June 12, 2024 Longchang Chemical

Preparation and application of acrylic resin for ultrafine powder coatings

The polyacrylate resin and its ultrafine powder coating were prepared, the structure of polyacrylate resin was characterised by infrared spectroscopy, thermogravimetric analysis, differential loss scanning calorimetry, etc., the properties of the powder coating and coating film prepared in this way were tested, and the comminution, electrically charged, fluidity, storage stability and construction properties of the ultrafine powder coating were investigated; and the application prospects of the ultrafine powder coating were also looked forward to.

1、Introduction

With the environmental problems becoming more and more serious, green coatings are getting more and more attention and importance. Powder coating is a new type of solvent-free 100% solid powder coating, which has aroused extensive interest from countries all over the world due to its features of low pollution, high efficiency, excellent performance, energy and resource saving, and powder recyclability.

Among them, acrylic resin-based powder coatings are low-toxicity products with a series of advantages: excellent decorative, outdoor weathering, aging, corrosion and pollution resistance, high surface hardness, good flexibility, has been widely used in automotive home appliances and other fields, and in the future, acrylic powder coatings will become one of the main varieties of automotive decorative topcoats.

Ultrafine powder coatings due to particle size and its distribution with ordinary powder coatings and performance differences and special features, such as coatings with thin coating, good surface flatness and gloss, and liquid coatings to achieve similar results, making ultrafine powder coatings to meet the more stringent requirements for powder coatings in various fields for the promotion and application of powder coating to further expand the development of space.

Acrylic ultrafine powder coatings have excellent performance, will have good prospects for development and huge market demand, therefore, the study of acrylic ultrafine powder coatings is of great significance.

2、Experimental part

2.1 Experimental raw materials

Methyl methacrylate (MMA), butyl methacrylate (BMA), glycidyl methacrylate (GMA), cyclohexyl methacrylate (CHMA), isobornyl methacrylate (IBOMA), azobisisobutyronitrile (AIBN), and dodecadecanedioic acid (DDDA), were all analytically pure; benzene and toluene were chemically pure.

2.2 Synthesis of acrylic resin

In this experiment, acrylic resin was synthesised by homogeneous solution polymerisation. Before polymerisation, all the monomers used were removed from the polymerisation blocking agent by distillation under reduced pressure. Methyl methacrylate (MMA), butyl methacrylate (BMA), glycidyl methacrylate (GMA), cyclohexyl methacrylate (CHMA), and isobornyl methacrylate (IBOMA) were mixed, and a small part of the monomer mixture was poured out and reserved for subsequent use; the initiator, azobisisobutyronitrile (AIBN), was added to the remaining monomer mixture and stirred until complete dissolution.

Toluene was added to a four-necked flask, heated to 80°C and refluxed at a constant temperature for 0.5 hours. Passed into the N2 for protection, dropwise addition of initiator monomer mixture 2h, holding reaction 0.5h. Additional dropwise addition of the remaining monomer mixture 0.5h, dropwise addition is completed, holding reaction 1.5110 reaction ends to obtain the polyacrylate resin solution containing toluene.

The above product is poured into a single bottle while hot, with a rotary evaporator at 80 ℃ / 0.098MPa under the vacuum degree of basic evaporation of all solvents, the polyacrylate resin is poured on the surface of the dish, placed in a vacuum drying oven drying for 24h can be obtained clean white polyacrylate resin.

2.3 Preparation of ultra-fine powder coatings

The preparation of ultra-fine powder coatings need to use ultra-fine grinding and grading system, the equipment used by the ACM325 ultra-fine mill, SCX400 ultra-fine classifier, high-efficiency cyclone dust collector, pulse bag filter and centrifugal fan. The preparation steps of the ultrafine acrylic powder coating are as follows:

(1) The polyacrylate resin is initially crushed;

(2) Pre-mix the polyacrylate resin, dodecanedioic acid (DDDA), levelling agent and other additives;

(3) The mixed materials are melted and extruded in a twin-screw extruder;

(4) After cooling, the extruded film and A1203 in the crusher for crushing and mixing;

(5) Extrude the above materials for the second time and press the tablets;

(6) Add 0.5%, 3% A1203 in the ultrafine grinding system for crushing and grading;

2.4 Coating preparation

After degreasing the surface of the substrate with acetone, sandpaper was used to remove rust and wipe clean, and then put into the blower oven for 2 min. then, the electrostatic spraying process and equipment for the preparation of acrylic ultra-fine powder coating. Put the pre-treated sample plate into the powder spray cabinet, use the corona discharge electrostatic spray gun to spray it, keep the sample plate vertical after spraying, and put it into the blower oven for curing, and then leave it at room temperature for 24h for performance testing.

