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Adhesives Ingredients

Silane Modified Polymers (SMP): An Alternative to PU Polymers

Vladimyr Wolan – Apr 13, 2020

This article was first published in 2017 and was revised in 2020.

Silane Modified Polymers: An Alternative to PU PolymersThe 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

H-Bonds in a DNA MoleculeNatural 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.

Polyurethane Bond

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.

Hydrogen Bonding in DIDP

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.

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.

Silyl Modified Polymers in Adhesives, Sealants & Coatings for High-performance & Safety

View All Commercial Silane Modified Hybrid Polymer Grades

Polymer Processing View Starting Point Formulations Based on SMP
Polymer Application Stay Updated with SMP Developments in the Industry

17 Comments on "Silane Modified Polymers (SMP): An Alternative to PU Polymers"
vladimyr W Nov 20, 2017
Dear Vasil Please contact sales@wolantech.com . We have Polymers that can make coatings that are isocyanate free, and we are getting shore A90 hardness, and pencil hardness of 6H. Solvent is difficult to eliminate but can be done .
Vasil L Nov 10, 2017
hello there, can we communicate with vladimyr regarding his silane/can be used as a substitute for hdi in top coat of 100micron/dft for 2k pu/1k pu?
vladimyr W Oct 3, 2017
Dear Apurba, Yes, you are correct that one part adhesives or sealants rely on moisture to pass through the cured material. A 35 mm thick sealant joint would be a problem, and I suggest the profile is not the correct one for the application. There are times when a 2 part adhesive or sealant needs to be used.
Apurba M Aug 31, 2017
Curing rate is also an issue as sealing compound. we tried a depth of 35 mm and found the material remained soft below 20 mm in 7 days. Higher catalyst may solve the problem at the cost of UV resistance.
vladimyr W Jun 9, 2017
Dear Darmesh, The SPUR silane terminated polyurethanes have some urea groups, and viscosity is high . Our Polymers use some patented additives to reduce the hydrogen bonding in the Polymer to have a workable product. Our DMS10000 Polymer is methyl dimethoxy silane terminated . The Polymer can be processed with wet PCC and GCC. The big advantage is that with the hydrogen bonding available, plasticizing with DIDP or Mesamoll is no problem, and the plasticizer does not leach.
Dr. Dharmesh D Jun 2, 2017
What about the adhesion performance between STPE (Silan Terminated Polyether) Vs. SPUR (Silan Terminated Polyurethane) on different substrates? What is the advantage of SPUR over STPE / MS ?
Dr. Dharmesh D Jun 2, 2017
I would like to know the difference between STPE (Silan Terminated Polyether) Vs. SPUR (Silan Terminated Polyurethane).
Alberto P Jun 1, 2017
So, dear colleagues, could you show me some formulations (That They work!) of S'PUR type? Thanks
vladimyr W May 23, 2017
Dear Roberto, these issues are known and formulation methods and additives are used to minimise the alkali and UV attack on SMP Polymers, which also affects most organic polymers. High catalyst levels in some of these formulations is a cause for UV attack, as are the commonly used PU polymers. The trimethoxy silane terminated polymers, as well as the alpha silane Polymers use very little or no catalyst and have excellent exterior performance .
roberto m May 20, 2017
SMP, nucleophilic attack of silicon and depolymerization phenomena in sealants and adhesives. These phenomena have caused controversy, civil and criminal cases like windscreens that are detached while the car is traveling. Many sealants manufactured with the famous raw mnaterials "genio-sil" (or somethings like that, an smp technology, and other famous brands) have depolymerized after few month or and 5 years after exposure at outdoor environment or on alkaline substrates such as portland cement. So, an elastic and tough elastomer turns into a plastic and sticky gum similar to a chewing gum. The problem arises, even when the indication of the raw material producer is respected. I think that we must pay attention to the problems of nucleophilic attack of silicon and act with specific molecules to cancel this depolymerization effect. It is sufficient to add from 0.5 to 1% of specific molecules.

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Omya: Paints & Coatings
For stronger adhesives stick to Nynas
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