F. پاکسازی توسط حلال و چربیزدایی توسط بخار

حلالهای آلی مورد استفاده در پاکسازی سطح عموما از نوع هیدروکربنهای کلرینه شده غیر اشتعال زا از قبیل: تری کلرو اتیلن (تری کلر)، پری کلرو اتیلن (پرکلر) و 1،1،1تری کلرواتان (اتیلن کلروفرم) می باشند. اکثر حلال های آلی به علت سمیت، اشتعال پذیری، اثر بخشی کافی بعنوان محلول پاک کننده اولتراسونیک یا از نظر سهولت بازیابی غیر قابل قبول می باشند. خواص مورد نیاز یک حلال آلی جهت استفاده بعوان پاک کننده عبارتند از :

1 پاک کننده موثر برای انواع آلودگیهای آلی.

2. غیر خورنده باشد.

3 اشتعال ناپذیر باشد.

4. سمیت آن پایین باشد.

5. بوی ملایم یا متوسط داشته باشد.

6. در دمای محیط خشک شود.

7. سطح را بدون تشکیل پیوند با آن ترک کند.

8. .سطح را بدون برجای گذاشتن هیچ نوع ماده مخربی برای اوزن (ODP)، ترکیبات هالوژنه شده، آب، کتونها، ترکیبات آروماتیک یا هر یک از 189 ترکیب لیست شده توسط سازمان هوای پاک ایالات متحده آمریکا بعنوان  آلاینده های خطرناک هوا  

SOLVENT CLEANING & VAPOR DEGREASING

Th eorganic solvents used for cleaning applications are the non-flammable chlorinated hydrocarbons: trichloroethylene (Trichlor), perchloroethylene (Perchlor) and 1,1,1 trichloroethane (Ethyl chloroform) 54-56.Most organic solvents are considered unacceptable for reasons of toxicity, flammability, effectiveness as ultrasonic mediums, or ease of recovery. required solvent properties as follows:

1. Effective cleaner for variety of organic soils.

2.Noncorrosive.

3. Nonflammable.

4.Low toxicity.

5.Mild or moderate odor.

6.Dries on ambient temperature.

7.Leaves no bondable surface.

8.Leaves no compounds with ODP, halogenated compounds, water,

ketones, aromatics or any of the 189 compounds listed by the US Clean

Air Act as Hazardous Air Pollutant.

Unlike aqueous cleaners, the solvent cleaners function by dissolving soil: oil, grease, wax, asphaltic materials, resins and gums.  They are used for hard surface cleaning of all metals, glass and certain plastics. Since these solvents are also relatively toxic and possess rapid evaporation rates when they are used in vapor degreasing equipment, the inhibited or stabilized forms are used. This is because these may be recovered by distillation and automatic control of these factors must be exercised.

These solvents may be used however, in properly designed degreasing tanks, as a last resort either for temperature sensitive applications or for those applications where particulate matter is the major contaminant. In the event that removal of solvent soluble residue is also involved, the contaminated solvent may be reclaimed by distillation in an auxiliary still.  Because of its low toxicity in comparison to Trichlor or Perchlor, ethyl chloroform (H₃C-CCl₃) is commonly selected for solvent cleaning in non-vapor degreasing applications.

Although one must neither recognize the chemical activity of a metal in the selection of a cleaning procedure, this factor is virtually non-existent in solvent vapor degreasing, since neither etching nor staining of the part will be experienced. Thus, when other factors are compatible to this method of cleaning, solvent vapor degreasing is a natural choice in cleaning a variety of materials. Neither the shape nor the fragility of a part is considering being a governing factor in vapor degreasing since the parts may be cleaned thoroughly without being subjected to a mechanical action. This method will realize good penetration even in the deepest recess. Where the soil is heavy or the part is so light as to limit the amount of flushing by vapors, immersion and/or spraying with liquid solvent can be performed within a degreasing unit prior to the vapor rinse. Since vapor degreasing may be performed on an automated basis, it is frequently preferred when high productivity is involved.

