Method of making rubber

Rubber is made by mixing the rubber ingredients at elevated temperature in rubber mixing equipment to obtain a mixed rubber compound. The mixed rubber compound is cryogenically ground into a powder by the application of a cryogenic material into the grinder. A further ingredient is added to the mixed rubber compound powder in a mixer while the further ingredient and mixed rubber compound are in a dry condition to obtain a resulting mixture which is later vulcanized. In one embodiment the further ingredient is fragile ingredients. In another embodiment the method is used for making non-solvent contact adhesive.

BACKGROUND OF THE INVENTION 
Various techniques have been attempted to improve and expand the properties 
of rubber. It would be desirable if techniques could be provided which 
would permit the incorporation of or increase in the amount of certain 
ingredients that can be added to the rubber formulations to achieve 
distinct properties. It would also be desirable if rubber making 
techniques could be used for producing an adhesive without the need for a 
solvent. It would further be desirable if rubber making techniques could 
be achieved which permits lower vulcanization temperatures to be used. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide rubber making techniques which 
fulfills the above desires. 
A further object of this invention is to provide such techniques which can 
utilize conventional rubber mixing and forming equipment to achieve such 
desires. 
In accordance with this invention the basic rubber ingredients are mixed 
under conditions of heat in a conventional manner to obtain the desired 
mechanical strength. The mixed ingredients are ground in the presence of a 
cryogenic material to obtain a powdered mix. In one embodiment of the 
invention fragile ingredients are mixed with the powdered rubber and the 
mixture is then vulcanized. In another embodiment of the invention a resin 
reaction product is mixed with the rubber compound in the presence of a 
cryogenic to obtain the powder with the resultant rubber powder being 
components of a contact adhesive. The powder is applied to two substrates 
and then heated to activate the adhesives so that the substrates can be 
secured together when pressed into contact with each other.

DETAILED DESCRIPTION 
The present invention relates to the improvement in rubber making 
techniques as described in co-pending application Ser. No. 533,342 filed 
Sep. 25, 1995, now abandoned, all of the details of which are incorporated 
herein by reference thereto. The invention is based upon the recognition 
that prior attempts at improving rubber making techniques have had 
shortcomings which the invention overcomes. In particular, the invention 
modifies the conventional rubber making techniques by allowing the rubber 
to be made in powder form which allows the incorporation of added 
ingredients such as fragile ingredients which would not survive 
conventional rubber mixing and forming equipment or the incorporation of 
resin reaction products which would adapt the rubber to be used as a 
solvent-free contact type adhesive. 
Powdered rubber has been known since at least the early 1970's when its use 
was being promoted as an energy saving way to mix conventional rubber 
materials. The process never was widely used because the rubber parts 
produced using the powder mixing process did not have the mechanical 
strength which was obtained using the traditional mixing processes. Active 
development in producing homogeneous mixtures failed to provide materials 
of suitable physical strength. This was due to the failure to achieve the 
intimate association between the fillers and the rubber polymer. The 
reinforcement interaction of fillers with rubber is particularly evident 
with carbon black but is also evident to a smaller degree with non-black 
(mineral fillers). This reinforcement reaction appears to require heat and 
often shear to be accomplished. Heat is certainly not available during 
cryogenic mixing and the type of shear produced in conventional rubber 
mixing is not present in the cryogenic processes. 
The present invention utilizes conventional rubber making equipment wherein 
the basic ingredients are mixed under conditions of heat in suitable known 
mixers so as to obtain the desired mechanical strength. The mixed 
ingredients are then ground in the presence of a cryogenic material to 
form a powder. The advantage of utilizing a cryogenic material is that it 
makes the rubber ingredients brittle thereby readily permitting the rubber 
compound to be made into powder. Thus, the invention has as an advantage 
that the rubber compound is mixed in conventional rubber equipment under 
conditions of heat prior to being cryogenically ground and can then be 
made into a rubber material with mechanical properties typical of those 
obtained with conventional manufacturing procedures that do not use 
powdered rubber. One significant advantage of the new process is that 
after the rubber compound is powdered, fragile ingredients can be added 
which would not survive the conventional mixing processes. The addition of 
these ingredients provides rubber materials with physical properties 
previously unattainable from rubber. These properties include: 
unique electrical properties useful for producing materials for electronic 
shielding and microwave absorption (carbon fibers); 
very high extensive modulus produced from large amounts of chopped textile 
fibers (e.g. KELVAR.RTM.). These fibers can be added in quantities which 
would cause too high a viscosity build up in conventional mixers; 
light weight materials by the addition of glass or plastic miscrospheres; 
very durable materials by the addition of strengthening agents which are 
too abrasive to be mixed in conventional equipment. 
