The advantages of networking

Transcription

1 The advantages of networking Poly(vinylidene fluoride)-acrylic IPN structures for solvent and waterborne coatings. Kurt A. Wood. Poly(vinylidene fluoride) (PVDF) is a semi-crystalline fluoropolymer which is unique in its ability to be compounded with acrylic resins. Interpenetrating network (IPN)-type resin blend morphologies with around 70wt% PVDF have particular performance advantages. The exterior weatherability is excellent and the inherently thermoplastic nature confers significant advantages in gloss retention and chalk resistance. The outstanding outdoor weatherability of "Kynar 500" PVDF solvent paints is well demonstrated by over thirty years of commercial use on premier architectural structures [1]. Like many fluoropolymers, PVDF is semi-crystalline, but it is unique among fluoropolymers in its versatility in coatings applications, because of its compatibility with a certain class of conventional resins, and many common ketone and ester solvents. Commercial PVDF paints have a hybrid binder [2]. Typical solvent formulations contain 70-80wt% PVDF and 20-30wt% of a miscible acrylic resin (e.g. a methyl methacrylate-ethyl acrylate copolymer). These are baked to achieve a semi-crystalline interpenetrating network (IPN)-type morphology with a highly crystalline pure PVDF phase, and an amorphous phase containing all the acrylic plus some of the PVDF. The PVDF polymer itself is highly inert, and is completely resistant to UV-A and UV-B radiation [3]. In this context, for highly weatherable paints, it is the acrylic polymer which must be considered the "weak link" of the binder-even though the acrylics which are used are among the most weatherable non-fluorinated organic polymers. The particular mass ratios in the vicinity of 70:30% PVDF/acrylic are preferred in part because they give an excellent balance of properties for typical architectural applications. Blends at levels of 70% PVDF or higher are able to maintain the pure PVDF phase as a dominant component in the coating film (Figure 1), contributing to the excellent weathering and chemical resistance observed at these proportions. Below 70% PVDF, the amount of pure PVDF phase drops off dramatically, and the binder properties are dominated by the amorphous phase, which is predominantly acrylic. Pigment choice is also important As might be expected, to achieve highly weatherable architectural paints with a PVDF-acrylic binder, the choice of pigment is also important. Highly stable inorganic metal oxide pigments are preferred for these applications. These pigment grades should not undergo any kind of chemical or photochemical change during the application process or outdoor weathering, which would lead to colour shifts. With stable pigment systems of this sort, the strategy for maintaining paint colour durability can shift to a focus on maintaining the integrity of the paint binder. Catastrophic binder degradation around surface pigment particles (chalking) becomes the dominant mechanism for colour change (Figure 2). One reason why 70% PVDF coatings have been so successful, is that their inherent resistance to chalking is excellent, so that even very weathered PVDF coatings, at 20 years or more Florida exposure, show only very modest colour change compared to their original colour. Starting from the same base raw materials (70% PVDF, 30% miscible acrylic, and inorganic pigment), it is possible to generate coating films with an IPN-type binder resin morphology in nearly every general coating class: waterborne coatings, powder coatings and true solution coatings, in addition to solvent dispersions. However, comparing the solvent-borne and water-borne dispersion routes, the details of how the IPN morphology is achieved are considerably different, because of the differences in the film formation process. Nevertheless, in both cases, the coating resin structures which are obtained have outstanding weatherability properties. In particular, the PVDF resin protects the acrylic resin from photochemical attack. Moreover, on a macroscopic, visible level, the resulting coatings have an exceptional degree of resistance to chalking, especially when compared to thermoset coatings such as polyesters or even many other fluorinated resins. In commercial PVDF paints, the PVDF is applied as a non-aqueous dispersion, using latent solvents such as isophorone [4] (Figure 3). The acrylic co-resin is dissolved in the solvents. To generate the hybrid IPN binder structure, the coating is baked at temperatures above 200 C. These temperatures are above the PVDF crystalline melting point ( C). Assisted by the latent solvents, the two resin components form a miscible alloy, and upon cooling, form the IPN structure as some of the PVDF crystallizes into spherulites. The PVDF spherulites grow from the melt in a dendritic fashion, and the remaining resin material, which forms the amorphous phase, is interleaved between the crystalline lamella. This amorphous material contains all the acrylic, plus some of the PVDF. Some fraction of individual PVDF polymer chains bridge the interface between the amorphous and crystalline regions, forming effective labile crosslinks which reinforce the overall structure. This complex morphology, with structure at several different length scales, is responsible for the advantageous balance of properties seen for 70% PVDF systems (e.g. high solvent resistance coupled with high flexibility). Film formation and IPN generation in waterborne systems Since the film formation process is considerably different for latex and solvent dispersions, a different strategy is used to generate IPN films in PVDF latex systems. PVDF-acrylic latex dispersions form films at room temperature, in a manner analogous to other kinds of latexes, such as acrylics, where the film formation process has been extensively studied [5, 6]. The standard model for latex film formation involves three steps: 1) evaporation of bulk water and latex particle packing; 2) particle deformation with collapse of the interstitial voids between latex particles; 3) the interdiffusion of polymer chains across "membranes" between the residual particles leading to full mechanical properties. Since the internal latex particle morphology (distribution of resins within each latex particle) is substantially unaffected by the early stages of latex film formation, it is highly desirable to begin with an IPN-type internal latex particle morphology, if the desired goal is an IPN structure in the final paint film. Latex systems incorporating both PVDF and acrylic resins at a 70:30 weight ratio, similar to that used in commercial solvent dispersions, can be generated in a variety of latex particle morphologies. In particular, using a seeded emulsion polymerization approach, either core-shell or IPN-type structures can be generated. The core-shell

