d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206
Available online at www.sciencedirect.com
jo u rn al hom epa ge : www.int l .e lsev ierhea l th .com/ journa ls /dema
he interaction of various liquids with long-term dentureoft lining materials
en-Chien Liaoa, Gavin J. Pearsonb, Michael Bradenc, Paul S. Wrightb,∗
Department of Prosthetic Dentistry, Tri-Service General Hospital and School of Dentistry, National Defense Medical Center, Taipei,aiwanFormerly Queen Mary, University of London, London, UKQueen Mary, University of London, Institute of Dentistry, Barts and The London School of Medicine and Dentistry, London, UK
r t i c l e i n f o
rticle history:
eceived 6 January 2011
eceived in revised form
4 April 2012
ccepted 25 April 2012
eywords:
a b s t r a c t
Objectives. To study the uptake of liquids, representative of those encountered orally, by long-
term denture soft lining materials, and analyze the data in terms of appropriate theories.
Methods. Four proprietary and one experimental soft lining material were investigated, and
the weight change presented as a function of time in both aqueous and organic fluids over
the course of a year. A separate experiment determined the equilibrium swelling in ethanol
of poly(ethyl methacrylate) and poly(methyl methacrylate).
Results. Uptake date for the five soft lining materials in various aqueous solution, coconut oil
and HB307 are reported. The experimental value for the equilibrium swelling of poly(ethyl
enture
ong-term soft-lining materials
ood simulating liquids
heory
methacrylate) and poly(methyl methacrylate) in ethanol was reported to indicate the solu-
bility parameter of the system.
Significance. The results have been analyzed by relevant theoretical models, which have been
shown to explain the experimental data.
emy
have been made of these systems [6–15]. One disadvantage◦
he term soft lining refers to the lining of a denture withn elastomeric type material. Such materials, being easilyeformable, will absorb energy during biting [1], and distributehe loads evenly over the whole denture bearing area, thusessening the deformation of the oral mucosa. This reduceshe discomfort when such loads exceed the ability of theissues to support them and potentially reduces resorptionf the residual bone [2]. Clearly, it is highly desirable that
uch materials do not degrade in the mouth, i.e. do not losetrength or compliance, nor become detached from the den-ure. The most obvious mechanism of compliance loss is with
∗ Corresponding author at: Institute of Dentistry, Turner Street, Londonel.: +44 0020 7882 8656; fax: +44 0020 7377 7064.
of Dental Materials. Published by Elsevier Ltd. All rights reserved.
the so-called soft acrylics, where compliance is achieved byincorporation of a plasticizer, and subsequently lost when theplasticizer leaches out [3]. Some experimental materials havebeen described which use a polymerisable plasticizer [4,5].
Another approach has been to make a preformed, heatcuring dough, comprising a main elastomer from the rub-ber industry, e.g. polyisoprene, butadiene, styrene copolymers,doughed with a higher methacrylate monomer (n-butyl andabove). In this way, it was hoped that the high strength ofsuch elastomers would be of benefit. A number of studies
E1 2AD, UK.
was that prolonged immersion in water at 37 C produceda marked deterioration in mechanical properties. This wasattributed to peroxide catalyzed scission of the double-bonds
in diene copolymers, the residual peroxide polymerizationcatalyst being the source. Hence brominated butyl rubber wasinvestigated; butyl rubber is a copolymer of ∼98% isobutylmethacrylate and 2% isoprene [16]. The brominated versionwas used because it is easier to peroxide cross-link [15].
The loss of strength in water can also, a priori, be due eitherto certain aspects of water uptake, or the interaction of organicliquids in the diet such as products containing vegetable oilsor ethanol. High water uptake can be experienced in otherwisehydrophobic elastomers, with consequent loss of strength, bythe presence of water soluble components. Thus Braden andWright observed water uptake values in a silicone rubber softliner of >60% [17]. More recent studies are those of Mante et al.on the effect of aqueous solutions on hardness, and Leite et al.on the effect of various beverages on hardness [18,19].
