Brief description ofthe invention

The invention is relating to photo initiators for radiation curing, such as UV curing, comprising liquid rnixtures of benzene derivatives prepared from chemical mixtures comprising technical amyl alcohols.

Background to the invention

UV curing is regarded as a modern, cost efficient and environmental-friendly way of "drying" coatings, lacquers, adhesives, printing inks, resists, etc. Unlike conventional industrial curing using heat and organic solvents, no heat is necessary and there are no harmful organic solvents evaporating during the UV curing. In UV curable formulations, polymerizable monomers, oligomers, and binders are together making up the liquid matrix. When exposed to UV light this liquid matrix is cured, i.e. photochemically converted into solid polymer. In order to perform UV curing, the UV light has to be absorbed by photochemically active centers which need to be present in the formulations. By using so called photo initiators, UV curing takes place within a fraction of a second.

UV curing of acrylates by free radical chemistry is the most frequently used UV curing technology. Free radicals are generated in UV light by photo initiators. These free radicals are attached to acrylic unsaturations in the UV curable formulations giving fast polymerizations, i.e. UV curing.

One major limiting factor for the use of UV curing is the high costs of some ofthe components, especially the photo initiators.

Another important factor, which limit the use of UV curing, is the smell ofthe cured polymer. This smell is often derived from a) initiators not sufficiently incorporated into the cured polymer, and b) photo degradation products from the initiators.

Several leading initiators available on the market are solids. However, liquid initiators are nomally preferred by formulators due to handling advantages in the manufacturing ofthe curable formulations.

Unsubstituted benzophenone is the most frequently used photo inititator. The main reason is its comparatively low price and its curing efficiency. However, the said benzophenone has a serious drawback in respect of its smell. Products cured by benzophenone in UV light can often be recognized by the smell of residual benzophenone. This smell is a problem in food packaging. In packaging applications wherein the smell cannot be tolerated, other types of photo initiators, considerably more expensive, than benzophenone have to be used. Such a replacement ofthe cheap benzophenone with more expensive ones increases the price ofthe curing formulations.

Known as photo initiators are also methyl, methoxy, and phenoxy substituted benzophenones. Non-toxic benzophenone derivatives, used as photo initiators, are produced from comparatively expensive aromatic raw materials. The said derivatives are considerably more expensive than conventional unsubstituted benzophenone.

Unsubstituted benzophenone is prepared e.g. from benzoic acid and benzene. 4-Methyl- benzophenone is a commercially available substance analogously prepared from 4-methyl- benzoic acid. These two substances are solids at room temperature.

2,4,6-Trimethylbenzophenone is a commercially available solid substance marketed as a liquid eutectic mixture with 4-methylbenzophenone. It is prepared from 1,3,5-trimethylbenzene and benzoic acid.

The unsubstituted conventional benzophenone cannot be replaced by substituted benzophenones according to prior art as the leading photo initiator in UV curing ofthe following reasons:

The smell restricts their use in such applications where residual smell cannot be tolerated.

UV curing is not sufficiently efficient compared to the curing with unsubstituted benzophenone with respect to the price differences.

Solid benzophenones do not lead to handling cost advantages.

Michler's ketone, a very efficient benzophenone, is a suspected carcinogen.

According to DE OS 24 58 345 alkyl esters of p-dimethylaminobenzoic acids are used as photo sensibility agents and according to EP 0003872 the compound 2-(n-butoxy)ethyl 4 dimethylaminobenzoate is known as photoactivator. However, nothing is specifically disclosed in said documents about the odourless liquid mixtures according to the invention comprising amyl esters of 4-(dimethylamino)benzoic acid prepared from the commercially available, inexpensive technical amyl and isoamyl alcohols.

The preparation ofthe substituted benzophenones according to prior art is well described in the literature. For example, condensation of aldehydes and activated aromatics give diarylalkanes (U.S. 2,848,509). In a subsequent oxidation step these diarylalkanes can be converted into diarylketones (U.S. 4,086,277).

It is previously known from U.S. 3,978,143 that substituted benzophenones prepared from 1, l-bis(alkylphenyI)-2,2-dinitroethylene can be used as photo initiator and drug intermediate.

Another frequently used aromatic photo initiator is ethyl 4-(dimethylamino)benzoate (short form EDMP). However, this efficient initiator has disadvantages due to its solid state, its comparatively high price, and its bad smell characteristics. EDMP is prepared from ethanol. Other structurally related initiators are e.g. isoamyl 4-(dimethylamino)benzoate, and amyl 4- (dimethylamino)benzoate, prepared from isoamyl and amyl alcohols respectively. Said compounds are comparatively expensive for the manufacturer as they require pure alcohols as starting materials. Therefore, there is a market need for new, liquid, efficient photo initiators, low in price and having good smell characteristics.

