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Hyperbranched polymers in modifying natural plant fibers and their applications in polymer matrix composites - A review Zhanying Sun J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.9b03436 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Journal of Agricultural and Food Chemistry

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Hyperbranched polymers in modifying natural plant fibers and their

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applications in polymer matrix composites - A review

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Zhanying Sun*

4 5 6

College of Material Science and Engineering, Hebei University of Science and

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Technology, Shijiazhuang 050018, China

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Hebei Key Laboratory of Material Near-Net Forming Technology, Hebei University of

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Science and Technology, Shijiazhuang 050018, China

10 11 12 13 14 15 16 17 18 19 20 21

* Corresponding author:

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Zhanying Sun, College of Material Science and Engineering, Hebei University of

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Science and Technology, 70 Yu Hua Dong Road, Shijiazhuang 050018, China

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Email: [email protected]

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ABSTRACT

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Natural plant fibers have been widely used in agricultural and forest industries, and

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even in automobile industry, especially for producing fiber reinforced polymer matrix

29

composites. However, the low mechanical properties of composites remain the key

30

problem in the applications. Hyperbranched polymer has lots of advantages such as low

31

viscosity and high reactivity, etc. Multi-reactive end groups of hyperbranched polymers

32

are ideal for modifying natural plant fibers to achieve better interface bonding between

33

fiber and resin matrix. This article reviews some research advances in hyperbranched

34

polymer-modified natural plant fibers and summarizes the applications of the modified

35

fibers in polymer matrix composites with particular focus on the chemical modification

36

of fibers and interface bonding.

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KEYWORDS:

natural

fiber

composites;

hyperbranched

39

modification; interface bonding; mechanical properties.

40 41 42 43 44 45 46 47 48 49 50

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polymer;

chemical

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Journal of Agricultural and Food Chemistry

INTRODUCTION

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Natural plant fibers have abundant resources in the nature with a series of

53

advantages such as low cost, low density, environment-friendliness, natural

54

degradability etc. In recent years, fabricating composites with the natural plant fibers

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has been becoming one of the focuses in the field of composite research, which has

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significant economic and social benefits and is of great significance for increasing the

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added value of agricultural products and promoting the development of agricultural and

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forest industries.1- 4 For example, AL-Oqla et al. 5, 6 analyzed the potential of agro waste

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fibers (coir, date palm, flax, hemp, jute, kenaf and sisal) for automotive industry using a

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decision making model. And they found that the flax fiber showed the best result for

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automotive industry. They also found that the impact strength is the major evaluation

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criterion in evaluating natural plant fiber composites for interior parts in the automotive

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industry. Kalagi et al.

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composites in wind turbine and discussed the main factors affecting mechanical

65

performance of natural fiber composites. They pointed out that fiber type, matrix

66

selection, interfacial strength are all the main factors. In the recent paper, Keya et al.

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systematically summarized the applications of natural fiber composites, including

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aerospace, automotive, sports, musical instruments, construction materials, packaging

69

materials, etc.

7

studied the application of natural fibers reinforced polymer

8

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However, up to now, natural plant fiber composites have not gain a large market

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yet due largely to the fact that their mechanical properties cannot meet the requirements.

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The mechanical properties of the natural plant fiber composites are determined by plant

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fiber, resin matrix, and the interface bonding between the fiber and the resin. 2

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Natural fiber

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Natural fibers play an important role in reinforcing composites. Its reinforcing

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effect mainly depends on the aspect ratio, mechanical properties and loading of the

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fibers. 9 In general, natural fibers are grouped into three types: plant fibers, animal fibers

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and mineral fibers.

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composites due to their abundant source, easy extraction, low cost and superior

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properties. The main types of plant fibers are jute, ramie, flax, sisal, etc.

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mechanical properties compared with synthetic fibers are summarized in Table 1. 12

10

Among these fibers, plant fibers are widely used in polymer

11

Their

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Plant fibers are mainly composed of cellulose, hemicellulose and lignin. The

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content of cellulose contributes most to the mechanical properties of fibers. 13 Moreover,

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there are a lot of hydroxyl groups on the surface of plant fibers, thus some functional

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groups can be introduced onto fiber surface through appropriate modification. However,

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compared with synthetic fibers, plant fibers have poor heat resistance, lower thermal

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decomposition temperature, and the temperature higher than 240 oC will have adverse

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effects on the mechanical properties of fibers.

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heat resistance of the plant fibers can be greatly improved by the metal particles

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modification. 14

11

Recently, it has been reported that the

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About the fiber selection for natural plant fiber composites, the availability of

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waste fibers, moisture content, cost, the period of renewal, and their thermal and

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mechanical properties must be noted. 15

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Polymer matrix

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The polymer matrix mainly protects the fibers from the external environment, and

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it mainly acts to transmit stress in the composite material. Among the natural fiber

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composite materials, the selection of resin matrix mainly considers the heat resistance of

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the fiber. Therefore, the matrix currently used in natural fiber composites mainly

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includes polypropylene, polyethylene, polystyrene, poly(vinyl chloride), epoxy,

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polyester, polyurethane, phenol–formaldehyde and rubber, etc.

