Polymer Material Selection: Defining the Part Requirements as Common Starting Point

Hello and welcome to a new post. Today we discuss the starting point in polymer material selection. 

In general there are several procedures for material selection such as the Ashby methodology or my own developed funnel approach. 

However, all the selection processes and procedures should have the definition of the part requirements as their common starting point. 

How to do it the best? 

Figure 1: Polymer Material Selection - Estimation of product requirements as common starting point

Covering effectively the product requirements, a combination of functionality questions and selection factor questions can support you to achieve this (Figure 1).

First we have to ask some questions on the functionality of the part. Following questions can help us with this assessment:

-What are the performance requirements (structural, etc.)?

-Do you want to combine multiple parts or functions?

-What will be the structural load of the part (static, dynamic, cycling, impact, etc.)?

-What will be the environmental impact on the part (chemical, temperature, time)?

-What is the expected lifetime of the product?

With the collected answers we can define the part requirements as accurately as possible. Together with the understanding of the differences of thermoplastics (amorphous and semi-crystalline) and thermosets we have an understanding of the performance, thermal and mechanical properties, as well as chemical resistance and processing differences of thermoplastics and thermosets. 

Selection factors - checklist for your material selection 

There are more detailed lists , however in this post we cover the six essential questions on material selection factors. 

1. What is the service environment of your part?

-what is the operating temperature: high, low, duration, thermal expansion

-exposure to chemicals, solvents, lubricants, salt

-exposure to water and humidity

-UV stability for use in outdoor / indoor environment

2. What are the regulatory requirements?

-flammability rating needed such as UL 94 at different wall thickness

-food contact

-fulfillment of medical standards

-any other regulation such as IP 44 for electrical devices

3. What types of load at which service temperature need to be fulfilled?

-continuous load represented by Young modulus and creep resistance

-intermittent load represented by tensile strength

-impact load represented by impact strength

-fatigue represented by cycles to failure for example over a Wöhler curve

4. Other considerations such as: 

-dimensions and tolerances which need to be met

-electrical properties such as CTI, electrical breakdown strength

-wear and friction 

-thermally conductive materials with or without electrical isolation

-aesthetics and colour (relevant for application with food contact, and toys)

-painting and printing 

-life time needs 

5. What is the processing and fabrication method?

-injection moulding, extrusion, thermoforming

-assembling by using screws, laser welding, or adhesives

-secondary operations

6. What are the economic and commercial considerations

-useful to make when material short-list is available

Conclusions

The combination of questions on part functionality and selection factors will help to facilitate your polymer material selection, together with fundamental data in a systematic way. 

Thank you for reading and #findoutaboutplastics

Greetings,

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature:

[1] GE Plastics - Product Guide

[2] https://www.grantadesign.com/education/students/charts/

[3] https://www.findoutaboutplastics.com/2020/11/plastic-part-failure-part-2-antidote.html

Plastic Part Design Properties for Engineers - CTE/ CLTE of Polymers, Mineral Fillers and Metals

Hello and welcome to a new blog post. In a previous post we discussed the coefficient of linear thermal expansion (CLTE) of high performance polymers such as PPS, PEEK and PVDF already. 

In this post we will introduce the CLTE of commodity polymers, mineral fillers and metals with the focus on how to control CLTE in an optimal way. 

Optimizing the CLTE of polymer compounds - how to do it? 

Comparing the CLTE values of metals, minerals and polymers (Figure 1) we notice that polymers have a factor 3 to 5 times higher CLTE in comparison to metals. Comparing the polymer CLTE with minerals the factor is with 10 even higher [1]. Based on this fact, lowering the CLTE of polymers can be done by mixing them with minerals. Also it is important to use isotopic fillers which result in equal mechanical properties and CLTE in the x,y, and z directions. 

Figure 1 compares the CLTE of unfilled and filled plastics as well as minerals and metals. 

Figure 1: CLTE of polymers, minerals, and metals.

Example of talc filled Polypropylene 

Figure 2 shows the CLTE of PP in comparison with 10 w% and 20 w% talc filled PP. The CLTE could be reduced from 150 to 95 x 10-6 K-1 (36%). 

