Multimodal fluoropolymer dispersions

Progress in Organic Coatings 48 (2003) 310–315

Multimodal fluoropolymer dispersions T. Poggio a,∗ , V. Kapeliouchko a , V. Arcella b , E. Marchese b a

AUSIMONT S.p.A., p.le Donegani 5/6, 15047 Spinetta Marengo (AL), Italy b AUSIMONT S.p.A., v.le Lombardia 20, I-20021 Bollate (MI), Italy

Abstract Fluoropolymers are continuously increasing their application fields in many industrial areas because of their unique properties, such as heat and chemical resistance, high purity, lubricity, dielectric properties etc. [Encyclopedia of Chemical Technology, vol. 11, 3rd ed., Wiley, New York, 1980, p. 1]. This paper describes very promising results obtained thanks to the Ausimont proprietary technology of microemulsion polymerization in formulating bimodal and multimodal fluoropolymer dispersions with superior film forming behavior. Very carefully size-controlled nanoparticles, ranging from 10 to 60 nm, allow to obtain blend with 200–250 nm particles that not only do have highly improved film formation properties, but also show outstanding film characteristics with respect to conventional 200/110 nm bimodal PTFE latexes. Moreover, the high flexibility in selecting polymer characteristics such as molecular weight, particle shape, monomer composition and melting properties allows to have sintered films with a denser and tougher structure characterized by superior mechanical and barrier properties. © 2003 Published by Elsevier B.V. Keywords: Multimodal dispersions; Fluoropolymers; Nanolatexes; Packing

1. Introduction The outstanding PTFE characteristics, such as chemical and thermal resistance are mainly determined by the high bond energy of C–C and C–F bonds, that also implies a very low molecule polarity and cohesive energy. Nevertheless this extremely convenient combination of properties is accompanied by very low film forming capability of the polymer dispersion. Indeed, the very high melting temperature always leads to formation of the film below the minimum film forming temperature of the polymer, so the film structure is controlled by forces acting during water evaporation and particle coalescence, although limited by the polymer high melt viscosity, that occurs during sintering as a distinct step. To overcome these problems the bimodality concept, well known from numerous rheological investigations [1,2] on polymer dispersions and from ceramics technology [3,4], was applied. Cited studies showed that bimodal dispersions display a lower viscosity at equal volume fraction of particles, or requested viscosity at a higher volume fraction with respect to monomodal dispersions [5]. It is reasonable that this behavior can help the non-coalescing PTFE dispersed particles to get a better packaging during water evaporation. ∗ Corresponding author. E-mail address: [email protected] (T. Poggio).

0300-9440/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0300-9440(03)00100-0

This approach was also applied with some result to fluoropolymer dispersions [6], and PTFE bimodal dispersions consisting of large particles with an average diameter of about 240 nm and small particles, obtained by low conversion conventional emulsion polymerization, with an average particle size ranging from 30 to 70% of the large particles. Authors had presumed that PTFE bimodal dispersions form films characterized by the so-called Horsfield packing [7], in which small particles have a diameter of about 0414 of the bigger one, to properly fill the structure voids. Indeed the bimodality effect at lower size ratio (e.g. below 0.3) is difficult to be investigated because such small particles cannot be obtained from conventional PTFE emulsion polymerization [8,9]. It was determined [8] that PTFE particles formed at the earliest stage of conventional PTFE dispersion polymerization have a diameter of about 72–76 nm. Ausimont has developed a technology [10,11] of perfluorinated microemulsion polymerization which permits to obtain PTFE particles from 10 to 60 nm. This technology, using aqueous microemulsions of perfluoropolyethers (PFPE), enables fluoropolymer nanolatexes to be obtained on industrial scale with high solids content, narrow size distribution and relatively low surfactant content. PFPEs are commercially available from Ausimont under the trade name Galden® and Fomblin® . Galden® and Fomblin® Y are manufactured at −40 ◦ C by UV photooxidation of hexafluoropropylene (HFP). The products obtained consist,

