Influence of Screw Extrusion Expansion Drying on Structure and Properties of Natural Rubber

doi:10.3969/j.issn.1000-2561.2022.12.017

Abstract

Abstract:

Natural rubber is mainly dried by hot air. However, natural rubber is a poor conductor of heat and has strong self-adhesion, which leads to low drying efficiency, sticky rubber and entrapment in hot air drying, and adversely affects the performance of the product. Screw extrusion expansion drying process, a new drying method, has been widely used in the synthetic rubber industry due to the high drying efficiency, high automation level and stable process. And the drying process also has a good application prospect in the production of natural rubber (NR). In this paper, the structure of NR prepared by traditional hot air drying and screw extrusion expansion drying (respectively denoted as NT and NCT) was compared, and the performance characteristics of NCT were analyzed to explore the feasibility of screw extrusion expansion drying process in the production and processing of natural rubber. For raw rubber, the screw extrusion expansion drying process could reduce the thermo-oxidative degradation of the NR molecular chain during the drying process because of its low drying temperature and short drying time, so NCT had higher initial plastic value, Mooney viscosity and gel content than NT. The screw extrusion drying process had little effect on the non-rubber components, so the change range of the non-rubber components such as protein and volatiles was low. For the pure compound vulcanizate, NT exhibited a faster cure rate than NT. The contribution of the internal network structure of the vulcanizate was fitted using the Mooney-Rivlin equation. The effective crosslink density and the number of chemical crosslink networks after NCT vulcanization were relatively low. The conventional mechanical properties such as tensile strength, tear strength and modulus of NCT were lower than those of NT, and NCT exhibited higher fatigue temperature rise and higher degree of creep during dynamic loading. For carbon black vulcanizate, the conventional mechanical properties of NCT and NT were not significantly different., this may be due to the faster vulcanization rate of vulcanization accelerator N-tert-butylbenzothiazole-2-sulphenamide. However, NCT had longer fatigue life and better dynamic fatigue properties, which means the screw extrusion drying process could improve the dispersion of carbon black.

Key words: natural rubber, screw extrusion expansion, drying, network structure

CLC Number:

Q949.748.5

CHEN Guojing, WANG Bingbing, LUO Ting, LI Gaorong, ZHANG Fuquan, LIAO Jianhe, LIAO Lusheng. Influence of Screw Extrusion Expansion Drying on Structure and Properties of Natural Rubber[J]. Chinese Journal of Tropical Crops, 2022, 43(12): 2545-2553.

Figures/Tables 14

Tab. 1 Physical and chemical properties of NR

项目Item	NT	NCT
氮含量/%	0.43	0.42
挥发分/%	0.61	0.52
凝胶含量/%	14.1	17.5
塑性初值	34.5	41.0
塑性保持率	75.0	76.8
门尼粘度	71.8	80.7

Tab. 2 Cure properties of unfilled compounds

项目 Item	NT	NCT
焦烧时间/min	1.3	1.8
最佳固化时间/min	12.3	14.8
最小扭矩值	1.3	1.4
最大扭矩值	6.0	5.3
扭矩差	4.7	3.8
硫化速率	9.1	7.7

Fig. 1 Tensile behavior for unfilled vulcanizates

Tab. 3 Mechanical properties for unfilled vulcanizates

项目 Item	NT	NCT
绍尔硬度	43	39
回弹率/%	79.3	76.8
拉伸强度/MPa	23.1	19.7
撕裂强度/(kN·m^-1)	30.1	27.3
100%定伸应力/MPa	0.81	0.74
300%定伸应力/MPa	1.8	1.3
扯断伸长率/%	785	858

Fig. 2 Rubber network structure parameter a: Gc and Ge of samples; B: Vc and αu of samples.

Tab. 4 Rubber network structure parameters and conformation entropy change

项目 Item	NT	NCT
交联模量/MPa	0.36	0.22
缠结模量/MPa	0.23	0.31
结晶临界应变α_u	3.89	5.37
有效交联密度/(10^-4 mol·cm³)	1.51	1.28

Fig. 3 Cyclic reciprocating tensile of 100%-500% incremental strain samples A: 100%-500% strain cyclic stretching; B: Energy loss during cyclic stretching.

Tab. 5 Entropy change parameters of unfilled vulcanizates

项目 Item	NT	NCT
交联点间链段质量/(g·mol^-1)	6067.75	7166.81
链波动半径/nm	22.9	19.7
缠结点间连段数	911.6	676.3
熵变/(J·cm^-3·K^-1)	5797.8	14699.3

Fig. 4 Cyclic reciprocating tensile of 500% fixed strain specimen A: 500% Fixed strain cyclic stretching; B: Energy loss during cyclic stretching.

Fig. 5 Dynamic fatigue testing curves of unfilled vulcanizates A: Temperature rise curves; B: Creep curves.

Tab. 6 Mechanical properties of carbon black vulcanizates

项目Item	CB-NT	CB-NCT
绍尔(A)硬度	65	64
拉伸强度/MPa	28.5	28.2
撕裂强度/(kN·m^-1)	56.0	63.7
扯断伸长率/%	554	563

Fig. 6 Network structure parameters of carbon black filled vulcanizates

Fig. 7 Dynamic fatigue testing curves of carbon black filled vulcanizates A: Temperature rise curves; B: Static stiffness retention curves.

Tab. 7 Panye effect of carbon black filled vulcanizates

样品 Sample	初始模量G'_∞/Mpa	100%应变G'₀/Mpa	差值 ?/Mpa	佩恩效应?G''/Mpa
NT	0.56	0.49	0.07	-
NCT	0.49	0.41	0.08	-
CB-NT	1.87	0.90	0.97	0.90
CB-NCT	1.81	0.92	0.89	0.82

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