Article pubs.acs.org/JPCB
Study of Polymer Particles Suspensions for Electrophoretic Deposition M. Federica De Riccardis,*,† Virginia Martina,‡ and Daniela Carbone† †
ENEA - Unità Tecnica Tecnologie dei Materiali Brindisi (UTTMATB), C.R. Brindisi, SS. 7 APPIA KM 706, 72100 Brindisi, Italy CETMA, Brindisi, SS. 7 APPIA KM 706, 72100 Brindisi, Italy
‡
ABSTRACT: Recently a great interest has been expressed in electrophoretic deposition (EPD) of polymers, both as particles and as chains. It is generally accepted that also for polymer particles, the Derjaguin−Landau− Verwey−Overbeek (DLVO) theory is valid, therefore, in principle, polymer suspensions suitable for EPD could be easily obtained by dispersing polymer particles in an aqueous or nonaqueous medium. Nevertheless, this work demonstrated that in order to obtain good quality deposits based on poly(ether ether ketone) (PEEK) and poly(tetrafluoroethylene) (PTFE), some additives have to be used. In the case of PEEK, a dispersant providing citrate anions was successfully used, whereas for PTFE a steric suspension stabilization was reached by adding polyvinylpyrrolidone (PVP). In such a way, codeposition of PEEK and PTFE was achieved. The efficiency of the EPD process was demonstrated by means of differential scanning calorimetry (DSC) measurements. A thermal program consisting of heat/cool/heat cycles at a low rate was used in order to evaluate the crystalline amount of each polymer in the deposits. In order to explain the obtained results, it needed to also consider the dimension and structural characteristic of the polymer particles.
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INTRODUCTION In recent years, electrophoretic deposition (EPD) has been successfully applied to polymers, also together with ceramic particles.1−5 Some efforts have been made in order to understand the interparticle interactions and the behavior of polymer particles when they were inserted in suspensions, more or less concentrated.6,7 Particular interest was also focused on electrostatic interactions and on the validity of the Derjaguin− Landau−Verwey−Overbeek (DLVO) theory7,8 when the suspension liquid was nonaqueous.9,10 On the other hand, several papers reported on the effect of an electric field on polymer chains, investigated by simulating the interface growth and the density profile of a polymer deposit on an impenetrable substrate.11−13 Nevertheless, the actions of the additives are not clear, for example, a dispersant or a deflocculant, in a suspension where the polymer is in the form of a solid particle with a finite dimension rather than as a long chain. However, the considerable number of published papers on this subject demonstrates the large interest in the polymer deposition process, as the use of polymers is forever increasing, especially in the fields of bio-, nano-, and hybrid materials.14−16 Among polymers used for coating applications, a high performance polymer such as poly(ether ether ketone) (PEEK) is particularly attractive because it has excellent mechanical, chemical, and thermal properties in respect to other polymers. In spite of its outstanding properties, PEEK exhibits a high friction coefficient in dry sliding, which restricts its tribological applications. In order to reduce the friction coefficient of PEEK © 2012 American Chemical Society
and therefore to increase its range of applications, some solid lubricants, such as glass and carbon fibres, graphite or carbon nanotubes,17−20 as well as polymers with low surface energy, such as poly(tetrafluoroethylene) (PTFE),21−23 were added to the PEEK matrix. Being interested in low friction coatings, we studied the possibility of obtaining PEEK-based coatings by the use of the EPD process, and also by mixing PEEK with other materials, both organic and inorganic. With respect to other deposition techniques such as thermal spraying or printing processes,24−31 EPD does not induce polymer degradation, but produces coatings with low roughness, and is able to create patterned or miniaturized devices. Recently, EPD has demonstrated to be a suitable method to obtain polymer coatings based on PEEK.1−3,32,34,35 Corni2,3 has obtained good quality coatings by using EPD suspensions where PEEK particles were dispersed in ethylic alcohol without any additive. On the contrary, we observed a certain precipitation in PEEK powder suspensions when a dispersant was used.32 This result is probably due to different PEEK powder characteristics. In order to get a good stabilization of an ethylic suspension containing PEEK particles, we used a mixture of citric acid and triethylamine (hereinafter referred to as CT), which has Special Issue: Electrophoretic Deposition Received: May 28, 2012 Revised: September 13, 2012 Published: October 5, 2012 1592
dx.doi.org/10.1021/jp3051752 | J. Phys. Chem. B 2013, 117, 1592−1599
The Journal of Physical Chemistry B
Article
already been successfully used by the authors as dispersant in ceramic suspensions.33 Regarding PTFE, to the knowledge of the authors, the application of EPD to PTFE suspensions was reported in only two papers.4,5 In one case, a commercial aqueous suspension of PTFE was used; in the other one, acetone was used as dispersion medium. In both the cases, PTFE was mixed with inorganic powders (ruthenium oxide and titanium oxide, respectively), which assisted the polymer particles in the deposition. As a relevant result, we prepared and used a suspension where only PTFE powder was present. In order to make possible EPD of PTFE, an organic dispersant, polyvinylpyrrolidone (PVP), was used. In this paper we reported the study of electrophoretic polymer suspensions, by investigating the effect of CT and PVP on the stabilization on PEEK and PTFE suspensions, respectively. Moreover, the efficiency of the EPD process was evaluated by means of a thermal analysis of deposited coatings, performing a specific cycle of measurements. The proposed procedure is innovative and could be generally adopted to determine the amount of different polymers in a compound.
Table 1. Composition of the Investigated Deposits and Voltage Applied for Deposition name
% PEEK suspension
% PTFE suspension
voltage (V)
PEEK MIX3 MIX1−100 MIX1−75 MIX1−50 MIX1−25 MIX2 PTFE
100 80 50 50 50 50 20 0
0 20 50 50 50 50 80 100
100 100 100 75 50 25 100 100
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RESULTS AND DISCUSSION Suspension Optimization. PEEK is a polyaromatic semicrystalline thermoplastic polymer (typically with 30−40% crystallinity), formed by three aromatic rings and a carbonyl group. It is thermally stable up to 580 °C, with a glass transition temperature equal to 145 °C, a crystallization temperature equal to 250 °C, and a melting temperature equal to 340 °C. Polytetrafluoroethylene is a hydrophobic fluorocarbon compound, with a melting temperature equal to 325 °C. It is highly nonreactive, because of the strength of carbon−fluorine bonds, and so it is often used in containers for reactive and corrosive chemicals and as a lubricant. The chemical structure of PEEK and PTFE was shown in Figure 1.
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EXPERIMENTAL DETAILS PEEK powder (semicrystalline, particle size
