Flame-resistant aqueous epoxy impregnating compositions containing nonionic surface active agents

Aqueous emulsions of epoxy resins, which can be used to treat glass cloth used in the preparation of printed circuit board laminates, are produced by combining an epoxy resin and a flame retardant phenol, such as tetrabromobisphenol A, with a selected nonionic surface active agent, such as an alkylaryl polyether alcohol, monomethylol dicyandiamide curing agent and, if needed, a catalyst.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing aqueous, high solids 
emulsions of epoxy resins, which are not solvent based, and which are 
suitable as impregnating resins for use in the preparation of laminates, 
for example, printed circuit boards. 
State and Federal air pollution control regulations have become 
increasingly stringent, and could significantly affect the use of 
solvent-based epoxy compositions for industrial use. Other than 
regulations, the disadvantages of solvent-based epoxy compositions include 
solvent evaporation costs, the possible toxicity of some of the residual 
solvent retained in the cured material, and the cost and availability of 
solvents useful for epoxy dilution. To overcome the problems associated 
with solvent-based epoxy compositions, many systems have been converted to 
completely aqueous systems. These epoxy formulations contain a multitude 
of components which are necessary to obtain the desired properties. 
Hosoda, in U.S. Pat. No. 3,983,056, recognizes air pollution problems 
caused by solvents, and discloses a room temperature curable aqueous epoxy 
resin paint, obtained by combining a mixture of bisphenol A epoxy resin 
with a novolac, dimer or trimer acid epoxy type, or a methyl substituted 
bisphenol epoxy, 0.1% to 20% by weight, based on epoxy, of a nonionic 
surface active agent, preferably a polyoxyethylene benzylated phenyl 
ether, preferably a polyamide curing agent, and a molybdic acid salt. 
Hosoda, in U.S. Pat. No. 4,073,762, teaches the use of a polyamide curing 
agent derived from a dimer acid and a polyether diamine to be used with 
the paint composition disclosed above. The combination of these compounds 
is essential to obtain this emulsified paint. The paint is used, for 
example, as a rust preventive primer. 
Zentner, in U.S. Pat. No. 4,222,918, teaches the use of dicyandiamide 
dissolved in water as a curing agent for an emulsion of epoxy, anionic 
polycarboxylic emulsifying agent, and thermoplastic resin, such as 
polyvinyl formal or polysulfone. However, dicyandiamide has limited 
solubility in aqueous systems and is not very effective in the preparation 
of higher weight solids water emulsions. The use of a dicyandiamide 
derivative, such as monomethylol dicyandiamide, helps eliminate the 
solubility problem and enables one to prepare a wider range of emulsions. 
The preparation of these compounds, in organic solvents, is disclosed by 
Alvino et al., in U.S. Pat. No. 4,327,143. This solvent based system is 
not able, however, to utilize energy efficient infrared "B" staging 
operations without potential flammability problems. Substitution of water 
for solvent in the Alvino et al. compositions, to decrease potential 
flammability problems, may result in unstable mixtures with phase 
separation of water. 
The need for energy efficient and non-polluting epoxy based high solids 
resin systems, that are also non-flammable, stable in and compatible with 
water, and possess suitable electrical and mechanical properties required 
by certain laminates, such as printed circuit boards, has not been met by 
these prior art compositions. 
SUMMARY OF THE INVENTION 
The above need has been met and the above problems solved by providing a 
high solids, emulsified, aqueous impregnating composition comprising: (1) 
epoxy resin, (2) flame retardant phenol, (3) a selected nonionic surface 
active agent, (4) a latent curing agent, preferably monomethylol 
dicyandiamide, (5) water, and if needed, (6) a catalyst, such as 
benzyldimethylamine. The useful weight ratio of epoxy:flame retardant 
phenol:nonionic surface active agent solids:latent curing agent is about 
(100):(1 to 100):(13 to 25):(2 to 10). Water is usually combined with the 
nonionic surface active agent, curing agent, and catalyst to provide 
aqueous solutions. 
