Phosphonated lipids as primary plasticizers for PVC with improved flame retardancy

Keywords: bio-based, flame retardant, lipids, plasticizer, PVC

Bio-based plasticizers bearing moieties exhibiting flame retardancy properties (dimethyl (methyl oleate)phosphonate (PMO), diethyl (methyl oleate)phosphonate (PMO2), dimethyl (methyl linoleate)phosphonate (PML), dimethyl (dimethyl oleate)phosphonate (PDE)) were synthesized from methyl oleate, methyl linoleate and oleic diacid through free-radical addition of dialkyl phosphonates. Soft PVC were prepared from PMO, PMO2, PML and PDE primary plasticizers and compared to materials containing usual diisononyl phthalate DINP.

Rheological behavior, thermal stability, mechanical properties and flame resistance performances of these PVC films were carefully investigated. The presence of the phosphonate group into the plasticizer structure explained the best thermal stability of the PVC films based on phosphonated plasticizer than that of based on DINP. Regarding all these results, PMO and PDE can be considered as efficient flame retardant primary plasticizers for PVC and can substitute phthalates in soft PVC based materials.

Introduction

Poly(vinyl chloride) (PVC) is the second widespread manufactured and consumed thermoplastic in the world 1,[START_REF] Boudhani | Viscoelasticity and Mechanical Properties of Reactive PVC Plastisols[END_REF] owing to its low cost and its versatility in term of properties. The applications extend from highly rigid PVC (window profiles, computer housings, outdoor products, containers, blister packages) to flexible PVC (soft PVC) (wires and cables, flooring, wall coverings, roofing, food-wrap films, bottle caps, medical products, toys) [START_REF] Saeki | Technical progresses for PVC production[END_REF] . Despite these advantages, the high melt viscosity of PVC induces a low processability. Additives including heat stabilizers, lubricants... are required to enhance the processing performance and the enduse properties. The most widely used additives are plasticizers [START_REF] Sejidov | Esterification reaction using solid heterogeneous acid catalysts under solvent-less condition[END_REF][START_REF] Da Silva | Polyvinylchloride (PVC) and natural rubber films plasticized with a natural polymeric plasticizer obtained through polyesterification of rice fatty acid[END_REF] defined as "a substance or material incorporated in a material to increase its flexibility, workability or distensibility" by the council of the IUPAC (International Union of Pure and Applied Chemistry). The main candidates for PVC are the phthalate series even if since the early 1980s, they have been controversed due to their possible toxicity and their migration from PVC products [START_REF] Marcilla | Migrability of PVC plasticizers[END_REF] that represent a potential risk to human health [START_REF] Sunny | Use of Polymeric Plasticizers in Polyvinyl Chloride to Reduce Conventional Plasticizer Migration for Critical Applications[END_REF] . To solve this problem, researches focused on alternative plasticizers based on citrate, adipate, azelate, sebacate, benzoate and other polymeric plasticizers, etc. [START_REF] Crespo | Substitution of di(2-ethylhexyl) phthalate by di(isononyl) cyclohexane-1,2-dicarboxylate as a plasticizer for industrial vinyl plastisol formulations[END_REF][START_REF] Banu | Contributions to characterization of poly(vinyl chloride)-lignin blends[END_REF][START_REF] Bocque | Petro-Based and Bio-Based Plasticizers: Chemical Structures to Plasticizing Properties[END_REF] . Recently, an interest for natural-based plasticizers grew, especially for modified fatty acid derivatives (epoxidized oils [START_REF] Bouchareb | Effects of epoxidized sunflower oil on the mechanical and dynamical analysis of the plasticized poly(vinyl chloride)[END_REF][START_REF] Lardjane | Migration of new bio-based additives from rigid and plasticized PVC stabilized with epoxidized sunflower oil in soil[END_REF][START_REF] Bueno-Ferrer | Characterization and thermal stability of poly(vinyl chloride) plasticized with epoxidized soybean oil for food packaging[END_REF][START_REF] Karmalm | Thermal stability of poly(vinyl chloride) with epoxidised soybean oil as primary plasticizer[END_REF][START_REF] Lv | Epoxidation of Methyl Oleate and Subsequent Ring-Opening Catalyzed by Lipase from Candida sp[END_REF] , acetylated fatty esters 16,[START_REF] Fogassy | Catalyzed ring opening of epoxides: Application to bioplasticizers synthesis[END_REF] , fatty acid polyesters [START_REF] Omrani | Synthesis of a bio-based plasticizer from oleic acid and its evaluation in PVC formulations[END_REF] , sulfonyl fatty esters, [START_REF] Jia | Synthesis of a novel polyester plasticizer based on glyceryl monooleate and its application in poly(vinyl chloride)[END_REF] glycerol esters [START_REF] Suárez Palacios | Low-molecular-weight glycerol esters as plasticizers for poly(vinyl chloride)[END_REF] etc.), citrates [START_REF] Gil | Evaluation of the effects of biobased plasticizers on the thermal and mechanical properties of poly(vinyl chloride)[END_REF] and sugar derivatives [START_REF] Yin | Oligomeric isosorbide esters as alternative renewable resource plasticizers for PVC[END_REF][START_REF] Adhikari | Effect of plasticisers in the moisture migration behaviour of low-amylose starch films during drying[END_REF] .

Additional properties such as UV resistance, electrical conductivity or fire retardancy must often be required for industrial applications. PVC is self-extinguishing since its chemical structure contains more than 50 % of chlorine. But, most of the organic additives are flammable and affect its fire resistance justifying the addition of flame retardant compounds.

The most commonly flame retardant additives are antimony oxide, zinc borate, magnesium hydroxide, aluminum trihydrate and halogenated and phosphorus based compounds [START_REF] Sain | Flame retardant and mechanical properties of natural fibre-PP composites containing magnesium hydroxide[END_REF][START_REF] Pál | Plastics, their behaviour in fires[END_REF] .

Recently, Jia [START_REF] Jia | Properties of poly(vinyl chloride) incorporated with a novel soybean oil based secondary plasticizer containing a flame retardant group[END_REF] described a PVC secondary plasticizer based on soybean oil containing a flame retardant phosphate moiety. Despite a good limiting oxygen index (LOI), the resulting soft PVC materials using these bio-based plasticizers exhibit lower mechanical properties and plasticizing efficiency than those using common observed with phthalates. Furthermore, the use of a primary plasticizer is needed to reach acceptable properties. Phosphorylation of lipids has already been widely described, including the synthesis of phosphorylated castor oil in presence of chlorodiphenylphosphine [START_REF] De Espinosa | A straightforward strategy for the efficient synthesis of acrylate and phosphine oxide-containing vegetable oils and their crosslinked materials[END_REF] or phosphoryl chloride [START_REF] Liu | Phosphoester cross-linked vegetable oil to construct a biodegradable and biocompatible elastomer[END_REF] as described by De Espinosa and Liu, respectively. Guo detailed an alternative route between phosphoric acid and the epoxide rings of epoxidized soybean oil (ELO) [START_REF] Guo | Hydrolysis of Epoxidized Soybean Oil in the Presence of Phosphoric Acid[END_REF] whereas Jia investigated the reaction epoxyphosphorylated derivatives using diethyl phosphate with ELO [START_REF] Jia | Properties of poly(vinyl chloride) incorporated with a novel soybean oil based secondary plasticizer containing a flame retardant group[END_REF] to design a secondary plasticizer for PVC. Knigh and Swem, 30 and Sasin 31 described the grafting of phosphorylated derivatives on vegetable oils by free-radical addition of dialkylphosphonates onto the lipidic double bonds. Based on this approach, a corrosion inhibitor additive was recently synthesized by Millet using dimethylphosphite on methyl oleate [START_REF] Millet | Improvement of corrosion protection of steel by incorporation of a new phosphonated fatty acid in a phosphorus-containing polymer coating obtained by UV curing[END_REF] .

In this paper, we investigated the synthesis of primary PVC plasticizers combining a lipidic structure and phosphorus based moieties on the basis of our expertise on the chemical modification of triglycerides into lipidic polyols [START_REF] Pham | Lipidic polyols using thiol-ene/yne strategy for crosslinked polyurethanes[END_REF][START_REF] Desroches | Synthesis of Biobased Polyols by Thiol-Ene Coupling from Vegetable Oils[END_REF] , polyamines [START_REF] Stemmelen | Plant oil-based epoxy resins from fatty diamines and epoxidized vegetable oil[END_REF][START_REF] Stemmelen | A fully biobased epoxy resin from vegetable oils: From the synthesis of the precursors by thiol-ene reaction to the study of the final material[END_REF] , amphiphilic (macro)molecules [START_REF] Rotta | Synthesis of Amphiphilic Polymers Based on Fatty Acids and Glycerol-Derived Monomers -A Study of Their Self-Assembly in Water[END_REF][START_REF] Travelet | Amphiphilic copolymers based on polyoxazoline and grape seed vegetable oil derivatives: self-assemblies and dynamic light scattering[END_REF][START_REF] Stemmelen | Synthesis and self-assembly of amphiphilic polymers based on polyoxazoline and vegetable oil derivatives[END_REF][START_REF] Pham | Various radical polymerizations of glycerol-based monomers[END_REF] . The main goal of our researches was to get primary plasticizer exhibiting plasticization closed to that of DINP and simultaneously, flame retardancy avoiding the addition of any additives in PVC during compounding. Phosphonated groups are now well known for their ability to extinguish fire [START_REF] Vahabi | Theoretical and empirical approaches to understanding the effect of phosphonate groups on the thermal degradation for two chemically modified PMMA[END_REF][START_REF] Sonnier | Relationships between the molecular structure and the flammability of polymers: Study of phosphonate functions using microscale combustion calorimeter[END_REF][START_REF] Tibiletti | Thermal degradation and fire behavior of thermoset resins modified with phosphorus containing styrene[END_REF] . Thus, four phosphonated vegetable oil derivatives (PMO, PMO2, PML and PDE) were synthesized by free-radical addition of dialkyl phosphonates onto lipidic double bonds (Figure 1) into C9 or C10 position (and C12 or C13 position for PML). Soft PVC films were prepared from these bio-based plasticizers before evaluating their plasticizing efficiency and fire resistance.

Experiment

Materials

Methyl oleate (MO, purity 98.5 %, see Figure 1SI), methyl linoleate (ML, purity 80.9 %, see Figure 2SI) and C18:1 ,diacid [(9Z)-octadec-9-enedioic acid] (purity 97.