2.5 Structural characterisation and performance test

(1) Structural characterisation of resin
Infrared spectroscopy (IR) was used to qualitatively analyse and identify the functional groups and chemical bonds that may be contained in the molecule and quantitatively determine the number. The specimens were prepared by the tabletting method by grinding a small amount of resin samples into a fine powder in an onyx mortar and mixing it well with dry potassium bromide powder, and then loaded into moulds for tabletting, and then scanned on an infrared spectrometer to collect infrared spectra.

(2) Resin property test
① Glass transition temperature (Tg)
Polyacrylate resins undergo sudden changes in properties when glass transition occurs. Differential scanning calorimetry (DSC) is a method to characterise the glass transition temperature with the increase of temperature and the change of heat flow. In this experiment, the glass transition temperature of the resin was determined by DSC method, and the thermal analyser used was DS02910 series product of the American company, and the test conditions are listed in the table below.

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②Thermal stability
Thermogravimetric analysis (TG) is a method to measure the change of mass of a substance with temperature (or time), which reflects the thermal stability of a polymer chain through the change of mass due to oxidation, decomposition of side groups, breakage of the main chain, or structural change after being heated. In this experiment, TA-2000 series of thermogravimetric analyser was used to analyse the thermal stability of polymers, and the test conditions were as follows: scanning temperature range of 25~600℃, and heating rate of 10℃/min.

(3) Crushability test of ultrafine powder coatings
The particle size of the powder coating was analysed by the MS2000 laser particle size analyser of Malvern UK, and the average particle size of the product was determined to be less than 15 and less than 30 and the average particle size of the product.

(4) Coating film performance test
Appearance: visual inspection; mechanical properties: pencil method to measure the hardness, paint film scribing test to measure adhesion, paint film bending test (cylindrical axis) to measure flexibility, paint film impact tester to measure impact resistance.

3、Results and Discussion

3.1 Synthesis of acrylic resin

(1) Selection of polymerisation method
The molecular weight distribution of acrylic resin for powder coating should be as narrow as possible, while the molecular weight of the resin synthesized by suspension polymerization or emulsion polymerization is larger and the molecular weight distribution is wider, and at the same time, there will be water-soluble substances remaining in the resin, such as: dispersant, emulsifier, stabilizer and so on, and the trace impurities will affect the performance of the resin and lead to failure to meet the high quality of the requirements of the powder coatings, and thus the two methods are less frequently used.

Although there is no need to remove the solvent, the polymerisation system becomes more and more viscous as the reaction proceeds, and a lot of heat is released during the reaction, which makes it easy for violent polymerisation to occur and the reaction process is difficult to control.

The synthesis of acrylic resin mainly uses free radical polymerisation method, compared with the four major free radical polymerisation methods, due to the solution polymerisation of the reaction at reflux temperature, and nitrogen gas to protect, stirring and solvent reflux in the process of the reaction will remove the heat generated by the reaction, can effectively avoid the local temperature is too high or even violent polymerisation, it is easy to control the reaction temperature, the reaction is higher conversion, the system is more stable, and the molecular weight of the polymer is easy to control. The molecular weight of the polymer is easy to control. Although the solvent used in the solution polymerisation method is generally toxic, but it is easier to remove the solvent, so the resin synthesis method in this thesis is solution polymerisation.

(2) Selection of copolymerisation monomer
Acrylic resins are generally synthesized by five-member copolymerization, which requires hard monomer, soft monomer, crosslinking agent together at a certain temperature crosslinking polymerization. There are many types of monomers that can be used as raw materials for the synthesis of acrylic resins, and each monomer has different effects on the performance of the resin. The glass transition temperature of the resin can be changed by selecting the type of monomer and adjusting the ratio between the monomers to improve the crushing properties and anti-caking properties of the resin, as well as to improve the levelling of the coating.

Therefore, in order to ensure that the comprehensive performance of the target resin reaches the expected results, comprehensively consider the influence of various monomers on the resin properties, as well as the influence of the ratio of different monomer types on the glass transition temperature of the resin, in this paper, MMA was selected as the hard monomer, BMA as the soft monomer, and GMA as the cross-linking monomer, which introduced the epoxy group into the resin, and IBOMA was selected to reduce the viscosity of the polymer.

