For condensation polymers—such as PET, PC, PA, and PU proper resin drying is absolutely critical before processing. Why? These polymers contain reactive groups in their backbone (ester, amide, carbonate, or urethane) that can interact with even tiny amounts of moisture.
If not thoroughly dried, water can trigger chain scission during processing at high temperatures (250–300°C), leading to a significant drop in molecular weight (Mw) and, ultimately, a loss of part performance and properties.
Even as little as 0.01% water can have a major impact, so special care must also be taken with regrind material, which can introduce additional moisture into the system.
Figure 1: Polyester, Polyamide, Polycarbonate, and Polyurethane - Example of Chain Scission Due to Hydrolysis
As a rule of thumb, the molecular weight of condensation polymers should not decrease by more than 10%, since a 5% drop in Mw can already reduce properties by 10%.
In contrast, addition polymers like PE and PP are more tolerant to molecular weight reduction, showing only minor property losses (5% property drop).
Understanding and controlling moisture is key to ensuring the quality and durability of your finished parts and prevent part failure.
Hello and welcome to this post on design properties for plastics engineering. In this post we deep dive into the Comparative Tracking Index (CTI) of aliphatic Polyamides, Polybutylene terephthalate (PBT) as well as Polyphenylene sulfide (PPS) used for electronic components made out of plastic.
In previous posts we discussed the CTI of high performance polymers such as PPS and how we can improve it. The values shown in past posts were estimated according to the standard IEC-60112.
What is the Comparative tracking index (CTI) and why the CTI is important?
In general, when the plastic surface, which is the insulation material, carbonizes due to voltage exposure, a conducting path is formed and tracking occurs. Over time, the surface erodes and a conduction of electricity takes place continuously. The resistance to the occurrence of tracking and erosion is represented by the Comparative tracking index (CTI).
How is the CTI value changing if flame retardant additives and glass-fiber reinforced are added to PA and PBT?
Figure 1 compares the CTI values of Polyamide 6 (PA 6), Polyamide 6.6 (PA 6.6) and Polybutylene terephthalate (PBT) with and without reinforcements, as well as with and without halogen free flame retardants. For Polyamides and PBT, adding reinforcement is not leading to a decline in CTI performance. PBT shows a decline in CTI performance in case flame retardants are added.
Figure 1: CTI of PA 6, PA 6.6 and PBT with and without reinforcements, as well as with and without HFFR [1].
How is the CTI of PA, PBT, and PPS changing after heat aging and moisture influence?
Figure 2 [2] shows the results of the CTI measurements on untreated and treated PA 6.6, PBT, and Polyphenylene sulfide (PPS) samples (all with glass or glass/mineral reinforcement). For heat aging and moisture influence, the samples were exposed to 85°C at 85% relative humidity for 1,000 hours (in line with the international standards e.g. IEC 60068) . PA 6.6- GF33 wt% (Zytel® 70G33L) and PBT-GF30 wt% (DURANEX® CG7030) reached in the untreated test scenario the maximum achievable value of 600 V. PPS-(GF+MF) 65 wt% (TEDUR® HTR PPS 2465) reached 500 V in the untreated test scenario. After the heat aging and moisture treatment, PA 6.6 and PPS did not show a decline in CTI performance. The advantage of Polyamides is their molecular structure which enables an inherent resistance to tracking and erosion. Achieving a 500 V level with PPS needs for example a special additive modification which we discussed here. PPS has a CTI in the range of 250 V. In hot and humid environments, PBT showed a decline in CTI; however, it can still keep the CTI above 500 V.
Figure 2: CTI of PA 6.6, PBT, and PPS before and after heat aging and moisture treatment (85°C/85%RH/1000h) [2].
Additional influences on CTI performance - part surface structure
Apart from moisture and temperature influence, part surface influences the CTI performance of your plastic part too (Figure 3). In case of PPS (Tedur HTR), a highly polished surface can increase the CTI from 500 V to 550 V [3]. On the other hand, rough surface structures such as the K29 and K30 (Knauf Industries), decrease the CTI value from 500 to 450 V [3].
Figure 3: CTI of PPS and influence of different surface finishes onto the CTI value [3].
Conclusions
CTI plays an important role when designing electronic components such as busbars for traction motors and power electronics. Selecting the optimal polymer material which can withstand temperature, humidity, time and mechanical impacts is key in order to make your design compact and safe without having short-circuits in the long run.
