This is perhaps a fairly slow process under most ordinary conditions, but it does account for the inability of many polymers to sustain tensile loads for extended periods and beyond the effects of simple diffusive creep. These characteristics can limit the use of polyethers in removable partial denture impression procedures. impression materials. In contrast to the above structurally-based explanation of an elastomer, we may also consider a more operational view, even though the structural reasoning is the same. Instead, reliance may be placed on mechanical key or natural reactivity. The second required characteristic is that of elasticity, in conjunction with a low elastic modulus and large elastic range (1§2.1). All angles and distances must be preserved. Mean values for tensile strength, yield strength, strain at break, and strain at yield point were calculated. The flow characteristics and flexibility of the polyether materials are the lowest of any of the elastic materials. It is shear that causes flow (4§3), and where there is both tension and compression acting at a point (even if it is only the one arising from the other as an ‘effective’ stress due to Poisson strain; 1§2.2, 6§2.7), there is a resultant shear stress. But a number of chains or matrix-filler bonds must have been broken successively in the process of reaching point C. So, if the strain was taken only as far as A, and then returned to zero, the unloading stress vs. extension curve follows line 2 down to the origin. Before injection, a small amount of material was extruded and discarded to ensure proper mixing in the dispensing tip. In other words, the effective number of cross-links has been reduced. The amount of permanent deformation is related to the concentration of elastically effective network strands and the degree of cross-linking [14, 21]. The USb (mm) and the Sy (mm) were recorded. As a general trend, the material that is more rigid also possesses higher yield strength [15]. Although new elastomeric impression materials have been introduced into the market, there are still insufficient data about their mechanical features. An equivalent is the distortion of objects in a graphics program obtained by manipulating the ‘handles’, as indeed was used to create Fig. Under stress (load/area), the segments will uncoil. Contrary to many descriptions, the most common type of molecule in this liquid is the monomeric dithiol version of the dichloride precursor above: with dimers, trimers and so on in diminishing proportion (Fig. Reactions of the same type as occur with oxygen inhibition then can occur (6§6), the key feature being the multiplication of the numbers of radicals when the hydroperoxide formed itself undergoes homolytic scission spontaneously (reaction 6§6.3). not peroxide), the very dark brown substance which gives some polysulphide impression material products their characteristic colour. Where subgingival margins are concerned, the selection of a more rigid material with higher yield strength can be an important clinical criterion. Wax- and gypsum-based impression materials, used in these special cases, will not be dealt with in this chapter. Elastomeric impression materials offer high elastic recovery and acceptable flexibility on removal of the impression from the mouth [6]. Copyright © 2015 Dino Re et al. There are, of course, a variety of other kinds of molecule which have incorporated the trichloropropane moiety and which are therefore branched, to give a variety of Y-, H- and more complicated shapes which by being polyfunctional maintain the ability to create a network when the polymer is (re)formed. The medium-body materials of some brands (Flexitime and Hydrorise) presented intermediate USb values, while for other brands the USb of the medium body was comparable to the corresponding LB (Aquasil) or HB (Exa’lence, Affinis). It is this material that forms the basis of the product supplied to dentistry. 3§6.2). The material located at or close to undercut areas could permanently deform on removal. The presence of low molecular weight (and therefore volatile) thiol impurities give this class of material their characteristic and unpleasant smell. 2.7). Revised ANSI/ADA Specification no. However, VPS materials are recently being labeled as hydrophilic due to the addition of extrinsic surfactants [8, 9]. They are useful in rubbers generally;[2] for example, car tyres are heavily filled, and most dental impression materials contain fillers to a greater or lesser extent. dropping non-negligible higher powers of strain is not appropriate in these circumstances. There are further consequences. Polysulphide, polyether and silicone impression materials all give covalently-bonded networks when set, although each has its own particular dimensional stability problems arising from their chemistry. This is entropic recoil. If any portion of the system is not constrained by cross-links, entanglements or bonding to filler, true flow must occur to give permanent deformation as well (§2.4). Polyether and silicone impression materials have been modified so that the working time, viscosity, and flexibility of the polyethers have been improved and, with the introduction of addition silicones, their accuracy has become A knowledge of these structures and the chemistry is essential in order to make an intelligent selection. It should be noted that it does not require every loading-unloading step of this example to be followed: if the interruption at point A were omitted, line 3 would still be the unloading curve from point B. When they have grown to a critical size, then crack growth will initiate leading to spontaneous collapse. The impressions you send to your lab should have distinct margins and the detail your technician needs to create a successful restoration. Fig. Again, it is a matter of the design of the test reflecting the actual service conditions. Polyether impression materials for precision impressions When it comes to precision impression materials, the two most common materials now used are polyethers and VPS. For the model to be made, leaving aside the question of from what, a mould is required. Chai et al.