1. A process for preparing a low-odor hydrogel-forming acrylic acid polymer, which comprises the steps of:
a) preparing a polymeric hydrogel by free-radically polymerizing a monomer composition comprising at least 50% by weight of acrylic acid containing volatile saturated carboxylic acids selected from the group consisting of acetic acid, propionic acid and combinations thereof as impurities in a total amount of less than 400 ppm, by weight, based on the amount of acrylic acid, in an aqueous polymerization medium and converting said hydrogel into a particulate hydrogel or into hydrogel-forming powder; and
b) treating said particulate hydrogel or said hydrogel-forming powder with a crosslinking substance which, actually or latently, contains at least two functional groups capable of reacting with the carboxyl groups on the addition polymer.
2. The process as claimed in claim 1, wherein the acrylic acid polymerized in step a) is an acrylic acid obtained by a single or multiple stage crystallization of a crude acrylic acid having an acetic andor propionic acid content in the range from 0.1 to 5% by weight.
3. The process as claimed in claim 2, wherein the acrylic acid monomer of step a) is obtained by single or multiple stage crystallization of said crude acrylic acid at a temperature ranging from 0 to 13\xb0 C.
4. The process as claimed in claim 1, wherein the acrylic acid monomer of step a) is in the form of a partially or completely neutralized aqueous acrylic acid solution.
5. The process as claimed in claim 1, wherein the crosslinker in step b) is selected from compounds capable of forming ester groups with the carboxyl groups on the addition polymer.
6. The process as claimed in claim 1, wherein the monomer mixture to be polymerized in step a) comprises, based on its total weight,
from 50 to 99.99% by weight of acrylic acid as monomer A,
from 0 to 49.99% by weight of one or more monoethylenically unsaturated monomers B which are copolymerizable with acrylic acid, and
from 0.01 to 30% by weight of at least one crosslinking compound C.
7. The process as claimed in claim 1, wherein the acrylic acid monomer contains not more than 300 ppm of acetic acid, propionic acid or combinations thereof.
8. The process as claimed in claim 1, wherein the acrylic acid monomer contains not more than 200 ppm of acetic acid, propionic acid or combinations thereof.
9. A hydrogel-forming addition polymer obtainable by a process as claimed in claim 1.
10. A method for preparing a hygiene article comprising:
forming the hydrogel-forming addition polymer as claimed in claim 9 into a shape of said hygiene article.
11. Hygiene articles comprising an absorbent core which contains at least one hydrogel-forming addition polymer as claimed in claim 9.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
What is claimed is:
1. A control method of a lockup clutch comprising:
eliminating a loss stroke of a piston by a pre-charged hydraulic pressure;
increasing a force applied to the piston by further supply of the hydraulic pressure after the pre-charge;
controlling a slip rotation speed between a pump impeller and a turbine runner in a torque converter connected to an automatic transmission; and
determining a time for performing the pre-charge when a shift stage of the automatic transmission is under a neutral state and a vehicle is substantially stopped.
2. A control method of a lockup clutch according to claim 1, wherein the time for performing the pre-charge is determined based on a required time from starting the pre-charge of the hydraulic pressure until the slip rotation speed reaches equal to or less than a predetermined value.
3. A control method of a lockup clutch according to claim 1, wherein the time for performing the pre-charge is determined based on a required time from starting the pre-charge of the hydraulic pressure until a difference between the slip rotation speed under no-operation of the lockup clutch and the slip rotation speed during an operation of the lockup clutch becomes equal to or greater than a predetermined value.
4. A control method of a lockup clutch according to claim 1, wherein the time for performing the pre-charge is determined based on a required time from starting the pre-charge of the hydraulic pressure until increasing the rotation number of the turbine runner to be equal to or greater than a predetermined value.
5. A control method of a lockup clutch according to claim 2, wherein the pre-charge of the hydraulic pressure is performed for a predetermined time from a start of the pre-charge of the hydraulic pressure until immediately before elapsing of the required time.
6. A control method of a lockup clutch according to claim 3, wherein the pre-charge of the hydraulic pressure is performed for a predetermined time from a start of the pre-charge of the hydraulic pressure until immediately before elapsing of the required time.
7. A control method of a lockup clutch according to claim 4, wherein the pre-charge of the hydraulic pressure is performed for a predetermined time from a start of the pre-charge of the hydraulic pressure until immediately before elapsing of the required time.