Hello and welcome to a new post. During material selection you may realize that high strength, stiffness, excellent surface properties, low water uptake and good processing is needed which cannot be fulfilled with the selected aliphatic Polyamide, then Polyarylamide (PARA or MXD6) may be an excellent way forward.
Differences between PA and PARA (PA vs PARA)
Figure 1 shows the molecular structure of a Polyamide 6.6 and a PolyArylAmide, which makes the first major difference visible: the aromatic group in PARA. This difference has a major influence in properties. Table 1 compares the properties of PARA, PA 6.6, and PA 6 with a 50 wt% glass fiber loading, together with a PBT having a 30 wt% glass fiber reinforcement. PARA takes up 87% less water compared to PA 6.6 and contains high modulus levels also after moisture pick up. Table 2 shows the dimensional change after moisture absorption of PARA, PPA, and standard PA. It can be shown that 50 wt% glass fiber reinforced PARA changes only 0.32% after 24-hour water immersion (at 23°C), where else other semi-aromatic polyamides changed twice and standard polyamides four times as much compared to the PARA value. Apart from the low water uptake, PARA offers the best surface among all Polyamides due to its fine crystallization in the surface regions. This makes it a good choice for coating or painting applications. The high modulus comes from the fact that PARA is a fairly large molecule with its aromatic ring structures which entangle. Furthermore, this kind of crystallization allows during injection moulding to apply longer effective packing pressure and prevent sink marks.
Figure1: Comparison of the molecular structure of Polyamide 6.6 and PolyArylAmide (PA 6.6 vs PARA).
Table 1: property comparison of PARA, PA 6, PA 6.6 and PBT.
Table 2: dimensional change after moisture absorption of PARA, PPA, and PA after 24 hours at 23°C, ISO 62 test
Another major difference between PARA and standard Polyamides is the surface finish. Moulded parts in glass-fiber reinforced PARA achieve a low surface roughness value of 0.10 mu Ra (Figure 2) and standard Polyamides are around 0.25 mu. Mechanically polished steel has the equivalent surface roughness value as PARA. Among the Polyamides (aliphatic and semi-aromatic), PARA has the lowest surface roughness value. Reason is the fine crystallization of PARA in the surface layers of a moulded part.
Figure 2: surface roughness of glass-fiber re-inforced PARA and standard Polyamides
Differences between PARA and PPA
If we compare PARA to PPA (Polyphthalamide) we see one major difference: the location of the aromatic rest with the double bonds. PARA is build up via polycondensation of an aliphatic dicarboxylic acid (adipic acid) and an aliphatic diamine with aromatic ring (1,3-xylylenediamine; MXD)
-PPA: aromatic rest is coupled to C=O
-PARA: aliphatic rest is coupled to C=O
This difference is shown in Figure 3.
Among the high heat plastics, PPA and also PPS play an important role; PPAs based on 6T/6I/66 have a Tg of 123°C, a 6T/6I has even a Tg of 133°C which makes them useful for high heat applications.
PARA on the other hand has a Tg of 85°C and although you need for moulding tool temperatures of 120°C (for optimal crystallization), it is not a typical high heat plastic.
However, due to this small difference in the main chain, it has a high Young's modulus and makes it a perfect metal replacement material where optimal surface aspects are needed too. Here an example of a metal replacement for a children buggy where high strength, and stiffness as well as outstanding surface finish is needed.
Figure 3: differences in chemical structures of PARA and PPA.
Pros of PARA
-low water uptake (<1,5%)
-high dimensional stability which enables complex parts
-high stiffness and strength (metal replacement)
-very good flow properties (like PPS) and thick parts without sink marks are possible
-PARA has low thermal expansion and it is similar to glass making PARA a good candidate for automotive interior applications. Polymers such PC/ABS and PPS do not have such low thermal expansion.
-crystallisation in injection mould takes place slowly with a fine crystal structure. This has the advantage of filling the part even in the packing phase and having a part with outstanding surface appearance although it has a high glass fiber loading (up to 60 wt%).
-thin wall moulding down to 0.5 mm is possible too
Cons of PARA
-it is not a typical high temperature polymer such as PPS and PPA, however for many metal replacement and aliphatic Polyamide temperature resistance levels are sufficient. PARA with 60 wt% glass reinforcement can compete with a PPS - 65 wt% glass and mineral filling up to 150°C as shown here.
