Silanes and Other Coupling Agents, Volume 2

Kash L. Mittal

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000083156
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

This book embodies the proceedings of the Second International Symposium on Silanes and other Adhesion Promoters held in Newark, New Jersey, October 21--23, 1998. Silanes are the most popular and widely used coupling agents (or adhesion promoters) to promote adhesion between dissimilar materials in a variety of situations, e.g. coating technology, adhesive bonding, reinforced composites, etc. Since the first symposium on this topic in 1991, there had been a tremendous R&D activity in developing new and more effective adhesion promoters and in understanding and optimising the performance of available coupling agents. The technical program for the symposium contained 36 papers and reflected both overviews and original research contributors and the presenters hailed from academia, industry and other research laboratories. Many different aspects of coupling agents were discussed, and both fundamental and applied aspects were accorded due coverage. In addition to formal presentations, there were brisk and lively discussions throughout the symposium, and this event provided an opportunity for cross-pollination of ideas in the broad arena of adhesion promoters. This present volume contains 18 papers by experts from academia, industry and other research laboratories. All manuscripts were subjected to rigorous peer review and were suitably revised before inclusion in this volume. The book is divided into two parts as follows: Part 1. Silane Coupling Agents; and Part 2: Non-silane Coupling Agents/Adhesion Promotors. The topics covered include: silane adhesion prompters for hydrosilylation cure systems; sterically hindered silanes; study of silanes hydrolysis; adsorption of silanes on different substrates; interaction of water with silane films studied by neutron reflection; characterization of glass fiber sizings; silanes as dispersion promoters; corrosion protection of metals by silanes; surface 'Intelligraft' as a new class of adhesion promoters; hydroxymethylated resorcinol, sol-gels, and -diketone functionalised polymers as adhesion promoters; and plasma deposition of silanes.

Part 1

Silane Coupling Agents

Novel silane adhesion promoters for hydrosilylation cure systems

MICHAEL A. LUTZ, THERESA E. GENTLE, SUSAN V. PERZ and MICHAEL J. OWEN^*

Dow Corning Corporation, Midland MI 48686-0994, USA

* To whom correspondence should be addressed. Phone: [phone], Fax: [phone], E-mail: [email protected]

Abstract—Hydrosilylation offers a convenient, low-temperature, addition cure that is widely exploited in silicone sealants, adhesives and elastomers, and increasingly so in other polymer systems. One significant drawback of this cure system is the absence of polar groups, such as silanols, for reaction with hydroxylated oxide surfaces, which can result in poor adhesion. Numerous functional silanes have been proposed to improve matters. Preferably, these silanes should be incorporated as integral adhesion promoters rather than applied separately as primers. Cure and development of adhesion are interdependent phenomena in such mixed systems with compatibility and relative reaction rate considerations being paramount in product formulations.

We have found calculated solubility parameters to be an effective guide to the design of optimum combinations of wholly compatible cross-linkers and partially compatible adhesion promoters to enhance the adhesion of both silicones and hydrocarbon polymers such as poly (isobutylene). Adducts of tetraethoxysilane with unsaturated alcohols were found to be particularly effective. The tetraalkoxy substitution provides the most reactive functionality for bonding to hydroxylated substrates such as glass, aluminum and stainless steel. The unsaturation permits linkage to the polymer matrix and the partial compatibility is believed to allow an interpenetrating interfacial network to develop.

Keywords: Silane hydrosilylation silicones poly (isobutylene) solubility parameter

1. INTRODUCTION

One of the most useful ways of cross-linking poly(dimethylsiloxane) (PDMS) polymers to produce elastomers, sealants, adhesives and coatings is the hydrosilylation reaction where a silicon hydride group adds to an unsaturated group, generally vinyl or allyl, using a noble metal catalyst, typically a platinum complex. This approach has found widespread commercial use because of the following advantages:

1. One-part and two-part systems are possible.

2. Fast, controllable cure rate.

3. Reaction can be accelerated by heat.

4. No volatile by-products are produced.

5. Deep section/confined cure capability.

6. Good bond stability.

The general drawbacks of this approach, of which only the last is the subject of the present investigation, include:

7. Catalyst easily inhibited/poisoned.

8. Interfering side reactions.

9. Properties very stoichiometry dependent.

10. Poor adhesion without priming or other adhesion promotion scheme.

Curable poly(isobutylene) (PIB) is a class of cross-linked polymer that offers some interesting parallels to PDMS coupled with some intriguing differences. The use of hydrosilylation cure technology should confer similar cure control and processability as PDMS. This approach requires alkenyl functional PIB and a variety of such species have been described in the patent literature, for example, telechelic allyl terminated PIB [1]. The surface energy of PIB, deriving like PDMS from its pendent methyl groups, is also quite close to PDMS. However, due mostly to the more compact, constrained hydrocarbon backbone, it has much reduced gas permeability compared to PDMS. Also with a higher solubility parameter than PDMS it should offer more compatibility with organic materials, for example, the resultant sealant should probably be readily paintable by conventional solvent-based coatings. Some quantitative comparisons of PIB and PDMS along with low density poly(ethylene) (LDPE) and poly(tetrafluoroethylene) (PTFE) are given in Table 1. Characteristic pressure (p*) is probably the least familiar property: it comes from Flory’s equation-of-state and is a measure of intermolecular energy per unit volume. A perusal of this table readily helps understand the attractiveness of PIB in applications such as insulated glass sealants.

Table 1
Relevant fundamental polymer properties

	PDMS	PIB	LDPE	PTFE
Critical surface tension of wetting {mN/m}	24	27	33	18
Water contact angle {deg}	101	102	94	108
Solubility parameter {(cal/cm^3)^1/^2}	7.5	7.9	8.0	6.2
Glass transition temperature {K}	146	197	148	390
Characteristic pressure {J/cm^3}	341	448	485	362^a
Oxygen permeability {cm^3 (STP) cm·cm^−2·s^−1(cm Hg)^−1 × 10^10}	605	1.0	2.9	4.2

^a Data for perfluorohexane.

The solubility parameter (δ) is the square root of the cohesive energy density (CED) defined in Equation 1:

$\text{CED}=\frac{\text{energy of vaporization (Δ}{E}_v\text{)}}{\text{molar volume}(V)}$

(1)

The cohesive energy density is a measure of intermolecular forces and as such is related not only to solubility but also to surface tension, boiling point, glass transition temperature, etc. Both the energy of vaporization and the molar volume can be calculated to a first approximation by the summation of atomic and group contributions. A variety of such approaches are available, the simplest and the one we chose to use is the one introduced by Fedors [2].

2. ADHESION FUNDAMENTALS

Adhesion is a complex, multidisciplinary subject in which the nature of the interfacial region between the curing polymer matrix and the substrate to which it is applied is one very important aspect of the problem. It should be pos...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Part 1: Silane Coupling Agents
• Part 2: Non-silane Coupling Agents / Adhesion Promoters