Synthesis of crosslinkable acrylic latexes by emulsion polymerization in the presence of etherified melamine-formaldehyde (MF) resins

ELSEVIER Progress in Organic Coatings 28 (1996) 133-141

PROGRESS IN ORGANIC COATINGS

Synthesis of crosslinkable acrylic latexes by emulsion polymerization in the presence of etherified melamine-formaldehyde (MF) resins

Yun Huang, Frank N. Jones Coatings Research Institute, Eastern Michigan University, 430 West Forest Street, Ypsilanti, MI 48197, USA

Received 20 July 1994; accepted 9 August 1995

Abstract

The feasibility of making crosslinkable latexes by emulsion polymerization in the presence of etherified melamine-formaldhyde (MF) resins was demonstrated. In favorable cases the resulting latexes crosslinked at ambient temperatures to give tough, solvent-resistant films. High levels of acid catalyst are required. A monomeric, hydrophobic MF resin etherified with about five moles of methanol and one mole of isooctanol gave better overall results than monomeric MF resins etherified entirely with methanol, which are more hydrophilic, or with polymeric MF resins etherified with butanol, which are less miscible in acrylic copolymers with compositions similar to the crosslinkable latex.

Keywords: Crosslinkable acrylic latexes; Emulsion polymerization; Melamine-formaldehyde resins

1. Introduction

In 1978-1979 reviews Bufkin and Grave [ l-61 cited hun- dreds of references on crosslinkable latexes made by emulsion polymerization, and since then the literature on the subject has proliferated. The aim of most of this effort has been to improve the physical properties of coalesced latex films over the levels attainable with thermoplastic latexes. ’

While thermoplastic latexes dominate the market for archi- tectural paints, they lack the hardness, toughness and resistance to acids, bases and solvents required of many industrial and special purpose coatings. Crosslinking can improve these properties. A substantial fraction of the industrial and special purpose markets are still served by water-reducible coatings, which are also crosslinkable and can have the desired film properties plus the advantages of better film homogeneity and

’ Note on terminology. In this paper ‘latex’ means a dispersion of polymer particles in water. ‘Emulsion’ means a dispersion of liquid in water. ‘Emulsion polymerization’ is a process that starts with liquid monomers emulsified in water and ends with a latex. ‘Thermoplastic latex’ means a latex which forms a film by coalescence but cannot chemically crosslink. ‘Crosslinkable latex’ means a latex which is capable of chemically crosslinking after coalescence. ‘Waterborne coating’ means any coating in which most of the volatile medium is water. ‘Latex coating’ is a category of waterborne coatings in which the polymer is a latex made by emulsion polymerization. ‘Water-reducible coating’ is a second category of waterborne coatings in which the polymer is made as a solution in organic solvent and then water is added; the polymer usually forms a small-particle size dispersion, and thus qualifies as a latex, but it is not called a latex here.

0300-9440/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SDJO300-9440(95)00600-J

gloss. However, water-reducible coatings contain substantial levels of organic solvent. It is foreseeable that VOC regula- tions will eventually become so restrictive that the water- reducible coatings in use today will exceed allowable VOC levels. One of the alternatives will be crosslinkable latexes, which can be made with very low levels of solvent. Thus, there are powerful incentives to find crosslinkable latexes that can do the job of water-reducible coatings.

Self-crosslinking latexes can be made by copolymerizing a monomer with a crosslinking site, for example methallyl methacrylate, or a pair of reactive monomers, for example butoxymethyl acrylamide and hydroxyethyl acrylate, into a single latex [ 1,7]. However, this approach may present syn- thetic problems, and it limits the degrees of freedom subsequently available to the paint formulator. A more widely used approach has been to copolymerize one reactive monomer, for example a hydroxy or carboxy functional acrylate, into the latex and to add an external crosslinker, for example an amino, epoxy, aziridine, or /Shydroxyalkylamide resin, to it after the emulsion polymerization is completed [ 3,4,8]. Recent patent literature suggests that the crosslinkers most widely used in this approach are amino (aminoplast) resins, often etherified melamine-formaldehyde (MF) resins [ 9- 12].

MF resins are the class of crosslinkers most widely used in industrial coatings. As reviewed by several authors [ 13- 151, they are made by treatment of melamine with formal-

134 Y.Huang, F.N. Jones /Progress in Organic Coarings 28 (1996) 133-141

dehyde and subsequent etherification of the adduct with an alcohol. This procedure lends itself to production of a wide range of resins. Number average molecular weight ranges from less than 1000 to well over 10 000. Crosslinking characteristics depend on the degree of formylation, the degree of etherification, the alcohol selected and molecular weight. Of particular interest in this study is that the resins may range from hydrophilic, water-soluble materials to hydrophobic, water-insoluble resins. As described [ 131, hydrophilic char- acter is imparted by incomplete formylation, incomplete etherification, methanol as etherifying alcohol and low molecular weight, while complete formylation, complete etherification, high molecular weight and use of butanol as the etherifying alcohol can make the resins very hydrophobic.

