Effects of additives on properties of self-skinning polyurethane foam for automobile steering wheel

. Commercialized polyurethane foam products for automobile steering wheels have been committed to the development of products with comprehensive properties such as low density, high production efficiency and environmental protection. Based on this, this article has launched a series of experiments to explore the effects of different types of chain extenders, blowing agents and catalysts on the mechanical properties, cell structure, skin and reactivity of self-skinning polyurethane foams for automobile steering wheels. The results show that the hardness and tensile strength of the foam gradually decrease with the chain growth of small molecular chain extenders (ethylene glycol, 1.3-propanediol, 1.4-butanediol), and the elongation at break increases gradually. The foaming agent formic acid is more conducive than water as it favors the formation of small but compact cells, thus dense skin. The catalytic efficiency of non-reactive catalysts (A1, KC101 and DabcoEG) is higher than that of reactive catalysts (LED-103, ZR-50, DPA). The theoretical research for the development of high-performance and high-efficiency polyurethane foam products for automobile steering wheels.


Introduction
Polyurethane self-skinning foam, also known as integral skin molded foam, is polyurethane foam with a self-skinning structure formed by reaction injection molding [1]. The outer epidermis is a dense layer, and the inner has high resilience [2]. This polyurethane self-skinning foam is comfortable to the touch, and has the characteristics of high environmental protection and no pollution [3]. The material has been used in the mass production of automobile steering wheels in the market.
With the further improvement of the social environment's requirements for green and healthy products, a number of low volatile organic compounds content polyurethane foams for automobile steering wheels have been developed in recent years. The foams have good mechanical properties, but in terms of foam reactivity, it takes 3~5 min to achieve the curing and molding of the product, which is inefficient in mass production. Therefore, a series of performance experiments were carried out in this paper, which laid a certain foundation for improving the reactivity of polyurethane foam and developing polyurethane foam products with excellent mechanical properties and high reactivity.
Herein, different types of chain extenders, blowing agents and catalysts are used to explore their influence and changing rules on the mechanical properties, cell structure, skin and reactivity of polyurethane foams [4][5][6][7]. The main research work includes: the effect of chain extenders ethylene glycol, 1,3-propanediol and 1,4-butanediol on the hardness, tensile strength and elongation at break of polyurethane foam; foaming agents water and formic acid effects on polyurethane foam cells and skin [8][9]; reactive catalystsLED-103, ZR-50, DPA and non-reactive catalystsA1, KC101, DabcoEG effects on the reactivity of polyurethane foam [10][11].

Synthesis of polyurethane foam
The basic formula used in this study is shown in Table 1. Preparation of component A combined polyether: After weighing 160 g F3135, 40 g POP3630, 10 g chain extender (Ethylene glycol or 1, 3-Propylene glycol or 1, 4-Butanediol), 1.2 g catalyst (LED-103 or Al or ZR-50 or KC101 or DPA or DabcoEG), 1 g foaming agent (formic acid or water), mix and stir evenly.
B component is a modified isocyanate, W1361. Isocyanate index was used to calculate the ratio of the dosage of component A to component B. The calculation formula of isocyanate index is as follows: where (

Foam reactivity test
Using a stopwatch, observe and record the reactivity of polyurethane foam in a disposable plastic cup, including rise time (the time when the foam starts to grow), gel time (the time for the foam to be drawn) and debonding time (the skin does not change after the foam is cured and formed).

