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SEPARATIONS Pretreatment of Liquid Silicone Rubbers to Remove Volatile Siloxanes Michael A. Brook,*,† Hanns-Ulrich Saier,‡,§ Julia Schnabel,‡ Kaitlin Town,† and Michael Maloney‡ Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1, and Silcotech North America Inc., 54 Nixon Road, Bolton, Ontario, Canada L7E 1W2
Liquid silicone rubbers (LSR) are widely used to create devices with complex shapes for various commercial and consumer applications, because of their many beneficial properties including lubricity, thermal and electrical stability, and aesthetic feel. Regulatory bodies require postcure thermal treatment of silicone elastomers to remove volatile materials: the rate and efficiency of these processes depends on the specific elastomer properties (e.g., cross-link density). We examine in this paper the ability to remove volatiles before curing in the mold, a process that should be much less dependent on specific elastomer formulation. The thermal devolatilization efficiency, optionally under vacuum, of silicone elastomers prior to cure, was compared to different convection heating techniques postcure. Parts A (olefin-functional silicone and the catalyst) and B (Si-H functional silicone) were treated separately or mixed, and the ability to create parts and the requirement for postcure thermal devolatization (200 °C for 4 h) were determined. Themolysis precure permitted the removal of volatile species, but with several key caveats: (i) Loss of volatiles from part B, in particular, was accompanied (especially in moist atmospheres) by premature cure, likely due to cure mechanisms other than hydrosilylation and the thermal loss of inhibitors. Even without part A, the part B samples skinned over after a few hours. (ii) The pot life significantly decreased, particularly as volatiles were removed from part B. (iii) The efficiency of devolatilization can be detrimentally affected by transpirationsthe migration of volatiles from one silicone elastomer object to another via contact or gas-phase transfer. Thinner objects both lost and absorbed volatiles by contact and evapotranspiration more effectively than thicker objects. Precure treatment had little effect on the resulting elastomer properties. To establish if precure thermolysis is a viable route to devolatilization, it was determined that the surface/volume ratio of the object to be prepared should be considered, as this takes into account the relative proportion of both thin and thick sections of the complex object to be molded. In the case that the object consists primarily of thick objects, precure devolatilization of part A can be an effective way to mitigate the need for postcure thermal treatment. Introduction Silicones are appreciated for their beneficial properties, including lubricity, thermal and electrical stability, and aesthetic feel. Liquid silicone rubbers (LSR), also known as liquid injection molding (LIM) silicones, are widely used to create devices with complex shapes for various commercial and consumer applications.1 In general, LSR formulations from silicone suppliers use platinum cured hydrosilylation to crosslink silicones into elastomers, although alternative (or additional) cure mechanisms can also be used. Thus, a typical formulation will comprise a two-part system, in one part of which is found an olefin-functional silicone and the catalyst (part A); in the other is a Si-H functional silicone (part B). The specific manner in which the alkenyl and Si-H functional materials are formulated, catalyst loadings, catalyst inhibitors, use of fillers (typically, treated silica), and ultimate viscosity are all variables controlled by manufacturers. The majority of users of LSR silicones pump the two separate parts into a mixing chamber, * To whom correspondence should be addressed. Tel.: (905) 5259140, ext 23483. Fax: (905) 522 2509. E-mail:
[email protected]. † McMaster University. ‡ Silcotech North America Inc. § Current address: Saier Holding GmbH, Reutiner Strasse 7, Postfach 1160, 72275 Alpirsbach-Peterzell, Germany. Part of the Diplomarbeit thesis of H.-U.S.
