What is Polyimide? Why it’s being commonly used in the form of tapes, sheets, and PCB. Kapton Polyimide (PI) are building commercial polyimide tapes with cheap insulation characteristics. In the present article, we are going into detail of this commercially low-cost polymer and see why it’s being searched so much…
If you are into Polymers, you can study the latest Article in Polymer Section.
What is Polyimide?
Polyimides are polymeric materials formed by the condensation reaction of dianhydride and diamine monomers. These polymers contain two carbonyl (C=O) groups, covalently bonded with Nitrogen (N) atoms. The CO-NR2 functional group is called imine. With their inert chemical behaviour, high strength and stiffness, and, high range of operating temperatures, polyimides have a wide variety of applications for example,
- LCD devices
- Gas Separation membrane
- High-temperature polymer fuel cells
- Aerospace Industry
- Defense applications
This pretty much explains the reason we are asking a question, “What is Polyimide?”.
For more demanding engineering and harsh applications, where a high range of temperature along with strength and stiffness are required, the preferred choice is Polyamides. Thus, new grades of polyimides are developed to replace conventional materials.
Classification of Kapton Polyimides
Kapton Polyimides are classified on two bases:
- According to the composition of the main chain
- According to the type of interactions between the main chain
Composition of Main Chain
According to the composition of the main chain, there are three types of this polymer.
- Linear or Aliphatic Polyimides
These polyamides are derived as a result of the reaction of aliphatic monomers.
- Aromatic Heterocyclic Polyimides
They are formed by the reaction of aromatic monomers.
- Semi-Aromatic Polyimides
When one of the monomers is aliphatic and one is aromatic, the reaction yields a semi-aromatic Kapton.
Interaction between the Main Chain
Based on the interaction between the main chains, Kapton is classified as thermoplastics and thermosets. The thermosetting Kapton Polymer is responsible for the thermal stability, good chemical inertness, and excellent strength and stiffness of polyimides.
PI can be used for longer periods at 200oC whereas, at 480oC, they can be used for short-term applications.
PI is synthesized by a two-step process.
In the first step, pyromellitic dianhydride (PMDA) monomer reacts with a p-phenylenediamine (PDA) dianhydride monomer to form a poly (amic acid).
In the second step, this poly (amic acid) is dehydrated to form PI. This reaction is carried out in a Physical Vapor Deposition setup at a vacuum of 2×10-3 Pa. This formation of Kapton polymer follows the Second-order kinetics of polymerization reactions. However, many researchers have reported autocatalytic kinetics as well.
Autocatalysis occurs when one of the products of the first step reaction is a catalyst for the next reaction.
The poly (amic acid) is then exposed to high temperatures of up to 250oC to obtain Polyimide films.
The polymerization of polyimides (PI) is extensively limited by their infusible and insoluble nature: this is why Kapton Polymers were not mass produces until the 1960s.
To overcome this drawback, many researchers have experimented with structural modifications, for example, the addition of flexible linkages in the backbone that do not sacrifice the thermal stability of this thermosetting polymer.
DuPont was the first company to commercially produce a polyimide film known as Kapton in the late 1960s. Kapton was based on pyromellitic dianhydride and 4,4’diaminodiphenyl ether. It was produced by the two-step polymerization reaction.
Moreover, this thermosetting polymer can be functionalized by the incorporation of side chains. For example, Azobenzene-functionalized polyimides are used as actuators and piezoelectric.
Strength of Polyimides
A polymer that has a charge transfer complex (CTC) contains a donor and an acceptor. The donor has plenty of electrons that it can share with the acceptor.
In the respective thermosetting polymer, the Nitrogen atoms share their electrons with the carbonyl group. These units are tightly held by this CTC property of PI. The electrons are also shared between the chains of polymers, which hold them tightly so that they cannot move at the molecular level which renders their motion in the whole material. This is why PI offer high strength and stiffness.
It is also the reason why this polymer is thermally stable and has chemical resistance and low solubility.
One of the drawbacks of this property is the lack of processability of these polymers. Processability can be improved by the use of plasticizers. This is overcome by the introduction of more flexible monomers during polymerization. Kapton is synthesized from bisphenol-A and ether linkage to induce softness.
Thermal Gravimetric Analysis (TGA) is the most accurate method for the estimation of the thermal stability of any polymer. The general TGA analysis of PI shows no weight loss until 450OC and for Kapton, the weight loss starts around 500oC.
PI offers a wide range of useful properties including high mechanical toughness, excellent chemical resistance, and high thermal stability.
They are used as dielectric materials because they exhibit excellent dielectric properties and have a low coefficient of thermal expansion. As discussed earlier, different properties can be induced in this polymer by the incorporation of side chains into the backbone of the polymer, due to this property they have found applications in the medical industry as polyimide tubes.
