Author: Meite Polymer Wu Tianhua
1. Factors affecting the transparency of polymer materials
There are many factors that affect the transparency of polymer materials, such as processing technology, molecular weight size and distribution of polymers, etc., but the most important influencing factors are as follows:
1.1 Refractive index
For large logarithmic polymers, they are incomplete crystallization, that is, there are crystalline regions and amorphous regions inside them, but the refractive index of the two is not the same, and the incident light will be refracted and reflected at the interface of the crystalline region and the amorphous region, so that it cannot pass directly, it is milky white, opaque, such as PE, PA, and conversely, when the refractive index of the crystalline region and the amorphous region are the same, the polymer is transparent, such as poly-4-methyl-1-pentene.
1.2 Crystallinity
The higher the crystallinity of plastic products, the greater the anisotropy of their products and the lower their transparency. So when the crystallinity decreases, the transparency increases, such as those completely amorphous polymers, which are usually transparent, such as PMMA, PS, etc.
1.3 Crystal size
When the crystal size is smaller than the wavelength of visible light, light does not refract and reflect, so even crystallization does not necessarily affect the transparency of polymers. Therefore, whether it is a spherical crystal or a general crystal, the smaller the crystal size, that is, the finer the grain, the more conducive to the improvement of transparency.
1.4 Surface roughness
For transparent plastics such as PE, when the sample is thin, surface roughness becomes the main factor affecting light transmittance. If the surface is rough, the incident light scattering loss is more, and the light transmittance decreases.
Although there are many factors affecting the transparency of polymer materials, the research on the transparent modification of polymer transparent materials has been mainly focused on reducing the crystallinity and crystal size, and has achieved good modification effects, similarly, the transparent modification of PP is mainly from the crystallinity and spherical size of the two aspects of research.
The PP usually used is partially crystallized, and the crystallinity is about 50%-60%, so the aggregate structure of PP also exists in the crystalline region and the amorphous region at the same time. The amorphous zone is conducive to the passage of light, and after the light reaches the crystalline zone, because the crystal size is greater than the wavelength of visible light, the visible light is refracted and reflected, which is not conducive to the passage of light, thereby reducing transparency. Therefore, improving the transparency of PP is basically carried out from two aspects, one is to reduce the crystallinity, that is, to increase the range of amorphous zones; the other is to reduce the crystal size.
At present, the main methods to improve PP transparency include the addition of transparent agents, the synthesis of random copolymers, the synthesis of transparent PP by metallocene catalytic synthesis, the blending of increased permeability PP, and the process control to improve PP transparency.
2. The main methods of PP transparency improvement
2.1 Add clear agent
Ordinary PP usually crystallizes into spheroids with larger crystal size, and since the diameter of the spheroids is greater than the wavelength of visible light, the incident light is scattered, thereby reducing the transmittance. After the addition of transparent agent to PP, when PP melts and crystallizes, the transparent agent plays the role of crystal nuclei, so that the original homogeneous nucleation becomes heterogeneous nucleation, increases the number of crystal nuclei in the crystalline system, increases the number of microcrystals, and reduces the number of spheroids, so that the crystal size becomes finer and the resin transparency is improved.
At present, the addition of nucleating agent to PP is the simplest and most effective method of transparent modification of PP, according to the chemical structure and composition of the nucleating agent, transparent nucleating agent is usually divided into three categories: inorganic nucleating agent, polymer nucleating agent and organic nucleating agent.
Inorganic nucleating agents mainly include talc, kaolin, calcium oxide, etc., although the source is widespread and inexpensive, but due to poor compatibility with resin and poor dispersion, the nucleating agent itself will be turbid and non-homogeneous effects, so the degree of transparency is limited.
Polymer nucleating agents refer to some high melting point macromolecular compounds, mainly composed of polyvinyl cyclosilanes, cellulose aromatic esters, polyurethanes, etc., but their blending with resins is not good, and the use of technology is not mature, and has not yet formed commercial varieties.
