Comparative study on the influence of additives on interfacial adhesion strength between fibres and extruded polymer core via peel-off test

Various peel-off experiments can be used to determine the adhesion strength of various fibres. Among them was the ‘T’ type peel-off test, which measures the adhesions between fibres. The use of organic and inorganic fillers in flake or powder form will alter the adhesive properties of the binders as well as the adhesive strength between the fibres. In this study, the adhesion strength between three different varieties of glass fibres (C-Glass, E-Glass, and S-Glass fibres) with a PLA core has been investigated. As a binder, an epoxy resin was used, and the resin was modified with inorganic additives such as alumina, bentonite, and silicon carbide. Peel-off testing was used to examine the effect of additives on the bonding strength between a thermoplastic core and a synthetic fibre. The addition of inorganic compounds was found to reduce the adhesion strength between the fibres and the core. In order to evaluate the initial adhesion between the filaments and the core, nine specimens were fabricated without the addition of any additives and their strengths were determined. Maximum adhesion strength of 71.8 MPa was recorded for the neat resin. The adhesive strength after inclusion of additives was observed to decrease by 18.14%, and recorded maximum peel-off strength of 58.2 MPa. Also, the inclusion percentages of the additives were found to be the most influential factor in determining the composites’ adhesive properties. Validation tests were also conducted with the optimized results which demonstrated that the predicted and experimental results were in excellent agreement. Macro and micro approaches were employed to analyze the deformation mechanisms in order to gain a comprehensive understanding of how the additives affected the adhesion strength.


Introduction
Interfacial adhesion is crucial in determining the load-bearing capacity and overall strength of polymer composites.In most cases, the matrix of fibre-reinforced polymer composites will be resin, and it will be reinforced with fibres.In sandwich composites, the core will be made of a distinct material, such as a metal or thermoplastic, and will be covered on both sides by face sheets.The adhesion properties of the binder determine the bond between the main material and the face sheets.The adhesion properties of binders vary depending on the type of material and the surface condition of the adhering materials.Adhesion properties are affected by the contact angles and wettability of the resin on the surface of the material.Plasma-modified poketone films are tested for adherence using the T-test with polyurethane adhesive.After plasma exposure, DC current discharge on the cathode and anode surfaces causes hydrophilization and improves adhesion properties [1].GLARE fibre metal laminates are predominantly employed in aerospace applications.The primary disadvantage of GLARE laminates was delamination.The T-peel off test was conducted in accordance with ASTM D1876-08 standards, and the test results indicated that the anodizing procedure increases interlaminar shear strength and improves adhesion between glass fibre and aluminium surfaces [2].Anodized GLARE laminates showed improved peak loads, force-displacement curves and fracture toughness [3].Natural resins that are alternatives to polymers derived from petroleum are gaining popularity due to environmental concerns.Therefore, researchers are more focused on obtaining high-strength polymers from natural sources.As an alternative to polyester and vinyl ester polymers, a thermoset derived from sesame and castor oil was utilised.T-peel and lap shear experiments demonstrated that bio-resins are suitable substitutes for vinyl ester and polyester resins [4].Several materials are currently employed in armor applications, and the penetration resistance of polypropylene impregnated with shear-thickening fluid was examined.It was discovered that as the layer thickness increases, the resistance to penetration increases for projectiles with flat, elliptical, hemispherical, and conical profiles traveling at 500 mm min −1 [5].It was discovered that the adhesion and compatibility of modified micro fibrillated cellulose and polyvinyl alcohol layers were superior [6].The addition of triblock copolymer and functionalized CNT to epoxy adhesives improved shear strength.However, the addition of CNTs and triblock copolymers has no influence on the epoxy resin's thermal stability or glass transition temperature [7].After adding portunus sanguinolentus shell powder, jute-reinforced epoxy composites' mechanical and thermal characteristics improved due to the increased chitosan and mineral content [8].Thermal curing of adhesives used to bond metals causes distortions and variations.Targeted curing with radiofrequency electromagnetic fields can cure adhesives with minimal deformations and reduce residual stresses [9].Biodegradable polymer composites reinforced with luffa cylindrical and human hair fibre as well as incense stick ash infill exhibited improved thermal and water absorption resistance as well as excellent acoustic insulations [10].Natural fibres such as ramie fibres treated with amino silane-reinforced polymer composites containing OMMT.Nanoclay demonstrated superior adhesion properties compared to unmodified resin [11].By reinforcing structurally segregated silicon carbide nanowires within an epoxy matrix, thermal management system materials are produced.Through Agari's model, the formation of a heat-conducting network was discovered to be simple [12].The addition of Alumina Nano inserts to basalt fibre reinforced epoxy composites increased inter-laminar shear strength while decreasing tensile strength.The incorporation of alumina into the basalt/epoxy composite increased its resistance to attrition [13].The mechanical and thermal behaviour of epoxy toughened by phenol novolac and polyester resins with Nano-sized particulates of silica and bentonite.It was observed that the incorporation of nanoparticles enhanced the mechanical and physical properties, particularly the corrosion resistance [14].Silicon carbide's effect on glass fibre reinforced epoxy polymer composite thermal conductivity was examined.Thermal conductivity enhanced with silicon carbide concentration [15].Numerical models employing ANN were used to predict the optimal proportions of each component to develop a structural adhesive with high strength [16].As a result of the formation of polar groups on the surface, oxidative chemical treatment has been demonstrated to be an effective method for enhancing the peel adhesion strength between carbon fibres [17].A finite element model based on ASTM D1876 simulated a sandwich panel with honeycomb and dimpled cores.It was discovered that adhesive surface area has a greater impact on adhesion strength [18].Using the acoustic emission method, the adhesive strength of composite specimens was determined, and it was discovered that contaminated surfaces result in feeble bonding strengths [19].This study examined the impact of additives including alumina, bentonite, and silicon carbide on the interfacial adhesion between glass fibres and extruded thermoplastic core plates.Initially, three types of glass fibers were adhered to the polymer core without any additives to determine the most suitable type with superior adhesion.Subsequently, the three types of additives were incorporated at varying amounts and produced according to the experimental design.The impact of additives on various deformation mechanisms was analysed at both macro and micro scales.An empirical relation was established from the results to simplify the understanding of the engineering features.The study achieved its major goal by optimizing the type of glass fiber, additive, and the proportion of fillers added.This study reveals how additive inclusion impacts the adhesion strengths between a synthetic fibre and a solid polymer core, providing valuable insights for researchers in this field.

