USE OF POLYOL MIXTURES INCLUDING POLYCARBONATE DIOL FOR IMPROVING PERFORMANCE OF COATINGS OBTAINED FROM WATERBORNE POLYURETHANE DISPERSIONS

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1 USE OF POLYOL MIXTURES INCLUDING POLYCARBONATE DIOL FOR IMPROVING PERFORMANCE OF COATINGS OBTAINED FROM WATERBORNE POLYURETHANE DISPERSIONS Vanesa García-Pacios 2, José A. Jofre-Reche 2, Manuel Colera 1, Victor Costa 1, José Miguel Martín-Martínez 2 1 UBE CHEMICAL EUROPE, S.A. Polígono El Serrallo, Grao (Castellón), Spain. 2 Adhesion and Adhesives Laboratory, University of Alicante, Alicante, Spain. Abstract In this study, the synthesis of PUDs with different mixtures of polyether and polycarbonate diol of similar molecular weight was carried out and their performances as coatings on stainless steel were tested. The objective was improving the performance of polyether diol by blending with polycarbonate diol, avoiding their own limitations. To the best of our knowledge this is the first time that this study is carried out both at scientific and technical level. Ten PUDs obtained with mixtures of polycarbonate diol (polycarbonate of 1,6- hexanediol) and polyether diol (1,4-polytetramethylene glycol) (50, 25, 10 eq% polyether diol) with molecular weight of 1000 and 2000 Da, were synthesized and coatings on stainless steel 304 plates were prepared and characterized by measurements of drying time, yellowness index, Persoz hardness, Pencil hardness, gloss, cross-cutter adhesion and chemical resistance. Comparison of the coating properties was carried out before and after ageing test. Drying time of the PUDs was faster as the amount of polycarbonate diol in the polyol mixtures increased. Furthermore, the Persoz hardness of the coatings prepared from polyol mixtures increased by increasing their polycarbonate diol content. On the other hand, the addition of mixtures of polyether and polycarbonate diol in coating formulations decreased their yellowness index. After ageing, the retention of initian properties of coatings was enhanced when polyol mixtures including polycarbonate diol were used. Keywords: Coatings. Waterborne polyurethane dispersions. Polyol nature. Polycarbonate diol. Polyol mixture. Durability improvement.

2 INTRODUCTION Coating industry increasingly faces stricter environmental legislation and consumer concerns related to environmental impact. Coating technology has changed from traditional solvent-borne to environmentally friendly coatings. One example is the coating obtained from waterborne polyurethane dispersions (PUDs) as they minimize the content of organic solvents and excellent properties such as high hardness, adequate gloss and good chemical resistance can be reached. PUDs can be mainly used as flexible coatings for textiles and hard coatings for wood and metallic surfaces [1]. Waterborne polyurethane dispersions (PUDs) are becoming more commonly used as coatings since Du Pont Company commercialized in 1961 the first waterborne polyurethane dispersion. Depending on the requested properties, different polyols can be used in polyurethane synthesis to modify its segmented structure and thus its properties. The most common polyols used in polyurethane synthesis are polyester, polyether, polycaprolactone and polycarbonate diols. Waterborne polyurethane dispersion consists in linear thermoplastic polyurethane chains dispersed in water due to the presence of ionic groups in the structure (i.e. polyurethane ionomer), which act as internal emulsifier. Typically, an NCO-ended prepolymer ionomer is first prepared, which is readily dissolved in acetone. Afterwards, a chain extender, such as diamine, is added to react with the terminal NCO groups to increase the molecular weight of the polymer. For facilitating the dispersion of the polyurethane in water, the acetone has to be removed to produce phase inversion and thus obtain the polyurethane dispersion. Recent studies [2-9] demonstrated the influence of the ionic group content, soft segments, segmented structure, molecular weight of the polyol, nature and type of chain extender, and hard segments/soft segments ratio on PUDs properties. The soft segments chains incorporated in the PUDs are generally derived from a polyol with molecular weight of daltons. The first polyol used in the PUD synthesis was a polyester diol. Later, polyester diol was replaced by polyether diol because polyurethanes (PUs) obtained showed excellent hydrolytic stability, elastomeric properties and good low temperature characteristics. However, PUs obtained with polyether diol easily deteriorated upon exposure to heat and light. In order to try to

