This invention is concerned with improvements in and relating to polyamides and, more particularly, is concerned with polyamides suitable for use as resinous vehicles in printing inks, especially flexographic printing inks containing such polyamides.

Thermoplastic polyamides derived, inter alia, from so-called "dimer acids" have been used as components of printing inks for some years.

In particular such polyamides may be used as components of flexographic inks and may be formulated to meet the requirements of such inks, namely good adhesion to a wide variety of substrates, rapid drying, the use of solvents which will not attack rubber stereos, low print odour and good resistance properties. The polyamides may be used alone in flexographic inks or in combination with other resinous materials, especially nitrocellulose. In particular, thermoplastic polyamides of the sort described above may be used in flexographic inks used in the packaging industry, for printing on polyethylene and other extruded film. In this application, the polyamide may be used to impart good adhesion and flexibility in combination with nitrocellulose which will impart good gloss and hardness to the final ink.

Polyamide resins are also used in gravure inks for printing treated polyolefin, nylon and polyesters films. Their main advantages are that they show excellent pigment wetting properties, high gloss, excellent water and deep freeze resistance. The suitability of resins for use in overprint varnishes (high gloss lacquers on various packaging designs) largely depends on the skill of the formulator. All these resins make extremely good starting point materials due to their high gloss, toughness and flexibility, combined with good grease and water resistance.

Whilst commercially available polyamides have generally acceptable properties, these could advantageously be improved by increased solubility in alcohol (ethyl alcohol or industrial methylated spirits) and resistance to gelation in alcohol. GB-A- 1.055.676 discloses a polyamide, suitable as an adhesive, being a condensation product of a polymeric fatty acid having a

dimeric fatty acid content of at least 90%, and a diamine mixture comprising a major molar amount of ethyl ene diamine and a minor molar amount of an aliphatic diamine such as 2,2,4- trimethylhexamethylene diamine. GB-A- 1.052.575 discloses a reaction product, useful in the formulation of printing inks, of 1,2-propane diamine (and possibly ethylene diamine), a polymeric fatty acid and an acid selected from acetic, lactic, glycolic and formic acid.

It has now been found, in accordance with the present invention, that such polyamides having improved alcohol solubility and gelation characteristics may be prepared by including, in the diamine component, selected straight chain and branched chain diamines and by including a monocarboxylic acid as a chain terminating agent. According to one embodiment of the invention, therefore, there is provided a thermoplastic polyamide derived from the polycondensation of :

(i) a polybasic acid component comprising a polymeric fatty acid, and optionally, one or more other polybasic acids which may be aliphatic or aromatic in character,

(ii) a polyamino component comprising a mixture of at least a straight-chain lower diamine and at least a branched chain diamine, and (iii) at least a monocarboxylic acid. The polybasic acid component (i) preferably comprises only the polymeric fatty acid. The term polymeric fatty acid, as used herein, is intended to refer to the product of the polymerisation of ethylenically unsaturated monocarboxylic acids containing from about 8 to about 22 carbon atoms. These products may be obtained by the process described in US-A-3157681 and may be produced by a catalytic or non-catalytic polymerisation process. Generally unsaturated aliphatic monocarboxylic acids containing from 16 to 18 carbon atoms are preferred as starting materials and the most preferred starting materials are unsaturated aliphatic monocarboxylic acids containing 18 carbon atoms such as linoleic and oleic acids. After polymerisation in the presence or absence of a

catalyst, the resulting mixture contains predominantly dimeric fat acids, some trimeric and higher polymeric fat acids and some unpolymerised monomeric fat acids. Typically such polymer acids contain from about 5 to about 15% by weight of monocarboxylic acids, from about 60 to about 80% by weight of dicarboxylic acids and from about 10 to about 35% by weight of higher carboxylic acids. In view of the preponderance of the dimeric species, such polymeric fatty acids are frequently referred to as "dimer acid" and this terminology will be used in the following description. These mixtures may be fractionated by high vacuum distillation or solvent extraction to obtain dimer acid cuts of higher concentration if desired.

The diamino component (ii) preferably consists of diprimary diamines and, especially, of a mixture of at least a straight chain lower diamine selected from ethylene diamine and 1 ,3-propane diamine together with at least a long chain (i.e. C ₆ -C ₁₂ ) branched chain diamine. In such a mixture, the lower straight chain diamine suitably forms from about 5 to about 95 mole % of the diamino component, preferably from about 50 to about 70% thereof, the branched chain diamine forming the balance.

