Vacuum Assisted Resin Transfer Molding (VARTM) is open mold, out of autoclave (OOA) composite manufacturing process, with its distinguishing characteristic being the replacement of the top layer of a mold tool with a vacuum bag and also peel ply, distribution media are arranged in a sequence manner and the use of a vacuum to assist in resin flow. The process involves the use of a vacuum to facilitate resin flow into a fiber layup that has been arranged within a mold tool covered by a vacuum bag. After the impregnation occurred the composite part is allowed to cure at room temperature with an optional post cure sometimes carried out.

In general, all VARTM processes can be divided into three processing steps including material and tooling preparation, the infusion step and the post-infusion step (Figure 1). Each process step will influence final material quality in particular the fiber volume fraction and void content distribution. Capital and infrastructural requirements are less as compared to other methods.

The methodology consists of two steps:

Step 1: Surface treatment of fibers

Fibers were treated with NaOH alkali solution ordered from N₂O₃, Deutschland to reduce the inter-fibrillar region of the fiber by removing hemicellulose and lignin and further to reduce the cementing force between fibrils. This led to a more homogenous dispersion of the bio fiber in the matrix as well as increase in the aspect ratio of the fiber in the composite, resulting in an improvement in fiber reinforcement efficiency. For 30 minutes, the required amount of

fibers were treated with a 2 wt-% NaOH solution. The excess NaOH was then thoroughly washed from the fibers with water. The fibers were then allowed to cure for 24 hours at room temperature before being reinforced.

Step 2: Sample Preparation

The low temperature curing epoxy resin and corresponding hardener are mixed in a ratio of 100:40 by weight. A mold with the dimension 330 mm × 330 mm was used for casting the composite slabs. Table 1 shows the compositions that are varied for the Six (6) different specimens.

In the preparation step, the cutting of reinforcement fibers, flow media, peel ply, and the vacuum bag was carried out. The mold was prepared according to the required dimensions with the sealant tape. The fiber reinforcements were arranged with a stacking sequence. Peel ply was placed on both sides of the fibers. To ensure the fast and even distribution of resin over the fibers, the flow media was placed on top of the peel ply. This is followed by placing the entire setup in a vacuum bag as shown in Figure 2 and Figure 3.

The processing step entails adjusting the pressure in the vacuum bag as well as initiating a chemical reaction by adding a hardener to the resin that causes the resin to cure. For VARTM to create high quality composite parts it is crucial that air leakages are avoided. Air leakages can cause resin to improperly flow through the mold and also lead to the formation of air bubbles.

The main process of impregnating the fibers with resin occurs with the aid of a vacuum in the infusion step. The Specimen was cured for 24 h at room temperature followed by warm curing for 8 h at 60˚C.

After Post Curing, The Specimen slabs which are initially fabricated are cut down according to the test standards. The thickness of the composites

should be around 2.4 mm to match the requirements of the “Test conditions for fiber-reinforced plastic composites”. In order to achieve this thickness, the number of sheets was varied from 2 sheets for Flax specimen to 6 and 8 sheets for the Specialty Paper and Newspaper specimens respectively.

From m options (alternatives) A_i, each of which depends on n parameters (criteria) x_j whose values are expressed with positive real numbers x_ij. The best option should be selected.

Initially, the parameter values x_ij should be balanced according to the procedure of normalization. Suppose that A_ij (or) a_ij are the normalized parameter values. Then each option A_i is expressed as the point A_i ( a i 1 , ⋯ , a i n ∈ R ). Selecting the most optimal value a j ∗ ∈ { a 1 j , ⋯ , a m j } for every parameter x_j, we determine the positive ideal solution A * = ( a i ∗ , ⋯ , a n ∗ ) . The opposite is the negative ideal solution A o = ( a i o , ⋯ , a n o ) . The positive and negative ideal solution are also denoted by A⁺ and A^-. The decision on the order of options is made respecting the order of numbers.

Based upon the weights, the equation for the Weight Normalization Decision Matrix is given by

A i j ( or ) a i j = w j x i j ∑ x = 1 6 x i j 2 (2)

where w_j—Weight of the criteria and x_ij—Parameter(Property) values.

The positive ideal solution A⁺ (or) A* contains the largest numbers of the first, second and third column of A, and the smallest numbers of the fourth and fifth column of A.

The negative ideal solution A^- (or) A^o contains the smallest numbers of the first, second and third column of A, and the greatest numbers of the fourth and fifth column of A. The positive and negative separation measure is calculated by using the formulas.

s i + = ∑ j = 1 n ( A i j − A j * ( or ) A j + ) 2 (3)

s i − = ∑ j = 1 n ( A i j − A j 0 ( or ) A j − ) 2 (4)

where A_ij—Normalized parameter values, A⁺ (or) A*—Positive ideal solution and A^- (or) A^o—Negative ideal solution.

Relative Closeness Factor decides the best and worst solutions from the set of alternatives. The equation to calculate this given by

C i = s i − s i + + s i − (5)

where, s i + —Positive Separation measure and s i − —Negative Separation measure.

Finally, the ranking of attribute is given by highest closeness factor value to the lowest closeness factor.

