WO1988007059A1

WO1988007059A1 - Improved cellulose ether and its manufacture

Info

Publication number: WO1988007059A1
Application number: PCT/EP1988/000160
Authority: WO
Inventors: Serge Huybrechts; Alfons Detemmerman; Jozef De Pooter; Rudy Eduard Blyweert
Original assignee: Union Carbide Benelux N.V.
Priority date: 1987-03-14
Filing date: 1988-03-03
Publication date: 1988-09-22
Also published as: BR8806039A; JPH0643442B2; JPH01502675A; GB8706120D0; EP0304454A1

Abstract

A process for making a cellulose ether (eg HEC) having improved bioresistance is provided. The process consists of two stages, in a first stage cellulose is etherified in the presence of a boron containing compound to an MS value in the range 0.6 to 1.3; in a second stage the product of the first stage is etherified to an MS value in the range 1.6 to 3.0.

Description

IMPROVED CELLULOSE ETHER AND ITS MANUFACTURE

The present invention relates to a process for preparing cellulose ethers having an enhanced resistance to breakdown by biological agents such as enzymes and the like. In particular, the invention relates to a two step process for making bioresistant cellulose ethers which have a relatively low molar substitution (MS) level relative to those bioresistant cellulose ethers of the prior art.

Cellulose ethers are widely used as thickeners in a range of industrial applications, for example latex paints and the like. In these applications, the cellulose ether is susceptible over a long period of time to breakdown by biological agents. Breakdown of the cellulose ether results, in the case of paints, in an overall decrease in viscosity and hence a deterioration in the coating properties of the paint. Thus the paint loses its ability to cover any surface to which it is applied, has a greater tendency to drip or run and can undergo irreversible pigment sedimentation during storage.

It is already known to make cellulose ethers with improved bioresistance by carefully preparing it under rigorously clean conditions and by subsequently adding a biocide such as a mercury compound and this is done commercially. However because of environmental considerations, it is becoming more desirable to produce bioresistant materials which avoid or at least reduce the use of toxic biocides. Cellulose ethers are usually prepared from a cellulose source, e.g. wood cellulose or cotton cellulose, by a process which consists of (1) reacting the cellulose with aqueous alkali, e.g. sodium hydroxide, in a solvent to prepare an alkali cellulose intermediate and then (2) treating the alkali cellulose with an etherifying agent such as an alkylene oxide to produce a water soluble hydroxy functionalised cellulose ether.

A characteristic feature of some cellulose ethers is their Molar Substitution or MS value. The MS value defines the number of molecules of alkylene oxide which have reacted with each anhydro-glucose unit in the cellulose. In general terms the higher the MS value the better the solution properties of the cellulose ether although this had to be balanced against the cost of using larger amounts of the alkylene oxide.

It is known, for example from US 4,084,060 and references cited therein, that improvements In the bioresistancy of cellulose ethers produced by such routes can be achieved if the process is carried out in two stages. These two stage processes involve first making the alkali cellulose and etherifying to an intermediate MS level and second, reacting the intermediate product, after suitable purification or neutralisation, with further alkali and further etherifying agent to produce a bioresistant product having solution properties suitable for industrial application. When the cellulose ether is a hydroxy ethyl cellulose (HEC) the prior art teaches that the intermediate should have an MS value in the range 0.6 to 1.3 and that the final product should have an MS value in excess of 3.6 in order to produce a product having good bioresistance and good solution properties.

A two stage process has now been devised which produces a cellulose ether having good bioresistance and good solution properties at MS values of less than 3.6 and will produce a product having such properties at an MS value in the range 1.6 to 3.0. The process involves adding a boron containing compound in the first stage of the process.

By carrying out a two stage process using a boron containing compound in the first stage only It Is found that the bioreststancy properties of the final product at an equivalent MS value are superior to a two stage process not using boron; a one stage process using boron as described in EP 41364; or a conventional one stage process not using a boron containing compound. Accordingly, the present invention provides a process for the production of a cellulose ether which process comprises (1) in a first stage reacting cellulose with an alkali metal hydroxide and an etherifying agent to produce an intermediate product comprising a cellulose ether having an MS value in the range 0.6 to about 1.3 and (2) in a second stage reacting the intermediate product with further or residual alkali metal hydroxide and etherifying agent, to produce a cellulose ether having an MS value in the range 1.6 to 3.0 characterised in that in the first stage a boron containing compound is added. The alkali metal hydroxide used in the process is preferably sodium hydroxide, lithium hydroxide, potassium hydroxide or a mixture thereof. It is suitably employed as an aqueous solution preferably comprising between 5 and 55% by weight alkali metal hydroxide. The etherifying agent is preferably a compound which is able to produce a hydroxyalkyl cellulose ether on reaction with the alkali cellulose. Suitably, the etherifying agent is an alkylene oxide, preferably a lower alkylene oxide such as ethylene oxide, propylene oxide or one of the butylene oxides. Most preferably the lower alkylene oxide used is ethylene oxide.

As regards the cellulose used, this can be any convenient natural source of cellulose such as purified wood, cotton linters, jute, hemp, sisal and the like.

Although In principle any boron containing compound which is soluble in the first stage reaction medium can be used, it is preferable to use boric acid, a borate salt, for example borax, or an oxide of boron or a lower alkyl ester of boric acid e.g. trimethyl borate. It will be appreciated by those skilled in the art that in most cases the boron containing compound will be converted in the first stage reaction medium into the corresponding borate salt. Hence usually the cheapest source of boron will be used by those operating such a process.

