Hydrolyzable polymer additive-based silicone elastomers

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Specific properties of the erodible biocidal coatings

The physico-chemical properties of the water in which the coating is immersed are undeniably affecting the erosion process but in the following text the focus is done on the nature of the polymers. The mechanisms of CDP and SPC coatings are based on a hydration/ dissolution and hydration/hydrolysis processes, respectively. In other words, in one case the erosion is physically induced while in the other the erosion is chemically induced. In addition, depending on the nature of the hydrolyzable polymers, the coating can undergo either surface or bulk erosion [105–107]. Surface erosion occurs when hydrolytic cleavage of polymer chains is faster than the water penetration. On the opposite, bulk erosion occurs when the diffusion of water into the bulk material is faster than the hydrolysis process [105]. Many aliphatic polyesters such as poly(lactide) (PLA) and poly(ε-caprolactone) PCL undergo bulk degradation. The advantages of surface erosion are the linear erosion profile, which traduces a gradual surface renewal, and the conservation of the intrinsic mechanical properties. On the contrary, bulk erosion can lead to a massive swelling with no mass loss for a certain amount of time followed by a fast failure of the material (Figure I-11). The sudden material failure would drastically reduce the coating life time. Besides, bulk erosion generates porosity which can be source of hiding places for marine organisms. For antifouling applications, surface erosion is thus more suitable. A family of polymers that can undergo surface erosion is for example the poly(anhydride)s as long as the applied thickness is higher than 100 µm (Lcritical) according to Burkersorda et al. [108].

Other surface physico-chemical properties

Erodible coatings exhibit roughness values around 0.33 and 0.47 µm which are higher to that of FRCs (0.077-0.116 µm) [60]. This roughness can be source of additional frictional drag. Some values of friction increase rate (FIR) from commercial coatings are quoted in Table I-4. The self-polishing participates to the smoothing of the surface with immersion time which can limit the frictional drag of biocide-based AF coatings. The water contact angle of SPCs are around 80-90° [120,121]. Once immersed, SPCs can turn even more hydrophilic (down to 60°, [121]) due to the hydration and hydrolysis of the polymer which ends up more soluble in the water. The surface free energies of SPCs (not yet immersed in water) are usually between 33 and 36 mJ/m² (with P≈ 5-9 mJ/m²) against 23-26 mJ/m² [60]. γSP hydrophilicS and water swelling nature of SPCs (with ≤ 2 mJ/m²) for FRCs traducing the more γ

