Acrylic Polymers as Additives for Engine Oil: A Historical Perspective

Abstract

Oil undergoes temporary viscosity changes under operating conditions in engines. Therefore, engine oils usually contain polymeric additives called viscosity modifiers. These additives are oil soluble polymers; enable the oil to provide adequate hydrodynamic lubrication at high temperatures and good starting/pumping performance at low temperatures. Pour point depressants are additives which add to engine oil to lower/decrease the probability of wax argument formation under lower temperature conditions. The aim of this chapter is to present the historical synthesis of different types of acrylic polymers, there effect as lubricating oil additives (viscosity index improvers and pour point depressants). In addition, the mechanisms by which viscosity modifiers and pour point depressants work, and method of evaluation.

1. Introduction

Almost all commercial manufacturing and general-use equipment have mating surfaces that rub against each other, creating a lot of wear and tear. There is always resistance between the mating surfaces as a result of frictional reactions. As a result of wear and tear, material is removed from the top surface. Lubricants create a protective layer between mating surfaces, minimizing friction and, hence, wear and tear. Lubrication is a technique that is commonly used to maintain a protective layer between moving surfaces in order to reduce frictional effects and material degradation due to wear and tear [1, 2, 3, 4].

1.1 Lubricant uses

The main functions of the lubricant may be dominated as follow:

1. To reduce mating layer wear and tear by limiting direct surface-to-surface contact, particularly in metallic surfaces, by introducing a lubricating layer between mating layers [4, 5, 6].

2. To decrease frictional heat-induced material surface loss and metallic surface expansion.

3. To transport heat by acting as a heat dissipater or coolant [7, 8, 9, 10].

4. It adds to relative motion smoothness.

5. It lowers total maintenance expenditures.

6. It minimizes power losses in internal combustion engines [11, 12, 13], etc.

1.2 Lubrication methods

The concepts of lubrication can be elucidated using the mechanisms described below.

1.2.1 Use of thick films for lubrication

The moving/sliding faces are isolated with a thick layer of liquid, with the purpose of allowing for occasional top layer to layer contact. The lubricant layer fills the space at the irregularities of mating layers and creates a not-so-thin layer between them, preventing immediate mating among the top layers of the material in use. As a result, wear and tear is greatly reduced. The lubrication oil must be consistent during typical operation in machine parts, and it must also remain viscous enough to isolate the layers [14, 15, 16, 17].

1.2.2 Lubrication through the use of thin films

Maintaining a continuous layer of lubricant between the mating surfaces can be problematic in some instances. Then a process is used in which the region between layers sliding over one another is lubricated by an adsorbing substance that, according to its adsorption qualities, remains on the higher layers. This reduces friction between moving peak regions of mated surfaces. Adsorption can occur as a result of physical or chemical attributes [18, 19, 20, 21, 22].

1.2.3 Extreme pressure lubrication

When the moving/sliding surfaces are subjected to strong loading, a high temperature is reached. Under such conditions, liquid lubricating oils fail to adhere and may crumble or even evaporate. To solve these ridiculous circumstances, unique additives are added to mineral oils. These are referred to as exceptional weight included compounds. On metal surfaces, these extra chemicals form more intense films (capable of withstanding high loads and temperatures). The most fundamental contained compounds are regular mixtures containing dynamic radicals [23, 24, 25, 26].

1.3 Type of lubricants

Lube oils are generally classified depending on their condition, which is as follows;

1.3.1 Lubricants in liquid form or lubricating oils

There are three types of oils:

1.3.1.1 Animal and vegetable oils

They are derived from unrefined fatty oils and vegetable extracts. They are also called bio-lubricants [27, 28, 29, 30, 31, 32, 33, 34, 35]. They have a high degree of smoothness and, as a result, may stick to metallic surfaces for extended periods of time and under harsh conditions.

1.3.1.2 Mineral or petroleum oils

These are readily available and reasonably priced lubricants. They are relatively stable in regular operating settings and so commonly used. In general, bulkier chemicals are added to them to increase their oiliness. For example, oleic acid and stearic acid are added to improve oiliness [36, 37, 38, 39].

1.3.1.3 Blended oils

Because no individual oil provides all of the desired qualities, these are the most regularly utilized oils. These oils perform better and are often manufactured to order with the addition of larger molecular components [40, 41].

1.3.2 Semi-solid lubricants or grease

These are typically created by combining thickening additives with base oil. Grease can withstand heavy loads at low speeds but is a poor heat dissipater and is hence utilized in low temperature bearings [42, 43, 44, 45].