Synthesis of phosphonated diester (PDE)

In a first step, D18:1 diacid (DA) were dissolved in chloroform ([DA] = 0.3 mol.L -1 )

in presence of methanol (20 equiv). The mixture was vigorously stirred and p-toluenesulfonic acid monohydrate (0.2 equiv) was added. In a reverse Dean-Stark apparatus, the reaction mixture refluxed for 24 h. The organic solution was washed twice with distilled water and twice with brine, then dried over anhydrous MgSO 4 , filtered and evaporated under vacuum to remove solvent and excess methanol. The dimethyl ester (DE) was obtained in 98 % yield. The GC analysis were recorded on a Shimadzu GC-2010 Plus with RI detection.

Samples were injected using a Supelco column (300 × 0.25 mm). The analysis was performed between 100 and 300 °C with a flow rate of 10 mL/min using acetonitrile as eluent.

The HPLC-MS analysis were recorded on a TSP/surveyor MSQ+ instrument in APCI + mode. Samples were injected using a Nucleosil 100-5C4 column (150 × 4.6 mm). The analysis were performed at 22 °C with a flow rate of 1 mL/min using isocratic elution with 20/80 acetonitrile/water.

Fourier Transform Infrared (FTIR) spectra were recorded with a Perkin-Elmer Spectrum 100 spectrometer equipped with an attenuated total reflectance (ATR) crystal (ZnSe).

Size exclusion chromatography (SEC) was performed using a Waters 515 HPLC pump followed by a Water 2410 differential refractometer and a column set consisting of three 5µ PL-gel of 10 4 , 10 3 and 500 angstroms. Chloroform was used as eluent at 0.8 mL.min - 1 at 35 °C and 200 µL of sample solution were injected. Calibration of the SEC equipment was carried out with polystyrene narrow standards (Agilent Standards, peak molecular weight range 162-1 238 000 g.mol -1 ).

Calorimetric analyses were carried out using a Star1 differential scanning calorimeter (DSC) from Mettler Toledo ® in order to determine the plasticizers melting points and the glass transition temperatures of PVC films. 10 mg of product was poured into an aluminum pan that was consecutively placed in the measurement-heating cell. An empty pan was used as reference. These calorimetric experiments were achieved under air atmosphere, from -70 to 25 °C with a heating rate of 10 °C.min -1 for melting points, and from -100 °C to 150 °C with a heating rate of 15 °C.min -1 for glass transition temperatures.

The thermal stability of the plasticizers and the PVC films was examined using a Q50 thermogravimetric analyzer (TGA) from TA Instruments ® . The experiments consisted in registering the weight loss of the sample under air flow (25 mL.min -1 ) as a function of temperature from the ambient up to 500 °C with a heating rate of 10 °C.min -1 .

To determine the gelation temperature, rheological analyses were performed using a dynamic rheometer (MCR 102 from Anton Paar ® ) equipped with parallel plate geometry (upper plate diameter  = 25 mm and inner plate diameter  = 40 mm). The liquid PVC (plastisol)

formulation was inserted into the measurement plates at room temperature. The upper plate was lowered until contact with the sample with an average gap of 1 mm. The evolution gelation with temperature was monitored at an angular frequency of 6.28 rad.s -1 from the variation of the linear complew shear modulus (G*()= G'()+jG''(), j 2 =-1). Such method has been described in a previous work. [START_REF] Boudhani | Rheology of physically evolving suspensions[END_REF]

Sample preparation

A usual PVC plastisol formulation (around 30 wt.% plasticizer) consisted in mixing PVC, plasticizer and thermal stabilizer until obtaining a homogeneous mixture. The resulting paste was degassed under vacuum for 40 min. The mixture was spread out on a mirror or a glass plate, with a thickness of around 0.5 mm, using an Elcometer squeegee and heated at 190 °C in an oven for 10 min.

Sample characterization

Elongation at break and Young modulus of the films were measured on an Instron 3366L5885 tensile tester according to ISO 527-3:1995 standards. Films were cut into 10 strips of 1.5 x 10 cm and mounted between tensile grips with an initial grip spacing of 5 cm and cross-head speed of 200 mm.s -1 . The elongation at break is expressed as a percentage of the original length.

Cured plastisols' hardness was evaluated with a Shore A durometer (HBA 100-0 from Sauter).

Limited oxygen indices (LOI) were determined using an ILO Rheometrics apparatus.

Fire was ignited from strips of PVC films (5 x 17 cm) under of a growing amount of oxygen.

This test characterizes the combustion performance and is expressed by a unitless value. High numbers reveal great resistance to ignition since it means that higher amount of oxygen are needed to keep the flame alive.

Results and discussion

Phosphonation of vegetable oil derivatives

Herein, phosphonated lipidic derivatives (PMO, PMO2, PML and PDE) were synthesized by the free-radical addition of dimethyl phosphite (DMP) and diethyl phosphite (DEP) on methyl oleate (MO), methyl linoleate (ML) and diester (DE) (previously synthesized from the diacid D18:1 which is a dimer made from the metathesis of dec-9-enoic acid or oleic acid [START_REF] Behr | Metathesis applied to unsaturated lipid compounds[END_REF] ) as shown in Figure 1. Two peroxide-type initiators: di-tert-butyl peroxide (dtBP) and tert-butyl peroxypivalate (tBPP) were investigated since they are well known for their hydrogen abstraction ability in the case of hydrogen phosphonate. They yielded in quantitative conversion rate the phosphorylated lipids at 141 and 75 °C, respectively.

The syntheses of PMO, PMO2, PML and PDE plasticizers were monitored by FTIR spectroscopy with the appearance of the characteristic peaks corresponding to P-O-C symmetric bending vibration at 1048 cm -1 and P-O-C stretching vibration at 813 cm -1 , as well as the total disappearance of =C-H bonds absorption band at 3009 cm -1 . An additional 1 H NMR study was achieved on PMO, PMO2, PML and PDE and is shown in The chemical modifications of lipids were also monitored by SEC using chloroform as eluent (Figure 3). The shift of the SEC traces, before and after addition of phosphonates onto the unsaturations attested the total conversion into phosphorylated lipid as illustrated for DE.

As expected, the trace of the resulting product shifted towards the lower elution time corresponding to the higher molecular weight even if the difference coming from the addition of a phosphonate group corresponded only to 110 g/mol. The molecular weight at the peak of the phosphorylated lipid followed the order of the theoretical ones: PMO > PMO2 > PDE > PML. In each case, we can observe a large peak and a smaller one in the area of higher elution time attributed to oligomers. This oligomerization was observed with all the chemical modifications of lipidic derivatives when the reaction was performed at high temperature involving some oligomerization in radical way. This phenomenon was observed by Robin et al. 47 than DINP (3.2 Pa.s). This behavior is in favor of a better processability of these compounds during plasticization.

Plasticization

The usual plasticization of PVC has already been fully described by several authors as summarized in our recent review [START_REF] Bocque | Petro-Based and Bio-Based Plasticizers: Chemical Structures to Plasticizing Properties[END_REF] . The plasticization starts by mixing the small PVC particles into the liquid plasticizer, to obtain a paste called plastisol. The heating allows the solvation of PVC particles by the plasticizer [START_REF] Wang | Test methods for gelation of PVC plastisol[END_REF][START_REF] Semsarzadeh | Dynamic mechanical behavior of the dioctyl phthalate plasticized polyvinyl chloride-epoxidized soya bean oil[END_REF][START_REF] Hong | Effect of polymer-solvent interaction on gelation of polyvinyl chloride solutions[END_REF] then above the PVC glass transition temperature (85 °C), PVC particles absorb the plasticizer in a great extent and the plastisol becomes a solid paste [START_REF] Marcilla | Rheological study of PVC plastisols during gelation and fusion[END_REF][START_REF] Boudhani | Rheology and gelation kinectics of PVC plastisols[END_REF][START_REF] Kwak | Structural-changes of PVC plastisols in progress of gelation and fusion as investigated with temperature-dependent viscoelasticity, morphology, and light-scattering[END_REF] . The extension of the heating at 190 °C triggers the fusion of PVC micro-crystallites, high interactions between swollen plasticizer molecules and PVC polymer chains [START_REF] Lopez | Influence of crystallinity in the curing mechanism of PVC plastisols[END_REF][START_REF] Schartel | Phosphorus-based Flame Retardancy Mechanisms-Old Hat or a Starting Point for Future Development?[END_REF] producing, once cooled down, a flexible solid material. This complicated mechanism is based on a high compatibility between the plasticizer and PVC and explain why only few candidates are able to plasticize PVC. Several efficient lipidic plasticizers have already been reported but the question was to know if the modification of triglycerides by one or two phosphonate groups decreases the plasticizing effect of PVC? To study this aspect, our candidates were investigated as primary plasticizers of PVC. DINP plasticizer acts as reference with the elaboration of the sample A as shown in Table 1. Different soft PVC compositions based on our bio-based phosphonated plasticizers were prepared as described in the experimental part (samples B-E). To allow the comparison between the various formulations, 30 % plasticizer content, 2 % of stabilizer T652 and 50 g of PVC P70 were set for each ones.

Table 1. Composition of PVC plastisol formulations. As mentioned earlier, the gelation process of a plastisol is a complex process in which PVC particles and aggregates are swollen to form a gel structure. This process is governed by diffusion laws and highly depends on temperature and compatibility with the plasticizer. This phenomenon can be monitored by rheological measurements determining the evolution of the storage and loss moduli, G' and G" respectively, as a function of the curing temperature. The gelation temperature (T gelation ) of the plastisol can be defined as the crossover between complex modulus G'() and G''() [START_REF] Boudhani | Rheology of physically evolving suspensions[END_REF] . This study was performed for the samples A to E as depicted in Figure 4. The gelation temperature ranged from 46 to 73 °C with an intermediate value for PDE (sample E) at 58 °C and a lower and higher value for the samples A and C, respectively.

Sample

At the end of the analyses, all G' moduli are located between 1.10 5 and 1.10 6 Pa and show some differences between the various samples. These variations will be magnified for mechanical properties and discussed.

Properties of soft PVC containing phosphonated lipidic plasticizers

The thermal properties of the plasticizers and of the resulting plastisols were investigated by calorimetric study (DSC) as shown in Figure 5. The melting point of the plasticizers ranged from -43 and -2 °C for DINP and PDE, respectively. These additives are liquid at room temperature favoring the swelling of PVC particles and the final plasticization of this resin. The thermograms of plasticized PVC revealed that all the additives were miscible with PVC aggregates since no melting of free plasticizer was observed (Figure 5).

The plasticizing effect of the phosphonated lipids (PMO, PMO2, PML and PDE) on PVC films was confirmed by the decrease of the T g values in comparison to that of PVC close to 85 °C. The chemical structure of the phosphonated lipidic plasticizers had only a slight influence on the resulting material and final T g ranged between -10 and -7 °C for the samples B to E.

