Mechanical , Thermal and Crystallization Properties of Polypropylene ( PP ) Reinforced Composites with High Density Polyethylene ( HDPE ) as Matrix

Our work aims to evaluate a complete outlook of virgin high density polyethylene (HDPE) and polypropylene (PP) polyblends. Virgin PP of 20, 30 and 50 weight% is compounded with virgin HDPE. The properties like tensile strength, flexural strength, Izod impact strength are examined. Scanning electron microscopy (SEM) and polarised light microscopy (PLM) are used to observe the surface and crystal morphology. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) tests verify the non compatibility of both polymers. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques are used to study the thermal behaviour of composites. The results manifest co-occurring spherulites for polyblends; indicating the composite to be a physical blend of continuous and dispersed phases, but on the other hand PP improves the tensile and flexural properties of HDPE.


Introduction
Polymer composite is material of research in modern days.Thermoplastic polymers are of great interest due to their technical and commercial importance [1].
In general two or more polymers are melt blended to form a product as polyb-  [5].The component percentages are the primary factor influencing their physical properties [6].The manufacturing technique and operating conditions are second governing factor.
Among the thermoplastic polymers, PP possesses good mechanical strength.
In addition it has high chemical resistance, low cost and easy to manufacture.PP has wide application in automobile spare parts and as well as container [7].
HDPE is known for its large strength to density ratio due to its little branching.
Jia-Horny Lin et al. has reinforced HDPE to PP matrix and verified the noncompatibility of both polymers, but improves the impact strength of PP [14].
Souza et al found the effect of processing temperature and content of HDPE on interfacial tension of the PP/HDPE polyblend [15].Past studies show the compatibility of PP/HDPE polyblends depends on factors like processing temperature, polymer structure and blending ratios [15] [16] [17].Polymers with similar physical properties form polyblends with greater mechanical strength [18] [19] [20].The mechanical properties of the PP/HDPE polyblend decreases with increase in dissimilarity of melt flow index (MFI) [21], so we have investigated a complete prospects of PP reinforced HDPE polyblends with similar MFI manufactured by the help of twin screw extruder and injection moulding machines.In addition to mechanical properties; thermal behaviour of the composites are characterised by using DSC, TGA tests.Crystal morphologies are captured using PLM, SEM and X-RD techniques.Compatibility of both the thermoplastics are re-examined by study of molecular structure using FTIR.

Preparation of Composites
Polymers in the form of pellets are collected.The pellets are dried in a hot air oven at 60˚C for 8 hrs to remove moisture content followed by mixing of  1) and ( 2) respectively.
2 Speed 6 where, Z is Rate of straining at 0.01 mm/mm/min, L is span length (mm) and d is sample thickness (mm), max F σ is flexural strength (MPa), P is load (N), L is span length (mm) and b is sample width (mm).
Impact tests are conducted using a Izod and Charpy impactometer (IT 504 Plastic impact , Tinius Olsen ,USA ) with a V-notch cutter as per ASTM D256-A standard , possessing a pendulum energy of 13.70 J. Impact test specimens are prepared by cutting the flexural samples to a size of 63.5 mm × 12.7 mm × 3.2 mm with a V-notch of 45˚ and 0.25 mm depth.

Microscopy Test
Our investigation has used SEM (JEOL; JSM-6480 LV, Japan), Field emission SEM (Nova Nano SEM-450, USA) and PLM (Leica, DM750P, Germany).Morphology of samples is captured before and after fracture of impact test.Energy dispersive spectroscopy (EDS) analysis and carbon mapping test are conducted using FESEM at an operation voltage of 10 KV.Samples are gold coated before each test.PLM is used to observe the spherulite behaviour of the polyblends.A tiny sample is placed on a glass slide and melted at 200˚C (using the hot stage) followed by sandwiching the sample by placing a micro glass slide over it to Materials Sciences and Applications form a thin film.The sample is cooled at 5˚C/min (using cold stage) and spherulite morphologies are captured at 130˚C and 125˚C at magnification × 10.

