Studies on synthesis and characterization of aqueous hybrid silicone-acrylic and acrylic-silicone dispersions and coatings. Part II

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Studies on synthesis and characterization of aqueous hybrid silicone-acrylic and acrylic-silicone dispersions and coatings. Part II Janusz Kozakiewicza,*, Joanna Trzaskowskaa, Wojciech Domanowskia, Anna Kieplinb, Izabela Ofat-Kawaleca, Jarosław Przybylskia, Monika Woźniakb, Dariusz Witwickib, Krystyna Sylwestrzaka a b

Industrial Chemistry Research Institute, Department of Polymer Technology and Processing, Rydygiera 8, 01-793, Warsaw, Poland D&R Dispersions and Resins Sp. z o.o., Duninowska 9, 87-800, Włocławek, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Aqueous hybrid silicone-acrylic dispersions Aqueous hybrid acrylic-silicone dispersions Silicone acrylic/styrene copolymer Silicone-containing dispersion particles Coatings

Aqueous silicone-acrylic (SIL/ACR) and acrylic-silicone (ACR/SIL) dispersions with hybrid particle structure were synthesized, respectively, by polymerization of acrylic/styrene monomers in silicone resin dispersion (SIL) and of silicone monomers in two acrylic/styrene copolymer dispersions (ACR A and ACR B). A comprehensive study of the effect of SIL/ACR or ACR/SIL (w/w) ratio, i.e. of the silicone component content in the dispersion solids, on the properties of such hybrid dispersions, coatings and films was conducted. It was found that increasing the SIL/ACR ratio from 0 to 1/9, 1/6 and 1/3 (or decreasing ACR/SIL ratio accordingly) influenced quite significantly the appearance of hybrid dispersion particles and thus also the properties of dispersions (particle size, MFFT, Tg of dispersion solids) as well as the properties of coatings (contact angle, surface free energy, water resistance, water vapor permeability, hardness, impact resistance) and films (% swell in toluene, % of toluene-soluble fraction, tensile strength, elongation at break) made from these dispersions were affected. Based on the results of these investigations some dispersions may be selected for further testing as binders in architectural paints.

1. Introduction Aqueous polymer dispersions containing silicones are of great interest of both researchers and industry due to advantages they can offer if used as coating binders, especially if the dispersion particle has hybrid structure since it may result in synergistic effect leading to enhanced coating properties [1]. In the study reported in [2] we investigated the effect of method of synthesis of such dispersions on their properties as well as on properties of coatings and films obtained from them. In that study, dispersions with hybrid particle structure containing silicones were obtained through polymerization of acrylic/ styrene monomers with different composition in aqueous dispersions of silicone resins with different composition of starting silicone monomers and also through polymerization of silicone monomers of the same composition in acrylic/styrene copolymer dispersions with the same composition of starting acrylic/styrene monomers. It was found that the method of synthesis significantly influenced the morphology of hybrid dispersion particles what, in consequence, affected the properties of dispersions and corresponding coatings and films. While in all

⁎

dispersions, coatings and films obtained in that study the ratio (w/w) of silicone component (SIL) to acrylic/styrene component (ACR) remained at the constant level of 1/3, in this paper we present the results of investigations of the effect of SIL/ACR or ACR/SIL (w/w) ratio, i.e. of the silicone component content in the dispersion solids on the properties of such hybrid dispersions, coatings and films. It is obvious that the features of any polymer system consisting of two or more different polymers would depend on the share of individual components. The same was also proved in papers dealing with coatings obtained from aqueous dispersions with particles consisting of different polymers regardless of the method of dispersion synthesis – see the reviews in [1,3] and [4]. If one of these polymers was silicone the effect of its content on properties of dispersions and coatings could be very significant. In the study reported in [5] hybrid dispersions and corresponding coatings were obtained by emulsion polymerization of a mixture of acrylic monomers (butyl acrylate, acrylic acid, acrylonitrile and acrylamide) and silicone monomers (3-methacryloxypropyltrimethoxysilane, octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane) applying a wide range of different silicone/acrylic

Corresponding author. E-mail address: [email protected] (J. Kozakiewicz).

https://doi.org/10.1016/j.porgcoat.2019.105297 Received 30 May 2019; Received in revised form 25 July 2019; Accepted 26 August 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Janusz Kozakiewicz, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105297

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dispersions ACR A and ACR B, respectively. The compositions of monomers were appropriately designed to get Tg of dispersion solids at a level of ca. + 15 °C (ACR A) and at a level of ca. + 30 °C (ACR B). Unlike in synthesis of standard aqueous acrylic/styrene polymer dispersions ACR A and ACR B dispersions were not neutralized after polymerization process in order to ensure low pH value (ca. 3) that was needed to conduct subsequent polymerization of silicone monomers in the process of synthesis of hybrid acrylic-silicone (ACR/SIL A and ACR/ SIL B) dispersions.

