New applications of catalytic chain transfer polymerization to waterborne binders for automotive paint systems

Progress in Organic Coatings 45 (2002) 173–183

New applications of catalytic chain transfer polymerization to waterborne binders for automotive paint systems Jos Huybrechts a,∗ , Paul Bruylants a , Ken Kirshenbaum b , Jiri Vrana c , Jaromir Snuparek d a

Dupont Performance Coatings, Antoon Spinoystraat 6, 2800 Mechelen, Belgium b Dupont Performance Coatings, Troy, MI, USA c Research Institute, Pardubice, Czech Republic d University of Pardubice, Pardubice, Czech Republic Received 1 September 2001; accepted 1 March 2002

Abstract Catalytic chain transfer polymerization (CCTP) is a conventional free radical polymerization technique that allows the preparation of macromonomers in a one step process. Acid functional macromonomers can be copolymerized with acrylate backbone monomers to form graft copolymers that, after neutralization with a base, are water dispersible. Low molecular weight oligomers from CCTP act as chain transfer agents themselves for methacrylate monomers via addition–fragmentation mechanism and lead to (semi) AB-block copolymers. Group transfer polymerization (GTP) is another polymerization technique to make AB-block copolymers but economically less attractive for functional comonomers since they interfere with the initiation mechanism. Graft and AB-block copolymer dispersions offer advantages in waterborne coatings compared to linear polymers of the same overall composition and molecular weight. Examples discussed in this paper are pigment dispersants and dispersion resins in which the backbone or A-segment of the copolymer has specific groups to anchor to the pigment surface and the side chains or B-segment give both charge and steric stabilization. AB-block copolymers with one block water soluble or dispersible also function as copolymerizable surfactants in an emulsion polymerization process and allow the synthesis of surfactant-free emulsions with low amounts of hydrophilic groups. The catalytic chain transfer agents (CCTAs) used in CCTP do have a high chain transfer activity for methacrylate monomers at very low concentrations so that low molecular weight oligomers (e.g. dimers) can be made. This chain transfer activity is lost in an emulsion polymerization process if the CCTA is (partly) water soluble. The paper will further demonstrate the use of dimers in the control of molecular weight in emulsion copolymerization. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Block; Graft copolymers; Catalytic chain transfer polymerization; Waterborne binders; Automotive coatings

1. Introduction Catalytic chain transfer polymerization (CCTP) is a free radical polymerization technique in which specific cobalt complexes (Fig. 1) act as chain transfer agents (CTAs) and catalyze the transfer of hydrogen from a growing free radical chain to an olefin (Fig. 2). The cobalt complex is not consumed and the rate constant of catalytic chain transfer (CCT) is believed to be controlled by diffusion [1] so that low molecular weight polymers can be prepared with CTA on a ppm level. The overall process allows the synthesis of oligomers from methacrylates with an unsaturated end group. Oligomers with a terminal double bond behave as macromonomers and can be used to prepare graft, block and telechelic polymers [2,3]. Graft copolymers ∗ Corresponding author. Tel.: +32-15-441665; fax: +32-15-441510. E-mail address: [email protected] (J. Huybrechts).

are formed when a macromonomer is copolymerized with acrylic and vinylaromatic monomers. Block copolymers can be made when the macromonomer is polymerized in the presence of methacrylate monomers. The block copolymer formation is explained by an addition–fragmentation mechanism (Fig. 3). If the molecular weight of the macromonomer is sufficiently low (and the concentration of terminal double bonds high), the addition–fragmentation mechanism results also in an efficient chain transfer effect. Block copolymers can be prepared by living polymerization, such as anionic, cationic and group transfer polymerization (GTP). These techniques, however, have many disadvantages versus CCTP. Cationic polymerization is only possible with a limited type of monomers resulting in (co)polymers not useful for most of the automotive paint applications while anionic polymerization needs low temperature reaction circumstances difficult or impossible to realize in plant reactors. GTP is a

0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 0 4 2 - 5

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Fig. 1. Bis(borondifluorodiphenylglyoximato) cobaltate(II) (A) and isopropyl-bis(borondifluorodimethylglyoximato) cobaltate(III) (B).

Fig. 2. Catalytic cycles in CCTP.

Fig. 3. Addition–fragmentation mechanism.

technique comparable with anionic but can be run at higher temperatures. GTP (as anionic) cannot be done with functional (acid, hydroxyl) methacrylate monomers unless the functionality is blocked during synthesis and deblocked afterwards which is an economic disadvantage. GTP (Fig. 4) on the other hand has the advantage that it is a real living polymerization method resulting in block copolymers with narrow molecular weight distribution. In the last decade many new living free radical polymerization techniques have shown up in literature, such as living free radical (using stable nitroxyl radicals as initiators), radical addition–fragmentation chain transfer (RAFT) (with e.g. trithiocarbonates for addition–fragmentation chain transfer), atom transfer radical polymerization (ATRP) (based on bromides/copper(I) bromide complexes as initiators) all allowing the synthesis of polymers with precise control of architecture (graft, block, star, etc.) and molecular weight. An excellent summary of the status of commercialization of those new living free radical polymerization methods is given in [4]. Although promising, there remain still many practical problems to solve, such as conversion, color and catalyst removal, mostly related to the use of fairly high

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175

Fig. 4. Group transfer polymerization process.

