Design of high-voltage insulation parts for new energy electric drive systems

Therefore, in the material selection process, in addition to looking at the physical properties table at room temperature, it is more necessary to pay attention to the performance of key material parameters after aging under different temperatures and environments.

When it comes to aging, we first need to understand a very important indicator of the material - the glass transition temperature (Tg)

Glass transition temperature refers to the temperature corresponding to the transition from glass state to highly elastic state. Glass transition is an inherent property of amorphous polymer materials and is a macroscopic manifestation of the change in the form of polymer motion. **In layman's terms, below the glass transition temperature, the plastic is "frozen" and has almost no aging; above the glass transition temperature, the plastic material begins to age, and the higher the temperature, the more severe the aging. **Of course, the increase in temperature is not all bad, and sometimes it will increase certain parameters of the material, such as the tensile strength that we are concerned about.

In addition to the glass transition temperature, there is another parameter that directly affects the aging properties of the material. We will discuss this later.

As a high-voltage insulating part in an electric drive system, in some cases, we need to consider the oil resistance of the material. Due to the characteristics of polymer molecules themselves, oil solvent molecules can penetrate into the interior. The factors that affect penetration mainly include material structure, glass transition temperature, and crystallization properties. Therefore, when evaluating whether a plastic material is oil-resistant or not, you can start from the following two points.

• At the same temperature, the lower the glass transition temperature, the easier it is to penetrate

• Oil penetration occurs in amorphous areas, so materials with higher crystallinity are less susceptible to penetration

However, the cost of obtaining material aging data is relatively high, so material suppliers do not conduct aging data tests for each grade. Therefore, before these aging data are available, we generally use the material's heat deflection temperature (HDT) to represent the material's short-term heat-bearing capacity.

Heat distortion temperature is a popular term for load deflection temperature. It is a method of measuring the rigidity of plastics at high temperatures: under a certain load, the temperature is continuously increased at a certain speed until the sample shows the temperature at which the deformation is indicated.

High Voltage Insulation Plastic Properties - Additives

In order to meet different performance requirements, plastic materials are often added with various additives, just like adding various seasonings when cooking. The most common additive in plastic materials, and the one we are most familiar with, is glass fiber (inorganic non-metallic material, which can effectively improve the tensile strength of the material), which is the English letter GF after the material grade. There are many other additives, such as heat stabilizers/antioxidants/foaming agents/flame retardants.

There are two common problems with additives:

1. Price

Taking glass fiber as an example, GF40 material has a higher glass fiber content and higher tensile strength than GF30 material. Is it also more expensive?

In fact, the price of glass fiber is much lower than that of plastic substrate. More glass fiber means less substrate, so the material cost is lower. In addition, while the tensile strength is improved by glass fiber, other parameters will deteriorate, such as plasticity. This kind of side effect of one gaining while the other loses requires us to make a careful decision.

2. Flame retardant

Does the electric drive system need V0 flame retardant?

Before returning to this question, let's first take a general look at what other flame retardant grades there are besides V0. The flammability UL94 grade is the most widely used flammability performance standard for plastic materials. It is used to evaluate the ability of a material to extinguish after being ignited. UL94 has a total of 12 fire ratings, namely: HB, V-0, V-1, V-2, 5VA, 5VB, VTM-0, VTM-1, VTM-2, HBF, HF1, HF2.

Here we focus on HB and V0.

HB is the lowest flame retardant grade in the UL94 standard. The test method is to clamp one end of the test piece horizontally at 45±2 degrees to the horizontal, take an iron net, fix it horizontally 10±1mm below the test piece, and move the Bunsen burner (flame height of about 20mm) at 45 degrees to the other end of the test piece to contact 6mm of the sample, and move it away after timing for 30±1 seconds. When the test piece burns to the 25mm mark, start another timer. If it burns to the 25mm mark within 30±1 seconds, start another timer and move the Bunsen burner away.

