Polymer Bulletin (2021) 78:3489–3508 https://doi.org/10.1007/s00289-020-03260-x ORIGINAL PAPER The effect of acid aging on the mechanical and tribological properties of coir–coconut husk‑reinforced low‑density polyethylene composites David Olubiyi Obada, et al. [full author details at the end of the article] Received: 28 December 2019 / Revised: 28 December 2019 / Accepted: 11 June 2020 / Published online: 3 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The present study investigates the physical, thermo-mechanical and tribological properties of coir–coconut husk particulate-reinforced polymer composites sub- jected to a corrosive environment. The composites were prepared by the conven- tional facile hot compression molding method. The composite was immersed in a strongly acidic environment of pH 2.2 for a period of 3, 6, and 9 days. X-ray diffrac- tion (XRD) and scanning electron microscopy (SEM) analysis were used to eluci- date the structure and morphology of the composites. The thermal analysis using dif- ferential scanning calorimetry, water absorption, hardness, coefficient of friction and wear rate was performed as per the ASTM standards to characterize the as-prepared and aged composites. The experimental test results revealed that with an increase in acid aging time, the acid aged samples lost surface matrix such that the fiber was seen on the surface. The effects of corrosion seemingly reduced the crystallinity of the acid aged samples allowing amorphous regions to be trapped within the crystals. Water absorption of the samples increased with aging time due to inherent voids in the specimens as weight gain values were 5.27, 16.80, 19.33 and 19.91%, respec- tively for control and acid aged samples. Hardness values initially decreased with immersion time and increased which was attributed to the crystallinity of the speci- mens and to some extent the elemental carbon present in the specimens before and after aging. The measured hardness values of the control and acid aged composites were 2.98, 7.27, 14.40 and 9.07 HV, respectively. From the thermal analysis, it was noticed that the glass transition temperature (Tg) of the polymer shifts to higher tem- peratures as the aging time in the acidic medium increased, which can be attributed to cross-linking of the polymer chains. The control specimen shows higher coeffi- cient of friction (CoF) because they are more rigid than the acid aged samples, and hence under dry sliding can cause more friction leading to increased heat and CoF. Keywords Coir · Reinforcement · Tribological properties · Acidic medium · Thermal properties Vol.:(012134 56789) 3490 Polymer Bulletin (2021) 78:3489–3508 Introduction Natural plant fibers (NPFs) when utilized as reinforcing materials in making low-cost engineering materials generate huge interest which spans across several industrial applications. Environmental remediation efforts as well as continued consumer pressure have forced manufacturing industries like the automotive and construction sectors to continually search for novel materials which can replace the conventional non-biodegradable reinforcing materials such as glass fiber [1]. The advantages of natural fibers over conventional glass fibers stem from its strength, low cost, low density, reduced wear rate of tools, biodegradability amongst others [2]. Thermoplastics like low-density polyethylene (LDPE) are advantageous as compared to thermoset materials and of course act as alterna- tive matrix materials [3]. Nonetheless, the poor wettability and bonding of matrix with fiber, void inclusions and deficient processing techniques are key aspects that have hindered the use of thermoplastic as matrix. In this way, a lot of stud- ies are ongoing towards enhancing fiber/matrix adhesion by various fiber surface treatments as well as matrix modifications for increasing the characteristics of thermoplastic composites. In many industrial applications, these thermoplastics are sometimes exposed to aggressive environments which may influence the chemical and mechanical prop- erties of these polymers. The influence of exposure or attack of polymer materials to corrosive environments may be quite difficult to notice. It is possible that the material may seem normal but, in actual sense, may have become embrittled and the mechanical properties may have deteriorated. The degradation pathway could occur when the acid, salt or alkaline solution diffuses through the surface and reaches the laminates of the polymer or a situation where the solution penetrates the laminate through micro-cracks or other deficiencies/imperfections which may have resulted during the processing stage of the polymer [4]. Therefore, looking at the industrial applications of these polymers and to obtain suitable characteris- tics like, longer life, reduced friction and wear, enhanced strength and hardness, etc., both