Development Of Waterborne Polyurethane Adhesives (WPU) Technology For Shoes In China

Development history of shoe adhesive

modern

Because of the constant upgrading of shoe materials, synthetic leather, rubber plastic and soft polyvinyl chloride (PVC) are widely used.

Adhesive shoes with common neoprene adhesives are not able to meet the requirements because of their low adhesive strength and frequent gluing.

In 1970s, grafted chloroprene rubber was developed to improve the adhesive properties and wear fastness of shoes.

After introducing other acrylic monomers and polymers, the graft copolymerization of polychloroprene rubber can be more suitable for shoe making.

However, chloroprene rubber is not ideal for the plasticized PVC, high oil content styrene butadiene rubber, leather or rubber shoes with high oil content or high grease content, and the chlorine contained in the molecules is easy to hydrolyze and release hydrogen chloride, which has the disadvantages of corrosion and damage to human health, thus hindering the application of neoprene in the footwear industry at this stage.

In order to ensure the quality of shoes and reduce environmental pollution and improve personal safety, the developed countries in Europe and the United States gradually pformed their shoe adhesives from neoprene to polyurethane in the 80-90 's in the twentieth Century.

Although solvent based polyurethane adhesives have many advantages, they are still inflammable, explosive and toxic chemicals to the human body. Unsafe accidents often occur in shoe factories.

In 1970s, the development of waterborne polyurethane (WPU) shoe adhesive was carried out abroad, basically using water as the adhesive medium. At that time, due to poor product performance and stringent government environmental regulations, its R & D progress was slow.

80s is still in the experimental stage.

At the beginning of 90s, the environmental regulations of the European and American governments became increasingly stringent. The total volatile organic compounds (VOC) released from the shoe factories began to control. The research and development of the synthesis and application technology of waterborne polyurethane were enhanced. The products such as Bayer's Dispercoll U and Desmodur D had basically met the requirements for the production of sports shoes and gradually entered the industrial scale.

According to Bayer estimates, in 1998, the consumption of waterborne polyurethane adhesives for shoes worldwide was hundreds of tons. In 2000, it reached 5000 tons; in 2001, it reached 10000 tonnes and developed rapidly.

Technical progress of waterborne polyurethane adhesive for footwear in China

China is a big country in the production, consumption and export of shoes and boots, but it is not a powerful shoe making country. Its per capita consumption is also low.

In 2007, the output was about 10000000000 pairs, and exported 8 billion 170 million pairs, most of them were adhesive shoes, basically using solvent based adhesives, and the annual adhesive volume was about 300 thousand tons.

The development of China's shoe adhesives has also gone through the above stages. The development of waterborne polyurethane adhesive for shoes has started in the late twentieth Century, and about more than 20 units have been engaged in the research of synthetic technology and application.

At that time, the solid content of the products was low, the evaporation speed of the shoes was slow and the number of gluing times was much, which affected the implementation of the mechanized production process. The high cost restricts the wide adoption of the shoe making industry.

But insiders agree that waterborne polyurethane adhesive for shoes will be the direction of development in recent years.

In twenty-first Century, R & D work began to be active. At present, significant progress has been made, and some have entered the pilot stage.

In the early days, waterborne polyurethane was mainly used for processing foreign famous brand sports shoes by foreign funded enterprises and made according to the requirements of foreign companies.

In 2000, the consumption of water based polyurethane footwear for these processing enterprises was estimated at 2500-3000 tons, compared with 4000 tons in 2001, which basically depends on Taiwan funded enterprises in mainland China.

In recent years, the annual use of waterborne polyurethane adhesive for top four sneakers in China is higher than that of 20 thousand tons, and waterborne polyurethane materials need to be imported and expensive.

For many reasons, waterborne polyurethane technology in China has been developing slowly for a long time.

In twenty-first Century, especially in recent years, with the increasing demand for energy saving and emission reduction in the world, the requirements for environmental indicators of export shoes have been stricter and stricter. In addition, people's awareness of environmental protection has been gradually enhanced. The state has formulated mandatory standards for shoe adhesive: there are clear limits on benzene, toluene, xylene, total halogenated hydrocarbons, free isocyanates, hexane and VOC.

People with insight realize that China's footwear industry must shift from quantity growth to upgrading quality, grade, create brand names and increase added value, thus accelerating the research pace of high-performance waterborne polyurethane adhesive for shoes.

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1. Polyester type is more than polyether type.

The initial adhesion and final adhesion strength are the key technical indicators of the adhesive for sole and upper.

To achieve high initial viscosity, the molecular structure of the base resin is required to have high crystallinity.

The ether bond is easy to rotate and flexible, and has excellent hydrolysis resistance and low temperature resistance.

The more the number of methylene groups between the ether bonds and the less the side groups, the higher the crystallinity.

Therefore, polytetrahydrofuran polyurethane has higher adhesive strength than polyoxyethylene polyurethane.

In order to make the waterborne polyurethane adhesive for flexibility and hydrolysis resistance, polytetrahydrofuran is often used as resin raw material in China. Its relative molecular mass is 2000-2500. The shoe adhesive is made of PVC, leather and other shoe materials with a curing agent, and the effect is good.

