The key difference between physical and chemical cross linking is that physical cross links occur through weak interactions whereas chemical cross links form through covalent bonding.
Cross link formation is the bonding of one polymer chain to another. This link can form in one of two ways: physical and chemical method which involves ionic bonds and covalent bonds, respectively.
What is Physical Cross-Linking?
Physical cross linking is the formation of a bond between polymer chains through weak interactions. Most of the times, these interactions tend to be ionic bonds. E.g. sodium alginate gels form ionic bonds upon exposure to calcium ions. This cross linking involves bridge formation between alginate chains. Another common example includes the addition of borax to polyvinyl alcohol, which forms hydrogen bonds (weak interaction forces) between boric acid and the alcohol groups of the polymer. Some examples of substances that can undergo physical cross linking include gelatin, collagen, agarose, and agar-agar.
Generally, physical cross links are not comparatively stable mechanically and thermally. There is a class of polymers known as thermoplastic elastomers that tend to rely on physical cross linking in their microstructure. This cross linking gives the material stability, so they are widely useful in non-tire applications, e.g. snowmobile tracks, and catheters for medical use. This is because the physical cross linking is often reversible, and we can reform it through the application of heat.
What is Chemical Cross-Linking?
Chemical cross linking is the formation of a bond between polymer chains through covalent chemical bonds. These cross links form by chemical reactions that can be initiated through heat, pressure, change in pH, or irradiation.
As an example, chemical cross-linking occurs when an unpolymerized or partially polymerized resin is mixed with specific chemicals named cross linking reagents. This results in a chemical reaction that forms the cross links. Moreover, we can induce this cross linking in materials that usually are thermoplastic. That is through exposure to a radiation source such as an electron beam exposure, gamma radiation, or Ultraviolet radiation. E.g. we can use an electron beam processing for the crosslinking of the C type of cross linked polyethylene.
Vulcanization is another type of cross linking which is a chemical process. It can change rubber to the hard, durable material that is associated with car and bike tires. This step is named sulfur curing. It is a slow process that can be accelerated using accelerators.
What is the Difference Between Physical and Chemical Cross Linking?
In chemistry and biochemistry, cross linking is the process of the formation of bonds between polymer chains. The key difference between physical and chemical cross linking is that physical cross links occur through weak interactions whereas chemical cross links form through covalent bonding. Moreover, thermoplastic elastomers undergo physical cross linking while thermosetting polymers undergo chemical cross linking. In addition, physical cross linking has low durability while chemical cross linking has high durability. Another difference between physical and chemical cross linking is that physical cross linking is weaker than chemical cross linking.
The below infographic lists the differences between physical and chemical cross linking in tabular form.
Summary – Physical vs Chemical Cross Linking
The term cross linking is common in chemistry and biology. The key difference between physical and chemical cross linking is that physical cross links occur through weak interactions whereas chemical cross links form through covalent bonding.
1. “Physical Crosslinking.” ScienceDirect Topics, Available here.
1. “Vulcanization of POLYIsoprene V.2” By Jü – Own work (CC0) via Commons Wikimedia