Materials
The three main components of a composite are the matrix, reinforcement, and fillers or additives.Enhancing the physical and mechanical properties of composites, reinforcement makes the matrix tougher.Reinforcements typically consist of microfibers, shredded fibres, woven fibres, and fibre mesh.Filler in the form of Nano powders, micro powders, particulates, and foams are used to improve the composite's properties and to fill voids and cavities.Not all fibres are suitable for use with binders and additives.The matrix-fibre interfacial adhesion depends on the fibre's hydrophilic or hydrophobic nature.In certain instances, the infill and additives added to composites will have an effect on their adhesion properties.In this study, the interfacial adhesive strength between various varieties of glass fibres and a solid PLA core was assessed.Polylactic acid was selected as the thermoplastic material for the central core's fabrication.The solid core was manufactured using a 3D printer's extrusion process.To manufacture the central core, a WANHAO Duplicator 3D printer was used.Wol 3D in Mumbai supplied PLA filaments without any additives and with a standard diameter of 1.75 mm.The solid core was developed in SolidWorks and exported to STL.The design was then sliced using software provided by the printer manufacturer.During material extrusion, the process parameters, such as extrusion temperature, bed temperature, layer height, infill density, and fill pattern, are fixed as 220 °C, 50 °C, 0.10 mm, 100%, and a linetype fill pattern, because these parameters, among many others, were determined to be important.The printed PLA core specimen was displayed in figure 1. Ti-6Al-4V was printed and bonded to 3D-printed thermos plastic with carbon fibre reinforcement, and their lap shear strength was measured.It was discovered that bonding strength is dependent on the surface morphology of printed metal [20].The flexural and tensile strengths of a 3D-printed PLA core covered with glass fibre skin were determined.It was discovered that three layers of glass fibre skin provide superior flexural strength, while four layers provide superior tensile strength [21].For this particular work, glass fibre varieties C, E, and S are utilised.C-type glass fibres have enhanced chemical corrosion resistance [22], E-type glass fibres have good insulation against electricity [23,24], and S-type glass fibre has good strength attributes [25,26].Silicon is the primary component of glass fibre, and the efficacy of the fibres will vary depending on the addition of other elements.The glass fibres were acquired from Go green products, Chennai.Five, ten, and fifteen percent of alumina, bentonite, and silicon carbide were added to the binder.The micronsized particles are obtained from Bangalore's fine chemicals.
The incorporation of 1% Nano alumina into glass-nylon-6 hybrid composites improved matrix-to-filler adhesion and significantly increased fracture toughness [27].The addition of 5 vol% alumina to polyester composites improved the tensile and hardness properties, whereas increasing the amount of alumina resulted in a decrease in properties due to agglomeration and void formation [28].The addition of bentonite Nano particles increased the flexibility and rigidity of hemp-reinforced polymer composites during bending tests [29].Calcined bentonite in basalt epoxy composites exhibited enhanced microstructure and enhanced thermal stability [30].Silicon carbide addition to glass fibre composites increases the composites' thermal conductivity [15].E-glass fibre reinforced in epoxy polymer with silicon carbide and aluminium oxide was produced, and with a 2 wt% additive percentage, it exhibited improved chemical resistance [31].

