soft prosthesis materials based on powdered elastomers

Soft prosthesis materials based on powdered elas~mers

Sandra Pazker and M. Braden Dental School, The London Hospital Medical College, Turner Street, London El ZAD, UK (Received 6 June 1989; accepted 10 September 1989)

A new class of soft prosthesis material has been developed, based on the combination of a powdered elastomer and a mathacrylate monomer that polymerizes to an elastomer. Such systems are processable by conventional dental technology. This principle avoids the use of plasticizers. Natural rubber, butadiene styrane and butadiene ac~lonitrile elastomers have bean used, together with a number of higher alkyl methac~latas (C,&J) and 2-etho~athyl methac~late. Such systems hove been evaluated with respect to mechanical propa~ies, including tear strength. adhesion to denture base polyjmethyl methac~lata), water sorption and visco-elastic properties. A number of potentially viable systems have emerged, which may be useful in external prostheses. Many have high long-term water absorption, which makes questionable their long-term usefulness interorally. Nevertheless, there is still considerable scope for development in this area.

Keywords: Dental materiak elastomers, methacrylates, mechanical properties, water sorption

The difficulties in producing a satisfactory soft lining material for dentures and maxillofacial prosthesis materials are well documented’. Soft acrylics, whilst generally having satisfactory adhesion to poly(methyl methacrylate), harden due to loss of plasticizer and have poor mechanical properties and high water uptake. Indeed the latter is a problem with all elastomeric materials’. Most silicone materials have problems of adhesion.

Whilst there seems to be no ready solution to the problem of high water uptake, the development of soft acrylics with enhanced strength and containing no plasticizer seems to be worth invest;gating. Following prelimina~ work in this area3, more detailed work is now presented using powdered elastomers with higher methacrylate esters.

MATERIALS AND METHODS

Materials

The polymers used are summarized in Tab/es 7 and 2. It will be noted that these materials contain partitioning agents to keep the particles separate; these can be a problem in that some enhance water uptake.

The monomers are listed in fable 3. In some cases, ethylene glycol dimethac~late was added as a cross-linking agent.

Correspondence to Professor M. Braden.

Visco-elasticproperties. The real and imaginary parts of the complex shear modulus (G, and G2, respectively) and tan S were determined at three different frequencies in the range 0.1-0.5 Hz, using the torsional pendulum apparatus developed by Braden and Stafford4. For some of the materials, G,, G2 and tan S were determined over the temperature range of 20-80°C.

Bond strength to po~~~rnerhy~ methac~ia~e~. The peel strength in terms of the energy of separation was determine bythe method first described by Kendal’, subsequently used for soft lining materials by Wright’. Some materials were tested after immersion in water at 37°C for periods up to 6 month.

Tear strength. The ‘trousers’ test-piece for elastomers first described by Rivlin and Thomas7, subsequently used for dental elastomers, was used. Wright has described this method for soft lining materials* and Webber and Ryge for a number of materials’.

Ensite tests. Prelimina~ tests were carried out on thin strips of material, approximately 5.5 X 0.5 X 0.1 cm in size. Two lines were drawn across the width of the specimen, which was then suspended from a rigid support. The distance between the two lines was measured using a

8 1990 Butterworth-Heinemann Ltd. 0142-9612/90/070482-09

482 Biomaterials 1990, Vool 11 September

Table 1 Powdered natural rubbers

Product Manufacturer Description

Pulvatex Rubber Latex Ltd A powdered natural rubber containing an anti-oxidant

Crusoe (Standard) Harrison and Crossfield Ltd

A powdered natural rubber produced by the spray drying of natural rubber latex. Contains

Crusoe Fines

Crusoe MG.49

Harrison and Crossfield Ltd

Harrison and Crossfield Ltd

7-8% silica as partitioning agent

A by-product of the Crusoe (standard) production, containing

50% rubber and 50% silica A graft polymer of 5 1% natural rubber and 49% methyl methacrylate, spray dried as Crusoe (standard) but only containing

Harub 5CV. 6O.CP Harnson and Crossfield Ltd

l-2% silica as partitioning agent SMR CV 60 which has been crumbed with the addrtion of 3-4% silica as partitioning agent

Table 2 Synthetic elastomers

Product Manufacturer Descnption

Breon 1042 CAS BP Chemicals A butadiene/acrylonitrile copolymer Ltd of medium-high acrylonitrile.

