Degreasing Efficiency of Electroplating Pretreatment Process Using Secondary Alcohol Ethoxylate as Nonionic Surfactant

Abstract

In this study, the effect of the hydrophilic–lipophilic balance (HLB) number and cloud point (CP) of a secondary-alcohol ethoxylated nonionic surfactant on degreasing efficiency was investigated. A degreasing process was conducted for steel samples with different surfactants in a degreasing solution. The HLB number and CP increased with the increasing n of the hydrophilic ethylene oxide (OCH₂CH₂)_n group. For a constant temperature of the degreasing solution (30–80 °C), the degreasing efficiency was investigated as a function of degreasing time. The highest degreasing efficiency was observed near the cloud point of the surfactant, and the degreasing efficiency decreased significantly at temperatures lower and greater than the cloud point. A Hogaboom test was carried out to observe oil stains on the surface of samples. Additionally, the contact angle of the surface with water droplets was measured after degreasing with various surfactants.

1. Introduction

Electroplating is defined as the deposition of a thin layer of metal alloy onto the surface of metallic or non-metallic materials to improve their surface properties. It consists of pre-treatment, plating and surface treatment, and post-treatment processes. Degreasing is a crucial part of the pre-treatment whereby organic substances such as rust preventive rolling and cutting oils are applied to the surface of a metallic sample; hence, inorganic substances such as iron powder, abrasives, and dust are removed. Degreasing agents are classified as alkaline, organic solvents, or emulsion-based solvents, depending on the composition of the degreasing agent. The degreasing of a metallic surface is used for saponifying, emulsifying, penetrating, dispersing, and mechanical peeling. In the alkaline degreasing process, the saponification reaction occurs when heating animal or vegetable oils and fats with an aqueous alkali solution, resulting in a sodium salt from the fatty acid and glycerin. The soap produced in this reaction has emulsifying and dispersing power as a surfactant, and secondary emulsification and dispersion is provided by the aqueous alkali solution [1,2,3].

Alkaline degreasing is a cleaning process widely used across the industry before chemical conversion coating or plating. A variety of characteristics such as low surface tension, chemical and heat stability, low corrosiveness, high electrical conductivity, emulsification, non-toxicity, and usability in low concentrations are synergistically demonstrated by mixing various chemical substances, including a surfactant and alkali builder, in certain proportions.

The surfactant may increase the solubility of substances that are insoluble in aqueous solutions by forming micelles above a certain concentration; this phenomenon is known as solubilization. Research has been actively conducted to enhance the solubilization of substances insoluble in aqueous solutions. The solubilization by surfactant micelles has been widely applied in pharmaceutical, paint, cosmetics, and food industries. The location of solubilization is determined by the molecular structure of the solubilizer and solubilizate, and the hydrophilic–lipophilic balance (HLB) number. The factors influencing solubilization include chemical structure effects of the surfactant, such as the alkyl group length, functional group type and position, ionic effect, HLB number, solubilizate effects, concentration, temperature, and additives [4,5,6].

Atlas, a surfactant manufacturer in the 1950s, first used the HLB as an indicator to represent the properties of nonionic surfactants. Based on the work conducted by Atlas, Griffin proposed an equation to express the HLB number as a function of the weight fraction of a hydrophilic group of a nonionic surfactant (Equation (1)) and claimed that the optimal condition for oil emulsification corresponds to the weight fraction of a hydrophilic group in the emulsifying agent molecule. Several methods have since been proposed to calculate the HLB from the chemical structure of an emulsifying agent [7,8,9]; however, the equations are all based on the proportion of hydrophilic groups that constitute the molecules of an emulsifying agent as below.

$$HLB number=20 \left(1-\frac{M_0}{M}\right)$$

(1)

where M₀ is the molecular weight of the hydrophobic portion and M is the total molecular weight [10]. The aqueous solubility of the emulsifying agent increases with the increasing HLB, which increases the critical micelle concentration (CMC) value, i.e., the number of dissolved surfactants outside micelles increases as well. Conversely, a decrease in the HLB number induces the opposite effect. Since the introduction of the concept by Griffin, the HLB has been widely used to select emulsifying agents. However, the strong influence of temperature must be considered [10,11,12,13,14,15,16].