Among these resins,

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the thermoplastic resin has been widely used because of its recyclability and low

102

processing costs. However, its biggest disadvantage is its poor compatibility with fibers.

103

At present, the method of adding a compatibilizer is widely used to improve the

104

interfacial compatibility between the fiber and the resin matrix.

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The selection of matrix for natural fiber composites also considers the wide

106

availability of resins, excellent mechanical properties, and easy processing. Recently,

107

biodegradable resins have received increasing attention due to the demand for the

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environmental protection.

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polypropylene is a superior thermoplastic resin matrix, and epoxy is a good thermoset

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resin matrix for manufacturing natural fiber composites.

16

In the study of AL-Oqla et al.

17,

they found that

111

As mentioned above, impact property is a very important evaluation criterion for

112

the application of natural fiber composites in automotive industry. However, the impact

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properties of natural fiber composites are much lower than those of synthetic fiber

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composites. Particularly, the problem is noticeable under a large filling amount of plant

115

fibers.

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composites.

18, 19

Thus, it is the top priority to improve the impact properties of such

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The conventional method for improving the impact properties is to use resin with

118

better impact properties or add rubber or elastomer to the resin matrix. However, the

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addition will inevitably degrade the tensile and flexural properties of the composites,

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which has been demonstrated by many scholars.20,21

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Another major method for improving the impact properties is to select an

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appropriate fiber. For example, Mieck et al. 22 proposed in their article that the addition

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of other fibers conducive to energy absorption to the system would improve the impact

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strength of the composites. Jarukumjorn and Suppakarn

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have discovered that the

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impact properties of the composites can be improved significantly by introducing the

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glass fiber to the natural plant fiber composites. However, one major disadvantage of

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this method is that the introduction of the glass fiber considerably reduces the

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environment-friendliness of the composites. Canché-Escamilla et al. 24 have pointed out

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that the impact properties of the composites increase with the improvement of the

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mechanical properties of the fibers. However, in the two methods above, the selection

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of the resin matrix and fiber is usually restricted by various external factors such as

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market supply, fiber growth location etc. While the interface bonding between the fiber

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and the matrix may be regulated easily and thus is more likely to be promoted in the

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market of natural plant fiber composites.

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Interface bonding

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Interface bonding always remains one of the critical problems in the field of

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research on natural plant fiber composites. Many scholars are committed to research on

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the interface bonding between the fiber and the resin matrix.25-28 Kabir et al.

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Mohanty et al. 30 outlined the surface treatment methods for natural plant fibers. Li et al.

140

31

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methods include alkali treatment, silane treatment, acetylation treatment, benzoyl

142

treatment, acrylonitrile graft treatment, permanganate treatment, peroxide treatment,

143

isocyanide acid treatment, stearic acid treatment, sodium hypochlorite treatment, etc.

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The maleic anhydride grafted polypropylene has significant effects in improving the

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interface bonding of natural plant fiber composites as an interface compatilizer and

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substantially improves the tensile and flexural properties of the composites. However, it

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has great negative effects on the impact properties, which is considered by many

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scholars to be caused by excessively strong bonding of the interface.32 Thus, a new

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modification method is expected to be explored if various properties of the composite

29

and

also elaborated the chemical treatments of natural plant fiber surface. The major

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system (particularly the improvement of impact strength) are to be comprehensively

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considered.

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Based on the above fact, the proper control of the interface bonding can effectively

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guarantee the equilibrium among various properties of the composites. Zhou et al.

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prepared glass fiber composites with different interface bonding degrees by grafting

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rubber molecular chains of different lengths onto the fiber surface and discovered that

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the introduction of a proper interface flexible layer could improve the impact strength of

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the composite and also improve other mechanical properties to some extent. 33, 34

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Hyperbranched polymer

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The hyperbranched polymer is a highly branched polymer containing a large

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number of reactive end groups with a series of advantages such as less molecular

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entanglement, high solubility, low viscosity, easy film formation, high reactivity etc.35-37

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It has a large number of active end groups and controllability during the synthesis

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process and can easily form an interface flexible layer in the composite system. Ratna 38

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and DeCarli et al.

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can significantly improve the impact properties of the epoxy resin matrix composites.

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Gao and Yan 37 have also discussed the application of the hyperbranched polymer in the

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epoxy resin matrix composite and pointed out that the use of the hyperbranched

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polymer can improve the toughness of the composite without influencing the processing

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performance of the composite. Wong et al.

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polylactic acid matrix through the hyperbranched polymer and subsequently discovered

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that the toughness of the composite made from the hyperbranched polymer, polylactic

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acid matrix, and the flax fiber improves significantly.

