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Study of multivesiculated polyester particles synthesis by double emulsion process
European Polymer Journal 49 (2013) 664–674
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Study of multivesiculated polyester particles synthesis by double emulsion process Ângela Dias a,b,c, Joana Fidalgo a, João Machado b, Jorge Moniz c, Adélio M. Mendes a, Fernão D. Magalhães a,⇑ a
LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal CIN – Corporação Industrial do Norte, S.A., Av. Dom Mendo, 831, Apartado 1008, 4471-909 Maia, Portugal c Resiquímica – Resinas Químicas, S.A., Rua Francisco Lyon de Castro, 28, 2725-397 Mem-Martins, Portugal b
a r t i c l e
i n f o
Article history: Received 1 October 2012 Received in revised form 28 November 2012 Accepted 3 January 2013 Available online 11 January 2013 Keywords: Waterborne dispersions Particle size distribution Hollow particles Opacity
a b s t r a c t This work studies the synthesis of multivesiculated particles (MVPs) from a w/o/w double emulsion comprising water and unsaturated polyester dissolved in styrene. These particles can be used as opacifying agents in paint formulations, due to light refraction within the air-containing voids, after water evaporation. The effect of some key variables was analyzed: time of dispersion and concentrations of added amine, polyester and protective colloids. The final products were characterized in terms of particle size distribution, vesicle morphology and dry film opacity. All the studied variables were shown to play a role in the quality of vesiculation. The underlying mechanisms were proposed, allowing for a better understanding of this sparsely studied process. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Multivesiculated particles (MVPs) can be synthesized as waterborne dispersions of unsaturated polyester crosslinked with styrene, each particle containing innumerous water-filled vesicles [1]. Depending on the particle sizes, MVPs can have different applications. In a range between 0.5 and 40 lm they can be used as opacifying agents in waterborne coating formulations [1,2], for beneficial replacement of expensive titanium dioxide pigment. Upon drying, water quickly evaporates from the internal vesicles, creating air voids. The large difference in refractive indexes between the entrapped air and the polymer walls originates light scattering, contributing to the hiding power and white appearance of the coating. This is the same optical phenomenon that occurs in snow and sea foam, where the opacity and whiteness arises from the interaction of light with a multiplicity of interfaces and microvoids. MVPs can also be used as opacifiers in paper coatings [3] and molding com-
⇑ Corresponding author. Tel.: +351 225081601; fax: +351 225081449. E-mail address: [email protected] (F.D. Magalhães). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.01.004
positions [4]. Larger particle sizes may be used to impart special effects in paints (e.g. textures) [2]. Methods for production of MVPs can be roughly divided in two-step and one-step processes. The first is described in most of the existing patents [3–10]. Aqueous phase is first emulsified in an unsaturated polyester–styrene solution, in the presence of an amine (with at least 3 amino groups per molecule) [8]. This water-in-oil (w/o) emulsion is then dispersed in an aqueous solution containing poly (vinyl alcohol) (PVA) and hydroxyl ethyl cellulose (HEC), forming the final water-in-oil-in-water (w/o/w) double emulsion. Free radical polymerization is then initiated to form cross-linked polyester particles encapsulating multiple water domains, which form the vesicles. The one-step process is less described [1,2], but consists on a more straightforward approach. The unsaturated polyester–styrene mixture is first neutralized with an amine, and then is directly emulsified in the aqueous solution of protective colloids (PVA and HEC). The diffusion of water into the dispersed organic phase (Fig. 1a) originates stable droplets, forming the double emulsion (Fig. 1b). In the patent literature available, interpretation of this vesiculation process is limited to mentioning that the added
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Fig. 1. Double emulsion formation, in the one-step process: (a) water diffusion from the aqueous phase (water, PVA and HEC) into the emulsified organic droplets (polyester, styrene and amine); (b) w/o/w double emulsion; (c) stabilization of water droplets in organic phase by polyester salts.
amine forms hydrophilic polyester salts with the polyester’s carboxylic groups, which act as water emulsifiers in the organic phase (Fig. 1c). The early work by Horie and co-workers [11] on formation of w/o emulsions in polyester is cited as a reference describing this inverted emulsion mechanism. The final curing procedure consists in the reticulation of polyester with styrene by free radical polymerization, and subsequent obtention of solid vesiculated particles. The initiation process is based on an oxidation–reduction reaction, comprising cumene hydroperoxide, diethylenetriamine and ferrous sulphate. The amine forms a complex with iron ions, which then reacts with cumene to form peroxide radicals [12], as shown in Eq. (1), where R = C6H5C(CH3)2 and R0 = NH2CH2NHCH2.
ROOH þ R0 NH2 Fe2þ ! RO þ R0 NH þ Fe2þ þ H2 O
ð1Þ
The radicals RO initiate polymerization involving styrene and the double bonds of the unsaturated polyester. As a result, polyester chains cross linked by polystyrene are obtained (Fig. 2). The present work studies the influence of different factors on the one-step MVPs synthesis process, namely the time of dispersion of the organic phase in the aqueous medium, and the concentrations of amine, polyester, and protective colloids. A detailed analysis of the effect of this set of factors has not been reported in the published literature that deals with this process. The particles obtained are characterized in terms of size distribution, vesicle morphology and dry film opacity. This last property is an indirect evaluation of the quality of particle vesiculation, and it is relevant in the context of use of MVPs as paint opacifying agents. The purpose of this work is to obtain a clearer understanding of this complex and, to a certain extent, unfamiliar process.
in the range of 15–20 mg KOH/g and a Brookfield viscosity in the range of 1000–2000 mPa s. Styrene, poly (vinyl alcohol) (PVA, Mw > 205000 g/mol, degree of hydrolysis = 88%), hydroxy ethyl cellulose, ammoniacal ferrous sulphate and biocide were also provided by Resiquímica. Diethylenetriamine (DETA, 98% purity confirmed by titration with a hydrochloric acid aqueous solution) and cumene hydroperoxide (technical grade, 80%) were supplied by Sigma–Aldrich and used as received. 2.2. Multivesiculated particles production The single-step double emulsification formulation described in literature [1,2] was adopted as the reference. DETA was added to a solution of UP in styrene and this mixture was gradually added to an aqueous solution, containing PVA, HEC, and DETA, in a jacketed glass reactor (100 ml), under mechanical agitation (600 rpm with a 40 mm impeller). After adding all organic phase, stirring was maintained for a predefined dispersion time. Finally, additional water and the curing system (cumene hydroperoxide and ferrous sulphate) were added to the reactor. The cure stage lasted 5 h at room temperature. The temperature in the liquid medium was kept homogenous using an anchor stirrer at low rotation speed. At the end of this time, biocide was added and the dispersion was stirred to ensure homogeneity. The standard formulation described in the literature includes a small amount of titanium dioxide to provide wet film opacity (before water evaporation from the vesicles). After confirming in a set of tests that addition of this pigment did not affect the final dry film opacity, particle size, pH and whitening index, it was excluded from the formulation. 2.3. Multivesiculated particles characterization
2. Experimental 2.1. Materials The unsaturated polyester (UP) used was provided by Resiquímica (Mem Martins, Portugal) as 70 wt.% solution in styrene. According to the provider, it has an acid value
2.3.1. Brookfield viscosity measurements The viscosity of the polyester phase was determined using a Brookfield LVDV-III Ultra rheometer, at 20 °C. All the measurements were made using the same spindle (LV-2) at the same rotational speed (20 rpm). The viscosity of the final MVPs dispersions was determined under the
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Fig. 2. Simplified scheme of cure process.