concentration of metal-, sulfur- and nitrogen-containing raw materials. In this respect, CCTP is quite unique and broadly applicable since the cobalt complexes introduced on a ppm level do not suffer from those drawbacks. Furthermore, CCTP is applicable to a wide range of monomers since it is basically a free radical polymerization method. Acid functional macromonomers from CCTP act as copolymerizable ‘surfactants’ in an emulsion polymerization process which lead to graft copolymers in case the backbone monomers are mostly vinylaromatic or acrylic in nature. Such surfactant-free emulsions offer advantages in waterborne two component polyurethane coatings [5,6]. The overall concentration of a random macromonomer needed to stabilize an emulsion is high compared to the amount of surfactants used in traditional emulsion. This can be explained by the fact that random acid functional copolymers do not form micelles. The nucleation mechanism believed to take place is homogeneous in which the hydrophobic backbone monomers form graft copolymers with hydrophilic arms which stabilize the overall composition. Fairly high concentrations of such random macromonomers are needed because the copolymerization of macromonomer with backbone monomers is not efficient due to the low reactivity and availability of the vinyl end group. In waterborne paint applications, high amounts of hydrophilic, low molecular weight oligomers should be avoided since those may negatively influence the final paint properties. It would therefore be an advantage if polymeric structures could be made with more surfactant properties so that lower amounts have to be used in the stabilization of the overall polymer emulsion. AB-block copolymers with one block hydrophilic (polyethyleneoxide

(PEO)) and one hydrophobic (polystyrene) prepared anionically have been shown to form micelles [7]. It was further demonstrated that emulsions can be prepared with concentrations of AB-block copolymer in the range as typical surfactants are used. The efficiency of stabilization increases with decreasing molecular weight and increased amount of PEO content. Such AB-block copolymers with low molecular weight, high water solubility and low overall glass transition temperature (Tg ) (high PEO content) will not be bound to the particle and may affect final paint properties as hardness and humidity. CCTP is a unique technique to prepare a broad range of AB-block hydrophobic–hydrophilic oligomers varying in Tg and functionality with a terminal unsaturation to allow the oligomer to be bound to the final latex particle. A first part of this paper will demonstrate the ability of AB-block copolymers from CCTP to form stable emulsions in which the AB-block is anchored to the final particle via the terminal double bond. Lower amounts of AB-block are needed in an emulsion polymerization process when compared with a random macromonomer at the same molecular weight and composition. AB-blocks from CCTP are as effective as GTP blocks in particle nucleation. Block and graft copolymers offer advantages versus random as pigment dispersants. CCTP is a versatile technique to prepare such copolymers and allows to build in a variety of functionalities needed to be able to make the copolymer water dispersable, reactive versus crosslinkers and to interact with the pigment surface. Several factors in the overall copolymer structure can be varied as the ratio and molecular weight of backbone versus arm in the graft or A- versus B-block in the AB-block polymer structure.

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Another part his paper will give some practical examples of pigment dispersants and dispersion resins for waterborne automotive applications. Small particle size and low molecular weight, surfactantcontaining emulsions are difficult to prepare with conventional emulsion polymerization. The regular chain transfer agents, such as mercaptans have to be used in high concentrations to obtain low molecular weights which negatively affects emulsion properties as odor, toxicity and monomer conversation. Small particle size implies the use of high amounts of surfactant and those again result in high concentrations of water-soluble species in the final emulsion. The cobalt complexes from CCTP act as CTA in an emulsion process, however, those emulsions require the use of expensive water-soluble azo initiators since peroxides kill the activity [8]. The cobalt CTA should also be designed to have sufficient solubility in the latex particle. Oligomers from CCTP act as CTA through an addition–fragmentation mechanism and can be designed so as to control the water versus organic phase solubility. The chain transfer activity of methyl methacrylate (MMA) dimer and higher oligomers has been reported before [9]. The copolymerization of ␣-methyl styrene with styrene in the presence of cobalt CTA has also been studied [14]. Most of the studies of oligomers and cobalt CTA in emulsions were focusing on the control of molecular weight but never on particle size. Dimers from CCTP have some water solubility and high mobility and contribute to homogeneous nucleation during an emulsion polymerization process. The last part of this paper will show the results of a comparison between cobalt CTA of Fig. 1A, methyl methacrylate dimer and ␣-methyl styrene dimer in the control of particle size and molecular weight of a conventional emulsion.

2. Experimental 2.1. Synthesis All monomers, initiators and surfactants in the preparation of the acrylic copolymer emulsions were used from commercial sources without purification except for the block copolymer made with GTP where traces of water and inhibitor were removed by passing over a column. The cobalt CTA is sensitive to oxidation so polymerizations were run under nitrogen purge with azo initiators. Some typical examples are given below. Abbreviations used for monomers in the examples are as follows: BMA BA MAA HPMA MMA S AA

n-butyl methacrylate n-butyl acrylate methacrylic acid 2-hydropropyl methacrylate methyl methacrylate styrene acrylic acid

