Bio-based Phenolic Resin-Reinforced Butyl Rubber Damping Composites

The same amount additives were added to each sample with the sum mass of ZnO, stearic acid, TMTD, DM and S of 12.6 g when IIR was used 100 g. The rest amounts of the added reagents are shown in Table 1.Table 1

Formulation design of bio-based PF/NaH activation-modified butyl rubber damping material.

Sample code	IIR	Montmorillonite	LPF	NaH	DBA
IIR	100	–	–	–	–
IIR/M	100	40	–	–	–
IIR/M/LPF-10	100	40	10	–	–
IIR/M/LPF-15	100	40	15
IIR/M/LPF-20	100	40	20	–	–
IIR-6DBA/M/LPF-10	100	40	10	–	6
IIR-H2DBA/M/LPF-10	100	40	10	0.21	2
IIR-H4DBA/M/LPF-10	100	40	10	0.42	4
IIR-H6DBA/M/LPF-10	100	40	10	0.63	6
IIR-H8DBA/M/LPF-10	100	40	10	0.84	8
IIR-H10DBA/M/LPF-10	100	40	10	1.05	10

Stearic acid acts as a softening and plasticizing agent and facilitates the dispersion of other fillers. Zinc oxide acts as an activator of the rubber. TMTD and DM are rubber accelerators used to increase the rate of vulcanization of the rubber. Sulfur is used in the vulcanization of the rubber to cross-link the linear chains of rubber molecules into a network. Lignin phenolic resins are added to improve IIR related properties such as damping and tensility.
Figure 1 shows the flow chart for the synthesis of bio-based phenolic resins. In 250 mL three-necked flask equipped with stirrer, thermometer and reflux condenser, a certain proportion of phenol, NaOH and alkali lignin were respectively added and stirred well, and the temperature of the reaction system was raised to 90 °C and reacted for 90 min to obtain lignin phenol. The lignin mass accounted for 40% of the total mass of lignin and phenol, and the NaOH mass was 6% of the phenol mass. The reaction conditions of the lignin phenolic resin synthesis stage were consistent with the phenolic resin synthesis. When the temperature of the reaction system was reduced to 60 ℃, the formaldehyde was added, stirred uniformly and reacted for 60 min. After that, the temperature of the reaction system was increased to 90 ℃, and the reaction was carried out for 120 min with the molar mass ratio of phenol to formaldehyde of 1: 1.7. The reaction product was cooled to room temperature and then washed three times with respective anhydrous ethanol and deionized water in ultrasonic cleaner, and the product was finally put into vacuum oven and dried at 60 ℃ for 24 h. The product was dehydrated to obtain lignin phenolic resin (LPF).Figure 1

Synthesis of lignin phenolic resin.

Lignin was first phenolized by adding phenol under alkaline solution conditions, and formaldehyde was added under alkaline conditions after lignin was phenolized. Finally, the prepared bio-based phenolic resin was dried, crushed and kept for next preparation.
Figure 2 shows the flow chart for the preparation of damping composites. At a specific temperature, IIR and NaH were added into 60 mL Hacker’s torque rheometer, and after the torque time curve was stabilized, dibenzylidene fork acetone was added to continue the reaction for about 15 min (Fig. 3), and the modified rubber was obtained and set aside.Figure 2

Schematic diagram of the composite structure.

Figure 3

NaH activation modified IIR with introduced DBA.

The rubber added with dibenzyl fork acetone was plasticized on a double-roller opener, and the fillers stearic acid, zinc oxide, TMTD, DM, sulfur, lignin-based phenolic resin and montmorillonite were added in order and blended well. Stearic acid acted as softening and plasticizing agent and facilitated the dispersion of other fillers. Zinc oxide acted as activator of the rubber. TMTD and DM were rubber accelerators to increase the rate of vulcanization of the rubber. Sulfur was used in the rubber vulcanization to cross-link the linear chains of rubber molecules into network. Lignin phenolic resins were added to improve IIR related properties such as damping and ductility. The blended sample was left for 24 h to remove the air bubbles, and the raw rubber was prepared. Finally, 7–10 g of raw rubber was weighed and vulcanized on the plate vulcanizer to prepare the specimens of butyl rubber comprehensive temperature range damping composites. The vulcanization conditions were determined with vulcanization temperature of 160–170 ℃, vulcanization pressure of 10–12 MPa and vulcanization time of 14–18 min.
The isoprene monomer provides the molecular backbone of butyl rubber with an active point where the cross-linking reactions can take place. The most significant advantage of using reactive processing technology of rubber is that the chemical modification reaction can be carried out in general-purpose rubber processing equipment (e.g., torque rheometers, compactors). Short reaction process, simple preparation process and low experimental equipment requirements characterize this solvent-free ontology modification method. According to the principles of organic chemistry, sodium hydride does not react with alkanes. However, under certain conditions, it can react with hydrogen in the olefin double bond or the allyl position to produce carbon-negative ion. The typical reaction of carbon-negative ions is nucleophilic addition to compounds containing carbonyl groups^{26 (link)}. In this experiment, the NaH activator was selected to produce negative carbon ions on the molecular rubber chain. Then DBA, a compound containing carbonyl groups, was selected to introduce unsaturated double-bond functional groups and benzene rings with larger side groups to the main macromolecular chain. In the butyl rubber modification reaction process, the isobutylene structural unit is assumed not to participate in the reaction because the side methyl group on the isobutylene unit has low reactivity and does not participate in the reaction.