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Improved natural rubber processing and physical properties by use of selected compounding additives.

Natural rubber viscosity reduction

Natural rubber (NR) usually needs some degree of viscosity reduction in order to ensure good processability. This process is called mastication and is achieved by breaking polymer chains during the mixing stages.

Historically, a separate first mixing stage called premastication was used. This stage involved adding just NR or NR plus an active chemical called a chemical peptizer to the mixer and mixing for several minutes. Premasticated rubber was mixed together with fillers over several further mixing stages.

Chemical peptizers are materials usually based on a combination of a free radical scavenger and a so-called booster material, as an effective source of oxygen radicals that serve to increase oxidative chain scission at higher temperatures. Examples of typical chemical peptizers featured within this work include Struktol A 89 and Struktol A 86.

Natural rubber chain breaking mechanisms

Long NR chain material is broken down during the mixing process. At first, when the mixing temperatures are low, the high levels of shear force generated are sufficient to break the chains. Longer chains are statistically more likely to break due to this mechanical shearing action, since the higher degree of chain entanglement seen along longer chain material leads to more segment entrapment, and hence higher forces are applied. Shorter chains may more easily disentangle against the shear forces.

The high shear loads applied generate large amounts of heat. This heating softens the NR, which in turn reduces the degree of shear force. At a mixing temperature of about 120[degrees]C, mechanical chain breaking ceases to be the predominant mechanism and chemical chain breaking by oxidative attack becomes more significant.

Oxidative chain scission is a chemical chain breaking reaction where oxygen radicals randomly attack the NR chain, and although long chains will be broken, short chains can become even shorter during this process. This action tends to increase the molecular weight distribution of the NR. This is not good for subsequent NR properties because the high strength and elasticity found in NR is dependent upon the presence of long chain material. The booster material contained within many chemical peptizers serves to accelerate oxidative chain scission by acting as a source of oxygen radicals. Frequently, use is made of materials based on iron complexes for this purpose. Figure 1 shows the increase in molecular weight distribution that occurs due to an increased amount of short chain material following mastication with a chemical peptizer.

Any chain that has been broken, either by the high mechanical shear forces of the mixer at lower temperatures, or by the action of oxygen radicals at higher temperatures, has the possibility to react with sites on the NR chain. Branched polymer results from this action. Branched polymer is undesirable because it reduces the highly elastic nature of NR. In order to limit this possibility, a radical scavenger is included within many chemical peptizers. Formerly, the radical scavenger used was a pentachlorophenol derivative (PCTP). However, due to environmental concerns, dibenzamido-diphenyl-disulphide (DBD) is now almost exclusively preferred. The scavengers act to block free radicals created at the ends of the broken NR chains.

Modern mixing methods

The mixing process frequently used today is somewhat different from what was established. Demand for higher productivity at lower cost means that energy intensive multiple mixing stages are avoided wherever possible. The premastication mixing step is often not used. Instead, the raw NR together with a chemical peptizer is added to the mixer and mixed for a short period, in some cases for only 30 seconds, before the filler is added; we call this "in situ mastication." There are several different considerations relating to this type of mixing. Because the mixer is only half-filled due to the volume which will shortly be occupied by the fillers, the amount of shear and hence temperature increase at the start of mixing is much lower than achieved during traditional premastication. Often, temperature may not reach 120[degrees]C. Under this condition, oxidative chain scission will not occur to any significant degree. Of course, when we add carbon black, temperature does increase rapidly due to the increase in shear force generated within the now fully filled mixer. Oxidative chain scission can occur. However, carbon black is a good antioxidant and stabilizes many of the oxygen radicals generated by the booster material contained in the peptizer. The chemical peptizer is much less effective due to this action.


Limitations of chemical peptizers

Further problems also result from this "in situ" method of mixing. The dosage of chemical peptizer is generally very small; typically only 0.5 parts per hundred of rubber (phr) are used. The major active ingredient of most chemical peptizers is DBD, which acts to terminate the broken chains before they can react to form branched polymer. However, DBD is crystalline, with a melting point of between 136[degrees]C and 144[degrees]C, and does not melt during the start of in situ mixing due to the lower heat build up in the half-filled mixer. It is important, therefore, that the chemical peptizer is physically very well distributed in order to be effective. Of course, with such a small loading of peptizer in a half-filled mixer, good distribution is almost impossible. As a result, there is a much-increased chance that broken chains may recombine as branched polymer. Furthermore, the poor distribution of peptizer may lead to localized over-concentration resulting in areas of excessive chain breaking, especially if temperature variation within the large volume mixer is also poor. This condition, often referred to in the industry as over-mastication, results in regions of sticky polymer during the batch-off stage on an open mill. Considering all of these factors, although chemical peptizers can have a dramatic influence toward lowering viscosity, there are also many limitations to their usage. Adjustment of the mixing process is required in order to retain a separate mastication stage for best effect. Even with proper mixing, the resulting compound has inferior dynamic properties; the presence of shorter or more branched material gives rise to reduced elasticity, higher hysteresis and lower aging performance due to the action of the chemical peptizer.


