Introduction
The seat system is mainly composed of seat frame, seat polyurethane foam and seat cover. Seats are an important part of vehicle interiors, and the VOC volatilization of polyurethane foam has a greater impact on VOCs in the vehicle. Considering the harm of in-car air quality to human health, the country has currently issued a draft for comments on mandatory standards, and various OEMs are also formulating corresponding testing methods and parts restrictions to improve in-car air quality.
1VOC concentration requirements
Volatile organic compounds (VOC) refer to organic compounds that are easily volatilized at room temperature. The draft “Guidelines for the Evaluation of Air Quality in Passenger Cars” published on the official website of the Ministry of Environmental Protection has the requirements for the concentration of organic matter in the air in cars, as shown in Table 1.
Polyurethane foaming VOC sources and improvement methods
Polyurethane foaming VOC sources
Polyurethane rigid foam is a foam plastic with waterproof and thermal insulation functions formed by mixing A and B components in a certain proportion and undergoing a series of chemical reactions. Component A is a composite material composed of polyol (polyether or polyester) and additives such as foaming agent, catalyst, stabilizer, flame retardant, etc., commonly known as white material. The main component of component B is isocyanate, which is a brown liquid commonly known as black material. The raw materials used in polyurethane lead to VOCs from the following sources.
Polyol
Theoretically, trace amounts of polyols remaining in polyurethane materials will produce low-molecular aldehydes, alcohols, and carboxylic acids.
Isocyanates
Free isocyanate monomer exists in polyurethane foam. Due to its low boiling point and high vapor pressure, it will be slowly released during future use, producing volatile substances and endangering human health.
Catalyst
Tertiary amine catalysts can provide catalytic performance for the foaming reaction and gelation reaction of polyurethane, but most of them do not participate in the reaction. After the reaction is completed, part of the amine catalyst will volatilize during the production process, and the rest will be free in the foam. Due to the porous structure of polyurethane foam, the free tertiary amine catalyst will slowly dissociate to the foam surface and volatilize.
Foam stabilizer
The silicone stabilizer used in hard foam, commonly known as “silicone oil”, has a main structure of polysiloxane-alkylene oxide block copolymer. Its function is to increase the closed cell ratio of the cells and stabilize and homogenize the foam. Small siloxane molecules with low relative molecular weight will directly enter the finished product and do not participate in the foaming reaction. They can only remain inside the foam, increasing the volatilization of silicone VOCs in polyurethane foam.
Other additives
In order to maintain the characteristics of the product, various additives such as foaming agents, flame retardants, antioxidants, release agents and anti-aging agents are also added during the production of seat polyurethane foam. Due to the incomplete reaction, a small amount of residual free monomers and some low molecular weight substances are easily volatilized from the polyurethane foam.
Methods for improving VOC in polyurethane foam
Material selection
At present, isocyanate cannot be replaced in PU foam production. Appropriate measures can only be taken to reduce VOC, which are mainly reflected in the following aspects:
(1) Use polyester polyol to replace polyether polyol, which has higher activity and can reduce the amount of catalyst;
(2) The isocyanate is formulated using diphenylmethane diisocyanate (MDI) system and does not use toluene diisocyanate (TDI);
(3) The catalyst uses high-efficiency, low-volatility substances, such as triethylenediamine instead of dimethylcyclohexylamine as a medium-active amine catalyst;
(4) ReduceThe use of flame retardants, such as the introduction of flame retardant elements into polyols instead of flame retardants;
(5) Add aldehyde eliminator during the mixing process.
Processing
While improving the application of raw materials, the process adopts production process control and post-processing methods to further reduce VOC, mainly in the following aspects:
(1) The foaming pouring is changed from manual pouring to robot pouring, so that the raw materials are fully mixed, while ensuring that the dosage ratio of A and B materials is 1:3, the foaming temperature is 50℃~70℃, and the maturation time in the mold is 4min;
(2) The foaming exhaust of the foaming vacuum foaming machine is discharged outdoors through the pipeline, and the vacuum degree is guaranteed to be 0.6Mbar-0.7Mbar;
(3) The rolling foaming and opening machine has been rolled twice to open the foam, and the foaming is sufficient, and there is no gas residue in the foam;
(4) Foam the hanging chain, the curing time is at least 8 hours, and the hanging chain cycle time is about 24 hours;
(5) Adopt ventilation measures, such as increasing vacuum exhaust at the foaming station, adding an exhaust device on the top of the factory building, and opening the glass of the workshop windows intermittently.
Seat VOC test methods and results
Seat VOC test method
Sample requirements: Samples should be parts that should be kept within 15 days under normal conditions after being rolled off the production line. The test sample bag should be packaged with a volume of 2000L, and the samples should be placed according to the actual use conditions in the vehicle.
Pretreatment: The pretreatment of the sample before testing is in accordance with the standard environment in GB/T2918-1998 “Standard Environment for Condition Adjustment and Testing of Plastic Samples” (air temperature 23℃±1℃, relative humidity 50%±15% ) left for 24h.
Gas capture: After the sample is kept at a constant temperature of 65°C for 2 hours, it is sampled according to the device shown in Figure 1.
Figure 1 Simplified diagram of sampling device
Data analysis and processing: Measurement is carried out in accordance with the provisions of 5.2 in HJ/T400-2007 “Sampling and Determination Methods for Volatile Organic Compounds and Aldehydes and Ketones in Car Air”, and the volatile concentration C = compound measured in the header Calculate the quantity W/gas quantity Q collected by the collection tube.
Test results
Adopt the improvement method in this article and conduct multiple rounds of test tests at a third-party testing agency, and take the average value. The results are shown in Table 3.
Table 3 VOC test results of seats of a certain model
It can be concluded from Table 3 that the improvement method can improve the volatilization of VOC to a certain extent. Other similar seat polyurethane foam materials can be referred to. During the test, it was found that ambient temperature, production, storage and transportation methods have a certain impact on VOC testing. It is recommended to ensure appropriate temperature and good ventilation in the production workshop, and to change the packaging of plastic parts to non-woven fabrics during transportation.
Conclusion
“Energy saving, environmental protection, and safety” are the eternal themes of automobile development. As an important aspect of automobile environmental protection and safety, indoor air quality is increasingly valued by consumers. In-car air quality control must start from the source, with a responsible attitude towards consumers, fundamentally solve the problem of VOC in the car, and minimize its harm to passengers.