Organic nucleating agents mainly include sorbitol, phosphate and rosin nucleating agents, which have a good anti-permeability modification effect. Sorbitol nucleating agents have the aggregation property of self-physical polymerization and can be dissolved in molten PP to form a homogeneous solution. When the polymer is cooled, the transparent agent first forms a fibrous network through self-aggregation, which is not only evenly dispersed, but also has a very large surface area. With further cooling, PP first forms layered crystals under orientation, and then other PP segments are arranged along the fiber axis. Therefore, the nucleation density of PP is increased, so that PP forms uniform and refined spheroids, reduces the scattering and refraction of light, and increases transparency.
For organophosphate nucleating agents, alkylbenzene and PP resin in such compounds have good affinity, and through the action of PP backbone chain and benzene ring, PP forms a stable crystal with a regular spiral structure, which can significantly improve the mechanical properties of the material. For rosin-type nucleating agents, because of the carboxyl group in the molecule, instability is prone to heterogeneous rearrangement or oxidation, which can make PP grains fine, improve the crystallization temperature, shorten the processing cycle, increase transparency, and be non-toxic and odorless.
Recently, sorbitol nucleating agent is the most widely used in PP transparent modification of a class of transparent nucleating agents. The effect of 3,4-dimethylbenzylsorbitol (DMDBS) on the phase morphology, nucleation and optical properties of PP was found to increase with the increase of DMDBS when the DMDBS content was 0.2%-1%, and when the DMDBS content was greater than 1%, the transparency of PP had the opposite result, as shown in Figure 1.
Dehydroabial acid-type nucleating agent can greatly reduce the haze of PP, improve gloss, and improve mechanical properties. DEHYDROAZID: POTASSIUM DEHYDROAZILATE: Sodium dehydrocafide is 1:1:1 (molar ratio) of the cocrystal (1:1K:1Na) with the best modification effect when the dosage is 0. At 3%, the haze of PP decreased by 80% to 7.2%, and the gloss was increased by 35% to 134.1%, while having better mechanical properties. The addition of dehydroabicanate-type nucleating agent can greatly reduce the size of PP spheroids, especially 1:1K:1Na cocrystals, after 0.3% 1:1K:1Na cocrystals are nucleated, PP sphere grain size is less than l μm. The crystallization temperature, melting temperature and crystallinity of dehydroabidotic acid-type nucleating agent modified PP were also improved, and the nucleation modification efficiency of 1:1K:1Na cocrystalline was the highest.
Under the polarized display mirror, PP was observed to add silica sol, the spheroid size was smaller than that of the same content of nano SiO2, and the transparency effect of PP was the best at 0.15% of the silica sol addition, and its spheroid size was significantly smaller than that of the silica sol content of 0.05%. Its spheroids are also smaller in size compared to the optimal content of nano-SiO2. This shows that the haze value of PP is reduced, and the increase in transparency is greatly related to the size of the spheroids. Where the size of the spheroids is small, the crystallinity is high and the transparency is good.
When a variety of transparent agents are added at the same time, the synergistic effect between different transparent agents is shown, which greatly increases the pp crystallization rate, the spherical crystal size becomes smaller, the dispersion is more uniform, and the transparency of PP is significantly improved.
2.2 Synthesis of random copolymers
The crystallinity of polymers is determined by their molecular chain structure, and the more regular the polymer chain structure, the easier it is to crystallize. In the process of synthesizing PP, the addition of ethylene as a second monomer and propylene copolymerization, ethylene in the molecular backbone of the irregular arrangement, destroying the regularity of the PP molecular chain, with the increase of ethylene content, pp crystallinity gradually decreased, the spherical structure is destroyed, and even the formation of fine grains, to a large extent reduces the scattering and reflection of light, so that the transparency of PP products is improved at the same time, impact resistance is also improved.
