Research progress and analysis of water-based polyurethane binder

At present, in the packaging and printing industry, polyolefin films occupy the first place in the printing and packaging film base materials, such as biaxially oriented polypropylene (BOPP) film, polyethylene (PE) film, etc., by polyethylene terephthalate glycol. Ester (PET) film, nylon (PA) film, etc. Water-based polyurethane molecular chains contain more polar groups and have high surface tension. Therefore, WPU inks are suitable for surface coating of highly polar substrates such as PET and PA. BOPP, as an important printing substrate, has lower Surface energy, so WPU is difficult to wet on its surface, resulting in poor printing quality [2-4].

In order to improve the applicability of WPU ink to the substrate of BOPP film, the main methods currently used are: First, surface treatment such as corona treatment and coating treatment is performed on the film before printing, and polar groups such as carboxyl and hydroxyl groups are introduced to the surface. , to increase the surface tension of the BOPP film, thereby improving the wettability and adhesion of the WPU ink; second, adding adhesion promoters to the water-based ink, such as silicone, chlorinated polypropylene, etc., can reduce the adhesion of the water-based ink. Surface Tension. The third is to carefully design the molecular structure of WPU to reduce the content of polar groups and surface tension in its molecular chain to achieve the goal of improving its printing quality on BOPP films. This is one of the more researched methods currently.

Silicone has the advantages of low surface energy, good biocompatibility, high thermal stability and oxygen resistance, and has been widely used in the modification of polyurethane materials [5]. Li et al. [6] studied the blending modification and in-situ modification of WPU emulsion with polyorganosiloxane and found that the use of physical blending method can more effectively reduce the surface energy of WPU. Taking advantage of the low surface energy of fluorine-containing compounds, introducing fluorine-containing groups into waterborne polyurethane molecules can effectively reduce the surface energy of water-based polyurethane and improve hydrophobicity. For example, Xu et al. [7] performed hydroxylation modification of dodecafluoroheptyl methacrylate (DFHMA) to synthesize EDFHMA, then reacted with alcoholized lactide to synthesize fluorine-containing glycol (PLPF), and then reacted with hexamethylene diol Polyurethane was prepared by isocyanate (HDI) reaction. Compared with the control group, the surface energy of WPU containing EDFHMA decreased by nearly 20 mN/m. In addition, relevant studies have shown that grafting long fat side chains into the WPU molecular chain can also reduce the surface tension of WPU, and during the film formation process of WPU, long fat side chains will aggregate to the film surface, which is beneficial to the interaction with low polarity materials. A similar compatibility effect occurs in the BOPP film, which improves the adhesion of WPU on the surface of the BOPP film. Based on this, Zhang et al. [8] used liquid polyester polyol BY3003 with long branched aliphatic chains to prepare WPU latex suitable for BOPP film printing. BY3003 makes the surface tension of the prepared latex not exceed 43 mN/m, while the surface tension of traditional WPU latex exceeds 55 mN/m. Therefore, the T-peel strength of inks made from these latexes is above 0.8 N/15 mm.

In addition, the degree of post-chain extension, dimethylol butyric acid content and NCO/OH molar ratio also have a significant impact on the latex and film properties of WPU, especially on the T-peel strength of the corresponding ink. By optimizing these factors, a water-based polyurethane emulsion with a surface tension as low as 39.6 mN/m and an adhesion fastness to BOPP films exceeding 95% was obtained, with a corresponding T-peel strength of ink as high as 2.05 N/15 mm [ 8] .

Inkjet printing has become an essential output method, and research on output devices and printing inks is also continuing to deepen. The printability of an ink is related to transfer and wetting properties such as viscosity, particle size and surface tension, and the coating properties are related to mechanical properties, hardness and aging resistance. In order to obtain WPU ink with excellent performance, Wang et al. [9] used an emulsion polymerization method with WPU as a seed to synthesize core-shell WPUA emulsions with different methyl methacrylate (MMA) contents. As the MMA content in WPUA increases, the average particle size and contact angle of WPUA increase, and the heat resistance and hardness of WPUA coating are enhanced. Inkjet printing inks prepared with WPUA emulsion as base resin show good printability. Yin et al. [10] used isophorone diisocyanate (IPDI), polyol, dimethylol butyric acid (DMBA) and 3,5-dimethylpyrazole (DMP) as raw materials to synthesize a series of block water-based Polyurethane (BWPU). DMP-terminated BWPU has good inkjet fluency and color fastness, and has great potential in digital inkjet printing industrial applications.