34.5-37% bound acrylonitrile Breon 1112 PVC BP Chemicals A butadiene/acrylonitrile copolymer

Ltd as above Breon NBR BP Chemicals A butadiene/acrylonitrile copolymer 465 CAS Ltd of high acrylonitrile. 40-42% bound

acrylonitrile Cariflex 101 Stechler Ltd A butadrene/styrene random

copolymer SBS Plascoat A butadiene/styrene block

Systems Ltd copolymer. Maximum particle size 1 mm. No partitioning agent

travelling microscope. Weights of increasing value were suspended from the free end of the specimen and the extension was measured using the travelling microscope. The loading was carried out in a timed sequence and was the same for all specimens. The weight was added and left for 2 min, after which the extension was measured and the

Table 3 Available alkyl methacrylate monomers

Soft prosthesis materiaals: S. Parker and M. Braden

weight removed. The specimen was then left for 1 min to recover before the next loading. The weights were increased until the specimen fractured.

A second specimen of the same material was used to repeat the experiment, except, on this occasion, the loading was stopped short of the fracture point, after which the loading was reduced in the same timed sequence.

Plots of stress (load/cross-sectional area) versus extension ratio ,I = (1 + strain) were made for each of the specimens, from which the Young’s modulus and recover- ability of the material were evaluated.

A Nene tensile testing machine was used to determine the stress-strain relationships of some of the materials. A flat dumb-bell shaped specimen was used. The apparatus was calibrated by suspending known weights from the upper grip and noting the deflection on the chart recorder. The specimen was held vertically in the apparatus by two grips and the load was applied and automatically recorded on the chart recorder. The speed of separation of the two grips was approximately 0.1 cm s-‘. The extensron was measured using an infrared extensometer. Before testing, two triangles, their apices 2 cm apart, were imprinted on to the specimen using a silk screen. The two sensors of the infrared extensometer are locked on to these two marks and follow them as the specimen is extended. The extension is recorded on the X axis of the chart recorder. The extension was increased until fracture occurred, so that ultimate tensile strength and related percentage elongation could be determined. It is essential that the specimen fractures in the central narrower region of the specimen where the dimensions are known.

Waterabsorption. The specimen consisted of a thin sheet of the material to be investigated. It was first weighed to an accuracy of f 0.0002 g and then immersed in distilled water contained in a ground glass stoppered test tube. The tube was kept in a thermostatically controlled water bath at 37°C & 1 “C. At noted intervals, the specimen was removed, blotted on filter paper to remove excess water and weighed to the same accuracy as its initial weighing. This process was continued until there was no significant change in weight. The specimen was then transferred to a drying oven kept at

Monomer Formula Manufacturer Relevant propemes

0 2-ethoxyethyl methacrylate CH,-CH,-0-(CH,),-O-C-C = CH,

CH,

0 1 -tridecyl methacrylate CHs--(CH,),2-O-C-C = CH,

CHs

CH,-CH, 0 2-ethylhexyl methacrylate CH,--(CH,),-CH-CH,-O-C-C = CH,

CH3

0 Lauryl methacrylate CH,-(CH,),,-O-C-C = CH, (1 -dodecyl methacrylate) C”3

Nonyl methacrylate 0

CH,-(CH,),-O-C-C = CH,

CH3

ICI Chemicals Division or Ancomer Ltd. Manchester

Roehm’s methacryl ester from Cornelius

ICI Chemrcals Division

As emprcryl methacrylate from Albnght and Wilson Ltd. WhItehaven

ICI Chemrcals Division

T, of polymer = - 15”;

No data on T, of polymer. Poly(tetradecyl methacrylate). T, reported to be -76°C to 9”C**

Tg of polymer IS - 10°C (brittle pomt)**

Tg of polymer IS -65”C**

T, of poly(octyl methacrylate) vaned -70°C to -2O”C**

* Sartomer Monomer (197 1). ** W.A. Lee and R.A. Rutherford (1975).

Biomaterials 1990, Vol 11 September 483

Soft prosthesis materials: S. Parker and M. Braden

37°C k 2°C and again at intervals taken out and weighed. It would have been ideal to continue the cycle of absorption and desorption until the two were reproducible. However, in the time available, up to 3 yr, none of the materials had attained this situation. In fact the first absorption cycle of most of the materials lasted a year or more.

