chemistry lubricants

7/29/2019 Chemistry Lubricants

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LUBRICANTS

Friction:Re sistance observed bet w ee n t w o

m o v in g o r slid in g su r fa ce s cr e at in g w e a r

a n d t e a r.

Lubricant:

A ny su bst an ce in tro du ce d b et w e e n t w o

m o v in g o r slid in g su rfa ce s w it h a vie w t o

r e du ce t h e f rict io n a l r e sist a n ce b e t w e e n

t he m is k no w a s a lu br ica nt .

Lubrication:The proce ss of int roducing lubricant

b et w e e n m o v in g/ slid in g p ar ts is k no w n

aslubricat ion

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Fun ct ion s o f Lub rican to To r e d uce f rict io n a l r e sist a n ce b e t w e e n su r fa ce s a n d

re d uce d efo rm a tio n, w e ar a nd t e ar b et w e e n m o vin g/ slid in gsurfaces.

o To re du ce lo ss o f e ne rgy in t he fo rm o f h ea t (Co ola nt ).o To re d uce w a st e o f e n ergy i. e. , t o in cre ase e ff icie n cy o f

machines.o To r ed u ce ir re gu la r e xp a nsio n o f m e t als.o To re du ce w e ld in g o f t he t w o su rfa ce s.o To re du ce o r avo id ro ugh re lat ive m o t io ns o f m o vin g /

sl iding part s.o To re d uce r un nin g a nd m a in te n an ce co st o f t h e m a ch in e .o To re d uce t h e le a ka ge o f ga se s u nd e r h igh p re ssu re like a

se a l o r Te f lo n .


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M e cha nism s of Lub ricat ion

1 . Flu id -f ilm (o r) Th ick-f ilm (o r)

Hydrodynam ic lubr icat ion

2 . Bo u nd ar y lu b rica t io n (o r)Thin- f ilm lubr icat ion

3. Extre m e p ressure lubricat ion


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1 . Fluid Film / Thick-Fi lm / Hydro dyna m ic Lubricat ion

(~ 100 0 )

Characteristics:

o Th e su rfa ce s a re se pa rat ed b y a t h ick-f ilm (at le ast

1 00 0 t hick) an d h en ce t he re is n o d ire ct su rface t osurface contact .

o N o w e ld in g o f ju n ct io n s.

o Since th ick f ilm lubr icant covers/ f ills the i rregu lar it ieso n t h e b ot h su rfa ce s, t h ere is n o d ire ct co nt act

b et w e e n m at eria l su rfa ce s a nd so t h e w e ar is re du ce d.


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In Fluid Film Lub ricat ion ,th e Lub ricat ing prop ert ies de pe nd o n:

o V iscosity o f lubr ican t (Lubr ican t shou ld be

of low v iscosity ).

o Thickness of lubr icant layer.

o Re la t ive ve lo cit y a n d a re a o f m o vin g/ slid in gsurfaces.


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Example: Shaft running (Journal bearings)

Lu b rica nt o il co ve rs t h e ir re gu la rit ie s o f t h e sh aft a s w e ll a s t h ebea ring surfaces.


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o Exam ples w here f luid f i lm lubricat ion is used are :

i) So p h ist ica te d in st r um e n t s

i i) Light m achines l ike w atches, guns, sew ing m achines etc .

o Exa m ple s for f lu id - film lubr ica nts a re :

Hyd roca rbon oilsa re co n sid e r ed t o b e sa t isf act o r y lu b r ica n t s.

To m a in t ain visco sit y t h ro u gh o u t life cycle s lo n g ch a in p o lym e rs a reu se d a s b le n d e rs w it h n o r m a l h yd r oca r b on s o ils.

Sm a ll a m o u n t o f u n sa t ura te d h yd ro ca rb o n s p re se n t in h yd ro ca rb o n

o ils p ro d u ce d f ro m p e t ro le u m f ra ct io n s, w h ich ca u se s o xid at io n a n d

p r o du ce gu m m y like p r o du ct s. H e n ce , a n t i- oxid a n t like a m in o p h e n o la re u se d in jo u rn a l b e ar in gs w it h lu b rica nt


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M echan ism of f lu id f ilm lubr icat ion


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2 . Bou nd ary Lub ricat ion / Thin -f ilm

LubricationCharact e rist ics o f t h in f ilm lubrica t ion are :

o H igh viscosity-inde x.

o Re sist a nce t o h e at a n d o xid at io n .

o Good Oi liness.

o Lo w p o u r-p o in t .