Method of composite specimen fabrication
The T-peel test was conducted in accordance with ASTM D1876 to evaluate the peel strength between fibres or between the filament and the core [32,33].The specimens must be developed according to the dimensions specified in the standards.In this study, since the peel strength between the fibres and the thermoplastic core must be determined, the fibres were bonded to both surfaces of the thermoplastic core [34].Using a mechanical stirrer, the binder was altered by adding the specified proportions of additives while agitating continuously.The study's variable input parameters are outlined in table 1, and the specimens are fabricated with the selected glass fibre and additive binder mixture listed in table 2.
Care must be taken during mechanical stirring, as insufficient stirring will result in non-homogeneity, agglomeration, and gelation of the binder mixture as the temperature increases during stirring.Especially during binder modifications involving a greater quantity of compounds, greater care must be taken.Since a single layer of fibre must be laid over the core, the conventional hand layup technique, which involves a layering process, was employed.Higher percentages of additives will result in extremely opaque binders, which will cause layering problems.When distributing the binder on the core, less dense binders will spread more easily, and layer thickness can be maintained consistently.The central core was deposited on a flat surface, and the top surface was cleaned and coated with an additive-modified epoxy resin.Layer thickness of the binder must be uniformly maintained, and one layer of any type of glass fibre must be placed over the core and pressed with rollers.Rolling action ensures proper wetting of the fibres and dissipates trapped air, resulting in enhanced fibre-to-core adhesion.After rolling, the setup was left to cure for six hours, after which the other side of the core was positioned and the process was repeated.In accordance with the standards, the length of the core was 152 mm, and the fibre will be entirely bound to the core, with an additional 76 mm of fibre on both sides for holding it in the fixtures.Each partially remaining fibre was adhered with insulation membranes for improved grip in the grippers.Due to the 3D printer's limited print volume, the dimensions of the specimen were reduced.Before testing, the specimens were conditioned for seven days at 23 °C and 50% relative humidity.The fabricated specimens for peel-off test with different glass fibre and additive combinations are displayed in figure 2.