The operation that is to follow the cleaning step is also a selection factor. Thus, vapor degreasing is ideally suited to the cleaning of work prior to inspection or subsequent processing where the work is best handled dry. Where the work is to be wetted in the next operation, i.e., part will be plated, vapor degreasing is frequently used to remove soil economically and to minimize contamination of subsequent treating solutions. The normal cleaning sequence used in vapor degreasing consists of vapor clean, plus vapor rinse and dry. The initial vapor cleaning treatment is conducted in the vapor zone over the boiling sump. It offers the advantage of elevated temperature melting and dissolving actions plus the beneficial concentration of gross contaminants in one section of the degreaser.

Cleaning is next conducted in continuously distilled solvent for the final removal of greases, oils and particulate matter. Removal of particulate matter from the solvent is effected by either intermittent or continuous filtration. The temperature in the sump is maintained thermally well below the boiling temperature of the solvent to effect a second condensation of pure solvent on the work piece upon its withdrawal from the degreaser. For this treatment, the part is suspended in the hot vapor until it attains the vapor temperature, thus emerging from the degreaser as a clean dry part. Very slow exit speed, 3 ft/min, must be used to prevent vapor from escaping into air, and to ensure that no liquid solvent remains on the part.

The selection of a vapor degreasing solvent is made based on the physical properties of the soil, the solvent and the substrates involved. For example, Perchlor is specifically used where high temperature solvency is required as in the removal of high melting point waxes.  Trichlor on the other hand is the more popular of the two solvents because it offers greater solvency at a lower operating temperature; hence, a lower operating cost. For certain temperature sensitive operations, the use of ethyl chloroform may be justified because of its lower boiling temperature (74⁰C/165^oF).

Where selective solvency is a requirement, as in the cleaning of assembled devices containing plastic or painted components, the use of trichloro-trifluoroethane is recommended. Its low boiling point (48 ⁰C/118 ^oF), low Kauri Butanol value (31)( The Kauri-butanol value ("Kb value") is an international, standardized measure of solvent power for a hydrocarbon solvent, and is governed by an ASTM standardized test, ASTM D1133. The result of this test is a scaleless index, usually referred to as the "Kb value".) low surface tension (19.6 dynes/cm) and low toxicity (1000ppm) combine to fulfill a need not met by the chlorinated solvents. Regardless of the solvent selected, none will remove water-soluble contaminants. These must be removed in a solvent emulsion or in an aqueous cleaning treatment before or after vapor degreasing.

The popularity of vapor degreasing originates from the rapid solvent cleaning action achieved and the fact that subsequent drying operations are eliminated. The latter is particularly important for intricately shaped parts where the presence of residual water cannot be tolerated. Vapor degreasing by itself does not usually insure that the part is sufficiently clean for the final electroplating step.

حلال­های امولسیون شونده و پاک کننده­های امولسیونی

به نظر می رسد در مقایسه با سایر روشهای پاکسازی همچون پاکسازی توسط حلال، چربی زدایی با بخار و فرایندهای پاکسازی با محلول های قلیایی تمایل کمتری به روش پاکسازی توسط پاک کننده های امولسیونی وجود دارد. این مسئله ممکن است ناشی از فقدان آگاهی از ویژگی های این طبقه از مواد پاک کننده و مزایای به دست آمده از آنها باشد، به ویژه هنگامی که از آنها بعنوان یک پیش تمیز کننده در حذف آلودگیهای سخت پاک شونده استفاده می شود.

EMULSIFIABLE SOLVENTS & EMULSION CLEANERS^57,58

There appears to be some reluctance toward the use of emulsion-type cleaners, as compared to the more widely accepted methods of solvent cleaning, vapor degreasing, and alkaline cleaning procedures. This may be due to a lack of knowledge of the characteristics displayed by this class of cleaning materials and the advantages gained, particularly when it is used as a pre-cleaner in the removal of stubborn contaminants.