One other critical factor affecting degreasing efficiency is cloud point, which refers to the temperature transforming an isotropic micellar state into a two-phase state of surfactant solution. The cloud point of an alcohol ethoxylate is influenced by HLB, the number of ethylene oxide, and hydration state [17].

Many studies of the HLB of surfactants have been actively conducted in the pharmaceutical, paint, cosmetic, and food industries, but no research on HLB is sufficient in the electroplating field, especially for the pre-treatment process. Meanwhile, to the best of our knowledge, there has been no study focused on the influence of cloud point on degreasing efficiency. In this study, the effect of the HLB number and cloud point (CP) of secondary alcohol ethoxylate nonionic surfactant (Equation (2), where R, R1, R2, R3, and R represent the alkyl group of RCOOR′) on its degreasing performance was evaluated. A typical degreasing process for steel was applied before electroplating, and the degreasing efficiency was evaluated for various surfactants and degreasing temperatures (30–80 °C).

(2)

2. Materials and Methods

2.1. Materials

The raw metal specimen (100 mm × 65 mm × 0.3 (t) mm hull cell cathode) was immersed in a 500 mL beaker containing commercial degreasing solution. The beaker was placed in an ultrasonic cleaner filled up with water for 5 min at 60 °C. Subsequently, the specimen was rinsed with deionized water and dried. Degreasing was performed by immersing the steel specimen, which had been lubricated with hydraulic oil and then dried, into 1 L of the manufactured degreasing solution for 1–5 min at 30–80 °C [10]. Wear-resistant hydraulic oil (GS Caltex-Kixx RD HD 46) was applied uniformly onto the dry steel specimen and then dried before use. For the experimental degreasing solution, NaOH (20 g/L), Na₂O·SiO₂-9H₂O (0.5 g/L), secondary alcohol ethoxylate (SE) nonionic surfactants with 3, 7, 9, 12, or 20 moles of ethylene oxide (EO) (0.5 mL/L), and another commercial stabilizing agent (0.5 g/L) were mixed in deionized water. The basic physical properties of the SE surfactants used in the experiments are presented in

Table 1. Physical properties of the studied secondary alcohol ethoxylate nonionic surfactants.

Surfactant	HLB Number	Moles EO	CMC (ppm at 25 °C)	Cloud Point (of 1 wt%) (°C)	Pour Point (°C)
SE-3	8	3	Insoluble	Insoluble	−46
SE-7	12.1	7	38	37	1
SE-9	13.3	9	52	60	9
SE-12	14.5	12	107	89	−8
SE-20	16.3	20	315	>100	6

.

2.2. Degreasing Efficiency

Degreasing efficiency is commonly used for evaluating the degreasing performance and was calculated here using Equation (3).

$$\mathrm{Degreasing} \mathrm{efficiency}=\frac{\mathrm{Amount} \mathrm{of} \mathrm{degreased} \mathrm{oil} \left(\mathrm{g}\right)}{\mathrm{Amount} \mathrm{of} \mathrm{deposited} \mathrm{oil}}\times 100\%$$

(3)

The specimen weight was measured using an electronic scale (Shimadzu, AUX220, Kyoto, Japan). The weight of the degreased hydraulic oil was calculated by measuring the difference between the weight of the dried specimen before and after degreasing (i.e., the difference between the weight of the specimen that has been cleaned with ultrasonic waves and was then dried, and the weight of the specimen that had hydraulic oil applied and was then dried). The degreasing solution was maintained at a constant temperature using a digital constant-temperature bath (LKLAB Korea, LB-WD522, Namyangju-si, Gyenggi-do, Korea).