There are two methods by which the emulsion can be created. In the fusion 
method, the epoxy resin is combined with the flame retardant phenol, 
preferably tetrabromobisphenol A at about 25.degree. C. Then these two 
ingredients are heated to from about 80.degree. C. to 100.degree. C. over 
a 20 to 30 minute period, to form a reaction product consisting 
essentially of a homogeneous brominated epoxy solution, when 
tetrabromobisphenol A is used. This reaction product is then cooled and 
combined with the selected nonionic surface active agent. Monomethylol 
dicyandiamide or dicyandiamide dissolved in water is then added 
incrementally under constant stirring at from about 40.degree. C. to 
55.degree. C. 
The preferred method is in-situ addition, which mixes the selected nonionic 
surface active agent and the epoxy resin, then heats the mixture up to 
from about 80.degree. C. to 90.degree. C. When the mixture is at about 
85.degree. C., the flame retardant phenol is then added until it dissolves 
in about 1/2 to 5 minutes, after which the admixture is immediately cooled 
so that no reaction product is formed. Then monomethylol dicyandiamide or 
dicyandiamide dissolved in water is added in increments at from about 
40.degree. C. to 55.degree. C. A catalyst, such as benzyldimethylamine may 
also be introduced. Glass cloth or other porous substrate material may be 
impregnated with the emulsion and "B" staged in a convection oven. A 
plurality of these impregnated substrates can then be heat and pressure 
consolidated to form a laminate. 
This composition provides superior mechanical and electrical properties, 
and imparts increased flame resistance to cured, impregnated substrates 
such as glass cloth used for printed circuit board laminates. This 
composition, utilizing a nonionic surface active agent, reduces surface 
tension between water and hydrophobic components of the admixture, 
allowing a stable epoxy-water emulsion having a solids content of from 
about 50% to about 70%. The manufacture of the composition also can use 
less energy because energy efficient infrared radiation can be used to "B" 
stage impregnated substrates, and the use of such infrared radiation can 
allow much faster line speeds. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Epoxy resins that are suitable for the water-based emulsion of this 
invention include diglycidyl ethers of bisphenol A, diglycidyl ethers of 
bisphenol F, polyglycidyl ethers of a novolac, glycidyl esters, hydantoin 
epoxy resins, cycloaliphatic epoxy resins, and diglycidyl ethers of 
aliphatic diols, all of which are commercially available and described in 
detail by Smith, et al., in U.S. Pat. No. 4,273,914. The preferred epoxy 
resin is a liquid diglycidyl ether of bisphenol A having an epoxy 
equivalent weight of from about 180 to 215. This preferred epoxy resin 
provides a more controlled reaction, and provides superior mechanical and 
electrical properties in laminates. 
Any halogenated phenol may be used in the resin mixture to impart flame 
resistance to the cured product, such as chlorinated phenols, however, 
tetrabromobisphenol A (TBBPA) is particularly effective. These materials 
chemically react with the other components to provide an internal flame 
resistance. Other flame retardant materials such as Sb.sub.2 O.sub.3 and 
Al.sub.2 O.sub.3.3H.sub.2 O do not chemically react with the other 
components, and are not as effective as flame retardant agents. 
The useful class of nonionic surface active agents is a nonionic alkylaryl 
polyether alcohol having the following general structural Formula: 
##STR1## 
where A is an alkyl group having from 5 to 10 carbons, preferably from 8 
to 9 carbons, y=1 to 2, and z=15 to 70, preferably 25 to 70. Below z=15, 
difficulty in forming a stable emulsion will be encountered. The preferred 
nonionic surface active agents are those prepared by the reaction of octyl 
phenol or nonyl phenol with ethylene oxide or propylene oxide. They are 
usually in the form of a 60% to 80% aqueous solution. The nonionic surface 
active agent should comprise from about 13 to 25 parts solids based on 100 
parts by weight of epoxy resin. If below 13 parts are used, there is an 
increase in viscosity, separation, and foaming. Anionic or cationic 
surface active agents are not useful in this invention. 
Monomethylol dicyandiamide, the preferred curing agent, provides moisture 
resistance and prevents crystallization and void formation in the cured 
product. This curing agent has increased solubility in aqueous solutions 
and thus allows higher solids weight products to be formed than if 
dicyandiamide is used. Dicyandiamide can be used, however. These are both 
latent curing agents which initiate cure at above about 155.degree. C., 
i.e., they are not capable of curing at room temperature but will cure at 
reaching a certain elevated temperature, such as 160.degree. C. Room 
temperature curing agents are not useful because "B" staged laminates 
could not be produced. The useful weight ratio of (epoxy resin):(flame 
retardant phenol):(nonionic surface active agent solids):(monomethylol 
dicyandiamide or dicyandiamide latent curing agent) is from about (100):(1 
to 100):(13 to 25):(2 to 10). The monomethylol dicyandiamide or 
dicyandiamide is usually added as an aqueous solution. 