typically in a single screw extruder, which is kept at room temperature or lower to prevent adventitious cure. The mixture is then injected into a heated mold (∼180 °C) where initials shape holdingscure occurs. Subsequently, after ejection from the mold, a secondary thermal process may be used both to complete cure and optionally to remove volatiles. Silicones are normally synthesized by all manufacturers using the equilibration polymerization of cyclic oligomers in the presence of acid or basic catalysts, and end groups (Figure 1).2 In the case of dimethylsilicones, the polymer concentration at equilibrium is about 85%, with the residue consisting of unreacted cyclic oligomers.3 Even when care is taken to optimize the process, and when the majority of the low molecular weight materials are removed by vacuum, most silicones contain measurable amounts of cyclic oligomer starting materials. Residual oligomeric materials can be disadvantageous. They are somewhat mobile within the silicone, and can migrate to a device interface.5 Postmanufacture release from the elastomer can therefore change surface as well as mechanical properties, including tensile strength, tear strength, elongation, compression set, etc.6 The latter changes are a consequence of changes in swelling that result from loss of silicone “solvents” within an elastomer. In the special case where silicones may come into intimate human contact, regulatory agencies also require volatile oligomer
10.1021/ie061666q CCC: $37.00 © 2007 American Chemical Society Published on Web 11/08/2007
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Figure 1. Silicone equilibration, adapted from ref 4.
concentrations to be limited. For example, American7 and European agencies require test data demonstrating that commercial devices release small quantities of low molecular weight materials. As an example, the German regulatory agency (BfR), with one of the most stringent requirements, clarifies that a conditioned silicone device must release less than 0.5% by weight of silicone materials on heating at 200 °C for 4 h.8 The release of low molecular weight silicones from silicone elastomers requires that the silicones diffuse from the core of the device to the surface, from which evaporation can take place. The rate of this process will depend on the characteristics of the silicone and on the geometry of the object (e.g., thickness, total surface area).9 To better understand and optimize these evaporative processes, a study was undertaken in which parts A and Bsseparately or mixedsof an LSR silicone were thermally treated, optionally under vacuum, prior to cure (or in the case of A + B, during and after cure). The effectiveness of devolatization in a precure heating process is compared with different convection heating techniques postcure. In all cases, the nature of the volatiles lost and the rate and magnitude of loss were correlated with the processing and resulting materials processing consequences that result from the loss of the volatiles. Experimental Section Materials. LIM 6061 (General Electric Silicones, now Momentive Silicones) was provided by Silcotech. The LSR manufacturer reports zero shear viscosities of between 250 000 and 300 000 cP. After cure, and heating for 4 h at 200 °C, the following specifications are reported by GE: tensile strength, g900 psi, ∼6.2 N mm-2; max elongation, g350%; and shore hardness, ∼60. Instrumentation and Analytical Procedures. 1. Nuclear Magnetic Resonance. The 1H NMR spectra were recorded on a Bruker AC-300 (300 MHz) or Bruker AC-200 (200 MHz) spectrometer. Chemical shifts are reported with respect to tetramethylsilane as standard, set to 0 ppm, or CHCl3 at 7.24 ppm. 2. Gas chromatography (GC) separations were performed on a Hewlett-Packard 5890 gas chromatograph with FID detector, equipped with a DB-1 capillary column (30 m × 0.53 mm i.d. × 0.5 µm film, J & W Scientific) and HP integrator (3393A). The temperature profile used was the initial temperature, 80 °C for 2.5 min; then 20 °C/min for 8.5 min; and the final temperature for 250 °C for 5 min. 3. Gas chromatography/mass spectrometry was performed using a Micromass GCT (GC-EI/CI time-of-flight mass spectrometer), equipped with a DB-XLB capillary column (30 m × 0.25 mm i.d. × 0.25 µm film, J & W Scientific, Serial No. US3122942H). The same temperature profile as described above for the GC was used. 4. Thermal Volatilization. Two related processes were used to follow the loss of volatile silicones. In the first, a Bu¨chi Glass Kugelrohr Oven, B-585, equipped with a cold finger cooled to -78 °C was used. Heating at temperatures up to 200 °C was used with sample rotation (10 rpm). Only one sample at a time, or multiple small objects, could be treated in the Kugelrohr
