Integrated Skin Polyurethane Systems

Integrated Skin Polyurethane Systems By varying components such as polyol, chain extenders, crosslinkers, isocyanate and other agents, polyurethanes with vastly different properties can be obtained. For example, very soft, rigid, semi-rigid, flexible, and ultra-rigid foam polyurethanes can be molded into desired shapes by adjusting these inputs and other factors. By the late 1960s, polyurethane use had become widespread in industries such as automotive, furniture and footwear. Many of the polyurethanes we use today were patented during these years, and many of their applications and methods were discovered during this period. While foam structure was needed for softness, comfort and lightness, a protective skin had to be added to the foam structure for strength and durability. Initially, additional materials such as ABS film, PVC film, leather and synthetic leather, which was gaining popularity during these years, were added to the mold together with polyurethane to obtain skin. These types of applications continue to be used in many areas today. However, the need to obtain both foam structure and skin in a single process for production efficiency led to successful research in a short time. Low-density internal structure and dense, compact skin structure could be obtained in the mold in a single pass. This polyurethane, which enabled obtaining materials of different densities simultaneously in one process, was called integral skin polyurethane or self skin polyurethane, although the latter term is less common. Integral skin polyurethane was obtained in the 1960s by increasing mold temperatures and the amount of raw material poured into the mold. A colder mold than necessary produced thick skin, while a hotter mold produced thin skin. However, this meant increased costs and longer molding times. Following continued research, integral skin polyurethane could be obtained with molding times as short as 2 minutes using a mixture of low-molecular-weight, high-hydroxyl-number polyols and large-molecular-weight, low-reactivity polyols with blowing agents. When this mixture combines in the mixing head of the polyurethane machine and is poured into the mold, the viscosity of the final mixture rises rapidly, and thanks to the rising temperature from heat released during the reaction, blowing agents evaporate and begin forming foam. Due to high viscosity and well-regulated flow, as the mold fills, the resulting polyurethane has a foamy interior and an outer skin 1-5 mm thick. For the formation of integrated skin polyurethane, the selection of blowing agents and mold temperature, as well as catalysts, modified silicone surfactants and their proportions were equally important. This method continues to be used today. From the 1960s to now, due to regulations, awareness and later environmental impacts, catalysts containing heavy metals have been replaced with safer alternatives, and volatile inputs have been replaced with non-volatile new versions. When the ozone-reducing effects of blowing agents became apparent over time, alternatives that do not harm ozone began to be used. The blowing catalyst that initiates the reaction for foam formation enables the mold to fill, viscosity to increase, and exothermic heat causes blowing agents to boil, while the gelling catalyst protects the formed structure and enables low-activity high-molecular polyols to react with remaining isocyanates. The silicone surfactants used reduce the surface tension of this mixture, ensuring that the resulting foams are uniform in size and structure. The mold used in integrated skin polyurethane production must close well, withstand pressure up to 1.2 atm, and not allow insufficiently cured polyurethane to escape from its walls. When this mixture and conditions are provided, although the effect of mold temperature decreases, the optimum mold temperature has been found to be higher than the boiling temperature of the blowing agent and preferably between 40 and 65°C. Molding time ranges from 2 to 5 minutes depending on mold shape and size. MDI prepolymers are very commonly preferred as isocyanate. MDI is preferred because it can be successfully modified for many different polyurethanes. In integrated skin polyurethane, surface quality of the skin is as important as skin thickness. These polyurethane parts are visible components in automotive, furniture, medical equipment, sporting goods such as headrests, armrests, handles, steering wheels, gear knobs and floor mats. The surfaces of these long-lasting parts can be preferred in matte, semi-matte, glossy and different colors. As the mold release agents used in the 1960s did not provide the desired surface smoothness, smooth surfaces were obtained using 0.5 mm thick deep-drawn polystyrene films, but today with improved in-mold paints and mold release agents, smooth-surfaced skins in desired colors can be obtained without the need for these films. Teknik Kimya Donatım A.Ş. offers liquid colorants, mold release agents, in-mold paints, touch-up paints and mold cleaning products for integral skin foam polyurethanes.

Sources

• Wirtz, H. (1969). Integral Skin Urethane Foam Molding. Journal of Cellular Plastics, 5(5), 304-309. • Gupta, V. K., & Khakhar, D. V. (1999). Formation of integral skin polyurethane foams. Polymer Engineering & Science, 39(1), 164-176. • Dow Integral Skin PU Systems General Info, (Special thanks to Gizem CAKİR ASLAN) İlker Akça Technical Director Teknik Kimya