PU synthetic leather industry

PU synthetic leather

PU is the abbreviation of the word polyurethane in English and is a type of polymer. PU synthetic leather, short for polyurethane, is artificial leather made by coating or bonding liquid PU resin onto a carrier (usually fabric). Coated PU, vegan/vegan leather, vegan PU, synthetic leather, artificial leather, artificial leather, and so on—any synthetic leather composition coated with PU or bonded onto a carrier can be called PU synthetic leather.

Structure

Top layer (skin layer): A PU layer with leather texture or customer-specified patterns, called the “skin layer” or “fabric.”

Intermediate (foam layer): A foam body with a microporous structure, called the “foam layer” due to its bubble structure.

Backing: This is the carrier of the aforementioned PU and PU synthetic leather, called the “base layer,” “base layer,” “base material,” or “base fabric,” usually a type of fabric, so it is mostly called the “base fabric.”

PU synthetic leather structures are generally divided into surface layer + base fabric or surface layer + puff layer + base fabric. The latter offers a thicker or fuller feel.

Water-based PU VS.S. Solvent-based PU

Although water-based PU and solvent-based PU are both PU synthetic leathers, they are not the same. Here, we will explain the distinction between the two based on the PU synthetic leather industry. Note: Water-based PU is actually an abbreviation for water-based polyurethane. In English, water-based PU has many terms, such as water-based polyurethane, waterborne polyurethane (waterborne PU), aqueous polyurethane (aqueous PU), and sometimes simply simply called water-based PU. In academic research, it is sometimes also referred to as waterborne polyurethane dispersions (for the following reasons).

In the field of PU synthetic leather, solvent-based PU is a system that uses the organic solvent “dimethylformamide” as the solvent to dissolve PU particles (i.e., polyurethane particles) into liquid PU resin (see PU resin). Dimethylformamide is dimethylformamide in English, abbreviated as DMF or DMFA (CAS No.68 – 12 – 2, see DMF dimethylformamide). Solvent-based systems consume a lot of water during the process of producing PU foam layers, and DMF solvents mix into this water, generating large amounts of industrial wastewater contaminated by DMF (see the wet/wet process and curing molding of solvent-based PU synthetic leather). In contrast, water-based PU systems disperse PU particles in water as emulsified liquid resin, so they do not use DMF and are DMF-free. Water is an inorganic, clean liquid, and the PU particles in water-based PU are dispersed and emulsified in water, not dissolved in water, so water is not an organic solvent. Therefore, the industry refers to this type of PU system as water-based PU rather than solvent-based PU for distinguishing between them. The foam layer of water-based PU synthetic leather does not require much water or water mixed with DMF organic solvents to produce.

DMF dimethylformamide

Dimethylformamide is a toxic organic solvent, abbreviated as DMF or DMFA (CAS No. 68-12-2). It can damage human organs and may also cause reproductive problems and cancer. At the same time, it is also a VOC, a volatile organic compound, and may cause TVOCs to exceed the standard safety values for certain customers.

Water-based PU synthetic leather

The raw material for water-based PU synthetic leather is liquid, DMF-free waterborne polyurethane resin, also known as water-based PU resin (see PU resin). We first apply liquid water-based PU resin onto release paper (see release paper) and dry it, allowing the texture of the release paper to set onto the PU during drying to create the surface layer of PU synthetic leather. Next, we mechanically mix air with another water-based PU resin to create a microporous structure, which is mechanically foamed and then coated on the back of the surface, heated and dried to form a solid foam layer resembling a thin sponge. Then, we bond a layer of base fabric onto the foam layer to create PU synthetic leather with a surface layer + foam layer + backing fabric structure, and then peel off the entire structure with release paper (see PU synthetic leather structure). Sometimes, depending on customer requirements, products don’t need a very thick feel or thickness, so instead of making a foam layer, we simply bond the top layer to the base fabric. In the industry, this type of surface layer + base fabric structure has been commonly called “dry PU” since the era when solvent-based PU was dominant. Therefore, in Dayang Plastics’ water-based systems, it is called “water-based dry PU” (see dry/dry process processing). Here, our PU resin coating method is called “transfer coating.” This is because PU is first coated on release paper, then bonded onto the ‘rotating’ base fabric. At the same time, Dayang Plastics does not use DMF in both raw materials and processes, so our PU products do not contain VOCs of DMF volatile organic compounds, nor do they have residual DMF migration and precipitation. Moreover, unlike traditional solvent-based PU wet processes, our foam layer process does not use water and does not produce the same batch of industrial wastewater as solvent-based wet processes (see the wet/wet process and curing molding of solvent-based PU synthetic leather). As a result, we eliminate the need for maintaining tank water levels, heating, and washing products. Compared to solvent-based processes, energy and water consumption are greatly reduced, making it more sustainable and environmentally friendly.