tribological and processing conditions must be examined [5]. Some- times, it becomes extremely important to study polymers and polymer-based composites in different conditions to obtain the combination of good mechanical and tribological properties [6–10]. According to Kalacska [11], the effect of fric- tion on the wear of engineering polymers is complex; therefore, the condition of the system is imperative to accurately evaluate the friction and resulting wear [12]. A few studies have elucidated the physical and mechanical properties of natu- ral fiber-reinforced polymers (NFRPs) subjected to a corrosive medium. Sindhu et al. [13] studied the degradation of mechanical properties of coir/polyester and glass/polyester composites under different aggressive conditions. The mechanical properties declined when samples were aged in water, acid solution and environ- mental weathering. The resistance of basalt fibers in alkaline solution was noticed to be better than that of glass fibers; while, the acid resistance was found to be poorer in a study conducted by [14, 15]. Amaro et al. [16] conducted experiments 1 3 Polymer Bulletin (2021) 78:3489–3508 3491 on the degradation of mechanical properties after subjecting glass fiber-reinforced polymer (GFRP) samples in HCl and sodium hydroxide (NaOH) solutions. It was concluded that the mechanical properties (flexural and impact) reduced with an increase in exposure time with the effect of mechanical properties deterioration in alkaline solution more pronounced than that noticed for the acidic solution. Trip- athy et al. [17] studied the mechanical properties and interfacial properties of jute fiber-filled epoxy resin. It was observed that the moisture intake by natural fibers, insufficient adhesion between untreated fibers and the polymer matrix, led to fiber pull-out with time [18]. Despite the number of studies on fiber-reinforced poly- mer composites under investigation, and the continued usage of fiber-reinforced polymers (FRPs) in addition to the ambiguities in environmental conditions, etc., the degradation of FRPs in terms of their physico-mechanical, thermal, tribo- logical properties, etc. when subjected to aggressive environments has not been examined in detail. Thus, the aim of the current work is to investigate the physico-mechanical, ther- mal and tribological behaviour of coir–coconut husk particulate-reinforced LDPE composite aged in an acidic medium. The wear characteristics of the produced com- posites were subjected to tribo-contacts for a specific time duration. Towards that, an investigative approach of sliding wear behaviour was explored under a pin-on-disk experimental setup followed by a variety of materials characterization techniques to establish a relationship between the mechanical properties of these composites and their respective tribological performance. Materials and methods Materials The low-density polyethylene (LDPE) used as matrix was scavenged around Zaria metropolis in Nigeria as it is a common material used for packaging sachet water. Coir was obtained from the fibrous husk of coconut which was sourced from a fruit market in Zaria, Nigeria. The sulphuric acid (H2SO4, CP Grade 98%) was obtained from Steve Moore Chemicals Ltd, Zaria, Nigeria, while the NaOH (10 M in H 2O) used for fiber treatment was sourced from Merck & Co. Composite fabrication and exposure conditions The coir was washed with water to remove dirt and dried in open air at ambi- ent temperature (27 °C). The dried coir was treated with 5% wt. NaOH solution (10 M in H2O) and washed to remove retained alkali from the fibers. The coconut husk was sun dried for 72 h before converting it into powder using a hammer mill, while the polyethylene was shredded using a plastic shredding machine. The vari- ous compositions of the coconut husk powder, coir and LDPE were formulated and subjected to thermal treatment for 30 min at a temperature of 180 °C utilizing a two-roll mill. The composites were further thermoformed by a hot press aided 1 3 3492 Polymer Bulletin (2021) 78:3489–3508 Fig. 1 a Process flow chart showing the applied fabrication route of the composites. b Schematic of the composite consolidation by a fabricated steel mold. The temperature condition was 80 °C with 15 min of preheat, 2 min of compaction pressure and 5 min of cooling (see Fig.  1). Each sample of the composite fabricated contained 70%, 15% and 15% by weight of recycled low-density polyethylene, coir and coconut husk powder, respectively [19]. The acid aging was done by immersing the polymer samples in an acid media (H2SO4) of pH 2.2 for different time periods, i.e., 3, 6 and 9 days (hereafter named 3PA, 6PA and 9PA, control sample—NP). The choice of H 2SO4 was as a result of work carried out by [20] where investigations on aging of glass fiber- reinforced composites showed that H 2SO4 had a more pronounced effect