Chapter 16 Thermosetting Resins

Thermosetting resins are those that change irreversibly under the influence of heat from a fusible and soluble material into one which is infusible and insoluble through the formation of a covalently crosslinked, thermally stable network.

Comparison of Major Coatings Technologies

1. Phenolic and Amino resins

Phenolic resins are mainly used in plywood and other construction materials. It normally contais wood flour as a filler and intended for applications involving low working stresses, e.g. knobs, covers, handles.  Variations include fine powders for high gloss finishes, and controlled particle size materials for accurate automatic molding machines.

 

Phenolic resins are among the oldest of plastics and were the first to be commercially exploited.  They are used principally in reinforced thermoset molding materials. 

A phenolic resin is a low molecular weight polymer (or oligomer) produced from phenol and formaldehyde in a condensation reaction.

There are two major types of commercial phenolic resins:

• Novolacs or indirectly hardening resins are thermoplastic like (2 steps resin) before curing.
• Resols or directly hardening resins are thermoset resins (1 step resin).

For the production of phenolic moulding compounds, the most frequently used types of resin are solid novolacs.

   
Characteristics 

Mechanical properties:  High modulus - outstanding resistance to deformation under load.  Good compressive strength. Limitations are that forms are hard and brittle.  Impact strength not high except with cord, fabric, glass and butadiene filled grades.  Hence high breakage rates during transportation have been reported.

Thermal properties:  Good dimensional stability over wide temperature range.  Limitations are that upper temperature limit 150 C (continuous) and 220 C for short-term operation.  This can be extended depending upon fillers used.

Electrical properties:  Mineral filled grades used for highest electrical properties (non conducting material).  Limitation is the tendency to form tracking paths at high voltages.

Environmental properties:  Chemical resistance to common solvents, weak acids and most detergents.  Very low water absorption.  Limitation is that it is not stable in presence of alkalis or strong oxidizing acids.  Surface finish adversely affected by hot, wet conditions, especially wood-filled grades.

Processing:  Injection molding and transfer molding grades available. 

Miscellaneous:  Low cost and large amount of application experience.


 

Amino Resins:

Amino crosslinkers (melamine-formaldehyde and urea- formaldehyde resins) are used in thermoset coatings to chemically link the molecules of the primary film-former into a three-dimensional polymer network. This is accomplished through reacting the amino resin with functional groups on the film-former and simultaneous self-condensation with other amino molecules.

Amino resins react readily with primary and secondary hydroxyl, carboxyl and amide-functional polymers. For this reason, aminos are generally used in paint systems based on acrylic, polyester, alkyd or epoxy vehicle resins. Amino resins are also used as additives in urethane systems to improve overall performance in certain applications.

Amino Synthesis

The amino resins are synthesized by melamine with formaldehyde in the presence of a catalyst.

Amino resins are versatile, low-cost crosslinkers for todays high-performance thermoset coatings. A wide variety of amino products are available that give formulating chemists latitude in selecting the optimum product for an application. Choosing the best crosslinker requires knowledge of its structure, including functional groups available to participate in reactions and molecular weight.

2. Unsaturated polyester resins (omitted)

3. Epoxy resins

The first part of the two-part epoxy you bought from market is a low-molecular-weight polymer with epoxy groups at each end. Its structure looks like this:

In these prepolymers n can be a high as 25, but the diepoxy in the two-part epoxy adhesive is more likely a small molecule with two epoxy groups, like one of these little fellows:

The second part is a diamine, which looks like this:

When you mix the two parts together, the diepoxy and the diamine react, and join together, in such a way that links all the diepoxy and diamine molecules together, like this:

What this gets us is a crosslinked network, that looks something like this:

When this happens, the result is a hard substance that can be very strong, but not processable. It can't be molded into shape, or even melted.

We make low molecular weight polymer by reacting bisphenol A with epichlorohydrin with some NaOH thrown in as a catalyst.

Epoxy resins made great adhesives, and are one of the few adhesives that can be used on metals. But they're also used for things like protective coatings, and as materials in things like electronic circuit boards and for patching holes in concrete pavement. Epoxies are also used to make composites -- any material made of more than one component.

4. Silicon Polymers

Silicones are used for a lot of things. They can be elastomers and lubricating oils. The caulking in your bathroom is probably made of a silicone. Silicones are also used to make the heat resistant tiles on the bottom of the space shuttle.

Silicones are inorganic polymers, that is, there are no carbon atoms in the backbone chain. The backbone is a chain of alternating silicon and oxygen atoms. Each silicone has two groups attached to it, and these can be any organic groups.

The picture below shows methyl groups attached to the silicon atoms. This polymer is called polydimethylsiloxane. It is the most common silicone. Polymethylphenylsiloxane and polydiphenylsiloxane are also popular with the kids these days.

Poly(dimethyl siloxane)

"Polysiloxane" is the proper name for silicones. But when they were discovered it was thought that they had "silicone" groups in the backbone chain. When the real structure was discovered, it was too late, and the name stuck.

Silicones make good elastomers because the backbone chain is very flexible. The bonds between a silicon atom and the two oxygen atoms attached to it are very flexible. The angle formed by these bonds can open and close like a scissors without much trouble. This makes the whole backbone chain flexible.

Polydimethylsiloxane does something really strange when you mix it with boric acid, or B(OH)₃. The mixture is soft and pliable, you can mold it into any shape easily with your fingers. But it is also very bouncy. What's more, push it gently and it gives way, but hit it hard with a hammer and it cracks! Strangely, if you spread it over newspaper, and pull it away, it gets printed with a mirror image of the newspaper text. No industrial use was ever found for his wonder material, but tons of it has been sold as toy called Silly Putty.

The crosslinked silicone resins are used primarily as insulating varnishes, impregnating and encapsulating agents, and in industrial paints. A part to be coated is typically dipped into the resin solution, and drained or scraped free of excess resin. The solvent is allowed to evaporate, and the resin is cured in an oven.

Homework:

Compare the structures and properties of each polymers.