These low T g suggested a convenient solubility of these plasticizers in PVC. The thermal degradation of PMO, PMO2, PML, PDE and DINP plasticizers as well as PVC plastisol formulations A to E were studied by TGA and are reported in Figures 6 and7, respectively. These thermogravimetric analyses were performed to highlight the plasticizers behaviors at the temperature corresponding to the curing of the plasticized PVC, that is to say 190°C. First of all, all the plasticizers decomposed according to the same three-stage thermal degradation profile with a fast degradation around 300 °C except for PDE which degraded later. At the curing temperature, only 2% of DINP were lost by evaporation versus 3-6% for the monophosphorylated lipidic plasticizers. The diphosphorylated one, PML, suffers a little more important degradation with 10% lost. Another piece of information deduced from the TGA study was the formation of a char layer around 15% observed at 500 °C for the plasticizers bearing phosphonate groups. This effect is well known for phosphonate materials at high temperature [START_REF] Rotta | Synthesis of Amphiphilic Polymers Based on Fatty Acids and Glycerol-Derived Monomers -A Study of Their Self-Assembly in Water[END_REF][START_REF] Schartel | Phosphorus-based Flame Retardancy Mechanisms-Old Hat or a Starting Point for Future Development?[END_REF] and missing for DINP. The degradation of dimethyl or diethyl phosphate into phosphonic acid triggered the formation of carbonization zones protecting the compound from further degradation. In conclusion, PMO2 owned the worst thermal stability compared to all the other bio-based plasticizers with a faster degradation. By contrast, PMO and more particularly PDE possessed the best thermal stability with the slowest degradation and the most important char layer formation at the end of the analysis. Finally, all soft PVC plasticized with the lipidic phosphonated compounds gave 27 % of char at 500°C whereas only 16% were produced by DINP. Plasticizers drastically impact the mechanical properties of PVC. The plasticizer efficiency is currently investigated by the tensile properties and Shore hardness [START_REF] Coltro | Performance evaluation of new plasticizers for stretch PVC films[END_REF] . Herein, all samples with a plasticizer content of 30% have similar Shore A values in the range of 89-99 as summarized in Table 2. They also gave good mechanical properties with an elongation at break ɛ ranged between 290% and that of DINP value (sample A) as shown in Figure 8. The influence of the plasticizer content on the glass transition temperature and the mechanical properties was deeply investigated for the two best candidates, PMO and PDE (Figure 9 and Table 3). As expected, T g values decreased with plasticizer content increasing.

This effect is magnified with PMO plasticizer going from -10 to -50°C for 30 to 50 % PMO content, respectively. Moreover, tensile properties (Young modulus and the tensile strength at break) logically decreased with the plasticizer content. Inversely, the elongation at break increased with the plasticizer content, especially for 50 % in plasticizer. PMO (full lines) gave higher Young modulus and tensile strength as well as lower elongation than PDE (dotted lines) while this trend slightly reversed for high percentage. At last, the combustion performances of plasticized PVC films with PMO, PDE and DINP have been evaluated by the LOI test. It is first important to have in mind that rigid PVC possesses an oxygen index of 50 [START_REF] Hilado | Flammability Handbook of Plastics[END_REF] . As said before, this is due to the presence of chlorine. The addition of DINP lowers this value up to 23. Samples B and E both gave higher LOI values than the reference DINP with 27. Knowing that the materials exhibiting a LOI value above 26 are considered as efficient flame retardants with a self-extinguishing behavior [START_REF] Liu | Phosphorus-containing polyaryloxydiphenylsilanes with high flame retardance arising from a phosphorus-silicon synergistic effect[END_REF][START_REF] Pan | Fire retardant mechanism analysis between ammonium polyphosphate and triphenyl phosphate in unsaturated polyester resin[END_REF] , it can then be concluded that PMO and PDE are effective flame retardant plasticizers for PVC.

Conclusion

In this work, four phosphonated lipidic derivatives (PMO, PMO2, PML and PDE) were successfully synthesized in a one step process from methyl fatty (di)ester by free-radical addition of dimethyl phosphite or diethyl phosphite onto the double bonds. A rheological study revealed the diffusion of these bio-based additives into the PVC particles and their possible use as primary plasticizers in soft PVC films. PVC samples with similar thermal properties than DINP ones and better flame retardant properties were obtained. The presence of phosphonate groups allowed the formation of a char layer and the protection of the material against larger degradation. Mechanical tests highlighted a good plasticizing efficiency with a high elongation at break around 350 % and a lower Young modulus and tensile strength, especially PDE and PMO. Thus, PMO and PDE could efficiently substitute phthalates as primary PVC plasticizers with additional flame retardancy properties.

Figure 1 .

 1 Figure 1. General chemical synthesis of PMO, PMO2, PML and PDE using MO, ML and

Figure 2 .

 2 The peak located at 0.8 ppm corresponding to the terminal methyl protons of fatty esters (-CH 3 or H a ) was taken as reference for the conversion calculation whereas the triplet at 2.3 ppm corresponding to the fatty ester protons [-(CH 2 )-CO-] (H b ) was used for the PDE plasticizer. The typical peak attributed to methyl ester protons (CO-O-CH 3 ) was located at 3.66 ppm. The addition of phosphorylated derivatives was monitored by the appearance of the -P-CH-signal at 1.6 ppm as well the doublet at 3.70 ppm corresponding to the phosphonate group (-P-O-CH 3 ) for PMO, PML and PDE products or the peaks at 1.3 ppm (-P-O-CH 2 -CH 3 ) and 4.1 ppm (-P-O-CH 2 -) for PMO2.

Figure 2 . 1 H

 21 Figure 2. 1 H NMR spectra of PMO, PMO2, PML and PDE.

Figure 3 .

 3 Figure 3. SEC chromatograms of PMO, PMO2, PML and PDE (left); of PDE related to

Figure 4 .

 4 Figure 4. G' (full lines) and G'' (dotted lines) as a function of temperature for various PVC plastisol formulations (5 °C/min).

Figure 5 .

 5 Figure 5. DSC thermograms of soft PVC corresponding to formulations A to E (10 °C.min -1 ).

Figure 6 .

 6 Figure 6. TGA analyses of plasticizers under air.

Figure 7 .

 7 Figure 7. TGA analysis of cured PVC plastisols under air.

  Among the phosphonated lipidic plasticizers, PMO and PDE (samples B and E respectively) lead to the highest elongation values. Moreover, PDE (sample E) has the highest plasticizing efficiency and flexibility compared to DINP and other bio-based plasticizers samples. Indeed, sample E exhibits lower Young modulus (9 related to 11-15 MPa) and tensile strength. In summary, the results show that the phosphonate group has an impact on the final properties of the plastisol. Hence, increasing the length of the phosphonate ester alkyl group or increasing the number of the phosphonate groups linked to the fatty chain diminishes the mechanical and thermal properties. Moreover, raising the number of carboxylic ester improve these properties.

Figure 8 .

 8 Figure 8. Tensile properties for soft PVC films with a plasticizer content of 30%.

Figure 9 .

 9 Figure 9. Tensile strength related to elongation for various plasticizer contents in PMO (full

  H NMR (300 MHz, CDCl 3 , δ ppm): 1.24-1.37 (m, 22 H, CH 2 and CH 2 -CH-P), 1.58-1.62 (m, 5 H, CH 2 -CH 2 -C=O and CH 2 -CH-P), 2.26-2.31 (t, 2 H, CH 2 -CH 2 -C=O), 3.65 (s, 6 H, CH 3 -O-C=O), 3.70-3.73 (d, 6 H, CH 3 -O-P). 31 P NMR (300 MHz, CDCl 3 , δ ppm): 37.46.

	C=O), 1.99-2.01 (m, 4 H, CH 2 -CH=CH-CH 2 ), 2.27-2.32 (t, 2 H, CH 2 -CH 2 -C=O), 3.66 (s, 6
	H, CH 3 -O-C=O), 5.32-5.35 (m, 2 H, CH=CH).

1 H NMR (300 MHz, CDCl 3 , δ ppm): 1.25-1.30 (m, 16 H, CH 2 ), 1.59-1.64 (m, 4 H, CH 2 -CH 2 -In a first step, phosphonated DE was dissolved in chloroform ([DE] = 0.3 mol.L -1 ) in presence of methanol

(20 equiv)

. The mixture was vigorously stirred and p-toluenesulfonic acid monohydrate (0.2 equiv) was added. In a reverse Dean-Stark apparatus, the reaction mixture refluxed for 24 h. The organic solution was washed twice with distilled water and twice with brine, then dried over anhydrous MgSO 4 , filtered and evaporated under vacuum to remove solvent and excess methanol. Dimethyl (dimethyl oleate)phosphonate (PDE) was yielded in 97 % (102.1 g, 0.227 mol). 1 2.3. Methods Nuclear magnetic resonance (NMR) spectra were recorded on a RMN BrukerAvance I 300 MHz. The chemical shifts were expressed in part per million (ppm), where (s) means a singlet, (d) a doublet, (t) a triplet, (m) a multiplet and (dd) a doublet of doublet. Chemical shifts ( 1 H NMR) were referenced to the peak of residual CHCl 3 at 7.26 ppm. Chemical shifts ( 13 C NMR) were referenced to CDCl 3 at 77 ppm.

Table 2 .

 2 Mechanical properties of PVC films.

	Sample	Elongation at break ɛ (%)	modulus E (MPa)	Tensile strength at break (MPa)	Shore A hardness
	A (ref.)	405 ± 10	14 ± 0,8	20 ± 1	93 ± 2
	B	320 ± 28	15 ± 0,4	20 ± 1,8	89 ± 2
	C	300 ± 30	15 ± 2,2	19 ± 1	94 ± 1
	D	290 ± 31	11 ± 1	18 ± 2,6	99 ± 1
	E	365 ± 13	9 ± 0,5	18 ± 1,1	90 ± 2

Table 3 .

 3 Influence of PMO and PDE content in PVC on mechanical properties.

	Plasticizer	%	Tg(°C)	Elongation at beak (%)	Young Modulus E (MPa)	Tensile strength at (MPa) break	Shore A hardness
		30	-10	320 ± 28	15 ± 0.4	20 ± 1.8	89 ± 2
	PMO	40	-30	340 ± 26	9 ± 0.5	14 ± 1.9	92 ± 1
		50	-50	480 ± 17	3 ± 0.3	7 ± 0.7	83 ± 2
	PED	30	-10	365 ± 13	9 ± 0.5	18 ± 1.1	90 ± 2

Acknowledgement

The authors thank the "Fonds Unique Interministériel" for financial support. They are thankful to J. Monbrun, V. Henryon of Activation and Oléon.