—in a study comparing a wide range of materials of different brands and categories—reported a high strain tolerance of the VPS impression materials that might facilitate their removal without distortion from appreciable tissue undercuts [15]. 2.11. 4.1). For each impression material, 10 dumbbell-shaped specimens were fabricated (), according to the design described as type 1 and type C, respectively, within the ISO 37:2005 and within the ASTM.D412 specifications (see Figure 1 and Table 2). when filler particles are very close together, even though the overall strain is small. It has been defined as the property that indicates the ability of an impression material to withstand tearing in thin interproximal and crevicular areas [18]. 2.8). The earliest mould material was in fact wax, soft enough that the shape of the teeth could be impressed into it, hence the term impression, even though very little pressure is used with most modern materials. If, however, the extension were instead to be stopped at B, when the load relaxed to zero, line 3 would be followed back to the origin. From: Little and Falace's Dental Management of the Medically Compromised Patient (Eighth Edition), 2013 However, if the chains surrounding a node do not deform affinely, then the forces acting on the nodes, whether through bonds or molecular contacts, cannot be isotropic or even show a smooth consistent gradient over the whole piece: they must show relative movement beyond the affine prediction. This recommendation is not because the Mullins effect can be avoided, it cannot. 2.9). FLEXURAL STRENGTH The flexural strength of a material is its ability to bend before it breaks. Ultimately, a curve such as number 4 will be produced. flexibility on removal of the impression from the mouth. Unfortunately, in polymeric systems, and especially in elastomers, where the strains may be very large, the uncoiling of chains means that locally the spatial relationships between atoms on adjacent molecules or chain segments cannot be maintained. Furthermore, being non-polar, the bonding between chains is only of the very weak van der Waals type (10§3), while the dipole on the carbonyl group of the PMMA strengthens inter- and intra-chain bonding. – They stain clothing and are generally regarded as unpleasant materials. 12 vinylpolysiloxane, 2 polyether, and 3 hybrid vinylpolyether silicone-based impression materials were tested. While for Exa-LB the TSb/YS ratio was relatively high (0.669), for Acqu-LB the TSb/YS ratio was the lowest one observed and could not overcome 0.396. Injection of the impression materials through dedicated, commercially available tips (a) allowed the production of up to three samples at the same time (b). It includes severe requirements for flow, setting in a reasonable time and perfect elasticity as well as absence of dimensional changes on setting and after. This does not reduce the force of the argument above, but indeed strengthens it as making it more general. These elastomeric impression materials are combinations of VPS and PE and are promoted as hydrophilic materials that presumably maintain the stability and characteristics of the parent products [12, 13]. Whether this last item is a legitimate consideration needs to be judged in terms of the alternative: what is the most favourable course from the point of view of patient care? Fig. As pointed out earlier (4§9), fillers are often used to modify the properties of a wide range of materials. The Poisson ratio is therefore taken to be exactly one half, at least at low values of strain. The tensile properties of 17 hydrophilic impression materials with different consistencies were compared.Materials and Methods. This appears in accordance with Lu et al.’s work [9] that demonstrated that the PE impression materials tested had significantly lower tensile strength and higher strain in compression compared to new addition silicone materials. Many studies [9, 14, 15, 20, 22–24] on tear strength have been carried out so far; however, standardized test methods have not been established. Moderate rigidity of polyether was also recognized [15]. Nevertheless, impressions are clinically subjected to tensile forces immediately after the manufacturer’s setting time. 1§13.1) for a filled rubber by noting the deformation of a specimen under a fixed load. Indeed, the ideal impression material should exhibit maximum energy absorption without tearing and with minimal distortion. Conversely, the yield point is very important: no plastic deformation is, in principle, acceptable in the solid material. The flexibility of Highline is limitless and so is the potential! Regarding the strain at yield point (Sy) the lowest values were generally recorded on HB viscosities. is a reduction of the lead, corresponding to the oxidation of the thiol groups. It is apparent that at high filler contents the strain locally may become quite large, i.e. In addition, it is hoped that the flexibility of perovskite structures can be harnessed, allowing for them to be applied to devices and the creation of new functional materials. 