The crosslinking agent is an isocyanate-free and metal-free crosslinker and is at least a dimerised di-acetal, completely compatible with organic solvents, intended to be used at temperatures above 0°C in industrial coatings, in self or acid-catalysed systems, at temperatures under 1200C in both solvent borne systems and water borne systems. According with the examples described below, some of the crosslinkers presented in a suitable form, according to the instant invention may be used in water-borne film forming compositions in combination with anionic dispersions based on acrylic resins, polyesters, alkyds which are hydroxyl functional at a level of 50 to 150 mgKOH/g solid resin, then performing the function as both crosslinker and coalescent solvent. The organic film forming composition using a crosslinker as described by the present invention requires no more addition of a preservative or decontaminant, this function being accomplished by the crosslinker itself.

The crosslinking system as described by the present invention is based on an di-aldehyde selected from the di-aldehyde family having an even number of carbon atoms in the chain connecting the two aldehyde groups, presented as an oligomer, mostly cyclic dimers and/or trimers, and hydrogen active compounds.

Products in this class are not included in Dr.Erich Karsten's "Lackrohstoff Tabellen" as commercial products.

Prior art Increasing concern related to formaldehyde emission, using and disposing of urethane crosslinkers, oxirane crosslinkers, peroxides, aziridine and metal crosslinkers such as zirconium and zinc compounds has initiated the development of alternative systems, and different approaches to solve the problem of improving the quality complying with the request of harder safety regulations have been made. Systems based on Michael- and pseudo-Michael reaction, polyoxazolidone crosslinked with polymers containing anhydride functional groups are known to be tested on the market with the major deficiency of a short pot-life and high sensivity to the humidity of the atmosphere, aspects that make these systems mostly unreliable.

Systems based on acetalated compounds chemistry have been previously described in relation with: 1. oven ( over 1200C ) drying coating system in E P 0255608 2. thermosetting composition based on methylene diethers in US Pat 3200099 3. melamine formaldehyde coating composition which gives off reduced levels of formaldehyde in US Pat 5155170 The systems previously described have certain limitation because of the way the crosslinker is formulated, which either requires temperatures well above room temperature or give limitations with respect to recoatability due to the reactivity of the crosslinker, usually present as a functionalised carbonyl derivative, often as polyacetal. The systems may give strong variations in gloss. Another risk of the quoted systems is a certain yellowing when drying and/or a colour change on stained wood surfaces, especially when textile dyestuff are used in the stain formulation.

Moreover, the fact that the crosslinker is presented as a polyacetal or an acetalated carbonyl compound, gives different reactivities, and it is therefor difficult to reproduce the degree of crosslinking and respectively the interval of time which allows the stackability and recoatability.

Description of the invention The present invention relates to a crosslinking system which gives off no aldehyde emission from the dried film, based on acetal chemistry, capable to provide a network polymer coating at temperatures between 0 and 120"C with or without acid catalyst. According to the present invention, the storage stability of the crosslinker is secured by the use of mono-alcohols, selected from methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, depending of the aimed drying temperature and the required drying time, while the reactivity and reproducibility of the respective quality is secured by preserving the active crosslinking species at least as dimerised carbonyl compounds by using controlled amounts of aliphatic hydrocarbons related, as previously stated to the required drying temperature, in the organic film forming composition.

The instant invention relates to a crosslinking system based on acetalised carbonyl compound, presented at least as a dimerised carbonyl compound, which eliminates the gloss variation when cured at different temperatures below 120"C with or without acid catalyst. The present invention eliminates the compatibility risks, which may occur when common acetals or poly-acetals, i.e. as those derived from pentaerythritol or tri-methylol-propane, are used in combination with organic film formers.

The present invention introduces a new crosslinking control at low temperatures i.e.

between 0°C and 50"C for solvent borne coatings by using a combination of acid catalyst and aliphatic hydrocarbons. The present invention eliminates the risks of eventual looses in acetalated compound by using carbonyl dimers and trimers which preferably react with hydrogen active groups or other carbonyl derivates in the presence of a suitable catalyst.

The present invention relates to a crosslinker based on acetalated derivates from di- functional carbonyl compounds presented as oligomers, at least as dimers or trimers preferably in cyclic form, derived from linear aliphatic or aromatic di-aldehydes with an even number of carbon atoms connecting the two functional groups, that may be used in a film forming composition, based on a combination with water borne or solvent borne hydroxyl functional polymers, stabilised with mono-alcohols and aliphatic hydrocarbons which gives a high build coating on porous substrate providing a reduction of the solvent released compared to the classical systems and to anionic polymer dispersions when used in water borne systems.

