Ch. 1 POLYMERS

              1. POLYMERS

 POLYMERS :
  A large molecule built up by small, simple chemical units.

 PROPERTIES :

1. Melt Viscosity :
       It is the measure of the rate at which chains can move relative to each other. The higher the molecular weight, greater the melt viscosity.

2. Yield Strength & Modulus :
       Many polythenes have the yield strength below 2000lbf/sq.inch (i.e.14MPa)While the nylon may have the value of 12000lbh/sq.inch (83MPa).

3. Specific Gravity :
      It is the mass per unit volume.
 
      Amorphous hydrocarbon polymers generally have sp.gravity of 0.86-1.05.
      Where large atoms are present e.g.chlorine atoms,the sp.gravity  is higher,such as
       PVC-1.4.

4. Impact Strength :
      Familarity with a given plastic material under normal conditions of use leads to it being considered as either a brittle or a tough material.
     Thus polystyrene, unmodified unplasticised PVC are brittle where as L.D.P.E. & plasticized PVC are tough.

 PRINCIPLES OF THE PROCESSING OF THE PLASTICS :
             
            A  large part of polymer processing technology can be summed up in the statement : “Get the shape & then set the shape .”
           
 Such objects can be shaped by the following general techniques :

1. Deformation of polymer melt either thermoplastic or thermosetting processes operating in this way include extrusion, injection moulding & calendaring and form in tonnage terms.
2. Deformation of the polymer in the rubbery state of importance in vacuum forming, pressure forming & warm forging techniques.
3. Deformation of a solution usually either by spreading or by extrusion as used in making cast film & certain synthetic fiber & filaments.
4. Deformation of a suspension.This is great importance with rubber latex &the other lattices & with the PVC paste.
5. Deformation of lower molecular weight polymer or polymer precursor such as in the casting of acrylic sheets & glass reinforced laminates.
6. Machining operations.
                           The first five of these techniques involve deformation & this has to be followed by some setting operation which stabillises the new shape.In the case of  polymer melt deformation,this can be affected by cooling of thermoplastics & cross linking f thermosetting plastics and the similar can apply to deformation in the rubbery state.
 MELT PROCESSING OF THERMO-PLATICS :
              In order to realise the full potential of process, it is necessary to consider the following factors –
1. Hygroscopic behavior of polymer compound.
2. Granule characteristics.
3. Thermal properties that influence the melting of the polymer.
4. Thermal stability.
5. Flow properties.
6. Thermal properties that affect the cooling of polymer.
7. Crystallisation.
8. Orientation.

 Hygroscopic Behavior :
                   It is essential that the polymer compounds shall be free of water & other low boiling solvents. A
        small volume of water can generate steam which will trend to be trapped within the compound during
        the production stage.This will expand on decompression of melt in latter stage of process leading to
        voids in the finished product.Such voids ar sometimes flattened out through shear during the polymer
        flow, leading to reflecting surfaces known as “mica marks”.

                Sometimes the water may just be present on the surface of compound & easily removed.In the
        other cases, water may be absorbed in the body of the polymer & long drying period is necessary.

                Thus the higher processing temperature, the lower is the tolerable level of water in the
         compound.Since higher temperature will generate higher volume of  steam with a fixed mass of water.

 Granule Characteristic :  
                    All one time it was quietly common practice to extrude & mould granules of varring shapes &    
size that had been obtained by breaking of sheet between rotating 7 stationary blades.It was subsequently
found that the use of granules of more regular shape & even size can lead to much higher throughput
rates of extruders & much more even heating and hence better control in flow properties in all the process.

 Thermal properties influencing polymer melting :
                 Polymer compounds vary considerably in the amount of heat required to bring them up processing temperatures. These differences arises not so much as a result of  differing processing temperatures but because of different specific heats. Crystalline polymers additionally have a latent heat of fusion of the crystalline structure which has to taken in to account.
                The specific heat is independent of   the temperature. It is therefore necessary to use an average
   figure for the specific heat over the range from room temperature to the processing temperature.

