Oil & Adhesiv

7/27/2019 Oil & Adhesiv

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Oil Viscosity - How It's Measured and Reported

According to the Society of Tribologists and Lubrication Engineers (STLE), viscosity

is one of an oils most important physical properties. It is often one of the first

parameters measured by most oil analysis labs because of its importance to oil

condition and lubrication. But what do we really mean when we talk about an oils

viscosity?

A lubricating oils viscosity is typically measured and defined in two ways, either

based on its kinematic viscosity or its absolute (dynamic) viscosity. While the

descriptions may seem similar, there are important distinctions between the two.

An oils kinematic viscosity is defined as its resistance to flow and shear due to

gravity. Imagine filling a beaker with turbine oil and another with a thick gear oil.

Which one will flow faster from the beaker if it is tipped on its side? The turbine oil

will flow faster because the relative flow rates are governed by the oils kinematic

viscosity.

Now lets consider absolute viscosity. To measure absolute viscosity, insert a metal

rod into the same two beakers. Use the rod to stir the oil, and then measure the force

required to stir each oil at the same rate. The force required to stir the gear oil will be

greater than the force required to stir the turbine oil. Based on this observation, it

might be tempting to say that the gear oil requires more force to stir because it has a

higher viscosity than the turbine oil. However, it is the oils resistance to flow and

shear due to internal friction that is being measured in this example, so it is more

correct to say that the gear oil has a higher absolute viscosity than the turbine oil

because more force is required to stir the gear oil.

For Newtonian fluids, absolute and kinematic viscosity are related by the oils

specific gravity. However, for other oils, such as those containing polymeric viscosity

index (VI) improvers, or heavily contaminated or degraded fluids, this relationship


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does not hold true, and can lead to errors if we are not aware of the differences

between absolute and kinematic viscosity. For a more detailed discussion on absolute

versus kinematic viscosity, refer to the article Understanding Absolute and

Kinematic Viscosity by Drew Troyer.

Capillary Tube Viscometer Test Method

The most common method of determining kinematic

viscosity in the lab utilizes the capillary tube viscometer

(Figure 1). In this method, the oil sample is placed into a

glass capillary U-tube and the sample is drawn through thetube using suction until it reaches the start position

indicated on the tubes side. The suction is then released,

allowing the sample to flow back through the tube under

gravity. The narrow capillary section of the tube controls

the oils flow rate; more viscous grades of oil take longer

to flow than thinner grades of oil. This procedure is described in ASTM D445 and

ISO 3104.

Because the flow-rate is governed by resistance of the oil flowing under gravity

through the capillary tube, this test actually measures an oils kinematic viscosity. The

viscosity is typically reported in centistokes (cSt), equivalent to mm2/s in SI units,

and is calculated from the time it takes oil to flow from the starting point to the

stopping point using a calibration constant supplied for each tube.

In most commercial oil analysis labs, the capillary tube viscometer method described

in ASTM D445 (ISO 3104) is modified and automated using a number of

commercially available automatic viscometers. When used correctly, these

viscometers are capable of reproducing a similar level of accuracy produced by the

capillary tube manual viscometer method.

Figure 1. Capillary

Tube Viscometer
http://www.machinerylubrication.com/Read/294/absolute-kinematic-viscosityhttp://www.machinerylubrication.com/Read/294/absolute-kinematic-viscosityhttp://www.machinerylubrication.com/Read/294/absolute-kinematic-viscosityhttp://www.machinerylubrication.com/Read/294/absolute-kinematic-viscosityhttp://www.machinerylubrication.com/Read/294/absolute-kinematic-viscosity


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Stating an oils viscosity is meaningless unless the temperature at which the viscosity

was measured is defined. Typically, the viscosity is reported at one of two

temperatures, either 40C (100F) or 100C (212F). For most industrial oils, it is

common to measure kinematic viscosity at 40C because this is the basis for the ISO

viscosity grading system (ISO 3448). Likewise, most engine oils are typically

measured at 100C because the SAE engine oil classification system (SAE J300) is

referenced to the kinematic viscosity at 100C (Table 1). Additionally, 100C reduces

the rise of measurement interference for engine oil soot contamination.


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Rotary Viscometer Test Method

A less common method of determining an oils viscosity

utilizes a rotary viscometer. In this test method, the oil is

placed in a glass tube, housed in an insulated block at a

fixed temperature (Figure 2). A metal spindle is then

rotated in the oil at a fixed rpm, and the torque required to

rotate the spindle is measured. Based on the internal

resistance to rotation provided by the shear stress of the

oil, the oils absolute viscosity can be determined.

Absolute viscosity is reported in centipoise (cP),

equivalent to mPas in SI units. This method is commonly

referred to as the Brookfield method and is described in

ASTM D2983.

