1. Introduction to PET
Polyethylene terephthalate (PET) is an excellent thermoplastic with high melting temperature and glass transition temperature, good heat resistance, creep resistance, fatigue resistance and friction resistance. Abrasion, electrical insulation and chemical resistance are widely used in the fields of synthetic fibers, films, engineering plastics, medicine and daily necessities. PET is inexpensive, and it began to be used as an engineering plastics application in the 1960s. It has been used in the automotive, electrical, electronics, household appliances and machinery industries.
PET has a slow rate of crystallization (homogeneous nucleation) due to the presence of a rigid benzene ring in the molecular chain and a short flexible carbon chain segment, and low crystallinity during rapid cooling (such as injection molding), resulting in mechanical strength of the product. , rigidity (dimensional stability) and poor heat resistance. In the actual injection molding process, it is usually necessary to keep the mold temperature above 100 ° C for a long time, in order to give sufficient time for the PET melt to fully crystallize at a relatively slow cooling rate, so the processing energy is high, the molding cycle is long, and the production The high cost limits the engineering applications of PET in the automotive, electrical and construction sectors.
Therefore, in order to give full play to the high performance and low cost advantages of PET, it must be crystallized and modified so that it can be rapidly formed at a low temperature, and the crystallinity is relatively high. The most economically feasible improvement method is to add a crystal nucleating agent or Crystallization accelerator.
2. PET nucleation and crystallization
PET molecules have high structural regularity and strong crystallization ability, but because of their large molecular chain rigidity and high glass transition temperature, which hinders the movement of their molecular chains, PET is only a semi-crystalline substance at the melting point. The PET forms crystals in the range between the glass transition temperature and the melt, and if it cools rapidly, it forms a transparent amorphous structure. The amorphous PET has a density of 1.33 g/cm3 and the fully crystalline PET has a density of 1.445 g/cm3. Like other polymers, PET can only partially crystallize, and it is difficult to achieve complete crystallization.
The difference in crystallization rates of different polymers is due to the different activation energies required for molecular chain diffusion into the crystal lattice. Generally, the simpler the structure of the chain, the higher the symmetry, and the higher the crystallization speed. The polyethylene has a simple structure, good symmetry, and high crystallization speed, and it cannot obtain a completely amorphous state even if it is quenched under liquid air. The -CO- group on the macromolecular chain of PET reduces the symmetry, and the benzene ring in the main chain increases the rigidity of the molecular chain, which hinders the movement of the chain and affects the diffusion of the molecular chain. The speed, and thus the crystallization rate, is much slower. Even when compared with PBT, which is also polyester, the crystallization rate is much slower. This is mainly because PBT has two methylene groups in the molecular chain than PET, which makes PBT more flexible than PET, so the crystallization rate is higher. PET is fast.
Nucleation of the polymer includes primary nucleation and secondary nucleation. Primary nucleation is divided into homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation is caused by the thermal motion of the melt molecular segment itself to produce an ordered chain or folded chain as a crystal nucleus. The nucleus is continuously formed throughout the crystallization process, so the size of the crystal nucleus developed is different. The required temperature is usually relatively low; heterogeneous nucleation is centered on foreign impurities, polymer crystals, artificially added dispersed small particles or walls of the container, and the adsorbed polymer segments are ordered to form crystal nuclei. Heterogeneous nucleation is less affected by temperature and can occur at higher temperatures. Therefore, the addition of a nucleating agent (promoting heterogeneous nucleation) can significantly promote the crystallization of PET.
Finally, the crystallization of polymers differs from the crystallization of low molecules in many ways. In addition to the influence of its structure, polymer crystallization is also affected by many external factors, such as temperature, molecular weight and molecular weight distribution, additives and other factors.
For PET, the higher the molecular weight of the PET, the higher the melting temperature of the PET, and the worse the crystallization performance. For a given grade of PET, appropriately increasing the extrusion temperature and adding a nucleating agent are beneficial to the growth of the nucleus, which can be significantly improved. The mechanical properties of the final product; in addition, when the specific PET grade is selected, PET with lower diethylene glycol (DEG) content has better crystallization properties; due to industrial synthesis, catalysts are generally used, mainly including Mn, Zn, Pb, Cd, Mg, Ca, Ce, Co, Li, Na, and Sb, etc., while the catalyst with lower melting point is the most active nucleating agent, and can also be used as an optional reference standard.
