Degradation and cracking materials of plastic polymers

Degradation of plastic polymers/plastic cracking and plastic decomposable materials

Due to the complex external factors encountered in the actual industrial production and use of polymer materials, the forms of polymer cracking are various, such as thermal cracking, mechanical cracking, photolysis, radiation cracking, oxidative cracking, biological cracking , chemical cracking, etc. Different types of cleavage can occur simultaneously. In the production, processing and use of polymer materials in the natural environment, the air will inevitably come into contact, and the problem of oxidative cracking is the most common problem. How to control these factors to prolong the structural strength and service life of the product has always been an important task in dealing with the polymer process and the use of the product. In recent years, with the diversified development and research of synthetic materials, especially the rapid development of the plastics industry, the application and development of plastic polymer materials has been advancing in two distinct directions. Cleavage, the other is to accelerate the degradation and cracking of polymer materials under certain conditions.

The way of strengthening and strengthening to avoid degradation and cracking is the research and development of the molecular chain of the material to strengthen its performance and efficiency. Longevity may enhance the physical properties of polymer materials to delay degradation or cracking time,

Second, in order to solve the environmental pollution problem of solid waste, as most countries in the world have issued various levels of plastic restriction orders, the research and development of degradable materials/decomposable materials has become an important direction for the long-term sustainable development of the polymer materials industry. Nowadays, people have developed biodegradable, photodegradable, thermally degradable and chemically degradable polymer materials.

Under the influence of external environmental factors such as heat, force, oxygen, water, light and radiation, or in the raw material manufacturing process, injection molding process and subsequent processing and assembly processes, polymer materials/polymers are often in the relevant environment. The chemical change process of degradation and cracking occurs. The possible cause of degradation or poor solution is the chain scission, cross-linking reaction, structure or side group change of the polymer chain in the plastic polymer, or the comprehensive reaction between them.

Degradation of polymers generally refers to the degradation of polymers under the action of physical factors (such as heat, ultraviolet rays, high-energy radiation, mechanical force) and chemical factors (such as oxygen, ozone, corrosive media and chemical drugs). Denaturation, which involves the breaking of molecular chains.

Chain scission type: Random chain scission and Chain depolymerization

Random chain scission

Random chain scission means that the scission occurs at any weak point along the polymer backbone, and the average degree of polymerization of the reaction product is lower than that of the original sample. The cracking under the action of chemical factors is mostly random chain scission, such as the ozonolysis of various unsaturated rubbers.

Chain depolymerization

Chain depolymerization can be regarded as the reverse process of chain polymerization, which means that once a certain part or both ends of the molecular chain is broken, the monomer will be continuously released in the manner of negative growth reaction. Usually, the cracking that occurs under the influence of physical factors is often a chain depolymerization, such as the thermal depolymerization of polymethyl methacrylate.

Whether these two cleavage occur independently or simultaneously depends mainly on the structure, properties and cleavage conditions of the polymer chain.

At present, the terms polymer degradation and polymer cracking are often used to describe the process of polymer materials gradually losing their inherent properties under the action of various external factors. This process includes both chain scission and cross-linking reactions. Variations, such as the distribution of average molecular weights and their molecular sizes, structure formation processes of gels and entanglements, side chain branching and cyclization of molecular structures, etc. Oxidative crystallization of high molecular polymers, and side chain scission, etc.

Degradation type of plastic polymer

Thermal degradation, thermal cracking

The degradation reaction caused by high temperature heating for too long during molding is called thermal degradation. This is a free radical chain depolymerization reaction, the speed of which increases with the increase of temperature. First, some chemical molecular chains on the main chain of macromolecules are broken and the initial free radicals grow, and then an active center and chain are generated according to different degrees. Reactions such as knot transfer, shortening of main or side chains, and molecular chain termination lead to the formation of different degradation products. When processing plastics, it is easy to cause polymer plastics to degrade accidentally, especially when using hot runner molds and product types that require high temperature production. In the thermal degradation process, the weakest chemical bond in the molecule will start first, and the reaction rate will increase as the temperature continues to increase. Therefore, in the process of extrusion of raw materials, pretreatment of molding raw materials, and molding processes, thermal degradation or even thermal cracking may occur to a certain extent.

Force degradation, force cracking

In the raw material process, due to high-pressure mixing and extrusion, and in the molding process, the shear stress and high-strength molecular tensile stress due to crushing, high-speed stirring, extrusion, and injection will cause the molecular chain to break. The phenomenon that causes the relative molecular weight to decrease is called force degradation and force cracking. It is often accompanied by the release of heat. If the heat energy and power energy cannot be released in time, the accumulated heat may cause thermal degradation and thermal cracking. According to the relevant experimental results, the higher the relative molecular weight, the higher the applied stress will aggravate the force degradation; and with the increase of the molding temperature or the addition of plasticizers, the degree of force degradation can be weakened by comparison.

Oxidative degradation, oxidative cracking

At room temperature, most plastic polymers/polymers can react slowly with oxygen. Some weaker parts of chemical molecular chains often produce extremely unstable peroxidation structures, which are easy to decompose free radicals, resulting in depolymerization, which is called oxidative degradation. During the molding process, it rapidly intensifies with the action of heat, which is called thermal oxidative degradation in production. This thermal oxidation degradation rate is different due to the structure of the polymer. For example, the unsaturated carbon chain polymer is easily oxidized due to the double chain on the main chain, and its thermal oxidation rate is much faster than that of the saturated carbon chain polymer. . In addition, the thermal oxidation rate is also related to the oxygen content of the environment, the heating temperature and the heating time. The enhancement of each of the above factors will accelerate the degree of oxidative degradation. Therefore, the temperature and time must be strictly controlled during molding to avoid oxidative degradation and cracking of materials due to overheating.

Water degradation, water splitting, hydrolysis

When the molecular structure of the polymer contains chemical groups that are easily hydrolyzed, such as amide groups (—CO—NH—), ester groups (—CO—C—), nitrile groups (—C—N—), ether groups (-C-O-C-), etc., or containing groups that can be hydrolyzed after oxidation, will be degraded by water. If these groups are on the main chain of the polymer, the performance of the hydrolyzed polymer will not be as good as before; if these groups are on the branches of the molecular chain, the effect of degradation will be less. When the polymer material is melted by thermal energy, if water molecules containing hydrogen and oxygen are added, the molecular chain of the plastic polymer can easily reach the conditions of hydrolysis in this environment, thereby causing water degradation and water splitting. In order to avoid the influence of hydrolysis during molding, a completely dry pretreatment must be taken, which is more important for materials with strong hygroscopicity and strong polarity (such as polyester, polyether and polyamide, etc.).

In product development, the selection of materials is a major focus. In the selection of materials and the development process, Yeh Her Yow Plastic Co., Ltd. (YHY) can assist in the development and production. Appearance, use environment, product strength requirements, etc., provide you with the most suitable material selection.