Rubber processability tests

With respect to stream rheological effects in rubber processing, and despite all the restrictions discussed above, it seems nevertheless that the key information is how the nonlinear viscoelasticity is related to the processing behavior of mbber compounds. Such information can be deduced from the appropriate test procedure with the RPA, providing one considers the capabilities of the instmment to provide nonlinear viscoelastic data. 

In addition, some rubbers, for example natural rubber and polychloroprene, stiffen at low temperatures by partial crystallisation. This is a gradual process continuing over many days or weeks and is most rapid at a particular temperature characteristic of each polymer, for example -25°C for natural rubber. Hence, tests intended to measure the effect of crystallisation must detect changes in stiffness or recovery after periods of ageing at a low temperature. 

FIGURE 7.5 Smoke emission from selected polypropylene compounds filled with 50wt % of filler. Tests undertaken according to UITP E4. (From Hornsby, P.R. and Watson, C.L., Plast. Rubber Process. Appl., 11, 45, 1989. With permission.)... 

Dynamic-mechanical testing of cross-linked samples are often carried out with high precision on specimen strips in torsion mode, e.g., with a Rheo-metrics Dynamic Analyzer II (RDA) with a sample size of 28x10x2 mm. Here, temperature-and strain sweeps are performed in a displacement range from 0.01% to about 5% strain and a frequency range between 0.1 and 100 Hz. Dynamic mechanical testing of uncross-linked samples can be made, e.g., with a Rubber Process Analyzer RPA 2000 (Alpha Technologies) from 0.28% to 350% strain at various frequencies and elevated temperatures. 

Processability tests differ from all the other physical properties included in the book by irtue of being concerned with properties of relevance to the forming of materials and not the performance of the finished material or product. Chapter 8 deals with the processability tests in two parts, for rubbers and plastics respectively. 

This chapter deals with processability tests for the two general polymer categories, rubbers and plastics. Not surprisingly, there are many similarities in the processing of the two types of material, but there are also many differences arising from their structural differences, differences in the processing methods and because of the separate development of the two industries. International and national standards for the two types of material are separate, and generally the apparatus used is specific to one material type. Consequently, rubbers and plastics are dealt with here sequentially, and it is left to the interested reader to contrast and compare the appoaches taken. 

Various processability tests are used to predict how well a rubber compound will process. However the process" can vary greatly. Table 1 shows the diverse categories of rubber processes and some of the quality characteristics and concerns associated with each process. The natures of these processes vary because of differences in applied shear rates, temperatures, residence times, etc. Changing a given compound property can improve performance in one or more of these processes but could hurt the performance in another process. This is why it is important to look at all processes in a manufacturing operation when implementing compound changes. 

This next section reviews and compares the different processing tests that arc used in the rubber industry today. These process tests either directly or indirectly measure one or more of the ten fundamental rubber properties just discussed. 

Another very fast and effective method for measuring rubber processing properties is to perform a stress relaxation test. A stress relaxation decay curve can quickly quantify the viscoelastic properties of both raw rubbers and mixed stocks. The Maxwell model, shown in Fig. 35. illustrates this principle with a spring and dashpot in series [125]. A sudden... 

Sezna, John. The use of processability tests for quality assurance. Rubber H nrld. p. 23 (January 1989). 

Ratio of the velocity of light at a specified wave length in air to its velocity in a substance under examination. The refractive index can be determined by test method ASTM D642, using a refractometer and a monochromatic light source. Refractive index is an excellent test for uniform composition of solvents, rubber process oils, and other petroleum products. It may also be used in combination with other simple tests to estimate the distribution of naphthenic, paraffinic, and... 

Many widely used Tests of adhesion can be applied to the rubber to metal bonds Blister test. Fracture-mechanics test specimens. Non-destructive testing of adhesively-bonded structures. Peel tests. Shear tests. Tensile tests and Wedge test. This particular article is concerned with those aspects that are of practical concern in the rubber-processing industry. 

Some of the more important tests are detailed below. Others, e.g. tensile tests and hardness tests are routine in any rubber processing. However, details may be found in the standard literature (ref. 2). 

Recently, a group of Indian researchers developed a new elastomer product based on NR/BR blend with reclaim rubber (RR) from ground rubber tires (GRT). The reclaiming was carried out by tetra methyl thiuram disulfide (TMTD) in the presence of spindle oil, a paraffin-based rubber process oil. Thermogravimetric analysis of RR, NR/BR and different NR/BR/RR vulca-nizates was carried out in order to measure the thermal stability of the vulca-nizates. Isothermal ageing test of fresh rubber RR composites showed that the ageing performance of RR containing vulcanizates are superior than that of the fresh rubber vulcanizates, which do not contain any reclaimed rubber. 

Processability of elastomers, whether in mixing, extrusion, moulding or curing is, and always has been, a problem for the processor. The concept itself is difficult to define. In fact, rubber processability is a subjective concept in that it depends on what you want to do with the material, and in what equipment, whether it processes well or badly. Furthermore, there has been a lack of simple, easy tests that can measure processability. Generally, if the test is simple it does not correlate directly with processability, and, if it does correlate, it is not... 

Most of the processability testing which is carried out in the rubber industry is performed on the product of mixing. There are three types of test (a) tests aimed at determining the state of mixedness of the compound, (b) tests of some fundamental property believed to be related to processing behaviour in subsequent operations, and (c) tests attempting to simulate those operations. Only tests of type (a) will be considered here. 

Results are presented of high strain testing for the characterisation and quality assurance of NR using the RPA 2000 rubber process analyser. Such testing was found to be more effective in detecting processability ditferences than traditional processing tests such as Mooney viscosity or Wallace plasticity. 24 refs. 

---