Dipartimento di Chimica e Chimica Industriale
Corso di Laurea magistrale in Chimica Industriale
Curriculum materiali
Flame retardant properties of polyolefin
compounds with high loading of micrometric
mineral fillers
Supervisor:
Prof. Giacomo Ruggeri
Industrial Supervisor:
Dott. Camillo Cardelli
Examiner:
Dott. Dario Puppi
Anno Accademico 2019-2020
Candidate:
Michela Meucci
INTRODUCTION ... 1
1.1 Cables design and fire safety ... 1
1.2 Flame retardant systems for polymers ... 3
1.3 HFFR (Halogen Free Flame Retardant) solutions ... 8
1.3.1 Polymers ... 10
1.3.2. Flame retardant inorganic fillers ... 15
1.3.3. Additives ... 19
1.4 Flame retardant properties of compounds and cables ... 22
2. Objectives of the work ... 27
3. Experimental part ... 29
3.1 Materials ... 29
3.2 Instruments and methods ... 30
4. Results and discussion ... 37
4.1 Flame retardant properties of polyolefin compounds with high loading of micrometric mineral filler ... 37
4.2 Formulation for cable application (case study) ... 38
4.2.1 Polymers ... 38
4.2.2 Fillers ... 42
4.2.3 Compatibilization ... 46
4.3 Mechanical properties of the highly filled POE compounds ... 54
4.4 Flame retardant properties of the highly filled POE compounds ... 60
4.4.1 Limited Oxygen Index (LOI) ... 60
4.4.2 Vertical burning test ... 62
4.5 Design of Experiment (DoE) ... 67
4.5.2 Evaluation of a principal component analysis (PCA) influence on behaviour of the system ... 67
4.6 Study of the crystallization behaviours of compounds on the mechanical properties ... 78 5. Conclusion ... 88 REFERENCES ... 88 Appendix I ... 95 Appendix II ... 99
1
Introduction
1.1 Cables design and fire safety
“Plastics – the Facts 2018. An analysis of European latest plastics production, demand and waste data.” is a statistic suggest that the consumption of polymer-based materials has been increasing rapidly in recent years.(Fig. 1)
Fig. 1 Polymer production in million tons in 2016-2017.1
Production increased from 335 million tons in 2016 to 348 million tons in 2017, this is because polymers are used in many application fields: medicine, construction, automotive, electronics, etc.
The increase in the use of polymers is due to the synergy of their properties, such as low density, low thermal conductivity, and high electrical resistance unit with the low cost of materials and ease to process. Most of polymers, however, suffer from thermal degradation and are highly flammable which increases fire hazards when used in practical applications, thus reducing their use.
Flame retardants prevent or delay the combustion of materials and are indispensable in the protection of plastic products, electric devices, construction materials or textiles. The most important application for flame retardants polymers is the construction sector, plastics cables are used in the construction of new buildings; but also, the transportation
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industry is one of the most important application and the sector of electrics and electronics is expanding.
Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses. Cables consist of three major components: conductors, insulation, protective jacket. The design of individual cables varies according to application. Today, there are available three ranges of voltages for cables:
• Low-voltage design often those used in industrial settings are rated to 2 kV or lower
• Medium-voltage designs range from 2 kV to 68 kV • High-voltage cables are those rated above 68 kV
A typical, single-conductor, low-voltage design is usually constructed of the following:5 • An aluminium or copper conductor
• Insulation • Jacketing
The conductor and insulation shields are necessary to provide a homogeneous electric field to carry fault currents and for touch protection of the high voltage part. This safety mechanism prevents possible discharges that can damage the cable, equipment or worse yet, cause injury.
LV cables normally have a low electric field around the conductor; therefore, screening is not mandatory, but it can be helpful to carry fault currents and to give touch protection to the cable. A screening function can also be provided by metallic armour.
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1.2 Flame retardant systems for polymers
The flammability of a material is not an intrinsic property, like its density or heat capacity, but is dependent on the fire conditions. Changing the material composition, for example by the addition of a fire-retardant filler, will also change its reaction to fire behaviour and will reduce the dripping tendency. There may be an induction period (involving smouldering) before flaming ignition takes place, then a rise in temperature until ventilation-controlled burning occurs (usually 800–1000 °C)8, followed by decay as fuel is consumed, shown schematically in Fig. 3
Fig. 3 Stages in a fire.9
The burning of plastic materials is a process that includes many stages, but there are the three essential stages required to initiate the combustion: 10
• Heating, the enough temperature of the solid polymer is raised either due to an external heat source such as radiation or a flame, or by thermal “feedback”. • Thermal decomposition or pyrolysis is and endothermic process which requires
the input of enough energy to satisfy the dissociation energies of any bonds to be broken (200-400 kJ/mol).
• Ignition, during this phase the impact of the heat on a polymeric material causes an increase in temperature. If a sufficiently high temperature is reached, then chemical bonds break, and volatile fragments are produced. These gases react with the oxygen in the air and form visible flames.
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In the presence of a source of an enough heat, polymer will decompose and evolving in flammable volatiles. They combine with an oxidative atmosphere and depending on the flammability limits of the polymer and temperature, ignition (either flash or autoignition) occurs, and flames will subsequently release gaseous products, smoke, and heat.
Fig. 4 Schematic representation of a polymer combustion cycle.11
Then a combustion cycle will be established, in the presence of a source of sufficient heat, polymers will decompose evolving flammable volatiles and produce the H2–O2 scheme propagating the fuel combustion by branching:12
H· + O2 OH· + O· (Eq.1) O· + H2 OH· + H· (Eq.2)
The main exothermic reaction which provides most of the energy maintaining
OH· + CO CO2 + H· (Eq.3)
Pure polymeric materials degrade via one or more of the following simple mechanisms:13 • End chain scission, individual monomer units successively cleaved from chain
end
• Random chain scission, scissions occur at random locations along the polymer chain
5
• Chain stripping, atoms, or groups not part of the polymer backbone is cleaved off • Cross linking, bonds created between polymer chains
Us National fire Incident Reporting System shows that around 15% of deaths are caused by smoke, which can contain different asphyxiant gases causing incapacitation, soot, which limits visibility, acid gases able to irritate eyes and nasal passage and to cause respiratory pain and inhibition of breathing.
Fig. 5 Causes of fatal residential building fires in 2018 (2000 fatal fires estimated)14
Based on this concept, different types of fire retardants (FRs) have been designed and developed by various researchers to decrease or delay the flammability of polymers. Moreover, the flame toxicity is reduced.
Since the end of the last decade, different types of FRs have been used for the synthesis of FR polymers either as chain extenders or monomers. A FR does not burn but prevents or delays the flammability of the material in which it is incorporated by releasing non-flammable gases or by forming char during burning or by interfering with radical propagation into flame. There are several ways in which the combustion process can be retarded by physical action:9
• By cooling: endothermic reactions cool the material.
• By forming a protective layer: obstructing the flow of heat and oxygen to the
polymer, and of fuel to the vapour phase.
• By dilution: release of water vapour (H2O) or carbon dioxide (CO2) may dilute the radicals in the flame so it goes out.