polymer w/w ratios. It was found that water absorption of the coatings decreased and contact angle increased with increase in silicone/acrylic polymer w/w ratio from 0 to 50% while in some other earlier paper [6] it was stated that silicone content in such hybrid dispersions (also obtained by direct emulsion copolymerization of acrylic and silicone monomers) should not be higher than 20% due to “reatardation of the polymerization and limitation of the conversion”. Similar conclusions were made based on the results of another study [7]. Positive effect of silicone content on water resistance and surface hydrophobicity of coatings was also confirmed in [8] and [9] while the authors of two other papers [10,11] reported that not total silicone content, but rather the part of silicone grafted onto acrylic polymer is responsible for such enhancement of properties, including also thermal stability and mechanical properties. As it was proved in [10], if the content of the grafted silicone was changed the water distribution in the drying coats was different what distinctly affected the drying time. The effect of silicone content on the properties of hybrid dispersions, specifically on dispersion stability and coagulum content was investigated in a study described in [12] where 3-methacryloxypropyltrimethoxysilane was polymerized in aqueous acrylic polymer dispersion. It was confirmed that only certain amount of silicone could be grafted on acrylic polymer to produce core-shell particle structures and further increase in silicone part content led to poor dispersion stability and increased coagulum content due to formation of separate crosslinked silicone particles. It was also proved in the other studies [13,14] that increasing the content of silicone part in a hybrid dispersion led to the increase in dispersion particle size. This phenomenon was explained by condensation of silanol groups formed during hydrolysis of alkoxysilane groups in silicone monomers that is more probable at higher silicone content in the system. However, when the hybrid dispersion was made by polymerization of acrylic monomers in aqueous polydimethylsiloxane (PDMS) dispersion [15] the opposite effect was observed, i.e. the particle size decreased with increase in silicone/acrylic polymer ratio and it was explained by “shrinking of PDMS chains in the hydrophobic core”. The examples of studies where the effect of silicone content on properties of silicone-containing aqueous dispersions with hybrid particle structure as well as the corresponding coatings and films show that though some conclusions in that regard concerning specific systems could be made there was no systematic study published were that subject was investigated in greater detail. In this study we attempted to fill that gap.

2.2. Synthesis of silicone resin dispersions (SIL) and hybrid silicone-acrylic (SIL/ACR A and SIL/ACR-B) and acrylic-silicone (ACR/SIL A and ACR/ SIL B) dispersions Synthesis of silicone resin dispersion (SIL) applied as starting dispersion for emulsion polymerization of acrylic monomers to obtain SIL/ ACR hybrid dispersions was conducted following the procedure described in [16] using a mixture of silicone monomers with the following composition : D4 - 84.0%, MTES - 9.5%, VTES - 6.5%. DBSA was acting as both surfactant and D4 ring opening catalyst. The reactions which proceeded during the process of synthesis of SIL dispersion comprised ring opening polymerization of D4 and hydrolysis of ethoxysilane groups of MTES and VTES leading to formation of silanol groups capable for further condensation. The product of these reactions was aqueous dispersion of partly crosslinked silicone resin. The process of synthesis of silicone resin dispersion (SIL) is shown in Fig. 1. After distillation of ethanol under vacuum no free VTES or MTES were detected by GC in the resulting SIL dispersion, though small amounts of D4 (ca. 0.8%) and ethanol (ca. 0.2%) were still present. The conversion of VTES and MTES in that process was practically 100% while the conversion of D4 was rather low due to cyclic to linear oligosiloxanes thermodynamic equilibrium and varied from ca. 30% for ACR/SIL = 3/1 to 80-90% for ACR/SIL = 9/1. Synthesis of hybrid silicone-acrylic (SIL/ACR A and SIL/ACR B) by polymerization of acrylic/styrene monomers in SIL dispersion and of hybrid acrylic-silicone (ACR/SIL A and ACR/SIL B) dispersions by polymerization of silicone monomers in acrylic/styrene polymer dispersions ACR A and ACR B was conducted as described in [2]. The conversion of monomers in this process was close to 100% since the concentration of monomers in the final dispersions was very low (below 0.01%). The ratio (w/w) of silicone polymer and acrylic polymer components (SIL/ACR) was set at 1/9, 1/6 and 1/3 what corresponded to 10%, 143% and 25% of silicone component in hybrid dispersions solids. It was essential that the composition and concentration of surfactants remained the same in all SIL/ACR and ACR/SIL dispersions despite the SIL/ACR or ACR/SIL ratio, so their properties (and properties of coatings obtained from them) could be compared.

2. Materials and methods 2.1. Starting materials Octamethylcyclotetrasiloxane (D4) was obtained from Momentive. Other silicone monomers, i.e. vinyltriethoxysilane (VTES) and methyltriethoxysilane (MTES) were obtained from Evonic. Surfactants - dodecylbenzenesulphonic acid (DBSA) and Rokanol T18 (non-ionic) were obtained from PCC Exol and Emulgator E30 (anionic) was obtained from Leuna Tenside GmbH. Other standard ingredients used in synthesis of dispersions (sodium acetate, sodium hydrocarbonate, potassium persulphate and aqueous ammonia solution) were obtained from Standard Lublin. Biocide used to protect dispersions from infestation was Acticide MBS obtained from THOR. Starting acrylic/styrene copolymer dispersions (ACR A and ACR B) characterized by different glass transition temperatures (Tgs) were supplied by Dispersions&Resins. Monomers applied in synthesis of ACR A and ACR B dispersions were butyl acrylate (BA) obtained from ECEM, Arkema, styrene (ST) obtained from KH Chemicals, Helm, acrylic acid (AA) obtained from Prochema, BASF and methacrylamide (MA) obtained from ECEM, Arkema. Acrylic and styrene monomers were used as received as mixtures designated as A and B with compositions corresponding to compositions of monomers applied to synthesize

2.3. Characterization of dispersions All dispersions were characterized by: - Solids content, wt% – percentage of sample mass remaining after drying for 1 h at 80◦C followed by 4 h at 125 °C. - pH – using standard indicator paper and Elmetron Pehameter CP411 with ERH-11S electrode - Viscosity – using Bohlin Instruments CVO100 rheometer, coneplate 60 mm diameter and 1 deg measuring device, shear rate 600 s−1 - Coagulum content - after filtration of dispersion on 120 mesh net the solids remaining on the net were dried and weighed. Coagulum content (wt%) was calculated from equation mc/mdx100% where mc was mass of dry coagulum remaining on the net and md was mass of dispersion. - Acrylic and styrene monomers, ethanol and D4 content – by GC (HP 5890 series II apparatus, FID detector) - Mechanical stability – lack or occurrence of separation during rotation in Hettich Universal 32R centrifuge at 4000 r.p.m. for 90 min 2

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Fig. 1. Process of synthesis of silicone resin dispersion (SIL) by emulsion polymerization of silicone monomers.

