CONOCIMIENTO Y PARTICIPACION DE LAS FAMILIAS EN OTROS ESPACIOS PUENTE

Reinforced polymers are often employed in industrial applications in order to improve the mechanical properties and enable the replacement of heavier metals. Therefore, composite polymers have also been widely studied from the perspective of the relationship between processing parameters and filler content with the resulting morphology and foam properties.

As shown in Table 2.4, several research works have been carried out on clay additives, above all with PA6 as matrix material, as it is one of the most effective filler for this polymer [152]. Yuan and Turng [153] analyzed the effect of different content of montmorillonite (MMT) on the cell structure and crystallinity of microcellular foams, as well as the influence of some injection parameters on the physical foaming with N2 as blowing agent. Fillers act as

nucleating agents giving rise to uniform structures, with high cell density and small cells. The filler dispersion is improved due to the expansion of the foaming agent. The crystallinity phase is reduced due to the clay nanoparticles, although the reinforcement effect is predominant and the tensile and impact properties are enhanced as compared to the non-filled material. This effect is clearly evinced as the clay content and shot volume increase, and with a medium level of supercritical fluid (SCF).

The same filler has been used in experiments with a wide variety of materials. Most of these studies reported the same beneficial effect of clay on cell density, cell size, cell distribution and mechanical properties aforementioned above. Nevertheless, experiments conducted with PLA [154] reported a higher degradation of the polymer with the increase in filler content, reducing cell density, tensile and impact properties. However, in PLA/PHBV blends [155], clay raises the crystallinity levels of PHBV phase, and therefore, the morphology and mechanical properties are improved. Clay particles are polar materials, so compatibilizers like maleic anhydride are needed in order to be employed with polyolefins, such as LDPE [156]. In some investigations with PP [157] was also found that ramified structures lead to more uniform morphologies, with higher cell densities and lower cell size.

Talc and glass fiber are common fillers reinforcing polymers, such as PP in automotive applications. Yetgin et al. [158] reported an increase in cell density and tensile strength and modulus of foamed PP by adding 20% of talc, but also bigger cells and a decrease in the impact strength and elongation at break. The optimal weight percentage of glass fiber and processing parameters enhancing cell structure and mechanical performance of PP foams was reported by Xi et al. [159].

Rubber particles are effective impact modifier for many polymer systems. According to Xin et al. [160], the content of chemical foaming agent is the most influencing variable on the cell structure and tensile properties of PP foams. Rubber particles act as nucleating agents and increase the viscosity of the melt PP, enabling the formation of fine morphologies with high cell densities and low cell sizes. However, their efficiency as nucleation agents in PA6 resulted lower than nanoclay, as reported by Yuan et al. [152].

Natural fillers offer also many advantages due to environmental issues. Wood fibers are light and stiff and can be processed with microcellular polymers. In different studies [161- 163], the authors studied the effect of chemical foaming agent, processing variables and fiber types on PP foaming. High weight reduction ratios (up to 30%) and smoother surfaces than solid parts were obtained with hard wood fibers and exothermic chemical agents. Medium melt temperatures and finer fibers promoted more uniform cell distributions and smaller cell diameters. The most influencing variable was the foaming agent content. The mechanical properties decreased with the apparent density, although an improvement can be obtained with maleic anhydride as compatibilizer agent.

In some experiments conducted on recycled PP, Xie et al. [164] optimized the processing temperatures and pressures of chemical foaming to achieve fine cell structures, increasing the impact resistance as compared to the non-foamed material. Microcellular wood fiber reinforced polymer composites obtained by different processes (batch, injection molding, extrusion, and compression molding process) and their properties have been reviewed by Faruk et al. [165].

Table 2.4. Main research works on foaming injection molding conducted with filled polymers.

Author Filler Polymer Blowing

agent

Analyzed properties

Yuan and Turng [153]

(2005)

Montmorillonite PA6 SCF N2 Microstructure, tensile,

impact, dynamic mechanical, crystallization

Guo et al. [157] (2007)

Nanoclay PP Chemical Microstructure, dynamic

mechanical Hwang et al.

[156] (2009)

Montmorillonite LDPE SCF N2 Microstructure, thermal,

tensile Hwang et al.

