Dispersant (meth) acrylate copolymers having excellent low temperature properties

Polyalkyl (meth)acrylate copolymers comprising from about 12 to about 18 weight percent methyl methacrylate; from about 75 to about 85 weight percent of a C.sub.10 -C.sub.15 alkyl (meth)acrylate; and from about 2 to about 5 weight percent of a nitrogen-containing dispersant monomer provide excellent low temperature properties to lubricating oils.

TECHNICAL FIELD
 This invention relates to novel dispersant (meth)acrylate copolymers having
 excellent low temperature properties in a wide variety of base oils. The
 present invention also relates to the use of these copolymers as viscosity
 index improvers for lubricating oils.
 BACKGROUND OF THE INVENTION
 Polymethacrylate viscosity index improvers (PMA VII's) are well known in
 the lubricating industry. Many attempts have been made to produce PMA
 VII's that have the desired balance of high temperature and low
 temperature viscometrics, as well as the required shear stability for a
 given application. Obtaining suitable low temperature performance has
 become even more difficult recently with the movement away from API Group
 I base oils and the increased utilization of Group II and Group III base
 oils. Further, refiners who blend with different base oils desire a single
 product which performs effectively in all of these different base oils.
 The present invention is directed to novel dispersant (meth) acrylate
 copolymers which exhibit excellent low temperature performance in a wide
 variety of base oils.
 U.S. Pat. No. 5,112,509 teaches a method for making a methyl
 methacrylate-lauryl methacrylate copolymer. The '509 patent does not teach
 the copolymers of the present invention, which contain a dispersant
 monomer.
 SUMMARY OF THE INVENTION
 The present invention is directed to novel dispersant poly (meth)acrylates
 and their use as viscosity index improvers for lubricating oils.
 The polyalkyl (meth)acrylate copolymers of the present invention comprise
 units derived from:
 (A) about 12 to about 18 weight percent methyl methacrylate;
 (B) about 75 to about 85, weight percent of a C.sub.10 -C.sub.15 alkyl
 (meth)acrylate; and
 (C) about 2 to about 5, weight percent of a nitrogen-containing dispersant
 monomer.
 DETAILED DESCRIPTION OF THE INVENTION
 The present invention is directed to polyalkyl (meth)acrylate copolymers
 comprising units derived from:
 (A) about 12 to about 18 weight percent methyl methacrylate;
 (B) about 75 to about 85 weight percent of C.sub.10 -C.sub.15 alkyl
 (meth)acrylate(s); and
 (C) about 2 to about 5 weight percent of a nitrogen-containing dispersant
 monomer.
 The polyalkyl (meth)acrylate copolymers of the present invention comprise
 the reaction products of:
 (A) from about 12 to about 18, weight percent methyl methacrylate;
 (B) from about 75 to about 85, weight percent of C.sub.10 -C.sub.15 alkyl
 (meth)acrylate(s); and
 (C) from about 2 to about 5, weight percent of a nitrogen-containing
 dispersant monomer.
 As used herein, C.sub.10 -C.sub.15 alkyl (meth)acrylate means an alkyl
 ester of acrylic or methacrylic acid having a straight or branched alkyl
 group of 10 to 15 carbon atoms per group including, but not limited to,
 decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate,
 lauryl (meth)acrylate, myristyl (meth)acrylate, dodecyl pentadecyl
 methacrylate, and mixtures thereof.
 The alkyl (meth)acrylate comonomers containing 10 or more carbon atoms in
 the alkyl group are generally prepared by standard esterification
 procedures using technical grades of long chain aliphatic alcohols, and
 these commercially available alcohols are mixtures of alcohols of varying
 chain lengths in the alkyl groups. Consequently, for the purposes of this
 invention, alkyl (meth)acrylate is intended to include not only the
 individual alkyl (meth)acrylate product named, but also to include
 mixtures of the alkyl (meth)acrylates with a predominant amount of the
 particular alkyl (meth)acrylate named.
