Polyurethane elastomer production process
A. Overview of polyurethane elastomer
The so-called elastomer refers to the glass transition temperature is lower than room temperature, elongation at break > 50%, after the withdrawal of external force recovery is relatively good polymer materials, while the glass transition temperature is higher than room temperature polymer materials called plastic. In the elastomer, its elongation at break is larger (>200%), 100% fixed elongation stress is smaller (such as <30Mpa), the elasticity is better can be called rubber. So elastomer is a class of polymer material more widely than rubber.
Polyurethane elastomer, also known as polyurethane rubber is a relatively special category of elastomers, with a wide variety of raw materials, a variety of formulations, and a wide range of adjustable. Polyurethane elastomer hardness range is very wide, as low as Sauer A10 below the low modulus rubber, as high as Sauer D85 high impact rubber elastic material. Therefore, polyurethane elastomer performance range is very wide, is a class of polymer materials from rubber to plastic.
Second, the main raw materials of polyurethane elastomers
The raw materials used in polyurethane elastomers are mainly three categories, namely oligomeric polyols, polyisocyanates and chain extenders (crosslinkers). In addition, sometimes, in order to increase the reaction speed, improve the processing performance and product performance, it is also necessary to add certain matching agents. Only the raw materials used in the production of polyurethane saddles are described specifically below.
Reaction process: polyol reacts with diisocyanate to make low molecular weight prepolymer; by chain expansion reaction, high molecular weight polymer is generated; then appropriate cross-linking agent is added to generate polyurethane elastomer. The process flow is as follows.
2.1 Oligomeric polyol
The average functionalities of oligomeric polyols for polyurethane are low, usually 2 or 2~3. The relative molecular masses are 400~6000, but the commonly used ones are 1000~2000. They play a very important role in the synthesis of polyurethane resins. Generally, the physicochemical properties of polyurethane can be adjusted by changing the type, molecular weight, functionalities and molecular structure of polyol compounds.
2.1.1 Polyester polyol
Polyester polyol, referred to as polyester, is one of the most important raw materials for polyurethane elastomers. It is made of dicarboxylic acid and polyol condensation, the most commonly used dicarboxylic acid is adipic acid, the most commonly used polyol are ethylene glycol, propylene glycol, butylene glycol, diethylene glycol. In addition, some special polyesters also use pentanediol, ethylene glycol, trimethylolpropane, glycerol and other polyols. Due to the many varieties of polyols available, so the molecular structure of polyesters are diverse, and there are more varieties of grades. In order to get the end-hydroxy polyester, must use an excess of polyol and dicarboxylic acid reaction. Generally use intermittent method to produce polyester. The reaction process is divided into two stages: esterification reaction and ester exchange reaction. Production equipment is similar to 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 gas tightness of the whole system is very strict. The stirring shaft of the condensation kettle can use end mechanical seal. The charging sequence is to add polyol and coordination agent first, followed by adipic acid, and then nitrogen charging. The esterification reaction starts from heating up to 220~250℃ and is basically completed after about 1h. This stage is mainly the atmospheric pressure dehydration process, generating low molecular polyester and condensation water. When the temperature rises to about 135 ℃ esterification reaction is the most intense, generating a large number of condensation water. Due to the evaporation of condensation water will occur a large number of bubbles rising, with 1,4-butanediol and 1,6 – ethylene glycol as raw material bubbles are particularly intense. At this time, the heating power should be adjusted in time to control the condenser water speed to prevent a large amount of water vapor from taking a large number of molecules of polyol out of the fractionation condenser. After the intense reaction, maintain the appropriate water discharge rate, and gradually increase the reaction temperature to 220~250℃. When the acid value drops to about 30mgKOH/g or the water output is about equal to the theoretical water, the esterification is difficult to continue because there is little hydroxy acid in the mixture, and the water output basically stops and the esterification reaction stage is basically finished.
The oligomeric polyol selected in this polyurethane saddle is polyester polyol, the brand is ODX-218, molecular weight 2000.
2.2 Polyisocyanate
There are many varieties of polyisocyanates, but there are only two kinds of polyisocyanates with the largest output, namely, diphenylmethane diisocyanate (MDI) and its polymer polyphenyl poly(methylene) polyisocyanate (PAPI) and toluene diisocyanate (TDI). For the polyisocyanate we use TDI-100 from Bayer, Germany.