With the present invention it is possible to make sponge rubber containing 
fibers for added strength or improved electrical properties. When sponge 
is produced in conventional processes, the addition of fibers causes a 
concentration of the gas and blisters rather than a homogeneous cell 
structure. In accordance with the technology of this invention unexpanded 
plastic balloons are added to the powder and allowed to expand during the 
vulcanization process. 
With the invention it is also possible to make adhesives which do not 
require the use of solvents, especially neoprene contact adhesives. 
Adhesives based on neoprene have been used for over sixty years for 
providing fast strong adhesive bonds between many different substrates. 
The key to the rapid formation of these bonds is the interaction between 
neoprene and the product which results from the reaction of magnesium 
oxide and tertiary butyl phenolic resin. When two films of neoprene 
contact adhesive are placed together a high strength bond is formed almost 
instantaneously because the two films act as one without having to be 
melted together. Just the surfaces of the films need to touch for the 
strong bond to be formed. In almost all prior applications, the adhesive 
film has been deposited from a solvent because the adhesive is usually 
manufactured in a solvent and the solution of the adhesive is easy to 
apply by brush, spray or other liquid handling techniques. Activating the 
films using solvent or heat has been used since the early days of this 
technology. With the invention the unique response of these materials to 
heat activation has been utilized but employing it to produce a 
solvent-free adhesive has not been thought of before. 
For about thirty years researchers have been trying to develop a neoprene 
contact adhesive which does not require the use of solvents. The 
increasing awareness of toxicity problems and the fire hazards make 
solvent based adhesives very difficult to use today. A solvent-free 
contact adhesive product is achieved with the invention by using the 
powder process wherein the two basic components produced in a conventional 
way, then powdering the two components and mixing them as powders. 
However, the mixing operation can be carried out during grinding. 
In the various practices of this invention, the first step would be to mix 
the basic rubber ingredients using traditional mixing processes which 
involve heat in order to obtain the desired mechanical strength. The 
ingredients are then formed into a powder in the presence of a cold 
hardening agent. 
The cold hardening agent could be any suitable cryogenic such as dry ice or 
liquid nitrogen, with liquid nitrogen being the preferred cryogenic 
material. In accordance with this invention the grinding of the rubber 
takes place in the cold atmosphere to make the rubber brittle and to avoid 
the rubber massing back together. 
In one practice of the invention liquid nitrogen is sprayed into the 
grinder. It is preferable to pre-chill the rubber in liquid nitrogen just 
prior to being placed in the grinder. The amount of liquid nitrogen would 
depend on the type of grinder and the particle size that is desired which 
would be, for example, from 0.10 to greater than 100% by weight of N.sub.2 
which can be used. Usually, the amount of N.sub.2 needed is from 1 to 3 
times the weight of the rubber. 
Any suitable grinding equipment can be used. Preferably the grinder has a 
port or ports where the liquid nitrogen is introduced. The invention has 
been successfully practiced using an attrition mill. Other types of 
grinding equipment that could be used include hammer mills, pin mills and 
Fitz mills. 
Using powdered rubber allows the incorporation of, or the increase in the 
amount of certain desirable ingredients that can be added to rubber 
formulations to achieve unique properties. 
After the rubber is ground, the selected ingredient(s) is mixed into the 
powder and then vulcanized. In most cases, the rubber compound is mixed in 
conventional rubber processing equipment prior to grinding. 
The use of powder mixing allows the incorporation of fragile ingredients 
(e.g. micro-balloons) which would not survive conventional rubber mixing 
and forming equipment. 
Using the powder process also allows the incorporation of large amounts of 
certain materials (e.g. chopped fibers) whose concentration would be 
limited by viscosity buildup in conventional techniques. 
The orientation, or grain, imparted to rubber during conventional rubber 
processing can be eliminated or greatly reduced using powder mixing. 
Achieving random orientation of fragile materials (e.g. carbon fibers) in 
a vulcanized rubber part is also possible using powder. 
Examples of such desirable materials include polyaramid fibers (e.g. 
