Analysis of Factors Affecting Molecular Weight of Polybutyl Acrylate Leveling Agent
1. Introduction: The role of leveling agent: When the powder coating is melted and leveled, there are two interfaces of “liquid phase” and “liquid-solid phase”. The main molecular chain of the leveling agent can form a very thin monomolecular layer on the surface of the melt, which provides a uniform surface tension in the “liquid phase” and helps eliminate paint diseases such as pinholes and shrinkage holes; at the same time, the side of the leveling agent The chain ester group can controllably reduce the surface tension of the melt coating, which is helpful for the wetting and adhesion between the coating and the surface of the “liquid-solid phase” of the coated workpiece, and at the same time provides good wetting and spreading of the substrate.
Polybutylacrylate is one of the most commonly used low-cost materials for powder coatings. It shows strong surface activity, the main reason is that it has a relatively neat “—C—C—” main chain with resin-phobic properties, and the side chain is “butyl ester group” with resin-friendly properties; such a structure enables it to Free orientation at the interface, the part of the ester bond that is compatible with the liquid phase is inward, the main chain floats on the interface to form a uniform monolayer, and the surface tension can be uniform. The degree of polymerization “n” reflects the size of the molecular weight.
2. What is the most reasonable molecular weight of the liquid flow?
According to the reports of domestic and foreign experts, from the surface tension, the flexibility of the main chain, the wettability of the substrate, the wettability and encapsulation of the pigment filler, the mobility of the molecules, and the operability of the synthesis process, the formation of From the perspectives of gloss, plumpness and distinctness of the coating, the conclusions generally tend to the following indicators: Mn=4000-5000; Mw=7000-10000, molecular weight distribution polydispersity Mw/Mn=1.5-2.2, This is the most ideal molecular structure index of polybutyl acrylate.
Molecules above 20,000 are difficult to be compatible with the system because the molecules are too large, and tend to lose light or extinction; below 4,000 and above 20,000 flow molecules, the less the proportion, the better. A point worth emphasizing: Mn=3000-10000, molecular weight distribution Mw/Mn=1.5-7.0 is also available; but occasionally there will be problems, such as: too much content of small molecules and super-molecules, pinholes and loss of light will appear , not anti-interference and so on. Molecules below 4000, because the molecules are too small and the compatibility is too good, this will lose the leveling property, causing all small ripples when looking at the light; as shown below:
3. Synthetic materials: Once the molecular weight range is determined, the remaining work is to find raw materials and determine the process around this goal. The main materials are: n-butyl acrylate, xylene solvent, azo AIBN catalyst, molecular weight regulator. In order to reduce costs, 3%-5% styrene can be added for copolymerization; some companies also use 2-ethylhexyl acrylate, Modified with vinyl trimethicone, hydroxyethyl acrylate, octyl acrylate or isooctyl acrylate. Mainly to reduce surface tension to improve anti-jamming performance, these are beneficial innovations. In order to reduce costs, some factories directly add dioctyl terephthalate, isooctyl terephthalate, diethyl fumarate, diethyl maleate, epoxy Soybean oil, etc., the general dosage is about 20%. These practices are not conducive to leveling and anti-interference.
The choice of solvent is varied. In the earliest stage, toluene (boiling point 110.63 °C) was used, but due to the problems of toxicity and low boiling point, xylene (138.5~141.5 °C) was gradually used. Ethylene glycol monomethyl ether (or ethylene glycol monomethyl ether) is also a very ideal solvent. Its toxicity is lower than xylene, and its boiling point is 124.6 °C. It is not widely used in China, and the method of mixing it with xylene as a solvent abroad is very worthy of reference. If the boiling point is too low, the molecular weight of the synthetic leveling agent is too large, which tends to defoaming, but the capacity loses light and fogs. Solvents with a high boiling point are not suitable for use because the solvent evaporation temperature is too high, which is easy to oxidize the finished product to yellowing and form macromolecules. At present, the mainstream in China is to use mixed xylene as a solvent. The raw materials are affordable, cheap, low in toxicity, easy to store, easy to operate, and safe.
