Selected Processing Aids in NBR and CR compounds: How do you know your processing aid is working

Last month we brought to you Why one should use Dispergum Zinc soaps as a processing aid. As a follow on to that this month we address the all-important question: How do you know your processing aid is working.  What follows are various testing methods, for both NBR and CR, for the processes used for conversion to finished rubber products.

Introduction

The processes to convert NBR and CR into finished rubber products is by

1. Extrusion
2. Compression
3. Transfer or
4. Injection Moulding Process

NBR and CR are the compounds most frequently converted.  However, their flow characteristics are not always satisfactory for such processing steps.  Improved processing can be obtained by the addition of procession aids. Specially developed chemical products, used in small concentrations, are able to substantially modify the processing characteristics of these rubber compounds without noticeably affecting mechanical cured rubber properties.

Preparation of the compounds

The following products were added in amounts of 1:3 and 5 phr respectively to poorly processing, unplasticized NBR and CR compounds

Dispergum L : Zinc soaps of defined unsaturated fatty acids
Deoflow S : Calcium soaps of defined saturated fatty acids
Deoflow A : Mixed fatty alcohols and fatty acid esters
Deosol VE : Aqueous emulsion of selected fatty alcohol ester and higher molecular waxes.

The base formulations were as follows

*only in compounds for the evaluation of curing characteristics and cured rubber properties.

TEST METHODS

EVALUATION OF PROCESSING AIDS IN NBR COMPOUNDS

1. High pressure capillary rheometer
These tests under varying shear rate conditions showed Dispergum L to impart the lowest viscosities over the whole shear rate range and the highest volume outputs over the entire pressure range, followed by Deoflow A.  Interestingly, the addition of 5 phr Dispergum L was able to increase the extrusion volume at a pressure of 200 bar more than fourfold, i.e. from 15 to 66 mm3s-1

This shows that Dispergum L should impart optimum processing properties to NBR compounds for all technological processes concerned.

2. Mooney viscosity test
Here Dispergum L brought about the most pronounced viscosity decrease, with Delflow A a close second.  Dispergum L equally led to the most pronounced lowering of mixing energy followed by Deoflow A.  In the compound studied, the addition of 3 phr Dispergum L heled save 15% and 5 phr Dispergum L even 25% mixing energy compared tot eh control compound without any processing aid.

3. Transfer moulding tests
Again, here characterized by intermediate shear rates, Dispergum L achieved an increased of injected compound weight by 119% and Deoflow A by 63%.

In the table below we show the Effect of processing aids (5 phr) on the transfer moulding of a NBR compound in a spiral mould at 150 degrees centigrade

4. Extrusion Tests
This is at higher shear rates; the expectations once more were fully confirmed.  In the high pressure capillary rheometer tests, Dispergum L allowed to obtain the lowest viscosity levels of all processing aids tested: in the extrusion experiments, energy consumption was most drastically reduced by this product (22% at 3 phr, or 35% at 5 phr),, with Delflow A somewhat less effective (40% at 5 phr). Zinc stearate situated itself at a much lower level of effectiveness.

When torque and back pressure were readjusted to match the level of the control compound by increasing the screw speed, the compound containing 3 phr Dispergum L impressed by an extrusion rate of 750 cm/min for the control, i.e. an improvement by 127%.  Surface smoothness and edge precision of a Mini-Garvey profile were improved from a D1 rating (control) to an A4 level.

The table below shows the effect of processing aids on extrusion results in a mini extruder

5. Injection moulding
The results as discussed so far allowed one to expect optimum effectiveness of Dispergum L also under injection moulding conditions which is high shear rates.  But the high pressure capillary rheometer results, in fact, did not predict such a dramatic increase in injected compound volume as was effectively found in the injection moulding experiments.  In the NBR compound studied, the addition of 5 phr Dispergum L was able to increase the injected volume by 500%.

A study was made of the effect of raw rubber viscosity on the flow properties of compounders with and without processing aids.  By way of definition, the tests aimed at determining the pressure necessary to obtain, in a given time interval, an output of 2 cm3 of rubber compound from a Rheovulkameter instrument.  This pressure can be taken as an index of the flowability of a compound, with lower figures indicating more favorable flow properties.  The results confirm that raw rubber viscosity does exert a marked influence on the flow characteristics (changing from a Mooney 80 to a Mooney 30 grade reduced the required pressure by half), but the effects of processing aids are still considerably more beneficial.  Flowability of a given NBR grade was increased most dramatically by Dispergum L, in one case by a factor of 2.7, while Delflow A gave somewhat smaller improvements and DOP plasticizer showed comparatively little effectiveness.

Such a favorable action of processing agents offers the possibility to rubber compounders to adjust required processing properties using a reduced number of individual NBE elastomer grades.

6. Curing characteristics
Dispergum L is distinguished by a certain delaying action on scorch and full cure, an observation which cold lead to the assumption that the remarkable improvement of the flow properties of NBR compounds by this processing agent might be due, at least in part, to such a retarding effect. In a separate study on TMRD-EV cured NBR compounds, Dispergum L on the the other hand, gave evidence of a slight activation and acceleration of cure (t90 at 150 degrees C was decreased from 6.3 to 4.4 mins) and never the less the presence of 5 phr Dispergum L increased the injected volume in transfer moulding tests by a factor of 4.8.  The other processing aids exert only a minor influence on the curing properties of NBR compounds.

In concentrations upto 5 phr, the processing agents affected cured rubber physicals only to a ver limited degree.  In certain cases, 5 phr of Dispergum L were able to reduce the compression set (72h/100 degrees C) by as much as 10 percentage points.

