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M. Bajpai, V. Shukla

(MADHU-BAJPAI@Rediffmail.com) and

(Vipin_1222@Rediffmail.com) are at the Oil and Paint Department, H.B.

Technological Institute, Kanpur 208002, India.

, and are at Jaydeep Polycon(P) Ltd., Kanpur,

India.

Oligomers, Diluents, Monomers, Photoinitiators, UV curing

The market for ultraviolet curing technology has been growing at double-digit

rates in the last 10 years. The main reason for such a rapid technological growth

of UV curing is its unique process characteristics, which allow UV-coating to be

applied on virtually any substrates, including plastic, metal, composite, wood,

paper, leather, vinyl, glass, magnetic recording tape and even human teeth.

The original driving forces behind the commercialisation of UV-technology

were energy saving and freedom from solvents. These benefits are complemented

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(v) Light stabiliser

(vi) Thermal stabiliser

(vii) Colourants, plasticisers and additives (Berner et al., 1978;

Papas, 1973; Ledwith, 1976)

Some of the more important ingredients are now detailed as follows.

Oligomers (radiation curable binders)

Resins used in UV-curing include the following classes,

(i) Unsaturated polyester/acrylated polyester

(ii) Acrylated epoxy resin

(iii) Acrylated urethanes (both aliphatic and aromatic)

(iv) Acrylated silicone resins

(v) Acrylated polyethers

(vi) Acrylated melamines

(vii) Acrylated oils

(viii) N-vinyl urethanes

(ix) Thiolene system

a) Acrylated polyesters

Polyester acrylates can be produced in a wide range of viscosities

and reactivities to adapt to end uses in printing inks, wood and paper

coatings. One of the major advantages offered by polyester acrylates

over the other prepolymers is their low viscosity. For example, the basic

recipe reported by Rybny (Reference???) has a viscosity of 1:660 (This

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value is ususual. Please check!) centipoise at a molecular weight of

around 1000 compared with an epoxy acrylate which will be 5-10 time

higher in viscosity. As the molecular weight of the polyester decreases,

its acrylated counterpart becomes more monomer like. It should be

appreciated from the low molecular weight polyester, the presence of

pure di-acrylate from the glycol used in the preparation of the base

polyester becomes a statistically significant probability. One of the major

problems associated with the use of the low molecular weight polyester

acrylates is the reduction in reactivity and the increased surface inhibition

observed and found that this can be compensated for by a number of

techniques, e.g. incorporation of pendent aromatic group and ether

grouping within the polyester backbone. It is possible to prepare

polyester acrylate using trans-esterification technique with an acrylic

monomer such as ethylacrylate. (Micheli, 2000; Hara, 2002)

Acrylated polyesters are relatively cheaper compared with other

acrylated prepolymers. By varying the molecular weight of polyester, it is

possible to obtain acrylates from low viscosity to hard solids, at ambient

temperature. The compatibility of acrylated polyesters with other

prepolymer is good. As such, acrylated polyesters can be used in many

formulations. Polyester acrylates are mainly used in UV roller coat

varnishes for paper and board and in UV wood coatings.

(b) Acrylated epoxies

Epoxy acrylates are extensively used in UV curable litho inks and

varnishes, roller coating varnishes for paper and board, printed circuit

board, wood and plastic coatings. Epoxy acrylates offer good all-round

properties combined with high cure rate. Both aromatic and aliphatic

epoxies and epoxy novolacs are used. (Aggrawal and Maithani, 2002)

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The reaction of an epoxy group with acrylic or methacrylic monomers

gives rise to an epoxy acrylate (or methacrylate). There is a wide range

of epoxy acrylate available including acrylates of DGEBA (full name

needed), acrylates of epoxidised oils such as soya or linseed and acrylates

of epoxy novolacs.

Epoxy acrylates have high skin irritancy. As such, careful processing

and formulation is essential in preparation of epoxy acrylate. Very low

acid values are necessary since, unlike other coating resins such as alkyds

or polyesters, any residual acid is present as molecular weight (e.g.

