Waterborne anionic polyurethanes and poly(urethane-urea)s: influence of the chain extender on mechanical and adhesive properties

Polymer Testing 19 (2000) 939–952

Material Behaviour

Waterborne anionic polyurethanes and poly(urethane-urea)s: influence of the chain extender on mechanical and adhesive properties Marcia C. Delpech a, Fernanda M.B. Coutinho a

a, b,*

Instituto de Macromole´culas Professora Eloisa Mano, IMA/UFRJ, C.P. 68525, Rio de Janeiro, RJ, CEP: 21945-970, Brazil b Departamento de Processos Industriais, IQ/UERJ, Rio de Janeiro, RJ, Brazil Received 27 August 1999; accepted 21 October 1999

Abstract Polyurethane and poly(urethane-urea) aqueous dispersions based on 4,4⬘-dicyclohexylmethane diisocyanate (H12MDI), poly(propylene glycol) (PPG) and dimethylolpropionic acid (DMPA) were synthesized. Three types of chain extenders were used, hydrazine (HYD) and ethylenediamine (EDA), producing poly(urethane-urea)s and ethylene glycol (EG), polyurethanes. The dispersion was performed before or after the chain extension reaction, depending on the extender employed. The dispersions were prepared with and without the addition of acetone after the prepolymer synthesis and neutralization steps. The length of soft segment and NCO/OH ratio were varied. Some mechanical properties of cast films obtained from the aqueous dispersions, the characteristics of coating application on a wood surface and their adhesive properties were evaluated.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction The continuous reduction in costs and the control of volatile organic compound emissions are increasing the use of aqueous-based resins, motivating the development of polyurethanes dispersed in water [1–3]. These products present many of the features related to conventional solventborn coatings with the advantage of presenting low viscosity at high molecular weight and good applicability [2,4]. Polyurethanes can be tailor-made and have versatile applications. Basically, linear thermoplastic * Corresponding author. Tel.: +55-21-270-1035; fax: +55-21-270-1317. E-mail address: [email protected] (F.M.B. Coutinho)

0142-9418/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 6 6 - 5

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polyurethanes are synthesized by the prepolymer reaction of a diisocyanate and a polyol (mainly polyethers and polyesters) [1,2]. If a diol of low molecular weight reacts with the –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 –NCOterminated prepolymers will form urea linkages. In this case, poly(urethane-urea)s, which are the most important class of polyureas [5], are produced [6]. These copolymers show reduced plasticity in comparison to homopolyurethanes [7]. The resulting polyurethane 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 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 [7,8]. The hard segments act as physical crosslinks and, as a consequence, the physical [8], mechanical [9–11] and adhesive [12] properties depend strongly on the degree of phase separation between hard and soft segments and interconnectivity of the hard domains [8]. The urethane linkages in polyurethanes can serve as H-bond acceptor and donor. In polyether-based polyurethanes, the urethane –NH can bond to either the polyether –O– linkage or the urethane –C=O groups [13]. In the case of poly(urethaneurea) formation, there is an additional –NH from urea linkage participating in the interactions. Polyurethanes, in general, present good adhesive properties due to their elastomer properties, enhanced by the soft segments of the polyol, and by the polar character of the urethane groups [12]. The applications of polyurethane adhesives include substrates such as glass, wood, leather, plastics, rubber, metals, concrete and ceramic [4,10,14–16]. Polyurethane aqueous dispersions are becoming an increasingly important class of materials in the surface coatings industry. Their application includes areas such as construction, automotive, packing, transportation, electronics, textiles, tape, paper and footwear [14,17]. In this work, some mechanical properties of cast films obtained from polyurethane and poly(urethane-urea) aqueous dispersions were evaluated. The performance of the dispersions as coatings for wood is discussed.

2. Experimental 2.1. Reagents The following reagents were used without further purification in waterborne polyurethane and poly(urethane-urea)s synthesis: dibutyltin dilaurate (DBTDL), Aldrich; dimethylolpropionic acid (DMPA), Aldrich; ethylene diamine (EDA), 98.8%, PA-ACS, Reagen; ethylene glycol (EG), Vetec; isophorone diisocyanate (IPDI), 98.6% [18,19], Hu¨lls; hydrazine (HYD), 64%, Bayer; poly(propylene glycol) (PPG): (P1) Voranol 2110 Mn (VPO)=1300, hydroxyl number=56.25 [20] and (P2) Voranol 2120, Mn (VPO)=2450, hydroxyl number=106.50 [20], dried under vacuum, at 120°C [2,21], Dow; and triethylamine (TEA), Union Carbide.

