Plastic modification methods of polymer materials

In recent years, with the rapid development of my country's automobile, home appliance, communication and electronic product industries, it is difficult to meet the application requirements for plastics with a single variety or nature. Therefore, in order to meet the needs of different uses, chemical or physical methods are used to change some properties of plastic products to achieve the intended purpose, which is plastic modification.

Plastic modification is generally divided into chemical modification and physical modification. Plastic modification technology is much easier than synthesizing a new resin, especially physical modification, which can be carried out in general plastic molding processing factories, which is simple and easy to implement and has fast results.

Plastic chemical modification technology and method 

The chemical modification of polymer materials is generally carried out in the synthesis stage, especially in coatings, adhesives, thermosetting resins and other industries. Because these types of materials mostly involve cross-linking and curing in the application process, different expected effects and material properties can be achieved by introducing functional groups with different reactivity or structural characteristics. For plastics, chemical modification is more common in resin synthesis factories, especially in the synthesis and production of special resins. Of course, some plastic molding processing factories such as cable factories will also carry out chemical modification.

Graft modification

Grafting modification refers to a modification method that achieves the required properties by chemically bonding appropriate branched chains or functional side groups on the polymer macromolecular chain. The graft copolymerization reaction must first form active graft points, and various polymerization initiators or catalysts can provide active species for graft copolymerization, and then generate graft points. The active point is in the middle of the polymer chain segment, and the graft copolymer is formed after polymerization. 

The performance of graft copolymers depends on the composition, structure, length and number of branches of the main chain and branch chain. The long-chain branched graft is similar to a blend. Through copolymerization, two polymers with different properties can be grafted together to form a graft with special properties, ABS resin. Grafts with short and many branches are mostly used as compatibilizers, such as MAH and GMA grafts. Therefore, the graft modification of polymers has become a simple and effective method to expand the application fields of polymers and improve the properties of polymer materials.

 

Block Copolymerization Modification

Block copolymerization modification is a special polymer prepared by linking two or more polymer segments with different properties. According to the number of segments that make up the block copolymer, it can be divided into: Diblock copolymers, such as styrene-butadiene copolymers; Triblock copolymers, such as styrene-butadiene-3-chloropropene copolymer; Multi-block copolymers, etc.

According to whether the alternating polymerization of various segments is regular or not, it can be divided into regular block copolymers and random block copolymers. This kind of polymer has controllable molecular weight, narrow molecular weight distribution, and designable molecular structure and composition. It is one of the most meaningful and challenging research tasks in the field of polymer research. Block polymers with specific structures exhibit different properties from simple linear polymers, as well as many random copolymers and even mixtures of homopolymers. It can be used as thermoplastic elastomer, blending compatibilizer, interface modifier, etc. Widely used in various fields such as biomedicine, construction, and chemical industry.

Since the block copolymer has the original characteristics of each polymer that composes it, such as glass transition temperature, transparency, chemical resistance, hydrophilicity, hydrophobicity, etc., products for different purposes can be obtained.

Industrially produced block copolymers include the following: Thermoplastic elastomers have both rubber properties and thermoplastic properties; The molded products of SBS block copolymer, SB star block copolymer and polyether polyester multi-block copolymer can be used in the automobile industry and as shoe soles, adhesives, coatings, sealing materials, hot melt adhesives, etc.; Polyester polyurethane multi-segment copolymer can be used as elastic fiber; Styrene-butadiene star block copolymers can be used as toughened plastics; Propylene oxide-ethylene oxide two-stage copolymer can be used as a surfactant, which can emulsify aqueous or non-aqueous solutions according to the ratio of hydrophobic and hydrophilic segments.

Cross-linking modification 

Cross-linking modification refers to the process of connecting linear or branched polymer chains with covalent bonds to form a network or body-shaped polymer. It usually refers to chemical cross-linking, which is generally achieved through polycondensation and polyaddition reactions. The vulcanization of rubber is the most typical case. After the linear polymer is moderately cross-linked, its mechanical strength, elasticity, dimensional stability, and solvent resistance are all improved.

Crosslinking is also commonly used in the modification of plastics. For example, polyethylene (PE) cross-linking technology is one of the important means to improve its material properties. The cross-linked modified PE can greatly improve its performance, not only significantly improve the mechanical properties of butyl PE, environmental stress cracking resistance, chemical corrosion resistance, creep resistance and electrical properties, etc., Moreover, the temperature-resistant grade is significantly improved, which can increase the heat-resistant temperature of PE from 70°C to above 100°C, thus greatly expanding the application field of PE. At present, cross-linked polyethylene has been widely used in pipes, films, cable materials and foam products. The cross-linking methods of polyethylene mainly include radiation cross-linking, silane cross-linking and peroxide cross-linking. 