The synthesis of the different polymers is done in a glass reactor equipped with a mechanical stirrer, thermometer, inert gas inlet, addition funnels and condenser. 2.1.1. Example 1: AB-block BMA//MAA = 62//38 by weight from CCTP An amount of 150 g of deionized water, 0.3 g of azo initiator VA-044 (2,2 -azobis(N,N -dimethyleneisobutyramidine) dihydrochloride from WAKO) and 13 mg of cobalt(III) CTA (Fig. 1B) dissolved in 2 ml acetone were added to a 500 ml reactor and heated under nitrogen at about 55 ◦ C. An amount of 74 g of MAA mixed with 7.5 mg of cobalt(III) were added over about 1 h. After the feed, the reactor contents were held for 30 min and the MAA macromonomer was isolated by evaporation of water. The conversion was about 90% and the number average molecular weight (Mn ) determined from NMR was 1204, which has a degree of polymerization (DP) of 14. An amount of 200 g of MAA macromonomer of above was dissolved in 1000 ml of isopropanol and heated at 80 ◦ C reflux under nitrogen purge. An amount of 4.01 g of Vazo 64 from Dupont (azobis(isobutyronitrile)) was added followed by the addition of 326.1 g of BMA over 5 h. After every 90 min during the feed, an additional 2 g of Vazo 64 was added. After the feed, the mixture was held for another 150 min. The conversion was over 95%, Mn = 1390 and weight average molecular weight Mw = 4450. 2.1.2. Example 2: AB-block BMA//BMA/MAA = 47.5//23.7/28.8 by weight from CCTP An amount of 261.7 g of isopropanol, 63 g BMA, 77 g MAA and 0.1 g of cobalt(II) CTA (Fig. 1A) were heated at reflux under nitrogen. An amount of 252 g of BMA, 308 g MAA, 0.4 g cobalt(II) CTA, 5 g Vazo 52 from Dupont (2,2 azobis(2,4-dimethylpentannitril)) and 189.5 g of methylethylketone (MEK) were added over 4 h. Then 10 g of MEK was added to rinse the feed tank and the mixture was held for 60 min at reflux. Finally, the reactor contents were thinned with 233 g MEK. The macromonomer solution had 47.2% solids, viscosity Z1 + 1/2 (Gardner–Holdt), Mn = 2380 and Mw = 4190. An amount of 735 g of the macromonomer solution was heated at reflux under nitrogen with 44.2 g MEK. Then 332.5 g of BMA, 5 g Vazo 64 and 25 g of MEK were added over 4 h at reflux. After rinsing the feed tank with 10 g MEK, 0.5 g of Vazo 64 dissolved in 4.5 g of MEK was added as a shot followed by 5 g of MEK rinsing. The mixture was held for an additional 60 min at reflux before diluting with 106 g MEK to 55% theoretical solid content. The AB-block copolymer had 55.5% solid content at viscosity Z1 + 1/2 with a molecular weight of Mn = 4370 and Mw = 6200. 2.1.3. Example 3: AB-block BMA//BMA/MAA = 47.5//23.7/28.8 by weight from GTP An amount of 350 g of tetrahydrofuran and 1 g of p-xylene was charged to the reaction flask to which

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300 ml of a 1 M solution catalyst tetrabutyl ammonium m-chlorobenzoate (TBACB) in acetonitrile were added. Initiator, 1,1-bis(trimethylsiloxy)-2-methyl propene (20 g) was injected. A feed of 300 ml of 1 M TBACB in acetonitrile was added over 150 min and a second feed of 61.1 g BMA and 136 g of trimethylsilyl methacrylate added over 20 min. Two hundred minutes after the start of the feed, more than 99% of the monomers were reacted. Next 121.8 g of BMA was added over 30 min. After 400 min, 55 g of dry methanol was added and distillation started. An amount of 112 g of solvent (98 g theoretical of methoxytrimethylsilane (bp = 54 ◦ C)) was removed. The AB-block copolymer had a solid content of 51% with Mn = 4000 and Mw = 4250. 2.1.4. Example 4: MMA dimer from CCTP An amount of 500 ml of MMA was mixed with 500 ml of acetone and the blend was purged with argon for 2 h at 72 ◦ C. Then 170 mg of cobalt(II) complex and 500 mg of Vazo 64 were added and the blend kept for 2 h after which the unreacted MMA and acetone were distilled off. Under 0.004 bar at 53 ◦ C, 150 g of MMA dimer was obtained. 2.1.5. Example 5: Synthesis of a hydroxyl functional acrylic emulsion with composition BMA/BA/HPMA = 80/10/10 by weight An amount of 200 g of deionized water, 1 g of sodium bisulfite and 2.35 g Disponil AES 60 (an alkylaryl polyglycol ether sulfate sodium salt from Henkel) were heated to 80 ◦ C under nitrogen purge. A monomer pre-emulsion (emulsified at high speed) consisting of 640 g of BMA, 80 g BA, 80 g HPMA, 6 g ammonium persulfate, 44.7 g Disponil AES 60 and 525 g of water were fed over 180 min at 80–82 ◦ C. The mixture was held for an additional 60 min at 85–87.8 ◦ C after which 2.44 g of t-butyl hydroperoxide in 5 g of water were added. An amount of 2.44 g of sodium formaldehyde sulfoxylate dissolved in 20 g of water were added over about 1 h and the mixture held for an additional hour at 85–87.8 ◦ C. In this standard recipe, 2 wt.% surfactant was used on monomer concentration. 2.1.6. Example 6: Synthesis of a low molecular weight latex with MMA dimer as chain transfer agent An amount of 20 g of a 10% aqueous solution of Empicol ESB 70 (lauryl ethoxy sulfate from Rhodia) and 353 g of water were heated in a reactor to 85 ◦ C. Then 20 g of MMA and 0.4 g of MMA dimer from example 4 were added and the mixture stirred for 5 min. The reactor contents were initiated by addition of 25 g of a 2 wt.% aqueous solution of potassium persulfate. Then 80 g of MMA with 1.6 g of MMA dimer were added over 30 min and the reaction mixture kept for an additional 2 h. 2.2. Analytical Particle size distribution was determined by means of dynamic light scattering using photon correlation spectroscopy.