TDI is made from toluene as the basic raw material, nitrated with a mixture of nitric acid and sulfuric acid to generate dinitrotoluene, then dissolved in methanol, hydrogenated and reduced to toluenediamine (TDA) under Raney catalyst and hydrogen pressure of 15~20Mpa, and made by photogasification.
The first stage of toluene nitration generates a mixture of three nitrotoluene isomers in the o-, para-, and m-positions, and their contents are 55%~60%, 35%~40%, and 2%~5%, respectively. The content of the isomers was almost independent of the reaction conditions. The above mixture of mononitrotoluene was subjected to non-nitration to produce 2,4-dinitrotoluene and 2,6-dinitrotoluene in the ratio of 80/20, which was then reduced and photochemically produced 80/20 TDI (T-80). If the mononitrotoluene mixture is now separated by crystallization to produce pure o- and para-mononitrotoluene, then a second nitration and reduction and photochemical respectively to produce 65/35TDI (T-65) and pure 2,4-TDI (T-100). The production process is shown in the following diagram.
Chemical reaction equation.
The reaction of isocyanate is obtained by having organic acid ester with potassium cyanate as follows.
R2SO4+KCNO→2RNCO+K2SO4
TDI products T-80 accounts for most of the products, mainly for soft foam, accounting for about 31.5% of the production of soft foam, followed by polyurethane coatings, adhesives and elastomers, T-100 production is mainly used for the production of polyurethane prepolymer and polyurethane elastomers. T-65 is basically not produced by T-80 instead.
2.3 Chain extender and cross-linking agent
In the synthesis of polyurethane elastomer, the chain extender is involved in the chemical reaction, and the polymer molecule grows and extends; the cross-linking agent participates in the chemical reaction, not only makes the polymer molecule grow and extend, but also can produce branching in the polymer chain to produce a certain mesh structure and cross-linking reaction. Most of the chain extenders are diols or diamine compounds. Alcohols and amine compounds above the binary level have the dual functions of chain extension and cross-linking.
Polyurethane elastomer preparation of chain extender and cross-linking agent are required to have certain requirements, in particular, the water content of less than 0.1%, if the index is not reached are to be processed.
Generally used binary amine chain extenders are aromatic, the most commonly used is 3,3′-dichloro-4,4′-diphenylmethane diamine (trade name MOCA).
MOCA is a chain extender and cross-linking agent for polyurethane elastomers, especially for cast polyurethane elastomers are generally used. The chlorine atom substituent on the amino-adjacent benzene ring in the structure of MOCA increases the density of the amino electron cloud and reduces the reaction rate of the amino group with isocyanate, thus extending the kettle life, which is extremely important for cast polyurethane elastomer products.
When we process cast products, we usually control the amount of MOCA to 90% or less of the theoretical amount, the purpose of which is to make the processed products have a considerable crosslink density to improve the performance of the products such as compression permanent deformation and swelling resistance.
The MOCA selected for this polyurethane elastomer is Suzhou Xiang Yuan II MOCA.
2.4 Other additives
Additives are important raw materials for the rubber industry, although the amount is small, but the role is very large, polyurethane elastomer from synthesis to processing applications are inseparable from the additives. There are many kinds of polyurethane elastomer additives, which can be added in appropriate amounts according to the different requirements of the products. The following is a brief description of the main additives used in polyurethane saddles.
2.4.1 Mold release agent
It is the production of polyurethane elastomer products can not be separated from the operating aids. Polyurethane is a strong polar polymer material, it has a strong bond with metal and polar polymer material, so it is difficult to release the product from the mold without the release agent.
There are four kinds of mold release agents commonly used.
The first type is silicone rubber, silicone ester, with toluene, methylene chloride, trichloromethane, gasoline and other solvents dissolved into a solution, rubbed or sprayed in the mold. Silicone oil can also be used as a release agent, but it is not ideal for hot press vulcanization.
The second category is the new product with water as solvent.
The third category is the release agent used under normal pressure, like liquid paraffin, vacuum pump oil, petroleum jelly, etc.
The fourth type of release agent is the internal release agent.
2.4.2 Coloring agent
Polyurethane elastomer products colorful, beautiful and generous appearance relies on coloring agents. There are two kinds of colorants, organic dyes and inorganic pigments, organic dyes are mostly used in thermoplastic polyurethane products, decorative beautification of injected parts and extruded parts. There are generally two ways to color elastomer products: one is to grind pigments and other additives and oligomeric polyol into a color paste master batch, and then mix the right amount of color paste master batch with oligomeric
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