S M Mirabedini
email: m.mirabedini@utoronto.ca

I Dutil

L Gauquelin

N Yan

R R Farnood

Preparation of self-healing acrylic latex coatings using novel oil-filled ethyl cellulose microcapsules

Keywords:

Novel oil-filled microcapsules were prepared by introducing a phase separation method using ethyl cellulose as a shell-forming containing rapeseed oil. The prepared oil-filled microcapsules were evaluated by optical microscopy, scanning electron microscopy and particle size analysis. Results showed that spherical microcapsules with a diameter of 10 to 45 m and a rough porous shell were obtained. Carboxylated styrene/butadiene copolymer latex films containing various levels of these microcapsules were subjected to various levels of pre-elongation and their tensile properties were examined. The addition of oil-filled microcapsules resulted in a significant improvement in the modulus, strain-to-break, and toughness of the films. The self-healing mechanism of latex films was examined through the colorimetric measurements of the release of dye-containing following the pre-elongation of the samples. These measurements confirmed that pre-elongation of samples resulted in the release of oil within the latex films, hence plasticizing the surrounding polymeric network and partly restoring the mechanical properties of the pre-elongated films.

Introduction

Water-based polymeric coatings are used in a wide variety of applications ranging from decorative wall paints to packaging materials. However, due to the low strain-to-failure exhibited by these materials, unexpected damages and cracks often incur during their service life [START_REF] Tong | Preparation and characterization of novel melamine modified poly (urea-formaldehyde) self-repairing microcapsules[END_REF]. In many cases, physical and mechanical damages of polymeric coatings occur within the manufacturing and converting processes. In addition, water-based coatings usually provide relatively poor barrier properties against gases, high sensitivity to water and moisture, and inferior mechanical properties [START_REF] Andersson | Preparation and incorporation of microcapsules in functional coatings for self healing of packaging board[END_REF]. Some of conventional methods for improving these properties include; increasing the coating thickness [START_REF] Burnett | The relationship between hardness and scratch adhesion[END_REF], applying multiple coating layers [START_REF] Tong | Preparation and characterization of novel melamine modified poly (urea-formaldehyde) self-repairing microcapsules[END_REF], adding functionalized nanoparticles [START_REF] Mirabedini | Weathering performance of the polyurethane nanocomposite coatings containing silane treated TiO2 nanoparticles[END_REF][START_REF] Mirabedini | Effect of various combinations of zirconia and organoclay nanoparticles on mechanical and thermal properties of an epoxy nanocomposite coating[END_REF][START_REF] Díez-Pascual | Mechanical electrical properties of carbon nanotube/poly(phenylene sulphide) composites incorporating polyetherimide and inorganic fullerene-like nanoparticles[END_REF], increasing the crosslinking density of polymer [START_REF] Chattopadhyay | Thermal and mechanical properties of epoxy acrylate/methacrylates UV cured coatings[END_REF] and introducing functionalized core-shell latexes into the coating formulation [START_REF] Pedraza | Effect of functional monomer on the stability and film properties of thermosetting core-shell latexes[END_REF].

Among the aforementioned available methods [START_REF] Burnett | The relationship between hardness and scratch adhesion[END_REF][START_REF] Mirabedini | Weathering performance of the polyurethane nanocomposite coatings containing silane treated TiO2 nanoparticles[END_REF][START_REF] Mirabedini | Effect of various combinations of zirconia and organoclay nanoparticles on mechanical and thermal properties of an epoxy nanocomposite coating[END_REF][START_REF] Díez-Pascual | Mechanical electrical properties of carbon nanotube/poly(phenylene sulphide) composites incorporating polyetherimide and inorganic fullerene-like nanoparticles[END_REF][START_REF] Chattopadhyay | Thermal and mechanical properties of epoxy acrylate/methacrylates UV cured coatings[END_REF][START_REF] Pedraza | Effect of functional monomer on the stability and film properties of thermosetting core-shell latexes[END_REF], application of healable materials in coating formulations shows great potential for improving the durability and enhancing bulk mechanical properties of the coating layers. Such self-healing coatings possess the ability to repair in response to damage in the material [START_REF] Blaiszik | Self-healing polymers and composites[END_REF].

In 2001, White and co-workers [START_REF] White | Autonomic healing of polymer composites[END_REF] introduced a novel method to repair the mechanical properties of thermosetting polymers following crack propagation through the micro-encapsulation of self-healing materials. Repairing has been obtained via the addition of urea-formaldehyde microcapsules containing dicyclopentadiene (DCPD) healing agent and through the ring opening polymerization of DCPD in the presence of Grubbs' catalyst [START_REF] Rule | Wax protected catalyst microspheres for efficient self healing materials[END_REF]. Micro-encapsulation is an approach that consists of isolating a dispersed phase from an external medium by surrounding or coating it with a protective sell material having sizes typically ranging from submicron up to 1 mm [START_REF] Hennequin | Synthesizing microcapsules with controlled geometrical and mechanical properties with microfluidic double emulsion technology[END_REF]. The microcapsules containing healing agents have the capability of self-repairing upon crack propagation [START_REF] Brown | Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite-Part II: In situ self-healing[END_REF][START_REF] Murphy | The world of smart healable materials[END_REF]. As the healing agent flows within the crack lines, it comes into contact with the active materials inducing polymerization and hence sealing up the crack [START_REF] Kessler | Self-healing structural composite materials[END_REF]. The repairing of mechanical damage is particularly useful to coatings or composites where subsurface damage occurs that is difficult to detect and fix [START_REF] Kessler | Self-healing structural composite materials[END_REF][START_REF] Patel | Autonomic healing of low-velocity impact damage in fiber-reinforced composites[END_REF][START_REF] Moll | A self-sealing fiber-reinforced composite[END_REF][START_REF] Jin | Fracture and fatigue response of a selfhealing epoxy adhesive[END_REF].

A variety of self-healing chemistries have been introduced to enhance the interaction between the healing agent and a polymeric matrix. These include epoxy [START_REF] White | Autonomic healing of polymer composites[END_REF][START_REF] Mc Ilroy | Microencapsulation of a reactive liquid-phase amine for self-healing epoxy composites[END_REF][START_REF] Yuan | Self-healing polymeric materials using epoxy/mercaptan as the healant[END_REF], isocyanate [START_REF] Yang | Microencapsulation of isocyanates for self-healing polymers[END_REF] and solvent welding [START_REF] Caruso | Solvent promoted self-healing epoxy materials[END_REF] siloxane chemistries for vinyl ester matrices [START_REF] Cho | Polydimethylsiloxane-based self-healing materials[END_REF] and poly dimethyl siloxine (PDMS) [START_REF] Keller | Torsion fatigue response of self-healing poly(dimethylsiloxane) elastomers[END_REF].

An alternative to self-healing using chemical bonding agents is self-healing by plasticizing mechanism. This approach is a promising method for enhancing mechanical properties of polymeric coatings via increasing tensile strength and elongation at break.

In the current study, rapeseed oil was encapsulated in ethyl cellulose microcapsules using a robust phase separation route and the mechanical properties of the water-based latex coatings containing these oil-filled microcapsules were investigated. Latex films containing 1-3 wt% microcapsules and two microcapsule sizes were studied. The deformation and disruption of latex films containing microcapsules filled with colored rapeseed oil were examined under applied tensile load using color coordinate measurements.

Experimental

Materials

Ethyl cellulose (EC), rapeseed oil (RO), sodium dodecyl sulfate (SDS), Sudan Red 7B solvent dyestuff, and ethyl acetate were purchased from Sigma Aldrich (Oakville, Ontario, Canada). A non-ionic surfactant, Dynol 604, was provided by Air Products USA. A commercial carboxylated styrene-butadiene latex, Styronal ND 656 was supplied by BASF Corporation. This latex is an emulsion-type latex commonly used for paper coating applications for its filmforming properties, resistance to water and moisture, and relatively rapid drying with low porosity [START_REF] Schuman | Dispersion coating with carboxylated and cross-linked styrene-butadiene latices: 2. Effects of substrate and polymer characteristics on the properties of coated paperboard[END_REF]. All chemicals were analytical reagent grade and used as received.

Microcapsules preparation

Microcapsules with oil to ethyl acetate ratio of 70:30 were prepared via a two-stage solvent evaporation method [START_REF] Andersson | Preparation and incorporation of microcapsules in functional coatings for self healing of packaging board[END_REF] following the methodology reported elsewhere [START_REF] Mirabedini | Preparation and characterization of ethyl cellulose-based core-shell microcapsules containing plant oils[END_REF][START_REF] Es-Haghi | Preparation and characterization of pre-silane modified ethylcellulose-based microcapsules containing linseed oil[END_REF]. The flowchart for the synthesis of microcapsules is provided in Fig. 1. In brief, in the first stage, ethyl cellulose powder was dissolved in ethyl acetate (5 wt%) under magnetic stirring for at least 2 h at ambient temperature. RO and 0.005 wt% of red solvent dyestuff were then added to the above-mentioned solution and system was magnetically stirred for further 30 min. In the second stage, the solution was added drop wise to an aqueous solution of 1 wt% SDS and 0.05 wt% • C for about 90 min. These microcapsules were washed with deionized water and freeze dried at -60 ± 5 • C and 6.5 mPa for characterization. In addition, to examine the effects of oil encapsulation, a control sample was prepared where no oil was added in the synthesis procedure, producing solid ethyl acetate particles.

Microcapsules characterization

Multisizer 3 particle size analyzer (Coulter Counter, FL, USA), equipped with a 280 m aperture tube, was used to determine the average particle size and size distribution of microcapsules. Microcapsules (0.05 g) were dispersed into 250 ml deionized water using a stirring bar for about 10 min before particle size measurement.

The morphology and shape of microcapsules were evaluated using a Leica DMLA optical microscope (Leica Microsystems, Germany). The surface and morphology of microcapsules were also evaluated using a scanning electron microscope (HITACHI S-2500, Japan). The vacuum freeze-dried microcapsules [START_REF] Mirabedini | Preparation and characterization of ethyl cellulose-based core-shell microcapsules containing plant oils[END_REF] were mounted on a conductive stage and sputter coated with a thin layer (∼10 nm) of gold for 90 s under Ar atmosphere and SEM micrographs were recorded in the secondary electron mode at 10 kV.

Mechanical properties

Various wt% of oil-filled microcapsules were added to the latex suspension and magnetically stirred for 30 min. The suspension was then applied on the degreased glass plate using a standard coating rod with a wet film thickness of 200 ± 10 m. the latex samples were then dried at 50 ± 5 • C for about 30 min.

