One of the major innovations of the past century has been the introduction and wide adoption of plastics for many day-to-day applications. Previously we relied on traditional materials like metal, glass, or even textiles. Plastics resist environmental degradation over time, are generally safe for human beings, are economical and widely available, and are produced with a wide variety of material properties that allow adaptation to many different applications.
But the term “plastics” covers a multitude of different types. They may be thermoplastic or thermosetting. They may have a crystalline or amorphous structure. There are commodity plastics and engineering plastics. They may be modified by fillers and reinforcements, as well as many proprietary ingredients.
Plastics come in families, they have relatives, and the whole group includes materials that are as different as cats are from dogs. All of this leads to as many as 100,000 different plastics materials available on the global market, differing in major or minor ways from one another and from the materials they may replace.
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The 11 most important classes of thermoplastic are (arguably):
- Polyethylene terephthalate (PETE or PET)
- Polyethylene (PE)
- Polyvinyl Chloride (PVC)
- Polypropylene (PP)
- Polystyrene (PS)
- ABS (Acrylonitrile Butadiene Styrene)
- Polylactic Acid (PLA)
- Polycarbonate (PC)
- Acrylic (PMMA)
- Acetal (Polyoxymethylene, POM):
- Nylon (Polyamide, PA)
Even within each class, there are significant differences. There are two major classes of nylon (6 and 6/6), as well as many other forms (11, 12, 6/10, 6/12, etc) which have different properties that may be useful in specific circumstances.
Material data sheets
Material Data Sheets are a great starting point for choosing a material. They provide an amazing amount of information at a glance. Every polymer supplier will provide a basic data sheet describing the properties of a particular grade. And most suppliers, on request or through websites such as UL Prospector, will provide additional and more relevant property data.
Each property value relates to a well-understood, repeatable, reliable test related to international standards. This makes comparison with other grades of similar plastics from a range of suppliers much easier. But these easy comparisons must also be interpreted with care.
Limitations to Material Data Sheets
The standard test methods cited in Material Data Sheets refer to a highly stylised test under a specific set of conditions (temperature, load, time, etc). These conditions are unlikely to correspond to product service conditions. As a result, material data sheets are at best a snapshot of the potential of a particular grade. Very few of the values can be used directly by design engineers in their development calculations.
So, as with all types of engineering knowledge, it takes background knowledge on what the numbers mean to interpret that data and a good engineer to evaluate the properties and choose the right material for a design.
Sadly, there is no standard presentation for a Material Data Sheet. They may differ in the way that the data is presented and the properties which are included. Some extra information, in addition to what is listed in a Material Data Sheet, may require further digging.
For example, when designing food products, FDA guidelines and information from the manufacturer will indicate whether or not the proposed material is food safe. In addition, one should also consult Material Safety Data Sheets (MSDS), which look at health hazards.
Similarly, if the material should come into contact with other materials, how will the properties be altered? Outgassing of some adhesives, for example, will cause brittle cracking in acrylics and polycarbonates.
Wider materials selection
Comparing materials on the basis of data sheets alone may be a useful exercise in comparing plastics which differ from one another in a minor way. But where materials are significantly different, and may fail in completely different ways, a straight comparison, even if possible, may not lead to a meaningful result.
The wider materials selection process must include:
- How to select the most economical materials in design which meet the specification
- Knowing the minimum levels of certain properties which the chosen material must exhibit
- Understanding how different classes of materials, and the different materials within them, are likely to fail
- Differentiating between the wide range of polymer materials, their properties and applications
- Interpreting technical information provided by material suppliers (material data sheets)
- Appreciating the environmental benefits of plastics
- Setting up appropriate real-time and accelerated testing to confirm performance of the early shortlisted materials
As an example, let’s consider tensile strength (or more correctly ‘tensile stress at break’), which appears on almost all Material Data Sheets.
Tensile properties
Although tensile stress is a useful property for metals, it is of less value when dealing with thermoplastics. It may give a useful first comparison to compare strength between materials. For example, ABS is a strong plastic and has a UTS of 40MPa; steel has a UTS of 400MPa – 10 times as strong as ABS for the same size. That doesn’t mean that designers should abandon plastics for steel, but it does mean that parts of the same strength will need to be much thicker.
