Chemically Assisted Degradation of Polymers
Encyclopedia
Chemically assisted degradation of polymers is a type of polymer degradation
that involves a change of the polymer
properties due to a chemical reaction with the polymer’s surroundings. There are many different types of possible chemical reactions causing degradation however most of these reactions result in the breaking of double bonds within the polymer structure.
by ozone particles. Ozone
is a naturally occurring atmospheric molecule that is produced by electric discharge or through a reaction of Oxygen
with solar radiation. Ozone is also produced with atmospheric pollutants reacted with ultraviolet
radiation. For a reaction to occur, ozone concentrations only have to be as low as 3-5 parts per hundred million (pphm) and when these concentrations are reached, a reaction occurs with a thin surface layer (5 x10-7 metres) of the material.
The ozone molecules react with the rubber which in most cases is unsaturated (contains double bonds), however a reaction will still occur in saturated polymers (those containing only single bonds). When reaction occurs, scission of the polymer chain (breaking of double covalent bonds) takes place forming decomposition
products:
Chain scission increases with the presence of active Hydrogen
molecules (for example, in water) as well as acids and alcohols. Along with this type of reaction, cross linking and side branch formations also occur by an activation of the double bond and these make the rubber material more brittle. Due to the increase in brittleness due to the chemical reactions, cracks form in areas of high stress
. As propagation of these cracks increases, new surfaces are opened for degradation to occur.
in PVC
(Peacock). Solvolysis occurs when a Carbon-X bond, with X representing a halogen, is broken. This occurs in PVC in the presence of an acid
species. Active Hydrogen atoms will remove a Chlorine atom from the polymer molecule, forming Hydrochloric acid
(HCl). The HCl produced may then cause dechlorination of adjacent Carbon atoms. The dechlorinated Carbon
atoms then tend to form double bonds, which can be attacked and broken by ozone, just like the degradation of rubbers described above.
may occur without the presence of the acidic catalyst that causes degradation of PVC. During hydrolysis
water acts as the reactive catalyst instead of the acid. It causes degradation mainly at high temperature and pressure during processing.
In this process the water molecule will attack the C-O ester bond, splitting the polymer in half. The water molecule will then dissociate, with one Hydrogen atom forming a carboxylic acid group on the Carbon atom with the double bonded Oxygen, while the remaining atoms form an alcohol on the other chain end. These reactive products may also cause further degradation of the polymer chain.
This chain scission lowers average molecular weight of the polymer, decreasing the number and strength of intermolecular bonds as well as the degree of entanglement. This will increase chain mobility, decreasing strength of the polymer and increasing deformation at low stresses.
This barrier involves addition of a material into the polymer blend during fabrication of the polymer. Due to this, the barrier addition must have a suitable solubility, must be economically feasible and must not hinder the production process. For the barrier to be activated, the addition must diffuse to the surface and so a suitable diffusivity is also required.
There are four theories on how these types of barriers protect the polymer material:
Of these theories, the Scavenger Theory is the most common and most important. However, more than one theory can act at the same time and the theory that takes place depends on the protective materials, the polymer and surrounding environment.
Polymer degradation
Polymer degradation is a change in the properties—tensile strength, colour, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts...
that involves a change of the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
properties due to a chemical reaction with the polymer’s surroundings. There are many different types of possible chemical reactions causing degradation however most of these reactions result in the breaking of double bonds within the polymer structure.
Degradation of rubber by ozone
One common example of chemically assisted degradation is the degradation of rubberRubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
by ozone particles. Ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
is a naturally occurring atmospheric molecule that is produced by electric discharge or through a reaction of Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
with solar radiation. Ozone is also produced with atmospheric pollutants reacted with ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation. For a reaction to occur, ozone concentrations only have to be as low as 3-5 parts per hundred million (pphm) and when these concentrations are reached, a reaction occurs with a thin surface layer (5 x10-7 metres) of the material.
The ozone molecules react with the rubber which in most cases is unsaturated (contains double bonds), however a reaction will still occur in saturated polymers (those containing only single bonds). When reaction occurs, scission of the polymer chain (breaking of double covalent bonds) takes place forming decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
products:
Chain scission increases with the presence of active Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
molecules (for example, in water) as well as acids and alcohols. Along with this type of reaction, cross linking and side branch formations also occur by an activation of the double bond and these make the rubber material more brittle. Due to the increase in brittleness due to the chemical reactions, cracks form in areas of high stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
. As propagation of these cracks increases, new surfaces are opened for degradation to occur.
Degradation of poly(vinyl) chloride (PVC)
Degradation can also occur as a result of the formation, and then breakage of double bonds, such as solvolysisSolvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
in PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
(Peacock). Solvolysis occurs when a Carbon-X bond, with X representing a halogen, is broken. This occurs in PVC in the presence of an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
species. Active Hydrogen atoms will remove a Chlorine atom from the polymer molecule, forming Hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
(HCl). The HCl produced may then cause dechlorination of adjacent Carbon atoms. The dechlorinated Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms then tend to form double bonds, which can be attacked and broken by ozone, just like the degradation of rubbers described above.
Degradation of polyester
Degradation of polyesterPolyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
may occur without the presence of the acidic catalyst that causes degradation of PVC. During hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
water acts as the reactive catalyst instead of the acid. It causes degradation mainly at high temperature and pressure during processing.
In this process the water molecule will attack the C-O ester bond, splitting the polymer in half. The water molecule will then dissociate, with one Hydrogen atom forming a carboxylic acid group on the Carbon atom with the double bonded Oxygen, while the remaining atoms form an alcohol on the other chain end. These reactive products may also cause further degradation of the polymer chain.
This chain scission lowers average molecular weight of the polymer, decreasing the number and strength of intermolecular bonds as well as the degree of entanglement. This will increase chain mobility, decreasing strength of the polymer and increasing deformation at low stresses.
Protection against chemically assisted degradation
Both physical and chemical barriers can be used to protect a polymer from chemically assisted degradation. A physical barrier must provide continuous protection, must not react with the polymer’s environment, must be flexible so that stretching may occur and must also be able to regenerate (after wear processes). A chemical barrier must be highly reactive with the polymer’s surroundings so that the barrier reacts with the environmental conditions rather than the polymer itself.This barrier involves addition of a material into the polymer blend during fabrication of the polymer. Due to this, the barrier addition must have a suitable solubility, must be economically feasible and must not hinder the production process. For the barrier to be activated, the addition must diffuse to the surface and so a suitable diffusivity is also required.
There are four theories on how these types of barriers protect the polymer material:
- Scavenger Theory: the protective layer reacts with the ozone rather than the polymer.
- Protective Film Theory: the protective layer reacts with the polymer producing a thin film on the polymer surface which is inert and cant be penetrated.
- Re-linking Theory: the protective layer causes broken double bonds to be reformed.
- Self Healing Theory: the protective layer reacts with degraded polymer chains to form low molecular weight material which forms an inert film on the surface.
Of these theories, the Scavenger Theory is the most common and most important. However, more than one theory can act at the same time and the theory that takes place depends on the protective materials, the polymer and surrounding environment.
See also
- Indication of weather degradation
- Environmental stress crackingEnvironmental stress crackingEnvironmental Stress Cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. Environmental stress cracking may account for around 15-30% of all plastic component failures in service.ESC and polymer resistance to ESC have been studied...
- Weather testing of polymersWeather testing of polymersWeather testing of polymers is the controlled polymer degradation and polymer coating degradation under lab or natural conditions.Just like erosion of rocks, natural phenomena can cause degradation in polymer systems. The elements of most concern to polymers are Ultraviolet radiation, moisture and...
- Photo-oxidation process
- Thermal degradation of polymersThermal degradation of polymersThermal degradation of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer can begin to separate and react with one another to change the properties of the polymer...
- Use of stabilisters to enhance weathering resistance
- Polymer degradationPolymer degradationPolymer degradation is a change in the properties—tensile strength, colour, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts...
- Ozone crackingOzone crackingCracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking...
- UV degradationUV degradationMany natural and synthetic polymers are attacked by ultra-violet radiation and products made using these materials may crack or disintegrate . The problem is known as UV degradation, and is a common problem in products exposed to sunlight...
- HydrolysisHydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...