oven: a specially designed receptacle held a glass vial of ∼2.4 cm diameter × 5.5 cm height (inner dimensions). Alternatively, a VWR vacuum oven, Model No. 1415M, with interior dimensions of ca. 45 cm × 45 cm × 155 cm, interior volume ∼0.32 m3, with two sample trays was used. The unit was operated at ambient pressure, ambient pressure with nitrogen gas flow through (up to 3 L min-1), or under vacuum (∼1 Torr). The oven temperature was normally set to 180 °C, although 200 °C was used in some cases to mimic the regulatory assay.8 Up to about 60 samples could be simultaneously treated in the oven. The total sample weight within the oven was typically 100 g, and never exceeded 360 g. In early experiments, samples of parts A, B, and/or A + B of different thicknesses in vials, or as free-standing samples, were placed in the oven, both in direct contact and without direct contact. As will be discussed below, the presence of mixed samples in the oven (e.g., parts A with B, etc.) led to contamination or sample crossover issues. Thus, unless explicitly noted, the results below describe the weight losses when only one type of sample was present in the oven. 5. Thermogravimetric analysis was performed using a Netzsch STA-409 Luxx. Samples in an Al2O3 crucible were heated at 10 °C min-1 in an air atmosphere from room temperature to 400 °C. 6. Rheometric measurements were taken using an ARES (TA Instrument, 2KFRTN1 transducer, maximum torque 2000 g cm, HR actuator ) 0.001 rad/s, with a Windows 95 based RHIOS software). The device is a modular, controlled strain, parallel plate rheometer (plate diameter 25 mm); all tests were done in oscillating mode. Two types of tests were run: (1) temperature frequency sweep (structure viscosity), from 60 to 180 °C, frequency 0.1-100 s-1, strain 0.25 (part A or B, Figure 7); (2) temperature ramp test (cure velocity test) from 30 to 150 °C, heating rate 30 °C/min-1, frequency 10 s-1, strain 0.25 (A + B, Figure 8). 7. Tensile tests were performed using an Instron Model No. 3366 table-mounted materials testing system, capacity 10 kN. Dog bones were prepared according to DIN 53504, 5.1.2, S2. It is critical to form dog bones without bubbles, as these have a negative influence on tensile test results. To achieve this goal, while using small-scale protocols that mimic traditional manufacturing processes, raw or heat pretreated LSR components were filled using an aluminum “syringe” which simulates the processes in an injection molding machine. Compounds were weighed, mixed, and finally injected in the dog-bone mold (mold dimensions are according to DIN 53504, Table 3, S2; for mold design, see Supporting Information). The mold is detachable from the syringe for heating/curing purposes. The cold mold was placed on a preheated hot plate (at 250 °C) for 20 min. The samples were allowed to cool at room temperature. Elongation to break tests were performed following the requirements of DIN 53504, 6.4.1, with test speeds of 200 mm/min, at room temperature. Moduli of elasticity (E-moduli) were calculated in accordance with EN ISO 527-1:1996. Film Preparation. 1. Vials. Initially, thin films were formed by hand spreading the silicones on the interior surface of a vial ∼2.4 cm diameter × 5.5 cm height (inner dimensions). However, as noted below, devolatization is affected by subtle variations in film thickness. Therefore, a “syringe” tool was designed to reproducibly deposit silicone films of known thickness on the inner vial wall. A plug of silicone was delivered to the base of the vial using a normal syringe. Subsequently, a central plunger (which has an access port to permit activation by air pressure, or release of overpressure during stamping) was used to extrude the silicone along the vial walls (see Supporting
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Figure 2. Thermolysis of samples: (A) TGA ambient pressure heating at 10 °C/min. (B) Extended heating in a Kugelrohr oven. (C) Evaporation velocity while heating of samples in a glass vial (only 1/2 surface area exposed; 2.25 mm thick; surface/volume ) 0.42 mm-1; 180 °C; ∼1 Torr).
