RHEOGRAPH: Interpretation And Applications

cure performance of different accelerators

Evaluating Effect of Different Accelerators on cure

Referring to Figure 1.2 above we may postulate that:

• Compound A is very scorchy (scorch time is too short) – lower processing safety.
• Compound D with  sulphenamide accelerator shows the longest scorch time and hence longer cure time but with almost similar degree of cross-linking (modulus).
• So compound D is more suitable for large size rubber article to allow ample time for mold filling.
• Compound C with shorter scorch and cure time may be suitable for small rubber article.

Rheograph Analysis

{Content of this section is based on information available from Future Foundation (India) Technical Data Sheet.}

We can compare the results from rheometer testing for  graphical and statistical analysis. The statistical analysis can also help in setting control parameters of stocks.

Analysis reports are:

a. Specimen runtime report

b. Statistical analysis report

c. Graphical analysis report

d. Statistical Quality Control report

Specimen Runtime Report

Points to be followed for monitoring batch-to-batch consistency (Future Foundation)

Values of minimum torques, scorch times and cure times of different batches may be used to monitor batch-to-batch consistency.  Should these values for a particular batch fall outside the specified ranges the compound might require some reworks prior to final moulding and curing.

Statistical Analysis Report

Using ODR data for monitoring batch-to-batch consistency (Future Foundation)

From the figure above it is obvious that values of initial, minimum and maximum torques as well as scorch times, induction period and optimum cure times are indicatives of compound batch quality and thus are very useful for statistical analysis of batch-to-batch consistency.

Terminologies Related to Cure Curves

Scorch:

Premature formation of crosslink network rendering the compound elastic (no longer deformable).

Delayed Action Cure:

A curing system that employs an accelerator or combination of accelerators or a retarder that could delay the reactions which cause the formation of crosslinks, and thaus retarding the scorchiness (prolong the scorch time) of the compound.

Scorch and delayed action cure curves

The scorch time for  compound A is too short. Thus it is said to be very very scorchy. The compound has a tendency to scorch or cure prematurely be-fore reaching all extremities of mould cavities.

Compound B is a delayed action cure in which the onset of cross-linking reaction is delayed thus retarding the scorchiness or prolong the scorch time of the compound.

State of Cure a. The level of crosslink density in a rubber compound. b. Commonly expressed as a percentage of the maximum attainable cure (or crosslink density) for a given cure system. Optimum Cure a. The amount of x-linking that results in the maximum modulus. b. A cure condition to achieve an acceptable compromise between a number of desired properties or the optimum value of a selected property. c. Cure time t_c90 is derived to achieve an optimum state of cure. Undercure

Cure curve of undercure compound

A state of vulcanization less than the optimum state of cure.  This is evidenced by tackiness or inferior physical properties of the vulcanizate such as: a.     inferior tensile strength, b.     inferior modulus, c.     inferior compression set. Overcure A state of cure at a cure time longer than the optimum cure time.  As a result of overcure the vulcanisate: a.     may continue to harden with a loss in tensile strength and elongation, or b.     may soften with a loss in modulus, tensile strength and elongation (due to reversion).

Overcure due to reversion

Effects of Overcure on Vulcanizates

A) Natural rubber vulcanizate:

• Tensile strength & modulus decline on overcure.

B) Styrene butadiene rubber vulcanizate:

• Does not show any reduction in tensile strength & modulus on overcure.

C) Nitrile rubbers vulcanizates:

• Do not show any reduction in tensile strength & modulus but exhibit pronounced improvement in compression set on overcure.

CURE MEASUREMENT AS QUALITY TOOL

Curemeters

• The course of vulcanization could be closely followed using curemeters.
• The working principle of a curemeter is to apply a cyclic strain to a test piece and measure the associated force.  In doing so the curemeter measures the development of torque as a function of time at a constant predetermined temperature.
• It is actually a measure of the increase in the stiffness (modulus) of the test piece with respect to time.

