Notes

Understanding Polyurethanes
The plastic materials that are referred to as polyurethanes comprise a group of polymers that differ from other plastics because there isn't a monomer made of urethane, as well as the polymer is usually produced during the manufacturing process of the particular item.

Polyurethanes are created by an exothermic reaction between alcohols that have at least two reactive hydroxyls (-OH) molecules (diols triols or polyols) and isocyanates which have multiple reactive isocyanates (-NCO) for each molecule (diisocyanates and polyisocyanates). In the case of diisocyanates, it reacts with a diol.
The group created through the reaction between two molecules is referred to as the "urethane linkage'. It is the most important component that makes up the polyurethane molecule.

Polyurethanes and their uses the physical properties, as well as the chemical structure of a polyurethane, depending on the chemical structure and structure of initial reactants, in particular, that of 1 and 2 groups, and in particular the R 1 and 2 groups. The properties of the polyols are characterized by their relative molecular mass and the number of functional groups reactive per molecule, and also the molecular structure influence the properties of the final polymer and, consequently, how it will be used.



Polyurethanes Uses

There is a significant distinction in the manufacturing of the majority of polyurethanes and the production of many other plastics. Polymers like poly(ethene) and poly(propene) are made in chemical factories and are available as powders, granules or films. They are made of these by heating up the polymer and creating it into a shape with pressure and then cooling. The properties of the final products are almost entirely dependent on the properties of the initial polymer.

Polyurethanes on the contrary, are typically integrated straight into final products. The majority of polyurethanes made are made up of huge blocks of foam that are cut into cushions or as thermal insulation. The chemical reaction may be carried out within moulds and result in creating, for instance, a car bumper, computer casing, or a panel for a building. It could happen when the reactants in liquid form are sprayed onto a surface for building or coated onto the fabric.


The effects of regulating polymer properties and density of the polymer result in the development of a wide variety of materials and polyurethanes can be employed in many different ways (Table 1.).

Polyurethanes can be either rigid or rubbery in any density, say 10, kg 3, or 100kg 3. The variety of properties that are available to the designer as well as the manufacturer is broad. It is evident in the numerous and diverse ways in the polyurethanes they are used for.

The annual production of polyurethanes
World 17.9 millions tonnes 1,2
Europe 3.5 millions tonnes 3
1. In 2015. Estimated to be 19.0 as well as 26.4 million tonnes by 2021 and 2016 and 2021, respectively. Research and Markets, 2016.
2. It is estimated that in the present report, China holds well more than 50 percent of the total capacity. HIS Markit 2014
3. The Facts on Plastics, 2016 PlasticsEurope in 2016,

Production of polyurethanes since polyurethanes are created by the reaction of an isocyanate and a polyol, the section is split into three components:
A) the production of Isocyanates
B) the production of polyols
C) the production of polyurethanes

(a) Producing isocyanates
While a variety of aromatic and alphatic polyisocyanates are available but two have particular industrial significance. Each one of them comes with variations and together form the foundation of around 95% of the polyurethane s. They include:


TDI (toluene diisocyanate or methylbenzene diisocyanate)
MDI (methylene diphenyl diisocyanate or diphenylmethane diisocyanate).
TDI first invented initially, but it is now utilized predominantly in the manufacturing of low-density, flexible foams to make cushions.

The diisocyanate mixture referred to as TDI includes two isomers:



The basic material is methylbenzene (toluene). It reacts with mixed acids (nitric and sulfuric) two isomers of the compound nitromethylbenzene (NMB) make up the primary product.



If this mixture is nitrated further, a mixture of dinitromethylbenzenes is produced. It is in the industry they are referred to as their names of nil, such as 2,4-dinitrotoluene as well as 2-dinitrotoluene (DNT). 80percent is 2,4-DNT, and 20% is 2,6 DNT:



The dinitrobenzene mixture is then reduced to analogous amines.



The chemical amines, commonly referred to in the market as Toluene Diamines or TDA, are heated using carbonyl chloride (phosgene) to make diisocyanates. This procedure can be performed through the liquid phase using chlorobenzene, a solvent, at around 350 K.



In other words, these reactions are executed in the gas phase by dissolving the diamines in the gas phase at approximately 600 K before combing them in carbonyl chloride. This is an ecological and economic enhancement compared to the liquid phase procedure since no solvent is required.

In any of the processes, the reagent used is dinitro compounds in an isomeric mix that are with 80 percent 24% and 20% of 2,6 thus the final product is a mixture of diisocyanates with similar proportions.

MDI refers to the mix of three isomers shown in Figure 3. They are separated through distillation.


The polyol's choice, the quantity of reacting hydroxyl groups per polyol, and the size and flexibility in its structure ultimately control the amount of cross-linking between the molecules. This has a major impact in the physical properties that are exhibited by the polymer.

An example of a molecule composed of 2 hydroxyl groups (ie an extended chain diol) is one that is made of epoxypropane (propylene oxide) in the process of the interaction of propane-1,2-diol (which itself is made from epoxypropane through hydrolysis):

A polyol that has three hydroxyl groups is made through propane-1,2,3 triol (glycerol) along with epoxypropane and could be represented in this modelized structure:

Soya bean oil has triglycerides that are unsaturated and long-chain saturated carboxylic acids that, following hydrogenation, may, upon reaction with epoxypropane, create polyols that are suitable for the production of a broad range of polyurethanes. Using biopols ensures that at a minimum, a part of the polymer originates from renewable sources.

A widely used polyurethane is made of TDI as well as a polyol that is derived from epoxypropane.

The adjacent long-chain molecules are connected at intermediate points if the polyol contains at least two reacting hydroxyl groups. This creates a stronger polymer structure with better mechanical properties, which are exploited when developing rigid polyurethanes. This is why a diisocyanate, like MDI or TDI interacts with 3 hydroxyl groups, for instance, one derived from propane-1.2,3-triol and epoxyethane, is crosslinked and creates a stiff and thermostable polymer.

Alongside polyisocyanates and polyols, the production of polyurethanes requires different chemicals for control the polyurethane-forming reaction and create the correct properties in the final product.

Practically, all polyurethane systems are comprised of certain, but not all, of the systems described in Table 2.

Additives that are used in the production of polyurethanes.


Foamed polyurethanes
If the two liquids interact to form, a solid polymer is created. The polymer can have the properties of being elastic and rigid. It could also contain gas bubbles to create cells or foam.

When making foamed polyurethane, there are two ways to create a gas within the liquid mixture that is reacting. Chemical blowing is a type of chemical blowing makes use of water that could have already been added to the polyol that reacts with a portion of the polyisocyanate molecules to produce carbon dioxide.

Alternately (physical blowing) or physical blowing, a liquid with an extremely low boiling point such as pentane is mixed with the polyol. It is an exothermic reaction; therefore, as it proceeds, the mixture heats up and vaporizes the pentane.

A small amount of air is released through the polyisocyanate-polyisocyanate mix and polyol. This is the source of nucleation seeds for the numerous gas bubbles created in the polymer. The heat causes the bubbles to grow so that the chemical reaction converts the liquid into solid polymer and the gas pressure will not allow any additional expansion.


Fabric and flexible foam and fabric can create an enveloping cushion, as well as the rigid foam and sheet construction materials (e.g. steel sheet, plasterboard, plywood) can be used to create construction insulation panels.


Read More: https://www.armorthane.com/protective-coating-products/polyurethane/