Short fiber thermoplastics

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Thermoplastics containing short fiber reinforcements were first introduced commercially in the 1960s. [1] The most common type of fibers used in short fiber thermoplastics are glass fiber and carbon fiber [2] . Adding short fibers to thermoplastic resins improves the composite performance for lightweight applications. [1] In addition, short fiber thermoplastic composites are easier and cheaper to produce than continuous fiber reinforced composites. [1] This compromise between cost and performance allows short fiber reinforced thermoplastics to be used in myriad applications.

Contents

Mechanical properties

Mechanical properties of short fiber reinforced composites depend critically on the fiber length distribution (FLD) and the fiber orientation distribution (FOD). [3] In particular, the strength of short fiber reinforced composites increases with the increase of the mean fiber length and with the decrease of the mean fiber orientation angle (angle between the fiber axis and the loading direction). [3] [4] The elastic modulus (E) of misaligned short fiber reinforced polymer composites depends on the distributions of fiber lengths and orientations within the composite structure. [5] In general, the composite elastic modulus increases with the decrease of the mean fiber orientation angle and with the increase of the fiber orientation coefficient; and the elastic modulus increases with the increase of mean fiber length when the mean fiber length is small. When the mean fiber length is large, it has nearly no influence on the elastic modulus of short fiber reinforced composites. [5]

Aspect ratio

An important characterizing parameter of short fiber composites is the aspect ratio (s) defined as the ratio between the length (l) and the diameter (d) of the fibers used as reinforcement:

The value of s can vary depending on fiber type and design, assuming values from approximately 50 to 500. [6] Aspect ratios can affect properties such as the strain to failure and toughness. A higher aspect ratio will result in lower values of strain at failure and toughness, due to angular particles inducing crack formation. [7]

Void formation

Short fiber reinforced composites are used increasingly as a structural material because they provide superior mechanical properties and can be easily produced by the rapid, low-cost injection molding process, by extrusion and with spray-up technique. [8] An important issue for short fiber thermoplastic composites is void formation and growth during production processes. It has been shown that voids tend to nucleate at fiber ends, and their content depends on processing conditions, fiber concentration, and fiber length. [8] For example, in an injection molding process bubble growth is suppressed by cooling the material under pressure. Density measurements confirm a much lower void content (-1%) in the injection-molded samples in comparison with the extrudates. [8] Another factor playing an important role in void formation is the cooling rate. While the melt is cooled external surface layers solidify first. These layers restrain the contraction of material within the melt. This leads to internal voiding. As a result, slower cooling rates decrease void content in the composite. Finally, in an extruded structure, longer fibers result in higher void contents. This unexpected behaviour [8] is due to the overcoming of other factors like viscosity, extrusion pressure and shear rate, which make the analysis on this phenomenon very complicated.

Simulations and modelling

Short fiber thermoplastics can be modelled as a matrix with fiber inclusions. [9] According to the inclusion model, the stress within the material is proportional to the product of inclusion volume fraction and the stress within a single inclusion. [10] In other words, the stress within the composite is proportional to the fiber volume fraction and the stress on a single fiber. Using Mean Field Theory and the Mori-Tanaka model, the stresses within a short fiber thermoplastic can be modelled computationally. [9] Assuming the matrix is a newtonian material, the creep from an applied shear stress can be approximated from equilibrium thermodynamics. [11] This will yield information about the composite's rheological response.

Applications and processing

Short fiber reinforced thermoplastics have a broad range of applications due to fiber reinforcement properties. [2] Short fiber thermoplastics are able to withstand up to 30,000 psi of applied tensile load and have an elastic modulus on the order of 2 x 106 psi. [1] They are ideal for applications for which toughness is of critical importance, high volume production is involved, and long shelf life and scrap recycling are important issues. [1] With all of these performance capabilities, one of the greatest advantages to using short fiber reinforced thermoplastics is their ease of processing and reprocessability. [1] [12] Ease of processing has been the key factor to the widespread use of short fiber reinforced thermoplastics. [2] Effective processing techniques and the ability to recycle scrap offer significant cost reductions that compare to those of thermoset compounds and metals. Because of this, short fiber reinforced thermoplastics are desired in the electrical and electronic, automotive, oilfield, chemical process, and defense industries. [1] Although short fiber thermoplastics have progressed considerably over the years and have a secured spot in a colossally-sized market, further refinement of compounding and process technology along with improvements in part design could allow the performance window of these materials to widen significantly, allowing them to be used for more applications in the future.

