Polyvinyl chloride solvent weld primer composition

An improved solvent welding primer is described for use particularly in the assembly of polyvinyl chloride pipe and fittings to themselves or polyvinyl chloride pipe or fittings to acrylonitrile-butadiene-styrene pipe or fittings. The primer consists essentially of an organic solvent containing an amount within the range of from about 0.5 percent by weight to about 2.5 percent by weight of an unplasticized polyvinyl chloride resin dissolved in the solvent. The solvent is a mixture of tetrahydrofuran and dimethylformamide having a weight ratio of THF to DMF within the range of from about 1.64:1 to 1:1.

BRIEF SUMMARY OF THE INVENTION 
With the advent of plastic pipe and fittings as acceptable replacement for 
metallic and ceramic materials, new methods of joining such components 
have been developed. One such method of joining polyvinyl chloride (PVC) 
to itself or to acrylonitrilebutadiene-styrene (ABS) is the use of solvent 
welds following the recommended practices in ASTM D-2855 for PVC to PVC 
joints, and ASTM D-3138 for PVC to ABS. PVC resin requirements are listed 
in ASTM D-1784. Solvent cements are covered in ASTM 2564 for PVC, and for 
PVC to ABS in ASTM D-3138. All these ASTM references are incorporated 
herein by reference and made a part thereof. 
Forming joints of PVC by solvent welding requires reasonable care. PVC pipe 
and sockets are made by extrusion of hollow tubes of the PVC composition 
for a desired end use. Sockets are made on the end of a length of pipe by 
expanding one end with a die under heat and pressure which also imparts a 
slight taper on the socket bore to enable the pipe nose to be inserted and 
pushed down to an interference fit enabling the solvent welding materials, 
i.e., primer and solvent cement, to form a sealing bond between the pipe 
nose and socket. 
With the production speeds required today to produce an economically 
competitive PVC pipe product line, processing conditions of heat and 
pressure have been intensified to a point where the surfaces of the pipe 
and particularly sockets have become highly compressed and less 
susceptible to solvent attack, and in most cases, particularly for 
pressure uses, primers are mandatory for usable solvent weld joints. The 
primers, when applied to the part surfaces both penetrate and dissolve the 
coated area of the respective parts. The sockets, since they have been 
subjected to additional heat and pressure are very difficult to solvate 
sufficiently to form a leakfree joint. In addition, with presently 
available commercial primers, it is almost impossible to keep the pipe and 
socket in a consistent state of solvation long enough to complete a 
satisfactory joint in reasonable time under ambient conditions of 
temperature, wind and humidity in the field. This problem is aggravated as 
pipe diameters increase. Completed lines were subject to leakage before 
and after trench burial resulting in costly claims and repairs and loss of 
customers. Considerable other problems have arisen in actual practice when 
making solvent welded joints, such as the necessity of keeping the mating 
surfaces clean and dry while forming the joint. Moreover, since the 
presently available primers are composed of pure solvent, runs and sags of 
the primer coating occur causing uneven distribution and solvation of the 
welded elements. This leads to voids in the joint, starved welds and poor 
bonds which ultimately causes leaks in joints during line testing or 
actual service. 
The presently claimed invention was effected to overcome these deficiencies 
which have been overcome in actual field trials and in commercially 
fabricated pipe lines under various ambient conditions with temperatures 
from 40.degree. F to 110.degree. F, 100% humidity and strong winds of 
30-40 mph. 
It is therefore an object of this invention to provide a novel composition 
for ensuring the formation of improved solvent welded joints in plastic 
pipe construction. 
The object of this invention has been achieved by the discovery of an 
improved solvent welding primer composition consisting essentially of (1) 
an amount within the range of from about 0.5 percent by weight to about 
2.5 percent by weight of polyvinyl chloride homopolymer having an inherent 
viscosity of 0.80 - 0.95 and a specific gravity of 1.4 in solution in (2) 
a solvent mixture consisting essentially of tetrahydrofuran (THF) and 
dimethylformamide (DMF), the weight ratio of THF to DMF being within the 
range of from about 1.65:1 to 1:1. The preferred ratio is 1.11 THF:1.0 
DMF. All ratios tested were found to give much better field performance 
than presently available commercial primers. 
While use of cyclohexanone has been suggested in some of the ASTM 
specifications previously listed, it has been found that it has a tendency 
to have poorer release from the fabricated joints and cause excessive 
internal solvation resulting in weakening of the joint parts and is not 
considered as acceptable.

EXAMPLES 1-5 (% BY WEIGHT) 
The following are suitable solvent welding primer compositions for use with 
polyvinyl chloride pipe: 
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Example 
Ingredients 1 2 3 4 5 
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Tetrahydrofuran 51.7 51.2 52.2 64.1 49.3 
Dimethylformamide 46.8 46.3 47.3 34.4 49.2 
PVC homopolymer, 
(inherent visc. 0.90-.95) 
1.5 X X X 1.5 
PVC homopolymer, 
(inherent visc. 0.80) 
X 2.5 X X X 
PVC homopolymer, 
(inherent visc. 0.92-.98) 
X X 0.5 X X 
PVC homopolymer, 
(inherent visc. 0.87-.93) 
X X X 1.5 X 
(PVC - polyvinyl chloride) 
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The normal application of the solvent welding primer compositions of this 
invention is to apply one coat thereof to the spigot of the joint and two 
coats to the socket. The spigot then receives one coat of a suitable 
solvent cement on top of the primer. Solvent cement is not applied to the 
socket. The joint is usually completely in sixty seconds or less depending 
on size and ambient temperature and humidity conditions. 
While the preferred embodiments of the composition and process of this 
invention pertain to the solvent welding of polyvinyl chloride pipe, the 
scope thereof is intended to include the solvent welding of all 
thermoformed resin pipe systems by means of compatible resinated primers 
and solvent cements.