177

Measurements was done on a Coulter N4 plus (Coulter corporation) for the nucleation experiments with block copolymers. For the other experiments a Brookhaven BI90 particle size analyzer was used. All samples were diluted with deionized water and filtered. Molecular weight distributions were measured in a dilute tetrahydrofuran solution by gel permeation chromatography (GPC) using polystyrene as standards for calibration (instrument from Waters with refractometer as detector). Absolute molecular weights were determined by GPC coupled with multiangle light scattering (GPC-MALLS). The combination of GPC with a MALLS photometer (miniDAWN from Wyatt Technology Corporation) enables to determine the absolute molecular weight distribution and averages without column calibration as well as the root mean square (RMS) radius distribution. The slope of the plot of RMS versus molecular weight gives information about the polymer architecture (linear versus branched). A slope of 0.5–0.6 corresponds to linear polymers while less than 0.5 indicates the presence of branched molecules. 3. Results and discussion 3.1. AB-block copolymers as copolymerizable ‘surfactants’ Emulsion polymerization is a free radical polymerization technique in which monomers, dispersed in water and stabilized by surfactants, are converted to a polymer. Surfactants are surface active agents, consisting of hydrophilic (polar) and hydrophobic (non-polar) segments. When the surfactant concentration reaches a threshold value, i.e. the critical micelle concentration (CMC), the surfactants form aggregates (mostly spherical) which are known as micelles. The initiator is water soluble and forms hydrophilic free radical species, which propagate in the water phase to a certain degree of polymerization before being capped by the micelles, which is the main locus of polymerization. In the beginning of the emulsion polymerization process (nucleation stage), monomer is converted to polymer in only a fraction of the micelles and particles grow. Surfactant molecules from the water phase diffuse to the surface of the particles till the surfactant concentration reaches a value below the CMC. In this first stage the number of particles (loci) reaches a constant value. In the second stage of the polymerization process, monomer is transported from droplets to the loci of polymerization where the concentration is kept constant. A drop in polymerization rate characterizes the last stage since monomer droplets have disappeared. It is well known that emulsion polymerization can be run in the absence of surfactants. The growing oligomers from water-soluble initiator fragments will become surfactants to stabilize nucleating and growing particles (homogeneous nucleation). In waterborne applications where acrylic emulsions are part of the binder system, the surfactants often give problems related to the water solubility (humidity resistance)

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and surface activity (adhesion, wetting) of those species. Furthermore, acrylic emulsions based on copolymers with a high glass transition temperature (Tg ) needed to get sufficient physical drying and early hardness development, are not film-forming unless high amounts of plasticizers or coalescing solvents are used. Plasticizers or coalescing solvents remain in the coatings film as volatile materials since they are not reactive versus crosslinkers, such as polyisocyanates and melamine resins used in automotive coatings. High acid and/or hydroxyl-containing acrylic copolymers are water soluble and are film-forming out of the water phase even at high Tg since water hydroplasticizes the copolymer. Surfactant-free emulsions can be prepared from such low molecular weight, random copolymers if high amounts are used on total polymer composition. The copolymer stabilizes the latex particle through adsorption but may also be linked chemically through unsaturated reaction sites. A way of generating chemical grafting sites on the stabilizing copolymer, is e.g. through reaction of a glycidyl functional monomer (glycidyl methacrylate) with the acid functional copolymer. This way is not ideal since macromonomers will have a statistical distribution of polymerizable groups where the non-functional will not have a double bond to link to the particle and the more than one functional result in crosslinking and (micro or macro) gel formation. CCTP is a way to prepare the same type copolymers in a one step process and guarantees one end double bond per polymer chain. Random macromonomers with terminal double bond have been shown to function as copolymerizable, polymeric surfactants at lower concentrations than comparable copolymers at the same composition and molecular weight [5], however, the concentrations needed are still higher than typically used in a surfactant-containing emulsions. AB-block copolymers (A-block water soluble and B-block hydrophobic) should give more ‘surfactant’ behavior compared with random copolymers at the same overall composition and molecular weight. The purpose of the first part of this study was to find out if random macromonomers differ from AB-block structures in particle nucleation efficiency at low concentrations. In this comparison, two surfactant-containing emulsions were included as a reference for an AB-block oligomer as described

Table 1 Surface tension (mN/m) as a function of concentration for a random, AB-block macromonomer compared with two surfactants Concentration

