Improving Latex Performance by Using ... - ACS Publications

Chapter 13

Improving Latex Performance by Using Polymerizable Surfactants 1

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Esteban Aramendia , Maria J. Barandiaran , Jose C. de la Cal , Jo Grade , Trevor Blease , and Jose M . Asua Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch013

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Institute for Polymer Materials (POLYMAT) and Grupo de Ingeniería Química, Departamento de Química Aplicada, Facultad de Ciencias Químicas, The University of the Basque Country, Apdo. 1072, ES-20080 Donostia-San Sebastián, Spain Uniqema, Everslaan 45-B-3078 Everberg, Belgium 2

Latexes with superior properties were produced by using a new alkenyl-functional polymerizable surfactant (surfmer). These latexes have a better freeze-thaw stability, and form films with less water absorption and lower vapor and water permeability. These improvements were due to the chemical bonding of the surfmer to the polymer that minimized its migration. Results obtained by using a mathematical model show that the reactivity ratio of the monomer with respect to the surfmer is the key parameter for an adequate incorporation of the surfmer and such a value should be between 1 and 5.

Surfactants play a crucial role in the production of emulsion polymers. They are very important for the nucleation of latex particles, emulsification of the monomers and stabilization of the polymer particles during the polymerization and shelf life of the product. However, they can also have an adverse effect during application. For example, when the latex is coated at high application speeds the surfactant may desorb and cause destabilization, and when mixed

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© 2002 American Chemical Society

In Polymer Colloids; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch013

169 with pigments in paints the surfactant may migrate to the pigments destabilizing the latex particles. The presence of the surfactant can also have adverse effects on the film properties, as the surfactant may migrate through the film to the filmair interface affecting gloss and to the film-substrate interface reducing adhesion. Also it can concentrate in pockets increasing the percolation by water. In polymer recovery by coagulation the surfactants can have negative effects, for example, in waste-water treatment. These drawbacks are due to the mobility of the surfactant molecules during the film formation process. Therefore, a promising way to reduce the negative effects of surfactants is to use polymerizable surfactants (surfmers). A surfmer is a surfactant with a polymerizable double bond in its structure. Upon polymerization, these compounds become bound covalently to the polymer material so that desorption from the polymer particle and migration through the polymer film is impeded. Surfmers have attracted much attention in both open (1-14) and patent (1520) literature as improvements in product quality by using surfmers have been reported (16-31). Asua and Schoonbrood (32) have recently reviewed the publications concerning the use of surfmers in heterophase polymerization in terms of the mechanisms relevant in the process. The authors concluded that there is no general formula for a conventional emulsifier that works well in every system and gave general guidelines to choose or design a specific surfmer and to apply it in an appropriate way. In particular, they proposed that the reactivity ratio of the surfmer with respect to the monomers should be close to zero to avoid surfmer homopolymerization, and the reactivity ratios of the monomers with respect to the surfmer should be between 0.5 and 10 to avoid early polymerization of the surfmer, which would otherwise result in burying of the surfmer, and to achieve a high degree of incorporation at the end of the process. The authors stressed that these were rough guidelines because other factors may affect the observed reactivity. This paper addresses two important aspects of surfmer technology. The first one deals with latex performance improvements achieved using a new nonionic surfmer. The second is related to the choice of the reactive group of the surfmer.

Improvement of Latex Performance The performance of latexes prepared using a new alkenyl-functional nonionic surfmer (Maxemul 5011, Uniqema) was compared with that of the latexes prepared with the same monomers but stabilized with conventional nonyl phenol ethoxylated nonionic surfactants (NP20 and NP30).



