Permanently antistatic polyacrylate articles and process for making them

Permanently antistatic polyacrylate resin articles, antistatic polyacrylate resin compositions, and process for making them are described. The antistatic articles and compositions are obtained by reacting pendant ester groups on the surface of polyacrylate resin articles or on the chains of the polyacrylate resin compositions with dialkanolamines. The dialkanolamines used are typically dialkanolamines with lower alkanol groups, i.e. groups containing 1 to 4 carbon atoms. A preferred polyacrylate resin composition is poly(methyl methacrylate). A preferred alkanolamine is diethanolamine.

BRIEF SUMMARY OF THE INVENTION 
Permanently antistatic polyacrylate resin articles, antistatic polyacrylate 
resin compositions, and process for making them are described. The 
antistatic articles and compositions are obtained by reacting pendant 
ester groups on the surface of polyacrylate resin articles or on the 
chains of the polyacrylate resin compositions with dialkanolamines. The 
dialkanolamines used are typically dialkanolamines with lower alkanol 
groups, i.e. groups containing 1 to 4 carbon atoms. A preferred 
polyacrylate resin composition is poly(methyl methacrylate). A preferred 
alkanolamine is diethanolamine. 
BACKGROUND OF INVENTION 
Polyacrylates, made by homopolymerizing acrylic and substituted acrylic 
ester monomers, are versatile resins. Of particular importance among them 
is poly(methyl methacrylate) made by free-radical polymerization of methyl 
methacrylate monomer as shown below. 
##STR1## 
Poly(methyl methacrylate) is used to make the socalled "organic glass" 
tradenamed Plexiglas.RTM. plastic by the Rohm & Haas Company. It is a hard 
and fairly rigid material which can be sawed, carved, or worked on a 
lathe. When heated above its glass transition temperature, poly(methyl 
methacrylate) is a tough, pliable, and extensible material that is easily 
bent or formed into complex shapes that can be molded or extruded. It 
finds many applications in which its shatter-resistance, high optical 
clarity, and workability make it preferred over glass. Among its important 
applications are airplane windows, electronic displays, safety shields, 
signs, see-through cabinet doors, automotive taillights, complex camera 
lenses, magnifiers, and reducers. 
Though poly(methyl methacrylate) is probably the most prominent member of 
the acrylate polymers, other acrylates also find a wide range of 
commercial applications such as in coatings, adhesives, textile and paper 
sizes, etc. The choice of the polymer composition depends upon the 
intended use. Thus acrylic esters of higher alcohols impart softness or 
rubbery character, and the alpha-methyl group in methyl methacrylate 
imparts stability, hardness, and stiffness to the resultant polymers. 
Copolymers of acrylic esters with other ethylenically unsaturated monomers 
such as styrene, acrylonitrile, and vinyl acetate can also be made and 
they offer interesting properties and applications. 
Versatility not withstanding, the polyacrylate resins suffer from one major 
disadvantage--they are very much prone to develop static when rubbed 
against other materials including air. Though static accumulation is by no 
means an unique property of polyacrylates, because all plastics by virtue 
of their insulative properties by and large are susceptible to static 
buildup, the polyacrylates are among those that are most susceptible. Thus 
the surface resistivity of poly(methyl methacrylate) is 1.times.10.sup.17 
to 2.times.10.sup.18 ohms/square which is higher than that of other 
plastics which typically lie in the 5.times.10.sup.13 to 1.times.10.sup.15 
ohms/square range. 
Static presents many problems to plastics. Among them are dust attraction, 
electrical shock, fire hazard through spark generation, etc. Dust 
attraction by static in plastic is a major nuisance, but more than that, 
when one tries to wipe off this tenaciously adhering powder by buffing, it 
acts as an abrasive and scratches the surface of the plastic. Thus, one 
may start with a perfectly transparent Plexiglas.RTM. plastic object but 
with use, soon end up with one that is haxy and opaque. 
The recognition of the nuisance and other problems associated with static 
in plastic is not new and various approaches for controlling static 
buildup have been described in the literature. The most important of these 
approaches consists of treating the outer surface of the plastic with an 
antistatic compound, called an antistat in short. The Modern Plastics 
Encyclopedia lists 170 commercial antistatic additives. Another approach 
involves adding conductive pigments such as carbon black or metal fibers 
to the bulk of the plastic. 