The boron containing compound may be added either directly to the first stage reaction medium, or it may be introduced with one or more of the other reactants. For example In some cases it is convenient to dissolve the boron containing compound in the aqueous solution of alkali metal hydroxide and introduce it in this way. The boron containing compound is suitably added so that the weight ratio of the boron containing compound to cellulose In the first stage is In the range 0.002 to 0.3. The preferred range is 0.02 to 0.2.

The first stage of the process can be carried out for example as described in GB 688,486 with the exception that the boron containing compound is added. The reaction mixture can take the form of a paste or a slurry depending on the solvent to cellulose ratio. If a solvent is used it is preferably a lower aliphatic alcohol such as isopropanol or tertiary butanol. The first stage is preferably operated at a temperature in the range 15 to 100°C. During the first stage of the process, the cellulose is converted into an intermediate cellulose ether having an MS value in the range 0.6 to 1.3 via an alkali cellulose. After completion of the first stage, the reaction mixture can either be neutralised (partially or completely) with acetic acid and then fed to the second stage or it can be purified. This purification can be achieved by, for example, neutralising the first stage product with acetic acid and subsequently washing it with a polar solvent to extract the boron.

During the second stage, the first stage product is contacted with further or residual alkali metal hydroxide, further alkylene oxide and solvent, to produce a cellulose ther having an MS value in the range 1.6 to 3.0 preferably 1.8 to 2.6. The conditions used are similar to those used for the first stage but preferably with a lower alkali: cellulose ratio (0.05-0.15). At the end of the second stage, the final cellulose ether product is separated from the reaction mixture and purified. The invention is illustrated by the following examples.

Bioresistancy Tests

In order to evaluate the bioresistance of cellulose ether samples a 1% solution in water of each cellulose ether was made up and innoculated with lppm of the enzyme cellulase (ex Merck No. 2312 20mIU/mg or Fluka No 22180-45mIU/mg) . The viscosity of the solution was measured at time t=0 and at time t=1hr to determine the percentage viscosity retention. All tests were conducted at 25°C and pH 5.8 Example 1

A bioresistant hydroxyethyl cellulose was prepared using a two step process under slurry conditions, (a) Stage 1

65g (dry weight) of wood cellulose was introduced into a 2 litre stirred, laboratory bench reactor together with 975g of an isopropanol/water mixture containing 12.2% by weight water. The reactor was evacuated and flushed with nitrogen, and then 108.8g of aqueous sodium hydroxide (22.1% NaOH by weight) was added. Prior to addition, 11. lg of borax was dissolved in the aqueous sodium hydroxide. The mixture was agitated at a temperature below 25°C.

After 30 minutes, 39.0 of ethylene oxide, calculated as giving an MS level of 1.1 was added. The reactor temperature was increased to a final temperature of 80°C at a rate of 0.75°C per minute and held at this temperature for a further 30 minutes. At the end of this time, the mixture was neutralised with 39.7 of acetic acid (100%), and then cooled to 25°C. The crude intermediate HEC was washed with a further 3 litres of the isopropanol/water mixture. The MS value of the intermediate HEC was measured as being 1.02. (b) Stage 2

50g of the intermediate HEC (as a dry cake) and 750g of fresh isopropanol/water mixture were introduced into a clean laboratory reactor. After evacuation and nitrogen flushing 18. lg of aqueous sodium hydroxide (22.1% NaOH), without added sodium tetraborate, was added and the mixture again agitated for 30 minutes below 25°C. added and the reaction continued as in stage 1. After 30 minutes at 80°C the mixture was neutralised with 6.6g of acetic acid (100%) cooled and washed with 3 litres of isopropanol/water. The HEC product was finished by treatment with about one litre of isopropanol/water containing 3g of glyoxal and 0.6g of acetic acid. The product was dried at 60°C for 40 minutes. Properties of HEC Produced

MS value = 2.17 % viscosity retention (1 hour) = 72

Comparative Test A

Example 1 was repeated except that the sodium tetraborate was omitted from the first stage. Properties of HEC Produced MS value = 2.00

% viscosity retention (1 hour) = 52 The MS value of the intermediate HEC In this experiment was 0.92.

Comparative Test B Example 1 was repeated except that the second stage was omitted and that the amount of ethylene oxide used in the first stage was doubled. The product HEC obtained after this one step process using sodium tetraborate had the following properties:

MS value = 2.02 % viscosity retention (1 hour) = 36

Comparative Test C

Comparative Test 3 was repeated except that the sodium tetraborate was omitted from the test. Properties of HEC Product MS value = 2.00

% viscosity retention (1 hour) = 30

Claims

1. A process for the production of a cellulose ether which process comprises:

(1) in a first stage reacting cellulose with an alkali metal hydroxide and an etherifying agent to produce an intermediate product comprising a cellulose ether having an MS valve in the range 0.6 to about 1.3

(2) in a second stage reacting the intermediate product with further of residual alkali metal hydroxide and etherifying agent to produce a cellulose ether having an MS valve in the range 1.6 to 3.0 characterised in that in the first stage a boron containing compound is added.

2. A process as claimed in claim 1 wherein the etherifying agent is selected from the group consisting of ethylene oxide, propylene oxide and butylene oxide.

3. A process as claimed in claim 1 wherein the boron is removed after the first stage.

4. A process as claimed in claim 1 wherein the cellulose ether has an MS valve in the range 1.8 to 2.6

5. A process as claimed in claim 1 in which the first stage is carried out in the presence of a solvent selected from the group consisting of isopropanol, tertiary butanol and mixtures thereof.