**3.1 Definition**

222 Thermoplastic Elastomers

interactions among factors. Both methods are extensively explored in recent research work (Zhao et al., 2003; Liou & Chen, 2006; Michaeli et al., 2007b; Sha et al., 2007b; Theilade &

Variotherm mould cycling along with the high injection temperature could significantly improve the weld line strength of the micro tensile test samples (Theilade & Hansen, 2007). Moreover, at higher mould temperature viscosity of the polymer melt significantly decreases and therefore requires less injection pressure and speed (Despa et al., 1999). Even in the absence of the variotherm cycling, higher mould temperature was reported to promote better cavity filling for a wide range of the tested polymers (Zhao et al., 2003; Sha et al., 2005; Michaeli et al., 2007b; Sha et al., 2007a; Sha et al., 2007b; Tofteberg & Andreassen, 2008). In the case of over-molded micro needles, an increase of bonding strength has been accomplished by the combination of the high holding pressure, high mould and low melt temperature (Michaeli et al., 2007b). An attempt of the empirical correlation of the part quality with process parameters has been proposed by Tofteberg & Andreassen (2008). In their study, replication of the micro features of Cyclic Olefin Copolymer (COC) and Poly(methyl methacrylate) (PMMA) has been significantly improved with an increase of the mould and melt temperature. It is also worth mentioning that with an increase of the micropart complexity the mould temperature factor was found out to prevail over the other

Premature solidification of polymer may be reduced, to some extent, by injection at high injection speed. The latter is reported to assist in filling of the micro pins, having on the other hand, an adverse effect on the surface finishing (Sha et al., 2007a). The positive effect of the injection speed has also been confirmed by the other researchers. High injection speed was decisive for high-quality replication of the micro walls as well as its interaction with the injection temperature (Theilade & Hansen, 2007). When moulded with cold runners, it is not rare to encounter that volume of the feeding system may contain several times the volume of the micropart. Such discrepancies make precise metering unlikely, increasing the probability of incomplete filling and invalidating the holding pressure effect (Zhao et al., 2003). Nonetheless, some difficulties related to the metering precision and process fluctuations can be attenuated by applying higher holding pressure (Liou & Chen, 2006). Although with the holding pressure increases improvement of filling the micro and submicron structures has been reported, it seems uneasy to differentiate single holding pressure

influence from its interaction with the mould temperature effect (Liou & Chen, 2006).

their new properties will eventually influence rheological behavior at micro scale.

In the recent research, discrepancies in rheological behavior of the different grades of plastics have been widely reported. In order to guarantee the proper filling and acceptable surface quality, the easy flow grades should be preferred (Despa et al., 1999; Zhao et al., 2003; Michaeli et al., 2007b; Tofteberg & Andreassen, 2008). Adhesion forces between polymer and mould tool may vary significantly for different polymer grades and should be accounted for successful demolding of the microparts. The latter may be achieved by fine surface finishing of the mould cavity, positive draft angles and by using release agents (Wu & Liang, 2005; Grave et al., 2007). The variety of factors involved into the transformation of the polymer melt within the microcavity makes the interpretation of the cause-effect relationship uneasy task even for neat (unfilled) thermoplastics. Furthermore this relationship may become substantially more complicated for enhanced thermoplastics as

Hansen, 2007; Tofteberg & Andreassen, 2008).

process parameters (Attia et al., 2009).

Thermoplastics are long chain polymers than can be structurally amorphous or semicrystalline. These polymers possess long chains, where macromolecules are bonded through the weak van der Waals forces. Their general properties are toughness, resistance to chemical attack and recyclability, i.e. they can be re-processed as many times as needed, till their degradation, due to processing. Despite that the continuum search for better and cheaper materials has been guiding the scientists to the development of new thermoplastics with enhanced properties, such as better resistance to water and UV; better mechanical properties (toughness, stiffness); and enhanced electrical conductivity. These new class of thermoplastics are known as "enhanced thermoplastics".

