Silicone Resins and their Combinations E-Book

Wernfried Heilen

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Serie: European Coatings LIBRARY
• Sprache: Englisch

Seitenzahl: 130

Veröffentlichungsjahr: 2015

Beschreibung

Get a concentrated overview of the chemistry and technology of silicone resins and a deep insight into their use from an industrial point of view: this completely revised and expanded edition reports on most recent developments and points out the outstanding properties of silicones for coatings. Essential for any formulator of competitivemodern paint systems! Get a concentrated overview of the chemistry and technology of silicone resins and a deep insight into their use from an industrial point of view: this completely revised and expanded edition reports on most recent developments and points out the outstanding properties of silicones for coatings. Essential for any formulator of competitivemodern paint systems!

Leseprobe

Wernfried Heilen

Sascha Herrwerth

Silicone Resins and their Combinations

Cover: lunamaria/Fotolia.com

Wernfried Heilen and Sascha Herrwerth

Silicone Resins and their Combinations

Hanover: Vincentz Network, 2015

European Coatings LIBRARY

ISBN 978-3-86630-697-4

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European Coatings // LIBRARY

Wernfried Heilen

Sascha Herrwerth

Silicone Resins and their Combinations

Foreword

Because of their molecular structure, silicone resins and silicone combination resins are used in numerous industrial applications, particularly as binders, for formulating coatings. The linking of silicon and oxygen atoms to form a stable basic framework, in which the free valencies of the silicon are saturated by hydrocarbon groups, results in outstanding properties which cannot be achieved with other products. The many possible combinations of the silicone building blocks are reflected in the impressive diversity of silicone chemistry and the resultant products.

The present book (2nd edition) is intended to provide a concentrated overview of the chemistry and technology of silicone resins and their use from an industrial viewpoint.

It aims to report on current developments in the field of silicones for coatings and gives those approaching the subject an overview of the most important areas of application.

Since publication of the 1st edition of this book almost ten years ago, some areas of application have seen further technological developments and these have driven significant advances.

For example, silicone combination resins are being increasingly used in anti-corrosion coatings to cope with extreme weather conditions and temperatures.

Alkoxy-silyl functional urethane resins are used in high-tech coatings to increase scratch resistance.

New types of acrylic dispersions in combination with functional polysiloxanes enable the formulation of “below-critical” emulsion coatings with similar properties to “above-critical” silicone resin coatings.

These and further innovations are described in the 2nd edition of this book and reflect the current state of the art in this field.

Since this field will continue to develop, I am pleased that I have been able to win my colleague Dr. Sascha Herrwerth to join me as co-author for this new edition.

I would like to thank our colleagues Dr. Michael Ferenz, Dirk Hinzmann and Dr. Berendjan de Gans for numerous technical discussions.

Thanks are also due to Evonik for making available company literature and permission to reprint extracts as well as for technical and material resources.

Wernfried Heilen

Essen, July 2014

In line with the publisher’s guidelines, the authors have identified trademarked product names by enclosing them within quotation marks “ ”.

