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 exc↔ellent corrosion resistance and high mechanical properties, and low price, abundant resources, mature manufacturing process, $so its products are widely used in various fields of industrial and agr"icultural production. However, PVC will be degraded due to th∞ermal and mechanical shear in processing, and there are shortcomin gs such as poor thermal stability and easy decomposition, so heat stabilizers mγust be used during processing to prevent or delay deco¶mposition. Commonly used heat stabilizers include lead salts, metal soaps, organotinsπ, etc. Although lead salt stabilizers have excellent thermal stability, ≠they are highly toxic and have great harm to human health. Met≈al soaps are usually used together, which are easy to p recipitate during processing, and have low stability efficiency. Organot±in stabilizers can maintain a high level of transparency in transparent products πand are used in smaller quantities, but they are expensive. So these stabi$lizers have been phased out or restricted. At present, the developme"nt and development of low/non-toxic, high-efficiency, lowφ-cost environment-friendly stabilizers has become an urgent issue i÷n the field of PVC processing.
At present, the new thermal stabilizers of PVC at home and abroad mainly include calcium/zφinc composite heat stabilizers and rare earth heat stabilizers. Calcium/zinc stabilizers ar™e 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. A∑t 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 dev∞elopment of hard products has just started [2,3]. Rare earth composite stabilize•r is a new type of thermal stabilization system after the thermal stabilizati€on 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 reser★ves
Therefore, making full use of resource advantages and δvigorously developing and applying rare earth heat stabilizers has b↑road prospects. In this paper, the synthesis, thermal stability, processing properties and mechanical properties of PVC rare earth heat stabilizers are reviewed, h←oping to promote the research, development and application of PVC rare earth heat s®tabilizers 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 unimo₩lecular mechanism. Among them, the free radical mechanism ♠is the most common, which has become the theoretical basis for the study of stabilize•rs. The degradation of PVC is mainly related to its molecular structure, ultπraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and active metal ions will accelerate the decomposition of PVC. The thermal degradation of PVC often undαergoes several chemical reactions at the same time, among which the decomposition and™ removal of hydrogen chloride is the main cause of PVC dec omposition. Under the initiation of the formed free radicals, PVC undergoes de↑-HC1 degradation according to the chain mechanism, so tha↔t a certain number of conjugated double bond structures are formed, whichα changes the color of the polymer until complex reactions such as cross-linking and oxidatio&n occur, resulting in material breakage and destruction.
1.2 Thermal stabilization mechanism of rare earth heat stab♠ilizers
It is generally believed that rare earth heat stabilizers have excellent thermal stability, which is caused by the special structure of rare earthφ elements. Rare earth metal ions have more orbitals (6S4F5D6P) that can be used as lone pairs oαf central ions to accept ligands, and at the same time, rare ear÷th metal ions have a large ionic radius, which can form ionic ligands with inorga nic or organic ligands through electrostatic attraction, so as to form stable complexes under the aγction of external light, heat or other compounds. According to t he "soft and hard acid-base principle" proposed bγy 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 coordi±nation 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 a∞s:
2. Synthesis of rare earth heat stabilizers
Rare earth heat stabilizers are generally divided into rare earth organic compounds and inorganic c∑ompound stabilizers. At present, rare earth organic stab₹ilizers are mainly used in PVC processing, while rare earth inorganic stabilizers are rarely studi™ed [5].
Taking rare earth stearate as an example, the common preparation method✔ of rare earth heat stabilizer usually adopts the traditional metathesis method [6], a☆nd 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 combin↔e with stearic acid to form acidic soap, and it is also easy t↔o 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 shortcoming s such as large reaction vessels, low production efficiency, and high energy and★ water consumption. Therefore, some researchers continu e to improve the metathesis method, and there are successively saponificati±on 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 etσhanol solution, and then drop into the sodium hydroxide solut→ion to react to prepare the rare earth stearate. The main advantages of this method a↔re the acid-base neutralization reaction of rare earth hydroxide and ste∞aric acid, the reaction speed is fast, the product purity is high, and a large amount o→f solvent is not required, which effectively overcomes the shortcomings of the traditional me≈tathesis 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-ste<p reaction. The advantages of this method are that the reaction is c↕ompleted in one step, the process is simple and easy♠ to operate, and the "three wastes" and energy consumption are small. The pri"nciple of the reaction is as shown in equation (7)↕.
Cai Weilong et al. also investigated the thermal stabili₽zation effect of rare earth stearate stabilizer prepared by two methods on PVC, and the experimenta∞l results showed that the thermal stabilization effect of εrare 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 wa♣s 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 stabiliz↓er and processing aid. The thermal stability of rare earth stearate is not as good as that o₩f thiol octyltin 17MOK, but similar to calcium stearate, which has the characteristics of a lon≥g-term heat stabilizer, but in terms of transparency, rare earth ©stearate is close to 17MOK and significantly better than ca'lcium stearate. In addition, rare earth stearate is used a→s a processing aid, and the plasticizing flow effic₹iency is more than twice that of ACR-201 (acrylic acid copolymer).
