PU Knowledge

Sometimes, there is confusion between water-based PU v.s. solvent-based PU. We will explain in terms of the PU synthetic leather industry. Note: water-based PU stands for water-based polyurethane, aka waterborne polyurethane (waterborne PU), aqueous polyurethane (aqueous PU), and sometimes in layman’s terms, water polyurethane (water PU). Scholastically, it is sometimes called aqueous polyurethane dispersion (see reason below).

In the industry, solvent-based PU is a system using dimethylformamide (DMF or DMFA, CAS No. 68-12-2, see DMF) as solvent in dissolving polyurethane particles to make liquid PU resin solution (see PU Resin). The solvent-based system uses tanks of water that become contaminated with DMF to create PU leather’s foam layer (see Wet Process and PU Coagulation for Solvent-Based PU Leather). On the other hand, the water-based PU system disperses polyurethane particles in water as a liquid PU resin emulsion, so it’s DMF-free. Water is not an organic solvent because the PU particles are dispersed or emulsified, not dissolved. In addition, since water is an inorganic, clean liquid, people refer to this type of PU as water-based PU, not solvent-based. Water-based PU leather’s foam layer does not need tanks of water or DMF-infused water for production (see Water-Based PU Leather).

Water-based PU leather is made of liquid-state DMF-free water-based polyurethane resin (see PU Resin). First, we create a PU skin layer by coating the resin on a release paper (see Release Paper), drying the resin into a solid PU skin layer imprinted with the paper pattern. Next, we mechanically mix air with another batch of water-based PU resin to give it a microporous cellular structure, coat it onto the back of the skin, and heat-dry it to create a solid sponge-like foam layer. Then, we laminate (adhere) a backing onto the foam layer, creating PU leather with skin + foam + backing structure, peeling it off from the paper (see PU Leather Structure). Sometimes, we don’t need much dimensional volume in PU leather, so we don’t add the foam, and just laminate the skin with backing directly. The skin + backing structure is called dry-process PU, so we call this dry-process water-based PU (see The Dry Process). The way we coat PU layers here is called “transfer coating” because they are first coated on the release paper, then “transferred” onto the backing through lamination. We use no DMF, leaving no DMF-based VOC or migratable DMF remnants in PU. Also, unlike the wet process in solvent-based PU (see Wet Process and PU Coagulation for Solvent-Based PU Leather), our foam layer involves no water tanks that would lead to industrial wastewater. Without the water tanks and washing, we consume much less energy and water resource than solvent-based PU.

Different from us, there are other schools of water-based PU leather that use water tanks which create industrial wastewater. They use water-based PU resin, scholastically called aqueous polyurethane dispersions, as the raw material. However, they’re essentially “wet process water-based PU” or “wet water PU,” similar to the wet process in solvent-based PU (see Wet Process and PU Coagulation for Solvent-Based PU Leather). One of the methods coats alkaline water-based PU resin on a backing and immerse it in water tanks with acids. The acid-base reaction solidifies / coagulates the PU resin and forms the PU leather’s foam layer on the backing. This reaction yields industrial salts in the tanks and takes a long time, 5-10 minutes sometimes. Also, to maintain appropriate pH levels in the tanks, water and acids are constantly replenished. This consumes a lot of water and energy and creates a lot of industrial wastewater with unnatural pH and salts. Even though this process uses DMF-free water-based PU resin, it still contains the negative environmental impacts of the solvent-based system—industrial wastewater and energy & water consumption. Another method impregnates (see Impregnation) textile into water-based PU resin to soak the textile completely with PU, heat-dry the textile, then immerse it in caustic solutions to remove extra PU. That also creates industrial wastewater and caustic solution problems. We do not use these methods: they go against our belief at creating a green, eco-friendly, vegan PU solution with good functional and physical performances.

In the coated PU leather industry, PU resin is usually a liquid-form resin consisting of polyurethane polymer chain particles dissolved in solvent as a solution, or dispersed in water as emulsion / dispersion. When the resin is dried with heat, the polyurethane polymer chains crosslink with one another, forming a 3D net-like structure at the molecular level, which becomes a solid PU layer. Traditionally since the 1960s-1970s, the solvent is an organic, toxic solvent dimethylformamide DMF (CAS No. 68-12-2, see DMF). In DMF-based PU resin solution, the PU is hydrophobic, and constitutes 20-40% of the resin as its “solid content.” On the other hand, in the newer water-based technology, water-based PU polymers have hydrophilic functional groups at the molecular level so that the PU particles disperse in water as an emulsion, instead of dissolving in DMF. The industry calls these two resin types solvent-based PU or wet PU (see Wet Process and PU Coagulation for Solvent-Based PU Leather) v.s. water-based PU. There are also PU resins with less solvent percentage and more solid content, making the resins more viscous. These are called high-solid PU, which contains a smaller percentage of DMF than traditional solvent-based PU, but it sometimes requires dilution with solvent during PU leather production. We have a product similar to high-solid PU, but with no organic solvent, and close to 100% solid content and no DMF required at production; we call it PPU.

