In general, PVC has bulky chlorine groups that are easily cleaved by external energy, such as heat or light. Upon the cleavage of chlorine, radicals are generated at the cleavage sites and immediately form double bonds. Such an increase in double bonds causes a change in color of PVC. As the temperature increases, double bond formation rapidly increases, so that PVC undergoes a change in color into a red color, and then into a black color.
Due to such weak heat resistance of PVC, processing and molding PVC are allowed only when a stabilizer reinforcing the heat resistance is added to PVC. As such stabilizers for PVC, metal soap stabilizers (e.g. Ca-stearate, Zn-stearate, Ba-stearate, Mg-stearate, etc.), tin (Sn)-based stabilizers, lead (Pb)-based stabilizers are widely used. Other wax and additives are also used in combination with inorganic fillers.
In the past, lead (Pb)-based, cadmium (Cd)-based or barium (Ba)-based stabilizers were frequently used for these purposes. However, use of non-toxic stabilizers prevails nowadays due to the harmful nature of heavy metals and environmental pollution problems. In general, calcium stearate and zinc stearate have been widely used as non-toxic stabilizers. They have been used in combination with a hydrotalcite-like compound, because they cannot provide sufficient heat resistance when used alone.
The hydrotalcite-like compound is a metallic compound having two or more metal double layers. Since the hydrotalcite captures anions in the interlayer, it shows an excellent ability of capturing free chlorine cleaved from PVC, and thus effectively inhibits degradation of the heat resistance of PVC caused by chlorine.
In addition, among the currently used metal soap-type stabilizers, zinc stearate is effective for initial heat resistance and calcium stearate is effective for long-term heat resistance. The zinc stearate stabilizer prevents initial coloration because zinc rapidly captures chlorine generated at the initial time. For these reasons, zinc oxide or hydroxide has been used in preparing hydrotalcite to provide a hydrotalcite-like compound wherein divalent zinc is added to the combination of magnesium/aluminum. However, in this case, zinc exists in a crystallized form as the base of the unique hydrotalcite double layer structure. Thus, the crystal structure is unstable and the particle size cannot be controlled with ease. Moreover, the hydrotalcite-like compound does not exist in an ionic structure capable of rapidly capturing chlorine, and is not effective for chlorine capture.
In general, hydrotalcite compounds have a structural formula represented by the following Formula (1):M(II)XM(III)Y(OH)N(Am−)Z.nH2O  (1)
Wherein M(II) is a divalent metal selected from Mg2+, Ni2+ and Zn2+; M(III) is a trivalent metal selected from Al3+, Fe3+, Cr3+ and Co3+; and Am− is an anion selected from CO32−, OH−, NO3−, SO42− and halides. Since the hydrotalcite compounds represented by the above structural formula have a double layer structure, they are also called LDHs (layered double hydroxides) or MMLHs (mixed-metal layered hydroxides). Recently, the crystal structure has been modified at high temperature or under other conditions. The hydrotalcite compounds may also be utilized without crystallization water (wherein n=0).
In addition, materials like hydrotalcite have been prepared from various combinations of different metals. Those materials are hydrotalcite derivatives and also called hydrotalcite-like minerals. However, the hydrotalcite-like minerals provided with excellent capturing ability still have a disadvantage in that they cannot provide sufficient initial coloration resistance and medium- and long-term heat resistance when used as stabilizers for PVC. Under these circumstances, there has been an imminent need for a novel type hydrotalcite compound capable of providing excellent initial coloration resistance and medium- and long-term heat resistance.