Rheometer Torque & Tensile Modulus

Usefulness of Curemeter

• Research & Development

1. Assigning compound specification and standard
2. Designing preliminary compound, selecting specific ingredients and determining dosage of each ingredient.
3. Re-designing the formulation recipe to achieve quality target

• Quality Control

Monitoring consistency of batch quality

• Process Control

Production Control Test Equipment due to its capability of detecting minor changes.

• Usefulness of Curemeter

1. Checking Technical Viability of New Ingredients
2. Cure curves can assist prediction of any effect of adding new ingredient into existing formulation.

Different Types of Curemeters

• There are three major categories of curemeters commercially used in the rubber industry:

1. Oscillating Disk Rheometer (ODR)
2. Moving Die Rheometer (MDR)
3. Rubber Process Analyser (RPA).

Oscillating Disk Rheometer (ODR)

Oscillating Disk Rheometer is the most ‘traditional’ kind of rheometer.

rubber sample covering ODR rotor

ODR Cure Test

1. The sample is placed between the two dies.
2. A rotor, placed in the middle of the dies oscillates with either ± 3 degrees or ± 1 degree at 1.7 Hz oscillation rate.
3. This action exerts a shear strain on the test piece and the torque (force) required to oscillate the disc depends upon stiffness (shear modulus) of the rubber compound.
4. The stiffness of the specimen compound increases when crosslinks are formed during cure.
5. An ODR gauges the amount of torque (twisting force in deci-Newton meter, dNm) needed to oscillate a rotor within the rubber sample.
6. To maintain a given amplitude (1^o or 3^o) of rotor oscillation at a given temperature requires sufficient increase in torque.
7. The increase in torque is proportional to an increase in low strain modulus of elasticity of the rubber compound due to the formation of cross-link structure.
8. The torque values are plotted against time to give a so-called rheometer chart, rheograph or cure curve.

Typical Cure Curve

Description of Rheometer Curve

• The processing safety phase, during which the viscous (plastic) behaviour of the compound is dominating shows rubber processability and therefore furnish useful indications about the fluency in the moulds.
• During the curing phase of the curve, the crosslinking 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 be produced and to the kind of moulding process.
• The last part of the curve is an indication of the physical properties of the compound.
• The maximum value of torque obtained is related with:

1. the final level of cross-linking,
2. the quality of the polymer used
3. the filler used
4. the compounding process.

• A decrease in torque after the maximum has been reached indicates a reversion of crosslinking process in the compound.
• Reversion refers to the loss of crosslink density as a result of non-oxidative thermal aging.  It is characterised by the time required for a defined drop in the rheometer torque as measured from the maximum observed torque.

Features from a Cure Curve

M_H  =  Maximum torque   ║  M_L  =  Minimum torque

t_s2   =  Time to reach 2 unit increase in torque above minimum (at M_L + 2 units above it) {induction time}

t_s5    =   Time to reach 5 unit increase in torque above minimum (at M_L + 5 units above it) {scorch time}

t_c90  =  Cure time at which 90% of cure has taken place.

=  Time for the torque to increase from the beginning  of the test to the value              equivalent to 0.9(M_H − M_L)+M_L

Calculating Cure Time

Calculating cure time at 90% optimum cure from this curve.

                                   M_L = 1.8 dNm

                                   M_H = 8.7 dNm

 Torque at 90% cure        = 0.9(M_H − M_L)+M_L

                                         = 0.9(8.7-1.8) + 1.8 = 8.0 dNm

Extrapolate to time axis t_c90 = 5 minutes i.e. cure time is 5 minutes.

