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7/29/2019 More Rhe Omd

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USE OF RHEOMETERSFOR RUBBER

COMPOUNDS CONTROL


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The preparation of rubbercompound is influenced by

several factors that influence theresults obtained:


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Variability in the quality of the ingredients:

PolymerVulcanizing agentCure activatorCure accelerator

Fillers ( reinforcing, diluents.)Softener/ process acid /tackifierPlasticizerProtective agent (s)Miscellaneous ingredients ( pigments, blowing agent, etc.)


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Variability originated by the weighing system (automatic or manual)used for the ingredients

Variability originated by the environmental conditions (temperatureand Humidity) which influence the dispersion and the activity levelof the ingredients.

Variability originated by the mixing parameters (time andtemperature)

In addition, the characteristics of the compound changesignificantly when the product is stored.


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All the influence factors described above has a considerable effecton the characteristics of the compound produced

In many cases the variations are small and dont have a significanteffect on the results obtained after the transformation of thecompound into finished products.

In any case, compound producers, even with automatic weighingsystem, statistically produce between 1% and 5% of non-conforming compounds that require additional rework activity tocorrect the defects.


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It is very important that the non-conforming compounds areidentified as quickly as possible in order to:

- remove the reason of the defect and avoid the production of morenon-conforming products- prevent the non-conforming products from being transferred to thenext production stage

One of the biggest differences between rubber and other productsis the fact that many of the defects in the compound (ex. Dispersionproblems or mistakes in the dosage of accelerators) can beinvisible during the transformation process and be detected only

when the final product is used. For this reason, the final cost for anon-detected defect can be very high.


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Normally a test of the compound made with a Rheometer permitsto identify most of the possible problems in a rubber compound.

The test only takes a few minutes (generally between 3 and 5) andis normally done on 100% of the batches.

This instrument permits to measure the change of the properties ofa rubber compound during the course of vulcanization.Rheometers permit to apply a cyclic strain to a test piece and theassociated force is measured.

The test is carried out at a predetermined constant temperatureand the measure of stiffness is recorded continuously as a function

of time


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MOVING DIE RHEOMETER

Moving Die Rheometer is considered the mostmodern version of Rheometer.The sample is placed between two dies.

The lower dies oscillates with 0.5 degreesThe upped die is connected to a torque sensor tomeasure the torque response of the rubber at thedeformation.Typically the working temperature is set between

170C and 190C (100 120C for platinum curedsilicon)Plastic film (typically Polyamide or Polyester) isplaced between the dies and the samples toprevent contamination of the dies during the test ofdifferent products.

Thermal recovery is very fastThe instrument has very high repeatability andreproducibility.


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WHICH KIND OF RHEOMETER ?

The Moving Die Rheometer and the Oscillating DiskRheometer produce different test curves and results.

Even if the results are numerically different, they arestrongly correlated and provide the same informationabout the sample tested.

Useful indication for the choice of the rheometer are:-in case you need to compare your results with yoursupplier or your customer it is useful to have the samekind of instrument-In case you need to test different kind of polymer it isuseful to use plastic film to prevent contamination (MDRheometer)

-Oscillating Disk Rheometer are normally slightly cheeperboth for purchase and for maintenance.-Moving Die Rheometer have higher repeatability and theinfluence of the operator is lower-Moving die Rheometer can be combined with automaticsample loading systems.


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PROCESSING

PHASE

RESULTS OBTAINED WITH A RHEOMETER

The PROCESSING phase, during which the viscous (plastic) behaviour

of the compound is dominating shows rubber procesability and

therefore furnish useful indications about the fluency in the moulds.

Meaningful results are:

ML, (least torque)

tS1, tS2: Scorch time: time required for the increase of 1 (ts1) or 2 (ts2)

points from Minimum Torque. This number is an indication of the time

required for the beginning of the process of cross link ing


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During the curing phase of the curve, the cross-linking process evolutes.

The shape and the slope of the curve are very important because the

curing curve must be designed according to the thickness of the finished

part to produce and to the kind of transformation process.

Typical data that can be obtained from the curve are:

t10: time corresponding to 10% curing

t50: time corresponding to 50% curing

t90: time corresponding to 90% curing. This result is considered the

optimum vulcanization time

Note: more data about the slope of the curve can be obtained.

CURING PHASE



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The last part of the curve is an indication of the physical proprieties of the

compound.

The maximum value of torque obtained is related with:

-the final level of cross-linking,

-The quality of the polymer used

-The filler used

-The compounding process.

The typical result is:

- MH: Maximum Torque

After the Maximum Torque has been reached the degradat ion of the compound can

be measured: the reversion is an indication of the resistance of the compound at

high temperature and can be useful for the design of the transformation process

-TR_x: Time for the torque reduct ion of x points

PHYSICAL

PROPERTIES



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SCREENSHOT OF RHEOMETER SOFTWARE

Gibitre rheometers permit to:- Store data and curves of the products tested- Prepare a statistic of the results stored and create tolerance limits for future tests- Compare each results with the expected tolerance limits for the product

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