Polyester initiators: opportunities overlooked?

A cure for tough economic times: Several potential cost-saving technologies that have been largely ignored for years.

If reinforced-plastics/composites fabricators look to their peroxide catalyst suppliers for help in raising their productivity and cost-competitiveness, they better not wait for discovery of some new "miracle molecule." Suppliers say they're still hampered by an EPA-imposed regulatory straightjacket that has brought a dead halt to production of any new peroxide chemistries since 1986.

The good news is that there is a backlog of numerous catalyst technologies developed in the 1970s and '80s that have been all but ignored by most fabricators. The rigors of the now-fading recession should provide all the more incentive for processors to take another look at some catalysts and formulation technologies that have been languishing on the shelf, despite their potential for reducing costs and speeding production.

Here's an updated look at some of those under-utilized technologies that could benefit spray-up, casting, SMC/BMC, pultrusion, RTM and filament winding. This article sprang initially from discussions with Elf Atochem, which provided most of the supporting technical data. Subsequent discussions with other major peroxide suppliers confirmed the general consensus that much existing catalyst technology remains under-exploited.

ROOM-TEMPERATURE CURING

For room-temperature spray-up, as well as casting of cultured marble or polymer concrete, sources at both Elf Atochem and Witco's Argus Div. say an "old" initiator is coming back into style. Ketone peroxides based on 2,4-pentanedione are coming to be recognized once again for their advantages over both conventional MEKPs and BPO emulsions, says Elf Atochem technical service director Ronald B. Gallagher. Pentanedione peroxide is a true liquid, unlike BPO, so no equipment changes are needed to spray it. It contains a low amount of water--too much for gel coats, perhaps, but much less than fire-resistant BPOs. And yet it's very safe: it is fire resistant, has a high flash point, and can be stored at room temperature like BPO and MEKP. Like BPO, it carries no DOT yellow label; and it is lower in toxicity and corrosiveness than MEKP. Unlike BPO, 2,4-pentanedione uses standard cobalt rather than high levels of amine promoters to get a fast cure--this means that it works in standard resins formulated for MEKP, and there is no unwanted color development from the amine.

TABLE 1--PENTANEDIONE PEROXIDE VS. BPO & MEKP
(POLYESTER PLUS 50% CALCIUM CARBONATE)
                        40%        2,4       2,4
Peroxide                BPO(a)     PD(b)     PD(b)      MEKP(c)
Phr                     2.0        0.50      0.75      0.5
Market Price,           2.70-      3.05-     3.05-     1.60-
  $/lb                  3.30       3.55      3.55      2.35
Avg. Cost/
  100 lb Resin          6.00       1.65      2.48      0.99
SPI Test, 77 F
  Gel, min              13.9       14.0      5.7       7.1
  Cure, min             23.6       20.0      10.3      17.6
  Peak, F               218        220       238       176
Barcol, 1/8-in. Castings
  15 min                0          0         20-25     0
  30 min                5-10       5-10      25-28     0
  60 min                25-30      10-15     25-30     0
  6 hr                  25-30      15-20     25-30     0-5
  24 hr                 25-30      20-25     25-30     10-15
a With 0.2 phr DMA.
b Lupersol 224 (4% active oxygen) with 0.15 phr Cobalt-6, 0.05%
DMA.
c Lupersol DDM-9 with same Cobalt/DMA promoter system.
Source: Elf Atochem.

But most important, pentanedione peroxide offers faster Barcol or "green strength" development than either alternative, helping to reduce styrene emissions and permitting faster demold times. This factor may overcome historical resistance to the cost of the initiator. As shown in Table 1, pentanedione peroxide is less expensive to use than BPO but more expensive than MEKP. However, shorter mold cycle times could more than make up for the added cost.

Fully 10 years ago, Elf Atochem (then called Lucidol) proposed a novel initiator system utilizing t-butyl perbenzoate (TBPB), normally used for elevated-temperature molding, in place of either MEKP or BPO for room-temperature curing. A key difference is in the use of a promoter system combining copper and iron chlorides and mercaptobenzothiazole (MBT). Although Atochem's data show that this approach could offer cost savings and/or productivity enhancements (Table 2), it fell upon deaf ears. FRP fabricators appeared unwilling to try such a radically different approach, despite a number of ancillary benefits besides cost:

* It's a liquid, unlike BPO, and has much lower viscosity than MEKP, which could mean lower pumping pressures and easier atomization.

* Peroxide assay of TBPB won't drift in storage, unlike MEKP.

* TBPB contains no plasticizer nor water--matters of concern in electrical-grade laminates and gel coats.

* It's said to be safer than ketone peroxides because of its high flash point and low sensitivity to metal contamination. Like MEKP and BPO, it requires no refrigerated storage, though it is a yellow-label product like MEKP.