Polyimide tubes provide flexibility, high tensile strength, biocompatibility, low friction, transparency, tight tolerances, thin walls, smooth surface, and column strength.
|Polyimide Mechanical Properties|
|Elastic Modulus (GPa)||1.3-4|
|Yield Strength (MPa)||15-230|
|Elongation at Fracture||200% (Room temperature)|
|Strength at fracture (MPa)||72-120|
|Flexural Modulus (GPa)||2.48-4.10|
|Hardness, Rockwell R||123-129|
The dielectric properties of PI can also be tailored by incorporating CF3 in the dianhydride monomer of PI. This will result in low dielectric constant, and optically transparent PI.
|Polyimide Electrical Properties|
|Electrical Conductivity||1.5x1017 (ohm-cm)|
|Dielectric Strength||22 - 27.6 kV/mm|
|Electrical Resistivity||1x10-16 (ohm-cm)|
Strengths and drawbacks of Kapton Polyimide
- High mechanical strength and toughness
- Superior thermal stability
- High flexural modulus
- Radiation and chemical resistant
- Wear resistance
- Easily tailored properties for required applications
- Flame and combustion resistant
Besides, there are also some drawbacks associated with PI.
- Expensive processing and manufacturing costs
- High-temperature requirement during the processing
- Post-treatment like annealing at specific temperatures is required
- Sensitive to acid and alkali attack
What is Polyimide and Its use in Composites
PI can be used as a matrix for the fabrication of composite materials to further enhance its properties and applications. PI/Carbon Nanotube composite is fabricated for applications in energy storage devices. However, the service temperature of composites depends on its matrix thus, these composites cannot be used at very high temperatures i.e. >500OC.
PI has numerous applications ranging from the aerospace industry, automotive industry, medical industry to adhesives.
Kapton is a commonly used polyimide film which is used as an insulator in electronic circuits. It can remain stable from -269OC to 400OC.
Other applications in electronics include cables and insulating films. Nowadays, PI films are popular as a source of production of renewable energy. They are used as a substrate material in amorphous-Silicon and CIGS solar cells because of their high thermal stability and mechanical properties. The research in solar cells is concentrated on increasing the efficiency and flexibility of solar cells, PI films provide high efficiency, flexibility as well as stability at higher temperatures.
One of the widely used applications of PI in electronics is that of Printed Circuit Boards. Polyimide PCBs are widely used because they are flexible, lightweight, heat, and chemical resistant.
Owing to their wear resistance they are also used to make bearings for structural applications.
Recently PI foams have received much attention because of their excellent properties. PI foams were originally designed for NASA as an insulator for rocket fuel and as an acoustic damper.
For US Readers, you can read more on foams by looking into the book mentioned below;
PI foams offer:
- Low outgassing (a problem in spacecraft and vacuums)
- Thermal stability
- Acoustic performance
- Fire resistance
Other uses of PI foam include ducting, ducts and pipe insulation, and strengthening of hollow components while staying lightweight at the same time.
The PI tape is an adhesive material used for insulation of electrical circuits, for example, coil and capacitor insulation. It can withstand harsh conditions and temperatures. In August 2018, the astronauts on the international space station (ISS) faced a small leak in the Soyuz spacecraft; they used Polyimide Tape to repair the leak temporarily.
Polyimide vs. Polyamide
Kapton polymer is largely compared and confused with polyamide. Although the difference between polyimide and Polyamide is huge. You can see the difference in clear Polyimide VS Polyamide. Polyamide aromatic group is called aramid fiber which is used in a large area of applications. One of commercially used aramid is KEVLAR. The KEVLAR is a common application of polyamide which is being used as Bulletproof vests.
|Polyimides are the polymers of imide linkages||Polyamides are the polymers of amide or peptide linkages|
|Dianhydride and diamine monomers||diamines and dicarboxylic acid|
|Kapton||Kevlar and Nylon|
|Elastic Modulus (GPa)|
|Flexural modulus (GPa)|
|Insulation, Lamination, Coatings, structural adhesives||Radiator header tanks in cooling systems, ignition parts, engine covers|
Commercially Available Grades of Polyimide
Grade: DuPont™ Kapton® HN
This is a general-purpose PI grade. It is used in applications which require excellent properties over a wide range of temperature.
Grade: DuPont™ Kapton® FPC
This grade of PI has excellent adhesion and dimensional stability because it is treated on both sides. It is used in applications which demand low shrinkage and excellent adhesion. This grade is also used for printed circuit boards.
Grade: VESPEL® PI SP1
This grade is manufactured by Angst+Pfister. It is a brown colored, high strength PI grade.
Grade: DuPont™ Kapton® MT+
This grade offers 3 times the thermal conductivity of standard HN grade PI.
Grade: TECASINT 2011 natural
This grade is manufactured by Ensinger Plastics. It is one of the purest natural grades of polyimide. It offers high elastic modulus, rigidness, and hardness.
Grade: DuPont™ Kapton® 150PRN411
This grade is used for the insulation of copper wires.
Grade: DuPont™ Kapton® PV9100 Series
This graded series is very flexible thus they find their applications as a substrate for thin-film solar cells.
Grade: DuPont™ Pyralux® APR
This grade of PI offers excellent thermal resistance. It is used in the aerospace industry and military.