**2. Ingredients for water–based PUD**

80 Polyurethane

Waterborne PUD are fully-reacted PU systems produced as small discrete particles. 0.1 to 3.0 micron, dispersed in water to provide a product that is both chemically and colloidally stable, which only contains minor amounts of solvents and does not emit VOC. Polymeric structure of waterborne PUD is formed by usually reacting an excess of aliphatic isocyanates (mainly IPDI or HDI based), with a polyol or a mixture of polyols to form a prepolymer containing the so called soft segment. The polyols are generally polyesters, polyethers, or polycarbonates. The hard segment is generally formed by chain extending the prepolymer with short chain diamines and from the short chains containing ions. Due to incompatibility between the two segments of the polymer chain, the hard segment separates and aggregates into domains that act as reinforcing fillers to the soft segment. The degree of phase separation as well as the the concentration of the hard segments are contributing factors to the good properties of PUD. PU backbone with a minority of the repeat units contains pendant acid or tertiary nitrogen groups, which are completely neutralized or quarternized, respectively, to form salts. Such ionomeric groups are absolutely necessary for the formation of dispersions, because they act as internal surfactants, and are not incorporated in the chain of the solvent-based PU. Ionic centers in the hard segment generally favor segregation and cohesion within the hard segment domains due to their strong electrostatic forces and thermodynamic incompatibility with the polymer matrix. Water-based PUD can be divided into two classes. The first group consists of polymers stabilized by external emulsifiers, and second one achieves stabilization by including hydrophilic centers in the polymer. Such hydrophilic centers may be one of the three types: non-ionic, cationic groups and anionic groups. These hydrophilic groups fulfill the function as internal emulsifiers and make possible to produce stable water/based emulsions. Water–based PUD can be classified into anionic, cationic and nonionic systems (Rothause & Nechtkamp, 1987; Kim et al., 1996).

Several processes have been developed for the synthesis of PUD. All of these have in common the first step, in which a medium molecular weight polymer (the prepolymer) is formed by the reaction of suitable diols or polyols (usually macrodiols such as polyether or polyester) with a molar excess of diisocyanates or polyisocyanates. In this reaction mixture, an internal emulsifier is added to allow the dispersion of the polymer in water; this emulsifier is usually a diol with an ionic group [carboxylate, sulfonate, or quaternary ammonium salt) or a nonionic group poly(ethylene oxide)]. The internal emulsifier becomes part of the main chain of the polymer. The critical step in which the various synthetic pathways differ is the dispersion of the prepolymer in water and the molecular weight build-up. The most important dispersions are emulsifier-free ionomer dispersions. The resulting dispersions are mainly anionic or non-ionic, that have the potential for wide variations in composition and property level. They can be obtained by different processes, however, the earliest process to prepare the aqueous PUD is known as acetone process and this process has remained technically important so far (Hepburn, 1992; Oertel, 1985). Within the last three decades several new processes have been developed such as prepolymer

The facts that aqueous/water PUD have become increasingly important for industrial and

mixing process, hot melt process and ketamine/ketazine process.

academic research in recent years is due to the following reasons:

The basic components used to build up PUD include long–chain polyether, polyester or polycarbonate polyol, diisocyanate, aromatic or (cyclo)aliphatic, low molecular weight glycol and /or diamine, bis–hydroxycarboxylic acid and a neutralization base. In general, an excess of diisocyanate is treated with a long-chain linear polyol, bis-hydroxycarboxylic acid and other low-molecular-weight glycol to form an isocyanate-terminated prepolymer with a segmented structure. In this polymer, the long-chain polyol units form soft segments, and the urethane units-built up from diisocyanate, glycol and bis-hydroxycarboxylic acid form hard segments. The pendant carboxylic acid groups are neutralized with base to form internal salt group containing prepolymers that can be easily dispersed in water. The microphase separation between the incompatible soft- and hard-segment sequences contributes to the unique properties of PUD. The PU chains with NCO terminating groups can be extended with glycol forming urethane groups. Chain extenders are low molecular weight, hydroxyl and amine terminated. Aliphatic isocyanates: hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and (4,4'–diisocyanatodicyclohexylmethane (H12MDI), improve thermal and hydrolytic stability, resistance to UV degradation and they do not yellow (Bechara, 1998).

Aliphatic diisocyanates are less reactive than aromatic ones and they must be used with certain catalysts. Aromatic isocyanates: methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI) and 1,5–naphthalenediisocyanate (NDI) on the other hand, provide for toughness but yellow upon exposure to UV light. Although early water dispersed PU resins heavily utilized TDI, there is a high tendency to shift to aliphatic diisocyanates or to the aromatic diisocyanates with NCO groups not directly attached to an aromatic nucleus (Gunduz & Kisakurek, 2004).

