Film Formation in Waterborne Coatings - ACS Publications - American

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Chapter 9

Role of Interdiffusion in Film Formation of Polymer Latices

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Albrecht Zosel and Gregor Ley Polymer Research Laboratory, BASF AG, 67056 Ludwigshafen, Germany

The mechanical strength offilmsfrompolymer latices develops by the interdiffusion of chain segments and the formation of entanglements across particle boundaries duringfilmformation. Measurements of the fracture energy of poly-n-butylmethacrylatefilms,tempered above the glass transition temperature for different times, show a transition from brittle to tough (yielding) behaviour with a strong increase of thefractureenergy at short temper times, which is followed by an increase proportional to the square root of the temper time until a constantfractureenergy is reached after long times. The film strength is correlated to the interdiffusion length determined by small angle neutron scattering of deuterated films. The formation of interparticular entanglements is hindered in latices with crosslinked particles, as has been studied with a series of latices, crosslinked with various amounts of a bifunctional monomer.

In most applications polymer latices are transformed into coherent, transparent films which generally have a considerable mechanical strength comparable to that of films from polymer solutions or melts. The film formation of polymer latices has accordingly found wide-spread interest, as a great part of the end-use properties of emulsion polymers develops during this process. Thefilmformation of latices can be regarded as a three stage process consisting of 1. the concentration and packing of the latex particles by evaporation of water, 2. the deformation of particles, and 3. the coalescence of particles by interdiffusion of chain ends and segments across particle boundaries and the formation of entanglements.

0097-6156/96/0648-0154$15.00/0 © 1996 American Chemical Society

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Interdiffusion in Film Formation of Polymer Latices

155

It is supposed that the mechanical strength of latexfilmsdevelops during this third step, which will be the subject of this paper. The research on diffusion accross polymer polymer interfaces started with the pioneering work of Voyutskii [1]. In the last two decades, the investigation of the roleof interdiffusion in the healing and welding of polymer polymer interfaces has experienced a tremendous progress, mainly connected with the work of de Gennes [2], Kausch [3], Tirrell [4] and Wool [5]. The interdiffusion in latex films has also been studied during the last years, mainly by SANS [6-8] and by steady state fluorescence decay measurements [9]. The work presented here deals with the development of mechanical strength during this third stage of film formation. Other studies of the mechanical behaviour during film formation have been published by Klein, Sperling and coworkers [8] and by Eckersley and Rudin [10]. The development of film strength by interdiffusion and the formation of interparticular entanglements will be increasingly hindered in latices with crosslinked particles, especially when the mean molecular mass between crosslinks, Mc, becomes smaller than the mean molecular mass between entanglements, Me. This has been studied withfilmsformedfromlatices with crosslinked particles [11,12]. A crucial point in studying interdiffusion during film formation is the unambiguous definition of the beginning of the interdiffusion phase. As interdiffusion in particles above the glass transition temperature starts immediately upon particle contact, the time "zero" for interdiffusion may be different at different parts of the particle surface. Furthermore, it may be different in different regions of the drying film, as it is well known that often a dryingfrontpropagates through a film in the compaction stage. One way to overcome this difficulty is to carry out thefirsttwo steps of the film forming process below the glass transition temperature and subsequently to heat up the film to an annealing temperature above Tg, so that the starting time of the interdiffusion process is well defined. This strategy has been applied in this study. Experimental Part Samples. For this purpose we used model emulsion polymers of n-butyl-methacrylate (BMA). The latices were prepared by batchwise emulsion polymerization with sodium laurylsulphate as emulsifier, using a standard recipe at 80 °C. They had typically a solid content of about 30% and a mean particle diameter of about 60 nm. More details on the latex synthesis are given in a previous publication [6]. PBMA latices are well suited for studies of interdiffusion as PBMA has a glass transition temperature of 29 °C, determined by dynamic mechanical analysis with afrequencyof 1 Hz, but nevertheless forms coherent, brittlefilmsat 23 °C. No significant interdiffusion takes place at room temperature owing to the low segmental mobility of the polymer, as has been shown earlier by SANS studies of the same samples [6]. Interdiffusion thus starts only on annealing thefilmsat temperatures above T . The particles of the BMA homopolymer are uncrosslinked. In order to study the effect of crosslinking on interdiffusion, copolymers of BMA and the birunctional monomer methallyl-methacrylate (MAMA) with molar concentrations of MAMA g

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FILM FORMATION IN WATERBORNE COATINGS

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between 0 and 2% were prepared. The B M A / M A M A copolymer latices have crosslinked particles with a crosslink density increasing with the concentration of the bifunctional monomer. Experimental Methods. The latexfilmswere characterized by measurements of the dynamic shear modulus as a function of temperature andfrequency,using a dynamic mechanical analyzer with a parallel plate geometry [12]. The glass transition temperature of PBMA, the mean molecular mass between crosslinks, Μς, and the entanglement length Me are determinedfromthe dynamic modulus which consists of the storage modulus G' and the loss modulus G". The mechanical strength of the films is characterized by tensile tests at 23 °C which measure the tensile strength σ , i. e. the tensile stress at break, and the strain at break, ε . The energy per unit volume, W , which is necessary to break the sample, is calculated by integration of the stress strain curve: Β

