Basic Concept of Waterborne Polyurethane

Waterborne polyurethane resin has gradually replaced solventborne and become an important direction for the development of the polyurethane industry. Waterborne polyurethane can be widely used in coatings, adhesives, fabric coatings and finishes, leather finishing agents, paper surface treatment agents and fiber surface treatment agents.

This project has been funded by the National Natural Science Foundation of China and more than 10 years of research and development, has a mature anionic self-emulsifying polyurethane emulsion and cationic self-emulsifying polyurethane emulsion synthesis and modification of the technology, can provide 1 ton / day production capacity of waterborne polyurethane production of a complete set of processes and equipment technology. The project can design and adjust the formula of waterborne polyurethane to meet the requirements of actual use according to the needs of users, and can combine with nano-hybridization technology to prepare high-performance waterborne polyurethane. According to the particle size and appearance can be divided into polyurethane aqueous solution (particle size <0.001 micron, transparent appearance), polyurethane aqueous dispersion (particle size: 0.001-0.1 micron, semi-transparent appearance), polyurethane emulsion (particle size >0.1 micron, turbid appearance);

Depends on the hydrophilic group of the nature of the charge, the water-based polyurethane can be divided into anionic water-based polyurethane, Cationic waterborne polyurethane and nonionic waterborne polyurethane. Among them, the anionic type is the most important, which is divided into two categories: carboxylic acid type and sulfonic acid type.

Waterborne polyurethane can be divided into polyether, polyester and polyether, polyester hybrid according to the different synthetic monomers. In accordance with the selection of diisocyanate, waterborne polyurethane can be divided into aromatic and aliphatic, or specifically divided into TDI-type, HDI-type and so on.

Waterborne polyurethane can be divided into one-component waterborne polyurethane and two-component waterborne polyurethane according to the form of product packaging.

The whole synthesis process of waterborne polyurethane can be divided into two stages. The first stage for the pre-gradual polymerization, that is, by the oligomer diol, chain extender, waterborne monomer, diisocyanate through the solution of gradual polymerization to generate the relative molecular mass of l000 waterborne polyurethane pre-polymer; the second stage for the dispersion of the pre-polymer in the water after the neutralization.

Waterborne PU because of its environmental role, although the history is not long, but the development is very rapid.

Waterborne polyurethane including polyurethane aqueous solution, aqueous dispersion and aqueous emulsion three kinds, for the binary gelatinous system, polyurethane (PU) particles are dispersed in a continuous water phase, also known as waterborne PU or water-based PU. Due to the diversity of the polyurethane raw materials and formulas, the development of waterborne polyurethane in the development of 40 years or so, people have been studied in a number of kinds of preparations and preparations of formulas. There are many varieties of waterborne polyurethane, which can be categorized in many ways.

⒈ to the appearance of points

Waterborne polyurethane can be divided into polyurethane emulsion, polyurethane dispersion, polyurethane aqueous solution. The most practical application of polyurethane emulsions and dispersions, collectively referred to as waterborne polyurethane or polyurethane emulsions in this book.

Peake according to the form of use

Waterborne polyurethane adhesive can be divided into two categories of one-component and two-component according to the form of use. Can be used directly, or without crosslinking agent to get the required performance of waterborne polyurethane called one-component waterborne polyurethane adhesive. If used alone can not get the required performance, must add crosslinking agent; or general single-component waterborne polyurethane crosslinking agent can be added to improve adhesive properties, in these cases, waterborne polyurethane main agent and crosslinking agent of the two on the composition of the two-component system.

3 to the nature of hydrophilic groups

Based on the polyurethane molecule side chain or main chain contains ionic groups, that is, whether or not is an ionic bonded polymer (ionomer), waterborne polyurethane can be divided into anionic, cationic, non-ionic. Waterborne polyurethane containing anions and cations is also known as ionomer-type waterborne polyurethane.

(1) anionic waterborne polyurethane can be subdivided into sulfonic acid type, carboxylic acid type, with the side chain containing ionic groups mostly. Most of the waterborne polyurethanes contain carboxyl chain extender or sulfonate chain extender to attract carboxyl ion and sulfonic acid ion.

(2) cationic waterborne polyurethane generally refers to the main chain or side chain containing ammonium ions (generally quaternary ammonium ions) or sulfonium ions of waterborne polyurethane, the vast majority of cases are quaternary ammonium cation. The preparation of waterborne polyurethanes containing ammonium ions in the main chain is generally based on the use of tertiary amine group-containing chain extenders, tertiary amines and secondary amines by the action of acid or alkylation reagents, forming hydrophilic ammonium ions. Ammonium ions can also be formed through the reaction of amino-containing polyurethanes with epichlorohydrin and acids.

