Effect of Soft Segment of Polyurethane Elastomer on Its Mechanical Properties
Urethane elastomer (PUE) is mainly synthesized from oligomer polyols, polyisocyanates and chain extenders as raw materials. Its mechanical properties can be adjusted in a wide range. It can be synthesized by selecting suitable raw materials according to needs to prepare products that meet the needs. Generally speaking, oligomer polyol constitutes the soft segment of polyurethane, polyisocyanate and chain extender constitute the hard segment, and microphase separation occurs between the soft segment and the hard segment, so that PUE has excellent performance. There are many classification methods for PUE: according to its processing technology, it can be divided into three types: casting type, thermoplastic type and mixing type; according to the chemical reaction process, it can be divided into one-step method and two-step method; Alcohol types can be divided into polyether type and polyester type.
1 Experimental part
1.1 Raw materials and instruments
Tetrahydrofuran homopolyether polyol (PTMG): Mn=1000 and Mn=2000, Japan Mitsubishi Chemical Corporation, industrial product; Polycaprolactone polyol (PCL): Mn=2000, Japan Daicel Chemical Industry Co., Ltd., industrial Products; polybutylene adipate diol (PBA, Mn=1000, 2000), polyethylene adipate diol (PEA, Mn=2000): Yantai Huada Chemical Co., Ltd., industrial products; Polycarbonate diol (PCDL): Mn=2000, Asahi Kasei Chemical Co., Ltd., industrial product; 4, 4' – Diphenylmethane diisocyanate (MDI): Yantai Wanhua Polyurethane Co., Ltd., industrial product; 1, 4 – Butanediol (BDO): Beijing Chemical Plant, analytical grade; Dibutyltin dilaurate (T12): Tianjin Ruijinte Chemicals Co., Ltd., reagent grade.
Electronic universal testing machine: Model CMT6104, Shenzhen Xinsansi Measurement Technology Co., Ltd.; Rubber Durometer: Model LX-A, Yingkou Experimental Materials Factory.
1.2 Synthesis of PUE
1.2.1 Prepolymer preparation
Dehydrate the oligomer polyol under vacuum at 100~110°C for 2h, cool to 50~55°C for use; pour the melted MDI into a three-necked bottle, when the MDI temperature drops to 50~55°C, add the metered For good oligomer polyols, first react naturally for 30 minutes without heating, and then stir and heat for 2 hours at 80~85 °C, take samples to analyze the mass fraction of NCO, and end the reaction when the analysis value is close to the design value. The prepolymer can be obtained, which is sealed and stored for later use.
1.2.2 Elastomer preparation
Weigh an appropriate amount of prepolymer, add BDO and T12 in proportion, stir quickly and evenly, pour it into a preheated 120°C flat mold, and press and cure for 30 minutes when it reaches the gel point. The mechanical properties were tested after curing for 18h in an oven at room temperature for 24h.
1.3 Test method
NCO content is determined according to HG/T2409-1992; Shore A hardness is determined according to GB/T531.1-2008; 300% tensile stress, elongation at break and tensile strength are determined according to GB/T528-2009; tear strength is determined according to GB/T531.1-2008 GB/T529-2008 determination.
2.1 The effect of oligomer polyols on the mechanical properties of PUE
Different types and molecular structures of oligomer polyols have a direct impact on the mechanical properties of PUE. Use polyether polyol and polyester polyol with the same relative molecular mass (both 2000), keep the mass fraction of soft segment at 55%, synthesize prepolymer with MDI, chain extension coefficient [n(OH) in the system: n(NCO )] is 1, the prepolymer is chain-extended with BDO, and the mechanical properties of the prepared PUE are shown in Table 1.
It can be seen from Table 1 that when the w(NCO) of the prepolymer is the same, the mechanical properties of the PUE whose soft segment is PCL, PBA, PEA and PCDL are generally better than those whose soft segment is PTMG. This is because PCL, PBAPEA and PCDL are all polyester polyols with ester groups inside, while PTMG is a polyether polyol with ether groups inside, and ester groups are more polar than ether groups, making the interior of the polyurethane easier Hydrogen bonds are generated, and the mechanical properties are better. In polyester PUE, because PCDL contains carbonate group, the polarity is the largest, and the obtained PUE has the highest degree of crystallinity in the soft segment, so its hardness and tensile strength are the highest, and the elongation at break is the lowest; the content of ester group in PCL The lowest Shore A hardness and tensile strength, and the highest elongation at break; PBA and PEA have more PEA ester groups, but because the chain extender uses BDO, the number of carbon atoms between the two hydroxyl groups is 4, and PEA has 2 less carbon atoms than PBA. The structure of PBA and BDO is more similar, the resulting polyurethane is more regular, and the crystallinity is stronger. Therefore, the performance of PUE synthesized by PBA as a soft segment is better than that of PEA.
