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Does Silicone Melt? How Does Silicone Break Down?

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Skye Rhodes

Does silicone melt?

Silicone is the 14th periodic table element that makes up about 25% of the total earth crust, it’s present in combined form because, in nature, it doesn’t exist in free form.

Silicone compounds are very unique because of their excellent intra-atomic bonding, they are very strong compounds with great stability and strength, hence why they’re widely used in industries.

Silicone compounds are mostly present in the polymer form of poly(dimethylsiloxane), these polymers have a large number of unique properties – they’re thermally stable, hydrophobic, nonreactive, resistant to ozone, UV, and oxidation.

They also have a low surface tension with a high range of dielectric constant, and because of these desirable properties, they are widely used in many industries, however, silicone shouldn’t be confused with silicon.

Silicon is the raw form from which all other types of silicones are derived, it’s derived from Quartz (SiO2) and carbon, while silicone is an oxygen compound of silicon.

But the big question remains – “does silicone melt?”, and if it does, what’s the melting temperature of silicone?

Melting Of Silicone

For silicone to be made, the reaction between SiO2 and carbon must be done at a temperature of about 1700 degrees Celsius, and this reaction is generally not so expensive.

SiO2 + C → Si + CO2

Silicone contains high intra-atomic forces that are stronger than any other type of metallic and ionic forces.

Silicone is a large polymer compound that has alternating atoms of silicon and oxygen.

The Si-O-Si bond contains stronger covalent forces than the C-C bond that require very high energy to break down, this is as a result of their thermal durability and high resistance against multiple factors. 

Silicones melt and burn, but they require a temperature of approximately 1414 degrees Celsius to break the bonds between them.

The melting temperature of silicone also depends upon the type of silicone compounds present, and this can either be silicone oil or silicone rubber. 

However, there are many other polymers present in nature including nitrile and acrylate, which are known for their thermal stability, these polymers mostly start to melt at temperatures between 70 to 150 ⁰C.

On the other hand, siloxanes can withstand temperatures within the range of 200 to 250 ⁰C, and siloxanes start to burn at their melting point.

So, to avoid the combustion of silicone, different types of flame retardant elements are added during the production of these polymers.

These additives not only protect the material from combustion but also enhance the durability of the polymer. 

In their combined form, polymers are made to be less sensitive to high temperatures, and this makes them even fitter to be applied in industrial or medical use.

They are only reactive under extremely acidic or alkaline environments, or in the presence of excessive oxidants. 

The bond length between the Si-O-Si makes them move differently in a low energy configuration state, however, this provides them with their low surface tension. 

Degradation Of Silicone

The most important property of silicone is its durability because this also makes it quite resistant to other environmental factors.

Most of the polymers can resist the natural degradation process, and this might make them unsafe for the environment, moreover, some of these compounds undergo evolutionary changes that make them resistant to natural degradation.

But the degradation of the material is a natural process that is to be carried out, hence, harsher conditions are created for silicone to be degraded.

There are multiple ways through which the material can be degraded: 

  1. Biodegradation
  2. Chemical degradation
  3. Thermal degradation

Biodegradation is different from chemical degradation in the sense that, the biodegradation is carried out under the action of microorganisms, or it involves the process of biotransformation which requires enzymatic action.

Enzymes play a catalytic role in the degradation process that is specific for each type of reaction.  

Microorganisms like bacteria, fungi, algae, and protozoa are mainly involved in the process of natural degradation.

These organisms secrete the enzymes which break down the bonds between the complex compounds, and it then converts them into the simpler compounds that can be reused by nature.

In the case of chemical degradation, artificial reagents are used to make the bonds weak so that they can be degraded by nature.

These chemicals make the bonds weak and then gradually break them, after breaking the bonds, they also remove the monomer units from the polymer.

These chemicals are synthetically designed according to the different types of bonds present in the polymer. 

Chemical degradation either removes the terminal compound or internal groups by hydrolyzing their bonds.

Sometimes, chemical degradation proves to be more effective but there are a large number of side effects associated with using this process, moreover, they are also quite expensive.

However, the most problematic side effects are that they release toxins or hazardous substances into the ecosystem.

Biodegradation Of Silicone

Synthetic polymers have a characteristic strength and they can withstand any type of biological degradation techniques, hence they become a burden on nature because of their unreactivity.

The pure form of these siloxane polymers is resistant to microbial degradation, while the hybrid or modified form of these polymers has some sort of antimicrobial characteristics. 

Due to the unique structure of the siloxanes, they are unable to cross the cellular membrane.

The hydrophobic property of the outer methyl group protects the Si-O bond from any type of enzymatic activity, however, partial hydrolysis of siloxanes is carried out by some microbes.

These microbes secrete enzymes that adhere to the surface of the siloxanes, and this leads to the removal of the methyl groups on the surface of polymers.

And this shortening of the surface polymers makes them vulnerable to partial hydrolysis.