Contents

1 Introduction

1.1 Organo-siloxanes and organo-polysiloxanes

1.2 Chlorosilanes – the building blocks for silicone resins

1.3 Manufacture of the resin intermediates

2 Silicone resins

2.1 Pure silicone resins

2.1.1 Methyl-silicone resins

2.1.2 Methyl/phenyl-silicone resins

2.2 Silicone combination resins/silicone resin hybrids

2.2.1 Silicone-modified polyester resins

2.2.2 Silicone-modified alkyd resins

2.2.3 Silicone-modified epoxy resins

2.2.4 Silicone-modified polyacrylate resins

2.3 Alkoxy-silyl modified resins and alkoxy-silyl modified isocyanate crosslinkers

2.4 Radiation-curable silicone resins

2.4.1 Acrylic-functional silicone resins

2.4.2 Epoxy-functional silicone resins

2.5 Room temperature vulcanizing silicone resins (RTV resins)

2.6 Waterborne silicone resins

3 Examples of applications of silicone resins

3.1 Silicone resins for heat resistant coatings and corrosion protection above 300 °C

3.2 Silicone-modified aromatic polyester resins for heat resistant coatings up to 250 °C for decorative coatings

3.3 Applications at normal temperatures

3.3.1 Silicone-modified aliphatic epoxy resin for versatile coating applications

3.3.2 Acrylic- and epoxy-functional silicone resins as UV-cured release coatings

3.4 Silicones and silicone resins in building conservation

3.4.1 External water repellency

3.4.2 Internal water repellency

3.4.3 Architectural coatings

3.4.4 Silicate emulsion coatings and renderings

3.4.5 Emulsion based coatings with silicate character (SIL coatings)

3.4.6 Siloxane architectural coatings with strong water-beading effect

3.4.7 Silicone resin coatings and renderings

3.4.8 Below-critical PVC formulated exterior coatings

3.4.9 Photocatalytic architectural coatings

4 Outlook

5 Glossary

5.1 Façade protection theory according to Künzel

5.2 sd-value

5.3 w-value

5.4 Definitions of PVC and CPVC

6 Analysis of silicone polymers

6.1 NMR spectroscopy

6.2 IR spectroscopy

6.3 Wet analysis

7 Literature

Authors

Index

1 Introduction

The outstanding properties of silicone resins and silicone-containing combination resins in the form of solutions, liquid resins and emulsions make them extremely versatile. The versatility of silicone resins stems largely from the fact that the organosiloxanes, on which they are based, can be modified in many ways and combined with numerous organic polymers and different functionalities. Thus the unique properties of the organo-siloxanes, such as high surface activity, chemical inertness and thermal stability can be combined with the properties of the chosen organic functionality or polymer. Applications range from impregnations and weather-resistant exterior coatings for building conservation, through heavy duty anti-corrosion protection to high temperature resistant coatings. Acrylic-functional silicone resins and epoxy-functional silicone resins are reactive resins, which are used as UV-curable coatings for paper and as additives for radiation curing printing inks and wood lacquers.

1.1 Organo-siloxanes and organo-polysiloxanes

The starting products required to manufacture silicones, R3SiOH (silanols), R2Si(OH)2 (silane diols), RSi(OH)3 (silane triols) and Si(OH)4, are obtained by hydrolysis of the corresponding halogen compounds R3SiCl, R2SiCl2, RSiCl3 and SiCl4[1]. The latter can be prepared by addition of alkyl halides RX to very pure silicon (> 98 %) in the presence of a copper catalyst[2].

This industrial process today generally known as Müller-Rochow Synthesis is also termed “direct synthesis” since organo-chlorosilanes are manufactured directly from elemental silicon (Figure 1.1). This process was discovered by Müller and Rochow in the early 1940’s and was the starting point for silane and silicone chemistry to grow and become economically viable on a large scale.

Figure 1.1: Organo-chlorosilanes manufactured by Müller-Rochow-Synthesis

Table 1.1: Typical product distribution for the reaction of silicon with methyl chloride (Müller-Rochow-Synthesis)

The various by-products formed in this reaction can be separated by distillation. The most important products by volume are methy-chlorosilanes produced by “Direct Synthesis” with methyl chloride and elemental silicon[3]. Longer chain alkyl halides result in low yields of product. Table 1.1 shows a typical product distribution for the reaction of silicon with methyl chloride.

Although the Grignard process is more complex than direct synthesis, it permits, for example, manufacture of mixed aryl/alkylchlorosilanes, i.e. silanes with different organic groups at the same silicon atom[4]. This can be of interest where the polysiloxane end product is required to have specific properties. Besides direct synthesis, the Grignard process investigated by Kipping[5] is thus still of considerable industrial interest (Figure 1.2).

Hydrolysis of dichlorodialkyl silanes R2SiCl2 leads to dihydroxydialkyl silanes R2Si(OH)2. However these are not stable and condense immediately to polymeric silicones (R2SiO)n with the elimination of water.