Cure Time & Cure Rate

• Cure time is a time required by the sample to reach a desired state of cure, usually to reach 90% of the maximum cure.
• Cure rate is the rate at which x-linking & the development of compound stiffness (modulus) occur after scorch point.  This correlates with a rise in the value of torque with time.  It is indicated by the slope of a linear part of the rising curve.
• Use of accelerator also increases cure rate.
• Cure rate Index  =  100/(t_c90 – t_s2)

PEROXIDE VULCANIZATION

• Organic peroxides as curing agent.
• Especially for saturated rubbers which do not contain any reactive group capable of forming x-links.
• Peroxide does not enter into the polymer chains but produces radicals which form C-to-C linkages with adjacent polymer chains.
• Eg. dicumyl peroxide, zinc peroxide, benzoyl peroxide, 2,4-chlorobenzoyl peroxide & 2,5-bis(t-butylperoxy)-2,5-dimethylhexane.
• Applicable to both diene (NR) and saturated (silicone, urethane, ethylene-propylene, etc.) rubbers.
• The mechanism of crosslinking using peroxides is a homolytical one. At the beginning of the vulcanization process, the organic peroxide splits into 2 radicals, according to the equation :

RO:OR → 2RO^•

• The free radicals formed as a consequence of the decomposition of the peroxide, abstract hydrogen atoms from the elastomer macromolecules, converting them into macroradicals.

~CH₂C(CH₃)=CHCH₂ + RO^• → ROH + ~CH₂C(CH₃)=CHHC^•~

• The resulting macroradicals react each other by forming carbon – carbon intermolecular bridges.

Side Reactions

• Simultaneously with these crosslinking processes, side reactions, which reduce vulcanization (crosslinking) yield, occur. Thus, peroxides can react with the components of the compound, i.e., antioxidants, plasticizers, extenders, etc., and can be deactivated
• Other side reaction can take place to the radical centers formed on the elastomer backbone. These radicals can disproportionate leading to a saturated molecule and an unsaturated one.

Mechanism of Peroxide Vulcanisation

• The mechanism of vulcanization (crosslinking) with peroxides is exemplified below involving natural rubber and benzoyl peroxide :

Peroxide Curing Mechanism

 Curing Agent & Vulcanizate Properties

• Dicumyl peroxide performs cross-linking of NR, SBR, nitrile rubber, resulting in vulcanizates with good cold and aging resistance.
• Higher tensile strength of the vulcanisates obtained with peroxides as vulcanizing agents, can be obtained by the addition of small amounts of sulfur, amines or unsaturated compounds.
• By comparison with the vulcanization with sulphur and accelerators, peroxides produce a lower reaction rate and the resulting vulcanizates have a lower tensile strength, scorching tendency and unpleasant smell.

Availability

• Most peroxides are available as:

1. liquid (90% – 98% active),
2. powders (40% – 50% active), or
3. pastes made from silicone fluids and gums (20% – 80% active) to facilitate handling and dispersion

Comparison with other x-link systems

• C-C crosslinks, peroxide initiated.
• The material has a tensile strength about 40% that of the S_x network. Again, this is probably due to the immobility of the carbon-carbon bond.

Strength of different cure systems

• Advantages of Peroxide Cure:

1. Applicable to both saturated & unsaturated rubber.
2. Transparent products due to the absent of sulphur that causes staining of the vulcanisates.
3. C-C x-link is more stable and thus has good ageing resistance.
4. Excellent resistance to compression set at high temperatures (70^o – 100^oC) due to the absence of ZnO and Stearic acid (activators in S-vulcanization).
5. No x-link reversion.

• Disadvantages of Peroxide Cure

1. Scorch rather easily (Scorchy).
2. Slow cure rate with no delayed-action.
3. Long cure times for completion to obtain the best heat resistance.
4. Poor hot tear strength.
5. Incompatibility with chemical antiozonants.
6. Exposure of rubber compound to the air (oxygen) gives a sticky rubber surface.

VULCANIZATION SYSTEMS

Vulcanization Systems

1. Conventional Cure system
2. Efficient Vulcanizing (EV Cure) system
3. Semi-EV Vulcanizing (Semi-EV Cure) system which also includes sulphurless (sulphur donor) cure.
4. Non-Sulphur Vulcanization System

• Conventional, EV and semi-EV systems are based on sulphur/accelerator ratio and applicable only to NR as well as isoprene & butadiene based synthetic rubbers such as SBR, NBR, IIR and EPDM (unsaturated rubbers).