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2.2. Synthesis 2.2.1. Prepolymer formation and neutralization Prepolymer anionomers were prepared in the absence of solvent at 100°C by reacting, for 2 h, PPG, DMPA and H12MDI, in the presence of the catalyst DBTDL. The –NCO end groups were determined by using a standard dibutylamine titration method [19]. The carboxylic groups of DMPA were neutralized by reaction with TEA, at 40–50°C. After the neutralization, the –NCOterminated prepolymers were separated in two parts. In one part, acetone was added (10% w/w) and the resultant mixture was submitted to high stirring for 15 min. 2.2.2. Dispersion in water and chain extention reaction The next step to which both prepolymer parts were submitted was dependent on the chain extender type used. When the extender had –NH2 groups (HYD and EDA), both parts (prepolymer containing acetone and pure prepolymer) were firstly dispersed in deionized water, at 20°C, and then submitted to chain extension reaction. The reaction systems were maintained at 35°C, for 1 h, in order to complete the extension reaction. When the chain extender had –OH groups (EG), the extension reaction was carried out before the dispersion, in order to avoid the competition between the hydroxyl groups of the extender and the water. After the addition of EG, the reactions were maintained at 45°C, for 1 h. Then, water was added and the systems, with and without acetone, were kept at 30°C, for 1 h more. Even in the presence of acetone, the viscosity of the reaction medium was very high, increasing for lower NCO/OH ratios. 2.3. Methods Films were obtained by casting the aqueous dispersions on leveled surfaces and allowing them to dry at room temperature, for 7 days, and then at 60°C, for 12 h [2]. When the chain extenders were HYD and EDA, poly(urethane-urea)s were formed and casting was performed on glass plates. When the chain extender was EG, the aqueous dispersions had to be cast on teflon surfaces due to the high adhesiveness observed to the glass surface, making demolding impossible. After demolding, the films were kept in a desiccator to avoid moisture contact. The tensile test performed was an adaptation of ASTM-D 412-83, D 638-84 and D 882-83 standard methods [2]. This adaptation was performed because the films obtained were very thin (varying in the range from 0.3 to 0.8 mm) and, consequently, not appropriate to be cut as dumbbells. Tensile test bars (5×70 mm2) were cut from aqueous dispersion cast films. A tensile test machine INSTRON 4204, equipped with a 1 kN load cell and pneumatic grips, was used at a crosshead speed of 50 cm/min. The adhesive properties of the dispersions as coatings for wood were evaluated according to standard tests [22,23]. This method uses a class of apparatus known as pull-off adhesion testers. In this work, a pull-off adhesion tester Elcometer 106 number 2 (1–7 MPa scale) was used. This is portable and permits the application of a concentric load to a single surface so that coatings can be tested even though only one side is accessible. Aluminum fixtures were adhered on the dried coated surface by using a bi-component standard adhesive. The tester was coupled to the