The V0 test is also called a 50w vertical combustion test because the combustion energy is about 50w. The test method is to have a Bunsen burner (flame height of about 20mm) contact the center point below the sample and burn for 10±0.5 seconds, then move the Bunsen burner away at a speed of 300mm/s, at least 150mm away from the test piece, and record the first spontaneous combustion time. After the spontaneous combustion stops, immediately carry out the second combustion, and after burning for 10±0.5 seconds, move the Bunsen burner away, and record the second spontaneous combustion time and the red-hot time after the flame is extinguished.

From the test specifications and required limits, the V0 flame retardant grade is higher than the HB grade, which is why the new energy vehicle industry has one of the flame retardant V0 requirements. For such requirements, the original HB flame retardant materials cannot meet the new requirements, and measures must be taken to improve the flame retardancy. The most common method is to add flame retardants. So we can see that the same category of materials, such as PPA, has different grades of HB flame retardant and V0 flame retardant. At the current level of technology, the side effects of flame retardant additives are huge compared to glass fiber. Not only will the tensile strength and toughness be greatly reduced (especially at the weld mark), but most flame retardants are easily decomposed at high temperatures, releasing acidic substances that are corrosive to metals.

The burning of new energy vehicles has always been a focus of attention, and the social impact it brings is huge, which to a certain extent has promoted the attention paid to improving the flame retardancy of plastic parts in the industry.

Let’s get back to the question we want to discuss: Does the new energy electric drive system require V0 flame retardancy?

Only when the three conditions of combustibles, combustion-supporting materials, and ignition sources are met at the same time, can combustibles burn. For electric drive systems, the sealing level is generally IP67, and the internal parts are mainly made of metal. From this perspective, the severity, frequency, and detection of electric drive systems caused by material flame retardancy failure are relatively low. Of course, for design, it is a process of continuous improvement. There is no best choice, only better choices.

After weighing the factors of price, performance, manufacturing, etc., at the current stage, I believe that from an engineering perspective, electric drive system technology does not need to have V0 flame retardancy as a hard requirement. Of course, having V0 capability would be the best.

High Voltage Insulation Plastic Characteristics - Material Selection

There are many types of plastic materials. Taking BASF's plastic product pyramid as an example (there are slight differences among different companies), there are more than 40 types.

There are five common types in electric drive systems: PA6, PA66, PPA, PPS, and PBT.

Before analyzing the properties of these materials, let's take a look at their molecular formulas. Of course, without professional knowledge or relevant homework, it is bound to be confusing. So in this confused situation, if you need to choose a material that you think has the best aging resistance, which one would you choose? (Remember the previous question: In addition to the glass transition temperature, there is another parameter that directly affects the aging performance of the material. We will discuss it later.)

You can try to choose. We won’t keep you in suspense. The answer is the third PPS.

PPS is a polyether plastic. The first impression given by its molecular formula is that it is extremely rigid and stable like a "turtle shell". This "turtle shell" is benzene. Benzene has properties different from those of saturated compounds. It is not easy to add, not easy to oxidize, and the carbon ring is extremely stable. These properties are collectively called aromaticity (non-polar or weakly polar).

PBT also has a "turtle shell", so it should be very stable? Yes, the scientific name of PBT is polybutylene terephthalate, which is an aromatic polyester and also has the aromaticity mentioned above.

PA6 and PA66 are the nylons that we often mention in our daily life (polyamide is commonly known as nylon). If aromatic materials are "stifling" and stable, then nylon is "outgoing" and compatible.

Perhaps some readers have noticed that among the five common plastics mentioned above, why are there only four molecular formulas? Where is the remaining one?

PPA is quite special, sometimes called "PA6T". PPA is a blend of polymers formed by polycondensation of isophthalic acid, terephthalic acid, adipic acid and hexamethylenediamine. It is a semi-crystalline semi-aromatic nylon. The most common blend is PA6T/66, which is both aromatic and nylon, so the comprehensive performance of PPA is very excellent. Of course, the price is not cheap, after all, it is a special nylon.