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structurally, of randomly distributed repeating units. The synthetic procedures allow great flexibility as far as the nature of the ending group is concerned. In this connection, both –CF3 and functional –CF2 COOH end groups are available, giving rise to a full class of neutral PFPE oils and monofunctional PFPE derivatives of relatively low molecular weight. Monovalent salts (e.g. NH4 + ) of the carboxyl derivatives behave as surfactants. In the early 1980s it was discovered that, in combination with PFPE oils, these surface active agents very easily form oil in water (O/W) microemulsions. These systems have been successfully applied to the polymerization of TFE with the aim of obtaining PTFE nanodispersions having a controlled particle size distribution in the range of 10–60 nm. It has been shown that such PTFE nanodispersions could be used as ultralow-k dielectric materials for integrated circuits insulation [12,13]. In this work, we summarized results obtained in a systematic study of the effect of nano-PTFE/standard-PTFE particle size ratio on film formation, film structure and technological characteristics of PTFE coatings. A preliminary report of secondary effects due to the nanopartilces polydispersity and shape is also given.

2. Experimental section 2.1. Latexes preparation and characterization For the experimental work described in the present paper a quasi-spherical PTFE dispersion (PV20D) has been used (see Table 1). This dispersion has been obtained by conventional emulsion polymerization using an anionic perfluorinated surfactant (ammonium salt of perfluorooctanoic acid—APFO), the desired shape has been obtained by copolymerizing a small amount (0.02 mol%) of perfluoropropylvynilether (PFPVE). The emulsion, stabilized by an alkylphenolpolyethoxylate, was then concentrated up to a solid content of 72 wt.% in order to have appropriate possibility for blend formulation. The nano-PTFE polymerization was carried out in de-ionized water using a perfluoropolyether (PFPE) microemulsion [10,11]. This microemulsion is constituted of 50–100 Å PFPE oil droplets, stabilized by a PFPE surfactant layer that allows the microemulsion to have very high kinetic stability under polymerization dilution conditions. The

Table 1 PTFE and nano-PTFE dispersion characteristics Dispersion

Particle size (nm)

Comonomer

Particle shape

Solid content (wt.%)

PTFE-PV20D N20 N40 N40R N100

260 20 42 40 100

PFPVE MDO MDO None MDO

Spherical Spherical Spherical Rod-like Spherical

72 22 30 29 27

311

very high number of stable droplets, up to 1020 droplets/l, allows to control very carefully the particle nucleation and growth. So it is possible to tune polymer particle size (10–100 nm) and shape (L/D 1-100) by varying the microemulsion amount and by using specific comonomers and chain transfer agents, at high reaction conversion. Table 1 reports the characteristics of PTFE nanoparticle dispersions. Particle sizes were measured by means of a Brookhaven Instruments photon correlation spectrometer (PCS) equipped with a BI 9000 correlator and a BI 200 SM goniometer and argon laser light source having a wave length of 5145 nm by Spectra Physics. The latex specimens to be subjected to measurements, were suitably diluted with bidistilled water and filtered on 0.2 ␮m millipore filter. Fig. 1 shows TEM pictures of the primary particles of the two 40 nm nanodispersions, obtained by microemulsion polymerization. 2.2. Preparation and characterization of bimodal dispersions The PTFE dispersions, polymerized as above, were used to prepare blends having bimodal particle size distribution. Table 2 summarizes the compositions and the value of particle diameter ratio (small particle/big particle = d/D) which characterize each bimodal mixture. In order to study the effect of nanodispersion polydispersity some “broad particle size distribution” nanolatexes were prepared blending the three dispersion N20, N40 and N100 as shown in Table 3. All the bimodal dispersion were properly formulated in order to get final compositions equivalent to conventional PTFE latexes used in coating applications; surfactant and salts were equalized at the same concentration, leveling the differences of composition of the starting components. The final compositions were 58 wt.% of PTFE and 3.5 wt.% of surfactant. The dispersions compositions were determined by a gravimetric method: 2 g of dispersion were heated in an oven at 105 ◦ C for 2 h, to eliminate water and then at 400 ◦ C for 10 min to decompose the surfactant. The weight of the final residue and the difference between sample weight at 105 and 400 ◦ C, referred to the initial dispersion weight, give the polymer and surfactant contents, respectively. 2.3. Preparation and characterization of coatings The PTFE dispersion blends, prepared as described above, were applied as a top-coat onto flat aluminum sheets, previously treated with a common polyaminoimide-based PTFE primer, to have a two layer coating system. The coating was than dried at 100 ◦ C for 2 min and sintered at 420 ◦ C for 10 min. The structural properties of the films formed by the studied polymer blends were evaluated by atomic force microscopy (AFM) using a Thermomicroscopes Autoprobe CP in non-contact mode with Ultralever tips with a nominal

