01 several transparent resins
(1) PMMA
PMMA is an excellent transparent material with a transmittance of up to 93%. Its disadvantages are small refractive index, poor surface hardness, high moisture absorption, poor solvent resistance and birefringence. PMMA cannot be used as high-grade optical materials.
(2) PC
The transparency of PC is excellent, the transmittance can reach 93%, the heat resistance and impact resistance are excellent, the refractive index of PC is 1.586, which is higher than PMMA.
Its disadvantages are: low hardness, slightly larger birefringence and higher than PMMA and glass. PC products can be used for high-grade optical materials after being coated with wear-resistant layer.
(3) TPX
TPX belongs to crystalline polymer, but there is no difference in refractive index between crystalline and non crystalline areas, so it has small birefringence, good transparency, heat resistance, drug resistance and low moisture absorption. The disadvantages are low hardness and large shrinkage.
(4) CR-39
CR-39 is one of the most commonly used high-grade optical materials, and its lenses have accounted for more than half of the total lenses in the United States. CR-39 is a diallyl diglycol carbonate polymer, which is a thermosetting plastic and can be cast. It has high transmittance, good hardness, impact resistance, heat resistance, low birefringence and is suitable for lens production. Its main disadvantages are poor wear resistance and small refractive index.
The refractive index of CR-39 can be increased to 1.546 by copolymerization with high refractive monomers such as diallyl phthalate, and the wear resistance of CR-39 can be improved by coating.
(5) J.D series optical resin
J. D is a derivative copolymer of PES, which is composed of diene polyphenylene ether sulfone, styrene and methyl methacrylate. It is also a thermosetting resin and can be cast. J. The refractive index of D is up to 1.62, the hardness is up to 6h (Rockwell hardness is 332), the cost is only 1 / 2 of PMMA and 1 / 6 of CR-39. D is an optical material that can compete with CR-39.
02 principle of transparent modification of plastics
The transparency of plastics is related to the crystallinity and crystal structure of the products. Therefore, the transparency can be improved by controlling the different crystal morphology of the products.
1. Reduce crystallinity
The higher the crystallinity is, the greater the isotropy is and the lower the transparency is. In the specific molding process, the crystallinity can be reduced by controlling the molding temperature. For example, increasing the processing temperature and reducing the mold temperature can reduce the crystallinity, thus improving the transparency of products.
2. Control crystallization quality
The same resin with the same crystallinity has different crystal quality and transparency. The crystal quality that can affect the transparency mainly includes:
(1) crystal form
Taking PP as an example, it has five different crystal forms: α, β, γ, δ and pseudo hexagons. The increase of β and pseudo hexagons is beneficial to the transparency of products. In the specific molding process, quench can produce more quasi hexagonal crystal forms, adding β crystal nucleating agent can obtain high content of β crystal forms.
(2) Spherulite content
Spherulite has a great influence on the transparency of the products. The higher the content of spherulite, the worse the transparency of the products. The generation of spherulites is related to the cooling temperature, and the probability of spherulites produced by sudden cooling is small.
(3) Crystal size
No matter it is spherulite or general crystal, the smaller the crystal size is, the finer the crystal is, and the better the transparency is. In the specific molding process, the crystal size can be reduced by low temperature processing, sudden cooling, stretching and adding nucleating agent.
(4) Crystal regularity
The more regular the crystal structure is, the better the transparency will be. The more regular the crystal structure is, the better the crystal regularity will be
03 formulation design of transparent modification of plastics
The transparent modification of plastics is a method to improve the transparency of transparent resins by adding small molecules.
1. Add nucleating agent
Adding nucleating agent is the most effective way to increase the transparency of transparent resin. Nucleating agent is a kind of small molecular substance that can promote crystallization. It can play the role of crystal nucleus in resin, make the original homogeneous nucleation become heterogeneous nucleation, increase the number of crystal nucleus in the crystal system, increase the number of microcrystals, reduce the number of spherulites, so that the crystal size becomes smaller and the transparency of products is improved.
There are many kinds of nucleating agents. As transparent modified nucleating agents, transparent nucleating agents are generally selected, mainly α and β nucleating agents, of which β nucleating agent has the best effect.