6 Or chemical action:
• Reaction in the gas phase: the radical reactions of the flame can be interrupted
by a flame retardant. The radical concentration falls below a critical value, and the flame goes out. The processes that release heat are thus stopped, and the system cools down. However, interfering with the flame reactions often results in highly toxic and irritant partially burnt products, including CO, which generally increase the toxicity of the fire gases while reducing fire growth.
• Reaction in the solid phase: the flame retardants work by breaking down the polymer, so it melts like a liquid and flows away from the flame (“runaway effect”). Although this allows materials to pass certain tests (like American UL-94 V2 vertical fire test), sometimes fire safety is compromised by the production of flammable drops (like in the European EN 50399 for cables).
- Char formation: better solid-phase flame retardants are those which form a layer
of carbonaceous char on the polymer surface. This can occur, for example, by the fire retardant removing the side chains and thus generating double bonds in the polymer (like do, for example, PVC by releasing HCl and PVAc by releasing acetic acid). Ultimately, these form a carbonaceous layer by forming aromatic rings. Char formation usually reduces dripping and prevent part of organic substrate from combustion.15
The basic FR mechanism varies depending on the type of FR used, chemical structure, and interactions between the polymer and FR. Based on their mechanism, FRs are classified into four categories:11
• Gas phase, showing activity by releasing non-flammable gases into the flame
zone area to decrease the heat and oxygen levels, which are responsible for flame propagation. The quantity of airborne components is low because FRs limit the heat released into the flame zone area. When polymers containing mineral fillers as FRs are exposed to heat or flame, the mineral fillers start to decompose and release non-flammable gases, such as CO2, H2O, NH3, PO⋅ and some acids, resulting in a reduced oxygen content and cooling the flame zone area.
• Endothermic, showing FR activity by forming endothermic radicals that absorb
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For example, hydrated fillers when incorporated in polymers decompose endothermically at around 200-400°C and decrease the temperature to a value less than that required for polymer degradation or combustion.
• Solid phase, like phosphorus acting as solid-phase FRs. When exposed to flame,
they form the corresponding acids, which are capable of fast dehydration and form an insulating char layer on the surfaces of polymer materials. The char layer can protect the substrate from oxygen attack and heat transfer, which is necessary for flame propagation.
• Intumescent: the incorporation of blowing agents in synergy with highly efficient char-forming solid phase compounds causes swelling behind the surface layer and provides much better insulation under the protective barrier. The same technology is used for coatings for protecting wooden buildings and steel structures.
Fig. 6 The 5 effects of flame retardant mechanism16
There are two commonly used methods to achieve flame retardancy in polymers:17 • Additive type, in this method, FR polymers are prepared via physical mixing.
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bonding between the additives and polymer. The FR additives are dispersed evenly throughout the polymer matrix via physical interactions, such as H-bonding, van der Waals forces, and ionic interactions. Most additive FRs are not compatible with polymers and may experience phase separation and leaching. As there are no covalent bonds between the polymers and additive FRs, this method is not effective in improving the FR properties of polymers. Therefore, in order to increase the FR activity, large quantities of FR additives should be incorporated, which adversely affects the mechanical properties of the polymers. To overcome to this issue, surfactants and coupling agents are frequently used increasing dispersion and homogeneity of polymer and FR system.
• Reactive-type methods, using this method, FR polymers are synthesized via the
introduction of reactive FR additives into the polymer in the form of a monomer or polymer precursor. Consequently, reactive FRs are more effective in improving the FR activity of polymers compared to additive-type FRs owing to the presence of covalent bonds between the FR compounds and polymers; under such conditions, the FR additives are neither phase-separated nor leached out.
1.3 HFFR (Halogen Free Flame Retardant) solutions
The main classification of FR additives is between:
• Halogenated Flame Retardants like chloroparaffins, organic brominated FR in
combination with antimony trioxide, halogenated liquid phosphates, …
• Halogen Free Flame Retardant (HFFR) like metal hydroxides, ammonium
polyphosphate, phosphorous based FR, melamine-based salts, …
In recent years, the halogenated have been supplanted by HFFR due to the growing interest in the toxicity and bioaccumulation problems of halogen.14
In addition, halogenated flame retardants show following disadvantages:
• Release heavy black smoke, which can prevent people from leaving a burning building.
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• Both the halogenated organic compounds and the most common synergists (ATO and chloroparaffin) are suspect from the point of view of the impact on the environment and from the point of view of health and safety.
For this reason, flame retardant halogen-free compounds (HFFR) have gained more attractiveness and have been studied by many researches. The HFFR additives are further divided in different classes based on their nature and composition:11
• Mineral fillers
• Boron-based (B-based) flame retardants • Phosphorus-based (P-based) flame retardants • Nitrogen-based (N-based) flame retardants • Silicon-based (Si-based)
• Intumescent
Main characteristics required to the HFFR compounds are: • High flame retardancy
• Low smoke emission • Low toxicity of smoke • No corrosive gases • Low calorific load
Low hazard cables produced with HFFR compounds are mainly used for buildings (hospitals, airports, schools, and commercial centres), public transportation (subway, railways, ships airplanes) and industrial areas (power plans).
The main components of HFFR polyolefin compounds for cables are:
• Polymers
Polar PE-copolymers: Ethylene Vinyl Acetate (EVA), Ethylene Methyl Acrylate (EMA)/ Ethylene Ethyl Acrylate (EEA)/ Ethylene Butyl Acrylate (EBA).
Non-polar PE-copolymers: ULDPE (otherwise called “POE” corresponding to Poly Olefin Elastomers) based on C2-C8 or C2-C4 copolymers Flexible PP copolymers, EPR/EPDM, LLDPE, metallocene LLDPE, PP rigid copolymers.
10 • Flame retardant fillers
Fine precipitated fillers: Aluminium trihydrate (ATH), Magnesium hydroxide (MDH)
Ground fillers: coarse ATH, natural MDH (brucite), Huntite/Hydromagnesite. • Other fillers: Aluminium monohydrate (Boehmite), CaCO3, MgCO3, Zinc
borate, nanofillers
• Coupling agents: Maleic anhydride grafted polymers, amino silane, vinyl silane
• Lubricants/processing aids: Silicon gum, silicon oil, EVA wax, stearic
derivatives.
• Stabilizers/antioxidants: Hindered phenols, phosphites, thioesters, metal deactivators and acid scavengers.
1.3.1 Polymers
The organic component of halogen-free flame retardant polymer compounds mainly uses ethylene-vinyl acetate (EVA)18 copolymers as polymer bases and ULPDE/POE like ethylene-octene or ethylene-butene copolymers, produced by metallocene catalysis. Both these polymeric families are thermoplastic semi crystalline having low crystallinity. EVA decomposes thermally in two stages: a first stage is the loss of acetic acid which produces an unsaturated polymeric residue; the second stage is the decomposition of this chain unsaturated polymer (similar to that of PVC) (Fig. 7).19
11
Fig. 8 Analysis TGA of Engage 8450.15
In this work of thesis will be used as polymeric matrix ULDPE/POE, provided for R&D by DOW.