2 materials have a semi-crystalline PVDF fluoropolymer core and an acrylic polymer shell. The materials with an IPN-type structure contain a relatively homogeneous distribution of the acrylic and a PVDF copolymer within each latex particle, although there is still a small amount of residual PVDF crystallinity left in these materials. In both cases, acrylic polymers were chosen which are thermodynamically miscible with the PVDF fluoropolymer resin [7, 8], so that the latex and film morphology are determined primarily by kinetic rather than thermodynamic factors. The intimacy of the polymer mixing in the actual materials is determined by various analytical techniques. One of the more elegant and direct techniques uses cross-polarization NMR to directly detect different nuclei in different resins which are within Angstroms of each other. For the case of latex film formation under ambient conditions (below the PVDF crystalline melting point), it appears to be the case that the latex particle structure is maintained during the particle compaction and deformation stages of film formation (Figure 4). For the case of a core-shell particle morphology (hard fluoropolymer core/soft acrylic shell), film formation tends to lead to a "filled" film morphology (acrylic continuous phase with fluoropolymer inclusions) [9]. In this case, since the component resins are thermodynamically miscible, baking at temperatures above the fluoropolymer melting point can create a more homogeneous polymer morphology. For the case where the latex particles have an IPN structure, a relatively homogeneous IPN film morphology is generated during film formation at room temperature, as the original IPN morphology is maintained. Two mechanisms explain outstanding performance There are at least two separate mechanisms which can be invoked to explain the outstanding outdoor weatherability of PVDF-acrylic coatings, when compared with coatings based on non-fluorinated resins, and both are linked to the IPN morphology of the coating films. First, there is a protective effect exercised by the PVDF resin, whereby the photochemical degradation rate of the acrylic co-resin is dramatically reduced, relative to the rate of degradation that would occur in a neat acrylic film. Second, on a macroscopic level, after extended outdoor exposure times, the resistance to chalking is dramatically better in PVDF-based coatings compared to other resin types, particularly thermoset resins such as polyesters and even thermoset fluoropolymer systems. This is attributable both to the thermoplastic nature of the PVDF coatings, and to the enrichment of the highly weatherable PVDF in the top layers of the coating as it weathers. The ability of PVDF resins to protect the acrylic co-resin against photochemical attack can be demonstrated directly in gravimetric experiments using clear coats. Accelerated tests using UV-B radiation are particularly effective for measuring this, since the UV-B radiation, centred at 313nm, is rather aggressive toward acrylic resins, while PVDF resins are not affected at all by these solar UV wavelengths. In such a test unit, the weight loss of an acrylic clear coat on a 10 x 15cm panel will typically be on the order of tens of milligrams/1000 hours exposure. Compounded into a PVDF clear coating at a 30wt% level, the weight loss rate is reduced practically to zero, indicating that the acrylic photodegradation rate on an equivalent mass basis is reduced by at least an order of magnitude, when it is compounded in with the acrylic. The same effect can be observed in outdoor exposure experiments. In one test, a series of PVDF coatings weathered in Arizona, a high UV environment, showed no measurable loss of coating thickness after fifteen years exposure (Figure 4). As might be expected, to achieve a