Plastic is an important organic synthetic polymer material and is widely used. However, the "white pollution" caused by waste plastics is becoming more and more serious. If we can understand the composition and classification of plastics in detail, it will not only help us scientifically use plastic products, but also facilitate the classification and recycling of plastics, and effectively control and reduce "White pollution". There are many kinds of plastics, and the structure and composition of plastic molecules are complicated. If you accidentally classify errors, how can we identify the types of plastics with simple methods? First, we must master the knowledge of identifying different plastics. The following describes the simple identification method of plastics:
1. Plastic appearance identification
By observing the appearance of plastics, it is possible to initially identify a large category of plastic products: thermoplastics, thermosets or elastomers. Generally, thermoplastics have both crystalline and amorphous types.
Crystalline plastic | The appearance is translucent, opaque or opaque, only transparent in the state of the film, hardness from soft to horny; |
Amorphous | Generally colorless, completely transparent when no additives are added, hardness is harder than horny rubber (additives such as plasticizers are often added at this time); |
Thermoset plastic | Usually contains filler and is not transparent, such as transparent when it is free of filler; |
Elastomer | Rubbery feel with a certain stretch ratio |
Visual identification
It refers to the use of human perception to experience some of the intuitive features of plastic. Look at the appearance with eyes: transparent? Translucent? Opaque? How is the color (when not dyed)? Put it in the water, float? Sink? Use the nose to smell: Is there any smell? What smell? Hand touch: Smooth or rough? Feeling cold Still hot? Use your fingernail to scratch it. Is there any trace? Is it hard or soft to stretch it by hand? Is it toughness and elasticity? Drop the plastic, listen to its sound, loud? Crisp? Or low? Easy Broken? or tough? Through these sensory checks, you can identify which kind of plastic.
PE
The raw material of polyethylene LDPE is white wax, transparent; HDPE is white powder or white translucent granular resin. Floating in the water, odorless and tasteless, with a waxy smooth feel, traces after scratching, the film is soft and stretchable. LDPE is soft, extensible, bendable, but easy to break; MDPE, HDPE are harder, have better rigidity and toughness, and have a lower sound.
PP
The polypropylene raw material is white waxy and translucent, floating in water, odorless and tasteless, smooth in hand, no trace after scratching, bendable, not easy to break, good tensile strength and rigidity, and bright sound.
PS
Polystyrene standard glass-like transparency; impact-resistant dull, sinking in water, odorless and tasteless, smooth hand, brittle, easy to break with nails and metal sound, commonly known as "sounding gum".
ABS
Milky white or beige, amorphous, opaque, dull, sinking in water, odorless, tasteless, tough, hard, and rigid. Not easy to break, the sound is crisp.
PVC
Polyvinyl chloride products vary depending on the plasticization and filling conditions, and some are opaque. It sinks in water, and it is soft and can be bent when heated to 50 °C. Soft products will sag, some have elasticity, hard products such as doors and windows, sewer pipes, etc.
PA
The polyamide (nylon) raw material is milky white, such as colloid. When heated to above 250 ° C, it becomes watery. Sinking in the water, odorless and odorless surface, hard and hot, will not break when gently hammered, the sound is low.
PMMA
Polymethyl methacrylate (plexiglass), glass-like transparent, beautiful appearance. It sinks in the water, is odorless and tasteless. It can be bent freely when heated to 120 °C. It can be processed by hand, hard and not easy to break.
PTEE
White waxy, low transparency, smooth, non-combustible, non-absorbent, and weather resistant. Sinking in the water, odorless and tasteless,
Lubrication, low sound.
PU
There are five kinds of forms, such as foam, elastomer, paint, synthetic leather and adhesive, which have different forms, sinking in the water and floating in some. It is odorless and tasteless, varies with the shape, and has a low sound.
PC
The polycarbonate raw material is a white crystalline powder, light yellow to amber, transparent solid, and the product is nearly colorless. It is a high-grade insulating material, odorless, tasteless, metallic, hard, strong in bending, impact resistant, tough, and loud.
2. Heating identification
The heating characteristics of the above three types of plastics are also different, and can be identified by heating.
Thermoplastic | Softens when heated, easy to melt, and becomes transparent when molten, often pulls out the filament from the melt, usually easy to heat |
Thermoset plastic | Before heating to the chemical decomposition of the material, keep its original hardness not softened, the size is stable, and the carbonization to decomposition temperature |
Elastomer | When heating, no flow occurs until the chemical decomposition temperature, and the decomposition temperature is decomposed and carbonized. |
The softening or melting temperature range of commonly used thermoplastics is shown in the table.