3 overcome these drawbacks, polycarbonate diols (PCDs) were considered as an alternative [10] that has been shown to be very effective. Polyurethanes synthesized with PCDs show better hydrolytic stability, high elastomeric properties and better behaviour at low temperature than those obtained with polyether polyols [10], To the best of our knowledge, there are few studies regarding on synthesizing and characterizing waterborne polyurethane dispersions derived from mixtures of polyols of different nature and including polycarbonate diol intended for using as coatings. Therefore, the main objective of this work is the study of the properties and stability of coatings obtained from PUDs synthesized with mixtures of polyols of similar molecular weight but different nature (polyether and polycarbonate diol) with different rate, in order to compare and evaluate the improvement respect those polyurethanes prepared from the pure polyols. EXPERIMENTAL Materials and synthesis procedure The UH-100 polycarbonate diol based on 1,6 hexane diol (UBE Chemical Europe S.A. Castellón, Spain) and PTMG-1000 polyether polyol (Sigma Aldrich ST. Louis MO, USA), both with molecular weight 1000 Da, were used as polyols, as weel as polyols with the same nature but higher molecular weight, 2000 Da (UH-200 and PTMG-2000). The polyols were dried at 80ºC and 5 Torr for 3 hours before use. The diisocyanate used in the PUD synthesis was isophorone diisocyanate (IPDI, 98 wt% purity) (mixture of cis/trans isomers) which is liquid at room temperature. Diethyleneglycol (DEG, 99 wt% purity) and dimethylolpropionic acid (DMPA, 98 wt% purity) were used as short diol and internal emulsifier, respectively, without further purification. Triethylamine (TEA, 99 wt% purity) was used as neutralization agent, and monohydrated hydrazine (HZ, 99 wt% purity) was used as chain extender. Deionised water was used as dispersing phase, and high purity acetone (99.5 wt% purity) was also used in the PUD synthesis. Preparation of the polyurethane coatings In a first step, ten aqueous polyurethane dispersions obtained with mixtures of two different polyols and with two values of molecular weight in each case were prepared by the acetone process, with an NCO/OH ratio of 1.5 and 39 wt% solids content. The DEG content was set to 0.5 wt% and that of DMPA to 5 wt% (both with respect to the prepolymer). The polyol, IPDI, DEG and DMPA were added to a glass jacketed reactor equipped with a mechanical stirrer to obtain the prepolymer. The reaction was carried out

4 at 80ºC under nitrogen atmosphere. When the amount of residual NCO groups reached the desired value (it was determined by n-dibutylamine titration), the prepolymer was dissolved in acetone at 45-55ºC by continuous stirring at 450 rpm until complete dissolution and then triethylamine (TEA) aqueous solution was added to neutralize the carboxylic acid moieties of the DMPA. Afterwards, the chain extender (hidrazine) was added to complete the reaction with the unreacted NCO groups. Then, water was added and the mixture was stirred at 900 rpm. Finally, the residual acetone was removed in a rota vapour unit at 50ºC and 300 mbar for 60 minutes. Then, these waterborne polyurethane dispersions were applied on stainless steel 304 plates by means of a metering rod of 200 µm. The polyurethane coatings were obtained by drying at room temperature for 72 hours. Polyurethane coatings were about µm thick. Experimental techniques Drying time. The drying time was measured as the time necessary to leave a trace by an indenter on a film of freshly applied PUD. The PUDs were applied on a microscope glass slide of 25 x 700 mm g PUD was applied on the glass slide by means of a metering rod of 200 µm. The drying time was measured according to ASTM D standard in BK Drying Time Recorder DT-BK3 (Shean Instruments, Kingston, UK). The values obtained were the average of two replicates. Thickness. The thickness of the polyurethane coating on stainless steel plates 304 was measured according to ISO 2808:2007 standard in Neurtek Positector 6000 instrument (Neurtek, Eibar, Spain). The measurements were carried out at 23 ºC and 50% relative humidity. The values obtained were the average of twenty replicates. Persoz hardness. The Persoz hardness of the polyurethane coatings on stainless steel 304 pieces (dimensions of 60 x 50 x 2 mm) was measured according to ISO 1522:2006 standard in 707 Konig (707 KP) instrument (Shean Instruments, Kingston, England). The values obtained were the average of three replicates. Gloss. The gloss of the polyurethane coatings on stainless steel 304 pieces (dimensions of 60 x 50 x 2 mm) was measured according to ISO 2813:1994 standard in a Micro-TRI-Gloss instrument (BYK Gardner GmbH, Geretsried, Germany). Gloss is measured by directing a constant intensity light beam at a fixed angle (60º) on the coating surface and measuring the amount of reflected light. The values obtained were the average of three replicates. Yellowness index. The yellowness index of the polyurethane coatings on stainless steel 304 pieces (dimensions of 60 x 50 x 2 mm) was measurement according to ASTM