The catalyst concentrations may be varied over a broad range to provide 
effective cure conditions dictated by the process. The benzyldimethylamine 
(BDMA) solids may be varied from about 0.2% to about 0.6% of the weight of 
the epoxy resin. The preferred range is from about 0.4% to about 0.6% 
based on the weight of the epoxy resin. Potassium carbonate dissolved in 
water may be added in an amount of from about 0.03% to about 0.12% based 
on the weight of the epoxy resin. The potassium carbonate is used for 
catalyzing the emulsified impregnation composition. Acid salts are not 
useful in this invention, adding ionic impurities which have a deleterious 
effect on the electrical properties of the final laminate. 
The fusion product, which is made by heating the epoxy resin plus the flame 
retardant phenol, at from about 80.degree. C. to 100.degree. C. over a 20 
to 30 minute period to form a reaction product, is made water compatible 
by mixing in the nonionic surface active agent at 50.degree. 
C..+-.5.degree. C., using a variable speed air stirrer with a suitable 
impeller to insure thorough mixing. A monomethylol dicyandiamide-water or 
dicyandiamide-water solution is then added at about 50.degree. C. to the 
emulsion in about 10 percent aliquots, and stirring is continued until the 
system is homogeneous. When the blend is homogeneous, a second 10 percent 
volume of monomethylol dicyandiamide-water or dicyandiamide-water solution 
is added and again stirred until homogeneous. In the in-situ process, the 
nonionic surface active agent is added to the epoxy and the mixture 
temperature is raised to about 85.degree. C., followed by flame retardant 
phenol addition at about 85.degree. C. for up to about 5 minutes without 
cooling, so that a reaction product is not formed. This admixture is then 
cooled to about 50.degree. C. and the latent curing agent-water solution 
is then added in increments. 
During the initial additions of water with the curing agent, a 
water-in-resin emulsion is formed and the system becomes increasingly 
thixotropic. After about one-half of the monomethylol dicyandiamide-water 
or dicyandiamide-water solution is added, an inversion to a resin-in-water 
emulsion occurs with an abrupt drop in viscosity. When this point is 
reached, the remainder of the monomethyl dicyandiamide-water or 
dicyandiamide-water solution is added continuously under moderate 
stirring. The usual addition is in ten, 10% increments. If the latent 
curing agent-water solution is added all at once, an emulsion will not 
form. If added in increments larger than 25% increments, the emulsion will 
tend to be unstable. 
Properly prepared, the emulsion will have a white color, a low viscosity, 
and a droplet size of from about 3 to 9 microns. Some settling may occur 
after about twenty-four hours, but the settled material is easily 
reintroduced into the emulsion by hand stirring. To catalyze the 
emulsified impregnating composition, K.sub.2 CO.sub.3 (potassium 
carbonate) dissolved in water, and benzyldimethylamine (BDMA) are added 
and thoroughly mixed. The solids content of the admixture is usually from 
about 60% to 70% in water. 
Glass cloth is treated and "B" staged, i.e., dried but not completely 
cured, at around 155.degree. C. in a convection oven, or the glass cloth 
may be passed under an infrared radiation source to "B" stage it. The 
resin content of the glass cloth ranges from about 35% to about 50% by 
weight. A laminate can be prepared from a plurality of treated glass cloth 
sheets with good impregnation of the glass cloth by the emulsion. This 
laminate can be finally cured and used as a heat and flame resistant 
printed circuit board, which can be clad with a copper conductor sheet 
applied to at least one surface, or have conductive copper circuits 
applied thereto.