2.3Mo metal synthetic transparent PP
Transparent PP resins produced from metallocene catalysts with a light transmittance of up to 94% comparable to PS provide new opportunities for PP to replace other materials and open up new areas, and begin to challenge the Ziegler-Natter catalysts that have dominated the market for 30 years.
However, from the current market point of view, the proportion of PP produced with metallocene catalysts is still very small. With the deepening of research and development, the application field of METALlocene catalytic PP products will expand, and the second generation of metallocene catalysts will produce some new homopolymers, impact copolymers and random copolymers, which will accelerate the development of the metallocene PP market.
2.4 Blend increases permeability PP
With the development of modern science and technology, human beings have higher and higher requirements for the properties of polymer materials, and single polymers often cannot meet the requirements of use due to their own performance limitations. In order to obtain polymer materials with excellent comprehensive performance, in addition to continuing to develop and synthesize new types of polymers, the use of existing polymers to prepare new materials by methods such as blending, copolymerization, filling, and reinforcement has also become one of the important methods for obtaining polymer materials with excellent performance. In particular, polymer blending modification technology, which is simple and easy to implement, can adapt to both small production scale and large-scale production, and has become the main method for preparing new performance polymer materials.
The transparency of blended polyphase materials is often affected by blurriness or reduced clarity, which is mainly due to differences in refractive indexes and surface roughness between polymers. The common indicators for evaluating the transparency of materials are haze and light transmittance, and generally the higher the transmittance, the lower the haze. For polymer blends, the main influencing factors of haze and transmittance are the particle size and relative refractive index of the dispersed phase, and the similar refractive index and the reduction of the particle size of the dispersed phase are conducive to reducing scattering, reducing haze and improving light transmittance.
In many applications, however, surface phase roughness is the primary source of scattering that limits transparency. It is found that the relative molecular mass distribution, molecular chain microstructure, melt elasticity and processing and molding all have an impact on roughness, and the surface roughness and light transmittance that determine the haze are related to the crystallization of the polymer surface.
The optical transparency of the blend is not a simple average of the transparency of each component, for example, when the unformed transparent PET and transparent PS blend, due to the different refractive index between the two phases, the resulting film is highly hazed and opaque, while on the other hand, the SBR/PS blend leads to the occurrence of phase separation due to poor compatibility, so that the dispersion phase particle size increases, close to the wavelength of visible light, and serious light scattering occurs, so it is also opaque. Therefore, in order to improve the transparency of the blending system, there are usually two alternative pathways, one is to make the composition of the mixture have a similar refractive index between them, and the other is to make the particle size of the dispersed particles smaller than the wavelength of visible light.
In extrusion blending engineering, particle size is a function of shear rate, interfacial tension, matrix viscosity, dispersion phase viscosity, and rubber content. In blending engineering, a change in shear rate can change the particle size, but this change will be lost during subsequent injection molding processing. Only by stabilizing the structure against condensation can the above methods be used to obtain stable and reasonable particle sizes, such as the dynamic crosslinking of rubber in the interface grafting reaction and blending engineering.
Although reducing the size of the fendang particles so that they are smaller than the visible wavelength can improve the transparency of the blend, the dispersion phase particles are too small and tend to reduce the toughness. The best way is to select components with similar refractive indexes, if the refractive index of the two components is equal, then regardless of the morphological structure, the blend is always transparent, such as methyl methacrylate, butadiene, styrene terpolymer (MBS) type resin.
In order to improve the impact resistance and transparency of PP, it is of practical significance to blend PP, LDPE and EDPM. When LDPE is blended with PP, pp crystallization can be prevented and the PP crystallization rate is greatly reduced, but in the PP crystallization process, when the addition of LDPE is at least 10%, the size of PP spherical crystals decreases. This is because LDPE is partially compatible with PP, while a small amount of LDPE is fully compatible when blended with PP, and ethylene propylene rubber at the phase interface, through the co-crystallization of the combination with the PP matrix, can improve the compatibility of PP and LDPE, refine the spheroid radius.