Information). Finally, the plunger was withdrawn with compensating air pressure to obviate issues with adhesion between the injector and the silicone. Film thicknesses of 2.25, 4.25, and 6.25 mm (smaller wall thicknesses are not possible due to vial design) were prepared from single components A and B or mixed rubber A + B dependent on the used stamp size. An additional set of molds was created to prepare for thinner, more rigid, cured rubber systems (see Supporting Information). For these, only mixed A + B silicones could be applied to the mold surface. Following cure, the films could be removed from the mold such that both internal and external faces of the cylindrical elastomer were exposed during devolatilization. With these systems, a much larger range of elastomer wall thicknesses could be prepared: films of 0.2, 0.5, 1.0, 2.4, 4.3, and 6.2 mm were produced; surface/volume ratios (s/v ratio) from 0.12 to 10.07 mm-1 are accessible. Between the use of these two molds, it was possible to prepare samples that had comparable volumes and wall thicknesses, but in some cases with only one-half to one-third the exposed surface area (the remaining surface area in the vial was in contact with a glass surface from which it is presumed evaporation does not occur). 2. Characterization of Volatiles from the Kugelrohr Oven. As noted above, highly volatile species were trapped in the cold finger, and less volatile species were isolated from the room
temperature collection vessel, following Kugelrohr distillation: one sample (sample wall thickness ) 2.25 mm; T ) 180 °C; P ) 1.3 mbar) of each of parts A, B, and A + B were heated. Samples were collected and characterized each 30 min over 4 h. The weight changes for the individual or combined parts A and B are shown in Figure 2B. The same data, plotted as evaporation velocity (wt % min-1) is provided in Figure 2C. The materials collected were approximately equally distributed between the cold finger and the bulb, with slightly more material in the cold finger, particularly at longer heating times. The two fractions collected from the Kugelrohr oven were characterized by GC and GC/MS (at different time periods over 5 h heating). Representative GC profiles of these compounds are shown in Figure 3. A calibration curve was created from available silicone standards, which are normally limited to D3D6. Very little D4 or D5 was found in any of the samples. Higher molecular weight materials were characterized by correlating to the lower molecular weight standards, using mass spectrometry, with incremental masses derived from monomer homologues (e.g., D7 (MW 518) ) D6 (MW 444) + 74 (Me2SiO)) or as matches from the library that accompanies the instrument. The most volatile species present in any significant concentration, D6-D8, were found in the sample from the cold finger. In
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Figure 3. (A) GC profile component A (blue, cold finger; red, bulb). (B) GC profile component B (blue, cold finger; red, bulb).
Scheme 1
addition, there were small amounts of higher cyclic and linear siloxanes. The higher MW fraction, found in the bulb, also had cyclics but of higher MW. The functional group profiles of the volatiles were established using 1H NMR (see Supporting Information). Three-dimensional graphs were calculated and plotted using Mathematica.10 The raw data and the algorithms used in calculating Figures 4-8, 10, and 11 are provided in the Supporting Information. Results LSR silicones are available in a wide variety of different formulations that differ in cure chemistry, filler, etc., which ultimately lead to different physical properties of the elastomer, such as Shore hardness. We chose to examine the processing of a representative silicone LIM 6061, from General Electric Silicones,11 which, when prepared according to the manufacturer’s instructions, results in an elastomer with a Shore hardness of 60. This compound is representative of the range of LSR materials, all of which contain volatiles, of different Shore hardnesses sold by Momentive Silicones and the other main suppliers of silicones, including Wacker, Dow Corning, Shinetsu, and Bluestar Silicones. The material is a classical platinum (addition) cured polymer in which a hydrosilylation reaction links vinyl groups found on some silicone chains with Si-H groups found on other chains (Scheme 1). The material is provided as a viscosity-matched two-part material that is mixed in equal volumes prior to injection into the mold: part A contains vinyl-containing silicones and the platinum catalyst; part B also contains vinylcontaining silicones, and the Si-H containing silicones. Both parts are filled with treated fumed silica to give the final product enhanced strength.