A procedure commonly described for making melamine crosslinked latexes involves (i) emulsion copolymerization with monomers including a hydroxy-functional acrylic fol- lowed by (ii) mixing the MF resin into the resulting latex [ 9-121. This procedure has, in some cases, proven workable, but it poses problems. If a water-soluble MF resin is selected, most of it will remain in the aqueous phase of the latex with the potential for non-uniform crosslinking and perhaps chemical instability. If, on the other hand, a hydrophobic MF resin is used it may separate, emulsify with stray surfactant and/ or associate with latex particles - at the surface if it is incompatible with the latex and perhaps slowly suffusing the particles if it is compatible. This situation might also cause non-uniform crosslinking, mechanical instability, and a situation where the characteristics of the latex change with time as the system slowly seeks equilibrium. The patent literature suggests that, in practice, moderately hydrophilic MF resins, such as fully formylated, fully methylated low molecular wzight types, are chosen [ 9-121.

The questions of how such melamine resins partition among the continuous phase, the latex particles and the inter- phase, and of how fast the equilibrium is established appear unanswered. Grawe and Bufkin classified such materials as ‘interfacially crosslinked coatings’ on the assumption that the “microscopic crosslink density is heterogeneously dispersed and increases in an outward direction from the particle core to the shell area” [ 41. This assumption is plausible, but it is hard to prove, and there may be substantial differences in the degree of heterogeneity from case to case, possibly making it difficult to obtain reproducible final properties.

This paper describes an alternative process for making crosslinkable latex/MF resin blends and an initial study of the properties of the products as coatings. In this process emulsion polymerization is effected in the presence of the melamine resin. We are not aware of any previous published reports of such a process. We will show that: (i) free-radical initiated chain polymerization can be effected in the presence of MF resin; (ii) premature crosslinking can be prevented by controlling pH; (iii) the process works best when the MF resin is moderately hydrophobic and is compatible with the particular latex polymer being made; (iv) acidified cast films of the best products crosslink with surprising speed, even at

ambient temperatures, to give impact and solvent resistant films. We will speculate that rapid crosslinking and toughness and the good film properties result from the miscibility of certain MF resins in the latex particles leading to facile and relatively uniform crosslinking. Further, we speculate that this method may prove attractive for use in very low VOC coatings, as the MF resin may act as a non-volatile coalescing agent.

2. Experimental

2.1. Materials

Materials were used without further purification. Butyl acrylate (BA), methyl methacrylate (MMA), styrene (Sty), toluene, 2-mercaptoethanol(2-ME), 2-butanone (MEK) ,2- heptanone (MAK), 2-butoxyethanol, ammonium persulfate, tert-butyl hydroperoxide, 2,2’-azobisisobutyronitrile (AIBN) and sodium hydrogen carbonate were purchased from Aldrich. Hydroxyethyl acrylate and Amberlite 200 CH, an H+ ion form cation exchange resin, were obtained from Rohm & Haas.

‘Triton’ surfactants were supplied by Union Carbide: QS-44 is a phosphate anionic surfactant; X-305, X-405 and X-705 are octylphenoxypolyethoxyethanol non-ionic surfactants; X-200 is a sodium salt of an alkylaryl polyether sulfonate; X-301 is a sodium salt of an alkylaryl polyether sulfate. ‘Duponal’ WAQE is sodium lauryl sulfate, supplied by Witco.

Etherified melamine-formaldehyde (MF) resins were ‘Resimenes’ obtained from Monsanto. Trade literature and a personal communication [ 16,171 describe them as follows: R-747 is a fully formylated, ‘monomeric’ MF resin that is etherified entirely with methanol. It is a standard commercial resin of the hexakis( methoxymethyl)melamine (HMMM) type. RF-4518 and CE-6550 are ‘mixed ether’ resins. RF- 45 18 is similar to R-747 except that about one-sixth of the -OCH, groups are randomly replaced by -iso-O&H,, groups. CE-6550 has about 30 mol% of the methyl groups replaced with n-butyl groups. These mixed ether resins have exceptionally low levels of -NH and -NCH20H groups. BM- 75 12 and BM- 1612 are ‘polymeric’ resins that are etherified entirely with n-butanol. BM-7512 is fully formylated and fully etherified; it has very low levels of -NH and -NCH,OH groups and a molecular weight at the high end of the commercial range. BM-1612 is also almost fully butylated, but it is less formylated than BM-75 12 and has a lower molecular weight.

2.2. Preliminary experiments

2.2.1. Solution polymerization in the presence of MF resin To estimate the effects of MF resin on free radical polym-

erization, solventborne acrylic resin was prepared by free radical solution polymerization [ 181, (A) in the absence and

Y.Huang, F.N. Jones/Progress in Organic Coatings 28 (1996) 133-141 135

(B) in the presence of MF resin R-747. (A) A solution of 7.5 g of styrene, 15 g of MMA, 27.5 g of BA, 0.4 g of AIBN, and 0.3 g of 2-ME in 14 g of 2-heptanone (MAK) was added dropwise over 5 h to a four-neck, 250 ml flask containing 14 g of MAK under N2 with stirring at 100 “C. The flask was equipped with a temperature controller, a reflux condenser, a dropping funnel, a gas inlet, a heating mantle and a mechanical stirrer. After addition was complete, the solution was stirred for 4 h at 100 “C. (B ) The above procedure was repeated with the addition of 15 g of MF resin (R-747) to the solution that was added to the flask.