substantial protective effect from the PVDF resin, the extent of resin mixing is crucial. Positive results are obtained from the use of thermodynamically miscible acrylic resins, with a very intimate degree of mixing to attain an IPN-type structure. One demonstration of this can be seen by comparing the weight loss of clear coats of PVDF-acrylic latex dispersions, generated with different resin morphologies. The weight loss rate for films with an IPN-type structure (bi-continuous resin phases) is about an order of magnitude lower than the rate for a film with a core-shell structure (acrylic continuous phase with PVDF inclusions) [10]. Furthermore, when core-shell type films are annealed at temperatures above the PVDF melting point in order to generate a miscible, IPN type structure (similar to the film formation process for solvent dispersion coatings), the lower weight loss rate is obtained. The exact reasons for this PVDF protective effect are not known, but it is reasonable to suppose that at a minimum, physical protection is involved - i.e. the superior barrier properties of the PVDF resins to agents such as water and oxygen would shield those acrylic resins in the interior of the film. As the above latex results indicate, this protective effect is seen in both high and low crystallinity PVDF systems. PVDF outperforms acrylic and urethane resins in colour change tests Industry standards for colour change and chalking in architectural coatings can be found in specifications generated by industry groups such as the American Architectural Manufacturers Association (AAMA). A number of the AAMA specifications have five or ten year requirements for the gloss and colour retention, and the chalking resistance, for actual painted building components in South Florida exposure testing. Figure 5 shows some representative five year Florida weathering results, for a series of colour matched commercial spray coatings from the same coatings manufacturer, using a variety of different resin chemistries. The conventional acrylic and urethane resins can be seen to be highly weathered, and are showing considerable chalking. Chalking is beginning at the five year point for a crosslinked fluoropolymer coating based on a fluorinated ethylene-vinyl ether (FEVE) polyol, while no chalking is observed for the PVDF coatings. These results can be understood in terms of the effects that photooxidation has on the underlying structure of the paint binder in each system. For thermoset coatings, such as conventional polyesters, "superpolyesters", acrylic-urethanes, and FEVE polyol-based systems, the binder integrity is maintained, and pigment particles are secured, through the crosslinked network structure. As photooxidative events near the coating surface gradually disrupt the network connectivity, eventually a point is reached where the network integrity is lost, and catastrophic failure of the coating binder occurs. Macroscopically, these events manifest themselves in a sudden loss of gloss (sigmoidal gloss retention curve), and dramatic colour change coupled with chalking, as the binder is no longer encapsulating pigment particles at the surface. For thermoplastic coatings such as PVDF coatings, the effect of photooxidative events is considerably different. Since the integrity of the coating binder is attained through labile polymer entanglements, photooxidative events which lead to chain scission and eventual loss of low molecular weight fragments do not inherently disrupt the binder's integrity. Gloss loss occurs gradually, through the progressive roughening of the coating surface as it contracts as binder material is lost from the near surface regions of the coating. A catastrophic loss of properties does not occur, although chalking will still occur, along with colour change,