Conventional Sulphur Cure System

• It contains high proportions of sulphur (2.0-3.5 phr).
• low proportions of accelerator (0.4-1.2 phr)
• accelerator to sulphur ratio is 0.1-0.6
• sulphur to accelerator ratio > 1.0
• (low accelerator to sulphur ratio = high sulphur to accelerator ratio)

Characteristics of Conventional Cure System

• Network will be mainly polysulphidic (above 65%), which are thermally unstable.
• Fair degree of wasted sulphides and main chain modification.
• Excellent mechanical strength
• High set
• Poor heat & ageing resistance

Typical crosslink structures in Conventional System

EV Sulphur Cure System

• Contains very little sulphur (0.4-0.8 phr)
• High proportion of accelerator (2-5 phr)
• Accelerator to sulphur ratio is 2.5-12
• Efficient use of sulphur
• Mainly monosulphidic bonds (75% mono & 25% disulphidic)

di- and mono-sulphidic x-link

Sulphurless Cure in EV System

• Can employ sulphurless cure where there is no elemental sulphur present.
• In sulphurless cure the sulphur available for cross-linking is donated by partial decomposition of sulphur containing accelerators (sulphur donors).

Sulphur Donating Accelerators

Properties of EV Cure Vulcanizate

• low set and slightly lower mechanical strength
• Excellent oxidative ageing resistance
• Excellent heat and reversion resistance
• Poor flex-fatigue properties – not suitable for dynamic applications.

Semi-EV Sulphur Cure System

• Sulphur levels are intermediate between conventional system and EV system.
• Used for  a compromise in cost and/or performance.
• Particular application in NR where a compromise between heat ageing and fatigue life is sought after.

Sample Formulation

Formulations for Sulphur Vulcanization Systems

CROSSLINK DENSITY IN RUBBER VULCANIZATES

Crosslink Density

• Crosslink density is defined as the number of crosslink points per unit volume.
• The value of crosslink density may be in the order of 10^-3 to 10^-5 mol/cm³ for a typical rubber material, corresponding to 15 to 1500 monomer units between the crosslinks.
• The elastic force of retraction, elasticity, is directly proportional to the crosslink density.
• Reversion is due to the loss of crosslink density as a result of non-oxidative thermal ageing.

Qualitative Crosslink Density

Rheometer Curves

The torque values of the rheometer curve tells us the dgree of crosslink density in the vulcanizates. The higher the torque the greater the crosslink density.

This enable us to qualitatively compare crosslink densities developed from various compounds should we be able to have their curves printed on a single plot of torque versus time.

Chemical & Physical X-link Density

• Crosslink density can further be divided into two: chemical and physical crosslink density.
• Chemical crosslink is the contribution of the pure chemical crosslinks that result when a rubber material is vulcanized.
• The physical crosslink density is composed of both chemical crosslinks and chain entanglements, and is the crosslink density that is physically measured by e.g. stress-strain measurements.

Crosslink Density and Vulcanizate Properties

• Crosslink density is fundamental for polymeric networks, as it determines many physical properties of the resulting material [Coran, 1994].

dependence of properties on crosslink density

OVERVIEW OF RUBBER VULCANIZATION

Vulcanization is the process whereby viscous and tacky raw rubber is converted into an elastic material through the incorporation of chemical crosslinks between the long molecular chains.  As a result, loose individual rubber chain molecules are linked to each other by atomic bridges composed of sulfur atoms or carbon-to-carbon bonds.

Crosslink Formation in Sulfur Vulcanization

Sulphur, accelerator, zinc oxide & stearic acid constitute the ‘cure system’.  Zinc oxide reacts with stearic acid to form zinc stearate and together with the accelerator they speed up the rate at which sulfur vulcanization occurs.  With sulfur alone, the curing process might take hours. With this curing system, it can be reduced to minutes.