same conditions. However, the spindle type was chosen depending on measured torque percentage, which should preferentially be close to 50%. All readings were taken after 20 s stabilization time. 2.3.2. Contrast ratio measurements Opacity is the ability of a film to optically obliterate the substrate where it is applied. One method for evaluating this property, commonly used by paint industry, is known as contrast ratio (CR). It gives a useful qualitative evaluation of the quality of vesiculation. In this method, a film with a controlled wet thickness (100 lm) is applied on a Leneta 2A opacity chart fixed on a vacuum table, covering the black and white background portions of the chart. This film is left to dry for 24 h. The reflectance on the black and white portions is then measured using a GretagMacbeth Coloreye 3100 spectrophotometer, at wavelengths between 400 and 700 nm. The equipment software computes the ratio between the two reflectance values, which corresponds to the CR value. A completely opaque film would have a CR of 100%, meaning that the substrate is fully hidden by the coating.
performed without sonication. Special care was taken to perform the analysis immediately after removal from the reactor. 2.3.4. Scanning electron microscopy Particle morphology and internal vesiculation were observed by scanning electron microscopy (SEM), using a JEOL JSM-6301F, Oxford INCA Energy 350 equipment. Thick MVPs films were applied on 1 cm2 glass slabs, dried for 24 h and then fractured in liquid nitrogen. This resulted in fracture a fraction of the MVPs present, allowing for observation of the internal vesiculation structure. Before being analyzed, samples were sputtered with gold/platinum using a K575X Sputter Coater by Quorum Technologies. 2.3.5. Optical microscopy The size and morphology of polyester droplets along emulsification time was analyzed by optical microscopy using an Olympus IX 51 inverted optical microscope. The samples were previously diluted in water to allow better droplet individualization. 3. Results and discussion
2.3.3. Particle size distribution The particle size distribution of the waterborne MVPs dispersions was measured on a Beckman Coulter LS230 light scattering system. Dispersions of cured MVPs were diluted and sonicated for 20 min to eliminate agglomeration. Measurements with uncured dispersions were
3.1. Reference formulation MVPs produced according to the reference formulation have a solids content of 24 wt.%. The aqueous dispersions present low syneresis and no sediment after 1 month
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storage at room temperature. The average dry film contrast ratio (CR) is 89%. Fig. 3 is a representative SEM image of a dry film fractured under liquid nitrogen, showing uniformly vesiculated particles, with well-defined spherical alveoli with thin polymer walls, mostly sized between 0.15 and 1.2 lm. Particle size distribution (PSD) measurements showed an average particle size of about 10 lm. Some small holes are seen on the external particle surfaces, corresponding to collapse of the thin walls, probably during drying.
3.2. Effect of dispersion time In the reference synthesis procedure, after completing addition of the organic phase to the aqueous phase, the dispersion is left under mechanical stirring for 20 min. In order to evaluate the influence of this dispersion time on the resulting organic droplets, samples were collected after dispersion times of 0 min, 5 min, 20 min, 1 h and 5 h, and analyzed by optical microscopy (Fig. 4). Right after complete addition of the organic phase (Fig. 4a), a wide variety of droplet sizes is visible (from about 1 lm to several tens of lm). However, as the dispersion time progressed, the droplet sizes decreased and became more homogeneous (Fig. 4b). The image corresponding to the dispersion time of 1 h is not shown since it is qualitatively similar to the one for 20 min (Fig. 4c). It is interesting to note that the internal morphology of the droplets is apparent in these images. Fig. 4a already shows the presence of a water-in-oil emulsion within the organic phase. The droplet size distributions measured for the collected samples are shown in Fig. 5. In agreement with the optical microscopy results, there is a significant decline in particle sizes during the first 5 min of agitation, together with a gradual evolution towards a narrower distribution – between about 2 and 12 lm, attained after 20 min of dispersion. For longer times, the distribution does not change very significantly. The initially broad particle size distribution is due to the presence of droplets with different ages in the liquid
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medium. Droplets formed at the beginning of organic phase addition have been sheared to smaller sizes, while the last droplets added are still large. As mechanical agitation proceeds, all droplets evolve towards more uniform sizes. The collected samples were cured in order to obtain a suspension of solid particles. Note that it was previously confirmed that the droplet and particle size distributions are essentially the same before and after cure. Table 1 shows the average particle sizes, Brookfield viscosities, and dry film contrast ratios measured for the cured dispersions. Fig. 6 shows representative SEM images of the corresponding fractured dry films, showing the internal particle structures. Apparently well vesiculated particles are seen for dispersion times up to 1 h (Fig. 6a–c). For 5 h, however, the presence of monovesiculated particles was observed (Fig. 6d). From Table 1, the best opacity result (CR = 89%) was obtained for 20 min dispersion time, which corresponds to the reference condition. A value almost as high was obtained for 5 min dispersion, indicating that internal vesiculation was already well established at this time, as confirmed when comparing figures Fig. 6a and b. The opacity decreased for MVPs cured after 1 h dispersion and became quite low after 5 h, indicating that the quality of vesiculation was negatively affected. After this long dispersion time, many water-in-oil droplets end up coalescing into single domains, due to deficient long-time stability, originating monovesiculated particles like the one seen in Fig. 6d. The presence of small bright droplets in the liquid phase after 5 h dispersion, shown in Fig. 4d, was also an indication of the worsening in vesiculation quality. Average vesicle sizes measurements were based on several SEM images taken for the samples collected up to 1 h dispersion time. These are shown in Fig. 7. Despite the large scattering of the data, there seems to be a tendency for increasing vesicle size as time progresses, which is probably associated to progressive water intake into initially formed vesicles. The decrease in viscosity of the cured MVPs dispersion along dispersion time (Table 1) may be due to decreasing concentration of dissolved protective colloids in the continuous phase. The observed lowering in droplet size with dispersion time implies higher interfacial area and hence higher colloid adsorption. In summary, during the dispersion time shear forces induced by mechanical agitation cause rupture of larger organic droplets, and the system evolves towards an equilibrium size distribution. Formation of water droplets within the organic phase occurs very rapidly – in the first minutes of dispersion these are already visible by optical microscopy. However, some water intake seems to proceed along dispersion time, increasing vesicle sizes. The waterin-oil droplets are not stable for long dispersion times, and end up coalescing, significantly decreasing the number of vesicles per particle. 3.3. Effect of amine addition
Fig. 3. SEM image of a MVPs film fractured in liquid nitrogen (magnification 6000) produced under reference conditions.
A key component in the synthesis of MVPs is the amine (diethylenetriamine – DETA) added to the organic phase. Fig. 8a and b show optical microscopy images of organic
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Fig. 4. Optical microscopy images of organic phase emulsions in aqueous medium (magnification 1000), produced at different dispersion times: (a) 0 min; (b) 5 min; (c) 20 min; (d) 5 h. Reference dispersion time corresponds to 20 min.
Fig. 5. Variation of organic droplet size distribution with dispersion time. Reference dispersion time corresponds to 20 min.
Table 1 Influence of dispersion time on average particle size (lm), contrast ratio, CR (%) and Brookfield viscosity (cP) at 20 °C, of cured MVPs.
a
Dispersion time
Average particle size (lm)
Brookfield viscosity, 20 °C (cP)
0 min 5 min 20 min 1h 5h
48.4 16.4 10.2 7.2 6.2
a
b
3730 1580 535 137
86 89 81 49
CR (%)
Phase separation occurred after the curing time. An uniform dry film could not be formed for measurement of contrast ratio. b
droplets produced with and without amine addition, respectively. In the second case, not only vesiculation decreased significantly but also much larger droplets were formed. In addition, complete phase separation occurred soon after stopping agitation, indicating that the oil-inwater dispersion became highly unstable. The hydrophilic acid–base ion-pairs formed in the reaction between the polyester’s carboxyl end-groups and DETA’s amino groups are considered to be responsible for the formation of the water-filled vesicles within the organic phase [11]. The amine is therefore essential for formation of the water-in-oil-in-water double emulsion, where water droplets are stabilized within the dispersed organic phase by polyester chains terminated by carboxylate anions in the water/oil interface. The presence of a few vesicles in Fig. 8b, where no amine was added to the organic phase, can be justified by the addition of a small amount of DETA to the aqueous phase, used as part of the redox curing system. The observation that removal of DETA originates an unstable organic phase dispersion, despite the presence of protective colloids in solution, was somewhat surprising. This indicates that the amine also plays a role in the oil-in-water stabilization mechanism. This can be understood considering that a fraction of the neutralized carboxyl end groups are positioned at the external surface of the organic droplets, creating a dense water solvation layer surrounding the hydrophilic ion-pairs. This provides an oilin-water stabilization mechanism similar to the existing in aqueous polyurethane dispersions (PUDs), where the polymer chains contain amine-neutralized pendant carboxyl groups [13–15].
