Polymer Degradation, Chemistry tutorial

Introduction 

Polymer degradation includes all the actions which when happened on a polymer will decrease the quality of the polymer, practically, the ageing of rubber, the deterioration of meat, the rotting and burning of wood are instances of degradation of polymers that have continually challenged man's control over atmosphere. Chemically, degradation is a reaction involving rupture of chemical bonds in the major chain of the macromolecules. Depending on the kind of chemical bond (covalent or ionic) three mechanisms of polymer degradation are possible, namely; radical, ionic and iono-radical. Deterioration in the physical property of a polymer due to exposure to sun and rain is recognized as weathering.

Polymer degradation

Polymer degradation is a transform in the properties - tensile strength, colour, shape, and so on., of a polymer or polymer-based product under the influence of one or more environmental factors these as heat, light or chemicals such as acids, alkalis and several salts. These changes are generally undesirable, such as cracking and chemical disintegration of products or, more rarely, desirable, as in biodegradation, or deliberately lowering the molecular weight of a polymer for recycling. The changes in properties are frequently termed 'aging'. In an ended product these change is to be avoided or delayed. Degradation can be helpful for recycling/reusing the polymer waste to prevent or decrease ecological pollution. Degradation can as well be induced deliberately to assist structure determination. Polymeric molecules are extremely huge (on the molecular scale), and their unique and helpful properties are mainly a consequence of their size. Any loss in chain length lowers tensile strength and is a primary cause of premature cracking.

Nowadays there are primarily seven commodity polymers in use: polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate or PET, polystyrene, polycarbonate, and poly(methyl methacrylate) (Plexiglass). Such make up nearly 98% of all polymers and plastics encountered in daily life. Each of these polymers has its own characteristic modes of degradation and resistances to heat, light and chemicals.

Polyethylene, polypropylene, and poly (methyl methacrylate) are sensitive to oxidation and UV radiation, while PVC may discolour at high temperatures due to loss of hydrogen chloride gas, and become very brittle. PET is sensitive to hydrolysis and attack by strong acids, while polycarbonate depolymerizes rapidly when exposed to strong alkalis.

Degradation therefore, can be classified into physical, chemical and biological depending on the agent causing the bond rupture in the chain. Physical degradation falls into thermal (heat), mechanical, photolytic (light), ionizing - radiation and fire (flame) degradation. Chemical degradation happens under the action of diverse chemical agents. The most significant kinds of chemical degradation are oxidative, hydrolysis, acidolysis, and alcoholysis. Biological degradation is caused via enzymes produced through animals that attack the molecules and degrade them.

Ionizing-Radiation Degradation

Many polymers are vulnerable to degradation and cross bonding upon exposure to elevated energy ionization radiation these as X-rays, gamma rays, and electromagnetic waves through the energy of visible light to provide lower molecular weight molecules. Sometimes, such results can be utilized to a benefit. Several polymers as polystyrene and polysulfone are extremely radiation resistant, but others like polypropylene will readily degrade upon irradiation. The primary event in radiation damage is the ejection of a high - energy electron.

                                              R 749_aro.jpg R+ + e

Photolytic Degradation

This is the kind of degradation that happens under the action of light. The degree of photochemical degradation based on the wavelength of the ultraviolet light (quantum energy), the intensity of radiation, the conditions of the experiment and the structure of the polymer.

Thermal Degradation

Thermal degradation is the diverse chemical and physical changes that occur when polymers are heated. In common vinyl polymers are particularly susceptible to thermal degradation that can happen either in chain scission, (route A) involving the breakage of the backbone bonds to yield free radical segments or via non-chain scission, (route B) involving the removal of a small molecule from a substituent group and subsequent double bond formation.

Instance: For a vinyl polymer, we have such routes-

                                   H

                                   1498_vertical.jpg

                       ~CH2 - C? + ?CH2 - CH - CH2~        Random chain scission mechanism.

                                  1498_vertical.jpg                   1498_vertical.jpg

                                  R                    R

                                                A

 

                            ~CH2 - CH - CH2 - CH - CH2~

                                        1498_vertical.jpg                1498_vertical.jpg

                                       R                  R

                                                 B                                        

                            ~CH2 - CH - CH2 - CH = CH~ + RH      Non-chain scission mechanism.

                                         1498_vertical.jpg

                                         R

Hydrolytic Degradation

Step-growth polymers as polyesters, polyamides and polycarbonates can be degraded mainly by hydrolysis to give lower molecular weight molecules. The hydrolysis takes place in the presence of water enclosing an acid or a base as catalyst. Polyamide is sensitive to degradation by acids and polyamide mouldings will crack when attacked by strong acids. For instance, the fracture surface of a fuel connector showed the progressive growth of the crack from acid attack to the final cusp of polymer. The problem is recognized as stress corrosion cracking, and in this case was caused via hydrolysis of the polymer. 

Oxidative Degradation

This is degradation through atmospheric oxygen and other oxidants. The resistance of a polymer to oxidizing agent based on its structure and primarily on the presence of willingly oxidizable groups and bonds in the macromolecule. That is, oxidizable carbon-carbon molecular compounds and unsaturated hydrocarbons, for example, natural and butadiene rubbers. Light and heating intensify oxidative degradation. The presence of double bonds in the major chains is more active during oxidative degradation than double bonds in side chain vinyl groups. Ozone is a energetic oxidizing agent. In the light, ozone degrades natural rubber strongly. Carbon-carbon saturated polymer compounds (polyethylene, polystyrene) are more resistant to oxidation. Polymers exposed to ozone are found to crack on stretching. 

             Instance of ozone oxidation:

301_Ozonoide.jpg

Galvanic action

Polymer degradation via galvanic action was 1st explained in the technical literature in the year 1990. This was the discovery that "plastics can corrode", for instance polymer degradation might occur through galvanic action similar to that of metals under assured conditions. Usually, when two dissimilar metals such as copper (Cu) and iron (Fe) are put into contact and then immersed in salt water, the iron will undergo corrosion, or rust. This is said a galvanic circuit where the copper is the noble metal and the iron is the active metal, for instance, the copper is the cathode or positive (+) electrode and the iron is the anode, or negative (-) electrode. A battery is formed. It follows that plastics are made stronger via impregnating them through thin carbon fibers only a few micrometers in diameter recognized as carbon fiber reinforced polymers (CFRP). This is to produce substances that are high strength and resistant to elevated temperatures. The carbon fibers act as a noble metal similar to gold (Au) or platinum (Pt). When put into contact through a more active metal, for instance by aluminum (Al) in salt water the aluminum corrodes. 

Though in early 1990, it was reported that imide-linked resins in CFRP composites degrade when bare composite is coupled by means of an active metal in salt water environments. This is since corrosion not only occurs at the aluminum anode, but also at the carbon fiber cathode in the form of a very strong base through a pH of about 13. This strong base reacts with the polymer chain structure degrading the polymer. Polymers influenced comprise bismaleimides (BMI), condensation polyimides, triazines, and blends thereof. Degradation happens in the form of dissolved resin and loose fibers. The hydroxyl ions produced at the graphite cathode attack the O-C-N bond in the polyimide structure. Standard corrosion protection procedures were established to stop polymer degradation under most situations.   

Table: Effect of Environmental Agents on Polymers

358_Effect of Environmental Agents on Polymers.jpg

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