polymer_technology

پلیمریزاسیون آلکنها ( پلی اتیلن سبک و سنگین , پلی پروپیلن , پلی وینیل کلراید , پلی تترافلوئورو اتیلن

THE POLYMERISATION OF ALKENES


This page looks at the polymerisation of alkenes to produce polymers like poly(ethene) (usually known as polythene, and sometimes as polyethylene), poly(propene) (old name: polypropylene), PVC and PTFE. It also looks briefly at how the structure of the polymers affects their properties and uses.


Poly(ethene) (polythene or polyethylene)

Low density poly(ethene): LDPE

Manufacture

In common with everything else on this page, this is an example of addition polymerisation.

An addition reaction is one in which two or more molecules join together to give a single product. During the polymerisation of ethene, thousands of ethene molecules join together to make poly(ethene) - commonly called polythene.

The number of molecules joining up is very variable, but is in the region of 2000 to 20000.

Conditions

Temperature: about 200°C
Pressure: about 2000 atmospheres
Initiator: a small amount of oxygen as an impurity


Note:  If you want the mechanism for this reaction, you can get it by following this link to the mechanism section of the site.

Use the BACK button on your browser if you want to return to this page.



Properties and uses

Low density poly(ethene) has quite a lot of branching along the hydrocarbon chains, and this prevents the chains from lying tidily close to each other. Those regions of the poly(ethene) where the chains lie close to each other and are regularly packed are said to be crystalline. Where the chains are a random jumble, it is said to be amorphous. Low density poly(ethene) has a significant proportion of amorphous regions.


Note:  Various sources quote estimated values for the proportion of low density poly(ethene) which is crystalline. These vary from 50% to 75%. I have no idea what the correct value is!


One chain is held to its neighbours in the structure by van der Waals dispersion forces. Those attractions will be greater if the chains are close to each other. The amorphous regions where the chains are inefficiently packed lower the effectiveness of the van der Waals attractions and so lower the melting point and strength of the polymer. They also lower the density of the polymer (hence: "low density poly(ethene)").


Note:  Follow this link if you don't understand van der Waals forces.

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Low density poly(ethene) is used for familiar things like plastic carrier bags and other similar low strength and flexible sheet materials.


High density poly(ethene): HDPE

Manufacture

This is made under quite different conditions from low density poly(ethene).

Conditions

Temperature: about 60°C
Pressure: low - a few atmospheres
Catalyst: Ziegler-Natta catalysts or other metal compounds

Ziegler-Natta catalysts are mixtures of titanium compounds like titanium(III) chloride, TiCl3, or titanium(IV) chloride, TiCl4, and compounds of aluminium like aluminium triethyl, Al(C2H5)3. There are all sorts of other catalysts constantly being developed.

These catalysts work by totally different mechanisms from the high pressure process used to make low density poly(ethene). The chains grow in a much more controlled - much less random - way.

Properties and uses

High density poly(ethene) has very little branching along the hydrocarbon chains - the crystallinity is 95% or better. This better packing means that van der Waals attractions between the chains are greater and so the plastic is stronger and has a higher melting point. Its density is also higher because of the better packing and smaller amount of wasted space in the structure.


Note:  Despite the names, HDPE is only about 3% denser than LDPE! HDPE has a density of about 0.95 g cm-3 compared with about 0.92 for LDPE.


High density poly(ethene) is used to make things like plastic milk bottles and similar containers, washing up bowls, plastic pipes and so on. Look for the letters HDPE near the recycling symbol.


Poly(propene) (polypropylene): PP

Poly(propene) is manufactured using Ziegler-Natta and other modern catalysts. There are three variants on the structure of poly(propene) which you may need to know about, but we'll start from the beginning with a general structure which fits all of them.


Note:  You should check your syllabus to find out exactly how much you need to know. It is pointless getting bogged down in this if you don't need to! If you are studying a UK-based syllabus and haven't got a copy of that syllabus, follow this link to find out how to get one.

Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page later.



The general structure

If your syllabus simply mentions the structure of poly(propene) with no more detail, this is adequate.

The trick is to think about the shape of the propene in the right way:

Now line lots of them up in a row and join them together. Notice that the double bonds are all replaced by single bonds in the process.

In a simple equation form, this is normally written as:


The three variations on this structure

You have got to remember that the diagrams above are 2-dimensional. Real poly(propene) chains are 3-dimensional. There are three different sorts of poly(propene) depending in how the CH3 groups are arranged in space.

