Types Of Rubber Footwear Manufactured Today Biology Essay

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Fabric tops are not limited to cotton ducks, black, white or brown. Today, patterns and colors of all descriptions in many of the new synthetic fibers are now utilized. Sneakers and casuals have gained widespread use in the replacing of leather shoes not only for summer wear, but for all seasons. The selection of last shapes and widths that are offered are similar to those of leather footwear. Orthopedic features such as rigid supporting counters, steel shanks, and arch supports, combined with lightness in weight and washability make them even more practical than leather for many uses, particularly in the field of women's footwear where this type offers better protection than the thin soled, low slung ballet type shoes.

In the waterproof and protective footwear field we have a similar diversification. The development of new polyesters and chemicals has enabled the industry to engineer their products for recreation, business wear and everyday living. The synthetics have produced footwear resistant to solvents, static cracking, heat and light. A full rainbow of colors are produced to meet the consumer requirements for style and utility.

One of the most recent developments in the footwear industry is the automatic rotary table rubber injection molding machine. This process is a combination of injection and compression molding.

At the beginning of each cycle, the whole injection unit moves forward axially, forcing the preheated compound through the nozzle. Simultaneously, the injection unit retracts so that a layer of compound is distributed over the length of the lasted shoe. The volume of compound

distributed in each part of the mold cavity is controlled by varying the speed of retraction. When the injection unit is sufficiently retracted, the sole mold is closed, and the clamp pressure is applied. The reciprocating screw then moves backward, masticating the amount of compound required for the next cycle, and the rotary table moves the next station in front of the injection unit. The cycle is from 2 to 5 minutes depending on the curing temperature. Although this machine was developed for PVC, it is being used with a variety of elastomeric compounds. The upper materials used include fabric, leather and plastic.

Compounding Requirements

The best polymer for handmade footwear is still Hevea rubber because of its excellent green tack and thermoplastic quality. These properties are most important since the lamination of parts is largely affected with hand rolling and a little plastic flow is needed to seal these joints to make them waterproof.

Since most of the parts of rubber footwear are not molded, the surface of the stock as it is calendered must be free from all defects and must be compounded so as to retain calender embossing in the operations that follow. Proper polymer combinations and filler selection, along with the correct balance of new gum to scrap will maintain this finish.

High styrene and other high melting point resins, organic colors, flock and some accelerators are often master batched in order to improve their dispersion when added to the final mix.

A proper balance maintained between the cure rate of the various parts of the shoe, particularly in heavy goods where there is a great variation in thickness between upper and outsole. "Started" soles will result if this rate

is not proper.

The acceleration system must permit cures at relatively low temperature and yet not cause scorching at processing or storage temperature. Thiazole type accelerators land themselves to footwear compounding since they yield safe cures with good plateau, Guanadines are effective secondary accelerators.


The manufacturing of rubber footwear involves the use of the same basic mill room equipment as in other rubber industries. Beyond this process however, there Is a multitude of operations such as fabric and rubber cutting, stitching, pre-cementing and fitting of various upper insole and outsole components. All of these parts in the various shoe sizes (a range generally from a child's 2 to men's 16) must arrive to the making conveyor in proper sequence for the final assembly.

Upper compounds are generally warmed up on feed mills along with cutting scrap. The ratio of new gum to scrap is determined by the plasticity requirements of the stock. Certain parts, such as unlined uppers can tolerate very little scrap, lest "sagging" take place during vulcanization. Soling stocks are warmed up on mills or in a Banbury along with cutting room scrap. All stocks are water cooled when calendered in order to obtain maximum shrinkage before the cutting operation. If shrinkage takes place after the cutting operation, distortion would result. In order to retain both surface clarity and tack, calendered upper stocks are run into hammocks, beaded reels and spools. Smaller parts such as toe caps, heel pieces and gum stays are cut using a toggle press cutting method. These parts are then laminated to fiber carrying boards or are booked. Soling is calendered and cut into three or four foot lengths and stored on cleated plywood or fiber boards in order to allow it to shrink completely before cutting. Larger gum parts such as boot legs are cut by hand with a hot knife at the time the rubberized lining fabric is laminated to it on a flat conveyor. Heavy parts such as outsoles are generally cut with a Wellman cutting machine.

