Saturday, 29 October 2022

The Important Role of Additives: Enhancing Polymer Properties for High Performance Applications (Part 2)

Hello and welcome to a new post. Today we continue with the second part of our plastic additives series. In this post we discuss how fillers change the properties of polymer compounds and how we can use this during material selection

Here you can read part 1.

General effects caused by fillers

Table 1 shows the effects on Elastic-modulus, elongation, toughness, flames resistance, and dimensional stability caused by different fillers such as glass fibers, UV-stabilizers, and flame retardants. UV-stabilizers, flame retardants (organic and inorganic), and anti-statics show a negative impact on the modulus of elasticity, elongation and toughness. 

Table 1: overview of effects caused by different additives [adapted from 2]

Modification of properties by using isotropic, flaky, and fiber shape additives

Now with the know-how of Table 1 it is possible to influence certain properties by using different additive geometries. In general, isotropic fillers have the same behavior in x-, y-, and z-direction and improve the dimensional stability of your final part. Platy shaped fillers are very good in the x- and y-direction however not so good in z-direction. Fibers are only good in one direction and show a fair behavior in the remaining two. 

Table 2: overview of property modification by filler geometry [3]

Example on warpage control of semi-crystalline polymers

In Table 1 and Table 2 we discussed how to improve the dimensional stability of polymers. In this example we have semi-crystalline polymer with a glass fiber loading and we would like to have a better hand on the warpage control of the final part. The glass fiber loading causes a different shrink rate in x-direction than in y-direction leading to warping of the part. If we take a semi-crystalline polymer with isotropic filler loading, shrink rate in x- and y-direction are the same, however the final part will show a low strength. The key is to combine glass fibers with isotropic filler in order to obtain a flat part with good strength properties. 

How to do it? 

A common compounding solution is to use 15 weight-% glass fiber and combine it with 25 weight-% mineral or beads. 

Example improvement of wear resistance of amorphous polymers

In case you consider an amorphous polymer such as Polycarbonate (PC) for applications which need to have a certain level of wear resistance (for example gears), an effective way is to use PTFE as a lubrication additive. This is demonstrated in Table 3, where 15 weight-% PTFE is added to a PC. Both, the wear factor and the dynamic coefficient of friction could be reduced. 

Table 3: wear improvement of amorphous polymers

Example Polyamide 6.6 (PA 6.6): filler vs. reinforcement

In general, by using fillers we can have a good shrink control, improve the modulus of elasticity, and heat distortion. Impact resistance will decrease and strength will remain the same. Using reinforcements, strength and modulus will improve, together with heat distortion. Regarding impact resistance, reinforcements will make brittle resins tough and tough resins brittle. Table 4 compares an unfilled PA 6.6 with a 40 weight-% talc filled and a 40 weight-% glass fiber reinforced PA 6.6 to better illustrate the main differences of filler and reinforcement. 

Table 4: PA 6.6 - filler vs. reinforcement

In the next part we discuss how to improve the conductivity (thermal and electrical) of polymers. 

Here you can jump to part 1 and here to my posts on flame retardants, as well as CTI improvement strategies, involving additives. 

Thanks and #findoutaboutplastics

Greetings, 

Herwig 

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

Subscribe to my Polymer Material Selection book launch page 

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

Literature: 

[1] Gächter and Müller: Plastics Additives

[2] DuPont and Biesterfeld Interowa - Design with plastics, 2011

[3] Chris DeArmitt: https://phantomplastics.com/plastic-materials-training-from-a-top-expert/learn-about-filled-plastics/

[4] https://youtu.be/h7iUC9-JdiU

[5] https://youtu.be/nLGcSszTaTs

[6] https://www.youtube.com/watch?v=1pPx1YbGDBA


Thursday, 27 October 2022

Processing of Engineering Thermoplastics: PBT and its crystallization behavior

Hello and welcome to a new post. Today we discuss considerations for injection moulding of Polybutylene terephthalate (PBT). In previous posts we discussed the DMA behavior of PBT and other thermal properties.  

General properties of PBT

PBT belongs, together with PET, to the class of Polyesters. Polyesters are made by polycondensation reaction under the release of water. Unreinforced PBT can be used from -60°C up to +110°C and reinforced PBT up to +200°C. In case PBT is exposed to hot water above 60°C it shows hydrolysis reactions (degradation reaction). It has good electrical properties such as electrical breakdown strength / dielectric strength at higher temperature (aliphatic Polyamides show much lower dielectric strength at higher temperature). Also the Comparative Tracking Index (CTI) is high with PBT and therefore it is often used in electrical and electronics applications (housings, connectors, busbars, e-Mobility). PBT has good mechanical and stress cracking properties. It burns with yellow-orange sooty flame. It needs to be protected against UV and to reach a certain flame classification, flame retardants need to be added. 

