65/35 TDI (T-65) and pure 2,4-TDI (T-100) can be obtained. The production process is shown in the figure below:

Chemical reaction equation:

The reaction of isocyanate is obtained by organic acid ester and potassium cyanate:

R2SO4+KCNO→2RNCO+K2SO4

T-80 accounts for the vast majority of TDI products, and is mainly used in soft foam plastics, accounting for about 31.5% of the output of soft foam plastics, followed by polyurethane coatings, adhesives and elastomers. The output of T-100 is mainly used in the production of polyurethane prepolymers and polyurethane elastomers. T-65 is basically no longer produced and has been replaced by T-80.

2.3 Chain extenders and crosslinkers

Chain extenders and crosslinkers are additives with different chemical effects. In the synthesis of polyurethane elastomers, chain extenders participate in chemical reactions to make polymer molecules grow and extend; crosslinkers participate in chemical reactions to not only make polymer molecules grow and extend, but also to produce branches in the polymer chain, produce a certain network structure, and perform crosslinking reactions. Generally, chain extenders are mostly diols or diamine compounds. Alcohols and amine compounds with more than two hydrations have the dual functions of chain extension and crosslinking.

There are certain requirements for the chain extenders and cross-linking agents needed in the preparation of polyurethane elastomers, especially the water content must be less than 0.1%. If this indicator is not met, treatment must be carried out.

The diamine chain extenders generally used are all aromatic, and the most commonly used one is 3,3'-dichloro-4,4'-diphenylmethanediamine (trade name MOCA).

MOCA is a chain extender and crosslinker for polyurethane elastomers, and is particularly used for cast polyurethane elastomers. In the structure of MOCA, the chlorine atom substituent on the amino ortho-benzene ring increases the electron cloud density of the amino group, reduces the reaction rate between the amino group and isocyanate, and thus prolongs the life of the kettle, which is extremely important for cast polyurethane elastomer products.

When we process cast products, we usually control the amount of MOCA to around 90% of the theoretical amount. The purpose is to make the processed products have a considerable cross-linking density to improve the compression permanent deformation and swelling resistance of the products.

The MOCA used for this polyurethane elastomer is Suzhou Xiangyuan II MOCA.

2.4 Other additives

Additives are important raw materials in the rubber industry. Although the amount used is small, the effect is very significant. Polyurethane elastomers cannot do without additives from synthesis to processing and application. There are many types of polyurethane elastomer additives, which can be added in appropriate amounts according to the different requirements of the product. The following briefly describes the main additives used in polyurethane saddles.

2.4.1 Release agent

It is an indispensable operating aid in the production of polyurethane elastomer products. Polyurethane is a highly polar polymer material, and it has a strong bonding force with metals and polar polymer materials. Without a release agent, the product is difficult to remove from the mold.

There are four commonly used release agents:

The first category is silicone rubber and silicone ester, which are dissolved in toluene, dichloromethane, chloroform, gasoline and other solvents to form a solution, which is then rubbed or sprayed on the mold. Silicone oil can also be used as a release agent, but it is not ideal during hot pressing vulcanization.

The second category is new products that use water as solvent.

The third category is release agents used under normal pressure, such as liquid paraffin, vacuum pump oil, vaseline, etc.

The fourth type of demoulding machine is the internal demoulding agent.

2.4.2 Colorants

Polyurethane elastomer products are colorful, and their beautiful appearance depends on colorants. There are two types of colorants, organic dyes and inorganic pigments. Most organic dyes are used in thermoplastic polyurethane products to decorate injection parts and extrusion parts. There are generally two ways to color elastomer products: one is to grind pigments and other additives and oligomer polyols into color paste master liquor, then stir and mix an appropriate amount of color paste master liquor and oligomer polyols evenly, and then react with isocyanate components after heating and vacuum dehydration to produce products, such as thermoplastic polyurethane color granules and colored paving materials; the other method is to grind pigments and other additives and oligomer polyols or plasticizers into color paste or color paste, heat and vacuum dehydrate, and package for standby use. When using, add a little color paste to the prepolymer, stir evenly, and then react with the chain extender crosslinker to cast into products. This method is mainly used in MOCA vulcanization system. The pigment content in the color paste is about 10%-30%, and the amount of color paste added to the product is generally less than 0.1%.