The ratio of cyclic to linear compounds and the chain length of the linear siloxanes as well as the properties of the resultant polymer can be controlled by the hydrolysis conditions: basic catalysts and high temperatures favour the formation of high molecular weight linear polymers. Acidic catalysts result in the formation of low molecular weight polymers and cyclic oligomers.

Figure 1.2: Manufacture of mixed aryl/alkylchlorosilanes by the Grignard synthesis

A large proportion of polysiloxanes is currently manufactured by ring-opening polymerization (equilibration). For this, cyclic tetramers and pentamers are manufactured from dichlorodimethyl silane with the help of ionic initiators. The cyclic oligomers are separated from the reaction mixture and used for the ring-opening polymerization.

The equilibration can be acid or alkaline catalysed. After deactivation of the catalyst, 10 to 20 % of cyclic siloxanes, mostly tetramers and pentamers, can be removed by vacuum distillation. If the catalyst is not deactivated, the removed cyclic siloxanes are permanently regenerated until the linear polymers are completely used up.[6]

Since the basic composition R2SiO corresponds with the formula of organic ketones R2CO, they were called silico-ketones (silicones) by their discoverer (F. S. Kipping), a name which was then extended to the entire class of organo-silicone oxygen compounds.

The term siloxane, derived from the expression Sil-Oxan for the Si-O-Si-bond, is however more accurate. If one or more organic groups, such as methyl, phenyl, octyl or aminoalkyl, are attached to each silicon atom, the compounds are called organo-siloxanes. Such monomers are used as building blocks to manufacture silicone polymers, i.e. organo-polysiloxanes (Table 1.2).

Table 1.2: Building blocks for polysiloxanes

The fact that various siloxane units in the molecule can be combined with each other leads to a huge diversity of compound types[1]. The following are examples of the compounds which have been synthesized:

1.   A monofunctional siloxane unit reacts once with a similar unit resulting in a hexaorgano-disiloxane:

Figure 1.3

2.   Closed rings result when difunctional units combine. The smallest known ring contains three siloxane units; rings with four or five siloxane units are easiest to create (Figure 1.4).

Figure 1.4

3.   Trifunctional siloxane units linked together generally yield randomly, three-dimensional crosslinked molecules (Figure 1.5).

Figure 1.5

Under certain conditions, however, small cage-like structures with four, six and twelve siloxane units, which can be considered polycyclic, have been found.

4.   A combination of mono and difunctional siloxane units leads to linear polymeric siloxanes with a huge variety of chain lengths determined by the ratio of di to monofunctional units (Figure 1.6).

Figure 1.6

Similar processes can lead to linear high polymers (Figure 1.7).

Figure 1.7

5.   The combination of mono- and tri- or tetrafunctional siloxane units results in low molecular weight structures such as shown in Figure 1.8.

Figure 1.8

6.   Linking di and trifunctional siloxane units generally leads to macromolecules, which are usually networks, if they have a high T-unit content (Figure 1.9) while an excess of D-units results in chains with a low degree of crosslinkage (Figure 1.10).

Figure 1.9

Figure 1.10

On the other hand, D- and T-units can congregate in limited numbers to form low molecular weight structures (Figures 1.11):

Figure 1.11

7.   Finally, the reaction of di- and tetrafunctional units yields not only strongly, crosslinked groups of molecules (as with the combination of di and trifunctional components) but, under specific conditions, spiro compounds (Figure 1.12):

Figure 1.12

In line with their functionality, the D-unit is used as a chain or ring former, the M-unit as a stopper and the T- and Q-units as crosslinkers.

1.2 Chlorosilanes – the building blocks for silicone resins

Silicone resins are highly crosslinked siloxane systems. The crosslinking components are introduced with tri- or tetra-functional silanes. Only a few silanes have attained practical importance as resin building blocks and can be easily manufactured on a large scale. These are: methyl-trichlorosilane, phenyl-trichlorosilane, dimethyl-dichlorosilane, phenylmethyl-dichlorosilane, diphenyl-dichlorosilane, trimethyl-chlorosilane and tetrachlorosilane[7]. Methyl-trichlorosilane is the most important monomer by volume in silicone resin technology. Normally combinations of various silanes are used in the synthesis of silicone resins[8].