3. Crystallization modification of PET by nucleating agent
The nucleating agent improves the crystallization property of PET mainly by playing the role of heterogeneous nucleation. If a nucleating agent is present in the system, the spherulites formed during the cooling of the melt are small and large, and the corresponding internal stress is also small. Dispersion, which improves the application performance of the product.
Therefore, by adding a nucleating agent, the following objectives can be achieved:
Increasing the initial crystallization (nucleation) temperature of PET and prolonging the temperature range of crystallization;
Suppresses the formation of large spherulites that cause PET to be brittle;
Induction produces a small, regular crystalline structure.
There are many kinds of nucleating agents, which are mainly divided into organic nucleating agents, inorganic nucleating agents, polymer nucleating agents and composite nucleating agents.
(1) Inorganic nucleating agent
The inorganic nucleating agent is basically an inorganic filler commonly used in polymers. The inorganic nucleating agent is equivalent to the small particles of the second phase present in the melt in the PET during the crystallization process, and the particles are infusible in the high temperature region. In the state of cooling, the PET molecular chain is centered on these particles, adsorbed onto the particles and arranged in an orderly manner to form a core. Therefore, when these small-molecular inorganic substances act as heterogeneous nucleating agents, the activation energy required for PET formation of the core is reduced, and for the subsequent crystal growth process, that is, the PET molecular segment is adsorbed on the surface of the crystal nucleus and enters the crystal. The process of the grid has little effect.
However, due to the poor interfacial bonding ability between the inorganic filler and the PET matrix, if the inorganic material is directly mixed with the PET under molten conditions, it is not easily dispersed uniformly in the PET matrix, and agglomeration is likely to occur. Therefore, the inorganic filler generally needs to undergo surface modification. Sexual processing.
Common PET inorganic filler nucleating agents are clay, oxide and hydroxide, inorganic salts, in addition to Si3N4, carbon nanotubes/graphite, zinc powder and pyrophyllite.
Clay nucleating agent
Montmorillonite is a layered inorganic material. This material is rich in raw materials and inexpensive. It can also effectively improve the barrier properties of the material when added to the polymer. It can be used to prepare good packaging materials after compounding with PET. Tudor, other clay materials such as kaolin, rector, etc., can also prepare well-dispersed nanocomposites, and can effectively improve the crystallization, thermal and barrier properties of PET.
Montmorillonite (MMT) is typically added to PET using a melt intercalation process. For example, chlorohexadecane-triphenylphosphine (CHDTPP) is used as a modifier of MMT, and then OMMT is formed, and then PET / OMMT nanocomposites can be obtained by melt intercalation. The results show that the prepared PET/OMMT The crystallization rate of the composite material is 4-5 times that of pure PET, which can greatly reduce the mold temperature during injection molding. When the montmorillonite mass fraction is 1%, the mold temperature of the composite material can be reduced to about 80 ° C, while pure PET Must be at 130%; not only that, PET/OMMT can be directly combined with glass fiber without nucleating agent and toughening agent to obtain engineering plastics with excellent mechanical properties and heat resistance.
Hydrotalcite (HT) can also be applied to the crystallization modification of PET. Studies have shown that when HT mass fraction is 0.5%-1.0%, HT is better as a crystal nucleating agent for PET; it is prepared by twin-screw melt blending method. The crystallization properties of the PET/attapulgite (AT) composites were also significantly improved.
Oxide nucleating agent
The SiO2 nucleating agent is one of the most used inorganic filler nucleating agents. As a heterogeneous nucleating agent for PET, the nucleation effect is best when the addition amount is about 2%, and the total mechanical properties of the obtained PET are the best when the addition amount is about 0.2%. When adding the same mass fraction, SiO2 has better heterogeneous nucleation effect on PET than MMT. Similar to most inorganic nucleating agents, excessive addition of SiO2 will agglomerate in PET matrix, which is not conducive to PET crystallization. Therefore, in order to achieve a better nucleating effect of SiO2, it is necessary to improve its compatibility and dispersibility with the PET substrate. The well-dispersed SiO2 inhibits the growth of PET spherulites due to the large number of nucleation sites, thus increasing the crystallization rate of PET while increasing its transparency.
PET/nano-titanium dioxide composites can be prepared by second-order melt blending. The results show that nano-TiO2 particles have the function of nucleating agent in PET matrix resin, which can significantly improve the crystallization temperature and crystallization rate of matrix resin. The effect of nano-Ti02 on PET crystallization has a significant enhancement effect on the matrix. The yield and tensile strength of the material increase by about 25% in the range of 3%. A small amount of nano-TiO2 can increase the toughness of PET to a certain extent. The notched impact strength of the material increased by about 10% in the content range of %; at higher content, the nano-Ti02 has obvious damage to PET toughness. The composite material has good comprehensive mechanical properties when the Ti02 content is 1%.