However, applied loads during product service life are more likely to be in compression or in bend than in pure tension… with a few notable exceptions.
Secondly, rupture under an applied tensile load is very seldom the cause of product failure in a plastics component. A more likely cause is unacceptable deformation or distortion. That is an entirely different property, related to the tensile or flexural modulus (stiffness) and component geometry.
Another reason for not reading too much into tensile strength data is that the value listed in data sheets is a snapshot, derived from a standard test. This is carried out under a specific combination of temperature and strain rate. Changing the temperature and strain rate can significantly alter the result, as can the presence of flaws and imperfections in the material.
Flexural properties
Overall, it is prudent to pay more attention to the values of modulus (stiffness) and impact data rather than tensile strength. However, data for impact can also give a false sense of security and needs careful interpretation.
Here, we use a test of flexural properties – and in many materials other properties can be approximated from the same measurement. However, in plastics, it is best to perform a three-point flexural strength test, where a bar of the material is placed across two supports, with a hydraulic head pressing in the centre, which measures flexural strength and flexural modulus.
In homogenous materials, such as most steels, flexural strength and modulus will be the same as the tensile strength and modulus, but plastics are hardly ever the same throughout the cross section. Because of injection moulding, the material will form a skin of material that doesn’t match the rest of the material.
Also, when material surface roughness or surface imperfections are present, they can result in a much lower flexural strength than tensile strength. This is prevalent with parts produced with 3D printing (SLS or FDM). When prototyping with these materials, take care when designing parts that flex.
Temperature effects
Temperature, both high and low, has a major influence on the way that different materials fail. Steel becomes very brittle at low temperatures, while at high temperatures, plastics get soft. In both cases, this is covered by materials testing, using notched and unnotched impact testing for metals and certain brittle polymers, and measurement of heat deflection temperature and glass transition temperature for polymers to be used at higher temperatures.
Glass transition only applies to amorphous materials: materials that don’t have a crystalline structure (such as polycarbonate and polystyrene), as opposed to crystalline polymers like nylon and polypropylene. When an amorphous material gets warm, it goes from being brittle to being rubbery, and that happens at the glass transition temperature. Designers will not want their rubbery parts to become brittle, nor their hard parts to become rubbery.
Further reading:
- Plastics Materials Selection: Narrowing Down the Options
- WEBINAR: Expert Plastic Material Selection Techniques
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Hey,
Can I find a version in Russian?
Hi Ilya,
I’m sorry, we don’t have the resources to translate articles into Russian.
Sorry.
Angie
Content Manager, Prospector Knowledge Center
Although I agree with the author overall, one missed point is that polymer material datasheets are generally MARKETING documents. Thus, the data printed tends to be best of the samples tested under the specific test conditions.
Also, the user of these data sheets must also consider the Dry As-molded vs. pre-Conditioned material characteristics when tested.
Thanks for sharing this important clarification with the world.
Dear Mr Andy Pye,
I work for a Croatia Company Industrooprema and we are interested for material which is used in electricity for example cover clamp covers, and covers for owerhead lines .
From my point of wiev it is wery good that there is such a page where you coud find souch information.
Excellent
Hi Andy ,
Can you clarify what do you mean by the sentence below –
“So, as with all types of engineering knowledge, it takes background knowledge on what the numbers mean to interpret that data and a good engineer to evaluate the properties and choose the right material for a design.
Sadly, there is no standard presentation for a Material Data Sheet. They may differ in the way that the data is presented and the properties which are included. Some extra information, in addition to what is listed in a Material Data Sheet, may require further digging. “
I would like to make an standar for data sheets some times including process recomendation but not in others, please including the same orden in each data sheet
Best regards
Thanks Michael for adding to the debate!
Couldn’t agree more! Little has changed over 40 years. Data sheets are partly sales documents, which is a shame. The metals companies are a bit better at this than the polymer suppliers.
I believe his intent was to clarify that even when the data sheet provides you with specifics about the raw material, practical expertise or knowledge about the “end product’s” environment and usage will be of equal importance to the validity of the Spec Sheet’s impact in the design process.