Wet water-based PU

There is another water-based PU synthetic leather on the market, similar to traditional solvent-based wet processes, where PU is solidified using a sink (see the wet/wet process and curing process for solvent-based PU synthetic leather), which generates industrial wastewater. This process also uses liquid water-based PU resin as the raw material. However, since this process is almost equivalent to solvent-based wet PU, it should be called “wet/wet water-based PU,” or conversely, “water-based wet/wet PU.” Some also call it the “water-based drainage PU.” One such method involves applying alkaline water-based PU resin foam onto a base cloth and then immersing it in acid-containing water tanks. This causes acid-base neutralization in the water, solidifying the water-based PU resin into the solid PU foam layer on the base fabric of PU synthetic leather. Acid-base neutralization reactions produce industrial salt in the tank, and the reaction time can sometimes last 5–10 minutes. Furthermore, to maintain the pH pH of these acid-base neutralization tanks and sustain the neutralization reaction, water and acids in the tank must be continuously replenished during the process. This consumes large amounts of water and energy, and produces large amounts of industrial wastewater containing salts and non-neutral (abnormal pH values). Even if such processes do not use DMF or solvent-based PU resin, they still bring negative environmental impacts to solvent-based PU: industrial wastewater, water consumption, and energy consumption. Another wet water-based PU process involves immersing the fabric, the base fabric, in liquid water-based PU resin (see impregnation), filling the fabric’s structure with resin, then heating and drying it, and finally immersing it in a corrosive solution to remove excess PU resin. However, this method also causes issues with industrial wastewater and corrosive solutions. At Dayang Plastics, we do not use these “wet” processes: when we move toward green, eco-friendly, vegan, high-performance, and physically strong PU solutions (commonly referred to in the industry as “high physical properties”), they run counter to our direction.

PU resin

In the PU synthetic leather industry, PU resin generally refers to a liquid resin in which the polymer chain of PU polyurethane is dissolved in solvents or dispersed in water. The former is in a solution state; The latter is a dispersed emulsion state. When we heat and dry the resin, the PU polyurethane polymer chains interlink (chemically called “cross-linking”), forming a 3D three-dimensional network molecular structure that becomes a solid PU layer. Originating from the development of solvent-based PU in the 1960s and 1970s, the solvent for the former solution resin is usually organic, toxic DMF, namely dimethylformamide (CAS No. 68-12-2, see DMF dimethylformamide). In DMF solvent-based resin solutions, PU is a hydrophobic molecule that makes up 20%-40% of the solid content in the resin, also known as solid content or solid content (sometimes the term “solids content” is derived from Japanese). In the industry, these resins are called solvent-based PU, oil-based PU (hydrophobic and water-repellent called ‘oil-based’), while this type of PU synthetic leather is referred to as wet PU, wet PU, sewage PU, or wastewater solidifying PU (see the wet/wet process and curing molding of solvent-based PU synthetic leather). On the other hand, in the more innovative water-based PU technology compared to traditional solvent-based PU, the water-based PU polymer contains hydrophilic functional groups, and the PU resin particles disperse like emulsions in water rather than dissolved in DMF solvents. The industry refers to this as water-based PU. There are also other types of PU resin in the industry. For example, “high-solids PU” with high solid content and lower solvent content, making it viscous. Compared to traditional solvent-based PU, high-solidity PU contains less organic solvent in DMF, but when processed into PU synthetic leather, it is sometimes diluted with organic solvents to a usable viscosity. Dayang Plastics offers products similar to high-solidity PU, but they do not have the organic solvent content of high-solidity PU, so the solids content is almost 100%, and no DMF or solvents are needed during processing; This Dayang plastic product is called PPU.