As shown in Figure 4 and Table 2, the flexural strength and modulus increased with an increase in the weight percentage of fiber. As Figure 4 depicts the values of flexural strength, clearly composite of paper reinforcement with 40% fiber content has the highest values i.e., 174 MPa for S11 and 154 MPa for S21 respectively, when compared to the flax-composites with same composition. As the fiber content has been reduced to 30%, the percentage difference of flexural

strength between S12 and S02 is 19.32% and S22 and S02 is 4.20% accordingly.

The results of tensile test are presented in Figure 5 and Table 3. In compared to other samples, the tensile strength of Specialty Paper with 40% fiber content was quite high. Tensile strength improved by 16.82% when compared to, according to the test findings. The yield strength defines the maximum load that a sample can bear. In comparison to Flax S02 (30% Fiber), Specialty paper S12 improved yield strength by 20%. There is not much difference in Tensile Strength between S21 (40% fiber) to S01 and S22 (30% fiber) to S02. The tensile strength was 125 MPa for S11, 120 MPa for S12, 108 MPa for S21, 107 MPa for S01, 100 MPa for S02 and 99 MPa for S22. The percentage of increase in tensile strength is due to the uniform dispersion of resin between the fibers by holding them together.

Figure 6 depicts impact strength. The energy absorption rate increased as the fiber content decreased. As a result, the energy absorbing capability of the paper composite is increased, enhancing the composite’s toughness. Specimen S22, possessing the highest impact strength. It absorbed 40.6 kJ/m² of energy which is 4.90% more than the energy absorbed by S02 with 70% of resin content. Whereas, S21 (60% resin) absorbed 0.5% of more energy than S01 which is the lowest of all. The difference in Impact strength between S12 (70% resin) and S02 is

3.25% and that of S11 (60% resin) and S01 is 9.12% respectively.

As seen in Figure 7, specimen S12 with 70% resin content possessed the highest

shore hardness value of 88.6 Shore. As the matrix content has been increased, the hardness of the composites has been raised. The specimen with high matrix content showed the highest hardness values. The noticeable hardness difference can be observed from S11 to S01 as 8.13%, There was not that much difference in shore hardness values of composites with 60% resin content i.e., In comparison, specimens S21 to S01 and S22 to S02 difference of 1.57% and 2.62% was observed respectively. The improvement in this property is particularly due to the incorporation of resin to fiber.

Figure 8 depicts that the fiber content is more. The moisture content has been the highest. Specimen S01 has the highest moisture content with 31.39%. Whereas, the Specimens S21 and S22 has 18.75% and 2.04% moisture than S01 and S02 respectively. When the moisture is present in any composite it affects the mechanical properties due to degradation of the interfacial bond between the fiber and the matrix, so here the same happened with flax reinforced composites they achieved less mechanical properties when compared to paper reinforced composites.

According to this, the best alternative would be the one that is closest to the positive-ideal solution and farthest from the negative ideal solution. TOPSIS main aim is to selecting the top ranked alternative and comparing it with all ranks in this set of simulations. In Figure 9, all the composite materials are compared based on the TOPSIS method and ranking has been done. The Decision Matrix, Weight Normalized Decision Matrix, Ideal Positive and Ideal Negative Solution, Separation Measure, Relative Closeness Value and Ranking are tabulated in Tables 4-8.

In the above Table 4, all the properties that have been achieved were accumulated and named as Decision Matrix. To achieve the Weight Normalized Decision Matrix, the weight percentage is distributed equally among all the properties

i.e., w_j = 1/5 and x_ij refers to value of each property that has been achieved. The values w_j and x_ij are substituted in Formula (2) to get Table 5.

The positive ideal solution A⁺ (or) A* contains the largest numbers of the first, second and third column of A, and the smallest numbers of the fourth and fifth column of A. The negative ideal solution A⁻ (or) A^o contains the smallest numbers of the first, second and third column of A, and the greatest numbers of the fourth and fifth column of A (refer Table 6).

TOPSIS originally used Euclidean distances or Separation measure (Table 7) to compare alternatives with their PIS (Positive Ideal Solution) and NIS (Negative Ideal Solution), with the selected option having the smallest distance from the PIS and the greatest distance from the NIS. To achieve this, use Formula (3) and Formula (4). Here A_ij represents weight normalized decision matrix value of individual property (refer Table 5) and A⁺ (or) A, A⁻ (or) A^o has been presented in Table 6.

Relative Closeness values Table 8 can be achieved by substituting the positive and negative separation measure values in Formula (5). Based upon the relative closeness factor values, ranking has been done accordingly from highest to lowest.

The results can be summarized as follows:

· From the samples with different compositions, the specialty paper composite with epoxy resin has a very high Young’s modulus of 7980 MPa.

· The flexural modulus was more in specialty and newspaper reinforced epoxy composites with 6860 MPa and 5960 MPa respectively.

· The composites with newspaper and epoxy resin have higher impact strength with a value of 40.6 kJ/m² and flax composite has least impact strength with a value of 32.87 kJ/m²

· The specimens with high matrix content showed the optimal hardness values.

· The composites with flax and epoxy resin have the high moisture content with 14.5% and 13.98%.

· Based on TOPSIS, Ranking of the Composites are S11 (1^st rank), S12 (2^nd rank), S21 (3^rd rank), S22 (4^th rank), S01 (5^th rank), S02 (6^th rank).