This machine cuts a beveled edge so that the sole will lay at proper angle with the upper of the shoe with a minimum of strain at the edge. The soles are stored in special fabric leaved books to prevent distortion and pressing. All of these engraved gum parts are generally calendered on a (19"-30") four roll inverted L type calender in which the offset top engraved roll is removable to allow design changes. Foxings, bindings and other strips are cut into proper width at the calender using either a "self-cut" engraving roll or conventional wheel knives properly spaced, and are boarded or booked. Colored stripes are generally laminated to foxings at this point also. A 60" four roll lining calender is generally used to rubberize sheetings, ducks, net and fleece linings, friction coat fabrics, and also to sheet rag stock. Rag is a mixture of downgraded rubber stocks that are recompounded along with reclaim refined rubberized fabric scrap and whiting. This stock is stiff and is used for counters and insole parts.


Some of the basic fabrics used in footwear include, array, enameling, hose, boot and number ducks, osnaburgs, drills and twills, sheetings, nets and fleeces.

Additionally in the fabric shoes, we use various prints, corduroys, suedes, and hopsacking materials cotton, rayons, woolens, nylon, Dacron, sisal, hemp and many blends.

It is very important that these materials be free frcm copper and manganese and also that they have optimum amounts of sizing where required. Insufficient sizing in the backing fabrics will cause limpness and wrinkling when combined to the face fabric. An excess will cause lasting wrinkles in the making operation. An excess starch in the face fabric will also adversely affect the adhesion to the rubber foxing,etc.

The tennis upper fabrics are "combined" using SBR adhesive applied with a roller or doctor blade and subsequently laminated dry through a can dryer. Most uppers are two-ply although three-ply is also used to some extent in the direct molded tennis shoe. The amount and type of deposit will determine the hand of the upper. There must be a balance between adhesion and "breathability."

The face fabrics must be checked for wettability, crock resistance bleeding, washablity and adhesion. Some of these deficiencies can be compensated for by modification of foxing latex. Others like bleeding crocking and shrinkage can be remedied only by the fabric finisher Color stability in vulcanizing must be predetermined. Certain fabrics and braids cannot be vulcanized in an ammonia cure. Others may require lower than normal temperature.

Unlike waterproof uppers, tennis fabrics are plied up on a cutting table, generally 24 plies, 12 face down, 12 face up. These are cut with double arm clicker die presses so that each cut will produce 12 pair cuts. The cutting scrap is ground into flock for use as a stiffener in rag compounds.

Cements and Latices

Most rubber parts in both waterproof and term footwear are cemented at the calendaring operation with rubber cement. This cement is usually sheet rubber compounded with tackifiers and curatives, and dissolved rubber solvent. Tennis tops are machine cemented lasting. Lasted tennis shoes are dipped into compounded rubber latex to form a base on which to adhere the parts. Colo pigments, curatives, tackifiers and agents are used to obtain good adhesion and appearance.


Shoes arriving at the end of the making line are loaded onto monorai vulcanizer cars which are rolled into place after final inspection of the cure cycle.

Cure cycles will vary from 45 minutes to 1.5 hours depending on the type of footwear and the efficiency of the vulcanizer. In all cases 20 to 30 lbs. of compressed air is introduced to prevent blister formation from air entrapped between parts and also within heavy rubber part blowers circulate the air to minimize cold spots.

On heavy items like boots, a vacuum cure is run where air pressure is reduced inside the boot in order to compress parts and minimize the occurrence of pin holes at the seams.

If ammonia is to be used, three to five pounds of anhydrous ammonia is injected after the air has been introduced. Curing temperatures used vary from 260F to 300F. Ammonia cure produces a glossy tack free surface.

