Silane – A Multifunctional Compound in Plastics

Silanes are versatile! They function as coupling and dispersing agents for fillers in rubber & plastics formulations, as polymerization modifiers for polypropylene synthesis, and as crosslinking agents for polyethylene homopolymers & copolymers. Due to the unique properties of silanes, they are used to enhance performance and processes in the plastics and rubber industries. Learn more about the chemistry of silanes and which mode of mechanism & filler treatment do they follow based on their functionality. Also, check out the benefits of using silanes along with their applications.

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Silane Crosslinking Agents

Silane crosslinking process: the organofunctional group of the silane can react, and bond to, the polymer backbone. Residual moisture activates the silane’s alkoxy groups to the active silanol form which react with each other, liberating moisture and forming siloxane bonds between the polymers. The resulting Si-O-Si crosslink is extremely durable, offering excellent weather, UV, temperature, chemical and moisture resistance.

The cross-linking agent used in RTV silicone systems consists of a species that can be represented as R-Si-X3 (typically used in one-component systems) or Si-X4 (typically used in two-component systems). The R is an organic group such as methyl, ethyl, or vinyl, phenyl, and the X is a moisture hydrolysable group. A simplified cure mechanism for a one-component RTV silicone adhesive sealant is showing below.

Figure A: Reaction of crosslinker with polymer ends:

Figure B: Reaction of crosslinker-capped polymer end with moisture:

Figure C: Reaction of resultant polymer end with another polymer:

Repeated hydrolysis and reaction of resultant polymer end groups lead to full cure with the elimination of HX as a by-product of the condensation reaction.

The acetoxy cure system is the most common RTV system, and it has been used for the longest period of time. However, the by-product is acetic acid, and this could be corrosive to metal substrates or undesirable because of the odor. The alkoxy cure systems produce a by-product that is noncorrosive and has an unobjectionable odor. The acetoxy, alkoxy, and oxime chemistries are all prevalent today. The characteristic of these cure systems are summarized in the table below:

Characteristics of Various RTV silicone Cure Systems

RTV cure system

Characteristics

Acetoxy

Relatively fast cure time and short tack-free time. Good adhesion.

Alkoxy

Longer talk-free time and slower cure than acetoxy. By-product produced is noncorrosive and without objectionable odor. Adhesion is not as good as acetoxy.

Oxime

Low corrosion behavior but somewhat longer tack-free and cure times than acetoxy or alkoxy.

In one-component systems, the crosslinker is added to the filled silicone polymer and immediately reacts with the polymer as indicated in figure A. The reaction results in the formation of two moisture-hydrolyzable reactive sites at each end of every polymer chain. Once reacted in such a manner, the product is ready for packaging. It must be kept away from moisture or moisture vapor to avoid the subsequent curing steps and to provide long shelf life.

Once applied and exposed to ambient moisture, two adjacent polymer chains will react through the hydrolyzable reactive sites as show in figure B. The cross-linking will continue until all cross-link sites have been completely consumed. The resulting molecule is a highly cross-linked network with good elasticity.

The cure of an RTV two-component silicone sealant occurs in a similar manner. Most often an alkoxy silane crosslinking agent and a catalyst are packaged together leaving the siloxane as the second part. A reactive metal catalyst such as dibutyltin dilaurate is generally used to begin the curing reaction. The components must, of course, be kept dry to provide adequate shelf life. Once the two components are mixed, the hydrolysis reaction begins. After this occurs, the cross-linking reaction may be accelerated by exposure to slightly elevated temperatures.

Cross-linking of either one-component or two-component RTV silicone systems at room temperature may be accelerated by the use of catalysts at low levels. The catalyst is usually a tin octoate or dibutyltin dilaurate. The rate of crosslinking is a function of catalyst concentration and its chemical nature. Catalyzed systems are especially useful in forming a quick-dry skin that is often desirable in an outdoor applications where the weather and elements cannot be controlled.

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Acetoxy Silane Crosslinker

Oximine Silane Crosslinker

Alkoxy Silane Crosslinker

SiSiB® PC7930 MTA

SiSiB® PC7130 MOS

SiSiB® PC5131 MTMS

SiSiB® PC7950 ETA

SiSiB® PC7220 DMOS

SiSiB® PC5132 MTES

SiSiB® PC7960 VTA

SiSiB® PC7500 VOS

SiSiB® PC5420 TEOS

SiSiB® PC7970 PTA

SiSiB® PC7400 TOS

SiSiB® PC5424 TEOS-40

SiSiB® PM2080

SiSiB® PC7600 POS

SiSiB® PC5430 TPOS

SiSiB® PM3070

SiSiB® PC7133 Methyl MIBKO Silane

SiSiB® PC6110 VTMO

SiSiB® PM7030

SiSiB® PC7530 Vinyl MIBKO Silane

SiSiB® PC6151 Enoxy

SiSiB® EM7030

SiSiB® PC7410 Tetra MIBKO silane

SiSiB® PC8151 Enoxy

SiSiB® PC7131 Methyl Acetoxime Silane

SiSiB® PC7531 Vinyl Acetoxime Silane

SiSiB® PC7160 Methyl 2-PO Silane

SiSiB® PC7560 Vinyl 2-PO Silane

SiSiB® MT9010 / MT8515 / MT8020

SiSiB® MV6733 / MV8020

SiSiB® VT5545 / VT6535 / VT8020 / VT8515

Silane – A Multifunctional Compound for Plastics

Silane Crosslinking AgentSilane Crosslinking Agents