[154] (2009)

Montmorillonite PLA SCF CO2 Microstructure, thermal,

tensile Rizvi and

Bhatnagar [166] (2011)

Montmorillonite PP SCF N2 Microstructure, tensile,

flexural, impact

Zhao et al. [155] (2013)

Nanoclay PLA/PHBV SCF N2 Microstructure, tensile,

dynamic mechanical Srithep and

Turng [167] (2014)

Nanoclay PET SCF N2 Microstructure, tensile,

rheology, thermal

Wang et al. [168] (2016)

Nanoclay TPU SCF N2 Microstructure, thermal,

rheology, filler dispersion, tensile

Michaeli et al. [132]

(2009)

Talc PP SCF CO2 Microstructure, impact

Yetgin et al. [158] (2013)

Talc PP Chemical Microstructure, tensile,

impact Yetkin et al.

[169] (2013)

Talc PP/EPDM Chemical Microstructure, tensile,

Table 2.4. Continued.

Author Filler Polymer Blowing

agent

Analyzed properties

Ameli et al. [170] (2013)

Talc PLA SCF N2 Microstructure, thermal,

tensile Edwards et al.

[141] (2004)

Glass Fiber PP SCF N2 Microstructure, thermal,

viscoelasticity, flexural, impact

Bian et al. [171] (2012)

Glass Fiber PP SCF N2 Shrinkage, warpage

Xi et al. [159] (2014)

Glass Fiber PP SCF N2 Microstructure, tensile,

flexural, impact Roch et al. [172] (2014) Long Glass Fiber PP SCF N2 / Chemical Flexural, impact Roch et al. [173] (2015)

Long and Short Glass Fiber PA6 SCF N2 / Chemical Microstructure, tensile, flexural, impact Ameli et al. [174] (2013)

Carbon Fiber PP SCF N2 Electrical conductivity,

dielectric permittivity, electromagnetic interference

shielding effectiveness Hwang et al.

[175] (2014)

Carbon Fiber PBT SCF N2 Microstructure, tensile,

impact, electrical conductivity, electromagnetic interference shielding effectiveness Pilla et al. [176] (2007) Carbon nanotubes

PLA SCF N2 Microstructure, tensile,

thermal, dynamic mechanical-thermal Arjmand et al. [177] (2014) Carbon nanotubes

PS Chemical Microstructure, electrical conductivity, dielectric permittivity Yuan et al. [152] (2005) Core Shell Rubber and nanoclay

PA6 SCF N2 Microstructure, tensile,

impact

Xin et al. [160] (2010)

Rubber powder PP Chemical Microstructure, tensile

Zhang et al. [178] (2011)

Rubber powder PP Chemical Microstructure, rheology,

Table 2.4. Continued.

Author Filler Polymer Blowing

agent Analyzed properties Yoon et al. [179] (2005) CaCO3 PP SCF N2 Microstructure Hwang and Hsu [180] (2013)

Silica PP SCF N2 Microstructure, tensile

Li et al. [181] (2013) Carbon nanotube, nano- montmorillonite, talc

PEI SCF N2 Microstructure, tensile,

flexural, dielectric constant, thermal conductivity

Bledzki and Faruk [182]

(2005)

Wood fiber PP Chemical Microstructure, surface

roughness, impact, odor concentration Gosselin et al.

[183] (2006)

Wood fiber HDPE/PP Chemical Microstructure, density

Bledzki and Faruk [184]

(2006)

Wood fiber PP Chemical Microstructure, tensile,

flexural, odor concentration

Bledzki and Faruk [161]

(2006)

Wood fiber PP Chemical Microstructure, tensile,

flexural, impact

Bledzki and Faruk [162], [163] (2006)

Wood fiber PP Chemical Microstructure, density,

tensile, flexural, impact

Gwon et al. [185] (2012)

Wood fiber HDPE Chemical Microstructure, density,

flexural Xie et al.

[164] (2012)

Wood fiber PP Chemical Microstructure, impact

Kramschuster et al. [186]

(2007)

Recycled paper fibers

PLA SCF N2 Microstructure, tensile,

thermal, dynamic mechanical

Ding et al. [187] (2016)

Cellulosic fiber PLA SCF N2 Microstructure, rheology,

thermal Pilla et al.

[188] (2009)

Flax fibers PLA SCF N2 Microstructure, tensile,

Table 2.4. Continued.

Author Filler Polymer Blowing

agent

Analyzed properties

Zafar et al. [189] (2016)

Willow fiber PLA SCF N2 Microstructure, tensile,

flexural, impact, thermal, crystallization

Chapter 3: