Environmental Engineering and Management Journal May/June 2008, Vol.7, No.3, 337-342

http://omicron.ch.tuiasi.ro/EEMJ/

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SMART SOLUBLE GRAFTED POLYSILOXANES WITH POTENTIAL

APPLICATIONS IN WATERBORNE PAINTS

Iulian Nor1, Ioana Moleavin1, Constanţa Ibănescu1, Viorel Sandu2

Nicolae Hurduc1∗

1“Gh. Asachi” Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Natural and Synthetic Polymers, 71 Mangeron Blvd., 700050, Iasi, Romania 2National Institute of Materials Physics, 105b Atomistilor Str., 077125, Magurele,Romania

Abstract The synthesis of some grafted polysiloxane and azo-polysiloxanes with potential applications as rheology modifiers or surfactants for waterborne paints is reported. Nowadays it became very important to design environmentally friendly systems exhibiting the best end-uses properties. Due to their complex architecture and their sensitivity to UV radiation the studied polymeric systems exhibit very interesting rheological properties. All the polymers containing the azobenzenic groups in the side chains are able to interact with the UV/VIS light, due to the photo-isomerization processes. Due to this property, the azo-based polymers are able to generate photo-stimuli response accomplished by strong supramolecular properties modifications. Keywords: ATRP, rheology modifier, soluble polysiloxanes, sustainability, waterborne paints,

∗ Author to whom all correspondence should be addressed: nhurduc@ch.tuiasi.ro

1. Introduction

Waterborne paints have been evaluated as alternatives to solvent-based paints. The volatile organic compound (VOC) content of waterborne paints is significantly lower than conventional solvent-based paints, thereby reducing VOC emissions (Chang et al, 1999; Clausen et al. 1991; Silva et al. 2003). Waterborne (or latex) paints are composed of synthetic resins and pigments that are kept dispersed in water by surfactants (http://p2library.nfesc.navy.mil/P2_Opportunity_Handbook/4_7.html). They also contain small amounts of coalescing solvents (Bardage and Bjurman, 1998). Waterborne paints dry by evaporation of the water. The coalescing solvents allow the resin particles to fuse together (coalesce) as the water evaporates to form a continuous coating. Waterborne paints reduce VOC emissions and worker exposure to hazardous air pollutants. These paints can also reduce the amount of hazardous waste generated, depending on the type of paint used (Rusu and Mindrila, 2005; Rusu et al., 2003).

The wellness and sustainability trends will have a decisive influence on the future and, because

of this, on the behavior of consumers. One of the major goals of sustainability is to maintain an optimal balance between increases in manufacturing output, and a clean and safe environment. Sustainability will be one of the main drivers for innovation in order to allow the technical industries to care for the well-being of consumers in a safe and healthy environment. Thus, one area in which significant efforts and progress have been made is in the development of “green” or sustainable chemistry (Brezet and Silvester, 2004; Gavrilescu, 2003).

However, the recent European Union (EU) directive on volatile organic compounds (VOCs) could become an obstacle for paints and varnishes manufacturers. The EU introduced this directive (2004/42/CE) with the specific aim of reducing emissions of VOCs resulting from the use of organic solvents in certain paints, varnishes and vehicle refinishing products. Following this legislation, coating companies are more or less compelled to reduce solvent emissions by improving efficiency in using them or by substituting products that contain less or no solvent.

"This has increased the market for water-borne, high solids and solvent-free paints and affected

“Gh. Asachi” Technical University of Iasi, Romania

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the demand for solvent-borne paints in all sectors," notes Frost & Sullivan (http://chemicals.frost.com) Research Analyst Vinayak Rao. Upon his opinion: "The growth in water-borne paints acts as a driver for rheology modifier types such as cellulosics and synthetics, which are widely used in water-borne formulations" (http://www.prnewswire.co.uk/cgi/news/release?id=152117). Additionally, rheology modifier manufacturers, like others, are feeling the pressure from customers to create more innovative, custom-designed, high-quality products and to provide enhanced customer service and technical product support (Fernando et al., 2000; Kästner, 2001). Formulation of waterborne industrial coatings requires skillful balancing of the various formulation components to achieve the desired balance of film properties, application characteristics, package stability and substrate protection (Fox, 2000). Coatings formulators often attempt to minimize the use of water-soluble materials such as rheology modifiers. It is generally believed that water-soluble ingredients cause water sensitivity, which reduces the durability of the coating. This article reports synthesis and examines some azo-polysiloxanes, hydrophobically modified polymers applying Atom Transfer Radical Polymerization (ATRP), useful as rheology modifiers for acrylic based waterborne paints. 2. Experimental

The materials used in Atom Transfer Radical Polymerization (ATRP) reactions were as follows: all monomers and solvents (Aldrich) distilled under vacuum before the polymerization; CuCl and 2, 2’-bipyridyl purified by recrystallization, dried under vacuum and stored in argon or nitrogen atmosphere. In a typical polymerization, CuCl (0.6 mmol) was placed in a carefully dried 50mL-flask. The monomer and 2, 2’-bipyridyl were added under stirring and the system was degassed by three freeze-pump-thaw cycles. The flask was heated at the reaction temperature under nitrogen atmosphere. At the end of the reaction, the polymerization mixture was diluted with CH2Cl2 and the catalyst was removed by passing the solution through alumina. The clear solution was then precipitated in methanol, filtered and dried under vacuum.

