Waterborne polyurethane adhesive characteristics and classification (on)

Aqueous polyurethane adhesive refers to the adhesive formed by polyurethane dissolved in water or dispersed in water, also known as water-based polyurethane adhesive.

1. Features

In addition to the advantages of polyurethane, waterborne polyurethane adhesives have unique characteristics compared to solvent-based polyurethane adhesives.

(1) Water is used as a medium, non-toxic, non-flammable, non-hazardous, non-hazardous, with small odor, does not pollute the environment, saves energy, and is suitable for substrates that are easily eroded by organic solvents.

(2) The viscosity is low, and it can be adjusted with water-soluble thickener and water. It is easy to operate and the residual glue is easy to clean.

(3) Can be mixed with a variety of water-based resins, which will help improve performance and reduce costs, but pay attention to ionic properties and acidity and alkalinity to prevent condensation.

(4) Contains carboxyl, hydroxyl and other groups, can participate in the reaction under appropriate conditions, produce cross-linking, improve performance

(5) Poor wettability to non-polar substrates

(6) Slow drying, low initial viscosity, and poor water resistance.

2, classification

Can be classified in many ways

First, according to the structure of polyurethane

(1) According to the raw materials and structure can be divided into polyurethane emulsion, vinyl polyurethane emulsion, polyisocyanate emulsion, closed polyurethane emulsion.

(2) Polyurethane emulsions can also be subdivided into polyurethane emulsions and polyurethane-urea emulsions. The latter refers to emulsions in which a polyurethane prepolymer is dispersed in water while being extended by water or diamine chain extension.

(3) According to the molecular structure, it can be divided into linear polyurethane emulsion (thermoplastic) and cross-linked polyurethane emulsion (thermosetting). Cross-linked type is divided into internal cross-linked and diplomatic link.

Second, according to the nature of hydrophilic groups

(1) Anionic Most aqueous polyurethane adhesives introduce carboxyl or sulfonic acid ions by chain extension with carboxyl groups or sulfonates.

(2) Cationic type generally refers to an aqueous polyurethane containing ammonium ions (mostly quaternary ammonium ions) or sulfide ions in the main chain or side chains.

(3) Non-ionic molecules contain non-ionic hydrophilic segments or hydrophilic groups.

(4) There are both ionic and non-ionic hydrophilic segments or groups in the mixed molecular structure.

Third, according to the preparation method

(1) Self-emulsification method and external emulsification method.

(2) Prepolymer method, acetone method, melt dispersion method.

(3) Ketimine-methoxone nitrogen method.

Fourth, according to the use of classification

According to the use form can be divided into one-component and two-component waterborne polyurethane adhesive.

3, preparation of waterborne polyurethane

The preparation of water-borne polyurethanes is different from the methods used for general synthetic resin emulsions. It is generally necessary to pre-react oligomeric glycols (or polyols) and diisocyanates to produce prepolymers of relatively high mass or high molecular weight. Polyurethane, after which it is dissolved or emulsified in water using a phase transfer method. The key to preparing a polyurethane emulsion is to choose the appropriate dispersion method.

1. Preparation Method of Waterborne Polyurethane

Waterborne polyurethanes are prepared in the following ways.

(1) Preparation of NCO-terminated polyurethane prepolymers from oligomeric glycols, diisocyanates, or low molecular chain extenders, or preparation of high molecular weight polyurethanes in organic solvents, in emulsifiers and high shear Emulsification. It is called forced emulsification method.

(2) Polyethylene oxide diols (or blends of PEO-containing copolymer diols, and other oligomer polyols) of low to medium molecular weight as oligomer diol starting materials, and diisocyanates ( And chain extenders) Prepare polyurethanes or other prepolymers and redisperse in water.

(3) First prepare NCO prepolymers containing hydrophilic chains or groups such as PEO, and then react with and emulsify with sodium bisulfate water; prepolymers can also be blocked with ketoxime or caprolactam The reaction and emulsification in water form a closed polyurethane emulsion. Also known as blocked isocyanate method.

(4) The NCO polyurethane prepolymer reacts with a polyamine to prepare a polyurethane-urea-polyamine (PUUA) which is then emulsified in an aqueous solution. Or the PUUA and epichlorohydrin adducts are emulsified in an acidic aqueous solution to give a cationic polyurethane emulsion. PUUA reacts with dibasic anhydride and emulsifies in alkaline aqueous solution to obtain anionic polycondensation

Urethane emulsion.

(4) A polyurethane or its prepolymer is prepared by using a chain extender containing a carboxyl group, a sulfonic acid group or a tertiary amine group, neutralized, and an ionic polyurethane is prepared and emulsified.

(5) The amine group of polyurethane reacts with formaldehyde, or the NCO-containing polyurethane prepolymer reacts with excess triethanolamine to render the polyurethane with a hydrophilic hydroxymethyl group.