312

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Fig. 1. TEM micrographs of nano-PTFE.

force constant of 2.8 N/m. In order to obtain smoother surfaces for the AFM analysis, the dispersion blends were deposited by spin coating onto glass plates. After the deposition the substrate was spun at 6000 rpm for 2 min, the film was then baked as previously described. The coating was characterized by measuring: Table 2 Composition ranges of PTFE/nano-PTFE blends Experiment set

Nano-PTFE

Nano-PTFE concentration (wt.% dry polymer)

Size ratio, d/D

1 2 3 4

N20 N40 N40R N100

1–2–5–7–10 1.4–2.8–4.2–6.1–8–11.5 1–2–5–7–10–15 5–7–10–15

0.077 0.162 0.154 0.423

• Critical cracking thickness. The top-coat formulation is applied on the aluminum plate in order to obtain a thickness gradient ranging from 20 to 60 ␮m. After sintering the coating is examined with an optical microscope (Leitz SM-LUX at 10× magnification) and the coating thickness corresponding to the cracks occurrence is indicated as the critical cracking thickness (CCT) of the system. • Scratch resistance and film hardness. A primerized aluminum plate is uniformly coated with the top-coat formulation under analysis at a thickness of 40–45 ␮m. After sintering, the panel is submitted to a scratch test using a pneumatic boom bearing a 1 mm diameter ball fixed at the bottom of an orthogonal rod. The pneumatic motion allows the ball to be moved onto the coated surface along

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Table 3 Broad particle size distribution nano-PTFE and blend composition range Experiment set

Average diameter (calc.)

Weight ratio (N20–N40–N100)

Nano-PTFE concentration (wt.% dry polymer)

5 6 7

80 32 40

10–20–70 70–20–10 20–70–10

2–5–7 2–5–7 2–5–7

CCT increase (µm)

35 30 25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

nano-PTFE (% b.w. dry polymer) 110nm

40nm

20nm

Fig. 2. Critical cracking thickness (CCT) increase of bimodal blend (CCT of PV20D dispersion alone = 30 ␮m).

80 mm, at a speed of about 50 mm/s. The vertical rod is progressively loaded with 50 g increments, starting from 500 g, and the maximum load bome by the coating represents its scratch resistance. The failure of the coating is indicated by a short circuit obtained when the ball, piercing through the coating, gets in contact with the plate that is connected to a 12 V electric circuit. The test is carried in air at room temperature, in water at 100 ◦ C and in vegetable oil at 180 ◦ C.

3. Results and discussion The analysis of film forming properties of the studied PTFE dispersions has been carried out by comparing the critical cracking thickness of the bimodal blends monomodal dispersions. In the following figures the data are reported in terms of CCT increments as a function of nano-PTFE concentration on total amount of polymer. Fig. 2 shows how nanoparticles of different size enhance the maximum film thickness that can be applied without cracks formation. The best results are obtained using dispersion N40, constituted of spherical particles, having a diameter 0.162 of the size of big particles, smaller particles are less effective while particles having d/D = 0.42 does not allow to obtain any increase in critical cracking thickness of the top-coat formulation. This behavior suggest that PTFE particles during drying should bring forth random close structure, which is theoretically best filled by small particles having d/D = 0.155 [1].