Advances made on erodible antifouling coatings at the research state

Since 2010, researchers have further broadened the choices of polymer binders for erodible antifouling coatings. Their objective was to explore novel copolymer microstructure to design efficient AF erodible coatings containing lower amounts of biocides. In the following section, four categories of recent polymers used for AF coatings are reported: (i) polymers with hydrolyzable main chains, (ii) polymers with hydrolyzable pendant groups, (iii) polymers combining (i) and (ii), and (iv). hydrolyzable polymer networks.
Biodegradable polyesters can be considered as promising binders for antifouling paints to prevent major environmental pollution [121–124]. They usually possess the following characteristics to be suitable for paint applications:
– Solubility in most common organic solvents;
– Compatibility with fillers and other additives;
– Controlled degradation;
– And molecule release.
Solubility and compatibility of hydrolyzable polymers in the paint formulations are usually not problematic as long as the solvent and additives are not source of undesired chemical reactions. Xylene, ethylene acetate, methyl isobutyl ketone or THF are the most employed solvents in paints [125–129].
Biodegradable polyesters have been extensively studied by Réhel’s and Zhang’s groups. Fäy et al. studied coatings degradation to compare the antifouling efficacy of surface erodible-coatings and bulk erodible-coatings [105]. The poly(phtalic acid-co-ricinoleic acid-co-isophtalic acid ester anhydride) (P(PA-co-RA-co-IPA) copolymer (Figure I-13, A) exhibited a surface erosion and displayed the second higher erosion rate behind poly(methyl methacrylate-co-butyl methacrylate) (PMMA-PBMA)/rosin-based coating (with bulk erosion process) in distilled water. However, the field tests revealed P(PA-co-RA-co-IPA) was totally eroded after 2 months due to the different physico-chemical properties of the natural seawater (pH, salinity and presence of microorganisms). Poly(ε-caprolactone-co-δ-valerolactone) P(CL-co-VL) (Figure I-13, B) and PMMA-PBMA rosin-based paints (with bulk erosion process) were more suitable antifouling binders. As mentioned previously in § I., the surface erosion is more adapted than bulk erosion for antifouling applications but as it was proven by Faÿ et al., there can be a huge difference of coating behaviors between laboratory erosion tests and real condition tests.
Copolymers such as PCL-b-PDMS-b-PCL were also investigated for antifouling applications (Figure I-13, C) [130]. Their particularity relies on the dual hard/soft nature of the triblock copolymer. The central PDMS block was intended to decrease the PCL crystallinity and also add fouling release properties thanks to the silicone chemistry. By changing the polymer crystallinity, its erosion rate was highly increased. Acrylic polyurethane hybrid materials were also studied as a novel self-polishing resin with a uniform erosion rate of 8-10 µm/month [131]. The self-polishing behavior is allowed by the central block of the poly(acrylic acid)-block-polyurethane-block-poly(acrylic acid) (PAA-b-PU-b-PAA) copolymer (Figure I-13, D). Another particularity of this binder was the addition of an ammonium salt-based acrylate monomer acting as a biocide.
Bressy’s group synthesized block and random copolymers of poly(methyl methacrylate- tert-butyldimethylsilyl methacrylate) (P(MMA-TBMSiMA)) and compared their erosion rate to a TBT-based-coating reference in dynamic conditions [112]. The resulting erosion rates were very similar to the TBT-coating for diblock copolymers containing 21-27 mol.% of hydrolyzable silylated groups with an erosion of 0.37-0.38 µm/day. The glass transition temperature of PMMA was essential to maintain a good mechanical stability and good film properties of the resulting coatings. Its presence also aimed for adjusting the erosion profiles. Bressy’s group has also studied the erosion profiles of trialkylsilyl polymethacrylate copolymers [112]. Some examples of poly(tri-alkylsilyl methacrylate)s with hydrolyzable side groups are presented in Table I-5. Hydrolytic degradations of the side groups is strongly dependent on the size of the alkyl group linked to the silicone atom [120]. The steric hindrance of the pendant moiety is responsible for lower hydrolysis kinetics. For instance, poly(bis(trimethylsilyloxy)methylsilyl methacrylate) (PMATM2) is less hindered than poly(triisopropylsilyl methacrylate) (TIPSiMA) and thus degrades faster.

Elaboration of conventional silicone elastomers

Silicone elastomers are a class of organic-inorganic materials with an alternation of silicon-oxygen atoms in the main chain. Each silicon atom carries two alkyl pendant groups, usually methyl groups. The elastomer material is obtained through the crosslinking of high molar masses polysiloxane chains carrying reactive end-chain functions such as –hydroxyl, -amino, -alkoxy, vinyl and hydrosilylated functions essential for the curing process. Most of silicone elastomers are prepared using three different crosslinking routes: (i) peroxide-induced free radical reactions of vinyl-functionalized polysiloxane oil (High Temperature Vulcanization), (ii) condensation reactions of hydroxyl, amino- or alkoxy-terminated polysiloxane oil (Room Temperature Vulcanization, RTV), and (iii) hydrosilylation or addition cure of vinyl and hydrosilylated polysiloxane oil [151]. The crosslinking of silicone elastomers can be very challenging for the FRCs manufacturers given that the conditions of application on ship hulls are not always ideal (for example at very low temperature).

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Peroxide-induced crosslinking of polysiloxane

High consistency rubbers (HCRs) are made of very high molar mass vinyl-based PDMS (50,000 to 200,000 g/mol) [152]. HCRs are crosslinked via free radical reactions at high temperature. A typical drawback of the peroxide cure reaction is the formation of a heterogeneous crosslinking due to the random distribution of vinyl groups [153]. The network may contain numerous dangling chains and entanglements between neighboring chains which can weaken the elastomer mechanical properties [152]. Additional fillers can be required to improve mechanical properties

Condensation cure system

The condensation cure is a nucleophilic substitution reaction occurring on the silicon atom in presence of a catalyst (Figure I-21). To form crosslinks, a hydroxyl-terminated polysiloxane is reacting at room temperature with small quantities of tri- or tetrafunctional silanes (RSiX3 or SiX4) such as the ones presented in Table I-7. The amount of crosslinking agents as well as the nature of their leaving groups will influence the crosslinking density and the cure rate, respectively. Low molecular weight alcohols are generated during this process.

