Polymer Materials: Properties, Processing, and Applications

Posted on Dec 20, 2024 in Geology

Introduction to Polymers

Much of organic natural products consist mainly of complex macromolecules that are repeated systematically. Certain groups of atoms with identical provisions, called monomers, are connected by covalent bonds. Because of cellular activity, thousands of monomer molecules are linked in polymeric macromolecule aggregations, which causes the so-called natural polymers or biopolymers. Chemical technology has allowed us to modify the molecular structure of these materials for their best use and also to produce simple polymers by polymerization of synthetic chemicals that have proved useful and are now mass-produced, sometimes with advantages, by replacing natural polymers and even other materials. Today, what no one disputes is that these materials have emerged as a key factor in industrial development experienced in recent years by sectors such as agriculture, construction, food, medicine, telecommunications, and transport.

General Characteristics of Plastics

Molecular Structure of Polymers

Macromolecules that constitute polymeric materials are formed by sequences between carbon atoms that can insert atoms of silicon, nitrogen, oxygen, and sulfur (among others), covalently bonded, forming what is called a molecular chain that can be linear or reticulated. Polymers obtained from identical monomers are known by the name of homopolymers. By contrast, when two different monomers give rise to a polymer, the molecule has new, different properties from any of the monomers separately. This body is called a heteropolymer (copolymer) and has properties intermediate between the constituent monomers.

Molecular Weight and Average Degree of Polymerization

A peculiarity of plastic materials is that the molecular weight of macromolecules is not the same because the number of molecules that are grouped into the polymerization reaction (degree of polymerization) is determined by random circumstances. There is, therefore, a statistical distribution of molecular weights of the molecules that form a polymer sample, defined by an average molecular weight, an average degree of polymerization, or an index of heterogeneity as a reference to characterize it. This is very important in determining all the physical properties of plastics.

Morphology of Organic Polymers

The properties that characterize organic polymers (consisting mostly of carbon and hydrogen) not only depend on the structure or molecular configuration and the statistical distribution of molecular weights but also on the spatial arrangement or conformation of the molecular chains. Unlike the configuration or molecular structure, it cannot be changed without breaking and re-establishing covalent bonds between atoms of each macromolecule. The shapes of these molecules can be modified by physical means: mechanical efforts and thermal variations. We can say that the characteristics (mechanical, thermal, chemical, etc.) of a part or product made of a plastic material depend not only on the macromolecular structure of the raw material used but also on the process of transformation it has undergone and, consequently, its crystallization.

Procurement Process Sculpture: The Polymerization

Carothers distinguished two types of polymerization: those performed by condensation of monomers (polycondensors), which produce simple molecules as byproducts of the reaction, and those produced by the addition of two monomers (polyaddition) in which such products are not formed. The resulting polymers of the first type are called polycondensates (polyesters, polyamides, phenolic resins, etc.), and the second type are called polyadducts (polyethylene, polybutadiene, polyurethane, for example). For plastic materials with certain characteristics, which meet in each case the conditions for their use, the polymerization process should be properly controlled. This is achieved with the help of agents involved in the reaction that serve different functions. These agents can be classified into the following groups:

  • a) Catalysts: Serve to activate the polymer reaction.
  • b) Retardants: Slow down the action of the polymer.
  • c) Inhibitors: Completely stop the reaction.
  • d) Chain Transfer Agents: They cause the formation of bonds between chains at specific points.

Polymerization Process

The chemical process called polymerization is the process to obtain the polymer from a given quantity of monomers. For the polymerization reaction to occur, the intervention of certain substances that act as catalysts is required, in whose presence the monomer becomes a polymer. Every polymerization or polymer reaction takes place in three phases or periods:

  • Induction Phase: In this period, the reaction proceeds slowly, activated by the catalyst, until a conversion of 10%.
  • Propagation Phase: The reaction rate increases rapidly until a conversion of 90%.
  • Termination Phase: The process slows down, and the conversion of the polymer ends.

Depolymerization

The reverse process of polymerization, i.e., the decomposition of the plastic material by the action of external agents, is known as depolymerization. This is an unwanted process by which plastic loses some (if not all) of its properties. To avoid depolymerization, products are added to counteract the action of destructive agents that cause it. The main destructive agent that contributes to depolymerization is the action of heat: all plastics decompose or depolymerize with the action of heat upon reaching a temperature (characteristic of each plastic) called the decomposition temperature.