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fixture and the upper part was turned until the fixture was pulled off [22]. The value was read from the scale and the fixture surface was observed. Measurements were limited by the strength of adhesion bonds between the loading fixture and the specimen surface or the cohesive strength of the substrate. The adhesion was considered satisfactory when the rupture took place in the substrate. In this case, the surface of the fixture becomes covered with a layer of wood. Due to the destructive nature of the test the wood chosen had to be very hard (ipeˆ, from a native Brazilian tree). Test specimens measuring (20×20×1) cm3 were rubbed with sandpaper and the water dispersions were applied with a paint brush. The dispersions were applied on the substrate and dried at room temperature, for 7 days. Then four aluminum fixtures for each coating were fixed on the surface of the coated substrate with the standard bi-component adhesive. The systems were maintained at room temperature for another 7 days, for complete evaporation of the volatiles from the adesive. A test adaptation, in which the adhesive was not applied on the coating, was also performed in order to verify the adhesion of the coating directly on the metal fixture. In this case, the dispersions were applied, the fixtures were immediately put over the surface and the system was kept at room temperature to dry for 14 days. 3. Results and discussion The polyurethane and poly(urethane-urea) aqueous dispersions synthesized in this work were one-component and resistant to ultraviolet radiation, due to the aliphatic nature of the diisocyanate. Polyurethanes with a solid content of about 35% and a 2% w/w content of ionic groups were obtained in the presence of acetone (acetone in reaction—AR) or as pure dispersions (PD), by varying the molecular weight of the polyol (P1, P2 and P12 series, the last one including both polyols in the reaction) and the ratio between the equivalent-number of –NCO groups of the diisocyanate and the equivalent-number of –OH groups of the hydroxylated compounds: (NCO/OH) ratio, which was varied from 1.5 to 3.1. For ratios lower than 1.9, the dispersions corresponding to the P1, P2 and P12 series were only obtained when acetone was added due to the high viscosity of the prepolymer. The viscosity of the dispersions varied from 10 to 50 cP, making their application on wood surfaces easy. After drying, the coated surfaces, depending on the NCO/OH ratio, the molecular weight of the soft segment and the chain extender utilized, presented different characteristics: brilliant surface with tack, brilliant surface slightly sticky or without tack and opaque surface without tack. 3.1. Characteristics of the films Table 1 presents the characteristics of the dried polyurethanes and poly(urethane-urea)s films, obtained from P1, P2 and P12 series. All AR aqueous dispersions cast films were more flexible in comparison with those obtained from pure dispersions (PD). The P2 aqueous dispersions cast films were more plasticized than the P1 and P12 series. When the chain extender was EG, the AR aqueous dispersion did not form a film but a very tacky surface. The films from dispersions with HYD and EDA as chain extenders were less flexible than

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Table 1 Characteristics of films formed from cast polyurethane aqueous dispersions Characteristics of the films

Chain extender

NCO/OH ratio

Transparent, homogeneous aspect, without imperfections

HYD

1.5–2.1

EDA EG HYD

1.5–1.9 1.5–3.5 2.3

EDA HYD EDA HYD and EDA

2.1 2.5–2.9 2.3 3.1

Transparent, heterogeneous (“spiderweb”) aspect but entire Transparent and brittle Opaque and brittle

those from EG dispersions, probably due to the presence of urea groups formed by the reaction of –NCO end groups with –NH2 groups. The diol extender formed urethane groups by the reaction with –NCO end groups. Urea groups present two nitrogen atoms suitable to form hydrogen bonds, whereas urethane groups present only one nitrogen in this condition. So, the presence of urea groups produced an increase in the crystallinity of the material and, consequently, in its rigidity and brittleness. The EDA group films presented brittleness at lower NCO/OH ratios than HYD group films. The segment –CH2–CH2– from EDA, located between the two urea groups, probably imparted more flexibility to the hard segment and was also more compatible with the PPG-based soft segment. In this case, the miscibility between the phases, hard and soft, was improved, considering also the possibility of hydrogen bonding interaction between the PPG ether linkage and urethane and urea linkages, and the crystallinity of the whole system increased [24]. On the other hand, HYD provided a more polar character to the hard segments and, in this case, the miscibility between soft and hard segments may decrease [25]. 3.2. Mechanical properties The mechanical behavior of thermoplastic elastomers is dependent on the intermolecular interactions between their hard segments [26]. Figs. 1–3 show stress–strain curves, Fig. 4 presents the variation of modulus of elasticity with NCO/OH ratio and Figs. 5 and 6 show elongation at break value variation with NCO/OH ratio. Fig. 1 shows the stress–strain curves of films obtained from casting of aqueous dispersions based on PPG P1, with the addition of acetone and chain-extended with HYD. It can be observed that the increase in NCO/OH ratio increased the mechanical resistance of the material because there is an increase in the degree of interchain hydrogen bonding, which leads to the formation of more rigid films. Fig. 2 presents the influence of the length of soft segment and the presence of acetone in the reaction. The shorter segment (P1) provides higher mechanical resistance and the films obtained

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Fig. 1. Stress–strain curves of cast films obtained from aqueous dispersions synthesized with P1, HYD as chain extender, in the presence of acetone (AR).