 The nitrogen-containing dispersant monomers suitable for use in the present
 invention include dialkylamino alkyl (meth)acrylamides such as,
 N,N-dimethylaminopropyl methacrylamide; N,N-diethylaminopropyl
 methacrylamide; N,N-dimethylaminoethyl acrylamide and
 N,N-diethylaminoethyl acrylamide; and dialkylaminoalkyl (meth)acrylates
 such as N,N-dimethylaminoethyl methacrylate; N,N-diethylaminoethyl
 acrylate and N,N-dimethylaminoethyl thiomethacrylate.
 In a preferred embodiment, the polyalkyl (meth)acrylate copolymers of the
 present invention consist essentially of the reaction products of (A), (B)
 and (C). However, those skilled in the art will appreciate that minor
 levels of other monomers, polymerizable with monomers (A), (B) and/or (C)
 disclosed herein, may be present as long as they do not adversely affect
 the low temperature properties of the fully formulated fluids. Typically
 additional monomers are present in an amount of less than about 5 weight
 percent, preferably in an amount of less than 3 weight percent, most
 preferably in an amount of less than 1 weight percent. For example, the
 addition of minor levels of monomers such as C.sub.2 -C.sub.9 alkyl
 (meth)acrylates, hydroxy- or alkoxy-containing alkyl (meth)acrylates,
 ethylene, propylene, styrene, vinyl acetate and the like are contemplated
 within the scope of this invention as long as the presence of these
 monomers do not adversely affect the low temperature properties of the
 copolymers. In a preferred embodiment the sum of the weight percent of
 (A), (B) and (C) equals 100%.
 The copolymers may be prepared by various polymerization techniques
 including free-radical and anionic polymerization.
 Conventional methods of free-radical polymerization can be used to prepare
 the copolymers of the present invention. Polymerization of the acrylic
 and/or methacrylic monomers can take place under a variety of conditions,
 including bulk polymerization, solution polymerization, usually in an
 organic solvent, preferably mineral oil, emulsion polymerization,
 suspension polymerization and non-aqueous dispersion techniques.
 Solution polymerization is preferred. In the solution polymerization, a
 reaction mixture comprising a diluent, the alkyl (meth)acrylate monomers,
 a polymerization initiator and a chain transfer agent is prepared.
 The diluent may be any inert hydrocarbon and is preferably a hydrocarbon
 lubricating oil that is compatible with or identical to the lubricating
 oil in which the copolymer is to be subsequently used. The mixture
 includes, e.g., from about 15 to about 400 parts by weight (pbw) diluent
 per 100 pbw total monomers and, more preferably, from about 50 to about
 200 pbw diluent per 100 pbw total monomers. As used herein, "total monomer
 charge" means the combined amount of all monomers in the initial, i.e.,
 unreacted, reaction mixture.
 In preparing the copolymers of the present invention by free-radical
 polymerization, the acrylic monomers may be polymerized simultaneously or
 sequentially, in any order. In a preferred embodiment, the total monomer
 charge includes from 10 to 20, preferably 12 to 18, weight percent methyl
 methacrylate; 70 to 89, preferably 75 to 85, weight percent of at least
 one C.sub.10 -C.sub.15 alkyl (meth)acrylate; and 1 to 10, preferably 2 to
 5, weight percent of a dispersant monomer.
 Suitable polymerization initiators include initiators which disassociate
 upon heating to yield a free radical, e.g., peroxide compounds such as
 benzoyl peroxide, t-butyl perbenzoate, t-butyl peroctoate and cumene
 hydroperoxide; and azo compounds such as azoisobutyronitrile and
 2,2'-azobis (2-methylbutanenitrile). The reaction mixture typically
 includes from about 0.01 wt % to about 1.0 wt % initiator relative to the
 total monomer mixture.
 Suitable chain transfer agents include those conventional in the art, e.g.,
 dodecyl mercaptan and ethyl mercaptan. The selection of the amount of
 chain transfer agent to be used is based on the desired molecular weight
 of the polymer being synthesized as well as the desired level of shear
 stability for the polymer, i.e., if a more shear stable polymer is
 desired, more chain transfer agent can be added to the reaction mixture.
 Preferably, the chain transfer agent is added to the reaction mixture in
 an amount of 0.01 to 3 weight percent, preferably 0.02 to 2.5 weight
 percent, relative to the monomer mixture.