The current study has involved representative proprietarymaterials from the soft acrylic and silicone types, and anexperimental material based on brominated butyl rubber. Thetest liquids included aqueous, ethanol containing, and paraf-finic liquids. However a major objective of this paper was theapplication of relevant theories to the results obtained.
2. Materials and methods
2.1. Materials
The materials used are listed in Table 1 and further detailsare listed in Table 2. Eversoft and Vertex are examples of theso-called soft acrylics. Ufi-gel and Molloplast b are siliconerubbers, the latter having dispersed polymethyl methacrylate(PMMA) domains, as indicated by infra-red spectroscopy. Var-ious suggestions have been made as to its exact composition[20,21].
The brominated butyl rubber (BBR), doughed with n-butylmethacrylate and heat cured was an experimental material[15].
2.2. Methods
2.2.1. Sample preparationA polyether impression material was used to make sheetsnominally 1 mm thick, from which 20 mm diameter discs werecut using a cork borer. These discs were invested in dentalstone using conventional dental techniques. On setting, theflask was separated and the discs removed to leave a moldready for the preparation of specimens. Discs were preparedfor each of the materials using the manufacturer’s instruc-tions.
In the case of the brominated butyl rubber, the monomerliquid was made up with 1% lauryl peroxide (w/w) as initiatorand n-butyl methacrylate monomer containing 1% ethyleneglycol dimethacrylate (wt/vol) as a cross linking agent. Laurylperoxide was used instead of benzoyl peroxide, because thedecomposition in the former case is lauric acid, and benzoic
acid in the latter. Lauryl peroxide has a much lower solubil-ity in water, and consequently will have much less influenceon water uptake [20]. 100 g brominated butyl elastomer wasdoughed with 100 ml of the monomer liquid described above.
( 2 0 1 2 ) e199–e206
The curing cycle comprised 2 h at 74 ◦C, followed by 30 min at100 ◦C.
2.2.2. Water and fluid absorption characterizationAll specimens were processed according to the manufactur-ers’ directions. A total of 42 specimens were constructedfor each denture soft lining material. The specimens werethen randomly divided into seven groups of six specimens.Specimens were preconditioned after manufacture by stor-ing in a desiccator at 37 ± 1 ◦C. The specimens were removedfrom the desiccator and then after immediately weighing,were weighed at regular intervals until a constant weight wasachieved. All readings were taken to an accuracy of ±0.0002 gon an AE Mettler electronic balance (Metler-Toledo Ltd, Leices-ter, UK). This initial weight (W0) was noted. After weighing,each specimen was immediately transferred to a wide mouth,amber, screw topped glass jar containing 50 ml of a food sim-ulating liquid conditioned to 37 ◦C. The immersing liquidsselected were distilled water (DW), artificial saliva (AS) (com-position shown in Table 3) [22], 3% aqueous acetic acid (3AA)(EC Food Contact Legislation, 2000), 10% ethanol (10E), 50%ethanol (50E), coconut oil (CO) and HB307 (HB) (FDA, 2002).Each glass jar was then stored in an incubator (LABHEATModel RLCH0400, Boro Labs Ltd, Berkshire, UK) at 37 ± 1 ◦C.Each specimen was removed at predetermined time intervalsusing tweezers and carefully blotted to remove excess surfaceliquid using filter paper prior to weighing. The weights werethen recorded. Initial intervals between weighing were shortbut subsequently were increased. The fluid was unchanged forthe duration of the experiment but was topped up after eachmeasurement to maintain a fixed volume.