These are called isotactic, atactic and syndiotactic poly(propene). The commonly used version is isotactic poly(propene).

Isotactic poly(propene)

A bit of the isotactic poly(propene) chain looks like this:


Note:  Dotted lines show bonds going back into the screen or paper, and wedge shapes show them coming out towards you. If you aren't very happy about the various ways of drawing organic structures it might be worth following this link before you go on.

Use the BACK button on your browser to return to this page.



This very regular arrangement of the CH3 groups makes it possible for the chains to pack close together and so maximise the amount of van der Waals bonding between them. That means that isotactic poly(propene) is quite strong either as a solid object or when it is drawn into fibres.

This is the common form of poly(propene) which is used to make plastic crates and ropes amongst many other things. Look for the letters PP near the recycling symbol.

Atactic poly(propene)

In atactic poly(propene) the CH3 groups are orientated randomly along the chain.

This lack of regularity makes it impossible for the chains to lie closely together and so the van der Waals attractions between them are weaker. Atactic poly(propene) is much softer with a lower melting point.

It is formed as a waste product during the manufacture of isotactic poly(propene) and its uses are limited. It is used, for example, in road paint, in making roofing materials like "roofing felt", and in some sealants and adhesives.

Syndiotactic poly(propene)

Syndiotactic poly(propene) is a relatively new material and is another regularly arranged version of poly(propene). In this case, every alternate CH3 group is orientated in the same way.

This regularity means that the chains can pack closely, and van der Waals attractions will be fairly strong. However, the attractions aren't as strong as in isotactic poly(propene). This makes syndiotactic poly(propene) softer and gives it a lower melting point.

Because syndiotactic poly(propene) is relatively new, at the time of writing uses were still being developed. It has uses in packaging - for example, in plastic film for shrink wrapping food. There are also medical uses - for example, in medical tubing and for medical bags and pouches. There are a wide range of other potential uses - either on its own, or in mixtures with isotactic poly(propene).


Poly(chloroethene) (polyvinyl chloride): PVC

Poly(chloroethene) is commonly known by the initials of its old name, PVC.

Structure

Poly(chloroethene) is made by polymerising chloroethene, CH2=CHCl. Working out its structure is no different from working out the structure of poly(propene) (see above). As long as you draw the chloroethene molecule in the right way, the structure is pretty obvious.


The equation is usually written:

It doesn't matter which carbon you attach the chlorine to in the original molecule. Just be consistent on both sides of the equation.

The polymerisation process produces mainly atactic polymer molecules - with the chlorines orientated randomly along the chain. The structure is no different from atactic poly(propene) - just replace the CH3 groups by chlorine atoms.

Because of the way the chlorine atoms stick out from the chain at random, and because ot their large size, it is difficult for the chains to lie close together. Poly(chloroethene) is mainly amorphous with only small areas of crystallinity.

Properties and uses

You normally expect amorphous polymers to be more flexible than crystalline ones because the forces of attraction between the chains tend to be weaker. However, pure poly(chloroethene) tends to be rather hard and rigid.

This is because of the presence of additional dipole-dipole interactions due to the polarity of the carbon-chlorine bonds. Chlorine is more electronegative than carbon, and so attracts the electrons in the bond towards itself. That makes the chlorine atoms slightly negative and the carbons slightly positive.

These permanent dipoles add to the attractions due to the temporary dipoles which produce the dispersion forces.


Note:  If you aren't happy about the various sorts of intermolecular forces, it is important to follow this link.

If you don't understand about electronegativity and polar bonds, then follow this one as well.

Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page.



Plasticisers are added to the poly(chloroethene) to reduce the effectiveness of these attractions and make the plastic more flexible. The more plasticiser you add, the more flexible it becomes.

Poly(chloroethene) is used to make a wide range of things including guttering, plastic windows, electrical cable insulation, sheet materials for flooring and other uses, footwear, clothing, and so on and so on.


Poly(tetrafluoroethene): PTFE

You may have come across this under the brand names of Teflon or Fluon.

Structure

Structurally, PTFE is just like poly(ethene) except that each hydrogen in the structure is replaced by a fluorine atom.


