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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
One of the very few books devoted to the chemistry, materials and processing of liquid silicone rubber The scientific literature with respect to liquid silicone rubber is collected in this monograph. The text focuses on the fundamental issues such as properties, curing methods, special materials, as well as the latest developments, and provides a broad overview of the materials used therein. In particular, materials and compositions for liquid functional rubbers are discussed. Methods of curing and special properties are also described, such as tracking and erosion resistance, adhesion properties, storage and thermal stability. Methods of curing are precision casting, hybrid additive manufacturing, peroxide curing, ultraviolet curing, liquid injection moulding, or hot embossing. The book includes applications including automotive and underwater applications, electrical and optical uses, as well as medical uses.
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Seitenzahl: 426
Veröffentlichungsjahr: 2019
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Contents
Cover
Preface
Chapter 1: Materials
1.1 History
1.2 Properties
1.3 Special Materials
References
Chapter 2: Methods
2.1 Special Curing Methods
2.2 Hydrosilylation Catalysts
2.3 Recoating Methods
2.4 Shaped Elastomeric Bodies
References
Chapter 3: Automotive and Underwater Applications
3.1 Automotive Applications
3.2 Underwater Vehicles
References
Chapter 4: Electrical and Optical Uses
4.1 Electrically Conductive Silicone Rubber
4.2 High-Voltage Insulation
4.3 Silicone Rubber Composite Insulators
4.4 Electromagnetic Wave Absorber
4.5 Suppression of Surface Charge
4.6 Heat Dissipation Devices
4.7 Optical Fiber Sensor
4.8 Optical Semiconductor Device
4.9 Light-Emitting Devices
4.10 Capacitance Sensors
4.11 Dielectric Elastomer Transducers
4.12 Solar Cells
4.13 Portable Electronic Devices
4.14 Cable Accessories
4.15 Electrophotography
4.16 Secondary Battery Pack
4.17 Pressure and Temperature Sensor
4.18 Piezoresistive Device
4.19 Proton Exchange Membrane Fuel Cells
4.20 Light-Emitting Diodes
4.21 Recycling of Used Composite Electric Isolators
4.22 Triboelectric Nanogenerator for Wearable Electronics
4.23 Large Specific Surface Area Electrodes
4.24 Casing
References
Chapter 5: Medical Uses
5.1 Sensors for Medical Application
5.2 Materials for Medical Instruments and Uses
5.3 Biomaterials
5.4 Pharmaceutical Compositions
References
Chapter 6: Other Uses
6.1 Non-aqueous Organic Product Sensor
6.2 Synthetic Leather
6.3 Two-Part Curable Composition
6.4 Microchannel Thermocured Silicone Rubber
6.5 Dry Cleaning of Surfaces
6.6 Adhesive Tapes
6.7 Capsules for Beverages
6.8 Usage for Toner
6.9 Acoustic Applications
6.10 High Temperature Gas Line Heater System
6.11 Cosmetic Compositions
6.12 Silk Fibers
6.13 Elastic Silicone Rubber Belt
6.14 Recycling and Devulcanizing
6.15 Mobile Robots
References
Index
End User License Agreement
Guide
Cover
Table of Contents
Begin Reading
List of Illustrations
Chapter 1
Figure 1.1
Methyltrimethoxysilane and Vinyltrimethoxysilane.
Figure 1.2
Alcohols.
Figure 1.3
Tensile strength of various crosslinked LSR systems as a function of silica…
Figure 1.4
Tensile elongation break of various crosslinked LSR systems as a function of…
Figure 1.5
3-(2-Bromoisobutyramido)propyl(trimethoxy)silane.
Figure 1.6
Volatile silicon-oxygen components (19).
Figure 1.7
Octamaleamic acid-POSS.
Figure 1.8
SEM photograph of a tubular cured product (22).
Figure 1.9
Fillers.
Figure 1.10
Hydrolyzable methylpolysiloxane (44).
Figure 1.11
Organohydrogenpolysiloxanes (44).
Figure 1.12
Triazine compound (44).
Figure 1.13
1,2,2,6,6-Pentamethyl-4-(vinyldiethoxysiloxy) piperidine.
Figure 1.14
Polyhedral POSS compounds (53).
Figure 1.15
Melamine poly(phosphate).
Figure 1.16
Boron bridge-opening chemicals.
Figure 1.17
Silanol-terminated poly(siloxane)s.
Figure 1.18
Formation of Si-O-Si bridges via exchange of functional groups and subsequent…
Figure 1.19
Synthesis of urethane-containing silane (68).
Figure 1.20
Synthesis of (
γ
-diethylureidopropyl)allyloxyethoxysilane (69).
Figure 1.21
3-Aminopropyltriethoxysilane.
Figure 1.22
Hexamethyldisilazane.
Figure 1.23
Vulcanizing mechanism of phenyl silicone rubber (75).
Figure 1.24
Crosslinking agents.
Figure 1.25
Organic foaming agents.
Figure 1.26
Micrograph of the silicone foam (78).
Figure 1.27
Hydrosilylation reaction (79).
Chapter 2
Figure 2.1
Curing retarders.
Figure 2.2
Karstedt platinum complex.
Figure 2.3
Apparatus for 3D printing (20).
Figure 2.4
Methyldiethoxysilane.
Chapter 3
Figure 3.1
Cyanurate- and isocyanurate-based compounds.
Figure 3.2
Turbocharger hose (1).
Figure 3.3
Adhesion aids (6).
Figure 3.4
Amino silanes.
Figure 3.5
Curing retardants.
Figure 3.6
Adhesion promoters (9).
Figure 3.7
3,5-Dimethyl-1-octyn-3-ol.
Figure 3.8
Organophosphazenes (10).
Figure 3.9
Fluorosilicone gums (13).
Figure 3.10
2,5-Dimethyl-2,5-di(
tert
-butyl-peroxy)hexane.
Figure 3.11
N,N
‘-di-
sec
-Butyl-
p
-phenylenediamine.
Figure 3.12
Airbag device (20).
Figure 3.13
Adhesion aids (20).