Radiation crosslinking is to irradiate polyethylene products, such as polyethylene sheaths, films, and thin-walled tubes coated on wires, with gamma rays and high-energy rays for crosslinking. By controlling the radiation conditions, cross-linked polyethylene products with a certain degree of cross-linking can be obtained. Cross-linked polyethylene produced by radiation cross-linking has the following advantages: Cross-linking and extrusion are carried out separately, the product quality is easy to control, the production efficiency is high, and the scrap rate is low; No additional free radical initiators (such as peroxides, etc.) are required during the crosslinking process, which keeps the cleanliness of the material and improves the electrical properties of the material; It is especially suitable for small-section and thin-wall insulated cables that are difficult to produce by chemical cross-linking. However, radiation crosslinking also has some disadvantages, such as the need to increase the accelerating voltage of the electron beam when crosslinking thick materials; For the crosslinking of circular objects like wires and cables, it is necessary to rotate them or use several beams of electron beams to make the irradiation uniform; the one-time investment cost is considerable; the operation and maintenance technology is complex. This method requires a large investment in equipment and better protective facilities, and is most suitable for preparing thin cross-linked products. 

Silane crosslinking is the use of vinyl silane containing double chains to react with molten polymers under the action of initiators to form silane grafted polymers. In the presence of a silanol condensation catalyst, the polymer is hydrolyzed when it encounters water, thereby forming a networked oxane chain cross-linked structure. Silane cross-linking technology has greatly promoted the production and application of cross-linked polyethylene due to its simple cross-linking equipment, easy-to-control process, low investment, high cross-linking degree and good quality of finished products. In addition to polyethylene and silane, catalysts, initiators, antioxidants, etc. are also required for crosslinking. Compared with other methods, polyethylene products obtained by silane crosslinking method have the following advantages: Less equipment investment, high production efficiency and low cost; The process is highly versatile, suitable for all densities of polyethylene, and also suitable for most polyethylene with fillers; not limited by thickness. 

Peroxide crosslinking generally uses organic peroxides as crosslinking agents, which decompose under the action of heat to generate active free radicals. These free radicals generate active points on the polymer carbon chain, and generate carbon-carbon crosslinking to form a network structure. This technology requires high-pressure extrusion equipment to allow the cross-linking reaction to proceed in the barrel, and then uses a rapid heating method to heat the product to produce a cross-linked product.

 

Plastic physical modification technology and method 

Filling, flame retardant, blending, fiber reinforcement, etc. are commonly used plastic physical modification technologies and methods.

Category	Subdivision	Consumers	Applications
Flame retardant resins	Flame retardant HIPS resin, flame retardant PP resin, flame retardant ABS resin, etc.	Home appliances, lighting factories and other manufacturing enterprises	Housings, internal parts, peripheral equipment, etc. of various products
Reinforced toughening resins	Weather-resistant toughened PP special material	Automobile parts and other manufacturing enterprises	Home appliances and automotive interior parts
	Glass fiber reinforced thermoplastics	Computer accessories, mechanical parts, power tools, lamps and other manufacturing enterprises	Computer accessories, mechanical parts, power tools and lamp parts
Plastic alloys	PC alloy, PVC alloy	Electrician, computer, auto parts factory and other manufacturing enterprises	Automotive dashboards, automatic equipment, home appliance housings, building materials, etc
	PA alloy, PET alloy	Automobiles, home appliances, power tools and other enterprises	Auto parts, home appliance parts, power tool parts, etc
Functional masterbatches	HIPS toughened flame retardant masterbatch	TV, audio and other manufacturing enterprises	Electrical housing

The purpose of plastic modification is to:

Improve the comprehensive performance of plastics;

Improve the mechanical properties of plastics, such as strength, low temperature toughness, etc.;

Improve the heat resistance of plastics;

Improve processing performance;

Reduce absorbency and improve product dimensional stability;

Improve the flame resistance of plastics;

Realize the functionalization of materials and improve their performance;

Reduce costs and more.

 

Plastic Blend Modification 

Plastic blends generally refer to blends of plastics and plastics, and blends of elastomers such as rubber in plastics. Blends of plastics and plastics are commonly referred to as alloys, while blends of plastics and a small amount of elastomers such as rubber are commonly referred to as toughened plastics. Typical plastic modified blends such as PC/ABS alloys, super-tough nylon. 

Although plastic blending modification belongs to physical modification, in fact, in plastic blends, there are inevitably a small amount of chemical bonds between different polymer macromolecular chains. For example, in the process of melt mixing under the action of strong shear, macromolecular free radicals may be generated due to the shearing action, resulting in the formation of a small amount of block or graft copolymers.