Tensile properties of free-standing latex films containing various levels of RO:EC microcapsules were evaluated according to ASTM D 638 standard. Rectangular tensile specimens (0.15 mm × 15 mm × 100 mm) were prepared by cutting the latex film to shape. The experiments were carried out on a Universal computerized system for material testing machine (Sintech 1). The crosshead moved with a constant speed of 50 mm min -1 at 23 ± 2 • C. At least seven individual replicates were tested for each formulation.

Color measurements

To visualize the oil released during the mechanical testing of latex films, after being subjected to different elongation percentages (0, 25, 50, 100 and 150%), color measurements were performed according to ASTM D 65 standard using a X-Rite 530 spectrodensitometer. Total color change, E, as a function of pre-elongation % a NLa, R and EC in sample coding stand for Neat latex film, RO:EC microcapsules, non-oil filled microcapsules, respectively. The first digit symbol represents the wt% of each type of microcapsule embedded in latex film and the second digit corresponds to mixing speed of either 800 or 1400 rpm. 1.0EC-800 and 1.0EC-1400 show latex films containing 1 wt% oil free microcapsules prepared at mixing speed of 800 and 1400 rpm, respectively.

E * ab = ( L * ) 2 + ( a * ) 2 + ( b * ) 2 (1)
here L* is on the black-white (L* = 0 for black, L* = 100 for white) axis, a* is on the red-green (positive values are red, negative values are green) axis, and b* is on the yellow-blue (positive values are yellow, negative values are blue) axis.

Results and discussion

Microcapsules characterization

It has been reported that functionality, mechanical properties and appearance of microcapsules are affected by their size [START_REF] Brown | Fracture testing of a self-healing polymer composite[END_REF][START_REF] Rule | Effect of microcapsule size on the performance of self-healing polymers[END_REF][START_REF] Behzadnasab | Preparation and characterization of linseed oil-filled ureaformaldehyde microcapsules and their effect on mechanical properties of an epoxy-based coating[END_REF]. Also, in the case of self-healing coatings, the amount of healing agent available for delivery to the crack area depends on the microcapsule size [START_REF] Behzadnasab | Preparation and characterization of linseed oil-filled ureaformaldehyde microcapsules and their effect on mechanical properties of an epoxy-based coating[END_REF].

The particle size distribution plots of the RO microcapsules and the volumetric average (d m ) and the coefficient of variation of particle size (CV) are shown in Fig. 2. With increasing mixing rate from 800 rpm to 1400 rpm, the average diameter of microcapsules decreased from 34.0 m to 18.8 m while the coefficient of variation (CV) of particle size remained almost constant at about 8.1-8.5%. These results reveal that the average diameter of microcapsule can be controlled by varying mixing speed.

Shell wall integrity, aggregation phenomena, and microcapsule shape and size were also evaluated by optical microscopy and scanning electron microscopy (SEM). The optical micrographs of microcapsules prepared at mixing speed of 800 rpm and 1400 rpm are shown in Fig. 3. The prepared microcapsules were polydispersed spherical particles ranging in size from about 5 to 50 m without any inter-capsule bonding. No free oil was visible in these micrographs, suggesting that oil was well encapsulated in the shell material [START_REF] Tong | Preparation and characterization of novel melamine modified poly (urea-formaldehyde) self-repairing microcapsules[END_REF][START_REF] Mirabedini | Preparation and characterization of ethyl cellulose-based core-shell microcapsules containing plant oils[END_REF].

Fig. 4 shows optical micrographs of latex films representing the range of morphologies of prepared microcapsules observed in this study. The morphology of microcapsules mainly depends on the core material and the deposition process of the shell. Based on their morphology, microcapsules may be classified as mononuclear (or core-shell), multinuclear and matrix [START_REF] Murphy | The world of smart healable materials[END_REF]. In mononuclear microcapsules core material is surrounded by a continuous shell. Multinuclear microcapsules on the other hand contain a number of small droplets of core material embedded within a single shell. Finally, in matrix microcapsules, the core material is distributed uniformly in the shell material. It is expected that a more efficient self-healing property of microcapsules within the coating film can be achieved by mononuclear morphology.

SEM micrographs of the freeze-dried microcapsules are shown in Fig. 5. With increasing mixing speed, shell structure became more porous and the number of "holes" appeared on the surface of microcapsules increased. The porous surface structure of microcapsules is expected to affect the self-healing properties of the latex films. Microcapsules with higher porosity and surface holes, could release the oil under a relatively lower level of mechanical stress. However, to avoid premature oil release, it is necessary that oil-filled microcapsules withstand mechanical stresses occurred during the processing of latex films. On the other hand, for non-porous microcapsules, breakage of the microcapsule is the initiating event for oil release and the repairing process. Therefore, it is essential to synthesize microcapsules with appropriate shell morphology, wall thickness and mechanical properties [START_REF] Samyn | Modifications of paper and paperboard surfaces with a nanostructured polymer coating[END_REF].

Mechanical properties

Summary of the mechanical properties of latex films samples are given in Table 1. The results plotted in Fig. 6 show that with the addition of microcapsules, yield stress and yield strain initially increased before decreasing again at high microcapsule contents. However, statistical analysis revealed no significant difference between the mechanical properties of the samples up to 2 wt% microcapsule addition levels (Table 2).

Stress-strain curves for latex films containing 1 wt% RO microcapsules with and without pre-elongation are plotted in Fig. 7. These samples exhibited a three-part stress-strain plot: an elastic region, a plastic region, and a strain-hardening region. Compared to the neat latex films (NLa), the addition of oil-containing microcapsules resulted in a more extended plastic region. Furthermore, the latex films containing microcapsules prepared at 800 rpm exhibited a better mechanical properties compared to those containing microcapsules prepared at 1400 rpm. Modulus, toughness and elongation at break of microcapsule containing samples after 100% and 150% pre-elongation are plotted in Fig. 8. As expected, increased levels of pre-elongation generally deteriorated the mechanical properties of samples. However, this effect was less pronounced for the latex films containing RO microcapsules.

In particular, microcapsules prepared at 800 rpm better restored the mechanical properties of latex films after pre-elongation. The dependency of mechanical properties on the microcapsule size has been reported for polyurethane composite [START_REF] Landon | The influence of particle size on the tensile strength of particulate-filled polymers[END_REF][START_REF] Blaiszik | Nanocapsules for self-healing materials[END_REF], where smaller particles had less of an effect on the tensile properties of the composite film [START_REF] Landon | The influence of particle size on the tensile strength of particulate-filled polymers[END_REF].

To better examine the oil release within the latex films during the elongation process, optical micrograph images were taken at various elongation levels of latex films. As discussed before, in order to better visualize the oil release in these experiments, a red dyestuff was added to the rapeseed oil. According to Fig. 9, as the specimen was elongated, the oil was released and spread in the surroundings polymeric matrix. After removing the tensile force, the specimen retracted to its initial dimensions but the released oil; as characterized by the spreading of darker regions in the image area, remained within the polymer matrix. Close examination of the samples showed that the oil release mainly occurred via oil penetration through the porous wall of microcapsules and without capsule rupture.

Colorimetric analysis

The oil release properties from microcapsules were characterized by evaluation of changes in the color coordinates of samples. The results of color measurements (Table 3) show that for all microcapsules embedded samples, a* values (redness) increased significantly after pre-elongation up to about 50%, then followed by a gradual increase at higher elongation %. Specimens containing 2 wt% of microcapsules prepared at mixing speed of 800 rpm had a higher increase in a* value, due to more colored oil released from the bigger microcapsules within the polymeric matrix. The smallest a* and the highest L* value were observed for un-filled latex specimens. This is attributed to the fact that the latex film is colorless, transparent and has no particles. For all specimens, L* values were decreased by addition of microcapsules. Specimens containing 2 wt% of capsules revealed higher reduction in L* value, due to more light scattering by the particles. L* values were decreased with increasing elongation %, resulting of microcapsules rupture, possible stress-whitening and better orientation of polymeric chains, therefore more lights are scattered through the latex film.

Fig. 10 shows that the total color change ( E) of various latex samples increased rapidly with increasing the pre-elongation from 25% to 50%. The color change, E had a similar trend compared to redness, a*, and increased after pre-elongation. Specimens containing 2 wt% of microcapsules prepared at mixing speed of 800 rpm had a higher increase in E value. A higher value of E is indicative of more oil being released within the sample. The specimens containing 2 wt% of larger capsules revealed maximum color changes ( E = 1.15) after 150% pre-elongation, while the E for the specimen containing 2 wt% of smaller capsules was 1.01. Unfilled coating revealed the lowest color change ( E = 0.323) after 150% preelongation.

These color measurements confirm the release of oil was released among the polymeric network and the surrounding latex matrix with increasing the % elongation. The released oil could act as a healing agent and restore mechanical properties of the latex film by filling the micro-cracks generated during the tensile testing of the sample.

Conclusion

In this study, new self-plasticizing microcapsules with enhanced mechanical and healing properties of latex film were prepared by introducing a robust solvent evaporation method using ethyl cellulose as a shell-forming polymer containing rapeseed oil. These results suggest that the oil was released from the capsules under mechanical stress into the microcapsules surrounding through capillary force, and sliding of molecular chains on each other, resulting in improved mechanical properties through plasticization of the latex films.

The results also reveal that latex films containing larger microcapsules, prepared at mixing rate of 800 rpm, showed slightly better tensile strength properties in second and third tensile measurements runs, compared with those counterparts having smaller microcapsule.

The improvement in the mechanical properties of latex film was due to the release of oil within the latex network that plasticized the surrounding polymeric network thus restoring the integrity of the film.

Fig. 1 .

 1 Fig. 1. Microencapsulation procedure.

Fig. 2 .

 2 Fig. 2. Volume fraction (%) and the average diameter of 70:30 RO:EC microcapsules: (a) 1400 rpm (dm = 18.8 m, C.V. = 8.1%), and (b) 800 rpm (dm = 34.0 m, C.V. = 8.5%).

Fig. 3 .

 3 Fig. 3. Optical micrographs of microcapsules prepared at mixing rate of: (a) 800 rpm and (b) 1400 rpm.

Fig. 4 .

 4 Fig. 4. Optical micrographs of latex film containing microcapsules with different morphologies.

Fig. 5 .

 5 Fig. 5. Scanning electron micrographs of RO:EC microcapsules synthesized at mixing rate of (a) 800 and (b) 1400 rpm.

Fig. 6 .

 6 Fig. 6. Tensile properties of latex films containing various wt% of RO filled microcapsules prepared at mixing speed of: (a) 800 and (b) 1400 rpm.