Epikote 1001 and 1004) have been prepared but are not used widely.

(Please check this sentence to ensure that it makes sense.) Epoxidised

oils can also be acrylated to give good flexibility, lower viscosity, good

pigment wetting properties and very low skin irritancy. Molecular weight

of acrylated oils falls in the region of 800.

Epoxy novolac acrylates are harder materials and have superior

resistance properties to the standard epoxy acrylates. Epoxy novalac

acrylates have found use in screen printing application and in UV solder-

resist formulations for the electronic industry. (Randell, 1986)

(c)Acrylated urethanes

Reaction between hydroxyl and isocyanate groups proceeds efficiently at

low temperature without the evolution of volatile by-products. The well

known properties of urethanes, such as hardness, chemical resistance,

toughness and light stability can be built into the acrylated, radiation

curable prepolymers. Acrylated urethane can be formed by the reaction

of 2 moles of EHA (Ethylhydroxylacrylate) with one mole of a diisocyanate.

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The product is highly viscous and produces cured films which are very

hard and inflexible but also highly chemical resistant.

Colour retentive properties of the acrylated urethane achieved will

depend on the type of isocyanate used. Two main classes of isocyanate

are available namely, aromatic isocyanates, e.g. toluene diisocyanate and

aliphatic isocyanates, e.g. isophorone-di-isocyanate. Aromatic

isocyanates have higher viscosities and poorer colour properties. The

poor colour properties are seen in the prepolymer as supplied, as well as

in the relevant coatings (poor light and heat stability). Improvement of

the flexibility of acrylated urethanes may be achieved by chain extension

using long chain diol to produce a higher molecular weight isocyanate

functional prepolymer which is subsequently capped by a hydroxy acrylic

monomer. (Roffey, 1982)

(d) Acrylated silicones

Incorporation of silicone into radiation curable prepolymers is of interest in

a number of areas. Silicones are well known for their release properties,

heat and weather resistance. The advantages of silicone acrylates in the

protection of optical fibres are afforded by their excellent flexibility and

extensibility properties, particularly at low operating temperatures. At

present state of development, silicone acrylates appear to be far more

sensitive to air inhibition than other UV curable systems and curing under

inert gas is often recommended.

(e) Acrylated polyethers

Polyether acrylates are lower viscosity resins compared with the polyester

type and are relatively inexpensive to prepare. In the acrylation of

polyether however, a trans-esterification technique is used to prevent the

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polyether links from degrading. Polyethers may also be reacted with the

isocyanate groups and this is often an easier method to introduce

polyether linkages into an acrylated system since the problems of

removing the by-products of trans-esterification such as ethanol are

eliminated.

The reaction of ethylene or propylene oxide with a polyol in the

presence of basic or acidic catalysts such as BF3 or NaOH will give a

polyether. Where secondary hydroxyl is available, incomplete

etherification may occur. The total degree of etherification depends upon

the ratio of propylene oxide to polyol. Many other common polyethers

have been acrylated. Amongst them are ethoxylates and propoxylates of

trimethylolpropane, pentaetythritol and polyethers or 1,4-butane diol.

These form the basis of the new generation of low viscosity, low toxicity

monomers which are rapidly gaining importance. (Roffey, 1982)

(f) Acrylated oils

Normally, acrylated oils are derived from natural products such as castor

or fish oils. All acrylated oils contain triglyceride oils. Generally, acrylated

oils are highly flexibility or soft, due firstly to their flexible aliphatic acrylic

backbone and secondly to their low level of acrylic unsaturation.

The oil that has the highest consumption, about 90% of the acrylated

oils in the market is soyabean, due to its relatively low cost and ready

availability. (Please check to make sure that this sentence makes sense.)

