UNIVERSITÀ DI PISA
DIPARTIMENTO DI CHIMICA E CHIMICA INDUSTRIALE
Corso di Laurea Magistrale in Chimica Industriale
Tesi di Laurea Magistrale
Rheology and surface properties of polyolefin compounds
with high loading of micrometric mineral fillers
Relatori
Andrea Pucci
Camillo Cardelli
Controrelatore
Celia Duce
ANNO ACCADEMICO 2019-2020Candidato
Sara Haveriku
Contents
1.
Introduction ... 1
1.1 Fillers as flame retardant ... 1
1.2 Polyolefin composites highly filled with flame retardant ... 8
1.2.1 Fillers for polymeric composites ... 10
1.2.1.1 Mineral flame retardants ... 10
1.2.1.2 Carbonates and hydroxycarbonates ... 14
1.2.1.3 Borates ... 16
1.2.2 Surface treatment of the filler ... 17
1.3 Types of polyolefins ... 22
1.3.1 EVA ... 23
1.3.2 POE ... 26
1.4 Coupling agents and processing aid agents ... 28
1.5 Rheological behaviour of polymers ... 30
1.6 Rheological behaviour of highly filled composites ... 36
1.7 Determinations of the rheological properties ... 38
1.7.1 Capillary Rheometer ... 39
2.
Objectives of the work ... 42
3.
Experimental part ... 44
3.1 Materials ... 44
3.2 Instruments and methods ... 47
4.1 Mechanical properties and MFI ... 57
4.1.1 Dosage of coupling agent ... 57
4.1.2 Variation of filler loading and Limited Oxygen Index (LOI) ... 60
4.1.3 Change of polymer partner for EVA28 ... 63
4.1.3.1 Polyethylene grades to combine with EVA28 ... 63
4.1.3.2 Other polyolefins as partner for EVA28 ... 66
4.1.4 Different filler partners for n-MDH... 68
4.1.4.1 n-MDH vs s-MDH ... 76
4.1.5 Role of silicon gum into formulation ... 78
4.2 Rheological properties... 80
4.2.1 n-MDH vs s-MDH ... 81
4.2.2 Change of polymer partner for EVA28 ... 86
4.2.2.1 Polythylene grades to combine with EVA ... 86
4.2.2.2 Other polyolefins as partner for EVA28 ... 88
4.2.3 Combination of n-MDH with other fillers ... 90
4.3 Surface analysis of extruded compounds ... 92
4.3.1 Change of polymer partner for EVA28 ... 93
4.3.1.1 Polyethylene grades combined with EVA ... 93
4.3.1.2 Other polyolefins as partner for EVA28 ... 96
4.3.2 Combination of n-MDH with other fillers ... 98
4.3.2.1 n-MDH vs s-MDH ... 98
4.3.2.2 Combination of n-MDH with other natural fillers ... 100
4.3.2.3 Combination of n-MDH with other synthetic fillers ... 101
4.3.3 Effect of silicon ... 102
6.
References ... 108
7.
Appendix (characterization of raw materials) 120
1
1. Introduction
1.1 Fillers as flame retardant
Originally mineral fillers were introduced into polymeric materials in order to reduce the total cost of the compounds. Nowadays, it is ascertained that fillers enhance also specific properties of the materials and for this reason the term functional filler has been introduced23: there are a lot of fillers used for different applications due to their functionalities. One of the most important factors, still influent on the industrial use of filler for polymer application, is their cost, different for each filler type and affected by raw material, production, transports and their specific properties. A wide variety of fillers can be found, which are different for their properties and origin. Their behaviour in the polymeric composite is deeply influenced by many factors, such as morphology, colour, refractive index, presence of impurities, density, hardness, moisture content, thermal stability, modulus, surface chemistry and toxicity. The morphology of fillers can be considered the key for understanding their performance in polymers and it includes
particle size, shape, surface area and particle packing capacity. All these properties
are related to the primary particles composing the filler, but it must be taken into account that fillers can easily form assemblies defined as aggregates, with very strong interaction among the particles, and agglomerates, characterized by weaker interactions. The tendency of the filler to assembly influences strongly the dispersion in the compound, even if it can be possible to break down aggregates and agglomerates by increasing the processing conditions severity and so dispersion energy24 (Fig. 1).
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Fig. 1 Idealised view of the way that filler particles disperse and of the different forms of particle
types that might be encountered
The size, the specific surface area and the shape of the particles are strictly correlated each other. The most used parameter is the particle size and the distribution, but often the irregularity of the shapes makes its determination quite difficult and not so meaningful.
The concept of equivalent spherical diameter (esd) is diffused for size identification and it corresponds to the diameter of a sphere having the same volume of the particle; the main defect of its use as indicative parameter is that fillers with different particle characteristics (porosity, surface roughness, shape, etc) can have the same esd. In technical datasheets, the particle size and distribution (PSD) are usually given by the average diameter values of particles at different percentage (indicated as D10, D50, D90).
With few exceptions, mineral fillers do not have a unique particle size, but exhibit a range of sizes, the shape of the distribution being determined by the nature of the filler and the method of production. It must be recognised that fillers with the same average particle size can have widely differing distributions and it is the distribution that is generally more important in determining the effects in polymer composites. As an example of the effect of particle size distribution, the strength of thermoplastics seems to be critically affected by the presence of small proportions of material much above 10 microns25.
Specific surface area (SSA) is also frequently used as a defining parameter for the
filler types encountered in thermoplastics applications24. This is the amount of surface possessed by one gram of the filler material, and in the present context usually lies in the range 2–20 m2/g. Its determination gives an idea of the surface available for polymer in
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composite: higher specific surface area commonly corresponds to more possibility of interaction points between filler and matrix. It is deeply related to the particle size of the filler and usually higher is the size and smaller is the surface area.
In some cases, this information is calculated from the particle size information, assuming a particular particle shape (usually spherical), but it is today more usually measured by techniques such as gas adsorption26 or dye adsorption27. In the adsorption methods an adsorbing molecule of known covering power is deposited in a controlled way such that a monolayer is present on the filler surface and from the amount adsorbed the specific surface area of the filler can be calculated. The most rigorous method is the BET procedure using nitrogen as the adsorbing molecule. This is a very reproducible procedure but requires relatively expensive equipment not always possessed by filler producers. The value determined by the BET method can exceed that from dye adsorption as the nitrogen molecule is considerably smaller than a dye molecule and hence is able to access surface that is in small pores and not determined by the dye. In using data measured on a filler powder, it must be remembered that particle breakdown, generating additional surface, may occur during compounding.
The specific surface area is obviously related to the particle size distribution of the filler and can be used as a guide to this. For materials of the same density and shape a higher specific surface area means a smaller particle size, but again it must be remembered that two distinctly different particle size distributions can give rise to the same value for the specific surface area and so it is not a unique property.
