Research on the application of hyperbranched waterborne polyurethane acrylate in 3D printing

 

Ultraviolet light (UV) curing technology is a new high-efficiency, energy-saving and environmentally friendly new technology developed in the 1960s. % to 15% annual growth rate. Compared with traditional natural drying or thermal curing coatings, light curing coatings have the advantages of fast curing speed, energy saving, excellent film performance, and wide application to substrates. Among the materials used in UV curing technology, polyurethane acrylate (PUA) has excellent comprehensive properties. It is a photosensitive resin that is widely used and studied at present. It has high adhesion and high wear resistance of polyurethane resin, and has acrylic acid. The resin’s cold and hot water resistance, corrosion resistance and good flexibility. Among them, waterborne polyurethane acrylate and WPUA have the advantages of excellent mechanical properties, safety and reliability, good compatibility, and no pollution. However, water-based PUA will lead to poor water resistance, decreased mechanical properties, and poor optical properties. Therefore, it must be diluted with active monomers before use to adjust its viscosity and improve fluidity. Although active diluents have low volatility, they are harmful to the environment. The pollution is small, and it becomes part of the coating film after curing, but it has a strong odor, is irritating to the skin and respiratory system, and has a negative impact on the safety, hygiene and long-term performance of the product. These shortcomings also hinder the application of WPUA in various industries. promotion and application in the field. Therefore, the improvement of waterborne urethane acrylate is of great significance, among which hyperbranched modification is the current development direction.

 

There have been many studies on the synthesis and application of hyperbranched polyurethanes. Johansson et al. have synthesized a series of hyperbranched polyurethane acrylates. Such multibranched polymers have low viscosity, high solubility, fast light curing, and good thermal stability. and other advantages, can avoid or reduce the use of reactive diluents, these characteristics make it have many advantages in the application of UV-curable coatings. Asif et al. synthesized a series of new hyperbranched waterborne polyurethane acrylates with good thermal stability and low viscosity by introducing some of the hydroxyl groups on the hyperbranched polyester into the acidic groups of the acrylate. The hyperbranched modification of WPUA endows WUPA with better physical and chemical properties and mechanical properties, which can be better applied to photocuring 3D printing.

 

1 Hyperbranched Modification of Waterborne Polyurethane Acrylates

 

1.1 Structure and properties of hyperbranched polymers

 

1.1.1 Definition and Introduction

 

Hyperbranched polymers can be described simply as polymers with a highly branched structure, which is different from both branched polymers and dendrimers. That is, its degree of branching is greater than that of the branched polymer and less than that of the dendrimer.

 

Like dendrimers, hyperbranched polymers are reactions that introduce two or more active groups into potential branched active sites in each repeating unit, but the difference is: hyperbranched polymers are more Dispersed, not every repeating unit is fully involved in the reaction, whereas dendrimers have a regular and monodisperse structure. Dendritic polymers have a complete structure, so they need to be synthesized through complex and precise multi-step reactions, and each step needs to be separated and purified, so the cost is very expensive, which is not conducive to industrialized production. In contrast, hyperbranched polymers can be synthesized by “one-step method” or “quasi-one-step method”, no purification or little purification is required during the reaction process, the production process is simple, the price is cheap, and its properties are similar to those of dendrimer. The polymers are similar, so they have great potential in industrial applications.

 

According to the structural characteristics of synthetic monomers, generally speaking, the synthesis methods of hyperbranched polymers can be divided into the following three categories: ① ABx(x>1) type monomer self-condensation polymerization; ② multi-branched ring-opening polymerization; ③ self-condensation vinyl polymerization. Some people also regard the method of hyperbranched polymer obtained by copolymerization of multiple functional monomers (such as A2 B3 monomer copolymerization) as a separate class, which is called the multifunctional monomer copolymerization method. Among the above methods, AB2 type monomer In vivo self-condensation polymerization and multi-branched ring-opening polymerization have been studied and applied more. At present, people have synthesized hyperbranched polyesters, hyperbranched polyethers, hyperbranched polyamides, hyperbranched polyurethanes and other hyperbranched polymers using the above methods. Among them, hyperbranched polyester is one of the most important members of the hyperbranched polymer family. It has early synthesis, mature technology and strong applicability, and is the only product with pilot-scale industrial production. The series of macromolecular modified 3D printing filaments are its typical representatives.