Additive Substances of Plastics

In order to improve the qualities of plastic, reduce weight, improve their moldability, and/or give them color, substances of various kinds, known as additives, are usually added. These are classified as:

  • a) Fillers: The addition of mineral fillers in plastic compounds provides increased stiffness, tensile strength, and impact resistance, low thermal distortion, and improved fire behavior. The most common fillers used are sawdust, textiles, glass fibers, asbestos fibers, and minerals.
  • b) Colors: These may be dyes, which actually alter the color of the resin, or colored pigments, whose presence imparts the desired color. As prerequisites for colors, we quote:
    • Compatibility with the plastic resin to be mixed.
    • Scattered in them.
    • Stability at high temperatures that can be achieved in the processing of the polymer (200 to 300 ºC) and extreme environmental conditions.
  • c) Plasticizers: With them, the flow of plastic during molding is increased and controlled, acting as internal lubricants. The amount of plasticizer is usually small because plasticizers almost always affect the stability of the finished product through gradual loss during the natural aging of the plastic.
  • d) Lubricants: They are also used in small amounts to improve moldability and facilitate the removal of parts from their molds.
  • e) Stabilizers: Their mission is to counteract the effects of external destructive agents, preventing depolymerization of the plastic material, such as antioxidants.
  • f) Hardeners: These substances accelerate the hardening of plastics or increase their hardness.
  • g) Solvents: The effect obtained is similar to that of plasticizers, but their action is more energetic, converting the solid plastic material into a liquid.
  • h) Reinforcing Fibers: With reinforcing fibers, the mechanical properties of plastics are improved, mostly due to the large length of these fibers.
  • i) Foaming Agents: To make them lighter and swollen for some specific applications.
  • j) Flame Retardants: They are added to reduce flammability and obstruct the entry of oxygen.
  • k) Antistatic Agents: They avoid the accumulation of electrostatic charges that make handling difficult. It also prevents dust from collecting on plastic objects.

Classification of Polymers

This classification refers to their fundamental characteristics derived from their molecular structure and behavior with variation of temperature and with different solvents:

  • Thermostable
  • Thermoplastic
  • Elastomers

Although in this classification, there is a degree of overlap.

Thermoplastics

The most significant feature that gives its name to thermoplastic polymers is that they soften, leading to flow if subjected to heating, returning to being solid and hard when the temperature drops. This behavior allows them to be molded an indefinite number of times; it is sufficient to heat them until they soften, become sticky, and are introduced into a mold so that, when cooled, they take the shape of the mold. Among them are PVC (Polyvinyl Chloride), Nylon, Polystyrene (expanded and solid), Polyethylene, Polypropylene, PTFE (Polytetrafluoroethylene).

a) Polyolefins

The polymers and copolymers of olefins of greater use are derivatives of ethylene and propylene. Polyethylene has excellent resistance to most solvents and chemicals and is tough and flexible over a wide range of temperature applications. The propylene derivatives have a softening point higher than that of polyethylene and are stronger and stiffer.

b) Vinyl and Acrylic Polymers

  • PVC:
    • Rigid PVC: It is very fragile and eventually ages with a consequent loss of strength and increased fragility.
    • Plasticized PVC: Mechanical resistance and low electrical resistivity. Useful as a coating.
  • Polystyrene (PS) and Copolymers: It is clear, colorless, hard, and tough, but fragile. It softens at 90-95 ºC, and at 140 ºC, it is a viscous liquid, which makes it suitable for injection molding processes. It has the disadvantage of being attacked by many solvents and not being age-resistant outdoors.
  • Polyacrylonitrile (PAN), Polyacrylate, and Polymethyl Methacrylate (PMMA): Good light transmission and resistance to the action of solar radiation. Just as good optical properties.

c) Linear Polyamides and Polyesters (Synthetic Fibers)

High degree of crystallinity, water absorption ability. They are suitable for the manufacture of synthetic fibers.

d) Special Thermoplastic Resins

These plastics replace metals in the manufacture of mechanical parts and machine elements with an advantageous relationship between mechanical strength and weight. Besides offering the ability to be machined and polished, they also have high resistance to chemical corrosion, good electrical properties, and extraordinarily small coefficients of friction with metals.

Thermosets

These polymers do not soften or flow as much as the temperature rises, coming soon to decompose before they flow. That is why this type of plastic cannot be molded repeatedly. Therefore, they are suitable for instruments that have to work at a certain temperature without changing their shape.

a) Phenolic Resins

Good dimensional stability, heat resistance, and mechanical properties.

b) Urea Resins

They offer greater tensile strength and hardness than phenol but are less resistant to heat and humidity.

c) Melamine Resins

Products are obtained with a good surface finish, high heat resistance, hardness, and lower water absorption than previous resins.

d) Polyester Resins

These resins, once treated, are infusible and insoluble, with good transparency, high refractive index, high dimensional stability, good mechanical properties, and good resistance to chemicals.

e) Epoxy Resins

High resistance to water, solvents, acids, and bases, as well as most chemical agents.

f) Polyurethane Resins

They come in flexible or rigid forms.

Elastomers

Elastomers have the unique characteristic that at room temperature, they can be stretched to at least twice their original length and recover very quickly immediately after the demise of traction. Although elastic between very large margins, they do not obey Hooke’s law. Elastomers, depending on their nature, may be natural rubber, synthetic rubber, or thermoplastic elastomers.

Polymers Derived from Cellulose

Cellulose is the most abundant natural polymer in nature. In its purest form, it is found in cotton (80%) in the form of fibers covering the seeds. The largest derivatives of cellulose include nitrocellulose, cellulose acetate, cellulose ethers, and regenerative cells.