The main parameter introduced for the determination of particle shape is the aspect ratio, which is useful for distinguishing anisotropic and isotropic particles. It represents the ratio of the length to the diameter of a rod, or of the thickness to the diameter of plates and its value is 1 for a sphere; for filler characterized by irregular shape it is not easy to obtain an average significant value.
As mentioned above, particle shape is an important factor determining the use of fillers in thermoplastics. Anisotropy, or aspect ratio, is particularly important, being valuable in improving factors such as stiffness and heat distortion temperature. Despite this, shape itself is generally only roughly described in filler literature and specifications. This is due to the difficulty of carrying out meaningful measurements quickly and at
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reasonable cost. One relatively simple approach that is sometimes effective is to compare the specific surface area as determined by nitrogen with that calculated from the particle size distribution assuming spherical particles. If these two values agree then the particles are assumed to have a low aspect ratio, whereas if the measured value is greater than the predicted one, the filler is either porous or has some degree of anisotropy. This concept is illustrated in Fig. 2:
Fig. 2 Schematic illustration of the ways in which the BET specific surface area can differ from that
derived from particle size measurements: All particles have a similar equivalent spherical diameter but in: a the particle is solid; b the particle is porous; c the particle has a rough surface; d the particle is
anisotropic24
Both particle size and shape play an essential role in determining how the particles can pack together.
One of the main aspects influencing filler properties, is their origin: the use of natural or synthetic fillers can involve different production devices and final product properties due to their different characteristics23,28,29.
Synthetic fillers
Synthetic fillers are commonly produced by precipitation and usually synthetic routes are chosen when the desired natural mineral is not available and/or high purity, particular shapes and sizes are required than that obtainable for natural fillers; obviously the cost of
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the final product is higher due to the necessary raw materials, production routes and final properties30.
Among the synthetic fillers carbon black is the most widely used especially for elastomer applications due to its electrical properties and surface capacity of interacting strongly with elastomer molecules without the use of coupling agent treatments. The main properties required to the carbon black incorporation are colour and electrical conductivity31.
Synthetic silica can be found as precipitated silica and fumed silica, different for production method, cost and thus application32. Precipitated silica is not expensive and it has been used mostly for rubber applications (tyres and footwear), while fumed silica is one of the most expensive fillers because it is produced by controlled combustion of compounds like silicon tetrachloride, so its use is restricted to silicone elastomer materials.
Precipitated calcium carbonate (PCC) is used mostly as "speciality" polymer filler, even if the main use is for paper production and PVC windows profiles. Natural calcium carbonate is usually quite pure and white (and very cost competitive), so the production of synthetic calcium carbonate is necessary when very fine sizes and narrow PSD are required.
Aluminium trihydroxide (ATH) is obtained by synthetic routes from gibbsite-containing rocks as bauxite. Bayer process is the first to be used and still the most important (about 60% of ATH fire retardant market by volume) for coarse form and fine precipitated grades8. The process consists of extraction of gibbsite from bauxite ores, followed by precipitation. The obtained ATH is a coarse form characterized by high particle size (up to 750 μm) and high presence of aggregates29. These are milled to
produce finer size form of Bayer hydrate (5-20 μm of particle size), suitable for polymer application. The production of higher quality ATH products follows similar chemical steps of Bayern process with the use of purer reagents and controlled precipitation conditions in order to obtain directly required particle shape and size, avoiding milling step. It represents one of the principal polymer fillers (650000 tonnes a year): the main applicative uses of ATH are for flame retardant composites based on elastomeric, thermosetting and thermoplastic matrices (90% of the usage) and in solid surface because
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its composites are characterized by good performance and mostly desirable aesthetic quality.
Magnesium hydroxide can be commercially found as both natural and synthetic filler, where natural MDH (=from natural mineral named Brucite) thanks to certain number of natural deposits of brucite in China and in Russia. The two main synthetic process are hydropyrolysis and hydrothermal, which both go through a solution stage (magnesium chloride) for purification and then the treatment of purified solution by using different methods. The main uses of synthetic magnesium hydroxide are for pharmaceuticals, water treatment and for conferring flame retardant properties to polymeric materials. Its performances are quite similar to ATH, but the synthetic production processes are significantly more expensive and so it is usually used only when more thermal stable filler is necessary, or, instead of synthetic MDH, the natural milled Brucite is used.
Natural fillers
Natural fillers come from natural mineral23, 29, which must be abundant, low cost, colourless, inert and easily convertible in filler with the required particle size. The raw materials are obtained by mining, followed by a variety of processes depending on the desired final properties, such as comminution, purification, classification, calcination, surface treatment and drying. The main problem is that the commercial form of fillers from natural sources may still contain significant quantities of other minerals as well as other constituents, so it must be considered if they can be deleterious for the intended application.
The most important mineral fillers used are carbonates, clays and talc, while other silicates are also of interest. Several carbonate minerals are known with some having potential for use as mineral fillers, although only a few are of industrial importance in plastic and rubber applications.
Calcite (calcium) and dolomite (calcium-magnesium) are the main carbonate fillers and are very widespread, being exploited in many countries. The other carbonate mineral of any importance is, in fact, a mixture of two carbonates: hydromagnesite (a hydrated basic magnesium carbonate: Mg5(CO3)4(OH)2∙4H2O) and Huntite (CaCO3∙3MgCO3).
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A large, relatively pure deposit is found in Greece and in Turkey, and a smaller one in Texas, USA, with the former being exploited as a filler for paint and paper applications, and as a flame-retardant filler in plastics and rubber.
Ground calcium carbonate (GCC) is widely used in a variety of polymers and the main applications are in poly(vinyl chloride) (PVC), polypropylene, polyethylene, elastomers and unsaturated polyester. Due to the high purity at which it can be found in nature, it is generally very white, and its price usually increases with whiteness. Calcite is the most used crystal form of calcium carbonate, with usually isotropic particle structure of the filler. It can be produced from three different mineral sources such as chalk, limestone and marble: the origin influences deeply the final properties of the filler because they are characterized by different hardness and purity.
Natural magnesium carbonate is natural magnesite, which is mostly used for refractory application. It is characterized by higher Mohs hardness (4) and specific gravity (2.9-3) in comparison to calcite: these are often considered a disadvantage especially because of the higher milling costs for production and more severe wearing of processing equipment (mills and extruders). It is also used as flame retardant filler in PVC, since the patent of Pirelli Cavi on 1974 and in ethylene-vinyl acetate rubber (EVA).
Huntite/hydromagnesite is a natural physical blend of magnesium calcium carbonate and magnesium hydroxy-carbonate: the ratios vary between 40 and 30% of hydromagnesite and 60-70% huntite and specific gravity depends on the ratio. The main deposits are present in Turkey and Greece and the level of impurities is very low33, giving a very white mineral. The blend has gained importance for flame retardant properties in polymer application, but it is mostly appreciated for reinforcing effect in rubber compounds due to their lamellar shape form and high surface area of particles. As micronized magnesite, also huntite hydromagnesite is also used as flame retardant filler in PVC for cables and in elastomeric compounds based on chlorinated polyethylene (CPE), EVA, and blend of nitrile-butadiene rubber with PVC (NBR/PVC).