1H-NMR spectra were recorded on a Bruker ACE-200 spectrometer (200 MHz) or a Bruker AC-500 spectrometer (500 MHz). Molecular weights were determined vs. PS standards by SEC (Waters GPC equipped with a refractive index detector) using 5 microstyragel columns (100, 500, 103, 104, 105 Å) (Taran et al. 2006).

In a typical reaction for the azo-polysiloxanes synthesis, 0.3 g of polysiloxane were dissolved in 6 mL DMSO and mixed with the corresponding amount (based on the chloromethyl groups content) of sodium salt of 4-(phenylazo)phenol and 0.5-0.7 g tetra-butyl-

ammonium hydrogen-sulphate. The mixture was then introduced in a flask and heated 5h at 90ºC. The polymer was precipitated in methanol and washed 5-6 times with methanol to eliminate the unreacted products. The polymer was dried under vacuum. The azo-polysiloxane obtained in a first step (as previously described) containing unreacted chlorobenzyl groups (40%) was quaternized with triethyl and tributyl amines in solution (reaction times between 5 and 7 hours). 3. Results and discussion

All the investigated polymers were obtained starting from a polysiloxane with chlorobenzyl groups in the side-chain, by one or two step reaction. A series of homo- and grafted polymers with poly(chloromethyl styrene) or poly(chloromethyl styrene-co-styrene) side-chains were synthesized. Furthermore, these polymers were modified with azobenzenic groups in order to obtain some particular rheological behavior induced by the conformational changes of azo-groups (consequence of the isomerization process). All the polymers containing the azobenzenic groups in the side chains are able to interact with the UV/VIS light, due to the photo-isomerization processes supposing configurational changes from the trans- to the cis-form. This change in configuration is accomplished by significant geometrical and dipole-moment (from 0.1 to 3.5 D) modifications (Fig. 1).

Fig. 1. Geometries corresponding to the tras- and cis-

isomer Due to this property, the azo-based polymers

are able to generate photo-stimuli response accomplished by strong supramolecular property modifications. In our case, the azo-polymers can generate photo-sensitive micelles (if they are used as surfactants) or rheological “smart” additives, able to modify the solutions viscosity as a consequence of the UV light interaction. The grafted copolymers were modified with different amines in order to generate quaternary ammonium groups, able to induce water solubility properties. The azo-polysiloxanes were obtained in a two-step reaction, starting from a polysiloxane with chlorobenzyl groups in the side-chain. In the first step, the polysiloxane was modified with 4-hydroxyazobenzene (50-75% substitution degree) and, in the second one, the unreacted chlorobenzyl groups were quaternized with triethyl- or tributyl-amines in solution (CH2Cl2) at 30°C (Fig. 2).

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Fig. 2. Reaction scheme

Fig.3. 1H-NMR spectrum of a polysiloxane grafted with chloromethylstyrene

All these systems are able to aggregate/disaggregate under UV-VIS irradiation. The chemical structures of the modified polysiloxanes were confirmed by 1H-NMR spectroscopy as illustrated in the Figs. 3 to 6.

In the case of the polysiloxanes grafted with chloromethylstyrene (Fig. 3) or styrene / chloromethylstyrene (Fig. 4) the signals corresponding to the –Ø-CH2Cl groups are present at 4.5 ppm. At 0 ppm there is the signal corresponding to methyl groups connected to the Si atom and in 0.5

– 2.8 ppm domain the signals corresponding to methylenic moieties can be observed. The signals corresponding to the aromatic side-chain rings are situated at 6.1 – 7.1 ppm. For the azo-polysiloxanes modified with different amines, the polymer substitution degree was monitored using the signals corresponding to the –CH2Cl groups (4.5 ppm) that shift to 5.0 – 5.1 ppm after the bonding of the azobenzene groups (Figs. 5 and 6).

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Fig.4. 1H-NMR spectrum of a polysiloxane grafted with styrene and chloromethylstyrene

Fig.5. 1H-NMR spectrum of a polysiloxane with chlorobenzyl groups in the side-chain (a) and azo-polysiloxane (b)

Fig.6. 1H-NMR spectrum of a polysiloxane modified with azo-phenol and quaternary ammonium groups

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Table 1 presents some characteristics of the

modified polysiloxanes obtained by ATRP reactions and Table 2 shows some features for the azo-polysiloxanes modified with different types of amines.

