PBT is a crystalline linear saturated polyester prepared by polycondensation reaction of terephthalic acid (PTA) or dimethyl terephthalate (DMI) and 1,4-butanediol (BDO). It has good mechanical properties, its symmetrical molecular structure can achieve tight stacking, has high crystallinity, and can be quickly crystallized at low temperatures. PBT parts are easy to flow forming and short molding cycle during processing, which can reduce production costs, and PBT has the advantages of moisture resistance, wear resistance, oil resistance, etc., and the creep is also small.
Since PBT contains both crystalline and amorphous parts, it is easy to modify it by adding other substances. However, PBT also has defects such as flammability, large precipitation of small molecules in contact with refrigerant, insufficient dielectric properties, and easy warping of thin-walled parts, which limit its application scope. In order to make up for the lack of performance of pure PBT resin, there have been some modification studies on PBT resin.
1. Research status of modification of PBT engineering plastics.
In recent years, related enterprises have developed various new technologies and new products, and PBT engineering plastics have developed in the direction of high performance, functionalization and diversification. In response to the needs of the industrial field, the functionality of PBT is improved through modification, which is favored by the market. At present, copolymerization modification, inorganic material filling modification, nanocomposite technology, blending modification and other methods are mainly used at home and abroad to improve the comprehensive performance of PBT. The research on the modification of PBT materials mainly focuses on high strength, high flame retardant, low warpage, low precipitation, and low dielectric. In terms of mechanical properties.
The tensile strength, flexural strength and flexural modulus of pure PBT resin are low, which cannot be widely used in the industrial field, and it needs to be modified to improve the mechanical properties. Glass fiber has the advantages of strong applicability, simple filling process and low cost. The addition of glass fiber to PBT gives full play to the original advantages of PBT resin, and the tensile strength, bending strength and notched impact strength of PBT products have been significantly improved.
In addition to glass fibers, other fibers can also be introduced to improve the mechanical properties of PBT. Zeng Deming et al. used short-cut basalt reinforced PBT resin, and the basalt could be well compatible with PBT after the coupling agent, which effectively improved the mechanical properties of PBT composites.
Flame retardant properties.
The vertical combustion grade of pure PBT can only reach HB level, which is flammable and continuously drips during combustion, and the flame is easy to spread, so its application in automobiles, electronic appliances and textiles is limited. Halogenated flame retardants and halogen-free flame retardants are often added to modify PBT for flame retardant modification. Halogenated flame retardants release toxic fumes containing hydrogen halide when burned, which is harmful to human health and the ecological environment, and some halogenated flame retardants have been banned in the European Union. Phosphorus-based flame retardants and inorganic flame retardants are mainly used in the flame retardant modification of PBT. When using inorganic flame retardant modification, too much content will lead to the decline of the mechanical properties of the material; However, phosphorus-based flame retardants do not have this defect, and have the excellent characteristics of low smoke, low toxicity and high flame retardant. Phosphorus-based flame retardants often work synergistically with nitrogen-containing compounds to achieve more efficient flame retardant systems. Phosphorus-based flame retardants generate phosphoric anhydride during the combustion process to dehydrate and carbonize combustibles, and the carbon layer can reduce heat conduction, delay or prevent the generation of combustible gases, and phosphoric anhydride forms a melt covering the surface of combustibles after being heated, hindering the release of flammable gases. In addition to traditional flame retardants, nanofillers are added to PBT materials to improve their flame retardant properties and anti-dripping properties without damaging their processability. The commonly used nanomaterials are mainly nano-metal-oxide polymers and carbon-like nanocomposites.
Warpage deformation aspect.
The molecules of PBT materials are relatively easy to slip, easy to orient and crystallize, resulting in a large shrinkage rate of the material, which causes the warping and deformation of PBT parts, especially large thin-walled parts. For glass fiber reinforced PBT, due to the anisotropy of the added glass fiber, the shrinkage rate of the material in different directions in the injection molding is different, which increases the warping deformation of the workpiece, which not only affects the surface quality and installation performance of the plastic product, but also affects the strength of the plastic. For the warping of PBT parts, in addition to improving the shape of the parts, the design of the mold and the molding process parameters, the PBT materials can also be modified to slow down the warpage deformation. In recent years, the warpage deformation of PBT materials has been improved mainly by inorganic filling and co-mixed gold. Inorganic filling includes single inorganic filling and combined filling with glass fiber, and the inorganic substances used for filling mainly include talc, mica, wollastonite, glass beads, kaolin, calcium sulfate whiskers, etc.
In addition, amorphous polymers such as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), and styrene-acrylonitrile copolymer (SAN) do not crystallize during the injection molding process, and can effectively improve the shrinkage of PBT after blending with PBT.
precipitation performance.
Due to the inability of the raw materials of PBT materials to react completely in the production process, small molecules, oligomers, etc. are generated, and the products made of unmodified PBT materials are precipitated under certain conditions, which affects the utility of the workpiece. When PBT material is applied to the muffler, motor coil skeleton and air conditioning insulation skeleton inside the refrigerator compressor, due to the special working conditions, a large number of small molecule substances are precipitated, which are dissolved in the refrigerant (freon, difluorodichloromethane), which is easy to block the refrigeration pipe and make its refrigeration fail.