Fig. 7 .

 7 Fig.7. Stress-strain plots for latex films containing 1 wt% of RO microcapsules prepared at 800 and 1400 rpm mixing speeds, before and after 100% pre-elongation.

Fig. 8 .

 8 Fig. 8. Tensile strength test results for latex films containing 1 wt% EC:RO microcapsules (A) before elongation, (B) After 100% pre-elongation, (C) after 150% pre-elongation.

Fig. 9 .

 9 Fig. 9. Optical micrographs of latex films containing 1.0 wt% EC:RO microcapsules during various elongation percentages and after recovery.

Fig. 10 .

 10 Fig. 10. The color difference, E, versus the pre-elongation for latex films containing RO:EC microcapsules.

Table 1

 1 Summarized mechanical properties of latex films containing oil-filled microcapsules.

	Sample coding a	Elongation at break (%)	Young modulus (MPa)	Yield stress (MPa)	Yield strain (MPa)	Toughness (MPa)
		±14	±0.25	±0.15	±0.17	±0.23
	NLa	191	4.77	0.74	2.70	3.84
	0.5R-800	222	5.59	1.40	3.03	4.36
	1.0R-800	204	5.47	2.19	4.38	5.85
	2.0R-800	154	4.75	1.15	2.00	2.68
	3.0R-800	145	4.08	0.87	1.49	1.55
	1.0EC-800	98	5.00	2.35	3.80	3.15
	0.5R-1400	187	5.08	1.82	3.28	4.18
	1.0R-1400	201	5.74	2.05	3.38	4.65
	2.0R-1400	211	5.42	2.23	3.08	3.10
	3.0R-1400	153	3.79	1.80	2.60	2.61
	1.0EC-1400	117	5.16	2.40	3.85	3.05

Table 2 P

 2 -value results obtained from t-test statistical evaluation for mechanical properties of latex films containing various amounts of microcapsules.

	A) Microcapsules synthesized at mixing speed of 800 rpm
	-	NLa	0.5R-800	1.0R-800	2.0R-800	3.0R-800	1.0EC-800
	NLa	-	0.004	0.15	0.001	3.7 ×10 -5	2.6×10 -8
	0.5R	0.003	-	0.10	8.3×10 -6	3.0 ×10 -6	1.7 ×10 -8
	1.0R	0.14	0.38	-	0.001	6.3 ×10 -8	1.5 ×10 -8
	2.0R	0.46	0.006	0.005	-	0.41	1.1 ×10 -8
	3.0R	0.02	3.7×10 -4	2.3×10 -4	0.03	-	2.7 ×10 -8
	1.0EC	0.22	0.02	0.03	0.34	0.008	-
	B) Microcapsules synthesized at mixing speed of 1400 rpm
	-	NLa	0.5R-1400 1.0R-1400 2.0R-1400 3.0R-1400 1.0EC-1400
	NLa	-	0.62	0.14	0.03	0.001	1.0×10 -7
	0.5R	0.06	-	0.10	0.02	0.001	1.8×10 -6
	1.0R	0.01	0.07	-	0.22	2.5×10 -5	3.4×10 -8
	2.0R	0.01	0.11	0.57	-	2.3×10 -5	1.7×10 -7
	3.0R	0.13	0.02	0.003	0.004	-	0.001
	1.0EC	0.05	0.47	0.23	0.44	0.01	-

*: Elongation at break point (horizontal), Young's modulus

(vertical)

Table 3

 3 Color coordinates changes of specimens before and after various elongation percentages.

	Sample coding	Pre-elongation (%)	Average color coordinates		Total color changes
			L *	a *	b *	E	STD
	NLa	0	90.13	-1.43	3.90	0	0
		25	90.05	-1.40	3.80	0.132	0.061
		50	90.04	-1.37	3.85	0.119	0.037
		100	89.98	-1.39	3.84	0.166	0.073
		150	89.85	-1.37	4.05	0.323	0.081
	1.0EC-800	0	73.00	-1.15	2.10	0	0
		25	72.85	-1.18	2.04	0.166	0.012
		50	72.77	-1.17	1.98	0.262	0.07
		100	72.67	-1.20	2.13	0.330	0.065
		150	-1	-	-	-	-
	1.0R-800	0	79.80	12.35	-0.70	0	0
		25	79.65	12.98	-0.72	0.643	0.1
		50	79.48	13.13	-0.68	0.842	0.052
		100	79.26	13.21	-0.67	1.016	0.034
		150	79.20	13.27	-0.472	1.121	0.1
	2.0R-800	0	82.50	10.70	-0.09	0	0
		25	82.47	11.29	-0.13	0.595	0.116
		50	82.30	11.53	-0.17	0.854	0.112
		100	82.08	11.61	-0.12	1.027	0.149
		150	81.92	11.69	-0.13	1.149	0.121
	1.0R-1400	0	72.79	25.80	-4.40	0	0
		25	72.71	26.26	-4.33	0.474	0.052
		50	72.60	26.45	-4.32	0.683	0.056
		100	72.51	26.54	-4.27	0.801	0.174
		150	72.35	26.59	-4.23	0.926	0.109
	2.0R-1400	0	73.90	25.05	-4.10	0	0
		25	73.81	25.47	-4.09	0.433	0.075
		50	73.71	25.75	-4.06	0.726	0.106
		100	73.55	25.84	-4.13	0.860	0.169
		150	73.39	25.93	-4.11	1.013	0.133

1

Specimens disrupted for the elongation grater that 100%.

Acknowledgments

Authors wish to gratefully acknowledge support from Pulp & Paper Centre at the University of Toronto as well as Iran Polymer & Petrochemical Institute.


Oleksii Shtehin

Sébastien Seguy
email: s.seguysebastien.seguy@insa-toulouse.fr

Vincent Wagner
email: vincent.wagner@enit.fr

Yann Landon

Gilles Dessein
email: g.desseingilles.dessein@enit.fr

Michel Mousseigne
email: m.mousseignemichel.mousseigne@univ-tlse3.fr

Low-frequency chatter genesis during inclined surface copy-milling with ball-end mill: Experimental study

Keywords: Ball-end milling, Low immersion milling, Stability analysis, Chatter

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INTRODUCTION

Appearance of oscillations of different nature during machining is one of the main factors, which restrain growth of productivity. Genesis of regenerative vibrations (chatter) is the most highlighted problem of machining dynamic over the last decades. It caused by its complexity on the one part and significant influence on quality and efficiency on the other part.

Regenerative oscillations in machining are well known for almost a hundred years; its general mechanics in the case of turning was described rigorously by [START_REF] Tobias | Theory of regenerative machine tool chatter[END_REF]. The theory of regenerative tool chatter supposes that self-oscillation during cutting process appears and supported due to previous (one period earlier) cut trace on machined surface. [START_REF] Altintaş | Analytical prediction of stability lobes in milling[END_REF] described a classical method of stability machining conditions prediction for the case of interrupted cutting like milling. This method makes possible to determine pairs of spindle speed rotation and axial depth of cut for which end-milling process will be stable. Analytical prediction of milling using stability lobes diagrams was improved also for ball-end milling by [START_REF] Altintaş | Analytical prediction of stability lobes in ball end milling[END_REF]. Low radial immersion and high interruption of ball-end milling process are the reasons which demand investigate this kind of cutting as a highly nonlinear dynamics process.

Stability of high interrupted cutting also may be predicted by temporal finite element analysis (FEA), as it was shown by [START_REF] Bayly | Stability of interrupted cutting by temporal finite element analysis[END_REF]. A solution of temporal FEA is presented as a discrete map that contains current and previous position and velocity at the beginning and at the end of each element, which eigenvalues are used to determine stability. Two other methods of analytical stability identification of up-milling and down-milling are described by Insperger et al. (2003a) and then validated experimentally by them [START_REF] Mann | Stability of up-milling and downmilling, part 2: experimental verification[END_REF] in the case of end-milling for single-degree-of-freedom system. Hence, it was shown, that period-doubling instability is typical for high-interrupted low radial immersion milling process. Time of cut, in turn, also is small and may be even less than period of natural oscillations of the system, especially when high-speed milling. Furthermore, Insperger et al. (2003b) presented analytical investigations of multiple frequency model of high-speed milling to identify chatter frequencies; the study is reinforced by experiments.

A numerical method of chatter modeling during low radial immersion milling was presented by [START_REF] Campomanes | An improved time domain simulation for dynamic milling at small radial immersions[END_REF]. Model that is used is an improved kinematics model. Cutting forces, surface topology and chatter stability may be numerically predicted inclusive of nonlinear effects, that is almost impossible do analytically. Time-domain simulation ensures good results when dynamic model is quite simple and when non-periodic processes take place in the case of multi-frequency solution too, as it has been shown by [START_REF] Merdol | Multi frequency solution of chatter stability for low immersion milling[END_REF]. In general, regenerative effect and its correlation with nonlinear dynamics of high-speed milling are explained by the loss of contact between the work-piece and the tool, which leads to secondary Hopf bifurcation [START_REF] Stépán | Nonlinear Dynamics of High-Speed Milling Subjected to Regenerative Effect[END_REF]. Two methods of stability boundaries predictedzero thorder approximation and semi-discretizationwere compared by [START_REF] Gradišek | On stability prediction for low radial immersion milling[END_REF] for the case of low radial immersion milling. The disagreement between these methods grows up if radial immersion decreases. Period doubling bifurcation takes place when radial immersion is extremely small and was predicted only by the semi-discretization method. Thus, it was found that two chatter types occur during low radial immersion milling (period doubling bifurcation and quasiperiodic Hopf bifurcation) and both of them must be taken into account for accurate prediction of chatter free milling conditions. Analytical models of milling force, part and tool deflection, and form error predicting for high performance flat-end milling were presented by Budak (2006a). These models make possible an optimizing of feed rate, depth of cut and spindle speed, but only when process is stable. For chatter suppression models were also improved then (Budak, 2006b), analytical milling stability model is based on a Floquet's theorem. A low rigidity parts often require more complex dynamic models than single-degree-of-freedom model. Such three-dimensional model for the calculation of the stability lobes is presented by [START_REF] Campa | Selection of cutting conditions for a stable milling of flexible parts with bull-nose end mills[END_REF]. The model enables to predict accurate values of radial and axial depth of cut and spindle speed. As demonstrated [START_REF] Faassen | An improved tool path model including periodic delay for chatter prediction in milling[END_REF], more precise machining process modeling, particularly change from the uncertain circular tool path to the certain trochoidal tool path, allows increasing stability lobe diagrams accuracy, as well. Presented models show good results in practice of flat-end milling cases but they have less usability during low radial immersion ball-end milling.