Dry-process processing

The dry/dry process is a PU synthetic leather manufacturing process. Manufacturers apply a layer of solvent-based or water-based liquid PU resin onto textured release paper, then heat and dry it, while the PU resin is then dried and cross-linked on the paper (see PU resin). When PU resin dries on paper, it solidifies into an elastic PU layer, while the texture of the release paper is set on the PU surface. This PU layer is the “surface layer” or “fabric” (see PU synthetic leather structure). In DMF solvent-based PU resin, although DMF volatilizes with heat, some residue still remains on the surface layer. In water-based PU systems, water evaporates with heat, and since the resin and process itself do not use DMF, there is no residual DMF solvent on the PU surface. After the surface layer is dried, manufacturers use adhesives to bond the surface layer to a carrier or substrate, which is usually the textile, namely the base fabric. Next, the manufacturer will peel off the PU surface layer + base fabric self-release paper. The material that combines this surface layer with the base fabric is PU synthetic leather. As described above, this process does not use water, so it is called the “dry” or “dry” PU process. PU synthetic leather produced by this process can be called “dry/dry PU.” Although this process can be used in both solvent-based and water-based PU systems, traditional solvent-based PU has a long history of using this process, so it has become customary to use “dry/dry PU” for DMF solvent-based PU with a surface layer + backcloth structure. However, Dayang Plastics’ “dry/dry PU” is water-based PU. Note: Since PU is first coated onto release paper and then transferred onto the base fabric, this PU coating method is called “transfer coating.”

Solvent-based PU synthetic leather

Since the 1960s and 1970s, PU practitioners have been using dry processes (see dry/dry process) and wet processes (see wet/wet process and curing molding of solvent-based PU synthetic leather) to produce PU synthetic leather. At that time, the main raw material for PU synthetic leather was solvent-type liquid PU polyurethane resin, composed of particles formed by hydrophobic PU polymer chains dissolved in DMF organic solvents (see PU resin, DMF dimethylformamide). When this liquid resin is heated and dried, DMF and other volatile organic compounds (VOCs) evaporate, while PU resin forms a solvent-type solid, elastic PU layer (see PU resin).

In solvent-based dry processes (see dry/dry process), manufacturers apply this DMF solvent-based PU resin onto release paper, heat and dry it to form a solid PU surface layer, and then use solvent-based adhesives to bond this surface layer to the base fabric (see PU synthetic leather structure). This surface layer + backing structure is a type of PU synthetic leather, known in the industry as “dry/dry PU,” and after the drying process, some residual DMF residue may still be detected. Sometimes, PU synthetic leather requires a richer feel or thickness, so manufacturers use the so-called “wet process,” also known as “PU solidification,” adding a layer of foam between the top layer and the base fabric (see the wet/wet process and curing process for solvent-based PU synthetic leather). This process not only uses DMF solvent-based PU resin as the raw material for PU synthetic leather but also employs a large amount of DMF in the process itself, making it one of the processes for solvent-based PU synthetic leather.

wet process and curing molding of solvent-based PU synthetic leather

In the wet process, to create the foam layer of solvent-based PU synthetic leather (see PU synthetic leather structure), manufacturers apply DMF solvent-based PU resin solution onto the base fabric. Because liquid resin is directly applied onto the base fabric, this PU coating method is called “direct coating,” rather than the “transfer coating” of the dry process (see dry/dry process).

