As a leading company in POM, Polyplastics has released grades with a variety of attributes till now. We have produced a lineup of several grades which further enhance the sliding properties that are the strong point of POM. Recently the electrification of cars has brought about greater needs for noise reduction, and there are also more cases in which sliding grades are needed. At the same time, fuel economy is still as important of a keyword as it has ever been, and all types of parts now need to be more lightweight and space-efficient. In these circumstances, sliding grades need not only sliding properties such as noise reduction, but also durability in high-load and high-weight conditions which employ thin-walling to achieve lighter weight and better space efficiency.

Sliding grades that Polyplastics has produced in the past have mainly used polymers that have good flowability (Table 1), taking moldability into consideration. Since these polymers have low molecular weight they offer good flowability, but the flipside to that is declining long-term characteristics such as creep and fatigue properties. Since soft compounds are also used in various sliding improvement agents that are added to facilitate sliding, mechanical properties (tensile strength, elastic modulus, etc.) decline as the amounts of these additives increase. For that reason, typical sliding grades tend to have lower short-term and long-term mechanical properties such as initial strength, elastic modulus, and creep properties, in order to deliver sufficient sliding properties. However, since there is now increasing demand for durability as mentioned above, we are introducing DURACON^® POM WW-09 which aims to balance mechanical properties with sliding properties.

DURACON WW-09 delivers better mechanical properties by using high molecular weight polymers and employing our own original molecular design and polymerization technologies to refine spherulites^※, in order to achieve high initial strength and creep properties. Illustration 2 shows the results of spherulite observations for a standard grade (M25-44) and high-strength grade (WW-09) of DURACON, taken through a polarizing microscope. We can see that the spherulites are more refined for the high-strength grade than they are for the standard grade. Since the addition of sliding improvement agents typically results in declining mechanical properties, we optimized the types, combinations, volumes, and other attributes of sliding improvement agents to successfully impart sliding properties while also maintaining mechanical properties on par with standard grades.

Table 1 : Comparison of values for the properties of major DURACON^® POM sliding grades (Full size)

※Spherulites are aggregations of crystals which have grown into spheres which can be seen in crystalline polymers. The boundaries of the spherulites are mechanically weak, so stress gets concentrated on these boundaries during warping and causes fractures to occur. Refining the spherulites and dispersing the boundaries can discourage the concentration of stress, resulting in better strength and elastic modulus.

DURACON WW-09 is a grade with (1) high strength which has (2) good creep properties and (3) good sliding ability (Illustration 2). It also has moldability on par with high-viscosity type standard grades. Here we will explain the points which represent the strengths of
WW-09.

3-1. Mechanical properties

As you can see in Table 1, WW-09 has highly-rated mechanical properties compared to existing sliding grades, and it maintains physical properties close to those of high-viscosity standard grade M25-44. As we have already mentioned, strength tends to decline in existing sliding grades when materials are changed to impart sliding properties, and there have been times when this has had to be compensated for in product design, load reduction, and other cases. However, there is little concern about this with WW-09 because its properties are close to those of standard grades.

3-2. Creep properties

Since WW-09 uses high-viscosity type polymers, it shows better creep properties than other sliding grades (Illustration 3). Just as it possesses the short-term properties mentioned above, it also has long-term durability, so it can be used in product designs close to those of standard grades in applications where loads are continuously applied over the long term.

3-3. Sliding properties

WW-09 shows the same good sliding properties as other sliding grades, but one of the characteristics of this grade is that it takes into consideration noise reduction during sliding.

Illustration 4 shows the surface pressure at which noise occurs during sliding for different grades against the same material. The polymer in POM has good sliding properties by itself, but it has the negative characteristic of being prone to noise occurrence when sliding against other POM materials. This characteristic also shows up in the data, as non-sliding grade M25-44 produced noise even at the low surface pressure of 0.1 MPa. On the other hand, sliding grades such as AW-09 produce no noise even at surface pressures up to 3 MPa, but noise then occurs at surface pressures higher than that. NW-02 has extremely good noise reduction as it makes no noise even at 9.8 MPa which is the maximum surface pressure measured, but there are probably only a limited number of actual applications with this much surface pressure. WW-09 is designed for balance with mechanical properties in mind, anticipating a maximum of 5 MPa of surface pressure which can typically occur in practical conditions, and with noise resistance at surface pressures higher than that.

4. Summary

DURACON WW-09 is a new grade that can simultaneously deliver satisfactory sliding, strength, and creep properties. It offers great potential for use when there are concerns about fractures under high loading conditions, when there is insufficient wall thickness, and in parts which also require sliding properties for aspects such as sliding sounds or wear. Since it has mechanical properties on par with standard grades, it can also be more lightweight and space-efficient than existing sliding grades.

Polyplastics offers grades that cater to needs of our customers, with an extensive lineup of DURACON sliding grades in addition to WW-09 which we have introduced here.

As a specialized manufacturer of engineering plastics, we will continue pushing forward with the development of materials that meet market needs.

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9th May 2019