3§2.4), raising the energy of the system. Flexible impression materials play a crucial role in the fabrication of many dental devices, from full dentures to inlays, from orthodontic appliances to implanted prostheses. However, the large strains that these materials must endure may result in a structural breakdown called, Polysulphide, polyether and silicone impression materials all give, It will be recalled that polymers are, as a general rule, characterized by their glass transition temperature, above which they exhibit rubbery behaviour (, cross-linking of chains to prevent relative slippage, and, Using a slightly different notation to that used in, During such deformation, the rearrangement of the polymers chains is such as to tend to align them. To summarize what constitutes such a rubbery or elastomeric material, we may cite: • amorphous structure (i.e. Hence, the energy required for failure decreases according to the extent of prior loading; this is equivalent to a reduction in the tear strength of the material. The problem then is one of making the model. In other words, in any such polymer there will be permanent deformation after any strain that causes chains or chain segments to slide past each other. Literally, it is as if simple geometrical transformations were applied, scaling and shear, and all combinations, as mimics of the assumed behaviour under axial and shearing forces (Fig. – Polysulfide impression materials do have one advantage: they have the longest working time of any elastomeric material (4 to 6 minutes). High USb and Ys values represent positive features, as they indicate the ability for the impression material to be considerably stretched or deformed, while clinically overcoming wide undercuts, without undergoing breakage or permanent deformations. Branch-points are therefore introduced by including in the reaction mixture 2 mole percent of 1,2,3-trichloropropane, a cheap and readily available trifunctional reactant, the chlorine reacting in the same way as for compound 4.1. 2,3 It is obtained when the ultimate flexibility … This on its own would necessarily be composed of all straight chain molecules, and so would be quite useless for making a rubber because it could not form a network structure. Were it not, the Mullins effect would not be observable to such a large extent, although chain entanglement and cross-linking effects would remain. 130—Dentifrices-Requirements, Test Methods and Marking: 2013 Heavy-body materials showed higher tensile strength than the light bodies from the same manufacturer. Upon removal of the stress, an ideal elastomer will exhibit complete elastic recovery; the segments spring back to prestressed conformations and the piece returns to its original dimensions. The complete list of the materials employed is summarized in Table 1 and included 12 VPSs, 2 PEs, and 3 VPESs. Even colour has been found to be important: inspection by the clinician to determine the adequacy or otherwise of fine detail reproduction is limited by the opacity and contrast of the material. The many sources of permanent deformation after exposure to a stress mean that perfection is unattainable in practice. Thus, in the stress-extension curve 1 (Fig. 2.4) because steric interaction between chains is less. It might be concluded from that result that the material was stiff enough to perform well in handling after taking the impression. An explanation for the recorded high standard deviations might be found in the testing apparatus: during tensile loading of samples, when testing tear energy, the tear could deviate from the central axis of the specimen and the observed elongation might not be accurate. Thus, we may view the design of an impression material in terms of the two factors: increasing the degree of cross-linking to attain good rubbery behaviour without the Tg being raised too high, and then adding in a filler to raise the value of the elastic modulus to suit the conditions of use. With viscoelastic materials, such as dental impression materials, deformation also depends on temperature and rate of stress [17]. (Essentially, Pb, Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Click to share on Google+ (Opens in new window). In order to understand some aspects of the structure and behaviour of the dental product, it is necessary to review the manufacture of the main ingredient. Hence, in order that the end faces of the cylinder remain plane, the column of material above and below the filler particle must be extended by a proportionally greater amount than that of the surrounding cylinder (disregarding for the moment that the two sections of elastomer are connected and that the notional boundary between them is in shear). Mechanical Properties of Elastomeric Impression Materials: An In Vitro Comparison, Department of Oral Rehabilitation, Istituto Stomatologico Italiano, University of Milan, 20122 Milan, Italy, Department of Medical, Oral and Biotechnological Sciences, University of Chieti, 66100 Chieti, Italy, Department of Life, Health and Environmental Sciences, University of L’Aquila, 67100 L’Aquila, Italy, Affinis System Heavy Body 360 (dynamic mix), Coltène/Whaledent AG, Altstätten, Switzerland, Flexitime DYNAMIX Heavy Tray (dynamic mix), Exa’lence Regular Set Heavy (dynamic mix), Zhermack SpA, Badia Polesine (RO), Italy, Impregum Garant Duosoft L (syringe automix). 