(6) The polyurethane prepolymer is prepared and ionized using oligomer polyols containing carboxyl, sodium sulfonate, or tertiary amine groups and emulsified in water.

2. Preparation of carboxylic acid polyurethane emulsion

Polyurethane prepolymers are synthesized from oligomeric diols, diisocyanate-containing dihydroxy compounds, and emulsified in water. A commonly used carboxyl-containing chain extender is dimethylolpropionic acid (DMPA).

The specific synthesis method is to prepare a prepolymer with an NCO content of 10.7% by reacting 400 g of polyether polyol with XDI, and then chain-extending with DMPA 83 g and 56 g of 1,4-butanediol in acetone to obtain a 50% concentration. Polyurethane solution. Neutralized with 50 g of triethylamine, emulsified with water to a polyurethane emulsion having a solid content of 30%. The emulsion was added with 3% glycerin polyglycidyl ether as a cross-linking agent to obtain a water-based polyurethane adhesive for food flexible packaging composite films.

3, sulfonic acid polyurethane emulsion

A polyurethane prepolymer was prepared by reacting 2000 g of poly(ethylene adipate neopentylglycone neopentyl glycol) diol (hydroxyl value 56 mgKOH.g-1) and HDI 289 g in 5500 ml of acetone for several hours under heating. Further, about 550 g of an aqueous solution containing 47.5 g of sodium ethylenediamine-sodium ethanesulfonate was added, and the mixture was stirred and emulsified, acetone was distilled off under reduced pressure, and the mixture was diluted with water to obtain a polyurethane solution.

4, cationic polyurethane emulsion

5, non-ionic water polyurethane

Bisphenol A was used as a starting polyurethane polyglycol 396 g, polyoxypropylene triol 890 g, TDI 365.4 and toluene 165 g to make a polyurethane prepolymer. 4 g of nonionic surfactant (polyoxyethylene glycol and bisphenol A epoxy resin as raw materials) was dissolved in 70 g of water, added to 110 g of the above prepolymer diluted with 30 g of toluene at 100 C, stirred vigorously, and added with 2- In 4.7 g of cold water containing 4.9 g of methylpiperazine and 1.6 g of morpholine, the prepolymer was emulsified and chain extended to obtain a polyurethane emulsion. The film has a tensile strength of 14.2 MPa, an elongation of 420%, and a hardness (Shore A) of 67.

4, performance

The micro-particle size of the polyurethane emulsion has a great influence on the appearance, stability, film formation, wettability, film properties and adhesive strength of the emulsion. The smaller the particle size, the more transparent the emulsion is. The finer the particles of the emulsion, the better the various properties of the emulsion.

The surface tension is an important factor influencing the wettability of the aqueous polyurethane to the substrate. The surface tension of waterborne polyurethane is generally 40~60mN.m-1. In order to effectively bond the low energy surface, wetting (leveling agent), such as 15% organic fluorine surfactant (Megafac F-813), can be added. Reduce the surface tension of polyurethane emulsions.

The main medium of waterborne polyurethane is water, which is usually less volatile than organic solvents. Therefore, the drying speed is slow. The minimum film forming temperature of waterborne polyurethane adhesive is about 00C, because it does not contain emulsifiers, and it can form shiny, uniform, and toughness when dried at room temperature. film. The dry film formation of the emulsion is related to the humidity and temperature of the air.

In general, the waterborne polyurethane has a lower solid-state strength than the solvent-based polyurethane, but the performance of the waterborne polyurethane can be comparable to a solvent-based one through the choice of raw materials, formulations, and processes, such as BASF's anionic polyether type. The properties of waterborne polyurethane composite film adhesives have reached the level of two-component solvent-based polyurethane adhesives.

Waterborne polyurethanes are generally composed of water-containing groups, especially polyurethanes containing ionic groups. If salt-forming reagents can escape when dried, the carboxyl-hydrophilicity will be weak and the film will obtain hydrophobicity; if it is a thermoplastic polyurethane, salt-forming reagents Can not escape, then the carboxyl hydrophilic groups remain, then the glue film poor water resistance. Water resistance can be improved by heat treatment and using a cross-linking agent. Waterborne polyurethanes have more polar groups, such as urethane bonds, urea bonds, ion bonds, and have good adhesion to polar substrates and porous substrates, especially for soft PVC containing plasticizers. Excellent adhesion, but there are differences in the bonding strength of different substrates.

The storage stability of polyurethane emulsions is related to the microparticle size and the hydrolysis resistance of polyurethanes. The emulsion is usually centrifuged at 3000 r.min-1 for 15 min after centrifugation. If no precipitation occurs, it can be considered as a 6-month storage stability period. If the polyurethane is poor in hydrolysis resistance, it will degrade slowly during storage, produce carboxyl groups, lower the pH, and cause the emulsion to coagulate. Freezing and long-term high temperatures should be prevented during storage.