Insect Repellent

1. Field of the Invention

This invention relates to insect repellents, and in particular to an insect repellent in which the durability of N,N-diethyltoluamide as a repellent ingredient is increased.

2. Description of the Prior Art

Most mosquitoes among various vectors inhabit warm or torrid zones, but could climatically inhabit a cold district. The mosquitoes attack the exposed skin surface during field work, and often cause infectious diseases and endemic diseases.

In order to repel such insect, many studies on insect repellents have been made and, as a result, a variety of compounds possessing a repellent effect have been found. Among them, N,N-diethyltoluamide is the most suitable for practical use in human beings and finds wide usage at present in view of the durability after application and the strength of effect.

However, N,N-diethyltoluamide has the defect in that the compound is absorbed from the skin into the human body when it is applied to the body surface, and consequently, loses its repellency. N,N-diethyltoluamide becomes more durable when employed in increased amounts, but use of such large amounts gives a sticky feeling to the body and is not practically acceptable. As one attempt to inhibit the absorption from the skin into the body, a method has been reported which comprises forming on the skin a film insoluble in N,N-diethyltoluamide and subsequently coating N,N-diethyltoluamide on the film. In this instance, no satisfactory results are obtainable because the film adheres poorly to the skin.

In order to surmount the above noted difficulties, the present inventors have made various studies, with their attention directed to the fact that N,N-diethyltoluamide has a carbonyl group which can act as a proton acceptor and that the energy of a hydrogen bond formed with the carbonyl group is far smaller than that a covalent bond. As a result of the studies, they have found that the repellent effect of N,N-diethyltoluamide is not decreased in the least and that the absorption from the skin into the body is significantly inhibited by forming a complex with a proton donor.

Based on this finding, the present invention has been attained.

The accompanying drawing shows the relationship between the amount of digallic acid and the residual ratio of N,N-diethyltoluamide.

Suitable proton donors according to this invention are aromatic compounds in which the aromatic rings are substituted directly by hydroxyl and/or carboxyl groups, which proton donors have the formulae (I) and (II), ##STR1## wherein R represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted aromatic ring, a halogen atom, --NH.sub.2, --SO.sub.3 H, --COOM, --OM (M is an alkali metal) or --CHO. R' represents R"--COO--, R"--O--, R"--OOC--, R"--OCNH--, R"NHCO-- or R"NH-- (R" is a substituted or unsubstituted alkyl group or a substituted or unsubstituted aromatic ring); k, l, m and n are integers from 0 to 6, respectively, wherein m+n.gtoreq.1; and p, q, r and s are integers from 0 to 8, respectively, wherein R+s.gtoreq.1.

These proton donors may be further divided as follows:

(1) The compounds in which the monocyclic aromatic compounds are substituted by hydroxy groups [n=0 in the formula (I)]: ##STR2##

(2) The compounds in which the monocyclic aromatic compounds are substituted by carboxyl groups [m=0 in the formula (I)]: ##STR3##

(3) The compounds in which the monocyclic aromatic compounds are substituted by hydroxyl and carboxyl groups [both m and n are 1 or more in the formula (I)]: ##STR4##

(4) The compounds in which the polycyclic aromatic compounds are substituted by hydroxyl groups [s=0 in the formula (II)]: ##STR5##

(5) The compounds in which the polycyclic aromatic compounds are substituted by carboxyl groups [r=0 in the formula (II)]: ##STR6##

(6) The compounds in which the polycyclic aromatic compounds are substituted by hydroxyl and carboxyl groups [both r and s are 1 or more in the formula (II)]: ##STR7##

In order to produce a pest repellent according to the invention, N,N-diethyltoluamide may be mixed with any one of the above proton donors. Preferably, the mixing ratio of both compounds is of such a value that the number of N,N-diethyltoluamide molecules is the same as that of the proton donor groups. In practice, the weight in grams of the proton donor for a given weight of N,N-diethyltoluamide used is 0.5 to 2.0 times the value of Q which is calculated from the following equation: ##EQU1##

The pest repellent according to the invention can be used in the generally accepted forms such as a solution, lotion, cream, gel, stick, paste, aerosol spray, foam and powder. The content of N,N-diethyltoluamide should be in the range of 1 to 50% by weight, preferably 2 to 30% by weight.

Having generally described the invention, a more complete understanding can be obtained by reference to certain specific examples, which are included for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Hair was clipped out of the trunks of three hartley strain quinea-pigs, female, weighing 500 to 600 g, to which area the test compositions were applied in an amount of 0.2 mg N,N-diethyltoluamide per/cm.sup.2. At 3 hours after application, N,N-diethyltoluamide was recovered from the skin surface with acetone and the residual amount was determined by gas chromatography.

The results obtained are shown in Table 1. The recovery ratio in the Table indicates the weight percent of N,N-diethyltoluamide recovered at 3 hours after application relative to that recovered immediately after application.

Using Composition B containing digallic acid as a proton donor, the residual ratio of N,N-diethyltoluamide at 3 hours after application was determined in the same manner as in Example 1. The results obtained are shown in the accompanying drawing.

Guinea-pigs, treated with Composition C and the Control Composition of Example 1, in the same manner as in Example 1, were fixed on a holder and placed in a cage in which 30 mosquitoes were released. The number of the mosquitoes that sucked blood within 3 minutes was counted. The results obtained are shown in Table 2.

The proton donors shown in Table 3 were combined with N,N-diethyltoluamide and the residual ratio of N,N-diethyltoluamide (DET) at 3 hours after application was determined in the same manner as in Example 1. The results obtained are shown in Table 3.