Experimentations
The analyses were conducted in accordance with the ASTM D1876 procedure [33].The specimen's peel-off strength was evaluated using an Instron 8801 model UTM.The grippers retain the free portions of the fibres.The grippers were moved until the initial tension on the specimen was applied, and then the load was reset.A constant load was applied in opposite directions at 2 mm min −1 cross head speed [35].As per the standards, the maximum loading rate must fall between 15% and 85% of the upper limit of the loading range [33].The peel speed also affects the peel-off strength of the polymer composites [36].Similar to the tensile test, the T-peel off test was conducted.On the computer, the peel-off strength will be displayed directly.In addition to the L27 tests, nine specimens were also individually fabricated with three distinct varieties of glass fibres and epoxy resin.Three specimens of each glass fibre type were made to test the fibre and core peel-off strength without fillers.This will provide a unique understanding of how the addition of filler affects the adhesion strength between the core and the fibre.In a study, the surface roughness of the substrate was found to enhance adhesion energy during the peel-off test [37].In polyamide and epoxy-based carbon fibre composites, silane coupling treatments  on the carbon fibres enhanced interfacial adhesion and prevented delamination [38].The test apparatus for the T peel-off test is depicted in figures 3(a) and (b).The specimen was inserted into the UTM, and the two tapewrapped ends of the fibres are held between the grippers.After gripping the fibres, the grippers are initially moved apart to provide initial tension, ensuring that the load acts equally on both sides.Figure 3(b) depicts the horizontal placement of the specimen, which indicates that the load is distributed evenly on both sides.Uneven handing will produce inadequate outcomes.

Regression analyses
The relationship between a dependent variable and one or more independent variables can be modeled using regression analysis.It helps analyse and quantify variables and make predictions or inferences from data.The objective is to find the best mathematical model that describes the variables' interaction.The regression model's goodness of fit and significance can be assessed using statistical measures like R2 and p-values.These measurements examine interactions between variables and model performance.Regression analysis is used to study and forecast outcomes in economics, finance, social sciences, marketing, and healthcare.It aids in variable analysis and decision-making.A full factorial design (L27 array) with three input variables and three levels was used for this investigation.The effects of several factors and their interactions on a response variable are studied using a full factorial design in statistical analysis and research.Each factor can be modified at many levels in a full

Results and discussion
To compare the effect of additive incorporation, nine specimens are fabricated and tested without the addition of additives, and their peel-off strengths are measured.The average peel-off strength values of the three varieties of glass fibres, C, E, and S, used in combination with epoxy resin are tabulated in table 3 and illustrated in    The peel strength for each of the 27 tested trials is listed in table 4, and the average peel strength of specimens fabricated with the addition of various additives is listed in table 5.The first of the twenty-seven trials observed the highest strength of 58.2 MPa, while all other trials recorded lower values.The first sample was produced with 'C' type glass fibre containing 5% alumina.In contrast to the previous result, which was obtained without the addition of additives, 'S' type glass fibre was observed the highest strength in this experiment.The difference between the peel-off strengths of each fibre type is minimal, indicating that the variation in fibre type has minimal effect on the peel-off strength.Among inorganic additives such as alumina, bentonite, and silicon carbide, alumina with 5 wt% was the most effective, and it was clear that any inorganic additive addition would reduce the interfacial adhesion between the fibre and PLA core.
Among the specimens tested, the alumina-included specimens had the highest recorded strength, and it was evident from table 5 that increasing the additive inclusion decreases the adhesion between the fibre and the core.In table 5, the average peel-off strengths that were incorporated with various additives at varying inclusion percentages are displayed, as shown in figure 5.The additives function as walls between two carbon atoms and inhibit cross-linking between the atoms, resulting in a decrease in adhesion and adhesive strength.The additive percentages were increased beyond 15 weight percent and tested.The adhesion between the fibre and the core was observed to be very inadequate, resulting in a significant decrease in strength.In addition, when compared to the peel-off strength of alumina, the specimens containing bentonite and silicon carbide exhibited inferior results.The fibers exhibited superior adhesion properties with the PLA core, as observed by having no evidence of fiber delamination and the fracturing of fiber strands.If there were insufficient adhesion means, fibers would delaminate from the bonded surface, but there would be no fiber damage.This behaviour was observed in nearly all additive combinations as well as additive-free samples.As soon as fiber breakage begins, the composite's loadbearing capacity decreases, leading to a decline in loads, which is interpreted by the UTM as specimen failure.Figures 6(a), and (b), which depict the bentonite and silicon carbide-added specimens, respectively, provide evidence of the fiber breakdown and shearing behaviour.