The present invention relates to a crosslinker presented as oligomers, at least cyclic and/or linear dimers and/or trimers of di-carbonyl compounds acetalated with mono functional alcohols as derivatives from linear aliphatic or aromatic di-aldehydes having an even number of carbon atoms in the chain connecting the two carbonyl groups such as glyoxal, succin aldehyde, adipic aldehyde, octandial, phtal-aldehyde, terephtal-aldehyde etc.

The present invention eliminates the use of an other coalescent solvent in water borne acrylic dispersions based on hydroxyl functional polymers having a glass transition temperature over and a minimum film forming temperature of 25"C and makes the system suitable for industrial coating application at pH below 7.

The present invention more specifically relates to a crosslinker composition obtained by acetalisation in situ of polymerised di-aldehydes at least dimers and trimers derived from di- aldehyde compounds with an even number of carbon atoms connecting the two functional groups presented as such or as water solution with: alcohols selected from ethanol, n-propanol, iso-propanol, butanol, iso-butanol, sec-butanol and/or hydroxyl functional polymers in the presence of the above mentioned alcohols, selected from: 1. polyesters oil-free or oil modified with a hydroxyl functionality of 80-180 mg KOflig and a acid number of max. 25 mg KOH/g, 2. polyethers such as DGEBA-ethers with. an epoxy-equivalent between 0. 18 and 0.027 equiv./l00g,(DGEBA = di-glycidil ether of bisfenol A) 3. cellulose derivates with an elementary molecular weight of 263.7 and a substitution degree of 2.26, 4. hydroxyl functional vinyl or acrylic polymers with a hydroxyl equivalent weight of 500-2000, 5. hydroxyl functional aldehyde/ketone resins with a hydroxyl number of 60-150 mg KNOWS, 6. hydroxyl functional fumed or precipitated silica having a functionality of 800-1200mgKOH/g in suitable mono-functional alcohol.

The present invention relates to a crosslinking system based on a family of carbonyl modified compounds presented as acetals derived from hydroxyl functional organic compounds, such as those described above, which at the state of dilution and respective solvent balance in the formulation, with respect to the polarity of the system and of the pH level, shows preferably cyclic functional structures and optionally linear structures, consisting of at least a dimer as showed below: where: n= 0, 2, 4, 6,....

Rl=P2=H, CH3, C2H5, C3H7, C4H9,...,HSi205 R3=R4=polymer backbone, H, CH3, C2H5, C3H7, C4H9,...

The present invention relates to an organic crosslinker which embodies the di-functional carbonyl compounds with an even number of carbon atoms connecting the functional groups presented as cyclic and linear oligomers at least as dimers and/or trimers in acetalated form, formulated according to the instant invention which eliminates the problems quoted above such as varied reactivity, poor colour retention, gloss variation, low compatibility with fatty acid derivates, low compatibility with low polarity solvents.

The present invention relates to an organic crosslinker which may be used to perform good anti-blocking properties at ambient temperature, good substrate protection, good stain resistance, without increasing the ground emission of aldehydes which generally lies on 0.05mg/m3, experimentally determined well below 0.005mg/m3, per square meter and hour.

The present invention relates to an organic crosslinker composition which at the commercial fullness of the dried film ensures a diminished volume of solvent release into the atmosphere with at least 35%, and at most 50% by volume, compared with standard nitro-cellulose systems, acid curing systems, polyurethane systems at an application viscosity between 57 mPas and 250 mPas and a non-volatile content of 15 to 45% by weight.

The invention is illustrated by means of the following examples describing the preparation of the crosslinker according to the instant invention.

Example 1 In a round-bottom four-necked flask, equipped with a mechanical stirrer, nitrogen inlet, thermometer, condenser and dropping funnel, was placed 322g ethanol 99.9% and 7 moles glyoxal (as polymer containing ca. 20% water by weight ) were added under stirring, heated and maintained at 80"C under nitrogen blanket until a clear yellow-pale solution was obtained. The crosslinker obtained as described is active in combinations with hydrogen functional compounds in organic film forming compositions.

Example 2 In a round-bottom flask as described in example 1, 51 8g n-butanol are added and under nitrogen blanket, 51 0g polymerised glyoxal containing 20% water is added under stirring, and heated at 65 - 80"C under stirring and an over pressure of 0. 1-0.2 atm. until a clear solution is obtained (viscosity 53-62 seconds DIN 4mm cup). The crosslinker as such may be used in formulation of organic coatings to comply to electrostatic applications requirements as well as in combination with anionic dispersions for water borne coating systems.

Example 3 In a similar equipment as described above are placed 4.38 moles ethanol, 1.8 moles iso- propanol and under stirring 7 moles glyoxal polymer containing ca.20% by weight water is depolymerised at 65"C to obtain a crosslinker active in film forming compositions based on hydroxyl functional oil modified oligoesters.

