Learn more about the science behind thermoplastic composites with these quick-read articles on composite materials.
As a general rule, polymer materials are created from many monomers bonded together in chains.
When these materials bond, they take up physical characteristics and properties of their own, such as hardness, tensile strength, and viscosity.
They also tend to remain amorphous, meaning they are made out of randomly coiled and entangled chains like worms in a can. This gives them low melting points and some solvent penetrability.
Their low melting points make them easy to thermoform, which is why they are often used in injection molding applications. Their translucent nature makes them good materials in optical products such as ski goggles as well.
This amorphous polymer class of materials includes thermosets and polystyrene. However, some plastics can be partially crystalline, meaning some of their structure is not completely random.
This partially crystalline structure gives them a higher melting point (the temperature at which the molecular forces are overcome) than their amorphous colleagues, but also one or more glass transitions (the temperature above which their molecular flexibility is increased).
They are also more opaque than their amorphous counterparts. This semi-crystalline category includes polymers such as: polyamides (nylons), polyesters, and some polyurethanes. They are often referred to as semi-crystalline plastics.
This distinction between amorphous and partially crystalline characteristics have a tremendous impact on their applications and manufacturability.
Polymers can be seen as repeating long chains of bonded monomers. These polymers can also be amorphous or semi-crystalline in nature.
These distinctions have direct consequences for the usage, manufacturability and recycling of the polymer in the advanced composite and plastic industries.
A composite material is a combination of two or more distinct materials that, when bonded together, achieve better properties not possessed by any of its components acting on its own. The two more talked about components of a composite material are its fiber and the matrix.
The fiber is usually a stiff material that will give the composite material its strength and rigidity, like carbon fiber for example.
The matrix serves to bind its fibers together as well as protect them from harm. Another word commonly used to describe the matrix is resin. This matrix can be either a thermoset or a thermoplastic.
Thermosets are the most commonly used for composites and they are all based on amorphous polymers. They exist out of the cross-linking of their amorphous molecular structure, which creates a rigid final structure and therefore will not be able to be recycled or reformed after they are cured.
Most thermoplastics, on the other end, are semi-crystalline in nature. Because their crystalline structure is linear and not chemically cross-linked together like a thermoset, their bonds will be weaker than those of a thermoset.
These weaker bonds can be broken by the application of heat. When the heat source is removed, the molecules “freeze” in their new position as those bonds are restored, allowing thermoplastics to be re-molded or recycled.
These differences can be best illustrated below:
Thermoplastics are made out of long molecule chains composed of bonded monomers. Additionally, most thermoplastics have a semi-crystalline structure, and therefore have a weaker molecular bond than their thermoset counterparts.
This weaker molecular structure, in turn, provides thermoplastics with some interesting characteristics compared to thermosets.
One of the main characteristics is that they are more ductile than thermosets and therefore yield a higher impact strength as the impact energy is absorbed by plastic deformation of the matrix. Under impact thermosets would rather break.
The way to break those thermoplastic bonds beyond this simple ductility is to apply heat, and a lot of it.
While thermosets can flow at temperatures of 350°F/180°C, thermoplastics require temperatures approaching 700°F/370°C. This also means that the production equipment will be different depending on what type of material is being processed.
It is important to note that elevated temperature cures for thermosets only serve to speed up the chemical reaction, not break molecular bonding. Thermoplastics require this high heat to actually break molecular bonds.
When it comes to usage temperature, thermoplastic usage temperature is lower than that of thermosets because they are more susceptible to material creep at high temperature.
Usage temperature for thermoplastics must also be be gauged carefully as not to exceed their processing temperature. A safety margin must always be applied. It is best to discuss these temperatures and margins with your material manufacturer when including thermoplastics in your designs.
An important factor to take into consideration when thinking about matrix materials is matrix viscosity.
Viscosity is the measure of a fluid’s resistance to deformation at a given rate. It’s a fluid’s resistance to flow under shear stresses.
Viscosity of a fluid can be informally seen as a fluid’s thickness. For example, honey has a higher viscosity than water.
The common unit used to express absolute viscosity is “centipoise”. A centipoise is 1/100 of a Poise and 1/1,000 of a Poiseuille. It is named after the French physician Jean Louis Poiseuille based on his 1846 paper documenting his research into the flow resistance of liquids.
The centipoise is the preferred measure for industry because water happens to have a viscosity of 1.002 centipoise (cP), making it easier for mental comparisons between fluids. Here are some fluids’ cP at 68°F/20°C:
• Acetone 0.3
• Water 1.002
• Pancake syrup 2,500
• Ketchup 50,000
• Peanut butter 250,000
• Tar 3x 10^10
As we can see from these numbers, the lower the cP, the more easily the material will flow. But, it is important to know that temperature has a major influence on a fluid’s viscosity as well. When heated, fluids exhibit lower viscosities. This relationship between heat and viscosity is critical when computing molding parameters in composites manufacturing.
Paradoxically, it is interesting to note that the viscosity of gases rises with temperature.
The viscosity of thermoplastics at room temperature is very high, as they are solid (usually in the form of pellets), giving them a massive advantage compared to thermosets when it comes to shelf life. Thermoplastics can be softened with heat, melted, and reshaped (post-formed) as many times as desired without any major loss in their properties.
For composites manufacturers, this heat/pressure (flow) relationship is very useful when the properties of a resin do not yield the right viscosity outright. They can increase temperature and pressure parameters to lower the resin’s viscosity to fit the application or, in this case, the required resin flow. This also explains why so many thermoplastics manufacturers advertise the heat (processing temperature) and pressure capabilities of their equipment to demonstrate their process flexibility.
Meanwhile, thermosets have a low viscosity to start with as they are liquid. That viscosity can be lowered with heat but because of the internal chemical reactions in the material they reach their “gel point” and by end of cure they reach extremely high viscosity value because they turned into a solid mass that will not be recyclable (post-formed).
Their low melting points make them easy to thermoform, which is why they are often used in injection molding applications. Their translucent nature makes them good materials in optical products such as ski goggles as well.
This amorphous polymer class of materials includes thermosets and polystyrene. However, some plastics can be partially crystalline, meaning some of their structure is not completely random.
This partially crystalline structure gives them a higher melting point (the temperature at which the molecular forces are overcome) than their amorphous colleagues, but also one or more glass transitions (the temperature above which their molecular flexibility is increased).
They are also more opaque than their amorphous counterparts. This semi-crystalline category includes polymers such as: polyamides (nylons), polyesters, and some polyurethanes. They are often referred to as semi-crystalline plastics.
This distinction between amorphous and partially crystalline characteristics have a tremendous impact on their applications and manufacturability.