EXPERIMENTAL STUDY OF TEMPERATURE EFFECT AND CURING TIME ON THE SHEAR STRENGTH OF ADHESIVE JOINTS BY POLYVINYL PYRROLIDONE PVP K30

This research represents an experimental work to study the effect of temperature and curing time on the shear strength of adhesive joints by polyvinyl pyrrolidone PVP k30. In this work, shear strength of the adhesive made from dissolving 8 grams of polyvinyl pyrrolidone (k30) in 50 ml of ethanol alcohol high purity 99.9% was studied. This study focused on the effect of temperature and curing time under pressure of a fixed rate of 5 MPa. Therefore, the effect of five temperatures (200, 215, 225, 235 and 250) °C for different times (10, 20 and 30) minutes under the influence of the same pressure in all cases was studied by taking pictures of samples using Scanning Electron Microscope (SEM). It was inferred that the best resistance obtained at a temperature of 225 °C and time of 10 minutes, reaching overhead hanging to 1669 N, that there is an increase in the resistance of the adhesive with the increase in temperature and up to 225 °C, and then there is a decrease in the resistance and up to 250 °C at the time of 10 minutes.


INTRODUCTION
Successful bonding of parts requires an appropriate process. The adhesive must not only be applied to the surfaces of the adherents, but the bond should also be subjected to the proper temperature, pressure, and hold time (Ebnesajjad and Landrock, 2007). Starch-based adhesives were developed by hydrolyzing starch with PVA under alkaline and acidic conditions at various treatment temperatures. The results showed that the joint strength of starch adhesives reached a maximum value at 40 °C. When the treatment temperature was 55 °C, the crystalline of treated starch was the lowest and the thermal resistance also the weakest; it decreased by 10.1% and 13.6%, respectively compared with untreated starch. Obvious erosion could be observed from the SEM images of treated starch (Yu et al., 2015). The temperature affects on the behavior of two adhesive systems (acrylic and epoxy) and on the capacities of adhesively bonded lap shear joints. The temperature influence is quantitatively described for short-term loading over a service range of temperature from -20 °C to +40 °C. The quantitative description is done by proposing the partial factors and the conversion factors that take the temperature effect into account. This influence is also dealt with for long-term loading to describe the shear creep behavior of the adhesive materials used. Consequently, the time-to-failure of the bonded lap shear joints due to the creep phenomenon of the adhesives under three applied stresses at room temperature is predicted. Moreover, the estimation of time-to-failure is extended to be used for other shear stress levels. The temperature influence as well as the efficiency of using adhesivebonded joints in lightweight galvanized steel constructions is also illustrated by giving a practical example of strengthening cold-formed "C" section girders. Comparisons between the two adhesive systems for all cases are given (Sahllie, 2015).
The combined effect of the temperature and test speed on the tensile properties of a high temperature epoxy adhesive was investigated. Tensile tests were performed at three different test speeds and various temperatures (room temperature (RT) and high temperatures (100ºC, 125ºC and 150ºC)). The glass transition temperature (Tg) of the epoxy adhesive investigated is approximately 155ºC. The ultimate tensile stress linearly decreased with temperature (T) while logarithmically increased with the loading rate, which is in the accordance with the Airing's molecular activation model (Doina, 2011). The test of mechanical properties of as list of adhesives was conducted at different temperatures from room temp. to 100 K. This material was candidated to use in spaceflight hardware assembly in one of NASA programs, the material properties which measured were Young's modulus, Poisson's ratio, and the coefficient of thermal expansion. The aim of the work was to improve the shear strength of adhesive joints 56 Abbas A. Diwan et al. by using best value of temperature and heating time at certain pressure (Cease et al., 2006). The cohesive-zone model approach was used to study fracture mechanics in lap-shear joints, the authors investigated the limit of using linear-elastic fracture mechanics. Experimental and numerical study were done on a joint system of aluminum and commercial adhesive, using finite element solve with ABAQU5.8 code software. By employing data of the maximum traction-separation law, most researchers studied the adhesive joint fracture behavior by linear elastic fracture mechanics when the substrates remain elastic, and when the adherents deformed plastically, they gave a focus on transition for the zone between the elastic and plastic regions (Kafkalidis and Thouless, 2002). The motive behind the present work is to produce an adhesive material for metals and is characterized by strong high-cut and bearing high temperatures up to 250 o C as the wholesale production is cheap compared to other types of adhesives.

Materials
Polyvinyl pyrrolidone (PVP. K30) has linear formula [C6H9NO]n which a molecular weight M.W 40,000 was used in this work as glued material with a molecular weight 40000 gm.mol -1 .
It has viscosity 26 cps and was tested by using viscosity measurement device for 0.
Where : Cx is the concentration mol. L -1 , and nx is the number of moles. V = 50 ml volume of solvent , g = 8 gm. is the mass of PVP M.W = 40000 gm. mol -1 is the molecular weight.
After substitute the above quantities then Cx = 0.004 mol. L -1

Preparation of adhesive joint
The single shear lap joints of adherent were Copper specimens with dimension (25.4 mm * 1.6 mm * 90 mm) having a contact area 330 mm 2 of the adhasive joint that was prepared according to ASTM D1002 (ASTM, 2001), as shown in the Fig. 1. Kufa Journal of Engineering, Vol. 8, No. 2, 2017 57  The copper specimens were pressed under pressure 5 MPa and heating temperatures (200, 215, 225, 235, and 250) o C for various times (10, 20 and 30) minutes by using press heating system, as shown in the Fig. 2. Fig. 3 show the pressed copper adherents.
Pressure 5 MPa was selected depending on some references and conducting some laboratory tests. First, these tests were carried out at pressure less than 5MPa, but it was found that the adhesion of metallic specimens did not occur when adding adhesive material under this pressure at the same conditions, i.e. at temperature range 200 -250 o C. Consequently, other test were then performed at pressure higher than 5MPa, and it was found the adhesion took place without adding the adhesive material due to pressure increase at temperature range 200-250 o C.
The temperature range 200-250 o C was selected to be in consistent with the adhesive material properties. Where, the melting point of PVP is 172 o C according to the melting temperature test, while the decomposition temperature reach to 483 o C according to TGA test, so the required temperature is with in this range.
Curing times (10,20,30) minutes were selected to compare among them and since increasing the curing time in a test for more than 30 minutes caused burning of the adhesive material, formation of a black adhered region, the existence of cracks in the adhesive material structure and weakness in shear strength.

Shear test
The shear strength of the joint was measured by using a universal tensile testing machine by pulling apart the two plates with a rate of 1 mm/min -1 , as it can be seen on the scheme presented in Fig. 4.   been plotted with weight loss as a function of the temperature for the reference PVP precursor with a heating rate of 10 °C/min in the temperature range from 40 to 800 °C. It is clear that the initial weight loss from the TGA curve is 12% in the temperature range 40-87.9 °C, this is due to loss of OH content. In the Differential Thermal Analysis (DTA) curve, two exothermic peaks were observed at 483.92 °C and 678.08 °C, respectively. The sharp and strong exothermic peak at 483.92°C is due to the combined effect of combustion of organic residuals and the decomposition of PVP and which is well above the heating temperature employed in the present work .These results approximate to which in (Sivaiah et al., 2010).        Kufa Journal of Engineering, Vol. 8, No. 2, 2017 63 This     Table 1 and