39

have pointed out that the addition of the hyperbranched polymer

40

have improved the brittleness of the

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Based on the unique properties and potential application values of hyperbranched

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polymers, this paper outlines the studies on the surface modifications of natural plant

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fibers by the hyperbranched polymers and summarizes the progress on the applications

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of the modified fibers in the polymer matrix composites.

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MODIFYING NATURAL PLANT FIBER BY HYPERBRANCHED POLYMER

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With an extremely complex structure, the natural plant fiber is mainly composed of

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cellulose, hemicellulose, and lignin. The proportions of the three main compositions are

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not the same depending on different types of plant fibers. However, the most important

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composition in the natural plant fiber is cellulose, which determines the mechanical

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properties of the fiber itself. A higher content of cellulose would lead to better

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mechanical properties of fiber. Generally, in terms of chemical constitution, the

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cellulose is a linear chain polymer of β-D-anhydroglucopyranose units connected via

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the β-1,4-glycosidic bonds without any branch structure.

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groups are distributed in the glucose unit. Their chemical activity varies depending on

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different positions and different degrees of accessibility of the hydroxyl groups.

41

An abundance of hydroxyl

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There are many published articles on the modification of nano-SiO2, carbon

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nanotube, carbon fiber, glass fiber, graphene and other reinforcements by the

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hyperbranched polymers. 42-52 While the modification of the natural plant fiber is mainly

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concentrated in the textile field and its application in the polymer matrix composites is

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very limited. The effects of various hyperbranched polymers on the modification of

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different reinforcements are systematically summarized in Table 2. 42-64

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The modified method for the natural plant fiber by the hyperbranched polymer can

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be divided into “grafting-to’’ method and “grafting-from’’ method. In the “grafting-to’’

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method, the hyperbranched polymer is first synthesized and then directly grafted onto

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the surface of the plant fiber via chemical reactions. In the “grafting-from’’ method, the

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monomer stepwise polymerization growth method is used to achieve stepwise growth

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into hyperbranched polymer on the fiber surface. Generally, the hydroxyl groups on the

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fiber surface are activated before the initiation of surface polymerization. Usually, the

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silane coupling agent is used. For natural plant fibers, both methods have their

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respective advantages and disadvantages. The “grafting-to’’ method has a simple

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treatment process, commercial availability of the hyperbranched polymers, a short cycle

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of fiber treatment, and minor fiber damage. However, the steric-hindrance effect of the

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hyperbranched polymer will reduce the efficiency of modification by grafting on the

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fiber surface. The “grafting-from’’ method can achieve high graft density due to the

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easy access of the reactive groups to the chain ends of the growing polymers.41

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However, this method has a very complex treatment process, a long period of fiber

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treatment, and large fiber damage and requires repeated separation and washing when

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grafting hyperbranched polymers of different generations. Some research progress will

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be reviewed in the following paragraphs with a focus on the two modification methods.

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“Grafting-from’’ method

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Although the “grafting-from’’ method has some disadvantages mentioned above,

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there are lots of researches on this method due to its high graft density. For example,

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Dadkhah Tehrani and Basiryan

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crystallites in the methanol solution using ethylenediamine and then grafted the

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hyperbranched polyamide on the cellulose crystallite surface using the repeated

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reactions between the methyl acrylate and ethylenediamine through the divergent

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synthesis method. The reaction lasted for 24 h at 70 °C. The infrared and nuclear

220

magnetic analysis showed that the hyperbranched polyamide was successfully grafted

221

onto the nano cellulose crystallite surface. The atomic force microscope also showed

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that a globular covering formed on the surface of the cellulose crystallite. The

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aggregation of the cellulose crystallites was also improved following the hyperbranched

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polyamide modification.

65

performed surface amination of the nano cellulose

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Hassan

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reported that two amino-terminated hyperbranched polymers, ie,

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hyperbranched poly(propylene imine) and hyperbranched polyamide could be grafted

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on the cellulose fiber surface using the “grafting-from’’ method. In the two approachs

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for the hyperbranched polymer grafting, the acrylonitrile was first used to modify the

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cellulose to synthesize the cyanoethyl cellulose under the NaOH alkaline condition;

230

then the cyano group was hydrolyzed into amino in the BH3/THF (Borane

231

tetrahydrofuran complex) solution. Thereafter, the formed amino reacted with

232

acrylonitrile and the cyano hydrolysis performed again in BH3/THF solution. The two

233

types of reactions could be repeated to graft the hyperbranched poly(propylene imine)

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of different generations onto the cellulose fiber surface. The grafted amino-terminated

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hyperbranched polyamide was obtained by the cyanoethyl cellulose, which was

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hydrolyzed into aminated cellulose. Michael addition reactions and amidation reactions

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occurred between the acrylic ester monomers and multi-amino chemical compounds.