EXAMPLE 1 
An emulsified impregnating composition formulation was prepared using the 
fusion process. To a 2,000 ml round bottom flask equipped with a stirrer, 
thermometer, condenser and N.sub.2 inlet tube, was added 912 g. (2.28 
parts) of an epoxy resin having an epoxy equivalent weight of from 193 to 
203 and a viscosity at 25.degree. C. of from 3,000 cps to 7,000 cps. (sold 
commercially by Shell Chemical Co. under the tradename Epon 829) and 400 
g. (1.00 part) of tetrabromobisphenol A (TBBPA). The mixture was heated 
and stirred under a N.sub.2 blanket until the temperature reached 
90.degree. C., over a 20 minute period, and all the material dissolved to 
form a homogeneous solution reaction mixture. At this point the reaction 
mixture obtained was analyzed and found to contain 3.75% oxirane oxygen 
according to the analytical procedure described in the Handbook of Epoxy 
Resins by Lee and Neville, Chapter 4-17. This percentage corresponds to an 
epoxide equivalent (EEW) of 427. 
To 100 g. of the above reaction product, which was heated to 50.degree. C., 
was added 10 g. of a nonionic surface active agent, an octylphenol 
polyether alcohol, where A=8 carbons, y=1 and z=30 in Formula (I) 
hereinbefore described (sold commercially by Rohm and Haas Co. under the 
tradename Triton X-305). The ingredients were mixed thoroughly at about 
1,000 rpm. The mixture was opaque white in color and homogeneous. To this 
mixture at 50.degree. C. was added 5 g. of dicyandiamide dissolved in 140 
ml of tap water, in 14 ml increments, approximately 10% of the total 
volume. During the addition, the mixture was constantly being stirred. 
A very viscous mixture was obtained after the first 2 to 3 portions of the 
dicyandiamide-water were added. Upon further addition of the 
dicyandiamide-water solution, the emulsion inverted to give a watery 
emulsion. After inversion, the remainder of the dicyandiamide-water 
solution was used. A white homogeneous emulsion then was obtained. No 
separation occurred after about 2 days at room temperature. This emulsion 
provided a useful low viscosity, aqueous, epoxy impregnating composition. 
EXAMPLE 2 
An emulsified impregnating composition formulation was prepared from the 
following ingredients, using the in-situ process: 
______________________________________ 
Epon 829 epoxy resin 100.00 g. 
Tetrabromobisphenol A (TBBPA) 
51.43 g. 
Triton X-305 nonionic surface 
21.60 g. 
active agent (70% aqueous solution) 
(15.1 g. solids) 
Tap H.sub.2 O 100.00 g. 
Monomethylol Dicyandiamide* 
3.77 g. 
______________________________________ 
*N--CyanoN'--Hydroxymethyl Guanidine 
The nonionic surface active agent, an octylphenol polyether alcohol, where 
A=8 carbons, y=1 and z=30 in Formula (I) hereinbefore described, (sold 
commercially by Rohm and Haas Co. under the trade name Triton X-305) was 
thoroughly mixed at room temperature with an epoxy resin having an epoxy 
equivalent weight of from 193 to 203 and a viscosity at 25.degree. C. of 
from 3,000 cps. to 7,000 cps. (sold commercially by Shell Chemical Co. 
under the trade name Epon 829). The mixture was heated to from 80.degree. 
C. to 90.degree. C. and TBBPA was added and stirred until it all dissolved 
over a 1 to 5 minute period. The mixture was not cooked to provide a 
reaction product. The clear amber resin was immediately cooled to from 
40.degree. C. to 50.degree. C., and then a monomethylol 
dicyandiamide-water solution was added in 10 percent increments, with 
stirring at about 1,000 rpm. 
Initially a white viscous mixture was obtained after the first incremental 
monomethylol dicyandiamide-water solution addition. The consistency of the 
emulsion after successive additions of monomethylol dicyandiamide-water, 
changed from viscous to creamy to watery. After the emulsion reverts from 
a viscous to a watery mixture, the remainder of the monomethylol 
dicyandiamide-water solution was added at a reduced stirring speed. A 
white, homogeneous, approximately 60% solids fluid emulsion was obtained. 
No separation occurred after two days at room temperature. 
The Brookfield viscosity at 23.degree. C. measured from 70 cps. to 80 cps. 
To this emulsified impregnating composition was added 0.093 g. K.sub.2 
CO.sub.3 dissolved in 0.18 g. H.sub.2 O, and 0.45 g. benzyldimethylamine 
(BDMA). The ingredients were thoroughly mixed. No separation of the 
emulsion occurred after 10 hrs. This provided a useful low viscosity, 
catalyzed, aqueous, high solids, epoxy impregnating composition, where the 
nonionic surface active agent was effective to lower surface tension 
between the hydrophobic epoxy component and the water, and allow a stable 
emulsion. 
