Study and characterization of LDPE/Polyolefin elastomer and LDPE/EPDM blend: effect of chlorinated water on blend performance

Abstract The free chlorine present in water which is used as a disinfectant is reported to reduce the life of the polymeric material. The objective of this work is to study the influence of chlorine concentration on low-density polyethylene (LDPE) and blends of LDPE with ethylene butene copolymer (EBC) and ethylene propylene diene terpolymer (EPDM). The LDPE blend with EBC and EPDM were tested with water containing 50, 500, and 5000 ppm chlorine under static condition for 500 h at 25 and 80 °C. It has been seen that at 5000 ppm chlorine concentration, the mechanical properties of LDPE, LDPE/EBC blend, and LDPE/EPDM blend changed drastically and a significant reduction in the elongation at break was found for LDPE, LDPE/EBC, and LDPE/EPDM blend. LDPE/EPDM shows stable modulus value for 5000 ppm as 80 °C. Chemical changes in the aged sample were studied by Fourier transform infrared spectroscopy (FTIR) where an increase in the O–H and C=O peaks were observed. The thermal characteristics of LDPE, LDPE/EBC blend, and LDPE/EPDM blends were investigated using DSC and TGA which shows that the melting temperature and crystalline melt temperature remains unchanged while percent crystallinity increases slightly. Scanning electron microscope showed that there was the formation of microcracks and cavities on the fracture surface of LDPE, LDPE/EBC blend, and LDPE/EPDM blend after exposure to a higher concentration of chlorine indicative of degradation. Furthermore, the Chlorine resistance of LDPE/EPDM blend at 5000 ppm chlorine concentration is much higher than that of pristine LDPE.


