What is the progress of the research on waterborne polyurethane modification?

The full name of polyurethane (PU) is polyurethane, and the molecular chain contains characteristic groups such as urethane group (-NHCOO-)
.
PU is widely used because of its advantages of wear resistance, low temperature resistance and good flexibility
.
At present, solvent-based products account for a large proportion in the PU industry, but solvent-based PU contains volatile organic compounds, so its application is limited
.
With the enhancement of people's awareness of environmental protection, water-based polyurethane (WPU) has developed rapidly
.
Using water as the dispersion medium, WPU has the advantages of being environmentally friendly, non-toxic and non-flammable, and has been successfully used in automotive interiors, building materials and leather processing
.
Although WPU has outstanding advantages, it also has many shortcomings
.
The low solid content of WPU emulsion leads to its slow drying speed; in addition, WPU products have poor water resistance and heat resistance, and low bonding strength, so they need to be modified to improve their comprehensive properties
.
Scholars at home and abroad have done a lot of work on WPU modification.
The author summarizes the modification research of WPU in the past 10 years from the aspects of heat resistance, water resistance, adhesion and solid content
.

1.

 WPU is relative to solvent-based PU, which is a binary colloid system in which PU particles are dispersed in water
.
The raw materials for the preparation of WPU include polyisocyanates, hydroxyl-terminated compounds, catalysts, hydrophilic chain extenders, cross-linking agents and salt-forming agents
.
According to appearance and particle size, WPU can be divided into emulsion, water dispersion and aqueous solution; according to the properties of hydrophilic groups, WPU can be divided into anionic, cationic, amphoteric and non-ionic; according to the synthetic raw materials, WPU can be divided into For polyester, polyether and hybrid
.

2.

PU is difficult to dissolve or disperse in water, and its raw material polyisocyanate can react with water, so WPU cannot be directly prepared in water, but can be synthesized by external emulsification method and internal emulsification method
.
The external emulsification method refers to the method of adding PU prepolymer or solution into water for high-speed stirring and dispersion in the presence of an emulsifier to obtain an aqueous dispersion or emulsion
.
The internal emulsification rule is to introduce a hydrophilic group or a hydrophilic segment into the PU molecule, and directly disperse it in water to form a stable emulsion without an emulsifier
.
The WPU emulsion prepared by the external emulsification method has a wide particle size distribution and poor storage stability, so the internal emulsification method is usually used
.
Internal emulsification methods include prepolymer method, acetone method, melt dispersion method and end-group protection method
.

3.
Modification methods to improve the performance of WPU

3.
1 Modification method to improve heat resistance: The molecular chain of WPU contains ionic groups, and the molecular chain has a linear structure with low degree of cross-linking, so the heat resistance is poor
.
The heat resistance of WPU can be improved by the following methods:

(1) Nanomaterial modification
.
Taking advantage of the special properties of nanomaterials, nanomaterials such as nano-SiO2 and attapulgite are introduced into the polymer matrix by blending method and in-situ polymerization method to improve the heat resistance of WPU
.

(2) resin composite modification
.
Through mechanical blending, graft copolymerization, etc.
, it is compounded with resins with excellent heat resistance such as acrylates and epoxy resins to improve the degree of crosslinking and heat resistance
.

(3) Ultraviolet (UV) cross-linking modification
.
The UV cross-linking agent such as azide compound is introduced into the molecular chain of WPU, and the cross-linking reaction occurs under the irradiation of ultraviolet light
.

The nano-SiO2 modified WPU adhesive was prepared by in-situ polymerization, and the effect of nano-SiO2 concentration on the adhesive properties was studied by Fourier transform infrared spectrometer and particle size distribution analyzer
.
The results show that the heat resistance of the modified WPU is greatly improved.
When the mass fraction of nano-SiO2 is 2.
0%-2.
5%, the performance of the adhesive is the best, and it can also show high performance when the operating temperature is as high as 175℃.
adhesive properties
.

(1) Resin compound modification
.
By compounding with other resins with good water resistance, the degree of crosslinking is increased
.

(2) Design the rough surface structure
.
A rough structure with low surface energy was formed on the surface of the WPU film to enhance the surface hydrophobicity
.