(3) non-ionic waterborne polyurethane, that is, waterborne polyurethane that does not contain ionic groups in the molecule. Nonionic waterborne polyurethane preparation methods are: ① ordinary polyurethane prepolymer or polyurethane organic solution in the presence of emulsifiers for high shear forced emulsification; ② made of molecules containing nonionic hydrophilic chain segments or hydrophilic groups, hydrophilic chain segments are generally low and medium molecular weight polyethylene oxide, hydrophilic groups are generally hydroxymethyl. (4) mixed type polyurethane resin molecular structure at the same time with away from the type and non-ionic hydrophilic groups or chain segments.

S⒋ Polyurethane raw materials

By the main type of oligomer polyol can be divided into polyether type, polyester type and polyolefin type, etc., which refers to waterborne polyurethanes made of polyether polyol, polyester polyol, polybutadiene glycol, etc. as oligomer polyol, respectively. There are also polyether-polyester, polyether-polybutadiene and other mixed polyurethane isocyanate raw materials can be divided into aromatic isocyanate type, aliphatic isocyanate type, alicyclic isocyanate type. According to the specific raw materials can also be subdivided, such as TDI type, HDI type, and so on.

Be careful to divide the polyurethane resin according to its overall structure

(1) According to the raw materials and structure can be divided into polyurethane emulsions, vinyl polyurethane emulsions, multi-isocyanate emulsions, closed polyurethane emulsions. Polyurethane emulsion refers to the emulsion formed by dispersing polyurethane prepared in the usual way in water with oligomer polyol, chain extender and diisocyanate as raw materials. Vinyl urethane emulsion generally refers to the emulsion formed by adding isocyanate to an aqueous solution or emulsion of vinyl resin, and is a two-component system. Polyisocyanate emulsion refers to the emulsion formed by emulsifying polyisocyanate containing hydrophilic groups in water, or dispersing organic solution of polyisocyanate in water containing emulsifier, which is also a two-component ready-to-use and ready-to-mix system, with a shorter application period. Enclosed isocyanate emulsion is a polyurethane emulsion containing enclosed isocyanate groups in the molecule, which is a stable one-component system. In the preparation of polyurethane emulsion Division introduced closed isocyanate groups, can also be made into closed isocyanate group content of high emulsion for and other emulsion system **** mixing, crosslinking, water evaporation after heating crosslinking.

(2) polyurethane emulsion can also be subdivided into polyurethane emulsion and polyurethane - urea emulsion, the latter refers to the polyurethane pre-polymer dispersed in water at the same time through the water or diamine chain expansion and the formation of emulsion, essentially generating a polyurethane - urea, but due to the pre-polymer dispersal method of preparation is more common, customarily known as the polyurethane emulsions are predominant.

(3) according to the molecular structure can be divided into linear molecules of polyurethane emulsions (thermoplastic) and crosslinked polyurethane emulsions (thermosetting). Crosslinking type can be subdivided into internal crosslinking and external crosslinking. Internal crosslinked polyurethane emulsions are synthesized to form a certain degree of branched crosslinked molecular structure, or the introduction of thermally reactive groups, it is a stable one-component system. Diplomatic cross-linking is the addition of a cross-linking agent to the emulsion that can react with the groups in the polyurethane molecular chain, and it is a two-component system.

Optional classification according to the method of waterborne polyurethane

There are various classifications according to the method of preparation. Examples are as follows.

(1) Self-emulsification method and external emulsification method

The self-emulsification method, also known as the internal emulsification method, is a method in which a hydrophilic component is contained in the urethane chain segments, and thus a stable emulsion can be formed without the need for an emulsifier.

The external emulsification method, also known as the forced emulsification method, requires the addition of an emulsifier to obtain an emulsion if the molecular chain contains only a small amount of hydrophilic chain segments or groups that are not sufficient for self-emulsification, or if it contains no hydrophilic components at all.

In comparison, the emulsion prepared by the external emulsification method, due to the residue of hydrophilic small molecule emulsifiers, affecting the performance of the cured polyurethane film, while the self-emulsification method eliminates this drawback. The preparation of waterborne polyurethane is dominated by the ionic self-emulsifying method.

(2) pre-polymerization method, acetone method, melt dispersion method

Self-emulsifying waterborne polyurethane the most commonly used methods of pre-polymer dispersion method and acetone method. Pre-polymer method is to guide the hydrophilic component in the pre-polymer, to get a certain viscosity range of the pre-polymer, emulsification in water at the same time chain growth, the preparation of stable waterborne polyurethane (waterborne polyurethane - urea).

