Effect of volume fraction of ramie cloth on physical and mechanical properties of ramie cloth/UP resin composite

LEI Wen(雷 文), LEI Wen-guang(雷文广), REN Chao(任 超)

College of Information Science and Technology, Nanjing Forestry University, Nanjing 210037, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Ramie cloth/UP resin composite was formed at 0.2 MPa and cured at room temperature for 24 h and treated at 80 ℃ for 2 h. The physical and mechanical properties of the composites with different volume fractions of ramie cloth were studied. The results show that, with the increase of the volume fraction of the ramie cloth, densities of the composites become greater and greater, though all lower than the theoretical values, the linear shrinkage during the formation decreases from 1.20% of the original UP resin to 0.18% of the composite with 30% of ramie cloth in volume, all the composites also absorb more water than UP resin casting, greater volume fraction of the fiber, more water will be absorbed, but the increase in water absorption becomes smaller and smaller with time. As regards some mechanical properties, the tensile strength, flexural strength, flexural modulus and impact strength are all improved when more ramie fiber is added. Compared with those of pure UP resin casting, the mechanical properties are increased by 93.93%, 76.20%, 190.18% and 227.26% respectively when the volume fraction of the ramie cloth in the composite is 30%. The differential scanning calorimetry results show that only one peak will appear for the sample without or with less ramie fiber while two peaks will appear when more ramie cloth is added.

Key words: ramie cloth; UP resin composite; density; linear shrinkage; mechanical properties; thermal analysis

1 Introduction

In recent years, natural fibers have attracted the attention of scientists and technologists because of the advantages that these fibers provide over conventional reinforcement materials, and the development of natural fiber composites has been a subject of interest for the past few years[1-4]. These natural fibers are low-cost, biodegradable and non-abrasive, unlike other reinforcing fibers, and also present low densities and high specific properties. In addition, they are readily available and renewable[5].

   Ramie fiber is a typical kind of natural fibers, its tensile strength is about 870 MPa[6], substantially lower than that of glass fiber, but its modulus is of the same order, especially when the specific modulus is considered, ramie fiber shows value that is comparable to or better than that of glass fiber. In addition，China is the largest producer of ramie fiber in the world[7]. Thus composite from plastic with ramie fiber is not only of low cost, low density，renewable，natural and less abrasive during processing but also easy to get, consequently it is a potential green material to be used in many areas such as automotive, packaging and construction industries.

2 Experimental

The polymer used in this study was a UP resin, supplied by Jingling DSM Resin Co, Ltd. The catalyst MEKP and promoter cobalt naphthenate were provided by Zhejiang Huangyan Jiaoken  Chemicals Co, Ltd, and Changzhou Qianjin Chemicals Co, Ltd, respectively.

The reinforcement used was ramie fiber cloth supplied by Changzhou Wujin Ramie Knitting and Weaving Company. The reinforcement was dried at 105 ℃ for 2 h and then cooled to room temperature before using.

The composite was formed by pouring the resin with promoter and catalyst and the ramie cloth into the cavity of the mold, then moulding at 0.2 MPa and room temperature. The studies on the density, water absorption and mechanical properties of the composite were performed following the standard methods from ASTM. In order to carry out the shrinkage test, pieces of resin casting and composites were formed in a steal mould, the shrinkage (s) of each sample was calculated by the equation s=[(l-l₀)/l₀]×100%, where l and l₀ stand for the lengths of the cavity of the mould and the sample respectively.

To investigate the effect of ramie content on the thermal properties of the composite, TG analysis and dynamic DSC scans of the samples were performed at a heating rate of 10 K/min under N₂ atmosphere to produce the decomposition process.

3 Results and discussion

3.1      Effect of volume fraction of ramie cloth on density of composite

Table 1 shows the densities of ramie fiber reinforced UP resin composites. The density of pure UP resin casting is also shown in Table 1. It can be concluded that the density increases with the  increase of the fiber content. The main reason may be that the density of the ramie fiber(1.50 g/cm³)[7] is greater than that of the matrix polymer(i.e. 1.230 8 g/cm³, the measured density of pure UP resin casting in Table 1). When the resin is reinforced with ramie cloth, its density certainly becomes greater. But it can be also seen from Table 1 that, the densities of composites are slightly smaller than the theoretical ones according to the rule of mixture, the reasons might be the less efficient dispersion of ramie fiber in UP resin and perhaps some porosities due to cellulose fine decomposition or vapor emission during processing[8]. In addition, some additives of lower densities are used for the weaving of ramie cloth, and thus the density of the ramie cloth is really lower than that of natural ramie fiber, when we calculate the density of the composite, the density of the reinforcement is that of natural ramie fiber but not that of the cloth. This will certainly leads to greater densities than the real ones.