A large amount of natural fillers is characterized by layered crystal structure, where sheets of different composition are stacked together by van der Waals forces. The main are talc, mica and clays (like kaolin), which are widely used especially as reinforcement agents for polymer application due to their general high aspect ratio, high specific surface
8
of particles and the good dispersion can be achieved in compound. In particular in the last decades nanosized layered clays have gained interest: they are composed by silicate stacks, potentially able to separate down in sheets with a thickness on the order of nanometres. The incorporation of such fillers in polymers have been widely studied especially in order to maximize their dispersion in the matrix: it is reported that nanocomposites with good dispersion show improved stiffness, heat distortion, gas barrier and flame retardancy of polymeric materials only with few percentage incorporated34.
1.2 Polyolefin composites highly filled with flame retardant
Polymeric composite materials are defined as polymers filled with solid particulate or fibrous fillers of organic and inorganic nature. They are heterophasic polymer systems, composed by two or more phases, which can interact each other. Lipatov35 classified the polymeric composite materials in three main systems according to the types of the components introduced in the polymer matrix:
• polymers filled with particulate or fibrous mineral and organic fillers, such as talc, chalk, carbon black, silica, polymeric powders, etc;
• reinforced polymers with continuous reinforcing inorganic and organic fibres, which are distributed in definite way in the matrix;
• polymer blend of polymeric components which are not thermodynamically compatible and form different phases with definite distribution of the phase separation regions
The final properties of composites are the result of the composition, structure and influence of single components properties together with the interphase phenomena. This means that introducing filler into a polymeric material cannot be considered only a method of modification of polymer, but it represents the creation of new material with different properties. In particular fillers introduced are chosen in order to obtain materials with improved specific properties, such as mechanical reinforcement, thermal stability and flame retardancy.
As reported in the previous paragraph, filler size and filler structure, together with the level of dispersion, are the main factors affecting the final properties of the composites.
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Even the amount of filler in the composites influences greatly the properties and it depends on filler characteristics and functionality: filler with very high specific surface area such as nanofiller can give the desired mechanical and gas barrier properties with very low percentage (usually less than 10 wt%) of dispersed filler34, while for compounds with flame retardant fillers (microsize particles), such as hydroxides, high amounts are necessary for obtaining acceptable level (usually more than 50 wt%)36.
Polyolefins are among the most interesting polymeric matrices for highly filled composites from the applicative point of view: especially polypropylene and polyethylene (and copolymers) based composites offer a variety of uses in different fields of application (construction, electric and electronic applications, transportation, etc) involving different types of functional filler.
The interactions, which take place in a particulate highly filled polymer, are
particle-particle and particle-particle-matrix interactions. Particle-particle-particle interaction is of great
importance in influencing the dispersion of the filler in highly filled composites, even if it is often neglected; this kind of interactions are mainly determined by the size and distribution of the filler particles. Usually more importance is given to particle-matrix interaction as interfacial adhesion between filler and polymeric phase. In fact, it is considered the most influent factor responsible for mechanical behaviour of polymer composite.
The understanding of these interactions is crucial for the final properties of the composites as they can be really problematic: for example, in the case of carbon black37, kaolin38, etc, high particle-particle interactions can create strong aggregates of filler, which are difficult to destroy also at high shear of processing and that are detrimental especially for the mechanical properties of composites.
Filler-matrix interactions can also be crucial for the thermal behaviour of polyolefin composites, as it has been reported by a variety of studies 39,40 mainly on polypropylene
composites. Polymer morphology and the structure aspects such as crystallinity, size of crystallites and spherulites can be changes deeply by the incorporation of filler. This effect is due to the nucleating activity of fillers, which depends on many factors, including surface energy, ion spacing and state of aggregation. The incorporation of high amount
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of inorganic fillers takes to the strong increase of nucleating sites, which influence the crystallization level and so the internal morphology of the composite.
The addition of rigid particles to polymers influences a few mechanical effects41, such as stiffness, stress, modulus, elongation, creep resistance and fracture toughness. Obviously all these parameters are dependent on the properties of the reinforcing particles and in literature it is possible to find a variety of theories and equations developed in order to describe how these parameters affect the macroscopic mechanical behaviour of filled polymers42.
1.2.1 Fillers for polymeric composites
1.2.1.1 Mineral flame retardants
Mineral fillers are inorganic non-reactive compounds that are added to the polymer during the final stages of processing to reduce the flammability of the finished product. The filler particles are usually under 10 μm in diameter, and sometimes in the submicron range. The particles are blended into the polymer and must be uniformly dispersed to ensure consistent flame retardant properties throughout the polymer. Most polymers require a high loading of filler to show an appreciable improvement to their flammability resistance, and the minimum volume content is usually about 20% and the average content is typically 50 to 60%22. Fillers should only be used in polymers that are chemically compatible, otherwise the mechanical properties and environmental durability of the material can be severely degraded. Fillers can have other adverse effects on the properties, including an increase to the viscosity and a reduction to the gel time of the polymer melt which makes processing more difficult. Many filler materials gradually break-down when exposed to moisture by hydrolysis, and this degrades their flame retardant action. Despite these problems, fillers are often used because of their low cost, relatively easy addition into the polymer, and high fire resistance. It is important to note that fillers are rarely used alone, but instead are used in combination with other flame retardants (such as organohalogen or organophosphorus compounds) to achieve a high level of flammability resistance.
To date43, a fairly simplistic view of their mode of action has been generally assumed. Mineral filler fire retardants (FRs) decompose endothermically, with the release of inert
11
gases or vapour, resulting in a fire retardancy effect. In order to be effective, this decomposition must occur in a narrow window above the polymer processing temperature, but at, or below, its decomposition temperature. In practice most of the suitable materials are group II or III carbonates or hydroxides. Four effects can be considered to contribute to its fire retardancy:
• Heat capacity of the filler prior to decomposition
• Endothermic decomposition, absorbing heat and therefore keeping the surrounding polymer cooler
• Production of inert diluent gases absorbing heat from the flame. Flaming reactions require a critical concentration of free radicals to be self-sustaining. If this concentration falls sufficiently, through temperature reduction or dilution, for example by the release of water or carbon dioxide, flame extinction will occur.
• Accumulation of an inert layer of inorganic residue on the surface of the decomposing polymer.
This will shield it from incoming radiation, and act as a barrier, to oxygen reaching the fuel, flammable pyrolysis products reaching the gas phase, and radiant heat reaching the polymer. Aluminium hydroxide (ATH), magnesium hydroxide (MDH), zinc borate, and hydroxycarbonates are the most commonly used mineral flame retardants. These inorganic fillers impart flame retardancy through a direct physical flame retardant action. Generally, they retard the flame and suppress the smoke through an endothermic reaction, releasing non-flammable molecules (H2O and CO2) and providing charring through
decomposition by-products, such as alumina (Al2O3) from ATH and magnesia (MgO)
from MDH.
