Understanding Polymer Degradation and Failure Mechanisms #
Polymer materials, especially plastics, are widely used in industrial production and daily life. However, their performance and longevity are often challenged by a range of external factors, leading to various forms of degradation and cracking. This article explores the mechanisms behind polymer degradation, the types of cracking that can occur, and considerations for material selection to enhance product durability and sustainability.
Overview of Polymer Degradation #
In real-world applications, polymer materials are exposed to complex environments. As a result, they may undergo different forms of cracking and degradation, such as:
- Thermal cracking
- Mechanical cracking
- Photolysis
- Radiation-induced cracking
- Oxidative cracking
- Biological degradation
- Chemical cracking
These processes can occur independently or simultaneously, often influenced by factors like heat, mechanical stress, light, oxygen, water, and radiation. Among these, oxidative cracking is particularly common due to inevitable contact with air during production and use.
Controlling these factors is crucial for prolonging the structural integrity and service life of polymer products. The plastics industry has responded by developing materials in two main directions: enhancing molecular structures to resist degradation, and creating polymers that degrade more readily under specific conditions to address environmental concerns.
Approaches to Polymer Longevity and Sustainability #
-
Strengthening Molecular Chains: By modifying the molecular structure, polymers can be made more resistant to degradation, thereby extending their useful life and improving physical properties.
-
Developing Degradable Materials: In response to global efforts to reduce plastic waste, research has focused on biodegradable, photodegradable, thermally degradable, and chemically degradable polymers. These materials are designed to break down more easily under environmental conditions, supporting sustainable development.
Mechanisms of Polymer Degradation #
Degradation in polymers typically involves chemical changes such as chain scission, cross-linking, or alterations to side groups. These changes can be triggered by:
- Physical factors: Heat, ultraviolet light, high-energy radiation, mechanical force
- Chemical factors: Oxygen, ozone, corrosive substances, chemical agents
The result is often a reduction in molecular weight and a loss of the material’s inherent properties.
Types of Chain Scission #
-
Random Chain Scission: Breakage occurs at random weak points along the polymer backbone, reducing the average degree of polymerization. This is common in chemical degradation, such as the ozonolysis of unsaturated rubbers.
-
Chain Depolymerization: This is the reverse of polymerization, where breakage at specific points or chain ends leads to the continuous release of monomers. Physical factors like heat often cause this type of degradation, as seen in the thermal depolymerization of polymethyl methacrylate.
Whether these processes occur independently or together depends on the polymer’s structure and the conditions it is exposed to.
Types of Degradation in Plastic Polymers #
Thermal Degradation (Thermal Cracking) #
Prolonged exposure to high temperatures during molding can cause thermal degradation, a free radical chain depolymerization reaction that accelerates with increasing temperature. The weakest chemical bonds break first, leading to a cascade of reactions that shorten molecular chains and produce various degradation products. This is a particular concern in processes involving hot runner molds or high-temperature production.
Mechanical Degradation (Force Cracking) #
High-pressure mixing, extrusion, and mechanical stresses during processing can break polymer chains, reducing molecular weight. This process, known as force degradation, is often accompanied by heat release. If this heat is not dissipated, it can further accelerate thermal degradation. Higher molecular weight polymers are more susceptible to force degradation under stress, but the effect can be mitigated by increasing temperature or adding plasticizers.
Oxidative Degradation (Oxidative Cracking) #
At room temperature, most polymers react slowly with oxygen, forming unstable peroxidation structures that decompose into free radicals and cause depolymerization. During molding, heat accelerates this process, known as thermal oxidative degradation. The rate of oxidation depends on the polymer’s structure (e.g., unsaturated carbon chains oxidize faster than saturated ones), as well as environmental oxygen content, temperature, and exposure time. Strict control of these factors during processing is essential to prevent oxidative damage.
Hydrolytic Degradation (Water Splitting) #
Polymers containing hydrolyzable groups (such as amide, ester, nitrile, or ether groups) are susceptible to degradation by water, especially if these groups are part of the main chain. Hydrolysis can significantly impair material performance. To prevent this, thorough drying of raw materials is necessary, particularly for hygroscopic and polar polymers like polyester, polyether, and polyamide.
Material Selection and Product Development #
Selecting the right material is a critical step in product development. Factors such as appearance, operating environment, and strength requirements must be considered to ensure optimal performance and longevity. Yeh Her Yow Plastic Co., Ltd. (YHY) offers expertise in material selection and development, assisting clients in choosing the most suitable polymers for their specific applications.