In the process of plastic blending modification, especially incompatible blending systems, adding compatibilizers or reaction compatibilization measures to improve the interfacial bonding of blending components will also generate a certain degree of chemical crosslinking.

 

Plastic/"Filler" Fill Modifications

Plastic filling modification refers to a modification method in which various fillers are added to the plastic carrier to obtain certain expected properties. This method is the earliest modification method. It has obvious modification effect, simple process and low cost, so it is widely used.

Filling modification is not limited to reducing costs by adding various cheap inorganic powders such as calcium carbonate and talcum powder or organic powders such as wood powder. Moreover, special functions such as flame retardancy, electricity, light, magnetism, heat conduction, and antibacterial can be improved by adding metal powder, metal oxide, inorganic phosphorus, organic halide, organic phosphide, organic silicon, and nitride. 

The "filler" here also includes various plastic additives, such as organic carboxylic acids, sorbitol and other nucleating agents to improve the rigidity and transparency of the material; Antistatic agents to improve the antistatic performance of plastics; plasticizers, thermal stabilizers, lubricants and processing aids to improve the processing performance of materials; graphite, MoS2, etc. to improve the wear resistance of plastics, etc.

 

Plastic/Fiber (Whisker) Reinforced Modification

Reinforced modified plastics are composed of two parts: plastic carrier and reinforcement material. The reinforcement material mainly used in engineering is glass fiber and its products. In addition, there are carbon fibers, carbon nanotubes, metal fibers, ceramic fibers, whiskers and so on. These reinforcing materials can significantly improve the rigidity and hardness of plastics, and significantly improve the dimensional stability and heat resistance of materials. For reinforced plastics, the interface bonding between the reinforcing material and the plastic carrier, the dispersion of the reinforcing material in the plastic, the processing temperature, the diameter of the fiber, the type of the reinforcing material, etc. will all affect the final properties of the material. 

At present, a large number of reinforced plastic parts have replaced the original metal in automobiles and home appliances. In terms of electronic products, ABS/carbon fiber, PA/carbon fiber are used in the casings of various notebooks and cameras on the market, and the screen back covers of mobile phones. In terms of transportation, high-speed trains, manned spacecraft, military aerospace, etc., reinforced plastics have also become "darlings", and we can all find them.

 

Multi-phase composite modification of plastics

In actual production, modified plastics are more of a multi-phase composite system, such as enhanced flame retardant systems, enhanced toughening systems, toughened weather-resistant systems, etc., and are often the comprehensive application of several modification technologies. But no matter which kind of modification technology is used, the interface bonding between blending components is the key point, and the interface modifier plays a vital role. The distribution and morphology of the dispersed phase in the continuous phase also have a huge impact on the properties of the final material. 

In addition, the modification of plastic products, such as surface modification, also belongs to the technical means of plastic modification. Plastic surface modification refers to a type of modification method that changes the surface properties of plastic products by physical or chemical methods. Plastic surface modification differs from other modifications in two ways: One is that its modification is limited to the surface of the product, and its internal properties do not change; The second is that its modification is carried out after the primary molding process of plastic products, which belongs to the secondary processing modification. 

Plastic surface modification methods include improving surface gloss, surface hardness, surface wear resistance and friction, surface barrier; improving plastic adhesion, printing, etc. to improve the surface tension of plastics, etc. Taking plastic electroplating as an example, only the abs coating fastness can meet the requirements for plastic varieties without surface treatment. Especially for polyolefin plastics, the coating fastness is very low, and surface modification must be carried out to improve the bonding fastness with the coating before electroplating can be performed. Corona, plasma surface activation, etc. are commonly used methods for plastic surface modification.

 

The realization of high performance of modified plastics is not only closely related to the composition of materials, but also inseparable from advanced mechanical processing equipment and technology. The high-speed mixers, parallel twin-screw extruders, and injection molding machines commonly used in my country's modified plastics have made great progress in terms of function and continuous operating life. At the same time, there is also fierce competition for low prices and low profits. Nowadays, my country is developing in the direction of energy-saving society, environment-friendly society, sustainable development strategy, scientific development concept, energy saving and emission reduction, and innovative country. Plastic processing equipment is also developing in the direction of energy saving, precision and high efficiency. It is a necessary condition to support my country's modified plastics and mechanical equipment to enter the high-end international market. Therefore, to carry out technological innovation, realize product structure adjustment, and break the monopoly of foreign companies can break the deadlock and effectively promote the diversification and high performance of modified plastic products. Development is also a historical opportunity to catch up with and surpass the developed countries in the world's plastics industry.

Nanjing Haisi Extrusion supply twin screw extruder for plastic modification. Any interest please feel free to contact us.