2. Adopt new hydrophilic agent.

At present, the most commonly used self emulsifying anionic hydrophilic agent is DMPA. It has the structure of new anhydrodiol, which can give the product certain heat resistance and water solubility. Its relative molecular mass is low, and its usage is small. Carboxyl group is blocked by 2 hydroxymethyl blocks, which is not easy to react with isocyanate and maintain its hydrophilic function. It contains 2 active hydroxyl groups, which can react with isocyanate, which is introduced into the main chain molecule of carbamate, and plays an internal emulsifying role.

But it does not dissolve with polyols and other reactants, melting point is high, it is generally necessary to pre dissolve N- methyl pyrrolidone (NMP) in order to make the reaction uniform and stable, and NMP boiling point high, will remain in the product.

In recent years, NMP has been reported to be irritating and toxic.

In view of the fact that almost all the waterborne polyurethanes in the market today contain NMP, the working group of the European Union has decided that in the future, any formula with NMP content above 5% is classified as toxic.

Some large foreign companies have been searching for alternatives, or changing the formula of raw materials, that is, using Dihydroxymethyl butyric acid (DMBA) which can be dissolved in higher temperature polyols as a hydrophilic agent, thereby greatly reducing the dosage of NMP, or even not.

At present, there are DMBA production units in China, but the price is more expensive.

The preparation and application of High Performance Waterborne Polyurethane multipurpose sulfonic acid hydrophilic agent abroad has been relatively mature. Some companies have already supplied various brand products.

Sulfonate can form stronger Coulomb force and make the interaction force between molecules stronger. The lower hardness of waterborne polyurethane has higher tensile strength and tear strength.

Sodium sulfonate of waterborne polyurethane is a strong acid and strong base salt, which can promote the formation of waterborne polyurethane particles to disperse Yu Shuizhong steadily, and has excellent storage stability.

In recent years, domestic scholars have focused their attention on the preparation of waterborne polyurethane with sulfonic acid, even sulfonic acid and carboxylic acid ploughing hydrophilic chain extender, in order to get better performance and lower cost of shoe adhesive.

3. Use of mixed isocyanates.

The products of TDI and MDI Aromatic isocyanates are low in cost and high in adhesive strength, but yellowing easily. They are not suitable for making light colored shoes.

In recent years, isocyanate of isophorone diisocyanate (IPDI) or IPDI and I, 6- six methylene diisocyanate (HDI) was used.

The latter can be used to produce waterborne polyurethane adhesive for low temperature, even lower than 45 degrees, which is required by the shoe line.

In addition, it is also resistant to temperature and yellowing.

4. Use nanometer additives.

The introduction of inorganic nanoparticles can improve the glass pition temperature of adhesives, that is, increase heat resistance. The hydroxyl groups on the surface of nanoparticles can react with polar groups in organic polymer chains to form internal crosslinking and enhance adhesive strength.

The addition of micron inorganic particles can improve the mobility and sizing of adhesives, so that the adhesive is evenly coated and the amount of glue is reduced. It can also increase the force between the adhesive and the surface of the adhesive, so that the two can be closely integrated.

The two types of additives play a synergistic role in adhesives. Nano organic montmorillonite and micronuum dioxide, nano and micron SiO2 are commonly used. The adhesive can be well glued to PU, PVC, EVA, rubber and leather.

5. Use auxiliaries.

Waterborne polyurethane adhesive for shoes is environmentally friendly with water as its medium. However, the water has poor solubility and wettability on the surface of the hydrophobic sticky material, and the surface tension of the water is too large. Its colloid is hard to penetrate into the material pores.

The volatilization of water is much lower than that of solvents, and the drying time is long. If heating and drying are used, when the adhesive is sticky, especially the sticky porous shoe material, the absorption and drying of the substrate are out of harmony, resulting in discontinuity and non-uniformity of the adhesive layer, which greatly affects the adhesive effect.

Therefore, it is very important to improve the adhesiveness of the adhesive body, but the penetration problem is not resolved, which will always affect the smooth application of waterborne polyurethane adhesive.

In recent years, various auxiliaries have been developed in foreign countries to try to solve this problem.

Various auxiliaries studied include: wetting agents that give the wetting action between the adhesive and the substrate, such as polysiloxane or hydroxyl polysiloxane, which can reduce the surface tension of the adhesive and make it easy to flatten on the surface of the substrate so as to obtain even and smooth film leveling agents, such as polyether modified siloxane. Besides, there are also antioxidants to prevent PU from thermo oxidative degradation.

In conclusion, the development and utilization of waterborne polyurethane adhesive for shoes has been effective in China. Some units have already carried out pilot tests. Some of them have been built with a small scale of production equipment. Most of them are mixed polyester polyols, sulfonic acid and carboxylic acid based internal emulsifiers, which react with IPDI (or HDI mixed) isocyanate to form prepolymer, neutralize and disperse in water, and then extend the chain with ethylenediamine to produce waterborne polyurethane adhesive for shoes.

The non volatile content of adhesive can reach more than 50%, the activation temperature is close to 50 degrees, and the storage stability is good. It can stick various shoe materials firmly, and has certain heat resistance.

The shortage is that in order to produce high quality and high solid content products, a small amount of solvent is still used in the process.