Polymer performance - in a tough world

TWI Bulletin, May - June 2004

Nature has been designing tough, hard and abrasion resistant polymeric materials for millions of years in the form of ivory, shell, horn and skin. The science of equally durable man-made polymers has just begun.

Since graduating for his bachelor's, master's, and doctorate, Sam Rostami has gained 20 years in the field of polymer science and technologies. He is experienced in research, new product development and characterisation of multi-component materials. He is co-editor and author of a book on 'Multi-component polymeric systems', author of five chapters in technical books, over 60 published technical papers and eight patents.

Sam Rostami examines some of the similarities between natural and man made materials particularly high performance polymers that are designed for applications under extreme conditions. Like their natural counterparts they often contain chemical functional groups such as those listed in Table 1.

Table 1 Functional groups commonly present in polymer chains.

High performance polymers - a long look back

The most active era for the development of high performance polymers was between the 1960s and the 1980s. In these three decades, high performance and melt processable thermoplastic polymers such as semi-crystalline poly(ether ether ketone), poly(ether ketone), poly(phenylenesulphide) and amorphous polymers such as poly(arylsulphones) and poly(ether imide) were developed.

Additionally, the high performance family of polyimide thermosetting polymers such as Kapton ^TM and Upilex ^TM films, and Vespal ^TM moulding compound, with unprecedented tensile strength and modulus, and low coefficient of thermal expansion were introduced.

High modulus polyamide-imide fibre, Kevlar ^TM , and rigid rod polymers, such as Poly(benzoxazoles), poly(benzimidazoles) and poly(benzthiazoles), were also developed in these decades. semi-mouldable polyamide-imide polymers such as Torlon ^TM and PMR-15 (Polymerisation of Monomeric Reactants to yield oligomer with molecular weight of 1500 g/mol), also emerged.

High performance polymers - today and tomorrow

The high performance terminology refers to unusual stability upon exposure to harsh environments and to performances that surpass the usual bulk polymers. The primary contributing factor to high performance is the chemical bond strength among the constituent molecules in the chain, followed by secondary factors such as hydrogen bonding, polar interactions, van der Waals forces, etc. Dissociation energies for typical primary bonds are shown in Table 2.

Table 2 Dissociation energies for some common primary bonds

The strength of polymers is often enhanced by reinforcing them with mineral additives, such as particulate, short or continuous carbon or glass fibres.

The high performance, semi-crystalline thermoplastic polymers such as poly(ether ether ketone) and poly(phenylene sulphide) find high and low temperature engineering applications in aerospace, transport, medical, nuclear and oil recovery industries. These polymers are shaped at temperatures greater than 350°C, and are usually stable up to 500°C for short periods of time.

The high performance thermoset polymers such as polyimides, polyamide-imides and polybismaleimides are also suited for aerospace, electronics and other high performance applications. These thermosetting polymers undergo thermal decomposition at very high temperatures without significant melting.

Harsh environments

Harsh environments may consist of one or any combination of the following elements:

• Very high or low temperatures.
• Aggressive chemicals.
• Radiation.
• Stress.
• Humidity.
• Electrical fields.
• Outer space.

Properties of high performance polymers

The following properties are collated from product literature and the World Wide Web.

Chemical resistance

Only semi-crystalline thermoplastic polymers possessing more than 20% to 30% crystallinity or highly cross-linked thermoset polymers can offer substantial resistance to chemical attack. For example, Table 3 shows chemical resistance ratings of PEEK to some chemicals at 200°C.

Table 3 Chemical resistance rating of PEEK at 200°C.

The majority of other semi-crystalline and/or cross-linked high performance polymers are highly resistant to ordinary chemicals. They are, however, less resistant to hot strong acids. For example, concentrated sulphuric acid dissolves Kapton and PEEK.

The resistance to organic chemicals decreases as the temperature and/or applied stress increases. Chemicals readily attack uncross-linked amorphous polymers.

High temperature resistance

Technically, the glass transition and crystalline melting temperatures, Tg and Tm respectively, influence the onset of thermal decomposition of polymers. In practice, the heat distortion temperature (HDT), continuous service temperature (CST) and ductile-brittle transition temperature are used an indicative of the thermal resistance of polymers.

In most practical applications, polymers are used in the temperature range of -50°C to +250°C. However, they can be used over a much wider temperature range, -269°C to 500°C, for short periods of time.

Some data for typical high performance thermoplastic and thermosetting polymers are shown in Table 4. Beyond 500°C, the majority of the primary and secondary bonds in high performance polymers dissociate, causing thermal degradation and decomposition.

Table 4. Selected thermal properties of some un-reinforced high performance polymers

Humidity and water vapour resistance

Polyolefins, fluorinated and silicon polymers are hydrophobic, whereas other polymers absorb moisture to various degrees. The water molecule can either interact with the functional groups in the polymer chains, to form H-bond, and/or purely diffuse into its free volume. Due to the affinity of water molecules to amide and amine functional groups, polymers containing these groups are generally more susceptible to moisture attack. The susceptibility to moisture attack can be assessed by a simple water uptake measurement, as shown in Table 5.

Though the water uptake can be used as an indicator of the moisture sensitivity of polymers, the permeability is probably a more reliable variable to use. Permeability, P, is a product of the diffusivity (or diffusion coefficient), D, and the solubility, S. The diffusion coefficient is considered as the velocity of the penetrant into polymer and the solubility corresponds to the amount of penetrant sorbed by the polymer. D is concentration dependent whereas S is concentration independent. Both parameters are temperature and pressure dependent.

Table 5 Water uptake of some un-reinforced, high performance polymers after 24 hours immersion

A constant permeability rate is achieved after the penetrants have fully diffused and sorbed in the polymer. It is a selective process that varies from one polymer to another and from one penetrant to another.

The permeability data for high performance polymers are scarce. Table 6 contains water, carbon dioxide and oxygen permeability values for some polymers.

Table 6 Moisture and gas permeability of some un-reinforced, high performance polymers

Addition of mineral fillers, such glass beads or fibres, may reduce moisture and gas permeability of polymers, provided the interface between the polymer chains and the fillers is engineered appropriately.

Resistance to electromagnetic radiation

Most polymers can withstand low-level exposure to radiation. However, those polymers that contain C = C double bonds and/or amine groups in their molecular structures are potentially susceptible to UV degradation.

The dielectric constant and loss tangent of some high performance polymers at 1 MHz are given in Table 7.

Table 7 Dielectric and loss tangent of some un-reinforced, high performance polymers at 1 MHz

Polymers with low dielectric constant, <3, and very low loss tangent, <0.004, will be transparent to high frequency electromagnetic radiation, i.e. in the GHz frequency range. Long exposure to X- and

Fire resistance

There are several ways to assess the fire resistance of polymers, including:

• Underwriters Laboratories 94, UL94
• Limited Oxygen Index (LOI)
• Smoke density and toxicity of the combustion gases
• Cone calorimetry
• Smoke tunnels

The UL94 is small-scale test designed solely to test the burn rate of polymeric materials used as a part of an end product. LOI, indicates the concentration of oxygen required to sustain burning of a plastic for three minutes. Polymers with high LOI values are less likely to burn. As an example, the LOI values and the UL94 rating for some high performance polymers are compared in Table 8.

Table 8 UL94 rating and LOI values for some un-reinforced high performance polymers

Polymers will eventually burn on prolonged heating at very high temperatures, greater than 600°C. In this situation, the critical factors are the type of gases, their concentrations and the smoke density that are produced.