Polyurethane structure and properties

There are not many kinds of basic structures of polyurethane, which are generally called systems in the industry; such as MDI system foam or prepolymer, TDI system sponge, polyester type sponge, polyether type or even special tetrahydrofuran type polyether prepolymer or thermoplastic granules, etc.

First of all, the system generally refers to the main isocyanate types in the formulation, such as the common TDI system and MDI system, of course, there are also special NDI system and aliphatic isocyanate system, etc., the common ones are basically TDI or MDI, after determining the system, according to the different oligomers are broadly divided into polyether polyurethane and polyester polyurethane, and then subdivided can have caprolactone, tetrahydrofuran, carbonate and other polymerized oligomer types, from the reactivity can also be further distinguished, today we mainly look at the differences and properties from the molecular structure.

TDI system due to its high NCO mass fraction, or smaller specific gravity often brings lower density to the sponge, so ultra-low density sponge formulations are often less TDI monomer, the shortcoming is that due to the small specific gravity of the small molecules are easy to volatilize and lead to poor control of VOC, in CASE applications there is also a certain amount of odor.Because the molecule itself does not have the high degree of symmetry of MDI also leads to its glass transition temperature is relatively lower, in the rebound and Young's modulus is relatively weak, in the elastomer is required to use aromatic amine curing agent to increase the strength to meet the requirements of use, which can also extend the operating time makes manual pouring possible, the same for foam products, or also have enough fluidity to play a certain delayed effect.

At present, due to environmental protection and the cost advantage under the expansion of MDI capacity, more and more polyurethane formulations use MDI system as the basis, and this trend is more obvious in foreign countries. Modified MDI is mostly liquid at room temperature, which also brings convenience for downstream replacement of TDI in the process, but the process control of MDI system is more demanding than TDI, which makes the manual process more difficult, among which the measurement and operation time are more demanding and more suitable for equipment or assembly line operation. In terms of physical properties, MDI system is generally preferred to TDI system because MDI has high glass transition temperature, in addition to the double benzene ring also provides a stronger intermolecular force for the polymer, which in turn enhances the glass transition temperature effect; from the monomer molecules to contrast, it is more symmetrical regularity brought about by a higher degree of crystallinity caused by, can also be understood as a harder hard segment of polyurethane contributor to enhance the island structure.

Polymer polyol is generally polyester and polyether, the former is a little more expensive and the production capacity is relatively small. Mainly used in CASE, polyester foam is relatively small. Polyester type relative to polyether type, often with better strength, generally more wear-resistant; and the latter is often more resistant to weathering and hydrolysis, tear strength is weak and elongation is high, of course, this is all relative, each formula has a suitable balance to meet the actual use of the requirements to reflect the added value of the formula. Sometimes, in order not to lose the resilience or elongation, but also want to increase the strength, people will consider tetrahydrofuran polyether, the same does not lose strength and want to resist hydrolysis, will consider caprolactone and other special polyether. In the foam is mainly polyether, choose different propylene oxide ethylene oxide content and different starting agents of polyether to meet the foam requirements, also will use grafted POP polyether to assist in strengthening, etc.. Foam sponge has a high and low density, there are also different points according to the system, but also according to the rebound and strength of the points, the common mattress sponge is mostly ordinary sponge, seat high rebound sponge or U-shaped pillow memory sponge, etc..

Isocyanates are now also a lot of mixed, especially MDI varieties are the most, some are even considered prepolymers such as 8019, some are different MDI isomer ratios, and some are mixed together with TDI. And in addition to copolymer POEO, some manufacturers also embed siloxane units and acrylic units, etc., which retain the parent properties of polyurethane itself, but also extend certain special functions.