Structure, Properties, Classification, Modification, Application and Molding Of PMMA

I. Structure and Properties of PMMA

1. Molecular structure and properties

Polymethyl methacrylate, English name polymethyl methacrylate, PMMA, commonly known as plexiglass, acrylic, by MMA monomer and a small amount of acrylate copolymerization of non-crystalline plastics, with good transparency, optical properties, weathering, chemical resistance, impact resistance and aesthetics and other characteristics, is known as the [Queen of Plastics" advanced materials, products include molded plastic, extruded sheet and cast sheet. It is an advanced material known as the [Queen of Plastics", and its products include molded plastics, extruded boards and casting boards. Its density is 1.18-1.19g/cm3, refractive index is small, about 1.49, light transmittance of 92%, haze is not more than 2%, is a high-quality organic transparent materials.

PMMA structural formula is as follows:

Where n = 10 000~12 000.

According to the different polymerization mechanism, the molecular structure of poly(methacrylic acid methyl vinegar) has three different configurations, i.e. the U-configuration, the full homo-congruent configuration, and the inter-congruent configuration, which is generally the random configuration, but there exists the short-range order arrangement that is isolated from each other (the regularity of the configuration).

2. Aggregate Structure and Characteristics

Due to the large volume of COOCH3 side group functional groups in the molecular chain of poly(methyl methacrylate), it is difficult for the molecular chain to be arranged into a crystalline structure, and it is generally in an amorphous form, but there are some crystalline configurations in the tensile oriented products.

PMMA performance characteristics:

1) Physical properties

PMMA has excellent optical properties, light transmittance of up to 92%, 10% higher than inorganic glass; colorless, almost does not absorb visible light, can transmit 270nm of ultraviolet light, coloring is good, and in the role of the heat almost does not change color, fading, refractive index of 1.49, the surface reflectance is not greater than 4%, high surface gloss; PMMA relative density is small, 1.17-1.20, only half of the inorganic glass. The relative density of PMMA is small, 1.17-1.20, only half of inorganic glass.

2) Mechanical properties

PMMA has high mechanical strength at room temperature, and is little affected by temperature, but when close to the softening point and glass transition temperature, the strength decreases sharply; PMMA surface hardness and scratch resistance is poor, and the impact toughness is low, often need to use rubber modification; high water absorption, large dimensional shrinkage.

3) Thermal properties

PMMA has a large coefficient of thermal expansion, and the dimensions change greatly due to temperature.

4) Electrical Properties

PMMA in a very high frequency range, its power factor decreases with the increase of frequency, suitable for long-term outdoor electrical appliances; has good arc resistance and resistance to leakage, surface resistance, high electrical insulation.

5) Chemical resistance

PMMA is resistant to strong acids, alkalis, inorganic salts, oils and fats, aliphatic hydrocarbons and so on.

6) Aging resistance

PMMA has excellent weathering resistance, long-term exposure in the outdoor, its transparency and gloss changes are very small.

II. Classification of PMMA

1. Classification method

According to its synthesis method, poly(methyl methacrylate) can be classified into four kinds, namely, body casting method, suspension polymerization method, solution polymerization method and emulsion polymerization method, among which, the most important ones in the industry are the body casting method and suspension polymerization method, and the ones used as the plastic materials are mainly prepared by these two methods.

1) Body polymerization method

Ontology polymerization method is in the MMA monomer add plasticizer, mold release agent, color and a small amount of initiator and other additives, stirring temperature to the appropriate temperature for pre-polymerization, to be the viscosity of the material up to about 2Pa-s, after cooling, filtration, metering, and then the slurry mold, exhaust, sealing, and in the drying room or the water tank in the section of the temperature-controlled polymerization, demolding in the higher than the glass temperature of the condition of the heat treatment, that is, the desired PMMA casting profile.

2) Suspension polymerization method

Suspension polymerization method is to use water as the medium, under mechanical stirring, with the help of polyvinyl alcohol, sodium polymethacrylate and other dispersants, the MMA dispersed in water in the form of small balls, adding magnesium sulfate and other stabilizers (to prevent the polymerization of the beads sticking together), and the use of oily initiator (eg, BPO) to carry out the polymerization of the segmental heating, and then filtered, washed, dried, that is, to obtain molded plastic powder.

