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, injection, extrusion, 3D printing and other molding processes, and are mainly used in the manufacture of optical components.
Transparent engineering plastics mainly include polyolefins, such as cyclic olefin polymer (COC or COP); Polyester, such as polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycarbonate (PC), etc; Polysulfone, such as polysulfone (PSF), polyether sulfone (PES), etc; Polyamide (PA), such as transparent nylon; Transparent fluoroplastics, such as polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) copolymer, transparent polyimide (PI), etc.
In practical applications, transparent engineering plastics can be used in optical engineering as engineering plastics alone, or as the matrix of transparent composite materials.
In the traditional application field, transparent engineering plastics can be used as lenses in the manufacturing of optical components such as glasses and lenses, as transparent components (lights, portholes, interiors, etc.) in the manufacturing of automobiles and aircraft, as transparent thermal insulation materials (TIM) in the construction field, and as transparent consumables in additive manufacturing (3D printing) and other fields.
In emerging fields, transparent engineering plastics can be used as transparent substrates for LED lighting, photocatalytic wastewater degradation devices, and optical components for flexible electronics, flexible solar cells, flexible sensors and other devices. Therefore, the research and development of transparent engineering plastics has received extensive attention in recent years.
"Transparent" is a high value feature for most engineering plastics, especially optical engineering plastics end products. Amorphous engineering plastics often have good optical transparency. For highly crystalline materials, especially for products with high thickness, such as plastic injection parts, crystallization often leads to the refraction of light, which degrades the transparency of products.
In order to make crystalline engineering plastics transparent, the general method is to make the cell size smaller. Smaller crystals avoid refraction of light. 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 material with slow crystallization. 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 another example, for polyamide (nylon) materials, amorphous nylon is truly transparent and will not crystallize under normal molding conditions. However, to achieve transparency, semi crystalline nylon 6 engineering plastics often need rapid cooling speed and thin wall design.
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.
In a word, for pure engineering plastics, the main factor that affects its optical transparency is the crystallization characteristics 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, light scattering caused by the mismatch of refractive index between polymer matrix and inorganic reinforcement 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 molding conditions.
（1）Properities of polymer materials
As mentioned above, for polymer materials, although there is no absolute correspondence between their crystallinity and optical transparency, generally speaking, transparent polymer materials are amorphous in terms of molecular aggregation characteristics.
The crystalline properties of polymer materials are essentially heterogeneous, and the crystalline region within the structure itself is composed of relatively perfect unit cells, but there are also amorphous regions caused by branching, random conformation and other defects.
The anisotropic composition and structure of the crystalline state result in three kinds of refractive index fluctuations in crystalline polymer materials, namely polymer body/air interface, crystalline/amorphous interface, and crystalline/crystalline interface.
The above refractive index fluctuations often cause significant light scattering on the surface of crystalline polymer materials, thereby affecting the transparency of crystalline polymer materials.
In addition, for polymer materials with aromatic conjugated units, if there are relatively strong intramolecular or intermolecular charge transfer (CT) interactions in their internal structures, it is easy to form "charge transfer complexes (CTC)" between electron donors and electron receptors.
In this process, the visible light will be significantly absorbed due to the charge transition and transfer, so the color and transmittance of polymer materials will be degraded.
（2）molding conditions
As mentioned above, for engineering plastics with low crystallinity or slow crystallization rate, rapid cooling (similar to "quenching" in metal material processing) can produce more and finer defects and more crystalline aggregates in the material, which is conducive to maintaining good transparency of the material.
However, the forming process conditions are limited by the crystallization characteristics of materials and the level of Tg. For example, for high crystalline polymer materials with fast crystallization rate and low Tg, the cooling rate is limited by factors such as heat transfer rate and the release of crystallization heat, so it has little influence on the crystallization characteristics of the final materials. In addition, rapid cooling may cause undesirable stress in engineering plastic products.
In addition, defects such as holes on the surface or inside of engineering plastics during molding, extrusion and injection molding will also affect the optical transparency of materials. Porous defects in polymer materials may become light scattering centers.
Although these holes are small in size and have little impact on the transparency of polymer materials under normal conditions, when crystalline polymer materials are oriented by external tensile force, these holes are often the key factor that causes the material to "whiten" and sharply reduce the transparency.
To sum up, the factors affecting the transparency of engineering plastics come from the dual effects of body materials and processing parameters. To design and develop high-performance transparent engineering plastics, it is necessary to give full play to the favorable factors and minimize the impact of adverse factors.
