Zinc oxide heat stabilization

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

Zinc oxide (ZnO, wurtzite structure) eliminates oxygen on heating to form nonstoichio-metric colored phases, Zni+xO with x < 70 ppm. ZnO is almost transparent and is used as white pigment, polymer stabilizer, emollient in zinc ointments, creams and lotions, as well as in the production of Zu2Si04 for TV screens. A major application is in the rubber industry to lower the temperatures and to raise the rate of vulcanization. Furthermore, it is an n-type semiconductor (band gap 3.37 eV) and shows piezoelectric properties, making zinc oxide useful for microsensor devices and micromachined actuators. Other applications include gas sensors , solar cell windows and surface acoustic devices. ZnO has also been considered for spintronic application because of theoretical predictions of room-temperature ferromagnetism . 

Inorganic additives for rubber compounds also include materials that enhance performance under various accelerated stress conditions. Zinc oxide is an effective heat stabilizer for some types of elastomers. Iron oxide, lead compounds, barium salts, and specially treated clays, such as kaolinite, add performance margin in wet aging conditions. 

The white pigments of choice are led by titanium dioxide, and it too has excellent heat stability. Lithopone, zinc sulfide, zinc oxide, and antimony oxide are additional white pigments of choice. 

Paint Stability. Table III shows that paints containing the higher levels of zinc oxide had a greater viscosity drop after heat aging, and gelled after three freeze-thaw cycles. At 0.5 eq of zinc per COOH unit, both the zinc oxide and zinc ammonium carbonate paints were stable. 

Table IV shows that paints containing the higher levels of zinc ammonium complexes also generally showed the greatest viscosity drop on heat aging. Table IV also shows that these paints had poorer freeze-thaw stability. Close attention to formulation techniques, such as the judicious choice of dispersants and surfactants, can minimize or eliminate this problem in zinc oxide-containing latex paints.

Mark K 136 in combination with zinc oxide and barium stearate stabilizes the formulation and acts as a kicker, giving a very fine homogeneous eell structure. Produetion of sponge leather is a two stage proeess in which calendering is done at 155-180°C and foaming takes place in a heating ehannel at 200-230°C. 

Iron titanates are formed from combinations of iron(II) oxide and Ti02- These formulations are commonly modified by additions of iron(III) oxide, zinc(II) oxide, and aluminum(lll) oxide. Like the cobalt titanates, these are inverse spinels stmc-tures. Iron titanate pigments yield light yellow-brown to dark reddish-brown hues. In many cases these pigments exhibit greater heat stability than zinc ferrite or iron oxide browns, and are generally used in applications for this purpose. 

Three parts of zinc oxide are sufficient for cure, but five parts are sometimes recommended and it may be effective in enhancing heat resistance (Fig. 13). Current theory explains that zinc oxide produces C—C crosslinks, ensuring excellent heat stability most of the zinc oxide forms, during cure, zinc halide, which may be the actual curative and ingredients inhibiting the conversion of zinc oxide to zinc halogenide delay the cross-linking reactions. 

In glass, zinc oxide reduces the coefficient of thermal expansion, thus making possible the production of glass products of high resistance to thermal shock. It imparts high brilliance of luster and high stability against deformation under stress. As a replacement flux for the more soluble alkali constituents, it provides a viscosity curve of lower slope. Specific heat is decreased and conductivity increased by the substitution of zinc oxide for BaO and PbO. 

Both one-part and two-part neoprene sealants are made most commonly using general purpose neoprenes of the GN and W types. Two-part compositions contain 25-30% neoprene plasticized with materials such as dioctyl sebacate or resinous plasticizers, stabilized with phenolic antioxidants, filled with reinforcing pigments such as carbon black and hard clays, and modified with cure-rate regulators and acid acceptors such as zinc oxide and magnesium oxide. Cure accelerators such as polyamines (i.e., tetraethylene pentamine) are used at 5-10 phr based on neoprene. Heat-reactive phenolic resins are also effective. One such composition is shown in Table 12. 

The advantages just mentioned are attributed to the ionic crosslinking of the carboxyl groups with zinc oxide. As the ionic crosslinks have little heat stability the vulcanisates have only moderately good compression set behaviour at elevated temperatures. The various ways in which XNBR can be crosslinked have been reviewed by Brown. For a number of years the commercial importance of XNBR was limited by the scorching which tends to accompany the crosslinking action of the ZnO. One solution to the problem is that provided by Hallenbeck, who coated the particles of the ZnO with zinc sulphide or zinc phosphate. 

Metal soaps of barium, cadmium, lead, zinc, and calcium obviously can react with HCl. The addition of a zinc soap (or zinc oxide) to poly(vinyl chloride) gives a material that can be heated at 175°C for 10 or 15 min with scarcely any discoloration, as opposed to a control with no zinc that turns yellow or brown as a result of the conjugated unsaturations and oxidized structures. However, once the zinc is converted in large measure to zinc chloride, there is a very rapid and copious evolution of HCl and the remaining material is blue-black and brittle. The zinc soap, which was a stabilizer, is converted to zinc chloride, a rapid and efficient degradation catalyst. 

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