Under normal circumstances, the two parts are pumped from drums into an extruder barrel where mixing, at ambient temperature or lower, occurs prior to injection into a heated mold. Depending on object thicknesses and geometries, typical cycle times of less than 90 s are required.12 Following release from the mold it is common, particularly for automotive and medical market suppliers, to postcure heat the molded materials. This ensures the complete cross-linking of the silicone and, as noted above, also serves as a mechanism to volatilize low molecular weight materials found in the original materials. Both processes change the ultimate materials’ properties, but in different directions. The objective of the current research is to determine the ability of various thermal precure treatments to remove volatiles in comparison with the traditional postcuring processes, to correlate the type of treatment with the amount and type of volatiles removed (e.g., by gas chromatography/mass spectrometry (GC/ MS)), and to establish the properties of the elastomers prepared. All precure devolatilization processes on un-cross-linked material were thus normalized against traditional postcure treatments in which the cured objects were heated at 200 °C at ambient pressure in a ventilated oven with air flow for 4 h:8 these tests were undertaken following g48 h storage in a desiccator with a drying agent.13 Devolatilization Processes. Preliminary determination of the volatile content of parts A and B were undertaken using thermogravimetric analysis (TGA). As shown in Figure 2A, two profiles were exhibited by the silicones. Samples that had not been treated thermally showed extensive weight loss over the temperature range 100-400 °C. By contrast, the weight loss profiles of precure heated parts A or B, and the A + B sample which was thermally heated postcure, were essentially flat until the temperature approached 300 °C, at which point the thermal
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Figure 4. (A) Weight change as a function of sample thickness and wall surface area for differently long postcured (1 (blue), 2 (red), 4 h (green)) samples. (B) Ability of objects to gain weight by contact transpiration as a function of wall thickness (blue, thick; green, thin).
decomposition of methylsilicones starts.14 These data show that while parts A and B initially contain volatile silicones, they are not created as a consequence of hydrosilylation cure. Although the data shown in Figure 2A suggests that the volatiles which are initially evaporated are present at similar concentrations in the parts A, B, and A + B, these data are not representative of the entire sample. The short-term sensitivity to temperature of the loss of volatiles from parts A, B, and A + B was determined using TGA. Figure 2 demonstrates that the short-term release of volatiles can be increased by increasing the heating temperature. For example, when the temperature was increased from 200 to 250 °C, the initial rate of weight loss increased by 40% for component A, 18% for component B, and 21% for A + B. Heating parts A, B, or A + B under vacuum or in a vented oven over extended periods of time effectively removed volatiles. In all cases, there was an initial burst in loss of material, as reflected in the rate data just presented, followed by a slower loss profile (Figure 2B). That is, the velocity of volatile loss decreases with heating time (Figure 2C). Somewhat surprisingly, the efficiency of loss from parts A, B, or A + B in a vented oven, conditions more closely related to normal industrial practice, was comparable to that with a vacuum oven or Kugelrohr oven (see Supporting Information). As discussed below, this points to the importance of the surface area of the object, as well as the boiling point of the volatiles, in determining the rate of volatile loss. A key observation of this study is that the amounts of volatiles present follow the order part B > part A > parts A + B: ∼2 wt % could be removed from part A after extensive heating and ∼4 wt % from part B, and parts A + B (without preheating) showed losses of only ∼1.5% (Figure 2B). Chemical Composition of