KEVLAR.RTM. fibers), carbon fibers, metal coated carbon or textile fibers, 
ceramic fibers, large amounts of nylon, cotton, polyester, etc. up to 200% 
more of the additives than the rubber, with 0.1-100% by weight being 
preferred. In addition, glass or plastic beads, and many other products 
that can not survive standard rubber manufacturing techniques can also be 
added. 
The use of such additional ingredients provides many desirable properties 
including high impact strength, abrasion resistance, controlled 
conductivity, controlled density, ability to float, weight reduction, 
enhancement of acoustical performance, enhancement of microwave absorption 
and increased flex resistance. Where, for example, KEVLAR.RTM. fibers are 
used, 5-200% by weight of the fibers compared to the rubber is added from 
an equal size to a much bigger size than the rubber with the size being up 
to 2 inches in length. The invention has been practiced grinding to 30 
mesh but can be either larger or smaller. 
A significant advantage of the present invention is in a reduction of the 
temperature needed for vulcanization. In the normal rubber vulcanization 
process the vulcanization of the rubber has been from 8 to 10 minutes at 
350.degree. F. The present invention permits the vulcanization temperature 
to be lowered to about 190.degree. F. which is significantly below the 
traditional minimum of about 275.degree. F. This is possible because 
vulcanization materials can be added to the powder that are too active to 
be added during the normal rubber processing. This allows low melting 
products such as plastic beads to be vulcanized into a rubber part. The 
rubber can be vulcanized in accordance with the invention, in practical 
times at these low temperatures in a range of, for example, 190.degree. 
F.-250.degree. F. The vulcanizing time could be in the range of 10 minutes 
to 2 hours. The thickness of the rubber part would determine the time and 
temperature requirements for such vulcanization. 
Where the invention is used to permit the incorporation of fragile 
ingredients the following steps summarize the practice of the invention: 
1. Mix all but fragile ingredients in standard rubber mixing equipment 
(high shear). This allows the normal reinforcement from carbon black and 
other fillers, as well as providing a homogeneous mixture. Any suitable 
known mixing equipment can be used. 
2. Cryogenically grind the mixed rubber compound into powder in the 
presence of partioning agent (i.e. talc). 
3. Mix the powdered rubber with the fragile ingredients in a low shear 
mixer while the fragile ingredient and rubber compound powder are in a dry 
condition. (A "V" blender can be used.) 
4. Vulcanize the resulting mixture in a standard rubber mold. The mold 
should be filled with the mixed powder in such a way as to minimize air 
entrapment and orientation of fibrous materials. A Teflon coated mold is 
preferred to using a metal surface mold which requires mold release. It is 
important to avoid high shear rubber processes after the fragile 
ingredients are added to prevent breakage. 
5. Other materials can be added in the powder mixing stage, such as: 
(a) highly reactive vulcanizing agents; 
(b) selected plasticizers to provide softer materials than can be processed 
on standard rubber equipment; 
(c) high concentrations of fibers and other materials which would cause too 
much viscosity increase to be processed in standard rubber equipment; and 
(d) abrasive materials which would damage standard rubber equipment. 
The use of powdered rubber also permits production of an adhesive without 
the need for a solvent. In this practice of the invention the adhesive 
ingredients are mixed as powders and then heated to form a film. This 
practice appears most useful (but is not limited to) a neoprene 
contact-bond adhesive. In this type of product, both substrates are coated 
and then bonded after heat activation. A flame sprayer can be used to 
apply the adhesive and to heat activate the bond when the substrates are 
combined outside of controlled factory conditions. 
When producing a neoprene contact-bond adhesive, as described above, the 
metal oxide reaction product of the p-tertiary butyl phenolic resin is 
made available in a dry (non-solvent) form. This can be achieved by 
obtaining reacted resin from the resin manufacturer, or by reacting the 
base resin and isolating the dry product. 
In the making of such an adhesive the powdered rubber ingredients would 
also be produced by grinding the rubber ingredients in the presence of a 
cryogenic. 
The following is an example of the composition of such an adhesive. 
