US3481828A

US3481828A - Bleaching of wood pulp with thiourea dioxide and zinc sulfate

Info

Publication number: US3481828A
Application number: US531339A
Authority: US
Inventors: John Harry Wallice Turner; Pavel Krug
Original assignee: Hardman and Holden Ltd
Current assignee: Borax Europe Ltd
Priority date: 1965-03-05
Filing date: 1966-03-03
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02
Also published as: FI43527B; NO118091B; GB1079135A; SE306660B

Description

Unitcd States Patent Int. cl. D21c 3/20 vs. Cl. 162-42 3 Claims ABSTRACT OF THE DISCLOSURE The invention consists in a method of bleaching wood pulp, in which a bleaching composition is used containing 0.2% to 1.0% of thiourea dioxide and 1% to 5% of zinc sulphate, based on the weight of dry wood substance in the pulpto be bleached.

This invention relates to the bleaching of wood pulp. It is particularly concerned with the bleaching of mechanical wood pulp, in which the efiiciency of conventional bleaching agents, such as sodium'hydrosulphite and sulphoxylate is impaired due to the fact that such substances decompose at an early stage during the bleaching process. The invention is however also applicable of chemical wood pulp.

to the bleaching If +30 "According to the invention, wood pulp 1s bleached by.

means of a bleaching composition. which contains thiourea dioxide as the active bleaching-agent.

When thiourea dioxide is used as the active bleaching agent, there is greatly increased efficiency per unit quantity as compared with conventional materials, and also the bleaching effect obtained is superior; brightness in creases of from 5 to 10 points as measured by the calcium block brightness test can be achieved-and the brightness has greater permanency.

The effectiveness of the bleaching composition containing thiourea dioxide can be further enhanced by an addition of zinc sulphate.

The invention may conveniently; be carried out by bleaching the wood pulp in a bath which contains 0.2%- 110% of thiourea dioxide and approximately five times this quantity of zinc sulphate, based on the weight of dry wood substance in the pulp to be bleached.

Optical bleaching agents may if desired be incorporated in the bleaching composition.

Sequestering agents such as poly'phosphates and ethylene diamine tetra-acetic acid may also be incorporated in the bleaching composition.

With some paper pulps, the process of bleaching with thiourea dioxide, in the presence or absence of zinc sulphate, proceeds with greater efiiciency 'at increased pH values depending upon the source of the pulp. Suitable adjustments of pH can be achieved by the addition of sodium silicate.

.The process of bleaching wood ,,pulp or similar fibrous material with thiourea dioxide may be combined with other'treatments designed to impart specific properties to the finished product. The reduction of flammability of ICC paper boards by treatment with solutions of borates or phosphates is well known; unfortunately, during the process the borate or phosphate solutions become coloured by organic materials in the wood pulp and the treated material becomes discoloured. Discolouration can be greatly reduced, if the fire retarding treatment is carried out simultaneously with a bleaching process with thiourea dioxide. 'Qptimum bleaching efiects are obtained by using 0.05-3.00% weight/volume of thiourea dioxide in the flame retarding treatment solution.

The following are examples illustrating the method of the invention:

EXAMPLE 1 Ground wood pulp containing 20 g. dry wood substance were treated in 100 cc. of a bleaching bath containing in solution 0.04 g. of thiourea dioxide and 0.2 g. of zinc sulphate. The initial temperature of the bath was 40 C., and the temperature was raised within 20 minutes to the boil, at which it was maintained for a further 20 minutes. After rinsing a bleached wood pulp with superior brightness was obtained.

' EXAMPLE 2 Ground wood pulp (1,000 g. containing 200 g. dry wood substance) was mixed with rapid stirring with 4,000 ccs. watercontaining 1 g. pent-sodium tripolyphosphate (sequestering agent) and sodium silicate solution sufi'icient to bring the pH of the bath to 6.3-6.5; the temperature of the bath was raised. to C. and 0.5 g. thiourea dioxide and 2 g. zinc sulphate (ZnSO -7H O) added. Bleaching was carried out at 70 C. for one hour. After rinsing a bleached pulp was obtained showing a bright ness increment of 5 points.

EXAMPLE 3 was drawn. 0n drying the board showed considerable surface discolouration. The second pad was treated with a 14% solution of sodium polyborate containing in addition 0.25% i(w./v.) thiourea dioxide. On drying considerably less discolouration occurred than was the case with no thiourea dioxide. The third pad was treated with v a 14% polyborate solution containing 1% thiourea dioxide;

the beneficial effects were even greater, and the board produced was even paler than the fourth pad which was not treated with a fire retardant.

What is claimed is:

1. A process for bleaching wood pulp, which comprises bleaching the wood pulp in a composition containing from 0.2% to 1.0% of thiourea dioxide and 1% to-5% of zinc sulphate based on the weight of dry wood subtance in the pulp to be bleached.

2. A process as claimed in claim 1, in which the bleaching composition contains, in addition, a sequestering agent.

3. A process as claimed in claim 1, in which the bleaching is carried out simultaneously with a fire retarding treatment using a solution containing a fire retarding agent selected from the group consisting of borates and phosphates.

References Cited UNITED STATES PATENTS 5 10/1941 Morgan 252-186 X 10/1944 Loughborough 162-72 X 1/1949 Zcehuisen 162-72 X 6/1958 Sprout 162-78 X 11/1958 Hawkinson et a1. 162-78 10 2,872,482 2/1959 Provoost 260552 v 2,956,898 10/1960 Fleck 8-1 X 354,477 12/1886 Just et a1 162-79 X 3,384,534 5/1968 Kindron et a1. 162-71 S. LEON BASHORE, Primary Examiner US. Cl. X.R.