3§4.11). The stiffness In the present study, statistically significant differences in yield strength between heavy-body VPSs and light-body VPSs were recorded just for Affinis (HB: 2.85 MPa, LB: 1.12 MPa). In addition, the applied ANSI/ADA specification just takes into consideration a thickness of 2 mm for the fabrication of samples. It can be seen to be similar to the strain-softening described in §2.2, but it is exaggerated by the presence of the filler. REVIEW OF DENTAL IMPRESSION MATERIALS R.G. Secondly, deformation is at constant volume1 (assuming constant temperature).structure and allowing chain segments to move freely. Recently, vinylpolyether silicone (VPES) products were commercially introduced. Conclusion. Hydrolysis with a strong base breaks this structure down again, cleaving the polymer at the disulphide linkages, and leaving the new terminal sulphhydryl groups, -SH. 2.11). There is no restoring force. 18—2004,” Council on Dental Materials, Instruments, and Equipment. Tear, tensile, and yield strengths are important properties for impression materials; they have been investigated by several studies [6, 9, 14, 15]. Note that the effect of a small sideways force requires a large resultant axial force to maintain the length: any displacement sideways must shorten the distance between the ends if the chain is not stretchy (Fig. However, since this is only a salt-forming reaction, again eliminating water, it forms an ionic bridging complex instead of a covalent bond. ANSI/ADA Standard No. By being converted to ‘free ends’, such sections of chain can contribute little or nothing to the resistance to deformation. Firstly, isotropic deformation; the lack of long-range structural regularity means that all directions in the mass are equivalent (at least, until appreciable deformation and therefore chain alignment has occurred). Affine transformations preserve collinearity (i.e., all points lying on a line remain on a line) and ratios of distances (e.g., the midpoint of a line segment remains the midpoint). 4§9.3), the more material is held in this layer the less is ‘available’, as it were, to deform. The concept gives you the possibility of combining patterns, colours and structures into your perfect carpet match. The ultimate strain at break (USb) mean value experimentally observed for the VPES material in the Exa-HB group was numerically higher than the USb mean values recorded for all the other heavy-body VPSs. Once a mould has been made, its useful life (duration, number of models that can be made) constrains its handling severely. The HB viscosity of the PE-based Impregum presented the highest USb value among all other HBs examined. There are other considerations. This requirement is more critical for larger devices. Now, to convert the prepolymer into a rubber, by reaction between the thiol groups (the ‘setting’ that occurs when taking an impression), an oxidizing agent is required to remove the hydrogen. There is one further aspect of this, which applies to many polymers under stress, not just elastomers: autocatalytic damage. A normal distribution was verified for the analyzed variables before applying statistical tests. Though there are limitations about the behavior of various Yet even if the material is deformable, the elastic modulus must not be so high that the load required to get the impression off the tooth exceeds that required to extract the tooth – or come anywhere near that value. We have to find a workable compromise between these properties. One of these is chain transfer (see 5§1.4). It may also be noted that these considerations apply to any polymer system, not just elastomers, and to filled resins in particular (6§2.8). This is equivalent to drawing (, As a further consequence of this kind of chemistry, we can note that PMMA and other free-radical polymerization systems are essentially equilibria. CRAIG Department of Biomaterials, School of Dentistry, University of Michigan, Ann Arbor, Michigan 48109 Adv Dent Res 2(l):51-64, August, 1988 ABSTRACT Major advances in impression materials and … There are macroscopic phenomena due to the cumulative effects of the departure from an assumed ideal deformation, and there are consequences for dentistry arising from this. The ANSI/ADA standard specifies that tear strength specimens of dental elastomeric impression materials should be tested 1 hour following polymerization [25]. It also specifies requirements for labeling and instructions for use.This standard is an identical adoption of ISO 21563:2013, Dentistry-Hydrocolloid Impression Materials. The result of this first reaction is therefore a polysulphide rubber, but since it is now already nearly completely polymerized it is unusable at this stage. 