The study found that a small amount of LDPE and rosin-type nucleating agent was added to PP at the same time, compared with the addition of rosin-type nucleating agent alone, the crystallization rate of PP was greatly improved, and the generated spherical crystals not only had a smaller radius, but also the semi-warp distribution was more uniform, and the transparency of PP was improved as shown in Figure 2.
Blending with nylon 6 and PP can improve the transparency of PP. Because PA6 is less compatible with PP, maleic anhydride grafted PP (MAPP) is used as a compatibilizer, and MAPP is easy to react with unspent amino acids of polyamide. In the process of polymer cooling, PA6 crystallizes first, providing a heterogeneous crystal nucleus for the crystallization of PP, that is, PP attaches to the PA6 grain to crystallize out-of-phase. Compared with homogeneous crystallization, the nucleation density at this time is greatly improved, the crystallization speed is accelerated, and the spherical ruler becomes smaller.
In general, the product obtained by blending propylene copolymer with high molecular weight rubber is opaque, but the product of mixing with low molecular weight rubber is low in impact strength, but the transparency of the material is very high.
However, improving transparency through blending also has significant limitations. Because it requires not only two or more substrate materials to have good compatibility, but also requires their refractive index to be similar, otherwise it is difficult to achieve transparent modification, so the method has developed slowly, and few people are currently conducting research.
2.5 Process control improves PP transparency
Process control, like the addition of clear agents, is an effective way to improve pp transparency. Among them, the process conditions that have the greatest impact on transparency are mainly processing temperature and cooling temperature.
(1) Processing temperature
The lower the molding processing temperature of the plastic, the more original crystal nuclei remain in the melt, which plays the role of adding a nucleating agent to make the crystal size smaller, thereby improving transparency.
(2) Cooling temperature
The lower the cooling temperature, the faster the cooling, the melt quickly passes through the crystallization zone, the lower the crystallinity, the more conducive to the improvement of transparency. At the same time, the smaller the size of the spherical crystal, which is conducive to the improvement of transparency, especially for PP, the lower the cooling temperature, the greater the content of the quasi-hexagonal crystal form in PP, and the higher the light transmittance.
(3) Control the molding orientation
In general, the orientation in molding will increase birefringence, but when the sorbitol hyaluron modifies PP, the molecular orientation that occurs in the transparent PP cortex obtained by injection is conducive to reducing light scattering and improving light transmittance. Therefore, the orientation must be reasonably controlled so that birefringence and light scattering are minimized.
In the injection molding process, the temperature of the mold, the melt temperature, and the increase in injection pressure will lead to a decrease in the optical performance of the product; the packing time is at a suitable size, the optical performance is optimal, and the cooling time is too short or too long, which will cause the optical performance of the product to decline.
In summary, because the anti-reflection process of adding transparent agents is simple, and the effect is obvious, so for a long time in industrial production, the method of adding transparent modifiers is mainly used to improve the transparency of PP, and the transparent modifiers with better effects are mainly organophosphates and sorbitol antidiaphers, but the aldehydes released by the decomposition of sorbitol at the processing temperature have odor and toxicity, and the cost of organophosphate anti-permeable agents is too high, which is 2 to 3 times that of sorbitol, and the compatibility with resin is limited, and the dispersion is poor. Under normal conditions, it is not easy to mix, and it is easy to cause defects on the surface of the product, so the use of this method is greatly limited.
At present, the latest technology for the international research and production of transparent PP is the transparent PP obtained by direct synthesis using The Mawkin Tree catalyst. The PP obtained by this method not only has excellent transparency and good comprehensive mechanical properties, but due to the technical difficulty, it is only owned by a small number of large foreign enterprises, and the cost is too high, so it is difficult to popularize.
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