 By way of example and without limitation, the reaction mixture is charged
 to a reaction vessel that is equipped with a stirrer, a thermometer and a
 reflux condenser and heated with stirring under a nitrogen blanket to a
 temperature from about 50.degree. C. to about 125.degree. C., for a period
 of about 0.5 hours to about 8 hours to carry out the copolymerization
 reaction.
 In a further embodiment, the copolymers may be prepared by initially
 charging a portion, e.g., about 25 to 60% of the reaction mixture to the
 reaction vessel and heating. The remaining portion of the reaction mixture
 is then metered into the reaction vessel, with stirring and while
 maintaining the temperature of the batch within the above describe range,
 over a period of about 0.5 hours to about 3 hours. A viscous solution of
 the copolymer of the present invention in the diluent is obtained as the
 product of the above-described process.
 To form the lubricating oils of the present invention, a base oil is
 treated with the copolymer of the invention in a conventional manner,
 i.e., by adding the copolymer to the base oil to provide a lubricating oil
 composition having the desired low temperature properties. Preferably, the
 lubricating oil contains from about 1 to about 20 parts by weight (pbw),
 preferably 3 to 15 pbw, most preferably 5 to 10 pbw, of the neat copolymer
 (i.e., excluding diluent oil) per 100 pbw base oil. In a particularly
 preferred embodiment, the copolymer is added to the base oil in the form
 of a relatively concentrated solution of the copolymer in a diluent. The
 diluent includes any of the oils referred to below that are suitable for
 use as base oils.
 The copolymers of the present invention typically have a relative number
 average molecular weight, as determined by gel permeation chromatography
 using polymethyl methacrylate standards, between 5000 and 50,000,
 preferably 10,000 to 25,000.
 The molecular weight of the alkyl(meth)acrylate polymer additive must be
 sufficient to impart the desired thickening properties to the lubricating
 oil. As the molecular weight of the polymers increase, the copolymers
 become more efficient thickeners; however, the polymers can undergo
 mechanical degradation in particular applications and for this reason,
 polymer additives with number-average molecular weights (Mw) above about
 50,000 are generally not suitable for certain applications because they
 tend to undergo "thinning" due to molecular weight degradation resulting
 in loss of effectiveness as thickeners at the higher use temperatures (for
 example, at 100.degree. C.). Thus, the molecular weight is ultimately
 governed by thickening efficiency, required shear stability, cost and the
 type of application.
 Those skilled in the art will recognize that the molecular weights set
 forth throughout this specification are relative to the methods by which
 they are determined. For example, molecular weights determined by GPC and
 molecular weights calculated by other methods, may have different values.
 It is not molecular weight per se but the handling characteristics and
 performance of a polymeric additive (shear stability, low temperature
 performance and thickening power under use conditions) that is important.
 Generally, shear stability is inversely proportional to molecular weight.
 A VII additive with good shear stability (low SSI value) is typically used
 at higher initial concentrations relative to another additive having
 reduced shear stability (high SSI value) to obtain the same target
 thickening effect in a treated fluid at high temperatures; the additive
 having good shear stability may, however, produce unacceptable thickening
 at low temperatures due to the higher use concentrations.
 Conversely, although lubricating oils containing lower concentrations of
 reduced shear stability VI improving additives may initially satisfy the
 higher temperature viscosity target, fluid viscosity will decrease
 significantly with use causing a loss of effectiveness of the lubricating
 oil. Thus, the reduced shear stability of specific VI improving additives
 may be satisfactory at low temperatures (due to its lower concentration)
 but it may prove unsatisfactory under high temperature conditions. Thus,
 polymer composition, molecular weight and shear stability of VI improvers
 must be selected to achieve a balance of properties in order to satisfy
 both high and low temperature performance requirements.
 The finished lubricating oil composition may include other additives in
 addition to the copolymer of the present invention, e.g., oxidation
 inhibitors, corrosion inhibitors, friction modifiers, antiwear and extreme
 pressure agents, detergents, dispersants, antifoamants, additional
 viscosity index improvers and pour point depressants.
