Why Does PP Usually Not Require Drying, While PA Must Be Dried?

On the injection molding floor, the arrangement of the drying step often confuses people: PP (polypropylene) pellets are poured out of the bag, sometimes without even passing through a dryer, and can be dumped straight into the hopper for molding. PA (nylon) pellets, on the other hand, must be placed in a desiccant dryer for several hours in advance, and after drying they must be used quickly—if left too long, they need to be dried again. Both are plastics—why such a big difference?

The answer lies in the molecular structure of the two materials. PP and PA have completely different attitudes toward water—PP and water "ignore each other," while PA and water are "inseparable." This difference determines whether moisture can enter the interior of the pellets and whether drying is a mandatory step.

How Does Water Enter Plastic? Polarity Is the Key

The process by which plastic pellets absorb moisture is essentially the process of water molecules diffusing into the interior of the pellets. Whether this diffusion can occur and how fast it happens depends on whether there is "attraction" between the plastic's molecular chains and water molecules.

Water molecules are polar molecules—one end carries a positive charge and the other carries a negative charge. If the plastic's molecular chains also have polar groups, water molecules will be attracted to these groups, diffuse into the free volume between molecular chains, and even form hydrogen bonds with the polar groups. This process requires no external pressure; it occurs spontaneously under normal temperature and humidity conditions, with the speed depending on the material's hygroscopic tendency.

If the plastic's molecular chains are non-polar, containing only carbon and hydrogen with no polar groups, water molecules have no affinity for them and can hardly diffuse into the interior of the pellets. Moisture can at most attach to the pellet surface, forming a thin film of water, or condense in the gaps between pellets.

The fundamental difference between PP and PA lies in whether there are polar groups on the molecular chains.

PP: Non-Polar Molecular Chains, Water Cannot Get In

PP's molecular chain is composed of carbon atoms and hydrogen atoms, with propylene as the repeating unit. There is no oxygen, nitrogen, or other atom carrying polar groups along the entire molecular chain—it is a pure hydrocarbon chain. This structure makes PP non-polar.

Non-polar molecular chains have no affinity for water molecules. Water molecules on the surface of PP pellets are like water droplets on wax paper—they cannot wet the surface, let alone diffuse into the interior. PP's water absorption rate is typically below 0.02%, and this moisture almost entirely remains on the pellet surface or in the gaps between pellets, not dissolved inside the material.

Because of this, PP usually does not require dedicated drying before injection molding. If the pellet surface has condensed some moisture due to a humid storage environment, a short blow with an ordinary hot-air dryer, or even just a little preheating with hot air in the hopper, is enough to carry away the surface moisture. If the storage environment is dry, it can even be used without drying.

There is another reason PP can be molded without drying: even if a small amount of surface moisture is carried into the barrel, it does not cause serious problems. PP's processing temperature is typically between 200-280°C, and water vaporizes at this temperature. However, because PP's melt viscosity is relatively high, a small amount of water vapor is insufficient to form obvious silver streaks on the product surface. Moreover, PP's molecular chains do not have groups like ester bonds or amide bonds that are easily hydrolyzed—water will not cut PP's molecular chains at high temperatures. So PP has a high tolerance for moisture.

PA: Polar Amide Groups, Water Actively "Bores" In

PA's molecular chain contains amide groups (-NH-CO-). Amide groups are polar—the nitrogen and oxygen atoms have lone pair electrons and can form hydrogen bonds with water molecules. This means there is a strong affinity between PA's molecular chains and water molecules.

PA's moisture absorption process works like this: water molecules are first attracted by the amide groups on the pellet surface, forming hydrogen bonds; then these water molecules become new "attraction points," pulling in more water molecules; water molecules gradually diffuse toward the interior of the pellets along the free volume between molecular chains, eventually reaching a certain moisture content even in the pellet core. This process occurs at room temperature, and not slowly. PA's water absorption rate is typically between 1%-2%, with some grades even higher—far above PP's 0.02%.

What is more troublesome is that PA's amide groups not only absorb water but are also prone to hydrolysis at high temperatures. When moisture-containing PA enters the barrel, at processing temperatures of 250-290°C, the amide bonds are attacked by water molecules, molecular chains break, and molecular weight decreases. The consequences of hydrolysis are product brittleness, reduced strength, and silver streaks and bubbles on the surface.

