Classification of water-based polyurethane adhesives

Due to the diversity of polyurethane raw materials and formulations, many preparation methods and formulations have been developed in the development of water-based polyurethane for about 40 years
.
There are many varieties of water-based polyurethane, which can be classified by various methods

1.

Waterborne polyurethane can be divided into polyurethane emulsion, polyurethane dispersion, and polyurethane aqueous solution
.
The practical application is polyurethane emulsion and dispersion, which is collectively referred to as water-based polyurethane or polyurethane emulsion in this book, and its appearance classification is shown in Table 5.
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com
.

Table 5 Form classification of waterborne polyurethane

Waterborne polyurethane adhesives can be classified in a number of ways:

2.

A.

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

2.

A.

B.
Cationic type: generally refers to the water-based polyurethane containing ammonium ions (mostly quaternary ammonium ions) or sulfur ions on the main chain or side chain
.

C.
Non-ionic: The molecule contains non-ionic hydrophilic segments or hydrophilic groups
.

D.
Mixed type: both ionic and non-ionic hydrophilic segments or groups exist in the molecular structure

3.

4.
According to the overall structure of polyurethane resin
(1) According to the raw materials and structure, it can be divided into polyurethane emulsion, vinyl polyurethane emulsion, polyisocyanate emulsion and closed polyurethane emulsion
.
Polyurethane emulsion refers to an emulsion formed by dispersing polyurethane prepared by common methods in water with oligomer polyol, chain extender and diisocyanate as raw materials
.
Vinyl polyurethane emulsion generally refers to the emulsion formed by adding isocyanate to vinyl resin aqueous solution or emulsion, which is a two-component system
.
Polyisocyanate emulsion refers to the emulsion formed by emulsification of polyisocyanate containing hydrophilic group in water, or dispersion of organic solution of polyisocyanate in water containing emulsifier.
It is also a two-component ready-to-use system with a short pot life
.
Blocked isocyanate emulsion refers to a polyurethane emulsion containing blocked isocyanate groups in the molecule, which is a stable one-component system
.
In the preparation of polyurethane emulsion, blocked isocyanate groups are introduced, and an emulsion with a high content of blocked isocyanate groups can also be made, which can be used for blending with other emulsion systems to play a cross-linking effect.
After the water evaporates, heat cross-linking.   

(2) Polyurethane emulsion can also be subdivided into polyurethane emulsion and polyurethane-urea emulsion.
The latter refers to the emulsion formed by dispersing polyurethane prepolymer in water and extending the chain by water or diamine, which essentially generates polyurethane-urea.
, but because it is more common to prepare by the prepolymer dispersion method, it is usually called polyurethane emulsion
.
 
  

(3) According to the molecular structure, it can be divided into linear molecular polyurethane emulsion (thermoplastic) and cross-linked polyurethane emulsion (thermosetting)
.
Cross-linking type can be subdivided into internal cross-linking and external-linking type
.
The interlinked polyurethane emulsion forms a certain degree of branched and cross-linked molecular structure during synthesis, or introduces thermally reactive groups, and it is a stable one-component system
.
Crosslinking is the addition of a crosslinking agent that can react with the groups in the polyurethane molecular chain in the emulsion.
It is a two-component system.   

5.
According to the water-based method of polyurethane,
there are various classifications according to the preparation method
.
An example is as follows
.

(1) Self-emulsification method and external emulsification method
Self-emulsification method, also known as internal emulsification method, refers to a method in which the polyurethane segment contains hydrophilic components, so that a stable emulsion can be formed without an emulsifier
.

(2) Prepolymer method, acetone method, melt dispersion method and
self-emulsifying method are commonly used methods for preparing waterborne polyurethane, including prepolymer dispersion method and acetone method
.
The prepolymer method is to introduce a hydrophilic component into the prepolymer to obtain a prepolymer with a certain viscosity range, which is emulsified in water while the chain is extended to prepare a stable water-based polyurethane (water-based polyurethane-urea)
.

