Polyurethanes - What Goes Into PUs?

Topics Covered

Background

Selection of a Polyurethane

Raw Materials

Isocyanates

Polyols

Polyethers

Polyesters

Pre Polymers

Common Additives

Catalysts

Chain Extenders

Blowing Agents

Flame Retardants

Pigments

Fillers

Basic Polyurethane Chemistry

Background

A PU is made by mixing together the ingredient chemicals (isocyanate and polyol see later) in predetermined proportions, which then react to form the polymer.

Uniquely, PUs utilise simultaneous polymerisation and shaping of the part.

The production of consistent end products depends on mixing, in precise ratio, the ingredient chemicals and maintenance of the appropriate processing temperatures. As the liquid isocyanate and polyol react to form the PU, the liquid mix becomes increasingly viscous eventually forming a solid mass. The reaction is exothermic and therefore heat is involved.

Other ingredients will be included in the polyol blend, for example the catalyst which controls the rate at which the liquid mixture reacts to become solid.

There are no hard and fast rules for obtaining the optimum PU end product, success is due to good formulation selection with well chosen and appropriate processing parameters and mould geometry. The process by which liquid polymers are converted to elastomeric or glassy solids is fundamental to the manufacture of PU products.

Selection of a Polyurethane

There are a number of steps:

Consider the requirements which the application will demand of the PU with respect to chemical and physical properties.

Based upon an understanding of what controls these properties select a few candidate PU systems. The properties of a PU are largely controlled by the chemical nature of the system and how it is processed so it is prudent to consult specialist suppliers and processors at this stage.

Establish that the converter can process the proposed system on existing plant. The important processing characteristics of the system will include viscosity, pot life, reactive mix ratio control, demould time and process temperature.

Undertake preliminary tests, make prototypes, conduct field trials and obtain customer approval.

Raw Materials

lsocyanates

Many commercial grades of isocyanates used for making PUs are aromatic in nature. Each isocyanate will give different properties to the end product, requiring different curing systems and, in most cases, different processing systems. An important property of an isocyanate is its functionality, i.e. the number of isocyanate groups (-NCO) per molecule. For cross linked PU applications the average functionality of the isocyanate is usually a little over two. The higher functionality isocyanates are used for special applications. When a di-functional isocyanate is used with a di-functional polyol a long linear PU molecule for elastomeric applications is formed. The common isocyanates used to make PUs are shown in figure 1.

Figure 1. Typical isocyanates

Many PU products, such as flexible foams, are made with toluene diisocyanate (TDI).

The other main isocyanate used is methylene diphenyl diisocyanate (MDI), the most widely used MDI product is `Crude MDI' with a functionality of about 2.8.

A monomeric derivative of MDI, called 'Pure MDI', with a functionality of 2 can be distilled from Crude MDI. `Pure MDI' is a solid at ambient temperatures and is usually modified to a liquid form for ease of handling.

The modified isocyanates and isocyanate pre-polymers with special reactivity characteristics are used when it is impractical to use the more conventional isocyanates. Such derivatives are formed from the reaction of the isocyanate with compounds such as amines, diols or triols.

Polyols

There are two main types of polyols used in the PU industry, polyethers and polyesters. Typical polyols used are shown in figure 2.

 

Figure 2. Typical polyols.

Polyethers

The more widely used polyethers have a relatively low molecular weight in the range of 500 to 3000 and are manufactured from propylene oxide (PO) and ethylene oxide (EO).

PO is the major constituent of the polyol, whereas EO is only included in small amounts to modify the properties of the polyol.

The functionality of the polyether polyol (number of active hydroxyl groups per molecule) can be varied and is normally 2 for elastomers, approximately 3 for flexible foams and up to 6 or more for rigid foams.

Polyesters

The polyester polyols are typically produced by the condensation reaction of a diol such as ethylene glycol with a dicarboxylic acid.

Polyester polyols tend to be more expensive, are usually more viscous and difficult to handle but develop PUs with superior tensile, abrasion, flexing and oil resistance properties. Consequently they are used to make PUs for more demanding applications.

A disadvantage of polyester based PUs is their lower hydrolysis resistance.

Pre polymers

In a pre polymer system, the polyol and isocyanate (either a polyester or a polyether) are reacted to give a pre polymer that may be either a liquid or a waxy solid.

The reactant ratios used ensure the pre polymer contains isocyanate groups at the chain ends. The pre polymer can, when required, be chain extended to give a high molecular weight cross linked product.

Common Additives

Catalysts

Catalysts have a key role in PU production being required to maintain a balance between the reaction of the isocyanate and polyol.