POE are a relatively new class of polymers and it can replace for several polymers including ethylene propylene rubbers (EPR or EPDM), ethylene vinyl acetate, styrene-block copolymers (SBCs) and plasticised poly vinyl chloride (PVC). POE 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 have emerged as a leading material for automotive exterior and interior applications (primarily in thermoplastic olefins TPOs via impact modification of polypropylene), wire and cable, extruded and moulded goods, film applications, medical goods, adhesives, footwear and foams.21
POE are compatible with most olefinic materials and are available in a wide range of grades to meet the most demanding processing and performance needs. POE is available with properties ranging from almost totally amorphous to semi-crystalline and low to very high molecular weight.22
POE are often chosen over alternatives because they are:
• Suitable for thermoplastic or crosslinked (peroxide or moisture-cure) applications, either as the main polymer or as a value-enhancing ingredient in compound formulations.
• In pellet-form for use in both batch and continuous compounding operations, providing superior elasticity, toughness, and low temperature ductility.
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• Designed to optimise processing and end-use performance.
• Saturated polymers providing excellent thermal stability and UV resistance. • Recyclable (when not crosslinked)
Common features of these classes of polymers is the ability to withstand a large percentage of filler (up to over 65% by weight), often necessary to achieve flame retardant properties, thanks to their low crystallinity.23
POE are copolymers of ethylene with up to 20% mol of other comonomers like 1-butene, 1-hexene, and 1-octene, are made by a variety of processes gas phase, slurry phase, solution, or high-pressure processes. Production variables for these polymers include various possible catalyst combinations, comonomer selection and polymerization post-treatments. These basic combinations allow for a wide range of polymers that can be used in a huge number of applications. PE is often considered first for use in any application because of it excellent cost/performance value such as low density, easy recyclability, and processability.25
Late 1950s to late 1960s, DuPont, Union Carbide (UCC) and The Dow Chemical Company (Dow) worked separately on developing copolymers of ethylene with α-olefins such as 1-butene, 1-hexene, and 1-octene. The processes employed were a low-pressure slurry process, the gas phase process, and the solution-phase process. The copolymers created have densities and crystallinity with linear backbones without any long chain branches. As a result, these linear copolymers, classified as linear low-density polyethylene (LLDPE), have much better mechanical performance than LDPE but do not process through fabrication equipment as easily as LDPEs. With the better mechanical performance, new and higher value packaging and durable applications were developed, such as heavy-duty shipping bags, food.
The development of metallocene catalysts (homogeneous single site catalysts) made possible the synthesis of copolymers with structure and properties completely different from those of traditional linear polyethylene’s.
In general, the physical properties of semicrystalline polymers, such as polyethylene, depend on a set of different variables, for example, molecular weight and molecular
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structure. In the case of ethylene/α-olefin copolymers, the α-olefin content plays an important role on these properties.26
Ethylene/α-olefin copolymers obtained by metallocene catalysts show well defined structures, homogeneous comonomer distribution and narrow molecular weight distribution if compared with copolymers obtained by traditional Ziegler–Natta catalysts.27
The performances of this material are mainly influenced by the type and by the amount of comonomer used and by the distribution of the short chain branches (SCB) introduced by the comonomer into the linear polymer chain. Many studies28 talk about the effect of branching on the crystallization behaviour and properties of ethylene/α-olefin copolymers. In these cases, it is generally accepted that the methyl branches are included in the crystalline lattice but branches higher than propyl are not able to enter the crystalline phase, creating a thin interphase inside the amorphous domain.29 This intermediary region presents some organization and some crystallinity. The tendency for segregation of short branches depends on their size, and the interfacial layer is extended with an increase in the contents of short branches.
It is known that copolymers of ethylene and α-olefins with long chains, such as 1-octadecene, might show a different behaviour from that usually found for ethylene/short α-olefins copolymers. In fact, long branches may participate in the crystallization process, favouring the crystallinity and, it is possible that long branches can crystallize, forming crystallites with different sizes when compared with the main crystallizable chain. The general model accepted for the polyolefin crystallization is the folded chain lamella. Methyl side chains may be incorporated into lamella, but medium and long chains are excluded. In copolymers with higher comonomer content, the average distance between two side chains might influence the lamellar thickness. Thus, copolymers with low crystallinity and long side chains can develop additional crystallinity when submitted to strain. Such behaviour arises from the shear at the interface, allowing the reordering and crystallization of the side chains, as shown in Fig. 9.
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Fig. 9 Schematic representation of the stress-induced orientation at the interface.
The presence of short branches disturbs the crystallization kinetics, giving rise to copolymers with different properties. It was demonstrated that the degree of crystallinity and the melting point of the ethylene/a-olefin copolymers decrease as the comonomer content is increased.30
Fig. 10 show the stress versus strain curves for several ethylene/a-olefin copolymers with different comonomer contents, all the copolymers exhibit smaller stress values as the comonomer content increases (and crystallinity decreases), indicating that the mechanical response of semicrystalline polymers is really determined by the sample crystallinity.
Fig. 10 Stress/strain curves of ethylene/1-octene copolymers with different comonomer contents.
In the case of ethylene/1-octene copolymers, the presence of the comonomer significantly decreases the degree of crystallinity and highly branched chains may lead to molecular
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segregation. Both the degree of crystallinity and the molecular segregation have great influence on the thermal and mechanical properties of ethylene copolymers.
Fig. 11 Stress/strain curves for ethylene/a-olefin copolymers with 5% comonomer contents.
Fig. 11 illustrates the curves for ethylene copolymers with about 5% of comonomer contents, comparing the curves to those for linear comonomers (1-hexene, 1-octene, and 1-octadecene) and shows that longer comonomers have lower resistance to strain. The ethylene/4-methyl-1-pentene copolymer shows an intermediate behaviour between the ethylene/1-octene and ethylene/1-octadecene copolymers. The isopropyl group in the branch is highly efficient in decreasing the crystallinity and, consequently, the resistance to strain.
So, the use of single site metallocene catalysts opened the possibility to tailor the properties and the structures of these copolymers and opened the possibility to obtain a novel LLDPE resin class. Metallocene-based catalysts permits to incorporate high amount of comonomer with a resulting polymer with narrow molecular weight and composition distributions31.
1.3.2. Flame retardant inorganic fillers
Metal hydroxides (e.g., Al(OH)3 and Mg(OH)2), have many advantages in fact they turn out to be halogen-free, environmentally friendly, non-toxic and not volatile, no corrosive
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or toxic decomposition products and they also reduce the emission of toxic fumes and the consequent damages that can derive from them. 32
Metal hydroxides due to the presence of hydroxyl groups, can release free water to dilute oxygen in the burning area and to cool down the pyrolysis zone during their endothermal decomposition. For example, on heating to T>200 °C Aluminium Hydroxide (Al(OH)3), it decomposes into 65% alumina oxide and 35 % water. 33
2 Al(OH)₃ Al₂O₃+ 3H₂O (Eq.4) ΔH = + 1,1 kJ/g (Eq.4)
Magnesium dihydroxide follows the same decomposition reaction of ATH, but it occurs at higher temperature and with lower water release (31%):
Mg(OH)2 MgO+ H₂O (Eq.4) ΔH = + 1,45 kJ/g (Eq.5)
It starts decomposing at around T>300°C, and thanks to its thermal stability it can be incorporated in polymers needing high temperature to be processed. Also, they are reported to show a diluting in the gas phase and form protective metal oxide layers on the burning surface of the polymer after the water-release mode of action has been exhausted (which decreases the heat feedback to the pyrolyzing polymer). However, a well-documented drawback of metal hydroxides is the high loading levels required for adequate flame retardancy (till 70% in weight), which often leads to processing difficulties and deterioration on other critical polymer characteristics.12
Tab. 1 Main inorganic fillers used with formula, decomposition temperature and decomposition enthalpy
Name Formula Start of decomposition (°C) Enthalpy of decomposition (J/g)
ATH Al(OH)3 180-200 1300
MDH Mg(OH)2 300-320 1450
Huntite/
17
The fillers can be synthetic or natural. The synthetic fillers are very white due to the absence of impurities and, when they are produced by fine precipitation, they have a regular granule shape and size (crystalline structure). On the other side, micronized the natural fillers are obtained by mining, selection and milling of natural minerals like Brucite and Huntite/Hydromagnesite: they show irregular granules in size and shape and also have a colour tending to grey / brown due to the impurities that may be present in the mineral.34
General effects of fillers on polymer ignition and combustion: 35 a) dilution, reducing the amount of fuel available for combustion. b) change of the heat capacity and thermal conductivity of material. c) thermal effects such as reflectivity and emissivity.