1.3.3 Lubricants in solid form

These are to be used when even grease cannot tolerate the temperature and pressure, when contamination must be avoided, when combustible lubricants are undesirable. They keep the lubricating film persistent even in conditions that grease cannot. To improve their adhesion to metallic layers, these are manufactured in powder form, dry form, and as coalescent [46, 47, 48, 49, 50, 51, 52, 53, 54].

1.4 The role and importance of engine oil

In addition to decreasing friction and wear, the engine lubricant is required to aid in sealing, cooling, protecting components against corrosion, cleaning surfaces of deposits, and transporting particles in suspension to the oil filter. All of this must be accomplished while fulfilling customer expectations for low costs and, in some circumstances, considerable intervals between oil changes.

The lubricant base stock, which accounts for 75 to 85 percent of the oil by volume, is a combination of hydrocarbons chosen to offer a starting point for viscosity and lubrication performance. The molecules that make up this base stock might be refined straight from crude oil or generated through chemical processing (synthetic lubricants).

In either scenario, the hydrocarbon compositions may be very similar, with the chemical makeup being more tightly controlled in synthetic oils. Depending on the hydrocarbon sources utilized to make the synthetic oil, it may also be free of the undesired sulfur and ash found in crude oils and which are costly to remove. The remaining lubricating oil is an additive package made up of a variety of chemical compounds chosen to give the anticipated lubrication performance [55].

1.5 Additives to engine oil

Engine oils are composed primarily of base oil, with the majority consisting of friction modifier additives used to increase performance. Typically, additives reduce wear, prevent oxidation, aid in dispersion, and add detergents to the base, which helps to keep the engine clean and improves viscosity index. Furthermore, the engine oil must maintain proper viscosity across a wide operating temperature range [56, 57, 58, 59, 60].

These features are carried by the additives added to base oil, which are necessary for the engine to run smoothly and efficiently. Oil is now produced with the stringent demands of today’s engines in mind. The amount of additives ranges from 5 to 30% from total engine oil, Figure 1.

Figure 1.

Main components of engine oil.

This, without a doubt, makes them tough and costly to create, but they can now be tailored to meet the application they are required to serve for engine safety and economical fuel efficiency. They can also be engineered to be exceedingly stable at both low and high temperatures. We will discuss viscosity index improvers and pour point depressants for lube oil.

1.5.1 Viscosity index improvers

The viscosity of hydrocarbon lubricants varies dramatically with temperature — roughly. Two orders of magnitude between cold ambient and full-load oil operating temperature for an unaltered mixture. Because of this reliance, ensuring adequate oil flow under all operating conditions and effectively balancing supply and leakage rates for acceptable lubrication of important surfaces is particularly difficult. Long-chain polymers called viscosity index improvers coil up at low temperatures and uncoil as the temperature rises [61, 62, 63].

As temperatures drop, the viscosity of ordinary petroleum oil increases, making it flow more slowly; conversely, as temperatures rise, the oil thins out and flows more freely, Figure 2. When there are large fluctuations in ambient temperature, it is often advantageous to use an oil whose viscosity remains as close to the ideal value as feasible despite the temperature fluctuation.

Figure 2.

Mechanism of viscosity index improvers.

The “viscosity index” (V.I) is the rate at which viscosity changes with temperature. A liquid’s viscosity is more consistent the greater its V.I. Depending on the source of the crude [paraffinic crude oils have the greatest natural viscosity index], lubricants extracted from crude oil by simple distillation can have a wide range of viscosity indexes. Viscosity index can be calculated according to ASTM D -2270-10 [64].

However, by adding a viscosity improver or viscosity modifier to an oil, the V.I can be increased. Long chain polymers with a very high molecular weight, such as polyisobutylene, polyacrylates or polymethacrylates, are commonly used as viscosity index improvers.

1.5.1.1 Mechanism of viscosity index improvement

Viscosity index improvers work by enhancing viscosity at high temperatures proportionally more than at low temperatures. Their behavior in oil shows that at higher temperatures, they distend or stretch, limiting flow and giving the oil more viscous qualities [62]. In doing so, they compensate to a significant extent for the oil’s tendency to thin out when heated. Increasing the molecular weight of a polymer increases its volume in an oil solution. Viscosity improvers respond to temperature in the same way that springs do. When cold, the molecules in the V.I improver contract and expand or thicken when heated. These changes in the physical properties of the V.I improver serve to adjust for variations in the basic oil stock. As a result, the oil’s temperature stability is improved throughout a wide temperature range [65].