Preparation of materials. In preliminary experiments, 1% benzoyl peroxide was dissolved in the monomer; subsequently, it was added to the powdered elastomer as 2% Lucidol CH50 (Akzo Chemicals Ltd), a 50/50 benzoyl peroxide/ cyclohexyl phthalate master batch, by ball milling.

Preliminary experiments were carried outtodetermine which monomers would dough with which polymer. Subsequent curing was by standard heat-cured dental techniques.

RESULTS

Doughing properties

Tab/e 4 summarizes the doughing characteristics of natural rubber-based powders. The butadiene/acrylonitrile rubbers would only dough with 2-ethoxyethyl methacrylate, presumably due to them both being highly polar. Both block and random copolymers of butadiene/styrene doughed with all the monomers listed in Table 3.

Visco-elastic properties

Tab/es 5-9 summarize data for all the natural rubber systems studied. The Pulvatex-nonyl methacrylate system proved to be so dependent on powder/liquid ratio that the results are also presented in Figure 1.

Tab/es 10-l 7 give data for synthetic elastomers, including the effect of water immersion at 37°C. The effect of temperature on the two mixes given in Tables 12 and 13 was investigated and plots of G, and tan 6 against temperature for both can be seen in Figure 2.

Figure 3 plots visco-elastic properties as a function of temperature for the Pulvatex natural rubber-nonyl methacrylate system.

Bond strength to poly(methyl methacrylate)

Data for the butadiene/acrylonitrile-2-ethoxyethyl methacrylate system are given in Tab/e 78.

Table 4 Gelling properties

Rubber Z-ethoxyethyl methacrylate

2-ethylhexyl methacrylate Harub 5CV.6O.CP

Crusoe (standard) No gelling Very slow gelling to a sticky dough

Crusoe Fines No gelling Rapid gelling to a sticky dough

Crusoe MG.49 Gelled in 6 min No gelling into a sticky dough

Harub 5CV.6O.CP No gelling Very slow gelling to a sticky dough

Table 5 Visco-elastic properties of the Pulvatex/2-ethylhexyl methacrylate system

Powder/liquid (g/ml) G, (MN m-*) Tan 6

0.30 27.8 0.37 0.40 15.4 0.39 0.45 12.5 0.39

Table 6 Visco-elastic properties of the Pulvatex/nonyl methacrylate system

Powder/liquid (g/ml) G,(MN m-*) Tan 6 Test temperature (“C)

0.200 25.800 0.45 23 0.250 2.400 0.42 23

3.010 0.47 22 5.460 0.46 21

0.275 4.510 0.55 26 0.300 (i) 5.100 0.41 23

(ii) 0.350 0.11 21 0.369 0.15 23

0.325 0.306 0.1 1 26 0.350 0.404 0.09 22

0.439 0.18 21 0.400 0.104 0.19 21

0.200 0.20 24

0.317 0.25 25

Table 7 Visco-elastic propeflies of the Pulvatex/l -tridecyl methacrylate system

Powder/liquid (g/ml) G,(MN m-*) Tan 6 Time (s)

0.30 0.467 0.090 5.3 0.35 0.139 0.089 8.8 0.40 0.1 78 0.074 6.7

Table 8 Visco-elastic properties of the Pulvatex/laoryl methacrylate

system

Powder/liquid (g/ml) G,(MN m-‘) Tan 6 Time (s)

0.30 2.10 0.17 6.77

0.35 0.25 0.06 7.19

0.40 0.052 0.12 1 1.63

Table 9 Visco-elastic properties of the natural rubber materials other than Pulvatex

Powder Monomer Powder/ G, (MN m-*) Tan 6 liquid (g/ml)

Crusoe (standard)

Crusoe (standard)

Harub 5CV.6O.CP

Crusoe Fines

Crusoe (standard)

Harub 5CV.6O.CP

Crusoe Fines

Crusoe (standard)

Harub 5CV.6O.CP.