6 days under water were also examined. Water vapor permeability – according to ASTM F1249. TotalPerm 063 (Extra Solution) apparatus was used. Tests were conducted at 23 °C for 0.35 mm thick film. Fomblin perfluorinated grease from Solvay Solexis was applied to seal the test vessels. Moreover, coatings applied on PET film were examined for surface structure by X-ray Photoelectron Spectroscopy (XPS) using ULVAC/ PHYSICAL ELECTRONICS PHI5000 VersaProbe apparatus and by Atomic Force Microscopy (AMF) using RHK-VHV350 apparatus working in a “tapping mode” with PPP-RT-NCHR nanosensors.

was considered as good stability - Average particle size (nm), particle size distribution and zeta potential (mV) – light scattering method using Malvern Zeta Sizer apparatus - Particle appearance – STEM (Scanning Transmission Electron Microscope) Hitachi 2700, dispersions were diluted 1000× with water for taking pictures. High Angle Annular Dark Field (HAADF) mode also called “Z-contrast” was applied for processing the images reproduced in this paper. - Minimum Film Forming Temperature (MFFT) – according to ISO 2115 using Coesfeld apparatus equipped with temperature gradient plate. Temperature range : −3 °C to +50 °C. - Glass transition temperature (Tg) of dispersion solids – by DSC (TA Instruments Q2000 apparatus), heat–cool–heat regime, 20◦C/min

2.5. Characterization of films - % swell in toluene and % of toluene-soluble fraction - ca. 0.12 g samples of 0.35 mm thick film were weighed and placed in 40 ml toluene contained in closed glass cups and left for 20 h at 23 °C. Then the samples were taken out, delicately dried with filter paper and weighed. % swell was calculated from the equation : % swell = m1-m0/ m1x100%, where m0 = mass of the sample before test and m1 = mass of the sample after test. For determination of percentage of toluenesoluble fraction the films were weighted before immersion in toluene and then after complete drying at room temperature (till no weight change was noticed). % of toluene-soluble fraction was calculated from the equation : % of toluene soluble fraction = m1-m0/m1x100%, where m0 = mass of the sample before test and m1 = mass of the sample after test. - mechanical properties (tensile strength and elongation at break) – determined for 0.2 mm thick films using Instron 3345 testing machine according to EN-ISO 527-1 at a speed of 50 mm/min.

2.4. Characterization of coatings Coatings were produced from dispersions by applying them on glass (for testing contact angle, hardness, adhesion or water resistance), aluminium plates (for testing elasticity) or on steel plates (for testing impact resistance and cupping) using 120μ applicator. Drying was carried out for 30 min at +50 °C and then the coatings were seasoned in a climatic chamber at +23 °C and 55% R.H. for 72 h. Since no continuous coating could be obtained in this procedure for SIL/ACR B 1/3 the relevant dispersion was dried at +8 °C for 2 h and then seasoned as above. The resulting coatings were characterized by: - Contact angle (water) – according to EN 828:2000, using KRUSS DSA 100E apparatus. - Pendulum hardness (Koenig) – according to EN ISO 1522 - Adhesion to glass – according to EN ISO 2409 - Elasticity – according to EN ISO 1519 - Impact resistance (direct and reverse) – according to EN ISO 62721, using Erichsen Variable Impact Tester Model 304 - Cupping – according to EN ISO 1520, using Erichsen Cupping Tester - Water resistance – glass Petri dishes of 50 mm diameter were filled with distilled water and placed upside-down on the coating, so the coating was covered with 7 mm thick layer of water. Assembles prepared this way were left for 72 h and appearance of coatings was examined for the bubbles size (S0 - no bubbles, S2-S5 - low to high size of bubbles) and density (0 - no bubbles, 2–5 low to high density of bubbles) according to EN ISO 4628-2. Changes of coating appearance after

3. Results and discussion 3.1. Effect of silicone content on properties of dispersions All SIL/ACR and ACR/SIL dispersions synthesized in our study were mechanically stable. Free acrylic/styrene monomers content in SIL/ ACR dispersions was below 0.01% while ACR/SIL dispersions contained no free acrylic/styrene monomers. Ethanol and D4 content in both SIL/ ACR and ACR/SIL dispersions was at the level of 0.1% and 0.4% respectively. The other properties of dispersions obtained using different SIL/ACR or ACR/SIL ratio are shown in Table 1. Designation of dispersion in the table reflects the method of its synthesis and the 3