The PTFE chains tend to pack well and PTFE is fairly crystalline. Because of the fluorine atoms, the chains also contain more electrons (for an equal length) than a corresponding poly(ethene) chain. Taken together (the good packing and the extra electrons) that means that the van der Waals dispersion forces will be stronger than in even high density poly(ethene).


Note:  There won't be any dipole-dipole attractions between the chains in addition to dispersion forces (unlike PVC). The fluorines are arranged regularly around the carbon backbone. Although each bond is very polar, overall they cancel each other out.

(In fact, there might even be some repulsions because the outsides of the chains all consist of slightly negative fluorine atoms, but these obviously aren't enough to have a significant effect on the strong dispersion forces. I haven't been able to find any mention of this either in textbooks or on the Web.)

You might like to read about CCl4 (a simple example of a non-polar molecule containing polar bonds) on the main page about electronegativity (different from the link above).

Use the BACK button on your browser to return to this page.



Properties and uses

PTFE has a relatively high melting point (due to the strength of the attractions between the chains) and is very resistant to chemical attack. The carbon chain is so wrapped up in fluorine atoms that nothing can get at it to react with it. This makes it useful in the chemical and food industries to coat vessels and make them resistant to almost everything which might otherwise corrode them.

Equally important is that PTFE has remarkable non-stick properties - which is the basis for its most familiar uses in non-stick kitchen and garden utensils. For the same reason, it can also be used in things like low-friction bearings


Note:  Despite an extensive Web search, I haven't found any convincing explanation for the non-stick properties of PTFE at the molecular level. Many sources talk about it in terms of surface tension or surface energy, which actually beg the question. Unless you can explain the origin of these in terms of attractions or repulsions at the molecular level, it seems to me that you aren't actually explaining anything just by using a posh-sounding term!

If you are interested, you will find a discussion of how far I've got in coming up with an answer to this on a page about the non-stick properties of PTFE in the section of questions that I can't answer to my satisfaction!


reference of this text is: http://www.chemguide.co.uk/

Ammar Ghasemian Azizi ; ammar5ghasemian@yahoo.com or

azizi@fmplastics.nl Tel : +98 912 386 2365

+ نوشته شده در  یکشنبه بیست و چهارم آذر 1387ساعت 0:7  توسط Ammar Ghasemian Azizi  | 

پلی اتیلن ترفتالات ( PET ) و بررسی ای بر پلی استر ها

POLYESTERS


This page looks at the formation, structure and uses of a common polyester sometimes known as Terylene if it is used as a fibre, or PET if it used in, for example, plastic drinks bottles


Poly(ethylene terephthalate)

What is a polyester?

A polyester is a polymer (a chain of repeating units) where the individual units are held together by ester linkages.

The diagram shows a very small bit of the polymer chain and looks pretty complicated. But it isn't very difficult to work out - and that's the best thing to do: work it out, not try to remember it. You will see how to do that in a moment.

The usual name of this common polyester is poly(ethylene terephthalate). The everyday name depends on whether it is being used as a fibre or as a material for making things like bottles for soft drinks.

When it is being used as a fibre to make clothes, it is often just called polyester. It may sometimes be known by a brand name like Terylene.

When it is being used to make bottles, for example, it is usually called PET.


Making polyesters as an example of condensation polymerisation

In condensation polymerisation, when the monomers join together a small molecule gets lost. That's different from addition polymerisation which produces polymers like poly(ethene) - in that case, nothing is lost when the monomers join together.

A polyester is made by a reaction involving an acid with two -COOH groups, and an alcohol with two -OH groups.

In the common polyester drawn above:

The acid is benzene-1,4-dicarboxylic acid (old name: terephthalic acid).

The alcohol is ethane-1,2-diol (old name: ethylene glycol).



Now imagine lining these up alternately and making esters with each acid group and each alcohol group, losing a molecule of water every time an ester linkage is made.

That would produce the chain shown above (although this time written without separating out the carbon-oxygen double bond - write it whichever way you like).


Note:  This does NOT describe the way the actual reaction happens - it is a way of working out the structure of the polymer. The chemistry of the reaction is more complicated than this (see below).

The diagram shows a slightly shorter bit of chain than the corresponding one at the top of the page. However, it is exactly consistent with the loss of water from the last diagram. It was impossible to include another ethane-1,2-diol in that diagram for space reasons. If any of this offends you, draw it again yourself so that everything matches! In fact, it would be good practice to draw a bit of chain starting from a few more monomers.