Chatter prediction in ball-end milling is a lot more complicated issue than in flat-end milling due to higher complexity of machining dynamics. A method of chatter prediction based on accurate modeling of cutting forces was proposed by [START_REF] Kim | Prediction of chatter in NC machining based on a dynamic cutting force model for ball end milling[END_REF]. Dynamic cutting force model considers tool runout and the penetration effect as well. There was proposed to detect chatter from the frequency spectrum of calculated cutting forces. Precise spectrum analysis may demand signal decomposition to clean original signal from noises and unnecessary frequencies. One of the common methods of signal decomposition consists in its wavelet transform. This method is developing for the last thirty years and it may be used for chatter identification in some cases of flat-end milling, as is shown by [START_REF] Yao | On-line chatter detection and identification based on wavelet and support vector machine[END_REF]. More complex methods of chatter prediction, which demand big data computing were proposed then. For instance, [START_REF] Butcher | Analysis of milling stability by the Chebyshev collocation method: algorithm and optimal stable immersion levels[END_REF] proposed high-performance approximation technique based on Chebyshev collocation method.

The main advantage of this technique is that it can be applied to the systems with non-smooth coefficients of delay-differential equations (like high-interrupted milling dynamics). [START_REF] Wan | A unified stability prediction method for milling process with multiple delays[END_REF] proposed a unified method of stability prediction during milling with multiple delays. The effects of the runout and the pitch angles of the cutter are considered with this method. It makes possible to analyze the asymptotic stability for variable cutting conditions (feed rate, radial immersion, helix angle) as well. It is also noticed that feed per tooth has great effect on the stability lobes when cutter runout occurs in the case of flat-end mill machining. Multiple modes influence on the stability lobes is examined by [START_REF] Wan | Study on the construction mechanism of stability lobes in milling process with multiple modes[END_REF] using the semi-discretization method. The lowest envelop method (LEM) is used for the ultimate stability lobe prediction. The accuracy of the prediction is increased by the considering the effects of multiple modes, not only the most flexible mode. The experimental verification is carried out for flat-end mill. [START_REF] Yang | Chatter prediction for the peripheral milling of thin-walled workpieces with curved surfaces[END_REF] investigated chatter prediction for the curved surfaces peripheral milling of the thinwalled workpieces. The proposed method of the stability analysis is based on structural dynamic modification scheme and takes into account the effect of material removal upon the in-process workpiece dynamics. It was verified experimentally during milling of two thin-walled workpieces with curved surface. The verification results show that stable milling zones can be accurately predicted by this method.

Ways of improvement of process stability with nonlinear components are different. The one of the most common ways consists in spindle speed variation. It also showed good results in stability control during flat-end milling of thin walled structures when high-speed machining occurs, as it was described by [START_REF] Séguy | On the stability of highspeed milling with spindle speed variation[END_REF]. [START_REF] Tangjitsitcharoen | Analysis of chatter in ball end milling by wavelet transform[END_REF] proved that on-line chatter detection during ball-end milling may be successfully achieved with wavelet transform.

At the same time, the combination of wavelet and Hilbert-Huang transforms can simplify and improve accuracy of chatter identification, as it has been shown in the case of flat-end milling [START_REF] Cao | Chatter identification in end milling process using wavelet packets and Hilbert-Huang transform[END_REF]. Usually, such in-cutting chatter detection methods are mandatory because of high dynamic complexity of ball-end milling, especially when machining takes place on 5 axis machines, where lead and tilt angles must be exactly taken into account for uncut chip and cutting force computation, as [START_REF] Mousseigne | Predicting the dynamic behaviour of torus milling tools when climb milling using the stability lobes theory[END_REF] showed. A chatter free calibration method for determining cutter runout and cutting force coefficients in ball-end milling was proposed by [START_REF] Yao | A chatter free calibration method for determining cutter runout and cutting force coefficients in ball-end milling[END_REF]. Varying time delay and tooth runout effect are essential factors of milling dynamics investigation, with ball-end mills among other, which was described by [START_REF] Zhang | An efficient approach for milling dynamics modeling and analysis with varying time delay and cutter runout effect[END_REF]. Applying of innovative algorithms which use neural networks for instance [START_REF] Lamraoui | Chatter detection in milling machines by neural network classification and feature selection[END_REF] demand carry out big data computing, from the one part, but it achieves high accuracy of stability prediction of nonlinear processes, from the other part. Another one algorithm that is based on ensemble empirical mode decomposition (EEMD) was proposed by [START_REF] Cao | Chatter identification in end milling process based on EEMD and nonlinear dimensionless indicators[END_REF].

In recent times a number of complex adaptive control systems to avoid chatter in 5-axis freeforms milling were developed, like an automatic adjustment of tool axis orientations for 5 axis milling machine proposed by [START_REF] Sun | Chatter free tool orientations in 5-axis ball-end milling[END_REF].

Warped surfaces machining with ball-end mills was analysed by [START_REF] Shtehin | Stability of ball-end milling on warped surface: semi-analytical and experimental analysis[END_REF]. The proposed model takes into account nonlinear effect of radial allowance on the contact angle during stability lobes plotting. The aim of the current study is to expand experimentally investigation of warped surface copy-milling with ball-end mills. Previous researches don't give clear information and experimental validation of dominant frequencies changing for different machining conditions like cutting direction, lead angle, spindle speed and depth of cut.

Understanding of these interrelations is necessary for intelligence adaptive control system development.

For this aim cutting forces and dynamics during inclined surfaces ball-end milling are analyzed in the 'Cutting Forces and Dynamics' section. In the 'Experimental Setup' section test setup of massive single-degree-of-freedom system is presented. In the 'Analysis and Discussion' section there are presented results of cutting tests which were carried out with different conditions: cutting direction (upward down-milling and downward up-milling), spindle speed rotation in a range from 5852 rpm to 8008 rpm, depth of cut values in a range from 0.10 mm to 0.50 mm, angle of inclination values in a range from 22.5° to 52.5°. Finally, general analysis of stability loss specifics for diverse cases of ball-end milling has been done. During copy milling of warped (sculptured) surface with ball-end mill the machining of different areas of this surface may be approximate analyzed as machining of number of inclined surfaces with different values of inclination angle. The inclination angle is identical to the lead angle ξ as the angle between cutter axis and surface normal.

CUTTING FORCES AND DYNAMICS

Angle of contact

The angle of contact between the tooth cutting edge of ball-end mill and the uncut chip is various in the case of copy milling of warped surface, and it depends on the values of depth of cut 𝑎 𝑝 , radial depth of cut 𝑎 𝑒 , lead angle ξ, radius of the tool 𝑟 (Fig. 2).

Fig. 2 Downward up-milling of inclined surface with ball-end mill

Its value results as a difference of exit angle ϕ 𝑒 and start angle ϕ 𝑠 , which may be determined from equations (1) for upward up-milling, (2) for upward down-milling, (3) for downward upmilling and (4) for downward down-milling [START_REF] Shtehin | Vyznachennya kutiv vrizannya ta vykhodu pry obrobtsi pokhylykh poverkhon sferychnymy kintsevimi frezamy [Definition of start and exit angles in ball end milling of inclined surfaces[END_REF].

{ ϕ 𝑠 = π 2 -arctg √1-𝐾 2 𝐾•sinξ ϕ 𝑒 = π 2 + arctg ( 𝑎 𝑒 𝑟•sinξ √ 4- 𝑎 𝑒 2 𝑟 2 ) ; (1) { ϕ 𝑠 = π 2 -arctg ( 𝑎 𝑒 𝑟•sinξ √ 4- 𝑎 𝑒 2 𝑟 2 ) ϕ 𝑒 = π 2 + arctg √1-𝐾 2 𝐾•sinξ ; (2) { ϕ 𝑠 = 3π 2 -arctg √1-𝐾 2 𝐾•sinξ ϕ 𝑒 = 3π 2 + arctg ( 𝑎 𝑒 𝑟•sinξ √ 4- 𝑎 𝑒 2 𝑟 2 ) ; (3) { ϕ 𝑠 = 3π 2 -arctg ( 𝑎 𝑒 𝑟•sinξ √ 4- 𝑎 𝑒 2 𝑟 2 ) ϕ 𝑒 = 3π 2 + arctg √1-𝐾 2 𝐾•sinξ . (4)
In these equations coefficient 𝐾 = 1 -

𝑎 𝑝 𝑟 .
Thus, contact patch is variable for every area of machining surface with dissimilar value of lead angle ξ.

Cutting force components

There is an essential difference between total cutting force 𝑓 𝑐 and its components for different machining conditions of warped surface ball-end milling. When lead angle is changing, values redistribution in a pair of radial and axial components occurs, even if its absolute value is the same for a constant depth of cut 𝑎 𝑝 . Normally radial component 𝐹 𝑟 increases and axial component 𝐹 𝑎 decreases during lead angle increasing, as it is shown on a Fig. 3. The effective average radius of the tool 𝑟 𝑒𝑓,𝑎𝑣 is difference for the same ξ (Fig. 4). (5)

In Eq. ( 5) ψ ℎ𝑙 is a helix angle of cutter.

Dynamic model

Schematically the single-degree-of-freedom (SDOF) system during ball-end milling is shown on a Fig. 5. The equation of oscillation of SDOF-system in a cut direction 𝑦 is:

{ 𝑦̈(𝑡) + 2ζ 𝑠 ω 𝑛 𝑦̇(𝑡) + 𝑑(𝑦̇) + ω 𝑛 2 𝑦(𝑡) = 𝐹 𝑦 (𝑡) 𝑚 , 𝑤ℎ𝑒𝑛 𝑓𝑜𝑟𝑐𝑒 𝑖𝑠 𝑎𝑐𝑡𝑖𝑛𝑔 𝑦̈(𝑡) + 2ζ 𝑠 ω 𝑛 𝑦̇(𝑡) + ω 𝑛 2 𝑦(𝑡) = 0, 𝑤ℎ𝑒𝑛 𝑓𝑜𝑟𝑐𝑒 𝑖𝑠𝑛 ′ 𝑡 𝑎𝑐𝑡𝑖𝑛𝑔 (𝑓𝑟𝑒𝑒) ; (6)
where 𝐹 𝑦 (𝑡)cutting force in the 𝑦 direction; 𝑚modal mass; ω 𝑛natural modal radial frequency; ζ 𝑠natural modal structural damping ratio; 𝑦(𝑡)instantaneous perturbation;

𝑑(𝑦̇)general nonlinear damping. Modal stiffness 𝑘 𝑧 along the axis 𝑧 may be neglected, so

𝑘 ≈ 𝑘 𝑦 .
The equation of motion of SDOF-system Eq. ( 6) has two different representations when cutting and when non-cutting. When tooth slices into uncut chip, the total cutting force 𝑓 𝑐 starts acting.

