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
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.