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M echanism of Boun dary Lubr icat ion/ Thin- f ilmLubrication

Th is Lu b rica t io n t a ke s p la ce d u e t o :

o A d so r pt io n o f lu b rica t in g o ils t o b o t h su r fa ce s b y

physica l / chem ica l m eans.o Th e a dso rb ed laye rs o n t he b ot h m e ta l su rfa ce s

ca rr y t h e a pp lie d lo ad .

o Co -e ff icie nt o f fr ict io n, f = 0 .0 5 - 0 .1 5 a nd d ist an ce

b et w e en su rface s is t o b e t he o rd er o f t he d ist an ce

o f t h e a sp e rit ie s.


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For boundary lubrication, the lubricantmolecule should have:

(i) Long hydrocarbon chain with polar groups.

(ii) Polar groups promote spreading and orientationover the metallic surfaces at high pressure.

(iii) Lateral attraction between the chains.

(iv) Active groups or atoms, which can formchemical linkages metal or other surfaces.


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Exam ples of Bou nd ary lub ricat ion

Ve ge ta ble a nd a nim a l o ils (glyce rid e s o f h igh e r fat t y a cid s& t h eir so ap s).

o Th e se o ils e it h e r p hysica lly ad sorb e d to m e t a l surface s or re act

ch e m ica lly a t t h e m e t a l su r fa ce s.

o

A lt h o u gh t h e se o ils p o sse s gr e at e r a d h e sio n p r op e r t y a n d o ilin e ss, y ett hey t en d t o b re akd ow n at h igh t em p erat ure s. H en ce , fat ty acid s are

a dd e d t o m in e ra l o ils t o im p ro ve t h e o ilin e ss.

Grap hite an d M o lyb de nu m disu lp hide alo ne o r o ilsu sp e nsio n m ay b e u se d b e ca use :

o They have Low internal fr ict ion

o They can bear / w ithstan d com pression

o They are th erm al ly stable


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M echan ism of Bounda ry Lubricat ion


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3 . Extre m e-p ressure Lub ricat ion When moving/sliding surfaces are under very high pressure

and speed, a high local temperature is attained. In such conditions, liquid lubricants fail to stick and may

decompose and even vaporize. To avoid this, special additives are added to mineral oils. These

are called extreme-pressure additives.

Mechanism The extreme-pressure additives form on metal surfaces

more durable films, capable of withstanding very high loadsand high temperatures.

Fe-Chloride (mp)= 650oC, Fe-Sulphide (mp)= 1100oC

Examples: Organic compounds containing chlorine, sulphur and phosphorus.

o These compounds react with metallic surfaces, at prevailing high

temperatures, to form metallic chlorides, sulphide or phosphides.

o These metallic compounds possess high melting points.


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CLASSIFICATIO N O F LU BRICAN T

Based on Physical sta te , lub rican t s areclassifie d a s:

a ) Lubr icat ing oils o r liqu id lub rican t sb) Sem i solid lub rican t s or grea sesc) Solid lubricants

d ) Synt he t ic lubrican t se) Lub ricat ing em ulsions


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1. Lub ricat ing oils or l iqu id lub rican t s

Purpose:o Provide a continuo us f luid f i lm .o Provide a cool ing betw een th e surfaces.o Act as a sea ling agen t .o Act as corro sion pre vent ing m ate rials.

Prope rt ies of l iqu id lubricant s:o Low pre ssure i .e. , high b oil ing po int.o Ade qu ate viscosity fo r pa rticular ser vice con dition s.o Low cloud & pour point .

o Stabi lity to heat & decom posit ion a t t he o perat ing term s.o High oxidat ion resistan ce.

o N on-corrosive prop ert ies.


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Typ e s o f Liqu id Lu brican t s

a) Anim al and Vegetab le oi ls:- Usable under very h igh tem perature and heavy load. Disad vant ages of its usages are

1. Costly2. U nde rgo oxidation e asi ly in cont act w ith a ir and form s

gum m y and acidic prod ucts, and get th ickened .3. Tend ency to hydrolyze in cont act w ith m oist-a ir or aqu eou sm e d i u m .

So, t hey a re u sed as blending agent s w ith othe r m inera l o ils.

b) M inera l o i ls or pet ro leum o i ls:

- They are obta ined by dist i lla t ion of petroleum .o Length of hydrocarbon chain var ies bet w een 12 t o 50 carbon ato m s.o Short er- chain oi ls have low er v iscosi ty than t he longer- chain

hydrocarbons.


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Liquid lubricants are most widely used lubricants

because they are

1. Cheap

2. Available in abundance

3. Quite stable under service conditions.

o But they have poor oiliness charactercompared toanimal and vegetable oils.

o So, high molecular weight compounds like oleicacid, stearic acid are used to overcome this problem.

c) Blended oils:

o No single oil serves as the most satisfactory lubricantfor many of the modern machines. Hence, additivesare used to improve the properties. These blendedoils give desired lubricating property required for amachinery.