This is what I meant further up the page by working the structure out rather than remembering it. The structures of both monomers are easy to remember. If you line them up and remove water as I have shown, the structure follows automatically.




Manufacturing poly(ethylene terephthalate)

The reaction takes place in two main stages: a pre-polymerisation stage and the actual polymerisation.


Warning!  This manufacturing process is only currently required by one UK A level Exam Board (WJEC). If you don't need to know about this, skip over the next bit. It is really confusing, because it doesn't relate easily to the way we have used to work out the structure of the polyester. The overall result is the same, but it happens by a much more complicated process.


In the first stage, before polymerisation happens, you get a fairly simple ester formed between the acid and two molecules of ethane-1,2-diol.

In the polymerisation stage, this is heated to a temperature of about 260°C and at a low pressure. A catalyst is needed - there are several possibilities including antimony compounds like antimony(III) oxide.

The polyester forms and half of the ethane-1,2-diol is regenerated. This is removed and recycled.


Note:  Notice the way the polymer is drawn. This is the minimum amount of chain that you can draw to show the repeating unit.



Hydrolysis of polyesters

Simple esters are easily hydrolysed by reaction with dilute acids or alkalis.

Polyesters are attacked readily by alkalis, but much more slowly by dilute acids. Hydrolysis by water alone is so slow as to be completely unimportant. (You wouldn't expect your polyester fleece to fall to pieces if you went out in the rain!)

If you spill dilute alkali on a fabric made from polyester, the ester linkages are broken. Ethane-1,2-diol is formed together with the salt of the carboxylic acid.

Because you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole!

For example, if you react the polyester with sodium hydroxide solution:


Note:  Hydrolysis of esters is covered in detail on another page in this section.


reference of this text is : http://www.chemguide.co.uk/

Ammar Ghasemian Azizi ; ammar5ghasemian@yahoo.com or

azizi@fmplastics.nl ; Tel : +98 912 386 2365

+ نوشته شده در  شنبه بیست و سوم آذر 1387ساعت 23:55  توسط Ammar Ghasemian Azizi  | 

کولار و بررسی ای بر پلی آمید ها

Kevlar is an extremely strong material that derives its strength from its weave. It is woven like tiny spider webs. Stephanie Kwolek and Herbert Blades created this special material in 1965 for the Dupont Company. Since then it has been used in a number of ways.

Kevlar is a special way of weaving a liquid into a solid. This is called an aramid weave. Aramid fibers tend to be difficult to corrode, resistant to heat, and have no melting point. Aramid fibers like Kevlar may be slightly corrosive if exposed to chlorine.

Because Kevlar is light, it is the premium choice for bulletproof vests. A variant of Kevlar called nomex is fireproof and may be used by fireman or people responding to disaster situations.

Kevlar has different types of weaves, and the weave that makes a fabric-like material for vests is called Kevlar 29. Kevlar 29 may also be used in brake pads, or to replace asbestos. It is also is a major part of the composition of body armor.

Kevlar has two other types, Kevlar, and Kevlar 49. Kevlar may be used to replace rubber items like tires. Kevlar 49 is extremely strong and can replace the more traditional materials used for a boat hull, or be used in simple items like bicycle frames.

Currently, one of the most interesting applications of Kevlar is its use in shelters for protection against tornadoes. The material is used in a shed-like structure that can be placed in a garage. Tests show it can deflect large materials at speeds of up to 250 mph (402.32 kph). In areas with frequent tornadoes, Kevlar shelters may become the best way to protect against strong tornadoes.

Under great compression Kevlar can buckle, and in some cases, be pierced. For example, people quickly found a way to make bullets that could pierce bulletproof armor. These are illegal for sale to consumers. However, a nation’s army, to provide additional strength in ground combat, may use them.

In general, however, Kevlar offers many opportunities for protection and for replacement of materials more likely to corrode. Thus one can expect still more Kevlar products in the future.

What is KEVLAR made of?

 

Kevlar is a synthetic (person-made) material known as a polymer. Some other common synthetic polymers include Nylon, Teflon, Lycra, and polyester. A polymer is a chain made of many similar molecular groups, known as monomers, that are bonded together. To get a better picture of this, imagine that you are looking at a long locomotive train. Each identical boxcar could represent a monomer and the whole train would represent the polymer chain.

