INTRODUCTION TO ENGINEERING MATERIALS - POLYMERS- PART 6

Audio: Introduction to engineering materials – polymers- part 6

In our introduction to engineering materials series, we covered a whole range of different materials. As we progress through the series, we are going to look into the polymers. In today’s world, it is practically impossible not to use products made out of polymer materials. Products made out of plastics are an integral part of our everyday modern life, and we are using them probably more than we are aware of.

Due to the great properties of plastics, processability, and scalability, as a mechanical design engineer, you will for probably use plastic materials in your design. For example, you could use raw plastic material sheets, films, standardized parts, adhesives, coatings, or even design a completely new product from the plastic material.

However, before we go to the plastic selection process, manufacturing techniques, and design rules, we first must learn what plastic materials are. In this article, we will learn what plastic materials are, how they are classified, the importance of plastic materials, and some of the most used plastic materials today.

Introduction to polymers

You all probably heard about plastic. In the world we live in, it is hard not to know about it. We are drinking water from the plastic bottles; we are watching the TV that has plastic housing and changing the channels with the remote control made from plastics. We sit on the plastic chairs in our garden while kids play with the plastic toys. But what exactly is plastic?

The building material of plastic is a polymer. Polymers are chemical compounds comprised of a large number of monomers (structural units) linked together to form a long-chain high molecular weight polymer. The origin of polymers can be natural (silk, cotton, etc.) and synthetic (paints, fibers, films, etc.).

Usually, you will hear different terms like plastic, polymer, resin, elastomer, reinforced plastic…

Polymers can be separated into plastics and rubbers.

The term plastic (or plastic material) usually is denoted as polymers containing additives, fillers, and/or reinforcements that can be shaped into a product.

As already stated, the term polymer refers to a building material of plastic.

The resin is a natural polymer, while resin found in plastic is a synthetic polymer, and it is referred to as plastic resin. During your work in the industry, you will often hear different plastic materials referred to as “resin.” These resins are usually part of the material family, and within a material family, different chain lengths or molecular weight grades could occur. In addition, these resins typically have various additives already included addressing the diverse needs of an application or environment. So, do not get confused when someone refers to commercial plastic material as resin.

Elastomers are materials with high elasticity (like rubber).

Reinforced plastic or plastic composites are reinforced with fibers and whiskers to improve the mechanical properties of products.

Plastic polymers classification

As previously mentioned, polymers can be separated into plastics and rubbers. The plastic polymers can be further divided into thermoplastics and thermosetting polymers.

Thermoplastics materials are materials that, when heated, get soft and can melt, but when they are cooled, they resolidify and hardens again. These materials can be recycled, but some loss of mechanical properties could occur due to the processing. Today on the material market, various thermoplastic material types and grades can be found, ranging from materials with rigid properties to materials with elastomeric properties.

We can further divide the thermoplastic materials in several ways, but the most important one is based on the material structure (morphology). Based on the material structure, we can classify thermoplastic materials as amorphous, semi-crystalline, and liquid crystalline. Each structure defines the performance capability of thermoplastic material.

Examples of thermoplastic materials are Polycarbonate (PC), Polymethyl methacrylate (PMMA), Polypropylene (PP), Polyamide (PA), etc.

Thermosetting materials (or thermosets) during the processing chemically react and form a cross-linked chain network. When heated, the polymer chains tend to degrade, and due to that, thermosets are not directly recyclable (addition processing required).

Examples of thermosetting materials are epoxy resins, a variety of elastomeric materials, etc.

Elastomers, also known as rubbers are polymers with extreme elastic extensibility when subjected to the relatively low mechanical stress.

Considering that 70% of produced synthetic polymers are thermoplastics,In the further text, we will focus more on thermoplastic materials.

Material blends

So far, we have discussed thermoplastic materials derived as individual material formulations. But in order to cover extended usage and environmental requirements, two or more resins (plastic polymers) can be blended together. Blending two resins is a physical mixture of two (or more) resins where the physical and mechanical properties are somewhere between the base materials (in some cases, blended material properties can exceed those of base materials).

An example of blended resin is Polycarbonate/acrylonitrile butadiene styrene (PC – ABS).

Additives

Generally speaking, the mechanical design engineer rarely will work with the material supplier to tailor and develop a new polymer material. Instead, the mechanical design engineer will work with a material supplier to choose the best commercially available material that fits the intended use and environment.