There are different techniques to manipulate polymeric materials in order to obtain enhanced properties, such as: (a) by modifying their molecular structure, the hard and soft segments. Through this technique, the molecular structure of the polymer is changed by different combinations of: chain flexibility and hard segments, chain entanglement, the orientation of different segments, the hydrogen bonds and their intermolecular interactions. Through the modifications of the hard/soft segments ratios of the polymer, it is possible to obtain different physical, thermal and mechanical properties (Chattopadhyay & Raju 2007); (b) the incorporation of plasticizers, which are substances, usually plastics or elastomers, that are incorporated in a material in order to increase its flexibility, workability or extensibility, and modify the thermal and mechanical properties (Wang et al. 1997; Rahman & Brazel 2006); (c) throughout the incorporation of particles into a polymeric matrix (Tjong 2006). Some of the most commonly used reinforced particles are: CaCO3, glass, carbon fibres, and in the last years, carbon nanotubes have been frequently used due to their extraordinary thermal, electrical and mechanical properties (Wang et al., 1998; Coleman et al., 2004).

### **3.2 The effect of incorporating carbon nanotubes (CNTs) into thermoplastic materials**

The incorporation of carbon nanotubes (CNTs) in thermoplastic materials has been one of the hottest topics in materials science in the last years, since their discovery by Iijima (1991), essentially due to the extraordinary properties of CNTs

CNTs can be classified has single-walled carbon nanotubes (SWCNT), which consist on a single grapheme sheet wrapped into a cylindrical shape and are characterized by a small diameter (0.4 - 3 nm) and lengths up to centimeters; and multi-walled carbon nanotubes (MWCNT) that detain a number of grapheme layers coaxially rolled together to form a cylindrical tube, and their outer diameter ranges from 1.3 - 100 nm and their length can be as long as tens of micrometers (Baughman et al., 2002). It has been reported that CNTs possess an elastic modulus in a range of TPa (Yu et al., 2000) and detain high aspect ratios (>100),

Microinjection Molding of Enhanced Thermoplastics 225

method has the advantage of the agitation of the CNTs powder in a solvent that could facilitate the de-aggregation and dispersion. The in-situ polymerization has been used because the obtained composite presents a good dispersion of CNTs into polymeric matrixes. This method is particularly important for the preparation of insoluble and thermally instable polymers that cannot be processed by solution or melt processing. The main disadvantages of this method are the time consumed and the complexity involved in the process to obtain the composite material. The melt processing method is commonly used for thermoplastic polymers. The advantages of this technique are the speed and simplicity, and essentially, its compatibility with standard industrial techniques (Andrews et al., 2002;

The melt mechanical technique is characterized for the melting of the thermoplastic and then the addition of CNTs by shear mixing. However, the processing conditions should be optimized for each thermoplastic and desired composite, because CNTs can affect melt properties, of the pure thermoplastic, such as viscosity (Potschke et al., 2003). As it was mentioned earlier, Fonseca et al. (2011) has prepared ultra-high molecular weight polyethylene (UHMWPE) reinforced with CNTs through a variation of the melt mechanical processing technique. Previously to melt processing, they used the mechanical ball-milling to mix the raw UHMWPE with the chemically treated CNTs. After mixing the powder composite was processed by compression molding technique. The results have shown a homogenous distribution of the CNTs into the polymeric matrix (before and after melt processing), and an enhancement of the main mechanical properties, such as the elastic

The incorporation of CNTs in polymeric matrices has been extensively studied in last years. Different polymeric matrices have been considered, such as PMMA (Jia et al., 1999; Gorga & Cohen, 2004), polystyrene (PS) (Qian et al., 2000; Yang et al., 2005), polypropylene (PP) (Dondero & Gorga, 2006; Bao & Tjong, 2008), polyurethanes (PU) (Koerner et al., 2005; Xiong et al., 2006; Zhang et al., 2011) and polyethylene (PE; high density polyethylene - HDPE, ultrahigh molecular weight polyethylene - UHMWPE) (Ruan et al., 2003; McNally et al.,