 A single Kevlar polymer chain could have anywhere from five to a million segments bonded together. Each Kevlar segment or monomer is a chemical unit that contains 14 carbon atoms, 2 nitrogen atoms, 2 oxygen atoms and 10 hydrogen atoms.

Chemists represent a Kevlar monomer like this:

            
Kevlar is a polymer (of ~20-30 units) of the monomer

H[-NH-C6H4(para)-NHCO-C6H4(para)CO]n
where C6H4 is the benzenoid ring; the n=number of monomers and where the linear polymer is cross polymerized with Hydrogen bonding between NHCO of one cross chain with other similar parts of adjacent chains.
 

POLYAMIDES


This page looks at the structures, formation, hydrolysis and uses of the polyamides, nylon and Kevlar.


What are polyamides?

Polyamides are polymers where the repeating units are held together by amide links.

An amide group has the formula - CONH2. An amide link has this structure:

In an amide itself, of course, the bond on the right is attached to a hydrogen atom.


Note:  If you know any biology or biochemistry, you may have come across this called a peptide group. If you are interested in the presence of this group in proteins, you could follow this link.

Use the BACK button on your browser to return to this page.




Nylon

In nylon, the repeating units contain chains of carbon atoms. (That is different from Kevlar, where the repeating units contain benzene rings - see below.) There are various different types of nylon depending on the nature of those chains.

Nylon-6,6

Nylon-6,6 is made from two monomers each of which contain 6 carbon atoms - hence its name.

One of the monomers is a 6 carbon acid with a -COOH group at each end - hexanedioic acid.


Note:  When you count the carbons don't forget to include the ones in the -COOH groups.


The other monomer is a 6 carbon chain with an amino group, -NH2, at each end. This is 1,6-diaminohexane (also known as hexane-1,6-diamine).

When these two compounds polymerise, the amine and acid groups combine, each time with the loss of a molecule of water. This is known as condensation polymerisation.

Condensation polymerisation is the formation of a polymer involving the loss of a small molecule. In this case, the molecule is water, but in other cases different small molecules might be lost.

The diagram shows the loss of water between two of the monomers:

This keeps on happening, and so you get a chain which looks like this:


Note:  This chain is much easier to work out than to remember. Learn the structures of the monomers, and then practice writing them down and removing water from them as shown above.


Nylon-6

If you are doing UK A level, you are unlikely to need the structure of nylon-6. I am including it to show that it is possible to get a polyamide from a single monomer.

Nylon-6 is made from a monomer called caprolactam.

Notice that this already contains an amide link. When this molecule polymerises, the ring opens, and the molecules join up in a continuous chain.


Kevlar

Kevlar is similar in structure to nylon-6,6 except that instead of the amide links joining chains of carbon atoms together, they join benzene rings.

The two monomers are benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene.

If you line these up and remove water between the -COOH and -NH2 groups in the same way as we did with nylon-6,6, you get the structure of Kevlar:


Making nylon-6,6

Making nylon-6,6 industrially

Nylon-6,6 is made by polymerising hexanedioic acid and 1,6-diaminohexane exactly as shown further up the page.

Because the acid is acidic and the amine is basic, they first react together to form a salt. That is then converted into nylon-6,6 by heating it under pressure at 350°C.

The two monomers can both be made from cyclohexane.

  • Oxidation of the cyclohexane opens the ring of carbon atoms and produces a -COOH group at each end. That gives you the hexanedioic acid.

    Some of that can then be converted into the 1,6-diaminohexane.

  • The acid is treated with ammonia to produce the ammonium salt.

  • The ammonium salt is heated to 350°C in the presence of hydrogen and a nickel catalyst. This both dehydrates the salt and reduces it to the 1,6-diaminohexane.


Making nylon-6,6 in the lab

In the lab, it is easy to make nylon-6,6 at room temperature using an acyl chloride (acid chloride) rather than an acid.

The 1,6-diaminohexane is used just as before, but hexanedioyl dichloride is used instead of hexanedioic acid.

If you compare the next diagram with the diagram further up the page for the formation of nylon-6,6, you will see that the only difference is that molecules of HCl are lost rather than molecules of water.


Note:  If you are interested, the reaction between acyl chlorides and amines is explored on another page.

If you follow this link, use the BACK button on your browser to return to this page.