Why use azobisisobutyronitrile AIBN as initiator instead of dibenzoyl peroxide BPO? Two factors are mainly considered: First, the type and amount of initiator can affect the relative molecular weight and molecular structure of the polymer, thereby affecting the chemical and physical properties of the resin. Azobisisobutyronitrile (AIBN ) and dibenzoyl peroxide (BPO) have different decomposition kinetics, respectively. The chain transfer constant of AIBN is small, the decomposition rate is not much different in different solvents, the activity of free radicals is less than that of benzene radicals, the chain transfer reaction is less, and the obtained polymer has a narrower relative molecular mass distribution. AIBN is used as the initiator. The agent can keep the resin viscosity in a more suitable range. However, after BPO is decomposed, it is easy to generate benzene radicals with higher activity, and benzene radicals are easy to capture hydrogen atoms on the monomer or polymer molecular chain, resulting in branching and cross-linking, broadening the relative molecular mass distribution, and reducing the viscosity of the polymer. significantly increased. The second is: the end group formed by the polymerization initiated by azo AIBN is (CH3)3C–, which has better outdoor durability; while the end group formed by the BPO-initiated polymer is a benzene ring, which has poor outdoor weather resistance and will lead to coating film after a long time. Yellowing and aging. It is generally believed that when the mass fraction of initiator is greater than 4%, the mechanical properties, chemical properties and thermal stability of the polymer will be adversely affected.
Practice has proved that in the process of synthesizing polybutyl acrylate leveling agent, the following important points need to be controlled well:
1) The ratio of the total amount of monomer to solvent ≈ 1: (1.0—1.2): This ratio has a great influence on the polydispersity of molecular weight. Although the whole reaction process is a “starvation” reaction, the solvent is in an excess state, but with the The concentration of each component in the solvent changes greatly due to the increase in the amount of monomer dripping and the formation of reactants. Surface tension negatively affects the leveling and fullness of the coating.
2) Dropping speed: Uniform and gradual dropwise addition is advantageous. The chemical reaction is carried out in an instant. Once the reaction conditions are reached, the reaction can be completed within one ten thousandth of a second. It is very important, especially not to act in a hurry and be casual. The polydispersity of molecular weight is greatly affected by it.
3) Influence of water content in monomers and solvents: both monomers and solvents will contain ≤5‰ of water, although not much, but because the solvent is recycled, the water will continue to accumulate. A small amount of water enters the reaction kettle with the solvent or monomer, and azeotropes with xylene after heating, consuming a large amount of azo AIBN, increasing the cost, and affecting the molecular weight distribution, which is not conducive to later use.
4) Timing and rate of adding azo: The purpose of this operation is to make the unreacted small molecules or monomers continue to react, preventing the unreacted small molecules from entering the solvent when the solvent is evaporated later, and affecting the next reaction. The monomer concentration ratio and molecular weight distribution of the kettle. Generally, one-tenth of the total amount of the initiator can be used.
5) The heat preservation time and solvent evaporation time have little effect on the molecular weight, but the temperature should not be too high, and the time should not be too long. The low temperature vacuum evaporation of the solvent is beneficial to the chromaticity of the final product. The lower the temperature and the shorter the time, the more favorable it is, of course, the premise is that the solvent should be fully evaporated.
6) Three-stage filtration: that is, the solvent and monomer should be filtered when entering the reaction kettle and the dripping storage tank; The filter element or filter screen should be cleaned and replaced regularly, which is conducive to the filtering of gelled particles or impurities. Although these operations have little effect on the molecular weight distribution, they are absolutely essential to ensure high quality.
7) Through the above operation control, we carried out GPC gel chromatography test on the produced products, and obtained the molecular weight data as follows: number average molecular weight 4457; mass average molecular weight 7879; molecular weight distribution 1.7679; relatively ideal.
Conclusion: Butyl acrylate leveling agent is the most widely used variety of powder coatings. On this basis, other monomers are added for modification, and many varieties are derived. Their synthesis processes are similar to this. Among them, the molecular weight and its distribution are related to The size of the surface tension affects the most critical factor. It does not involve its modification and derivatives here, and I only express my personal opinion on the most basic part. It is inevitable that there will be disputes and even fallacies.
There are two points of view that this paper wants to express: First, the molecular weight of the leveling agent has a fundamental impact on the surface tension and leveling. The second is: there are many synthetic factors that affect the size of the molecular weight. The operation details and the digitization of the operation are the fundamental factors of quality assurance. Although the positioning of many data may not be correct at the beginning, and the wrong can be corrected through practice, these items must be present. Continuous improvement is the only way to control the quality to the extreme!