Volatile Fractions. Low molecular weight materials were quantified and characterized using a Kugelrohr oven (Figure 2B). Samples of parts A, B, or A + B of various thicknesses were heated under vacuum (∼1 Torr) at 180 °C. Two fractions for each sample were collected: fluid collected in the bulb cooled at room temperature, and lower molecular weight materials collected in a cold finger at -78 °C. 1H NMR and gas chromatography/mass spectrometry (GC/MS) were used to characterize the volatiles. Over time, constituents released from part A drift to higher MW, while the distribution of MW in the two fractions derived from part B remains essentially constant (Figure 3, see Supporting Information for the profile from A + B and the overlay of all three graphs). Thus, there is a higher fraction of low molecular weight materials in A than in B. Approximately half of the volatiles recovered, found in the cold finger, were primarily low molecular weight cyclics D6 ((Me2SiO)6)-D8 ((Me2SiO)8) as
shown by both GC and GC/MS. It is noteworthy that insignificant quantities of the volatile cyclics D4 (Me2SiO)4) and D5 (Me2SiO)5) were found in these samples. Higher molecular weight cyclics were found in the bulb cooled at room temperature. Other silicone fragmentsslikely from linear siliconess were also present in both fractions. In the absence of standards for higher molecular weight silicones, their structures are inferred from the peak profiles in the mass spectra, showing daughter ions of mass loss 74, which are characteristic of dimethylsilicone polymers (Me2SiO) (e.g., a peak at amu 518 is inferred to be D7 (Me2SiO)7 ) D6 (MW 444) + 74 (Me2SiO)), or from functional monomers 60 amu (MeHSiO), or 86 amu (Me(H2CdCH)SiO).15 More convincing evidence that the volatile fraction also included functional groups (Me(H2CdCH)SiO; MeHSiO) came from 1H NMR studies (see Supporting Information). These showed that the volatiles, both condensed at room temperature and at -78 °C, from either part A or part B contained vinyl groups (a cluster of peaks ranging from 5.7 to 6.3 ppm), and that SiH groups (broad singlet centered at 4.7 ppm) were found in part B. Factors Affecting Devolatilization. 1. Wall Thickness and Surface Area (s/v Ratio). For a given temperature (200 °C) very large differences in devolatilization efficiency were initially observed as a function of wall thickness. Thinner objects exhibited significantly greater ability to lose volatiles both preand postcure. Describing the evaporation process using the object wall thickness as one parameter initially appeared to be a suitable approach. However, the available surface area for evaporation is also key. To demonstrate this, cylindrical samples of a given thickness and volume were prepared in vials where the outer and bottom surface areas were in contact with glass or, alternatively, removed from the vial such that the exposed surface area was increased by a factor of 2-3. As can be seen from Figure 4, total rates of evaporation are a function of both surface area and wall thickness. Most manufactured LSR silicone objects are complex and have a variety of wall thicknesses within the same object.16 It was found, heuristically, that an object geometry is sufficiently explained by the ratio of environmentally exposed surface area over volume (s/v ratio) rather than by thickness alone. Thus, the trend to greater weight loss from objects either with thinner walls or with greater exposed surface area can be seen in Figure 4. The s/v ratio is generic in that it implicitly considers all silicone thicknesses in a given object irrespective of object design. 2. Evaporation and Contact Transpiration. Experiments were performed to optimize the devolatilization process. Two
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Figure 5. Weight loss of part A as a function of wall thickness and heating time. (A) No part B was present in the oven. (B) Noncontacting samples of part B were present in the oven. (C) Overlay of A and B.
Figure 6. (A) Net weight change in the object (after 4 h at 200 °C) as a function of precure heating weight loss. (B) Cross section taken through line delineated by red dots.