The third component (iii) from which the polyamide is formed is, as noted above, a monocarboxylic acid, especially an aliphatic monocarboxylic acid containing from 2 to about 18 carbon atoms, most preferably from 2 to 4 carbon atoms such as acetic, propionic and isobutyric acids. It acts, of course, as a chain stopper and the amount thereof relative to the dimer acid will effectively control the molecular weight of the final polyamide. Typically, the monomeric fatty acids suitably provides from about 10 to about 70 mole % of the total carboxyl functionality of the mixture, preferably from about 45 to about 65 mole % thereof. In general, it is preferred to use acid and amine components in substantially stoichiometric amounts.

The polyamides of the invention are produced in a conventional manner, i.e. by polycondensing the acid and amine components at elevated temperature, e.g. from about 80 to about 230°C, whilst distilling off water evolved during the condensation reaction. Reaction is suitably carried out until the reaction mixture

has a maximum amine value of about 4 mgKOH/g and a maximum acid value of about 4 mgKOH/g.

The resultant polyamides may be used in printing inks, especially flexographic inks to give products having good adhesion, scratch resistance, water resistance and deep freeze resistance whilst, at the same time, having improved solution properties as indicated by a lower gel temperature and a lower gel recovery temperature.

Accordingly, another embodiment of the invention provides an ink or varnish comprising a polyamide as defined above dissolved in a solvent (typically an alcoholic solvent or an ester solvent such as an acetate) and also containing in the case of an ink, a colorant such as a pigment or dyestuff. Typically such an ink or varnish may also contain other resinous material especially nitrocellulose and, in this latter instance, the weight proportion of polyamide to nitrocellulose is suitably from about 0.1 : 1 to about 20: 1.

EXAMPLES 1 - 6

Polyamides were prepared, following the process described above, from the components listed in Table 1 hereinafter.

O ω ro ro en o n o Oi ϋl

TABLE 1

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6

Dimer acid 707.34 696.36 690. 1 1 579.1 1 699.06 663.65

Propionic acid 1 19.20

Acetic acid 95.13 79.12

Isobutyric acid 88.72 140. 12 133.08

Phosphoric acid 0.07 0.07 0.07 0.07 0.07 0.07

Antifoam 0.04 0.04 0.04 0.04 0.04 0.04

2,2-dimethyl- 99.91 180.08 pentamethylene diamine

2,2,4-trimethyl 1 19.55 327.22 hexamethylene diamine

2-methylpenta-methylene 88.1 1 diamine

2-butyl-2-ethylpenta- 1 ,5- 134. 1 1 diamine

Ethylene diamine 71.44 12.44 70.60 67.08

1 ,3-propane diamine 86.85 38.98

Topanol CA 2.00 2.00 2.00 2.00 2.00 2.00

1000.00 1000.00 1 1000.00 1000.00 1 1000.00 1000.00

Typical formulations for inks containing the above polyamides are as follows : a) White Flexographic ink

% Weight

Titanium Dioxide 37.00

Nitrocellulose 4.50

Polyamide 14.00

Crayvallac 62 2.00 n-Propanol 14.40

Industrial Methylated Spirit 17.60

Ethyl acetate 10.50

100.00 b) Black Flexographic ink

% Weight

Pigment Black No.7 13.20

Alcohol soluble Nitrocellulose 10.30

Polyamide 16.90

Crayvallac 62 0.80 n-Propanol 18.50

Industrial Methylated Spirit 21.20

Ethyl acetate 13.20

Plastisiser 5.90

100.00

* Dispercel Carbon black F-A - Runneymede Dispersions Ltd c) White Gravure ink

% Weight

Titanium Dioxide 34.00

Nitrocellulose 6.00

Polyamide 12.80

Crayvallac 62 1.60 n-Propanol 13.00

Industrial Methylated Spirit 20.40

Ethyl acetate 12.20

100.00

The polyamides of Examples 5 and 6 were incorporated into Cyan, Magenta and White ethanolic flexographic inks. By way of comparaison, a conventional polyamide was incorporated in the same inks. The gelation characteristics of the inks were evaluated to give the following Gel temperature /recovery values (°C).

Ink Polyamide

Conventional Example 5 Example 6

Cyan -8/-6 -12/- 10 -11 /-9

White -8/-7 -11/-10 -11/-10

Magenta -10/-8 - 12/-11 -12/- 11