(1) Alpha nucleating agent
① Sorbitol system: this kind of nucleating agent has good nucleation effect, but it is unstable at high temperature, easy to decompose, and produces odor, embossing and whitening, which is not suitable for food packaging. The main varieties are dibenzylidene sorbitol and so on.
② Phosphate system: the nucleating effect of this kind of nucleating agent is not as good as sorbitol system, but it has good heat resistance, can withstand temperature above 400 ℃, and can be used for food packaging. The main varieties are aromatic phosphoric acid, phosphate ester and phosphoric acid.
③ (substituted) aryl carboxylates: evolved from the early traditional varieties of benzoic acid, sodium benzoate and oxalic acid. The representative variety is p-tert-butylbenzoic acid hydroxy aluminum, the nucleation efficiency is lower than sorbitol and phosphates, but the cost is low.
④ Dehydrogenated rosin acid soaps: a new type of a-nucleating agent for PP resin developed in Japan in 1999. Although its performance is not as good as sorbitol and phosphates, it has a low cost and is mainly used for the modification of PP.
(2) β nucleating agent
The main varieties are: quinoacridone (E3B) such as polyaniline yellow, organic binary acid and IIA metal oxides such as heptanoic acid and calcium stearate (is-cxa, hydroxide and its salt blend series, aromatic amine compounds such as TM series, nu-100, etc.
The resins modified by adding nucleating agent can be crystalline polymers such as PE, PP, PA and pet. The amount of nucleating agent changed parabola with the light transmittance, that is, the light transmittance increased with the amount of nucleating agent added, and decreased with the amount of nucleating agent added after reaching the maximum light transmittance. This is mainly because there are too many nucleating agents, reducing the degree of crystal size can not offset the effect of the increase of crystallinity, but make the light transmittance decrease.
PP is a typical crystalline polymer, which is very sensitive to the addition of nucleating agent and can be widely used for transparent modification. The details are as follows:
The crystallization rate of PP is slow, the crystal nucleus is few, the size of spherulite is large (50-70 μ m), which is larger than the wavelength of visible light 390-760 μ M. the incident light is reflected, which reduces the transparency. To increase the transparency, it is necessary to improve the crystallization quality. The specific methods are as follows:
(1) Add nucleating agent
① Sorbitol: sorbitol is the best transparent nucleating agent for PP, its biggest feature is that it can be dissolved in PP melt to form homogeneous solution, and has a unique network nucleation mechanism. The specific varieties are millad3905, millad3940, millad3988, and the specific performance comparison is shown in table 8-2.
② Organophosphates: not as transparent as sorbitol, but with good heat resistance, decomposition temperature as high as 400 ℃, and small odor, which can be used in food. The disadvantages are poor dispersion and high price, mainly including markna-10, markna-11 and makna-12 of Asahi Chemical Corporation of Japan
③ The amount of all kinds of nucleating agents mentioned above: inorganic, sorbitol and phosphate are less than 0.01%, 0.1% ~ 0.3% and 0.01-3% respectively.
(2) Adding high melting point polymer
PP can be added with resins such as pa.pom, but due to poor compatibility, compatibilizers need to be added. If 2% PA6 and 3% PP-g-MAH were added to PP, the transparency was 90%.
In addition, pet, PA and PLA can also be used to improve their transparency.
2. Add antifogging agent
The addition of antifogging agent can eliminate the condensation phenomenon caused by the temperature difference of transparent materials, so as to improve their transparency. Antifogging agents are mainly some surfactants.
3. Blending other transparent plastics
The transparent resin is mixed with other resins to improve the transparency. The principle is similar to that of nucleating agent. The blends play a role of heterogeneous nucleation, thus reducing the size of crystallization.
The following are some examples of transparent plastic modification formula design:
(1) PP blend PA6 transparent formula
PP: 95%; maleic acid modified
PP:3%;PA6:2%。
The formula has a light transmittance of 90%, haze of 3% and high gloss.
(2) Transparent formulation of PS blend butadiene / styrene copolymer
PS: 50.04%; butadiene / styrene block copolymer: 49.96%.
The light transmittance of the formula can reach 95%.
(3) Transparent PP formula
PP: 99%; antioxidant 215; 0.5%; Di (p-methylbenzylidene) sorbitol: 0.5%.
The light transmittance and haze of the formula can reach 40% and 37%.
(4) LDPE: 100 phr; transparent nucleating agent: 8 phr; EVA: 60 phr.