Figure 2: CLTE of PP and talc filled PP

Example comparison CLTE of glass and glass/mineral filled semi-aromatic Polyamide (PPA)

Another example shows the impact of  combined filling of glass fiber with mineral filler on the CLTE. A 33 wt% glass-fiber reinforced PPA is compared to a 66 wt% glass-fiber and mineral filler PPA. In the temperature range of 0°C to 100°C, the CLTE could be lowered 30% in flow direction and 28% in transverse direction. In the temperature segment from 100°C to 200°C, the reduction in flow direction is 40% and in transverse direction 40% too (Figure 3).

Figure 3: Comparison CLTE of glass and glass/mineral filled semi-aromatic Polyamide (PPA) [4].

In conclusion

CLTE can be a critical requirement for enclosed or metal overmoulded parts. Modification of polymers with certain fillers to reduce the CLTE needs to be kept in mind during polymer material selection. 

Thank you for reading and #findoutaboutplastics

Greetings

Herwig Juster

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature:

[1] https://phantomplastics.com/fillers-for-cte-clte-modification-of-plastics/

[2] https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

[3] https://www.findoutaboutplastics.com/2020/04/design-properties-for-engineers_28.html

[4] Syensqo, Amodel(R) Product Guide

Highly Filled PP Compounds - Materials for Improved Flame Retardancy, Cost, and Functionality

Highly Filled Polypropylene Compounds 

Hello and welcome to a new blog post. Today we discuss highly filled PP compounds as enabler materials for improved flame retardancy, cost, and functionality.

Some time an industry colleague said to me that if the application does not have a high temperature and precision demand, most of them will end up made out of Polyolefins (PP, PE): “PP can do the job”.

And we see more and more engineering polymer replacements (PA, PC, PBT) by PP with fillers (functional or non-functional). Therefore, let us have a look at highly filled PP compounds. 

Which types of highly filled Polypropylene are there and what are some major application fields for highly filled Polypropylene?

As a first overview we can cluster them in three groups: 

-Calcium carbonate filled PP: main drive is cost out and improved dimensional stability; calcium carbonate belongs to one of the most used inorganic fillers which increases the modulus of elasticity. Loading levels of 30 parts per hundred result in 11 % by volume. 

-Mineral filled flame retardant PP: main driver  is to comply with more stringent flame retardant requirements; Magnesium Hydroxide (MDH) can be used as flame retardant. 

-Graphite filled PP: main driver are Electrostatic discharge (ESD) applications such as fuel connectors, and fuel cell applications.

What filler levels can be realized?

For reaching certain stringent flame retardancy ratings with PP compounds, filler levels of flame retardant filler MDH can be 60-65% by weight. This would result in a composite density of 1.45 g/cm3. However, the balance need to be found between property reduction and processing capability. 

How to increase filler amounts to improve performance?

New technologies such as the patented PlastFormance technology (Guest interview here) allows loading a base polymer up to 80% weight and keeping still good flow capabilities for injection moulding. 

Examples of PP replacing engineering polymers

Also, we see talcum filled PP (min. 20% by weight) replacing ABS and long-glass fiber filled PP replacing short glass fiber PA.

Key for highly filled compounds is the understanding of filler shape, their size and how their surface is treated to be able to make a proper bonding with the base polymer. Using such compounds helps to enlarge your repertoire for polymer material selection. 

Thank you for reading and #findoutaboutplastics

Greetings,

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature:

[1] https://phantomplastics.com/highly-filled-plastic-for-reduced-cost/

[2] Introduction to Polymer Compounding  Raw Materials, Volume 1 by Natamai Subramanian 

Polymer Material Selection: What are PTFE free alternatives for friction and wear compounds?

Hello and welcome to a new blog post in which we discuss if there are PTFE free alternatives for lubrication.

Why replace PTFE in friction and wear materials?

In the regulation "EU 2019/1021" of the European Union, Perfluorooctanesulfonic acid (PFOS) and Perfluorooctanoic acid (PFOA) are already listed and restricted in use. Further restrictions are under discussion and alternatives to replace materials such as PTFE gain traction. 

PTFE is due to its chemical composition an extremely stable polymer with a low static and dynamic coefficient of friction. Using it in small amounts as additive for friction and wear materials is advantageous. Also, self-lubrication properties are possible. On the other hand, the fluorine is subject to more stringent regulation and the PTFE containing compounds are more aggressive during processing (mould deposits, corrosion and fumes). 

