Resin is a substance that can be either natural or synthetic, characterized by high viscosity. Natural resins are transparent, highly flammable, and typically range in color from yellow to brown. They are obtained from the exudations of trees such as pine, often resulting from damage caused by factors like fire, wind, lightning, etc. Synthetic resins, on the other hand, differ from natural ones in terms of chemical properties and manufacturing methods.

These materials easily dissolve in organic solvents like ether but are insoluble in water. The primary distinction between natural and synthetic resins lies in their origin and production methods. In modern industries, natural resins have largely been replaced by synthetic ones. Synthetic resins can further be categorized into thermoplastic and thermosetting types based on their response to heat.

Types of Resin:

1. Natural Resin:
   - Example: Rosin (or colophony)
   - Chemical Formula: C20H30O20
   - Obtained naturally from tree exudations, it has a low cost but exhibits relatively weak resistance to drying and chemicals.

2. Ester Gum:
   - Chemical Formula: C3H5(OH)3
   - Produced from the reaction of rosin with glycerol, it is a synthetic resin with improved color stability and adhesive properties compared to natural resins.

3. Pentarzine:
   - Derived from the reaction of pentaerythritol with rosin.
   - Has a higher melting point, good thermal stability, and produces high gloss when cooked with various oils.

4. Coumarone-Indene (Coumar):
   - Obtained from coal tar distillation, these resins are polymers of complex cyclic compounds like coumarone and indene.
   - Neutral and ideal for aluminum surfaces due to resistance against corrosive agents.

5. Pure Phenolic Resins:
   - Synthesized from the reaction of phenol with formaldehyde.
   - Two types: one used in baked oil coatings and the other sold as a solution in a solvent, both with excellent water resistance and durability.

All phenolic products are prone to color changes and yellowing over time. The choice of resin depends on the specific application and desired properties, such as resistance to water, chemicals, and durability.

6- Modified Phenolic Resins
This product is obtained by combining ester gum with pure phenolic substances. These materials exhibit good resistance to water, alkalies, and chemicals, and due to the presence of ester gum, they also have good adhesion. This type of resin dries well, providing a high gloss. These products are suitable for floors, pavements, decks, doors, and any surface that requires quick drying and can withstand yellowing.

7- Maleic Resins
These resins result from the reaction of an anhydride acid with a multifunctional alcohol such as glycerin, in the presence of rosin gum or ester gum. This substance is compatible with nitrocellulose and possesses good lacquer properties. Maleic resins dry quickly, maintaining good color retention. They are ideal for use in quick-drying and white coatings. This category of resins should be used with short-chain oils, as using them with long-chain oils tends to prolong drying time.

8- Alkyd Resins
This product is obtained from the reaction of a polybasic acid such as phthalic acid or an anhydride acid with a multifunctional alcohol like glycerin and pentaerythritol. It is possibly the most widely used resin in commercial paints and many industrial coatings. Alkyd resins modified with a high percentage of drying oils are commonly used in commercial paints and are known as long oil or medium oil alkyds. Those modified with a lower percentage of oil or with non-drying oils are used in industrial paints, enamel coatings, and lacquers, known as short oil alkyds or non-drying alkyds. Generally, the higher the percentage of glyceryl phthalate or resin content, the faster the drying time, increased brittleness, and improved baking properties. Other properties depend on the type of modifying oil and the type of multifunctional acid used.

Properties:
In general, alkyds have excellent drying properties and, in addition to that, exhibit excellent flexibility and durability. When modified with non-drying oils or with oils with good stability, such as soybean oil, they have excellent color stability. Alkyds show outstanding gloss and color retention in alkyd coatings. In baking enamels, alkyds are often combined with other resins like urea and melamine to create excellent layers.

The resistance properties of alkyds, while good, are not comparable to those of pure phenolic materials and do not even match those of modified phenolic materials. However, if high resistance properties are not necessary, alkyds are unparalleled in terms of overall properties. Therefore, alkyds are ideal for all types of interior, exterior, marine, and a significant percentage of industrial coatings.

9- Amino Resins
These materials are mainly copolymers of formaldehyde with urea or melamines. Since the resulting film is very brittle and has low adhesion to metal surfaces, they are compatible with resins containing carboxyl and hydroxyl groups, such as various alkyds and epoxies, and impart desirable properties to the resulting films. Urea type has more solubility than melamine type, but melamines exhibit a more pronounced hardening effect.

In general, amino resins are used in combination with other resins, especially in the preparation of enamel coatings for household items such as refrigerators. Also, one type of automotive paint is alkyd-melamine baked enamel, used to increase hardness and water resistance.

- Urea Resins:
Derived from short oil alkyds rich in phthalic (as mentioned earlier), these are used in combination with urea and melamine in baking enamels. Urea resins can only be used in types of baking enamels as they undergo polymerization into a solid form under heat. These resins, resulting from the reaction of urea with formaldehyde, form hard, relatively brittle, and colorless layers. Their brittleness and weak adhesion are corrected by combining them with alkyd resins or plasticizers. They exhibit excellent color stability and good resistance to alcohol, grease, oils, and many corrosive substances. Moreover, they provide excellent final coatings for metal surfaces such as refrigerator exteriors, metalware, automobiles, and toys.

- Melamine Resins:
These products, made from melamines (a cyclic compound) and formaldehyde, perform better than urea resins. Melamine resins cure faster or at lower temperatures, producing a harder, more durable, glossy, and heat-resistant layer. Although more expensive, they are preferred for creating high-quality white coatings due to their shorter curing cycle, resulting in whiter and more color-stable layers.

10- Vinyl Resins
This product consists of copolymers of polyvinyl chloride and polyvinyl acetate. These are available in the market as white powder, which needs to be dissolved in strong solvents such as esters and ketones. Alternatively, pre-dissolved versions in the mentioned solvents are also available. To achieve an acceptable layer, it is necessary to add plasticizers to make them more flexible.

As mentioned, this product is a copolymer of polyvinyl chloride and polyvinyl acetate. Dissolving chloride is challenging, but it is highly resistant to chemicals, acids, alkalis, and solvents. Acetate lacks this resistance but has higher solubility. The copolymer of these two substances exhibits exceptional resistance to corrosive agents, chemicals, water, alcohol, and acids and alkalis. These products provide excellent coatings for cables, swimming pools, barrels, construction structures, or any surface requiring high resistance.

Vinyl protective coatings have the greatest versatility among other resin types. They exhibit good resistance throughout the pH range, excellent resistance to water, and perhaps the lowest tendency to chalk in comparison to any other substance. Their layers are thermoplastic, ensuring ease of recoating. The main drawback of vinyl resin coatings is their limited heat resistance. They become sensitive to temperatures between 66 to 82 degrees Celsius. Additionally, they have a noticeable tendency to become brittle due to interlayer contamination. Layers with higher tensile strength and adhesion become peelable when applied on a contaminated surface. Hence, compared to alkyd and epoxy coatings, they have fewer advantages.