In different studies where scientists tried to observe the degradation of siloxanes through the aquatic diatoms, it was seen that these diatoms didn’t show any hydrolytic effect on the siloxanes.

Liquid siloxane compounds also didn’t show any degradation during the diatomic-based decomposition. 

In nature, the siloxanes are slowly hydrolyzed by CO2 and inorganic elements like silicates, most times, silicone enter into the soil for degradation through the sludge.

In the soil, these silicone compounds are hydrolyzed into simple soluble compounds and then ultimately converted into monomer units.  

Silicone hydrolysis takes months or even years because of the presence of moisture in the soil, however, they can be hydrolyzed more easily in dried soil.

The hydrolyzed products of silicone have no hazardous effect on the growth of crops.  

When silicone is present in the soil that contains sludge, it’s exposed to the catalytic activity by soil minerals.

These catalytic components start the slow depolymerization of the complex silicone compounds and convert them into simple natural components. 

Siloxanes Removal Through Biofiltration

Biogas contains volatile methyl siloxanes, these silicate-based derivatives are deposited on the biogas power plants and it’s very costly to remove them.

Biofiltration is a cost-effective and eco-friendly procedure that helps to remove these silicate derivatives.

Over the last many years, biogas is used for the generation of energy and it’s produced by the combustion of organic waste material.

Many by-products are also generated along with the biogas, these byproducts include methane, CO2, CO, N2, and H2, unfortunately, these byproducts damage the equipment which often requires an expensive repair.

Besides the production of these common byproducts, many other contaminants are also produced during the utilization of biogas, these contaminants include halides, hydrogen sulfide, and most important the silicone compounds.

Different sources of biogas contain different amounts of VMS, depending on the source from which the biogas is generated.

Fermentation biogas contains a lower amount of silicate compounds than landfill gas, which is enriched with volatile methyl silicates.

During the process of biogas combustion, siloxane compounds are converted into silicone compounds, and because it’s difficult to remove the layer of silicone formed by simple chemical or other mechanical processes, they accumulate on the equipment and affect their functionality.

The siloxane can be degraded if subjected to biofiltration when biogas is being treated with some specific types of microorganisms. 

Biotrickling filtration plants are used for the treatment of siloxane polluted air.

The effective species of microbes that were observed to degrade siloxane are Pseudomonas, Xanthomonadacea, Mesorhizobium, Zooglea, and Rhodanobacter. 

Solar Degradation Of Silicone

Volatile silicones can be degraded in the open atmosphere by sunlight, while siloxanes can be washed out into rainwater by the sunlight-induced reaction. 

Degradation By Clay Minerals

The siloxanes degradation process is different in different environmental conditions, i.e, the rate at which silicone will be degraded in clay soil will be different from that of loamy soil. 

However, the rate of silicone degradation in various soil is not yet predicted due to the tremendous soil variations, this inability is because the mechanism of the soil degradation process is still pretty much not clear.   

When we disposed of silicone into the soil along with sludge, it comes in contact with soil minerals, and these minerals have a catalytic effect on the degradation process.

Till now, we still don’t know which minerals are involved in this degradation process, neither is there any clear natural or biological mechanism of synthetic polymer (siloxanes) degradation. 

This crucial structure of the siloxanes makes it difficult to predict any degradation mechanism for them, moreover, most of the biodegradation studies on siloxanes so far are in vitro studies.

Siloxanes have high molecular weight, and they are insoluble in water, hence their degradation has to be by atmospheric volatilization.

Most of the liquid siloxanes are degraded in the soil, but only if the soil is enriched with the different catalytic enzymes.

Many minerals (like kaolinite clay) and microbes can hydrolyze siloxane due to their catalytic effect

Re-arrangements and conversion of the complex polymers into simple cyclic monomer units are achieved via the hydrolysis of the Si-O bond.

Dry particles of soil show enhanced catalytic behavior on silicone hydrolysis, moreover, the formation of low molecular weight siloxanes and the Silanols are observed in montmorillonite clay.

Since the soil contains different decomposing factors, minerals play a more crucial role in siloxanes degradation than other human components whose catalytic effect on siloxane is significantly lower.

Although it’s not yet clear which minerals are actively involved in this catalytic activity, some compounds present in the soil are currently being studied.

These compounds include gibbsite, nontronite, kaolinite, goethite, and allophanes, some of which show higher activity than others.

Dry soil is more favorable for the degradation of siloxanes, but the hydrolysis process can also be done in wet soil; it’s also observed in freshwater.

In dry soil, the half-life period for hydrolysis is about 4 to 28 days while in the case of wet soil, it may take 1 year to hydrolyze about 0.5% of the PDMS.

Degradation by clay minerals is a slow process because of moisture present in the soil.

This process is quite fast in dry conditions, but naturally, the soil remains damp and moist so the natural degradation process proceeds very slowly.

