Polymer Polarity

A polymer is considered polar if the it’s charge can be measured as positive or negative.  If a polymer has no charge, we say it is non-polar.  Some examples of polar plastics are PA, PC, PMMA, and ABS.  The most common non-polar plastics are PP, PE, SEBS, PS and PTFE.
Significant research has been done on how to modify the polarity of different polymers.  This information is very technical and goes beyond what needs to be known by polymer sales representatives.  To keep things simple, I’m going to focus on two questions that we recently heard from our customers.
 I know polyethylene is a non-polar resin.  Is there any time when it behaves like a polar resin?  Why?
How can I help a non-polar material like SEBS adhere to a polar material like PC?
Question #1
OK, so when and why is PE or more specifically LDPE polar?  Research has shown the when producing LDPE film, they LDPE film is slightly polar right as it comes out of the T-die.  To understand why we’ll need to know a little bit about oxidation and carboxyl groups.  Put simply, oxidation is the combination of a substance with oxygen.  As the LDPE exits the T-die oxidation occurs and forms carboxyl groups.  A carboxyl group is a functional group found in organic compounds in which a carbon atom is doubly bonded to an oxygen atom.  Because oxygen has the greater affinity for electrons, it acquires a partial negative charge, becoming electron-rich; the carbon atom of the carbonyl group thus becomes electron-deficient, acquiring a partial positive charge.
This unique feature allows LDPE to bond with polar substrates and makes LDPE a popular choice for many lamination applications. 
Question #2
How can I help a non-polar material like SEBS adhere to a polar material like PC?
Typical SEBS grades will not adhere well to polar surfaces such as PC.  However, if maleic anhydride is grafted onto the ethylene butadiene mid-blocks, these functional groups will allow modified SEBS to adhere to polar materials.  This is especially useful for soft touch applications where over-molding is often used.  Because anhydride groups are extremely useful polar reactive sites, modified SEBS can allow for successful cohesion without the use of adhesives.  Contact Kraton Polymers for more information about SEBS or visit our website to see what we can offer.