Polymer Degradation Flashcards
What types of polymer degradation exist and what inhibits or retards it?
Bond scission, chemical transformations, formations of new functional groups
Polymer stabilisation
What are biotic and abiotic degradation?
Biotic is through biodegradation - gut microbes, digestion, mineralisation etc
Abiotic is through physicochemical - photo and thermal, hydrolysis etc
What factors affect abiotic degradation rate?
Environmental - sun light, water, temperature, oxygen level
Polymer nature - crystallinity, chemical composition, molecular weight, type, additives, production method
What factors affect polymer stability?
Chemical structure:
- higher branching = higher rate of oxidation as there are more tertiary hydrogen atoms
- presence of double bonds which are more reactive so oxidation rate increases
- increased polymer hydrophobicity = increased degradation rate
Chemical composition:
- bond energies are different
- tertiary allylic bonds are weaker than primary so weakens the molecule as degradation starts with scission of weakest bond
Tacticity:
- more crystalline polymers are more resistant to oxidation
- syndiotactic PVC is more stable than ordinary PVC
Physical and morphological:
- oxidation is limited in amorphous phase of semi-crystalline polymers and propagates to crystalline regions which destroys the crystalline order
Internal mechanical stresses
Additives:
- many increase oxidation rate
- stabilisers are often used in plastic manufacturing to decrease degradation rate
Molecular weight:
- in solids, no significant effect
- in solution, degradation rate decreases with molecular weight
How do a range of environmental factors affect polymer degradation?
Sunlight - increased light intensity, increased photo-oxidation rate, increased degradation rate
Temperature - increased temp, increased abiotic degradation rate (reaction rate duplicates per 10C increase)
Humidity - chain scission massively increases at higher humidity
What are the types of polymer degradation?
Thermal
Thermo-oxidative
Photo
Photo-oxidative
Mechanical
Hydrologic
Chemical
Irradiation
Pyrolysis
Biological
How does thermal degradation occur?
At high temps, components of the long chain backbone of polymer can separate (molecular scission) and react with one another to change the properties of the polymer
Usually affects molecular weight and typically causes reduced ductility, chalking, colour changing, cracking etc
What accelerates thermo-oxidative degradation and what’s used to reduce it?
Accelerated by stress and exposure to reactive compounds
Antioxidants are used to prevent thermal oxidation at low temps
How does photo-oxidative degradation occur?
Photons are absorbed which excite the electrons in polymers
Excited electrons react with oxygen molecules in environment, passing energy to them
Highly reactive singlet oxygen can initiate chemical reactions in polymer structure forming free radicals
Radicals react with polymer chains causing scission of the polymer backbone, resulting in shorter polymer chains
Physical changes occur such as embrittlement, cracking and loss of strength
How does hydrolysis degradation occur?
Hydrolysis reaction rate is affected by molecules mobility, susceptibility of polymeric chemical bonds to water attack, and hydrophobicity or philicity.
How does photo-thermal degradation occur?
Chemical bonds in main polymer chain are broken by light or heat to produce free radicals (formed when UV light breaks C-H bonds on the polymer backbone)
For this to happen, the polymers must contain unsaturated chromophoric groups that absorb light energy
PE and PP don’t contain unsaturated double bonds so might be expected to be immune to photo-initiated degradation, however small amounts of external impurities in the structure can allow for some photo-initiated degradation
To propagate, free radicals react with oxygen and forms a peroxy radical - leads to chain scission or crosslinking
Autoxidation is brought about by reactions with oxygen at normal temperatures, without intervention of flame or electric spark
What terminates photo degradation?
Termination of the radical reaction occurs when inert products are formed by combining two radicals
These products are more susceptible to photo initiated degradation as they have unsaturated double bonds
Molecular weight is reduced so material gets brittle and is more susceptible to fragmentation which increases surface area available for reactions
How are plastics with a C-C backbone degradated?
High molecular weight so must be broken down by abiotic degradation to smaller pieces before biotic degradation takes place
In biotic degradation, polymer is converted to monomers and these are mineralised.
How is PE degradated?