In 1998, Schadler et al. (1998) studied the effects of the dispersion of 5 wt% of MWCNT in an epoxy resin, and the results have shown an increment of the modulus in tension and in compression of about 20% and 24%, respectively. Jia et al. (1999) prepared PMMA/CNT composites with different weight fractions of CNTs. The composites were prepared by an improved in-situ process and the results reveal an enhancement of the mechanical properties as well as an increase of the heat deflection temperature, as the CNT loading increases. However, for weight percentages higher than 7% the mechanical properties of the PMMA/CNT composite starts to decrease and for 10 wt% the composite becomes brittle. Later, Gorga & Cohen (2004) presented results consistent with the previous one. They studied the mechanical properties of MWCNT/PMMA as a function of nanotube orientation, length, concentration and type. The results reveal a good dispersion of the MWCNT into PMMA matrix for weight percentages ranging from 0.1 – 10 wt%, although, for loadings higher than 5 wt% there was evidence of aggregation. The orientation of MWCNT in PMMA seems to be the better way to improve the toughness of the material. Qian et al. (2000) mixed 1 wt% MWCNT to polystyrene (PS) and the results have shown an increase of about 36 - 42% in elastic modulus and about 25 % in the tensile strength, for the

Potschke et al., 2003).

modulus and toughness.

2005; Kanagaraj et al., 2007; Fonseca et al., 2011).

due to it small diameter (in the range of nanometers) and long length (as long as 100 micrometers). However, the incorporation of CNTs into a polymeric material is quite difficult due to the Van der Waals forces that tend to clump nanotubes together, leading to a poor dispersion of them into polymeric matrix. For an effective reinforcement there are essentially four important requirements: 1) the CNTs must have high aspect ratio; 2) they must be uniformly dispersed; 3) the alignment of CNTs through a preferable direction, and 4) should be verified interfacial stress transference. The distribution of CNTs into polymeric matrix could be the most important parameter to obtain an enhanced thermoplastic reinforced with CNTs. The CNTs must be individually dispersed and should the coated by polymeric matrix, in order to achieve efficient load transfer to the nanotube network, which is of utmost importance. The aggregation of CNTs is generally accompanied by a decrease in strength and modulus.

In order to improve the dispersion of the CNTs, and to prevent the agglomeration, many techniques has been developed, such as, ultrasonic activation, the addition of surfactants and chemical treatment with surfuric and nitride acids (Esumi et al., 1996; Gong et al., 2000; Deng et al., 2002; Safadi et al., 2002). The present authors applied a chemical treatment, with sulphuric and nitride acids in a proportion of (3:1) to modify the surface of CNTs and the results has shown that CNTs maintain their physical integrity (diameter, length) and that they are no longer entangled as previously to chemical treatment (Fig.4).

Fig. 4. Scanning electron microscopy of the (a) non-treated CNTs and (b) chemically treated CNTs.

There are essentially three methods to incorporate CNTs into polymeric matrixes: 1) solution mixing or film casting of suspensions of CNTs in dissolved polymers; 2) in situ polymerization of CNTs-polymer monomer mixture; and 3) melt mechanical mixing of CNTs with polymer (Jia et al., 1999; Haggenmueller et al., 2000; Jin et al., 2002). Recently, Fonseca et al. (2011) proposed a simpler technique based upon the general methodology described by method 3, where the CNTs and polymeric matrix are prepared by mechanical ball-milling.

The solution mixing is probably the most applied method to produce polymer nanotube composites. The basis of this method is the mixing of the CNTs and the polymer in a suitable solvent before its evaporation occurs and, thus, forming a composite film. This

due to it small diameter (in the range of nanometers) and long length (as long as 100 micrometers). However, the incorporation of CNTs into a polymeric material is quite difficult due to the Van der Waals forces that tend to clump nanotubes together, leading to a poor dispersion of them into polymeric matrix. For an effective reinforcement there are essentially four important requirements: 1) the CNTs must have high aspect ratio; 2) they must be uniformly dispersed; 3) the alignment of CNTs through a preferable direction, and 4) should be verified interfacial stress transference. The distribution of CNTs into polymeric matrix could be the most important parameter to obtain an enhanced thermoplastic reinforced with CNTs. The CNTs must be individually dispersed and should the coated by polymeric matrix, in order to achieve efficient load transfer to the nanotube network, which is of utmost importance. The aggregation of CNTs is generally accompanied by a decrease in