What are PTFE free alternatives for friction and wear compounds?

Ultra-high-molecular-weight Polyethylene UHMW-PE is known for its high abrasion resistance. By using UHMW-PE dispersed in a base polymer similar coefficient of friction values can be achieved. 

There are several methods to estimate the material wear resistance. The sand slurry test (ISO 15527) is a good test to estimate the wear resistance of thermoplastics. In this test, the specimen is rotated 200 - 2400 RPM for 3 hours inside a box which contains the abrasive material (aluminum oxide or silica). The loss of mass is calculated after the test and compared to other material specimens. UHMW-PE has a coefficient of friction of 0.25, which is higher than the coefficient of friction of PTFE (0.1), however the wear resistance is 8 times better than PTFE.

Apart from the sand slurry test, there is the taber abrasion test. The specimen is exposed to an abrasive wheel for 1000 cycles and weight loss of a material is measured. Also in this test, UHMW-PE performs well and outperforms PTFE (Table 1). 

Table 1: Taber abrasion test - results of different polymers and steel

What are some inherently wear-resistant polymers?

Apart from UHMWPE, there are polymers which are inherently wear-resistant: 

-Polyketone (POK)

-Polyoxymethylene (POM)

-Ultra-high molecular weight polyethylene (UHMWPE)

-Polytetrafluoroethylene (PTFE)

-Polyamide (PA)

-Polyamide-Imide (PAI)

-Polybenzimidazole (PBI)

-Polyetheretherketone (PEEK)

In conclusion, from an polymer material selection point of view, UHMW-PE is an alternative material for applications that need excellent sliding properties as well as excellent wear resistance. Downside is that UHMWPE cannot be used for high temperatures as it is the case with PTFE. For high temperatures, PEEK can be a good, however more expensive alternative. Furthermore, flame retardant properties can be achieved by mixing UHMW-PE (wt 80%) with PTFE (wt 20%). 

Another PTFE filler alternative is Hexagonal boron nitride (hBN) which can offer a fluorine- and micro plastic-free replacement. The very good lubricating properties of hBN come from its crystal structure. We discussed this in detail with Michaela Schopp - Product Manager at Henze BNP AG in this guest interview here.

Recently i received  good feedback from one of our community members regarding an additional filler to replace PTFE: Molybdenum Disulphide (moly filler) - MoS2.

Adding moly filler as an additive to your polymer compound, low coefficient of friction, abrasion and wear resistance, as well as excellent low and high temperature properties can be achieved. Furthermore, molybdenum disulphide can be used to reduce the impact of other additives, such as glass fibers, on the tribology.

Example Tribo-Polyamide without PTFE

In this example I present potential solutions to have a Tribo-Polyamide without the us of PTFE as additive:

• PA66 with Graphite/MoS2: Graphite and Molybdenum Disulfide (MoS2) are common alternatives to PTFE, providing self-lubrication. Studies indicate that PA66 composites with graphene/graphite can reduce wear volume, with 0.1% graphene showing superior tribological performance, creating a protective transfer film.
• Silicone-Modified PA66: Lubricant-modified PA66 (often using silicone/polysiloxane) provides enhanced,, PTFE-free, wear resistance.
• Carbon Fiber Reinforced PA66: While primarily for strength, carbon fiber additives can improve wear resistance in PA matrixes. 

Thank you for reading and #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://www.umco.de/de/blog/artikel/PFAS-Einschraenkungen.html

[2] http://www.sugison.com/div-eng/_shared/files/PE-UHMW%20compare%20to%20PTFE.pdf

[3] https://www.plastix-world.com/ptfe-free-self-lubricating-compound/

[4] https://www.corzan.com/en-us/piping-systems/specification/abrasion-resistance

[5] https://www.brad-kem.com/moly-filler-for-plastics/#:~:text=Plastics%20Manufacturers%20make%20moly%20filled,Abrasion%20and%20wear%20resistance

[6] TECACOMP PA66 TRM XS black 1014962: https://www.ensingerplastics.com/en-sg/compounds/tecacomp-pa66-trm-xs-black

[8] https://www.asahi-kasei-plastics.com/de/products/leona/pfas-free/