3D printing, also known as additive manufacturing technology, is the most representative molding technology in current intelligent production. It has the advantages of strong processability and high efficiency. It can be customized according to different needs and is suitable for equipment processing with complex structures. Manufacturing, it has broad application prospects in the fields of aerospace, offshore equipment manufacturing and biomedicine. Compared with traditional polyurethanes, most WPUs have poor mechanical properties, rheological properties, thermal stability and electrical conductivity, and have poor hydrolysis strength in humid environments. In order to overcome the above shortcomings, inorganic fillers such as carbon nanotubes, clay or graphene are usually introduced into the WPU matrix to form organic-inorganic hybrids, thereby improving its performance [11-13].

Vadillo et al. [14-15] improved the performance of new polycaprolactone-polyethylene glycol (PCLPEG) water-based polyurethane urea (WBPUU) ink in direct writing 3D by adding cellulose nanocrystals (CNC) in situ as a rheology modifier. Properties in printing technology that can improve the printability and shape fidelity of 3D structures, as well as improve the mechanical and thermal stability of the resulting parts.

Chen et al. [16] developed an in-situ synthesis method to modify WPU (WPUCNF) by using cellulose nanofibrils (CNF) to improve its printability. Adding CNF during the emulsification process reduces the size of WPU nanoparticles and increases the viscosity of the suspension. In addition, additional CNF was added to prepare WPUCN/CNF composite ink, which showed excellent printability in various shapes of printing structures such as honeycombs, wood piles, or human ears.

The inherent shortcomings of polyurethane such as high melting point and slow degradation rate hinder its application in 3D printing tissue engineering. In view of this, Feng et al. [17] developed a 3D printable amino acid-modified biodegradable water-based polyurethane (WBPU) using a water-based green chemical process. By controlling the content of the hydrophilic chain extender, the printed block has controllable degradation and does not cause the accumulation of acidic products. It is envisioned that it can be used as a biological alternative material for tissue engineering.

Currently, 3D printing methods can only create static objects and do not involve any functional changes in intrinsic or extrinsic properties, while 4D printing is defined as the use of 3D printing technology to create materials with active structures that respond to external forces such as heat, magnetism, or light. Stimulated, the material is able to change over time to change the printed 3D shape. There are two main types of polymer materials used for 4D printing: responsive hydrogels and shape memory polymers (SMP). Among various SMPs, polyurethane displays a variety of properties that make it an excellent candidate for 4D printing. For example, in 2019, Su et al. [18] studied the formation of water-based polyurethane coating-based composites as 4D printing precursors by adding carboxymethyl cellulose (CMC) and silicon oxide (SiO2) nanoparticles to the coating.

Fused deposition modeling (FDM) is a rapid prototyping method used on 3D printers. In order to prepare WPU materials with excellent comprehensive properties and use them for surface protection of FDM printing products. In order to simultaneously improve the mechanical properties and waterproofness of the WPU membrane, Zhang Jing et al. [19] used in-situ polymerization and surface fluorination to prepare a halloysite nanotube/water-based polyurethane (AHNTs/WPU) composite membrane. The water contact angle increased. As large as 114.5°, it shows better hydrophobicity. A WPU composite film is formed on the surface of FDM. Experimental results show that it can improve the waterproofness and mechanical properties of the sample, and has an obvious surface protection effect.

Recently, Zheng Ling et al. [20] used silane coupling agent KH550 to carry out covalent bond functional modification of carbon black (CB), obtained KH550 modified CB, and prepared KH550/CB/WPU composite materials. The CB The addition significantly improves the thermal stability of WPU. The modified CB content was selected to be 3% for the preparation of 3D printing ink. Compared with other non-3D printing products, its conductive properties were improved by 1 to 2 orders of magnitude.

In addition, compared with traditional linear macromolecules, the three-dimensional spherical structure of hyperbranched polymers has abundant end groups and lower viscosity, which can provide more modification sites [21] and is therefore widely used in optical applications. Cured coatings, 3D printing photosensitive resin and other fields. Zhang Dongqi et al. [22] prepared hyperbranched water-based polyurethane acrylate by esterifying hyperbranched polyester polyol containing 16 terminal hydroxyl groups with succinic anhydride and reacting with the isocyanate group of isocyanate ethyl acrylate to introduce double bonds. Then, using it as the matrix resin, a series of 3D printing water-based photosensitive resins were prepared by compounding it with the reactive diluting monomers acryloylmorpholine and polyethylene glycol diacrylate. The prepared 3D printing devices have better printing properties. Accuracy.