2.3. Hogaboom Test after Degreasing

The Hogaboom test is conducted by plating companies where an intermediate inspection is performed after pre-processing, but before painting or plating work. This step is performed to exclude nonconforming items from being painted or plated, thus preventing defects in advance. Pre-processed products are immersed in a copper sulfate plating solution for 15 s and determined to be acceptable if the product appearance has a uniform copper color and unacceptable if the color is nonuniform (spots) or if plating was incomplete.

To prepare the plating solution used in the Hogaboom test, 30 g/L of copper sulfate (CuSO₄·5H₂O) was completely dissolved in 200 mL/L of pure water, and then 30 mL/L of sulfuric acid (H₂SO₄; specific gravity of 1.84) was added; the obtained liquid was diluted with pure water. A 5 L batch of the copper sulfate was produced, and 500 mL was used at a time. After degreasing, the steel specimen was immersed in the copper sulfate plating solution for 15 s and then cleaned and dried to observe its appearance. To prevent the copper sulfate plating solution from being contaminated by any remaining hydraulic oil on the steel specimen that had not been removed by degreasing, the plating solution was used only once for each steel specimen.

2.4. Contact Angle Measurements

A contact angle goniometer (Rame-Hart, Model 250, Succasunna, NJ, USA) was used to measure the contact angle of the steel specimen after degreasing to compare the degreasing performance of the different surfactant solutions and degreasing conditions. After degreasing for 1 min in a degreasing solution with a temperature of 40 °C or 80 °C, the contact angle of each steel specimen was measured at the same location using the same amount of deionized water.

2.5. Foaming Power Test

To test the foaming power of each SE surfactant degreasing solution, 60 mL of each degreasing solution was placed in a 100 mL measuring cylinder. The foam was generated after shaking up and down 10 times. The height of the foam was measured immediately after, 30 s after, and 60 s after shaking.

3. Results

3.1. Degreasing Efficiency over Time

Increasing the number of moles of hydrophilic EO in the SE nonionic surfactants increases the HLB number. The effects of changing the number of moles of EO on the degreasing efficiency of the steel specimens were examined for a constant surfactant content and temperature. Using the variation of samples before and after degreasing, the degreasing efficiency was calculated using Equation (3) as mentioned in Section 2.2. Figure 1 shows the degreasing efficiency over time for each type of SE surfactant in the degreasing solution at temperatures of 30 °C and 40 °C. Apparently, the degreasing efficiency increased along with the extension of duration. At 30 °C, with a degreasing time of 2 min, the degreasing efficiency of SE-9 and -7 were 97.9% and 91.6%, respectively, which are higher than those of SE-12 and SE-20 (90.6% and 87.9%, respectively), while SE-3 had the lowest degreasing efficiency of 73.3%. This trend in degreasing efficiency was observed for all measured times (1–5 min). At 40 °C and a degreasing time of 2 min, the degreasing efficiency of SE-9 and -7 were 99.1% and 98.3%, respectively, which is higher than the efficiencies of SE-12 and -20 (97.5% and 90.3%, respectively), while the degreasing efficiency of SE-3 was the lowest at 85.0%. Therefore, the degreasing efficiency decreased in the order of SE-9, SE-7, SE-12, SE-20, and SE-3, at both 30 °C and 40 °C.

Figure 2 shows the degreasing efficiency over time for each type of SE surfactant at degreasing solution temperatures of 50 °C and 60 °C. Overall, when the temperature increased over 50 °C, the degreasing efficiency significantly increased for all surfactants and reached maximum after 2 min. At 50 °C and a degreasing time of 1 min, the degreasing efficiency of SE-9 was 97.1%, which was higher than those of SE-12 and SE-20 (95.9% and 93.3%, respectively); the degreasing efficiency of SE-7 was 90.3%, which was lower than that of SE-12 and SE-20, unlike at lower temperatures, while the degreasing efficiency of SE-3 was still the lowest at 78.0%. This order was maintained for all measured degreasing times (1–5 min). At 60 °C and a degreasing time of 1 min, the degreasing efficiency of SE-12 and SE-20 were 98.4% and 97.2%, respectively, which are higher than the efficiencies of SE-9 and SE-7 (95.9% and 94.4%, respectively), while the degreasing efficiency of SE-3 was the lowest at 87.7%. This order was maintained for all degreasing times.