In order to improve the dispersion of the CNTs, and to prevent the agglomeration, many techniques has been developed, such as, ultrasonic activation, the addition of surfactants and chemical treatment with surfuric and nitride acids (Esumi et al., 1996; Gong et al., 2000; Deng et al., 2002; Safadi et al., 2002). The present authors applied a chemical treatment, with sulphuric and nitride acids in a proportion of (3:1) to modify the surface of CNTs and the results has shown that CNTs maintain their physical integrity (diameter, length) and that

Fig. 4. Scanning electron microscopy of the (a) non-treated CNTs and (b) chemically treated

There are essentially three methods to incorporate CNTs into polymeric matrixes: 1) solution mixing or film casting of suspensions of CNTs in dissolved polymers; 2) in situ polymerization of CNTs-polymer monomer mixture; and 3) melt mechanical mixing of CNTs with polymer (Jia et al., 1999; Haggenmueller et al., 2000; Jin et al., 2002). Recently, Fonseca et al. (2011) proposed a simpler technique based upon the general methodology described by method 3, where the CNTs and polymeric matrix are prepared by mechanical

The solution mixing is probably the most applied method to produce polymer nanotube composites. The basis of this method is the mixing of the CNTs and the polymer in a suitable solvent before its evaporation occurs and, thus, forming a composite film. This

they are no longer entangled as previously to chemical treatment (Fig.4).

strength and modulus.

CNTs.

ball-milling.

method has the advantage of the agitation of the CNTs powder in a solvent that could facilitate the de-aggregation and dispersion. The in-situ polymerization has been used because the obtained composite presents a good dispersion of CNTs into polymeric matrixes. This method is particularly important for the preparation of insoluble and thermally instable polymers that cannot be processed by solution or melt processing. The main disadvantages of this method are the time consumed and the complexity involved in the process to obtain the composite material. The melt processing method is commonly used for thermoplastic polymers. The advantages of this technique are the speed and simplicity, and essentially, its compatibility with standard industrial techniques (Andrews et al., 2002; Potschke et al., 2003).

The melt mechanical technique is characterized for the melting of the thermoplastic and then the addition of CNTs by shear mixing. However, the processing conditions should be optimized for each thermoplastic and desired composite, because CNTs can affect melt properties, of the pure thermoplastic, such as viscosity (Potschke et al., 2003). As it was mentioned earlier, Fonseca et al. (2011) has prepared ultra-high molecular weight polyethylene (UHMWPE) reinforced with CNTs through a variation of the melt mechanical processing technique. Previously to melt processing, they used the mechanical ball-milling to mix the raw UHMWPE with the chemically treated CNTs. After mixing the powder composite was processed by compression molding technique. The results have shown a homogenous distribution of the CNTs into the polymeric matrix (before and after melt processing), and an enhancement of the main mechanical properties, such as the elastic modulus and toughness.

The incorporation of CNTs in polymeric matrices has been extensively studied in last years. Different polymeric matrices have been considered, such as PMMA (Jia et al., 1999; Gorga & Cohen, 2004), polystyrene (PS) (Qian et al., 2000; Yang et al., 2005), polypropylene (PP) (Dondero & Gorga, 2006; Bao & Tjong, 2008), polyurethanes (PU) (Koerner et al., 2005; Xiong et al., 2006; Zhang et al., 2011) and polyethylene (PE; high density polyethylene - HDPE, ultrahigh molecular weight polyethylene - UHMWPE) (Ruan et al., 2003; McNally et al., 2005; Kanagaraj et al., 2007; Fonseca et al., 2011).

In 1998, Schadler et al. (1998) studied the effects of the dispersion of 5 wt% of MWCNT in an epoxy resin, and the results have shown an increment of the modulus in tension and in compression of about 20% and 24%, respectively. Jia et al. (1999) prepared PMMA/CNT composites with different weight fractions of CNTs. The composites were prepared by an improved in-situ process and the results reveal an enhancement of the mechanical properties as well as an increase of the heat deflection temperature, as the CNT loading increases. However, for weight percentages higher than 7% the mechanical properties of the PMMA/CNT composite starts to decrease and for 10 wt% the composite becomes brittle. Later, Gorga & Cohen (2004) presented results consistent with the previous one. They studied the mechanical properties of MWCNT/PMMA as a function of nanotube orientation, length, concentration and type. The results reveal a good dispersion of the MWCNT into PMMA matrix for weight percentages ranging from 0.1 – 10 wt%, although, for loadings higher than 5 wt% there was evidence of aggregation. The orientation of MWCNT in PMMA seems to be the better way to improve the toughness of the material. Qian et al. (2000) mixed 1 wt% MWCNT to polystyrene (PS) and the results have shown an increase of about 36 - 42% in elastic modulus and about 25 % in the tensile strength, for the