Figure 3.14
Organopolysiloxane.
Figure 3.15
Reactor (27).
Figure 3.16
Buoyancy control device (29).
Chapter 4
Figure 4.1
N
-(
β
-Ammoethyl)-
γ
-aminopropyltriethoxysilane.
Figure 4.2
Ligands for catalysts (19).
Figure 4.3
Experimental setup for leakage current observation (22).
Figure 4.4
Applied voltage viz. leakage current (22).
Figure 4.5
Acetoxysilane.
Figure 4.6
Functional silane groups.
Figure 4.7
Synthesis of a functional silane with hindered amine.
Figure 4.8
Ionic liquids (49).
Figure 4.9
3-Methylpyridine.
Figure 4.10
Loop heat pipe (schematic) (52).
Figure 4.11
Optical semiconductor device (55).
Figure 4.12
Triazole compounds.
Figure 4.13
Siloxane monomers.
Figure 4.14
Peroxides.
Figure 4.15
Scorch inhibitors.
Figure 4.16
Tetrakis[methylene(3,5-di-
tert
-butyl-4-hydroxyhydro-cinnamate)]methane.
Figure 4.16
4,4′-Thiobis(2-methyl-6-
tert
-butylphenol).
Figure 4.16
Tris(2,4-di-
tert
-butylphenyl)phosphite.
Figure 4.16
2,2,4-Trimethyl-1,2-dihydroquinoline.
Figure 4.17
Cross-sectional views of the production process (78).
Figure 4.18
Assembly for preparation (79).
Figure 4.19
Process cartridge (80).
Figure 4.20
Pressure dependence of resistance (85).
Figure 4.21
Temperature dependence of resistance (85).
Figure 4.22
Weight changes under temperature cycling (99).
Figure 4.23
Shore A hardness under temperature cycling (99).
Figure 4.24
Electronic device (107).
Chapter 5
Figure 5.1
Sketch of a sensor (1).
Figure 5.2
Laminated sensor (4).
Figure 5.3
Injection vulcanization molding device (8).
Figure 5.4
Drugs.
Figure 5.5
Preparation of a curable silicone rubber composition (55).
Figure 5.6
Suture Sleeve (63).
Figure 5.7
Cyclic silicones.
Figure 5.8
Silicones with branched side chains.
Figure 5.8
Silicones with branched side chains.
Figure 5.9
Emollients.
Figure 5.10
Hydrophobic solvents.
Chapter 6
Figure 6.1
p-Aminobenzoic acid.
Figure 6.2
Adhesive tape (20).
Figure 6.3
Capsule schematic (21).
Figure 6.4
Peroxides.
Figure 6.5
Reaction regulators (23).
Figure 6.6
Heater element.
List of Tables
Chapter 1
Table 1.1
Properties of various silica samples (12).
Table 1.2
Crosslinking enthalpy and curing onset temperature of silicon rubber composites…
Table 1.3
Ingredients for pressure-sensitive adhesive compositions (23).
Table 1.4
Preferred properties of the composition (28).
Table 1.5
Boron bridge-opening chemicals (57).
Chapter 2
Table 2.1
Curing retarder compounds (3).
Table 2.2
Recommended formulations for hard silicone elastomers (22).
Table 2.3
Recommended formulations for soft silicone elastomers (22).
Chapter 3
Table 3.1
Fluorosilicone rubber composition (1).
Table 3.2
Adhesion aids.
Table 3.3
Amino silanes (7).
Table 3.4
Curing retardants (8).
Table 3.5
Components of sponge-forming liquid silicone rubber (25).
Table 3.6
Components used (26).
Chapter 4
Table 4.1
Effect of A-MQ content on the mechanical properties of the ALSR samples (7).
Table 4.2
Applied voltage and leakage current under dry conditions (22).
Table 4.3
Applied voltage and leakage current under wet conditions (22).
Table 4.4
Silica particle contents in the composites (31).
Table 4.5
Ionic liquids (49).
Table 4.6
Silicone rubbers for cold shrink splices (75).
Table 4.7
Antioxidants for cold shrink splice compositions (75).
Table 4.8
Specific examples of ingredients (77).
Chapter 5
Table 5.1
Properties of silicone depending on CNT loading (1).
Table 5.2
pH after
γ
-irradiation (64).
Table 5.3
pH after
γ
-irradiation (64).
Table 5.4
Hydrophobic solvents (81).
Chapter 6
Table 6.1
Swelling of a Silastic EP4412 silicone rubber film (1).
Table 6.2
Swelling of a Silastic 9280/30E silicone rubber film (1).
Table 6.3
Liquid silicone rubber composition (2).
Table 6.4
Properties dependent on mixing ratio (3).
Table 6.5
Components of heater system (32).
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Liquid Silicone Rubber
Chemistry, Materials, and Processing
Johannes Karl Fink
This edition first published 2019 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2019 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-63133-0
Preface
The scientific literature with respect to liquid functional rubbers is collected in this monograph. The text focuses on the basic issues and also the literature of the past decade. The book provides a broad overview of the materials used therein.
In particular, materials and compositions for liquid functional rubbers are discussed. Also, methods of curing and special properties are described, such as tracking and erosion resistance, adhesion properties, storage and thermal stability. Methods of curing are precision casting, hybrid additive manufacturing, peroxide curing, ultraviolet curing, liquid injection molding, or hot embossing. Special fields of applications are automotive and underwater applications, electrical and optical uses, and medical uses.
The text is usable for a lot of different audiences, such as education in university, but also for researchers and practitioners in the fields of electrical engineers, automotive engineers and researchers in medical fields.
How to Use This Book
Utmost care has been taken to present reliable data. Because of the vast variety of material presented here, however, the text cannot be complete in all aspects, and it is recommended that the reader study the original literature for more complete information.
Index
There are three indices: an index of acronyms, an index of chemicals, and a general index. In the index of chemicals, compounds that occur extensively are not included at every occurrence, but rather when they appear in an important context. When a compound is found in a figure, the entry is marked in boldface letters in the chemical index.