(3) Decorate the surface with low surface energy materials
.
Fluorine has a shielding effect, the lowest polarizability, and the strongest electronegativity, which endow fluorinated WPUs with low surface free energy and improved water resistance
.
WPUs double modified with 3-aminopropyltriethoxysilane (KH550) and epoxidized soybean oil (ESO) were synthesized
.
The results show that the epoxy groups in ESO react with -NCO groups under acidic conditions to form a cross-linked structure; the reaction of KH550 with -NCO groups can further increase the cross-linking degree, and the fatty acid glycerides in ESO are hydrophobic , so the water resistance is greatly enhanced
.
When the mass fraction of ESO and KH550 were 4% and 2%, respectively, the comprehensive performance of the modified WPU reached the best
.
Hydroxyl-terminated poly(fluoroalkyl methacrylates) were synthesized to prepare fluorinated WPUs
.
The results show that the long fluorinated side chains migrate more easily to the surface, and the enrichment factor of fluorine surface is 10-18 times higher than that of the bulk
.
When the mass fraction of fluorine is 0.
1%, the contact angle of the film to water increases from 60° to 101°; when the mass fraction of fluorine is 0.
5%, the contact angle of the film to water and diiodomethane is 108°, respectively and 84°
.
WPU with excellent water resistance can be obtained by changing the fluorine content
.
FPMA/SiO2/WPU hybrid films with rough surface structure were prepared using fluorinated polymethacrylate (FPMA), modified SiO2 and WPU
.
The results show that fluorine is enriched on the surface of the hybrid membrane, the incompatibility of FPMA and WPU enhances the phase separation of the soft and hard segments of WPU, and the interfacial tension shrinkage caused by the phase separation leads to the surface roughness
.
After adding modified SiO2, the rough surface becomes a striped structure, and the synergistic effect of surface fluorine enrichment and rough surface structure endows the hybrid film with superhydrophobicity
.
Synthesis of fluoroalkyl-terminated hyperbranched WPU by grafting reaction of perfluorohexylethanol
.
The results show that the fluorine-containing segments are easy to migrate and accumulate on the surface of the film.
The water contact angle and water absorption of the unmodified film are 77.
8° and 136.
2%, respectively.
The water contact angle and water absorption of the modified film are The new fluoroalkyl-terminated hyperbranched WPU can effectively improve the water resistance of the films
.

Results and discussion

Modification method to improve adhesion performance: The ionic groups on the molecular chain of WPU destroy its integrity, resulting in poor adhesion performance of WPU adhesives

(1) A chemical bond is formed between the filler and the polymer matrix to increase the cohesive strength of the WPU, resulting in greater cohesion
.
   

 (2) Add an appropriate amount of high-efficiency curing agent
.

(3) Blending or copolymerizing with other high-performance resins
.

(4) Use terminal hydroxyl compounds or polyisocyanates with special chemical structures
.
Ruanpan et al.
synthesized a porous clay with an amino-functionalized heterostructure and used it as an additive for WPU adhesives
.
The results show that the porous clay can form a stable urea bond between the polymer and the filler, so that the cohesive strength is improved, and the bonding performance of the adhesive is significantly enhanced.
The optimum mass fraction of the porous clay is 2%
.
A series of adhesives were synthesized by mixing different WPUs with aliphatic or aromatic isocyanate curing agents
.
The results show that with the increase of curing agent content, the adhesive force first increases and then decreases; the optimal number ratio of -NCO groups to urethane/urea groups of the curing agent depends on the type of curing agent and the hydrophilic chain extension.
agent content; at the optimum amount ratio, aliphatic curing agents have stronger adhesion than aromatic curing agents
.

Polystyrene (PS) modified WPU adhesives were prepared by in-situ polymerization of styrene in poly(1,4-butylene adipate) glycol (PBA)

Modification method to increase the solid content: The specific heat capacity and latent heat of evaporation of water are high, so the drying speed of WPU is slow and energy consumption is high.
The drying speed can be accelerated by increasing the solid content
.
However, the increase of solid content will lead to the increase of emulsion viscosity, poor fluidity, increase of particle size and decrease of stability, etc.
Therefore, it is necessary to prepare stable emulsions with high solid content through molecular structure design and other methods
.
The solid content of WPU is related to its chemical composition, particle size distribution and particle state.
The following three methods can increase the solid content:

(1) Blend or copolymerize with water-based resin with high solid content
.

(2) The synergistic use of anionic and nonionic hydrophilic chain extenders can obtain WPU containing fine particles, and WPU can still maintain emulsion stability under the condition of low content of hydrophilic groups
.

(3) Increase the peak number of particle size distribution and control the diameter ratio and volume percentage of large and small particles, small particles will be effectively filled into the gaps between large particles, thereby increasing the maximum filling amount of particles and increasing the solid content
.
Using 2,2-dimethylolpropionic acid and sodium 2,4-diaminobenzenesulfonate as hydrophilic chain extenders, a WPU prepolymer was prepared, and then added to ethylene-vinyl acetate polymer emulsion for emulsification , The sulfonic acid type WPU adhesive was obtained
.
The results show that the WPU co-emulsified with the polymer emulsion is uniformly dispersed and spherical, with a solid content of up to 50%, and the peel strength and heat resistance of the adhesive are improved
.

A high solid content WPU was synthesized through the synergistic effect of 2,2-dimethylolpropionic acid and polyethylene glycol, and the effects of the relative molecular weight of polyethylene glycol and the connection relationship of hydrophilic groups on the stability of WPU were studied.