170 50 wt% solids vinyl acetate / butyl acrylate (85/15 by weight) latexes were prepared through semicontinuous emulsion copolymerization with both Maxemul 5011 and NP20 using the recipe given in Table I. The latexes were subjected to freeze-thaw cycles (-20 °C, 11 hrs / 1 hr heating, 23 °C, 11 hrs, 1 hr cooling). It was found that the latexes prepared with Maxemul 5011 resisted 5 freeze-thaw cycles whereas those prepared with NP20 failed after 2 cycles. A possible reason for the superior performance of the latex stabilized with the surfmer is that during the freezing process as ice crystals are formed, the polymer particles are restricted in a smaller volume, and hence they are in close contact. The pressure created by the ice forces the interpénétration of the hairy layers formed by the surfactant covering the polymer particles. The interpénétration of the hairy layers increases the energy of the system causing the repulsion of the polymer particles, and hence maintaining their stability. However under conditions occurring during the freezing process, the pressure created by the ice may be sufficient to displace the conventional surfactant along the surface of the particles, leaving uncovered zones that are closer to other particles. This results in particle coagulation. When the latex is stabilized by the polymerizable surfmer, the surfmer cannot be displaced along the particle surface because it is bound to the polymer. Therefore, no parts of the polymer particle are left unprotected and the particles remain stable.

Table I. Formulation Used to Prepare the Vinyl Acetate / Butyl Acrylate Latexes 8

Initial charge

Pre-emulsion

Initiator stream

Mop-up

Water VAc/BA KPS K C0 Nonionic surfactant / Maxemul 5011 Water VAc/BA Maxemul 5011 KPS K C0 Water Water t-BHP SFS 2

3

2

3

99.30 12.50 0.63 0.25 12.51 268.97 437.50 18.93 1.80 1.80 120.00 25.00 0.45 0.36

A l l amounts are in grams


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Films with a wet thickness of 200 μπι were cast from these latexes on polyester and allowed to dry for 7 days at 21°C and 50% relative humidity (R.H.). The films were compared for water absorption upon immersion in water. Figure 1 shows that the film cast from the latex stabilized with Maxemul 5011 showed much better water resistance than that cast from the latex containing the conventional emulsifier. The difference increased with the time of immersion in water. The higher water sensitivity showed by the film stabilized with the conventional emulsifier may be due to the migration of the emulsifier during the film formation that results in both channels through which the water can percolate and segregated pockets of emulsifier where water can accumulate.

c ο ο (0 η ta k.

S w

Figure 1. Water absorption of the vinyl acetate /butyl acrylate copolymer films. (O) Maxemul 5011; (·)ΝΡ20

50 wt% solids methyl methacrylate / butyl acrylate (50/50 by weight) latexes were prepared by semicontinuous emulsion copolymerization using the recipe given in Table II. Both Maxemul 5011 and NP30 were used in these polymerizations. Films from these latexes were cast onto a glass surface and kept at room temperature overnight. The films were observed by atomic force microscopy ( A F M , NanoScope Ilia), working in contact mode. Figure 2 presents the A F M micrographs of the film produced from the latex stabilized with NP30. The untreated film (Figure 2a) shows the presence of large amounts of surfactant that migrated to the film-air interface during the film formation. This emulsifier disappeared when the film was rinsed with water (Figure 2b). Figure 3 shows the A F M micrographs of the film cast from the latex stabilized by Maxemul 5011.


172 No evidence of surfactant migration was observed in the untreated film (Figure 3a) and no significant differences were found after rinsing with water (Figure 3b). Table II. Formulation Used to Prepare the Methyl Methacrylate / Butyl Acrylate Latexes 8


SEED

Water MMA BuA AA KPS NaHC0 Anionic surfactant SEEDED POLYMERIZATION Initial Charge Seed Water MMA BuA KPS Nonionic Surfactant / Maxemul 5011 Feed 1 MMA BuA AA Feed 2 KPS Nonionic surfactant / Maxemul 5011 Water Feed 3 Water Na S 0 3

2

2

5

1485.01 68.91 68.91 1.76 6.00 0.90 17.16 156.50 59.27 11.28 11.28 0.056 0.67 212.71 212.71 8.98 0.91 12.79 143.75 100.00 0.69