-it has not the best UV resistance capabilities, however with proper additives is possible to fulfil certain Automotive UV standards.
-Other application fields include air vents in car interior and buggy parts.
In conclusion, if you need a combination of low moisture uptake, high dimensional stability (complex parts), excellent surface, together with outstanding stiffness & strength, and very good flowability than PARA is the material of choice.
Hello and welcome to a new blog post. Today with the topic of flame retardants, starting with an overview and then discussing as an example effective flame retardants for Polyamides.
Why do we need flame retardants in plastics?
In general, adding flame retardants to your polymer compound formulation helps to prevent the immediate start of fire or slowing the growth of fire of your material. This in turn helps to fulfill a certain burning classification such as the UL V0 at a certain material thickness. The material requirement list of your application should consider flame rating needs since it will be easier later during polymer material selection to not miss such an important detail.
Do all polymers need them?
Aliphatic polymers need them to achieve a desired level of UL V0. Semi-aromatic polymers such as PPS do not need them since the benzene rings enable an intrinsic flame retardancy. As a rule of thumb the higher the aromatic amount (benzene) the better the flame retardancy level of your polymer compound.
Overview of the 3 major systems
There are three major systems used in the plastics industry: nitrogen-phosphorus systems, halogenated systems, and metal-hydroxide systems.
Nitrogen-phosphorus systems are halogen free and show a lower smoke emission compared to halogenated flame retardants. Furthermore they do not decrease the mechanical properties of your base polymer too much. Usually, adding the flame retardants results in a lowering of properties. Downside of this system is the narrow production window, water solubility and poss surface aesthetics.
Halogenated systems are very good flame retardants and can be used at low concentration levels, together with a wide production window. Major disadvantage is the use of halogens (pay attention to local regulations) and during combustion it develops a lot of smoke emissions, together with the release of free radicals. Also, stabilization against weathering is not possible.
Metal-hydroxide systems are halogen free and stabilization towards weathering is possible. During combustion, this system only releases water. Downside is the high concentration of flame retardant needed, and lower mechanical properties as a result.
Figure 1 summarizes the advantages and disadvantages of the different flame retardant systems.
Figure 1: Comparison of the advantages and disadvantages of the different flame retardant systems
Example: Use of flame retardants in Polyamide PA 6
Alumina Trihydrate (ATH) is a widely used flame retardant and can be a starting point for Polyamide 6. The decomposition temperature of ATH is around 180°C and it releases water. However, the compounding and processing temperature of PA 6 is between 230°C and 290°C which leads to an activation of the decomposition of ATH. Therefore we need an alternative to safely bring PA 6 onto a certain flame rating. The solution is in Magnesium Hydroxide (MDH) which has a decomposition temperature of 330°C. Apart of MDH, boron zinc oxide and organophosphorus salt can be used for high performance Polyamides such as PPA and PARA.
Hello and welcome back to a new post. Today we discuss the water and moisture uptake of aliphatic short and long chain Polyamides. In a previous post I discussed the water uptake for high performance polymers - check it out here. Here you can find a collection of all my "Design Properties for Plastics Engineering" posts.
Properties of Polyamides
In general, Polyamides are often used as engineering material due to their high thermal stability, very good strength and hardness, combined with high mechanical damping characteristics and good chemical resistance. However, all Polyamides are hygroscopic due to the polar amide groups which form hydrogen bonds with water. Water absorption (at a given temperature and relative humidity) is proportional to the amount of amorphous part of the Polyamide. As a consequence, the water acts as a plasticizer and lowers the mechanical properties. At higher temperatures, hydrolysis can take place too.
How much is the water uptake of aliphatic Polyamides?
Figure 1 shows the water uptake situation of the most used aliphatic Polyamides at equilibrium in 50% relative humidity and atequilibrium in complete saturation.
Figure 1: Water uptake data of most used aliphatic Polyamides
Long chain aliphatic Polyamides such as PA 6.10, PA 6.12, PA 11, and PA 12 show a lower water absorption compared to PA 6, PA 6.6, and PA 4.6. Higher dimensional stability, together with low variation in the properties during ambient humidity changes are the result. Major reason for the lower water uptake is the relatively long hydrocarbon chain length (limiting the amide groups to form hydrogen bonds with water).