XRD and FTIR
In order to analyse any new phase formations after blending the polymers and to understand the chemical structure of the polyblends; the XRD (Philips, PW1720, USA) and FTIR (Perkin-Emler Spectrum 100, USA) techniques are utilised.X-ray scanning is done within a diffraction angle (2θ) range of 10 -90˚ with Cu Kα radiation at 40 KV and 30 mA.The rate of scanning is 10˚/min and at λ = 0.154 nm.The IR Spectroscopy is observed between the waveband of 450 to 4000 cm −1 .

DSC and TGA Analyses
The polyblends thermal behaviour is analysed using a DSC (Perkin-Elmer DSC 7, MA, USA) and TGA (Perkin-Elmer TGA, MA, USA) analysers.The DSC tests are performed under nitrogen flow rate of 50 ml/min.Polymer samples of around 10 mg are scanned at a heating rate of 10˚C/min from ambient temperature to 200˚C.The samples undergo three thermal cycles.Heating, cooling and reheating under the same condition to follow an identical thermal history for all polymer blends.The degree of crystallinity (X C ) of the polyblends is evaluated by Equation (3) where,

Tensile, Flexural and Impact Strengths
Tensile strength results are shown in Figure 2 there by weakening the impact property.

Phase Analysis
The chemical and crystal structure of HDPE/PP polyblends are analysed by XRD and FTIR. Figure 5 and Figure 6 reports the XRD and FTIR results.For PP all the peaks lies between 2θ of 15 to 30˚, which are α form of PP.The peaks are corresponding to crystalline lattices [23].Two diffraction peaks for HDPE are observed between 20 to 30˚ diffraction angles, comprising of orthorhombic crystals [24] [25].Reinforcing PP to HDPE does not produce any new peaks, only shortening of peaks for HDPE/PP polyblends are seen.So combination of PP with HDPE is only a physical mixing with no alternation of chemical structure.Table 2 shows the frequency ranges of different functional groups of PP and HDPE polymers, with assigned vibration type.The FTIR spectra reveals, the peaks of HDPE/PP composites confirms to those of virgin HDPE and PP matrices.

Thermal Behaviour
From the DSC study the melting temperature (T m ) of PP and HDPE are 168.6 and 134.6˚C respectively.Table 3 reports the detailed results of melting temperature and melt enthalpy (      The superscript ab corresponds to cite HDPE and PP respectively. composite to be a physical mixture of both the polymers.The existence of PP in HDPE does not alter the melt peak temperature significantly.The weight loss of a polymer with respect to time or temperature is usually predicted by using TGA technique.The thermal degradation is an irreversible process.Our work focused to predict the degradation temperature (T D ).It is defined in our project as; the temperature at which the weight loss of the polymers just starts to fall immediately.Figure 8 shows TG/DTG thermogram sketches of our prepared polymers.The results obtained via TG analysis on polymers are revealed in Table 4.All the samples undergo a single degradation step.The inflection point (I P , at which the rate of weight change with temperature is maximum) and residual weight % are also reported in Table 4.It is evident from the results that, the thermal behaviour of the binary polyblends differ marginally; may be due to similar density and MFI.

Surface Behavior
The surface morphology of polymers before fracture is reported in Figure 9.A    absorbed falls with augmentation of PP particles (see Figure 4), resulting the cracks to prevent bloating due to exerted strain.
Crystal structures of the polyblends during solidification from molten stage are reported in Figure 12.Spherulites are large and spherical for PP conforming to the results reported by jia-Horng Lin etal [14], and so called ring spherulites for HDPE.PP forms an overlapped layer in the polyblend and hence an incomplete spherulitic growth is resulting for the polymer composites.Spherulites stalk over each and cannot reach to complete form.