Crusoe Fines

2-ethylhexyl methacrylate





1 -tridecyl

methacrylate

1 -tridecyl methacrylate

1 -tridecyl methacrylate

Lauryl methacrylate

Lauryl methacrylate

Lauryl methacrylate

0.36 9.530 0.42

0.60 1.300 0.33

0.66 13.100 0.37

1 .oo 6.900 0.29

0.38 56.200 0.34

0.60 0.474 0.13

1 .oo 0.193 0.15

0.40 6.980 0.17

0.60 0.636 0.14

1 .oo

0.40

0.175

9.070

0.18

0.19

4.84 Biomaterials 1990, Vol 11 September

Soft prosthesis mater/a/s: S. Parker and M. Braden

Tab/e 10 Visco-elastic propelties of Breon elastomer-2-ethoxyethyi methaclylate systems, benzoyl peroxide in the monomer

Elastomer Visco-elastic property Percent (%) by volume of ethylene glycol dimethacrylate in monomer -_ _____

3 5 10 15 20 30

Breon 1112 PVC G, (MN mm*) 1.94 i .7a 7.52 29.40 27.80 18.90 Tan 6 0.58 0.52 0.42 0.29 0.28 0.23

Breon 1042 CAS G, (MN m-‘) 3.81 10.40 9.66 5.19 Tan 6 0.37 0.34 0.31 0.29

Breon NBR 465 G, (MN mm*) 1.48 7.66 3.8 1 7.47

Tan 6 0.45 0.36 0.37 0.30

I

Figure 7 The effect of powder/l/quid ratio on the visco-elastic properties of Pulvatex (NRJ-based materials. 2-ethylhexyl methacrylate: G, (0), tan 6 (0): nonyl methacrylate: G, (0). tan 6 (H); 1-tridecyl methactylate: G, (A), tan 6 (A).

Tear strength

Tear strength data for the butadiene/acrylonitrile system are

given in Table 19.

Tensile properties

Typical stress-extension ratio plots are given in Figures 4-9.

(Note that extension ratio A = 1 + strain.)

Water sorption

Figures lo-12 show data for the butadiene/acrylonitrile

systems, which are typical of elastomer uptake as a whole.

Tab/e 1 1 Visco-elastic properties of Breon 1042 CAS-2-ethoxyethyl methacrylate system, benzoyl peroxide ,n the powder

Percent (%) by volume of ethylene glycol G,(MN mm’) Tan fi

dimethacrylate in monomer ___-___

30 12.10 0.25 20 6.66 0.31 10 2.20 0.36

Table 12 Effect of water absorption on the visco-elastic propertIes of Breon 1042 CAS plus 70% 2-ethoxyethyl methacwylate and 30% ethylene glycol dimethacrylate, with benzoyl peroxide in the monomer

Treatment G,(MN mm2) Tan 6

lnltial 9.7 1 0.27 4 wk In water at 37°C 10.70 0.25 3 d dryng at 37°C 15.50 0.23

Table 13 Effect of water absorption on the wsco-elastz properties of &eon 1042 CAS plus 90% Z-ethoxyethyl methacrylate and 10% ethylene glycol dimethacrylate, with benzoyl peroxide in the powder

Treatment G,(MN mm*) Tan S

lnltial 1.85 0.35 4 d in water at 37°C 2.15 0.38 3ddryngat37”C 5.87 0.32 4 wk In water at 37°C 2.69 0.35 5 d dryng at 37°C 5.48 0.33

Table 14 Visco-elastic propemes of the Cariflex 701/Z-ethoxyethyl methaclylate system

Percent (%) by volume of ethylene glycol G, (MN m -‘) Tan 6 dimethacrylate in monomer

--__

3 0.91 0.54 5 2.00 0.47

10 2.09 0.37 15 1.82 0.36 20 1.66 0.33 30 3.72 0.3 1

Table 15 Visco-elastic propenies of the Cariflex 10 l/2-ethylhexyl methacrylate system

Percent (%) by volume of ethylene glycol dlmethacrylate in monomer

G, (MN m-*) Tan fi

___.

10 0.63 0.22 20 2 95 0.31 30 2.20 0.28

Biomaterials 1990, Vol 11 September 485


Table 16 Visco-elastic proper of Cariflex 10 1 plus various monomers

Monomer G, (MN m-*) Tan 6

Nom/l methacrylate 0.390 0.34 Lauryl methacrylate 0.102 0.31 1 -tridecyl methacrylate 0.430 0.20

Table 17 Visco-elastic properties of the SBS- I-tridecyl methacrylate system

Powder/liquid (g/ml) G, (MN m-*) Tan 6

0.9 1.51 0.12 1 .o 1.14 0.15

10

2 :

1.0

01 20 30 60 70 00

&ore 2 The effect of temperature on the visco-elastic propefiies of the Breon 1042 (butadiene/acrylonitrile)-2-ethoxyethyl methacrylate systems. 70% 2-ethoxyethyl methacrylate (benzoylperoxide in the monomer): G, (0). tan 6 (e); 90% Z-ethoxyethylmethacrylate (banzoylperoxide in the powder): G, (O), tan 6 (m).