While less common than kinematic viscosity, absolute viscosity and the Brookfield

viscometer are used in formulating engine oils. For example, the W designation,

which is used to denote oils that are suitable for use at colder temperatures, is based in

part on the Brookfield viscosity at various temperatures (Table 2).

Based on SAE J300, a multigrade engine oil that is designated as SAE 15W-40 must

therefore conform to the kinematic viscosity limits at elevated temperatures according

to Table 1 and the minimum requirements for cold cranking as shown in Table 2.

Viscosity Index

One other important property of an oil is viscosity index (VI). The viscosity index is a

unitless number, used to indicate the temperature dependence of an oils kinematic

viscosity. It is based on comparing the kinematic viscosity of the test oil at 40C, with

the kinematic viscosity of two reference oils - one of which has a VI of 0, the other

with a VI of 100 (Figure 3) - each having the same viscosity at 100C as the test oil.

Figure 2. Rotary

Viscometer


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Tables for calculating VI from the measured kinematic viscosity of an oil at 40C and

100C are referenced in ASTM D2270.

Figure 3. Determination of Viscosity Index (VI)

Figure 3 shows that an oil that has a smaller change in kinematic viscosity with

temperature will have a higher VI than an oil with a greater viscosity change across

the same temperature range.

For most paraffinic, solvent-refined mineral-based industrial oils, typical VIs fall in

the range of 90 to 105. However, many highly refined mineral oils, synthetics and VI

improved oils have VIs that will exceed 100. In fact, PAO-type synthetic oils

typically have VIs in the range 130 to 150.

Viscosity Monitoring and Trending

Monitoring and trending viscosity is perhaps one of the most important components

of any oil analysis program. Even small changes in viscosity can be magnified at

operating temperatures to the extent that an oil is no longer able to provide adequate

lubrication. Typical industrial oil limits are set at 5 percent for caution, and 10


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percent for critical, although severe- duty applications and extremely critical systems

should have even tighter targets.

A significant reduction in viscosity can result in:

Loss of oil film causing excessive wear

Increased mechanical friction causing excessive energy consumption n Heat

generation due to mechanical friction n Internal or external leakage

Increased sensitivity to particle contamination due to reduced oil film

Oil film failure at high temperatures, high loads or during start-ups or coast-

downs.

Likewise, too high a viscosity can cause:

Excessive heat generation resulting in oil oxidation, sludge and varnish build-

up

Gaseous cavitation due to inadequate oil flow to pumps and bearings

Lubrication starvation due to inadequate oil flow

Oil whip in journal bearings

Excess energy consumption to over- come fluid friction

Poor air detrainment or demulsibility

Poor cold-start pumpability.

Whenever a significant change in viscosity is observed, the root cause of the problem

should always be investigated and corrected. Changes in viscosity can be the result of

a change in the base oil chemistry (a change in the oils molecular structure), or due to

an ingressed contaminant (Table 3).


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Viscosity changes may require additional tests, such as: acid number (AN) or Fourier

transform infrared spectroscopy (FTIR), to confirm incipient oxidation; contaminant

testing to identify signs of water, soot or glycol ingress; or other less commonly used

tests, such as the ultracentrifuge test or gas chromatography (GC), to identify a

change in the base oil chemistry.

Viscosity is an important physical property that must be monitored and controlled

carefully because of its impact on the oil and the oils impact on equipment life.

Whether measuring viscosity onsite using one of many onsite oil analysis instruments

capable of determining viscosity changes accurately, or whether sending samples

routinely to an outside lab, it is important to learn how viscosity is determined, and

how changes can impact equipment reliability. A proactive approach must be taken to

determine the condition of the equipments lifeblood - the oil!


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Many factors need to be considered when devising meaningful viscosity tests for

adhesives and sealants.

Most people in the adhesives and sealants industry are aware of viscosity and realize

that it characterizes the way a material will flow. More technically speaking, viscosity

refers to a materials resistance to flow; it can be measured in various ways, dependingon the nature of the application. An evaluation of how the material is being processed in

manufacturing or how the end user will try to apply it is the basis for determining the type

of viscosity test that should be performed.

Measuring Caulk ViscosityImagine a caulk gun. The squeezing force required to expel the material out of the nozzle

is important to know. If the caulk is not sufficiently viscous, too much will come out andsome will be wasted. In addition, the caulk that does come out probably wont hold its

position on the substrate. Should the caulk be too viscous, it may not come out at all. In

either case, the material is rejected and the customer is unhappy.

Quality control (QC) in manufacturing requires that a test method be established to

predict this type of problem before it happens. The way to simulate the guns squeezing

action on the caulk is through an extrusion test. An instrument called a texture analyzer-

with an extrusion cell-is ideally suited for this type of test. This type of instrument, also

sometimes called a universal tester, essentially applies compression or tension to a

material. In this case, the instrument simulates the process of pushing the material

through a tube and out of the nozzle.