Block

Random

Polystep AU-5

Disponil AES60

0.01 0.01 0.05 0.0265 0.25 0.50 1 5 10

65.5 65.3 – 58.3 51.5 49.3 47 43.7 42.3

53.9 43.4 – 40.1 35.6 34.3 33.2 31.9 –

– 39 30.1 28 30.9 30.9 31 – –

– – 41.2 40 – 40.6 40.5 40.3 40

in example 1. (Disponil AES60 is an alkylaryl polyglycol ether sulfate sodium salt and Polystep AU-5 is a polymerizable surfactant from Stephan, described as an allylamine salt of lauryl alcohol sulfate.) A random macromonomer with composition BMA/MAA = 62/38 was prepared with cobalt(III) CTA (Fig. 1B) in a one step process. Mn was comparable with AB-block oligomer example 1 (1370) with Mw (2900) and polydispersity somewhat lower. The macromonomers were dried, neutralized with potassium hydroxide (KOH) (stoichiometric on MAA) and inverted in deionized water to form a stable oligomer dispersion at 25% solids. The degree of polymerization of MAA in the A-block is about 14. Poly-MAA at this Mw neutralized with KOH is fully water soluble while the DP of the hydrophobic poly-BMA segment was about 12. In Table 1, the surface tension of the two reference surfactants is given compared with the random and block oligomers. The CMC values of the commercial surfactants are below 0.1 wt.% and correspond to literature values (Polystep AU-5, 0.07%; Disponil AES60, 0.07%). The surface tension of both AB-block and random macromonomer level off at a concentration of about 1%. It is not proven if such low molecular oligomers really form micelles or if there is just some agglomeration into aggregates or clusters. The oligomers were investigated for nucleation efficiency in a typical recipe as in example 5. The reactor was charged with AB-block and random macromonomer dispersion

Fig. 5. Initial particle nucleation of Disponil AES60 (- - -), Polystep AU-5 (—) and AB-block oligomer (· · · ) (particle size (nm) as a function of time (min)).

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Table 2 Measured solids, molecular weight distribution Mn , Mw (×1000), d, particle size (PS) and RMS radius of latex of example 5 polymerized at different solids level Surfactant

Polystep AU-5 AB-block Polystep AU-5 AB-block Disponil AES60 AB-block Disponil AES60

Solids

29.8 29.8 39.2 40 40 50.7 51.8

GPC

GPC-MALLS

Mn

Mw

d

Mn

Mw

d

68 56 43 53 45 49 55

284 283 250 340 301 249 344

4.2 5 5.8 6.4 6.6 7.1 6.2

128 108 127 113 149 138 140

488 701 429 765 486 747 562

3.8 6.5 3.4 6.8 3.3 5.4 4

(25%) so that the amount was 3% on monomer (concentration well above the CMC) and particle size was measured as a function of time (Fig. 5). Disponil AES60 and Polystep AU-5 were included as a reference with 0.05% on total monomer in reactor charge. The results of Fig. 5 show that the AB-block oligomer is active in particle nucleation. The random oligomer resulted in a bimodal particle size distribution after 5 min feed with larger particles above 3000 nm. The AB-block copolymer prepared by GTP (example 3), with a somewhat different composition as the AB-block from CCTP was also confirmed to be effective in particle nucleation resulting in a slightly lower particle size (50 nm after 15 min). In a next study, the hydroxyl functional latex of example 5 was prepared at different solids level (30, 40 and 50%) using the different stabilizers. The composition of this latex is BMA/BA/HPMA = 80/10/10 by weight and could serve as a model for a latex used in an OEM water-based basecoat (Tg = 14 ◦ C, OH value 39). The latex is a hydrophobic type with some hydrophilicity coming from HPMA (introduced to create sites for crosslinking with, e.g. melamines/polyisocyanates) which is a water-soluble, hydroxyl functional monomer. It is expected that HPMA contributes to particle nucleation through a homogeneous mechanism (poly-HPMA is soluble in water to a DP about 5). Formulators of waterborne paints want the binders at the most favorable solids/viscosity since higher solid content should result in shorter drying times since less water has to evaporate. The purpose was to find out if the AB-block

rw (nm)

PS (nm)

0.42 0.26 0.38 0.24 0.40 0.23 0.42

– 137 191 155 – 183 163

oligomer is grafted on the polymer particle through the terminal vinyl bond. If the macromonomer copolymerizes with the backbone monomers, one expects to form a highly branched polymer with broad Mw distribution. GPC is not a good technique to determine exact molecular weights of highly branched polymers inherently coupled to the fact that molecular weights are separated through retention on a column and calibration done with linear standards. A highly branched polymer occupies less volume than a linear one at the same overall molecular weight so retention in the pores of a GPC column will be different. GPC-MALLS does not suffer from this drawback and allows the determination of exact molecular weight as well as the RMS radius of gyration. Table 2 summarizes the results of the GPC and GPC-MALLS measurements of latex of example 5 at different solids level and stabilized with Disponil AES60, Polystep AU-5 and AB-block oligomer. The measured solids level of different samples is close to the theoretical one indicating high conversion. All lattices were very low in coagulum, good mechanical and electrolyte (NaOH 10%) stability and gave transparent films on drawdowns. The molecular weight GPC versus GPC-MALLS indicates the lattices stabilized with the AB-block oligomer to be highly branched with substantial higher Mw and lower RMS radius. Figs. 6 and 7 give the plots of differential molecular weight fraction versus molar mass and the RMS radius versus molar mass for AB-block, Disponil AES60 and Polystep AU-5 in the latex at 40% solids level.

Fig. 6. Differential weight fraction vs. molar mass in latex of example 5 at 40% solids: Disponil AES60 (䉬), Polystep AU-5 (䊉); AB-block oligomer (+).

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Fig. 7. RMS radius vs. molar mass in latex of example 5 at 40% solids: Disponil AES60 (䉬); Polystep AU-5 (䊉); AB-block oligomer (+).

