Polymer Technology

Discuss during which stages of the melt process the orientation of the material can be increased.

- When exiting the spinnert - During cold-drawing

Describe three ways to reduce die swell.

- increase the length of the die - increase temp. - decrease MW

Which process parameters can be varied in order to minimize shrinkage during injection molding?

- pressure holding - gate size - hold-on time

How can you avoid melt fracture defects in an extruder?

- streaming the die - increase the melt temp. - choose a resin with low MW

Brielfly explian the three main sections of an extruder screw!

1. Feed section - efficient transport. 2. Compression section - melting by internat (friction, due to compress) and extrernal heating. 3. Metering section - final mixing by shearing

Explain the term toughness.

The energy needed to break a unit volume of material.

Discuss the physical parameters that lead to melting of a polymer resin in a single screw extruder? Which one of these parameters is typically varied? How is this parameter varied?

The extruder screw is characterized by: - length (detemines degree of homogenisation) - barrel diameter (determines capacity) A high L/D ratio gives good melting and mixing. Length is typically varied depending on the melting behaviour.

Define the term induction period.

The initial slow stage of a chemical reaction, after the induction period the reaction accelerates.

Explain the term melt strength of a polymer melt.

The maximum tension that can be applied to a melt without breaking.

Explain what you have to do to create a really strong fiber on a molecular level?

- Chains must be fully oriented and extended. - Entanglements being removed. In order for orientation the fiber is streched. Often heating is applied to increase the mobility in the crystalls.

How can you avoid shark-skin defects in an extruder?

- Decrease the RPM in the extruder

How can mold temp. be used to prevent weld lines?

- Increaste the mold temp.

Give examples of three applications where biodegradability is a good property.

- food-compost bags - adsorbable stiches - Covers of plants while the plants are growing

Suggest three materials/processing parameters that can be varied to avoid "shark skin".

- heating the die - regulating pressure and speed - higher MWD

Briefly describe the technique Film Blowing

A "tube" is extruded upwards through a die. The "tube" is inflated with air inflating the melt. The material undergoes biaxal orientation.

Discuss how the aspect ratio of an electrically conducting filler influences the percolation thershold in a polymer nanocomposite.

A higher aspect ratio gives a lower percolation thershold and vice versa.

Define the term "nanocomposite"!

A multiphase solid material where one of the phases have 1,2 or 3 dimensions, less than 100nm.

Describe Sheet molding compounds (SMC)!

A premade thermoset copound is formed by high pressure in a closed mould. Crosslinking is initiated by heating to 150-160°C.

Describe injection molding.

A screw inside an extruder rotate in a heated barrel to mix the polyer. The screw also work as a plunger that puches the melt into the mold.

What type of extruder must be used if a sensitive polyer such av polyoxymethylene (POM) is to be extruded?

A twin-screw extruder transports he melt without relying on the friction which are preferable for POM.

Describe the possibilities that are available for both durable and biodegradable plastics. What are the A and D for plastics regarding recycling? Can it differ from country to country?

A: Less littering, less waste, energy recovery, less need for new raw-materials. D: Hard for an untrained eye to separate the plastic types, can't mix all plastic types due to impurities In ore sounthern countries it may to easier for biodegradable materials to degrade due to sun and UV-light. In Scandinavia it takes more energy to warm up a house and is therefore more importnat to recover energy.

Describe what went wrong with the composition of the ABS polymer: - Melt at much lower temp.?

ABS contains more polybutadien than normal

Describe what went wrong with the composition of the ABS polymer: - The ABS did not melt at normal temp.

ABS contains more polycrylonitrite than ususal

Propose one strategy that can be used to compatibilize nanoparticles with a polymer matrix.

Add fillers and/or reinforcing agents. This will reduce the cost and increase stiffness, strength and usage temp.

Briefly discuss one approach that permits to increase the HDT of PE.

Adding fillers or reinforcements will increase HDT since it makes the material stiffer whereas Tm is not affected.

How does the melt viscosity vary with increasing MW.

An increase in MW will increase the shear viscosity and decreases the length of the Newtonian plateau. Entanglements will drop the Newtonian plateau faster.

How can you obtain polymers that are biodegradable? Is there a differnece in sustainability between types of biodegradable polymers?