Metal hydroxides are the most used family of Halogen Free Flame Retardants thanks to their benefits (Fig. 3).
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Fig. 3 Metal Hydroxide flame retardants benefits21
These mineral compounds are used in polyolefins, TPE, PVC, rubbers, thermosets and can also be used in some engineering polymers (such as polyamide).
The most widely used28 FR is aluminium hydroxide (Al(OH)3), (commonly referred
to as alumina trihydrate (ATH) and incorrectly formulated as Al2O3∙3H2O, even though
it is neither an alumina, nor a hydrate44, decomposes to form alumina (Al2O3) with the
release of water. Al(OH)3 breaks down endothermically (ΔH = +1.17 kJ/g) forming water
vapour (34 %), diluting the radicals in the flame, while the residual Al2O3 (66 %)builds
up to form a protective layer.
2 𝐴𝑙(𝑂𝐻)3 (𝑠) 180−200 °𝐶→ 𝐴𝑙2𝑂3 (𝑠) + 3 𝐻2𝑂 (𝑔)
ATH is a nonabrasive white powder that loses 34.6% of its weight by 350°C but begins losing weight at slightly above 200°C for the filler obtained by precipitation method, and sometimes as low as 180°C for ground versions. By 300°C it is essentially anhydrous alumina. Careful control of time and temperature can produce alumina monohydrate or Böhmite, AlO(OH), sometimes expressed as Al2O3∙H2O. It has been
proposed for use, by itself or with Mg(OH)2, in polymers processed at high temperatures.
ATH, Böhmite and Mg(OH)2 and their dehydration products are non-toxic for humans
and the environment.
There are two main categories of ATH, ground (g-ATH) and ultrafine precipitated (p-ATH)45. Both have similar thermal properties but differ in size and surface area. p-ATH
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usually raises the viscosity of elastomeric formulations undesirably, whereas g-ATH typically yields lower compound viscosities and improved processing of composites. In addition, the surface area of g-ATH particles is typically higher than that of p-ATH particles with the same average particle size. Higher surface area provides better flame retardancy and smoke suppression in elastomeric formulations.
Due to the relatively low decomposition temperature of ATH, it is typically used in PVC, EVA, and low density polyethylene (LDPE)-based wire and cable compounds, which have processing temperatures lower than 180°C46.
At present magnesium hydroxide (Mg(OH)2, MDH) is used at a much lower volume
than ATH, however, in some applications it competes; in applications above the water-loss range of ATH (i.e., around 200 °C) it can be used where ATH cannot.
MDH retards flame in the same way as ATH, except that decomposition occurs at higher temperatures (above 330 °C). The mineral decomposes into 69% magnesia and 31% water with an enthalpy of water release equal to 1.356 kJ/g. The endothermic decomposition reaction is
2 𝑀𝑔(𝑂𝐻)2 (𝑠) 330 °𝐶→ 2 𝑀𝑔𝑂 (𝑠) + 2 𝐻2𝑂 (𝑔)
Owing to its higher decomposition temperature, MDH is more often used in polymers with higher processing temperatures, such as polypropylene (PP), high speed PVC and EVA. Although MDH is currently used at lower volumes than ATH, MDH can be used in some applications where ATH cannot, including applications in which the processing temperatures are above water-loss range of ATH. MDH is generally more expensive than ATH but is more effective at the same weight percentage in imparting fire performance47. It must be noted that the flame retardant action of magnesium di-hydroxide is very effective up to 400 °C. Beyond this temperature, the exothermic character of degradation predominates. Metallic hydroxides (ATH and MDH) may also have a catalytic effect on the combustion of the carbonized residues produced, which would explain the incandescence phenomena observed during several flame retardant tests48.
14 1.2.1.2 Carbonates and hydroxycarbonates
All carbonates release CO2 at high temperatures but only magnesium and calcium
carbonates release it below 1000 °C, with magnesium carbonate presenting the lowest release temperature (550 °C). In addition, it was reported that carbonates lowered the flammability only through dilution. Therefore, carbonates are generally used as inert bulk fillers for polymers23.
Even though hydroxycarbonates are less widely used than other conventional flame retardants, they remain an alternative to metal hydroxides. In addition to the release of water, natural magnesium carbonate (magnesite) and synthetic magnesium hydroxycarbonate (hydromagnesite) also break down endothermically due to the liberation of CO2 at high temperature.
The thermal decomposition of hydromagnesite (4MgCO3∙Mg(OH)2∙4H2O or
5MgO∙4CO2∙5H2O) will release both H2O and CO2 at temperatures around 320–350 °C.
The total endotherm is about 800 J/g and H2O and CO2, account for about 53% of the
total material loss. These flame retardants reportedly have flame retardant efficiency similar to metal hydroxide in PP, LDPE/EVA, EVA, and poly(ethylene-co-ethyl acrylate) (EEA)23,49,50.
With a slightly higher decomposition temperature and different type of chemistry than metal hydroxides, hydroxycarbonates may be a suitable flame retardant for specific applications.
As well as ATH and MDH, naturally occurring mixtures of hydromagnesite (Mg5(CO3)4(OH)2.4H2O) and huntite (Mg3Ca(CO3)4) have similar potential as FRs.
Previous authors have discussed the decomposition of huntite and hydromagnesite6–8. Huntite particles have a platy morphology and the particles are usually about 1 μm or less in diameter, much smaller than hydromagnesite particles. It thermally decomposes between about 450 °C and 750 °C in two stages, releasing only carbon dioxide, leaving a solid residue of magnesium oxide and calcium oxide.
The thermal decomposition of mixtures of these minerals, through endothermic release of carbon dioxide and water, has led to several studies showing their potential applications, including FR additives for polymer compounds. The endothermic decomposition of hydromagnesite coincides with the temperature range at which polymeric materials, such
15
as ethylene vinyl acetate and polyethylene, thermally decompose. This is a good indicator that hydromagnesite has potential to perform well as an FR. Huntite decomposes between about 450 °C and 750 °C, a temperature range where most of the polymer has completely volatilised. This has led to a suggestion that huntite has little more influence than an inert diluent filler in terms of fire retardancy. It has been argued by the current authors9 that
the evidence in the literature does not back up this assertion. Recent work10 demonstrates that both huntite and hydromagnesite contribute significantly to the FR properties of polymer compounds. These data, together with the average values of heat capacities of the filler, its residue, and its gaseous decomposition products have been used to estimate the heat absorption by the filler, the residue, the evolved water vapour and carbon dioxide and the decomposition endotherm, shown in Fig. 4. The selection of mineral fillers is matched to the current work, including those fillers for which experimental data is reported. As the data have been calculated in energy units, the contribution to the individual fillers may be compared in absolute terms. Fig. 4 shows the energy absorption per gram of each of the processes undergone by the filler.