Important during material selection is the consideration of the behavior of Polyamides when they are exposed to water (part immersion) or humid environment. The part dimensions need to be still kept within the specified tolerance. If a lower water uptake material with high dimensional stability compared to Polyamide, Polyketone can be a good alternative. Figure 2 compares the water absorption at saturation level of Polyamide PA 6.6 and Polyketone (23°C; weight-%). Polyketone reaches the saturation level at 2.1 % weight increase, where else PA 6.6 at 8.5 weight-%.
Figure 2: Comparison water absorption of PA 6.6 and PK [4].
Hello
and welcome to this three part series on bio-based polyamides. Since
biopolymers are among the fastest growing segment in the polymer industry it is
worth having a closer look at selected new polymers, such as the PA 5.6. In the
first part we will discuss some definitions and then turn the focus towards
Polyamide 5.6 and 5T. In part two we have a look at bio-based PA 6.6 and in the third
part onto bio-based high performance polyamides.
Definitions
and some basics
In this training video I review the chemistry of Polyamide 6 and Polyamide 6.6, discuss
the properties and applications of Polyamides and look at their global demand
and producers.
In
general, Polyamides are semi-crystalline condensation polymers with repeating
amide (–CO–NH–) links in their backbone. The number after the prefix ‘PA’
results from the number of carbon atoms between the amide groups. Furthermore,
there are Monadic (AB) and Dyadic (AABB) Polyamides. AB-Polyamides have a
single repeating lactam with an amine reactive group and as a ‘B’ component a
carboxylic acid group. AABB-Polyamides are created by the reaction of diamine
and a diacid. For them, the first number after the prefix ‘PA’ is the diamine
and the second number describes the diacid. Now back to our bio-based
Polyamides
Chemical
structure and production of bio-based PA 5.6
Pentamethylene
diamine, which is needed for PA 5.6, can be made from biomass or sugar. This is
enabled by using microorganisms and in 2013 the company Cathay Industrial
Biotech was able to increase the efficiency of amino acid decarboxylase by 100
times during the biological fermentation processes. As an enabler, a
gene engineering technique was applied. This can be considered as the
breakthrough for industrial up-scale of bio-sourced pentamethylene diamine
(commercial name: C-BIO N5). Condensation reaction of the green diamine with a
diacid (petrol based or bio-sourced) will lead to a full or partially
bio-sourced Polyamide 5.6 (commercial name: Terryl™), depending if the diacid
is bio-based too. Cathay claims that 8% less diamine is needed with C-Bio N5.
Cathay
and Toray too hold patents on Polyamide PA5T. In general, the melt temperature
of 5T is lower compared to PA6T and glass transition temperature of 5T is 141°C
and therefore slightly higher (Tg 6T = 138 °C) which results in an improved
thermal stability. Due to the high amide group concentration of PA 5.6 and
PA5T, water absorption is higher compared to PA6T and PA 6.6.
Properties
of PA 5.6 compared to PA 6.6 and PA 6
In the
table below major thermal and mechanical properties of PA 5.6, in comparison to
PA 6 and PA 6.6 are shown. The comparison indicates that PA 5.6 is more similar
to PA 6.6 than to PA6.
Table 1: Property Comparison of Petrol-Based vs. Bio-Based Polyamides
Processing
and applications
Processing
of bio-based PA 5.6 can be done via the melt fiber spinning route for yarns and
textiles as well as injection moulding for engineering parts. Since some
internal H and O sites are free (compared to PA 6.6 where all internal H and O
sites are bound), an easier dyeing is achieved. Furthermore, the higher
moisture absorbance increased the comfort of wearing. Injection moulding
compounds reinforced with glass fiber, enter different industries such as
automotive, electrical, and industrial.
Environment,
Health, Safety and Value Proposition
The
bio organism fermentation approach described in the first section represents a
safer route to produce monomers and polymers due to lower temperature, low
pressure and much less toxic raw materials as well as by-products. This in turn
makes the whole process more environmentally friendly, reduces the carbon
footprint and also energy requirements. The value proposition of bio-based
plastics in general is that by switching the monomer sourcing base from petrol
to bio-based plant feedstock an material with intrinsically zero carbon footprint
is obtained. The obtained polymers are not necessarily biodegradable and the
optimal end-of-life option (for example circular economy approach) needs to be
further developed.
Also, PA5X (X=6, 10, 12, 13, 16, 18) and PA56T are all already commercially available for global market applications which allows them to be the tomorrow's choice to cover sustainable, renewable and environmental demands.