Acknowledgements
I am indebted to our university librarians, Dr. Christian Hasenhüttl, Gerlinde Iby, Margit Keshmiri, Friedrich Scheer, Christian Slamenik, Renate Tschabuschnig, and Elisabeth Groß for their support in the literature acquisition. In addition, many thanks to the head of my department, Professor Wolfgang Kern, for his interest and permission to prepare this text.
I also want to express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with herein. This book could not have been otherwise compiled.
Last, but not least, I want to thank the publisher, Martin Scrivener, for his abiding interest and help in the preparation of the text. In addition, my thanks go to Jean Markovic, who made the final copyedit with utmost care.
Johannes Fink
Leoben, 16th May 2019
Chapter 1Materials
1.1 History
The history of silicone rubber, its development and technological progress has been described (1–3). Its first commercial development was in 1944. Pioneering research on silicon opened the door to the development of silicone polymers and silicone rubber. The substitution of two methyl groups on silicon was present in the first examples of silicone rubber and still is the predominant organic group in commercial silicone rubber today (1).
Silicone rubbers have filled a need in the marketplace because of their combination of unusual properties not found in other rubbers. The alternating inorganic main-chain atoms of silicon and oxygen, and the two pendant organic groups, primarily methyl, provide strong chain bonds, backbone flexibility, ease of side-group rotation, and low inter- and intramolecular forces (1).
Also, industrial organosilicone materials, their environmental entry and their predicted fate have been described in a monograph (4). Furthermore, a comprehensive overview of the issues of organosilicon compounds has been presented. Commercial products, such as sealants, adhesives, and coatings are reported (5,6).
1.2 Properties
1.2.1 Tracking and Erosion Resistance
Silicon rubber (SR) also offers excellent electrical performance under contaminated environments. However, a pristine silicone rubber has low thermal properties and this may cause tracking and erosion failure due to severe dry band arcing and ohmic heating on the insulating surface (7).
The effect of material thermal characteristics on the tracking and erosion resistance of silicone rubber filled with micron-sized alumina trihydrate, aluminum nitride and boron nitride particles was investigated (7). Composites with different loading were synthesized by dispersing the particles in pristine room temperature vulcanized (RTV) silicone rubber and an IEC 60587 inclined plane test (8) was conducted to evaluate tracking and erosion resistance. Apart from physical parameters and leakage current, an infrared thermal imager was used to measure the surface temperature distribution during the course of the inclined plane test. Experimental results showed tracking and erosion resistance is significantly enhanced with addition of boron nitride particles followed by alumina trihydrate.
Aluminum nitride composites exhibit a poor tracking and erosion resistance, similar to pristine silicone rubber. It has been concluded that the addition of boron nitride composites improves the ability to impede the tracking and erosion process, the reasons being better thermal stability and enhanced thermal conduction in the discharge region. On the other hand, infrared analysis revealed that thermal accumulation is remarkably higher in aluminum nitride composites, which promotes dry band arcing and results in tracking and erosion failure (7).
To improve the tracking and erosion resistance performance of addition-cure liquid silicone rubber (LSR) without alumina trihydrate, addition-cure LSR samples with different fumed silica mass fraction and platinum catalyst concentration were prepared (9).
The tracking and erosion resistance performance of the samples were evaluated. Also, the thermal decomposition products were detected. Based on the comparative analysis of thermal decomposition characteristics, the morphology, crystal structure, and improvement mechanism were assessed.
The results of this study showed that both increasing the concentration of the platinum catalyst and the mass fraction of fumed silica can improve the tracking and erosion resistance performance, whereas the former was more effective. Further analysis indicated that the platinum catalyst played a very important role in the thermal decomposition process of addition-cure LSR. Increasing the platinum catalyst content can not only promote the crosslinking reaction between the methyl groups, but also can suppress backbone decomposition (9).
1.2.1.1 Impact of Various Fillers
The influence that various fillers having different sizes, from 0.3 µm to 18 μm, and surface modifications (unmodified, modified by the material supplier and in-situ modified during compounding) have on the erosion resistance of high temperature crosslinked silicone rubber composites was analyzed. The particles used were aluminium trihydrate, alumina (Al2O3) and silica (SiO2). The main focus was on aluminium trihydrate fillers since they have the ability to release water at elevated temperature.
By a simple water storage test under defined conditions the water uptake for different composites was analyzed to assess the effectiveness of an in-situ modification. The inclined plane test according to IEC 60587 (8), and the high voltage, low current dry arc test according to IEC 61621 (10), were used to determine the erosion resistance of the different samples.
For the inclined plane test, an adapted evaluation model was applied. This test is known to have a wide scatter in the case of material formulations, which are on the borderline of passing or failing the test and therefore show substantial erosions on certain samples. The scatter could be reduced by evaluating the eroded volume by using samples only, which showed a limited erosion length. It was found for aluminium trihydrate, that larger particles show slightly better results than smaller particles. This can be explained by the formation of boehmite AlO(OH) for the larger particles, which causes a release of the bound water over a wider temperature range.
This effect could be confirmed by thermogravimetric analysis (TGA). The surface modification of the particles with vinyltrimethoxysilane and methyltrimethoxysilane, c.f. Figure 1.1, did not improve the erosion resistance significantly, but reduces the water uptake to a large extent, which is advantageous for the retention of the hydrophobicity. In order to achieve a low erosion rate, high filler loadings are essential (11).
Figure 1.1 Methyltrimethoxysilane and Vinyltrimethoxysilane.
1.2.2 Enhancing Strength
Silica is the most widely used filler to reinforce LSR, but the high viscosity of a LSR/silica suspension significantly limits its processing flexibility (12).
To balance the processability and the reinforcing efficiency of LSR/silica systems, two kinds of enols, i.e., propenol and 1-undecylenyl alcohol, and a saturated alcohol, 1-undecylic alcohol, were employed to modify the silica surface. The compounds used here are shown in Figure 1.2.
Figure 1.2 Alcohols.
