A friend of mine recently expressed a bit of sadness that the most exciting periods of new polymer development may have be over. This appears certainly to be true if you look at the emergence of new polymers over time (Figure 1). The introduction of fully synthetic polymeric materials based on new monomers started in the 1920s and passed through a maximum in the 1950s. After that only a few new polymers were introduced and even fewer were successful in the market. There, of course are several explanations for this and there are also several reasons why this does not mean that innovation has stopped or even that it is dying out.

Figure 1: Emergence of new polymers. 1 (SpecialChem Fig. Ref.: New polymer history.)

A principal reason for the "hat" shaped curve in Figure 1 is that low sales prices both for standard and engineering plastics erect a high barrier to the introduction of new polymers, since the market launch of a new material is estimated to cost $100 million USD. ² Today it appears unlikely that any new polymer with a volume production potential will be introduced industrially.

However, I told my friend that bright days are still ahead despite the bleak outlook projected in Figure 1. Because of the cost of developing and introducing new polymers, significant innovation is still occurring by "engineering" current polymers for improved properties to meet specific market applications. As a result many of the innovations that will take place in polymeric materials for adhesives and sealants will be in the areas of:

• Physical modification of current polymers morphology by highly selective catalysts and processing technologies
• Blends, composites, and multilayer forms to minimize cost and provide improved performance properties.

Polymer architecture and property profiles are tailored as a function of catalyst structures and process conditions. The scope of single site catalyst technology has been expanded well beyond that of metallocene-based systems to produce novel families of 1-olefin, styrene, diene, and cyclo-olefin polymers ranging from engineering plastics to flexible elastomers. Progress in catalytic olefin polymerization has led to the development of environmentally friendly and versatile polymeric materials.

Blending is a way of producing barrier materials using only limited amounts of expensive barrier resin in an inexpensive matrix material. In general the properties of the blend will be between those of the components, but in some cases surprising synergies have been noticed. For a blend, the optimal properties generally are dependent on the morphology.

Blending can also be used to improve the processing properties of conventional polymers. New polymers such as linear low-density polyethylene (LLDPE) are largely replacing low-density polyethylene due to their improved physical and mechanical properties. However, they are more difficult to process and are prone to melt fracture at certain processing conditions. Polymer processing additives based on fluorocarbon chemistry are effective in reducing the apparent viscosity of high molecular weight LLDPE and improving their processing characteristics.

The ultimate "engineered" material is perhaps those that are based on nanotechnologies. Adhesives can be a relatively large and important application for nanotechnology. Materials constructed from nanotechnology have been found to provide unexpected and valuable properties. The properties that are most improved by nanotechnology include tensile, heat distortion, modulus, toughness. These property improvements may even be of such high value that they can justify the early price of nanomaterials.

The most commercially and technically interesting nanotechnology-based product developed for adhesives are polymer nanocomposites. These are polymeric compounds that consist of discrete fillers on the order of a few nanometers and with immense surface areas. These compounds can be processed using conventional conversion equipment. Nanocomposites represent a radical alternative to traditional filled polymers and polymer blends. The enhanced properties that these materials can provide are both considerable and surprising. The surprise comes from the fact that by being so small the nanofillers are the same size range as the polymer molecules and react directly with them. As a result, the material properties are affected by the laws of atomic physics rather than behaving as traditional bulk materials.

When compared to other nanomaterials, nanocomposites are relatively low cost. Low volume additions INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (1-5 weight percent) of highly anisotropic, high aspect ratio nanoparticles, such as layered silicates, provide mechanical property enhancements with respect to the virgin polymer that are comparable with those achieved by conventional filler loadings of 15-40%. This results in significant processing advantages and reduced cost potential due to down-gauging of cross-section