Β

B

A crosshead speed of 1.67 mm/s and a sample length of 24 mm were employed in the tensile tests. Film Formation of Uncrosslinked Latices Films were formedfromthe PBMA latex without bifunctional monomer at 23 °C, and were tempered above Tg, i. e. at 90 °C, for different times after film formation. Specimens for the tensile tests were cut before the samples cooled down to room temperature. As the unannealedfilmsare very brittle they had to be heated to about 50 °C for a short time of less than 2 min in order to cut specimens. Some examples for stress strain plots are given in the left part of Figure 1 for the uncrosslinked PBMA. The untempered film shows a rapid increase of σ with increasing tensile strain ε and brittlefractureat a low strain of about 4-10'. After an annealing time of 5 min already, thisfracturebehaviour changes completely. A steep stress peak is observed at low strains, too, but it is followed by yielding which gives rise to a high ultimate strain of about 3. In the case of yielding, the area under the stress strain plot is much higher than for brittlefracture.That means that the fracture energy increases drastically. At an annealing time of 5 min or less, thus, a transition from brittlefractureto tough or yielding behaviour is observed. Longer annealing times only gradually change the shape of the stress strain characteristics. In Figure 2 thefractureenergy is plotted versus the temper time at 90 °C for the uncrosslinked PBMA showing a strong increase by more than two orders of magnitude at annealing times below 5 min in correspondence with the transition from brittle to tough behaviour. This transition is followed by a further gradual increase of Wg which is not finished in the time interval of 360 min, shown in Figure 2. Annealing at a lower temperature, e. g. 50 °C, results in a slower increase of the fracture energy with the tempering time.

9. ZOSEL & LEY

Interdiffusion in Film Formation of Polymer Latices

annealing time

0% MAMA

2% MAMA

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0 min

5 min

60 min

360 min

Figure 1: Stress-strain diagrams for poly-n-butylmethacrylate with 0 and 2% methallylmethacrylate after different annealing times

30mJ/mm

3

•i—i-

20-

10-

100

200 t

300

m

i

n

400

-

Figure 2: Fracture energy W B per unit volume for uncrosslinked P B M A as a function of the annealing time at 90 °C

157

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FILM FORMATION IN WATERBORNE COATINGS

Hahn, Ley and coworkers carried out SANS experiments onfilmsfromsimilar latices at the Institute Laue - Langevin, Grenoble, France [6,7]. From measurements of the radius of gyration of a small amount (1%) of deuterated particles in dependence on the annealing time, the interdiffusion length can be evaluated and correlated with the increase of thefractureenergy. In Figure 3 thefractureenergy W B and the interdiffusion depth d are plotted versus the square root of the annealing time t at 90 °C. The penetration depth is proportional to t , as predicted by the reptation theory of de Gennes [13]. Wg increases by about two orders of magnitude within the first 5 min, as already shown in Figure 2, then gradually increases linear with t and seems to level off with a constant value. At present, there is no theory to relate the mechanical strength of the interface to the arrangement of chains in the interface. The models of de Gennes, Prager and Tirrell, and Kausch agree that thefractureenergy should increase proportional to t , too, what is verified by various experimental studies and the work, presented here. If we assume that no considerable interdiffusion occurs during film formation below Tg, the untempered film is supposed to consist of a packing of the hard latex spheres, which has a certain mechanical strength, i.e. tensile stress at break, due to intermolecular forces such as van der Waals forces acting across the boundaries between the packed particles. However, it lacks a measurable toughness, i.e. the ability to store and to dissipate energy during deformation which is related to the slippage and the disentanglement of chain molecules. This toughness requires the interdiffusion of chain segments. That means that thefirststeep increase of W B and the transition from brittle to toughfracturecannot be caused by a further "dry" sintering but should be attributed to the beginning of the interdiffusion process, which givesriseto an interdiffusion depth of about 2 nm after 5 min at 90°C, as estimated from the SANS data in Figure 3. This compares to about 3 nm for the length of an extended chain of the molecular mass between entanglements, Me. A value of 2.1 10 g/mole has been evaluated for Mefromdynamic mechanical measurements. 1/2

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1/2

1/2

4

It has been shown by molecular modelling of polymer aggregates in solution [14] that chain ends and short chains are present in a surface layer of the aggregates with a higher concentration than in the "bulk" of the aggregates. As this material diffuses faster than the average weight molecules, it possibly contributes to the fastriseof film toughness at short annealing times, too. The further slow increase of W is caused by the progressive interdiffusion and the formation of entanglements of the long molecules across the boundaries of the former particles. At least, thefractureenergy becomes constant at an interpénétration depth of about 40 nm, which has the same order of magnitude as the radius of gyration of the PBMA molecules for which a weight average molecular mass of about 5· 10 g/mole has been found. B