 

1.1.2 Structure and Characteristics

 

Similar to the traditional linear polyester, the main segment of the hyperbranched polyester molecule is also an ester group (-COO-), but compared with the traditional linear polyester, the hyperbranched polyester has a highly branched structure, molecular There are cavities, a large number of end-group functional groups and other structural features

 

The above structural characteristics make hyperbranched polyesters have some characteristics that linear polyesters do not have, which are summarized as follows:

 

(1) Good fluidity and low viscosity

 

Generally speaking, only small molecular fluids can be regarded as Newtonian fluids. Compared with linear polyesters, hyperbranched polyesters have a more compact molecular structure and have a three-dimensional three-dimensional structure similar to a sphere, so they often exhibit Newtonian fluid behavior.

 

(2) It is not easy to crystallize and has good film-forming properties

 

The flexible segments and polar carbonyl groups contained in linear polyesters make some linear polyesters easy to crystallize, such as PET, PBT, etc. Due to the highly branched structure of hyperbranched polyester, the degree of regular arrangement of molecular chains is greatly reduced, thereby significantly reducing its crystalline properties. This characteristic of hyperbranched polyester is very important for applications that require high transparency. In addition, hyperbranched polymers also make it easier to form films due to their good flow properties.

 

(3) Versatility and high reactivity

 

The large number of functional groups present at the end of the hyperbranched polyester can be of different types such as hydroxyl, carboxyl, etc., which in itself makes the hyperbranched polyester suitable for different applications. In addition, most of these functional groups have high reactivity, and new types of hyperbranched polyesters can be obtained by modifying and modifying these terminal functional groups, which further broadens the application.

 

(4) Good solubility

 

Linear polyester is generally difficult to dissolve in traditional solvents due to its generally high molecular weight and serious entanglement of molecular chains. For hyperbranched polyesters, due to the introduction of a highly branched structure, under the same molecular weight, the solubility in organic solvents is significantly improved.

 

(5) Good weather resistance

 

Traditional linear polyesters often have strong water sensitivity, easy hydrolysis and poor weather resistance due to the easy exposure of the ester groups in the molecular chain to the air. The hyperbranched structure of hyperbranched polyester can embed the ester group in the molecular chain, effectively preventing the ester group from directly contacting with moisture in the air, thereby reducing the probability of hydrolysis.

 

Due to the existence of these characteristics, the use of hyperbranched polymers in UV-curable waterborne polyurethane acrylate systems can effectively increase the double bond content of the system, thereby effectively improving the UV curing rate, as well as the mechanical properties of the cured film; on the other hand, in the same Under the solid content, the viscosity of the system can be significantly reduced, which is beneficial to construction and saves energy consumption.

 

 

1.2 Hyperbranched modification of waterborne polyurethane acrylate

 

There are still many reports of hyperbranched resins used in UV systems, and a review by Chattopadhyay and Raju published in Progress in Polymer Science in 2007 has a good summary. But its applications in waterborne UV curing systems are few and far between. The work done by Professor Shi Wenfang of the University of Science and Technology of China and her doctoral student Asif is one of the representatives.

 

Asif et al. first modified the terminal hydroxyl groups of the second-generation Boltorn hyperbranched resin with succinic anhydride, and then added glycidyl methacrylate dropwise to the above modified product to prepare a product with an acrylic acid structure at the end, and then added glycidyl methacrylate to the above modified product. A UV-curable waterborne polyurethane system was obtained after the neutralization and water dispersion steps. They found that the higher the content of salt-like structure in the structure, the better the water solubility. Adding a small amount of water or increasing the temperature can make the system viscosity decrease rapidly. In addition, in the presence of photoinitiators, the UV curing rate showed an upward trend with the increase of the content of acrylic groups in the structure. Asif et al. also carried out similar modification on the synthesized hyperbranched polyester, and found that the viscosity of the WPUA system with hyperbranched structure was much lower than that of the commercial linear waterborne polyurethane product EB 2002. The crosslinking density and thermal stability have a great influence.

 

In the UV curing waterborne coating system, the photoinitiator is generally oil-soluble and has poor compatibility with the waterborne system, resulting in low curing speed and poor curing effect. On the other hand, the small molecule photoinitiators are often not fully consumed during the curing process, and will remain in the cured film or migrate to the surface of the cured film, affecting its mechanical properties. To this end, Chen Mengru et al. grafted acryloyl groups, carboxyl groups and photosensitive groups on the ends of hyperbranched polyesters through chemical modification methods to obtain UV-curable water-based hyperbranched polyesters containing photosensitive groups. The system of agents was compared. The results show that the system can act as a macromolecular initiator to initiate and cure waterborne coatings without the addition of photoinitiators, and the initiation effect is better than that of traditional UV-curable waterborne coatings with small molecular initiators.