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The Michael addition reaction was a reaction between the double bond of the acrylic

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ester and the amino of chemical compounds. The amidation reaction was the reaction

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between the ester group introduced by the Michael addition reaction and the

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ethylenediamine. The above two reactions were repeated to graft hyperbranched

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polyamide of different generations onto the cellulose fiber surface. The research

243

exhibited that the graft yield was high in the first generation and the yield decreased

244

significantly with the increase in graft generations. The heat resistance of the modified

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cellulose material declined somewhat with the increase in the hyperbranched polymer

246

introduced.

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Xiao et al. 61 first extracted cellulose crystallite from the sisal fiber, then performed

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the silane modification on the cellulose crystallite at 60-90 °C using γ-Aminopropyl-

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triethoxysilane, and finally allowed the modified cellulose crystallite and 3,5-

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diaminobenzoic acid to react for 6h at 100°C under the protection of nitrogen. As a

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result, hyperbranched aromatic polyamide was synthesized on the fiber surface. The

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infrared spectrum and XPS analysis showed that the hyperbranched aromatic polyamide

253

was successfully grafted onto the cellulose crystallite surface and the dispersity of the

254

grafted cellulose crystallite in the aqueous solution was improved.

255

Lu et al.

62

used the γ-aminopropyl-triethoxysilane to modify the surface of the

256

sisal cellulose crystallite, and dispersed the modified sisal cellulose crystallite into the

257

N-methyl pyrrolidone. Then, 3,5-diaminobenzoic acid, pyridine, and triphenyl

258

phosphite were added for reacting and producing intermediate products under the

259

protection of N2. Subsequently, they continued to react with methylbenzene-2, 4-

260

diisocyanate, and 4, 4′-(β-hydroxyl oxyethyl) biphenyl for producing the hyperbranched

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biphenyl liquid crystal graft modified sisal cellulose crystallite. The repulsive

262

interaction between the graft modified nano sisal crystallite particles enabled the

263

modified sisal cellulose crystallites to well disperse in the epoxy resin matrix, thus

264

improving the mechanical properties and thermal performance of the epoxy resin

265

composites.

266

Very recently, Sun et al. 57 first introduced the amino groups onto the fiber surface

267

by γ-Aminopropyl-triethoxysilane modification of the sisal fiber and then grafted the

268

hyperbranched polyamide onto the fiber surface by repeated reactions between the

269

methyl acrylate and the ethylenediamine (Figure 1). As shown in this figure, with the

270

reaction proceeding, the amino content increased significantly. However, the steric-

271

hindrance effect of hyperbranched polyamide also increased gradually, which affected

272

the grafting efficiency of the fibers. The infrared and XPS analysis exhibited that the

273

hyperbranched polyamide was successfully grafted onto the surface of the sisal fiber.

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With the increase in the graft generations, the element N increased significantly. The

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TGA result showed that the heat resistance of the fiber degraded with the increase in the

276

number of graft generations.

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“Grafting-to’’ method

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Grafting hyperbranched polymers onto the surface of natural plant fiber by using

279

the “grafting-to’’ method mainly concentrated in the textile field. For instance, Zhang et

280

al.

281

excellent anti-bacterial property and washing durability. They first synthesized the

282

hyperbranched polyamide with the amino as the terminal group with the

283

diethylenetriamine and the methyl acrylate as the raw materials. Then, the

284

hyperbranched polyamide further reacted with the AgNO3 aqueous solution to prepare

285

silver nanoparticles with amino groups. Finally, the hyperbranched polymer with the

286

silver nanoparticle as the core (HPSN) was grafted onto the oxidized cotton fabric

287

(Figure 2). The analyses of the anti-bacterial activity test and laundering durability test

288

exhibited that the method could enable the cotton fabric to demonstrate an excellent

289

anti-bacterial property and laundering durability. In Figure 2, it is noted that oxidation

290

will have a negative impact on the fibers. Attentions must be paid to the concentration

291

of oxidant and oxidation time, otherwise the mechanical properties of the fibers will be

292

reduced by excessive oxidation. In addition, the concentration of HPSN and treatment

293

time also have an important effect on the grafting efficiency. The steric-hindrance effect

294

of hyperbranched polymers is also an important factor affecting the grafting efficiency.

295

53,55

described an approach for fabricating anti-bacterial cotton fabrics with an

Subsequently, they

54

deposited the nano ZnO on the cotton fabric in situ with the

296

hyperbranched polyamide and Zn(NO3)2 as the raw material and obtained cotton fabrics

297

with different contents of nano ZnO by the optimized finishing process. The cotton

298

fabric finished by the in-situ deposited nano ZnO not only had an excellent anti-

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ultraviolet property but also presented a good anti-bacterial property and laundering

300

durability.

301

Zhang et al.