In the lab, this reaction is the basis for the nylon rope trick.

You make a solution of the hexanedioyl dichloride in an organic solvent, and a solution of 1,6-diaminohexane in water. You carefully float one solution on top of the other in a small beaker, taking care to get as little mixing as possible.

Nylon-6,6 forms at the boundary between the two solutions. If you pick up the boundary layer with a pair of tweezers, you can pull out an amazingly long tube of nylon from the beaker.


Note:  The exact details of this experiment depend on which organic solvent you use for the hexanedioyl dichloride. This will affect which layer is at the top and which layer at the bottom, depending on the densities of the two solutions. For specific details, you should find this experiment in most organic practical books at this level, or a Google search for "nylon rope trick" will throw up lots of variations.

You will find a Google search box on the Main Menu. Don't forget to set it to search the whole web and not just this site.




Hydrolysis of polyamides

Simple amides are easily hydrolysed by reaction with dilute acids or alkalis.

Polyamides are fairly readily attacked by strong acids, but are much more resistant to alkaline hydrolysis. Hydrolysis is faster at higher temperatures. Hydrolysis by water alone is so slow as to be completely unimportant. Kevlar is rather more resistant to hydrolysis than nylon is.

If you spill something like dilute sulphuric acid on a fabric made from nylon, the amide linkages are broken. The long chains break and you can eventually end up with the original monomers - hexanedioic acid and 1,6-diaminohexane.

Because you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole!


Note:  Hydrolysis of amides is covered in detail on another page in this section.



Uses of polyamides

Nylon

Apart from obvious uses in textiles for clothing and carpets, a lot of nylon is used to make tyre cords - the inner structure of a vehicle tyre underneath the rubber.

The fibres are also used in ropes, and nylon can be cast into solid shapes for cogs and bearings in machines, for example.

Kevlar

Kevlar is a very strong material - about five times as strong as steel, weight for weight. It is used in bulletproof vests, in composites for boat construction, in lightweight mountaineering ropes, and for lightweight skis and racquets - amongst many other things.


reference of this text is : http://www.chemguide.co.uk/ and  http://www.wisegeek.com/ and http://www.lbl.gov/ 

Ammar Ghasemian Azizi ; ammar5ghasemian@yahoo.com or

 azizi@fmplastics.nl Tel : +98 912 386 2365

+ نوشته شده در  شنبه بیست و سوم آذر 1387ساعت 23:18  توسط Ammar Ghasemian Azizi  | 

پلی اتیلن با جرم مولکولی فوق العاده بالا موسوم به UHMWPE

WHAT IS UHMW-PE? (Ultra High Molecular Weight Polyethylene)

UHMW Polymer is a Linear Polyethylene with a molecular weight in the range of 3,000,000 to 6,000,000. This value represents the "average molecular weight". Therefore, UHMW Polymers have a molecular weight average 10 times that of conventional high density polyethylene resins. The higher molecular weight is what gives UHMW Polymers a unique combination of characteristics making it more suitable for many applications where lower molecular weight grades fail.

UHMW MECHANICAL PROPERTIES :

PROPERTY ASTM TEST UNITS METRIC (U.S) .030" .060" .125"
Density D792 gm/cc 0.93 0.93 0.93
Tensile Strength @ Yield D638 MPa(psi) 23(3300) 20(2964) 22(3227)
Tensile Strength @ Break D638 MPa(psi) 53(7740) 49(7056) 44(6373)
Elongation @ Break D638 % 460 463 466
Youngs "E" Modulus D638 MPa (psi x 105) 725(1.05) 731(1.06) 672(0.97)
Izod Impact Strength (23oC) D256 (1) J/m(ft-lb/in notch)      
Izod Impact Strength (-40oC) D256 (1) J/m(ft-lb/in notch)      
Hardness Shore "D" D2240 65 65 65
Abrasion Resistance          
Water Absorbtion  D570 % Nil Nil Nil
Relative Solution Viscosity D4020 dl/gm 2.3 - 3.5 2.3 - 3.5  2.3 - 3.5
COEFFICIENT OF FRICTION
Static     0.16 0.16 0.16
Dynamic     0.14 0.13 0.14

(1) Izod Impact Strength: Samples have two(15° +/- 1/2°) notches on opposite sides to a depth of 5mm.