variables were examined: the effect of constitutionally different samples in the same oven (e.g., parts A and B), and the effect of samples contacting each other. In the latter case, the effect of wall thickness was also examined. During these experiments, two unanticipated parameters that were key to devolatilization were observed: cross contamination of part A with part B can occur due to either solid/solid contact (contact transpiration) or gas/solid exchange (evapotranspiration). Contact: Random Touch Transpiration. Initially, devolatilization experiments of parts A, B, and/or A + B were simultaneously undertaken in either the vented, vacuum, or Kugelrohr oven. However, early experiments, particularly in the Kugelrohr oven where random contact between different objects occurred during sample rotation, demonstrated that not only was weight lost during heating (blue curve), but also some objects were able to absorb silicones such that the rate of weight loss was significantly reduced. The ability of an object of a given thickness to gain weight is shown in Figure 4B (green curve). In this case, the objects were uniform cylinders, and films thinner than 1 mm were particularly prone to increasing weight. It can be anticipated that objects of a different design will undergo contact transpiration more effectively at thin objects. Therefore, in subsequent experiments the material type (A, B, A + B) and also the wall thickness of samples were kept constant within a given experiment and samples were not allowed to come into direct contact with each other. Evapotranspiration. Exchange of materials through gasphase transpirationsdesorption and resorption of volatiless changed the cure profile of both parts A and B. For example, when samples of part A alone were heated in the oven, the rate of weight loss as a function of heating (Figure 5A) was dramatically different from that in the case when part B was also present in the oven (Figure 5B) even if the different samples were not touching; a superposition of the two curves is shown in Figure 5C. This demonstrates that material from the two parts
exchange within the oven even when not in direct contact, a fact confirmed by visual inspection. If noncontacting objects derived from parts A and B, respectively, were simultaneously heated in an oven, skins formed on the outer surfaces of both types of parts as a consequence of premature cure. Therefore, in subsequent experiments, to avoid adventitious cure, parts of only one type (either A, B, or A + B) were cured in the oven at one time. The supplier requires the LSR to be mixed in a 1:1 ratio of the two components. Potential volatile loss during preheating can thus be calculated as (volatile loss A + volatile loss B)/2 for a mixed silicone. However, this method does not consider the different properties of part A, which have little effect on the final processing or elastomer properties, and of part B, which do (see below). To optimize the formulations, samples of part A preheated for different periods of time were mixed with preheated samples of part B and the weight loss of the vulcanized silicone was measured after 4 h heating at 200 °C in a vented oven. For a given sample with an s/v ratio of 0.42 mm-1, which was chosen as a representative value within the range of those that were studied, the optimal heating protocols to minimize volatile concentration in the final elastomer A + B, obtained by regression analysis, are shown by the red dots in Figure 6: the targeted mixed weight loss (A + B) of 0-2.15% assumes that part A can release a maximum of 1.8 wt % and part B 2.5 wt % of volatiles. The tests showed that residual amounts of volatiles in the vulcanized silicone are most efficiently reduced (for example, up to about 0.4 wt % in the final product) by preheating part A only. This benefit comes without the accompanying processing problems (e.g., premature cure) that can be incurred when part B is preheated (see below). Rheology. The behavior of the siliconessindividual parts or combinedswas examined rheometrically as a function of thermal history. Two main tests were performed: a study on the effect of preheating on vulcanization speed and the effect of preheating on the structure viscosity of LSR.
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Figure 7. Significant difference in shear between the two silicone parts: (A) part A, (B) part B, and (C) overlay of parts A and B.
Figure 8. Cure profile as a function of preheating and actual mixture temperature. The change in the minimum viscosity as a function of removal of volatiles is indicated by the red line.
The shear behavior of the two untreated parts as they were heated in an air atmosphere over about 30 min is shown in Figure 7. During the precure heating experiments with part A alone, it was noted that the changes in viscosity as a function of time and heating temperature were insignificant (Figure 7A). By contrast, part B exhibited large increases in shear viscosity after longer heating times (Figure 7B, see also Supporting Information). However, as silicones are highly structure viscous materials, even large changes in zero shear viscosity of the single component had little impact on the processability of the mixed parts: at low shear rates the viscosity dropped to processable shear viscosities.17 At longer heating times, and particularly in the presence of moisture, part B “skinned over”, undergoing partial cure from the upper surface down. This is consistent with a secondary cure mechanism initiated by moisture and heat, in addition to the platinum cured cross-linking that occurs when parts A and B are mixed.18 To test if the secondary cure protocol is associated with the presence of moisture, the heating protocols were repeated in an oven under various conditions. Silicone films of A, B, or A + B with a thickness of 2.25 mm were introduced into a vial. The vials were placed in the preheated vacuum oven at either a temperature of 180 °C, with a pressure of ∼1 Torr, or 200 °C, with ambient pressure and a flow of nitrogen (the nitrogen was introduced after first exchanging the environment in the oven chamber from air to nitrogen by a series of purge/refill cycles), or vacuum. Skinning over occurred most rapidly with part B in a heated oven in air (
