What is polymer degradation?

The term polymer degradation[1] refers to the processes induced by sunlight, heat and other atmospheric agents that lead to a modification of the polymer structure, which are normally accompanied by a decrease in the mechanical characteristics of the material. Due to complexity of the chemical structure, environmental agents will act according to:

Factors Affecting Polymer Stability

 

  • Chemical structure
  • Chemical composition (type of bonds and what sort of arrangement; the higher the dissociation energy, the more difficult its degradation)
  • Other factors of chemical structure and composition
  • Steric factors
  • Resonance stabilization
  • Tacticity
  • Physical and morphological factors
  • Internal mechanical stresses
  • Contaminants
  • Impurities (compounds added as catalysts)
  • Physical structure of the material (degree of crystallinity, compaction of the chains, etc.)

Environmental agents or factors causing degradation are classified into[2]:

Chemical agents

  • The most important is oxygen, since all polymers react with it, fundamentally, at high temperatures (thermoxidation or autoxidation).
  • Ozone
  • Water (hydrolytic degradation), catalyzed by acids and bases
  • Enzymatic processes (biodegradation

Agents or energy factors

  • Temperature (thermal degradation and pyrolysis (in the presence of nitrogen and absence of oxygen);
  • Ultraviolet radiation (photodegradation). If there is joint action of ultraviolet rays with oxygen, we are talking about photooxidation.
  • High energy radiation (X-Ray) is known as Ionizing Degradation
  • Mechanical Energy causing Mechanical Degradation.

A polymer can have various physical or chemical changes[3]:

1.- PHYSICAL: discoloration, loss of surface gloss, formation of cracks, greasy surfaces, surface erosion and loss of properties such as resistance to traction-deformation

2.- CHEMICALS: chain breakage (scission), changes in lateral substituents, crosslinking reactions, other chemical reactions etc.

Each external agent that leads to polymer degradation results in a different degradation mechanism (and modifies the polymer structure)..

 

Types of polymer degradation

The degradation of polymers usually starts at the outer surface and penetrates gradually into the bulk of the material. The following table shows a summary of the different types of degradation depending on the external agent acting on the polymer

Table 1: External agents vs type of degradation (1).

 

Degradation mechanisms

THERMAL-OXIDATIVE DEGRADATION [4]

It consists of the attack of the active oxygen on the polymer; deep down, it is an organic oxidation-reduction reaction. As in thermal degradation, oxygen causes free radicals in the polymer, which can give rise to all kinds of degrading side reactions. In general, diene or tertiary carbon polymers are the least resistant to radical oxygen due to the reactivity of aryl and tertiary carbons. This type of degradation has been widely studied in polyolefins and clearly depends on the O2 concentration. In a first stage, oxygen fixes on the susceptible carbons in the chain, and a peroxide is formed which decomposes to acetone or aldehyde

Mechanism of thermal-oxidative degradation (1)

Ways of polymer thermal degradation

  • Depolymerization
  • Random chain scission
  • Side group elimination

Remedy

  • Protecting with stabilizers
  • Protecting polymers with radical scavenger stabilizers
  • Use of stronger bonds if it possible

Common antioxidant include

  • Hydroperoxide decomposers.
  • Radical scavengers.
  • Singlet oxygen, (1O2), quenchers.

 PHOTODEGRATION [5]

Polymers will change over time when exposed to UV radiation. These changes are the result of light-induced homolytic fission of chemical bonds (photolysis) and photo-oxidation. This process is based on the fact that the energy of ultraviolet light from sunlight is greater than the binding energy of the C-C and C-H molecular bonds and therefore break the molecular chains, reducing their molecular weight and mechanical properties.

Factors Causing Photodegradation:

Mechanism of photo-oxidative degradation of polymers[6]:

Two mechanisms have been proposed to explain the photooxidation of polymers in conformity with similar observations made on low molecular weight compounds. One proceeds through direct reaction of singlet oxygen with the substrate while the other involves the production of radicals and subsequent reaction with oxygen.

(I)The singlet oxygen mechanism of oxidation

It has been clearly demonstrated that many photosensitized oxidation reactions proceed with participation of oxygen in an electronically excited singlet state. Singlet oxygen exhibits several specific reactions and the one that has been most often invoked in the photooxidation of polymers is the formation of a hydroperoxide by oxidation of an olefin containing an allylic hydrogen, and which could further decompose and lead to chain scission and formation of a terminal of carbonyl group.

