Jan 16, 2023

Using Phenolic Resins to Enhance Rubbers and Adhesives

Reading Time: 4 minutes

Phenolic resins are used to tackify, reinforce or help cure rubber compounds. They are known for their heat resistance, hardness, dimensional stability, and chemical and electrical resistance.

What are Phenolic Resins?

Phenolic resins are a type of thermoset polymer, meaning they cure in an altered form from their uncured state and cannot be re-melted like thermoplastics. They are synthesized from the condensation reaction of phenol with formaldehyde in the presence of a catalyst.

When compared with other thermoset polymer resins like polyester, urethane, epoxy, and melamine, phenolic resins can better withstand heat and harsh chemicals. They are compatible with SBR, butyl, natural rubber and nitrile rubber.

Phenolic resins have low phenol content, and are compatible with many solvents including alcohols, higher alcohols, ketones, esters, aromatic hydrocarbons, chlorinated hydrocarbons, aliphatic hydrocarbons and oil.

Phenolic tackifier resins

Tackifier resins are added to rubber compounds to help keep different components in part or component together until they are cured. Phenolic tackifier resins are the high performance tackifiers when compared to competing technologies.

Creating tack in a rubber compound is a surface phenomenon that relies on contact between the surfaces and formation of chemical bonds

Adding phenolic tackifier resins to rubber compounds enables the surfaces to make better contact because chemical bonds are formed based on intermolecular forces of attraction.

Better tack means the resin can be used at lower levels – the optimum level is typically around 3-5 phr. Lower levels produces rubber compounds with reduced heat build up, which is especially useful for synthetic rubber compounds, which have low levels of inherent tack. Phenolic tackifier resins have pros and cons.

Pros:

• High initial tack
• Better tack retention
• Better heat and humdity performance
• Lower Loading Levels Required
• A process aid during mixing
• Inert in finished product

Cons

• Higher Cost
• Discoloration upon Light Exposure

Phenolic Reinforcing Resins

Phenolic resins are used in rubber compounds to provide increased hardness, toughness, stiffness, abrasion resistance and tear resistance. These effects occur during curing when reinforcing resins form a three-dimensional cross-linked network.

These resins can be used when different properties are required in the same application. For example, when a timing belt is attached to a steel pully, different rubber compounds are required. The rubber compound next to the steel pulley needs a hard rubber compound, while the rubber used in the circumference of the belt requires a flexible compound and fiber reinforcing.

Phenolic reinforcing resins are added at 10-30 phr depending on the formula. Crosslinking occurs at 115-120°C and masterbatch temperature determines which type (with or without methylene donor) should be used.

Pros:

• Good reinforcing properties
• Easily compounded – acts as plasticizer until cured
• Modified for ideal solubility
• Can be provided as a powder pre-mixed with methylene donor
• Allows addition of higher filler levels

Cons

• Higher Cost than Fillers

Phenolic Curing Resins

Phenolic curing resins are typically used for tires and engineered parts like belts and hoses. They are used to crosslink unsaturated elastomers using methyl or bromo-methyl groups for reactivity. They require a halogen donor for activation and form a crosslinked network with the rubber when cured. These phenolic resins are typically used Reduce thermoplasticity, improve abrasion resistance, increase tensile strength and improve chemical resistance.

Unlike sulfur and peroxide curing systems, phenolic curing resins form covalent carbon-carbon bonds, which improves rubber performance. This is ideal for repeated heat cycles such as butyl curing bladders. These curing resins act as a plasticizer until the cure temperature is reached. Brominated varieties provide rapid cure without a hydrogen donor.

Phenolic curing resins are typically added at around 10 phr. They require high temperature for curing in the range of 180-200 °C.

Pros

• Phenolic cure form strong covalent bonds
• Resulting product has improved moisture and temperature resistance
• Easily compounded – acts as plasticizer until cured

Cons

• Higher cost than sulfur
• Higher temperature cure required
• Longer cure time required

Resistance to heat and chemicals

Phenolic resins are able to withstand high temperatures with minimal deformation and creep when they are subjected to severe environments and high mechanical loads. That means that cured phenolic resins maintain dimensional stability and structural integrity in severe conditions.Phenolic molding compounds are sometimes used as additives to provide fire resistance for the final product because they are resistant to steam and able to withstand temperatures as high as 550 °F.

Because they are compatible with many solvents, phenolic resins have been popular within transit and architectural businesses. When applied, they emit low smoke. Some phenolic resin blends meet the expectations of UL fire ratings.

Properties enhanced with phenolic resins

Phenolic resins impart increased hardness and tensile strength. For example, they would be an ideal material to reinforce pump impellers because those mechanical parts are subject to intense mechanical and thermal stress.

They can also improve the tear, abrasion and chip resistance of rubbers and enhance adhesives used for rubber, shoes, upholstery, leather, and tape. These resins are also used in rubber goods, enamels, coatings, varnishes, and convey belting.

Applications

Phenolic resin was traditionally used for electrical applications like circuit boards and knife handles. When used in vulcanized products like tires, phenolic resins not only impart tear resistance, they improve tire traction on roads.

These resins are also used as binders in friction materials such as clutch discs and brake pads because of its high heat tolerance. Phenolic resins are used in a wide range of applications, including electronics, ballistics, mine ventilation, offshore water pipe systems, aerospace, rail and mass transit. Phenolic resins are also good for high-temperature applications where parts must meet fire safety standards.

Polyols & Polymers’ POLYTONE^® AP Series contains both heat-reactive and non-heat reactive thermoplastic options:

Heat-reactive

• POLYTONE AP 111 is formulated for use in Neoprene rubber adhesives. It imparts low initial viscosity and viscosity stability to adhesives of this type. It can also be used with the adhesive grades of chloroprene where good bond strength at room and elevated temperatures are required.
• POLYTONE AP 113 is mainly used for the crosslinking or curing of butyl rubber in the tire industry. It is also used in adhesives based on butyl rubber.

Non-heat reactive

• POLYTONE AP 120 E has excellent alcohol solubility. It is quick-drying, has excellent color retention, and is resistant to weathering, alkalis, water and abrasion. It is designed to develop high tack levels in rubber compounds that are used in tire construction and mechanical goods. It is used in tung oil-based varnishes processed at 450 °C. It can also be used to add surface hardness to polyamide-based inks.

POLYTONE AP 121 designed to develop high tack levels in rubber compounds that are used in tire construction and mechanical goods. This resin provides tack in elastomers that are usually difficult to tackify.

For assistance selecting the right adhesive for your, call HM Royal.