 Base oils contemplated for use in this invention include natural oils,
 synthetic oils and mixtures thereof Suitable base oils also include
 basestocks obtained by isomerization of synthetic wax and slack wax, as
 well as basestocks produced by hydrocracking (rather than solvent
 extracting) the aromatic and polar components of the crude. In general,
 both the natural and synthetic base oils will each have a kinematic
 viscosity ranging from about 1 to about 40 cSt at 100.degree. C., although
 typical applications will require each oil to have a viscosity ranging
 from about 2 to about 20 cSt at 100.degree. C.
 Natural base oils include animal oils, vegetable oils (e.g., castor oil and
 lard oil), petroleum oils, mineral oils, and oils derived from coal or
 shale. The preferred natural base oil is mineral oil.
 The mineral oils useful in this invention include all common mineral oil
 base stocks. This would include oils that are naphthenic or paraffinic in
 chemical structure. Oils that are refined by conventional methodology
 using acid, alkali, and clay or other agents such as aluminum chloride, or
 they may be extracted oils produced, for example, by solvent extraction
 with solvents such as phenol, sulfur dioxide, furfural, dichlordiethyl
 ether, etc. They may be hydrotreated or hydrorefined, dewaxed by chilling
 or catalytic dewaxing processes, or hydrocracked. The mineral oil may be
 produced from natural crude sources or be composed of isomerized wax
 materials or residues of other refining processes.
 Typically the base oils will have kinematic viscosities of from 2 cSt to 40
 cSt at 100.degree. C. The preferred base oils have kinematic viscosities
 of from 2 to 20 cSt at 100.degree. C.
 The American Petroleum Institute has categorized these different basestock
 types as follows: Group I, &gt;0.03 wt. % sulfur, and/or &lt;90 vol %
 saturates, viscosity index between 80 and 120; Group II, .ltoreq.0.03 wt.
 % sulfur, and .gtoreq.90 vol % saturates, viscosity index between 80 and
 120; Group III, .ltoreq.0.03 wt. % sulfur, and .gtoreq.90 vol % saturates,
 viscosity index&gt;120; Group IV, all polyalphaolefins.
 Group II and Group III basestocks are typically prepared from conventional
 feedstocks using a severe hydrogenation step to reduce the aromatic,
 sulfur and nitrogen content, followed by dewaxing, hydrofinishing,
 extraction and/or distillation steps to produce the finished base oil.
 Group II and III basestocks differ from conventional solvent refined Group
 I basestocks in that their sulfur, nitrogen and aromatic contents are very
 low. As a result, these base oils are compositionally very different from
 conventional solvent refined basestocks. Hydrotreated basestocks and
 catalytically dewaxed basestocks, because of their low sulfur and
 aromatics content, generally fall into the Group II and Group III
 categories. Polyalphaolefins (Group IV basestocks) are synthetic base oils
 prepared from various alpha olefins and are substantially free of sulfur
 and aromatics.
 Synthetic base oils include hydrocarbon oils and halo-substituted
 hydrocarbon oils such as oligomerized, polymerized, and interpolymerized
 olefins (such as polybutylenes, polypropylenes, propylene, isobutylene
 copolymers, chlorinated polylactenes, poly(1-hexenes), poly(1-octenes) and
 mixtures thereof); alkylbenzenes (including dodecyl-benzenes,
 tetradecylbenzenes, dinonyl-benzenes and di(2-ethylhexyl)benzene);
 polyphenyls (such as biphenyls, terphenyls and alkylated polyphenyls); and
 alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their
 derivatives, analogs, and homologs thereof, and the like. The preferred
 synthetic oils are oligomers of alpha-olefins, particularly oligomers of
 1-decene, also known as polyalpha olefins or PAO's.
 Synthetic base oils also include alkylene oxide polymers, interpolymers,
 copolymers, and derivatives thereof where the terminal hydroxyl groups
 have been modified by esterification, etherification, etc. This class of
 synthetic oils is exemplified by: polyoxyalkylene polymers prepared by
 polymerization of ethylene oxide or propylene oxide; the alkyl and aryl
 ethers of these polyoxyalkylene polymers (e.g., methyl-polyisopropylene
 glycol ether having an average molecular weight of 1000, diphenyl ether of
 polypropylene glycol having a molecular weight of 100-1500); and mono- and
 poly-carboxylic esters thereof (e.g., the acetic acid esters, mixed
 C.sub.3 -C.sub.8 fatty acid esters, and C.sub.12 oxo acid diester of
 tetraethylene glycol).