55,56

studied in detail the effects of the amino-terminated

302

hyperbranched polymer on the dyeability and laundering durability of the cotton fabrics.

303

They first synthesized the amino-terminated hyperbranched polyamide by the

304

polycondensation method with the diethylenetriamine and the methyl acrylate as the

305

raw materials and selectively oxidized the cotton fabric with sodium periodate. The

306

resulting aldehyde groups on the oxidized cotton fabric reacted with the amino groups

307

of the hyperbranched polyamide to produce the grafted cotton fabric. The research

308

found that the dyeability and laundering durability were improved and the anti-bacterial

309

property was significantly strengthened following the amino hyperbranched polymer

310

modification.

311

Zhao et al.

58,59

first synthesized the amino-terminated hyperbranched polymer,

312

then selectively oxidized the flax fiber with sodium periodate, and finally grafted the

313

synthesized amino-terminated hyperbranched polymer to the fiber surface for improving

314

the dyeability and anti-bacterial property of the flax fiber. The result exhibited that the

315

sodium periodate selectively oxidized the flax fiber to generate active aldehyde groups.

316

The aldehyde groups on the oxidized flax fiber surface could be covalently bound to the

317

amino groups on the surface of the amino-terminated hyperbranched polymer. The flax

318

fiber modified with the amino-terminated hyperbranched polymer could present

319

improvements on dyeability and satisfactory color fastness as well as an excellent anti-

320

bacterial property and anti-ultraviolet property.

321

In a recent publication, Wang et al.

66

prepared a new amino-terminated

322

hyperbranched polymer with diethyl malonate, methyl acrylate, and diethylenetriamine

323

by a two-step procedure. Subsequently, the hyperbranched polymer was used to modify

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the ramie fabric. Based on the test, it was found that the hyperbranched polymer was

325

successfully grafted onto the ramie fabric surface, and the dyeability and color fastness

326

of the modified ramie fabric increased.

327

To improve the quantity of the active hydroxyl groups on the fiber surface and thus 67

328

improve the compatibility between the fiber and the resin matrix, Yang et al.

329

synthesized the poly 3-methyl-3-oxetane methanol hyperbranched polymer (HBPO)

330

using the 3-methyl-3-oxetane methanol (MOM) and the boron trifluoride diethyl ether

331

(BF3OEt2), then grafted the hyperbranched polymer onto the cellulose fiber surface

332

(Figure 3), and studied its various properties. The result exhibited that the increase in

333

the number of hydroxyl groups on the cellulose fiber surface was very significant. The

334

growing number of hydroxyl groups could be easily controlled by controlling the

335

amount of the monomer added, which laid a solid technical basis for subsequent

336

functionalization of the cellulose surface. However, in Figure 3, MOM monomer self-

337

polymerization could affect the grafting efficiency, so it is necessary to remove the

338

HBPO physically adsorbed on the surface of the fibers. In addition, the effect of steric-

339

hindrance of hyperbranched polymers on the grafting efficiency of fibers should also be

340

considered.

341

In a recent patent article, Lu et al.

63

first

published an approach for preparing sisal

342

cellulose crystallites modified by hyperbranched polyester. With malic acid as the main

343

material, they synthesized a new carboxyl-terminated hyperbranched polyester by the

344

polycondensation reaction and then performed a graft reaction with the sisal cellulose

345

crystallites to obtain the sisal cellulose crystallite modified by hyperbranched polyester

346

grafting. The research found that the modified sisal cellulose crystallites were able to

347

uniformly disperse in the epoxy resin due to the synergistic effects of the sisal cellulose

348

crystallites and the globular hyperbranching.

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In addition, several studies also showed that hyperbranched polymer grafted

350

cellulose fiber demonstrated potential value of application in the fields of biosensors,

351

catalysts, and drug delivery systems. For example, Pohl et al. 68 reported the preparation

352

of the 6-deoxy-6-(1,2,3-triazolo)-4-hyperbranched polyamide cellulose by treating 6-

353

deoxy-6-azido cellulose dissolved in dimethyl sulfoxide (DMSO) with the amino group

354

terminated propargyl-hyperbranched polyamide dendron at ambient temperature. Khan

355

et al.

356

carboxyl functionality and incorporated them into ethyl cellulose.

357

APPLICATION OF HYPERBRANCHED POLYMER IN NATURAL PLANT

358

FIBER COMPOSITE

69

synthesized amidoimide dendrons having branched alkyl periphery and focal

359

The natural plant fiber composites can be divided into thermosetting composites

360

and thermoplastic composites by resin matrix. For the thermosetting composites, the

361

fiber and the matrix can be easily mixed uniformly for molding as the resin matrix has

362

low viscosity. For the thermoplastic composites, it is hard to uniformly mix the fiber

363

and the matrix as the resin matrix has high viscosity, particularly when the nano fiber is

364

used. The molding method selected for producing composites and their products must

365

also meet the basic requirements for material properties, product quality, economic

366

benefits etc. Usually, the hyperbranched polymers can reduce the viscosity of the

367

composite system due to their advantages of less intermolecular entanglement, low

368

viscosity etc., which is extremely conducive to molding processing. Besides, their

369

surface contains a large number of reactive terminal groups, which is very conducive to

370

the improvement of the compatibility of the interface between the fiber and the matrix.

