Sheet-molding compounds are prepared from unsaturated polyester resin and styrene. A filler like MgO or Mg(OH)2 is added to the solution to thicken it and give the final product a higher modulus. The thickening is probably caused by a combination of ionomer formation and the filler itself. The polyester resin and styrene are cured (crosslinked) by free radical chemistry to give a tough, stiff polymer composite. If the inorganic filler is replaced with glass fiber prepreg , you have the formulation for FiberglasTM. The glass prepreg is used to give the material more stiffness. While the material is a composite, the matrix is an IPN. The combination of polymers helps to get the desired mechanical properties. SMCs are used in alot of appliance housings and panels for vehicles. One specific example is Corvette body panels, which are made from FiberglasTM.
A semi-IPN based tetrafluoroethylene and silicone is used as a wound dressing material. It is sold under the trade name Silon� from BioMed Sciences. The fluoropolymer provides support and reinforcement for the silicone, which is soft and pliable. The combination of the two biocompatible materials yields a film that is very flexible and tough. A picture of it is shown below. The material can be made into thin (15-750 microns) films which allow oxygen and moisture to diffuse through the material. Thus the material is "breathable", which is important in wound care applications. More information on this material can be found at the BioMed website.
IPNs can be designed to absorb mechanical energy if one component is a rubbery material (low Tg) and the other is a rigid, glassy material (high Tg). When these two materials interpenetrate, you generate an interphase that has a glass transition temperature between that of the two components. This is important for energy absorption because, generally speaking, polymers absorb the most energy when they are near their glass transition temperature. If the two components can be made semi-compatible, as is the case in an IPN, you end up with a material that can show a glass transition spanning over 100�C range. This is a pretty dramatic effect that can easily be seen with dynamic mechanical analysis. Click here to learn more about this technique. One example of this approach is the full-IPN composed of rubbery vinyl polymers and phenolic compounds developed by Hitachi Chemical.
As already mentioned in the History of IPNs Page, homo-IPNs based on styrene make excellent ion-exchange materials for applications such as water-purification. Rohm and Haas are well established in this product area. Homo-IPNs made of polymethylmethacrylate are used as artifical teeth. Artificial teeth can be made of porcelain or polymers. Polymers are much tougher than porcelain and resist chipping. A problem with polymers such as crosslinked PMMA is that certain foods (like salad oil) can plasticize the material, which severely decrease mechanical properties. Homo-IPNs of methylmethacrylate swell less than crosslinked polymers and therefore aren't plasticized as easy.
Rimplast Thermoplastic IPNs are made by reaction of functionalized silicone in a thermoplastic melt, such as Nylon 6,6. The silicone reacts to form a thermoset material that interpenetrates the thermoplastic. This process has alot of things going for it. The silicone and thermoplastic can be injection molded and react to form the IPN in the mold. This type of process is known as Reaction Injection Molding (RIM), which is a fast and efficient way to make parts. You can also see that its in the name of the material. The IPN shows increased tensile and flexural strength, as well as better creep resistance. Did you ever hear about those plastic gears that are self-lubricating, so they don't require any oil? Well the silicone in these materials acts as a lubricant, which lowers the surface wear on gears made of Nylon. As an additional bonus the silicone acts as internal mold-release agent, which means that the injection molded parts are easier to remove from the mold after they have cooled. Petrarch System Inc. are just one of companies that use this process.
Rubber-based IPN latexes with surface functionality have been used as impact modifiers in thermoset resins, especially those based on styrene and unsaturated polyesters. These latexes typically have a core-shell morphology. One specific example uses crosslinked butyl acrylate as the rubbery core and methyl methacrylate and glycidyl methacrylate as the shell material. The latex is collected and mixed with styrene and methacrylic acid. The methacrylic acid groups react with the glycidyl groups so that the latex particles have a bunch of functional groups over their surfaces. Unsaturated polyester is added to the mixture and the whole thing is polymerized to form a styrene-unsaturated polyester IPN matrix with rubber particles integrated throughout the matrix. If this situation seems familiar to you, you're one step ahead of the game. This is the same type of trick used to toughen plain ol' polystyrene by making HIPs, except the impact modifier is butadiene.
Polypropylene- EPDM thermoplastic IPNs are tough plastics used in automotive parts, such as tires, hoses, gaskets, belts, and car bumpers.
How about taking a
little quiz on the material on this page to see how well you learned it.
Return to Ye Olde IPN Shoppe