Introduction
Traditionally, metallic pipes have been used in hot and cold water distribution systems. These materials were found to corrode faster due to contact with chlorinated water where chlorine is used as a disinfectant, which is a strong oxidizer and can increase the rate of corrosion. 1,2 Originally, free chlorine was the predominant disinfectant used because it was "effective, economical, and easily available. " 3 Now a days, polymeric material has replaced traditional material, mostly because of its low cost, excellent chemical resistance, high strength with low weight, easy processing etc.
Chlorine resistance is the strength of a material to protect against chlorine attack or reaction. A material with low chlorine resistance generally results in a mass loss, increase in surface hardness and loss of mechanical strength. When testing a material for chlorine resistance, important factors include temperature, concentration, exposure duration, and mechanical stress. It has been reported that many polymers exposed to chlorinated water degrade at rates that can adversely influence the properties and, hence, the function of the polymer. 4,5 Thus, polymer-based materials used in the handling of chlorinated water (e.g. purification membranes, pipes, and pump parts) often require frequent replacement due to degenerative effects initiated by exposure to chlorinated water. The past several studies have reported the effect of chlorinated water on polymers, mostly polyethylene. By exposing the polyamide Nylon 66 to chlorinated water, Cl 2 was found to be the key reactive intermediate responsible for polyamide degradation. 6 In addition to chlorine concentration, temperature and pH are important parameters but have not been adequately defined. Additionally, a wide variety of materials are available to the water industry, ranging from nitriles (NBR), neoprenes, EPDMs, Polytetrafluoroethylene (PTFE), Polysulphone (PSU), Polypropylene (PP), and Polyvinyl chloride (PVC). Each material undergoes some unique reaction when exposed to free chlorine or chloramines. Some have been observed to resist degradation, while others degrade rapidly. 7,8 Additional results are available from Solvay technical bulletin that compare the performance of Polysulfone polymers and polyacetals in chlorinated environments. 9 The following materials have been evaluated: Polyvinylidene fluoride (PVDF) (Solef® 1010), Ethylene ChloroTriFluoroEthylene (ECTFE) (Halar® 350 LC), Polyphenylsulfone (PPSU) (Radel® R-5000), Polysulfone (PSU) (Udel® P-1700), Silane Crosslinked Polyethylene (PEX-b) (Polidan® T-A/SL), and Polyamide (PA12). 10,11 Many antioxidants (AO) were also reported that increased the chlorine stability of Polyethylene. The effect of chlorinated water on high-density polyethylene pipes stabilized with a combination of phenolic and phosphite antioxidants like phenol and phosphite show that chlorinated water rapidly degraded the antioxidants. 12,13 The loss of antioxidant activity could be the result of either extraction into the inner medium or chemical consumption by reaction with various active species formed in the aging medium and is found to be a prevalent step during aging. Subsequently, the polymer undergoes chain scission which then leads to a loss in mechanical strength and ultimately a reduction in the life of pipes as can be seen for PE pipes exposed to chlorinated water.
Hassinen et al. 12 have observed that antioxidant is consumed rapidly even far into the pipe wall, due to diffusion of chlorine species into the solid plastic material. Colin et al. 14 showed a rapid consumption of a phenolic antioxidant in a 1.2 mm thick layer near the inner pipe wall, after which there was no propagation of the antioxidant. These authors suggested that chlorine dioxide reacts not only with the antioxidant but also with polyethylene, at a considerable lower rate. However, Stevens and Seeger 15 provided evidence that the reactivity of saturated hydrocarbons to chlorine dioxide is strictly zero. Dear and Mason 16 found that chlorine can penetrate polyethylene in amorphous regions without reacting with the polymer chains. In 1957, Russell 17 reported that autoxidation of aryl hydrocarbons and peroxy radicals was a cause of the failure of polymer materials. Hence, the deterioration of polyethylene may not be due to the disinfectant but to the by-products (radicals) of chlorine or chlorine dioxide dissolving in water.
Polymer blends play a significant role in preparing high-performance materials. Properties of blends are mainly determined by its structure. The study on binary polymer blends containing crystalline polymer has received more attention from both scientific and technological viewpoints. For blends, the miscibility, morphology, and crystallization behavior have been widely investigated. [18][19][20] Blending of two or more different polymers makes it possible to achieve various property combinations of the resulting material-mostly in a more cost-effective way than by synthesis of new polymers. The blend offers the wide range of properties like excellent chemical resistant, good weatheralibity, high thermal resistant, good electrical insulation properties.
The Polyolefin Elastomer (POE)-ENGAGE is a premium impact modifier for a wide variety of plastics and Thermoplastic elastomer (TPO) applications. Blends using ENGAGE resins exhibit significant improvements in impact strength and a better balance of properties with addition levels ranging from 5 to 30%. 21,22 Several new EPDM-based products were reported that have been formulated specifically for chloramine resistance in aggressive water utility applications. EPDM rubber is ideal for outdoor applications because of its excellent resistance to ozone, oxidants, and severe weather conditions. Within the water industry, EPDM is widely used for O-rings, valve seats, flat gaskets, and pond liners. More recently, peroxide-cured EPDM is being used where chloramines resistance is important. 8 All over the world a lot of work has already focused on the performance of LDPE, HDPE, MDPE, Nylon, Poly (sulfone) and crosslinked-PE in a chlorinated environment. 23 There is no open literature available on the performance of a blend of LDPE with an elastomeric material in chlorinated water. Recently, polyethylene molecular structure has been modified by the addition of co-monomer (octene) in order to enhance intrinsic thermomechanical properties without material cross-linking. 24 In the present work, the effect of concentration of chlorine at a different temperature on the properties of polymer (e.g. elasticity, weight change, hardness and tensile strength, crystallinity and surface morphology) of LDPE and its blends are studied.
pre-drying at 80 °C for 8 -10 h. Injection molding (Boolani machinery India Ltd, Mumbai, India) was done maintaining temperature profile of 190, 200, and 210 °C from the hopper to the injection nozzle, respectively. Standard ASTM-based samples for tensile testing (Type IV) were obtained from injection molding. Formulation of LDPE/EBC and LDPE/EPDM has been given below. Batches having optimum properties were selected for further study. The initial properties have been provided in Table 1. In the sample code "LDPE" stands for Low-density polyethylene, EBC stands for ethylene butene copolymer (Engage), and "EPDM" stands for Ethylene propylene diene terpolymer. The summary Testing method and characterization performed are shown in Figure 1.

