Polyvinyl chloride (PVC) is one of the five major general-purpose plastics, and its annual sales volume in the world ranks second only to PE among thermoplastics. PVC has excellent corrosion resistance and high mechanical properties, and low price, abundant resources, mature manufacturing process, so its products are widely used in various fields of industrial and agricultural production. However, PVC will be degraded due to thermal and mechanical shear in processing, and there are shortcomings such as poor thermal stability and easy decomposition, so heat stabilizers must be used during processing to prevent or delay decomposition. Commonly used heat stabilizers include lead salts, metal soaps, organotins, etc. Although lead salt stabilizers have excellent thermal stability, they are highly toxic and have great harm to human health. Metal soaps are usually used together, which are easy to precipitate during processing, and have low stability efficiency. Organotin stabilizers can maintain a high level of transparency in transparent products and are used in smaller quantities, but they are expensive. So these stabilizers have been phased out or restricted. At present, the development and development of low/non-toxic, high-efficiency, low-cost environment-friendly stabilizers has become an urgent issue in the field of PVC processing.
At present, the new thermal stabilizers of PVC at home and abroad mainly include calcium/zinc composite heat stabilizers and rare earth heat stabilizers. Calcium/zinc stabilizers are the most active R&D field of composite stabilizers in recent years, and such heat stabilizers have been developed abroad that can be used for soft and hard products. At present, most of the calcium/zinc stabilizers produced in China are in liquid form [1], which are only used for soft products, and the research and development of hard products has just started [2,3]. Rare earth composite stabilizer is a new type of thermal stabilization system after the thermal stabilization system of PVC processing such as lead, metal soap, organotin, etc., which was first developed by China in the 80s of the 20th century. Rare earth composite stabilizer has the characteristics of non-toxic, high efficiency and good comprehensive performance, and is suitable for all kinds of PVC products. China is rich in rare earth resources, accounting for 8O of the world's total reserves
Therefore, making full use of resource advantages and vigorously developing and applying rare earth heat stabilizers has broad prospects. In this paper, the synthesis, thermal stability, processing properties and mechanical properties of PVC rare earth heat stabilizers are reviewed, hoping to promote the research, development and application of PVC rare earth heat stabilizers in universities, research institutes and enterprises.
1. Thermal stabilization mechanism of rare earth heat stabilizers
1.1 Thermal degradation mechanism of PVC
The degradation mechanisms of PVC mainly include free radical mechanism, ionic mechanism and unimolecular mechanism. Among them, the free radical mechanism is the most common, which has become the theoretical basis for the study of stabilizers. The degradation of PVC is mainly related to its molecular structure, ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and active metal ions will accelerate the decomposition of PVC. The thermal degradation of PVC often undergoes several chemical reactions at the same time, among which the decomposition and removal of hydrogen chloride is the main cause of PVC decomposition. Under the initiation of the formed free radicals, PVC undergoes de-HC1 degradation according to the chain mechanism, so that a certain number of conjugated double bond structures are formed, which changes the color of the polymer until complex reactions such as cross-linking and oxidation occur, resulting in material breakage and destruction.
1.2 Thermal stabilization mechanism of rare earth heat stabilizers
It is generally believed that rare earth heat stabilizers have excellent thermal stability, which is caused by the special structure of rare earth elements. Rare earth metal ions have more orbitals (6S4F5D6P) that can be used as lone pairs of central ions to accept ligands, and at the same time, rare earth metal ions have a large ionic radius, which can form ionic ligands with inorganic or organic ligands through electrostatic attraction, so as to form stable complexes under the action of external light, heat or other compounds. According to the "soft and hard acid-base principle" proposed by Pesa-son [4], hard alkali chloride ions and strong acid rare earth metal ions are easy to form stable complexes. Therefore, rare earth metal ions (RE) has a strong coordination complexation ability with the chlorine atom (C11) on the PVC chain, and [C1CHC1-]- and Cl- are no longer involved in the catalytic reaction of HC1 decomposition, so they play a certain stabilizing role in PVC. The stabilization mechanism can be expressed in a chemical model as:
2. Synthesis of rare earth heat stabilizers
Rare earth heat stabilizers are generally divided into rare earth organic compounds and inorganic compound stabilizers. At present, rare earth organic stabilizers are mainly used in PVC processing, while rare earth inorganic stabilizers are rarely studied [5].
Taking rare earth stearate as an example, the common preparation method of rare earth heat stabilizer usually adopts the traditional metathesis method [6], and the reaction is carried out in two steps, such as equations (1) and (2). The metathesis method reacts in an aqueous medium, due to the large gelatinization energy of sodium soap, small solubility, and viscous solution, it is easy to combine with stearic acid to form acidic soap, and it is also easy to be salted out. To obtain highly pure rare earth salts of stearate, the reaction must be carried out slowly under dilution conditions. In production, there are shortcomings such as large reaction vessels, low production efficiency, and high energy and water consumption. Therefore, some researchers continue to improve the metathesis method, and there are successively saponification method, improved metathesis method and one-step method.