2.2 Effect of relative molecular weight of oligomer polyol on mechanical properties of PUE
The same kind of oligomer polyols with different relative molecular weights are used to synthesize polyurethane, because the size of the soft segment is different, and the influence on the mechanical properties of PUE is also different. Use polyether polyol and polyester polyol with different relative molecular weights, keep the same w(NCO), synthesize prepolymer with MDI, use BDO as chain extender, and the chain extension coefficient is 1. The mechanical properties of the prepared elastomer are shown in Table 2.
As can be seen from Table 2, whether it is polyester PUE synthesized by PBA or polyether PUE synthesized by PTMG, when the w(NCO) of the prepolymer is the same, with the increase of the relative molecular weight of the oligomer polyol, the PUE will be reduced. The hardness, 300% tensile stress, tensile strength and tear strength decreased, while the elongation at break increased. When the relative molecular mass of the oligomer polyol increases, the polarity of the prepared PUE decreases, which is not conducive to the formation of hydrogen bonds, the degree of microphase separation between the soft and hard segments is weakened, and the related hardness, modulus and strength are reduced; The relative molecular mass of oligomer polyols increases, which weakens the interaction between hydrogen bonds, reduces the crystallization tendency of soft segments, maintains the high elasticity of PUE, and increases the elongation at break.
2.3 Effect of NCO content on the mechanical properties of polyurethane elastomers
Different NCO content of prepolymer will lead to different content of soft segment of synthetic PUE, thus affecting the mechanical properties of elastomer. Firstly, polyether polyol and polyester polyol of the same relative molecular mass were used to synthesize prepolymers with different NCO values with MDI. The physical and mechanical properties are shown in Table 3.
It can be seen from Table 3 that whether it is polyester PUE synthesized by PBA or polyether PUE synthesized by PTMG, with the increase of prepolymer w (NCO), the hardness, 300% tensile stress, tensile strength and tearing strength of PUE increase. The breaking strength increases, while the elongation at break decreases. This is because the higher w(NCO) in the prepolymer, the higher the content of polyisocyanate, and at the same time, the amount of chain extender will increase, which will increase the hard segment content of PUE and increase the hardness; w(NCO) in the prepolymer will increase. ), the urethane groups increase, the polarity of the polyurethane becomes larger, and the degree of microphase separation increases to increase the modulus and strength; the increase of w(NCO) makes it easier to form hydrogen bonds inside the polyurethane, and the hydrogen bond interaction increases. large, so the elongation decreases.
2.4 Influence of chain extension coefficient on mechanical properties of PUE
If the chain extension coefficient is different, the amount of chain extender used will be different, which will lead to different content of the synthetic PUE soft segment, thus affecting the mechanical properties of PUE. Using polyether polyol and polyester polyol of the same relative molecular mass, keeping the same w(NCO), synthesizing a prepolymer with MDI, then changing the chain extension coefficient, and using BDO to extend the chain of the prepolymer, the prepared PUE mechanics The performance is shown in Table 4.
It can be seen from Table 4 that whether it is polyester PUE synthesized by PBA or polyether PUE synthesized by PTMG, when the w(NCO) of the prepolymer is the same, with the increase of the chain extension coefficient, the hardness of PUE, 300% The tensile stress, elongation at break, tensile strength and tear strength increased first and then decreased, and the mechanical properties were the best when the chain extension coefficient was 1.00. This is because when the chain extension coefficient is 1.00, the polyurethane elastomer tends to form a long-chain molecule with the highest degree of linearity, and the PUE has the best mechanical properties at this time. When the chain extension coefficient is lower than 1.00, due to insufficient amount of chain extender, macromolecular long chains cannot be formed, and the polyurethane performance is low; when the chain extension coefficient is greater than 1.00, due to excessive chain extension agent, it can meet the chain extension requirements. There are residues, and these residues act as plasticizers to degrade the mechanical properties of the PUE.
Conclusion
① When the prepolymer w (NCO) is the same: the mechanical properties of PUE whose soft segment is PCL, PBA, PEA, and PCDL are generally better than those whose soft segment is PTMG; Hardness, 300% tensile stress, tensile strength and tear strength decreased, while elongation at break increased; with the increase of chain extension coefficient, the hardness, 300% tensile stress, elongation at break, tensile strength of PUE The strength and tear strength showed a trend of increasing first and then decreasing, and when the chain extension coefficient was 1.00, the mechanical properties were the best.
② With the increase of prepolymer w(NCO), the hardness, 300% tensile stress, tensile strength and tear strength of PUE increased, while the elongation at break decreased.