Accelerated aging methods
Aging solutions were prepared according to ASTM D6284a with reagent water to form a 4% sodium hypochlorite. 25 Water pH was adjusted using NaOH and HCl. The first method (referred to as the 50 ppm method) involved adding 4% NaOCl directly to reagent water to achieve 50 ppm as Cl 2 free available chlorine concentration. The Same procedure was followed for the preparation 500 and 5000 ppm chlorinated solution. 26 Several die-cut dumbell shaped LDPE, LDPE/EBC, and LDPE/ EPDM samples were placed in separate 1 L glass bottles with polypropylene caps. Chlorine solution was added and sealed bottles were stored vertically at 25 and 80 °C oven in the dark. On Day 21 (500 h), the samples were removed from the chlorinated solution and placed in reagent water and stored in the dark at room temperature. After 24 h of soaking in reagent water, all remaining samples were removed and dried in an oven at 60 °C for 24 h. Tensile properties after aging were measured according to ASTM D638. Physical properties were also measured after a specific interval of time e.g. Hardness, melting point, percent crystallinity, surface structure, and percent mass change. Test specimens were routinely inspected for any visual change.

Characterization techniques Percent mass change
Aged sample was dried in an oven for 24 h at 80 °C before weight measurement. Mass variation was calculated by the equation , where M f is the final mass and M i is the initial mass.

Mechanical tests
Dumbell-shaped samples were subjected to a tensile test to determine tensile strength, % elongation as well as Young modulus using LLOYD UTM with 50 kN load. Cross head speed was set to 50 mm/min. Sample dimension was 50 mm × 5 mm × 2 mm. Numbers were derived from three replicas of the same samples producing close results.

Hardness
Polymer hardness was measured with a shore D hardness test according to ASTM D2240 using a Zwick 7206 Hardness Tester. Three measurements were taken for each sample and then averaged to ensure a uniform hardness value representative of the sample as a whole.

Differential scanning calorimetry (DSC)
To analyze the melting and crystallization behaviors of neat polymers and their blends, differential scanning calorimetry (DSC; TA Q100, USA) was used. The samples were heated from room temperature to 200 °C at 10 °C min −1 and kept for 2 min at this temperature to erase thermal history. Then to study the crystallization process the blends were cooled to 0 °C at a controlled rate of 10 °C min −1 . After holding the samples at 0 °C for 2 min, the melting tests were run at a heating rate of 10 °C min −1 from 0 to 200 °C. The second melting curve was recorded and used to calculate the crystallinity. The weight of each sample was 5 mg. Percent crystallinity was calculated as where ΔH m is the heat of fusion of sample and ΔH m100% is the heat of fusion of 100% crystalline PE taken as 293 J/g. 27

Thermogravimetric analysis
Thermogravimetric analysis (TGA) was performed by using Perkin Elmer Pyris 1 at the heating rate of 20 °C/min from ambient temperature to 700 °C in the air atmosphere. In this technique, the mass of the substance and thermal decomposition of polymer blend were measured as a function of temperature.

Figure 1 Summary of testing sample preparation with an accelerated aging test to characterization details
At 25 °C weight gain in the sample was lower as compared to 80 °C. At 80 °C, the weight gain was more and this phenomenon matched with data reported by Marshall J. As the concentration of chlorine was increased from 50 to 5000 ppm, the weight gain also increased due to the oxidation of the material. Weight Change within ±0.6% was statistically significant and in an acceptable range. 9 Test results, which are summarized in Figure 2, confirmed that the LDPE, LDPE/EBC, and LDPE/ EPDM have very good resistance to cold and hot chlorinated water. LDPE/EPDM showed a slight but statistically significant weight gain at 5000 ppm chlorine concentration due to sorption of water by EPDM and oxidation at a higher temperature. 28

