Why Do Thermoplastics and Thermosets Work Together?
For centuries, humans have used a variety of polymers such as tars, oils, resins and gums. However, the Industrial Revolution ushered in the modern era of polymer as a material. Since then, synthetic polymers have become a necessity of modern life.
In the 1830s, Charles Goodyear created a process called vulcanization that produced a natural rubber. The first successful synthetic thermoplastic material was celluloid – a hard plastic created from nitrose cellulose, which became available in the 1870s. The family of polymers known as thermoset has its beginning in 1906, when Belgian chemist Leo Baekeland patented the material he named Bakelite – a combination of phenol and formaldehyde. This thermoset costs less to make than celluloid. The ability to mold Bakelite quickly made it very appealing for mass production, especially for automotive, industrial, electrical and mechanical parts.
Over the next several decades, the development of polymers moved at a snail’s pace. During the 1930s, new polymer materials, such as neoprene, polystyrene and nylon, began to replace Bakelite. The new plastics laid the foundation for an explosion in polymer research and the quest for thermoplastic and thermoset materials, including the development of Liquid Silicone Rubber (LSR), across a broad range of industries.
Today, designers of plastic components and products must consider a variety of characteristics in order to select the most qualified material for the particular application. These factors include mechanical, thermal and electrical requirements of the material.
Nonetheless, for many applications, designers can combine the two materials to take advantage of the best features each material have to offer. Understanding the unique qualities of each material can help designers and engineers make well-informed decisions about product design and material selection.
Thermoplastic and thermoset have different properties and applications. The main difference between the two materials has to do with the ability to reverse the solidification process and remelt thermoplastics into a liquid.
Thermoplastics vs. Thermosetting Plastics
The polymerization process creates the polymers used for making plastics. The fabrication process for thermoplastic and thermoset start outs with the same raw materials, such as ethylene and propylene, made from crude oil. The crude oil contains hydrocarbons, which make up the monomers. During a cracking process, various hydrocarbons produce monomers – such as styrene, vinyl chloride and acrylonitrile – that are used in plastics.
The polymerization process splits a monomer into two identical parts, or half-bonds. Each part has an unpaired or free electron. This produces a free radical that combines with other half bonds to make whole bonds. The process repeats itself numerous times. Eventually, it results in the formation of millions of polymer chains or a large polymer.
During the polymerization reaction or curing, millions of separate polymer chains grow in length simultaneously, until the monomers have been exhausted material manufacturers can add predetermined amounts of hydrogen or another chain-stopper to create polymers with a consistent chain length. The chain length or molecular weight is critical for determining the characteristics of the plastic, as well as its processing attributes. Increasing the chain length determines characteristics such as increased toughness and creep resistance.
Other characteristics of plastic include lightweight, waterproof, noncorrosive, nontoxic, stress/crack resistance, melt temperature, melt viscosity and manufacturability of the material. These properties are what make thermoplastic and thermoset plastics suitable material choices for applications across a variety of industries, including:
- Automotive
- Aerospace
- Toys
- Telecommunication equipment
- Computers
- Sports equipment
- Household appliances
- Construction material
- Office equipment and supplies
- Product packaging
- Medical equipment and devices
The widespread availability and use of synthetic plastics make it easy to take the material for granted.
Thermoset Plastics
Thermoset encompasses a category of materials, such as rubber, that sets or cures into a particular shape through the application of heat or chemical interaction. The curing process, or vulcanization, creates an irreversible chemical reaction that makes permanent connections called cross-links. You can visualize these cross-links as chemical bridges, which gives the vulcanized polymer a three-dimensional structure that makes the material more rigid prior to curing. After the initial heat forming, and once the thermoset cures, it cannot be reheated or otherwise remolded.
The fabrication process for thermosets differs from thermoplastics. Thermoset cures in two stages involving the material supplier and the molder. For example, the thermoset plastic known as phenolic undergoes partial polymerization at the supplier. The supplier reacts under intense heat and pressure and stops the chemical reaction at the stage of most of the linear chain formations. The final stage of vulcanization occurs in the molding press, where the unreacted portion of the phenol liquefies under heat and pressure, which creates a crosslinking reaction between molecular chains.
Types of Thermosetting Materials
Traditional thermoplastics consist of vicious liquids that have a susceptibility to creep and deformation, which makes them unsuitable for many applications. Along with other characteristics, thermosets offer product designers chemically robust materials with a surface hardness and heat resistance, which exceeds that of thermoplastics. Material options include:
-
- Epoxy resin (Epoxide) – Epoxy is hard, has chemical resistance, is a good electrical insulator, and unless reinforced, it can be brittle. It is often used to fabricate adhesives, for bonding of other materials, and casting and encapsulation. It is also employed for printed circuit boards and surface coatings.
- Melamine formaldehyde (MF) – A stiff, hard and strong material that resists some chemicals and stains. Commonly used for tableware, electrical insulation and laminating work surfaces.
- Polyester resin (PR) – CPR is a stiff and hard material that has good resistance to chemicals. It is brittle unless laminated. Works as a good electrical insulator, for car bodies and boats, and the bonding of other materials.
- Urea formaldehyde (UF) – UF is stiff and hard, but also brittle. Common uses include serving as an electrical insulator and for electrical fittings, handles and control knobs, and adhesives.
- Phenol formaldehyde (PF) – Known as Bakelite, artificial pigments can be added to this colorless polymer to produce a wide range of different colors. Used for dark-colored electrical fittings and parts for domestic appliances, bottle tops, kettle handles and saucepan handles.
- Silicone – This category of thermoplastic material has a variety of material options that include High Consistency Rubber (HCR) and LSR. Silicone is odorless, colorless, water-resistant, chemical-resistant, oxidation-resistant and thermal-resistant. Silicones are used for a broad range of products, including lubricants, adhesives, gasket and seals, dishware and medical equipment.