than the other acids used in their experiments. The experiments conducted in this study were done to investigate the short-term effect of the exposure of the fabricated coir-reinforced polymer composite to an acidic medium. Table 1 presents the for- mulation of fabricated composites. Table 1 Formulation of specimens with exposure time to sulphuric acid Specimens code LDPE (Wt%) Coconut husk pow- Coir (Wt%) Exposure to der (Wt%) sulphuric acid (days) NP 70 15 15 – 3PA 70 15 15 3 6PA 70 15 15 6 9PA 70 15 15 9 1 3 Polymer Bulletin (2021) 78:3489–3508 3493 Materials characterization Scanning electron microscopy (SEM) Scanning electron microscopy examination was conducted to reveal the mor- phological features of the polymer specimens utilizing an ultra-high vacuum and high-resolution FEI Xl-30 scanning electron microscope equipped with an EVEX EDS (Energy-dispersive spectrometer) and operated at an accelerating voltage of 15 kV. The surfaces of the composite samples were sputter with gold before their morphological observation. Since the composites are nonconductive, the speci- mens were sputter coated with an ultrathin (5 nm) coating of gold, using a JFC- 1100E ion sputtering device (JEOL, Tokyo, Japan). X‑ray diffraction (XRD) analysis Structural investigation of the composite samples was conducted on a Shimadzu XRD-6000 (Shimadzu, Japan) X-ray diffraction instrument. The X-ray beam was a Cu Kα (λ = 0.1540 nm) radiation, with operating conditions at 40 kV and 30 mA. The diffraction profiles were collected at a scanning rate of 2°/s in the 2θ range of 10°–60°. Differential scanning calorimetry (DSC) The melting behaviour of the polymer composites was analyzed on a Metler Toledo differential scanning calorimetry instrument. The temperature ranged from 30° to 200 °C and the heating rate was 10 °C/min. For the test, approximately 25 mg of sample was placed in an aluminum crucible which was equipped with a pierced and sealable lid and the specimen was tested under nitrogen atmosphere. Water absorption tests The effect of water absorption on the control and acid aged composites (composite specimens NP, 3PA, 6PA, 9PA) was investigated in accordance with BS EN ISO 62:1999 standard [21]. First, the composite specimen was dried in an oven at 60 °C and then allowed to cool to room temperature before weighing them to the near- est 0.1 mg. This protocol was repeated until the mass of the composite specimens reached a constant value. Water absorption tests were then conducted by immersing the specimens in de-ionised water bath at 27 °C for different time durations. After immersion for 24 h, the composite specimens were taken out from the water and all surface water was cleaned with a clean dry cloth. The specimens were reweighed to the nearest 0.1 mg within 1 min of removing them from the water. The specimens were weighed regularly at 24, 48, 72, 96, 120 and 144 h exposure times. 1 3 3494 Polymer Bulletin (2021) 78:3489–3508 Hardness Hardness measurements of the as-prepared composites were made on an FIE MV1- PC Vickers Indentation Hardness Tester by applying a load of 0.3 kg/f for a dwelling time of 10 s. Three indentations were made across the center of each specimen. For each sample, hardness measurement was conducted before acid aging to obtain an initial hardness value. After each aging period, the hardness tests were conducted to obtain hardness values and a comparison was made. The hardness indentation meas- urement procedure was conducted three (3) times and an average hardness value was determined under each scenario. Friction and wear evaluation The friction and wear tests were conducted on a pin-on-disc type tribometer (DRTB, 70090) with the working principle depicted in Fig. 2. Before each test, a pre-rubbing process was carried out to ensure a full contact of the pin and disc surfaces. All the composite specimens were thoroughly cleaned. The friction and wear tests were per- formed at room temperature (25 ± 5 °C) in ambient atmosphere (relative humidity (RH): 50 ± 10%). Applied normal load was 5 N, the rotation speed of discs 10 cm/s, and the sliding distance was 15.71 m. To ensure the comparability of testing results for the different specimens, the parameters of the friction tests are set to be the same. Specifically, the testing radius was set to be 5 mm, and the testing time was set as 159  s. As this study focused on the friction behaviour at the inception of sliding, the low testing speed and short testing time are chosen to prevent the over abrasion of surface roughness and the increase of temperature. The wear after CoF tests was measured by the weight loss of tested samples using an analytical balance (preci- sion: 0.001 mg). The wear rate (K, m m3/N-m) was calculated according to the fol- lowing equation: Fig. 2 Working principle of the pin-on-disc tribometer. FN normal force, v velocity, s friction trace, r