Dry process is a type of PU leather production process. Manufacturers coat a layer of solvent-based or water-based liquid PU resin on a release paper (aka casting paper), a special paper with pattern, and dry the resin with heat so that the PU resin dries and crosslinks on the paper (see PU Resin). As it dries on the paper, it solidifies into an elastic PU layer “casted” with the paper pattern. This is the “skin” layer (see PU Leather Structure). In DMF solvent-based PU resin, despite the DMF evaporating with the heat, some remains in the PU skin. In water-based PU, the water evaporates, and no DMF is ever in the skin. Next, the manufacturers can use an adhesive to laminate (adhere) the skin layer with a “substrate,” typically a textile substrate referred to as “backing.” After that, they peel the PU skin + backing off from the release paper. This skin-on-backing material is the PU leather. The process involves no wetness or water, so it is called the “dry process” or “dry PU process.” Though the process can be used for both solvent- and water-based PU, since it has long been used on DMF-based PU, people usually use “dry PU” or “dry process PU” to mean DMF solvent-based PU with skin + backing structure. Note: the way PU is coated here is “transfer coating” because PU is first coated on release paper, then “transferred” onto the backing through lamination.

Traditionally, since the 1960s and 1970s, PU leather has been manufactured using the dry process (see The Dry Process) and wet process (see Wet Process and PU Coagulation for Solvent-Based PU Leather). The main raw material is liquid-state solvent-based polyurethane resin consisting of hydrophobic PU polymer chain particles dissolved in the solvent DMF (see DMF). When this liquid resin is dried at high temperature, DMF and other VOCs evaporate while the PU resin forms a solid, elastic PU layer (see PU Resin). In the solvent-based “dry process” (see The Dry Process), manufacturers coat this DMF-based PU resin on release paper and dry it at high temperature into a solid PU skin layer then laminate it with a backing using solvent-based adhesive (see PU Leather Structure). This skin + backing structure is the PU leather, called the “dry process PU,” or “dry PU.” Detectable levels of DMF can remain after the heat-drying. However, sometimes, PU leather needs more thickness or volume when you touch and grasp it (texture when touching and grasping is sometimes called hand feel). Therefore, manufacturers use the wet process, or PU coagulation (see Wet Process and PU Coagulation for Solvent-Based PU Leather), to add a PU foam layer between the skin and backing.

Manufacturers create solvent-based PU leather’s foam layer (see PU Leather Structure) by coating a layer of DMF-based PU resin solution on a backing. Since the liquid resin is coated directly on the backing, this is “direct coating,” not the “transfer coating” in the dry process (see The Dry Process). Then, they immerse the resin-coated backing into heated water tanks filled with 15% DMF. This induces DMF in the PU resin to exchange with water: DMF leaves the resin solution and dissolves into the water osmotically, while water fills the micro pore-like spaces left by the DMF in the PU. As the solvent DMF leaves this coated layer of resin solution, the resin particles stop maintaining a dissolved liquid state, so they coagulate into a solid layer. After coagulation, the coagulated PU + backing will enter more heated water tanks to wash away the DMF as much as possible. Then, it will be dried in oven to drive away the water and remaining DMF and VOC. After the water evaporates from the PU layers’ micro pore-like spaces, the layer becomes a microporous PU with tiny cells, the foam layer of solvent-based PU leather. The above process uses many water tanks for coagulation and washing, so it is called the “wet process” or “PU coagulation,” and PU leather with this kind of foam layer is called “wet PU,” “wet-process PU,” or “coagulated PU.” The PU layer created has no surface pattern, so the manufacturers use this PU-coated backing as a substrate in the dry process and laminate it with a skin layer to obtain surface pattern. The result is PU leather with a patterned skin, wet-process foam layer, and backing. A lot of DMF is used, so a lot remains in the PU leather, sometimes exceeding 500-1000 ppm, becoming a source of VOC and migratable DMF in PU product. Moreover, a lot of DMF has dissolved in water, creating toxic industrial wastewater. With multiple heated water tanks, the entire process also consumes a lot of water and energy.

The word “impregnation” in the PU leather industry describes the process of dipping textile into liquid resin for the resin to seep into the textile, forming a layer over the textile to bind the textile fiber together or prepare for the PU coagulation process (see Wet Process and PU Coagulation for Solvent-Based PU Leather). In layman’s terms, “impregnation” is sometimes called “dipping.” Usually, heat-drying or coagulation is followed.

Release paper is also called casting paper. It serves exactly that purpose: to cast certain patterns onto a coated resin surface. The paper is embossed with a pattern, similar to how synthetic leather is embossed (see Embossing). In other words, the paper has these patterns imprinted from the embossing, and, if seen from a horizontal cross-section, the paper’s profile has troughs (valleys) and peaks forming a thin 3D pattern’s “mold.” When liquid resin is coated over these troughs and peaks and dried into solid layer, the pattern becomes molded, or imprinted, onto the resin surface. Of course, this is not the only way PU leather attain surface patterns. Other methods like embossing, etc. is available at different technical thresholds and costs.