(II)The free radical mechanism of oxidation

The radical mechanism of photooxidation of polymers proceeds through a chain reaction (initiation, propagation and termination steps)

Photoinitiation step

Internal and/or external chromophoric groups absorb light and produce low molecular weight radicals (R.) and /or polymeric macro radicals (P.)

Propagation step

Low molecular radicals

Termination step

The radicals formed in the degradation of polymers can be terminated by numerous different combination reactions between two polymer radicals, in which inactive products are formed

Where : PH = Polymer, P• = Polymer alkyl radical, PO• = Polymer oxy radical (Polymer alkoxy radical), POO• = Polymer peroxy radical (Polymer alkylperoxy radical), POOH = Polymer hydroperoxide, OH• = hydroxy radical

Mechanism of photo-oxidative degradation and UV –stabilizer types(2)

 

Stabilizers

The different types of stabilizers can be classified according to their mechanisms of action in the photostabilization process into:

  • UV absorber and light screeners.
  • Hydroperoxide decomposers.
  • Radical scavengers.
  • Singlet oxygen, (1O2), quenchers.

THERMAL DEGRADATION (HIGH TEMPERATURE)

Heat leads to hemolytic disruption of covalent bonds in the chain or side groups, caused by increased temperature. After the link is broken, the reactions that occur depend on the activity of each radical (obviously, the higher the temperature, the greater the degradation). It also influences the viscosity of the melt, since local shear stresses can increase the temperature of the mass. Many thermal degradations are autocatalytic, so the polymer degrades faster if it is already partially degraded.

 

Stabilizers

The different types of stabilizers can be classified into:

  • Type Ba/Cd
  • Type Ba/Cd/Zn
  • Type Ba/Zn
  • Type Ca/Zn
  • Type Ca/Zn
  • Type Ba/Cd/Pb
  • Type Lead complex

HYDROLYTIC DEGRADATION[7]

When the material comes into contact with an aqueous medium, the water penetrates the polymer matrix and causes swelling, rupture of intermolecular hydrogen bonds, hydration of the molecules and finally the hydrolysis of unstable bonds.

Hydrolysis cleavage of functional groups can occur in both main chain groups and side substituents. However, the concept of polymer degradation is associated with a decrease in molecular weight, so it is necessary for the main chain to break at various points. Ability of polymers to degrade by hydrolysis is given by difference in electronegativity of atoms in polymer chain or side groups. Heteroatoms in polymer chains provide higher ability for hydrolytic degradation.

Depends on

  • Repulsive interactions with ions
  • Availiabity of reacting bonds
  • Physical parameters: swelling, transport of ions along/inside the polymer chain

BIODEGRADATION[8]

This term is applied when the transformations and deterioration of the polymer are due to the action of living organisms: the process is catalyzed by the action of fungi, bacteria, etc. and enzymes secreted by them. When happening in aqueous media, sometimes biodegradation and hydrolytic degradation go hand in hand.

Mechanism

1. Biodegradation begins with colonization of the polymer surface by bacteria and fungi. The bonding to its surface depends on factors such as surface tension, porosity, and surface texture. Compact polymers are less biodegradable since enzymes are less accessible to hydrolyzable groups.

2. Enzyme-based hydrolysis of plastics involves two steps: Firstly, the enzymes attach to the polymer substrate followed by hydrolytic division (Fig. 1). Degradation products of polymers like oligomers, dimers, and monomers are much low in molecular weight. This oligomers suffers a new degradation by the enzymes by the microorganism

3.1 Under aerobic conditions, oxygen is used as an electron acceptor by the bacteria followed by the synthesis of tinier organic compounds, and thus, CO2 and water are produced as end products

3.2Under anaerobic conditions, polymers are crushed down in the absence of oxygen by microorganisms. Sulfate, nitrate, iron, carbon dioxide, and manganese are used as electron acceptors by anaerobic bacteria

Common antibiodegradants include

  • Biocides

MECHANICAL DEGRADATION [9]

Mechanochemistry encompasses all the chemical reactions that take place as a consequence of the application of a stress to the polymeric material, related to the machinery used in the processing and transformation stage. Effects that occur:

  • Modification of the initial structure (the tension weakens the interaction forces of the chains, modifying the conformation of the chains and their relative disposition) breaking the covalent bonds;
  • Susceptible to ozone attack;
  • The link breaking zone seems to be more likely in the center of the chains than at the ends. Longer chains are more susceptible to breakage.