EXAMPLE 3 
The same emulsified impregnating composition was made as described in 
Example 2, except that the amount of monomethylol dicyandiamide was 
increased from 3.77 g. to 6 g. To the resulting emulsion was added 0.093 
g. K.sub.2 CO.sub.3 in 0.186 g. H.sub.2 O, and 0.45 g. BDMA. No separation 
of the emulsion occurred after 20 hrs. The gel time of this emulsion 
measured 30 minutes at 153.degree. C. The viscosity of the emulsion ranged 
from 60 cps. to 66 cps. at 25.degree. C. Additionally, 0.65 g. of BDMA was 
added to the emulsion without separation. The gel time was remeasured and 
found to be 20 minutes. These emulsions provided useful epoxy impregnating 
compositions. 
EXAMPLE 4 
The following ingredients were mixed as described in Example 2, to provide 
Samples A through F, to determine the effect of concentration of Triton 
X-305 nonionic surface active agent on emulsion preparation: 
______________________________________ 
Amount, Grams 
A* B* C D E F 
______________________________________ 
Epon 829 
100 100 100 100 100 100 
TBBPA 51.43 51.43 51.43 51.43 51.43 51.43 
Tap H.sub.2 O 
100 100 100 100 100 100 
Mono- 6 3.77 6 3.77 6 3.77 
methylol- 
Dicyan- 
diamide 
Triton 14 17.3 21.6 28.1 30 32.4 
X-305** 
(Triton (9.8) (12.1) (15.1) 
(19.7) 
(21.0) 
(22.7) 
solids) 
______________________________________ 
*Comparative sample 
**70% aqueous solution 
Emulsions were prepared from all of the above Samples as described in 
Example 2, and the following observations were made regarding emulsion 
quality, stability, and viscosity as set forth in TABLE 1 below: 
TABLE 1 
______________________________________ 
Emulsion 
Quality** Stability 
______________________________________ 
A* Poor None. Separated into layers 
E Excellent &gt;1 day. Homogeneous 
C Excellent &gt;1 day. Homogeneous 
D Excellent &gt;1 day. Homogeneous 
B* Poor &lt;5 hrs. Separated into two layers 
F Excellent &gt;1 day. Homogeneous 
______________________________________ 
*Comparative sample 
**Based on color, stability and viscosity. 
As can be seen, the addition of Triton X-305 solids at 9.8 and 12.1 parts 
by weight per 100 parts epoxy yielded poor results. The viscosity of the 
emulsions was measured using a Brookfield viscometer at 25.degree. C.: 
______________________________________ 
Emulsion Viscosity Range, cps. 
______________________________________ 
A* -- 
B* -- 
C 60 to 100 
D 60 to 120 
E 60 to 130 
F 200 to 500 
______________________________________ 
These emulsions provided very fluid impregnating compositions. 
EXAMPLE 5 
Emulsion Samples C, D, E, and F, prepared in Example 3, were catalyzed by 
adding BDMA and K.sub.2 CO.sub.3 in the following amounts: BDMA solids 
0.45%, K.sub.2 CO.sub.3 0.093%, and H.sub.2 O 0.186%. These percentages 
are based on the total weight of the epoxy resin. K.sub.2 CO.sub.3 was 
dissolved in H.sub.2 O before adding it to the emulsion. After adding 
these catalysts, the stability of the emulsion was recorded. No separation 
of the emulsion was observed after 20 hours. The gel times at 153.degree. 
C. of these emulsions were measured and ranged between 27 to 30 minutes, 
showing good setup properties. 
EXAMPLE 6 
In this experiment several different nonionic surface active agents were 
used to determine their effectiveness in emulsion formation. The resin 
composition was as follows: 
______________________________________ 
Epon 829 100 g. 
TBBPA 51.43 g. 
Tap H.sub.2 O 100 g. 
Monomethylyol- 6 g. 