Modified silica samples were prepared via surface esterification of silica with alcohols by the reflux method. The calculated amount of precipitated silica (Rhodia Z-142), xylene, and alcohol were added into a reaction flask. Then a solid-liquid extractor containing sufficient CaH2 and a condenser with a CaCl2 drying tube were mounted on the reaction flask. The mixture was heated to 160°C and refluxed for 24 h. Finally, the samples were filtered and washed with ethyl acetate at least five times and dried in a vacuum oven at 120°C for 8 h.
The carbon contents, alkoxyl surface densities, and bound rubber contents of various silica samples are shown in Table 1.1.
Table 1.1 Properties of various silica samples (12).
Modifier
None
1-Undecylic alcohol
1-Undecylenyl alcohol
Propenol
Obviously, the bound rubber content is largely reduced after silica surface modifications, suggesting that silica poly(dimethyl siloxane) (PDMS) interaction is effectively lowered by silica surface modification. Comparing different modifiers, there is not much difference between 1-undecylic alcohol modified silica and 1-undecylenyl alcohol modified silica, while bound rubber content for propenol modified silica is a little higher. It seems that modifiers with higher carbon chain lengths are more effective in lowering the PDMS silica interactions. Though propenol has a little higher density on the silica surface, it is the combined effect of grafting density and modifier chain length that determines the effect of silica surface modifications.
Various rheological tests were carried out to investigate the processability as well as filler networking and the crosslinking processes of the modified systems.
Polymer/filler composites are known to exhibit a complex rheological behavior that reflects interactions among components in the system. In PDMS/hydrophilic silica suspensions, there are strong interactions between PDMS and silica and among silica, mainly via hydrogen bonding, which would cause a drastic increase in viscosity or reduction in the mobility of PDMS, consequently deteriorating the processing properties of LSR.
The effect of silica surface modification on viscosity of the suspension is believed to be determined by the combined effects of the type of grafting enol (saturated or unsaturated), grafting carbon content (or carbon chain length), and grafting density
Tensile tests were also adopted to verify the reinforcing effect. Tensile strength of various crosslinked LSR systems as a function of silica content are shown in Figure 1.3. Also, the tensile elongation break of various crosslinked LSR systems as a function of silica content are shown in Figure 1.4.
Figure 1.3 Tensile strength of various crosslinked LSR systems as a function of silica content (12).
Figure 1.4 Tensile elongation break of various crosslinked LSR systems as a function of silica content (12).
The investigated systems follow a similar trend: First, the elongation increases gradually with the content of silica, and then decreases because the rigidity of the filler begins to dominate the property. Comparing the systems reinforced with different kinds of silica at the same silica content, their orders of elongation at break (from high to low) are listed as follows: 1-Undecylenyl alcohol modified system, untreated silica reinforced system, propenol-modified system, and 1-undecylic alcohol modified system (elongations of the last two systems are very close). For instance, at the silica content of 2.5%, where tensile strengths of various systems are similar, the average elongations of the reinforced rubbers at the above sequence are 268%, 249%, 231%, and 225%, respectively.
In summary, it was found that surface modification of silica by 1-undecylenyl alcohol could significantly reduce the viscosity of its suspension with LSR. Also, the mechanical strength of LSR could be largely enhanced by six times with 10% modified silica (12).
1.2.3 Surface Treatment
1.2.3.1 Prevention of Microwrinkles
Attempts have been made to prepare a homogeneous film on a PDMS surface and to eliminate microwrinkles from the surface (13). Hydroxy groups were generated on a PDMS surface using different methods.
Because the hydroxylation process changes the chemical composition of the PDMS surface, resulting in a cracked surface, the selection of the best method for surface treatment with minimized surface microwrinkles and cracks was tried.
The results obtained from scanning electron microscopy showed that using the pulsed ultraviolet-ozone radiation method with a controlled duration time, ozone treatment, continuous ultraviolet-ozone treatment using a glass filter, and water media in ultraviolet-ozone treatment was more effective than other methods evaluated in the study to prevent microwrinkles.
Also, the results obtained from contact angle measurements and attenuated total reflectance Fourier transform infrared (FTIR) spectroscopy revealed that the ultraviolet-ozone treatment in the presenee of a water medium created more hydroxy groups in comparison to other methods (13).
1.2.3.2 Antifouling Surfaces
Biomimetic Antifouling Surface. Despite the distinct advantages of PDMS for biomedical applications, because of its hydrophobic nature, this material suffers from non-specific protein adsorption and platelet adhesion and activation when used as a blood-contacting material (14).
To confer hydrophilicity and biomolecules repelling characteristics, well-defined and high-density poly(2-hydroxyethyl methacrylate) brushes were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP) on the PDMS substrate (14).
First, the PDMS surface was activated using an ultraviolet/ozone wet treatment in water media to introduce hydroxy moieties without scarifying the surface property, resulting in a crack-free SiO2 surface. Then, 3-(2-bromoisobutyramido)propyl(trimethoxy)silane, c.f. Figure 1.5, the active atom transfer radical polymerization initiator, was immobilized on the ultraviolet/ozone-treated PDMS surface to prepare a thin layer of hydrophilic poly(2-hydroxyethyl methacrylate) brush on PDMS substrate, exhibiting excellent protein and platelet resistance. Poly(2-hydroxyethyl methacrylate) brushes supply a biomimetic feature by combining antifouling properties due to hydrophilic characteristic with bioactive properties resulting from the presence of high density hydroxy groups, which can be subsequently used for the conjugation of biomolecules.
Figure 1.5 3-(2-Bromoisobutyramido)propyl(trimethoxy)silane.
The results of the study indicated that grafting of poly(2-hydroxyethyl methacrylate) chains on the PDMS surface enhances the surface wettability, which leads to a decrease in non-specific protein adsorption and platelet adhesion compared to the bare PDMS substrate. The adhered platelets on the poly(2-hydroxyethyl methacrylate)-tethered PDMS substrate maintain their normal round morphology. In addition, the conjugated gelatin macromolecules on the tethered poly(2-hydroxyethyl methacrylate) chains promote the adherence and growth of human umbilical vein endothelial cells via ligand-receptor interactions (14).