 Another suitable class of synthetic lubricating oils comprises the esters
 of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic
 acids and alkenyl succinic acids, maleic acid, azelaic acid, subric acid,
 sebasic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic
 acid, alkylmalonic acids, alkenyl malonic acids, etc.) with a variety of
 alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol,
 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoethers,
 propylene glycol, etc.). Specific examples of these esters include dibutyl
 adipate, diisobutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl
 fumarate, dioctyl sebacate, diisooctyl phthalate, diisooctyl azelate,
 diisooctyl adipate, diisodecyl azelate, didecyl phthalate, diisodecyl
 adipate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid
 dimer, and the complex ester formed by reacting one mole of sebasic acid
 with two moles of tetraethylene glycol and two moles of 2-ethyl-hexanoic
 acid, and the like. A preferred type of oil from this class of synthetic
 oils are adipates of C.sub.4 to C.sub.12 alcohols.
 Esters useful as synthetic base oils also include those made from C.sub.5
 to C.sub.12 monocarboxylic acids and polyols and polyol ethers such as
 neopentyl glycol, trimethylolpropane pentaerythritol, dipentaerythritol,
 tripentaerythritol, and the like.
 Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or
 polyaryloxy-siloxane oils and silicate oils) comprise another useful class
 of synthetic lubricating oils. These oils include tetra-ethyl silicate,
 tetraisopropyl silicate, tetra-(2-ethylhexyl) silicate,
 tetra-(4-methyl-2-ethylhexyl) silicate, tetra-(p-tert-butylphenyl)
 silicate, hexa-(4-methyl-2-pentoxy)-disiloxane, poly(methyl)-siloxanes and
 poly(methylphenyl) siloxanes, and the like. Other synthetic lubricating
 oils include liquid esters of phosphorus containing acids (e.g., tricresyl
 phosphate, trioctylphosphate, and diethyl ester of decylphosphonic acid),
 polymeric tetra-hydrofurans, poly-.alpha.-olefins, and the like.
 Lubricating oils containing the copolymers of the present invention may be
 used in numerous applications including automatic transmission fluids,
 continuously variable transmission fluids, manual transmission fluids,
 hydraulic fluids, crankcase applications and shock absorber fluids.
 Depending upon the intended end use of the lubricating oil formulations,
 the shear stability of the copolymer can be adjusted by controlling the
 amount of initiator and/or chain transfer agent present in the reaction
 mixture.
 For example, in automatic transmission fluid applications it may be desired
 to have a highly shear stable lubricating fluid. In an embodiment of the
 present invention, automatic transmission fluids are prepared by adding to
 a base oil a copolymer of the present invention and a detergent/inhibitor
 package such that the fluids have a percent shear stability index (SSI) as
 determined by the 20 hour Tapered Bearing Shear Test in the range of 1% to
 about 80%, preferably 2 to 20%. The 20 hour Tapered Bearing Shear Test is
 a published standard test entitled "Viscosity Shear Stability of
 Transmission Lubricants" and is described in CEC L-45-T-93 and is also
 published as DIN 51 350, part 6.

EXAMPLES
 Table 1 sets forth the compositions of various representative and
 comparative viscosity index improvers prepared to demonstrate the
 effectiveness of the polymers of the present invention. All amounts are in
 percent by weight based on the total amount of monomer charged to the
 reactor (i.e., excluding initiator and chain transfer agent).
 The general procedure used for preparing the polymethacrylates in Table 1
 was as follows: To a 2 liter resin kettle fitted with an overhead stirrer,
 a thermocouple, a sparge tube and a condenser was charged the total
 monomer charge listed in Table 1 for each polymer. The stirrer was set at
 300 rpm and the temperature was increased to 40.degree. C. The sparge tube
 was replaced with a nitrogen blanket and the temperature was increased to
 about 78.degree. C. Then, lauryl(dodecyl) mercaptan as a chain transfer
 agent was then added, followed by AIBN (azobisisobutyronitrile). The
 mixture was heated and stirred for 4 hours at 78.degree. C. The
 temperature was then increased to about 104.degree. C. for 1.5 hours to
 decompose any residual catalyst. Diluent oil was added to arrive at 80%
 polymer solution by weight and stirring and heating continued at about
 70-80.degree. C. for 1 hour. The reactor was cooled and the various
 polymer solutions were then stored at room temperature until testing.