After a period of 52 weeks, specimens were removed fromsolution, weighed and then desorbed in an incubator (Gal-lenkamp Durastat Type 3, LTE Scientific Ltd, Oldham, UK) at37 ± 1 ◦C. Specimens were weighed at regular intervals until aminimum weight was reached (Wd). Percentage weight changeand percentage solubility were calculated as a percentage ofthe initial weight. Real percentage uptake was calculated asthe sum of percentage weight change and percentage solubil-ity, and desorption diffusion coefficients by the application ofsolutions of Fick’s equations [23]:
% Uptake =(
Wt − W0
W0
)× 100 (1)
% Solubility =(
W0 − Wd
W0
)× 100 (2)
Real % Uptake = % Uptake + % Solubility (3)
where W0 = initial weight, Wt = weight at time t and Wd = finalminimum desorbed weight. The diffusion coefficient was cal-culated from the slope of the linear parts of the plot [24]:
Distilled water DW Aqueous foods (control) Queen Mary, University of LondonArtificial saliva AS Saliva Fusuyama [22] formulation3% Acetic acid 3AA Aqueous and acidic foods (EC standard) BDH Chemical Co.10% Ethanol 10E Aqueous and low-alcoholic foods BDH Chemical Co.50% Ethanol 50E High-alcoholic foods BDH Chemical Co.
wla
S
w
D
F
e
Coconut oil CO Fatty foods
HB307 HB Fatty foods
here Mt = mass uptake at time t; M∞ = mass uptake at equi-ibrium; D = diffusion coefficient; thickness = 2l. This predicts
linear plot with respect to t(1/2) of slope (S):
= 2(
D
�l2
)1/2(5)
hich rearranged gives the diffusion coefficient:
= (S2�4l2)16
(6)
rom which D is calculated from S.Eq. (4) is valid for the earlier stages of uptake; the relevant
quation for the uptake up to equilibrium is [23]:
Mt
M∞= 1 − 8
�2
∞∑0
1
(2n + 1)2exp
{− (2n + 1)2Dt
4�l2
}(7)
Coconut oil from Cocosnucifera,C1758, Sigma Chemical Co. USANATEC GmbH, Hamburg, Germany
2.2.3. Solubility parameters of PMMA/ethanol andPEM/ethanolIn the case of PMMA, a rectangular strip of “Perspex”(5.5 × 2 × 0.1 cm) was weighed, immersed in ethanol, andweighed until constant weight was reached, then desorbedto constant weight to give the equilibrium uptake C0 (g/cm3),from which v2 (Eq. (1)) was calculated. A similar sheet ofPEM was made by making a dough of PEM powder and ethylmethacrylate monomer, which was processed in the normalway to give a heat cure product.
3. Results
Fig. 1 gives the uptake data for BBR in various aqueous solu-tions.
Fig. 2 gives corresponding data in for coconut oil, andHB307, which is largely coconut oil based.
Fig. 3 gives Shore hardness vs. storage time for BBR incoconut oil and HB307.
e202 d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206
Uptake in aqueous solutions by
Brominated Bu tyl/n bu tyl methacrylate
systems
0
5
10
15
20
25
30
0 800600400200
t½(min s½
)
% U
pta
ke
DW
AS
AA
90/10
50/50
Fig. 1 – Uptake in aqueous solutions by brominatedbutyl/n-butyl methacrylate systems.
Brominated butyl rubber in coconut oil and HB307
0
50
100
150
200
250
0 800600400200
t½
(mins ½
)
% u
pta
ke
Fig. 2 – Brominated butyl rubber in oil and HB307.
Effect of immersion of brominated butyl rubber
based lining on Hardness
0
5
10
15
20
25
30
35
40
45
1000010001001010.1
time(hours)
Sh
ore
hard
ness
coc oil
HB307
Fig. 3 – Effect of immersion of brominated butyl rubberbased lining on hardness.
Water uptake of Ufigel from various aqueous
solutions
0
0.2
0.4
0.6
0.8
1
1.2
8006004002000
t½(mins ½)
Mt/M
∞
DW
AS
3%AA
10%eth
50% eth
theory
t½ fit
Fig. 4 – Water uptake of Ufi-gel from various aqueoussolutions.
Fig. 4 gives the uptake of Ufi-gel in various aqueous solu-tions, in the form of Mt/M∞ vs. t(1/2), in order to check thevalidity of Eq. (7) for DW. The theoretical plot was obtainedby substituting the value for D obtained from Eq. (6) in Eq. (7).Typically M∞ was ∼0.3% (w/w).