d) formation of solid residue.
e) slowing down the rate of diffusion of oxygen and pyrolysis products. f) influence on polymer melt rheology (reduction of dripping).
Fine precipitated Aluminium Hydroxide (ATH)
Aluminium Hydroxide is a powder manufactured by precipitation process.
It’s the main FR filler in cable industry for HFFR cables compounds for insulation and sheathing. Most used grade in cables is d50 = 2 μm with surface area 4 m2/g. Uncoated grades are typically used, even if some coated grade is available.
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Milled Aluminium Hydroxide (ATH)
Milled Aluminium Hydroxide is produced by milling the coarse ATH grades of about 60-100 μm, obtained as an intermediate of the Bayer process, which is the main and currently most used method to produce alumina from bauxite. Most used grade is d50 = 15 - 20 μm with area 4-6 m2/g. Fine milled grades at d
50 = 1 - 3 μm result highly hygroscopic and with very high surface area (>10 m2/g) with relevant problems of viscosity into final HFFR compounds.36
Fig. 13 SEM image of Milled Aluminium Hydroxide.20
Magnesium Hydroxide (MDH)
MDH is a crystalline powder manufactured either from different minerals by a chemical process (synthetic MDH), by precipitation from seawater (seawater MDH) or by grinding processes of naturally occurring brucite (natural milled MDH).
Compared to ATH, all types of MDH offers several advantages, including higher decomposition temperature, reduced smoke levels and reduced acidity of the smokes. Most of these performances come from high surface and high basic reactivity MgO generated by thermal decomposition of Mg(OH)2. Synthetic magnesium hydroxide is the most expensive mineral flame-retardant filler. It is available with different granulometry d50 = 0,7 – 3,5 μm with surface area 3-12 m2/g. It is used in very high demand applications and used in all polyolefins where the production can reach very high temperature, like 250 °C.
19
Fig. 14 SEM image of Natural Magnesium Hydroxide.15
The typical composition of high quality Milled MDH is 90-94% MDH, 6-8 % Magnesite/Calcite, <2% other minerals. It is available in different granulometry from 1,5 – 200 μm, with surface area 1-13 m2/g. It can be found with surface coated grades with stearic acid and silanes. In HFFR cable compounds for sheathing is used in EVA and in POE, alone or in combination with other fillers like ATH and CaCO3.37
Fig. 15 SEM image of syntethic Magnesium Hydroxide.15
1.3.3. Additives
This type of ingredients represents a key element for the success of the compound’s formulations.
To produce appropriately dispersed polymer compound, it is generally necessary to add a coupling agent to have interaction between fillers and polymer chains.38 A typical way is to attach a grafting agent, such as maleic anhydride, onto the polymer to ensure adequate dispersion and for reducing the surface tension between the surface of the fillers and the polymeric matrices. In this way, comes out a system of polymers and fillers with adhesion between the phases, that favours a good homogeneity and good mechanical
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properties. In the case of polyolefins loaded, two are the most common classes of coupling agents:
• Silanes: these are liquid compounds that react chemically with the surface of the hydroxides by condensation and with the polymer chains by radical grafting (in the case of vinyl silanes) or dipolar interaction (in the case of amino silanes).
Fig. 16 Vinyl silane coupling action (P=base polymer; M=Al or Mg corresponding to aluminium trihydroxide or magnesium hydroxide).39
Fig. 17 Amino silane coupling action (P=base polymer; M=Al or Mg corresponding to aluminium. ) trihydroxide or magnesium hydroxide). 39
• Functional polymers: these are real polymers in the form of granules that have inside them reactive groups (e.g. maleic anhydride) copolymerized or grafted.
Fig. 18 Compoline coupling action (P=base polymer; C=Compoline; M=Al or Mg corresponding to aluminium trihydroxide or magnesium hydroxide). 39
21
In this research a coupling agent is used as a process additive, FUSABOND™ N525 is a maleic anhydride modified POE, supplied by Dow.
The processing aids are indispensable to favour the workability of the compounds that contain large quantities of mineral fillers, where the viscosity would be extremely high. Three categories of processing aids are generally used:
• Internal lubricants: oils or waxes chemically compatible with the polymer
mixture based on mix. Their use reduces the viscosity of the compounds by plasticizing the polymer chains.
• External lubricants: these are additives incompatible or partially compatible with the polymeric matrix and therefore subject to migration at the external surface during processing; this phenomenon reduces metal - compound friction into machines on which its “slides” with greater ease. Benefit of this is also the improvement of surface smoothness of extruded cables.
• Stearin acid / metallic stearates: these are very versatile additives mostly
incompatible with polyolefins and which in a sense have an intermediate action with respect to the two classes described above. They are also pre-applied on the surface of the inorganic fillers (as organic coating on filler particles) by improving the dispersion of filler inside the compounds and greatly reducing particle-particle interactions.
Stabilizers are used to prevent or slow down the termo-mechanical degradation occurring
during compounding and extrusion processes. Stabilizers can be classified into antioxidant, primary and secondary stabilizers, metal deactivators and photo stabilizers. Antioxidants are used to delay or specifically suppress the chemical alterations that occur in the polymer due to oxidation reactions in the presence of oxygen from the air. Primary stabilizers act in order to stop the propagation reactions of radicals forming non-reactive species, while secondary stabilizers act on the products of the reaction of free radicals with oxygen. Photo stabilizers are specific for delaying chemical reactions initiated by UV radiation on the surface of polymeric materials. Since the reactivity of the stabilizers depends on the species with which they react and on the boundary conditions, the choice is weighted on the basis of different discriminating factors such as: the type of polymer,
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the polymerization process, the type and the exposure time at which it will be subjected the product, the intended use, the type of processing and any stress to which it may be subjected.40
In this researcher was used Irganox 1010, that is a pentaerythritol tetrakis[3-[3,5-di-tert-butyl-4-hydroxyphenyl] propionate. It is a sterically hindered phenolic primary antioxidant. It is odourless and tasteless. It is highly effective stabilizer. It has good compatibility, high resistance to extraction and low volatility.