A l l amounts are in grams The methyl methacrylate/butyl acrylate copolymer films were also compared for vapor and liquid water permeability. In order to measure the vapor permeability, the films were used to close a small chamber (the lower part is about 2 cm , while the whole system is about 4 cm ) containing 1.2 cm of water. The chamber was placed on a balance and the loss of weight due to vapor diffusion through the film was recorded. Data were normalized by means of the following equation: 3

3

WVTR=

,

B

x

L x

8.64xl0

3

5

(1) 2

where WVTR represents the water or vapor transmission rate (g mm/m day), Β is the slope of the weight loss vs time curve, L the film thickness, and A the area of the cell. In Polymer Colloids; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


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Figure 2. AFM micrographs of the film produced from methyl methacrylate / butyl acrylate latex stabilized with Ν Ρ30. (a) untreated film; (b)film rinsed with water.



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Figure 3. AFM micrographs of the film produced from methyl methacrylate / butyl acrylate latex stabilized with Maxemul 5011. (a) untreated film; (b)film rinsed with water.


175 Each measurement was carried out three times and the data were found to be reproducible within 10%. The main source of error in these kinds of experiments is related to the thickness of the films.


Table III. Permeability of MMA/BuA Films Surfactant NP30

WVTR 17.21

VTR 183.50

Maxemul 5011

14.12

34.31

Film permeability by liquid water (WTR) was determined in a similar way, but placing the chamber upside down, maintaining the film in direct contact with the water. In this system, the loss of weight was due to the evaporation at the external surface of the film of the water that diffused through the film. Table III shows that both vapor and liquid water permeability were higher for the film cast from the latex stabilized by the conventional surfactant, the difference being enormous for the liquid water permeability. The formation of percolation channels and segregated surfactant pockets is the reason for the higher liquid and vapor permeability of the film containing the conventional surfactant. It is worth pointing out that a high water penetration is a serious drawback for using these latexes for exterior paints because they will not protect the substrate.

Choice of the Reactive Group of the Surfmer In the previous section it has been shown that the use of surfmers can lead to latexes of superior performance. The advantages of these latexes are related to the fact that the surfmer is bound to the polymer, namely to its lack of mobility. However, the lack of mobility may be a serious drawback during the polymerization process. Thus, if the surfmer reacts early in the process it may become buried inside the polymer particle as the polymer particle grows. Ideally, the surfmer should not react early in the process to avoid being buried, but complete incorporation into the polymer at the end of the process is desired (52). The key parameter controlling the polymerization of the surfmer is its intrinsic reactivity. Asua and Schoonbrood (32) gave a rough guideline for the reactivity ratios of the surfmer and monomers ( r - 0 and 0.5 < r < 10). This guideline was mostly based on scattered experimental results and there are indications that they may not be accurate (33). In order to check the recommended values for the reactivity ratios, a mathematical model was developed for emulsion gM

m



176 polymerization involving surfmers. The model incorporates the following mechanisms: i) The surfmer adsorbs on the surface of the polymer particles according to an adsorption equilibrium isotherm. ii) The surfmer polymerizes in an outer shell of thickness δ , that is a parameter of the model. iii) Due to particle growth, the polymerized surfmer may become buried. The polymerized surfmer may resist burying by migrating towards the particle surface. The driving force for this migration is the water affinity of the hydrophilic part of the surfmer. Viscous drag is the resistance to migration. The relative rates of particle growth and surfmer migration determine the fate of the polymerized surfmer. These considerations are accounted for in the model by considering that the surfmer does not migrate but that all of the polymerized surfmer located in an outer shell of thickness δ is active in stabilizing the polymer particles. A small value of 5 (in the range of 0.5 nm) means that the polymerized surfmer does not migrate towards the surface. On the other hand, a value in the range of 5 nm represents an important migration. iv) Stability is provided by both the adsorbed unreacted surfmer and by the polymerized surfmer located in an outer shell of thickness δ . ν) A radial concentration profile may exist in the polymer particle due to the surface anchoring effect (34,35). 2

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Table IV. Parameters of the Model dpsee

Improving Latex Performance by Using ... - ACS Publications

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