The main difference between the acetone method and the prepolymer method is that in the acetone method, the polyurethane is first prepolymerized into a prepolymer with a larger molecular weight.
Since the prepolymer with a large molecular weight has a high viscosity, it must be diluted to reduce the viscosity; while the prepolymer method A small amount of acetone and other solvents can be added or not added as needed
.
The concepts of the two overlap, and some emulsification methods belong to both the acetone method and the prepolymer method
.
Melt dispersion method is also known as melt dispersion method, prepolymer dispersion formaldehyde chain extension method
.
Pre-synthesize the terminal NCO group prepolymer containing tertiary amine group (or ionic group), and then react with urea (or ammonia water) in the bulk system to form polyurethane biuret (or terminal urea group containing ionic group) ) oligomer, and add chloroamide to continue the reaction in a high temperature molten state and continue quaternary amination
.

Raw materials for waterborne polyurethane preparation
1.
Oligomer polyol: polyether diol, polyester diol, polyether triol, polybutadiene diol, acrylate polyol, etc.

The oligomer polyols commonly used in the preparation of waterborne polyurethane adhesives are generally polyether diols and polyester diols, and sometimes small varieties such as polyether triols, low-branched polyester polyols, and polycarbonate diols are also used.
Oligomeric polyols
.
Polyether polyurethane has good low temperature flexibility, good water resistance, and the price of commonly used polyoxypropylene glycol (PPG) is lower than that of polyester glycol.
Therefore, most of the waterborne polyurethane research and development in China is based on polyoxypropylene glycol.
Main oligomer polyol raw material
.
The mechanical strength and hydrolysis resistance of the polyurethane prepared from polytetrahydrofuran ether glycol are good, but its high price limits its wide application
.   

Polyester polyurethane has high strength and good adhesion, but because the hydrolysis resistance of polyester itself is worse than that of polyether, polyester waterborne polyurethane prepared from general raw materials has a shorter storage stability period
.
However, by using a hydrolysis-resistant polyester polyol, the hydrolysis resistance of the water-based polyurethane adhesive can be improved
.
The mainstream products of foreign polyurethane emulsion adhesives and coatings are polyester
.
The flexibility of the aliphatic non-regular structure polyester is also good, and the one-component polyurethane emulsion adhesive prepared from the regular structure crystalline polyester diol, the adhesive layer is thermally activated and bonded, and the initial strength is high
.
The water-based polyurethane made of aromatic polyester polyol has high adhesion to metals, RET and other materials, and has high cohesive strength
.   

Other oligomer diols, such as polycarbonate diol, polycaprolactone diol, polybutadiene diol, acrylate polyol, etc.
, can be used in the preparation of waterborne polyurethane adhesives
.
Polycarbonate polyurethane has good hydrolysis resistance, weather resistance and heat resistance, and is easy to crystallize.
Due to its high price, its wide application is limited
.
 
  

2.
Isocyanates: TDI, MDI, IPDI, HDI, etc.

Diisocyanates commonly used in the preparation of polyurethane emulsions include aromatic diisocyanates such as TDI and MDI, and aliphatic and alicyclic diisocyanates such as TDI, MDI, HDI: MDI
.
Polyurethanes made of aliphatic or cycloaliphatic diisocyanates have better hydrolysis resistance than polyurethanes made of aromatic diisocyanates, so waterborne polyurethane products have good storage stability
.
Foreign high-quality polyester water-based polyurethane is generally made of aliphatic or alicyclic isocyanate raw materials, while in China, limited by the variety and price of raw materials, most of them only use TDI as the diisocyanate raw material
.

Polymethylene polyphenyl polyisocyanates are generally used in the preparation of vinyl polyurethane emulsions and isocyanate emulsions
.
 

3.
Chain extender: 1,4-butanediol, ethylene glycol, hexanediol, ethylenediamine, etc.
 

4.
Water: distilled water, ionized water 

5.
Hydrophilic chain extender: dimethylol propionic acid (DMPA), dihydroxy half ester, sodium ethylenediaminoethanesulfonate, diethylenetriamine, etc.
 

6.
Salt-forming agent: HCL, acetic acid, propylene oxide

7.
Solvent: acetone, methyl ethyl ketone, toluene, etc.

8.
Emulsifier : polyethylene oxide-propylene oxide copolymer 

9.
Cross-linking agent: epoxy resin, melamine-formaldehyde resin, polyisocyanate

10.
Thickener: carboxymethyl cellulose, hydroxymethyl cellulose,
etc.