2.2.2. Miscibility of MF resins with a solution acrylic resin Using the procedure described above, 25 g of MMA, 21 g

of BA, 4 g of 2-HEA and 0.3 g of AIBN, were polymerized in 20 g of toluene at 80 “C. The monomer composition of this solution polymer was the same as the composition used to make crosslinkable latexes, described subsequently.

To estimate miscibility of this polymer with MF resins, solutions of 10 g of acrylic resin, 3 g MF resin and 5 ml of mixed solvents (acetone, toluene and/or n-butanol) were prepared in a 100 ml round bottom flask. All solutions were transparent. Solvent was removed at reduced pressure using a rotary evaporator at maximum temperature of 40 “C. Mixtures that gave transparent residues with no appearance of phase separation were judged miscible. Mixtures that gave translucent or opaque residues were judged immiscible.

2.3. Emulsion polymerization in the presence of MF resins

After a number of preliminary experiments a modification of a conventional emulsion polymerization method [ 191 was adopted. The monomers, methyl methacrylate (MMA) , butyl acrylate (BA) and hydroxyethyl acrylate (HEA), are rep- resentative of monomers used in weatherable coatings. Var- ious MF resins were used. Five recipes, detailed in Table 1, were studied. Recipes a-d contain MF resin. Recipe e was designed for comparison studies in which MF resin is added after polymerization. Two polymerization procedures gave useful results. Both involve formation of a seed latex. In Procedure I, the seed latex was formed from 15% of the acrylic monomers and 15% of the MF resin with normal stirring. In Procedure II the seed latex was formed from 15% of the acrylic monomers and all of the MF resin, and the seed latex ingredients were emulsified in a dispersator before introduction into the flask. Best results were obtained with Pro- cedure II, recipe c, so it is described in detail.

2.3.1. Procedure for recipe c Emulsion polymerization was effected in a 500 ml four-

necked flask immersed in a constant temperature water bath and equipped with a mechanical stirrer (armed with a 48 mm crescent shaped Teflon blade), a reflux condenser, a gas inlet and two graduated dropping funnels. A 1.25% initiator solution was prepared by dissolving 0.25 g of ammonium persulfate in 20 g of deionized water. A monomer solution of 25

Table 1 Emulsion polymerization recipes

Recipe

a b c d e

Monomers and water (wt.%) MMA 17.70 BA 14.80 HEA 2.80 MF resin 5.30 Water 59.40

100

Other components (phr a on polymer solids) Buffer: NaHCO, 1.30

Surfactants WAQE 1.20 QS-44 x-305 2.80

Initiators

(NK)&Qs 0.44 (CH,),COOH

T, (talc.) (“C) 10

17.70 17.70 14.80 * 14.80 2.80 2.80 5.30 5.30

59.40 59.40 - - 100 100

1.30 1.30

1.20 1.20 1.20 1.20 2.80 2.80 2.80 2.80

0.44 0.44 0.44 0.10 0.10

10 10 25

21.20 11.30 2.80 5.30

59.40 100

1.30

20.00 16.80 3.20

60.00 100

1 .oo

0.14

10

a phr = parts per hundred.

g of MMA, 21 g of BA and 4.0 g of HEA was prepared. 15% of the monomer solution and 7.5 g of the MF resin were dissolved in a metal beaker and then 64 g of deionized water, 0.7 g of QS-44, 1.6 g of X-305, 0.75 g sodium bicarbonate, 0.01 g of t-butyl hydroperoxide and 30% of the ammonium persulfate solution were added. An emulsion was formed with high speed stirring using a dispersator. This emulsion was quickly charged into the reaction flask with continuous stirring. The emulsion was stirred at 120 rpm for 0.5 h at 85 “C bath temperature. Then a solution consisting of 0.04 g of t-butyl hydroperoxide dissolved in the remaining 85% of the acrylic monomers was added through one dropping funnel at about 0.4 ml min- ’ while the remaining 70% of the ammonium persulfate solution was added through the other dropping funnel at about 0.1 ml min-r. Addition was complete in about 2 h. stirring was continued for 2 h at 85 “C. The latex was cooled to ambient temperature and filtered through tared cheesecloth. The coagulum and grit were dried and weighed. The non-volatile content of the latex was tested by ASTM D 4758-87, and its pH was measured with pH paper.

Procedure II was used for recipe d except that 30 g of MMA and 16 g of BA were used. Procedure II was used for recipe b except that t-butyl hydroperoxide was not used. Pro- cedure I was used for recipes a and e; it was essentially the same as Procedure II except for the differences noted above. A modification of Procedure I in which 85% of the acrylic monomers and MF resin were emulsified in water before addition to the flask did not give reproducible results.

2.4. Formulation and curing studies

Unpigmented latexes were cast as films and cured.

136 Y.Huang, F.N. Jones/Progress in Organic Coatings 28 (1996) 133-141

2.4.1. p-TSA catalyst Varying amounts (0.5-3.0 wt.%) of p-toluenesulfonic

acid (p-TSA) catalyst were added to latexes made from recipes a-d, and pH was measured with pH paper. Coatings were drawn down on 0.32” X 3” X 6” R-36-P phosphate pretreated dull matte steel panels (Q-Panel company) using a No. 24 or No. 36 wire-wound draw-down bar. The panels were baked at elevated temperatures in a forced air oven or were kept at ambient temperature. Dry film thicknesseses were 10-15 pm.