frictional radius 1 3 Polymer Bulletin (2021) 78:3489–3508 3495 Δm K = , LP where ∆m is the weight loss (g), L is the sliding distance (m), P is the applied load (N) and ρ is the density of polymers (g/cm3). The density was measured as per ASTM D792 standard, which was primarily based on Archimedes principle. Results and discussion Morphological studies of raw materials and synthesized composites before and after corrosion The surface analysis of raw materials (Fig. 3) shows the morphology of as-received coir, coconut husk powder and LDPE matrix. The surface roughness of coir (Fig. 3a) supposedly induces better adhesion between fiber and matrix due to mechanical interlocking [22, 23]. The structure of the coconut husk powder was observed to be solid in nature but irregular in size which proved valuable for use as a filler to enhance the mechanical properties. The scanning electron microscopy (SEM) image of the LDPE (Fig. 3c) shows the typical smooth surface as corroborated by [24]. SEM images of the surface of the composite specimens before and after acid aging are shown in Fig. 4. It can be observed that the control specimen (sample not aged in acid solution) showed no signs of fiber pull-out which can sometimes hap- pen by reason of a response to an aggressive environment. The acid aged samples (3PA, 6PA and 9PA) show fiber pull-out which indicates gradual material degrada- tion and poor adhesion between coconut coir and LDPE matrix. This assertion is also in line with works carried out by Feng et al. [25] and Wu et al. [26]. A possible explanation for this is the debonding effect which occurs between the coir and the LDPE matrix. The sample exposed to the acid solution for 9 days (9PA) exhibited more debonding tendencies in addition to a change in color (from light gray to dark gray) and a cross-sectional shape (luster) was noticed. It was evident that the aged samples gradually lost surface matrix such that the fiber was visible on the surface. Although these changes were only observed on the surfaces, it is possible to specu- late upon the extent of corrosive effects on the samples. Fig. 3 Scanning electron micrographs of a coir, b coconut husk particulate, c LDPE 1 3 3496 Polymer Bulletin (2021) 78:3489–3508 Coir Fiber (coir) pull-out (a ) (b ) (c ) (d ) Coir Fiber (coir) pull-out Fig. 4 Scanning electron micrographs of composites a’ NP (control sample); acid aged polymers (b’) 3PA, c’ 6PA and d’ 9PA Fig. 5 EDS analysis of control sample (NP), acid aged polymers 3PA; 6PA; and 9PA 1 3 Polymer Bulletin (2021) 78:3489–3508 3497 Energy‑dispersive spectroscopy (EDS) of specimens before and after corrosion EDS analysis in the zones corresponding to the control specimen (NP) and acid aged specimens (3PA, 6PA and 9PA) is shown in Fig. 5. According to the EDS analysis, for all zones, the EDS measurement showed increase and decrease in the oxygen band. The relative number of oxygen atoms in the polymer specimens is assumed to be present as oxide and could help to determine approximate metal- oxide constituent ratio. There is also a possibility that the gradients of oxygen bands is an indication of the formation of a thin oxide film at the fiber/matrix interface [27]. In addition, all the zones correspond to the composite in which the corresponding carbon bands increased and decreased. This means that the high oxygen band as noticed for 3PA may have reduced the carbon band which may reduce the composite solidity. This is because carbon-based materials have shown great adaptability and can be chemically joined with other carbon-based materials which consist of a range of different elements to form strong covalent bonds [28]. The elemental content of calcium, potassium and iron was evident in some of the zones. This confirms that some metal ions were deposited on the surface of the composites. The presence of coir and coconut husk particulates should give rise to silica and aluminum peaks. It was assumed that the calcium ion of the samples after acid aging (3PA, 6PA and 9PA) percolated or leached out which was due to the corrosion effect. Also, a look at the 9PA zone showed that all metals ions (K, Ca, Fe) had also leached out [29]. This is a further confirmation that after 9 days of acid aging, as noted by SEM analysis, more degradation as a result of the cor- rosive environment took place. Fig. 6 XRD pattern of polymers before and after acid aging. C + LDPE represents coir-reinforced poly- mer; and Q represents quartz 1 3 3498 Polymer Bulletin (2021) 78:3489–3508 XRD analysis of composite before and after corrosion To study the corrosive effects in the composite caused by the acid immersion, the X-ray diffraction analysis was conducted on the polymer composites before and after acid aging. The XRD profiles are presented in Fig.  6. It can be seen that there were relatively low reflections for, most notably, 6PA and 9PA at the