Acetone method belongs to the solution method, which is to dilute or dissolve the polyurethane (or prepolymer) with an organic solvent and then emulsify it. In the presence of solvents, prepolymers and hydrophilic chain extender for chain expansion reaction to generate higher molecular weight polyurethane, the reaction process can be added according to the need for solvents to reduce the viscosity of polyurethane solution, so that it is easy to stir, and then add water for dispersion, the formation of emulsions, and finally evaporate the solvent. The solvent is mostly acetone and methyl ethyl ketone, so it is called acetone method. The advantages of this method is acetone, methyl ethyl ketone boiling point is low, and water mutual compatibility, easy to recycle, the whole system is uniform, easy to operate, due to reduce the viscosity at the same time also reduces the concentration, is conducive to the emulsification of high molecular weight before the preparation of prepolymer or polyurethane resins, the emulsion obtained

film properties than simple prepolymer method is good. While the prepolymer method due to viscosity limitations, in order to facilitate shear dispersion, the molecular weight of the prepolymer can not be too high, may affect the waterborne polyurethane performance, for example, high viscosity is difficult to emulsify, particle size, emulsion stability; prepolymer molecular weight of small NCO group content is high, the formation of urea bonding after emulsification of the urea bond is more hard, the film is hard, lack of softness.

The main difference between the acetone method and the prepolymer method is that in the acetone method, the polyurethane is first prepolymerized into a larger molecular weight prepolymer, due to the high molecular weight prepolymer viscosity, it must be diluted to reduce the viscosity; while the prepolymer method can be used according to the needs of the small amount of solvents, such as acetone, can be added or not added. The two concepts have inter*, some emulsification methods are both acetone method and prepolymer method. Melt dispersion method, also known as melt dispersion method, prepolymer dispersion formaldehyde chain expansion method. Pre-synthesis containing tertiary amine groups (or ionic groups) end NCO group prepolymer, and then with urea (or ammonia) in the body system reaction, the formation of polyurethane bicontracted diureas (or ionic groups containing end urea group) oligomers, and the addition of chlorinated amides in the high-temperature molten state to continue the reaction, continue to quaternary amination.

Polyurethane bis-conjugated diurea ionomer has enough hydrophilicity, plus acid dilute aqueous solution to form a homogeneous solution, and then react with formaldehyde aqueous solution for hydroxymethylation, hydroxymethyl-containing urethane strict contraction of diurea can be diluted with unlimited water in the 50-130 ℃, the formation of stable emulsions. When lowering the pu value of the system, it can carry out condensation reaction in the dispersed phase to form high molecular weight polyurethane. Containing ionic groups of terminal NCO prepolymer to form terminal urea-based or diureido polyurethane oligomer, then directly in the molten state emulsified in water, and then add formaldehyde aqueous solution for hydroxymethylation and chain expansion reaction.

(3) binary amine direct chain expansion and ketoimide - keto nitrogen method

In the prepolymer dispersion method, if the use of water-soluble dibasic primary amine chain expander chain expansion, due to the fast reaction speed of a NCO and a NH2, it is not easy to get a micro-fine and homogeneous emulsion, ketoimide or keto nitrogen can be used to solve this problem. Ketoimine - ketolian nitrogen method refers to the prepolymer and the ketone protected diamine (ketoimine system) or hydrazine (ketolian nitrogen system) after mixing, and then dispersed with water, dispersion process, ketoimine, ketolian nitrogen hydrolyzed at a certain rate, releasing free diamine or hydrazine and dispersed polymer particles reacted to get the waterborne polyurethane - urea has good performance. Waterborne polyurethane coatings are a class of coatings based on waterborne polyurethane resin and using water as the dispersion medium. The waterborne polyurethane coatings modified by crosslinking have good storage stability, film mechanical properties, water resistance, solvent resistance and aging resistance, and have similar properties with traditional solvent-based polyurethane coatings, which is an important development direction of waterborne polyurethane coatings. The varieties mainly include several varieties of thermosetting polyurethane coatings and waterborne polyurethane coatings containing closed isocyanate.

(1) thermosetting polyurethane coatings. Cross-linked polyurethane can increase its solvent resistance and hydrolytic stability. Polyurethane aqueous dispersion in the application with a small amount of additional crosslinking agent mixed with the composition of the system called thermosetting waterborne polyurethane coatings, also called diplomatic waterborne polyurethane coatings. The cross-linking agent used is mainly multi-functional aziridine, amino resin (trichloramine resin) or special epoxy resin. The use of aziridine, the general dosage of 3% -5% of the mass of polyurethane, there is a very good crosslinking film generation;