 

2 Application of hyperbranched waterborne polyurethane acrylate

 

2.1 Light-curing 3D printing photosensitive resin

 

The photosensitive resin for light-curing 3D printing needs to be sprayed at high temperature and cured at room temperature, and has certain requirements for viscosity. In addition, the resin needs to have low volatility, good jetting and rheology, no sedimentation, blocking phenomenon, curing After that, the resin is required to have high precision and good mechanical properties. Therefore, it is very important for the development of 3D printing technology to make full use of the characteristics of various photosensitive resins, master the properties of resins, and improve the performance of 3D printing products by modifying resins.

 

Different photosensitive resins have different properties and different application ranges. Before use, it is necessary to comprehensively consider whether the properties of the photosensitive resin (such as viscosity, shrinkage, hardness, chemical stability, etc.) are suitable for 3D printing technology. For its shortcomings, try to modify it by physical or chemical methods to make it suitable for 3D printing. Product performance is not significantly affected. At present, there is still a lot of research and development space for the modification of photosensitive resins. In addition, some photosensitive resins may have more than one synthesis method, and the most suitable synthesis method should be selected based on factors such as energy consumption, price, environmental protection, feasibility and actual operating conditions.

 

Polyurethane acrylate has good flexibility, high wear resistance, strong adhesion and good optical properties, but the comprehensive performance of water-based polyurethane acrylate used to produce environmentally friendly products is not ideal, which affects its scale of use, resin coloring stability, viscosity, Strength, hardness, hydrophobicity, hydrophilicity, thermal stability, etc. all need to be enhanced by modifying the molecular structure. The hyperbranched modification of water-based polyurethane acrylate can significantly reduce the viscosity and surface tension of the resin, increase the solubility, film-forming performance, low temperature flexibility of the resin, reduce the application of organic diluents, and be beneficial to the protection of the environment. Improving the application of waterborne urethane acrylate photosensitive resin in 3D printing is of great significance for the hyperbranched modification of waterborne urethane acrylate photosensitive resin.

 

The research on photosensitive resins for photocuring 3D printing at home and abroad mainly focuses on:

• the properties and applications of different photosensitive resins. By studying various properties of photosensitive resins (such as viscosity, hardness, curing rate, compression resistance, etc.), select resins with corresponding properties to obtain ideal 3D printing products.
• Modification of photosensitive resin. By modifying the photosensitive resin, the influence of the small molecule photoinitiator on the photosensitive resin system is reduced.
• Development and innovation of new materials. The rapid development of this field can only be promoted by developing new resins on the basis of theoretical research on the synthesis and modification of original photosensitive resins.

 

 

2.2 Other applications

 

Hyperbranched silicone-modified urethane acrylates can also be used in the medical field. British medical device manufacturer Aortech International uses hyperbranched silicone-modified urethane acrylate for a new artificial heart valve and explores its potential for use in a range of implantable human devices, polymerizing urethane acrylate with silicone, hyperbranched Combined with materials, it has good durability, flexibility and safety.

 

Now there is research to use polysiloxane hyperbranched urethane acrylate copolymer in the field of liquid crystal. Liquid crystal polysiloxane urethane acrylate has both the properties of liquid crystal and the elasticity of rubber, has good film-forming properties, and can be made into various liquid crystal film.

 

3 Outlook

 

In recent years, with the improvement of the synthesis process of hyperbranched urethane acrylate photosensitive resin, the application of hyperbranched waterborne urethane acrylate photosensitive resin in the field of photocuring 3D printing has become more extensive. But there is still a lot of research space: (1) When hyperbranched waterborne polyurethane acrylate photosensitive resin is used as a photocurable 3D printing material, reactive diluents need to be added, which will have an impact on the environment during its curing process, which should be further reduced or Avoid the use of reactive diluents, and find a reagent with lower volatility and can well adjust the viscosity of the system instead of reactive diluents; (2) Research on the modification of hyperbranched urethane acrylate photosensitive resin, and adjust the system from raw materials Viscosity, physical and chemical properties, photocuring properties and film-forming properties can further meet the needs of photocuring 3D printing, thereby reducing the use of reactive diluents; (3) Try to bond hyperbranched water-based urethane acrylate and photoinitiator, Reduce the use of small molecule photoinitiators, thereby increasing the photocuring rate.

 

4 Conclusion

 

The hyperbranched modification of urethane acrylate can further improve its fluidization properties, and a large number of end-group active functional groups in the hyperbranched system make it have better reactivity. Moreover, the non-entanglement between hyperbranched molecules greatly reduces the viscosity of hyperbranched urethane acrylate, thereby improving the rheology of the system, thus making hyperbranched urethane acrylate more widely used.

 

Light-curing 3D printing technology has the advantages of fast speed, strong applicability, high degree of automation, and easy control. These advantages determine that the study of hyperbranched waterborne polyurethane acrylate photosensitive resin is of great significance. The widespread use of 3D printing technology will also promote photosensitive resins. Towards diversification and high performance.