Most of the commercially available materials are already to some degree modified by additives. The additives are used to improve the properties that the base polymer lacks.

Additives include but are not limited to:

Antimicrobials – prevents deterioration of plastic when the material is exposed to microbiological attack.
Processing aids – used to improve the processability of plastics by increasing the flowability.
Antioxidants – oxidation of polymer can result in loss of material properties. Antioxidants prevent the polymer from reacting with oxygen.
Antistats – prevents the build-up of static electric charge.
Fillers or reinforcements – used to enhance mechanical properties of plastics (fillers are usually mineral-based, and reinforcements are usually fiber-based)
UV stabilizers – prevents deterioration of plastic when the material is exposed to UV light.
Flame retardants – to prevent ignition or spread of flame in plastic material.
Colorants – used to change the base material color to the desired color.
Plasticizers – are used to make plastic softer and more flexible.

If you are interested to learn more about the additives, you can read this article.

Importance of plastics

A large number of conveniences in the modern world are achieved using plastic materials. Plastic products can be found in almost every part of our lives, from the exterior and interior car parts and airplanes to toys, PCs, laptops and other consumer products, medical equipment, food containers, beverage containers, exterior and interior housing decoration, and many many more.

The plastic materials are easy to process, and the plastic products can be designed with simple or complex shapes. Moreover, the possibility of achieving high production quantities with relatively small costs and versatility of the plastic makes the skill of developing plastic products desirable for every mechanical design engineer.

You probably heard about the impact of plastic on our environment. Plastic materials need anywhere between twenty to a few hundred years to decompose. So we should keep developing more environment-friendly materials (like bioplastics) and use recyclable material in our product design. We should also keep pushing our governments to provide the infrastructure for plastic disposal, educate each other on environmental impact and adopt better consumer habits and behavior. Unfortunately, I feel that plastic is getting a bad reputation in today’s world. Always keep in mind that it does not matter how good materials we develop and what laws we have to protect our environment, plastic did not do this; humans did:

Different types of plastic materials

There are many different commercially available plastic resins on the market. These commercial plastic resins are already a mixture of one or more plastic polymers and additives. Usually, these materials are trademarked by a material manufacturer. Let us now look into the most used material types.

Polyamide or Nylon (PA)

Polyamide, better known as Nylon, is a material highly resistant to temperature, chemicals, and electricity with excellent mechanical properties: high melting temperature, good wear resistance, less operating noise, etc. Therefore, it is considered high-performance plastic.

Nylon is a material that can be processed by all standard melt processing manufacturing techniques.

Usage: automotive industry, consumer goods, electrical and electronics devices, food processing machinery parts, packaging machinery parts, ropes wheels, rollers, bushings, bearings, gears, etc.

If you would like to know more about Nylon, you can read this article.

Polymethyl methacrylate (PMMA)

PMMA, better known as Acrylic, is a transparent plastic material with outstanding mechanical properties: strong, stiff, and excellent resistance to weather conditions. Acrylic is transparent as glass but at half the weight and has better impact resistance than glass.

Acrylic can be processed by injection molding, extrusion, thermoforming, and casting. Also, it can be welded with processes like hot-blade, hot-gas, ultrasonic welding, and spin welding.

Usage: windows, shelves, fixtures, clear pipes, lightning equipment, smartphone displays, outdoor and indoor signs, architecture, transportation, decorations, etc.

If you would like to know more about Acrylic, you can read this article.

Polycarbonate (PC)

PC is a rigid plastic with excellent strength, stiffness, and impact resistance. In addition, this transparent plastic has excellent optical properties. It is widely used in blends with other materials like PC – ABS.

Polycarbonate is mainly processed with injection molding.

Usage: cartridges, oven doors, chocolate molds, cosmetic packaging, tail lamp housings, CDs, chassis, frames, light pipes, different business machine components, plugs, outdoor lighting, etc.

If you are interested to read more about Polycarbonate, you can read this article.

Acrylonitrile butadiene styrene (ABS)

ABS is an opaque plastic material with excellent impact resistance, high rigidity, and scratching resistance that makes him an ideal choice for various structural applications. In addition, it has a low melting temperature with outstanding thermoforming characteristics.

ABS is usually processed by injection molding and extrusion.

Usage: machine housings, machine panels, home appliances housing and different parts, instrument panels, food packaging, handle, chain covers, TV backplates, computer and laptop housings, copy machine housing, etc.