Mechanical properties
Effect on tensile strength after exposure to chlorine A summary of the tensile strength test results was shown in Figure 3(a) and (b) for 50, 500, and 5000 ppm chlorine concentration exposure at two different temperature conditions. As shown in Figure 2, the mass gain has been observed for exposure to 5000 ppm because of water absorption and this can change the macroscopic mechanical behavior of the blend. 29 When water is absorbed, there is increased mobility of the polymer chains, resulting in lower strength, modulus, and percent elongation. Tensile strength reduces from 16 to 12 MPa for unexposed and exposed specimen of LDPE and from 14 to 7 MPa for unexposed and exposed specimen of LDPE/EBC. While LDPE/EPDM show lower rate of reduction in tensile strength ranges from 14 to 12 MPa for exposed specimen (Figure 3(a) and (b). At higher temperature (80 °C) reduction in tensile strength is greater compared to normal temperature (25 °C). The tensile strength of the native LDPE is 16 MPa. After only 500 h of exposure at 80 °C, the strength is reduced near to half, approximately 9 MPa. Approximately 50% of the tensile strength reduction is also found in the case of LDPE/EBC and LDPE/EPDM at harshest condition (5000 ppm at 80 °C).
Effect on percent elongation after exposed to chlorine Percent elongation at break is shown in Figure 4(a) and (b) for unexposed specimens and specimens exposed to different (2 mm thick) were recorded on a Bruker-Alpha's Platinum Attenuated total reflection (ATR) model. The sample was characterized using ATR attachment within 500-4000 cm −1 .

Surface morphology by scanning electron microscopy
The samples of LDPE, LDPE/EBC, and LDPE/EPDM blends fractured in liquid nitrogen were characterized by scanning electron microscopy (SEM) using FEI Quanta 200 ESEM model. Both unexposed and exposed to chlorine sample were analysed.  Effect on modulus after exposure to chlorine Young's Modulus data for unexposed and exposed LDPE, LDPE/EBC and LDPE/EPDM blends samples are shown in Figure 5(a) and (b). The plot shows a linear decrease in modulus with respect to the concentration of chlorine for both unexposed and exposed samples.

Percent mass changes
As shown in Figure 5(a) and (b), an LDPE is started with a modulus of 600 MPa, but over time this will predictably concentration and temperature conditions. There was, however, great variability in elongation at break results and a clear brittle failure mode for 5000 ppm as 80 °C LDPE and blends specimens in comparison to ductile fracture for unexposed samples of LDPE and its blends. The reduction in percent elongation was caused by chain scission and chlorinated water induced oxidation. As with the percent elongation data, there is significant reduction found for LDPE ranges from 150 % to 50% for unexposed and exposed specimens. Also in the case of LDPE/EBC and LDPE/EPDM blend, the percent elongation value reduce approximately to 50% at harshest conditions. At 50 ppm concentration of chlorine and temperature of 25 °C,