The second approach is often undesirable and is particularly so with 
polyacrylates such as Plexiglas.RTM. plastic since it reduces 
transparency, aesthetic appeal, and deteriorates physical properties of 
the plastic. This approach is also undesirable because it increases bulk 
conductivity and as such minimizes one of the major attributes of plastic 
over metal, namely its insulative property. 
The major problem with the use of topical antistats, on the other hand, is 
that the treatments are nondurable and the antistatic protection is lost 
in the course of normal use of the article or when the articles are 
washed. 
In certain instances the antistats are directly incorporated into the bulk 
of the polymer to impart permanent antistatic properties. An example is 
the use of lauryl diethanolamide, the structure of which is shown below, 
##STR2## 
as an internal antistat in polyethylene film used for electronid 
packaging. 
Internal antistats are used in relatively large concentrations and they 
work by gradual migration to the surface to form a thin equilibrium layer 
on the surface of the article, due to their inherent incompatibility with 
the resin. Even though internal antistats are used to provide permanent 
protection against static, they do not always work reliably or as desired. 
For instance, when polyethylene films containing lauryl diethanolamide are 
washed with water, the antistatic protection is lost and does not reappear 
for a long time presumably due to the loss of the surface layer via 
washing and the relatively slow rate of migration that helps to form a new 
layer. 
Still another problem with internal antistats is that they act as 
plasticizers and as such deteriorate the physical properties of the 
plastic. Thus, with plastics like poly(methyl methacrylate) with 
relatively low glass transition temperatures (110.degree.-115.degree. C.), 
further decrease in softening temperature through plasticization is 
particularly undesirable. 
When optical clarity is of prime value, as is in the case of Plexiglas.RTM. 
poly(methyl methacrylate) plastic, the use of internal antistats is 
additionally undesirable because they reduce such clarity. 
THE INVENTION 
The present invention provides a process for rendering polyacrylate resin 
articles permanently static-free. More importantly, the permanent 
protection from static is achieved without sacrificing any of the bulk 
properties of the resin article. 
The resin compositions that are used in making the antistatic polyacrylate 
resin articles of this invention are polymers of acrylic or substituted 
acrylic esters or their copolymers with other ethylenically unsaturated 
monomers and are represented by the following formula: 
##STR3## 
wherein R is either hydrogen or an alkyl group containing from 1 to 2 
carbon atoms; R' is an alkyl group containing from 1 to 4 carbon atoms; 
R'' is phenyl, cyano, or acetoxy group; n.sub.1 +n.sub.2 =1 wherein 
n.sub.1 has a value ranging from about 0.2 to 1 to n.sub.2 has a value 
ranging from 0 to 0.8; and where x is an integer ranging from about 100 to 
about 10,000,000. 
As can be seen from the above description, the resins that are useful for 
this invention derive at least 20% of their chemical composition from an 
acrylic or substituted acrylic ester of the following formula: 
##STR4## 
where R and R' are as described above. 
A preferred resin composition is poly(methyl methacrylate). 
The polyacrylate resin articles contemplated in this invention can be a 
powder, pellets, or preformed objects made from the resin by rolling, 
molding, extrusion, cutting or other operations common to thermoplastic 
processing. An example of a preferred object is a commercial grade 
Plexiglas.RTM. plastic sheet of 0.16 to I5 cm thickness (Plexiglas is a 
registered tradename of the Rohm & Haas Company for poly(methyl 
methacrylate) sheets); an example of a preferred powder is 
commercial-grade poly(methyl methacrylate) granulated powder; and an 
example of preferred pellets are the commercial-grade 3.2 mm molding 
pellets of poly(methyl methacrylate). It is to be understood, however, 
that objects of other configuration, powders that are finer or coarser, 
and pellets of other sizes and of other polyacrylate resin compositions 
described herein before can also be used. 
According to the present invention, the polyacrylate resin articles of the 
above description are rendered permanently static-free by reacting them on 
the surface with dialkanolamines represented by the following formula: 
##STR5## 
wherein R.sub.1 OH and R.sub.2 OH are alkanol groups containing from 1 to 
4 carbon atoms. 