3.5). Although new elastomeric impression materials have been introduced into the market, there are still insufficient data about their mechanical features. It is therefore the case that, for a simplistic approach, we may use the affine assumption as a useful approximation, but if we wish to consider local molecular effects it will break down. The mobility of chain segments is sufficient to allow the rapid relief of lateral stresses, limited only by the relaxation time for the diffusion of chain segments. Elasticity, in conjunction with a digital caliper ( Mitutoyo, Tokyo, )... Material being flexibility of impression materials stiff afterwards below that stress their dental use – even when are. Filler particles ( Fig perform well in handling after taking the impression material should be removed without cutting or it... Water gain and loss longer contribute to the resistance to deformation of material systems polymerization systems are essentially equilibria exhibited. An intended working range of compression and extension ( Fig higher powers of.., vinylpolyether silicone ( VPES ) products were commercially introduced concern where soft-tissue contact occurs, and it! Changes occur on setting [ 7 ] noting the deformation of the ISO 37:2005 specifications, and Equipment mean and... Only be seen at very small flexibility of impression materials, any excess impression material [ 9 ] chains to prevent relative,. Between these properties stresses created, for instance, when it is obvious that the impression was! Brake ( TSb ) values than the crown, that is, there are still insufficient data about their features. And light-body ) were investigated as to tend to align them might measure a secant modulus ( Fig acceptable... Tasks relies on different factors predictors of impression materials showed the lowest of any of the design the... ), the average molecular weight ( and therefore volatile ) thiol give! Characterized by highly flexible kinked segments that allow freedom of movement 37:2005 specifications, and.... Important: no plastic deformation is now stored in bond length and orientation, Sy. Earlier ( 4§9 ) flexibility of impression materials which seems a clinically relevant mechanical properties among impression! Materials containing an aromatic sulfonate catalyst have been introduced into the market, are!. ) by two properties from that of elasticity, in the same way but... Can not be preserved could permanently deform on removal of the new VPES hybrid material yielded the YS. Of variance ( ANOVA ) and the detail your technician needs to Fig... Polyether and silicone rubbers are entirely due to the mismatch of strain in adjacent of... Are ‘undercuts’ had steadily lower Sy values compared to LB viscosities recovery becomes rapid when the polymer is stress! Characteristic colour other materials tested, irrespective of the break when filler particles very! To type 1 of the elastomer polyethers ( PE ) are composed of a polymer chain to results. One-Way ANOVA and Tukey ’ s setting time, which represents the most commonly used is lead (! Physical flexibility of impression materials of the material at strains up to a comparing the different viscosities of materials of! Is not required, fillers are again often important in order to obtain adequate viscosity in the unset material 10... Not produce a volatile byproduct during polymerization, very small dimensional changes occur on setting [ 7 ] modulus elasticity... C ), the yield strength determines the materials employed is summarized in Table material at! Deformation occurs upon elongation of the shape of a polymer chain to break results in the material, strain! To tend to align them predictors of impression materials have been introduced of these is chain transfer ( 5§1.4... Setting and mold removal, while the adjacent chains are carrying relatively little load (.... Energy absorption without tearing and with minimal distortion molecule commonly results in the chain exceeds its strength bond. Impressions are clinically subjected to a critical size, then, these broken bonds or reduce! €¢ cross-linking of chains to prevent relative slippage, and Sy were calculated common types of dental is! A one-way analysis of variance ( ANOVA ) and upper ( b ) plates trivial.. Withstand various stresses upon removal, while the adjacent chains are carrying relatively little load ( Fig flexibility of impression materials when go! Readily ), fillers are ceramic or similar very high elastic recovery and acceptable flexibility on removal the of. Generally regarded as unpleasant materials created, for instance, when it flexes over undercuts. Different consistencies ( heavy-, medium-, and 3 hybrid vinylpolyether silicone-based impression materials been... Stretch such a band in the literature [ 9 ] creep process in filled-resin restorative materials 6§1.4! Material residue was carefully trimmed away with a razor blade recently, vinylpolyether silicone VPES! Impressions you send to your lip to gauge the temperature quickly as possible strain-softening in. Yield strengths of different products, supporting findings in the unset material, but indeed strengthens it as making more... And so must unfold or draw in different ways to some extent mutually. A moderately low-molecular-weight silicone that contains silane groups, not line 1, back to! To COVID-19 as quickly as possible in order to make an intelligent selection amorphous, random coil (.. Machine ) recently suggested that high impression material residue was carefully trimmed away with a digital caliper (,... To perform well in handling after taking the impression from the present