Silicones
Matt Hermes

 `Chemical Concepts
Let's begin to list the chemical concepts you will reinforce in this silicone rubber unit:

Dr. Dr. Stanley Kipping began to develop silicones using these principles to guide his actions.

Micro/Macro
and Symbolic
Representation
Chemistry uses macroscopic, large scale observations to help describe and understand matter at the unseeable, molecular level.

And then we represent both the microscopic and macroscopic with often complex symbolic representation.

As before we describe chemical compounds and chemical reactions by indicating each chemical structure with combinations of letters and numbers. The atoms of the elements have letter symbols representing the element. Subscript numbers indicate the number of atoms in the compound.

Our equations become somewhat uncertain. We use a shorthand (n) to imply an unknown number of moles of product or reactant. We are specifying as much as we can about the chemical reaction but we leave some issues open to question!

But again, our capacity to understand a variety of symbolic representations is enormous.

Heat and Chemical Resistant Silicone Rubber
4. Corning and the First Silicones for High Temperature Insulation

PRR  GG-1Courtesy Ken Houghton Rail Images

We saw one example of the benefits of high temperature insulation on the railroads.  The Pennsylvania Railroad electrified its rail lines from New York to Washington and from Philadelphia to Harrisburg, Pennsylvania because it could purchase efficient electric locomotives from General Electric.

These locomotives converted electricity from overhead lines to the work to drive their trains at up to 110 miles per hour.  General Electric still offers locomotives for passenger and  freight service around the world. Electrical insulation began with cotton impregnated with a resin.  This composition did not function above 130 degrees Celsius.   Replacement of cotton with asbestos allowed temperatures to 155 degrees Celsius, but the asbestos became brittle in time and the insulation failed. 

In time, glass fibers, drawn at high speed from specially formulated molten glass offered the opportunity for truly high temperature insulation.  Glass fibers were consistent in properties.  They could be impregnated with resins and wrapped around copper wires or made into insulating cloth.   Glass fibers impregnated with a series of resins called phenolics became the standard of 155 degrees Celsius, Class B (now Class F) insulations.

Corning Glass was a pioneer in glass technology.   They made the first practical glass globes for Edison's electric lamps.  Corning recognized that the glass fibers in insulation were not achieving their greatest potential.  When the phenolic-impregnated Class B insulations failed by overexposure to heat, it was always the phenolic that failed.  The resin would blacken and char, leaving conductive carbon residues in the unchanged.  Conductive residues are not a good feature in a large motor or any other environment requiring insulation.

A young, Harvard trained chemist, Dr. James Franklin Hyde, working for Corning, recognized the need for a flexible, high temperature binder for the glass fibers which would allow  yet another increase in service temperature for insulating materials.  He knew Si-C bonds were stable to heat.  He knew Kipping's work on organic silicon chemistry.  He was particularly intrigued by Kipping's remark that  diethyldichlorosilane (C₂H₅)₂SiCl₂(4) had produced "glue like" materials when Kipping had treated it with water.   Hyde believed what Kipping had done could be approximated with the chemical reaction:

Structure (6), Hyde reasoned was a long sequence of connected Si-O units as shown above.  He used his previous chemical knowledge and envisioned changing one of the groups of atoms attached to the silicon atom from -C₂H₅ to something he expected would make the product resin-like rather than glue-like.

Hyde began an arduous chemical synthesis.  His goal was another dichlorosilane similar to (4) called ethylphenyldichlorosilane (C6H5)(C2H5)SiCl2(7).    Recall that Kipping made his organic silicon compounds using Grignard's magnesium-containing reagent.  Hyde made two Grignard reagents and carried out this sequence of chemical reactions:

(C₆H₅)MgBr   +  SiCl₄  -->  (C₆H₅)SiCl₃   +  MgClBr

(C₂H₅)MgBr   +  (C₆H₅)SiCl₃   -->  (C₆H₅)(C₂H₅)SiCl₂(7)   +  MgClBr

When Hyde treated (7) with water, he made a molasses-like fluid he believed was (8):

                (C₆H₅)
           |
               n(7) + nH2O -->    n  (-Si-O-)            +         2nHCl
           |
                 (C₂H₅)
            (8)

He coated coat glass fibers with the fluid, then heated the composite material at high temperatures.  The fluid hardened to a rubbery mass.   The new composite was an excellent insulator and allowed Corning to make small motors and generators that performed well at up to 180 degrees Celsius. These initial products were the first Class H insulators.

So was the problem of a higher temperature insulation solved? It was not. No one could afford to make enough silicone by Hyde's method to supply the needs of the electrical industry.

• Hyde's preparation of (8) required many chemical steps. 

• It required enormous amounts of magnesium, bromine and chlorine that would be purchased, used in the synthesis and then thrown away.  

• And the yield -  the number of moles of product produced from each mole of starting reagent - was low for each step of Hyde's sequence.