Table 18 Peeling energies for Breon 1042-2-ethoxyethyl methacrylate

system

Treatment Peeling energy (kJ m-‘)

As processed 14.18 After 24 wk in water at 37°C 7.06

DISCUSSION

Preliminary observations

A number of doughing materials were possible from the systems studied, all of which could be fabricated into elastomeric products without the use of plasticizers.

30 60 70 101

Figure 3 The effect of temperature on the visco-elastic properties of Pulvatex-nonyl methacrylate system. 0.275g/ml: G, (0). tan S (0); 0.325 g/ml: G, (01, tan 6 Im).

Table 19 Rupture properties of the Breon 1042 plus 2-ethoxyethyl methacrylate material

Tear rate (cm e-l) Tear energy (kJ m-*)

0.050 33.20

0.053 30.09 0.059 35.95

0.066 33.77

0.084 32.02

10 20

Extemlon Ratlo ,+

Figure 4 Stress-extension ratio data for Breon 1042-2-ethoxyethyl methacrylate systems. (0) 30% ethyleneglycol dimethacrylate. (0) 20% ethyleneglycol dimethacrylate, IA) 10% ethyleneglycol dimethacrylate.

486 Biomaterials 1990, Vol 11 September


L Ertemian Ratio X

figure 6 Stress-extension ratio data for Cariflex (butadiene/styrene)-based systems. (X) 2-ethoxyethyl methacvlate, (A) 2-ethhylhsxyl methscryylate. (01 nonyl methacrylate, (0) lauryl methacrylate, (0) I-tridecyl methacrylate.

/&_#+-+---- -n--+------m -“.‘“_a

4

Figure 7 Stress-exrension ratio data for Crusoe (NR)-based systems. (A) 2-ethylhexyl methacrytate, (El) Jauryl methacrylate, f 0) 1 -tridecyl methscwlate.

Biomateriafs 1990, Vof f I September 487


10 11 12 13 Ext&on Rail’, 16 17 18 19 20

X

Figure 8 Stress-extension ratio data for Harub 5CWOCP (NR)-based systems. (A) 2-ethylhexyl methacwlate. (0) law1 methacrylate, (0) I-tridecyl

methacwlate.

%

12 Extension Raho X

Figure 9 Stress-extension ratio data for various butadiene-acrylo nitrile

polymers with 2-ethoxyethylmethacrylate. (A/ &eon 1112 PVC. (0) Breon 1042 CAS, (0) Breon NBR 465 CAS.

Visco-elastic properties

The materials studied gave compliant materials, with a wide range of shear moduli in the range 0.052-56.0 MPa; Clarke and Braden” showed that current proprietary soft lining materials were in the range 0.2-2.0 MPa. The higher moduli in the current work were either with high amounts of ethylene glycol dimethacrylate or with 2-ethoxyethyl methacrylate.

In those cases where the effects of water immersion at 37°C was studied, there was some hardening, probably due to post-curing. Most materials are relatively insensitive to temperature.

What was very apparent with natural rubber in some cases, particularly Pulvatex with nonyl methacrylate, was the critical dependenceof modulus on rubber content (Figure 7). There appeared to be a discontinuity at a powder/liquid ratio of 0.3 g/ml, possibly due to a phase inversion.

In general, the powdered elastomer-based materials had much lower tan 6 values than conventional soft acrylics, i.e. they were much more elastic.

Bond strengths

The butadiene/acrylonitrile-2-ethoxyethyl methacrylate materials had bond strengths comparable with those quoted by Wright for other materials6 (4.47-23.0 kJ/m’).

Tear strength

Again, the tear strength of the butadiene/acrylonitrile systems was much higher than most other materials, judging by Wright’s values7 (0.8-2 1.1 kJ/m*).

Tensile properties

The tensile properties compared well with published data for soft acrylics. It was surprising that materials based on natural rubber were not stronger, as natural rubber unfilled vulcanizates are extremely strong because of the ability to crystallize on stretching. The best results in the extension to break were obtained with the butadiene/styrene block copolymers.

As strength depends to some extent on cross-linking of the elastomer phase, it could be reasoned that the added peroxide will, apart from initiating polymerization, cross-link the elastomer’ and produce graft polymersg.