Antifouling Marine Paints. Antifouling marine paints are topcoats intended to prevent the attachment of animals or plants to the lower parts of the hulls of ships (15). They are used for reasons of safety, maintaining the maneuverability of ships, reducing fuel consumption, combating corrosion and weighing-down of structures.
Biofouling is a major problem resulting from the immersion of materials in marine environments. The prevention of this phenomenon represents a considerable maintenance cost. Specifically, the formation of biofouling occurs during immersion in seawater, where a layer of organic and inorganic molecules is adsorbed to the surface of the material extremely rapidly. This layer of adsorbed material, or biofilm, serves as a mediator for the adhesion of the bacteria present in suspension in the marine environment.
This colonization of the surface by marine bacteria is rapid and a stationary state is reached after a period of a few hours to a few days. Finally, other marine organisms colonize the surface, the adherent bacteria recruiting these other organisms. All these live organisms attached to the surface constitute the biofouling or fouling. The adhesion of marine fouling concerns any structure immersed in the sea: ships, pipelines, cooling towers and circuits, harbor structures, marine sensors, aquaculture systems, etc. The damage caused is considerable and diverse. Specifically, the structures become coated, for example, with organisms which have a negative effect on the performance levels of the structures. In particular, for the hulls of ships, the incrustation of various marine organisms increases the friction between the hulls of the ships and the seawater, which reduces the speed and can lead to greater fuel consumption (15).
Thus, the bottom of a ship which is not protected by an antifouling system can, after less than six months spent at sea, be covered with 150 kg of fouling per square meter. In order to avoid economic loss, and also in order to more successfully inhibit corrosion phenomena, antifouling paints, the objective of which is to prevent or notably reduce the soiling due to the incrustations of marine organisms, are applied to the immersed parts of the structures exposed to water.
The principle of antifouling paints is based on the controlled release of the active substance at the interface between the surface and the seawater. The effectiveness of the paint is maintained as long as the concentration of active substance released at the surface is effective and regular. Most antifouling paints therefore contain a biocidal product which is most commonly an organometallic compound (based on tin, copper or zinc) or an organic compound (fungicide, algicide, bactericide) which prevents adhesion of the marine soiling owing to the toxic activity thereof (15).
Tributyltin, which is very effective, was therefore the biocide most commonly used in antifouling paints, but this product, its degradation molecules and its metabolites proved to be seriously and sustainably polluting. For these reasons, the International Maritime Organization prohibited the use of tin-based antifouling paints. The antifouling paints used today are mainly based on copper-containing compounds and/or on synthetic chemical compounds, but also are based on silicone polymers. With regard to the copper-based paints, although they are less toxic than tin salts, they are virtually always formulated with a massive proportion of cuprous oxide. However, they are effective only against the marine fauna, and, in order to combat the growth of algae, it is essential to add herbicides, which can pose new threats to the environment.
These silicone-based paints forming an antifouling coating are very innovative:
They are completely friendly to the marine environment: no metal waste, and
They improve the glide of ships, thus reducing by 1 to 5% their fuel consumption and therefore their greenhouse gas emissions.
An antifouling topcoat based on a silicone elastomer can contain fluids which improve the antifouling effect, in particular (15):
Methylphenylpolysiloxane oils (16),
A hydrocarbon-based liquid compound, for example a polyolefin,
A plasticizer,
A lubricating oil,
Liquid paraffins and waxy masses of the petrolatum type,
A thermoplastic polymer such as poly(vinyl chloride),
A vinyl chloride/vinyl acetate copolymer, or
Cationic, anionic, nonionic or amphoteric surfactants.
In order to form the silicone elastomer coating, the silicone formulations generally used involve a silicone oil, generally a reactive poly(dimethyl siloxane) with hydroxylated endings, which optionally prefunctionalize with a silane so as to have alkoxy ends, a crosslinking agent and a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and other optional additives such as bulking fillers, adhesion promoters, and dyes (15).
The catalysts which are used are based on tin, titanium, or amine or compositions. Catalysts based on tin and on titanium are very effective (17).
1.2.3.3 Bilayers for Gas Separation
Thin film composites of poly(2-hydroxyethyl methacrylate) and poly(methyl methacrylate) (PMMA) chain-tethered poly(vinylidene fluoride) (PVDF)-PDMS were prepared as a gas separation membrane (18). PDMS was coated on the PVDF support using a dip coating method. Poly(2-hydroxyethyl methacrylate) and PMMA were then grafted on PVDF-PDMS substrate by atom transfer radical polymerization.
The PVDF-PDMS-poly(2-hydroxyethyl methacrylate) and PVDF-PDMS-PMMA trilayer membranes were studied by attenuated total reflection FTIR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle measurement, and X-ray photoelectron spectroscopy (XPS). The results of separation tests indicated that the CO2/N2 selectivity of PVDF-PDMS-poly(2-hydroxyethyl methacrylate) and PVDF-PDMS-PMMA thin film composites increased by ~2 and ~3 times, respectively, compared to the solvent-extracted PVDF-PDMS support (18).
1.2.4 Adhesion Properties
1.2.4.1 Combinations with Thermoplastic Polymers
The application field of combinations of thermoplastic polymers and LSR is remarkably wide, ranging from household consumers to the automotive sector (19). For the application of such combinations, the adhesion is a deciding factor.
In the case of LSR, tempering for improving its mechanical properties is used. However, there is no profound knowledge on annealing the combinations, thus making the influence on adhesion unclear.
The influence of different tempering temperatures (80°C, 100°C, and 120°C) and times (1, 3, 6, and 9 h) on the adhesion between the thermoplastic poly(butylene terephthalate) (PBT) and poly(amide) 12) and LSR was studied. The results showed that post-tempering influences the single components.
In the case of PBT, post-crystallization already occurs at 80°C, which is reflected, for instance, by increasing the degree of crystallinity by about 22%. LSR showed a post-crosslinking and the release of volatile components. Analyzing the tempering impact on adhesion, the peel resistance of LSR and PBT decreased around 23% at 80°C. The covalent bonds at the boundary layer were weakened due to the post-crystallization and the release of volatile components.