Polymer Sp.
Gravity Sp.Heat
J/g°c Latent heat
J/g Avg.moulding temp. °c Total heat reqd. J/cm3
ABS 1.01 1.46 - 225 305
Polycarbonate 1.20 1.26 - 300 422
Polyeheylene 0.91-0.96 2.31 209 220-280 610-740
Polystyrene 1.05 1.34 - 200 250
PVC (unplasticised) 1.44 1.00 - 180 230
polypropylene 0.90 1.92 100 250 487
 
      The heat for melting can be generated externally,in which case heat transfer distances should be kept to a minimum & a temperature distribution will depend on the thermal conductivity, or internally either by a high frequency heating process or by mechanical mechanical working can provide a significant contribution.
The amount of frictional heat generated increases with the rate of working & with the polymer viscosity.

 Since the melt viscosity decreases with the increasing temperature,the rate of frictional heat decreases with increase of temperature once the polymer is in the molten state.

 In some polymer processing, the frictional heat generated exceeds the total requirement.So that provision has to be made for cooling facilities around the main heating chamber, be it an extruder barrel or an injection moulding cylinder.

Thermal Stability :

 Polymer vary inoromously in their thermal stability. Before attempting to process,any specific polymer compound, its thermal characteristics to be considered

Stability at elevated temperature in absence of oxygen; i.e. the period of heating at typical processing temperature.

Stability at elevated temperature in presence of oxygen.

If the product is unstable, how are the polymer properties affected?

What degradation products, if any are given off ?

Is degradation catalysed by any metals which could be present in the processing machinery?

 Is degradation catalysed by any other material with which the polymer might come in the contact.
                           
Some materials are able to withstand quite lengthy ‘thermal histories’ i.e.intensity (temperature ) & the duration of heating.

Polyethylene & polystyrene may reprocessed a number of times with little more than a slight decolouration & in case of polyethylene deterioration in electrical insulation properties.
But PVC requires incorporation of stabillisers & even may discolour and give off hydrochloric acid,later having a corrosive effect on many metals.

At the same time some metals have a catalytic effect on these polymers so that care has to be taken in construction of barrel,screw & the other metal parts liable to come in contact with the polymers.

 SOME POLYMERS & THEIR SHORT FORMS IN DAILY USE :
PVC Polyvinyl chloride
FRLS Fire retardant low smoke
HFFR Halogen free flame retardant
LDPE Low density (high pressure) polyethylene
HDPE High density (Low pressure) polyethylene
XLPE Cross linked polyethylene
EPR Ethylene Propylene rubber
HEPR High modulus/grade Ethylene Propylene rubber
PP Polypropylene
PS Polystyrene
HIPS High Intensity Polystyrene

 SOME PROCESSES OF POLYMERS :

 EXTRUSION :
         Material is pumped with the screw pump, through a die to give a product of constant cross-section.

 INJECTION MOULDING :
          Material is pumped by a screw pump to the front end of the injection cylinder with the screw moving to the rear in order to provide the space for the material, then the screw moves forward as the injecting the molten material into a relatively cool mould in which the material sets.

 EXTRUSION BLOW MOULDING :
          The extruder tube is inflated in the mould while still above softening point.

 CALENDERING :
           Softened material is flattened out into sheet between roles.

 Most Common Materials in Pipe Systems :

• PE-Polyethylene

Characteristics:
•  Ductile behaviour
•  Widely available in extensive pipe grades
•  Easy to handle – pellet form
•  Easy to extrude
•  Good possibilities for recycling
•  Tolerant to broad working temperature range
•  Flexible allowing coiling to reduce jointing in long runs
•  Can be jointed by welding
•  Material is bought in pellet form already compounded
•  MDPE
•  HDPE –80 -100
•  Relatively highwall thickness’required for pressure

• PVC –Poly vinyl Chloride

Characteristics:
•  Can show brittle behaviour
•  Flexible compounding possibilities
•  Pellet or powder form
•  Corrosive
•  Typically scrap is not re-used in pressure pipes
•  Brittle when exposed to cold temperatures
•  Rigid –cannot be coiled
•  High strength –low wall thickness in relation to pressure
•  Linear Oriented PVC –extremely high strength
•  Difficult to test