Plastic variety | Softening or melting range / °c | Plastic variety | Softening or melting range /oc |
Polyvinyl acetate | 35~ 85 | Polyoxymethylene | 165~185 |
Polystyrene | 70~115 | Polypropylene | 160~170 |
Polyvinyl chloride | 75~90 | Nylon 12 | 170~180 |
Polyethylene | |||
Density 0.92/ cm3 | 110 | Nylon 11 | 180~190 |
Density 0.94/ cm3 | About 120 | Polychlorotrifluoroethylene | 200~220 |
Density 0.96/ cm3 | About 130 | Nylon 610 | 210~ 220 |
Poly-1-butene | 125~ 135 | Nylon 6 | 215~225 |
Polyvinylidene chloride | 115~140 (softening) | Polycarbonate | 220~ 230 |
Plexiglass | 126~ 160 | Poly-4-methylpentene-1 | 240 |
Cellulose acetate | 125~175 | Nylon 66 | 250~260 |
Polyacrylonitrile | 130~ 150 (softening) | Polyethylene terephthalate | 250~260 |
3. Solvent treatment identification
Thermoplastic | Swelling occurs in the solvent, but is generally insoluble in the cold solvent. In the hot solvent, some thermoplastics will dissolve, such as polyethylene dissolved in xylene; |
Thermoset plastic | Insoluble in the solvent, generally does not swell or only slightly swell; |
Elastomer | Not soluble in solvents, but usually swells. |
Solubility of commonly used plastics
Plastic solubility | ||
polymer | Solvent | Non-solvent |
Polyethylene | Para-xylene 1, trichlorobenzene 1 | Acetone, ether |
Poly-1-butene | Decane 1, decalin 1 | Lower alcohol |
Isotactic polypropylene | ||
Atactic polypropylene | Hydrocarbons, isoamyl acetate | Ethyl acetate, propanol |
Polyisobutylene | Hexane, benzene, carbon tetraoxide, tetrahydrofuran | Acetone, methanol, methyl acetate |
Polybutadiene | Aliphatic and aromatic hydrocarbons | |
Polyisoprene | ||
Polystyrene | Benzene, toluene, chloroform, cyclohexanone, butyl acetate, carbon disulfide | Lower alcohol, ether (swelling) |
Polyvinyl chloride | Tetrahydrofuran, cyclohexanone, ketone, dimethylformamide | Methanol, acetone, heptane |
Polyvinyl fluoride | Cyclohexanone, dimethylaminocarboxamide | Aliphatic hydrocarbons, methanol |
PTFE | Insoluble | |
Polyvinyl epoxide | Benzene, chloroform, methanol, acetone, butyl acetate | Ether petroleum ether, butanol |
Polyethylene isobutyl ether | Isopropanol, methyl ketene, chloroform, aromatic hydrocarbons | Methanol, acetone |
Polyacrylate and polymethacrylate | Trichlorotetramine, acetone, ethyl acetate, tetrahydrofuran, toluene | Methanol, ether, petroleum ether |
Polyacrylonitrile | Dimethylaminocarboxamide, dimethyl sulfoxide, concentrated sulfuric acid water | Alcohols, ether, water, hydrocarbons, |
Polyacrylamide | water | Methanol, acetone |
Polyacrylic acid | Water, dilute base, methanol, dioxane, dimethylaminocarboxamide | Hydrocarbons, methanol, acetone, ether |
Polyvinyl alcohol | Water, dimethylformamide 1, dimethyl sulfoxide | Hydrocarbons, methanol, acetone, ether |
Cellulose | Aqueous copper ammonium hydroxide, aqueous zinc chloride, aqueous calcium thiocyanate | Methanol, acetone |
Cellulose triacetate | Acetone, chloroform, dioxane | Methanol, ether |
Methyl cellulose (trimethyl) | Trichloromethane, benzene | Ethanol, ether, petroleum ether |
Carboxymethyl cellulose | water | Methanol |
Aliphatic polyester | Trichloromethane, formic acid, benzene | Methanol, ether, aliphatic hydrocarbons |
Ethylene terephthalate | M-cresol, o-chlorophenol, nitrobenzene, trichloroacetic acid | Methanol, acetone, aliphatic hydrocarbons |
Polyamide | Formic acid, concentrated sulfuric acid, dimethylaminoformamide, m-cresol | Methanol, ether, hydrocarbons |
Polyurethanes (not crosslinked) | Formic acid, γ-butyrolactone, dimethylaminoformamide, m-cresol | Methanol, ether, hydrocarbons |