2. Characteristics and selection of various types

Body polymerization method is usually used to manufacture PMMA plates, rods, tubes and other casting products, high molecular weight, high mechanical and thermal properties of the product, but poor fluidity, can not be extruded, injected and other ways of forming; suspension polymerization method to get the lower molecular weight of the PMMA molding powder, can be extruded, injected and other methods to make a number of precision products.

III. PMMA modification and application

1. PMM modification methods

(1) polymerization process modification

Polymerization process modification of a kind refers to the PMMA casting process through the stretching direction to obtain directional plexiglass. The method is the PMMA casting plate is heated to 105- 110 ℃, in the specialized stretching equipment for two-way stretching, to reach 50-70% of the degree of stretching after stopping the stretching, cooling and shaping.

Another polymerization modification method is to use the α-position methyl substituent of methyl acrylate as the monomer for polymerization, commonly used α-position substituent for halogen such as Cl, F, Br or cyano(-- CN) polymerization to obtain poly α-halogenated acrylic acid and poly α-cyano acrylic acid methyl ester. α-Halogenated acrylic acid methyl ester can be prepared through the body and solution polymerization, the polymerization of the product of high transmittance, density (1.40), 1.49 g/cm3, 1.40 g/cm3, 1.49 g/cm3, 1.40 g/cm3), 1.49 g/cm3, 1.49 g/cm3. 1.49 g/cm3), surface hardness, scratch resistance, mechanical strength are higher than PMMA, at the same time improve the heat resistance and corrosion resistance to organic solvents, the disadvantage is poor weather resistance. Poly α-cyanoacrylic acid methyl ester can be used free radical or anionic triggered by the body or emulsion polymerization, the product heat resistance than PMMA has improved significantly, the heat distortion temperature of 157 ℃ (PMMA for 60 ~ 102 ℃), Vicat softening point of 168 ℃: (PMMA for 113 ℃), processing temperature is high, easy to decompose in the molding process, so it is not suitable for processing according to the molding method of thermoplastics in general, often using casting. Molding is often used casting.

(2) Blending modification

In the polymethyl methacrylate prepolymer add fish scale powder or alkali lead carbonate, after stirring uniformly poured into the mold vibration polymerization, get pearlescent plexiglass; PMMA polymer can also be blended with PC, because of the two components of the resin with each other to present layers to get pearlescent color.

Pearlescent plexiglass general performance with ordinary plexiglass is basically the same, but more bright and Colorful pearlescent color.

(3) Co-polymerization modification

1) Copolymer of methyl methacrylate and methyl acrylate

Good fluidity, good film-forming properties, but the mechanical strength and performance is not as good as PMMA.

2) Copolymer of methyl methacrylate and ethyl acrylate

Its softening temperature is 180℃, can be used for a long time at 70-80℃, and has tensile, adhesive, flexible and elasticity at room temperature, can be injection molding, cast film.

3) Copolymer of methyl methacrylate and butyl acrylate

The copolymer has good water resistance, dielectric properties and melt flow.

4) Copolymer of Methyl Methacrylate and Methacrylic Acid

The heat distortion temperature is higher than PMMA, up to 120-140℃, and other mechanical properties are good.

5) Methyl methacrylate and dimethacrylic acid - glycol ester crosslinked or with dimethacrylic acid glycol ester copolymer

Its casting sheet's silver resistance and tensile strength are higher than PMMA, but its molding temperature and tensile orientation temperature are also increased, and its water absorption rate is increased.

6) Methyl methacrylate and α-methyl styrene copolymer (MMA/α-MS) copolymer

The refractive index is greater than PMMA, and the operating temperature is 11-22°C higher than that of heat-resistant PMMA. It is less fluid, but can be injection molded, extruded, and thermoformed.

7) Methyl Methacrylate and Butadiene Copolymer

The copolymer is translucent, low water absorption, high surface hardness, and 5 times tougher than PMMA in terms of impact resistance.

8) Methyl Methacrylate and Styrene Copolymer (MMA/S)

Its heat deflection temperature is 10-20°C higher than that of heat-resistant PMMA, and it is a heat-resistant transparent material with strong coloring power and good melt fluidity.