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Fig. 6. SEM images of MVPs films fractured in liquid nitrogen (magnification 6000), produced at different dispersion times: (a) 5 min (b) 20 min, (c) 1 h (d) 5 h. Reference dispersion time corresponds to 20 min.
Fig. 7. Influence of dispersion time on average vesicle size of MVPs. The error bars represent standard deviations based on at least 100 size measurements.
The influence of DETA on particle size, vesicle morphology and dry film opacity was studied in more detail. The amount of amine added is represented here in terms of stoichiometric degree of neutralization (n), defined as the molar ratio of basic groups in the amine to acid groups in the polyester. Note that DETA has two basic primary amino groups (–NH2) and the secondary amino group (–NH) has no basic character. The relation between dry film opacity
and average particle size, for different degrees of neutralization (from 0.5 to 2.1), is shown in Fig. 9. The reference conditions correspond to n = 1.2. A significant increase in film opacity was observed until n = 1.2. For this lower range of neutralization values, adding more amine increases the concentration of hydrophilic acid–base ion pairs improving water retention in the organic phase. This promotes vesiculation and hence higher opacity. Note that at equimolar neutralization (n = 1) not all acid end groups are yet deprotonated, and thus addition of excess amine further improves internal vesiculation, until n = 1.2. Past this point, film opacity increased more gradually. Particle sizes decreased continuously with amine addition, but a significant drop was observed after n = 1.2. This may be an indication that after neutralizing the majority of carboxyl groups, additional amine migrates to the droplet’s external interface, promoting additional stabilization of the organic phase dispersion and hence contributing to reduce droplet sizes. This may involve interaction with the protective colloids (PVA and HEC) adsorbed at the surface. Therefore, it may be suggested that for degrees of neutralization above 1.2 the film opacity does not increase due to improved particle vesiculation, but to the lowering in particle sizes, which may originate more effective packing and light scattering within the dry film.
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3.4. Effect of polyester concentration Different concentrations of polyester in the organic phase were tested, in the range between 30 wt.% and 57 wt.% (g of polyester/g of organic phase 100). These were obtained by adding different amounts of styrene to the polyester solution. The total amount of organic phase dispersed in the aqueous medium was maintained constant to ensure the same final solids content. The reference polyester concentration corresponded to 48 wt.%. The neutralization index was maintained constant, equal to the reference value (n = 1.2). The results are grouped into two concentration ranges: values above and below 40 wt.%. As shown in Fig. 11, increasing the concentration of polyester between 40 and 57 wt.% increased the average particle size from 6 to 20 lm. This can be attributed to the corresponding sevenfold increase in organic phase viscosity, as shown in Fig. 12. Higher viscosity of the dispersed phase implies more difficult droplet break-up. The influence of polyester concentration on the dry film opacity and its relation with the average particle sizes are presented in Fig. 13. Initially, contrast ratio increased
Fig. 8. Optical microscopy of organic phase emulsions in aqueous medium (magnification 1000) produced with: (a) 1 wt.% DETA added to the organic phase (reference condition); (b) no amine.
Differences in internal particle structure were followed by SEM imaging. Two representative examples are shown in Fig. 10. For the lower degree of neutralization tested (n = 0.5), Fig. 10a shows a fractured particle that is an example of deficient vesiculation: one single large vesicle in the center and a thick polymer wall with a few voids. For a degree of neutralization of 2 (Fig. 10b), one observes the same type of vesiculation as for the reference conditions: small vesicles separated by thin walls, uniformly distributed within all particles.
Fig. 9. Relation between polyester neutralization with DETA and contrast ratio of MVPs films (%), and average particle size (lm).
Fig. 10. SEM images of MVPs films fractured in liquid nitrogen (magnification 6000), for degrees of neutralization of 0.5 (a) and 2 (b).
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rapidly, reaching a maximum for the reference polyester concentration (48 wt.%). Vesiculation improved as the concentration of hydrophilic acid–base ion-pairs increased within the organic droplets. Above this value, a slight decrease in contrast ratio is observed, which may be related to the increasing particle size becoming a determinant factor. As discussed previously, dry film opacity seems to be favored by lower particle sizes. Internal particle vesiculation for a polyester concentration of 57 wt.% is similar to the one obtained at the reference conditions, as shown in Fig. 14. For polyester concentrations below 40 wt.%, a distinct set of results was obtained. Despite the lower viscosity of the organic phase, the particle size distributions shifted towards larger sizes and became significantly broader as polyester concentration decreased (Fig. 15). The dry films applied for this concentration range had a heterogeneous appearance, showing agglomerated areas with non-uniform thickness, not allowing contrast ratio measurements. For the lowest concentration tested (30 wt.%), the cured dispersion became a highly viscous mass and application on an opacity chart was not even possible. Fig. 16 shows a SEM image of a fractured film obtained for a polyester concentration of 38 wt.%. Particles are not well individualized and are enveloped in an unshaped coalesced polymer mass. Some vesiculation is visible, but is surrounded by a thick layer of compact polymer. These results show that the stability of the organic dispersion was compromised by reduction of polyester concentration below a critical level. If the amount of neutralized end groups available at the external surface of the organic droplets is too low, their stabilization is negatively affected. This confirms the importance of polyesteramine hydrophilic sites as both water-in-oil and oil-inwater stabilizers.
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Fig. 12. Influence of polyester concentration (g of polyester/g of organic phase 100) on Brookfield viscosity of organic phase at 20 °C.
Fig. 13. Influence of polyester concentration (g of polyester/g of organic phase 100) on final dry film opacity and MVPs average particle size (lm). The reference concentration value corresponds to 48%.
3.5. Effect of protective colloids concentration The protective colloids used in MVP production were PVA and HEC, in a concentration of 2.3 and 0.43 wt.% (g of colloid/g of aqueous phase 100), respectively, for the reference conditions. The effect of varying both concentrations separately was studied.
Fig. 14. SEM image of MVPs produced with a polyester concentration of 57% (g of polyester/g of organic phase 100).
Fig. 11. Influence of polyester concentration (g of polyester/g of organic phase 100) on MVPs average particle size (lm). Reference value corresponds to 48%.
Concerning PVA, the tested concentrations in the continuous aqueous phase were 1.6, 2.1, 2.3, 2.5, 3.0 and 3.5 wt.% (corresponding to 30, 10, +10, 0, +30 and +50% in relation to reference concentration). For all cases,
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Fig. 15. Influence of low polyester concentration in organic phase (g of polyester/g of organic phase 100) on MVPs average particle size (lm).
Fig. 17. Influence of PVA concentration on polyester droplets size distribution. Reference concentration corresponds to 2.3%.