Both from fossile and renewable resources. I think the difference in sustainability is between the two types. The renewable polyerms that are biodegradable are more sustainable!

Explain the following manufacturing methods and explain what types that are produced. c. Pultrusion

Continuous manufacturing technique. Cost effective for mass production of linear composite Tubes, ski poles, radio masts

Sketch the expected stress-strain behaviour and specify the yield point and break point.

DRAW PICTURE!

What are the elongational viscosity of a polymer? How does it differ from the shear viscosity? It is important when manufacturing films through film blowing. Why?

Elongation viscosity - resistance to enlongation WITHOUT shear, which is involved in shear viscosity. It is important due to ensure that the melt can be stretched without bursting

Which process would you select to avoid any thermal degradation when adding pigment during compounding? Why?

Extruder, due to its excellent mixing properties and gentle product handling.

Explain the differnece between a filler and a reinforcing agent.

Filler: reduce the cost Reinforcing agent: improve the mechanichal properties of the material.

Suggest modifications for a high temp. application polyphenylene sulphide. The E-modulus should be 50% higher

Fillers increases hardness and E-modulus and decreases elongation and ultimate strength.

Explain the following manufacturing methods and explain what types that are produced. b. GMT followed by compression molding

GMT: 1. Preheated blanks placed on mould 2. Mould begins to close - materials flow 3. Mould closed - material flow stopped 4. Premoulding Compression molding: Cost effective method for long series

σBr, εBr and Vicat T HDPE vs. LDPE

HDPE: core crystalline, higher Tm - higher Vicat T LDPE: lower crystallinity, lower Tm - higher σBr & εBr a

How does the tacticity affect HDT on a polymer?

HDT increases with crystallinity. In order for a polymer to be crystalline it has to have a regular structure (e.g. isotactic)

Discuss the molecular configuration of an triblock copolymer of polybutadiene and polystyrene that displays elastic at room temp.

HIPS = mixture of polybutadiene and PS. 25% PS - glassy (Tg = 100°C) 75% Polybutadiene - rubbery

Describe what went wrong with the composition of the ABS polymer: - Normally: very high stiffness - Now: less transparent and slightly softer

Have less polystyrene in ABS

What is envirmomental stress cracking? How does it differ from ordinary polymer degradation?

If a material has residual stress after molding, cracks can be formed in a material when exposed to chemicals. Normally: A chemical attack or molecular degradation.

Explain the term weld line.

If there is more than one gate into the mold it is invitable that the melt fronts meet. The cooler the melts are when they meet, the less plastic will "weld" together.

Explain why die swell occurs.

In the die, the molecules are less oriented. When the melt comes out from the die, the molecules will relax to random coils. This will lead to expansion perpendicular to the machine-direction -> die swell

What will happen to the curve (Q.22) if we change the MW and MWD of our polymer? Why? Will it affect the processing?

Increasing MW -> incease viscosity -> decrease length of Newtonian plateau. It will need more time in the extruder in order to get it mixed well -> increase of energy consumption.

Suggest modifications for application of PA 6 with low friction but in contact with water gets too low modulus.

Introduce organic groups to the polymer. The water uptake decreases with increasing amount of CH3-groupos.

Explain the main function of plasticising additive for a polymeric material. Briefly discuss the two mechanisms.

It improves the mechnical properties of the material. 1. Spacers that decrease the energy required to rotate bonds. 2. Break interactions between polymer chains by creating links between the plasticiser and the polymer.

What is melt fracture in an extrusion process?

It is generally not on the surface but throughout the material. Caused by a turbulent and discontinous flow as the melt enters the die

What is the most important property of a polymer regarding its possibility of being spun to a fibre?

It must be semicrystalline

Brielfy discuss the origin of "shark skin" formation in a extruder.

It occurs from extruding the resin too fast, forming a surface layer that is overstreched.

At which point during the tensile deformation do you obtain the highest possible degree of uniaxial orientation?

Just before the break point. There's a fibrillar structure in the material.

Describe how Kevlar is made and get its strength!

Kevlar is an aramid fiber meaning it's created by solution spinning. Its strength comes from strong intermolecular forces between the chains.

Which of HDPE and LDPE has the higher HDT. Discuss structual diffeences and their impact on relevant transition temperatures.