It consists of about 50% linoleic acid, 25% oleic acid, 10% palmitic and

linolenic acid and about 5% stearic acid. These acids contain a relatively

high level of unsaturated groups which can be oxidised by reaction with

peracids and hydrogenperoxide. On the other hand, even though they are

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less used than soyabean oils, linseed oils are used extensively due to their

reasonable cost and relatively higher level of unsaturation that revents

(What does revents mean?) in a higher number of epoxy groups.

Acrylation is usually performed with acrylic acid under similar conditions

to epoxy acrylate manufacture. (Webster, 1996) The viscosity and the

degree of unsaturation of acrylated oils are lower than epoxy acrylate and

consequently, it is not necessary to use a reactive diluent to decrease

viscosity.

(g) Thiolene system

Free radical addition of mecaptans to olefins has been known for many

years. As presently understood, mercaptan olefin polymerisation occurs

according to given reaction. (Please check this sentence to ensure that it

make sense.)

W. R. Grace company carried out much of the original work in this

area and they consequently hold many relevant patents. There are two

advantages to the thiol/polyene system. Thus,

1) they are non-air inhibited, and

2) flexibile cured films can be obtained from relatively low

viscosity mixture without the need to incorporate a diluent. Polyfunctional

thiol compound gives very tough abrasive - resistant coatings which are

ideal for applications such as flooring coatings.

It is also possible to modify acrylic UV curable formulation with

polythiol polymer to improve their properties. (Irene, 1996)

A comparison of general film properties of coatings derived from

acrylated oligomers are given in Table 1.

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Diluents/monomers

As all the currently available oligomers (e.g. acrylated epoxy, acrylated

urethane etc.) are too viscous to be applied through conventional coating

equipment, most formulators dilute the oligomer down to application

viscosity (from 10,000 cps to 100 cps depending on the method of

application) (Pelgrims, 1978). The diluents used to give system a

workable viscosity have to be crosslinkable. To obtained a low viscosity, a

high level of monomer has to be added. However, earlier literatures

indicated that to retain good reactivity in the mixture, as little monomer

as possible should be used. As little monomer as possible should be

added to a system, also for toxicological reason, to avoid skin irritation

which is caused by a high level of monomer. (Neerbos, 1978; Durgval,

2002)

It is obvious that, with a higher functionality monomer, the cure

rate of the formulation will increase. A monomer is selected for a system

based upon its

photoresponse,

contribution to the properties of the photopolymerised film

properties,

relative volatility,

odour and toxicity,

solvation efficiency, and

cost

Various diluents are detailed as follows.

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(iii) Iso decyl acrylate (IDA)

This is good viscosity reducer, being less volatile than EHA. It is reported

to increase flexibility owing to long aliphatic chains.

(iv) Iso bornyl acrylate (IBA)

A strong odour is the main disadvantage of this monomer, which has low

toxicity and volatility, imparts a hardness comparable to MMA, but with

the fast cure rate of acrylate, and has a low shrinkage rate.

(v) 2-Hydroxy ethyl acrylates (HEA)

Although it is widely used at present, it high toxicity has forced people to

formulate around this material.

(vi) 2-Hydroxy propyl acrylate (HPA)

It is good reducer but highly toxic.

(2) Diacrylates

Diacrylates have relatively stronger odour, are skin irritants and

carcinogenic. Methacrylic analogues are also available but they suffer

from the disadvantage of having low reactivity as a result of oxygen

inhibition and also exhibit strong odour. The only major area where the

methacrylates are used is in the UV-adhesive industry. Following are

examples of diacrylate diluents/monomers.

(i) 1,4-Butanedioldiacrylate (BDDA)

BDDA is widely used in wood coatings.

(ii) 1,6-Hexanedioldiacrylate (HDDA)

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With its lower volatility, HDDA is increasingly replacing neopentyl glycol

diacrylate, although both are suspected skin irritation sensitisers. HDDA

are relatively good viscosity reducers.

(iii) Neopentylglycoldiacrylate (NPGDA) (Some description should be

given here.)

(iv) Diethyleneglycoldiacrylate (DEGDA) (Some description should be

given here.)