Fig. 18. Influence of PVA concentration (g of PVA/g of aqueous phase 100) on final dry film opacity and MVPs average particle size (lm). Reference value corresponds to 2.3%. Fig. 16. SEM image of MVPs film fractured in liquid nitrogen (magnification 6000), produced with a polyester concentration of 38% (g of polyester/g of organic phase 100).
the MVPs dispersions produced were within the same range of viscosities and presented similar stability (low syneresis and no sediment after several weeks storage). Fig. 17 shows the particle size distributions obtained. Results for concentration of 2.1 and 2.5 wt.% were not presented since the curves overlapped with the reference. As expected, as colloid concentration was increased, smaller particles were obtained, showing PVA’s influence on droplets stabilization. For the highest concentration, 3.5 wt.%, a second peak appeared, for sizes of about 20 lm, which probably corresponds to formation of agglomerates. The effect of PVA concentration on final dry film opacity and average particle sizes is presented in Fig. 18. Beyond a concentration of 2.1 wt.%, the opacity started to decrease significantly, even though particle sizes also tended to decrease. This result was unexpected, since internal vesiculation, and hence film opacity, should not be directly affected by the concentration of PVA in the continuous phase. Fig. 19 shows representative SEM images of fractured particles corresponding to the lowest and highest PVA concentrations tested. For the lower concentration (Fig. 19a)
the fractured particle is uniformly vesiculated. But for the highest PVA concentration (Fig. 19b) deficient vesiculation is visible, with a reduced number of voids within particles, and large regions of dense polymer. This shows that the observed lowering in contrast ratio with decreasing PVA concentration was due to inefficient vesiculation. It is interesting to note that in Fig. 19a the particle outer surface is seen to be porous, having sub-micrometric holes probably due to collapse of thin vesicle walls. This fact is common to all previously described MVPs productions. However, in Fig. 19b the particle surfaces are distinctly not porous, due to existence of a compact PVA film surrounding the cured particles. It is hypothesized that increasing PVA concentration in the aqueous phase originates a dense layer of adsorbed colloid at the droplets surface, so that water diffusion into the organic phase is hindered. The vesiculation process is therefore delayed and when curing is performed, proper vesiculation has not yet been attained. The HEC concentrations tested were 0.30, 0.39, 0.43, 0.47 and 0.56 wt.% (corresponding to 30, 10, 0, +10 and +30% in relation to reference concentration). The final dispersion viscosities and stabilities did not change significantly for the different productions.
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Fig. 21. Influence of HEC concentration (g of HEC/g of aqueous phase 100) on final dry film opacity and MVPs average particle size (lm). Reference concentration corresponds to 0.43%.
higher HEC concentration, 0.56 wt.%, which yielded distinctively smaller particle sizes. Fig. 21 shows that the influence of HEC concentration on dry film opacity and average particle sizes is not as
Fig. 19. SEM images of MVPs (magnification 6000) produced with different PVA concentrations (g of PVA/g of aqueous phase 100): (a) 1.6%; (b) 3.5%.
Fig. 20. Influence of HEC concentration (g of HEC/g of aqueous phase 100) on particle size distribution. Reference concentration corresponds to 0.43%.
Cured particle size distributions are shown in Fig. Results for 0.39 and 0.47 wt.% are not shown because distribution curves almost overlapped with the one 0.43 wt.%. The largest deviation obtained was for
20. the for the
Fig. 22. SEM images of MVPs produced with different HEC concentrations in aqueous phase (g of HEC/g of aqueous phase 100): (a) 0.30%; (b) 0.56%.
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strong as in the case of PVA. The contrast ratio does not change significantly, except for the higher HEC concentrations, where it decreases slightly. Representative SEM images for the lowest and highest concentrations tested are shown in Fig. 22. For 0.30 wt.% HEC concentration, the particle internal morphology is similar to the reference results. However, for the highest concentration (0.56% wt), some regions of dense, poorly vesiculated polymer are visible in some particles, indicating this is the reason for the lower measured film opacity. As in the case of PVA, discussed above, HEC is not expected to interfere directly with the vesiculation process. A possible explanation is again that high concentration of HEC adsorbed at the droplet surface may hinder water diffusion into the organic phase and consequently delay vesicle formation. 4. Conclusions The work performed allowed for a better understanding of the process of formation of cross-linked polyester multivesiculated particles. As the polyester/styrene phase is dispersed in the aqueous medium, diffusion of water into the organic phase originates growing water-in-oil droplets. If a sufficient long dispersion time is allowed, these end up coalescing, eventually forming a homogeneous water core surrounded by an organic shell. During the initial stages of dispersion, organic droplets decrease in size due to mechanical shearing, even though water is being absorbed and vesicle size is apparently increasing. The added amine (DETA) was shown to play a crucial role in the process of formation of the water-in-oil-inwater double emulsion, affecting both the quality of vesiculation and particle size. The first effect is well described by the mechanism suggested by Horie and co-workers for inverted emulsions in polyester/styrene solutions [12]: the polyester salts formed by combination of the amine with the carboxyl end-groups are responsible for the emulsification of water within the organic phase. On the other hand, we suggest that the contribution of the amine to the stabilization of the organic droplets in the aqueous medium involves a mechanism equivalent to the one observed in aqueous polyurethane dispersions (PUDs). The polyester concentration in the organic phase affects the amount of carboxyl end-groups available for stabilization of the double emulsion. At too low concentrations, well defined particles cannot be obtained and, in addition, vesiculation is incipient. Higher concentrations allow formation of homogeneously vesiculated particles, whose average particle sizes tend to decrease with increasing
polyester concentration, probably due to higher viscosity of the dispersed phase. Increasing PVA concentration in the continuous aqueous phase leads to a decrease in particle size, as a consequence of more efficient steric stabilization. Surprisingly, vesiculation is negatively affected by high PVA concentration, probably because this hinders water diffusion into the organic droplets. Varying HEC concentration shows similar consequences, but to a lower extent. Acknowledgements Funding for this work was provided by FEDER, through Programa Operacional Factores de Competitividade – COMPETE, and by national funding through FCT – Fundação para a Ciência e a Tecnologia, in the framework of project PTDC/EQU-EQU/112151/2009. Ângela Dias thanks CIN – Corporação Industrial do Norte, S.A., Resiquímica – Resinas Químicas, S.A., and FCT for PhD grant SFRH/BDE/ 33432/2008. References [1] Terblanche J. The development of vesiculated beads: faculty of Engineering at Stellenbosch University; 2003. [2] Engelbrecht JF, De Wet-Roos D, Smit AC, Cooray B. Vesiculated polymer particles. US patent: 10/529764; 2006. [3] Ferguson LD, Hayes PC, Macas TS. Vesiculated polymer granules. US patent: 4917765; 1990. [4] Jonathan Banford PD. Polymer granules and compositions containing them. US patent: 5055513; 1991. [5] Ritchie PJA, Serelis AK. Vesiculated polyester granules. US patent: 2003/0040557 A1; 2003. [6] Kershaw RW, Lubbock FJ, Polgar L. Vesiculated polymer granules. US patent: 3933579; 1976. [7] Karickhoff M. Vesiculated beads. US patent: 4489174; 1984. [8] Gunning RH, Henshaw BC, Lubbock FJ. Process of making porous polyester granules. US patent: 3879314; 1975. [9] Gillan J, Kershaw RW. Process of preparing vesiculated cross-linked polyester resin granules. South African patent: 70/8592; 1974. [10] Geoffrey Willison JH. Bead polymerization process. US patent: 4363888; 1982. [11] Horie K, Mita I, Kambe H. Copolymerization of unsaturated polyester with styrene in inverted emulsion. J Appl Polym Sci 1967;11(1):57–71. [12] Razumovskii SD, Medvedev SS. Kinetics of the reaction between cumene hydroperoxide and triethylenetetramine in presence of iron salts in aqueous solutions. Russ Chem Bull 1958;7(8):944–51. [13] Nanda AK, Wicks Da, Madbouly Sa, Otaigbe JU. Effect of ionic content, solid content, degree of neutralization, and chain extension on aqueous polyurethane dispersions prepared by prepolymer method. J Appl Polym Sci 2005;98:2514–20. [14] Galgoci EC, Hegedus CR, Snyder JM, Lawson DC, Lindenmuth DL. Urethane-acrylic hybrid polymers. AIR PRODUCTS; 2001. [15] Chinwanitcharoen C, Kanoh S, Yamada T, Tada K, Hayashi S, Sugano S. Preparation and shelf-life stability of aqueous polyurethane dispersion. Macromol Symposia 2004;216:229–40.
