A basic understanding of block copolymers is crucial in the study of adhesive formulation. In particular, styrenic block copolymers (SBC) are a vital component in a wide range of hot melt adhesives. SBC’s have a characteristic structure that makes them ideally suited for many adhesive applications. As the name implies, SBC’s are made of blocks of distinct polymers that each contribute to the properties of the polymer. Figure 1 shows the typical structure of a linear SBC. The styrene end-blocks are hard polymers that add strength to a HMA, but do not contribute to the tack and adhesion properties. The mid-blocks are composed of rubbery polymers such as polyisoprene, polybutadiene, ethylene-butene copolymer and others. The mid-block combined with tackifiers and plasticizers produce the adhesive properties.
Figure 1: Schematic Drawing of a Linear Styrenic Tri-Block Copolymer
SBC’s are produced in several variations of this basic structure. A di-block copolymer can be visualized as half a tri-block molecule. These polymers can be mixed with tri-block polymers to increase tack at the expense of some cohesion. Radial and star block copolymers have several arms branching from a central attachment point each terminated with a styrene end-block.
Block copolymers are useful because the end-blocks and mid-blocks are completely incompatible polymers. If one tried to mix, for example, polystyrene with polyisoprene, the result would be a useless compound that would separate like oil and water. In a block copolymer, however, these incompatible phases are chemically bonded together and cannot separate. The best they can do is to arrange themselves into domains on a microscopic level. In this tiny segregated world, styrene end-blocks clump together with other end-blocks as well as they can, while the rubbery mid-blocks tangle together like rubber bands connected to bowling balls. Since each molecule of a tri-block polymer has styrene on each end, the styrenic domains “glue together” several molecules simultaneously as shown in Figure 2. This association of end-blocks makes the entire mass of molecules act as one crosslinked molecule with flexibility and adhesive properties contributed by the mid-blocks and a strengthening skeleton provided by the styrene.
This explains the behavior of a block copolymer at ambient temperatures. The situation changes, however, at high temperatures. At about 100°C the styrene end-blocks melt and become liquid. When this happens the crosslinked structure of the material breaks down. As the temperature rises, the liquefied end-blocks act as a plasticizer for the system and the viscosity drops rapidly. This explains SBC’s value as components for hot melt adhesives. At low temperatures the styrene blocks reinforce the relatively low molecular weight mid-blocks. At high temperatures the styrene acts to soften and liquefy the system allowing it to be applied as a low-viscosity liquid. Furthermore, as the compound cools the styrene domains reform and the strength is regenerated each time the heat cycles. Block copolymers, therefore, are the perfect base for hot melt adhesives: they are strong, rubbery solids as ambient temperatures, but become viscous liquids at high temperatures and this change of state is reversible.
Figure 2: Domain Structure of a Solidified Styrenic Tri-Block Copolymer
The resins used to make SBC-based hot melt adhesives fall into two categories: end-block modifiers and mid-block modifiers. An end-block modifying resin is generally one with enough aromatic structure that it only associates with the styrenic end-block domains. If the resin has a softening point lower than styrene, it will cause the end-blocks to melt at a lower temperature reducing the melt viscosity and application temperature of the compound. If the resin has a softening point higher than styrene, it tends to reinforce the crosslink structure and provide higher temperature resistance to the HMA at the cost of higher application temperature and viscosity. Since these resins associate only with the end-block phase of the adhesive, they generally do not contribute to the tack and adhesion of the system. Mid-block modifying resins are tackifiers that associate only with the rubbery block and contribute to the tack and adhesion of the system. Since using a mid-block dilutes the styrenic portion of the polymer, they tend to decrease the crosslink density and cohesion of the system. It is also possible to formulate a tackifier with partial compatibility with both end- and mid-blocks. These materials are useful in balancing the properties of the system or have the necessary compatibility with some types of mid-block polymers. Aromatic/aliphatic C5/C9 resins, for example, are usually required for successful tackification of styrene-butadiene-styrene (SBS) block copolymers.
The selection of a tackifier for a block copolymer depends on the composition of the mid-block. Styrene-isoprene-styrene block copolymers are the most readily tackified and are most compatible with C5, hydrogenated C5, and hydrogenated C9 tackifiers. SBS block copolymers are somewhat more difficult to tackify, are less stable than SIS, and require either C5/C9 hybrid or rosin ester tackifiers. SBS block copolymers are more readily available and cost less than SIS. Styrene-ethylene-butene-styrene (SEBS) block copolymers have fully saturated mid-blocks for the ultimate in stability and low color, but are the most difficult SBC’s to tackify. SEBS block copolymers require fully hydrogenated C5 or C9 tackifiers to produce a useful adhesive.