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Table 1 Properties of SIL/ACR and ACR/SIL hybrid dispersions obtained using different SIL/ACR or ACR/SIL ratio. Designation of dispersions

pH

Solids content %

Coagulum content %

Viscosity at 23 °C mPas

Average particle size nm

Particle size distribution nm

Polydispersity

Zeta potential mV

Minimum FilmForming Temperature (MFFT) °C

Tg (dispersion solids) °C

ACR A SIL SIL/ACR SIL/ACR SIL/ACR ACR/SIL ACR/SIL ACR/SIL ACR B SIL/ACR SIL/ACR SIL/ACR ACR/SIL ACR/SIL ACR/SIL

6.2 6.3 5.8 6.3 6.3 6.3 6.3 6.3 6.2 6.3 6.5 6.3 6.3 6.3 6.3

51.1 19.3 42.2 50.5 50.6 43.2 50.3 47.7 51.5 42.3 50.81 50.76 42.0 50.4 48.29

< 0.1 0.00 0.06 0.05 0.03 0.38 0.13 0.02 < 0.1 0.11 0.19 0.1 0.21 0.054 5.23

94 2 20 72 137 140 148 162 98 24 127 143 61 259 346

105.7 121.7 143.8 166.2 144.7 111.7 120.4 105.4 112.2 140.3 175.4 151.7 114.4 123.1 98.7

74–119 104–157 69–160 62-161 112-187 71–131 101-134 95-110 108–135 69–190 133-203 73-186 98–133 106-144 88-118

0.112 0.121 0.074 0.066 0.057 0.096 0.059 0.058 0.066 0.072 0.052 0.042 0.071 0.052 0.058

−51.0 −50.9 −57.7 −51,2 −58.3 −55.2 −53.2 −58.5 −56.0 −59.5 −56.9 −57.6 −55.0 −58.8 −58.5

11.4 N.A. 7.3 10.6 8.1 −0.5 −0.5 −0.2 32.7 26.2 25.3 29.4 16.0 16.0 19.9

+17.17 −117.33 −126.66 +17.66 −130.36 +17.24 −132.27 +16.47 −130.54 +14.58 +14.97 +17.15 +32.36 −129.09 +30.87 −133,09;+32,25 −135,71 +35,75 −126.33 +27.98 +28.72 +30.28

A A A A A A

1/3 1/6 1/9 3/1 6/1 9/1

B B B B B B

1/3 1/6 1/9 3/1 6/1 9/1

than that of acrylic/styrene dispersion what suggests that indeed hybrid particles were obtained. This finding was confirmed by TEM investigations – see 3.1.2. It is also clear that the average particle size of SIL/ACR dispersions was higher than that of ACR/SIL dispersions for all SIL/ACR or SIL/ACR ratios what most probably resulted from different appearance of dispersion particles when dispersions were obtained by different methods see discussion on appearance of dispersion particles in 3.1.2 and in [2]. Another interesting finding from Fig. 2 is that the average particle size was generally higher for higher SIL contents for both SIL/ACR and ACR/SIL dispersions and for both ACR A - based and ACR - B based dispersions. That finding is consistent with the results reported by the other authors [5] who, however, obtained hybrid SIL/ACR dispersions by different method, namely direct copolymerization of acrylic and silicone monomers. Based on that finding it can be concluded that increase in the average particle size with increase in silicone content in hybrid SIL/ACR dispersions is a general rule. Nevertheless, in dispersions investigated in our study a maximum was observed for ca. 15% of silicone in dispersion solids what could suggest that the change in appearance of dispersion particles occurred at that specific SIL/ACR (1/6) or ACR/SIL (6/1) ratio. This assumption was confirmed by the results of particle appearance studies described in 3.1.2.

composition of its solids. E.g. designation SIL/ACR A 1/9 corresponds to hybrid dispersion synthesized by polymerization of acrylic/styrene monomers with composition ACR A in silicone resin dispersion applying the ratio (w/w) of silicone monomers to silicone dispersion solids equal to 1/9 (w/w) while designation ACR/SIL B 3/1 corresponds to hybrid dispersion synthesized by polymerization of silicone monomers in acrylic/styrene polymer dispersion ACR B applying the ratio (w/w) of acrylic/styrene dispersion solids to silicone monomers equal to 3/1. As it can be noted from Table 1 all hybrid dispersions had solids content equal to ca. 50% and pH equal to ca. 6. The coagulum content was found to be quite low, especially for SIL/ACR dispersions, as compared to the relevant values reported for other hybrid silicone/ acrylic dispersions [17–21] and viscosity was rather low (20–350 mPa s). 3.1.1. Particle size and particle size distribution As it can be noted from Table 1 the particle size of hybrid dispersions was generally close to 100–150 nm and particle size distribution was quite narrow. The effect of SIL/ACR or ACR/SIL ratio (i.e. silicone content in the dispersion solids) on average particle size of SIL/ACR and ACR/SIL hybrid dispersions is shown in Fig. 2. The results of average particle size measurements of SIL/ACR and ACR/SIL hybrid dispersions as compared to the corresponding acrylic/ styrene copolymer dispersions shown in Fig. 2. demonstrate that regardless SIL/ACR or ACR/SIL ratio and method of hybrid dispersion synthesis the particle size of hybrid dispersions was generally higher

3.1.2. Particle appearance The particle appearance observed for different contents of silicone in dispersion solids for both SIL/ACR and ACR/SIL dispersions is shown in Fig. 3. It can be noted from Fig. 3 that the appearance of dispersion particles depended very much not only on SIL/ACR or ACR/SIL ratio but also on the method of dispersion synthesis and on type of acrylic/ styrene copolymer (A or B). The TEM images are not perfect because of coalescence of the particles, but nevertheless certain conclusions can be made. In the case of SIL/ACR A dispersions smaller silicone particles were contained in bigger acrylic/styrene copolymer particles while in the case of SIL/ACR B dispersions the single dispersion particle consisted of a number of mixed tiny silicone and acrylic/styrene copolymer particles. In both cases the increase in silicone content did not lead to distinct changes in particle appearance. In the case of ACR/SIL dispersions the particle appearance changed significantly with increase in silicone content and for ACR/SIL A 3/1 dispersion a very interesting “embedded sphere” structure was observed where silicone part formed a sphere trapped in the mass of acrylic/styrene copolymer. However, such structure was not present in ACR/SIL B 3/1 dispersion.

