This article was first published in 2017 and was revised in 2020.
The formulators of adhesives, sealants and coatings have a huge range of polymers and chemicals to use. They need to know the basics of how polymers crosslink and hydrogen bond, and form a useful product. Also, they need to balance the advantages and disadvantages of each polymer type.
A challenge in today’s markets is suppliers not disclosing the disadvantages of their product. However, it's one of the key things to know prior formulations. Every synthetic polymer has its
strengths and limitations. Price is not the only consideration in choosing a polymer.
For example, polyurethane (PU) is the most widely used polymer in adhesives and sealants industry. Yet it faces some drawbacks that formulators must be aware of. This article will help you find a safer solution to polyurethanes.
But before that, let's take a quick look at polymer bonding.
Bonding in Polymers
Natural and synthetic polymers are based on carbon atoms with a mixture of oxygen, nitrogen and other atoms combined in a huge variety of forms. The forms are governed by the way atoms can form a covalent bond and the number of electrons available for a strong bond.
The most famous polymer we have in nature is the DNA molecule.
Note the hydrogen bonds, which hold the polymer together, until time comes to replicate the DNA instructions and a special molecule or enzyme comes along and breaks the hydrogen bonds in the process that occurs in nature all the time.
In the 20th century, the art of synthetic polymer construction was developed using chemical factories and metal catalysts in a cruder reaction than found in nature. The
polymers are mostly based on carbon and oxygen atom chains, with some electronegative atoms attached in various ways.
Shown below is a table on atom electronegativity:
| Atom |
Electronegativity |
| F |
4.0 |
| O |
3.5 |
| Cl |
3.0 |
| N |
3.0 |
| Br |
2.8 |
| I |
2.5 |
| S |
2.5 |
| C |
2.5 |
| H |
2.1 |
Atom Electronegativity
It clear from the table that: Fluorine is the most electronegative, and used in PTFE polymers.
Now let's turn our attention towards PU polymers.
Design of Polyurethanes
PU polymers are very successful products used in the
adhesive, sealants and paint industries. These polymers contain
a urethane group that has a nitrogen atom in the chain with a very electropositive hydrogen atom branched on the chain.
There is also a carbon atom as shown in the figure below with a bonded oxygen atom also branched. The result is two atoms sticking out of a polymer chain, and a hydrogen bond is formed with another adjacent chain. The oxygen has a negative charge and the hydrogen a positive charge.
These hydrogen bonds:
- Add strength to the polymer matrix
- Are able to reform during sealant elongation, and
- Can form a bond with a plasticizer that has a C=O or carbonyl group in its chain structure
This we have with
DIDP and other plasticizers of a similar structure. This is important, as without hydrogen bonding, plasticizers will leach out of a sealant composition and can have poor adhesion to bonded surfaces after some time.
H-Bonding in Plasticizer
Problems with Uncured PU Sealants and Adhesives
In uncured PU sealants and adhesives, the polymer has
unreacted N=C=O groups, which are called
isocyanate groups. These groups are very reactive, and in various forms are very harmful to humans who get the sealant on their skin, or inhale some of the vapors.
This is a
big issue as many of these PU products also contain solvents. So we have a very good polymer product when cured, however in the uncured state,
it is very nasty (No arguments about this problem).
Silane Terminated Polymers: A Safer Solution
Our innovative chemists have taken a very good polymer design and
eliminated the nasty NCO group, by adding a silane molecule, which in a sealant crosslinks by a reaction of a
silanol Si-OH, to form a
Si-O-Si bond. This is most easily done with an isocyanato silane, which is shown below.
Structure of Isocyanato Silane
The great result with
using an isocyanato silane to terminate a PPG chain, is that we react the NCO group with the OH group on the PPG or polypropylene glycol chain. And we still get the urethane group, with the N-H and the C=O groups that hydrogen bond across the polymer chains. Also, hydrogen bond with any added plasticizers that contain the C=O groups.
The toxic NCO groups should be fully reacted. Our chemists make sure
there are no unreacted NCO groups by adding a little alcohol that cleans up any unreacted NCO. The tradesman has a product he applies with no N=C=O groups present.
We now have a family of polymers that
cure a different way to cured PU Polymers. However, they have the hydrogen bonding across chains and a main crosslinking bond that is a
Si-O-Si as we see in silicone sealants and rubbers.
Explore Silyl-modified Polymers (SMP) Options in Our Material Database >>
This is good, however formulators of sealant and membranes need to be aware there are also polymers in the market that are formed by a different process. These polymers do not have the C=O and N-H groups that form the hydrogen bonds. These
polymers rely mainly on the silane bond, and there can be a major issue when common plasticizers are needed. These polymers need to use a special plasticizer molecule.
Adhesives & Sealants Professionals - Stay Alert!
Limit the use of isocyanates &
switch to silane hybrid prepolymers vs PU in your adhesives, sealants, coatings by achieving a better balance of mechanical properties (
strength, elongation, viscosity control…). Take the course "
Silyl Modified Polymers in Adhesives, Sealants & Coatings for High-performance & Safety" to design high-performing, safer formulations with in-depth understanding of
silanes.
Silane Modified Hybrid Polymer Grades for Adhesives and Sealants
View a wide range of silane hybrid polymers available today, analyze technical data of each product, get technical assistance or request samples.
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