Nano-MgO also has a good nucleation effect on PET. Studies have shown that when nano-MgO is used as a nucleating agent, it is injection molded at a mold temperature of 60 ° C or 80 ° C. The impact strength, bending strength, maximum bending force of the obtained sample, The flexural modulus of elasticity is slightly better than when using nano-SiO2 as a nucleating agent. In the case where nano-MgO was separately added, when the mold temperatures were 60 ° C and 80 ° C, respectively, the tensile strength, impact strength, and flexural modulus of the sample did not differ much. And at the mold temperature of 60 ° C, the product does not stick and warp during the injection molding process, and can be smoothly demolded. This shows that after the addition of the nano nucleating agent, it is possible to further reduce the temperature of the injection mold.
Inorganic salt nucleating agent
Calcium carbonate, barium sulfate, talc, mesoporous molecular sieves (MMS) and barite are commonly used inorganic salt nucleating agents. Among them, talc powder has better nucleation effect on PET than calcium carbonate, clay, SiO2 and TiO2, and the crystallization performance is optimal when the addition amount is 5%.
Adding nano-BaSO4 with a mass fraction of 1% can increase the peak temperature of PET 6.8 °C, indicating that BaS04 has a better heterogeneous nucleation effect. The nucleation effect of BaSO4 on PET has an optimum amount with the addition of BasO4. When the mass fraction is 2%, the mechanical properties of PET/nanoBas04 composites are optimal. Compared with the pure PET samples, the tensile strength and flexural strength increased by 16% and 18.6%, respectively, and the tensile elastic modulus and flexural modulus increased by 32% and 14%, respectively, and the notched impact strength decreased slightly.
(2) Organic small molecule nucleating agent
Organic nucleating agents are mainly Na, Li, Ba, Mg, Ca salts of monobasic acid, Na, K, Ca salts of benzoic acid, Mg, Zn salts of aromatic hydroxy sulfonates and organophosphorus compounds, among which effects Preferred are sodium carboxylate and potassium carboxylate. For example, sodium ion-containing polymers are considered to be the most effective nucleating agents in the PET industry, with commercial PET nucleating agents as shown in the table below.
The nucleation mechanism of organic nucleating agents is mainly related to their chemical structure. These nucleating agents also play a role in chemical nucleation while performing heterogeneous nucleation, which not only improves the nucleation ability but also improves the nucleation. The growth rate, that is, they provide a heterogeneous nucleus while also reducing the diffusion free energy of the PET molecular chain diffusing into the crystal lattice.
(Substituted) Sodium benzoate and magnesium stearate are different from other heterogeneous nucleating agents. They dissolve in PET melt at high temperatures, and PET reacts with the above sodium carboxylic acid salt at high temperatures to form PET. -COONa substance, which forms ion clusters between PET melts with ionic end groups. The ion clusters will become nucleating agents in the melt, and the molecular chains will be arranged in a rapid crystallization. On the other hand, Due to the molecular chain cleavage during the reaction, the local relative molecular mass is lowered, and the crystallization of the whole system is promoted by the local crystallization, and the two aspects work together to greatly increase the crystallization rate. However, the nucleophilic substitution reaction between sodium benzoate and PET causes degradation of the resin. One possible method is to add a chain extender to supplement the molecular weight of PET. Commonly used chain extenders mainly include epoxides, isocyanates, acid anhydrides, oxazolines and the like.
Studies have shown that the addition of sodium benzoate nucleating agent can shorten the crystallization induction period, reduce the crystallization activation energy and increase the overall crystallization rate of PET. However, the increase of crystallinity decreases with the increase of the addition amount, which is not conducive to the stability of the properties of the blended materials. Therefore, when sodium benzoate is used in PET, it is necessary to pay attention to the amount, and it also needs to be used together with other modifiers.
In addition, sodium sulphate containing long saturated aliphatic chain (C26-C32) is also a good PET nucleating agent. Studies have shown that the longer the alkyl chain of the sodium sulphate nucleating agent, the crystallization of PET The faster the rate, and the higher the nucleation ability of sodium sulphate with longer carbamate segments than the (substituted) sodium benzoate.
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