Microinjection Molding of Enhanced Thermoplastics 227

increase in strain energy density of about 150% for the composites as compared with pure UHMWPE. They also reported an increase of about 140% in ductility and up to 25% in tensile strength. An analysis by nanoindentation and atomic force microscopy (AFM) of UHMWPE/MWCNT composites has been reported by Wei et al. (2006). They have observed a decrease of the friction coefficient with MWCNT content increase. Recently, Fonseca et al. (2011) prepared UHMWPE/CNT composites, with different volume fractions of CNTs, through mechanical ball-milling and processed by compression molding. In this work, for this specific materials and mould geometry, it was optimized the time of mixture, in order to obtain an homogeneous distribution of CNT into polymeric matrix, as well as the processing conditions. The results reveal that mechanical ball-milling is well suited for mix the CNTs with UHMWPE. CNTs were well dispersed and did no loose their physical integrity (as can be shown in Fig. 5). These scanning electron microscopy (SEM) pictures shown different CNT/HDPE powder composites mixed for different time (15, 45 and 60 min). Though the SEM pictures and the tensile tests, it was observed that the optimized time of mixture was for 45 min. To process the mixed composites, it was used the compression molding technique, and the cycle was optimized (Fig. 6 (a)). The SEM analysis of the processed composites has shown that CNTs were well dispersed into the polymeric matrix (Fig. 6(b)). The tensile tests of these composites have shown an enhancement on the mechanical properties of about 20% on the elastic modulus, for 0.2 % vol. of CNTs, and about 80% for higher concentrations (0.4 to 1% vol.). Similar results were reported by other groups (Wang

Fig. 5. SEM pictures of the 0.2% vol. MWCNT composite mixed for 15, 45 and 60 min.

Fig. 6. (a) Optimized compression molding cycle applied for processing the composites; (b) SEM picture of the processed CNT/UHMWPE composite with 1% vol. fraction of CNTs.

The round circle gives evidence for the presence of the CNTs.

et al., 2005; Mierczynska et al., 2007).

PS/MWCNT composite as compared with pure PS. Yang et al. (2005) obtained an enhancement in microhardness, of about 40%, for the composites of PS/CNT with CNT loading lower than 1.5 wt%. They also observed a considerably decreasing in wear rate and friction coefficient as CNT loading increases. Dondero & Gorga (2006) studied the mechanical properties and the morphology of the MWCNT/PP composites, prepared by melt mixing, as a function of nanotube orientation and concentration. The results reveal an enhancement in toughness and in modulus of about 32% and 138%, respectively, for 0.25 wt% CNT loading, as compared with pure PP. The effects of loading rate of CNT on thermal and mechanical properties was studies by Bao & Tjong (2008), for PP/CNT composites. The x-ray diffraction (XRD) reveals that the presence of CNT did not influence the crystal structure of PP/CNT composites and differential scanning calorimetry (DSC) shows that the glass transition (Tg) and the activation energy (ΔE) increases with the increase of the amount of CNTs, demonstrating that the mobility od the polymer chains is reduced with the presence of CNTs. The tensile tests reveal an enhancement on Young's modulus from 1570 MPa for pure PP to 2107 MPa for 0.3 wt% of CNTs loading.