5

Film Formation of Latices with Crosslinked Particles The mean molecular mass between two crosslinks, Mc, and between two entanglements, Me, can be calculated from the storage modulus G' according to the theory of rubber elasticity [15]. Mc was additionally determined by swelling

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measurements in tetrahydrofurane. It followsfromFigure 4 which shows Mc as a function of the molar concentration of MAMA that Mc equals Me at a molar concentration of about 1.5%. Stress strain measurements were carried out withfilmsof the PBMA latex with 2% MAMA, i. e. above this "critical" concentration, following the same procedure as for the uncrosslinkedfilms.In therightpart of Figure 1, stress strain plots are shown for films, annealed 60 and 360 min, resp. It follows that the PBMA with 2% MAMA exhibits brittlefractureup to the longest annealing times without any significant change of the very lowfractureenergy. This conclusion can also be drawn from Figure 5 showing thefractureenergy of the uncrosslinked and the crosslinked PBMA in dependence on the annealing time. For thefilmscrosslinked with 2% MAMA, the fracture energy is by more than two orders of magnitude lower than in the case of the uncrosslinked PBMA up to an annealing time of 360 min. Thesefilmsremain extremly brittle upon annealing. The highly crosslinked particles cannot perform any significant interdiffusion, they are only deformed by Van der Waals attractions. Figure 6 shows thefractureenergy as a function of the temper time at 90°C for films with MAMA concentrations of 1 and 1.5% resp. together with the results for 0 and 2% MAMA. The PBMA with 1% MAMA exhibits exclusively brittle fracture until 15 min, bothfracturetypes at 30 min, and yielding at 60 min and longer times. For the samples with 1.5% MAMA brittle as well as toughfractureare observed between annealing times of 60 and 360 min. In the Figure the data points are plotted for tough behaviour only. Films with the MAMA concentration near the critical value, thus, show some kind of transition between bothfracturetypes. It follows, thus, that interdiffusion leading to entanglements is essential for the toughness of latexfilms.Measurements on uncrosslinked films from a latex with a mean molecular mass smaller than the entanglement length fit into this scheme. This can be concluded from Table 1 which gives a summary of the measurements and shows the low mechanical strength offilmswith a M < Me which is due to brittle fracture of this polymer. w

Conclusion As a conclusion we propose the following model for the development of mechanical strength of the uncrosslinked latex film: 1. Very quick formation of a continuous, though very brittlefilmby sintering of the particles, increasing the area of close contact between the particles. 2. Rapid transitionfrombrittle to toughfractureby fast interdiffusion of chain ends and small chains in the interparticular boundaries, and the formation of the first entanglements. 3. Slow development of thefinalmechanical strength by interdiffusion and entanglements of the long chain molecules. These processes are more and more slowed down and hindered with increasing MAMA concentration. Stage 2 and 3 of this model are absent in the formation of filmsfromcrosslinked particles with Mc < Me.

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FILM FORMATION IN WATERBORNE COATINGS

Figure 3: Fracture energy W B and interdiffusion length d for uncrosslinked P B M A , plotted against the square root of the annealing time at 90 °C

0,1

0,2

0,4

0,6

A

swelling in THF

Ο

shear modulus G'

1

6

%

10

Figure 4: Mean molecular mass Μς between crosslinks for B M A / M A M A copolymers as a function of the M A M A concentration

9. ZOSEL & LEY

Interdiffusion in Film Formation of Polymer Latices

0% MAMA

30 mJ/mm

161

3

ΟΗ

-

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20

10

2% MAMA 100

400

300

200

Figure 5: Fracture energy W of P B M A with 0 and 2% M A M A as a function of the annealing time at 90 °C B

30 m J/mm

0%MAMA 3

•t—i-

20

- · -

1% 1,5 ,5%

10

2% 100

300

200 t

m

j

n

400

-

Figure 6: Fracture energy W of P B M A with various concentrations of M A M A versus the annealing time at 90 °C B

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FILM FORMATION IN WATERBORNE COATINGS

Table 1 : Mechanical strength of films from P B M A latices Sample

ε

Β

W mJ/mm" B

3

MPa

Characterization of sample

Uncrosslinked P B M A M

w

» Me, untemp.

M » Me tempered M < Me, ··

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w

w

9 12 4

0.04 3.3 0.07

0.25 31 0.2

M

w

= 5 · 10 g/mole

M

w

= 2 · 10 g/mole

13

1.9

20

1%MAMA, Mc = 3 · 10 g/mole 2% M A M A , Mc= 1.5 · 10 g/mole

5

4

Crosslinked P B M A Mc > Me, tempered

4

10

Mc