The most preferred alkanolamine is diethanolamine which has the following 
formula: 
##STR6## 
Another preferred dialkanolamine is diisopropanolamine and it has the 
following formula: 
##STR7## 
According to the present invention, the polyacrylate resin article and the 
dialkanolamine are reacted in the presence of excess of the dialkanolamine 
to provide adequate reaction on the surface of the article. Since the 
resin is not soluble in the dialkanolamine, very little reaction, if any, 
is believed to take place inside the bulk of the resin article. It is 
believed that the dialkanolamine reacts with some or all of the ester 
groups protruding out of those segments of polymer chains that are exposed 
to the surface of the article, resulting in its permanent modification. 
This is shown schematically as follows: 
##STR8## 
wherein R', R.sub.1 and R.sub.2 are as described before. 
It is not clearly understood why the surface modified polyacrylate resin 
composition does not hold static electricity. It is being speculated here 
that the alkanol groups hold a thin layer of water through hydrogen 
bonding and this conductive layer dissipates static charge as soon as it 
is developed as shown by the following schematic. 
##STR9## 
Since the reaction otherwise tends to proceed only slowly, it is often 
conducted in the presence of a catalytic amount of a basic catalyst such 
as alkali metal alkoxide which increases the reaction rate. Ordinarily, 
the alkali metal alkoxide contains from 1 to about 4 carbon atoms. 
Frequently the alkali metal alkoxide contains from 1 to about carbon 
atoms. Preferably the alkali metal alkoxide is alkali metal methoxide. The 
alkali metal alkoxide is frequently sodium alkoxide or potassium alkoxide. 
Sodium alkoxide is preferred. The particularly preferred alkali metal 
alkoxide is sodium methoxide. Other basic catalysts may be used when 
desired. 
The ratio of the weight of the basic catalyst introduced to the weight of 
the dialkanolamine is ordinarily in the range of from about 0.0001 to 
about 0.05, although greater ratios may be used when desired. Frequently 
the ratio is in the range of from about 0.0005 to 0.001. 
The reaction is obviously a heterogeneous reaction in which the insoluble 
resin article is either stirred or soaked in the liquid phase or exposed 
to the vapor phase of the dialkanolamine. 
The reaction is frequently conducted at temperatures in the range of from 
about 70.degree. C. to about 180.degree. C., although greater or lesser 
temperatures may be used when desired. Often the reaction temperature is 
in the range of from about 80.degree. to about 150.degree. C. In many 
cases, the reaction temperature is in the range of from about 80.degree. 
C. to about 120.degree. C. From about 80.degree. C. to about 105.degree. 
C. is preferred. 
The pressure under which the reaction is conducted may vary widely. The 
pressure may be below, at, or above ambient atmospheric pressure. In most 
cases, the reaction is conducted at about ambient atmospheric pressure. 
The reaction may be conducted continuously, semi-continuously, batchwise, 
or semi-batchwise as desired. 
Usually the reaction is a neat reaction, although substantially inert 
solvent may be used if desired. 
In most cases, the reaction is conducted under substantially anhydrous 
conditions. 
The surface-modified polyacrylate resin articles provided by this invention 
have permanent antistatic properties and find many uses. They are 
particularly useful for applications where static is undesirable, as is 
the case with most applications of such resins. The articles of this 
invention, when they are resin powders, for instance, can be handled with 
less annoyance and hazard in subsequent manufacturing steps than their 
static-prone counterparts. When these articles are preformed articles, 
they become less prone to dust pickup and, being substantially free from 
dust-related damage, provide longer useful life. Airplane windows, 
automotive taillights, etc. made from static-free surface-modified 
poly(methyl methacrylate) compositions of this invention retain their 
transparency longer since one major cause of loss of transparency in these 
cases is known to be dust-induced abrasive damage. 
The invention is further described in conjunction with the following 
examples which are considered to be illustrative rather than limiting.

EXAMPLES 1-5 
Preparation of Permanently Static-Free Poly(methyl methacrylate) Sheets 
First, an intimate mixture of diethanolamine and sodium methoxide catalyst 
was made as follows. 
250 g of diethanolamine was weighed into a three-necked flask fitted with a 
thermometer, a mechanical stirrer, and a drying tube. A solution of 0.25 g 
of sodium methoxide in 10 ml of anhydrous methanol was then added to the 
flask. The flask was heated in a heating mantle with stirring and the 
temperature was allowed to rise slowly to 100.degree. C. As the 
temperature increased, methanol escaped through the drying tube into the 
atmosphere. When no more methanol was escaping (approximately 30 min. at 
100.degree. C.), the mixture was cooled to room temperature and stored in 
a sealed bottle. 