During this type of test, the extrusion cell is filled with the sample material. A small

circular opening in the bottom of the cell represents the nozzle diameter through which

the caulk material will be expelled. A disc-shaped plunger is brought down into contact

with the material in the top of the cell. The test method is to move the plunger at a

defined speed down into the cell. The material is expelled out of the opening in the cell

bottom. A load sensor in the instrument monitors the resistive force experienced by the

plunger as it pushes downward into the material. The key data recorded is the force as a

function of time and distance, which provides a complete characterization of the

squeezing action required to extrude the caulk out of the gun.

Technically speaking, this type of instrument is not a viscometer, but it nonetheless gives


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a measure of the caulks viscosity. The output parameter is the force required to extrude

the caulk. This correlates to how much work the user will perform to get the caulk out of

the gun. It is crucial to select a test method that correctly characterizes what is happening

to the material. Since many adhesive and sealant materials are applied from a gun of

some type, the texture analyzer becomes a relevant tool to perform the tests.

Shear RateOne key concept that is often not understood is that a materials viscosity is not a single

point measurement; it typically depends on a number of factors. Take the previous

example of the caulk. The speed at which the plunger pushes into the material will have a

direct effect on the force resistance that is measured. The faster the plunger moves into

the caulk, the higher the force that will be measured. Therefore, the rate of shearing

action on the caulk is greater and may result in different viscosity values.

The primary challenge to understanding viscosity is to recognize that shear rate plays a

key role in determining the resistance to flow. As previously stated, it is necessary to

think about how the material will be processed or handled when in use. The shearing

action is what should be analyzed in order to arrive at a relevant test method for

measuring viscosity.

For liquid glues, the use of a rotational viscometer running at different speeds simulates

what is happening to the material when stored in a bottle or applied to a substrate. The

shearing action of the glue sloshing around in the bottle is relatively low, while the

application process for placing the glue can be rather high. Once the glue is applied, the

shearing action is again pretty low, because only gravity is causing it to spread out

farther. This assessment of how the glue is used leads to the selection of the appropriate

viscometer.

Dynamic ViscosityRotational viscometers are perhaps the most popular tool for measuring viscosity. The

spindle, when inserted into the test material, rotates at various discrete speeds, thereby

shearing the material at precise shear rates. The viscometer measures the amount of

torque resistance imparted by the material against the rotating spindle at each speed.

The torque measurement is quantified as the shear stress acting on that portion of the

spindle surface that is immersed in the material. The measured parameter (torque) and

the control parameter (spindle speed) are combined into an equation that defines

dynamic viscosity as the ratio of shear stress to shear rate:


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Dynamic Viscosity = Shear Stress

Shear Rate

Centipoise (cP) is the unit of measurement most often used in North and South Americato quantify rotational viscosity; the milli-Pascal second (mPa.s) is typically used outside

the Americas (1 cP = 1 mPa.s). This one-for-one equivalency minimizes potential

problems when comparing data between multinational manufacturing plants.

Distilled water is the benchmark reference for comparing the viscosity of all other

materials. The National Institute of Standards and Technology (NIST) specifies that the

viscosity of water is 1cP when measured with a capillary viscometer at 20

Shear RateThe choice of shear rate is determined by analyzing how the material is processed.

Imagine that the material is sandwiched between two plates separated by a fixed

distance. If the bottom plate is kept stationary and the top plate moves at a defined

velocity, then the shear rate is the ratio of the moving plate velocity (V) to the distance

separating the plates (X):

Shear Rate = V

X

The reciprocal second (s-1) is the unit of measurement for shear rate. This approach to

quantifying shear rate assumes that the fluid behaves in a uniform (laminar) way, as

shown by the arrows in Figure 2. The layers of molecules in the material remain together

in the same plane and slide over each other in such a way that the closer they are to the

moving plate, the faster each layer moves.

The relevant shear rate(s) for an application can easily be computed by applying theabove equation. When placing adhesive on a substrate with a scraper, the substrate and

the scraper represent the two plates. To lay down a bead of adhesive 1 mm thick in an

automated operation where the substrate is moving at 50 cm/sec, the shear rate is 500

sec-1. This becomes one of the shear rates that should be used in a QC test.


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Figure 3. Flow Curves for Viscosity vs. Shear Rate

Flow BehaviorWith the above concept concerning relevant shear rates in mind, it is possible to see

more clearly that viscosity may not be a single number for a given material. The idea is to

test a material with a viscometer at different rotational speeds (i.e., different shear rates)and see what viscosity values are measured.