2. Application of PMMA

Directional plexiglass is widely used in airplanes, automobiles, ships, windshields, window glass, composite safety glass and other protective, explosion-proof, screen protection, bulletproof and other occasions; poly α - halogenated methyl acrylate and poly α anoxy acrylic acid methyl ester is mainly used in supersonic aircraft window glass or general plexiglass heat resistance and mechanical strength can not meet the requirements of the occasion.

Pearlescent plexiglass is a kind of high-level decorative materials, can be used for machinery, instrumentation, construction, light industry and other aspects of the decorative materials of literature, education and publicity, etc., PMMA/PC blended pearlescent plastics do not contain toxic copper inorganic substances, suitable for food and cosmetic containers, and automotive interior decoration, household appliances, decoration, etc..

(1) Methyl methacrylate and methyl acrylate copolymers are mainly used as [organic films" for electron beam tubes in television sets;

(2) The copolymer of methyl methacrylate and ethyl acrylate is used as an electrical insulating material;

(3) the body polymerization of methyl methacrylate and butyl acrylate copolymer for the manufacture of safety glass interlayer, suspension polymerization of copolymers can be used as tooth powder (a self-coagulating material), dentures, false eyeballs, but also used as a protection of the motor coil windings, transformer structure materials;

(4) methyl methacrylate and methacrylic acid copolymer used for high-speed flight transparent materials and heat-resistant plexiglass;

(5) methyl methacrylate and dimethacrylic acid - ethylene glycol ester crosslinked or with dimethacrylic acid glycol ester copolymer is also mainly used for high-speed flight transparent materials and heat-resistant plexiglass;

(6) Methacrylic acid methyl sound and α-methyl styrene copolymer (MMA / α-MS) used for a certain degree of transparency, heat resistance and strength of the parts, such as transparent pipes, speculums, oil cups, optical lenses, equipment signage, automotive lamps, instrumentation parts and so on;

(7) Methyl methacrylate and butadiene copolymer is used for high-impact plexiglass products, such as motor cover for motorboat outfitting, machine casing, frames, containers and so on;

(8) methyl methacrylate and styrene copolymer (MMA/S) used for pens, clocks and watches, automobiles, aircraft, ships, instrumentation parts, medicine, education, daily necessities, optical lenses, gas masks, mirrors, lamps, lights, billboards, windshields and decorative parts.

Fourth, PMMA molding process

1. Process conditions of various processing methods

l) The body polymerization of PMMA slurry, can be made by casting method of plates, rods, tubes and other profiles. 2) suspension polymerization to get powder, granules, suitable for general injection, molding, extrusion, hollow forming and other plastic processing methods.

2) Suspension polymerization to get powder, granules, suitable for general injection molding, molding, extrusion, hollow forming and other plastic processing methods. PMMA melt viscosity is more sensitive to temperature than most thermoplastics, the processing temperature must be strictly controlled, and at the same time, due to its oxidation in the processing temperature close to the viscosity is high, in order to improve its mobility, can be modified.

3) PMMA profiles can be mechanically processed, such as sawing, milling, drilling and dumpling holes, etc.; and can be bonded with solvent-based adhesives, but also with acrylate adhesives and fabric-reinforced acrylate plastics, polyester tape bonding; transparent products made of PMMA can be used inlaid connection, edge flexible connection and rigid connection method assembled on the metal skeleton.

2. Pretreatment and post-treatment process

PMMA's moisture absorption rate of 0.3%, in the processing of molding heat will lead to blistering products, so in the heat before processing need to pay attention to drying, especially in the use of recycled materials need to pay more attention. As polymethyl methacrylate processing process is prone to internal stress, so the molding of the product should be appropriate treatment to eliminate internal stress. Eliminate internal stress is an effective method of annealing, annealing time and annealing temperature can be determined according to the thickness and structure of the product. Usually the thicker the part, the higher the required annealing temperature, the longer the annealing time. The annealing process should not affect the structure of the part. Generally, annealing at 120°C for 2 h or 90°C for 6 h is sufficient.

3. Technical safety

The body of cast Plexiglas is non-toxic, in sawing, drilling or molding temperature may release a small amount of methyl methacrylate gas, in the processing room should be equipped with an air exchange device.