Macro scale deformation analysis
Macro deformation study will elucidate how adhesion strength varies with different types of additives and different amounts of additive incorporation [39].Figure 7 displays macroscopic images of specimens manufactured with varying inclusion percentages of three types of adhesives.Previous research demonstrated the deterioration of the composites caused by inherent processes and environmental influences, as illustrated in figure 8.The degrading process of the polymer composites can be analyzed at macroscopic, mesoscopic, and  microscopic scales.When a model examines a research from the micro to macro level, it is referred to be a bottom-up method [40].
This study examines how additives impact the adhesion strength between the fibre and the polymer core by analysing deformations occurring on the fibre and at the interface between the fibre and the core.The macro analysis explains the various deformations that happened in the fibres and provides reasons for the changes in adhesion strength caused by filler additions.The main deformations observed are interfacial debonding, interface cracking, fibre pull-outs, core deformation, fibre breaking, and delamination.Interfacial debonding occurs when the adhesion between the core and the fibre at the interface breaks down.It causes the core and reinforcement to separate, leading to adhesive and cohesive failures.Micro fractures develop at the interface between the fibre and the core as a result of fibre pull-outs and the adhesives getting brittle due to the presence of additives.The primary cause of fibre pull-outs is the lack of compatibility between the reinforced fibres and the matrix or adhesive.Gaps between fibres and adhesive occur due to their lack of compatibility, causing fibres to be dragged out from the interface under load, resulting in gaps due to fibre loss.When the core is weaker in certain instances, core deformation happens because of increased adhesion strength.The adhesives penetrate into the core through the layer gaps, and when the load is applied, the fibres will dislodge the deposited adhesives from the cores.This phenomenon will damage the central core and reduce its strength, ultimately causing it to fail.Fibre breakdown happens because of increased adhesion strength between the adhesive and the reinforced fibres.Load application results in the stretching of the fibres, and improved adhesion between the fibre and the adhesive causes the fibres to break.Delamination is caused by the lack of compatibility between the fibre face sheet and the central core.Inadequate adhesion between the adhesive on the face sheet and the central core will cause the face sheet to separate from the core, a process known as delamination.This issue causes an uneven distribution of load between the face sheet and the core, resulting in the failure of either component.
Figure 9 displays the average adhesion strength in relation to varying percentages of each additive included.Increased incorporation of additives was found to decrease adhesion strength.The highest adhesion strength was observed at 5% inclusion of additives, while the adhesion strength decreased with 10% inclusion of additives for alumina and bentonite fillers.At 10% inclusion, the silicon carbide filler exhibited superior adhesion strength compared to the other two fillers.Alumina addition demonstrated the strongest adhesion strength at a filler addition rate of 15% compared to the other two fillers.The inclusion of filler was found to inhibit the adhesion at the interface between the core and the face sheet.It was observed by macro analysis that different levels of filler addition result in varying deformations in the face sheets and cores.A detailed explanation was provided regarding the various types of deformations that occurred at different levels of additive inclusion.Polypropylene self-reinforced composite T-Peel test By increasing the draw ratio, the adhesion strength decreased and impact resistance increased [42] Polypropylene sheet, steel sheet, wire mesh Peel and lap shear stress Resistance welding of steel sheet and wire mesh provided better bond strength and vacuum heating interlocked mesh with the polypropylene [43] Polypropylene, graphene Nano platelets Peel test Inclusion of 0.062 wt% of graphene Nano particles increased the interfacial strength of the composites [44] Maleic anhydride grafted Polyethylene, carbon fiber

T-Peel test
The composite showed increased peel strength even at 90 °C [45] Thermoplastic elastomer, thermos-responsive shape memory polymer, and nylon