Impact resistance, direct and reverse, was determined with a Gardner impact tester using ASTM D 2794-84. Film thickness was measured by a Mikrotest III thickness gauge (De Felsko Corporation).

3. Results and discussion

3.1. Preliminary experiments

2.4.2. Cation exchange resin (CER) catalysis An alternative method of acidification of the latex was to

pass it through an Amberlite 200 ion exchange resin packed in a glass column with a porous frit at the bottom. A rate which yielded a latex pH of about 2 was used. Films were prepared as described above.

2.4.3. Post-addition of MF resin MF resins were added to reference latex e, using a method

described by DeLong and Shepherd [ 201. MF resin (5 g) was dissolved in 5 ml of 2-butoxyethanol. Water (10 ml) was added dropwise to this solution under high speed agita- tion with a dispersator. The resulting solution or dispersion was added to the latex over 5-10 min. The weight ratio of latex solids to MF resin was 87 / 13.

A requirement for successful emulsion polymerization in the presence of MF resins is that the MF resins do not drastically interfere with free radical initiated polymerization. Further, it was hypothesized that the most desirable situation would be to use MF resins that are soluble in the monomers (all are) and miscible with the latex particle. It was also hypothesized that the best chance of obtaining uniform crosslinking would occur when the MF resin is miscible with the particles but has only limited solubility in water. Water soluble MF resins are likely to partition largely in the continuous phase.

Little information regarding these characteristics of MF resins has been published, so preliminary experiments were carried out. Solution polymerization and solution-made polymers were used as models.

In reporting results, formulations are designated by codes of the form Mxxxx-Y-z-# in which Mxxxx specifies the MF resin used, Y specifies the emulsion polymerization procedure (I or II), z specifies the recipe used to make the acrylic latex, and # specifies the wt.% of p-TSA catalyst (as a % of resin solids) added, when applicable. For example, M747-II- c-l specifies a formulation made from ‘Resimene’ 747 by Procedure II using recipe c and containing 1 wt.% of p-TSA.

2.5. Evaluation methods

Molecular weights and the molecular weight distribution of solventborne acrylic resins were estimated by gel permeation chromatography (GPC) using polystyrene standards for calibration. The GPC apparatus was equipped with a Waters model 5 10 pump, a Waters model R-401 differential refractometer detector and three Waters 105, lo3 and 100 A pore size Styragel columns. The eluting solvent was tetra- hydrofuran at a flow rate 0.9 ml min-‘. Viscosities were determined at 25 “C with a Brookheld viscometer.

3. I. 1. Solution polymerization in the presence of MF resin Free radical initiated polymerization was effected at 100

“C (A) in the absence and (B) in the presence of a widely used MF resin to estimate the effects of MF resin on the process. A chain transfer agent was used to limit molecular weight. Gel permeation chromatography (GPC) of (A) showed a single peak with relative M, =9400 and M,/ M, = 2.0. GPC of (B) showed two slightly overlapping peaks. The long retention-time peak was virtually superim- posable on the chromatogram of the MF resin. The shorter retention-time peak, attributed to the acrylic copolymer, had relative M, = 13 800 and M,,,/M, = 1.7. Viscosities at 25 “C of 56 wt.% solids solutions in MAK were: (A) 330 and (B) 190 mPas.

Non-volatile contents were determined gravimetrically using ASTM D 4758-87 for latexes and ASTM D 2369-87 for solventborne and water-reducible resins. Both methods involve heating at 110 “C in a forced air oven.

Thus, it appears that the MF resin was largely unaffected by free-radical initiated chain polymerization in solution, and that the characteristics of the acrylic polymer were only mod- estly affected by the presence of the MF resin. This result shows that at least one condition for successful emulsion polymerization in the presence of MF resins can probably be met.

3.1.2. Miscibility of MF resins with an acrylic resin Coating hardness was determined by pencil hardness A crosslinkable acrylic copolymer with the same compo-

(ASTM D3363-74) and was designated as the hardest pencil sition to be used for emulsion polymerization was made in that failed to mar the coating surface. Solvent resistance was solution and used to determine miscibility with MF resins. In determined by rubbing the surface of the coating with non- this case no chain transfer agent was used. MF resins R-747, woven paper (‘Kim-Wipes’) saturated with acetone or RF-4518 and CE-6550 appeared entirely miscible with a methyl ethyl ketone (MEK) , and noting the number of dou- crosslinkable acrylic polymer, forming transparent, appar- ble rubs at which the surface of the coating was marred. ently homogeneous solutions. On the other hand, MF resins

Y.Huang, F.N. Jones/Progress in Organic Coatings 28 (I996) 133-141 137

BM-75 12 and BM-1612 were obviously immiscible, forming translucent mixtures. The three MF resins that appear miscible are all mixtures of monomeric and oligomeric species; &i, values are probably under 1000. The immiscible resins, BM-75 12 and BM- 1612, have substantially higher molecular weights; being fully butylated, they probably also have lower solubility parameters than the miscible resins.

