What are the conventional identification methods for PVC?

Date:2021-02-01 11:37:09 星期一

C onventional PVC identification methods are generally […]

C onventional PVC identification methods are generally summarized into the following three categories, namely:
(1) Identification of combustion method:
Softening or melting temperature range: 75~90°C;
Burning situation: self-extinguishing after ignition;
Burning flame status: yellow up and green with smoke; after leaving the fire: extinguished from the fire; smell: pungent sour.
This method is the easiest and most direct, and is generally preferred.
(2) Identification of solvent treatment:
Solvent: tetrahydrofuran, cyclohexanone, ketone, dimethylformamide;
Non-solvents: methanol, acetone, heptane.
By adding the suspected PVC plastic to the above solvent, observe the dissolution of the plastic to determine whether it is PVC. After the solvent is heated, the dissolving effect will be more obvious.
(3) Proportion method:
The specific gravity of PVC is 1.35~1.45, generally around 1.38. The difference in specific gravity or the method of determining specific gravity can be used to distinguish PVC from other plastics. However, because PVC can add plasticizers, modifiers and fillers to make the PVC become very different in specific gravity, and the difference in softness and hardness is very large. At the same time, due to the addition of some ingredients, many properties of PVC plastic will change, resulting in The effect of our commonly used identification methods is not obvious, and even the phenomenon has changed, making it impossible to make accurate judgments. For example, in terms of density, plasticized polyvinyl chloride (containing about 40% plasticizer) is 1.19~1.35; while PVC hard products have increased to 1.38~1.50. If it is a highly filled PVC product, the density sometimes exceeds 2.
In addition, it can also be determined by determining whether the material contains chlorine, but because vinyl chloride copolymers, neoprene rubber, polyvinylidene chloride, chlorinated polyvinyl chloride, etc. all contain a high proportion of chlorine, they must be colored by pyridine Reaction to identify. Note that before the test, the sample must be extracted with ether to remove the plasticizer. Test method: Dissolve the sample taken with ether and benzene in tetrahydrofuran, filter out the insoluble components, add methanol to make it precipitate, and extract the first 75 Below the degree of dryness. Do not need 1mL of pyridine to react with a small amount of dried sample. After a few minutes, add 2 drops of methanol solution of 5% sodium hydroxide (1g sodium hydroxide is dissolved in 20mL methanol), and immediately observe the color, 5min And observe again after 1h. Different chlorinated plastics can be identified based on the color.
In daily life, PVC and PE plastic film (bag) need to be distinguished from more contact. Simple method:
(1) Touch method
It feels lubricious to the touch, and the surface looks like a layer of wax (chemically called wax feel). This is a non-toxic polyethylene film bag, while the PVC film feels a little sticky.
(2) Dithering method
When shaking with hands, the sound is crisp, and the thing that floats easily is a polyethylene film bag. And the low sound of shaking with hands is a polyvinyl chloride film bag.
(3) Combustion method
It is flammable in case of fire, the flame is yellow, paraffin-like oil drips when burning, and there is gas from candle burning. It is a non-toxic polyethylene film bag. If it is not easy to burn, it will be extinguished after leaving the fire, and the flame will be green and it is a PVC film bag.
(4) Immersion method
The plastic bag is immersed in water and pressed into the water by hand. Polyethylene that can rise to the surface and polyvinyl chloride that sinks into the water (polyethylene has a density less than water, and polyvinyl chloride has a density greater than water; at room temperature, they are about 0.92g/cm3 and 1.4g/cm3).
You can also take a piece of copper wire and burn it into red in the fire. Then, the copper wire is brought into contact with the plastic film used for the test to cause a chemical change, and then the copper wire dipped in the plastic component is put back into the flame. At this time, you need to observe carefully. If there is a colorful, dazzling green flame, it means that this plastic material contains chlorine and belongs to the polyvinyl chloride material.