Fig. 2. Stress–strain curves of cast films obtained from aqueous dispersions synthesized with NCO/OH ratio=1.9 and EG as chain extender.

from dispersions synthesized with the polyols mixture (P12), as expected, showed an intermediate behavior. When the length of the soft segments increased (P2), the elastomeric character of the samples increased. An increase in the molecular weight of the soft segments led to a higher phase segregation [27]. Moreover, there is a lower proportion of hard segments and, as a result, in hydrogen bonding interactions [27,28]. The presence of acetone in the reaction caused a decrease in the mechanical resistance of the

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films. The results indicate that acetone was probably retained among the chains even after the process of drying. The acetone carbonyl may participate in hydrogen bonding with the urea and urethane linkages contributing to a decrease of the structural regularity of hard segments, decreasing the mechanical resistance [29–31]. Therefore, acetone seems to act as an internal plasticizer. This behavior was not observed when acetone was added to the pure dispersion just before casting, as presented in a previous work [2]. Fig. 3 shows the influence of the chain extender on the mechanical behavoir of cast films from pure (PD) aqueous dispersions synthesized with NCO/OH ratio=1.9 and with soft segments of different lengths. In relation to the soft segments, the behavior follows the same pattern observed and discussed in Fig. 2. The chain extenders containing –NH2 groups (HYD and EDA), which formed poly(urethaneurea)s, resulted in more rigid films due to the nature of urea linkages, that increase the hydrogen bonding interactions. Film bleaching was observed during the stretching process and the stress– strain curves presented a concave form. These are factors that indicate the occurrence of a crystallization process during the test [31]. The mechanical resistance was, consequently, higher than that observed for the films obtained from EG dispersions, containing only urethane linkages. The EDA films showed higher mechanical resistance than HYD ones. This result corroborates the indication of a higher miscibility between hard and soft phases than the HYD films. Fig. 4 shows the variation of modulus of elasticity with NCO/OH ratio. The AR film behavior is presented. The same pattern was observed for PD films. An increase in the modulus values can be noticed and, consequently, in the mechanical resistance, by increasing the NCO/OH ratio and decreasing the soft segment length. There was a remarkable increase in modulus for EDA films showing again the lower elastomeric character of those films and probably the occurrence of higher interaction between the hard and soft segments. Figs. 5 and 6 present the variation of elongation at break with NCO/OH ratios. Fig. 5 shows AR films and Fig. 6 compares AR and PD film behavior, for P12 series. The elongation at break

Fig. 3. Stress–strain curves of cast films obtained from aqueous dispersions synthesized with NCO/OH ratio=1.9 and in the absence of acetone (PD).

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Fig. 4. Variation of modulus of elasticity with NCO/OH ratio for cast films obtained from aqueous dispersions containing acetone (AR).

Fig. 5.

Variation of elongation at break with NCO/OH ratio for AR cast films.

decreases as the NCO/OH ratio increases and the length of the soft segment decreases, as can be observed in Fig. 5. The elastomer character is higher when the chain does not present urea groups, i.e. when the chain extender is EG. Following the same behavior discussed above, EDA films showed the lowest elastomer character and, consequently, the highest rigidity. The presence of acetone provided a marked increase in elongation at break for EG and HYD films. However, this increase was not observed for EDA films. In this case, if there were really a higher miscibility between hard and soft segments than for HYD and EG films, the plasticizing effect for EDA/AR films would not be so remarkable in comparison with the other two extenders-based films, because the hard segments were interacting more with PPG-based soft segments.

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Fig. 6. Comparison between curves of elongation at break variation with NCO/OH ratio for films obtained from pure dispersions (PD) and containing acetone (AR).

3.3. Characteristics of the coatings The characteristics of the coatings were gathered according to the NCO/OH ratio and chain extender type, as follows. 앫 NCO/OH=1.5 Poly(urethane-urea): HYD and EDA as chain extenders: the surfaces were homogeneous and brilliant. The coatings obtained from the dispersions based on polyol P2 showed slight tack, which did not occur with the other series. Polyurethane: EG as chain extender: the surfaces were homogeneous, brilliant, very tacky, especially the coating obtained from the P2 series. 앫 NCO/OH=2.3 Poly(urethane-urea): HYD as chain extender: the surfaces were homogeneous and brilliant, without tack, whichever the polyol used in the synthesis. EDA as chain extender: for P1 series, the surface was opaque and entire but presented a slightly cracked aspect. For P12 and P2 series, the coatings formed brilliant and entire surfaces but with a “web” effect. Polyurethane: EG as chain extender: the surfaces were homogeneous, brilliant, with little tack, especially the coating corresponding to the P2 series. 앫 NCO/OH=3.1 Poly(urethane-urea): HYD as chain extender: the surfaces were homogeneous and entire. The coatings obtained from the P1 series presented no tack and were completely opaque. P12 and P2 series formed coatings with homogeneous surfaces slightly brilliant and without tack. EDA as chain extender: for the P1 series, the surface was opaque and completely cracked. For P12 and P2 series, the coatings were entire but showed a “web” effect and were slightly brilliant, without tack. Polyurethane: EG as chain extender: the surfaces were homogeneous, brilliant and without tack.

