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Polyurethane Raw Materials - What Goes Into PUs?

Background
A PU is made by mixing together the ingredient chemicals (isocyanate and polyol, see later) in predetermined proportions. These 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.

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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 room temperature 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.

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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 are classed as PPG (polypropylene glycol) polyols. A special polyether is PTMEG, polymerised from the ring-opening polymerising of tetrahydrofuran. PTMEG has better physical properties that PPGs but are significantly more expensive. It is mainly used in applications where high heat build-up might be a problem.
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 than PPGs but cheaper than PTMEG, 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.

Polycaprolactones
Polycaprolactones bridge the gap between PTMEG and polyesters. It is manufactured from caprolactone and has a ether type backbone with ester linkages. This implies that they are very useful in applications where neither a PTMEG or polyester could be usedl.

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Prepolymers
In a prepolymer system, the polyol and isocyanate (either a polyester or a polyether) are reacted to give a prepolymer that may be either a liquid or a waxy solid. The reactant ratios used ensure the prepolymer chain is terminated with isocyanate groups. A prepolymer is classified as a true prepolymer if it has an NCO of 12% or less. A prepolymer with an NCO of between 12 and 25% is defined as a quasi-prepolymer. This would mean that (strictly speaking) prepolymers are isocyanates. These prepolymers can now be extended or crosslinked with -OH or -NH terminated molecules to give a high molecular weight polyurethane.

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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 (DBTDL) and stannous octoate. Tin catalysts are used to catalyse micro cellular elastomers and reaction injected moulded (RIM) systems. Mercury based catalysts are also used and although many producers shy away from supplying mercury based systems, this catalyst is still the best for the application. This is because most catalysts chase the water-isocyanate reaction whereas mercury prefers the polyol-isocyanate reaction. It also provides the formulator with a flat viscosity build-up and sharp increase at gel with excellent throughcure properties. Most other catalysts give a sharp and fast (initial) viscosity increase with weak troughcure properties. The latter catalysts are most used in combonation with others to overcome their shortcomings.
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 can be classified as chemical blowing agents or physical blowing agents. Physical blowing agents are usually low boiling point liquids that are volatilised by the heat generated by the exothermic reaction between the isocyanate and polyol. Chemical blowing agents react with other substances and produce a gas. Water is the most common as it reacts with the isocyanate and forms carbon dioxide.
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 carbon dioxide 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. An example would be PU foams used in furniture. 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 although without any adverse affect on the physical properties. To produce coloured PUs pigment 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.

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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. The formulator must therefore take into account every possible reactive ingredient. PUs may have a widely varying structure depending on the type of isocyanate and the type of reactive hydrogen components present in the formulation. The presence 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 crosslinking 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 crosslinked polymers.

Thermoplastic polyurethanes (TPUs) are an exeption. They are (in theory) co-polymers of a hard PU and a very flexible PU. Microphase segregation takes place in the hard phase (figure 3).

Figure 3. Microphase separation of their hard segments

The clusters of hard PU (the thicker lines in figure 3) act as 'pseudo crosslinks' and allow the material to behave as an elastomer. When the temperature is raised the clusters disassociate and the material starts to flow. When cooled again, the clusters reform and the material again exhibits elastomeric properties. Although these materials show elastomeric behaviour at room temperature, they can be processed as thermoplastics (at elevated temperatures). Hence the name, thermoplastic polyurethane (TPU) elastomers.

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Info adapted from Azom.com

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