Morphology

Injection molding is a traditional cost-effective method for manufacturing of short-fiber thermoplastics. The processing conditions such as mold temperature and pressure as well as filling time, the part geometry, position and number of injection gates are main factors influencing distribution of fibers. [12] As a result, depending on the total thickness of the manufactured parts as well as the distance from mold wall, different fiber orientation distributions can be observed. In a thin layer in mid-thickness fiber orientations are preferably perpendicular to the mold flow direction, while in two near wall thicknesses fibers are preferably in line with the mold flow direction. [4]

Self-heating

An aspect of thermoplastics which distinguishes them from metallic materials is their time dependent properties as well as relatively low melting temperatures. As a result, the frequency at which a load is applied or rate of applied load is a determining factor on mechanical properties of such materials. Due to low thermal conductivity of thermoplastics, the generated heat due to energy dissipation under applying load results in self-heating or thermal degradation. In short fiber thermoplastics, the frictional heating between fiber and matrix as well as a higher intensity of stress near fiber ends increase the degree of self-heating. [13]

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<span class="mw-page-title-main">Composite material</span> Material made from a combination of two or more unlike substances

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.

<span class="mw-page-title-main">Young's modulus</span> Mechanical property that measures stiffness of a solid material

Young's modulus is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Young's modulus is defined as the ratio of the stress applied to the object and the resulting axial strain in the linear elastic region of the material.

<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system.

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Wood-plastic composites (WPCs) are composite materials made of wood fiber/wood flour and thermoplastic(s) such as polythene (PE), polypropylene (PP), polyvinyl chloride (PVC), or polylactic acid (PLA).

<span class="mw-page-title-main">Representative elementary volume</span>

In the theory of composite materials, the representative elementary volume (REV) is the smallest volume over which a measurement can be made that will yield a value representative of the whole. In the case of periodic materials, one simply chooses a periodic unit cell, but in random media, the situation is much more complicated. For volumes smaller than the RVE, a representative property cannot be defined and the continuum description of the material involves Statistical Volume Element (SVE) and random fields. The property of interest can include mechanical properties such as elastic moduli, hydrogeological properties, electromagnetic properties, thermal properties, and other averaged quantities that are used to describe physical systems.

<span class="mw-page-title-main">Filler (materials)</span> Particles added to improve its properties

Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity, etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler that are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Short Fiber Reinforced Blends are partial case of ternary composites, i.e. composites prepared of three ingredients. In particular they can be considered as a combination of an immiscible polymer blend and a short fiber reinforced composite. These blends have the potential to integrate the easy processing solutions available for short fiber reinforced composites with the high mechanical performance of continuous fiber reinforced composites. The performance of these complex, ternary systems is controlled by their morphology.

Fiber volume ratio is an important mathematical element in composite engineering. Fiber volume ratio, or fiber volume fraction, is the percentage of fiber volume in the entire volume of a fiber-reinforced composite material. When manufacturing polymer composites, fibers are impregnated with resin. The amount of resin to fiber ratio is calculated by the geometric organization of the fibers, which affects the amount of resin that can enter the composite. The impregnation around the fibers is highly dependent on the orientation of the fibers and the architecture of the fibers. The geometric analysis of the composite can be seen in the cross-section of the composite. Voids are often formed in a composite structure throughout the manufacturing process and must be calculated into the total fiber volume fraction of the composite. The fraction of fiber reinforcement is very important in determining the overall mechanical properties of a composite. A higher fiber volume fraction typically results in better mechanical properties of the composite.

<span class="mw-page-title-main">Cellulose fiber</span> Fibers made with ethers or esters of cellulose

Cellulose fibers are fibers made with ethers or esters of cellulose, which can be obtained from the bark, wood or leaves of plants, or from other plant-based material. In addition to cellulose, the fibers may also contain hemicellulose and lignin, with different percentages of these components altering the mechanical properties of the fibers.

Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

Olimunllum® CF/PEEK is a thermoplastic composite material containing a quasi-isotropic endless carbon fiber reinforcement and a semi-crystalline thermoplastic polymer matrix from the Polyaryletherketone (PAEK) family.

