Paste curing

Paste adhesives are supplied as either one- or two-component adhesive systems. They can be used in applications where pressure cannot be applied. Some two-part pastes cure at room temperature after the appropriate proportions are mixed. Epoxy, urethane, and acrylic adhesives are all available as paste adhesives. 

Fig. 8.8 Relations between compressive strength (log scale) and porosity data from various sources for Portland cement pastes cured at ordinary temperatures (open circles), hot pressed Portland cement paste (filled circle), autoclaved pastes of Portland cement, sometimes with added sulphur (filled squares) and autoclaved pastes of Portland cement with 50% (open squares) or 30"/o (cross) of added pfa. After Feldman and Beaudoin (F36).

Subsequent studies have shown that K is not related to the total water porosity but depends on the volume and connectivity of the larger pores. Mehta and Manmohan (M75) and Nyame and Illston (N16) found linear relations between log and estimates of the maximum continuous pore radius, obtained from MIP, and other quantities derived from the pore size distribution and degree of reaction. A high proportion of the flow appears to be through pores wider than about 100 nm. Typical values of log K for mature pastes cured at ordinary temperatures range from around - 13.4 at w c 0.3 to around — 11.8 at w/c 0.7 (M75,N16,G67,M76,H47). K increases with temperature (G67,M76). 

Fig. 9.6 Pore size distributions for pastes of Portland and composite cements (vv/s = 0.45. curing temperature 21 C), determined using mercury intrusion porosi-metry. CID. CIY Portland cement pastes cured for 1 day and 1 year. S2D. SIY Portland-blastfurnace slag cement (70% slag) pastes, cured for 2 days and I year. F.3D, FI Y pfa cement pastes (13% CaO in fly ash) cured for 3 days and 1 year.. After Feldman (F34).

DTA (K32,025) and XRD (A29) studies indicate that cement pastes cured at temperatures of or approaching lOO C contain little or no detectable hydrated aluminate phases, the observation of hydrogarnet noted above thus being unusual. Kalousek and Adams (K32) considered that the Al , 864 and other ions were incorporated into a Phase X , which possibly included all the oxide components of the cement. In pastes cured normally, much of the AFm phase is probably poorly crystalline and intimately mixed with the C-S-H (Seetion 7.3.1). This tendency may be increased if hydration takes place more rapidly. 

Figure 8.2b shows the X-ray diffraction patterns for 3BS pastes cured at 90 °C and 98% RH for 4 h. Under these conditions, 3BS is converted into 4BS but only partially. The ratio between the crystal phases formed is 3BS/4BS = 0.83. 

Figure 8.2c presents the X-ray diffraction patterns for the paste cured at 90 °C for 1 h and then steam treated for another hour. This paste contains a well pronounced 4BS crystal phase. Hence, introduction of steam into the curing chamber facilitates the conversion of 3BS particles into 4BS ones. With this technology, a small quantity of Pb9(C03)6(OH)6 is also formed in the paste. 

Pore volume distribution by radius in (a) 4BS paste prepared at 80 °c and (b) same paste cured at... 

The 3BS paste cured at 35 °C for 48 h contains fairly small pores with the largest total pore volume. The total pore volume vs. pore radius curves for pastes undergone two-step 3BS 4BS conversion feature 2 inflection points, which suggests formation of two groups of pores with pronounced mean pore radii. This is related to the formation of large- and small-sized 4BS crystals, respectively. 

Pore volume distribution by radius for 3BS and 4BS pastes cured under different conditions [16],... 

Paste Group Battery Number Phase Composition of Paste Curing Conditions Pore Volume (cm g ) Mean Radius ( tm) BET Surface, 2 -1> (m g ) Solid Phase Density (g cm ) 1, A kg at 80 Ah kg PAM... 

The 3BS cured paste has higher solid phase density than that of the pure 3BS phase. Such a high value can be obtained if the paste contains considerable amounts of PbO and Pb. Except for the paste cured at 90 °C for 4 h the pastes of Group III (Table 8.1), produced from the same initial 3BS paste by two-step curing with steam treatment, have densities about 6.50 g cm after curing. The decrease in solid phase density indicates that conversion of 3BS crystals into 4BS ones occurs through incorporation of water in the crystals. There is no other substance in the paste that would reduce the crystal density. 

Figure 8.23 shows the interface paste/CL2 layer of a plate prepared with 3BS paste cured at 90 °C for 3 h and then treated with steam for another 2 h. Carbon dioxide is introduced into the curing chamber with the water steam [16]. The shape of the crystals formed under these conditions is typical of plumbonacrite and cerussite crystals. Obviously, the technology of 3BS conversion into 4BS through purging with water steam, i.e. introduction of CO2 from the air, leads to formation of hydrocarbonates at some sites of the paste/CL2 interface. 

Paste curing and drying Atmospherically dependent processes... 

Batteries produced with 3BS positive pastes cured at 90 °C, whereby the 3BS paste converts into 4BS, have good power output performance. However, the cycle life of these batteries is shorter by about 30% than that of the batteries with 4BS pastes. 

Differential Scanning Calorimetric (DSC) Measurements of Soaked Pastes Cured at 90 °C... 

Fig. 5.35 Strength vs. porosity relationship (porosity calculated as a ratio of pore volume to total volume of sample) (according to [80]). Portland cement paste cured at 25 °C empirical formulas ...

Fig. 5. 36 Compressive strength (marked on the curves, in MPa) of paste as a function of porosity and the spatial arrangement of grains. P—T lines show the results for the increasing finest fractions of quartz. AB, CD, EF lines relate to the pastes cured at room temperature, autoclaved or to the eement-fly ash mixture respectively, (according to [82])...

In a paste cured for a short period of time, the cracks induced during bending occur in the vicinity of the paste interface with reinforcement, while in the old samples— along the contact plane [51]. This observation is not fulfilled for copper and brass, which proofs a better cement paste adhesion, of chemical nature, to these metals (Fig. 6.20). 

The same results were obtained by Ghorab and Kisher [158]. Also Sylla [159] found the reduction of ettringite content in cement pastes after 3 h of thermal treatment at temperature of 60 °C this phase disappears almost entirely after this period when the temperature of curing rises to 80 °C. The same results were found by Odler et al. [160]. It found also good relation with the composition of solution, which in the pastes subjected to thermal treatment at temperature of 90 °C have a diSerent sulphate ions concentration (Fig. 6.38). The concentration of these ions is maintained on the high level after 28 days of paste matured in water and is quicker reduced after this period [151]. However, even after 90 days it is four times higher than in the paste cured all the time in water at ambient temperature. 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

Room temperature curing paste. Cure at 149-1771C. 

A preaccelerated, flexible highly thixotropic, structural bonding paste cured with either MEKP or DBP. 

EXPOSURE IN 100% RELATIVE HUMIDITY AT 125 F ACID ETCHED 6061-T6 ALUMINUM -K) TWO PART ROOM TEMPERATURE CURED EPOXY (INITIAL SHEAR STRENGTH 2800 PSIj - ONE PART VINYL-PHENOLIC PASTE CURED AT 350°F FOR 15 MINUTES AT 100 PSI (INITIAL SHEAR STRENGTH 3400 PSI]... 

Highest Tg Chemical resistance Past cure speed CN2600 Acrylated Acrylic ... 

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