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In the production of plastic products, the possible waste can be divided into solid, liquid and gas three categories. Among them, the sources of gas waste are more, the harm is bigger. Mainly includes the organic volatile gas (VOC) and the inorganic volatile gas two major categories.
When VOC reaches a certain concentration, there is a huge impact on human health. Facing the policy pressure, consumer’s demand, reduces the plastic product VOC content to become the material enterprise’s first important matter. Today we’ll talk about some of the common ways to remove VOC.
What are VOC gases?
VOC is short for Volatile organic compound, which is highly volatile in the temperature range of 50-260 °C. Common volatile organic compounds include: benzene (such as benzene, toluene, xylene, ethylbenzene) , alkanes (tetradecane) , aldehydes (formaldehyde) , alcohols (ethanol) , ethers, esters (ethyl acetate) , ketones (acetone) .
These VOC gases are generally invisible to us, and some of the odors you can smell are not entirely the gases themselves. The main sources of taste of these organisms include:
(1) formaldehyde flavor produced by POM decomposition;
(2) HCl produced by decomposition in the production of PVC, acid taste produced by water;
(3) vinyl acetate-vinyl acetate copolymer in the process of vinyl acetate monomer residual release of vinegar, etc. .In the processing of plastic products, in addition to raw resin and additives added by the modification, the following materials will also produce VOC gas:
01 printing ink
(1) Solvent ink in the printing, drying and other processes will produce benzene, ketone and ethyl acetate and other VOC pollution.
(2) water-based ink organic solvent content is low (about 20%) , so the use of the process (drying) less VOC.
Acrylic acid and polyurethane adhesives in common use are still mainly organic solvents of ethyl acetate.
03 Surface coating
Coating can eliminate all kinds of surface defects of plastic products, and give products with conductive, anti-static, wear-resistant and other functions, can also be dark plastic products into light-colored products. Commonly used are acrylate or polyurethane coatings.
04 release agent
Commonly used demoulding machine will contain CH2CL2, CH3CHCl and other organic solvents.
How is VOC gas removed?
01 The resin with low residual amount of monomer was selected
The resin with low residual monomer content should be selected as far as possible. For example, medical PVC resin VC monomer content is very low, VC content can reach 1 ppm (mg/kg)(normal value of 5) ; automotive interior panel with PP, must choose low-molecular volatile content resin.
02 Used higher molecular weight additives
As far as possible to choose high molecular weight additives, preferably polymer additives, to ensure that the processing temperature will not volatilize and external migration.
(1) if the coupling agent changes to a compatibilizer, such as a grafted polymer or a maleic anhydride polymer, but the grafted part has a certain odor, but the maleic anhydride graft part has a certain odor.
(2)The lubricant uses the pentaerythritol stearate (PETS) and so on; the dispersant uses the polyethylene wax (PE-WAX) which the molecular weight is more than 5000.
Sainuo PETS with good internal and external lubrication, it can improve the thermal stability of products, at high temperature with good thermal stability, low volatility, good demoulding and flow performance.
(3) as far as possible to choose high molecular type of flame retardant, such as brominated polystyrene, brominated epoxy resin and so on.
03 add adsorbent
It refers to the use of adsorbent capture or adsorption resin matrix residual organic small molecules to control the VOC content requirements. The adsorption method can be divided into chemical reaction adsorption and physical adsorption.
04 control processing conditions
(1) low temperature production: under the premise of ensuring the full plasticization of products, lower processing temperature, thereby reducing the production of small molecule decomposition substances.
(2) multistage vacuuming: increasing the times of vacuuming can reach 3 or even 4 levels to remove the small molecule volatiles in time.
05 to reduce the use of organic solvent-based materials
(1) water-based coatings: water as a dispersion medium instead of volatile organic solvents, the current technology also needs to contain about 20% ether and alcohol solvents, only a low VOC coatings, not no VOC coatings.
(2) solid powder coatings: no organic solvents, zero VOC emissions.
(3) photo-curing coatings: film-forming materials are formed by photo-curing reaction, which can reduce the use of organic solvents, but often need to contain about 30% of organic solvents.
(4) water-based adhesives and water-based mold release agents: currently available for organic solvents in low-content products.
06 high temperature dry steaming
After the plastic matrix is transported to the VOC treatment system through a twin-screw extruder and pelletizer, and heated at 80-120 °C, the VOC substances (benzene, toluene, ethylbenzene and other hydrocarbon substances) adsorbed by the plastic particles are evaporated as clean as possible, the time of dry steaming depends on the temperature, high temperature, less time, generally about 3 hours. At present, large modification enterprises basically use this method.
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