There are two distinct types of emulsion cleaners. One is the stable emulsion that is a single-phase solution type, whereas, the other is termed a diphase emulsion cleaner. The latter type, as its name implies, is a two phase solution type in which neither phase is completely or permanently emulsified in the other. Within the class of stable emulsion cleaners, if we are to be technically correct, a further division may be made in that both, emulsifiable solvent and emulsion cleaners are available. Both of these stable emulsion types are frequently considered synonymous; however, there is a difference, even though it is very slight. Basically, both are of the same formulation, but an emulsion cleaner is one that is diluted with water, whereas, the emulsifiable solvent cleaner is used without the addition of water. In general, the emulsifiable solvent and the emulsion cleaner will perform in a similar manner. However, the effectiveness of the cleaner is reduced in proportion to the water content of the mixture.

i.Emulsion cleaners contain water, an emulsifier and a petroleum hydrocarbon, typically kerosene. The commercial products contain the solvent and emulsifier who is added to water in varying proportions prior to use. Emulsifier is usually a higher alcohol or wetting agent. Additional effectiveness may be obtained by addition of surface active agent. The normal dilution is one part concentrate to ten parts water. When used as an ultrasonic medium, the tap water makeup is first thoroughly degassed. The concentrate is then added to effect the desired dilution. The emulsion cleaners are used at an elevated temperature of 50-70 ^oC (~120-160 ^oF) and are preferably followed by a hot water rinse. These cleaners function well as ultrasonic mediums yielding bath uniformity difficult to achieve in either aqueous or solvent media alone.

Emulsion cleaners are formulated to combine the functions of both aqueous cleaners and solvent cleaners to yield a cleaner that possesses both detergency and solvency powers. For this reason, they are very well suited for the removal of gross contaminants that are otherwise difficult to remove in a single cleaning treatment. The cleaner residue from an emulsion cleaner is a thin oily residue that offers temporary rust protection to the parts after this cleaning operation. The presence of this oily residue is a distinct advantage for in-progress (intermediate) cleaning, but its presence serves to prohibit the use of emulsion cleaners for final cleaning operations.

ii. In the emulsifiable solvent cleaning, unlike with the emulsion cleaners, no water is involved. Water does enter into the cleaning operation. However, it is used in the subsequent pressure rinsing operation, which serves to emulsify and remove both the loosened soil and the emulsifiable solvent. These effects are made possible by virtue of the emulsifiable solvent formulation.

The commercial emulsifiable solvents may be used in the undiluted condition or diluted with the appropriate type of hydrocarbon solvent prior to use. Since the resultant solvent assumes the characteristics of the solvent base, both flammability and/or toxicity must be considered. Those based on petroleum hydrocarbons must be treated with caution when used as ultrasonic cleaners, as the flash points of the emulsifiable solvent are being the limiting factor.

The use of emulsifiable solvents based on the non-flammable chlorinated solvents is more acceptable for ultrasonic applications, since these may be thermally degassed prior to use as an ultrasonic medium without the danger of fire. The same considerations in respect to toxicity apply to the emulsifiable solvents as for the chlorinated hydrocarbon solvents.

When cleaning prior to plating, alkaline cleaning should always be used after emulsion cleaning to remove thin oil film left by the emulsion.

54. P.M. Peck, Met. Finish., 96(4)36(1998).

55. J.B.  Durkee, Prod. Finish., June 1984, pp.58 .

56. P.D. Skelly: Cold and Heated Solvent Cleaning Systems, in: “Handbook of Precision Cleaning”, B.

Kanegsberg ed., CRS Press, Boca Raton, FL (2001), pp.265-272.

57. Emulsion Cleaning  in : “Surface Engineering”, Vol.5, pp.49-54, ASTM International, Material Park,

Oh,(1994).

58. D. Myers, “Surfactant Science and Technology”, Chapter 6 (Emulsions), VCH Publishers, NY (1988).

تهیه شده در واحد پژوهش و گسترش جلاپردازان پرشیا (JP)

آذر 96

Prepared by research and development unit of jalapardazan (JP)

December 2017