Table of contents :

Chapter I State-of-the-art
I.1. Introduction
I.2. Marine fouling
I.2.1. Impact of biofouling on human activities
I.2.2. Biofouling as a process
I.3. Overview of the antifouling strategies
I.3.1. Biocidal antifouling strategy
I.3.2. Biocide-free antifouling coatings
I.4. Focus on biocidal antifouling coatings
I.4.1. Commercial biocidal antifouling coatings
I.4.1.1. Description of the biocidal coating composition
I. Insoluble-matrix coating
I. Soluble-matrix coating
I. Self-polishing coatings
I.4.1.2. Specific properties of the erodible biocidal coatings
I. Erosion process
I. Release process
I. Other surface physico-chemical properties
I.4.2. Advances made on erodible antifouling coatings at the research state
I.5. Focus on fouling release coatings
I.5.1. Commercial fouling release coatings
I.5.1.1. Description of the FRC composition
I. Elaboration of conventional silicone elastomers
I. Peroxide-induced crosslinking of polysiloxane
I. Condensation cure system
I. Addition cure system
I.5.1.2. Improvement of the FRCs
I.5.1.3. Specific properties of FRCs
I. Surface properties
I. Contact angles
I. Surface free energy
I. Smoothness
I. Bulk properties
I. Softness
I. Coating thickness
I. Ecotoxicity in the aquatic environment
I.5.2. Advances made on FRCs at the academic research state
I.6. Hydrolyzable polymers as potential candidates for novel hybrid FRCs
I.6.1. Hydrolyzable polymers
I.6.1.1. Linear hydrolyzable polymers
I.6.1.2. Hydrolyzable networks
I. Hydrolyzable in the polymeric chains
I. Hydrolyzable in the crosslinks
I.6.2. Kinetics of hydrolysis
I.7. Conclusion
I.8. Bibliographical references
Chapter II Hydrolyzable polymer additive-based silicone elastomers
II.1. Introduction
II.2. Hydrolyzable polymer additives
II.2.1. Requirements for the hydrolyzable polymer additives
II.2.2. Preparation of polymer additives
II.2.3. Physico-chemical properties of the hydrolyzable polymer additives
II.2.4. Hydrolysis mechanisms of the polymer additives
II.2.4.1. Hydrolysis mechanism of PCL-based polymers
II.2.4.2. Hydrolysis mechanism of PMATM2
II.2.5. Wettability properties of the hydrolyzable polymer additives
II.3. Preparation of silicone-based coatingsII.3.1. Choosing a model system
II.3.1.1. Choice of the PDMS oil
II.3.1.2. Choice of the crosslinker
II.3.1.3. Choice of the catalyst
II.3.2. Blend with additives
II.3.2.1. Choice of the additive amount
II.3.2.2. Choice of the solvent
II.3.2.3. Formulation process
II.4. Characterization of coatings
II.4.1. Mass loss test
II.4.2. Surface properties
II.4.2.1. Preliminary study on PDMS
II.4.2.2. Roughness measurements
II.4.2.3. Contact angle measurements and surface free energies
II. Dynamic water contact angles
II. Surface free energy
II. Modified time lag method
II.4.2.4. Infrared spectroscopy
II.4.3. Bulk properties
II.4.3.1. Differential scanning calorimetry analyses
II.4.3.2. Elastic modulus
II.5. Conclusion
II.6. Bibliographical references
Chapter III Hydrolyzable PDMS/polyester hybrid networks
III.1. Introduction
III.2. Hydrolyzable copolymers
III.2.1. Preparation of the hydrolyzable copolymers
III.2.1.1. Synthesis of poly(D,L-lactide-co-glycolide)
III.2.1.2. Synthesis of PLGA-b-PDMS-b-PLGA
III.2.1.3. Poly(ε-caprolactone)-based polymers
III.2.2. Physico-chemical properties of the (co)polymers
III.2.2.1. General description of the polymers
III.2.2.2. Wettability of the polyester-based polymers
III.3. Elaboration of PDMS/polyester hybrid networks
III.3.1. Preparation of polyester macrocrosslinkers
III.3.1.1. Preliminary investigations
III. Alkoxysilanization reaction of polyester-based polymers
III. Influence of the alkoxysilane functions
III.3.2. Formation of PDMS/polyester hybrid networks
III.3.2.1. Preliminary investigations
III. Co-solvent
III. Molar mass of the polyester-based polymer
III. Catalyst amount
III. Formulation preheating
III. Curing conditions
III. Conclusion on the optimal parameters