1.5.2 Pour point depressants

All oils contain dissolved wax; however, the percentage of dissolved wax varies depending on the source of the oil. As the temperature dropped, the wax particles began to interlock like a sponge, attracting oil molecules into its microscopic pockets and so impeding oil transport [66, 67, 68].

Many approaches were employed to reduce the amount of wax in the generated oil, ranging from mechanical agitation to the inclusion of chemicals known as pour point depressants. The interaction mechanism of action of pour point depressants in oils is explained theoretically. Examples of these theories include adsorption, co-crystallization, nucleation, and improved wax solubility [69].

A good pour point depressant additive must have the following structural characteristics:

• It must have a polymeric structure,

• It must have both waxy and non-waxy components,

• It must have a comb structure

• It must have a balanced molecular weight distribution.

1.5.2.1 What is the mechanism of action of a pour point depressant?

Pour point depressants work by modifying the wax – crystal bond. The main question is how they work based on crystal size? This can happen through one of two ways:

1) adsorption onto the surface of newly formed crystals, or 2) co-crystallization with the precipitating wax [70, 71, 72, 73, 74, 75]. Pour point can be evaluated using ASTM D 97–17 [76].

1.5.3 Rheological characteristics

Polymer molecules are mostly hydrocarbon compounds. When dissolved in oil, they form a random coil. Under significant shear stress, the polymer molecules will separate into two or more polymeric particles. Polymers with higher molecular weights are more resistant to distortion and mechanical degradation, whereas polymers with sufficiently low molecular weight may not even undergo permanent shearing. Because the sheared polymer molecules have a sufficiently low molecular weight to be resistant to further breakdown, the degradation process is self-limiting [77, 78, 79].

1.6 Acrylic polymers

Acrylic polymers are “polymers based on acrylic acid, its homologues, and derivatives.” Acrylic acid, methacrylic acid are the most common commercial polymers in this class, as are acrylic acid esters, methacrylic acid esters, acrylonitrile, acrylamide, cyanoacrylates, and copolymers of these compounds. Styrene–acrylonitrile, acrylonitrile-butadiene-styrene terpolymers, as well as acrylonitrile-butadiene-butadiene terpolymers [80, 81, 82].

Acrylate polymers are easily polymerized (Homo-polymers, co-polymers and terpolymers) using free radical polymerization, where polymerization occur according to free radical polymerization; using initiator such as benzoyl peroxide, H₂O₂ … etc. the general mechanism of acrylate polymerization is illustrated at Figure 3.

Figure 3.

General mechanism of acrylate polymerization.

The incorporation of acrylic polymers with other green ingredients such as (jojoba oil [83, 84, 85, 86, 87], sunflower oil [88, 89, 90], castor oil [91] … etc.) has been reported. Acrylic polymers have been evaluated as motor oil additives (viscosity index improvers, pour point depressants, anti-wear, and anti-friction … .etc.) as tabulated at Table 1.