COMPARISON OF DYNAMIC COEFFICIENT OF FRICTION
ON POLISHED STEEL

PROPERTY DRY WATER OIL
UHMW-PE .10 - .22 .05 - .10 .05 - .08
NYLON 6 .15 - .40 .14 - .19 .02 - .11
NYLON 6/6 .15 - .40 .14 - .19 .02 - .11
NYLON/MoS2 .12 - .20 .10 - .12 .08 - .10
PTFE .04 - .25 .04 - .08 .04 - .05
ACETAL COPOLYMER .15 - .35 .10 - .20 .05 - .10



THERMAL PROPERTIES

PROPERTY ASTM TEST UNITS METRIC (U.S) .030" .060" .125"
Crystalline Melting Range Polarizing °C(°F) 136(276) 134(273) 134(273)
Crystallinity D3417-96 % 48 47 50
COEFFICIENT OF LINEAR EXPANSION
20 - 100 °C D696 TBD TBD TBD TBD
-20 to -100 °C D696 TBD TBD TBD TBD



ELECTRICAL PROPERTIES

PROPERTY ASTM TEST UNITS METRIC (U.S) .030" .060" .125"
Volume Resistivity D357 Ohms/cm 5.9544x10^7 1.4516x10^7 > 2.000x10^13
Dielectric Strength D150 KV/cm (V/mil) * * 142
Dielectric Constant  D150   2.481 2.454  2.542
Surface Resistivity 1% Carbon Black D257 Ohms 10^3 10^3 10^3
Static Decay 1% Carbon Black   Seconds < .01 < .01 < .01
DISSIPATION FACTOR 
At 50Hz D150   0.0594 0.0213 0.0082
At 10KHz D150   0.1085 0.0690 0.0022
At 5MHz D150   0.1035 0.2340 0.0034

* No reading could be taken due to material thickness.


reference of this text is : http://www.crownplastics.com/

Ammar Ghasemian Azizi : ammar5ghasemian@yahoo.com or

azizi@fmplastics.nl or call +98 912 386 2365

+ نوشته شده در  جمعه بیست و دوم آذر 1387ساعت 18:50  توسط Ammar Ghasemian Azizi  | 

انواع تخریب در پلیمر ها

تخریب پلیمر ها از دیر زمان مورد شناخت بوده است . از بین رفتن سلولز در چوب , لاستیک در تایر ماشین ها و ترک خوردن و زرد شدن فیلم های نقاشی , از معدود مثال های معمولی هستند که می توان نام برد . انواع فرآیند های تخریبی , بسته به شرایط محیطی که یک پلیمر در آن مورد استفاده قرار می گیرد تا تاریخچه ساخت و ساختمان پلیمر مربوطه فرق می کنند و این مسائل و مسائل موثر دیگر همگی نقش مکملی را در کنترل مرحله تعیین کننده سرعت کلی تخریب ایفا می کنند .

تخریب و اکسایش پلیمر های طبیعی اگر چه مدت هاست که شناخته شده اند اما گونه هایی که در طبیعت رخ می دهند پیچیده هستند . پشم و سلولز از این نظر انگشت نما هستند به طوری که درک ما از مکانیزم های اصلی تخریب آنها بسیار اندک است و اطلاعات کمی راجع به آنها داریم . با ورود استر های سلولزی مسائل تخریبی دیگری مثل قابلیت شعله وری و ناپایداری از نظر هیدرولیز شدن و رها شدن اسید ها آشکار گردیدند . در حقیقت هرگونه درک اساسی در مورد مکانیزم این فرآیند ها فقط طی سالهای اخیر حاصل شده است . با ورود ترموپلاستیک های مصنوعی جدید , مسائل جدید بسیاری نیز مطرح شدند زیرا ک هر یک در نوع و چگونگی تخریب با یکدیگر متفاوت بودند . به عنوان مثال در حالی که پلی متیل متاکریلات ( PMMA ) در دماهای بالا تقریبا به طور کامل ( ۱۰۰ ٪ ) به مونومر خود واپلیمریزه ( Depolymerise ) می شود , پلی وینیل کلراید ( PVC ) به یک مکانیزم سلسله وار ( Unzipping mechanism ) تخریب میشود و تولید مواد پلیمری سیر نشده و بخارات اسیدکلریدریک ( HCl ) میکند ( به علت ضعیف بودن پیوند سیگمای موجود بین کربن و کلر در back bone اصلی زنجیر ) . پلی تترا فلوئورو اتیلن ( PTFE ) موسوم به تفلون بسیار پایدار است و اغلب در وسایل پخت و پز غیر چسبنده مورد استفاده قرار میگیرد ( به پایداری در دمای پخت و پز توجه کنید ) .