There may be many other environmental factors that affect the degradation process just like moisture. 

The siloxane components are insoluble in water but when they are hydrolyzed, they are converted into water-soluble compounds like dimethyl silane diol (DMSD). 

DMSD has multiple fates, some of its particles are completely hydrolyzed and converted into CO2 and soluble silicates, some particles are volatile in the atmosphere, while others remain bonded with the soil particles.

Chemical Degradation

A large number of siloxanes are used in industrial applications for their many desirable properties.

Siloxanes are favorable because they are thermostable, can act as a lubricant, and serve as an adhesive material.

Siloxanes have an important biochemical effect because they’re resistant to acidic, basic, and oxidative environments, moreover, another advantage that they have is that their covalent bonds are extremely difficult to break.

Their unique properties may be regulated by changing the chain length of the polymer and by using different functional groups on the chain.  

Without modifications, it’s very difficult to use silicone compounds for industrial purposes.

Siloxanes are converted into their derivatives and they can then be treated with heat and other chemicals – can’t be treated if not modified.

Siloxanes have a complicated structure, however, their chemical and physical degradation results in the formation of different irreversible compounds.

Their derived structures do not share the same benefits as the original structure.

Chemical degradation of siloxanes can be carried out by different processes like:

  • Headspace gases
  • Ionizing radiation
  • Thermal cycling
  • Exposure to atmosphere
  • Exposure to acid
  • Exposure to base

The chemical degradation process is accelerated in the presence of moisture because it may speed up the hydrolytic process.

Sulfuric acid or potassium hydro-oxide serves as catalytic reagents for siloxane degradation.

The hydrolysis process can be carried out in the presence of water or ethanol which serves as a solvent to speed up the catalytic reaction, the main aim of this mechanism is to remove cyclic monomer units from the polymer chain of PDMS. 

Since the hydrolysis of PDMS is not an easy process, it always starts with the surface polymer removal, and many factors such as temperature, pH, stirring during the reaction, and the type of solvent, may affect the hydrolytic process.

Quantum Based Molecular Dynamics

Atmospheric exposure provides different stimulants for mechanistic insights.

Quantum-based molecular dynamics provide the stimulants for the chemical degradation of siloxanes, however, dephenylation is carried out by the combined processes of the steered dynamics and DFTB. 

The excited phenyl group is seen in the different reactions, and the presence or absence of water doesn’t influence it.

These excited phenyl groups then initiate the hydrolytic process by removing the hydrogen from the intrachain group, and this produces the resultant benzene compounds.

An excessive amount of water can promote the formation of benzene and prevent the intrachain cyclization of the phenolic groups.

Silanols and benzene are formed by the hydrolytic reaction. 

Degradation By Hydrofluoric Acid

Hydrofluoric acid selectively breaks the Si-O bond to degrade the siloxanes.

Upon the degradation, crystallization of the siloxanes is enhanced and then, they are removed as a monomer compound.

In this degradation process, the regulation of acidity and temperature is very significant.

Thermal Degradation Of Siloxanes

PDMS has an industrial application where it serves as an insulator for many electronic devices.

Many years ago, the hybrid form of siloxane was designed and termed polysilalkylenesiloxane (PSAS), this hybrid form has alternative siloxane and alkylene units.  

The thermal degradation of the siloxanes is mainly carried out by the anionic ring-opening of the complex structure of siloxane.

The resultant compounds that are obtained from thermal degradation are mainly deformed structures.

Low-temperature thermal degradation mainly yields cyclic monomer units, while the methane and oligomer units are formed by high-temperature thermal degradation of siloxane. 

The thermal degradation of PDMS usually starts from 340 C, and if we add the thermal retardant mixture to the PDMS, then it may enhance the thermal stability of siloxane. 

The thermal degradation of these polymers is carried out by the combined reaction of thermal volatilization analysis (TVA) and thermogravimetric analysis (TGA) reaction.

The degraded products of PDMS and hybrid siloxanes are quite different when they’ve been thermally degraded.

PDMS yields condensed products with high-quality volatile compounds, which are described as D3 to D6 cyclic oligomer compounds of siloxane.

While the thermal degradation of PSAS yields low-quality condensed volatile compounds which are described as D3 to D6 cyclic and linear siloxane oligomers.

Final Thoughts

Silicone is a unique element in nature that is present in a compound form, this element is not present in free form.

It has many important characteristics, hence it has been widely used in different industries, however, people are still not sure if silicone melts or not, hence the question – does silicone melt?

This compound is very stable because its covalent forces are stronger than any other type of ionic and metallic forces, hence it’s very difficult to break the siloxane because of its strong intra-atomic covalent bindings. 

They are highly resistant to acidic, basic, and oxidative environments.

They also have a high melting point, thus they can melt and burn, but only at extremely high temperatures.

Hence, there’s no need to worry about if your silicone product will melt, however, you should use the product according to its temperature requirement.

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