• PP -Polypropylene

Characteristics:
•  High end product
•  Good resistance to temperature
•  Pellet form
•  Easy to handle
•  Ductile material
•  Excellent surface quality

• PB -Polybutylene

Characteristics:
•  Requires cross linking
•  Good resistance to temperature
•  Pellet form
•  Easy to handle
•  Ductile material
•  Excellent surface quality

Introduction to PVC pipes

                       Introduction


Plastic : A material which contains as an essential ingredient an organic substance of high to ultrahigh molecular weight, is solid in its finished state, and at some stage of its manufacture or processing can be shaped by flow. The two general types of plastic are thermoplastic and thermosetting.

There are many types of plastic pipe, both rigid and semirigid. Under normal conditions, rigid plastic pipe can be supported using conventional supports with the spacing half that used with steel pipe or as recommended by pipe manufacturers.

The support of plastic pipe or tubing should be continuous if, owing to the nature of the plastic, it will become flexible from elevated temperatures or from line contents. The continuous support can be in the form of a light angle or channel into which the plastic pipe is laid.

In some cases, wear shoes or pads should be added to plastic pipe where it may rub against steel supports. The use of wear pads will prevent the abrasive action caused by thermal movement, thus preventing damage to the pressure boundary.

It is suggested that recommendations of the manufacturer of the specific plastic
pipe also be followed.

Plastic piping is made from either of two basic groups of synthetic materials, thermoplastic and thermosetting. Thermoplastics can be softened and reshaped repeatedly by the application of heat. In contrast thermosetting materials are irreversibly set, or cured, or hardened into a permanent shape during factory manufacture.

Once hardened into their final shape, thermosetting products cannot be softened and therefore may not be reshaped by heating. Thermoplastic materials include minimal reinforcements, whereas thermosetting resins are almost always combined with reinforcements (such as glass fibers) and sometimes fillers (such as sand) to produce structurally integrated composite constructions.

Due to its excellent material characteristics and its outstanding price / performance ratio, until today PVC is the most used material worldwide for pipes or profiles made from plastics.

As the market is very competitive manufacturers of RPVC/UPVC pipes or profiles have been exposed to increasing pressure on prices and costs for years.

At the same time higher and higher demands are made on the product quality.

Plastic Pipe and Fittings :

General.  Plastic pipe is manufactured in a great variety of compositions, many of which are suitable for plumbing systems. The applicable code is the most important factor in selecting the type of plastic pipe for any specific purpose. All plastic pipe, components, and jointing methods used in potable water systems must be approved by the NSFI. Plastic pipe must be closely integrated with the selection of hangers and the entire pipe support system.

The advantages of plastic pipe include excellent resistance to a wide variety of chemical and waste effluents, resistance to aggressive soils, availability in long lengths, low resistance to fluid flow, and low initial cost. Disadvantages include poor structural stability (requiring additional supports), lower pressure ratings at elevated temperatures, susceptibility of some types of plastics to physical changes due to exposure to sunlight, low resistance to solvents, and production of toxic gases released upon combustion of some types of plastics.

Three designations are used to express pressure rating and wall thickness: schedule (dimensions are outside-diameter-controlled, matching iron pipe size); standard dimensional ratio (SDR) (a pressure rating only); and dimensional ratio (a pressure rating only using nonstandard dimensional ratios). Where the pressure rating is the prime consideration, the abbreviation PR is used.

Polyvinyl Chloride (PVC) Pipe:  PVC is used for potable water and drainage systems. It is one of the most widely used of the plastic pipes. It has a low pressure and temperature rating and very poor resistance to solvents.

PVC pipe and fittings must conform to the following standards:
1. ASTM D 1785, PVC Plastic Pipe, Schedules 40, 80, and 120
2. ASTM D 2241, PVC Pressure-Rated Pipe (SDR Series)
3. ASTM D 2466, PVC Plastic Pipe Fittings, Schedule 40
4. ASTM D 2467, Socket-Type PVC Plastic Pipe Fittings, Schedule 80
5. ASTM D 2665, PVC Drain, Waste and Vent Pipe and Fittings

Chlorinated Polyvinyl Chloride (CPVC) Pipe :  CPVC is used for potable water and drainage systems. It has the same characteristics as those of PVC and is used where a stronger piping system with higher pressure and temperature ratings is required.