The two key classes of polyols are polyethers and polyesters. Polyester polyols have been largely used in PUD paints as they exhibit outstanding resistance to light and aging. There are four main classes of polyester polyols: linear or lightly branched aliphatic polyester polyols (mainly adipates) with terminal hydroxyl groups, low molecular weight aromatic polyesters for rigid foam applications, polycaprolactones, polycarbonate polyols. Polyether polyols are susceptible to light and oxygen when hot, however, they improve water dispersion, and impart chain flexibility. These are made by the addition of alkylene oxides,

usually propylene oxide, onto alcohols or amines which are usually called starters or 'initiators'. Polyether based on propylene oxide contains predominantly secondary hydroxyl end–groups. Secondary hydroxyl end–groups are several times less reactive with isocyanates than primary hydroxyl groups and for some applications, polyether based only on propylene oxide may have inconveniently low reactivity. The primary hydroxyl content may be increased by a separate reaction of the polyoxypropylene polyols with ethylene oxide to form a block copolymer with an oxyethylene tip.

Thermal Analysis of Polyurethane Dispersions Based on Different Polyols 83

Soft segment in hard domain

> − OOC<sup>−</sup>

N<sup>+</sup>-COO<sup>−</sup>

N+ − OOC-N<sup>+</sup>

triethylamine are preferably used. The structure of waterborne anionic PUD is illustrated in

Most commonly used catalysts in PU chemistry are tertiary amine catalysts and metal catalysts, especially tin catalysts. Tertiary amines are catalysts for the isocyanate–hydroxyl and the isocyanate–water reactions. Organotins are the most widely used, however organomercury and organolead catalysts are also used but have unfavourable hazardous

Hard domain

**Scheme 1.** Structure of waterborne anionic PUD (Tawa & Ito, 2006)

Macropolyol

<sup>N</sup>+ DMPA and counterion


**2.1. Various methods for preparing water–based PUD** 

polyamines to the water–based prepolymer dispersion.

The most important process is the prepolymer mixing process that has the advantage of avoiding the use of a large amount of organic solvent. In this process hydrophilically (carboxylate molecule) modified prepolymer is directly mixed with water. If the mixture viscosity is too high, a small amount of a solvent such as N–methyl pyrrolidone can be added before the dispersion step. Chain extension is accomplished by the addition of di– or

Hydrogen bonding

Reacted diisocyanate Urethane or urea linkage

Soft domain

The acetone process can be considered the link between the solvent synthesis and the prepolymer mixing process. In effect, the prepolymer is synthesized in a hydrophilic organic solvent, for example acetone solution and afterwards it is subsequently mixed with water. The hot melt process explains the process of obtaining a PUD by the reaction of NCO– terminated ionic modified prepolymer with, for example ammonia or urea resulting in a prepolymer with terminal urea or biruet groups, respectively. The terminal urea or biruet prepolymer is methylolated with formaldehyde and mixed with water, forming dispersion

Scheme 1.

properties.

Hard segment in soft segment

In the choice of polyol for PU application, selected polyols must be competitive with other polyols and also enable the final PU product to be cost competitive with other materials in the end application.

In a typical anionic PUD process, anionic groups (carboxylic and sulfonic) are introduced along the length of the polymer chain by using hydrophilic monomers or internal emulsifiers. As the hydrophilic monomer, dimethylol propionic acid (DMPA) is the most widely used acid, which has two hydroxyl groups, therefore, it can be one of the main constituents of the PU backbone. DMPA improves the hydrophilic property by serving as the potential ionic center with N–methyl pyrrolidone as the co–solvent. Tartaric acid (TA) can also be used, but it usually results in branching. However, TA improves the mechanical properties of PU paint. Study has shown that the particle size of dispersion depends on the content of DMPA. Therefore, increased amount of DMPA leads to more ionic centres in the PUD backbone and thereby increasing hydrophilicity of the polymer and hence reductions in particle size (Dieterich, 1981; Jacobs & Yu, 1993; Rosthauser & Nachkamp, 1986).