5 D standard in a Minolta CR410 Colorimeter (Ramsey, USA). The values obtained were the average of three replicates. Chemical resistance against ethanol. The chemical resistance of the polyurethane coatings on stainless steel 304 pieces (dimensions of 60 x 50 x 2 mm) against ethanol were obtained according to the modified ISO 4210:1979 standard. The test consists in applying cotton fully embedded in ethanol on the polyurethane coating surface for 10 minutes at room temperature. The deterioration of the coating was analyzed according to a scale of 1 (full removal of the coating) to 5 (intact coating). The values obtained were the average of two replicates. Cross-cutter adhesion. This test was used to measure the adhesion of the polyurethane coatings according to ISO 2409:2007 standard. The cross cutter adhesion was measured in a multi blade cutting device (Sheen Instruments Ltd, Surrey, England) and a Scotch adhesive tape was used. The values obtained were the average of three replicates. Thermal degradation: The heat resistance of the PU coatings on stainless steel 304 pieces were studied according to ISO 3248:1998. Test pieces were heated at 120ºC for 15 days in an oven and the resulting changes in several properties were determined. Weathering: The weathering resistance of the PU coatings on stainless steel 304 pieces were studied according to ISO 11507:2007. Test pieces were treated in QUV/se weathering chamber (Q-LAB, Ohio, USA) by exposing to several cycles consisting of exposition to UV light (2.14 kj) for 102 min at 45ºC and 65 % relative humidity followed by 18 min under UV light (0.37 kj) at 45ºC and 65 % relative humidity under continuous exposition to water spray for 10 days. The resulting changes in several properties were determined. RESULTS AND DISCUSSION During coalescence of PU particles after PUD application on steel surface (thickness controlled to 200 µm) there were observed noticeable differences in drying time depending of the type of polyol, mainly for that PUD based on polyether polyol, whose lenght of drying time was much higher than the others, as it can be seen in Table I. It must be noted that drying time of PUD based on polycarbonate diol, although is quite shorter, is even higher than those corresponding to mixtures of polyols, which could be due to a better coalescence process when soft segments of different nature

6 are present in the dispersion; it occurs for polyurethanes based on polyols with different size, although drying time was slightly longer when molecular weight is higher. About final thickness of coating obtained after drying, it was similar in any case, being around 50 microns; however, stability of thickness after harsh conditions showed different behaviours (Figure 1), producing a marked loss of thickness when most component of the polyol was a polyether, and on the other hand almost no changes in thickness were observed for a polyurethane based in polyol 100% PCD. Slight swelling phenomena were observed for coatings based on mixtures of polyols in which polycarbonate diol was the most component, being this swelling more perceptible for polyols with higher molecular weight. Table I. Values of drying time of PUD s as function of percentage of PCD in the base polyol. PCD (wt%) M w = 1000 Da Drying time (h) M w = 2000 Da 0 >6 > <2 <2 Thickness (µm) PU coating Thermal degradation Weathering UH200 (wt%) M w = 10 3 Da M w = Da Figure 1. Final thickness of PU coatings of polyurethane with different polyether-polycarbonate polyols ratio, after different environmental conditions. About colour and brightness properties of surface of coatings, yellowness index and gloss at 60 degrees values were measured both 72 hours after coating application, and after ageings by thermal degradation and weathering, respectively. Figure 2 shows curves of yellowness index after different ageing processes, as function of PCD weight

7 content in polyurethane formulation. It can be seen there low values of yellowness index both for polyurethane coatings before and after weathering, and they were almost the same for any polyol composition; however, after ageing by thermal degradation it was produced a huge increasing in yellowness of samples, being much higher when a polyether polyol was the main polyol, and the higher was the polycarbonate diol content, the lower was the yellowness index after thermal degradation. That behaviour of coatings against ageing conditions is the same for molecular weight of polyols of 1000 and 2000 Da. In the case of gloss, as it is shown in Figure 3 there were no high differences in gloss of coatings with different polyol composition, both before and after ageing produced by thermal degradation; however, after ageing by weathering, gloss property strongly drops, except for that case of PU with polyol based in polyether and polycarbonate in the same rate and molecular weight 1000 Da, whose gloss reduction was lighter. Yellowness index PU coating Thermal degradation Weathering UH200 (wt%) M w = 10 3 Da M w = Da Figure 2. Yellowness index of PU coatings of polyurethane with different polyether-polycarbonate polyols ratio, after different environmental conditions. 100 PU coating Thermal degradation Weathering 80 Gloss 60ª UH200 (wt%)

