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

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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 

Interested to talk with me about your polymer material selection, sustainability, and part design needs - here you can contact me 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.

New to my Find Out About Plastics Blog – check out the start here section

My YouTube channel

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.

Thank you for reading and #findoutaboutplastics

Greetings, 

Herwig 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.

!NEW! Ultra and High Performance Polymer Selection - new online course coming soon - join the waiting list

New to my Find Out About Plastics Blog - check out the start here section

🎤My Polymer Expert Talks Podcast

📹My YouTube channel

My personal website

My dedicated site for Polymer Material Selection

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