Mechanical degradation(8)

Chemical degradation[10]

Chemical degradation refers exclusively to processes, which are induced under the influence of chemical reagent (e.g. acids, bases, solvents reactive gases, etc.)

Radiolytic degradation [11]

When polymeric materials are subjected to high energy radiation (e.g. gamma radiation) changes are observed on their molecular structure, mainly chain scission, which leads to reduction in molar mass

Ozone degradation[12]

The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond

The immediate result is formation of an ozonide, which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on the chain molecular weight or degree of polymerization, the higher the chain length, the greater the mechanical strength (such as tensile strength). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes a circuit and the product separates or fails.

The carbonyl end groups which are formed are usually aldehydes or ketones, which can oxidise further to carboxylic acids

Common antiozonants include:

Flame degradation [13,14,15]

When polymeric materials are subjected to flames (e.g. burning process) changes are observed on their molecular structure, mainly chain scission, which leads to reduction in molar mass

How materials burn?

The fire triangle demonstrates that three factors must coincide in order for anything to burn

  • Fuel
  • Heat
  • Air (oxygen)

The fundamental parameters governing a fire are

Combustibility: will a material burn?

Ignitability: if it is combustible, how and when will it ignite?

Spread of flame: once ignited how quickly will the flames spread?

Heat release: what will be the rate and total amount of heat released?

Solid materials do not burn directly: they must first be decomposed by heat (pyrolysis) to release flammable gases.

Visible flames appear when these flammable gases burn with the oxygen (O2) in the air.

If solid materials do not break down into gases, then they will only smolder slowly and often self extinguish, particularly if they “char” and form a stable carbonaceous barrier.

The gas flame itself is maintained by the action of high energy radicals (that is H. and OH. in the gas phase) which decompose molecules to give free carbon which can react with oxygen in the air to burn to CO2, generating heat energy.

Common flame inhibitors include

  • Flame retardants

What are flame retardants

  • Materials or substances that inhibit or slow down the growth of a fire
  • The term “flame retardant” does not refer to a specific class of chemical
  • It describes the function of retarding a flame
  • They are incorporated in different materials to reduce the risk of fire either by:
  1. Providing increased resistance to ignition
  2. Acting to slow down combustion
  3. And thereby delay the spread of flames

Retardation mechanisms

The basic mechanisms of flame retardancy vary depending on the specific flame retardant and the substrate. Additive and reactive flame-retardant chemicals can both function in the vapor (gaseous) or condensed (solid) phase.

Endothermic degradation

Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various carbonates and hydrates such as mixtures of huntite and hydromagnesite. The reaction removes heat from the substrate, thereby cooling the material. The use of hydroxides and hydrates is limited by their relatively low decomposition temperature, which limits the maximum processing temperature of the polymers (typically used in polyolefins for wire and cable applications). 

 

Thermal shielding (solid phase)

A way to stop spreading of the flame over the material is to create a thermal insulation barrier between the burning and unburned parts:

Flame retardants can cause a layer of carbon (charring) on the polymer’s surface. This occurs, for example, through the dehydrating action of the flame retardant generating double bonds in the polymer. These processes form a carbonaceous layer via cyclizing and cross-linking processes cycle.

In intumescence, the amount of fuel produced is also greatly diminished and char rather than combustible gases is formed. The intumescent char, however, has a special active role in the process. It constitutes a two-way barrier, both for the hindering of the passage of the combustible gases and molten polymer to the flame as well as the shielding of the polymer from the heat of the flame. The  intumescent systems are based on the application of 3 basic ingredients:

  • a “catalyst” (acid source),
  • a charring agent and
  • a blowing agent (Spumific).

Additives combining the last three ingredients leading to intumescent effect are commercially available. However, intumescent formulations can simply be developed and are more suitable than some commercial grades for some specific applications. Table below summarize usual catalyst, charring and blowing agents.