The advantage of "air" cures is that less pigmentation is required particularly for white compounds, as the ammonia has a yellowin effect. Shoes cured in this manner are slightly tacky to the touch. Multicolor braids and fabrics are often cured in air to retain brightness particularly if one or more of the colors react adversely with the ammonia.

Synthetic UpperUpper CompoundNatsyn80Air Dried sheets80Solprene 120520SBR 100920Hard Clay60Whiting60Whiting60Hard Clay60Light Process Oil13Pepton 220.25Velsicol X-306Process Oil15Zinc Oxide5Velsicol X-305Stearic Acid0.6Zinc Oxide5Anthicheck Wax0.15Stearic Acid0.5AGERITE SUPERLITE0.25Anthicheck Wax0.16DOTG0.5AGERITE SUPERLITE0.25ALTAX0.9DOTG0.5Sulfur2.15ALTAX1.0Sulfur2.25Total248.55Total249.91Sp.gr1.43Sp.gr


Brown Crepe SBR 1009

Hard Clay Pepton 22 Process Oil Velsicol X-30 Zinc Oxide Stearic Acid Retarder





90 10

1 5


85 0 5 3


1.25 0.15

3 6 6


Brown Crepe

High Styrene


SBR 1009

Hard Clay


Process Oil

Velsicol X-30

Zinc Oxide

Stearic Acid







25 20 85 80 15 10







Total Sp.Gr

204.70 1.35

Total Sp.Gr

310.30 1.46

Top Grade Molded Sole

Brown Crepe SBR 1703 Solprene 1205 Hard Clay Silene EF

Press Scrap (Ground)

Pepton 22

Process Oil

Velsicol X-30

Zinc Oxide

Stearic Acid






70 25 10 80 25 30

0.15 16 10







SBR 1507 Hi Sil 215

Diethylene Glycol Light Process Oil

Zinc Oxide Stearic Acid AGERITE SUPERLITE Retarder CAPTAX


100 40


10 2







Air Dried Sheets 100.00

Pepton 22 0.30

Rosin K 5.00

Zinc Oxide 10.00

Retarder 0.20


UNADS 0.02

ALTAX 0.35

Sulfur 2.00

Pine Tar 1.00

(Dissolve in Rubber

so vent)

Soles and Heels

The last decade has witnessed revolutionary technical changes in the shoe industry. A result of this has been a gradual but persistent decrease in the production of old style conventional sole and heels.

Sole and heel producers, however, have not lost position as a result of this. Annual tonnage of their products has increased substantially due to the ingenuity of chemists and engineers in developing new materials and processes.

Today non-leather soles account for well over 70 per cent of shoe bottoms, as compared to 60 percent a decade ago. This increasingly high level of acceptance has been reached not only through demonstrated better service and comfort, but also through improved finished product appeal, a factor so essential according to current merchandising standards.

This steadily increasing acceptance of other than leather shoe bottoms indicates clearly how well the sole and heel industry has met challenge of these basic technological changes. The most important of these that need some elaboration are as follows:

Direct Molded Footwear

Unisoles - Nuclear and Polyvinyl Chloride

Printed Soling Sheets

Micro-Cellular Soling-Cushion-type and Firm (LeatherĀ­like)

A. Direct Molded Footwear (DMF)

This process involves the molding of rubber soles and heels directly to the shoe upper. Conceived in the early 1900's in Germany direct molded or vulcanized footwear did not reach actual production proportions until the depression of the 30s in Europe - in the United States in the mid '50's.

DMF production continues to show annual increases, in women's casuals, tennis, basketball shoes and sneakers. Also to some degree this process has been adopted by the U.S. Army.

Efficient, economical operation of expensive DMF equipment demands much faster curing cycles than conventional soles and heel. In addition to providing cycles of two minutes at 300 F. DMF compounds must be fast and free-flowing, without any incipient scorch which would preclude a strong permanent bond to the shoe upper.

