How Polyurethane is made


How Polyurethane is made

Polyurethanes are linear polymers that have a molecular backbone containing carbamate groups (-NHCO2). These groups, called urethane, are produced through a chemical reaction between a diisocyanate and a polyol.

Polyurethanes, also known as polycarbamates, belong to a larger class of compounds called polymers. Polymers are macromolecules made up of smaller, repeating units known as monomers. They consist of a primary long-chain backbone molecule with attached side groups. Polyurethanes are characterized by carbamate groups (-NHCO 2 ) in their molecular backbone.

Like polyurethane, are produced by reacting monomers in a reaction vessel. A step—also known as condensation reaction is performed. In chemical reaction, the monomers that are present contain reacting end groups.

A diisocyanate (OCN-R-NCO) is reacted with a diol (HO-R-OH). First step of reaction results in the chemical linking of the two molecules leaving a reactive alcohol (OH) on one side and a reactive isocyanate (NCO) on the other. These groups react further with other monomers to form a larger, longer molecule.

This is a process which yields high molecular weight materials at room temperature. Polyurethanes have important commercial uses typically contain other functional groups in the molecule including esters, ethers, amides, or urea groups.

Raw Materials:
A variety of raw materials are used to produce polyurethanes. These include monomers, prepolymers, stabilizers which protect the integrity of the polymer, and colorants.




Isocyanates: Key reactive materials required to produce polyurethanes are diisocyanates. These compounds are characterized by a (NCO) group, which are reactive alcohols. Widely used isocyanates employed in polyurethane production are toluene diisocyanate (TDI) and polymeric isocyanate (PMDI). TDI is produced by chemically adding nitrogen groups on toluene, reacting these with hydrogen to produce a diamine, and separating the undesired isomers.

PMDI is derived by a phosgenation reaction of aniline-formaldehyde polyamines. In addition to these isocyanates, higher end materials are also available. These include materials like 1,5-naphthalene diisocyanate and bitolylene diisocyanate. These more expensive materials can provide higher melting, harder segments in polyurethane elastomers.

Polyols: The other reacting species required to produce polyurethanes are compounds that contain multiple alcohol groups (OH), called polyols. Materials often used for this purpose are polyether polyols, which are polymers formed from cyclic ethers. They are typically produced through an alkylene oxide polymerization process. They are high molecular weight polymers that have a wide range of viscosity. Various polyether polyols that are used include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. These materials are generally utilized when the desired polyurethane is going to be used to make flexible foams or thermoset elastomers.

Additives: Some polyurethane materials can be vulnerable to damage from heat, light, atmospheric contaminants, and chlorine. For this reason, stabilizers are added to protect the polymer. One type of stabilizer that protects against light degradation is a UV screener called hydroxybenzotriazole. To protect against oxidation reactions, antioxidants are used. Various antioxidants are available such as monomeric and polymeric hindered phenols. Compounds which inhibit discoloration caused by atmospheric pollutants may also be added.




The Manufacturing Process:
While polyurethane polymers are used for a vast array of applications, their production method can be broken into three distinct phases. The bulk polymer product is made. Polymer is exposed to various processing steps. Polymer is transformed into its final product and shipped. This production process can be illustrated by looking at the continuous production of polyurethane foams.

Polymer reactions:
At the start of polyurethane foam production, the reacting raw materials are held as liquids in large, stainless steel tanks. These tanks are equipped with agitators to keep the materials fluid. A metering device is attached to the tanks so that the appropriate amount of reactive material can be pumped out.

A typical ratio of polyol to diisocyanate is 1:2. Since the ratio of the component materials produces polymers with varying characteristics, it is strictly controlled.

The exchanger adjusts the temperature to the reactive level. Inside the pipes, the polymerization reaction occurs. By the time the polymerizing liquid gets to the end of the pipe, the polyurethane is already formed. End of the pipe is a dispensing head for the polymer.

Processing:
The dispensing head is hooked up to the processing line. For the production of rigid polyurethane foam insulation, a roll of baking paper is spooled at the start of the processing line. This paper is moved along a conveyor and brought under the dispensing head.

As the paper passes under, polyurethane is blown onto it. As the polymer is dispensed, it is mixed with carbon dioxide causes it to expand. It continues to rise as it moves along the conveyor.




After the expansion reaction begins, a second top layer of paper is rolled on. Additionally, side papers may also be rolled into the process. Each layer of paper contains the polyurethane foam giving it shape. The rigid foam is passed through a series of panels that control the width and height of the foam bun. As they travel through this section of the production line, they are typically dried.

At the end of the production line, the foam insulation is cut with an automatic saw to the desired length. The foam bun is then conveyored to the final processing steps that include packaging, stacking, and shipping.

Quality Control:
Ensure the quality of the polyurethane material, producers monitor the product during phases of production. These inspections begin with an evaluation of the incoming raw materials by quality control chemists. Test various chemical and physical characteristics using established methods. Some of characteristics that are tested include the pH, specific gravity, and viscosity or thickness. Appearance, color, and odor may also be examined. Manufacturers have found that by strictly controlling the quality at the start of production can ensure that a consistent finished product will be achieved.

After production, the polyurethane product is tested. Polyurethane coating products are evaluated in the same way the initial raw materials are checked. Characteristics like dry time, film thickness, and hardness are tested. Polyurethane fibers are tested for things such as absorbency,elasticity and resilience. Polyurethane foams are checked to ensure the proper resistance, density and flexibility.


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