Fig. 19 Chemical structure of Irganox 1010.41
1.4 Flame retardant properties of compounds and cables
In the past two decade, the CPR Construction Products Regulation, (EU 305/2011), has been one of the most discussed topics that has involved the whole world of construction products, including of course that of cables, being recognized by the EU for their importance in case of fire. The purpose of this regulation is to guarantee the free movement of construction products in the European Union by adopting a harmonized technical language capable of defining its performance and essential characteristics.45 Electric cables are rarely the cause of a fire, but when they are involved, they can constitute an element of danger in reason for their high quantity and their diffusion in all areas of the building. The Construction Products Regulation (CPR) is a European law directly applicable that imposes immediately duties and rights to citizens of the Union and / or to Member states. The CPR Regulation establishes the basic requirements and harmonized essential characteristics that all the products designed to be permanently
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installed in the works of civil engineering (e.g. homes, buildings industrial and commercial, offices, hospitals, schools, subways, cc.) must guarantee for the scope. All cables permanently installed in the constructions be they for the transport of energy or data transmission, any voltage level and with metal conductors or optical fiber, must be classified according to the classes of the relative environment of installation.
Basic requirements of the works of construction for a service life economically adequate: • Mechanical and established resistance
• Safety in case of fire
• Hygiene, health, and environment • Safety and accessibility in use • Protection against noise
• Energy saving and retention of heat • Sustainable use of resources natural
So, in accordance with the European Construction Products Directive 89/106/EEC, cables and lines are classified as construction products.
Fig. 20 Cable classification scheme. Classification criteria are mandatory requirements and additional classifications are optional requirements.
There are seven Euro classes: Aca, B1ca, B2ca, Cca, Dca, Eca and Fca, with Aca having the highest performance and Fca having the lowest. These Euro classes reference several fire test-standard, specifically EN 50399, EN 60332‐1‐2 and EN ISO 1716. This new standard
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provides new and innovative fire reaction test and standard specifies, for all electrical cables subject to the CPR regulation, the equipment and test procedures for evaluating the reaction to fire. In the past, cables and lines were classified / evaluated using several standards. After the CPR was established, all test methods were summarised in EN 50399. The purpose of the test is to verify the cable's reliability in not spreading flame / fire and to measure thermal release, the production of smoke and incandescent drops / particles. The measured parameters are:
• Fire spreading – FS
• Thermal release rate – HRR • Total thermal release – THR • Fire growth index – FIGRA • Smoke production rate – SPR
• Total smoke production – TSP (CPR parameters: S1 /S2 /S3)
• Production of drops/ inflamed particles (CPR parameters: D0 /D1 /D2)
The method used is the cable is exposed to a burner for a test time of 20 minutes. During this time the flame spread and burning droplets/particles are observed and when the test is finished, fire damaged length is measured. Flue gases emitted during testing are collected for analysis and from these data the emitted fire effect and smoke formation can be calculated. The burner's output is 20.5 or 30 kW, depending on which fire class the cable is expected to achieve. The cable to be tested is mounted in 3.5-meter lengths on a test ladder. The lengths are mounted with spacing and the total width for cables and spaces is approximately 30 cm. During the fire test, the ladder with the cables is placed vertically against a wall in a combustion chamber. The cables are facing inwards, away from the wall. In front of the ladder the burner is placed at 7.5 cm from the cable lengths.
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Fig. 21 Schematic representation of a fire chamber according to EN50399.46
In the EN 50399 there are a definition of the necessary parameters:
• Flame spread H and/or FS (flame spread; damaged length) [m]: after any
burning or glowing of the cables or lines has stopped or was extinguished, the test sample must be wiped clean. Any soot must be neglected if the original surface is undamaged after the soot was wiped away. Softening or any deformation of the non-metal material must be ignored as well. The flame spread is measured as the extent of damage. It is measured from the lower edge of the burner to the start of charring in metres with two decimal points. The start of charring is determined as follows: a sharp object must be pressed against the surface of the cable or line. If the surface turns from elastic to brittle (friable), this marks the start of charring. • Heat release rate HRR (heat release rate) [kW]: thermal energy, released in a
time unit by an object during combustion under defined conditions. The peak value of the HRR is defined as the maximum value of the HRRav (t), without the burner output; it is determined during the entire flame exposure time between burner ignition tb and the end of the flame exposure time (tb + 1200s). In order to neglect the burner output when calculating the HRRav(t) value, the burner output curve must be used during routine calibration. The heat release of the burner is defined as the average HRR during the last 5min of the burning time during routine calibration.
• Total heat release THR1200s (total heat release) [MJ]: the total heat release
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a period from burner activation (tb) until the end of flame exposure (tb + 1200s). If the test had to be stopped beforehand, because the HRR was too high, this value must not be calculated, but a detailed report must be prepared. Negative values of the HRR, without burner, must not be taken into consideration within the integral. • Smoke production rate SPRsm60 (smoke production rate) [m²/s]: this means
the smoke production per time unit. The peak value of the SPR is defined as the maximum value of the SPRav(t); it is determined during the entire flame exposure time, i.e. between tb and tb + 1200s).
• Total smoke production TSP1200s (total smoke production) [m²]: the total
smoke production value is calculated as integrated value of the SPR over a period from burner activation (tb) until the end of flame exposure (tb + 1200s). If the test had to be stopped beforehand, because the HRR was too high, this value must not be calculated, but a clear indication must be documented. Negative values of the SPR must not be taken into consideration within the integral.
• Fire growth rate FIGRA (fire index growth rate) [W/s]: the maximum of the
quotient from HRR and time.
Fig. 22 Trend of the fire growth rate.
• Occurrence of burning droplets / particles: material separating from the test
specimen during the test and continuing to burn during a minimum time, as described within this test method
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2. Objectives of the work
The aim of this research is the study of the mechanical properties of highly filled polyolefin compounds used to produce electrical flame-retardant electrical cables.
The dependence of the properties of the final compounds on the variation of the properties of the used ingredient will be investigated.
Five polymers have been selected from the commercially available ethylene-octene copolymers, the so-called polyolefin elastomers (POE), characterized by different density and Melt Flow Index (MFI), while the 1-octene comonomer content remains constant. A third variable will be explored regarding the formulation’s recipe and that is the percentage of micronized natural mineral used ad filler.
The reference formulation studied is based on natural magnesium hydroxide, obtained from brucite (mineral widely used in the flame-retardant sector) and ethylene-octene copolymers (POE) as the main polymeric matrix.
The optimal formulation will be identified following a statistical approach for the prediction of the final properties of the compound according to the type of polymer matrix and the amount of flame-retardant filler.
During the last decades, statistical approaches have been applied even to the analysis of flame-retardant properties to provide efficient framework for systematically gaining information about the behaviour of polymer systems in case of fire.
Concerning this research, statistical approach will been applied especially for the prediction and for the evaluation of mechanical properties of compound and a specific flame retardant system due to the need of finding rapid, indicative and representative methods required by both scientific and applicative research.
There are many polymers which are commercially available with wide range of properties, as shown in the introduction, the creation of this model using a DoE (Design of Experiment) allows to simplify the choice of the matrix as it allows to predict the flame retardant and mechanical properties of the compounds. Specifically, DOE is known to be a powerful statistics-driven tool for designing and analysing experiments.