3 when there is enough binder surface erosion to leave free pigment particles at the surface. In 70% PVDF systems, the excellent chalk resistance and colour retention must be attributed not only to this thermoplastic character of the binder, but also more specifically to its molecular structure. Since the PVDF component of the binder is inherently insensitive to degradation, the extent of shrinkage or erosion of the coating which could occur during natural weathering should primarily be limited to the 30% acrylic which is in the coating. As long as there is an IPN binder morphology, with PVDF dominating a continuous phase, differential loss of the acrylic would be expected to leave pigment particles near the surface encapsulated by the resin, with the surface resin component being enriched in PVDF. This kind of surface enrichment has been observed in a number of experiments, and explains why chalking and colour change in PVDF coatings can be so minimal, even after many years of South Florida exposure. - Interpenetrating network (IPN) morphologies with around 70wt% PVDF give coatings with excellent exterior weatherability. - Beginning from either solvent or aqueous dispersion formulations, IPN-type resin blend morphologies can be obtained using blends of PVDF and miscible acrylics, through careful design of the resin components, and by taking into account the film formation mechanism. The author: -> Kurt A. Wood is a Principal Scientist in the Technical Polymers Division at Arkema, Inc.'s laboratories in King of Prussia, Pennsylvania, USA. He has been working in "Kynar" PVDF coatings research since He received a PhD in Physical Chemistry from the University of California, Berkeley. This paper was presented at the 8th Nurnberg Congress, Creative Advances in Coatings Technology, Nuremberg, Germany, 25/26 April 2005 IPN morphologies with 70% PVDF perform particularly well Beginning from either solvent or aqueous dispersion formulations, IPN-type resin blend morphologies can be obtained using blends of PVDF and miscible acrylics, through careful design of the resin components, and by taking into account the film formation mechanism. IPN morphologies with around 70wt% PVDF have particular performance advantages. The exterior weatherability of these compositions is excellent due to several factors. In intimate IPN blends, the presence of the PVDF component reduces the photooxidation rate of the acrylic component. Colour retention and chalk resistance is also excellent, compared to thermoset systems such as conventional polyesters and systems made with fluorinated polyols, because any photooxidative loss of the acrylic component leaves the pigment particles still encapsulated by PVDF resin. Moreover, any gloss loss which occurs is gradual rather than catastrophic, due to the inherently thermoplastic nature of the PVDF coating. References [1] "Kynar 500" PVDF for coatings has been in continuous commercial production since it was introduced by the Pennsalt Company in 1965 [2] S.R. Gaboury, K.A. Wood, Surf. Coat. Intl. Part B: Coatings Trans B4 243 [3] J.-L. Philippart, N. Siampiringue, A. Strassel, J. Lemaire, Makromol. Chem [4] R.A. Iezzi, "Fluoropolymer coatings for architectural application," in Modern Fluoropolymers; John Wiley & Sons Ed., 1997; Chap. 14, 271 [5] M.A. Winnik, "The Formation and Properties of Latex Films", in Emulsion Polymerization and Emulsion Polymers; (John Wiley and Sons, 1997; Chapter 14, 467 [6] P.A. Steward, J. Hearn, M.C. Wilkinson, Adv. Coll. Interface Sci [7] W. Veeman, Makromol. Chem Macrol. Symp [8] D. Hourston, I. Hughes, Polymer [9] K.A. Wood, L. Hedhli, J. Willcox, J. Coatings Technology 2002(924) [10] K.A. Wood, C. Cypcar, L.. Hedhli, J. Fluorine Chem Results at a glance - Poly(vinylidene fluoride) (PVDF) is a semi-crystalline fluoropolymer which is unique in its ability to be compounded with acrylic resins.

4 Figure 1: Proportion of pure PVDF phase in blends of PVDF and miscible acrylic at different weight ratios.

5 Figure 2: Comparison of colour matched "Kynar 500" and polyester powder coatings, 12 year South Florida exposure.

6 Figure 3: A schematic representation of the film formation process for "Kynar 500" solvent dispersion coatings.

7 Figure 4: Evolution of the coating thickness of "Kynar 500" coatings, exposed in Arizona for 15 years.

8 Figure 5 : Comparison of weathering and chalking for a series of colour-matched commercial spray coatings from the same coatings manufacturer, after five years South Florida exposure (South 45)* * The bottom half of each panel is washed annually, while the top remains unwashed and is used for the chalk test. The original colour can be seen at the very top of the panel.