1. Technical Field
The present invention relates to a polyester resin with high resistance to hydrolysis and a toner comprising the same. More particularly, the present invention concerns a polyester resin prepared by adding an anti-hydrolysis agent to a reactant comprising an acid selected from an alkylester of aromatic dibasic acid or a mixture of an alkylester of aromatic dibasic acid and an alkylester of aliphatic dibasic acid; an aromatic diol; and an aliphatic diol, which has its acid value of 0.1 to 2.0 mgKOH/g and the hydroxyl radical value of 200 to 600 mgKOH/g, and a toner produced by using the polyester resin as a binder resin.
2. Background Art
Image formation has been traditionally accomplished by way of electrostatic recording, magnetic recording, toner jet and so forth. While there have been developed several methods and apparatuses for settling toner images on sheets such as papers, currently the most typical method is hot-pressing. The hot-pressing is advantageous to process rate and accuracy because a heat roller operates to compressively contact with a toner image. To form such a quick image, proposed is an art improving low-temperature fixability of toner by employing as a binder the polyester resin which has a low softening point. However, in the preparation of the polyester resin with the lower softening point, the required use of the smaller molecular weight results in a low degree of polymerization. The polyester resin with a low degree of polymerization has high acid value to degrade chargeability and moisture resistance, and has low glass transition temperature to deteriorate the blocking resistance.
For a binder resin, mostly required as cardinal functions are blocking resistance during the toner preservation and chargeability heavily affecting image formation. With respect to the blocking resistance, it is important for a toner-specific binder resin to maintain relatively high glass transition temperature without an effect of blocking even if temperature is high during the toner preservation. But if the glass transition temperature is too high, it is disadvantageous to low-temperature fixability and pulverizability thereof. The chargeability most seriously affecting image formation is represented in the dimensions of a quantity of electrified charges and moisture resistance. A quantity of electrified charges can be determined by measuring the charges of a toner and a toner-specific binder resin through a visual receptor of a copier or printer. In the toner-specific binder resin, charge quantity can be controlled by means of its monomer and molecular structure. Moreover, for the charge stability of the resin, it is necessary to ensure good moisture resistance so as to retain a constant charge quantity during a printing operation and prevent a decrease in the charge quantity of the resin at very humid conditions.
There have been disclosed technical manners in preceding documents by which the aforementioned functions for a binder resin can be accomplished, e.g. controlling acid values and blocking resistance by means of monofunctual isocyanate (refer to Japanese Patent Publication No. 2000-275902), enhancing moisture resistance and blocking resistance by means of monofunctual amine (refer to Japanese Patent Publication No. 11-092553), and adjusting acid values with an equivalence ratio of hydroxyl and acid radicals and monocarboxylic acid (refer to Japanese Patent Publication No. 2007-004149).
However, since the polyester resins produced from the former applications are kinds of acids whose terminals are made of carboxylic acids, their acid values are high while their hydroxyl radical values are low, incurring higher rates of moisture contents. Thus, toners produced from those resins are easily hydrolyzed to deteriorate durability and image quality, so that it is difficult to preserve them for a long term.
The present inventors have found out that when a polyester resin is left at room temperature, its softening temperature may be lowered with hydrolysis by hydrogen bonding between ester linkages in the chain of the polyester resin and/or carboxyl terminals of the resin and water. Accordingly, the inventors intended to develop a polyester resin with excellent hydrolysis resistance, filming resistance, moisture resistance, electrical characteristics and image stability through controlling of its acid value and hydroxyl radicals value thereby lowering its moisture content and through preventing hydrolysis of the resin with addition of anti-hydrolysis agent thereto.
An acid value and a hydroxyl radical value of a polyester resin generally affect moisture content thereof, thereby significantly effect on occurrence of hydrolysis by moisture during production of a toner with the polyester resin and on image formation and storage stability of the toner. Enhanced anti-hydrolysis of a polyester resin can be accomplished to reduce its moisture content by lowering its acid value and hydroxyl radical value. For controlling acid and hydroxyl radical values of a polyester resin, the following ways have been used: (1) varying a composition ratio of acidic and alcoholic components; (2) changing a reaction rate of esterification and polycondensation; and (3) employing a monomer other than acidic or alcoholic components.
The method (1) is carried out by using an excess of one component to adjust an acid value of a polyester resin. But, its commercial application may heavily increase product costs because an excess of the monomer must be consumed. The method (2) is adversely influential to physical properties of a polyester resin owing to change of the reaction rate, and the method (3) increases product costs and adversely affect the physical properties of a polyester resin owing to a new monomer further added thereto.