This result suggests, but does not assure, that MF resins R- 747, RF-4518 and CE-6550 will be miscible with the latex particles. The latex polymer is expected to have a higher molecular weight and to be less homogeneous than the solution polymer.

3.1.3. Water miscibility of MF resins MF resin R-747, a ‘monomeric’ methyl ether type, has

modest or partial solubility in water. Its solubility is difficult to measure because different species, of differing solubility, are present. MF resins RF-45 18 and CE-6550 are etherified with mixtures of methanol and isooctanol(45 18) or of methanol and n-butanol (6550). They appear immisicible with water, although they probably contain some molecules that are soluble. Resins BM-75 12 and BM- 16 12, which are etherified entirely with n-butanol, are water insoluble. Thus, a range of water solubilities is available for study.

3.2. Emulsion polymerization in the presence of MF resins

The acrylic monomer composition selected for initial study was similar to the one used by Hahn (example 1 of Ref. [ 111) . MMA and BA are common monomers for latex coatings for outdoor use, and HEA is a widely used hydroxy functional monomer to incorporate crosslinking sites. Pro- portions were adjusted to a MMA/BA/HEA weight ratio of 50/42/g so that the theoretical Tg, calculated by the well known Fox equation, would be 10 “C. These proportions were used in recipes a-c and e. For recipe d a 6013218 weight ratio was used to give a calculated Tg of 25 “C.

T, of the acrylic latexes (T,, copo,ymer) was estimated by the well-known Fox [ 211 equation:

1

T g, copolymer

= F + F + . . . g. a a b

where W,, W,... are the weight fractions of monomers a, b,... in a copolymer and Tg, a, Tg, b... are the Tg values, in K, of homopolymers of the respective monomers.

Initial emulsion polymerizations were plagued by formation of excessive grit and coagulum, shown by FT-IR to consist primarily of MF resin. Anionic surfactants ‘Triton’ X-200, X-301, QS-44 and ‘Duponol WAQE’, and non-ionic surfactants ‘Triton’ X-305, 405 and 705 were investigated, as were various buffers, initiators and process conditions. Coagulum and grit were minimized by using a combination of anionic and non-ionic surfactants, by buffering with sodium bicarbonate to pH 5-6, and by aiming for a solids content of about 38wt.%. These steps led to Procedure I, recipe a as described in Section 2 and Table 1.

With Procedure I and recipe a, the amount of coagulum and grit depended strongly on the type of MF resin used. As shown in Table 2, resin RF-45 18, a water insoluble resin that appeared miscible with polymer gave the lowest grit and coagulum. Also shown is solvent rub data, described below, which indicates that only this MF resin gave acceptable levels of crosslinking. The lowest coagulum levels achieved, about 3 wt.%, are undesirably high for latex production.

Since Procedure I and recipe a gave satisfactory results only with mixed ether MF resin RF-45 18, process refine- ments were explored. A phosphate salt surfactant, QS-44, was substituted for sodium lauryl sulfate because it provides additional buffering action [ 191. The potential advantages of adding the acrylic monomers as an emulsion were assessed. However, monomer emulsions containing MF resin were quite unstable, and it proved difficult to assure a uniform feed throughout the two hour addition period.

A second initiator which is soluble in both water and organic media, t-butyl hydroperoxide, was also introduced. This change reduced grit and coagulum and improved con- version.

The most satisfactory methods found are described in Sec- tion 2 as Procedure II and recipes c and d. In this procedure 15% of the monomers and all of the MF resin were pre- emulsified and used to make a seed latex, and the balance of the monomers was added as a homogeneous solution. The pre-emulsions were made with intensive mixing and were continuously agitated before and during polymerization. With this procedure, coagulum and grit was, in most cases, reduced to l-3 wt.%.

With Procedure II it proved possible to get fairly satisfactory results with butylated polymeric MF resins as well as with mixed ether MF resins. Grit and coagulum were reduced to below 4%. The effect of changing procedures and recipes on crosslinkability are shown in Table 3.

The polymers in all these latexes were soluble in common solvents such as acetone. This indicates that little premature reaction of the MF resins with the hydroxyl groups of the acrylic occurred during emulsion polymerization. This is not surprising, as fully alcoholated MF resins are known to require strong acid catalysis to react with hydroxyl groups at

Table 2 Coagulum formation and solvent resistance with different MF resins

Formulation Weight % of coagulum and grit in latex

Acetone rub resistance

I-e- 1 3.2 M747-I-a- 1 5.4 M4518-I-a-1 3.0 M1612-I-a-l 8.7 M7512-I-a-l 8.7

Film thickness: 10 pm. Bake: 150 “C/30 min. Catalyst 1 .O% (solids basis) p-TSA.

4 40

200+ 20 10

138 Y. Huang, F.N. Jones /Progress in Organic Coatings 28 (I 996) 133-141

Table 3 Effect of process changes on emulsion polymerization in the presence of a butylated MF resin

Formulation Acetone rub resistance

150 “C, 30 min 140 “C, 30 min

M7512-I-a-1 M7512-II-b-1 M75 12-B-c- 1

Film thickness: IO pm,

10 10 140 25 200 130

the temperatures used for polymerization [ 13,141. Buffering to pH above 5 is adequate to prevent premature reaction.