8 M w = 10 3 Da M w = Da Figure 3. Gloss value of PU coatings of polyurethane with different polyether-polycarbonate polyols ratio, after different environmental conditions. Hardness of coatings were evaluated using Persoz hardness and Pencil hardness measurements as quantitative and qualitative methods, respectively. Persoz hardness of coatings were very similar if there was polyether polyol in their formulation, with any value of molecular weight, but coatings based on PU from 100% PCD showed remarkable higher value of hardness (Figure 4). After ageing processes, no important changes in hardness were observed on those coatings obtained from polyurethanes containing polycarbonate diol as single polyol or blended with polyether diol,, but polyether based PU coatings showed a strong and the strongest variation in hardness after thermal degradation and weathering conditions. Table II shows values of pencil hardness, which indicate hardness increasing of PU based on polyether polyol, in the same way of Persoz hardness; however, it is observed a remarkable increase in hardness of coatings with 10 wt% and 25 wt% of PTMG after ageing in weathering conditions. Persoz hardness (N oscillations) PU coating Thermal degradation Weathering UH200 (wt%) M w = 10 3 Da M w = Da Figure 4. Persoz hardness values of PU coatings of polyurethane with different polyether-polycarbonate polyols ratio, after different environmental conditions.

9 Table II. Values of pencil hardness of PUD s as function of percentage of PCD in the base polyol and PCD (wt%) molecular weight. Pencil hardness (M w = 10 3 Da) PU coating Thermal degraded Wheather Pencil hardness (M w = Da) PU coating Thermal degraded Wheather 0 6B 2B - 6B 4B B 6B 6B 6B 6B 6B 75 5B 6B F 6B 4B 3B - 4B 90 4B 4B F 6B 4B 5B 100 4B 3B 4B 6B 3B 4B Finally, chemical resistance of coatings measured by wetting with ethanol for 10 minutes was the property which shows more differences between coatings based on polyols with different molecular weight. Polycarbonate based PU was the most resistant to ethanol compared with polyols of 1000 Da of molecular weight, but there weren t perceptible differences when molecular weight was 2000 Da. On the other hand, stability against ethanol was quite good in any case for lower values of Mw, however chemical resistance dropped after ageing conditions for higher values of molecular weight of polyol, except in the polyurethane based on PCD 100 wt%. Chemical resistance (a.u.) PU coating Thermal degradation Weathering Chemical resistance (a.u.) PU coating Thermal degradation Weathering UH100 (wt%) UH200 (wt%) M w = 10 3 Da M w = Da Figure 6. Chemical resistance to ethanol mesurements of PU coatings of polyurethane with different polyether-polycarbonate polyols ratio, after different environmental conditions.

10 CONCLUSIONS The polyol nature determined the properties of the PU coating properties. The polyurethane polycarbonate diol coating showed better coating and surface properties than the polyether based polyurethane coatings. However, mixtures of polyols with small amount of polyether polyol improves slightly some of these properties, such as hardness stability after ageing conditions or drying time of polyurethane dispersions. In general, the thermal stability of the polyurethanes increased as the amount of polyether in the polyols mixture increased. Before degradation the Persoz hardness and Pencil hardness values of the polyurethane coatings increased as the amount of polyether in the polyols mixture decreases due to the higher amount of carbonate groups in the polyurethane. Moreover, after thermal degradation the polyurethane obtained with PTMG-100 showed the highest modification of the Persoz hardness, compared to other coatings. Yellowness index of the polyurethane coatings was similar before degradation and after weathering, but after thermal degradation the yellowness index decreased as the amount of polycarbonate diol in the polyol mixture increased. The presence of the polycarbonate in the structure of the PUs increases the chemical resistance before ageing for polyol with low molecular weight. ACKNOWLEDGMENTS Financial support of UBE CHEMICAL EUROPE is acknowledged. Contact people: Dr. Manuel Colera ( m.colera@ube.es). REFERENCES [1]. D.K. Lee, Z. D. Yang, H.B. Tsai, R.S. Tsai, P.H. Chen, Polymer Engineering and Science 49, (2009). [2] S. Zhang, W. Miao, Y. Zhou, Journal of Applied Polymer Science 92, (2004). [3] A. K. Nanda, D. A. Wicks. Polymer 47, (2006). [4] S. C. Wang, P. C. Chen, React Reactive and Functional Polymers 67, (2007). [5] S. Yamasaki, D. Nishiguchi, K. Kojio, M. Furukawa, Polymer 48, (2007). [6] S. Zhang, H. Jiang, Y. Xu, D. Zhang, Journal of Applied Polymer Science 103, (2007).

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