(3) Triacrylates

(i) Pentaerythritoltriacrylate (PETA)

PETA is widely used in the printing inks since it gives rapid cure response.

However, PETA is a severe eye irritant and suspect to be carcinogenic.

(ii) Trimethylolpropanetriacrylate (TMPTA)

TMPTA has a low volatility and is widely used in printing inks.

(4) Tetracrylates

Pentaerythritoltetracrylate (Some description should be given here.)

(5) Pentacrylates

Dipentaerythritol (monohydroxy) pentaacrylate (Some description should

be given here.)

(c) Allylic Monomers

(1) Triallyl cyanurate (Some description should be given here.)

(2) Trimethylol propane trially ether (Some description should be given

here.)

Plasticising diluents

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the cure rate. A suitable photoinitiator system must first have a high

absorption in the emission range of the light source, usually a medium

pressure mercury lamp. In addition, the excited states thus formed must

both have a short lifetime to avoid quenching by oxygen or the monomer,

and split into reactive radicals or ionic species with the highest possible

quantum yield.

The selection of photoinitiator mixture depend on a number of

factors including

required line speed with given curing system,

coating thickness,

transparency of the coating material, presence of pigments and filler,

surface properties to be obtained such as hardness and glass,

required non yellowing, odourless, low volatility good thermal stability,

required non toxic, low migration and cost effective,

required high absorption in the region of activation, and

required high quantum yield for free radical formulation.

A distinction is necessary between the term photoinitiator and

photosensitiser, which are often commonly incorrectly regarded as

interchangeable. A photoinitiator absorbs the incident light directly and

split up or fragments to form free radicals. These than attack the

monomer (generally by hydrogen atom abstraction) to initiate the

photopolymerisation reaction. In contrast, a photosensitiser absorbs the

incident light but does not fragment itself, instead transfers the energy it

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state and radicals of the surrounding monomer and benzohydrophenone

are produced. The monomers polymerise whereas the

benzohydrophenone radical reacts with another benzohydrophenone

radical to form benzopinacol or it may react with another radical to form

an electro neutral and saturated compound. (Barner et al., 1978; Gamble,

1976; Hulme, 1976)

This later compound can be considered an independent primary

photoinitiator as being used as a synergist or secondary photoinitiator.

Amines such as small amounts of triethylamine have been found to

enhance the photopolymerisation rate in benzophenone/acrylate system

probably via the formation of the exciplex, which gives rise to free

radicals.

Michlers Ketone has markedly improved light response sensitivity

for most UV systems when used with mixture with benzophenone, benzil,

benzoin alkylethers and/or amines, pre-ground mixtures of the solids to an

inks appear to be more efficient than it they are added individually and

milled straight into ink.

(2) Bnzoin alkyl ethers

To obtain better performance, modification of the benzoin molecules has

thus induced alkylation. These compounds are the benzoin alkyl ethers,

methylolation has also been tried.

As opposed to benzoin which undergoes fragmentation from the

excited triplet state, these were thought to fragment from singlet excited

states. This may be the cause of benzoin exhibiting higher quantum

efficiencies for fragmentation in monomers having high triplet energies

than it would in low triplet energy monomers, where triplet-triplet energy

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transfer may be in direct competition with fragmentation of the excited

benzoin. Quantum yields for the photo - initiation by benzoin derivatives

are often in the region 0.2 - 0.3. Later work however, indicates a reactive

triplet state.

Norrish type 1 cleavage of benzoin, benzoin alkyl ethers and

alkylated benzoins will form benzoyl radicals. Which can be expected to

be reactive entities a efficient photoinitiators. Photoinitiation is though to

proceed as follows. (Allen and Edge, 1990)

The two radicals are formed, benzoyl radical and alkoxy benzyl

radical, are of different reactivity.