#	Components			ratio	Initiator	Solvent	Function with Lube Oil	Refs.
	Monomer1	Monomer2	Monomer3
1	Octylacrylate, dodecylacrylate, tetradecylacrylate, hexadecylacrylate	vinyl acetate	—	1:1	BPO	Toluene	VIIs; PPDs	[92, 93]
2	Octylacrylate, Dodecylacrylate, Tetradecylacrylate, Hexadecylacrylate	maleic anhydride	—	1:1	BPO	Toluene	VIIs; PPDs	[92, 93]
3	Octyl acrylate, dodecyl acrylate, tetradecylacrylate, hexadecylacrylate	1-octene, 1-dodecene, 1-tetradecene	—	1:1	BPO	Toluene	VIIs; PPDs	[93, 94]
4	Octylacrylate, decylacrylate, dodecylacrylate, tetradecylacrylate, hexadecylacrylate	styrene	vinyl pyrrolidone	Different ratios	BPO	Toluene	VIIs; PPDs	[95]
5	2-ethyl-hexyl methacrylate	vinyl acetate	—	1:1	BPO	Toluene	VIIs	[96]
6	octene, dodecene, tetradecene, and octadecene	Butyl acrylate	—	1:1	BPO	Toluene	VIIs; PPDs	[97]
7	dodecylacrylate, tetradecylacrylate, and hexadecylacrylate	jojoba oil	—	2:1	BPO	non	VIIs; PPDs	[83]
8	dodecylacrylate, tetradecylacrylate and hexadecylacrylate	Jojoba	vinyl acetate	1:1:1	BPO	non	VIIs; PPDs	[84]
9	dodecylacrylate, tetradecylacrylate, hexadecylacrylate	Jojoba	vinyl pyrrolidone	1:1:1	BPO	non	VIIs; PPDs	[85]
10	decylacrylate	Jojoba	dodecylacrylate	1:1:1	BPO	non	VIIs; PPDs	[86]
	decylacrylate	Jojoba	tetradecylacrylate
	decylacrylate	Jojoba	hexadecylacrylate
	dodecylacrylate	Jojoba	Tetradecylacrylate
	dodecylacrylate	Jojoba	Hexadecylacrylate
	tetradecylacrylate	Jojoba	hexadecylacrylate
11	dodecylacrylate, tetradecylacrylate, hexadecylacrylate	Jojoba	1-dodecene, 1-tetradecene, 1-hexadecene	1:1:1	BPO	non	VIIs; PPDs	[87]
12	dodecyl acrylate, and isodecyl acrylate	α- pinene	—	Different molar ratios	BPO	toluene	VIIs; PPDs	[98]
13	Alkyl acrylate	N,N-Dimethylacrylamide	—	1:1, 1:2 and 2:1	AIBN	toluene	VIIs; AW; AF; AC.	[99]
14	Sunflower oil	octylacrylate/decylacrylate/ dodecyl acrylate	styrene	1:1:1, 2:1:1, 3:1:1	AIBN	toluene	VIIs; PPDs; AW	[88]
15	Sunflower oil	Methylmethacrylate; /decylacrylate	styrene	different mole fraction	BPO	toluene	PPDs; VIIs	[89]
16	Maleic Anhydride	C10-C18 alkylacylate	—	1:1	BPO	Dry benzene	PPDs	[100]
15	Maleic Anhydride-	n-alkylacrylates	N-butylmaleimide	1:1:1	H2O2	benzene	PPDs; VIIs; AW; EP	[101]
16	Myristyl acrylate	—	—	—	BPO	Thermal: Toluene; Microwave: non	VIIs; PPDs	[102]
17	Isodecylacrylate and Isooctylacrylate	styrene	—	different mole fraction	BPO	toluene	VIIs; PPDs	[103]
18	different esters of acrylic acid	styrene	—	different mole fraction	BPO	xylene	PPDs	[104]
19	Dodecylacrylate	Hexadecylacrylate,	styrene	different mole fraction	BPO	acetone	VIIs; PPDs	[105]
20	n-decyl methacrylate, alkylacrylates C16-C20, alkylacrylates C18-C26,	N- (n-octyl) acrylamide, N- tert-nonyl acrylamide, N(tert-dodecyl) acrylamide	—	different mole fraction	AIBN	toluene	VIIs; PPDs	[106]
21	castor oil	dodecyl acrylate	—	different mole fraction	AIBN	Non	VIIs; PPDs; AW	[91]
22	Methyl Methacrylate	—	—	different mole fraction	BPO	toluene	VIIs; PPDs	[107]
	Methyl Methacrylate	Styrene
23	Dodecyl methacrylate	—	—	Different molar ratios	AIBN	toluene	PPDs; VIIs	[108]
	Dodecyl methacrylate	Vinyl acetate
24	n-alkyl acrylates	maleic anhydride	—	Different molar ratios	H2O2		PPD	[109]
25	methyl methacrylate and decyl acrylate	sunflower oil	—	Different molar ratios	BPO	non	VIIs	[90]

Table 1.

Application of acrylate polymers in petroleum sector as engine oil additives.

2. Conclusions

In this chapter we summarize the important uses of acrylate polymers, copolymers, and terpolymers and their uses as viscosity index improvers, pour point depressant, anti-wear, corrosion inhibitors, and rheology modifiers for engine oil. We can conclude the important of this category of polymers in the petroleum sector.

Acknowledgments

I would like to acknowledge this work to my Mother, and my Sons (Abd El-Rahman, Mostafa, and Faty).

Conflict of interest

I have no conflict of interest.

ASTM	American Society for Testing and Materials
VIIs	Viscosity index improvers
PPDs	Pour Point Depressants
AW	Anti - wear
AC	Anti- Corrosion