Elsevier

Polymer

Volume 107, 19 December 2016, Pages 249-262
Polymer

PDMS-based self-replenishing coatings

https://doi.org/10.1016/j.polymer.2016.11.026Get rights and content

Highlights

  • PDMS low-surface-energy blocks to build self-replenishing hydrophobic coatings.

  • PDMS acting as attractive eco-friendly alternative for low surface energy coatings.

  • Study of network flexibility, dangling chains segregation and surface reorganization.

  • Characterization of initial and damaged surfaces with water CA and XPS.

Abstract

For the first time, we report the use of PDMS low surface energy blocks to build self-replenishing hydrophobic coatings. As the use of 8-fluorinated-carbon (Teflon-like) end groups (Rf8) has been perceived as a serious concern in industry due to its potential harm for health and environment, alternatives are sought. Poly(dimethyl siloxanes) (PDMS) have become an attractive eco-friendly alternative as low surface energy building blocks for self-replenishing surfaces with low environmental impact. Hence, new coating systems with PDMS-based chain ends were investigated. Following the design criteria for self-replenishing coatings, several important aspects have been characterized, such as network flexibility, dangling chains surface segregation and surface reorganization ability. After demonstrating that all the design criteria have been met, the self-replenishing behavior after intentional damage was examined via characterizations with water contact angle and XPS measurements. Based on the results, we conclude that the chemical composition at the damaged surfaces was recovered with a clear PDMS enrichment, but that the hydrophobicity recovery is dependent on the environmental temperature. This extraordinary temperature dependence seems to be due to the hindrance on full reorientation of the dangling chains by the PCL spacer crystals formed at the air interface. The effect of PDMS block molecular mass Mw and PCL spacer length are discussed in terms of surface segregation, reorganization and self-replenishing behavior.

Introduction

Low surface energy materials are indispensable components to obtain coating surfaces with advanced properties such as hydrophobic [1], [2], [3], easy-to-clean [4], [5] and anti-fouling behavior [6]. The most well-known low surface energy material is probably polytetrafluorinated ethylene (PTFE). For coating applications, PTFE-based materials can be used as additives to enhance the surface hydrophobicity, for instance, via a self-stratification approach, so that the bulk properties of the coatings remain unchanged. Low molecular mass fluorinated Teflon-like polymer chains -(CF2)nF (with n = 6–10) are usually preferred in coatings, due to less incompatibility issues while providing an equivalent level of hydrophobicity as compared to high molecular mass PTFE [7].

The intrinsic problem of self-stratified hydrophobic coatings is their vulnerability to surface damage. The hydrophobicity can be significantly reduced due to the removal of the surface segregated fluorinated layer upon surface damage. Therefore, a self-replenishing concept has been developed to address this issue by using a 8-fluorinated-carbon group (-(CF2)8F, or Rf8) tethered to crosslinked polyurethane networks via a polycaprolactone (PCL)-based spacer. In this proof of principle research, the Rf8 groups could drive the dangling chains (DC) preserved in the bulk to reorient towards the new interfaces created by the damage, so that the surface hydrophobicity could be self-repaired [8] (Scheme 1). Based on further investigations by a combined experimental-simulation approach, three basic requirements were identified to design self-replenishing hydrophobic surfaces [9], [10]. One of them is the presence of low surface energy chain ends (such as the Rf8 block) offering the driving force for the re-orientation of the dangling chains and recovery of the low surface energy on the newly created surfaces.

Self-replenishing hydrophobic surfaces are of high potential for hydrophobic coating applications as this can extend their service life-time. However, the use of Rf8 blocks is under great regulation pressure due to its harm to health and environment [11], [12]. Therefore, finding new and eco-friendly low surface energy groups to substitute Rf8 blocks is a crucial step to push self-replenishing hydrophobic coatings further to real applications. In recent decades, organosilicone-based materials have become an attractive alternative material for coatings [13]. Among this category of materials, polydimethylsiloxanes (PDMS) are known for their low surface energy [14], [15], which provides interesting properties, such as low friction, low adhesion [16], [17], [18] and hydrophobicity [19], [20]. Moreover, PDMS is also known for its chemical stability and low environmental impact, making it a promising option to substitute the Rf8 blocks to obtain “green” self-replenishing hydrophobic coatings. Finally, while the hydrophobicity of fluorinated groups depends on the amount of fluorination on the carbon atom [21], the hydrophobicity of PDMS has been found to be dependent on its weight percentage and molecular mass (Mw) [22], [23].

Surface segregation and reorganization ability of chain ends are amongst the most important prerequisites to enable self-replenishing behavior. For PDMS chain ends, the surface segregation has been studied in crosslinked networks [22], [24], block copolymers [19], [25] and their blends [25]. It has also been shown that PDMS can reorganize with changing environmental factors, e.g., temperature [26], [27] and contact medium [28], [29], and such behavior was found to be dependent on the PDMS weight percentage and average molecular mass [19], [26]. Tezuka et al. [19] for instance, revealed that in a PDMS-PU block copolymer, the PDMS reorganization is reversible and the kinetics are highly dependent on the length of the PDMS block. For Mw above 2400 Da and at identical siloxane content, the longer PDMS chains respond slower to environmental changes than the shorter ones [26]. This reversible reorganization could also be monitored by the dynamic recovery contact angle (DRCA) method via sequentially changing the contact medium between water and air, repeatedly. As investigated by Esteves et al. [24], the surface hydrophobicity of poly(styrene-maleic anhydride) crosslinked networks grafted with PDMS chain ends dropped, when the sample was immersed in water for a long period to reach its equilibrium, but autonomously recovered to the original value when exposed to air again, within tens of minutes. In order to achieve self-replenishing coatings, it is required to have a polymeric-spacer bonded to the PDMS block to improve its compatibility with the binder. In this way, PDMS low surface groups could be preserved in the bulk of the coatings. A compatibilization effect via a pre-bonded polymeric block of PDMS in the PU crosslinked networks has been reported by Rabnawas et al. [30]. For the self-replenishing purpose, a linear chain structure is preferred in order to improve the PDMS mobility span, so that a larger range of surface reorganization is available to enrich the surface with the PDMS blocks, whenever surface damage occurs. However, neither the surface segregation/reorganization of PDMS-based dangling chains connected to a PCL spacer (mimicking the Rf8-based dangling chains), nor their self-replenishing ability have been studied so far.