Physical peptizers

In order to overcome the problems associated with chemical peptizers, many processors have turned to the use of so-called physical peptizers. The process makes use of a polymer interchain lubricant product. In this case, no chemical action towards polymer chain breaking occurs. The effect of such an additive is to reduce the degree of polymer interchain forces, especially at higher shear rates. Long polymer chains are more able to disentangle against high shear forces due to this increased interchain lubrication. This results in an increased flow property of longer chain material. Of course, this is an ideal situation, and an improved balance between processability and product properties may result. As a lower level of chain breaking occurs, the more aggressive nature of the chemical peptizer, especially towards the production of short chain material, is avoided. Physical peptizers, frequently based on metal soaps and zinc soap products, are widely cited as extrusion additives in many NR and NR blend compounds. However, for many applications the effect of the zinc soap is not sufficient. The higher levels of viscosity occurring during mixing stages may still result in poor processing and lead to scorch, and therefore the need for further viscosity reduction remains. Nevertheless, many lubricant products are available and their use in NR containing compounds is quite widespread.

Examples of physical peptizers include Struktol A 60 and Struktol A 50P (zinc soap), and Struktol EF 44 (zinc potassium soap).

Modified viscosity reducing additive

In order to achieve adequate viscosity reduction without the need for separate pre-mastication, while also retaining the good dynamic properties of NR, a different approach is called for. Mechanical mastication without use of a chemical peptizer is the preferred method for chain breaking, but improvement of this rather slow and inefficient process is required.

Struktol HT 105 is based upon the technologies of both chemical and physical peptizers. HT 105 contains DBD, but no iron complex or other booster is included. A different method to combine DBD within a new carrier system during the production of HT 105 was developed. The result of this change is that dispersion of liT 105, and hence that of DBD in the polymer, occurs much faster and to a higher degree than is normally observed with chemical peptizers. DBD becomes effective much earlier in the mixing cycle and at lower mixing temperatures. As a result, the degree of branched polymer formation is much lower. The reduced chemical activity due to the absence of the booster is more than compensated by the improved DBD dispersion, and by the presence of the carrier material. The carrier, based upon a metal soap, functions as a highly efficient interchain lubricant, promoting good physical peptization within the polymer.


In order to demonstrate the relative action on NR of each class of viscosity reducing chemical, mixing using a Brabender Plasticorder Model PLE 2000 was undertaken. This small mixer provides measurement of the reaction torque needed to turn the rotors at constant speed. A mixer torque curve (figure 2) comparing HT 105 with a chemical peptizer and a physical peptizer lubricant shows that chemical peptizer results in a much greater reduction in mixing torque than either the mechanical shearing action or the action of an interchain lubricant. HT 105 results in a similar torque reduction to that of a chemical peptizer, even though no booster material is present.

Viscosity reduction in the presence of carbon black

The fact that HT 105 does not contain booster is an advantage when mastication in the presence of carbon black (in-situ mixing) is used. In this case, oxygen radicals generated by the booster material contained in a conventional chemical peptizer are largely de-activated by the carbon black. By contrast, with HT 105 the better dispersion of DBD achieved at lower temperatures allows for better termination of flee radicals at broken chain ends, and hence a reduction in long chain branching. More emphasis is placed upon mechanical mastication, when shear forces are at their highest. This is further supplemented by the presence of a lubricant material, the purpose of which is to act as a carrier and dispersant for the DBD, and later as an interchain lubricant within the polymer.

Mixing of formulations containing a chemical peptizer compared to HT 105, together with several controls using mechanical mastication with different mixing time, was carried out. The details are given in table 1. This mixing method makes use of the in-situ mixing technique as described previously. This method is quite representative of the type of mixing used in many production applications.

Mixing was done using a laboratory internal mixer having tangential rotors, Model Type WP GK 1.5N. A single stage mastication and filler dispersion stage was used. Table 2 details the mixing procedure together with mixing temperatures and energy consumption recorded. Curative addition was achieved using an open mill in each case.

Processability was determined using both Mooney viscosity measurement and extrusion performance under constant head pressure by means of a 10D laboratory scale cold-feed extruder. Rheological property measurement was made by means of a Monsanto MDR reometer. Table 3 details the results.

Vulcanized physical properties and measurement of hysteresis as tangent delta (Tan [delta]) at 70[degrees]C using an Eplexor viscoelastic dynamic tester in extension mode at 10 Hz were also recorded (table 4).


Using the in-situ mixing method provides for only a short mastication time of approximately one minute before carbon black addition. The resulting Mooney viscosity data confirm that under this condition, chemical peptizer is less efficient than new additive HT 105. Significantly better extrusion output obtained with HT 105 compared to a chemical peptizer further confirms the improved efficiency; in this case, the presence of a lubrication package in HT 105 is of additional benefit. The rheological data indicate no significant difference in terms of cure speed for any variant. The reversion resistance time [t95.sup.r] demonstrates the poor result for the chemical peptizer, while improvement is seen in the case of HT 105. This improvement is primarily due to the stabilizing effect conferred by the presence of zinc soap within the lubricant system. Such influence is expected when using zinc soaps. By contrast, reversion resistance is reduced by the presence of the chemical peptizer. Vulcanizate properties are largely unaltered by use of either a chemical peptizer or the new additive HT 105 in this study. However, the vulcanizate viscoelastic dynamic property tangent delta, which is often taken as an indication of the hysteresis loss for rubber compounds, is significantly reduced in the case of HT 105; the chemical peptizer resulted in a slight increase in tangent delta value, relative to that of the similarly mixed control compound.