The analysis of data will be conducted using a CAT (Chemometric Agile Tool) software used for the creation of experimental design of the case.
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The compounds will be prepared with a twin-roller mixer. Crystallinity will be characterized by DSC, mechanical properties will be measured by tensile dynamometer, while rheological properties will be studied with Melt Flow Index (MFI).
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3. Experimental part
3.1 MaterialsPolymers
Tab. 2 Raw materials
Name Trade name Supplier Density(g/cm3) (g/10min) MFI
MFI measured (g/10 min)
Co-monomer
POE Engage 8100 Dow 0.870 1 0.9 Octene
POE Engage 8200 Dow 0.870 5 4.3 Octene
POE Engage 8003 Dow 0.885 1 0.9 Octene
POE Engage 8480 Dow 0.902 1 0.8 Octene
POE Engage 8450 Dow 0.902 3 2.8 Octene
Fillers
Tab. 3 Raw materials
Ingredient Trade name Supplier D50 (μm) (mBET 2/g) Dosage (%)
Natural Magnesium Hydroxide ECOPIREN 3.5 Europiren 3.5-4 11-13 60
Additives
Tab. 4 Raw materials
Ingredient Trade name Supplier Density *1 MFI*2 Density *3 Molecular Weight
(g/cm3) (g/10min) (g/mL) (g/mol)
Coupling agent FUSABOND N525 Dow 0.88 3.7 - -
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3.2 Instruments and methods
Polymer compounding and relative properties analyses (apparent density, Melt Flow Index (MFI) and mechanical analysis) and the fire tests (Limiting Oxygen Index and vertical burning test) have been carried out at IPOOL S.r.l. laboratory.
Mechanical analysis
The tensile properties were determined by using a Lloyd Instruments LS 500 dynamometer at a crosshead speed of 250 mm/min. The width and thickness of the tensile test specimens were respectively 3.0 mm and 2.0 mm ± 0.2 mm, and the stretched length was 20 mm ± 0.5 mm (according to the standard ISO 37 type 2). 47
Fig. 23 Shape of dumb-bell test pieces.
Five samples for each test were usually analysed to obtain reproducible results and determine average values. Young's modulus was calculated as the slope at the beginning of the stress/strain curve by using the software interfaced with the instrument.
31 Thermal degradation analysis (TGA)
Thermogravimetric analyses were carried out using a TGA Q500, TA instruments. Samples of 10-15 mg were placed in Al2O3 crucibles and the runs carried out in high purity N2, flowing at 60 cm3/min. The used heating rates were 20°C/min over the range 50-950°C.
Fig. 25 TGA TA Instrument
Differential scanning calorimetry (DSC)
Differential Scanning Calorimetry, or DSC, is a thermal analysis technique that looks at how a material’s heat capacity (Cp) is changed by temperature. A sample of 10 mg is heated or cooled and the changes in its heat capacity are tracked as changes in the heat flow. This allows the detection of transitions such as melts, glass transitions, phase changes and curing.
The program used is:
1) Hold for 5.0 min at 30.00° C
2) Heat from 30.00° C to 200.00° C at 20.00° C/min 3) Hold for 5.0 min at 200.00° C
4) Cool from 200.00° C to 30.00° C at 10.00° C/min 5) Hold for 5.0 min at 30.0° C
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Fig. 26 Pyris 6 DSC
Melt Flow Index (MFI)
MFI is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length (diameter 2.095 mm and length 8 mm) by a pressure applied; the operating temperature and the weight of the cylinder are fixed by the standards (for us CEI EN 60811-511) according to the type of material tested.
Fig. 27 Melt flow index instrument
Polymer compounding
All the composites were melted mixed via twin-roll mill (calender) at the constant temperature of 140°C for 10 min. At first the polymer matrix was melted for 1 minute and then the fillers and additives were added as a mixture. Before mixing the fillers were heated at 100°C for 8 h to get them dried.
33 Carver press
It’s a manual hydraulic press, mainly used for the preparation of polymer films. Consisting of two plates that can be heated and maintained at a constant temperature (temperature higher than the melt temperature of compound). Equipped with a lever with which the plates are brought into contact and a specific pressure is applied, using a barometer that provides the psi value.
Fig. 29 Carver press instrument
Density
Specific gravity is a measure of the ratio of mass of a given volume of material at 23°C to the same volume of deionized water. The specimen is weighed in air then weighed when immersed in distilled water at 23°C using a sinker and wire to hold the specimen completely submerged as required to the standard method ASTM 2792-00.
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑆𝑆𝑔𝑔𝑔𝑔 = 𝑚𝑚 𝑚𝑚𝑤𝑤
𝑎𝑎− 𝑚𝑚𝑤𝑤𝑑𝑑𝑤𝑤
Where:
mw: mass of the specimen in water
ma: mass of the specimen in air
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Fig. 30 Specific gravity instrument
Limiting Oxygen Index
A burner flame was applied on the top of a vertically oriented bar, which was in a test column with a mixture of oxygen and nitrogen flow. LOI value represents the minimum concentration of oxygen (%) in the gas mixture necessary to support the combustion of the material, which was initially at room temperature. Initial concentration of oxygen is chosen arbitrarily. The used specimens were cut with dimension of 10 × 6 × 3 mm according to the standard ASTM D2863.
Fig. 31 Limit Oxygen Index instrument
Vertical burning test
The measurements were carried out on samples with dimension of 190×100×1.5 mm, with a graduation line at 150 mm. The specimen was fixed at a specimen holder and a Bunsen burner flame was applied at the bottom of the sample for 15 seconds. The time necessary for the top of the flame to reach the graduation line was recorded (t1). The
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modified set up of this test was used for determination of characteristic burning time parameters and for evaluation of the physical stability of the materials during combustion.
ImageJ
Vertical burning measurements were acquired after video recording using the ImageJ program.
CAT (Chemometric Agile Tool)
CAT (Chemometric Agile Tool) is the software used for the data analysis for DOE (Design of Experiment).
Dynamic mechanical analysis (DMA)
Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behaviour of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus. The temperature of the sample or the frequency of the stress are often varied, leading to variations in the complex modulus; this approach can be used to locate the glass transition temperatureof the material, as well as to identify transitions corresponding to other molecular motions.
