Materiali da costruzione avanzati - Parte 3

Polymers

Polymeric materials: pipeline, windows frame, polymer that replace the glass (polycarbonate), thermal insulator (EPS).

Characteristics

• Insulator (thermal, electrical, acoustic).
• Light weight (low density).
• Low cost, reason why we have large use.
• Chemical resistance, for example container of acid solution are made by plastic.
• Workability, shape that we want in an easy way, the change of shape do not need high energy, for this reason the cost is low.
• Low resistance to fire (fire smoke), polymer came from oil and for this reason burn very easily, we have problems related to the production of smoke.
• Low mechanical resistance (fibers), problem of low mechanical properties except for the fiber.
• High coefficient of thermal expansion, can give problem when we have change in temperature.
• Low resistance to solvent.
• No long term performance, short period of durability, is not easy to reach 15 years of service life.

Distribution of the global plastics use by application

44% of plastic is used for packaging, there are a lot of advantage in the use of plastic materials, for example for the protection of food or for the protection of brittle material with the expanded polymers.

The 18% is used for building and construction, for this reason is important for us to know very well the properties of polymers.

Polymers are organic materials (mainly [C] and [H], also [O], [N] ...).

Organic means that contain carbon but they also contain other material.

Classic shape of polymers, polymers have connections with different macromolecules, the bond inside of macromolecules are very strong and they are called covalent bonds.

The problem is the connection among the different macromolecules because we have weak bond (like hydrogen bonds) this is due to the low mechanical properties.

Composed of macromolecules: very large molecules formed by the repetition of a small group of molecules (composed of thousands of covalently bonded atoms).

Structure

Macromolecules.

Composed of many repeating subunits (monomers) we have to use in order to produce macromolecule, we have to transform the subunit in monomer, we destroy the double bond to create macromolecule.

Identify the polymer.

Created via polymerization, technique to produce macromolecules.

Monomers can have:

• The same chemical structure (homopolymers).
• Different chemical structure (copolymers) (acrylonitrile butadiene styrene (ABS), styrene/butadiene co-polymer (SBR), nylon).

ABS used for chair, SBR is rubber and it is used for a lot of applications like for nylon.

Polymer: combination of macromolecules (weak bond among them) (name = poly + monomer).

For example the Ethylene became Polyethylene, that have 4 molecules of hydrogen, if one of these molecules change in Chloride we have the Polyvinyl-chloride.

Properties: depend on:

• Chemical structure (monomer type).
• Chain length.
• Macromolecule structure, affect the properties of material.
• Molecular organisation in the solid state (disordered or partially ordered) if we want to have solid polymer the macromolecules have to have big dimensions.

Low molecular means slow chain and give problem related to the bins, we have liquid form.

If we want to have solid we have to increase the length of macromolecules.

The un-crosslinked has only weak bond, the branched generally limit the movement of macromolecules.

The crosslinked has the production of some strong bond among macromolecules, these are present only in some specific position, this covalent bond increase the mechanical properties.

We have areas in which we have amorphous structure and some areas in which we have crystals that are in order.

This crystalline part help to increase some characteristics.

We have not ordinate structure, is not so strong but we can have transparent materials.

Types of polymers

Polymers can be divided in 3 types:

• 1) Thermoplastic polymers —> un-crosslinked amorphous semicrystalline.
• 2) Thermoset polymers —> crosslinked.
• 3) Elastomers —> thermosets, slightly crosslinked, or thermoplastic.

The advantage of elastomers is the easily change of the shape, they can have an incredible deformation and they can recover completely the deformation in elastic way.

These are used for wheels (ruote) and they have advantage also related to the adhesion.

All the polymers are marked (like in the bottom of bottle).

It is very important the specific temperature because polymers are very sensitive to temperature because this change their behavior.

Different for amorphous or semicrystalline (very important: knowledge of the working temperature) the working temperature depend from polymer to polymer.

Glass temperature is the temperature in which we consider to work glass to change easily the shape.

The glass temperature change the viscosity that consent to have a malleable material without reach the liquid form.

After the glass temperature the material became rubber.

In semicrystalline we have both melting temperature and glass temperature.

The polymers shape remain solid until the melting temperature.

Glass and melting temperature give information about the correct use.

Transparency is a property only of amorphous structure but some semicrystalline structure (like plastic bottle) are transparent, this depends on the dimensions of crystal parts.

There is not an incredible resistance in temperature for polymers, it is not easy to determine glass temperature.

When we start to reach the Tg we start to change the shape, the temperature that is before Tg is called HDT (Heat Distortion Temperature).

There is test in which we apply a load in a small sample.

Initially we have no problems in deflection but when we start to increase the temperature we start to have deflection, this point is at the HDT.

Processing technologies

To obtain polymers at a specific shape we have to start with macromolecules.

The producers use pellets that have spherical or cylindrical shape, or use liquid or powder, to produce polymers.

Start from pellet, heat the material at a temperature higher then Tg for amorphous and higher than Tm for semicrystalline.