peak positions of the major reflections of coir-reinforced LDPE. Typically, crys- talline peak position of coir-reinforced LDPE composites is located at 21.39° [30]. Composite samples subjected to the acidic medium ( H2SO4) compared with that obtained before exposure showed reduced peak intensities at characteris- tic peak positions of the composites as it is evident from the intensity of counts particularly for the reflections of (C + LDPE) indicating poor fiber–matrix adhe- sion. Reflections before acid aging showed higher intensity comparatively, which can be mainly attributed to the formation of well-bonded interface between the fiber and the matrix. The corrosion effect seemingly reduced the crystallinity of the acid aged polymer samples allowing more amorphous regions to be confined within the crystals. This infers that the degree of structural order in the composite had been affected which expectedly would have an influence on the mechanical and physical properties. XRD analysis was used to calculate the crystallinity of the samples and the results show that NP, 3PA, 6PA and 9PA has crystallinity values of 48.71%, 27.86%, 45.49%, and 50.89% respectively. In general, the higher the degree of crystallinity, the better the mechanical properties of the composite [31]. The more crystalline a polymer, the more regularly aligned is its chains. The crystallinity of the polymer increases strength, because in the crystalline phase, the intermo- lecular bonding is more significant. Hence, the polymer deformation can result in higher strength leading to oriented chains. This was consistent with the observed hardness test results (see Fig. 10). Fig. 7 DSC thermogram of control specimen (NP), acid aged polymers (3PA, 6PA and 9PA) 1 3 Polymer Bulletin (2021) 78:3489–3508 3499 Table 2 DSC data of uncorroded polymer (NP), corroded polymers (3PA, 6PA and 9PA) Specimens code Onset (°C) Melting tem- Endset (°C) Glass transition Enthalpy (J/g) perature (Tm) (Tg) (°C) NP 115.59 122.27 125.68 36.70 − 35.98 3PA 112.96 124.43 127.63 38.36 − 149.90 6PA 111.60 125.01 129.10 38.60 − 95.77 9PA 113.26 122.84 126.42 38.78 − 116.55 Differential scanning calorimetry analysis of specimens before and after corrosion The thermal behavior of the polymer composites was investigated by DSC via stepwise ramping as illustrated in Fig.  7 and Table  2 which was slowly heated through its melting temperature. As the melting temperature is reached, an endothermal peak appeared because heat must be favorably added to the sam- ple to continue this isothermal process. It is obvious that the melting tempera- ture (Tm) and glass transition temperature (Tg) of the polymer shifts to higher temperatures as the aging time in the acidic medium extends up to 6  days and decreased gradually. The melting temperature is highly dependent on the strength of crystal bonds. Thus, the value of the glass transition temperature depends on the molecular characteristics that affect chain stiffness and melting temperature as well. The degradation caused by the acid aging partially decomposed the polymer into fragments with higher molecular weight [42–44]. This degradation noticed for the acid aged specimens has the tendency to cause cross-linking of the poly- mer chains and a slight increase in glass transition temperature as compared to the control sample (NP). The cross-linking restricts the motion of the chains and increases the thermal stability of the polymer. The Tg of the specimens increase with aging time, while the melting temperature was simply a function of the period when the strength of the crystal bonds were weakened by the application of heat. In addition, an increase in Tm and Tg can be linked to possible sec- ondary crystallization, same size of the primary crystals and the relaxation pro- cess of amorphous regions of the semi crystalline composite. The enthalpies of transformation during the DSC tests have been determined by measuring the area between the curve and the baseline. The melting enthalpy calculated by integrat- ing the areas (J/g) under the peaks is estimated by − 35.46, − 149.90, − 95.77 and − 116.55 J/g for NP, 3PA, 6PA and 9PA, respectively. This is because above the melting point and corresponding raised temperature, the changes in enthalpy produce a negative change in the free energy for melting. The results show that melting enthalpy of the polymer composite after 6  days of acid aging is lower than after 3 days of same and increased on immersion for 9 days. This trend can be caused by the composite surface contribution which is associated to surface area–volume ratio and bond breaks. Essentially, the atoms at the free surface of the polymer matrix exhibit a different background than atoms in the bulk of the composite. On the other hand, it is well known that the mechanical properties of 1 3 3500 Polymer Bulletin (2021) 78:3489–3508 polymers are