The incorporation of CNTs into polyurethanes (PU) has been studied by many groups. Koerner et al. (2005) incorporate different volume fractions of CNTs (from 0.5 to 10 vol.%) into thermoplastic PU. The results reveal an enhancement in electrical conductivity, and in mechanical properties including the modulus and yield stress. Chen et al. (2006) reported an enhancement in elastic modulus, from 4.96 MPa for pure PU to 135.1MPa for the composite with higher weight fraction of CNTs (17.7 wt%). On the other hand, the maximum for tensile strength was occurred for the composite with 9.3 wt% of CNT, decreasing for higher concentrations, which can be ascribed to the increased frequency of localized clusters or aggregations. A study on the thermal properties of the PU/CNT composites was performed by Xiong et al. (2006) and the results reveal an increase on the glass transition temperature of the composite of about 12ºC, as compared with pure PU. Results of the tensile testes also reveal an improvement of the mechanical properties of the composites as compared with pure polymer.

The reinforcement of polyethylene (PE, HDPE, UHMWPE) with CNTs is probably the most studied issue in what concerns to the enhancement of thermoplastics, due to its enormous range of applications. McNally et al. (2005) reinforced PE with CNTs with weight fractions ranging from 0.1 to 10 wt% by melt blending. The results have shown that the addition of CNTs did not affect the temperature of melting (Tm) of the PE, however, the temperature of crystallization (Tc) increases for the composite with 10 wt% of CNTs. This indicates that the CNT have a nucleation effect on PE. These composites have shown a decreasing in toughness with increased CNT addition, which can be associated with the nucleation of CNTs into polymeric matrix. On the other hand, Kanagaraj et al. (2007) has shown a good load transfer effect and interface link between CNTs and high density polyethylene (HDPE). The results of tensile tests have shown a linear increase of Young's modulus (with maximum of about 22% for a volume fraction of 0.44 % CNT). This linearity is suggested to be due to a good load transfer effect and interface link between CNT and HDPE. The thermal analysis reveals that the melting point and oxidation temperatures of CNT/HDPE composites are not affected by the addition of CNTs, although the results reveals an increasing on crystallinity of the composites. Ruan et al. (2003) reported an enhancement of toughness in UHMWPE films with the addition of 1 wt% MWCNT. Their results reveal an

PS/MWCNT composite as compared with pure PS. Yang et al. (2005) obtained an enhancement in microhardness, of about 40%, for the composites of PS/CNT with CNT loading lower than 1.5 wt%. They also observed a considerably decreasing in wear rate and friction coefficient as CNT loading increases. Dondero & Gorga (2006) studied the mechanical properties and the morphology of the MWCNT/PP composites, prepared by melt mixing, as a function of nanotube orientation and concentration. The results reveal an enhancement in toughness and in modulus of about 32% and 138%, respectively, for 0.25 wt% CNT loading, as compared with pure PP. The effects of loading rate of CNT on thermal and mechanical properties was studies by Bao & Tjong (2008), for PP/CNT composites. The x-ray diffraction (XRD) reveals that the presence of CNT did not influence the crystal structure of PP/CNT composites and differential scanning calorimetry (DSC) shows that the glass transition (Tg) and the activation energy (ΔE) increases with the increase of the amount of CNTs, demonstrating that the mobility od the polymer chains is reduced with the presence of CNTs. The tensile tests reveal an enhancement on Young's modulus from 1570

The incorporation of CNTs into polyurethanes (PU) has been studied by many groups. Koerner et al. (2005) incorporate different volume fractions of CNTs (from 0.5 to 10 vol.%) into thermoplastic PU. The results reveal an enhancement in electrical conductivity, and in mechanical properties including the modulus and yield stress. Chen et al. (2006) reported an enhancement in elastic modulus, from 4.96 MPa for pure PU to 135.1MPa for the composite with higher weight fraction of CNTs (17.7 wt%). On the other hand, the maximum for tensile strength was occurred for the composite with 9.3 wt% of CNT, decreasing for higher concentrations, which can be ascribed to the increased frequency of localized clusters or aggregations. A study on the thermal properties of the PU/CNT composites was performed by Xiong et al. (2006) and the results reveal an increase on the glass transition temperature of the composite of about 12ºC, as compared with pure PU. Results of the tensile testes also reveal an improvement of the mechanical properties of the composites as compared with