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Study of multivesiculated polyester particles synthesis by double emulsion process Ângela Dias a,b,c, Joana Fidalgo a, João Machado b, Jorge Moniz c, Adélio M. Mendes a, Fernão D. Magalhães a,⇑ a
LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal CIN – Corporação Industrial do Norte, S.A., Av. Dom Mendo, 831, Apartado 1008, 4471-909 Maia, Portugal c Resiquímica – Resinas Químicas, S.A., Rua Francisco Lyon de Castro, 28, 2725-397 Mem-Martins, Portugal b
a r t i c l e
i n f o
Article history: Received 1 October 2012 Received in revised form 28 November 2012 Accepted 3 January 2013 Available online 11 January 2013 Keywords: Waterborne dispersions Particle size distribution Hollow particles Opacity
a b s t r a c t This work studies the synthesis of multivesiculated particles (MVPs) from a w/o/w double emulsion comprising water and unsaturated polyester dissolved in styrene. These particles can be used as opacifying agents in paint formulations, due to light refraction within the air-containing voids, after water evaporation. The effect of some key variables was analyzed: time of dispersion and concentrations of added amine, polyester and protective colloids. The final products were characterized in terms of particle size distribution, vesicle morphology and dry film opacity. All the studied variables were shown to play a role in the quality of vesiculation. The underlying mechanisms were proposed, allowing for a better understanding of this sparsely studied process. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Multivesiculated particles (MVPs) can be synthesized as waterborne dispersions of unsaturated polyester crosslinked with styrene, each particle containing innumerous water-filled vesicles [1]. Depending on the particle sizes, MVPs can have different applications. In a range between 0.5 and 40 lm they can be used as opacifying agents in waterborne coating formulations [1,2], for beneficial replacement of expensive titanium dioxide pigment. Upon drying, water quickly evaporates from the internal vesicles, creating air voids. The large difference in refractive indexes between the entrapped air and the polymer walls originates light scattering, contributing to the hiding power and white appearance of the coating. This is the same optical phenomenon that occurs in snow and sea foam, where the opacity and whiteness arises from the interaction of light with a multiplicity of interfaces and microvoids. MVPs can also be used as opacifiers in paper coatings [3] and molding com-
⇑ Corresponding author. Tel.: +351 225081601; fax: +351 225081449. E-mail address: [email protected] (F.D. Magalhães). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.01.004
positions [4]. Larger particle sizes may be used to impart special effects in paints (e.g. textures) [2]. Methods for production of MVPs can be roughly divided in two-step and one-step processes. The first is described in most of the existing patents [3–10]. Aqueous phase is first emulsified in an unsaturated polyester–styrene solution, in the presence of an amine (with at least 3 amino groups per molecule) [8]. This water-in-oil (w/o) emulsion is then dispersed in an aqueous solution containing poly (vinyl alcohol) (PVA) and hydroxyl ethyl cellulose (HEC), forming the final water-in-oil-in-water (w/o/w) double emulsion. Free radical polymerization is then initiated to form cross-linked polyester particles encapsulating multiple water domains, which form the vesicles. The one-step process is less described [1,2], but consists on a more straightforward approach. The unsaturated polyester–styrene mixture is first neutralized with an amine, and then is directly emulsified in the aqueous solution of protective colloids (PVA and HEC). The diffusion of water into the dispersed organic phase (Fig. 1a) originates stable droplets, forming the double emulsion (Fig. 1b). In the patent literature available, interpretation of this vesiculation process is limited to mentioning that the added
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Fig. 1. Double emulsion formation, in the one-step process: (a) water diffusion from the aqueous phase (water, PVA and HEC) into the emulsified organic droplets (polyester, styrene and amine); (b) w/o/w double emulsion; (c) stabilization of water droplets in organic phase by polyester salts.
amine forms hydrophilic polyester salts with the polyester’s carboxylic groups, which act as water emulsifiers in the organic phase (Fig. 1c). The early work by Horie and co-workers [11] on formation of w/o emulsions in polyester is cited as a reference describing this inverted emulsion mechanism. The final curing procedure consists in the reticulation of polyester with styrene by free radical polymerization, and subsequent obtention of solid vesiculated particles. The initiation process is based on an oxidation–reduction reaction, comprising cumene hydroperoxide, diethylenetriamine and ferrous sulphate. The amine forms a complex with iron ions, which then reacts with cumene to form peroxide radicals [12], as shown in Eq. (1), where R = C6H5C(CH3)2 and R0 = NH2CH2NHCH2.
ROOH þ R0 NH2 Fe2þ ! RO þ R0 NH þ Fe2þ þ H2 O
ð1Þ
The radicals RO initiate polymerization involving styrene and the double bonds of the unsaturated polyester. As a result, polyester chains cross linked by polystyrene are obtained (Fig. 2). The present work studies the influence of different factors on the one-step MVPs synthesis process, namely the time of dispersion of the organic phase in the aqueous medium, and the concentrations of amine, polyester, and protective colloids. A detailed analysis of the effect of this set of factors has not been reported in the published literature that deals with this process. The particles obtained are characterized in terms of size distribution, vesicle morphology and dry film opacity. This last property is an indirect evaluation of the quality of particle vesiculation, and it is relevant in the context of use of MVPs as paint opacifying agents. The purpose of this work is to obtain a clearer understanding of this complex and, to a certain extent, unfamiliar process.
in the range of 15–20 mg KOH/g and a Brookfield viscosity in the range of 1000–2000 mPa s. Styrene, poly (vinyl alcohol) (PVA, Mw > 205000 g/mol, degree of hydrolysis = 88%), hydroxy ethyl cellulose, ammoniacal ferrous sulphate and biocide were also provided by Resiquímica. Diethylenetriamine (DETA, 98% purity confirmed by titration with a hydrochloric acid aqueous solution) and cumene hydroperoxide (technical grade, 80%) were supplied by Sigma–Aldrich and used as received. 2.2. Multivesiculated particles production The single-step double emulsification formulation described in literature [1,2] was adopted as the reference. DETA was added to a solution of UP in styrene and this mixture was gradually added to an aqueous solution, containing PVA, HEC, and DETA, in a jacketed glass reactor (100 ml), under mechanical agitation (600 rpm with a 40 mm impeller). After adding all organic phase, stirring was maintained for a predefined dispersion time. Finally, additional water and the curing system (cumene hydroperoxide and ferrous sulphate) were added to the reactor. The cure stage lasted 5 h at room temperature. The temperature in the liquid medium was kept homogenous using an anchor stirrer at low rotation speed. At the end of this time, biocide was added and the dispersion was stirred to ensure homogeneity. The standard formulation described in the literature includes a small amount of titanium dioxide to provide wet film opacity (before water evaporation from the vesicles). After confirming in a set of tests that addition of this pigment did not affect the final dry film opacity, particle size, pH and whitening index, it was excluded from the formulation. 2.3. Multivesiculated particles characterization
2. Experimental 2.1. Materials The unsaturated polyester (UP) used was provided by Resiquímica (Mem Martins, Portugal) as 70 wt.% solution in styrene. According to the provider, it has an acid value
2.3.1. Brookfield viscosity measurements The viscosity of the polyester phase was determined using a Brookfield LVDV-III Ultra rheometer, at 20 °C. All the measurements were made using the same spindle (LV-2) at the same rotational speed (20 rpm). The viscosity of the final MVPs dispersions was determined under the
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Fig. 2. Simplified scheme of cure process.