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Additivesused are:

a)Oiliness- carriers:o Coconut oil, caster oil, and palmitic, stearic and oleic acids.

b)Extreme-Pressure additives such as:

o Fatty esters or acids which form oxide film with metal surface.

o Organic materials containing sulphur.o Organic chlorine compounds.

o Organic phosphorous compounds.

o Some times lead (Pb) compounds could be used as high pressure

lubricants.c)Pour-point depressing additives:

o Phenol, condensation product of chlorinated wax with

naphthalene.


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d)Viscosity index improvers : hexanol

e)Thickeners : Polystyrene or polystyers

f)Antioxidants or inhibitors : Aromatic phenolic or amino compounds

g)Corrosion preventers : Phosphorous or Antimony organic

compounds

h)Abrasion inhibitors : tricresyl phosphate

i)Antifoaming agents : glycols and glycerol

j)Emulsifiers : sodium salts of sulphonic acid

k)Deposit inhibitors : detergents such as salts of phenol and

carboxylic acids


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2 . Sem i-So lid Lub rican t s o r Gre ases

o Sem i solid consist in g of a soap dispe rsed th rou gho uta l iqu id lub ricatin g oil.

- M ay be Petroleum oi l or synthe tic oi l w i th a specif ic add it ive.

Preparat ion:Sapo nif icat ion of fat (such as ta llow or fatt y acid) w i th a lkali

(l ike lim e, caustic sod a et c.,)

Add it ion o f the a bove to hot lubr icat ing oi l und er agitat ion

o To in cr e ase t h e h e a t r e sist a n ce o f gr e ase , in o r ga n ic so lidt h icke n in g a ge n t s ( like f in e ly d iv id e d cla y, b e n t o n it e , co llo id a lsilica , ca r bo n b lo ck e t c. ,) a re a d d e d .

o H a ve h igh e r sh e ar o r f rict io n al r esist a nce t h a n o ils a n d h e n cesu p p or t m u ch h e avie r lo a d at lo w e r sp ee ds.


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Applications of Greases:

o When oil cannot remain in place due to high load, low speed,

intermittent operation, sudden jerks etc.o Work at high temperature

o When external contamination may create problem

o When dripping or spurting of oil is undesirable

Types of greases:

o Ca based greases or cup-greases (up to 80oC & water resistant)

o Soda-based greases (up to 175oC and not water resistant)

o Lithium-based greases (up to 15oC and water resistant)

o Axle- greases lime with resin and fatty acids (water resistant &

less delicate equipment under high loads and at low speed)

o Graphite greases (rail axle boxes, machine bearings, tractor

& wire ropes)

o Soap stone (Steatite is relatively soft and may feel soapy when

touched, hence the name and rich in Mg).


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3 . SO LID LU BRICAN TS

Solid lub ricant s are u sed w he n:

o Ot her lubricant s can no t b e u sed

o Cont am inat ion und esirable

o Too h igh tem perat ure o r load a re involved

o Com bu stib le lub ricant s no t accep tab le

Exam ple s of solid lub rican t s used are :a) Graphite (or )

b ) M olybdenum disu lphidec) Sandstone , m ica, ta lc, etc.


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Exam p les o f So lid Lub rican t s

1.Graphite:o Very soapy in tou ch

o N on in f lam m ableo N ot oxidized in air below 37 5Co Oil + graphite o ild a g (internal

com bustion engines)o Water + Graphite aquadag

- Em ulsi fy ing agent ( tannin)

( food stu ffs indu str y)o Grease + graphite

graphite greases (at h igher t em p.)(It works upto 800o C

in High Load and High Temp. conditions, ex. in bearings,)

Talc


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Exam ples o f So lid Lub rican t s

2 . M olybdenum d isu lph ide :o Low coeff icient o f fr ict ion (0 .005 -0.01)o Stab le in air up t o 4 00 C

San dsto ne , talc or m ica are also used a s solid lub ricant s.

Sa

ndwitch-likestr


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4. Synthetic Lubricants

Petroleum lubricants + specific additives meets most drastic

and severe conditions

Temp. range ~ -50oC to 250oC

Low freezing point

High viscosity-index

Non-inflammable

High flash point

High temp. stability

High chemical stability

Examples:

Polymerized hydrocarbons: polyethylene, polypropylene,

polybutylene ~ molecular ranges of 500 to 50,000 Da


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Polyglycols & related compounds: polyethylene glycol,

polypropylene glycol, polyglycidyl ethers & higher polyalkylene

oxides in rubber-bearings and joints.

polyglycidyl ethers & higher polyalkylene oxides in roller bearings

of sheet glass manufacturing machines

Oxides are not used at high temp. due to thermal decomposition

Organic amines, imines & amides:

- Low pour points

- High viscosity-index

- Temp. range: -50oC to 250oC

Silicones:

-Up to 200oC and applicable at low temp. lubrication process

- Starts cracking at 230oC

- High viscosity-index


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Fluorocarbons:- Not decomposable by heat i.e. temp. resistance- Chemically resistant- Non-oxidizable- But reacts with molten sodium

5. Lubricating Emulsions

A fairly dispersion of two immiscible liquids, the one being dispersed as

fine droplets in the other

The disperse phase/the internal liquid that is broken into droplets (1-6m)

The dispersing phase/external/continuous phase is the surrounding liquid

Emulsifier/emulsifying agent:

Oil in water emulsion: 25.98% oil + 74.02% water (3-20% water-soluble

emulsifying agent as sodium soap; sodium & potassium salts of

sulphonic acids) for cooling of cutting tools and for rust protection

Water in oil emulsion: 74.02% oil + 25.98% water (1-10% water soluble

Emulsifying agent like alkaline-earth soap e.g., calcium stearate)


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Properties of Lubricants1. Viscosity:

The property of a l iquid or f lu id by v i r tue of w hich i t of fersresistan ce to i ts ow n f low .

- V iscosity should not be too low or too h igh .

(viscosity is inversely pro po rt iona l to te m pe ratu re)2. Flash - Points and Fire - Points :

Flash Point :The low est te m pe rat ure at w hich t he oil lubricant give s off e noughvapour t hat ignit es for a m o m ent , w h en a t iny flam e is brought ne arit .

Fire Point :The low est tem perature a t w hich the vapour o f the o il burncont inuo usly for at least f ive secon ds, w hen a t iny f lam e is broughtnea r i t . In m ost cases the f i re-points are 5 to 4 0 oC higher t han th eflash-po ints. I t is m ea sure d by using Pensky-m arte ns app aratus.


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3. Oiliness:

A measure of its capacity to stick on to the surfaces of

machine parts under conditions of heavy pressure or load.

o For high pressure - high oiliness oil should be used.

o Important for extreme Pressure lubrication

4. Cloud and Pour points:

When an oil is cooled slowly, the temperature at which it

becomes cloudy or hazy in appearance, is called

its CLOUD POINT.

The temperature at which the oil ceases to flow or pour, is

called its POUR POINT.

5. Volatility:

o Good lubricant should have low volatility.

o It is measured by Vaporimeter.


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6. Emulsification:

o The property of oils to get intimately mixed with water formingan emulsion.

o Emulsions have a tendency to collect dirt, girt, foreign materialetc., causing abrasion and wearing out of the lubricating partsof the machinery.

o A good lubricating oil should form an emulsion with water

which breaks off quickly.

7. Carbon residue:

Normally lubricants consist of high % of carbon containingcompounds.

o Lubricants decompose due to raise in temp. and depositcarbon creating problems to :a) IC engines and b) Air compressors.

o A good lubricant should deposit least amount of the carbon .


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8. Corrosion stability:

Corrosion Test:

o A polished copper strip is placed inside a lubricating oil for

a specified time and temperature and then checked for any

tarnishing .

o To prevent or retard corrosion effect of lubricating oils,

additives such as Phosphorous, Arsenic, Antimony,

Chromium, Bismuth or Lead are added.

9. Decomposition stability:

o Lubricating oils must be stable to decomposition at the

operating temperatures by :

a. Oxidation: To prevent it anti oxidant or inhibitor are used.

b. Hydrolysis: Moisture in oils causes hydrolysis of esters

c. Pyrolysis : At high temperature


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10. Aniline point: (A.P.)

o The minimum equilibrium solution temp. for equal volumes of

aniline and oil sample.

o A good lubricating oil should have higher aniline points (A.P)

o Higher A.P means higher % of paraffinic hydrocarbons and

hence lower % of aromatic HC.

(Aromatic HC dissolves natural rubbers and few synthetic

ones)

11. Precipitation Number:

o The percentage of asphalt present in oil after centrifuging oilsolution in petroleum ether.

o Precipitation Number is used to differentiate the different

classes of lubricants.


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12. Ash Point:

o For used oil it is important to get an idea as to how much

abrasion and wear it may cause

13. Saponification number:o Number of milligrams of KOH required to saponify 1g of oil.


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14. Mechanical stability:

o At very high pressures of operation, the stability of a

lubricant is judged by four balls extreme pressure

lubricant test.

To check maximum load that can be carried safely by a

lubricant

15. Neutralization Number:

o Is a scale to determine the amount of acidic or basicconstituents of an oil.

o Acid Number: Amount of KOH required in milligramsto neutralize the fatty acids in 1g of oil.

o Good lubricating oil acid number value < 0.1o More than 0.1 indicates oil gets oxidized and

corrodes the engine

chemistry lubricants

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