(3) Selection and dosage of initiator
The commonly used initiators for polyacrylate resin synthesis are azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO). Among them, the normal use temperature of BPO is 70, 100 ℃, and the use temperature of AIBN is 60, 80 ℃. AIBN is preferred in the synthesis of acrylic resin for the following reasons:

① BPO is easy to induce decomposition reaction, the primary radicals are easy to capture the hydrogen, chlorine and other atoms or groups on the macromolecular chain, and then the introduction of branched chains on the macromolecular chain to make the molecular weight distribution broader; AIBN decomposition of free radicals generated by the activity of the smaller than the BPO, generally no induced decomposition reaction, so that the molecular weight of the polymer obtained from the distribution of the narrower;
② Benzoyl radical decomposition for highly active benzene radical initiated polymerisation, the polymer end group is poor outdoor durability, the coating film will yellow for a long time; and AIBN initiated polymer end group is (CH3)3C-, good outdoor durability;

③ The two free radicals C6H5C00- and C6H5 produced by the decomposition of BPO will undergo a coupling reaction, which inactivates most of the initiator and reduces the efficiency of the initiator.

④ At 60, 100 ℃, the half-life of AIBN is shorter than that of BPO, indicating a high reaction rate, and the peroxide residue will lead to oxidative yellowing of the resin.

The amount of initiator is also critical. Too little, resulting in polymer molecular weight is too large, the resin melt viscosity is too high, processing performance is bad, based on the resin coating leveling is poor, and the formation of the film is prone to orange peel phenomenon; initiator dosage is too large, the molecular weight of the polymer is small, although easy to process, but the coating film’s mechanical properties and impact resistance deterioration.

(4) Selection of solvent
AIBN does not induce decomposition reaction, so the solvent for the initiator decomposition rate is very small. Therefore, only the boiling point of the solvent and the chain transfer ability on the molecular weight and its distribution of the impact. The solvents commonly used in the synthesis of acrylic resin are benzene, toluene, xylene and butyl acetate, etc., while the toxicity and cost of xylene are higher, so benzene and toluene are chosen as mixed solvents. Among them, benzene has a boiling point of 80°C and plays the role of reflux, while toluene plays the role of chain transfer.

The glass transition temperature (Tg) of the resin is directly related to the storage stability of the powder coating, the higher the Tg, the better the storage stability, but the Tg is too high will make the processing performance of the powder coating as well as the decrease of levelling, so the Tg of the resin used for powder coating needs to be adjusted appropriately, and the Tg of the polyacrylate resin used for powder coating is generally in the range of 40-100℃, and the more optimized range is 40-60℃. The glass transition temperature of the copolymer can be used to make a preliminary design of the Tg of the polyacrylate resin through the Fox equation to better guide the experiment.

3.2 Performance analysis of ultrafine powder coatings

(1) Crushability
Ultrafine powder coatings and ordinary powder coatings production process is similar, mainly including the pre-mixing of raw materials, melt extrusion, cooling and crushing, fine crushing and grading sieve, product packaging and other processes. Only in the degree of crushing and grading and the selection of flow-assisting agent is different.

The experimental results show that the powder coating particle size less than 15μm accounted for more than 80%, less than 30m accounted for more than 90%, and the average particle size is smaller, below 10μm. This indicates that the system has a better effect on the crushing and grading of acrylic powder and reaches the level of ultrafine. It also indicates that the ACM impact pulveriser with internal classification and the SCX ultrafine classifier are feasible for the new process route of pulverisation and classification for the preparation of ultrafine powders. This ultrafine powder crushing and grading system can well meet the requirements of the product on particle size and yield after multiple processes such as ultrafine crushing, coarse grading and fine grading.

(2) Fluidisation
Ultrafine powder coating with a small particle size, the particle mass itself is reduced, the relative surface area increases, the inter-particle force (mainly Van der Waals force) greatly enhanced, it is very easy to form agglomerates. In the electrostatic spraying process into the fluidisation problems caused by difficulties, easy to block the pipeline, storage stability is not good, clusters lead to particle size increase and lose the excellent performance of ultra-fine powder. Therefore, it is necessary to solve the fluidisation problem of ultrafine powders to eliminate the limitation of the promotion and application of ultrafine powders.

According to the current literature, the main method to improve the fluidisation of ultrafine powder is to introduce some guest particles in the main ultrafine powder, which are much smaller than the ultrafine powder itself, as a flow-assisting agent, in order to change the interaction force between the particles of the ultrafine powder, so that the ultrafine powder is easy to be dispersed, and play a role in improving the fluidisation.

Common flow-assisting agents include alumina, aluminium hydroxide, calcium oxide, silicon dioxide, zinc oxide, wrong oxide, platinum trioxide, titanium dioxide, ornamental dioxide, tungsten trioxide, and aluminium silicate, and a combination of at least two of these substances will improve the fluidisation properties of the ultrafine powder coating. Therefore, it is necessary to select the type, particle size, and addition ratio of the added nanofluidising agent. The flow aid should not be added in excess or the coating properties will be affected, and the type of flow aid also has an effect on the fluidisation effect and other properties of the coating.