Abiotic oxidation by exposure to UV is initiation
During propagation, autoxidation by complex radical reactions takes place leading to the formation of oxygenated low molecular weight fragments
Random chain scission and cross linking affect molecular weight, material becomes brittle and subject to fragmentation
How is PP degraded?
PP has a lower stability than PE because every other carbon atom in the backbone is tertiary which are more prone to abiotic attack compared to PE which are all secondary carbons
Reaction mechanisms similar to PE
How is PS degraded?
Susceptible to outdoor weathering
Polymer is irradiated with UV light which excited the nearest C-H bond, and forms a radical, causing cross-linking and chain scission
How is PVC degraded?
PVC is least stable of the popular polymers as it is highly sensitive to UV radiation so photo degradation usually occurs
As PVC monomers only have saturated chemical bonds, impurities are required for photo initiation by UV.
When exposed to UV, de-chlorination is first step which forms conjugated double bonds in a polyene polymer and hydrocholoric acid
Rate of decolorisation is increased under aerobic conditions, in presence of HCl and for low molecular weight polymers
Humidity, mechanical stress, other chemicals and high temps enhance the rate of degradation
Formed unsaturated C-C double bonds are less stable to photo degradation so backbone breaks into smaller fragments
How is PET degraded?
Either by photo or hydrologic degradation.
Radical reactions form a hydro peroxide. Chain scission then happens.
How is PU degraded?
PU have very large and complex chains with carbon oxygen and nitrogen.
Ester bond in backbone is more susceptible to degradation.
Either by photo, hydrolysis or biodegradation
Radicals leads to hydroperoxides
How does biodegradation happen in polymers?
Organic materials can be degraded aerobically (with oxygen) or anaerobically (without oxygen)
Breakdown of large polymers to carbon dioxide (mineralisation) requires several organisms:
- one breaks down polymer into monomers
- one that uses monomers and excretes simpler waste products
- one that uses exceeded wastes
What is biomineralisation?
Organic matter is converted into minerals
How do you make PE biodegradable?
Modify its crystalline level, molecular weight and mechanical properties that are responsible for PE resistance to degradation
Can be achieved by improving PE hydrophilic level and reducing its polymer chain length by oxidation to be accessible for microbial degradation
Additives starch and prooxidant are used in making biodegradable PE
Starch blend PE has a continuous starch phase that makes the material hydrophilic and is catalysed by amylase enzymes
Microorganisms can easy access attack and remove this part
Prooxidant blend PE can be photo or chemically degraded
Discuss 4 types of polymer degradation and how to identify them.
Oxidation Degradation:
- polymer reacts with oxygen in presence of heat light or catalysts. Oxygen diffuses in and attacks polymer chains, forming free radicals and chain scission
Thermal Degradation:
- polymer chains breakdown at high temps, can lead to cleavage bonds, molecular weight reduction and generate volatile degradation products
Photo Degradation:
- polymers exposed to UV light can cause bond breakage and generation of free radicals
Hydrolysis:
- water molecules make hydrolysis reactions possible which oxidase the surface of the metal
Describe the mechanisms of thermal degradation on PVC?
1) under thermal stress, PVC can undergo dehydrochlorination where HCl is eliminated from polymer chains which forms double bonds
2) can involve chain scission as polymer chains undergo cleavage which shortens the chains which reduces molecular weight and mechanical strength
3) volatile degradation products are formed such as vinyl chloride monomers, HCl, C02 - can affect physical properties, colour and odour of the degraded PVC
How do bond energy, crystallinity, hydrophilic/phobic, double bonds etc affect stability against polymer degradation?
Bond energy - stronger covalent bonds tend to be more stable - more energy to break them
Crystallinity - crystalline have high order and closely packed which hinder diffusion of oxygen, water etc., amorphous lack order and are more mobile with weaker bonds making them more susceptible to diffusion and thermal degradation
Hydrophobic nature - phobic repel water so better hydrolytic stability
Double bonds - more susceptible to oxidation and photo degradation as they have unsaturated sites which are open to free radical attacks