______________________________________ 
PREFERRED 
RANGE 
______________________________________ 
Neoprene 100 100 
antioxidant 2 0-8 
magnesium oxide 
8 2-16 
tertiary butyl 
phenolic resin 45 10-200 
zinc oxide 5 0-20 
water 0.5 .1-5 
______________________________________ 
The above components would be prepared by 
(a) mixing with normal rubber equipment 
neoprene 
antioxidant 
zinc oxide 
sometimes 1/2 of the MgO 
(b) mixing in toluene (or other suitable solvent(s)) 
tertuary butyl phenolic resin 
magnesium oxide (sometimes 1/2) 
water 
Part (b) is dried (i.e. solvent evaporated) and added to part (a) during 
grinding or blended in after grinding. 
For large scale production part (b) could be obtained from a resin 
supplier, thus avoiding the need for a solvent. 
The advantage of this process is that the flammable, toxic, and expensive 
solvents are eliminated. 
Part (a) is mixed on a 2 roll mill or in an internal mixer. Part (b) is 
usually mixed overnight in a churn for production. 
In the laboratory, part (b) is mixed in a can on rolls overnight. This 
process can usually be carried out at ambient temperature. 
If the neoprene "contact bond" adhesive is made using standard techniques, 
part (a) and part (b) are mixed in a solvent after part (a) has been dry 
mixed. The adhesive film is then applied by brushing, spraying, roller 
coating, etc. the adhesive is applied to both substrates to be bonded. The 
adhesive films on the substrates can be brought together just before the 
films are completely free of solvent thus forming a "Bond upon Contact" or 
"Contact Bond". If the films dry completely they will not bond when 
contacted. To reactivate the bond the two films can be coated with a thin 
layer of solvent (solvent reactivation) or one or both films can be heated 
above 160.degree. F. (above 200.degree. F. is preferred) and the films 
will then bond when brought together. This procedure is called heat 
activation or heat reactivation. 
In using the powder process of this invention, the powdered adhesive is 
usually applied to both substrates. The adhesive can be applied and then 
heated to form a film. Infrared heaters are preferable but microwave 
heating could also be used. Microwave heating can be enhanced by adding 
microwave active materials to the adhesive, the simplest additive being 
carbon black. 
The adhesive film can also be applied and formed into a film 
simultaneously, using a powder flame spray technique. This technique is 
now used to apply thermoplastic coatings on metal. The powder is blown 
through a flame which melts the powder so that it forms a film when it 
hits the substrate. The substrates can be bonded before the adhesive film 
cools off (&lt;160.degree. F). Alternatively, the films on both substrates 
can be allowed to cool. Then one or both films on the two substrates can 
be heat reactivated or heat activated using the flame from a flame sprayer 
or any other means to raise the temperature above 160.degree. F. 
In practice, the films are usually heated above 200.degree. F. so they can 
be brought together before they cool below 160.degree. F. The two films 
could also be "solvent activated" although it is preferred to avoid all 
solvents if possible. 
The following summarizes the steps in the making of a non-solvent adhesive: 
1. Mix all the adhesive ingredients except the resin in conventional rubber 
mixing equipment (Banbury, mill, etc.) 
2. Prepare the magnesium oxide reaction product of the tertiary butyl 
phenolic resin (for neoprene contact bond adhesives only). This process 
can be carried out in bulk by the resin manufacturer. Alternatively, the 
process can be carried out in a small amount of solvent which can be 
recovered when the reaction product is isolated and dried. 
3. Cryogenically grind the adhesive compound to a fine powder-either alone 
or in the presence of the resin reaction product or another type of resin. 
4. Mix the ingredients as dry powders. This step may not be necessary if 
all the ingredients have been ground together. 
5. Apply the powder to the two substrates to be bonded. 
6. Heat the powder on the substrates to form an adhesive film on each 
substrate. 
7. Bring the two films together to form the adhesive bond. 
In conventional techniques, the adhesive compound and resin are mixed in 
solvent and applied to the two substrates in the solvent and allowed to 
dry before bonding. This conventional technique allows flammable, toxic 
and costly solvents to escape into the atmosphere. The use of flame 
spraying to apply the adhesive powder and form a film at the same time 
with the invention offers a practical method of combining materials on a 
large scale. 
Many other methods for applying the adhesive powder to the substrates and 
heating it to form a film can be used. The preferred method for any 
specific application will depend on the type, size and, shape of the 
substrates to be adhered. 
A neoprene contact bond adhesive is most likely the major use of this 
invention, but it can be used for many other types of adhesives. 
As can be appreciated the present invention thus has wide application in 
expanding the properties of rubber products.