Fig. 5 gives the uptake of Molloplast b in various aqueoussolutions.
Fig. 6 gives the weight change of Molloplast b and Ufi-gelin coconut oil. Note both graphs have a common plot up to∼200 min(1/2), with a maximum at ∼50 min(1/2), with the twoplots diverging beyond this point. Similar behavior is notedwith HB307.
Fig. 7 gives the uptake of Eversoft and Vertex Soft in variousaqueous solutions.
Water uptake of Molloplast b from
aqueous solutions
0
0.5
1
1.5
2
2.5
3
3.5
8006004002000
t½(mins ½)
% U
pta
ke
DW
AS
3%AA
10%eth.
50%eth.
Fig. 5 – Water uptake of Molloplast b from aqueoussolutions.
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206 e203
Absorption of Coconut Oli by
Molloplast b and Ufi-gel
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
8006004002000
t½(mins½)
% W
eig
ht
Ch
an
ge
MB
Ufi-gel
Fig. 6 – Absorption of coconut oil by Molloplast b andUfi-gel.
Eversoft and Vertex in 50/50
ethanol/distilled water
-20
-15
-10
-5
0
5
10
8006004002000
time (mins)
% W
eig
ht
ch
an
ge
Eversoft
Vertex
F
4
4
Iwqasason
4Shtns
Table 4 – Comparison of the interaction between Ethanoland PMMA and PEMA.
Ethanol PMMA PEMA
% (w/w) uptake 4.5 35.5
ig. 7 – Eversoft and Vertex in 50/50 ethanol/distilled water.
. Discussion
.1. Brominated butyl rubber
n the current work, the brominated butyl polymer was mixedith n-butyl methacrylate to form a dough, which was subse-uently heated to give a cured product. The fact that it formed
homogeneous dough suggests that the elastomer is dis-olving, at least in part. However, when the product is cured,nd the n-butyl methacrylate momomer polymerizes, phaseeparation is likely, because of the decrease in the entropyf mixing. Hence the interaction of the material with liquidseeds to take this into account.
.1.1. Uptake from aqueous solutions (Fig. 1)ince the structure of brominate butyl is largely paraffinic, it isydrophobic. Hence a water uptake of all of the aqueous solu-
ions of >10% is initially surprising – the water uptake of poly-butyl methacrylate is ∼1%. However, the presence of wateroluble moieties is the likely cause, and reflects the findings of
Muniandy and Thomas for natural rubber [24,25]. The fact thatthe uptake is essentially the same for artificial saliva indicatesthat the osmolarities of the impurities is very much greaterthan that of the artificial saliva.
Clearly, the uptake is diffusion controlled, as the plot is lin-ear with respect to t(1/2); the fact that the plot is linear overthe whole time period of the experiment (∼340 days) indi-cates the uptake is less than half that at equilibrium [23]. Theactual uptake at the end of the time period is greater for thesolutions containing ethanol and acetic acid, particularly thelatter. Whether this reflects a higher diffusion coefficient inthese cases or an actual increase in the equilibrium uptake isimpossible to discern.
4.1.2. Uptake from coconut oilFig. 2 shows the uptake from coconut oil to be diffusion con-trolled; from the linear t(1/2) region, the slope was determinedand used to calculate the diffusion coefficient from (see Eq. (6)above), giving D = 1.07 × 10−13 m2 s−1.
The equilibrium concentration of coconut oil is ∼180%,which gives a volume fraction of polymer (vp) of 0.357. Theuptake of non-cross linked polymers is governed by the FloryHuggins equation [26,27]:
� = RT[ln(1 − v2) + v2 + �v2
2
](8)
where � = chemical potential, R = universal gas constant,T = temperature (K), v2 = volume fraction of polymer, � = thesolubility parameter. If � < 0.5, the polymer and solvent arecompletely miscible. For cross-linked polymers, a fourth termis added between the square brackets, namely �2V1v
1/32 /Mc.