PBT precipitates are mainly small molecule oligomers of the resin itself and a small amount of additives contained therein. Using glass fiber and high-viscosity resin, filling a certain amount of adsorbent can reduce the precipitation of PBT. At present, the precipitation modification of PBT is mainly carried out in the form of adsorption of inorganic porous materials and the addition of capping agent, chain extender, 1,4-cyclohexanedimethanol (CHDM) and other chemical methods. Dielectric properties.
In the application of PBT materials in the fields of integrated circuits and electromagnetic shielding, its dielectric properties play an important role in signal transmission speed and signal loss. In recent years, more stringent requirements have been put forward for the dielectric properties of insulating materials, requiring that the dielectric constant of insulating resin materials does not exceed 28。However, the dielectric properties of pure PBT materials cannot meet the communication requirements, so it is of great significance to develop a PBT material with low dielectric constant and low dielectric loss.
At present, the dielectric properties of PBT are mainly modified by filling and blending copolymers with low dielectric constant, and the commonly used fillers are PTFE powder and hollow glass beads. Carbon nanotubes also have a positive effect on the dielectric properties of PBT materials, but the dielectric constant and dielectric loss of the materials are increased due to the high content. With the rapid development of electric vehicles, its high-voltage transmission has higher requirements for the dielectric strength of connector materials, although PBT has good arc resistance and is easy to achieve high-speed molding, but the dielectric strength is low, and it is impossible to achieve high-voltage transmission on electric vehicles. The dielectric properties of PBT are modified by conductor or ceramic fillers to improve the dielectric strength of the material to ensure the safety of electric vehicles.
2. Application of PBT modified engineering plastics.
2.1. Automotive field.
With the gradual development of plastic instead of steel, more and more non-ferrous metals and alloy materials are replaced by plastics. PBT has good chemical corrosion resistance, stress crack resistance, wear resistance, weather resistance, aging resistance and high strength performance, and is widely used in automotive exterior parts, such as wiper handle brackets, bumpers, door handles, rearview mirror housings, underbody panels, body side panels, radiator fans, radar penetration covers, corner grilles and lighting components.
2.2. Electronic and electrical fields.
PBT is widely used in the field of electronic appliances due to its characteristics of low dielectric, low warpage, high flame retardant, high toughness, aging resistance and environmental protection. Such as the shell of electronic computers, igniters, electrical switches, copiers, skeletons of transformers, parts of baking machines, electric iron covers, etc. In addition, because the modified PBT has excellent dielectric properties and is easy to process, it can be used for the bottom cover, shell and spool of electrical appliances.
2.3. In the field of machinery and equipment.
Due to its high flame retardancy and heat resistance, PBT is widely used in the field of machinery and equipment, such as cams, gears, camera parts, electronic watch housings, mercury lampshades, and various buttons. Common coil skeletons require materials with high insulation breakdown strength to avoid electrical breakdown during use; When it is used in refrigerators and other components, it also needs to have low precipitation to prevent the precipitation of small molecules from failing mechanical parts.
PBT is widely used in production coil skeletons due to its excellent low dielectric and low precipitation properties. Because PBT has the advantages of excellent flame retardant performance, good fluidity and easy molding, it can be used in the production of cooling fans, such as computer CPUs, power supplies and motors and similar radiator fans.
2.4. Communications.
PBT is widely used in the field of communication due to its good dielectric, formability and dimensional stability, as well as low coefficient of linear expansion. In radio communication, Fe3O4 nanoparticles are added to PBT composites to increase their consumption of electromagnetic waves, realize magnetic shielding function, reduce the harm of electromagnetic radiation to the human body, and are used as plastic substrates for basic components of high-power communication equipment.
2.5. Textile field.
Due to the long flexible part on the basic chain link of PBT, PBT fiber shows superior flexibility and elasticity, the elasticity at room temperature is equivalent to that of rubber, and the elasticity is not affected by the surrounding environment, with fine three-dimensional curl, suitable for the production of elastic fashion (such as elastic denim) and elastic composite fabrics (such as elastic yarn for clothing) and other high elasticity and good hand feel textiles.
There are several different modification methods for some performance defects of PBT engineering plastics in specific fields. Through filling, blending, preparation of nanocomposites and other modification methods, PBT has excellent high strength, high flame retardancy, low warpage, low precipitation and low dielectric properties, which can meet its requirements in automobiles, electronic appliances, optical fibers, textiles and other fields. In the future, PBT materials that meet the requirements of low-carbon environmental protection and high-quality materials in various industries should be developed. By improving the modification technology to accelerate the development of functional PBT products, a variety of modification schemes can be used comprehensively, avoiding the disadvantages caused by a single modification method, and developing PBT materials with high added functions. Focus on expanding the application of PBT materials in thermal conductivity, biological fields and electromagnetic shielding, so that PBT materials can be applied in more and more fields.