The reason for the differences in the degreasing efficiency between the degreasing solution temperatures was related to the CP. A nonionic surfactant becomes soluble when hydration occurs at the oxygen position of ether groups bonded to the EO chain, which is a hydrophilic group. The number of ether bonds increases as the length of the EO chain increases; therefore, the solubility in water improves as the degree of hydration increases. Precipitation occurs at the Krafft point related to the hydrophobic part of a surfactant molecule, and at the cloud point system which separates into a concentrated and dilute surfactant phase. The temperature at which emulsification occurs is referred to CP, where the CP of nonionic surfactants is higher because the percentage of the hydrophilic part of the CP is higher [18,19,20,21]. The degreasing efficiency decreases at temperatures above the CP as the solubility of the surfactants in water is reduced, while it is optimal when temperatures approach the CP. Therefore, temperatures of 50 °C and 60 °C yielded higher degreasing efficiency than temperatures of 30 °C and 40 °C, as these temperatures were closer to the CP of most surfactants, except SE-3.

Figure 3 shows the degreasing efficiency over time for each type of SE surfactant at degreasing solution temperatures of 70 °C and 80 °C. It could be observed that the degreasing efficiency was reduced for SE-3, -7, -9, and -12 as the temperature was above the CP of these surfactants. Moreover, the degreasing efficiency could achieve a maximum value at 1 min due to the activation at high temperatures. At 70 °C, with a degreasing time of 1 min, the degreasing efficiency of SE-12 and SE-20 reached 100%, while that of SE-9 and SE-7 were 96.5% and 95.1%, respectively, and SE-3 had the lowest degreasing efficiency of 91.9%. This order was observed for all degreasing times. At 80 °C and a degreasing time of 1 min, the degreasing efficiencies of SE-20 and SE-12 were 100% and 97.6%, respectively, while those of SE-9 and SE-7 were 95.1% and 93.7%, respectively, and SE-3 had the lowest value of 88.2%. This order was maintained for all measured degreasing times.

3.2. Hogaboom Results after Degreasing

The Hogaboom test was conducted at 40 °C and 80 °C, which are representatives of low and high temperature ranges. Figure 4 presents images of the specimens degreased with the various degreasing solutions (1–5 min at 40 °C) before and after the Hogaboom test. The areas marked in yellow were not well degreased, resulting in poor plating after the Hogaboom test (uneven colored stain). For SE-3, degreasing was still incomplete after 5 min of degreasing, which caused hydraulic oil to be left on the specimen; furthermore, spots and incomplete plating were observed on the specimen after the Hogaboom test. For SE-7 and SE-9, hydraulic oil was completely removed after 4 min of degreasing which created a clean specimen surface, and the specimen was uniformly copper colored after the Hogaboom test. For SE-12, the specimen had a clean surface after 5 min of degreasing and a uniform copper color was observed after the Hogaboom test. Although most of the oil weight was removed from the surface relating to SE-20 after 5 min (Figure 1), an oil stain was observed on the surface of the sample after the Hogaboom test. This could be explained by the fact that the temperature of 40 °C was lower than the CP of SE-20, and hence, an oil stain still remained on the surface. Figure 5 presents images of the specimens degreased with the various degreasing solutions (1–5 min at 80 °C) before and after the Hogaboom test. For SE-3, SE-7, and SE-9, degreasing was still incomplete after 5 min of degreasing, which caused hydraulic oil to be left on the specimen. Furthermore, spots and incomplete plating were observed on the specimens after the Hogaboom test. For SE-20 and SE-12, clean surfaces were observed after 2 and 3 min of degreasing, respectively, resulting in uniform copper-color surfaces after the Hogaboom test. As 80 °C was above the CP of SE-7 and SE-9, the oil stain could not completely eliminate from the surface. Meanwhile, 80 °C was approaching the CP of SE-12 and SE-20, and thus, clean surfaces without oil stains were obtained.