In
this post I discuss the difference between bio-based content vs. bio-based
carbon content and in this post helpful standards for composting of
biodegradable polymers. In the second part we discuss bio-based Polyamide 6.6 –
stay tuned!
Thank
you for reading and #findoutaboutplastics
Greetings
Herwig
#materialselection #polymerengineering #biobased
Interested to talk with me about your plastic selection and part design needs - here you can contact me
Hello and welcome to this post on reviewing key engineering plastics. Today, we have a closer look at polyamide 6 and polyamide 6.6.
Similarly to the last time with ABS, we review the chemistry including the simplified petrochemical flowchart, discuss the properties and applications of polyamides and look at their global demand and producers. I will provide you some price indications as well.
Here you can find the youtube video of the review:
Before we start with the chemistry I would like to spend some words on the history of polyamide. Wallace Carothers, working 1930 at DuPont Company, developed together with his assistants the two most widely used synthetic polymers of the 20th century: nylon or polyamide and neoprene (synthetic rubber). We will focus in this review on polyamide 6 (PA 6) and polyamide 6.6 (PA 6.6). Although they may seem quite alike they are different e.g. in terms of glass transition and melting temperature as well as water uptake. The latter is higher for PA 6.
Polyamide chemistry
PA chemistry and simplified flow chart:
Where do PA 6 and PA 6.6 have their roots?
PA 6 is made by ring-opening polymerization of e-amino caprolactam which is obtained over the benzene route (from benzene over cyclohexane to caprolactam). Cyclohexane is first converted to its oxime. An oxime is a chemical compound with a carbon-nitrogen double bond. The word oxime is a combination of the words oxygen and imine. Treating the oxime with acid initiates the so-called Beckmann rearrangement to obtain caprolactam. The global demand on caprolactam are 5 milllion tons.
PA 6.6 can be polymerized using hexamethylenediamine and adipidic acid. Both have 6 carbon atoms leading to the nomenclature of 6.6. HMDA can be obtained over three routes: from adipidic acid route, from hydrogenation of acrylonitrile, and from hydrocyanation of butadiene.
Polyamide - Simplified Flow Chart
Poylamide properties
PA 6 shows good toughness at relatively high (80°C) and low temperatures as well as resistance to repeated impacts. This is rounded up with good resistance to abrasion and wear. Chemically, PA 6 exhibits resistance to many organic solvents, oils and gasolines.
In comparison, Polyamide 6.6 can be used at higher continuous service temperatures (100°C – 120°C) combined with a better retention of stiffness, tensile properties, and shape at high temperatures.
Polyamide - Properties
PA Capacity and Global demand
The total consumption of Polyamides in 2016 was around 7.5 Million Tons. PA 6.6 has a consumption of 2.4 million tons and PA 6 of 5.1 million tons. 70% of the global PA 6.6 consumption was used in the automotive market. Half of the consumption of PA 6 is used for producing fibers.
Polyamide - Capacity and Global Demand
Geographically you can state that Asia is the largest market for PA 6 and it is preferred when flexibility and barrier properties are important. North America is the largest market for PA 6.6 and is mainly selected for engineering thermoplastic applications due to a higher melting point.
Important to note is that both Nylons are interchangeable for most applications.
Polyamide - Capacity and Global Demand
PA Price to performance
Polyamides are forming the base of engineering thermoplastics and have a price range of 2.5€/ kg for base grades and high heat Nylons can reach up to 6.5€/kg.
Polyamide - Price to Performance
PA End uses
As previously shown, PA 6 is more selected for fiber applications such as tire cords, and filaments for fishing lines. PA 6.6 with a glass fiber reinforcement of 30% is used in many automotive applications where the combination of elevated temperature, toughness and long life time is needed. However, there are many other industry fields, where the properties of Nylon 6.6 can play a key role.
Important to note is the water uptake of PA 6.6 which can be 2.5% at 50% relative humidity. PA6 has with 2.8% a bit higher water uptake. The water uptake results in dimensional changes, reduces the yield stress 29% of its original value. Elongation will be increased with the factor of 5 and toughness will be doubled. Overall stiffness can decrease almost 60%.
Just to name a few of the typical applications: belts, guitar strings, car engine upper intake manifold and engine support mounts, and the famous Fischer wall plug, introduced in 1958 by inventor Arthur Fischer.
This was a review on polyamide, an important engineering thermoplastic used in many applications.