The most common flow behavior is called pseudoplastic (or shear thinning), where a

materials viscosity decreases with increasing shear rate. Figure 3 is a rheogram that

illustrates the behavior for a liquid adhesive tested at relatively low shear rates. Most

adhesive and sealant materials exhibit pseudoplastic behavior; for example, autobody

fillers are highly viscous materials with obvious pseudoplastic flow. Figure 4 shows that

the apparent viscosity decreases dramatically as the shear rate increases to 100 s-1,

then decreases more gradually above that shear rate.

Figure 4. Viscosity vs. Shear Rate in an Autobody Filler

A related issue that affects measured viscosity values is the length of time the shearingaction is applied to the material. When a material is sheared at a constant rate and the

measured viscosity decreases with time, the flow behavior is called thixotropic. Many

adhesive and sealant materials exhibit thixotropic behavior; it is important to consider

whether the viscosity recovers to its original value once the shearing action stops, and

whether this type of behavior is wanted or expected. Once the bead of glue has been

placed on the substrate, does the viscosity recover so that the material holds its shape

and doesnt spread out like water on a flat surface?

Temperature is yet another parameter to consider when measuring viscosity. As

temperature increases, most materials exhibit a decrease in viscosity. Therefore, the

temperature at which the QC check is performed should be defined in order to ensureconsistent results. When new adhesives or sealants are formulated, it is customary to

perform a temperature profile test. The material is tested at a constant shear rate while

the temperature cycles between a minimum and maximum value. The resulting graph

provides QC with a reference chart for expected viscosity values at different

temperatures.


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Figure 5. Yield Stress Flow Curve for Floor Adhesive at Room Temperature

Yield StressSome users may wish to apply an adhesive to a particular area and have it stay put (hold

its position) until pushed with a blade or trowel. Perhaps a second piece is then pressed

into place on the adhesive layer. Flooring adhesive is an example of this type of

application. The property corresponding to that behavior is called yield stress and it

represents the amount of applied force at which a solid material begins to flow like a

liquid. The yield strain is the degree of sample deformation that results from the applied

yield stress. These two values appear on a stress-strain curve at the yield point.

One easy method for testing the yield stress of adhesive/sealant materials is to use a

rotational viscometer running at a very low speed (e.g., 0.01-1 rpm). The instrument uses

a vane spindle immersed in the material and running at a constant speed. The calibrated

spring inside the viscometer winds up, applying increasing force to the vane spindle, but

the sample resists moving. The sample then begins to deform slightly, until its structure

breaks down and it starts to flow. The measured torque value is converted into a stress

value (in Pascals or dyne-cm), and this defines the yield stress for the material.

Figure 5 shows the type of flow curves that can be generated when measuring yield

stress with a viscometer. In this case, the instrument is specifically configured to measure

yield stress and present the data in the appropriate scientific units for this parameter.

Four curves show good repeatability for the yield stress of this particular material. The

test is easy to run and typically takes less than one minute.

The controlled stress rheometer is an alternative instrument for measuring yield stress. If

the quantity of material available for testing is limited, the cone/plate system is the best

choice. Practically speaking, the cone/plate system is also ideal if temperature control is

required; the time required to bring the sample to temperature is minimized when using

cone/plate. The test method is to run a shear ramp in which increasing torque is applied

to the spindle until it begins to rotate. The torque value where the rotation of the spindle

commences is the yield stress.

Controlled stress rheometers are more expensive instruments than benchtop

viscometers. If the budget allows for the controlled stress rheometer, this is the preferred

approach, because the instrument can also perform the viscosity flow curve test and

check for pseudoplastic behavior, thixotropy, and recovery of material after being

sheared.


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Figure 6. Cure Test Data for Epoxy Material

Curing TestOne final thought on viscosity testing applies to materials like epoxies, where viscosity

builds with time as the adhesive gains strength. The objective is to know the endpoint of

the reaction (i.e., the final strength of the epoxy and how long it takes to get there). A

curing algorithm can be used with a standard digital viscometer that is equipped with a

special program that enables the instrument to change speeds automatically.

The viscometer measures viscosity continuously as the spindle rotates initially at a high

speed (e.g., 50 or 100 rpm). When the measured torque reaches 95% of capacity, the

viscometer downshifts its speed by an order of magnitude and the test continues withoutinterruption. This process repeats itself several times while the viscometer continues to

report the increasing viscosity value of the epoxy (see Figure 6). At the conclusion of the

test, the final viscosity value and the time needed to reach it are reported.

SummaryMany factors must be considered when devising meaningful viscosity tests for adhesives

and sealants. Careful thought regarding how the material is processed in manufacturing

or applied by the end user will result in specifying tests that are more relevant for an

effective QC program.

If they are not sure about the best approach to take, adhesives and sealants

manufacturers should contact an instrument manufacturer and review the test plan with

them. Instrument manufacturers are the experts on getting the most out of their

equipment and can help ensure that the most appropriate tests are run for a given

application.

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