Its nature is quite complex, so total cutting force in fact has linear and nonlinear components.

Usually nonlinear component may be neglected because of its irrelevant effect on the When tooth disengages workpiece, system goes to the state of free damped oscillations. These oscillations are generated with disengaging (an impact). They have only linear (structural) damping component.

In accordance with the classical approach of stability analysis in machining the system loses its stability at frequency values close to the natural frequency of the system. However, in the case of low immersion milling, which is typical for ball-end machining, stability loss may take place at lower frequencies. It is a case of period-doubling bifurcation, when system oscillates with tooth pass frequency: 𝑓 𝑇.𝑃. ;

1 2 𝑓 𝑇.𝑃. , …also called flip bifurcation.

EXPERIMENTAL SETUP

For understanding of mechanics of chatter genesis in the warped surface ball-end milling with small radial immersion it has been performed a number of tests with variable depth of cut, lead 6) and analyzed on a PC (7) using MATLAB software. The sample rate was 10 kHz.

Fig. 6 The experimental setup for low-frequency chatter examining

To maintain the experimental integrity on each inclined surface the valleys of necessary depth were machined before. It made possible to provide machining with a pure up-milling and downmilling.

It was found that the dominant mode of the SDOF-system has natural frequency 𝑓 𝑛 = 1542 𝐻𝑧 (period of oscillation equals ~6.5•10 -4 s). Dynamic parameters of the SDOF-system are presented in a Table 1. Depth of cut (radial allowance) was in a range from 0.10 mm to 0.50 mm. Radial depth of cut was calculated for each test as 𝑎 𝑒 = 𝑟√1 -𝐾 2 . Feed-per-tooth value was constant for all tests 𝑓 𝑧 = 0.10 𝑚𝑚.

Surface roughness was measured with a non-contact optical measurement machine Alicona InfiniteFocus in a cutting direction.

ANALYSIS AND DISCUSSION

Stability lobes validation

Detailed analysis

Analysis of these tests showed that downward up-milling is much less stable than upward downmilling. Spectral analysis of unstable cutting showed that stability loss takes place with 11a), respective 6 decomposition levels (Fig. 11b), machined surface photos (Fig. 11c) and profile diagrams (Fig. 11d) show the areas of unstable cutting. It is noticeable, that low-frequency chatter has a very negative effect on machined surface.

Roughness measurement zones in [mm] (Fig. 11d) were obtained by feed-per-tooth (𝑓 𝑧 = 0.10 𝑚𝑚) and spindle speed rotation values and compared with waveforms in [s] (Fig. 11a). In this way it has been found out that 20 mm length of machined surface conforms to 0.9 s in the case of 𝑛 = 6636 𝑟𝑝𝑚 and it conforms to 0.8 s in the case of 𝑛 = 7584 𝑟𝑝𝑚. 12a). The length of surface area with a large roughness is 16 mm. In the case of machining with 𝑛 = 7584 𝑟𝑝𝑚 maximum roughness deviation 𝑅 𝑚𝑎𝑥2 = 17.5 𝜇𝑚 (Fig. 12b). The length of surface area with a large roughness is 12 mm. Although 𝐹 𝑟 ÷ 𝐹 𝑎 ratio during downward milling is smaller than during upward milling, total cutting force may be greater in 2 times or more when downward milling because of substantially smaller effective cutting velocity. Because of it effective radial cutting force component is bigger during downward milling with the same lead angle value.

During downward up-milling tooth slices into uncut chip starting from zero chip thickness. Thus, contact stress between tooth cutting edge and uncut chip leads to deflection of the most non-rigid element of technological system without real shearing of uncut chip. While mill rotates uncut chip thickness increases. Therefore, shearing may occur suddenly. It leads to the leap of cutting force magnitude and its direction. Cutting tooth may even disengage workpiece before real cutting starts, especially if depth of cut is very small. Probably, genesis of low-frequency chatter has such nature.

While machining is in progress, oscillation energy is accumulated in the system, so the system goes to the steady state of parametric oscillation with a frequency value close to the natural frequency of the system.

Further research intends ascertain genesis of low-frequency chatter and its transition to the natural frequency vibrations (steady state).

CONCLUSIONS

In this paper the genesis of low-frequency chatter during warped surfaces machining with ballend mills was investigated. More than two hundred cutting tests of inclined surface with different inclination angle, depth of cut and spindle speed rotation values were carried out. These tests showed, that low-frequency chatter has a very negative effect on machined surface, which gets worse, when angle of inclination growth in a range from 22.5 ° to 52.5 °. However, this effect is not so strong during down-milling in contrast with up-milling. It also was experimentally proved, that machining with downward direction is less stable than upward machining during the same depth of cut values.

Two cases of machining with angle value of 52.5 ° and depth of cut 0.50 mm were analyzed. In the first case spindle speed was 6636 rpm and in the second case it was 7584 rpm. Lowfrequency chatter took place in both cases. Roughness measurements showed that when spindle speed increased maximum roughness deviation Rmax decreased in 2.5 timesfrom 42.8 μm (n = 6636 rpm) to 17.5 μm (n = 7584 rpm). The length of machined surface area with a large roughness became shorterfrom 16 mm in the first case to 12 mm in the second case.

Nevertheless, low-frequency chatter during downward up-milling does not take place during machining with n = 6636 rpm and ap = 0.50 mm, when lead angle equals 37.5° and less due to decreasing of radial cutting force component.

Clear low-frequency chatter analysis demanded vibration signal decomposition, that has been done by wavelet transform (mother wavelet was Daubechies wavelet db4). It made possible to analyze initial signals without high-frequency components, which have large magnitudes but immaterial effect on machining. Poincaré map of decomposed signals (6 th decomposition level) showed that just period-doubling of tooth pass frequency oscillation has a huge effect on machined surface.

The supposition, that the development of vibrations with low frequencies while magnitude is small is caused by sudden nature of shearing process have been made. This in turn leads to the leap of magnitude of cutting force and to the changing of its direction.

NOMENCLATURE

LIST OF TABLES

Table 1 Dynamic parameters of the SDOF-system

Fig. 1

 1 Fig. 1 with flat-end mill (a) and ball-end mill (b), respectively.

Fig. 3

 3 Fig. 3 Radial and axial cutting force components changing during lead angle increasing

Fig. 4

 4 Fig. 4 Radial and axial cutting force components changing depending on machining direction

Fig. 5

 5 Fig. 5 Single-degree-of-freedom system during ball-end milling: a) oscillation model; b) self-interruption area

  angle and spindle speed values. The experiments were carried out for upward down-milling (approximate the most preferable case) and for downward up-milling on CNC milling machine Victor VCenter-85. The experimental setup is shown on a Fig. 6. It includes solid carbide ball-end mill Ø8 mm (1) with two flutes, flexible in the 𝑦 direction base plate (2) with the workpiece (3) on its top made of AISI 1045 Steel with inclined surface (4). On the back side of this plate the vibration sensor DN-4-M1 (piezoelectric accelerometer with a range up to 12.4 kHz) is attached (5). Its axis is parallel to the Y-axis of CNC milling machine. Signal from the sensor is converted with AD/DC-converter L-Card E14-440 (

  performed with spindle speed values in a range from 5852 rpm to 8008 rpm. It made possible to maintain effective cutting velocity in a wide rangefrom 50 m/min (when ξ = 22.5 °) to 160 m/min (when ξ = 52.5 °).

Fig. 7

 7 Fig. 7 Predicted stability lobes for downward up-milling with ξ = 52.5 °

Fig. 8 Fig. 9 Fig. 10

 8910 Fig. 8 Experimental acceleration for downward up-milling with lead angle of 52.5° and depth of cut 0.50 mm for two spindle speeds (6636 rpm and 7584 rpm): a) initial signal; b) 5 decomposition level; c) 6 decomposition level

Fig. 11

 11 Fig. 11 Roughness of machined surface for downward up-milling with lead angle of 52.5° and depth of cut 0.50 mm for two spindle speeds (6636 rpm and 7584 rpm): a) initial signal; b) 6 decomposition level; c) machined surface photo; d) machined surface roughness

Fig. 12

 12 Fig. 12 Roughness of machined surface for downward up-milling with lead angle of 52.5°, depth of cut 0.50 mm and spindle speed: a) 6636 rpm; b) 7584 rpm

Fig. 2

 2 Fig. 2 Downward up-milling of inclined surface with ball-end mill

Table 1

 1 Dynamic parameters of the SDOF-system

	Parameter	Value
	Natural frequency 𝑓 𝑛	1542 Hz
	Modal stiffness 𝑘 𝑦	12.1 •10 6 N/m
	Modal damping ζ 𝑠	2.75 %
	A number of tests were carried out with five values of lead angle: ξ =
	[22.5 °; 30 °; 37.5 °; 45 °; 52.5 °]. Tests with lead angle values less than 22.5

°

were not taken into account because of too small effective cutting velocity values.

  Hz𝐹 𝑡 , 𝐹 𝑟 , 𝐹 𝑎 tangential, radial and axial cutting force components, N

	ζ 𝑠		natural modal structural damping ratio, dimensionless
	ξ		lead angle, ˚
	ϕ 𝑠		start angle, ˚
	ϕ 𝑒		exit angle, ˚
	ψ ℎ𝑙		helix angle of the cutter, ˚
	ω 𝑛 = 2π𝑓 𝑛 natural modal radial frequency, s -1
	𝑎 𝑒		radial depth of cut, mm
	𝑎 𝑝		depth of cut, mm
	𝑓 𝑐		total cutting force, N
	𝑓 𝑛		natural modal frequency, Hz
	𝑓 𝑇.𝑃. tooth pass frequency, 𝐾 𝑡 tangential cutting force component, N/mm 2
	𝑘 𝑟		radial cutting force component coefficient, dimensionless
	𝐾 = 1 -	𝑎 𝑝 𝑟	ball end mill indentation ratio (it characterizes the relation between depth of cut and ball end mill radius values), dimensionless
	𝑘		modal stiffness, N/m
	𝑚		modal mass, kg
	𝑛		spindle speed, min -1
	𝑟		ball end mill radius, mm
	𝑟 𝑒𝑓		effective radius of the cutting tool, mm
	𝑉 𝑐,𝑒𝑓		effective cutting velocity, m/min

ACKNOWLEDGEMENTS

Authors wish to gratefully acknowledge METALLICADOUR -ASSAT (Tarbes, France) for the help in surface roughness measurements.

LIST OF FIGURES CAPTIONS