* It is much less corrosive to skin than MEKP.

* Use of a different promoter system means lower color.

* It provides long pot life of several days, unlike MEKP, without sacrificing cure speed.

Note in Table 2 that initiator cost is lowest with TBPB; total cure system cost is marginally higher than for MEKP, but cure time is half as long--a major advantage. Although cure rate is about the same as with BPO, cure system cost is significantly lower with the new system.

TABLE 2--T-BUTYL PERBENZOATE IN ROOM-TEMPERATURE CURE
                          40%
Peroxide                  BPO        TBPB        MEKP
 phr                      2.0        0.66        1.32
Promoter, phr
 DMA                      0.4        --          --
 Co-6                     --         --          0.17
 MBT                      --         0.46        --
 1% Cu|Cl.sub.2~          --         0.26        --
 45% Fe|Cl.sub.3~         --         0.26        --
$/100 lb Resin
 Catalyst(a)              6.00       2.00        2.61
 Promoter(b)              1.36       1.38        0.65
 Total                    7.36       3.38        3.26
Gel Time, min(c)          9.0        6.5         8.5
Cure Time, min            12.3       12.0        24.1
Gel/Cure
 Interval, min            3.3        5.5         15.6
Peak Exotherm, F          237        181         147
Barcol, 24 hr             0          0           0
Flex. Mod., psi           100,000    100,000     83,800
a Average of market price range.
b List price.
c Clear castings at 86 F.
Source: Elf Atochem.

FOR SMC/BMC

Relatively few matched-metal die molders have taken advantage of the benefits offered by t-amyl peresters and perketals, in the view of Atochem sources. T-amyl perbenzoate, especially, offers a "largely untapped reservoir of benefits," says Gallagher. Compared with workhorse t-butyl perbenzoate, TAPB offers faster cure plus longer flow. According to Gallagher, it would require a complex inhibitor/promoter system to accomplish the same result with TBPB.

Peter Spoor, peroxide business manager for Akzo Chemicals, is less sanguine about the long-term potential for t-amyls in view of their higher prices and "marginal benefits," as he puts it.

Molders may be able to judge whether or not those benefits are marginal from Table 3. It shows that catalyst cost is 23% lower with TAPB to achieve the same cure time as with TBPB. Table 3 also shows that at the same catalyst cost, cure time is 10-13% shorter with the t-amyl version. In addition, the table shows that perketals are even more efficient and cost-effective than peresters, with a t-amyl perketal surpassing the t-butyl version on both scores. Perketals have the advantage of not requiring refrigerated storage and of offering longer catalyzed shelf life and faster cures than do peresters; but peresters still outshine perketals where long flows are important.

Besides Elf Atochem, at least two other believers in the potential of t-amyl peresters are Norac and Aztec Catalyst Co. Norac sales manager Ken Weber believes that some of the "more advanced SMC molders" are about to convert to t-amyls to get better productivity than with standard two-component perbenzoate/peroctoate systems, and with less tendency to pregel. Aztec is also betting on t-amyls; it offers the peroctoate commercially now and plans to introduce t-amyl perbenzoate and peroxyneodecanoate by early '93.

Sources at both Aztec and Akzo say they foresee a possible trend to lower molding temperatures for SMC/BMC, partly to save on energy costs. This would stimulate interest in lower-temperature initiators, probably as a "kicker" in a blend with a higher-temperature peroxide to finish the cure. Blends of TBPB with t-butyl peroctoate reportedly are already the most common system in SMC/BMC today, but some peroxide suppliers think molders should look into the productivity advantages of other types of dual-catalyst systems. If perester blends are good, then perester/perketal combos are even better, according to both Aztec and Elf Atochem spokesmen. (Those firms and Akzo offer premixed perester/perketal blends.)

And Aztec is recommending an old "new" option: TBPB with BPO. Such systems date back to the infancy of SMC, but Aztec is adding a new wrinkle. The company will introduce late this year what senior marketing development manager Stephen C. Isaacs, jr. says will be the first non-shock-sensitive, relatively safe-to-handle "dry" BPO powder. This specially treated product will be free-flowing and contain no water, so it will be readily soluble in styrene and liquid resins unlike standard "wet" BPO. Comparing this new BPO with peroctoate as the low-temperature initiator in a dual-catalyst system, Isaacs says, "Although cost may be comparable to or slightly higher than peroctoate, it offers the advantages of ambient-temperature storage and non-yellow-label shipping requirements." (Akzo sources caution that use of BPO at SMC/BMC mold temperature may run a risk of surface defects from volatile decomposition byproducts.)

FOR PULTRUSION

Atochem's Gallagher says some pultruders have caught on to both major cost-saving potential and greater convenience by replacing an expensive solid percarbonate initiator with a liquid percarbonate or perester costing under half as much. Akzo's Perkadox 16 peroxydicarbonate has long been a workhorse in pultrusion (typically as part of a two- or three-catalyst system), despite its high price and being a solid that must be predispersed.