To identify the volatile components of the LSR, a gas chromatography coupled with mass spectrometry measurement was done. The test specimens were tempered in hermetic bags to catch the volatile components.
The volatile components mainly consisted of silicon-oxygen connections, such as siloxanes, silanes, and silanols, such as trimethylsilanol, disiloxane, cyclotritrisiloxane, and cyclotetrasiloxane, c.f. Figure 1.6.
Figure 1.6 Volatile silicon-oxygen components (19).
During tempering, the volatile components are released. The amount of volatile components increased during varying tempering times, but their chemical composition remained the same while altering the temperature (19).
1.2.4.2 Octamaleamic Acid-POSS
The addition of octamaleamic acid-polyhedral oligomeric silsesquioxane (POSS), c.f. Figure 1.7, nanoparticles to SR composites was investigated for enhancing the adhesion properties of reinforcing fibers (20). The content of octamaleamic acid-POSS was examined as the experimental parameter.
Figure 1.7 Octamaleamic acid-POSS.
As reference a 5 phr silica-filled SR compound was used. The peroxide curing characteristics of composites was determined using differential scanning calorimetry (DSC) and a moving die rheometer.
It was found that octamaleamic acid-POSS retarded the curing time and decreased the crosslinking density. DSC analysis indicated that the crosslinking reaction started at a lower temperature as the octamaleamic acid-POSS concentration increased. The thermal mechanical analysis results showed that the melting point decreased with the addition of octamaleamic acid-POSS.
The crosslinking enthalpy and the curing onset temperature of SR composites are shown in Table 1.2.
Table 1.2 Crosslinking enthalpy and curing onset temperature of silicon rubber composites (20).
The crystallinity of SR decreased with increasing octamaleamic acid-POSS content. The thermal stability of SR composites obtained from TGA apparently improved with the addition of octamaleamic acid-POSS. Scanning and transmission electron microscopy showed that the octamaleamic acid-POSS distribution was homogeneous at lower contents, but some agglomerates were seen when the content of octamaleamic acid-POSS increased.
Tensile tests showed that comparable mechanical properties were achieved for SR/octamaleamic acid-POSS composites with respect to reference composite.
It could be demonstrated by H-adhesion tests that the utilization of octamaleamic acid-POSS in the SR composites improved the adhesion of the matrix to a Rayon fiber (20).
1.2.4.3 Improvement of Adhesion Strength
Polymeric hard/soft combinations consisting of a rigid, thermoplastic substrate and an elastomeric component offer many advantages for plastic parts in industry (21). Manufactured in one step by multi-component injection molding, the strength of the thermoplastics can be combined with sealing, damping or haptic properties of an elastomer. Bonds of self-adhesive LSR on high performance thermoplastics, such as poly(ether ether ketone) or poly(phenylene sulfide), are especially interesting for medical applications due to their outstanding resistance properties.
To ensure good adhesion between the two components, surface treatments from an atmospheric pressure plasma jet and a Pyrosil® flame were applied. The chemical changes on the thermoplastic surfaces were verified by water contact angle measurement and XPS.
A plasma treatment causes a decline in water contact angle, indicating the formation of functional groups, especially –OH, on the surface. XPS measurements confirm the increase of oxygen on the surface. Thus, the number of functional groups on the thermoplastic surface is enlarged by plasma treatment, leading to stronger bonding to the organofunctional silanes of the self-adhesive silicone rubber.
A thin layer of silanol groups is created by the Pyrosil flame on the thermoplastic substrates, which could be verified by XPS. A hydrophilic behavior of the coated surface is noticed. Both surface modification methods lead to enhanced adhesion properties of self–adhesive LSR on thermoplastic surfaces. This could be confirmed by 90° peel tests of the injection-molded composites, which led to an increase in the peel force by the surface modification techniques used (21).
1.2.4.4 Tackiness Reduction
A millable silicone rubber is widely used for producing rubber components because it can be mixed with a coloring agent with a simple device such as a twin-roll mixer and can be molded by simple equipment such as a press, and especially because its shape can be retained even in an uncured state. It is widely used in extrusion molding of tubes, gaskets, etc. (22).
However, the unique tackiness that the surface of a cured silicone rubber has is likely to cause the problem of blocking of surfaces of its molded products (22). Forming minute irregularities on the surfaces can prevent the blocking, and in molded products formed using a mold, the blocking can be easily prevented by roughening the surface of the mold. However, in extrusion molding and coating which do not use a mold, it is difficult to control the surface state.
A silicone rubber composition has been described that has improved moldability, in particular, extrusion moldability, while ensuring that a cured product obtained therefrom has sufficient strength. The silicone rubber composition contains (22):
A100 part by mass base polymer consisting of a poly(organo siloxane diol) whose viscosity at 25°C is 1 to 100 Pa s and a poly(organo siloxane) whose viscosity at 25°C is 0.2 to 40000 Pa s, with a ratio of the last compound of 20 to 100% by mass to the whole composition, the base polymer having a viscosity of 5 to 20000 Pa s at 25°C and having an alkenyl group content of 0.001 to 0.3 mmol g–1. Furthermore, a 10 to 50 part by mass silica powder whose specific surface area is 50 to 400 m2g–1, a 1 to 10 part by mass organosilazane, and a catalytic amount of a curing agent.
Several special examples for such compositions have been detailed (22). When such a composition is extruded at a rate of 2 m min–1 by a screw extruder and thereafter cured into a tubular cured product, the tubular cured product preferably has, on a surface, 1 to 300 pieces/0.01 mm2 granular protrusions whose maximum diameter measured in a scanning electron microscopic image is 0.1 µm to 30 μm. A SEM photograph (5000X) of the surface of the silicone rubber extrudate is shown in Figure 1.8.
Figure 1.8 SEM photograph of a tubular cured product (22).
1.2.5 Pressure-Sensitive Adhesive Film
Silicone pressure-sensitive adhesives have the excellent heat resistance, freeze resistance and electrical properties inherent in silicone and maintain adhesion without impairing these properties (23). Thus, they can be widely used as pressure-sensitive adhesives where a high level of reliability is required.
