层状与超分子插层结构热稳定剂的组装及结构和性能研究/SUPRAMOLECULAR ASSEMBLY, STRUCTURE AND PROPERTIES OF

2018-11-20 16:30:30

CL PVC HCl LDHs 层板



聚氯乙稀(PVC)树脂是一种应用广泛的热塑性塑料,但其热稳定性很差,必须加入一定数量的热稳定剂以提高其热稳定性。目前使用的热稳定剂主要以铅盐类、金属皂类、有机锡类等含重金属品种为主,会给人类健康带来长期危害,随着环保意识的加强,世界各国逐步开始对重金属进行限制甚至禁止使用,因此无毒、无污染、复合和高效成为PVC热稳定剂的发展方向。
层状双金属氢氧化物(LDHs)是一种典型的阴离子型层状化合物,由于其层板表面具有碱性,可以吸收PVC热分解释放出的HCl,同时层间CO32-可以和Cl-进行离子交换,从而达到吸收HCl、抑制PVC自催化分解和稳定PVC的效果。由于LDHs的特定性能及其层板主体和层间客体的可调控性,以及无毒、制备简单等特点,使得将LDHs及其超分子插层产物作为PVC热稳定剂具有良好的应用前景。
本文利用成核/晶化隔离法制备了MgAl-CO3-LDHs,研究了其对PVC的热稳定效果。结果表明MgAl-CO3-LDHs对PVC具有较好的长期热稳定作用,添加2phr可以使PVC的热稳定时间达到103min以上;MgAl-CO3-LDHs与硬脂酸钙和硬脂酸锌具有良好的协同热稳定作用,复合使用可以克服MgAl-CO3-LDHs对PVC初期着色性差的缺点。根据LDHs的结构特点,进一步研究发现调变层板主客体会影响羟基与HCl的反应速率和离子交换反应的进行,对PVC产生不同的热稳定效果。
LDHs主体层板中Mg元素具有较小的电负性,减小其含量,有利于降低层板对羟基的作用力,增强层板对HCl的吸收;Zn元素具有较大的电负性,并且可以通过置换PVC中不稳定的烯丙基氯抑制其自催化分解的进行,因此将其引入LDHs层板可以达到改善PVC热稳定性的目的。层板中二价和三价金属离子比例的改变,会导致层板电荷密度的改变,减小它们的比例能够增大层板电荷密度,有利于增强Cl-进入层间的驱动力,加速离子交换反应,提高PVC的热稳定性。因此本文对LDHs层板主体进行调控,调整层板元素比例并将Zn元素引入层板制得MgZnAl-CO3-LDHs,考察了其对PVC热稳定性的影响。结果表明,减小Mg/Al比提高了层板对HCl的吸收,加快了离子交换的进行,从而改善了PVC热稳定性。根据LDHs结构特点,确定Mg/Al=2为最佳比例。将Zn元素引入层板,显著提高了PVC的初期着色性,但过量的Zn含量会引起“锌烧”现象,综合考虑以Mg:Zn:Al=3:1:2为宜。研究表明,MgZnAl-LDHs对PVC具有良好的热稳定作用,并且在增强、增韧、阻燃、抗小分子迁移、雾度和透光性、改善PVC加工性能等方面具有良好的效果,多项指标和综合性能优于传统铅盐和有机锡等热稳定剂,有可能替代现有产品成为新一代的PVC热稳定剂。
不同LDHs层间客体和PVC热分解产生的Cl-具有不同的交换性,并对层板羟基产生不同的作用力,影响到LDHs层板对HCl的吸收性能。马来酸根具有较大的空间尺寸,将马来酸根与LDHs进行超分子插层组装,可以增大LDHs层间距,减弱和层板的作用力,增强层板羟基与HCl的反应活性,提高对HCl的有效吸收。同时马来酸分子中具有双键,可以和PVC分解产生的共轭双键发生环加成反应,破坏PVC分解的自催化反应。因此本文调控LDHs层间客体,制备出具有不同无机阴离子的MgAl-LDHs,又将马来酸根阴离子引入层间得到具有新型结构的MgAl-maleate-LDHs,考察了它们对PVC热稳定性的影响。结果表明不同层间阴离子对层板羟基具有不同作用力,较小的作用力有利于提高层板对HCl的吸收和PVC的初期着色性,层间为CO32-的LDHs可以和Cl-发生离子交换反应达到吸收HCl的目的。马来酸根和LDHs进行超分子插层组装后,大大降低了层板羟基受到的作用力,增强了层板对HCl的吸收,其层间马来酸根还可以在层板反应消耗后和PVC分子中的共轭双键发生环加成反应,阻止双键的延续,显著提高PVC老化过程中的热稳定性和着色性。
在上述工作基础上,进一步同时调控LDHs层板主体和层间客体,将Zn元素引入层板,将马来酸根阴离子引入层间,得到了具有新型结构的MgZnAl-maleat-LDHs。通过考察对PVC热稳定性的影响,结果表明马来酸根和MgZnAl-LDHs进行超分子插层组装后大大增强了层板对HCl的吸收,层间马来酸根也可以通过环加成反应阻止双键延续,显著提高了PVC热老化过程中的热稳定性和着色性。
PVC分解生成的Cl-需要及时有效地迁移到LDHs表面并被其吸收,才能达到提高PVC热稳定性的目的。本文通过研究LDHs晶粒大小及其分散状态对PVC热稳定性的影响,发现在一定添加量情况下LDHs具有最大有效粒径,在此基础上提出了LDHs对PVC有效热稳定作用半径的概念,同时还研究了LDHs不同添加量对PVC热稳定性的影响,得出了LDHs在一定粒径情况下所需的最小添加量。
为深入研究LDHs对PVC热稳定作用的机理,将具有不同层间阴离子的LDHs粉体直接与PVC粉体进行充分地物理混合,在180±1℃条件下进行热老化实验,于不同时间取出对其进行XRD、IR等测试,直观模拟和动态表征LDHs与PVC的作用过程,研究LDHs在不同热老化阶段的变化,得出LDHs对PVC的热稳定作用过程如下:
1)PVC受热脱出不稳定Cl,按链式机理释放出HCl并在PVC体相中向四周扩散,加速PVC的分解;
2)LDHs层板羟基与扩散到其表面的HCl发生反应,阻止了PVC自催化分解;
3)Cl-克服扩散阻力进入LDHs层间,与层间阴离子进行交换,生成层间为Cl-的LDHs;
4)Cl-进入层间后,LDHs层板与HCl的反应减弱,对PVC热稳定作用降低。



Polyvinylchloride (PVC) resins are a type of thermoplastic that have a wide variety of applications. However their thermal stability is very poor and addition of one or more stabilizers is required during processing. The main classes of thermal stabilizers in current use are lead salts, metal soaps and organo-tin compounds. These all have disadvantages in terms of toxicity, environmental pollution and/or high cost. As a result of the worldwide increase in environmental awareness and the increasingly stringent limits on the use of heavy metals, attention is currently being focused on thermal stabilizers that are non-toxic and environmentally-friendly, as well as being economical.
Layered double hydroxides (LDHs) or hydrotalcite-like materials are a class of anionic clays. LDHs have basic properties and their CO32- counterions can be exchanged by Cl-, which should make it possible to absorb HCl formed during thermal dehydrochlorination of PVC, and thus inhibit the autocatalytic degradation of the polymer. The unique structure and properties of LDHs together with their non-toxic and odorless nature, as well as their low cost, make LDHs and their intercalates very promising for application as PVC stabilizers.
In the work described in this thesis, MgAl-CO3-LDHs were prepared by a method involving separate nucleation and aging steps. Their thermal stabilization effect on PVC was studied. The results showed that MgAl-CO3-LDHs have good long-term stabilizing effect on PVC and that the stability time can exceed 103 min at 180±1 ºC when the added ratio is 2 phr (parts per hundred resin). LDHs and Ca(st)2/Zn(st)2 (st = stearate) mixtures have a synergetic effect on the stability of PVC which can overcome the problem of bad early coloring of PVC observed with MgAl-CO3-LDHs as the sole stabilizer. Further research indicated that modulating the nature of both the host layers and interlayer guests can have a significant effect on both the reaction rate with HCl and the ion exchange process.