Example 4 In a four-necked round bottom flask equipped with nitrogen inlet, mechanical stirrer, thermometer, dropping funnel and a Dean-Stark trap with condenser 241.5g succin-aldehyde 50% in water were placed. Under stirring and progressive heating 240g butanol is added and azeotropic distillation of water begins. The process is continued until ca. 200g water is distilled. The mixture is then cooled down and a crosslinker derived from a carbonyl compound is obtained. The crosslinker as such may be used in organic film forming compositions where flash point temperature over 23"C is required.

Example 5 In a similar equipment as described in example 4, 518 g n-butanol are added and under nitrogen blanket and stirring 5 1 0 g polymerised glyoxal containing 20% water are added. The mixture is heated up progressively to 11 70C; then 25 g paraffin solvent with boiling temperature is added carefully and maintained under azeotropic circuit until a clear solution is obtained and approximately 55 g water is distilled. A crosslinker is obtained appropriate for use in organic film forming compositions containing glyoxal oligomers, at least dimers and trimers acetalated nvith n- butanol in a molar ratio of 1 to 1.

Example 6 In a similar equipment as previously described above 284g soybean oil are charged and heated at 2000C under nitrogen blanket; then 0.006g LiOH are added under stirring and further 131 g pentaerythritol 85% in small portions while the temperature is raised to 260"C within 2 hours.

The batch is maintained at this temperature until 1 g reaction mixture is completely compatible with 6g ethanol at 20-230C. At this point the batch is cooled down to 2000C and 1 77g phtalic anhydride is added and the reaction is continued at 2000C until the acid number of less than 25mg KOH /g is attained. The resin obtained as such is then cooled down to 150"C and let down with 1 90g butyl acetate to a non-volatile content of ca. 75%. At 80"C, 1 10g polymerised glyoxal 80% is added and further 69g ethanol and maintained at this temperature and over pressure of 0.2 until the reaction batch is a completely clear liquid. The crosslinker obtained may be used in combinations with cellulose esters to obtain a film forming acid curing composition.

Example 7 In an equipment, similar to that described above, 335g dehydrated castor oil fatty acids are added and under stirring is added further 195g pentaerythritol. The system is purged with nitrogen and progressively heated up to 2200C to remove the reaction water, to a conversion of ca. 80%.

Then the ester is cooled down to 1700C and 180g phtalic anhydride is added under stirring and further 20.5g propylenglycol are added to complete the reaction to a conversion of 90-95%. The oligoester is then cooled at a temperature of 85-90"C and 1 80g poly-glyoxal is added and maintained at this temperature to obtain a clear acetalated compound that may be used as a crosslinker in combination with other film forming polymers to obtain coatings crosslinkable at a temperature under 50"C in acid catalysis or by backing at a temperature of 80"C for 30 min.

Example 8 In an four-necked flask equipped as above 310g dehydrated castor oil fatty acids are added and purged under stirring with nitrogen while heated up to 200"C whenl49g pentaerythritrol are added and heated further to 2500C to obtain an ester. When the acid number is under 10mg KNOWS the reaction mixture is cooled down to 1 900C and 81.9 g phtalic anhydride is added and under the temperature of 1300C 135g benzoic acid is added and the reaction goes on to an acid number of 30mg KOKIg The oligoester is cooled down to 1 000C and 1 54g n butanol is added and further 200g succin-aldehyde as 50% solution in water and maintained at 119-125"C under azeotropic distillation until the water is completely removed. Thus a crosslinker aimed for film forming compositions is obtained reactive in acid catalysis at room temperature or by forced drying an hour at max. 80"C.

Example 9 In a round-bottom flask as described in example 1, 51 8g n-butanol are added and under stirring 50g hydrophobic silica with a hydroxyl functionality of 800-1200mgKOH/g and a average particle size of 3-4pm is added and dispersed 30 minutes, and then under nitrogen blanket, 510 g polymerised glyoxal containing 20% water is added under stirring, and heated at 800C under stirring and an over pressure of 0. 1-0. 2 atm until a hazy, free of coarse particles, solution is obtained. The crosslinker as such may be used in solvent bome formulations of organic coatings to comply to electrostatic applications requirements or in water borne film forming formulations free from any other coalescent solvent.

Example 10 In a similar equipment as described above 280g tall fatty acids are added and heated up to 200"C under nitrogen blanket and stirring. At 200"C, 134g pentaerythritol is added under stirring and the temperature is then increased progressively up to 2500C and maintained until the acid number decrease under 10mg KNOWS. A mono-ester is then obtained with a viscosity of 200-600 mPas. The batch is then cooled down at 50"C and 2l7g polymerised glyoxal of purity 80% is added under stirring. Further 1 83g n-propanol is added rising the temperature at 800C and maintaining the batch at this status until a clear solution is obtained. The crosslinker derived from a carbonyl functionalised fatty acid mono-ester is completely compatible in organic film forming compositions that show good reactivity at room temperature in the presence of an acid catalyst.