The reinforcement of polyethylene (PE, HDPE, UHMWPE) with CNTs is probably the most studied issue in what concerns to the enhancement of thermoplastics, due to its enormous range of applications. McNally et al. (2005) reinforced PE with CNTs with weight fractions ranging from 0.1 to 10 wt% by melt blending. The results have shown that the addition of CNTs did not affect the temperature of melting (Tm) of the PE, however, the temperature of crystallization (Tc) increases for the composite with 10 wt% of CNTs. This indicates that the CNT have a nucleation effect on PE. These composites have shown a decreasing in toughness with increased CNT addition, which can be associated with the nucleation of CNTs into polymeric matrix. On the other hand, Kanagaraj et al. (2007) has shown a good load transfer effect and interface link between CNTs and high density polyethylene (HDPE). The results of tensile tests have shown a linear increase of Young's modulus (with maximum of about 22% for a volume fraction of 0.44 % CNT). This linearity is suggested to be due to a good load transfer effect and interface link between CNT and HDPE. The thermal analysis reveals that the melting point and oxidation temperatures of CNT/HDPE composites are not affected by the addition of CNTs, although the results reveals an increasing on crystallinity of the composites. Ruan et al. (2003) reported an enhancement of toughness in UHMWPE films with the addition of 1 wt% MWCNT. Their results reveal an

MPa for pure PP to 2107 MPa for 0.3 wt% of CNTs loading.

pure polymer.

increase in strain energy density of about 150% for the composites as compared with pure UHMWPE. They also reported an increase of about 140% in ductility and up to 25% in tensile strength. An analysis by nanoindentation and atomic force microscopy (AFM) of UHMWPE/MWCNT composites has been reported by Wei et al. (2006). They have observed a decrease of the friction coefficient with MWCNT content increase. Recently, Fonseca et al. (2011) prepared UHMWPE/CNT composites, with different volume fractions of CNTs, through mechanical ball-milling and processed by compression molding. In this work, for this specific materials and mould geometry, it was optimized the time of mixture, in order to obtain an homogeneous distribution of CNT into polymeric matrix, as well as the processing conditions. The results reveal that mechanical ball-milling is well suited for mix the CNTs with UHMWPE. CNTs were well dispersed and did no loose their physical integrity (as can be shown in Fig. 5). These scanning electron microscopy (SEM) pictures shown different CNT/HDPE powder composites mixed for different time (15, 45 and 60 min). Though the SEM pictures and the tensile tests, it was observed that the optimized time of mixture was for 45 min. To process the mixed composites, it was used the compression molding technique, and the cycle was optimized (Fig. 6 (a)). The SEM analysis of the processed composites has shown that CNTs were well dispersed into the polymeric matrix (Fig. 6(b)). The tensile tests of these composites have shown an enhancement on the mechanical properties of about 20% on the elastic modulus, for 0.2 % vol. of CNTs, and about 80% for higher concentrations (0.4 to 1% vol.). Similar results were reported by other groups (Wang et al., 2005; Mierczynska et al., 2007).

Fig. 5. SEM pictures of the 0.2% vol. MWCNT composite mixed for 15, 45 and 60 min.

Fig. 6. (a) Optimized compression molding cycle applied for processing the composites; (b) SEM picture of the processed CNT/UHMWPE composite with 1% vol. fraction of CNTs. The round circle gives evidence for the presence of the CNTs.

Microinjection Molding of Enhanced Thermoplastics 229

high precision tolerances are not wear free. On the contrary, the molding surface wear can be even much more critical than in conventional molding. The wear out of the molding tools creates demolding problems, compromising the molding finish quality, speeding up the corrosion of the tools and resulting in costly maintenance stops. Polymer abrasion, adhesion and corrosion are the catalysers of these mechanisms. Furthermore, the increasing usage of polymers reinforced with glass fibers, minerals, or even carbon nanotubes, enhance the

Coatings technologies have been strongly developed in the last decades, as did their application on mould and die tools. Ion implantation and unbalanced magnetron sputtering PVD (Bienk & Mikkelsen, 1997), High Velocity Oxy Fuel (HVOF) and Atmospheric Plasma Spraying (APS) (Gibbons & Hansell, 2008), diamond-like carbon and silicon carbide (Griffiths et al., 2010), nanostructured TiB2 (Martinho et al., 2011), Oxide coating (Alumina) and Nitride coatings (AlN, CrN, NiCr(N), TiN) (Navabpour et al., 2006), among other surface engineering coating have been tested in order to evaluate their performance to avoid