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

A void or a pore is three-dimensional region that remains unfilled with polymer and fibers in a composite material. Voids are typically the result of poor manufacturing of the material and are generally deemed undesirable. Voids can affect the mechanical properties and lifespan of the composite. They degrade mainly the matrix-dominated properties such as interlaminar shear strength, longitudinal compressive strength, and transverse tensile strength. Voids can act as crack initiation sites as well as allow moisture to penetrate the composite and contribute to the anisotropy of the composite. For aerospace applications, a void content of approximately 1% is still acceptable, while for less sensitive applications, the allowance limit is 3-5%. Although a small increase in void content may not seem to cause significant issues, a 1-3% increase in void content of carbon fiber reinforced composite can reduce the mechanical properties by up to 20%

Transfer molding is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. Furthermore, unlike injection molding the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding process.

In materials science, a polymer matrix composite (PMC) is a composite material composed of a variety of short or continuous fibers bound together by a matrix of organic polymers. PMCs are designed to transfer loads between fibers of a matrix. Some of the advantages with PMCs include their light weight, high resistance to abrasion and corrosion, and high stiffness and strength along the direction of their reinforcements.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.

References

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  3. 1 2 Fu, S.; Lauke, B. (1996). "Effects of fiber length and fiber orientation distributions on the tensile strength of short-fiber-reinforced polymers". Composites Science and Technology. 56 (10): 1179–1190. doi:10.1016/S0266-3538(96)00072-3.
  4. 1 2 Mortazavian, Seyyedvahid; Fatemi, Ali (2015-04-01). "Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites". Composites Part B: Engineering. 72: 116–129. doi:10.1016/j.compositesb.2014.11.041.
  5. 1 2 Fu, S.; Lauke, B. (1998). "The elastic modulus of misaligned short-fiber-reinforced polymers". Composites Science and Technology. 58 (3–4): 389–400. doi:10.1016/S0266-3538(97)00129-2.
  6. ↑ Hull, D.; Clyne, T. W. (1996-08-13). An Introduction to Composite Materials. Cambridge University Press. ISBN   978-0-521-38855-9.
  7. ↑ Oréfice, R. L.; Hench, L. L.; Brennan, A. B. (2001). "Effect of particle morphology on the mechanical and thermo-mechanical behavior of polymer composites". Journal of the Brazilian Society of Mechanical Sciences. 23 (1): 1–8. doi: 10.1590/S0100-73862001000100001 .
  8. 1 2 3 4 Vaxman, A.; Narkis, M.; Siegmann, A.; Kenig, S. (1989). "Void formation in short-fiber thermoplastic composites". Polym Compos. 10 (6): 449–453. doi:10.1002/pc.750100609.
  9. 1 2 Kaiser, J.; Stommel, M. (2014). "Modified mean-field formulations for the improved simulation of short fiber reinforced thermoplastics". Composites Science and Technology. 99: 75–81. doi:10.1016/j.compscitech.2014.05.010.
  10. ↑ Mori, T.; Tanaka, K. (1973). "Average stress in matrix and average elastic energy of materials with misfitting inclusions". Acta Metallurgica. 21 (5): 571–574. doi:10.1016/0001-6160(73)90064-3.
  11. ↑ Ghosh, T.; Grmela, M.; Carreau, P. J. (1995). "Rheology of Short Fiber Filled Thermoplastics". Polymer Composites. 16 (2): 144–153. doi:10.1002/pc.750160206.
  12. 1 2 Mortazavian, Seyyedvahid; Fatemi, Ali (2015-01-01). "Fatigue behavior and modeling of short fiber reinforced polymer composites: A literature review". International Journal of Fatigue. 70: 297–321. doi:10.1016/j.ijfatigue.2014.10.005.
  13. ↑ Mortazavian, Seyyedvahid; Fatemi, Ali; Mellott, Stephen R.; Khosrovaneh, Abolhassan (2015-10-01). "Effect of cycling frequency and self-heating on fatigue behavior of reinforced and unreinforced thermoplastic polymers". Polymer Engineering & Science. 55 (10): 2355–2367. doi:10.1002/pen.24124. ISSN   1548-2634.
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