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Increasing the stability of polymers against degradation

Oxidative stability on polymer structure

Polymers with saturated bonds are more stable than unsaturated polymers. Because unsaturated bonds are easily attacked by O2.

Linear polymers are more stable than branched polymers. Because linear polymers tend to crystallize. In crystalline polymers, polymer molecules are packed well. So, O2 molecules cannot easily enter the polymer structure. That means the degradation is low. But in branched polymers, there are branches that are connected to the backbone. Due to these branches, the polymer tends to form an amorphous structure. An amorphous structure is easily entered by Oxygen. Therefore, the degradation of the amorphous polymer is fast. Crystalline polymers are more stable than branched polymers.

 Polymers that have electron-withdrawing groups such as -CN, Carbonyl, Peroxy, -OR have low stability. Because such groups lower the bond energy of adjacent C-H bonds. It causes the degradation of the polymer.

BondBranch dissociation energy (KJ mol-1)
CH3-H104
C2H-H98
(CH3)2CH-H44.5
(CH3)3C-H91
CH2=CH-H104
CH2=CH-CH2-H85
C6H5-H104
H3[COCH2-H]92
CH3CH(OH)-H90

The stability of a polymer against degradation can be increased in two ways. Stability can increase either by adding stabilizers or modifying the polymer structure.

1.Modifing the polymer structure

Polymer structure can be modified chemically by adding or removing functional groups. Polymers with unsaturated bonds and branches have low stability. By removing unsaturation and reducing branching can increase stability.

Functional groups such as -CN, -OR, carbonyl, peroxy, reduce the stability of the polymer. By removing their stability can be increased. When the polymer backbone is directly connected to an aromatic group as a pendant group, it increases the reactivity of adjacent Hydrogen. That means it decreases the stability. But when aromatic groups are in the backbone of the polymer, that causes to increase the stability.

But when aromatic groups are connected indirectly to the backbone it increases the stability. Because of the protection of the bulky group, the polymer cannot be attacked by another group. That is called the “Shielding effect”. Completely aromatic polymers such as 'polyparaphenylene' are very thermal stable. If the polymer has fused ring structures on the backbone of the polymer has high thermal stability.

Atoms such as O, S, N increase the reactivity of the adjacent hydrogen atom. So, these atoms are removed.

If H is replaced by an F (Fluorine) atom, it increases the thermal stability. That is why PVC is more stable than polyolefin. Neoprene which has a Cl atom in its repeating unit is more stable than natural rubber.

Ladder polymers

Ladder polymers have high thermal-oxidative stability. Although a bond breaks, it can heal easily. Because the polymer chains are very closer to each other.

Ladder Polymers - INCREASING THE STABILITY OF POLYMERS
Figure 01: Ladder Polymers

To parallel polymer chains connected that frequent and regular intervals by short chains to form crosslinked ladder structure. They show unusual resistance to degradation even though the structure shows normal reactivity toward oxygen. These crosslinks prevent cleavage into segments.

aldol type reaction
Figure 02: Aldol Type Reaction

Organo silicon polymers

Oregano silicon polymers are polymers that have silicon in the backbone. They have high thermal-oxidative stability.

This polymer has unusual stability due to the thermal oxidative stability of the ligan combined with the inorganic backbone structure. Each silicon atom in polysiloxane is surrounded by a bulky planner ligan. These have high thermal-oxidative stability. By removing reactive sites and by replacing alkyl groups with phenyl groups increase the thermal stability.

But few modified polymers are applicable. They, themselves have a lot of other drawbacks. The main drawback is the modification is a costly method. It increases the cost of the final product also. Due to their less solubility and high softening temperature processing is very difficult. And, chemical modifications can alter essential properties of the material (polymer loss its original property.)  So, adding stabilizers is the most feasible way to increase the stability of polymers.

2.Using stabilizers against oxidation

Stabilizers which are also known as Antioxidants are added to the polymer to prevent degradation. Generally, there are two types of antioxidants based on their chemical structure.

  1. Hindered phenol
  2. Aryl amines
aryl ammines
Figure 03: Aryl Ammines
hindered phenol
Figure 04: Hindered Phenol

Several factors should be considered when selecting a stabilizer.

  1. Cost
  2. Application – Intended use
  3. Environmental factors which material going to be exposed.
  4. Toxicity
  5. Volatility
  6. Solubility – solubility of the stabilizer in the polymer in melting.
  7. Staining
  8. Leachability
  9. Durability

A stabilizer can be a short-term stabilizer or a long-term stabilizer.

Short term stabilizers

When polymer synthesis, processing, and fabrication, a polymer is subjected to high temperatures. So, polymers are susceptible to thermal-oxidative degradation at these stages. Therefore, stabilizers need to be added to protect the material at these stages. so, the short-term stabilizers are used to protect the polymer in polymer synthesis and fabrication.

Stabilizers in long term aging.

Additional stabilizers are required for storage and application stages. These stabilizers can add during the fabrication step.

Mechanism of stabilization

Wo different classes of antioxidants are available according to the mechanism which increases the stability.