3.3. Contact Angle

The contact angle, which is measured using a standard droplet, refers to the angle formed when liquid and vapor are in a thermodynamic equilibrium on a solid surface, and it represents the wetting ability of a specific liquid on a specific solid surface. Deionized water is most commonly used for measuring the contact angle of a solid surface; a contact angle greater than 90° indicates a hydrophobic surface [22], whereas a contact angle below 90° indicates a hydrophilic surface. Figure 6 shows the measured water contact angles of the metal sheets that were degreased for 1 min with various degreasing solutions at 40 °C. Since the shape of the water droplets was horizontally asymmetrical, the average of the left and right contact angles was calculated. All degreased surfaces were slightly hydrophilic, where better degreasing of the hydraulic oil from the steel specimen was observed as a lower contact angle. In terms of the contact angle, the hydrophilic property of the surface decreased in the following order: SE-9, SE-7, SE-12, SE-20, and SE-3. As 40 °C was close to the CP of SE-7 and SE-9, the surfaces involving SE-7 and SE-9 showed better the hydrophilic property than other ones. Therefore, the degreasing efficiency at 40 °C decreased as follows: SE-9, SE-7, SE-12, SE-20, and SE-3.

Figure 7 shows similar contact angle images for the samples degreased at 80 °C. As for the specimens degreased for 1 min at 80 °C, these surfaces were hydrophilic, and the contact angle increased (hydrophilic property of the surface decreased) in the following order: SE-20, SE-12, SE-9, SE-7, and SE-3. The surfaces involved SE-12 and SE-20 showed better the hydrophilic property than other ones, because 80 °C was near the CP of these surfactants. Therefore, the degreasing efficiency at 80 °C decreased as follows: SE-20, SE-12, SE-9, SE-7, and SE-3.

3.4. Foaming Power

The foam generated when using a degreasing solution with a high concentration of surfactant has the advantage of adsorbing and transporting grease and debris. However, the generation of excessive foam can make it difficult to manage the plating bath, and foam exiting the bath with the product after degreasing can affect subsequent processes. Therefore, it is important for the degreasing solution to maintain a low foaming power. Figure 8 shows the foaming height up to 60 s of settling time after shaking up and down. Solutions SE-3, -7, -9, -20, and -12 had foam heights of 4, 39, 44, 55, and 65 mm, respectively, immediately after stirring, and then decreased to 4, 33, 39, 43, and 55 mm, respectively, after 30 s, and to 4, 30, 37, 41, and 52 mm, respectively, after 60 s. Excluding SE-3, which had almost no foam since it was an insoluble surfactant, the tendency to foam increased in the order: SE-7, SE-9, SE-20, and SE-12.

4. Conclusions

In this study, the effects of the HLB number and cloud point (CP) of SE nonionic surfactants on degreasing efficiency was explored. For the same concentration of surfactant, the hydrophilicity of the solution increased with the increasing number of hydrophilic EO groups as the HLB number increased. The degreasing efficiency of the solutions was temperature-dependent because of the different CPs of surfactants. The Hogaboom test confirmed that SE-9 and SE-7 exhibited cleaner surfaces at 40 °C, and SE-20 and SE-12 showed less oil stains at 80 °C, than other ones. Based on the contact angle of the surface with water droplets, it could be concluded that the hydrophilic property decreased in the order: SE-9, SE-7, SE-12, SE-20, and SE-3; at 80 °C, it decreased in the order: SE-20, SE-12, SE-9, SE-7, and SE-3. Excluding SE-3, which is an insoluble surfactant that generates almost no foam, the tendency to foam increased in the order of: SE-7, SE-9, SE-20, and SE-12.