For a perfectly elastic solid, the resulting strain and the stress will be perfectly in phase. For a purely viscous fluid, there will be a 90° phase lag of strain with respect to stress. Viscoelastic polymers have the characteristics in between where some phase lag will occur during DMA tests. When the strain is applied and the stress lags, the following equations hold:
• Stress: σ = σ0 sin (tω + δ) • Strain: ε = ε0 sin (tω) where
ω is frequency of strain oscillation,
t is time,
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The storage modulus measures the stored energy, representing the elastic portion, and the loss modulus measures the energy dissipated as heat, representing the viscous portion. The tensile storage and loss moduli are defined as follows:
• Storage modulus: • Loss modulus: • Phase angle:
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4. Results and discussion
4.1 Flame retardant properties of polyolefin compounds with high loading of micrometric mineral filler
Five different POE (elastomeric polyolefin) samples provided by DOW were used in this thesis. The information regarding the polymers was provided to us through the technical data sheets which show the different grade over a wide range of densities and melt flow rates. In our case only ethylene copolymerized with octene comonomer were used. The mechanical properties of the compound are very important for the final application, for the cable sector. The difficulty in producing flame retardant compounds using mineral fillers lies in using a polymer that can accept large quantities of filler (up to 60-65%) but still maintain good mechanical properties as the basic polymer matrix. Since there is a large variety of polymers on the market with different properties between them, it is difficult to predict which ones to use to have good properties in the final compound (flame retardant and mechanical properties). Hence the idea of this study, that is to obtain a model with which to predict the properties of the final compound, varying the characteristics of both the polymer matrix and the formulation. As reported in the introduction, the mechanical properties can vary greatly depending on the type of matrix used, in the case of POE, both depending on the variation of the comonomer used in the polymerization, and on varying the density and MFI of the polymer. In the proposed model, three different variables are taken into consideration, the density and MFI of the main polymer and the percentage of filler within the formula.
The Design of Experiments (DoE) method can be adapted to offer a practical way for studying, modelling, and characterizing the influence of the pertinent parameters involved in the response of flame-retardant compound. Indeed, the DoE method has been successfully introduced in industrial systems and research and has built its principles from statistical and mathematical methods.48
Substantially, the DoE method is used to design new industrial products based on both a set of experimental trials and a statistical analysis process in order to optimize the settings of a manufacturing process, to improve its performances, or to predict and characterize its behavioural model.49
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Based on a few experiments in a strict closed study domain of input parameters variation, DoE appears as an alternative method for evaluating the significant factors, correlation between factors and their influence on the response of the system. The method does not require to know the physical model of the studied process. By cons, other physical methods, which can vary only one parameter at a time, are not able to measure the correlation between different input parameters that influence the system response.
The DoE method allows to predict the self-effects as well as the interactions between the different variables involved in the experiment strongly minimizing the number of experiment trials without influencing accuracy of the response
4.2 Formulation for cable application (case study)
As reported in the introduction, a typical recipe of a flame-retardant compound consists of polymers, flame retardant fillers, coupling agents and stabilizers.
In this research, the basic recipe that is used for all formulations is reported in the table (each formulation is repeated with five different polymers as polymer matrix and with the variation of the charge content):
Table 1 Typical recipe used
4.2.1 Polymers
The available grades of polyolefin elastomers that have as comonomer octene are shown in the following Table 222:
FORMULATIONS
POE (polymer)
POE-g-MAH (coupling agent) Natural Magnesium Hydroxide (filler)
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Table 2 Commercially available polymers
To model any system, the DoE is concerned with a set of input variables that can modify a specific output variable named by a response of the system. The DoE leads to deduce a mathematical model of factorial design of the response as a function of input factors that can vary in a bonded study domain limiting the input parameters variations.
In the present work, one can stand out the characterization, the predictive modelling, and the study of the behaviour flame retardant compound by using the DoE technique. We consider in our study, as input parameters of the established predictive model, both variations of density, MFI, and the variation of the filler. For the output responses we consider the mechanical and flame-retardant properties.
In the DoE method theory, an experimental domain is geometrically represented by the input factors and output responses as indicated on Fig. 32.
Trade name Density (g/cm3) MFI (g/10min) Comonomer
Engage 8842 0.857 1 Octene Engage 8180 0.863 0.5 Octene Engage 8130 0.864 13 Octene Engage 8150 0.868 0.5 Octene Engage 8100 0.870 1 Octene Engage 8200 0.870 5 Octene Engage 8400 0.870 30 Octene Engage 8452 0.875 3 Octene Engage 8411 0.880 18 Octene Engage 8003 0.885 1 Octene Engage 8401 0.885 30 Octene Engage 8440 0.897 1.6 Octene Engage 8480 0.902 1 Octene Engage 8450 0.902 3 Octene Engage 8402 0.902 30 Octene Engage 8540 0.908 1 Octene
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Fig. 32 The experimental factorial design presentation in reduced values.
Orthogonally factor axes define this experimental domain represented on a graduated axis. To standardize the units, the axis graduations can be with original units or in reduced values. The domain of the study is limited between two important levels: “lower level” and “upper level”. The intersection of the factor levels gives an “experimental point”.51 So, we have selected for our study from the various Engages on the market, available by Dow, those that have a variability of MFI and density at the top of the experimental domain plus one at the centre. Considering as an experimental domain, a range of MFI between 1-5 g/10 min, while density 0.870 – 0.902 g/cm3.
Polymers with MFI equal to 30 are excluded, because for this application it is not used, as it leads to low mechanical properties, for this reason are considered those with an MFI between 1 and 5.
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Table 3 Selected raw materials
Engage
grade Density*(g/cm3) 1 Density(g/cm3) MFI* 2
(g/10min) MFI measured(g/10 min) Co-monomer
8100 0.870 0.870 1 0.9 Octene 8200 0.870 0.870 5 4.3 Octene 8003 0.885 0.883 1 0.9 Octene 8480 0.902 0.900 1 0.8 Octene 8450 0.902 0.902 3 2.8 Octene *1 ASTM 2792-00 *2 21,6 Kg @ 190°C 0,865 0,870 0,875 0,880 0,885 0,890 0,895 0,900 0,905 0 1 2 3 4 5 Engage 8100 Engage 8200 Engage 8003 Engage 8480 Engage 8450 MFI (g/ 10mi n) Densità (g/cm3)
Fig. 33 MFI variation as a function of density.
As you can see from the graph, the points of the experimental domain do not appear to be exactly at the corners of the square and in the middle of it, this is because commercially
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there were no polymers available from the same supplier (same technology, same catalyst, same production plant) that could meet these requirements.
4.2.2 Fillers
A wide variety of fillers can be found, which are different for their properties and origin, as reported in the introduction. Their behaviour in the polymeric composite is deeply influenced by many factors, such as morphology, presence of impurities, density, hardness, moisture content, thermal stability, modulus, and surface chemistry.
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. Nowadays the principal fire-retardant fillers for composites of industrial application are aluminium hydroxide (ATH), magnesium hydroxide (MDH) huntite/hydromagnesite mixture (Hu/Hy). The relative fire-retardant performance depends strongly on the nature of the filler, on its origin, on the chemical characteristics of the filler and of the polymer, together with the polymer-filler interactions can be achieved.
As reported in the Introduction, the presence of fillers completely changes the rheological and mechanical properties of polymer matrix.
From the comparison between the SEM micrographs Magnifin H5 (commercial synthetic magnesium hydroxide obtained by crystallization from solution) has a regular hexagonal shape, while Ecopiren (natural magnesium hydroxide) is characterized by irregular shapes with different sizes. The BET of Ecopiren is 11-13 m2 / g while that of synthetic magnesium hydroxide is 4-6 m2 / g, this difference between the surface areas results in a different behavior within the compound which results in mechanical properties very different endings.
The physical properties of the fillers are reported in Table 4, among which their size distribution D50 the specific surface area measured by BET method are reported from literature. 52,53
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Table 4 Properties of the inorganic fillers.