Cai Weilong et al. [8] introduced the preparation process of improved metathesis method and one-step method, and compared the two processes and products. The improved metathesis method is to add a pre-made rare earth nitrate solution to the stearate ethanol solution, and then drop into the sodium hydroxide solution to react to prepare the rare earth stearate. The main advantages of this method are the acid-base neutralization reaction of rare earth hydroxide and stearic acid, the reaction speed is fast, the product purity is high, and a large amount of solvent is not required, which effectively overcomes the shortcomings of the traditional metathesis method. The reaction principle is as follows in equations (5) and (6).
The one-step method uses hydrogen peroxide as a catalyst, absolute ethanol as a solvent, and stearic acid and lanthanum oxide to obtain a stearate rare earth product through a one-step reaction. The advantages of this method are that the reaction is completed in one step, the process is simple and easy to operate, and the "three wastes" and energy consumption are small. The principle of the reaction is as shown in equation (7).
Cai Weilong et al. also investigated the thermal stabilization effect of rare earth stearate stabilizer prepared by two methods on PVC, and the experimental results showed that the thermal stabilization effect of rare earth stearate prepared by one-step method and rare earth stearate prepared by improved method was comparable to that of PVC, which was slightly better than that of traditional metathesis products, and the effect was significantly better than that of zinc stearate and calcium stearate of metal soaps, and was close to the thermal stabilization effect of organotin.
3. Performance of rare earth heat stabilizers
3.1 Thermal stability
Wu Maoying et al. [9] synthesized RH
1. Rare earth stearate, which has the role of both heat stabilizer and processing aid. The thermal stability of rare earth stearate is not as good as that of thiol octyltin 17MOK, but similar to calcium stearate, which has the characteristics of a long-term heat stabilizer, but in terms of transparency, rare earth stearate is close to 17MOK and significantly better than calcium stearate. In addition, rare earth stearate is used as a processing aid, and the plasticizing flow efficiency is more than twice that of ACR-201 (acrylic acid copolymer).
Yang Zhanhong et al. [10] carried out alkaline treatment of rare earth stearate according to the concept that increasing the metal content of metal soap can increase thermal stability, and synthesized two products: basic rare earth monostearate (or rare earth monostearate) and basic rare earth distearate (or rare earth distearate), and the rare earth content in the products was increased, reaching 31.01% and 19.53%。 Zeng Dongming et al. [11] also prepared rare earths of stearate, citrate, laurate and malate according to this principle, and studied the thermal stability of each rare earth product, and found that the thermal stability of rare earth malate was the best, and its long-term thermal stability was comparable to that of organotin. The study also found that the compound of rare earth malate and zinc stearate has a good synergistic effect, and the sulfite-rare-earth-zinc composite system also has a good synergistic effect.
Wu Maoying et al. [l2] found that monoester maleate rare earth is similar to stearate rare earth, and also has the function characteristics of long-term heat stabilizer, but monoester maleate rare earth has a strong ability to inhibit PVC coloring, and the transparency of monoester maleate rare earth is better than that of stearate rare earth, which is similar to thiol octyltin 17MOK Very close.
In particular, it should be pointed out that the composite heat stabilizer composed of monoester maleate rare earth has a wide range of applications and high cost performance, which is not only suitable for soft products, but also can be used for the processing of semi-rigid PVC products.
Liu Jianping[l3]
The study also shows that monoester maleate rare earth has a good thermal stability effect on PVC, and the thermal stability improves and gradually stabilizes with the increase of the amount added. In addition, in terms of impact performance and tensile properties of PVC, the use of monoester maleate rare earth is slightly higher than that of stearate rare earth.
Wu Maoying et al. [14] developed a high-purity epoxy fatty acid rare earth with a well-preserved epoxy group. Similar to stearate rare earth, epoxy fatty acid rare earth has the thermal stability characteristics of long-term heat stabilizer, but its stable specimen is lighter in the later stage of heating, indicating that epoxy fatty acid rare earth has better long-term thermal stability. In addition, the long-term thermal stability of epoxy fatty acid rare earths is complementary to the outstanding initial thermal stability of thiol organotins, and there is a synergistic effect between the two.
Liu Guangye et al. [15] studied the stabilizing effect of rare salicylate through thermogravimetric loss, and the results showed that the stabilizing effect of rare salicylate exceeded that of lead stearate and cadmium stearate, which were commonly used. Han Huaifen et al.[16] It is believed that salicylic acid mixed rare earth salts have a good thermal stabilizing effect on PVC, which is significantly better than stearate. When the stabilizer is used together, the stearic acid system has a good synergistic effect, but the salicylic acid rare earth system does not, and the salicylic acid rare earth and organotin have a positive synergistic effect in a certain ratio range, which lays the foundation for the rare earth stabilizer to partially replace the expensive organotin stabilizer.