TABLE 3--T-AMYL VS. T-BUTYL IN SMC/BMC
(POLYESTER WITH 50% CALCIUM CARBONATE)
1. Cost-Performance at Equal Activity (3.1-min Cure)
                 Cost,               Avg.    Relative
Peroxide         $/lb       Phr      Cost      Cost
t-Butyl          2.70-
 Perbenzoate     3.30       1.0      3.00      1.00
t-Amyl           3.75-
 Perbenzoate     4.20       0.58     2.31      0.77
2. Activity Comparison at Similar Cost
                        Cat.
                        Cost,   Cure
@ 275 F,    Cure
@ 300 F,                                      Peroxide         Phr    $
min   % Change   min %   Change
TBPB             1.0    3.00    3.1   --         2.4     --
TAPB             0.75   2.99    2.8   -10        2.1     -13
t-Butyl
  Perketal(a)    0.68   3.03    2.5   -19        1.9     -21
t-Amyl
  Perketal(b)    0.56   2.88    2.3   -26        1.8     -25
a Lupersol 331-80B.
b Lupersol 531-80B.
Source: Elf Atochem North America.
TABLE 4--FAST-CURE SYSTEM FOR RTM
Formulation:
 Vinyl Ester Resin             100 Parts
 Perester
  (e.g., Perbenzoate)          1.0
 2,4-Pentanedione              0.4
 6% Cobalt                     0.4
Mold Temp.       160 F       180 F       200 F
Gel, min         1.7         1.2         0.8
Cure, min        2.5         2.0         1.6
Peak, F          430         440         450
Source: Elf Atochem North America

Elf Atochem recommends as alternatives three liquid initiators that reportedly offer the same pot life as Perkadox 16, equal or faster cures, and potential use-cost savings ranging from 55% to 77%. They are t-butyl and t-amyl peroxyneodecanoates (Lupersol 10M75 and 546M75, respectively) and a percarbonate, Lupersol 223M75. The only drawbacks, according to Gallagher, are that these liquids require storage below 14 F, vs. 68 F for Perkadox 16; and t-butyl peroxyneodecanoate, in particular, is recommended more for thin-section pultrusions than thick ones, owing to its higher exotherm.

FOR RTM

A key goal in this fast-growing area of composite technology is to speed up mold cure cycles to improve the economics of the process for large-scale production. Both Akzo and Witco's Argus Div. see 2,4-pentanedione peroxide playing a significant role here. Argus' peroxide technology manager Ronald A. Pastorino says a product such as USP-240 used at the same phr level as MEKP can give 50% reductions in cycle time, with faster build-up of green strength to permit quick demolding, and no yellow-label designation. The promoter system may need adjustment--less cobalt than for MEKP, and perhaps no amine. Although pentanedione peroxide costs more than MEKP, it's competitive in cost with BPO and faster, as well.

For really fast cures in RTM (and also in some filament winding and pultrusion), Elf Atochem recommends combining 2,4-pentanedione (the ketone itself, not the peroxide) with cobalt as promoters for a perester such as perbenzoate that doesn't require refrigerated storage. Table 4 shows the speed of such a system. Despite such fast cures, the promoter-catalyst system reportedly can provide three to four days' catalyzed pot life.

Akzo sources say that this approach is widely used in Europe. The company offers two preblends (Trigonox 93 and 42pr) of peresters with pentanedione.

More New Products

For corrosion-resistance: Argus' new CHP-5 is a specially formulated cumene hydroperoxide that's said to be nonfoaming in vinyl esters and also to provide faster gel and cure than standard cumene hydroperoxide. Also new from Argus is HPC-9, a blend of MEKP and hydroperoxides that lowers foaming and provides a more controlled cure--with less cracking, crazing and shrinkage--in vinyl esters and polyester/urethane hybrids.

Akzo's Trigonox 239A is a cumene hydroperoxide solution that's said to be nonfoaming in vinyl esters and to produce low exotherm and shrinkage.

Norac introduced Norox MCP, a blend of MEKP and a hydroperoxide designed to keep exotherm down in thick sections, like flanges. It's also low-foaming in vinyl esters.

Pourable/pumpable BPOs: Norac's new Benox L-40 LV, is a low-viscosity 40% suspension in a non-phthalate plasticizer, aimed at densified castings and RTM.

Reichhold recently modified its Superox 46744 40% emulsions for lower viscosity and easier mixing into densified castings.

And Aztec plans to introduce late this year a non-aqueous, free-flowing powdered BPO that will be both non-shock-sensitive (thus relatively safe to handle) and easily soluble in styrene and resins. This pourable powder will be aimed at RTM, pultrusion, and even SMC/BMC.