3.3. Crosslinking at elevated temperatures

In a preliminary experiment, it was shown that the M45 18- I-a latex did not crosslink appreciably in 30 min at 150 “C when no acid catalyst was added; solvent resistance was only 5 rubs, similar to unbaked or low-baked material. Addition of 0.5% of p-TSA reduced the pH from 6 to 5 and catalyzed good cure (200 rubs) at 150 “C but not at 130 “C. It was concluded that 1% of p-TSA catalyst is the minimum for satisfactory cure rates. This relatively high catalyst level for MF resin cured coatings is presumably necessary to at least partly neutralize residues of the sodium bicarbonate buffer.

The crosslinkable acrylic/MF latexes with 1 .O% of p-TSA catalyst were cured at different temperatures and conditions to compare curing behavior. Solvent resistance, hardness and impact resistance were used to evaluate the coating films. Solvent resistance (acetone or methyl ethyl ketone rub resistance) is a rough, qualitative indication of the degree of crosslinking in the coating. While solvent rub resistance is influenced by Tg, molecular weight and, probably, other fac- tors, it can provide useful comparisons for similar polymers. With the materials studied here, a film that can withstand 200 solvent rubs is considered highly crosslinked, and an un- crosslinked film will withstand 3-5 rubs. Intermediate levels indicate partial or inhomogeneous crosslinking. Impact resistance and pencil hardness were also used as indicators for cure.

Table 4 Film properties of latexes made with RF-45 18 at different bake temperatures

AS shown in Table 2, latex M45 18-I-a- 1, prepared with a mixed ether MF resin, crosslinked much better at 150 “C than Procedure I latexes made from fully methylated or fully butylated MF resins. The substantial difference supports the hypothesis that miscibility of the MF resin in the polymer and insolubility in water are desirable characteristics.

As shown above, Procedure II makes it possible to obtain well crosslinked films with all MF resins studied, including butylated types. It affords improved results with mixed ether MF resin RF-4518, as shown in Table 4. While results are indistinguishable at high temperatures, the advantages of Pro- cedure II are evident when the films are cured at 120 and 110 “C.

The appearance of the cured films was similar to that of films made from many latexes - the gloss was low and the films were not transparent. In some cases the films were yellowish on steel panels, but they were colorless on glass.

3.4. Catalyst studies; crosslinking at ambient temperature

As expected, the latexes studied here, including those made with recipe d (T, = 25) form films by coalescence at ambient temperature. It is conjectured that the MF resins function to some extent as coalescing aids. Presumably MF resins must be compatible with the acrylic polymer to be effective in this respect.

While the latexes studied above coalesce at ambient temperature, crosslinking is very slow with 1% catalyst. Since many of the potential uses of crosslinkable latexes require cure at ambient or mildly elevated temperature, ways to accel- erate the reaction at lower temperatures were sought.

The behavior of films containing 2, 3 and 4% p-TSA is shown in Table 5. Latexes M4518-II-c-3 and M4518-II-c-4 crosslinked readily at low bake temperatures in 30 min and within 1 day at ambient temperature. The films displayed good solvent resistance, good impact resistance and reason- able (HB) pencil hardness. Note that in order to obtain cure at ambient temperature the pH of latex must be reduced to the 2-2.5 range.

While these results are encouraging, there is concern that the very high levels of catalyst necessary for ambient tem-

Formulation Bake temperature (“C for 30 min)

150 130 120 110

M4518-I-a-1 Acetone rub resistance Pencil hardness Impact resistance (D/R)

M451R-II-c-1 Acetone rub resistance Pencil hardness Impact resistance (D/R)

200 + 200 130 80 H H H H 1601160 160/160 160/160 160/160

200 + 200 + 200 + 200 + H H H H 160/ 160 160/160 160/160 160/ 160

Film thickness: 10 pm

Y.Huang, F.N. Jones/Progress in Organic Coatings 28 (I 996) 133-141 139

Table 5 Film properties of latex M45 18-11-c at different acid catalyst levels

Formulation/properties PH Bake temperature (“C)

80 70 60

Ambient (1 day)

M4158-II-c-2 MEK rub resistance Pencil hardness Impact resistance (D/R)



4 150 HB 160/160

2.5 200+ HB 160/160

2.0 200 + HB 160/ 160

110 85 40 HB B B 160/ 160 160/ 160 160/160

200 + 200 + 200 + HB HB HB 160/160 160/ 160 160/160

200 + 200 + 200 + F HB HB 160/ 160 160/ 160 160/ 160

Film thickness: 15 pm. Bake time: 30 min.

perature cure would leave residues in the film that might impart humidity sensitivity and reduce weatherability. An alternative method of reducing pH was demonstrated by Hahn [ 111, who made latexes more acidic by passing them through a protonic cation exchange resin (CER) . In our experiments the pH of the latexes was reduced to about 2.5 by slowly percolating them through a column packed with Amberlite 200 CH CER. Crosslinkable latexes M747-II-c, M6550-II-c and M4518-11-d acidified in this way were compared with those acidified by addition of 3% of p-TSA with the results shown in Table 6.