Benzoyl radicals are the main cause of polymer chain initiation,

alkoxy benzyl radicals are less reactive and partially dimerise. Benzoin

ethers lead to poor pot stability probably owing to the activated hydrogen

in the position of the ether group. Ethers with this structure readily react

with oxygen forming hydroperoxides. This intermediate may result in

thermal stability. Particularly in the presence of transition metals which

are often present in filler materials. The substituted pattern of benzoin

ether can also effect the pot life, shot chain alkyl or non branched ether

such as benzoin methylether, being the worst. A reasonable compromise

between reactivity and shell life is benzoin isopropyl ether. Stabilisers can

also be incorporated as along as cure rate is not effected.

(3) Thioxanthone and derivatives

Many photoinitiators can be regarded as derivatives of benzophenone in

particular thioxanthane and its substituted compounds. The table over

leaf(Which table is this? Descriptions, such as table over leaf, table

above and table below, should never be used in scientific writing.

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Instead, all tables should be numbered and quotation should alwasys be

made based on the table number, to avoid ambiguity.) shows some of

these which can be seen to have a derivation from the formula.

[Y may occupy any one of the positions 1, 2, 3, 4. (Please check and

modify this sentence.) If it represents a Cl-atom replacing on hydrogen

atom, or it may itself be a hydrogen atom] (Please check and modify this

sentence.)

Substituted thioxanthones have been developed for white

pigmented coating as two chief problems occur with these relatively thick

coatings. These problems are,

i) The pigment can reflect or absorb incident UV light, diminishing the

available light to the photoinitiator.

ii) The opacity of thickner pigmented films can result in a poor

through cure (Pappas, 1973).

Titanium dioxide has high reflectivity in the visible range of

spectrum and strong adsorption at wavelength

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6) Cost effectiveness

Of particular value in application are

(i) 2-chlorothioxanthone (2-CTX)

(ii) 2-isopropyl thioxanthone

The latter has good stability (as it is a liquid) in many solvents and

former is however, a solid. 2-CTX probably reacts by a hydrogen

abstraction methods and has absorption bands at 260 nm and 385 nm.

Some synergistic agent (or photoactivators) often used with thioxanthone

and its derivatives (or any other convenient photoinitiator responsive to

these compounds) are:

a) Ethyl para-dimethyl amino benzoate

b) Ethyl ortho-dimethyl amino benzoate

c) 2(n-butoxy) ethyl-para dimethyl amino benzoate

(4) Benzil ketals

Benzil ketals represent a versatile family of photoinitiators. 2,2-dimethoxy

2-phenyl acetophenone (DMPA) is one of the most important commercial

photoinitiators of the ?cleavage family. Its mode of action is thought to

be primarily a Norrish type I cleavage. This ester formation is strongly

temperature dependent.

Here, acetophenone is a cage collaps product of benzoyl and methyl

radical. The pot stability of this compound relative to benzoin ethers is

reported to be for greater. (Hageman, 1985)

The major drawback of this compound is its considerable yellowing.

Acrylate based inks and coating for metal, paper, plastic and wood

commonly make use of these types of photoinitiators.

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(5) Acyl phosphine oxide

Acylphosphine oxides were introduced some year ago as a new class of

-??? cleavage photoinitiators, derived from DEAP by replacing C-

H by P=O alkoxy by aryl groups. A relatively high oxygen inhibition may

decrease their reactivity in the curing of thin films.

Mono-acyl phosphine (MAPO e.g. Lucirin TPO) and bis

acylphosphine oxides such as BAPOI (Irgacure 1700 and 1800) and BAPO

2 (Irgacure 819) have absorption band in the near UV/visible region, and

so are specially indicated for use in pigmented system. Additionally

acylphosphine oxides bleach on irradiation, hence there is a decrease of

absorptivity in the near UV-visible range and radiation can penetrate into

deeper layers. Acylphosphine oxides produce little yellowing immediately

after curing and no long term exposure, therefore, they have been used in

applications where low yellowing is required as in white and ale inks. Acyl

phosphine oxide possess short-lived excited state and present low

quenching characteristics being suitable in styrene-based coating for the

furniture manufacturing industry (Studer and Roniger, 2001).