Fig. 4 Absolute estimation of heat adsorbed by potential FR mineral fillers (labels such as
HU40HM60 show the % huntite (HU) and hydromagnesite (HM) in the mixture)43
The higher decomposition temperature of MDH and particularly the greater contribution of the filler, increase its energy absorbing capacity by about 250 J g−1, compared to aluminium hydroxide. Comparing aluminium and magnesium hydroxide, it is evident that the difference between their relative effects arises from the higher
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decomposition temperature of Mg(OH)2, giving a larger contribution to the heat capacity
of the undecomposed filler, but a smaller contribution from the heat capacity of the residue, and from the heat capacity of the greater volume of water vapour released by the Al(OH)3 – even though the energy for such a release is almost identical for both fillers.
For the hydromagnesite, huntite and HU43HM57 mixture (43% huntite – 57% hydromagnesite), which has been proposed as an alternative to Al(OH)3, it can be seen
that the higher decomposition temperature of huntite gives a greater filler contribution, although overall HU43HM57 absorbs less energy than either ATH or MH.
1.2.1.3 Borates
Borates are another family of inorganic additives with flame retardant properties and have a synergistic effect with halogenated and not halogenated polymers51. In halogen-free systems, borates not only can be used alone in the silicon rubber but can also be used in combination with ATH or MDH in polymers such as EVA, PE, EPDM, and acrylics as an effective flame retardant and smoke suppressant. Among them, zinc borates such as 2ZnO∙3B2O3∙3.5H2O are the most frequently used. Their endothermic decomposition
(503 kJ/kg) between 290 and 450 °C liberates water (15%), boric acid and boron oxide (B2O3). There is also commercially available natural milled calcium borate, named
Colemanite, with formula Ca2B6O11·5H2O,mostly used in bitumen roofing membranes52.
The B2O3 formed softens at 350 °C and flows above 500 °C leading to the formation
of a protective vitreous layer (low melting glass forming additive). In the case of polymers containing oxygen atoms, the presence of boric acid causes dehydration, leading to the formation of a carbonized layer. This layer protects the polymer from heat and oxygen. The release of combustible gases is thus reduced6. In addition, unlike metal oxide, the dehydration product of metal hydroxide, which is known to cause glowing combustion in polyolefin, the zinc borate is known to suppress the glowing combustion. In 1985, RTM (Rio Tinto Minerals - formerly US Borax) first reported that zinc borate and ATH can form a porous and ceramic like hard layer at temperatures above 550 ◦C (during the combustion of cross-linked EVA)53. This hard layer acts as a thermal insulator for heat
transfer and it can prevent short-circuiting and sparking, as well as protect the underlying material. Bourbigot et al54. reported that partial replacement (5%) of ATH with zinc
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borate in EVA resulted in a significant increase in oxygen index from 42 to 51.5% and a significant reduction of heat release rate (HRR), delayed time to ignition, and reduction in smoke evolution. Recently, Shen et al.55 observed that partial substitution of MDH with fine particle size of zinc borate (5–10%) in EVA resulted in improvement in HRR, electrical properties, physical properties (excluding tensile strength), and the processability, as evidenced by a reduction in torque during compounding. The fire performance of the zinc borate and metal hydroxide interaction can be further improved with other additives such as silicone, melamine (MEL) polyphosphate, red phosphorous, and nanoclay56.
1.2.2 Surface treatment of the filler
Surface modification of fillers to give improved properties to a polymer composite is a topic that has received enormous attention over the last 30 years. Improvements in mechanical properties, dispersion of the filler (which leads to improved properties), improved rheology and higher filler loading have all been reported to accrue from rendering the surface more hydrophobic and hence compatible with the polymer or by enabling the filler to bond covalently, through hydrogen or ionic bonds to the polymer; or by changing the physical nature of the interface so that energy absorption can occur23. High filler loadings are needed to achieve the required flame retardancy in compounds and this can deeply deteriorate mechanical and rheological properties of compounds. One of the most common routes for improving the mechanical properties of polymer compositions containing fillers, flame retardant or not, is through surface treatment of the inorganic phase.
Surface treatment can have other advantages such as improved production, reduction in moisture content, chemical protection, reduced dustiness and increased bulk density24. The use of a filler coating can have different effect on flame retardant properties of the composites depending on the nature of the filler, of the treatment and of the polymer matrix used.
Commercially, many manufacturers of fillers offer surface coated grades, including the use of fatty acids or their derivatives in order to alter the surface energy of the filler
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promoting wet-out by thermoplastic melts and improved filler dispersion; silane coupling agents with alkyl, vinyl, methacryl, epoxy, amino functionalities usually are reported as improving tensile properties57 because they can considerably enhance filler to polymer bonding.
Surface treatment is a common method for changing both particle-particle and particle-matrix interactions especially for polyolefin compounds. It takes to combined
modification of composite properties and it is often used for evaluating the presence and the degree of interactions: the starting point is that hydrophilic characteristic of mineral fillers makes it difficult to combine with most polyolefin materials, which are usually hydrophobic. The compound used for the treatment (coupling agent, surfactant etc.) must be selected according to the characteristics of the components and the goal of the modification. This latter is very important, surface modification is often regarded as a magical tool which can solve all problems of processing technology and product quality24. The modification of filler surface with low molecular weight organic species contributes in preventing sedimentation and in reducing agglomeration (aid for dispersion), taking to the desired properties, especially for wire and cabling application systems58. Four main classes of modification can be recognized59:
• non-reactive treatment by using amphoteric surfactants
• reactive treatment such as coupling agents, mostly organosilanes;
• compatibilizers based on functionalized polymers like maleic anhydride (MA) grafted polymers (polymer-g-MA);
• encapsulation of filler with elastomers or use of functionalized elastomers during composite processing60;
This section is focused on the treatment’s effects on flame retardants properties and some examples are reported as follows. The most of them are on PP, where the detrimental effect on mechanical properties of high filler loading is usually the highest.
Stearic acid is one of the most used treatment, especially for calcium carbonate and it has shown great contribute to mechanical properties of highly filled composites. The principle of the treatment is the preferential adsorption of the polar group of the surfactant onto the surface of the filler. The high energy surfaces of inorganic fillers can often enter special interaction with the polar group of the surfactant. Preferential adsorption is
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promoted in a large extent by the formation of ionic bonds between stearic acid and the surface of CaCO3, but in other cases hydrogen or even covalent bonds can also form.
Surfactants diffuse to the surface of the filler even from the polymer melt, which is further proof for the preferential adsorption61. Because of their polarity reactive coupling agents also adsorb onto the surface of the filler. If lack of reactive groups does not make possible chemical coupling with the polymer, these exert the same effect on composite properties as their non-reactive counterparts62,63,64.
One of the crucial questions of non-reactive treatment, which, however, is very often neglected, is the amount of surfactant to use. It depends on the type of the interaction, the size of the treating molecule, its alignment to the surface and on some other factors. Determination of the optimum amount of surfactant is essential for the efficiency of the treatment. Insufficient amount of surfactant does not bring about the desired effect, while excessive amounts lead to processing problems as well as to the deterioration of the mechanical properties and appearance of the product65,66.