Thermal properties
The melting temperature, crystallization temperature, and percent crystallinity data of LDPE, LDPE/EBC, and LDPE/EPDM are shown in Table 4. Exposure for 5000 ppm at 80 °C in LDPE and LDPE/EPDM had a major influence on the crystallinity profile. There was no influence of chlorine on melting peak temperature of LDPE, LDPE/EBC, and LDPE/EPDM blends. Oxidation of polyolefins in the solid state led to an increase in crystallinity. 16,[30][31][32] DSC is typically used as a quick screening tool to check any structural changes in the material when heating up the sample. For semi-crystalline materials, this is used to measure the amount of energy needed to crystallize. No changes are seen for the unexposed material whereas a slight shift in crystallization temperature is seen after aging. This is an indication that the crystalline structure of the material has some what changed.
The shape of the melting peak has changed and an enlargement is observed essentially at a lower temperature after exposure to chlorine for a long time. The observed broadening of the melting peak on the 5000 ppm-aged sample at 80 °C is probably due to differences in the crystallite sizes. It is also interesting to note that the melting peak temperature was lower by (3 °C) after aging (500 and 5000 ppm aged samples). Hassinen et al. reported that the melting peak temperature of degraded HDPE was 3 °C lower than the un-degraded HDPE (after 438 h of aging at 3 ppm and 105 °C). These results suggest that the 5000 ppm aged sample had degraded. 12 An increase in mass crystallinity had already been reported as an indicator of a polyethylene (HDPE, LLDPE, and PEX) degraded layer. 12,[33][34][35][36] Literature agrees that the crystalline component is preserved during aging, and therefore only the amorphous component is degraded leading to an increase in crystallinity. 12,37 degrade to a modulus of 350 MPa. Similarly, for the case of LDPE/EBC and LDPE/EPDM modulus data ranges from 350 to 200 MPa. Surprisingly, the difference for modulus value was insignificant between 25 and 80 °C for LDPE/EPDM blend ( Figure 5(b)). LDPE/EPDM blend shows constant modulus value ranges from 300 to 400 MPa at a temperature of 80 °C for exposed specimens. Tables 2 and 3 compare the surface hardness degradation values for the blend evaluated in this study when exposed to a varying concentration of chlorine solution. In applications where long-term elastomer surface hardness is critical, it is important to select a material that will have limited loss of hardness over time. An application of this in the water utility industry would be in cases considering wear resistance as an essential part. However, in water utility applications where hardness is not a critical factor, material that exhibits a loss of hardness with time may be suitable, provided they meet other critical performance requirements.

Hardness
As observed from Tables 2 and 3, the hardness value in case of pure LDPE increases after exposure to a higher Table 3 Hardness value before and after exposure to chlorine at a temperature of 80 °C Sr. no Sample name Before 0 ppm 50 ppm 500 ppm 5000 ppm  1  LDPE  57  61  60  62  64  2  LDPE/EBC 70/30  61  63  62  63  63  3  LDPE/EPDM 70/30  68  69  55 59 59   Before  403  458  486  446  483  502  466  491  507  5000 ppm 80 °C  457  490  516  459  489  507  464 491 508 at 446 °C and after exposure at 459 °C. Also, in the case of LDPE/EPDM before exposure degradation took places at 466 °C and after exposure at 464 °C. Pure LDPE polymer decomposes more rapid compared to LDPE/EBC and LDPE/EPDM blend with increasing temperature in both cases before and after exposure to chlorine. This showed that there is no effect of chlorine on the thermal properties of blends.

Fourier Transform Infrared Spectroscopy (FTIR)
Changes in the chemical structure of the polymer can be studied by infrared spectroscopy. Polymer oxidation is shown by