CPVC pipe and fittings must conform to the following standards:
1. ASTM F 441, CPVC Plastic Pipe, Schedules 40 and 80
2. ASTM D 2846, CPVC Plastic Hot and Cold Water Distribution Systems
3. ASTM F 439, Socket-Type CPVC Plastic Pipe Fittings, Schedule 80

Polypropylene (PP) Pipe: This material is widely used for chemical drainage piping systems. PP pipe and fittings are manufactured from flame-retardant material and are available in Schedule 40 or 80. Joining methods include solvent cement joints, threaded joints, or mechanical-type joints. (Only Schedule 80 can be
threaded.)

Polyethylene (PE) Pipe: It is widely used for underground fuel gas and foundation drainage piping. It is joined by socket and butt heat fusion. .

Acrylonitrile-Butadiene-Styrene (ABS) Pipe : ABS is widely used as drainage pipe and is available in Schedules 40 and 80 with plain or socket ends. Joints are made by either solvent cement or threaded connections. Only Schedule 80 can be threaded.

ABS pipe and fittings must conform to ASTM Standard D 2661, ABS Schedule 40 Plastic Drain, Waste, and Vent Pipe and Fittings.

IN BRIEF
• What Is the Manufacturing Process for PVC Pipes?

1. Chemical Combination :
Polyvinyl chloride (PVC) is created from a combination of vinyl and plastic. Experiments to create a vinyl chloride polymer were first done in the mid-1830s, but the solid product created in the lab was simply an oddity, without any apparent use at that time. Additional experimentation in the 1880s reproduced a solid that withstood high heat exposure, but it was ignored in commercial applications due to the inflexibility of the product. Not until the 1920s did scientists attempt to use co-polymers of vinyl chloride. These were easier to produce and created an extremely durable substance. The first experimental pipes made from a co-polymer of polyvinyl chloride were first produced in 1932. Three years later, commercial production began. The first commercial pipes were used in applications to transport water, sewage and waste water, and for the movement of chemicals.

2. Preparing Ingredients :
The first step in manufacturing PVC pipes is to prepare the ingredients. PVC is created in a chemical process that bonds the vinyl and chloride. The process involves the polymerization of monomer vinyl chloride (VCM). Most manufacturers use suspended polymerization that involves use of a polymerization reactor to mix chemicals and a polymerization initiator. The resulting PVC resin is suspended in water and then removed for degassing and water removal, which is accomplished by use of a centrifuge. The product is then dried and turned into a granulated dust for transportation to the manufacturing plant, where it will be heated to the melting point. Early manufacturing melted the dry powder as a sheet on a roll mill. Once this was done it was molded into a solid product that was ready to be rolled.

3. Additives :
Early PVC production created a rigid and stiff product. As a result of experimentation with other polymers and oil products during the 1950s and the subsequent decades, the PVC product improved dramatically. While formulas for PVC are patented by individual companies, most modern PVC ingredients include various types of stabilizers and lubricants to facilitate processing. Colors are also added during the manufacturing process to indicate the appropriate use of the piping. Dark grey pipe is used for industrial pressure applications, white and blue pipes indicate cold water uses, and green is used for sewer applications.

4. Extrusion :
While rolled PVC was produced beginning in the 1930s, no extrusion process to form PVC pipe was perfected until the 1950s and 1960s. Modifications to such processes are still underway, but today there are two predominant types of PVC pipe--cellular core and solid wall. Three separate layers are extruded to create the cellular core pipe, with hard outer layer walls sandwiching a cellular core center. All three layers are immediately incorporated into one pipe during the manufacturing process. Solid wall PVC pipes are formed in a single manufacturing step. PVC pipe is extruded to meet industry-standard 10 and 20 foot lengths. Pipes are tested for compliance with industry and government standards for durability and the ability to withstand pressure.