A primer on natural fibers: Sustainable reinforcements for plastics

The use of natural fibres as reinforcing fillers in plastics is becoming increasingly popular among plastics processors, compounders and their customers, within the European Union and around the world. This trend has been driven by environmental concerns, increased recyclability and a move towards more sustainable products and processes. This paper, offered to PlasticsToday by Vanessa Gutiérrez Aragonés, one of the plastics experts in the compounding department of Spain's Aimplas plastics technical institute, offers insights into the fibers in use, their advantages and disadvantages, as well as some tips on compounding and processing plastics reinforced with these fibers.

Natural fibers: A definition for plastics processors

Natural fibers can be classified according to their origin, which can be vegetable, animal or mineral. The fibers cited in this paper used in the plastics industry come from vegetable sources, and most of them are part of agricultural wastes. They are composed of microfilaments of cellulose (70-75%) and hemicellulose (15-20%), bonded together by a matrix that can be pectin or lignin (3%). The later components degrade at a relatively low temperature, which limits the use of these fibers in thermoplastics.

The natural fibers currently employed to reinforce plastics are:         

Kenaf (Hibiscus cannabinus)

Hemp (Cannabis sativa)

Sisal (Agave sisalana)

Cotton (Gossypium hirsutum)

Many other fibers are also used, such as rice husk, jute (Corchorus olitorius) or flax (Linum usitatissimum), and even coconut, banana and pineapple [Ed. Note: Read our recent article on the use of coconut fibers to reinforce plastic parts in Ford vehicles.]. All of these fibers are highly sensitive to heat and some of them can only be compounded with polymers that have a low processing temperature, such as LDPE (low density polyethylene) and TPS (thermoplastic starch).

The degradation temperatures of some of the fibers mentioned above are presented in Table 1. You'll note that cotton, kenaf and hemp fibers have the higher resistant to temperature. However, the use of these fibers in thermoplastic compounds is still limited to medium processing temperature polymers, such as HDPE (high density polyethylene), PP (polypropylene) and PLA (polylactic acid). The use of natural fibers in engineering thermoplastics is not possible yet, since the processing temperature of these plastics typically exceeds 250ºC. At temperatures close to 200ºC the degradation of the lignin is already appreciable through a dark coloration of the final material and a smell like burned wood.

The degradation of the fibers can occur either during compounding or in the later transformation processes, such as injection molding or extrusion. For this reason there has to be careful control of the processes parameters, especially temperature and shear rate.

Table 1. Degradation temperatures of some natural fibers

Reinforcing plastics with natural fibers

Here we'll consider plastics derived from petrochemicals and those based on renewable-resource materials. The main difficulty when compounding oil-based plastics, such as polyolefins, with natural fibers is to achieve a proper wetting of the fiber. Only if there is a good adhesion between the polymer and the fibers can the reinforcing effect be achieved. Due to the high processing viscosity of the polymers and their non-polar nature, this wetting often proves difficult. Nevertheless, given the relatively low price and reasonable processing temperatures of commodity thermoplastic such as PE and PP (virgin or recycled), there is a growing interest in using them as the matrix polymer for natural fibers. Unmodified polyolefins, however, will not have a proper adhesion with the fibers by applying consolidation forces alone.

Natural fibers will only act as reinforcement if compatibilizers are used. An interface between the fiber and matrix should correct the natural incompatibility of both materials, which is due to the hydrophilic nature of the natural fibers and the hydrophobic nature of the polymer. A commonly used compatibilizer is the respective polyolefin modified with maleic anhydride. A small amount of this compatibilizer, added to the polymer when compounding it with the fiber, will lead to much higher strength in the resulting material. Other types of coupling agents used as a polyolefins' compatibilizer with natural fibers are organosilanes and organotitanates, but these usually are more expensive because they are added directly to the fiber surface by means of some previous process.