Interestingly, the use of extended mixing time in an attempt to make greater use of mechanical chain scission was largely ineffective; only a small reduction in Mooney viscosity is obtained and this does not translate into big improvement in extrusion output. These data suggest that this method is not efficient and must be considered against the considerably higher energy consumption associated with the longer mixing time required.


Struktol HT 105 as a viscosity reducing chemical for NR containing formulations is of benefit; improved processability is obtained even during short mixing cycles. Premastication is not required due to the fact that the product retains function when carbon black is added. Further more, the use of HT 105 as a replacement for a chemical peptizer or compared to the use of no additive results in improved viscoelastic dynamic properties.

by Colin Clarke and Manfred Hensel Schill + Seilacher
Table 1--formulations

 Mech. mast. Mech. mast. Mech. mast.
 3'30" 4'00" 4'30"

NR (TSR 5) 100 100 100
Struktol HT 105
Chemical peptizer
N-234 55 55 55
Aromatic oil 5 5 5
Zinc oxide 3.5 3.5 3.5
Stearic acid 1 1 1
6PPD 2 2 2
TMQ 1 1 1
Wax 1 1 1
Sulphur 1.5 1.5 1.5
TBBS 1.5 1.5 1.5

 Struktol Chemical
 HT 105 peptizer

NR (TSR 5) 100 100
Struktol HT 105 2
Chemical peptizer 0.2
N-234 55 55
Aromatic oil 5 5
Zinc oxide 3.5 3.5
Stearic acid 1 1
6PPD 2 2
TMQ 1 1
Wax 1 1
Sulphur 1.5 1.5
TBBS 1.5 1.5

Table 2--mixing procedure and mixing data for
in situ mixing method

 First pass--internal mixer GK 1.5 N,
 start 60[degrees]C, 65 1/min.
Start of mixing 0' NR, chemical peptizer or
 Struktol HT 105
Ram raise 1'15" 1/2 black, zinc oxide, oil
Ram raise 2'30" Antioxidants, stearic acid, 1/2 black
Ram raise 3'00" Sweep
Compound type Control Control Control Struktol Chemical
 HT 105 peptizer
Dump time (mins.) 3'30" 4'00" 4'30" 3'30" 3'30"
Dump temperature 122 135 164 115 124
Energy consumption (kJ) 2,122 2,537 3,076 1,915 2,103

Table 3--Mooney viscosity and rheological measurement

 Control Control Control
 3'30" 4'00" 4'30"

Mooney viscosity 108 107 104
 ML 1+4 100[degrees]C
Extrusion speed 1.01 1.07 1.18
 (m min.-)
Torque ML (dNm) 4.98 4.54 4.37
Torque MH (dNm) 20.75 20.82 21.07
tc 10% (mins.) 2.39 2.26 2.37
tc 90% (mins.) 4.09 4.02 4.12
Reversion resistance 10.53 10.23 10.38
 [t95.sup.r] (mins.)

 Struktol Chemical
 HT 105 peptizer

Mooney viscosity 91 96
 ML 1+4 100[degrees]C
Extrusion speed 2.18 1.61
 (m min.-)
Torque ML (dNm) 3.49 4.24
Torque MH (dNm) 19.39 21.27
tc 10% (mins.) 2.10 2.15
tc 90% (mins.) 4.27 3.80
Reversion resistance 14.60 9.68
 [t95.sup.r] (mins.)

Table 4--vulcanizate viscoelastic and dynamic properties

 Control Control Control
 3'30" 4'00" 4'30"

Durometer hardness A 67 67 66
 (Sh. U)
Tensile strength (MPa) 27.5 26.3 27.4
Elongation at break (%) 563 531 537
Modulus 100% (MPa) 2.3 2.3 2.6
Modulus 300% (MPa) 12 12.1 12.9
Rebound 23[degrees]C (%) 40 40 40
Abrasion loss DIN 123 114 119
Tangent delta 70[degrees]C 100 97 96
 (as %)

 Struktol Chemical
 HT 105 peptizer

Durometer hardness A 65 67
 (Sh. U)
Tensile strength (MPa) 28.9 27.5
Elongation at break (%) 576 522
Modulus 100% (MPa) 2.4 2.7
Modulus 300% (MPa) 12.4 13.6
Rebound 23[degrees]C (%) 40 39
Abrasion loss DIN 126 122
Tangent delta 70[degrees]C 92 102
 (as %)
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Author:Clarke, Colin; Hensel, Manfred
Publication:Rubber World
Date:Nov 1, 2009
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