III.3.2.2. Final coatings formulations
III.3.3. Elaboration of self-crosslinked polyester-based networks
III.3.4. Residue extraction test
III.3.5. Thermal degradation of the networks
III.4. Conclusion
III.5. Bibliographical references
Chapter IV Characterization of the PDMS/polyester hybrid networks
IV.1. Introduction
IV.2. Physico-chemical properties at ti=0
IV.2.1. Surface physico-chemical properties
IV.2.1.1. Microstructure of the coating surface
IV.2.1.2. Surface chemistry
IV.2.1.3. Surface micromechanics
IV.2.2. Thermo-mechanical bulk properties
IV.2.2.1. Thermal properties of the networks
IV.2.2.2. Viscoelastic properties of the networks
IV. Viscoelastic behavior at ambient temperature
IV. Influence of the temperature on the viscoelastic properties of the networks
IV.2.3. Summarization of the key findings of the surface/bulk physico-chemical properties
IV.3. Physico-chemical properties during immersion
IV.3.1. Static immersion
IV.3.1.1. Mass loss test (and water uptake test)
IV.3.1.2. Roughness
IV.3.1.3. Wetting properties
IV. Dynamic contact angle measurements
IV. Surface free energy
IV.3.1.4. Mechanical properties
IV.3.2. Dynamic immersion
IV.4. Conclusion
IV.5. Bibliographical references
Chapter V Antifouling and fouling release properties of the coatings
V.1. Introduction
V.2. Static field test
V.2.1.1. Antifouling efficacy duration
V.2.1.2. Identification of the type of fouling organisms
V.2.1.3. Fouling release properties
V.3. Biological assays
V.3.1. Biological assay on A. amphitrite cypris larvae
V.3.1.1. Settlement of A. amphitrite on the hydrolyzable additive-based PDMS coatings .
V.3.2. Bioassays on diatoms and algal spores
V.3.2.1. Diatoms Navicula incerta
V. Initial attachment of diatoms on the PDMS/polyester hybrid networks
V. Removal of diatoms on the PDMS/polyester hybrid networks
V.3.2.2. Ulva spores
V. Green alga Ulva rigida
V. Green alga Ulva linza
V.4. Conclusion
V.5. Bibliographical references
Chapter VI Experimental section
VI.1. Synthesis protocols
VI.1.1. Controlled radical polymerization
VI.1.2. Ring opening polymerization
VI.1.3. Silanization reactions
VI.2. Preparation of polymer additive films
VI.3. Analyses of the polymers
VI.3.1. Nuclear Magnetic Resonance Spectroscopy
VI.3.1.1. PMATM2
VI.3.1.2. PLGA
VI.3.1.3. PCL
VI.3.1.4. PLGA-b-PDMS-b-PLGA
VI.3.1.5. PCL-b-PDMS-b-PCL
VI.3.1.6. Alkoxysilane-terminated polymers
VI.3.1.7. 1H NMR kinetics of the hydrolysis of trialkoxysilane functions
VI.3.2. Size exclusion chromatography
VI.3.3. Physico-chemical properties
VI.3.3.1. Contact angle measurements
VI. Dynamic contact angles
VI. Surface free energy
VI.3.4. Thermal properties
VI.3.4.1. Differential scanning calorimetry analysis
VI.3.4.2. Thermogravimetric analysis
VI.4. Formulation of coatings
VI.4.1. Formulation recipe of hydrolyzable additive-based PDMS coatings
VI.4.2. Formulation recipe of the PDMS/polyester hybrid networks-based coatings
VI.5. Characterization of the PDMS-based coatings
VI.5.1. Physico-chemical properties
VI.5.1.1. Contact angle measurements
VI. Modified time lag method
VI. Dynamic contact angles
VI. Surface free energy
VI.5.1.2. Roughness measurements
VI.5.1.3. Infrared spectroscopy
VI.5.2. Residue extraction test
VI.5.3. Scanning electron microscopy
VI.5.4. Confocal laser scanning microscopy
VI.5.5. Thermo-mechanical properties
VI.5.5.1. Dynamic mechanical analysis
VI.5.5.2. Differential scanning calorimetry analysis
VI.5.5.3. Thermogravimetric analysis
VI.5.5.4. Atomic force microscope
VI.6. Erosion properties
VI.6.1. Water uptake test
VI.6.2. Mass loss test
VI.6.3. Thickness loss test
VI.7. Antifouling properties
VI.7.1. Field test
VI.7.2. Bioassays
VI.7.2.1. Larval barnacle culture and settlement assay of A. Amphitrite Cyprids
VI.7.2.2. Settlement and removal assays of diatom N. incerta
VI.7.2.3. Settlement and removal assays of Ulva spores
VI.8. Bibliographical references


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