Conclusion
Our project promisingly combines PP with HDPE.The dispersion of PP in    conventional methods are adopted for preparing the HDPE/PP blends with low manufacturing cost, the composite blends may find suitable application areas.
of HDPE or PP in the blend, thalpy corresponding to melting of 100% crystalline HDPE or PP and Ø = weight fraction of HDPE or PP in the blend.In TGA test, polymer samples with masses of approximately 10 mg are heated from atmospheric temperature to 600˚C, at heating rate of 10˚C/min and nitrogen flow rate of 50 ml/min, to observe their degradation behaviour.Data corresponding to (a).The maximum value (≈35 H. Sutar et al.DOI: 10.4236/msa.2018.95035506 Materials Sciences and Applications MPa) of tensile strength is resulted from PP where as the HDPE matrix bears a tensile strength of ≈22 MPa.Reinforcement of PP to HDPE improves the tensile strength due to formation of brittle polyblends as observable in Figure 2(b).The magnitude of tensile modulus at break point is reported in Figure 2(c).The polyblends of 50 HDPE/50PP shows the maximum (≈146 MPa) value of tensile modulus.The experimental outcomes for flexural tests are reported in Figure 3. Flexural strength improves (See Figure 3(a)) and a value of ≈23 MPa is observed for all the composite blends.Figure 3(b) reveals the PP added polyblends bear more extension properties when compare to HDPE.Data pertaining to the flexural modulus are reported in Figure 3(c); indicating the PP content increases the flexural modulus; as PP to be a separate phase in the polyblend and HDPE as continuous matrix.The impact strength of polymers are expressed in three different ways and reported in Figure 4.The results corresponding to impact strength in Joule (J) is reported in Figure 4(a); indicating a maximum value for HDPE where as attributing a minimum value to 50/50 polyblend.Energy absorbed during impact per unit thickness of sample is manifested in Figure 4(b).The impact energy absorbed per unit cross sectional area, perpendicular to load; also shows a similar trend as visible in Figure 4(c).Reinforcement of PP particles to HDPE matrices contracts the stress concentration and the plastic deformation property is lost; f H ∆ ) of all the polymer type.The HDPE/PP polyblend bears two melt points as shown in Figure 7(a); indicating the polyblend to be a co-occurrence of both HDPE and PP.The results authenticate the polymer H. Sutar et al.DOI: 10.4236/msa.2018.95035507 Materials Sciences and Applications

Figure 2 .
Figure 2. Tensile properties of the polymer composites, (a) tensile strength at yield; (b) load against extension; (c) tensile modulus at break.

Figure 7 (
Figure 7(b) shows the temperature (T c ) and enthalpy
Figure 11(b) shows the fractured surface for virgin PP is flat owing to brittle fracture.Fractured virgin HDPE specimen results a wrinkled and aggregative exterior.The ruptured surface of the polyblends is also irregular, owing to their toughness.The reinforcement of PP to HDPE smoothens the surface as visible in Figures 2(c)-(e).The impact energy HDPE improves tensile and flexural strengths.The results show that a 50 wt% PP increases the tensile strength of the composite by 29%, and is maximum among the polymer blends.The magnitude of the flexural strength for all the polyblends are close to 23 MPa and improved by 44%.The XRD, FTIR and DSC tests prove the polyblend to be a combination of two dispersed matrices.No changes in chemical structure are observed, confirming the composite to be a physical blending.PLM tests authenticate; reinforcement of PP particles to HDPE retards the crystal growth and spherulites lap over.TGA tests disclose the degradation characteristics; showing a maximum degradation temperature and weight loss for polymer blends is for composite with 50 wt% PP.Because of the Materials Sciences and Applications

Table 1 .
Physical properties of polymers.
(span = 16 times of thickness) at an extension up to 5%.The speed of the test and flexural strengths are calculated according to Equations (

Table 2 .
IR spectra analysis reports.

Table 4 .
TG/DTG results of prepared polymers.