If you would like to learn more about ABS, you can read this article.

Polycarbonate/acrylonitrile butadiene styrene (PC – ABS)

The name itself suggests that this material is created from PC and ABS material blend. The properties of PC – ABS depends on the ratio of PC to ABS and additives in the mixture. The PC – ABS plastic is rigid and heat resistant material with high heat resistance and dimensional stability over time. All this makes PC – ABS one of the most widely used plastic materials globally.

PC – ABS is mainly processed by injection molding.

Usage: dashboard components, control consoles, housings for different devices like medical instruments, housings and structural parts for computers and copiers, lamp housings, TV housing components, terminals, etc.

If you are interested to read more about PC – ABS, you can read this article.

Polyethylene Terephthalate (PET)

PET is a general-purpose plastic material used in our everyday life. This material has good mechanical strength and stiffness and excellent insulating properties. PET is also FDA approved, and it can be blended with various other polymers. All these together make PET important material in our everyday life.

PET can be easily processed by injection molding, extrusion, blow molding, and thermoforming.

Usage: rigid and flexible packaging for food and beverages, films, fabrics, waterproofing cable barrier, etc.

If you are interested to read more about PET, you can read this article.

Polyoxymethylene (POM)

POM is a plastic material used to produce high-precision parts due to its excellent dimensional stability (high lubricity). It is a very strong, rigid, and stiff material.

POM can be processed by injection molding, extrusion, blow molding, rotational casting.

Usage: mechanical gears, sliding and guiding elements, bearings, housing parts, screws, nuts, gun parts, knife handles, etc.

If you are interested to read more about POM, you can read this article.

Polypropylene (PP)

Polypropylene is a low-cost, rigid, and FDA-approved plastic material. It has high chemical and moisture resistance.

PP is a material that can be processed by all standard melt processing manufacturing techniques.

Usage: food packaging, hot beverage cups, flip-top bottles, reusable water bottles, prosthetic sockets, fabrics, piping system, etc.

If you would like to know more about Polypropylene, you can read this article.

Polyetherimide (PEI)

Polyetherimide is a semi-transparent, high-strength and quality plastic material. It has high heat resistance, flame retardancy, and broad chemical resistance.

PEI is usually processed by injection molding and extrusion.

Usage: transmission components, headlight reflectors, lamp sockets, electrical switches, electrical motor parts, etc.

If you would like to know more about PEI, you can read this article.

Thermoplastic elastomer (TPE)

A thermoplastic elastomer is a plastic material with thermoplastics and thermoset rubber properties. It has great fatigue and heat resistance and is easy to process.

TPE can be processed by injection molding, extrusion, blow molding, calendaring, film extrusion, and thermoforming.

Usage: buttons and button panels, toys, sporting goods, spinal orthoses, medical devices, household goods, consumer products, etc.

Thermoplastic elastomers can be divided into different classes, each having a slight difference in its chemistry, hence, different properties. The most used ones are:

Thermoplastic polyurethane (TPU)
Thermoplastic rubber (TPR)
Thermoplastic vulcanizate (TPV)

If you are interested to read more about TPEs, you can read this article.

Silicone rubber

Silicone rubber is a transparent elastomer material with excellent mechanical properties, high-temperature resistance, and outstanding insulation properties.

In its uncured state, silicone rubber is in the form of a highly adhesive gel or liquid. To get the silicone rubber in solid-state, it must be cured, vulcanized, or catalyzed. Liquid silicone rubber (LSR) can be processed by injection molding of LSR.

Usage: in automotive for light guides, in healthcare for catheters, medical tubing, respiratory masks, in consumer goods for headsets, mascara brushes, earbuds, etc.

If you would like to know more about Silicone rubber, you can read this article.

Closing words

This article covered the basics of plastic materials and introduced you to some of the most used plastic materials on the market. As a mechanical design engineer, you will most likely not get into the chemistry and processes of creating the new plastic resin. However, you will work closely with the material supplier to get the suitable material for the intended use and the environment. In order to get to that point, you have to know what information to request and what questions to ask to find what you need.

As we move forward with the topic of engineering materials, I would suggest you read the engineering material selection. It will give you an excellent overview of the engineering selection process and requirements for choosing the suitable material for your intended design.

Now you have an excellent overview of polymers you could encounter as a mechanical design engineer. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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