T-Peel test
The composite achieved better peel strength with 70 A Shore TPU [46] Carbon fiber, AW 6082-T651 aluminium alloy T-Peel test Adhesives with brittle nature showed good peel strength performance and roller peel test was more suitable to access the adhesion quality of metalpolymer composites [47] GFRP, Enguard BP72A adhesive T-Peel test Inclusion of synthetic amorphous silica into vinyl ester resin resulted in better peel strength [32] Thermoplastic polyurethane knitted type, thermoplastic elastomer T-Peel test TPU yielded higher peel strength than TPE and polymers showed better bonding with single knitted fabrics than plain woven fabric [48] Toluene diisocyanate (TDI) grafted graphene oxide (GO) and amino terminated poly(butadiene-acrylonitrile) (ATBN)

T-Peel test
The peel strength increased with 25 wt% of ATBN, 17 wt% of TDI, and 0.5 wt% of GO, when compared to neat epoxy resin [49] When alumina and bentonite are added at 5%, fiber damage occurs.When alumina and bentonite are added at 10%, or when alumina is added at 15% and silicon carbide at 5%, core deformation happens.Fiber pull-outs happen with 10% and 15% alumina addition and at 5% silicon carbide addition.Interfacial debonding happened at 5% and 15% inclusion of bentonite, while delamination occurred at 15% inclusion of bentonite and silicon carbide.Adding 10% alumina and bentonite, 15% alumina, and 5% silicon carbide causes core deformation.Table 6 listed previous research findings on adhesion strength of various materials obtained from different types of peel off testing.

Regression analysis results
The results of regression analysis and analysis of variance were tabulated in table 7. The purpose of the regression analysis was to identify the input parameter that has the greatest impact on the output.In this regression analysis, the three input parameters were considered, and the Fischer value was used to ascertain the significance of each of them.The percentage incorporation of additives has the greatest effect on peel-off strength and adhesion between PLA and glass fibre based on the Fischer value.Other two parameters, such as glass fibre type and additives, have minimal effect on output responses.The P-value determines the significance of each input parameter; if the P-value is less than 0.05, the parameter is considered significant.The significance of the model is determined by the regression coefficient (R-Squared) value, which must be above 95%.This model's regression coefficient was near to 95%, indicating that the model was statistically significant.The regression equation was shown in equation (1), which indicates the empirical relation between the peel off strength and the selected input factors.
The influence of the input parameters and their interdependence can be comprehended from figures 10 and 11, which depict the main effect and interaction plots, respectively.The main effect plot was generated using the larger is better logic, and the plot with the greatest deviation indicates the greatest impact on the output.The main effect graph indicates that the percentage of additives added has a greater impact on the output response.Based on the interaction graph, it was determined that all three input parameters are interdependent.Figure 9 displays the maximum response surface plot with the two most influential input parameters, type of glass fibre and percentage addition of additives.For optimum peel-off strength, it was evident from the surface plot that 'S' type glass fibre with 5% additive inclusion must be used.The plot shown in figure 12 also shows the empirical relation between the peel off strength and the two most influential parameters.For the peel off strength to be maximum, the filler addition must be minimum.
A probability distribution chart, also known as a density plot, shows the probability distribution of a continuous quantity.It presents a graphical representation of the data composition.Figure 13(a) displays a  general normal distribution plot with the mean value at the center and the standard deviation values on the Xaxis.Figure 13(a) displayed the distribution curve separated by standard deviation percentage, with probability density represented on the Y axis.The adhesion strengths of the 27 trails were plotted on a normal distribution curve, and it was observed that all data points were within the 95% confidence interval.Figure 13(b) displayed the minimum, average, and maximum values on the normal distribution curve.Figure 13(c) displays both the right and left side tail values, indicating the range within which the results fall within a 95% confidence level.Figure 13(d) displays an empirical cumulative distribution function plot illustrating the convergence and proximity of the results to the normal probability curve.Understanding the distribution of the results derived from the predicted curve was accomplished in 13e by displaying the normal probability plot.From the obtained results, a mean value of 54.32 and a standard deviation of 2.78 were calculated.