Trade name Supplier D50
(μm)*1 (mBET 2/g) *2
Synthetic magnesium
Hydroxide Magnifin H5 Huber 1.5 4-6
Natural Magnesium
Hydroxide ECOPIREN 3.5 Europiren 3.5-4 11-13
*1 D
50: The portions of particles with diameters smaller and larger than this value are 50%. Also known as the medium diameter.
Measured by laser diffraction method according to ISO 13320.
*2 BET: Determination of the overall specific external and internal surface area of disperse or porous solids measuring the amount of
physically adsorbed gas (N2) according to ISO 9277.
Fig. 34 Micrograph SEM Ecopiren 3.5
Fig. 35 Micrograph SEM Magnifin H5
The micrographs show that natural magnesium hydroxide particles are characterized by irregular shape, it is also possible to evaluate the presence of particles with very different sizes for these natural fillers. The analysis of the synthetic fillers points out the regular shape of the particles together with size uniformity shows plate-like particle with hexagonal base.
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Size distribution data confirm the qualitative information obtained by the micrographs of the fillers. The natural fillers show a wider distribution of the particle size than the synthetic fillers. This also leads to a different availability of the surface to interact: due to the irregular shape, that of ground brucite is less available while the regular surfaces and the high surface area of the synthetic magnesium hydroxide lead to a greater interaction between charge and polymer. This result was expected due to the different production method of the fillers. The synthetic ones are obtained by precipitation method, where almost the same growth of the particles can be reached regulating all the parameters (temperature, time, etc). In this study Ecopiren 3.5 is used as a filler, but formulations containing also synthetic magnesium hydroxide have been carried out and are shown in Table 5:
Table 5 Formulations recipes
1d Magnifin H5
Ingredient Trade name % %
Engage 8003
Ecopiren 3.5 Engage 8003 Magnifin H5
POE d=0.885 MFI 1 Engage 8003 36.8 36.8
POE-g-MAH d=0.88 MFI 3.7 Fusabond N525 3 3
Natural Magnesium Hydroxide Ecopiren 3.5 60 60
Stabilizer Irganox 1010 0.2 0.2
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From the prepared mixtures have been characterized the specimens according to the standard ISO 37 type 2 were obtained and tested on the dynamometer, the values shown in the table below were obtained.
Table 6 Properties (density, MFI, Tensile Strength, Elongation at break, Young’s modulus) of the sample
FORMULATIONS 1d Magnifin H5
Properties Engage 8003 + Ecopiren 3.5 Engage 8003 + Magnifin H5
Calculated Density (g/cm3) 1.44 1.44
Density (g/cm3) *1 1.46 ± 0.02 1.41 ± 0.01
MFI (g/10min) *2 3.8 ± 0.2 3.8 ± 0.19
Tensile Strenght (MPa) *3 11.8 ± 0.3 12.9 ± 0.3
Elongation at break (%) *3 233 ± 15 257 ± 26
Young’s modulus (GPa) 0.27 ± 0.02 0.46 ± 0.03 *1 ASTM 2792-00 *2 21,6 Kg @ 190°C *3 ISO 37 Type-2 0 50 100 150 200 250 300 0 2 4 6 8 10 12 14 Str es s (M Pa ) Strain (%) Engage 8003 + Magnifin H5 1d (Engage 8003 + Ecopiren 3,5)
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As can be seen from the SEM micrographs (Fig. 34, Fig. 35) and from the characteristics of the two fillers shown in the Table 6, Magnifin has more surface area available than Ecopiren. A larger available surface area means a greater interaction between polymer and mineral filler and therefore a greater tensile modulus as can be seen from the stress-strain graph. So, if a comparison is made between natural and synthetic magnesium hydroxide, evaluating the quality / price ratio, it is preferable to use natural magnesium hydroxide (in this case Ecopiren 3.5), compared to the synthetic one, which certainly offers better performance, from the point of view of mechanical and rheological properties of the final compound, but it has a definitely higher cost (4 times more) than Ecopiren 3.5.
4.2.3 Compatibilization
Six different formulations were prepared using Engage 8003 as a polymer matrix (d = 0.885 g / cm3 MFI 1 g / 10 min 2.16 kg @ 190 ° C) and with a percentage of Fusabond between 0-5%, to evaluate the best percentage of maleic coupling, such as to improve the mixing between the various components and improve the mechanical properties of the final product.
Table 7 Recipe formulations
Once the compounds were prepared in a roller mixer (calender) at 160 º C, the next day samples were prepared (ISO 37 type 2) and tested at the dynamometer; the obtained results are reported in the Table 8. The gray colour comes from the natural colour of brucite ("off-white") and the presence of Irganox 1010.
FORMULATIONS 1a 1b 1c 1d 1e 1f
Ingredient Trade name % % % % % %
POE d=0.885 MFI 1 Engage 8003 39.8 38.8 37.8 36.8 35.8 34.8
POE-g-MAH d=0.88 MFI 3.7 Fusabond N525 0 1 2 3 4 5
Natural Magnesium Hydroxide Ecopiren 3.5 60 60 60 60 60 60
Stabilizer Irganox 1010 0.2 0.2 0.2 0.2 0.2 0.2
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Fig. 37 Dumb-bell specimen
Table 8 Properties (density, MFI, Tensile Strength, Elongation at break, Young’s modulus) of prepared samples 1a-1f. PROPERTIES 1a 1b 1c 1d 1e 1f Coupling (Engage 8003) 0% 1% 2% 3% 4% 5% Calculated density 1.44 1.44 1.44 1.44 1.44 1.44 Density 1.4 3± 0.01 1.46 ± 0.02 1.46 ± 0.02 1.46 ± 0.01 1.45 ±0.01 1.44 ±0.01 MFI*1 7.4 ± 0.4 5.3 ± 0.3 4.9 ± 0.2 3.8 ± 0.2 3.1 ± 0.1 2.9 ± 0.1 Tensile Strength (MPa) 11.3 ± 1.9 10.7 ± 2 11.4 ± 0.1 14.4 ± 0.3 16.3 ± 0.2 17.3 ± 0.1 Elongation at break (%) 518 ± 117 322 ± 188 180 ± 16 232 ± 15 226 ± 18 216± 20 Young’s modulus (GPa) 0.11 0.23 0.25 0.29 1.02 1.01
48 0 50 100 150 200 250 300 350 400 0 2 4 6 8 10 12 14 16 Str es s (M Pa ) Strain (%) 1A (0%) 1B (1%) 1C (2%) 1D (3%) 1E (4%) 1F (5%)
Fig. 38 Stress-strain curves of Engage 8003 with different content of agent of compatibilization.
From the graphs (Fig. 38) it is possible to observe as to the increase of the coupling agent percentage in the formulations correspond to a change in the curve shape with reduction of the elongation at break and a corresponding increase of tensile strength.
0 10 20 30 40 50 60 0 2 4 6 8 10 12 14 Str es s (M Pa ) Strain (%) 1a (0%) 1b (1%) 1c (2%) 1d (3%) 1e (4%) 1f (5%) 0 2 4 6 8 10 0 2 4 6 8 10 12 14 Str es s (M Pa ) Strain (%) 1a (0%) 1b (1%) 1c (2%) 1d (3%) 1e (4%) 1f (5%)
Fig. 39 Enlarged scale for the graph at Fig. 49
