Autohesion is the spontaneous adherence between two unvulcanized surfaces of an elastomer. Only certain types of elastomers exhibit this self-tack characteristic. Such elastomers are used to formulate semi-structural contact adhesives and self-fusing sealants. Elastomers showing autohesive characteristics are widely used in contact adhesives as well as for sealing electrical wires due to their ease of application. Another important use is the sealing of food packaging. Here, cold seal adhesives have a huge market and add significant value to the end product.
Autohesives bond only to themselves. This differs from conventional pressure sensitive adhesives that are applied to one surface and bond to non-coated surfaces by means of tack and viscoelasticity. It also differs from heat seal adhesives that also require coating on only one surface but need a source of heat and pressure to make a bond.
 What are your recommendation to enhance polymer resins adhesion properties? Share your thoughts with your Peers! |
When two pieces of uncrosslinked elastomer is contacted under light pressure, some fraction of the molecular chains from one surface will come into intimate contact with those on the opposite surface. Progressive molecular contact will be realized as the surface regions flow under the action of interfacial forces and applied pressure.
In regions where the molecular contact is complete, diffusion of molecules from one substrate into another will occur creating a cohesive bond. Several theoretical models have been developed to represent this process. They show the following interdependencies.
Autohesion can also depend heavily on surface roughness. Smooth surfaces will, of course, provide better autohesion since the intermolecular contact area is maximized. Contamination on one of the surfaces can prevent autohesion.
There are several critical material parameters that affect the degree of autohesion for any polymeric material. These include rheology, microstructure, and molecular weight. Of these, molecular weight is perhaps the most important. For any elastomer type, there is a molecular weight range in which joint strength as the result of autohesion is maximized. At very low molecular weight ranges, all elastomers will rapidly form an autohesive bond, but the cohesive strength of the resulting weld will be low. At very high molecular weight ranges, the cohesive strength of the joint will be high, but the interdiffusion will be slow and limited. Only at intermediate molecular weight ranges is there an optimal situation relative to both adhesion and cohesion. Table 1 provides measured autohesive strength for several polymers that were used at near optimum molecular weight conditions.
Table 1: Measured Autohesive Data for Several Polymers1 |
The base polymer that is used for autohesion must be sufficiently plastic to form a bond to itself under the application of slight pressure alone. However, it also must be sufficiently hard to resist bonding to another substrate during storage, pressure, etc. The polymers that appear to provide the best autohesion characteristics are natural rubber, polyisoprene, butyl, and certain formulations of silicone rubber.
Natural rubber latex was the first adhesive showing autohesive characteristics, and it was widely used in spite of several well recognized disadvantages, such as poor oxidation resistance and discoloration. Attempts to replace natural rubber with synthetic rubbers to alleviate these problems have proven to be difficult except for several notable elastomer families (Table 2).
|
加载中...