Injection moulding of PBT: Dos and don'ts

Drying before moulding: min. 3 hours at 120°C; this ensures optimal mechanical properties in your final part at a later stage. Remaining humidity should be not more than 0.04%. If there is 0.1% humidity left, tensile strength may be reduced up to 12% from the optimum value. I created a table with the different maximum moisture levels after resin drying to ensure proper processing of 36 polymers.

Residence time: max. 8 to 10 min, otherwise material degradation will take place; thin wall parts optimal residence time is around 5 minutes and for thick wall parts around 3 minutes; if you are around 1 to 2 minutes, risk is given that there are not molten plastic pellets. 

Packing: long packing is beneficial to prevent shrinkage cavities. 

Injection speed: as high as possible, especially for thin wall parts. 

Tool temperatures: PBT needs lower tool temperatures due to the high crystallization rate of the polymer. Mould temperatures between 30-60°C are sufficient.  If you produce precision parts, mould temperatures up to 120°C are better, since the post shrinkage will be negligibly small. 

Post shrinkage: low mould temperatures up to 0.3% and with high tool temperatures between 0.01 and 0.022%.

Melt temperatures: In table 1 an overview on the different processing temperatures is given.

Table 1: PBT processing temperatures

Conclusions

During the design and material selection phase, polymers are selected due to their specific properties which they can bring to the table. However, if certain things such as proper drying before processing, crystallization rate and residence time during processing are not considered, the final part will not have the desired optimum properties. 

Thanks 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 

Subscribe to my Polymer Material Selection book launch page 

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

Literature: 

[1] http://www.kunststoff-meister.de/mediapool/132/1320009/data/PBT.pdf

[2] https://www.plastverarbeiter.de/wp-content/uploads/migrated/docs/348_27934.pdf


Tuesday, 11 October 2022

The Important Role of Additives: Enhancing Polymer Properties for High Performance Applications (Part 1)

Hello and welcome to a new blog post. Today’s topic is the important role of polymer additives and since it is a broad topic I will split it into several parts. 

In the first part we discuss the types of additives and things to consider when you use additives in material- and application development.  Having a basic understanding of fillers and what they can enable in a plastic compound is important for polymer material selection too. 

What can additives do for your polymer?

The limited usage range of certain polymers can be improved by incorporating additives into the polymers. Thinking of polyolefins, without adding antioxidants, not even processing would be possible in a proper way. Another example is rigid PVC which can be made flexible by adding plasticizes. Adding glass fibers to Polyamide 6 increases the heat distortion temperature (HDT), as well as strength and stiffness. Using color pigments allows the PC to be moulded in several colors and PP can be made radiation stable by adding free radical scavengers. Conductive fillers turn polymers from an isolator to a material which can be used as a heat sink. Another example is the chemical compound Piperine which is responsible for the spicy appearance of black pepper. It is added during fuel line extrusion for cars to prevent biting of cables due to animals. If they bite, it will start hurting and they will immediately stop. 

Additives can turn brittle polymers such as Polystyrene into a toughened polymer by using impact modifiers. Also can additives improve processing, dimensional stability, and strength. Furthermore, they can improve thermal, radiation, and light stability. They are able to improve the flame retardant level and turn for example Polyesters flame retardant and improve long-term aging, scratch resistance, aesthetics, and biocompatibility.  Another example is that additives can improve electrical properties such as the Comparative Tracking Index (CTI)

Polymer material performance: the effects of additives in an overview

Table 1 shows the different additive classes and their effect in the final plastic compound. Flame retardant additives we discussed previously already here

Table 1: Overiew of different additive classes and their effect in the final plastic compound.

What do we need to consider when we use additives? 

The different effects of various additives on the final compound need to be considered by the formulators. Physical and chemical properties, toxicity, sterilization as well as biocompatibility (in case of health care applications) must be evaluated. Also checking the shelf life of the new material compound needs to be considered since the used additive may cause some side reactions which lead to a decrease in properties. 

In the next part we discuss how to modify the properties of compounds by using different shaped fillers. 

Thanks and #findoutaboutplastics

Greetings, 

Herwig 

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

Subscribe to my Polymer Material Selection book launch page 

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

Literature:

[1] Gächter and Müller: Plastics Additives

[2] Sastri - Plastics in Medical Devices: Properties, Requirements, and Applications