The results in Table 6 show that the two methods of acidification yield similar crosslinking rates. In these particular experiments the results obtained with MF resin M-747 were superior to those obtained from MF resin CE-6550. The results with MF resin RF-4518 are not directly comparable because a higher Tg acrylic resin was used. With a comparable acrylic resin, MF resin RF-45 18 gives the fastest crosslinking (compare Tables 5 and 6).

As is predictable, use of high levels of acid to make ambient temperature cure results in a sacrifice in package stability. At pH about 2.5 these crosslinkable acrylic/MF latexes gelled in l-3 days at room temperature. Thus, they would be practical only in circumstances where acid catalyst can be added or CER treatment effected within a few hours of application.

3.5. Reactivity comparisons of crosslinkable latexes

As shown above, crosslinkable latexes made from monomeric methylated and mixed ether MF resins R-747, RF-45 18 and CE-6550 can cure at ambient temperature if acidified to pH 2.5. However, latexes M7512-II-c-3 and M1612-II-c-3 did not crosslink nearly as readily. They required bake temperatures of 140 “C to obtain good solvent resistance even with high levels of acid catalyst. Thus, it was judged that latexes made from polymeric, butylated MF resins are poorly

suited for applications of this type. Perhaps their immiscibility with the polymer particles causes non-uniform crosslinking at temperatures < 140 “C, and as a consequence, study of these materials was discontinued.

Further comparisons of the cure rates of the more reactive MF resins are shown in Table 7. Two effects can be distin- guished, the effect of the type of MF resin and the effect of acrylic resin Tg. With acrylics of theoretical Tg = 10 “C (recipe c) the apparent cure rates are in the order A-M45 18-11-c- 3 > M747-II-c-3 > M6550-11-c-3, consistent with results described in Tables 2 and 6. The same order is maintained with an acrylic of theoretical Tg = 25 “C, but the cure rates are much slower.

From results with the five MF resins studied it can be inferred that miscibility of the MF resins with the acrylic latexes is a critical factor in cure rate. Immiscibility is a strong impediment to formation of good crosslinked films, appar ently outweighing differences in chemical reactivity of the MF resin. Insolubility of the MF resin in water facilitates latex synthesis, but it does not appear to be a requirement for good crosslinking as long as the MF resin is miscible with the polymer. These observations suggest, but do not prove, that the MF resins such as R-747 that are somewhat water miscible can diffuse to a considerable extent into the latex particles before crosslinking is far advanced. If so, the gra- dients of crosslink density postulated by Grawe and Bufkin [ 41 would be minimized and film integrity should benefit.

The ideal situation would probably be for the MF resin to reside primarily in the latex particle before coalescence, pref- erably as a solution in the acrylic polymer. At best this situation could lead to virtually homogeneous crosslinking. It might also improve long-term stability of the latexes.

It was expected that higher Tg latex might slow cure but would improve the film hardness. Table 7 shows evidence of a strong retardation of cure rate at ambient temperature, but crosslinked films of M45 18-11-d ( Tg = 25 “C) were not meas-

140 Y.Huang, F.N. Jones /Progress in Organic Coatings 28 (1996) 133-141

urably harder than those of M4518-II-c ( Tg = 10 “C). As expected, higher Tg caused lower impact resistance. It was concluded that higher T, is a net disadvantage for ambient temperature cure films, although it might be advantageous for baked coatings. A recent study by Zosel and Ley [ 71 may shed some light on the poor performance of the higher Tg polymer. They showed that as crosslinking proceeds it pro- gressively retards interdiffusion and entanglement of polymer molecules from different latex particles. It follows that low T, and MF resins that aid in coalescence might favor good film properties by enabling extensive interdiffusion and entanglement before crosslinking is far advanced.

3.6. Comparison with conventional crosslinkable latexes

A conventional crosslinkable acrylic latex (recipe e) was prepared with the same monomer composition as recipes a- c. Direct blending of the most reactive MF resin, RF-4518, into this latex caused coagulation. This problem was largely overcome by pre-emulsifying the RF-45 18 in water; when emulsified RF-4518 was blended into the latex with intense stirring an apparently uniform dispersion was produced; a small amount of coagulum was removed by filtration, p-TSA catalyst was added, and films were prepared. Film properties are compared with those of latex M45 18-11-c-3 in Table 8. It can be seen that the latter cured much faster at ambient temperature. The conventional RF-45 18 crosslinkable latex gave harder films than M4518-II-c-3 at 70-80 “C bake temperatures.

It is known that MFresins with very low levels of unreacted ‘NH and ‘NCH20H groups, such as RF-45 18, are the most / / reactive of any fully formylated, fully alkylated MF resins in high solids coatings. The results suggest that this feature of RF-4518 carries over to latex coatings, facilitating cure at ambient temperature.

3.7. VOC considerations

If, as the results suggest, the new process lends itself to making latex particles in which water-insoluble MF resins are dissolved, it may have practical advantages over conventional methods. Fast reacting MF resins can be used and would be partially protected from hydrolysis if they reside in the latex particles rather than in the aqueous phase. Further, MF resins that are miscible with the acrylic polymer are predicted to function as coalescing aids, accelerating coalescence and molecular entanglement before crosslinking is far advanced.