The combination of very complex PU chemistry and diverse processing and moulding conditions make great demands of the catalyst. Its main function is to exploit the diverse reactions to create a product with the desired properties.

There are two main classes of catalyst used in PU production.

Organometallics are used to accelerate the reaction and formation of urethane linkages and hence promote rapid curing.

The most popular organometallic catalysts are tinbutyltin dilaurate and stannous octoate. Tin catalysts are used to catalyse micro cellular elastomers and reaction injected moulded (RIM) systems.

Amines are the other major class of catalysts widely used in the making of PU foams. Some amine catalysts promote crosslinking whilst others assist in controlling the foam's cell structure.

Chain Extenders

Chain extenders are reactive low molecular weight di-functional compounds such as hydroxyl amines, glycols or diamines and are used to influence the end properties of the PU.

The chain-extender reacts with the isocyanate to affect the hard/soft segment relationship and therefore the modulus and glass transition temperature (Tg) of the polymer. The Tg provides a measure of the polymer's softening point and some indication of the safe upper limit of its working temperature range.

Blowing Agents

Cellular or foamed PUs are manufactured by using blowing agents to form gas bubbles in the reaction mixture as it polymerises. They are usually low boiling point liquids which are volatilised by the heat generated by the exothermic reaction between the isocyanate and polyol.

Rigid foams yield sufficient exothermic heat from the reaction to allow foam expansion in association with the blowing agent.

Flexible PU foams are usually blown by the C02 generated by the reaction of water and isocyanate (or in association with methylene chloride). Blowing of the foam can also be accomplished by the direct injection of air or gas into the foam. Chloroflurocarbons (CFCs) have been used as blowing agents but their effects on the ozone layer have led to restrictions of their use and they are being replaced by more environmentally acceptable alternatives such as pentane.

Flame retardants

Certain end use sectors now take greater account of possible 'worst scenarios' in materials selection.

These considerations will include the effects of smoke and toxic decomposition products on people, property and equipment. PU foams used in furniture are an example which spring to mind. Fire retardancy can be achieved by the addition of fluorine, chlorine, bromine or iodine compounds to the polyol. Solid compounds such as melamine and aluminium trihydrate are also important flame retardants.

Materials and products are continuously evolving and developing and the trends are now to lower smoke and fume generation, and in the much longer term `lower toxicity'. There is an increasing commitment to tougher requirements and in certain sectors of the PU industry this has led to the development of low or halogen free systems.

Pigments

Many PUs tend to yellow in the light albeit without any adverse affect on the physical properties. To produce coloured PUs pigmented pastes are added to the polyol formulation. The pigments, both inorganic and organic, improve the light stability of PU products.

Fillers

As with other polymers the use of fillers in PUs will yield products with modified performance. Calcium carbonate and glass fibres are most commonly used. The former primarily to make cheaper formulations, the latter are of growing interest in reaction injection moulding (RIM) technology (see later).

Basic Polyurethane Chemistry

The simplest PU is linear in which the hydroxyl compound and the nitrogen compound each have a functionality of two. This can be represented by the following:

Isocyanate + Polyol = Polyurethane

The isocyanate can react with different chemical groups, so the final properties of the polymer will vary according to the reaction route taken.

Therefore the formulation of a PU must take into account every possible reactive constituent. PUs may have a very widely varying structure depending on the type of isocyanate and the type of reactive hydrogen components present in the formulation.

The presence or otherwise of the various groups along the urethane linkage will control the end properties of the polymer.

The curing of a PU can be regarded as the formation of a network, also called crosslinking, the extent or degree of cure is often expressed as the crosslink density.

The extent of cross linking may vary and will be reflected in the final properties of the PU, ranging from longer, linear chains of flexible elastomers and foams to the rigid, heavily cross linked polymers. Thermoplastic polyurethanes (TPUs) are a particular case. They are effectively co-polymers of a hard PU and a very flexible PU in which microphase segregation of the hard phase occurs (figure 3).

Figure 3. Microphase separation of their hard segments.

The clusters of hard PU, shown as thicker lines, act as ‘pseudo cross-links’ and allow the material to behave as an elastomer. When the temperature is raised the clusters disassociate and the material can be made to flow, when subsequently cooled the clusters reform and the material again exhibits elastomeric properties.

Thus these materials show elastomeric behaviour at room temperature, but can be processed as thermoplastics. Hence the name of the material class, thermoplastic urethane elastomer.

 

Primary author: Brian Lees

Source: Materials Information Service

 

For more information on Materials Information Service please visit The Institute of Materials.

 

Date Added: Mar 26, 2001 | Updated: Jun 11, 2013
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