Polyoxymethylene | Γ-butyrolactone 1, dimethylformamide 1, benzyl alcohol 1 | Methanol, ether, aliphatic hydrocarbons |
Polyethylene oxide | Water, benzene, dimethylformamide | Aliphatic hydrocarbons, ether |
Polydimethylsilyl oxide | Trichloromethane, heptane, benzene, ether | Methanol, ethanol |
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4. Density identification
Different types of plastics have different densities. The method of measuring density can be used to identify plastics, but in this case, the foamed products should be separated because the density of the foam is not the true density of the materials. In the actual industry, there are also different plastics used to sort plastics. The density of commonly used plastics is shown in the following table:
Density / (g / cm3) | material | Density / (g / cm3) | material |
0.80 | Silicone rubber cavity (filled with silica to 1.25) | 1.19~1.35 | Plasticized polyvinyl chloride (approximately 40% plasticizer) |
0.83 | Polymethylpentene | 1.20~1.22 | Polycarbonate (bisphenol A type) |
0.85~0.91 | Polypropylene | 1.20~1.26 | Crosslinked polyurethane |
0.89~0.93 | High pressure (low density) polyethylene | 1.26~1.28 | Phenol formaldehyde resin (unfilled) |
0.91~0.92 | 1-polybutene | 1.26~1.31 | Polyvinyl alcohol |
0.9~0.93 | Polyisobutylene | 1.25~1.35 | Cellulose acetate |
0.92~1.00 | natural rubber | 1.30~1.41 | Phenol formaldehyde resin (filling organic materials: paper, fabric) |
0.92~0.98 | Low pressure (high density) polyethylene | 1.30~1.40 | Polyvinyl fluoride |
1.01~1.04 | Nylon 12 | 1.34~1.40 | celluloid |
1.03~1.05 | Nylon 11 | 1.38~1.41 | Polyethylene terephthalate |
1.04~1.06 | Acrylonitrile-butadiene-styrene copolymer (ABS) | 1.38~1.50 | Hard PVC |
1.04~1.08 | Polystyrene | 1.41~1.43 | Polyoxymethylene (polyoxymethylene) |
1.05~1.07 | Polyphenylene ether | 1.47~1.52 | Urea-melamine resin (with organic filler) |
1.06~1.10 | Styrene-acrylonitrile copolymer | 1.47~1.55 | Chlorinated polyvinyl chloride |
1.07~1.09 | Nylon 610 | 1.50~2.00 | Phenolic plastics and aminoplasts (with inorganic fillers) |
1.12~1.15 | Nylon 6 | 1.70~1.80 | Polyvinylidene fluoride |
1.13~1.16 | Nylon 66 | 1.80~2.30 | Polyester and epoxy resin (with glass fiber) |
1.10~1.40 | Epoxy resin, unsaturated polyester resin | 1.86~1.88 | Polyvinylidene chloride |
1.14~1.17 | Polyacrylonitrile | 2.10~2.20 | Polytrifluoro-vinyl chloride |
1.15~1.25 | Cellulose acetylbutyrate | 2.10~2.30 | PTFE |
1.161.20 | Polymethylmethacrylate | ||
1.17~1.20 | Polyvinyl acetate | ||
1.18~1.24 | Cellulose propionate |
a solution commonly used for density identification of plastics
Type of solution | Density (25oc) / (g / cm3) | Preparation method | Plastic (product) type | |
Floating in solution | Submerged solution | |||
water | 1 | Polyethylene, polypropylene | Polyvinyl chloride, polystyrene | |
Saturated salt solution | 1.19 | 74ml water and 26g salt | Polystyrene, ABS | Polyvinyl chloride |
58-4% alcohol solution | 0.91 | 100ml water and 140ml 95% alcohol | Polypropylene | Polyethylene |
55-4 alcohol solution | 0.925 | 100ml water and 124ml 95% alcohol | High pressure polyethylene | Low pressure polyethylene |
Calcium chloride aqueous solution | 1.27 | 100g calcium chloride (industrial) and 150ml water | Polystyrene, plexiglass, ABS polyethylene | Polyvinyl chloride, phenolic plastic |
5. Pyrolysis test
Pyrolysis test identification is a method in which the plastic is heated to a pyrolysis temperature in a pyrolysis tube and then the pH value of the evolved gas is tested using litmus paper or pH test paper.