Table 1 Densities of composites

3.2      Effect of volume fraction of ramie cloth on curing shrinkage of composite

The linear shrinkages of ramie cloth/UP resin composites are shown in Fig.1. Generally, the linear shrinkage of the composite decreases with the increase of the ramie fiber content, and the shrinkage decreases faster at the beginning. This tendency is similar with that of other inorganic particles filled polymer composite[9].

The theory of “free volume” tells us that, the volume of  polymer is composed of two parts, one is that occupied by the molecular chains of the polymer themselves, the other is the free space among different molecular chains, and these spare spaces are inside the polymer randomly. When UP resin cures, the space will shrink, at the same time, the double bonds (molar volume is 25.0 cm³/mol) will be turned into single bond(molar volume is 18.9 cm³/mol). This will also cause shrinkage. When the ramie cloth is used to form composite with UP resin, just like other inorganic particles, the hard ramie fiber will decrease the linear shrinkage[9].

Fig.1 Linear shrinkage of different composites

3.3    Effect of volume fraction of ramie cloth on water absorption of composite

Many porosities may exist on the surface of the composite, porosities may arise from the emission of organic materials and/or the poor adhesion between UP resin and ramie cloth. Porosities will absorb water or moisture from humid environment. Generally, under the same conditions, the less the water absorbed, the shorter the equilibrium of absorbing water is reached, the longer life the material will have.

Water absorption of different ramie cloth reinforced UP resin composites is shown in Fig.2. The greater the

Fig.2 Variation of water absorption with sinking time

fiber volume fraction, the more the water will be absorbed, at the same time, the time to reach water absorption equilibrium is prolonged. The result agrees with that of hemp fiber reinforced UP resin composite[10], which in turn proves the existence of porosities.

3.4    Effect of volume fraction of ramie cloth on mechanical properties of composite

The effects of ramie cloth content on the tensile strength, flexural strength and flexural modulus of ramie cloth/UP resin composites are graphically shown in Figs.3-5, respectively. As shown in Figs. 3 and 4, both the tensile and flexural strengths are generally increased with a rising content of ramie fiber, and the modulus is also increased when more cloth is used to form composite, which means the stiffness of UP resin can be improved.

The impact strength was also measured with unnoted

Fig.3 Tensile strength of composites

Fig.4 Flexural strength of composites

specimens in this study. Izod unnoted impact strength values of ramie cloth/UP resin composites can also be improved when more ramie cloth is used, as shown in Fig.6.

Fig.5 Flexural modulus of composites

Fig.6 Impact strength of composites

3.5    Effect of volume fraction of ramie cloth on thermal property of composite

Fig.7 shows the TG curves of different materials. It can be seen that the initial decomposition temperature(t_i) of UP resin casting, composites with volume fraction of 10%, 20% and 30% ramie cloth is 333.9, 336.2, 338.6 and 348.2 ℃ respectively, which means that t_i gets higher and higher with the increase of ramie cloth content in the composite. So it can be concluded that, complexing with ramie cloth can improve the thermal stability of UP resin.

DSC scanning obtained from dynamic measure- ments for the composites with different volume fractions of ramie cloth is shown in Fig.8. It is noticed that the composites absorb some heat for the first stage at about 50-150 ℃, the heat absorbed here may be used to

Fig.7 TG curves of different composites

Fig.8 DSC curves of composites

remove the water or the low molecular weight substances from the sample. Then, from about 330 ℃ to 420 ℃, another one or two peaks of heat absorbance arises for the second stage for each sample, the temperature at the peak for the pure UP resin casting and the composite with 10% of ramie cloth is 365.5 ℃ and 378.9 ℃ respectively, while two peaks appear for both the composites with 20% and 30% of ramie cloth whose temperatures at the right peaks are accordingly 383.7 ℃ and 384.9 ℃. The appearance of two peaks may be due to the weak bonding between the cloth and UP resin matrix, and it may also be the result that the complex has changed the mechanism of the melting or thermal decomposition of the sample. The temperatures at peaks in the DSC curves differ from each other, and the number of the peaks turns from one for the samples without or with less ramie cloth into two for the samples with much more ramie cloth. All the results show that the melting or decomposition of the composite may happen differently with the increase of ramie cloth content.

4 Conclusions

The physical properties of the ramie cloth reinforced UP resin composites are different from those of pure UP resin casting, some mechanical properties such as tensile strength, flexural strength, flexural modulus and impact strength are improved significantly. All these mean that ramie cloth is a good reinforcement material for UP resin. Besides these, the composites will decompose from about 330 ℃ to 420 ℃, and the mechanism of the decomposition may be different from that of UP resin casting. The absolute values of the mechanical properties are not very high yet, the composite can only be used as some parts without over-loads.

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(Edited by YANG You-ping)

Foundation item: Project(G2002-21) supported by the Funds for Importing High Grade and Overseas Personnels of Nanjing Forestry University

Corresponding author: LEI Wen; Tel: +86-25-81777389; E-mail: nfuleiwen@163.com