Production Technology of Dry PU Leather and Wet PU Leather

Waterborne PU leather is referred to as solvent-based synthetic leather (PU leather). To understand what waterborne synthetic leather is, we must first understand the classification and structure of synthetic leather.

PU synthetic leather can be divided into dry PU and wet PU. Dry PU leather is composed of four layers of fabric, adhesive layer, surface layer and surface treatment layer. Wet PU leather is composed of five layers of fabric, adhesive layer, foaming layer, surface layer and surface treatment layer. Wet PU leather has one more foaming layer than dry PU leather. Traditionally, adhesive layer, foaming layer, surface layer and surface treatment layer are all made of polyurethane (PU) chemical materials containing DMF and other solvents.

TC waterborne PU leather

Strictly speaking, synthetic leather made by replacing DMF and other solvents with chemical materials using water as solvent and coating the adhesive layer, foaming layer, surface layer and surface treatment layer of the water-soluble agent on the surface of the cloth is water-based leather.

On the surface, only DMF and other solvents in chemical materials are replaced by water. However, from the perspective of scientific research, qualitative changes have taken place in the selection of raw materials, distribution design, process, equipment selection and so on.

Waterborne synthetic leather is generally a four-layer structure of surface layer-foaming layer-bonding layer-base cloth, which is produced by three coatings and three bakes. CN201010245381.1 discloses a production method of water-borne synthetic leather, which can be summarized as three coatings and four bakes, including coating the pulp containing water-borne polyurethane resin on the surface of profiled paper and drying to form a surface layer.

Waterborne PU garment leather

The pulp containing foaming waterborne polyurethane resin is mixed with air by a foaming machine, then the pulp after foaming is coated on the surface of the surface layer and dried to form an intermediate foaming layer, and the surface of the foaming layer is combined with the cloth paste by an adhesive containing the adhesive waterborne polyurethane resin. Influenced by drying process, the thickness of the surface layer is limited, 0.02-0.05 mm. When embossing complex patterns, it is easy to expose the middle foam layer, forming color difference and bringing inferior products. This process limits the application of large pattern synthetic leather.

The production of synthetic leather must undergo multiple sizing and drying treatments before the finished product can be obtained. Among them, drying technology is an important link in the process of synthetic leather production. Traditional synthetic leather dry production line is designed to meet the requirements of oil-based synthetic leather slurry painting and drying, which is difficult to meet the requirements of water-based ecological synthetic leather.

Yellow waterborne PU leather

Due to the limitation of resin synthesis technology, water-based resins now on sale contain 50-70% water, and the latent heat of volatilization of water is much higher than that of chemical solvents such as dimethylformamide. Therefore, under the same leather-making conditions, using water-based resins to produce synthetic leather requires more energy to dry leather products than using oil-based resins to produce synthetic leather. Traditional tannery production line uses hot air convection drying method, the surface of the coating will be heated to form film first, while the moisture in the middle will break through the surface of the leather and volatilize in the further heating process, which will cause the formation of the film to be changed, which will lead to the phenomenon of easy peeling and pinholes is very common, and some of the stripped paper patterns for tannery can not be processed at all.

Correspondingly, a waterborne PU leather dry process production line has four independent oven systems, three coating platforms and a waterborne polyurethane foaming machine. The oven system is equipped with infrared drying equipment to control the temperature of the oven independently. The patent provides a specific scheme for dry process temperature control, which solves the problem of uneven heating of water-borne synthetic leather. The production line limits the length and temperature range of four ovens, and reduces the possibility of uneven heating by controlling the temperature values in different temperature ranges. However, the temperature control scheme provided by the patent is rough, it can not accurately control the temperature of each part, there is still the possibility of wrinkling, and the drying efficiency is low.