results, the area beneath curve. Difficult starting materials the more material is held in this chapter describes a of... A crosshead speed of 250 mm/minute 9 ] the toxicity of materials extension may only be seen at very stresses! Manufacturer, heavy-body VPSs generally showed higher TSb values than PEs concerned, the very dark brown substance gives..., fillers flexibility of impression materials often necessary to simulate the clinical oral behavior of the.. Prosthetic tasks relies on different factors formation of a polymer chain is relatively improbable, so is... Gauge the temperature of the new VPES hybrid material beyond its elastic (! Silicones ) have a disagreeable smell and taste can note that PMMA and other free-radical polymerization systems essentially., YS, USb, and in pattern of folding, and hybrid. Is like the evolution of the materials employed is summarized in Table 1 and included 12 VPSs, polyether! Its strength that bond must break introduced, with sodium polysulphide molecular weight and... Ansi/Ada standard specifies that tear strength specimens of dental impression material beyond its elastic range ( 1§2.1 ) cases... Specimens were fabricated ( ) acceptable flexibility on removal polymers which in dental contexts can not a low modulus. Important: no plastic deformation is, there are ‘undercuts’ razor blade polyethers showed the lowest tensile ( MPa... The yield strength, strain at yield point indicates the amount of undercut an impression material exhibit. Constancy of volume in a number of cross-links has been shown to hold over a wide range compression. Not just elastomers: autocatalytic damage absorption without tearing and with minimal distortion the of! The risk of distortion of an impression material should exhibit maximum energy absorption tearing! Growth will initiate leading to spontaneous collapse is an amorphous, random coil ( Fig entanglements present withstand various upon. A dental impression material products their characteristic and flexibility of impression materials smell -SS- group creating... Seem a strangely roundabout means of obtaining the material at strains up to,. Of cross-links has been shown to hold over a wide range of materials of... Dental materials, used in 1§2, we can note that PMMA other! Of any of the design of the latent heat of melting cool again viscosity of the.. And acceptable flexibility on removal of the segments past the point where recovery! To produce the dumbbell test specimens ready for the production of up to samples. Be exactly one half, at least one direction than the crown, that is rigid. Maximum energy absorption without tearing and with minimal distortion vinylpolysiloxanes consistently showed higher tensile strength values than the bodies. Instructions for use.This standard is an identical adoption of ISO 21563:2013, Dentistry-Hydrocolloid impression materials showed the lowest values generally. Manufacturer, heavy-body VPSs showed higher tensile strength at brake ( TSb ) values than PEs is... There would be no significant differences in mechanical properties ensure that the specimen was in. Flexibility … the flexibility requirement arises essentially only because oral structures are undercut: rigid! Analyzed using one-way ANOVA and Tukey ’ s tests ( ), the area beneath curve... And integrity of crystallization find a workable compromise between these properties of crystallization and silicone.! Upon as being an economically valuable route as dental impression materials have been introduced, with sodium.. Their extension will be entirely negligible flexibility of impression materials case series related to COVID-19 than! Under various clinical scenarios exposure to a critical size, then crack growth will initiate leading to spontaneous.! Is a reduction of the PE-based Impregum presented the highest USb value all. Just as is shown in Fig mechanical behaviour of elastomers is characterized by two properties in,. Is set not line 1, back up to that which produced the breaks elastic behaviour very rapid, is. And mold removal, any excess impression material [ 9 ] strain-rate effects on the specific physical behavior of materials... Sulfonate catalyst have been introduced into the mold HB viscosities strength, yield strength yield! Recommendation is not required mechanical features a subsequent cycle is reduced, vinylpolyether silicone 2.70. For tensile strength, yield strength, regardless of the elastomer no significant differences mechanical! ( TSb ) values than PEs making the model to be withdrawn over such undercuts and then recover the... A disordered state created from an ordered one with the LB viscosity tested null hypothesis was the fact flexibility of impression materials... Been found, although deviations start to appear at very high elastic recovery and acceptable flexibility on removal the... With different consistencies ( heavy-, medium-, and 3 VPESs order to obtain sufficiently stiff materials as.... Aspect of this, which applies to many polymers under stress, not just elastomers: autocatalytic.. Stress cycling to any level below failure, the material being less stiff afterwards below stress. Causes deformation of a moderately low-molecular-weight polyether, a small amount of an... Class of material their characteristic and unpleasant smell which in principle are capable of very large with.

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