How is Silicone Oil Manufactured? A detailed Overview.

You might have heard the phrase “We are living in a plastic world” because plastic is everywhere, incorporated into almost every aspect of our lives. Here at Hengyi Tek, we have our phrase “We are living in a silicone world”, silicone has been an integral part of our daily chores such as using silicone chips in our computers and automotive vehicles. Silicone has an excellent reputation for its existence in various physical forms which gives it a headstart over its counterparts. Silicone exists in the solid state as the general public knows it. Silicone also exists in the gel-like material such as in silicone implants. The last known physical state of silicone to a common is liquid silicone which is often termed silicone oil or silicone fluid. Although it is not commonly known that silicone also exists in liquid form, the uses of silicone oil or silicone fluid are very crucial and are used in several industries.

You might be pondering about the manufacturing process of silicone oil, but before that, it is a must that we talk briefly about the uses of silicone oil and its properties.

 Let’s discuss the later part first, the properties of silicone oil:

Properties:

The linear structure of silicone fluids (polydimethylsiloxanes) is primarily responsible for all the beneficial properties exhibited by them.

High Molecular Weight:

It is one of the very few materials which maintains a liquid consistency at such high molecular weight making it useful.

Stable Viscosity-Temperature profile:

High molecular weight and the linear structure helps silicone oil maintain a constant or minor change in viscosity over a wide range of temperatures. This is one of the most important properties which makes it the best for various uses.

Thermal and Oxidative Stability:

Silicone oil is semi-inorganic which makes it somewhat resistant to thermal degradation and oxidative degradation. While organic fluids are particularly prone to both kinds of degradations.

Doesn’t absorb UV and X-Rays:

Silicone oil is also used in the medical and healthcare industry so this property of inability to absorb UV and X-rays comes in handy while being used in medical settings.

Low Surface Tension:

This property is of particular importance for its use in various industries.

Low Vapor Pressure.

Low Volatility.

High Flash and Ignition Point.

High Specific Resistance.

Water Insolubility.

Dielectric Properties.

Now that we have discussed the properties briefly, let’s move on to discuss the uses of silicone oil in brief:

Uses of Silicone fluid: 

The properties mentioned above provide silicone fluids with the liberty of usage in several industries. Silicone fluids are used as lubricants and release agents in their pure forms. The use of silicone oils as lubricants has been around for a while, especially in hydraulic aircraft engines. The food industry owes a bundle of thanks to silicone fluids as it is used as releasing agent in cooking sprays and for baking purposes. The textile industry has also seen a major boom in the use of silicone fluids for smooth touching in fabrics. The paint industry is also one of those reaping benefits from this magical fluid.

How to Manufacture Silicone oil?

Before diving into the exact process of how silicone fluids are made. We must develop an understanding of the structure of silicone and its various compounds. Commonly used Silicones are polymeric compounds of silicone having a silicone-oxygen chain with both of these at alternating positions (Si-O-Si), this alternating chain of silicon and oxygen atoms makes up the backbone of Silicone compounds.

Depending upon the extent to which a silicone compound can bear functional groups, they can be classified as monofunctional, difunctional, trifunctional, and tetrafunctional. The structural formulas for all of them are given below:

Monofunctional Silicone Compounds:

They are also known as end groups. The degree of Polycondensation and viscosity of the polymer depends upon the monofunctional group present in organo-poly siloxane which limits the chain length affecting these parameters.

Molecular Formula:                              R3SiO1/2

Starting Silane:                                     R3SiCl

Difunctional Silicone Compounds:

Difunctional functional groups tend to have a higher degree of molecular chains.

Molecular Formula:                                  R2SiO2/2 

Starting Silane:                                         R2SiCl2 

Trifunctional Silicone Compounds:

These often tend to produce 3-D structures which are cross-linked.

Molecular Formula:                                  RSiO3/2 

Starting Silane:                                         RSiCl3 

Tetrafunctional (Quaternary) Silicone Compounds:

Molecular Formula:                                  SiO4/2 

Starting Silane:                                         SiCl4 

Mono and difunctional silicone compounds are the primary components for linear chain silicone oils, so our discussion will be limited to these two.

Another categorization of organosilanes is based on the type of substituent (R) present in the structure of the silicone oil.

Dimethyl Siloxane: 

When methyl (CH3) is present as a substituent on two positions in a linear chain of siloxane, it is called dimethyl siloxane or dimethyl silicone oil. The majority of organosilanes have dimethyl silicone in their structure. Trimethylsilyl-terminated dimethylpolysiloxane is also a major contributor to organosilanes.

