Polyphenylene Sulfide (PPS) - Complete Guides

Overview

Polyphenylene sulfide (PPS) is a semi crystalline, high temperature engineering thermoplastic. It is rigid and opaque polymer with a high melting point (280°C). It consists of para-phenylene units alternating with sulfide linkages. 

PPS offers an excellent balance of properties like:

And, it can be easily processed as well as its toughness increases at high temperatures.

These assets make Polyphenylene sulfide a chosen alternative to metals & thermosets for use in automotive parts, appliances, electronics and several others applications.

Polymerization of Polyphenylene Sulfide (PPS)

The first commercial process for PPS was developed by Edmonds and Hill (US patent 3 354 129, Yr. 1967) while working at Philips Petroleum under the brand name Ryton. 

Today, all commercial processes use improved versions of this method. PPS is produced by reaction of sodium sulphide and dichlorobenzene in a polar solvent such as N-methylpyrrolidone and at higher temperature [at about 250° C (480° F)].

In the original process developed by Philips, the product obtained has a low molecular weight and can be used in this form for coating applications. To produce molding grades, PPS is cured (chain extended or crosslinked) around the melting point of the polymer in the presence of a small amount of air. This curing process results in:

• Increase in molecular weight

• Increased toughness

• Loss of solubility

• Decrease in melt flow

• Decrease in crystallinity

• A darkening in color (a brownish color in contrast to this linear PPS grades are off-white)

Over the period of time, modification to the process have been reported to eliminate curing stage & develop products with improved mechanical strength.

Regular PPS is an off-white, linear polymeric material of modest molecular weight and mechanical strength. When heated above its glass transition temperature (Tg ~85°C), it crystallizes rapidly. Main three types of PPS include:

Linear PPS

• The MW of this polymer is nearly double as compared to regular PPS.

• The increased molecular chain length results in high tenacity, elongation and impact strength

Cured PPS

• Obtained from heating of regular PPS in the presence of air (O2)

• Curing results in molecular chain extension & formation of some molecular chain branches increases the MW and provides some thermoset-like characteristics

Branched PPS

• Has higher MW than regular PPS

• The backbone of the extended molecule has extended polymer chin branched from it

• Branched PPS has improved mechanical properties, tenacity and ductility

Key Properties of Polyphenylene Sulfide (PPS)

In the above section, we discussed about the general characteristics of Polyphenylene sulfide. It is also important to mention that there are several other properties of Polyphenylene sulfide that should be considered before employing it for a specific application. Let’s discuss all the properties of PPS in detail…

Crystal Structure and Physical Properties

PPS is a semi-crystalline polymer. 

• The unit cell is orthorombic (a=0.867 nm, b=0.561 nm, c=1.026 nm)

• The heat of fusion for an ideal PPS crystal was calculated as 112 J/g

• Depending from thermal history, molecular weight and cross-linked status (linear or not) the degree of crystallinity ranges from 0.30 to 0.45%

• Amorphous and crosslinked PPS can be prepared by:

• Heating the material above the melting temperature

• Cooling it to around 30°C below the melting temperature, and

• Holding it for several hours in the presence of air

Knowledge about the crystallization behavior of PPS is very important to understand the recommended processing parameters. The following table shows the phase transition temperatures of PPS. Ranges depend on mol. weight and curing status (linear or crosslinked). 

Dimensional Stability

PPS is an ideal material of choice to produce complex parts with very tight tolerances. The polymer exhibits an excellent dimensional stability even when used under high temperature and high humidity conditions.

Electrical Properties

PPS has excellent electrical insulation properties. Both the high-volume resistivity and insulation resistance are retained after exposure to high-humidity environments. It has a less pronounced O2 sensitivity and can be conveniently doped to get high conductivity.

Thermal Properties and Fire Resistance

PPS is a high-temperature specialty polymer. Most of the PPS compounds pass UL94 V-0 standard without adding flame retardant. PPS can be resistance to 260°C for short time and used below 200°C for a long time.

Mechanical Properties

PPS has high strength, high rigidity and low degradation characteristics even in high temperature conditions. It also shows excellent fatigue endurance and creep resistance.

Chemical Properties

PPS has good chemical resistance. If cured, it is unaffected by alcohols, ketones, chlorinated aliphatic compounds, esters, liquid ammonia etc. however, it tends to be affected by dilute HCl and nitric acids as well as conc. sulphuric acid. It is insensitive to moisture and has good weatherability.

PPS has however, a lower elongation to break, a higher cost and is rather brittle. Today, PPS is available in different forms and grades such as compounds, fibers, filaments, films and coatings. 

Modification of PPS Properties

There is a great number of PPS compounds in the market. Due to the chemical robustness of the polymer, a great variety of fillers and reinforcing fibers and combinations of these can be applied. 

PPS resin is generally reinforced with various reinforcing materials or blended with other thermoplastics in order to further improve its mechanical and thermal properties. PPS is more used when filled with glass fiber, carbon fiber, and PTFE.

Many grades are available including:

• Unfilled Natural

• 30% and 40% glass filled

• Glass mineral filled

• Conductive and Anti-Static Grades

• Internally lubricated bearing grades

However, on the market PPS-GF40 and PPS-GF MD 65 are established as standard compounds. These two have the overwhelming market share.

As you can see, the mechanical properties of reinforced grades differ significantly from the unfilled neat polymer. The typical property values for reinforced and filled grades fall in the range as shown in the table below. 

Typically neat polymer grades are used for fibers and films, whereas filled/reinforced grades are used for a great variety of applications in thermally and/or chemically demanding environment. 

Further PPS-based nanocomposites can also be prepared using carbon nanofillers (expanded graphite (EG) or ultrasonicated EG (S-EG), CNTs) or inorganic nanoparticles. Due to insolubility of PPS in common organic solvents, most PPS-nanocomposites have been prepared by melt-blending approach. One of the main reasons for adding nanofillers to PPS is to improve its mechanical properties to meet the increasingly high demand of certain applications.

Further, different additives are used to alter PPS properties.

• In order to lower the melt flow i.e. achieve high viscosity, additives such as alkali metal silicate, alkali metal sulfite, amino acids, oligomers of a silyl ether may be added. 

• During polymerization, if calcium chloride is added, the molecular weight will increase.

• The impact resistance can be improved with the inclusion of block copolymers in initial reaction

• Sulfonic acid esters along with a nucleating agent would improve the crystallization rate

• With the addition of an alkali metal or alkali earth metal dithionate in the mixture, they would increase the heat stability and lower the crystallization temperature

UNINKO modified PPS compounds series includes glass/mineral/carbon/stainless steel fiber reinforced grade, flame retardant grade, anti-static grade, ESD protection grade, electrically/thermally conductive grade, wear resistant grade, UV resistant grade, EMI shielding grade, etc broad range, with excellent performances, and widely used in the automotive and consumer electronics, mechanical engineering, home appliances and other industries, etc.

(Source:omnexus.specialchem.com)