Table 2: Intumescent  components

Dilution of gas phase

Inert gases (most often carbon dioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, and slowing the reaction rate.

Gas phase radical quenching

Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide or, if used in the presence of a synergist like antimony trioxide, antimony halides. These react with the highly reactive H· and OH· radicals in the flame, resulting in an inactive molecule and a Cl· or Br· radical. The halogen radical is much less reactive compared to H· or OH·, and therefore has much lower potential to propagate the radical oxidation reactions of combustion.

Synergism with Antimony trioxide (Sb2O3)

To be efficient the trapping free radicals needs to reach the flame in gas phase. Addition of antimony trioxide allows formation of volatile antimony species (antimony halides or antimonyoxyhalide) capable to interrupt the combustion process by inhibiting H* radicals via a serie of reactions proposed bellow. This phenomenon explains the synergistic effect between halogenated compounds and Sb2O3. For most applications, these two ingredients are present in the formulations.

Common flame retardants include

Flame retardants are activated by the presence of an ignition source and are intended to prevent or slow the further development of ignition by a variety of different physical and chemical methods.

Action of flame retardants
Most effective chemical action of flame retardants

  •  The reaction in the gas phase:
    …… where the flame retardant interrupts the radical gas phase combustion process resulting in a cooling of the system,  a reduction and suppression of the supply of flammable gases
  • The reaction in the condensed phase:
    …… where the flame retardant builds up a char layer, smothering the material and inhibiting the oxygen supply, thereby providing a barrier against the heat source or already ignited flame from another source
Less effective physical action of flame retardants, can take place by

  • Cooling: where the additive or chemically–induced release of water, cools the underlying substance to a temperature that is unable to sustain the burning process
  • Coating: where the substance is shielded with either a solid or gaseous layer, protecting it against the heat and oxygen required for combustion to take place
  • Dilution: Chemically inactive substances and additives turn into non-combustible gases which dilute the fuel in the solid and gaseous phases of the fire cycle

 

Tables

[1] Anexo B: Introducción a los polímeros

[2] http://fr.polymerinsights.com/home/mechanisims

[4] https://paxymer.se/introduction/

Imagenes:

[1)https://www.slideshare.net/ozagaurang/thermal-degradation-ppt-of-polymers

[2]https://www.slideshare.net/sirris_be/invisible-but-functional-uvprotecting-coatings

[3]https://www.slideshare.net/IEIGSC/presentation-on-photo-degradation-and-photo-stabilization-of-polymers

[5] https://slideplayer.com/slide/4641345/

[6] doi.org/10.1007/s11356-018-1234-9

[7] https://slideplayer.com/slide/4641345/

[9

] www.wikipedia.com

[10] https://www.firerescue1.com/fire-products/apparatus-accessories/articles/what-is-a-fire-triangle-4HSY7X5xagWZR5KQ/

[11] www.stahl.com

[12] https://www.sciencedirect.com/science/article/abs/pii/S0950423018304509

[13]: www.stahl.com

[14] www.stahl.com

[15] www.stahl.com

[16]www.stahl.com

[17] http://fr.polymerinsights.com/home/mechanisims)

[18] https://www.sciencedirect.com/science/article/abs/pii/S1359836818336151

[19]https://polymer-additives.specialchem.com/interests?previouspage=%2fselection-guide%2fflame-retardants-for-fire-proof-plastics

[20]Synergism with Antimony trioxide (Sb2O3): http://fr.polymerinsights.com/home/mechanisims)

References

[1] What is polymer degradation?

[2] https://www2.ulpgc.es/hege/almacen/archivos/file00008915

[3] Anexo B: Introducción a los polímeros

[4] https://www.slideshare.net/ozagaurang/thermal-degradation-ppt-of-polymers

[5]Photodegradation: https://www.slideshare.net/sirris_be/invisible-but-functional-uvprotecting-coatings

[7] https://slideplayer.com/slide/4641345/

[8]  doi.org/10.1007/s11356-018-1234-9

[12]www.wikipedia.com

[13] www.wikipedia.com

[14] www.stahl.com

[15] http://fr.polymerinsights.com/home/mechanisim

 

 

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