dependent on the glass transition temperature, due to their visco- plastic nature [32]. Crystal solidification can also occur, which usually leads to the increase of strength of the polymers and, therefore, enhancement in their mechanical properties. The peak breadth of the DSC thermograms for the control and acid aged specimens which varied a bit can be primarily related to the dimen- sion and amount of perfection of the polymer crystals which also has an influence on the mechanical properties. Water sorption analysis of specimens before and after corrosion The weight gained by the control and acid aged samples after immersing in water at room temperature (25 °C) for a total period of 144 h as shown in Fig. 8 is 5.27, 16.80, 19.33 and 19.91%, for NP, 3PA, 6PA and 9PA, respectively. The water uptake process for all specimens except NP, which hardly absorbs any water, is linear during start and then slows and reaches saturation with time, fol- lowing a Fickian diffusion process. Both the initial water absorption trend and the maximum weight gained due to water absorption increases for all the composites samples. This phenomenon can be explained by considering that hydrophilic fiber swells and there is a tendency of fiber pull-out allowing for an increased diffusion of water through voids formed by fiber/matrix degradation. As a result of fiber swelling through water penetration, the tendency for micro- cracking/voids of the matrix is enhanced. The high cellulose content in coir fur- ther contributes to more water penetrating into the interface through the micro- cracks creating swelling stresses which can as well cause composite failure [33]. The water molecules actively degrade the interface, subsequently causing debonding between the fiber and the LDPE matrix [34]. The effect of the acid aging caused large voids in the composite due to fiber pull-out. The SEM images in Fig. 9 support this explanation. Fig. 8 Water absorption for control sample (NP), acid aged polymers (3PA, 6PA and 9PA) 1 3 Polymer Bulletin (2021) 78:3489–3508 3501 Small voids Large voids Large voids Fig. 9 Scanning electron micrographs of composites showing voids responsible for water penetration Fig. 10 Hardness values for control sample (NP), acid aged polymers (3PA, 6PA and 9PA) Hardness testing results of specimens before and after corrosion Hardness data collected for the control and acid aged polymer samples have been illustrated in Fig. 10. For condition 1 (control sample), it is observed that hardness value was recorded as 12.98 HV. For condition 2 (3-day acid aging), a decrease in hardness value is much more pronounced (7.27 HV). In conditions 3 and 4 (6 and 1 3 3502 Polymer Bulletin (2021) 78:3489–3508 9-days acid aging), after 6 and 9 days of immersion in the acid solution, hardness values increases to 14.40 and 19.07 HV, respectively. It is possible to conclude that the measured hardness of the composite samples increases after immersion for 6 and 9 days, in comparison with the NP and 3PA samples. This means that higher acid exposure time seems to promote a significant change in terms of the composite hardness values. This tendency agrees with Banna et al. [35] where this similar ten- dency in hardness value variations was observed. The reason for the low strength of 3PA can be ascribed to the fact that at initial acid aging, the chains of the polymer are irregularly arranged and this makes the polymer chains loosely bonded by weak van der Waals forces making the chains move easily, thus a resultant low strength, although crystallinity is present. In case of more aging time (6PA and 9PA), the chains become large and hence are entangled, giving strength to the polymer. Another possible explanation can be related to the EDS results (see Fig. 5) where a reduction of the carbon band in the 3PA spectrum may have been responsible for the reduced hardness reported. An increase in carbon bands for 6PA and 9PA equally suggests a corresponding increase in hardness values for these composite speci- mens. This explanation is buttressed by the fact that carbon-based materials exhibit excellent characteristics such as high strength, high density, and high hardness [28]. Friction and wear tests The friction coefficient μ as a function of time t for the different polymer samples (NP, 3PA, 6PA and 9PA) under a normal load (5 N) is presented in Fig. 11. The friction coefficient was obtained based on the normal load which was measured by a load cell. The coefficient of friction (CoF) quickly increased from 0 with a slight nose-up peak in the curve. After reaching this peak, it gradually declined to a rather stable value for all composite specimens. The nose-up peak values which were noticed in the curve refer to the static friction coefficient; while, on the other hand, the dynamic friction values denote stability as far as the coefficient of Fig. 11 Variation