(6) -????Acyloxime esters

A Norrish type I cleavage reaction occurs to form a benzoyl radical and

another which undergoes further cleavage.

A useful (0-acylated-????;-oximino ketone) derivatives

that has been used in 1-phenyl-1, 2-propane dione-2-(0-ethoxy carbonyl

oxime) ????

It is a white/off white crystalline solid, odourless stable for at least

one year at room temperature and soluble in many solvent such as

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trimethoxyol propane triacrylate, 1-6-hexanedioldiacrylate, MMA (Methyl

methacrylate), hexane, CCl4, acetone, ethanol, methanol, toluene.

(7) Acetophenone derivatives

Dialkoxy acetophenone can be regarded as benzoin ethers. The most

popular used is DEAP (diethoxy acetophenone). DEAP undergoes a

Norrish type II cleavage yielding a biradical as the chain initiating species.

If no reactive double bond are available, the biradical undergoes

initial coupling to form an oxetanol intermediate which disproportionates

thermally to acetaldehyde an ????-ethoxy acetophenone.

Chlorinated acetophenone derivatives

Substituted di- and tri-chloroacetophenone undergo photolysis. Such as

p-tert-butyl trichloroacetophenone (in the general formula below for this

type of compound R = (CH3)3C. ????-cleavage is the prime

reaction. X may be either an H or another CI-atoms.

The liberated chlorine radical is particularly highly reactive and

initiates polymerisation efficiently especially ionic curable binders. A

disadvantage is that this radical can form hydrochloric acid by hydrogen

abstraction from a hydrogen donor and this presents an obvious

disadvantage in many areas. Such as metal decorating and may also lead

to poor pot stability. (Pappas and McGinniss, 1978; Holman and Oldering,

1988)

????

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[All references should contain the full title of the paper/book/proceedings and the

range of pages. (see example below) All references should be listed according the

the surname of the first author, in alphabetical order. If in doubt, please refer to

the Pigment & Resin Technology journal.]

Roffey, C.G. (1986), Title?, Journal of Oil & Colour Chemist Association,

Vol. 69 No. 11, pp. 288 - ??.

2. Berner, G; Kirchmayr, R and Rist, G; Journal of Oil & Colour Chemist

Association; 1978, 61, (4), 105.

3. Pappas, S P; Progress in Organic Coating, 1973, 74, (2), 333.

4. Ledwith, A; Journal of Oil & Colour Chemist Association, 1976, 59, 157.

5. Micheli de P.; Journal of Oil & Colour Chemist Association, 2000, 83, (9),

457.

6. Hara, K O; Journal of Oil and Colour Chemist Association, 1985, 68, (4),

254.

7. Aggrawal, D. and Maithani, A.; Paint India, 2002, 52, (4), 43.

8. Randell, D R; Radiation Curing of Polymer, 1986, 121 (P) Great Britain

by Whitstable Litho Ltd., The Royal Society of Chemistry, Burlington

House, London.

9. Roffey, C G; Photopolymerisation of Surface Coatings, 1982, 146 (P), A

Wiley-Interscience Publication, John Wiley and Sons.

10. Roffey, C G; Photopolymerisation of Surface Coatings, 1982, 156 (P), A

Wiley-Interscience Publication, John Wiley and Sons.

11. Webster, G; Journal of Oil and Colour Chemist Association, 1996, 76,

(5), 215.

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Cured film

properties

Acrylated oligomers

Acrylic Polyester Urethan

e

Epoxy

Tensile strength Low Moderate Variable High

Flexibility Good Variable Good Poor

Chemical resistance Low Good Good Excellent

Hardness Low Moderate Variable High

Non yellowing Excellent Poor Variable Moderate to poor

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Monomers

Properties

Average

mol.

Wt., Mn

Viscosity,

cP

Acrylic

functionality,

FA (What is

FA?)

Acid

value

BP

(What

is BP?)

(0C,

at y

mm

Hg)

Primary

irritation

index

Neopentyl

glycol

diacrylate

212 8 2

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acrylate