Braden et al. [28] have shown that Eq. (8) containing thecross-linking term can be written in terms of the Elastic Mod-ulus (E) of the polymer, whence the � is given by Eq. (9):
� = − ln(1 − vp) + vp + Ep(1 + C2/C1VLv1/3p /3RT)
v2p
(9)
where C1 and C2 are the parameters of the Mooney RivlinStored Energy function for the deformation of an elastomer[29]. In this case � has been calculated for the C2/C1 ratio of0, 0.5, and 1. E was calculated from the Shore hardness of thematerial [30]:
E(MPa) = 0.0981(56 + 7.66s)0.1375(254 − 2.54s)
(10)
where E is Young’s modulus and s is the Shore hardness.The value of E so calculated from a Shore hardness of 44.6
is 2.00 MPA; this substituted in Eq. (9) gives the � values givenin Table 4. Fig. 3 plots hardness against storage time for BBR.It is clear that the hardness decreases with time, in Youngs
Modulus terms, when the Hardness is 20; this is equivalent toa Youngs Modulus of 0.73 MPa, i.e. a fall of 63.5%.
All of these are <0.5, meaning that coconut oil is a solventfor brominated butyl rubber; this is hardly surprising giventhe paraffinic character of polyisobutene and coconut oil. Itshould also be noted that the parameter � for polyisobuteneand octane is ∼0.4 [31]. Some circumspection is necessary withthe application of the above theory to the actual system used,i.e. brominated butyl rubber doughed with n-butyl methacry-late. When this system is cured, it is not at this stage knownwhether the poly (n-butyl methacrylate) phase separates, orforms an interpenetrating network. However, with low TG ofpoly (n-butyl methacrylate) and the similarity of its solubilityparameter (16.58 [J m−3](1/2)) to that of n-dodecane, which isclosely related to lauric acid (16.0 [J m−3](1/2)) [31,32], the mainconstituent of coconut oil, suggests that it will dissolve rapidlyin the coconut oil, and hence not limit the swelling of thebrominated butyl system.
4.1.3. Uptake from HB307The uptake from HB307 is very much higher than that forcoconut oil (240% c/f 180%), a major constituent of HB 307;this is reflected in the extreme physical degradation in HB307beyond two months, to the point where Shore hardness valuescan no longer be determined.
The constituents of HB307 are predominantly those ofcoconut oil, with two differences. HB307 contains 0.7% diglyc-eride and <0.4% monoglyceride. Clearly the question iswhether these small quantities can cause the effects exhib-ited. Both glycerides are hydrophilic, and their solubilityparameters indicate they will be immiscible with brominatedbutyl rubber, hence are water soluble impurities, and hencewill promote absorption of water from the atmosphere. Itis suggested the subsequent droplet growth around thesedroplets in an already highly swollen system, and the stressesgenerated therein, may lead to the mechanical breakdownnoted above. These observations emphasize the importanceof using clinically relevant solutions for evaluation purposes.
4.1.4. General observationsThe substantial uptake noted in aqueous solutions and incoconut oil based systems, are clearly major disadvantagesof brominated butyl rubber, which may well apply to otherhydrocarbon based elastomers.
4.2. Silicone polymers
4.2.1. Uptake from aqueous solutionsBoth Ufi-gel and Molloplast b (Figs. 4 and 5) show very sim-ilar plots, when the data is plotted as Mt/M∞ vs. t(1/2); forthe linear t(1/2) region, the data for the various aqueous solu-tions fit a common straight line at the earlier values of time,from which the diffusion coefficients could be calculated fromEq. (6). These were 4.2 and 4.5 × 10−13 m2 s−1 for Molloplast band Ufi-gel respectively. These near identical results indicatethat the uptake of water is through the polymer matrix, even
though Molloplast b contains poly(methyl methacrylate), pre-sumably in discrete domains. M∞ values in the range 0.3–0.5%are low, although much greater then the solubility of water inpolydimethyl siloxanes.
