The main factors that affect the performance of plastic parts during use include temperature, chemicals, radiation, and time, which we refer to as the four major killers of plastic part performance.

Killer 1: Temperature

All thermoplastic materials, including PC, ABS, PBT, and nylon, will soften and melt at a certain temperature. The melting temperature of different plastics varies. However, even at lower temperatures, long-term exposure of thermoplastic materials to heated environments can have a significant negative impact on their performance.

The primary reason is that heat can cause the breakage of plastic molecular chains, resulting in a decrease in plastic molecular weight and a decrease in performance. The decrease in performance is mainly reflected in elasticity and toughness, and other performance will also be affected.

▲ Molecular chain breakage

The temperature at which plastic materials begin to degrade depends solely on the chemical group of the polymer and the chemical mechanisms involved (oxidation, depolymerization, etc.). Sometimes, this degradation can be reduced by adding heat stabilizers. However, degradation still occurs; It only occurs at a lower rate at higher temperatures.

If the ambient temperature is too high, it can cause plastic parts to deform, melt, and even cause a fire.

Killer 2: Chemicals

Like many other materials, thermoplastic is also very susceptible to chemical attacks. When we think of chemicals, the first thing we think of is odorous and corrosive substances such as acids, solvents (such as paint and paint diluents, acetone and toluene), gasoline and fuel, or detergents and cleaning agents.

However, there are also chemicals in all kinds of things we encounter in our daily life, from sunscreen lotion to moisturizing cream, to lipstick, and even water.

We often consider water to be an inert substance, but for certain substances such as pig iron, contact with water can immediately cause a chemical reaction.

Fortunately, most thermoplastic materials do not react chemically with water.

But there are also some thermoplastic materials, such as nylon, that can absorb water. This absorption process is completely reversible, causing the material to expand and also acting as a plasticizer, making the material harder, more flexible, and more ductile, while reducing its mechanical strength.

▲ Retention rate of bending modulus of nylon of different grades after water absorption

Some plastics, such as PBT, are prone to hydrolysis at high temperatures. This is because PBT contains ester bonds, which can break when placed in water at temperatures higher than its glass transition temperature. The acidic environment formed by hydrolysis accelerates the hydrolysis reaction, resulting in a sharp decline in performance.

I once used a PBT plastic, and after 1000 cycles of double 85 testing, the inside of the plastic part had almost turned into flour. With just a slight force, the part could be broken open.

Of course, methods such as adding hydrolysis stabilizers to PBT can avoid this problem. There are already many commercial water-soluble PBT plastics available.

Whether thermoplastic materials will be subjected to chemical attacks or to what extent they will be affected depends on three major factors.

The first and most important factor is whether the plastic reacts with this chemical substance. It may be completely unaffected by this chemical substance. It may also remain unaffected at low temperatures, but may be affected by exposure to high temperatures.

The second factor is the relative concentration of chemical substances; Is exposure long-term constant or intermittent; And the duration of exposure.

The third factor is the chemical mechanism. Is this chemical substance acting as a plasticizer? If so, is it reversible or permanent? Will this chemical substance cause oxidation reactions, plastic degradation, or just surface discoloration?

CPVC pipe ruptured due to contact with incompatible sealant

Killer 3: Radiation

Another ultimate use condition that affects thermoplastic materials is radiation. Most people believe that the term radiation is related to radioactivity, which is a substance that releases particles and energy during nuclear decay. But radiation is a broader term that describes the process of electromagnetic waves propagating in space.

Electromagnetic waves are a form of energy composed of electric and magnetic fields. These waves can have wavelengths as small as 1 picometer (10-12 meters) and as large as 100 megameters (106 meters, or 1000 kilometers). This wavelength range is commonly referred to as electromagnetic spectrum, starting from gamma rays (less than 10pm), including X-rays, ultraviolet rays, visible light, infrared rays, microwaves, and radio waves.

The energy carried by these waves decreases as the wavelength increases. Gamma rays carry the most energy, followed by X-rays, and then ultraviolet rays. In physics, electromagnetic waves are collectively referred to as "light" waves, although the term "light" is commonly used to describe visible light, which is composed of electromagnetic waves with wavelengths approximately between 390 and 750 nanometers.

When choosing thermoplastic materials, we sometimes care whether they and their additives will block electromagnetic waves of a given frequency or transmit them without loss. For example, in optical applications, we typically want all light in the visible spectrum to be transmitted without considering other wavelengths. Alternatively, for sunglasses, we may want to block a certain amount of visible light or wavelengths within the ultraviolet range. Alternatively, in electronic shielding applications, we may wish to prevent the transmission of electromagnetic waves within a certain frequency band of the radio frequency (RF) spectrum.

However, we also need to consider the impact of electromagnetic waves on the plastic polymer itself. Basically, we input energy into the polymer matrix, especially at the low end of the spectrum (through gamma rays of ultraviolet radiation). If the polymer is transparent to these waves, energy will pass through. However, if the polymer blocks this transmission, energy will be absorbed or converted into heat, leading to polymer molecular chain breakage.

Changes of PS plastic polymer film before and after UV irradiation

One of the reasons why sunlight causes such damage to materials (not just thermoplastic materials) is that it contains electromagnetic waves not only in the visible spectrum, but also in the infrared and ultraviolet spectra. Long term and continuous direct sunlight means that materials absorb a large amount of energy, which usually has harmful effects. For example:

Household appliances turn yellow under long-term exposure to fluorescent lamps

The car dashboard ruptured under long-term sunlight exposure

Killer 4: Time

As the saying goes: Time is a pig killing knife, the knife is deadly!

For plastic parts, the same applies!

Over time, especially when combined with one or more factors, it almost always leads to a loss of plastic material properties. In fact, most of the test data used to evaluate environmental impacts is created using time as a variable.

For example, high-temperature aging tests are used to evaluate the consequences of long-term exposure to high temperatures, and regular measurements of certain mechanical properties (such as tensile strength) can reflect changes in performance over time.

In a similar way, weather resistance testing is typically used to evaluate the long-term effects of exposure to outdoor environments. These tests typically involve a combination of temperature, chemical, and radiation (primarily ultraviolet) effects measured over several days, weeks, months, or years.

And these tests may include different factors depending on the application area of the plastic parts: for example, weathering tests in certain areas need to target high heat and high ultraviolet radiation in dry environments, while weathering tests in certain locations target high humidity and high ultraviolet radiation in subtropical environments, sometimes with the addition of salt spray effects. Although these tests are usually conducted with a certain acceleration factor, their purpose is to predict the long-term performance of plastic parts after months and years of exposure.

To evaluate the impact of exposure to one of the above conditions, it is necessary to measure and compare the performance data of plastic parts before and after exposure. Because any changes in performance data will be obvious, it is easy to predict the impact on performance.