**3. Thermal analysis of PU**

Thermal analysis techniques have been used for many years in many scientific and industrial laboratories for studying the thermal decomposition of polymeric materials. Among them thermogravimetry (TG) is one of the most common since the mass of a sample is easy to measure accurately and valuable information regarding the nature of the process can be extracted from a mass loss against time or temperature plot. The thermal decomposition of PU (their degradation attributed to absorbed thermal energy) is important phenomenon from both fundamental and industrial applications (Pielichowski et al., 2004). The understanding of degradation processes allows determination of optimum conditions for designing PU in order to obtain high-performance polymer materials. Fundamental research has established that the thermal decomposition of PU is a complex heterogeneous process and consists of several partial decomposition reactions (Scaiano, 1989). The study of the decomposition of PU is particularly difficult since they degrade with the formation of various gaseous products and a number of decomposition steps are typically observed in thermogravimetric analysis (TGA) experiments. Some authors claim that the study of the thermo-degradation behavior of PU at high temperatures provides a fingerprint of the material that has to do not only with the characteristics of the original material, but also with its processing and the final quality of the end use products (Prime et al., 1988). The thermal stability of a material is defined by the specific temperature or temperature-time limit within which the material can be used without excessive loss of properties (Chattopadhyay & Webster, 2009). With respect to commercial applications, the investigation of thermal decomposition processes has two important aspects. The first concerns the stabilization of a polymer to obtain novel materials with a desired level of thermal stability that will be able to fulfill the demands of contemporary materials engineering. The second is to understand material behavior at higher temperature as well as to obtain characteristic thermal decomposition data.

A waterborne PUD can be defined as a binary colloidal system in which PU particles are dispersed in a continuous aqueous medium. PUD are usually prepared as low molecular weight NCO-terminated prepolymers for ease of dispersion. Then, diamines are generally 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 chemistry.

84 Polyurethane

extension or cross–linking was obtained.

changes and addition of electrolytes.

**3. Thermal analysis of PU** 

hydrazine, which then reacts with the prepolymer.

to obtain characteristic thermal decomposition data.

spontaneously. By polycondensation (lowering the pH, increasing the temperature), chain–

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

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

Thermal analysis techniques have been used for many years in many scientific and industrial laboratories for studying the thermal decomposition of polymeric materials. Among them thermogravimetry (TG) is one of the most common since the mass of a sample is easy to measure accurately and valuable information regarding the nature of the process can be extracted from a mass loss against time or temperature plot. The thermal decomposition of PU (their degradation attributed to absorbed thermal energy) is important phenomenon from both fundamental and industrial applications (Pielichowski et al., 2004). The understanding of degradation processes allows determination of optimum conditions for designing PU in order to obtain high-performance polymer materials. Fundamental research has established that the thermal decomposition of PU is a complex heterogeneous process and consists of several partial decomposition reactions (Scaiano, 1989). The study of the decomposition of PU is particularly difficult since they degrade with the formation of various gaseous products and a number of decomposition steps are typically observed in thermogravimetric analysis (TGA) experiments. Some authors claim that the study of the thermo-degradation behavior of PU at high temperatures provides a fingerprint of the material that has to do not only with the characteristics of the original material, but also with its processing and the final quality of the end use products (Prime et al., 1988). The thermal stability of a material is defined by the specific temperature or temperature-time limit within which the material can be used without excessive loss of properties (Chattopadhyay & Webster, 2009). With respect to commercial applications, the investigation of thermal decomposition processes has two important aspects. The first concerns the stabilization of a polymer to obtain novel materials with a desired level of thermal stability that will be able to fulfill the demands of contemporary materials engineering. The second is to understand material behavior at higher temperature as well as

A waterborne PUD can be defined as a binary colloidal system in which PU particles are dispersed in a continuous aqueous medium. PUD are usually prepared as low molecular weight NCO-terminated prepolymers for ease of dispersion. Then, diamines are generally 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 mode of degradation (Dulog & Storck, 1996).

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 products formed.