Fig. 2. The effect of SIL/ACR or ACR/SIL ratio (i.e. silicone content in the dispersion solids) on average particle size of SIL/ACR and ACR/SIL hybrid dispersions. 4

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Fig. 3. Particle appearance observed by TEM for SIL/ACR and ACR/SIL dispersions with different silicone content (i.e. different SIL/ACR or ACR/SIL ratio). Since the images were taken in HAADF mode the silicone component of the particles is represented by whiter shade.

5

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Fig. 5. The results of DSC studies conducted for SIL/ACR dispersion solids obtained from dispersions synthesized using different SIL/ACR ratios. The results obtained for ACR and SIL dispersion solids are also presented for comparison.

Fig. 4. The effect of SIL/ACR or ACR/SIL ratio (i.e. silicone content in the coating) on MMFT of SIL/ACR and ACR/SIL hybrid dispersions.

3.1.3. Minimum film-forming temperature (MFFT) The results of MFFT determinations of SIL/ACR and ACR/SIL dispersions are shown in Fig. 4. It is clear from Fig. 4 that modification with silicone resin resulted in a beneficial decrease in MFFT of hybrid SIL/ACR and ACR/SIL dispersions as compared to the corresponding acrylic/styrene copolymer dispersions. This finding is rather obvious assuming much lower Tg of silicone resin than that of acrylic/styrene copolymer. It can also be noted that the observed decrease was much more distinct for ACR/SIL than for SIL/ACR dispersions. The reason for that difference can be the different appearance of hybrid dispersion particles – see 3.1.2. Specifically, in SIL/ACR dispersions silicone component was trapped inside acrylic/styrene copolymer component, so its influence on MFFT could not be very significant. Another observation form Fig. 4 is that the MFFT values decrease only up to a certain content of silicone in dispersion and further increase in SIL/ACR ratio (or decrease in ACR/SIL ratio) does not lead to lower MFFT.

Fig. 6. The results of DSC studies conducted for ACR/SIL dispersion solids obtained from dispersions synthesized using different ACR/SIL ratios. The results obtained for ACR and SIL dispersion solids are also presented for comparison.

3.1.4. Glass transition temperature (Tg) of dispersion solids Based on the results of Tg determinations (see Table 1) it can be noted that Tg of silicone part of SIL/ACR hybrid dispersions solids regardless SIL/ACR ratio was significantly (almost 20 °C) lower than Tg of starting SIL dispersion solids. This phenomenon can only be explained by occlusion of small amounts of cyclic oligosiloxanes in hybrid dispersion particles. Such occlusion could occur because in these particles silicone resin was, in fact, trapped in the particle (coated with a layer of acrylic/styrene copolymer) what was confirmed by TEM investigations – see 3.1.2. Then, the occluded cyclic oligosiloxanes as well as unreacted silanes with silanol groups that were formed by hydrolysis of alkoxysilanes could act as plasticizers for silicone resin. The observed decrease in Tg of silicone part of hybrid dispersion solids for SIL/ACR dispersions as compared to Tg of starting silicone dispersion solids is further demonstrated in Fig. 5 where the results of DSC studies conducted for these dispersions are presented. The explanation presented above can be assessed by the fact that for ACR/SIL hybrid dispersions such phenomenon was observed (see Fig. 6) only for dispersion with high (ACR/SIL = 3/1) silicone content where the particle structure was much more homogeneous than in the case of SIL/ACR = 1/3 dispersions. In the particle structure of ACR/SIL = 3/1 silicone resin was “hidden” inside acrylic/styrene copolymer particle. The same observation was made in our earlier studies of hybrid SIL/ ACR and ACR/SIL dispersions and the corresponding coatings [2]. It is interesting that while two distinct Tgs were observed for silicone part and acrylic/styrene copolymer part of ACR/SIL dispersion solids when they contained more silicone (ACR/SIL = 3/1) only a single Tg was observed for hybrid ACR/SIL dispersions solids with lower silicone content. Explanation of this phenomenon can be also based on

differences in dispersion particle appearance since for lower silicone contents the uniform or “fruit-cake” type dispersion particles structures were found. 3.2. Effect of silicone content on properties of coatings and films The properties of coatings and films made from SIL/ACR and ACR/ SIL hybrid dispersions obtained using different SIL/ACR or ACR/SIL ratio are shown in Table 2. Designation of dispersions is the same as used in Table 1. 3.2.1. Surface properties It is well known [1,2,22] that modification of aqueous acrylic/ styrene copolymer dispersions with silicones results in significant changes in surface properties of the corresponding coatings, specifically in increase in contact angle and decrease in surface free energy because silicone component migrates to the coating surface. In our investigations described in this paper we attempted to assess how silicone content in the coating affected this phenomenon. When the contact angle (CA) and surface free energy (SFE) determined for coatings made from SIL/ACR and ACR/SIL dispersions and corresponding acrylic/styrene copolymer dispersions were plotted against the silicone content in the coating it became clear that the higher was silicone content the higher was CA and the lower was SFE – see Fig. 7 and Fig. 8, respectively. As it can be seen in Fig. 7 and Fig. 8 this effect was generally quite significant (even with only 10% of silicone in the coating) and was generally more distinct for coatings made from ACR/SIL dispersions than from SIL/ACR dispersions, but for the latter higher CA values (or 6