After the forming there is a fast cooling, we can consider to heat again these materials and reform for recycling.

Generally with every heating process we reduce a little bit the dimensions of macromolecules and so a decrease in mechanical properties, for this reason we can not recycled in an infinity way the polymers, but we can add new pellets to replace the “old” macromolecules and maintain the performances.

Start from liquid or powder, we put this in former that give the shape and then we have to stimulate reaction or we can use temperature to have crosslinking, with this is not possible to change the shape after the forming and so is not possible the recycling.

For recycled these materials we can reduce in powder the formed material and mix this powder with other new powder to produce new polymers.

We have not a lot of crosslinking, this give the characteristics of easy modification.

We start from pellet, we can give shape and after production we have to create crosslinking.

The vulcanization is due to the presence of sulphur.

Additives are used to modify the characteristics of materials.

Mechanical properties

Problems of Elastic modulus (E) it is very low, also tensile stress is slow.

The limit is the use due to low mechanical properties.

Advantage is the ductility.

Polymers have low density.

Depends on:

• Type of polymer, related to the connections between the macromolecule.
• Temperature, Tg and Tm, we have to know the work temperature of polymers.
• Load application speed, we can have a very easy change in the mechanical properties, for example we can have no deformation with a fast load application and very big deformation with a slow load application.

Viscoelastic behaviour

If we consider a load apply faster we have two different yield mechanisms, brittle and ductile.

The shape of stress - strength curve is strange.

When we apply load we have an area in which change the color because there is the formation of necking.

The random shape of macromolecules became order in the necking, this increase the mechanical properties in that point.

We can increase the necking area with the application of load.

Crazing is microscopic gap, we have a less transparency of material due to the fracture of fibrils.

For amorphous: before Tg we have solid state, when we reach the Tg there is a decrease in E modulus, after Tg we reach the rubber state.

Material change the viscosity and then we have something that is similar to liquid.

For semicrystalline: we have the solid state before the Tg, after this we have a small decrease in the E modulus, then we reach the Tm, at this point we have rubber state and after Tm material become liquid.

We have crosslinking that help material to maintain the solid state also after Tg.

We can not destroy crosslinking.

Degree of crosslinking, recover completely the shape.

Elastomers: amorphous with low Tg (rubber at Tamb) + slightly crosslinked.

Elastomers can support a very high stress.

Macromolecule have inside some strong bonds that consent to maintain fix the shape of materials.

For elastomers we start from thermoplastic material.

If there are cross-linking the materials recover completely their shape, come gli elastici.

Without cross-linking the material do not recover the initial shape, come il pongo.

The behavior of material at a temperature lower than glass temperature we have the brittle behavior.

This is the reason why these materials are not used when the temperature outside is very low, this is also reported on the standards.

When temperature is closed to glass temperature there is the change in behavior, so the material has plastic ductile behavior, there is plastic deformation and non completely recover of the shape.

If temperature is higher than glass temperature, for thermoplastic there is an incredible plastic deformation, for crosslinked there is the completely recover of shape.

Working temperature

The working temperature is temperature between a minimum temperature in which the material is too brittle, till a temperature that is 20 Celsius degree lower than glass temperature.

We can use the materials also in the rubber state only for special materials like for non structural applications as bitumen.

Advantage that material remain stable till melting temperature so there is the capability of material in a range between the temperature in which material is too brittle till a temperature 20 Celsius degree lower the melting temperature.

Higher range of applications in structural.

Working temperature depends on the degree of crosslinking, we can use material in range that is also after glass temperature because we have solid material also in rubber state.

With high number of crosslinking the change in behavior is negligible.

No possible use this material closed to glass temperature.

Viscoelastic behaviour: creep and relaxation

Creep refers to the gradual deformation that occurs in a material when a constant stress or load is applied over an extended period of time.

When a viscoelastic material is subjected to a constant stress, it initially undergoes immediate elastic deformation.

However, over time, the material continues to deform, exhibiting a time-dependent response.

This time-dependent deformation is known as creep.

In general, higher stress levels and elevated temperatures accelerate creep deformation.

Creep can be observed in various materials, such as metals, polymers, and even geological materials like rocks and soils.

Relaxation is the process by which a viscoelastic material reduces its internal stress over time when subjected to a constant strain.

When a viscoelastic material is deformed or strained and then held at a constant strain, it initially exhibits a stress response.

However, with time, the stress decreases, and the material relaxes.

This relaxation process occurs as the internal molecular or structural rearrangements within the material gradually reduce the stress.

In summary, creep and relaxation are two fundamental aspects of viscoelastic behavior.

Creep refers to the time-dependent deformation that occurs under a constant stress, while relaxation describes the time-dependent reduction of stress under a constant strain.

Increase load increase the effect related to viscoelasticity.

To reduce stress I have to increase the cross section because of the formula (θ = F / A).

Other properties

Thermal properties.

Low thermal conductivity —> thermal insulator (expanded).