(2) waterborne polyurethane coatings containing closed isoamino acid ester. The film-forming raw material of the coating consists of two parts: a polyisocyanate component and a hydroxyl-containing component. Polyisocyanate is closed by phenol or other compounds containing monofunctional groups of active hydrogen atoms, so that the two parts can be combined and not reacted to become a single-component coatings, and has good storage stability. Polyisocyanate components and phenol, malonate, caprolactam and other sealing agents to generate urethane bond reaction, and ammonia ester bond in the case of heating and cleavage to generate isocyanate, and then with hydroxyl components to generate polyurethane reaction. Therefore, the film formation of closed polyurethane waterborne coatings is the use of different structures of ammonia ester bond differences in thermal stability, to the more stable ammonia ester bond to replace the weaker ammonia ester bond. There are many types of sealers, but aromatic isocyanate waterborne polyurethane paints mainly use phenol or cresol. Aliphatic waterborne polyurethane paint is not used phenol to avoid discoloration, can be used ethyl lactate, caprolactam, diethyl malonate, acetylacetone, ethyl acetoacetate, etc.;

3) room temperature curing waterborne polyurethane coatings. For some heat-sensitive substrates and large-scale parts, can not be heated crosslinking, must use room temperature crosslinking waterborne polyurethane coatings. The U.S. Air Products and Chemicals Corporation reported that the performance of waterborne end-hydroxy polyurethane prepolymer/acrylate blends, especially hydroxyacrylate blends, can be improved by combining them with water-dispersible polyisocyanates. This type of waterborne polyurethane coatings, the use of specially formulated polyisocyanate crosslinking agent, that is, containing (-NCO) end group of the isocyanate prepolymer, dispersed in a variety of hydroxyl-containing polymers after hydrophilic treatment and dispersions, and a variety of hydroxyl-containing polymers aqueous dispersions composed of polyurethane coatings capable of being cured at room temperature in aqueous coatings;

(4) light-curing waterborne polyurethane coatings. Light curing waterborne polyurethane coatings using electron beam radiation, ultraviolet radiation, high-intensity radiation triggered by the low activity of the polymer system to produce crosslinking curing, in the form of ultraviolet light curing. Unsaturated polyester polyol is first used to prepare prepolymer, and then the conventional method of introducing particle groups, after hydrophilic treatment to produce in the main chain with a double bond polyurethane aqueous dispersion, and then with soluble high activity alkoxy triacrylate monomer, photosensitizer and other additives to get the light-curing waterborne polyurethane coatings;

5 (5) the third generation of waterborne polyurethane coatings (PUA). Polyurethane (PU) emulsion and polyacrylic acid (PA) emulsion with its solvent-based products, compared with cheap, safe, non-combustible, non-toxic, non-polluting and other advantages. Pure PA emulsions exist wear resistance, water resistance and chemical resistance defects, a single PU emulsion also has some shortcomings, such as stability, white thickening and film light retention is poor, high solid content, the application range is not wide, etc. PU and PA in the nature of the complementary role of PU and PA composite emulsions have the advantages of the two, with wear-resistant, corrosion-resistant and bright, soft and flexible, water and mechanical properties, good weatherability and so on. PUA composite emulsion has the advantages of wear resistance, corrosion resistance and brightness, softness and elasticity, water resistance and mechanical and mechanical properties, good weather resistance, etc. Therefore, it is regarded as the third generation of waterborne polyurethane, and has become a development trend of today's coating. Application range

Waterborne PU dispersions have been used in large quantities in the fields covered by general-purpose solvent-based PU, and have been successfully applied in industries such as light textiles, leather processing, coatings, wood processing, building materials, papermaking and adhesives.

Leather industry in the processing of PU emulsion coated leather, with high gloss, feel good, abrasion resistance, not easy to break, good elasticity, low temperature resistance and flexural performance, etc., to overcome the acrylic resin finishing agent, "hot viscous cold brittle" defects.

In addition, the textile coating has a wide range of applications. Water-based PU on the textile film is good, high bonding strength, can give the fabric a soft, full-bodied feel, improve the fabric wear resistance, wrinkle resistance, resilience, permeability and heat resistance.

Waterborne PU than organic solvent-based PU applications, low cost, pollution-free, easy to handle, good adhesion, in the adhesives and coatings industry has a very good future. PU ionic polymer on the surface of the natural and synthetic rubber have a very good adhesion, can be used in the manufacture of footwear.

Waterborne PU is mainly used as furniture paint, electrophoretic paint, electrodeposition coating, architectural coatings, paper treatment coatings, fiberglass coatings, etc. In addition to this waterborne coatings also have some special uses, such as used as an intermediate coating film for safety glass to make non-shattering safety glass, which is widely used in automobiles, airplanes, ships or aerospace instruments.

Waterborne dispersions are mainly used as metal coatings, such as cationic electrodeposition coatings are widely used in automotive primers to improve the corrosion resistance of the car body.