Water vapor permeability g/ m2/24 h

28.1 56.5

40.1

31.4

45.0

37.8

35.1

15.6

64.5

28.4

25.3

34.6

31.3

27.0

Designation of dispersions

ACR A SIL/ACR A 1/3

SIL/ACR A 1/6

SIL/ACR A 1/9

ACR/SIL A 3/1

ACR/SIL A 6/1

ACR/SIL A 9/1

ACR B

SIL/ACR B 1/3

SIL/ACR B 1/6

SIL/ACR B 1/9

ACR/SIL B 3/1

ACR/SIL B 6/1

ACR/SIL B 9/1

> 5(S5) 5(S2)Medium whitening 4(S3) Large whitening 4(S2) Large whitening 0(S0)Light whitening 2(S5)Medium whitening 0(S0)Medium whitening 5(S2)Medium whitening 5(S2) Light whitening 5(S4) Medium whitening 4(S3) Medium whitening 0(S0) Medium whitening 0(S0)Medium whitening 3(S2)Large whitening

Water resistance after 72 h %

7

1037

0,023

0,185

0,127

0,050

0,154

0,156

0,085

0,458

Sample deteriorated

1440

905

723

643

561

1547

1228

1050

0,02

0,023

707

703

1156 591

Swell in toluene %

0,02

0,04

0.08 0,04

Hardness (Koenig)

Sample deteriorated

39.2

39.6

17.3

17.0

25.9

31.2

24.3

43.4

58.6

14.9

17.7

21.5 15.6

Toluene-soluble fraction%

45,9

58,0

91,5

72,3

68,6

81,1

34,5

45,8

93,3

95,2

73,8

80,6

30,1 82,7

Contact angle (H2O) °

2

1

5,9

3,9

0

0

2

2.0

2.0

15.7

5.0

11,8

2 9,8

Impact Resistance (direct) J

0

1

19,6

0

2

0

0

19,6

19,6

19,6

19,6

19,6

19,6 19,6

Impact Resistance (reverse) J

Table 2 Properties of coatings and films made from SIL/ACR and ACR/SIL hybrid dispersions obtained using different SIL/ACR or ACR/SIL ratio.

9,6

10,4

10,9

10,5

10,8

10,9

10,7

10,4

10,9

11,0

10,8

11,1

10,7 11,6

Cupping mm

passed

passed

passed

passed

passed

passed

failed

passed

passed

passed

passed

passed

passed passed

Elasticity (rod diameter2 mm)

4

5

5

5

4

5

5

2

5

2

5

2

5 3

Adhesion to glass

7,83

4,74

3,05

1011

8,21

4,33

12,00

1,05

1,02

0,75

2,22

1,11

4,23 2,13

Tensile strength MPa

333,75

808,52

1015,22

244,41

62,08

11,01

340,49

1039,82

1410,79

1851,15

966,07

765,35

999,95 773,28

Elongation at break %

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from SIL/ACR and ACR/SIL dispersions is shown in Figs. 11 and 12. It is clear from Fig. 11 that water resistance was significantly enhanced in coatings produced from hybrid dispersions containing silicone as compared to unmodified acrylic/styrene copolymer (ACR) dispersions even if silicone content in the coating was very low (10%). This effect was much more distinct for coatings obtained from ACR/SIL dispersions than for coatings obtained from SIL/ACR dispersions what can be explained by the fact that in the latter coatings silicone that could stop water penetration was much more “hidden” inside the acrylic/styrene copolymer particles – see 3.1.2. It can also be noted that water resistance was generally much better for coatings obtained from SIL/ACR A and ACR/SIL A dispersions than for coatings obtained from SIL/ACR B and ACR/SIL B dispersions. 3.2.3. Percentage of swell in toluene and percentage of toluene-soluble fraction The percentage of swell of the polymer film in a solvent may be used for estimation of the level of crosslinking density of the polymer. In our studies toluene was selected since it is good solvent for both polymers that constituted the hybrid dispersion particle, i.e. acrylic/styrene copolymer and silicone resin. The results shown in Fig. 13 indicate that the hybrid polymer material that was formed in synthesis of SIL/ACR dispersions by emulsion polymerization of acrylic/styrene monomers in starting SIL dispersion was apparently more resistant to solvent than the acrylic/styrene copolymers of ACR dispersions even for quite low silicone content. This finding may suggest that some grafting of acrylic/ styrene monomers on starting SIL dispersion particles has occurred. In the case of synthesis of ACR/SIL dispersions by emulsion polymerization of silicone monomers in starting ACR dispersions this effect was observed only when silicone content was quite high what indicated that it resulted rather from the fact that silicone resin itself was much more resistant to solvent than silicone-acrylic hybrid. These assumptions were confirmed by the results of determination of percentage of toluene-soluble fraction -see Table 2. For SIL/ACR dispersions values of that parameter were lower than for ACR dispersions what conformed that some grafting of acrylic/styrene monomers on silicone resin occurred during polymerization process. For ACR/SIL dispersions values of that parameter were higher than for ACR dispersions what suggested that no grafting was present.