  1. Preventive antioxidants
  2. Chain breaking antioxidants

Preventive antioxidants

“Antioxidants which absorb radiations, heat, and reject metal catalysts and decrease forming ROOH are called preventive antioxidants”

Preventive antioxidants inhibit or retard the formation of free radicals in the initiation step (retard the oxidation process) They interrupt the chain process by interrupting alkyl radicals (R*) or peroxy radicals (RO*). This type of antioxidant actually slows down the oxidation without changing the mechanism.

There are two categories of preventive antioxidants

01.Metal ion deactivators

Metal ions in the polymer can form unstable condition complexes with hydroperoxide followed by electron transfer reactions which will produce peroxy radicals. Two reactions have occurred when degrading a polymer that has metal ions.

Metal ion deactivators

Reaction 1 follows by strong reducing agents and reaction 2 follows by oxidizing agents. Degradation can reduce by preventing these reactions. Metal ion deactivators prevent the above two reactions by strongly complexing the metal ion to its maximum coordination number or by stabilizing its valance site or by precipitation of metal ion.

02.Peroxide decomposes

These types of preventive antioxidants prevent degradation by converting hydroperoxides into stable inert products.

 Chain breaking antioxidants

Chain-breaking antioxidants interrupt the propagating cycle by reacting with free radicals (R* or RO*). This reaction can be an addition, coupling, hydrogen transfer, or electron transfer reaction. This type of antioxidant introduces a new termination reaction. Then the machine will be more complex. This mechanism is called retarded antioxidant mechanism.

retarded antioxidant mechanism

After the optimum level, antioxidants help to decompose. That is called the “Pro oxidation effect”

rate of degradation
Figure 05: Rate of Degradation vs Time

By mode of action, chain-breaking antioxidants can be divided into three categories. 

  • Free radical traps.

Free radical traps are substances that have the capability to react with free radicals and form stable radicals. These radicals do not re-initiate the degradation. Later, these radicals can undergo dimerization, disproportionation, or reaction with a second alkyl radical to form a stable product.

  • Carbon black

Carbon black is an effective inhibitor. Its structure is an agglomerate of complex condensed polycyclic aromatic rings with hydrogen and reactive functional groups around the edges. They can trap free radicals by additional mechanisms and convert them into stable radicals while terminating their kinetic chain.

carbon black
Figure 06: Carbon Black
  • Nitroxides

Alkyl nitroxides, aryl nitroxides, pyridine type nitroxides are used. alkyl nitroxides are less practical compared to aryl nitroxides. Aryl nitroxides are the most stable and better antioxidants that can react with both alkyl (R*) and peroxy (ROO*) radicals.

  • Di-phenyl ammines and Hydroxyl ammine.

When radicals are present hydrogen can react with radicals and form stable radicals.

  • Electron donors

This type of antioxidant donates an electron to the radicals and form a salt.

Example: - tertiary amines

Electron donors
  • Hydrogen donors

Hydrogen donors are the very common antioxidants in the industry for thermal oxidation.

Examples: - hindered phenols and secondary arylamines.

Phenols and arylamines have reactive hydrogen. This hydrogen will be donated to the free radicals and form stable radicals. These stable radicals are also used as trap radicals.

Hydrogen donors

COMBINATION EFFECT OF STABILIZERS AGAINST TO THERMAL OXIDATION

There are many different types of stabilizers available today which may function in different ways. As an example, if a stabilizer is selected to inhibit the initiation process, or to interrupt the propagation step, combining them may be beneficial in many instances.

  • Additive effect

When stabilizers mix with each other, they will retain their character. So, the combined effect is additive.

Example: -

Additive effect
  • Antagonistic effect

Antagonistic stabilizers show a lower total effect than sum of the individual effect.

Examples: - Carbon black + secondary aryl ammine

                    Carbon black + alkylated phenol

Secondary aryl amines or alkylated phenols with carbon show antagonistic effects. Carbon black has holes and functional groups on the surface. So, it can absorb other stabilizers. This makes less availability of stabilizer.

  • Synergistic effect / Synergism

Synergistic stabilizers show a greater total effect than the sum of the individual effect. There are two types of synergism as “Homo-synergism” and “Hetero-synergism”. Homo-synergism involves two stabilizers of unequal activity operated by the same mechanism. Hetero-synergism generates by the cooperating effect of two or more antioxidants acting by different mechanisms. Hetero-synergism can be observed when mixing chain-breaking antioxidants and preventing antioxidants.

There are some antioxidants that have two or more kinds of antioxidants activity associated with the same molecules. It is called “Auto-synergism”

Examples: -

  • Carbon black – carbon black can act as either a radical trapper or hydrogen donor at the same time

Read more about polymer degradation and stability here.


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References and Attributes

Figures:

Cover Image Attribute

1. Image by Various-Photography from Pixabay

2.Image by William Iven from Pixabay

3.Image by Yao Charlen from Pixabay

4. Image by ClassicallyPrinted from Pixabay

5. Image by James Wheeler from Pixabay


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