LDPE have lower HDT due to it have lower crystallinity than HDPE.

Can another composite be used to avoid the problems described in Q.17?

Nanocomposites can be used. Nano-clay acts like a fibre and increases strength, stiffness and HDT without making the processing harder.

How does the fibre length impact the fibre orientation in a composit?

Long and continuous fiber are mostly oriented in difined directions, making the material anisotropic. Small fibres are commonly random oriented, giving isotropic materials.

Two polymers (polyamide and fibers). Diffenrence in length of fibers. What will be the difference in strength and stiffness?

Long and continuous fibers are mostly oriented in defined directions giving high strength to weigth ratio. Best strength/stiffness is obtained in the fiber with direction 0°.

What does the Molecular Weight and Molecular Weight Distribution tell us for a polymer?

MW: Decreasing the MW decreases the zero shear viscosity -> increases the length of the newtonian plateau. MWD: A broader MWD shortens the newtonian plateau and the shear viscosity drops faster.

Describe how a stress relaxation test curve is constructed.

Make tension experiments at different temperatures. With a temp. shift factor it is possible to make a master-curve.

Briefly describe the term master-batch.

Master batched is used as an additive to colour plastics or refine other properties in the plastic.

You are suppose to design a laminate with no differnece in the in-plane tensile strength. Explain your choice!

Maybe like a plywood, 0°,45°,90°,45°,0° and so on...

Descibe melt spinning

Melt spinning (PP, PE, PA) An extruder feeds several spinnerets which is a die with many small holes. When the melt is pressed out from the spinneret the melt is cooled down with an air stream. Most common and used to make fibers.

Describe Transfer molding.

Molding material is preheated and loaded into a pot. A plunger is used to force the material from the pot, through channels and into the mold cavities.

Draw the structure of PE, PVC, PS and PA and explain how the molecular structure difference affect the properties.

PE: linear, HDPE, LDPE gives different properties. PVC: Atactic structure -> mainly amorphous. High density PS: Bulky -> Brittle. Amorphous PA: Restricts movement -> increased tensile strength. Close packing -> Increased crystallinity

For which application is a plasticiser required?

Plasicisers will lower the Tg and hence, increase flexability and softness and also make the material less brittle.

You have two polymers will filler added, one isotactic and one atactic. How do you expect the HDT to differ from the two composites?

The atactic polymer will have lower HDT then the isotactic polymer du to crystallinity.

How come that the same polymers can show different properties on a strain-stress curve?

Polymers are sensitive to temperature and "rate". If the tensile test is done at low temp. the polymer will be stiffer and if the temp. is high the polymer will be softer and more ductile. If the test is performed at high rate the polymer will have a more brittle behavior.

What are polyolefins?

Polymers produced from an alkene.

How does fillers change a polymer material?

Primary reason is to reduce cost. - Increase in E-modulus and hardness - Decrease in elongation at break and ultimate strangth Fillers are more polar than most polymers -> surface treatment to improve the blending properties.

What will happen if we add filles to the polymer in Q.22?

Primary reason to use fillers: decrasing cost. Increased hardness and E-modulus and decreased eleongation at break.

What do you mean with polymers being pseudoplastic? Draw a curve!

Pseudoplastic = shear thinning

How does fibers (reinforcing agents) change a polymer material?

Reinforcing agents are used to improve mechanical properties. - Increase stiffness, strength, usage temp. - Decrease shrinkage

Define the term reinforcement in the context of polymer composites.

Reinforcment is the discontinuous phase in a material that aims to improve its properties in some way.

Explain the following manufacturing methods and explain what types that are produced. d. Resin transfer molding (RTM)

Resin transfer molding is an intermediate volume molding process for producing composites. In RTM, resin is injected under pressure into a mold cavity. This process produces parts with two finished surfaces.

What thermal transition temperatures most strongly influence the HDT of an: 1. amorphous 2. semicrystalline polmer

Semi-crystalline: depends on the crystallinity. At high crystallinity the HDT can be affected at Tm

How are the in-plane tensile strength affected by fiber orientation.

The best strength is obtained in the fiber direction (0°). The lowest strength is obtained at 90°. FIGURE!!