Typical silicone pressure-sensitive adhesive compositions contain (23, 24):
A diorganopolysiloxane having at least two alkenyl groups in a molecule,
An organopolysiloxane containing R
3
SiO
1/2
units and SiO
4/2
units in a molar ratio of the R
3
SiO
1/2
unit to the SiO
4/2
unit of from 0.6 to 1.7, wherein R is a monovalent hydrocarbon group having 1 to 10 carbon atoms,
An organopolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule,
An inhibitor,
A hydrosilylation catalyst, and
A solvent.
The silicone pressure-sensitive adhesive compositions may be applied to micro gravure coatings in order to obtain a thin pressure-sensitive adhesive layer on a substrate film. However, the aforementioned silicone pressure-sensitive adhesive compositions cannot form the pressure-sensitive adhesive layer exhibiting proper adhesion and good anti-scratch properties on the substrate film (23).
A silicone composition has been developed that exhibits proper viscosity and can form a pressure-sensitive adhesive layer that exhibits proper adhesion and good anti-scratch property on a substrate film (23). Another objective was to provide a pressure-sensitive adhesive film whose pressure-sensitive adhesive layer exhibits proper adhesion and a good anti-scratch property.
The compositions contain the ingredients shown in Table 1.3. These components will be explained in detail later on.
Table 1.3 Ingredients for pressure-sensitive adhesive compositions (23).
No.
Description
A
Diorganopolysiloxane having at least two alkenyl groups in a molecule, and having a viscosity at 25°C of from 10,000 to 1,000,000
mPa s,
in an amount of from 60 to 80 mass based on a mass of the composition
B
A diorganopolysiloxane having at least one alkenyl group in a molecule, and being raw rubber-like at 25°C or having a viscosity at 25°C of more than 1,000,000
mPa s,
in an amount of more than 0%, but not more than 10% based on a mass of the composition
C
An organopolysiloxane resin containing (R
1
3
SiO
1/2
)
x
(SiO
1/2
), where R
1
is halogen-substituted or unsubstituted monovalent hydrocarbon group free from an alkenyl group and x is a number from 0.5 to 1.0, in an amount of from 0.5% to 20%
D
An organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule, in a quantity that provides from 0.1 to 10 moles of the silicon-bonded hydrogen atoms in this component per 1 mole of the total alkenyl groups in the composition
E
Silica fine powder in an amount of from 0.5 to 5% based on a mass of the composition
F
A hydrosilylation catalyst in a quantity that accelerates hydrosilylation of the composition
The alkenyl groups in component (A) can be exemplified by vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl groups and are preferably vinyl groups. Component (A) can be exemplified by dimethylvinylsiloxy-endblocked dimethylpolysiloxanes, dimethylvinylsiloxy-endblocked dimethyl siloxane-methylvinyl siloxane copolymers, trimethylsiloxy-endblocked dimethyl siloxane-methylvinyl siloxane copolymers, partially branched chain dimethylpolysiloxane with molecular chain ends terminated by dimethylvinylsiloxy and trimethylsiloxy, trimethylsiloxy-endblocked partially branched chain dimethyl siloxane-methylvinyl siloxane copolymers.
Component (B) is another diorganopolysiloxane having at least one alkenyl group in a molecule. The alkenyl groups in component (B) can be exemplified by vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl groups and are preferably vinyl groups. The non-alkenyl Si-bonded organic groups in component (B) can be exemplified by alkyl groups and aryl groups and are preferably methyl and phenyl groups.
The pressure-sensitive adhesive film is similar to that shown in Table 1.3.
The silicone composition exhibits a proper viscosity and can form a pressure-sensitive adhesive layer that exhibits proper adhesion and good anti-scratch property on a substrate film. Furthermore, the pressure-sensitive adhesive film has a pressure-sensitive adhesive layer exhibiting proper adhesion and good anti-scratch property (23).
The pressure-sensitive adhesive film is produced by applying the silicone composition onto the surface of the substrate film, and then forming a pressure-sensitive adhesive layer on the surface of the substrate film by curing the composition at room temperature or with heating. Curing with heat is preferable. Heating should be carried out at a temperature above 50°C, preferably within the range of 80 to 200°C (23).
1.2.6 Storage Stability
A composition prepared by blending a resin microparticulate catalyst with a mixture of an organopolysiloxane and a crosslinking agent cannot secure long-term storage stability in a mixed state at around room temperature depending on the type of the resin making up the resin microparticulate catalyst, so that problems of a significant increase in viscosity of the mixture and progress of curing of the mixture are caused (25).
In addition, it takes time to crosslink the mixture when curing the mixture by heating, so that the composition could show a low crosslinking reactivity. Until now, there has not been a technique for obtaining a silicone rubber composition that has both excellent storage stability in a mixed state and excellent crosslinking reactivity during heating.
A silicone rubber composition has been developed that contains an organopolysiloxane, a crosslinking agent, and a microcapsule type of catalyst that is made of microparticles of a resin and a crosslinking catalyst encapsulated in the microparticles.
Examples of the organopolysiloxane include an alkenyl group-containing organopolysiloxane, a hydroxyl group-containing organopolysiloxane, a (meth)acryl group-containing organopolysiloxane, an isocyanate-containing organopolysiloxane, an amino group-containing organopolysiloxane, and an epoxy group-containing organopolysiloxane.
The alkenyl group-containing organopolysiloxane is used as a main material for an addition curing type silicone rubber composition. The alkenyl group-containing organopolysiloxane is crosslinked by a hydrosilyl crosslinking agent in addition reaction with the hydrosilyl crosslinking agent. While proceeding even at room temperature, this addition reaction is promoted under heating. Thermal curing by this addition reaction is preferably performed at at 100–170°C.
The preparation of microcapsule type catalysts can be done as follows (25):
Preparation 1–1: A toluene solution of a platinum catalyst, containing 3% by mass of platinum metallic atoms, a coating resin for microparticulation, and toluene were mixed at a mass ratio of 0.6:5:95, and the thus-prepared solution was dropped into a water solution of a surface acting agent to prepare an emulsion. Then, the toluene was distilled and removed under reduced pressure and the emulsion was filtered to obtain microparticles for each catalyst that contain the coating resin and the platinum catalyst. As platinum catalyst, platinum chloride (IV) manufactured by Furuya Metal Co., Ltd. was used.