of friction coefficient for control sample (NP), acid aged polymers (3PA, 6PA and 9PA) 1 3 Polymer Bulletin (2021) 78:3489–3508 3503 friction values for all composite specimens is concerned. Increasing and decreas- ing tendencies of the dynamic friction coefficient with the test time are observed for NP, 3PA and 6PA which can be ascribed to the abrasion of rough surfaces and the increase in temperature which contributes to the fluctuating values of the CoF observed. From the curve, it is noticed that after the initial period of rubbing, the value of friction coefficient increased steadily to 0.225, 0.224, 0.200, and 0.250 for NP, 3PA, 6PA and 9PA samples, respectively. The friction coefficient for control sample (NP) further increased steadily to 0.318 over a  duration of about 2.5 min, while there was a decrease in the fric- tion coefficient of the 9PA sample to 0.190. The friction coefficient for the 3PA and 6PA samples increased and decreased over the test duration of about 2.5 min with end CoF values of 0.222 and 0.216, respectively. The possible explanation for the observed values is that at the initial stage of rubbing, friction is low and the factors responsible for this low friction are the presence of a layer of for- eign material, for instance, moisture, oxides, etc. on the surface of the polymers. The deposited layer of foreign materials break up and the bare and clean surfaces come in contact which in turn increases the adhesion force between the contact surfaces. In the same vein, due to the ploughing effect and the roughening of the surface, the friction force in most cases increases with duration of rubbing and may reach a certain steady state value which allows the values of friction coef- ficient to remain constant for the rest of the testing time [36]. It should be noted that “Ploughing” occurs when two bodies in contact have different hardness. The roughness or unevenness on the harder surface may penetrate into the softer sur- face and produce grooves on it, if there is relative motion. According to Jia et al. [8], the constant rubbing between two materials causes an increase in heat due to the roughness of the surfaces when they come in contact. The wear after the CoF test measured by an analytical balance were NP-1.8631  g, 3PA-1.8336  g, 6PA- 1.7454 g and 9PA-1.6535 g. The results show progressive wear on the samples with test time. Fig. 12 Wear rate for control sample (NP), acid aged polymers (3PA, 6PA and 9PA) 1 3 3504 Polymer Bulletin (2021) 78:3489–3508 Figure 12 shows the relation between testing time and wear rate (Eq. 1) of the various polymer composites. As test time increased, the wear rate increased for 3PA and 6PA which is due to the aggressive effect of the acid on the samples before test- ing which may have eroded the surface of the specimens and resulted in increased absorption of the acidic solution. As the acid aging time increased, the hardness of the polymers increased (Fig. 10). As earlier discussed (see “Hardness testing results of specimens before and after corrosion”), higher acid exposure time seemed to pro- mote a significant change in terms of the composite hardness values. In this way, the fracture toughness is expected to increase, and as the fracture toughness is propor- tional inversely with wear rate, then there is a decrease in wear rates as noticed for the 9PA specimen. The relationship between sliding velocity and wear rate is complex because the process of wear includes deformation which deals with two surfaces. In this way, there is no unique trend between wear rate and sliding velocity and experiments has shown these inconsistent trends depending on the nature of the materials and operat- ing conditions [37–41]. It can be further explained that the control specimen (NP) shows higher CoF but not necessarily a consequent increase in wear rate. The possible reason for this increase in CoF for the control sample may be due to molecular motions [8] of the sliding surfaces under dry lubrication because the control specimen absorbed no liq- uid which may have served as a lubricant. Therefore, an increase in heat generation takes place and leads to higher CoF. But in case of 3PA, 6PA and 9PA, the acid which penetrated the voids/micro-cracks can be said to have served as a lubricant and in turn have reduced the CoF as observed in Fig. 11. The control specimens are more rigid than the corroded specimens and hence under dry sliding can cause more friction leading to increased heat and CoF with a subsequent increase in PV value. The possible increase and decrease of CoF and wear rate of the samples under test conditions can also be linked to the reason that, when the PV value is small (nose- up peak in the curve), the thermal mobility of NP, 3PA, 6PA, and 9PA molecules are weak because the evolving heat by virtue of friction is relatively low, which infers that the