To carry out the reaction, two 4".times.4" Plexiglas.RTM. poly(methyl 
methacrylate) plates cut from 2.5 mm thick sheets were dipped in 75 g of 
diethanolamine containing the sodium methoxide catalyst in a 6" diameter 
petri dish, which was placed on a 12".times.12" glass plate. This petri 
dish was then covered with a larger petri dish and the edge of the cover 
dish on the glass plate was sealed with silicone calking to ensure that no 
moisture could leak into the reaction chamber. The entire assembly was 
then heated in an oven set at a desired temperature. 
After the appropriate reaction time, the assembly was opened and the three 
plexiglas test plates removed from under the diethanolamine, washed first 
with cold, running tap water and then with distilled water. They were then 
wipe-dried with Kleenex.RTM. tissues, conditioned for at least 124 hours 
in the 15% relative humidity chamber, and tested for static buildup 
behavior. The reaction was repeated on separate plates at different 
temperatures for different periods of time. 
In all, three sets of this reaction were carried out--one at room 
temperature for 48 hours, one at 80.degree. C. for 6 hours, and one at 
100.degree. C. for 10 hours. These constitute examples 3 through 5. 
Examples 1 and 2 are untreated controls--example 1 representing a 
Plexiglas.RTM. plastic as received and example 2 is the same except that 
the sample was thoroughly washed with water before drying. Both control 
samples were also conditioned at I5% relative humidity for at least 24 
hours before testing for static. 
Measurement of Static 
The propensity for static buildup in the test plates was measured by using 
a modification of the popular cigarette ash test (F. H. Steiger, Text. 
Res. J. 28, 721, 1958). The latter consists of rubbing the object against 
another surface and then holding it over a shallow dish containing 
cigarette ash; the presence of static charge manifests itself by vigorous 
jumping of ash onto the charged surface. 
The modification adopted here consisted of replacing cigarette ash with 
fireplace ash that had been stored under 15% relative humidity for at 
least 24 hours before using. The objects used were 4".times.4" 
Plexiglas.RTM. poly(methyl methacrylate) test plates of examples 1 through 
5. 
Static was generated by rubbing the test plates against daily newspapers 
that were also conditioned at 15% relative humidity for at least 24 hours 
before using. Fifteen medium pressure twelve-inch strokes were used for 
each sample. The samples were immediately held over the fireplace ash 
contained in a shallow dish. 
Depending upon the quantity of ash picked up, the test plates were rated on 
a scale of 0 to 4, with 0 representing no static pickup of ash and 4 
representing heavy pickup. 
A similar test was conducted with screened and conditioned house dust 
collected from a home vacuum cleaner in place of fireplace ash to 
represent real life dust conditions. The propensity for dust pickup was 
again rated on a scale of 0 to 4. 
Results 
The results of static buildup in the Plexiglas.RTM. poly(methyl 
methacrylate) test plates are shown in the following table. 
______________________________________ 
Ash/Dust Pickup Tests 
(0 = no static, 4 = heavy static) 
Ratings: 
Sample # Reaction Temp./Time.sup.1 
Ash/Dust 
______________________________________ 
1 Control - No Treatment 
4/4 
2 Control - Washed With Water 
4/4 
3 Room Temp. (24.degree. C.)/48 Hrs. 
4/4 
4 80.degree. C./6 Hrs. 
1/1 
5 100.degree. C./10 Hrs. 
0/0 
______________________________________ 
.sup.1 With diethanolamine in the presence of sodium methoxide catalyst. 
As can be seen from the above table, the sample that was reacted at room 
temperature for 48 hours did not show any appreciable improvement over the 
controls, indicating that no or minimal reaction took place under these 
conditions. The sample that was reacted at 80.degree. C. for 6 hours 
showed an appreciable improvement in static elimination, and that which 
was reacted at 100.degree. C. showed a total elimination of static. 
The antistatic performance exhibited by 4 and 5 was permanent since the 
samples were thoroughly washed with water before drying, conditioning, and 
static measurement. 
Except for differences in ash/dust pickup behavior, the plates of examples 
1 through 5 looked and behaved the same way.