Introduction
Seat system mainly consists of seat skeleton, seat polyurethane foam and seat cover. Seat as an important part of the vehicle interior, its polyurethane foam VOC volatilization on the vehicle VOC impact is relatively large. Considering the danger of air quality in the car to human health, the country has released a draft of mandatory standards for comment, and the OEMs are also developing corresponding testing methods and parts restrictions to improve the air quality in the car.
1VOC concentration requirements
Volatile organic compounds VOC (Volatile Organic Compounds), refers to organic compounds that are easily volatile at room temperature. The Ministry of Environmental Protection’s official website released a draft for comment “passenger car interior air quality evaluation guidelines”, for automotive interior air organic concentration requirements, see Table 1.
Polyurethane foam VOC sources and improvement methods
Polyurethane foam VOC sources
Polyurethane rigid foam is formed by a series of chemical reactions after mixing A and B component materials in certain ratio, which has the functions of water-proof and heat insulation. A component material is a combination of polyol (polyether or polyester) and additives such as foaming agent, catalyst, stabilizer and flame retardant, commonly known as white material. B component material is mainly composed of isocyanate, a brown liquid, commonly known as black material. The raw materials used in polyurethane, resulting in VOC have the following sources.
Polyol
Theoretically, the residual trace polyols in polyurethane materials produce low molecular aldehydes, alcohols, and carboxylic acids.
Isocyanates
Free isocyanate monomers are stored in polyurethane foams, and because of their low boiling point and high vapor pressure, they are slowly released during later use, producing volatile substances that are hazardous to human health.
Catalysts
Tertiary amine catalysts can provide catalytic properties for polyurethane foaming and gelation reactions, but most do not participate in the reaction. After the reaction is completed, part of the amine catalyst will be volatilized during the production process, and the rest will be free in the foam. Due to the porous structure of polyurethane foams, the free tertiary amine catalysts will slowly evaporate to the foam surface.
Foam stabilizer
Silicone stabilizer used in rigid foam, commonly known as “silicone oil”, the main structure is polysiloxane-olefin oxide block copolymer, the role is to improve the closed pore rate of the bubble, so that the foam stability, homogenization. The small molecules of silicone with low relative molecular mass will directly enter into the finished product and do not participate in the foaming reaction, but can only remain inside the foam, which increases the volatilization of silicone VOC of polyurethane foam.
Other additives
In order to maintain the characteristics of the product, a variety of additives such as foaming agents, flame retardants, antioxidants, mold release agents and antioxidants are added during the production of seating polyurethane foam. Due to the incomplete degree of reaction, a small amount of residual free monomer and some low relative molecular mass substances can be easily volatilized from the polyurethane foam.
Polyurethane foam VOC improvement method
Material selection
At present, isocyanate is not yet replaceable in the production of PU foam, only appropriate measures can be taken to reduce VOC, mainly in the following areas.
(1) The use of polyester polyol instead of polyether polyol, which has higher activity and can reduce the amount of catalyst.
(2) Isocyanate using diphenylmethane diisocyanate (MDI) system formulation, not using toluene diisocyanate (TDI).
(3) The catalyst adopts high-efficiency, low-volatility type substances, such as triethylenediamine instead of dimethylcyclohexylamine and other active amine catalysts.
(4) Reduce the use of additives, such as the introduction of elements with flame retardant effect in polyol instead of flame retardant use.
(5) Adding aldehyde eliminators in the mixing process.
Process treatment
While improving the application of raw materials, the process uses production process control and post-treatment to further reduce VOC, mainly in the following areas.
(1) foam pouring is changed from manual pouring to robotic pouring, which makes the raw materials fully mixed, while ensuring that the dosage ratio of A and B materials is 1:3, the foaming temperature is 50℃～70℃, and the in-mold maturation time is 4min.
(2) The open foam exhaust of the foaming vacuum opening machine is discharged into the outdoors through a pipe, and the vacuum level is guaranteed to be 0.6Mbar-0.7Mbar.
(3) crushing foam opener after two crushing open foam, open foam fully, no gas residue in the foam.
(4) foam on the suspension chain, maturation time of at least 8h, suspension chain cycle time of about 24h a week.
(5) the use of ventilation measures, such as foaming stations to increase the vacuum emissions, the top of the plant to increase the exhaust device and the workshop window glass was intermittently opened.
Seat VOC test method and results
Seat VOC test method
Sample requirements: the sample should be kept in the usual state after the production line within 15 days of the parts, test sample bag using 2000L volume for packaging, the sample is placed in accordance with the actual use of the state of the car.
Pretreatment: The sample is pretreated according to GB/T2918-1998 “Standard Environment for Conditioning and Testing of Plastic Specimens” in the standard environment (air temperature 23℃±1℃, relative humidity 50%±15%) for 24h before the test.
Gas trapping: After the sample is kept at a constant temperature of 65℃ for 2h, the sample is sampled according to the device shown in Figure 1.
Figure 1 sketch of sampling device
Data analysis and processing: Measurements were made in accordance with the provisions of 5.2 of HJ/T400-2007 “Methods for Sampling and Determination of Volatile Organic Compounds and Aldehydes and Ketones in Vehicle Air”, and calculated according to the volatile concentration C=the amount of compound measured by the collection tube W/the amount of gas collected by the collection tube Q.
Test results
Using the improved method in this paper, and in the third-party testing organization, multiple rounds of experimental tests were conducted and the average value was taken, and the results are shown in Table 3.
Table 3: VOC test results of a model seat
From Table 3, it can be concluded that the improvement method can improve the volatilization of VOC to a certain extent, and other similar seat polyurethane foam materials can be referred to. During the test, it was found that the ambient temperature, production storage and transportation methods have some influence on the VOC test, and it is recommended to ensure a suitable temperature in the production workshop, good ventilation, and change the plastic parts packaging to non-woven fabric in transportation.
Conclusion
“Energy saving, environmental protection, safety” is the eternal theme of automotive development, the air quality in the car as an important aspect of automotive environmental protection, safety, more and more attention by consumers. To control the air quality in the car, we should start from the source, with a responsible attitude to consumers, to solve the problem of VOC in the car fundamentally, and minimize the harm to the occupants of the car.