Zuiderduin et al.67 reported that calcium carbonate particles treated with stearic acid showed a larger increase in impact strength than the untreated in PP composites. They attributed the effect to the improved dispersion due to the lower particle-particle interaction induced by organic coating. Even the tensile properties were modified and specifically in Fig. 5 the reduction effect of stearic acid treated calcium carbonate on tensile strength of PP composites is reported: higher is the filler surface coverage and lower is the obtained strength. Adhesion and strong interaction, however, are not always necessary or advantageous to prepare composites of desired properties; plastic deformation of the matrix is the main energy absorbing process in impact, which decreases with increasing adhesion68,69,70.
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Fig. 5 Effect of non-reactive surface treatment of a CaCO3 filler with stearic acid on the tensile
strength of PP composites: (◯) non-treated; (△) 75% surface coverage; (▢) 100% surface coverage24
Oyama et al71 investigated the properties of composites composed of isotactic
polypropylene (iPP) and MDH superficially modified by dodecylphosphate (DP) and dodecanoic acid (DA) as reported in the Fig. 6.
Fig. 6 Representation of organic reagents adhering on the MH surface [(a) MH-DA and (b) MH-DP]
As flame retardant results they obtained that the incorporation of only 1.8 wt% of DP in the total weight of the iPP/MH-DP composite was highly effective in reducing the flammability of iPP by lowering the peak of HRR to 39% in (70/30) iPP/MH- DP composite compared with that of neat iPP (Fig. 7). Since effects of DA, with the same
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dodecyl chains, were not significant, they concluded that the phosphate group in DP functions as a flame retardant agent.
Fig. 7 Change in HRR during combustion of (70/30) iPP/MH composites in air [···, neat iPP;- · -,
iPP/MH;- - -, iPP/MH-DA; —, iPP/MH-DP]
Polymeric compatibilizers are also widely used for improving interfacial adhesion in composites, especially because they are of great practicableness in industrial compounding. Azizi et al.72 analysed the effect of PP-g-MA on the mechanical properties of PP/CaCO3, PP/talc and their mixture. They point out that the incorporation of
compatibilizer took to the increase in tensile strength and in tensile modulus compared to the composites without compatibilizer. Moreover, impact strength increased with compatibilizer in both the case of talc and carbonate, due to the better dispersion of the fillers, which allows the impact energy to be more uniformly distributed. The importance of interphase interdiffusion and entanglement density is clearly demonstrated by another experiment made by Felix and Gatenholm73, who introduced maleic anhydride modified
PP (MA-PP) into PP/cellulose composites and achieved an improvement in yield stress. The increase was proportional to the molecular weight of MA-PP (Fig. 8). Maximum effect of functionalized PP was found with fillers of high energy surfaces74,75,76 or with those capable of specific interactions, e.g. ionic bond with CaCO377 or chemical reaction
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Fig. 8 Effect of interdiffusion of the functionalized polymer with the matrix on the mechanical
properties of PP/cellulose composites. Molecular weight of MA-PP: (◯) non-treated, (△) 350, (▽) 4500, (▢) 3.9 x 104 73
1.3 Types of polyolefins
A variety of flame retardant polymers have been developed over the past twenty years, and many of these are suitable for use in fibre composites. The incorporation of bromine, chlorine or phosphorus into the molecular structure of a polymer is the most common method used to improve the flammability resistance of thermoset resins and thermoplastics. The incorporation of nano-sized particles into a resin is another approach for improving fire resistance. Polymer nanocomposites are rapidly emerging as an important class of flame retardant materials, and several other methods can be used to produce flame retardant polymers, including chemical modification of the molecular network structure by graft copolymerisation79.
The common matrices for the incorporation of the natural flame retardant fillers are thermoplastic polyolefin materials like polypropylene (PP) and polyethylene (PE), together with its copolymers, such as ethylene-vinyl acetate (EVA), ethylene-alkyl acrylate (EMA,EBA and EEA), metallocene copolymer ethylene-octene (mULDPE) and ethylene-propylene rubber (EPR/EPDM). Due to their intrinsically better compatibility with ATH or MDH, olefin copolymers such as EVA and acrylate copolymers are easier
23
to process with these and other polar mineral additives than are the purely hydrocarbon polymers.
Markets for building/construction, electronic consumer products, motor vehicles, and textiles have shown an important growth of FR polyolefin compounds based on polyolefin and it is not surprising thanks to their affordability and properties which make them appropriate choices for many applications where possible ignition threats exist80.
Polymers with flame retardant properties could also be used as blends to enhance the flame retardancy of other polymeric materials; the major criteria being compatibility and miscibility4.
1.3.1 EVA
Fig. 9 Ethylene-vinyl acetate
Ethylene vinyl acetate (EVA) copolymer (Fig. 9) is widely used in numerous applications in biomedical, food industry, construction, transport, and electrical engineering areas such as drug delivery device81, hot-melt adhesive82, packaging83 and essentially electric cable material84, etc. However, the high flammability of this polymer reduces its use for various applications. Thus, EVA must be protected using flame retardant (FR) agents. Various FR have been used to improve flame retardancy of EVA. Halogenated flame retardants (HFRs) have been extensively used over the past 40 years. Nevertheless, many studies reported by various authors indicate that HFRs exhibit health and environmental risks85,86. Moreover, they lead to the production of halogenated dioxins and furans during combustion. For that purpose several HFRs have been restricted by the Stockholm Convention on persistent organic pollutants87 and the use of halogenated compounds has started to be reduced in Europe. In the recent years, inorganic
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fillers (particularly precipitated aluminium trihydroxide) have been widely explored especially for EVA in wire and cable industry88,89.
EVA is a random copolymer of ethylene and vinyl acetate (VA) segments, typically formed via free radical polymerization. EVA generally contains 1-50% of the VA comonomer along the carbon chain backbone. The presence of the bulky polar pendent, VA, provides the ethylene backbone an opportunity to manipulate the end properties of the copolymer by varying and optimizing the VA content90. The low VA content
copolymers (e.g., 9 wt%) are essentially modified LDPE. They have a reduced regular structure compared to the higher VA content EVA copolymers. The enhanced intermolecular bonding between VA ether and carbonyl linkages is promoted by increased polarity due to high VA content91,90.
Typically, this thermoplastic resin is copolymerized with other resins like LDPE and LLDPE or it is part of a multilayer film. In blends and copolymers, the percentage of EVA ranges from 2% to 25%. It enhances clarity and sealability of olefins (LDPE/LLDPE) whereas a higher percentage of EVA is often used to reduce the melting point and to improve the low temperature performance92.
The thermal degradation of EVA and the modes-of-action of metallic hydroxides are well documented. EVA decomposes by a two steps mechanism, as we can see from the TGA analysis in Fig. 10.