The larger particle size of the latex with increasing amount of AB-block macromonomer could be explained by the water-soluble monomer HPMA. At a lower overall water content the partitioning of the water-soluble monomer over water and particle phase is different (more in the particle phase) so that less water-soluble monomer is available for homogeneous nucleation. The film drawdowns of the lattices stabilized with AB-block versus Disponil AES60 at 50% solids level showed a difference in water absorption in favor of the AB-block (27% for Disponil AES60 versus 14.5% for AB-block). 3.2. Control of molecular weight and particle size of lattices with dimer oligomers from CCTP The film-formation properties of a coating based on a latex are determined by several factors. Small particle size and low molecular weight improve the coalesce of the particles after the water has evaporated. The particle size of a latex is related to the number of particles formed. For micellar nucleation, Smith and Ewart derived an equation relating the number of particles N formed to surfactant (S) and initiator concentration (I) [10]: N ∼ S 3/5 I 2/5 Increasing surfactant and/or initiator concentration to get smaller particle size increases the amount of hydrophilic components in the final formulation which often negatively influence paint properties. The degree of polymerization DP is proportional to the rate of polymerization Rp that is related to the number of particles N: Rp = nkp [M]

N Na

In this relation kp is the propagation rate constant, [M] the monomer concentration in the particle, Na the Avogadro’s number and n is the average number of radicals per particle. Increasing the number of particles will result in an increase in polymerization rate and thus molecular weight,

which is opposite of the film-forming properties. Classical CTAs, such as halogens and sulfur derivatives to lower molecular weight are not very efficient in emulsion polymerization unless used at high concentrations levels which affect latex properties as color, odor and monomer conversion. Use of certain long chain mercaptans, such as CTA many times result in larger particle size because of more monomer droplet nucleation. The cobalt complexes of Fig. 1 are efficient in the control of the molecular weight of an emulsion if the solubility in the particle phase is sufficiently high and when used with azo initiators. Oligomers from CCTP do not suffer from those disadvantages. The chain transfer activity of ␻-unsaturated oligo(methyl methacrylate) has been studied extensively and proven to be dependent not only on the propagating monomer but also of the oligomer chain length [9]. The chain transfer constant of ␻-unsaturated oligo(methyl methacrylate) for MMA, which is the ratio of chain transfer over propagation rate, increases from 0.007 for dimer to 0.13 for trimer to 0.23 for tetramer. The activity of a CTA in an emulsion polymerization process is more complicated than the chain transfer activity itself and is also determined by the partition coefficients of the CTA in water versus particle, the diffusion rate constant of oligomer entry and exit in and out of the particle and the copolymerization reactivity ratios of oligomer and propagating monomer. Those factors are determined by water solubility, molecular weight of the oligomer and homopolymerization versus crosspropagation rate constants. The value of ␻-unsaturated oligo(methacrylates) in the control of molecular weight of emulsion copolymers was already recognized and exploited in a series of Dupont patents [11]. In those patents, the oligomers were used in lattices with surfactants well above the CMC and no effects on particle size were reported. In this second part, we intended to compare the activity of dimers in emulsions with low surfactant concentration. Dimers were chosen since it was expected that such low molecular weight species would show high mobility between aqueous and particle phase to influence particle size through homogeneous nucleation [16]. In this study ␣-methyl styrene dimer was compared with methyl methacrylate dimer for an emulsion polymerization of styrene (S) and MMA according to example 6. ␣-Methyl styrene dimer has been described as CTA before [12] but its use so far was limited since the existing methods of preparing this dimer are expensive. It has been demonstrated recently that ␣-methyl styrene dimer can be prepared from CCTP with high yields [13]. In Tables 3 and 4 the results of the study are reported to demonstrate the efficiency of dimer CTAs versus cobalt CTA (Fig. 1A) in a recipe as in example 6. Cobalt CTA (Fig. 1A) has a higher solubility in an organic phase compared to water [15]. The results from Table 3 show that dimers and cobalt CTA are effective in lowering the molecular weight and particle size of a poly-MMA emulsion. This efficiency is lost when peroxide initiator is used in combination with cobalt CTA. An addition of 2% MMA dimer to a formulation has the

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Table 3 Emulsion polymerization of MMA according to example 6 in the absence and presence of MMA dimer and cobalt CTA (Co, Fig. 1A)a Reaction parameters 85 ◦ C

(A) 0.56% Na2 S2 O8 , no CTA, 1% surfactant, (B) 0.5% azocarboxy K-salt, no CTA, 2% surfactant See (A), 60 ppm Co See (B), 30 ppm Co See (B), 60 ppm Co See (D), 2% MMA dimer (C) 0.5% azocarboxy Na-salt, no CTA, 6% surfactant, 85 ◦ C (D) 0.5% K2 S2 O8 , no CTA, 2% surfactant, 85 ◦ C a

Particle size (nm)

Mn

Mw

d

78 71 63 54 52 49 47 64

168000 20500 155000 25700 5800 12800 128000 10400

569000 604000 350000 48000 10000 32000 479000 320000

3.4 2.9 2.3 1.9 1.7 2.5 3.8 3.05

Azocarboxy K-salt or Na-salt is 4,4 -azobis(4-cyanopentanoic acid) neutralized with KOH or NaOH, respectively.