371

Compared with the “grafting-from” method, the “grafting-to” method is more

372

advantageous in the application of composite materials due to the simple preparation

373

process. Table 3 systematically summarizes the molding processing method for the

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composite after the hyperbranched polymer is introduced to the composite system. 40, 57,

375

61-64, 70-76

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introduction of hyperbranched polymer on various properties of the natural plant fiber

377

composites.

378

Polypropylene composite

379

The following paragraphs will summarize in detail the effects of the

Lu et al.

64

synthesized a new lubricant to improve the compatibility of the

380

interface between the lignin and the polypropylene resin using the hyperbranched

381

polyamide and oleic acid. The research exhibited that the impact strength of the

382

composite increased by 52.3% after addition of the lubricant; the flexural strength and

383

the flexural modulus increased by 63.6% and 10.6%, respectively; the melt index

384

increased by 234%; the lignin fiber dispersity was also improved significantly. In

385

addition, the heat resistance of the composite was also improved. The addition of the

386

lubricant also improved the crystallinity and crystallization rate of the polypropylene.

387

In another study of Lu et al. carboxyl-terminated

70,

the authors modified a hyperbranched polyester,

388

introduced

groups,

and

prepared

389

hyperbranched polymer. Subsequently, they prepared the sisal fiber/polypropylene

390

composite with the carboxyl-terminated hyperbranched polymer as the compatilizer and

391

tested its mechanical properties. In the research, it was found that the impact strength

392

and flexural strength of the composite increased by 21.5% and 9.7%, respectively after

393

the addition of 2 wt% carboxyl-terminated hyperbranched polymer. The scanning

394

electron microscope photographs showed that the interface bonding of the composite

395

was very firm after the addition of the carboxyl-terminated hyperbranched polymer. The

396

X ray diffraction showed that the crystal structure of the polypropylene was not

397

influenced. Moreover, the carboxyl-terminated hyperbranched polymer could also

398

improve the water resistance of the composite.

16 ACS Paragon Plus Environment

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carboxyl-terminated

Page 17 of 42

399

Journal of Agricultural and Food Chemistry

Quite recently, Sun et al.

57

prepared the sisal fiber/polypropylene composite with

400

the melt blending method using the sisal fiber with its surface grafted with the

401

hyperbranched polyamide and the polypropylene resin and tested its mechanical

402

properties. The results revealed that the tensile, flexural, and impact strength of the

403

composites were improved considerably with the poly(amidoamine) dendrimer grafting

404

treatment. For the 2.0 generation treatment with the poly(amidoamine) dendrimer, the

405

tensile, flexural, and impact strength of the composites at 30 wt% fiber loading

406

increased by 29%, 13%, and 54%, respectively.

407

Polylactic acid composite

408

Wong et al. 40 used a hyperbranched polyester to toughen the composite system for

409

improving the brittleness of the flax fiber/polylactic acid composite. The research found

410

that the elongation at break of the composite reached 314% when a 50% volume

411

fraction of hyperbranched polyester was added to the composite system. The addition of

412

the hyperbranched polyester improved the interlaminar fracture toughness of the

413

composite. When a 10% volume fraction was added to the composite system, the

414

interlaminar fracture toughness increased about by one fold; when a 50% volume

415

fraction was added to the composite system, the interlaminar fracture toughness was

416

250% of the fracture toughness without the addition. The scanning electron microscope

417

photographs exhibited that the interface bonding of the composite was strengthened

418

after the addition of the hyperbranched polyester.

419

In a recent study, Moshiul Alam et al.

71

improved the compatibility of the

420

interface of the palm fiber/polylactic acid composite using a hyperbranched polyester as

421

the interface compatilizer. The infrared spectroscopy analysis showed that the

422

hyperbranched polymer reacted with the fiber or the polylactic acid to form a good

17 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

423

interface. The tensile property and impact strength of the composite were improved

424

significantly.

425

To improve the impact property of polylactic acid, Mohanty and Bhardwaj 72 used

426

the hyperbranched polyester to modify the polylactic acid and prepared the composite

427

with the hemp fiber. The research result indicated that the elongation at break of the

428

composite increased significantly compared with that of the composite subject to no

429

hyperbranched polyester modification but both the tensile strength and the tensile

430

modulus decreased somewhat.