Dicyandiamide 
______________________________________ 
The nonionic surface active agents (described by tradenames and in relation 
to Formula (I) hereinbefore described), were added as follows: 
______________________________________ 
Y in For- Z in For- Grams 
mula (l) mula (l) Series (Solids) 
______________________________________ 
Triton 1 30 Octyl 15.1 
X-305 (A = 8 carbons) 
Triton 1 40 Octyl 15.1 
X-405 (A = 8 carbons) 
Triton 1 70 Octyl 15.1 
X-705 (A = 8 carbons) 
Triton 1 40 Nonyl 15.1 
N-401 (A = 9 carbons) 
Tergitol 
1 40 Nonyl 15.1 
NP-40 (A = 9 carbons) 
Tergitol 
1 4 Nonyl 15.1 
NP-4* (A = 9 carbons) 
______________________________________ 
*Comparative sample 
The procedure used to prepare the emulsions was the same as described in 
Example 2. The emulsion characteristics with the various emulsifiers are 
described in TABLE 2 below: 
TABLE 2 
______________________________________ 
Emulsifier Emulsion Properties 
______________________________________ 
Triton X-305 
Stability &gt;24 hrs. 
Viscosity, 23.degree. C. = 40 to 120 cps. 
Triton X-405 
Slight separation of layers. Restirred 
manually to form homogeneous emulsion. 
Viscosity, 23.degree. C. = 300 cps. 
Triton X-705 
Homogeneous emulsion. No separation 
after 24 hrs. 
Viscosity, 23.degree. C. = 1,100 cps. Initial 
foam formation which dissipated after 
a short time 
Triton N-401 
Stability &gt;24 hrs. 
Viscosity, 23.degree. C. = 800 cps. Initial 
foam formation which dissipated after 
a short time. 
Tergitol NP-40 
Stability &gt;24 hrs. 
Viscosity, 23.degree. C. = 800 cps. 
Tergitol NP-4* 
No emulsion formed. 
______________________________________ 
*Comparative sample 
Additional comparative emulsifier samples, sorbitan laurate, a lipophilic 
nonionic surface active agent (sold commercially by ICI United States 
Inc., Atlas Division, under the trade name Span 20); polyoxylthylene 
sorbitan laurate, a hydrophilic nonionic surface active agent (sold 
commercially by ICI United States Inc., Atlas Division, under the trade 
name Tween 20), and mixtures thereof were tried using comparable amounts 
as described above, and the products could not be emulsified. These 
additional comparative emulsifiers have a formula completely different 
from that shown in Formula (I). 
EXAMPLE 7 
Using the resin prepared as described and catalyzed in Example 2, glass 
cloth (Style 7628 A-100) was impregnated therewith, and then "B" staged 
for 7 minutes at 155.degree. C. in a convection oven, to form a prepreg. 
The resin content of the glass cloth ranged from 35% to 50%. A laminate 
was prepared by pressing an 8 ply stack-up of the treated glass cloth 
prepregs (6".times.6") at 420 psi, and at from 160.degree. C. to 
180.degree. C., for 55 minutes. The laminate produced was a unitary bonded 
laminate, that was homogeneous in color with good impregnation of the 
glass cloth by the resin. Both single and double side copper clad 
laminates were prepared using this same procedure by adding 1 oz./sq.ft. 
copper foil exterior sheets to the stack-up. 
EXAMPLE 8 
Laminates of emulsion impregnated glass cloth were made from nine 
12".times.18" plies of prepreg and one exterior sheet of 1 oz./sq.ft. 
copper foil about 1 mil thick. The glass cloth and laminating procedures 
were similar to that used in Example 6. The emulsified impregnating 
composition was similar to that used in Example 2, except that 5 g. of 
monomethylol dicyandiamide was used, and 82 g. of tap water was used, to 
give a 65% solids composition. Samples were tested using the standard test 
procedures for flame-resistant FR-4 laminates. The results are reported in 
TABLE 3 below: 
TABLE 3 
______________________________________ 
Dielectric 
Breakdown, Dielectric 
Dissipation 
kV* Constant** 
Factor*** 
______________________________________ 
50 5.23 0.0211 
______________________________________ 
*NEMA Standard = &gt;45. 
**NEMA Standard = &lt; 5.4 
***NEMA Standard = &lt;0.035. 
As can be seen, the copper clad circuit board type laminates are well 
within NEMA standards. Additionally, a piece of laminate was immersed into 
molten solder (260.degree. C.) for 30 seconds with no blistering or 
flaming.