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 understand the structure-property relationship. The properties of PUD are mainly determined by the interactions between the hard and soft segments, and by the interactions between the ionic groups. The ionic group content, solids content, segmented structure, molecular weight of the macroglycol, the type of chain extender and the hard/soft segments ratio, determine the properties of PUD.

Thermal Analysis of Polyurethane Dispersions Based on Different Polyols 87

this group of materials (Chang et al., 1995). These thermoplastic elastomers generally are not very thermally stable, especially above their softening temperatures (Wang & Hsieh, 1997), and their mechanism of thermal degradation is very complex due to the variety of products formed. Commonly, it presents a bimodal profile where the first mode is related to the hard segments of PU. Usually, at a low heating rate, the degradation process results in differential weight loss (DTG) curves with several peaks, which is an indication of the complexity of the

Poly(propylene glycol) (PPG) ), (Mn = 1000, hydroxyl value 111 mg KOH/g, dried under vaccuum, at 120 oC), isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA), were obtained from Aldrich Chemical Co. 1-methyl-2-pyrrolidone (NMP), dimethyl formamide (DMF) and triethyl amine (TEA) were received from Merck (Darmstadt, Germany). Ethylene glycol (EG) and propylene glycol (PG) obtained from Zorka Co.(Šabac Serbia). Dibutyltin dilaurate (DBTDL), was supplied by Bayer AG. Zirconium catalyst (ZrCat) was supplied by King Industries Inc., Norwalk, CT, USA. Manganese catalyst (MnCat) has been used in the reactive diluent with a metal content of 0.4% (Stamenković et al., 2003; Cakić et al., 2006 b). Water–based PUD from PPG with selective catalyst were prepared using the prepolymer method has been described in detail in our previously work

In the first step, PPG and DMPA were dispersed in DMF to obtain a homogeneous mixture and heated at 70 oC. IPDI and DBTDL were added to the homogenized mixture at 80 oC. An NCO/OH equivalent ratio of 3.0 was used. Hard : Soft segment ratio was defined as a ratio between IPDI weight and polyol weight in the starting formulation and is calculated as 1.5. The reaction times were determined by the dibutyl amine back titration method. After obtaining completely NCO terminated prepolymer, the mixture was cooled to 60 oC and the carboxylic groups were neutralized with TEA (DMPA equiv) dissolved in NMP. In second step the chain extension was carried out with EG or PG. The selective catalysts, ZrCat or MnCat, at concentration of 2% relative to the resin solids, have been added to the reaction solution. Water was added to the mixture and stirred to obtain dispersion of organic phase in water. The waterborne PUD contains 40 wt% solids (Cakić et al., 2009). Films were prepared by casting the waterborne dispersions on leveled surfaces and allowing them to dry at room temperature, for 7 days, and then at 60 oC, for 12h (Coutinho, 1996, 2003). When chain extenders were EG and PG, the PUD had to be cast in Teflon surfaces due to the high adhesiveness observed on the glass surface, making demoulding impossible. After demoulding, the films were kept into a desiccator to avoid moisture.and polyurethanes

Degradation profile of waterborne PUD is influenced by the variation of chain extender presented in Fig.1. It was verified that the thermal stability was influenced by chain

degradation (Delpech & Coutinho, 2000).

**4.1. Experimental** 

(Cakić et al., 2009).

were formed.

**4.2. Results and discussion** 

In the following sections, we will review typical results to demonstrate the utility of TGA in deducing the structural and bonding information about waterborne PUD based on different polyols. The thermal stability of PU and poly(urethane –urea)s cast films with anionomer character, obtained from waterborne dispersions and based on isophorone diisocyanate (IPDI), dimethylolpropionic acid (DMPA), poly(propylene glycol) (PPG), polycarbonate diol (PCD) and glycolized products obtained from recycled poly(ethylene terephthalate)(PET) is also compared. Three types of chain extenders were used: ethylene glycol (EG), propylene glycol (PG) and ethylene diamine (EDA). The effect of type of polyols, chain extender, type of catalyst, ionic content, length of soft segment, hard segment content and the presence of urea or urethane linkages on the thermal stability of the waterborne anionic PUD are discussed.