Properties of Plastics

Thermal Properties

When you lower the temperature of a polymer in a fluid state, it reaches a point known as the glass transition temperature (Tg), where polymeric materials undergo a marked change in properties associated with the virtual cessation of movement locally. In general, the main requirement to be useful as a plastic material at room temperature is that the glass transition temperature or melting temperature is well above room temperature. By contrast, the essential requirement for a high polymer to be used as a rubber or elastomer is that its glass transition temperature is well below this ambient temperature. The behavior of thermoplastic materials before the fire or in direct contact with glowing bodies is characterized by measuring the resistance to ignition or incandescence. There are certain products that interfere with the chain reaction leading to combustion. They are called flame-retardant additives.

Chemical Properties

The attack of chemicals is often internal, characterized by softening, thickening, and loss of material strength. A general rule of chemistry, which states that”related materials attract and repel different” helps us to predict the chemical resistance of polymers. Thus, a polymer is more soluble in a solvent with a similar chemical structure than in another of a different chemical structure. The action of solvents on polymers may lead to different effects: dissolution, swelling, permeability, environmental stress fracture, and cracking. Both solvents and acids, bases, and strong oxidizers may be regarded as the greatest enemies of plastics.

Electrical Properties

As for their response to electricity, we can consider polymers as electrically insulating materials, but their composition can be adjusted to allow some conductivity.

Sensory Properties

We define as sensory properties those which we perceive through the senses, such as color and texture.

Mechanical Properties

The shape of the curves of traction (stress per unit area/elongation) in the same material depends on the temperature at which they register. Indeed, there is a temperature boundary, which delimits two areas that have very different mechanical properties; this is what we called the glass transition temperature.

  • a) Impact Resistance
  • b) Hardness
  • c) Fatigue
  • d) Dimensional Stability
  • e) Compliance
  • f) Torsional Strength and Hardness

Optical Properties

The most interesting optical properties of plastic materials are related to their ability to transmit light, take color, and have a brightness, which gives the objects made a visually aesthetic high quality. The outstanding properties are:

  • a) Transmission and Reflection of Light
  • b) Absorption of Light and Color
  • c) Photodegradation: To avoid the detrimental effect of light and radiation, absorbing pigments or fillers that absorb radiation near the surface are added, thereby protecting the interior.

Permeability to Gases and Vapors

The permeability of a plastic is a fundamental property that determines not only the ability to act as a”barrie” against gases and vapors, preventing their spread through them, but it also affects the deterioration of plastic against certain agents.

Applications of Plastics

The use of plastics as raw material in different sectors of the industry has displaced other traditional materials, becoming the most used (in containers, for example). Spain ranks seventh in world rankings for the use of plastics. Applications for more widespread use of plastics:

a) Packaging Sector

The requirements to be met by a material to be useful as a package are:

  • It must protect products against environmental agents, including air, water, humidity.
  • It must not modify the properties of the content.
  • It must avoid losing the flavor of the product.
  • It must protect against light radiation.
  • It must protect against compression and impact.
  • It must comply with existing health standards.

Plastics meet these requirements due to the barrier effect presented by not allowing gas exchange with the inside or the outside. They also have the advantage that the components that will not migrate are the additives containing products, so any possibility of contamination of material caused by these additives is ruled out. Besides the advantages seen before, they have the following qualities that make them the most used materials in the packaging industry:

  • Their lightness makes it possible to decrease the dead weight of each packaged product, resulting in energy savings in transport and an increase in the amount of product transported per trip.
  • Ability to be screen printed, avoiding the use of different materials for labeling.
  • Fit like packaging of products as diverse as food and appliances, so their versatility is high.

b) Agricultural Sector

These applications can be classified into two groups: The first group includes the application directly in agricultural production, and the second group would include the plastics used in marketing and consumption and not involved in production.

c) Hospitality

The application of plastics in medicine focuses not only on the field of surgery but long ago joined pharmacology. In the field of surgery, their application focuses on the use of hearing aids (hard or soft), made with biocompatible materials called, which do not have rejection by the recipient organism.

d) Construction

From soft thermoplastic materials, through hard thermoplastics, to compounds, all have applications in almost all aspects of construction.

e) Automotive

Plastics are coming with great force in this sector due to the lightness, strength, toughness, and ease of molding posed by these materials. In addition to their ideal characteristics, we must note the possibility of recycling, the same factor that is increasingly considered by manufacturers.

Methods of Identification of Plastics

To identify an unknown plastic, the most immediate thing to do is to determine whether this is a thermoplastic, a thermoset, or an elastomer. If it is tough and fibrous, it is a reinforced thermosetting. If heat is applied and the material softens and flows, this is a thermoplastic. If you apply heat and the material softens and is destroyed, this is a thermoset. The most common method of identification is the destructive method of identification by combustion. This method is done by burning a piece of material and examining the characteristics of combustion (flame, smoke, odor). Comparing these characteristics with those of a reference table provides sufficient information to approach the kind of material that can be treated. Before the purpose of identification by combustion, it is advisable to make a series of preliminary tests of key data to obtain the type of plastic that we intend to analyze. These preliminary tests, chemical analysis, are: Determination of density, refractive index, iodine value, hydroxyl, carbonyl index, solubility, acidity and saponification indices, melting points and softening, transparency, flexibility, and performance from the impact.