Influence of the type of chain extender on the mechanical properties of foam
Different types of chain extenders have different hydroxyl values. In order to keep the total hydroxyl value of the chain extenders added in the combined polyether unchanged, the addition amount of different types of chain extenders is adjusted accordingly. By changing the types of chain extenders, the foam samples with a density of 350 kg/m 3 weretested for performance under standard laboratory conditions. The results are shown in Table 2.
As can be seen from Table 2, with the growth of the molecular chain of the chain extender, the hardness and tensile strength of the polyurethane foam gradually decrease, and the elongation at break gradually increases. When the hydroxyl value content of the combined polyether and the isocyanate index keep the same, the content of the hard segment (urethane group generated when the isocyanate reacts with the hydroxyl group) in the molecular chain of the polyurethane foam remain unchanged. While with the chain growth of small molecule chain extender, the spacing of the urethane groups in the hard segment group gradually enlarges, the distribution of the hard segment in the molecular chain changes from centralized to disperse. Meanwhile the more enhanced flexibility of the molecular chain is observed, which indirectly increases the soft segment. Therefore, the hardness and strength of the polyurethane foam gradually decrease, and the elongation at break increases.

Influence of foaming agent types on foam cells and skin
In this part, we mainly compared the influences of foaming agent water and formic acid. After a series of reactions of water molecules and formic acid molecules with isocyanate, both of the two foaming agents can generate 2 carbon dioxide molecules. Therefore, it is necessary to keep the amount of the substance added to the foaming agent constant. By changing the type of foaming agent, a foam sample block with a density of about 350 kg/m 3 was obtained, and then the changes of the cells and skin were observed and compared. The results are shown in Table 3.
It can be seen from Table 3 that when foaming with water, the cells of the foam are larger, loosely arranged, and the skin is poor; when foaming with formic acid, the foam of the foam is small and closely distributed, and the skin is relatively dense. Although the amount of CO 2 gas produced by the foaming agent water and formic acid is the same, the cell structure is quite different. It could be explained as follows, in the process of combining the polyether, some formic acid and amine catalysts undergo complex reaction to form salts (reversible reaction), with the progress of the polyurethane foam reaction, the complexed formic acid is gradually released, so the generation of CO 2 gas is also gradually carried out. Compared with water foaming, the cells are smaller, and the arrangement is tighter. The epidermis is also denser.

Effect of catalyst types on foam reactivity
Keeping the same amount of catalyst, by changing the type of catalyst, mainly compare the reactive catalyst with the non-reactive catalyst, the polyurethane foam cup cells were prepared, and then the reactivity was observed and compared. The results are shown in Table 4. It can be seen from Table 4 that in terms of the reactivity of polyurethane foam, the non-reactive catalyst has better effect, and is faster than the reactive catalyst in the rise, gelation and debonding time. Because the molecular chain of the reactive catalyst is introduced. The active hydrogen group can react with the -NCO group in the isocyanate to be combined with the polymer chain of the polyurethane foam. Compared with the free non-reactive catalyst, its reactivity is significantly lower.
However, non-reactive catalysts are more volatile in polyurethane foam products. The introduction of reactive catalysts can effectively reduce the VOC content of polyurethane foams. Therefore, while improving the foam reactivity, it is necessary to consider other properties of the foam to comprehensively design the catalyst.

Conclusions
In this paper, by comparing different types of chain extenders, blowing agents and catalysts, the effects and changing laws on the mechanical properties, cell structure, skin and reactivity of polyurethane foams were studied. The results are shown as follows: First of all, the small molecule chain extenders include ethylene glycol, 1.3-propanediol and 1.4-butanediol, with the growth of their molecular chains, the hardness and tensile strength of polyurethane foam gradually decrease, and the elongation at break increases. Secondly, compared with water, using formic acid foaming, polyurethane foam has small cells, compact cell structure, dense skin crust and better touch feeling. Thirdly, compared with reactive catalysts A1, KC101 and DabcoEG, non-reactive catalysts A1, KC101 and DabcoEG are more helpful to improve the reactivity of polyurethane foam.
In the field of polyurethane foam for automobile steering wheel, this paper lays a theoretical research foundation for the development of polyurethane foam with better performance, summarizes the effects and influences of different kinds of additives, further standardizes and optimizes the formulation and development system of polyurethane foam for automobile steering wheel, and has certain guiding significance for the product research of automobile steering wheel, improving production efficiency and reducing production cost.