1. Cross-linking: R I

[CH,-C = CH-CH2], + X0. - R I

-CH2 -C = CH-CH- + XOH 0

R I

2. -CH,- C =CH-CH-- 0

R I

-CH, -C = CH-CH- I

-CH, -C = CH-CH- I R

488 Biomaterials 1990. Vol 11 September


Figure 10 WaterabsorptiorYdesorption data on Breon 1112 PVC-2-ethoxyethylmethacrylate systems. Absorption: 1st (0). 2nd (0). 3rd (A); desorption: 1st

(*I, 2nd (WI.

Figure 11 Water absorption/desorption data on &eon 1042-2-ethoxyethyl methacrylate systems. Absorption: 1st (0). 2nd /O), 3rd (a J; desorption: 1st (a), 2nd (m), 3rd (A).

2. Grafting:

R

I

CH3

-CH,-C = CH-CH- + CH2 = C-

0 I

cool3

R

-CH,-C-CH-CH-

r _-L7

CH,

ROOC-C-CH3

L___.~ I I l n

Water sorption

As found by Wright” and Parker and Braden’, water

sorption was high and prolonged due to the mechanism

described by the latter authors. The high water loss in the

butadiene/acrylonitrile materials was somewhat disturbing.

As there was no plasticizer, the question arises as to what is

being extracted. The manufacturers give the residue acrylo-

nitrile content as < 1 p.p.m. One of the manufacturers of 2-

ethoxyethyl methacrylate” has used a warning that the type

of ether linkage in this monomer is related to that in

chemicals which are tetragenic. Recently, Aiken13 has

shown that implanted poly(2-ethoxyethyl methacrylate)

produces a number of pathological responses in experimental

animals. Hence, the possibility of hydrolysis of this polymer

in both cases cannot be overlooked.

Biomaterials 1990. Vol 11 September 489


Figure 12 Water absorption/desorption data on Breon NBR465-2-ethoxyethyl methacrylate systems. Absorption: 1st (O), 2nd (0). 3rd (A); desorption:

I@). 2nd Iw).

1st

CONCLUSIONS

Soft acrylic materials based on powdered elastomers and higher methacrylate esters, yielded a range of elastomeric products, processable by conventional dental technology.

A wide range of moduli, all with generally low loss tangent values are possible. Reasonable strengths were obtained, particularly with butadiene/acrylonitrile rubbers and bonding to PMMA seemed to present no problems. However, high water sorption and the dependence of such materials on the use of 2-ethoxyethyl methacrylate makes them biologically suspect. Hence such materials could not be used in an implant or interoral situation. There is, however, clearly much scope for developing the principles described in this work.

REFERENCES

1 Wright, P.S., Soft lining materials: Their status and prospects,.! Dent. 1979,58, 1801-1807

10

11

12

13

Parker, S. and Braden. M., Water absorption of methacrylate soft lining materials, Biomaterials 1989, 10, 91-95 Parker, S. and Braden. M.. New soft lining materials, J. Dent. 1982, 10. 149-l 53 Braden, M. and Stafford, G.D., Visco-elastic properties of some denture base materials, J. Dent. Res. 1968, 74, 519-523 Kendall, K., The adhesion and surface energy of elastic solids,./. Phys. D. 1971,4, 1186-1195 Wright, P.S., Characterisation of the adhesion of soft lining materials to poly(methylmethacrylate). J. Dent. Res. 1982, 61, 1 OOZ- 1005 Rivlin, R.S. and Thomas, A.G., I characterisation energy for tearing, J. Polym. Sci. 1953, 10, 291-318 Wright, P.S., Characterisation of the rupture properties of denture soft lining materials, J. Dent. Res. 1980. 58, 614-619 Webber, R.L. and Ryge, G., The determination of the tear energy of extensible materials of dental interest, J. Biomed. Mater. Res. 1968, 2.28 1-296 Clarke, R.L. and Braden. M., Visco-elastic properties of soft lining materials, J. Dent. Res. 1970, 51 No. 6, 1525-l 528 Wright, P.S. and Braden, M., Water absorption and water solubility of soft lining materials for acrylic dentures, J. Dent. Res. 1983, 62, 764-768 Imperial Chemical Industries, Mond Division. Chemical Safety Sheet 2-ethoxyethyl methacrylate Aiken, A., A physico-chemical biological study of systems based on poly(Z-ethoxyethyl methacrylate) for use in oral and maxillo-facial surgery, Ph.D. Thesis, University of London, 1988, pp 1 16-l 22

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