Liu Yuejian et al. systematically studied the thermal stability of carboxylate rare earths [17-19]. Studies on static and dynamic thermal stability have shown that carboxylate rare earths have excellent thermal stability, which is comparable to that of organic thiol tin on PVC. They compared carboxylate rare earth and organotin pairs using artificial aging the photoaging and stabilization effect of PVC showed that the ability of carboxylate rare earth to remove HC1 was better than that of organotin, but the oxidation resistance was not as good as that of organotin, but the composite stabilizer of the two had a synergistic effect. Carboxylate rare earths also have the effect of promoting the gelation of PVC.
From the above, compared with the traditional metal lead salts or metal soap heat stabilizers, the thermal stability performance of different rare earth heat stabilizers is equivalent to or has been surpassed, and some varieties even reach the effect of organotin heat stabilizers. What's more, rare earth heat stabilizers have excellent synergistic effects with other stabilizers, and can be used efficiently through compounding.
3.2 Processing performance, mechanical properties and others
Rare earth stabilizers have a plasticizing effect on PVC because the plasticization process of PVC compounds is actually PVC powder particles (~1O0μm) is broken into primary particles (~1 μm) and finer particles (~O. 1 μm) the strong interaction between the rare earth atom Reδ+ jitter and the chlorine atom C1δ- in the PVC molecule can enhance the transfer of force (especially the shear force), thereby promoting the gelation of PVC. It is not difficult to understand that the more immediate effect of this interaction is to effectively improve the compatibility of rare earth compounds with PVC. Therefore, the transparency of rare earth compounds~PVC system is better [20]. Experimental studies by Wu Maoying [12], Liu Yuejian et al. [21], Hu Shengfei et al. [22], and Zhang Yonghua et al. [23] all prove the above views.
Due to the special structure of rare earth elements, rare earth heat stabilizers can improve the mechanical properties of PVC products. Hu Shengfei et al. [22] and Zhang Yonghua et al. [23] compared rare earth composite stabilizers with composite lead salts, and found that the impact strength, flexural strength or tensile strength of materials using rare earth stabilizers were significantly higher than those of materials using lead salts, and the dimensional stability was good. On the other hand, the increase of inorganic fillers does not affect the performance of its products. Liu Jianping et al. [13] also compared the differences in mechanical properties between monoester maleate rare earth and stearate rare earth stabilizer. In comparison, the impact performance and tensile properties of monoester maleate rare earth stabilizers are slightly higher.
Rare earth heat stabilizers can also be made into rare earth multifunctional composite stabilizers with other additives, which have the effects of coupling, compatibilizing, and toughening PVC systems [20], improving material fluidity and improving the aging resistance of PVC [18]. Hu Luguang et al. [24] applied rare earth composite multifunctional stabilizer to the processing of micro-foaming plates, and used Brabender plasticizer to measure the plasticization performance, and the experimental data showed that with the increase of the amount of rare earth stabilizer, the plasticization temperature, torque and homogenization section temperature of the system decreased, which indicated that the use of rare earth stabilizer improved the fluidity of the material, had a strong internal and external lubrication effect, and the processing rheological performance was better than that of the composite lead series. Hu Luguang et al. further used scanning electron microscopy to study the structural properties of microfoamed sheets, and found that the gradual increase in the amount of rare earth stabilizers brought about the enhancement of toughening and coupling can lead to the recovery of mechanical properties, and the tensile and impact strength are also slightly improved. The authors point out that the rare earth composite multifunctional stabilizer is suitable for the rapid extrusion of larger products, and is PVC-U the production of micro-foamed sheets results in better processing flow properties.
4. Outlook
Rare earth heat stabilizer has the advantages of non-toxic, high efficiency and high cost performance, and the multi-functional rare earth composite stabilizer made by using synergistic effect can be widely used in the processing of soft and hard products such as PVC profiles, pipes, plates, artificial leather, transparent products, etc., suitable for extrusion, injection molding, calendering, blow molding and other processing technologies, and has low quantity, high efficiency, good processing performance, excellent light and heat stability and weather resistance in product processing, and has the effects of coupling, solubilization and toughening, which fully meets the development requirements of environmentally friendly plastic additives.
PVC has a wide range of applications, including soft products such as artificial leather and plastic shoes, and hard products such as profiles, pipes, and plates. In 2005, China's total PVC output reached 6.492 million tons, and the apparent consumption of PVC was 7.918 million tons, with an average annual value of 10 the rate of growth around the right. Taking building pipes as an example, by 2010, there were 80 building drainage pipes in the country plastic pipes will be used. The research and development of PVC heat stabilizer and the development of PVC industry are closely related, so make full use of China's rich rare earth resources to vigorously develop, produce and apply rare earth heat stabilizer, which has huge economic and social benefits.