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Considering the molecular weight of the soft segment of polyurethane chains and its content on the reaction, the characteristics of the coatings showed that an increase in the length of the polyol and in their content on the reaction (lower NCO/OH ratios) led to a higher tack [32] and promoted surface brightness. For the coatings obtained from the P1 series, the surfaces were more opaque and with little or no tack. It was observed that the development of tack and brightness was related to the length of the soft segment and its content in the polymeric chains. Considering the effect of the NCO/OH ratio, it can be observed that the characteristics of brightness and stickiness increased as that ratio decreased. That effect was more accentuated for the P1 series, where the soft segment was shorter. The effect of the chain extender in the polymer chains showed that EG, even at the highest NCO/OH ratio (3.1), provided a more flexible character than HYD and EDA because of the presence of only urethane groups, where the hydrogen bonding is not so strong as it is when urea linkages are present. The coatings obtained from poly(urethane-urea)s presented higher rigidity that increased with the increase of NCO/OH ratio and led to brittle coatings, especially those with the shorter soft segment (P1 series). The chain extender EDA produced more brittle coatings than HYD. 3.4. Adhesive test Table 2 presents the nature of the failure expressed as the percentage area and site of fracture in the system under test in terms of adhesive, cohesive or adhesive/cohesive failure. The adhesion between coating and substrate is considered satisfactory when the disruption of the system occurs due to cohesive failure of substrate [22,23]. In Table 3 the average values of the breaking strength obtained for all the systems (AR and PD) are listed. Two sets of tests are reported: one is the standard test, where an adhesive was applied between the aluminum fixture and the dried coating, and in the other one, the fixture was put in direct contact with the wet coating. In general, the adhesive tests provide low sensitivity measurements and give only comparative characteristics, but from results obtained in this work some conclusions were reached. The P1 series (shorter soft segment) produced the highest values of breaking strength, for all systems. Lower values of breaking strength, obtained for P12 and P2 series, can be attributed to the higher molecular weight of the soft segment causing a hindrance for the coating diffusion through the porosity of the wood, resulting in more superficial and less adherent films. Furthermore, the degree of hard segments is lower for P12 and P2 series, in comparison with the P1 series, leading to a lower degree of hydrogen bonding interaction with wood hydroxyl groups. It was expected that the coatings based on EG as chain extender would present higher values of breaking strength. As discussed previously, those films cast on glass plates could not be demolded, in contrast to films based on HYD and EDA. However, in adhesion tests it was observed that the chain extender did not have a significant influence on the adhesion of the coatings in wood. The values of breaking strength and the nature of the failure were close for the three types (based on HYD, EDA and EG). As a whole, by comparing coating formation characteristics and the results obtained in adhesion tests, the chain extender HYD apparently produced the best coatings. Considering the standard tests (in which the adhesive was applied), a tendency to lower values of breaking strength with the increase of the NCO/OH ratio was observed. There was also a

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Table 2 Nature of the general types of failure observed on the systems described according to standard tests [22,23] NCO/OH ratio

1.5

Adhesive present in the test Site where failure 䉬 wood occurred

Nature of failure

䉬 cohesive failure of the substrate

Absence of adhesive in the test Site where failure 䉬 wood (苲30%) occurred 䉬 fixture/coating interface (苲70%) Nature of failure 䉬 cohesive failure of substrate 䉬 adhesive failure between fixture and coating

2.3

3.1

䉬 wood (苲40%)

䉬 adhesive/coating interface (苲70%) 䉬 adhesive/coating interface 䉬 coating/wood interface (苲40%) (苲30%) 䉬 coating/wood interface (苲20%) 䉬 cohesive failure of 䉬 adhesive failure between substrate adhesive and coating 䉬 adhesive failure between 䉬 adhesive failure between adhesive and coating coating and substrate 䉬 adhesive failure between coating and substrate 䉬 wood (苲10%) 䉬 fixture/coating interface (苲90%) 䉬 cohesive failure of substrate 䉬 adhesive failure between fixture and coating

