What are the factors that affect the transparency of engineering plastics?

Feb 16, 2023

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Introduction to transparent engineering plastics

Transparent engineering plastics generally refer to a class of engineering plastics with excellent optical transparency, low yellowness index and haze, which can be processed by molding processes such as molding, injection, extrusion, 3D printing, etc., and mainly used in the manufacturing of optical components.

Transparent engineering plastics mainly include polyolefins, such as cyclic olefin polymers (COC or COP); Polyester, such as PMMA, PET, PBT, PEN, PC, etc; Polysulfones, such as polysulfone (PSF), polyethersulfone (PES), etc; Polyamide (PA), such as transparent nylon; Transparent fluoroplastics, such as poly (vinylidene fluoride hexafluoropropylene) (PVDF HFP) copolymer, and transparent polyimide (PI).

In practical applications, transparent engineering plastics can be used as engineering plastics alone in the manufacturing of optical components, and also as the matrix of transparent composite materials in optical engineering.

In the traditional application fields, transparent engineering plastics can be used as lenses in the manufacturing fields of optical components such as glasses and lenses, as transparent components (lights, portholes, interiors, etc.) in the fields of automobile and aircraft manufacturing, as transparent thermal insulation materials (TIM) in the construction field, and as transparent consumables in the fields of additive manufacturing (3D printing).

In the emerging field, transparent engineering plastics can be applied to the transparent substrate of light-emitting diode (LED) lighting, photocatalytic wastewater degradation devices, and optical components of flexible electronics, flexible solar cells, flexible sensors and other devices. Therefore, the research and development of transparent engineering plastics have received extensive attention in recent years.

Main factors affecting the transparency of engineering plastics

“Transparency” is a characteristic with high value for most engineering plastics, especially optical engineering plastics terminal products. Amorphous engineering plastics often have good optical transparency, while for highly crystalline materials, especially products with high thickness, such as plastic injection parts, crystallization often leads to light refraction, thus degrading the transparency of products.

In order to make crystalline engineering plastics transparent, the commonly used method is to reduce the cell size. Smaller crystals can avoid causing light refraction. In addition, the light transmittance of some semi-crystalline engineering plastics can also be improved through additive technology.

For PET, unless special additives are added to promote crystallization, PET itself is also a slow crystallization material. Amorphous PET is transparent and hard, and will soften (~80 ℃) at the glass transition temperature (Tg).

However, if the material is heated to 120~130 ℃, it tends to become turbid due to the formation of crystals. For example, for polyamide (nylon) materials, amorphous nylon is truly transparent and will not crystallize under normal molding conditions, but semi-crystalline nylon 6 engineering plastics often need fast cooling speed and thin wall design to achieve transparency.

If the thickness of the product exceeds 1~1.5 mm or the mold temperature is high when cooling, these materials will begin to appear turbidity related to crystal formation.

To sum up, for pure engineering plastics, the main factor affecting their optical transparency is the crystalline properties of polymer bulk. For polymer/polymer blends, the phase separation and refractive index difference caused by the polarity mismatch between components are the main reasons that affect their optical transparency.

For polymer/inorganic composite engineering plastics, the light scattering caused by the mismatch of refractive index between polymer matrix and inorganic reinforcing material is the main factor affecting its optical transparency. In short, the transparency of engineering plastics is closely related to the properties of polymer materials and processing conditions.