So PA must be dried for two reasons: first, it absorbs a lot of water and absorbs it quickly; second, after absorbing water, it not only produces bubbles but also undergoes chemical degradation. These two reasons combined together make PA drying not an "optional step" but a "mandatory step."

PA's drying requirements are also quite strict. It typically needs to be dried at 80-100°C for 2-4 hours, with moisture content controlled below 0.1%. After drying, if PA is exposed to air, it will quickly reabsorb moisture, so it needs to be used immediately or stored sealed. This is also why PA drying is often scheduled for the last moment before injection molding.

The Degree of Hygroscopicity Determines the Necessity of Drying

Comparing PP and PA side by side clearly shows the influence of hygroscopicity on the necessity of drying.

PP's water absorption rate is below 0.02%—moisture stays only on the surface, does not enter the interior, does not trigger hydrolysis, and does not produce obvious silver streaks. Drying is "optional" and can even be omitted in some cases.

PA's water absorption rate is between 1%-2%—moisture enters the interior, triggers hydrolysis at high temperatures, and produces silver streaks, bubbles, and performance decline. Drying is "mandatory," and strict moisture content standards must be met.

PC's water absorption rate is between 0.15%-0.35%—although not as high as PA, PC's ester bonds are extremely sensitive to hydrolysis, so PC must also be strictly dried, with moisture content requirements even stricter than PA (below 0.02%).

ABS's water absorption rate is between 0.2%-0.4%—it contains butadiene components and requires drying, but the requirements are not as demanding as PC.

PET's water absorption rate is between 0.2%-0.5%—ester bonds are sensitive to hydrolysis, processing temperature is high, and drying requirements are very strict.

It can be seen that the necessity of drying depends not only on the level of water absorption but also on the material's sensitivity to hydrolysis. PA absorbs a lot of water, so it must be dried; PC does not absorb the most water, but its hydrolysis sensitivity is extremely high, so it must also be dried; PP absorbs very little water and does not hydrolyze, so it usually does not require drying.

What Does Moisture Do During Injection Molding?

Understanding the necessity of drying also requires knowing what problems moisture causes during injection molding.

For PA, moisture does two things after entering the barrel: physical vaporization and chemical hydrolysis. Physical vaporization produces water vapor, which forms bubbles in the melt. During filling, the bubbles are stretched and, after cooling, form silver streaks on the product surface. Chemical hydrolysis cuts PA's molecular chains, molecular weight decreases, and the product's mechanical properties—impact strength, tensile strength, wear resistance—decline across the board. These two things happen simultaneously and aggravate each other.

For PP, even if a small amount of surface moisture is carried into the barrel, the amount of water vapor is very small, and PP's melt strength is relatively high, so bubbles are not easily stretched to the point of forming obvious silver streaks. More importantly, PP has no amide bonds or ester bonds, and water will not cut PP's molecular chains at high temperatures, so performance does not decline due to hydrolysis. PA drying protects not just appearance, but the molecular chains themselves. Moisture is removed before entering the barrel, hydrolysis does not occur, molecular weight does not decrease, and product performance is guaranteed. PP does not need this protection because its molecular chains are inherently not afraid of water.

The Drying Methods Are Also Different

PP and PA differ not only in "whether to dry" but also in "how to dry."

If PP is only slightly damp on the surface, an ordinary hot-air dryer at 80-100°C for one to two hours is sufficient. The dew point of the drying air does not need to be very low, because moisture is only on the surface and easy to remove.

PA must use a desiccant dryer. The dew point of an ordinary hot-air dryer is affected by ambient humidity and may be only around 10°C during humid seasons, so moisture inside the pellets cannot diffuse out. A desiccant dryer cools the air below the dew point, condenses out the water, and then heats it and sends it to the hopper. The dew point can be lowered to -40°C or even lower, greatly increasing the moisture concentration difference between the inside of the pellets and the air, so that moisture can be effectively driven out.

PA's drying temperature also cannot be too high. PA's melting point is around 220°C (varying by grade), and drying temperature is typically controlled at 80-100°C to prevent the pellets from softening and agglomerating during drying. Drying time must also be sufficient to allow internal moisture enough time to diffuse to the surface.

Ultimately, Molecular Structure Determines the Necessity of Drying

PP does not require drying, and PA must be dried—the fundamental reason lies in the different molecular structures of the two. PP is a non-polar hydrocarbon chain: water cannot get in, and it does not hydrolyze. PA is a polar amide chain: water actively bores in, and at high temperatures it cuts the molecular chains.