The chain extension step also has a high influence on the properties of the resin produced, not only due to the structure and concentration of the extender but also due to process variables that influence the particle size distribution. Chain extenders are difunctional glycols, diamines or hydroxyl amines. If a diol of low molecular weight reacts with the NCO terminated PU chains in the chain extension reaction step, urethane linkages will be formed but if a diamine is used as chain extender, the NCO terminated PU chains will form urea linkages. The higher density of hydrogen bonds of polyurea hard segments is responsible for the improved mechanical properties of polyurea and PU/urea products. Typical chain– extending agents are as follows: water, diethylene glycol, hydroquinone dihydroxyethyl ether, bisphenol A, bis(hydroxyethylether), ethanolamine, hydrazine, ethylene diamine. Aliphatic diamines such as hydrazine or ethylene diamine are used as chain extenders in processes directed to preparing waterborne PUD. In the chain extension step, it is most important to control the extremely fast reaction between NCO groups and NH2 groups accompanied by the viscosity rise. Molecular weight of PUD increases by the formation of urea linkages through the chain extension step and it is the most important step to determine the molecular weight of PUD (Kim, 1996; Delpech & Coutinho, 2003).

In the synthesis of PUD, to neutralize the carboxyl and/or sulfo groups, are used agents that contain one or more bases and to form internal salt groups containing prepolymers that can be easily dispersed in water. During the neutralization, carboxyl and/or sulfo groups serve for anionic modification or stabilization of the PUD. Tertiary amines and in particular triethylamine are preferably used. The structure of waterborne anionic PUD is illustrated in Scheme 1.

Most commonly used catalysts in PU chemistry are tertiary amine catalysts and metal catalysts, especially tin catalysts. Tertiary amines are catalysts for the isocyanate–hydroxyl and the isocyanate–water reactions. Organotins are the most widely used, however organomercury and organolead catalysts are also used but have unfavourable hazardous properties.

**Scheme 1.** Structure of waterborne anionic PUD (Tawa & Ito, 2006)

82 Polyurethane

the end application.

usually propylene oxide, onto alcohols or amines which are usually called starters or 'initiators'. Polyether based on propylene oxide contains predominantly secondary hydroxyl end–groups. Secondary hydroxyl end–groups are several times less reactive with isocyanates than primary hydroxyl groups and for some applications, polyether based only on propylene oxide may have inconveniently low reactivity. The primary hydroxyl content may be increased by a separate reaction of the polyoxypropylene polyols with ethylene

In the choice of polyol for PU application, selected polyols must be competitive with other polyols and also enable the final PU product to be cost competitive with other materials in

In a typical anionic PUD process, anionic groups (carboxylic and sulfonic) are introduced along the length of the polymer chain by using hydrophilic monomers or internal emulsifiers. As the hydrophilic monomer, dimethylol propionic acid (DMPA) is the most widely used acid, which has two hydroxyl groups, therefore, it can be one of the main constituents of the PU backbone. DMPA improves the hydrophilic property by serving as the potential ionic center with N–methyl pyrrolidone as the co–solvent. Tartaric acid (TA) can also be used, but it usually results in branching. However, TA improves the mechanical properties of PU paint. Study has shown that the particle size of dispersion depends on the content of DMPA. Therefore, increased amount of DMPA leads to more ionic centres in the PUD backbone and thereby increasing hydrophilicity of the polymer and hence reductions

in particle size (Dieterich, 1981; Jacobs & Yu, 1993; Rosthauser & Nachkamp, 1986).

determine the molecular weight of PUD (Kim, 1996; Delpech & Coutinho, 2003).

In the synthesis of PUD, to neutralize the carboxyl and/or sulfo groups, are used agents that contain one or more bases and to form internal salt groups containing prepolymers that can be easily dispersed in water. During the neutralization, carboxyl and/or sulfo groups serve for anionic modification or stabilization of the PUD. Tertiary amines and in particular

The chain extension step also has a high influence on the properties of the resin produced, not only due to the structure and concentration of the extender but also due to process variables that influence the particle size distribution. Chain extenders are difunctional glycols, diamines or hydroxyl amines. If a diol of low molecular weight reacts with the NCO terminated PU chains in the chain extension reaction step, urethane linkages will be formed but if a diamine is used as chain extender, the NCO terminated PU chains will form urea linkages. The higher density of hydrogen bonds of polyurea hard segments is responsible for the improved mechanical properties of polyurea and PU/urea products. Typical chain– extending agents are as follows: water, diethylene glycol, hydroquinone dihydroxyethyl ether, bisphenol A, bis(hydroxyethylether), ethanolamine, hydrazine, ethylene diamine. Aliphatic diamines such as hydrazine or ethylene diamine are used as chain extenders in processes directed to preparing waterborne PUD. In the chain extension step, it is most important to control the extremely fast reaction between NCO groups and NH2 groups accompanied by the viscosity rise. Molecular weight of PUD increases by the formation of urea linkages through the chain extension step and it is the most important step to

oxide to form a block copolymer with an oxyethylene tip.