䉬 no adhesion between the fixture and the coating

significant difference in the nature of the failure (Table 2). The coatings obtained from reactions with NCO/OH ratio=1.5 presented higher adhesion to the substrate and the failure occurred in the wood. That result showed the adhesion between adhesive/coating and coating/substrate (wood) was lower than the cohesion of the substrate. All the coatings obtained from reactions with NCO/OH ratio=3.1 showed adhesion failure in adhesive/coating and coating/substrate interfaces, the latter mainly when EDA was used as chain extender. The higher degree of hard segments probably hindered the accommodation and diffusion of the polyurethane chains in the porosity of the substrate, decreasing the coating adhesion. Considering the tests performed with no adhesive, i.e. with the fixture fixed directly on the surface of the coating, it was observed, as expected, that the values of breaking strength decreased. The results showed significant differences taking into account the NCO/OH ratio. The results presented in Tables 2 and 3 show that the coatings produced from reactions with NCO/OH ratio=1.5 produced a satisfactory adhesion to the substrate and also some adhesion to the metal fixture surface. Taking into consideration the presence of solvent in the reaction (AR and PD systems), the values of breaking strength were nearly the same and there was a tendency towards slightly higher values for PD coatings. As observed in the mechanical evaluation, the presence of acetone decreased the resistance of the tested films.

3.5 2.0 1.0 3.0 2.0 1.5 3.0 2.0 1.0

7.0 6.0 5.0

6.5 6.0 5.0

5.5 4.5 4.0

– – 5.5

– – 6.0

– 6.5 6.0

– – 2.0

– – 1.5

– 2.5 2.0

5.0 5.0 3.0

5.0 5.0 4.0

5.0 5.0 3.0

2.0 1.0 no

no 0.5 no

2.5 1.5 1.0

6.5 6.5 5.5

6.0 5.0 5.0

5.0 4.0 3.5

NCO/OH=2.3 PDc ARb test ARc PDb test fixt./coat. adh. fixt./coat. adh.

2.0 2.0 1.5

no 0.5 no

2.5 2.0 1.0

5.0 4.5 3.0

5.0 4.0 4.0

5.0 4.5 3.5

no no no

no no no

0.5 no no

6.0 4.5 3.0

5.0 4.5 4.0

5.5 4.5 4.0

no no no

no no no

0.5 no no

NCO/OH=3.1 PDc ARb test ARc PDb test PDc fixt./coat. adh. fixt./coat. adh. fixt./coat.

AR: acetone present on the dispersion; PD: dispersion without acetone; test. adh.: presence of the standard test adhesive between the fixture and the coating; fixt./coat.: fixture in direct contact with coating; no: the fixture did not adhere to the substrate. b Average values of four measurements. c Average values of two measurements. Obs: blank test without the coating (fixture+adhesive+wood)=5.5 MPa.

a

HYD P1 P12 P2 EDA P1 P12 P2 EG P1 P12 P2

Breaking strength (MPa) Ratio NCO/OH=1.5 PDb test Soft segment ARb test ARc adh. fixt./coat. adh.

Table 3 Average values obtained in the pull-off adhesion testa

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4. Conclusions The mechanical properties of the cast films and the adhesive properties of the coatings studied in this work were dependent on the combination of the constituents of polymer chains (length of soft segment, NCO/OH ratio, type of chain extender and presence or not of solvent in the synthesis). The mechanical resistance, i.e. increasing values of stress with strain and modulus of elasticity, was favored, in general, by increasing NCO/OH ratio and decreasing length of soft segment. The aqueous dispersions in which acetone was added during the synthesis (AR) resulted in films with lower mechanical resistance in comparison with the dispersions cast films produced without addition of acetone (PD). Poly(urethane-urea)s cast films obtained from aqueous dispersions chain-extended with ethylenediamine (EDA) showed higher mechanical resistance than those in which hydrazine (HYD) was the chain extender. On the other hand, polyurethanes cast films, based on ethylene glycol (EG) as chain extender, showed higher elastomeric character. Although polyurethane coatings generally present better adhesive properties than poly(urethaneurea) coatings, it was observed that, for the systems studied, there was not a significant difference. It was observed that the shorter soft segment led to systems with more adequate characteristics for a better adhesion. It was verified that the surfaces showing tack were not necessarily the ones with better adhesion. According to the properties of coating formation and values of breaking strength, it was observed that hydrazine (HYD), used as a chain extender, contributed significantly to the formation of homogenous and entire coatings, without tack.

Acknowledgements The authors thank Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) for financial support, Centro de Pesquisas da Eletrobra´s (CEPEL) and Dow Quı´mica SA.

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