This particle packing corresponds to the best film forming ability of the bimodal dispersion and seems to be quite sensitive to structure interference. Indeed particles with almost the same d/D ratio (=0.154) having a rod-like shape, N40R, seems to be less effective in increasing CCT. Fig. 3 reports a comparison between N40 and N40R showing that the two nano-dispersions are different also in terms of efficiency determined from the slope of the first part of the curves (effect at low amount of small particles added). The effect of nano-PTFE dispersion polydispersity, investigated in the set of experiments 5–7, is reported in Fig. 4 using the behavior of dispersion N40 as reference. It is immediately apparent that polydispersity results in a lower effectiveness of small particles (lower limit value of CCT) and the best results are still obtained with polydisperse 40 nm (calculated diameter) nano-PTFE. As for the “80 nm” particles, they show a lower effect on CCT, even with respect to their N40 content (2.8% when 7% of nanodispersion is added). This result is in general agreement with the observed negative effect of polydispersity of the small particles. A whole comparison among the different kind of nanoparticles, in terms of both efficacy and efficiency, is given in Table 4 that clearly shows that spherical PTFE having a d/D = 0.162 with respect to the big particles permit to get best results in terms of film forming performances. The impact of bimodality on film structure and film properties was also investigated. The surface structure of sintered PTFE films were compared by AFM analysis and the results

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CCT increase (µm)

35 30 25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

nano PTFE (% b.w. dry polymer) 40nm sph

40 nm rod

Fig. 3. Effect of nanoparticles shape on CCT increase of PTFE blends (CCT of PV20D dispersion alone = 30 ␮m).

CCT increase (µM)

35 30 25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

nano-PTFE (%b.w. dry polymer) 40nm

80nm (broad)

32nm (broad)

40nm (broad)

Fig. 4. Effect of nanoparticle polydispersity on CCT increase of PTFE blends (CCT of PV20D dispersion alone = 30 ␮m).

are reported in Table 5 in terms of line profile analysis showing a significant improvement of the surface roughness. Fig. 5 shows the micrographs of films with no nanoparticles and containing 20% of N40: the closer particle packing that is formed during film drying results in a more compact film Table 4 Film forming properties (analysis of CCT data reported in Figs. 2–4) Dispersion ID

N20 N40 N40R N100 32 nm broad 40 nm broad 80 nm broad

d/D

0.077 0.162 0.154 0.423 0.123 0.154 0.308

CCT increase Efficacy (% of PV20D CCT)

Efficiency (slope)

60 100 80 0 67 87 50

3.5 6.9 4.2 0 4.2 13 2.6

structure which is maintained after sintering giving a less porous coating. These differences in film structure also allow to have better coating performances as shown in Table 6. We can see from the table that coating containing 10% N40 (d/D = 0.162) has better appearance and higher scratch resistance even at high temperature. Such performances are not reached with N100 nanoparticles that have d/D = 0.423, although they show better behavior than monomodal dispersion.

Table 5 Film roughness measured by AFM analysis Microparticles

Rms (A)

Average (A)

0 10% N40 20% N40 30% N40

384 337 214 188

306 262 169 147

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polymerized in microemulsion. These coatings show improved performances in terms of film formation, appearance and mechanical strength due to their more compact and less porous structure. The experimental work described in this paper demonstrates that PTFE bimodal dispersions have optimal film forming and particle packing behavior at a size ratio of about 0.15, and no significative improvements in film forming characteristics are found by using small particles with size ratio above 0.3 [14,15]. These results are in good correspondence with the results obtained with polystyrene and polymethyl methacrylate latexes [1]. Authors had found that PS and PMMA bimodal latexes behavior can be described using random close packing model having characteristic pore size dimension of about 0.155 related to large particles size. The best rheological properties were obtained at 0.128 size ratio, meanwhile small particles with size ratio about 0.36 gave a negative effect. Further studies on multimodality effect [16] under the applicative and rheological point of view are now in progress. References

Fig. 5. AFM micrographs of monomodal and bimodal (d/D = 0.162) sintered films.

Table 6 Coating characteristics Gloss

PV20D 10% N40 10% N100

23 28 24

Scratch hardness (gr.) 25 ◦ C

100 ◦ C

180 ◦ C

1000 1500 1100

750 1100 850

500 750 600

4. Conclusions High performance coatings have been obtained from novel PTFE bimodal dispersions prepared by blending emulsion polymerized latexes (260 nm) with nanoparticles dispersions

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