Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 13:15

The formation of covalent bonds which hold portions of several polymerA large molecule containing a large number of repeating units; a substance formed from such molecules. chains together is called cross-linking. Extensive cross-linking results in a random three-dimensional network of interconnected chains, as shown in the figure. As one might expect, extensive cross-linking produces a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. which has more rigidity, hardness, and a higher melting pointThe temperature at which a solid becomes a liquid. Also called freezing point. than the equivalent polymer without cross-linking. Almost all the hard and rigid plastics we use are cross-linked. These include Bakelite, which is used in many electric plugs and sockets, melamine, which is used in plastic crockery, and epoxy resin glues.

A cross-linked polymer. For purposes of clarity, hydrogen atoms and side chains have been omitted, and only the carbon atoms in the chains are shown. Note that the cross links between chains occur at random.

Below is a video of the formation of Polyurethane Foam.

Polyether polyol, a blowing agent, which adds a gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. to the mixtureA combination of two or more substances in which the substances retain their chemical identity. to produce a foam, silicone surfactant, and a catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction. is mixed with a second liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container contains a polyfunctional isocyanate. The polyol and the polyfunctional isocyanate react to form polyurethane - a very hard substance when dried. The general reaction is shown below:

isocyanate groupThose elements that comprise a single column of the periodic table. Also called family.             hydroxyl groupThe functional group of an oxygen atom bonded to an hydrogen atom, -OH; found in alcohols.                        urethane linkage

In the reaction in the video, each R1 group has multiple isocyanate groups; the reactants are polyfunctional. Thus there is a high degree of cross-linking in the polyurethane. This causes the foam to become rigid after cooling.