same effect on particle size as a triple increase in surfactant concentration. The cobalt CTA also allows narrowing down the molecular weight distribution. The chain transfer activity of cobalt complexes for S is several orders of magnitude lower than for methacrylates since there is no methyl group available for hydrogen abstraction [17]. As shown in Table 4, MMA dimer also has a limited effect on molecular weight. This can be explained by the differences in reactivity ratio (r1 /r2 homopolymerization versus crosspropagation) of MMA dimer (monomer 1) and S (monomer 2) [18]: r1 = 0.29,

r2 = 1.46

The growing styrene radical prefers to add S molecule versus MMA dimer, which is much different versus a copolymerization of S and MMA, where the reactivity ratios indicate alternating additions (r1 = 0.46, r2 = 0.52). ␣-Methyl styrene dimer is much more efficient as CTA for S and shows the same effect on both molecular weight and particle size distribution as MMA dimer does for MMA emulsion polymerization. 3.3. Structured polymers as pigment dispersants for waterborne coatings Polymeric dispersants adsorb onto pigment surfaces during a dispersion process. There are several mechanisms by which the dispersants anchor to the pigment surface. The choice of anchor groups depends on the surface characteristics of the pigment. In most cases, acid or basic groups are effective as anchoring groups for inorganic pigments but more specific H-bonding functionalities may be used for organic pigments. The polymer chains are sticking out from the pigment particle and form a molecular layer around the

pigment, which prevents the particles from reagglomeration through steric stabilization. Structured polymers like graft and AB-block copolymers do a better job as pigment dispersants compared to random copolymers if the polymer is designed in such a way that one part of the molecule contains the anchoring groups and the other part provides for the steric stabilizing barrier. The design of a pigment dispersant for waterborne applications is more difficult since the stabilizing polymer chain has to be water soluble or dispersible. If the water solubility is obtained through anionic (or cationic) groups, the stabilizing mechanism is both electrostatic and steric. In anionically stabilized binders, the acid groups on the polymer chain will compete also for the pigment surface. The anchoring part of the polymeric dispersant must on the other hand not be too polar to be water soluble, needs to be specific to adsorb selectively on the pigment surface but also avoiding self-aggregation to micelles. Block and graft copolymers prepared by esterification reaction of hydroxyl functional PEOs (water soluble) with backbone prepolymers (to anchor to pigment surface) have been evaluated versus random structures [19]. PEO in paint formulations many times result in problems as compatibility, lower hardness (because of low Tg ) and humidity resistance. CCTP is a unique technique to make structured polymers with functional monomers (hydroxyl, amine, acid) which can be placed on the polymer segments of choice. In a first set of experiments, it was tried to find out if an AB-block structure prepared by a real living polymerization technique (GTP) compared with a semiblock from CCTP. In Table 5, block GTP and block CCTP were structures prepared according to examples 2 and 3. In this table, a commercial random copolymer was included as a reference as well as a graft dispersant from CCTP. All copolymers were

Table 4 Emulsion polymerization of S according to example 6 in the absence and presence of MMA dimer/␣-methyl styrene dimer Reaction parameters (A) See See See

80 ◦ C,

0.5% K2 S2 O8 , 1.5% surfactant (A), 2% MMA dimer (A), 0.5% ␣-methyl styrene dimer (A), 2% ␣-methyl styrene dimer

Particle size (nm)

Mn

Mw

d

78 70 58 54

20000 40000 22500 9700

313000 121000 70200 28900

2.61 3.03 3.12 2.97

182

J. Huybrechts et al. / Progress in Organic Coatings 45 (2002) 173–183

Table 5 Random, graft and AB-block copolymer as pigment dispersants from different synthetic processes Architecture

Process

Acid arm (%) or A-block

Total acid

Random Block Block Graft

Free radical GTP CCTP CCTP

0 37 37 30

10 13 13 11.5

Table 6 Tint strength and average flocculation rating of dispersants of Table 5 for three different pigments Architecture

Iron oxide

Perrindo maroon

Phthalocyanine blue

Flocculation average

Random Block GTP Block CCTP Graft CCTP

100 29 104 93

104 – 81 79

96 73 84 87

4 0 2 0

prepared with standard monomers selected from methacrylates (BMA, MAA) and acrylates (BA, AA). The dispersants of Table 5 were evaluated in a typical dispersion for a waterborne basecoat. The dispersants are neutralized with 2-amino-1-propanol and optimum pigment and dispersant concentrations are determined by viscosity measurements. At a given pigment concentration, the optimum amount of dispersant needed is the lowest viscosity in the plot of viscosity versus percentage of dispersant. At this concentration, dispersant and any excess ends cover the pigment surface up in solution. The quality of the pigment dispersion can be judged versus many procedures and test conditions. Two important parameters are the tint strength and flocculation rating. The tint strength is a number, which measures the hiding power of the dispersion. The higher the number, the better the dispersion and the lower the amount of pigment dispersion needed in the final paint to give sufficient hiding power at low film builds. The flocculation rating (the lower, the better) is a relative number, which measures the strength by which the dispersant is anchored to the pigment surface in a paint environment. Many times, coalescing solvents, such as butylcellosolve used in the paint formulation displace the dispersant from the pigment surface and cause the pigments to flocculate resulting in a loss of hiding power. Table 6 gives the tint strength values and average flocculation ratings for the dispersants of Table 5 evaluated