431

Epoxy resin composite

432

Recently, Xiao et al. 61 fabricated the composite using the sisal cellulose crystallite

433

modified by the hyperbranched aromatic polyamide and the epoxy resin (Figure 4) and

434

studied the heat resistance and mechanical properties of the composite. The result

435

demonstrated that, relative to the pure epoxy resin, the impact strength, tensile strength,

436

Young's modulus, and toughness of the cellulose crystallite composite modified by the

437

hyperbranched aromatic polyamide increased by 83.4%, 34.7%, 25%, and 178.3%,

438

respectively. Moreover, the heat resistance of the composite also increased significantly.

439

The bonding of the interface between the fiber and the resin was also improved. As can

440

be seen in Figure 4, the most critical steps are the uniform introduction of the amino

441

groups onto the fiber surface by the modification of silane coupling agent and the

442

efficient grafting of the hyperbranched aromatic polyamide. Therefore, the reaction

443

efficiency of these two steps directly affects the mechanical properties of the composite.

444

In addition, it should be noted that the grafting of the hyperbranched polymer has a

445

certain negative impact on the thermal stabilities and mechanical properties of the fibers,

446

and as the grafting generation increases, the steric-hindrance effect leads to a decrease

447

in the grafting rate of the hyperbranched polymer. Therefore, in order to prepare high-

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Page 19 of 42

Journal of Agricultural and Food Chemistry

448

performance composite materials, it is necessary to control the grafting generation of

449

the hyperbranched polymer and the appropriate reaction time.

450

In the study of Lu et al.

62,

they grafted the sisal cellulose crystallite with the

451

hyperbranched biphenyl liquid crystal and used this product to modify the epoxy resin.

452

The result showed that both the mechanical properties and thermal property of the

453

composite were improved somewhat. The impact strength of the cellulose crystallite

454

composite containing about 1wt% cellulose crystallite modified with the hyperbranched

455

polymer increased by 55% compared with that of the pure epoxy resin. The thermal

456

deformation temperature increased by about 4°C.

457

Another study of Lu et al.

63

also exhibited that the sisal cellulose crystallite

458

modified by the carboxyl-terminated hyperbranched polyester had good interface

459

compatibility and bonding strength with the epoxy resin, thus significantly improving

460

the comprehensive properties of the composite. When the amount of sisal cellulose

461

crystallite modified by the hyperbranched polyester was only 5 wt% of the epoxy resin,

462

the impact strength of the composite increased to 28.3 kJ/m2, while the impact strength

463

of the pure epoxy resin was 17.5 kJ/m2. The result also showed that the initial

464

temperature for thermal decomposition increased by 15°C.

465

Urea formaldehyde resin composite

466

Essawy et al.

74

first modified the urea formaldehyde resin adhesive using the

467

hyperbranched polyamide and tested its bonding strength with the wood board. The

468

research found that the bonding strength was significantly improved with the addition of

469

the hyperbranched polyamide; its water-absorbing quality and the release amount of

470

formaldehyde also decreased significantly. Subsequently, they also modified the urea

471

formaldehyde resin adhesive using a hydroxyl-terminated hyperbranched polymer and

472

tested its bonding strength. The research also revealed that the addition of the

19 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

473

hyperbranched polymer could also improve the bonding strength and decrease the

474

water-absorbing quality and the release amount of the free formaldehyde. 75

475

To address the problem of solidification defect of the melamine urea 73

476

formaldehyde, Zhou et al.

477

formaldehyde resin using the hyperbranched polyamide of different generations,

478

prepared composites using this material and the wood fiber, and studied the bonding

479

strength and water-resistant quality of the composite. The result exhibited that the

480

bonding strength and the water-resistant quality of the composite were improved

481

somewhat after it was modified by the hyperbranched polyamide. This modification

482

method also reduced the production cost of the composite and relieved the problem of

483

environment pollution caused by the release of formaldehyde.

484

modified the synthesis process of the melamine urea

In a recent publication, Amirou et al.

76

synthesized three hyperbranched

485

polyamides of different structures, used them to modify the melamine urea

486

formaldehyde resin, fabricated the composite with this material and the wood board,

487

and studied the bonding strength and water-resistant quality of the composite. The test

488

results showed that the hyperbranched polyamides of different structures had different

489

bonding strength and water-resistant quality for the composite. Reasonable control of

490

the structure of the hyperbranched polymer could obtain composites with excellent

491

comprehensive properties.

492

In summary, compared with the synthetic fiber, the natural plant fiber has complex

493

surface characteristics, poor heat resistance, and polydispersity of mechanical

494

properties, which would make the graft modification of the hyperbranched polymers

495

more complex. Preparing natural plant fiber composites of high performance is an

496

important measure for expanding its application field. The improvement of the interface

497

bonding degree of the composite has become one of its key issues. The hyperbranched

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Page 21 of 42

Journal of Agricultural and Food Chemistry

498

polymers have been increasingly widely used in the material modification technique due

499

to the existence of abundant active end groups. Grafting different types of

500

hyperbranched polymers onto the plant fiber surface can introduce a large number of

501

active groups and create more regulation space between the fiber and the resin matrix,

502

thus making it possible to fabricate composites that meet various requirements.