Crosslinking Reaction Scheme

The following reaction scheme may be written for sulfur vulcanization:

From compound (II) or by reshuffling the crosslinks from (III), intramolecular bridges (cyclic structures) may form.

(Adapted from http://trip.cc.tut.fi/portfolio/english.html)

Network maturing and competing side reactions also occur which do not lead to effective crosslinks. For example thermal decomposition may lead to the following reactions :

1. R-S_x-Ac → Cyclic sulphides + Dienes + ZnS (Degradation)
2. R-S_x-Ac → R-S_x-y-Ac + S_x-1 (Desulphuration)
3. R-S_y-R → R-S-R + S_y-1 (Monosulphidic crosslinks)
4. R-S_x+y-R + Ac-S_x-Ac → R-S_x-R + Ac -S_y+z -Ac  (sulphur exchange)

Only crosslink structures which form bridges between rubber chains are useful as stress-bearing members contributing to elasticity and strength of rubber vulcanizates. These are polysulphidic, disulphidic and monosulphidic crosslinks.

Polysulphidic Crosslink

This is a crosslink structure in which two rubber chains are bridged by a chain of 3 or more sulphur atoms.  It is a weak and labile crosslinks which is not thermally stable. However it does have high influence on mechanical properties of the rubber vulcanizates.

Disulphidic Crosslink

In disulphidic crosslink structure two sulphur atoms bridge the two rubber chains. The proportion of disulphidic crosslinks is seldom more than 20 – 30% in both conventional and efficient vulcanization (EV) systems.

Monosulphidic Crosslink

This is a crosslink structure in which two rubber chains are bridged by a chain of one sulphur atom. It is a thermally stable structure with high influence on reversion resistance and ageing.

Non-Crosslink Structures

Intra-chain cyclic sulphides, pendant sulphidic group with accelerator fragments, vicinal crosslinks and the formation of conjugated diene and triene on the rubber backbone chain do not contribute to elasticity and strength of the vulcanizates. These unwanted structures waste the sulphur.

Determinant of Crosslink Structure

The crosslink structure so produced depends on:
1. The nature of the rubber
2. Ratio of sulphur to accelerator
3. Vulcanization temperature

COMPOUNDING INGREDIENTS (02)

Processing Additives

Processing additive is a compounding ingredient which improves the processability of a rubber compound. Various types of processing additives are available for various functions. They are available, to name a few, in the forms of lubricant, dispersing agent, wetting agent, plasticizer, blowing agent, factice, softener, tackifying agent etc.

BRIEF DESCRIPTION OF PROCESSING ADDITIVES

Processing Additive	Application	Examples
Chemical peptizer	Reduces rubber viscosity  by chain scission	2,2’-Dibenzamidodiphenyl-  disulphide
Physical peptizer	Reduces rubber viscosity  by internal lubrication	Zinc soaps
Dispersing agent	Improves filler dispersion  Reduces mixing time  Reduces mixing energy	Mineral oils, Fatty acid esters  Metal soaps/Fatty alcohols
Lubricant	Improves compound flow  and release	Mineral oils/Metal soapsFatty  acids/Fatty acid esters      Fatty acid amides/Resin  blends
Tackifier	Improves green tack	Hydrocarbon/Phenolic resins
Homogenizing agent	Improves rubber blend  compatability. Improves  compound  uniformity	Resin blends
Stiffening agent	Increases compound  hardness	High styrene resin rubber  Phenolicresins, Trans-  Polyoctenamer
Mold release agent	Eases product release  from mold.  Decrease  mold contamination	Organosilicones                Fatty acid esters              Fatty acid amides            Metal  soaps
Softener	Lowers compound  hardness	Mineral oils
Plasticizer	Improves product  performance at low and  high temperatures	Aromatic di- & tri-esters  Aliphatic diesters, Alkyl &  alkylether mono-esters

COMPOUNDING INGREDIENTS (01)

There are five (5) major categories of compounding ingredients. They are elastomer as a major component, curatives for crosslink formation, antidegradants for protection, fillers for reinforcement and cost dilution and processing additives for ease of processing.