for three different pigments, one inorganic iron oxide and two organic Perrindo maroon and phthalocyanine blue. It is clear from the results of Table 6 that there is no specific advantage using GTP versus CCTP in the block polymer structure. Furthermore, there are no overall specific advantages using structured polymers versus random. From those results, one might conclude an optimum dispersant to be a graft structure for flocculation rating and random for tint strength. The graft dispersant of Table 5 was modified in which acid percentage in the arm and the polarity of the backbone was varied. In this study, the molecular weight of the macro was kept the same (Mn = 2000, Mw = 3800) and as well as the overall molecular weight of the graft copolymer dispersant (Table 7). The acid used in the backbone is acrylic acid while the macromonomer/arm is based on methacrylic acid. The compositions are chosen such that the macromonomer is more water soluble or dispersible versus backbone. The dispersants were evaluated for the same pigments as showed in Table 6. The results are summarized in Table 8. The best overall rating for the balance of tint strength and flocculation is dispersant with a graft copolymer structure 3 where the backbone is polar and modified with acid groups. To compare again the effect of a structured polymer versus a random, both dispersants were used to formulate a titanium dioxide white pigment dispersion. The graft dispersant 3 from Table 7 allowed to prepare a dispersion with 73% pigment load versus 60% with the random. Furthermore, the ratio dispersant/pigment was 2.15 for graft and 5 for random indicating there is substantial less dispersant needed for a structured polymer. The particle size distribution of both pigment dispersions is given in Fig. 8. The dispersion stabilized with a random copolymer dispersant shows significant amounts of pigment aggregation and flocculation versus graft which affords a much more even particle size distribution.

Table 8 Tint strength and average flocculation rating of dispersants of Table 7 for three different pigments Architecture

Iron oxide

Perrindo maroon

Phthalocyanine blue

Flocculation average

Graft Graft Graft Graft

93 109 107 106

79 101 101 102

87 97 111 110

0 2.3 1 2.3

1 2 3 4

Table 7 Graft copolymer dispersants from CCTP with different ratio arm/backbone and polarity of the backbone Architecture

Process

Acid arm (%)

Total acid

Backbone/arm

Backbone

Mn /Mw

Graft Graft Graft Graft

CCTP CCTP CCTP CCTP

40 40 40 30

12 12 18 9

70/30 70/30 70/30 70/40

Apolar Polar Polar + acid Polar + acid

6500/22600 5900/20100 6600/22900 7500/27700

1 2 3 4

J. Huybrechts et al. / Progress in Organic Coatings 45 (2002) 173–183

183

Fig. 8. Particle size distribution of a titanium white pigment dispersion stabilized with a random (A) and a graft (B) copolymer dispersant.

4. Conclusions

References

CCTP is a powerful technique to prepare acid functional graft and AB-block copolymers and macromonomers in a conventional free radical process. AB-block macromonomers function as surfactants during emulsion copolymerization at concentrations much lower than random macromonomers. Graft copolymers from CCTP are superior pigment dispersants which allow to formulate of pigment millbases with high pigment load and low dispersant content. The cobalt CTA from CCTP are effective CTAs in emulsion copolymerization lowering, particle size molecular weight and polydispersity at low concentrations. Dimers from CCTP do show the same effect and can be used with less expensive peroxide initiators. AB-block copolymers from GTP with narrower molecular weight distribution and control of structure do not give advantages versus CCTP in the above studies.

[1] D. Kukulj, T.P. Davis, Macromol. Chem. Phys. 199 (1998) 1697. [2] A. Gridnev, W.J. Simonsick Jr., S.D. Ittel, J. Polym. Sci. A: Polym. Chem. 38 (2000) 1911. [3] A. Gridnev, J. Polym. Sci. A: Polym. Chem. 38 (2000) 1753. [4] Proceedings of the Conference on Commercialization of Controlled Polymer Synthesis, Cambridge, MA, USA, 4–5 December 2000. [5] J. Huybrechts, P. Bruylants, A. Vaes, A. De Marre, Prog. Org. Coat. 38 (2000) 67. [6] J. Huybrechts, et al., US Patent 5 936 026 (1999). [7] G. Riess, Colloids Surf. A: Physicochem. Eng. Aspects 153 (1999) 99. [8] D.M. Haddleton, D.R. Morsley, J.P. O’Donnell, S.N. Richards, J. Polym. Sci. A: Polym. Chem. 37 (1999) 3549. [9] D.S. Harrison, The chemistry of ␣-unsaturated oligomers and polymers, Master’s Thesis, Swineburn Institute of Technology, June 1988. [10] W.V. Smith, R.H. Ewart, J. Chem. Phys. 16 (1948) 592. [11] C.T. Berge, et al., US Patent 5 362 826 (1994). [12] W. Masafumi, et al., US Patent 5 637 644 (1997). [13] A. Gridnev, US Patent 6 294 708 (2001). [14] D. Kukulj, J.P.A. Heuts, T.P. Davis, Macromolecules 31 (1998) 6034. [15] D. Kukulj, T.P. Davis, R.G. Gilbert, Macromolecules 30 (1997) 7661. [16] M.C. Grady, R.R. Matheson Jr., J. Comput.-Aided Mater. Design 3 (1996) 296. [17] D. Kukulj, T.P. Davis, Macromol. Chem. Phys. 199 (1998) 1697. [18] K.J. Abbey, G.M. Carlson, D.L. Trumbo, M.J. Masola, R.A. Zander, J. Phys. Sci. A: Polym. Chem. 31 (1993) 3417. [19] E. Reuter, S. Silber, C. Psiorz, Prog. Org. Coat. 37 (1999) 161.

Acknowledgements The authors would like to thank Huub Van Aert from Agfa Gevaert for the study of cobalt CTA and dimers in emulsion polymerization. They are also grateful to Alexei Gridnev, Charles Berge and Mike Grady for the many discussions in the field of CCTP.