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In the first step there is the loss of acetic acid (300-350 °C), resulting in the formation of unsaturated polyenes (Fig. 11).
Fig. 11 Thermal degradation mechanism of EVA (step 1): release of an acetic acid molecule94.
The second decomposition step involves random chain scission of the remaining material, forming unsaturated vapor species (~430 °C), such as butene and ethylene95. Deacetylation proceeds through β-elimination of the vinyl acetate groups present in the EVA molecules96, with up to 100% conversion to polyethylene macromolecules containing polyene sequences having up to four conjugated double bonds. These crosslinking reactions lead to the formation of a char-like residue. Therefore, the initial polyene formation is of interest as it represents the represents stage of char formation95.
Metallic hydroxides based on aluminium and magnesium are frequently used to improve the fire performance of EVA. They release water during heating through an endothermic decomposition which allows heat absorption and therefore polymer cooling. Water release leads to fuel dilution in vapor phase delaying ignition. In some cases, fillers also promote charring. Since high loadings of fillers must be used, the formation of an inert layer on the surface of decomposing polymer may improve flame retardancy through a so-called barrier effect36. Layer formation is assigned to various possible mechanisms. The main one is the ablation of polymer, which makes the surface richer in inorganic fillers97.Another one is related to fillers migration to the polymer surface during heat exposure98.Nevertheless it must be kept in mind that the accumulation of fillers at the surface is much more complex due to additional mechanisms including agglomeration, change in particle orientation, exfoliation induced by temperature and decomposition (in the case of montmorillonite) and bubbling99,100.
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1.3.2 POE
Polyolefin elastomers (POEs) are a relatively new class of polymers that emerged with recent advances in metallocene polymerisation catalysts. POEs represent one of the fastest growing synthetic polymers; and POEs can be substituted for a number of generic polymers including ethylene propylene rubbers (EPR or EPDM), EVA, styreneblock copolymers (SBCs) and PVC101. Polyolefin elastomers are compatible with most olefinic materials. They are excellent impact modifiers for plastics, and they offer unique performance capabilities for compounded products. Polyolefin elastomers have proven their viability in flexible plastics applications and are used in a variety of industries. Further advances in application development, product design and manufacturing capabilities will provide increasing opportunities for the future.
Polyolefin elastomers are copolymers of ethylene and another alpha-olefin such as butene or octene. The metallocene catalyst selectively polymerises the ethylene and comonomer sequences and increasing the comonomer content will produce polymers with higher elasticity as the comonomer incorporation disrupts the polyethylene crystallinity. Furthermore, the molecular weight of the copolymer will help determine its processing characteristics and end-use performance properties with higher molecular weights providing enhanced polymer toughness102.
Metallocene catalysts consist of cyclic ligand such as cyclopentadienyl, indenyl, and fluorine combined to a transition metal in a sandwich structure. Homogeneous metallocene catalysts can be used to prepare stereoregular polyolefins having constant chemical composition and narrow molecular weight distribution that results from the catalysts having a single type of polymerization site103,104. Other advantages of POEs include good impact resistance, easy colorability, high flexibility, and recyclability. The elastomers are usually light-weight low density polyethylene that be easily mixed, formed and processed on plastic or rubber equipment. POEs also offer unique performance capabilities for compounded products.
However, POEs suffer from some drawbacks such as low modulus, low strength, and low thermal stability, which hinder the effective use of these elastomers.
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In the early nineties, Dow Chemical Company have introduced this new family of polyolefins based on ethylene and 1-octene using a homogeneous Constrained Geometry Catalyst Technology (CGCT)105 under the tradename of ENGAGE™ Polyolefin
Elastomers (POE).
POE has hard (crystalline) and soft (amorphous) phases for physical crosslink that makes good processability of thermoplastic and rubber elastic. So, it has better physical properties than conventional synthetic rubber-like ethylene propylene diene rubber (EPDM) and styrene-ethylene-butylene-styrene rubber (SEBS). It is generally used as polymer modifiers because of its excellent thermal stability, weather resistance, ageing resistance, and high toughness.
There are a large variety of POEs depending on the type of a-olefin. Generally, a co-monomer such as propylene, 1-butene, 1-hexene, or 1-octene is used with ethylene (Fig. 12) and the crystalline structure of the POE is affected by the co-monomer type and content: the crystallinity is affected by the number of branched chains (content of the comonomer unit) and their shape (linear or cyclic). With increasing content on hexene or octene the crystallinity decreases. The influence of the linear comonomers on the crystallinity is as strong as that of the cyclic comonomer. The effect of the different comonomers on the properties depends on their chemical structure. The incorporation of aliphatic comonomers reduces the crystallinity resulting in reduced tensile strength106.
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1.4 Coupling agents and processing aid agents
To improve performance properties in a polymer composite, the polymer-filler interface properties can be modified to improve compatibility between the polymer and filler particle23,59. The interface properties can be modified by reacting fillers with the following three major types of coupling agents: fatty acids, alkoxysilane, and maleic anhydride grafted polymers.
Fatty acids or salts of fatty acids are commonly used for treating metal hydroxides108. For fatty acid surface treatment, it was postulated that organic salt can form on the surface of magnesium hydroxide. This treatment is believed to reduce the adhesion between polymer and magnesium hydroxide as well as to improve dispersion of magnesium hydroxide.
Alkoxysilane, with a general formula of RSi(OR)3, contains one organic group (R)
and three alkoxy groups (OR). The organic group can be inert or exhibits vinyl or amino or epoxy functionalities to produce a strong bond with the polymer matrix. On the other hand, the alkoxy group can react with hydroxyl on the metal hydroxide surface to produce a Si-O-Mg bond. A silane coupling agent will act as a link between an inorganic substrate (such as metal, mineral) and ad organic material (such as organic polymer, coating) to bond, or couple, the two dissimilar materials together like in Fig. 13.
Fig. 13 Silane coupling mechanism109
Furthermore, the remaining alkoxy groups can also undergo hydrolysis and condensation reactions to polymerize silane on the filler surface. Silane treatment of magnesium hydroxide in the ethylene vinyl acetate (EVA) composite has been found to increase tensile strength and elongation relative to an untreated composite.
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Maleic anhydride grafted polyethylenes or polypropylenes are also used to help improve adhesion between polyolefins and fillers. It was found that carboxyl groups on maleic anhydride grafted polypropylene interacted with hydroxyl groups from the magnesium hydroxide surface. The addition of maleic anhydride grafted polyethylene was found to impart an interlayer between magnesium hydroxide particles and polypropylene matrix, which resulted in reducing stress whitening of the cable insulation compound108.
Since most common fillers have no inherent reactivity with inert, nonpolar polyolefins, reactive dispersion aids and coupling agents are essential. Without proper dispersion and bonding to the matrix, fillers and fibers added to polyolefins are greatly limited in improving a compound’s properties—or they may even degrade essential properties, acting simply as contaminants in the polymer110.
