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Low fuel swelling PPS DURAFIDE(R) 6165A7S

DURAFIDE® PPS 6165A7S retains the inherent mechanical properties of conventional materials, while markedly suppressing swelling due to fuel, it is used in automobile parts than come into contact with fuel and oil, appliance parts thancome into contact with coolant and oil, and parts in hot water supply systems, pumps and water supply applications.

1. Introduction
PPS (polyphenylene sulfide) possesses a structure of alternating benzene rings and sulfur atoms, and it exhibits extremely good heat resistance, flame retardance, mechanical properties, dimensional stability, and chemical-resistance. Its melting point is 280–290 °C, and its continuous usage temperature is 200–240 °C.
There are no organic solvents that dissolve PPS at less than 200 °C and a major feature of the resin is thus its chemical resistance that is on a par with fluororesins.
As is widely known, there are cross-linked, semi-cross-linked, and straight chain (linear) types of PPS, and while linear PPS (DURAFIDE is our company's trademark) boasts superior toughness and weld strength compared with cross-linked PPS, linear PPS formerly had the drawback of extensive flash formation but this disadvantage has been ameliorated via compounding technology.
Linear PPS features vastly improved toughness and on this account, utilization has widened to include applications such as film and fiber where it is impossible to employ cross-linked PPS.
Among these, PPS fiber can be processed into multifilament, monofilament, and nonwovens and applied as various filters and conveyor belts.
In addition, PPS exhibits excellent stability and compatibility towards organic and inorganic fillers and for this reason, it is possible to add various fillers in high loadings. On this account, in addition to unfilled materials, active grade development is underway with the objective of conferring new functionalities through addition of fillers.

2. Background of low fuel swelling grade development
Because PPS possesses excellent chemical resistance and hydrolysis resistance, it is used in automobile parts that come into contact with fuel and oil, appliance parts that come into contact with coolant and oil, and parts in hot water supply systems, pumps and water supply applications. Highly filled grades such as DURAFIDE 6165A6 are applied particularly in automobile parts that come into contact with fuel and oil but demands are intensifying with regards to fuel swelling resistance properties. In order to respond to these needs, DURAFIDE 6165A7S has been developed and marketed as a low fuel swelling PPS with markedly improved fuel resistance and here we introduce it. DURAFIDE 6165A7S retains the inherent mechanical properties of conventional materials, while markedly suppressing swelling due to fuel. Adoption is proceeding in the market, and further expansion in sales is anticipated in the future.

3. Fuel swelling mechanism
3-1. Why does the problem of swelling occur?
PPS is a resin that features excellent chemical resistance. There are no organic solvents that dissolve PPS at temperatures lower than 200 °C, and it exhibits chemical resistance on a par with fluororesins. In addition, it is a resin with one of the lowest fuel permeabilities (Figure 1) and on this account, it is widely used in parts that contact fuels such as fuel pump parts. However, accompanying advancing part performance, minute dimensional variations (swelling) that occur when trace amounts of fuel permeate the resin are becoming an issue.

Figure 1 : Comparison of fuel permeability of various resins

(CE-10, 60°C, sample thickness : 1 mm)
* Evaluation method : Differential pressure met (solution contacts with sample)
* Test environment : Bone-dry state (no presence of moisture)

3-2. Where does fuel penetrate PPS?
PPS is a type of crystalline resin and it possesses a crystalline phase and an amorphous phase, and studies carried out at our company to date have proved that the fuel component ingresses into the amorphous phase alone, and there is no permeation into the crystalline phase (Figure 2). Figure 3 shows a wide-angle X-ray chart prior to and after immersion of a sample in fuel and it can be seen that the peak position does not change at all. A significant quantity of energy is required in order for the fuel component to enter the crystalline phase and break down the crystal structure and it is thought that the fuel selectively permeates the amorphous phase.