Description ofthe invention

According to the present invention, low cost organic raw materials are used in the manufacturing of liquid, efficient photo initiators having good smell characteristics to be used in radiation curing, especially in UV curing.

In the present description and claims the term "substituted olefins" comprises such olefins as esters of acrylic and methacrylic acid. The term "radiation curing" relates mainly to UV curing but also curing in visible light may be appropriate.

"Technical amyl alcohol" refers to commercially available technical products consisting mainly of n-pentanol and methyl- 1-butanols. "Technical isoamyl alcohol" refers to commercially available technical products, mainly fusel oil from ethanol fermentation, consisting mainly of isoamyl alcohol and methyl- 1-butanols.

It has by the present invention surprisingly been shown that mixtures of benzene derivatives chemically prepared from inexpensive, impure, technical organic raw materials and selected from the group consisting of technical amyl alcohols, and technical isoamyl alcohol, are liquid and act as photo initiators in radiation curing of free radical curable systems, such as substituted olefins. The liquid mixtures are especially preferred as photo initiators in UV curing.

The invention relates, according to one embodiment, defined in claims 1 to 4, to liquid mixtures of amyl esters of 4-(dimethyIamino)benzoic acid having good smell characteristics when used as photo initiators in radiation curing, especially UV curing. Said esters are prepared from technical amyl alcohol or from technical isoamyl alcohol.

Another embodiment, defined in claims 5 to 6, relates to photo initiators comprising the liquid mixtures as defined in claims 1 to 4 and a third embodiment, defined in claim 7, to the use of technical amyl and isoamyl alcohols for the preparation of liquid amyl esters of 4- (dimethylamino)benzoic acid having good small characteristics.

We have also found that amyl alcohols selected from the group consisting of technical isoamyl alcohol and technical amyl alcohol can be used as organic low cost raw materials for the

manufacturing of amyl 4-(dimethylamino)benzoates. Products consisting of esters derived from technical isoamyl alcohol are below named LADMP and the products derived from technical amyl alcohol ADMP. The different esters present in LADMP and ADMP are derived from the different alcohols present in the alcohol raw materials as used. We have found that IADMP and ADMP have the following unknown characteristics:

IADMP and ADMP are initiators in UV curing. Their efficiency is ofthe same magnitude as EDMP.

In this application IADMP and ADMP have improved smell characteristics relative to EDMP.

IADMP and ADMP are liquid at ambient temperature in contrast to the conventionally used EDMP, which is a solid.

Technical isoamyl alcohol, viz. fusel oil, is a by-product from ethanol fermentation. It is normally discharged thus being a cost ofthe fermentation process. It has by the present invention been shown that said fusel oil is an excellent inexpensive raw material for IADMP.

To summarize, use ofthe above-mentioned raw materials gives photo initiators with superior properties compared to the presently leading initiators ofthe same categories. Furthermore, use of these low cost raw materials give room for comparatively low production costs of IADMP and ADMP.

The scope ofthe invention is as defined in the claims.

Chemical transformations

The preparation ofthe esters of 4-(dimethylarnino)benzoic acid, as shown below, is not previously known and can be described with the following formula scheme:

Step i: 0 ₂ N- -C0 ₂ H + ROH C^N- -C0 ₂ R + H ₂ O

\\ //

Step 3: + 2 hfeO

where R represents an alkyl or branched alkyl group containing 5 carbon atoms.

Step 1 is an esterification catalyzed by a strong acid like sulfuric acid or an organic sulfonic acid, such as p-toluenesulfonic or methanesulfonic acid.

Step 2 is a reduction of a nitro compound to the corresponding primary amine using hydrogen as reducing agent along with a reduction catalyst such as a nobel metal, for example palladium, on a support, or nickel, metallic or on a support.

Step 3 is a reductive alkylation using formaldehyde as alkylating reagent with use ofthe same catalysts as in step 2. Formaldehyde is added as a solution in water or in an alcohol.

These chemical transformations are in principle previously known, but have not until now been applied to the present substrates.