The selection of a coupling agent can have an important effect on how well the filler is dispersed and bonded to the matrix, how much can be incorporated, and on how easily the compounding/ converting process runs111,112.
The overarching reason for using processing aid additives usually has more to do with reducing overall processing times and costs than with enhancing properties of the compound. Even though polyolefins are relatively easy to process, molding operations still can benefit from processing aids that decrease the viscosity of the melt by lubricating the polymer internally, or by simplifying demoulding by lubricating the surface of the resin.
Thus, processing aids are essentially tools that reduce the time and energy to plasticize the melt, completely fill the mould, and expediently allow a part to be extracted. Such tools are also helpful for solving processing problems that can reduce an operation’s productivity113.
Internal processing lubricants are somewhat soluble in the polymer and allow polymer chains in the melt to slide against one another with minimal friction. This lubrication decreases melt viscosity and reduces the screw torque and processing energy required for mixing and plasticization.
These processing aids are effective at 0.1-2% concentrations when added during compounding, or even when added as pellet concentrates to dried resin right before the
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processing. Some basic families of flow-enhancing aid chemistries include polymeric additives, such as silicones, fluoropolymers (e.g., PTFE), or metallocene-catalysed polyolefin polymers or oligomers; these are relatively expensive internal lubricants, but they tend to be heat resistant and resist migrating after processing, and tend not to reduce impact strength or other properties as much as other processing aids (and they may even improve the physical properties of the resin).
1.5 Rheological behaviour of polymers
Synthetic polymers are widely used in the plastic industry to produce goods for many different applications. In order to give the plastic object its desired shape, polymers are typically melted, processed in the liquid state, and then solidified. One of the crucial aspects in the engineering of polymer processing operations is therefore the study of the flow behaviour of the polymeric liquids. However, differently from ‘simple’ liquids like water, polymeric liquids behave in an extremely complex way (Newtonian or non-Newtonian fluids). The science of flow or deformation of complex fluids is called rheology (from the Greek words rheo and logia meaning flow and study of, respectively)114.
Rheology is the science dealing with the deformation and flow of materials. For polymers, understanding the deformation and flow, both in the extruder and die, is critical to optimum operation of the extrusion process115.
Rheological studies reduce the flow complexity to a set of basic simple flows, since complex flows can be considered as a combination of simpler ones. Flow is the permanent deformation of a liquid with an applied force. This can also be interpreted as a strain (deformation) occurring as a result of a stress (force/area). In the case of polymers, flow occurs when the chains are caused to slide past one another freely116.
Flow occurs in at least one of four ways. These four types of flow are pressure flow, drag flow, shear flow, and elongation flow. A liquid may experience one or more of these types of flow at the same time. The first two types are driving mechanisms for flow to occur. The last two types describe how the fluid deforms during flow. Stress accompanies the driving forces and deformation is quantified by strain.
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There are two driving mechanisms that can cause flow to occur. These two mechanisms are known as pressure flow and drag flow. If neither of these is present in a system, there will be no flow present either.
There are also two flow fields in which polymer deformation occurs. These fields are shear flow and elongational flow.
The set of rheological measurements in simple, well-defined flow conditions constitutes the so-called rheometry, and includes both shear and extensional flows, the latter being encountered in many shape-forming operations like fiber spinning, blow molding, etc.
Pressure flow is experienced when the flow front is moving due to pressure differences. Fluids flow from a high pressure toward a lower pressure in a system. A common example of this is found in the water faucet. Within the plastics industry, pressure flow is found in injection-molding applications. The polymer is forced into the mould using pressure flow.
Drag flow is induced by a surface dragging over a fluid. This is similar to a boat causing water to flow as it travels. In the extrusion industry, the screw rotating causes the polymer to flow by dragging the polymer along the barrel.
Shear flow is defined as a flow field in which adjacent fluid elements are moving at different velocities. Polymers experience friction as they flow along a stationary object, whether it is the barrel or part of a mold. The polymer next to the stationary object is moving at a much slower rate than the polymer in the center of the channel or far from the wall.
Elongational flow occurs when a fluid is being stretched in the direction of flow. This action takes place through the acceleration of the fluid elements along a streamline. The most common occurrence of this in the plastics industry is found as polymer is stretched by a puller as it exits an extrusion die.
Shear behaviour was pointed out. Fig. 14 shows a cube or a volume element that is undergoing a shear deformation due to a force, F, that is applied as shown to the upper surface which has a cross-selection area of A.
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Fig. 14 Schematic of an element undergoing deformation by shear117
The resulting shear stress, τ, is defined as 𝜏 = 𝐹
𝐴
Equation 1 Shear stress
Assuming this degree of deformation is not great, one can define the associated shear strain, γ, as
𝛾 = 𝑋 𝑌
Equation 2 Shear strain
Relative to Fig. 14 one can view this process as sliding of layers of material (fluid) across one another.
Another important parameter that is used extensively is that of shear rate, Y., which is also known as the velocity gradient. This is given as
𝛾̇ = 𝑑𝑥 𝑦𝑑𝑡= 𝑑γ 𝑑𝑡 (𝑠 −1 𝑟𝑒𝑐𝑖𝑝𝑟𝑜𝑐𝑎𝑙 𝑠𝑒𝑐𝑜𝑛𝑑𝑠) Equation 3
The significance of this term is relate to how fast is the shearing process, how high is the shear rate, (or how high is the velocity gradient) relative to the speed or rate at which molecular motions can occur for a given set of conditions.
Fig. 15 attempts to illustrate the realistic range for the values of 𝛾̇ for such polymer process as injection moulding, etc. It is possible to observe that the range is rather large, values from 1-104 s-1 are allowed for different polymer processes. Note that high speed injection molding tends to be the upper shear rate values while compression molding is
33
an example of a low shear rate process. Also included in this figure are the general shear rate ranges for various types of instruments used for measuring rheological properties of fluids.
Fig. 15 Range of shear rate that typically are involved with the general types of rheometers and are
also observed in polymer processing117
In expressing the resistance of a material to flow, the term viscosity (symbolized by η) is the common indicator used. This parameter relates the observed shear stress to the imposed shear rate. One of the basic laws of flow extends from Newton’s law of viscosity which is
𝜏 = 𝜂𝛾̇
Equation 4
where it assumes η to be independent of 𝛾̇; hence the law is linear in nature. Also, it is noted that η = τ/𝛾̇= (Force/Area) ∙ time = Pa∙s where represents the accepted SI units.
Many low molecular weight systems display flow behaviour in laminar flow ( Fig. 16) that is describable by Equation 4 and hence are called Newtonian fluids.
Fig. 16 Laminar flow 118 Fig. 17 Turbolent flow 118
Instead, polymer melts and solutions often deviate from this linear behaviour. With Newton’s law of shear viscosity (Equation 4) we realize that a plot of shear stress, τ, versus shear rate, 𝛾̇, provides a straight line whose slope will be the viscosity (Fig. 18 line