Explain how the size of the gate through which the mold cavity is filled influences the degree of shrinkage. How can the size of the gate be used to minimize shrinkage during injection molding?

Several gates into the mold will cause weld lines due to that the melt points will mett. The size of the gates determines the temperature of the melt into the mold. With a large gate the temp. will be higher -> more shrinkage.

Explain shear viscosity and melt viscosity.

Shear viscosity is a material resistance to shear flow. Melt viscosity is a constant at low shear or frequencies. The viscosity in this region is known as Newtonian.

Describe two methods to make non-woven materials. One where you use long fibers and one when you use short fibres.

Spunbond process (long fibers) - polymer granula is melted in an extruder and fed out through a die. The filament are being pressed out, solidifies and is mechanically streched. The filament is then is then laid down on a formation wire. Carding (short fiber) - Produce a continuous web or sliver by passing the fibers between differntially moving surfaces covered with card clothing.

What is stress relaxation and fatigue?

Stress relaxation: How polymers relieve stress under constant strain Fatigue: The weakening of material caused by repeatedly applied loads -> lower tenacity

What is the diffenrence between temporary and permanent crosslinks?

Temporary: Temporary networks that disentangle under strain. Permanent: E.g. crystalline -> can be removed by heating above Tm.

Which transition temp. is most strongly affected by addition of plasticisers?

Tg will be lowered.

Assume that you are restricted to injection molding as prosessing method and would like to fibre reinforce PA6. Your first choise is to use glass fibres. Discuss the main consequences this have for the final product.

The addition of glass fibers to PA6 will increase the ultimate strength and the HDT. This gives a stronger material that is less sensitive to higher temp. It can be hard to process PA6 if the fibers are too long.

Discuss which of the following properties strongly influence the melt strength of a polymer: - MW - tacticity - degree of long-chain branching

The melt strength is related to molecular chain entanglements and polymers resistance to untangling under strain. The mechanical properties that "prevent" untangling are molecular weight. If the MW increases the melt strength is improved at low shear rates.

Define the term percolation thershold in the context of electrically conducting polymer composites.

The minimum filler content in a polymermatrix after there is no significant change in the electrical properties of the composites.

Describe Compression molding.

The molding material is placed in an open cavity. The mold is closed and pressure is applied until the molding material has set. - rainboots

Explain wet spinning!

The polymer is in a solution. The solvent is being evaporated and the polymer solidified by precipitation in a non-solvent.

Expalin dry spinning!

The polymer is transferred to a liquid state by dissolving it in a solvent. The solvent is pushed through the spinneret into a tower where hot air is blown.

Why does shinkage occur during injection molding? Is shrinkage more severe for semi-crystalline or amorphopus polymers.

The temp. of the melt is hot when it goes into the mold. When the temp. of the melt decreases, the melt contracts due to thermal contractions. Amorphous polymers have very low shrinkage.

Explain the term heat deflection temperature.

The temperature at which a polymer deformes under a specific load.

When is the fatigue properties of a polymer important to consider? What is the "fatigue endurance limit"?

They are important to consider when the material is going to be used in applications where it is going to be exposed to repeated stress (cogwheel) FEL: Maximum stress below which fatigue never takes place.

Why do elastomeric rubbery have rubbery properties and how is this achived? Which is the main advantage with the thermoplastics elastomers compared to ordinary rubbers?

They have rubbery prop. due to that they consist of copolymers of hard and soft segments, where the hard segments act as crosslinks. With increased temp. the hard segments can melt. A: They can be remelted due to lack of crosslinking. Easy to process.

Suggest modifications for a PP chosen as a suitable solution for storage containers. You would like to increase the resistance towards gasoline.

To increase the resistance towards gasoline (organic solvent), polar groups can be introduced in the chain. The organic gasoline wouldn't want to mix with PP then.

Descibe solvent spinning

Wet and dry spinning The polymer can't be kept as a melt instead it is dissolved before spinning.

Many plastics are based on blends. Discuss advantages/disadvantages of such polymer blend with regard to sustainability use of of plastics.

When mixing the polymers it can improve the mechanical properties (strength, durability etc.). This can increase the life time of the product which is good from an enviromental point of view. However, the mechanical properties must be realistic to the usage, without being exaggerated (unnecessary material etc.)