By reducing or eliminating the need for coalescing aids the MF resins could help reduce VOC emissions. It is noted that by-product alcohol will add to VOC as the crosslinking proceeds, but because of the high molecular weight of latex polymers relatively low levels of crosslinker are required. Thus, with optimizaiton, very low VOC formulations appear possible.

Table 6 Film properties of coatings catalyzed by p-TSA and after cation exchange resin (CER) treatment

Temperature Catalyst MEK rub Pencil resistance hardness

Impact resistance (D/R)

M747-II-c (T8 = 10) 80 “C p-TSA (3%)

CER 70°C p-TSA (3%)

CER 60 “C p-TSA (3%)

CER Ambient p-TSA (3%)

(1 day) CER Ambient p-TSA (3%)

(3 days) CER Ambient p-TSA (3%)

(7 days) CER

M6550-II-c (T, = IO) 80 “C p-TSA (3%)






(7 days) CER

M-4518-U-d (T, = 25) 80 “C p-TSA (3%)






(7 days) CER

200 + HB 160/160 200 + HB 160/ 160 200 + HB 160/ 160 200 + HB 160/160 200 + HB 160/160 1so+ HB 160/160 120 HB 160/ 160 120 HB 160/ 160 160 HB 160/160 160 HB 160/ 160 200 HB 160/160 180 HB 160/ 160

200 + 200 + 200 + 200 + 200 + 200 + 15 20 25 20 65 70

HB (B) 160/160 B 160/ 160 B 160/160 B 160/160 B 160/160 B 160/160 N/A N/A N/A N/A N/A N/A N/A N/A B N/A B 160/ 160

200 + 200+ 200 + 200 + 150 200 + 50 40 200 + 150 200 + 200 +

F 160/ 160 F 160/160 F 160/160 HB 160/160 HB 160/ 160 HB 160/ 160 B 40/30 B 160/ 160 HB 50125 HB 160/ 160 HB 160/ 160 HB 160/160

4. Summary and conclusions

The results show that emulsion polymerization in the presence of MF resins is practical with careful selection of process conditions. The presence of MF resins does not drastically interfere with free-radical polymerization, and premature crosslinking can be minimized by controlling pH at above 5.

This novel process yields crosslinkable latexes that coalesce readily without coalescing aids and crosslink within a day at ambient temperature, although high levels of catalyst must be added. In preliminary tests the films had excellent solvent and impact resistance and fair hardness.

The new process has potential advantages over the conventional method of making the latex first and adding MF resins later. It works best with MF resins that are insoluble in

Y.Huang. F.N. Jones/Progress in Organic Coarings 28 (1996) 133-141 141

Table 7 Cure rates of different MF resins with acrylic resins of Ts 10 and 25

Formulation Time (days)

T8 (“C)

Film properties

MEK rub resist. Pencil hardness Impact resist. (D/R)

M45 18-11-c-3 M45 18-11-d-3 M747-II-c-3 M6550-II-c-3

M45 18-11-c-3 M45 18-11-d-3 M747-II-c-3 M6550-II-c-3

M45 18-11-c-3 M45 18-11-d-3 M747-II-c-3 M6550-II-c-3

I 1 1 1

3 3 3 3

7 7 7 7

10 200 + 25 50 10 120 IO 15

10 200 + 25 200 + 10 160 10 20

10 200 + 25 200 + 10 200 10 60

HB 160/160 B 40/30 HB 160/160 N/A N/A

HB 160/160 HB 160/160 HB 160/ 160 N/A N/A

HB 160/160 HB 60/30 HB 160/ 160 B 160/ 160

pH of latex: 2.5. Film thickness: 10 pm.

Table 8 Comparison of the film properties of two types of crosslinkable latexes

Properties Bake temperature (“C for 30 min )

80 70 60 Ambient

(1 day)

M4518-II-c-3 MEK rub resistance Pencil hardness Impact resistance

M4518-I-c-3 MEK rub resistance Pencil hardness Impact resistance

200 + 200 + 200 + 200 + HB HB HB HB 160/ 160 160/160 160/160 160/160

200 + 200 + 180 40 F F HB 2B 160/160 140/160 160/160 110/160

water and miscible with the latex particles; it is speculated that such MF resins reside primarily in the latex particles, acting as coalescing aids and promoting rapid formation of a relatively uniform film.

The preliminary study reported here leaves many unanswered questions about scientific aspects of the process and its practical utility. Emulsion polymerization and coalescence of relatively simple thermoplastic latexes are very complex processes that are not fully understood. Crosslinkable latexes have additional complexities as discussed by Zosel and Ley [ 71. Further studies are needed to illuminate some of these questions.

Acknowledgements

Financial support of this study by the Monsanto Chemical Company is gratefully acknowledged.

References

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Monsanto Chemical Co., Springfield, MA, USA, [ 171 L.W. Hill and J.C. Baatz, personal communication, Jan 1992. [18] V.R. Kamath, J. Coat. Technol., 59 (746) (1987) 51. [ 191 Anon.,Emulsion polymerizationofacrylic monomers, Tech. Bull. CM-

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Documents

Synthesis of crosslinkable acrylic latexes by emulsion polymerization in the presence of etherified melamine-formaldehyde (MF) resins