Thermal conductivity increase, increasing crystallinity.

High coeff. of thermal expansion, polymers have coeff of thermal expansion higher than the other materials.

Optical properties.

Many polymers with amorphous structure (PMMA, PC and PS) are transparent (refractive index similar to that of glass).

Crystalline areas in a polymer reduce transparency (reflection and refraction phenomena; LDPE vs HDPE).

Electrical properties.

Generally insulator (conductive polymers).

Durability

Degradation can cause variations in the mechanical, thermal, optical and electrical characteristics, alteration of the color (yellowing or chromatic variation) or material failure.

Polymers change color when there is the start of degradation, there is decrease of properties, these are caused also to environment.

Aging

One of the possible degradation, usually we have this in thermoplastic polymers, during production there is fast cooling and this give structure not so equilibrate structure so during the movement inside of structure in time there is the change of material structure and became more rigid.

Amorphous part below Tg.

Fast cooling, non-equilibrium structure.

In time —> equilibrium condition.

> density, > rigidity, e low of the Volume.

Reversible: high T above Tg.

Thermal and chemical degradation.

Small cracks and material become opaque.

Biological degradation

Some bacteria are able to destroy also carbon bond, generally in time there is the continuous reduction in the dimension of macromolecule, so in time become micro-plastic.

Fire

Self-extinguishing materials (they do not start the fire), important that when we burn material we need to have the collapse of material but not the propagation of fire, to check this we can make test of burning material.

Problem: smoke and substances released.

Distribution of the global plastics production

Performance and price.

Polymers identification.

Types of polymers.

Polymers in building

Polyvinyl chloride (PVC) is a commonly used material in construction due to its versatility, durability, and cost-effectiveness.

PVC is a synthetic polymer made from vinyl chloride monomers.

It can be rigid or flexible, depending on the formulation and additives used during its production.

• 1 Pipes and Fittings.
• 2 Window Frames and Profiles.
• 3 Roofing Membranes.
• 4 Siding and Cladding.
• 5 Flooring.
• 6 Insulation.

PMMA, which stands for polymethyl methacrylate, is a transparent (PLEXIGLASS) thermoplastic polymer commonly known as acrylic or acrylic glass.

It is widely used in construction for various applications due to its optical clarity, impact resistance, weatherability, and ease of fabrication.

• 1 Windows and Skylights.
• 2 Architectural Glazing.
• 3 Lighting Fixtures.
• 4 Signage and Displays.
• 5 Noise Barriers.
• 6 Bathtubs and Shower Trays.
• 7 Roofing and Canopies.

PMMA is favored in construction for its optical clarity, versatility, and weather-resistant properties.

However, it is worth noting that PMMA is less resistant to scratching than glass and can be prone to yellowing over time when exposed to UV radiation.

Proper maintenance and cleaning procedures should be followed to maintain its appearance and longevity.

Polycarbonate is a durable and versatile thermoplastic polymer that is widely used in the construction industry due to its excellent impact resistance, transparency, and weatherability.

• 1 Windows and Skylights.
• 2 Roofing and Canopies.
• 3 Greenhouses.
• 4 Skylights and Atriums.
• 5 Safety Glazing.
• 6 Noise Barriers.
• 7 Facades and Curtain Walls.
• 8 Partitions and Dividers.

Polycarbonate's versatility and performance properties make it a popular choice in construction for applications where transparency, impact resistance, and design flexibility are essential.

It is important to select the appropriate polycarbonate grade and thickness based on the specific requirements of the construction project.

Polymeric foam

Polymeric foam is a type of foam material that is widely used in the construction industry due to its lightweight, insulation properties, and versatility.

There are various types of polymeric foams utilized in construction, including the following:

• 1 Expanded Polystyrene (EPS): EPS foam, often referred to as styrofoam, is a lightweight and rigid foam material. It is commonly used for insulation in walls, roofs, and foundations.
• 2 Extruded Polystyrene (XPS): XPS foam has a more closed-cell structure, which provides improved moisture resistance. XPS foam is commonly used in below-grade applications, such as insulating foundation walls and insulating inverted roofs.
• 3 Polyurethane (PU) Foam: PU foam is a versatile and high-performance foam material that offers excellent insulation properties. It can be used in the form of spray foam insulation or rigid foam panels.
• 4 Polyisocyanurate (PIR) Foam: PIR foam is a type of rigid foam insulation that provides excellent thermal insulation. It is widely used in roofing applications, including flat roofs and pitched roofs.
• 5 Polyethylene (PE) Foam: PE foam, often in the form of closed-cell foam sheets or rolls, is used for a variety of construction applications. It can be employed as a cushioning material, vapor barrier, soundproofing material, or as an underlayment for flooring installations.

Each type of foam has its specific characteristics, including insulation properties, moisture resistance, and compressive strength.

The choice of foam material depends on factors such as the application, building design, required thermal performance, and local building codes and regulations.

Polymeric sealants are commonly used i