same conditions. However, the spindle type was chosen depending on measured torque percentage, which should preferentially be close to 50%. All readings were taken after 20 s stabilization time. 2.3.2. Contrast ratio measurements Opacity is the ability of a film to optically obliterate the substrate where it is applied. One method for evaluating this property, commonly used by paint industry, is known as contrast ratio (CR). It gives a useful qualitative evaluation of the quality of vesiculation. In this method, a film with a controlled wet thickness (100 lm) is applied on a Leneta 2A opacity chart fixed on a vacuum table, covering the black and white background portions of the chart. This film is left to dry for 24 h. The reflectance on the black and white portions is then measured using a GretagMacbeth Coloreye 3100 spectrophotometer, at wavelengths between 400 and 700 nm. The equipment software computes the ratio between the two reflectance values, which corresponds to the CR value. A completely opaque film would have a CR of 100%, meaning that the substrate is fully hidden by the coating.
performed without sonication. Special care was taken to perform the analysis immediately after removal from the reactor. 2.3.4. Scanning electron microscopy Particle morphology and internal vesiculation were observed by scanning electron microscopy (SEM), using a JEOL JSM-6301F, Oxford INCA Energy 350 equipment. Thick MVPs films were applied on 1 cm2 glass slabs, dried for 24 h and then fractured in liquid nitrogen. This resulted in fracture a fraction of the MVPs present, allowing for observation of the internal vesiculation structure. Before being analyzed, samples were sputtered with gold/platinum using a K575X Sputter Coater by Quorum Technologies. 2.3.5. Optical microscopy The size and morphology of polyester droplets along emulsification time was analyzed by optical microscopy using an Olympus IX 51 inverted optical microscope. The samples were previously diluted in water to allow better droplet individualization. 3. Results and discussion
2.3.3. Particle size distribution The particle size distribution of the waterborne MVPs dispersions was measured on a Beckman Coulter LS230 light scattering system. Dispersions of cured MVPs were diluted and sonicated for 20 min to eliminate agglomeration. Measurements with uncured dispersions were
3.1. Reference formulation MVPs produced according to the reference formulation have a solids content of 24 wt.%. The aqueous dispersions present low syneresis and no sediment after 1 month
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storage at room temperature. The average dry film contrast ratio (CR) is 89%. Fig. 3 is a representative SEM image of a dry film fractured under liquid nitrogen, showing uniformly vesiculated particles, with well-defined spherical alveoli with thin polymer walls, mostly sized between 0.15 and 1.2 lm. Particle size distribution (PSD) measurements showed an average particle size of about 10 lm. Some small holes are seen on the external particle surfaces, corresponding to collapse of the thin walls, probably during drying.
3.2. Effect of dispersion time In the reference synthesis procedure, after completing addition of the organic phase to the aqueous phase, the dispersion is left under mechanical stirring for 20 min. In order to evaluate the influence of this dispersion time on the resulting organic droplets, samples were collected after dispersion times of 0 min, 5 min, 20 min, 1 h and 5 h, and analyzed by optical microscopy (Fig. 4). Right after complete addition of the organic phase (Fig. 4a), a wide variety of droplet sizes is visible (from about 1 lm to several tens of lm). However, as the dispersion time progressed, the droplet sizes decreased and became more homogeneous (Fig. 4b). The image corresponding to the dispersion time of 1 h is not shown since it is qualitatively similar to the one for 20 min (Fig. 4c). It is interesting to note that the internal morphology of the droplets is apparent in these images. Fig. 4a already shows the presence of a water-in-oil emulsion within the organic phase. The droplet size distributions measured for the collected samples are shown in Fig. 5. In agreement with the optical microscopy results, there is a significant decline in particle sizes during the first 5 min of agitation, together with a gradual evolution towards a narrower distribution – between about 2 and 12 lm, attained after 20 min of dispersion. For longer times, the distribution does not change very significantly. The initially broad particle size distribution is due to the presence of droplets with different ages in the liquid
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medium. Droplets formed at the beginning of organic phase addition have been sheared to smaller sizes, while the last droplets added are still large. As mechanical agitation proceeds, all droplets evolve towards more uniform sizes. The collected samples were cured in order to obtain a suspension of solid particles. Note that it was previously confirmed that the droplet and particle size distributions are essentially the same before and after cure. Table 1 shows the average particle sizes, Brookfield viscosities, and dry film contrast ratios measured for the cured dispersions. Fig. 6 shows representative SEM images of the corresponding fractured dry films, showing the internal particle structures. Apparently well vesiculated particles are seen for dispersion times up to 1 h (Fig. 6a–c). For 5 h, however, the presence of monovesiculated particles was observed (Fig. 6d). From Table 1, the best opacity result (CR = 89%) was obtained for 20 min dispersion time, which corresponds to the reference condition. A value almost as high was obtained for 5 min dispersion, indicating that internal vesiculation was already well established at this time, as confirmed when comparing figures Fig. 6a and b. The opacity decreased for MVPs cured after 1 h dispersion and became quite low after 5 h, indicating that the quality of vesiculation was negatively affected. After this long dispersion time, many water-in-oil droplets end up coalescing into single domains, due to deficient long-time stability, originating monovesiculated particles like the one seen in Fig. 6d. The presence of small bright droplets in the liquid phase after 5 h dispersion, shown in Fig. 4d, was also an indication of the worsening in vesiculation quality. Average vesicle sizes measurements were based on several SEM images taken for the samples collected up to 1 h dispersion time. These are shown in Fig. 7. Despite the large scattering of the data, there seems to be a tendency for increasing vesicle size as time progresses, which is probably associated to progressive water intake into initially formed vesicles. The decrease in viscosity of the cured MVPs dispersion along dispersion time (Table 1) may be due to decreasing concentration of dissolved protective colloids in the continuous phase. The observed lowering in droplet size with dispersion time implies higher interfacial area and hence higher colloid adsorption. In summary, during the dispersion time shear forces induced by mechanical agitation cause rupture of larger organic droplets, and the system evolves towards an equilibrium size distribution. Formation of water droplets within the organic phase occurs very rapidly – in the first minutes of dispersion these are already visible by optical microscopy. However, some water intake seems to proceed along dispersion time, increasing vesicle sizes. The waterin-oil droplets are not stable for long dispersion times, and end up coalescing, significantly decreasing the number of vesicles per particle. 3.3. Effect of amine addition
Fig. 3. SEM image of a MVPs film fractured in liquid nitrogen (magnification 6000) produced under reference conditions.
A key component in the synthesis of MVPs is the amine (diethylenetriamine – DETA) added to the organic phase. Fig. 8a and b show optical microscopy images of organic
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Fig. 4. Optical microscopy images of organic phase emulsions in aqueous medium (magnification 1000), produced at different dispersion times: (a) 0 min; (b) 5 min; (c) 20 min; (d) 5 h. Reference dispersion time corresponds to 20 min.
Fig. 5. Variation of organic droplet size distribution with dispersion time. Reference dispersion time corresponds to 20 min.
Table 1 Influence of dispersion time on average particle size (lm), contrast ratio, CR (%) and Brookfield viscosity (cP) at 20 °C, of cured MVPs.
a
Dispersion time
Average particle size (lm)
Brookfield viscosity, 20 °C (cP)
0 min 5 min 20 min 1h 5h
48.4 16.4 10.2 7.2 6.2
a
b
3730 1580 535 137
86 89 81 49
CR (%)
Phase separation occurred after the curing time. An uniform dry film could not be formed for measurement of contrast ratio. b
droplets produced with and without amine addition, respectively. In the second case, not only vesiculation decreased significantly but also much larger droplets were formed. In addition, complete phase separation occurred soon after stopping agitation, indicating that the oil-inwater dispersion became highly unstable. The hydrophilic acid–base ion-pairs formed in the reaction between the polyester’s carboxyl end-groups and DETA’s amino groups are considered to be responsible for the formation of the water-filled vesicles within the organic phase [11]. The amine is therefore essential for formation of the water-in-oil-in-water double emulsion, where water droplets are stabilized within the dispersed organic phase by polyester chains terminated by carboxylate anions in the water/oil interface. The presence of a few vesicles in Fig. 8b, where no amine was added to the organic phase, can be justified by the addition of a small amount of DETA to the aqueous phase, used as part of the redox curing system. The observation that removal of DETA originates an unstable organic phase dispersion, despite the presence of protective colloids in solution, was somewhat surprising. This indicates that the amine also plays a role in the oil-in-water stabilization mechanism. This can be understood considering that a fraction of the neutralized carboxyl end groups are positioned at the external surface of the organic droplets, creating a dense water solvation layer surrounding the hydrophilic ion-pairs. This provides an oilin-water stabilization mechanism similar to the existing in aqueous polyurethane dispersions (PUDs), where the polymer chains contain amine-neutralized pendant carboxyl groups [13–15].