 The monomers used to prepare the polymethacrylates were methyl methacrylate
 (MMA), butyl methacrylate (BMA), lauryl methacrylate (LMA), cetyl-eicosyl
 methacrylate (CEMA) and/or dimethylaminopropyl methacrylamide (DMA). The
 weight percent of the monomers used to prepare polymers VII-1 to VII-7 are
 set forth below in Table 1.
 TABLE 1
 PMA Composition
 Mn
 MMA BMA LMA CEMA DMA (approx.)
 VII-1* 10.7 82.6 3.1 3.6 11,000
 VII-2* 13.8 79.6 3 3.6 11,000
 VII-3* 11.3 85.1 3.6 11,000
 VII-4 14.2 82.1 3.7 11,000
 VII-5* 14.4 77 4.9 3.7 11,000
 VII-6 15 81.4 3.6 18,000
 VII-7 17.9 78.4 3.7 13,000
 *Polymers outside the scope of the present invention.
 Table 2 sets forth some properties of the various base oils used in
 evaluating the low temperature performance of the polymers of Table 1.
 TABLE 2
 Base Oil Properties
 Group I.sup.1 Group Group
 API Class SNO 70 SNO 100 Group II III(1) III(2)
 VI 93 105 114 120 125
 Pour Point (.degree. C.) -21 -15 -21 -27 NA
 Paraffinic (%) 59.9 64.8 51.4 66.2 76.1
 Naphthenics (%) 33.7 33.7 48.3 32.4 23.8
 Aromatics (%) 6.4 1.5 0.3 1.4 0.1
 Sulfur (%) 0.21 0.01 &lt;0.01 &lt;0.01 &lt;0.01
 .sup.1 The Group I base oil was a mixture of approximately 45 wt. % SNO 70
 and 55 wt. % SNO 100
 NA Not available or not measured
 To demonstrate the low temperature properties of the copolymers of the
 present invention, lubricant compositions were prepared containing the
 identical type and amount of detergent/inhibitor package. No pour point
 depressant was added. To demonstrate the effectiveness of the polymers of
 the present invention across a wide variety of base fluids, four different
 base oils were used. Details of the base oils are set forth in Table 2.
 The polymers were added to the oil in an amount such that the finished
 lubricants had a kinematic viscosity at 100.degree. C. of approximately
 7.6 cSt. The low temperature properties of these fluids were tested
 according to ASTM D 2983 and the Brookfield Viscosity (cP) at -40.degree.
 C. is reported in Table 3.
 TABLE 3
 Low Temperature Performance (Brookfield Viscosity (cP) at -40.degree. C.)
 Group I Group II Group III(1) Group III(2) Avg.
 VII-1* 34075 DNT DNT DNT --
 VII-2* 52150 DNT DNT DNT --
 VII-3* 37350 25075 15510 33250 28296
 VII-4 30400 21850 14810 18320 21345
 VII-5* 32950 33975 15920 35225 29518
 VII-6 24750 16660 12520 13790 16930
 VII-7 31700 21750 16440 20025 22479
 *Comparative Examples
 DNT Did Not Test
 It is clear, from the above Table 3, that lubricant formulations comprising
 the polymethacrylate viscosity index improvers of the present invention
 (VII-4, VII-6 and VII-7) exhibit superior low temperature properties
 across the range of base oils compared to polymethacrylate viscosity index
 improvers outside the scope of the present invention (VII-1, VII-2, VII-3
 and VII-5) as evidenced by the superior Brookfield Viscosity results.
 This invention is susceptible to considerable variation in its practice.
 Accordingly, this invention is not limited to the specific
 exemplifications set forth hereinabove. Rather, this invention is within
 the spirit and scope of the appended claims, including the equivalents
 thereof available as a matter of law.
 The patentees do not intend to dedicate any disclosed embodiments to the
 public, and to the extent any disclosed modifications or alterations may
 not literally fall within the scope of the claims, they are considered to
 be part of the invention under the doctrine of equivalents.