Tuesday, 28 July 2020

Holiday Experiences: How Plastics Protect Everyday Life in Mexico’s Yucatán Peninsula

Holbox, Mexico - bottle collection box in the shape of a fish

Before the Covid-19 pandemic stroke in Europe, especially in Italy, Spain, France and Germany, we were far away on holiday in Mexico. It was end of February 2020 when our holiday started and two weeks later, Italy and Germany announced the restriction of going out to flatten the infection curve. The Covid crises spread all over Europe causing the healthcare systems to be highly exhausted.

During our holiday, I made several observations on how plastic-based products facilitate the life of people in areas where jungle dominates and infrastructure is limited. We visited the island of Holbox, located in the north east of Yucatan surrounded by the Gulf of Mexico. It is connected to the mainland over a fair, starting from Chiquila.

Holbox, located in the north east of Yucatan surrounded by the Gulf of Mexico

As you can imagine, things in a well-established system on the mainland, can work on an island completely different. Drinking water supply is a major challenge on islands. Here, plastics packaging plays a major role to ensure fresh drinking water supply. In the example of Holbox island, drinking water supply is ensured by a daily truck loaded with 10, 18 up to 20 liter water bottles coming from Chiquila. Once arrived on the island, the truck drives to the major square and people all over the island come to pick up such bottles to be used in their homes. They also bring their used bottles back for recycling purposes. Most of the bottles are made out of Polyethylenterephthalate (PET). The wall thickness is increased compared to a standard 1 liter PET bottle, since more mechanical strength needs to be provided considering transportation as well as hot climate aspects. One time we saw a boy who just had picked up such an 18 liter bottle and was carrying it home on his bicycle.

Drinking water supply is ensured by a daily truck loaded with 10, 18 up to 20 liter PET water bottles

There are also creative ways on how to collect the PET bottles. One example is the collection box in the shape of a fish which is made of a metal net, similar to a fence. It should remind that littering is harming the sea and its animals. A picture of the collection box with myself is at the beginning of this post.

Rain water tanks
Apart of PET bottles filled with fresh water, one can also see on most houses allover Mexico, rain water collection tanks. Those are mainly provided by a company called Rotoplas. However, rain water collection is not the only reason to have such plastic tanks. Most water pipes are gravity fed or very low pressure fed for the distribution of water to and within the house. Therefore, most homes pump additional water into roof tanks as well. This allows having a good pressure for showering and doing the laundry, as well as other activities. The tanks are made by rotational moulding and in most cases Polyethylene (PE) powder is used. Rotational moulding allows you to make hollow plastic parts. The moulding tool represents the outer wall of your final product. The tool is loaded with the PE powder and closed. The tool starts rotating and will be heated at the same time. The PE melts and distributes along the tool walls to form the outer layer of the product. After cooling down, the tool is opened and the product which is in this case a rain water collection tank can be used. The big advantage is to make hollow parts.
Water collection tank on the rooftop of houses.

In both cases, plastic products provide access to fresh water for many people in an efficient, durable, save and thus cost effective way. This is valid if you think of utilizing other materials such as glass, metal or wood for the described purposes.

On our returning trip from Mexico to Germany, Coronavirus was already spreading. The north of Italy was heavily affected, as well as Germany, followed by France, Spain and the UK. However, plastics are playing a crucial role in fighting the Coronavirus.

Following are a few examples how plastics are protecting us during the pandemic and beyond:
 - Packaging of food: especially vegetables and fruits are exposed to liquids released when sneezing and blowing your nose.
 - Single-use plastic bags: the one time use prevents to spread germs and viruses. - Medical devices protecting equipment: face shields, masks, ventilator components, intubation boxes are examples where plastics play a crucial role.
 - 3D printing filaments: all over the economy, companies provide their additive manufacturing resources to produce e.g. the face shield holding frame.

Although there are several plastic bans on the way such as the single-use plastics ban directive of the European Union [1], it could be demonstrated in several life cycle assessment studies that plastics are the greenest solution [2-4]. In these studies, e.g. single-use plastic bags are the best and cotton based bags are the worst due to a very energy-intensive life cycle. 

My food for thought: we do not have a waste problem but a littering problem. This means that people are to blame for throwing away the plastics into the nature and not the materials itself. 

Thank you for reading and till next time! 

Best regards,

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[1] https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=LEGISSUM:4393034&rid=1
[2] https://phantomplastics.com/plastics-the-environment/ https://belu.org/when-the-answer-to-your-anti-plastics-challenge-is-in-fact-plastic/
[3] Civancik-Uslu, et.al.: Life cycle assessment of carrier bags and development of a littering indicator, Science of the Total Environment 685 (2019) 621–630
[4] Our World in Data, Danish Environmental Protection Agency, 2018

Friday, 24 July 2020

Design Properties for Engineers: Dynamic Mechanical Analysis (DMA) of High Performance Polymers

In this blog post, we have a closer look at the dynamic mechanical analysis (DMA), a thermo-analytical method that estimates the viscoelastic properties of a given material over the course of different temperatures. It steps away from a single point view toward a multipoint data view which is beneficial for polymer material selection tasks. 