deformation of the samples’ molecule would not be able to respond to external forces, so the CoFs are increased at this point. As the frictional heat gradu- ally increases, the mobility of polymer chains in the 3PA, 6PA and 9PA samples gradually increases, and then the deformation of these composite specimens is trig- gered by the external forces, so the corresponding CoF and the wear rate gradually decrease (as in the case for 9PA). This is not the case for the NP sample which because of the rigidity and the comparatively reduced erosion of the surface of the polymer did not have enough time to respond to the external forces during the test time, hence a steady increase in the CoF. Conclusions This work studied the physico-mechanical and tribological responses of fabricated coir/coconut husk powder/LDPE composite after aging in sulphuric acid ( H2SO4) for 3, 6 and 9 days. The following conclusions can be drawn: 1 3 Polymer Bulletin (2021) 78:3489–3508 3505 1. It was observed that the control sample (NP) showed no signs of fracture or fiber pull-out which can happen by reason of a response to stress or an aggressive environment. The acid aged samples show fiber pull-out which indicate gradual material degradation and poor adhesion between reinforcement and matrix 2. From the structural point of view, it can be seen that there were relatively low reflections for, most notably, 6PA and 9PA due to the effect of acid aging at the peak positions of the major reflections of coir-reinforced LDPE. 3. The water uptake process for all specimens except NP is linear in the beginning, then slows and approaches saturation with test time, following a Fickian diffusion process. This phenomenon can be explained by considering that hydrophilic fiber swells and there is a tendency of fiber pull-out allowing for an increased diffusion of water through voids formed by fiber/matrix degradation. 4. The degradation noticed for the acid aged specimens has the tendency to cause cross-linking of the polymer chains and a slight increase in glass transition tem- perature as compared to the control specimen (NP). 5. From the curve, it is noticed that after the initial period of rubbing, the value of friction coefficient increased steadily to 0.225, 0.224, 0.200, and 0.250 for NP, 3PA, 6PA and 9PA samples, respectively. The friction coefficient for control specimen (NP) further increased steadily to 0.318 over a test duration of about 2.5 min, while there was a decrease in the friction coefficient of the 9PA sample to 0.190. The friction coefficient for the 3PA and 6PA samples increased and decreased over the test duration of about 2.5 min with end CoF values of 0.222 and 0.216, respectively. The control samples are more rigid than the acid aged samples, and hence under dry sliding can cause more friction leading to increased heat and CoF. 6. Higher acid exposure time seemed to promote a significant change in terms of the composite hardness values. The fracture toughness is inversely proportional with wear rate; hence, there is a decrease in wear rates as noticed for 9PA sample. Acknowledgements Authors acknowledge the Department of Mechanical Engineering and Materials and Metallurgical Engineering, Ahmadu Bello University, Zaria, Nigeria for providing facilities to carry out this study. DDA acknowledges the support of the University of Ghana BANGA-Africa programme. Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Compliance with ethical standard Conflict of interest The authors declare no conflict of interest 1 3 3506 Polymer Bulletin (2021) 78:3489–3508 References 1. Bledzki AK, Gassan J (1999) Composites reinforced with cellulose based fibres. Prog Polym Sci 24(2):221–274 2. Bolton J (1995) The potential of plant fibres as crops for industrial use. Outlook Agric 24(2):85–89 3. 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Biomaterials 21(16):1659–1668 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 1 3 3508 Polymer Bulletin (2021) 78:3489–3508 Affiliations David Olubiyi Obada1  · David Dodoo‑Arhin2,3 · Abdulrahman Jimoh1 · Abdulrrahman Abdullahi1 · Naresh Dayaram Bansod4 · Msuega Jnr Iorpenda1 · Md. Osim Aquatar5 · Adetunji Rasheed Sowunmi1,6 · Moshood Yemi Abdulrahim1 · Christy Yiye Abraham1 · Ezekiel Otor Ochuokpa7 * David Olubiyi Obada obadavid4@gmail.com; doobada@abu.edu.ng 1 Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria 2 Department of Material Science and Engineering, University of Ghana, Legon, Ghana 3 Institute of Applied Science and Technology, University of Ghana, Legon, Ghana 4 Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok, Thailand 5 Academy of Scientific and Innovative Research (AcSIR), Environmental Materials Division, CSIR-NEERI, Nagpur, India 6 National Universities Commission, Maitama, Abuja, Nigeria 7 Department of Metallurgical and Materials Engineering Ahmadu, Bello University, Zaria, Nigeria 1 3