Elastomer is the key ingredient and thus always at the top of the formulation list. It can be a single elastomer or a blend of two or more different rubbers.

Curatives

Curatives are the main ingredients which play an important role  in the the formation of crosslink structure in a vulcanized rubber. The curatives are classified as:

• Vulcanizing/curing agents such as sulphur, peroxide etc.
• Activators such as zinc oxide and stearic acid.
• Accelerators such as CBS, TMTD, MBT, DPG etc.
• Scorch retarders or prevulcanization inhibitors such as salicyclic acid, NDPA, CTP etc.

Curing agents are chemicals which impart three dimensional network  or crosslink structure to the rubber molecular chains. In doing so they react with active sites in the rubber to form crosslinks between the chains.

Activators in rubber formulation are typically metal oxides (zinc oxide), and fatty acids (stearic acid). Together they form rubber soluble complexes with the accelerators making them better able to react with sulphur to develop cross-links.

Accelerators are used in small amounts to increase the rate of cure and the efficiency with which sulphur is utilised in crosslinking.  Accelerators also control the onset and extent of reaction between sulphur and rubber.

Retarders are added to the rubber formulation in order to reduce the tendency of a rubber compound to vulcanize prematurely (delay the onset of cure) by increasing  scorch delay.

Antidegradants

Degradation of rubber products is caused by attacks of oxygen, ozone and weathering on the rubber molecular structure.  Rubber vulcanizates become brittle when they age.  Aging is caused by a severe attack of oxygen and is accelerated by heat.

An antidegradant is a compounding ingredient added into a rubber compound to retard the deterioration caused by oxidation, ozone, light or combination of these. Hence antioxidants are added to protect the rubber against oxygen attack while antiozonants are added to protect the rubber against ozone attack.

Fillers

Fillers are materials that when added to a rubber formulation will either lower the compound cost or improve vulcanizate properties .

Fillers can be classified into three categories, which are as follows:

Diluents or extenders which are intended primarily to occupy space and are mainly used to lower the formulation cost. This type of filler is also called non-reinforcing filler with particle between 1,000 and 10,000 nm (1 to 10 μm).

Functional or reinforcing fillers are those with particle size range from 10 to 100 nm (0.01 to 01 μm)which when added to the rubber compound improve the modulus and failure properties (tensile strength, tear resistance and abrasion resistance) of the final vulcanizate.

In-between these two types are semi-reinforcing fillers with particle size range from 100 to 1000 nm (0.1 to 1μm).

In rubber-manufacturing industry fillers are generally classified into black (carbon black) and non-black or white fillers. These black or non-black fillers thus may come as diluents, reinforcing or semi-reinfrocing fillers depending on their particle sizes.

Adding Ingredients into Internal Mixer

ADD IN (Internal Mixer)

Duration	Operation on Internal Mixer
0’	Set the mixer temperature to achieve the discharge conditions (110 to 125oC dump temperature). Close the discharge gate, start rotor.
½’	Raise ram.  Add the rubber. Lower ram.
½’	Raise ram.  Add peptiser.  Lower ram.
1’	Raise ram.  Add stearic acid. Lower ram.
1½’	Raise ram.  Add ZnO  and one-half the filler. Lower ram.
1½’	Raise ram.  Add the remainder of the filler. Lower ram.
1’	Raise ram.  Add the sulphur. Clean the mixer throat & the top of the ram. Lower ram.
1’	Adjust the mixer to give 110 to 125oC dump temperature.
	Dump the compound onto a 2-roll mill.

SHEET OUT (Open Mill)

Duration	Operation on Open Mill
2’	Set the nip size to 8 mm and maintain the roll temperature at 70 ± 5oC.
1’	Set the nip size to give a sheet with minimum thickness of 6 mm. Pass the sheet through the rolls 4 times, folding it back onto itself each time.

• Allow, at least 24 hours, for maturation after which the compounded sheet is ready for subsequent processes (moulding /curing).