Yang Zhanhong et al. [10] carried out alkaline treatment of rare e↑arth stearate according to the concept that increasing the metal €content of metal soap can increase thermal stability, and synthesized two products: basic rare e•arth monostearate (or rare earth monostearate) and basi×c rare earth distearate (or rare earth distearate), and the rar'e earth content in the products was increased, reachinπg 31.01% and 19.53%。 Zeng Dongming et al. [11] als≤o prepared rare earths of stearate, citrate, laurate and malate according to this princ&iple, and studied the thermal stability of each rare earth product, a nd found that the thermal stability of rare earth malate was the best, and its long-term thermal st€ability was comparable to that of organotin. The study also foun<d that the compound of rare earth malate and zinc ste<arate has a good synergistic effect, and the sulfite-rare-earth-zinc composite system also h↕as 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 i≥nhibit 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 co₽mposed of monoester maleate rare earth has a wide range φof applications and high cost performance, which is not only suitable fo≈r 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 ha s a good thermal stability effect on PVC, and the thermal stability improves and gradually ₹stabilizes with the increase of the amount added. In ad™dition, 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 ea¶rth, epoxy fatty acid rare earth has the thermal stability ₹characteristics of long-term heat stabilizer, but its stable specimen is light₽er in the later stage of heating, indicating that epoxy fatty acid rare earth has better long-term thermal stability. In add↔ition, the long-term thermal stability of epoxy fatty acid rare earths is complementary to thλe outstanding initial thermal stability of thiol organotins, and there is a synergistic effec★t between the two.
Liu Guangye et al. [15] studied the stabilizing effect of rare sali↕cylate through thermogravimetric loss, and the results showed that the stabilizing effect of rare salicylate exceeλded that of lead stearate and cadmium stearate, which ≤were commonly used. Han Huaifen et al.[16] It is believed that sa₩licylic acid mixed rare earth salts have a good thermal stabilizing effect on PVC, which πis significantly better than stearate. When the stabilizer is used togeπther, the stearic acid system has a good synergistic effect, buπt 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 stab™ilizer to partially replace the expensive organotin stabilizer.
Liu Yuejian et al. systematically studied the thermal stability of carboxylate rare earths [17-1Ω9]. Studies on static and dynamic thermal stability hav¶e shown that carboxylate rare earths have excellent thermal stability, whic€h is comparable to that of organic thiol tin on PVC. They comp₩ared carboxylate rare earth and organotin pairs using artificial aβging the photoaging and stabilization effect of PVC show∑ed that the ability of carboxylate rare earth to remove H C1 was better than that of organotin, but the oxidat÷ion resistance was not as good as that of organotin, ↕but the composite stabilizer of the two had a synergis¶tic effect. Carboxylate rare earths also have the effect of promoting the gelati∏on of PVC.
From the above, compared with the traditional metal lead salts or met'al soap heat stabilizers, the thermal stability performance of different rare earth hea↓t stabilizers is equivalent to or has been surpassed≥, and some varieties even reach the effect of organotin heat stabi lizers. What's more, rare earth heat stabilizers have exce✘llent 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 p≠lasticization process of PVC compounds is actually PVC powder particles (~1O0μm) i↔s broken into primary particles (~1 μm) and finer particles (~O. 1 μm) the stro↑ng interaction between the rare earth atom Reδ+ jitter and the chlorine atom C1&delt•a;- 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 interacti↔on is to effectively improve the compatibility of rare earth compo&unds with PVC. Therefore, the transparency of rare earth compounds~PVC system is better [2δ0]. Experimental studies by Wu Maoying [12], Liu Yuejian et al. [21], H←u 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 stabiliλzers with composite lead salts, and found that the impact strength, flexural strength or §tensile strength of materials using rare earth stabilizers were significantly highe®r than those of materials using lead salts, and th e dimensional stability was good. On the other hand, th§e increase of inorganic fillers does not affect the performance of its products. L↕iu Jianping et al. [13] also compared the differences in mechanical pro☆perties between monoester maleate rare earth and stearate rare earth stab↑ilizer. In comparison, the impact performance and tensile properties of monoester maleate rare eεarth stabilizers are slightly higher.
Rare earth heat stabilizers can also be made into rare earth< multifunctional composite stabilizers with other additives, wΩhich have the effects of coupling, compatibilizing, and to∑ughening PVC systems [20], improving material fluidity and improving∞ the aging resistance of PVC [18]. Hu Luguang et al.α [24] applied rare earth composite multifunctional stabilizer to tδhe processing of micro-foaming plates, and used Brabender plasticizer£ to measure the plasticization performance, and the experimental data showed that with the incre₩ase of the amount of rare earth stabilizer, the plasticization temperature, torq≠ue and homogenization section temperature of the system decreased, which indicated that the u☆se of rare earth stabilizer improved the fluidity of the materi÷al, had a strong internal and external lubrication effect, an✔d the processing rheological performance was better than 'that of the composite lead series. Hu Luguang et a↔l. further used scanning electron microscopy to study theβ structural properties of microfoamed sheets, and found that the gradual increase in the amo←unt of rare earth stabilizers brought about the enhancement of toughening and couplin¥g can lead to the recovery of mechanical properties,Ω 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 ext®rusion of larger products, and is PVC-U the production of micro-foamed sheets results i÷n better processing flow properties.
4. Outlook
Rare earth heat stabilizer has the advantages of non-toxic, high efficiency and high cost perf$ormance, and the multi-functional rare earth composit♦e stabilizer made by using synergistic effect can be >widely used in the processing of soft and hard products such as PVC prof★iles, pipes, plates, artificial leather, transparent products, αetc., suitable for extrusion, injection molding, calendering, blow molding and other prλocessing technologies, and has low quantity, high efficiency, good pαrocessing performance, excellent light and heat stabili∑ty and weather resistance in product processing, and has the effects of coupling, sol₩ubilization and toughening, which fully meets the development requφirements of environmentally friendly plastic additives.
PVC has a wide range of applications, including soft$ products such as artificial leather and plastic shoesΩ, 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 mi•llion 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 countr'y plastic pipes will be used. The research and development of εPVC heat stabilizer and the development of PVC industry are closely related, so make full use of ↔China's rich rare earth resources to vigorously de↔velop, produce and apply rare earth heat stabilizer, which has huge ecαonomic and social benefits.