Some other types of methyl silane are considered important such as

Methylhydrogensiloxanes:

This type of silicone has hydrogen in place of one of the methyl groups present on the difunctional unit.

Phenymethylsilicone Fluids:

The methyl group present in the backbone of dimethyl silicone is replaced by phenyl groups. They tend to show somewhat different extents of properties

Different properties can be achieved by changing the proportion of phenyl to methyl groups present on the backbone of silicone oil structures.

There are a lot of modifications that can be incorporated into the alternating silicone oil backbone and there is a significant number of silicone oils that are produced each year.

Glycolfunctional Siloxanes are another modified silicone that is water soluble, their silicone backbone contains glycol chains consisting of ethylene oxide or polyepoxide.

The use of silicone oils in the aqueous environment depends upon what kind of bonding they have for silicone with adjacent atoms. Bonds can be formed between silicone, oxygen, and carbon such as Si-O-C, these types of bonds are subjected to hydrolysis, also the bonds can be formed between silicone and carbon such as Si-C, the best property exhibited by this type of linkage is that they are unsaponifiable.

The procedure for the production of Silicon Fluids/Silicone Oil: 

We have already stated that our focus in this article will be the production method or manufacturing method of dimethyl silicone oil as it is one of the basic and most important forms. 

There are a few basic reactions that take place in the production of dimethyl silicone fluids, these reactions are considered to be of fundamental importance in all organo-silicone compound chemistry.

Before going into details of all these fundamental reactions, let’s list them all:

‌Müller-Rochow Synthesis

‌Hydrolysis, Methanolysis, Condensation

‌Polymerization Reaction

‌Equilibration Reaction

Transesterification Reaction

Hydrosilylation Reaction

Direct Synthesis Reaction

Now Let’s discuss all of them in a little detail:

‌Müller-Rochow Synthesis

Methylchlorosilanes are produced with the help of this reaction. These are cheaper to produce in terms of finances. Silicone and methyl chloride (in gaseous form) are primarily required for the reaction. A high temperature of 260-320°C is required with the presence of copper as a catalyst. Reactions take place in a gas/solid state. The equation of the above-said reaction is shown below, and it leads to the production of dimethyldichlorosilane as follows:

Si + 2CH3CI → (CH3)2SiCl2

‌Hydrolysis,methanolysis, and Condensation.

These three steps take place after the synthesis of organocholosilanes. Hydrolysis and methanolysis generally take place before condensation; both have organocholosilanes as their targets. After hydrolysis and methanolysis, the next step is condensation which leads to silane production. During these three processes, the acid chloride of silicic acid reacts with water and methanol. Details of the reaction are given below in the following:

(CH3)2SiCI2+ 2H2O→(CH3)2Si (OH)2 +2HCI 

n(CH3)2Si(OH)2→HO一[(CH3)2SiO]n一H + (n一1) H2O

nSi + 2nCH3OH →rnHO一[(CH3)2SiOH]x一 f (n一rn)H20 

The above reaction depicts the condensation of dimethyl dichlorosilanes after hydrolysis had already taken place. During this reaction, the elimination of water from silanol groups results in the production of high molecular siloxanes. The biggest drawback of hydrolysis reaction is that it results in a considerable amount of HCL gas production which leads to pollution. Methanolysis, on the other hand, is pollution free in this regard and fact more economical because the chlorine produced can be reused in the production of methyl chloride. 

Polymerization Reaction:

Out of many ways to produce silicone oils, one is called the polymerization reaction. As the name suggests, polymerization reactions result in the production of high molecular linear polysiloxanes from those siloxane rings which are free from hydroxyl groups.

Equilibration Reaction:

It is one of the mega players in the production of silicone fluids. The main goal of the reaction is to homogenize the siloxane mixtures which have a different molecular weight. This reaction is catalyzed in such a way that the final siloxane mixture is a gaussian molecular weight mixture. Silicone fluids with desired viscosity can be drawn out using an equilibration reaction and also stable uniform distribution can be achieved using this reaction.

Direct Synthesis Reactions:

In this type of synthesis, the reactants are either silicone or SiH containing silanes and chlorobenzene. After silicone and chlorobenzene react to produce phenylchlorosilanes which are repeatedly hydrolyzed to produce silicone oil.

Hydrosilylation reaction: 

It is a catalytic reaction in which noble metals act as a catalyst and they add ω-terminated olefinic molecules to SiH silanes.

Transesterification Reaction: 

Alkosilyl groups are transesterified by a silanol group-containing siloxanes which yield silicone oil.

Insertion Reactions are also one way to produce silicone oil.

All these processes can be used to produce silicone oil. Some of them are more economical and more frequently used but that is not our concern for today.