Fig. 7. The effect of silicone content on CA (water) of coatings made from hybrid SIL/ACR and ACR/SIL dispersions.

Fig. 8. The effect of silicone content on SFE of coatings made from hybrid SIL/ ACR and ACR/SIL dispersions.

3.2.4. Water vapor permeability Expected improvement of water vapor permeability of the coating is one of the key reasons why the modification of acrylic/styrene copolymer dispersions used as binders architectural paints with silicone is recommended. In our study described in [2] we proved that for 25% silicone content in the particles of hybrid ACR/SIL or SIL/ACR dispersions water vapor permeability of the coatings increased significantly. It was therefore interesting to see whether lower silicone contents (10% and ca. 15%) would also influence this parameter to large extent. The results presented in Fig. 14 show that for both coatings made SIL/ACR and ACR/SIL dispersions water vapor permeability increased distinctly with increase in silicone content reaching the maximum value for 25% of silicone in the coating. Specifically, for the coating obtained from SIL/ACR B dispersion the increase was quite spectacular. In this case water vapor permeability increased from ca. 15 g/m2/24 h measured for the coating obtained from unmodified acrylic/styrene copolymer dispersion up to ca. 25 g/m2/24 h and ca. 65 g/m2/24 h for coatings containing 10% and 25% silicone, respectively. These results suggest that even relatively low silicone content in coatings produced from silicone/acrylic dispersions with hybrid particle structure will be enough to achieve distinct increase in water vapor permeability. These results are, however, quite different from findings of other study [23] where hybrid dispersions were synthesized by polymerization of acrylic monomers in PDMS dispersion modified with methacryloyloxypropyl trimethoxysilane (MATS). The authors of that paper found that increasing the amount of MATS resulted in decrease of water vapor

Fig. 9. Differences in Si content on the surface of coatings made from hybrid SIL/ACR dispersions obtained using different SIL/ACR ratio.

lower SFE values) are observed at lower SIL/ACR ratios. These results suggested that the surfaces of coatings which contained more silicone were more enriched with silicone what was proved by XPS studies – see Fig. 9. The apparent differences in the structure of coating surface depending on SIL/ACR ratio can be seen on AFM images presented in Fig. 10. While the surface of coating made from unmodified ACR B dispersion (c) seem to be rather smooth the round-shaped structures with ca. 100 nm size are emerging from the surface of coatings made from SIL/ACR B dispersions modified with silicone (a) and (b) and are much more abundant for the coating that contains more silicone (a). 3.2.2. Water resistance The effect of silicone content on water resistance of coatings made 8

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Fig. 10. AFM images of the surface of coatings made from hybrid SIL/ACR B dispersions obtained using different SIL/ACR ratio (a) 1/3 and (b) 1/9 as compared to the surface of coating made from unmodified ACR B dispersion (c).

Fig. 11. The effect of silicone content on water resistance of coatings made from hybrid SIL/ACR A and ACR/SIL A dispersions.

permeability and explained that phenomenon by formation of more crosslinked polymer structures with increase in MATS content. In our study the probability of formation of crosslinked structures was quite low and therefore we observed better water vapor permeability for coatings containing more silicone.

Fig. 12. The effect of silicone content on water resistance of coatings made from hybrid SIL/ACR B and ACR/SIL B dispersions.

presented in Fig. 15. It is obvious from the results shown in Fig. 15 that coating hardness was much affected by modification of coatings with silicone. For coatings made from SIL/ACR A and ACR/SIL A hybrid dispersions this effect was not so clear because hardness of coating made from unmodified acrylic/styrene copolymer dispersion (ACR A) was very low.

3.2.5. Mechanical properties 3.2.5.1. Hardness. The effect of silicone content on hardness (Koenig) of the coatings made from SIL/ACR and ACR/SIL dispersions is 9

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Fig. 13. The effect of silicone content on % swell in toluene determined for films made from hybrid SIL/ACR and ACR/SIL dispersions. Swell in toluene determined for SIL dispersion solids was 202%. Since ACR/SIL B sample containing 10% silicone was completely deteriorated after soaking in toluene (see Table 2) that experimental point could not be represented on the graph.

Fig. 16. The effect of silicone content on impact resistance (direct) of films made from hybrid SIL/ACR and ACR/SIL dispersions.

adhesion) and generally improved for coatings made from ACR/SIL or SIL/ACR dispersions, but no clear distinct effect of silicone content in the coating on adhesion to glass could be observed. 3.2.5.3. Impact resistance and cupping. As it can be noted from Table 2 both direct and reverse impact resistance of coatings made from SIL/ ACR and ACR/SIL dispersions was determined. Based on these results it can be anticipated that modification with silicone did not affect the impact resistance (reverse) of coating made from ACR A dispersion simply because already for that unmodified coating the maximum value of impact resistance (reverse) was measured. However, in the case of coating made from ACR B dispersion that was quite brittle, i.e. zero impact resistance (reverse) was measured, modification with silicone could help much, but only when silicone content in the coating was quite high (25%). As it can be noted from the results presented in Fig. 16 the effect of silicone content on impact resistance (direct) was quite distinct. For coatings made from SIL/ACR A and ACR/SIL A dispersions impact resistance (direct) started to increase really significantly above 10–15% of silicone content in the coating. However, for coatings based on silicone-modified ACR B this increase was observed only for coatings made from ACR/SIL B dispersions while for coatings made from SIL/ ACR B dispersions and more silicone resulted in lower impact resistance (direct). This phenomenon could result from distinct phase segregation that might occur in these coatings when silicone content was high. The phase segregation (i.e. silicone particles were mostly separated from acrylic/styrene copolymer particles) occurred in SIL/ACR B = 1/3 and SIL/ACR B = 1/6 samples and did not occur in SIL/ACR A = 1/3 and SIL/ACR A = 1/6 samples. The reason for that phenomenon most probably was that the composition of monomers in ACR part was different. Actually, for ACR B there was more styrene in the monomers mixture what resulted in higher Tg (ca. 30 °C as compared to ca. 15 °C for ACR A). As it can also be noted from Table 2 the cupping test did not reveal any differences between coatings made from unmodified acrylic/ styrene copolymer (ACR) dispersions and the coatings made from SIL/ ACR or ACR/SIL dispersions. The reason may be the relatively high values of cupping resistance obtained in this test for unmodified coatings, so modification with silicone did not result in increase of these values.