Thermogravimetric analysis
To examine the thermal stability of above-mentioned polymers and blend, TGA ( Figure 6) data under nitrogen flow were obtained, and the results have been summarized in Table 5. The LDPE, LDPE/EBC, and LDPE/EPDM blend polymer showed a single step of decomposition. If initial decomposition temperature at which 10% weight loss were considered were assumed to be a measurement of thermal stability, LDPE, before exposure, decomposes at 403 °C, while, after exposed, decomposes at 457 °C. However, in the case of a blend of LDPE/EBC before exposure degradation took places  corresponding to hydroxyl was found significant for aging at 500 and 5000 ppm and is shown in Figure 7(a-f ).
The C=O bond was also analyzed (Figure 8(a-f ).). The C=O bond has also been reported as an LDPE oxidation indicator. For the C=O bond, the same consideration as for the OH bond can be taken into account. the appearance of hydroxyl (~3400 cm −1 ) shown in Figure 7 and carbonyl (~1700 cm −1 ) peaks shown in Figure 8. 29,32,38,39 In particular, Colin et al., 40 for the pipe sample exposed to chlorine dioxide containing water show an increase in hydroxyl absorption as well as an increase in carbonyl absorption which is in agreement with our data. The growth of peak changing by over 50 percent, and tensile strength and modulus dropping by approximately 10 percent. For example, LDPE/EBC and LDPE/EPDM at high concentration experienced relatively small changes in performance at a temperature of 25 °C. From this it can be concluded that the blends of LDPE/ EBC and LDPE/EPDM had excellent resistance to hot and cold chlorinated water. Most materials followed anticipated trends of increasing degradation with increasing temperature across all concentration levels, some anomalies were observed. However, exposure to chlorinated water at higher temperature and concentration of chlorine can result in formation of cracks observed from SEM images and also the loss of material from a LDPE, LDPE/EBC and LDPE/EPDM surface were found. For the LDPE/EPDM material, a constant modulus was measured after the 500 h exposure in high concentration at 80 °C. In addition to the reduction in elongation, hardness increased (2-9%) as concentration increased. FT-IR data indicate that upon exposure to higher temperature and chlorine concentration, carbonyl-and hydroxyl-forming reaction products are formed. FT-IR analysis revealed that all transmittance indices showed the same general trend of increasing the chlorine concentration. The FT-IR and mechanical data for LDPE/EBC and LDPE/ EPDM blend are in agreement, revealing the effectiveness of the both EBC and EPDM. TGA and DSC showed that there is no effect of chlorine on the thermal properties of blends.
FT-IR has been used to evaluate potential chemical organic changes in a material. The right section of the graph is used as a fingerprint area from a material and can be used to identify a material. There are some major changes visible for LDPE, LDPE/ EBC, and LDPE/EPDM at higher concentration and temperature around 1,700 cm-1, which is an indication of oxidation. 41 The correlation shows it to be a carbonyl absorption peak, which should not be present at all. It may have been formed by oxidation, which may have occurred by several possible mechanisms, but especially by exposure to chlorine-induced oxidation and this has been in agreement with data reported by K. Karlsson et al. in 1993. 42 Morphology SEM was used to evaluate the extent of degradation in the sample exposed to chlorine. SEM offers a hyper-magnification of the surface and can allow observation of the surface imperfections, such as micro-cracks. 43 A superficial light layer is formed on the surfaces of the sample diped in chorine solution. SEM examination of nitrogen fractured surface of unexposed and exposed samples showed that the surface roughness increases due to chlorine pitting as observed from Figure 9(a-f ). Figure 9(a) showed micrographs of LDPE before aging, in which no crack and smooth surfaced was observed. While the Plenty of cracks on the rough fractured surface were prominent in the case LDPE after exposure as shown in Figure  9(b). It also exhibited discontinuous path with cavities and some flow lines observed in LPDE after exposed to 5000 ppm at 80 °C. Figure 9(c) showed micrographs of LDPE/EBC 70/30 blend, in which surface became relatively smoother and the number of cavities was less compared to LDPE samples before aging due to the compatibility of blend and softness in some part of EBC (Figure 9(a)). LDPE/EBC blend exposed to chlorine concentration of 5000 ppm and temperature of 80°C showed increases in number of cracks with an increase in surface roughness which indicate that at higher concentration of chlorine starts oxidative degradation of LDPE/EBC blend. In the case of LDPE/EPDM, micrographs (Figure 9(e)) show same morphology as in the case of LDPE and LDPE/EBC before aging. Figure 9(f ) represents the fractured surface of LDPE/EPDM sample exposed at 5000 ppm at 80 °C. After aging LDPE/EPDM blend of 5000 ppm at 80 °C, showing that the size of the cavity was increased with surface roughness.

Conclusion
This paper includes weight loss, surface hardness, mechanical properties, FT-IR, DSC, and surface morphology data for LDPE, LDPE/EBC (70/30), and LDPE/EPDM (70/30) that immersed in hot chlorinated water. For exposure time up to 500 h in hot chlorinated water, LDPE/EBC and LDPE/EPDM showed drastic degradation in mechanical properties, as well as an increase in the surface roughness, was observed from SEM. The results show that at the highest concentration (5000 ppm), large reductions take place in performance due to temperature and chlorine exposure effect. While the temperature was amplified at high concentrations, LDPE exhibited the least degradation, particularly with respect to percent elongation and change in hardness. At 80 °C and at 5000 ppm concentration, all performance parameters were greatly reduced, with bulk parameters