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Polyphenylene Sulfide (PPS): A Comprehensive Guide on High Heat Plastic

Polyphenylene sulfide (PPS) is a high-performance, engineering thermoplastic characterized by an unusual combination of properties. These properties range from high temperature performance to dimensional stability and excellent electrical insulation properties. But, what makes PPS a high performance plastic in several sectors? What it is made up of? Get detailed technical information on Polyphenylene sulfide.

Overview

What is Polyphenylene Sulfide (PPS)?

What is Polyphenylene Sulfide (PPS)?

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:

Key Properties of Polyphenylene Sulfide (PPS) Polymer

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.


What is PPS Made From?

What is PPS Made From?

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)].

Synthesis of PPS


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)

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

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 UL94V-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. 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.

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.


How to Optimize PPS Properties?

How to Optimize 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:


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.

Property (Unit) Test Method Unfilled Glass Reinforced Glass-Mineral Filled*
Filler Content (%)   - 40 65
Density (kg/l) ISO 1183 1.35 1.66 1.90 - 2.05
Tensile Strength (Mpa) ISO 527 65-85 190 110-130
Elongation at Break (%) ISO 527 6-8 1.9 1.0-1.3
Flexural Modulus (MPa) ISO 178 3800 14000 16000-19000
Flexural Strength (MPa) ISO 178 100-130 290 180-220
Izod notched Impact Strength (KJ/m2) ISO 180/1A   11 5-6
HDT/A (1.8 Mpa) (°C) ISO 75 110 270 270
Typical Mechanical Properties of PPS and PPS Compounds
Data from Product brochures: DURAFIDE®, Polyplastics; Ryton®, Solvay
* depending on filler ratio Glass / Mineral

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. 


  • High Temperature Plastics: How to Well Control Melt Temperature

  • 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


Processing Conditions for PPS

Processing Conditions for PPS

PPS resins (base polymer, glass-reinforced, and mineral/glass-reinforced systems) are typically used for injection molding, extrusion blow molding, and extrusion applications.

Polyphenylene Sulfide can be processed at temperatures in the range of 300 to 350°C. The processing of PPS can be hampered at times due to its high melting point.

If the PPS used is of the filled grade, the upper processing temperature should be used to avoid any kind of wear and tear of the barrel, screw and screw tip.


Pre-drying


Pre-drying at 150-160°C for 2-3 hrs or 120°C for 5 hours is recommended to enhance molded products appearance and prevent drooling.

For carbon fiber filled grades, pre-drying is particularly important, because moisture absorption is high due to the carbon fibers.


Injection Molding


  • Because of its low viscosity, mold tightness has to be checked
  • Cylinder temperature: 300-320°C
  • Mold temperature: 120-160°C, to obtain good crystallization and minimize warping
  • Injection pressure: 40-70Mpa
  • Screw Speed : 40-100 rpm

» View All PPS Grades Suitable for Injection Molding

But in order to have higher productivity, a mold temperature of 50°C followed by post crystallization at 200°C is possible, but it is not recommended for applications requiring high dimensional stability.


Extrusion


Extrusion PPS grades are commonly available for fiber and monofilament production as well as tubing, rod and slab.

  • Drying conditions: 121° for 3 hrs
  • Mold Temperature: 300-310°C
  • Melt Temperature: 290-325°C


Key Applications

Key Properties

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