1.2.7 Thermal Stability
A platinum catalyst and a nitrogen-containing silane were introduced into a silicone rubber to improve the thermal stability (26). The effects of Pt and nitrogen-containing silane on the thermal stability and degradation mechanism of silicone rubber were investigated by thermogravimetry (TG), TG-FTIR spectrometry, SEM and other methods.
A significant synergism was found between Pt and nitrogen-containing silane for improving the thermal stability of the silicone rubber. When 6.67 ppm of Pt and 1.4 phr of nitrogen-containing silane were introduced, the temperature of 10% and 20% weight loss under nitrogen atmosphere were respectively increased by 36°C and 119°C. The residue weight at 900°C was doubled to 68% in the presence of the Pt/nitrogen-containing silane.
The synergistic mechanism might be that the nitrogen atom coordinated with Pt and improved the catalytic efficiency of Pt. Additionally, nitrogen-containing silane preserved the catalytic activity of Pt under air atmosphere. Thus, the Pt/nitrogen-containing silane efficiently catalyzed the thermal crosslinking and suppressed the degradation of silicone chains. Moreover, it revealed that the presence of Pt/nitrogen-containing silane protected silicone chains from oxidation. Thus, the unzipping depolymerization by silanol groups was reduced significantly (26).
1.2.8 Hydrophobed Pyrogenic Silica Filler
In order to prevent the undesirable influence of the silanol groups on the mechanical properties of a silicone rubber, it is necessary to render the surface of the pyrogenic silica hydrophobic (27).
A silicone rubber composition has been described, which is characterized by its containing a structurally modified hydrophobic pyrogenic silica as filler (28).
The silanized, structurally modified silica has vinyl groups fixed to the surface, hydrophobic groups such as trimethyl silyl or dimethyl silyl and/or monomethyl silyl groups additionally being fixed to the surface. The preferred properties are shown in Table 1.4.
Table 1.4 Preferred properties of the composition (28).
Property
BET surface area
Average primary particle size
pH
Carbon content
DBP value
The dibutyl phtalate (DBP) value in Table 1.4 is defined as the volume of dibutyl phtalate absorbed by 100 g of black pigment (29, 30).
1.2.9 Superhydrophobic Materials
1.2.9.1 Superhydrophobic Surfaces
The fabrication of large-scale superhydrophobic surfaces for commercial applications is challenging due to certain limitations. A simple and inexpensive method has been developed to fabricate superhydrophobic surfaces on silicone rubbers (31).
Templates with different rough structures were prepared first. Silicon carbide particles of (63, 21, 15, and 10.5) μm in diameter were obtained by using test sieves of 80, 240, 600, 800, and 1200 mesh, respectively. A uniform layer of resin binder-epoxy resin was sprayed on the inner surface of a cubic mold with dimensions of 3cm×3cm×3cm after treating with a silane coupling agent N-β-aminoethyl-γ-aminopropyl trimethoxysilane. The prepared silicon carbide particles were evenly sprayed on the inner surface of the mold and dried at room temperature. Then rough silicon rubber samples were prepared through a conventional molding process. Liquid silicone rubber was poured into the mold, the mold was taken off after consolidation, and thus silicone-rubber surfaces with different surface morphologies were obtained (31).
Rough microstructures were prepared on the inner surfaces of the molds and then sample superhydrophobic surfaces on silicone rubbers with different surface roughness were achieved using the standard molding process.
Furthermore, the effects of roughness on the wettability were investigated. The results showed that by controlling the roughness, the fabricated surfaces exhibited a static contact angle of 150.9° and a sliding angle of 8°. Finally, the property of hydrophobicity recovery for the silicone-rubber samples was studied. The surfaces of the samples could recover well after a sandblasting experiment. The proposed method is low cost, environmentally friendly and suggests promising industrial applications (31).
1.2.9.2 Flashover Characteristics
The influence of the low adhesion of superhydrophobic surface on flashover characteristics under wet conditions has been studied (32).
The samples are prepared as follows (32, 33):
Preparation 1–2: A sheet of glass with dimensions of 25 mm ± 75 mm ± 1 mm was put into a mold. The mold was 25 mm ± 75 mm ± 2 mm. Then, a liquid one-component room temperature vulcanized silicone rubber was poured into the mold. After the liquid silicone rubber cured, the common silicone rubber sample was acquired. The thickness of room temperature vulcanized coating was about 1 mm.
The common silicone rubber sample was similar to glass insulators coated by room temperature vulcanized coating. Then a mixture of ethyl acetate, liquid silicone rubber and SiO2 nanoparticles was sprayed on the common silicone rubber sample. After the silicone rubber cured, a superhydrophobic surface was formed on the silicone rubber sample. Here, a superhydrophobic silicone rubber sample was acquired. The contact angle of superhydrophobic surface was larger than 150° and the sliding angle was less than 2°.
The flashover experiments were done with copper foils with 0.05 mm thickness that were used as electrodes. The copper foils were glued onto the samples. The distance between the electrodes was 2.5 cm. Then the electrodes were connected to an AC high voltage source. The mode of increase of voltage was a step-by-step test. If the samples withstood a voltage for 2 s without failure, the voltage would increase in steps of 0.2 kV. The voltage would be increased until flashover occurred. The whole process was recorded by a digital camera.
The flashover experiments were conducted under two different wet conditions. One was placing a constant volume droplet on the silicone rubber. The other one was that silicone rubber was wetted by salt fog.
It was found that the adhesion between water droplets and a superhydrophobic surface was very low, because of the presence of air cushion. Accordingly, water droplets were easy to slide on a superhydrophobic surface under the effect of electric filed. The sliding of droplets could provide a longer insulation path before the flashover occurred. The results of the study showed that the flashover voltage could be improved greatly on a superhydrophobic silicone rubber surface (32).