On the whole, Dispergum L unquestionably stands out as the best suited processing aid for NBR compounds with Deoflow A a good second choice.  Deoflow S tends to bloom from NBR compounds, and therefore cannot be recommended.  Deosol VE only shows very itte effectiveness.

With the addition of 3 to 5 phr Dispergum L, the following improvements can be obtained in the processing of NBR compounds

• Mixing Mixing energy reduced by 25%
• Transfer Moulding Injected volume increased by approx..120@
• Extrusion Extrusion energy reduced by upto 35%
• Extrusion rate increased by approx. 125%
• Extrudate quality improved from D1 to A4
• Injection Moulding Injected volume increased by upto 500%
• Injection time reduced (e.g. by 83%)

EVALUATION OF PROCESSING AIDS IN CR COMPOUNDS

Studies of CR compounds in the high-pressure capillary rheometer under varying shear rates identified Deoflow S as the product to give the lowest viscosities and the highest extrusion volumes under low shear rates, followed by Deoflow A.  However, in the higher shear rate region this relationship was inversed, with Deoflow A generating the lowest viscosities and highest extrusion outputs.  Under high shear rate conditions, a t a pressure of 200 bar, the addition of 5 phr Deoflow A was able to increase volume output from 1.8 to 7.3 mm3s-1, i.e. by a factor of more than 4.  Dispergum L suffers in CR compounds from the relative stiffening caused by the presence of zinc ions, and only causes a slight decrease of viscosity at low shear rates, with practically no effect at all under high shear rate conditions.

The extrapolation of these high pressure capillary rheometer results to technical industry processes at varying shear rates allows to predict the flow behaviour of compounds under industrial processing condition in various type of equipment.

1. Mooney viscosity tests
These tests are characterized by low shear rates and the absence of wall slip effects, showed the strongest decreased of viscosity for Delflow A, although the high pressure capillary rheometer studies have indicated Deloflow S as the most effective processing aid for viscosity reduction in the low shear rate range. This discrepancy can be explained by the exudation tendency of Deoflow S in CR compounds (a reason to disadvise its use), with the result of pronounced slipping on the walls, e.g. of the capillary rheometer, whereas Deoflow A is distinguished by preferentially internal lubrication effects.

Likewise, Deoflow A came out as the most effective processing aid in mixing processes, i.e. at intermediate shear rates.

2. Transfer moulding
As expected here, studies confirmed the relative rates of effectiveness of the individual processing aids as established by high pressure capillary rheometer tests in the intermediate shear rate region (101 – 102s -1)

The table below shows the Effects of processing aids (5 phr) in the transfer moulding of CR compounds in a spiral mould at 150 degrees C.

Dispergum L in CR compounds can lead to melt fracture already at intermediate pressures, which makes the measurement and assessment of flow processes difficult if not impossible, and also causes delayed onset of cure.  Except for this product, the other processing aids came out in the expected order of effectiveness.  Just because of the stronger wall slip effects, Deoflow S gave the highest increase of extrusion output, followed by Deoflow A. In view of the objectionable exudation tendency of Deoflow S, it is Deoflow A that has to be put forward the processing aid (here with a 27% increase in injected compound volume).

3. Extrusion tests
Here, at higher rates should principally give similar ratings of the individual processing aids as found in the high-pressure capillary rheometer tests inte sear rate region above 102s-1. Such expectations were fully confirmed by the actual experiments. Deoflow A, which is the high pressure capillary rheometer generated the lowest viscosity level, brought about the strongest decrease in extrusion energy consumption. With 5phr of Deoflow A in the CR formulation studied, savings of about 20% in energy uptake could effectively be realized.

In the table below we see the effect of processing aids on the extrusion results in a mini extruder.

*in this case because of feeding problems the screw speed could not be raised further i.e. to the level of the control compound.

4. Injection moulding
Here the trials, at still higher shear rates, gave slightly different results from expectation. Deoflow S and Deoflow A caused nearly equivalent increases in injected volume, while Deoflow A rather was predicted to give the more pronounced improvements. The wall slip effects due to the bleeding of Deoflow S should explain the observation. 5 phr of Deoflow A were able to increase the injection volume of the CR compound studied by more than 175%.

Deoflow A furthermore, exerts a favorable influence on the curing characteristics and a minimum effect on cured rubber physicals of CR compounds in particular with respect to high temperature compression set. As Deoflow A does not present a risk of exudation, this product has to be regarded overall as the best suited processing aid for CR compounds.

The following improvements can be obtained in the processing of CR compounds with the additions of 5 phr Deoflow A

• Mixing Mixing energy reduced by approx. 5%
• Transfer Moulding Injected volume increased by approx. 27%
• Extrusion Extrusion energy reduced by approx. 20%
• Extrusion rate increased by approx. 58%
• Extrudate appearance improved from D3 to B7
• Injection Moulding Injected volume increased by approx. 15%
• Injection time reduced by approx. 65%

Conclusion

For each individual processing aid, the non-diluted active ingredient will always offer the highest degree of effectiveness. Therefore, Dispergum T, Dispergum N or Dispergum C will be less pronounced improvements of processing properties in line with their relative content of active substance compared to Dispergum L. A similar relationship holds true for Deoflow D in comparison with Deoflow A.

I R Tubes Pvt. Ltd. is a leading specialty chemical suppliers for the chemical industry. Contact I. R. Tubes on info@irtubes.com or Call: 9689927193 for more information

Raju Jethmalani
IRTubes Pvt. Ltd., Pune

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