Thursday, 28 May 2020

The Secret of High Performance Polymers: Why They Can Handle High Heat and Harsh Chemicals?


In this blog post, we explore the reasons why high performance polymers can handle high heat and harsh environmental conditions over a long period of time. 

Definition of high performance polymers 
There are several definitions for high performance polymers. One good way to define high performance polymers is over the Underwriters Laboratory (UL) Relative Thermal Index (RTI). According to the UL 746B, high heat polymers need to withstand a continuous use temperature of 150°C for 100,000 hours (approx. 11 years), while retaining at least half of the initial properties afterwards. 

Polymers such as PPS and PEEK inherently fulfill this requirement. Conversely, PPA’s need to be mechanically reinforced and thermal stabilized so that their continuous use temperature can rise from 130°C to 150°C. Most PPA’s have a continuous use temperature of 130°C. 

Fundamental structure- property relationships 
For better understanding the high heat resistance, we go back to the basic structure of a polymer such as a Polyethylene (PE). The main backbone consists out of carbon-carbon bonds and on the carbons, hydrogens are also bonded. This linear macromolecule has a maximum temperature resistance of ca. 80°C and continuous use temperature of 50°C.

If we replace now the carbon in the main chain with a phenyl group, which is an aromatic cyclic group of atoms with the formula C6H5 we obtain the Polyparaphenylene (PPP).

PPP has a temperature resistance of 500°C and is a linear macromolecule made out of benzene building blocks. Aromatic structures result in high macromolecule stiffness. Aromatics in the backbone are a main driver to obtain high heat and chemical resistance. The detailed look at the structure of benzene reveals that the double bonds are not statically localized, i.e. electrons move along the carbon cyclic structure, which is expressed by the ring in the structural formula. This together with inherent molecular stiffness supports stability at high temperatures and in contact with chemicals. 

7 basic building blocks of high performance polymers
The high thermal resistance of PPP has one major downside, i.e. it makes it unsuitable for all melt-based processing techniques such as injection moulding and extrusion.
However, the integration of heteroatoms such as Oxygen, Nitrogen, and Sulfur in a polymeric aromatic-based structure can change this. Following, common chemical groups which are used to make melt-processable high performance polymers are described.

1. Diphenyl ether group: In this case, oxygen is the linkage of two phenyls. Diphenyl ether groups are used for example in Polyaryletherketones (PAEK’s).

2. Diphenylsulfone group: Here, sulfur is double-bonded to oxygen as well as bonded to phenyls. Diphenylsulfone groups are the main building block for Polysulfone (PSU), Polyethersulfone (PESU) and Polyphenylsulfone (PPSU).

3. Diphenylketone group: Oxygen is bonded over a double bond to carbon resulting in a carbonyl group. Together with the diphenyl, it forms the ketone group. The ketone group is the second crucial element for obtaining Polyetheretherketones (PEEKs).

4. Diphenylsulfide group: Here, sulfur is linked to phenyls and forming the sulfide group. It forms the basis of Polyphenylensulfide (PPS).

5. Imide group: It consists out of two acyl groups (R-C=O) bounded to nitrogen. It is the base element of Polyimides (PIs), Polyamideimides (PAIs), and Polyetherimides (PEIs).

6. Terephthalic acid (TPA) and Isophthalic acid (IPA): It is used as precursor for making Polyethylene terephthalate (PET). It also forms the monomer for Polyphthalamides (PPAs). Two carboxyl groups are attached to a benzene in a 1,4 or 1,3 configuration.

7. Fluor-carbon group: the fluor-carbon bond is the most stable single bond with 485 kJ/mol bonding energy (in comparison, carbon-carbon bond has 350 kJ/mol). Additionally, the fluor atom is much larger compared to the carbon forming a protecting layer around the carbon-carbon main chain. This explains to the same extent the high chemical and thermal stability of fluoropolymers such as PTFE and PVDF.

Chemical resistance of high performance polymers 
As a rule of thumb, the chemical resistance of polymers decreases with increasing temperature, i.e. increasing molecular mobility. In this case, high performance polymers have inherent advantages compared to commodity or engineering polymers. I made a table comparing all major high performance polymers with chemical resistance.

Conclusions
A key element to achieving high temperature and chemical resistance is the inclusion of aromatic structures. The combination of the latter with various heteroatoms such as carbon, oxygen and sulfur impart flexibility to the resulting polymer macromolecule, which enables melt processing. The use of melt processing techniques enable high performance polymers to be economically used in several high-end applications such as airplanes, automotive, oil and gas, and chemical processing industries.

If you want to use high performance polymers for your application and you need support to choose the optimal grade, I am glad to help. Reach to out to me here.

A further blog post on high performance polymers on my blog you can read here.

Thank you for reading! 
Till next time! 
Herwig Juster

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Literature:
Johannes Fink - High Performance Polymers, Plastics Design Library

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