#### **2.1. Various methods for preparing water–based PUD**

The most important process is the prepolymer mixing process that has the advantage of avoiding the use of a large amount of organic solvent. In this process hydrophilically (carboxylate molecule) modified prepolymer is directly mixed with water. If the mixture viscosity is too high, a small amount of a solvent such as N–methyl pyrrolidone can be added before the dispersion step. Chain extension is accomplished by the addition of di– or polyamines to the water–based prepolymer dispersion.

The acetone process can be considered the link between the solvent synthesis and the prepolymer mixing process. In effect, the prepolymer is synthesized in a hydrophilic organic solvent, for example acetone solution and afterwards it is subsequently mixed with water.

The hot melt process explains the process of obtaining a PUD by the reaction of NCO– terminated ionic modified prepolymer with, for example ammonia or urea resulting in a prepolymer with terminal urea or biruet groups, respectively. The terminal urea or biruet prepolymer is methylolated with formaldehyde and mixed with water, forming dispersion

spontaneously. By polycondensation (lowering the pH, increasing the temperature), chain– extension or cross–linking was obtained.

Thermal Analysis of Polyurethane Dispersions Based on Different Polyols 85

used to increase the molecular weight by reaction with the terminated NCO end groups (chain extension). The presence of ionic species in PUD has a considerable effect on the physical properties. PUD are now one of the most rapid developing and active branches of

PU are synthesized by the prepolymer reaction of a diisocyanate and a polyol (mainly polyethers and polyesters). If a diol of low molecular weight reacts with –NCO-terminated prepolymers in the chain extension reaction step, urethane linkages will also be formed but if a diamine is used as chain extender, the reaction between the –NH2 groups and the –NCO terminated prepolymers will form urea linkages. In this case, poly(urethane-urea)s, which are the most important class of polyureas, are produced. These copolymers show reduced plasticity in comparison to homopolyurethanes. The resulting PU or poly(urethane-urea) chains consist of alternating short sequences forming soft (flexible) and hard (rigid) segments. The soft segments, originated from the polyol, impart elastomeric characteristics to the polymer. The hard segments are mainly produced by reacting the isocyanate and the chain extender. They are polar and impart mechanical properties to PU. The hard segments contain the highly polar urethane linkages. Due primarily to interurethane and urea hydrogen bonding, the two segment types tend to phase-separate in the bulk, forming microdomains. The hard segments act as physical crosslinks and, as a consequence, the physical, mechanical and adhesive properties depend strongly on the degree of phase separation between hard and soft segments and interconnectivity of the hard domains. The urethane linkages in PU can serve as H-bond acceptor and donor. In polyether-based PU, the urethane –NH can bond to either the polyether –O– linkage or the urethane –C=O groups. In the case of poly(urethane-urea) formation, there is an additional –NH from urea linkage participating in the interactions (Delpech & Coutinho, 2000). The degradation of thermoplastic PU has been extensively studied, and a number of reviews are available (Lu et al., 2002; Fambri et al., 2000). Thermal degradation of ester- and ether-based thermoplastic PU is performed under vacuum, air and nitrogen, allowing investigators to determine the

Polyester-based thermoplastic PU exibit rapid degradation in air and nitrogen, indicating that a nonoxidative mechanism is involved. In contrast, the significantly improved thermal stability of ether-based PU under vacuum and nitrogen indicates that the oxidative process plays a major role in the decomposition of ether-based thermoplastic PU. In general, the ester-based PU normally exibit better thermal and oxidative stabilities than the ether-based ones. The mechanism of thermal degradation of PU is very complex due to the variety of

It is proposed that the thermal degradation of thermoplastic PU is primarily a depolycondensation process, which starts at about 200 oC (Cakić et al., 2006 a). The first stage of decomposition is because of degradation of hard segments and starts at about 200 oC and at ~ 360-380 oC, while the second step of degradation is because of degradation of soft segments and ends above 480 oC. Waterborne PU should exhibit some different features in thermal degradation due to their unique chain structure, for example, salt-forming groups. Therefore, it is necessary to analyses their thermal degradation behavior to

PU chemistry.

mode of degradation (Dulog & Storck, 1996).

products formed.

Ketamine and ketazine process explains the process of obtaining a PUD by reaction of NCO–prepolymers containing ionic groups mixing with a blocked amine (ketamine) or hydrazine (ketazine) without premature chain extension. These mixtures can be emulsified with water even in absence of co–solvents. The reaction with water liberates the diamine or hydrazine, which then reacts with the prepolymer.

Non–ionic dispersions are obtained similar to ionomer dispersions if the ionic centre is replaced by lateral or terminal hydrophilic ether chain. The temperature of dispersing process has to be kept below 60 °C. Non–ionic dispersions are stable towards freezing, pH changes and addition of electrolytes.