Figure 2 : Schematic diagram of polymer structure and fuel permeation

Figure 3 : Wide-angle X-ray chart of
DURAFIDE® 6165A6 sample

(M15 regular gasoline, 60°C)

3-3. Crystallinity and fuel swelling properties
As mentioned previously, fuel permeates the amorphous phase of the PPS resin, and it does not penetrate the crystalline phase, Accordingly, the higher the crystallinity and the lower the proportion of amorphous phase, the more advantageous the material can be said to be with regards to fuel swelling resistance. Figure 4 shows the fuel swelling properties (mass variation) of an amorphous PPS molded at a low mold temperature and a crystalline PPS thoroughly crystallized at a high temperature mold (150 °C). It can be seen that the swelling rate of amorphous PPS is markedly faster compared with crystalline PPS. PPS is broadly classified as either linear or cross-linked depending on its molecular structure and DURAFIDE PPS is adopted as a linear polymer with excellent toughness. Normally, linear polymers tend to have a
Figure 4 : Comparison of crystalline and amorphous polymer fuel swelling behavior
(Un led PPS, sample thickness = 0.8 mm,
fuel C, 80 °C)
* Amorphous : Processing at mold temperature of 50 °C,
* Crystalline : Processing at mold temperature of 150 °C
* Fuel C : Toluene:iso-octane = 1 : 1
higher degree of crystallinity compared with cross-linked polymers. Accordingly, linear polymers with high degrees of crystallinity and a low proportion of amorphous phase are thought to be better suited as low fuel swelling PPS materials.

3-4. Molecular mobility of amorphous phase and fuel swelling properties
We have mentioned that the fuel component permeates the amorphous phase of PPS resin and in this instance, the fuel is influenced by the molecular movement of the polymer and permeates and diffuses from the resin surface layer into the resin interior. The polymer's molecular mobility can be suppressed through modifications to the polymer backbone, additive components and other compounding technologies. Control of the molecular mobility of the amorphous phase is one important approach in improving the fuel swelling resistance properties.

4. Properties of low fuel swelling material
As mentioned above, a linear polymer with high crystallinity and excellent low fuel swelling properties has been adopted together with improved compounding technology to restrict the molecular mobility of the amorphous phase and as a result, we have succeeded in developing DURAFIDE 6165A7S featuring a markedly improved fuel swelling rate compared with traditional materials. As indicated in Figure 5, the swelling rate of DURAFIDE 6165A7S has been reduced by half compared with the conventional material DURAFIDE 6165A61. DURAFIDE 6165A7S exhibits comparable mechanical properties to DURAFIDE 6165A61, while melt viscosity that is an indicator of flowability is lower compared with DURAFIDE 6165A61. Because flowability is improved, the material is ideal for thin-wall shapes and complex shapes (Table 1)
Figure 5 : Fuel swelling properties
(Fuel C 80°C, 0.8-mm thickness sample, pretreatment at 180°C × 2 hr)
* Fuel C : Toluene:iso-octane = 1 : 1

Table 1 : Mechanical properties of DURAFIDE®6165A7S
Method (ISO)
 Melt viscosity (310°C,1000/sec)
Pa · s
 Tensile strength
 Strain at break
 Flexural strength
 Flexural Modulus
 Sharpy Impact Strength
 Temperature of
deflection under load (1.8MPa)

As shown above, not only has the fuel swelling rate of DURAFIDE 6165A7S been drastically reduced: the material also boasts excellent flowability and mechanical properties. Improved performance of existing parts and improved long-term reliability is anticipated through use of DURAFIDE 6165A7S. Furthermore, potential will be expanded in applications that contact with fuel and organic solvents where until now substitution of metal part by resins has been difficult and total cost reduction and lightweighting can be anticipated. Polyplastics intends to propose the use of DURAFIDE 6165A7S not only in parts used in the vicinity of fuel, but also in various other applications.

5. Conclusions
The superior properties inherent in PPS have been recognized by the market, and its scope of application has been steadily broadening. Sales volumes for PPS have been steadily recovering since the Lehman Shock, and stable growth of around 5–10% per annum can be anticipated in the future. In particular, considerable demand is expected in the automobile sector for auto-electrical parts accompanying the development of hybrid vehicles and the key point in adoption in such critical parts is thought to be ensuring long-term reliability. DURAFIDE 6165A7S introduced in this paper is a resin possessing extremely good chemical resistance and its application can be anticipated in parts that contact water, hot water, acids, alkalis and other chemicals in addition to parts that contact fuels. We plan to propose the use of DURAFIDE 6165A7S in chemical-resistant/fuel-resistant applications, and we also anticipate that we will be able to apply the technology cultivated in development of this material for the future development of new materials. We fully intend to actively promote material development in order that we can continue to respond to market requirements.

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