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Fig. 6. SEM images of MVPs films fractured in liquid nitrogen (magnification 6000), produced at different dispersion times: (a) 5 min (b) 20 min, (c) 1 h (d) 5 h. Reference dispersion time corresponds to 20 min.
Fig. 7. Influence of dispersion time on average vesicle size of MVPs. The error bars represent standard deviations based on at least 100 size measurements.
The influence of DETA on particle size, vesicle morphology and dry film opacity was studied in more detail. The amount of amine added is represented here in terms of stoichiometric degree of neutralization (n), defined as the molar ratio of basic groups in the amine to acid groups in the polyester. Note that DETA has two basic primary amino groups (–NH2) and the secondary amino group (–NH) has no basic character. The relation between dry film opacity
and average particle size, for different degrees of neutralization (from 0.5 to 2.1), is shown in Fig. 9. The reference conditions correspond to n = 1.2. A significant increase in film opacity was observed until n = 1.2. For this lower range of neutralization values, adding more amine increases the concentration of hydrophilic acid–base ion pairs improving water retention in the organic phase. This promotes vesiculation and hence higher opacity. Note that at equimolar neutralization (n = 1) not all acid end groups are yet deprotonated, and thus addition of excess amine further improves internal vesiculation, until n = 1.2. Past this point, film opacity increased more gradually. Particle sizes decreased continuously with amine addition, but a significant drop was observed after n = 1.2. This may be an indication that after neutralizing the majority of carboxyl groups, additional amine migrates to the droplet’s external interface, promoting additional stabilization of the organic phase dispersion and hence contributing to reduce droplet sizes. This may involve interaction with the protective colloids (PVA and HEC) adsorbed at the surface. Therefore, it may be suggested that for degrees of neutralization above 1.2 the film opacity does not increase due to improved particle vesiculation, but to the lowering in particle sizes, which may originate more effective packing and light scattering within the dry film.
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3.4. Effect of polyester concentration Different concentrations of polyester in the organic phase were tested, in the range between 30 wt.% and 57 wt.% (g of polyester/g of organic phase 100). These were obtained by adding different amounts of styrene to the polyester solution. The total amount of organic phase dispersed in the aqueous medium was maintained constant to ensure the same final solids content. The reference polyester concentration corresponded to 48 wt.%. The neutralization index was maintained constant, equal to the reference value (n = 1.2). The results are grouped into two concentration ranges: values above and below 40 wt.%. As shown in Fig. 11, increasing the concentration of polyester between 40 and 57 wt.% increased the average particle size from 6 to 20 lm. This can be attributed to the corresponding sevenfold increase in organic phase viscosity, as shown in Fig. 12. Higher viscosity of the dispersed phase implies more difficult droplet break-up. The influence of polyester concentration on the dry film opacity and its relation with the average particle sizes are presented in Fig. 13. Initially, contrast ratio increased
Fig. 8. Optical microscopy of organic phase emulsions in aqueous medium (magnification 1000) produced with: (a) 1 wt.% DETA added to the organic phase (reference condition); (b) no amine.
Differences in internal particle structure were followed by SEM imaging. Two representative examples are shown in Fig. 10. For the lower degree of neutralization tested (n = 0.5), Fig. 10a shows a fractured particle that is an example of deficient vesiculation: one single large vesicle in the center and a thick polymer wall with a few voids. For a degree of neutralization of 2 (Fig. 10b), one observes the same type of vesiculation as for the reference conditions: small vesicles separated by thin walls, uniformly distributed within all particles.
Fig. 9. Relation between polyester neutralization with DETA and contrast ratio of MVPs films (%), and average particle size (lm).
Fig. 10. SEM images of MVPs films fractured in liquid nitrogen (magnification 6000), for degrees of neutralization of 0.5 (a) and 2 (b).
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rapidly, reaching a maximum for the reference polyester concentration (48 wt.%). Vesiculation improved as the concentration of hydrophilic acid–base ion-pairs increased within the organic droplets. Above this value, a slight decrease in contrast ratio is observed, which may be related to the increasing particle size becoming a determinant factor. As discussed previously, dry film opacity seems to be favored by lower particle sizes. Internal particle vesiculation for a polyester concentration of 57 wt.% is similar to the one obtained at the reference conditions, as shown in Fig. 14. For polyester concentrations below 40 wt.%, a distinct set of results was obtained. Despite the lower viscosity of the organic phase, the particle size distributions shifted towards larger sizes and became significantly broader as polyester concentration decreased (Fig. 15). The dry films applied for this concentration range had a heterogeneous appearance, showing agglomerated areas with non-uniform thickness, not allowing contrast ratio measurements. For the lowest concentration tested (30 wt.%), the cured dispersion became a highly viscous mass and application on an opacity chart was not even possible. Fig. 16 shows a SEM image of a fractured film obtained for a polyester concentration of 38 wt.%. Particles are not well individualized and are enveloped in an unshaped coalesced polymer mass. Some vesiculation is visible, but is surrounded by a thick layer of compact polymer. These results show that the stability of the organic dispersion was compromised by reduction of polyester concentration below a critical level. If the amount of neutralized end groups available at the external surface of the organic droplets is too low, their stabilization is negatively affected. This confirms the importance of polyesteramine hydrophilic sites as both water-in-oil and oil-inwater stabilizers.
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Fig. 12. Influence of polyester concentration (g of polyester/g of organic phase 100) on Brookfield viscosity of organic phase at 20 °C.
Fig. 13. Influence of polyester concentration (g of polyester/g of organic phase 100) on final dry film opacity and MVPs average particle size (lm). The reference concentration value corresponds to 48%.
3.5. Effect of protective colloids concentration The protective colloids used in MVP production were PVA and HEC, in a concentration of 2.3 and 0.43 wt.% (g of colloid/g of aqueous phase 100), respectively, for the reference conditions. The effect of varying both concentrations separately was studied.
Fig. 14. SEM image of MVPs produced with a polyester concentration of 57% (g of polyester/g of organic phase 100).
Fig. 11. Influence of polyester concentration (g of polyester/g of organic phase 100) on MVPs average particle size (lm). Reference value corresponds to 48%.
Concerning PVA, the tested concentrations in the continuous aqueous phase were 1.6, 2.1, 2.3, 2.5, 3.0 and 3.5 wt.% (corresponding to 30, 10, +10, 0, +30 and +50% in relation to reference concentration). For all cases,
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Fig. 15. Influence of low polyester concentration in organic phase (g of polyester/g of organic phase 100) on MVPs average particle size (lm).
Fig. 17. Influence of PVA concentration on polyester droplets size distribution. Reference concentration corresponds to 2.3%.