Fig. 14. The effect of silicone content on water vapor permeability of coatings made from hybrid SIL/ACR and ACR/SIL dispersions.

Fig. 15. The effect of silicone content on hardness of films made from hybrid SIL/ACR and ACR/SIL dispersions.

In the same time it was very significant for coatings made from SIL/ACR B and ACR/SIL B hybrid dispersions – a drop from 0.5 to ca. 0.2 was observed even with only 10% silicone in the coating. Such distinct effect could have been expected taking into account that silicone resin is quite soft and even small amount of such soft modifier in the coating may soften the whole body of coating significantly. However, since SIL/ ACR and ACR/SIL hybrid dispersions are foreseen to be applied as binders for architectural paints the lower hardness of the binder may not necessarily result in worse performance of the pigmented paint.

3.2.5.4. Tensile strength and elongation at break. The effect of silicone content on tensile strength and elongation at break of the films made from SIL/ACR and ACR/SIL dispersions is presented in Figs. 17 and 18 respectively. It can be noted from Fig. 17 that tensile strength of the films obtained from SIL/ACR and ACR/SIL dispersions decreased very much with increase in silicone content. This phenomenon can be explained by the presence of rigid and tough part (acrylic/styrene copolymer) and

3.2.5.2. Adhesion to glass. As it can be seen in Table 1 adhesion to glass determined for coatings made from both ACR A and ACR B dispersions was rather poor (according to ENISO 2409 value of 5 means the worst 10

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Hybrid dispersions synthesized in our study were mechanically stable and have solids content around 50%, small particle size (100–180 nm), narrow particle size distribution and very low coagulate content. Differences in dispersion particle appearance for dispersions synthesized with different silicone content or using different method were identified and it was anticipated that properties of coatings and films could be influenced by these differences. Detailed investigations of coating surface properties by XPS and AFM revealed that silicone migrated to coating surface what was reflected in increased surface hydrophobicity. Based on the results of the study described in this paper certain hybrid dispersions can be selected for further examination as binders in architectural paints. Since the performance of paints was found to be very good even when hybrid dispersions containing only 10% of silicone part were used as binders we believe that there would be no techno-economic obstacles in their commercialization.

Fig. 17. The effect of silicone content on tensile strength of films made from hybrid SIL/ACR and ACR/SIL dispersions.

Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. Acknowledgments This research was funded by Polish State R&D Centre (NCBiR), grant number PBS/B1/8/2015. The Authors wish to thank dr Piotr Bazarnik from Warsaw University for conducting TEM studies, dr Janusz Sobczak and dr Konstantin Nikiforow from Polish Academy of Sciences for conducting XPS and AFM studies, respectively. The assistance of Colleagues from Industrial Chemistry Research Institute in testing mechanical properties of films, water vapor permeability and dispersion particle size distribution is also acknowledged.

Fig. 18. The effect of silicone content on elongation at break of films made from hybrid SIL/ACR and ACR/SIL dispersions.

much more soft and elastic part (silicone resin) in the hybrid structure of the dispersion particles which coalesced to form the films in the process of drying of the dispersions. It could be then expected that elongation at break of the films would increase with increase in silicone content. However, it is interesting that such assumption appeared to be true only with regard to the films produced from ACR/SIL dispersions while for the films produced from SIL/ACR dispersions elongation at break (see Fig. 18) remained at approximately the same level up to 10% of silicone and after started to decrease slightly with increase in silicone content. This phenomenon can only be explained by specific structure of the film made from SIL/ACR dispersions where silicone part was totally covered by acrylic/styrene copolymer part because of such hybrid particles structure - see 3.1.2. If the silicone content was low, silicone did not influence the elongation at break at all while for higher silicone contents the presence of mechanically weak silicone part could lead to faster break during testing. For films made from ACR/SIL dispersions elongation at break increased with increase in silicone content most probably because silicone and acrylic/styrene copolymer parts were mixed up well in hybrid dispersion particles.

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4. Conclusions The results of investigations of the effect of silicone content on properties of aqueous hybrid SIL/ACR and ACR/SIL dispersions synthesized, respectively, by emulsion polymerization of acrylic/styrene monomers in silicone resin dispersion and by emulsion polymerization of silicone monomers in acrylic/styrene copolymer dispersion as well as on properties of coatings and films made from such dispersions proved that modification with silicone positively influenced several important features of these dispersions, coatings and films. The substantial conclusion is that the properties which are most important from the point of view of application of these hybrid dispersions as binders for architectural paints, namely MFFT, surface hydrophobicity, water vapor permeability and water resistance of coatings could be significantly improved even for relatively low silicone content (10%) while mechanical properties of coatings and films were not much deteriorated. 11

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