In this paper, self-replenishing hydrophobic coatings based on PDMS low surface energy groups are reported. First, new dangling chains with PDMS chain ends were synthesized, with different number average molecular mass (Mn) of PDMS (from 500 to 2000 g/mol) and degree of polymerization (DP) of a PCL spacer (16 and 32), following the design requirements for self-replenishing surfaces. For the purpose of comparison with the model system based on dangling chains containing Rf8 blocks previously reported [8], a thermal curing system was chosen for these new PDMS-dangling chains polyurethane-based hydrophobic coatings. Second, the surface segregation of the DC-PDMS in non-cured coating mixtures was investigated by surface tension measurements, and the surface hydrophobicity of the cured coatings was evaluated by contact angle measurements. Third, the surface reorganization of the DC-PDMS in the PU crosslinked networks was assessed by the DRCA method, as previously reported [24]. As all the design criteria for a self-replenishing surface were met, the self-replenishing ability was examined by damaging the coating surfaces via cryo-microtoming and characterizing the hydrophobicity recovery of the damaged surfaces. The self-replenishing ability dependence on the PDMS block Mn and the differences observed upon comparison with the model Rf8-based dangling chains are discussed.

Section snippets

Materials

PDMS with a mono-hydroxyalkyl terminal group with different Mn of approximately 500, 1000 and 2000 g/mol (denoted as PDMS500, PDMS1000 and PDMS2000, respectively) were kindly offered by BYK-Altana (Wesel, Germany) and dried with molecular sieves (4 Å) prior to use. ɛ-Caprolactone (ɛ-CL, purity 99%) for synthesizing the dangling chains containing the PDMS chain ends was purchased from Alfa Aesar and distilled at reduced pressure and high temperature (∼ 170 °C) before use. Fumaric acid was

Synthesis of PDMS-based dangling chains (DC-PDMS)

The first step to obtain self-replenishing coatings with more environmentally friendly components was to synthesize dangling chains based on PDMS chain ends. The mono-OH functional PDMS blocks enable to grow a PCL spacer from one end by ROP of ɛ-CL, forming PDMS-PCL diblock polymers. Fumaric acid, a weak acid and insoluble in the monomer, was used as the catalyst to carry out ROP via the activated monomer mechanism, in order to minimize side reactions. Two different ɛ-CL/PDMS mole ratios were

Conclusions

Following the design requirements for self-replenishing hydrophobic coatings, the typical 8-fluorinated carbon end group (Rf8) used in low surface energy dangling chains was replaced by a more environmentally friendly PDMS block. After successfully synthesizing dangling chains containing PDMS as chain ends (DC-PDMS), polyurethane (PU)-based coatings were prepared by thermal curing. For these coatings, the surface segregation ability of DC-PDMS was demonstrated for both its liquid non-cured

Acknowledgement

This research forms part of the research programme of the Dutch Polymer Institute (DPI), project #758 (SER-LED).

References (43)

  • X.F. Wang et al.

    J. Colloid Interf. Sci.

    (2008)
  • R.E. Banks et al.

    Process Saf. Environ. Prot.

    (1998)
  • K. Tokuhashi et al.

    J. Fluor. Chem.

    (2004)
  • I.D. Gunbas et al.

    Prog. Org. Coat.

    (2013)
  • S. Holberg et al.

    Prog. Org. Coat.

    (2014)
  • R.D. van de Grampel et al.

    Prog. Org. Coat.

    (2002)
  • Y. Zhang et al.

    Polymer

    (2015)
  • D.G. Neerinck et al.

    Thin Solid Films

    (1996)
  • X.P. Wang et al.

    Polymer

    (2007)
  • D. Chandler

    Nature

    (2005)
  • L.C. Gao et al.

    J. Am. Chem. Soc.

    (2006)
  • X.M. Li et al.

    Chem. Soc. Rev.

    (2007)
  • R. Blossey

    Nat. Mater.

    (2003)
  • K.S. Liu et al.

    Annu. Rev. Mater. Res.

    (2012)
  • A.G. Nurioglu et al.

    J. Mater. Chem. B

    (2015)
  • T. Dikić et al.

    Adv. Mater.

    (2012)
  • A.C.C. Esteves et al.

    Macromolecules

    (2013)
  • A.C.C. Esteves et al.

    J. Chem. Phys.

    (2014)
  • D. Graiver et al.

    J. Polym. Environ.

    (2003)
  • E.F. Hare et al.

    J. Phys. Chem.

    (1954)
  • P. Somasundaran

    Encyclopedia of Surface and Colloid Science

    (2006)
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