The choice of chlorosilanes for a particular silicone resin determines its characteristics. Prediction of specific resin properties as a function of composition frequently fails since processing and curing conditions influence the final molecular configuration and related characteristics.

However, some generalizations can be made:

Methyl-trichlorosilane leads to high hardness and rapid curing but also brittleness, poor pigmentability and poor compatibility with organic resins. This is due to the fact that the methyl-trichlorosilane contains the smallest amount of carbon of all chlorosilanes, except for the less used tetrachlorosilane, and is much closer to inorganic silicates than, for example, dimethyl-dichlorosilane or phenyl-trichlorosilane.

The use of phenyl-trichlorosilane results in resins with high thermal stability, good pigmentability, increased compatibility with organic resins but in a high degree of crosslinking and comparatively brittle products of low thermoplasticity as the methyl-trichlorosilane. In practice phenyl-trichlorosilane is not used alone.

Flexibility is achieved primarily by the concomitant use of dimethyl-dichlorosilane but at the cost of sacrificing hardness at elevated temperatures. This applies particularly to dimethyl-dichlorosilane, but also, to a lesser extent, to phenylmethyl-dichlorosilane and diphenyl-dichlorosilane. In the synthesis of silicone resins, cyclic siloxanes, such as D4 and D5, can also be used instead of dichlorosilane derivatives to achieve flexibility.

Table 1.3: Effect of silanes on the properties of silicone resin films

Trimethyl-chlorosilane is often used as a monofunctional silane in combination with trichloro- or tetrachlorosilane derivatives to obtain more stable resin end products and a lower degree of crosslinking, which leads to less brittle coatings with lower susceptibility to cracking under thermal stress. Because of its mono-functionality, trimethyl-chlorosilane is used as an end blocking agent in silicone resin synthesis. Reproducible incorporation of trimethylsilyl groups in the polymer frequently poses considerable difficulties because trimethyl-chlorosilane exhibits comparatively low reactivity during hydrolysis/condensation. Hexamethyl-disiloxane can be used instead of trimethyl-chlorosilane, in silicone resin synthesis.

Higher aliphatic groups (usually butyl or octyl) increase compatibility with organic polymers but significantly impair heat resistance.

Aminoaliphatic groups (aminopropyl- or aminoethyl aminopropyl groups) lead to specific properties in water repellency applications for the construction business.

Methylvinyl dichlorosilanes can be used to incorporate reactive vinyl-groups in the silicone resin structure. The vinyl functionality can be used for further crosslinking of the silicone resins by addition curing.

All organo-oligosiloxanes used on a large scale have the general formula (Figure 1.13) and are obtained primarily by hydrolysis of organo-chloro- or organoalkoxy-siloxanes.

Hydrolysis of chlorosilanes with alcohols under partial condensation leads to oligosiloxanes or low molecular weight alkoxy-functional siloxanes. The degree of condensation n is of major importance.

Figure 1.13: General formula for organo-oligosiloxanes

The functional groups OR2 (where R2 stands for hydrogen or an alkyl group with one to four C-atoms), obtained by hydrolysis or alcoholysis, contribute decisively to the properties of the silicone resin pre-products. These functional groups are responsible for the reactivity and the stability of the intermediate and silicone resin end products. They also govern the functional properties of the cured coating film.

1.3 Manufacture of the resin intermediates

The first step in preparing silicone resin intermediates consists of formulating an appropriate mixture of organo-chlorosilanes in an aqueous medium to obtain organo-oligosiloxane intermediates by hydrolysis and partial condensation[9].

Almost all industrial silicone resins contain methyl- or phenyl groups as Si-C bonded organic groups R1 (see Figure 1.13). Other organic groups such as higher alkyl, substituted alkyl- or aryl- but also vinyl-groups are quantitatively unimportant but are used in some resins as modified structural components.