Table 1.Characteristics of the grafted polysiloxanes containing poly (CMS) or

poly (styrene-co-CMS) side chains

Spl no.

Macro initia-

tor

Feed ratio

St/CMS (%

mol)

T (°C)

Reaction time (h)

Copolym St/CMS (% mol)

Mn PDI

1 cyclic 70/30 120 8 72/28 6750 1.6 2 cyclic 66/34 120 7 62/38 4800 1.8

3 linear 0/100 110 7 - 4840 4.03

4 linear 0/100 130 3.5 - 4540 3,96

(Mn- molecular weight; PDI-polydispersity index)

Table.2. Some characteristics of the synthesized azo-polysiloxanes

Spl. no.

Substituent type Substitution degree (%)

Mn

1 Azobenzene triethyl amine

40 58

6900

2 Azobenzene tributyl amine

40 57

7050

(Mn-molecular weight); These polymers prepared by ATRP reactions

or by a simple SN2 substitution reaction are suitable to be used as additives in some waterborne paints and coatings, while the azo-polysiloxanes modified with quaternary ammonium salts can be used as surfactants for the waterborne paints. As a consequence, their aggregation capacity in water has been studied by the use of pyrene fluorescence method (Lin and Alexandridis, 2002).

To calculate the critical aggregation concentration (cac) value, the first (named I1 – Fig 7) and the third (named I3 – Fig 7) absorption peaks corresponding to the fluorescence pyrene spectrum were used. In aqueous solution, the I1/I3 ratio value is situated around 1.70-1.75 in agreement with the literature data (Kalyanasundaram and Thomas, 1977).

In the case of amphiphilic samples, for concentrations lower than 10-3 g/L, no aggregation process was evidenced, the I1/I3 ratio being ~ 1.7 (Fig. 8). Increasing the azo-polysiloxane concentration, the pyrene was progressively incorporated in the hydrophobic core of the aggregates formed in the aqueous solution. As a result the reduction of the I1/I3 ratio value was noticed. This reduction is not sharp, as in the case of low-molecular amphiphiles. The cac values were estimated from the first inflexion point on the curves and correspond to 2·10-3

g/L. These relatively low values may be explained by the presence of azobenzenic hydrophobic groups with

a high aggregation tendency on the polysiloxane side-chain.

Fig. 7. The water/pyrene solution fluorescence spectrum.

Fig.8. Plot of the I1/I3 ratio as a function of the azo-polysiloxane concentration corresponding to sample 1

(Table 2)

The evolution of viscoelastic parameters as a function of time provided information about the influence of conformational modifications (under UV irradiation) on rheological properties of the studied systems. Fig. 9 presents the evolution of the viscoelastic parameters as a function of time, for a 3% solution of azo-polysiloxane grafted with S/CMS. For the non-irradiated 3% solution in toluene no isomerization was observed, but for the same sample exposed to UV irradiation, the response of the material was recorded after 250 s for a period of 200 s. In this case an increase of G’, G” and η* over 4 orders of magnitude is to be noticed.

The changes of the viscoelastic parameters can be associated with the configuration changes induced by the trans-cis isomerization undergone by the azobenzene groups. The variation of the viscoelastic parameters may have a significant influence on the properties of the waterborne paints so these polymers can be used as “smart” additives. The rheological properties of the paint determine the sprayability, flow and leveling, and sag resistance. The steady shear rate viscosity of paint is the sum of the elastic or storage modulus G’ and the viscous or loss modulus G. The elastic modulus represents resistance to flow and the spring-like character of the fluid.

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Fig.9. Evolution of viscoelastic parameters as a function of time (3% concentration of an azo-polysiloxane grafted with styrene/chloromethyl styrene in toluene, non-irradiated sample comparatively with UV irradiated sample 15 minutes)

(200C).

The loss modulus represents the flow of the fluid without memory effects. Increasing the elasticity in a coating improves the sag resistance, but reduces the flow and leveling. Reducing the elasticity of a coating should improve sprayability, and may improve gloss. 4. Conclusions

In the present study the possibility to obtain a series of grafted polysiloxanes and azo-polysiloxanes able to be used as additives for waterborne paints was reported. The polysiloxanes grafted with styrene and styrene/chloromethyl styrene are more suited as rheological modifiers for waterborne paints, while the azo-polysiloxanes modified with quaternary ammonium groups proved to be the choice as surfactants in waterborne paints. The rheological investigations evidenced important solution properties modification as a consequence of the UV irradiation. Acknowledgement This work was supported by the Romanian National Authority for Research and Science under the Project CEEX 45/2006 (COMPOSPIN). The DSC studies and molecular simulations were effectuated on the Interdisciplinary Research Platform for Multifunctional, High Performance Polymers (Contract no. 69/2006 CNCSIS). References Bardage S., Bjurman J., (1998) Adhesion of waterborne

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