در دیدی کلی , انواع فرآیندهای تخریبی را که پلیمر ها در حین مصرف روزمره متحمل آنها میشوند را میتوان به صورت زیر تقسیم بندی کرد :

1- گرمایی :

این نوع تخریب هنگام فرآیند کردن و یا به کار گیری پلیمرها در دما های بالا رخ میدهد و ممکن است با اکسایش توام باشد ( Thermoxidation Degradaton ) و یا اینکه بدون دخالت اکسیژن صورت گیرد ( Thermal Degradation ) .

2- مکانیکی ( Mechanical Degradation ) :

این نوع تخریب بر اثر وارد شدن نیرو و شکست فیزیکی زخ میدهد که میتواند با شکست زنجیری نیز توام باشد .

۳- مافوق صوتی ( Ultrasonic Degradation ) :

استفاده از صوت با فرکانس های خاص می تواند باعث شود ک زنجیر های یم پلیمر مرتعش شده و به پاره شوند .

۴- آبی ( Hydrolytic Degradation ) :

این فرآیند در پلیمرهایی رخ می دد ک دارای گروه های فعال حساس به آب می باشند , به ویژه آنهائی که رطوبت زیادی به خود میگیرند .

۵- شیمیائی ( Chemical Degradation ) :

در این مورد مواد شیمیائی خورنده و یا گازهائی نظیر اکسیژن ( O2 ) و اوزون ( O3 ) میتوانند به عوامل ساختمانی یک پلیمر حمله نموده و باعث پاره شدن زنجیر مولکولی و اکسایش آن شوند .

۶- زیستی ( Biological Degradaton ) :

این مسئله فقط مخصوص معدودی از پلیمر ها است که حاوی گروه های فعال خاصی هستند و توسط موجودات ذره بینی مورد حمله قرار میگیرند .

۷- تشعشعی ( Radiation Degradation ) :

در برابر نور خورشید و یا تشعشعات پر انرژی , پلیمر و یا ناخالصی های موجود در پلیمر , اشعه را جذب کرده و موجب واکنش های تخریبی و در نتیجه آن افت خواص پلیمر میشود . در مورد تشعشعات پرانرژی , زنجیر های مولکولی پلیمر مستقیما پاره میشوند .

تغییرات خواص :

تغبیرات خواص در یک پلیمر در اثر تخریب شدن را میتوان به انواع فیزیکی و شیمیائی تقسیم بندی نمود :

۱- فیزیکی : کاش در وزن مولکولی  , کاهش خواص مکانیکی پلیمر ( کاهش در مقاومت کششی , مقاومت ضربه ای و ... ) , کاهش ازدیاد طول در نقطه پارگی , افت شفافیت , ایجاد ساییدگی در سطح و ایجاد ترک های ریز دز توده پلیمر .

۲- شیمیائی : تغییر در ساختمان شیمیائی , به وجود آمدن عوامل فعالی نظیر گروه های سیر نشده , هیروکسیل , کربونیل , و هیدرو پراکسید ها که میتوانند سبب افت خواص الکتریکی پلیمر شوند .

دوستان گرامی ,  این مطلب با رفرنس کتاب << مبانی تخریب و پایدارسازی پلیمر >> تالیف " نورمن اس. آلن & مایکل ادج " به ترجمه کامل و خوب << خانم دکتر هما عاصم پور >> هستش که ویرایش کاملی روی این مطلب استفاده شده از جانب بنده اعمال شده و سپس در این پست درج کردم که شما دوستان عزیز میتوانید برای اطلاع بیشتر راجع به تخریب پلیمرها , به این کتاب رجوع کنید . ای کاش در واحدهای درسی رشته مهندسی پلیمر , درسی به نام " مکانیزم های تخریب پلیمر ها " اضافه بشود که مطلبی بسیار بسیار مهم است و نیازمند آگاهی ما دانشجویان رشته پلیمر به آن می باشد .

+ نوشته شده در  شنبه دوم آذر 1387ساعت 21:5  توسط Ammar Ghasemian Azizi  |