Microscopic analyses
Figure 14 displays images of scanning electron microscopy.The specimen with a PLA core, glass fibre, and alumina additives is depicted in figure 14(a).The additives adhered to the surface of the fractured fibres are displayed.The bentonite particles adhered to the surface of the sheared fibres during the peel-off test, as depicted in figure 14(b).As the adhesion between the fibre and the core was sufficient, the fibre was prevented from detaching from the surface of the core, which would have resulted in fibre fracture.In figure 14(c), (a) combination of glass fibre, PLA core, and silicon carbide additive is depicted, with visible agglomeration of silicon carbide in some locations, but improved adhesion between the fibre and the core.Figure 14(d) depicts the composite surface manufactured with silicon carbide under 100X magnification to ensure the bidirectional structure of the fibre and the uniform distribution of additives.Even after fibre fracture, additives adhere to the surface of the fibre, indicating compatibility between the fibre and the additives, and the phenomenon of fibre breakage indicates the adhesion compatibility between the fibre and the core surface.

Validation test
Figure 15 depicts the software-predicted responses, which indicate that the input parameter must be chosen as A3B1C for maximal peel-off strength to be achieved.'A3' denotes the 'S' variety of glass fibre, 'B1' denotes the alumina additive, and 'C1' denotes the addition of 5 wt% of additives to the binder.On the basis of these predicted input responses, three specimens are fabricated and their peel-off strengths are evaluated.Table 8 shows the average of the three measured peel-off strengths and the percentage difference between the expected and observed responses.The error rate was 1.45%, which is within the acceptable range and indicates a reasonable correlation with the predicted data.In figure 16, the same information is depicted graphically for comparison and clarity.

Conclusions
This research aims to elucidate the fundamental transformations that take place in the adhesion between the fibres and a polymer core, which are joined together using a ceramic-modified adhesive.With the addition of additives to a polymer resin containing fibres, the adhesive strength was altered.There was a decrease in adhesion between the polymer core generated by an extrusion process and the glass fibres.The causes of the decrease in adhesive strength and the most influential factor in determining adhesive properties are elucidated in detail.The findings of the peel-off test were analysed using macroscopic and microscopic methods to investigate the deformation mechanisms in the fibre, core, and their interface.In the case of composites with additives, specimens with alumina-blended binder exhibited superior performance.For superior results, the amount of additives must be kept to a minimum.During the testing of twenty-seven specimens and three specimens for result validation, the S glass fibre with alumina-blended binder demonstrated the highest adhesive strength, and the fibres fractured due to improved adhesion of fibres to the core.The optimal input parameters were determined to achieve a peak peel strength of 57.52 MPa using 'S' type glass fiber and including 5 wt% alumina as an additive.The experimental results correlate effectively with the predicted results.This study's findings will be beneficial for researchers working on the development of functional composites.The work can be expanded  by integrating various types of fibres, additives, and core materials.Furthermore, future research should explore the rate of improvement in additional characteristics resulting from the presence of additives.

figure 4 .
figure 4.Among the three varieties of fibres, 'S' type glass fibre produced the highest peel strength when combined with a PLA solid core.Other types 'E' and 'C' recorded 67.4 MPa and 65.6 MPa, respectively.Throughout all of the experiments, the test conditions were kept constant.

Figure 5 .
Figure 5. Average peel-off strength values of each additive added specimens.

Figure 7 .
Figure 7. Macro deformation analysis of specimens with different additive inclusion percentages.

Figure 8 .
Figure 8. Example of a model to predict the durability of the composite materials [40].

Figure 9 .
Figure 9. Relation between the adhesion strength and the additive content.

Figure 12 .
Figure 12.Surface plot for maximum peel-off strength.

Figure 13 .
Figure 13.(a) Parts of normal distribution plot, (b) Distribution plot with minimum and maximum strength values, (c) Normal distribution plot with 95% confidence level, (d) Empirical cumulative distribution function plot, (e) Probability plot.

Table 1 .
Variable input parameters and their levels.

Table 2 .
L27 trials and their selection levels.

Table 3 .
Peel-off strength of specimens fabricated with neat resin.

Table 5 .
Average peel-off strength for various additive inclusions.

Table 6 .
Results of previous literature results for comparison.

Table 7 .
ANOVA table with model summary.