Examples

Example 1

(Preparation of ADMP)

In a 5 L flask were introduced 1337 g of p-nitrobenzoic acid, 920 g of technical amyl alcohol (consisting of about 70% of n-pentanol and about 30% of 2-methyl-l -butanol and 3-methyl-l- butanol), 675 g of toluene and 18 g of 96% sulfuric acid. The stirred mixture was heated to reflux and water was collected in a Dean-Stark separator with the temperature gradually rising from 127 °C. Water separation was continued under slight vacuum until no more water

separated. After cooling to 20 °C the mixture was washed with brine an the organic phase was washed with sodium hydrogen carbonate until neutral reaction.

A 75 % toluene solution ofthe p-nitrobenzoic ester prepared above was reduced in a 1 L Parr autoclave with use of hydrogen with palladium on carbon as catalyst. The reduction took place at 90 °C and 5 bars pressure. After completed reduction more catalyst was added and reductive methylation took place using a slight excess of formaline (40% formaldehyde) at a total pressure of 10 to 11 bars and 130-150°C. After completed methylation the mixture was cooled and the catalyst was separated. The water phase was discarded and the organic phase was washed with water. Toluene and other low-boiling components were removed by distillation at 120-150 °C /5-30 mmHg.

The residue was distilled twice. The main fraction distilled at 155-159 °C/0,3 mmHg. The product was called ADMP 151/27C.

Example 2

(Preparation of ADMP)

Example 1 was repeated using an excess of 4-nitrobenzoic acid. The excess was removed by alkaline extraction before reduction. Evaporation of toluene from the washed methylated solution gave the end product ADMP 151/27B.

Example 3 (Preparation of IADMP) p-Nitrobenzoic acid, 1055 g, was esterified as described in Example 1 starting from 636 g of technical isoamyl alcohol (fusel oil), 525 g of toluene and 28,5 g of p-toluenesulfonic acid monohydrate.

The temperature during the esterification rised from 86 to 117°C. More toluene was added

(700 mL) and the mixture was allowed to cool to 20 °C. The organic layer was washed with water, sodium hydrogen carbonate solution, and brine. Toluene and other low-boiling components were removed in vacuum to give crude 4-nitrobenzoic ester mixture.

This mixture was dissolved in toluene to give an approximatively 75 % solution. It was then reduced and methylated as described above in Example 1. The low-boiling components were removed by distillation at 120-150 °C/5-30 mmHg. The main fraction was collected at 159-164

°C/0,8 mmHg. This material was called IADMP 151/27 A.

Example 4

(Evaluation of ADMP and IADMP as photo initiators)

ADMP and IADMP from examples 1, 2, and 3 were compared with each other and with

EDMP.

The photo initiators were evaluated in a blue pigmented UV curable printing ink formulation. 2 grams m of each formulation was applied on a corona treated white polyethylene film. The

UV curing was performed with a 120 W/cm Fusion UV lamp at two different speeds (80 and

120 m/min). The cured inks were evaluated with regard to ethanol rubs, surface dryness, scratch resistance, tape resistance, and smell.

The blue pigmented inks were formulated accordingly: 32 % polyester acrylate 30 % tetrafunctional acrylate 10 % amino modified polyetheracrylate 0,5 % stabilizer

4 % photo initiator (Irgacure 184) 1,5 % photo initiator (ITX, isopropylthioxanthone) 16 % blue pigment 1,5 % polyethylene wax 2 % filler

2,5% of total initiator to be tested (ADMP, IADMP or EDMP) The test results of 4-(dimethylamino)benzoic esters are collected in Table 1.

Table 1 . Test results of 4-(dimethylamino)benzoic esters.

sample raw material preparation properties uv curing surfac scratch ethano tape smell mode e I speed drynes resistan rubs resistanc characteristic m/min s ce e s

IADMP technical isoamyl alchohol (fusel oil) distilled liquid at room 80 4 3 3 5 3 151/27 A temperature

ADMP 151/27B technical amyl alcohol (pentanol isomer not distilled liquid at -24°C 80 5 3 3 5 3 mixture)

ADMP 151 /27C technical amyl alcohol (pentanol isomer distilled liquid at -24°C 80 5 4 3 5 4 mixture)

EDMP solid, melting point 80 3 4 3 5 2 65°C

IADMP technical isoamyl alchohol (fusel oil) distilled liquid at room 120 4 3 2 5 3 151/27A temperature

ADMP 151 /27B technical amyl alcohol (pentanol isomer not distilled liquid at -24°C 120 4 2 2 5 3 mixture)

ADM 151 /27C technical amyl alcohol (pentanol isomer distilled liquid at -24°C 120 3 2 2 5 4 mixture)

EDMP solid, melting point 120 3 2 1 5 2 65°C