503

In the future, first, the fiber modification technology should still be vigorously

504

developed to gradually improve the heat resistance of natural plant fibers and reduce the

505

cost of modification by the hyperbranched polymer. Second, the types of hyperbranched

506

polymers should be developed and the steric-hindrance effect of hyperbranched

507

polymers should be reduced by suitable technique. Finally, attentions should be paid to

508

the development of compatibilizers based on the functionalization of hyperbranched

509

polymers to improve the mechanical properties of composites, especially the impact

510

properties of composites.

511 512

FUNDING

513

This research was supported by the National Natural Science Foundation of China

514

under Grant [31300475] and the Natural Science Foundation of Hebei Province under

515

Grant [E2016208083].

516 517

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fibers grafted with hyperbranched poly(3-methyl-3-oxetanemethanol). Cellulose

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composites modified with carboxyl terminated hyperbranched polymer.

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Plast., Rubber Compos. 2013,42,361-366.

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(71) Moshiul Alam, A. K. M.;Beg, M. D. H.;Reddy Prasad, D. M.;Khan, M. R.;Mina,

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M. F. Structures and performances of simultaneous ultrasound and alkali treated oil

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palm empty fruit bunch fiber reinforced poly(lactic acid) composites. Composites

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Part A 2012,43,1921-1929.

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(72) Mohanty, A.; Bhardwaj ,R. Hyperbranched polymer modified biopolymers, their

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biobased materials and process for the preparation thereof. U.S.Patent 7,579,413

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B2, August 25, 2009.

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(73) Zhou, X.;Essawy, H. A.;Pizzi, A.;Li, X.;Pasch, H.;Pretorius, N.;Du, G.

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Poly(amidoamine)s dendrimers of different generations as components of

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melamine urea formaldehyde (MUF) adhesives used for particleboards production:

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what are the positive implications? J. Polym. Res. 2013,20,267.

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urea-formaldehyde wood adhesive system using dendritic poly(amidoamine)s and

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their corresponding half generations. J. Appl. Polym. Sci. 2009,114,1348-1355.

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urea formaldehyde wood adhesive system using different generations of core-shell

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modifiers based on hydroxyl-terminated dendritic poly(amidoamine)s. J. Appl.

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30 ACS Paragon Plus Environment

Page 30 of 42

Page 31 of 42

Journal of Agricultural and Food Chemistry

747 748

FIGURE CAPTIONS

749

Figure 1. Surface grafting hyperbranched polyamide onto sisal fiber. 57 © Taylor &

750

Francis. Reproduced by permission of Taylor & Francis. Permission to

751

reuse must be obtained from the rights holder.

752

Figure 2. Grafting of silver nanoparticle onto cotton fiber by hyperbranched polyamide.

753

53

754

must be obtained from the rights holder.

755

© Elsevier. Reproduced by permission of Elsevier. Permission to reuse

Figure 3. Surface grafting multihydroxyl hyperbranched polyether onto cellulose fiber.

756

67

757

must be obtained from the rights holder.

758

© Springer. Reproduced by permission of Springer. Permission to reuse

Figure 4. Schematic diagram for fabricating composites. 61 © RSC Publishing.

759

Reproduced by permission of RSC Publishing. Permission to reuse must be

760

obtained from the rights holder.

761 762

TABLE CAPTIONS

763

Table 1. Mechanical properties of natural and synthetic fibers. 12

764

Table 2. Effect of hyperbranched polymer on the modification of different

765 766 767

reinforcement. Table 3. Method for preparing natural fiber composites with the hyperbranched polymer added.

768 769 770 771

31 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

772

Page 32 of 42

Table 1. Mechanical properties of natural and synthetic fibers. 12 Fiber

Density

Diameter

Tensile

Young’s

Elongation at

(g/cm3)

(μm)

strength

modulus (GPa)

break (%)

(MPa) Flax

1.5

40–600

345–1500

27.6

2.7–3.2

Hemp

1.47

25–500

690

70

1.6

Jute

1.3–

25–200

393–800

13–26.5

1.16–1.5

1.49 Kenaf





930

53

1.6

Ramie

1.55



400–938

61.4–128

1.2–3.8

Sisal

1.45

50–200

468–700

9.4–22

3.0–7.0

PALF



20–80

413–1627

34.5–82.5

1.6

Abaca





430–760





Oil palm

0.7–

150–500

248

3.2

25

EFB

1.55

Cotton

1.5-1.6

12–38

287–800

5.5–12.6

7.0–8.0

Coir

1.15–

100–460

131–220

4–6

15–40

1.46 E-glass

2.55