DMA can be measured according ASTM D4065-94. Here, a fixed frequency of oscillation of 1 Hz and heating rate of 2°C/minute are defined. When performing the DMA it is important to cover the glass transition area for amorphous polymers as well as the melting point for semi-crystalline polymers. Results of a DMA are the storage or elastic modulus (E’), the loss or viscous modulus (E’’) and the tangent of the phase angle delta (E’/E’’). 

In the figure below, the storage modulus vs. temperature behavior of different high performance amorphous polymers is shown. They all show a significant drop in modulus in the glass transition region. Among the amorphous high performance polymers, PAI has with 275 °C the highest glass transition temperature. A continuous use temperature of 260°C is feasible. 

Storage modulus vs. temperature behavior of different high performance amorphous polymers 

In the next figure, DMA curves of semi-crystalline high performance polymers are shown. We can see a decrease in the modulus at the glass transition temperature. However, mechanical properties can be retained until the crystalline melting temperature.

PPS shows a glass transition of 88 °C and is fully melted at 270°C. From 80°C to 200°C the retained mechanical strength is still sufficient.

 DMA curves of semi-crystalline high performance polymers

Fluoropolymers have an excellent low temperature performance. This allows e.g. their usage as sealing gaskets at cryogenic temperatures where flexibility is needed. Storage modulus performance is high enough to provide the necessary mechanical performance, at low temperatures (down to -200°C) as well as at high temperatures (up to 250°C).

DMA curves of fluoropolymers

Thank you for reading!
Best regards and #findoutaboutplastics


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Monday, 13 July 2020

Metal replacement with High Performance Polymers: How to Design for Equivalent Part Stiffness?

In this blog post, I show you how to substitute a metal part by high performance polymers. In this case the replaced metal is magnesium.

There are two approaches on how to substitute metals with other materials such as Polyphthalamide (PPA), a high performance polymer. In the first approach, we increase the cross-sectional thickness to provide increased stiffness. In the second approach, increased stiffness is achieved by adding ribs. 

First approach: changing wall thickness

In general, deflection is proportional to the load and length of the part. Furthermore, deflection is inversely proportional to the modulus of elasticity and moment of inertia. For the first approach we need the deflection (Y) equation of a beam with a uniform distributed load F (Equation 1). Both ends of the beam are fixed. Then, we equate the deflections of metal and plastic (Equation 2). FL3 is a constant since load (F) and length (L) remain the same. This leads to Equation 3. Now, we can insert the modulus of elasticity E for magnesium, which is 44.8 GPa. As PPA, we have selected Amodel® AS-1145 HS (45% glass fiber; impact modified; 6T/6I/6.6). As a result, the magnesium part stiffness is 3.25 times higher in comparison to PPA.

Metal replacement: first approach - changing the wall thickness

To achieve the same stiffness with the plastic part we have to increase the moment of inertia (I). This can be done by inserting Equation 5 (moment of inertia for rectangular section) into Equation 4. “b” is the width which is kept constant and “d” is the thickness of the section. We resolve the equation for “d” (assumption: the magnesium part has d = 2.54 mm) and as a result we get the new thickness of the PPA part, which is 3.76 mm.

The PPA part will be 48% thicker than the magnesium part and will achieve the same stiffness.

Metal replacement: first approach - changing the wall thickness

Second approach: adding ribs 

Another way to increase the moment of inertia is by adding ribs to the plastic part. This will decrease the wall thickness and weight significantly.

We start with a 3.76 mm thick plate design made out of the same material, Amodel AS-1145. Therefore, the modulus of elasticity with 13.8 GPa remains for the plate and the ribbed design the same. If the moment of inertia of the ribbed design is the same as for the plate design, then the ribbed part will show equivalent deflection and/or stiffness.

We will assign 25.4 mm for the width “b”. Now, we can calculate the moment of inertia for the plate design (Equation 8). Based on the rib design (figure below) we can then calculate the moment of inertia for the rib structure (Equation 9).

Metal replacement: second approach - adding ribs

As a result, the ribbed design will be 9.5 times stiffer than the previous calculated PPA plate design and the original magnesium plate design which was 2.54 mm thick. Reducing the height of the rib by half would still result in a part which is twice as stiff as the magnesium part. 


When replacing metal by high performance plastics, adding ribs will reduce the thickness and weight of your new part. Furthermore, stiffness can be increased dramatically, which allows for new part applications or even thinner parts in total.

Furthermore, modern CAE software is able to add automatically a rib structure by applying topology optimization tools.

My other posts with metal replacement:
Metal-2-Plastic in Automotive - How to do it (incl. Youtube Training Video) 
Metal Replacement with Polyarylamide (PARA) for Single-Use Surgical Instruments

Thank you for reading!

Herwig Juster

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