Commonly used plastic pyrolysis products litmus and pH test paper test results | ||
Litmus paper | ||
red | Basically no discoloration | blue |
pH test paper | ||
0.5~4.0 | 5.0~5.5 | 8.0~9.5 |
Halogen-containing polymer polyvinyl ester cellulose ester polyethylene terephthalate phenolic resin polyurethane elastomer unsaturated polyester resin fluoropolymer hard fiber board polysulfide | Polyolefin polyvinyl alcohol polyvinyl formal polyvinyl ether styrene polymer (including: styrene-acrylonitrile copolymer) 2 polymethacrylate polyoxymethylene polycarbonate linear polyurethane phenolic resin epoxy resin cross-linking Polyurethane | Polyamide ABS polymer polyacrylonitrile phenol and cresol resin amino resin (aniline-melamine- and urea-formaldehyde resin) |
1 Slowly heat the pyrolysis tube 2 Some samples show weak alkalinity | ||
6. Combustion test identification
The combustion test identification method is to use a small fire to burn a plastic sample, observe the flammability of the plastic in the fire and outside, and pay attention to the method of identifying the plastic type by the drop form and odor of the molten plastic after the flameout.
Combustion performance | Flame state | Gas odor | material |
Not burning --- | —————— | Irritant (hydrofluoric acid, HF) | Polysilicone polytetrafluoroethylene, polychlorotrifluoroethylene polyimide |
Flame retardant, extinguished after leaving the flame | Bright, with black and yellow, the edge of the flame is green, shiny, black smoke yellow, gray smoke, orange, blue smoke | Phenol, formaldehyde ammonia, amine, formaldehyde hydrochloric acid --- charred animal keratin | Phenolic resin amino resin chlorinated rubber, polyvinyl chloride, polyvinylidene chloride (no flammable plasticizer) polycarbonate silicone rubber polyamide |
Burning in flames, leaving the flames to slowly extinguish or still burn | Yellow, shiny, material decomposition, orange, orange, black smoke, yellow, blue and yellow edges, blue in the center | Phenol, charred paper is irritating, damages tracheal charred rubber, fresh aromatic irritating (isocyanate) paraffin | Phenolic resin polyvinyl alcohol polychloroprene polyethylene terephthalate polyurethane polyethylene, polypropylene |
Easy to ignite, continue to burn after leaving the flame | Shining, black smoke yellow, black smoke dark yellow, a little black smoke dark yellow, black smoke shining, the center is blue and sparks | Has a strong pungent phenolic aroma, and then tastes acetic acid burnt rubber aroma, fruity formaldehyde | Polyester resin (glass fiber reinforced) epoxy resin (glass fiber reinforced) polystyrene polyvinyl acetate rubber polymethyl methacrylate polyoxymethylene |
Easy to ignite, continue to burn after leaving the flame | Deep yellow faint sparks light green, let out sparks orange bright and strong | Acetic acid and butyric acid acetic acid charred paper nitrogen oxides | Butyl acetate cellulose cellulose cellulose nitrocellulose |
Combustion characteristics of commonly used plastics
name | English | Combustion situation | Burning flame state | After the fire | odor |
Polypropylene | PP | easily | Melt dripping, yellow and blue | Smoke continues to burn | Paraffin wax |
Polyethylene | PE | easily | Melt dripping, yellow and blue | Continue to burn | Paraffin burning smell |
Polyvinyl chloride | PVC | Hard to soften | There is smoke on the yellow | Extinguished from the fire | Irritating sour |
POM | POM | Easy to melt and drip | Yellow, blue, smokeless | Continue to burn | Strongly stimulating formaldehyde |
Polystyrene | PS | easily | Soften foaming orange yellow, thick black smoke, charcoal | Continue to burn the surface of oily light | Special vinyl smell |
nylon | PA | slow | Melt dripping | Foaming slowly extinguishes | Special wool, nail odor |
Polymethylmethacrylate | PMMA | easily | Melted foaming, light blue, white, smokeless | Continue to burn | Strong fruity smell, rotten vegetable flavor |
Polycarbonate | PC | Easy, softening and foaming | Have a small amount of black smoke | Extinguished from the fire | No special taste |
PTFE | PTFE | Not burning | —— | —— | A pungent smell of hydrogen fluoride is broken down in the fire |
Polyethylene terephthalate | PET | Easy to soften and foam | Orange, with a small amount of black smoke | Slowly extinguished from the fire | Sour |
Acrylonitrile-butadiene-styrene copolymer | ABS | Slowly soften and burn without dripping | Yellow, black smoke | Continue to burn | Special smell |
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