Next, the manufacturer immerses the base fabric coated with this PU resin into a heated water tank containing 15% DMF solvent. This guides DMF out of the PU resin and displaces it with water in the tank: DMF leaves the PU resin solution layer and, through the principle of concentration gradient difference, dissolves from the high-concentration PU resin coating into tank water with lower DMF concentration. The PU resin layer that loses DMF leaves tiny pores, which the tank water enters through the tank to fill them. When these DMF solvents leave this PU resin solution, the solvent can no longer maintain the solution state and solidifies into an elastic solid PU layer.

After PU solidification and molding, the material formed by the solidified and microporous PU layer and the base fabric is fed into a more heated water tank to wash out the residual DMF as much as possible. Next, the manufacturer heats and dries it to minimize residual DMF and VOC volatile organic compounds with thermal energy. After the moisture evaporates, the tiny pores filled by the initial moisture remain in the solidified PU layer, forming a microporous PU layer, which is the foam layer of solvent-based PU synthetic leather.

The above process uses a large number of sinks in solidification forming and material washing, hence it is called the “wet process,” sometimes referred to as the “PU solidification molding process,” but the former is a Chinese term, while the latter is usually used when translated into Chinese for more academic terms in English literature. PU synthetic leather made with a base fabric + foam layer structure through this process is known as “wet/wet PU,” but it is also referred to as “sewage PU” or “sewage solidifying PU.” The PU layer produced by this process has no texture, so manufacturers must use this foam layer + base fabric structure as the carrier for the dry process, bonding a PU surface layer to it. This allows PU synthetic leather with surface texture, wet foam layer, and base fabric through surface texture.

Because the above process uses a large amount of DMF solvent, some DMF remains in PU synthetic leather, sometimes exceeding concentrations of 500–1000 PPM, and is also a source of VOCs or DMF that may migrate and precipitate in PU products. Furthermore, a large amount of DMF dissolves into tank water during the process, generating a large amount of toxic industrial wastewater. Moreover, many of these heating sinks also consume a large amount of water and energy.

impregnation

The term “impregnation” in the PU synthetic leather industry refers to a method or process. “impregnation” involves immersing a substrate or base fabric in liquid resin, allowing the resin to penetrate the microstructure of the base or base fabric, forming a layer of resin for the base fabric to bind yarn, or to prepare for the wet PU solidification process (see the wet/wet process and solidification molding of solvent-based PU synthetic leather). The official English term for “impregnation” is “impregnation,” but the more non-academic term is “dipping.” Typically, after impregnation, the material is dried or sent to the next stage of wet PU solidification and molding.

Release paper

The function of release paper is reflected in its composition: when the paper is “peeled off” from the solidified resin layer, the surface texture of the paper is “shaped” onto the resin surface. The texture of this paper comes from the embossing process, similar to the texture of PU synthetic leather (see embossing). In other words, the paper surface has lines left by the embossing process. From the side view, you can see the undulations and unevenness across the paper’s cross-section, forming a thin 3D three-dimensional “mold.” When liquid PU resin is applied to this release paper, it acts like the liquid resin fills the paper with a “mold” of textured texture. Afterwards, when the resin is dried to form a solid PU layer, this layer cures as if it were in the mold, preserving the texture of the release paper. Of course, this is not the only method for producing surface textures of PU synthetic leather; there are other methods, such as embossing, each with different technical requirements and considerations.

Embossing

The surface texture of PU synthetic leather can be achieved by applying PU resin onto release paper with the desired texture, but other methods can also be used. Embossing involves heating a metal wheel engraved with the desired texture and pressing it onto the surface layer of PU synthetic leather, then pressing the pattern onto the surface of the PU synthetic leather. Because it passes through heat pressing, it is called “embossing.”

2 Lamin skin bonding and peeling the skin

Erlang leather is the lower layer after being horizontally shaved and split into multiple layers of animal leather. This lower layer is not the upper surface of animal leather and lacks the natural animal leather grain of genuine leather. Erlang leather manufacturers bond Erlang leather with a artificially made, textured surface layer, which is usually a PU film layer. In the industry, this PU film is called “peeled skin,” which is bonded onto the laminated skin through adhesive resin.