Consider a typical melt spinning process of a semi-crystalline polymer. Which step of the process does the tensile deformation experiment simulate?

When the polymer goes out from the die as spinning fillaments.

Describe the term thermoplastic elastomer.

a class of polymers with both termoplastic and elastomeric properties.

Give one advantage/disadvantage for the polymers: a. polypropylene b. polyamide 6,6 c. PTFE (teflon)

a. A: No stress cracking D: Brittle below 0°C b. A: Very strong in fiber direction D: Sensative towards polar solvents c. A: Withstand high temp. intervals D: High density

Explain the following manufacturing methods and explain what types that are produced. a. Prepreg lay-up

a. A technique employed to create mechanical pressure on a laminate during its cure cycle. Aerospace and racing industries.

How has the following polymers been modificated? a. PE that could not be dissolved in water b. PP with traces of ethylene copolymer c. A fiber reinforced PP with silicon detected

a. Crosslinking with a radical process. (increase strength, stiffness). b. PP is very brittle. That's why you can copolymerize it with PE. (lower crystallinity, Tm). c. Silicone come from silane (cupling agent). (Good adhesion between matrix and fiber).

Discuss possibilities and difficulties with recycling of the following materials: a. PVC from buildings b. Disposable PLA c. Bottles made from PE;PP;PET and PVC

a. D: will contain additives -> not possible to get monomer back. A: is a thermoplastic -> can be reused. b. D: often sensitive to water A: Renewable source and is biodegradable c. D: Difficult to sepparate the plastics from each other. A: thermoplasitcs -> can be reused.

Injection molding. Replace PS with HDPE. Components did not fit. a. How and why did the HDPE differ from PS? b. What should your company really have done? c. Is there a "quick fix" to decrease the problems that have aroused?

a. HDPE is a semicrystalline polymer with quite high degree of crystallinity. Crystalline polymers suffer from mold shrinkage when taken out from the molding machine an the temp. is dropped. PS is completly amprphous and have a very low mold shrinkage. b. Compensate for the mold shrinkage by changing the molt size (expensive!). c. Adding fillers to the polymer solution which can reduce mold shrinkage.

A reinforcing agent: a. What are the primary functions? b. How do the mechanical properties of the polymer change after the addition?

a. Improve the mechanical properties of a material. b. The mechanical stress is tranferred to the fiber which can handle the stress. Increases stiffness, strength and usage temp. and to decrease shrinking.

Explain why the following properties are improved for nanofillers in a polymer nanocomposite! a. tensile strength b. no loss in transparency c. barrier properties

a. The nanofillers act as fibers. No dimensions leads to larger surface area which gives larger possibility for interaction. b. The nanofillers are too small to scatter visible light. c. Fills out the nanocomposite which makes it more compact.

Fillers: a. What are the primary functions? b. How do the mechanical properties of the polymer change after the addition?

a. To reduce the cost, increase hardness and stiffness and to decrease elongation and ultimate strength. b. increase in stiffness and decrease in ultimate strength and in elongation at break.

a. Fishing rod b. Ski/snowboard goggles c. kitchen ware with living hinge d. sole for hiking boots Isotactic polypropylene, glass fibre composite, polycarbonate, styrene butadiene rubber.

a. bendable, non-brittle, low density E.g. Glass fibre Composite b. transparant, shatter resistant E.g. Polycarbonate c. Strong and sturdy, no stress cracking E.g Isotactic polypropylene d. abrasion resistant E.g Styrene-Butadien rubber

a. Lapton casing b. bungee cord c. a cogwheel d. biodegradable camping cutlery polyamide, polyester, synthetic rubber, polycarbanate, polyethylene.

a. resist bumps, shockproof, thought E.g. polycarbonate b. elastic deformation, tensile strength E.g. synthetic rubber c. thoughness, stiffness E.g. polyamide d. biodegradable at "room"-temp E.g. PLA

a. police riot shield b. bottle for engine oil c. shoe sole d. plastic toy car polyamide, synthetic rubber, polyethylene, polysulfone, polycarbonate.

a. transparant, low density, shatter resistant E.g. polycarbonate b. cheap, low water permability E.g. polyethylene c. abrasion resistant E.g. synthetic ruber d. thougness, durability E.g. polyamide