Chemical vapor deposition (CVD) of polycrystalline diamond, in microcrystalline and nanocrystalline morphology, detain a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. It possesses a high mechanical hardness and wear resistance, high thermal conductivity and is very resistant to chemical corrosion. Its properties and their optimization by means of the deposition process, in order to fulfill the application requisite have been investigated by several research teams (Ahmed et al., 2006; Das & Singh, 2007; Gracio et al., 2010). Some successful work has already been performed on the evaluation of diamond coating on molding tools in special for micro-featured tools (Neto, 2008b, 2008c, 2009). As refereed above, in this sub-chapter, the application and evaluation of CVD diamond thin films as a surface engineering technique to improve operation and durability of microinjection mould tools will be

Polycrystalline diamond, in microcrystalline or nanocrystalline morphology, detains a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. It detains an extreme mechanical hardness (ca. 90 GPa) and wear resistance, one of the highest bulk modulus (1.2 × 1012 N.m−2), the lowest compressibility (8.3 × 10−13 m2 N−1), the highest room temperature thermal conductivity (2 × 103 W m−1 K−1), a very low thermal expansion coefficient at room temperature (1 × 10−6 K)

Most of these properties are attractive for the application on cavities and mould tools; nevertheless coating an entire cavity with polycrystalline diamond is presently an utopia. CVD systems are considerably size limited due to the means of activating (thermal, electric discharge, or combustion flame) the gas phase carbon-containing precursor molecules.

A second problem related to the usage of CVD on mould tools is concerned with the fact that diamond cannot be directly coated onto ferrous substrates, the widest used row material to produce mould tools. Carbon, the precursor element of diamond, easily diffuses into the ferrous matrix, leaving behind no matter to start the diamond nucleation process.

and is very resistant to chemical corrosion (Das & Singh, 2007; Grácio et al., 2010).

abrasive power of polymers.

highlighted.

**4.2 Diamond coatings** 

molding surface wear and assist the demolding process.

The enhancement of UHMWPE properties is of utmost importance, essentially for medical applications, as UHMWPE is largely used for that purpose. However, the incorporation of CNTs into UHMWPE, for medical applications, requires special attention in what concerns to toxicity and biocompatibility of these composites. Reis et al. (2010) describe the response of human osteoblasts-like MG63 cells in contact with particles generated from UHMWPE/CNT composites. The results show the absence of significant elevation of the osteolysis inductor IL-6 values, pointed out for possible use of this superior wear-resistant composite for future orthopaedic applications.

### **3.3 The rheology properties and molding tools**

As it was discussed earlier, with the addition of particles to thermoplastics it is possible to modify the physical, thermal and mainly the mechanical properties of the raw thermoplastic. With these enhanced thermoplastics it is of utmost importance to evaluate the relationship of the new materials with the processing parameters, and optimize them, especially when new processing techniques are arising, like microinjection molding. In the bibliography it is possible to read some studies on microinjection molding of neat polymers (Chien, 2006; Sha et al., 2007). Few studies did consider polymer compounds containing fillers, such as glass fibers, glass particles, nanoceramic materials and carbon nanotubes (Huang et al., 2005; Huang, 2006; Hanemann et al., 2009; Abbasi et al., 2011). Huang et al. (2005) study the moldability and wear particles of composites of polymeric matrices reinforced with nanoceramic particles. Their rheology analysis reveals that with increasing nanoparticle loading the shear viscosity increases, meaning that, in microinjection moulding, high pressure is needed to produce high quality parts with micro-features.

Abbasi et al., (2011) prepared PP/CNT and PC/CNT composites and studied the effects of processing conditions on its structure, mechanical properties and electrical conductivity. They concluded that the high deformation values of the microinjection molding only slightly changed the overall crystallinity due to the short cycle time of process. They also observed that the crystals were all oriented in the flow direction. Other interesting result is that the type of processing strongly affects the electrical conductivity of the composites.