Bio-based plastics reinforced with natural fibers

A clear trend in many applications and across a variety of industries is the preference of bio-based or biodegradable plastics over traditional non-biodegradable oil-based plastics, with environmental reasons so far the strongest driver of this trend. Aliphatic polyester PLA (polylactic acid) is the most commonly used bioplastic because of its availability, processability and relative low price; however, this plastic lacks good mechanical properties and a reinforcing fiber is needed for many applications. If a synthetic fiber is used to reinforce it, i.e., glass fibers, the biodegradability of the composite will not be achieved and some potentially harmful product (glass fiber) will remain in landfills. Using natural fibers as reinforcement means the biodegradability of the compound is not permanently impacted.

The main advantage of using aliphatic polyester as a matrix is its inherent polarity, which eliminates the need for a compatibilizer. But, on the other hand, these polymers trend to degrade very easily during processing, especially in the presence of moisture, which can be up to 5% in many natural fibers.

Thermoplastic starch is also employed as a matrix for compounding with natural fibers, due to its low processing temperature and high hydrophilicity, which makes it very compatible with the fibers.

Tips for successful compounding of natural fibers

The compounding process is usually done in a co-rotating twin-screw extruder, adding the polymer through the main hopper and the fiber through a side feeder, after the melting zone. One of the biggest issues during compounding is feeding the fibers into the extruder. This is cause by two main problems:

The fibers have a very low apparent density, and as a result are very difficult to feed into the extruder.

The high agglomerating tendency of the fibers, due to the presence of strong hydrogen bonds, reduces the effective specific surface of the fibers and their distribution in the polymeric matrix.

One way to increase the apparent density of the fibers is to pelletize them. In the new form the fibers can be fed into the extruder using conventional feeding systems. But not all fibers can be pelletized; for instance rice husk disintegrate due to their extreme fragility, resulting in a powder that can't be densified.

Once inside the extruder, the pelletized fibers have to be dispersed and distributed in the polymer matrix in order to realize good properties and appearance in the final material. The dispersion of the fibers is achieved through the shear forces caused by the screws of the extruder. The amount of dispersion and distribution of the fibers is regulated by the configuration of the individual screw elements in the extrusion barrel. When using a high shear configuration on the screw lights, good dispersion of the fibers can be obtained, but too much shear can break the fibers and even cause degradation.

The fibers not only modify the mechanical properties of the polymer, but also modify its rheology, increasing the viscosity of the melt. This increase can be valued if the final compound is going to be extruded into profiles, in which a high viscosity is required. For this reason polymers with medium melt flow index have to be used (≈ 5 gr/10min), in order to achieve a good wetting of the fiber as well as a final compound that can be processed via extrusion or thermoforming. When compounding with higher MFI polymer (> 8 gr/10min), poor melt strength is observed, i.e., the melt strand is broken when it is stretched. However, if the compound will be used for injection molding, then this high rate is necessary.

The increase in the melt viscosity will also depend on the type and amount of fiber used. For instance, when compounding a modified starch with 10% reinforcement of flax fiber, the viscosity is increased 146% compared to the uncompounded starch. But if 10% hemp reinforcement is used, the melt viscosity is increased by 353%.

How much fiber to use?

The amount of fiber admissible in the polymer will be determined by several factors:

Post-processing of the polymer/fiber compound. In this case, for low MFI processes, e.g., profile extrusion, the fiber content can be up to 30%.

Type of polymer matrix. If the polymer and the fiber are not compatible, then the fiber content is limited by the amount and efficiency of coupling agent used.

Final application of the compound. The amount of fiber added to the polymer will determine the level of improvement of the mechanical properties of the final product, but up to a limit. Only a certain amount of reinforcement is achievable, so over a critical content the fiber will not act as a reinforcing agent but as filler. Once this happens, the additional fiber can even negatively affect the mechanical properties of the material, due to a higher interaction between the fibers and a subsequently lower interaction between the polymer matrix and the fibers.

Pros & Cons of natural fibers

As stated earlier, there are many good reasons to use natural fibers as reinforcement agents in thermoplastic polymers. However, there also are several challenges when compounding with these materials. This pros and cons are summarized in table 2.

Table 2. Pros & Cons of compounding with natural fibers