Problemas jurídicos y administrativos

addition of a methyl group adjacent to the ammonium group. 59

An extensive study of complexation by macrocyclic multidentate compounds showed that of over 50 such

compounds 21-crown-7 gave the strongest complexation. Although the cavity in such a compound appears to be rather too large for the diazonium ion, a degree of

flexibility may be assumed to be present in rings of this size. It was found that complexation is promoted by

electron-donating substituents in aromatic rings forming part of the macrocycle.

Electronic factors were found to have a considerable effect on the binding of 18-crown-6 ether to

arenediazonium salts. As para substituents in the

benzenediazonium ion become increasingly electron donating the stability of complexes with 18-crown-6 progressively declines. Indeed complex stability is almost completely

lost when the para substituent is N(CH3 ) 2 or N(C2H5)2. This

was presumed to be due to the delocalisation of positive charge away from the diazonium group (as discussed in section 1.8.2). Thus the compounds with the more effective positive charge on the nitrogen atoms of the diazonium ion form the stronger complexes. This has implications for the effectiveness of this kind of complexation for the

stabilisation of diazodiphenylamine salts as used in

lithographic systems, as they contain a substituted amino group para to the diazonium ion.

61

A particularly interesting study by Bartsch and Juri has cast doubt on the original "host"-"guest" relationship

in that similar stabilisation of diazonium ions has been observed by open, none-ring polyethylene glycol dimethyl

ethers. These ethers have structures Me(OCH2CH2)nOMe, and

are commonly known as glymes. In diglyme, n = 2, and in triglyme n = 3, etc. Kinetics of thermolysis of p-tert- butylbenzenediazonium tetrafluoroborate in dichloroethane at 50 °C were followed by UV spectroscopy, and the effects of adding in turn ten equivalents of each of a series of glymes in which n = 2, 3, 4, 5, 6 , 7, 8 , 9 or 10 were

observed. Little change was observed until n = 5 when the decomposition rate constant was observed to fall, showing increased stabilisation. For the next value in the series the rate constant decreased again until at n = 7 the rate rose sharply and then fell again.

An explanation of these values was obtained from the study of models which showed that the structures

approached the form of a ring and that in heptaglyme all eight oxygen atoms lie in a plane without significant van der Waals repulsion or angular strain. In octaglyme,

repulsion and strain exist in any conformation allowing nine oxygen atoms to lie on a plane within a pseudo-cyclic cavity. In nonaglyme and decaglyme, a pseudo-cyclic cavity is formed by eight of nine oxygen atoms and the others stand in an arm passing over one face of the cavity. In order to assess the effect of glymes with even larger molecules, experiments were made with the dimethyl ether

of Carbowax 1000, a polymer with structure

MeO (CH2CH20 ) 2 2-2 3Me/ which gave a decomposition rate constant

about half that of decaglyme. Although incorporation of 18-crown^ether was even better at promoting stability, the results on ethers of Carbowax are of interest because of their potentially lower cost.

1.9 INVESTIGATIVE STRATEGY

The importance of diazo compounds to the operation of positive and negative lithographic printing plates has been explained above. In order to improve the lifetime of printing plates it is first necessary to investigate the processes that occur during plate and plate coating aging. Once the major processes and chemical reactions that give rise to the observed deterioration in performance have been identified, these can be examined in further detail.

Given that lithographic printing plates and the coatings from which they are manufactured contain a complex matrix of chemicals, the initial investigations concentrated on simple one or two component systems. These were examined after they had decomposed at various

elevated temperatures. The experiments were designed so as to provide sufficient data to be able to elucidate the reaction mechanisms that lead to the production of the decomposition products observed.

It was intended to begin with simple monomeric diazo compounds in simple matrices and then to extend the work to an understanding of the behaviour of monomeric and polymeric diazo formaldehyde condensates in full coating systems.

Once an understanding of the reaction mechanisms had been obtained, the work was to be directed towards the overall goal of extending the lifetime of the coating during plate manufacture and of the printing plate produced. It is of course equally important that any changes in coating formulation do not detract from the photoreactivity of the coating system. To this end the

effect of photochemical as well as thermal decomposition was investigated.

The investigative strategy can be summarised as follows:

1. Examine plates and coatings that contain monomeric diazo compounds for factors influencing the rate of decomposition.

2. Analyse plates and plate coatings for degradation products formed under controlled conditions.

3. Investigate the effects of heat on diazo compounds in isolation.

4. Extend this treatment to thermal decomposition in

simple matrices of compounds found in lithographic plates and coating solutions.

5. Using a range of diazo compounds develop mechanistic guidelines for the processes involved.

6 . Based on the information gathered, attempt to improve

the thermal stability of the diazo system by use of certain coating additives and elimination of commonly encountered impurities.

7. Extend the scope of the investigation to photochemical decomposition in lithographic plate systems and to

polymeric diazo compounds of the type used in negative working plates.

2 RESULTS AND DISCUSSION

2.1 INITIAL INVESTIGATIONS

Monomeric diazo compounds are used in positive working plates. Plates and plate coatings of this type were therefore chosen for initial investigation. A simple plate formulation consisting of a cresol novolak, diazo ester, colour change dye and acid release diazonium salt was used.

The complete formulation was :

Cresol novolak 75% w/w

Diazo ester 20% w/w

Colour change dye 4% w/w

Diazonium salt 1% w/w

Cresol novolak is a cresol formaldehyde condensate as

shown earlier in structure (6 ). The resin used in this

work is a commercially available grade from Bakelite, Germany.

Some printing plates incorporate the quinone diazide as an ester of the novolak polymer. Preparing the ester using a diazo acid chloride will preferentially react with the monomers in the novolak polymer. This has the

advantage of removing residual free cresols left over from the condensation reaction. This is preferable in

commercial plates as the presence of cresols can be detected as a characteristic smell during plate

processing. Such odours represent only low concentrations of cresols but are perceived as an avoidable health hazard by print customers.

For this work the added complication of analysing the range of polymer decomposition products that would be

formed is a major drawback, hence the simple mixture rather than novolak ester has been used.

The diazo ester is a 1,2-naphthoquinone diazide-5-

sulphonic acid ester of 2 ,4-dihydroxybenzophenone with

the structure given below (16) . The ester is primarily the bis ester but a small amount (about 15%) of the mono

ester is also present depending on the relative amounts of reactants used. 0 SO

₂