( 2 0 1 2 ) e199–e206
Beyond the linear region, again the plots for the varioussolutions are similar, with the exception of the 50% ethanol inthe case of Ufi-gel; the reason for this is not clear. If the diffu-sion coefficients obtained are substituted into Eq. (5), it can beseen that the experimental data lags behind the theoreticallypredicted values. This is symptomatic of the diffusion coeffi-cient decreasing with increasing concentration and is a wellknown feature of the diffusion of water in silicone polymers[33].
Overall, the amount of water absorbed in the two materi-als is ∼1.5–3% (w/w). Considering that the solubility of waterin silicone polymers is 30 mol m−3, i.e. 0.054% [34] it is clearthat the bulk of the water uptake is due to other causes, prob-ably the presence of water soluble moieties in the filler. Thisfeature could be important if higher levels of water solublemoieties are present. This could result in high enough osmoticpressures within droplets to cause failure. Indeed Amsden hasutilized this phenomenon to produce drug delivery systems[35,36].
4.2.2. Uptake of coconut oil based liquids (Fig. 6)Both of the silicone materials give very similar and unusualplots for both coconut oil and HB307. A very rapid increase inweight is evident, but this rapidly reverses leaving ultimately anet uptake of 0.5–1.0%. This suggests the possibility that somesoluble material is being extracted.
4.3. Soft acrylics
The results for water/ethanol mixtures are drastically differ-ent from the behavior of both materials in distilled water itself,and artificial saliva (Fig. 7). In spite of the fact that that Ever-soft is a visco-elastic gel, and Vertex is a heat cured system,the plots are remarkably similar in character. There are threedistinct stages:
(i) A rapid increase in uptake, suggestive of absorption ofethanol from the ethanol/water mixture.
(ii) A subsequent reversal of the weight change process,indicative of an extraction process, presumably theextraction of the phthalate plasticizer, which is misciblewith ethanol.
(iii) A further reversal, indicative of an uptake process.
This apparently complex behavior indicates that twoprocesses are going on simultaneously, i.e. extraction of plas-ticizer and the absorption of ethanol by the PEM component.
Considering first Eversoft, the weight change from the max-imum weight loss to the final weight (+) change is ∼20%.However this is based on the original weight of the sam-ple, whereas the remaining material is just the original PEMswollen by ethanol. Assuming a powder/liquid content of2.5/1, this indicates that the PEM contains ∼30% ethanol.Separate experiments on a PEM polymer per se (see Materi-als and Methods) gave an equilibrium uptake of ethanol of35.5% (Table 4), indicating the above estimate of ∼30% rea-
sonable. Calculations of the Flory Huggins parameter (see Eq.(8)) for the PEM-ethanol system gave 0.6502. Parallel exper-iments on poly(methyl methacrylate) (PerspexTM) gave a �
value of 2.66, in reasonable agreement with published values
30,31]. This corresponds to an equilibrium uptake of ethanoly PMMA of 2.11%. This comparative data for the interac-ion of ethanol with PMMA and PEMA respectively, showsuantitatively why PEM is used in tissue conditioner type for-ulations. It also indicates that ethanol containing liquids
ould well have a deleterious effect on other materials basedn poly(ethyl methacrylate), such as some denture reliningnd temporary crown and bridge materials.
. Conclusions
he uptake of water or organic liquids by the elastomerssed as soft lining materials can be analyzed in terms ofhe Muniandy and Thomas theory for the effect of wateroluble moieties on the water uptake of elastomers and thelory Huggins equations for the interaction of organic liquidsnd polymers. The former is of particular interest, in that itxplains the high water uptake of some essentially hydropho-ic polymers.
cknowledgements
ased on a thesis submitted to the University of London inartial fulfillment of the requirements for the PhD degree.
Dr Wen-Chien Liao was supported by the Tri-Service Gen-ral Hospital and National Defence Medical Center, Taipei,aiwan.
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