Many manufacturers are realizing the benefits of replacing traditional metal materials with thermosets. Thermosets have the characteristics necessary to meet demanding product specifications. They also have a longer lifecycle and reliability. The material provides excellent value through improved performance at a lower cost.
Product designers, engineers and managers have an ongoing mission to discover materials that have the properties and capabilities to meet very demanding performance requirements, as well as to save time and money. The use of LSR has been trending up since its introduction in the 1960s, and has supplanted thermoplastic and other silicone materials for use in aerospace, food and beverage, automotive construction, communications, consumer products and more because of its superior durometer, elongation and modulus tear qualities.
Thermoset Processing Methods
Thermoset plastic products are typically produced by heating liquid or powder within a mold and allowing the material to cure into its hardened form. The molder can remove the part from the mold before it cools. The reaction used to produce thermosetting plastic products involve a chemical interaction between specialized materials.
- Compression Molding – This method requires the preheating of the material, which is fed into an open, heated mold cavity. With the mold closed, pressure forces the material into contact with the mold until solidification. It is commonly used for projects that require molding intricate components in high volume.
- Transfer Molding – Prior to molding, the thermoset is measured and inserted into the machine; it is then preheated and loaded into a pot. A plunger forces the material from the sprues – or channels – and runner system into the mold, which heats the material above its melting point to create flow through the cavity. The mold remains closed as the material flows into the mold, but opens to release the part form the sprue and runner.
- Liquid Injection Molding – This method uses Liquid Silicone Rubber (LSR), which contains polymers with lower molecular weight. This means shorter chains and better flow. The LSR is fed, under medium pressure, to a static mixer and then into the hopper port on the barrel. To prevent friction and pressure from the curing material, the screw is rotated and the barrel cooled. Although it is not a requirement, the screw allows for some additional mixing to take place.
The depth of the screw flight decreases at the end of the screw nearest the mold, which compresses the LSR. A check valve at the end of the screw facilitates high-pressure injection, by moving the screw forward – employing the screw like a plunger. This process is commonly used for products that require high precision, including seals, electrical connectors and medical applications.
Advantages of Thermosetting Materials
Many companies appreciate the high strength-to-weight ratio – thermoset components are up to 35 percent lighter than steel parts of equal strength. This extremely strong material that offers ease of manufacturing and thermosetting materials provides a host of other characteristics at a low cost, including:
- More resistance to extreme heat and high temperatures compared to thermoplastic
- Excellent resistance to corrosives and solvents
- Electrical insulation
- Thick to thin wall capabilities
- Fatigue strength
- Excellent adhesion
- Tailored elasticity
Although thermoset cannot be reheated and remolded, materials can be repurposed for other applications. For example, polyurethane foam can be shredded into small flakes and used for fabricating carpet underlayment.
Thermoplastic Materials Properties
Thermoplastic begins in pellet form, which becomes pliable when processed with heat above its melting point. As the heat increases, the material becomes softer and more fluid. The fluidity of the molten material allows for its injection, under pressure, from a heated cavity into the cool mold, and the material solidifies into the shape of the mold. The process does not require a chemical cure.
The transformation of the thermoplastic encompasses a completely different physical process, which can be reversed with the reapplication of heat. The formation of thermoplastic material occurs during the viscous or melted phase – heating, forming and cooling the material into the final shape.
Based on the chemistry of thermoplastic, the material can exhibit many of the same behavior characteristics associated with rubber. It can also have the strength of aluminum. Some thermoplastics retain their properties at 100 degrees F, while other thermoplastic materials can withstand temperatures as high as 600 degrees F. At room temperature, some thermoplastics do not have a known solvent. They function as excellent electrical and thermal insulation. The addition of metal or carbon to thermoplastic composites can make them electrically conductive, such as:
- Polycarbonate
- Polyester
- Nylon
- PEEK
- ABS alloy
Some of the benefits of thermoplastic include:
- Rubbery or hard crystalline surface options
- Attractive finishing options
- High-impact resistance
- Chemical resistance
- Environmentally friendly manufacturing
- Remolding/reshaping capabilities
- Highly recyclable
The different thermoplastic resins have unique characteristics that offer various performances. Most materials commonly offer high strength, easy bendability and resistance to shrinkage. The most common method used to manufacture plastic components is plastic injection molding. During this process, the plastic can be heated into a molten liquid which is cooled and solidified into the final product. Dried, pelletized material, color or other additives can be added to the hopper and fed into a heated barrel.
Two-Shot Injection Molding: Combine Thermoset and Thermoplastic
Often, many manufacturers require a high-volume manufacturing process that combines aesthetics with high precision and consistency. LSR 2-Shot allows for the creation of injection-molded components by applying two different materials – thermosets such as LSR and thermoplastics – into different locations in the same mold. This proven process works well for the placement of a thermoset LSR cover in or over a hard thermoplastic substrate. For example, in the medical industry, a surgical instrument can have a base fabricated from rigid thermoplastic material, but the handle is made of soft-grip silicone.
The advantages of this technique include:
- Design more complex parts
- Improved product quality
- Improved material bonding
- Cost reductions
- Reduction in assembly time
The key to overmolding LSR onto a thermoplastic substrate successfully is to use the right combination of materials. The designer must factor in the shrinkage difference between the two materials, and the thermoplastic must have the thermal capability within temperatures of 300 degrees F or more.
Contact SIMTEC
Generally, each material has unique attributes that make suitable for different applications. It is important to collaborate with a company that is well-versed in the intricacies of material selection and the appropriate process to ensure successful design and production. To learn more about material selection for your project, or to request a quote, contact a SIMTEC representative.