The present invention relates to blocked water-soluble isocyanate compounds.

Isocyanates are of particular value in the treatment of plant fibres used as reinforcing materials in synthetic resin composites in that the isocyanates react with the fibre surface in a manner which improves the water resistance of the bond formed between the fibre and the synthetic resin matrix. Moreover the level of modification of the fibre surface can be controlled so as to enable plant fibres to perform as effectively as man-made fibres w hen used as reinforcing materials. It is advantageous, in the early stages of such a treatment, to be able to handle the isocyanate in a water soluble form in which the isocyanate group is blocked with a protecting group and then to remove the protecting group when reaction with the fibre is to take place. However, existing blocked isocyanates require relativeh high temperatures to remove the blocking group. Furthermore there is the disadvantage that the preparation and handling of such blocked isocyanates involves use of toxic solvents.

Thus, for example. Guise (Λust. .1. Chem.. 25:2583-2595( 1972)) has described the reaction of isocyanates with sodium or potassium metabisulphite to form blocked isocyanates in aqueous 1.4-dioxane alone or in admixture with other solvents. The preparation of a blocked form of 1.6-hexamethylene diisocyanate ( HDI) is described and this compound requires a temperature of 167"C lo enable the isocyanate groups to be unblocked.

According to the invention, therefore, there is provided a method of forming bisulphite adducts of aliphatic isocyanates bearing an aromatic moiety by mixing an aqueous solution of a bisulphite adduct forming agent with a solution of an isocyanate bearing an aromatic moiety in a substituted pyrrolidinone containing a catalytic amount of a substance which catalyses the bisulphite adduct forming reaction, the

isocyanate being kept well dispersed so that the adduct is formed as a solution in the aqueous phase.

We have found, and this forms the basis of the present invention, that water- soluble blocked aliphatic isocyanates bearing an aromatic moiety may be formed in good yields by reacting aqueous solutions of a adduct forming agent bisulphite with isocyanates dissolved in a substituted pyrrolidinone in the presence of a catalyst such as di-butyl tin chloride. This process avoids the u.se of 1 ,4-dioxane which is difficult to use on a large scale because of the precautions required to protect those carrying out reactions with such a solvent.

The blocked compounds as produced by the above method can be deblocked at lower temperatures than blocked HDI.

The solvent is most preferably an N-substituted pyrrolidinone, most preferably a N-alkyl substituted pyrrolidone. particularly N-alkyl substituted 2-pyrrolidinone. The most preferred solvent for use in the invention is l -methyl-2-pyrrolidinone (also known as N-methyl pyrrolidinone).

The bisulphite adduct forming agent may for example be a sulphite, hydrogen sulphite or metabisulphite. most preferably in the form of an alkali metal salt. The preferred alkali metals are sodium and potassium. It is preferred that the bisulphite adduct forming agent is sodium metabisulphite or potassium metabisulphite.

The catalyst may for example be an organotin compound, e.g. a dialkyl tin halide. Preferably the catalyst is dibutyl tin dichloride. Other catalysts which may be used include l,4-Diazobicyclo[2.2.2 |octane (DABCO) as well as secondary and tertiary amines.

It is preferred that the reaction be conducted in the presence of triethanolamine which, we have found, improves the yield of the bisulphite addition product, possibly by acting as a co-solvent to introduce the isocyanate to the aqueous phase of the mixture. Other co-solvents may be used for this purpose.

The proportions of isocyanate and bisulphite adduct forming agent used for the reaction are preferably such that the ratio of the number of moles of potential sulphite groups present to isocyanate groups is in the range ( 1 -2): 1 (i.e. the number of moles of potential sulphite groups used are equivalent to the number of moles of isocyanate groups or exceed the latter by no more than a twofold excess).

The proportions of water and substituted pyrrolidinone should be such that forced precipitation of the metabisulphite does not occur. T iethanolamine (or other amines found to have catalytic action) may be present in a proportion up lo 20% by volume of the reaction mixture.

The catalyst is preferably used in an amount up to 0.1% by weight of the total reaction mixture.

The amount of catalyst to be used should be such that the yields of the blocked compound are not substantially reduced by virtue of the isocyanate reacting with water present in preference to the metabisulphite. The amount of catalyst to be used in any particular circumstances can be easily determined by routine experimentation so as to secure a satisfactory yield.

The reaction is preferably carried out at a temperature not exceeding 15"C. Satisfactory yields are obtained when a temperature of about 5°C is used. Lower temperatures can be used but without any particular advantage and reaction time can be longer.

To carry out the method of the invention, the isocyanate is maintained well dispersed to ensure that the bisulphite adduct is formed as a solution in the aqueous phase. Any conventional method of stirring may be used lo achieve this purpose.

The isocyanates to be treated by the method of the invention comprise an aliphatic moiety to which the isocyanate group is bonded and further comprise at least one aromatic moiety. The aliphatic group to which the isocyanate group is bonded may, for example, be a -CMe ₂ - group which is also bonded lo an aromatic nucleus.

The method of the invention is particularly applicable lo the production of water-soluble forms of m-tetra-rnethylxylene di-isocyanatc (TMXDI) and of alpha- alpha-dimethyl meta-isopropenyl benzyl isocyanate (TMI), both of which can be used in the treatment of plant fibres such as jute. flax, hemp and linseed to improve fibre performance in composite materials. The resin of such a composite material may for example be a phenol-formaldehyde resin or a urea (or urea-melamine) formaldehyde resin.

The present invention also includes blocked isocyanates incorporating the groups of formula I or II:

Formula I Formula II

Preferably such blocked isocyanate compounds are alkali metal salts more preferably the sodium or potassium salts.

The blocked isocyanates of the present invention may be deblocked to free the isocyanate group or groups to react with plant fibres by simply heating to 130 degrees Celsius in the presence of the fibre. In the case of a pre-formed mat of plant fibres such as jute, one method of treatment consists of soaking the mat in a 25% w/w aqueous solution of adduct. The excess liquid is allowed to drain off and the treated mat dried to a moisture content of about 12% al about 70 degrees Celsius and the mat is then heated while being pressed in a press to a temperature where the blocking group is removed and the isocyanate is free to react with the fibre surface.

The following Examples illustrates but do not limit the invention:

Example 1

The compound shown in formula I (blocked TMXDI) was prepared in the following manner: Sodium metabisulphite (190g) was dissolved in 530g of water. The clear solution obtained was placed in a flask provided with a stirrer and cooling means.

l-Methyl-2-pyrroIidinone (200g) was added to the solution along with 120g of m tetramethylxylene di-isocyanate. 3.5g of triethanolamine and O. l g of dibutyl tin dichloride. The mixture was maintained at 5 degrees Celsius with continued stirring for seven days at which time the reaction mixture had become one aqueous suspension. After filtering this mixture, the residue contained the desired blocked isocyanate and the filtrate, after evaporating and adding excess acetone yielded a further crop of blocked isocyanate. The combined yield was 1 80g of the sodium salt of the blocked isocyanate.

Example 2

The compound shown in formula II (blocked TMI) w as prepared in the following manner: Sodium metabisulphite ( 1 g) was dissolved in water (53g) lo give a clear solution. To this was added l -methyl-2-pyrrolidinone (20g). dimethyl-m- isopropenyl benzyl isocyanate (TMI. 9.4g). triethanolamine (3.0g) and O. l dibutyl tin dichloride. The mixture was stirred vigorously for seven days al 5"C alter which the mixture consisted of a cloudy suspension. The product was separated from the cloudy suspension using the same steps as in example 1 , and l Og of blocked dimethyl-m- isopropenyl benzyl isocyanate of the formula II were obtained.