Quality Assurance of a Plain Bearing Made from a Polymer - METTLER TOLEDO

Quality Assurance of a Plain Bearing Made from a Polymer

Introduction

A consumer goods manufacturer suddenly began to receive an increasing number of complaints about a product. The cause was traced back to an injection-molded part made of plastic that was used as a plain bearing. Detailed investigations showed that nothing had changed in the product except that the supplier of the plain bearings was new. Preliminary DSC measurements in an external laboratory indicated that the material used for the production of the good and the bad plain bearings was the same polymer. Other factors were therefore thought to be the cause of the problem. A quick solution to the problem did not seem possible.

 

DSC Analysis

The DSC measurements were performed using a DSC 1 equipped with an FRS5 sensor and an intracooler cooling accessory.

 

Melting Point

More precise information about the cause of the problem was obtained by examining the DSC measurements in more detail. As is frequently the case in routine DSC analysis, the melting point was used for polymer identification. A polymer, of course, melts over a rather wide temperature range. The melting point is defined as the peak maximum (the so-called peak temperature) of the melting peak and is used to characterize the polymer. It is assumed that different types of polymers exhibit different melting points. In practice, processed polymeric materials often show slightly different melting points as a direct result of production conditions. The sample is therefore usually cooled under defined conditions and the second heating measurement used to identify the material.

In the present case, the heating and cooling measurements were performed at 10 K/min in the temperature range 30−220 °C. The sample mass was about 10 mg. The measurement curves of the second heating runs are shown in Figure 1. It can be seen that the peak temperatures of the good (Tm = 166.8 °C) and bad quality material (Tm = 166.1 °C) are practically the same. Effectively, there is no difference between the melting points of the two materials.

DSC of plain bearing materials 

 

Enthalpy of Fusion

The DSC measures not only the characteristic melting point but also the enthalpy of fusion, ΔH. This quantity is obtained by integrating the area under the melting peak. In this case, evaluation of the melting peaks yielded values of 178 J/g for the good quality material and 88 J/g for the bad quality material. This result shows that the two bearings are in fact made of different materials. Possible reasons for the different enthalpies of fusion could be 

  • the use of different polymers or
  • the use of a filled polymer for the bad bearing

A DSC measurement alone cannot distinguish between these two possibilities. However, the values obtained for the melting point and the enthalpy of fusion point to the possible types of polymer involved. 

If the filler content is insignificant, the two values Tm = 166 °C and ΔH = 88 J/g are typical for polypropylene (PP). Another polymer that melts in the same temperature range and that typically has an enthalpy of fusion of about 160 to 180 J/g is polyoxymethylene (POM). The melting point of pure POM is about 10 K above the measured value. However, materials are often used that have been modified through copolymerization. The melting points of such materials are somewhat lower and agree well with the measured values. Copolymerization also improves the mechanical properties of the POM. In summary, the results of the DSC measurements lead one to think that the bad quality material is PP and the good quality material POM. 

 

TGA Analysis

This possibility can be confirmed by measuring the filler content of the two materials using TGA. A METTLER TOLEDO TGA/DSC 1 instrument was used for these experiments. Samples of about 5 mg were heated from 40 to 800 °C at 20 K/min. At 600 °C, the reactive gas was switched from nitrogen (inert) to synthetic air (oxidative). The possible vaporization of volatile components and the degradation of the polymer are measured as steps in the TGA weight loss curve. 

Conclusions

The measurement data obtained from the DSC and TGA measurements is summarized in Table 1.

The conclusion from these results is that the bad quality material is polypropylene (PP), a low-cost plastic whose mechanical properties are not adequate for this particular application.

The degradation temperature and the enthalpy of fusion indicate that the good quality material is polyoxymethylene (POM). For a homopolymerizate, the melting point is about 10 K too low. The material is therefore most probably a copolymerizate based on POM. These materials have exceptionally favorable mechanical properties such as hardness and strength, good surface hardness, a low coefficient of friction and low abrasion, and good dimensional stability.

Polymers cannot be identified solely on the basis of the melting point obtained in a DSC measurement. The enthalpy of fusion provides important additional information that should also be used to aid identification.

A complementary TGA analysis provides information about the composition of the samples. The degradation temperature of many materials is a characteristic property and should be taken into account in the interpretation.

If the thermal events of interest occur at temperatures sufficiently above room temperature, the DSC curve can be measured directly using the TGA/DSC 1. 

Quality Assurance of a Plain Bearing Made from a Polymer | Thermal Analysis Application No. UC 306 | Application published in METTLER TOLEDO Thermal Analysis UserCom 30