Fig. 18. Influence of PVA concentration (g of PVA/g of aqueous phase 100) on final dry film opacity and MVPs average particle size (lm). Reference value corresponds to 2.3%. Fig. 16. SEM image of MVPs film fractured in liquid nitrogen (magnification 6000), produced with a polyester concentration of 38% (g of polyester/g of organic phase 100).
the MVPs dispersions produced were within the same range of viscosities and presented similar stability (low syneresis and no sediment after several weeks storage). Fig. 17 shows the particle size distributions obtained. Results for concentration of 2.1 and 2.5 wt.% were not presented since the curves overlapped with the reference. As expected, as colloid concentration was increased, smaller particles were obtained, showing PVA’s influence on droplets stabilization. For the highest concentration, 3.5 wt.%, a second peak appeared, for sizes of about 20 lm, which probably corresponds to formation of agglomerates. The effect of PVA concentration on final dry film opacity and average particle sizes is presented in Fig. 18. Beyond a concentration of 2.1 wt.%, the opacity started to decrease significantly, even though particle sizes also tended to decrease. This result was unexpected, since internal vesiculation, and hence film opacity, should not be directly affected by the concentration of PVA in the continuous phase. Fig. 19 shows representative SEM images of fractured particles corresponding to the lowest and highest PVA concentrations tested. For the lower concentration (Fig. 19a)
the fractured particle is uniformly vesiculated. But for the highest PVA concentration (Fig. 19b) deficient vesiculation is visible, with a reduced number of voids within particles, and large regions of dense polymer. This shows that the observed lowering in contrast ratio with decreasing PVA concentration was due to inefficient vesiculation. It is interesting to note that in Fig. 19a the particle outer surface is seen to be porous, having sub-micrometric holes probably due to collapse of thin vesicle walls. This fact is common to all previously described MVPs productions. However, in Fig. 19b the particle surfaces are distinctly not porous, due to existence of a compact PVA film surrounding the cured particles. It is hypothesized that increasing PVA concentration in the aqueous phase originates a dense layer of adsorbed colloid at the droplets surface, so that water diffusion into the organic phase is hindered. The vesiculation process is therefore delayed and when curing is performed, proper vesiculation has not yet been attained. The HEC concentrations tested were 0.30, 0.39, 0.43, 0.47 and 0.56 wt.% (corresponding to 30, 10, 0, +10 and +30% in relation to reference concentration). The final dispersion viscosities and stabilities did not change significantly for the different productions.
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Fig. 21. Influence of HEC concentration (g of HEC/g of aqueous phase 100) on final dry film opacity and MVPs average particle size (lm). Reference concentration corresponds to 0.43%.
higher HEC concentration, 0.56 wt.%, which yielded distinctively smaller particle sizes. Fig. 21 shows that the influence of HEC concentration on dry film opacity and average particle sizes is not as
Fig. 19. SEM images of MVPs (magnification 6000) produced with different PVA concentrations (g of PVA/g of aqueous phase 100): (a) 1.6%; (b) 3.5%.
Fig. 20. Influence of HEC concentration (g of HEC/g of aqueous phase 100) on particle size distribution. Reference concentration corresponds to 0.43%.
Cured particle size distributions are shown in Fig. Results for 0.39 and 0.47 wt.% are not shown because distribution curves almost overlapped with the one 0.43 wt.%. The largest deviation obtained was for
20. the for the
Fig. 22. SEM images of MVPs produced with different HEC concentrations in aqueous phase (g of HEC/g of aqueous phase 100): (a) 0.30%; (b) 0.56%.
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strong as in the case of PVA. The contrast ratio does not change significantly, except for the higher HEC concentrations, where it decreases slightly. Representative SEM images for the lowest and highest concentrations tested are shown in Fig. 22. For 0.30 wt.% HEC concentration, the particle internal morphology is similar to the reference results. However, for the highest concentration (0.56% wt), some regions of dense, poorly vesiculated polymer are visible in some particles, indicating this is the reason for the lower measured film opacity. As in the case of PVA, discussed above, HEC is not expected to interfere directly with the vesiculation process. A possible explanation is again that high concentration of HEC adsorbed at the droplet surface may hinder water diffusion into the organic phase and consequently delay vesicle formation. 4. Conclusions The work performed allowed for a better understanding of the process of formation of cross-linked polyester multivesiculated particles. As the polyester/styrene phase is dispersed in the aqueous medium, diffusion of water into the organic phase originates growing water-in-oil droplets. If a sufficient long dispersion time is allowed, these end up coalescing, eventually forming a homogeneous water core surrounded by an organic shell. During the initial stages of dispersion, organic droplets decrease in size due to mechanical shearing, even though water is being absorbed and vesicle size is apparently increasing. The added amine (DETA) was shown to play a crucial role in the process of formation of the water-in-oil-inwater double emulsion, affecting both the quality of vesiculation and particle size. The first effect is well described by the mechanism suggested by Horie and co-workers for inverted emulsions in polyester/styrene solutions [12]: the polyester salts formed by combination of the amine with the carboxyl end-groups are responsible for the emulsification of water within the organic phase. On the other hand, we suggest that the contribution of the amine to the stabilization of the organic droplets in the aqueous medium involves a mechanism equivalent to the one observed in aqueous polyurethane dispersions (PUDs). The polyester concentration in the organic phase affects the amount of carboxyl end-groups available for stabilization of the double emulsion. At too low concentrations, well defined particles cannot be obtained and, in addition, vesiculation is incipient. Higher concentrations allow formation of homogeneously vesiculated particles, whose average particle sizes tend to decrease with increasing
polyester concentration, probably due to higher viscosity of the dispersed phase. Increasing PVA concentration in the continuous aqueous phase leads to a decrease in particle size, as a consequence of more efficient steric stabilization. Surprisingly, vesiculation is negatively affected by high PVA concentration, probably because this hinders water diffusion into the organic droplets. Varying HEC concentration shows similar consequences, but to a lower extent. Acknowledgements Funding for this work was provided by FEDER, through Programa Operacional Factores de Competitividade – COMPETE, and by national funding through FCT – Fundação para a Ciência e a Tecnologia, in the framework of project PTDC/EQU-EQU/112151/2009. Ângela Dias thanks CIN – Corporação Industrial do Norte, S.A., Resiquímica – Resinas Químicas, S.A., and FCT for PhD grant SFRH/BDE/ 33432/2008. References [1] Terblanche J. The development of vesiculated beads: faculty of Engineering at Stellenbosch University; 2003. [2] Engelbrecht JF, De Wet-Roos D, Smit AC, Cooray B. Vesiculated polymer particles. US patent: 10/529764; 2006. [3] Ferguson LD, Hayes PC, Macas TS. Vesiculated polymer granules. US patent: 4917765; 1990. [4] Jonathan Banford PD. Polymer granules and compositions containing them. US patent: 5055513; 1991. [5] Ritchie PJA, Serelis AK. Vesiculated polyester granules. US patent: 2003/0040557 A1; 2003. [6] Kershaw RW, Lubbock FJ, Polgar L. Vesiculated polymer granules. US patent: 3933579; 1976. [7] Karickhoff M. Vesiculated beads. US patent: 4489174; 1984. [8] Gunning RH, Henshaw BC, Lubbock FJ. Process of making porous polyester granules. US patent: 3879314; 1975. [9] Gillan J, Kershaw RW. Process of preparing vesiculated cross-linked polyester resin granules. South African patent: 70/8592; 1974. [10] Geoffrey Willison JH. Bead polymerization process. US patent: 4363888; 1982. [11] Horie K, Mita I, Kambe H. Copolymerization of unsaturated polyester with styrene in inverted emulsion. J Appl Polym Sci 1967;11(1):57–71. [12] Razumovskii SD, Medvedev SS. Kinetics of the reaction between cumene hydroperoxide and triethylenetetramine in presence of iron salts in aqueous solutions. Russ Chem Bull 1958;7(8):944–51. [13] Nanda AK, Wicks Da, Madbouly Sa, Otaigbe JU. Effect of ionic content, solid content, degree of neutralization, and chain extension on aqueous polyurethane dispersions prepared by prepolymer method. J Appl Polym Sci 2005;98:2514–20. [14] Galgoci EC, Hegedus CR, Snyder JM, Lawson DC, Lindenmuth DL. Urethane-acrylic hybrid polymers. AIR PRODUCTS; 2001. [15] Chinwanitcharoen C, Kanoh S, Yamada T, Tada K, Hayashi S, Sugano S. Preparation and shelf-life stability of aqueous polyurethane dispersion. Macromol Symposia 2004;216:229–40.