Fast cures are obtained by a judicious combination of ALTAX and METHYL or ETHYL 21 MATE, or AMAX, CAPTAX and UNADS. Free flowing compounds require liberal amounts of oils and plastizers, such as light process oil, REOGEN and PLASTOGEN.

B. Unisoles

This is the term given to integrally-molded sole and heel combinations made in exact sizes and widths, once this unit is cemented to the shoe upper, it is ready for the shoe box, which eliminates many labour steps of finishing, heel attaching, inking, etc., for the shoe manufacturer.

Since these soles have to have the same "heft" or gauge on the edge as the sole, plus welting, of a stitched shoe they also result in a longer waring bottom. This means a higher quality shoe at lower cost, a tough combination to beat.

Unisoles, produced out of both rubber-resin compounds and polyvinyl chloride have been highly successful in juvenile shoes, and in men's shoes to a lesser degree.

Unisoles have become a highly significant, permanent fixture in the shoe industry, replacing a large amount of conventional soles and heels.

C. Printed Soling Sheets

Over the years, with steadily increasing labor costs in the sole and heel industry, there has been a trend to soling sheets rather than individually molded soles, in the thinner guages.

Since women's shoes constitute over 50% of shoe production and since soles for women's shoes are thin-gauge, this has meant a steadily increasing production of soling sheets .

Within the past few years, soling sheets, printed to give the appearance of a high-grade leather sole, have been imported from Germany. Acceptance by shoe manufacturers was immediate and sole and heel manufacturers procured equipment to duplicate these types of finishes.

The soling sheets are first printed pith rotogravur

rolls engraved to give a leather like pattern. In a continuous process, the sheet is then covered with a clear film of topcoat which can be formulated to give various levels of gloss. The topcoat is usually urethane or epoxy resin, which, when cured, makes the prints impervious to cleaning solvents used in the shoe factory.

This pre-finishing of soling sheets results in substantial savings to the shoe manufacturer, since the extra cost to hire for the pre-finished soling sheet is considerably less than his own cost of finishing the bottoms. The shoe manufacturer also ends up with a uniform, more pleasing appearance on his shoe bottoms.

DĀ» Micro-Cellular Soling, Soft (Cushion-Like) and Firm


Soft Cushion-Like

This type soling was first produced in the early '50's and gained immediate acceptance. Used in heavier gauges, this Micro-Cellular Soling gave a soft, cushiony, resilient walk to shoes of all types.

It was first predicted to be a more or less cyclical product but instead has become a permanent fixture it styling of shoes. It is estimated that 15% of all manufactured today carry these soft Micro-Cellular Soles.

Initially, this type soling was made only in sheet form but recently it has been produced in the heavier gauges in integrally molded sole and heel combinations which have become very popular on men's hunting and work boots.

This integrally molded shank effect results in a perfectly flat bottom usually with a rib design.

These soles are initally cured in design molds followed by oven treatment to eliminate residual shrinkage . Soles are then die-cut to a standard size.

Firm (Leather-Like) Micro-Cellular Soling

This type of soling has been a recent development in soling sheet and has been hastened by the desire of the shoe manufacturer for lighter weight in his shoe bottom.

Whereas the soft, cushion type is expanded to specific gravity of less than .70, the firm type is only partially expanded to a specific gravity range of 0.9 to 1.10.

There has been considerable usage of this firm cellular soling to date ,and it is predicted that it will continue to grow.

^"tirade e**i

Jan Standard


SBR 1507 ~ _ N M Black

Budene SO

SBR 177S 50

WH Tire Reelain - 40

Black MB 1805 17

Zinc Oxide 11


Zeolex 23

Zeo 45 * 3


Mapico Red 3 40

HAF Black 50


Stearic Acid "

Resinex 100 REOGEN

Panarez 6-210 15 ---

Carbowax 2


Light Process Oil 12 10

ALTAX 2 1.75

DOTC 1.50 1

Sulfur 2:50 ______2.50

1.19 1.35

Sd . Gr

""Total ~7 ^216.50 281.25