Magnesium has low polarizing power and replacing it by zinc increases the polarization of the hydroxyl group. This leads to a higher affinity for Cl-, which in turn should inhibit the autocatalytic degradation of PVC and therefore improve its stability. In the work described in this thesis, the host layers of LDHs were tailored by adjusting the relative amounts of layer cations, and by incorporation of Zn2+ into the layers in order to afford MgZnAl-CO3-LDHs. The results showed that decreasing the Mg/Al ratio has the effect of increasing the adsorption of HCl by the layers while enhancing the driving force for Cl- to enter the interlayers and exchange with counterions. As a consequence, the stability of PVC was increased. The optimal molar ratio of Mg:Al was found to be 2:1. The incorporation of Zn2+ in the layers has the effect of improving the resistance of PVC to early coloring, although too large a content can result in “zinc carbonation”. The optimal ratio of metal elements in the layers was found to be Mg:Zn:Al=3:1:2. MgZnAl-LDHs give a substantial increase in the stability of PVC with concomitant enhancement of strength, elasticity, fire retardancy, resistance to small molecule migration and processability of the composites. MgZnAl-LDHs are superior to traditional lead salts, organo-tin and first generation LDHs in many respects, which makes it viable to replace current stabilizers with this novel material.
Different interlayer guests have varying strength of interaction with the hydroxyl groups; weak interactions favor adsorption of Cl- by the layers via a process of ion exchange. Maleate is a large anion and supramolecular assembly by intercalation of maleate into LDHs leads to an increase in the interlayer spacing and decreases the interactions with layer hydroxyl groups. The reaction rate between HCl and the layers is thus accelerated and the stability of PVC is significantly enhanced. By virtue of its double bond, maleate can also give rise to a cycloaddition reaction with conjugated double bonds formed in the dehydrochlorination of PVC. The autocatalytic degradation of PVC is thus inhibited and the stability enhanced. In the work described in this thesis, the interlayer guests of LDHs were varied in order to give MgAl-LDHs with different inorganic counterions as well as novel MgAl-maleate-LDHs formed by intercalating maleate into the interlayers. After testing their thermal stabilizing effect on PVC, the results showed that weaker interaction between counterions and layer hydroxyl groups leads to the enhancement of Cl- adsorption by the layers. Supramolecular assembly by intercalation of maleate into LDHs decreases the force acting on the layer hydroxyl groups and accelerates the adsorption of HCl. After the layers have been destroyed by reaction with HCl, maleate will react via its double bond with the conjugated double bonds formed by degradation of PVC. The stability of PVC is thus enhanced.
Based these results, the identity of host layer cations and interlayer anions in LDHs were simultaneously adjusted by the incorporation of Zn and intercalation with maleate in order to give novel supramolecular MgZnAl-maleate-LDHs which leads to a further significant improvement in the thermal stability and resistance to discoloring of PVC.
According to the established mechanism, the thermal stability of PVC can only be enhanced if the Cl- first migrates to the LDH surface and is then adsorbed there. The effect of LDHs particle size and their dispersion conditions on the thermal stability of PVC was studied. The maximal effective particle size was calculated and the concept of Maximal Effective Stabilizing Radius of LDHs was proposed. The effect varying the amount of added LDHs on the thermal stability of PVC was investigated and the minimum required amount of LDHs with a given particle size was calculated.
Thermal aging tests were carried out under 180±1 ºC after physically mixing LDH powders containing different counterions with PVC powder. The interaction of LDHs and PVC was probed by studying the mixture after different thermal aging times by XRD and FT-IR. The mechanism of thermal stabilization of PVC by LDHs can be summarized as follows:
1)HCl given off by PVC at high temperatures diffuses through the PVC and accelerates the degradation;
2)Layer hydroxyl groups of LDHs react with HCl that migrates to the surface of LDHs and thus inhibit the autocatalytic degradation of PVC;
3)LDHs with Cl- as counterions were then formed after Cl- entered into the galleries of LDHs and exchanged with the original counterions;
4)After intercalation of Cl- between the layers, affinity of the layers for HCl decreases leading to a reduction in thermal stability of the PVC.