Through comparison, A1203 was found to be the most effective, and A1203 was selected as the flow aid. In the production of ultrafine powder coatings, in the crushing process added 0.5%, 3% of the nanoparticles of A1203, making the ultrafine powder fluidisation performance is good, and improve the storage stability.

(3) Chargeability
Ultrafine powder quality is small resulting in not easy to powder, in order to improve the powder rate, the theory should be added to some electrifying agent. However, in practice, it is found that a low powdering rate improves the selectivity of spraying, that is, the particle size of the particles on the spray is similar, and the coating thickness obtained is more uniform.

Ordinary coarse powder recycling powder in the high content of fines, repeated use will be holding, spit powder and other fluidisation problems, usually need to be recycled powder and new powder mixed at a certain ratio. Ultra-fine powder has solved the fluidisation problem, so the recycled powder can be used normally even if the particle size is too fine. Powder coating powder can be recycled after spraying, and its powder rate is good, the powder rate of ordinary coarse powder can reach more than 95%, and the powder rate of ultra-fine powder coating is more than 98%, which avoids the waste of resources.

(4) Construction performance
Comparison of the comprehensive performance test results of the coating and film is shown in the table below.

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It can be seen from the above table:
Appearance: The long wave on the surface of the coating film formed by ultra-fine powder is much lower than that of ordinary coarse powder, which largely eliminates the phenomenon of orange peel inherent in powder coatings. The surface of the coating film formed by ordinary powder coatings is not flat enough, and the surface gloss of the coating film formed by ultra-fine powder coatings is much higher, which can meet the high decorative requirements.

Mechanical properties: thin coating of fine powder and thick coating of coarse powder have the same effect in adhesion, corrosion resistance, etc. Thin coating of fine powder has better pencil hardness and impact resistance. Under the same thickness, the coating formed by fine powder has better corrosion resistance.

Levelling: ultra-fine powder coating particle size is smaller, after solving the problem of agglomeration, it is not easy to appear the problem of hanging, the fluidisation performance is very excellent, compared with the ordinary coarse powder, the formation of the coating film is more flat.

Construction performance: ultra-fine powder coatings can form thinner coatings due to smaller particle size, so covering the same area of the substrate, not only the amount of raw materials is greatly reduced and the surface roughness is also significantly reduced. Even if a very rough substrate is covered with ultrafine powder coating, there will be no obvious orange peel, which cannot be done with ordinary coarse powder.

And the thin coating of ultrafine powder dries faster, saves time and shortens the construction week after 2 or 3 layers of ordinary coarse powder and ultrafine powder are sprayed, the ordinary coarse powder has no covering power problem because of the thick coating, and the thin coating of ultrafine powder appears to have insufficient covering power, so you can choose to apply the appropriate thick coating or choose the pigment with strong covering power, but you should pay attention to the amount of pigment added should not be too much, or else it will be unevenly fused phenomenon.

(5) Storage stability
Powder acrylic resin is easy to store and transport, the transport cost is lower than solvent-based acrylic resin, storage and transport process security. However, there are some common disadvantages of powder coatings, such as paint storage and transport process pressure or moisture resulting in bonding, need to be kept at low temperatures and dry powder.

Powder acrylic resin is more easily accepted by the construction unit, and some models of solid acrylic resin has thixotropic, made of paint and ordinary latex paint has the same effect of opening the can and construction performance. High-grade solid acrylic resin, because the main monomer is methacrylate, in the ultraviolet radiation will not degrade, so its weather resistance is more prominent. Resin thermal stability of 170 ℃ or more, individual varieties up to 260 ℃ which is difficult to reach the ordinary solvent-based thermoplastic acrylic resin.

4, Conclusion

In summary, ultra-fine acrylic powder coatings and coating has a series of advantages: small pollution; good light and colour retention, excellent decorative; electrostatic coating effect is good, can be thinly coated; spraying efficiency is high, the powder can be recycled; good adhesion, without primer; heat resistance, maroon resistance, chemical resistance, not easy to turn yellow; good physical and mechanical properties.

Ultra-fine powder coatings can be widely used in all areas where powder coatings are used, and can meet more stringent requirements, such as highly decorative coating requirements in the automotive field, high weathering requirements for outdoor products, corrosion resistance requirements in the field of ships and containers, decorative and economic requirements for furniture and home appliances, and ultra-thin coating requirements for fine instrument components, and so on.

The environmental protection, economy and superior performance of ultrafine acrylic powder coatings will also make its application fields expanding, its extensive development prospects and huge market potential, will bring a new round of development opportunities for the powder coating industry.

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